BANCROFT
LIBRARY
THE LIBRARY
OF
THE UNIVERSITY
OF CALIFORNIA
f A, Economic Geology, 56
Professional Paper No. 42 Series \
( B, Descriptive Geology, 65
DEPARTMENT OF THE INTERIOR
UNITED STATES GEOLOGICAL SURVEY
>J^
CHARLKS I). \VALCOTT, DIRECTOR
GEOLOGY
OF THE
1WOPAH MINIM DISTRICT, NEVADA
BY
1 SFtJRR
WASHINGTON
GOVERNMENT PRINTING OFFICE
1 it 5
CONTENTS.
Page.
LETTER OF TRANSMITTAL 19
OUTLINE OP PAPER 21
INTRODUCTION 25
Location 25
Topography 25
Discovery 25
Development 26
Treatment of ores 28
Water supply 28
Fuel and power 28
CHAPTER I. GENERAL GEOLOGY 30
Description of the rock formations 30
Pre-Tertiary limestone and granite 30
Tertiary lavas 31
Andesi tes 31
Earlier andesite (hornblende-biotite-andesite) 31
Appearance 31
Original composition 31
Present altered condition 31
Location 32
Later andesite (biotite-augite-andesite) 33
Appearance 33
Composition and alteration 33
Location 34
Relation to earlier andesite 35
Distinction from earlier andesite 35
Rhyolites and dacites 36
Interrelation of rhyolites and dacites 36
Simultaneous eruptions 36
Heller dacite 37
Location 37
Age of Heller dacite 38
Nature of Heller dacite 38
Microscopic characters 39
Fraction dacite breccia 39
Location 39
Thickness 39
Conditions of eruption 39
3
4 CONTENTS.
CHAPTER I. GENERAL GEOLOGY Continued. Page.
Description of the rock formations Continued.
Tertiary lavas Continued.
Rhyolites and dacites Continued.
Fraction dacite breccia Continued.
Relative age 40
Microscopic characters 40
Tonopah rh yolite-dacite 41
Appearance 41
Microscopic characters 41
Alteration near contacts 41
Distinction between the northern and the southern areas 42
Age and origin 43
Brougher dacite 44
Location 44
Volcanic necks 44
Contact phenomena 44
Dikes from main masses 44
Included basalt : 45
Vestiges of cinder cones 45
Formation of the present Brougher dacite 46
Flow structure and other phenomena 46
Faulting due to Brougher dacite eruptions 47
Tuff dikes near contacts 47
Mineral composition 48
Oddie rhyolite 4!)
Location 49
Contact phenomena of Oddie-Rushton neck 49
Contact phenomena of Ararat neck 49
Smaller necks 50
Relative age of Oddie rhyolite 60
Mineral composition 50
Latest rhyolite or dacite 51
Location 51
Age and origin 51
Mineral composition 51
Siebert tuff (lake beds) 51
lacustrine origin 51
Size of the lake 52
< )rigin of lake basin 52
Thickness of sedinu-nti 53
Conditions during deposition 53
Explosive eruptions of the lake period 54
Uplift terminating lake period 54
Basaltic eruptions 55
Regional tilting accompanying uplift 55
CONTENTS. 5
CHAPTER I. GENERAL GEOLOGY Continued. Page.
Description of the rock formations Continued.
Tertiary lavas Continued.
Basalt 55
Location 55
Relations and composition of basalt of Siebert Mountain 56
Age 56
Chemical composition of lavas 56
Transitions in silica content 56
Analyses of Tonopah lavas 57
Chemical composition of the dacite-rhyolite series 57
Differences and relations 57
Comparison with Eureka and Washoe dacites and rhyolites 58
Retention of the term dacite 58
Rhyolitic nature of both dacites and rhyolites 59
Determination according to a quantitative classification 59
Varying composition in different vents 60
Theory of differentiation of Tonopah lavas from a uniform type. 61
Pseudomorphs in rhyolite 61
Character of pseudomorphs 61
Magmatic origin of pseudomorphs 62
Hornblende in Tonopah lavas 62
Derivation of rhyolite and basalt from intermediate magma 63
Statement of theory 63
Rhyolite-basult differentiation theory tested by analyses 63
Complementary nature of dacites and later andesites 64
Statement of differentiation theory 66
Summary of geological history 66
Age of the rocks at Tonopah 68
Place of Tonopah lavas in (ireat Basin volcanic history 68
Probable Neocene age .". . . 69
Infusoria in the Siebert tuffs 69
Comparison of Siebert tuffs with Miocene Pah-Ute lake deposits 70
Conclusion 71
Principles of faulting 72
Criteria of faulting 72
Siebert tuff boundaries 73
Dikes along fault-zones 73
Boundaries of lavas 74
Erosion fault-scarps 74
Scarp phenomena west of Brougher Mountain 75
Description of zigzag scarps 75
Zigzag scarps explained by faulting 77
Consequences of explanation 77
Zigzag fault-scarp on Tonopah-Sodaville road 77
Origin of zigzag fault-scarps 78
Origin of zigzag faults 79
6 CONTENTS.
CHAPTER I. GENERAL GEOLOGY Continued. Page.
Principles of faulting Continued.
Intrusions controlled by intersecting fractures 79
Corroboration of conclusions 79
Accuracy of fault mapping 80
Faulting due to volcanic action . 80
Application of principles beyond map 80
Suggested explanation of Great Basin Tertiary deformations 81
End of volcanic epoch 82
CHAPTER II. MINERAL VEINS 83
Veins of the earlier andesite 83
Period of mineralization 83
Nature of circulation channels 83
Veins due chiefly to replacement 84
Portions of veins due to cavity filling 85
Cross walls and ore shoots 85
Nature of mineralizing agents 85
Primary ores 86
Locality 86
Composition 86
Minerals 86
Quartz 86
Adularia 86
Sericite 87
Carbonates 88
Silver sulphides 88
Silver chloride 88
Chalcopy rite 88
Pyrite 88
Galena 88
Blende 88
Gold 89
Analyses of primary sulphide ores 89
Summary of vein minerals 90
Oxidation 90
Depth of oxidation 90
Cap-rocks as protection from oxidation 90
Silver chloride in oxidized zone of veins 91
Analysis of oxidized ore 92
Comment on the ore analyses 92
Formation of gypsum by oxidizing waters 94
Secondary sulphides 94
Pyrite, argentite, and native silver 94
Argentite, polybasite, and chalcopyrite in druses 95
Comparison of secondary sulphides at Neihart and Tonopah 95
Evidence favoring secondary deix>sition of sulphides by descending waters 96
CONTENTS. 7
CHAPTER II. MINERAL VEINS Continued. Page.
Veins of the Tonopah rhyolite-dacite period 96
Characteristics of Tonopah rhyolite-dacite veins 97
Age of Tonopah rhyolite-dacite veins 99
General restriction of veins to rhyolite-dacite 99
Effect of waters producing the Tonopah rhyolite-dacite veins on earlier-formed veins. 100
The calcitic veins of Ararat Mountain 101
The rhyolite of Ararat a volcanic plug 101
Flow brecciation near contact 102
Fissure- veins in the rhyolite plug 102
Fissures due to movement after consolidation 104
Paragenesis of vein materials 104
Composition of vein-forming waters 104
CHAPTER III. PRESENT SUBTERRANEAN WATER 105
Water encountered in mining operations 105
Outcropping water-zones 107
Distribution and explanation- of water zones 107
Usual absorption of precipitation by rocks 107
CHAPTER IV. PHYSIOGRAPHY 109
Origin of the range of hills 109
Sketch of Tertiary and Pleistocene erosion 109
General features 109
Measures for the amount of material eroded 110
Features of erosion in arid climates Ill
Precipitation in region near Tonopah 112
Dependence at Tonopah of topographic relief upon rock resistance 113
Effects of faulting upon the topography 114
CHAPTER V. DESCRIPTIVE GEOLOGY OP MINES AND PROSPECTS 115
The known earlier andesite veins 115
Mizpah vein system 115
Mizpah vein 115
Extent of vein ; 115
Limitation of vein by Mizpah fault 115
Limitation of vein by Burro fault 115
Limitation of vein by Siebert fault 115
Vein structure 117
Effects of transverse premineral fractures 119
Cross walls 119
Branching veins 119
Origin of ore shoots 119
Post-mineral faults and fractures 122
Vein composition 122
Secondary nature of ore minerals 124
Eearrangement of values during oxidation 124
8 CONTENTS.
CHAPTER V. DESCRIPTIVE GEOLOGY OF MINES AND PROSPECTS Continued. Page.
The known earlier andesite veins Continued.
Mi/pah vein system Continued.
Geology of the Desert Queen shaft 125
Intrusive nature of rhyolite contact 125
Variable attitude of Mount Oddie intrusive contact 126
Mizpah vein in Desert Queen workings 126
Formations encountered in the lower workings 127
The Burro veins 127
Vein structure 127
Strength and extent of the Burro veins 129
Valley View vein system 129
The Valley View veins on Mizpah Mill 129
Cross veins and allied phenomena 129
Vein structure and origin 130
Ore in the vein 132
The Valley View vein system underground 132
Veins in the Valley View workings 132
The Valley View fault 133
Veins in the Stone Cabin workings 135
Veins in the Silver Top workings 136
The Stone Cabin-Silver Top veins a part of the Valley View vein group... 137
Correlation of veins in different mines 137
Effect of the Valley View fault 137
Hypotheses to explain fault movement 137
Amount of vertical separation of Valley View fault 138
Fraction No. 1 vein 140
Discovery and development 140
Nature and relations of the Fraction veins 140
The northeast (Fraction ) fault system 141
The northwest faults 144
Cause of faulting 146
Composition of vein 146
Fraction No. 2 workings 147
Rocks exposed in shaft 147
Faulted vein-fragment 147
Tonopah rhyolite-dacite 148
Wandering Boy veins 149
Relative elevation of fault blocks containing Valley View and Wandering
Boy veins 149
Relation of Valley View and Wandering Boy veins 149
Opposing dips of the veins probably original 150
Change of dip shown by comparison of the Valley View and the Stone
Cabin 150
Wandering Boy and Valley View conjugated veins.. 151
CONTENTS. 9
CHAPTER V. DESCRIPTIVE GEOLOGY OF MINES AND PROSPECTS Continued. ' Page.
The known earlier andesite veins Continued.
Valley View vein system Continued.
Wandering Boy veins Continued.
Outcrops of Wandering Boy veins 152
Representation of outcropping veins underground 152
Fault systems in the Wandering Boy 152
Displacement of the Wandering Boy fault 153
Cross faulting on the 300-foot level 154
Effects of cross faulting ideally considered 157
Application of principles to Wandering Boy cross-faults 161
The vein dip as a factor in the problem 162
Correlation of veins in Fraction and in Wandering Boy 162
Faults not corresponding to the main systems 163
Relative age of Fraction and Wandering Boy faults 163
Ore in Wandering Boy veins 163
Veins of Gold Hill .' 164
Gold Hill a fault block 164
Nature of Gold Hill andesite 164
Alteration of Gold Hill andesite 165
Enumeration of Gold Hill veins 165
Production of Good Enough vein 166
Vein structure 166
Gold Hill shaft 166
Tonopah and California workings 167
Section exposed in workings 167
California fault 167
Veins .- 167
Montana Tonopah vein system 167
Montana Tonopah mine 167
Absence of veins in the later andesite 167
Vein on the 392-fcot level 167
Branch vein on the 460-foot level 169
Connection of branch vein with Montana vein 170
Brecciated structure in the Montana vein 170
Crustification in the Montana vein 171
Conditions of formation of Montana vein 172
Faults on the 460-foot level 1 72
Veins on the 512-foot level 173
Easterly pitch of ore bodies 1 75
Tonopah rhyolite-dacite in the Montana Tonopah 175
North Star workings 177
Section passed through 177
Veins '. 1 78
Faulting 178
10 CONTENTS.
CHAPTER V. DESCRIPTIVE GEOLOGY OF MINES AND PROSPECTS Continued. Page.
The known earlier andesite veins Continued.
Montana Tonapah vein system Continued.
Midway workings 179
Later andesite in shaft 179
Typical early andesite in shaft 179
Tonopah rhyolite-dacite in shaft 179
Formation exposed by drifting 179
Veins in the Midway 180
Tonopah Extension mine 181
Contact of earlier and later andesites. .'. 181
Veins in the earlier andesite 182
Veins in the Tonopah rhyolite-dacite 183
Other exploratory workings, wholly or partly in early andesite 184
West End workings 184
Outcrop of West End fault 184
Rhyolite intrusion along fault 184
Character of andesite above 220-foot level 185
Character of andesite on 220-foot level 186
Correlation of andesites in West End and Fraction workings 186
Extension of correlation to the Wandering Boy and to Gold Hill 186
The West End andesite probably earlier andesite 187
Contact between earlier and later andesites 187
Place and character of contact 187
Nature of similar contacts elsewhere 187
Tonopah rhyolite-dacite 188
Earlier andesite at bottom of shaft 188
MacNamara workings 189
Later andesite at surface 189
Character of andesite on 200-foot level 189
Correlation of MacNamara and West End andesites 189
Contact of earlier and later andesites 190
Tonopah rhyolite-dacite and included veins 190
Explorations on veins at the contacts of the Oddie rhyolite 191
Wingfleld tunnel 191
Boston Tonopah shaft 192
M iriam shaft 193
Desert Queen shaft 193
Shafts at the unmineralized contact of the Oddie rhyolite 193
Hi-lmont shaft 193
Rescue shaft 194
Explorations on veins at the contact of the Tonopah rhyolite-dacite 194
Mizpali Extension shaft 194
I>ater andesite at top of shaft 194
Rhyolite and rhyolite-dacite in shaft 195
Veins at contact of Tonopah rhyolite-dacite 195
CONTENTS. 11
CHAPTER V. DESCRIPTIVE GEOLOGY OF MINES AND PROSPECTS Continued. page.
Explorations on veins at the contact of the Tonopah rhyolite-da'cite Continued.
Mizpah Extension shaft Continued.
Correlation of the rhyolitic rocks in the shaft 196
Age of the veins 197
King Tonopah shaft 19"
. Geological situation 19"
Vein materials 197
Nature of rock inclosing vein materials 197
Correlation of veins with other occurrences 198
Belle of Tonopah shaft 198
Geological conditions 198
Veins 198
Shafts at the unmineralized contact of the Tonopah rhyolite-dacite 199
Butte Tonopah shaft 199
Little Tonopah shaft 199
Shafts at the contact of the Brougher dacite 200
Big Tono shaft 200
Molly shaft 200
Shafts wholly or chiefly in dacitic tuffs 200
New York Tonopah shaft 200
Fraction Extension shaft 201
Geological section 201
Fault 202
Tonopah City shaft 202
Geological section 202
Indicated displacement of fault blocks 202
Ohio Tonopah shaft 202
Dacite tuffs in shaft 202
Later andesite in shaft 203
Solid Tonopah rhyolite-dacite 203
Characteristics of the rhyolite-dacite 204
Mineralization ., 204
Pittsburg shaft 204
Red Kock shaft 204
Shafts entirely or chiefly in later andesite 205
Halifax shaft 205
Golden Anchor shaft 205
CHAPTER VI. ROCK ALTERATION CONNECTED WITH MINERALIZATION 207
Alteration of the earlier andesite 207
Alteration of earlier andesite, chiefly to quartz, sericite, and adularia 207
Alteration of hornblende and biotite 207
Relations of pyrite and siderite 208
Alteration of soda-lime feldspar to quartz and sericite 208
Alteration of soda-lime feldspar to adularia 208
Alteration of the groundmass 209
12 CONTENTS.
CHAPTER VI. ROCK ALTERATION CONNECTED WITH MINERALIZATION Continued. page.
Alteration of the earlier andesite Continued.
Alteration of earlier andesite, chiefly to quartz, sericite, and r.dularia Continued.
Advanced stage of alteration 209
Occurrence of kaolin 209
Alteration of earlier andesite, chiefly to calcite and chlorite 210
Transitions between alteration phases of earlier andesite 210
Different alterations the effect of the same waters 210
Refractoriness of potash feldspars 211
Meaning of adularia and albite as gangue minerals 212
Study of typical specimens 213
Microscopic descriptions 213
Earlier andesite from lower part of Siebert shaft 213
Earlier andesite from Tonopah and California shaft 213
Earlier andesite from Fraction No. 2 shaft 214
Earlier andesite from near Mizpah Hill 214
Earlier andesite from near Mizpah vein 214
Typical earlier andesite from Mizpah Hill 215
Earlier andesite from hanging wall of Mizpah vein 215
Ore material of Mizpah vein 216
Analyses of described types 216
Differences of phases expressed by diagrams 217
Study of alterations indicated by analyses 217
Alteration of earlier andesite from lower part of Siebert shaft 217
Alteration of earlier andesite from California and Tonopah shaft 220
Alteration of earlier andesite from Fraction No. 2 shaft 221
Alteration of earlier andesite from near Mizpah Hill 223
Alteration of earlier andesite from near Mizpah vein 224
Alteration of typical andesite from Mizpah Hill 225
Alteration of earlier andesite from wall of Mizpah vein 225
Alteration of earlier andesite to vein material 226
Maximum alteration located along the vein zones 226
Composition of mineralizing waters in the vein zones 227
Relation of adularia to sericite as alteration products
Formation and occurrence of adularia 228
Conditions required for the formation of adularia 228
Adularia as a metamorphic mineral 229
Adularia in veins 229
Chemistry of the alteration of soda-lime feldspar to adularia 230
Formation and occurrence of muscovite 231
Conditions required for the formation of muscovite 231
Muscovite as an alteration product 231
Distinct conditions required for muscovite and for biotite 232
The sericitic variety of muscovite 232
- Fluorine neoewary to the formation of mica 232
Chemistry of the alteration of soda-lime feldspars to sericite 233
CONTENTS. 13
CHAPTER VI. ROCK ALTERATION CONNECTED WITH MINERALIZATION Continued. Page.
Alteration of the earlier andesite Continued.
Changes in rarer constituents during alteration of earlier andesite 233
Resume of effects of mineralizing waters 234
Changes in waters as a consequence of rock alteration 235
Propylitic alteration of earlier andesite 236
Final composition of mineralizing waters 237
Alteration of the later andesite 238
Study of typical specimens 238
Nearly fresh later andesite from Mizpah Extension shaft 238
Nearly fresh later andesite from Halifax shaft 239
Entirely altered later andesite from North Star shaft 239
Entirely altered later andesite from Montana Tonopah shaft 240
Analyses of described types of later andesite 241
Differences of composition expressed by diagrams 242
Comparison of later andesite with Washoe and Kureka rocks 244
Degree of alteration of freshest Tonopah later andesite 244
Principles of studying alteration of later andesite 245
Alteration of later andesite from North Star shaft 246
Alteration of later andesite from Montana Tonopah shaft 247
Siderite as an alteration product -48
Scarcity of epidote as an alteration product 250
Composition of altering waters 250
Period of alteration of later andesite 250
Alteration mainly antecedent to faulting 250
Relation of alteration to vein formation 251
Exudation veinlets in later andesite 251
Metalliferous veins in later andesite 251
Conclusion 251
Alteration of the Oddie rhyolite 252
CHAPTER VII. ORIGIN OF MINERAL VEINS 253
Origin of the mineralizing and altering waters 253
Antithesis between waters and associated rock 253
Theory of atmospheric origin of hot springs 254
Theory of magmatic origin of hot springs 254
Characteristics of Nevada hot springs 256
Coupling of hot and cold springs 256
The Devil's Punchbowl 257
Amount of present and recent hot-spring action . 257
Origin of extinct hot springs at Tonopah 258
Connection with volcanic eruptions 258
Consequences of antithesis between rocks and waters 258
Meaning of nature of metals in veins 258
Nature of solfataric action 260
Minerals deposited around fumaroles 261
Conclusions as to genesis of Tonopah ores 261
14 *V CONTENTS.
Page.
CHAPTER VIII. INCREASE OF TEMPERATURE WITH DEPTH 263
Method of. measurement 263
Temperatures in the Mizpah Extension and the Ohio Tonopah 263
Temperatures in the Montana Tonopah 264
Temperatures in Mizpah Hill workings 264
Thermal surveys on the Comstock 265
Comparison of Comstock and Tonopah data 265
CHAPTER IX. COMPARISON WITH SIMILAR ORE DEPOSITS ELSEWHERE 267
Veins of Pachuca and Real del Monte, in Mexico 267
Other similar mineral districts in Mexico 269
The Comstock lode 270
Silver City and De Lamar districts, Idaho 271
Relation of the described districts to Tonopah 273
The petrographic province of the Great Basin 274
Extension of the Great Basin petrographic province into Mexico 274
Probable still further extension of the Great Basin-Mexico petrographic province 275
A metallographic province coextensive with the petrographic province 276
Origin of shoots or bonanzas in the veins of this nietallographic province 276
Existence of a major Pacific Tertiary petro-metallographic zone 278
ILLUSTRATIONS.
PLATE I. Topographic map of Tonopah mining district:.
II. A, B, View from near eastern corner of area mapped on PI. I, looking southwest;
C, D, Tonopah and surroundings, looking west from Mount Oddie; E, F,
Panorama, looking south from Butler Mountain 24
III. Map of mining claims, adapted from map of Booker and Bradford, Tonopah 26
IV. Heller Butte '.. . 38
V. A, B, Butler Mountain; C, D, View, looking northwest from a point between Rushton
Hill and Golden Mountain; E, F, View north from Butler Mountain 44
VI. A, Brougher Mountain and Tonopah, seen from Mizpah Hill; B, Butler Mountain
from east base, showing columnar dacite above and stratified Siebert tuffs below... 46
VII. Diagram showing relative displacement of fault blocks and their relation to the dacite
necks 48
VIII. Diagram showing relative provinces occupied by the Brougher dacite and the rhyolite
necks, and the relation of the fault lines to the former 50
IX. A, Siebert Mountain from the northeast; B, Mount Oddie from the northwest 52
X. Face of Siebert Mountain from the southeast 54
XI. Geologic map of Tonopah mining district 56
XII. Diagrammatic map showing two parallel zigzag south-facing scarps 76
XIII. Fragment of Montana vein, actual size, showing crustification 84
XIV. Map showing the chief veins of Ararat Mountain and their restriction to the white
rhyolite plug 102
XV. A, Recent basaltic cone near Silver Peak; B, East front of Quinn Canyon Range,
showing wash apron typical of the region 112
XVI. Geologic map of productive portion of the Tonopah mining district, showing out-
cropping veins and regions developed by underground workings 114
XVII. Outcropping veins of Mizpah Hill 116
XVIII. Siebert shaft, Tonopah Mining Company 118
XIX. Horizontal plan of veins as developed in midsummer of 1903 on Mizpah Hill, on
the plane of the 300-foot level of the Mizpah 120
XX. Cross sections showing structure of Mizpah vein . 122
XXI. Horizontal plan of veins and faults in Wandering Boy and Fraction 300-foot
levels, together with projected position of the main Fraction and Wandering
Boy faults at this level 162
XXII. Horizontal plan of veins and faults in the Montana Tonopah 512-foot level 172
XXIII. A, B, C, Pyrite and siderite in Tonopah andesite; D, E, F, Adularia in early
andesite and veins 208
XXIV. Diagrams to show the changes in composition brought about by the alteration of
the earlier andesite 218
15
16 ILLUSTRATIONS.
Page.
FIG. 1. Vertical section of shaft about 1,600 feet east of Tonopah and California shaft,
showing Fraction dacite breccia and interbreccia tuffs 40
2. Vertical sketch section of trench just west of Brougher Mountain, showing Tonopah
glassy rhyolite-dacite, intrusive into Fraction dacite breccia 41
3. Vertical section of part of tunnel north of Brougher Mountain and southeast of
Ohio Tonopah shaft 43
4. Vertical sketch section showing contact of intrusive dacite with tuff, southwest base of
Butler Mountain 44
5. Vertical section showing contact of Golden Mountain dacite with Siebert tuff (lake
beds) east of Golden Mountain 45
6. Horizontal plan showing eddying in the cooling lava of a volcanic (dacite) neck;
plotting of strong flow structure on top of eastern shoulder of Golden Mountain. 46
7. Vertical sketch section of mud dike at dacite contact at a point on the east side
of Butler Mountain 48
8. Vertical sketch section taken at a point on the east side of Butler Mountain,
showing mud dike in lake beds 49
9. Vertical cross section of southeast side of Siebert Mountain, showing relations of
Siebert tuffs (lake beds), basaltic flow and agglomerates and Brougher dacite 53
10. Ideal cross section of Tonopah rocks 71
11. Cross section of water runway (usually dry) of Plate XII 76
12. Map showing outcropping veins of Tonopah 84
13. Rhyolitic veins (later period) in Tonopah rhyolite-dacite, 814-foot level, Desert
Queen shaft, showing irregularity and lack of persistence (horizontal plan) 98
14. Cross section of outcropping fissure vein in Ararat rhyolite neck near margin,
Reptile claim, north of Boston Tonopah shaft 102
15. Vertical cross section of outcropping fissure vein 20 feet west of section shown in
fig. 14 103
16. Vertical cross section of a portion of Mizpah vein as exposed in the Oddie shaft,
showing reverse dip near the surface 116
17. Vertical cross section of Mizpah vein along Brougher shaft and inclines 116
18. Vertical cross section of Mizpah vein, Oddie and McMann lease, showing diverging
walls 11"
19. Detail sections from Mizpah vein, showing the effect of pre-minend cross fractures. 119
20. Sections showing the splitting of the Mizpah vein 120
21. Diagram showing the distribution of the richer ores in Mizpah vein 121
22. Sketch of faulted quartz veinlets in andesite, 300-foot level, Mi/,pah, just south of
the Valley View shaft 122
23. Horizontal sketch plan of [>ortion of the Mizpah vein in stopes east of Lease ">:.'.
about 70 feet from surface, showing probable compensating faulting 123
24. Reproduction of drawing of model showing the principal post-mineral fractures and
faults olwerved in the Mizpah mine workings 123
25. Horizontal d'mgra mtic plan of Mi/.pah vein as exposed in the Oddie and McMann
lease, 20 to 30 feet Iwlow the surface 124
26. Horizontal plan of mine workings, showing the relation of the vein in the Desert
Queen workings to that on the corresponding level of the Mizpah mine 12ti
l!7. S.-.-tioiis slmwinu the structure of the Burro No. 1 vein l- s
ILLUSTRATIONS. 1 7
Page.
FIG. 28. Sections showing the structure of the Valley View veins 131
29. Cross sections of the Valley View vein 133
30. Vertical section on plane of Siebert and Valley View shafts 134
31. Cross section of veins in Stone Cabin workings 135
32. Sketch of vertical cut on the east wall of the Silver Top 120-foot level, 3 feet south
of main vein, showing splitting and reuniting of a minor vein 136
33. Horizontal plan of veins in Valley View and Stone Cabin workings on the plane
of the Mizpah 200-foot level, to show the probable connection between the chief
veins on the two sides of the Valley View fault 138
34. Plotting of the strike of the faults in the Fraction workings 141
35. Horizontal plan of vein and faults on the 237-foot level, Fraction No. 1 workings.. 142
36. Horizontal plan showing vein and faults on the 300-foot level, Fraction No. 1 work-
ings 142
37. Horizontal plan showing veins and faults on the 400-foot level, Fraction No. 1 work-
ings 143
38. Stereogram showing nature of movement along the main northeast faults in Frac-
tion No. 1 workings 144
39. Cross section of Fraction No. 1 vein 145
40. Horizontal plan of veins and faults exposed on the 300-foot level, Fraction workings,
showing the relation of the vein fragment in the Fraction No. 2 to the vein on
the corresponding level of Fraction No. 1 147
41. Hypothetical vertical cross section of the Valley View vein system before faulting
and erosion 151
42. Plan showing outcropping veins near the Wandering Boy and their probable rela-
tion to the veins encountered underground 153
43. Vertical section on the Wandering Boy shaft, showing the main Wandering Boy
fault 154
44. Horizontal plan of 115-foot level, Wandering Boy workings, showing minor vein
and Wandering Boy fault 155
4o. Vertical section along east drift, 300-foot level, Wandering Boy mine, showing fault-
ing of vein 155
46. Vertical section along south drift, 300-foot level, Wandering Boy mine, showing
faulting of vein 155
47. Vertical section showing short crosscut to east near south end of south drift, 300-foot
level, Wandering Boy, showing faulting of vein 155
48. Horizontal plan of Wandering Boy, 300-foot level, showing fragments of vein and
cross faults, with the general trend of equal displacement 156
49. Stereogram showing the results of cross faults equally spaced and of equal throw . 157
50. Diagram showing horizontal plan of equal and equally spaced faults belonging to
two systems intersecting at right angles 158
51. Diagram showing course of line of equal faulting for two systems of faults intersect-
ing at right angles and having uniform displacements, the spacing being uniform
within each system but different for each system 159
52. Diagram showing the diverse courses of lines of equal displacement which are the
result of two systems of equal faults intersecting at right angles but unequally
spaced 159
16843 No. 4205 '->
18 ILLUSTRATIONS.
Page
FIG. 53. Diagram showing the line of equal displacement when the fault systems are oblique
to each other instead of being at right angles, the conditions otherwise being like
those in fig. 50 160
54. Diagram showing the effect of cross faults when the faults of one system have twice
the displacement of those of the other system 160
55. Diagram showing trend of zones of equal displacement with given directions of
downthrow 161
56. Section of Good Enough shaft, Gold Hill 166
57. Cross section of Good Enough vein, Gold Hill, as exposed in openings just west of
shaft, showing same characteristics as in fig. 56 167
58. Horizontal plan of faults and vein on the 392-foot level of the Montana Tonopah.. 169
59. Vertical section along north drift, 392-foot level, Montana Tonopah 169
60. Horizontal plan showing veins and faults on the 460-foot level of the Monana
Tonopah 170
61. Sketch showing face of ore of the Montana vein on the west drift, 460-foot level,
Montana Tonopah mine 171
62. Horizontal plan to show relations of the Mizpah and Montana veins on the 400-
foot level of the Mizpah 173
63. Vertical cross section (sketched) of cross wall limiting chief ore shoot of Montana vein
below, as displayed on the 512-foot level of the Montana Tonopah 174
64. Vertical cross section (sketched), showing effect of curving and branching faults
on MacDonald vein in stopes above the 615-foot level on the Montana Tonopah. 174
65. Vertical cross section (sketched), showing effect of curving and branching faults
on MacDonald vein in stopes above the 615-foot level on the Montana Tonopah. 175
66. Cross section showing geology exposed by Montana Tonopah workings 176
67. Section on plane of Desert Queen and North Star shafts 177
68. Section showing geology exposed by Midway workings 180
69. Diagrammatic vertical cross section of Tonopah Extension vein 182
70. Map showing principal earlier andesite veins now developed underground within the
main productive area; shown on the horizontal plane of the Mizpah 500-foot level. 183
71. Vertical section through MacXamara and Tonopah Extension shafts 191
72. Vertical sketch section of shallow trench just north of Belmont shaft, showing
contact of the Oddie rhyolite intrusion with the later andesite 193
73. Diagram to show changes in amount* of commoner elements during stages of altera-
tion of earlier andesite 218
74. Diagram showing relative proportions of the less common elements during the stages
of alteration of the earlier andesite 234
75. Diagram showing changes in composition during alteration of the later andesite 242
76. Diagram showing changes in composition during alteration of the later andesite 243
77. Plotting of temperature observations in the Ohio Tonopah, Mizpah Extension, and
Montana Tonopah mines, showing increase of temperature with depth 265
78. Vertical cross sections of ore bodies or bonanzas in De Lamar district, Idaho; Corn-
stock lode, Nevada; and Cristo vein, Pachuca, Mexico 277
LETTER OF TRANSMITTAL.
DEPARTMENT OF THE INTERIOR,
UNITED STATES GEOLOGICAL SURVEY,
Washington, D. C., March 27, 1905.
SIR: I transmit herewith the manuscript of a report on the Geology of the
Tonopah Mining District of Nevada, by J. E. Spurr, and recommend its publication
as a professional paper.
The geological problem presented in this district is one that could not have
been solved except by a trained petrographer, since the igneous rocks that carry
the vein deposits have been largely covered by practically barren flows of more
recent eruptives; hence the very careful and thorough study of the district made
by Mr. Spurr can hardly fail to be of great practical value to the miner, as
well as of scientific interest to the student of ore deposits.
Very respectfully,
S. F. EMMONS,
Geologist in Charge of Section of Metalliferous Deposits.
Hon. CHARLES D. WALCOTT,
Director United States Geological Survey.
19
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OUTLINE OF PAPER.
Ore deposit* were discovered in the Tonopah mining district, Nevada, in April, 1900, by James L.
Butler. The town of Tonopah soon had a population of several thousand. The climate is arid
and the water supply scanty.
The rocks of the mining district are all of immediate volcanic origin, with the exception of a series
of water-laid tuffs, which represent the accumulations of fine volcanic detritus in a Tertiary lake.
All the rocks are of Tertiary age, probably Miocene-Pliocene.
The first eruptions of this volcanic epoch, as displayed at Tonopah, were andesite. Two andesites
have been distinguished the younger or earlier andesite and the later andesite, which is slightly
more basic than the earlier andesite. Subsequently rhyolite and dacite eruptions occurred at inter-
vals for a long time and produced several of the formations mapped, which include tuffs and flows.
The rhyolite and dacites are closely connected in every way. In one of the latest periods of eruption
these lavas produced the volcanoes whose necks, left in relief by the erosion of the surrounding softer
material, now form the hills around Tonopah.
The water-laid fine tuffs were deposited in this rhyolite-dacite volcanic epoch at a time when the
eruptions had ceased temporarily. The lake basin may have been formed by a sinking of the crust
consequent upon the long-continued volcanic outpourings. The epoch of the deposition of the lake
beds was closed by an uplift accompanied by regional tilting. A little basalt was then thrown out
from volcanic vents, and cones of agglomeratic dacitic material were formed, whose once liquid necks
are now represented by the isolated hills.
The area occupied by the dacitic volcanic necks is coextensive with the region of observed com-
plicated faulting. Study leads to the conclusion that this faulting was initiated chiefly by the intrusion
of these necks. After the intrusion and subsequent eruption there was a collapse, a sinking of the
various vents. The still liquid lava in sinking dragged down with it adjacent blocks of the intruded
rock. (
The silica content of the lavas shows a fairly regular transition between the different types, but
there is a marked break in general composition between the andesite-basalts on the one hand and the
rhyolite-dacites on the other. In some of the most siliceous rhyolites there appear to be numerous
pseudomorphs after hornblende, which consist of fresh rhyolitic groundmass and indicate that the
hornblende had been dissolved and replaced by the magma. In the dacitic phases of the rhyolite-dacite
fresh hornblende is occasionally found. In the audesites, especially the earlier phase, hornblende is
abundant. In the basalt there is abundant hornblende, but it is often pseudomorphosed by magmatic
action into aggregates marked by crystals of iron oxide. It is concluded that in both the highly
siliceous (rhyolitic) and in the least siliceous (basaltic) magmas, hornblende was developed as a first
crystallization, which was unsuited to later conditions. A change of magmatic composition since the
first crystallization is inferred, arid the original magma is thought to have been intermediate or
andesitic. This theory of magmatic segregation is tested by comparison of analyses, and bears the
21
22 OUTLINE OF PAPER.
test well. The theory is reached that an original magma of composition similar to that of the earlier
andesite has split up by differentiation, first into more basic andesite (later andesite) and siliceous
dacite, and later, by a continuation of the process, into siliceous rhyolite and basalt.
The structure is so complicated that no general cross sections have been made. Some interesting
information on faulting has, however, been obtained, chiefly from mine workings. The faults are
reversed or normal, straight or curved, perpendicular or flat. Many varieties of movement are
illustrated by them.
The most important mineral veins occur in the early andesite, and do not extend into the overlying
rocks. These veins have been formed, chiefly by replacement, along narrow-sheeted zones, and have
all the characteristics of true veins. Transverse fractures have determined the position of cross walls
and ore shoots by limiting and concentrating the circulation. The mineralization was probably
caused by hot ascending waters immediately after the earlier andesite eruption. The primary
ores have a gangue of quartz, adularia, and some sericite and carbonates, and contain silver
sulphides such as argentite, polybasite, and stephanite silver selenide, gold in a yet undetermined
form, chalcopyrite, pyrite, and some galena and blende. The depth of oxidation is irregular. In the
ore of the oxidized zone no important changes in the amount of gold or silver, as compared with the
primary ore, has been proved to take place. The ore near the surface is not a truly oxidized ore,
however, but is an intimate mixture of original sulphides (and selenides), together with secondary
sulphides, chlorides, and oxides. Secondary sulphides include argentite and pyrargyrite.
The Tonopah ore deposits, when compared with others, find their closest resemblances in the
Comstock in Nevada and in the Pachuca and other districts in Mexico, while the Silver City and De
Lamar districts in Idaho are also similar in many respects. These deposits all occur in Tertiary lavas,
chiefly andesitic. The writer has previously described the Great Basin region as forming part of a
great petrographic province, and later it has been shown that this province extends into Mexico, and
may reach much farther northeast and southwest. The similarity of the ore deposits in the district
mentioned indicatee that there is a metallographic province, which coincides in part at least with the
petrographic province.
A series of veins, of small importance commercially within the Tonopah district, was formed after
the eruption of one of the members of the rhyolite-dacite series the Tonopah rhyolite-dacite. These
veins may be large, but are usually low grade or barren. They frequently contain a greater proportion
of gold than the earlier andesite veins, and have other distinguishing characteristics. In some cases
the waters accomplishing this latter mineralization probably attacked and concentrated the ores in the
earlier andesite veins.
A series of veins of still less importance was formed after the eruption of one of the later members
of the rhyolite-dacite series a siliceous rhyolite, which makes up some of the hills near Tonopah.
One of these, Mount Ararat, a denuded volcanic neck, is traversed by fissure veins, carrying very
little values. These veins are restricted to the neck, and the openings they fill were evidently formed
by an upward movement of the plug after consolidation.
Part of the earlier andeeite ia profoundly altered, chiefly to quartz, sericite, and adularia. Other
portions are altered chiefly to calcite and chlorite. These alteration phases are transitional into one
another, and were evidently caused by the same waters. The maximum effect of these waters was
the formation of the mineral veins along their circulation channels. Near the veins they effected the
quartz-sericite-adularia alteration, and penetrating farther away they effected the calcite-chlorite
alteration. The discussion of these processes is followed by the detailed study of analyses of
typical specimens. The conclusion is drawn that the mineralizing waters were charged with an excess
of silica, and probably of potash, together with silver, gold, antimony, arsenic, copper, lead, zinc, and
OUTLINE OF PAPER. 23
selenium; that they also contained carbonic acid and sulphur, with some chlorine and fluorine; but
that they were noticeably deficient in iron.
The alteration by thermal waters of the later andesite is also discussed. By comparison of analyses
and by microscopic studies it is concluded that the waters which produced the alteration were highly
charged with carbonic acid and sulphureted hydrogen, and contained magnesia, iron, and lime.
The advent of the waters is believed to have followed the eruption of the white siliceous rhyolite
above referred to.
The composition of the mineral waters in the two cases above referred to does not seem to
correspond with that of the volcanic rocks whose eruption their advent followed. The eruption of
andesite was followed by the advent of siliceous ad potassic waters, poor in iron; the eruption of the
rhyolite by waters rich in lime, magnesia, and iron. This antithesis may have some bearing on the
origin of these waters. There are two theories of the origin of hot springs atmospheric and
magmatic. In the dry Nevada region there are cold springs which give evidence of magmatic origin,
while most of the hot springs show no connection with atmospheric precipitation. The meaning of
the nature of the metals in the-Tonopah veins is also discussed. The conclusion is reached that the
waters which produced the veins were largely given off from the congealing lava below.
The temperature in the Tonopah mines shows an abnormally rapid increase with depth,
comparable to that in the Comstock.
The water encountered by underground workings is very irregularly distributed. Some of the
shafts have reached a depth of over 1,000 feet without encountering any general body of ground
water, yet along certain steeply inclined fracture zones water is found sometimes quite near the
surface. These water zones are widely spaced and occur only in brittle rocks. They are probably
reservoirs bottomed by impervious clay seams. The porous rocks, such as the volcanic breccias,
absorb the precipitation like a sponge, and no water has yet been encountered in them.
The relief of the range of hills in which Tonopah lies is primarily due to the volcanic
accumulations. These Tertiary volcanic rocks have been eroded and much material has been
transported from the hills into the adjoining desert valleys. In arid climates erosion is more general
than in moist climates, and as a result the relief is determined to a much greater degree by the
relative hardness of the rocks. This feature is beautifully illustrated at Tonopah. The complicated
faulting has had very slight effect upon the topography.
U. 6. GEOLOGICAL SURVEY
VIEW FROM NEAR EASTERN CORNER OF Af
'
.-
PANORAMA, LOOKING SOL
PROFESSIONAL PAPER NO. 42 FL-
APPED ON PLATE I, LOOKING SOUTHWEST.
NG WEST FROM MOUNT OC
"ROM BUTLER MOUNTAIN.
GEOLOGY OF THE TONOPAH MINING DISTRICT,
NEVADA.
By JOSIAH EDWARD SPURR.
INTEODUOTIOK
Location. Tonopah (see PI. I) is situated in Nye County, Nev., near the
Esmeralda County line. It lies south of Belmont and about 60 miles east of
Sodaville, on the Carson and Colorado Railway. During the last year a railroad
has been constructed to connect it with the Carson and Colorado Railway at
Rhodes, a short distance south of Sodaville.
Topography. Tonopah is situated in the western part of what has l>een called
the Great Basin region. In this region parallel north-south mountain ranges and
low, irregular hills and mesas, having also in general a north-south alignment,
alternate with broad, flat, or gently sloping valleys. On account of the ariditj' of
the climate the valleys and low hills are bare, save for scattering desert shrubs,
chiefly sagebrush, while higher up, on the mountains, there is a more abundant
vegetation.
At Tonopah the topography is typical of volcanic areas. Numerous isolated
or connected irregular hills denuded volcanic necks rise from a rolling plain.
The town lies about 6,000 feet above sea level, and the top of Butler Mountain,
the highest point near the town, has an altitude of 7,160 feet (PI. II).
Discovery. In April, 1900, James L. Butler, a resident of Belmont, left that
place, with a camping outfit packed on burros, to travel toward the mining camp
called the "Southern Klondike"" and to prospect the neighboring countn*. The
Southern Klondike lies about 10 miles south of the present Tonopah, and Butler's
trail lay over the site of the present camp. Observing the ledges of white quartz
cropping on Mizpah Hill, he broke off specimens, which he gave to the assayer at the
Southern Klondike camp to be examined. So little did these samples indicate the
values that the assayer let them lie a while in his shop, and then, not seeing any
A camp which attracted some attention at the time referred to, but which is now practically deserted.
25
26 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
definite prospect of financial benefit from the work, threw them outside into his
waste pile.
On his return journey to Belmont, Butler broke off more samples from the same
ledge. In Belmont he went to his friend, T. L. Oddie. a young lawyer and miner,
and asked him to have them assayed, promising him a share of the claims should
they turn out to be worth anything. Mr. Oddie sent the samples to an assayer
in Austin, offering him in turn a share in any possible forthcoming results as
compensation for the work. After a considerable delay the Austin assayer
reported values of from $50 to $600 per ton in silver and gold. Mr. Butler did
not act promptly on this news, and the report coming to the Southern Klondike
camp, a party, including the assayer who had thrown out the ore and who had
subsequently fished out the rejected specimens from his waste pile and assayed
them with surprising results, started out to locate the veins. They wandered
around within half a mile of the locality, but. confused by the similarity of the
low isolated mountains, they could not find the veins and were compelled to
return. Finally, on August 27, 1900, Mr. Butler, accompanied by his wife, drove
out from Belmont, and together they located the ledges in due form.
Mr. Butler gave T. L. Oddie, W. Brougher, and several others interests in
the original eight claims which he located, now the property of the Tonopah
Mining Company. In doing the location work two tons of ore were sorted out
and shipped to Selby's smelting company. This netted about $600, and from that
time the property has paid for its own development, a fact of which the locators,
who started in with a joint capital of $25, are properly proud.
Development. In order to prove the value of the property, Mr. Butler gave
leases, the lessee to pay 25 per cent royalty on the ore extracted. Some leases
were given in December, 1900, and over a hundred more in the spring of 1901.
Some of them proved enormously remunerative, and it is estimated that, before the
end of 1901, the lessees extracted ore to the value of about $4,000,000. When the
leases expired, in January, 1902, the result had been relative^- of so little profit
to the owners that no more were given. In the meantime the property had been
sold to Philadelphia capitalists and reorganized as the Tonopah Mining Company.
This company began development work, shipping only enough ore to pay for the
expenses of development and the installment of a proper plant until the present
season (1904), when much larger shipments have been made.
It is a fact worthy of record that the leases given by Mr. Butler were verbal,
not a scrap of paper being used, and that even when such arrangements proved
relatively unprofitable to the mine, as above stated, the agreements were observed
to the letter by Mr. Butler, who, on selling the control of the mine, expressly
U. S. GEOLOGICAL SURVEY
LITTLE TONY */P 4 LEUTUEN
MAP OF MINING CLAIMS, ADAPTED FROM
PROFESSIONAL PAPER NO. 42 PL. Ill
OF BOOKER AND BRADFORD, TONOPAH.
DEVELOPMENT. 27
stipulated for the fulfillment of all his promises. A similar spirit, worthy of
emulation by all engaged in mini ig practice, was observed in other respects.
The Austin assayer, for example, received $32,000 for the assay which he made.
With the proof that considerable quantities of high-grade ore existed at
Tonopah a the camp soon filled up with the usual stirring, excited population of a
new mining camp. A writer in the Anuuai Report of the Director of the Mint,
on the Production of Precious Metals in 1901, quaintly remarks, speaking of the
conditions in 1902:
"Tonopah supports 32 saloons, 6 faro Barnes, 2 dance houses, 2 weekly news-
papers, a public school, 2 daily stage lines, 2 churche?, and other elements of internal
prosperity. It is a very orderly community, and there has been but one stage rob-
bery thus far."
In the center of the town the Fraction shaft, starting in unmineralized soft
volcanic rock, sunk down and encountered some rich ore at a depth of several
hundred feet. This fired the imaginations of the prospector and the promoter with
the idea that ore underlay the surface formations everywhere and was to be had
for the sinking. Claims a long distance away from the real discoveries were in
demand, though they showed no surface indications. To hold these claims, samples
assaying something in gold and silver were diligently sought for, and in some cases
it was only an obliging or careless assayer that saved the day. Companies were
organized, treasury stock was advertised and sold, and shafts were started in many
different places. Four out of five of the shafts or tunnels that were actually begun
were desperately forlorn hopes, to speak conservatively, while many companies,
especially some who were a considerable distance from the discoveries, may safely
be classed as swindles. Others again aid this included most of those near the
camp proper were the honest investments of earnest men (PI. III).
In the winter of 1902-3 rich ores were discovered in the ground of the Montana
Tonopah shaft, which had been sunk several hundred feet through the overlying
barren andesite. Later on, other shafts also encountered ore at a considerable
depth, notably the Desert Queen shaft, the North Star, and the Tonopah Extension.
These, however, are all close to the original discoveries, and no important finds have
been made in the outlying territory. On this account, in the summer of 1903, a
decided dullness set in. Many of the most important prospecting and exploration
workings were closed down on account of lack of funds or too faint encouragement,
and the era of reckless and feverish investment and activity was closed, at least for
the time being.
"The name ia Indian, and means water brush, a desert shrub whose presence points to moisture in the soil
beneath.
28 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
Treatment of ores. The conditions of mining, reducing, and transportation,
which will be of great importance to the future prosperity of the camp, have not
yet been finally determined, though progress has been made. Several million
dollars' worth of ore has been marketed, but at a great cost, for only ore containing
gold and silver to the value of $100 per ton or more was profitable up to the time
of the completion of the railroad. This ore had to be hauled 60 miles in wagons,
and shipped to smelters in California or Utah. Some of the delay in definitely
settling upon more economical ways of reduction has been caused by practical
*. " experiments that have been carried on. It seems to have been finally decided,
however, that smelting is the best method, since any milling process does not
recover the full values. A railroad lately finished from Tonopah to Rhodes, a
point south of Sodaville on the Carson and Colorado Railway, has made trans-
portation to the smelters cheaper.
Water supply. The water problem is an interesting and vital one to any
enterprise in this arid region. At first water was brought into camp on the backs
of burros, from wells in the valley a number of miles to the east. Subsequently
water was developed by wells in the hills about -i miles north of the camp, and
led in by pipes. The supph r , however, was not abundant. Borings in the bottom
of one of the desert vallej^s near b} 1 , called Rye Patch, have developed a great deal
of water. Rather unexpectedly, also, some of the prospecting shafts in the camp
have struck an abundant supply of water, though others are quite dry. Altogether,
therefore, it appears that there is abundant water for domestic, mining, and milling
purposes.
fuel and power. The power problem is also important. Coal has not been
much used in Tonopah, although since the railroad has been completed the cost is
not so great as former!}'. For domestic purposes wood has been used. A variety
of scrubby pine (pine nut, pinyon) grows in the mountains and is cut and hauled
20 miles or more to Tonopah. Of course this is expensive. Some of the hoists of
the mines have been run by steam engines fired with this wood, while others have
used gasoline. The balance of favor at present seems to lie with the wood-burning
engines in regard both to efficiency and cheapness. In the White Mountain Range,
about 00 miles in an air line west from Tonopah, are many mountain streams which
have a great fall and on which an abundance of electric power could be generated.
The harnessing of this water power and the transmission of the electricity seems
feasible if it can be made profitable.
Coal is found about 40 miles west of Tonopah, in the north end of the Silver
Peak Range, and also in Tertiary strata in the mountains farther north. It is a
lignite, or at best a very light bituminous coal. It has been thus far rejected by
FUEL AND POWER. 29
those considering the power problem on account of its great content of ash. Not
all the seams, however, are of the same character; some coal can be found which is
without an extraordinary ash percentage. This is in part a coking coal and might
be efficient. The generation of gas from these coals and the use of this gas as a
fuel is also a possibility which should be carefully considered. While undoubtedly
the material is not high grade, it is worthy of being considered in a region where
other sources of power are so costly.
Crude petroleum, chiefly from southern California, has more recently come into
favor as a fuel.
CHAPTER I.
GENERAL GEOLOGY.
DESCRIPTION OF THE ROCK FORMATIONS.
PRE-TERTIARY LIMESTONE AND GRANITE.
In the immediate vicinity of Tonopah the rocks are all Tertiary volcanics or
tuffs. Eight or 9 miles south of the camp, however, there is limestone, very likely
of Cambrian or Silurian age, which is intruded by granitic rock. Limestones and
granites occur also several miles north of Tonopah, and at intervals between
Tonopah and Belmont. At Belmont the limestone, which is intruded by granite,
is known to be Silurian. From 20 to 40 miles west of Tonopah, on Lone
Mountain and the Silver Peak Range, both Cambrian and Silurian limestones are
cut into by granite.
At Tonopah occasional limestone and qnartzite fragments and more abundant
blocks of granite (often pegmatitic in structure) occur in the volcanic breccias.
Their position shows them to be blocks which were hurled out from volcanoes.
Thus it is shown that at an uncertain depth below the present surface the ascend-
ing lavas broke through rocks of this character. In every case noted these inclu-
sions were in extremely glassy, generally light-yellow volcanic breccia having the
composition of rhyolite-dacite." Three out of four localities are also on the borders
of areas of a peculiar dacite, considered probably the oldest dacite of the region
(Heller dacite), though whether this fact has any further significance is not clear.
At the northeast base of Heller Butte in this glassy Heller dacite there are
inclusions of angular granitic blocks, often several feet in diameter. At the west
base of the butte another bowlder of siliceous granitic rock was found in the
dacite. A fragment of the same rock was found on the borders of the Heller
dacite in the southeast part of the area mapped, southwest of the fork in the
road that runs southeastward from Tonopah. A similar fragment was found in
glassy rhyolite-dacite at the south base of Ararat Mountain. All these fragments
were probably derived from a single underlying granitic mass.
Fragments of altered limestone were noted in dacite breccias, especially in the
vicinity of the New York Tonopah shaft.
a These two rocks are intimately allied and associated in the Tonopah district, and in their glassy phases are often not
easily distinguishable one from another.
30
THE BOCK FORMATIONS. 31
TERTIARY LAVAS.
ANDE8ITE8.
EARLIER AJJDESITE (HORNBLENDE-BIOTITE-ANDESITE).
Of the Tertiary volcanics, which occupy all of the Tonopah district proper,
andesite appears to be the oldest. The writer has called this andesite the earlier
andesite to distinguish it from a subsequently erupted rock of very similar composi-
tion. In the camp it is often called the "lode porphyry," since in it the most
valuable veins lie.
Appearance. The earlier andesite has never been found in even an approxi-
mately fresh state, but is decomposed in varying degrees, sometimes only moderately,
often intensely. The freshest specimens are a light colored, dense, finely porphy-
ritic rock, with small glistening feldspar phenocrysts showing on a fresh fracture.
They have a greenish tinge, due to the presence of chlorite and similar secondary
minerals, if they are from the deeper unoxidized mine levels, and a yellow tinge
from iron oxide if they come from nearer the surface. On further alteration the
earlier andesite usually has become lighter colored and more siliceous, and at first
glance altogether resembles a rhyolite; by another process of alteration, especially
when there was a somewhat greater abundance of original ferromagnesian silicates,
the rock has become green of various shades.
Original composition. From microscopic study it appears that the original
fresh rock was a hornblende-biotite-andesite, of medium composition. The struc-
ture is tine porphyritic, with relatively sparse phenocrysts in a glassy groundmass
containing many microlitic crystals and frequently showing original flow structure.
The phenocrysts were mostly feldspar, hornblende, and biotite, occasionallv quartz.
Hornblende and biotite were about equal in amount, sometimes one predominating,
sometimes another, and frequently one occurring in a given rock specimen almost
to the exclusion of the other. Pyroxene (probably augite) was apparently rela-
tively rare. The ferromagnesian minerals as a whole were not abundant, and the
rock had a rather siliceous character. The feldspar was typically andesine-oligoclase
(as determined in the fresher rock), though some of the feldspars ranged from ortho-
clase to labradorite, the basic varieties being more abundant. The feldspar crystals
are typically small, slim, and simple (i. e., not compound). Apatite in small crystals
is abundant, and zircon is frequent.
Present altered condition.- In the ordinary altered condition these minerals are
often completely transformed. No actual biotite or hornblende has been found
in these rocks, although several hundred specimens have been studied micro-
scopically. These minerals are represented by their decomposition products
quartz, sericite, pyrite, siderite, and hematite, sometimes chlorite and calcite.
32 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
Frequently their former presence is attested only by the greater abundance in
certain areas of ferritic minerals, which form a rude pseudomorph after the
original crystal. Sometimes only the outline of the original is preserved, and
rarely the original lines of cleavage can be traced. Often, on the other hand,
the outline has been lost, and the decomposition products are bunched together
so rudely that the primary mineral can only be guessed at.
The feldspar also is sometimes so completely altered to a felt of secondary
minerals, entirely similar to those resulting from the decomposition of the ground-
mass, that its former existence can not be determined without careful observation.
If viewed by reflected light the outlines of the feldspar crystals can sometimes
be seen. Frequently the secondary minerals within the area of the original feld-
spar are of slightly coarser grain than those without. The feldspar has altered
essentially to quartz and sericite. Alteration of the feldspar to adularia or valen-
cianite (a variety of orthoclase) is also widespread and important. The decompo-
sition products not infrequently include kaolin, and occasionally calcite, chlorite,
and epidote.
The groundmass undergoes the same decomposition processes as the porphy-
ritic crystals, becoming generally a felt} 7 aggregate which is composed of secondary
quartz and sericite, but which includes some pyrite, siderite, and limonite, and
sometimes a little kaolin. By a rarer process of alteration chlorite and calcite
are formed.
As a result of these alteration processes the rock is usually more or less
completely altered to an aggregate which is composed of quartz and sericite, and
which usually includes some pyrite and siderite, and frequently adularia, kaolin,
and the iron oxides. Chlorite and calcite are not so common, but one or both of
these minerals may be very abundant. They indicate a process of decomposition
different from the ordinary. Chlorite may occur in a rock without calcite, and
vice versa. In one specimen studied, quartz and chlorite were the chief products
of decomposition; in another, quartz, sericite, and chlorite. As a rule, however,
the rocks may be divided according to their processes of decomposition, as
follows:
1. Quartz-sericite-adularia-pyrite-siderite rocks; most abundant, and most closely connected
with the metalliferous veins.
2. Quartz-sericite-kaolin-iron oxides rock; not infrequent; probably a modification of No. 1.
Usually plainly associated with some fault or other underground water channel.
3. Chlorite-calcite rock; not associated with the ores.
Location. The earlier andesite outcrops in only a limited area, being chiefly
confined to Mizpah Hill and Gold Hill. It has been proved to occur extensively,
however, underneath later lavas.
ANDESITES. 33
LATER AXDESITE ( BIOTITE-AUGITE-AXDE8ITE).
Appearance. The later andesite is much like the earlier andesite, but is
slightly less siliceous. It is often found nearly fresh, and is in other places
profoundly decomposed, but the general process of decomposition is usually
different from that of the earlier andesite. Typically it is a rock of medium
dark color, mottled with crystals of feldspar and biotite, and sometimes with
pyroxene. It has generally been more or less altered and has turned dark green.
Near the surface the red of the oxidized iron combines with these colors to form
a characteristic rich purple. In some places the rock has been thoroughly altered
to calotte, chlorite, serpentine, quartz, siderite, and pyrite, and other secondary
minerals, and in other places has been so thoroughly leached as to be soft and
white.
Composition and alteration, The porphyritic crystals or phenocrysts are
larger than in the earlier andesite, and are also much more abundant. There is
usually a graded crystallization, the crystals varying from very large size by
easy transitions down to tiny ones, which pass into the microlitic groundmass.
These crystals consist chiefly of feldspar, biotite. augite. and hornblende.
The feldspar occurs as stout crystals, which have an irregular form caused
by complex twinning or intergrowth. When fresh enough the species may
be determined to be predominantly between andesine and labradorite, although
there are more calcic and more sodic varieties, varying between oligoclase and
bytownite. The feldspar is therefore more calcic than in the earlier andesite,
where it is predominatingly oligoclase-andesine. It is usually altered more or
less completely to calcite, chlorite, and quartz. Any one or two of these
alteration products may be scant or absent, and chlorite, kaolin, and zeolites
may be present.
Biotite, which occurs in good-sized crystals, is usually bleached to a light-
green or transparent color, or is partly or wholly recrystallized to muscovite,
pyrite, calcite, and siderite, and occasionally a chloride aggregate. Triangular
skeletons of rutile (sagenite webs) are included in the biotite, and are left free
by its decomposition. The siderite, evidently derived from the breaking up of
the iron silicate in the biotite, general!}' occurs intimately throughout the crystal,
along cleavage lines, etc., while the pyrite is usually conlined to the outside or
the outer edges of the crystals.
The augite is pale green and is usually altered. The alteration products
vary considerably, but are generally serpentine, chlorite, siderite, pyrite, calcite,
and quartz. Kaolin and the zeolites also sometimes occur.
The hornblende is not abundant, and is almost always entirely altered. The
decomposition products are very similar to those of the augite, and include
16843 No. 4205- 3
34 GEOLOGY OK TOJSOPAH MINING DISTRICT, NEVADA.
chlorite, quartz, siderite, frequently calcite, and sometimes sericite, kaolin, and
zeolites. Small apatite crystals occur, in part as inclusions in the phenocrysts.
Magnetite and specular iron occur as primary minerals, often abundantly. In
several cases an isotropic cloudy material of a brilliant green color, suggesting
chromium or nickel, was observed in thin sections; and to this some of the rocks
owe, in part at least, their peculiarly vivid color. At times this secondary
substance seemed to be derived from the augite, but in one section it was
plainly derived from the magnetite, for it formed rims around the magnetite
crystals. As analysis showed a trace of nickel, it is probable that the magnetite
contains some nickel oxide." Siderite also occurs as rims around the magnetite
and as pseudomorphs after it.
Siderite and pyrite are more abundant than in the early andesite. They are
usually intimately associated, and their relations are interesting. Frequently they
seem to have been contemporaneous in origin, and to have formed side by side
without inconvenience. As stated above, however, the siderite is more intimately
disseminated through the mass of the primary ferruginous mineral (biotite,
augite, or hornblende) whence it is derived than is the pyrite. Occasionally the
pyrite is altered to siderite along its margins, but in many more cases the siderite
has unmistakably altered to pyrite along its borders. A delicate set of changes
is thus indicated. The intimate association of the siderite with the primary
minerals, its frequent replacement by pyrite along the borders, and the evident
alteration of the carbonate to the sulphide show that in general a period of
pyritization succeeded one of carbonization, or, if both were contemporaneous,
the period of pyritization was longer.
The groundmass when fresh is brown glass, sometimes spherulitic, or it is
microlitic with brown glass cement. Feldspar, pyroxene, and magnetite microlites
may sometimes be recognized. The groundmass alters, like the phenocrysts, to
quartz, chlorite, serpentine, siderite, pyrite, calcite, sericite-like aggregates, and
occasional zeolites and epidote.
In general the decomposition products of the rock are typically quartz, chlorite,
calcite, pvrite, and siderite, but occasionally portions altered chiefly to quartz and
sericite-like aggregates* may be found.
Location. The later andesite outcrops in only the northeastern portion of the
area mapped, for in the southwestern portion, as a result of relative subsidence
attendant upon faulting, only higher beds are exposed. It occurs in depressions
<iln magnetite some of the ferrous iron is rarely replaced by nickel; thus a variety from Pregratten, in the Tyrolese
Alps, in a schistose serpentine, gave 1.76 per cent nickel oxide (NiO), together with traces of the oxides of manganese,
chromium, and titanium.
t For some information on the real nature of these sericite-like aggregates see p. 240. It appears probable that
hydrargillite and talc form a large part of these masses.
ANDKSITES. 35
between hills of rhyolite and dacite, because it is less resistant to erosion than
these rocks.
Relation to earlier andesite. The later andesite directly overlies the earlier
andesite, and though in many underground workings and probably at every outcrop
the contact is a fault contact, caused by movements subsequent to the eruption of
the later andesite, yet in several shafts one andesite has been found apparently
lying undisturbed in its normal position upon the other. Such was the case in the
Midway, the West End, and the Tonopah Extension shafts. In these places the
contact was marked by a band of decomposed breccia, or even clay, yet there was
no good evidence of faulting. The quartz veins of the earlier andesite extend up to
this contact in full strength and then abruptly disappear. Most likely the earlier
andesite was deeply eroded and the veins were exposed before the later andesite was
poured out, and possibly the decomposed clay or breccia zone represents the result
of surface decomposition and disintegration before the later-andesite period.
Distinction from earlier andesite. The earlier andesite and the later andesite
are usually sufficiently distinct in appearance to permit identification in the field.
The later andesite is generally darker; on account of the greater amount of iron
present it has the characteristic strong coloration mentioned above. The earlier
andesite is characteristically finer grained than the later, and contains smaller
and less abundant porphyritic crystals. The porphyritic feldspars in the earlier
andesite are usually slim, of simple form, and almost rectangular, while those
of the later andesite are apt to be stout and complex as a result of twinning.
In the later andesite crystals of fresh or bleached biotite can usually be seen;
in the earlier andesite they occur more rarelv.
Similar characteristics serve, as a rule, for the microscopic determination.
The phenocrysts of ferromagnesian silicates augite, biotite, and hornblende and
their pseudomorphs or decomposition products are usually more abundant in the
later andesite. The typical alteration of the earlier andesite is to quartz, sericite,
and a little pyrite; that of the later andesite is to chlorite, quartz, calcite, siderite,
and pyrite. While the character of the alteration is a valuable help in diagnosis,
it is not by any means a sure test, for in some cases the processes of alteration
have been apparently almost exchanged."
On account of the similarity in the original composition of the earlier and
later andesites it is frequently very difficult, either from field or from microscopic
study, to refer a specimen to the proper age. Often this economically important
question is decided by tracing the doubtful phase into some decided phase in
the same rock body.
nit is probable, however, that the sericite-like aggregates in the altered later andesite are composed largely of
minerals like hydrargillite, talc, kaolin, etc., rather than of sericite. See pp. 240-241.
36 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
KHYOLITES AND DACITES.
INTERRELATION OF RHYOLITES AND DACITES.
The rhyolites and dacites at Tonopah are closely bound together in every
way in chemical and mineralogical composition, in areal distribution, and in
manner and time of eruption. In fact, they can be best understood if considered
as portions of the same great magma, split up, as the author would like to assume,
b} 7 internal segregation or magmatic differentiation. These lavas constitute tran-
sitions between the two types (rhyolite and dacite) named above, and the dacite
itself is a very siliceous" one, barely deserving distinction from the rhyolites
were it not necessary to emphasize the distinction between it and the still more
highly siliceous rhyolite which forms some of the hills of the region, such as
Oddie and Ararat. Moreover, although the rocks of Butler, Brougher, Siebert,
and Golden mountains are distinctly of the dacitic type, and so fairly classed
together and distinguished from the rhyolite, yet different hills (being denuded
volcanic necks and so representing separate vents) show different phases. Golden
Mountain, for example, is made up of a lava which, both in the field and under
the microscope, seems to be more closely allied to the near-by rhyolite than to
the dacite of the more distant eminences in the lower or southwestern half of
the mapped area, such as Brougher Mountain. Chemical tests bear out this
impression in large measure. The fine-grained border facies of this Golden
Mountain intrusion, being glassy with sparser feldspar phenocrysts than the
normal type, is indistinguishable, without chemical analysis, from similar rhyolite.
The glassy dikes which extend from the main mass are of the same character.
Many of the small dacite-rhyolite flows, erupted at an earlier period than the
volcanic necks, are similarly tine grained, and difficult to classify exactly as dacite
or rhyolite without numerous and altogether useless chemical tests. It is prac-
tically certain that many of these are transitions between the two extreme but
closely related types.
SIMULTANEOUS ERUPTION'S.
The eruption of dacite and rhyolite, which succeeded that of the andesite,
extended over a long period and was characterized by many variations in the
rhyolite and dacite. The observed phenomena favor the conclusion that different
vents were in a state of eruption nearly or quite simultaneously, each one
contributing its characteristic rock, and that the notable alternation^ of different
kinds of lava are due rather to the temporary inactivity of some of the vents
than to any real change in the character of the magma in the supply basins.
In order to describe better the geologic history and the economic geology a
number of subdivisions have been made in the dacite-rhyolite series.
Fur the use of the term "dacite" set 1 pp. 58-59.
DACITES. 37
HELLER DACITE.
Location. Heller Butte, a small, steep manielon near the town of Tonopah
(PI. IV), is made up of a dacite containing numerous included fragments. At first
it was considered to be of the same class and age as the larger buttes, such as
Butler and Brougher mountains, and since the latter are denuded volcanic necks
it was thought to represent a smaller contemporaneous vent. Afterward, however,
it was recognized that the marked abundance of inclusions, the unusually abundant
glassy groundmass, and the fact that the porphyritic crystals are frequently larger
than those of the dacite of the larger mountains were characteristic features
of this particular rock. Later, other grounds favoring its assignment to a quite
different and earlier period were discovered.
Heller Butte has a height of 150 to 200 feet and a steep conical form, elliptical
at the base. Its rock is vesicular glassy dacite, which contains inclusions of
pumiceous material, frequently of later andesite, and occasionally of coarse
siliceous granite. The inclusions of andesite and granite are sometimes large
angular bowlders, several feet in diameter. The form of the butte seems to be
governed by platy structure. It is steep and slopes away in curves on all sides.
On the northeast and southeast sides the lava is cut off from the Fraction dacite
breccia and the Siebert tuffs by faults, along which are intruded glassy dikes sent
off from the Mount Golden mass of Brougher dacite. On the western side the lava
of the butte seems to dip under the nearly horizontal Fi'action dacite breccia.
The Tonopah City shaft. 800 feet west of the Heller dacite area last referred
to, passed thi-ough 300 feet of the partly fragmental, loose Fraction dacite breccia
to solid, glassy dacite of the Heller type, which continued for 200 feet more to
the bottom. The contact in the shaft could not be seen by the writer on account
of the tight lagging, but it seems most likely that the order is normal and that
the lower formation is the older. Rounded and subangular inclusions of the later
andesite, having the appearance of waterworn pebbles, are frequent in the Heller
dacite of this shaft, and are more abundant toward the bottom. There are found,
also, smaller rounded quartz pebbles, which are accounted for on the hypothesis
that this lava was a flow, which ran over and caught up pebbles from an older
surface-gravel deposit.
Near the southeastern edge of the area mapped are other outcrops of lava,
which have the same peculiar phases the abundant glassy groundmass and the
numerous inclusions of foreign materials as the lava at Heller Butte. Here,
northeast of the main road that crosses the valley, running between Butler and
Golden mountains, is a small, smooth mamelon, or symmetrical cone, about 20 feet
high and 80 feet in diameter, that resembles in a general way Heller Butte, and is
composed of the same lava. This cone has a concentric, platy structure parallel
38 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
to its surface, and to this it evidently owes its form. It is adjoined on the west
by a long tongue of similar lava, like that surrounding Heller Butte, and the
whole is surrounded by the friable Fraction dacite breccia.
Farther west, just on the west side of the road, is a similar dome-like hill that
rises out of a limited irregular area of the same dacite and is surrounded by the
Fraction dacite breccia. Still farther west, a short distance, there is a projecting
ridge of Heller dacite, capped by a mamelon 5 feet high. The platy structure slopes
away from the ridge on all sides.
These three similar cones are aligned in a northeast-southwest direction.
Age of Heller dacite. The lava of Heller Butte has been faulted. It is thus
older than the later intrusive Brougher dacite, which makes up the important hills
of the district (see p. 44), and was erupted before the general faulting. It has been
found in normal contact only with the Fraction dacite breccia, which circumstance,
so far as it goes, favors an age either immediately before or after this formation.
On the southeast side of Heller Butte the lava of the butte is separated from the
Siebert tuffs by a fault contact. Examination of what is practically the same fault
(the California fault) a few hundred feet farther north, where the tuffs are brought
into contact with the Fraction dacite and the earlier andesite, both of which are
known to-be older than the tuff, shows that the tuff block is downthrown, and
that the Heller dacite belongs to a lower' horizon than the tuff. The fact that
nowhere has any of this Heller dacite been found between the Fraction dacite breccia
and the overlying formations would further restrict the probabilities; and the fact
that the dacite of the butte appears to dip under the Fraction dacite breccia near
Heller Butte and reappear beneath the breccia in the Tonopah City shaft favors
the final assignment of the Heller dacite to a period preceding the formation of
the Fraction dacite breccia. The inclusions of later andesite in the Heller dacite
again fixes the dacite as later than the andesite, and the place of the Heller dacite
may be held to be between the later andesite and the Fraction dacite breccia.
Nature of Heller dacite. The glassy groundmass of the Heller dacite indicates
cooling at or not far from the surface, and the apparently waterworn pebbles
included in the dacite in the Tonopah City shaft suggest that this portion of the
lava was a flow. At the same time the presence of inclusions of granitic rocks
(sometimes in bowlders several feet in diameter), as well as of the later andesite
near Heller Butte, shows that the lava rose directly from depths below the granite
and passed through this rock and the already erupted andesites on its way up. A
vent or volcanic neck is thus suggested and the topographic forms of Heller Butte
and the similar smaller buttes described with platy structure parallel to their
surface offer the same suggestion.
Summarizing the evidence and inferences, it appears that the eruption of
the later andesites was followed by an interval of rest and erosion; and that the
DACITES. 39
beginning of the dacite-rhyolite eruptions was signalized by the appearance of the
Heller dacite, which formed numerous small cones along lines of weakness and was
poured forth in relatively limited quantities.
Microscopic character*. Under the microscope the Heller dacite shows a
brown glass groundmass, which is sometimes spherulitic and which contains
numerous porphyritic crystals, nearly always broken, of quartz, feldspar, and
biotite. It resembles the Brougher dacite. Striated and unstriated feldspars are
about equally represented. The latter are probably in large part orthoclase,
while in one slide examined striated feldspars proved to be andesine.
FRACTION DACITE BRECCIA.
Location. A considerable part of the southern half of the area mapped is
covered with a soft brownish or greenish rock of volcanic origin. This rock is
sometimes solid, is occasionally dimly horizontally layered or packed, is at times
definitely stratified, and even contains well-bedded tuffs. The material is dacitic,
essentially like the Heller and the Brougher dacite. It does not occur in the
relatively elevated northwestern half of the area mapped but in the southeastern
half it spreads far beyond the map limits and occupies large portions of the low
areas between the hills.
Thickness. This formation varies in volume, but is frequently several hundred
feet thick. Perhaps the greatest thickness actually demonstrated is at the New
York Tonopah shaft, which is 745 feet deep and is entirely in this formation,
except for intrusive bodies of the Tonopah rhyolite-dacite or included fragments
of earlier rocks.
Conditions of eruption. In places the dacite belonging to this formation is
nonfragmental and of the nature of a flow. But it is invariably soft and friable.
It grades into a common type where it is often difficult to decide whether or
not the rocks are of fragmental character. They often consist of broken, close-
packed, medium-sized fragments of more or less pumiceous dacite, but under the
microscope show no signs of fragmental origin. An explanation of their origin
that accounts for their different features is that these rocks were partly or
entirely volcanic mud flows, in which the highly pumiceous and aqueous lava
was mingled with such an excess of heated waters that it was partly broken and
ground up in the course of the flowing. Rock of this nature grades with no
sharp line into thick, unstratitied accumulations of brownish or greenish pumice
fragments, which are of considerable size, and which grade into similar masses
of smaller pieces. In some parts of such deposits a rude stratification or layer-
ing may be observed, and occasionally there are thin layers of well-stratified tuff
(fig. 1). These pumice accumulations point to explosive eruptions. In them are
40
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
found fragments, some of which are several feet in diameter, of the earlier andesite,
of the later andesite, and of andesite and dacite tutf, which were probably hurled
out of the volcanoes in blocks during these eruptions.
We may therefore reason that the period of the formation of the Fraction dacite
breccia was one of considerable volcanic activity, though not necessarily prolonged.
The volcanoes exploded repeatedly, producing
showers of pumice and ash and rapid subaerial
accumulations of these materials on and near the
slopes, while the flows were scanty and so mixed
with water as to be often nearly or quite mud
flows. . The upper part of the formation, as seen
in the New York Tonopah shaft, in the north-
west side of Siebert Mountain, and elsewhere, is
more fragmental than the lower portion. Some
solid lava flows appear interstratified with these
upper fragmental deposits, but they belong
rather to the Tonopah glassy rbyolite-dacite than
to the Fraction dacite breccia.
Relative age. A number of data are of value
in the determination of the relative age of this
formation. It is clearly younger than the later
andesite, for it sometimes rests upon this forma-
tion and typically contains abundant inclusions
of it. On the other hand, it is frequently cut
by dikes of the Tonopah rhyolite-dacite (tig. 2).
As already stated, it overlies the Heller dacite
in the Tonopah City shaft, and is most likely
younger than it. Therefore it is probably imme-
diately between the Heller dacite and the Tono-
pah rhyolite-dacite.
Microscopic characters. Microscopically the
rock of the Fraction dacite breccia is a biotite-
dacite, substantially of the same composition as
the Heller dacite and the Brougher dacite. The
groundmass is brown glass, often felty, and fre-
quently very vesicular. As porphyritic crystals (usually broken) it contains quartz,
relatively sparse biotite, and feldspar, both striated and unstriated. The striated
crystals are relatively considerably more abundant than in the Tonopah rhyolite-
dacite. One determination showed andesine-oligoclase.
4 feet
FIG. 1. Vertical section of shaft about 1,600 feet
east of Tonopah and California shaft, showing
Fraction dacite breccia and interbreccia tuffs.
(1) Finely stratified tuff; (2) sandstone com-
posed of angular and rounded fragments of da-
clteglass; (3) stratified rock, largely made up
of pumice fragments; (4) soft dacite, broken
and containing pumice fragments, probably
a mud flow; (n) like 4, but containing little
pumice.
RHYOLITES AND DACITES.
41
TOXOPAH RHYOLITE-DACITE.
The Tonopah rhyolite-dacite occupies a large part of the area mapped. It
occurs in large unbroken areas in the northern corner and in numerous broken
and separated areas, bounded by faults, in the western corner.
Appearance. The rock has many different aspects in the tield, gray, bright
red, black, and white being among the colors represented. Fine brecciation is
frequently observable, while in many cases the rock is glassy, dense, and charac-
terless, especially near the contacts of the intrusive masses, or in the thin sheets.
Under the microscope, however, the characters are much more uniform.
Microscopic characters. Characteristically sparse and small phenocrysts occur
in a glassy, sometimes partly microcrystalline brown, gray-brown, or yellowish
groundmass. The rock often possesses flow structure, is rarely pumiceous or
slaggy, and frequently shows autobrecciation. Angular fragments of broken glass,
included in a cement of similar glass, and other phenomena indicate that the lava
moved while stiffening.
Scale
20 feet
FIG. 2. Vertical sketched section of trench just west of Brougher Mountain, showing Tonopah rhyoliie-dnoite (6), intru-
sive into Fraction dacite breccia (at.
The porphyritic crystals consist of feldspar, biotite, and quartz, which occur
in the order named. Small unstriated blunt crystals of orthoclase are always
predominant among the feldspars, though striated and more elongated crystals are
frequent. Optical determination of these shows that they range from andesine to
albite, andesine-oligoclase being the most frequent phase. Quartz crystals are
abundant in some phases, in others rare, and in many are wanting, especially in
the more glassy phases. Fresh biotite crystals are frequently present though
rarely abundant. They are usually small in size.
A pseudomorph of iron oxide (specular iron?) after hornblende was observed
in one case, the original hornblende having been resorbed by the dacitic magma.
A single small crystal of augite was found in one of the slides, out of several
hundred examined. Small original crystals of specular iron are often observed.
Alteration near contacts. Silicification and the production of secondary minerals
is widespread, especially near contacts where the rhyolite is intrusive into older
4:2 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
rocks, in which places the alteration has been accomplished mainly by hot-spring
action succeeding the intrusion. Secondary quartz, pyrite, and sometimes siderite
are the chief results, with exceptionally epidote and adularia and very rarely a
little calcite. The quartz ma}' form veinlets in the rock as viewed under the
microscope, and these silicifications may increase in importance till they form
large quartz veins. Vesicles lined with chalcedony were noted in one instance.
Some of the Tonopah rhyolite-dacite presents, when altered by the processes
above referred to, a close resemblance to certain highly altered and silicified phases
of the earlier andesite. Field work, however, seems to leave little doubt as to the
nature of such types, as they can be traced into unequivocal rhyolite-dacites.
These altered and silicified rhyolite-dacites, especially those which contain no
quartz phenociysts, differ from the similarly altered earlier andesites in the scarcity
and smallness of the phenociysts, in the predominating stout, blunt form of the
feldspars, which indicates orthoclase where the alteration is so great that no
determination can be made, and in the absence or scarcity of apatite.
Distinction between the northern and the southern areas. The general character
and relations of the Tonopah rhyolite-dacite differ considerably north and
south of an east-west line across the middle of the area mapped. This line,
probably a fault line (see PI. X[), runs up the main gulch along which the road
to town passes and into the town. To the north the dacite is always intrusive,
as its contacts prove. To the south the petrographic characters are in general
the same as to the north, but the geologic relations are more complicated. In
many cases the dacite is evidently intrusive, while in other places it occurs in
sheets that alternate with pumiceous tuffs and have all the appearance of flows.
Under the microscope also new features present themselves and indicate that
manv of these rocks are probably fragmental. In thin sections of such rocks
the autoclastic glassy dacite has been finely broken mechanically and the
fragments are intersected by dense kaolinic matter into which iron has infiltrated.
The material seems to be an unassorted accumulation of angular fragments,
which resulted from a shower of dacitic ash 'and lava fragments during and after
explosive eruptions.
The southern part of the area mapped has in general been depressed below
the northern half by faulting, and here internal faulting has been much more
active than in the other part (see p. 47). This depressed tract exactly corresponds
with the area of intermingled Tonopah rhyolite-dacite dikes, flows, and tuffs. In
the relatively elevated northern portion of the area mapped the surncial formations
have been largely worn away, and only the intrusive portions of the Tonopah
rhyolite-dacite are left, while in the southern portion the corresponding flows and
tuffs, as well as the feeding dikes, remain.
RHYOLITES AND DACITE8. 43
Age and origin. There is a great deal of evidence concerning the age of
the Tonopah rhyolite-dacite. In the northern part of the area mapped this
formation is intrusive into the earlier andesite, and in many places into the later
andesite. In the southern half of the area it contains numerous inclusions of
later andesites, as well as probable earlier andesites, vein quartz, and granitic
fragments. It is often intrusive into or overlies the Fraction dacite breccia, which
therefore in general seems to be older (fig. 3).
Above the Fraction dacite breccia proper is a series of coarse, pumiceous tuffs
which are rudely layered and rarety well stratified, and in which Tonopah rhyolite-
dacite sheets are often interbedded, with no sign of intrusion. This shows that
the flows were poured out intermittently and alternated with explosive eruptions,
which caused the great intervening accumulation of pumice and the 3 r ellow ash
derived from its disintegration. Occasionally also, but not commonly, thin sheets
of the same rhyolite-dacite are found in the lower part of the waterlaid Siebert
tuffs, which ovejlie the pumiceous unassorted tuffs and breccias and their inter-
calated Tonopah rhyolite-dacite flows.
/-;-;-;,-,;-;_< ;'ih%'^v''-""^^^^
,-'"' Scale
o 5 10 20 30 feet
FIG. 3. Vertical section of part of tunnel north oi Brougher Mountain and southeast of Ohio Tonopah shaft, showing
(1) Tonopah glassy rhyolite-dacite overlying Fraction dacite breccia; (2) faulting subsequent to both; (31 contact
dipping down and in general faulted down toward the volcanic neck of Brougher Mountain (mouth of the tunnel
is about 800 feet distant from the border of this neck).
The geologic position of the Tonopah rhyolite-dacite is, then, pretty clearly
fixed. The eruptions of Fraction dacite breccia were soon followed by those of the
Tonopah rhyolite-dacite, which mingled with the Fraction dacite, as indicated by
numerous observations where these rocks are intimately associated. The eruptions
of Fraction dacite became subordinate to those of the Tonopah rhyolite-dacite
and were probably chiefly explosive, contributing material to the brown pumice
beds, which are often several hundred feet thick and which alternate with the
Tonopah rhyolite-dacite flows. At this period the Tonopah rhyolite-dacite
eruption was at its height, though for some time subsequently, after the formation
of the Tertiary lake and the accumulation of the Siebert tuffs therein, scanty
flows were occasionally and locally emitted. Near Rushton Hill, however, pebbles
of glassy Tonopah rhyolite-dacite in a conglomerate at the base of the Siebert
tuffs indicate that the older Tonopah rhyolite-dacite flows contributed by erosion
their material to the upbuilding of the tuffs, at the same time that the last
tardy flows of the same lava were being brought forth.
44
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
BROfC.HER DACITE.
Location. This rock forms most of the important hills, the others being
composed mainly of rhyolite of the same age and origin as the dacite. The dacite
hills are Butler, Brougher, Siebert, and Golden (Pis. V and VI).
Volcanic necks. These eminences represent the necks of former volcanoes or
.the columns of lava which rose from the abyssal regions to the surface. The
Brougher Mountain neck (as mapped) is roughly circular, though slightly
elongated. Butler Mountain is ellip-
tical, Siebert Mountain is irregular,
while Golden Mountain is elongated
and irregular. Butler, Brougher,
and Golden are all elongated in an
east- west direction.
Contact phenomena. The proof of
their origin is found, chiefly in their
contact phenomena. The contacts
are usually marked by a belt of
dacite, which appears in the hand
specimen as a black glass and which
is shown by the microscope to be a
glassy phase of the dacite. This
band is generally several feet thick,
and locally as much as a hundred
feet. Powerful flow lines, parallel
to the contact, are usually observed,
and not infrequently the actual con-
tact with the intruded rock can be
seen. The contacts are typically
vertical, but they are by no means
regular. They frequently dip out
from the mountains, and perhaps
more frequently into them, and are
often wavy (figs. 4 and 5). The earlier andesite, the Fraction dacite breccia,
the Tonopah rhyolite-dacite, the Siebert tuffs, and the basalt are at various
places intruded along the contact of these dacite necks, and thus the age of the
dacite is established. The intruded rocks are usually hardened and silicified
near the contact, and contraction cracks in them are coated with chalcedony.
Dike* from main masse*. The contacts are irregular in detailed horizontal
plan, and tongues are frequently sent out into the intruded mass. Along faults
5 10
Scale
20
40 feet
Flo. 4. Vertical sketch section showing contact of intrusive
dacite with tuff, southwest base of Butler Mountain, a, Da-
cite; 6, dacite glass, contact face; e, tuff, broken at contact.
U. S. GEOLOGICAL SURVEY
VIEW LOOKING NORTHWEST FROM A POI
PROFESSIONAL PAPER NO. 42 PL. V
EN RUSHTON HILL AND GOLDEN MOUNTAIN.
RHYOLITE8 AND DACITES.
45
these tongues have sometimes penetrated a considerable distance, and there the
lava forms dikes, sometimes thinning to a very great degree or showing only in
occasional outcrops as "'intermittent" dikes. The lava in these dikes is glassy,
like the contact phase of the main mass.
Included basalt. Inclusions of basalt were found in the Brougher dacite in
several places. In two places dikes sent off from the Golden Mountain neck
along fault planes contain augite-hornblende-basalt, like that in place on Siebert
Mountain. Besides augite and hornblende, anorthite and labradorite-bytownite
feldspar were recognized in these inclusions. At the north base of Butler Moun-
tain, within the conspicu-
ous hollow there, the da-
cite is packed full of in-
clusions of similar basalt.
Vestiges of cinder
canes. At various points
around the base of Butler
Mountain, close to the in-
trusive neck, is a coarse
volcanic agglomerate of a
kind not seen at any
greater distance from the
mountain. It consists of
large angular blocks of
volcanic rocks, alternating
with finer breccia and ash.
On the south side of But-
ler Mountain this material
has a thickness of about 200 feet, and contains bowlders up to 5 feet in diameter.
These bowlders consist of lava resembling the Tonopah rhyolite-dacite. Imme-
diately adjacent to the intrusive Brougher dacite contact on this side of the
mountain there was noted a bowlder of similar Tonopah rhyolite-dacite that was
30 feet in diameter and lay in the Sietiert tuffs, as if it had dropped into them
when they were soft mud. On the north side of the mountain similar agglomer-
ates were observed, and here the blocks were chiefly of glassy dacite. This
indicates an accumulation of volcanic cinders and bombs, and their localization
around the base of Butler Mountain shows that this was the site of a cinder
and bomb cone, which was built up as a result of the resumption of volcanic-
activity at the close of the tuff period and perhaps following the slight basaltic
eruptions (manifested within the area mapped only on Siebert Mountain). On
10 feet
FIG. 5. Vertical section showing contact of the Golden Mountain dacite, glassy
along the margin, with Siebert tuff (lake beds). Location due east of Golden
Peak. Dotted outlines indicate a prospecting pit sunk in the tuff at the
contact.
46
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
the southwest slope of Brougher Mountain, also, a coarse agglomerate was
observed in one place; and it is very likely that this and other of the mountains
of this group have had a similar history.
Formation of the present Brougher dacite. After these explosions a column
of lava rose and filled the vent. It is not likely that this lava ever overflowed,
for no traces of flows have been found, and from such important vents the lava,
if poured out, would be sufficient in quantity not to have been wholly swept
awav bv erosion.
Scale of feet
50 100
Fio. 6. Horizontal plan showing eddying in the cooling lava of a volcanic (dacite) neck; plotting of strong flow*truc.
ture on top of eastern shoulder of Golden Mountain; attitude of flow planes nearly vertical, usually dipping 70
to 90 north, sometimes dipping south.
/'low structure and other phenomena. The flow structure in these necks was
carefully observed, and the conclusion was reached that only at the contact do
the flow lines indicate the direction of the original flow. Away from the contact
the lines follow all imaginable curves (fig. 5). It is plain that after the rapid
cooling of the glassy lava near the contacts the liquid material standing in the
neck circulated and eddied extensively before cooling. There may b seen in
U. S. GEOLOGICAL SURVtY
*!'-* 7'^vw **.
' ;:*. : >^.>v:'
S&$$
.4. BROUGHER MOUNTAIN AND TONOPAH. SEEN FROM MIZPAH HILL.
/J BUTLER MOUNTAIN FROM EAST BASE, SHOWING COLUMNAR DACITE ABOVE AND STRATIFIED SIEBERT TUFFS BELOW.
RHYOLITES AND 0ACITES. 47
these mountains columnar jointing, small gaping cracks caused by the stretching
of the nearly cooled lava, caves formed by the collapse of highly vesicular lava,
platy structure or parting parallel to the contacts, and other interesting volcanic
phenomena.
Faulting due to rougher dacite eruptions. The Brougher dacite is confined to
the southern half of the area mapped. This general dacite area is also coextensive
with the region of observed complicated faulting, and a connection between the
dacite intrusion and the faulting is suggested. The faulting occurred subsequent
to the eruption of all the rocks older than this dacite, while the dacite is unaffected
by it. This complexly faulted southern half of the area is also downsunken in
comparison with the little-faulted northern portion. Near the dacite necks the
observed faults are rather more numerous than elsewhere, and in many instances
the blocks adjacent to the dacite have been downsunken in reference to blocks
farther awav (PI. VII). From these intrusive necks the faults run in a roughly
radiating fashion and seem to follow no regular system of trend (PI. VIII).
Detailed study of the contact phenomena of the dacite shows that the minute
faults in the tuffs at these points generally have their downthrown side next the
dacite.
From these facts the following conclusions have been reached. The faulting
was chieflv initiated by the intrusion of the massive dacite necks (the rhyolite
necks were probably not so bulky)." After this intrusion and subsequent eruption
there was a collapse and a sinking at the vents. As the still liquid lava sank it
dragged downward the adjacent blocks of the intruded rock, accentuating the
faults and causing the described phenomena of downfaulting in the vicinity of the
dacite.
In reference to this phenomenon of subsidence around volcanic vents Scrope*
wrote:
"It would appear, however, that in some cases the eruption of volcanic matter
is accompanied by the subsidence not only of the column of lava which had risen
within the vent, but also of the neighboring surface rocks themselves. Several of
the cinder cones of New Zealand, as described by Mr. Heaphy, have been thrown
up on a line of fault in the Tertiary strata whose upcast forms the sea cliff, and
show a clear synclinal depression of the elsewhere horizontal beds, on either side
toward the eruptive vent.''
Tuff dikes near contacts. At some points along the contact of the Butler
Mountain neck with the Siebert tuffs, particularly on the south and east sides, sand
and tuff dikes are observed. They are composed of yellow tuff and included frag-
a In the North Star and Desert Queen mine workings, along the southeastern part of Mount Oddie, for example, the
dip of the lower contact of the rhyolite into the mountain is very flat.
>> Volcanoes, p. 225.
48
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
ments of the glassy contact phase of the dacite, and are intrusive into the Siebert
tuffs. Sometimes these dikes are composed mainly of glassy dacite fragments,
sometimes of clear sand. They often follow the exact intrusive contact and are
never far from it (tig. 7). Detailed study shows that these breccias are truly dikes
that have been injected in a plastic condition (fig. 8). This injection followed the
intrusion, and the intrusive material was probably a mixture of ascending hot
waters, consequent upon the eruption, with tuff and dacite fragments.
Mineral composition. Microscopical \\ the Brougher dacite shows a brown,
glassy groundmass, which is sometimes finely crystalline and contains frequently
Scale
so feet
FIG. 7. Vertical sketch section of dacite contact at a point on the east side of Butler Mountain; a, gray dacite; b, glassy
dacite, autociastic; c, dike of friable, partly consolidated detritul sand and angular fragments composed of material
derived from the tufl and from dacite glass; d, finely stratified Siebert tuffs (lake beds); e, coarser layer of tuffs; /, faults.
The sketch shows clastic dikes consequent upon marginal fissuring around the dacite, and dowufaulting toward the
contact.
broken porphvritic crystals of quartz, orthoclase, andesine or andesine-oligoclase,
biotite, and occasionally hornblende and augite. Magnetite and specular iron occur.
In the field the Golden Mountain dacite was judged to be more siliceous than
that of the other mountains, and this observation has been borne out by microscopic
and chemical analysis. It shows, indeed, a close relation to the Oddie rhyolite.
However, the Golden Mountain rock is distinguished as dacitic by the greater
abundance of porphvritic crystals, the frequent presence of elongated plagioclase
feldspars, the greater amount of biotite, the characteristic brown, glassy ground-
H, and the occurrence of occasional augite.
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RHYOLITES AND DACITES.
ODDIE RHYOLITE.
Location. A white siliceous rhyolite makes up Mount Oddie (PI. IX, B) and
Rushton Hill, and extends irregularly in .spurs and lobes away from their bases.
A similar rhyolite occurs on the summit in an irregular area at the northwest
base, but not on the slopes of Ararat Mountain, and in small patches around the
north base of Brougher Mountain.
Contact phenomena of Oddie- Rushton neck. By the same method of reasoning
applied to the Brougher dacite necks, the conclusion is reached that Mount Oddie
and Rushton Hill are also the necks of ancient volcanoes. On Mount Oddie and
Rushton Hill the rhyolite is intrusive. At man}- points along the contact there
is a vertical flow structure in the rhyolite and a platy structure parallel to it.
The rhyolite of Rushton Hill,
at its contact with the later
andesite near the Rescue
shaft, dips at an angle of 45 :
to 60 C away from the hill.
The Rescue shaft passed into
this rhyolite and has con-
tinued in it several hundred
feet up to the time of latest
information.
Near the contact the
rhyolite is frequently gl;i>s\-
and resembles the Tonopah
rhyolite-dacite; it has also
been silicitied in many places
subsequent to its eruption.
The rhyolite of Oddie
and Rushton hills sends out
irregular lobes into the surrounding rocks, which by reason of their superior
hardness, as compared with the intruded later andesite, form ridges. These also
are characterized by vertical flow lines and platy structure. As a whole the
intrusion is elongated in an east-west direction, parallel to the previously noted
elongation of the dacite necks.
Contact phi'wmiena of Ararat neck. The slopes of Ararat Mountain are
formed by the Tonopah glassy rhyolite-dacite, which has already been described.
The top, however, is of rhyolite like that of Mount Oddie, and the contact
between the two is sharp. The white rhyolite at the top of the mountain has
a roughly circular outline. At its margin it is brecciated, sometimes profoundly,
16843 No. 4205 4
Scale
to
20 -feet
FIG. 8. Vertical sketch section taken at a point on the east side of Butler
Mountain, 100 feet below the contact of Butler dacite and tuff, showing
dike of light-brown, semi-consolidated sand, of volcanic origin, containing
angular fragments of dacite glass, intrusive into Siebert tuffs (lake beds).
50 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
and contains large veins, filled chiefly with calcite, which do not extend into the
Tonopah rhyolite-dacite. That this brecciated and veined white rhyolite is
intrusive is shown by the fact that it includes large blocks of the later andesite,
where it comes in contact with that rock on the southwest side of the mountain.
The brecciation of the white rhyolite near its contact with the Tonopah rhyolite-
dacite is of a nature between a flow breccia and a friction breccia. It indicates
clearly that movement continued in this uprising column of lava after hardening
and stiffening had begun, so that the cooled portions were broken and dragged
onward in a jumbled mass by the still viscous upward-flowing lava. The upward
strain, continued after further hardening, resulted in marked sheeting, and even
in gaping fissures, which were filled with calcite and other minerals by the
waters which circulated through them after the eruption.
Smaller necks. The small areas of this rhyolite near the base of Brougher
Mountain are also probably necks. They are circular or roughly elliptical and
of relatively small size. One just northeast of Brougher Mountain is about 400
feet by 150 feet in dimensions, and a shaft has been sunk 200 feet in it without
encountering any change in the character of rock.
Relative age of Oddie rhyolite. The later rhyolite is intrusive into the later
andesite at many points into the Fraction dacite breccia near Brougher Mountain,
into the Tonopah glassy rhyolite-dacite breccia at Ararat Mountain and Brougher
Mountain, and into the Siebert tuffs on the east side of Rushton Hill.
The faults near Mount Oddie and Rushton Hill, which sometimes show great
displacement, seem to cease on reaching the rhyolite, like the faults that reach
the dacite necks. At the West End shaft a column of this rhyolite has appar-
ently ascended the fault plane which runs through the shaft. Therefore the
rhyolite is younger than all the other formations excepting the Brougher dacite,
and is also younger than the faulting. It is of apparently about the same age
as the Brougher dacite, and, as has been explained, is of the same nature and
origin. It is probable that the rhyolite and the siliceous Brougher dacite vol-
canoes were contemporaneous, and that adjacent vents gave outlet to slightly
differing lavas. The petrologic relationship of the rhyolites to the dacites will
presently be pointed out.
Mineral coiiipositi{>n.~ Examined microscopically, the rhyolite shows scattered
jx>rphyritic crystals in a generally fine-grained microgranular groundmass con-
sisting mainly of quartz and feldspar. The porphyritic crystals consist of quartz,
orthoclase, and occasional plagioclase, one determination of which shows andesine.
Biotite is a sparse accessory. Original magnetite and sphene have been noted.
On decomposition the rocks yield as secondary minerals quartz and sericitc.
sometimes kaolin.
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RHYOL1TES AND DAC1TES. 51
LATEST RHYOLITE OR DACITE.
Location. A few thin sheets of glassy rhyolite-dacite, which are of very
little importance, do not clearly seem to be correlatahle with the other volcanic
formations described. One of the small areas of this lies on the south side of
Mount Oddie. This rock is a black, very glassy, thin flow, overlying a coarse
stratified tuff made up of small fragments of glass. It also overlies the later
andesite in such a way as to indicate that the tuffs may have been eroded in
places from the andesites before the glassy sheet was poured out.
Similar lava occurs around the base of Brougher Mountain. On the north
side, immediately overling the tuff, is a thin bed of such lava. There seems to
be a slight unconformity between the two. Near by, the glassy lava seems to
rest on the Tonopah glassy rhyolite-dacite, which normally underlies the Siebert
tuffs, suggesting again that the tuff was eroded before the advent of the lava.
Age and origin. These flows may have been emitted from the volcanoes of
Butler, Brougher, and Oddie mountains during their earlier history, while the
cinder cones were being built up, or as the writer is inclined to believe, mainly
during their later history and so subsequent to the eruption of the Brougher
dacite. They are not observed to be more than a few feet thick. In places
small amounts of similar lava seem to have ascended as dikes, especiallv along
faults. Where it occurs as dikes, however, it may be difficult to distinguish it
from some of the glassy rhyolite-dacite lavas of other periods.
Mineral composition. Microscopically the lava resembles closelv the Tonopah
rhyolite-dacite. In a groundmass of brown glass there are porphyritic crystals
of quartz, orthoclase, striated feldspar, and biotite.
SIEBERT TOFF (LAKE BEDS).
LACt'STRIXE ORIGIN.
The white stratified tuffs form a conspicuous feature "of the geology near
Tonopah. As a rule they are beautifully and uniformly bedded, and composed of
well-assorted material. Where beds of conglomerate occur the pebbles are per-
fectly rounded. Since these sediments do not vary in character for thicknesses
of several hundred feet, it is plain that they were laid down in a large body of
standing water that lasted for a considerable length of time. That this body
was a lake is indicated by numerous general considerations derived from the
study of the geology of the surrounding region and by the presence of numerous
fresh-water infusoria in some of the strata. In contrast to the general regular
stratification, cross-bedded strata may occasionally be found, and .at one place
markings like those made by rills on a sandy shore were noted. These are
probably shore and delta features.
52 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
Size of the lake. The quantity of sediment which accumulated in this lake
shows that it was deep, and if it had a proportionate areal extent it must have been
a very important geographic feature, of which only a very small part was included
in the area mapped.
Origin of lak<- Ixmht. The lake came into existence at the close of the most
active period of the Tonopah rhyolite-dacite eruptions. These lavas, as well as those
of the preceding Fraction dacite-breccia eruptions, were poured out on a land sur-
face. The formation of the lake was due to a depression of the crust, forming an
inclosed basin, or to a climatic change with increased rainfall, or to both com-
bined. It is at least certain that there was such an inclosed basin, and while
it may have been due to unknown causes, a hypothesis to account for it is
suggested.
The extensive, active, and long-continued dacitic eruptions, which are attested
by the Heller dacite, the Fraction dacite breccia, and the Tonopah glassy rhyolite-
dacite not only poured out or showered upon the surface a great bulk of lava,
but emitted an enormous volume of gas and steam, which mingled with the
atmosphere. At the close of the active eruptions there ensued a period of com-
parative rest, as is indicated by the presence of fine-grained and undisturbed
white tuffs, which were deposited for the most part slowly. As the incompletely
occupied spaces left by the violent eruptions were filled the crust subsided of
its own weight and the basin was formed. That such collapse occurs around
centers of volcanism, consequent on the relief obtained by outbreaks, has been
proved by European geologists.
Sir Archibald Geikie. in his study of the ancient volcanic rocks of Great
Britain, refers to the plateau of Antrim in the north of Ireland, as follows:
. . . Hence the original area over which the iron ore and its accompanying
tuffs and clays were laid down can hardly have been less than 1,000 square miles.
This extensive tract was evidently the site of a lake during the volcanic period,
formed by a subsidence of the floor of the lower basalts. . . . For a long time
quiet sedimentation went on in this lake, the only sign of volcanic energy during
that time being the dust and stones that were thrown out and fell over the water
basin or were washed into it by rains from the cones of the lava slopes around.
It may here be remarked that the tendency to subsidence in the Antrim plateau
seems to have characterized this region since an early part of the volcanic period.
The lake in which the deposits now described accumulated was entirely effaced and
overspread by the thick group of upper basalts. Hut long after the eruptions had
ceased a renewed sinking of the ground gave rise to the sheet of water which now
forms Lough Neagh."
Lough Neagh, which occupies the deepest part of this hollow and covers about
one-eighth of the whole area of subsidence, is the largest sheet of fresh water in the
British Isles.*
A m-i.-nt Vulcanoeo of Great Britain, vol. '2. p. 21*. ' Op. clt., p. 1 1.-.
LAKE BEDS.
53
We may conceive that after the cessation of the outflows of basalt the territory
overlying the lava reservoir that had been emptied would tend to subside, partly by
ruptures of the crust, producing faults, and partly by a downward movement of a
more general kind. "
The same writer remarks, in his summary of observations: *
There seems to have been commonly a contraction and subsidence of the
material in the vents, with a consequent dragging down or sagging of the rocks
immediately outside, which are thus made to plunge steeply toward the necks.
Within the area shown on the Tonopah map a similar subsidence, due beyond
question to the causes mentioned, has been proved by the writer to have followed
the dacite outbreak which brought the formation of the tutf and the lake period
to a close (p. 47).
NW
SE
FIG. 9. Vertical cross section of southeast side of Siebert Mountain, showing relations of Siebert tuffs (lake beds), basaltic
flow and agglomerates, and Brougher dacite. a, finely stratified Siebert tuffs with occasional layers of rounded pumice
fragments or waterworn lava; b, basaltic agglomerate with bombs, capped by solid basalt; c. basalt; d, Brougher
dacite, intrusive neck, d', glassy marginal facies of dacite.
THICKNESS OF SEDIMENTS.
On account of the complex faulting of the district the maximum thickness of
the Siebert tuffs can not be given. On the east slope of Siebert Mountain
(PI. IX, ^1), however, an unbroken section about 600 feet in thickness is exposed
(fig. 9). As neither the bottom nor the top was seen, it is likely the maximum
is much more than 600 feet.
CONDITIONS DURING DEPOSITION.
The Siebert tuffs rest sometimes on the earlier andesite, as in the Tonopah and
California shaft; on the later andesite, as southwest of Mount Oddie; or more often
on the closely connected Fraction dacite breccia and the Tonopah rhyolite-dacite,
a Ancient Volcanoes of Great Britain, vol. 2, p. 460.
t>Op. cit., p. 473.
54 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
as is usually the case in the southern half of the area mapped. These facts show-
that before the deposition of the sediments considerable active erosion stripped
off the debris of the earlier dacite-rhyolite eruptions and bared the underlying
andesites. It is not unlikely, however, that the land which adjoined the lake and
which contributed the sediment was vigorously worn away, and that the sediments
were extended over this eroded region as a result of a rise in the lake or of a
shifting of its boundaries due to crustal movements. This idea is strengthened
by the fact that a careful macroscopic and microscopic study of the materials in
the tuffs proves that they were derived mainly from the erosion of the glassy
dacites and rhyolites. Pebbles in the tuffs, besides those of the rock,s just men-
tioned, are frequently of the later andesite, well rounded.
EXPLOSIVE ERUPTIONS OF THE LAK PERIOD.
It is probable that the quiet of the lake's existence was occasionally slightly
disturbed by small local eruptions of rhyolitie or dacitic material. Stratified
beds composed of rounded waterworn pumice fragments are sometimes found
between fine-grained strata. The imperfect bedding shows that they were
deposited more hastily than most of the strata, and each bed probabty represents
an explosive eruption. Sometimes angular fragments of obsidian occur in the
pumice. Moreover, thin sheets of Tonopah rhyolite-dacite, similar to the main
masses, are sometimes found within the tuff series.
UPLIFT TERMINATING LAKE PERIOD.
At one point on the northeast side of Siebert Mountain the tuff at its contact
with the Siebert dacite body, which is here intrusive, contains a conglomerate
f xmi which may be made significant inferences as to the conditions prevailing at
the time of its formation. This conglomerate is made up of rounded pebbles up
to 4 inches in diameter, most of which are composed of the Tonopah rhyolite-dacite,
but some are of later andesite. In it was found a fragment of silicified wood
over a foot long. This conglomerate is exposed for only about 50 yards. It dips
with the inclined tuffs, but is not continuous; in fact, it occupies a channel in
the tuffs. The change between the tuff and the conglomerate is abrupt and
complete, indicating a sudden change of conditions. All this suggests that these
pebbles are old river gravels. If this is true, the tuffs were uplifted at the close
of the lake period and became land. Immediately thereafter important outbreaks
of lava occurred, and the hypothesis may be formulated that the accumulation of
the lava beneath tin- future vents produced the uplift. A river, probably flowing
from the north (where the later andesite is now and was then exposed), brought
down the pebbles to this bed. That the banks of the stream were wooded is
shown by the now silicitied fragment.
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a
r . .: \ - ' \ '
3 (0 >
a CD >
BASALT. 55
BASALTIC ERfPTIOXS.
The conditions thus suggested could not have lasted for a long time, for at a
short distance from the conglomerate, at about the same horizon (on the east side of
Siebert Mountain near the summit), the white tuffs are overlain by l>eds of yellow
pumice breccia, full of fragments of black, slaggy basalt, a rock not known to have
been previously erupted. Small hollow spheres of pumice (lava bubbles) are
present. Some layers are made up entirely of large, angular fragments of scori-
aceous basalt. Over this lies a bed of black basalt 40 or 50 feet thick. This rude
accumulation of pumice and scoria; appears to lie unconformably on the tuffs, for it
is nearly horizontal, while the tuffs have a decided dip to the west; and the same
breccia appears at several other points on the mountain in contact with different
horizons of the tuffs. The uplifted tuff's of the same age as the river conglomerate
were probably tilted bodily to the west by a continuation of the disturbing uplift,
and after this tilting new volcanic vents were opened and there occurred a violent
explosion which scattered a relatively slight amount of basaltic material. This
explosion was followed in the neighborhood of Siebert Mountain b\- the welling
out of a thin sheet of slaggy basalt. On Brougher Mountain also a volcanic breccia
overlies the tuffs, but here no basalt is exposed.
KKdlOXAL TIl.TIXli ACCOMPANYING UPLIFT.
The uplift which preceded the explosions was not local. The westward dip of
the tuffs on Siebert Mountain is not essentially different from their general attitude
wherever found in the area mapped. There is a notably persistent north-south
strike, and a westward dip averaging perhaps 20, independent of local phenomena
These local phenomena bring about variations in the attitude; for example, near the
great Butler Mountain neck, where, as will be presently explained, the rocks have
been faulted and dragged down at the contact, there are places where the tuff is
locally folded so that it dips toward the mountain.
HASALT.
LOCATION.
Basalt in place occurs in only one small area within the district mapped
near the top of Siebert Mountain (PI. XI), although it was observed in three other
places, close to the area. Near Tonopah, on the road from Sodaville, low hills of
vesicular lava stand on the edge of the wash-covered desert valley. This lava is
an augite-olivine-basalt, containing augite and reddish altered olivine in a micro-
litic groundmass consisting of feldspar, augite, olivine, and magnetite.
The top of a broad, black mountain just north of Ararat Mountain is alos
covered with basalt of the kind just mentioned. A determination of one of the
feldspars here showed anorthite. Similar lava forms the hill east of Golden
Mountain and overlies the Fraction dacite breccia.
56 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
RELATIONS AND COMPOSITION OK BASALT OF SIEBERT MOUNTAIN.
Particulars concerning the age of these two occurrences can not be given,
but the basalt on Siebert Mountain has been more carefully studied than that
north of Ararat Mountain. The white tuffs which make up the bulk of the
mountain are overlain by a breccia of yellow pumice containing fragments of
scoriaceous basalt. This breccia probably rests unconformably on the tuffs, which
are tilted, and is overlain by a flow of vesicular basalt 40 or 50 feet thick. This
flow extends southwest of the mountain, beyond the limits of the area mapped.
Basalt inclusions occur also in the Brougher dacites (see p. 45).
Under the microscope this basalt shows small porphyritic crystals in a fine
holocrystalline groundnrass consisting chiefly of feldspar and augite. The porphy
ritic crystals are predominating pale -green augite, brown hornblende partly or
wholly altered to iron oxide by magmatic reactions, and feldspar.
AGE.
This basalt overlies the tuffs unconformably, so it must have been erupted
subsequent to the tilting. It and the tuff are intruded by the neck of dacite
which outcrops all over the summit, and which by its resistance to erosion has
created the mountain. On the east side of the mountain a fault has displaced
the basalt flow and the tuff, but has not affected the dacite (Pis. X, XI).
CHEMICAL COMPOSITION OF LAVAS.'
For the purpose of comparison the analyses of the fresh rocks of the district
have been assembled in the accompanying table. To represent the earlier andesite,
since no fresh specimen is available, an ideal type of hornblende-mica-andesite
(p. 217) has been substituted, practically identical with the analyses of the least
altered earlier andesite except as to the amount of silica. The knowledge obtained
by these analyses, though valuable, is only fragmentary, and more investigation
would certainly show a greater variation.
TRANSITIONS IN SILICA CONTENT.
The analyses have been arranged according to their silica content, which shows
the following differences: Between the basalt and the later andesite about 3 per
cent; between the earlier and the later andesites approximately 6 per cent; between
the earlier andesite and the least siliceous dacite about 6 per cent; and between
this dacite and the siliceous rhyolite about 5 per cent. This transition of silica
content is, then, fairly equable, but considering the analyses as a whole there is
a marked break between 4 and 5 that is, between the andesite-basalts on the one
hand and the dacite-rhyolites on the othe.r. The same break is shown, even
more plainly, in the iron and magnesia content, and, to a less degree, in the lime
percentage.
'jj:
\
\.
z
a.
z
o
11.
O
IT
CL
a
3
O
O
o
o
UJ
O
to
j
CHEMICAL COMPOSITION OF LAVAS.
57
A rather characteristic difference between the dacites and the rhyolites is the
predominance of potash over soda in the latter; and in this particular the inter-
mediate character of the Tonopah rhyolite-dacite is also seen.
Analyses of Tonopah
i.
2.
3.
4.
5.
6.
7.
8.
9.
10.
SiO 2
53.94
56.26
57.51
62. 16
71.71
72.31
73 00
75 56
75 66
76 57
ALO,--
16.18
16. 55
16.45
14.00
13.79
Fe s O,
5.56
3.20
3.27
1.06
1.54
FeO
1.17
2.02
2.71
.51
.26
MgO ..
2.78
2.30
2.20
.43
.56
CaO
7.32
5.07
6.06
4. 13
2 25
1 08
1 55
1 16
47
Na,O
3.89
3 25
2 76
4 07
3 21
2 56
3 50
4 20
1 70
96
K 2 O ..
2.09
3.43
2.81
3.45
4.41
4.66
4 71
4.50
4 94
5 81
H,O-
2.07
1.45
44
H 2 O+
2.61
2.56
1.15
1.38
TiO,
.73
.80
.28
.27
P.(X--
.32
.30
.07
.07
99.43
98.32
99.59
99.75
97.10
a Analyses 1, 7, 8, 9. and 10 are by Dr. E. T. Allen; analysis 2 by Dr. \V. F. Hillebraiul: analyses 3, 5, and 6 by Mr.
George Steiger.
1. Basalt, Siebert Mountain (specimen 168). This basalt is not typical chemically, containing
only 2.37 less silica than the andesite, analysis Xo. 2. It appears to fall, more accurately
considered, into the group intermediate between the basalts and the andesites, for which the
writer has proposed the name tileutite. For the same reasons that are given later for not using the
term latite, however, the name basalt will be retained.
2. Augite-biotite-andesite (later andesite), Halifax shaft (specimen 349).
3. Augite-biotite-andesite (later andesite), Mizpah Extension shaft (specimen 225).
4. Hornblende-biotite-andesite (earlier andesite). (See p. 217.)
5. Mountain dacite, Brougher Mountain (specimen 359).
6. Glassy Tonopah rhyolite-dacite, 2,700 feet north of King Tonopah shaft (specimen 661).
7. Mountain dacite, Butler Mountain (specimen 368).
8. Mountain dacite, Golden Mountain (specimen 388).
9. Rhyolite, Belmont shaft, Rushton Hill (specimen 376).
10. Rhyolite, G. & H. tunnel, Mount Oddie (specimen 337).
CHEMICAL COMPOSITION OF THE DAC1TE-RHYOLITE SERIES.
Differences and relations. The volcanic rocks which have been described as
dacites and rhyolites often differ markedl}' in composition as well as in age. For
example, the rock of Brougher and Butler mountains is quite different from that
of Mount Oddie, as is evident to every one, be he geologist or not. Yet the two
rocks are closely related and there are transitions between them, as represented,
for example, in the rock in parts of Golden Mountain.
58
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
Comparison with Eureka and Washoe dacite* and rhyolites. It is important
to ascertain the position of the Tonopah rocks with reference to (1) dacites and
rhyolites which have been described by Becker and by Hague and Iddings from the
neighboring and closely related districts of Washoe and Eureka (for these districts
and their rocks will often be compared with Tonopah in the present report), and
(2) to the system of igneous rocks as a whole. The comparison with the Washoe
and Eureka rocks is shown by the following analyses," which are arranged
according to silica content.
Analyses of rlacite and rhi/olite from Tonopah ami other districts in Nevada.
i.
2. i 3.
4.
5.
6.
7. 8.
9 -
10.
11.
12.
SiO 2 ....
A1,O H ...
67.03
16.27
69.96 71.71
15.79 14.00
72.31
13. 79
73.00
73.07
11.18
73. 09 73. 91
14.47 15.29
75. 56
75.66
75.69
12.26
76.57
Fe,O,
2.50 1.06
1.54
2.30
FeO
3.97
51
.26
2. 99 . 89
2.93
MizO
1 19
.64 43
56
39
CaO....
3.42
1.73 2.25
1.08
1.55
2.02
1.13 .77
1.16
.47
1. 13
Na,O - . .
K 2 O....
TiO,
2.93
3.96
.58
3. 80 3. 21
4.12 4.41
28
2.56
4.66
.27
3.50
4.71
1.19
6.84
2.77 3.62
5. 07 4. 79
4.20
4.50
1.70
4.94
3.01
4.74
.96
5.81
PA -
.23
07
.07
: .07
.06
SO,
i
1. Dacite, Eureka, Nev.
2. Dacite, Washoe, Nev.
3. Brougher dacite, Brougher Mountain, Tonopah (specimen 359).
4. Tonopah rhyolite-dacite, Tonopah (specimen 661).
5. Brougher dacite, Butler Mountain, Tonopah (specimen 368).
6. Rhyolite, Washoe, Nev.
7. Rhyolite, Eureka, Nev.
8. Rhyolite, Eureka, Nev.
9. Brougher dacite, Golden Mountain, Tonopah (specimen 388).
10. Rhyolite, Rushton Hill, Tonopah (specimen 376).
11. Rhyolite, Eureka, Nev.
12. Rhyolite, Mount Oddie, Tonopah (specimen 337).
There is a close relation between Nos. "2 and 3, dacites from Washoe and
Tonopah (Brougher Mountain). The.se rocks are plainly almost identical, and
suggest the general correlation of the dacites of the two districts, although the
high silica content of No. i, dacite from Tonopah (Golden Mountain), has caused
it to )>e placed in the table l>etween a Eureka rhyolite and a Tonopah rhyolitr.
Retetitum of t lie tit'in dacite. The analyses represent a series of closely related
rocks which show a transition from No. 1, which has nearly the composition of an
"The Eureka und Waahoe analvxes are taken from Mon. I'. 8. Geol. Survey, vol. 20. pp. 261,
CLASSIFICATION OF RHYOLITIC BOCKS. 59
andesite, to No. 12, an extremely siliceous and potassic rhyolite/' Separation of this
series into dacites and rhyolites is evidently largely arbitrary; hut the dacites and
rhyolites of Tonopah appear to be roughly comparable to those of Eureka and Washoe,
and as they are on the whole distinct rocks (in spite of the transitions) it is desirable
to have separate field names for them. For this reason it seems advisable to the
writer to retain the field name dacite for the less siliceous and alkalic of the dacite-
rhyolite rocks at Tonopah. 6
Rhyolitic nature of loth dacites and rkyoJite*. To determine the position of the
Tonopah dacite-rhyolites in the system of igneous rocks the writer has compared
their analyses with similar analyses. As almost all comparable rocks have been
classed as rhyolites, this designation would apply to these rocks, and there would be
no distinction between the white siliceous rock of Mount Oddie and the darker rock
of Brougher Mountain. If the region had been mapped without strict accuracy and
detail, therefore all these phases would probably have been included together and
mapped collectively as rhyolites, and the significance of their relations would have
been lost sight of.
Determination according to a quantitative classification. The word rhvolite is
part of the old-established classification, and its meaning is indefinite and inexact.
Undoubtedly the most notable attempt at an exact classification of igneous rocks is
that recently made by Cross, Iddings, Pirsson, and Washington/' Their own char-
acterization of the sj'stem is as follows:
"This system is a chemico-mineralogical one. All igneous rocks are classified
on the basis of their chemical composition, and all rocks of like chemical composi-
tion are grouped together. The definition of the chemical composition of a rock is
expressed in terms of certain minerals capable of crystallizing from a magma of the
given chemical composition, and the expression is quantitative."''
a Such rooks have been called tordrillite by the writer. Am. Geologist, vol. 25, p. 230.
("Since the classic work done in Nevada by Zirkel, Hague and Iddings, Becker, and others, some further division in
petrographic nomenclature has been made in rocks similar to those which they studied. Brogger has given the name
monzonite to granular rocks occupying an intermediate chemical position between granites and diorites. This group
therefore is made up of rocks which previously were classified either as granitesor diorites. Dr. F. L. Ransome has followed
out this idea and assigned a special name latite to extrusive rocks having a monzonitic composition. This new division is
made upof rocks previously classified as rhyolites, dacites, and andesites The Sierra Nevada volcanic province whose latites
were described by Dr Ransome is probably part of the same petrographic province as that in which Nevada lies (Spurr.
3. E., Jour. Geol., vol. 8, No 7, p. 638). Latites, indeed, are abundant in Nevada, and have there been described by the
writer; and mouzonitesare also present (Spurr, J. E . Bull U.S. Geol Survey No. 208, pp.53, 59, 73, 92, 108. 118, 122.126. Hl.lstij.
The latites correspond to a part of the dacites and andesites described by the earlier investigators in the region, as previously
pointed out by the writer (Spurr, J. E., Jour. Geol., vol 8. no. 7, p. 643). Thus a number of the dacite and andesite analyses
given for the Washoe and Eureka rocks would to-day be doubtless classified as latite by most petrographers.
Under the newer nomenclature and subdivision, therefore, the rhyolitic series at Tonopah would pass with decreasing
silica.increasing lime, and attendant changes to&lntite rather than a docile, and this is theclassification which the writer would
use were the Tonopah district an independent problem. Actually, however, the correlation of these Tonopah lavas with
those already described at Washoe and Eureka, as well as other parts of Nevada (Spurr, J. E., Jour. Geol., vol. 8, no. 7, pp.
621-646), is a highly important feature of the investigation; and most of the previous work on this region has been stated
simply in terms of basalt, andesite, dacite, and rhyolite. Thus the writer would be compelled to reorganize completely the
literature of the province in order not to introduce more confusion than illumination, and this task he does not at present
feel able or anxious to undertake.
< Quantitative Classification of Igneous Rocks, 1903.
<* Washington, H. S., Chemical analyses of igneous rocks: Prof. Paper U. S. Geol. Survey, No. 14, p. 47.
60
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
Rocks of different mineralogical but similar chemical composition are riot dis-
tinguished, therefore the classification is one of magmas, and is especially valuable
in discussions of the relation of magmas.
The Tonopah dacite and rhyolite analyses (the last six in the table on p. 57)
were classified according to this system. The results are as follows:
Position of Tonopah rhyolites and (Incites in the quantitative classification.
No. anal,
in table
on p. 57.
Speci-
men
No.
,
Field name.
Locality.
Class.
Order.
Rang.
Subrang.
Name.
5
359
Dacite
Persalane.
Quardofelic.
Domalkalic.
Sodipotassic .
Toscanose.
368
tain.
do
do
do
do
Do.
g
388
dacite.
do
...do ..
do
...do...
do
Do.
6
661
1 mile N. of King
do
Quarfelic . . .
do
....do
Tehamose.
9
376
rhvolite-
da'cite.
Rhvolite ..
Tonopah shaft.
Belmont shaft
do
do
do
l>opotassic. ..
Magdeburgose.
10
337
do
G. and H. tunnel,
Mount Oddie.
do....
do
do
lo
Do.
Thus it is seen that all the Brougher dacite falls under one subrang, toscanose;
the rhyolite falls under a quite distinct order, rang, and subrang, magdeburgose;
while the Tonopah rhyolite-dacite is in the same order as the rhyolite (though nearly
in the same order as the Brougher dacite) and otherwise like the Brougher dacite:
so that it falls into the subrang tehamose.
These divisions correspond to the natural divisions; and the classification is
evidently in this case a suitable one.
It may be added that the dacite from Washoe, Nev. (analysis No. 2 in table on
page 58), is classified by Washington" as toscanose, like the Tonopah dacites, and
rhyolite from Eureka, Nev. (analysis No. 7, p. 58), as mihalose (near dellenose).*
It is of the same order and rang as the Tonopah rhyolite-dacite of Tonopah, but
of a dopotassic subrang, like the Tonopah rhyolite, and is, therefore, intermediate
between these two Tonopah rocks.
Varying composition of lavas in different vents. The transition phases of the
dacite-rhyolite are not limited to small areas, but are represented by large masses;
so that there is no fixed point, either theoretical!}' or in the field, where one can be
separated from the other. Each vent, now represented by a more or less separated
and isolated volcanic plug, seems to have ejected nearly homogeneous lavas that
differed slightly in composition from the lavas from neighboring vents. Thus the
wilica content in the dacite-rhyolite series was least in the Brougher Mountain vent,
and increased successive!}' in Butler Mountain, Golden Mountain, Rushton Hill, and
Mount Oddie. The difference between the lava of Brougher Mountain and that of
Mount Oddie is very considerable. When compared with the Brougher Mountain
nOp. clt., p. 167. &0p. cit., p. 181.
DIFFERENTIATION OF LAVAS FROM A UNIFORM TYPE. 61
lava, the lava of Mount Oddie shows an increase of 4.86 per cent silica and of 1.41
per cent potash; and a decrease of 2.25 per cent soda, and probably 2 per cent lime.
The course followed by this gradual change from Brougher Mountain to Mount
Oddie by way of Butler Mountain, Golden Mountain, and Kushton Hill, is almost
circular; and while more extended knowledge is desirable, it has probably a signifi-
cance, for, as already explained, all these vents belonged to the same period, though
they were not necessarily absolutely contemporaneous. They may well have been
successive centers of outbreak in the order given.
THEORY OF DIFFERENTIATION OF TONOPAH LAVAS FROM A UNIFORM TYPE.
PSEUDOMORPHS IX RHVOLITE.
Character of pseudomorphs. The description of the first specimen of rhyolite
analyzed, as seen under the microscope (column 10 in table on p. 58), is as follows:
Specimen 376, from Belmont shaft, 50 feet down. This rock shows to the naked
eye small fresh crystals of orthoclase (sanidine), quartz, and biotite in a pinkish-
white groundmass. Abundant small, dull-white spots often have crystalline form,
and seem to play the part of phenocrysts.
Under the microscope the rock is seen to be fresh. The sanidine shows
sometimes Carlsbad twinning; it is often broken, and may be partly resorbed by
the magma. The quartz is frequently in dihexahedral crystals, rounded and
invaded by the resorbing magma. The biotite is fresh, in small crystals, and in
very small amount. The groundmass is a fine microgranular aggregate of quartz
and feldspar.
Some of the dull-white spots noticed in the hand specimen are without
crystal outlines, while others have sharp outlines. Inspection of u number of
longitudinal and cross sections leads to the conclusion that the forms are probably
those of hornblende. The material, however, is evidently pseudomorphous, for
it is a fine transparent aggregate of low single and double refraction, which
under high powers is seen to be spherulitic. It separates itself from the rest of
the groundmass chiefly by its greater fineness. In several cases small biotite
crystals were observed in this aggregate, as large as many in the rest of the
rock, and these were clustered together with a tendency to a diverging or radial
arrangement.
The description of the second specimen of rhyolite analyzed is as follows:
Specimen 337, from face of G. and H. Tunnel, Mount Oddie, contains larger
phenocrysts than usual of quartz, orthoclase, and a little biotite in a fine microgran-
ular groundmass of quartz and orthoclase. The feldspar is glassy and fresh sani-
dine. The biotite contains apatite crystals, which are clear, not smoky like those
of the andesites.
62 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
After the observations made on the areas apparently pseudomorphous after
hornblende in No. 376, similar areas were looked for in this rock. They were
at first not evident, but some definite though irregular areas of a fine aggregate
similar to the pseudomorphs referred to were found. On close observation,
however, faint but distinct crystal forms shaped like those of No. 337 were dis-
tinguished. The area occupied by these forms is surrounded by a border of
similar fine aggregate, running irregularly off into the rest of the rock, which
so obscures the crystal-like outline that it would not have been detected save for
the observations made on No. 337. This aggregate is somewhat coarser than in
No. 337, and its nature can be determined. It is semispherulitic and semigranular,
and differs from the rest of the groundmass only in being slightly finer grained
and containing a little more biotite. It is a fine mixture of quartz, orthocluse
(sanidine), and biotite. Very small idiomorphic crystals, or phenocrysts, of sani-
dine form part of the aggregate. It thus appears that the original hornblende
(in part pyroxene?) substance has been replaced by rhyolitic material.
Jfugmativ or'ujin of psevdomorphs. Since these pseudomorphs in No. 337 are
often in direct contact with perfectly glassy sanidine, they must be of magmatic
origin and must have been formed before or during the consolidation of the
rhyolite. It is probable that they represent hornblende, which was an earlv
mineral to crystallize and was afterwards decomposed by the siliceous magma
and pseudomorphosed to biotite and the fine aggregate. The process was plainly
a partial replacement of some material by others, for no mineral containing lime
in any quantity resulted. Indeed, it is somewhat difficult to determine where
the lime went to, for the analysis of the rock shows only so much lime as is
commonly contained in orthoclase. It seems difficult to explain .such a process
as this without supposing a chemical change in the magma.
HOUXHLEXDE IX TOXOI'AH LAVAS.
No hornblende or augite has been found in the white Tonopah rhyolites.
In the Tonopah rhyolite-dacite no fresh hornblende was seen, but there was
found in it one pseudomorph after hornblende, marked by crystals of specular
iron, the hornblende having been resorbed by the magma (p. 41). In the glassy
Tonopah rhyolite-dacite also only one small crystal of augite was seen out of very
many thin sections examined. In the Brougher dacite hornblende is rare, but has
been occasionally found. A specimen of dacite from Golden Mountain, at a point
south of the top of Mount Oddie, showed a single fresh hornblende crystal. This
Golden Mountain dacite is, as shown by the analyses (p. 58), closely related to the
near-by Oddie rhyolite, so that, as has already been mentioned, the two must be
considered as variations of a single magma. Augite is rare in the Hrougher dacite.
DIFFERENTIATION OF LAVAS FROM A UNIFORM TYPE. H3
but is occasionally met, more often than hornblende. In all the dacites and
rhyolites,, the dark mineral is almost exclusively biotite. The earlier andesite, on
the other hand, contains abundantly both hornblende and biotite, with some augite,
while the later andesite contains much augite and biotite, with some hornblende.
The basalt, again, contains abundantly both augite and hornblende, the latter often
partly or wholly resorbed by magmatic action and pseudomorphosed into aggre-
gates of iron-oxide crystals. No biotite is present. The presence, or evidence of
the former presence, of hornblende is thus shown in nearly all the Tonopah
volcanics, from the very siliceous to the very basic, and emphasizes their consan-
guinity. But the number of hornblende crystals (it is possible that some of these
pseudomorphs were also after augite) indicated by the pseudomorphs above
described as having been originally present in the unconsolidated rhyolitic magma
is large, being equaled only in the earlier andesites and the basalts.
DERIVATION OF KHYOLITE AND HASA l,T FROM INTERMEDIATE MAKMA.
Statement of theory. The Oddie rhyolite is considerably separated from the
earlier andesites in age, while it was nearly contemporaneous with the basalt of
Siebert Mountain. In this basalt the partly corroded and pseudomorphosed
hornblende crystals indicate that the hornblende was an earlier crystallization, not
entirety stable under the later conditions of the magma, which produced naturally
iittgite. That is to say, both the highly siliceous rhyolitic magma and the basic
basaltic magma developed, as first mineral, hornblende, which in each case was
unsuited to later conditions; the magma of the rhyolite became more siliceous and
alkaline, so that biotite was formed as the dark mineral, and that only sparingly;
the magma of the basalt became more basic and calcareous, so that abundant
augite was formed. If this is so, then these two magmas at the time of the Hrst
hornblende crystallization must have been more nearly intermediate in nature and
approached each other more closely; and as they were erupted at nearly the
same locality they may possibly have been nearly or quite the same. Such a
common intermediate magma might have a composition like that, for example,
of an andesite. These considerations would harmonize with the hypothesis that
the writer adopted several years ago, that the contemporaneous "complemen-
tary" rhyolites and basalts of the Great Basin region were the results of the
splitting up of a magma of intermediate composition. a
jRhy elite-bandit differentiation theory tested by analyses. Complete analyses of the
basalt and of the Oddie rhyolite were, unfortunately, not made; one partial analysis
of each shows the relative amounts of silica, lime, and the alkalies. These analyses
may be compared in considering the theory that the basalt and the rhyolite are the
"Succession and relation of the lavas of the Great Basin: Jour. Geol., vol. s, pp. 621-616.
64
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
two parts of an original andesitic magma. The average of the anatyses of these
rocks resembles the analysis of the type of hornblende-mica-andesite, taken as a
standard in default of any fresh andesite of this kind in Tonopah (p. 217).
Comparison of the means of the analyzes of rhyolltic and basaltic rocks of Tonopah with those of andesitic rocks.
1 (376).
2 (168).
I
3.
4.
5.
6.
SiO 2
75.66
53. 94
64.80
62.16
65. 13
65 68
CaO.
.47
7.32
3.89 '
4.13
3.62
3.50
NajO
1 70
3.89
2.79 !
4.07
2 93
3 20
K 2 O
4.94
2.09
3.51
3.45
3.96
3 37
1
1. Siliceous rhyolite, Belinont shaft.
2. Basalt, Mount Siebert.
3. Average of 1 and 2.
4. Average type of andesite.
5. Andesitic pearlite, Eureka."
6. Mica-andesite, Washoe. ''
COMPLEMENTARY NATURE OF DACITES AND LATET ANDESITES.
The fact that the rhyolite and basalt of the district were nearly contemporaneous
and probably complementary, and were perhaps derived from an original magma
like that of the earlier andesite, suggests that the later andesites and dacites, whose
eruptions in a general way intervened c between those of the earlier andesite and
of the rhyolite-basalt, may also be complementary and represent an earlier stage
in the differentiation.
There is available a single complete analysis of the dacite made from a typical
specimen of the Brougher dacite '' of Brougher Mountain (No. 359). There are, as
before stated, two complete analyses of the fresh later andesites (Nos. 225 and 349,
p. 57). To determine how far the dacite and later andesite may be complementary,
these analyses have been added together and halved.
The average of No. 349, perhaps the freshest specimen of later andesite, and of
No. 359 (dacite) is given in column 1 of the following table. The average of two
analyses of fresh later andesite (Nos. 349 and 225) was averaged with the dan to
analysis. The result is given in column 2.
o Won. V. S. Oeol. Survey, vol. 20, p. 264.
' Ibid., p. 282.
This applies lo the Heller daelte, the Fraction dacite breccia, and the Tonopah glassy rhyolite-dacile. The Brougher
dacite In an exception, Immediately succeeding the basalt eruption of Mount Siebert, and being probably nearly contem-
poraneous with the Odillc rhyalite.
rfSlnce this part of the re|x>rt was written, an analysis was made of the glassy Tonopah rhyolite-daeite (No. 661)
north of the King Tonopah shaft, as given on p. 57. This analysis has not been introduced into these calculations, sine-i-
ll offers no new but only corroboratory evidence concerning conclusions here set forth. This will appear from thu
following average of the glassy Tonopah rhyollle-dacite (So. 661) with fresh later andesite (No. 849).
HIO,. 64.28: AI,O,. 14.98; Fe,O a . 3.55; FeO, 0.71; MgO, 1.67; CaO. 3.07: Na-O. 2.95; K;O, 4.04.
COMPLEMENTARY NATURE OF DACITES AND ANDESITES. 65
Mean composition of Tonopah daciteg and later andesites compared with composition of early andesite.
i.
2.
3.
4.
SiOj
63.98
64.29
65.68
65 13
A1,O...
15.09
15.18
15.87
15.73
Fe,0,
3.31
2.72
1.78
2.24
FeO
.84
1.05
1 25
1 si;
MgO
1.60
1.48
1.79
1 49
CaO
3.66
3.90
3 50
3 62
NajO
3 15
3 08
3 20
2 93
K 2 0.
3.92
3.76
3.37
3 96
To compare these results with known rocks, the nearest analyses of Washoe
and Eureka rocks are also given above. No. 3 is mica-andesite from Washoe,"
already twice referred to; No. 4 is andesitic pearlite from Eureka. These two
rocks from Eureka and Washoe are among those which are regarded (p. 219) as
closely similar to the earlier andesite of Tonopah.
By comparison of the different analyses it is seen that the dacite and the
later andesite of Tonopah added together produce an andesite of intermediate
composition, such as is usually a hornblende-andesite or a hornblende-mica-
andesite. Moreover, the amounts of silica, lime, soda, and potash in this average
are strikingly like those in the average of the partial analyses of basalt and
rhyolite, as is shown by the following table:
Analyses of siliceous andesite compared with mean analysis of rhyolite and basalt and mean analysis of dacite
and later andesite.
I
a.
b.
Si0 2
64 80
63 98
64 29
65 68
CaO
3.89
3 66
3 90
4 27
3 50
Na,O
2 79
3 15
3 08
4 08
3 20
KjO
3.51
3 92
3 76
3 17
3 37
1. Average of rhyolite and basalt (Nos. 376 and 168, p. 64).
2. Averages of later andesite and dacite (see table above).
3. Earlier andesite, Tonopah (p. 216).
4. Mica-andesite, Washoe.
Further averages of the silica, lime, soda, and potash of the dacite and later
andesite may be had by combining with the andesite analyses the partial analyses
of the dacite (No. 368) from the east end of Butler Mountain and of the dacite
"Mon. U. S. Geol. Survey, vol. 20, p. 282.
16843 No. 4205-
66 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
(No. 388) from the south side of Golden Mountain. If each of these is combined
separately with the later andesite analysis, No. 349, the result is as follows:
Comparison of mean analyses of daeites and andesites.
1 (349 and
368).
2 (349 and
388).
3.
SiO,
64.63
65.91
65.15
CaO ..
3.31
3 11
3 60
NajO
3.37
3.62
3.29
K Z O
4.07
3.96
3.83
1, 2. Averages of later andesite and dacite.
3. The average of the fresh later andesite specimens 349 and 225 is averaged with the average
of the three dacite analyses, 359, 368, and 388.
STATEMENT OF DIFFERENTIATION THEORY.
These considerations suggest that an original magma of composition similar
to that of the earlier andesite has split up by differentiation, first into a more
basic andesite (later andesite) and a siliceous dacite, and later, by continuation of
the process, into a siliceous rhyolite and a basalt, as follows:"
Intermediate andesite.
Basic andesite. Siliceous dacite.
Basalt. Siliceous rhyolite.
SUMMARY OF GEOLOGICAL, HISTORY.
Previous to the Tertiary period, Paleozoic limestone, intruded by granitic
rocks, occupied this region. With the Tertiary began a period of volcanism,
attended by the accumulation of lake sediments and subaerial deposits in inclosed
basins. These deposits began in the Eocene, and beds belonging to this epoch
are found near Tonopah, though not within the area mapped.
About 8 miles north of Tonopah and 1 mile west of the little mining camp
of Ray the writer found a series of folded gravels, tuffs, lavas, and some white,
thin limestones carrying numerous Eocene fossils. These were sent to Dr.
W. H. Dall for determination, who remarks:
"According to the literature the fresh-water beds from which these fossils
came have been referred by Doctor White and Meek to the Wasatch, or Bear
River Laramie, Eocene, which is believed to be nearly the equivalent of the
lower Eocene or Chickasawan marine Eocene (Lignitic of old authors) of our
southeastern coastal plain. The species are:
"This corresponds with the scheme for the general succession of IMVMS In the Orcat Basin, as outlined by the
writer (Jour. Qeol., vol. 8, p. 643), and reaches the same conclusion that is already arrived at from independent
considerations. It coincides, as the writer has previously pointed out, with the luw previously deduced by Iddings
from ntudy of the volcanic* of the Oreat Basin and other regions (Bull. Phil. Soc. Wash., vol. 12, p. 145).
SUMMARY OF GEOLOGICAL HISTORY. 67
" Vivipara, close to if not V. couesi; Planorbis utahensis Meek; Ancylus 3 sp. ;
and a small bivalve, probably a Corbicula, but which I suspect to be the same as
Sphserium idahoense Meek. The specimens are merely internal casts, but if they
are really Corbicula may prove to be C. occidetitalis Meek. Their condition is
too imperfect to be certain even of the genus, but the form closely approaches
that of the figures of S. idahoense.''''
These overlie the Paleozoic limestones near Ray. Similar beds were noted
at several places between Ray and Sodaville. They are probably continuous with
a part of the Tertiary deposits of the Silver Peak and Monte Cristo mountains."
The oldest of the Tertiary rocks within the area of the Tonopah map are
probably early Miocene, and so far as known the volcanic manifestation began
with an eruption of andesite. In this andesite were formed fracture zones, along
which heated waters ascended and deposited the valuable veins of the region.
Another extensive eruption of similar but slightly more basic andesite followed,
and then there was probably a period of volcanic rest and of denudation.
Eruption was resumed by the outbreak of volcanoes, which alternate!}- ejected
siliceous dacite and poured out volcanic mud and frequently pumiceous lava. Some
of the material may have been accumulated in water; most of it was probably
deposited upon the land. Later, more glassy dacite of a slightly different
composition ascended from below in irregular channels and poured out on the
surface as thin sheets, or exploded and formed tuffs. Heated ascending waters
followed the intrusive contacts of this lava and formed a group of quartz veins
which contain gold and silver, but which are less important as regards strength
and values than the veins formed after the eruption of the earlier andesite.
As these dacite-rhyolite eruptions quieted down a lake was formed in a basin,
which may have been due to a depression of the crust consequent upon the
previous copious eruptions. In this lake there accumulated quietly several
hundred feet of sediments, with occasional light showers of ash from volcanoes,
and, in the lower portions, some thin flows of dacite lava. Then the beds were
lifted and became dry land. This uplift may have been due to the accumulation
of additional volcanic material beneath this portion of the crust. Streams began
to cut into the lake beds, the uplift was continued, and the whole district was
tilted bodily to the west at an average angle of 20. After this there were
renewed outbursts, from probably new vents, which doubtless, corresponded, in
part at least, to the present mountains. On Brougher and Butler mountains
explosive eruptions occurred, the material being dacitic, like that immediatelv
preceding the lake deposits. Cones of ash, cinders, and bombs were built up,
and there were occasional very thin and scant}' glassy flows. On Siebert
a Turner, H. W., Twenty-first Ann. Kept. U. S. Geol. Survey, pt. 2, pp. 192-244; Spurr, J. E., Bull. U. S. Geol.
Survey No. 208, PI. 1, and pp. 105-106, 185.
68 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
Mountain there was an explosive outburst of basaltic material, followed by a
thin basalt flow. Subsequently columns of liquid lava welled up and stood in
the vents of the volcanoes, but did not outflow. Some of these were composed of
dacite, some of rhyolite. As these columns cooled, heated waters rose along
their contacts and deposited chalcedony and other minerals, and mud dikes were
injected into the soft intruded rocks. The explosive outbreaks and the intrusion
of these large necks must have broken the rocks into blocks and displaced the
blocks, for at this time many faults were formed.
On the cessation of this dacite- rhyolite period of volcanic activity there was
a collapse or depression around the vents. This sinking took place largely along
the fault planes, and was especially prominent around the volcanic necks, which
as they sagged dragged down blocks of the intruded older rocks with them.
Since this time, which was probably somewhere in the Pliocene, erosion has
been active, stripping away the debris covering from the dacite-rhyolite necks,
and leaving them as hills, and in general removing the surface layers from the
hills to the desert valleys.
AGE OF THE ROCKS AT TONOPAH.
It is known that all these volcanic rocks are of Tertiary age. They belong
to a series of lavas which occupy a large part of the Great Basin and whose
Tertiary age has been established.
Place of Tonopah lavas in Great Basin volcanic history. Some years ago"
the writer attempted to classify the known facts concerning the nature and
succession of the lavas in this region. He found that in many places the same
lavas occur in much the same relative quantity, have nearly the same mineralogical
composition, and give evidence of about the same relative age. Moreover, where
two or more of these lavas are found close together, their order of succession is
in general much the same, although at any given place certain members of the
series may be lacking. In no one locality has the complete succession, as
indicated by the correlation of all the sections, been observed; but in order to
find it, gaps in one place may be filled from observations in another.
The result of this comparison was the separation of the Tertiary lavas into
five successively erupted groups, as follows:
1. Rhyolites.
2. Hornblende-biotite-pyroxene-andesites, followed by dacites.
3. Rhyolites, Homi-tinies accompanied by basalts.
4. Pyroxene-andantes.
5. Basalts, sometimes acc'omi>amed by rliyolites.
aSuccemlon and relation of lava In the Ureat Basin region: Jour. Geol., vol. 8. No. 7. pp. 621-646.
AGES OF THE BOCKS. 69
At Tonopah the succession of lavas, as above worked out, may be expressed
as follows:
(a) Hornblende-biotite-andeaite.
Biotite-augite-andesite.
(b) Dacites and rhyolites, with a little basalt.
These may be assumed to coincide with 2 and 3 of the above general grouping.
Probable Neocene age. In the comparative study above referred to" available
data were accumulated for determining roughly the age of the different groups
with reference to the standard divisions of geologic time and to the different
periods of Tertiary lakes as defined by King in his .-ummary of the results of
the Fortieth Parallel Survey. The eruption of group No. 2 (the hornblende-
biotite-pyroxene-andesites, followed by the dacites) occurred between the end of
the Eocene and the latter part of the Miocene, and was contemporaneous with
the Miocene lakes, while that of No. 3 (rhyolites, sometimes accompanied by
basalts) extended from the latter part of the Miocene well into the Pliocene, to
the time of the beginning of the Pliocene Shoshone Lake. On the assumption
that the correlation of the Tonopah lavas above given is correct, the andesites,
both earlier and later, would belong to the first half of the Miocene and to the
Miocene lake period; while the dacites, rhyolites, and basalts would extend from
near the middle of the Miocene into the Pliocene, and would be partlv
contemporaneous with the latter part of the Miocene lake.
INFUSORIA IN THE SIEBERT TUFFS.
In the white tuffs at the east base of Siebert Mountain a stratum, not
distinguished in the field from the more ordinary white rhyolitic or dacitic tuff,
was shown by the microscope to be entirely made up of minute diatoms or infu-
soria. These were recognized by the writer as probably similar to species described
by Mr. King as occurring in the deposits of the Miocene lakes of Nevada. At
the time the recognized succession of lavas did not seem compatible with this
idea, and the thin section was referred to Dr. Rufus M. Bagg, jr., for examina-
tion. Subsequently, it is proper to add, new discoveries as to the lava succession
removed the difficulties in the way of considering the deposits Miocene.
Doctor Bagg's report follows:
"The material submitted me from Tonopah, Nev., for examination consists of
innumerable diatoms which belong almost exclusively to two species, Helosira
granulala, L. W. Bailey, and Melosira varians, Ag., the latter being considerably
less abundant than the former.
"Jour. Geol., vol. 8, No. 7, p. 637.
70 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
"This species, Melosira granulata, is synonymous with Ehrenberg's Gallionella
granulata, and other synonyms for the species are Melosira punctata, Gallionella
marchica, G. procera, and G. tenerrima.
"I can discover no species in the material sent me which would limit the deposit
to the Miocene age, for the most abundant form, M. granulata, is living to-day in
the Para River, South America, and elsewhere, as well as occurring fossil in
Tertiary deposits.
"There is nothing to prevent this deposit from being regarded as Pliocene if
stratigraphical evidence warrants this view, but the deposit was laid dow-n in fresh
water. In addition to the two species above given there are a few forms of Coscino-
dlscus radiatm."
COMPARISON OK SIEBERT TUFFS WITH MIOCENE PAH-UTE LAKE DEPOSITS.
Miocene deposits have been described b3' King in western Nevada" between
the one hundred and seventeenth meridian and the Sierra Nevada. These deposits
are always upturned, dipping from 10 to 25, and they are frequently cut through
and overflowed by basalt. They are usually made up of volcanic materials, and
are several thousand feet thick. They contain beds of white and yellow infusorial
silica, and on the northeast point of the Kawsoh Mountains, where the strata are
tilted, eroded, and covered by caps of basaltic rock (as on Siebert Mountain in
Tonapah), the following species were most abundant:
Gallionella granulata.
Gallionella sculpta.
Spongolithis acicularis.
These also were recognized:
Pinnubaria ilia-quails, and
Coscinodiscus radiatus.
The age of these beds is determined more especially by molluscan and
mammalian fossils, found elsewhere.
These beds, therefore, are of the same character as the Siebert tuff at
Tonopah, which was deposited in the rhyolite-dacite period, and suggest that
the lake in which the tuffs were deposited is identical with the Miocene Pah-Ute
Lake of King. 6 The tilting and amount of erosion of the Tonopiih white tuffs
prevents any correlation with the Pliocene lake (Lake Shoshone)'' beds, whose
distribution frequently bears a close relation to the present topographic basins,
and which are little disturbed.
a U.S. Geol. Expl. Fortieth Har., vol. 1, p. 412etseq. l> Op. cit., p. 454. cOp. cit.. p. 466.
AGES OF THE VOLCANIC BOCKS.
CONCLUSION.
71
It may be provisionally concluded that the volcanic rocks at Tonopah, from
the earlier andesites to the Brougher dacites and the rhyolites, were erupted
between the early Miocene and some time in the first half of the Pliocene.
The following, then, is the sequence of events as deciphered for the vicinity
of Tonopah (fig. 10):
Hypothet- Hypothet- Earlier Later Dacite
ical deep- ical deep- andeatte. andesite. breccia,
seated seated
granite, limestone.
Tonopah Lake beds. Faults,
rhyollte-
daelte.
Later da- Earlier ande-
cite and site veins
rhyolite {lesser veins
intrii- belonging to
slons. other periods
not repre-
sented).
FIG. 10. Ideal cross section of Tonopah rocks. (This section does not represent any particular place, and is simply
intended to illustrate the geologic conditionsa-s described in the text.)
Sequence of formations and erent-s in the vicinity of Tonopah.
Earlier andesite.
Fracturing.
Vein formation. Primary minerals, quartz, adularia (valencianite), carbonates of lime,
magnesium, and manganese, stephanite, polybasite, argentite, silver selenide, galena, pyrite,
chalcopyrite, etc. Values good; gold and silver, silver predominant.
Erosion.
Later andesite.
Probable erosion.
Heller dacite.
Fraction dacite breccia.
Tonopah, rhyolite-dacite breccias, flows, and dikes, intermingled with slightly stratified or
unstratified pumiceous or tuffaceous fragmental material.
Vein formation. Primary minerals, quartz, pyrite, barite. Values usually relatively low;
gold and silver, gold apt to predominate.
72 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
Erosion.
Siebert tuffs (lake beds) deposited, with an occasional thin dacite flow.
Elevation of tuffs.
Tilting.
Basalt.
Chief faulting. Affects eyerything preceding.
Rhyolite intrusion (Ararat, Oddie, Rushton hills).
Vein formation. Primary minerals, quartz, chalcedony, calcite, siderite, pyrite, etc. Values
low; gold and silver, gold apt to predominate.
Brougher dacite intrusion (Butler, Brougher, Golden, Siebert mountains).
Mineralization (chalcedony, manganese). Values slight to insignificant. Mud veins.
Erosion.
Latest rhyolite-dacite flow (slopes of Oddie and Brougher).
Erosion.
PRINCIPLES OF FAULTING.
The chief recognized faulting of the district has already been described (p. 47)
as attendant and consequent upon the Brougher dacite intrusion. The writer deems
it unnecessary to attempt to describe separately the evidence and effect of each
fault. Their locations and the general nature of their displacement are shown on
the areal geology map. Their underground courses and intersections are doubtless
complicated, and their study would constitute a geometrical problem in three
dimensions for the solution of which there are in most cases no sufficient data. On
account of the irregular thickness and extent of each of the volcanic formations
at Tonopah, projection far beyond actual observation can not safely be made; so
no general cross sections have been constructed.
Valuable observations on faulting have been made underground, however, in
some of the mines, especially where veins have afforded measures of displacement.
It has been found impracticable to separate the account of such faulting from the
discussion of the veins which they affect, so the reader is referred to such
discussions, particularly to those concerning the Fraction, Wandering Boy, Valley
View, Mizpah, and Montana Tonopah workings (pp. 115-176).
CRITERIA OF FAULTING.
It is worth while to record the manner in which the structure has been
worked out in this complicated region. Although the region mapped embraces
only about 6 square miles, and outcrops are very nearly continuous, several months
of study were necessary to reach an approximately satisfactory solution of the
areal geology. Ideas concerning the structure were successively exchanged for
newer ones as fact after fact was brought to light. The existence of faulting was
strongly suspected, from topographic; evidence, from the time of arrival in the field,
FAULTING. 73
but the final results proved that in every case the faults assumed from such
evidence were not faults, while the ultimate discovery of numerous and important
faults was due to careful study of the rocks.
When by close examination and correlation of facts the complicated and often
closely related rocks were satisfactorily separated into stratigraphic units, after
numerous unsatisfactory attempts, the most important step toward the elucidation
of the geologic history and structure had been taken. But still the most extreme
caution was necessary, for while the local geologic column was probably historically
correct for the whole district, there were many local gaps and irregularities. As
there were several periods of apparently active but irregular erosion between
volcanic outbursts and as the distribution of many of the members of the series
was limited and irregular it seemed that any member might rest directly upon &ny
older one, the intervening ones being unrepresented, while a few hundred yards
away the represented succession would be different. For similar reasons it was not
possible to reckon upon any constant thickness for any formation; in one place it-
might be a few feet thick, in others hundreds. So the ordinary stratigraphic
criteria of faulting were very inconclusive.
SIEBERT TUFF BOUNDARIES.
The key to the problem, undoubtedly was the determination of the geologic
position of the Siebert tuff, which consists of characteristic finely stratified thick
beds. In working out the structure the first thing done was to carefully follow
the limits of these Siebert tuff areas. It was found that in most cases these were
separate; they reappear in different parts of the area mapped and are bounded on
several sides by straight lines. This fact immediately suggested the existence of
numerous intersecting faults.
Where a rectilinear boundary of a Siebert tuff area ran transversely to the
strike of the beds, a fault was evident, in case the contiguous rock was not
intrusive. In the case of a surface formation, like the Fraction dacite breccia,
this evidence was conclusive, and parts of the majority of detected faults were
followed in this way. Similarly, if a fault was parallel with the strike, and the
dip of the tuff would carry it below a contiguous rock (as the Fraction dacite
breccia, for example) which was known to be lower in the geologic column than
the tuff, the nature of the contact, as due to dislocation, was evident.
DIKES ALONG FAULT ZONES.
Another criterion, perhaps not so important, was developed by the discovery
that the Brougher dacite sent out dikes along some of the faults, as along the Cali-
fornia fault. (See map, PI. XVI.) This showed at once that the dacite reached its
present position essentially subsequent to the faulting (a conclusion which was other-
74 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
wise abundantly verified), and that the dikes running out from the volcanic centers
occupied at times fault zones. These dikes were then traced, and when they were
conspicuously straight and narrow their course was critically examined to deter-
mine whether it could possibly be a fault plane. Often such dikes are intermit-
tent, appearing only in small outcrops here and there along the line, with no
visible connection. Such a condition was still more strongly suggestive of a
fracture zone. Frequently the examination of the rocks on both sides of such a
line confirmed the suspicion of faulting, and important faults were discovered in
this way.
BOUNDARIES OF LAVAS.
A.S the knowledge of the different formations increased it became possible to
draw their boundaries with frequently great accuracy. Where these were recti-
linear, as in the case of the tuft's, and could not reasonably be interpreted as
normal contacts caused by the outcropping of inclined formations, and one for-
mation could not have been intruded into the other, faults were considered to be
indicated. Even in the case of two volcanic rocks, like the earlier and later
andesite on Mizpah Hill, the boundaries, though obscure and traceable with
difficulty on the surface, could finally be determined to be rectilinear, intersecting,
and probably due to faulting. In this case the veins afforded valuable evidence,
for their outcrops were cut clean off along the fault planes.
EROSION FAULT SCARPS.
As the perception of the real connection between the strati graph}' and structure
and the topography grew, the latter often became an efficient guide. The underlying
rocks have exercised a remarkably efficient control over the surface forms. Where
two rocks of unequal hardness are brought together by faults, the harder rock will
rise above the softer in a more or less perceptible scarp. With the exception of the
rhvolites and the Brougher dacite, and to a less degree of the silicified earlier
andesite, however, the difference in resistance of the rocks is not great. The
Fraction dacite breccia and the glassy Tonopah rhyolite-dacite in the southern part
of the area mapped are chiefly friable fragmental surface deposits, while the later
andesite disintegrates rapidly. The Siebert tuff is softer than the others, and
when sufficiently removed from the influence of a protecting harder rock, forms
flat, smooth areas, on whose boundaries fault contacts are apt to be marked by
slight but pronounced scarps, usually only a few feet high, since the adjacent
rock is apt to be very little harder. These slight scarps afford strong preliminary
evidence, and invite the closest searching after stratigraphic corroboration.
Nearly every topographic feature in the Tonopah district, however small, is
due to the nature of the underlying rock; thus many straight depressions or
slight valleys are probably due to the easier erosion of a fractured or faulted
FAULTING. 75
zone, as compared with the less fractured rock on each side. Such is probably
the case with the northeast depression at the southeast base of Brougher Mountain,
and with other creases in the surface.
SCARP PHENOMENA WEST OF BROUGHER MOUNTAIN.
Some especially interesting observations on the . surface configuration as an
indication of faulting were made in the comparatively flat area in the west part
of the district mapped, west and northwest of Brougher and Siebert mountains,
respectively. Here rhyolitic-dacitic breccias, chiefly detrital, are intermingled with
tuft's, so that they sometimes can be distinguished only with difficulty from the
main overlying Siebert tuff. Where the Siebert tuff is certainly distinguishable
the rectilinear intersecting boundaries show that complicated faulting has taken
place, but the mass of rhyolite-dacite breccias offered at first little suggestion as
to structural relations.
When this area is viewed from an eminence, as from Brougher Mountain or
from the hill west of Siebert Mountain, just beyond the area mapped, there is
seen a significant series of parallel ridges which were at once surmised to indicate
the presence of faults. From the hill last referred to, these slight scarps are seen
to bound areas which have rectilinear outlines, and which are plainly distinguished
in tint from one another, one being purplish, another reddish, and so on. A minute
study strengthened the conjecture that in this region there are complicated and
numerous intersecting faults. It was concluded that these faults brought into
juxtaposition the Tonopah rhyolite-dacite breccia, the Fraction dacite breccia, the
Siebert tuff, or different parts of any one of these, and that the resulting erosion
brought out the harder blocks, which were thus bounded by straight scarps, usually
of slight relief. The Tonopah rhyolite-dacite breccia, being harder, nearly always
occupies the relatively elevated portions, while the soft, Fraction dacitic breccia and
the Siebert tuff lie in the depressions. These depressions are covered with a slight
thickness of detritus, but prospect holes show in almost every case that they are
floored with the softer breccias. The straight boundary lines are strongly con-
trasted with the irregular unfaulted contact of the glassy Tonopah rhyolite-dacite
in the north corner of the area mapped.
DESCRIPTION OF ZIGZAG SCARPS.
One or two of the most interesting occurrences of these slight scarps were
made the subjects of especial study. Between Siebert and Brougher mountains
the flat area floored by the dacitic breccias and by the Siebert tuffs reveals to
the close observer certain straight lines, which are apparently slight ridges and
depressions in the detritus, but which are really closely underlain by the soft
bed rocks, though these outcrop only occasionally. In this area the occurrence
of a number of faults was proved by stratigraphic evidence, chiefly by the
76
GEOLOGY OK TONOPAH MINING DISTRICT, NEVADA.
rectilinear boundaries of the Siebert tuffs. The position of one such fault, marked
A on the accompanying diagram (PI. XII), was determined by stratigraphic
evidence for a part of its course, as will be noted by consulting the geologic
map (PI. XI). Eastward of this part, however, it is bordered apparently on both
sides by the tuff, yet along the continuation of the line established by strati-
graphic evidence there is on its north side a slight scarp about 10 feet high.
Just north of this scarp a similar scarp, of about the same height, and, like
the former one, facing to the south, runs in a straight line, but in a direction
more nearly east and west than the one first mentioned. Toward the east the
foot of this scarp is in the bottom of a narrow depression; toward the west,
where the depression broadens, the scarp lies on the north side. In this broader
portion, however, the other side of the depression has little or no scarp, is at a
maximum of 3 or 4 feet in height, varying from that to nothing, and has no
straight or rectilinear course (fig. 11). This first-mentioned scarp is continued
FIG. 11. Cross section of water runway, usually dry (c-d of PI. XII), showing bold, straight scarp on left, believed to be
consequent on faulting, and low, curved bank on right, believed to be due to occasional drainage.
farther west, but is set off en echelon, although the corners are slightly rounded;
the set-offs are always in a northerly direction and the main trend corresponds to
that of the straight scarp farther east. With a slight interruption, caused by the
incoming of a depression which is probably due to an unusually soft fault block,
this scarp continues northwestward beyond the area mapped, and can be followed
with the eye a considerable distance farther, toward the little eminence called
Table Mountain. A sighted line along the scarp near the western limit of the
map has a general direction of N. 65 W. On examination, however, the front
of the scarp, which has a uniform height of 10 or 15 feet, and which always
faces the south, is found to be continuously set off en Echelon in the same sense
and fashion as the portion farther east. The conditions are indicated in PI. XII.
The two chief alternating directions of the scarp faces are, (1) chief, N. 85 E.,
(2) minor (set-offs), N. 45 W. Along the whole of its course the relative depres-
sion to the south of the scarp is used as a runway for the occasional surface
waters, and can easily be mistaken for a depression due simply to erosion.
However, the south side of this depression does not partake at any point of the
peculiarities of the north side, being low and irregular in course, and without
US GEOLOGICAL SURVEY
PROFESSIONAL PAPER NO 42 PL XII
DIAGRAMMATIC MAP SHOWING TWO PARALLEL ZIGZAG SOUTH-FACING SCARPS
THE SOUTHERN ONE ABOUT 1O FEET HIOH.THE NORTHERN ONE 25 FEET HIGH
By, I.E. Spun-
Scale
IQOO 5OQ O 1OOO 2OOO rjOOOfVet
Contourmtei-val2O feet
FAULTING. 77
any definite continuous scarp. Moreover, the jogs in the scarp under considera-
tion can not he explained by stream erosion, for they are not at the entrance of
auxiliary gullies, the angle of the jog forming practically an unbroken wall.
ZIGZAG SCARPS EXPLAINED BY FAULTING.
The phenomenon described can hardly be explained except as controlled by
faulting, and two intersecting systems are indicated. Corroborative evidence of
this conclusion is present. Along the western portion of the scarp where
examined there occur at different points isolated outcrops of light-colored dike
rhyolite that has the characteristics of the Oddie rhyolite, and is distinct from
the glassy Tonopah rhyolite-dacite with which it is in contact. These dikes are
intermittent rather than continuous, but form distinct jogs parallel with the set-
offs of the scarp. It is known that this rhyolite sometimes forms dikes along
faults in this district and is later than the main faulting.
CONSEQUENCES OF EXPLANATION.
The chief or longer scarp faces are parallel to the straight scarp into which
the jogged scarp runs farther east (j?, in PI. XII), while the shorter or minor
faces are parallel with the slight scarps lying a short distance farther north,
limiting probable fault blocks, as already described. It appears, then, that the
jogged scarp is the result of two sets of intersecting faults, and from the figure
it is evident that when the dimensions of the jogs are diminished the course
of the resultant will approach a straight line, and indeed may do so to such
a degree as to be practically indistinguishable from such a line. By the pre-
dominance of one set of faulting over the other set the resultant line may lie in
any given direction and may be straight or curved. The line made by joining
the points of the sharp spurs along the scarp, indicating the general resultant
of the two systems of jogs, is parallel with the scarp first mentioned, which
lies farther east (A, in PI. XII). It is possible, therefore, that this last named
straight scarp may actually be a resultant of two intersecting systems, such as
have been described.
ZIGZAG FAULT SCARP ON TONOPAH-SODAVILLE ROAD.
On the north side of the main road which leads from Tonopah toward Soda-
ville, in the western part of the area mapped, a similar phenomenon was noted.
The road lies in a depression, on the south side of which there is an irregular,
undecided embankment consisting mostly of fragmental material and having a
height of about 10 feet. On the north side there is a sharp scarp about 25 feet
high, consisting of a continuous outcrop of solid, glassy Tonopah rhyolite-dacite.
On inspection this scarp shows well-marked rectilinear courses, forming steps
78 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
or jogs, although the detail is somewhat rounded by erosion. It runs chiefly in
two directions N. 60-70 E. and N. 30-40 W. This zigzag course, and the
absence of the scarp on the south side of the depression, as in the case of the
occurrence previously described, seem to indicate a complex fault fracture, and
the directions of the rectilinear components in each case are similar. In this
case also the indentations are not due to gulches, for there is usually not the
slightest depression at the top of the scarp, at the angles. The scarp continues
beyond the area mapped. The general trend (being the resultant of the two
directions noted) is almost exactly parallel to the similar scarp previously described.
ORIGIN OF ZIGZAG FAULT SCARPS.
From the general sum of knowledge concerning the relation of faulting to
topography in this district (see p. 114), it is inferred that probably these slight
scarps are due to differential erosion and mark the limits of fault blocks which are
slight^' harder than those contiguous. Their invariable slight relief strengthens
this idea. Similar scarps, which have been proved to have originated in this
manner, are characteristic of fault contacts in other portions of the area mapped.
The other possible hypothesis is that the faults are recent, and that the scarps
have formed as a result of direct displacement of the surface. In spite of the
fact that the probabilities seem to favor the first explanation, certain features
support the second. One of these is that scarps of this sort, like those just
described, sometimes have on each side material belonging to the same formation,
as the scarp marked B in PI. XII, which has tuff on both sides, or, as the scarp
last described, on the Tonopah-Sodaville road, which has the glassy Tonopah rhyo-
lite-dacite on both sides. If these surface features are due to erosion, the higher
block must be slightly harder than the lower and must represent a slightly more
resistant part of the formation. This indeed is true in the place last mentioned,
where the glassy Tonopah rhyolite-dacite in the area north of the road is the solid
intrusive lava, while the formation included under the same head in the region
south of the road is surface material, breccias and tuffs, and therefore more fragile
and more easily eroded. Another circumstance which also favors the idea of
direct displacement is that the two chief compound scarps just described both
face the south. It is known from independent evidence that the southern part of
the area mapped has been downthrown in respect to the northern part, so that a
slight continuation of the general movement into very recent times might result
in these south-facing scarps.
FAULTING. 79
ORIGIN OF ZIGZAG FAULTS.
Zigzag fault courses like those described may originate in two ways: (1) By
the intersection of independent fault systems which produce a zigzag line of equal
dislocation oblique to both the intersecting systems, as explained in the considera-
tion of the Wandering Boy fault (pp. 157-161); and, (2) by a simple fault whose
initial movement follows a zigzag course along previously existing fractures.
INTRUSIONS CONTROLLED BY INTERSECTING FRACTURES.
Rectilinear boundaries or rectilinear boundary scarps do not always indicate
faulting in the sense above described, where one of the rocks is intrusive. A case
is furnished by the outline of the Golden Mountain intrusion. As shown on the
map, the contact of the Golden Mountain dacite with the earlier andesite, on the east
side of Gold Hill, is so straight as to suggest the possibility of faulting. Moreover,
east of Gold Hill the long south contact of the same intrusion follows alternating
straight northwest-southeast and northeast-southwest courses, strongly suggesting
the resultant of two intersecting systems of faults, similar to the scarps already
described. But excellent evidence that the contact has not been faulted is present
in the band of dacite glass which represents the quickly chilled lava along the
margin of the intrusion, and which was found to follow the contact along its differ-
ent courses.
It appears that the straight western limit of the intrusive Brougher dacite along
Gold Hill, above referred to, has been determined by a preexisting fault, for the
continuation of this fault is evident near the California-Tonopah (California fault),
where a dike from the main dacite mass follows the fault zone. In this light, also,
it seems probable that the rectilinear courses and the set-offs regularly in the
same direction on the south side of the Golden Mountain indicate that the intrusive
contact was here also determined by a system of preexisting intersecting faults or
fractures.
CORROBORATION OF CONCLUSIONS.
A number of faults that were located on the surface by the methods above
given were subsequently found in mine workings and observed more closely and
satisfactorily. The Mizpah fault was recognized at an early stage in the investiga-
tion, both on the surface and underground. The Burro fault, distinguished and
followed with great difficulty at the surface, was subsequently developed under-
ground. The Wandering Boy and Fraction faults, first distinguished on the surface,
were subsequently found to be well exhibited in the mine workings.
80 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
ACCURACY OF FAULT MAPPING.
In this volcanic region faults can very often not be distinguished at all. This
is the case if similar rocks lie on both sides of a fault and other signs fail. There-
fore on the map some faults have been projected a reasonable distance and probable
connections made across spaces intervening between different fragments of what is
probably a single fault line. While the structure as finally depicted is undoubtedly
not strictly accurate in many details, the general features are well shown, and the
error, were a closer study possible, would undoubtedly be found to be not that too
many faults are represented, but that many have escaped detection.
FAULTING DUE TO VOLCANIC ACTION.
The faulting in this district is of extraordinary interest, for the origin,
time, and cause are clearly understood. It is rare that any explanation other
than a general unsubstantiated hypothesis can be applied to any particular case
of faulting. Here, however, it is plain that the faulting was the result of
adjustments of the crust to suit violent migrations of volcanic rock; that it
originated with the swelling up of the crust and its forcible thrusting up and
aside to make way for the numerous columns of escaping lava; and that after
the cessation of the eruptions it was continued by the irregular sinking of the
crust into the unsolid depths from which the lavas had been ejected. It can
readily be seen that all sorts of pressure (from below upward, lateral, and
downward, by virtue of gravity) must have been concerned in such movements,
and that the first faults were due rather to upward and lateral irregular thrusts,
while the later ones (in many cases along the same planes as the first) were
due to gravity. So reversed and normal faults are equally natural, and both
occur frequently.
APPLICATION OF PRINCIPLES TO REGIONS LYING BEYOND AREA MAPPED.
These observations are probably not of slight and local significance. The
faulting is intense, and the faults have frequently very great displacements,
amounting to many hundred feet at least. Moreover, considerable areas are
affected by subsidence or elevation connected with and in part, at least, accom-
plished by faulting, as, for instance, the relative depression of the southern part
of the area mapped (near the dacite necks), as compared with the northern portion.
The cause of these larger movements is plainly the same as that of the individual
faults. Evidently such phenomena are not confined to the area mapped, but
extend indefinitely beyond it. The writer at first looked upon the faulting at
Tonopah as exceptional and local, and not to be connected with ordinary
faulting in the Great Basin; but there now appears no reason for doubting
FAULTING. 81
that the phenomena within this small, carefully studied area are typical of the
unstudied similar volcanic region beyond the limits of the map.
The individual faults have been shown to have been minor, irregular
movements attending broader elevations or depressions; and the hypothesis has
been presented that at an earlier period the lake basin in which the Siebert
tuffs were laid down was formed by general subsidence of an area that was
occupied by earlier eruptive rocks (the earlier dacitic eruptions) and that this
basin was destroyed by a broad uplift which preceded the later dacitic outbursts.
There is little doubt that these earlier movements were attended by some faulting,
although such faults would be difficult of detection, especially in the presence of
the subsequent complicated faulting of the period of the later dacitic intrusions.
SUGGESTED EXPLANATION OF GREAT BASIN TERTIARY DEFORMATIONS.
The recognition (pp. 52, 70) of the facts that the lake in which the white tuffs
were laid down was a very large one, and that it very likely corresponds to the
great Miocene Pah-Ute Lake of King, gives a broader interest to this hypothesis
of its origin; and the hypothesis naturally extends itself to the other Tertiary
lake basins which preceded and followed the Pah-Ute.
In the great interior province in which Tonopah is situated, and which lies
between the Wasatch and the Colorado Plateau on the east and the Sierra Nevada
on the west, a number of successive lake basins of van-ing extent formed during
the Tertiary, as was first shown by King. These changing basins, of varying
shape and extent, were due to uneasy continual warpings (elevations and depressions)
which continued through the Tertiary period down to the present day. This
warping has been contemporaneous with folding and faulting, and all of this
crustal disturbance has been accompanied by volcanism.
"In general the period of deformation which lasted from the Mesozoic to the
present has been contemporaneous with volcanic activity. By far the most energetic
vulcanism, so far as we know, occurred in the Tertiary, beginning probably in late
Cretaceous or early Eocene and extending into the Pleistocene. Vulcanism and
deformation were, therefore, allied phenomena.""
In the earlier recognition of this coextension of the two phenomena of deforma-
tion and volcanism the writer's conception was that they were both the result of a
single unknown cause. In the light of the Tonopah studies, however, it seems fair
to admit that the former may have been the result of the latter, the effect of the
repeated accumulation and eruption of vast bodies of molten material, and the sub-
sequent subsidences and local adjustments.
aSpurr, J. E., Origin and structure of the Basin ranges: Bull. Geol. Soc. America, vol. 12, p. 248.
16843 No. 4205 6
82 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
CONTINUANCE OF VOLCANIC EPOCH.
Viewed in this or in other lights, there is small reason for believing that the
period of volcanism in this province is past. It rather appears that we are still in
it. The occurrence of recent almost undefaced basaltic craters at various points,
such as at Silver Peak (PI. XV, A), at Lake Mono, and in central Oregon, show
that the last eruptions occurred only a few hundred years ago, while the evidence of
enormous Pleistocene and recent elevation and subsidence, especially in the western
part of the region, near the Sierra Nevada," suggests the migrations of the molten
tide beneath the present crust.
a Spun, J. E., op. cit., p. 247, 248; also Bull. U. S. Geol. Survey No. 208, pp. 110, 129, 209, 210, etc.
CHAPTER II.
MINERAL VEINS.
VEINS OF THE EARLIER ANDESITE.
PERIOD OF MINERALIZATION.
The most important veins of the Tonopah district occur in the earlier andesite
and do not extend into the overlying rocks; hence, where the earlier andesite -is
not exposed at the surface the later rocks form a capping to the veins, and this
capping must be passed through before anything can be learned of the presence
or the nature of the veins beneath. This fact shows pretty plainly that the veins
were deposited before the eruption of the later andesite and immediately after
that of the earlier andesite, for the period of erosion between the two andesites
seems to have exposed the veins at the surface, indicating that they were formed
before this period or early in it. Indeed, there is every evidence that the veins
were formed by ascending hot waters succeeding and connected with the earlier
andesite intrusion, and that these waters had become inactive by the time of the
later andesites.
NATURE OF CIRCULATION CHANNELS.
The openings which afforded channels for these ascending waters were of the
nature of sheeted zones. The rock was complexly fractured, apparently soon
after cooling, and probably as a result of the stresses exerted by the still active
volcanic energy below. A major set of fractures extended in an east-west direc-
tion and zones of close-set parallel fractures attained a maximum thickness of
several feet. These became the chief channels of circulation. In places the
circulating waters divided into separate channels, which diverged and frequently
reunited, and many lateral channels were favorable to egress of the waters.
These channels, however, were apt to get poorer as the distance from the main
fracture zone increased.
The conditions above stated are clearly shown by a study of the veins of
Mizpah Hill and vicinity (fig. 12). The circulation channel now occupied by the
Mizpah vein may be taken as a type of the main fracture zones, and the diverging
Burro veins, dwindling as they increase their distance from the master veins,
represent the lateral channels. The splitting and reuniting is shown by the
structure of the veins at many points.
83
84
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
VEINS DUE CHIEFLY TO REPLACEMENT.
That the circulation channel was in practically every case a fracture zone and
not an open fissure is shown by the study of the veins, which reveals all stages
in the change from a fracture zone in porphyry to a solid quartz vein. In many
cases the vein consists simply of a zone of more or less altered andesite, not
essentially different, except, perhaps, for a somewhat greater silicification, from
N
I >
North Star Shaft a
Tonopah Extension Shaft
i MacNamara Shaft
Montana Tonopah Shaft B
Tonopah Mining Company
a Main Shaft
:.'lzpah Vein
West End Shaft
B Fractlon No.l Shaft
Fraction No.2 Shaft x^y
B Wandering Boy Shaft
Fraction NoJ Shaft
B Silver Top Shaft
Stone Cabin Shaft
Valley View
Vein Group
Gold Hill Shaft
Tonopah City Shaft
300 200 100
California
Tonop.'ih
" Shft_
FIG. 12. Map showing outcropping veins of Tonopah.
the andesite which forms the walls. This zone is cut by parallel fractures having
the same strike and dip as the walls, and the walls themselves are nothing more
than stronger fractures of the same kind. In the next stage, where part of this
fractured zone becomes altered to quartz, the main wall fractures have been the
most favorable for water circulation, so that sometimes a hanging-wall streak of
quartz and a foot-wall streak are found with only altered andesite between.
I-
;:
VEINS OF THE EARLIER ANDESITE. 85
Sometimes, also, either the hanging-wall or the foot-wall streak may be wanting.
Next, -streaks of quartz parallel with the walls may be found, or the quartz may
form a network in the andesite. Thus the process may be traced to the stage
where the whole of the andesite is replaced by quartz, forming a solid vein
several feet in width. As a rule, however, more or less decomposed andesite
forms part of the vein.
PORTIONS OF VEINS DUE TO CAVITY FILLING.
As exceptions there are found streaks of quartz, usually small, within the
vein, which show crustification and comb structure and thus bear evidence of
having been formed in cavities. These cavities, however, were often of irregular
shape and were not fissures, properly speaking, but spaces of dissolution, and
were the effect of the mineralizing waters themselves.
The largest example of a crustified vein is found in certain parts of the
Montana Tonopah workings, where the cavities were sometimes 2 or 3 feet in
diameter and gave rise to well-banded ores (PL XIII).
CROSS WALLS AND ORE SHOOTS.
The fractures transverse to the main system had a not inconsiderable effect
in determining the course of the ore solutions. Along important transverse
fractures it has been found that the vein frequently widens or narrows abrupth*,
the cross fractures playing the same part as the lateral wall fractures, even if not
to such an extent, and so earning the name of cross walls which has been given
them. To these cross walls, more or less pronounced, the division of the water
circulation along the main zone into columns of unequal importance was due, and
hence the mineralization accomplished by these waters was correspondingly
localized. It is probable that the recognized ore shoots or bonanzas had their
origin in this way.
NATURE OF MINERALIZING AGENTS.
That the mineralizing agent was water is evident from the character of the
vein and from the nature of the alteration of the wall rock. That its action was
probably connected with the earlier andesite eruption is shown by the fact that
it followed this and, at least so far as mineralizing activity was concerned, was
of limited duration, for its effects have not been determined in the succeeding
later andesite. It appears probable, therefore, that the mineralizing agents were
volcanic waters, such as are usually among the after effects of volcanic outbursts,
and that they were hot and ascending. A consideration of their effects, as dis-
played both in the veins and in the country rock, will throw further light on
their nature.
86 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
PRIMARY ORES.
LOCALITY.
The contents of veins lying near the surface have been transformed more or
less into new minerals minerals that are more stable under surface conditions;
the materials originally deposited from the mineralizing solutions must therefore
be sought in the unoxidized lower region. The Montana Tonopah veins carry
solid sulphide ores, primary and contemporaneous with the original quartz gangue
and very slightly altered, presenting strong contrast with the oxidized ores of
the Mizpah mine. Similar sulphide ores have been found in the North Star, the
Tonopah Extension, the Midway, and the Tonopah and California.
COMPOSITION.
MINERALS.
Quartz. In these veins the chief gangue mineral is quartz, frequently well
crystallized and translucent, but more usually rather fine-grained and dense, and
mixed with more or less aluminous material. This material, which will be described
later on, is a residue of the least soluble material of the earlier andesite. Under
the microscope the quartz has a characteristic structure, distinct from that of
ordinary crystalline vein quartz. Instead of the coarse interlocking grains com-
monly displayed by vein quartz, these veins usually show a mosaic in which the
grain varies enormously in size, ranging from very fine cryptocrystalline to very
coarse. Under the microscope the aluminous material proves to be very fine
muscovite (sericite). The quartz holds numerous fluid inclusions, which contain
bubbles, showing that the included material was in a state of vaporous tension
at the time of its inclusion or at the time of the vein formation, and that it has
contracted so as to fill only part of its original chamber upon the lowering of
the temperature. The inclusions are frequently densely packed and curiously
arranged. In some cases the interior of the crystals is clear, while the marginal zone
is packed with inclusions. Frequently the quartz has the rough retiform structure
which is due to the intergrowth of idiomorphic crystals starting from independent
crystallization centers, and which is often characteristic of quartz formed by
replacement." There are also coarser veinlets of quartz, later than the bulk of
the vein, which were introduced along cracks, and these in places show comb
structure.
Adularia. The nearly pure potash feldspar, adularia, a purer variety of
orthoclase, is a common gangue mineral. It is frequently very abundant, usually
in more or less idiomorphic crystals that show the characteristic rhombic cross-
section. It is intercrystallized with the quartz, which often incloses isolated
aSpurr, J. E., Mon. U. 8. Oeol. Survey, vol. 81, p. 218.
PRIMARY ORES OF THE EARLIER ANDESITIC VEINS. 87
idiomorphic crystals of it, showing the nearly contemporaneous deposition of the
two minerals. Its condition is fresh and glassy, and only when it has been locally
strained does it show cleavage cracks. That it has been deposited from solution in
the same way as the quartz and the metallic minerals of the veins is evident. Where
the adularia and quartz crystallize together the sharply idiomorphic feldspar,
included in the xenomorphic quartz, shows the former to have first crystallized, the
order being the same as in igneous rocks. The adularia, like the quartz, is sometimes
closely packed with liquid and gaseous inclusions.
For chemical determination a specimen (No. 254) from the Fraction vein,
which is made up of this mineral and quartz, finely intercrystallized, was ground.
The quartz was then removed, as far as possible, by the use of the Thoulet
solution. The best material thus obtained was analyzed by Dr. W. F. Hillebrand
of the United States Geological Survey.
Analysis of adularia and quartz.
SiOj ...................................................................... 75.28
Al 2 O 3 a .................................................................... 13.19
Na,O ...................................................................... 32
K 2 O ...................................................................... 10.95
99.74
Inspection of this analysis shows that the material is a nearly pure silicate
of aluminum and potassium, which, from its optical properties, can be only
orthoclase or adularia. The silica, however, is considerably too high, showing a
mixture of quartz. By calculating the amount of silica needed for orthoclase it
is found that about 28.8 per cent of it is present as free quartz, leaving as
components of the adularia
SiO 2 ...................................................................... 46.48
A1 2 O, ..................................................................... 13.19
NajO ...................................................................... 32
K 2 ...................................................................... 10.95
70.94
Recalculating this on a basis of 100 we have
Si0 2 ..................................................................... 65.52
A1 2 O, .................................................................... 18.59
Na,0 .......................... ' ........................................... 45
K 2 ............................................... 15.44
100.00
Sericite. Muscovite occurs in the veins only as a fine aggregate (sericite).
It usually is scattered through the vein, or is irregularly bunched in certain
areas. It has been found included in adularia.
a May contain traces of FejO 3 , etc.
88 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
Carbonates. A carbonate is sometimes found microscopically mingled with
the quartz as a gangue material, and has also been noted macroscopically. Doctor
Hillebrand has determined that this is composed of the carbonates of lime, iron,
magnesia, and manganese, in the proportions stated later on.
Silver sulphides. The principal metallic mineral of the ores is a black sulphide,
usually dense, fine grained, and intimately intermingled with quartz. As seen
under the microscope, this black sulphide has a typical blue-black color, and
often shows cleavage, but almost always lacks crystal outlines. In tiny cavities,
however, crystals form. These are usually the six-sided, tabular, striated crystals
characteristic of polybasite and stephanite. Partial analysis by W. T. Schaller
of such crystals from the Montana Tonopah crystals which may possibly be
secondary (see p. 95) showed appreciable amounts of antimony and copper, the
latter ingredients indicating that the mineral is polybasite rather than stephanite.
In such cavities argentite crystals also occur.
Silver chloride. What is apparently silver chloride (cerargyrite) is found in
some of the primary ores, interwoven with the primary sulphides in such a way
as to seem to denote contemporaneous crystallization. In thin sections of such
ores the chloride is apt to be more or less bunched, as is the sulphide, but the
two are occasionally intergrown, with clear-cut lines of demarcation, seeming to
denote independent and contemporaneous origin.
Chalcopyrite. Chalcopyrite in occasional small grains is often noted in the
primaiy ores, and is frequently so intergrown with the primary silver sulphide
and with the gangue minerals as to indicate its primary character. In quantity,
however, it is relatively unimportant.
Pyrite. Pyrite in the veins is comparatively scanty, much more so than in
the wall rock. In many thin sections of the ores it is not found at all; in others
it occurs in considerable amount. In the primary ores it is frequently intergrown
with the silver sulphide, with which it is evidently contemporaneous, though
usually less in quantity.
Galena. Galena has been noted in the high-grade sulphide ores of the
Montana Tonopah, where it is associated with silver sulphides, chalcopyrite, and
pyrite. A picked specimen from the 460-foot level which contained galena was
analyzed for the Survey by R. H. Officer & Co., of Salt Lake City, and showed
8.9 per cent lead, 5.08 per cent silver (1,481.8 ounces per ton), and 38.26
ounces gold.
Blende. What is probably zinc blende has been detected microscopically by
the writer in the primary ore of the Midway shaft. Zinc sulphide has been
detected chemically in the Montana Tonopah primary ores.
PRIMARY SULPHIDE ORES. 89
Gold. Gold is present in the average ore in the proportion of gold to silver
of 1:100 by weight. It has never been detected by the eye in the sulphide ores,
either in the hand specimen or under the microscope, though it has been found in
metallic particles both macroscopically and microscopically in the oxidized ores.
ANALYSIS OP PRIMARY SULPHIDE ORES.
Picked samples of rich primary sulphide ore were taken from the Montana
vein of the Montana Tonopah mine at depths ranging between -160 and 512 feet.
These were crushed and the sulphides were concentrated by panning. The analysis
of the concentrates by Dr. W. F. Hillebrand of the U. S. Geological Survey, is as
follows:
Analysis of concentrates of primary sulphide ore from Montana Tonopah mine.
Per cent.
Siliceous matter 15. 18
Gold 82
Silver 25.92
Lead t 6.21
Copper 1 . 32
Iron 9. 87
Manganese 1. 36
Zinc 5. 84
Selenium 2. 56
Tellurium None.
Arsenic 19
Antimony 92
Magnesia 1. 49
Lime 3. 70
Carbon dioxide 6. 34
Sulphur Not det
81. 72
The composition of the carbonate is as follows:
Per cent (in terms
of u hole analysis).
Lime carbonate (CaCO,) 6.71
Magnesia carbonate (MgCO 3 ) 3. 13
Iron carbonate ( FeCO 3 ) 2. 36=Fe =1. 14
Manganese carbonate (MnCO s ) 2.57=Mn=1.32
The whole of the manganese, therefore, exists as carbonate.
Doctor Hillebrand remarks:
"Prolonged boiling with hydrochloric acid decomposed all the sulphide except
pyrite (and chalcopyrite if present). Hot dilute nitric acid then dissolved the pyrite
and also considerable selenide of silver (and copper?). The residue remaining after
this treatment consisted, aside from quartz, of very malleable black scales and parti-
90 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
cles which showed under the microscope the corroding action of the reagents used.
When boiled with concentrated nitric acid, these black particles became golden in color,
and the solution contained little or no selenium, but of this last I am not positive.
So far as can be judged, the whole of the gold exists in the form of this malleable
black alloy, which is so high in silver that the latter can all be extracted by strong
nitric acid. The cause of the black color is not apparent, and it puzzles me not a
little."
SUMMARY OF VEIN MINERALS.
The principal minerals of the primary veins are, then, quartz, adularia, and
some sericite, carbonates of lime, magnesia, iron, and manganese, sulphides of
silver, antimony, copper, iron, lead, and zinc (sulphides occurring in the form of
argentite, stephanite, polybasite, chalcopyrite, pyrite, galena, and blende), silver
selenide, and gold in a yet undetermined form. The remarkable thing about the
metallic contents is the scarcity of the common elements and the abundance of the
rare ones.
OXIDATION.
The chief alteration of the rocks, as will hereafter be explained, is due to
the action of ascenaing underground waters. The effects of descending surface
waters are seen chiefly in oxidation and similar processes acting upon the altered
rocks. The oxidation or other alteration of metallic sulphides is the chief change,
and, on account of the universal presence of pyrite formed by hot-spring action,
this change can be observed both in the veins and in the country rocks.
DEPTH OF OXIDATION.
The depth to which this oxidation of pyrite has penetrated is exceedingly
irregular, being quite different in neighboring shafts, and is very variable in
different parts of the same workings. The difference plainly depends on the
porosity and fracturing of the rock. Where these are greatest the oxidizing
waters have penetrated farthest downward. Along veins the oxidation generally
penetrates much deeper than in the rock, so that the ores may be oxidized while
the country rock is pyritiferous. This is plainly due to the greater rigidity and
brittleness of the vein as compared with the rock, so that it has been more
fractured by .strains, and therefore offers a readier channel. Even in veins the
depth of oxidation is very irregular, dependent upon the amount of fracturing.
CAP ROCKS AS PROTECTION FROM OXIDATION.
The veins which outcrop are most deeply oxidized, as the Mizpah and Valley
View veins. The former is for the most part oxidized down to a depth of nearly
700 feet; the latter is oxidized at the lowest level developed (about 500 feet). At
a depth of 400 feet in both mines the vein is almost completely oxidized or
OXIDATION AND CHLOBIDATION. 91
otherwise altered by surface waters, while at 300 feet and below, in the Valley
View, the pyrite in the country rock is usually unaltered.
Where veins do not outcrop, but are covered with a blanket of overlying
rock, there is usually comparatively little oxidation. The ore in the Fraction, at
a depth of a little over 200 feet, is a sulphide ore; in this case the vein has been
protected by a covering of soft volcanic rock (Fraction dacite). Similarly, heavy
sulphide ores were found in the Montana Tonopah at a depth of about 460 feet,
the veins of this mine apexing under the later andesite, which is decomposed
and not readily susceptible of fracturing. The depth of general oxidation of the
country rock is only about 90 feet in the Montana Tonopah shaft, between 115
and 185 feet in the Wandering Boy, and a little over 200 feet in the Stone
Cabin. In the \Vandering Boy the vein is oxidized on the 300-foot level, while
the country rock is unoxidized.
A single fracture line often locally divides the oxidized from the unoxidized
ore and rock. This line of demarcation frequently coincides with a fault line, on
which account it was suspected that some of the oxidation might be earlier than
the faulting; but other considerations render it more probable that, by faulting,
rocks of different degrees of porosity and permeability are brought together and
thus the result is accomplished.
SILVER CHLORIDE IN OXIDIZED ZONE OF VEINS.
In the ores, the effects of oxidation are to change pyrite to limonite, and
also to deposit wad (oxide of manganese), which is formed from the manganese
carbonate in the primary ores; while horn silver (cerargyrite) becomes plentiful.
This abundance of horn silver, being characteristic of the oxidized zone, is evidently
due to the effects of chlorine contained in the surface waters. Silver bromides
and iodides also sometimes accompanj- the chloride. Free gold has been deposited.
The large quantities of the haloid metallic compounds in the weathered
portions of veins in the desert regions of America have been especially discussed
by Prof. R. A. F. Penrose, jr.," who suggests that they are probably due to
the arid climate which has prevailed in the present and during the more recent
geologic periods, and which has rendered the scanty ground waters saline. It is
suggested that these saline waters have accomplished this alteration.
At Tonopah it is regarded as probable that the primary ore contains some
silver chloride, and it is possible that the chloride therein contained may have
been concentrated in the zone of weathering, and may also have contributed to
the predominance of chlorides in this zone.
ojour. Geol., vol. 2, p. 314.
92 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
ANALYSIS OF OXIDIZED ORE.
Concentrates from a picked sample of thoroughly oxidized ore from the
300-foot level of the Valley View vein were found by Doctor Hillebrand to have
the following compositipn:
Analysis of oxidized ore from Valley View vein.
Siliceous matter 16. 53
Gold 62
Silver "62.54
Lead 32
Copper ." &.09
Iron '. 1.39
Manganese .07
Zinc - 10
Selenium .78
Tellurium None.
Arsenic .03
Antimony -15
Sulphur Not -det.
Total 82.62
Concerning this analysis Doctor Hillebrand adds:
"After extraction of all the silver chloride by ammonia the residue was boiled
with hydrochloric acid until silver no longer appeared in the filtrates. The insoluble
matter then consisted, aside from gangue, of a little pyrite, of the same black gold-
silver alloy found in the unoxidized ore, and of a pyritic-looking mineral, which
latter yielded to dilute nitric acid much silver and some selenium, leaving a residue
of gold."
COMMENT ON THE ORE ANALYSES.
Aside from the complex carbonate of lime, manganese, magnesia, and iron,
the analysis of the primary sulphide ore indicates (p. 89) the presence of a large
amount of silver sulphide argentite. Antimonial sulphides of silver, polybasite,
very likely stephanite, and smaller amounts of galena, blende, pyrite, and
chalcopyrite are also indicated. Of very great interest is the presence of a
considerable amount of selenium, which occurs, in part at least, as a silver
selenide, and the absence of its usually closely associated element tellurium.
The chemical form of the gold is 3 r et uncertain.
It is fair to assume that the oxidized ore in its primary sulphide state may have
had a composition somewhere relatively near that of the primary sulphide analyzed.
The two analyses may then be compared with the object of perceiving the changes
effected by oxidation. There is no element which can be considered as having
a 38.10 as lulpbldeg; 24.44 us chloride, selenitic, and alloy. 6 Mostly oxidized.
OXIDATION. 93
remained quantitatively unaffected during oxidation, so that merely the large rela-
tions can be glanced at. All the metals except silver and perhaps gold are present
in the oxidized ore in much diminished proportions. The lead, copper, and zinc are
present in small quantities. The manganese is now in the form of oxide, but very
little remains; the iron is in the form of oxide, with some residual or secondary
pyrite. There is much less gold in proportion to silver in the oxidized ore than in
the sulphide ore; but this may be fortuitous and depend on the specimen selected.
More than half the silver is in the form of sulphide, and from the very small quantity
of arsenic and antimony present this portion must be nearly all in the form of
argentite. The antirnonial silver sulphide is very probably pyrargyrite (ruby silver),
judging from microscopic observations. It is noteworthy that antimony and arsenic
are present in the same proportions to one another in both analyses. There is less
than a third as much selenium in the oxidized ore as in the sulphide ore, but the
discrepancy is not so great as in the case of lead, copper, manganese, zinc, arsenic,
and antimony; and this selenium seems to be still in the form of a silver selenide.
Therefore it is probable that during the process of oxidation the primary
carbonates were attacked by surface waters, and the lime and magnesia, together
with most of the iron and manganese, removed in solution. Some of the iron and
manganese remain as oxides. No important change in the amount of gold and silver
is proved. The argentite has largely remained unaltered, but the polybasite (and
stephanite if present) has probably been attacked, and much of the silver selenide.
Part of this silver has been reprecipitated with little change of position as secondary
argentite, not distinguishable from the primary argentite, while a large portion has
been altered to chloride by the action of chlorine contained in the shallow
underground waters. Most of the arsenic and antimony in the original polybasite
and stephanite has been removed in solution; the rest goes to form the secondary
sulphide pyrargyrite, as indicated by numerous field observations. The pyrite and
the chalcopyrite have been attacked. Most of the iron in these sulphides has been
removed; a small part remains as oxide, or rarely as residual or secondary pyrite.
Nearly all the copper has been removed, a little remaining in the probable form of
oxide.
It is thus seen that the so-called oxidized ore of the Tonopah district, like
that of man}' other deposits in desert regions, is really a modified ore consisting
of an intimate mixture of original sulphides (and selenides), together with
secondary sulphides, chlorides, and oxides. This case is without doubt character-
istic of the whole zone of oxidation from the outcrop downward, for the ores
throughout the zone are identical microscopically.
As to the reprecipitation lower down of materials dissolved in the process
of oxidation there is little light. The plainly secondary sulphides within the
94 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
sulphide zone are argentite and pyrargyrite, the latter always coating cracks or
cavities, with probably chalcopyrite. Possibly the copper of the secondary
chalcopyrite is formed by the action of copper solutions from above on primary
pyrite, but galena or blende have not been noted as secondary sulphides, and at
best are rare. Moreover, the secondary silver minerals argentite and pyrargyrite
are more abundant than secondary pyrite and chalcopyrite, and all these usually
occur on cracks in rich primary sulphides and not in barren or low-grade ore,
suggesting the derivation of the secondary minerals from this rich ore by lateral
secretion rather than an exotic origin.
FORMATION OF GYPSUM BY OXIDIZING WATERS.
Gypsum frequently occurs as veinlets or incrustations in both the earlier and
the later andesites where these are altered. It is more rare in the earlier
andesite, which has become highly silicified, and is abundant in the later andesite,
which has developed a large amount of calcite as a decomposition product. This
association with calcite suggests derivation from it, and the proximity in many
of these cases of partly oxidized pyrite indicates that the sulphuric acid derived
from the pyrite has wrought the change. The surface waters containing oxygen
would decompose the pyrite and form limonite (which is found near the surface)
and sulphuric acid. The latter would decompose the calcite (which itself was
formed by hydrothermal processes from the calcareous silicates of the andesite)
and produce gypsum and carbonic acid.
In the Fraction workings, at a depth of 400 feet and in the West End and
the MacNamara (the latter at 280 feet), fissures were tapped which contained a
heavy odorless gas that put out lights and necessitated temporary interruption
'of work. This gas was immediately dispersed by the ventilation, indicating that
the fissures were reservoirs and not outlets. The writer has not been able to
collect any of the gas, but in all these cases it was encountered near calcareous,
pyritiferous, and gypseous andesite, and it is likely that it may have been carbonic
acid, the final result of the reactions indicated, which accumulated in cavities.
SECONDARY SULPHIDES.
PYRARGYRITE, ARGENTITE, AND NATIVE SILVER.
Wherever observed macroscopically, pyrargyrite (ruby silver) and to a great
extent, also, argentite (silver glance) coat crevices which cut the primary ore and
are evidently of secondary deposition. These minerals were found in comparative
abundance in the Fraction, in the unoxidized ores on the 237- and 300-foot levels;
on the 237-foot level native silver occurred also, coating cracks, and also plainly
SECONDARY SULPHIDES. 95
secondary. In the Mizpah, ruby silver is rare, but it has been noted in the
250-foot level, where, from microscopic examination, it appeared that both ruby
and horn silver are secondary to the original black silver sulphide.
ARGENTITE, POLYBASITE, AND CHALCOPYRITE IN DRUSES.
In the Montana Tonopah, at a depth of about 500 feet, were found specimens
showing good crystals of argentite, polybasite (in part, perhaps, stephanite), and
chalcopyrite, often sitting free in cracks and little druses in the solid rich sulphide
ore. Evidently these minerals were formed subsequent to the solid ore, and the
silver seems to have been concentrated from the main mass and to have been
precipitated in the crevices. Secondary pyrite has also been noted, for example,
in the Fraction mine, sitting free upon quartz crystals which line druses in the
vein.
COMPARISON OF SECONDARY SULPHIDES AT NEIHART AND TOSOPAH.
At Neihart, Mont., Mr. W. H. Weed* has described polybasite and pyrar-
gyrite (ruby silver) incrusting impure galena, blende, pyrite, quartz, and
barite. These crusts are now forming in vugs and watercourses filled by
sluggish descending surface waters. The polybasite seems to be an alteration
product of galena, and in some cases pyrargyrite is undoubtedly derived from it.
Blende is also in some cases secondary. Argentite is probably present. Mr.
Weed explains the secondary precipitation by lixiviation of the ores by iron
sulphate, formed by oxidation of iron sulphide (pyrite).
The Tonopah occurrence is analogous, except that here satisfactory evidence
of the manner of deposition has not been found. There is little doubt that the
pyrargyrite and argentite found along cracks were formed subsequently to and
are probably derived from the primary ore. This primary ore is, however,
richer than that at Neihart; indeed, it consists largely of silver sulphide, in part
antimonial. For this reason the mode of occurrence of polybasite and argentite
in druses in the rich Montana Tonopah ore is not of such plain import. In the
Montana Tonopah it has been shown that during the period of primary deposition
the vein, after being filled, was crushed and reopened, and again cemented by
similar rich sulphides, somewhat richer apparently than those of the first deposition
(see p. 172); and the polybasite, argentite, and chalcopyrite in druses may mark
a third and final stage in the primary deposition. Also, chalcopyrite occurs in
the bulk of the ore as more or less definite seams, apparently somewhat later than
the rest, but not clearly of different origin.
a Probably argentite (see p. 92). STrans. Am. Inst. Min. Eng., vol. 30, p. 434.
96 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
EVIDENCE FAVORING SECONDARY DEPOSITION OF SULPHIDES BY DESCENDING WATERS.
On the other hand, the formation in the oxidized zone of limonite from pyrite
and of cerargyrite from sulphides affords evidence that the metallic minerals of
the ores have actually been dissolved and reprecipitated by surface waters, and in
several cases the occurrence of rub3 r silver (pyrargyrite) in cracks in these partially
oxidized ores shows beyond a doubt that it also is due to descending surface
waters. Moreover, some of the ores, when studied microscopically, show argentite
fringing cerargyrite, as if secondary to it. The iron sulphate necessary to the
solution of the silver sulphide has been present (as is shown in the alteration of
calcite to gypsum) and the silver has actually been dissolved, and such occurrences
of secondary sulphides as have been described would be the natural result. The
evidence therefore favors the view that these secondary sulphides in the oxidized
zone originated from descending surface waters and probably part, but not all, of
the sulphides in druses in the sulphide ore have a similar origin.
The characteristics of the superficial alteration of the ores are those which
naturally result from the climatic and topographic conditions." In all of the
mines discussed (yielding ores) standing ground water is lacking; at least, none
has been encountered up to the considerable depths attained (over 1,100 feet).
Therefore the alteration is spotty and incomplete, but extends irregularly to very
considerable depths in various places.
No definite secondary sulphide zone has been noted, the secondary sulphides
being associated with the predominant oxides, chlorides, etc., in the oxidized
zone and coating crevices in the primary sulphides.
VEINS OF THE TONOPAH BHYOLITE-DACITE PERIOD.
In many mine workings there are quartz veins of a certain class which are
large and may carry values, but which are to be separated from the principal ore-
bearing system. These are easily confounded with the veins of the earlier
andesite, just as the silicified Tonopah rhyolite-dacite, in which they usually occur,
may be confounded with certain highly silicified phases of the earlier andesite.
Such veins have been encountered in the Belle of Tonopah, the King Tonopah,
the Mizpah Extension, the Desert Queen, North Star, Montana Tonopah, Mizpah,
Midway, MacNamara, West End, Tonopah Extension, and Ohio Tonopah, and are
described in the detailed account of these mines. On account of their resemblance
to the earlier andesite veins they have been the object of a good deal of exploration
and development work, which, on the average, has been decidedly unprofitable.
In connection with the occurrence of such veins, which are described elsewhere
in the report in the mine descriptions, another occurrence, somewhat different
from the rest and having considerable interest, maj r be described.
"Spurr, J. E., Geology Applied to Mining, pp. 275-276.
VEINS OF TONOPAH RHYOLITE-DACITE. 97
Just beyond the western corner of the area mapped (PI. XI), opposite Siebert
Mountain, a group of three low hills rises above the plain. One of these
hills is capped by a patch of dacite, whose resistance to erosion has probably
caused the hill. The rest of this hill and all of the other two are composed of
white tuff mixed with beds of conglomerate, plainly referable to the white tuffs
of the area mapped. The origin of the two hills, which are composed entirely of
tuffs, is due to two elliptical areas where these tuffs and conglomerates have been
thoroughly silicitied and changed to a quartzite-like condition. Some mineral-
ization has accompanied the silicification. A random specimen of the silicified
material from the smaller of the two hills thus formed was reported to the
writer to have yielded on assay $8 in gold and no silver. This silicitication and
mineralization is evidently the work of powerful hot springs, and the elliptical
shape of the silicitied areas shows that the springs rose along pipe-like channels
and not along definite fractures. These deposits are probably of practically the
same age and origin as the veins in the Tonopah rhyolite-dacite.
CHARACTERISTICS OF RHYOLITE-DACITE VEINS.
The veins of this period are characterized by irregularity and by lack of
definition and persistence, though their size may locally be great. As a rule they
are elongated and have the appearance of veins, but can not be followed as far either
on the strike or dip as true veins may. They may disappear by scattering and
passing into a silicified wall rock, or may be cut off along a cross-wall fracture in
the same manner as some of the veins in the earlier andesite described on p. 85.
The quartz is as a rule dense and jaspery, and is white, gray, or black; it is therefore
usually of different appearance from the white quartz of the earlier andesite veins.
The veins are usually barren or contain only very small quantities of gold and silver,
except locally, as in the Desert Queen, where rich bunches of ore may occur, though
usually of limited and irregular extent (fig. 13). Like the veins of the earlier
andesite the rhyolite-dacite veins very frequently contain adularia, and in one case
probable albite was noted (see p. 197), a mineral which has not been detected in the
andesite veins. In the Ohio Tonopah barite has been found as a gangue mineral with
the rhyolite-dacite veins. This mineral has not yet been found in connection with
the earlier andesite mineralization. In the Desert Queen and the North Star, where
quartz of the rhyolite-dacite period has been cut by drifting, a green stain forms on
the walls, which is a basic copper sulphate. This phenomenon has not yet been
noted in connection with the earlier andesite mineralization. A characteristic of the
rhyolite-dacite veins, to which there are, however, numerous exceptions, is the
1684S No. 4205 7
98
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
300
4-oofeet
Fie. 13. Rbyolltlc veins (latur period) in Tonopah rh yolite-daclte, 814-foot level, Desert Queen shaft, showing irregularity
and lack of persistence. Horizontal plan.
VEINS OF TONOPAH RHYOLITE-DACITE. 99
greater ratio of gold to silver in them as compared to that in the earlier andesite
veins. In the earlier andesite veins the gold averages about two-fifths of the value,
the silver three-fifths, while in the rhyolite-dacite veins the gold is likely to exceed
this amount and sometimes occurs with practically no silver, although the proportion
is very changeable. Very often again the proportion of gold and silver is the same
as in the earlier andesite veins.
AGE OF TONOPAH RHYOLITE-DACITE VEINS.
These veins are younger than the Tonopah rhyolite-dacite, in which they
usually occur. In the mine workings referred to above this lava is a deep-seated
injection corresponding in age and composition to a great mass of surface breccias
and tuffs in the southern half of the area mapped. Even in the lower part of the
white tuffs or lake beds which succeeded the deposition of the volcanic ejectamenta
of this period there are intrusive sheets of the rhyolite-dacite. In this portion
of the tuffs occur the elliptical outcrops of the pipe-like deposits, formed by hot
springs in the hills west of Siebert Mountain. Thus the period of this mineral-
ization was, in broad terms, contemporaneous with the volcanic activity of the
Tonopah rhyolite-dacite period, and very likely persisted for some time after-
wards. These veins are plainly the results of ascending hot waters, and represent
the effects of the Tonopah rhyolite-dacite eruption. They have the same relation
to these eruptions that the earlier andesite veins had to the eruptions of the
earlier andesite.
The characteristic lack of definition and persistence in these veins as compared
with the veins in the earlier andesite shows that at the time they were formed no
definite fracture zones were available as channels, so that the ascending waters had
to force themselves up along irregular courses. This means that the faulting now
so characteristic of the district had not begun at the time of this mineralization,
and therefore that this mineralization ceased before the beginning of that period
of rhyolite and dacite injections and eruptions which is marked by the rhyolite
and dacite necks that form the hills around Tonopah. The mineralization is then
probably the same in time, nature, and origin as that at Gold Mountain, 4 miles
south of Tonopah," and very likely similar to that in the newly discovered camp
of Goldfields, about 28 miles south of Tonopah.
GENERAL RESTRICTION OF VEINS TO RHYOLITE-DACITE.
At first it seems strange that in underground workings like the West End*
the MacNamara, etc., these rhyolite-dacite veins do not extend into the earlier
andesite in which the rhyolite-dacite is intrusive. The fact that such veins
a Bull. U. S. Geol. Survey No. 218, p. 87.
100 GEOLOGY OB' TONOPAH MINING DISTRICT, NEVADA.
extend to the contact of the andesite and do not enter it, raises at first a doubt
as to whether the andesite is not really the younger rock instead of the older.
In some of the shafts mentioned the andesite is soft and very little silicified,
while the amount of silicification in the rhyolite-dacite is very great. However,
there is no doubt of the relative age of the rhyolite-dacite as given on p. 43,
and the reason for the described phenomenon appears upon reflection. The
rhyolite-dacite consists mainly of volcanic glass and was injected into the earlier
andesite after this was thoroughly decomposed and softened as the result of the
action of hot spring waters that accompanied and caused the principal minerali-
zation. Any slight subsequent strains in the earth resulting from volcanic action
shattered this fresh and glassy rock, but formed no fractures or fissures in the
soft adjacent andesite. The hot waters that rose immediately after the rhyolite-
dacite eruptions found almost their only channels in the fractured and fissured
glassy rock to which they owed their origin. Therefore the veins that they
formed are confined chiefly to this rock. Evidence of the correctness of this
explanation is furnished by the thick veins of this period that are found on the
contact of the rhyolite-dacite sheet with the overlying decomposed andesite. Such
veins are often found at this place and the accompanying silicification is very
great, but is almost invariably confined to the rhyolite-dacite near the contact.
Such, for example, is the situation in the Mizpah Extension, the MacNamara,
Tonopah Extension, and West End, and to a less degree in the Ohio Tonopah.
These things show that the ascending hot waters, circulating through the fractured
rhyolite-dacite, rose until at the contact with the overlying soft andesite they
found a practically impervious barrier, along whose lower contact they circulated
and deposited the materials which they held in solution.
Subsequent to this formation of quartz veins and attendant silicification,
similar differences between the rhyolite-dacite and the andesite with reference to
brittleness continued, so at the present day the silicified rhyolite-dacite is found
to be extremely faulted and fractured and to contain open fissures, features which
are not present to the same extent in the adjacent andesite.
EFFECT OF WATERS PRODUCING THE TONOPAH RHYOLITE-DACITE VEINS ON
EARLIER FORMED VEINS.
Although as a rule decomposed andesite seems to have presented a formidable
barrier to the circulating waters accompanying the Tonopah rhyolite-dacite, in
some places the waters must have traversed the andesite and found their way
along the andesitic veins. Indeed, it is along these brittle veins and the brittle
silicified adjacent andesite that fractures and fissures must have been most easily
formed at this period. In the case of the Tonopah Extension, as described
CALCITIC VEINS OF ARARAT MOUNTAIN. 101
elsewhere (see p. 182), the earlier andesite vein has been reopened and a new vein
of barren jaspery quartz formed along the hanging wall. This is probably due
to waters of the rhyolite-dacite period of mineralization. In the case just men-
tioned the new quartz is barren as compared with the old. It is evident, however,
that such solutions must have dissolved a great deal of the gold and silver contained
in the earlier veins, and naturally may have reprecipitated it elsewhere. In this
case the ores might be reprecipitated in a concentrated form. This very likely
has been the case in the Montana Tonopah, where, as described (see p. 171), the
original vein has been reopened and in the fissure thus formed minerals similar to
those in the older vein, but richer in gold and silver, have been precipitated in
crustified form. It is very likely that this was the work of the waters of the
rhyolite-dacite period, of the same kind and character as those to which the
barren quartz hanging-wall portion of the vein in the Tonopah Extension is
due.
Again, it is natural that such waters may have dissolved some of the metallic
contents of the older veins and, instead of precipitating them within these veins,
may have carried them out and deposited them elsewhere, as, for example, in the
veins of the rliyolite-dacite, forming bunches of high-grade ore in these usually
barren veins. This may be the explanation of the comparatively small amount
of rich ore found in the rhyolite-dacite veins, as, for instance, in the Desert Queen
and the MacNamara. These are practically the only cases of high-grade ore in
the district in veins of this period, and in both cases the veins are in the vicinity
of rich earlier andesite veins and the ores have a character altogether similar to
that of the earlier veins. Outside of the earlier andesite vein region, the veins
in the rhyolite-dacite have been found to be frequently large, but typically are
low grade or barren.
THE CALCITIC VKINS OF ARARAT MOUNTAIN.
THE RHYOLITE OF ARARAT A VOLCANIC PLUG.
The top of Ararat Mountain is composed of white rhyolite like that of Mount
Oddie. On the southwest side this is intrusive into the later andesite, and on
the other sides into the glassy Tonopah rhyolite-dacite, which is itself intrusive
into the later andesite. The area of white rhyolite is broadly ellipical in outline,
with the longer axis of the ellipse, as in the case of most of the other hills on
the map, lying in a general east-west direction (PI. XIV).
The contact, as is shown by the Wingfield tunnel and the Boston Tonopah
shaft and in other places, pitches steeply all around. The rhyolite is then in the
nature of a volcanic column or plug which has been forced up into the older
rocks, and which probably occupied the vent of an old volcano, now removed by
erosion.
102
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
FLOW BRECCIATION NEAR CONTACT.
Near the contact in many places the rhyolite is peculiarly brecciated, showing
great blocks jumbled together, with, however, rhyolitic matrix between. The dim
outline of these blocks and the rhyolitic matrix show that .they were formed when
the lava was in the process of cooling and only partly rigid. This brecciation
decreases away from the contact, but in places occupies a zone upwards of 100
feet wide. The breccia indicates that the plug was forced upward while cooling.
FISSURE VEINS IN THE RHYOLITE PLUG.
Many sharp fractures cut the rhyolite, increasing in number as the contact
is reached. These are chiefly parallel to the contact. They have been filled with
paraiiei 10 me contact, iney nave oeen nilea
m^mwsm^--^-^
m$mit:mmm.^'\ ::;'>>:
Rfe^Kili^:^
^pfStlla^'^'^f;
liliiSSfM^'-^^^
fif-r:i-51-:r-:-.--"H -,!-- V-J---L-----"-:L-- " , * , - '. - .-'-'.
P^lffi-^^W^
faiii^ft >3i AsV-4te
Z 32
e i _
5 feet
FIG. 14. Cross section of outcropping fissure vein In Ararat rhyolite neek near margin. Heptile claim, north of the
Boston Tonopah shaft. 1, Dark-brown calcite and siderite, mixed; 2, white calcite, beautifully banded; 3, quartz
mixed with calcite; 4, white rhyolite (wall rook).
material as described below, and constitute veins that are locally as much as 20 feet
thick, but are exceedingly irregular and nonpersistent. These veins conspicuously
follow the contact and are coterminous with it; they do not extend into the older
intruded rocks, but often run back into the rhyolite. A prominent line of veins,
as shown in PI. XIV, extends due north across the top of the hill, from the vicinity
of the Wingfield tunnel. These are fine examples of veins which have filled open
fissures.
U S GEOLOGICAL SURVEY
PROFESSIONAL PAPER NO 4.2 PL XIV
MAP SHOWING THE CHIEF VEINS OF ARARAT MOUNTAIN AND THEIR
RESTRICTION TO THE ODD IE RHYOLITE] PLUG
By >J. E.Spurr
IHO'i-
S oale
isoo Feel
Odcfir rtivitliip shtt *vr> in yrevti . veins in red .
CALOITIC VEINS OF ARARAT MOUNTAIN.
103
On the Reptile claim, above the Wingfield tunnel, an outcropping vein of this
material is beautifully banded, and consists of brown and white calcite and some
quartz (fig. 1-t)." Some assays of this are said to show a value as high as $20, all
in gold, but it is mostly barren. Several small veins near by are of the same
character. One of these distinctly shows quartz as a later deposit than calcite
(fig. 15). These veins have a general northerly trend, and the vein zone can be
followed all the way across the hill to the contact on the north, but no farther.
Each vein can be followed only a short distance, however, when it becomes con-
fused by reason of splitting, straggling, and thinning, while a lateral vein may
thicken up so as to become of predominating importance.
At the contact between the white rhyolite plug and the glassy Tonopah rhyolite-
dacite, on the east side,
an 8-inch vein of banded
white and brown calcite
and siderite, cementing
a fissure in the white
rhyolite, was observed.
This has a strike of N.
10 W. and a dip of 70
to the east.
On the oppo.site or
west side of the intru-
sive plug, near or at the
contact between it and
the later andesite, there
is a vein of beautifully
crustified crystalline cal-
cite, locally 20 feet
thick. The rhyolite on
one side of the vein has been silicified so as to form a pale-yellow jasper.
It will be noted from PI. XIV that these veins, although their position and
trend are governed to a large extent by the contact, have a general north-south
trend independent of it. This indicates that the chief strain at the time the fissures
were formed was in a direction nearly at right angles to the longest axis of the
elliptical horizontal cross section of the volcanic plug. This north-south trend is at
right angles to the principal trend of the ore-bearing veins in the earlier andesite,
formed at an earlier epoch (fig. 12, p. 84).
a Dr. W. F. Hillebrand kindly examined the dark-colored carbonate for the writer. He finds it essentially calcite,
with very small amounts of iron and manganese carbonates, a considerable amount of mechanically included hematite,
and some black manganese oxide.
-iz$<
3 feet
FIG. 15. Vertical cross section oi outcropping tissure vein, 20 feet west of section
shown in fig. 14. 1. Calcite with angular rhyolite fragments; 2, quartz; 3,
white rhyolite.
104 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
FISSURES DUE TO MOVEMENT AFTER CONSOLIDATION.
These fissures and fractures, judging from their distribution and direction,
plainly resulted from the continuation of the driving upward of the plug after con-
solidation was practically complete.
The movement thus indicated is like that which was manifested by the plug of
Mont Pele'e in Martinique subsequent to the late eruptions, when it was forced
upward after solidification, so as to tower several hundred feet in the air. Around
the base of such a plug as Pelee's, phenomena like those on Ararat must have taken
place.
The fillings are evidently the result of ascending hot water which followed the
channels thus opened and cemented them. That such large open spaces due to
rending could have been formed indicates that the spot was not very far distant
from the surface.
PARAGENESIS OF VEIN MATERIALS.
The substances deposited in the openings also are simple, as compared with
those of other periods of vein formation in the district. The alteration of the
rhyolite is confined to silicification and slight bleaching of the biotite. Some
of the specimens from the Wingfield tunnel show feldspar phenocrysts completely
altered to microcrystalline and cryptociystalline silica. In many cases this silici-
fication seems to have preceded the deposition of the carbonates, for the latter
are deposited in cavities upon the silicified rhyolite. In other cases, however,
the jaspery and chalcedonic quartz, which is often part of the fissure filling, is
plainly later than the carbonates. In several cases white calcite was observed
to be later than the dark or ferruginous calcite in origin.
COMPOSITION OF VEIN-FORMING WATERS.
No sericite was observed to be developed in the wall rocks, hence it seems
probable that the waters did not contain fluorine (see p. 232), or that their temperature
was very moderate, or both. Indeed, they do not give evidence of having contained
anything beyond silica, lime, iron, and manganese carbonates. Their content of
gold was small, for the veins are generally practically barren. No larger amount
of this metal is likely to have been present than has been detected in many hot
springs issuing at the surface. The presence of iron is contrasted with the
probable absence of iron in the solutions which produced the earlier andesite.
oHovey, E. O.. Am. Jour. Sci., 4th ser., vol. 16, pp. 269-281. Russell, I. C., Science, vol. 17, pp. 792-796; Am. Jour.
8cl., 4th ser., vol. 17, 1904.
CHAPTER HI.
PRESENT SUBTERRANEAN WATER,
WATER ENCOUNTERED IN MINING OPERATIONS.
The Desert Queen shaft is 1,114 feet deep. It is perfectly dry, except at
the contact of the rhyolite and later andesite at a depth of a little over 300 feet,
where water following the contact zone was encountered. Along the watercourse,
which strikes north and south and dips 60 east, the rocks have been altered to
clay. Fragments of rocks in the channel show fresh pyrite on cracks, indicating
that these waters have deposited the sulphide. The water tasted very slightly
astringent; when first encountered it was tepid, but afterwards it became cool.
The water was encountered in October, 1902, when the flow was about 3,000
gallons per twenty-four hours; it gradually diminished, till in six weeks it was
only 250 gallons, and later in the fall shrunk to 100 gallons. In the spring,
however, the flow increased to 250 gallons, and the water was cold.
These data show that the water of the contact zone was contained in a
comparatively small basin or reservoir, whose surface was quickly lowered, and
the increase in the spring with the melting snow indicates that this basin is fed
from the surface.
The Halifax shaft encountered water below 600 feet; at 640 feet the flow,
on July 17, 1903, was estimated by the manager at 12,000 to 15,000 gallons a
day, and on July 20 at 20,000 to 30,000, so it was necessary to stop work pending
the arrival of a pump.
A similar copious flow was encountered in the Rescue, situated just south of
Mizpah Hill. At a depth of 250 feet an estimated flow of 6,000 to 7,000 gallons
a day was encountered along a crevice in the rhyolite, striking northeast and
dipping southwest at an angle of about 40. Below this there was no water till
a depth of 300 feet was reached, at which depth more water came in along
fractures striking northwest and dipping northeast. When this second water zone
was struck the supply of water in the first was reduced, showing that the two
zones are connected. On July 10, 1903, the combined flow from the two was
about 8,000 gallons; on July 17 it was estimated by the manager to be from
25,000 to 30,000 gallons.
The Gold Hill shaft was dry to the bottom (490 feet), but a drift running
northward from the bottom struck water in fractures a short distance from the
105
106 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
shaft. The south drift was dry. The water here was estimated at one time to be
7,000 or 8,000 gallons a day.
The Belle of Tonopah shaft encountered water along fractures at a depth of
150 feet. This was drained, and another water seam was cut at 190 feet. The
rock is soft later andesite, very full of pyrite, indicating, as at the Desert Queen
shaft, that these waters deposit pyrite.
The Golden Anchor struck water at a depth of 130 feet and also farther
down along fractures. One fracture from which water issued, seen by the writer
at 200 feet, was perpendicular, and had a course of N. 70 W. This fracture
had been cemented by calcite and reopened. The Silver Top, east of the Golden
Anchor, encountered water at a depth of 180 feet.
The Mizpah Extension encountered water at a depth of 430 feet at the contact
of Oddie rhyolite and Tonopah rhyolite-dacite. The water runs on top of 14 feet
of wet clay, formed by rock decomposition. The water zone strikes N. 30 W.
and dip northeast at an angle of 40. At the time of the writer's inquiry, in
November, 1902, the flow was about 300 gallons a day. The shaft was sunk to a
depth of 800 feet without encountering any more water.
The other shafts in the district were quite dry at the time the writer made
his observations. Their depths at that time or soon afterwards were as follows:
Depths of dry shafts in Tonopah district.
Feet.
King Tonopah 300
Boston Tonopah 300
Behnont 340
North Star 1,050
Siebert 938
Valley View 700
Stone Cabin 400
Molly 468
Montana Tonopah 765
Midway 635
Tonopah Extension 485
MacNarnara 500
West End 780
Fraction 400
Wandering Boy 500
Tonopah and California 650
Tonopah City 500
Ohio Tonopah 756
BigTono 300
Fraction Extension 300
New York Tonopah 745
a A little seepage along a fault zone at a depth of 720 feet.
GROUND WATER. 107
OUTCROPPING WATER ZONES.
Previous to the discovery of the water in some of the shafts described the entire
water supply of the town of Tonopah was obtained from wells 4 miles to the
north, where geologic and topographic conditions are similar to those at Tonopah.
Here, in a distance of a half mile or more, along a small east-west valley, are a
number of wells, most of which reach water within 30 to 40 feet of the surface.
The wells are in solid later andesite, and the water circulates along a fractured
(probably faulted) zone. The trend of the water zone corresponds with that of
the valley, which has probably been eroded along this belt of fractures.
These water zones can often be recognized at the surface by the presence of
taller and greener vegetation or by plants requiring so much water that they
would not thrive under the usual arid conditions.
DISTRIBUTION AND EXPLANATION OF WATER ZONES.
The above data show that while some of the Tonopah shafts have reached
depths of over 1,000 feet (in the case of the Desert Queen over 1,100) no general
body of ground water has been encountered, though the rocks are extremely
fractured; yet along certain steeply inclined fracture zones water is found
sometimes quite near the surface and occasionally in considerable quantity. This
water is cool, is sufficiently nonmineral to be fair drinking water, and is
undoubtedly the storage of precipitation.
These water zones appear to be widely spaced. They have been noted only
in rigid and brittle rock rhyolite and andesite. They seem to occur especially
along intrusive contacts, where one rock has been shattered by the intrusion of
another. They are often, perhaps usually, accompanied by a clayey state of the
decomposed rock. This decomposed rock, while itself undoubtedly due to the
waters, now forms an impervious bottom or foot wall of the fractured zone and
keeps these waters from penetrating the underlying dry and fractured rocks.
Thus the water channel or basin has a dike-like shape. It appears probable that
similar clays may limit these water basins in depth, limiting the downward extent
of the zone-shaped basins, and thus explain why they are found sometimes so near
the surface in a region apparently without universal ground water.
USUAL ABSORPTION OF PRECIPITATION BY ROCKS.
In the southern half of the area shown on the Tonopah map (PI. XI), in the
depressed area capped by volcanic breccias, no water has been encountered, even in
shafts over 700 feet deep, although some shafts, as the Ohio Tonopah for instance,
have passed through the soft breccia to a rigid and fractured rock below.
Furthermore, in the breccia-covered region to the south, the writer does not
108 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
know of any water or water signs, while to the north, in the hard rock, water
zones outcrop in various places, both on and beyond the area mapped. The
explanation of this is probably that the porous breccias and tuffs absorb the
scanty precipitation like a sponge.
Even where rigid fractured rocks outcrop, the scanty descending water
normally sinks as through a sieve, using itself up in kaolinization, the formation
of limonite, and other hydration processes, and moistening the dry rocks with
interstitial water. Fresh rock taken from the Fraction and other shafts in frosty
weather was observed by the writer to steam vigorously in the cold air, though
the mines are perfectly dry. It is doubtful if there is enough of this water left
to form a standing body of ground water at any depth. Where, however,
kaolinization and other processes have formed clay seams, the water ma}" be
detained and even stored at any depth from the surface downward; and other
impervious rock materials may operate in the same way.
CHAPTER IV.
PHYSIOGRAPHY.
ORIGIN" OF THE RANGE OF HILIiS.
The area of the Tonopah map has been, from the dawn of its available
record in the middle Tertiary down to the present day. essentially a land surface,
save during the period when the white lake beds were deposited. At present
the region consists of isolated buttes (which are usually denuded volcanic necks),
and intervening depressions. These buttes are irregularly grouped, but occupy in
general a definite north-south belt, although this belt can not be distinguished upon
the small detailed map which accompanies this report. The belt becomes higher
on the north, where it is known as the San Antonio Range, and rather lower
toward the south, where it gradually loses its individuality. The character of the
rocks throughout is volcanic, and evidently a large part of the topographic relief
is due to the fact that this has been a chain of Tertiary volcanoes.
SKETCH OF TERTIARY AND QUATERNARY EROSION.
GENERAL FEATURES.
The Tonopah district, as limited by the mapped area, is in the central part
of this north-south topographic ridge. The surface run-off drains mostly to the
west, but in the eastern corner of the area mapped the slopes indicate that the
drainage is eastward. On both sides of this volcanic range are broad, flat, desert
valleys. On the west, which is reached by a moderate and regular though decided
slope down from Tonopah, is the east branch of Great Smoky Valley, and on the
east lies Ralston Valley. These general topographic conditions must have existed
during most of the period embracing the volcanic history of the region. Erosion
was steadily at work attacking the uplifted and outpoured rocks of the range,
and transferring them to the deep flanking valleys; and since much of the volcanic
material was loosely consolidated it must have been transported with extraordinary
rapidity, especially as periods of greater humidity than the present alternated with
the arid periods." Since the region was probably all this time without any outlet
to the sea, enormous amounts of detritus accumulated in the valleys, partly
"Spurr, J. E., Bull. Geol. Soc. Am., vol. 12, p. 250.
109
110 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
filling up these originally profound depressions. This process has continued up
to the present day, and is still going on, until the volcanic range in which Tono-
pah lies, like other ranges in the district, is flanked on both sides by nearlv level
stretches of waste veritable waste lakes which constantly rise as the degradation
of the mountains progresses. These waste lakes (kept level chiefly by the terrific
winds that travel up and down between the mountain ranges, sweeping the fine
material, unbound by moisture or by vegetation, before them) invade the deeper
mountain valleys and overflow the lower hills. Their surface portion consists of
Pleistocene subaerial accumulations, and it has been unwarrantably assumed that
this, material has a depth of thousands of feet, but observations by the writer in the
western part of the State lead to the conclusion that in many, perhaps most, cases
the Pleistocene cover is only a veneer, beneath which lie Tertiary accumulations."
MEASURES FOR THE AMOUNT OF MATERIAL ERODED.
Under the conditions sketched above a large amount of material must have been
stripped from the area of the Tonopah quadrangle and carried to the valleys.
Study of the local geology affords us more detailed data for this conclusion. The
thick volcanic agglomerates (chiefly dacitic), which occupy a large part of the
southern half of the area mapped and are probably upward of a thousand feet
thick, are not represented in the northern half. It is true that these are local
accumulations and may be essentially the remnants of bomb and cinder cones of
the earlier dacitic eruptions which occurred in the southern and not in the
northern region. Still, such material must also have fallen over the northern
half of the area mapped, even if the quantity was smaller; and it is only about
three-quarters of a mile from the New York Tonopah shaft, where nearly 800
feet of the dacite breccia has been passed through and the bottom not reached,
to the region east of Mizpah Hill, where the dacite breccia is missing. This
disappearance must be due to erosion, which, moreover, was accomplished before
or during the deposition of the lake beds (Siebert tuffs), for these in places south
and east of Mount Oddie lie directly upon the earlier andesitic rocks.
That part of the erosive work accomplished since the last important geologic
occurrences the intrusion of the volcanic necks and the faulting or since about
the beginning of the Pliocene (see pp. 69-70) can be estimated in a more detailed
way, since the evidences are not obscured by subsequent events. The volcanic necks
are much modified by erosion, and on the higher ones, as on Butler Mountain,
lateral drainage has pushed back and formed sharp dividing ridges. It is hard to say
how much the solid lava columns have been lowered, but the cinder and agglomerate
cones which once surrounded them have been swept away and only vestiges of them
aSpurr, J. E., Bull. U. 8. Oeol. Survey, No. 208, pp. 139-140.
EROSION IN ARID CLIMATES. Ill
remain (p. 45). These outer cones must have been very extensive in comparison
with the necks and must have covered the whole area of the quadrangle deeply.
There is a difference of 700 feet in elevation from the top of Butler Mountain to
the lowest point, near its base, where the dacite neck cuts the intruded rock, so
that 700 feet is less than the minimum possible thickness of the material that has
been removed between these two points.
A still better measure of the amount of erosion is supplied by a study of the
faulting. In general, the southern half of the area has been depressed by faulting
below the northern half by a distance which has not been closely measured, but
which is certainly many hundred feet; yet this differential movement has been
entirely compensated by erosion, and there has been stripped from the northern
half a crustal layer of a thickness equal to the sum of the amount of the displace-
ment and the thickness of the material removed from the southern half during
the same period. Similarly, the individual faults, elsewhere considered, show that
erosion has compensated for their dislocations.
FEATURES OF EROSION Ilf ARID CL.IMATES.
In the arid Great Basin region the conditions governing the origin of topo-
graphic forms are different from and more complicated than those which exist in
well-watered regions, where most of the reliable physiographic conclusions have
been formulated. It is therefore important that in a region where the topography
is well mapped and the geology fairly well understood, as in Tonopah, the origin
of the forms should be examined.
The writer has previously remarked that in the greater part of the arid Great
Basin region the effect of the scant moisture as an agency of erosion is equaled or
exceeded by disintegration, gravity, and wind action, with the result that in the
lower valleys leveling instead of dissection is brought about, and in the higher ones
dissection is much less marked than in moister regions." The general conclusions
reached by the writer concerning processes of erosion in the Great Basin region,
as expressed in an unpublished paper read before the Geological Society of
Washington in the spring of 1903, are as follows:
Climate controls not only the speed of erosion, but its manner. In moist
climates the precipitated moisture gathers into permanent bodies of running water,
a stream system is maintained, and erosion goes on chiefly along these lines. Thus
even those rocks which offer no differences in weakness are thoroughly dissected, not
because the materials in the valleys are less resistant, but because there the eroding
activity is concentrated. The disintegrated rock or soil, except along these naked
stream beds, is cemented with moisture and bound together by vegetation, and so
a Bull. Geol. Soc. Am., vol. 12. p. 237.
112 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
is fairly well armored against the attacks of erosion, which can make but compara-
tively slow progress.
In a truly arid region, where there are extremes of heat and cold, rock
disintegration at the surface is much more rapid. Streams are rare, transient,
and relatively unimportant, and stream erosion is slight compared with that of
moister regions. Yet erosion is active, so that in the Great Basin region even
moderately steep slopes are stripped of debris and consist of hard, unweathered
rock. The lack of vegetation renders the whole surface equally susceptible of
attack by frosts, thaws, rains, and snows, and the disintegrated material creeps by
the nearest way, in the form of a sheet, into the depressions. Thus the fronts of
many of the Basin ranges are bordered by a continuous apron of debris sloping
down into the center of the valley, an enormous mass of waste which is relatively
slightly increased by the alluvial fans at the mouths of the gulches (PI. XV, ).
In desert regions the more nearly equable distribution of the eroding agents
causes the differences in hardness of the attacked rocks to be far more prominent in
determining the lines of relief. In proportion as the aridity increases the topo-
graphic forms show more and more faithfully the resistance of the rocks. If the
rocks are folded and faulted the ridges will follow the lines of strike and of faulting.
In a country of igneous rocks a new element is introduced, but here also erosion
tends to preserve the original lines of structure. In intervals of moister climate
streams will cut gorges, a tendency which is probably antagonized in succeeding
arid periods.
It is proper to insist here that these distinctions apply to truly arid climates,
and are more applicable as the aridity increases. Semiarid regions, where violent
rains are not infrequent, have a different topography. The abundant waters of the
storms flow down the slopes in rushing torrents, which cut their beds all the more
deeply because the rock is naked and disintegrated as the result of the intervening
periods of aridity. A rugged, well-dissected topography may sometimes result,
often wrongly described as typical of arid regions.
PRECIPITATION" IX REGION NEAR TONOPAII.
Although violent rains sometimes occur at Tonopah, especially in the spring,
they are rare, and the region can not be classed as semiarid; it approaches more
nearly true aridity, and the channeling by torrents is not so important as the
universal downward working of disintegrated material.
No records of precipitation have been kept in Tonopah, for the town is only
a few years old. The observations made by the United Sates Weather Bureau
in Sodaville, 60 miles farther northwest, are as follows:
U. S. GEOLOGICAL SURVEY
PROFESSIONAL PAPER NO. 42 PL. XV
RECENT BASALTIC CONE NEAR SILVER PEAK.
B. EAST FRONT OF QUINN CANYON RANGE, SHOWING WASH APRON TYPICAL OF REGION.
RELATION OF RELIEF TO ROCK RESISTANCE. 113
Precipitation, in inches, at Sodarille, Nev.
1898 4.72
1899 2.30
1902 1.68
1903 2.16
Average for these four years (others not completely observed), 2.71 inches.
DEPENDENCE AT TONOPAH OF TOPOGRAPHIC RELIEF UPON ROCK
RESISTANCE.
After the great amount of erosion which the Tonopah district has undergone,
the relief is to-day determined in a very remarkable way by the character of
the rocks. The relief here is not like that resulting from the work of stable
and .strong streams, concentrating and almost monopolizing the erosion, pushing
back their systematic valleys from one rock formation into another and constantly
broadening their domains. It is rather like that produced by the warm breath
of the sun on a mass of ice and snow, where the softer material fades into the
air and the harder skeleton of ice protrudes above the surface.
The most prominent topographic features in the Tonopah district are the
denuded volcanic necks such as Butler, Brougher, Golden, Siebert, and Ararat
mountains, and Mount Oddie where the hard lava column has resisted erosion,
while the surrounding softer material has been worn down. The map shows how
closely the contours conform to the irregularities of the intrusion and to how
great a degree the difference of resistance has controlled even minor features of the
topography. Around the margins of the white (Oddie) rhyolite intrusions very
well-marked and closely set division planes parallel to the contact (platy structure)
render these border zones often more easily attacked. The outlying rhyolite
dikes also show this markedly, so that (as around Mount Oddie) such dikes, when
relatively narrow^ave been easily degraded to the level of the intruded rocks.
The smaller eminences are also almost always due to a harder intrusive rock, as,
for example, Heller Butte. The intrusive glassy Tonopah rhyolite-dacite in the
northern portion of the area mapped is evidently harder than the intruded later
andesite and occupies in general higher ground. Study of the map shows how
outlying bodies of this rhyolite-dacite are frequently responsible for hills and
ridges, while depressions have formed along the strips of later andesite flanked on
the sides by the rhyolite-dacite.
Mizpah Hill and Gold Hill an -. fault blocks whose relative relief is due to the
greater resistance of the silicified earlier andesite. of which they are made up, as
compared with surrounding rocks. On the east of Mizpah Hill, where the adjacent
rocks are the soft lake beds, the scarp is fairly well developed; on the west side the
16843 No. 4205 8
114 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
difference of resistance between the earlier and the later andesite is not great, so
that the slope is more uniform. The fact that the contours on the southwest corner
of the Mizpah Hill fault block are parallel to the contact of the softer lake beds
shows the minuteness with which the relief has been determined by the rock
resistance.
EFFECTS OF FAULTING UPON THE TOPOGBAPHT.
The effect of faulting on the topography in general is comparatively unimpor-
tant. The two earlier andesite hills above mentioned are the most conspicuous
cases where faulting has been (though indirectly) a factor. The volcanic (dacitic
and rhyolitic) agglomerates and tuffs, which, by the accidents of faulting, usually
adjoin faulted blocks of the Siebert tuffs in the southern half of the map, are
not much harder than these. Nevertheless, the lake beds (Siebert tuffs) are
undoubtedly the most easily eroded of all the formations, and areas occupied by
them are characterized for the most part by a smooth, flat surface, bounded
frequently by a slight scarp where the tuff adjoins more resistant rock. In most
cases, however, as is shown by the map, the tuff block is surrounded on all sides by
the harder blocks, and as there is no outlet for eroded material the tuff block can
not be now much below the harder blocks.
CHAPTER V.
DESCRIPTIVE GEOLOGY OF MINES AND PROSPECTS.
THE KNOWIST EARLIER ANDESITE VEINS.
MIZPAH VEIN SYSTEM.
MIZPAH VEIN.
EXTENT OF VEIN.
Limitation of vein Jyy Mizpah fault. The Mizpah vein has a strong outcrop
(PI. XVII), extending for a distance of about 800 feet in a nearly due east-west
direction. Toward the east end it is broken by a number of small faults, mostly,
it appears, with a north-south strike and an easterly dip, by which the vein is
offset, now in one, now in the other direction; and it is cut off abruptly by the
great northwest-southeast break, which may be called the Mizpah fault (PI.
XVI). This fault is clearly recognizable at the surface and in the underground
workings on the several levels (PI. XIX), as well as in the Desert Queen, the
Montana Tonopah, and the North Star workings. It has a moderate dip to the
northeast. Wherever the veins have been followed to this fault, they have been
found to be cut off abruptly; and the presence of a zone of clay due to rubbing
or trituration, and frequently of a drag of fragments from the quartz veins
along this zone, give evidence of a great movement subsequent to the vein
formation. On the upper side of the -fault, overlying the earlier andesite in
which the veins lie, is the later andesite. Since the later andesite is normally
above the earlier andesite, a normal fault, with a downthrow on the northeast
side, is shown.
Limitation of vein by Burro fault. On the west side the outcrop of the vein is
abruptly cut off by the Burro fault, beyond which the later andesite again outcrops.
This fault, traceable on the surface only by the use of the utmost skill, runs
northeastward. A break corresponding to this, and probably identical with it,
has been encountered underground in the west workings of the mine, farther
west than on the surface, showing that the fault dips northwestward.
Limitation of vein by Siebert fault. The vein normally dips north at an average
steep angle, but alternately flattens and steepens, and is even locally overturned
(fig. 16). It has been followed on the dip to a depth of nearly 700 feet, where it
115
116
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
is cut off by nearly flat fault, which dips west at a moderate angle. This may he
called the Siebert fault (fig. 17). On the upper side of the fault the rock is chiefly
light-colored, partly oxidized, silicified earlier andesite, mixed with much barren
quartz; on the lower side it is unoxidized and has a different appearance, and
though study shows it to be probably the earlier andesite there is much chlorite
and sometimes calcite among its decomposition products, thereby separating it
sharply from the andesite inclosing the veins, which has characteristically been
Surface
Surface
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o 10 20 30 40 so feet
H;. lf>. Vt-rticiil cross section of a portion of
Mi/pith vein as exposed in the Oddie shaft,
showing reversed dip near the surface.
Wm^&8&&i^&&?& m*.,-'"v
Scale
o so mo 200 300 feet
FIG. 17. Vertical cross section of Mizpah vein along
Brougher shaft and inclines.
altered to quartz and sericite. Below the 700-foot level in the Siebert shaft
(PI. XVIII) this rock in places is altered chiefly to quartz and sericite, and ev<-ii
contains silicified zones or quartz veins giving assays of a few dollars to the ton.
At a depth of about 935 feet there was encountered a body of dacitic or rhyolitic
rock resembling the rock in the lower part of the Mizpah Extension, and probably
referable to the Tonopah rhyolite-dacite; below this a vertical drill hole shows that
the same rock is continuous to nearly 1,400 feet from the surface, where the boring
was stopped.
U. S. GEOLOGICAL SURVEY
OUTCROPPING VEINS OF MIZPAH HILL
MIZPAH VEIN.
117
On the 700-foot level, south drift, above the Siebert fault, there was encoun-
tered a higher body of Tonopah rhyolite-dacite containing some barren quartz,
and similar rock occurs on an east drift on the same level. An east drift on the
500-foot level ran into a mass of the same formation.
VEIX STRUCTURE.
The Mizpah vein is usually several feet wide. Its walls are always earlier
andesite, which is generally completely altered to quartz, sericite, etc. The vein
may be succinctly described as a sili- ^ S
cified and mineralized sheeted zone in
the andesite. There are all stages of
transition from the sheeted altered an-
desite (PI. XX) to solid quartz. Both
extremes may be observed at many
places along the vein, and sometimes
not very far apart. More frequently
the vein is intermediate in character,
showing a variable amount of quartz in
the altered porphyry. Sometimes the
quartz forms parallel streaks or vein-
lets and sometimes it occurs reticulated
in the decomposed rock. Frequently
some of these small veinlets possess comb
structure, which shows that the3 r origi-
nated by deposition in open cavities; but
their frequently irregular branching and
their distribution indicate that these cavi-
ties were caused by solution by circulat-
ing waters and not by fracturing. Their
very existence proves that the main
zone did not originate by fracturing.
As a rule, however, even the small
veinlets give no evidence of having been deposited in cavities, but have evidently
been formed by a process of silicitication of the porphyry involving replacement,
the extreme of the process which has altered the andesite near the veins. This
profound alteration of the zone which became the vein was caused by close-set
parallel fractures, which marked this zone, and which afforded a favorable channel
for the silicifying and mineralizing solutions.
The main premineral fractures had therefore the course of the present vein,
striking east and west and dipping steeply north. Frequently, also, the walls are
locally not parallel (fig. 18).
Scale
10
20 feet
FIG. 18. Vertical cross section of Mizpah vein. Oddieand
McMann lease, showing diverging walls.
118
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
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Scale
40 feet
Scale ,
o?4 e e to 12 feet
Flo. 19. Detail sections from Mizpah vein showing the effect of premineral cross fractures. A, Sketched vertical cross sect ii m
of Mizpah vein at a point In the west workings (the whole vein sloped out). B, Sketched vertical cross section of
Mizpah vein, Oddie and McMann lease. C, Detail of Mizpah vein in the Golden big stope, about 160 feet from the
surface. Hori/xmtal plan along a crosscuttlng fracture vein; the vein abruptly increases in width from 2J to 10 feet.
Fracture dips 43 southwest, and the vein is here perpendicular. There is no evidence of movement along the cross-
cutting fracture, but it appears to be premineral. Its function as a fracture plane limiting the circulation is like that
of a wall. It may therefore be called a cross wall. />, Cross section of Mizpah vein, west face of big stope, Lynch
and Omcara workings, about KX) feet from the surface, illustrating the influence of crosscutting fractures on the
original ore. The ore, which was a solid mass east of here, is cut off ulong u premineral fracture plane (strike N. 56 E.,
dip 60 northwest), above which the vein is divided Into a foot-wall and a hanging-wall streak, with altered andesitc
between.
MIZPAH VEIN. 119
EFFECTS OF TRANSVERSE PREMIXERAL FRACTURES.
There were also minor fractures, among which some striking in a general north-
south direction, and dipping east, can be recognized.
Cross walls. These are shown by jogs in the vein following these planes, or, very
frequently, by a change from a highly silicified or mineralized condition to a less
altered one, while the vein zone is continuous and undisturbed. These jogs or offsets
may occur on both sides of the vein, and thus may simulate faults, from which they
are distinguished by the lack of evidence of movement; or they may be restricted to
one side of the vein (fig. 19), in which case they can not be mistaken. Sometimes the
vein may jog in opposite directions on the two sides of such a critical cross plane or
premineral fracture, and so become markedly larger or smaller (fig. 19, B).
Branching veins. Small veins which diverge from the main vein also testify to
these crosscutting premineral fractures.
Besides the north-south premineral fractures, there were other fractures
having a variety of strikes intermediate between that of the main vein zone and
the cross fractures. Among other things this is evidenced by the portions of
the veins which split up from the main vein and reunite with it. This splitting
and reuniting takes place in both a horizontal and a vertical direction, and the
general result can best be explained by illustrations (fig. 20). The veins thus belong
to the class of linked veins, and this same relation is exhibited on a larger scale
between some of the larger veins, and will be described.
The intersections of the minor veins with the vein zone seem, as a rule, to
pitch to the east also, like the main crosscutting premineral fractures.
Origin of ore shoots. The main crosscutting fractures, striking north and
south and dipping east, as above explained, in many places separate the highly
silicified and mineralized vein zone, often by a sharp division, from a portion
which has not been so much altered. These richer portions ma}- be considered
ore shoots; and while their internal size and richness are very irregular, a
careful plotting of the results of the assay chart" shows that the richer portions
of the vein may be separated into broad east-dipping shoots, of which there are
three within the developed vein. The internal distribution of the ore in these
shoots would make an interesting study if enough data were on hand.
Fig. 21 shows the shoot-like distribution of the richest portions. The space
between and beyond the shoots is, however, good ore. The company does not
desire to have the figures published, but it may be said that the amount of gold
and silver in those parts of the vein left blank on the diagram is fully equal to
that contained in the greater part of the ore produced by the Comstock during
a Kindly furnished to the writer by the Tonopah Mining Company.
120
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
Lynch frO'Meara shaft x x
Scale ,.
50 75 100 teet
FIG. 'A). Sections to show the splitting of the Mizpah vein. -4, Horizontal plan of portion of Mizpah vein as developed
on the 260-foot level, Mizpah mine, east of the Brougher shaft. 11, Horizontal plan of portion of Mizpah vein as
exposed in the Golden and Kendall and McMann leases, about 130 feet below the surface, showing splitting of solid
vein into foot-wall and hanging-wall seams. C, Horizontal sketch section of Mizpah vein, big slope. Lynch and
Omeara workings, showing splitting of vein in two. 1), Vertical sketch cross section of a portion of the Mizpah
vein at the Clark shaft, from the surface down. Junction of veins (a) pitches east on the vein at an angle of about 45.
E, Horizontal section of same, taken at 60 feet below the surface.
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M1ZPAH VEIN.
121
its best days." This whole plan, therefore, shows, in the large sense, a single
bonanza, comparable in size and richness to those of the Comstock (tig. 78, p. 277).
As a rule, within the mine, great size of the vein coincides with increased
richness, although exceptionally this is not true.
W
Surface
100
B
Scale
zoo
oo feet
FIG. 21. Diagram showing the distribution of the richer ores in the Mizpahvein. A, Distribution of the richer ores as indi-
cated by assays; black dots indicate assays above a certain figure; lower assays not indicated. Diagram of the Mizpah
vein projected on a vertical plane. The diagram indicates, roughly outlined, broad eastward-pitching shoots of rich
ore. B, Mizpah vein projected on a vertical section, showing slopes from which ore has been removed above 300-foot
level. Previous to the making of the assay plan (fig. 21, A) the distribution of these slopes indicated the eastward-
pitching richer shoots.
From what has been said, it is seen that the ore shoots are primary. Along
certain portions of the east-west fracture zone (those portions being governed by
north-south striking and east-dipping fractures) the circulation of mineralizing
aMon. U. S. Geol. Survey, vol.3, p. 10.
122
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
waters has been freer and the result greater. In these portions the channels must
have been more open, and since the main vein zone was a single set of fractures,
with fairly uniform conditions, the difference in degree of openness, influencing
circulation, must have depended largely on the cross fractures. In other words, it
appears likely that whore these cross fractures were most numerous rude east-dipping
columns or chimneys, speaking general!}', were fonhed, in which the circulat-
ing solutions were relatively concentrated.
POSTMINEEAL FAULTS AND FRACTURES.
Postmineral fractures and faults are com-
mon. Besides the great faults mentioned
there are continually encountered in the mine
many minor ones (figs. 22 and 23) which may
prove puzzling to the miner. Small faults
are very numerous in the workings in the
vicinity of the great Mizpah fault. These
faults usually strike north-south and dip
east, though they may have other attitudes.
Numerous postmineral fractures, along which
there has been no movement, have the same
general north-south strike and easterly dip,
while others have a variety of positions (fig.
24). Postmineral fractures parallel to the
vein are always present. In other . words,
the postmineral fractures in general have the
same directions as the premineral fractures, and stress subsequent to the ore
deposition has reopened the old wounds, which had been more or less completely
healed by the vein formation (fig. 25).
VEIN COMPOSITION.
The quartz of the vein is fine and cloudy. Poor quartz and rich quartz are
often much alike in appearance, save for a purplish tinge in the latter. Under
the microscope this tinge is seen to be due to disseminated particles of argentite.
This mineral is found from the outcrop of the vein downward, through all the
oxidized zone. Silver chloride is also very abundant, though usually, like the
other metallic minerals, it is determinable only microscopically. Orange and
yellow amorphous minerals were also observed, and surmised to be the combinations
of silver with chlorine, bromine, and iodine, and chemical examination of the speci-
mens showed the presence of all these elements. Free gold is sometimes observed,
Scale
z feet
FIG. 22. Sketch of faulted quartz veinlets in ancles.
ite, 300-foot level, Mizpah, just south of the Valley
View shaft.
u. s. GEOLOGICAL SURVEY
PROFESSIONAL PAPER NO. 42 Pl~ XX
mmz&ti
iPSPSI
'<ig5z!%*s*
- { >
' < tS> vV. \ V I > ,-.v 7;V. I J
Scale for sections (A)-(B)
CROSS SECTIONS, SHOWING STRUCTURE OF MIZPAH VEINS.
A. Sketch of Mizpah vein, vertical cross section, 300-foot level, west drift.
R. Cross section of portion of Mizpah vein in Tuscarora stopes, about 90 feet from the surface.
C. Vertical cross section of portion of Mizpali vein, 300-foot level, west drift. 200 feet west of shaft.
/>. Mizpah vein, 300-foot level, east drift, near main crosscut.
E. Sketch of vein in crosscut south from east drift, 300 foot level, Mizpah mine.
I rock, decomposed andesite; 6 = highly fractured, decomposed, and silicified andesite; c~ granular, gray, aluminous quartz (ordinary variety of these
veins); d clear quartz, filling cavities, often with corrb structure.
MIZPAH VEIN.
123
especially under the microscope. A slight copper stain has been reported on the
ore, but the writer has never seen any. Ruby silver and argentite sometimes occur
on cracks, but as a rule these minerals, if present, are not visible to the naked eye.
Scale
o 5 io 15 20 ?5 feet
Fio. 23. Horizontal sketch plan of portion of the Jlizpah vein in slopes east of Lease 52, about 70 feet from surface,
showing probable compensating faulting.
FIG. 24. Reproduction of drawing of model, showing the principal postmineral fractures and faults observed in the
Mizpah mine workings. The strikes of these fractures have been plotted through a center point on the top of the
cube, and the intersection of the fractures with the other faces of the cube has been drawn. The endless variety of
patterns which are made by the same systems of fracturing by their intersection with different planes is here shown.
A, Front view of block, looking down. B, Rear view of block, looking up.
Black manganese oxide is frequent and often concentrated in little vugs. Iron oxide,
the result of the alteration of pyrite, occurs, and sometimes pyrite itself, but this
mineral is much less abundant in the veins than in the wall rock.
124
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
SECONDARY NATURE OF ORE MINERALS.
It is probable that all these metallic minerals are nearly always secondary.
Ruby silver and argentite are often observed in this camp as secondary minerals
coating cracks, as well as horn silver (silver chloride) and the bromides and iodides.
The free gold appears probably secondary. In a few microscopic sections studied
argentite has formed as an alteration of silver chloride, itself probably secondary.
REARRANGEMENT OF VALUES DURING OXIDATION.
00 feet
All the ores in the mine are oxidized or semioxidized, for the zone of
oxidation goes down to the
600-foot level or below,
beneath which the vein is
cut off by the flat Siebert
fault.
The facts that the ores
in their present form are
FIG. 25. Horizontal diagrammatic' plan of Mizpah vein as exposed in the Oddie largely altered and that
and McMann lease, 20 to 30 feet below the surface. Of the crosscutting frac- ' J
tures (dotted lines) limiting the ore, as is shown, some are evidently premi- many postmineral frac-
neral fractures or cross walls, and some postmineral fractures. In the latter
case it appears probable that in some cases the postmineral fracture has origi- tUl'CS are present Suggest
nated by the continuation of movement along a premineral fracture.
the inquiry as to how tar
the values have been rearranged and concentrated during the alteration process.
A study of the assay plan of the mine failed to show any decisive change
at different depths in the relative proportion of gold and silver, the chief
metallic minerals present in the ores. A more accurate statement of this
investigation may be of interest.
Proportion of gold to silver in Mizpah vein.
Lifts.a
Number of
assays.
Percentage of
gold by weight.
Proportion of
gold to silver.
Fi*st 50 feet
22
1.00
1 to 100.
Second 50 feet
42
.86
1 to 116.
Third 50 feet
40
.85
Fourth 50 feet
57
.88
Fifth 50 feet
55
.88
-
Sixth 50 feet
72
.95
Seventh 50 feet
19
.77
Eighth 50 feet
8
.73
1 to 137.
"The word "lift" is here used to designate one of the horizontal zones into which the vein and mine have been
divided for the purpose of measurement. The use of the word is similar to that in speaking of the different "lifts" of
leather on a shoe heel, and the writer is under the Impression that the word in in use by mining engineers with the same
significance as given above.
In value the gold constitutes 25 to 30 per cent of the ore.
DESERT QUEEN SHAFT. 125
The percentage of gold may be smaller in the lower lifts, but the data are
not sufficient to support this idea, and a proportion similar to the average
(1:100) has been found in the shipments of primary sulphides from the rich
ores of the Montana Tonopah.
Moreover, the rich shoots seem, under microscopic stud} 7 , to be original,
though the ore is largely altered that is, the ore seems to have altered essentially
in place, without any thorough rearrangement. This may be ascribed in part to
the relatively scanty supply of surface waters in this arid region.
Some transportation, nevertheless, was inevitable, and it is probable that to
a minor degree the ores have been redeposited. The result has probably been
that values are more evenly distributed over the oxidized vein than they were
originally; and the vein has been enriched to some degree by the downward
penetration of minerals leached from the outcrop as it was eroded.
GEOLOGY OF THE DESERT QUEEN SHAFT.
The Desert Queen is the chief working shaft of the Belmont Company, and
the ores discovered in the workings from this shaft are usually referred to as
the Belmont ore bodies. The shaft is one of the deepest in the camp.
INTRUSIVE NATURE OF RHYOLITE CONTACT.
As shown on the map (PI. XVI), the Desert Queen shaft starts in the rhyolite,
on the southeast slope of Mount Oddie. It passes downward through this rhyolite
for 250 feet, below which it encounters a mass of dark-blue or brown clay,
containing harder residual bowlders of the later andesite. This has a thickness
of more than 50 feet in the shaft, and is evidently a broken and ground up
later andesite, altered to a clay by traversing waters. Below this water occurs
along a fracture.
This zone of movement strongly resembles a fault zone. However it is to
be noted that the movement has affected only the andesite and not the rhyolite,
that the rhyolite is not noticeably decomposed, and that there are no rhyolite
fragments in the breccia. This indicates rather that the disturbance was caused
bv the intrusion of the rhyolite into the andesite. The exact attitude of the
rhyolite contact could not be observed, but it may be assumed to be roughly
parallel to the watercourse just mentioned, which strikes north and south and
dips east at an angle of 60 D .
At the surface this rhyolite is in contact with the Siebert tuff lake beds
about 120 feet west of the shaft, as shown on the map; this contact strikes north
and south. Beneath the lake beds in this block lies the later andesite, as shown
for example in the Silver Top shaft, a short distance to the southwest. A line
126
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
drawn from this rhyolite-andesite contact at the surface near the Desert Queen
to the contact in the shaft has a general angle of dip of about 68. The contact
at the surface is evidently an intrusive
one, being on the western side of one
of the intrusive lobes which radiate
from the main rhyolite mass of Mount
Oddie.
Tonopah Mining Co.
SteOcrt shaft
\ M.zpah 500 ft leve
Desert Queen
I '3r
I Belmont 609 '
Mf uoor,,..*..
Scale
?oo
ofeet
VARIABLE ATTITUDE OF MOUNT ODDIE INTRUSIVE
CONTACT.
The steep dip of the contact be-
tween the andesite and the intrusive
rhyolite at this point is in contrast
with the flat portion of the same con-
tact in -the North Star shaft, where the
lower surface of the rhyolite is very
flat, dipping toward the mountain at
an angle not greater than 10, although
the later andesite shows the same brec-
ciation as at the contact in the Desert
Queen shaft, indicating that the rhyo-
lite is intrusive. This difference in dip,
however, is in accord with other obser-
vations made along the contact of the
rhyolite, all possible variation being
found, the contact being sometimes flat,
sometimes vertical, sometimes, indeed,
dipping away from the mountain rather
than toward it, but always showing the
intrusive character of the rock.
MIZPAH VEIN IN DESERT QUEEN WORKINGS.
The Desert Queen shaft cut the
Mizpah fault at 512 feet, and beneath
it the earlier andesite. At 500 feet a
Fio. 26. Horizontal planof mine workings, showing the rela- drift run a short distance north from
tlon of the vein In the Desert Queen workings to that on the 1 i i .1 \i- u i- 1,
corresponding level of the Mizpah mine. the * haffc cut the Mizpah fault again
and exposed an important vein (fig. 26).
The vein is sharply cut off by the fault on the east and is much sheeted and broken
by the fault movement, so that its course is not immediately evident. The general
BURRO VEINS. 127
trend, however, appears to be east and west, or perhaps more correctly N. 70 E.,
and it appears to have a steep northerly dip and a width of several feet. It is
somewhat dragged in the neighborhood of the fault. Some very rich ore was
found here, which was chiefly oxidized and contained a large amount of silver
chloride. The Mizpah fault here has its usual northwest strike and northeast dip,
but the dip is steeper than it is farther northwest, being from 35 to 45.
FORMATIONS ENCOUNTERED IN THE LOWER WORKINGS.
At a depth of 814 feet from the top of the shaft the rocks to the north and
west are extensively explored by drifting. These workings are almost entirely
in a white, dense rock which study shows to be Tonopah rhyolite-dacite. The
quartz masses characteristic of this formation were encountered, showing the
usual irregularities, nonpersistence, and barrenness; on the other hand, some
portions are exceptional in containing high-grade ore.
At a depth of 920 feet the shaft cut a sheet of white Oddie rhyolite, which
contained a. flat vein of some size, but showed no values of importance (see
p. 193). The bottom of the shaft, at a depth of 1,114 feet, is in the Tonopah
rhyolite-dacite. This rock is much like that at the bottom of the Siebert shaft.
THE BURRO VEINS.
On the south side of the Mizpah vein there are several weaker auxiliary veins.
Most of these converge and unite with the Mizpah on the surface at a point not far
west of the outcrop of the Mizpah fault. The principal ones have been called the
Burro veins, and they have been numbered 1, 2, and 3, No. 1 being nearest the
Mizpah.
These three veins are all branches of the system of which the Mizpah is the
trunk vein; on the surface the No. 2 and the No. 3 probably come together about
200 feet south of the Mizpah; this united vein joins the No. 1 just south of the
Mizpah, and unites with the trunk a very short distance farther northeast.
VEIN STRUCTURE.
These veins are all essentially silicifications of definite fracture zones in the
altered andesite. The zones average perhaps 4 feet in thickness, and along
them quartz has formed (almost entirely by replacement of the altered and
silicified andesite) to a varying degree, so that in places the vein zone may be
entirely of andesite, distinguishable from the wall rock only by its peculiar
and greater fracturing, and in other places may be entirely filled with quartz,
carrying good values in silver and gold. All intermediate stages are also seen
(fig. 27).
128
GEOLOGY OF TONOPAH MIKING DISTRICT, NEVADA.
Vein rone
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FIG. 27. Sections showing thestructurc of the Burro Xo. 1 vein: A, Vertical detailed sketched cross section of a portion of
Burro No. 1 vein at the surface, as exposed by a prospecting pit, at a point about 600 feet west of the probable junction
with the Mizpah vein. B, Vertical detailed sketched cross section of a portion of Burro Xo. 1 vein at the surface, as
exposed by surface workings, at a point about 128 feet west of section A. C, Vertical detailed sketched cross sec-
tion of Burro No. 1 vein, as exposed at the surface, at a point 6 feet east of section B, showing rapid thinning and
disappearance of quartz from the vein zone. D, Detailed sketched vertical section of Burro No. 1 vein at a point about
180 feet west of sections B and C, showing hanging-wall and foot-wall veins In the vein zone. E, Vertical detailed
sketched cross section of a portion of Burro No. 1 vein, as exposed at the surface by a prospecting pit, at a point about
150 feet west of section I), and near the farthest point west that the vein has been traced, showing vein zone with
only hanging-wall streak, and also the manner of dying out and disappearance of this class of veins. F. Horizontal
sketched plan of Burro No. 1 vein, uniting the two vertical sections B and C, showing the manner in which the
change takes place. In all these figures a filtered andcsite; b^q
VALLEY VIEW VEIN SYSTEM. 129
STRENGTH AND EXTENT OF THE BURRO VEINS.
Of the three Burro veins, that next the Mizpah, No. 1, is the strongest. No. 2
is next, and No. 3, the farthest away, the weakest; thus an evident dependence
on the main vein is shown. Moreover, No. 1 is strongest as it approaches its
junction with the Mizpah. Here it is at the outcrop composed of solid quartz 4 feet
wide, and appears to be as important as the Mizpah itself. To the west, however,
the quantity of quartz in the vein zone decreases till the vein is difficult to follow,
and very likely actually dies out. Vein No. 2 is not regularly mineralized and
has not the characteristics of a strong fracture zone. While in general it grows
stronger on approaching the Mizpah, the only place from which high-class ore
has been taken is several hundred feet west of its junction, where the volume
of quartz in the vein increases. No. 3 follows a definite fracture zone in the
andesite and ordinarily has very good walls. In this zone the quartz is mostly
in stringers, irregular and bunchy. High-grade ore was taken out only from
one small portion of the outcropping vein, that being opposite the productive
portion of No. 2. The relation of good walls to a strong vein is continually
shown. Good walls denote a strong fracture zone, which is a good channel for
mineralizing waters.
These veins have not been found to continue downward in general with the
same strength that they show on outcrop, and on the 300-foot level of the mine
they are represented only by weak silicifications or quartz seams, and not all of
them are with certainty identifiable.
VALLEY VIEW VEIN SYSTEM.
THE VALLEY VIEW VEINS ON MIZPAH HILL.
The Valley View vein outcrops, in its strongest portion, about 1,000 feet
south of the Mizpah vein. Its surface exposures are stronger and more compli-
cated than those of the Mizpah system, showing a number of veins which are
of various sizes, many of them being several feet thick. These are connected
by branches, so that the whole is interlaced. The general course is a little north
of east, practically parallel to the Mizpah vein, and the diiferent veins show a
tendency to fan out or diverge toward the west, as the Mizpah vein system does
to a more marked degree. The dip of the veins at the surface is nearly
perpendicular, some of them dipping north and some south, at angles usually
approaching 90.
CROSS VEINS AND ALLIED PHENOMENA.
Cross veins of considerable strength also occur, both on the east and on the
west side of the main outcrop, nearly at right angles to the main course. These
cross veins cut off the veins following the main course, though all are of the
16843 No. 4205 9
130 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
same age. On the east, the strong cross vein near the Stone Cabin shaft prob-
ably cuts off the complicated vein system in this direction; beyond this cross vein
the other veins will be found, for a space at least, abruptly of a different
character. Similarly the strong northwest-striking and northeast-dipping vein,
which heads off a number of the Valley View veins on the southwest side of
Mizpah Hill, seems to mark the boundary of a relatively poorer continuation of
the main vein system on the west (see PI. XVII). Nevertheless, some of the
veins escape and persist, and are found across the gulch, in outcrops and in
the workings of the Wandering Boy. The veins of the Fraction may be a con-
tinuation of this system.
Apparently the mineralizing solutions flowing along the east-west fracture
zones were deflected where the transverse fracture zones were strong enough to
control the circulation, and did not follow the old channel farther.
The same principle is shown by the numerous splitting and reuniting
branches, all running in the main direction of the vein system. Any of these
branches may divert the main strength of the vein along it and into a parallel
vein of the group.
This heading off of the main course of veins by crosscutting veins is entirely
analogous, though on a larger scale, to the crosscutting premineral fractures which
have produced the cross walls, as studied out on the Mizpah vein, and so have
brought about the localization of the ore deposits. The cross walls produce richer
shoots, both as regards quartz and precious metals, within the main fracture zone;
the cross veins cause relative differences in mineralization and vein formation
along portions of a belt of interlacing fracture zones which is similar to though
larger than that occupied by the main Mizpah vein. In the ordinary splitting
of the veins, as seen in both systems, the diverging branches have not so radically
different a direction from the main vein as have the cross veins, but have often
operated to deflect the solutions from the main fracture zone, and hence are called
vein robbers.
VEIN STRUCTURE AND ORIGIN.
On studying the different veins of the system, as exposed excellently in an
almost continuous series of surface openings, the fact that these veins are due to
the replacement, in varying degrees, of andesite by quartz along a zone of especial
fracturing is well illustrated. This is shown by the ever-changing amount of
replacement, at one point the vein zone being little more than fractured porphyry
and at another solid quartz, with all conceivable transitional stages represented
between, these points are illustrated by the sections forming fig. 28.
VALLEY VIEW VEIN SYSTEM.
131
Vein zone
Vein zone
Quartz veinlcts Quartz Quartz veinfets
Scale
s 10 feet
FIG. 28. Sections showing the structure of the Valley View veins; o, altered andesite; 6, quartz. A, Detailed vertical
section of one of the minor Valley View veins at surface. B, Detailed vertical cross section of the same vein as
A, taken about 30 feet east of it. C, Detailed sketched cross section of the same vein as A and B taken about 70
feet east of S. D, Vertical sketched cross section of the same vein as A, B, and C, at the surface, taken at a point about
50 feet east of C. E, Detailed sketched vertical cross section of a fracture zone near Valley View shaft, at surface,
showing the nature of the fracture zone, which by replacement may form a solid quartz vein. Even the small stringers
shown here have unquestionably originated by replacement of the andesite along mere cracks.
132 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
ORE IX TIIK VKIX.
As a rule the Valley View vein system contains a larger volume of vein
material than the Mizpah system, but a smaller amount of the precious metals.
Therefore the Mizpah has produced more ore than the larger vein. Considerable
ore of the kind locally considered low grade (up to $50 per ton, for example) has
been found in the Valley View veins, and some other portions have been found
to be very rich: this rich ore lies in masses, without, so far as yet developed, any
regular extension.
THE VAI-LEY VIEW VEIN SYSTEM UNDERGROUND.
Underground on the Valley View vein system are the workings from the main
Valley View shaft, those from the near-by Silver Top shaft (both of these shafts
belong to the Tonopah Mining Company), and those of the Stone Cabin shaft,
and, as before stated, outside of Mizpah Hill, probably the Wandering Boy and
Fraction workings.
VK1XS IN THE VALLKY VIEW WORKINGS.
Of these underground workings those of the Valley View are the most
extensive. There were levels at depths of 200, 300, 400, and 500 feet at the time
of the writer's last visit. Instead of the several parallel strong veins outcropping
at the surface, these workings show a single main vein, which is thicker than
any at the surface. Furthermore, while the surface veins are nearly perpen-
dicular, the underground vein has a dip to the north of less than 45. This
vein is tj or 8 feet or more thick in various places.
Other veins disclosed in the workings are weak and nonpersistent, though
locally they may be a few feet thick and may hold out promising indications.
Frequent quartz stringers, which may be so numerous in places as to form nearly
a network, occur; and plainly, from what is known of the general geology, these
scattered threads might at any point unite vertically or laterally and form a
decided vein, and thus account for the veins which outcrop and are not cut in
underground workings, or those encountered in one mine level and not in the
expected place in another. Many of these stringers are vertical, so that they
would merge with the flatter-lying main vein in a short distance. The general
situation would seem to be represented in rig. 2!. A strong east-west striking
and north-dipping vein (associated with parallel and crosscutting minor veins and
stringers, many of them nearly vertical) has given its strength near the present
surface to a number of vertical transverse fractures, so that the main vein splits
into a number of vertical ones.
On the oOo-foot level the vein is still strong. The crosscut on the 700-foot
level, however, did not encounter it, but passed through a body of white, dense
VALLEY VIEW VEINS UNDERGROUND.
133
rock, which microscopic study showed to be probably the Tonopah rhyolite-dacite.
This rock is a part of an intruded sheet, which would cut off the vein and termi-
nate it below, at least temporarily (tig. 30).
sw.
NE.
'!:' \^:>:^-'l^f':': ^'feK '~-' : ^^T^<: ~i''[- -",-'', '.\'' ' ' ','- <; '.' : : s -
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B
Scale
100 200 feet
Fie. 29. CroKsjieotions of the Valley View vein. A, Through the Valley View shaft. B, Cross section of Valley View rein
taken a short distance (averaging 200 feet) west of section A.
THE VALLEY VIEW FAULT.
Postmineral fractures are abundant in the mine workings, and u notable fault
occurs on the 200-foot level, by which the main vein is completely cut off and
lost on the east side. This fault has here a strike of N. 15 E. and a dip of
134
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
50 E. On the 300-foot level east the vein is likewise cut off by a broken and
fractured zone, with finally a .straight slip face running N. 28 E. and dipping
steeply east. These occurrences on the two levels represent probably the same
general fault or fault zone. This fault, which may be called the Valley View
fault, is therefore approximately parallel in strike and dip with the Stone Cabin
Valley View shaft
LATER ANDESITE Siebert shaft
Flo. 30. Vertical section on plane of Siebert and Valley View shafts.
fault, which bounds, on the east side, the earlier andesite of Mizpah Hill, and
separates it at the surface from the tuff formation (Siebert lake beds) on the west.
The Stone Cabin and the Silver Top workings, therefore, are on the east side
of the Valley View fault, or between the Valley View and the Stone Cabin faults,
while the Valley View workings are on the west side of the fault of the same name.
VALLEY VIEW VEIN SYSTEM.
VEINS IN THE STONE CABIN WORKINGS.
135
The Stone Cabin shaft had followed a strong vein from near the surface to a
depth of 400 feet at the time of the writer's last examination (fig. 31). This vein
varies in thickness from 1 to 8 feet, averaging perhaps 3 feet. It strikes N.
45 to 50 E., and is evidently one of the main east- west veins of the Valley
View system, which here swings around more to the north. From the surface to
the 200-foot level it dips steeply to the southeast, and thence to the 400-foot level
it is vertical.
About 30 feet east of it a parallel vein is encountered on the 100-foot level,
but it is so much broken up by small faults that it can not easily be followed.
These faults strike chiefly N. 25 to '40 W., and dip southeast at an angle of
FIG. 31. Cross section of veins in Stone Cabin workings.
55. They are probably auxiliary to the main Stone Cabin fault, which must
be close at hand, judging from its position in the near-by Silver Top workings.
On the 200-foot level the same vein is encountered, at the same relative position
with regard to the main vein. Here, being farther away from the eastward-
dipping fault, it is not broken. On the other hand, it is not so heavy as above,
and consists of two diverging branches, each 1 foot thick, which unite in the bottom
of the crosscut. On the 400-foot level, 200 feet below, this vein was not
recognized.
Thus there is a single nearly vertical strong vein in the Stone Cabin, as
in the Valley View workings, with another lesser vein parallel to it on the
136
GEOLOGY OK TONOPAH MINING DISTRICT, NEVADA.
east, which seems to grow weaker and tends to disappear in depth. The main
workings have been driven on the first vein.
Much of the ore is of low grade, and not much runs over $100 to the ton.
A considerable quantity of moderate grade ore has been found. This ore lies
largely in an ore shoot, which pitches steeply east on the vein, and which
was followed from near the surface to below the 400-foot level.
:V'-.'f:' / ->T < ?P of Adrift >./; /^j.-'.^
.
?Wj^i
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^.^.<v.-.v ->..>: i
iv-'/'-'j-'/' ~ : ^ .Vi ; H;v.i,
Fie. 32. Sketch of vertical cut on the east wall of the Silver Top 120-foot level, 3 feet south of mnlii vein, showing
splitting and reuniting of a minor vein.
VEINS IN THE SILVER TOP WOKKINOB.
The Silver Top shaft of the Tonopah Mining Company " starts on the oast side
of the Stone Cabin fault, in finely bedded white tuff, striking N. 20- W. and
dipping southwest. Below this is the later andesite to the bottom of the shaft,
There U another Silver Top shaft northeast of the Golden Anchor, as shown In the general r.iap.
VALLKY VIEW VEIN SYHTEM. 137
which is 120 feet deep. An easterly drift cuts the fault and passes into the earlier
andesite. On this drift are found the veins encountered in the Stone Cabin
workings (tig. 32). The chief vein here seems to run N. 50 to 70 E. and to
dip south at an angle of about 80. It is encountered, though in a broken-up
condition, just west of the fault, but it is undoubtedly cut off by this on the east.
To the southwest of this fault the vein lies in the Stone Cabin ground, and has
been developed by the workings on the 100-foot level of this mine for about 100
feet. Still farther southwest the vein comes again into the Silver Top ground,
and is followed southwest from the Stone Cabin ground for about 140 feet. At
somewhat over 100 feet southwest of the Stone Cabin ground the vein forks,
and at the end of the drifts both forks are cut off by a fault striking N. 22
W. and dipping eastward at an angle of 60. The vein is also developed by a
vertical winze HO feet in depth in the portion west of the Stone Cabin ground.
THE STONE CABIN-SILVER TOP VEINS A PART OF THE VALLEY VIEW VEIN OROl'P.
The second weaker and parallel vein noted in the Stone Cabin workings, to
the south of the main vein, appears also in the Silver Top workings, but has not
been developed.
The probable equivalents of both of these veins, which are shown in the
Silver Top and Stone Cabin workings, can be recognized on the surface, almost
immediately above, at the east end of the outcrop of the Valley View veins,
where they have the same characteristics that are given for the corresponding
veins underground. Even the forking of the vein in the Silver Top west drift,
as described above, corresponds with a similar forking of the corresponding vein
at the surface.
It is plain, then, that the veins in the Stone Cabin and the Silver Top belong
to the Valley View group; and that, as in the case of the Valley View mine,
this outcropping group resolves itself underground into a single strong and
persistent vein with parallel weaker veins.
CORRELATION OF VEINS IN DIFFERENT MINES.
If this is the case, why does the vein go down nearly vertically for a known
distance of 450 feet in the Stone Cabin and Silver Top, while in the Valley
View the vein dips north at an angle less than 45 for a known vertical depth
of 500 feet?
Effects of the Valley View fault. In approaching this problem we confront
first the fact that underground the veins of the Stone Cabin and Silver Top
have not been followed westward beyond a certain point, and that the Valley View
has not been traced eastward beyond a certain point. The veins are clearly cut off
138
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
by strong faults that strike north and dip east. Strangely enough these faults
have not been recognized on the surface, at the points where they should
outcrop, but of their importance underground there is no doubt.
The slip and cut-off at the west end of the Silver Top workings must be only
about 70 feet perpendicularly distant from the slip which cuts off the Valley View
vein on the east, on the 200-foot level. The two can be treated together, then, as
a fault zone, in which the main slip may or may not have been cut, but must either
be one of those cut or must lie in the narrow zone between them. Moreover,
the flat-dipping Valley View vein, which is thus cut off on the 200-foot level.
Valley View shaft
Store Cabin shaft
8OI
ZOO-ft. level, 7 feet below
Valley View ZOOft.level
50
200 feet
FIG. 33. Horizontal plan of veins in Valley View and Stone Cabin workings on the plane of the Mizpah 200-foot level,
to show the probable connection between the chief veins on the two sides of the Valley View fault.
would, if continued, almost exactly strike, at this level, the nearly vertical main
Silver Top ledge, which trends in the same direction (fig. 33).
Hypotheses to explain fault movement. The suggestion arises that the Silver
Top and the Valley View may be really parts of the same vein, and that faulting
is responsible for the remarkable differences in dip on the two sides of the
fault. Both are plainly the downward extension of the strongest portion of the
same outcropping vein system. We may at first consider the hypothesis whether
the fault has had a twisting movement so as to tilt the vein on one side more
than on the other. This difference in tilt, however, would be about 45, and
would involve such an extraordinary rotation of the rocks on one side of the
VALLEY VIEW VEIN SYSTEM. 139
fault that the truth of the hypothesis may well be doubted. Another possible
hypothesis may be formulated. Comparison of the vein in the main Valley View
workings and in the outcrops shows that near the surface the strong north-
dipping vein underground changes by branching into a number of vertical veins,
which are strong, yet not so strong as the main veins. In the Stone Cabin and
Silver Top workings these vertical veins extend far deeper than on the Valley
View side of the fault and no flat vein has been encountered. It follows as a
satisfactory explanation that the veins on the east have been dropped down by
the fault vertically, so that the upper vertical portions come opposite the lower
flat portion on the west.
It is not easy of explanation on either hypothesis why the fault has not been
recognized on the surface. Especially under the hypothesis of rotation or differential
tilting is this fact inexplicable, for such rotation must have been likewise manifested
at the surface as a great and sustained difference in the vein dips; whereas actually
no such change occurs, the steep, nearly vertical, dip being unvarying over the area
which the fault would naturally cut. If, however, according to the second hypothesis
of simple downward displacement, the movement is assumed to have been absolutely-
vertical, there is at least a possible explanation of the failure to detect the fault
namely, that in the surface portion of the group the vertical veins, broken by a
vertical fault, would not show any displacement, while below, where the. vertical
veins come opposite the flat ones, the displacement would be marked.
The probability of this latter hypothesis is strengthened by a consideration of
the main Stone Cabin fault, which has a general parallelism in strike and dip with
the Valley View fault, and lies about 250 feet horizontally east of it. This fault is
a normal one, having a heavy downthrow on the east side, bringing the tuffs, and
below these the later andesites, opposite the earlier andesite of Mizpah Hill on the
west. It is probable that a near-by parallel fault, like the Valley View fault, would
have a movement in the same direction. The Valley View fault is evidently much
the smaller of the two, and may, indeed, be considered auxiliary to the main
displacement.
Amount of vertical separation of Valley View fault. The amount of vertical
movement at the Valley View fault, on the basis reached above, would be something
over 400 feet. This affords some basis for understanding the movement on the
greater Stone Cabin fault, which may reasonably be expected to be several times
greater.
140 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
FRACTION NO. 1 VEINS.
DISCOVERY AND DEVELOPMENT.
The No. 1 Fraction shaft was sunk blindly in the outcropping dacite of the
Fraction dacite breccia in the fall of 1901, and was one of the first explorations
outside of Mizpah Hill and Gold Hill. The shaft was sunk to the depth of 238
feet by means of a horse whim. The shaft passed through 150 feet of soft dacite,
20 feet of crushed material probably representing the later andesite, several feet
of breccia indicating a probable fault zone, and ended in the earlier andesite. At
the depth of 238 feet, the rope not being long enough to sink any farther, drifting
was started, which, in 20 feet, cut a body of quartz that had a width of several
feet and showed some rich ore. Subsequent to this a great deal of development
work has been done, but the results have been unsatisfactory, the vein being very
badly faulted and there being very little rich ore.
NATURE AND RELATIONS OF THE FRACTION VEIN.
By looking at the detailed map of the mining district it will be seen that
the Fraction workings lie close to two faults which were drawn from surface
indications. A study of the underground workings indicates that the faulting has
been so intense and complicated as to defj' working out of the smaller details and
as to make the mining under these conditions practically hopeless, unless the ore
were very rich.
Apparently a single strong vein is represented in the Fraction workings. This
strikes in general east and west, but frequently north of west, and dips south at
varying angles. This vein is in line with the outcrops of the Valley View veins
across the gulch to the east on Mizpah Hill. It is possible, therefore, that it
belongs to the Valley View system. On the other hand, the Valley View and
the Fraction veins underground, when plotted on a given level, show no signs of
being part of the same body, following quite different lines. Indeed, the two
veins dip in opposite directions the Valley View to the north, the Fraction to the
south. The two are also separated, as shown on the map, by one or more faults
which makes correlation still more doubtful. If the Fraction is part of the Valley
View svstem, its vein, dipping in the opposite direction, might be considered as
making up with the Valley View vein a pair of conjugated veins. It is barely
possible, though perhaps not probable, that the fault movement has in the case
of the Fraction reversed the original dip by tilting the block in which the vein lies.
Some of the numerous faults which cut the vein have been exposed in the
mine workings, and such have been shown in the accompanying detailed plans
and cross section (PI. XXI).
FRACTION NO. 1 VEINS.
THE NORTHEAST ( FRACTION ) FAULT SYSTEM.
141
When the strikes of all the different faults observed in the workings are
plotted together, as in tig. 34, they are seen to run in almost every direction
without any fairly recognizable sj^stem. Considered as to their relative impor-
tance, however, systems are clearly traceable. The most important one is, perhaps,
that .striking in a general northeast direction and dipping, as a rule, southeast
at varying angles, perhaps approximating 45. By these faults the vein, as seen
on a horizontal plan, is moved to the north on the west side. There are many of
these, which distribute the faulting between them and constitute a fault zone
Km. 34. Plotting of tlie strike of the faults In tin? Fraction workings.
whose limits and total displacement are not known. If this fault zone had a
uniform dip it would reach the surface about where the fault line had been
independently drawn, from surface phenomena, up the gulch on the southeast side
of Brougher Mountain. This fault line, as will be seen, seems to be a direct
continuation of that bounding the earlier andesite of Mizpah Hill on the northeast,
but the fault movements probably do not correspond in the two localities.
On the 237- and 300-foot levels of the Fraction this northeast faulting has
divided the vein into a series of blocks of very limited extent horizontally, which
have l>een dragged apart and separated one from another, and, finally, the vein
has t>een lost on account of these faults, both on the east and on the west side.
142
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
On the 237-foot level there is a distance of about 200 feet between the portion
of the vein just north of and that south of the shaft, as exposed in the drifts,
but connecting bunches of quartz probably exist in the undeveloped country
to the southeast of the shaft (fig. 35). On
the 300-foot level the exploration has beer
Fraction No. I
shaft
\
'I
\ \
\ >
\
\\
\\
Scale
o 10 20 30 40 so feet
FraaionNoJ sh
Via. 36. Horizontal plan of vein aud limits on ine '>i
foot level, Fraction No. 1 workings.
Fio. 36. Horizontal plan showing vein and faults on
the 800-foot level. Fraction No. 1 workings.
more thorough, as far as it went, and the different steps of the faulting are almost
continuously shown (fig. 36). On the 400-foot level a single block of quart/, probably
belonging to the same vein, and bounded on all sides by faults, was found about
FRACTION NO. 1 VEINS.
143
300 feet south of the shaft (fig. 37). On account of the eastward dip of the
northeast-striking fault zone this bunch of quartz would lie to the west of the fault
and would correspond in position to the quartz near the shaft on the two upper
FIG. 37. Horizon uil plan showing veins and faults on the 400-foot level. Fraction No. 1 workings.
levels. This is shown also by the connection made between the 300- and the 400-foot
levels, where the relations of the vein on the west of the northeast-striking fault
zone are as shown in the accompanying cross section (fig. 39).
144
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
Abundant and strong striations on the fault planes of the northeast system,
together with the evidence afforded by minute faulting and stringers and b3'
the dragging of faulted veins, indicate that while the main movement was com-
plicated by numerous smaller ones, the general result was that the blocks on the
west side of the separate northeast faults were shoved northward past the blocks
on the east side, nearly horizontally, but with a slight downward plunge (tig. 38).
Fid. 3x. Stereogram showing nature of movement along the main northeast faults in Fraction No. 1 workings.
THE NORTHWENT FAl'l.TS.
There is also a well-marked system of faults striking north of west, sometimes
parallel to the veins, but generally cutting across them at slight angles. These
faults may have some connection with the northwest fault, which is shown on the
map as running just east of the Fraction workings (the Wandering Boy fault).
Thev have a great variety of dips, sometimes vertical and sometimes nearly
horizontal, with intermediate angles. An illustration of their effects is seen in
FRACTION NO. 1 VEINS.
145
the cross section (fig. 39), which is transverse both to the vein and to the faults.
.This crass section is taken along the series of inclined workings on the vein,
which run from a point about 60 feet above the upper level to below the lower
level. The portions actually exposed are indicated by solid lines, the intervening
portions are dotted. It will be seen that the vein follows a series of pronounced
rolls, steepening and flattening alternately. In the mine it is evident that these
rolls are the result of pressure and deformation in the rock, and are in the
nature of folds. On the two upper levels, at the sharp bend or apex of these
folds, as shown in the cross section, tangential fractures or slight faults leave the
Scale
30
mofeet
300-rt./evel
400-ft./e*el
FIG. 39. Cross section of Fraction No. 1 vein, along drifts and winzes.
vein and pass off into the surrounding andesite. Some of these become horizontal,
some nearly vertical, and both strike nearly parallel with the vein. Between the
300- and the 400-foot levels, a flat fault, striking and dipping in the same way as the
vein, has probabty the same origin as the flat tangential slips in the upper levels,
but is here of greater magnitude, so that the vein has actually been faulted con-
siderably along it. The incline from the 300- to the 400-foot level follows this
fault for some distance after the disappearance of the vein. The fault which
terminates the vein at its lower end in the cross section belongs to the northeast
system, and Is thus different from any other of the faults shown in the figure.
16843 No. 4205 10
146 GEOLOGY- OF TONOPAH MINING DISTRICT, NEVADA.
The deformation displayed in this section of the vein is analogous to the
monoclinal folding of strata, in which the fold passes into a fault if the deforma-
tion be carried farther than the stretching strength of the rocks. Since all the
veins in the Tonopah district have normally decided dips, ranging from vertical to
about 30, it may be believed that the flatter-dipping portions of the Fraction
vein as seen in the cross section have been deformed, and that the steeper portions
represent more nearly the original attitude. It appears then that the vein has
been deformed by movements acting in a nearly horizontal plane. These
movements have shoved the vein and the inclosing country rock to the north on
the upper side, and being distributed have caused rolls or folds which in places
break and form faults.
These west-northwest tangential faults are, however, not persistent^ parallel
to the veins, but may trend across them at a slight angle. The result is seen in
the western part of the workings on the 237- and 300-foot levels, as shown in
the figures. On the 300-foot level the west-northwest fault cuts out the vein
gradually. The vein runs parallel with the fault for some distance, appearing
and reappearing as lenses of quartz along the fault zone, until it entirely
disappears.
CAUSE OP FAULTING.
To explain this singularly intense, complicated, and peculiar faulting there
must be found a cause competent to thrust the blocks on the northwest side
of the northeast faults to the north in a nearly horizontal direction, and.
to shove the upper layers of rock and vein past the lower layers in a nearly
horizontal direction also. The volcanic neck of Brougher Mountain, whose
edge is only about 1,400 feet southwest of the Fraction No. 1 workings, has been
thrust up after the other rocks were erupted and the mineral-bearing veins
formed. Its smallest diameter in a north-south direction is about 1,200 feet, and
the examination of its contact zone shows that it probably extends downward in
much the same form as it appears at the surface, as a solid column of lava.
The intrusion of this column was probably competent to produce this complicated
faulting, and to exert the violent horizontal pressure indicated in the Fraction
workings. It has been determined independently that the faults of the region,
as a whole, came into existence at about the period of the intrusion of the
dacitic necks, of which Brougher Mountain is one. The conclusion arrived at
therefore falls in line with the general facts.
COMPOSITION OF VEIN.
A small quantity of rich ore was taken from the upper levels. This ore
showed ruby silver and argentite and in one case native silver, all in leaves or
films on cracks or crevices, evidently secondary. The rich quartz itself, as in
FRACTION NO. 2 WORKINGS.
147
other mines of the district, has a dull purplish color, due to the presence of fine
silver sulphide. Most of the quartz discovered, however, has proved to be of low
grade. Adularia (valencianite) is very abundant as a gangue material.
FRACTION NO. 2 WORKINGS.
ROCKS EXPOSED IX SHAFT.
The Fraction No. 2 shaft, which was sunk after the No. 1 shaft and became
the main working shaft, lies about 450 feet west-southwest of the No. 1 shaft and
is connected with it at the 400-foot level. The collar is slightly higher than that
of the No. 1 shaft, and the geologic section exposed is about the same. The shaft
passed through about 215 feet of soft dacite and about 8 feet of white breccia
(consisting in large part of rhyolite resembling the Oddie rhyolite) into the
earlier andesite. The contact of the overlying rocks with the earlier andesite
dips to the east at an angle of about 30, but this dip is probably only local.
Scale
Fraction No.2 shaft
300-ft. level
FIG. 40. Horizontal plan of veins and faults exposed on the 300-foot level, Fraction workings, showing the relation of
the vein fragment in the Fraction No. 2 to the vein on the corresponding level of Fraction No. 1.
FAULTED VEIN FRAGMENT.
At about 300 feet from the shaft a body of quartz was drifted on for a short
distance to the southwest. This quartz is a definite vein about 3 feet thick. It
strikes northeast and dips southeast at an angle of about 40. Some good assays
were obtained from it, although most of it was very low grade. On the north-
west side of the shaft this vein seems to be cut off by a flat fault that strikes a
little west of north and dips at a slight angle to the east. It is very likely that
this vein, which has not been very largely explored, may be part of the same
vein which is exposed in the Fraction No. 1 workings, although a plotting of the
vein on the corresponding levels in the No. 1 and No. 2 workings shows how
difficult it is to establish any definite connection (fig. 40). Only the size and
148 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
nature of the veins and the corresponding strike and dip warrant the above sug-
gestion, for the faulting is so complicated in this region that in any space actually
undeveloped by mining operations, little more than guesses can be made in many
cases.
TONOPAH RHYOLITE-DAC1TE.
The Fraction No. 2 workings lie mostly on the 400-foot level, and besides a
connection with the No. 1 shaft there is a drift running nearly 600 feet to the
north-northwest and more than 200 feet in the opposite direction. Only small
quartz veins, of no importance, occur in these workings. The rock encountered
is a rather dark-colored earlier andesite, sometimes considerably kaolinized, like
that encountered in the No. 1 workings. In the south drift from the shaft,
however, a white rock is encountered. This is solid at the end of the south drift,
and Ijetween this point and the shaft occurs as fragments and large bowlders up
to several feet in diameter in the darker andesite. The geologic features here
indicate that the breccia is due to movement in the rock, and this conclusion is
corrroborated by microscopic study. In this breccia are encountered several
strongly marked slip planes, which strike N. 30 or 40 E., and dip southeast at an
angle of 40 or more. These correspond in altitude to the northeast-striking and
southeast-dipping faults in the Fraction No. 1 workings, and it appears probable
that the hard white rock at the south end of the drift has been brought against
the darker and softer andesite of the north drift by means of this faulting. Some
perplexity has arisen concerning the nature and relation of these two rocks.
After study, however, the author is of the opinion that the latter rock is a phase
of the earlier andesite, while the white rock is a coarse-grained phase of the glassy
Tonopah rhyolite-dacite.
Microscopic examination shows that this hard, white rock is considerably
altered. The phenocrysts are of altered feldspar, in part andesine-oligoclase and
in part orthoelase; the}' arfe now largely changed to muscovite (sericite) and
adularia. Small original biotite crystals are thoroughly bleached. The glassy
groundmass contains veinlets of calcite and abundant pyrite. The chemical
analysis of the rock, by Mr. George Steiger, is as follows:
Analysis of altered Tonopah rhyolite-dacite.
[Specimen 2V9.J
SiO, 68.19
A1,O, 15.13
FeA 1.31 .
FeO 42
MgO 29
CaO 1.19
Na/) 3.13
K,O .66
TiO, 32
PA 15
VALLEY VIEW VEIN SYSTEM. 149
WANDERING BOY VEINS.
About 200 feet northeast of the Wandering Boy shaft there outcrop several
quartz veins whose position and course, an will be seen on the map (PI. XI), suggest
that they form a continuation of the Valley View system.
RELATIVE ELEVATION OK FAULT BLOCKS CONTAINING VALLEY VIEW AND WANDERING BOY VEINS.
The veins above mentioned are in earlier andesite, probably in the same fault
block as is Gold Hill. That Gold Hill is bounded on the north by a fault is shown
by stratigraphic evidence, for the Siebert tuff on the north is in rectilinear contact
with the earlier andesite on the south, indicating a very considerable displacement.
Along this fault line a valley has been eroded, up which the road runs. The fault
block north of this fault is bounded on the west by the Stone Cabin fault, which has
an upthrow on the west, bringing up the earlier andesite of Mizpah Hill. There-
fore the movement of the Stone Cabin fault compensates to a large degree for
that of the Gold Hill fault; and a prolongation of the Gold. Hill fault north-
westward between Mizpah Hill and the Wandering Boy finds the earlier andesite
corning together and lying on both sides of this prolongation. There is, however,
some reason for believing that the fault actually continues along this line, though
with much diminished displacement.
RELATION OK VALLEY VIEW AND WANDERING BOY VEINS.
According to the tentative conclusion stated in the last sentence above, the out-
cropping veins northeast of the Wandering Boy, if they are a part of the Valley
View system, are separated by a west- north west fault from the Valley View veins
of Mizpah Hill. They are represented on PI. XVII and on fig. 2 (p. 153). The
strike is northeast and the dip, like that of the Fraction veins, and unlike that of
most of the veins of Mizpah Hill, is to the south at angles of from 50 C to 75. In
size and course they are not unlike the westernmost outcrops of the Valley View
veins on the western edge of Mizpah Hill, about 300 feet away. The southerly dip,
also, is found represented in this portion of the Valley View outcrops, the western-
most veins dipping, at the surface, south at angles of from 70 to 80.
At a depth of a few hundred feet the veins which occur in the Wandering Boy
workings, and which are probably identical with those outcropping northeast of the
shaft, acquire a flatter dip 30 to -tO c to the south and thus correspond in dip
with the vein shown in the Fraction workings. On Mizpah Hill, however, the
Valley View veins, at a corresponding distance underground, have a similar dip of
about 30 in the opposite direction to the north. The veins in the two localities
can not be directly correlated, and their prolongations on a given uniform level
underground would be several hundred feet apart, though nearly parallel.
150 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
Opposing dips of the veins probably original. The reason for the opposing
underground dips of these veins, which have nearly the same line of outcrop
and a nearly identical dip at the surface, is not clear. As before stated, the
Wandering Boy and the Valley View veins seem to lie in different fault blocks,
being separated by a probable fault which runs along the road between them;
and it is possible that the faulting may have been of such a differential nature
as to partially revolve the block containing the Wandering Boy veins and to
reverse the dip. Evidence obtained both in the Wandering Boy and in the
Fraction demonstrates that the dip of a vein may be changed and even reversed
by faulting, and by accompanying deformation which corresponds nearly to
folding, but which is probably the result of an aggregate of small faults.
Against this interpretation is the fact that the steep south dip of the Wander-
ing Boy veins at their outcrop corresponds with the similar surface dips of the
heavy Valley View vein, which is the vein of the outcropping Valley View group
lying farthest east, and the one with which the Wandering Boy vein would naturally
be correlated. If the different dip of the veins in the two blocks is due to the
revolving of one block on another this difference should appear at the surface as
well as underground; that it does not is evidence rather in favor of the conclusion
that the displacement has occurred without any notable change in the attitude of
the veins aside from local and minor effects. In this- case it follows that the
veins of the Valley View system present, if the perplexing faulting were
eliminated, marked differences in dip, the main Wandering Boy vein dipping at
a moderate angle to the south, as the main Valley View vein does toward the
north.
Change of dip shown by the comparison of the Valley View and the Stone Cabin.
In this connection the studies already made on the Valley View veins are impor-
tant. It has been shown that the outcropping heavy vertical veins of this system
on Mizpah Hill do not persist, as demonstrated by the Valley View workings, to
a depth of as much as 200 feet, but are represented at this depth and below by a
strong vein dipping about 35 to the north. In the Stone Cabin and Silver Top
workings, however, a vein, which is certainly the continuation of the outcrop-
ping heavy Valley View vein, continues down almost vertically to a demonstrated
depth of over 400 feet, beyond which point exploration has not been made. This
portion of the vein is separated from the larger portion in the Valley View
workings by a fault, along which the displacement of the vein seems to have
been normal, so that the vertical portion shown in the Stone Cabin workings
has been dropped down below the north-dipping portion of the Valley View.
According to this the part of the main Valley View vein which has been
eroded to expose the present outcrop on Mizpah Hill must originally have extended
WANDERING BOY VEINS.
151
vertically up above the present surface for a distance of several hundred feet, at
least.
Wandering Boy and Valley View conjugated veins. If the conditions on the
west side of Mizpah Hill, where the Valley View veins approach the Wandering
Boy veins, are like those on the east end near the Stone Cabin, the Wandering
Boy block, if depressed, should have brought down the vertical portion of the
vein, a condition which is not found. What the relative movement of the two
blocks has actually been is not certain. Siebert lake beds are exposed in the
southwest corner of the Mizpah Hill block, and are assumed, from the topog-
raphy, to occur in the southeast corner, but have not been actually observed
FIG. 41. Hypothetical diagrammatic vertical cross section of the Valley View vein system (represented by its principal
and strongest vein) before faulting and erosion. The upper part is considered to b now represented in the Stone
Cabin and Silver Top workings and for a short distance below the outcrop of Mizpah Hill. The north vein is con-
sidered to be represented by the main vein in the Valley View workings, the south vein by that of the Wandering
Boy and Fraction.
there. This indicates that the Mizpah Hill block has been depressed, relatively
to the Gold Hill block, so that the Wandering Boy vein would represent an
originally lower portion of the Valley View vein system than the portion now
outcropping on Mizpah Hill. If this is so, the vertical portion of the Valley
View vein system should be expected to pass in depth to veins dipping south at
angles of 30 or 40. From the Valley View workings, however, it is known
that in depth the vertical veins here actually pass into north-dipping veins and
continue so several hundred feet downward, at least. The north-dipping and the
152 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
south-dipping flat veins, represented, respectively, in the Valley View and in
the Wandering Boy, are then probably not parts of the same vein, but represent
a pair of veins dipping at equal angles in opposite directions (fig. 41).
OUTCROPS OF WANDERING BOY VEINS.
The outcrop veins northeast of the Wandering Boy all have a northeast strike
and a southeast dip. As observed at the surface they are designated as 1, 2, 3,
and 4 on fig. 42.
REPRESENTATION OF OUTCROPPING VEINS UNDERGROUND.
The heaviest vein, No. 1, as there is reason to believe, may be the main
vein of the underground workings shown in the 300-foot level. The 8-inch vein
represented on the 300-foot level, northwest of the probable position of the main
vein at this point, may very well be the same as No. 2. The 6-inch vein followed
on the 115-foot level may perhaps also be No. 2, in spite of the fact that though
it has a southeast dip it lies almost directly over the supposed No. 2 vein on the
300-foot level. The general result of the faulting here has been to place the veins
in the lower levels in a position farther north, on the west side of the numerous
faults, than would be the case if the veins continued regularly downward. The
inclined shaft shown in the figure was inaccessible at the time of the writer's
visit, but drifts were run on two veins at distances of 65 and 95 feet from the
surface. It is likely, as shown in the figure, that the former was on the No. 3
vein." the latter on the No. 4.
FAULT SYSTEMS IN THE WANDERING BOY.
In the Wandering Boy workings the veins are thrown into great confusion
by faulting. Analysis of the disturbance leads to the conclusion that the faulting
can be referred to two major systems that of the Wandering Boy fault, which
strikes northwest and outcrops just east of the Wandering Boy shaft, and that
of the Fraction fault, which strikes northeast and whose outcrop is drawn on
the map as lying between the Fraction No. 1 and the Fraction No. 2 shafts.
In the Wandering Boy workings the Wandering Boy fault dips southwest at an
angle of approximately 50, while in the Fraction workings the Fraction fault
dips southeast at an angle of about 45. In the north corner of the block inclosed
by these two faults, therefore, the line of intersection of the faults pitches south,
and the faults rapidly approach as they go deeper. The estimated position of
these two faults on the 300-foot level is shown in PI. XXI, and may be compared
with the surface outcrops, as shown on the map (PI. XVI).
a Mr. J. M. Healy inform* the writer that the vein shown in the figure, as drifted on at the 66-foot level, was 3 feet
thick and of low grade.
WANDERING BOY VEINS.
DISPLACEMENT OF THE WANDERING BOY FAULT.
153
The Wandering Boy fault, as .shown on the map, separates the earlier andesite
on the northeast from the Fraction dacite breccia on the southwest. It has
Scale
o 10 20 40 co ao loofeet
FIG. 42. Plan showing outcroppinK veins near the Wandering Boy and their probable relation to the veins encountered
underground.
therefore a downthrow on the .southwest, and underground workings show that
it has a southwest dip, making it a normal fault (fig. 43). That the contact is
154
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
^V-^J- ^.'fracture' breccia
'' v>-/-'^'; ':*,-. I I "./ \/. r~. /
^i\ '^'-C'.K <?</. W-v/.
'Sv-'aoo-ff levei
pii^lfsp
^^Wl^'?^
:5^Pi;|^
Earlier :-;andesite A /'
sw
reallj- due to a fault of very great displacement is shown by the occurrence
underground along it of a thick friction breccia containing fragments of later
andesite, of granitic rock, and of the adjacent rocks.
On the 115-foot level the main Wandering Boy fault is well developed (fig. 44).
The small 6-inch vein followed on this level shows a repeated breakdown to the
southwest a.s the main fault is approached, a movement corresponding to the chief
normal faulting. Besides this there are horizontal grooves along the main fault
plane, and similar striations are found on it where it is cut in the shaft below at
a depth of 185 feet. Furthermore, on the 115-foot level, the vein is bent and
dragged to the northwest along the fault plane
(fig.44), and here the dip becomes north instead of
south, as normal. These phenomena show a hori-
zontal movement to the northwest on the southwest
side of the fault, and the reversal of the dip shows
some differential or torsional movement. The striae
on a fault plane indicate the last movement, the
records of previous and often more important move-
ments being erased by each new one. The combined
result of all the movements indicated, therefore, is
that the block on the southwest side of the fault has
moved downward, and also to a less degree (probably)
northwestward, along the fault plane. This hori-
zontal movement is also shown in the Fraction, where
the northwest faults (see p. 144) are probably auxiliary
slips related to the Wandering Boy system. In the
Fraction, especially on the 300-foot level, important
horizontal movement is registered by the striation.
100
so
Scale
o
CROSS FAULTING ON THE 300-FOOT LEVEL.
100 feet
Fiu. . Vertical section on the Wan
defing Boy shaft, showing the main
Wandering Boy fault.
Complicated faulting is shown in the 300-foot level
of the Wandering Boy. The main workings consist of
two drifts run at right angles, one running nearly east
and the other south. The vein shown in this level has a thickness of 3 or 4 feet,
strikes northeast or east-northeast, and dips southeast at an angle of 30 or 40.
The east drift, therefore, runs somewhat diagonally to the strike of the vein,
though more nearly along it, while the south drift also runs diagonally though
somewhat more across the strike (fig. 48). The vertical section along the east
drift is given in fig. 45, that along the south drift in fig. 46. Near the end of
the south drift a short east drift has been run, following a portion of the vein,
and the vertical section along this drift is given in fig. 47. In fig. 45 it is shown
WANDERING BOY CROSS FAULTS.
155
that the vein (which normally, following its strike and dip, would disappear from
the drift) is continually thrust up to the east by close-set faults, so as to persist
in the drift.
Workings.
25
100 feet
FIG. 44. Horizontal plan of 115-foot level, Wandering Boy workings, showing minor vein and Wandering Boy fault.
Judging from the section (tig. 45), most of the faults are apparently reversed
faults, while some are normal. In the south drift, the vein has been repeatedly thrown
FIG. 45. Vertical section along east drift, 300-foot level, Wandering Boy mine, showing faulting of vein.
up to the south by close-set, normal faults, as shown in tig. 46. There is no question
as to the identity of the fragments of vein in the east drift and those in the northern
half of the south drift, for the connection is nearly continuous.
The fragment shown in the south end of the south drift is 30 or 40
feet distant from the fragments farther north, and may represent a ||
closely parallel vein; on the other hand, it is identical with the
rt--n
It I fa
FIG. 46. Vertical section along south drift, 300-foot level, Wandering Boy mine,
showing faulting of vein.
ion.
FIG. 47. Vertical
section showing
short crosscut to
east near south
end of south
drift. 300 -foot
level, Wander-
ing Boy, showing
faulting of vein.
other vein blocks in size, strike, dip, and appearance, and there is no necessary
reason for separating it from them. The short drift running east on this southern-
most vein fragment shows conditions identical with those in the main east drift
(fig. 47), the vein being upfaulted to the east by close-set, apparentlv reversed
faults.
156
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
The situation is shown in horizontal plan in fig. -48, where the strikes of the
faults and the vein blocks may be studied and compared, as the dips may be
compared in the vertical sections. Here it is seen that the faults on the east drifts
have essentially a north-south course, some trending to the west of north, and
perhaps most of them to the east of north: and that those in the south drift are
T: i \
~~^=^^ 70 '
Scale
10 20
o feet
Flu. 48. Horizontal plan of WanderiiiK Hoy, 300-foot level, showing fragmentu of vein mill <-ross limits, witli tin-
general trend of equal displacement.
essentially east and west faults, though usually trending north of west. Therefore
the vein may be considered, for the sake of clearness, as cut by two intersecting
systems of faults, one striking north and south and the other east and west. The
vein is repeatedly upthrown on the east by the north-south system and on th
south by the east-west system.
EFFECTS OF CROSS FAULTING.
157
Effect* f CTOSS faulting ideally considered. In order to understand the
resultant effect of such intersecting faults, let us take a simplified example such
as is shown in the stereogram, fig. 49. This shows a rectangular block which
has been affected by two sets of vertical faults, striking at right angles to each
other. On the figure they are also represented as equally spaced and all having
the same displacement, thus giving to the example an ideal simplicity which is
FIG. 49. Stereogram showing the results of cross faults equally spaced and of equal throw.
probably rarely found in nature. The result of these intersecting faults, as is
seen, is that lines or planes of equal displacement are zigzag, being made up of
regularly alternating portions of each of the two fault-system planes, the length
of each of the component straight lines being determined by the spacing of the
faults, while the trend of the whole zigzag, and therefore of the lines of equally
displaced blocks, is diagonal to both the fault systems. In effect, the resultant
158
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
of these two intersecting directions of faulting has been a third diagonal system,
which represents the direction of equal faulting.
On a plane projection, with closer spacing of faults, or a greater number of
thorn shown on a smaller scale, the situation is again shown in fig. 50. It may
be remarked that to obtain such a resultant there need not necessarily be any
N
Downthrow
W
O
o
IE
FIG. 50. Diagram showing horizontal plan of equal and equally spaced faults belonging to two systems intersecting at
right angles, the north-south system having a regular downthrow on the east, and the east-west system a regular
downthrow on the south side. The heavy zigzag line represents one of the lines of equal faulting, the shaded squares
one of the zones or blocks of equal displacement.
correspondence between the direction of displacement (whether up or down) of
the two systems. If we reverse the movement of either of the fault systems,
for example, if in the figure (to be understood with the aid of the stereogram)
the north-south faults are downthrown to the west instead of to the east, a
EFFJ5OI8 OF CROSS FAULTING.
159
similar resultant faulting will be accomplished, but with a trend at right angles
to that depicted.
Downthrow
O
o
H-
IT
O
Line of equal
displacement
FIG. 51. Diagram showing course of line of equal faulting for two systems of faults intersecting at right angles and
having uniform displacements, the spacing being uniform within each system but different for each system.
11
F E
%s
Downthrow
Line of equal
displacement
Line of equal
displacement
FIG. 52. Diagram showing the diverse courses of lines of equal displacement which are the result of two systems of equal
.faults intersecting at right angles but unequally spaced.
From this simple case the variations and irregularities such as are usually met
in nature bring about endless changes. A few of these maj r be ideally deduced.
Fig. 51 illustrates a case of equal faulting in two systems which are at right angles,
160
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
the spacing of the faults within each system being equal, but that of one system
being different from the other. Fig. 52 represents a case similar in all respects, save
\
Downthrow
Downthrow
Line of equal
displacement
FIG. 53. Diagram showing the line of equal displacement when the fault systems are oblique to each other instead of
being at right angles, the conditions otherwise being like those in fig. 50.
that the spacing of the faults of both systems is irregular. Fig. 53 shows a case
similar to rig. 50. save that the fault systems are oblique instead of perpendicular.
If. now. the amount of displacement in the two fault systems is different, even
though it be constant within each system, blocks of equal displacement will no
N
Downthrow
Kui. .14. Diagram showing the effect of cross-faults when the faults of one system have twice the displacement of those
nl the other system. Here the north-south faults have double the displacements of the east-west ones. The shaded
blocks are blocks of equal displacement.
longer be connected, and therefore there will be no continuous line of equal
displacement. In rig. 54, for example, where the displacement of the faults is twice
EFFECTS OF CROSS FAULTING.
161
as great in one system as in the other, the isolated blocks of equal displacement will
be separated from one another, as are the starting and stopping squares of the
knight move on a chess board. If the displacement of one system is three times,
instead of twice, as great as the other, the blocks of equal displacement will
be removed (in the diagram) one square farther from one another, in a direction
parallel to the faults of greater displacement, and so on. If, again, the faults in
each system are unequal amotuj themselves in regard to their amount of displacement,
the fault blocks bounded by the two systems will be distributed in many apparently
irregular ways, and each block will appear as a separate unit that has moved
independently, rather than as the resultant of intersecting faults. Still, in all cases,
it appears to hold good that in general the zones of blocks of equal displacement,
roughly aligned though these ma\' be, will lie diagonally between the two fault
systems. Which diagonal it will be can be ascertained from the following diagram,
fig. 55:
Fra. 55. Diagram showing trend of zones of equal displacement with given directions of downthrow.
As illustrated in fig. 53, these conclusions hold good for faults striking obliquely
to one another as well as at right angles. They also hold good for faults which dip
obliquely instead of perpendicularly, and for cases where the dips in the two sets are
different in angle or direction, or both.
Application of principles to Wandering Boy cross faults. These deduced
general principles enable us to understand the result of the intersecting faults in
the Wandering Boy 300-foot level. The resultant of the east-west and the north-
south faulting is a northeast trend of equal displacement, as indicated on the figure,
16843 No. 4205 1 1
162 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
nearly parallel, as it happens, with the strike of the vein. Blocks lying in zones
with this general trend have been systematically elevated above adjacent parallel
zones lying to the northwest.
The vein, dip as a factor in th problem. In the case of the faulting on the
Wandering Boy 300-foot level, the problem takes on an added complexity, since the
available test of faulting is not the relative position of the displaced blocks, but rather
the position of the vein, whose plane is oblique to any of the planes of the fault blocks,
and whose present position is what we seek ultimately to understand. The strike of
the vein being nearly parallel with the trend of equal displacement, it results that if
the dip is toward the direction of resultant equal downthrow, then the two factors of
lowering the vein will be added and the fragments of the vein will gain depth faster
than the inclosing rock blocks. If, on the other hand, the dip is against the down-
throw, two factors of lowering the vein will be set off against each other. The vein
then will gain depth more slowly than the inclosing rock blocks, if the faulting has
a greater effect than the dip; will continue on a general horizontal plane, if the
faulting has an effect about equivalent to that of the dip; or will ascend, in spite of
the downfaulting, if the latter be sufficiently slight to have its effect overbalanced
by the dip. In the Wandering Boy 300-foot level, we have, as may be seen from
the sections, the second of these conditions. The dip is opposite to the downthrow,
and the angle of dip, the displacement, and the spacing of the faults are fortuitously
such (for a distance at least) that the one offsets the other, and the vein continues in
a horizontal zone. This explains why the long east and south drifts and the short
east crosscut from the south drift all encounter blocks of apparently the same vein;
and it follows that other blocks of the vein probably exist on this same level in
the angle between the two main drifts, and beyond the explored area as far as this
peculiar intersecting faulting and the balance of dip and displacement is maintained.
CORRELATION OK VEINS IN FRACTION AND IN WANDERING BOY.
PI. XXI, p. 140, shows the vein and faults of the corresponding 300-foot levels of
the Fraction and the Wandering Boy, together with the estimated position of the lines
of main faulting of both the Wandering Boy and the Fraction faults. It is here seen
that the northeast faults in the Wandering Boy, which form the majority of those
faults classed together, in describing the cross faulting on the 300-foot level, as north-
south faults, are parallel in strike, dip, and direction of displacement, with the chief
set of faults in the Fraction, and in strike at least with the main Fraction fault as
determined on the surface. These minor faults involve a movement, as seen on a
horizontal plane, to the north on the west side; as seen on a vertical section, down-
ward on the west side. The real movement has probably been a compound of these
two, as studied out in the Fraction workings. Along the main fault plane, then (if,
indeed, there is one, and the displacement is not rather distributed over many par-
*
X--
~J1L
t/>(/5
UJl
IS
WANDERING BOY FAULTS. 163
allel faults), the movement was undoubtedly similar to that of the minor faults, and
would bring the two portions of a faulted vein into somewhat the position that the
Wandering Boy and the Fraction veins, taken as a whole, occupy to each other. This
leads to the suspicion that the two occurrences were originally the same vein and
were separated by the Fraction fault. The veins in the two mines are similar in
strike, dip, size, and general characteristics. A fragment of the Fraction vein lying
farthest south on the 237-foot level and probably in a zone east of any exploration
on the 300-foot level has been plotted on the map. There is also shown its approx-
imate position on the 300-foot level, if it continues downward that far with the
observed dip. This fragment lies midway between the main portions developed in
the two mines, supporting the theory of the original identity of the veins.
FAULTS NOT CORRESPONDING TO THE MAIN SYSTEMS.
The northwesterly faults of the Wandering Boy 300-foot level are not so
closely related to the Wandering Boy fault as the northeasterly faults are to the
Fraction fault. Their trend is various, sometimes coinciding with that of the main
Wandering Boy fault, sometimes not. Their dip, as shown in fig. 48, is usually
steeply northeast, or in the opposite direction from that of the main fault, so that
while, like the main fault, they are normal, the downthrow is on the northeast
instead of on the southwest in accordance with the larger movement. Many of
them are, therefore, perhaps to be accurately regarded as independent minor faults.,
resulting from the combined stresses of the major displacements.
RELATIVE AGE OF FRACTION AND WANDERING BOY FAULTS.
The Fraction fault movement partakes essentially of the nature of thrust
faulting, and, as has been explained, seems to be due to the horizontal shove exerted
by the intrusion of the Brougher Mountain volcanic neck. The Wandering Boy
fault, on the other hand, is a normal fault, such as is ordinarily due to gravity; and
the fact that faulted blocks are downthrown on the south, in the direction of the
dacite volcanic centers, leads to the belief that the downthrow was a part of the
general downfaulting in the neighborhood of these volcanoes, which, as described
on page 47, probably took place subsequent to the last important outbursts as a
result of collapse due to the expulsion of a large bulk of material from the under-
lying region. According to this the Wandering Boy fault is slightly but distinctly
subsequent to the Fraction fault.
ORE IN WANDERING BOY VEINS.
Like most of the Fraction vein material, and much of the material in the Valley
View, most of the quartz in the Wandering Boy thus far developed is low grade, or
even practically barren. Good assays are obtainable, but even limited masses of
164 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
rich ore, like those which occurred in the Fraction, were not encountered to any
extent. Metallic minerals, other than a limited amount of iron, are not often noted
in the veins. Some ruby silver and argentite, like that in the Fraction, have been
reported, but were not seen by the writer.
VEINS OF GOLD HILL.
GOLD HILL A FAULT BLOCK.
The Gold Hill block is of especial interest, as being the only outcropping
block of earlier andesite besides the Mizpah Hill block. It is, roughly speaking,
a triangular area. It is bounded on the north and south by faults and on the
east by the intrusive dacite of Golden Mountain. The fact that the contact of
this dacite, as shown on the map, is nearly a straight line, suggests strongly the
idea that it has been determined by a preexisting fault. This idea is strengthened
by an inspection of the boundary just northeast of the Tonopah and California
shaft, where the intrusive dacite contracts to a narrow dike, which separates the
block in which the Tonopah and California shaft is situated from the Gold Hill
block. The former block has at its surface the white tuffs (Siebert tuffs) of the
lake beds, under which the Tonopah and California encountered the earlier
andesite. This block is therefore depressed with reference to the Gold Hill block,
and the dacite dike has been intruded along the fault plane.
NATURE OF GOLD HILL ANDKSITE.
The character of the andesite of Gold Hill has been the subject of critical
study. On the western extremity of the block at a point south of Mizpah Hill,
the andesite has the same peculiar appearance as at Mizpah Hill. Farther east,
toward the top of the hill, the andesite takes on a different appearance, being
darker and showing somewhat larger feldspar phenocrysts and frequent pheno-
crysts of altered but easily recognizable biotite. The latter kind of andesite
resembles in some ways the later andesite, and at one time aroused in the mind
of the writer the same doubt as to its affiliation that the andcsites of the Fraction,
West End, and MacNamara did. Critical study, however, established the following
points: That there is no real boundary between the typical Mizpah Hill variety
of andesite and the biotite-bearing andesite of the eastern part of the Gold Hill
block; that under the microscope the last-named phase showed many other char-
acteristics of the earlier andesite, while it was seen to contain, as ferromagnesian
phenocrysts, biotite to the practical exclusion of hornblende or pyroxene; and that
the Gold Hill andesite contained small but typical quartz veins like those of Mizpah
Hill. One of these Gold Hill veins has produced rich ore, although in limited
quantity. Moreover, while the Gold Hill shaft shows in its upper portion the
VEINS OF GOLD HILL. 165
peculiar characteristics of . the .surface andesite, in its lower portion it gradually
passes into fresher andesite, more like that of Mizpah Hill, and there is little
question that the two phases form parts of the same body. Therefore, it has been
concluded that there is here a phase of the earlier andesite which contains biotite
rather than hornblende, and which also has a somewhat coarser feldspar crystal-
lization. Similar phases can be found on Mizpah Hill, and even very close to
the Mizpah vein, and, as stated elsewhere, the rock can be matched in the Fraction
and neighboring shafts.
ALTERATION OF GOLD HILL ANDESITE.
The alteration of the Gold Hill andesite, as observed in surface specimens,
results in the formation of quartz, sericite and secondary orthoclase or adularia.
The plagioclase feldspars (oligoclase-albite) alter to orthoclase (adularia) and
sericite, or to sericite and quartz. The biotite is usually altered to muscovite .
and quartz. Occasional pseudomorphs of secondary minerals after hornblende
were detected, consisting chiefly of iron minerals (hematite, etc.). Numerous small
crystals of apatite occur. Practically the same characteristics are found in the
specimens from the Gold Hill shaft, with rather more pseudomorphs after horn-
blende and some chlorite as secondary mineral.
ENUMERATION OF THE GOLD HILL VEINS.
Gold Hill differs in an important manner economically from the Mizpah Hill
block, in its comparative poverty in mineralization. The veins are shown on
the map, but are narrow and weak. The most important outcropping vein may
be called the Good Enough vein, from the name of one of the claims. At one
point in the upper part of the Good Enough .shaft the vein has a thickness of
H feet, but diminishes farther down, and also laterally along the outcrop in both
directions, until it splits into diverging and unimportant stringers. This vein has
an east-northeast strike with a northerly dip at an angle of about 70. There
is a parallel vein 250 or 300 feet to the northwest, which dips in the opposite
direction, or to the southeast, at an angle of 70 or 80. This vein has a thickness
of 3 to 6 inches and is traceable across the hill. A number of other veins of the
same character are found, one of which runs southeastward from the Gold Hill
shaft, parallel to and just above the road. It strikes N. 60 W., has an average
thickness of <> inches, and is also evidently a weak vein. Veins of the same
character, nearly parallel to that last mentioned, occur on the .southwest side of
the road, in the same block, as shown on the map (PI. XVI). They are usually
several inches thick, but have not been traced far.
166
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
PRODUCTION OF GOOD ENOUGH VEIN.
The only place on the hill from which much ore has been obtained was from
one section of the surface portion of the Good Enough vein. According to the
Annual Report of the Director of the Mint on the Production of Precious Metals
for 1901, the vein had produced and shipped $15,000 worth of ore up to the
time of the publication of that re-
port. Not much further work has
been done on this ore body.
VEIN STRUCTURE.
The condition of the Good Enough
vein as seen in the chief working
shaft is shown in fig. 56. From the
standpoint of origin this vein is in-
teresting, as it shows plainly the
effect of fracture planes, in determin-
ing not only the walls, but in pro-
ducing a diminution in the size of
the vein and even a change of course.
There is no faulting in the section
shown in the figure, and the change
in size and dip of the vein is due
simply to the control of the original
mineralizing circulation first by one
and then by another set of fractures
(fig. 57). This is in accordance with
the observations made on the Miz-
pah Hill veins.
GOLD HILL SHAFT.
The Gold Hill shaft at the time
of the writer's visit was 490 feet
deep, in earlier andesite of an unusu-
fractures. ally fresh character for this district.
The workings consisted of crosscuts to the north and south at this level, of 30
feet each. There was another level at a depth of 300 feet, and a drift '20 feet
to the north and 50 feet to the south. The north drift at this level showed a
2-inch vein, running N. 80 W. and dipping north at an angle of 67.
Scale
10 20
30 feet
FIG. 56. Section of Good Enough shaft. Gold Hill. Lower out-
lines of shaft indicated by dotted lines. Shows eross section
of vein in early andesite, with minor cross walls. Also
shows the control of the size and direction of the vein by
dominating fractures, straight lines represent some of the
Surface
VEINS IN THE EARLIER ANDESITE. 167
TONOPAH AND CALIFORNIA WORKINGS.
SECTION EXPOSED IN WORKINGS.
The Tonopah and California shaft is situated several hundred feet southeast of
the Gold Hill shaft. It starts in the white stratified tuffs of the lake beds, which
here have a north-northeast strike and a westerly dip of about 20. According to
the report of the manager. 63 feet of these tuffs was passed through, and directly
beneath them was the earlier andesite. A short
distance south of the shaft the tuffs are thicker, as
a shaft has gone down 100 feet in them and has
not reached their lower limit.
Some quartz stringers were found in the ear-
lier andesite beneath the tuffs, at the depth of about
123 feet. At a depth of about 135 feet the shaft
enters a brecciated zone, which consists of softened
and broken earlier andesite and occasional bunches
of broken quartz. This continues down in the
shaft for about -40 feet. At a depth of 150 feet a
short drift runs southward in this broken zone.
The minor slips within this zone, have a north-south
strike and a dip of 30 to the east, and the bottom
of the zone has a similar strike and dip. Below
this there is hard earlier andesite. rather dark col-
ored, with occasional north-south slips and some
broken quartz stringers, evidently faulted. At a
depth of 450 feet a drift runs in a southeasterly direction for over 220 feet. There
is another level at a depth of H50 feet.
CALIFORNIA FAULT.
The broken zone described in the shaft is evidently a fault zone. Projected
on the same dip to the surface, this zone coincides with the outcrop of the fault
which separates Gold Hill from the block in which lies the top of the Tonopah
and California shaft. At the surface, however, this fault zone is occupied by a
dike of the Golden Mountain dacite, which is not present in the shaft. Evidently
the dacite is straighter than the fault or happens to be missing at this point.
According to this the shaft below the fault, that is to say, below 180 feet, is
in the Gold Hill block. Moreover, the east drift on the 450-foot level does not
run far enough to cut the fault, so that these workings are in the same block.
FIG. 57. Cross section of Good Enough vein.
Gold Hill, as exposed in opening just west
of shaft, showing same characteristics as in
fig. 56. Vertical lines in andesite are joints.
1H8 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
VEINS.
Some of the broken fragments in the fault zone show a small quantity of
material that is probably black silver .sulphide.
On the 450-foot level a small quartz vein, a few inches thick, with an east-
southeast strike, and a northerly dip (45 to 60), was followed. This has a gangue
of quartz, with some calcite, and contains pyrite. In some places good values
are shown. On the 650-foot level, a short distance south of the shaft, in very
dense, and tine-grained earlier 1 andesite, a ledge of 3 feet of mixed quartz and
altered andesite has been cut. This quartz contains argentite and shows some
good values.
MONTANA TONOPAH VEIN SYSTEM.
MONTANA TONAPAH MINE. '
ABSENCE OF VEINN IX THE LATER ANDESITE.
The Montana Tonopah shaft was sunk in the later andesite, on the northeast
or upper side of the Mizpah fault (PL XVI). It passed through 372 feet of the
later andesite before reaching the fault. Most of this rock was extraordinarily
decomposed and thoroughly bleached, while much was intensely brecciated, con-
taining hard bowlders in a clayey matrix, with strong fractures and slickensided
surfaces. This indicates a great deal of faulting, of which no measure could be
obtained.
Above the Mizpah fault only small veinlets of calcite and quartz were encoun-
tered, but 4 feet below the fault a heavy quartz vein in the earlier andesite was
encountered and followed in the shaft to a depth of 392 feet, where the first
mine level was made. The other main levels are at 460, 512, 612, and 765 feet.
The Mizpah fault was cut in a northeast drift on the 392-foot level, as shown
in fig. 58, at a point about 60 feet from the shaft; it was also encountered in
the 512-foot level, as shown in PL XXII. Its strike and dip are therefore fairly
well determined; the strike is about N. 55 W., and the dip is northeast, at an
angle of about 29. The later andesite has been found on the northeast or upper
side of this fault, at all depths thus far examined, both in this mine and in
neighboring OUCH.
This rock (the later andesite) has been extensively explored, both in this
mine (as in the drift on the 512-foot level connecting the Montana Tonopah and
North Star shafts) and in others, but no veins of size and value have been found,
nor anything that does not confirm the theory that the principal veins are older
than the later andesite.
MONTANA TONOPAH MINE.
169
VEIN ON THE 392-FOOT LEVEL.
The nature ana relations of the Montana Tonopah veins are best seen from
figures. Fig. 58 shows the upper or 392-foot level, and the plan of the vein
first encountered in the shaft at that level. The vein is about 3 feet thick, of
the normal Tonopah type, such as has resulted from a silieification and minerali-
zation of the rock along a zone of ^___
close-set fractures; the values in it are
moderate. It is sharply cut off on the
east by the Mizpah fault. Near the
shaft it is cut by a number of small
northeast faults, generally steep and
dipping in both directions. These faults
nearly always have brought about an
upthrow on the northwest side, so that
in horizontal plan the vein is offset to the
southwest on the southeast side. These
faults are both normal and reversed
(fig. 59). The vein dips northwest at
an average angle of 45- or 50.
This level, continued as a cross-
cut about 150 feet to the northeast,
cuts another vein, supposed to be the
Macdonald vein of the lower levels.
This vein strikes northeast and dips northwest at an angle of 40; it is from '2
to 4 feet thick and contains some good ore. Two portions of it, separated by
a northeasterly striking, southeast dipping (60) fault, are successively cut in the
drift. On account of this faulting the vein has not been much explored.
Shaft BRANCH VEIN ON THE 460-FOOT LEVEL.
1
At 440 feet the shaft cuts a
minor vein below the one iust
described (fig. 60). This vein is
about 4 inches thick at the shaft
and was followed a short distance
northeast along its strike. At a distance of about 25 feet it was represented only by
stringers 2 inches or less thick, and was not farther drifted upon. To the southwest
of the shaft, what is probably the same vein was followed a longer distance,
becoming stronger and being from 8 to 18 inches thick. The ore in this part of
the vein is often of high grade, consisting of black and white quartz, crustified or
FIG. 58. Horizontal plan of faults and vein on the 392-foot
level of the Montana Tonopah.
zo feet
Flo. 69. Vertical section along north drift. 392-foot level, Montana
Tonopah.
170
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
irregularly mingled. The black quartz owes its color to a large amount of
included black silver sulphide and other sulphides.
In this drift southwest of the shaft the vein dips to the northwest at an
angle of about 60.
CONNECTION OF BRANCH VEIN WITH MONTANA VEIN.
This vein was followed southwest on the strike and downward on the dip
to its junction with a larger and more important vein the Montana. At a
point a little over 40 feet southwest of the shaft, an incline on the vein went
down 38 feet to the Montana vein, while the same junction along the strike was
effected at a point over 100 feet southwest of the shaft. The Montana vein
o PO 20 30 40 so feet
Pie. 60. Horizontal plan snowing veins and faults on the 100-foot level of the Montana Tonopali.
strikes at this point generally east and west, and dips north at an angle of 45^ or
55, the dip being somewhat less than that of the smaller vein. The junction of
the two veins therefore pitches to the northeast at a comparatively low angle.
HHKCC1ATED STRt'CTUKE IN THE MONTANA VEIN.
The Montana vein as developed in this level was very strong. It was from 6
to 8 feet thick, being rather thicker than the average vein. It showed white
quartz with dark-colored portions and had often a brecciated structure. The
dark quartz, which contains a much larger amount of black silver sulphides than
the light-colored quartz, proves on assay to contain three times or more the
value of the white.
MONTANA TONOPAH MINE.
171
Examination of the breccia shows that frequently the black quartz occurs as
angular fragments cemented by the white, while in other places, perhaps in the
same exposure of the vein, fragments of the white quartz are cemented by the
darker and richer ore. The whole is a solid, substantial vein, both dark and
white quartz having every mark of primary origin. The only trace of movement
is in the brecciation of the dark and white quartz, as above described.
PIG. 61. Figure drawn from sketch, showing face of ore of the Montana vein on the west drift, 460-foot level,
Montana Tonopah mine. To illustrate fissure with crustified high-grade ores, subsequent to the formation of the
ordinary veins, but within the period of primary ore deposition; a, altered andesite, wall rock; b, main Montana
vein of ordinary type; c, subsequent fissure filling. Within this the dark streaks are rich black sulphide layers,
with quartz and carbonates between. In the central quartz band are druses lined with adularia crystals.
CBD8TIPICATION IN THE MONTANA VEIN.
Another allied peculiarity of this vein, as compared with other veins of the,
district, is that portions are regularly banded or crustified.
Such crustification is not characteristic of the whole vein, for the crustified
portion occurs surrounded by solid quartz possessing no banded or comb structure
172 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
and having all the characteristics of the typical quartz vein of the district. The
trustified vein also is not regular nor persistent, and seems to have filled uneven
clefts or openings in the main vein, which itself has every appearance of having
been formed by silicification along fracture zones in the way previously outlined
for the outcropping veins of Mizpah Hill (fig. 61).
CONDITIONS OF KOKMATION OF MONTANA VEIN.
The gangue and the metallic contents of the crustified vein are, however, of
exactly the same kind as those of the ordinary inclosing vein. There is no
reason to doubt that both portions of the vein are primary, like the different
depositions noted in the breccia ore. The phenomena indicate that, in this portion
of the vein at least, rock movement went on subsequent to the first ore depo-
sition and to the first cementation of the fractures by quartz, producing in places
a breccia, which was cemented with similar materials by vigorously circulating
mineralizing waters, and even forming irregular open spaces, in which the ores
and gangue materials were deposited in successive layers. It seems that the
movement continued even after the beginning of the deposition of some of these
crustified masses, for some of the breccia ores show fragments of very light and
of very black quartz, such as are characteristic of the crustified veins and not of
the ordinary type, intimately associated. The later part of the mineralization
thus indicated may have occurred at a period when the solutions were richer in the
metallic minerals than previously, for this portion of the vein is characterized
bv extremely rich ore, and some of the faces exposed in breaking down the vein
showed great masses of the black sulphides, constituting ore of a richness that
is rarely seen in such quantity.
KATLT8 ON THE 460-FOOT LEVEL.
As shown on fig. 60, the northeast branch vein is interrupted by a number of
minor slips or faults. On the east the Montana vein is sharply cut by northeast
faults having a southeast dip of about 35, and its eastward continuation has not
been found. The smaller faults of this series show that the result as seen in
horizontal section is an offset to the south on the east side. Such an effect might be
due to a variety of displacements; in this case the strong striae, pitching east at an
angle of 30 on the fault planes, show a diagonal downthrow on the east. According
to this, the continuation of the Montana vein should be offset to the south from the
present course.
The relative positions of the Montana and Mizpah veins at this level are shown
in fig. 62.
MONTANA TONOPAH MINE.
VEINS OX THK 512-FOOT LEVEL.
178
The Montana vein ha.s been followed from the 460-foot level up to the fault and
has been traced downward to the 512-foot level. The .situation on this level is shown
on PI. XXII. The vein marked on this diagram "Montana vein" has been shown,
by tracing the actual connection, to be the same as the vein on the 460-foot level.
On the northeast it grows less strong and definite on reaching the main north drift.
In the east drift a cross wall, striking nearly parallel with the vein, but dipping
460-ft level^
3ft.above400-ft.Mizpah/ '
N
Tonopah Mining Co-
main shaft
Mizpah 400-ft level
Scale
100 200
300 feet
FIG. 82. Horizontal plan showing relations of the Mizpah and Montana veins on the 400-foot level of the M i/pah.
in the opposite direction (to the south) constitutes the lower limit of this ore shoot.
Above this cross wall the ore has been continuously stoped out. Below it the walls
continue, and a good deal of quartz is present, but no rich ore has as yet been found
below this point on the vein (tig. 63).
A long north drift from the shaft, on the ol^-foot level, discloses two veins
parallel to the Montana. These are shown in PI. XXII. The one nearer the shaft
shows in the drift a '2-foot zone of quartz stringers with altered andesite between.
174
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
This zone contains some silver sulphides and some good ore, although it is largely
of low grade. The one lying farthest north has been called the Macdonald vein.
The Macdonald vein is a strong, rich vein having a strike a little north of
east, and a northerly dip varying from 45 to 65. It has been extensively
drifted on, on this level, and has produced a great deal of high-grade sulphide
s ore, of the same character as the high-class
ores of the Montana vein. It has been fol-
lowed down to the 615-foot level.
On both these levels and on the interven-
ing stopes this vein shows a complex fault-
ing, reminding one of the faulting that has
affected the Fraction vein. In a vertical
section such faults appear nearly parallel to
the vein, but curve and continually branch
and so become now steeper, now flatter in
FIG. 63. Vertical sketched cross section of cross wall
limiting chief ore shoot of Montana vein below, as
displayed on the 512-foot level of the Montana Tono-
pah. a, Rich sulphide ore, sloped out; 6, silicifled
andesite, some quartz and ore, no rich ore; c, Earlier
andesite, wall rock.
dip than the veins (tigs. 64, 65). If
straight this faulting would be like
that which has affected the vein of
the North Star, but the undulations
of the faults here in the Montana
Tonopah produce, in vertical section,
displacements of the vein to the north
on the under side of the faults. The
FIG. &J. Vertical cross section (sketched), showing effect of
line of faulting is not parallel in strike curving and branching faults on Mncdonald vein, in slopes
above the 615-foot level on the Montana Tonopnh.
or dip to the vein, though it sometimes
so appears in vertical section; in fact, the flat portions of the fault pianos pitch east
on the vein at moderate angles; and striiv along the faults show that the real
direction of movement has been to the east along this pitch. In horizontal sec-
tion, however, these faults are seen to curve and branch in as complicated a
manner as in the vertical section, producing an unrivaled complexity (PI. XXII).
zo feet
MONTANA TONOPAH MINE.
EASTERLY PITCH OF ORE BODIES.
175
There appears to be an easterly pitch to the chief ore shoot on the Montana
vein, as this has been developed in following down the vein from the 460- to the
51'2-foot level. Some of the richest ore in the 460-foot level lies vertically over
a relatively poor part of the 512-foot level, the rich ore in the latter level lying-
farther east.
On the Macdonald vein the ore shoots pitch to the east.
Ill I \ * / ' >^ I */ I / _ X ^ f > ' * / / V \'r.
^Ssy;^^^v^x^^^
y^";; ; /v\'/:V^V VA'.y,;/^;;,-'/^' ;-C;0-;o^ ,X\\>, N/.^/^v
;;v-v/;v^y> ^.^''V/'-'^Av',;';'^;^;^;/,^:^/^/^'/
-''>-. '-;if; 5 /vV/V'Vv:^/:^:^';/^'^/.;^;^;';^/-;^
:;,<',',^,f\/ /x>\vs>",y^ ^;,;',s;^>;'S,;\',;, ',*:^
>;. 1 ",V';y^ V^^^'V^,v;';vvXv:/:::vy/>v>^;''^:
'\,'c,',';:^- -<_/" / ,\ / >V^Vr\''/s v > / ^\\'^ / v^'/oVy'^X'>/ % V ''
Scale
ao 30
50 feet
Fiii. 6S. Vertical cross M^ction (sketched), showing effect of curving and branching faults ou Macdonald vein, in stoi>v
above the 615-foot level on the Montana Tonopah.
TONOPAH RHYOLITE-DACITE IN THE MONTANA TONOPAH.
At a depth of 560 feet the Montana Tonopah shaft passed downward from
the ordinarv earlier andesite which contains the veins to a dense rock, which
proves to be the glassy Tonopah rhyolite-dacite.
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
The same rock was encountered at the bottom of a winze, 2<> feet below the
west drift on the ;"> 12-foot level, where it seem* to cut off the Montana vein. It
Montana
-YAx v / - -> '/rAffi^^^\^^^. f j;-,V'/
v-# ' - ^^^8i^SS
- isl^iliM
300 feet
Kio. 66. t'ross wction showing guolofry exposed liy Montaim Tonopuh workings.
is also found on the 615- and 765-foot levels (fig. 66). As is usual in thi.s formation
irregular and bunchy quart/ veins are encountered, which sometimes yield good
assays, especially in gold; but no pay ore has yet been found.
MONTANA TONOPAH VEIN SYSTEM.
177
NORTH STAR WORKINGS.
SECTION PASSED THROUGH.
The North Star shaft was started in white rhyolite on the slope of Mount
Oddie (fig. 67). Below the rhyolite comes the later andesite, the contact being
practically horizontal and indicating the later age of the rhyolite. From this
contact down to a depth of about 720 feet the shaft is in the later andesite,
largely soft and decomposed. It is sometimes brecciated, indicating considerable
movement, and in places contains much secondary pyrite. At depths of about
KIG. 07. Section on plane of Desert Queen and North Star shafts.
720 to 740 feet the shaft cuts the zone of the Mizpah fault, which is characterized
by 20 feet or more of clay, formed by trituration and decomposition along the
fault. Beneath the fault the earlier andesite comes in.
Just above the bottom of the shaft, which is 1,050 feet deep, the Tonopah
rhyolite-dacite comes in. It is the same sheet which is encountered on the 814-
foot level of the Desert Queen.
The developments in the North Star consist of two drifts, at 950 and 1,050 feet.
The lower one of these levels is the more extensive, having a drift to the north
of over 500 feet. On the 950-foot level and on the north drift of the 1,050-foot
16843 No. 4205 12
178 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
level the rock is andesite, probably earlier andesite, largely altered to chlorite
and calcite, like that below the Siebert fault in the Mizpah shaft. The station at
the 1,050-foot level and a drift running southeastward from the shaft for over 200
feet are mainly in the Tonopah rhyolite-dacite. This rock is much silicified and
is in places cherty quartz. At the shaft and on the walls of the drift in this
formation there has formed, since the opening of the mine, a green coating. This
was determined by Dr. W. T. Schaller, of, the United States Geological Survey,
to be a basic copper sulphate, insoluble in water. The cherty quartz on which
this incrustation forms contains only traces of gold and silver. Since the mine is
perfectly dry the formation of this copper sulphate on the walls is interesting.
A similar incrustation forms on the quartz of the rhyolitic "veins on the 84rO-foot
level of the Desert Queen. It seems, so far as observed, to be a phenomenon
peculiar to the quartz of the Tonopah rhyolite-dacite and to have no connection
with gold and silver values.
On the.- 1,050- foot level in the earlier andesite a phenomenon was noted which
was not observed elsewhere 1 in the camp. This is the intrusion of one body of .
earlier andesite by another body of the same rock. The intrusive rock is tiner
grained than the rock which it cut, and near the margin showed flow structure.
The coarser intruded rock is of the biotite-bearing variety, while the intrusive
rock is of .v.ery similar composition and is very typical earlier andesite. This
occurrence is analogous to the finding in the Tonopah City shaft of dikes of
Heller dacite intrusive into a body of the same rock, and signifies successive injec-
tions of the earlier andesite, which may very well be of slightly different types
as regards composition.
VEINS.
On the 950 foot level, north of the shaft, a vein of quartz several feet thick
was cut in the earlier andesite. This has a general west-northwest strike and
a northerly dip of 45 or 50. This vein was cut also in the 1,050-foot level
and is developed by an incline between the two levels. Some ore has been shipped
from it, having the same characteristics as the ore of the Montana Tonopah; it
contains polybasite, ruby silver, etc., in a white quartz gangue. This is very
likely the same as the Montana vein of the Montana Tonopah.
FAULTING.
It has not been possible to follow this vein very far along the strike or dip
in any one place on account of faulting, which follows the vein very nearly in
strike and dip but curves and becomes oblique to it. On the 950-foot level this
fault nan a strike whicli is more northerly thitn that of the vein, and so has cut
out most of the vein, leaving only a wedge. On this level the fault is below the
vein, bufc in following the incline down to the level below, it is found to puss
through the vein and to go into the hanging wall, us shown in fig. 67.
MONTANA TONOPAH VEIN SYSTEM. 179
MIDWAY WORKINGS.
The Midway lies a short distance northwest of thte Siebert shaft, and
almost in line with and halfway between the Montana Tonopah and the Tonopah
Extension.
LATER ANDESITE IV SHAFT.
The surface at this point is composed of tlie typical later andesite. A
specimen taken a short distance from the Midway shaft has the characteristic
relatively fresh appearance, dark color, and large feldspars of this rock. Under
the microscope it is also typical, showing numerous phenocrysts crowded
together, these phenocrysts being mainly feldspars, often large and compound,
with pseudomorphs of serpentine after pyroxene.
The contact of this rock with the underlying earlier andesite is an obscure
one. This is a condition similar to that noted in other workings, such as the
West End and MacNamara, where, as described, the contact between the two
andesites could not be located in the shafts.
TYPICAL EARLIER ANDE8ITE IN SHAFT.
In the case of the Midway, as shown in the section (fig. 68), the contact
has been perhaps rather arbitrarily drawn at a depth of about 425 feet. From
this point to a point just below 475 feet in the shaft the formation is regarded as
probably all typical earlier andesite.
GLASSY TONOPAH RHYOLITE-DACITE IN SHAFT.
At a point in the shaft below 475 feet there is a change in the formation,
and the rock is quite uniform and of the same hard, siliceous nature and light-
green color as that at the main level of the Ohio Tonopah.
This rock contains jaspery quartz veinlets and fine quartz lines some of the
cavities left by the removal of pyrite and other crystals.
FORMATIONS EXPOSED BY DRIFTING.
The workings of the Midway consist of two levels at depths of 535 and 685
feet, the former having a north drift over 400 feet long and a south drift about
150 feet long, while the latter has a north drift nearly 700 feet long and a south
drift of about 150 feet. The formation in the upper level is entirely Tonopah
rhyolite-dacite, except at the end of the north drift, which passes through the
same contact as that encountered in the shaft and enters the earlier andesite.
The shaft passes downward through the body of rhyolite-dacite and enters earlier
andesite beneath it, of a type like that found on the 700-foot level of the Siebert
shaft. Similar andesite is encountered on the south drift of the (>35-foot level,
while the whole of the north drift on this level lies in the rhvolite-dacite.
180 GEOLOGY OF TONOPAH MINING DI8TBICT, NEVADA.
VEINS IN THE MIDWAY.
There are veinlets of calcite in the later andesite and these very often contain
pyrite. In the shaft at a depth of about -430 feet there are quartz stringers
containing pyrite. At a depth of 435 feet there is a short northwest drift, showing
a vein of black, jaspery quartz, which is barren and irregular.
A fragment of a vein was cut at 475 feet in the shaft. The vein was largely
barren but contained a rich bunch or shoot of original sulphide ore. This ore.
Flo. fi._ Section showing geology exposed by Midway wiirkint-s.
when examined microscopically, shows the typical structure of the productive
earlier andesite veins. The quartz has the usual varied grain, ranging from
inicrocrystalline to medium crystalline. There is scattered pyrite seeming to have
no relation to the values, which consist of black silver sulphide and silver chloride,
both of which are relatively abundant. The relation between these two is remark-
able, for the black sulphide forms rims around the chloride and in some cases is
found along cracks, showing that it was formed later than the chloride and is
very probably an alteration product of it. There is occasionally a little ruby silver,
TONOPAH EXTENSION MINE. 181
having the same relation to the chloride as does the sulphide. This black sulphide
may be either argentite or stephanite.
This quartz vein is much broken, so that the general strike and dip could not
l)e determined. It may very well be the extension of one of the veins developed
in the Montana Tonopah."
As usual in other parts of the camp, the Tonopah rhyolite-dacite in the
Midway contains a number of quartz veins which, however, are irregular, non-
persistent, and faulted, and are usually barren. The most important vein of this
class was encountered on the 535-foot level, a short distance south of the shaft.
This shows several feet of quartz, striking in a west-northwest direction and having
a steep dip. On the southeast this vein becomes irregular and passes into barren,
cherty quartz, which in turn disappears, turning to silicitied rhyolite-dacite.
Most of the vein is barren, but at one point 400 tons of ore, having a value of
$30 to the ton, were taken out and milled. A winze follows this vein to the lower
contact of the rhyolite-dacite with the earlier andesite, a short distance above the
635-foot level. The silicitication and the vein, however, cease at the rhyolite-
dacite contact and do not enter the earlier andesite, into which the rhyolite-dacite
is intrusive.
A second vein, having an east-west course, was encountered about 50 feet north
of the shaft on the 535-foot level. It dips about 65 to the south and is termi-
nated on the east by a fault, so far as explored. It is about 2- feet in thickness
and contains a little good ore, of which a few tons have been shipped; the rest
of the vein is barren. About 150 feet north of this last-named vein, on the same
level, there is smother 2-foot vein of white barren quartz, which has a west-south-
west strike and a" northerly dip of 80. This contains no ore, but only white,
barren quartz, although assays of from $20 to $30 can be had.
That these veins are nonpersistent is shown not only by the developments
upon this level, but by the fact that they are not found in the same formation
on the 635-foot level 100 feet below. Although this level runs through 650 feet
of rhyolite-dacite it encounters no strong and definite veins.
TONOPAH EXTENSION MINE.
CONTACT OF EARLIER AND LATER ANDESITE8.
The Tonopah Extension shaft starts in the later andesite and extends down about
183 feet to the contact of the earlier andesite (see fig. 71). This contact is marked
by 1 to 2 feet of soft, decomposed rock, and is very flat. Below it the earlier
andesite is ^ ory full of quartz veinlets. This phase of the earlier andesite resem-
bles in many places some of the phases of the later andesite, although just below
aSince the writer's visit more ore hu.s been fouud'in the Midway, in a drift at about this level.
182
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
the contact above referred to it is fairly typical. The contact is probably not
due to faulting, but is normal and indicates that the veins in the earlier andesite
outcropped at the surface at the time of the later andesite extrusion.
VEINS IN THE EARLIER ANDESITE.
At a depth of 230 feet, in the earlier andesite, a heavy vein was cut near the
shaft. This has been developed by levels at depths of 244 and 385 feet, and by an
incline between the levels. The general strike of the vein is west-northwest and
the dip north from 30 to 45. The vein is from 3 to 8 feet thick and shows shoots
of high-grade sulphide ore like that of the Montana Tonopah. So far as had been
developed at the time of the writer's visit, in November. 1904, the vein has not
been faulted.
tofeet
Fio. 69. Diagrammatic vertical cross section of Tonopah Extension vein. a. Altered earlier andesite, wall rock; 6,
typical white vein of earlier andesite period, containing black silver sulphides, with values of several hundred dollars
per ton; c, black, jaspery quartz of later introduction than original vein, of which it contains fragments. Values of
black quartz and fragments, 820 to $30 per ton.
An interesting phenomenon is displayed by the Tonopah Extension vein. Where-
ever it has been followed, a portion of the vein, generally that lying next to the
hanging wall, is of different character from the rest. The main body is composed
of white quartz containing black silver sulphides, and has exactly the same char-
acter as the other earlier andesite veins in the camp. The upper portion, however,
is of black or gray jaspery quartz, like so many of the veins in the Tonopah
rhyolite-dacite. Moreover, this portion contains angular fragments of the ordinary
quartz vein in such a way as to show conclusively that the jaspery quartz was of
later introduction than the main vein. Evidently renewed pressure reopened the
vein subsequent to the first ore deposition, and caused a new fracture or fissure,
TONOPAH EXTENSION MINE.
183
following in general the old hanging wall. Along this opening waters have cir-
culated and deposited jaspery quartz, cementing the broken fragments of the old
vein. On the 244-foot level the thickness of the jaspery subsequent quartz is about
li feet, while that of the typical antecedent quartz is about 3 feet. At the place
where the sketch (tig. 69) was made, the lower part has a value of about $600, while
the jaspery quartz has values of from $30 to $35. Moreover, it is probable that
these last-named values are in large part derived from included fragments of the
true vein, and also from the ruby silver which is sometimes found in cracks in the
jaspery quartz as well as in the true vein, this ruby silver being a secondary, mineral
derived from the primary ore.
The general character of this subsequent vein filling renders it highly probable
that this vein is of the same nature and period as the veins in the Tonopah
rhyolite-dacite. While the main vein was formed after the eruption of the earlier
O Midway shaft
North Star shaft
5 t?
j<* North Star vein [projected)
Wl 13 ' * Montana Tonopah shaft
^TONOPAH
Extension shall
Mac Namara shaft
TONOPAH MINING CO
Main shaft
Mizpah vein
J *
V
Belmont vein
West End shaft
Valley View shaft
Scale
500
Fni. 70. Map showing principal earlier andesite veins now developed undergound, within the main productive area:
shown on the horizontal plane of the Mizpah 500-foot level.
andesite, the subsequent tilling took place after the eruption of the rhyolite-dacite.
This main vein in the Tonopah Extension is probably identical with one of the
veins in the Mizpah or the Montana Tonopah. Very possibly it is the same as the
Montana vein, but this can not be definitely proved as yet.
VEINS IN THE TONOPAH RHYOLITE-DACITE.
The above conclusions as to subsequent filling are strengthened by certain
other occurrences in this same mine. On the 385-foot level a south drift from
the shaft cut the upper contact of a flat- lying north -dipping body of Tonopah
rhyolite-dacite. In this last-named rock, near the contact with the earlier andesite,
there is a great deal of silicification, amounting often to the formation of bodies
of pure jaspery quartz, of very irregular size and extent, and practically barren
184 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
for the most part. The main shaft passes through this contact between the
385-foot level and the bottom, which is at a depth of 485 feet, and from the
bottom a north drift runs out about 100 feet to the contact again. The heavy
silicitication resulting in the formation of jaspery barren quartz, especially near
the contact, is shown also on this level.
This contact was followed upward from the 385-foot level by means of an
incline for some distance, and showed more or less of the same rhyolitic quartz.
The dip of this silicified contact is less than that of the Tonopah Extension vein
in the earlier andesite, so that very likely these may come together at a greater
depth, in which case the barren jaspery portion of the Tonopah Extension vein
will unite with the similar quartz in the rhyolite -dacite, with which it has
undoubtedly a common origin. In this eventuality, however, the productive
portion of the Tonopah Extension vein may be cut off.
The relative position of the Tonopah Extension vein in regard to that of
other known veins of similar character is shown in fig. 70.
OTHER EXPLORATORY WORKINGS WHOLLY OR PARTLY IN EARLIER
ANDESITE.
WEST END WORKINGS.
OUTCROP OF WEST END FAULT.
As the map (PI. XVI) shows, the West End shaft is near the contact of the
Fraction dacite breccia on the southwest and the later andesite on the northeast.
This contact follows a straight line, and was judged, from a study of the surface
only, to be due to faulting. By projecting the known outcrops of the Gold Hill,
and the Wandering Boy faults it is seen that they would normally come together
in the vicinity of the West End shaft. Here they probably unite to form a
fault which is a direct continuation of the Gold Hill fault, and which is thought
to have been recognized farther on, in the line separating the later andesite
from the Fraction dacite breccia, in the vicinity of the MacNamara shaft. This
united fault may be called the West End fault. In general this fault appears to be
downthrown on the southwest, for the Fraction dacite breccia on this side is
younger than the later andesite on the northeast. Moreover, both the Gold
Hill and the Wandering Boy faults are downthrown on the southwest side.
RHYOLITE INTRUSION ALONG FAULT.
Near the West End shaft are seen rugged outcrops of dark-weathering
rhyolite, which belong to a dike or neck of rhyolite that has ascended along the
fault plane. Where encountered in the mine workings this rhyolite is white,
and of the same type as the rhyolite of Mount Oddie, and is probably of the same
age and origin.
WEST END WORKINGS. 185
The West End shaft when last visited by the writer was 780 feet deep.
The soft Fraction dacite, which forms the block on the southwest side of the
fault, is first encountered in the shaft, but at a depth of about 20 feet the Oddie
rhyolite comes in. The contact of dacite and rhyolite strikes N. 35 to 55 e W.,
or roughly parallel with the West End fault, and the dip is southwest at an
angle of aboijt 65, suggesting that the fault also dips in this direction and is
therefore normal. The contact is partly tight and partly separated by several
feet of breccia, containing fragments of rhyolite and of later andesite, with the
soft materials of the more fragile dacite. The rhyolite contact conies in on
the north side of the shaft and continues straight down to a depth of about
62 feet, where it passes out on the south side. The general dip of the rhyolite
dike is therefore to the south. At one or two places the rhyolite is evidently
intrusive into the dacite. The shaft passes downward through the upper contact
of the rhyolite with the breccia and traverses solid rhyolite for a short distance,
showing that here the thickness of the dike or neck is about 20 feet. On the
under contact of the rhyolite, at a depth of 84 feet, is green altered andesite,
which has been referred to the later andesite. At this contact also there is a
slight breccia.
The above phenomena are interpreted as indicating that the rhyolite ascended
along a fault plane, which in the upper part of the shaft separates the Fraction
dacite from the later andesite. The intrusion of this rhyolite caused some brec-
ciation of the rigid intruded rocks near the contact, and it is possible that some
subsequent slipping along the fault may have slightly brecciated the rhyolite itself.
As a rule, however, it has been ascertained that rhyolite of this sort is younger
than the faults and is little or not at all affected by them.
At a depth of 116 feet there is a zone of great movement and probable
faulting, in which the chief slips strike N. 10 W. and dip west at an angle of 25.
This suggests a northwesterly faulting.
CHARACTER OF ANDESITE ABOVE 220-FOOT LEVEL.
Below the lower rhyolite contact, at a depth of 84 feet, the shaft is in
andesite for some distance. All this andesite is extremely decomposed in conse-
quence of the proximity of faulting, and is therefore difficult to study. Below a
distance of perhaps 100 feet from the surface the character of the andesite has
occasioned much perplexity in the mind of the writer. The earlier and the later
andesites are so closely related that many times they have almost identical char-
acteristics, and it is difficult or impossible to discriminate them in the hand
specimen or under the microscope. A specimen taken in the shaft, at a depth of
116 feet, was judged to have the characteristics of the later andesite rather than
of the earlier andesite. Another specimen taken in the shaft, at a depth of 19t>
180 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
feet, was supposed to represent the same rock, for no sharp division had been
noted, but was judged, after microscopic study, to have rather the characteristics
of the earlier andesite. This specimen was altered to quartz, sericite. and pyrite.
CHARACTER OF ANDESITE ON 220-FOOT LEVEL.
At a depth of 220 feet from the surface, drifts were run 338 feet to the north
of the shaft and 285 feet to "the south. In both these drifts only andesite was
encountered and no general distinction was noted between the andesite in the
different parts of the drifts. In both drifts the rock strongly resembles certain
phases of the earlier andesite; in the south drift perhaps more than in the
north. This resemblance also holds good on microscopic study. Some sections
of the rock in the north drift showed occasional original phenocrysts of quartz,
such as are occasionally found in the earlier andesite. This original quartz was
found also in the specimen obtained in the shaft at a depth of 196 feet. On both
these drifts there was evidence of considerable movement, the general strike of
the slip or fracture planes being north and south and the dip west rather steeply.
The andesite when examined microscopically was found to be highly altered, the
chief alteration products being quartz, calcite, chlorite, serpentine, pyrite. siderite.
kaolin, and adularia.
COKKELATION OF ANDESITES IN WEST END AND FRACTION WORKINGS.
After studying the delicate question as to whether this rock is the earlier or
the later andesite the writer has satisfied himself that the andesite of the south
drift in the West End is identical with that shown in the long north drift from
the -100-foot level of the Fraction No. 2 shaft. The faces of the two drifts are
only about 250 feet apart in a straight line, but there may be, and very likely is.
intervening faulting. The writer was not able to distinguish between the general
type of the andesite in this north drift of the Fraction and the typical Fraction
andesite, which is often relatively dark and chloritic. In the Fraction No. 1
workings the andesite contains a large vein, carrj'ing in places at least good values.
EXTENSION OF CORRELATION TO THE WANDERING BOY AND GOLD HILL.
It seems to the writer, moreover, that the andesite in the Fraction No. 1 i>
identical with that in the Wandering Boy, which is more nearly the Mizpah Hill
type of earlier andesite. On following the chain still farther, the andesite in the
Fraction and that in the Wandering Boy seem to be identical and are probably
in the same fault block as the Gold Hill andesite. The rock of Gold Hill has
certain peculiarities which at one time caused the writer to study for some time
the question carefully as to whether or not it belonged to the earlier or later
WEST END ANDESITES. 187
andesite, thus bringing up again the question of the exact age, which has just
been raised with respect to what is probably the corresponding rock in the
West End. It was found, however, that the peculiarities which suggested the
correlation of the Gold Hill andesite with the later andesite, namely, the fre-
quently large-sized feldspars and the presence of biotite, could be paralleled in
specimens found in Mizpah Hill, even in the workings of the Mizpah mine, and
again in the Montana Tonopah, where there was no question as to the andesite
being other than the earlier andesite.
Moreover, in Gold Hill this andesite incloses veins having all the character-
istics of the veins found in Mizpah Hill, such as have not been found in the
undoubted later andesite. Therefore the evidence decidedly favors the conclusion
that the Gold Hill rock is the earlier andesite. If it is true, as has been con-
cluded, that the veins of the Wandering Boy and the Fraction were originally a
part of the Valley View system and that they were displaced by faulting, the
evidence grows still stronger.
THE WEST END ANDESITE PROBABLY EARLIER ANDESITE.
l. (
The writer is forced to the conclusion that the andesite exposed on the 200-
foot level of the West End belongs to the earlier andesite.
CONTACT BETWEEN EARLIER AND LATER ANDESITES.
PLACE AND CHARACTER OF CONTACT.
The conclusion that the rock on the 220-foot level is the earlier andesite
having been reached, the question comes up as to the line of demarcation between
the earlier andesite below and the later andesite above. Since the West End
fault probably dips southwestward and is normal, the shaft, after passing through
the fault and leaving the rhyolite, is in the block lying northeast of the fault,
which may be called the Midway block. This block is characterized at the sui'-
face everywhere by undoubted later andesite. It is, then, likely that the contact
between the later andesite and the earlier andesite occurs in the West End shaft
somewhere above 196 feet, and from considerations given it may be assumed,
temporarily at least, that it lies between 116 and 196 feet (see p. 185).
This assumption is rendered somewhat doubtful by the fact that no contact
was observed, but, on the other hand, the rock is thoroughly decomposed and
much disturbed by faulting, so that the presence of a contact would be obscured.
NATURE OF SIMILAR CONTACTS ELSEWHERE.
At another point where the writer has seen the contact between the over-
lying later andesite and underlying earlier andesite, in the same fault block, at
the Tonopah Extension, the contact is by no means striking, and could not be
188 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
distinguished if the rock was much decomposed or faulted. In the Tonopah
Extension this contact is at a depth of about 18-t feet from the surface and is
nearly flat.
Similarly in the Midway mine, which is very likely in the same block, the
contact between the overlying later andesite and the underlying earlier andesite
could not be definitely located, probably on account of the great decomposition
of the rocks at this place.
The earlier andesite in the Tonopah Extension, moreover, partakes very
largely of the characteristics of the Fraction andesite, and in many cases resem-
bles somewhat the later andesite, but is elsewhere quite typical, and contains
strong veins, which show in places high values and evidently belong to the earlier
andesite series of veins, so that there can be no doubt as to its identity.
TONOPAH RHYOLITE-DACITE.
Andesite similar to that on the 200-foot level continues down in the shaft to
390 feet, at which point a slight breccia is encountered, striking N. 70 C E. and
dipping northwest at an angle of 45. Below this a quartz vein is encountered,
with highly silicified Tonopah rlvyolite-dacite as its walls.
On the 500-foot level drifts run north and south about 300 feet in all. There are
also crosscuts. The whole is entirely in rhyolite-dacite. The rock is intensely
silicitied. being in places nearly solid quartz, and contains pyrite throughout, but
there are no definite veins. This quartz is barren, although assays of $51 or $2 have
been obtained in places. The rock is characteristically intensely fractured, and in
places contains open fissures running in a direction somewhat east of north. These
fissures when cut contain the heavy gas elsewhere referred to as being probably
carbonic acid (see p. !)4). The probable explanation is that the gas was formed in
the overlying soft andesite by the reaction of acids upon the contained calcite,
and by its weight sank into the fissures in the underlying rigid rhyolite-dacite
and there accumulated.
EARLIER ANDESITE AT BOTTOM OF SHAFT.
At a depth of about 680 feet in the shaft there is a sharp contact between the
rhyolite-dacite above and a fine-grained green variety of earlier andesite below.
This contact is said to dip east at an angle of about 40. The bottom of the shaft
is at a depth of 780 feet, and specimens taken from here and from below the contact
show earlier andesite of a type very much like that on the 700-foot level of the
Siobert shaft.
WORKINGS PARTLY IN EARLIER ANDESITE. 189
MACNAMARA WORKINGS.
LATER AXDE8ITK AT SURFACE.
The MacNamara shaft is situated a short distance northwest of the West
End, and probably in the same fault block. The geology partakes of the same
perplexing character as that described in the West End (see p. 184). The shaft
was first sunk to a depth of 200 feet, from which point drifts were run 50
feet to the north and about 300 feet to the south. The rock in which the shaft
started and which outcrops in the vicinity is undoubted later andesite, such as
covers the whole surface of this fault block.
CHARACTER OF ANDESITE ON 200-FOOT LEVEL.
The rock encountered on the 200-foot level differs in character very slightly
from that at the surface, except that the latter has the purplish color due to
partial oxidation, while the former has a green color characteristic of andesite,
containing a large proportion of chlorite as a result of subterranean alteration
processes. Also the andesite at the surface is decidedly fresher than that on the
200-foot level, where it is always highly altered.
CORRELATION OF MACNAMARA AXD WEST END AXDES1TES.
There would, however, be hardly sufficient reason for dividing the upper
and the lower andesite were it not that study and comparison make it seem clear
that the rock on the 200-foot level is practical!} 1 identical in characteristics with
that on the 220-foot level of the West End, which the writer, for reasons
previously given, is obliged to believe to be a phase of the earlier andesite rather
than of the later andesite.
The MacNamara rock can be matched almost exactly with specimens of the
West End rock. When studied under the microscope it is found to be altered
largely to chlorite and calcite, with pyrite, quartz, siderite. and sericite. If it is
the earlier andesite, therefore, it belongs to that phase which has altered to calcite
and chlorite rather than to that which has altered to quartz and muscovite. such
as the phase found on the 700-foot level of the Siebert shaft and below, which is
believed by the writer to have been formed usually at some distance from the
mineral-bearing veins rather than in their immediate proximity.
This rock contains calcite blotches and veinlets, and occasional stringers of
mixed quartz and calcite, one of which, it is claimed, afforded assays showing a
value of $2.
190 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
CONTACT OF EARLIER AND LATER ANDESITES.
Since it therefore seems necessary to distinguish between the andesite near
the surface and that on the 200-foot level, the question as to the line of contact
comes up. According to the conclusions arrived at this must exist, although it
is very difficult to distinguish it. From a study of the rock in the shaft and
from specimens taken there, the approximate boundary line has been placed at a
point 125 feet from the surface, where a change of formation was recognized by
the miners in sinking. This also would correspond fairly well with the conclusions
in respect to the West End, where the contact was placed between ll(i and 196 feet
from the surface, and with that in the Tonopah Extension, where it has been
placed at 184 feet from the surface.
TONOPAH RHYOLITE-UACITE AND INCLUDED VEINS.
At a depth of 285 feet a light-colored altered rock (Tonopah rhyolite-dacite)
was struck beneath the green andesite. At the contact, which strikes east and
west and dips north at an angle of 45, was a heavy zone of ground-up material.
The rock immediately beneath this breccia contained a barren quartz ledge, about
16 feet thick, striking and clipping nearly parallel with the contact, while beneath
this were numerous quartz stringers. This rhyolite-dacite proves on examination
to be entirely altered, chiefly to quartz and sericite. with probable kaolin. Original
phenocrysts consisted of small and rather sparse crystals of feldspar and biotite.
and in one case a small crystal of quartz. This rock is the same as that which was
found in the lower part of the neighboring West End shaft.
Besides the level at a depth of 200 feet, already described, there are levels at
depths of 855 and 500 feet. At the 355-foot level a drift runs a short distance
northwest of the shaft and encounters a heavy but irregular quartz vein, having a
general east- north east strike, and a moderate northwest dip. This vein, as shown
in the section (tig. 71), lies very nearly parallel with the upper contact of the
rhyolite-dacite and the earlier andesite. a short distance above. It consists of
white quartz, and also of gray and black jiispery quartz. It is in general barren,
but in places small assays have been obtained. It is cut by several faults, of
which the chief ones strike northeast and dip steeply southeast. The effect of
these seems to be in general to cause a movement as if the vein had been thrown
down on the southeast side. One of these faults, marked by a heavy drag of
quartz and rock breccia, has been followed by a drift for a few hundred feet to
the southwest. Near the end of the drift the fault splits and both forks have
been followed a short distance. In one of these branch drifts a small bunch of
ore, carrying very good values, is reported to have been found. Specimens of
this ore, shown to the writer, were composed of white quartz containing argen-
VEINS AT CONTACT OF ODDIE BHYOLITE.
191
tite, ruby silver, polybasite, or stephanite. This occurrence of bunches of -high-
grade ore, probably belonging to veins of the later rhyolite-dai-ite period, is
similar, to that of ore in veins of the same period in the Desert Queen.
At the 500-foot level, which is at the bottom of the shaft, the formation is
all rhyolite-dacite. It has been explored for a short distance north, south, and
west by drifts. No bodies of quartz of any importance were found, although a
drift along a northeast fault plane shows a breccia partly cemented by jasperv
quartz.
SE .,_.., . . NW
Tonopah Extension shaft
MacNamara shaft
Fin. 71. Vertical section through Mm-Samarn and Tonopah Extension shrfts.
EXPLORATIONS ON VEINS AT THE CONTACT OF THE ODDIE
RIIYOL.ITE.
WINGFIELD TUNNEL.
The Wingtield tunnel is situated on the southwest slope of Ararat Mountain.
It starts in later andesite near the contact of this rock with the Oddie rhyolite,
which forms the summit of the mountain, and passes from the later andesite
192 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
across the contact into the rhyolite. It is 160 feet long and runs N. 60 C E.
At the breast of the tunnel the rock is very much shattered Oddie rhyolite
containing 1 openings filled with brown iron-lime carbonate and white calcite.
From this point to the contact with the later andesite the rock is mostly a dense
rhyolite breccia of volcanic origin, the fragments being of very large size.
Strong open fractures striking N. 25 W. and dipping east at an angle of 60
are lined with white and brown carbonates, oxidized in places to iron oxide
and manganese oxide. Throughout the breccia, tilling all the interspaces, are
veinlets, filled chiefly with ferriferous carbonate and to a less degree with calcite
and chalcedony. Veins of smooth brown or bluish jasper, indicating silicification
of the rhyolite, have the same course and the whole breccia is largely silicitied.
Some of this material is claimed to run $8 or $9 to the ton, the values being all
in gold.
The contact of the andesite with the rhyolite is 70 feet from the mouth of
the tunnel, and strikes N. 35 W. and dips east at an angle of 50. The rhyolite
is plainly intrusive. The brecciation, fracturing, and silicitication of the rhyolite
increase in measure as the contact is approached. Near the mouth of the tunnel
two dikes of rhyolite breccia, one 6 inches thick and one 3 feet thick, lie in the
andesite. These are in general parallel to the main contact, but dip 50 in an
opposite direction. The fracturing and brecciation are confined to the rhyolite,
and are not notable in the later andesite, which, however, is highly decomposed
and crumbling, while the rhyolite is hard.
The evident interpretation of these phenomena is that this rhyolite column
was intruded into the andesite and that the upward movement continued after
the beginning of cooling. The result of this upward impulse was that the cooler
rhyolite for a zone of nearly 100 feet thick near the contact was intensely
brecciated while in n semisolid state. The upward pressure continued even after
further cooling, causing open fractures, mostly parallel to the contact, but
sometimes cutting across the rhyolite, as has been described elsewhere (p. 101).
Along these open fractures ascending hot waters, whose advent followed the
eruption, deposited iron and lime carbonates, silica, some manganese, and probably
some gold.
BOSTON TONOPAH SHAFT.
The Boston Tonopah shaft, lies 200 or 300 feet south of the Wingtield tunnel,
farther down the slope. At the time of the writer's visit it was 300 feet deep, 230
feet in the later andesite and the last 70 feet in white rhyolite like that constituting
the central plug. The contact between the andesite and the rhyolite in the shaft,
according to Mr. McCambridge, the superintendent, pitches northwest.
SHAFTS AT CONTACT OF ODDIE BHYOLITE.
MIRIAM SHAFT.
193
On the Miriam claim a shaft about -iO feet deep had been sunk at the time of
the writer's visit.. This shaft lies about 1,200 feet southeast of the Belle of Tonopah
and is at the contact of rhyolite and later andesite. It cuts at the top 30 feet of
brown decomposed later andesite and below this 10 feet of white rhyolite, which
is intrusive into the andesite. The rhyolite is typical and shows abundant quartz
and orthoclase phenocrysts with brown glassy groundmass. From some streaks
along this contact assays in gold were obtained, with no silver.
DESERT QUEEN SHAFT.
At a depth of 920 feet the Desert Queen shaft passed into the Oddie rhyolite,
the contact being flat. Twelve feet below this there was encountered a nearly flat
quartz vein, which is parallel with the rhyolite contact and consists of white or red
NE. sw.
Surface
Scale
5
10 feet
FIG. 72. Vertical sketch section of shallow trench just north of Belmont shaft, showing contact of the Oddie rhyolite
intrusion with the later andesite. a=Oddie rhyolite; 6=later andesite.
quartz carrying some pyrite. This quartz was 7 feet thick and had as a foot wall
the same body of rhyolite. The highest of several assays made showed 0.08 ounce
gold and 2.12 ounces silver, with a little galena and traces of arsenic and copper.
As this practically barren vein is within the Oddie rhyolite, it must be of later origin
than the rich veins in the earlier andesite.
SHAFTS AT THE UNMINEHALJZED CONTACT OF THE OUDIE RHYOLITE.
BELMONT SHAFT.
The Belmont shaft (distinct from the Desert Queen shaft, which is also on
the Belmont property) is situated on the north side of Rushton Hill. At the
time of the writer's last visit, in July, 1903, the shaft was 340 feet deep, all in
a For the description of the geology of the rest of the Desert Queen shaft, see p. 125.
16843 No. 4205 13
194 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
hard white rhyolite. It is located about 200 feet south of the contact of the
rhvolite with the later andesite. This contact is exposed in a short trench and
in a pit about 8 feet deep, and the rhyolite is seen to be intrusive into the
andesite, with an approximately perpendicular contact (fig. 72). This, together
with the depth of the Belmont shaft, indicates that it is being sunk in the
Rushton Hill neck (which is a part of and is connected with the Mount Oddie
neck) at a point where the contact is very steep.
RESCUE SHAFT.
The Rescue shaft is located south of Mount Oddie, about one-fourth of
a mile southeast of the Desert Queen shaft. It is near the contact of the white
rhyolite which makes up Mount Oddie and Rushton Hill with the later andesite.
The contact is exposed at the surface, about 120 feet north of the shaft, and
here has a general east-west strike and a southerly dip of from 45 to 60. The
contact is intrusive and there is some slight brecciation of the intrusive rock in
the bends of the lobes which jut into the intruded rock, showing squeezing of the
upflowing lava at these points.
The shaft, which starts in the later andesite, cuts the same contact as has
been described in outcrop, at a depth of 60 feet. This contact pitches in the
shaft about 45 to the south. From this point to a depth of 410 feet, which
the shaft had attained at the time of the writer's visit in November, 1904, the
rock was entirely white rhyolite of the Oddie Mountain type. From this it
will be seen that the shaft is being sunk in the intrusive rhyolite neck.
Water has been encountered in this shaft (see p. 105).
EXPLORATIONS ON VEINS AT THE CONTACT OF THE TONOPAH
RHYOLJTE-DACITE.
MIZPAH EXTENSION SHAFT.
LATER ANDESITE AT TOP OF SHAFT.
The Mizpah Extension shaft is sunk in the hollow between the two white
rhyolite intrusions of Mount Oddie and Ararat Mountain. The later andesite
outcrops between these two intrusions, and on account of its relative softness
has been worn away to form the depression separating the two hills. The
shaft was started in this later andesite, and continued in it down to a depth of
about 200 feet. The rock is of a general purplish color, with large white feldspars
and biotite phenocrysts. At a depth of about 200 feet, however, a variety of
this is tine grained, black, almost basaltic looking, and is fresher than the rest
of the rock, which is sometimes considerably decomposed.
MIZPAH EXTENSION SHAFT. 195
RHTOLITE AND RHYOLITE-DACITE IX SHAFT.
At a depth of 300 feet the andesite is in contact with an underlying typical
white rhyolite, like that of Mount Oddie. This contact strikes about N. 60 W.
and dips northeast at from 20 to 25. Both andesite and rhyolite have been
softened near the contact by circulating waters, so that their contact phenomena
are not observable. At a depth of about 430 feet in the shaft the rhyolite comes
in contact with a rock referred to the glassy Tonopah rhyolite-dacite. This
contact strikes N. 30 W. and dips northeast at an angle of 40, and is marked
by about 14 feet of wet clay, decomposed and containing bowlders. Some water
runs on top of this clay zone.
VEINS AT CONTACT OF TONOPAH RHYOLITE-DACITE.
Immediately below the contact, but in the Tonopah rhyolite-dacite, a large
quartz vein comes in. This vein is several feet thick, and has the same attitude
as the contact. Indeed, it appears to follow the contact, although it lies in the
rhyolite-dacite. At a depth of 500 feet a drift was run for the puipose of
developing this vein. The lower contact of the vein in the shaft (at 465 feet)
has a strike of N. 70 W. and a northeast dip of 45, but it is much natter
between this point and the point at which it was cut in the drift, where it
has, however, the same general strike. In this drift, which runs in an irregular
course for upward of 150 feet, the vein is displaced, not far from the shaft, by a
vertical fault having a strike of N. 45 E. The displacement of this fault is not
known, as the vein was not looked for on the southeast side. On the northeast side
it was drifted on for some little distance, and continued strong. This vein is an
ordinary quartz vein which is not very dissimilar in appearance from the average
vein in the earlier andesite, but which contains a notably large amount of pvrite.
It has locally a banded structure, which is probably due chiefly to replacement.
Nevertheless, the vein is ordinarily nearly barren, the highest assay obtained
having been about $12. The proportion of values differed from the ordinary
Tonopah vein in that they were about 75 per cent gold and 25 per cent silver.
At a depth of 505 feet in the shaft another quartz vein was encountered,
several feet thick, with characteristics like the one above. This vein has a strike
of N. 55 W. and a dip of 55 to the northeast. A specimen of the wall rock
taken immediately below this vein proved to be andesite, probably later andesite.
Therefore this vein appears to occur on the under contact of the Tonopah rhyolite-
dacite with the later andesite, while the first-mentioned vein occurs on the upper
contact of the same rhyolite-dacite body.
This rhyolite-dacite is similar to that which outcrops to the north of Ararat
Mountain, is like that discovered in depth in the Desert Queen and Siebert shafts.
196 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
and is very similar to some of the rock at Gold Mountain (4 miles south of Tonopah)
in the Tonopah Union shaft. It has a pyroclastic structure, with occasional pheno-
crysts of quartz and more common crystals of feldspar, chiefly orthoclase, which
are largely altered to quartz and muscovite. Small biotite crystals are also altered
to white mica by bleaching. The groundmass is a microfelsitic devitrified glass.
Some secondary adularia was observed.
From the lower contact of this rhyolite-dacite body the shaft passes through
later andesite again to a depth of about 620 feet, making the thickness of this
body of andesite traversed somewhat over 100 feet. At this point again there
is a contact between the later andesite above and Tonopah rhyolite-dacite below
similar to that just described. A short distance from the contact a vein of quartz
2 feet thick, containing pyrite and otherwise having the same characteristics of
the upper veins, was encountered in the rhyolite-dacite. This also seems to be
very nearly a contact vein. The bottom of the shaft, at a depth of 800 feet,
is still in the same rhyolite-dacite.
From the bottom of the shaft a drift was run due east 525 feet since the
visit of the writer. A specimen of the rock sent to the writer from the end of
the drift is rhyolite-dacite, like that at the bottom of the shaft, but a specimen
taken from an intermediate point in the drift is later andesite. Mr. C. E. Knox,
the president of the company which has conducted these explorations, reports that
the veins cut in the shaft were cut again in this drift in regular order. It is
probable, therefore, that the alternating bands of rock, striking northwest and
dipping southeast, were encountered in the drift also, with the exception perhaps
of the white rhyolite, which has not been reported as occurring in the drift. It is
interesting to note that the end of the drift has been carried somewhat past the
surface contact of the rhyolite-dacite with the later andesite perpendicularly above
on the slopes of Ararat Mountain.
CORRELATION OF THE RHYOLITIC ROCKS IN THE SHAFT.
As before stated, the contact phenomena were not observable in the mine on
account of alteration by circulating waters, but from what has been observed at
other points in the district it may be believed that here, too, the andesite' is the
older of the rocks exposed; that it has been cut by the Tonopah rhyolite-dacite,
and that the white rhyolite was the last of all and is also of an intrusive nature.
The form of the different igneous bodies underground must be very complex, and
it is difficult or impossible to even outline the connections between the similar
lavas. It seems likely, however, that the white rhyolite is connected with that
of Mount Oddie, and the Tonopah rhyolite-dacite with that around Ararat
Mountain.
KING TONOFAH SHAFT.
AGE OF THE VEINS.
The veins clearly belong to a period subsequent to the formation of the veins
in the earlier andesite, as shown by their having the Tonopah rhyolite-dacite for
a wall rock. The relatively high content in gold as compared with silver seems
to be very common in these post-andesitic veins connected with the dacite-
rhyolites.
KING TONOPAH SHAFT.
GEOLOGICAL SITUATION.
The King Tonopah shaft lies at the contact of the Tonopah rhyolite-dacite with
the later andesite. At many points along the irregular contact of these two rocks
phenomena were observed indicating that the rhyolite-dacite is intrusive into the
andesite. The rhyolite-dacite sends out intrusive irregular projections into the
andesite, and isolated dikes or necks appear in the andesite some little distance away
from the contact.
The shaft starts in the later andesite, and at a depth of 38 feet passes into
silicified rhyolite-dacite. The total depth of the shaft is 300 feet, and from the
bottom a drift was run to the north arid, at the time of the writer's visit, extended
48 feet from the shaft.
VEIN MATERIALS.
At a depth of 226 feet a zone of silicified rhyolite-dacite with quartz stringers
was cut in the shaft. It is several feet in thickness, but was practically barren of
values, the highest assay reported being only about $2. Some of this vein material
contains, besides quartz, abundant adularia, as is shown by microscopic study.
There is also some finely striated feldspar, which may be albite. Some of the
adularia shows the characteristic rhombic cross sections, and many of these crystals
are entirely inclosed in quartz.
NATURE OF ROCK INCLOSING VEIN MATERIALS.
The rock in which this material lies and in which the entire shaft and drift has
been driven below a depth of 38 feet is the Tonopah rhyolite-dacite. It is a glassy
lava made up for the most part of a glassy grounduiass, usually more or less devitri-
fied and altered to quartz, kaolin, and sericite aggregates. In some specimens
abundant fine adularia of secondary origin has been found in the groundmass.
Scattered small crystals of feldspar usually occur, but they are mostly nearly or quite
altered to sericite and sometimes to adularia. The blunt form of some of these
crystals shows probable original orthoclase, while some are more elongated, suggest-
ing a more basic species.
198 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
CORRELATION OF VEINS WITH OTHER OCCURRENCES.
The formation of pyritiferous quartz veins in this rock is therefore a contact
phenomenon near the edge of the intrusion, for the glassy rhyolite-dacite outcropping
away from the contact is usually quite fresh and unsilicified. This idea gains cor-
roboration from the fact that near by, at other points on the contact, namely, in the
vicinity of the Belle of Tonopah shafts and elsewhere, silicification and the formation
of veins has occurred. This vein is, then, of the same class as the veins at the contact
of the glassy Tonopah rhyolite-dacite in the Mizpah Extension and other shafts.
BELLE OF TONOPAH SHAFT.
GEOLOGICAL CONDITIONS.
The Belle of Tonopah is situated in the northern corner of the area mapped, about
1,600 feet west of the King Tonopah, on the irregular contact of the glassy Tonopah
rhyolite-dacite and the later andesite. At a number of places along this contact the
phenomena show that the rhyolite-dacite is intrusive into the andesite. Just south
of the Belle of Tonopah shaft a number of rhyolite-dacite dikes occur in the later
andesite near the contact. These are considerably decomposed and are accompanied
by small and nonpersistent quartz veins, which give assays showing generally small
and irregular quantities of gold, with little silver.
The Belle of Tonopah shaft starts in such a rhyolite-dacite dike, very close to the
contact, and passes downward through 20 feet of this material, when it enters the
later andesite. The contact of the two rocks strikes west-northwest and dips south-
west at angles ranging from 65 or 70. The contact is marked by a decomposed
zone, and the later andesite below is soft and is very full of pyrite which, however,
is quite barren.
At the time of the writer's visit the shaft was 230 feet deep, all except the
upper 20 feet being in the later andesite. Since then, in January, 1904, Mr.
A. C. Stock, the manager, has sent the writer a specimen of the rock from the
bottom of the shaft at a depth of 460 feet. This is later andesite.
The rhyolite-dacite in the upper part of the shaft resembles the rock from
the King Tonopah. It is highly decomposed, but has the structure of a nearly
glassy volcanic rock and contains many very small crystals, nearly all of which
seem to have been feldspar, with no original quartz. Now the whole rock is
altered to kaolin, chert, hematite, siderite, etc. This rock unquestionably belongs
to the glassy Tonopah rhyolite-dacite and is intrusive into the later andesite at
this place.
LITTLE TONOPAH SHAFT. 199
VEINS.
The quartz stringers found along the edges of the rhyolite dikes near the
edge of the shaft are stated by Mr. Stock to run as high as $13 in gold. These
consist of dark, rather dense quartz, carrying a great deal of pyrite. In the
shaft also small stringers have been found up to the time of the writer's visit,
generally striking parallel with the slips but dipping in the opposite direction,
and affording assays running up as high as $18, being all in gold. Mr. Stock
reports that at a depth of 440 feet a stringer was cut which gave an assay of
$39.60 in gold and $3.80 in silver, while at the bottom of the shaft (460 feet)
another stringer 2 inches thick gave an assay of $4.14 in gold and $6 in silver,
the latter being the first which showed preponderating silver values, the other
assays from the shafts of the neighborhood showing chiefly gold. This minerali-
zation is therefore comparable with the low-grade pyrite-bearing quartz veins,
with the values chiefly in gold, which occur at various other points in or near
the glassy Tonopah rhyolite-dacite near its contact. It is due to the action of
heated waters circulating along the contact, subsequent to the intrusion of the
rhyolite-dacite, and is of a different and later period from that of the veins in
the earlier andesite.
The abundance of pyrite in the altered later andesite seems to indicate that
the pyritization here, as probably in the case of similarly altered later andesite
on Mizpah Hill, is associated with present water courses. The pyrite, like that
of the later andesite on Mizpah Hill, is barren of gold and silver values.
SHAFTS AT THE UNMINERALIZED CONTACT OF THE TONOPAH
RHYOLITE-DACITE.
BUTTE TONOPAH SHAFT.
The Butte Tonopah shaft, at the eastern base of Ararat Mountain, was 35
or 40 feet deep at the time of the writer's visit. It was in the Tonopah
rhyolite-dacite, at the contact of this rock with the later andesite. This con-
tact, apparently vertical, is plainly and continuous!}- shown about 30 feet east of
the shaft. The rhyolite-dacite contains many inclusions of the andesite, and is
intrusive into it.
LITTLE TONOPAH SHAFT.
This shaft is located about 150 feet from the edge of the area mapped and
about one-half mile west of the Golden Anchor shaft. It is situated at the
contact of the glass}' Tonopah rhyolite-dacite with the later andesite. The shaft
starts in the rhyolite-dacite and runs down about 50 feet to the contact with the
andesite. The rhyolite contains fragments of the later andesite, and the contact
200 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
dips west at angles of 50 or 55. The shaft was, at the time of the writer's last
visit in the fall of 1904, about 585 feet deep, and the lower part was all in the
later andesite. At the surface the contact seen in the shaft outcrops 50 or 60 feet
east of the shaft. No mineralization was observed.
SHAFTS AT THE CONTACT OF THE BROTJGHER DACITE.
BIG TONO SHAFT.
This shaft is sunk in the intrusive dacite neck on the east side of Brougher
Mountain. It was started in the glassy contact phase of the neck at the very
contact with the dacite breccia, into which the neck is intrusive. The shaft is
somewhat over 300 feet deep, and is entirely in the Brougher dacite.
For a depth of about 50 feet the glassy phase persists in the shaft, below
which is the ordinary porphyritic phase. This indicates that with depth the shaft
departs from the contact, which at this point must pitch away from the mountain.
MOLLY SHAFT.
The Molly shaft is situated at the west end of Golden Mountain. It was
sunk in the summer of 1903 and was 468 feet deep when work was stopped. A
rough estimate of the section passed through, made by climbing down the some-
what tightly lagged shaft, was that the Brougher dacite occupied the upper two-
thirds, and the Fraction dacite breccia most of the lower third, with 25 feet of
later andesite at the bottom. There seems to have been some Tonopah rh3'olite-
dacite sheets in the Fraction dacite breccia. No water was encountered.
As shown on the map, the shaft lies about 250 feet east of the nearest point
of contact of the Golden Mountain intrusive dacite neck with the older rocks.
This contact, therefore, has here a pitch of about 45 toward the mountain, a
fact which is also indicated by the inward pitch of the outcropping contact and
by the flow structure in the dacite at the contact for some distance to the north
and east. The shaft has thus passed downward out of the dacite neck into the
older formations.
SHAFTS WHOLLY OK CHIEFLY IN DACITIC TUFFS.
NEW YORK TONOPAH SHAFT.
The New York Tonopah lies between Butler and Brougher mountains and
when last visited by the writer the workings consisted of only a shaft 745 feet
deep. At the point where the shaft was sunk the surface consists chiefly of
brecciated lavas and tuffs which have been referred to the glass}' Tonopah
rhyolite-dacite. However, the rocks belonging to this formation, when they are
chiefly fragmental, as they are here, are often not easily distinguishable, or
FRACTION EXTENSION SHAFT. 201
perhaps not at all, from the tuffs of the Fraction dacite breccia, which iu general
is considered to underlie the first named. In the shaft portions were passed
through which resemble the Tonopah rhyolite-dacite; these ma}' represent dikes,
especiall}' in the lower portion. As a whole, however, the shaft may be considered
to lie within the Fraction dacite breccia.
The first 150 feet is rather fine volcanic breccia, followed by 275 feet of frag-
mental tuff, light-colored and generally moderately coarse. This is horizontally
coarsely stratified and contains one bed, 1 feet thick, of finely stratified fine-grained
material. This passes gradually into a fine breccia and this into a vecy coarse
breccia containing included fragments up to 2 or 3 feet iu diameter. Most of
these inclusions are various phases of the later andesite, but some are probably
earlier andesite. Others are of dacite and tuff, much like the matrix. At the
time of the writer's examination the shaft was 475 feet deep, and had passed
through 50 feet of this coarse breccia. Specimens obtained from the shaft during
its progress farther downward showed that it remained in practically the same
material, some of the included bowlders of earlier and of later andesite being
several feet thick. The bottom of the shaft is in soft dacite that contains later
andesite inclusions. This dacite is much like that which caps the Fraction shafts.
The stratified tuffs referred to do not belong to the Siebert tuffs of the lake
beds, but are included in the Fraction dacite breccia. They are described else-
where as the interbreccia tuffs, and are found in the upper part of the Fraction
dacite breccia at various places in the district. The great thickness of the Fraction
dacite breccia, here shown, indicates that the block in which the New York Tonopah
lies has sunk down very considerably in respect to the blocks farther northeast
to those, for example, in which the Fraction shafts are situated. The breccias
and tuffs of the New York Tonopah are considered to be surface formations, formed
chiefly by explosive outbursts; and the included blocks of earlier rocks are con-
sidered to be fragments hurled out of the volcanoes at the time of the explosions.
FRACTION EXTENSION SHAFT.
GEOLOGICAL SECTION'.
This shaft is situated at the south base of Brougher Mountain, somewhat over
a thousand feet northwest of the New York Tonopah shaft. When visited by the
writer it was approximately 300 feet deep. On account of the tight lagging the
section of the shaft could not be observed, but a roughly estimated thickness of 75
or 80 feet of the white finely stratified tuffs of the lake beds was first passed
through. Below the tuffs the whole shaft is in hard gray or red brecciated lava,
belonging to the glass} 7 Tonopah rhyolite-dacite.
202 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
FAULT.
This same rhyolite-dacite outcrops about 35 feet east of the shaft, on the
farther slope of a little gull}-. A northeast fault running along this gully is thus
evidenced, and indeed is shown farther northeast up the hill slope. By this fault
the block in which the Fraction Extension is located is downthrown in respect to
that on the east side.
TONOPAH CITY SHAFT.
GEOLOGICAL SECTION.
The Tonopah City shaft lies on the outskirts of the town, about 1,100 feet
south of the Fraction No. 2. It was driven to a distance of 500 feet before work
was stopped. On the surface at this point is a very thin covering of black glassy
rhyolite or dacite (latest rhyolite-dacite flow), generally only a few feet thick, and
often broken up into bowlders rather than in place.
Practically none of this black lava is exposed in the shaft itself, the first solid
formation cut being the Fraction dacite breccia. The upper 100 feet of this was a
coarse breccia, evidently detrital, which contained large and small inclusions, mostly
of later andesite. From 100 to 300 feet the breccia was finer grained and denser,
and apparently had an explosive origin, being full of small, angular, white pulveru-
lent fragments, which are probably decomposed pumice. At a depth of 300 feet
solid Heller dacite (see p. 37) came in and continued for 200 feet to the bottom of
the shaft.
At a depth of 400 feet in the shaft this dacite was observed to be cut by
a dike of exactly similar material, the only difference being the presence in the
dike of a greater abundance of light-colored intrusions. This dike is 10 inches
thick and has a N. 70 W. strike and a dip of 75 to the northeast.
INDICATED DISPLACEMENT OF FAULT BLOCKS.
Since neither the earlier nor the later andesite was encountered in this shaft,
and the dacite breccia is so much thicker than in the Fraction shaft to the
north, it is plain that the fault block in which the Tonopah City is situated is
depressed relatively to that in which the Fraction shafts lie.
OHIO TONOPAH SHAFT.
DACITE TUFFS IN SHAFT.
The Ohio Tonopah is situated about l,f>00 feet west of the MacNamara shaft.
At this point the surface formation is a volcanic tuff due to dacitic outbursts.
Home of the harder portions are more clearly referable to the glassy Tonopah
rhyolite-dacite, while other portions, especially where the rock is softer, approach
OHIO TONOPAH SHAFT. 203
more closely the character of the Fraction dacite breccia. However, as has been
said in discussing these formations in general, there is much admixture, and on
account of the intimate relation of the two lavas the tuffs often can not be,
properly distinguished and separated.
The shaft is at present about 770 feet deep, and has a working level at
756 feet. Passing downward from the surface, the shaft passed through a con-
siderable thickness of the rhyolite-dacitic tuffs above referred to. These tuffs
continue down to about 485 feet. They are usually rather soft; under the micro-
scope they are plainly fragmental and are little assorted, indicating probably
showers of detritus from volcanic outbursts. On account of their original glassy
character and their subsequent decomposition (chiefly kaolinization) very few
definite characteristics can be distinguished. A specimen of one of the harder
portions, at 396 feet, however, showed, under the microscope, a glassy ground-
mass with phenocrysts of quartz, striated feldspar, orthoclase, and altered biotite.
In this slide the chief secondaiy minerals were calcite and muscovite.
LATER ANUESITE IN SHAFT.
From about 485 feet to 525 feet there is andesite having the appearance of
the later andesite. Well-marked slips near this contact indicate that it is very
likely a fault contact. One of these slips had a north-south strike, and a westerly
dip of 50.
SOLID TONOPAH RHYOLITE-DACITE.
Below the later andesite, from 525 feet to the bottom of the shaft, comes a
dense, siliceous rock, which is discussed elsewhere and is undoubtedly referable
to the Tonopah rhyolite-dacite.
At the contact of this rock with the overlying andesite movement is indicated
by the presence of 30 to 40 feet of ground-up material, which contains fragments
of hard rock and occasionally of quartz. The dip of this contact is northwest,
at an angle of about 25 C .
At the 756-foot level the ground has been extensively explored to the south,
north, and east by drifting, the main southeast drift running about 700 feet from
the shaft. The formation is almost entirely Tonopah rhyolite-dacite, character-
istically showing angular white fragments in a dense gray groundmass. The
brecciation indicated by these fragments took place before the cooling of the
rock. The only andesite shown on the level is a small patch about 150 feet
southeast of the shaft. This is a biotite-andesite, and may be either the earlier
or the later andesite. It has a sharp contact with the rhyolite-dacite, which is
probably intrusive into it. On the south side of the andesite patch, as exposed
in the drift, the contact dips north at an angle of about 55.
204 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
CHARACTERISTICS OF THE RHYOLITE-DACITE.
The specimens of the rhj-olite-dacite examined microscopically are of a highly
altered, very glass}' lava. The groundmass is glassy, often kaolinized. It is very
abundant, constituting nearly all the rock, and often shows marked flow structure.
The phenocrysts are rare and small, and consist chiefly of short, blunt feldspars,
biotite, and occasional very small quartz grains. Some of the feldspars are
striated. The feldspars are usually almost or entirely altered to kaolin, sericite,
and secondary adularia. The biotite is usually altered, sometimes to chlorite.
Secondary quartz and pyrite are usually common in the rock, and sometimes
there is calcite.
MINERALIZATION.
As a rule the rock is very much silicilied. Cracks in this rock are filled
with coatings of calcite, quartz, and pyrite, and excellent free crystals of barite.
Some streaks are considerably silicitied, and contain silver and gold, as is shown
by assay. Up to the time of this writing, however, no veins of importance have
been struck in this formation.
The chief veins are irregular, barren, and nonpersistent. They have a
general northeast or east strike, and die out along the strike by scattering into
the silicitied rock, or are cut off by faulting. At the upper contact of the
rhyolite-dacite with the patch of andesite above mentioned, there are 2 feet of
jaspery barren quartz, illustrating again the tendency of the rhyolitic quartz to
form at the upper contact of the rhyolite-dacite body, under the impervious
decomposed andesite, as elsewhere described in the discussion of the MacNamara,
the Tonopah Extension, the Mizpah Extension, and other mines.
Some faulting is shown in this level, the chief being in a north-northwest
direction, and indicating in places considerable displacement.
PITTSBURG SHAFT.
The Pittsburg shaft lies near the eastern edge of the area mapped, on the
south side of the main road which runs east out of Tonopah. It is not shown
on the topographic map, having been started since this was made. At the time
of the writer's visit, in November, 1904, it was 570 feet deep, all in volcanic
breccia, probably belonging chiefly to the Tonopah rhyolite-dacite period. This
formation contains some harder layers, which may be flows, but as a whole is to
be considered a surface formation, the product of volcanic explosions.
RED ROCK SHAFT.
This shaft lies about halfway between the Pittsburg and the Ohio Tonopah.
It was at the time of the writer's visit, in November, 1904, 230 feet deep in
volcanic breccia like the Pittsburg and the upper part of the Ohio Tonopah.
DESCRIPTIVE GEOLOGY OF MINES AND PROSPECTS. 205
SHAFTS ENTIRELY OR CHIEFLY IN LATER AXDESITE.
HALIFAX SHAFT.
The Halifax shaft was sunk in the depression lying just north of Golden
Mountain in the later andesite, just northeast of a probable fault line which
separates the later andesite on the northeast from the white tuffs on the southwest.
The shaft was 800 feet deep at the time of the writer's last notes in November,
1904, and was entirely in the later andesite. The andesite is very fresh fresher
than that examined in any other part of the district. The phases exposed in the
upper part of the shaft are very glassy, suggesting that they are near the upper
part of a flow, while those in the bottom are also relatively finer grained than
the rock exposed for most of the distance down the shaft. Much of this latter is
so coarse, with so great a development of phenocrysts compared with the quantity
of the groundmass, that it has in the hand specimen almost a granular texture.
Nevertheless the different phases all belong to a single mass.
At a depth of 200 feet in the shaft a drift was run a little east of south for
270 feet, and in the opposite direction for 100 feet along a heavy fault, which runs
parallel to the drift and dips west at an angle of 45 or steeper. Along this fault
plane there is a thick brecciated or ground-up zone 8 or 10 feet thick. The
strise indicate that the faulting was normal, the downthrow being on the west
side. The same conclusion is suggested by the difference in the texture of the
andesite on the sides of the two fault, that on the foot wall side being coarser
and almost granular, while that on the hanging wall is finer grained. It is probable
that the coarser textured andesite cooled at a somewhat greater depth than the
finer grained and, therefore, that this side has been relatively upthrust.
The shaft stays in this same granular andesite. for 50 feet below this level, when
another fault zone comes in, along which is also a clay seam. This dips 60 to
the west. Below this, hard and finer grained andesite comes in again and continues
downward.
This is one of the few shafts in the district which have struck a large flow
of water. Chiefly below 600 feet in the shaft, water was encountered, which rapidly
increased from 10,000 to 30,000 gallons a day, and owing to this the sinking of
the shaft was for a long time suspended (see p. 105). Some drifting is being done,
from the bottom, north and south, in the later andesite.
GOLDEN ANCHOR SHAFT.
The Golden Anchor shaft was started in the center of the later andesite area
west of the Midway. When last visited, in the middle of November, 1904, it
was 640 feet deep. At a depth of 400 feet a south crosscut runs 510 feet from
the shaft, and at a depth of 500 feet a north crosscut runs 463 feet. The upper
206 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
part of the shaft is in typical later andesite. On the 400-foot level the andesite
is of somewhat different character, being greenish and altered, but its characters
still indicate that it is probably the later andesite. On this level it contains some
calcite veinlets, but no quartz. On the 500-foot level the andesite is finer grained
than on the 400, and has some of the features of the earlier andesite, but there
is little doubt that it belongs to the same body as the 400-foot level, and the
balance of evidence is therefore in favor of considering it probably later andesite.
On this level there are some calcite stringers and some narrow quartz veinlets,
containing, however, practically no values. At a depth of 550 feet in the shaft
a change of formation was reported, and the material seen on the dump taken
from beneath this point is largely a dense, green, siliceous rock containing quartz
stringers. A specimen of this examined microscopically proved to be of the
glassy Tonopah rhyolite-dacite. Inspection of the dump indicates that this
rhyolite-dacite is mixed with some andesite, which may be either the earlier or
the later andesite, so far as microscopic characteristics go.
The above data indicate that at this point the later andesite is considerably
thicker than in the territory farther east. Indeed, its lower limit is uncertain.
CHAPTEE VI.
HOCK ALTERATION.
ALTERATION OF THE EARLIER ANDESITE.
The alteration of the earlier andesite by thermal waters has been profound,
indicating that these solutions were present in large quantity and were very
active.
ALTERATION OF EARLIER ANDESITE CHIEFLY TO QUARTZ, SERICITE, AND
ADULARIA.
On Mizpah Hill the andesite is entirely altered and has a siliceous, light-
colored rhyolitic appearance nearly everywhere, except in depth, where the
Mizpah shaft on the 700-foot level shows earlier andesite altered largely to
chlorite, separated from the quartz-sericite alteration by a fault, and, so far as
yet explored, marked by the absence of veins.
ALTERATION OF HORNBLENDE AND BIOTITE.
Various stages in the alterations are observable. The ferromagnesian minerals,
hornblende and biotite, have usually been completely destroyed. Their areas are
marked by liberally sprinkled pyrite crystals, by siderite, and often by some
sericite. Frequently the grouping of the iron minerals, which follows with more
or less clearness the well-known outlines of an original hornblende or biotite,
affords the only evidences of the former existence of these phenocrysts, at the
same time plainly showing the demarcation of the pyrite and siderite from the
original ferromagnesian minerals. In further stages of alteration the pyrite and
siderite have escaped from the confines of the original crystal and are scattered
through the rock; in this case they are usually less abundant, showing a leaching
of iron out of the rock as the silicification increases. It has been determined by
assay that the pyrite in these rocks does not contain appreciable amounts of gold
and silver, even close to the veins.
In other phases the ferromagnesian minerals have been entirely altered to
fine muscovite (sericite) and quartz.
The alteration of biotite has been sometimes not so complete as just sketched,
the mineral having been bleached and the separated iron represented by pyrite and
siderite.
207
208 GEOLOGY OF TONOPAH MINING DISTRICT. NEVADA.
RELATIONS OF PTRITE AND SIDERITE.
The relations of the siderite to the pyrite in these rocks have been carefully
studied. In some cases the siderite has been observed distinctly pseudomorphous
after the pyrite. Often the two exist side by side in such a way as to suggest
contemporaneous deposition, pyrite showing usually, and siderite frequently, some
characteristic forms (PI. XXIII). In observing the alteration of these minerals
from ferrotnagnesian crystals it has been repeatedly noticed that the carbonate had
more intimate relations with the original crystal than did the pyrite, the carbonate
occurring all through the decomposed mineral, while the pyrite was distinctly
confined to the outer zones.
ALTERATION OF SODA-LIME FELDSPAR TO QUARTZ AND SERICITE.
The feldspar phenocrysts, which are sometimes fresh enough to be determined,
are typically andesine-oligoclase, though sometimes they become more calcic.
Labradorite occasionallj' occurs. The}' are usually partly or completely altered.
The alteration to adularia is one of the most commonly observed changes, but
hardly so common as that to quartz and fine muscovite. These two last-named
minerals frequently form a pseudomorphous aggregate in the space occupied by the
original feldspar. With increasing alteration the outlines of these pseudomorphs
become more and more indistinct and finally indistinguishable. Even within the
veins, however, careful observation may often succeed in distinguishing the traces
of these original crystals in the highly silicified mass, for sometimes they are marked
by quartz that is relatively coarser grained than that in the groundnuts*, and
consequently they appear slightly lighter in transmitted light. These two processes
of alteration of the feldspar, either to adularia or to quartz and sericite, although
present in the same rocks, are not very commonly associated in the same specimens
and appear to be distinct. Occasionally the feldspar is altered to kaolin, as described
later.
ALTERATION OF SODA-LIME FELDSVAR TO ADULARIA.
The alteration of the soda-lime feldspar to adularia can be observed in all
its stages in different rock specimens. The alteration proceeds along the edges
and the cleavage cracks of the crystal, so that the brightly polarizing andesine,
somewhat turbid from decomposition, becomes reticulated with the fresh glassy
adularia, which shows markedly lower polarization colors (PI. XXIII). Character-
istic complete or incomplete crystals of adularia with rhombic outline frequently
form within the older crystal. In some cases the alteration is completely carried
out and the feldspar is completely pseudomorphosed to adularia, whose perfect
crystal outlines give the idea of a fresh primary crystal, but whose optical
U. S. GEOLOGICAL SURVEY
PROFESSIONAL PAPER NO. 42 PL. XXII
A, II, C.
D, E, F.
RELATIONS OF PYRITE AND SIDERITE IN TONOPAH ANDESITE.
ADULARIA IN EARLY ANDESITE AND VEINS.
ALTERATION OF THE EARLIER ANDESITE. 209
characters prove the truth of the change demonstrated in other cases by observed
transitions. Sometimes the alterations to adularia and to sericite go on side by
side, the original feldspar altering in part to one and in part to the other and
the two minerals sometimes forming an interlocking aggregate.
ALTERATION OF THE GROUNDMASS.
The microlitic, nearly glassy groundmass has been very largely decomposed
to or replaced by fine granular quartz, with fine muscovite (sericite), etc. The
quartz in the more highly silicitied specimens shows grains of larger growth and
is often segregated in bunches or veinlets. Pyrite and siderite are very commonly
disseminated throughout. Original zircon is frequently present. Sometimes
adularia can be made out as a portion of the fine secondary aggregate. Tiny
veinlets of adularia and others of quartz also seam the rock.
Apatite, usually brownish or yellowish and slightly pleochroic, is relatively
abundant, and not being easily attacked by the agents which have brought about
the alteration of the rest of the rock is very characteristic in the considerably
silicified phases.
ADVANCED STAGE OF ALTERATION.
In the advanced stages of alteration nearly all the iron has disappeared; the
similar alteration products of the feldspars, the ferromagnesian minerals, and the
groundmass merge to form a quartz-sericite aggregate. The quartz varies in grain
from microcrystalline or nearly cryptocrystalline to moderately coarse, a charac-
teristic applying also to the quartz of the mineral-bearing veins, which are
mostly the extreme alteration product of the andesite, as is shown by both field
and microscopic study. In these extreme phases the quantity of sericite becomes
less and that of the quartz more.
OCCURRENCE OF KAOLIN.
While kaolin is not an ordinary alteration product in the siliceous alteration
of the earlier andesite, it is frequently present. Specimens in which it has been
detected have usually been taken from near a fault or fracture, or other water
course connecting with the surface. Therefore the hypothesis has been formulated
that while the sericite is manifestly the work of the vein-forming solutions the kaolin
is the work of descending surface waters, and is probably of later origin, the
kaolinization attacking the unsericitized residual feldspar. Kaolin and sericite
are frequently found together in varying proportion.
16843 No. 4205 14
210 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
ALTERATION OF EARLIER ANDESITE, CHIEFLY TO CALCITE AND CHLORITE.
In the earlier andesite at points sufficiently remote from the important veins,
calcite and chlorite appear as distinct alteration products, which do not occur in
the rock nearer the veins and which take the place, partly or wholly, of the quartz
and sericite of the phases described above. This phase has a green color, growing
in depth of shade as the proportion of chlorite increases, and the rock has no
resemblance to the light-colored quartz-sericite alteration phases. Iron in the form
of pyrite and siderite is common to both phases, but while in the quartz-sericite
alteration it is characteristically in small quantity and diminishes with increasing
alteration, in the chlorite-calcite alteration it is abundant and remains so when
the rock is completely altered.
In this process of alteration the feldspar is usualry largely altered, chiefly to
calcite with a little quartz. Rarely the alteration is to quartz and epidote.
Original hornblende and pyroxene are always completely altered, usually to chlorite
(ripidolite) pseudomorphs. Biotite has been observed altered to sericite, with a
little calcite and hematite.
The groundmass is similarly altered to chloritic material, intermixed with
secondary quartz, etc.
TRANSITIONS BETWEEN ALTERATION PHASES OF EARLIER ANDESITE.
There are all transitions between the typical quartz-sericite alteration phase,
in which calcite and chlorite are always absent, and the typical calcite-chlorite
phase, in which quartz, and especially sericite, are decidedly subordinate. Thus
in a specimen from the 700-foot level of the Siebert shaft (from the same rock
mass as some of the typical calcite-chlorite phases) the feldspars are chiefly altered
to sericite, with a little chlorite; the hornblende and biotite crystals are altered
chiefly to chlorite; and while calcite is present, it is not prominent.
DIFFERENT ALTERATIONS THE EFFECT OF THE SAME WATERS.
The conclusion is thus reached that the chemical effects of the same mineralizing
waters became continually different as they penetrated to a greater and greater
distance from the circulation channels. Along these channels, which became veins,
the transformation or replacement of the rock by the addition of silica and the
sulphides of silver, antimony, etc., with gold and selenides, and by the complete
leaching out of soda and magnesia and the partial leaching out of lime and iron,
was profound. In the siliceous phase of the altered andesite near the veins a
similar alteration, though weaker, is recorded. The metals did not penetrate here,
but the partial replacement of lime, iron, magnesium, and soda by silica and potash
is present in all its stages. In the rock more remote from the vein channels the
ALTERATION OF THE EARLIER ANDESITE. 211
alteration has been often complete, yet there has been no very great increase
or decrease in the original elements. The original combinations of these elements
have been broken up, and hydrated silicates, with abundant carbonates and sulphides,
have formed, indicating only the presence of carbonic acid and hydrogen sulphide
in the altering waters. Since the quartz-sericite alteration of the earlier andesite
grades into the chlorite-calcite alteration by all possible stages, it is probable
that both were produced at the same time and by the same waters; and since
the transition from the quartz-sericite alteration to the metalliferous quartz
veins is similarly perfect, the waters are clearly those which have produced the
mineralization. Within the main circulation channels, therefore, these waters
introduced silica, potash, and the metallic sulphides, and abstracted other materials.
As they penetrated the rock away from these channels they ceased to deposit
metals, except possibly in trifling quantity," while the excess of silica and potash
was still deposited, failing with increasing distance. Finally, the changes in the
calcite-cblorite alteration show that only the common gases above mentioned, so
commonly present in surface hot springs, were left in the mineralizing waters,
which therefore had little to precipitate and small power to abstract.
The successive precipitation so plainly demonstrated probably took place by
reactions with the wall rock, which therefore acted as a screen for the traversing
solutions.
REFRACTORINESS OF POTASH FELDSPARS.
In arguing that the formation of potash minerals in the veins and in the wall
rock shows a relative excess of potash in the mineralizing waters, it must be taken
into consideration that potash feldspars are ordinarily more refractor}' to altering
waters than the soda-lime varieties. Comparison of analyses of fresh rocks and
of rocks altered by surface weathering usually show that the loss of soda is
greater than that of potash.* It is also true, as pointed out by Lindgren,"" that
one of the most prominent minerals formed by metasomatic processes in and
near veins is a potassium mica, such as muscovite, and that the most prominent
process brought about by the waters is the progressive increase of potash and
the decrease of soda. At the Boulder Hot Springs, described by Weed, rf sericite
and in one case adularia had been deposited from the waters, which contain
chiefly sodium sulphate, carbonate arid chloride, calcium carbonate, and silica;
no potassium is recorded. Near the Comstock lode, potash, as compared with
soda, is more important in the altered than in the fresh rock/ showing that
"Sampling of the Mizpah mine, under the direction of Mr. John Hays Hammond, showed that the earlier andesite
forming the walls of the vein runs in values from $0.50 to $2 a ton, as compared with many times that value in the vein.
6 Merrill, G. M., Rocks, Rock-weatliering, and Soils, p. 236.
eLindgren, W., Trans. Am. Inst. Min. Eng., vol. 30, p. 690.
dWeed, W. H., Twenty-first Ann. Kept. U. S. Geol. Survey, pt. 2, p. 246.
Lindgren, W., Trans. Am. Inst. Min. Eng., vol. 30. p. 647.
212 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
the fresh rock has been attacked more than the altered rock. Complete analyses
of the mine waters show chiefly carbonates of lime and magnesia in about equal
proportions, next, sulphate of magnesia and silica, a smaller amount of carbonate
of soda, about one-tenth as much carbonate of potash as soda, small amounts of
sodium chloride, and very small proportions of alumina and ferric oxide."
MEANING OF ADULARIA AND ALBITE AS GANGUE MINERALS.
While it might be inferred from this that ordinary waters, .even those
containing a large amount of soda and little or no potash, tend to produce potash
minerals in veins and owe their composition to the leaching out of the soda while
the potash is left behind, the fact remains that potash feldspar is contained, so far
as known, only in a relatively limited number of veins.
Soda feldspar or albite, a mineral as easily formed in the wet way as orthoclase,
occurs in a number of other veins and in rocks as the result of the alteration of
soda-lime feldspars, and, what is more interesting, of potash feldspars. Dr. G. L.
Gentil* has shown that in the granites of the Tofna basin in Algeria the soda-lime
feldspars have been largely transformed into albite, and the same phenomenon has
been described by other authors. On St. Gothard and other places in the Alps
albite has been described as pseudomorphous after adularia, and as occurring in
porous aggregates of fine c^-stals in the form of the original potash-feldspar
crystal. Comparative analyses of the feldspar's various phases of alteration show
that the original adularia contains very little soda and the resultant albite no
potash. Bischof <" explains this process of pseudomorphism as a decomposition of
the original adularia by waters into a perfectly soda-free adularia and a potash-free
albite. The potash, silica, alumina, and lime of the adularia were dissolved and
carried away, leaving the albite; in some cases the albite substance seems to have
been concentrated. Bischof suggests d that in some cases part of the adularia
has been transformed into albite by replacement. Also in localities in the
Riesengebirge in Austria small fresh albite crystals were observed in several
cases upon altered orthoclase, which was in part altered to muscovite/ Bischof
and Rose agree that the explanation of this is that the soda-feldspar has been
abstracted while the potash feldspar remains. Bischof remarks, " Such opposite
effects-'' presuppose beyond question, if not opposite, certainly different causes, i. e.,
different substances in solution in the waters."
a BecKer. G. F., Mon., U. 8. Geol. Survey, vol. 3, p. 162.
'- 1 i.-nlil. O. L., Review In Am. Geol., Apr., 1903, p. 264.
oBlnchof, Gustav, Chemische Geologic, vol. 2, p. 409.
dOp. clt., p. 412.
Op. cit., pp. 406, 407, 412.
/That In, In one case the adularia molecule was dissolved out, the albite molecule being insoluble; in the other the
albite molecule was diwolved out, while the adularia molecule was insoluble.
ALTERATION OF THE EABLIER ANDESITE. 213
Bischof" notes that albite occurs in quartz veins in gneiss in Sweden, and
F. A. Genth described it in pyritiferous gold quartz veins in California, 6 and it
has been noted as a common occurrence by subsequent observers/
It seems to the writer to be unquestionable that waters that deposit albite
without orthoclase in a vein are different from those which deposit orthoclase
without albite, and that the difference must consist in part in the relatively
greater quantity of soda in the waters in the first case -and of potash in the
second. The many observed instances in the earlier andesite at Tonopah of
complete pseudomorphs of adularia, quartz, sericite, etc., after soda-lime feldspars
show a process of replacement (not leaching and concentration), the soda and lime
being removed and potash and silica introduced. The waters which accomplished
these changes thus must have had abundant potash as well as silica in solution.
STUDY OF TYPICAL SPECIMENS.
MICROSCOPIC DESCRIPTIONS.
For the purpose of estimating more accurately the changes which have been
described as observed microscopically, a number of analyses were made and studied.
The specimens selected, arranged in their natui'al order, were as follows:
1. Earlier andesite (4O8) from Imoer part of Siebert shaft. Dense dark -green
rock, Siebert shaft, Mizpah mine, 670 feet from surface. Contains scattered
phenocrysts of rather small size in a fine microlitic groundmass, showing flow
structure. The microlites in the groundmass are chiefly feldspar. A little zircon
and apatite are present. Quartz grains also occur, of which some may be original.
Among the phenocrysts the feldspars are prominent. A determination in
another similar specimen near the same locality showed the species to be andesine-
oligoclase. They are largely altered to calcite with a little quartz. Abundant
pseudomorphs after hornblende, in which no trace of the original mineral remains,
consist of dark blue-green chlorite (ripidolite) with some specular iron. The
hornblende cleavage is still visible in the pseudomorphs. Pseudomorphs after
biotite consist of fine muscovite, with a little calcite and hematite.
2. Earlier andesite (358) from.Tonopah and California shaft. Green, but much
lighter than No. 1. Shows relatively sparse and small phenocrysts in a fine
microlitic groundmass, with much felty devitrified glass. Apatite is abundant.
Secondary chlorite occurs throughout the groundmass.
The feldspar phenocrysts have the optical characters of andesine, and are
only slightly attacked by decomposition. The ferromagnesian minerals are
Bischof, Gustav, Chemische Geologic, vol. 2, p. 412.
i> Genth, F A., Am. Jour. Sci., 2d series, vol. 28, p. 249.
c Ransome, F. L., Description of Mother Lode district: Geologic Atlas U. S., folio 63, V. S. Geol. Survey, 1900, p. 8.
214 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
entirely altered; pseudomorphs of chlorite after hornblende can be distinguished.
Numerous amygdule-like portions are lined with chlorite and filled with granular
quartz.
3. Earlier andesite (293) from Fraction No. 2 shaft, at depth of 218 feet.
Purple rock with v T hite feldspar phenocrysts. Phenocrysts rather abundant, but
relatively small, the feldspars being the largest. The groundmass is glassy and
microlitic, with flow structure. There is abundant magnetite, frequent apatite,
and occasional zircon.
The feldspars were determined as andesine; they are only partly altered
to fine muscovite. Pseudomorphs after original ferromagnesian minerals are
abundant, though small; biotite, pyroxene, and hornblende can be distinguished,
though no traces of the fresh minerals are left. The biotite has altered to
muscovite, with a small amount of siderite scattered through, and hematite
forming a zone around the edge. Rutile cr sagenite needles are included in the
biotite. Pseudomorphs after hornblende are of sericite or talc, with inclusions and
heavy rims of magnetite. Pseudomorphs after pyroxene or biotite are of quartz,
with a little calcite and hematite around the borders. Other pseudomorphs, which
are probabty after hornblende, but may be in part after pyroxene, consist of quartz
and sericite.
4- Earlier andesite (53) from near Mizpah Hill. Pale pinkish-purple ground-
mass, with white phenocrysts. This shows what was originally a microlitic glass}'
groundmass, now containing abundant secondary quartz and sericite, with dissemi-
nated fine limonite, hematite, and siderite. Pseudomorphs after biotite phenocrysts
are of muscovite, with a very little siderite. Other phenocrysts, possibly of
hornblende, are represented by pseudomorphs of quartz, sericite, and a little
siderite. Abundant pseudomorphs after feldspar are of clear, translucent mate-
rial, which appears isotropic, but which high magnification often resolves into a
fine aggregate, the grains of which may sometimes be made out as spherulitic.
This substance has a very low double refraction and also a low single refraction,
but the latter is apparently higher than that of balsam."
5. Earlier andexite (194) from Mispah mine, lease 86, 180-foot level, near Mizpah
vein. Rock of a light salmon-pink color. Shows several phenocrysts of feldspar,
whiter than the rest of the rock. No original mineral is left anywhere. The
groundmass, of which the fine microlitic glass} 7 composition and fluidal structure
o Some of this material was Isolated and analyzed by Mr. George Steiger, showing 62.1 per c ent SiO, 19 per cent A1 S O 3 .
and 4.8 per cent K 2 O. Sodium was absent. These figures correspond to a composition of about 28. 4 percent adularia, 30 per
cent kaolin, and 27.5 per cent silica. Water, probably contained in the kaolin and the silica, was not determined, and
was disregarded In the computations. The substance is therefore probably to be regarded as a colloidal mixture of these
three alteration products of the original soda-lime feldspar, In nearly equal parts. As bearing upon the change which
this feldspar has undergone, the proportions of the different constituents in rather siliceous nndesinc, such as we may
believe, from examination of fresher rock, that this altered feldspar originally wns, are given: SlOj, 60.36; A1 Z O 3 , 25.45;
CaO, 5.14; NajO, 7.63; K S O, 1.21. The change evidently has consisted mainly in a removal of the soda and lime, and a
substitution in part of exogenetlc potash.
ALTERATION OF THE EARLIER ANDES1TE. ' 215
may be distinguished, is altered to an aggregate of quartz and sericite, with a
little iron oxide. The pseudomorph.s after phenocrysts are frequent and well
denned. Numerous ones after feldspar form an aggregate of fine felty muscovite,
with a little quartz. Those after biotite consist of muscovite, with a little siderite.
Pseudomorphs after hornblende or pyroxene, or both, are barely distinguishable
from the groundmass. They consist of fine muscovite (sericite) and quartz, with
some siderite, which marks the outlines of the original phenocrysts. In this rock
the secondary quartz varies in grain, some areas becoming more coarsely crystalline.
6. Typical earlier andesite (398) from Mizpah Hill. Hard white rock with
small glistening feldspar crystals. This rock has a microlitic groundmass, show-
ing flow structure. It has the appearance of being unusually fresh, and fresh
striated feldspar can be seen in it. Nests of fine granular adularia and quartz
(both secondary) occur here and there. There is a little finely disseminated
siderite and limonite.
The feldspars are mostly altered to adularia. The original mineral has the
optical characters of an oligoclase, near andesine. The alteration of this to adu-
laria can be seen in all its stages. Polarized light brings out this change strongly,
the bright white of the soda-lime feldspar contrasting with the dark gray of the
potash feldspar. The latter penetrates the former irregularly and minutely, yet
with a fairly high power the characteristic crystal outlines (usually rhombic) of
the adularia can be distinguished. The process can be observed in all its stages
in different crystals, up to the complete pseudomorph. A little sericite accom-
panies the alteration in some cases. Traces of original ferromagnesian pheno-
crysts can be determined, but with difficulty. In one case a pseudomorph after
probable hornblende was of sericite, with apparently a little adularia and traces
of siderite.
7. Earlier andesite (143) from hanging wall of Mizpah vein, 300-foot level,
Mizpah mine. Light gray, nearly white rock, with uneven fracture and dull
luster.
This rock is so much altered as to be hardly recognizable. It consists of an
aggregate of quartz and fine muscovite, with small scattered pseudomorphs of
hematite after pyrite (the result of oxidation), and some siderite (?). The quartz
is irregular and is segregated throughout into areas and little veinlets, which are
of coarser grain than the quartz of the less altered rocks, while the muscovite is
apparently finer than usual. Original phenocrysts of feldspar are indicated by
pseudomorphou > areas characterized by different groupings of the quartz and
muscovite and freedom from iron, while others of ferromagnesian minerals are
marked by similar differences of grouping and by a relatively greater abundance
of the iron minerals. In all cases, however, the decomposition products are
similar. In many areas also the vestiges of the phenocrysts have been effaced.
216
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
8. Ore matei'ial (152) of Mizpah vein, 300-foot level, west drift. Shows in the
hand specimen dense quartz, intermixed irregularly with apparently kaolinic
material.
The microscope shows tine to moderately coarse granular and retiform quartz,
with much fine muscovite. The quartz contains inclusions. Intermixed with the
quartz in the finer-grained areas is adularia in characteristic rhombic-sectioned
crystals.
ANALYSES OF DESCRIBED TYPES.
Following are the analyses of these rocks by Mr. George Steiger:
Analyses of different phases of altered earlier andesite.
i.
2.
3.
4.
5.
6.
7.
8.
SiO 2
55.60
58.47
60.45
71.14
72.98
73.50
76 25
91 40
AI.O, .
16.70
16.85
17.78
15.24
14.66
14.13
12.84
4 31
Fe,0,
2.23
2.04
5.86
1.77
1.01
1 51
54
77
FeO .
3.51
3.12
.25
.26
.16
.26
33
11
MgO
2.60
3.84
1.55
.16
.33
.21
.56
18
CaO
4.27
1.35
1.04
.09
.18
.12
. 16
Na,O
4.08
4.30
3.58
.24
None.
.24
. 12
06
K.O..
3.17
3.14
2.11
6.31
6.03
5.11
3.20
1.68
H 2 O
.88
1.10
2.86
.85
.97
1.07
2 14
46
H,O+
3.06
3.59
2.93
2.87
2.95
2.81
3.17
98
TiO,
.72
.77
.81
.48
.44
.47
.37
.07
ZrOj
Undet.
.02
CO, .
2.76
.52
None.
None.
None.
None.
None.
None.
P,O<
.28
.35
.28
.05
.16
.09
12
04
SO 3
None.
None.
None.
.05
.17
None
Cl
None.
F
.12
Trace.
S
None.
.02
.03
None.
FeS,
.49
.06
NiO
(a)
MnO
Undet.
.26
()
(a)
(a)
()
( a )
.06
BaO
.12
.11
.07
.17
(a)
.19
.02
SrO
(a)
(a)
()
(a)
(a)
(a)
()
99.98
100.42
99.63
99.70
99.87
99.91
99.80
100.16
1. Lower part of Siebert shaft.
2. Tonopah and California shaft.
3. Fraction No. 2 shaft.
4. Near Mizpah Hill.
5. Near Mizpah vein.
6. Mizpah Hill.
7. Wall of Mizpah vein.
8. Mizpah vein.
a Not looked for.
ALTERATION OF THE EARLIER ANDESITE. 217
DIFFERENCES OF PHASES EXPRESSED BY DIAGRAMS.
The changes in the proportions of the various elements in the rocks can be
illustrated by diagrams in such a way as to be clearer than discussion. In tig. 73 the
proportions are represented by straight lines. As is usual and more accurate, the
proportions plotted are the quotient figures obtained by dividing the weight per-
centages by the molecular weights. The scale is 0.01 in the quotient figure = one-
fortieth inch (fig. 73).
The diagrammatic lines representing the different elements may be grouped
together for each analysis, and be arranged as radii of a circle, with lines connecting
the ends of the radii to form an irregular, polygon, forming a diagram slightly
modified from that used by Brogger" (PI. XXIV).
STUDY OF ALTERATIONS INDICATED BY ANALYSES.
ALTERATION OF EARLIER ANDESITE FROM LOWER PART OF SIEBERT SHAFT.
The proportions of the different constituents represented by the diagrams of
rock No. 1 (PI. XXIV) are practically identical with those in a fresh andesite. That
this is so is shown by the diagram (), prepared in a similar way to those referred
to above, by Prof. W. H. Hobbs, to illustrate the typical composition of andesite. 6
The analysis upon which this diagram was based was obtained by averaging seven
analyses of mica and hornblende andesites from the Eureka district, Nevada; Ouster
County, Colo.; Cartagena, Spain; the Siebengebirge on the Rhine; Panama; and
Colombia. The scale of the diagram has been adjusted by the writer to correspond
with the scale of his own. From this general correspondence it becomes apparent
that the profound alteration which rock No. 1 has undergone has resulted in
decomposing the original minerals and changing the constituent elements to new
minerals more stable under the new conditions that is, in the presence of the
permeating waters.
a Hobbs, W. H., Jour. Geol., vol. 8, pp. 1-31. 6 Op. cit., p. 23.
218
GEOLOGY Oif TONOPAH MINING DISTRICT, NEVADA.
Silica
Si0 2
Potash
K,0
2 _
4 I
5
6
7 _
Alumina
AI 2 3
Iron oxides
FeOand
2
3
4 _
5 .
6 .
8 '.
Magnesia
MgO
i
a
4 ~
3 .
6 .
7 .
8 .
2 _
3 _
4 .
6 .
8'
Soda
Na 2
Lime
CaO
1
2 _
4 ~
6 .
Scale-.. 01 (quotient figure) "^5 inch
Fig. 73. Uiagrum to show changes in amounts of commoner elements during stages of alteration of earlier andeslte.
U. 8. GEOLOGICAL SUftVEV
PROFESSIONAL PAPER NO. 2 PL. XXIV
DIAGRAMS TO SHOW CHANGES IN COMPOSITION BROUGHT ABOUT BY ALTERATION OF THE EARLIER AN DESITE
ALTERATION OF THE EARLIER ANDESITE.
219
A similar conclusion is reached by comparing the analysis of the Tonopah
rock with analyses of Eureka and Washoe andesites. For the purpose of com-
parison, the following table is presented:
Comparison of Tonopah unth Waihoe and Eureka rocks.
1. Tonopah.
2. Average
type mica-
hornblende-
andesite.
Washoe rocks.
Eureka rocks.
3. Horn-
blende-mica
andesite,
Mount Rose.
4. Horn-
blende-mica
andesite,
Cross Spur
quarry.
5. Mica-
andesite,
east of Wal-
ler Defeat
shaft."
6. Andesite-
pearlite,
south of
Carbon
Ridge.
7. Pyroxene-
andesite,
Richmond
Mountain.*
SiO 2
55.60
16.70
2.23
3.51
2.60
4.27
4.08
3.17
3.94
2.76
62.16
16.45
3.27
2.71
2.20
4.13
4.07
3.45
1.15
63.30
17.81
3.42
.83
2.07
5.12
4.27
2.26
63.13
16.00
4.34
1.52
2.07
4.45
3.87
2.65
65.68
15.87
1.78
1.25
1.79
3.50
3.20
3.37
65.13
15>73
2.24
1.86
1.49
3.62
2.93
3.96
61.58
16.34
A 1,O,
Fe.O,
FeO
6.42
2.85
5.13
2.69
3.65
MgO
CaO
Na,O
K,O..
H 2 O
CO
98.86
99.59
oTheae are the designations (riven by Hague, Mon. U. S. Geol. Survey, vol. 20, p. 282. The designations previously
given by Becker, Mon. U. S. Geol. Survey, vol. 3, p. 152, are 3 and 4, later hornblende-andesite; 5, mica-diorite.
6 Mon. U. S. Geol. Survey, vol. 20, p. 264.
The difference between the sums of the first two analyses is largely accounted
for by the difference in titanium, of which the Tonopah rock contains 0.72 per
cent and the average rock 0.23 per cent. When these are added the sums are
99.58 and 99.82 respectively.
It will be seen that there is a remarkable similarity in the amounts of the
bases present in the first two analyses. In the Tonopah rock more of the iron
is in the ferric condition, but the amounts of iron in the two rocks are almost
identical.
In the altered Tonopah rock the percentage of silica is about 6 less than in the
average type, while that of water is 2f greater. The Tonopah rock contains 2f per
cent of carbonic acid, which is lacking in the average type. Thus the increase of
6 in the percentage of water and carbonic acid in the Tonopah rock offsets the
increase of 64 in the percentage of silica in the average type. Since free primary
quartz is apparently rare in all these rocks," the silica is combined with the bases to
form the silicates, feldspar, hornblende, and mica; and since the amounts of the
bases are equal in both analyses, the original amount of silica was probably nearly
a For the Eureka type see Arnold Hague, Mon. U. S. Geol. Survey, vol. 20, p. 234.
220
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
the same; that is, the transformation of pyroxene, hornblende, and mica in the
Tonopah rock largely to calcite, chlorite, muscovite, and hematite was effected
without appreciable gain or loss of the bases, but some of the silica was abstracted,
its place being taken by water and carbonic acid, which entered into the decompo-
sition products mentioned." Therefore, since these waters abstracted instead of
precipitating silica, they were characterized by relative poverty in silica. They
were also carbonated. The lack of sulphur and sulphides in rock No. 1 also shows
the absence of sulphur combinations in the altering waters.
ALTERATION OF EARLIER ANDESITE FROM CALIFORNIA AND TONOPAH SHAFT.
Tonopah rock No. 2 and the average type may also be compared as to their
chief constituents:
Comparison of Tonapah rock No. 2 with average type.
Tonopah
rock No. a.
Average
type.
SiO 2
58.47
62.16
A1 2 0,
16 85
16 45
Fe 2 O s
2 04
3 27
FeO
3.12
2 71
MgO
3.84
2.20
CaO
1 35
4 13
Na,O . .
4.30
4 07
K 2 O
3.14
3 45
H 2 O
4.96
1.15
CO,
52
FeS 2
49
Total
99 08
99 59
The difference between the totals of these two analyses is again accounted
for mostly by the difference in titanium, the percentage of which in the Tonopah
rock is 0.77 and in the average rock 0.23. When these are added the total of the
Tonopah analysis is 99.85 and that of the average analysis 99.82.
The bases present correspond very closely, the only noticeable difference being
in the proportions of lime and magnesia. In the average type the percentage of
lime is three times as much, or 2.78 more, and that of magnesia is somewhat
more than half as much, or 1.64 less. If the lime and magnesia in each rock
are added together the percentage of these constituents is only 1.12 greater in
"It 1 only the water given off above 100 C. (H z O-(- in the analyses, p. 27) which can be considered chemically
combined. The rest(HaO-) is probably mainly hygroscopic, mechanically contained. In the average analysis com-
pared, aa well as the Washoe und Eureka analyses, however, this distinction is not made. Therefore all the water in
the Tonopah rock* Is considered together in comparing with these analyses, and the hygroscopic water in one is -up
posed to be offset by that in the other. Most of the water in the Tonopah analyses, it will be seen, is chemically
combined.
ALTERATION OF THK EARLIER ANDESITE.
221
the average type. This change is probably due to the alteration of the horn-
blende to chlorite, the lime being in part carried out of the specimen instead of
being entirely precipitated in place as carbonate, its place being taken by
magnesia. These changes are, however, mainly compensating, and probably
indicate a local rather than a widespread interchange. Apart from this the
correspondence of the bases is close. In the average fresh type, however, the
percentage of silica is 3.69 greater than in the Tonopah rock, and that of
water is 3.80 less, while the Tonopah rock contains 0.52 per cent carbonic acid.
The conclusion is the same as in comparing the first Tonopah rock, that in this
second specimen there is an increase of over 4 in the percentage of water and
carbonic acid, which has entered into the composition of chlorite and calcite,
while this gain has been compensated by a decrease of 3.69 in the percentage
of silica. The process of alteration, while not quite so far advanced, is similar
to that in rock No. 1, except that the lime has been abstracted and com-
pensated for by an increase in magnesia. The presence of sulphur in the
waters is indicated by the relatively small amount of iron oxide which has been
changed to pyrite, a change which did not take place in rock No. 1. The carbonic
acid present is only a fifth of that in rock No. 1, showing that in the case of rock
No. 2 the conditions were favorable to the acid acting as a solvent and trans-
porting the lime from the rock, rather than as a precipitant and entering into
the rock's composition. A poverty in lime in the circulating waters is the
apparent explanation.
ALTERATION OF EARLIER ANUKSITE FROM FRACTION NO. 2 SHAFT.
The comparison between rock No. 3 and the average fresh type may be
made as follows:
Comparison of Tonapah rock No. 3 with average type.
Tonopah
rock No. 3.
Average
type.
Si0 2
60.45
62. 16
Al 2 Oj
17 78
16 45
Fe 2 O 3
5.86
3.27
FeO
25
2 71
MgO . .
1.55
2.20
CaO
1.04
4.13
Na. 2 O
3 58
4 07
K.O..
2.11
3.45
H,O
5 79
1 15
CO,
None.
FeS,
.06
Total
98.47
99.59
222 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
As in the two former comparisons, the difference in the titanium determined
accounts for most of the difference between the totals of these analyses.
In this case the bases have been more plainly affected than in the first two.
The most noticeable change is, as before (in rock No. 2), the abstraction of lime,
whicli seems to have been carried farther than in rock No. 2. Yet in this case
the loss has not been compensated for by the deposition of magnesia which has
itself been abstracted, though not in so great degree so that the combined amount
of lime and magnesia in rock No. 3 is less than half what it is in the average
type. Simila-ly the alkalies have been extracted; the potash more than the soda.
The iron has become more oxidized, but its bulk remains the same. The proportion
of alumina has slightly increased, perhaps owing to the loss of weight of the rock,
caused by the removal of more material than was brought to replace it. In all,
the percentages of lime, magnesia, and the alkalies are 5.57 less in this rock than
in the average fresh type. There is also less silica, but the difference in percentage
is by no means so great as it was in rocks No. 1 and No. 2, being only 1.71.
In the Tonopah rock (No. 3) the percentage of water is 4.64 greater than in the
average type, carbonic acid is absent, and there is a very small amount of iron
sulphide. In this case, therefore, the gain in water, carbonic acid, etc., is by no
means offset by the loss of silica. The chief loss is plainly lime, magnesia, and
the alkalies, more particularly lime and next to that potash. In this case the
waters have extracted silica to a very slight extent only, and were therefore
solutions whose silica capacity was more nearly satisfied than in the case of rocks
1 and 2. The tendency to dissolve and carry away lime displayed in No. 2 was
more vigorous in this rock, and the same action was displayed in regard to magnesia
and the alkalies.
ROCK ALTERATION.
ALTERATION OF EARLIER ANDESITE FROM NEAR MIZPAH HILL.
Rock No. 4 may be compared with the average type thus:
Comparison of Tonopah rock No. 4 with average type.
223
Tonopah
rock No. 4.
Average
type.
SiO 2
71. 14
62.16
A1 2 O 3
15.24
16.45
Fe,O,
1.77
3.27
FeO
.26
2.71
MgO
.16
2.20
CaO
.09
4.13
Na.,0
.24
4.07
K..O
6.31
3.45
H 2 O..
3.72
1.15
CO, .
None.
Total
98.93
99.59
In this rock, as in rock No. 3, the removal of magnesia, iron, and soda
has gone on till only trifling quantities remain. In this rock also, the iron, which
was relatively free from attack in the first three specimens, has been partly dis-
solved, so that over half has been removed. Even the difficultly soluble alumina
has apparently lost a little, though this is doubtful. Of the metallic bases, iron,
lime, magnesia, and alumina, about 40" per cent has been removed, and of the
same, excluding alumina, about 73 per cent. On the other hand, while soda has
diminished, the amount of potash has increased, the increase of one nearly com-
pensating for the loss of the other. The silica also has increased largely.
In this case, then, the waters which altered the rock were charged with an
excess of silica and potash, which they deposited, attacking and dissolving all
the other components of the rock, the relative order of attack, dependent on
their solubility in the attacking waters, being lime, magnesia, soda, iron, and
alumina.
a In this case, as in many of the similar cases in the following pages, the percentages given are in terms of each
constituent. The reader will notice, however, that the percentages are elsewhere given in terms of the entire rock,
where such presentation has best lent itself to statement. This is the case with all of the figures on the preceding pages
and some in those which follow. The writer believes there will be no confusion brought about by the use of these two
methods of presentation; if any such should arise, a glance at the compared analyses will suffice for an explanation.
224
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
ALTERATION OF EARLIER ANDESITE FROM NEAR MIZPAH VEIN.
The alteration of No. 5 may be compared as follows:
Comparison of Tonopah rock No. 5 with arerage type.
Tonopah
rock No. 5.
Average
type.
SiO 2
72.98
62 16
AL,O S
14 66
16 45
Fe 2 O 3
1 01
3 27
FeO
. 16
2 71
MgO
.33
2 20
CaO
18
4 13
Na 2 O
None
4 07
K 2 O
6.03
3 45
H 2 O
3.92
1.15
Total
99 27
99 59
Here the same processes have been carried on as in rock No. 4, but more
thoroughly. As in No. 4, only tiny amounts of the magnesia and the lime are
left, while the soda has entirely disappeared. The removal of the more refractory
constituents alumina and iron has apparently proceeded farther than in No. 4.
Of the iron 80 per cent has been removed, against 70 per cent in No. 4; of the
alumina about 11 per cent, as compared with about 7 per cent in No. 4. On the
other hand, the silica has increased 17 per cent, as against 14 per cent in No. 4.
But the potash, while still showing an increase of 75 per cent over the normal
proportion in the type analysis, is somewhat less than in No. 4. It appears from
this (in conjunction with the succeeding analyses) that the increased activity of
the altering solutions, as indicated in the above figures, has begun to attack
some of the introduced potash and to replace it by silica, or, perhaps, rather
that the balance is more in favor of strong silicification than of the introduction
of potash.
ROCK ALTERATION.
225
ALTERATION OF TYPICAL EARLIER ANDESITE FROM MIZPAH HILL.
The relations of No. 6 are as follows:
Comparison of Tonopah rock No. 6 with arerage type.
Tonopah
rock No. 6.
Average
type.
SiO 2
73.50
62.16
A 1,O,..
14.13
16.45
Fe,O,
1.51
3.27
FeO
.26
2.71
MgO
.21
2.20
CaO
.12
4.13
Na 2 O
.24
4.07
K 2 O
5.11
3.45
H 2 O
3.88
1.15
Total
98.96
99.59
This shows the characteristic alteration of No. 5. with some further advances.
As in Nos. 4 and 5, the magnesia, lime, and soda are almost entirely eliminated.
The alumina is further reduced than in No. 5, 14 per cent of it having apparently
been abstracted, while the iron is slightly stronger. The decrease of the
excessive potash to make room for the increasing silica noted in No. 5 is here
carried further, No. 6 containing 0.92 per cent less potash than No. 5, and 0.52 per
cent more silica (in proportion of the whole rock composition).
ALTERATION OF EARLIER ANDESITE FROM WALL OF MIZPAH VEIN.
No. 7 may be compared as follows:
Comparison of Tonopah rock No. 7 with average type.
Tonopah
rock No. 7.
Average
type.
SiOj
76.25
62.16
A1 2 O 3
12.84
16.45
Fe,O,
.54
3.27
FeO
.33
2.71
MeO
.56
2 20
CaO ....
.16
4.13
Na 2 O
.12
4.07
K 2 O
3.20
3.45
H 2 O
5.31
1.15
Total
99.31
99.59
16843 No. 4205 15
226
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
This is an intensification of the alteration shown in the immediately preceding
analyses. The lime, magnesia, and soda are reduced to trifling quantities. The
refractory alumina and iron are further reduced than before, 22 per cent of the
alumina and 85 per cent of the iron having been removed. The substitution of
silica for potash (as well as the other elements) has made marked progress, the
percentage of potash being 1.91 less than in No. 6, and that of silica being 2.75
per cent more. In this way the excessive potash, caused in some of the preceding
cases by introduction of this element by the circulating waters, is here again
brought down to the original quantity.
ALTERATION OF EARLIER ANDESITE TO VEIN MATERIAL.
Rock No. 8 may be compared as follows:
Comparison of Tonopah rock No. 8 with average type.
Tonopah
rock No. 8.
Average
type.
SiO 2
91.40
62.16
A1 2 O 3
4.31
16.45
Fe,O,
.77
3.27
FeO
.11
2.71
MgO
.18
2.20
CaO
None.
4.13
Na 2 O
.16
4.07
K 2 O
1.68
3.45
H 2
1.44
1.15
Total
99.95
99.59
This shows the further continuation of the changes indicated in the preceding
analyses, the alumina and potash being gradually removed to make way for the
increasing silica.
MAXIMUM ALTERATION LOCATED ALONG THE VEIN ZONES.
The specimens thus examined, selected as being fairly well representative,
show an increasing intensity of alteration, beginning with only a slight modifi-
cation of the constituents of the decomposed rock and terminating with the
intense silicification which reaches its maximum in the quartz mineral-bearing
vein of the district. Considering the alteration from the standpoint of the altering
waters rather than the altered rocks, the order in this transition series is reversed,
for these changes have been brought about by solutions which circulated along
the fracture zones which are now largely transformed into veins, and penetrated
the adjoining rock so thoroughly that no even moderately fresh representative
ALTERATION OF THE EARLIER ANDESITE. 227
of this earlier andesite has as yet been found in Tonopah. The last stage of
alteration in the rock (in the vein zones) was then in a sense the first work of
the waters, and the first stage, remote from the main circulation zones, the last;
and although the transition as studied is gradual, it by no means follows that
the rock near the veins went through all of the stages represented, but may
have reached its present condition much more directly.
COMPOSITION OF MINERALIZING WATERS IN THE VEIN ZONES.
In the unoxidized quartz veins the predominating gangue mineral is quartz,
with frequent adularia, subordinate muscovite (sericite), and comparatively rare
carbonates of lime, magnesia, manganese, and iron. The metallic minerals are
most prominently silver sulphide, containing sometimes antimony, arsenic, etc.;
silver selenide, gold in some form, copper-iron sulphide (chalcopyrite), iron
sulphide, and probably silver chloride. The mineralizing waters were then charged
with an excess of silica, and also probably, as the comparative analyses indicate, of
potash, together with silver, gold, antimony, arsenic, copper, lead, zinc, selenium,
etc. They were noticeably deficient in iron, since they have removed this metal
from the vein zones and the adjacent rock, more and more completely in propor-
tion as their work has been thorough, and the iron left in the veins is clearly a
residuum. That they contained carbonic acid and sulphur is shown by the for-
mation of sulphides and carbonates, not only in the veins but in the altered rock.
That they contained some chlorine and fluorine, though not in excessive amounts,
is indicated by the presence of a little original silver chloride and by their work
in forming muscovite, as will presently be explained.
In the vein zone the maximum effect of these waters was a replacement of
nearly everything by precipitated silica. By a similar process of replacement
the sulphides, of which silver sulphide was the most prominent, were precipitated,
and the residue of the comparatively refractory iron was combined with free
sulphur to form pyrite. The residue of the comparatively refractor}- alumina
combined with the excessive silica and potash of the waters to form adularia
and muscovite (sericite).
RELATION OF ADULARIA TO SERICITE AS ALTERATION PRODUCTS.
It is necessary at the present point of the inquiry to investigate the conditions
of formation of adularia and of muscovite. Both are silicates of aluminum and
potassium, and both are conspicuous as secondary products in the altered andesite,
especially of the feldspar. The typical andesine-oligoclase alters sometimes to
adularia, sometimes to quartz and muscovite, sometimes to both. That one of these
products is not the alteration product of the other is shown by the fact that they
228 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
are often intercrystallized, each mineral being perfectly fresh. That, however,
they depend upon slightly different conditions for their formation is indicated by
the fact that some profoundly altered specimens show the feldspar almost entirely
altered to adularia without muscovite, while others show complete alteration to
quartz and sericite without adularia. Adularia requires more silica than musco-
vite, but its formation in preference to the latter does not necessarily depend on
this fact, for when muscovite is formed in these rocks an amount of free quartz
is separated out equivalent to the quantity which would have gone into the adu-
laria, as is shown by the analyses of rocks 5, 6, and 7, of which 5 and 7 are altered
chiefly to quartz and sericite, and 6 chiefly to adularia. This difference is not
shown in any way by the bulk analysis of the rocks, the relation of the elements
harmonizing in the two cases.
FORMATION AND OCCURRENCE OF ADULARIA.
CONDITIONS REQUIRED FOB THE FORMATION OK ADl'LARIA.
Adularia is a variety of orthoclase, which is a silicate of alum'num and potas-
sium. It is distinguished from ordinary orthoclase chemical^ by being nearly
pure," while ordinary orthoclase contains a variable and often large amount of
soda. Crystallographically adularia has usually an entirely different habit from
ordinary orthoclase, and this crystallographic difference is apparently controlled
by the difference in chemical composition. While ordinary orthoclase is one of
the commonest primary minerals in igneous rocks, especially in the more siliceous
varieties, the writer is not aware of adularia occurring in this way. On the other
hand it is known as a secondary mineral in metamorphosed rocks and in veins.
Still, experimental investigations do not seem to show any essential difference in
the conditions of formation.
Orthoclase, muscovite, and quartz are all minerals which have not yet been
artitically reproduced by the cooling of dry melts, in spite of many careful attempts.*
All these may, however, be formed in the presence of such agents as water, chlorides,
fluorides, boron compounds, tungstic acid, etc., without which they apparently can
not crystallize. These agents, so potent in the formation of minerals, but entering
into their composition slightly or not at all, are called "mineralizers" (agents
mineralisateurs).
Friedel and Sarasin heated a mixture of potassium carbonate, alumina, silica,
and water in a platinum-lined iron tube to about 500 C., for fourteen to thirty -eight
hours, and obtained tiny quartz crystals and rhomboidal tablets of feldspar. Similar
more abundant feldspar crystals were obtained by heating aluminum chloride,
" It usually contains, however, a little soda, lime, etc.
* Vogt, J. H. L., Mincralbildung in Silikatschmelzlosungen, p. 6.
ALTERATION OF THE EARLIER ANDESITE. 229
potassium silicate, a little potassium carbonate and water. Analysis showed that
this mineral had the composition of adularia mixed with a little quartz. The
feldspar crystals were sometimes of the ordinary orthoclase habit, and sometimes of
the adularia habit." The same investigators obtained orthoclase crystals by heating
potash, silica, and muscovite in water in the same apparatus as mentioned above,
and at the same temperature.
Calcite, in rhombohedral crystals, it may be remarked,* was also obtained
under similar conditions (temperature 500 C.) by heating precipitated calcium
carbonate and calcium chloride with water for ten hours.
ADULARIA AS A META.MORPHIf MINERAL.
Apart from the primary orthoclase in igneous rocks, secondary orthoclase,
due beyond question to attenuated watery solutions, distinct in every way from
rock magmas, has been often described as occurring in, nature. Van Hise* 1 showed
that clastic grains of orthoclase in sandstones on the north shore of Lake Huron
had been enlarged by a secondary similarly oriented growth. In St. Gotthard,
in the Alps, little druses in a fine-granular quartz-albite rock contain clear crystals
of adularia intercrystallized with calcite, both of which are younger than the
constituents of the rock. In some cases the adularia is provedly younger than
the calcite, and in one case it incloses older calcite and chlorite both water-
formed minerals showing that the feldspar originated as a precipitate from
solution. a In Chester County, Pa., orthoclase occurs in dolomite, indicating
that no intense heat was present at its formation. d In the metamorphosed zones
near the contact of intrusive igneous rocks it is frequent, as was shown by
Allport, and later by Teall," to be the case in altered lower Silurian slates in
England, and by Lessen in the Harz.
ADULARIA IS VEINS.
Adularia as a gangue mineral in veins has also been described a number of
times. In a vein in the Herzog Ulrich mine at Kongsberg, in Norway, Hausmann*
found adularia with quartz, pyrite, and dolomite. In veins in Schenmitz, in
Hungary, Wiser* found crystalline adularia associated with quartz, dolomite, pyrite,
chalcopyrite, blende, and gold. In the Lake Superior copper mines orthoclase
occurs in veins, associated with calcite and native copper; the feldspar, like the
other minerals, is plainly formed in the wet way, and was deposited later than
the copper and the calcite. Adularia occurs also in several places of special
a Bull. Soc. francaise de min., vol. 4. 1881, pp 171-175. Chemisches Centralblatt, 1892, vol. 1, p. 865.
bin connection with the occurrence of calcite and adularia in the same veins at Tonopah.
eCited by Zirkel. Lehrbuch d. Petrographie, vol. 1, p. 243.
d Bischof, Gustav, Chemische Geologie, vol. 2. p. 401.
'Cited by Bischof, Chemische Geologie, vol. 2. pp. 898-399.
230 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
interest because of their geographic and geologic relations to the Tonopah district.
It is found sparingly in the Apollo vein, Unga Island, Alaska (adularia or ortho-
clase). It has been described from the Valenciana silver mine, in the State of
Guanajuato, Mexico, where it was called valencianite. Lindgren has described
it as a common gangue mineral in the Silver City, Idaho, veins (see p. 272). These
ores are probably post-Miocene, and Mr. Lindgren gives reasons for considering
that the deepest ore bodies were formed at a distance of 700 to 2,000 feet below
the original surface. He therefore considers that the temperature at the time the
vein was formed can hardly have exceeded 100 C. a
At Boulder Hot Springs, Montana, 6 are springs of a temperature varying
from 120 to 164 F/ which contain a slight amount of sulphureted hydrogen,
sodium chloride, soda sulphate, and carbonates of soda, lime, and magnesia.
The granite through which they rise is altered in the vicinity of the springs,
the most notable products being sericite and kaolinite, the result of the alteration
of feldspar and quartz. Calcite does not occur in the altered rock, and has
apparently been carried out of it by the altering waters into the fissures, where
it has been deposited. Veins which have formed in this granite contain chiefly
medium-grained quartz, calcite, and stilbite, and a little adularia. These veins
contain slight but perceptible amounts of gold and silver.
At Cripple Creek, Lindgren found that adularia has been formed in the
granite near the veins, together with sericite and chlorite. Within cavities
produced by the removal of the granite, iron pyrite, fluorite, and tellurides have
been deposited.*
CHEMISTRY OF THE ALTERATION OF SODA-LIME FELDSPAR TO ADULARIA.
The chemistry of the change from andesine-oligoclase to adularia seems to be
fully explained by the following statements of Bischof," in speaking of observed
cases where adularia was altered to albite:
"The unequal effect of water upon different mineral substances is mainly based
upon the fact that it holds materials in solution, which decompose one mineral but
not another. Sodium chloride decomposes potassium silicate, and potassium chloride
and sodium silicate are formed. Thus waters which hold sodium chloride can
decompose potash feldspar, while it leaves soda feldspar undecomposed.
"In this way it is possible that such water may either change potash feldspar to
soda feldspar, or that it may take up and remove the alteration products of the
former. We can realize then how water containing sodium chloride (and this is
a Lindgren, W., Twentieth Ann. Kept. U. 8. Geol. Survey, pt. 3, pp. 165-167.
6 Weed, W. H., Twenty-first Ann. Kept. U. 8. Geol. Survey, pt. 2, pp. 236-248.
e By personal communication with Mr. Weed the writer learns there is evidence that these springs reach the boiling
temperature not many feet below the surface.
<l Lindgren, W., Trans. Am. Inst. Min. Eng., vol. 33, p. 589
' Blue hot, Gustav, Chemlsche Geologic, vol. 2, p. 411.
FORMATION OF MUSCOVITE. 231
rarely absent in waters) breaks up the potash silicate in the adularia and removes the
new-formed soda silicate with the separated alumina silicate, while the sodium
silicate contained in the adularia, with the combined alumina silicate, remains as
albite.
" On the other hand, potassium carbonate decomposes sodium silicate. It is
therefore possible that water containing potassium carbonate may either transform
soda feldspar into potash feldspar or that the alteration products of the former
may be taken up and removed. Such water brings about the opposite of that in
the first case.'" a
This explanation corresponds with the conclusion as to the excess of potash
in the mineralizing waters, derived from a comparison of the rock analyses.
FORMATION AND OCCURRENCE OF MUSCOVITE.
CONDITIONS REQUIRED FOR THE FORMATION OF MUSCOVITE.
Muscovite, as previously noted, has never been formed artificially by cooling
from dry fusion. Concerning its formation, as well as that of other micas, Doelter
observes:*
"Mica results from heating aluminum silicate with potassium fluoride or mag-
nesium fluoride; the fluorides seem to assist on the one hand because the fluoric
vapors which form bring about the crystallization, and so play the same part as in
the transformation of amorphous alumina into corundum; on the other hand the
influence is also chemical, since small quantities of fluorine enter into the composition
of the mica."
Brauns remarks : c
"Any mica can be easily formed if one melts any mineral containing its
elements with any fluoride at a temperature below 800 C. ; for in higher temper-
atures the micas are not stable."
Of the micas, biotite or magnesia mica is found in many volcanic rocks, such
as rhyolites, dacites, and andesites, while muscovite is not; neither does muscovite
occur in the deeper seated igneous rocks save in granites, where it is common, rf
and generally occurs together with quartz and potash feldspar/ Evidently, then,
muscovite demands for its formation special conditions not present in lavas or
in ordinary rock magmas and different from those necessary for biotite.
MUSCOVITE AS AN ALTERATION PRODUCT.
Muscovite is common as a secondary mineral the alteration product of many
other minerals, such as feldspar, nepheline, leucite, etc. and in these cases is
evidently the result of the action of waters, probably heated. It is very abundant
"The italics are the writer's (J. E. S.).
& Doelter, C., Allgemeine Chemische Mineralogie, p. 161.
o Brauns, R., Chemische Mineralogie, p. 247.
dRosenbusch-Iddings, Microscopical Physiography of the Rock-making Minerals, p. 2W.
r Brauns, op. cit., p. 301.
232 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
in the metamorphic rocks, such as the crystalline schists. It forms pseudomorphs
after orthoclase in tin veins," where it is associated with cassiterite, tourmaline, and
quartz, and owes its origin plainly to the action of water and other mineralizers,
among them undoubtedly fluorine; near the veins the granite is entirely altered
to a mixture of quartz and muscovite by the same processes. Weed has described
it as produced in granite by the action of hot-spring waters in Montana.*
DISTINCT CONDITIONS REQUIRED FOR MUSCOVITE AND FOR BIOTITE.
While muscovite is the alteration product of so many minerals, it seems
itself not at all subject to ordinary alteration, but is characteristically fresh, even
in highly decomposed rocks.'' Here again it shows its distinction from biotite,
which in rocks traversed by waters is easily altered to chlorite, iron oxides and
carbonates, quartz, epidote, etc., showing that its conditions of formation were
different. In many granites, indeed, muscovite and biotite have been found side
by side, and in these rocks the conditions for the formation of the two coincide,
but on the one hand stands the range of biotite into the more basic igneous
rocks and the lavas where muscovite does not occur, and on the other is the range
of muscovite among the minerals formed by circulating waters, where biotite
does not ordinarily occur. Plainly, then, the average or ordinary conditions under
which biotite forms are more heat and less water than muscovite, in whose
formation the evidence of comparatively little heat and abundant water is often
conclusive; and the upper extreme of the muscovite range overlies the lower
extreme of the biotite range only in the granites, a fact which affords some insight
into the conditions of formation of this rock.
THE 8ERICITIC VARIETY OF MUSCOVITE.
The fine-grained muscovite (which is often the secondary product of other
minerals and occurs as fine fibrous aggregates) is called sericite. Sericite, however,
does not differ from muscovite, and has the same relation to the coarser variety
(between which and it transitional grades of coarseness are often observable) that
the tine secondary quartz has to the coarser grains. For that reason the author
uses the words muscovite and sericite interchangeably in referring to the tine-
grained variety.
FLUORINE NECESSARY TO THE FORMATION OF MICA.
Not only has the presence of fluorine been shown to be necessaiy for the artifi-
cial reproduction of mica, but fluorine enters into the composition of the mineral,
Iwing most abundant in the best crystallized varieties.** The sericitic variety, then,
nRosenbiuch-Iddings, Microscopical Physiography of the Rook-making Minerals, p. 286.
'' Weed, W. H., Twenty-first Ann. Kept. U. S. Oeol. Survey, pt. 2, p. 247.
<-Zlrkel, Lehrbuch d. Petrographie, vol. 1. p. 340.
rfBischof. (justav, Chemische Geologic, vol. 2, p. 79.
ALTERATION OF FELDSPAR TO SERICITE. 233
m&y be assumed to have crystallized in the presence of a less potent amount
of fluorine, and indeed the analyses given by Dana" do not show any fluorine, while
the analyses given for ordinary rnuscovite sometimes do and sometimes do not show
it. To determine its presence in the altered Tonopah andesite, two tests for it were
made, in No. 2 and No. 8 (pp. 213, 216). No. 2 showed 0.12 per cent, No. 8 a trace.
No sericite was identified in No. 2, while No. 8 (the vein) contains it. The tests
therefore are not convincing as to the fluorine being contained in the mica, but
indicate its presence in the waters which altered the rock. No. 2, it may be noted,
now contains between three and four times as much water as No. 8.*
CHEMISTRY OF THE ALTERATION OF SODA-LIME FELDSPAR TO SERICITE.
The alteration of soda-lime feldspar by carbonated waters, according to
Rosenbusch, c may produce calcite, sericite, and quartz. If the former is carried
away by the permeating waters only quartz and sericite results, as in the case of
orthoclase. rf Where orthoclase is similarly altered, some potassium carbonate goes
into solution. Similarly Bischof ^ suggests, as an explanation for pseudomorphs
consisting largely of muscovite (sericite) after feldspar, such as he describes,
that part of the alkaline silicates of the feldspar was decomposed by carbonic
acid, their silica remaining and their alkalies being removed as carbonates; another
part of the silicate was removed as such; and the rest of the silicate went to
form the mica. In this way a mixture of mica and quartz originated.
The analyses of sericite pseudomorphs after feldspar, given by Bischof in
connection with his above-cited explanation, show in many cases the presence of
fluorine; whence the suggestion arises that though carbonic acid decomposes the
feldspar, it may still require the help of a small quantity of fluorine for the
decomposition products to crystallize as muscovite.
CHANGES IN RARER CONSTITUENTS DURING ALTERATION OF EARLIER
ANDESITE.
The evidence afforded by the rarer constituents of the rock is less trust-
worthy, on account of the small amounts present. The percentages of titanium,
barium, and phosphorus in the different rocks are represented in the diagram
forming fig. 74, the scale being ten times that employed for the commoner rock con-
stituents in fig. 73 (p. 218). It is here seen that the titanium behaves much like
the alumina, increasing with the increasing silica in the first three specimens, and
"System of Mineralogy, p. 618..
* Fluorine is abundant among the exhalations of cooling igneous rocks, is also found in many ordinary waters, in
spring waters, and even in sea water. (Bischof, Gustav, Chemische Geologic, vol. 2, pp. 86-89.)
c Elemente der Gesteinslehre, Stuttgart, 1898, pp. 70-71.
dThis change involves the substitution of potash for soda.
t Bischof, Gustav, Chemische Geologic, vol. 2, p. 743.
234 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
slowly decreasing with the increasing silica in the last. The phosphorus, though
present in still smaller amounts, behaves in much the same way, while the record
of the barium seems irregular. It appears, then, that even the resistant rutile
and apatite of the andesite were slowly attacked and in part dissolved by the
mineralizing waters. The amount of combined water in the different rocks does
not vary in any symmetrical way, and, indeed, remains nearly the same (about
3 per cent) except in No. 8. Carbonic acid was noted only in Nos. 1 and 2, but
microscopic analysis shows that siderite is usually present, often in very small
quantity, in most of the other rocks.
RESUME OF EFFECTS OF MINERALIZING WATERS.
The mineralizing waters, penetrating vigorously the rock on each side of
their main circulation channels, did not retain their metallic contents, which wei'e
all deposited in favorable places in the main
channels or in special lateral channels which
became lesser veins. However, they at-
tacked the rocks vigorously by virtue of
the carbonic acid, probably also sulphuric
acid, and perhaps to a less extent by the
acids of chlorine and fluorine. The ferro-
magnesian minerals were decomposed, the
lime and magnesia were taken into solution,
and the iron was mainly dissolved, but in
part was altered to iron sulphide by the sul-
phur in the waters. The feldspar was al-
tered, probably by potassium carbonate, to
adularia, or to sericite and quartz, the lime
and soda being taken into solution. Tocom-
8 pensate for these dissolved materials, silica
Scale:.OI(qyotientfigure)-iinch . , , , . , .
F,<,74.-Diagr a m showing relative proportions of the WaS deposited f ro m the highly charged
leas common elements in the various stages of altera- waters. So great W8S the llCCCSsitV of de-
tion ol the earlier andesite.
positing the silica that it probably takes the
place of part of the alumina, and also seems to have replaced even part of the
potash, though this is not certain. The waters, then, after passing through a rock
like No. 8, emerged poorer in silica and richer in all the other rock constituents.
On passing farther and traversing a rock like No. 7, the process was carried
further, though the excess of silica was not so great, and the capacity of the
solutions for the different rock materials became somewhat less. Hence the least
soluble, such as the alumina, was not so much dissolved, while lime, magnesia,
and soda were thoroughly extracted. On passing from rocks like 6, 5, and 4 the
CHANGES IN MINERALIZING WATERS. 235
process is continued with diminishing strength. Potash here is thrown down by
the waters, and its amount is greater than in the original rock. It might be
argued that in these rocks it may be a concentration, and that its percentage
increase is only apparent, and is due to its remaining constant while the volume
of the rock increases; but the decrease in the similar rocks 7 and 8 shows that
this can hardly apply. Again, it may appear that the increased potash in the
zone represented by 4, 5, and 6 was extracted from the zone represented by
7 and 8, and that the original waters did not necessarily contain much potash;
but the formation in the main vein zones of often large proportions of potash
minerals bespeaks an original large amount of this element, as noted on a
preceding page.
CHANGES IN WATERS AS A CONSEQUENCE OF ROCK ALTERATION.
The waters that traversed and altered this broad belt of rock," by the depo-
sition of silica and of potash, were themselves affected by the interchange and
emerged into the outer zones quite transformed. Rock No. 3 indicates that they
had no longer any excess of silica and that their solvent power was much
weaker. Still they dissolved part of the lime and magnesia in the rock, as well
as the alkalies, particularly potash. The fact that they dissolved potash shows
that they no longer contained an excess of this element. Rock No. 2, still
farther removed from the center of circulation, shows less change in the bases,
the alkalies being practically unaltered. The lime and magnesia have been
disturbed, but not to so great an extent as in No. 3. As in No. 3. much of
the lime has been extracted (though not so much as in No. 3), but while in No.
3 the magnesia also has been extracted, this constituent is relatively increased
in No. 4, and largely compensates for the loss of lime. Here, then, the
waters replaced some lime by magnesia and abstracted another part. The analysis
also indicates that some silica was abstracted. By this time, therefore, the waters
had so effectually precipitated the great excess of silica indicated by their first
effects (as, for example, in No. 8) that they were now able to take up fresh
silica from the rocks which they traversed instead of precipitating it. The
presence of carbonic acid and of sulphur is indicated by the pyrite and by the
analysis. The carbonic acid, though undoubtedly active as an agent in the altering
processes, was in the more highly altered types so hard pressed by the more
urgent silicification that it was free to form very little carbonate; but on the
aOn account of the small area of outcropping earlier andesite at Tonopah, the dimensions of these zones, such as
the zone ol siliciflcation, can not be given They are probably variable. The earlier andesite outcrops within the
limit of the map only on Mizpah Hill and Gold Hill, covering a maximum east-west extent of over 2.000 feet. Several
veins ouicrop in this distance, principally on Mizpah Hill. Nearly all of this andesite is siliciried in varying degrees,
the less altered specimens coming principally from underground workings in areas where the andesite does not
outcrop.
236 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
outer edge of the altered zone, as in No. 1, the case was different. Here calcite
was abundantly formed and, with abundant chlorite, makes up a good part of
the rock which now exhibits the typical "propylitic" alteration.
PROI'YLITIC ALTERATION OF EARLY ANDESITE.
Propylite was a name applied in 1867 by von Richthofen to certain early
Tertiary volcanic rocks of Nevada and California, especially to rocks observed near
the Comstock lode in Nevada. It was defined as being always porphyritic, and
very similar to porphyritic diorite, with oligoclase feldspars and dark-green fibrous
hornblendes, in a green groundmass which owes its color to small particles of fibrous
hornblende; as being very rich in mineral veins, and the earliest of the Tertiary
volcanic rocks. These definitions were accepted and new areas of propylite were
discovered by many prominent geologists. But Dr. G. F. Becker's work, published
in 1882, showed that the "propylites" near the Comstock were altered rocks
originally identical with fresh diorites, andesites, etc., from the same region; that
the characteristic supposed green fibrous hornblende was chlorite, a decomposition
product; and that this rock phase owed its association with mineral veins to the
altering mineral waters which produced the veins and this rock at the same time.
Other investigators have come to the same opinion, and the name propylite, as
signifying a rock type, has been dropped. It has, however, been sometimes used to
signify this especial form of alteration, and is in this sense characterized by Rosen-
busch as follows: 6
''The characteristic feature of the propylitic facies consists in the loss of the
glassy habit of the feldspars; in the chloritic alteration of the hornblende, biotite,
and pyroxene (often with an intermediate stage of uralite), with simultaneous
development of epidote; further, in alteration of the normal groundmass into
holocrystalline granular aggregates of feldspar, quartz, chlorite, epidote, and calcite,
and in a considerable development of sulphides (usually pyrite)."
Epidote has not been detected in the earlier andesite at Tonopah, and is rare in
the district in general; otherwise the rocks like 1 and 2 correspond to the "propy-
litic" phase. At the Comstock Becker c found epidote uncommon underground,
while abundant at the surface.
Mr. Waldemar Lindgren'' has considered gold and silver veins accompanied by
a "propylitic" alteration of the wall rock as a group, and has separated them from
another class (the sericitic and kaolimtic gold and silver veins) whose wall rocks
show characteristic alteration to sencite and kaolin. In a subsequent note he
remarks that "it is perhaps not advisable * * to retain the name propylitic
for the whole group, a.s some of them do not show alteration in typical form.''
a Mon. U. S. Geol. Survey, vol. , p 88, etc. ' Trans. Am. lust Mln. Eng., vol. 30. pp. 645-664. 668-666.
fcElemenlederGcstelnslehre, Stuttgart. 1898, p. 302. 'Ibid , vol. 33. p 798
o Mon. V. S. Geol. Survey, vol. 4, p 212.
ALTERATION OF THE EARLIER ANDESITE. 287
With this last conclusion the writer is in accord, for the Tonopah district seems to
show clearly that the distinctions between the two classes of veins are artificial, the
predominating alteration of the wall rock, whether to sericite and quartz, or to
chlorite, calcite, etc., depending not so much upon the original character of the
wall rock or the waters, as upon the abundance and intensity of the latter, and on the
size of the circulation channels; and in each case the vein materials may be the same.
The writer has already pointed out the close analogy of the Comstock and some
other districts to the Tonopah district; in some of the districts the one phase of
alteration is especially represented, in others the opposite extreme.
FINAL, COMPOSITION OF MINERALIZING WATERS.
The waters which accomplished the "propylitic" alteration of Nos. 1 and 2,
therefore, were capable by virtue of their carbonic acid, etc., of decomposing the
original minerals and forming new carbonated and hydrated minerals which were
more stable under the new conditions. They were not able to remove any large
quantities of the bases, with the exception of a slight amount of lime, magnesia,
and silica, and of the alkalies. The character of such waters would then be very
different from what it was when they were fresh from their channels of active
circulation. They were at first, if the reasoning is correct, highly charged with
silica and potash, with some carbonic acid and sulphur, and with silver and
gold and relatively small quantities of other metals. They would finally, as a
result of their interchange with the rocks which they have so profoundly altered,
be less highly charged with mineral substances and would contain soda largely in
excess of potash, important amounts of lime and magnesia, some iron, a little silica,
and a very little alumina; and at the best only traces of the rarer metals. The wall
rock in fact has, by its reactions with the mineralizing solutions, acted as a screen,
and has separated successively the different constituents from the waters. Similar
phenomena have been previously observed, and a chemico-physical explanation (the
hypothesis of osmotic action) has been offered." Dr. G. F. Becker remarks:
"On this hypothesis the concentration of ores in deposits would be largely due
to the fact of the lack of action between their solutions and the wall rocks; and the
decomposition of the country rock, so often observed near veins, would be due to
the absorption of solutions of gangue minerals by the walls. In short, there would
be a species of concentration by dialysis." 6
The writer's explanation, however, as indicated above, is of a purely chemical
character. He assumes that the ores of the veins did not penetrate far into
the wall rocks because they were all immediately precipitated in the main cir-
oBecker, G. P., Mineral Resources U. S. for 1892, D. S. Geol. Survey, p. 166; Eighteenth Ann. Kept. U. S. Geol. Survey,
pt. 3, p. 68. Lindgren, W., Trans. Am. Inst. Mln. Eng., vol. 30, p. 691.
i> Mineral Resources U. S. for 1892, U. S. Geol. Survey, p. 157.
238 . GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
dilation channels, just as the excessive silica did not penetrate to the "pro-
pylite" belt of the andesite because it was precipitated before it arrived there.
The evidence, elsewhere offered, that the veins themselves have formed chiefly
by replacement is plainly in favor of the writer's explanation.
If such changes take place within a space of a few hundred yards, more or
less, laterally from main circulation channels, they must take place also along
those channels upward (though they would require a much greater distance), for
such veins as those at Tonopah, where the channels were for the most part not
open fissures, but only zones of maximum fracturing in the rock, and the vein
formation involved intense replacement and interchange. When the waters which
accomplished this change emerged above they would be in the transformed condi-
tion described for the lateral moving waters emerging from the propylitic stage of
alteration that is, they would resemble the waters of many hot springs, or the hot
mine waters of the Comstock (see p. 212). It is not necessarily true that springs,
even hot springs, associated with mineral deposits have a composition similar to
that of the mineralizing waters. As the mineralized area is eroded the critical
area for mineralization will in many cases probably retreat lower down, and the
same interchange between water and rock will be effected at a lower level. When
such water reaches the surface, after flowing through and being again to some
degree affected by the ores and the altered rock (which were stable under the
conditions of original deposition, but now under different conditions are subject
to solution and redeposition), it will still contain the solutions resulting from the
mineralizing reactions, rather than those which accomplished the mineralization.
This may perhaps explain in part why, although the formation of veins by hot
springs has in man}' cases been pretty satisfactorily demonstrated, and many such
springs emerge at the surface at the boiling point or over, no satisfactory observa-
tion has as yet been made of such a spring depositing near its exit a definite and
typical vein.
AT/TERATION OF THE LATER ANDESITE.
The later andesite is not altered as much as the earlier andesite; it outcrops
over a much greater area, and is often found nearly fresh, save for the processes of
surface weathering, under which it disintegrates and decomposes easily. At many
places, both at the surface or underground, it is greatly decomposed. This alteration
is extremely irregular.
STUDY OF TYPICAL SPECIMENS.
Four analyses have been made to show the composition and alteration of the
later andesite. The rocks analyzed are described as follows:
1. Nearly fresh later andesite (225) from Mizpah Extension xhaft, 2^5 feet
down. Rock nearly black, dense, and basaltic looking. A very dark green dense
ALTERATION OF THE LATER ANDESITE. 239
groundiuass shows fresh crystals of feldspar and augite largely altered to
serpentine.
Under the microscope the groundmass is seen to be densely packed with
microlites of feldspar and augite partly altered in the same characteristic way as
the phenocrysts, which are to be next described. Magnetite is plentiful. Siderite
in small specks is scattered throughout in characteristic cloudy, semitransparent
white aggregates. Sometimes this mineral forms a rim around the magnetite,
showing derivation from it. In some cases there may be discerned characteristic
rhombic cleavage and even rhombic crystal outlines.
The phenocrysts vary in size from the microlites up to occasionally moderately
large crystals. They are of feldspar and colorless augite.
The feldspar is in general remarkably fresh. It is usually striated, and is
sometimes in complex forms. Two optical determinations by the Fouque method
showed, in one case andesine, in another labradorite. It is seamed and cracked,
and the cracks are filled with calcite and serpentine, evidently infiltration products.
In places the feldspathic substance is attacked and replaced by these minerals.
Idiomorphic colorless augite is abundant. Alteration to calcite and serpentine
is present in all stages, so that while some augite crystals are unattacked others are
completely transformed. Chlorite was not identified. Small apatite crystals were
noted as inclusions in the augitc.
2. Nearly fresh later andesite (SJfl) from Halifax shaft, 275 feet down.
Greenish rock, showing phenocrysts of glassy feldspar (altered along the outside),
greenish augite, and biotite.
Under the microscope the groundmass is glassy, with fine microlites of fresh
feldspar and augite, magnetite, micaceous hematite, and considerable cloudy kaolin.
Quartz (secondary?) is common.
The phenocrysts are relatively few. The feldspar is fresh, and one crystal
was determined as andesine. Sometimes it is altered to a cloudy white aggregate
of kaolin along its margin, and in one case a small crystal was completely altered to
calcite, kaolin, and quartz, the clear quartz forming an envelope for the rest of the
crystal. The fresh feldspar is cracked and infiltrated with micaceous hematite.
The augite is pale green; no alteration of it was noted.
Fresh brown biotite crystals sometimes have a border of magnetite.
3. Entirely altered later aiidesite (331) from North Star shaft, 305 feet down.
This has a general gray color, with dull-white altered feldspar phenocrysts; it
contains many small specks and seams of pyrite. Under the microscope it is seen
to be entirely altered. In the fine groundmass can be distinguished fine secondary
quartz and chalcedony, calcite, pyrite, siderite, and some zeolite needles.
The phenocrysts are also entirely altered. Pseudomorphs after biotite were
distinguished, consisting mainly of quartz and siderite. Numberless tiny crystals
240 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
are seen arranged in zones parallel to the rays of the pressure figure." These have
often the characteristic crystal form of siderite. They are translucent under high
powers, but under lower powers show in aggregate the white, cloudy appearance
characteristic of siderite. Between these siderite zones is quartz.
Pseudomorphs of calcite after pyroxene, with a few tin}' zeolite needles and
some siderite, were noted.
Pseudomorphs after feldspar consist of calcite and an aggregate of fibers
resembling in large part sericite, with some zeolite needles.
Pyrite and siderite are abundant, disseminated or in clusters. The siderite
frequently forms alteration rims around the pyrite. Aggregates of siderite some-
times show characteristic cleavage and even crystal outline.
Small smoky apatites occur in the pseudomorphs after biotite.
4- Entirely altered later andesite (219) from Montana Tonopah shaft. Type
for first 278 feet. Green pyritiferous rock, mottled with white feldspar pheno-
crysts, and with apparent kaolin coatings on joints.
Under the microscope the rock is seen to be entirely decomposed. The ground-
mass is a white, opaque aggregate containing quartz, some siderite, and much
cloudy material (which is very likely kaolin), with some chloritic material.
The feldspars are completely altered to pseudomorphs, made up of calcite
and a clear, colorless aggregate showing sometimes rather low interference colors,
while many fibers reach yellow, red, and even blue of the first order. The
individual grains are fine, and are often in the shape of vermicular strips, made of
fibers perpendicular to the long direction of the strips. Along these strips the
extinction is wavy, traveling from one end to the other, similar to the behavior
of spherulites. Also occasionally similar clear areas are nearly isotropic, low,
doubly refracting and faintly spherulitic, like the pseudomorphs after feldspar
described in specimen 53 (p. 214), where the material seems to be a kaolinic mixture.
Other areas are of low-refracting spherulitic material, resembling chalcedonic
silica.
Portions of this white pseudomorphous mixture, showing still the feldspar
cleavage, were separated from the rock, and were tested chemically by Mr. George
Steiger, of the United States Geological Survey. The calcite was leached out of
these pseudomorphs and the remainder was examined and found to contain, besides
considerable combined water, principally silica and alumina, with a small proportion
of magnesia, roughly estimated at about 4 or 5 per cent. The material therefore
appears to be a mixture of an aluminous mineral with some magnesian mineral,
probably talc, and with free silica.
aSee Rosenbusch-Iddlngs, Microscopical Physiography of the Rock-making Minerals, 2d. ed., p. 257.
ALTERATION OF THE LATER ANDESITE.
241
The optical characteristics above described indicate that the aluminous mineral
is probably largely hydrargillite, a while kaolin is also very likely present.
Abundant pseudomorphs after pyroxene consist chiefly of a pale green, very
faintly crystalline fibrous aggregate, which in part seems to be chlorite and in
part is certainly uralitic hornblende or actinolite.
The occasional biotite crystals are bleached and contain secondary quartz in
seams parallel with the cleavage.
To determine the character of the carbonates in this rock they were separated
and analyzed qualitatively. They were found to consist of an abundance of
siderite, though the larger part is calcite. No magnesium carbonate was present*
Analyses of described lypet of later andesite.
[Nos. 1 and 4 by Mr. George Steiger; Nos. 2 and 3 by Dr. W. F. Hillebrand.]
i.
2.
3.
4.
SiO 2
57.51
56 26
51 64
43
A1 2 0, . ..
16.55
16.18
15 58
16 49
Fe 2 3
3.20
5 56
16
2 86
FeO
2 02
1 17
58
6 31
MgO
2.30
2.78
2 79
6 19
CaO
6 06
5 07
6 25
5 i -,i|
Na,O
2.76
3.23
27
19
K 2 O
2 81
3 43
2 46
84
H 2 O-
1.45
2.07
2 56
3
H 2 O+
2 56
2 61
4 43
7 93
TiO 2
.80
. 73
73
89
ZrO.,
Trace ?
Trace ?
C0 2 :
1.91
62
4 24
4 19
PA
30
32
31
36
SO 3
None.
None
03
08
Cl
F
}
(0
(')
FeS 2
04
03
7 89
2 55
Cr 2 O 3
None
NiO
Trace
MnO
17
21
21
BaO
12
(d\
07
SrO
06
Trace
Li 2 O
Trace
m
100.44
100.47
100.13
100.57
a Rosenbusch-Iddings, Microscopical Physiography of the Rock-making Minerals, 3d ed., p. 351.
6 Determined by Mr. George Steiger, of the United States Geological Survey.
c Not looked for.
d Not estimated; very little.
16843 No. 4205 16
242
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
DIFFERENCES OF COMPOSITION EXPRESSED BY DIAGRAMS.
The four analyses may be represented by the Brogger diagram (fig. 75), in
the same manner as employed for the earlier andesite.
The diagrams show the principal elements of fresh rocks, and fulfill all
Scale:. 01 (quotient figure)-^ inch
KEY
Flo. 75. .Diagram showing changes in composition during alteration of the later andesite.
ordinary purposes for these, but in altered rocks the altering agents have fre-
quently entered into the rock and constitute an important part of its bulk. To
take cognizance of three of the most important of these agents in this case water,
ALTERATION OF THE LATER ANDESITE.
243
carbonic acid, and sulphur in the form of iron sulphide the writer has constructed
diagrams altered from the preceding, so that these may also be represented (fig.
76). Ten radii instead of eight are taken, representing the different elements as
KEY
Scale: .01 (quotient f igure) = A inch
Silican and water.OI *, inch
FIG. 76. Diagram showing changes in composition during alteration of the later andesite.
shown in the key. The arrangement of the elements differs from that in the
preceding diagram, the water, carbonic acid, and iron sulphide beiog grouped
together, as well as lime, magnesia and iron, and soda and potash. Silica is
assigned one radius instead of two, as in the preceding diagrams, and since its
244
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
quantity results in an impracticable length for this radius, it is represented on
half the ordinary scale. Water was so abundant in some of the analyses that it
has been represented in the diagrams on the same scale as silica for a similar
reason. Only the water given off above 106 C. has been represented, that being
chemically combined, while that given off below this point is mostly hygroscopic.
Otherwise the scale used is the same as for the preceding diagram.
COMPARISON OF LATER ANDESITE WITH WASHOE AND EUREKA ROCKS.
The first two analyses of nearly fresh rocks are similar to analyses of pyroxene-
andesites from the Comstock region and from Eureka, as shown in the following
table. Nos. 1 and 2 in the preceding table are here called A and B.
Analyses of andesites.
A.
B.
C.
D.
E.
SiO,..
57.51
56 26
56 71
56 40
61 58
A1,O...
16.55
16. 18
18 36
15 99
16 34
Fe,O,
3 20
5 56
3 26
FeO
2.02
1 17
6 45
3 82
6 42
MgO
2.30
2.78
3 92
3 54
2 85
CaO
6.06
5.07
6 11
6 98
5 13
Na 2 O
2.76
3.25
3.52
3 83
2 69
KjO
2.81
3 43
2 38
1 91
3 65
H,O-
1.45
2 07
HO-4-
9 *>
2A1
1.94
TiO,
.80
.73
1 14
68
CO 2
1.91
.62
PA--
.30
32
32
28
FeS,
.04
03
a 64
Loss on ignition.
A. Mizpah Extension shaft, Tonopah, Nevada.
B. Halifax shaft, Tonopah, Nevada.
C. Granular pyroxene-andesite, Eldorado, outcrop, Washoe, Nev."
D. Pyroxene-andesite, Sutro tunnel, Washoe, Nev.
E. Pyroxene-andesite, Richmond Mountain, Eureka, Nev. *
DEGREE OF ALTERATION OF FRESHEST TONOPAH LATER ANDESITE.
The freshest Tonopah specimens (A and B) show, not only under the microscope
but by the analyses, the beginnings of alteration more than do the Eureka and
Washoe rocks with which they are compared. The presence of carbonates, of a
greater amount of water, and of a small quantity of pyrite indicates that the
former have been somewhat attacked by waters containing oxygen, carbonic acid,
a Hague, A., Mon. t". 8. Oeol. Survey, vol. 'X. p. >.
6 Op. eit., p. '264.
ALTERATION OF THE LATER ANDESITE. 245
and sulphur, and there has resulted partial hydration, oxidation, carbonation, and
sulphuration. The minerals developed, as shown by the microscopic description,
are serpentine, siderite, calcite, kaolin, quartz, hematite, and pyrite. The consid-
erable degree of oxidation of the iron, as compared with C and E, is shown by
the analysis. There is no evidence, however, that this incipient decomposition has
been attended by any change in the relative amount of the rock constituents; it
was rather a rearrangement of the materials into new minerals that were more
stable under the new conditions.
PRINCIPLES OF STUDYING ALTERATIONS OF LATER ANDESITES.
No attempt has been made to follow the different stages of the alteration of
the later andesite by analysis, as in the case of the earlier andesite, although these
stages have been minutel} 7 studied under the microscope. Therefore, while the
first two analyses (p. 241) are of the freshest rocks obtainable, the last two, 3 and
4, are of entirely decomposed rocks. In 3 and 4 not only has the original mineral
composition, as shown by microscopic examination, been completely obliterated,
but in the process there has been an important change in the chemical composi-
tion of the rock as a whole. This is well illustrated by the diagrams forming
figs. 75 and 76.
It will be noted that in all four analyses the amount of alumina remains
practically constant. This oxide is perhaps the most refractory among rock
constituents, and computations in regard to loss or gain during rock alterations are
often based on the assumption that alumina remains unaltered. That it probably
does not exactly do this, under intense action, is shown by the study of the earlier
andesite analyses, where the percentages of alumina in the bulk analyses decrease.
The constancy of the alumina in the four later andesite analyses under consid-
eration, however, is taken to indicate that the alumina has not been noticeably
attacked by the alteration, and therefore that the comparison of the percentages
of the other constituents affords an approximately correct idea of the loss and gain.
246
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
ALTERATION OF LATER ANDESITE FROM NORTH STAR SHAFT.
To compare the completely altered rock No. 3 with No. 2 (which appears to
be the freshest of the rocks analyzed, and may he taken as representing nearly
the original composition of No. 3, except for the partial oxidation of the iron), the
two analyses are given together in the following table:
Analyses of later andesite.
Rock No. 2.
Rock No. 3.
SiO 2
56.26
51.64
A1 2 O,
16 18
15 88
Fe,O, .
5.56
.16
FeO
1 17
58
MgO
2 78
2 79
CaO
5.07
6 25
Na 2 O
3.25
.27
K 2 O
3 43
2 46
H 2 O
2.07
2 56
H 2 O+
2.61
4.43
TiO 2
.73
.73
CO,
.62
4 24
PA -
.32
.31
SOj
.03
FeS,
03
7 89
It is noticeable that both analyses show the same percentages of titanium,
another highly refractory substance, as well as of phosphoric acid. The phosphoric
acid is contained in the apatite, which resists decomposition very strongly. This
strengthens the belief that these percentages afford a measure of the change of
the other constituents.
Nearly all of the soda has been extracted, and the silica has been somewhat
attacked and removed. On the other hand, the magnesia is unchanged, as are
probably the lime and potash" and the iron. The loss of bulk of the rock occa-
sioned by the removal of the soda and silica is compensated by the addition of large
quantities of carbonic acid and sulphur, producing carbonates of lime and iron and
sulphide of iron. It will be noticed that most of the remaining iron oxide is in
the ferrous condition; this probably is present as siderite. No dark iron or mag-
nesian silicates were noted among the decomposition products. The amount of
lime present is in excess of the amount required to form calcite with all the
carbonic acid in the rock; indeed, a small portion of this carbonic acid is required
For these conclusions compare not only the foregoing Uible, but also the table on page 219, showing variations ot
frth rock of this kind.
ALTERATION OF THE LATER ANDESITE.
247
to form siderite with the ferrous oxide. There remains a small amount of lime
(about 1.35 per cent) which it is difficult to assign to any of the recognized minerals
except the zeolites, which therefore may be supposed to be chiefly lime zeolites.
As there are not present any recognizable colored minerals into which the
magnesia has been transferred from its original combination in the pyroxene and
the biotite, the magnesia is probably contained in one of the colorless minerals,
and the presence of talc in the sericitic aggregate which forms a large part of the
feldspar pseudomorphs is indicated, in accordance with the conclusions reached for
specimen No. 4 (see p. 240). At the same time the analysis indicates that in this
aggregate all or a large part of the original potash in the feldspar is now contained
in the form of sericite.
The sulphur trioxide shown in the analysis of No. 3 is probably contained in
gypsum, a mineral abundantly found elsewhere in this altered rock. It appears
to result from the action of waters containing sulphuric acid (derived from oxidation
of the pyrites) on the calcite. This is a recent process and one distinct from that
by which the main alteration was produced.
The waters which produced this main alteration were, therefore, highly charged
with carbonic acid and sulphur; they left these materials, with some water, in
exchange for soda and silica, which they carried away.
ALTERATION OF LATER ANDESITE FROM MONTANA TONOPAH SHAFT.
The relation which the altered later andesite from the Montana Tonopah shaft
(No. 4) bears to the fresh rock (No. 2) may be seen by comparing their respective
analyses, which follow:
Anali/ses of later andesite.
Rock No. 2.
Rock No. 4.
SiO 2
56.26
43
Al 2 Os
16.18
16.49
Fe,O,
5.56
2.86
FeO
1.17
6.31
MgO
2.78
6.19
CaO
5.07
5.69
Na/)
3.25
.12
K,O
3.43
.84
H 2 O
2.07
3
H,O+.
2.61
7.93
TiO,
.73
.89
COj
.62
4.19
P,(X .
.32
.36
so s
None.
.08
FeS 2
.03
2.55
248 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
In No. 2 and No. 4 again the close correspondence of the alumina, titanium,
and phosphoric acid the last two representing probably, respectively, the resist-
ant rutile needles (sagenite) in the biotite, and the apatite indicates that the
relative bulk of the rock has not been greatly changed by decomposition. The
fact, however, that the percentages of each of these constituents in No. 4 is
slightly in excess of those in No. 2 may be taken as indicating that a slight
reduction of density has taken place.
Like rock No. 3, rock No. 4 shows an almost complete loss of soda, and a
similar loss of silica, both these processes being carried further than in No. 3.
Like No. 3, the lime has not been noticeably affected. Unlike No. 3, most of the
potash has been removed, while the iron, which in No. 3 had not been noticeably
affected, is here present in quantity certainly largely exceeding the original
amount. The writer has computed the totul metallic iron present in the different
rocks as follows: No. 1, 3.82 per cent; No. 2, 4.81 per cent; No. 3, 4.24 per cent;
No. 4, 8.04 per cent. The magnesia, not noticeably affected in No. 3, is here
doubled. Therefore the waters removed soda, potash, and silica, and brought iron
and magnesia in partial compensation, the rest of the loss being compensated for
by the addition of large amounts of water, carbonic acid, and sulphur.
Judging from the microscopic analysis, the iron of this rock is chiefly
contained in pyrite, siderite, uralite, and chlorite; the magnesia in uralite, chlorite,
and talc. The alteration of augite to chlorite or uralite involves a relative increase
of magnesia and a decrease of lime. Dana, speaking of uralite pseudomorphs
after pyroxene, remarks:"
"The most prominent change of composition in passing from the original
pyroxene is that corresponding to the difference existing between the two species
in general; that is, an increase in the magnesium and a decrease in the calcium.
The change, therefore, is not strictly a case of paramorphism, though usually so
designated."
Discussing the alteration of feldspar the same writer remarks:*
"When the waters contain traces of a magnesian salt a bicarbonate or silicate
the magnesia may replace the lime or soda, and so lead to a steatitic change or to a
talc when the alumina is excluded."
Dana indexes this "steatitic mineral" as "magnesia aluminate."
SIDERITE AS AN ALTERATION PRODUCT.
The abundance of siderite in the altered later andesite is of some interest, as it
has not been often detected among the minerals resulting from hot-spring action/
It is almost always present as a decomposition product of the biotite, pyroxene,
aSyrtem of Mineralogy, 6th ed., p. 890. l>Op. cit., p. 820. cLlndgren, W., Trans. Am. Inst. Min. Eng., vol. SO, p. 607.
ALTERATION OF THE LATER ANDESITE. 249
magnetite, etc., and is nearly always closely associated with pyrite. Usually the
two occur intercrystallized, yet so clearly separated as to show contemporaneous
crystallization; sometimes, however, a rim of siderite around pyrite indicates later
crystallization for the former, if not its derivation from the pyrite; while quite
as often rims of pyrite around siderite indicate a reversal of this order of crys-
tallization, and sometimes the phenomena clearly indicate that the pyrite has
formed at the expense of the siderite (PI. XXIII). This is in harmony with
the conclusions arrived at that the rock has been altered by solutions at once
highly carbonated and sulphureted.
The siderite occurs usually as a cloudy, opaque or semitranslucent substance, of
a characteristic white color by incident light. It has indeed usually the appearance
of the mysterious substance called leucoxene by petrographers, and observed as the
decomposition product of ihnenite. In many examples of this mineral in the Tonopah
andesites, however, rhombic cleavage has been observed, and characteristic rhombic
crystal outlines. The nature of the mineral has also been determined by chemical
tests (p. 241).
Concerning similar siderite in the iron-bearing rocks of the Mesabi range in
Minnesota, the writer has made the following statement."
" It is to be noted that siderite * * * surrounds magnetite as a decomposi-
tion product, and is cloudy and without crystal form. It thus comes under the
group of decomposition products from magnetite called leucoxene. Rosenbusch
describes it as an alteration product of ilmenite, titaniferous magnetite, and rutile.
Concerning its nature he says: 6 'Its chemical composition is not the same in all
cases where it has been investigated, and has been considered the equivalent of a
variety of minerals (titanite, anatase, and siderite) by different observers.' In every
case where this mineral is present in these rocks, chemical tests show it to be
siderite, and no signs of titanium can be found either in it or in the magnetite
whence it is derived. The existence of this leucoxenic decomposition product
surrounding magnetite has sometimes been held as sufficient evidence that the
magnetite was titaniferous, but it is clear that it is not necessarily the case."
In the altered "propylitic" andesite of the Comstock lode, which in alteration
resembles very nearly the later andesite of Tonopah, Dr. G. F. Becker suspected
the presence of siderite. He remarks:'"
"* * * It seems certain that the black border of many hornblendes has been
attacked and has given place to a transparent mineral, which is more or less diffused
in and obscured by the groundmass. The natural supposition is that it is ferrous
carbonate."
aQeol. Nat. Hist. Survey Minnesota, Bull. No. 10, p. 84.
b Microscopical Physiography of the Rock-Making Minerals, by H. Rosenbusch. Translated by Joseph P. Iddings.
Second, revised edition, p. 165.
Mon. U. S. Geol. Survey, vol. 3, p. 215.
250 GEOLOGY OK TONOPAH MINING DISTRICT, NEVADA.
SCARCITY OF EPIDOTE AS AN ALTERATION PRODUCT.
Epidote, so common in similarly altered rocks elsewhere, is rare in the later
andesite at Tonopah, and where found is often in positions suggesting that the
conditions of alteration ma}- have been abnormal. For instance, bowlders of later
andesite in explosive volcanic ash and breccia not far from the contact of the Golden
Mountain dacite, east of Mizpah hill, show feldspar and biotite phenocrysts largely
altered to epidote. Also rare epidote was noted in one or two specimens from the
Halifax shaft. In a shaft sunk to "a depth of 60 feet in decomposed later andesite,
just west of the Siebert shaft dump, a specimen was collected which carried rather
abundant epidote. This, however, is exceptional, and the typical alteration seems
to be illustrated by the detailed descriptions and analyses given.
COMPOSITION OF ALTERING WATERS.
The waters which produced the widespread and often profound alteration of
the later andesite were then, as it seems, highly charged with carbonic acid and
sulphur and contained magnesia and iron. Since the3 r did not attack the lime in
the rocks, it is probable that they contained also this element in considerable
quantity. In the rock alteration observed they changed their composition chiefly
by the acquirement of the alkalies and silica. They were not ordinary cool ground
waters, but clearly hot-spring waters. The extensive carbonation and sulphura-
tion show this, as well as the formation of sericite and talcose material, uralite,
chlorite, serpentine, zeolites, etc. Thorough as their work was, their effects were
not so intense as in the case of the waters which affected the earlier andesite
in the vicinitj' of the veins, where the most insoluble elements were attacked.
Moreover, the chemical composition of the waters was evidently quite different.
PERIOD OF ALTERATION OF LATER ANDESITE.
ALTERATION MAINLY ANTECEDENT TO FAULTING.
The last and most altered specimen, No. 4, is, as already noted, the type in the
Montana Tonopah shaft between depths of 90 and 278 feet. Specimens taken at
various intervals show the persistence of this general type of alteration down to the
Mizpah fault, which was encountered at 376 feet. Immediately beneath the fault,
however, and in the rest of the workings, the earlier andesite was encountered,
completely altered to the quartz-sericite phase. In the Mizpah mine, also, it was
noted that earlier andesite altered to quartz and sericite was separated sharply by
the Mizpah fault from later andesite marked by the strong development of car-
bonates and sulphides. The indications are, therefore, that the faulting was not
only subsequent to the alteration of the earlier andesite (as is shown by the fact
that it faults the quartz veins), but that it was subsequent to the alteration of the
ALTERATION OF THE LATER ANDE8ITE. 251
later andesite, which occurred at a later period than that of the earlier andesite;
otherwise some trace or transition of the later andesite alteration would be found
on the earlier andesite side of the fault line.
RELATION OF ALTERATION TO VEIN FORMATION.
EXUDATION VEIXLETS IS LATER ANDESITE.
In the later andesite occur many veinlets of calcite, some of gypsum, and even
of quartz. They are almost always very small and nonpersistent, tilling cracks,
and are evidently mainly the product of lateral secretion or exudation from the
rock. The quartz generally has a chalcedonic or jasper}- look, as compared with
the quartz of the earlier andesite veins, although in some cases the resemblance
of the two varieties of quartz to one another may be close.
METALLIFEROUS VEINS IN LATER ANDESITE.
Some larger veinlets, probably of a different origin, are composed of quartz
or quartz and calcite, and contain pyrite. An assay" of such a bluish veinlet in
later andesite, from the east base of Mount Oddie, and near the contact of the
Oddie rhyolite showed only traces of gold and silver. It was noted that these
veinlets were especially characteristic of a zone in the later andesite near the
contact of the Oddie rhyolite.
Near the contact of the glassy Tonopah rhyolite-dacite at many points, as for
example, near the Belle of Tonopah shaft, there are numerous small veins of this
kind in the intruded later andesite. These veins gave variable but generally small
assays for gold and silver, the gold predominating. In the Mizpah Extension,
large veins of pyritiferou.s quartz were encountered in the later andesite, but
this was at or near the contact with Tonopah rhyolite-dacite, which is, it will be
remembered, of more recent date than the later andesite.
The pyrite in the altered later andesite is sometimes very abundant, and may
be segregated so as to be of striking appearance, and to suggest an ore; but assays
show in all cases that the mineral is barren of gold and silver.
CONCLUSION.
It thus appears probable that the more important quartz veinlets which
appear in the later andesite in places were largely formed under the influence of
solutions following the contacts of later intrusive rocks the rhyolites and rhyolite-
dacites. This being the case, it is likely that a large part of the rock alteration
just described may have been due to the same causes. The entirely altered
specimens 3 and 4, described and analyzed, were both near the intrusive contact
of the Oddie rhyolite, and in general the more altered portions appear to be in
<" By R. H. Officer & Co., Salt Lake City.
252
GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
the vicinity of the large subsequent igneous intrusions. It is therefore likely
that the alteration of the later andesite was largely produced by waters which
followed later, chiefly rhyolitic," intrusions into it.
ALTERATION OF THE ODDIE RIIYOLITE.
Some partial anah'ses were made, to show the composition of the fresh and the
altered white Oddie rhyolite. As a rule this rock is quite fresh, even when close to
the intensely altered earlier and later andesites. Sometimes, however, especially
along faults and watercourses, the rhyolite disintegrates and the feldspar is partly dis-
solved out, leaving cavities, while the scant biotite of the fresh rock has disappeared.
The partial analyses are as follows:
Analyses of Oddie rhyolite.
[By Dr. E. T. Allen.]
1 (376).
2(837).
3(227).
SiO 2
75.66
76.57
77 71
CaO
.47
Na^O
1.70
96
17
K 2 O
4.94
5 81
4 04
The first two analyses being of fresh rock, the difference in the chemical
composition is probably original. This difference was, indeed, noted in the field,
where the rhyolite of Rushton Hill (No. 1) was observed to have a slightly more
basic aspect than the rhyolite of Mount Oddie (No. 2), and to approach in
appearance the siliceous dacite of Golden Mountain near by. No. 3, however, is
Oddie rhyolite which was probably originally of a composition similar to No. 2,
and the chemical change undergone on alteration seems to have been a slight
increase in silica and a loss of the alkalies, especially soda.
The microscopic description of No. 3 is as follows:
3. (Specimen 227) Mizpah Extension shaft, 385 feet down. Hand specimen is
white and hard, but shows cavities due to the dissolution of feldspar phenocrysts.
There is no biotite. Under the microscope there are also no signs of biotite, and
the feldspars are entirely altered to a sericite aggregate, both in the phenocrysts
and in the groundmass. The phenocrysts consist of abundant quartz, with sericite
areas representing original feldspars, while the groundmass consists of an aggregate
of crystalline granular quartz, much coarser than in the fresh rock and sericite.
The size of the quartz grains in the groundmass is evidently due to enlargement by
the waters which produced the alteration, for crystal faces are frequent and such
idiomorphic grains frequently impinge upon the area of the original idiomorphic
feldspar phenocrysts, now altered to sericite.
' 'l in' glassy Tonopah rhyollKMlaclte Is a rhyolitic variety (p. 69.)
CHAPTER VII.
ORIGIN OF MINERAL VEINS.
ORIGIX OF THE MIXERALJZIXG AXD ALTERING WATERS.
ANTITHESIS BETWEEN WATERS AND ASSOCIATED ROCK.
In view of the composition of the waters which produced the veins and the
chief alteration of the early andesite, it has been argued that they were rich in
silica and potash and noticeably poor in the other common rock-forming elements.
They seem to have directly followed the earlier andesite eruption. In considering
the alteration of the later andesite in the vicinity of Mount Oddie, it has been
concluded that the waters which wrought the change were rich in magnesia, lime,
and iron, and low in silica and the alkalies; in this case the data seem to point
to the explanation that the waters followed the eruption of the Oddie rhyolite.
Both are concluded to have been hot-spring waters, which were active after volcanic
eruptions for a i - elatively short time, geologically speaking, and which differed in
composition as much as the rocks. If these conclusions are true, it is right to
notice an apparent antithesis in each case between the composition of the erupted
rock and that of the accompanying and succeeding hot solutions. The eruption
of the earlier andesite, a rock of intermediate composition, containing perhaps
about 60 per cent of silica, and about five times as much soda, lime, iron, and
magnesia as it does potash, was followed by the advent of waters which were
rich in the elements characteristic of extremely acid rocks (alaskites) namely,
silica and potash with the proportion of silica probably largely in excess of that
in these rocks and probably approximating that in feldspathic quartz veins of
granitic origin, as the composition of the Tonopah veins indicates. The eruption
of the Oddie rhyolite, a rock made up almost entirely of silica and potash, with
alumina, and only trifling quantities of magnesia, lime, and iron, was followed by
the advent of waters rich in these three last-named elements (which are charac-
teristic of basic rocks) and poor in the elements represented in the rhyolite itself.
Testing this latter conclusion, we may recall the calcitic veins of Ararat
Mountain, which are certainly directly due to hot solutions that ascended immedi-
ately after the eruption of the neck or plug of Oddie rhyolite (p. 101). It has been
shown that these waters give evidence of having contained chiefly lime, iron,
253
254 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
manganese, and silica. They have produced silicitication, and have deposited silica
in fissures, but the silica is usually greatly exceeded by the calcite (figs. 14, 15).
These waters then were also characterized by the materials of basic rather than of
acidic igneous rocks.
Along the contact of the dacitic rocks there has frequently been profound
alteration of the later andesite, but the process has not been studied sufficiently to
give definite conclusions. A specimen from the later andesite near the Molly shaft,
at the contact with the Golden Mountain dacite, is entirely altered to calcite and
quartz, the former unusually abundant, with siderite and pyrite, etc. At the Belle
of Tonopah shaft specimens of the later andesite near the contact with the glassy
Tonopah rhyolite-dacite are largely altered to calcite, together with quartz and
probable sericite; other specimens near here are more plainly silicified, but are
ferruginous. The glassy rhyolite-dacite itself, near the contact, is often silicified,
but shows frequently considerable epidote. Calcification as well as silicitication is
therefore suggested in all these instances.
Omitting, therefore, as without sufficient data, the consideration of the solu-
tions accompanying the rhyolite-dacites and referring only to the Oddie rhyolite
and the earlier andesite, the conclusions, if correct, may have a bearing on the
source of these solutions.
THEORY OF ATMOSPHERIC ORIGIN OF HOT SPRINGS.
There are two possible explanations of hot springs in general. One is that
atmospheric water, of which such a large quantity sinks below the surface,
becomes warmer in depth by the natural increment of temperature or in volcanic
regions by the residual heat of the rocks, and on finding a channel ascends toward
the surface as hot water, carrying with it materials which it has dissolved out of
the rocks on its passage. A physical objection to this theory is that surface
water could hardly work its way downward against pressure, to the depths neces-
sary to become highly heated. This has been met by the experiment of Daubre'e,
which showed that water would work itself downward through a solid marble
slab by capillarity, in spite of the resistance offered by a strong pressure on the
underside of the slab. It has been argued that by such capillary circulation
the supplies of hot springs may be replenished.
THEORY OF MAGMATIC ORIGIN OF HOT SPRINGS.
The other explanation goes back to the hypothetical origin of the atmos-
pheric or surface water at the period of the consolidation of the globe. Accord-
ing to the commonly accepted theory, when the molten or fluid earth stuff cooled
and was consolidated, a large part of the contained water was separated, and by
reason of its great mobility formed the oceans. Processes similar to those which
MAGMATIC ACTION OF HOT SPRINGS. 255
thus went on on a large scale in primeval times, it is argued, still go on when-
ever a body of magma consolidates; a large part of the water of this fluid material
is separated and expelled and most of it escapes to the surface as hot springs,
adding to the surface waters already originated by similar separations.
Of these two explanations, the former may seem more familiar and probable,
because of our acquaintance with ordinary surface waters and our lack of intimacy
with newborn magmatic waters. Yet the magmatic explanation is the only one
of whose possibility we have ocular demonstration. We have no such demon-
stration that surface waters can penetrate downward till they are heated far
above the boiling point and then rise again and emerge, and we can reach such an
idea only by a process of speculation which is not even logical reasoning. On the
other hand, the vast quantities of water vapor given off by lavas at many volcanic
centers afford proof that water is present in these unconsolidated magmas and
separates on cooling. Furthermore, the phenomena of contact metamorphism,
especially that connected with siliceous rocks, show, as has often been pointed out,
that in depth similar water vapor is expelled from cooling rock, even under great
pressure.
Volcanic activity has sometimes been ascribed to the infiltration of surface
water, which, on coming into contact with heated rocks below, causes explosions
and extravasations of lava; and the water given off from the cooling lavas is thus
thought to have a surface origin. Many facts, however, which can not be gone into
here are against this hypothesis. Concerning the steam given off at Vesuvius,
Prof. E. Suess remarks: a
u* * * j j s a (. j eas t; certain that the quantities of steam issuing from the
parasitic crater must have come from a zone in which the temperature equals or
exceeds the melting point of most rocks, and in which there can be no question of
porous or f ragmen tal rocks, and therefore no question of infiltration of vadose*
water."
That is, the principle of capillarity above referred to can not apply to rocks at
these great temperatures and can not explain the water in lavas.
When the upward movements in the lava bodies have ceased and a crust of
cooled and solid rock has congealed at the surface, consolidation will progress
downward. The aqueous vapor given off from this lower cooling lava will become
condensed to water on its passage through the cooled crust and will so emerge.
It seems, therefore, impossible to escape the conclusion that at least some hot
springs, the after-phenomena of volcanic activity, have the origin above described,
and contain newborn water separated from the magma. 6 '
aeog. Jour., vol. 20, p. 519.
6 Surface.
eSuch water has been called juvenile or primitive by Professor Suess, and hifpofjene by one of his translators, to
distinguish it from the shallow underground water derived irom the surface, or radose water, the latter term having
been proposed by Posepny in his essay on ore deposits.
256 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
Vadose or surface-derived descending water must meet and mingle with these
escaping magmatic waters, must change their composition and mitigate their heat,
and the mingled waters must in many cases emerge on the surface as warm
springs.
CHARACTERISTICS OF THE NEVADA SPRINGS.
The conception that the hot springs of the volcanic region of Nevada were
largely supplied by magmatic or primitive water from the cooling subterranean
lava was formed by the writer in the field in 1902, before reading Professor Suess's
paper, above referred to.
On account of the exceeding aridity of the Great Basin, there are, as a rule,
no flowing surface waters, the whole supply emerging from the ground as springs.
These springs are hot, warm, or cold. The cold springs usually emerge from
depressions, fault or fracture lines, and are especially found near the base of the
desert mountain ranges. They usually show two characteristics which indicate that
they are of vadose origin: (1) They fluctuate with the season, being abundant in
the spring and often becoming scanty or dry at the close of the summer, and (2)
they become more numerous and copious in the regions of greater precipitation
and very rare in the more arid portions. Near the Sierra Nevada and in the
region just east, which receives the overdrift from the Sierra precipitation in the
shape of relatively abundant snows and more frequent rains, the cold springs
emerging from the base of the mountains are numerous and so large as to
frequently form short streams, sufficing often for agriculture, and producing a
fringe of ranches along the mountain base, such as that which borders the eastern
base of the White Mountains in Fish Lake Valley. The hot springs, on the
other hand, so far as the writer's experience and information go, do not show
these characteristics of vadose origin; they show no change with the season and
are not noticeably associated with regions of greater precipitation. They are
noticeably associated with areas of volcanic rocks and are scattered all over these
areas, being often very vigorous in the heart of an arid region and sometimes
sufficiently copious to form short streams.
COUPLING OF HOT AND COLD SPRINGS.
It is a matter of frequent remark in this dry Nevada region that hot springs
and cold springs are frequently coupled together and emerge within a short
distance of each other. The writer has observed an instance of this at the village
of Silver Peak, 25 miles southwest of Tonopah, where a spring of nearly scalding
temperature and one at most lukewarm or tepid emerge from the edge of the
desert plain at the east base of the Silver Peak Range within a score of feet of
each other. These are evidently waters from diflerent sources, and their coupling
CHARACTERISTICS OF NEVADA SPRINGS. 257
must be ascribed to their having neighboring and probably parallel channels along
the same fracture zone. Decomposed rock along such a fracture zone would
form an effective barrier, preventing currents from mingling the waters and
averaging their temperatures. The cool water is evidently vadose, and probably
represents a part of the atmospheric waters which fall upon the Silver Peak
Range, while the hot waters have a distinct and vastly deeper origin. It is clear,
however, that in many similar cases the two currents of water must mingle,
appearing at the surface as springs of varying warmth and of composite origin.
In seeking to understand the nature of the Silver Peak hot springs the writer
learned from the inhabitants of the village a significant fact. According to them
the water of the hot springs is much hotter in winter and fall than in summer
and spring, so that in the former seasons much more cold water must be added
to bring it down to a temperature requisite for bathing. This indicates that the
temperature of the hot water is really modified by the cool vadose water, the
modifying being characteristic of the seasons when the melting of the snows
provides a considerable supply to the shallow underground circulation.
THE DEVILS PUNCHBOWL.
Mr. J. L. Butler, the discoverer of Tonopah and an old inhabitant of the
region, has described to the writer a hot spring in Monitor Valley, not far from
Belmont, which is 45 miles northeast of Tonopah. This spring occupies a cup-
shaped depression probably formed by sinter accumulations known as the Devils
Punchbowl. This depression is reported to be 30 feet in diameter and to be full
of hot water up to a point 30 feet below the top. The level of the water has
gone down 3 feet in thirty years and the water has become cooler. Formerly
more gas than at present was emitted, and occasional flames were seen. This
change is apparently a secular one, strikingly different from the seasonal variations
of vadose springs, and suggesting as a cause the diminution of volcanic energy
in this region of abundant Tertiary volcanics.
AMOUNT OF PRESENT AND RECENT HOT-SPRING ACTION.
Similar hot springs, some of them boiling, abound in the region and surround
Tonopah on all sides. Volcanic activity has been great in this province for a
prolonged period, lasting from the beginning of the Tertiary to within a few
hundred years ago. At Silver Peak is a small, undefaced basalt crater, which is
younger than the detritus of the valley, and can hardly be more than a few
hundred years old; and there are a number of other craters, such as those in and
near Lake Mono described by Russell which are comparatively recent. That
many of the hot springs which accompanied or followed the different manifestations
16843 No. 4205 17
258 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
of volcanic activity are now extinct is shown by the characteristic effects of these
springs in many localities, indicating that the number of such springs was probably
formerly greater than at present.
ORIGIN OF EXTINCT HOT SPRINGS AT TONOPAH.
CONNECTION WITH VOLCANIC ERUPTIONS.
At Tonopah evidence has been given to show that after several of the volcanic
eruptions waters ascended, altered the rocks, deposited new and removed old
material, and became extinct in a relatively short space of geologic time. If the
reasoning given in the preceding pages is correct, it is very difficult to explain
the totally different composition reasoned out for the waters at different periods
on the Itypothesis that the mineralizing waters were of atmospheric origin and
derived their material from solution of the rocks which they traversed. These
ascending waters followed channels practically side by side, if not in many cases
nearly the same, and it is most natural to suppose that the rocks which they
traversed were not greatly different.
CONSEQUENCES OF ANTITHESIS BETWEEN ROCKS AND WATERS.
A second important consideration is the apparent antithesis pointed out
between the contents of waters at different periods and the composition of the
lavas which they followed." There is indeed apparently a relation, but it is the
opposite one from what would result had the waters derived their mineral
contents from the leaching of these lavas by ordinary atmospheric waters. The
same difficulty presented itself to Professor Suess and many other investigators
in considering the origin of the Carlsbad Springs in Germany.* The amount of
soda and lime in these springs suggests that the bulk of the matter in solution
must be derived, not from the granite of the country, but from some unknown
source. The quantity of the water and the carbonic acid at Carlsbad were also
inexplicable on the hypothesis that the waters were of meteoric origin, and led
Professor Suess and others to believe that the waters and their contents were of
magmatic origin.
MEANING OF NATURE OF METALS IN VEINS.
A third consideration is the peculiar combination of materials in the waters
which produced the veins in the earlier andesite. Not only is the abundance of
silica and potash, together with the lack of sodium, magnesium, lime, iron, etc.
elements more characteristic of the andesite- difficult of explanation on the theory
of leaching from the traversed rocks, but also the presence of unusually large
aThe writer has at present no explanation of this antithesis to offer.
OSuesi, E., Geog. Jour., vol. 20, p. 617.
ORIGIN OF HOT SPRINGS. 259
quantities of the rare metals silver and gold, and unusually small ones of the
commoner ones copper, lead, and zinc. The amount of silver by weight in these
primary ores, so far as they have been developed, seems to exceed that of
either of the three last-named metals. No such results as this could be expected
were the metals derived from leaching of the andesite. Plainly some process of
separation and concentration has furnished the noble metals contained in the
mineralizing waters, separating them from the baser metals. Nickel is present in
the fresh later andesite (p. 34) and was detected in the fresh earlier andesite of
Eureka; yet this metal has not been detected in the ores in either camp. In the
rocks near the Comstock lode analyses conducted by Dr. G. F. Becker* showed
small quantities of silver and gold, whence it was concluded that the ores of the
lode had been derived from the wall rocks (by lateral secretion). But later
investigations on the subject of the presence of the precious metals in rocks
show that these metals are very frequently present in rocks not associated with
ore deposits, as well as in those that are; and the results of the assays tabulated
by Becker do not, to the writer's mind, indicate any connection between these
traces of metals and the ores of the Comstock lode. At Washoe, as at Tonopah,
the theory of leaching from wall rocks, or lateral secretion, indeed, leaves
unexplained the presence of silver and gold in such large quantities, relatively
to the commoner metals. The view concerning this problem at the Comstock,
expressed by von Richthofen,'' seems to the writer especially illuminating, and
applicable, as well, to the similar situation at Tonopah. Von Richthofen remarks:
" We have in the elements evolved during the first two periods of solfataras
namely, fluorine, chlorine, and sulphur all the conditions required for filling the
Comstock fissure with such substances as those of which the vein is composed.
Steam, ascending with vapors of fluosilicic acid, created in its upper parts (by
diminution of pressure and temperature, according to well-known chemical agencies)
silica and silicofluohydric acid, the former in solid form, the latter as a volatile gas,
which acts most powerfully in decomposing the rocks it meets on its course. The
chloride of silicon in combination with steam forms silica and chlorhydric acid.
Fluorine and chlorine are the most powerful volatilizers known, and form volatile
combinations with almost every substance. Besides silicon, the metals have a great
affinity with them. All those which occur in the Comstock vein could ascend in a
gaseous state in combination with one or the other of them. *
"It is a fact worthy of notice that there is scarcely a single chemical agent,
excepting fluorine and chlorine, which would not carry metallic substances into
a Since the above was written the important discovery of the presence of selenides has been made by Doctor Hillebrand.
See pp. 89, 90. Doctor Hillebrand remarks that the presence of selenides, and the absence of their closely associated
element, tellurium, indicated some unusual process of separation. Tellurides have been found at Goldfield, 28 miles
south of Tonopah, also in Tertiary volcanic rocks; and from that camp selenides have not yet been reported.
6Mon. U. 8. Geol. Survey, vol. 3, pp. 184, 155, 223.
c As examples, see Wagoner, Luther, Trans. Am. Inst. Min. Eng., vol. 31, pp. 798-810.
dMon. U. S. Geol. Survey, vol. 3, pp. 19, 20.
260 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
fissures in exactly or nearly the reverse quantitative proportion from that in which
they occur in silver veins. Iron and manganese are not only more abundant in
rocks, but also much more easily attacked and carried away by acids, than silver
and gold. The proportion of these to the former ought, therefore, to be still
smaller in mineral veins than it is in rocks, and lead and copper ought to be more
subordinate, if their removal from their primitive place had been effected by other
agents than fluorine and chlorine. Only these two will first combine with those
metals which are most scarce in rocks and relatively most abundant in silver veins,
and they are probably the only elements which have originally collected them
together into larger deposits, though these may subsequently have undergone
considerable changes, and water may have played altogether the most prominent
part in bringing them into their present shape."
NATURE OF SOLFATARIC ACTION.
Concerning the nature of solfataras, the following extracts are quoted from
Professor Bonney's Volcanoes (p. 52):
"In the intervals between the paroxysmal phases most volcanoes emit simply
steam, and all in their decadence pass through a longer or shorter period when it
alone is ejected. This is often termed the solfatara stage, from the crater of that
name in the Phlegrsean Fields. Like most of those in this district, the cone is low
and the crater wide; the floor is a level, sometimes marshy, plain, surrounded by
steep walls of ashy materials, perhaps a hundred feet in height. The last eruption
was in 1189, when a stream of trachytic lava was discharged from the southern side
of the crater; but now the sole sign of activit}', except some boiling puddles in one
part of the floor, is to be found at the foot of the crag on the side. Here, from a
fissure in the inclosing wall, something like the adit of a mine, a column of steam is
ejected to a height of 6 or 7 yards. The steam commonly is more than the
vapor of water. Such acids as hydrochloric or sulphuric are often present;" that
of the solfatara, as we can see from the sulphur abundantly deposited round the
aperture and the rotten condition of the adjacent rocks, is no exception to the rule.
No doubt the materials in and about a vent must undergo considerable chemical
changes when the volcano is passing through this stage in its history."
Professor Bonney finishes his summary of the description of volcanic eruptions
as follows (p. 62):
"An eruption is generally ushered in by earthquake shocks, is always associated
with explosions, and is frequently concluded by the emission of a considerable mass
of lava. Great quantities of water are discharged in the form of steam, and the
phenomena of an eruption are closely imitated by geysers. Other vapors also are
discharged, and the solfatara stage of a dying volcano commonly ends with the
exhalation of carbonic acid or some such gas; perhaps the last stage of all may even
be a cold mineral spring."
a The steam emitted from Vesuvius in January, 1876, was acid with these, particularly the former. Steel was rusted
and clothes were slightly altered in color in the course of an hour or two.
GENESIS OF TONOPAH ORES. 261
Professor Suess, in the essay referred to," thus describes the funiarolic activity
at Vesuvius:
"Turning now to the gases accompan3 r ing the eruptions. After steam, chlorine
and gases containing sulphur are the most important, and carbonic acid gas comes
next. Their occurrence follows a definite law. So far as it has been possible to
approach them, all fumaroles actually within vents contain steam; but the hottest
fumaroles (over 500 C.) on the surface of cooling lava streams, where approach is
easier, are dry. In the emanations from these high-temperature fumaroles are found
chlorine compounds, and along with them fluorine, boron, and phosphorus sub-
stances which are the first to disappear as the temperature of the f umarole sinks.
Sulphur persists longer, often combined with arsenic. Carbonic acid' is given off
freely till a much later stage, sometimes till the fumarole is comparatively cool,
notwithstanding that it is observed in the hottest dry fumaroles. Fumaroles in
different 'phases of emanation' may occur quite near one another. The steam of the
volcano can not be derived from vadous infiltration, for if it is, whence the carbonic
acid ? Both must come from the deeper regions of the earth. They are the outward
sign of the process of giving off gases which began when the earth first solidified,
and which to-day, although restricted to certain points and lines, has not yet come to
a final end."
MINERALS DEPOSITED AROUND FUMAROLES.
Around the orifices of the steam jets (fumaroles) at Vesuvius sulphides of arsenic
and mercury and chlorides of copper and lead have been deposited, showing the
efficacy of such gases in separating, dissolving, and precipitating these relatively
rare substances. Dana* quotes Mallet as authorit\ r for the statement that native
silver ore occurs rarely in volcanic ashes.
CONCLUSIONS AS TO GENESIS OF TONOPAH ORES.
The considerations above pointed out appear to the writer to indicate
strongly the following conclusions:
The Tonopah district was, during most of Tertiary time, a region of active
volcanism, and probably after each eruption, certainly after some of them,
solfataric action and fumarolic action, succeeded by hot springs, thoroughly
altered the rocks in many parts of the district. At the surface, during those
periods, the phenomena of fumarolic and solfataric action and of hot springs
were similar to those to-day witnessed in volcanic regions; but the rocks now
exposed were at that time below the surface. The veins fill conduits which were
formed by the fractures due to the heavings of the surging volcanic forces
below and along which the gases, steam, and finally hot waters, growing gradually
aSuess, E., Geog. Jour., vol. 20. p. 520. & System of Mineralogy, 6th ed., p. 20.
262 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
cooler, were forced, relieving the explosive energies of the subsiding volcanism.
The water and other vapors, largely given off by the congealing lava below,
carried with them, separated and concentrated from the magma, metals of such
kind and of such quantities as are present in the veins, together with silica and
other materials.
The nature of the metallic minerals in the veins in this case is believed to
depend largely upon the particular magma whence the emanations proceeded. In
the chief Tonopah veins this was the earlier andesite. Other factors, such as
relative depth, have evidently an important controlling influence.
CHAPTER VIII.
INCREASE OF TEMPERATURE WITH DEPTH.
Some measurements were made by Mr. Leon Dominian, field assistant, under
the direction of the writer, with a view to ascertaining the increment of temperature
with depth in this district.
METHOD OF MEASUREMENT.
The best opportunities were offered by the Mizpah Extension and the Ohio
shafts, both fairly deep shafts with (at that time) very little side workings and no
through system of ventilation. Holes were drilled dry into the rock at the sides
of the shafts at the points where the temperature was to be taken, deep enough
to take in the thermometers, which were especially procured for this purpose.
After the thermometer was inserted the hole was stopped up, and the reading
was taken after fifteen to twenty-five minutes in some cases twenty-four hours.
Check measurements were taken in every case. In the Ohio Tonopah the holes
were driven 18 inches; in the Mizpah Extension not so deep.
The Ohio Tonopah. shaft is perfectly dry. The Mizpah Extension encountered
a very little water on a contact zone at a depth of 300 feet, but is otherwise
quite dry.
TEMPERATURES
THE MIZPAHT EXTENSION AND THE OHIO
TONOPAH.
The results of the measurements of temperatures are given in the following
table:
Temperatures in Mizpah Extension and Ohio Tonopah shafts.
Feet below surface.
Temperature.
Rate of increase per 100
feet.
Depth required for in-
crease of 1 degree.
Mizpah Ex-
tension.
Ohio Tono-
pah.
Mizpah Ex-
tension.
Ohio Tono-
pah.
Mizpah Ex-
tension.
Ohio Tono-
pah.
100
Degrees F.
60.25
61.75
64
66.5
69
70.5
72
Degrees F.
60
61
62.5
64
66.5
69
74
78
Degrees F.
Degrees F.
Feet.
Feet.
200
1.5
2.25
2.5
2.5
1.5
1.5
1
1.5
1.5
2.5
2.5
5
7.6
66
44}
40
40
66
66
100
66
66
40
40
20
16}
300
400
500
600
700
766 (bottom Ohio Tonopah)
780 (bottom Mizpah Extension)...
73.5
1.9
53J
263
264
GEOLOGY OK TONOPAH MINING DISTRICT, NEVADA.
'
Mizptih Extension.
Ohio Tonopah.
Average increase
1 in 51.3 feet
1 iii 37 feet
TEMPERATURES
THE MONTANA TONOPAH.
Some observations were also taken in the Mizpah and in the Montana Tonopah
workings, but with a less range of depth. Those in the Montana Tonopah, however,
were taken at intervals along the vertical shaft, in holes drilled for the purpose,
and the thermometers were left in place 15 minutes, check readings corresponding
exactly. They are, therefore, worthy of confidence, and are given in the following
table:
Temperatures in Montana Tonopah shaft.
Feet below
surface.
Tempera-
ture.
Rate of in-
crease per
100 feet.
Depth re-
quired for
increase
of 1.
Degrees F.
Degrees F.
Feet.
317
64
460
68
2.8
36
600
70.5
1.8
56
Average increase, 1 in 43.5 feet.
Although the average increment of temperature (1 F. in 43.5 feet) for the
Montana Tonopah measurements differs from that shown by the Mizpah Extension
measurements (1 in 51.3 feet), comparison of the tables shows that the temperatures
for the corresponding levels in each case practically coincide.
These separate temperature measurements have been plotted as curves (fig. 77).
The Mizpah Extension curve, as shown, is distinct from the Ohio Tonopah curve,
while the Montana Tonopah curve coincides with the corresponding portion of
the Mizpah Extension.
TEMPERATURES IN MIZPAH HILL. WORKINGS.
Six measurements were taken in the Mizpah Hill workings, but under less
exact conditions. They were taken at various points in the drifts, and so at
variable distances perpendicularly from the surface, sometimes in drilled holes
and sometimes at the ends of unventilated drifts. These mines, however, had, at
the time of examination, a thorough system of ventilation (which the others did
not) and the measurements do not check exactly, and indicate that the temperature
was affected by air currents. They are therefore not published.
INCREASE OF TEMPERATURE WITH DEPTH.
265
THERMAL SURVEYS ON THE COMSTOCK.
During his study of the Comstock Dr. G. F. Becker made careful thermal
surveys along deep vertical shafts and along the Sutro tunnel, which runs in
and taps the vein. On plotting the temperatures taken in the shafts no indication
of curvature could be perceived, although the increment showed constant local
irregularities, and the line, plotted from point to point, was often zigzag. On
this account a straight line was assumed as expressing the relation of temperature
depth. The Sutro tunnel line, however, though also irregular in detail, shows an
unmistakable curve, clearly a conduction curve. It is to be noted, on the other
hand, that in the Sutro tunnel the temperature measures extended over a distance
20
30
Degrees Fahrenheit
40 50
60
80
90
Feet
100
200
300
400
500
600
700
SOO
900
1000
Ohio Tonopah
Mizpah Extension
FIG. 77. Plotting of temperature observations in the Ohio Tonopah, Mizpah Extension, and Montana Tonopah mines,
showing increase of temperature with depth. a=This part of the curve coincides with that of the Montana Tonopah.
of 11,000 feet, while the vertical shaft measurement did not extend more than
2,000 feet; and that any given 2,000 feet of the Sutro tunnel curve would not
by itself suggest a curved line.
COMPARISON OF COMSTOCK AND TONOPAH DATA.
Comparing the Tonopah and Comstock data, the temperature of 78 F.,
obtained in the Ohio Tonopah at 766 feet from the surface, was encountered in the
Forman shaft at the Comstock at about 900 feet; while the bottom temperature of
73.5 F. in the Mizpah Extension at 780 feet was encountered in the Forman at
between 600 and 700 feet. It seems likely, therefore, that the average increase at
266 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
Tonopah ma} 1 be as great as at the Comstock, where it is 1 F. for each 33 feet
vertical of extent.
The decided and characteristic curve in the Ohio Tonopah has no counterpart
in any of the vertical sections at the Comstock. It is probably, however, a local
deviation in a curve of vastly greater magnitude; though its form suggests a con-
duction curve, and it is possible that the extremely rapid increase of heat at the
bottom indicates the proximity of a local heat focus, such as a hot spring. The
larger and much less rapid conduction curve of the Sutro tunnel section is due to
the heat from a similar local focus the hot waters which rise along the lode.
CHAPTER IX.
COMPARISON WITH SIMILAR ORE DEPOSITS ELSEWHERE.
It is often advisable to study an ore deposit or a mining district not by itself
alone, but also in comparison with others. Similar districts often present informa-
tion, through their likeness or dissimilarities, concerning the nature, origin, and
future possibilities of the district under examination.
VEIXS OF PACHUCA AXD REAL DEL MOXTE, IX MEXICO.
Among the nearest anajogies to Tonopah yet described anywhere in the world
are the contiguous mining districts of Pachuca and Real del Monte, described by
Aguilera and Ordonez. "
These celebrated districts are 62 miles north of the City of Mexico, on opposite
slopes of the Pachuca Mountains, which bound the great valley of Mexico. The
mines support the city of Pachuca, which contains 35,000 people, most of whom are
actually engaged in mining. The ore deposits were discovered in 1522, and have
been worked almost continuously to the present day. Pachuca is the most important
mining district in Mexico, and is estimated to have produced since its discovery
3,500,000 kilos of silver. b
The geology is similar to that of numerous other mineral regions of Mexico.
The whole Pachuca Range is formed of Tertiary andesites, rhyolites, and basalts.
The andesites are of Miocene age and have a varied appearance, due to alteration,
the normal type being green and propylitic. The feldspar (labradorite) has often
been transformed to sericite, calcite, chlorite, epidote, and clayev products; the
pyroxene to chlorite, viridite, and epidote. The rocks are silicined near the veins,
so as often to resemble dacites or rhyolites, this alteration being due to the influence
of hot solutions during the formation of the veins. Rhyolites cover the andesites,
occurring as flows and dikes. The last eruptions were of basalt. The veins strike
east and west. Secondary veins branch out from the smaller ones, and splitting and
reuniting are common phenomena. The veins are more remarkable for constancy
and extension than for thickness. They seldom exceed 20 feet in thickness, while
they have a length of from 2 to 10 miles.
"Boletin del Institute geo!6gico de Mexico, Nos. 7, 8, 9: Trans. Am. Inst. Min. Eng., vol. 32. pp. 224-241.
f> About 112,000,000 ounces, valued at 8145,600,000 (1 oz.-about 81.30).
267
268 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
The quartz croppings carry pyrite and oxides of manganese. They are always
argentiferous, with- an appreciable amount of gold. They may be divided into two
zones, one overlying the other. The upper is composed of oxides (red ores) and the
lower of sulphides (black ores). The upper contains, besides iron oxide (always
auriferous), oxides of manganese and chlorides and bromides of silver; it has a
maximum downward extent of nearly 1,000 feet. The lower zone contains sul-
phides of lead, silver, etc. The lower limit of the upper zone corresponds to the
ground-water level.
Calcite is found only in small quantities. Of the sulphides, pyrite, galena,
and argentite were in most cases deposited simultaneously with the quartz. The
abundant manganese oxide in the upper zone is replaced in the sulphide zone by
a lesser quantity of the silicate, rhodonite. Pyrite is frequent in the mineralized
parts of the veins, and is also abundant in the country rock, but in the country
rock it does not contain even traces of the precious metals. Native silver has been
found at all depths; ruby silver has not been observed at Pachuca, but is found
at Real del Monte. a
Rich ores occur in certain parts of the veins called bonanzas, which are of
irregular form, frequently nearly elliptical. The bonanzas of the different veins
group themselves in a northeast-southwest zone nearly normal to the vein strike.
Some are in the oxidized, some in the sulphide zone; the former are more
numerous. In some cases bonanzas were encountered at the surface; in others
they were found in depth, where the vein was barren at its outcrop. The size
of the bonanzas varies; one of the largest was encountered at a depth of over 300
feet and was elliptical, the greatest axis being over 3,000 and the smaller 1,300
feet, with a thickness of 8 feet.
The veins become impoverished at great depths. At the bottom they change
to barren galena and blende, too poor to repay working. However, certain
developments lead to the belief that at still greater depth new bonanzas might
be found. Most of the mines are only about 1,300 feet or less deep; in only
one has a little work been done as deep as 1,800 feet.
This district is similar to Tonopah in the character and age of the wall rocks
(Miocene andesites); in the nature of the alteration of the rock near the veins
(silicification near the veins, propylitic alteration farther away); in the structural
characters of the veins, which form a splitting and reuniting group; in the
general character of ores (both oxide and sulphide), and of gangue, though
adularia as a gangue material and selenides as ores have not been recognized at
Pachuca; and in the occurrence of the rich ores in bonanzas, which seems to be
due to the intersection of transverse fractures with the main vein zone.
" Be<;k, Richard, Erzlagerstiittcn, 2d ed., p. '277.
COMPARISON WITH SIMILAR ORE DEPOSITS ELSEWHERE. 269
OTHER SIMILAR MIXERAL, DISTRICTS IX MEXICO.
The deposits of Pachuca are similar in many respects to many other Mexican
ores. J. G. Aguilera remarks concerning the ores of Mexico in general:
"The silver deposits proper are found in eruptive rocks. A very few are found
in sedimentary rocks, and in these the silver is accidental and variable in quantity.
Where silver veins occur in sedimentary rocks it is evident that they are related to
and dependent upon andesitic Tertiary eruptive rocks.""
"The majorit\ r of the silver- veins of Mexico are in hornblende- and
pyroxene-andesite. As examples of fissure veins in eruptive andesitic rocks, we mav
mention the following: In Zopilote, Tepic, the veins have a northwest course, and
consist of quartz, blende, and pyrite, sulphides of silver, and small amounts of
galena. At Topia the veins extend northeast-southwest, and contain galena, blende,
a very small amount of pyrite, argentite, and pyrargyrite with a gangue of quartz
and calcite. At the mines of Tecatitliin, Jalisco, the veins strike about N. 40 W.,
and dip 45 to the southwest. The gangue is quartz with a little calcite, carry-
ing sulphides and antimonides of silver, pyrite, and chalcopyrite. At Chinipas,
Chihuahua, the veins occur in diorite and hornblende-andesite. The strike is
northeast, or in some cases northwest. The vein filling is quartz with argentite and
pyrite, oxides of iron, and dendritic manganese. At Ajijic, Jalisco, the veins are in
hornblende-andesite, with an east-west strike; there is an oxidized zone, and as depth
is reached complex sulphides are encountered. At San Sebastian and Los Reyes,
Jalisco, the veins have a quartz gangue with some calcite, complex sulphides, and
tellurides of silver and gold, a very little galena, blende, and pyrite. The veins of
the Rosario mines and San Nicolas del Oro mine, Guerrero, are in hornblende-
andesite; their course is northwest, or in some cases northeast, and they contain an
oxidized zone. Below this is the sulphide zone, containing argentite, ruby silver,
pvrite, and a small amount of chalcopyrite. The gangue is quartz, carrying gold.
Some of the veins of Sierra de Tapalpa, San Jose del Amparo, and Rosario, etc.,
have a north-south course, and dip west; the gangue is quartz with some barite. In
the oxidized zone they contain the carbonates of copper, and beneath this grav
copper and stibnite occur. At Tlalchapa, Guerrero, the lodes have a northwest-
southeast course, dipping to the northeast. The vein-filling is quartz with argentite,
pvrite, and blende; occasionally the vein quartz contains calcite and, in addition to
the minerals named above, galena and chalcop\ T rite. At the mines of Chacoaco,
south of Fresnillo, the veins extend nearly north and south, and contain quartz with
marcasite and pyrite. Some of the veins strike northeast-southwest, and contain
quartz, pyrite, and sulphides of silver. The veins of Real del Espiritu Santo are
found in augite-andesite.
"In the pyroxene-andesites may be found the deposits of Pachuca, Real del
Monte, El Chico, Tepenene, Capula, Santa Rosa, in Hidalgo; the mines of Santo
Domingo, in Jalisco; and some of the mines of Noxtepec, Guerrero. Among the
veins in andesite may be mentioned those of the following mines: San Pable Analco,
which in the oxidized zone somewhat resembles those of Pachuca; the California
"Trans. Am. Inst. Min. Eng., vol. 32, p. 513.
270 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
mines, in which part of the veins strike northeast and dip southeast and others have
their course toward the northwest and dip northeast. The gangue is quartz, carrying
galena, pyrite, chalcopyrite, and tetrahedrite. In the San Rafael mine, Jalisco, the
veins have a course N. 25 W. In the mines of Hostotipaquillo the veins contain
calcite and quartz with some rhodochrosite, a small amount of pyrite and black blende,
argentite, galena, chalcocite, and chalcopyrite. In the oxidized zone they contain
native silver, carbonates of copper, and a very small amount of copper oxide. It
would be tiresome to enumerate all the silver veins of Mexico which occur in
andesites, but as has been said, the majority of the silver veins of the country are in
various species of this rock, which Humboldt designated as metalliferous porphyries."
Rarely similar veins are found in rhyolite. 6
Perusal of the instances mentioned above by Aguilera shows that the veins are
all closely alike, not only in regard to their country rock, but to their tilling.
THE COMSTOCK LODE.
Pachuca is about 2,000 miles southwest of Tonopah, but a similar analogous
deposit (the Comstock) lies 150 miles to the northwest.
The Comstock lode is a vein 4 miles long which has formed in Tertiary eruptive
rocks, chiefly andesites, along a fault line having a maximum displacement of 3,000
feet. At both ends it branches and so dies out. It strikes east of south and dips
east. It was discovered in 1859, and worked up till the present day, but most
actively from 1861 to 1880. Up to June, 1902, it had yielded $369,566,112.61 worth
of ore, of which about 42^ per cent was gold and 57 per cent silver/ The rocks of
the district in the order of their succession are, according to Hague and Iddings/
andesite, dacite, rhyolite, andesite, and basalt. The andesites are coarse grained in
depth (diorites and diabases). Near the lode, and for some distance away, in a space
about 5 by 2 miles, the country rock (chiefly andesitic) is extremely decomposed,
the period of alteration having succeeded an andesitic eruption. The hornblende,
augite, and biotite have altered to chlorite, pyrite, epidote, etc., the feldspar to quartz
and an undetermined white aggregate. This altered andesite is the famous "propy-
lite." The basalt, which is the latest rock of the district, has not been altered in the
same way as the andesites. The alteration of the rocks and the lode was due to
solfataric action which accompanied the faulting.
The lode material is quartz, certain limited portions of which contained large
quantities of silver and gold (bonanzas), while the rest is low grade. Calcite is
much less than quartz in amount and is generally insignificant. Most of the bullion
has been derived from a bluish quartz, like that at Tonopah, the color being mainly
"Trans. Am. Int. Min. Eng., vol. 32, pp. 515-516.
6 Ibid., p. 517.
e Becker, G. F.. Mon. f. S. Oeol. Survey, vol. 3, pp. 9, 11. Also Kept, of the Director of the Mint for 1901, p. 169.
>l Hague, A., and Iddlngs, J. R., Bull. U. S. Oeol. Survey No. 17. Doctor Becker's determinations anil succession are
somewhat different, a follows: Granite, diorite, quartz-porphyry, diabase, andesitc, basalt.
COMPARISON WITH SIMILAR ORE DEPOSITS ELSEWHERE. 271
due to disseminated argentite, which is the principal ore mineral and is accompanied
by gold, probably free. Bunches of stephanite, polybasite, and ruby silver were also
found. In the bonanzas, near the surface, chlorides and native silver occurred.
Frequently the ore grew base, and carried large quantities of galena, zinc blende, etc.
Pyrite occurs abundantly both in the altered country rock and in the ore. The
mineralizing solutions are thought to have derived their heat from volcanic rocks,
and thus the general phenomena are classed as due to solfataric action, but the
materials precipitated, including the ores, are thought to have been derived from
the decomposed wall rock.
The workable bodies or bonanzas represent the smaller portion of the lode.
The value of the ore in them ranges from $15 a ton to (very locally) several
thousand dollars. They are encountered at various depths, from the surface down
to 3,000 feet. The vein down to nearly 2,000 feet contained 16 workable ore
bodies, while below this level the ore has proved mostly low grade. One large
bonanza (that of the C. & C. and Con. Virginia) extends vertically from about
1,250 to 1,950 feet below the surface, and has a greatest diameter of about 1,100
feet. It yielded about one-tenth the product of the lode." The ore minerals
were chiefly stephanite, argentite, and gold, the latter probably free.
The source of the heated waters which are encountered in the mines, and
which are thought to have accomplished the rock alteration and ore deposition, is
concluded from thermal surveys to be not less than 2 miles deep, and the heat
and the active reagents, such as carbonic and sulphydric acids, are thought to
have had a volcanic origin, while the waters may have had an atmospheric source.
The waters above 800 feet had a temperature of about 70 F., while from about
1,000 feet down hot waters of above 100 F., rising under pressure, were
repeatedly encountered.
The Comstock district is similar to Tonopah in respect to the character and
age of the rocks in which the lode lies (Tertiar}- andesites), in their "propy-
litic" alteration, in the nature of the gangue and ore, and in the occurrence of
the rich ores in irregular " bonanzas." The chief distinction is that the Comstock
consists of a single very strong lode, while at Tonopah there are a number, of
less size.
SILVER CITY AXD DE 1AMAR DISTRICTS, IDAHO.
Another region having many striking peculiarities in common with Tonopah
lies about 400 miles due north of Tonopah. The districts of Silver City and De
Lamar (5 miles apart) are situated in the Ohwyee Range, in southwestern Idaho.*
The range has a granite core, almost covered by Miocene rhyolite and basaltic
n This ore averaged about $80 per ton, with silver at 31.29 per ounce.
fcLindgren, W., Twentieth Ann. Kept. U. S. Geol. Survey, pt. 3, pp. 107-188.
272 GEOLOGY OF TON OP AH MINING DISTRICT, NEVADA.
lavas. The ores were discovered in 1863. The total production to 1899 was
313,448 ounces gold and 10,540,000 ounces silver. The deposits are normal
fissure veins, chiefly in rhyolite. In one type the principal ore minerals are
small quantities of argentite and chalcopyrite, with a gangue of quartz and ortho-
clase (adularia). The proportion of gold to silver by weight averages 1:120.
In the other type scarcely any sulphides are ordinarily visible, though occasion-
ally pyrite, argentite, and pyrargyrite occur. The gangue is quartz, pseudo-
morphic after calcite or barite. The relation of gold to silver by weight is about
1 : 10. At De Lamar there is a strong silicification of the country rock near
the veins, with the formation of abundant pyrite and marcasite, and a little
sericite. Farther away from the veins the country rock is softer and more
pyritized. The veins strike northwest and dip southwest, both strike and dip
varying considerably. The system comprises ten veins, 20 to 80 feet apart. The
strike of these veins is such that parts of the group are like some of the radii
of a circle, as is the case at Tonopah, and each vein may join and fork in the
manner of linked veins, both horizontally and vertically. The width of the veins
is from 1 to 6 feet, averaging 3 or 4 feet. The rich ore occurs in large, contin-
uous bodies extending from the surface to a depth of a 1,000 feet, dipping
gently (20-30) southeastward along the plane of the vein. They are generally
about 200 feet long arid ordinarily 1 to 6 feet thick.
In other veins the ore bodies do not extend so deep, and, while having often
a generally definite course, are so irregular and discontinuous as to constitute
irregular bonanzas rather than definite shoots." No considerable ore shoots have
been yet found below 1,000 feet, though the veins remain strong. Cerargyrite,
pyrargyrite, and argentite occur locally (the latter being common to nearly all
the mines), as well as polybasite, proustite, native gold and silver.
Besides occurring in rhyolite, some of the veins are also in granite and basalt.
The rock alteration and the ore deposition are considered to have been accom-
plished by ascending hot waters, whose nature is indicated by the silicification of
the rhyolite and the formation of adularia, chlorite, and epidote. The period of
formation is post-Miocene. The veins extend along the strike sometimes for a
mile or so, but average less; they die out on botli ends. The ore at present mined
at De Lamar goes $14 in gold and $2 in silver; in 1872 the average value of the
ore mined was from $12 to $t>0 per ton in different mines.
The districts of Silver City and De Lamar just described are similar to Tonopah
in that the ores occur in Tertiary volcanics, and are probably in both cases post-
Miocene in age; to a striking degree in the character of the ores and gangue
materials; in the structural character of the veins, which form a group knit together
a Op. cit., p. 152.
COMPARISON WITH ^SIMILAR ORE DEPOSITS ELSEWHERE. 273
by branches; in the general character of the alteration of the wall rock; and in the
occurrence of the rich ores in irregular bonanzas. The chief difference is that the
wall rocks are mainly rhyolite and not andesite.
RELATION OF THE DESCRIBED DISTRICTS TO TONOPAH.
Of all the described ore deposits of North America, therefore, Tonopah appears
to be most closely related to many of the Mexican silver veins, and also to the
Comstock in Nevada and the Silver City-De Lamar veins of Idaho. With Pachuca,
as is seen, the relation is intimate, and Ordonez's description of the veins of this
district would do, with a very little change, for a report on the Tonopah veins.
The chief difference is in the occurrence of manganese silicate in depth at Pachuca.
which has not been found at Tonopah, " and also the less content of gold, with
the absence of ruby silver. Ruby silver, however, occurs in the cognate and
contiguous Real del Monte district; also gold in considerable quantity occurs
with silver in some of the Mexican districts of this type. Those enumerated bv
Aguilera* all occur in hornblendic andesite.
This group of veins is characterized by the following features: They occur
in Tertiary volcanic rocks of similar character in the different localities, being
chiefly Miocene andesites or rhyolites. They constitute strong masses or frequently
branching and "linked" veins of quartz, which have as gangue essentially quartz,
with frequently a little calcite, while adularia, barite, rhodochrosite, or rhodonite
may also be present in limited amount. The ore is characteristically a silver-
gold one, silver being usually predominant in the values in vaiying proportions,
though the relative value may be -reversed, and in some extreme cases either
metal may occur with little admixture of the other. In any case the abundance
of silver or gold, or both, in reference to lead, zinc, iron, etc., is characteristic.
Silver sulphides, especially argentite, also stephanite and polybasite (with ruby
silver) and gold, probably largely in the free state, are distinguishing features
in the great majority of cases. Tellurides c and selenides may also be present.
Pyrite, blende, chalcopyrite, and galena are usually present in varying quantity.
Where they become predominant the vein becomes relatively low grade.
Tetrahedrite, stibnite, and bismuthinite'' are also known to occur. The wall
rocks are characteristically much altered to quartz, sericite, chlorite, calcite,
epidote, pyrite, and sometimes adularia, etc. Frequently the rocks nearest the
veins are chiefly altered to quartz and sericite, those farther away to the softer
"propylitic" alteration, consisting of calcite, chlorite, pyrite, epidote, etc.
a Since the above was written manganese carbonate has been found in the sulphide ores at Tonopah. See p. 89.
6 Aguilera, J. G., Trans. Am. Inst. Min. Eng., vol. 32, p. 519.
oAt Goldfield, Nev., and Jalisco and Tepic in Mexico (Trans. Am. Inst. Min. Eng., vol. 32, p. 601).
<iAt Goldfleld. See Bull. U. S. Geol. Survey No. 260, p. 138.
16843 No. 4205 18
274 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
The rich ores occur in irregularly outlined portions of the lode called
bonanzas. These bonanzas are of limited extent both horizontally and vertically.
They are believed to have arisen as a consequence of the irregular intersection
of transverse fractures or tissure.s with the main vein channel, producing maximum
deposition in these portions. Intervening portions may be low grade or barren.
In the oxidized zone, silver chlorides and bromides, free gold, manganese
oxide, etc., occur.
THE PETKOGKAPHIC PROVINCE OF THE GREAT BASIN.
After a study of the lavas of the Great Basin region of Nevada in 1900 the
writer" came to the conclusion that the whole region "southward into the
Mojave Desert, together with a portion at least of the Sierra Nevada, constitutes
a petrographic province; that is to say, it is underlain by a single body of molten
magma, which has supplied, at different periods, lavas of similar composition to
all the different parts of the overlying surface. The limits of this subcrustal
basin, however, are not yet defined in any direction."
The general sequence of lavas, roughly outlined, was concluded to be as follows:
1. Rhyolite (Eocene).
2. Andesite (Miocene).
3. Rhyolite with occasional basalt (Miocene-Pliocene).
4. Andesite (Late Pliocene-Early Pleistocene).
5. Basalts and occasional rhyolites (Pleistocene).
EXTENSION OF THE GREAT BASIN PETROGRAPHIC PROVINCE INTO
MEXICO.
Later in the same 3'ear, Ordonez, in a stud}- of the rhyolites of Mexico 6 over
a northwesterly trending belt extending from the northern boundary southward
past the City of Mexico, found that the author's conclusions were also applicable
to this province. He writes as follows:
With very slight differences, which are without decisive importance, one may
say that everywhere the relative order of eruptions, judging from the composition
and structure of the rocks, has been the same. Let us here present the example of
the Great Basin of Nevada. Many ranges of that region show a succession strictly
comparable with that of Mexico.
The general succession is found to correspond with that given by the writer
above, and the rhyolites occupy the same position and are of the same age (Miocene-
Pliocene) as those under No. 3. The andesites, which preceded the rhyolites,
correspond with No. 2, and are Miocene. <'
ogpurr, J. E., Jour. Geol., vol. 8, 1900, p. (S38.
fcOrdoflez, E., Boletln del Instiluco geo!6gieo de Mexico, No. 14, p. 66.
"Op. oil., p. 67.
COMPARISON WITH SIMILAR ORE DEPOSITS ELSEWHERE. 275
PROBABLE STILL FURTHER EXTENSION OF THE GREAT BASIN-
MEXICO PETROGRAPHIC PROVINCE.
In 1902 the author" recalled his description of the petrographic province,
which includes the volcanic region of Nevada, and noted the work of Ordonez.
He also called attention to the fact that later developments showed similar lavas
of similar age and succession in localities in the State of Washington and on the
California coast. His statement was as follows:
" Without being in danger of carrying this correlation to excess I may point
out that the Pliocene olivine-basalts of the Sierra Nevada* are abundantly present
in Oregon and Washington; that the British Columbia basalts are approximately,
at least, of the same period/ and that throughout the whole of Alaska and into
the Bering Sea occur olivine-basalts of Pliocene age.''
" Again, the abundance of basic andesities (typically augitic, often hypersthene-
bearing, and verging toward basalts) all belonging to one epoch (very late
Pliocene-Pleistocene), in a continuous belt in Alaska, running the whole length of
the Aleutian Islands and peninsula, turning the same angle as the chief orographic
and topographic features, and running down the coast past Sitka; e the occurrence
of the same rocks, belonging to the same age, in Washington and Oregon
(Mount Rainier, etc.); the extension of the belt through the Sierra Nevada
and along the western part of the Great Basin; finally its extension into Mexico-''
this is all striking and deserves recognition. Moreover, this belt of late Pliocene-
Pleistocene augite (hypersthene) andesites extends through Central and South
America, in the Andes. 9 ' In Alaska and in the Andes some of the cones of this
epoch are still active, but the majority have become extinct.
"It appears, then, that the whole extreme western part of the western
hemisphere (the Pacific coast of the Americas) is a zone occupied by what (at some
periods, at least) is and has been a single petrographic province.
"It remains to be seen whether this province is not continued into Asia with
the change of erogenic trends in Alaska from northwest to southwest. The line of
late Tertiary-Pleistocene volcanoes, which extends along the Aleutian Islands to
Kamchatka, is represented by 15 or 20 cones in this peninsula; this line, following
the general erogenic trend, runs southwest through the Kurile Islands, the islands
of Japan, and the Philippines, into the East Indies. Andesites largely pyroxene
andesites, and frequently hypersthene andesites are characteristic of this chain also,
as far as the famous volcano of Krakatua."
ngpurr, J. E., Trans. Am. Inst. Min. Eng., vol. 33, pp. 332-333.
fcSpurr, J. E., Jour. Geol., vol. 8, No. 7, chart, p. 643.
cDawson, G. M., Ann. Kept. Geol. Nat. Hist. Survey Canada, vol. 3, pt. 1, p. 37, B; also, Trans. Royal Soc. Canada, vol. 8,
sec. 4, p. 15. i
<i Spurt, J. E., Geology of the Yukon gold district. Eighteenth Ann. Kept. U. S. Geol. Survey, pt. 3, p. 250.
< Spurr, J. E., Reconnaissance in southwestern Alaska, Twentieth Ann. Rept. U. S. Geol. Survey, pt. 7, map 13.
1 Ordonez, Ezequiel, Las rhyolitas de Mexico, Boletin del Institute geo!6gico de Mexico, No. 14, p. 66.
uZirkel. Lehrbuch d. Petrographie, 2d ed., vol. 2, pp. 831-832.
276 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
A METALLOGRAPHIC PROVINCE COEXTENSIVE WITH THE PETRO-
GRAPHIC PROVINCE.
In the paper above referred to the writer brought forward the idea of metal-
liferous provinces (perhaps better, metallographic provinces) characterized by the
presence of certain metals; and pointed out that these provinces may or may not
be closely identified with petrographic provinces, although they probabh T generally
are so, to a certain extent at least."
Unquestionably the close relation between the Nevada mineral districts, Tonopah
and the Cornstock, with the far more numerous array in Mexico, and the individuality
of this group as compared with other known veins of North America, shows a
metallographic province, which in this case coincides with a portion of the petro-
graphic province previously mentioned.
In this metallographic province ores occur in Miocene andesites in the great
majority of cases, and their formation followed soon after the eruption of these
rocks. In occasionally recurring cases (such as Silver City and De Lamar, Idaho,
and others) they appear in Miocene-Pliocene rhyolites, which succeeded the andesites.
In general, however, the Miocene andesites of this province are, as Humboldt
noted, the metalliferous formation par excellence, and if the conclusions which have
been arrived at regarding Tonopah are correct (which coincide with a number of
similar conclusions concerning other districts reached by other authors), the ore is
due to the after actions of the eruptions in the shape of fumaroles, solfataras, and
hot springs. Moreover, since similar manifestations (of fumaroles, solfataras, and
hot springs) follow most volcanic eruptions, it is probable'that the metals deposited
by the after processes at this period arose from an unusual proportion of them in
the andesitic magma; indeed, the very definition of a metallographic province
implies this. The existence of such metallographic provinces is evident; and the
theory of their origin, as propounded by the writer, is like that long entertained
by many petrographers for the origin of petrographic provinces namely, that
the}' are formed by magmatic segregation. *
ORIGIN OF SHOOTS OR BONANZAS IN THE VEINS OF THIS
METALLOGRAPHIC PROVINCE.
Light is thrown upon the origin of the shoots, chimneys, or bonanzas in
this class of veins by the studies of the influence of cross fractures on their
formation in Tonopah, and the similarity between these bonanzas and those at
Silver City and De Lamar, Idaho, the Cornstock and Pachuca (fig. 78). At De
Lamar the shoot or chimney form is evident, some of the bonanzas having been
"Trans. Am. last. Min. Kng., vol. 33, p. 33f>.
fcSpurr, J. E., Trans. Am. lust. Mln. Eng., vol. S3, p. 336.
COMPARISON WITH SIMILAR ORE DEPOSITS ELSEWHERE.
277
followed downward over a thousand feet, yet the local irregularity of the outline
is like that of the typical bonanza. At Tonopah a similar shoot-like form with
a definite pitch has been discerned, but the developments thus far made do not
(B)
FIG. 78. Vertical cross sections showing forms of ore bodies or bonanzas in districts similar to Tonopah. (A ) Vertical section
of Poor Man and Silver Cord veins, showing extent of rich ore body in De Lamar district; after Lindgren, Twentieth
Ann. Kept. U. S. Geol. Survey, pt. 3, p. 152. (B) Portion of projected vertical section of the Comstock lode, Nevada,
showing some of the chief bonanzas on the vein; adapted from Becker, Men. U. S. Geol. Survey, vol. 3, atlas.
(O Projected vertical section of a portion of the Cristo vein, Pachuca, Mexico, showing bonanzas on the vein: after
Aguilera and Ordonez, Boletin del Instituto geo!6gico de Mexico, Nos. 7, 8, and 9.
show so great a persistency as at De Lamar. At Tonopah the connection of the
shoots with cross fractures is evident, and the localization of the ore deposition
at intersections of especially fractured zones seems the correct explanation. It
278 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
is doubtful, however, if, when the bonanzas in the Tonopah veins shall have been
worked out, the shoot-like form will always be discernible; in the case of the
richer eastward-pitching shoots of the Mizpah vein, for example, the spaces
between the shoots should probabty be considered together with them, in the
larger sense, as parts of one great bonanza, whose eastward pitch and shoot-like
form would be less emphasized or not at all.
In the case of Pachuca, the bonanzas are irregular or roughly elliptical and
are not shoot like; yet the fact observed by Ordonez, that the bonanzas on the
different veins group themselves into a definite zone running transversely across
the strike, is hardly to be accounted for except by the explanation" arrived at
in the case of Tonopah, that the bonanzas are due to the influence of an intersecting
fracture system. At the Comstock the bonanzas are similar to those in Pachuca,
although no local evidence has been found explaining their origin.
The above explanation is readily acceptable for bonanzas that are elongated
into definite shoots, and are actually known to be associated with and dependent
upon cross fracturing, as in Tonopah; but it is hot so easily acceptable, perhaps, in
the case of wholly irregular bodies, such as those of the Comstock. Yet at Tonopah
the bonanzas are irregularly cut off, and do not continue indefinitely downward
on the pitch; and to this limitation the explanation of the controlling effect of
cross fractures must unavoidably be extended. Indeed, an inspection of the
platting of fig. 24, showing the principal observed faults and fractures in the
Mizpah mine, and a reflection that this is diagrammatic, while the real fractures
and their intersections will be much more varied and localized, shows that the
intersections of such mazes (such intersections constituting the tortuous channels
of most active circulation) with the main vein fractures will often be quite irreg-
ular will only approach a shoot-like form when dominated by some stronger set
of cross fracturing, and will cease to produce ore bodies or bonanzas of definite
form when there is no controlling fracturing, and now one fracture, now another,
invites and controls the circulation.
EXISTENCE OF A MAJOR PACIFIC TERTIARY PETROMETALLO-
GRAPH1C ZONE.
Some further notes may be added to the above references (see p. 275) to the
extension of the belt of late Tertiary -Pleistocene andesites.
In the region of Krakatua (situated between Sumatra and Java) the belt of
recent and active volcanism turns eastward and passes through the East India
Islands and adjoining island groups, paralleling the Australian coast, then curving
a Mr. 8. F. Emmons informs me, on reading the manuscript of this report, that the above explanation Imd been
adopted at l'iichm-H when he was there in 1901.
COMPARISON WITH SIMILAR ORE DEPOSITS ELSEWHERE. 279
southward extends through New Zealand. Still farther southward the zone
extends through the Macquarie Islands, and beyond this, in antarctic regions, in
Victoria Land, where are the volcanic cones of Erebus, Terror. Melbourne, and
Discovery, of which one Erebus is in almost continuous eruption.
The prolongation of the zone goes through the unexplored antarctic regions,
very near to the south pole, and on the other side there are Pleistocene and
recent volcanoes in the South Shetland Islands and other near-by land. Not far
beyond this the belt comes to Tierra del Fuego, a desolate volcanic region.
Thus the entire circuit of the earth has been made. This girdle, extending
around the world and measuring some 35.000 kilometers, has been called the
"circle of fire" by geographers, and is the theater of the world's most extensive
and active volcanic manifestations. Within this circle, in the Pacific Ocean, are
lesser volcanic belts." The major volcanic belt, when viewed on a globe or a
perpendicularly projected map, 6 has not a circular form, but rather that of a
great somewhat elongated rectangle, inscribed upon the sphere; the two longer
sides run northwestward and consist of the northwest American Pacific coast on
one side and the stretch from the Philippines to the south pole on the other;
the two shorter sides run northeastward and consist of that portion lying parallel to
the Asiatic coast line on the one side and that portion in and near the antarctic
regions on the other. This figure, however, is broken by irregularities consisting
of curves and angles; and the volcanic chains are characteristically arranged in
curves or "garlands,"'' though in many cases it may prove true that such
apparent curves are in reality combinations of straight lines, as is the case with
the changes of trend in the volcanoes of Java and Sumatra. <'
The Pleistocene-Recent volcanoes of the East Indies belt, which began their
activity toward the close of the Tertiary,' have emitted chiefly andesites with a
less amount of closely related basalt. Hornblende or pyroxene andesite. or both,
occur in Java, Borneo, Celebes, and neighboring islands. Most of the pyroxene
andesites have more hypersthene than augite.-'"
In New Zealand hornblende-andesites are common.'' Concerning the recent
lavas of the Macquarie Islands and other antarctic volcanic regions, there appears
to be little information; the lava of Mount Terror, in Victoria Land, is reported as
"basic."*
a See Reclus, Elisee, Nouvelle geographic universelle, vol. 14. pp. 41. 42: Suess, E., La face de la terre, Paris, vol. 2, p.
837; Bonney, Volcanoes, London, 1899. pp. 259-260: Ferrar, H. T., Geog. Jour., Apr., 1905, pp. 374, et seq.
& Reclus, op. cit., p. 43.
c Suess, E., op. cit., p. 339.
<t Bonney, Volcanoes, London, 1899, p. 226.
e Zirkel, Lehrbuoh d. Petrographie, vol. 2, p. 828.
/Zirkel, op. cit., pp. 615, 616, 828, 829.
orHutton, F. W., cited by Zirkel, op. cit., vol. 2, p. 618.
* Ferrer, H. T., Geog. Jonr., Apr., 1905, p. 375.
280 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
There appear, then, reasons for believing that the belt of very late Pliocene-
Pleistocene-Recent andesitic eruptions continues farther than suggested in the
writer's paper quoted above (p. 275), and even that they are characteristic of the
whole great "circle of fire;" and this uniformity seems to indicate a single
major petrographic province for this period, extending around the whole zone."
In some cases the analogy of the less-known Asian and Australasian portions
of this belt with the North American part is known to extend back of the Pleis-
tocene. In the East Indian archipelago, according to Zirkel, there was a general
eruption of pyroxene-andesite at the end of the Eocene or beginning of the
Miocene, since the early Miocene sediments already contain some andesitic material.
This period would correspond to group No. 2 of the scheme of succession presented
on page 68.*
In New Zealand the Hauraki Peninsula is made up almost wholly of Tertiary
igneous rocks, mostly andesites, with accompanying heavy deposits of volcanic
agglomerates; these andesites and accompanying tufl's and breccias are regarded
as of late Eocene and early Miocene age. In places they are covered by rhyo-
lites and rhyolitic tuffs of early Pliocene age. c These andesites and rhyolites,
respectively, fall into groups 2 and 3 of the scheme on page 68.
It is also probable that the coextension of the metallographic and the petro-
graphic provinces is greater than above established, for at many other points along
the belt of the petrographic province, in the Andes of South America (for example,
in Peru*), veins are reported having, so far as can be made out, a mode of occur-
rence, age, and composition similar to those of Mexico. The mines at Quespasia in
that country are in highly altered augite-andesite. The ore minerals are pyrargy-
rite, polybasite, and other rich silver ores, with galena and blende, and a little
copper pyrite and iron pyrite. In their richest portions they contained on an
average 2 per cent silver/ These richest portions in the Peruvian mines of this
type are like the Mexican bonanzas, and are called, in Peru, tajos.f
At Cerro de Pasco, also in Peru, the argentiferous formation is a metamor-
phosed Mesozoic sandstone intruded by altered andesite. The ore consists of
free silver, silver sulphides and antimonides, lead carbonate and sulphide, various
oThese andesites. constituting the most recent lava of this province, appear to be a distinctly later group in the
volcanic succession than the youngest (No. 6) enumerated in the scheme on p. 68. They may be designated as group
No. 6, Pleistocene and Recent, and the recurrence of lava of this composition, similar to Nos. 2 and 4 (early Miocene and
late Pliocene andesites, respectively), suggests the beginning of a new cycle of magmatic differentiation. \\ host- continua-
tlnn will bring about, for the fourth time In the history of this volcanic epoch, the eruption of basalts and rhyolites
similar to Xos. 1, 3, and 5. (See Spurr, J. E., Jour. Geol., vol. 8, No. 7. pp. 637-646.)
In the region near Tonopah there is one probable representative of these latest andesites. In Mono Lake, Cali-
fornia, 90 miles west of Tonopah, are ten or fifteen volcanic cones of very recent date, the lavas being in part hypersthene-
andeHite. In part rhyolltc. (Russell, I. C., Eighth Ann. Rept. U. S. Geol. Survey, pp. 374, 375, 377, 380.)
6 See Spurr, J. E., Jour. Cieol., vol. 8, No. 7, p. 637.
Park, James, elted by Lindgren, W., Eng. and Min. Jour., Feb. 2, 1906, p. 218.
' FuchH ct de Launay, Gltes metal 11 feres, vol. 2, p. 829.
Beck, Erzlagerstfitten, 2d ed., p. 277.
/ Fucha et de Launay, op. cit., vol. 2, p. 831.
COMPARISON WITH SIMILAR ORE DEPOSITS ELSEWHERE. 281
copper minerals, zinc, and iron pyrite. Twenty-seven miles from Cerro de Pasco
are veins in quartz-porphyry (rhyolite?). The ore contains, besides silver min-
erals, various copper minerals, galena, sphalerite, bismuthinite, and stibnite."
In view of the presence of selenium at Tonopah, the occurrence of this
element at other places along this Pacific petrographic province in America is
of interest. At Guanajuato, northwest of the city of Mexico, selenides, including
a sulpho-selenide of silver, occur in argentiferous veins in hornblende andesite. 6
At Tasco, 180 miles southeast of Guanajuato, crystallized selenide of silver
occurs/ In the South American Andes selenides occur at the Cacheuta silver mine,
province of Mendoza, Argentina, whose vein is in "trachyte."'' They include the
selenide of lead and copper, that of copper and silver, and others. The latter selenide
occurs also in the Chilean Andes, at Copiapo and Flamenco, and elsewhere/
It is also interesting, in regard to the speculations of the author above quoted
concerning the Asiatic prolongation of the petrographic province, to note that in
Japan veins of argentiferous quartz are being worked, which occur in the midst of
Tertiary eruptives, and which belong to the Comstock type.-'' Explicit information
concerning these has lately come to hand.* Tertian* and Quaternary volcanic rocks
are widely distributed in northern Japan. The Tertiary rocks include rhyolite
(as old as the beginning of the Tertiary), andesite, and basalt. Metalliferous veins
in Tertiary andesite and rhyolite are among the most important mineral resources
in Japan. The older andesites have often suffered alteration by mineral waters
and gases.
The Hoshino mines, in Hoshino-mura, Chikugo province, are in augite-andesite.
The deposits are quartz veins containing pyrite, blende, gold, and silver. The
Serigano mine, in Satsuma province, is in augite-andesite; the gangue is quartz, and
the metallic minerals are pyrite, chalcopyrite, gold, and silver. The Yamagano
district, between Satsuma and Osuini, is at present the most promising in the
country. Here are numerous veins in augite-andesite. The gangue is quartz, often
containing calcite and pyrite. The ore is native gold associated with argentite, and
rarely with chalcopyrite. The proportion of gold to silver is about 5 to 1. At the
Ponshikaribets mine, Shiribeshi province, the country rocks are Tertiary tuffs, cut
by andesite dikes. The gangue is rhodochrosite and quartz, the ores are auriferous
argentite, galena, chalcopyrite, and blende. The mine of Aikawa, in Sado province,
has had an enormous production. The veins are in augite-andesite and Tertiary
o Mason, Russell T., Eng. and Min. Jour., June 8, 1905, p. 1092.
fcTrans. Am. Inst. Min. Eng., vol. 32, p. 501. Dana, System of Mineralogy, 6th ed., p. 1025.
cDana, op. cit., p. 52.
<*Fuchs et de Launay, GHes m^talliferes, vol. 2, p. 832. The "trachyte" is probably andesite.
Dana, op. cit., pp. 53, 54. .
/ Fuchs et de Launay, op. cit., p. 832.
a Geology of Japan, Geol. Survey, Tokyo, 1902, pp. 18, 19, 118, 124-171.
282 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
tuff*. The gangue is quartz, with calcite, rarely with dolomite and gypsum.
The ores are chiefly native gold and silver, and argentite, associated with chalco-
pyrite, pyrite, blende, and galena; rarely stephanite, pyrargyrite, marcasite, and
arsenopyrite. At the Kosen mine, in Tajima province, the veins are connected with
"propylite'' dikes in granite. The gangue is quartz, the ore auriferous argentite,
with pyrite and galena. The Tasei mine, Tajima province, is in "propylite,"
rhyolite, and Tertiary tuffs. The gangue of the vein is quartz, with some calcite
and rhodochrosite. The ores are argentite and native gold and silver, with
chalcopyrite, pyrite, galena, blende, and malachite. At the Kanagase mine, not
far distant, the country rocks are similar; the gangue is quartz and calcite, and
the ores are chalcopyrite, bornite, pyrite. tetrahedrite, argentite, galena, stibnite,
pyrargyrite, blende, bismuth, and native silver and copper. At the Omori mine,
Iwami province, the rocks are bypersthene-quartz-andesite, andesite agglomerate,
and Tertiary strata. The ores are in veins and impregnation deposits. The gangue
is quartz; the ore native silver, argentite, siderite, malachite, and auriferous and
argentiferous chalcop3 r rite. The Okuzu mine, in Ugo province, is in Tertiary
tuff and augite-andesite. The gangue is quartz: the ore auriferous chalcopyrite,
with pyrite and rare blende. Silver is rare. At the Mizusawa mine, Ugo province,
the country rock is augite-andesite and Tertiary strata. The ore is a mixture of
barite. argentite, blende, galena, pyrite, quartz, calcite, chalcopyrite, and probably
stephanite. At the Tsubaki and Hachimori mines, Ugo province, veins in andesite
carry ores like the last named. At the Shirayama mine, Ugo province, veins in
Tertiary tuff and augite-andesite have a gangue of quartz and barite, and contain
argentiferous "galena, blende, pyrite, and chalcopyrite. At the Innai mine, Ugo
province, the country rock is Tertiary "propylite." the gangue is quartz and
rhodochrosite, the ore minerals stephanite, argentite, pyrargyrite, chalcopyrite,
pyrite, galena, and blende. At the Towada mine, in Rikuchu province, the vein
occurs in Tertiary tuff, associated with augite-andesite. The ore is auriferous
argentite and chalcopyrite in a clay and gypsum matrix. At the Omaki mine,
Ugo province, the country rocks are Tertiary tuffs and andesite. The ore is
argentite, silver oxide, copper and iron pyrite, and galena, with barite and g\'psum
as gangue minerals. At the Hisanichi mine, Ugo province, is a vein in Tertiary
strata and augite-andesite. The ore is galena, chalcopyrite, blende, and pyrite.
Many of the important metalliferous veins in northern Japan and Chugoku
are also in rhyolites. In the Kanahira mine, in Kananomura, Kaga province, the
veins are in rhyolite; the gangue is barite and quartz, the ores are native gold,
blende, and pyrite. At the Matsuoka mine, in Ugo province, the ore is a stockwork
at the contact of rhyolite with Tertiary strata; the ores are argentiferous galena,
blende, and pyrite, carrying gold. At the Handa mine, Iwashiro province, the
COMPARISON WITH SIMILAR ORE DEPOSITS ELSEWHERE. 283
veins are in rhyolite and Tertiary strata. The gangue is quartz with calcite and
amethyst; the ore is auriferous argentite, with blende; galena, pyrite, and native,
silver are sometimes found. At the Takadama mine, Iwashiro province, quartz
veins containing auriferous argentite occur in rhyolite and Tertiary strata. The
Kuratani mine, in Kaga province, contains veins in rhyolite and Tertiary tuffs.
The gangue is rhodochrosite, with barite and calcite; the ores contain argentiferous
galena, blende, pyrite, and jamesonite, and carry gold. At the Tagonai mine, Ugo
province, the veins are in Tertiary tuff, augite andesite, and rhyolite; the gangue
minerals are quartz and barite, the ores argentiferous galena, blende, and pyrite.
At the Hata mine, Ugo province, the rocks are Tertiary tutf and rhyolite; gangue
minerals are quartz, calcite, and barite; the ores are argentite, galena, pyrite, and
chalcopj'rite. At the Kuromori mine, Iwaki province, the vein is in rhyolite.
The gangue is quartz, often amethystine; the ore is argentite, with blende. At
the Kosaka mine, in Rikuchu province, the ore is an impregnation in Tertiary tutf,
with rhyolite and dacite intrusions; it consists of lead and copper carbonates,
copper sulphate, native copper and silver, and barite. At the Hatasa mine, Mino
province, the rocks are rhyolite (quartz-porphyry) and andesite. The veins consist
of quartz containing argentiferous chalcopyi'ite, galena, argentite, blende, and
pyrite. The Waidani mine, Bizen province, is in rhyolite; the ores are argen-
tiferous chalcopyrite, blende, and galena.
Besides the examples above cited, other veins of closely related types, but
often containing a larger amount of the baser ores (lead, zinc, and copper) than
the more abundant cases above, occur in or near Tertiary andesite or rhvolite.
Some information is available concerning certain East Indian ore deposits on
islands lying south of Japan along the belt characterized by similar Tertiary and
Pleistocene volcanics. In the whole of the Dutch East Indies, according to S. J.
Truscott," the gold (which is always accompanied by a larger amount of silver)
occurs in reefs, veins, and impregnation zones, in altered andesite (porphyrite),
or near the contact of such a rock with Devonian slates, in which slates there
are sometimes similar though less extensive occurrences. The ore deposition
probably took place in the Tertiary.
One of the principal productive centers in this region is the mine Redjang
Lebong, in the southwest part of Sumatra. Here the ore, which occurs in altered
andesite, has a gangue of fine-grained silica, with often some calcite. The gold is
finely disseminated and is rarely visible; it exists free and in combination with
silver, in the proportion of 1 to 10. At depth this silver probably exists as
sulphide, connected with pyrites. Bullion from this mine gives the following
analysis: Gold and silver, 91.52 per cent; selenium, 4.35; copper, 1.82; lead, 1.65;
zinc, 0.48; iron, 0.14; total, 99.96. Tellurium was not found.
a Trans. Inst. Min. Metal., vol. 10, pp. 52-73.
284 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
The similarity of Redjang Lebong to Tonopah has been commented upon by
Mr. Percy Morgan," judging from the writer's earlier description of Tonopah b
and from reports concerning Redjang Lebong. This similarity was also called
to the writer's attention by Mr. L. Hundeshagen, who has personally visited both
districts. The discovery of selenium in the Tonopah ores, in somewhat the same
proportion as indicated in the above analysis, subsequent to the comparisons
made by these gentlemen, strikingly strengthens the resemblance.
Five miles west of Redjang Lebong is a similar occurrence of gold ore in
altered andesite, at Lebong Soelit.
In southeastern Borneo gold occurs in altered andesite/
The northern arm of Celebes is gold bearing. The mine at Palehleh is in
altered andesite, often having a dioritic aspect. The ore contains pyrite, galena,
zinc-blende, and copper pyrite, with a little antimony and arsenic, and carries
gold and silver, of which the sulphides contain gold about 4 ounces and silver
12 ounces to the ton/' Forty miles west of Palehleh, at Soemalata, the ore is in
andesite or u porphyrite." f The ore is like that at Palehleh heavy sulphides with
some quartz gangue, more often feldspar. Ten miles west of Palehleh, at Denuki
Bay, are ores similar to those at Soemalata, but containing more quartz, in altered
andesite. Analysis of the sulphides shows zinc, 31 per cent; lead, 8 per cent;
copper, 1 per cent; gold, 5.3 pennyweights to the ton; silver, 4.9 ounces to the
ton; arsenic, 2 to 4 per cent; antimony, 4 to 6 per cent. On the south coast of
the peninsula, at Totok, are heavy auriferous quartz veins in altered andesite;
also 6 miles southwest of Totok, at Kataboenan, where the andesite has been
intensely silicified on each side of a central fracture, forming a wide mass of ore
of the following average composition: Gold, 4 pennyweights per ton; silver, 1
ounce per ton; sulphides, 6 per cent; vein quartz, 3 per cent; the remainder
being altered andesite.
Still farther along the Tertiary-Pleistocene volcanic zone lies New Zealand.
The late Eocene-early Miocene andesites of the Hauraki Peninsula, in the north
island of New Zealand, contain throughout veins bearing gold and silver. The
whole peninsula has produced $50,000,000. Near the veins the 'andesite has been
altered to calcite, chlorite, serpentine, quartz, and pyrite. The ore in the Thames
district is chiefly native gold alloyed with 30 to 40 per cent silver. Associated
minerals are dolomite, pyrite, chalcopyrite, zinc- blende, galena, stibnite and ruby
silver, arsenopyrite, and native arsenic/ Great masses of quartz are very low
grade, but bonanzas of very rich ore occur at the intersection of feeders with the
main vein.
a Eng. and Min. Jour., May 4, 1905, p. 862. rtTruscott, loc. cit., pp. 66-67.
t> Ibid., May 2, 1903. eTruscott, loc. cit., p. 68; also Suess, E., I>a face do la terre, vol. 8, p. 341.
oTriuicott, 8. J., los. cit., p. 63. /Lfndgren, W., Eng. and M!n. Jour., Feb. 2, 1905, p. 218.
COMPARISON WITH SIMILAR ORE DEPOSITS ELSEWHERE. 285
At Karangahake the ore is argentite, with a little pyrite and free gold, in
drusy, fine-grained quartz; stibnite and calcite, with some siderite and a little
nickel and cobalt, also occur. At the Waihi mine, which up to the end of 1903
had produced $15,000,000, the ores are in altered andesitic rock, and have been
covered by later rhyolitic flows. The oxidized quartz contains argentite and free
gold, with black oxide of manganese, and oxides of nickel and cobalt; the sulphide
ores contain pyrite and blende, with a little nickel, cobalt, and selenium. The
country rock is altered, the secondary products including pyrite, carbonate (calcite?),
and serpentine. In the veins and veinlets the gangue minerals are quartz, calcite,
and adularia.
There are two distinct flows of rhyolite overlying the andesite, of which the
older has a remarkable flow structure, giving it a brecciated appearance." There
has been a later period of mineralization, producing gold-bearing lodes in rhyolite.*
Mr. Lindgren calls attention to the striking similarity between the Waihi mine
and the De Lamar mine, in Idaho, described by him a mine already likened to
Tonopah by the writer (see p. 271). Mr. Morgan, judging from a personal knowl-
edge of Waihi and the writer's description of Tonopah, c calls attention to the close
resemblance of these two districts.
Tellurium occurs in some of the New Zealand districts, varying from traces
up to 12 ounces per ton, in picked samples. Samples from various districts show
the following types of ores in regard to gold, silver, and tellurium: Coromandel,
25 per cent mispickel, gold 200 ounces, silver 90 ounces, a little tellurium; Tapu,
2i ounces gold, 250 ounces silver, 7 ounces tellurium; Waiomo, gold 15 ounces,
silver 600 ounces, tellurium 12 ounces; Waiomo, Monawai, gold 2 ounces, silver
40 ounces, tellurium 4 ounces. No tellurium was detected in samples from
Waihi, Jubilee, Komata, Karangahake, and Great Barrier, in which the gold
and silver bore the following proportions: Gold 24 ounces, silver 760 ounces;
gold 8 ounces, silver 150 ounces; gold 600 ounces, silver 160 ounces; gold 2
ounces, silver 256 ounces; and gold 2 ounces, silver 200 ounces. Thus tellurium
has been found in a line stretching from Coromandel to Maratoto, but nowhere
to the east. rf
Besides the selenium noted above in the Waihi mine, Mr. Allen found selenium
in the ore at Great Barrier. In this New Zealand region selenium and tellurium
have not been proved to be present in the same district. This is especially
interesting in comparing New Zealand with the Nevada region, where selenium
"Compare the description of the Tonopah rhyolite dacite, p. 41.
ftLindgren, ut supra; also Morgan. Percy, Eng. and Min. Jour., May 4, 1905; Trans. Austral. Inst. Min. Eng., pp. 1&1-187.
cEng. and Min. Jour., May 2, 1903.
d Allen, F. B., Trans. Austral. Inst. Min. Eng., vol. 7, p. M.
286 GEOLOGY OF TONOPAH MINING DISTRICT, NEVADA.
without tellurium has been found at Tonopah, and tellurium without selenium at
Goldfield, 28 miles south.
Enough data has been given above to indicate the coordination of an inter-
esting set of phenomena. The greatest of the earth's oceans is rimmed by the
greatest of the earth's volcanic belts. This "circle of fire," whether it runs along
the coast of the mainland, as in the Americas, or along chains of islands, as in
the Asian and Australian regions, follows faithfully the Pacific-fronting outlines
of the continents of South America, North America, Asia, and Australia, and
demarks the continental from the oceanic areas. In the Asian, Australasian, and
Australian regions, indeed, the outlying islands rather than the continents have
been held, from a geological viewpoint, to represent the limits of the Pacific
Ocean. a Topographically the volcanic belt is also marked throughout its course
by a line of bold and towering mountains, the consequence of active and com-
paratively recent extravasation and uplift.
For the next step in coordination the data are not so complete, but our informa-
tion goes to show that remarkably similar lavas have been erupted from the active
and recently extinct cones which are ranged along this belt.
A still smaller fund of information is available for the next step, but we are
led to it by all that we can learn. It is that the ''circle of fire" existed as such
throughout most of the Tertiary, and, moreover, that the similarity of the more
recent lavas was paralleled by like similarities at the different earlier stages of
eruption. Roughly speaking, the idea is suggested that throughout the zone the
order, period, and nature of the different erupted lavas have been approximately
the same.
This belt also contains an extraordinary number of extraordinarily rich silver-
gold ores (as well as those of lead, copper, zinc, etc.). These ores are contained in
or associated with Tertiary andesites and to a less extent rhyolites (chiefly Miocene
andesites and Pliocene rhyolites); and wherever they occur the nature and propor-
tion of the ore and gangue minerals and the nature of alteration of the country
rock are uniform to a surprising degree. 6 Similar mineralizing solutions, dependent
upon the eruption of similar lavas at the same geological period, are attested.
The significance of the geographic coincidence of these different phenomena,
occurring on so stupendous a scale as to stand out unmistakably from the confusion
of detail of the world's geology, has yet to be thoroughly understood. These
geographically coinciding phenomena may be summed up as follows:
1. The borders of the earth's greatest ocean.
2. The most persistent of the earth's lofty and bold mountain belts.
aVon Drasche, P., cited by guess, E., La face de la terre, Paris, vol. 2, p. 339.
&At and near Schemnltz, in Hungary, ore veins and ore similar to those of this great Pacific province, and they
occur under simiar geologic conditions. Otherwise no good example outside of the province has come to the writer's
notice.
COMPARISON WITH SIMILAR ORE DEPOSITS ELSEWHERE. 287
3. The belt of the earth's most active and extensive recent vulcanism.
4. A belt showing .similar recently erupted lavas.
5. A belt showing similar lavas erupted during the Tertiary.
6. A belt of enormous and roughly uniform later Tertiary mineralization,
involving great concentration of silver and gold.
When it is considered that solutions accompanying (and presumably emanating
from) Miocene andesites (to a less extent Miocene-Pliocene rhyolites) in this
particular restricted zone have produced a very large proportion of the world's
available supply of the precious metals, the rare and special nature of the occur-
rences which have called these ore deposits into being becomes evident, and it
becomes impossible to entertain any explanation based upon processes uniformly
distributed throughout the world.
INDEX.
A.
Page.
Adularia, alteration of earlier andesite to 207-209
analysis of 87
character of 86-N7, 228
formation of, chemistry of 230-231
occurrence of 22, 229-230
explanation of 212-218
origin of 22,228,231
sericite and, relations of 227-228
view of 208
Aguilera, J. G., on Mexican ores 269-270 |
Albite, occurrence of, explanation of 212-213
Allen, E. T., analysis by 252 ]
Alteration. See Rock alteration.
Ancylus? sp., occurrence of 67
Andesite, earlier, alterations of 22, 31-32, 207-238
alterations of, diagrams showing 218, 234, 218
analyses of 57, 64, 65, 216
character of 21,31-32,63,164-165,253
classes of 32
later andesite and, comparison of 35
occurrence of 21, 32, 101, 164-165, 186-187
specimens of, description of 213-216
veins in 22, 83-%
age of 83
circulation channels in 831
cross walls in 85
mineralization of 85
minerals ^^ 90
view^I 208
mines ancrwospects in, description of 115-191
ores of ...." 86-90
origin of 84-85
oxidation of 90-94
Andesite, later, alteration of 23, 238-252, 254
alteration of, figures showing 242. 243
period of 250-252
analyses of 57, 66
character of 21,33-34,253
diagrams showing 242
earlier andesite and, comparison of 35
occurrence of 21, 34-35, 101, 179
origin of 22
shafts in ; 205-206
study of 245-248
veins in 261
Ararat Mountain, erosion on 113
rocks in 30,36,49,55,101,191-192,253-254
veins in 22,101-104
location of, map showing 102
Argentite, occurrence of 22,92,94-95
Arid lands, erosion in 111-112
16843 No. 4205 19
Page.
Bagg, R. M., jr., fossils determined by 69
Barium, alteration of 233-234
Basalt, age of 56
analysis of 57.64,65
character of 21, 56, 63
occurrence of 45, 55
origin of 21-22
Becker, G. F.. on mineralizing waters 237
on propylite 236
on sideritc 249
on the Comstock lode 259,265
Belle of Tonopah shaft, location of 198
rocks and veins in 198-199
water in 106
Belmont Company, property of 125
Belmont shaft, depth of 106, 19S
lava from, analysis of 57, 60, 64
location of 193
rocks in 30,61.193-194
section near, figure showing 193
Big Tono shaft, data concerning 200
depth of 106
Biotite, alteration of 207,232
Bischof , Gustav, on feldspar alteration 212-213, 233
Blende, character of 88
occurrence of 22
Bonney, T. G., on volcanoes 260
Booker & Bradford, map prepared by 26
Borneo, rocks of 284
Boston Tonopah shaft, depth of 106, 192
location of 192
rocks in 101,192
Brauns, D., on mnscovite 281
Brogger, W. C., diagram by 242
Brougher, W., mention of 26
Brougher dacite, analysis of 60, 65
areas of, diagram showing 50
character of 44-48,62-63
contacts of, shafts at 200
faulting due to 47
hornblendes in 62
occurrence of 44
origin of 46-47
Siebert tuffs and, relations of, figure showing 53
Brougher Mountain, character of 44
erosion on 113
faulting near : 75, 146
history of 67
lava of, analysis of 57, 58, 60, 65
origin of .' 146
rocks of 36,46,49-51,55,60-61
289
290
INDEX.
Page.
Brougher Mountain, section near, diagram showing.. 41,43
view of 46
Brougher shaft, section in, figure showing 116
Burro fault, discovery of 79
Burro veins, data concerning 127-129
structure of, section showing 128
Butler. Jas. L., gold discovered by 21,25-20
leases by 26
on Monitor Valley springs 257
Butler Mountain, character of 44, 110-111
elevation of 25
erosion on 113
eruptions on 67
lava from, analysis of 58, 60, 65-66
rocks of 36,45,55,60-61
section near, diagram showing 44, 48, 49
view from 24, 44
view of 44, 46
Butte Tonopah shaft, data concerning 199
C.
Calcite, alteration of earlier andcsite to 210
formation of 22, 229
veins of 101-104
California fault, location of 167
rocks at 38,79
Cambrian rocks, occurrence of 30
Cavities, filling of, veins formed by 85
Celebes, rocks of 284
Chalcopyrite, character of 88
occurrence of 22, 95
Chlorite, alteration of earlier and esite to 22,210
Cinder cones, occurrence of 45
Circle of fire, location of 279
ores of 286
rocks of 279-280, 286
Circulation of water, channels of 85
channels of, alteration in 210
Claims. See Mines.
Coal, character and occurrence of 29
Comstock, Nev., alteration at 211-212
ores of Tonopah and, comparison of 22,
270-271,273-274.278
section at, figure showing 277
underground temperatures at 265-266
figure showi ng 265
Contacts, phenomena of 44, 49, 79
Corbicula occidentalis. occurrence of 67
Coscinodiscus radiatus, occurrence of 70
Crislo vein, section of, figure showing 277
Cross faulting. .See Faulting, cross.
Cross walls, effects of 85
figure showing 173
occurrence of 119
D.
Dacite, analyses of 57, 58, 60, 65, i>6
character of 21, 36-51, S8-61
classification of 59-60
contact of, section showing 48
fault blocks and, displacement of, diagram show-
ing 46
occurrence of 21,36-51
origin of 22, 63
sections of, diagrams showing 40, 41, 44, 45, 48
shattsin 200-204
Siebert tuff and, contact of, figure showing 45
MI- w nl 46,48
Page.
Dall, W. H., fossils determined by 66
Dana, J. D., on pyroxene 248
Daubree, A., on underground water 254
De Lamar, Idaho, ores of 276-277, 285
ores of Tonopah and, comparison of 22, 271-274, 277
section at, figure showing 277
Depth, temperature and, relations of. See Temperature.
Desert Queen shaft, depth of 197
description of 125-127, 193
r eks in 125-12?! 193
section through, figure showing 177
values in 97
veins in, plan showing 13
water in 105, 125
workings at. plan of 126
Devils Punchbowl, hot springs of 257
Differentiation theory of origin of lavas 61-66
tests of, by analysis 64, 65, 60
Dikes, occurrence of 44-45, 73-74
section showing 49
Doelter, C., on museovite 231
Dominian, Leon, measurements by 263
E.
East Indies, rocks of 279-280, 283
Eocene rocks, fossils from 66-67
Eocene time, history in 66, 69
Epidote, occurrence of 250
Erosion, amount of 1 10-1 11
effects of 23
features of 111-112
progress of 109-111
rock resistance and, relations of 113-114
Eureka, Nev., andesite from, analysis of 219,244
daciteand rhyolite from, analysis of 58,65
F.
Fault blocks, displacement of, diagram showing 4R
Faulting, age of 72
character of 22, 141-14(1
dikes along 73-74
finding of 79-80
location of 115-116
occurrence of 37-38, 47, 75, 149, 164, 184
origin of 21, 47, 72, 80, 146
principles of 72-82
topographic features due to 74-79, 114
Faulting, cross, effects of 77-79
theory of , 157-161
diagrams illustrating 158, 159, 160, 161
Feldspar, alteration of 208, 21 1-1!] >, -JIM
alteration of, chemistry of 230-231
Fissures, origin of 104
Formations, geological, age of s-?2
description of 30-66
succession of 71-72, 274
Fossils, occurrence of 66-67, 69-70
Fraction dacite breccia, age of 40
character and occurrence of 39-40
section of, diagram showing 40
Fraction Extension shaft, depth of 106
location of 200
rocks in 201-202
Fraction faults, cause of 146
description of 141-146
discovery of 79
effects of 152
Wandering Boy fault and, age of 163
INDEX.
291
Page.
Fraction mine, faultsin 141-144,147
faults in, figures showing 141-145,147,162
gas in 94
gold discovered in 27, 140
oxidation in 91
plan of 162
rock from, analysis of 87
water in 108
shafts of, depth of 106, 140
earlier andesite from, alteration of 221-222
description of 214
analysisof 216,221
rocksof 147-148
silver in 94-95
veinsof 147-148 '.
plan showing 162
Fraction vein, character of 140,146
discovery of , 27,140
relations of Valley View vein system and 140, 187
Fracture zones, development of, stages of 84-85 j
Friedel and Sarasin, experiments by 228-229
Fuel, expense of 28
G. & H. mine, lava from, analysisof 67,60
rhyolite from 61
Galena, character of 88
occurrence of 22,88
GaJlionella granulata.-occurrence of 70
marchica, occurrence of 70
procera, occurrence of 70
punctate, occurrence of 70
sculpta. occurrence of 70
tenerrima, occurrence of 70
Gas, occurrence of 94, 188
Geikie, A., on Antrim rocks 52-53
Genth, F. A., on albite 212
jentil, G. L., on feldspar alteration : 212
Geological history, summary of 66-68, 109, 261-262
Gold, depth and, relations of 124-128
d iscovery of 21
occurrence of 22, 89, 123, 287
Gold Hill, erosion on 113
faulting at 164
rocksof 32,164-165
alteration of 165
sections of 166,167
veins of 165-166
Gold Hill fault, location of 164,184
Gold Hill mine, relations of 186-187
Gold Hill shaft, depth of 166
water in 105-106
Golden Anchor shaft, depth of 205
rocks in 206
water in 106
Golden Mountain, characterof 44
erosion on 113
lava of, analyses of 57, 58, 60, 66
cooling of, eddying in, figure showing 46
rocksof 36,45,55,60-fi2
section near, diagram showing 45
Good Enough shaft, section on 166, 167
Good Enough vein, characterof 165
output of 166
structure of 166
Granite, occurrence and character of 30
Page
Great Basin, deformation in 80-81
erosion in 111-112
lavas of, succession of 68-69, 274
metallographic province of 22,276-278
petrographic province of 22, 274-275
Gypsum, formation of 94
H.
Halifax shaft, later andesite from, analysisof. 57,241,244-247
later andesite from, description of 239
study of 244-248
location of 205
rocks in 205
water in 105,205
Healy, J. M., information from 152
Heller Butte, character of 87
erosion on 113
rocks in 30, 37
view of 38
Heller dacite, age of 38
character of 37-39
occurrence of 30, 37
Hillebrand, W. F., analyses by 87,89-90,92,103,241
History, geological, summary of 66-68, 109
Hobbs, W. H, diagram by 217
Hornblende, alteration of 207
occurrence of 63
I.
Igneous rocks, classification of 59
occurrence of 21
relative ages of 68-72
Infusoria, occurrence of 69-70
J.
Japan, rocksof 280-282
K.
Kaolin, occurrence of 209
Kawsoh Mountain, fossils from 70
King, Clarence, on Miocene deposits 69-70
King Tonopah shaft, depth of 106
lava from, analysis of 64
rocks and veins in 197-198
L.
Lake, ancient, location and history of 51-54, 67, 81
Lavas, analyses of 57
boundaries of 74
characterof 21.31-66,56
cooling of, eddying in, diagram showing 46
occurrence of 31-66
origin of, differentiation theory of 61-66
relations of, diagrams showing 40, 41, 43-45, 48, 49, 53
succession of 68-69
water in 255
Limestone, occurrence and character of 30
Lindgren, W., on propylite 236
Little Tonopah shaft, data concerning 199-200
Lode porphyry, mention of 31
See aim Andesite, earlier.
Lone Mountain, racks on 30
M.
Macdonald vein, characterof 174-175
occurrence of 169
section of, figure showing 174, 175
292
INDEX.
Page.
UacNamara mine, gas in 94
location of 189
shaft of, depth of 106
rocks in 189-191
section on 191
Magmatic segregations, theory of 21-22
Map of andesite veins in Mizpah workings 183
of Ararat Mountain 102
of Tonopah mining claims 26
of Tonopah mining district 21,114
of Tonopah outcropping veins 84
Map, diagrammatic, showing zigzag scarps 76
Map, geologic, of Tonopah mining district 56, 114
Melosira granulata, occurrence of 69-70
punctata, occurrence of 70
varians, occurrence of 69
Mesabi range, sideritein 249
Metallographic province, extent of 22, 276
ores of, origin of 276-278
Mexico, ores of, comparison of Tonopah ores and ... 267-270
petrographic province of 274-275
Mica, production of 232-233
Midway mine, location of 179
ores of, character of 86, 181
rocks in 35,179-180
section in, figure showing 180
shaft of, depth of 106
veins of 180-181
Milling, deficiencies of 28
Mineral veins. See Veins, mineral.
Mineralization, agents of 85, 253-264
origin of 22,258-262
period of 83,261
See also Water, hot, ascending.
Mines, descriptions of 115-206
plotof 26
Miocene time, history in 67-69
Miriam claim, description of 193
Mizpah Extension shaft, later andesite of, analysis
of 241,244
later andesite of, description of 238-239
study Of 244-245
lava of, analysis of 57
location of 194
rocks in 194-195
temperatures in 263-264
diagram showing 265
veins in 195-197
water in 106
Mizpah fault, discovery of : 79
location of 115,126-127,168-169,177
Mizpah Hill, alteration on 207
earlier "r^esite from, analysis of 216, 223, 225
description of 214-215
study of 223-225
erosion on 113-114
faulting on 74
gold found at 25-26
outcropping veins on, view of 116
rocks on and near 32,110,164
underground temperatures at 264
veins of 83
plan of 120
view from 46
Mizpah mine, earlier andesite from, analysis of.. 216, 225,226
earlier andesite from, description ol 214-215
study of 225-226
Page.
Mizpah mine, faults in, figure showing 122,123
ores of, character of 86, 132
rocks of 207
section in 173
silver in 95
veins of, diagram showing 122-124, 126, 183
Mizpah vein, alterations in 124-125
branching of 119
diagram showing 120
composition of 122-123
earlier audesite from, analysis of 216,226,226
description of 215-216
study of 225-221J
extent of 115-117
faults in 115-117
plan showing 123
fractures in 119-122
gold in 124-125
junction of Burro vein and 127
minerals of 124
occurrence of 126, 173-174
ores of, distribution of, diagram showing 121
oxidation of 90, 124-125
relations of, figure showing 173
sections of, diagram showing 116, 117, 119-124, 173
structure of 117
Mizpah vein system, description of 115-129
Molly shaft, data concerning 106,200
Monitor Valley, Nev., hot springs in 257
Mono, Lake, crater at 82, 257
Montana Tonopah mine, faults in 172
faults in, plan showing 168
galena in 88
gold of 27, 125
ores of 86, 95, 175
analysis of 89
oxidation in 91
sections in 169,170,171,174,175,176
shaft of, depth of 106
later andesite in, analysis of 241,247
description of 240-241
study of 247-248
temperatures in 264
diagram showing 265
veins of 85, 167-173
figures showing 169, 170, 171
formation of 172
structure in 169-170
Montana Tonopah vein system, description of 168-184
Montana vein, character of 170-172,174
faults on 172,174
occurrence of 170, 178
ore of, figure showing 84, 171
origin of 172
relations of, figu re showing 173
vein of, fragment of, view of 84
Muscovite, formation and occurrence of 231-233
Nelhart, Mont., silver sulphides at, comparison of
Tonopah sulphides and 95
Nevada, springs of 256-259
New York Tonopah shaft, depth of 106,200
location of 200
rocksin 30,39-10,110,200-201
New Zealand, rocks of 279-280, 284-286
Nickel, occurrence of 34
INDEX.
293
Page.
North Star mine, discovery of gold in 27
faultsin 178
ores of, character of 86
section in 177-178
figure showing '. 177
shaft of, depth of 106
later andesite from, analysis of 241, 246
description of 239-240
study of 246-247
rocksin 126,177-178
veins in 178
O.
Oddie, Mount, erosion on 113
lava from, analysis of 58,60
rocksof :. 36,49-51,53,60-61,101
view from 24
view of 52
Oddie, T. L., mention of 26
Oddie rhyolite, age of 50
character of 49-50
contact of, section showing 193
veins at 191-194
lava from, analysis of 57
occurrence of 49, 127
rhyolile from 61
Oddie shaft, section in, diagram showing 116
Ohio Tonopah shaft, depth of 106-203
location of . 202
minerals in 205
roctsof 107,202-205
section near, diagram showing 43
temperatures in 263-204
diagram showing 265
Ordonez, E., on Mexican ores 267-268,273
on petrographic province 274
Ore production, amount of --- 26,28
Ore shoots, origin of 85,119-122,276-278
Oregon, craters in 82
Ores, at Tonopah and elsewhere, comparison of.- .. 267-287
genesis of 261-262
treatment of - 28
Ores, oxidized, analysisof _ 92
Ores, primary, composition of 86-90
location of 86
Oxidation, agents of 90
depth of 22,90
effects of 90-94
process of 93-94
V.
Pachuca, Mexico, ores of Tonopah and, comparison of 22,
267-268,273,278
section at, figure showing 277
Pachuca Range, character of 267
veins of 267-268
Pacific petrometallographic zone, existence of 278-287
Pah-Ute Lake, deposits of, comparison of Siebert tuffs
and 70
Paleozoic limestone, occurrence of 66
Paragenesis of vein material 104
Pelee, Mont, plug of 104
Penrose, R. A. F. jr., on oxidized veins 91
Peru, rocks of 280
Petrcgraphic province, extent of 22, 274-275
Petroleum , use of 29
Petrometallographic zone. Pacific, existence of 278-287
Page.
Phosphorus, alteration of 233-234
Physiography of region, account of 109-114
Pinnubaria inaequalis, occurrence of 70
Pktsburg shaft, data concerning 204
Planorbis utahensis, occurrence of 67
Pliocene time, history in 68, 69, 110
1'olybasite, occurrence of 22,95
Power, use of 28
Propylite, definition of 236-237
Prospects, descriptions of 115-206
Pseudomorphs, character of 61-62
origin of 62
Pyrargyrite, occurrence of 22, 94-95
Pyrite, character of 88
occurrence of 22
relations of siderite and 208
view of rock specimen containing 208
Q.
Quartz, alteration of earlier andesite to 207-209
analysis of 87
character of 86
occurrence of 22
origin of 22
Quaternary erosion, sketch of 109-111
Quinn Canyon Range, view of 112
R.
Railroad, construction of 25, 28
Rainfall, absorption of 107-108
amount of 112-113
Ray, fossils near 66-67
Real del Monte, ores of Tonopah and, comparison of. 267-
268,273-274
Red Rock shaft, data concerning 204
Replacement, veins due to 84-S5
Reptile elaim, vein on 103
vein on, section of, diagram showing 102, 103
Rescue shaft, description of 194
water in 105
Rhynlitc, analyses of 57-58,60,64,65
areas of, diagram showing 46
character of 21,36-51, S9-SS
classification of 59-iiO
fissure veins in 102
flow brecciation in 102
hornblende in 62
occurrence of 21, 36-51
origin of 22,63
veins in 22
Richthofen, F. von, on mineralization 259-260
on propylite 236
Rock alteration, changes during, diagrams illustrat-
ing 218,234,242,243
maximum points of, location of 226-227
of the earlier andesite 207-238
of the later andesite 238-252
of the Oddie rhyolite 252
processes of 207-252
Rocks, altered, specimens of, analyses of 216
microscopic description of 213-216
phases of, diagrams showing 217-218
study of 217-226
Rosenbusch, H., on feldspar 233
on propylite 236
Rushton Hill, rocksof and near 43,49-50,57,60-61
294
INDEX.
8. Page.
San Antonio Range, location and origin of 109
Sarasin and Friedel, experiments by 228-229
Scarps, rock, origin of 74-79,113-114
view of 76
Schaller, W. T., analysis by 88,178
Scrope, G. P., on volcanic subsidence 47
Sedimentary rocks, occurrence of 21
Selenium, occurrence of '. 92, 281, 285
Sericite, adularia and, relations of 227-228
alteration of earlier andesite to 207-209
character of 87
occurrence of 22
origin of 22
Shafts, depth of 106-107
Siderite, formation of 248-249
relations of pyrite and 208
view of 208
Siebert fault, location of 115-117
Siebert Mountain, character of 44
erosion on 113
events on 67-68
faulting near 75
fossils at 69-70
lava from, analysis of 57, 64
rocks of 36,40,45,53-55
section near, diagram showing 53
veins near 97, 99
view of 52, 54
Siebert shaft, depth of 106
earlier andesite from, analysisof , 216
description of 213
study of 217,219-220
rocks in 116-117
section on, figure showing 134
view of 118
Siebert tuffs, boundaries of 73
character of 51
comparison of Pah-Ute Lake deposits and 70
erosion of 114
fossils in 69-70
occurrence of 21, 47-48
origin of 51-54
relations of, diagram showing 53
section of, diagram showing 45
view of 46
Silurian rocks, occurrence of 30
Silver, discovery of 21
occurrence of 91, 94-95, 123-124, 146, 164, 269, 287
Silver, horn. Hee Silver chlorides.
Silver chlorides, character of 88, 91
occurrence of 122, 180-181
Silver City, Idaho, ores of Tonopah and, comparison
of 22,271-274
Silver Peak, crater at 82,257
crater at, view of 112
hot and cold springs at 256-257
Silver Peak Range, coal in 29
rocks in 30
Hilver selenides, occurrence of . . .'. 22,92
Silver sulphides, occurrence of 22, 88, 94-%, 180-183
Silver Top sha ft, rocks in 125
section in, figure showing 136
veins in 136-137
relation of Stone Cabin vein and 137-139
relation of Valley View vein and 139
water in 106
Smelting, necessity for 28
Sodavllle, Nev.. rainfall at 112-113
Page.
Solfataras, action of, nature of 260-261
Spha-riuin idahoense, occurrence of 67
Spongolithis acicularis, occurrence of 70
Springs, hot, extinction of 258
origin of 23,254-2511
See also Water, hot ascending.
Steiger, George, analyses by 148, 216, 241
Stephanite, occurrence of : 22
Stock, A. C., aid from 198-199
Stone Cabin fault, extent of 139
Stone Cabin mine, ore of, character of 136
oxidation in 91
sections of 135, 138
shaft of, depth of lOti
veins in 135-136
relation of Silver Top vein and 137
relation of Valley View vein and 137-139
figure showing 138
Suess, E., on underground water 255, 258
on volcanoes 261
Sulphide ores, primary, analysis of 89-90
Sumatra, rocks of 283-284
Summary of paper 21-23
T.
Tellurium, occurrence of iix;
Temperatures at Comstock and Tonopah, comparison
Of 265-266
in the Mizpah Extension 263-264
in the Mizpah Hill mine 264
in the Montana Tonopah 264
in the Ohio Tonopah 263-264
measurements of, method of 263
on the Comstock 26n
relations of depth and 23, 263-26t;
diagrams showing 265
Tertiary rocks, character of 31-66
occurrence of 21, 31-66, 68
Tertiary time, history in 66, 109-110
Titanium , alteration of 233-234
Tonopah, character of 27
cross section at, figure showing ; 71
hot springs near 257-25S
lavas of, succession of 68-9, 274
location of 25
name of, meaning of 27
outcrops at, diagram showing 84
rocks of, age of 69-72
view of 27, 46
Tonopah and California mine, description of 167-168
fault in 167
ores of, character of 86
rocks in 53
section in 167
shaft of, depth of 106
earlier andesite from, analysis of 216,220
description of 213
study of 220-221
section near, diagram showing 40
veins in His
Tonopah City shaft, depth of 106,202
location of 202
rocks In 37-38,202
Tonopah Extension mine, discovery of gold in 27
rocks in 35,181-184
sections in, figures showing 182, 191
shaft of, depth of 106
vcinsin 182-184
INDEX.
295
Page.
Tonopah Mining Co., claims of, development of '26-27
shaft of, view of 120
Tonopah rhyolite-dacite, age of 43, 51
alteration in 41-42
analysis of 57,58,60,64,148
character of 41-13, 51, 59-l, 101
contact of, veins on 194-200
hornblende in 2
occurrence of 30,41,51,127
section of, figure showing 41
veins of 22,96-101
age of 99-100
character of 97-99
circulating waters in 100-101
diagram showing 98
limits of 99-100
Topography, character of 25
origin of 23,109,113-114
production of, by faults 74-75
relation of rocks to 113-114
Transportation, difflcultieH of 28
V.
Valley View fault, effects of 137-139
location of 133-134
Valley View mine, shaft of, depth of 106
shaft of, sections on and near, figures showing . . 122, 134
workings of 132-134
Valley View veins, correlation of 137-139
cross veins in 129-130
location of 129
onMizpah Hill 129-132
ores of, character of 132
origin of 130-131
oxidation in 90-91
oxidized ore of. analysis of 92
sections of 128,133-138
structure in 130-131
underground system of ; 132-139
in Silver Top mine 136-187
in Stone Cabin mine 135-137
Valley View vein system, description of 129-1C4
relations of Fraction vein and 140, 1S7
relations of Wandering Boy vein and 149-152, 187
section of, figure showing 151
Van Hise, C. R.. on orthoclase 229
Vein robbers, occurrence and character of 130
Veins, mineral, age of 71-72
branching of 119
character of .. 22, 83-101, 122-123
formation of 104
relations of alteration to 251-252
fractures in 119-122
in the earlier andesite 83-90
mines and prospects on, descriptions of 115-191
material of, paragenesis of 104
of Ararat Mountain : 101-104
of the Tonopah rhyolite-dacite period 96-101
origin of 84-85,102,104,253-262
outcrops of, map showing 116
Vivipara couesi, occurrence of 67
Volcanic epoch, continuance of S2
Page.
Volcanoes, accumulations from 23
character of 260-261
origin of 21
W.
Wandering Boy fault, discovery of 79
Fraction fault and, age of 163
location of 153
occurrence and character of 152-156, 161-163, 184
section showing 154, 155, 156
Wandering Boy mine, faults in 152-156, 161-162
faults in , figure showing 172
ore in 163
outcrops near, figure showing 153
oxidation in 91
plan of 172
relations of 186
section of 154, 155. 156
shafts of. depth of 106
Valley View veins in 130, 149-152
veins of : 1W2-163
plan showing 172
Wandering Boy veins, description of 149-164
dip of H9-151
Fraction vein and, relations of 162-163, 187
ores of 163-164
outcrops of 152
Valley View vein and, relations of 149-152
Wash apron, view of 112
Washoe, Nev., andesite from, analyses of 219, 244
dacite and rhyolite from 60
analyses of 58, 65
Water, supply of 28, 107
Water, underground, depth to 106-107
occurrence of 23, 105-107
origin of 107-108
zones of, 'occurrence of. explanation of 107
Water, descending, oxidation by 90
sulphides deposited by %
Water, hot, ascending, alteration by 207-252
alteration by, variation in 210-211
changes in 235-238
channels of 83
effect of 22,114,227.234-235
mineral composition of 22-
23, 104. 210-211. 227. 235-238, 250, 253, 2SN-25H
argument from 258-260
mineralization by 22-23,*% 99-101, 210-211
origin of 253-256
Water power, availability of 28
Weed, W. H., on Boulder Hot Springs 211
on silver sulphides 95
West End fault, location of 184
rocks along .' 184
West End mine, gas in 94
relations of 186-187
rocks in 35.50; 185-188
shaft of, depth of 106,185
workings of, description of 184-188
White Mountain Range, water power in 28
Wind, erosion by 110-112
Wingfield tunnel, location of 191
rocks in 101,104,191-192
Wood, occurrence of 28
o
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B 259. Report on progress of investigations of mineral resources of Alaska in 1904, by A. H. Brooks and others. 1905.
196 pp., 3 pis.
B 260. Contributions to economic geology, 1904; S. F. Emmons, C. W. Hayes, geologists in charge. 1905. 620 pp., 4 pis.
B 261. Preliminary report on the operations of the coal-testing plant of the United states Geological Survey at the Louisiana
Purchase Exposition, St. Louis, Mo., 1904; E. W. Parker, J. A. Holmes. M. R. Campbell, committee in charge. 1905.
172 pp.
B 26S. Methods and costs of gravel and placer mining in Alaska, by C. W. Purington. 1905. 27:1 pp., 42 pis.
PP 42. Geology of the Tonopah mining district. Nevada, by J. E. Spurr. 1906. 295 pp., -Jl |.K
SERIES B. DESCRIPTIVE GEOLOGY.
B23. Observations on the junction between the Eastern sandstone and the Kcwceiiaw series on Kcweenaw Point, Lake
Superior, by K. D. Irving and T. C. Chamberlin. 1885. 124 pp., 17 pis. (Out of stock.)
B 33. Notes on geology of northern California, by J.S. Dillcr. 1886. 23pp. (Out of stork.)
B 39. The upper beaches and deltas of Glacial Lake Agassiz, by Warren Upham. 1887. 84 pp., 1 pi. (Out of stock.)
B 40. Changes in river courses in Washington Territory due to glaciation, by Hailey Willis. 1887. 10 pp., 4 pis. (Out of
Ktock.)
B 45. The present condition of knowledge of the geology of Texas, by R. T. Hill. 1887. 94 pp. (Out of stock.)
B 53. The geology of Nantucket, by N. 8. Shaler. 1889. 55 pp., 10 pis. (Out of stock.)
B 57. A geological reconnaissance in southwestern Kansas, by Robert Hay. 1890. 49 pp., 2 pis.
K >. The glacial txmndary in western Pennsylvania, Ohio, Kentucky. Indiana, and Illinois, by G. F. Wright, with intro-
duction by T. C. Chamberlln. 1890. 112pp., 8 pis. (Out of stock.)
ADVERTISEMENT. Ill
B 67. The relations of the traps of the Newark system in the New Jersey region, by N. H. Darton. 1890. 82 pp. (Out of
stock.)
B 104. Glaciation of the Yellowstone Valley north of the Park, by W. H. Weed. 1893. 41 pp., 4 pis.
BIOS. A geological reconnaissance in central Washington, by I. C. Russell. 1893. 108 pp., 12 pis. (Out of stock.)
B 119. A geological reconnaissance in northwest Wyoming, by G. H. ElcJridge. 1894. 72 pp., 4 pla.
B 137. The geology of the Fort Riley Military Reservation and vicinity, Kansas, by Robert Hay. 1896. 35 pp., 8 pis.
B 144. Tlu- moraines of the Missouri Coteau and their attendant deposits, by J. E. Todd. 18%. 71 pp., 21 pis.
B 158. The moraines of southeastern South Dakota and their attendant deposits, by J. E. Todd. 1899. 171 pp., 27 pis.
B 159. The geology of eastern Berkshire County, Massachusetts, by B. K. Emerson. 1899. 139 pp., 9 pis.
B 165. Contributions to the geology of Maine, by H. S. Williams and H. E. Gregory. 1900. 212 pp.. 14 pis.
WS 70. Geology and water resources of the Patrick and Goshen Hole quadrangle!) in eastern Wyoming and western
Nebraska, by G. I. Adams. 1902. 50 pp., 11 pis.
B 199. Geology and water ^resources of the Snake River Plains of Idaho, by I. C. Russell. 1902. 192 pp., 25 pis.
PP 1. Preliminary report on the Ketchikan mining district. Alaska, with an introductory sketch of the geology of south-
.intiTii Alaska, by A. H. Brooks. 1902. 120 pp., 2 pis.
PP 2. Reconnaissance of the northwestern portion of Steward Peninsula, Alaska, by A. J. Collier. 1902. 70 pp., 11 pis.
PP 3. Geology and petrography of Crater Lake National Park, by J. S. Diller and H. B. Patton. 1902. 167 pp., 19 pis.
PP 10. Reconnaissance from Fort Hamlin to Kotzebuc Sound. Alaska, by way of Dall, Kanuti. Allen, and Ko\yak rivers,
by W. C. Mendenhall. 1902. 68 pp., 10 pis.
PP 11. Clays of the United States east of the Mississippi River, by Heinrich Ries. 1903. 298 pp., 9 pis.
PP 12. Geology of the Globe copper district, Arizona, by F. L. Ransome. 1903. 168 pp., 27 pis.
I'P 13. Drainage modifications in southeastern Ohio and adjacent parts of West Virginia and Kenutcky, by W. G. Tight.
1903. Ill pp., 17 pis.
B -IK Descriptive geology of Nevada south of the fortieth parallel and adjacent portions of California, by J. E. Spurr.
1903. 229 pp., 8 pis.
B 209. Geology of Ascutney Mountain, Vermont, by R. A. Daly. 1903. 122 pp., 7 pis.
ws TV Preliminary report on artesian basins in southwestern Idaho and southeastern Oregon, by I. C. Russell. 1903.
51 pp., '_' pl>.
PP 15. Mineral resources of the Mount Wrangell district. Alaska, by W. C. Mendenhall and F. C. Schrader. 1903. 71 pp.,
10 pis.
1'P 17. Preliminary report on the geology and water resources of Nebraska west <if the one hundred and third meridian.
by N. H. Darton. 1903. 69 pp., 43 pis. ,
B 217. Notes on the geology of southwestern Idaho and southeastern Oregon, by I. C. Russell. 1903. 83 pp., 18 pis.
B 219. The ore deposits of Tonopah, Nevada (preliminary report), by J. E. Spurr. 1903. 31 pp., 1 pi.
PP 20. A reconnaissance in northern Alaska in 1901, by F. C. Schrader. 1904. 139 pp., 16 pis.
PP21. The geology and ore deposits of the Bisbee quadrangle, Arizona, by F. L. Ransome. 1904. 168 pp., 29 pis.
WS 90. Geology and water resources of part of the lower James River Valley, South Dakota, by J. E. Todd and C. M. Hall.
1904. 47 pp., 23 pis.
PP 25. The copper deposits of the Kncampinent district, Wyoming, by A. C. Spencer. 1904. 107 pp.. 2 pis.
PP 26. Economic resources of the northern Black Hills, by J. D. Irving, with contributions by S. F. Emmons and T. A.
Jaggar, jr. 1904. 222 pp., 20 pis.
PP 27. A geological reconnaissance across the Bitterroot Range and Clearwater Mountains in Montana and Idaho, by
Waldemar Lindgren. 1904. 122 pp., 15 pis.
PP 31. Preliminary report on the geology of the Arbuckle and Wichita mountains in Indian Territory and Oklahoma.
by J. A. TarT, with an appendix on reported ore deposits in the Wichita Mountains, by H. F. Bain. 1904. 97 pp.,
K pis.
B 235. A geological reconnaissance across the Cascade Range near the forty-ninth parallel, by (.;. O. Smith and F. C.
Calkins. 1904. 103 pp.. 4 pis.
B 236. The Porcupine placer district, Alaska, by C. W. Wright. 1904. 35 pp., 10 pis.
B 237. Igneous rocks of the Highwood Mountains, Montana, by L. V. Pirsson. 1904. 208 pp., 7 pis.
B2H. Economic geology of the lola quadrangle, Kansas, by G. I. Adams, Erasmus Haworth, and W. R. Crane. 1904.
83 pp., 1 pi.
PP 32. Geology and underground water resources of the central Great Plains, by N. H. Darton. 1905. 433 pp., 72 pis.
WS 110. Contributions to hydrology of eastern United States, 1904; M. G. Fuller, geologist in charge. 1905. 211 pp., 5 pis.
B 242. Geology of the Hudson Valley between the Hoosic and the Kinderhook. by T. Nelson Dale. 1904. 63 pp., 3 pis.
PP H4. The Delavan lobe of the Lake Michigan Glacier of the Wisconsin stage ol glaeiation and associated phenomena, by
W. C. Alden. 1904. 106 pp., 15 pis.
PP 35. Geology of the Perry Basin in southeastern Maine, by G. O. Smith arid David White. 1905. 107 pp., 6 pis.
B 243. Cement materials and industry of the United States, by E. C. Eckel. 1905. 395 pp., 15 pis.
B 24(i. Zinc and lead deposits of northeastern Illinois, by H. F. Bain. 1904. 66 pp., 5 pis.
B 247. The Fairhaven gold placers of Seward Peninsula, Alaska, by F. H. Moffit. 1905. 85 pp., 14 pis.
B 249. Limestones of southwestern Pennsylvania, by F. G. Clapp. 1905. 52 pp., 7 pis.
B 250. The petroleum fields of the Pacific coast of Alaska, with an account of the Bering River coal deposit, by G. C.
Martin. 1906. 65 pp., 7 pis.
B 251. The gold placers of the Fortymile, Birch Creek, and Fairbanks regions, Alaska, by L. M. Prindle. 1905. 89 pp.,
16 pis.
WS. 118. Geology and water resources of a portion of cast central Washington, by F. C. Calkins. 1905. % pp., 4 pis.
B 252. Preliminary report on the geology and water resources of central Oregon, by I. C. Kusaell. 1905. 138 pp., 24 pis.
IV ADVERTISEMENT.
PP 36. The lead, zinc, and fluorspar deposits of western Kentucky, by E. O. TJlrich and W. 8. Tangier Smith. 1905.
218 pp., 15 pis.
fP 38. Economic geology of the Binghrtm mining district of Utah, by J. M. Boutwell, with a chapter on area! geology, by
Arthur Keith, and an introduction on general geology, by S. F. Emmons. 1906. 413 pp., 49 pis.
PP 41. The geology of the central Copper River region, Alaska, by W. C. Mendenhall. 1905.
B 254. Report of progress in the geological resurvey of the Cripple Creek district, Colorado, by Waldnar Lindgren and
F. L. Ransome. 1904. 36 pp.
B 255. The fluorspar deposits of southern Illinois, by H. Foster Bain. 1905. 75 pp., 6 pis.
B 256. Mineral resources of the Elders Ridge quadrangle, Pennsylvania, by R. W. Stone. 1905. 85 pp., 12 pis.
B 257. Geology and paleontology of the Judith River beds, by T. W. Stanton and J. B. Hatcher, with a chapter on the
fossil plants, by F. H. Knowlton. 1905. 174 pp., 19 pis.
PP 42. Geology of the Tonopah mining district, Nevada, by J. E. Spurr. 1905. 295 pp., 24 pis.
Correspondence should be addressed to
THE DIRECTOR,
UNITED STATES GEOLOGICAL SURVEY,
WASHINGTON, D. C.
SEPTEMBER, 1905.
LIBRARY CATALOGUE SLIPS.
[Mount each slip upon a separate card, placing the subject at the top of the
second slip. The name of the series should not be repeated on the series
card, but additional numbers should be added, as received, to the first
entry.]
Spurr, Josiah Edward, 1870-
. . . Geology of the Tonopah mining district,
g Nevada, by Josiah Edward Spurr. Washington, Gov't
print, off., 1905.
295, v p. illus., XXIV pi. (incl. front., maps) diagrs. 29* x 23 cm . (U. S.
Geological survey. Professional paper no. 42)
Subject series: A, Economic geology, 56; B, Descriptive geology, 65.
1. Geology Nevada.
Spurr, Josiah Edward, 1870-
. . . Geology of the Tonopah mining district,
1 Nevada, by Josiah Edward Spurr. Washington, Gov't
I print, off., 1905.
295, v p. illus., XXIV pi. (incl. front., maps) diagrs. 29J x 23 cm . (U.S.
Geological survey. Professional paper no. 42)
Subject series: A, Economic geology, 56; B, Descriptive geology, 65.
1. Geology Nevada.
U. S. Geological survey.
j Professional papers.
I no. 42. Spurr, J. E. Geology of the Tonopah mining
district, Nevada. 1905.
U. S. Dept. of the Interior.
see also
U. S. Geological survey.