STATE OF CALIFORNIA EARL WARREN. Governor DEPARTMENT OF NATURAL RESOURCES WARREN T. HANNUM, Director DIVISION OF MINES FERRY BUILDING. SAN FRANCISCO 11 OLAF P. JENKINS. Chief IN FRANCISCO SPECIAL REPORT 14 DECEMBER 1951 GEOLOGY OF THE MASSIVE SULFIDE DEPOSITS AT IRON MOUNTAIN SHASTA COUNTY, CALIFORNIA By A. R. KINKEL, JR.. and J. P. ALBERS Prepared in cooperation with the United States Geological Survey Digitized by the Internet Archive in 2012 with funding from University of California, Davis Libraries http://archive.org/details/geologyofmassive14kink GEOLOGY OF THE MASSIVE SULFIDE DEPOSITS AT IRON MOUNTAIN, SHASTA COUNTY, CALIFORNIA* By A. R. Kinkel, Jr.,** and J. P. Albbrs** OUTLINE OF REPORT Page jstract 3 traduction 3 sgional geology 4 Geologic formations 4 Plutonic rocks 6 Structure and geologic history 7 on Mountain mine 7 History and production 7 Formations in the mine area 8 Ore deposits 9 Character and distribution 9 Minerals of the primary ore 9 Distribution of metals 10 Disseminated copper ore 10 Structural features of the ore bodies 11 Hydrothermal alteration of the wall rocks 15 Summary of features controlling ore deposition 16 Oxidation and enrichment 18 Age of mineralization 19 teferences 19 Illustrations Page fiGURE 1. Map showing location of West Shasta copper-zinc district 5 2. Photo of pyroclastic beds in Balaklala rhyolite 6 3. Photo of Richmond mine plant 8 4. Photo of gossan quarry over Old Mine ore body 13 5. Plan of Number 8 mine ore bodies 14 6. Sections of Number 8 mine and Old Mine ore bodies 17 i*LATE 1. Geologic map of Iron Mountain mine area In pocket 2. Plan and longitudinal section of ore bodies In pocket 3. Iron Mountain mine plan of underground workings In pocket 4. Geologic cross-sections In pocket 5. Cross-sections of ore bodies In pocket 6. Restoration of Iron Mountain ore body In pocket ABSTRACT The Iron Mountain mine in the West Shasta copper-zinc dis- rict is in the rugged Klamath Mountains 15 miles northwest of ledding, California. The principal ore bodies are large lenses of nassive pyrite that contain chalcopyrite and sphalerite and minor mounts of gold and silver. The oldest rocks in the district are the andesitic flows and tyroclastie rocks of the Copley greenstone, which is probably Lower >r Middle Devonian in age. They are overlain by flows and pyro- lastics of the Balaklala rhyolite of Middle Devonian age, described iy Diller. Submergence during Middle Devonian initiated the depo- ition of shales, tuffs, and limestones of the Kennett and younger ormations. Later the rocks were folded, locally sheared, and intruded iy two large plutons. The massive sulfide deposits of the Iron Mountain mine occur a a porphyritic facies of the Balaklala rhyolite. Tuff and agglom- rate layers appear to have been unfavorable for ore deposition at iron Mountain. The ore bodies are composed almost entirely of ulfide minerals and are in sharp contact with rhyolite. The ore lodies range in size from a few thousand tons to more than 5,000,000 ons, but all are faulted parts of an originally continuous ore body hat was 4,500 feet long. All the massive sulfide ore contains some opper and zinc, but minable bodies of copper-zinc ore are closely ssoeiated with feeder channels along faults that existed prior to ormation of the ore. The main controls of ore deposition for the district are a ;ently plunging anticlinorium, favorable layers in the Balaklala •hyolite, and a thick cover of shale a few hundred feet above the ore ;one. The Iron Mountain ore body occurs where a steep feeder •hannel cuts folded beds near the crest of a large northeast-trending inticlinorium. INTRODUCTION * Published by permission of the Director, U. S. Geological Sur- ey. Manuscript submitted for publication April 1951. ** Geologist, U. S. Geological Survey. 2—47351 The Iron Mountain mine, which is owned and oper- ated by The Mountain Copper Company, Limited, is lo- cated in Shasta County, California, near the eastern edge of the Weaverville quadrangle. It is the southernmost mine in the West Shasta copper-zinc district, which ex- tends 8 miles northeastward from Iron Mountain (fig. 1). The mine is 17 miles by road northwest of Redding and lies at an altitude of 2600 feet in the rugged foothills of the Klamath Mountains at the north end of the Sacra- mento Valley. A hard-surfaced county road connects the Iron Mountain mine with U. S. Highway 299. The South- ern Pacific Railroad passes through Redding, and the ore from the mine is carried to a spur of the railroad by an aerial tramway. There is little residual soil in the area, but rock mantle masks the geology of many slopes. Good timber is scarce except at the higher altitudes, but a heavy growth of chaparral makes travel difficult over much of the area. The ore deposits of the Iron Mountain mine were discovered about 1865, but the mining of the massive sulfide deposits for their copper content did not begin until 1897. The development of separate bodies of sulfide ore led to the naming of individual ore bodies as different mines, although they were mined as part of one operation by The Mountain Copper Company, Limited. Thus the Old Mine, Number 8 mine, Hornet mine, etc. are separate, and were worked at different times, but all are part of the Iron Mountain mine. The more important publications of the many geolo- gists who have worked in Shasta County during the past 50 years are listed in the bibliography at the end of this report. The publications contain conflicting opinions on the origin of some of the rocks, and this divergence emphasizes the need for further study. This report is based on detailed surface and under- ground mapping at the Iron Mountain mine done during the summer of 1947. The description of the regional geology is based on 1 :24,000-scale mapping by the authors (194549) of about 200 square miles in the four 7|-min- ute quadrangles surrounding the mine, as part of an investigation of the geology of the West Shasta copper- zinc district and the relationship of the mineralization to the rocks and structure of this region. The work is part of a cooperative program between the California Division of Mines and the U. S. Geological Survey. The interest and cooperation of the staff of The Mountain Copper Company, Limited, have greatly facili- tated the mapping of the mine and the compilation of the records of old workings. Special thanks are due to C. W. McClung, general superintendent, T. P. Bagley, mine superintendent, R. K. McCallum, metallurgist of the mine staff, J. M. Basham, consulting engineer, L. T. Kett, general manager, and J. G. Huseby, assistant man- ager of the company. C. A. Anderson and R. S. Cannon, Jr., of the U. S. Geological Survey spent 10 days in the field with the writers and contributed many valuable (3) Special Report 14 suggestions. The writers are also indebted to E. H. Bailey of the Geological Survey for a review of the report and helpful suggestions on presentation of the underground data, and to R. A. Weeks and A. W. Postel of the Geological Survey for editing the report. REGIONAL GEOLOGY The rocks in the West Shasta copper-zinc district consist of a thick series of lava flows and pyroclastic rocks that are overlain by sedimentary formations. The Copley greenstone, named by Diller 'the Copley meta- andesite, consists of mafic flows and pyroclastics of probable Lower or Middle Devonian age, and is the oldest formation exposed in the district. It is here called the Copley greenstone because it is a greenish metamorphosed rock in which the primary ferromagnesian minerals have been altered to chlorite and epidote. Some units show andesitic textures, but other units contain basalt and mafic pyroclastic rocks. The Copley is overlain by silicic flows and pyroclastics of the Balaklala rhyolite of Middle Devonian age. The name "Balaklala rhyolite" proposed by Diller 1 had been abandoned by Graton, 2 but the authors are restoring the name because new evidence indicates that the formation is composed principally of extrusive rhyolite and pyroclastics. A subsidence oc- curred in Middle Devonian time, as the Balaklala rhyolite is overlain conformably by the Kennett formation which consists of tuff, shale, and limestone and is of Middle Devonian age. Uplift, followed by erosion, removed the Kennett formation from part of the area, but renewed subsidence initiated the deposition of a great thickness of sedimentary material, beginning with the shales, sand- stones, and conglomerates of the Bragdon formation of Mississippian age. No sedimentary formations younger than the Bragdon occur in the immediate vicinity of the copper-zinc district, but younger sedimentary rocks over- lie the Bragdon east of the district. The volcanic rocks of the Copley greenstone and Balaklala rhyolite are cut by two masses of intrusive rock, which according to Diller 3 intrude the Mississippian sedi- mentary rocks and according to Hinds 4 are overlain by Lower Cretaceous strata. The volcanic rocks and the Mississippian sedimentary rocks are folded and locally sheared. The major folds are broad, with moderate dips and low angles of plunge, but in places tight folds are present. Geologic Formations Copley Greenstone. The rocks of the Copley green- stone were principally mafic flows and pyroclastics, but also included minor amounts of shale and tuffaceous sedi- ments, as well as a few rhyolitic flows and pyroclastic rocks. Much of the Copley now consists of chlorite- epidote rocks, commonly schistose, in which few primary features remain. 5 As no fossils have been found in the tuffsjuid shales of the Copley, its age is not definitely 138) Mo""' J ' S " U - S - Ge °'- Survey ' Geo1 - AUas - Adding folio (no. •Diller, J. s.. op. clt., p. 6. v T 8 Sf -o^n^^^^s^ rM„«ri$; known, but its conformable relationship to the overlyin rocks suggests that it is probably of Lower or Midd Devonian age. Balaklala Rhyolite. The Balaklala rhyolite consist principally of silicic flows interlayered with coarse an fine silicic pyroclastics ; about one-fourth of the rhyolit is pyroclastic. Dikes and plugs that were feeders for the e- trusive material are included in the Balaklala rhyolite The flows and pyroclastics are abnormally rich in sod and silica. They are light-colored and are common! porphyntic, with phenocrysts of quartz and plagioclas' that range in size from 1 millimeter to 7 millimeters. Quartz phenocrysts are the most conspicuous mega scopically. The feldspar phenocrysts are altered to seri cite and clay minerals and blend with the groundmass Some of the rhyolite is amygdaloidal, and locally it con tains flow banding. Coarse rhyolitic pyroclastics and rhyo- lite tuffs are interbedded with the rhyolite flows, anc amygdaloidal andesite is present as thin flows in the lowei part of the Balaklala. Concerning the origin of the Balaklala rhyolite, there has been a major difference of opinion. A resume of the evidence of the origin is necessary here, because the mas- sive sulfide ore bodies in the West Shasta copper-zinc district are found only in the Balaklala rhyolite, and the distinction between an intrusive and an extrusive origin for this formation has an important bearing on conclu- sions regarding the geologic structure and the locus of ore bodies. Diller, 6 on the basis of his work in the Redding and Weaverville quadrangles, recognized the intrusive nature ot some of the rhyolite that occurs as dikes and sills but concluded that most of it consists of flows and breccias that iormed at the surface by volcanic processes. Graton 7 on the basis of mapping in the vicinity of the Iron Moun- tain and Bully Hill mines, concluded that the Balaklala rhyohte is an intrusive, possibly of laccolithic form Other geologists who worked in the area after Graton followed his interpretation and regarded the Balaklala as intrusive alaskite and alaskite porphyry. The writers have concluded that Diller was correct m assigning a volcanic origin to the Balaklala rhyolite, and that it consists of rhyolitic flows and pyroclastics, with some intrusive dikes and plugs that probably repre- sent the feeders for the ejected material. The interpre- tation of the surface origin of much of the Balaklala rhyo- lite is based on its internal features and on evidence seen at the contacts. Many types of contact relationships are found be- tween the Balaklala and the underlying Copley rocks, as listed below : r j , (1) The contact between the Balaklala rhyolite and the locaHilfp nSt °, n V iS a , n ° rmal de P osi tional -equmcT in many nin i ! ,, Pj 7 oclastlc and ellipsoidal lavas of the Copley are over am locally by as much as 50 feet of lenticular pyroclastic beds rhvoH? D man3 \ fragments of porphyria and nonporphyritic rhyohte m , subordinate matrix of mafic lava. The rhyolite fra^ rnents m these pyroclastic beds range from half an inch to several feet in dia meter. Some of the fragments have chilled borders • Diller, J. S., op. cit., p. 7. 'Graton L. C, op. cit., p. 87. 8 Hinds, N. E. A., op. cit., 1933 Massive Sulfide Deposits at Iron Mountain MASSIVE SULFIDE MINES .SUTRO ,STAUFFER J50LINSKY .MAMMOTH EARLY BIRD SHASTA KING .BALAKLALA .KEYSTONE .SPREAD EAGLE _STOWELL .SUGARLOAF .LONE STAR IRON MOUNTAIN EXPLANATION Biotite quortz diorite 1 '\V^ Albite gronite REDDING Poleozoic sedimentary rocks BolOklolO rhyolite A C v 5 Miles Copley greenstone Geology by AR.Kinkel, Jr and JPAIbers Figure 1. Map showing location of the West Shasta copper-zinc district and its generalized geologic setting. Special Report 14 i ■_• i in other areas near 1 1 • * - contacl a few rhyolite bombs occur hi rocks composed mainlj of bombs and fragments of ande- silir lava. (3) The basal layers containing rhyolite bombs and frag- ments in places grade upward into either porphyritic or non- porphyritic rhyolite; but in other localities they are in sharp contacl with overlying rhyolitic rocks. ill In some places the Balaklala rhyolite rests on the Copley greensi with a sharp contact, but thin amygdaloidal flows of andesite similar in appearance t" those of the underlying Copley occur in the Balaklala above the main contact. Lenses of volcanic debris composed principally of crystal chips, and fragments of porphyritic rhyolite also occur in the Copley greenstone several hundred feel below the main contact with the Balaklala. The presence of rhyolitic bombs and minor flows in the upper part of the Copley greenstone, and the occurrence of small mafic Hows in the lower part of the Balaklala rhyolite indicate that the change from predominantly malic to predominantly felsic lavas was not abrupt and that interlax ering of the two types of lava was Common at the contact. FlOURB 2. Pyroclastic beds in the Balaklala rhyolite ; beds of coarse pyroclastic rock overlain by shaly tuff with pyroclastic layers. Internal structures also provide evidence of the extrusive nature of the Balaklala rhyolite. At least 20 percent (if the Balaklala rhyolite contains either layers ol coarse volcanic fragments or bedded tuffaceous mate- rial. Evidence of rude sorting in volcanic breccias, graded bedding in the finer layers, the persistence and evenness of bedding in tuffs, the presence of unbroken shaly layers in the full', flow banding that parallels the fragmental layers in some of the rhyolite, interlayering of silicic and mafic material, layers of amygdaloidal rhyolite, and frag- ments of rhyolite in the andesitic flows all point to an effusive origin for much of the Balaklala rhyolite. Many massive structureless outcrops of the Balaklala rhyolite display no internal features that indicate mode of origin. Conclusive proof of the surface origin of the bedded material in the rhyolite is supplied by the presence of a fossil in a bed of crystal tuff near the top of the Balaklala. This fossil has been determined by Dr. D. II. Dunkle of the National .Museum as a fish plate f rom an euarthrodiran fish close to Titanichthys of Middle Devonian age. A small body of rhyolite porphyry shown in the southeastern part of the Iron Mountain map is believed to occupy one of the feeder channels for the extrusive rocks. Small bodies of intrusive breccia are found near the borders of this rhyolite porphyry plug that are quite different from the breccias of pyroclastic origin. Other steep tabular rhyolite porphyry masses that appear to be sills and dikes intruding the underlying Copley greenstone crop out along the southern part of the map area. Still other bodies of rhyolite intrude the Copley northwest of the mine. Kennett Formation. The Kennett formation of Mid- dle Devonian age does not occur in the mine area but is exposed northeast of the mine. A description is included in this report because the Kennett formation was depos- ited on the Balaklala rhyolite and at the contact there is further evidence of the surface origin of the Balaklala rhyolite. In addition, the Kennett is the lowest formation in the thick cover of shale that overlies the mineral dis- trict. It is composed predominantly of siliceous black shale but contains tuff and limestone. Diller 9 reports that the formation has a minimum thickness of 865 feet but may have been thicker because much of the Kennett in some areas was removed by erosion before the deposition of the younger sediments. Evidence that the Kennett formation rests conform- ably on the Balaklala rhyolite lies in the fact that inter- bedding of shale, shaly tuff, crystal and lithic tuff, fine pyroclastics, and flow-banded rhyolite is so common be- tween the two formations that it is usually difficult to draw 7 a contact between them. Some rudely bedded sandy debris and arkose, apparently derived from the erosion of Balaklala rocks, occurs locally. At a few places black shale of the Kennett formation lies directly on massive flows of Balaklala rhyolite. No rhyolite has been found that intrudes the strata of the Kennett formation. Such an occurrence in the lower part of the Kennett is not considered impossible because volcanic activity continued into Kennett time. Pyroclas- tics and flows of the Balaklala type occur several hun- dred feet above the base of the Kennett, and feeders for these may have cut the shale underlying the rhyolitic pyroclastic beds and flows. The fish plate, found in rhyo- lite tuff in the upper part of the Balaklala marks the change from volcanic conditions, in which flows predomi- nated, to primarily marine sedimentation and dates this change as occurring in Middle Devonian time. The Kennett formation is overlain by a great thick- ness of Paleozoic sedimentary rocks northwest of the mapped area. Plutonic Rocks Two plutons intrude Copley greenstone and Balaklala rhyolite in the West Shasta copper-zinc district south of the Iron Mountain area. The older and smaller of these plutons is albite granite, and it is exposed about | mile southeast of Iron Mountain. It crops out as a rudely ellip- tical stock over 18 square miles of the district. The younger pluton is biotite-quartz diorite, and it is exposed 7 miles southwest of Iron Mountain over an area of 17 square miles in the southwestern corner of the West Shasta dis- trict. It extends for many miles to the southeast and to the northwest out of the mapped area. Diller, J. S. ( op. clt., p. 2. Massive Sulfide Deposits at Iron Mountain Albite Granite. The albite granite varies consider- ably in texture and mineralogy at different localities, but it has many resemblances to the Sparta granite in Oregon •described by Gilluly. 10 The albite granite in most places is light colored and has a granitoid texture. Quartz and feldspar grains in the equigranular facies average about 2 millimeters in size, but much of the granite also contains quartz pheno- crysts. Combined quartz and plagioclase, in about equal proportions, make up more than 90 percent of the granite and altered ferromagnesian minerals less than 10 per- ' cent. The plagioclase now present is all ablite or albite- oligoclase, but saussuritic cores in some crystals suggest that the original feldspar may have been more calcic. Veinlets of quartz and albite replacing the rock, and secondary myrmekite and micrographic intergrowths of quartz and albite show that albitization is widespread in the rock. Much of the feldspar has been replaced by quartz. Although a little relict hornblende is present, the principal ferromagnesian minerals are chlorite and epi- dote. The albite granite appears massive and little sheared at most localities, but some deformation has occurred, particularly in parts containing numerous xenoliths of Copley greenstone. At these places the rock is mashed or sheared and has been altered to sericite schist. The albite granite intrudes the Copley greenstone and the Balaklala rhyolite. It has not been found in con- tact with the Devonian and younger sedimentary rocks in the area mapped. It transgresses the schistosity of the invaded rocks in many places, but is not itself* foliated except in small areas. On the basis of his mapping in the Redding quadrangle, Diller n determined its age tenta- tively as late Jurassic. Biotite-Quartz Diorite. The larger pluton ranges in composition from diorite to granodiorite. The rock has a granitoid texture, is light gray, and has a fresh appear- ance. It contains predominantly quartz, biotite, and zoned oligoclase-andesine and has less than 10 percent each of hornblende, orthoclase, and augite. A planar structure is developed throughout most of the intrusive. This structure is concordant in strike and dip along the margins of the intrusive with its steep con- tacts and is horizontal in the center of the instrusive, which suggests that the roof of the intrusive mass was only slightly above the present erosion surface. Biotite-quartz diorite intrudes both the albite granite and the shales of the Bragdon formation of Mississippian age. It is overlain nonconformably by Lower Cretaceous sedimentary rocks and must be late Jurassic in age. 12 Structure and Geologic History Rocks older than the Copley greenstone are not ex- posed in the West Shasta copper-zinc district, but gneiss and schist that are believed to underlie the Copley occur about 15 miles west of the mining district. 13 The Copley greenstone was a lava field that extended over at least 10 Gilluly, James, Replacement origin of the albite granite near Sparta, Oregon: U. S. Geol. Survey, Prof. Paper 175-C, pp. 67-81, a Diller, J. S., op. cit., p. 8. 12 Hinds, N. E. A., op. cit, 1934. w Hinds, N. E. A., op. cit, p. 81, 1933. 1000 and probably over several thousand square miles. 14 The Balaklala rhyolite was much less extensive, but as much of it has been removed by the present erosion cycle or lies under a cover of younger sediments, its original limits are not accurately known. The silicic volcanic rocks of the Balaklala apparently formed a volcanic highland on the Copley lava field but some mafic flows occur with the silicic flows. The deposition of mafic, Copley-type flows probably continued in adjoining areas during the formation of the highland of Balaklala rocks, as there appears to be an overlapping of Copley and Balaklala rocks near the edges of the highland. The Copley and Balaklala rocks were submerged in Middle Devonian time and the Kennett formation com- posed of siliceous shale and tuff with interbedded lime- stone was deposited upon the volcanic terrain. The Ken- nett formation is overlain north of the copper-zinc district by shale, sandstone, and conglomerate of the Bragdon formation of Mississippian age. An erosional uncon- formity occurs between the Kennett and Bragdon for- mations ; part of the Kennett has been eroded. An intru- sion of albite granite and a subsequent intrusion of bio- tite-quartz diorite cut all the rocks in the district. Both intrusions probably occurred in late Jurassic time. The major structural feature of the Shasta copper- zinc district is a northeast-trending, gently domed anti- clinorium that culminates about 5 miles northeast of Iron Mountain. Folding is moderate in the copper-zinc district ; the folds range from gentle near the axis of the anticli- norium to close in places along the flanks. The date of the major folding cannot be determined with certainty from the evidence in the Iron Mountain area. The folding oc- curred after Mississippian time but prior to the deposition of the Chico formation (Upper Cretaceous), and Diller 15 and Hinds 10 give evidence that the main period of fold- ing occurred in late Jurassic time. The rocks are region- ally metamorphosed, the mafic lavas being altered to chlorite-epidote rocks and the felsic lavas to sericitic and siliceous rocks. The rocks are commonly schistose, and at least part of the schistosity was formed before the intru- sion of the albite granite ; they were mineralized and fur- ther silicified after the intrusion of the albite granite. An angular unconformity is present between the older formations and the Chico formation of Upper Cre- taceous age that occurs to the south of the mapped area. The Chico strata are tilted to the south at low angles and are overlain by the Tuscan and Tehama formations of Pliocene age, which are composed of interbedded sedimen- tary and pyroclastic material. The Pleistocene gravels of the Red Bluff formation form a thin veneer on the older formations around the edges of the valleys. IRON MOUNTAIN MINE History and Production The first claims on the large gossan outcrops on Iron Mountain were staked in the early 1860 's and held for the future value of the gossan as iron ore. Silver ore was dis- covered in the gossan in 1879 and some development work " Diller, J. S., op. cit, p. 7. Hinds, N. E. A., op. cit, p. 86, 1933. Ferguson, H. G., Gold lodes of the Weaverville quadrangle, California : U. S. Geol. Survey Bull. 540, pp. 22-79, 1912. Averill, C. V., op. cit, p. 15, 1931. 15 Diller, J. S., op. cit, p. 10. " Hinds. N. E. A., op. cit. ,1934. Special Report 14 Figure 3. Iron Mountain mine. Richmond mine plant. and mining were done in the silver-rich portions. At that time little interest was shown in the disseminated chalco- pyrite and the massive sulfide ores that were encountered in the search for precious metals. It was not until 1895 when a thorough prospecting of Iron Mountain disclosed large bodies of copper-bearing sulfides, that the mineral possibilities of the region now known as the West Shasta copper-zinc district were recognized. T™ S iT Ver °/ eS W r e mi ," ed intermittently ™ the gossan at Iron Mountain from 1879 to 1897, when the present owners The Mountain Copper Company, Limited (for- merly Mountain Mines Company) began mining the mas- sive sufide ores for their copper content. The Old Mine ore body (pi. 2) was the first massive sulfide ore to be mi TLl avera ^ d 7 - 5 P er cent copper, 1.0 ounce of silver and 0.04 ounce of gold to the ton, but the ore of the Old Mine ore body was enriched by secondary copper min- erals. The zinc content of this ore body is reported by the mine staff to have been more than 2 percent, and may have been as much as 5 percent. The total production from the Old Mine sulfide lens was 1,608,000 tons of massive sul- fide ore. In 1907 a zone of disseminated chalcopyrite was found to underlie the Old Mine sulfide lens. Eight hun dred and twenty thousand tons of ore containing 3 5 per- cent copper, 0.001 ounce of gold, and 0.04 ounce of sifver was produced from this disseminated chalcopyrite zone (the Number 8 mine). The zinc content of the dTssem" nated copper ore is not known with certainty but it is reported by the mine staff to have been very low a V, 01 ^" 21 ! 10 ore has bee n mined from'the Richmond and the Mattie ore bodies, and minor copperhead™ sulfides occurred along the borders of the Hornet or! body. The flotation plant that treated the ore from the Richmond ore body is located near the nortal of t£« p- 7 mond adit. About 380,000 tons oS \ol was mined torn the" Richmond and Mattie ore bodies. This ore contained 2 percent copper and 3.5 percent zinc. The Brick Flat ore body containing copper-zinc ore has not been mined Mas sive pynte containing very little copper and zmc has been mined in large quantities from the Ilorne? and L ? chmo^S ore bod.es, and a large tonnage of pvrit * "til? remaius available for mining in the Richmond, Complex and S33 Flat ore bodies. Three million six hundred thousand ton of pynte has been mined at Iron Mountain ; this ore i used in the production of sulfuric acid. Copper has been produced by The Mountain Coppe Company, Limited, from direct smelting ore, from sulfide ore treated in a flotation plant, and from the leaching o pyntic ore that was mined for its sulfur content Th< Iron Mountain mine produced 197,951,738 pounds of cop per to the end of 1919 from direct smelting ore, but figure* are not available for the total copper production since that date as copper production was reported only by counties. After 1919, the principal periods of copper pro- duction from ore from Iron Mountain were in 1925 1928-1930, and 1943-1947. Minor copper production was maintained between these years by leaching of ore that was mined for the production of sulfur. No record is avail- able on the production of zinc from the Iron Mountain i mine. Gold and silver have been extracted from the gossan overlying the massive sulfide ore of the Old Mine ore body From 1889-1893, 38,000 tons of gossan was mined that contained 8 ounces of silver to the ton. The gold content of this ore is not known. From 1929 to 1942, 2,600,000 tons of gossan was mined that contained 8.3 ounces of silver and 0.073 ounce of gold per ton. Only small portions of the Iron Mountain mine have been accessible at any one time, and this has handicapped all geologists who have studied the ore bodies. An attempt is made m this report to record the information available on the location and mineralogy of the ore bodies mined many years ago, but it is recognized that many of the data are incomplete and fragmentary. The underground work- ings that were accessible at the time of the writers' study were the Richmond haulage level (2600-foot level) the grizzly floor under the northeast end of the Richmond ore body (2650-foot level), and the 2700-foot level. The levels of the Iron Mountain mine are numbered accord- ing to their elevation above sea level and are shown on plate 3. The Richmond Extension stopes at the southwest end of the Richmond ore body and some drifts and raises in the Complex ore body were also accessible. Diamond drill cores from the Brick Flat ore body were examined Information on the Hornet, Old Mine, Number 8 mine, Confidence-Complex, New Camden, and Mattie ore bodies was obtained by a compilation of old and incomplete mine maps. Much information on the old workings was lost • i noo d destr °y ed the engineering office at the mine in 1933. Formations in the Mine Area ♦i. ?S e Cople y greenstone, the Balaklala rhyolite, and the albite granite are the only rock units that occur in the immediate vicinity of the Iron Mountain mine. These are shown on the surface map and on the cross sections (pis 1 and 4). The writers have found that individual flows of porphyritic rhyolite of the Balaklala are characterized by approximate uniformity of phenocryst size. The stra- tigraphy within the Balaklala is mapped on such distinc- tive flows and on pyroclastic beds. For this reason, the porphyritic rhyolites in the Iron Mountain area are sub- divided as shown in the explanation of plate 1. Even though it is recognized that a rigid classification based on phenocryst sizes cannot be maintained everywhere be- cause of some heterogeneity within flows, the writers Massive Sulfide Deposits at Iron Mountain ,'lieve that the method of distinguishing individual flows phenocryst size is valid in the West Shasta copper-zinc strict. A change in phenocryst size can be correlated eitith other criteria used for distinguishing separate flows. Much of the rock in the mine area has been sheared, licified, chloritized, and argillically altered, making the distinction between rock types very difficult. Ore Deposits haracter and Distribution The two types of ore in the Iron Mountain mine are lassive pyrite bodies that contain chalcopyrite and spha- ;rite and zones of disseminated chalcopyrite and quartz- halcopyrite veins in schistose rock. The massive sulfide re is much more abundant than the disseminated ore. )isseminated ore occurs only in the Number 8 mine and he adjoining Confidence-Complex ore bodies. All other »re bodies in the Iron Mountain mine are of the massive ulfide type. The massive sulfide ore bodies differ in shape and tttitude. The Hornet ore body is nearly vertical. The Vlattie is a cigar-shaped, horizontal ore body whose faulted sxtension has not been located ; it may have been removed )y erosion. The rounded bottom of the erosion remnant >f the Old Mine ore body suggests that a large gently dip- )ing lens-shaped or synclinal mass was once present. The Richmond and Complex ore bodies, taken together, have i synclinal shape, and the Brick Flat ore body also may )e in part synclinal, although its shape is determined mly by rather widely spaced drill holes. The Number 8 nine ore bodies and the Confidence-Complex ore bodies figs. 5 and 6) are in zones of chalcopy rite-bearing seri- sitic, porphyritic rhyolite and along quartz-chalcopyrite reins on minor faults. Probably Iron Mountain contained about 25,000,000 ons of massive sulfide ore before the erosion of the upper >ortion of the Old Mine ore body. Minerals of the Primary Ore The principal ore minerals are pyrite, chalcopyrite, md sphalerite. The ore contains recoverable amounts of ;old and silver. Galena has been seen in a few specimens ;nd tennantite-tetrahedrite has been reported by the mine taff. Magnetite was not seen in the main ore bodies, but mall deposits composed of magnetite and specular hema- ite are closely associated with the ore bodies. The mag- tetite and hematite replace porphyritic rhyolite, but the ignificance of the presence of small bodies of these min- rals near the bodies of massive pyrite is not known. The »nly gangue minerals seen in the ores are very small imounts of quartz and calcite, both of which occur as nterstitial grains in the sulfide ore and as veinlets cut- ing the ore. Most of the massive sulfide ores contain 90 to 95 per- ent pyrite ; the silica content is remarkably low, the lornet ore body containing the least. Assays of 15 dia- aond drill holes in the main Hornet ore body that repre- ent 1400 feet drilled through massive sulfide ore aver- iged 2.68 percent silica and 48.6 percent sulfur. The Rich- mond ore body averaged about 5 percent silica. Small mreplaced or partly replaced ribs of porphyritic rhyolite ire found locally in the ore, but hydrothermal alteration tnd movement have usually transformed these into seri- itie or clayey gouges. Polished sections of the ore have not been studied by the writers, but the age relationships of most of the minerals have been determined megascopi- cally in the underground exposures. Pyrite. Pyrite is the predominant metallic mineral of the ore bodies of the Iron Mountain mine. The grain size ranges from less than 0.5 millimeter to 5 millimeters. A few crystals or clusters of coarse crystals attain a di- ameter of 1 centimeter. Typical ore is usually a fine- grained, yellow, metallic-looking mass of 1-millimeter pyrite grains containing a few irregular clumps, several inches in diameter, of more coarsely crystalline pyrite. Some euhedral pyrite is present, particularly in the coarse-grained varieties and in parts of the ore that con- tain unreplaced host rock, but most of the pyrite is anhedral. Pyrite specimens from different ore bodies differ somewhat in appearance, but the variation is no greater than that found locally in an individual ore body. Ore from the Hornet, Old Mine, and Complex ore bodies is somewhat finer grained than that from the Richmond and Brick Flat bodies. Banded ore occurred along the north- west wall of the Mattie and the southeast wall of the Hor- net ore bodies, and banded ore is found locally on the southeast wall of the Complex ore body. Such minor varia- tions, however, only serve to emphasize the uniform char- acter of the enormous masses of pyrite in the ore zone. Chalcopyrite. Chalcopyrite occurs throughout the pyrite bodies of the mine, but only locally is it present in sufficient quantities to be ore. Bodies of massive pyrite, mined for their sulfur content, contain only 0.5 to 1 per- cent of copper in chalcopyrite disseminations and vein- lets. The copper ore is comprised of the chalcopyrite-rich portions of the massive pyrite bodies. The ore has a more yellowish tint than the pyrite and contains irregular vein- lets and small lenses of chalcopyrite, which can be seen only in the high-grade portions of the copper-bearing mas- sive sulfides. In some ore the veinlets and lenticles of chal- copyrite are alined, and this imparts a banded or streaked appearance, but most of the copper ore shows no layering. Chalcopyrite ore in the Number 8 and Confidence-Com- plex ore bodies is not associated with massive pyrite. In these ores bodies chalcopyrite occurs as disseminations in schistose porphyritic rhyolite, as replacement bodies along faults, and as quartz-chalcopyrite veins along faults. Chalcopyrite is generally more abundant than pyrite in the disseminated copper ore. Chalcopyrite also occurs in quartz veins that fill fractures in the massive pyrite bodies. These veins vary in composition from quartz with a few specks of chal- copyrite, to chalcopyrite veins with a little quartz. They are rarely more than a few inches in width, and they cut the massive pyrite and silicified wall rocks. The disseminated chalcopyrite and the quartz-chal- copyrite veins are younger than the massive pyrite ore bodies, and it is probable that the main part of the chal- copyrite was introduced into the massive pyrite bodies at the time the quartz-chalcopyrite veins were formed in the massive sulfide ore and the disseminated chalcopyrite of the Number 8 mine was deposited. Sphalerite. Sphalerite occurs throughout the pyrite ore, but it is difficult to recognize in hand specimens except in high-grade zinc ore. It was seen in some of the ore at 10 Special Kepokt 14 the southwest end of the Richmond ore body, and it is reported by the mine staff to have been abundant in parts of the Mattie ore body and to have occurred locally in the Old Mine ore body. Tlie sphalerite is a fine-grained, dark pray to black variety, and contains a considerable amount of iron. It tends to be alined in layers and streaks, and where much is present as disseminations it imparts a gray color to the massive pyrite. Veinlets of sphalerite cut the massive pyrite locally, and chalcopyrite-sphalerite veinlets have been seen. The association of chalcopyrite and sphalerite suggests that the two minerals are probably in part con- temporaneous, although the mine staff reports that little or no sphalerite occurred with the disseminated chal- copyrite in the Number 8 mine. The sphalerite may have 1 n deposited during a shorter period of time than the chalcopyrite. Gold and Silver. No silver minerals or gold have been seen in the ore. Tennantite (or tetrahedrite) is re- ported to have occurred in the Old Mine ore body and may account for the silver content of the ore. Gold and silver occur in massive sidfide bodies and in gossan de- rived from massive sulfide ore; almost none is found in the disseminated copper ore or in the quartz-chalcopyrite veins. Quartz. The silica reported in assays of the Iron Mountain ore is almost entirely in the form of quartz. A little sericite is present in the ore, and a very small amount of unreplaced wall rock can be found locally near the ore boundaries and as gouge material in the ore, but silicates other than quartz in the ore probably amount to less than 1 percent of the insoluble material. Most of the quartz in the massive sulfide ore occurs as individual grains, as films between pyrite grains, or as small irregular bodies of mixed pyrite and quartz. A few quartz and quartz-calcite veinlets occur in massive pyrite ore, but these veinlets seldom exceed a few inches in width and several feet in length. The average silica content of the various massive sulfide ore bodies appar- ently ranges between 2.5 percent and 5 percent. The dis- seminated copper vein systems in the Number 8 mine contained much silica, however, in the form of quartz veinlets and silicified wall rock, and constituted a silice- ous copper ore. The quartz in the ore is both pre- and post-pyrite in age. Some of the quartz grains in the ore are unre- placed quartz phenocrysts from the porphyritic rhyolite. Other quartz-rich areas represent partly replaced silici- fied rhyolite. Some quartz may have been deposited with the massive sulfide bodies, but these bodies were also cut by quartz veins. Calcite. Calcite is the youngest of the hypogene minerals. It occurs in small amounts with quartz in veins, or less abundantly as small veinlets and irregular patches. Distribution of Metals The mineral content of the disseminated ore differs from that of the massive sulfide ore. The disseminated ore contains only pyrite and chalcopyrite in sericitic and siliceous rocks, and contains many chalcopyrite-bearing quartz veins. Pyrite is generally about equal in amount to chalcopyrite. Practically no sphalerite, gold, or silver occur in the disseminated ore. The massive sulfide ore is composed almost entirely of pyrite, but contains chf copyrite and sphalerite in small amounts distribub throughout the massive sulfide bodies, and local conce: trations of these minerals occur. Gold and silver occi only in the massive sulfide ore bodies. The distribution of copper and zinc in the massb sidfide ore is not well known in detail. However, as all tl larger concentrations of these metals were mined as bas. metal ore, the location of stopes that were mined for tl copper and zinc content of the ore shows the location ( these concentrations. The record is incomplete, as sma bodies of base-metal ore have been encountered at son: localities in the mine where such ore could not be mine separately from the massive pyrite. The copper-zinc ore is found principally along tl edges and bottoms of thick sulfide bodies, but it is no everywhere present along such boundaries and locall occurs in minable quantities throughout thinner or bodies. The known concentrations of copper and zinc i the massive sulfide ore at Iron Mountain are as follows : (1) Two-thirds of the Mattie ore body was mined for coppt and zinc ore, although some bodies of pyrite low in copper and zin were left in place. The average ore mined from the Mattie containe 2.25 percent copper and 3.5 percent zinc. (2) Small concentrations of copper ore occurred at the bol torn and at the top of the Hornet ore body, but these were no mined separately. The Hornet ore body averaged only 0.85 percen copper. (3) The northeast corner of the Complex ore body, near th Scott fault, contained copper-zinc ore, and small bodies were minei for those metals. The lowest portion of this part of the ore bod; contained the highest-grade ore. (4) Copper-zinc ore occurred along the bottom and west wal of the Complex ore body on the 2600-foot level. (5) The upper part of the Complex ore body, where it is cu by mine workings on the 3000-foot level, contains an appreciabli amount of copper but has not been mined. (6) The entire west end of the Richmond ore body, called th Richmond Extension, has been mined for copper and zinc. It wai the largest body of base-metal ore in the mine. (7) A large block of copper-zinc ore occurs in the Brick Flal massive sulfide body. The lower part of the eastern half of the ore body is reported to contain the best grade of ore. This ore has not been mined. No oxidation or secondary enrichment has occurred even though the top of this ore body is only 150 feet below the surface and the ore lies above the water table. (8) The Old Mine ore body is an erosion remnant and prob- ably represents the bottom of a mucli larger ore body. The Old Mine ore body contained the highest-grade copper ore yet found at Iron Mountain because the sulfides below the leached outcrop were en- riched by secondary copper minerals. Disseminated Copper Ore A body of disseminated chalcopyrite and pyrite un- derlies the Old Mine ore body. Where the disseminated ore lies beneath the Old Mine it was mined through the workings of the Number 8 mine, but its extension to the northeast of the Old Mine is known as the Confidence- Complex vein system. The location of this disseminated and vein-type copper ore is shown in plate 2. Information on the distribution of copper and the types of mineraliza- tion in the Number 8 and Confidence-Complex workings was obtained from the mine staff and from unpublished reports by G. F. Seager and O. H. Hershey. This section of the mine was closed down and partly filled in 1919, but portions of it were reopened for a short period in 1929-30. Two principal types of ore that may occur together or separately are present in the Number 8 mine. One type consists of chalcopyrite grains, veinlets, and fairly solid masses of coalesced chalcopyrite veinlets that replaced Massive Sulfide Deposits at Iron Mountain 11 ,'histose porphyritic rhyolite. Pyrite is subordinate in i jtnount to chalcopyrite and occurs as scattered anhedral rains. According to Seager 17 the pyrite is the earliest iietallic mineral, and is veined by chalcopyrite, quartz, | nd chlorite. Many small and discontinuous faults and iouge zones are present, and the largest ore bodies occur jt intersections of these gouge zones or fracture zones. L'he second ore type, quartz-chalcopyrite veins, is less bundant but locally occurs in the disseminated ore. The quartz-chalcopyrite veins of the Confidence- Complex vein system (the northeasterly continuation of he Number 8 mine ore) occur as fracture fillings. In he southwestern end of the Confidence-Complex work- ngs the ore zone contains both disseminated chalcopyrite !md quartz-chalcopyrite veins. The northeastern end of pe Confidence-Complex workings contains principally tjuartz-chalcopyrite veins along a fault that has formed (several inches to several feet of gouge. The only exposure of these veins seen by the writers is in the Complex adit on the 3000-foot level. There the vein is exposed for 80 feet and lies under a 1-foot fault gouge. The footwall of the vein is sharp and the wall rock is not replaced. One end of the exposed portion of the vein contains only quartz and chalcopyrite, but the other end consists of 2 feet of massive pyrite containing some chalcopyrite but no quartz. The pyrite ore in the vein is similar in appearance to that in the large massive sulfide ore bodies. There is apparently a gradation at this locality from quartz-chal- Eopyrite veins to massive pyrite ore along the strike of a single vein. The rocks adjoining the chalcopyrite ore bodies in the Number 8 mine contain secondary quartz, serieite, shlorite, and disseminated pyrite, but these minerals are more widely distributed than the chalcopyrite. Structural Features of the Ore Bodies Form. The massive sulfide deposits, which make up the bulk of the ore of the Iron Mountain mine, are snormous masses composed almost entirely of pyrite re- placing porphyritic rhyolite. Except in the vicinity of the Old Mine ore body, the wall rocks are virtually un- mineralized. The Hornet, Richmond, Complex, and Brick Flat massive sulfide ore bodies were one continuous body before they were displaced by the Scott and Camden faults. It also seems possible that the New Camden ore body is a faulted segment of the Complex ore body, but the rela- tionship of these two ore bodies is not well known. The longitudinal section, plate 2, suggests that the ore in the gossan area, which occurs up-dip from the Old Mine ore body and the Number 8 mine ore body, is a faulted portion of the Brick Flat ore body. It therefore seems probable that all the major ore bodies were one continuous deposit before post -mineral faulting. Isolated ore lenses, such as the Mattie and Okosh, which lie along the side of the main ore run, also occur but are small. Plate 6 is a reconstruction showing the probable shape of the massive sulfide ore body before faulting and erosion had destroyed its continuity. Dip-slip movement was assumed on faults to fit the ore bodies together and the lens-shaped habit of the ore bodies was assumed in estimating the amount of ore removed by erosion. 17 Unpublished report. In the vicinity of the Old Mine ore body, the ore zone occupied a considerable thickness of rock. The Number 8 mine ore was separated vertically from the Old Mine massive sulfide lens and from the sulfide gossan by as much as 300 feet of barren or slightly pyritized rock. The bands of pyritized rock and gossan are irregular in plan as shown on the surface map. The relation between the Old Mine ore and the Number 8 mine ore is illustrated in section B-B', figure 6, but north of this section, the gossan derived from massive sulfide ore occurs between the level of the Old Mine ore and the level of the Number 8 mine ore. The total thickness of the mineralized zone in the vicinity of the Old Mine ore body is at least 600 feet, meas- ured normal to the bottom of the Number 8 mine ore body. The mineralized zone, before faulting, lay on a gentle slope rising from the deepest ore in the Number 8 mine northward to a point above the present erosion surface east of Iron Mountain peak. From this point the Brick Flat and Richmond Complex ore bodies occupied a syn- clinal trough plunging gently northeast. Continuing northeastward through the Hornet ore body, the ore lies steeply along a fault zone. The total length of the original ore body before faulting must have been at least 4500 feet. The shapes of the Number 8 mine ore bodies are shown in figures 5 and 6. The ore in the Number 8 mine occurred along shear zones, particularly along intersect- ing shear zones or intersecting minor faults. The ore bodies are reported by the mine staff to parallel the schis- tosity of the replaced rock. Figure 5, which was compiled from stope plans, shows the shapes of the mined ore bodies. The material between ore bodies was in places mineralized rock that contained too little copper to be mined. Con- sequently, the plan shows the major ore runs but not the extent of mineralization. There are two main ore bodies in the Number 8 mine, and each is arcuate in horizontal section. This curvature of the ore bodies is best shown on the 2350-foot level in the east ore run and on the 2500- and 2610-foot levels in the west ore run (fig. 5). Sections A- A' and B-B' (fig. 6) illustrate the en echelon pattern of individual ore shoots and indicate that the thickest portions of most of the ore bodies correspond to marked changes in dip. Relation of Massive Sulfide Ore Bodies to Structures in the Host Rock. The ore deposits in the West Shasta copper-zinc district are found only in the Balaklala rhyo- lite, but within this formation their distribution is con- trolled mainly by folds and pre-mineral faults in the rocks, and to a lesser degree by the sheeting and schistosity of the rocks. The relationship between schistosity and folding throughout the region is not that of axial-plane cleavage, and many areas of folded schistosity are found. In general, schistosity is parallel to bedding at Iron Mountain, but schistosity is known to transgress bedding locally, and it is difficult to be certain of the relationship between bedding and schistosity in some areas. Schistosity is strongly developed only in small zones, the majority of the rocks being sheeted or slightly foliated. In strongly schistose areas, evidence of bedding is lacking and the relationship between bedding and schistosity is not known. Areas of regular schistosity are common, but in other areas folds in the schistosity do not have parallel axial planes and may plunge in any direction. The folding appears to be controlled at many places by difference in 12 competence of beds and by buttressing effects rather than by a regional pattern. The schistosity north of the Old Mine ore body 3S3E ?"? d , th u e C ° pley ^ reens tone and Balaklala rhyolite contact and the pyroclastic beds of the Balaklala ? beds Cft" - thG Pr 1Idism ™y be due t0 *eariS or beds into lenses m line with the schistosity. Little £S 1S W° ped in the Vicinit ^ of th * Richmond! Complex ore body, except near the ore contacts (The fhe ^ d £. M „?%? t r t T ° f thG ° re b ° dy -"hweK east of thi " T ; »^„ST? leX 1S / he St6ep P ° rti0n South " easi ot tne j fault.) In underground workings near th s ore body the schistosity parallels bedding, wTere the JSSfrtf ,S , n ? Wn ' but itS aline ™ent may be at variance With the schistosity at the surface. The block of Sound %Z£2l3£F m and the surface is not ac= " schisSsL re o 1 f a S" Shil l b - tWe l en the 0re bodies and the scmstosity of the rocks m the vicinity of the ore bodies masir:uia e :o st r d ; Schi ? tosity *»**^™*e a H , , e contacts is always parallel to the contact schistosity On the Ther^f ? n0t Paralld the re ^ ional pyrite that narallel t^! l^' ZOnes of disseminated in the Number 8 m n h fl e ^ and cbalc *- somewhat schistose and !1 a TOcks were P r obably the sulfides we^nTr^nS? n if < l 80Uge Z ° neS at the tim « -buttress and* 3^"ffl^^^^« beca^orJS^^.25^ j n rdatiVely few pla - "curtain" of massrv! suffix leavin * & 10 " to 2 °- foot wall rocks. Th^wa I rockf ^ ^ en he Sto P es and the when they are eSn^ • J ? mng the ° re Cave hadl Y ings, andUriS^^g^^^und wor^ caved areas, or where the wait ' obtamed only in exploration and develonmln* i ^ encoun tered in Special Report 14 ^ U ?S 7 \ brUP > alt 5° Ugh Some contac ts showing repl ment have been found. At most localities no visible cha can be seen m the massive sulfide ore as the contac approached, and the contact has massive sulfide on side, and unmmeralized soft, white claylike go U g e on other. This gouge is commonly more than a foot thick it grades from structureless white gouge affainst thp to strongly sheared, sericitized, porphyritfc XoHte the more solid wall rock is approached It constts highly sheared, altered, porphyria rhyolfte R e ] quartz phenocrysts can be found in the claySe portion the go„ge. Some gouge occurs along all observed Tcontac even though its thickness may be less than anTnch The gouge along the contact locally contains so, pyrite in the f ootwall of the Richmond ore body K th places the gouge contains less clay and is a strong tTn^ T7 t( t SChist The P ^ rite occ ^s in rude layer/ small euhedral pynte grains that show no evidence crushing. A narrow clay gouge senarat^ til • pyrite from the sheared, p^rltTzf d wXo ? The' banT, disseminated pynte in the gouge and sericite gffij lei the ore contact and extend several feet into the too wall diminishing in pyrite with distance from the or The mineralized layers have hazy boundaries and 1 are replacement of schistose material parallel ohe contact A few quartz vemlets or irregular bodies of more sXceou material may also parallel the ore contact, but n E pynte has been found in these zones. It should be empha i-ot mS£ at al ° ng ^ ° re C ° ntacts ^ « ep . * n a , few Realities, massive sulfide ore fingers int( ?i ^ oftot StoTf h 01 ? ^ ° f SChist ° sit ^ In thes'e PW x ioot to K a faulted synclme, as shown both by the shape of the Copley greenstone contact and by bedding in ^verlyui vW 'T matena1 -, ° re in the Number § mine ilwides? where changes in dip of the ore shoots occur and its SSSJSS^T^ C ° ntr01 by ^^ -mpleVin The massive sulfide ore contains few recognizable remnants of unreplaced rock. Exposures showTng i ncom pletely replaced rock can be seen in some of the quar^v benches and these exposures suggest that the re S ment favors massive porphyritic- rhyolite with 2- to 4 millimeter -quartz phenocrysts. Massive nonpruritic K hy0l . lte ' flow -bauded rhyolite, and rhyS vEn « breccia seem to be unfavorable host rocks in theT?l Mountain area, although ore occurs in vocanic breed" at other mines in the district. Some localities are f ™th the exception of those in the Old Mine ore body, all lie near the Camden fault,! again marking this fault as a feeder. These concentrations are found m the Mattie ore body, the southeast side of the Complex ore body, and in the Richmond and Brick 1-lat ore bodies along the north branch of the Camden, fault. The concentration of chalcopyrite in the bottom of some of the ore bodies adjoining the Camden fault also indicates that upward-moving, copper-bearing solutions were traveling along the fault. The amount or direction of the pre-mineral move- ment on the Camden and Sugar Loaf faults is not known It is believed to have been small because the ore bodies appear to have been continuous along one horizon before post-mineral faulting occurred. Massive Sulfide Deposits at Iron Mountain 17 Number 8 ne ore bodies Section A-A Number 8 ne ore bodies Sechon B-B EXPLANATION Gosson from massive sulfide ore Mossive sulfide ore Disseminated ore (in port projected to sections) Contact (Dosned where approximately located) Inferred contact Isometric fence diagram Datum is moon seo level Compiled from The Mountom Copper Company, Limited mops Geology by A R Kmkel.Jr ond J PAIbers Figure 6. Sections of Number 8 mine and Old Mine ore bodies. Location of sections is shown on Figure 5. 18 Oxidation and Enrichment Special Report 14 „,.,,/ 7" v " Gos l an - , Sm;l11 relicl nodules of massive pyrite have been found m gossan within 10 feet of the \Z\ '', • tl( \ u , PI 7 Part 0f the ^ uarr y- However, the high: rebel and broken, porous character of the ground i lin ° r le T/ M,ni , ,,l " ,(> leachin S »»™<> 2732 10.0 2720 10.0 2710 " 8.0 2682 8.0 2614 6.0 2601 __ 6.0 3.5 Mine ore bodv fs 0^7. ^^ ° f the g ° SSan 0ver the 01d she «, L y I ? ° UnCe Per t0n and that of the mas- TMs 1 T ? W the g ° SSan 1S ' 04 0Unce P er ton. dZna/T L? riChment CaUSed hy leachin g and oxi- dationof the sulfide minerals. The average weight of the 18 Unpublished report Massive Sulfide Deposits at Iron Mountain 19 san is 165 pounds per cubic foot and that of the sulfide is 275 pounds per cubic foot. Silver and zinc have been removed from the gossan r the Old Mine ore body, and although the silver ap- lirs to have been precipitated at the contact between tli gossan and the sulfides, no bodies of secondary zinc t'tnerals have been found. i e of Mineralization !H Pyrite seams and quartz veins containing pyrite and a alcopyrite have been found in all the rocks mapped in ib West Shasta copper-zinc district, but little informa- !< ^ exists that would indicate when the sulfide minerals ;re emplaced. However, the main body of biotite-quartz arite has been only slightly mineralized and the min- als are restricted to a few quartz-galena-gold-silver veins th some chalcopyrite in an area about 12 miles south Iron Mountain. It seems doubtful that these veins in 3 biotite-quartz diorite are genetically related to the assive pyrite type of mineralization. Sheared portions of e albite granite as well as the Devonian and Mississip- an sediments contain minor amounts of pyrite. It seems ossible that the massive base-metal type deposits re- laced the Balaklala before the biotite-quartz diorite was nplaeed but after the albite granite was emplaced. The massive sulfide deposits may be late Jurassic in age, but no direct evidence has been found to connect the base metal deposits genetically with any of the intrusive rocks. REFERENCES Averill, C. V., Preliminary report on the economic geology of the Shasta quadrangle : California Div. Mines Rept. 27, pp. 3-65. 1931. Diller, J. S., U. S. Geol. Survey Geol. Atlas, Redding folio (no. 138), 1906. Ferguson, H. G., Gold lodes of the Weaverville quadrangle, Cali- fornia: U. S. Geol. Survey Bull. 540, pp. 22-79, 1912. Gilluly, James, Replacement origin of the albite granite near Sparta, Oregon: U. S. Geol. Survey, Prof. Paper 175-C, pp. 67-81, 1933. Graton, L. C, The copper deposits of Shasta County, California : U. S. Geol. Survey Bull. 430, pp. 71-111, 1909. Hershey, O. H., The metamorphic formations of northwestern Cali- fornia : Am. Geologist, vol. 27, pp. 226-230, 1901. Hinds. N. E. A., Geologic formations of the Redding-Weaverville districts, northern California : California Div. of Mines Rept. 29, pp. 76-122, 1933. Hinds, N. E. A., Jurassic age of the last granitoid intrusives in the Klamath Mountains, California : Am. Jour. Sci., 5th ser., vol. 27, pp. 182-192, 1934. Seager, G. F., Petrology of the Balaklala chonolith, Shasta County, California (abstract) : Geol. Soc. America Bull., vol. 50, no. 12, part 2, pp. 1958-1959, 1939. Seager, G. F., Unpublished report in the possession of the Cali- fornia Division of Mines. printed >» California state printing office 351 7-51 2M EXPLANATION Q IOoiMO -hoc oporu.murtiy locoiea U.u(.B"0-n i'M, D. So«ntnm«n HJO 'S GEOLOGIC MAP OF THE IRON MOUNTAIN MINE AREA, SHASTA COUNTY, CALIFORNIA IRON MOUNTAIN MINE, PLAN AND LONGITUDINAL SECTION OF ORE BODIES Complto from mop* ol The Mountain Copper Compony, L Geology byAR Kinket,J< EXPLANATION Limil of slopes Open odd -£32_ Covad adil «v Outime of Okosh ore body Not Btop«d (outlined by drilling} c \ \ \ \ \ \ X'%feX- \ Outline Of Brick Flat X N \ ore body Not sloped N (outlined by drilling) tW A, IJ OuMm IRON MOUNTAIN MINE, PLAN OF UNDERGROUND WORKINGS GEOLOGIC CROSS SECTIONS. IRON MOUNTAIN AREA a i « 1 1 ur liKurunniM DEPARTMENT OF NATURAL RESOURCES •.-^fe ■ V "||- V 111 3000' Av-'/hi „ r. ^ 4 ■ .$?$&V?* ' '' ™ k C«*PM> fa&mtti ^ W&mm •"■ if; ■'■ : ■ «sJ '-'/v." '"''? ;\.V -.!-""- •re^^v'- .1 ^--. V - *MifeeftHtld we body 3000' " o \ '7 ■■■■"- / "CMspto- 1 Richmond \ o*SMy J 5 s « W « » fttiyoBt* Porenyndc rhyoliU Small (0-gmm) phtnocryilt Bolohlolc rhyolite Copley greenstone Goison Irom massive suilid Contact from drill holes or underground workings (Dashed where approximately located c projected to section) Fault, showing reli (Doshed where approximately located) Probable hull / Oip of bedding Dip ot foliation Nomenclature of foulls C Comden fault CS Comden Sooth fault CN Comden North tau II J "J"»oult S Scott fault CROSS SECTIONS OF ORE BODIES RESTORATION OF THE IRON MOUNTAIN ORE BODY BEFORE FAULTING AND EROSION