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 
 
 N FRANCISCO SPECIAL REPORT 29 FEBRUARY 1953 
 
 GEOLOGY AND ORE DEPOSITS 
 
 OF THE 
 
 AFTERTHOUGHT MINE 
 
 SHASTA COUNTY, CALIFORNIA 
 
 By JOHN 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/geologyoredeposi29albe 
 
GEOLOGY AND ORE DEPOSITS OF THE AFTERTHOUGHT MINE, 
 SHASTA COUNTY, CALIFORNIA* 
 
 By John P. Albers " 
 
 OUTLINE OF REPORT ABSTRACT 
 
 The Afterthought mine is in Shasta County, California, 24 miles 
 
 tract 3 northeast of Redding. It is primarily a zinc-copper mine. Between 
 
 ... „ 1905 and January 31, 1051, it produced 158,525 tons of crude sulfide 
 
 ore that assayed an average of about 1(5 percent zinc, 2.7 percent 
 
 tory and production 4 copper, 2 percent lead, 5.0 ounces of silver, and 0.04 ounce of gold. 
 
 The chief rocks in the mine area are folded shale and tuff beds of 
 
 eral geology 5 Triassic age, underlain by, and also intruded by, soda rhyolite and 
 
 riassic rocks 5 soda rhyolite porphyry. Rock contacts and bedding strike northwest 
 
 Bully Hill rhyolite 5 and in most places dip steeply northeast; a weak cleavage strikes 
 
 Pit formation 7 northwest and dips steeply southwest. Volcanic breccia of Pliocene 
 
 ertiary volcanic rocks 8 age overlies the older rocks uneonformably. Four types of faults 
 
 Tuscan tuff 8 are present — (1) low-angle thrust faults, (2) steep normal faults 
 
 alus and rock mantle 8 that strike northwest and dip southwest, (3) steep normal faults that 
 
 strike northeast, and (4) faults that dip northeast. An important 
 
 k alteration 8 anc ] unusual structural feature is the benchlike contact between soda 
 
 . o rhyolite and shale. 
 
 econdary~cTe~a"v~age"__ 8 Along certain zones the rocks have been carbonatized, pyritized, 
 
 i, o silicified or sericitized. 
 
 auits7nd7he"a7zo"ne7::::::::::::::::::::::::::: h , The ,° re ***** are ***** replacement deposits that occur (a) m 
 
 ench structure in sheared and fractured soda rhyolite, (b) as a replacement ot shale 
 
 along premineral fault contacts between soda rhyolite and limy 
 
 logic history _ 10 shale, and (c) in limy shale. About 15 sulfide bodies of minable size 
 
 are known in the mine. The largest of these, the Copper Hill No. 1, 
 
 bodies 10 now mined out, is estimated to have contained more than 50,000 tons 
 
 eneral features in of ore, but all the other ore bodies are considerably smaller. The chief 
 
 haracteristies of the ore 12 structural controls of the ore bodies are: sheared and broken zones in 
 
 tructural control of the ore bodies 14 the soda rhyolite ; high-angle faults between soda rhyolite and shale ; 
 
 [ineralogy and paragenesis 15 soda rhyolite benches; a reverse fault that dips northeast; and drag 
 
 angue minerals . 17 folds in the sedimentary rocks. The ore is a mixture of sulfides and 
 
 xidation and supergene enrichment 17 gangue minerals. The sulfide minerals are pyrite, sphalerite, chal- 
 
 enesis of the sulfide bodies 17 copyrite, galena, tetrahedrite, and bornite, with minor amounts of 
 
 , ,, , , luzonite, covellite, and chalcocite. Gangue minerals are calcite, 
 
 as favorable for exploration 18 quart _ and barite Oxidation extends only a few feet below the 
 
 All the known ore bodies in the mine, except the Afterthought 
 
 Illustrations shear zone ore bodies, are in a block that is GOO feet long, 500 feet 
 
 p a g e wide, and 700 feet deep. It is in this block that the irregular, bench- 
 
 e 1. Geologic map of the Afterthought mine area, like contact between soda rhyolite and shale is found, and several 
 
 Shasta County, California In pocket good prospecting areas along this contact in the upper part of the 
 
 2. Composite plan of the Afterthought mine work- mine are indicated. There is also evidence that the benchlike contact 
 
 ings In pocket between soda rhyolite and shale plunges southeast at a low angle; 
 
 3. Geologic plan of the 400 level, Afterthought therefore the writer believes that wildcat prospecting in the mine 
 
 mine In pocket area should be directed downward toward the southeast below the 
 
 4. Geologic plan of upper adits and 100, 200, 300, 5 °0 level. 
 
 450, 600, 700, and 800 levels of the After- INTRODUCTION 
 
 thought mine _____ In pocket The Afterthought mine is in Shasta County, California, 
 
 5. Geologic sections A-A', B-B', C-C, D-D', and tt c< tt- i. onn tt> c,a -i ±\ i e r» jr 
 
 E-E' of the Afterthought mine In pocket oa U " S - m ghway 299 E, 24 miles northeast of Redding. 
 
 6. Isometric diagram of the Afterthought mine In pocket Tlie property lies in sees. 10, 11, and 15, T. 33 N., R. 2 "W., 
 
 ,_. ■, T , . . , Mount Diablo base and meridian. It is near the eastern 
 
 ire i. index map showing the locations of the After- i in tj i m • t _ • _. /n _>• t \ 
 
 thought mine and the East Shasta and West end ° f tllC East ShaSta c ?PP er - zmc district. (See fig. 1.) 
 
 Shasta copper-zinc districts 4 The maximum relief within the mine area is about 800 
 
 2. Afterthought mine plant from u. S. Highway 299E. feet, and the canyon of Little Cow Creek, a permanent 
 
 Little Cow Creek in foreground 4 stream that flows southwest and provides an ample supply 
 
 S. Prismatic and brecciated Bu y Hill rhyolite near the e + c *i ; • *i l ■ + * l • 
 
 Afterthought mine _ __ p °* wa ^ er tor the mine, is the dominant topographic 
 
 4. Banded sulfide ore overlain by puddiiihead "breccia hi ~ feature. The mine plant and main haulage level are on 
 
 Copper Hill open cut _ G the east bank of Little Cow Creek at an altitude of 1,125 
 
 5. Longitudinal projection of the Afterthought mine, feet. The hillslopes surrounding the mine rise rather 
 
 showing relative positions of known ore bodies 11 . i . i ±- i? i onn * * i +i * i 
 
 6. Sketch showing sawtooth relationship between sulfide" steeply to an elevation of 1,800 feet, where the topography 
 
 and shale at the northwest end of 412 stope _ 12 becomes much more subdued, owing to the presence of 
 
 7. Photograph showing interfingering relationship be- a Tertiarv erosion surface capped bv a permeable volcanic 
 
 tween sulfide and shale at northwest end of the 120 _ tuff breccia. ( See pi. 1 ; fig. 2. ) 
 
 8. Sketch sho"wing"top"or4"20""o7e"body~nearitrnorth~" " The Aft e r thought mine is owned and operated by the 
 
 west end where it is controlled by drag-folded beds ._ 13 Coronado Copper & Zinc Co., and the property is said to 
 ^ Assay map of the 220 ore body __ 14 include approximately 1,800 acres of patented mineral 
 
 blication authorized by the Director U S Geological Survey claims consolidated into one block (Lilldberg, 1919, p. 3). 
 
 i ffi!' ed '" cooperation with th e California state Division of Of this total, less than 60 acres is underlain by mine 
 
 ' eologist, U. S. Geological Survey. workings. 
 
 (3) 
 
Special Report 29 
 
 Scole in Miles 
 
 Figure 1. Index map showing the location of the Afterthought 
 
 mine and the East Shasta and West Shasta 
 
 eopper-zine districts. 
 
 The mine is developed by nine levels extending through 
 a vertical distance of 720 feet. The 400 level, which is 
 
 the main haulage level, extends 2,910 feet into the hill. 
 At a distance of 1,450 feet from the portal a 329-foot shaft, 
 the No. 1 shaft, connects the 400 level with the surface. 
 Two underground shafts, one 300 feet deep, and one 400 
 feet deep, connect the 400 level with deeper workings. 
 The mine workings total 18,700 feet : 16,500 feet in drifts, 
 crosscuts, and stopes and 2.200 feet in raises and shafts. 
 (See pi. 2.) 
 
 At the time of this study 9,790 feet of the mine work- 
 ings were mapped. An additional 3,200 feet of workings, 
 now inaccessible, were mapped in 1946 by F. W. Stewart, 
 consultant for the Coronado Copper & Zinc Co., and 
 these maps were available to the writer. The remaining 
 5,710 feet of workings were inaccessible, and geologic 
 data on them are either inadequate or nonexistent. 
 
 The study of the Afterthought mine is part of a larger 
 project that includes studies of the entire East Shasta 
 and West Shasta copper-zinc districts (fig. 1). The field 
 
 work in the Afterthought mine area was begun in J 
 1949 and discontinued in September of the same y 
 Critical areas in the mine were re-examined and : 
 workings were mapped during November and Decea 
 1950. E. M. MacKevett assisted with the undergro 
 mapping, and operated the plane table during the 1 
 w r ork, and J. F. Robertson assisted with various phi 
 of the work during 1950. A. R. Kinkel, Jr., and W| 
 Hall of the U. S. Geological Survey spent two days nl 
 ping in the mine during 1949. 
 
 The writer wishes to acknowledge the many courtel 
 and fine cooperation extended by officials of the Coronl 
 Copper & Zinc Co., especially by Messrs. M. G. Grl 
 R. W. Moore, Lyttleton Price, K. C. Richmond, B.I 
 Stewart, and Jack Widauf . The underground photogreJ 
 that appear in this report were taken by M. G. Grl 
 Dr. James A. Noble of the California Institute of Tl 
 nology kindly loaned the writer nine polished sect] 
 of the ore for study. 
 
 HISTORY AND PRODUCTION 
 
 I 
 
 ! 
 
 The history of the Afterthought mine dates from alat 
 1862, when the Copper Hill claim was staked (Frank iii 
 Chappell, 1881, pp. 23-25). Apparently very little \t§ 
 was done on the claim until 1873, when H. M. Peck bo 
 the property. Peck and his employees at first cara 
 sacks of ore on their backs to the top of the hill, w 
 it was loaded on wagons and hauled to Stockton 
 Stockton it was transferred to ships that carried i 
 Baltimore, Maryland, and to Swansea, Wales. 
 
 About 1875 Peck erected a reverberatory furnace 
 attempted to smelt the ore. This venture was not sue 
 ful, and in 1903 he sold the property to the Great WesM 
 Gold Co. 
 
 The erection of a 250-ton blast furnace in 1905 resU 
 in the first successful reduction of the ore at the prop 
 Between 1905 and 1927 several different companies ( 
 ated the mine for short periods, and several difft 
 methods of extracting the metals from the ore were l 
 Although the ore carries a high percentage of zinc 
 considerable lead, only copper, gold, and silver wer 
 covered from the ore prior to 1925. 
 
 Figure 2. Afterthought mine plant from U. S. Highway 293. 
 Little Cow Creek in foreground. 
 
Afterthought Mine, Shasta County 
 
 ^he Coronado Copper & Zinc Co. purchased the After- 
 light property in 1946, and after new ore bodies had 
 
 m located by exploratory drilling, the company con- 
 lcted a 100-ton selective flotation plant. Mining was 
 *ted in October 1948 and continued until July 1949, 
 ?n the operation was stopped owing to a drop in the 
 ze of metals. In July 1950 the mine was reopened, and 
 ; operated continuously until August 1952. The crude 
 ide ore is ground to 94 percent minus 200 mesh, and 
 concentrates are made by selective flotation. One, a 
 per-lead concentrate, is shipped to a smelter at Tooele, 
 .h ; the other, a zinc concentrate, is shipped to a smel- 
 |at Great Falls, Montana. 
 
 iccurate records of the production of the Afterthought 
 ie extend back to 1905. Between 1905 and January 31, 
 1, the mine produced 158,525 tons of crude sulfide ore 
 ft assayed an average of about 16 percent zinc, 2.7 per- 
 
 et copper, 2 percent lead, 5.0 ounces of silver, and 0.04 
 
 ice of gold, 
 'ollowing is a summary of highlights in the history of 
 
 k Afterthought mine : 
 862 : The Copper Hill claim was staked. 
 373 : H. M. Peck bought the property for $6,000 and 
 
 .eied it the Peck mine. 
 
 ip75 : First local attempt was made to treat the sulfide 
 
 rby direct smelting in a reverberatory furnace (Tucker, 
 
 91. p. 425). 
 )03 : Great "Western Gold Co. acquired the property 
 
 n erected a 250-ton water-jacketed blast furnace, which 
 
 p*ated successfully from 1905 to 1907 (Lindberg, 1919, 
 
 )09 : Property was acquired by the Afterthought Cop- 
 eCo. (Tucker, 1924, p. 425). 
 
 H8-1919: Afterthought Copper Co. used the Har- 
 '(d process to reduce the ore. In this method the sulfide 
 i first pre-roasted in a reverberatory furnace, and then 
 •<ted by flotation (Averill, 1939, p. 174). This process 
 I not successful (Lyttleton Price, company engineer, 
 e onal communication) . 
 
 '25: The California Zinc Co. acquired the property 
 i erected an 8^-mile aerial cable tram to transport ore 
 •(i the Afterthought mine to the Bully Hill mill for 
 ■tment (Averill, 1939, p. 174). This tram operated 
 Ql 1927, when the mine was closed down. 
 
 .46: Coronado Copper & Zinc Co. acquired the prop- 
 rt, and diamond-drill holes totaling several thousand 
 Kjwere put down. New ore bodies were found. 
 . j48: A 100-ton flotation plant was constructed and 
 imining of new ore bodies found during the 1946 dia- 
 1 Id-drilling program was begun. 
 
 GENERAL GEOLOGY 
 
 ie Afterthought mine is in Triassic volcanic and 
 nentary rocks that belong to two formations— the 
 y Hill rhyolite and the overlying Pit formation. The 
 let between the two formations, and the bedding in 
 *it formation, strike about N. 45° W. and in general 
 teeply northeast. The mine is located on the northeast 
 
 of a large anticline whose axis appears to be almost 
 ;ontal. Most of the small folds in the mine area ap- 
 
 to be drag folds on the limb of this major structure, 
 eondary cleavage strikes northwest and dips steeply 
 west ; it is not everywhere present. At many places 
 
 
 throughout the East Shasta district the cleavage parallels 
 the axial planes of minor folds. 
 
 Numerous premineral faults and shear zones are found 
 in the mine area and more than 90 percent of them strike 
 northwest. No postmineral faults have been recognized, 
 but one premineral fault may have a slight amount of 
 postmineral movement. 
 
 At least five main types of hydrothermally altered rocks 
 occur in the mine area. Most conspicuous are the zones 
 of carbonatized rock, or "lime rock," which in a few 
 places are closely associated with sulfide bodies. Areas of 
 silicified and pyritized rocks commonly occur as halos 
 around sulfide bodies in the Bully Hill rhyolite. Sericite 
 and several clay minerals are rather widespread. 
 
 The highest hills in the vicinity of the Afterthought 
 mine are capped by the Tuscan tuff, a Pliocene volcanic 
 tuff breccia (Anderson, 1933, p. 223; Anderson and Rus- 
 sell, 1939, pp. 231-235) that rests unconformably upon 
 the older rocks. The Tuscan tuff has little economic sig- 
 nificance except insofar as it masks the geology of the 
 potentially ore-bearing older rocks. 
 
 Triassic Rocks 
 Bully Hill Rhyolite 
 
 General Description. The Bully Hill rhyolite was 
 named by Diller (1906, p. 8). Later Graton (1910, p. 82) 
 concluded that these rocks were intrusive alaskite and 
 alaskite porphyry and discarded the geographic name 
 "Bully Hill" as unnecessary. The writer, however, con- 
 siders these rocks largely extrusive soda rhyolite and soda 
 rhyolite porphyry and is, therefore, here restoring the 
 name "Bully Hill rhyolite" as a formal stratigraphic 
 term. A full discussion of the origin of the Bully Hill 
 rhyolite, including a description of the evidence leading 
 to the conclusions summarized above, is beyond the scope 
 of this report, but will be given in a future report on the 
 East Shasta copper-zinc district. 
 
 The formation name Bully Hill rhyolite is applied to 
 a sequence of volcanic and intrusive rocks that are com- 
 posed principally of quartz and sodic plagioclase with 
 little or no potash feldspar. The Bully Hill "rhyolite" is 
 therefore not a normal rhyolite, but rather a soda rhyo- 
 lite, or quartz keratophyre. The terms soda rhyolite, and 
 soda rhyolite porphyry will be used in this report, except 
 when the full formation name is used. 
 
 The Bully Hill rhyolite crops out in the western part of 
 the Afterthought mine area. It is a hard light-gray apha- 
 nitic rock that in some places contains quartz and feldspar 
 phenocrysts. In some parts of the area the Bully Hill 
 rhyolite is massive and has a weak cleavage ; in other parts 
 of the area it has a strong cleavage ; and in still other parts 
 it is brecciated. These differences are due partly to original 
 variations in the rock and partly to later changes wrought 
 by tectonic forces. The main types of Bully Hill rhyolite 
 that occur within the limits of the Afterthought map area 
 are described below. 
 
 Soda Rhyolite. Approximately 70 percent of the Bully 
 Hill rhyolite shown on plate 1 is a light-gray aphanitic 
 rock without quartz or feldspar phenocrysts. Except 
 where strongly sheared it is very hard, and it commonly 
 breaks with a smooth conehoidal fracture. Company en- 
 gineers who have logged the core of diamond-drill holes 
 that penetrated this soda rhyolite have described it as 
 "hard water colored rhvolite. " 
 
Special, Report 29 
 
 
 
 I 
 
 
 
 \ 
 
 i 
 
 i < 
 
 x:-' 
 
 Figure 3. Prismatic and breccia ted Bully Hill 
 rhyolite near the Afterthought mine. 
 
 Thin sections show the soda rhyolite to be composed 
 primarily of a fine feltlike mass of quartz and sodic plagio- 
 elase, with a few larger grains of albite as much as 0.1 mm 
 in diameter. Most of the soda rhyolite is seen under the 
 microscope to be somewhat altered to sericite and clay 
 minerals, and the degree of alteration is more or less pro- 
 portional to the intensity of shearing. 
 
 Soda Rhyolite Porphyry. About 30 percent of the 
 Bully Hill rhyolite in the Afterthought mine area has 
 phenocrysts of quartz or feldspar in a light-gray apha- 
 nitic groundmass, and is mapped as soda rhyolite por- 
 phyry. The phenocrysts range in size from 1 to 4 mm and 
 are irregularly distributed. In some areas the porphyry 
 contains several phenocrysts per square inch of rock sur- 
 face, whereas in other areas it contains an average of only 
 one or two phenocrysts per square foot of rock surface. 
 
 Contacts between the soda rhyolite facies and the soda 
 rhyolite porphyry facies are sharp in some places but more 
 commonly they are gradational. Possibly those bodies of 
 soda rhyolite porphyry whose contacts are sharply defined 
 were separate flows or intrusions, whereas those with gra- 
 dational borders may have formed in the inner part of a 
 thick body of silica-rich lava where the rate of cooling was 
 slow enough to permit the growth of phenocrysts. 
 
 Prismatic Structure. Small prismatic columns occur 
 at many places in both the soda rhyolite and the soda 
 rhyolite porphyry. Most columns are 1 to 2 inches in 
 diameter and are four-, five-, or six-sided. In some areas 
 the columns are fairly straight, but in other areas they 
 are crooked or irregular. In some places the straight col- 
 umns can be traced for a distance of 10 feet, hut the 
 crooked or irregular columns commonly fade out or grade 
 into a breccia within 2 or 3 feet. Most columns within a 
 given outcrop are oriented about the same direction, but 
 in another outcrop only a few feet away the columns may 
 be oriented in a different direction. Although the pris- 
 matic structure is developed iii both the soda rhyolite and 
 the soda rhyolite porphyry, the structure is most common 
 in the uppermost part of the soda rhyolite, within about 
 200 feet of its contact with the overlying Pit sedimentary 
 
 -*s. 
 
 
 v v v • She ' 
 
 '/ , 
 
 Figure 4. Banded sulfide ore overlain by puddinhead 
 breccia in Copper Hill open cut. 
 
 rocks. The prismatic structure is probably the result 
 contraction on cooling of the crystallizing rock. 
 
 Breccias. Four main varieties of breccia are founc 
 the Bully Hill rhyolite of the Afterthought mine a: 
 One of these is considered to be volcanic in origin, 
 others are probably both intrusive and tectonic, and 
 fourth appears to be tectonic. These varieties are de 
 nated as : volcanic breccia, prismatic breccia, tectonic 
 volcanic puddinhead breccia, and tectonic breccia. 
 
 The volcanic breccia is a coarse soda rhyolite breia 
 that consists of angular fragments of soda rhyolite! 
 much as a foot in diameter, surrounded by soda rhyct 
 matrix. It crops out along Little Cow Creek in the sou- 
 western part of the map area. The fragments are air 
 monly closely packed and constitute the bulk of the M 
 They are diversely oriented and are of two types of p 
 flow-banded soda rhyolite and massive soda rhyolite. 
 banding in the fragments is evidently the result of a o 
 variation, as no difference in grain size between the ac- 
 cent bands, or layers, is apparent in hand specimens 
 
 The boundaries of this breccia are in some pi* 
 sharply defined, but in other places the breccia grades I 
 large areas of unbrecciated soda rhyolite, either ft 
 banded or massive. Although examination of outcropil 
 this breccia yielded no certain clue as to its origin,* 
 writer interprets it as volcanic flow breccia. 
 
 The prismatic breccia is seen at many places in the r 
 area as patches of soda rhyolite with prismatic struct 
 These patches form islands in a sea of brecciated si 
 rhyolite (fig. 3). The prismatic structure fades into 1* 
 cia, and many of the fragments in the breccia haveU 
 same size, cross-sectional outline, and composition as* 
 adjacent coin inns. It seems obvious that this bre;U 
 formed when the small, brittle columns were brokerip 
 by mechanical processes into a jumbled mass of fragmt* 
 The breaking may have occurred while the soda rhyitt 
 was being intruded, or later as a result of tectonic st:| 
 The writer believes that most Bully Hill rhyolite 'ft 
 prismatic structure is intrusive because in some partw 
 the mine it appears to cut across beds of the Pit formal"- 
 
 
Afterthought Mine, Shasta County 
 
 uddinhead breccia is a local term applied by miners to 
 
 that consists of angular to sub-rounded soda rhyolite 
 
 ments in a matrix of dark-gray to black, soft, com- 
 
 ly slickensided argillaceous material (fig. 4). Many of 
 
 mod a rhyolite fragments have the same size and cross- 
 
 onal outline as prismatic columns in nearby areas, 
 
 they seem to be fragments of columns. 
 
 thin section of the shaly matrix of the puddinhead 
 
 cia shows that its dark color results from the presence 
 
 considerable amount of carbonaceous material. Other 
 
 tituents of the matrix are quartz and clay minerals. 
 
 writer concludes that the matrix is sheared mudstone. 
 
 he puddinhead breccia is distributed irregularly and 
 
 rs only in areas very near, but not alwavs adjacent 
 
 hale of the Pit formation. Its contacts with the shale 
 
 commonly sharp, and appear to be faults of small 
 
 Placement. Observed contacts between puddinhead 
 
 ■cia and massive soda rhyolite also are faults, but 
 
 ■acts with prismatic breccia are commonly gra- 
 
 iional. 
 
 'ie larsrest observed body of puddinhead breccia is on 
 u400 level a short distance northeast of the No. 1 
 lit. Here it has an outcrop width of 40 feet and a strike 
 »th of at least 140 feet. Other areas of puddinhead 
 Kcia occur on the surface in the glory hole and on the 
 D level near the No. 1 shaft. 
 
 'ie tectonic breccia occurs north of the mine buildings 
 ijl) ; it consists of sparse fragments of hard massive 
 •t rhyolite in a matrix of softer sheared soda rhyolite. 
 fc fragments are subrounded and slichtly elongate 
 n average less than an inch in diameter. They are some- 
 Ht lighter in color than the sheared matrix and there- 
 )i stand out in rather sharp contrast to it ; except for 
 idack of platy minerals in the fragments, there is no 
 if rence in composition between fragments and matrix. 
 
 ■ platy structure of the matrix fades into the frag- 
 I ts ; it does not wrap around them. Observed contacts 
 ereen this type of breccia and the unbrecciated rocks 
 r<jnd it are gradational. Brecciated zones cross contacts 
 efeen soda rhyolite and soda rhyolite porphyry. 
 
 (irjin of the Bully Hill Rhyolite. The orisrin of the 
 uy Hill rhvolite, like the origin of the Balaklala rhyo- 
 tfin the West Shasta copper-zinc district, has long 
 I a subject of controversy. Diller (1906, p. 8) described 
 if Bully Hill rhyolite as a series of flows alternating 
 il tuffs that dip beneath shale of the Pit formation; 
 i 1 he also states that it locally cuts the shale and en- 
 ibes its fragments. Fairbanks (1892, p. 32) likewise 
 ai noted the tuffaceous character of the rhyolite, and 
 vimtly he also considered it to be mainly extrusive. 
 
 ( aton (1910, p. 82), after larger-scale studies, con- 
 
 u;d that the "Bully Hill rhyolite" was intrusive 
 
 it the surrounding rocks, and he renamed it "alaskite" 
 
 "alaskite porphyry." Most geologists who have 
 
 ed in the area since Graton have followed his views 
 B -le, 1915, p. 69 ; Hinds, 1933, p. 107 ; Stewart, 1946 ; 
 Vii;er, 1946). 
 
 r e writer believes that most of the Bully Hill rhyolite 
 fi nated as an accumulation of siliceous volcanic rocks 
 ^t ided into the sea immediately prior to the deposition 
 f ie Pit formation. Moreover, the numerous beds of 
 
 ■ litic tuff interbedded with shale of the Pit formation 
 
 1 vidence that volcanism continued durin" Pit time. 
 
 Associated with the various extrusive faeics of the 
 Bully Hill rhyolite are several intrusive bodies, one of 
 which is llie prismatic soda rhyolite that intrudes the 
 lower part of the Pit formation in the Afterthought mine 
 area. The presence of these intrusive bodies does not in- 
 validate the extrusive origin thesis proposed above, be- 
 cause feeder dikes, sills, and other intrusive forms are to 
 be found in almost all volcanic areas. The local intrusions 
 of soda rhyolite that occur in the lower part of the Pit 
 formation are here regarded as shallow intrusive mani- 
 festations of the volcanic activity that continued while 
 Pit sediments were burying most of the old volcanic field 
 (pi. 5, sees. A A' and BB'). 
 
 Age and Thickness. The Bully Hill rhyolite has 
 yielded no fossils, but from its relationships with ad iacent 
 rocks it is 'considered to be middle or late Triassie in age. 
 
 The thickness of the Bully Hill rhyolite in the After- 
 thought mine area is probably about 1,100 feet. This 
 thickness is deduced from exposures about half a mile 
 southeast of the Afterthought mine where the Bully Hill 
 rhyolite lies with apparent cOnformitv between the older 
 Dekkas andesite and the younger Pit formation. Both 
 contacts dip northeast at a steep angle, and the thickness 
 of the intervening soda rhyolite is approximately 1,100 
 feet. 
 
 Pit Formation 
 
 General Description. The shales and tuffs that crop 
 out in the northeast portion of the Afterthought map 
 area belong to the Pit formation. The formation was first 
 described by Smith (1894, p. 592), who called it the Pitt 
 shales. Later, Diller (1906, p. 4) renamed it the Pit 
 formation, and described it as being composed largely of 
 dark-gray shale, thin-bedded sandstone, and many layers 
 of tuff, and conformably underlying the Hosselkus lime- 
 stone on the east and overlying the andesites and rhyo- 
 lites of the volcanic belt on the west. 
 
 The writer is in agreement with Diller 's statement, 
 and would add that the tuff layers are especially abundant 
 in the lower 500 feet of the Pit formation. It is this 
 tuffaceous lower part that crops out in the Afterthought 
 mine area; tuff underlies a considerably larger part of 
 the mapped area than shale, mudstone, and siltstone 
 (hereafter grouped as shale for convenience) (pi. 1). 
 
 Shale. The shale of the Pit formation is medium gray 
 to black and in the Afterthought mine area it is indis- 
 tinctly bedded. It effervesces slightly in dilute hydro- 
 chloric acid. It commonly has a prominent secondary 
 cleavage that in some places parallels bedding but more 
 commonly does not. Under the microscope the shale is 
 seen to consist predominantly of very fine-grained quartz 
 and kaolin, with dark-gray to black carbonaceous mate- 
 rial, and a few angular grains of detrital quartz 0.1 mm 
 and less in diameter. Calcite and clay minerals are present 
 as alteration products. The shale was especially hospitable 
 to mineralizing solutions, as shown by the fact that most 
 of the ore bodies in the Afterthought mine are replace- 
 ments of shale. 
 
 Tuff. More than half of the Pit formation underlying 
 the Afterthought mine area is tuff. The tuff occurs as beds 
 interlayered with shale, and commonly the individual beds 
 of tuff differ slightly from one another in color, grain 
 size, and composition. The most common variety of tuff is 
 light gray and poorly bedded, and consists mainly of fine 
 
 
8 
 
 Special Report 29 
 
 chlorite, crystals of quartz and feldspar, and small rock 
 fragments, generally less than half an inch in diameter. 
 Rome of the tTiff beds contain shale fragments up to a 
 foot long*. The tuff in a few of the beds is well layered, 
 sandy or gritty looking-, and is composed almost entirely 
 of broken crystals and small particles of rock. 
 
 Most of the tuff in the Pit formation is of rhyolitic com- 
 position ; but a relatively small percentage is dark gray or 
 brown, does not contain quartz crystals, and is probably 
 close to andesite in composition. The best exposures of tuff 
 occur near the northern edge of the napped area, along the 
 road to the Donkey mine. 
 
 In thin section the rhyolite tuffs are seen to consist of 
 broken fragments and whole crystals of quartz and plagio- 
 clase in a cryptocrystalline groundmass of fine chlorite 
 and calcite. In some thin sections the ghostlike outlines of 
 probable glass fragments can be seen, but the rock is so 
 altered that precise identification of these fragments is 
 not possible. 
 
 Age and Thickness. The Pit formation was indicated 
 to be of Middle and Late Triassic age by J. P. Smith 
 (1914. p. 4). The age assignment was based on fossils 
 collected from the upper part of the formation. 
 
 A thickness of about 2,000 feet was assigned to the 
 Pit formation by Diller (1906, p. 4). A thickness of about 
 500 feet is exposed in the Afterthought mine area. 
 
 Tuscan Tuff 
 
 Tertiary Volcanic Rocks 
 
 The top of the ridge that lies at the eastern edge of the 
 Afterthought mine area is covered by the Tuscan tuff, a 
 tuff breccia of Pliocene age, capped in some places by 
 flows of basalt. The Tuscan tuff is an extensive formation 
 that crops out along the eastern side of the Sacramento 
 River valley. It has been well described by Anderson 
 (1933) and Anderson and Russell (1939), who studied it 
 on a regional scale. 
 
 Talus and Rock Mantle 
 
 The bedrock geology in the Afterthought mine area is 
 masked by a thin cover of talus and soil on many hillslopes. 
 The presence of this material makes correlation of geology 
 between outcrops difficult, especially in the eastern part of 
 the map area, where the underlying rock consists of folded 
 lenticular beds of shale and tuff. 
 
 ROCK ALTERATION 
 
 Four alteration processes have affected the rocks in the 
 Afterthought mine area. The processes are carbonatiza- 
 tion sericitization, pyritization, and silicification. Pyritiza- 
 tion, silicification, and carbonatization appear to be 
 genetically related to the sulfide mineralization, whereas 
 sericitization apparently occurred at a somewhat earlier 
 date. 
 
 Carbonatization. Several tuffaceous layers of the Pit 
 formation and small masses in the Bully Hill rhyolite 
 liave been carbonatized, and the resulting calcareous rock 
 is known locally as lime rock. On the surface the lime 
 rock derived from tuff forms bold, dirty-gray outcrops, 
 and it contains rather coarse calcite crystals that have 
 incompletely replaced the various constituents of the 
 tuffaceous host rock. The amount of calcite in the rock 
 is extremely variable, and every gradation from slightly 
 
 carbonatized tuff to rock composed of more than 90 p 
 cent calcite can be found. The lime rock occurs in elong. 
 zones that generally parallel the bedding; some of 
 zones are as much as 100 feet wide and several huridi 
 feet long. 
 
 Smaller zones of lime rock are found between sulf 
 ore bodies and soda rhyolite wall rock ; in such places i 
 calcite seems to have replaced soda rhyolite, and althoui 
 these zones are generally much more limited in ext< 
 than the zones of carbonatized tuff, they are proba 
 more important as indicators of ore. 
 
 Thin sections of the lime rock show that the cal( 
 occurs as subhedral grains 1 to 5 mm in diameter a 
 as irregular patches as much as several centimeters 
 diameter. Calcite replaces all the principal constitue 
 of the rock — quartz, kaolin, and feldspar — apparent 
 without regard for mineral boundaries. It has been I 
 placed in turn by later quartz and pyrite, and vein'j- 
 of clay minerals cut across grains of calcite. 
 
 Sericitization. Almost all rocks in the mine area h a: 
 been slightly sericitized, and where the rocks are stronfi 
 sheared this type of alteration is prominent. The seric 
 alteration is not restricted to the mineralized areas 
 
 Pyritization. and Silicification. The Bully Hill rh 
 lite has been pyritized and silicified along some sh 
 zones and in a few areas where the rock is broken 
 brecciated. The pyritized areas are somewhat more 
 tensive than the silicified areas, but the two are clos 
 related. The pyrite occurs as separate subhedral crys 
 that are commonly arranged in layers or bands para 
 to the cleavage in the rhyolite. In silicified rock the qu< 
 occurs as radial growths that fan out from a center 
 granular quartz, or from a pyrite crystal, like sp( 
 from the hub of a wheel. 
 
 Copper-zinc ore bodies that occur in the soda rhyo" 
 principally those bodies below the 500 level, are i 
 rounded by a halo of heavily pyritized rock a few 
 to a few tens of feet thick. The weakly pyritized 
 silicified soda rhyolite on the 200 and 300 levels is i 
 shear zone located down dip from the 120, 122, and 
 ore bodies, and this zone may be a channel along which 
 sulfide-bearing solutions traveled. 
 
 STRUCTURE 
 Secondary Cleavage 
 
 Secondary cleavage, as distinguished from par 
 along bedding planes, is present in both the Bully il 
 rhyolite and the Pit formation, but it is not equally 
 veloped in all places. Most commonly it consist: 
 approximately parallel cleavage planes spaced a « 
 millimeters or a few tenths of a millimeter apart, •# 
 platy minerals developed along the cleavage planed 
 some zones the cleavage planes are so closely spaced id 
 platy minerals, principally sericite, are so abundant al 
 the rock has been called a schist. Contrasted with t» 
 strongly sheared zones are large areas where the rfl 
 have only an incipient cleavage, or no cleavage at-U 
 Every gradation between the two extremes can be fcflfc 
 in the Afterthought mine area. 
 
 The cleavage strikes northwest and dips an average 
 about 70° SW. It is parallel to the axial planes of mV 
 folds in a few places in the mine area and in many pi* 
 throughout the East Shasta district. In general, clea# 
 
 ill 
 in 
 
 . 
 
Afterthought Mine, Shasta County 
 
 isnost stronglv developed in the incompetent tnffs and 
 dies. Very siliceous rocks, especially the siliceous soda 
 rl olite with prismatic structure, commonly have no 
 fcvage at all. 
 
 Folds 
 
 "he shales and tuffs of the Pit formation were locally 
 Uormed during the intrusion of soda rhyolite and were 
 ar folded and squeezed during a period of orogeny. 
 m folds that resulted are grouped into three general 
 jes: overturned and recumbent folds, irregular asym- 
 n rical folds, and small asymmetrical drac folds. The 
 ismmetrical drag folds can be seen at several places in 
 h mine area, but folds of the other two types are in- 
 Sited only by the attitude of the bedding in various 
 >]?es. 
 
 •verturned and recumbent folds in the sedimentary 
 •alts near the contact between the Bully Hill rhyolite 
 m the Pit formation have wave lengths that ranp-e from 
 i ?w tens to a few hundreds of feet. These folds ap- 
 •Jllently formed in two stages. In the first stage, very 
 i ous soda rhyolite pushed its way into incompetent 
 e ments of the Pit formation. The intrusion was partly 
 ■02ordant and partly discordant : where concordant, the 
 e ments were wrapped around the intruding rhyolite ; 
 »lre discordant, the sediments were cut or torn by the 
 •faolite. The second stage in the formation of these 
 o s occurred in probable late Jurassic time, when the 
 cks were tilted and folded into their present form. The 
 n sive soda rhyolite acted more or less as a buttress, 
 .1 the result that sediments near the soda rhyolite con- 
 •I were squeezed and contorted into disharmonic struc- 
 
 D 'S. 
 
 . T ave lengths of the irregular asymmetrical folds aver- 
 Iless than a hundred feet. These folds are in harmony 
 vii the structural pattern of the reerion. and they ap- 
 Isntly formed during the period of deformation in 
 liable late Jurassic time. Overturned and recumbent 
 
 s and irregular asymmetrical folds are illustrated in 
 ■s section AA' on plate 5. 
 
 symmetrical drag folds exposed in the mine are de- 
 eped on the northeast flank of a large anticline. Wave 
 
 1 ths of the folds ranere from 3 to 12 feet. Axes of these 
 os plunge either northwest or southeast at angles of as 
 aih as 25°. The best folds can be seen in the No. 10 adit 
 I' the top of the B raise, on the 100 level a few feet west 
 I le collar of diamond-drill hole AU 21, and in the No. 
 : fit near the portal (pi. 4). 
 
 Faults and Shear Zones 
 
 our main classes of faults are found in the mine. They 
 
 1 from oldest to youncest: (1) low-angle thrust faults, 
 
 2 high-angle normal faults and shear zones that dip 
 odiwest, (3) normal faults that strike northeast, and 
 I faults that dip northeast. All the faults are pre- 
 ii'ral in age, and more than 90 percent of them strike 
 
 hwest. 
 
 w-Annle Thrust Faults. Most of the low-angle 
 Hst faults clip southeast or southwest at angles ranging 
 
 i 10° to 25°. The best exposures of these faults are 
 a <e soda rhyolite on the 200 and 300 levels, where the 
 h :s are clearly younger than the cleavage, but are offset 
 
 1 igh-angle faults. Calcite and quartz are found along 
 ' of the low-angle faults, and along nearly horizontal 
 
 fractures that occur at a few places in the mine. The 
 low-angle thrust faults and fractures are the result of 
 thrust movements during the later stages of orogeny. The 
 amount of displacement along most of these faults is 
 probably not more than a few feet. 
 
 IHgh-Anglc Normal Faults and Shear Zones that Dip 
 South west. Faults of this class strike northwest and have 
 dips that range from vertical to 45° S~W. In places these 
 faults are closely spaced, and the rock between the faults 
 is intensely sheared. These shear zones occur in soda 
 rhyolite, in sedimentary rock, and locally along the con- 
 tact between soda rhyolite and sedimentary rock. They 
 can be seen on every level in the mine but are most 
 prominent in the soda rhyolite on the old 100 level north- 
 east of the No. 1 shaft, and on the 500, 600, 700, and 800 
 levels. In all these places the shear zones are strongly 
 mineralized. On the 200, 300, and 400 levels the shear 
 zones are weakly mineralized. The most important of 
 these faults and shear zones are the 420, 220, and 122 
 faults, and the 450, Copper Hill, and Afterthought shear 
 zones. Displacement is difficult to measure because of the 
 lack of markers in the rocks. However, if the contact be- 
 tween the Bully Hill rhyolite and the Pit formation is 
 used as a marker horizon, the vertical displacement on 
 the 420 fault is about 250 feet, on the 220 fault 85 feet, 
 and on the 122 fault 150 feet (pi. 5. sec. BB'). The dis- 
 placements of this contact observed in a few places sug- 
 gest that most of the other faults in this class have a 
 normal movement measurable in feet or tens of feet. 
 
 Normal Faults that Strike Northeast. Less than 10 
 percent of the faults in the mine belong to this class. Most 
 of them dip southeast at moderate angles. They offset 
 faults of the first two classes but are themselves offset by 
 reverse faults that dip northeast. The Northeast fault, as 
 mapped on the surface, seems to be a normal fault with 
 a net displacement of more than 75 feet. Underground 
 measurements of the direction and amount of movement 
 alon<r the faults that strike northeast are scarce, but the 
 faults are regarded as normal faults of relatively small 
 displacement. 
 
 Faults that Din Northeast. The youngest faults strike 
 northwest and dip northeast. Faults of this class are few, 
 but two of thf>m — the Main fault and the 412 fault — are 
 of considerable economic importance. 
 
 The Main fault strikes N. 45° W., dips 45° to 60° NE., 
 and is the most continuous fault in the mine. In some 
 places, as on the 200 level, it is a shear zone as much as 
 12 feet thick ; in other places, as on the surface above the 
 No. 4 adit, it is a shaly breccia zone less than a foot thick, 
 bounded by slickensided surfaces. Mullions on the slicken- 
 sided fault surfaces plunge directly down the dip. The 
 Main fault is interpreted as a premineral reverse fault 
 with slight postmineral movement. 
 
 The age of the Main fault is deduced to be premineral 
 from the fact that calcite-quartz vein lets can be seen in 
 the fault zone where it is now exposed on the 200 level, 
 on the 300 level, and in the No. 7 adit; and also from the 
 fact that sulfide in the hanging wall of the Main fault 
 in the No. 7 adit is not broken or slickensided and appears 
 to have formed after the fault. The gangue minerals are 
 closely related to sulfide in other parts of the mine, so 
 the presence of calcite-quartz veinlets within the fault 
 zone suggests that the fault may have formed prior to 
 the deposition of sulfides. Indirect evidence leading to 
 
in 
 
 Special Report 29 
 
 the same conclusion lies in the fact that the Main fault 
 was explored at six different levels by hundreds of feet 
 of workings, now for the most part inaccessible ; although 
 no records exist for the inaccessible workings, it seems 
 valid to assume that at least small lenses of sulfide were 
 encountered along the fault zone in a few places. 
 
 A minor amount of postmineral movement on the Main 
 fault is postulated, because in a few areas the calcite- 
 quartz veinlets within the fault zone are somewhat 
 crumpled. 
 
 Evidence regarding the direction of movement along 
 the Main fault is scarce, but the fault appears to be a 
 reverse faujt with a dip slip of at least 450 feet. This 
 conclusion is based on the premise that the wedge-shaped 
 segment of Bully Hill rhyolite in the hanging wall of the 
 fault is an upthrown segment of the soda rhyolite nose 
 that forms the footwall of the fault below the 400 level. 
 Above the 400 level a block of sedimentary rock of the 
 Pit formation forms the footwall of the fault. This block 
 gradually widens toward the surface, because the average 
 dip of the contact between soda rhyolite and sedimentary 
 rock is less than the dip of the Main fault. Under such 
 circumstances, if the Main fault were a normal fault, there 
 would be no source for the soda rhyolite of the hanging 
 wall. 
 
 It is of course possible that above the present erosion 
 surface the contact between Bully Hill rhyolite and the 
 overlving Pit formation dipped at a higher angle than 
 the Main fault. Under these conditions the wedge of soda 
 rhyolite could have been dropped down from above with 
 normal movement on the Main fault. These relationships 
 are illustrated in plate 5, sections AA' through DD'. 
 
 The 412 fault is in shale in the general area of the 412 
 stope (pi. 5., sec, CO. It appears to be a weak fault, and 
 is marked by a zone of calcite-quartz stringers that ran<res 
 from a fraction of an inch to several inches in width. The 
 412 fault can be traced from the 300 level down to the 
 500 level, but above the 300 level it apparently dies out. 
 It is probably a normal fault of small displacement. 
 
 Bench Structure 
 
 In general, the contact between the Bully Hill rhyolite 
 and the Pit formation in the vicinity of the mine strikes 
 northwest and dips steeply northeast. Within the mineral- 
 ized area, however, the contact is alternately steep and 
 nearly horizontal, and assumes benchlike forms. Five dis- 
 tinct benches have been recognized. They range in width 
 from 20 feet to more than 200 feet; the length is some- 
 what greater than the width. The benches occur through 
 a vertical range of 500 feet. Most of them slope gently 
 toward the southeast. 
 
 Tn some places the bedding in the sedimentary rocks 
 adjacent to soda rhyolite follows the shape of the benches 
 and wraps around them like a carpet on a stairway, 
 whereas in other places beds of sedimentary rock dip 
 directly into a soda rhyolite bench and are truncated by 
 it. This relationship is seen beneath the 120 ore bodies 
 where the rocks are fairly well exposed in mine workings 
 (pi. 6). Here the beds of sedimentary rocks dip toward 
 a soda rhvolite bench for a distance of about 75 feet 
 across strike. At least one layer of shale projects down- 
 ward several feet into the soda rhyolite. Thus, the contact 
 does not have the shape of a low-angle fault surface. Tn 
 view of these contact features and the absence of low-angle 
 
 faults in this area, the writer concludes that the beds 
 sedimentary rock were truncated as a result of the int 
 sion of the soda rhyolite. Where the sedimentary ro< 
 were truncated or pushed aside during the intrusion 
 the soda rhyolite, contacts were irregular and angul 
 Later, during orogeny, these irregularities were modif 
 by tilting and faulting, and molded into their pres' 
 benchlike form. The bench structure is illustrated 
 plate 5, sections AA' through EE'. 
 
 A notable example of the bench structure is the bli 
 wedge of soda rhyolite that protrudes 150 feet into 
 sedimentary rocks between the 200 and the 400 lev 
 Both the top and the bottom of this wedge are nea 
 horizontal, and the blunt eastern end of the wedge is \ 
 tical. The beds of sedimentary rock appear to wrap aroi 
 this blunt soda rhyolite wedge, and thus are in the fci 
 of a crude recumbent fold. 
 
 GEOLOGIC HISTORY 
 
 The geologic history of the area around the Afv. 
 thought mine is summarized as follows : 
 
 (1) During middle or late Triassic time the Bully I 
 rhyolite, a soda rhyolite, was extruded into the sea. ' 
 basin of deposition was slowly depressed and the s< 
 rhvolite was covered by several thousand feet of shale f 
 tuff. 
 
 (2) Dikes and sills of soda rhyolite were intruded i 
 the Bully Hill rhyolite and into the lower part of the 
 formation, either while Pit and younger sediment 
 rocks were still being deposited or shortly afterwi 
 These intrusive bodies were emplaced at shallow de 
 and were later manifestations of the same igneous actn 
 that formed the Bully Hill rhyolite and the rhyolitic t 
 in the Pit formation. 
 
 (3) After middle Jurassic and prior to late Cretace 
 time the rocks were folded, faulted, and slightly m 
 morphosed. 
 
 (4) The rocks were hydrothermally altered along si 
 zones, and shortly afterward the sulfide ore bodies v 
 formed. 
 
 (5) The area was eroded during Cretaceous and e;y 
 Tertiary time, and a surface of rather low relief i 
 formed. On this surface the Tuscan tuff was deposd 
 during late Tertiary time. Finally, after still anolir 
 period of uplift, the present valleys were cut during Q 
 ernary time. 
 
 ORE BODIES 
 
 
 General Features 
 
 The ore deposits at the Afterthought mine are suJe 
 replacement deposits in shale and in sheared, altered si* 
 rhyolite. The ore is commonly banded, and is typical I 
 fine-grained mixture of pyrite, sphalerite, chalcopy^ 
 galena, tetrahedrite, and gangue minerals. Sulfide bo« 
 that replace shale are in general richer in sphalerite in 
 those that replace soda rhyolite. The ore bodies M 
 slightly elongate lenses that plunge southeast at an a*- 
 age angle of less than 10°. Some ore bodies are surroui!* 
 by a halo of heavily pyritized rock, and some are pal? 
 surrounded by a halo of carbonatized rock. The car fl- 
 at ized rock is most commonly found between sulfide bees 
 and soda rhyolite wall rock. It was formed partly I 
 replacement of soda rhyolite and partly as a replacerii! 
 of tuffaccous sediments. 
 
 
Afterthought Mine, Shasta County 
 
 11 
 
 EXPLANATION 
 
 1700 
 
 -1600 
 
 1500' 
 
 1400' 
 
 -1300' 
 
 1200 
 
 -1100 
 
 IOOO' 
 
 900 
 
 ^800' 
 
 700 
 
 200 
 
 Geology by J. R Albers, 1951 
 400 Feet 
 
 Talus and rock mantle 
 
 Ore body cut by 
 plane of section 
 
 Ore body in front of 
 plane of section 
 
 Ore body behind 
 plane of section 
 
 Surface profile on 
 plane of section 
 
 Contact between alluvium and 
 bedrock on plane of section 
 
 Mine workings on 
 plane of section 
 
 Mine workings in front of 
 plane of section 
 
 Mine workings behind 
 plane of section 
 
 Figure 5. Longitudinal projection of the Afterthought mine showing relative positions of the known ore bodies. 
 
12 
 
 Special Report 29 
 
 The metasomatic origin of the sulfide deposits is indi- 
 cated by banding in the ore that parallels bedding where 
 shale is the host rock and parallels cleavage where soda 
 rhyolite is the host rock ; by preservation in the banded 
 ore of drag folds and monoclinal flexures inherited from 
 the host rock ; by the presence of residuals of unreplaeed 
 shale within ore bodies — the bedding shows that these 
 shale residuals are oriented parallel to each other and 
 parallel to bedding in shale adjacent to the ore bodies ; 
 by interfingering, or sawtooth relationship of sulfide and 
 shale at the top, bottom, and ends of ore bodies; by grada- 
 tional contacts between some ore bodies and country rocks, 
 chiefly soda rhyolite ; and by the presence of disseminated 
 sulfide minerals in altered soda rhyolite and in lime rock. 
 
 Outlines of most of the 15 known ore bodies in the 
 Afterthought mine are shown in plan on plates 1, 3, 4, 
 and 6 and in section on plates 5 and 6 and in figure 5. 
 For convenience, all the ore bodies have been given a 
 name or a number. The Afterthought, Copper Hill No. 2, 
 600, 700a, and 700b, and 800 ore bodies occur in soda 
 rhyolite; whereas the Copper Hill No. 1, 120, 122, AS 6, 
 220, 412, 420, 450, AU 5, and AU 40 ore bodies occur in 
 shale or along the contact between soda rhyolite and shale. 
 If ore bodies existed in the Main fault zone between the 
 200 and the 400 levels they were probably in shale. The 
 minable ore bodies range in size from a few hundred tons 
 to more than 50,000 tons. In addition to the sulfide bodies 
 of minable size and grade there are numerous smaller 
 lenses Of sulfide and bodies of low-grade disseminated 
 sulfide. 
 
 Characteristics of the Ore 
 
 Sulfides are present in the Afterthought mine as mas- 
 sive ore that is commonly banded owing to thin layers 
 of differing composition ; as stringers that replace sheared 
 soda rhyolite or shale ; as disseminated grains in soda 
 rhyolite or shale ; and, rarely, as small vcinlets that fill 
 fractures. 
 
 The massive ore and banded ore consist of fine-grained, 
 intimate mixtures of pyrite, sphalerite, chalcopyrite, 
 galena, tetrahedrite, bornite, calcite. quartz, and barite, 
 with minor amounts of luzonite, covellite, and chalcocite. 
 The bands or layers in the banded sulfide ore average a 
 small fraction of a millimeter in thickness, but a few 
 bands are as much as 2 mm thick. Calcite and quartz 
 occur, in some places, as tiny veinlets oriented about 
 normal to the bandine in the sulfide : but more commonly 
 these minerals are intimately mixed with the sulfide, or 
 occur as veins along the walls of sulfide bodies, especially 
 along contacts between sulfide and soda rhyolite. Barite 
 is not visible in hand specimens of the ore. 
 
 Contacts between massive banded sidfide and shale 
 host rock are everywhere sharp, and no halo of dissemi- 
 nated or low-grade sulfide is present. The same condition 
 holds, in a few places, for contacts between massive sulfide 
 and soda rhyolite host rock. More commonly, however, 
 the massive sulfide that replaces soda rhyolite in the lower 
 levels of the mine is surrounded by a halo or casing of 
 pyrite. According to Stewart (1946, p. 15), replacement 
 of the host rock is nearly complete at the cores of these 
 ore bodies, but the degree of replacement decreases in all 
 directions away from the core, and massive sulfide grades 
 outward into zones of numerous parallel stringers of bar- 
 ren pyrite. Where zones of lime rock lie adjacent to mas- 
 
 FEET 
 
 Figure 6. Sketch showing sawtooth relationship betweei 
 sulfide and shale at the northwest end of the 412 stope. 
 
 sive sulfide, as near the southwest sides of the 120, .1 
 and 420 ore bodies, the contacts between massive sulle* 
 and lime rock are gradational and irregular. 
 
 In many places the contact between massive sulfide id 
 shale at the edges of sulfide bodies has, in cross sectnj 
 a sawtooth form. In such places, the shale layers incr 
 in width at the expense of the sulfide until the sulty 
 layer.s pinch out in sharp ends (fig. 6). Commonly 1 1 
 marginal sulfide layers are banded, and the banding tfli 
 to parallel the edges of an individual sulfide layer all lit 
 way to its apical contact with the shale. 
 
 Tn other places at the edges of the sulfide bodies lit 
 contact between sulfide and shale host rock may! 
 described as interfingering; that is, the shale widen! 
 the expense of the sulfide, but the ends of the sill 
 layers are blunt and rounded rather than sharp (fig )• 
 The banding in sulfide parallels the length of the finl 
 but is truncated by the contact at the blunt ends ofi 
 fingers. The banding parallels either bedding or cleajj* 
 in the shale host rock, and evidently was inherited 
 
 
Afterthought Mine, Shasta County 
 
 13 
 
 'tj host rock. Locally the edges of sulfide hodies replaced 
 filed shale beds and are partly controlled by them. In 
 sijh places the banding in the sulfide is concordant with 
 i (hape of the folded heds, and small drag folds may be 
 t ;eable in the sulfide for several inches inward from 
 tlj shale contact (fig. 8). Examples of this kind of band- 
 hi were seen near both ends of the 420 ore body, and 
 it he southeast end of the 450 ore body. 
 
 ILarge horses of unreplaced shale in the sulfide bodies 
 ■ not common, but thin septa of unreplaced shale are 
 [lisent at many places along the walls of sulfide bodies. 
 Cjnmonly these septa are only a fraction of an inch thick, 
 wreas (he intervening layers of banded sulfide have a 
 tfrkness several times as great. Because of their con- 
 
 i lity and the absence of disseminated sulfide minerals, 
 tike marginal shale septa are easily mistaken for the true 
 ■1 of an ore body. 
 
 'ontacts between sulfide and soda rhyolite host rock 
 ■many places have the same types of irregularities as 
 Ititacts between sulfide and shale described above, but 
 mv are more commonly gradational than sharp. Also, 
 tbjbanding in sulfide that replaces soda rhyolite parallels 
 livage rather than bedding; and in a few places where 
 th host soda rhyolite is weakly sheared the massive sul- 
 I is not banded. 
 
 Vhere closely spaced stringers or disseminations of 
 aides have replaced sheared soda rhyolite or shale the 
 
 
 Sulfide 
 
 Shale 
 
 2 FEET 
 
 4 FEET 
 
 RE 7. Photograph showing interfingering relationship between 
 sulfide and shale at the northwest end of the 120 stope. 
 
 FIGURE 8. Sketch showing the top of the 420 ore body near its 
 northwest end where it is controlled by drag-folded beds. 
 
 deposits are commonly not rich enough to be considered 
 as ore. This type of sulfide deposit is most commonly 
 found in sheared soda rhyolite, and intersections of these 
 mineralized shear zones with shale contacts are favorable 
 places to look for massive sulfide ore. Some zones of closely 
 spaced sulfide stringers in shale or in lime rock have an 
 average grade high enough to make them minable, and in 
 some places these zones pass into bodies of massive ore 
 within a short distance along strike. 
 
 The average assay of all ore mined up to January 31, 
 1951, was approximately 16 percent zinc, 2.7 percent cop- 
 per, 2 percent lead, 5.0 ounces of silver, and 0.04 ounce of 
 gold. This average represents the over-all grade of ore 
 mined from about 13 separate ore bodies. The grade of the 
 120, 122, 220, and 420 ore bodies approximated this aver- 
 age, but the grade of the 450 ore body was much higher, 
 whereas the grade of ore bodies in shear zones in soda 
 rhyolite was lower. Furthermore, the grade of ore within 
 each sulfide body is not uniform. As a general rule the 
 richest ore is near the center of the sulfide lenses and the 
 poorest is near the edges, but the type of rock replaced is 
 also a controlling factor. For example, the zinc content 
 of ore that has replaced shale is almost always higher than 
 the zinc content of ore that has replaced soda rhyolite, 
 but the copper content is about the same in both. Figure 
 9, an assay map of the 220 ore body, illustrates the changes 
 in grade that are common within ore bodies of the After- 
 thought mine. The sulfide has replaced shale except near 
 the southwest corner of the ore body, where partly car- 
 bonatized soda rhyolite was the host rock. The assays of 
 zinc are lower in samples 6, 7, 8, and 9 than in samples 
 3 and 4 taken near the middle of the stope, but the assays 
 of copper are not significantly different. Sulfide repre- 
 sented by samples 6, 7, 8, and 9 replaced soda rhyolite 
 and lime rock, whereas sulfide represented by all other 
 samples replaced shale and lime rock. 
 
14 
 
 Special Report 29 
 
 Structural Control of the Ore Bodies 
 
 Four structural features have played important parts 
 in the localization of ore bodies at the Afterthought mine. 
 They are shear zones in the Bully Hill rhyolite that dip 
 southwest ; the irregular benchlike contact between the 
 Bully Hill rhyolite and the Pit formation ; the Main fault 
 and the 412 fault; and drag folds in the sedimentary 
 rocks and bows, or convexities, in steep fault contacts 
 between soda rhyolite and shale. 
 
 The ore bodies below the 400 level are in sheared and 
 fractured zones in the soda rhyolite. These shear zones 
 strike northwest and dip southwest; the overlying shale 
 contact and the Main fault strike northwest but dip north- 
 east, and thus intersect and offset the shear zones. The 
 ore bodies are below these intersections. The lower part 
 of the 450 ore body is a replacement of soda rhyolite, 
 whereas the upper part appears to be a replacement of 
 shale. The southeast end of the 450 ore body is controlled 
 by a fold in the overlying shale that plunges about 40° 
 SE., and the- top of the 450 ore body at its northwest end 
 is bounded by the Main fault. The unusually high grade 
 of the 450 sulfide body may be a result of concentration 
 of the mineralizing solutions in that area, caused by the 
 two overlying structures. 
 
 The Afterthought open-cut sulfide lens is in a soda 
 rhyolite shear zone similar to the shear zones found below 
 the 400 level. Both the shear zone and the sulfide lens end 
 beneath the Northeast fault, and it is thought that the 
 fault was a rather impervious wall which deterred the 
 rising thermal solutions sufficiently to cause them to react 
 with and replace the underlying sheared and fractured 
 soda rhyolite. The No. 2 adit and the 400 southeast drift 
 explored lower parts of the Afterthought shear zone, but 
 the records do not show whether or not sulfide was en- 
 countered. A lenticular body of shale bounded by faults 
 was found in the No. 2 adit, and it is possible that a body 
 of sulfide was localized in this shale below the level of the 
 No. 2 adit. 
 
 The 420, AU 40, and 122 ore bodies are rather narrow 
 lenticular replacement bodies localized along irregular, 
 steep fault contacts between soda rhyolite and shale. The 
 sulfide formed mainly as a replacement of sheared and 
 broken shale, but the soda rhyolite and the discontinuous 
 screen of lime rock between the shale and the soda rhyolite 
 was replaced to some extent by sulfide stringers and dis- 
 seminations. The dip of the fault contact between shale 
 and soda rhyolite makes a roll at several places, from 
 northeast to southwest and back again ; the sulfide bodies 
 occur in places where this contact is convex to the south- 
 west and soda rhyolite overhangs shale. In such areas the 
 shale was apparently more fractured and therefore more 
 easily accessible to mineralizing solutions. In places where 
 the contact is convex to the northeast, a layer of mixed 
 calcite and quartz, or, rarely, a thin band of sulfide, marks 
 out the mineralized zone. The top of the 420 ore body is 
 controlled in part by drag-folded beds in the sedimentary 
 rocks (fig. 8). The 122 sulfide body is localized along a 
 steep, southwest-dipping fault contact between soda rhyo- 
 lite and shale, but several feet below it is a soda rhyolite 
 bench. The shale beds arc much broken and sheared, and 
 dip into the soda rhyolite bench. The top of the 122 ore 
 body may be partly controlled by drag-folded beds at the 
 northwest end. The AU 40 sulfide lens is localized along 
 
 Sample 
 number 
 
 Horizontal 
 width 
 
 Data furnished by the 
 
 Coronado Copper and Zinc Company 
 
 Figure 9. Assay map of 
 
Afterthought Mine, Shasta County 
 
 15 
 
 r<l in the structure similar to that which localized the 
 20>re body. The 122, 420, and AU 40 sulfide lenses all 
 like southeast at a verv low angle. 
 
 fie 120, 220, AS 6, and AU 5 ore bodies are localized 
 i tdimentary rocks above soda rhyolite benches. These 
 
 e>odies are not in contact with the soda rhyolite, but 
 •commonly separated from it by several feet of rela- 
 Hv barren sedimentary rock that dips into the benches. 
 
 Ksoda rhyolite on the 200 and 300 levels, down dip 
 
 the 120, AS 6, and 220 ore bodies, is locally sheared, 
 mcthe shear zones contain disseminations and stringers 
 ap-ite and chalcopyrite. Thermal solutions probably 
 
 aided along these shear zones and into the easily acces- 
 
 fcfbeds of shale that dip into the soda rhyolite benches. 
 \lt sulfide bodies were formed when the solutions 
 
 a led the shale is not evident. The chemical composition 
 f ie calcareous shale may have been an important fac- 
 >ri)ut a fold in the sedimentary rock about 75 feet above 
 
 ,e 20 and AS 6 ore bodies mav also have been influ- 
 
 tl. 
 
 lie AU 5 ore body is in sedimentary rock and lies in 
 
 eide synclinal structure that plunges southeast at an 
 # of about 35°. Diamond-drill hole AU 44, located 60 
 ^e 'northwest of the AU 5 ore body, penetrated soda 
 
 '3 lite at a relatively shallow depth, and from this infor- 
 lipn the writer infers that a trently dipping contact 
 
 it; soda rhvolite may project into the area beneath the 
 iT) ore body. 
 
 Be 412 sulfide lenses are in shale along the hanging 
 
 a of a premineral fracture. The reason for the locali- 
 aln of the individual lenses of sulfide is not known. 
 
 Te lar<rest bodv of sulfide in the Afterthought mine 
 
 adie Cooper Hill No. 1 ore body, now mined out. This 
 re odv appears to have had a length of about 400 feet, a 
 :irlh of at least 125 feet, and a maximum thickness of 35 
 >e The Copper Hill No. 1 ore body, now exposed only 
 i lie glory hole and for a short distance on the 100 level, 
 ratlin shale in the hanging-wall side of the Main fault 
 
 1 a soda rhyolite contact. It has a steep southwest 
 
 hat is essentially parallel to the dip of the contact 
 
 also parallel to bedding in the shale. The ore, where 
 
 sed in the glory hole, is banded parallel to bedding in 
 
 ■ hale host rock (fig. 4). The bottom of the ore body 
 
 < i parently a short distance above the 200 level where 
 
 ere zone intersects the Main fault. According to the 
 
 r's interpretation, sulfide solutions rose along the 
 
 fault, and in the vicinity of the soda rhyolite wedge 
 
 e fed upward into bedded shale and sheared soda 
 
 i>lite. This is a structurally favorable area because the 
 
 dip steeply southwest into the Main fault, and thus 
 
 easily accessible to the mineralizing solutions. 
 
 e Copper Hill No. 2 ore body was also in the hanging 
 
 ; i of the Main fault and replaced a portion of the soda 
 
 hvlite wedge. Only the remnant of the top of this ore 
 
 < is now exposed in the glory hole; at this exposure 
 
 I pears to be a very pyritic sulfide body about 20 feet 
 
 I that comes to a rather blunt top beneath two inter- 
 
 jng premineral fractures. A polished section of the 
 
 I Ie colled ed from the glory-hole exposure is composed 
 
 ( ore than 85 percent pyrite, less than 5 percent ehal- 
 
 'Orcrite, and about 10 percent gangue. A cross section 
 
 i'n by V. W. Stewart in 1946 suggests that the area 
 
 >pI;V the glory-hole exposure was sloped; if so, it must 
 
 >e isumed that a higher percentage of chalcopyrite was 
 
 present in the sloped area. Hence, the pyrite seen in the 
 glory-hole exposure is here considered as part of a pyritic 
 halo that surrounded a copper ore body. The highly pyri- 
 tized soda rhyolite in the hanging wall of the Main fault 
 on the old 100 level may be part of the same pyritic halo. 
 The small lens of sulfide exposed in the No. 7 adit is in 
 the hanging wall of the Main fault and has a structural 
 environment similar to that of the Copper Hill No. 2 ore 
 body. 
 
 Mineralogy and Paragenesis 
 
 The primary sulfide minerals composing the ore bodies 
 of the Afterthought mine are, in approximate order of 
 decreasing abundance, sphalerite, pyrite, chalcopyrite, 
 galena, tetrahedrite, bornite, and luzonite. Gangue min- 
 erals are calcite, quartz, and barite ; supergene minerals 
 are covellite, chalcocite, chalcopyrite ( ?), azurite, mala- 
 chite, and limonite. 
 
 The pyrite is in the form of cubes, pyritohedrons, and 
 anhedral grains that range in size from 0.05 mm to 5.0 
 mm. Where the mineral is abundant, as it commonly is 
 in and near ore bodies that replace soda rhyolite, the 
 individual grains may be so closely packed that in polished 
 section they appear as large patches of solid pyrite. On 
 the other hand, pyrite in high-grade banded zinc ore 
 appears in polished sections as sparse, small grains in large 
 areas of sphalerite, or as very small grains that are grouped 
 together, with much interstitial material, to form pyrite- 
 rich layers. Pyrite also occurs as disseminated grains 
 scattered through large areas of soda rhyolite, and as 
 granular layers a fraction of an inch thick that parallel 
 the bedding in shale. The pyrite is commonly strongly 
 corroded by sphalerite, but is not much replaced by 
 chalcopyrite. 
 
 The sphalerite is dark gray and probably rather high 
 in iron. In polished section it appears as large patches that 
 include both idiomorphic and irregular areas of all other 
 sulfide minerals. Sphalerite has commonly replaced pyrite ; 
 and in some polished sections it has a mottled appearance, 
 owing to the presence of numerous small, rounded, crudely 
 alined blebs of chalcopyrite. Such blebs of chalcopyrite 
 are interpreted as exsolution bodies formed by the unmix- 
 ing of a solid solution of sphalerite and chalcopyrite. 
 Much of the banded sulfide ore in polished section shows 
 sphalerite to contain very numerous, rather small, irreg- 
 ular, elongated patches of galena, or tetrahedrite, or both 
 minerals together. These patches are all elongated in the 
 same direction, and are largely responsible for the finely 
 banded appearance of the ore. In a few places sphalerite 
 appears in polished section as islands in a sea of galena, 
 but this relationship is not common because sphalerite is 
 generally much more abundant than galena. 
 
 Chalcopyrite appears in polished section as interstitial 
 fillings around pyrite grains; as irregular patches in 
 sphalerite; as large areas with islands of both pyrite and 
 sphalerite ; as small exsolution bodies in sphalerite ; as 
 small patches in bornite ; and, rarely, as discontinuous 
 septa between galena and sphalerite. Chalcopyrite also 
 occurs as small disseminated grains in gangue and in soda 
 rhyolite. In one polished section of ore from the 220 slope, 
 chalcopyrite appears as blades in bornite and is closely 
 associated with covellite. The writer is not certain whether 
 this is supergene chalcopyrite or whether it indicates an 
 exsolution intergrowth of chalcopyrite and bornite that 
 
16 Special Report 29 
 
 Table 1. Paragenesis of epigenetic minerals at the Afterthought mine. 
 
 (Dash lines indicate that few or no data on age relationship are 
 
 available . ) 
 
 HYPOGENE 
 
 SUPERGENE 
 
 Rock-alteration 
 stage 
 
 Sulfide -replacement and 
 vein-forming state 
 
 Kaolin 
 
 Cryptocrystalline quartz 
 
 Calcite 
 
 Montmorillonite and nontronite 
 
 Radial quartz and veins of milky quartz 
 
 Pyrite 
 
 Barite 
 
 1 
 
 
 Sphalerite 
 
 Luzonite 
 
 Chalcopyrite 
 
 Bornite 
 
 Tetrahedrite 
 
 Galena 
 
 Calcite -quartz veins 
 
 
 
 Covellite 
 
 Chalcocite 
 
 Secondary 
 chalc op yr.te 
 
 Azurite 
 
 Malachite 
 
 Limonite 
 
Afterthought Mine, Shasta County 
 
 17 
 
 a r were partly altered to covellite. In general, chalcopy- 
 i is somewhat more abundant in ore bodies that have 
 
 need soda rhyolite than in ore bodies that have re- 
 
 1 -d shale. 
 
 ornite appears in polished section as small veinlets 
 i as small, irregular patches in sphalerite. It commonly 
 hvs mutual boundary relationships with chalcopyrite, 
 I one polished section shows numerous islands of ehal- 
 o/rite in a sea of bornite. Bornite seems to be a minor 
 Istituent of sulfide bodies at all levels in the mine, and 
 ii he upper levels it is partly altered to supergene covel- 
 itand chalcocite. 
 
 etrahedrite is closely associated with galena. In pol- 
 <}rl section, it appears typically admixed with galena 
 s rregular patches interstitial to sphalerite, but in a 
 c places small areas of tetrahedrite occur without 
 ana. Less commonly tetrahedrite is interstitial to pyrite. 
 Vahedrite shows mutual boundary relationships with 
 ana; but where the two minerals occur as admixed 
 r s in sphalerite the tetrahedrite most commonly oc- 
 u es sheltered embayments or coves along the irregular 
 oidary of the area. 
 
 alena is interstitial to pyrite, sphalerite, and chal- 
 o- T rite, but shows mutual boundary relationships with 
 oiite and tetrahedrite. It also appears in polished sec- 
 I as irregular, alined patches in sphalerite and chal- 
 
 r rite. These patches of galena commonly coalesce, 
 ing the sulfide a banded appearance. 
 
 uzonite, a pink variety of enargite, was identified in 
 dished section of ore from the 122 stope. It is a rare 
 o;tituent of the ore and appears as anhedral grains in 
 h copyrite. 
 
 ovellite occurs in the upper ore bodies of the mine 
 s n alteration of bornite. As seen in polished section, 
 ; liaracteristically penetrates bornite along fractures 
 n grain boundaries. Covellite is not an abundant min- 
 
 1 and has not been seen as an alteration product of 
 h copyrite. 
 
 halcocite is rare, but it is intimately associated with 
 rllite in a few specimens. It seems to be most abundant 
 r the southeast end of the 120 ore body. Covellite and 
 hcocite, alon<r with azurite, malachite, and limonite, 
 rsupergene minerals. 
 
 Gangue Minerals 
 angue minerals, mainly calcite, quartz, and barite, 
 pintimately mixed with sulfide minerals. In polished 
 !< on the gangue minerals commonlv appear as large 
 Is enclosing euhedral grains of pyrite or small irregu- 
 ii iatch.es of the other sulfide minerals; less commonly 
 -i« appear as irregular patches in a sea of sphalerite 
 r lalcopyrite. The writer believes that the barite, calcite, 
 n part of the quartz were introduced, whereas the rest 
 E le quartz is probably residual. 
 
 tartz and Calcite Veins. In the Afterthought- mine 
 
 * te and quartz commonly occur as numerous small 
 
 _t s. Quartz and calcite occur together and appear to 
 
 e itergrown in most of the veins. Admixed calcite and 
 
 I tz. in veins as much as a foot wide, are common along 
 
 e contacts between soda rhyolite and shale. Less com- 
 
 K are veins of pure calcite in shale, or pure quartz in 
 
 J< rhyolite. 
 
 ie veins are small fissure-type veins that ranjre from 
 ■action of an inch to more than a foot in thickness. 
 
 Most of the veins strike northwest and dip steeply either 
 northeast or southwest, but a few nearly horizontal vein- 
 lets are present in sulfide and in highly fractured zones. 
 
 The veins were formed during and after the deposition 
 of the sulfides, and probably the fractures along which 
 the veins formed also served as channels for sulfide-bear- 
 ing solutions. The 420 ore body, for example, fades into 
 a network of quartz-calcite veinlets in shale at its north- 
 west end. Similarly, the 412 sulfide bodies are lenses of 
 sulfide that replace shale along the hanging wall of a 
 fracture filled with intergrown quartz and calcite. The 
 calcite-quartz vein itself continues for scores of feet be- 
 yond the 412 sulfide lenses but gives no other indication 
 of its close association with sulfide. Similar relationships 
 between unimportant-looking quartz-calcite veins and 
 small sulfide lenses are seen at other places in the mine. 
 
 Barite. Barite is closely associated with sulfide min- 
 erals in several of the Afterthought ore bodies. Although 
 its distribution is spotty, it seems to be generally more 
 abundant in ore bodies above the 200 level. According to 
 Brown (1916, pp. 760-761) the average ore, evidently 
 from the Copper Hill No. 1 ore body, contained 7.4 per- 
 cent barite. This is in agreement with a statement by 
 Diller (1904, p. 176) that the ore contains less than 5 per- 
 cent gangue, most of which is barite. 
 
 In a brief examination of polished sections of ore from 
 the upper level the writer has noted barite, along with 
 quartz, calcite, and unreplaced rock. Barite may also occur 
 in minor amounts at lower levels, but information is 
 inadequate. 
 
 The parakinesis of minerals associated with the ore 
 deposits is given in table 1. 
 
 Oxidation and Supergene Enrichment 
 
 The oxidized zone is very shallow, and supergene en- 
 richment of the sulfide ore bodies has been insignificant. 
 Sulfide ore in the Afterthought shear zone is capped by 
 only 2 feet of gossan, and the Copper Hill No. I sulfide 
 body, where now exposed in the open cut, could not have 
 had more than 3 or 4 feet of gossan capping. Minor 
 amounts of azurite and malachite, in addition to limonite, 
 are found in the surface zone of oxidation. According 
 to Brown (1916, p. 761) the oxidized ores of the Copper 
 Hill lode were very rich in gold, and were mined for their 
 gold content alone during the early days. Apparently 
 the workings from which the gold was mined consisted 
 only of surface pits and trenches. 
 
 Supergene sulfide minerals include covellite, chalcocite, 
 and secondary (?) chalcopyrite. These minerals occur as 
 deep as the 420 stope, but in such minor amounts that 
 they do not perceptibly increase the grade of the ore. The 
 supergene sulfide minerals were most abundant in the 
 west end of the 120 stope and near the east end of the 
 122 stope. 
 
 Genesis of the Sulfide Bodies 
 The sulfide bodies at the Afterthought mine were 
 formed after the period of orogeny in late Jurassic time 
 and before the deposition of late Cretaceous sedimentary 
 rocks. The mineral assemblage is characteristic of the 
 mesothermal class of mineral deposits, and from this as- 
 semblage the writer concludes that the ore bodies were 
 formed at moderate depth, probably 1 to 3 miles. Pre- 
 sumably, hydrothermal solutions emanated from a parent 
 magma somewhere below; but the nature of the magma, 
 
18 
 
 Special Report 29 
 
 
 as well as its depth, is conjectural. Whatever the original 
 source of the hydrothermal solutions, they probably made 
 their way upward through the earth's crust along sheared 
 and fractured zones in the Bully Hill rhyolite, and in 
 the sedimentary rocks of the Pit formation. Here and 
 there the physical condition of the host rocks was such 
 that the mineralizing solutions had relatively easy access 
 to the host rocks. "Where the temperature and composition 
 of the visiting solutions and host rock were favorable, a 
 chemical reaction occurred, and the host rock was slowly 
 replaced by sulfide minerals. The size of the resulting 
 sulfide bodies was dependent in part upon the volume of 
 host rock that was accessible to the solutions and also 
 upon the length of time that the solutions were active in 
 the area. 
 
 AREAS FAVORABLE FOR EXPLORATION 
 
 Known ore bodies, of which possible extensions may 
 be found, are : 
 
 1. Northwest extension of the Copper Hill No. 1 ore 
 body between the surface and the 200 level. According 
 to Lindberg (1919, p. 11) the northwest face of the old 
 100 level is in part massive sulfide ; and high-grade sulfide 
 appears on the dump of the No. 4 adit. 
 
 2. Downward extension of the Copper Hill No. 1 ore 
 body. There is no record of what was mined from this 
 ore body between the 100 and 200 levels. The ore body 
 has a maximum width of 35 feet on the 100 level, and 
 the favorable zone should extend at least half way to the 
 200 level before intersecting the Main fault. 
 
 3. Southeast extension of the 220 ore body. Drill-hole 
 data indicate that the soda rhyolite bench extends for 
 at least 100 feet farther to the southeast. The sedimentary 
 rocks probably dip southwest into the soda rhyolite bench, 
 and the area of this extension thus has the same geologic 
 environment as that above the 120 and 220 ore bodies. 
 
 4. Extension of the 412 mineralized zone southeast be- 
 low the 400 level, and northwest above the 300 level. 
 Bodies of sulfide along this zone, which is marked out by 
 veinlets filled with quartz and calcite, are apt to be small, 
 but there may be several of them. The 412 fault zone 
 projects downward toward the mineralized zone en- 
 countered in diamond-drill hole AU 44, and upward 
 toward the 220 ore body. 
 
 5. Southeast extension of the 450 ore body between the 
 500 and the 600 levels. The top of this ore body seems to 
 be controlled in part by an anticlinal fold in the shale 
 and plunges southeast at an angle of about 40°. From 
 the distribution of caved 500-level workings, and from 
 the presence of sedimentary rocks below the 500 level in 
 diamond-drill holes AU 4 and AU 5, the writer infers that 
 the mineralized zone continues to plunge southeast below 
 the 500 level. 
 
 6. Possible downward extension of the 800-level min- 
 eralized area. This is a strongly pyritized zone, which 
 includes a lens 15 feet wide and at least 40 feet loner that 
 assavs more than 4 percent copper (based on observations 
 by P. W. Stewart). Although the writer has not examined 
 these workings, it seems possible that the material exposed 
 on the 800 level may be part of the pyritic halo that 
 characteristically surrounds sulfide bodies in the soda 
 rhyolite shear zones. If so, the core of the sulfide body 
 is probably below the level. 
 
 70764 11-52 2M 
 
 *' 
 
 , 
 
 7. Almost all the ore bodies enclose layers of unreplaj 
 rock, and in some places these "false faces" mightf 
 mistaken for the true Avail of an ore body. Therefore 
 walls of all stopes, especially the shale wall, should 
 explored at regular intervals. 
 
 Geologically favorable blocks, other than extension if 
 known ore bodies, include the following: 
 
 1 . The block southeast of and below the 500-level w<i 
 ings is geologically favorable because a nearly horizo il 
 body of soda rhyolite underlies sedimentary rocks lfl 
 are known to be mineralized in diamond-drill holes A I 
 and AU 44. Furthermore, shear zones that probably seiJ 
 as feeder channels for the 420 and 450 ore bodies exti 
 downward into the soda rhyolite in this vicinity. 
 
 2. The block of sedimentary rocks southwest of the 
 3 shaft between the 400 and 500 levels is geologic If 
 favorable because the sedimentary beds dip south ■ 
 toward a mineralized fault contact with soda rhyoe. 
 This fault contact is the same one along which the 1 
 ore body is localized farther southeast. 
 
 3. Mineralized shear zones are probably present in i| 
 soda rhyolite below the 800 level. Bodies of high-gi 
 sulfide similar to the 700b ore body may reasonably b< 
 pected where such shear zones intersect the shale con' 
 
 REFERENCES 
 
 Anderson, C. A., (1933), Tuscan formation of northern Oalifoi 
 
 with a discussion concerning the origin of volcanic breccias: I 
 
 fornia Univ. Dept. Geol. Sci. Bull., vol. 23, no. 7, pp. 21511 
 Anderson, C. A., and Russell, R. D., (1939), Tertiary formatioB 
 
 northern Sacramento Vallev, California : California Jour. Bjj 
 
 and Geology, vol. 35, no. 3, pp. 219-253. 
 Averill, C. V., (1939), Mineral resources of Shasta County 
 
 fornia Jour. Mines and Geology, vol. 35, no. 2, pp. 108-191. 
 Boyle, A. G, (1915), The geology and ore deposits of the Bully] 
 
 mining district, California: Am. Inst. Min. Eng. Trans. 4: 
 
 07-117. 
 Brown, G. G, (1910), The counties of Shasta, Siskiyou, Tri ; 
 
 Shasta County: California Min. Bur. Rept. 14, pp. 74'° 
 Diller, J. S., (1904), Mining and mineral resources in the Rei 
 
 quadrangle. California, in 1903: U. S. Geol. Survey Bull.J 
 
 pp. 109-177. 
 Diller, J. S., (1900), U. S. Geol. Survey Geol. Atlas, Redding .li' 
 
 (no. 138), 14 pp. 
 Fairbanks, II. AV., (1892), Geology and mineralogy of 
 
 County : California Min. Bur. Rept. 11, pp. 25-53. 
 Frank, F. B., and Chappell, II. AV., (1881), History and bu: 
 
 directory of Shasta County, pp. 23-25. 
 Graton, L. G, (1910), The occurrence of copper in Shasta Cc 
 
 California: U. S. Geol. Survey Bull. 430-B, pp. 71-111. 
 Hinds. X. E. A., (1933) , Geologic formations of the Redding-W- 
 
 ville districts, northern California : California Jour. Mine 
 
 Geology, vol. 29, nos. 1 and 2, pp. 77-122. 
 Lindberg, C. O., (1919). Unpublished report on Afterthought 
 
 on file at Coronado Copper and Zinc Co., 1206 Pacific 51 
 
 Bldg., Los Angeles, California, 8 pp. 
 Smith. J. P., (1894), The metamorphic series of Shasta Cc 
 
 California : Jour. Geology, vol. 2, pp. 588-012. 
 Smith.V P., (1914), The Middle Triassic marine invertebrate f 
 
 of North America : U. S. Geol. Survey Prof. Paper 83, 2; 
 Stewart, F. W., (1940), Unpublished report on the Aftertli 
 
 mine, on file at Coronado Copper and Zinc Co., 1200 Pacific 5 
 
 Bldg., Los Angeles, California, 27 pp. 
 Tucker, AV. B.. (1924), Copper resources of Shasta County,! 
 
 fornia : California Min. Bur. Rept. 20, pp. 419-447. 
 AVisser, Edward, (1940), Unpublished report on the Aftertrj 
 
 mine, on file at Coronado Copper and Zinc Co., 1200 Pacific ~Sf 
 
 Bldg., Los Angeles, California, 15 pp. 
 
 printed in CALIFORNIA STATE PRINTING 
 
 'ill 
 
EXPLANATION 
 
 Tuscon luff 
 
 V 
 
 
 Pit formofion 
 Tips, shoit.Ttoi. luff 
 
 Bully Hill rhyolite 
 
 ■BO', soda rhfoli'f^brp, soda 'hyollU pcrpby 
 STRUCTURAL VARIETIES OF BULLY HILL RHYOL 
 
 
 1^ 1 "..'rvl 
 
 Rock wffipm^Nc^^ Pntmat.c fried 
 
 
 lv*\«| :<;'-X ; 'l 
 
 Br 
 
 HUlcrtlrtocclcMrct VMMc <,,k„o 
 
 
 \^4\ I-;' '•.'*! 
 
 
 Tronic D'fceio r«fome una .oleo 
 MINERALIZED ROCKS 
 
 
 1 '* •• 
 
 
 Gossan 
 
 
 1 1 
 
 
 Disseminated sulfides, mainly pyrile 
 
 
 Massive sulfide 
 
 
 Ml 
 
 
 Corbonotized rock (lime rock] 
 
 Contact, showing dip 
 (Dashed where oppro*imote(y located) 
 
 
 - , '. rrr^ 
 
 
 "Tj 
 
 v^f 
 
 
 
 °°~ — r ' ' ' 
 
 -^-f^ 
 
 
 . "\ 
 
 
 
 ^ , ■> 4 
 
 I 
 
 * - *» 
 
 ; 
 
 \ tip. 
 
 
 
 • ' • 
 
 * ' * " * 
 
 - *_ 
 
 ■7~~ ft 
 
 ,' 
 
 
 ' • 
 
 , 1 > 5 
 
 ' , 
 
 
 '* 
 
 T^K 
 
 4 4 • 
 
 * 
 
 -^ 
 
 
 
 '-'-ry 
 
 - 
 
 
 
 
 
 
 
 ft 
 
 
 
 
 
 
 O 
 O 
 
 
 
 Concealed loult 
 Probable fault 
 
 Strike ond dip of beds 
 
 Strike of vertical beds 
 
 Strike and dip of cleovoge 
 
 Strike of vertical cleovoge 
 
 Strike and dip of joints 
 
 Plunge of prismatic columns 
 
 Vertical shaft 
 
 Portal of adit 
 
 Trench or open cut 
 
 # 
 
 Geology by J PAIbers, 1949 
 
 GEOLOGIC MAP OF THE AFTERTHOUGHT MINE AREA, SHASTA COUNTY, CALIFORNIA 
 
EXPLANATION 
 
 m 
 
 Vertical 2 compartment she 
 Portal of odit 
 
 (Chevrons point down) 
 
 O 
 
 Elevation of workings 
 Caved i 
 
 compiled from Coronado Copper ond Zinc Company maps 
 
 COMPOSITE PLAN OF THE AFTERTHOUGHT MINE WORKINGS 
 
EXPLANATION 
 
 I formotion, undif 
 
 Bully Hill rhyolite, undifferentioted 
 
 STRUCTURAL VARIETIES OF BULLr HILL RHYOLITE 
 
 Disseminated sulfides, mainly pynte 
 
 Carbonotized rock (lime rock) 
 
 "•» 
 
 Strike ond dip of beds 
 
 Strike of verticol beds 
 
 Strike ond dip of cleavage 
 
 Plunge of prismatic column 
 
 Shaft at Surface 
 
 El 
 Bottom of snaft 
 
 (Chei 
 
 Foot of roise 
 Caved workings 
 
 Approximately horizontal diomond-drill hole 
 
 m 
 
 inclined diamond-drill hole projected to horizontal 
 
 I by the Coronodo Copper and Zinc Company 
 
 Geology by J. P Albers, 1951 
 
 GEOLOGIC PLAN OF THE 400 LEVEL, AFTERTHOUGHT MINE 
 
EXPLANATION 
 
 Pit formotion, unditferentioted 
 
 Bully Hill rhyolite, undifferentiated 
 
 STRUCTURAL VARIETIES OF BULLY HILL RHYOLITE 
 
 Geology ol 700 lfc*el Irom a, 
 F. W. Stewarts mop j" 
 
 Much coJo-W m cave' \ ^&„ fifl^Vfea*" 
 
 N5000 // TO * 
 No 3 snail v/ 
 
 
 sulfides, mainly pynte 
 
 -|- Massive suifid 
 
 Corbonatized rock (lime rock) 
 
 Fault, showing dip 
 (Doshed where opproiimotely locote 
 U,upthcown side-, 0, downlhrown sidt 
 
 Vertical fault 
 Shear zone, showing dip 
 Plunge of minor anticline 
 Plunge of minor syncline 
 Strike and dipof beds 
 Strike of verticol beds 
 Strike and dip of cieovoge 
 Plunge of prismatic columns 
 
 Shaft at surface 
 
 ■ 
 Shoft going above ond below levels 
 
 SI 
 Bottom of shaft 
 
 (Chevrons point down) 
 
 B 
 Foot of raise or winze 
 
 12 
 Head of roiseor winze 
 
 Chute 
 St ope 
 
 Oepression in floor 
 Logging or cribbing along drift 
 
 Coved workings 
 Horizontal diomond-drill hole 
 
 100 LEVELS 
 
 s those used by the Coronodo Copper ond Zinc Compony 
 
 800 LEVEL 
 
 0mv. T3S 
 
 Geology by J. P. Albers, 1951 
 
 GEOLOGIC PLAN OF UPPER ADITS, AND 100, 200, 300, 450, 600, 700, AND 800 LEVELS OF THE AFTERTHOUGHT MINE 
 
GEOLOGIC SECTIONS A"A\ B-B'. C-C', D-D', AND E-E' OF THE AFTERTHOUGHT MINE 
 
ISOMETRIC DIAGRAM OF THE AFTERTHOUGHT MINE