Nature, age, and genesis of quartz-sulfide-precious-metal vein systems in the Virginia City Mining District, Madison County, Montana by Marshall Morris Cole A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Earth Sciences Montana State University © Copyright by Marshall Morris Cole (1983) Abstract: In the Virginia City mining district, pre-Belt gneisses and the Late Cretaceous granitic Browns Gulch stock host numerous quartz vein systems. Hypogene mineralization is chiefly gold- and silver-bearing base-metal sulfides. The U.S. Grant 3-level vein system, N40°-50°E; 35°-50°,NW, 0.3 to 5.0 m wide, is contained in a shear zone exhibiting about 10 m of syn-ore right-lateral movement. The vein system is composed of elongate quartz lenses, quartz stringers, tabular quartz bodies, and variable amounts of crushed and altered gneiss. Altered wall rock gneiss exhibits early potassic alteration (microcline and possibly quartz) and a subsequent propylitic assemblage (carbonate, pyrite, quartz, chlorite, other phyllosilicates, and a zeolite?). Post-alteration mineralization occurs as pyrite, followed by variable amounts of contemporaneous galena, sphalerite, chalcopyrite, sparse tetrahedrite, and rare specular (?) hematite. Additionally, some (latest) sphalerite replaces pyrite, galena, and chalcopyrite. Observable gold is very rare. Quartz deposition is pre-, syn-, and post-sulfide mineralization. The Virginia City district is one of several districts in the Tobacco Root precious-metal mining region. The region is cored by the 77-72 m.y. old quartz monzonite Tobacco Root batholith. A regional zoning is present with respect to the batholith in the form of low silver-to-gold and high copper-to-silver ratios near the batholith, and high silver-to-gold and low copper-to-silver ratios far from the batholith (Virginia City district). It is proposed that ores of the district are of Latest Cretaceous to Early Tertiary age (70-60 m.y.B.P.), based on the occurrence of deposits in the Late Cretaceous Browns Gulch stock, the crosscutting of the El Fleeda 4-level vein system by a 51 m.y. old andesite plug, and the regional zoning with respect to the batholith. An epithermal precious-metal genesis model has been applied to the ores of the district. A geothermal convection cell powered by heat from Late Cretaceous plutonism, produced large-scale regional circulation of hydrothermal fluids at shallow crustal levels. These fluids collected (remobilized ?), transported, and deposited the ore constituents. NATURE, AGE, AND GENESIS OF QUARTZ-SULFIDE-PRECIOUS-METAL
VEIN SYSTEMS IN THE VIRGINIA CITY MINING DISTRICT,
MADISON COUNTY, MONTANA
by
Marshall Morris Cole
A thesis submitted in, partial fulfillm ent of the requirements fo r the degree
of
Master of Science
in ' ;
Earth Sciences
MONTANA STATE UNIVERSITY Bozeman, Montana
December, 1983 main lib .
C k 7 5 5 i i Cop. Si
APPROVAL
of a thesis submitted by
Marshall Morris Cole
This thesis has been read by each member of the thesis committee and has been found to be satisfactory regarding content, English usage, format, citations, bibliographic style, and consistency, and is ready fo r submission to the College of Graduate Studies.
/ Z- Y ) - £ { r 'C Date Chairperson, Graduate Committee
Approved fo r the Major Department
m 3 Approved for the College of Graduate Studies 'll&K-- IWI Date Graduate Dean I l i STATEMENT OF PERMISSION TO USE In presenting this thesis in partial fulfillment of the requirements for a master's degree at Montana State University, I agree that the Library shall make it available to borrowers under rules of the Library. Brief quotations from this thesis are allowable without special per mission, provided that accurate acknowledgment of source is made. Permission for extensive quotation from or reproduction of this thesis may be granted by my major professor, or in his/her absence, by the Director of Libraries when, in the opinion of either, the pro posed use of the m aterial is fo r scholarly purposes. Any copying or. use of the material in this thesis for financial gain shall not be allowed without my written permission. Signature Date « 2 / , / f ^ - 3 V ACKNOWLEDGMENTS This thesis was. funded in part by: The Montana Bureau of Mines and Geology; R and D Minerals, Missoula, Montana; Dr. David R. Lageson, professor at Montana State University; the Research-Creativity Program, Montana State U niversity; The St. Lawrence Mining Company, V irginia C ity , Montana. Great appreciation is expressed towards these people fo r th e ir generous contributions. The following individuals have donated th e ir s k ills , knowledge, and support in the compilation of th is thesis: Drs. Robert A. Chadwick, David W. Mogk, and David R. Lageson, professors at Montana State University; Clyde Boyer, geologist, Virginia City, Montana; Thomas. A. Callmeyer, field assistant and graduate student, Montana State University, Tefese ^ //^ X/z /ov^ ct™/ dlhc( ho uu i-teej-e. Cq)&. 4 rTAa I f If , v f/ Jz I cSeIL f vi TABLE OF. CONTENTS Page 1. LIST OF TABLES...... v i i i 2. LIST OF FIGURES ...... ix 3. ABSTRACT...... xi 4. INTRODUCTION...... • I Location, Access, and Physiography ...... I Purpose and Method of Study. , ...... 3 Previous Study ...... 3 5. . MINING HISTORY...... 4 6. REGIONAL GEOLOGIC SETTING ...... 5 7. GENERAL GEOLOGY OF THE VIRGINIA CITY MINING DISTRICT. . . 8 Rock T y p e s ...... 8 Structure. . ' ...... Tl 8. MINING GEOLOGY...... 13 Regional Setting: The Tobacco Root Precious-Metal Mining R eg io n ...... 13 Mining Geology of the V irg in ia C ity D is tric t ...... 16 9. MINING GEOLOGY OF SELECTED MINES OF THE VIRGINIA CITY DISTRICT ...... ’ ...... 20 U.S. Grant Mine ...... 20 . El Fleeda M in e ...... 36 Black Rock Mine...... i...... 38 Easton-Pacific Group ...... 38 Prospect Mine...... 40 St. Lawrence Mine ...... 40 Fork Mine ...... 42 10. GEOLOGIC HISTORY OF THE VIRGINIA CITY DISTRICT...... 44 V ii TABLE OF CONTENTS—Continued Page 11. CONCLUSIONS: AGE AND GENESIS OF QUARTZ-SULFIDE- PRECIOUS-METAL VEIN SYSTEMS IN THE VIRGINIA CITY MINING DISTRICT...... ■...... 46 Previous Interpretations ...... 46 Author's Interpretation...... 47 Age of Ore Deposits ...... 48 Genesis of Ores in the V irg in ia C ity Mining D is tric t . 51 12. REFERENCES CITED...... 59 13. APPENDICES...... 63 Appendix A Radiometric Dates (K-Ar Technique) ...... 65 Appendix B General Data on Some Tobacco Root Precious- Metal Deposits ...... 67 Appendix C Ore and Gangue Minerals of Tobacco Root Base- and Precious-Metal Deposits ...... 70 Appendix D Lode Production Figures fo r Tobacco Root Mining Districts (1901 ^l935) ...... 72 Appendix E Lode Production Figures for Selected Mines of the Tobacco Root Region (1901-1935) .... 74 Appendix F Lode Production Figures for Selected Mines of the Virginia City District (1901-1935). . . 76 v iii LIST.OF TABLES Table Page 1. Some of the quartz vein systems of the Virginia City D is tr ic t...... 18 2. Paragenetic sequence of hypogene a lte ra tio n and ore mineralization in the U.S. Grant 3-level...... 25 ix LIST OF FIGURES Figure Page 1. Mining districts and physiography of the Tobacco Root precious-metal mining region...... 2 2. General geology of the Tobacco Root precious-metal mining region ...... 6 3. Regional zoning of s ilv e r-to -g o ld and copper-to-silver ratios in the Tobacco Root precious-metal mining region . . 17 4. Cross-section of U.S. Grant 3-level vein structure ...... 22 5. View looking up-dip of vein structure on southeast wall of U.S. Grant 3-level d r i f t ...... 23 6. Unaltered gneiss from Browns Gulch...... 27 7. Slightly altered wall rock from the U.S. Grant 3-level, R. P. #8 .5 ...... 28 8. Strongly altered wallrock from the U.S. Grant 3-level, R. P. #8.5...... \ ...... 29 9. Strongly propylitized wallrock from the U.S. Grant 3-level, southwest of R.P;#8.5...... 30 10. A typical sample of quartz-sulfide ore and adjacent wallrock from the U.S. Grant 3-level ...... 31 11. Ore sample from the U.S. Grant 3-level ...... 33 12. Ore sample from the U.S. Grant 3-level ...... 33 13. Ore sample from the U.S. Grant 3-level...... 34 14. Ore sample from the U.S. Grant 3-level ...... 34 15. Ore sample from the U.S. Grant 3-leyel...... 35 16. Ore sample from the U.S. Grant 3 -le v e l ...... 35 17. Cross-section of the vein structure in the St. Lawrence 1 5 0 -le v e l...... 41 ______/S X LIST OF FIGURES—Continued Figure Page 18. Depositional model of epithermal precious-metals ...... 52 19. Epithermal precious-metal deposits of Montana ...... 58 xi ABSTRACT In the Virginia City mining district, pre-Belt gneisses and the Late Cretaceous granitic Browns Gulch stock host numerous quartz vein systems. Hypogene mineralization is chiefly gold- and silver-bearing base-metal sulfides. The U.S. Grant 3-level vein system, N40o-50°E; 35o-50°NW, 0.3 to 5.0 m wide, is contained in a shear zone exhibiting about 10 m of syn-ore rig h t-la te ra l movement. The vein system is composed of elongate quartz lenses, quartz stringers, tabular quartz bodies, and variable amounts of crushed and altered gneiss. Altered wall rock gneiss exhibits early potassic alteration (microcline and possibly quartz) arid a subsequent propylit i c assemblage (carbonate, p y rite , quartz, chlorite, other phyllosilicates, and a zeolite?). Post- alteration mineralization occurs as pyrite, followed by variable amounts of contemporaneous galena, sphalerite, chalcopyrite, sparse tetrahedrite, and rare specular (?) hematite. Additionally, some (latest) sphalerite replaces pyrite, galena, and chalcopyrite. Observable gold is very rare. Quartz deposition is pre-, syn-, and post-sulfide mineralization. The Virginia City district is one of several districts in the Tobacco Root precious-metal mining region. The region is cored by the 77-72 m.y. old quartz monzonite Tobacco Root batholith. A regional zoning is present with respect to the batholith in the form of low silver-to-gold and high copper-to-silver ratios near the batholith, and high silver-to-gold and low copper-to-silver ratios far.from the batholith (Virginia City district). I t is proposed th at ores of the d is tr ic t are of Latest Cretaceous to Early Tertiary age (70-60 m.y.B.P.), based on the occurrence of deposits in the Late Cretaceous Browns Gulch stock, the crosscutting of the El Fleeda 4-level vein system by a 51 m.y. old andesite plug, . and the regional zoning with respect to the batholith. An epithermal precious-metal genesis model has been applied to the ores of the district. A geothermal convection cell powered by heat from Late Cretaceous plutonism, produced large-scale regional circulation, of hydrothermal fluids at shallow crustal levels. These fluids collected (remobilized ?), transported, and deposited the ore constituents. I INTRODUCTION Location, Access, and Physiography The V irginia City mining d is tr ic t, located southwest of V irginia C ity , Madison County, Montana, is the southernmost d is tr ic t in the Tobacco Root precious-metal mining region (Fig. I). It may be sub divided into the Fairweather (located at Virginia City), Highland (3 km south of Virginia City), Summit (10 km south of Virginia City), Nevada (a t Nevada C ity ), and Browns Gulch d is tric ts . The district extends from Alder Gulch.on the north and east, to west of Browns Gulch, and to the flanks of the Gravelly Range on the south (Plate I ) . I t covers an area of approximately 90 square km. The Virginia City area is approachable via Montana Highway 287. Numerous d ir t and gravel roads provide access within the d is tr ic t. Winter access is d if fic u lt in the south part of the d is tr ic t but is re la tiv e ly easy in the north h a lf. Virginia City, elevation 1767 m, sits in a topographic low between two major mountain ranges. The Tobacco Root Mountains (summit eleva tions greater than 3000 m) are located to the north, and the Gravelly Range (summit elevations greater than 2900 m) to the south (Fig. I ) . West of Virginia City is the Ruby River Valley,elevation about 1525 m; to the east is the Madison River Valley, elevation about 1500 m. The major drainages of the district, Alder and Browns Creeks (Plate I), head in the southern portion of the district where they 2 ; . / . I d is tric t TOBACCO ROOT d is tr ic t MOUNTAINS ZRUBY RANGE Virginia City /. IOKM Vlrglnls\ ICIty d is tric t MONTANA ■ \ t Figures 1,2.3 GRAVELLY/ Madleon Co. RANGE Figure I . Mining districts and physiography of the Tobacco Root precious-metal mining region. Patterned areas are Late Tertiary and Quaternary unconsolidated or poorly consolidated sediments. Blank areas are older sedimentary, igneous, or metamorphic rocks. > 3 are deeply incised in the foothills of the Gravelly Range. Alder Creek flows north to Virginia City where it turns and flows northwest for 16 km until it joins the Ruby River. Browns Creek flows north until it joins Alder Creek at Nevada City (3.2 km northwest of Virginia C ity ). Purpose and Method of Study The major problem addressed in th is thesis is what processes are responsible fo r ore genesis in the minipg d is tric t? Precambrian meta- morphism and possible igneous a c tiv ity , along with Laramide plutonism and volcanism, play important roles in the geologic history of the d is tr ic t. Hydrothermal a c tiv ity related to e ith e r group of processes is capable of producing ore bodies. A detailed study involving above- and below-ground mapping, ore and wall rock microscopy, and K-Ar radiometric dating, were done in an attempt to determine the processes responsible for ore genesis. Previous Study Previous work in the mining district is minimal. Winchell (1914), Ta nsley, Schafer, and Hart (1933), and Lorain (1937) briefly described the general geology and various mines of the Tobacco Root region and the Virginia City district. The southern part of the district was mapped by Hadley (1969) in the 15 minute Varney quadrangle. A prelim inary bedrock map of the northern part of the district was published by Weir (1982). Vitaliano and Cordua (1979) mapped the southern Tobacco Root Mountains, located north of the mining district. 4 MINING HISTORY. Placer gold was discovered in Alder Gulch in 1863 approximately 0.4 km south of the present site of Virginia City. Within one year, auriferous and argentiferous quartz lodes were discovered and developed Most of the lodes were in secondarily enriched oxidized ores. In the late 19th century, these ores were exhausted and lower grade primary ■ ores were encountered. As a re su lt many of the mines were abandoned. Mines that continued to operate had contained relatively high-grade hypogene ores. Mining in the 20th century has been sporadic and never at the scale achieved in the late 19th century. ; Production figures from Lorain (1937) indicate the total value of lode gold from the entire Tobacco Root precious-metal mining region is.about 17 million dollars, including 2.5 million dollars from the V irg in ia C ity d is tr ic t. However, th is is minor when compared to 50 million dollars of placer gold froni Alder Gulch (Lorain, 1937). 5 REGIONAL GEOLOGIC SETTING . The Tobacco Root Mountains, a large northwest-plunging domal u p lift of multiple deformed and metamorphosed pre-Belt rocks (Mueller and Cordua, 1.976), is a major geologic feature in the region of the Virginia City district (Fig. 2). The mining district is located in the southernmost part of this pre-Belt te rra in . The Tobacco Root Mountains are cored by the Late Cretaceous (72-77 m.y.B.P.; Vitaliano and others, 1980) quartz monzonite Tobacco Root batholith (Fig. 2 ). This pluton crops out within a 310 square km area. It is thought to be genetically related to the Boulder batholith (Smith, 1970) located 22 km to the northwest (Fig. 2). This interpretation is supported by low Sr^/Sr^ values (indicative of a lower-crust or upper-mantle magma origin; Krauskopf, 1979) for the Boulder (Doe and others, 1968) and Tobacco Root (V ita liano and others, 1980) batholiths. Scattered throughout the pre-Belt rocks are numerous smaller p iutons sim ilar in age and composition to the Tobacco Root batholith (V italiano and Cordua, 1979). Numerous base- and precious-metal deposits are associated with these igneous bodies. Proterozoic, Paleozoic, and Mesozoic sedimentary rocks crop out on the north and west flanks of the Tobacco Root pre-Belt terrain (Fig. 2). Paleozoic, Mesozoic, and Early Tertiary sedimentary rocks crop out in the Gravelly Range. Cretaceous and T ertiary volcanic rocks 6 Boulder batholHh PCm Tobacco batholHh PCm TQa Figure 2. General geology of the Tobacco Root precious-metal mining region (modified from Ross and others, 1955). PCm are pre-B elt metamorphic rocks; PPM are Proterozoic, Paleozoic, Mesozoic, and Early Tertiary sedimentary rocks; KTi are Late Cretaceous plutonic rocks; KTv are Cretaceous and Tertiary volcanic rocks; TQa are Late Tertiary and Quaternary sediments. 7 are scattered about the region, the Virginia City basalt field being the largest. Unconsoliated and poorly consoliated Tertiary and Quaternary sediments are abundant in stream and riv e r valleys. \ 8 GENERAL GEOLOGY OF THE VIRGINIA CITY MINING DISTRICT Rock Types The mining district is almost entirely underlain by an assemblage of pre-Belt gneisses (Plate I). These rocks are thought to be the products of three metamorphic events (Mueller and Corduas 1976). The earliest event was probably of granulite facies and the sparse occur ence of diopside in the gneisses is thought to be a relict of that ' -- : event. A subsequent 2700 m.y.B.P. dynamothermal am phibolite-facies metamorphism formed the dominant mineral assemblage in these rocks. The th ird metamorphism was a 1600 m.y.B.P. thermal event that reset mineral ages (Wooden and others, 1978) in the Virginia City district. The dominant lithology in the mining d is tr ic t is q u a rtz -. ofeldspathic gneiss containing quartz, K-feldspar, perthite, and plagioclase with lesser amounts of hornblende, biotite, kyanite, epidote, garnet, and diopside. In outqrop the gneiss is light- colored and commonly exhibits foliation in the form of cm- to m-thick banding. Migmatite does occur but is relatively minor. Less common are amphibolites with abundant hornblende (or other amphiboles), plagioclase, and biotite with lesser amounts of quartz, feldspar, epidote and garnet. In outcrop they are dark colored and massive to strongly fo lia te d . 9 Rare garnet-rich units are present in either of the previously mentioned lithologies. Porphyroblasts rarely attain a diameter of 8 cm, with most less than 5 cm. Several small bodies (0.01-0.5 sq. km) of metamorphosed ultramafic rock crop out in the district. These rocks are strongly serpentinized and exhibit a white to dark green color in outcrop. Tan or brown meta-dolomite crops out discontinuously on the west - side of Browns Gulch and in upper Alder Gulch (Plate I). Thickness varies from 15 cm to greater than 20 m but is commonly less than I m. Minor amounts of impure q u artzite may occur adjacent to the. meta dolomite. Probably the meta-dolomite was a continuous layer of relatively uniform thickness but was deformed and attenuated by Precambrian orogenesis. The remnants of this marker u n it were used to recognize deformation in the metamorphic rocks. Excluding the meta-dolomite, thicknesses of the previously described rocks are unknown. Stratigraphic order is also unknown because metamorphism transposed all primary structures and textures. Unfoliated tabular pegmatites, commonly 3 or 4 m thick, crop out extensively within the Precambrian rocks (Plate I). They have sharp, discordant boundaries, and usually contain quartz, sericitized feldspars, and biotite or muscovite. Most pegmatites trend northwest or west-northwest. K-Ar radiometric dating indicates an age of 1572 m.y.B.P. ± 51 m.y. for biotite from a pegmatite in the U.S. Grant mine (Appendix I ) . 10 The Mississippian Madison Limestone is exposed in upper Alder Gulch (Plate I). The limestone is in thrust contact with the under lying rocks. An ore-bearing Late Cretaceous granitic stock is exposed in upper Browns Gulch (Plate I ) . This rock (the Browns Gulch stock) is assumed to be an outlier of, or genetically related to, the Tobacco Root batholith located 35 km to the north (Tansley and others, 1933). Composition is K-feldspar, quartz, plagioclase, and b io tite , with poorly developed graphic texture. A marginal pegmatite phase of similar lithology is well developed. In addition, the stock is pervasively faulted and fractured. > Numerous Early Tertiary (?) quartz-sulfide-precious-metal vein systems are present in the district. Poorly mineralized vein systems are more resistant to weathering and tend to crop out unlike most highly mineralized vein systems. All vein systems cut pegmatite dikes and fo lia tio n of metamorphic rocks. Post-mineralization Eocene and Oligocene volcanics, dominantly basalt (Chadwick, 1981), unconformably pverlie Early Tertiary gravels or Precambrian rocks along the east flank of the d is tr ic t. Two volcanic plugs w ithin the Precambrian te rra in cross-cut quartz-sulfide precious-metal vein systems. Marvin and Dobson (1979) indicate a whole rock K-Ar age of 51.1 m.y.B.P. ± I .2 m.y. fo r the andesite plug in the El Fleeda mine (Plates 2, 4; Appendix I ) . Late T e rtia ry and Quaternary sediments, including placer deposits, occur in Browns Gulch and Alder Gulch. 11 Structure Two episodes of folding were recognized in the metamorphic bedrock of the d is tr ic t. The f ir s t event, contemporaneous with amphibolite facies metamorphism, produced tig h t isoclinal folds with axial plane foliation. The second event coaxially (?) refolded this foliation into a northeast-striking, southeast-vergent, open antiform. This anti form is the dominant structure in the d is tr ic t (Plate I ) . The limbs of the fold are defined by a series of meta-dolomite outcrops, and attitudes of foliation in adjacent gneisses, in upper Alder Gulch and west o f Browns Gulch. The fracture system within the antiform may have had a strong control on the emplacement of pegmatites and quartz vein systems. Very few of these features were observed in the adjacent synform to the west. A multitude of fracture trends is present in the metamorphic rocks. The trends of the vein systems, pegmatites, shear zones, faults, joint patterns, and some stream valleys depict many of these patterns (Plate I) . The more common trends are N45°E, N45°W, N55°- 70°W, N6&°E, N80o-90°W, N-S, and N25°W. Evidence of faulting at the surface is observable at several locations. Three northwest-trending faults offset meta-dolbmite on the northwest limb of the antiform. Several northeast-trending faults o ffs e t the Madison Limestone and the underlying Precambrian rocks. In addition, the limestone is in thrust contact with the metamorphic rocks. 12 Faulting is abundant in underground workings although displacement is frequently less than I m. A major normal fa u lt in the Cornucopia mine offsets the U.S. Grant-Cornucopia group from the El Fleeda-Black Rock group (Plate 2 ). The fa u lt is comprised of a zone of fa u ltin g about 2 m wide with abundant slickensides on clay-covered surfaces. The presence of shear zones is evident both above and below ground. A northwest-striking shear in the northwest limb of the antiform exhibits right-lateral offset in meta-dolomite. This shear contains several blocks of gneiss and pegmatite "floating" in recrystallized "travertine-like", banded calcite. The nqrthwest- , --V 'I ' trending vein system of the Prospect mine appears to be in a shear zone (Lorain, 1937). The vein system of the U.S. Grant mine occupies a northeast-striking shear zone exhibiting right-lateral offset. Other vein systems occupying northeast-trending shear zones occur in the El Fleeda, Black Rock, and Fork mines. 13 MINING GEOLOGY ■ Regional S ettin g : The Tobacco Root Precious-Metal Mining Region The Tobacco Root precious-metal mining region is comprised of the Virginia City, Pony, Norris, Sheridan, Tidal Wave, and Renpva districts (Fig. 2). Although mines in the region are small and numerous, there is a strong similarity in the character of the ore deposits (Lorain, 1937). Most deposits in the region are quartz vein systems, composed of . one or more vein structures, occupying fractures in Precambrian metamorphics or Laramide g ra n itic intru&ives (see Appendix 2 for details). Although there is no sharp line of demarcation, vein structure morphology varies from tabular quartz bodies in clean-cut tension fractures to quartz lenses and stringers in strongly crushed and sheared fractures (Lorain, 1937). A few ore bodies occur as disseminations and replacements in fractured and altered walI rocks (Appendix 2). Examples are the Strawn mine (Tidal Wave d is t r ic t ) , the Broadguage-Tamarack group (Sheridan d istrict), the Mayflower mine (Renova d istrict), the Boss Tweed-Clipper group (Pony district), and the Atlantic and Pacific mine (Pony district; Tarisley and others, 1933; Lorain, 1937). Hypogene ore minerals common to a ll Tobacco Root precious- metal mining districts are gold, pyrite, galena, sphalerite, and 14 chalcopyrite. Other minerals in the ore deposits are listed in Appendix 3. Almost a ll near-surface portions of Tobacco Root ore bodies are enriched by oxidation (Lorain, 1937). . The depth and intensity of enrichment are variable but the same changes are evident in all enriched ores. These oxidized ores may contain secondary silver, lead, copper, zinc, and iron minerals at the expense of primary sulfides. Base- and precious-metal production figures of Tobacco Root d is tric ts and mines are presented in Appendices 4 and 5, respectively. The following factors indicate why these figures are in part nonrepresentative of true production and ore grade: 1) production records were not kept p rio r t o .1900 when mining a c itv ity reached its peak; 2) base-metal production was exclusively a by-product of precious-metal production, and base-metal content was recorded only when sufficient quantity was present to pay for their refinement; 3) selective mining methods were practiced (frequently in smaller mines) so only the richest ore shoots were mined; 4) some of the ores were m illed p rio r to smelting whereas . others were not; 5) production from many (especially smaller) mines was from ) • 1 shallow, oxidized ores that were relatively enriched compared to hypogene ores a t depth. 15 According to Lorain (1937) a nearly true measure of the average gold content of run-of-the-mine ore may be obtained by examining the production records of mines that m illed large tonnages of ore mined by non-selectiVe methods (i.e . Easton-Pacific group. Mammoth mine, and the Boss Tweed-Clipper group). Inspection of these mine records suggests a uniform gold grade in the Tobacco Root ores (excluding the unusually rich Mayflower mine in the Renova d istric t). Gold content of most major ore shoots varies from 0.25-0.33 ounces per ton of ore (Lorain, 1937), although silver, lead, zinc, and copper content vary considerably. Regardless of incomplete and somewhat inaccurate production records, silver-to-gold and copper-to-siIver ratios appear to have an inverse relationship. The Pony district (especially the Mammoth mine) is the locus of copper production, has the highest copper-to- si Iver ratio, and the lowest silver-to-gold ratio. At the opposite end of the spectrum is the V irg in ia C ity d is tr ic t. This d is tr ic t (especially the Easton-Pacific group.) is the locus of silver production, has the highest silver-to-gold ratio, and the lowest copper-to-siTver ratio. Other Tobacco Root districts appear to have intermediate silver-to-gold and copper-to-silver ratios. The abundance of lead and/or zinc in the Sheridan, Tidal Wave, and Renova districts may in part be due to zoning, however, the geochemical properties of (Precambrian or Paleozoic) carbonate rocks may be a major factor in localization of the lead- and zinc-bearing ores. Based on this data, a rough regional zoning of silver-to-gold and copper-to-silver ratios with respect to the Tobacco Root batholith 16 is evident (Fig. 3; Lorain, 1937). Highest copper-to-silver and lowest silver-to-gold ratios occur in and near the main exposure of the batholith (Pony and Norris? districts). Lowest copper-tp-silver and highest silver-to-gold ratios occur furthest from the batholith (Virginia City district). Intermediate ratios occur in districts at moderate distances from the b ath o lith . Mining Geology of the V irg in ia City D is tric t The precious-metal deposits of the Virginia City district e xh ib it a strong homogeneity in character and morphology. The deposits occur as quartz vein systems occupying fractures in Precambrian metamorphic rocks, except the Easton-Pacific group which occurs in the Brown Gulch stock. (Appendix 2; Plate I ) . These fractures are the primary (structural) control of ore deposition. Lode mines in the district may be roughly grouped into northeast- and northwest-striking vein systems (Table I; note the dominance of the northeast-striking vein systems), although a few strike north- south or east-west. Vein systems frequently transect foliation of metamorphic host rocks, however, some are subparallel ( i . e . the U.S. Grant mine: the s trik e of the vein system is approximately p arallel to the strike of foliation, but the dips differ). Pre-mineralization hydrothermal alteration is evident in the district. Although microscopic observation is limited, similar features are observable throughout in the form of an early potassic alteration (microcline and possibly quartz) cut by a later propylitic assemblage (carbonate, pyrite, quartz, ± chlorite). 17 R EN O VA 4 6 4 . 1.4 0 TIDAL ' 4 . 70 .1 0 6 Tobeece Aoet ^ S H E R ID A N V IR O If c m 1 6 . 36.1 Figure 3. Regional zoning of s ilver-to-gold and copper-to-silver ratios in the Tobacco Root precious-metal mining region. Numbers shown are s ilver-to-gold and copper-to-silver ratios, respectively, for each district. Ratios are calculated from total ounces of silver, total ounces of gold, and total pounds of copper fo r the period 1901-1935 (data from Lorain, 1937). The copper-to-silver ratio of Norris (0.48) is low possibly because greater than 70% of reported production is from oxidized ore. Data fo r primary sulfide ore production from the Norris district is available only from the Boaz mine (Appendix 5 ), with a s i l ver-to-gold ra tio of 0.97 and a copper-to-silver ratio of 3.96. The Boaz copper-to- silver ratio is congruent with the regional zoning, unlike the figure for the entire district. Dashed pattern is Late Cretaceous plutonic rock; dotted pattern is Late Tertiary and Quaternary sediment; blank area is Pre- cambrian through Early T e rtia ry metamorphic, sedimentary, or volcanic rock. 18 Table I. Some of the quartz vein systems of the Virginia City District. NORTHEAST-STRIKING NORTHWEST-STRIKING U.S. Grant: N40o-50°E; 35o-50°NW Prospect: N45°W; 75°NE Cornucopia: N45°E; 45°NW Easton-Pacific: N55°W; 70°NE El Fleeda: N40°-50qE; 30°-50°NW Mapleton: N50°W; 70°NE ' Black Rock: N50°E; 20o-30°NW St. Lawrence: N65°E; 50°NW Alameda: N70°E; 50°NW S ilv e r B ell: NSO0E; 45°NW Fork: N35°E; 20°SE Kearsage: N23°E; GS0NW M arietta: N45°E; 40°SE Hypogene mineralization consists chiefly of ubiquitous quartz and pyrite, and variable amounts of galena, sphalerite, chalcopyrite, and gold. Numerous other minerals alsq occur (Appendix 3 ). Secondary surficial enrichment (by oxidation) is important in the formation of high-grade ores in the Winnetka, Easton-Pacific, Prospect, Mapleton, Kearsage, Oro Cache, El Fleeda, Alameda, and other mines (Tansley and others, 1933). From 1901-1935, production was reported from more than 60 properties (Lorain, 1937). During the peak of mining activity (in the late 19th century) it was likely that a significantly greater number of mines were operating. 19 The vast m ajority of underground workings a re caved and inaccessible. At present, no mines are producing, although several are accessible and in operating condition. Most underground work conducted by the author was in the U.S. Grant 3-leveT. Other mines examined in varying detail were the El Fleeda, St. Lawrence, Cornucopia, Black Rock, and Alameda. Numerous surface exposures and mine dumps were also observed including the Prospect, Fork, and Easton-Pacific. In addition, some information about these mines was attained by examining available literature and consulting with local mining geologists. 20 MINING GEOLOGY OF . . SELECTED. MINES OF THE VIRGINIA CITY DISTRICT U.S. Grant Mine The U.S. Grant mine is located approximately 1.0 km south of Virginia City on the west flank of Alder Gulch (Plate I). The mine is composed of 3 le v e ls . The upper I - and 2 levels are abandoned and partially caved. The lower 3-level is relatively accessible and is the focus of underground study. The U.S. Grant has been one of the most consistent producers in the d is tr ic t, though never a very large one (Lorain, 1937; Appendix 6) The mine (in the 3 -le v e l) has been worked as recently as January, 1981 The 3-level is composed of a 65 meter-long adit in brecciated basalt, a 665 meter-long d rift in Precambrian gneiss, and numerous stopes and raises (Plate 3). Elevation of the portal, is approximately 1885 m. ' Composition of (unaltered) wall rock gneiss is dominantly hornblende-biotite-plagioclase-quartz-epidote-garnet, with Iesser amounts of a quartz-feldspar-perthite-plagioclase-hqrnblende-biotite- kyanite-garnet assemblage. Numerous pegmatites and several a p lite dikes transect wall rock foliation and are subsequently crosscut by the vein system (and the related a lte ra tio n halo). Pegmatite and aplite lithology is quartz-(sericitized) feldspars-biotite. L 21 Altered wall rocks contain variable amounts of hydrothermal and r e lic t metamorphic m inerals. The r e lic t mineral suite contains sericitized feldspars, biotite, quartz, and hornblende (?). Hydrothermal minerals are microcline, quartz, carbonate, pyrite, chlorite, other phyllosilicates, and rarely a zeolite (?). The 3-level vein system strikes N40o-50°E and dips SS0-SO0NW (Plate 3). The vein system is composed of two slightly overlapping vein structures. Vein structure #1 (VS-I) is present from reference point (R.P.) #2-#5 (Plate 3, Sheet I) beyond which it pinches out. Adjacent to and northwest of the pinch-out is vein structure #2 (VS-2). Unlike V S -I, VS-2 is not continuous, hence the secondary labels VS-2a, VS-2b, etc. This discontinuity is probably due to post-ore faulting of a continuous vein structure and may not imply the presence of more than one individual structure. Vein structure thicknesses vary from 0.3 to 5.0 m, but average 0.6 to 1.6 m (Figs. 4 & 5; Plate 3). Each structure is composed of elongate quartz lenses, tabular quartz bodies, quartz stringers, and variable amounts of crushed and altered gneiss. The quartz contains few cavity fillings because syn-ore deformation inhibits their development. The entire vein system is contained in a shear zone. Each vein structure appears to occur in a single shear of the shear zone. Shear fracturing is the primary (structural) control of ore deposition, and occurs in several major episodes. 22 Figure 4. Cross-section of U.S. Grant 3-level vein structure (VS-2). Dip approximately 50° northwest; thickness about 1.3 m. Located southwest of R.P.#6, Plate 3, Sheet I . Vein structure composed of an elongate quartz lens, quartz stringers, and crushed and altered wall rock ("X" p a tte rn ). Some wall rock (dot pattern) and pegmatite feldspar (line pattern) fragments in quartz lens. Sulfide mineralization (black shading) occurs as small seams and pockets in quartz. Wavy line pattern depicts wall rock not contained in vein structure. 23 Figure 5. View looking up-dip of vein structure on southeast wall of U.S. Grant 3-level drift. Located northeast of R.P.#9, Plate 3, Sheet I. Vein structure composed of an elongate quartz lens (blank area with thin-lined fractures) contain ing sulfide seams (black shading), wall rock fragments (dot pattern), and sparse pegmatite feldspar (line pattern). Wallrock (wavy lin e pattern) is crushed and a lte re d , and contains several quartz stringers. Thickness of vein structure is 0.8 to 1.0 m. 24 Most of the vein system (including adjacent wall rocks) was fractured and crusted by syn-ore, brittle, cataclastic shear. Right- la te ra l o ffs e t between hanging wall (HW) and footwalI (FW) has been documented by observing displacement of HW pegmatite and aplite dikes from corresponding FW portions (Plate 3 ). Sparse pegmatit ic feldspar was observed in the vein structure between o ffs e t HW and FW pegma tites (Plate 3; Fig. 5). The magnitude of wall rock deformation and lateral offset varies somewhat in the 3-level. At location I (R.P.#4.5; in the Only terminus exposed of either shear in the 3-level; see Plate 3) wall- rocks are not crushed or displaced. Lateral o ffset in uncrushed or slightly crushed wall rocks occurs at locations II (R.P.#4; 8.0 m offset), III (R.P.#8.5; 8.3 m offset), and IV (R.P.#18.5; 10.3 m ' offset). Lateral offset in strongly crushed wall rocks is present at locations V (R.P.#3; 12.0 m o ffs e t) and VI (R.P.#5; o ffs e t unknown). At location VII (R.P.#6) pegmatite blocks floating in crushed wall- rock suggest extreme deformation and preclude any determination of the magnitude of la te ra l movement. A two-stage alteration event is proposed by the author based on petrographic study of altered walI rocks. Potassic alteration consisting of microcline and possibly quartz is in part cut by a subsequent propyli tic assemblage containing carbonate, p y rite , quartz, chlorite and other phyllosilicates, and rarely a zeolite (?). Table 2 depicts the age relationships of these minerals. 25 Table 2. Paragenetic sequence of hypogene alteration and ore mineralization in the U.S. Grant 3-level. Major episodes of syn-ore shear are also shown. MINERAL WALLROCK ALTERATION ORE MINERALIZATION QUARTZ _U ______Ll_ MICROCLINE _JJ ______LI______li_ Il Il Il CARBONATE Il Il Il CHLORITE I I Il and other I I Il p h y llo s il- I I Il icates I I Il _u IL ______LI ZEOLITE I _ LI I PYRITE — _ L I GALENA — I CHALCOPY- Il Il RITE Il Il Il Il TETRAHE- Il Il Il DRITE Il Il Il Il Il HEMATITE — Il Il SPHALERITE Il Il Il Il ...... I I CO CO CO 3Z ZE =T mm^ m 7 0 yO 7 0 OZ O Z O Z 26 Variable amounts of microcline and possibly quartz (Figs. 6-8) occur as random, unoriented replacements of pre-existing gneiss. This conclusion is based on the following observations: 1) Metamorphic rocks not in spatial proximity to the 3-level vein system (or other vein systems) contain only sericitized feld spars but never non-sericitic microcline (Fig. 6). Microcline occurs (in variable amounts) only in wallrocks adjacent to and within the 3-level vein system (Figs. 7 & 8). Therefore, microcline is post- sen' citization and is likely a product of younger hydrothermal a c tiv ity . 2) Triple-point grain-boundary angles of approximately 120° between microcline and some quartz grains (Fig. 7) indicate textural equilibrium (Stanton, 1972, p. 234). K-Ar radiometric analysis was performed on the microcline by Geochron Labs. The sample contained a small amount of r e lic t perthite and K-feldspar (Fig. 8). An age of 311 m.y.B.P. ± 11 m.y. was attained (Appendix I) suggesting an admixture of Precambrian and Laramide feldspar. A la te r pervasive propylit i c a lte ra tio n assemblage occurs as microveinlets crosscutting, and as a matrix surrounding microcline, quartz, and relict minerals (Figs. 8 & 9). The shape and dimensions of the alteration halo are not known. The halo is probably more or less tabular (like the vein system). It is possibly no greater than 5 m wide on either side of the vein system and - in many places is less than I m. 27 M ineralization in the 3-level is almost e n tire ly hypogene, unlike the upper two levels where oxidized ore is more abundant. Major ore shoots (which appear to be controlled by shear fractu rin g ) occur as thin s u lfid e -ric h seams and pockets along the hanging wall or footwall contact (Figs. 5 & 10). Ore shoots may be continuous fo r tens of meters along s trik e , however, most are less. Petrographic study of polished-surface ore samples followed the criteria of Ramdohr (1969) in the identification of minerals and interpretation of mineral textures. Figure 6. Unaltered gneiss from Browns Gulch. Jjjj ae rlc ltlc ae rlcltlc K -te id a p a r quartz epldote b lo tlte plagloclaze hornblende I 28 Figure 7. Slightly altered wall rock from the U.S. Grant 3-level, R.P.#8.5. Note the addition of microcline and possibly quartz. Some biotite exhibits incipient propylitic alteration to chlorite, other phylIo siIicates, and pyrite. sericitlc K -fe ld s p a r quartz biotite • < * r chlorite/other // , y • s- • phyIloaillcates A y ' / microcline pyrite after biotite y Figure 8. Strongly altered wall rock from the U.S. Grant 3-level, R.P.#8.5. This sketch exhibits a well-developed potassic assemblage cut by a propyli t ic assemblage of carbonate, pyrite, and quartz. Propylitized biotite contains chlorite, other phyllosilicates, and pyrite. K-Ar age of the feldspar population in this sample is 311 m.y.B.P. ± 11 m.y. 30 Figure 9. Strongly propylitized wall rock from the U.S. Grant 3-level, southwest of R.P.#8.5. Fibrous mineral in lower half of sketch may be a zeolite. Quartz from vein structure is present in upper rig h t. aerlcltlc K -Ie ld s p a r quartz carbonate • • • y • • r microcline pyrite zeolite(?) 31 Figure 10. A typical sample of quartz-sulfide ore and adjacent walI - rock from the U.S. Grant 3 -le v e l. Wal I rock is crushed and altered, and contains quartz stringers. Quartz- sulfide ore has concentration of sulfides (black shading) along wall rock contact, and contains fragments of wall- rock (line pattern). Thin branching lines are fractures. 32 Hypogene ore m ineralization consists of abundant p y rite , moderate amounts of sphalerite and galena, minor chalcopyrite and tetrahedrite, and rare specular (?) hematite. Observed (bright- yellow) native gold is very rare. Microscopic flakes of gold occur in limonite (400x, oil imm.) and possibly in sphalerite (140x, oil imm.). Gangue is c h ie fly quartz with minor feldspar. The ore minerals in the 3-level exhibit relatively simple age relationships. Early quartz (Fig. 11) is followed by quartz and pyrite. Fracture fillings in pyrite contain quartz (Fig. 12); quartz, galena, chalcopyrite, and sphalerite (Fig. 13); or quartz, galena, chalcopyrite, sphalerite, tetrahedrite, and hematite (Fig. 14). Sphalerite appears in part to replace pyrite and galena (Fig. 14), or pyrite, galena, and chalcopyrite (Figs.15& 16). Post-sphalerite quartz occurs as microveinlets cutting sphalerite and other sulfides (Fig. 16). These relationships are depicted in Table 2. Secondary minerals in the 3-level ores include malachite, chrysocolla, coveilite , chalcocite, manganese oxide, (red) hematite, and lim onite. Reflection microscopy of ore samples exhibits chalcopyrite rimmed by chalcocite which is surrounded by cove11ite . Post-ore fa u ltin g (in the 3 -le v e l) is abundant though displacements are usually less than I m and frequently less than 0.3 m (Plate 3). Most of these faults crosscut the vein system,.hence post-ore internal deformation of the vein system is minimal. The U.S. Grant mine is adjacent and genetically related to the Cornucopia mine (Plate 2 ). The vein systems of both mines occur in a major northeast-striking shear zone or group of overlapping shear 33 Figure 11. Ore sample from the U.S. Grant 3-level. Early quartz ("Q") is surrounded by la te r quartz (white) and pyrite (b la c k ). Figure 12. Ore sample from the U.S. Grant 3-level. Pyrite with fractures filled by quartz. O C2 pyrite 34 Figure 13. Ore sample from the U.S. Grant 3-level. Pyrite with fractures filled by quartz, galena, sphalerite, and chalcopyrite. Figure 14. Ore sample from the U.S. Grant 3-level. Pyrite with fractures filled by quartz, galena, sphalerite, chalcopyrite, tetrahedrite, and hematite. Sphalerite, in part, appears to replace pyrite and galena. p yrlle galena sphalerite chalcopyrite tetrahedrite hematite 35 0 2 5 mm Figure 15. Ore sample from the U.S. Grant 3-level. Sphalerite is replacing galena, pyrite, and chalcopyrite. Figure 16. Ore sample from the U.S. Grant 3-level. Sphalerite is replacing pyrite, galena, and chalcopyrite. Post sphalerite quartz cuts across sphalerite and pyrite. ▲ 36 zones. This zone (or group of zones) also contains the El Fleeda-Homestake-Black Rock group. The U.S. Grant-Cornucopia group is offset from the El Fleeda-Homestake-Black Rock group by a recent normal fa u lt (N25°W, 810NE; Boyer9 personal comm., 1981; Plate 2 ). This fa u lt is observable at the southwest end of the Cornucopia I -le v e l. El Fleeda Mine The El Fleeda mine, located approximately 1.5 km south of Virginia City (Plate I), is composed of five levels and an inclined shaft. All workings except the 4-level are caved and inaccessible. The 4-level was worked as recently as the 1960's. It consists of a 105 meter-long a d it, a d r if t composed of two branches with a to tal length of 120 m, several stopes and raises, and an inclined shaft (Plate 4). Portal elevation is approximately 2000 m. Composition of unaltered wall rock gneisses is dominantly hornblende-bio tite-quartz-feldspar with a lesser amount of quartz- feldspar-biotite gneiss. Numerous granitic pegmatites cut wall rock foliation. The pegmatites are in turn cut by the vein system. Altered wall rocks exhibit potassic alteration in the form of abundant microcline and possibly quartz. B io tite is in part altered to chlorite and rare pyrite. Microveinlets of fine-grained mosaic quartz and hematized pyrite crosscut all previously mentioned m inerals. Carbonate is not present, however, this is probably due to lim ited sampling. 37 The 4-level vein system (like that of the U.S. Grant 3-level) occupies a shear zone. Average s trik e and dip of the system is N4Q°-50°E and 30o-50°NW (Plate 4 ). The vein system is composed of two vein structures that join along strike to the northeast. Maximum thickness of either structure is about 2 m. Each vein structure is composed of quartz lenses, quartz stringers, and tabular quartz bodies in crushed and altered wall rock. The quartz contains few open-space fillin g s because syn-ore deformation partly precludes their development. Considerable post-ore (including recent) faulting occurs within and adjacent to the vein system. As a re s u lt, footwall and hanging wall boundaries are disguised and difficult to locate. Syn-ore, b r it t le cataclastic shear displaced hanging wall and footwall portions of wall rock pegmatites, in addition to destroying their tabular form. Subsequent post-ore faulting has further displaced the pegmatites and surrounding wall rocks. As a result, analysis of pegmatite locations yielded ambiguous data fo r magnitude and direction of shear. Hypogene mineralization in the 4-level contains auriferous and argentiferous sulfides, dominantly pyrite with lesser amounts of galena, in a gangue of quartz. Bright-yellow gold occurs as flakes (1-2 mm across) in quartz, but is quite rare. Much of the ore is oxidized, especially where post-ore faulting is present in the vein system. In the v ic in ity of pegmatites, feldspar may be found in the ore. 38 A post-ore andesite plug crosscuts the vein system (Plate 4). Whole rock K-Ar radiometric dating indicates a 51.1 m.y.B.P. ± 1.2 m.y. age fo r the plug (Marvin and Dobson, 1979; Appendix I ) . This plug appears to separate the El Fleeda vein system from the Homestake-Black Rock vein system (Plate 2 ). Black Rock Mine The Black Rock mine is located 2.5 km southwest of Virginia City at an elevation of 2090 m (Plate I). Most of the workings are caved; however, access to one d r if t level is possible via a 20 meter-long ventilation raise. Approximately 80 m of drift are observable. Wallrock lithology is hornblende-biotite-quartz-feldspar gneiss. ! Pegmatites are lacking although they are abundant at the surface (Plates 1,2). The vein system, a ttitu d e N50°E; 20o-30°NW, is contained in a shear zone. It consists of a single vein structure composed of tabular quartz bodies 0.7 to 1.7 m thick (Lorain, 1937) and quartz stringers in fractured gneiss. Mineralization occurs as precious-metal-bearing sulfides, dominantly pyrite. Gangue is entirely quartz. Easton-Pacific Group The Easton-Pacific group, elevation approximately 2300 m, is located 7.5 km south of Nevada City in the headwaters of Browns Gulch 39 (Plate I ) . A ll underground workings are inaccessible due to recent open-pit operation on the claim. According to Winchell (1914), these mines are on the same vein system within the Browns Gulch stock. This group has yielded the largest tonnage of any property in the Virginia City district (Lorain, 1937; Appendix 6). Within the group, the Easton has been the major producer. The Easton claim was developed southeastward along the vein system, whereas the Pacific claim was developed towards the northwest. The Easton-Pacific vein system strikes N55°W and dips 70°NE. In the Easton mine the vein system is composed of two vein structures that jo in towards the southeast (WinchelI , 1914). Thickness of the vein structures varies from 0.5 to 2.0 or 2.6 m (Winchell, 1914). D etails of P acific vein system morphology are lacking in the available literature. Mineralization varies slightly amongst the two mines. In the Pacific mine antimonial silver sulfides and native gold and silver occur in quartz and iron oxides (Winchell, 1914). In the Easton mine the ore contains argentite, auriferous pyrite, native silver, tetrahedrite, native gold, sphalerite, and stibnite (Winchell, 1914). Gangue is chiefly quartz and feldspar. The silver-to-gold ratio is higher in the Easton than in the Pacific, and the ratio increases with depth in both mines (Tansley. and others, 1933). 40 Prospect Mine The Prospect Mine, elevation 1890 m, is located 1.5 km west of Virginia City (Plate I). All workings are dangerous and inaccessible. The vein system, attitude. N45°W; 75°NE, is in quartzofeldspathic gneiss. It is little disturbed by post-ore faulting (Tansley and others, 1933). Unlike other productive vein systems in the district, it is readily traced at the surface over the full length of the Prospect claim. The vein system appears to be composed of a single vein structure. It contains quartz lenses up to 2.3 m thick in sheared gneiss (Lorain, 1937). The principal hypogene mineralization consists of galena, chalcopyrite, and pyrite, with minor sphalerite and native gold. Petrographic study indicates 25% of observable microscopic gold is contained in quartz, with the remainder occurring in sulfides, mainly galena (Tansley and others, 1933). Although pyrite is not an important carrier of observable gold, it is assumed that pyrite contains gold as submicroscopic particles and/or as a lattice constituent. St. Lawrence Mine The St. Lawrence mine, elevation approximately 2000 m, is located about 4.0 km southwest of Nevada City (Plate I). Entrance to the mine is via a 50° inclined shaft. The following data is from observations by the author in the 150-level. 41 The St. Lawrence vein system, a ttitu d e N65°E; 50°NW, is contained in quartzofeldspathic gneiss. It is composed of two parallel vein structures that merge toward the northeast. Maximum distance between the two structures is about 20 m (Boyer, personal comm. 1982). The vein structures vary in thickness from 0.3 to 2.3 m. They are composed of tabular quartz bodies, exceeding I m in thickness, and quartz stringers in fractured and altered wall rock (Fig. 17). tabular quartz quartz stringer 30 cm Figure 17. Cross-section of the vein structure in the St. Lawrence 150-level. Sulfides (black) occur as pockets and blebs. 42 Altered wall rocks (in and adjacent to the vein system) contain a pervasive potassic alteration assemblage consisting of microcline and possibly quartz, -Subsequent p ro p ylitiza tio n occurs as massive dissemination-patches and microveinlets of carbonate and hematized pyrite. No chlorite is present probably because the original gneiss lacks biotite. Primary m ineralization occurs as blebs and large pockets of pyrite with some galena and minor chalcopyrite. Ore from the 150- level is only slightly oxidized. Each vein structure hosts a recent fault. Both faults exhibit approximately 60 m of left-lateral offset (Boyer, 1982, personal comm.). As a result, portions of the vein structures are fractured, crushed, and granulated. Fork Mine The Fork mine is located in Browns Gulch about 5 km south of Nevada City (Plate I). Elevation of the drift is about 1950 m. The vein system strikes N35°E and dips 20°SE (Lorain, 1937). Observations of mine dump samples indicate the vein system contains much altered and crushed gneissic wall rock, and possibly some pegmatitic feldspar. Microscopic examination of a single wall rock sample indicates intense p ro p ylitiza tio n in the form of massive carbonate and p y rite , with abundant b io tite ghosts containing c h lo rite and pyrite. According to Lorain (1937), a 0.7 meter-thick "quartz vein" is contained in a "crushed and sheared zone in gneiss". Ore samples 43 from the dump contain variable quantities of pyrite and galena in a gangue of quartz. 44 GEOLOGIC HISTORY OF THE VIRGINIA CITY DISTRICT Based on data from the southern Tobacco Root Mountains (Cordua, 1973; Mueller and Cordua, 1976; Burger, 1969; Wooden and others, 1978), data from maps of the mining district (Weir, 1982; Hadley, 1969), and field relationships observed by the author, the following geologic history has been proposed. Deposition of proto!iths and subsequent metamorphism p rio r to and at 2700 m.y.B.P. formed the pre-B elt rocks in the mining d is tr ic t. Deformation accompanying the 2700 m.y.B.P. metamorphism yielded tight isoclinal folds with axial plane foliation. Sometime after this deformation but prior to 1600 m.y. ago, coaxial (?) refolding of the pre-existing foliation produced the northeast-striking antiform of the district. About 1600 m.y.B.P., a thermal event reset mineral ages, and pegmatites of the district were formed. During the Paleozoic and the Mesozoic Eras, sedimentary rocks were probably deposited in the mining d is tr ic t, however, th is is not c e rta in . The Madison Limestone was observed in upper Alder Gulch, but was in thrust contact with the underlying Precambrian rocks. From Late Cretaceous to Mid Oligocene tim e, Laramide orogenic processes operated in the d is tr ic t and the surrounding region.; B r ittle , non-penetrative, low-temperature deformation in Precambrian crystalline rocks probably created new faults, fractures, and shears. 45 and reactivated many pre-existing weaknesses. Paleozoic and Mesozoic strata were thrust onto Precambrian rocks. Plutonism, volcanism, and hydrothermal activity were widespread throughout the area. The Browns Gulch stock and marginal pegmatite phase were injected, followed by the emplacement of quartz-sulfide-precious-metal ore bodies. Subsequent erosion and deposition of stream gravels was followed by the extrusion of voluminous quantities of Eocene and Oligocene volcanics. During Late Tertiary and Pleistocene time, erosion removed most of the volcanics, the underlying gravels, most of the Paleozoic and a ll Mesozoic s tra ta , and some of the Precambrian rocks. Sediments containing placer gold were deposited in the larger stream valleys. 46 CONCLUSIONS: AGE AND GENESIS OF QUARTZ-SULFIDE- PRECIOUS-METAL VEIN SYSTEMS IN THE VIRGINIA CITY MINING DISTRICT Previous Interpretations The genesis of quartz-sulfide-precious-metal vein systems in the Virginia City district is not obvious. Previous writers express ideas influenced mainly by the Lindgren magmatic-hydrothermal theory. Winchell (1914) stated the occurrence of Easton-Pacific ores in the Browns Gulch stock was not of chance but of genetic relation ship. He acknowledged th at vein systems hosted by Precambrian gneisses contained pyrargyrite, sylvanite or calaverite, nagyagite, Stibnite1 and tetrahedrite. This prompted him to say that vein system constituents "may have traveled a considerable distance from some parent igneous mass." However he then stated "the country rock of the ores is much broken and altered, and such conditions facilitate and bear witness to active circulation of meteoric waters. If such waters penetrated to sufficient depth....they might dissolve the materials of the ore deposits from the country rock on their downward and lateral journey and redeposit them in fissures..." In addition, he indicated the vein systems were definitely older than, and unrelated to. Tertiary volcanics in the district. Tansley and others (T933) also thought the occurrence of the Easton-Pacific group in the Browns Gulch stock suggested a genetic relationship. Subsequently, they stated it was probable that "the 47 Precambrian rocks of the region were underlain by monzonitic rocks which were responsible fo r much of the. fissuring and m ineralization of the area." Lorain (1937) implied an origin (of precious-metal ores in a ll Tobacco Root d is tric ts ) related to Laramide plutonism by his discussion of metal zoning with respect to the Tobacco Root batholith Additionally he stated the Easton-Pacific group was located in the Browns Gulch stock, and the M arietta, High-Up, and other mines were closely associated with it (Plate I). All other mines of the district are within several kilometers of the intrusive, with most of them between the main outcrop of the batholith and the stock. Weir (1982) mapped the bedrock of the north h a lf of the mining district, including outcrop patterns of quartz vein systems. He stated with uncertainty that the ore bodies were Tertiary in age though defintely older than the Tertiary volcanics. Author's Interpretation The following interpretation of age and genesis of quartz- sulfide-precious-metal ores in the Virginia City mining district is based on petrographic study and K-Ar radiometric dating of altered wall rocks in the U.S. Grant 3-level, information obtained via above- and below-ground mapping, and data from available lite r a tu r e . I t is assumed that the strong sim ilarity in character and morphology of . all mines in the district allows interpretations of data obtained from a single mine to be applicable to a ll. 48 Age of Ore Deposits I t is proposed that the age of emplacement of ores (in th e ir present form) in the mining d is tr ic t is between Latest Cretaceous and Early Tertiary time (70-60 m.y.B.P.). This interpretation is supported by several types of evidence. The occurrence of the Easton-Pacific group in the Late Cretaceous Browns Gulch stock indicates the ores bodies (of the district) must be younger than Late Cretaceous. It is reasonable to assume they are no younger than Early Tertiary because an andesite plug, with an age of 51.1 m.y.B.P. ± 1.2 m.y. (Marvin and Dobson, 1979), cuts the El Fleeda 4-level vein system (Plates 2, 4). Altered wall rocks from the U.S. Grant 3-level contain variable amounts of fresh, non-sericitized microcline, and, sericitized Precambrian feldspar (Figs. 7 &. 8). Microcline, because it lacks sericite, is post-sericitization. The youngest known age of any sericitic feldspar is 1572 m.y.B.P. ± 51 m.y. from the "Powder Mag" pegmatite. Therefore mi crocline is post-1572 m.y.B.P. This age interpretation is also supported by the crosscutting of all pegmatites in the 3-level by the microcline-bearing alteration halo. Based on the geologic history of the mining district, the only post- 1572 m.y.B.P. process capable of forming microcline is hydrothermal potassic a lte ra tio n related to Laramide igneous a c tiv ity . However, at present it cannot be determined if sericitization is a product of 1600 m.y.B.P. retrograde metamorphism, pre-micrpcline Laramide hydrothermal alteration, or post-pegmatite weathering. 49 An attempt was made by the author to determine the exact age of ore mineralization in the U.S. Grant 3-level vein system. It was assumed that alteration of wall rock was approximately synchronous with ore mineralization. This allowed the use of hydrothermal micro- cline for determination of ore body age. A sample of microcline- bearing altered wall rock (Fig. 8 is a petrographic sketch of the sample) was analyzed by Geochron Labs. The feldspar was removed and dated by K-Ar technique. An age of 311 m.y.B.P. ± 11 m.y. was attained. This age appears contradictory to the proposed Latest Cretaceous to Early Tertiary age. However, careful microscopic examination of the age-date sample reveals the wall rock feldspar population contains approximately 85% hydrothermal microcline and 15% Precambrian (variably sericitized) feldspar (Fig. 8 ). Therefore, this apparently 40 contradictory age (and the occurrence of excess Ar ) is due to contamination (Dalrymple and Lanphere, 1969; Schaeffer and Zahringer, 1966) of the Latest Cretaceous to Early Tertiary microcline sample by relict (1600 m.y.B.P.) feldspar. Consequently this date is a composite age of m icrocline and r e lic t feldspar. The previous in terpretation by the author appears plausible. Additionally, the alternative explanation of a Precambrian age presents problems. If the ore bodies are not the products of Latest Cretaceous to Early Tertiary hydrothermal activity but instead are the products of a Precambrian metamorphic related process, one would expect a maximum age of ore deposition and wall rock alteration of about 1572 m.y. To achieve a 311 m.y. age from 1572 m.y. old 50 microcline, 80% of the. Ar^ must escape the microcline lattice. Data for loss of Ar4® from microcline suggest a maximum of 20% (Schaeffer and Zahringer, 1966) or 20-30% (DalrympTe and Lanphere, 1969). Consequently it seems unlikely the apparent age of 311 m.y. is modified from a real age of 1572 m.y. It must also be noted that biotite from the "Powder Mag" pegmatite in the U.S. Grant 3-level (Plate 3, Sheet I , R.P. #6)' has a K-Ar date of .1572 m.y.B.P. ± 51 m.y. Biotite is known to retain Ar40 better than microcline, and considering the 1600 m.y.B.P. age fo r the la s t metamorphic event in the district, this biotite sample exhibits complete reten tion. Why ( i f microcline and b io tite are synchronous) would 40 the microcline lose an extreme amount of Ar while biotite exhibits total retention? The most likely explanation is that microcline is not 1572 m.y. old but is of Latest Cretaceous to Early Tertiary age. According to Lorain (1937), a rough regional zoning of copper- to-silver and s ilver-to-gold ratios occurs with respect to the Tobacco Root batholith (Fig. 3). This regional zoning appears to encompass a ll Tobacco Root mining d is tr ic ts , and therefore suggests a relationship between Tobacco Root precious-metal deposition and. the Tobacco Root batholith. Johns (1961), Tansley and others (1933), and Winchell (1914) report on various districts in the Tobacco Root region. They also suggest a possible genetic relationship between the batholith and precious-metal mineralization. If (most or all) Tobacco Root precious-metal deposits are of similar (batholith-related) genesis, they are likely of similar age. The occurrence of some deposits as fracture-fillings in the Tobacco Root batholith (77-72 51 m.y.B.P.; Appendix 2) and related rocks, and the unquestionable pre-51 m.y.B.P. age of the ores in the Virginia City district, strongly supports a Latest Cretaceous to Early Tertiary age for emplacement of (most or a ll) Tobacco Root precious-metal ores. Genesis .of Ores in the V irg in ia City Mining D is tric t In conjunction with the proposed Latest Cretaceous to Early Tertiary age of Virginia City precious-metal deposits, the following interpretation of genesis is suggested. Precious-metal deposits of the Virginia City district (and the Tobacco Root precious-metal mining region) appear to be the products at least in part of (Tobacco Root) batholith-related hydrothermal a c tiv ity . A dditionally, Precambrian metamorphism may be responsible for an in itial partial concentration of ore metals, though no evidence is available at present. A model depicting genesis of epithermal precidus-metal deposits ( Eimon and AnctiI , 1981; Buchanan, 1981) appears su itab le, with some modification, for the Virginia City deposits (Fig. 18). It must be noted that "epithermal" re fle c ts a genetic-class (Buchanan, 1981) and not a temperature-class of Lindgren (193.3). The following is a presentation of the basic concepts of this model. For more d etail re fe r to Buchanan (1981) or Eimon and Anctil (1981). The model encompasses a geothermal convection c e ll (driven by an igneous heat source; Taylor, 1973) that circulates a large . volume of hydrothermal fluids. The water in these fluids (as in rx> Figure 18 Depositional model of epithermal precious-metals Modified from Buchanan (1981) 53 documented in some deposits) is dominantly or entirely meteoric (Taylor, 1973). Episodic boiling of the fluids in the upper, near surface portion of the convection cell is responsible for metal deposition (Fig. 18). Precious-metals are deposited at and above the boiling le v e l, whereas base-metals are deposited a t and below i t . Reported temperatures of precious-metal deposition vary from 200o-300°C (Buchanan^ 1981) to IOO0-SOO0C ( Eimon and A n c til, 1981). In addition, numerous pre-mineralization fractures (frequently tensional) are required to transport, and accommodate deposition from, hydrothermal flu id s . Host rocks are dominantly volcanic or sedimentary Widespread propyli tic a lte ra tio n (c h lo rite , p y rite , montmoriI Io n ite , carbonate, and illite ) encloses erratically present inner alteration assemblages (ad u laric, p h y llic , a rg il l i e , a lu n itic , and s ilic ic ; Fig. 18). Also, most recognized epithermal deposits are of Tertiary age (Buchanan, 1981; Eimon and A n c til, 1981). Based on th is model, the following genesis has been proposed fo r the Virginia City deposits. A geothermal convection cell produced large-scale, regional circulation of hydrothermal fluids in the upper most portion of the crust. Heat from Laramide plutonism (Tobacco Root batholith and related rocks) powered the convection cell. Ore constituents were collected (in part remobilized ?) and transported by hydrothermal fluids. Numerous fractures, mostly within the northeast-striking antiform of the district, were invaded by the ore-bearing solutions." With time, fractures that exhibited proper physiochemical environments for ore deposition became the locus of mineralization. 54 At present it is not known if boiling of the ore-bearing fluids is the mechanism responsible for precious-metal.deposition in the Virginia City district. Additional data (i.e. fluid inclusion studies) is needed to make a definitive statement. The source of the ore constituents is not known, however, based on current literature, several statements may be made. Water in hydrothermal fluids is likely in part or entirely meteoric (Taylor, 1973). According to Boyle (1979) and T illin g and others (1973), any of the rock types in the d is tr ic t are adequate sources of gold. Therefore, Precambrian metamorphics and Laramide intrusives are possible source rocks. Nearby Laramide igneous rocks are possible sources fo r sulfu r (Whitney and Stormer, 1983). In the fu tu re , oxygen, sulfur, and lead isotope studies may provide valuable clues about the source of some ore constituents. The Virginia City district deviates from the model in several ways. Host rocks of the d is tr ic t are neither sedimentary or volcanic. In the U.S. Grant 3-level and the St. Lawrence 150-level, propylitiza- tion is present but appears to be restricted to a narrow irregular band on e ith e r side of both vein systems. In the U.S. Grant 3 -le v e l, the St. Lawrence 150-level, and the El Fleeda 4-level, microcline (and not adularia) is the feldspar present in the accompanying alteration assemblage. A dditionally, most of the ore-bearing fractures in the district are shear zones and not tension fractures. The occurrence of gold and s ilv e r with abundant base-metals suggests that d is tr ic t m ineralization is from the deeper portion of the model in the zone of overlap between base- and precious-metal 55 deposition. According to the model (Fig. 18), width of this overlap is about 50 m, however, most mines in the district span a considerably greater vertical distance. The question arises: why is there an apparent lack of major vertical change in hypbgene mineralization in the district? An answer may be that the location of the mechanism responsible fo r precious-metal deposition cannot remain constant in space and time (Buchanan, 1981). Vertical change of the location of this mechanism can explain large vertical intervals of mixed base- and precious-metal mineralization found in the Virginia City district and other epithermal deposits (Buchanan, 1981). A dditionally, genesis of these ores in the deep levels of the model may explain the limited extent of propylitization (Fig. 18). Another point to consider about district mineralization is the (former) occurrence of rich Alder Gulch placer deposits. These deposits yielded twenty-fold more gold than the combined production of all lode mines in the district. Undoubtedly, the source lodes for these deposits were of considerably higher grade than th e ir presently exposed and eroded roots. The regional zoning (of copper) with respect to the batholith appears to fit the epithermal model. Tobacco Root districts with high copper-to-silver ratios occur nearest the batholith and are possibly indicative of mineralization from the deepest portion of the model. Those districts further from the batholith exhibit mineraliza tion possibly indicative of less-deep portions of the model, based on moderate copper-to-silver ratios for these districts. The Virginia City district, with the lowest copper-to-silver ratio is possibly the 56 shallowest level of epithermal mineralization in the Tobacco Root region, though s till deep when compared to base-metal-free epithermal silver-gold deposits. The mineralization in the Virginia City district is similar to some of the epithermal deposits of Eimon and Anctil (1981) and Buchanan (1981) including Comstock (Nevada), Finlandia (Peru), and Guanajuato (Mexico). This sim ilarity helps support the author's interpretation of ore genesis. The following is a comparison of hypogene m ineralization in the U.S. Grant and the Comstock Lode. Throughout the Comstock Lode, dominant ore minerals (in order of th e ir abundance) are chalcopyrite, sphalerite, galena, argentite, and pyrite, with variable amounts of ruby silvers and (pale yellow) gold in a gangue of (dominantly) quartz ( Bastin, 1922). In the U.S. Grant 3-level dominant ore minerals (in order of abundance) are p y rite , sp h alerite, and galena, with minor amounts of chalcopyrite and tetrahedrite, and rare hematite and (bright-yellow) gold in a quartz gangue (Table 2). Microscopic study of ores from the Comstock Lode suggest contemporaneous deposi tion of the ore constituents during a single mineralization event, however, in some ores pyrite is earlier than, other metallic minerals (Bastin, 1922). Mineralization in the U.S. Grant 3-level is the product of a two-stage event where quartz and (auriferous ?) pyrite are followed by quartz, sphalerite, galena, chalcopyrite, tetrahedrite, hematite, and gold (? ). This comparison c le a rly depicts the strong sim ilarity in ore mineralogy and sequence of deposition between the Comstock Lode and the U.S. Grant 3 -le v e l. 57 Furthermore, Eimon and Anctil (1981) cite (not in their text but in a figure of epithermal g o ld -s ilve r deposits in Montana) V irginia City, Norris, Mineral Hill (part of the Pony district) and other gold-silver deposits in the Tobacco Root precious-metal mining region as examples of epithermal precious-metal deposits (Fig. 19). o FLATHEAD o LITTLE ROCKIES ©MARYSVILLE BAS,$ ©ELKHORN eRADERSBURG MINERAL HILL NORRIS ©VIRGINIA CITY Iadlson County Figure 19. Epithermal precious-metal deposits of Montana, Modified from Eimon and Anctil (1981). 59 REFERENCES CITED 60 REFERENCES CITED Bastin, E.S., 1922, Bonanza ores of the Comstock Lode, Virginia C ity, Nevada: United States Geological Survey B ulletin 735, p. 41-64. Boyle, R.W., 1979, The geochemistry of gold and its deposits: Geological Survey of Canada Bulletin 280, 584 p. Buchanan, L.J., 1981, Precious-metal deposits associated with volcanic environments in the southwest, _in Dickinson, W.R., and Payne, W.D., (eds.), Relations of tectonics to ore deposits in the southern Cordillera: Arizona Geological Society Digest, Vol. 14, p. 237-262. Burger, R.H. 3rd, 1969, Structural evolution of the southwestern Tobacco Root Mountains, Montana: Geolocial Society of America Bulletin, Vol. 80, p. 1329-1342. Chadwick, R.A., 1981, Chronology and structural setting of volcanism in southwestern and central Montana: Montana Geological Society 1981 Field Conference Guidebook, p. 301-310. Cordua, W.S., 1973, Precambrian geology of the southern Tobacco Root Mountains (ab stract): Dissertation Abstracts, Vol. 34, p. 3305B. Dalrymple, G.B., and Lanphere, M.A., 1969, Potassium-argon dating: San Francisco, W.H. Freeman and Company, 258 p. Doe, B.R., Tilling, R .I., Hedge, C.E., and Klepper, M.R., 1968, Lead and strontium isotope studies of the Boulder batholith, southwestern Montana: Economic Geology, Vol. 63, p. 884-906. Eimon, P .I., and Anctil, R.J., 1981, Epithermal precious-metal deposits Northwest Mining Association Convention, December, 1981, Spokane, Washington, 17 p. Hadley, J.B., 1969, Geologic map of the Varney quadrangle, Madison County, Montana: United States Geological Survey Map GQ-814, I :62,500 scale. Johns, W.M., 1961, Geology and ore deposits of the southern Tidal Wave mining d is tr ic t, Madison County, Montana: Montana Bureau of Mines and Geology B ulletin 24, 53 p. Krauskopf, K.B., 1979, Introduction to geochemistry (2nd ed.): New York, McGraw-Hill, 617 p. 61 Lindgrens W., 1933., Mineral deposits (4th ed„): New York, McGraw- H ill , 930 p. ’ Lorain, S.H., 1937, Gold lode mines of the Tobacco Root Mountains, Madison County, Montana: United States Bureau of Mines Information C ircular no. 6972, 74 p. Marvin, R.F., and Dobson, S.W., 1979, Radiometric ages: Compilation B, United States Geological Survey: Isochron/West, no. 26, p. 3-32. M ueller, P .A ., and Cordua, W.S., 1976, Rb-Sr whole rock age of gneisses from the Horse Creek, area, Tobacco Root Mountains, Montana: Isochron/West, no. 16, p. 33-36. Ramdohr, P., 1969, The ore minerals and their intergrowths: Oxford, Pergamon Press, 1175 p. Ross, C .P ., Andrews, D. A ., and W itkind, I. J . (com pilers), 1955, Geologic map of Montana: United States Geological Survey, 1:500,000 scale. Schaeffer, O.A., and Zahringer, J. (compilers), 1966, Potassium- argon dating: New York, Springer-Verlag, 234 p. Smith, J.L., 1970, Petrology, mineralogy and chemistry of the Tobacco Root batholith, Madison County, Montana (abstract): Disser tatio n Abstract, Vol. 31, p. 5429B. Stanton, R.L., 1972, Ore petrology: New York, McGraw-Hill, 713 p. Tansley, W., Schafer, P.A., and Hart, L.H., 1933, A geological reconaissance of the Tobacco Root Mountains, Madison County, Montana: Montana Bureau of Mines and Geology Memoir 9, 55 p. Taylor, H.P., 1973, 0 ^ /0 ^ evidence for meteoric-hydrothermal a lte ra tio n and ore deposition in the Tonopah, Comstock Lode, , and Goldfield mining districts, Nevada: Economic Geology, Vol. 68, p. 747-764. Tilling, R.J., Gottfried, D., and Rowe, J.J., 1973, Gold abundance in igneous rocks: Bearing on gold m ineralization: Economic Geology, Vol. 68, p. 168-186. V ita liano, C .J ., Kish, S ., and Towel I , D.G., 1980, Potassium-Argon dates and strontium isotopic values for rocks of the Tobacco Root b a th o lith , southwestern Montana: Isochrbn/West, no. 28, p. 13-15. 62 V italIano9 C.J ., and Cordua, W.S., (compilers), 1979, Geologic map of the southern Tobacco Root Mountains, Madison County, Montana Geological Society of America map and chart series'MC-31., 1:62,500 scale. Weir, K., 1982, Maps showing geology and outcrops of part of the Virginia City and Alder quadrangles, Madison County, Montana: United States Geological Survey map MF-1490, 1:12,000 and I :4750 scales. Whitney, J.A., and Stormer, J.C. dr., 1983, Igneous sulfides in the Fish Canyon Tuff and the role of sulfur in calcalkaline magma: Geology, Vol. 11, no. 2, p. 99-102. Winchell, A.N., 1914, Mining districts of the Dillon quadrangle, Montana, and adjacent areas: United States Geological Survey Bulletin 574. Wooden, J.L., V italiano, C.J., Koehler, S.W., and Ragland, P.C., 1978, The late Precambrian mafic dikes of the southern Tobacco Root Mountains, Montana: geochemistry, Rb-Sr geochronology and relationship to Belt tectonics: Canadian Journal of Earth Sciences, Vol. 15, no. 4, p. 467-479. APPENDICES 64 APPENDIX A RADIOMETRIC DATES (K-Ar TECHNIQUE)■ 40* SAMPLE K40, ppm Ar , ppm Ar40*/K 40 CONSTANTS AGE (m.y.) *** B io tite from 9.507 1.366 0.1437 Xe, - 4.72x10"10/yr 1572 ± 51 unaltered X e = 0.585x 10- l0 /y r Powder Mag pegmatite, K40ZK = 1.22xlO-4 g.Zg. 3-level U.S. Grant Mine *** Microcline 13.797 0.2729 0.01978 X b = 4.72x10-10Zyr 311 ± 11 . (with minor Xe = 0.585x10-10Zyr amount of Precambrian K40ZK = 1.22xlO-4g.Zg. feldspar and p erth ite) from altered wall rock, 3- level U.S. Grant Mine ** Andesite 2.567 0.0079 0.00307 X6 = 4.692x10-10Zyr 51.1 ± 1.2 plug, El . Xe= 0.581 xlO-10Zyr . Fleeda Mine (whole rock) K40ZK = 1.167xlO-4g.Zg. 40 *Ar refers to radiogenic argon **Data from M afvin and Dob son (1979) ***Data from Geochron Lab, Cambridge, Mas sachusetts 66 APPENDIX B GENERAL DATA ON SOME TOBACCO ROOT PRECIOUS-METAL DEPOSITS 67 DISTRICT MINE ATTITUDE HOST DEPOSIT MORPHOLOGY REF. PONY Mammoth E-W;60S PCgn QVS;LNS I Strawber E-W;50- PCgn QVS;TB I ry/Key 60N stone Boss Tweed- N60W; PCgn Rplcm-diss I C lipper 40SW Garnet N60E; 40-60SE QM QVS;TB I A tlan tic E-W; QM-K Rplcm- I & Pacific Steep PCgn diss NORRIS Galena N40E; QM QVS;STR 1,2 30NW Rosebud NE; QM QVS;TB I Steep Revenue Varies QM QVS;LNS 1,2 Group Bull Moose NS;25W QM QVS;TB I Boaz N40W;70NE PCgn QVS I Josephine N40W;50NE PCgh QVS I Montana N25-40E; PCgn QVS;LNS,STR I Boy 35NW I Madisonian N45E;.40NW QM QVS;LNS I Norwegian N20,40E; QM QVS;TB I v e rt. Betty Mae N45E; PCgn QVS;LNS,STR I 45-60NW Washing New Deal/ N60-70W; PCgn QVS;TB I ton sub- Heater 35-40SW d is tr ic t Missouri- F la t- PCgn-K QVS;TB ■ 1,2 McKee lying AS VIRGINIA Prospect N45W;75NE PCgn QVS;LNS I CITY Alameda N70E;50NW PCgn QVS;TB I U.S.Grant N40-50E PCgn QVS;LNS,STR1TB this 35-50NW paper El Fleeda N40-50E PCgn QVS;LNS,TB this 30-50NW paper St. Law N65E; PCgn QVS;TB this rence 50NW paper Winnetka N76E;50S PCgn QVS;LNS I Silver Bell N50E;45NW PCgn QVS 3 Black Rock . N50E; PCgn QVS;TB this 20-30NW paper Easton- N55W; QM QVS;TB I P acific 70NE Fork N35E;20SE PCgn QVS;TB,LNS I M arietta N45E; PCgn QVS;LNS,TB I 35-45SE 68 DISTRICT MINE ' ATTITUDE HOST. DEPOSIT MORPHOLOGY REF. SHERIDAN Lake Shore/ N30E; PCgn QVS 1,2 Gladstone 80NW A gitator N10E-50W PCm QVS;LNS I Broadguag&r N30E;45NW PCm Rplcm-diss I Tamarack. E-W;35NW Red Pine N20E;50NW PCm-K QVS;LNS I PCgn Lucky : E-W; PCgn? QVS I S trike 45-60N Fairview PCm P1STR1Rplcm I TIDAL Highridge N30E; PCgn-K QVS I WAVE 70-80SE SH Corn- N30E;70SE PCgn. QVS I cracker Mountain I PZLS QVS I View Hawkeye E-W; PZLS Rpl cm I 20-25N Pete and N35E QM QVS;TB I Joe 40-50NW Strawn Varies PZLS Rplcm I RENOVA Mayflower PZLS P,diss 1,2 KEY D PCgn = Precambrian gneiss 2) PCm = Precambrian marble • 3) PZLS = Paleozoic Limestone 4) SH = Paleozoic Shale 5) QM = Late Cretaceous intrusive rocks 6) AS = Andesite s ill 7) -K = deposit occurs in contact between the two rock types indicated Deposit Morphology: I ) QVS = quartz vein system/structure 2 ) LNS = quartz lens (es) 3) STR = quartz stringers 4) TB = tabular quartz bodies 5) Rplcm = replacement deposit 6 ) diss = disseminated deposit 7) P = pipe-shape deposit References: I = Lorain (1937) 2 = Tansley and others (1933) 3 = Boyer (personal comm.) 69 APPENDIX C ORE AND GANGUE MINERALS OF TOBACCO ROOT BASE- AND PRECIOUS-METAL DEPOSITS 70 MINERAL DISTRICT VIR,. CITY PONY SHERDN. TDLWV. NORRIS. RENOVA Argentite X X X iArseno- pyrite X X Azurite X X Barite X Bornite XXX C alcite X X X X X X Cerargy- r ite X Cerussite X X Chalcan- th ite ' Chalcocite X X Chal copy- r it e X X X X X X C hlorite X X Chryso- colla X X X Copper X Cuprite X X X Dolomite X X X Flourite X Galena X X X X X X Gold X X X X X X Hematite X X X X X X . Huebnerite . X Limonite X X X X X X . Magnetite X . X X Malachite X X X X X X Melaconite X X Melanterite X Microcline X Molybden ite X Nagyagite X Pyrargyrite % X Pyrite X X X X X X Pyrolusite X X X X X X Quartz X X X X X X S id erite X X S ilv er X X X Sphalerite X X X X X X S tib n ite X Sylvanite X X Tetrahedrite X X X X . Modified from Winchell (1914) 71 APPENDIX D LODE PRODUCTION FIGURES FOR TOBACCO ROOT MINING DISTRICTS (1901-1935) GOLD SILVER DISTRICT TONNAGE. T t l .oz oz/ton T t l .oz oz/ton Cu/Ag Ag/Au Cu (lb ) Pb (lb ) Zn (lb ) PONY 405,775 116,274 0.29 193,209 0.48 6.79 1.66 1,311,137 292,600 NR VIRGINIA 162,501 67,719 0.42 I ,107,064 6.81 0.05 16.34 55,449 . 178,018 NR CITY SHERIDAN 66,633 25,840 0.39 184,457 2.77 0.82 7.14 151,661 1,445,503 294,452 TIDAL 47,376 30,432 0.64 143,000 3.02 1.05 4.70 150,109 2,801,820 21,873 WAVE NORRIS* 56,200 54,716 0.97 124,345 2.21 0.48 2.27 60,162 154,368 NR ■ >70% ** of pro duction from oxidized ore RENOVA 15,227 16,674 1.10 75,628 4.97 1.40 4.54 106,202 1,005,792 NR REGIONAL TOTAL 753,712 311,655 ' 0.41 I ,699,003 2.25 1.08 5.45 1,834,720 5,878,101 316,325 Tonnage of each district is a sum (of variable proportion) of mining and direct shipping (usually oxidized) ores. NR = no production reported . *Includes Washington s u b -d is trict **This Cu/Ag is lower than expected probably because >70% of district production is from oxidized ores that do not express primary metal content. The Boaz mine production is exclusively from primary ore and Cu/Ag = 3.96; Ag/Au = 0.97. Both ratios are congruent with the regional zoning pattern. Data from Lorain (1937) 73 APPENDIX E LODE PRODUCTION FIGURES FOR SELECTED MINES OF THE TOBACCO ROOT REGION (1901-1935) MINE TONNAGE Au oz Ag oz Au/ton Ag/ton Ag/Au Cu lb . Cu/Ag Mammoth 214,149 53,377 113,938 0.25 0.53 2.13 1,008,111 8.84 (PONY) MILL (1905-1935) PRMY . . Boss 171,415 44,223 49,748 0.26 0.29 1.12 57,090 1.14 Tweed-Cl ipper MILL PRMY (PONY) (1904-1935) Easton-Pacific 70,049 22,091 759,148 0.32 10.84 34.4 NR (v.c.) MILL PRMY (1902-1935) OXDZD Revenue Group 26,095 15,842 16,878 0.61 0.64 1.07 NR (NORRIS) MILL (1901-1935) OXDZD Missouri-McKee 14,340 16,664 41,958 1 .16 2.92 2.52 NR (NORRIS-Wash. MILL •"-J subdstr.) OXDZD 4^ (1905-1935) Broadgauge- 8,976 5,055 380 0.56 0.04 0.07 NR Tamarack MILL (SHRDN) OXDZD (1908-1935) Lakeshore- 4,317 2,436 7,288 0.56 1.68 3.0 ' 7,154 0.98 Gladstone MILL (SHRDN) PRMY (1910-1929) Boaz 1,959 3,752 3,641 1 .92 1.85 0.97 . 14,425 3.96 (NORRIS) MILL (1902-1935) PRMY From Lorain (1937) MILL = production from milling (low-grade) ores NR = no production reported OXDZD = production from oxidized ores (grade not implied) PRMY = production from primary Numbers in parentheses show years of production (hypogene) sulfide ore APPENDIX F LODE PRODUCTION FIGURES FOR SELECTED MINES OF THE VIRGINIA CITY DISTRICT (1901-1935) 76 MINE TONNAGE Au oz Au/tbn Ag oz .. Ag/Au Easton- 70,049 22,091 0.32 759,148 34.4 P acific MILL (1902- PRMY/OXDZD 1935) U.S. 1,764 834 0.50 46,393 55.6 Grant SHIP (1908- - PRMY . 1926) Alameda . ? ? 1.64 ? 20.6 (1906- SHIP (33.9/ton) 1915) OXDZD Winnetka 3,322 3,205 0.96 4,747 1.49 (1909- ' SHIP 1932) OXDZD 236 0.27 ? ? MILL 811 PRMY M arietta I ,000- ? ? (1936) 1,300 tons/ 0.27 (1 .11/to n ) 4.11 month (1936) MILL PRMY DISTRICT 162,501 67,719 0.42 . 1 ,107,064 16.34 TOTAL From Lorain (1937) MILL = milling ores (low-grade) SHIP = shipping ores (high-grade) OXDZD = oxidized ores (no grade implied), production dominantly from PRMY = primary hypogene (sulfide) ores, production dominantly from Nevada City Virginia City BProspect Location of sample shown in figure 6 U . Sx-JsG rant H AIameda V y /B Cornucopi St. Lawrence Black Rock Bell Fork/ EastonK Vs W1 Pacific V K ear sage Plate I BEDROCK MAP OF THE VIRGINIA CITY MINING DISTRICT Scale ______ONE MILE Madison County, Montana 1:24,000 ONE KILOMETER Based on field work by M. Cole and data from Weir ( 1 98 2), Vitaliano and others (I 979), Hadley ( 1 96 9), and Tansley and others ( 1 933). Contour interval = 400 feet LEGEND / Tertiary volcanic rocks on Early Tertiary stream gravels. contact Includes some Quaternary gravels in lower Alder Gulch. contact, approximately located Tertiary volcanic plug fault Late Cretaceous granitic Browns Gulch Stock fault, approximately located normal fault, approximately located, ball on downthrown block Mississippian Mission Canyon Limestone thrust fault, teeth on upthrown block inferred overturned antiform axis, arrows show dip direction of limbs r: Cambrian through Mississippian sedimentary rocks t------shear zone Precambrian quartz-feldspar-mica pegmatite strike and dip of foliation (compositional layering) vertical foliation Precambrian meta-dolomite. Dots represent probable stratigraphic continuity. shaft Precambrian metamorphic rocks. Dominantly quartz-feldspar portal gneiss with some hornblende-biotite gneiss. Also minor gar- netiferous gneiss, quartzite, and meta-ultramafic intrusives. mine, caved stream paved road /y i 5 Cf /L' 4) r,' r j PLATE I GEOLOGY IN VICINITY OF THE U.S. GRANT MINE Virginia City Mining District, Madison County, Montana PLATE 2 Based on field work by M. Cole, a base map from R & D Minerals, Virginia City, and data from Weir ( 1 982) \ LEGEND <7 <3 Tertiary volcanic flows on Early Tertiary gravels <1 q a Tv Tertiary volcanic plug Precambrian pegmatite Cornucopia sh Precambrian meta-dolomite Precambrian gneiss, dominantly quartzofeldspathic Ti strike and dip of foliation (compositional layering) vertical foliation contact with dip J--- contact with dip, approximately located SI X jlX normal fault, ball on downthrown block approximate surface outcrop of vein system with dip vertical projection of mine workings portal a shaft st ream Contour Interval = 200 feet 4 X T SCALE 1:4750 I______I200 meters PLATE 2 1 2 3 4 5 6 7 8 9 PLAN MAP OF THE U.S. GRANT MINE NO. 3 LEVEL LOCATION OF SAMPLE SHOWN IN FIGURE 9 VIRGINIA CITY MINING DISTRICT MADISON COUNTY, MONTANA PLATE 3 SHEET 1 OF 2 VS-2c: elong qtz In 60 cm thick, several 2 .5 -5 .0 cm thick qtz strs, crushed and altered W R , several suit seams 2.5-5.0 cm thick, occasional suit pockets. G N E IS S : UNALTERED OR SLIGHTLY ALTERED , NOT STRIKE AND DIP OF FOLIATION (COMPOSITIONAL LAYERING) X / 7 Z V CRUSHED. HBgnrHORNBLENDE-PLAHBgn: PLAGIO- CLASE-BlOTlTE HORIZONTAL FOLIATION XX XAXX G N E IS S : ALTERED AND CRUSHED X X % XX BEARING AND PLUNGE OF LINEATION (SMALL ISOCLINAL FOLD AXES) vj V Xxx-XX 'I V J GNEISS: ALTERED, CRUSHED, AND OXIDIZED X- X-X-X FOLIATION WITH LINEATION PORTAL qtz: QUARTZ BASALT BRECCIA: CONTAINS TERTIARY VOLCANIC AND v v V V V PRECAMBRIAN METAMORPHIC CLASTS In: LENS 1 CM TO 1 M ACROSS, ALL STRONGLY OXIDIZED Ins: LENSES strs: STRINGERS PEGMATITE DIKE elong: ELONGATE suit: SULFIDE MINERALS ( d o m i n a n t l y p y r i t e ) APLITE DIKE WR: WALLROCK < ST: STORE # * « % RANDOMLY ORIENTED PEGMATITE BLOCKS R S: RAISE FFFF PEGMATITE FELDSPAR IN VEIN STRUCTURE I, TL, HI, etc.: LOCATIONS REFERRED TO IN TEXT VEIN STRUCTURE WITH DIP T u SCALE: 1 CM = 2.4 M INFERRED VEIN STRUCTURE T i - I------1 2.4 METERS FAULT WITH DIP I------1 6.0 METERS BASE MAP COURTESY OF R & D MINERALS, VIRGINIA CITY, MONTANA ~LL1 Vaol IJ~ LITHOLOGIC CONTACT WITH DIP GEOLOGY MODIFIED BY M. COLE ALL PLANAR FEATURES ARE PROJECTED AT WAIST LEVEL c 6 7 X & X * %X % F F F F .-Mfc T 75 V S - 2 e t thick :b cmI q r 90 t z PGAIE DIKE PEGMATITE P V L PAA FAUE AE RJCE A WIT LEVEL WAIST AT PROJECTED ARE FEATURES PLANAR ALL GNEISS: GNEISS: EN TUTR WT DIP WITH STRUCTURE VEIN GNEISS: ADMY RETD EMTT BLOCKS PEGMATITE ORIENTED RANDOMLY ETCL FAULT VERTICAL STRUCTURE VEIN INFERRED STRIKE AND DIP OF FOLIATION FOLIATION OF DIP AND STRIKE DIP WITH FAULT STRUCTURE VEIN IN FELDSPAR PEGMATITE OITO WT BAIG N PUG O LINEATION OF PLUNGE AND BEARING WITH FOLIATION some suit pockets. LEE AD CRUSHED AND ALTERED NLEE O SIHL ATRD NT CRUSHED. NOT ALTERED, SLIGHTLY OR UNALTERED HORNBLENDE- OCLASE- BI TE IT T IO -B T E N R A -G E S A L C IO G A L -P E TITE. D IO N E L E-B S B N LA R C O IO G :H n g G E-PLA D B N H LE B N R O gn:H B H S L IO IAL OD AXES) FOLD L LINA ISOC ALL (SM C OSTONA LAYERING) AL N SITIO PO M (CO 17 D N E G ln: ELONGATE elong: ts STRINGERS strs: t b I r TABULAR : suit: SULFIDE MINERALS MINERALS SULFIDE suit: t: QUARTZ qtz: R W ALLROCK WR: n: LENSES Ins: S RAISE RS: T STORE ST: n LENS In: AE A CUTS O R D IEAS VRII CT, MONTANA CITY, VIRGINIA MINERALS, D & R OF COURTESY MAP BASE I, TL, ec: OAIN RFRE T I TEXT IN TO REFERRED LOCATIONS etc.: , m CL: C = . M 2.4 = CM I SCALE: ELG MDFE B M COLE M. BY MODIFIED GEOLOGY ( y l t n a n i m o d J . METERS 2.4 . METERS 6.0 e t i r y p ) /L/ 3 7ST f k 7 $ ^ PLAN MAP OF THE I z I EL FLEEDA MINE / / / / I ST / Portal fault with dip; dashed where inferred ' 75 SCALE 1:240 vertical fault 6 meters contact with dip ALL PLANAR FEATURES PROJECTED AT WAIST LEVEL INCLINED WORKINGS ARE DEPICTED BY DASHED LINES Base map courtesy of R & D Minerals, Virginia City Geology modified by M. Cole MONTANA STATE UNIVERSITY LIBRARIES