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Nature, Age, and Genesis of Quartz-Sulfide-Precious-Metal Vein

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Nature, Age, and Genesis of Quartz-Sulfide-Precious-Metal Vein 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&deg,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 <S- Date Head, Major Department Approved for the College of Graduate Studies 'll&K-- IW I 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
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