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Majuba Hill) Mining District

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Majuba Hill) Mining District The Geology and Ore Deposits V of the Antelope (Majuba Hill) Mining District Pershing County, Nevada A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology by David Lee Stevens i • « June 1971 m in e s *-,B R A R y The thesis of David Lee Stevens is approved: a C.ZL. _______________i ry(. Thesis adviso^A University of Nevada Reno June 1971 CONTENTS Page Abstract ................................................ v INTRODUCTION ............................................ 2 Location and Access ...................... .. .... 2 Previous W o r k ....................................... 2 Purpose of Thesis .................................. 3 Acknowledgements..................................... 4 History and Production............................... 4 GEOLOGY....................................................7 General Statement .................................. 7 Regional Setting............ 8 STRATIGRAPHY........................................ 11 Triassic System(?) ............................ 11 Grass Valley (?) Formation...................... 11 Paleogeographic Implication.................... 15 IGNEOUS ROCKS .......................................19 Biotite Diorite (Kd) ................ =....19 Granodiorite (Kgm) ............................ 20 Andesite (Tha) ................................ 24 Older Rhyolite (Tr^) .......................... 25 Younger Rhyolite (Tr2) ........................ 26 Aplitic Rhyolite (Ta).......................... 29 Alkali-Olivine Basalt.......................... 31 UNCONSOLIDATED DEPOSITS...................................32 Ta l u s ...............................................32 Older Alluvium........................ 32 ii Younger Alluvium .................................... 32 STRUCTURE................................................. 34 Deformation of the Sedimentary Rocks ................ 34 Intrusion of the Granodiorite........................ 35 Intrusion of the Majuba Hill Plug, General Statement .36 Proposed Mechanism of Emplacement.................... 37 The Majuba F a u l t .....................................46 The Last Chance Fault.................................49 Faults and Photolinears of the District...............49 GEOLOGIC HISTORY..........................................53 ALTERATION.................................................55 Contact Metamorphism in the Sedimentary R o c k .........55 Propylitization of the Granodiorite...................55 Silification, Sericitization, and Tourmalinization of the Rhyolite..................................56 ORE DEPOSITS.............................................. 59 Arsenic Prospect.'....................................59 Last Chance M i n e .....................................60 Tin Mineralization, General Statement.................64 Majuba Hill M i n e .....................................68 Supergene Mineralization ............................ 78 Discussion of the Ore Deposits.......................80 APPENDIX.................................................. 82 REFERENCES................................................. 85 ILLUSTRATIONS Plate Page 1. Geologic Map of the Antelope Mining District........ In pocket 2. Interpretative Geologic Map of Majuba Hill.......... In pocket 3. Isometric Block Diagram of Majuba Hill............ In pocket 4. Geologic Map of the Middle Adit, Majuba Hill Mine . In pocket 5. Geologic Map of the Lower Adit, Majuba Hill Mine. In pocket 6. Geologic Map of the Last Chance Mine............... In pocket Figure 1. Location map of the Antelope Mining District................ 1 2. Photograph of linguoid current marks........................ 14 3. Photomicrograph of "ftgva (argillite).......................... 14 4. Photomicrograph of "Rgvq (quartzite) . ... ............. .16 5. Photomicrograph of monzonite porphyry ...................... 16 6. Photomicrograph of biotite monzonite porphyry .............. 22 7. Photomicrograph of rhyolite porphyry.........................22 8. Photomicrograph of rhyolite porphyry........................ 28 9. Photomicrograph of aplitic rhyolite.........................28 10. Photomicrograph of aplitic rhyolite.........................30 11. Photomicrograph of alkali-olivine basalt.....................30 12. Photograph of Majuba Hill looking northwest ................ 39 13. Photomicrograph of explosive breccia.........................42 14. Photomicrograph of breccia fragment.........................42 15. Joint rose, Majuba Hill intrusive ...........................43 iv Figure Page 16. Equal area contour, joint poles to lower hemisphere, Majuba Hill intrusive...................................... 44 17. Photolinear and fault rose, Antelope Mining district....... 50 18. Photomicrograph of arsenic ore, arsenic prospect........... 61 19. Photograph of Majuba Hill looking north .................... 69 20. Photograph of rhyolite dike contact with argillite......... 71 21. Photograph of tourmaline breccia........................... 71 22. Photomicrograph of cassiterite, Majuba Hill Mine............ 73 23. Photomicrograph of copper ore, Majuba Hill M i n e ............ 75 24. Photomicrograph of copper ore, Majuba Hill M i n e ............ 75 25. Photomicrograph of copper ore, Majuba Hill M i n e ............ 77 V ABSTRACT The Antelope (Majuba Hill) mining district is credited with 1.6 million dollars of production between 1905 and 1936. The Majuba Hill mine, in the center of the district, was the major producer with nearly 3 million pounds of copper. The presence of tin mineralization in the Majuba Hill mine makes the district anomalous among mining districts in the western United States. Sedimentary rocks of the district consist of argillites and quartzites of the Triassic "Grass Valley Formation". Early Cretaceous compressional deformation, and perhaps thrusting, produced the north­ easterly trending fabric of the beds and folds. Emplacement of inter­ mediate igneous rocks as large concordant intrusions and sill swarms during the mid-Cretaceous followed the main stage of deformation. During the Tertiary, a large explosive vent developed in response to violent volatile release from a buried magma chamber. Following volatile release, the upper siliceous portions of the magma chamber ascended the explosive vent. Upon reaching the surface, the viscous rhyolite magma welled into a large fan shaped extrusive dome. Sulfide mineralization in the throat of the vent was localized in a tensional fault produced by subsidence of the magma column. Mineralization in the district, outside of the Majuba Hill plug, appears unrelated to the rhyolite intrusion and may represent an earlier hydrothermal phase from the same parent magma. The interior of the Majuba Hill rhyolite plug is intensely altered to quartz, sericite, and tourmaline. Alteration is most intense where localized in intrusive breccias. The presence of high tempera­ vi ture ore and gangue minerals in a low pressure, near surface environ­ ment classifies the mineralization associated with the Majuba Hill plug as xenothermal. Mineralogically and genetically the Antelope mining district has many similarities with the tin districts of central Bolivia. 2 INTRODUCTION Location and Access The Antelope (Majuba Hill) mining district lies within township 32 and 33 north, range 30 and 31 east, M. D. B. M., in Pershing County, Nevada (Figure 1). Majuba Hill, which occupies the central portion of the district, is 34 miles due north of Lovelock and 18 miles west of Mill City. Access to the area is provided by county-maintained graded roads from Mill City to Sulphur, a mining district and siding on the Western Pacific Railroad. An unimproved road turns to the southwest to Majuba Hill 14 miles from the railroad crossing at Mill City. With the exception of the precipitous slopes of Majuba Hill, the area can be traversed by a four-wheel drive vehicle. Previous Work Knopf (1917) made the first published reference to the district approximately ten years after its discovery. Vanderburg (1936), in his Pershing County reconnaissance, briefly describes the geology of the copper-tin occurrence at Majuba Hill. The Geological Survey, in order to obtain more detailed information on the district, examined it in the spring of 1939 (Smith and Gianella, 1940), under an allotment of Public Works funds. Geologic Maps of Majuba Hill and of the mine workings were prepared by Ward Smith of the U. S. Geological Survey, and petrographic and mineralogic studies were made by V. P. Gianella of the Nevada Bureau of Mines. The area was again studied by the U. S. Geological Survey (Trites and Thurston, 1942) with emphasis directed 3 toward the minor uranium occurrence exposed in the middle adit of the Majuba Hill mine. The most recent work in the area is of Tatlock (1969), the 1:200,000 preliminary geologic map prepared in cooperation with the Nevada Bureau of Mines and published by the U. S. Geological Survey in the open file. The area was mapped in reconnaissance (1:24,000) in 1958 by the Southern Pacific Mineral Survey as an evalu­ ation of the mineral potential of land owned by the Southern Pacific Railroad Company. Purpose of Thesis The Antelope district presents an opportunity to study a class of ore deposit formed by relatively high temperature and low pressures. It is also unusual as one of the few hydrothermal tin occurrences in the western United States. The domestic shortage of tin invites a careful evaluation of all occurrences to define the environment and nature of the mineralization. Previous
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