Tertiary Volcanic Rocks, Hydrothermal Alteration, and Epithermal Precious Metal Deposits of the Patterson Mining District, Sweetwater Mountains, Mono County, California and Lyon County, Nevada
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The Sweetwater Mountains (Mono County, California and Lyon County, Nevada) contain a basement complex of felsic plutons of the Sierra Nevada batholith and overlying roof pendants and septa of metasedimentary and metavolcanic rocks that are unconformably overlain by Miocene-Pliocene volcanic rocks and Tertiary-Recent surficial deposits. The mountain range is situated in the Walker Lane, a zone of transtension and shear deformation that accommodates the transition of the Pacific-North American plate margin from a convergent margin to the north to a strike-slip margin to the south. Volcanic rocks of the Sweetwater Mountains belong to the western andesite assemblage, a subduction-related continental margin arc that erupted mostly within the Walker Lane in the southern part of the arc, forming the ancestral Cascade Mountains. The western andesite assemblage hosts both low-sulfidation and high-sulfidation epithermal mineral deposits related to ancestral Cascades volcanic activity.The Patterson mining district in the Sweetwater Mountains produced 4.5 Moz. of silver, 3,000 oz. of gold, and 35,000 lb. of copper during the 1880s-1940s from low-sulfidation epithermal silver-gold deposits that occur in veins and silicified breccias, mostly in Tertiary volcanic rocks. Goals of this study included producing a geologic map of the district and determining time-space relationships between volcanism and hydrothermal activity. The spatial and temporal relationships between Tertiary volcanism and hydrothermal mineral assemblages in alteration zones and precious metal deposits in the Patterson district were established by determining stratigraphy, structure, mineralogy, and ages of Tertiary volcanic rocks, silver-gold deposits, and zones of hydrothermal alteration. A geologic map and map of hydrothermal mineral assemblages of the district were produced. Tertiary volcanic rocks are high-K calc-alkaline and include small volume andesite flows, more voluminous dacite and rhyolite flows and domes, and associated tuffs, mudflows, volcaniclastic breccias, and block and ash flow deposits. Rhyolite and dacite units of the Sweetwater volcanic field, a late Miocene-early Pliocene felsic eruptive center, are ~6.6-5.3 Ma, based on dates of igneous minerals. Precious metal deposits and alteration zones formed during or after the later stages of felsic volcanism, based on dates of adularia and alunite that are ~5.6-5.1 Ma. Altered rocks and mineral deposits form three north-to-northeast trending zones which are interpreted to be high angle fault zones. Groups of precious metal deposits that form in these north-trending zones were named the Cameron belt, the Clinton belt, and the California Comstock belt by miners in the 1800s. These mineral belts exhibit different deposit styles, silver to gold ratios, geochemical signatures, mineralogy, and structure. Deposits of the Cameron belt contain the highest gold grades in the district in disseminated mineralized zones. Deposits consist of chalcedonic quartz, minor adularia and calcite, pyrite, and minor acanthite, chalcopyrite, galena, molybdenite, and sphalerite. Pyrite often contains micron to sub-micron scale inclusions of mostly acanthite and galena, and minor sphalerite and molybdenite. Multiple partly eroded sinter terraces, and a geochemical signature which includes high concentrations of arsenic and antimony, support near surface formation of these deposits and association with hot springs. The Clinton belt deposits exhibit high concentrations of silver in zones of stockwork veins. Mineralized rocks are spatially associated with a rhyolite porphyry dike and contain quartz, adularia, acanthite, hessite (Ag2Te), base metal sulfides, chlorargyrite, and iron tellurite minerals. The geochemical signature includes high concentrations of copper, molybdenum, lead, tellurium, and zinc. Deposits of the California Comstock belt consist of stockwork vein zones and 1-3 m thick veins which contain pseudomorphs of quartz after bladed calcite. Vein samples contain quartz, adularia, bladed calcite, electrum, acanthite with up to 4 weight percent selenium, and minor hessite, chalcopyrite, and galena, and exhibit high concentrations of mercury and selenium. Volcanic rocks and silver-gold deposits of the Patterson mining district belong to a small suite of ~7-5 Ma volcanic fields and low-sulfidation epithermal deposits in the Walker Lane. Volcanic rocks and low-sulfidation epithermal deposits of similar age to those in the Patterson district occur in the Como, Gilbert, Pine Nut, and Silver Peak districts. All of these volcanic rocks and precious metal deposits formed south of the southern edge of the subducting Farallon plate. In addition to the Patterson district, telluride minerals occur in other Walker Lane reported in precious metal deposits including veins in the Bodie, Bullfrog, Olingshouse, and Wahmonie districts. Future work in the Patterson district could include more detailed structural mapping, dates of emplacement of deposits of the Cameron and California Comstock belts, Re-Os dating of molybdenite in a proposed porphyry Mo system, identification of gold minerals in the Cameron belt, and determination of δD and δ18O values of gangue minerals in the Clinton belt deposits to determine if there was magmatic input in the system. Future work in the Walker Lane could include a more thorough investigation of low-sulfidation epithermal deposits which contain telluride minerals. Identifying the tectonic controls on telluride-bearing precious metal deposits in the western andesite assemblage could help understanding of Au-Ag-Te deposits worldwide.