Advanced argillic and sericitic alteration in the Buckskin Range, Nevada : A product of ascending magmatic fluids from the deeper yerington porphyry copper environment Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/t435gh581

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  • Jurassic rocks exposed in the Buckskin Range of the Yerington district, represent the upper 1 to 2 km of a large magmatic-hydrothermal system with porphyry copper deposits at 1-4 km depth. These rocks include, from deep to shallow, the Yerington batholith, the Artesia Lake Volcanics (intruded by the batholith), and the Fulstone Spring Volcanics. Hydrothermal minerals characteristic of advanced argillic, sericitic, and chlorite-bearing alteration assemblages have been detected in the Buckskin Range via PIMA infrared spectroscopy and hand-lens, together with selected X-ray diffraction, petrography, and microprobe analysis. A map of hydrothermal alteration assemblages covering a 2 km2 area was constructed mainly by PIMA identification of minerals on over 700 geographically located samples by comparison with mineral standards. Field relations suggest the high-level advanced argillic and sericitic alteration in the Buckskin Range is broadly contemporaneous and transitional downward into sericitic alteration and pyrite deposition in the porphyry copper environment. The spatial and temporal relationships of hydrothermal assemblages and veins in the Buckskin Range suggest a temporal evolution from very acidic, sulfide- and sulfate- rich fluids to weakly acidic, sulfur-poor hydrothermal fluids. Early acidic fluids produced feldspar-destructive, pyrite-rich assemblages containing quartz + alunite + pyrophyllite ± kaolinite ± dickite, or quartz + muscovite, and are restricted to the Artesia Lake Volcanics. These assemblages are cross-cut and overlain by porphyry dikes and the basal dacite flow of the Fulstone Spring Volcanics that are affected by sericite + hematite + chlorite alteration. A second fluid, possibly derived from circulating sedimentary brines responsible for Na-Ca alteration in the deeper porphyry copper environment, is responsible for feldspar-stable (albite and orthoclase) assemblages with calcite + chlorite + hematite in the overlying Fulstone Spring lavas. It is currently thought that these fluids exited the system along steeply-dipping, east-west-striking Jurassic faults that cut across advanced argillic and sericitic alteration and the lowest lavas of the Fulstone Spring Volcanics to produce hematite-magnetite-Au-Cu vein mineralization. Hydrogen and oxygen isotope compositions of muscovite (δD = -56 ⁰/₀₀, δ¹⁸O = 11.8 ⁰/₀₀) and pyrophyllite (δD = -59 ⁰/₀₀, δ¹⁸O = 9.7 ⁰/₀₀) are calculated to be in equilibrium at 350°C and 300°C with waters of δD = -24 and -25 ⁰/₀₀, and δ¹⁸O = 10.4 and 4.3 ⁰/₀₀, respectively. Sulfur isotope data give δ³⁴S values of 8.7-8.8 per mil for alunite (n=2) and - 4.7 per mil (n=1) for pyrite. In thin section, alunite typically occurs with pyrophyllite as 20 to 150-μm-long tabular grains in 10 to 50-μm-diameter quartz. These isotopic and petrographic data suggest that pyrophyllite and coarse-grained alunite has likely formed at 250°-300°C by the condensation of acidic hydrothermal fluids derived from magmatic vapors.
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