ADVANCED ARGILLIC AND SERICITIC ALTERATION IN THE BUCKSKIN RANGE, NEVADA: A PRODUCT OF ASCENDING MAGMATIC FLUIDS FROM THE DEEPER YERINGTON PORPHYRY COPPER ENVIRONMENT

Jurassic rocks exposed in the Buckskin Range, ~4 km west of Yerington, NV, represent the upper 1 to 2 km of the Yerington porphyry copper system and include, from deep to shallow, the Yerington batholith, the Artesia Lake Volcanics, and the Fulstone Spring Volcanics. Hydrothermal minerals characteristic of advanced argillic, sericitic, and chlorite-bearing alteration assemblages have been detected via infrared spectrometry on 700+ samples, X-ray diffraction, petrography, microprobe analysis, and hand-lens in a 2 km2 area mapped at 1:6000 in the central Buckskin Range. Field relations suggest the high-level advanced argillic and sericitic alteration in the Buckskin Range is broadly contemporaneous and transitional into sericitic alteration and pyrite deposition in the deeper (1-4 km) porphyry copper environment. The spatial and temporal relationships of hydrothermal assemblages and veins in the Buckskin Range suggest an evolution from very acidic, sulfide and sulfate-rich fluids to weakly acidic, sulfur-poor hydrothermal fluids. Feldspar-destructive, pyrite-rich assemblages of quartz + alunite + pyrophyllite ± kaolinite ± dickite, and quartz + muscovite are cross-cut and overlain by feldspar-stable assemblages with added sericite + hematite + chlorite ± calcite, reflecting a decrease in magmatic acids and sulfur input. Hydrogen and oxygen isotope data from muscovite and pyrophyllite have values of dD=-61 per mil and d18O=11.8 and 9.7 per mil suggesting that advanced argillic and sericitic alteration in the Buckskin Range may be caused by the same types of magmatic-dominated fluids that produced K-silicate and sericitic alteration in the deeper porphyry copper environment at Yerington. Sulfur isotope data give d34S values of 8.7 - 8.8 per mil for alunite and -4.7 per mil for pyrite. In thin section, alunite occurs as disseminations in microgranular quartz and as crystals surrounding leached pyrite cavities. These data suggest that alunites may represent a mixture of hypogene alunite formed by condensation of magmatic vapors in the subvolcanic environment and later alunite produced by supergene weathering of pyrite. These results provide a significant contribution to current models that link magmatic gases between shallow epithermal systems and deeper level porphyry copper deposits associated with volcanic arcs.