COMPOSITIONAL VARIATIONS IN HYDROTHERMAL WHITE MICAS AND CHLORITES IN A PORPHYRY CU SYSTEM AT YERINGTON, NV

Understanding subtle mineral variations in zoned hydrothermal alteration associated with the formation of porphyry-Cu deposits provides a potentially valuable vectoring tool for mineral exploration. Hydrothermal white mica and chlorite grains from Yerington, NV, a late Jurassic porphyry-Cu system extended and tilted ~ 80° to the west, vary spatially and record interactions with chemically distinct hydrothermal fluids over a vertical distance of ~3 km above the mineralized center. Central D-veins and selvages are dominated by muscovite (K=0.8-1.0 afu). Moving laterally away from main fluid pathways, phengitic muscovite ((Fe+Mg)^vi/sum Oct=0.18) is present, usually associated with chlorite, consistent with neutralization of the hydrothermal fluid accompanied by decreasing water/rock ratio. Also along D-veins, illite (K<0.8 afu) occurs, likely deposited by later, cooler, more acidic fluids utilizing existing paths. Moving up section, pyrophyllite indicates a transition to advanced argillic alteration and a more acidic hydrothermal fluid at shallow depths. In flanking propylitic alteration in the shallow levels, chlorites have lower octahedral Al (Al^vi/sum Oct<0.2) than more central sericitic assemblages. High trace element concentrations in muscovite (Ba>1000 ppm, Tl>2 ppm) and chlorite (Li>50 ppm) are also present at these levels compared to deeper parts of the hydrothermal system. Overall, the mineral chemical data suggest significant compositional variations in hydrothermal mica and chlorite that record large-scale wall-rock hydrothermal alteration and are key identifiers of specific hydrothermally altered zones associated with a porphyry Cu deposit.