Abstract

The precious and base metal deposits of the world-renowned Comstock district of western Nevada constitute one of several superimposed Miocene hydrothermal systems. In the district, Mesozoic metasedimentary and igneous rocks are overlain by Oligocene to Miocene ash-flow tuffs. A thick sequence of middle Miocene andesitic volcanic rocks and intrusions host the bulk of the hydrothermal alteration and ore deposits. Some of the magmatic events were directly associated with hydrothermal activity. Secondary biotite alteration locally affects preore dioritic intrusions. Preore barren quartz, alunite, pyrophyllite, and kandite alteration (high-sulfidation style) is zoned outward from mostly discontinuous, crudely radial, fractures associated with andesitic intrusions. Blanket-like cristobalite, alunite, and kaolinite alteration exposed at the periphery of the district may be linked to preore alteration near the paleowater table. Later, large volumes of subore-grade massive quartz veins and quartz-adularia stockworks were deposited in steeply to moderately east-dipping sets of north- and northeast-striking faults that also localized the major precious metal-bearing lodes. Quartz, chlorite, illite, and very localized muscovite alteration (deep low-sulfidation) developed coeval with vein deposition. Epidote-bearing propylitic alteration forms halos around the higher-temperature parts of the lodes. Adularia and rare bladed calcite indicate boiling during deep low-sulfidation hydrothermal activity, but little is apparently associated with ore deposition. The main stage Au-Ag-Cu-Zn-Pb ore was deposited late in the deep low-sulfidation system in irregularly spaced lenticular dilatant zones or structural intersections. Massive, nonbladed(?) calcite was deposited just below and in the lower parts of orebodies in the Comstock lode, but it was deposited in the upper parts of orebodies or was barren elsewhere in the district. Pliocene to Holocene reactivation of faults has disrupted the district, redistributing the relative positions of many of the orebodies and alteration assemblages.

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