Late Miocene Volcanic Rocks and Epithermal Precious Metal Deposits of the Patterson Mining District, Sweetwater Mountains, Mono County, California

The Sweetwater Mountains, California and Nevada, contain large volumes of hydrothermally altered rocks, including pre-Cenozoic metasedimentary and volcanic
rocks, Jurassic plutonic rocks, and late Cenozoic volcanic rocks from which 4.5 Moz.
of Ag and 3200 oz. of Au were produced. Most production was derived from low- and
intermediate-sulfidation, quartz-sulfide-adularia±telluride veins and silicified breccias in the Patterson mining district.
Felsic volcanic rocks of the Sweetwater volcanic field consist of late Miocene to
early Pliocene (~6.6–5.25 Ma) dacites and rhyolites. Hydrothermal activity associated
with Ag-Au deposits occurred during and after the later stages of felsic volcanism,
based on ~5.6–5.1 Ma adularia and hypogene alunite dates. Altered rocks and deposits form three north-to-northeast trending belts which are interpreted to be controlled
by high angle fault zones.
These mineral belts (Cameron, Clinton, and California Comstock, from east to
west) exhibit different deposit styles, vein textures, Ag to Au ratios, geochemical signatures, mineralogy, and structure. The Cameron belt consists of deposits interpreted to
have been formed near-paleosurface with higher Au concentrations than other belts,
with an average Ag: Au of 125:1. Precious metals are associated with disseminated
pyrite and chalcedony in altered tuff, dacite, and hydrothermal breccias. The Au-Ag
deposits have elevated As and Sb concentrations and occur <30 meters below sinter terraces and chemical sediments of finely bedded silica. The Clinton belt deposits have high concentrations of Ag and very little Au, with an average Ag: Au ratio of 1100:1. Stockwork quartz and adularia veins are spatially associated with a 5.89 ± 0.14 Ma rhyolite porphyry dike. Adularia yielded a 5.1 ± 0.0076 Ma date. Hypogene Ag minerals are acanthite and abundant hessite (Ag2Te). Other metallic minerals include pyrite, chalcopyrite, sphalerite, galena, minor sylvanite [(Ag,Au)Te2], petzite (Ag3AuTe2), supergene chlorargyrite and Fe tellurite minerals, and very minor Pb-Ag telluride. The presence of abundant base metal sulfides suggests a greater depth for deposits of the Clinton belt than those of the Cameron belt. The California Comstock belt contains both Au and Ag minerals hosted in stockwork and thick veins of banded quartz, adularia and bladed calcite. Metallic minerals include electrum, Se-bearing acanthite, and minor hessite, chalcopyrite, and galena.