TERTIARY STRATIGRAPHY, HYDROTHERMAL ALTERATION, AND PRECIOUS METAL DEPOSITS IN THE PATTERSON MINING DISTRICT, SWEETWATER MOUNTAINS, AND COMPARISON TO THE BODIE HILLS, CALIFORNIA-NEVADA

The Sweetwater Mountains (Mono County, CA and Lyon County, NV) contain a large volume of altered rock, including Triassic metasedimentary rocks, Mesozoic granitic rocks, and Miocene to Pliocene Tertiary volcanic rocks. The largest volume of Tertiary volcanic rocks in the range are high-K calc-alkaline to shoshonitic trachydacites and rhyolites, with volumetrically minor andesite, all a part of the western andesite assemblage, a subduction-related arc that erupted mostly within the Walker Lane. Epithermal Ag-Au deposits in the Patterson mining district occur in veins and silicified volcanic rocks. Total production since the 1860s is estimated at 3.4 Moz. Ag and 3,000 oz. Au.

Epithermal mineralization occurs in three north-trending belts of varying Ag:Au. The Comstock and Clinton belts are fault-controlled while the Cameron belt consists of shallow, disseminated deposits related to hot springs. Varying Ag:Au indicates different depths or time of formation of deposits. The Comstock belt (silver-gold) consists of anastomosing veins exposed on the surface that show boiling textures, with silver minerals trapped within the lattice of bladed calcite. The Clinton belt (silver only) consists mainly of stockwork veins of pyrite, chalcopyrite, galena, sphalerite, hessite and other tellurides, hosted mainly in chalcedonic quartz. The mineral deposits in the Cameron belt (gold rich) formed very close to the late Miocene paleosurface, with eroded sinter terraces indicating that hydrothermal fluids reached the surface and formed hot springs deposits at these locations.

The majority of volcanism and alteration occurred in the late Miocene to early Pliocene from 6-5 Ma. Mineralization and alteration are associated with and occur shortly after volcanic activity. Most of the stratigraphy and the age of hydrothermal activity in the Sweetwater Mountains are distinct from those of the adjacent Bodie Hills volcanic field, although both landforms exhibit elevations of Te in epithermal deposits. Enrichments of Te in epithermal deposits are associated with the last stages of subduction. Telluride-bearing epithermal deposits may be associated with oxidized calc-alkaline volcanic rocks, such as those observed in the Sweetwater Mountains, as well as with silica undersaturated alkaline rocks, as seen in Colorado and Fiji.