VOLCANIC STRATIGRAPHY, HYDROTHERMAL ALTERATION AND TELLURIDE-BEARING EPITHERMAL DEPOSITS OF THE PATTERSON MINING DISTRICT, SWEETWATER MOUNTAINS, CA-NV

The Sweetwater Mountains (Mono County, CA and Lyon County, NV) contain a large volume of altered rocks including Triassic metasedimentary rocks, Jurassic granitic rocks, and Miocene-Pliocene volcanic rocks, and epithermal Ag-Au deposits of the Patterson district. Most Tertiary volcanic rocks are high K calc-alkaline to shoshonitic trachydacites and rhyolites, and lesser andesite, of the ancestral Cascades magmatic arc. Hydrothermal alteration is widespread, with clay alteration grading into silica, illite and pyrite adjacent to Ag-Au deposits which 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 deposits form three north-trending belts of varying Ag:Au, suggesting different depths or times of formation. The Comstock and Clinton belts are fault-controlled while the Cameron belt consists of shallow, disseminated deposits related to hot springs. The Comstock belt (Ag-Au) consists of anastomosing veins of quartz, adularia, electrum, Se-bearing acanthite, chlorargyrite and minor hessite, and exhibit boiling textures. The Clinton belt (Ag only) consists mainly of stockwork veins of chalcedonic quartz, base metal sulfides, hessite, acanthite and chlorargyrite that are spatially associated with a 5.9 ± 0.12 Ma rhyolite porphyry dike. The Au-rich deposits in the Cameron belt formed very close to the late Miocene paleosurface, as evidenced by partially eroded sinter terraces. Base metal sulfides, acanthite, and electrum are disseminated as micron-scale grains in chalcedony, and included within arsenian pyrite in these shallow deposits.

The majority of rhyolitic volcanism occurred from 6-5 Ma, and hydrothermal activity occurred during and shortly after volcanic activity. Most of the stratigraphy and ages of hydrothermal activity in the Sweetwater Mountains are distinct from those of the adjacent Bodie Hills volcanic field, although both landforms contain telluride minerals in epithermal deposits. Silicic volcanic rocks and Ag-Au deposits of the Sweetwater Mountains are similar in age to a few < 7.5 Ma volcanic fields that host epithermal deposits, such as Silver Peak and Como. Some of these relatively young volcanic fields represent magmatism that lagged behind the northward migration of the convergent plate margin.