EXPLORATION OF A HIGH-SULFIDATION EPITHERMAL DEPOSIT WITHIN THE ANTELOPE VALLEY VOLCANIC CENTER, SIERRA COUNTY CALIFORNIA

Antelope Valley is interpreted to be an eroded andesitic-dacitic central vent complex located in the southern ancestral Cascade (SAC) arc, which was active between 30 and 3 Ma and formed through similar processes to the currently active Cascade arc. The core of Antelope Valley is eroded, exposing not only the volcanic stratigraphy but also the remnants of a mineralized (Cu-Au-Ag) high-sulfidation (HS) epithermal system. This study investigates the eruptive history of Antelope Valley and how it relates temporally and spatially to the HS epithermal deposit. The geology and alteration zones inside Antelope Valley were mapped at a scale of 1:24,000 and mapped geologic units were confirmed through petrographic analysis. Rock types include variations of andesite, dacite, volcaniclastics, basalt and hydrothermal breccia; and mapped alteration zones include propylitic, clay, pyrophyllite, quartz-alunite and vuggy-silica. Geologic mapping of nine-square miles revealed that Antelope Valley consists of alternating andesitic lava and pyroclastic flows that vary throughout the stratigraphy, suggesting an eroded stratovolcano. In the southern region of Antelope Valley are a series of porphyritic andesite and dacite hills surrounded by pyroclastic flow that may represent lava domes and hence resurgent volcanism. Vuggy-silica, quartz-alunite and pyrophyllite alteration appear as linear features on hilltops in the central core of Antelope Valley, whereas propylitic and clay alteration are found in drainages and on the valley floor. This suggests that the primary control on the hummocky topography is due to differential erosion rather than lava domes. The linearity of individual altered zones suggests that faults and fractures provided conduits for fluids. Host rocks are a tuff unit, porphyritic andesite, dacite and basalt; the alteration of basalt plugs suggest that epithermal alteration occurred during late stage volcanic activity. The alteration within the Antelope Valley volcanic center shows similarities to other HS epithermal systems (e.g. Goldfield, NV) within the SAC arc, in that it is primarily controlled by faults and fractures and hosted by an andesitic-dacitic stratovolcano with late stage, intrusive activity.