CHARACTERIZATION 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 that hosts the remnants of a mineralized (Cu-Au-Ag) high-sulfidation (HS) epithermal system. Geographic location and bulk rock geochemistry place this volcano within 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. Antelope Valley comprises an exposed volcanic stratigraphy, and the lowest stratigraphic flows have been hydrothermally altered, making it a prime location to study the relationship between HS epithermal systems and their volcanic setting. This study investigates the volcanic setting, the spatial distribution of alteration zones and the temporal relationship of volcanic activity to hydrothermal alteration using geologic mapping, geochemistry and ⁴⁰Ar/³⁹Ar geochronology. The geology and alteration zones inside Antelope Valley were mapped at a scale of 1:24,000 and field-determined geologic units were confirmed through petrographic analysis and geochemistry. Lava flows range from basaltic andesite to dacite and are interfingered with various volcaniclastic units of similar compositions. Mapped alteration zones include propylitic, clay, pyrophyllite, quartz-alunite and vuggy-silica. 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. The linearity of individual altered zones suggest that faults and fractures provided conduits for acidic fluids, these faults and fractures may be related to transtensional faulting within the Walker Lane Belt. Geologic mapping of nine square miles and geochemical analyses suggest that Antelope Valley is a stratovolcano that formed from a hydrous, oxidized, calc-alkaline magma which promoted the formation of an acid-sulfate type system, similar to other volcanically hosted epithermal deposits within the southern ancestral Cascade arc.