IDENTIFYING THE HOST ROCK OF A HIGH SULFIDATION EPITHERMAL SYSTEM IN ANTELOPE VALLEY, CA

Epithermal deposits exhibiting high sulfidation alteration are present throughout the southern ancestral Cascades, and are associated with Oligocene to Pliocene arc magmatism. Antelope Valley, near Loyalton CA, lacks a detailed geologic map and description of flow units, with previous work interpreting the area as an ancestral Cascade volcano. Andesitic and Dacitic lava flows within Antelope Valley are porphyritic with coarse plagioclase and hornblende. Locally, high sulfidation alteration comprises envelopes of graded alteration with linear trends. Vuggy silica represents the highest grade of alteration and forms the center of the altered regions, grading laterally to quartz-alunite, quartz-pyrophyllite, clay, and propylitic alteration. High sulfidation epithermal deposits form from geothermal systems where acidic fluids flow through volcanic rock, dissolving and replacing it. The fluid becomes progressively more neutral with rock interaction, forming a zoned pattern of alteration. Alteration zones produce topographic highs within the valley due to the high degree of silicification, which is resistant to erosion. As part of an overarching mapping project investigating the relationship of the alteration to the volcanic activity in the valley, this study aimed to determine which volcanic rocks host the epithermal alteration. Potential host rock samples included units located stratigraphically low in the valley, and outcropping near the alteration zones. Most primary igneous textures are absent in altered samples, but quartz ‘eyes’ are commonly observed (35-40 modal %) and are texturally similar to quartz phenocrysts (40 modal %) in unaltered local dacite. These observations were confirmed through petrography and geochemical analyses. Dacite commonly hosts argillic and advanced argillic grades of alteration, while propylitic alteration is mostly hosted by basalt plugs and breccia veins. The propylitic zones formed from the once acidic fluids traveling pervasively through the volcanic rock to alter distal volcanic rocks away from the main fracture zone. Identification of host rocks from altered rock samples will further constrain the timing of magmatic fluid migration, identify geochemistry of these fluids, and correlate alteration to other epithermal deposits in the ancestral Cascades.