Graduate Thesis Or Dissertation

Geology and mineral deposits of the Coyote Hills mining district, Lake County, Oregon

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  • The Coyote Hills are located about 46 kilometers north-northwest of Lakeview, Oregon, within the Basin and Range physiographic province. These hills represent a complex volcanic center of bimodal calc-alkaline igneous activity. The oldest rocks recorded in the Tertiary succession are horn-blende- bearing andesite and aphanitic basalt flows, laharic breccias, conglomerates, tuffaceous sandstones, and lithic wackes of the late Eocene to early Oligocene Lower Andesite formation. During middle to early late Oligocene time, voluminous eruptions of predominately basaltic andesite formed a large shield volcano. This unit, the Upper Basalt formation, was followed, after a short hiatus, by the Coyote Hills rhyolite of late Oligocene to early Miocene age. The Coyote Hills rhyolite represents a complex spectrum of multi-phase silicic volcanism and comagmatic near-surface plutonism. Magma compositions varied from dacite to rhyolite and include lava flows, volcanic plugs, a flow dome complex, and a hypabyssal quartz monzonite intrusion. Volcanic activity that post-dates formation of the bimodal Coyote Hills complex culminated with the lower slopes of the shield volcano onlapped by the middle Miocene Steens Basalt, the late middle to early late Miocene Plush tuff, and the late Miocene to early Pliocene Upper basalt. A prominent northwest and northeast-trending fault and fracture system formed after emplacement of the Coyote Hills rhyolite, and as early as late Oligocene to early Miocene in time. Basin and Range faults post-date the Steens Basalt and have caused some minor displacement of the younger rocks. Penecontemporaneous with silicic volcanism of the Coyote Hills rhyolite was a period of hydrothermal activity. Fluids ascended favorable structures, altered the surrounding country rocks, and deposited minor quantities of epithermal gold-silver-copper-mercury-molybdenum(?)- lead(?), and zinc(?) in structurally controlled quartz-pyrite veinlets and as disseminations. Because of the association of mineralization with silicic volcanics in time and space, it is concluded that the two processes were genetically related. The hydrothermal system in the Coyote Hills is related to the late stages of silicic volcanism. Evidence for a genetic relation includes the close association of rock type, and chemical and mineral zonations within the district. Geological and geochemical evidence that includes rock type and alteration patterns, and mineral and trace element zonations, collectively suggest that only the highest level of the hydrothermal system has been exposed. It is entirely possible that a large vein or disseminated-type deposit containing both base and precious metals is present at depth.
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