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CENOZOIC MINERAL DEPOSITS AND CENOZOIC IGNEOUS ROCKS OF NEVADA

Tertiary igneous rocks in Nevada are of two fundamentally different petrochemical types that are the result of two regional tectonic regimes active in western North America during the latter half of the Cenozoic. Many hydrothermal mineral deposits in Nevada clearly reflect these two tectonic regimes and their associated igneous activity. The older petrochemical igneous type is of late Eocene through Oligocene age, is intermediate calc-alkalic in chemistry, and is related to subduction of the Pacific (Farallon) plate beneath the continental North American plate. Magma generated by subduction of the Farallon plate produced skarn and distal disseminated mineral deposits in eastern and central Nevada such as Copper Canyon, McCoy, and Mineral Hill. This regional event is probably of most importance because heat from it produced hydrothermal systems throughout central Nevada that may be responsible for many mineral deposits including some disseminated precious metal deposits in the region. Magmas related to subduction in western Nevada are latest Oligocene and early Miocene in age and were derived from the stagnant Farallon plate that was no longer subducting beneath North America by this time. These magmas passed through the highly fractured Walker Lane region in western Nevada and produced widespread and pervasive alteration and the bonanza precious metal deposits in western Nevada at Aurora, Goldfield, Tonopah, and Comstock. The younger igneous type is of middle Miocene and younger age, is of basaltic or bimodal basalt-high-silica rhyolite composition, and is related to extension of the Great Basin by basin and range (horst and graben) faulting. In north-central, northwestern, and southwestern Nevada epithermal precious-metal and mercury deposits are related to this middle and late Miocene bimodal magmatism. Some deposits formed in this tectonic-magmatic environment include the McDermitt mercury, and the Sleeper, Seven Troughs, and Hog Ranch gold deposits in northwest Nevada; the Mule Canyon and Buckhorn gold deposits in north-central Nevada, and the Bullfrog gold district in the southwestern part of the state.