The geology of the Eldorado Mining District, Clark County, Nevada
The rocks of the Eldorado mining district, Clark County, Nevada, are of Precambrian, Tertiary and Quaternary age. Precambrian rocks include a heterogeneous series of gneisses and schists, younger garnetiferous granite gneiss and a youngest group of pegmatites. Paleozoic and Mesozoic rocks present in surrounding areas are absent in the district and Tertiary volcanics lie directly on the Pre- cambrian basement. Tertiary rocks consist of middle to late Tertiary flows and pyroclastic beds of andesite, basalt and rhyolite. These are assigned to three volcanic formations: the Patsy Mine volcanics; the Golden Door volcanics; and the Mount Davis volcanics. The Patsy Mine volcanics Is further subdivided into three members. Four Intrusive stocks of middle to late Tertiary age, but younger than the volcanic rocks, intrude the foliated basement rocks and volcanics. These are named the Nelson quartz monzonite, the Nob Hill and Aztec granodiorites, and the Keyhole leucogranite. The Nelson and Keyhole Intrusives appear to be structurally controlled. Dikes, sills and chonoliths of acidic to intermediate felsites and diabase are widespread throughout the district. Quaternary rocks and sediments consist of local Pleistocene flows of basalt, the Pleistocene Temple Bar conglomerate and widespread unconsolidated gravels and alluvium of Recent age. The Eldorado district is much disturbed structurally, Precambrian rooks are complexly folded and faulted. The district lies on the east limb of a broad northerly trending, asymetric fold, locally overturned to the east and expressed in the Tertiary volcanic rocks. The crest to trough distance of the fold is 13-15 miles, Numerous faults are present and from oldest to youngest include: east-west wrench faults with maximum displacements exceeding one mile; north-south normal faults with maximum displacements of several thousand feet; and northeast strike slip faults with displacements up to several hundred feet. Movement along the north-south faults has recurred in Pleistocene to Recent time. The Eldorado mining district has produced more than $5,000,000 since its discovery In 1857, primarily in gold and silver values. The silver:gold ounce ratio since 1907 has been about 23:l. The district has been active during periods of general economic favorability for production of precious metal ores in the United States. The years of greatest production were 1939, 1940, 1941 and 1942. Production was stopped in October 1942 by U.S. Government order WPB-L208 and has not been resumed. The economic mineral deposits occur within epithermal fissure veins within the margins of the Nelson quartz monzonite, in surrounding volcanic rocks and in foliated rocks of the Precambrian basement. Veins consist of brecciated fragments of wall rock cemented by calcite and quartz and contain native gold, silver, pyrite, galena, chalcopyrite and sphalerite. Secondary enrichment of silver has locally taken place near the surface, although most production has come from primary ores. The deepest workings in the district reach a depth of about 600 feet.
Missouri University of Science and Technology
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