Widespread Miocene Igneous Rocks of Intermediate Composition, Southern Nye County, Nevada
Rocks of intermediate composition form lava piles as much as 3000 feet thick and dikes, sills, laccolith-like masses, plugs, and stocks within an area of about 2000 square miles extending westward from the Belted and Reveille Ranges to the vicinity of Tonopah and Goldfield, Nevada. The age of the intermediate rocks is bracketed by two major ash-flow tuff units dated radio-metrically at 22.3 and 17.8 m.y. (H. H. Mehnert, 1966, written commun.; R. W. Kistler, 1964, written commun.). Where the dated tuffs are absent the age of the intermediate rocks is suggested by other tuff units or by lithology.
The rocks, all calc-alkalic, range in composition from andesite to quartz latite. Individual rock types — dacites, for example — are similar chemically and petrographically throughout the area. In some lava piles and intrusive complexes there is a trend to higher-silica types with decreasing age. At Gold-field, Tonopah, Gold Crater, Cactus Spring, and Gold Reed large volumes of such rocks are pervasively altered and are the chief hosts for precious-metal deposits.
The widespread eruption of rocks of similar composition within a restricted time interval suggests the presence of a broad substratum of magma that was tapped in numerous places. The limited range of chemical diversity is consistent with this idea, and is specifically not in keeping with withdrawal from small masses of magma formed anatectically. Available chemical, petrographic, and radiometric data suggest that similar Miocene igneous activity was characteristic of an area extending to west-central Nevada and the central Sierra Nevada.