Introduction Evaluation Prospects











Oligocene rhyolites are exposed in several ranges within the evaluation area. In the northern Simpson Park Mountains, Oligocene (36.8 Ma) flow-banded and auto-brecciated rhyolites with hypersthene andesite flow-breccias are exposed (Roberts and others, 1967). 38 to 33 Ma old rhyolites are present in the Eureka area as well. Rhyolite dike swarms dated at 36 Ma are also present in the southern portion of the Toquima Range (Kleinhampl and Ziony, 1985).

In the Grant Range near Currant, Oligocene rhyolite flows have been dated at 33 to 37 Ma (Armstrong, 1970; Marvin and others, 1973; Kleinhampl and Ziony, 1985). These aphanitic to porphyritic rhyolites are locally auto-brecciated and are about 2,500 feet thick, and are known as the Railroad Valley Rhyolite (Moores and others, 1968). This unit is a light-brown weathering, gray rhyolite which alters to a green, red, yellow, and purple color. These rhyolites are overlain by, and intertongue with red and green sandstones and conglomerates of the Blind Spring Formation. Similar flows are present at the same stratigraphic interval in the Horse and White Pine Ranges, but are dacitic and latitic in composition.

Oligocene rhyolite flows from 31 to 34 Ma old are locally present in the Hot Creek and Monitor Ranges and are commonly 150 to 200 feet in thickness (Kleinhampl and Ziony, 1985; Quinlivan and Rogers, 1974; Ekren, Rogers, and Dixon, 1973). These massive to flow layered, and devitrified flows are commonly gray to purple with local flows of rhyodacitic composition. In the Pancake Range, Oligocene rhyolite flows and breccias are light to medium-gray, brown, or red, flow banded to massive, phenocryst poor or rich units commonly 200 to 400 feet in thickness, with a discontinuous basal vitrophyre (Quinlivan and others, 1974). Phenocrysts often form 30 to 40 percent of these flows with about 30 to 40 percent quartz, 30 to 40 percent alkali feldspar, 25 percent potassium feldspar, and 1 to 5 percent biotite and hornblende (Quinlivan and others, 1974).

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Last modified: 09/12/06