Introduction Evaluation Prospects











North of the town of Battle Mountain at least 18 flows of basaltic andesite and olivine basalt, ranging from 10 to about 100 feet in thickness, have an aggregate thickness of more than 1000 feet along the south and west flanks of the Sheep Creek Range. These flows thin to the south where perhaps 250 feet of brownish weathering basaltic andesite holds up the northwest flank of the Shoshone Range (Stewart and McKee, 1977). Along the eastern portion of the Cortez Mountains these basaltic andesite flows are from 120 to 350 feet in thickness (Gilluly and Masursky, 1965), and similar flows cap the northeastern tip of the Simpson Park Mountains.

Both the basaltic andesite and olivine basalt are reddish-gray to black, platy, flow banded to massive, slightly vesicular lavas, with the thicker flows showing columnar jointing. Chemically however, the two are quite different. The olivine basalt compares closely with oceanic tholeiites, while the basaltic andesites are compositionally similar to the average andesite, suggesting the two are genetically unrelated (Stewart and McKee, 1977). The basaltic andesites are 14.8-16.3 Ma, and the overlying olivine basalts are about 10 Ma (Stewart and McKee, 1977). They dip gently east or southeast at 3 to 16 degrees (Gilluly and Masursky, 1965). In the Cortez Mountains, a swarm of altered basaltic feeder dikes as much as a mile long and 800 feet thick are scattered throughout the western portion the range and represent part of the plumbing system for these flows (Gilluly and Gates, 1965). In the Roberts Mountains, basaltic andesite and andesite flows up to 200 feet thick are underlain by 700 feet of rhyolite tuff and flow breccia and are overlain by thick flows of quartz latite (Merriam and Anderson, 1942).

In the Pinon Range, scattered nearly horizontal andesite and basaltic andesite flows are at least 1,000 feet thick. They are dark gray to black, dense and aphanitic with 10 to 15 percent small plagioclase or pyroxene phenocrysts (Smith and Ketner, 1976). These flows are younger than the Humboldt Formation and are considered Late Miocene in age.

North of the Pinon Range and south of Double Mountain in the North Fork Humboldt River drainage, pyroxene andesite, basaltic andesite and latite flows, tuffs, and breccias have been dated as Miocene (Barstovian) based on mammal bones in interbedded lenticular siltstones, and interfingering relationships with the 15 Ma old Jarbidge Rhyolite (Coats, 1985).

Late Tertiary hornblende and pyroxene andesites are exposed in the surrounding area in Elko County within the Double Mountain Southeast, The Narrows, Peko Peak, Mount Velma, and Owyhee Quadrangles (Coats, 1985). Unique andesites that appear to have partially assimilated granitic rocks are exposed in the Mount Velma, The Narrows and Peko Peak Quadrangles where they are locally interbedded with the Jarbidge Rhyolite. These flows contain partially resorbed granite xenoliths, resorbed crystals of quartz, potassium feldspar, and plagioclase, and rare nearly uniaxial pigeonite (Coats, 1985). Undated and unnamed hornblende and pyroxene andesite masses that may represent remnants of small andesitic cones are present in the Double Mountain Southeast, The Narrow and Peko Peak Quadrangles. These andesites are assigned to the Miocene by Coats (1985).

Miocene age basaltic andesite flows are also present in local exposures in the southern Pancake Range where up to 100 feet of black and brownish-gray aphanitic basalt is present (Ekren and others, 1972). In the southern Monitor Range, basaltic andesite plugs dikes and flows are believed to be no older than 15 Ma and are younger that the gold-bearing rhyolites of the Ellendale Mining district in that range (Kleinhampl and Ziony, 1985). Thin irregular and discontinuous Miocene andesite dikes and sills are also present in the southern Toquima Range (Kleinhampl and Ziony, 1985).

Quartz latite dikes and plugs dated at 18.7 +/- 0.8 Ma are present in several small exposures within the northern Reveille Range (Ekren and others, 1973). These Miocene dikes are light-gray, flow layered and contorted units with sparse crystals dominantly composed of plagioclase and biotite and lesser amounts of alkali feldspar and quartz.

Miocene andesite and quartz latite are exposed in the northern Pahroc and Golden Gate Ranges. These intermediate flows are flow layered, banded, and brecciated gray andesite and quartz latite, with phenocrysts of plagioclase as large as 1 cm, biotite up to 5 mm, and small phenocrysts of hornblende, clinopyroxene, and orthopyroxene. Flows are about 17-18 Ma and locally exceed 1,000 feet in thickness (Ekren and others, 1977).

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Last modified: 09/12/06