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Widespread massive and thin-bedded andesite and dacite flows and breccias are present within the southern Simpson Park, Toquima, Toiyabe and Monitor Ranges. Individual flows commonly cover less than 10 square miles and lava fields usually cover no more than 25 square miles (Kleinhampl and Ziony, 1985; Stewart and McKee, 1977). The Simpson Park lava field, however, covers about 100 square miles and is more than 2000 feet in thickness. Several major eruptive centers for these flows are present in the area, and include the Ninemile Peak area of the Antelope Range where these flows are almost 2,500 feet thick, and the Dobbin Summit area of the Monitor Range (Kleinhampl and Ziony, 1985).

These intermediate composition flows are dominantly black to gray, fine-grained to glassy, porphyritic flow and hypabyssal rock with phenocrysts of hornblende, biotite, and plagioclase. Chemically the rocks are dominantly high-potassium andesites (Taylor, 1969) containing as much as twice the potassium content of comparable andesites in western North America (Merriam and Anderson, 1942; Stewart and McKee, 1977). Rocks that are compositionally dacite and quartz latite are also represented in the Simpson Park, Toquima, and Toiyabe Ranges. Several age determinations give a range of Oligocene ages between 33.5 and 36.5 Ma (Stewart and McKee, 1977; McKee and Silberman, 1970). Similar andesitic to quartz latitic flow in the Hot Creek and Antelope Ranges are as much as 900 feet thick, and have been dated at 35.2 +/- 1.1 Ma (Kleinhampl and Ziony, 1985).

Medium to dark-gray Oligocene andesite flows and flow breccias from 200 to 1200 feet thick are present in discontinuous exposures within various portions of the Hot Creek Range and surrounding Squaw Hills, including the Tybo and Moores Station Quadrangles (Quinlivan and Rogers, 1974; Ekren and others, 1973). These flow layered and massive, pink, white, gray, and brown flows contain 5 to as much as 30 to 50 percent phenocrysts. Several flows contain an average of about 34 percent phenocrysts with 77 percent potassium feldspar, 18 percent pyroxene, and 5 percent opaques. The flow breccias contain pebbles and local boulders of andesite partially enclosed in a tuffaceous matrix (Quinlivan and Rogers, 1974).

In the Pancake Range, blue-gray and black weathering andesite flows and breccias from 700 to 1200 feet in thickness are present. Oligocene phenocryst-poor and rich, quartz latite flows are also present along the southwest flank of the range (Snyder and others, 1972). At least three phenocryst-poor, gray or purple latite flows up to 1,700 feet thick are present in the Lunar Crater area of the Pancake Range (Snyder and others, 1972). The upper flow is a flow banded to columnar jointed unit with about 15 percent phenocrysts. The middle flow is massive with a glassy flow breccia at the base and 10 percent phenocrysts. The upper flow has a perlitic top and flow-layered base contain 4 percent phenocrysts. The phenocryst-rich quartz latite flows are blue or brown-gray, flow layered units with a local vitrophyric top and 42 to 45 percent phenocrysts composed of 52 to 54 potassium feldspar, 15 to 18 percent quartz, 7 to 14 percent biotite, 7 to 8 percent hornblende, 4 to 5 percent alkali feldspar, 1 or 2 percent opaques and 1 or 2 percent orthopyroxene (Snyder and others, 1972). Similar dacitic to andesitic flows from 500 to 1,000 feet thick are also present in the northern Reveille Range (Ekren, Rogers, and Dixon, 1973).

Oligocene flows of andesite to quartz latite and minor rhyolite unconformably overlie the Indian Well Formation in the Carlin-Pinon Range, Cortez Mountains, and in the Elko East and West Quadrangles near Elko. In the Elko West Quadrangle these rocks have given K-Ar radiometric ages of 30.9 +/- 1.0 Ma and 32.5 +/- 1.1 Ma (Solomon and others, 1979). The andesites are dark gray to black flows with plagioclase and hypersthene phenocrysts in a cryptocrystalline glassy matrix. Basal andesite mudflow conglomerates, containing angular clasts of tuff and andesite in a brown-gray tuffaceous matrix, are locally up to 80 feet in thickness (Solomon and Moore, 1982a, 1982b). These flows are about 900 to 1,000 feet thick in the Carlin-Pinon Range area and about 350 feet thick in the Elko East and Elko West Quadrangles (Smith and Ketner, 1976, Solomon and Moore, 1982a, 1982b).

Andesitic and latitic flows and minor pyroclastic are exposed to the north in Elko County within the Adobe Summit area northwest of Elko, the Hanks Creek Southwest and Hot Springs Creek Quadrangles north of the Adobe Range, the Burner Hills, Little Rock Creek, Scraper Springs and Soldier Cap Quadrangles west of the Tuscarora Mountains, and in the Owyhee, Mountain City and Wild Horse Quadrangles and Cornucopia mining district (Coats, 1967; Coats, 1985). These rocks are essentially undated radiometrically (except near Elko), and thickness and lithologic/petrologic data are presently lacking. These volcanics are assigned to the Oligocene based upon regional relationships and the apparent relative correlation of several units (Coats, 1985).

In the Adobe Summit area northwest of Elko, hypersthene andesite and hornblende-pyroxene andesite flows are about 350 feet thick. Individual flows often show a basal mudflow conglomerate of poorly sorted, subrounded to angular clasts of tuff and andesite in a brown-gray tuffaceous matrix (Solomon and Moore, 1982a, 1982b). Similar hornblende-hypersthene andesite and basaltic andesites are irregularly exposed north of the Adobe Range in the Hanks Creek Southwest and Hot Springs Creek Quadrangles (Coats, 1985).

In the Little Rock Creek, Soldier Cap, and Scraper Springs Quadrangles west of the Tuscarora Mountains this unit includes dark-gray to grayish-red, pyroxene and hornblende-pyroxene andesite, which are locally underlain by glassy air-fall tuff. This unit may correlate with the andesites of the Cornucopia district (Coats, 1985). Near Rock Creek, biotite andesite and porphyritic hornblende-biotite andesite are also present, and may have been erupted from a subcircular vent about half a mile in diameter along the western edge of the Mount Blitzen Quadrangle (Coats, 1985).

In the Mountain City Quadrangle, thin hornblende andesites are overlain by widely distributed pyroxene andesite with augite and hypersthene phenocrysts. This unit is apparently overlain by the volcanic and sedimentary rocks of Jones Creek that reach a maximum thickness of 1,000 feet in the Mountain City and Owyhee Quadrangles (Coats, 1971). This sequence is composed of medium to dark-gray, porphyritic flows and tuffs of pyroxene andesite and lesser hornblende andesite, which are locally opalized, bleached and iron stained (Coats, 1971). Phenocrysts are abundant and are composed of plagioclase, quartz, hornblende, sanidine and biotite. Minor amounts of gray air-fall tuff and vent-filling breccia, and basal gravels are also present in the unit (Coats, 1985).

In the Cornucopia mining district, hydrothermally (propylitized) altered pyroxene andesite or hornblende-pyroxene andesite is the ore-bearing wallrock for precious gold and silver bearing veins (Coats, 1967). Some hornblende and biotite welded tuffs and tuff breccias are also present in this volcanic sequence. This Cornucopia sequence is exposed to the southwest in the Burner Hills between the Owyhee River and Rock Creek drainages (Coats, 1985).

Oligocene andesite and dacite older than 26 Ma are also present in White Pine and Lincoln Counties in the southern Egan, northern Pahroc and Seaman Ranges (Ekren and others, 1977). Hydrothermally altered dacites are grey-green porphyritic flows with plagioclase phenocrysts as large as 10 cm, and smaller mafic phenocrysts of pyroxene, biotite, and hornblende. The andesites are green with sparse plagioclase and pyroxene phenocrysts rarely exceeding 2 mm in length (Ekren and others, 1977).

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Last modified: 09/12/06