By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Nye County, founded in 1864, comprises more than 17,000 square miles in south-central Nevada and is studded with north-trending mountain ranges. The intermontane valleys are not drained, and streams terminate in sinks or salt-encrusted flats.
Sedimentary rocks of Paleozoic and Mesozoic ages are exposed in the mountains in the north; in the western ranges only Mesozoic formations are exposed. In the eastern and central parts of the county, the mountains are composed chiefly of Paleozoic rocks. Masses of granitic rocks of Jurassic and Cretaceous ages have intruded the older rocks and are exposed over large areas. Tertiary lava flows and intrusive rocks are abundant in the southern part of the county.
The most important mineral commodities of the county have been the precious metals. Some of the more successful gold districts are Bullfrog, Tonopah, Round Mountain, and Tybo, where gold and silver-bearing veins occur in Tertiary rocks. Another highly productive district is the Manhattan, where the highest yields were from veins in Paleozoic rocks.
Gold production for the county from 1903 through 1959 was 2,975,034 ounces - 298,593 ounces was from placers and 2,676,441 ounces was from lode mines.
The Bruner (Phomolite) district is in northwestern Nye County at lat 39"05' N. and long 117"46' E.
The district was founded in 1906 when small production was reported from the Paymaster mine. In 1936 the Penelas mine, the main producer of the district, was discovered (Krai, 1951, p. 26).
Production of gold from the district from 1936 through 1959 was 17,213 ounces. Earlier production could not be ascertained.
Tertiary rhyolite and andesites cover the area (Krai, 1951, p. 26). Metavolcanic rocks probably underlie the extrusives. The ore deposits are in quartz veins in the younger volcanic rocks with free gold associated with silver.
The Bullfrog district is in southern Nye County, 60 miles south-southeast of Goldfield. The principal town is Beatty.
The original claims were located in 1904; the customary rush ensued during which the settlements of Bullfrog, Bonanza, Beatty, and Rhyolite mushroomed and competed with one another for new settlers. Competition became so intense that three railroads served the area. The peak period was 1907-10 when $1,687,792 in gold and silver was produced (Krai, 1951, p. 29), mostly from the Montgomery-Shoshone mine. In more recent years activity declined to sporadic small-scale mining. Gold production from 1905 to 1959 was 120,401 ounces, and considerable silver has also been produced.
Ransome, Emmons, and Garrey (1910) described the geology and ore deposits in considerable detail. The oldest rocks in the district are contorted quartzites and mica schists of Ordovician age or older, and they are overlain locally by limestone, shale, and quartzite of Silurian age. Overlying the Paleozoic rocks is a sequence of Tertiary flows and tuffs, 6,000 feet thick, composed of 16 rhyolite units, five flows of basalt, one flow of quartz latite, and one flow of quartz-bearing basalt that caps the sequence. The entire area has been broken by normal faults that trend north to northwest, and the resulting fault blocks are tilted eastward. A late set of faults trends northeast. Ore deposits are mineralized faults or fault zones in the rhyolites. Many of the veins are simple but there are many zones of stringers or veinlets with ill-defined boundaries. The vein material consists of quartz and calcite and finely divided auriferous pyrite. The quartz is crustified and has a porcelaneous texture. Oxidized ore contains gold in limonite; the calcite is partly dissolved, and manganese oxide has been introduced. Varying amounts of silver may be alloyed with the gold.
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