This district is in the southern San Antonio Mountains near Tonopah, the county seat of Nye County and the largest town in the county.
Tonopah was predominantly a silver district, but it also yielded large amounts of gold. The first claims were staked in 1900, and by 1901 there was vigorous activity which lasted until the late 1940's; thereafter, production declined. The principal companies were the Tonopah Mining, the Tonopah Belmont Development, and the Tonopah Extension (Krai, 1951, p. 171).
Lincoln (1923, p. 186) listed the gold production of Tonopah from 1901 through 1921 as $30,360,903 (about 1,473,830 ounces). From 1901 through 1959, a total of 1,880,000 ounces of gold was produced.
The rocks exposed in the Tonopah district are all of Tertiary age and consist of a series of lava flows, volcanic breccias, tuffs, and intrusives that have been somewhat displaced from their original attitudes by extensive faulting. Seven formations recognized in the district (Nolan, 1935b, p. 13) are, from oldest to youngest: the Tonopah Formation (volcanic tuffs, breccias, flows), Sandgrass Andesite (dark lavas interlayered with the Tonopah Formation), Mizpah Trachyte (2,000-foot-thick flows and breccias overlying the Tonopah), Extension Breccia (tabular intrusive mass in west half of district), West End Rhyolite (sills as much as 600 feet thick that intrude the older formations), Fraction Breccia Member of Esmeralda Formation (volcanic breccia unconformably overlying ore bodies), and postore rhyolite (dikes and lenticular bodies that intrude the older rocks). Numerous faults cut the Tertiary rocks, and Nolan (1935b, p. 28-39) divided them into three general groups: the Halifax fault zone which strikes generally northward; the Tonopah, a compound fault that in cross section has a trace that is convex upward; and the youngest group which includes a fault which strikes northwest, another which strikes northeast, and a third which strikes north and dips west.
The ore bodies are replacement veins in faults or fractures (Nolan, 1935b, p. 409). The Tonopah fault is believed to have exerted a major control on the movement of mineralizing solutions. Ore bodies are found in all three groups of faults, but the Tonopah and Halifax seem to be more heavily mineralized. Hypogene ore contains electrum, argen-tite, polybasite, and pyrargyrite in a gangue of quartz, pink carbonate, barite, and altered wall-rocks. There has been some supergene enrichment locally, but the hypogene ore has been most important.
The Tybo (Hot Creek, Keystone, Empire) district is in northeastern Nye County, in the Hot Creek Range, 65 miles northeast of Tonopah and about 100 miles southwest of Ely (lat 38"23' N., long 116"23' E.). In the district, gold is a byproduct of ores mined primarily for their lead and silver content. The first mineral discoveries were made in 1865, and the town of Hot Creek was the population center during the early days (Krai, 1951, p. 189). The district prospered from the rich near-surface silver ores until 1888. The principal mine during this period was the Tybo mine. Despite several attempts at renewing operations, the district remained dormant from 1888 until 1929, when a concentrator was built that successfully separated the galena from sphalerite (Ferguson, 1933, p. 43-44). Gold production from 1872 to 1888 was 20,360 ounces (Ferguson, 1933, p. 43) ; from 1929 to 1958 it was 6,923 ounces.
The geology of the area was described by Ferguson (1933, p. 13-42). The rocks consist of Paleozoic sedimentary rocks and Tertiary fresh-water sediments, dikes, and flows. The Paleozoic rocks include Cambrian, Ordovician, and Silurian formations. These rocks were tightly folded and faulted; then Tertiary sediments and lavas were deposited. Dikes and masses of Tertiary quartz latite porphyry cut all the older rocks. The Esmeralda Formation, consisting of water-deposited tuffs of late Miocene age, overlies the latite masses, and it in turn is overlain by a series of dacite and andesite flows. There were several periods of faulting that began before Tertiary time and ended after extrusion of the post-Esmeralda lavas.
The ore bodies were deposited after the intrusion of the quartz latite porphyry. They are tabular replacement bodies along the 2-G fault, the oldest major fault in the area. The primary minerals consist of pyrite, sphalerite, argentiferous galena, chalcopyrite, pyrrhotite, and arsenopyrite. Quartz and calcite accompany the sulfides. The ores first mined were oxidized deposits within 300 feet of the surface.
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