Eureka County Nevada Gold Production |
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The structure, which is exceedingly complex, was considered by Hague (1892, p. 8-30) and Nolan (1962, p. 18-29) as a series of structural blocks separated from one another by faults of large displacements. Nearly all the ore bodies of the district are within the north-trending Prospect Ridge block, which is bounded on the east by the Hoosac fault and on the west by the Dugout Tunnel thrust and the Spring Valley, Sharp, and Cave Canyon faults. Recognized within the Prospect Ridge block are three thrust zones, two normal fault zones, and a transverse fault (Nolan, 1962, p. 18-26). The greater part of the deformation is thought to have occurred in the late Mesozoic, though the older structures were formed in Paleozoic time, and movement on some faults took place in Pleistocene or Recent time (Nolan, 1962, p. 27-29).
Ore bodies are grouped into five geographic groups or clusters, of which the most productive has been the Ruby Hill cluster. Most of the ore at Eureka has been mined from irregular replacement deposits in dolomite which consist of irregularly shaped masses of fine-grained anglesite, cerussite, plumbojarosite, mimetite, and galena and minor wulfenite, pyrite, arsenopyrite, hematite, sphalerite, calamine, smithsonite, calcite, aragonite, sider-ite, quartz, clay minerals, azurite, and malachite. Cerargyrite and native gold are present in small quantities. Gold ore from the Windfall mine is distinctive in that textures of the replaced dolomite have been preserved, and the dolomitic gangue has been converted to a "sand" by the mineralizing solutions. The Windfall ore is further characterized by relative absence of sulfides and their oxidation products (Nolan, 1962, p. 30-47).
LYNN DISTRICT
Located about 20 miles northwest of Carlin in northern Eureka County, the Lynn district was until 1962 a placer district. Gold was produced from placer deposits over a wide area, including Lynn, Simon, Rodeo, and Sheep Creeks. In 1962, however, a large lode deposit, the Carlin mine, was discovered. This mine is in a window in the Roberts Mountains thrust fault. The ore consists of very fine-grained gold, mostly less than 5 microns in size, in fractured and altered siltstone and limestone of Silurian and Devonian age below the thrust fault. In 1966, published reserves were 11 million tons of ore containing about 3.5 million ounces of gold.
Vanderburg (1936a, p. 83) reported total production of the district to about 1935 at $140,000, or approximately 6,800 ounces of gold. Total production through 1959 was only about 9,000 ounces. The opening of the Carlin mine in May 1965 brought about a large increase in production; during 1965 and 1966 the mine yielded almost 390,000 ounces of gold.
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