By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Eureka County was formed in 1873 from the eastern part of Lander County and from small parts of White Pine and Elko Counties. North-trending mountain ranges separated by wide valleys, typical of Great Basin terrain, are the predominant land forms in the county.
The first mineral discoveries were made in 1863 in the Cortez district, and the following year additional discoveries were made in the Eureka and Diamond districts. The Eureka district quickly emerged as the county's leading producer; the mining history of the county is largely a history of the Eureka district.
Gold production before 1902 is difficult to determine because of the practice in the early days of combining production of all metals and reporting it in dollar values. It can be stated, however, that a minimum of 1 million ounces of gold was mined from the Eureka district alone up to 1883 (Vanderburg, 1938a, p. 13). From 1902 through 1959 a total of 203,597 ounces of gold was mined in the county 9,618 ounces was from placers, and the remainder was a byproduct from silver ores.
The Buckhorn district is in the south end of the Cortez Mountains, 5 miles northeast of Cortez.
The production of the district, which is virtually that of the Buckhorn mine discovered in 1908, was 39,632 ounces in gold from 1910 through 1959. The Buckhorn was mined for gold, and silver was a byproduct.
The highest grade ore occurs along a nearly vertical fault that disrupts country rock composed of basalt and interbedded scoria. Both sulfide and oxide ore are present. The oxide ore consists of kaolinized breccia with high gold and silver values; the sulfide ore is primarily fine grained pyrite in talc-rich rock (Vanderburg, 1938a, p. 19-21).
The Cortez district is about 36 miles south of Beowawe at about the middle of the boundary line between Eureka and Lander Counties.
The district was founded in 1863 and was an active producer for many years. The principal mine is the Garrison mine. Silver has been the chief commodity, and lead, copper, and gold have been byproducts. From 1863 to 1903, a minimum of $500,000 (about 24,270 ounces) in gold was produced, and from 1902 to 1936, a total of 8,267 ounces of gold was produced (Vanderburg, 1938a, p. 23-24). Total recorded production of gold from 1863 to 1958 was about 48,720 ounces.
The country rock consists of Paleozoic limestone of probable Cambrian age, overlain by quartzite of probable Ordovician age (Emmons, 1910, p. 101-103). These were intruded by a mass of granite and by porphyry dikes. Ore occurs as replacement bodies in the limestone and in the quartzite. Most of it is in fissures parallel to the dikes. The ore minerals are galena, stibnite, pyrite, sphalerite, stromeyerite.
Gangue minerals are quartz and calcite. The galena is rich in silver. Ore such as this is reported (Emmons, 1910, p. 104) to carry from $3 to $15 per ton in gold. Some of the ore is oxidized and consists of silver chloride, copper carbonates, and iron and manganese oxides.
The Eureka district is located at the town of Eureka, in southeastern Eureka County.
The first locations were made in 1864, but little was produced until 1869, when large ore bodies were found on Ruby Hill (Hague, 1892, p. 6). The period of greatest production was 1871-88. Hague (1892, p. 6-7) estimated that $20 million in gold was produced from 1869 to 1883. During this period two mines emerged as the major producers - the Eureka Consolidated and the Richmond. After 1888 the major ore bodies were exhausted, the smelters were shut down, and the district entered a period of inactivity broken only by sporadic small-scale mining by lessees.
In 1905 the Richmond and Eureka properties were consolidated, and, after the old workings were rehabilitated, shipments of low-grade ore were made. But the low-grade ores could not be profitably mined under constantly increasing costs, and the workings were again closed except for small-scale leasing operations. Several unsuccessful exploratory ventures took place in 1919 and 1923. In 1937, however, the Eureka Corp., Ltd., discovered a new ore body. The Fad shaft was sunk to exploit this ore, but in 1949 work was halted at a depth of 2,500 feet because of the large flows of water that were encountered.
Other more recent exploration in the Adams Hill area was more successful; considerable ore was mined from the T. L. shaft. There has also been a renewal of activity in the Diamond mine, about 4 miles south of Eureka (Nolan, 1962, p. 2-3).
The production of the Eureka district is difficult to ascertain, but annual production has been tabulated by Nolan (1962, p. 56-59) from what he considered to be reliable sources.
Using Hague's (1892, p. 6-7) estimate of $20 million (967,585 ounces) in gold from 1869 to 1883 and Nolan's (1962, p. 58-59) data from 1884 through 1959, we arrive at an approximate total gold production of 1,230,748 ounces, which, in view of the generalized nature of Hague's estimate, should be rounded off to 1,230,000 ounces.
The rocks of the Eureka district consist of a thick section of Paleozoic sedimentary rocks, a Cretaceous sedimentary formation, and Cretaceous and Cenozoic igneous rocks.
The Cambrian formations, which in total are about 9,000 feet thick, are, in ascending order, the Prospect Mountain Quartzite, Pioche Shale, Eldorado Dolomite, Geddes Limestone, Secret Canyon Shale, Hamburg Dolomite, Dunderberg Shale, and the Windfall Formation (Nolan, 1962, p. 5-9). Rocks of Ordovician age are about 2,200 feet thick and consist of the Pogonip Group, Eureka Quartzite, and Hanson Creek Formation. The Devils Gate Formation of Devonian age, the Chainman Shale and Diamond Peak Formation of Mississippian age, and the Carbon Ridge Formation of Permian age complete the Paleozoic sequence.
Scattered outcrops of the Newark Canyon Formation of Early Cretaceous age unconformably overlie the older rocks (Nolan, 1962, p. 9-13). Igneous rocks of Cretaceous age consist of a plug of quartz diorite and a sill-like mass of quartz porphyry. Other igneous rocks range in age from Oligocene to late Tertiary or Quaternary and include hornblende andesite, rhyolite, rhyolite tuff, andesite, and basalt (Nolan, 1962, p. 13-17).
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).
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.