By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
The total recorded gold output of Lincoln County through 1959 was 556,800 ounces. The only major gold districts in the county are the Delamar, the largest gold producer, and the Pioche, which yielded considerable gold from silver-copper-lead-zinc ores from 1935 through 1959.
The Delamar district is in south-central Lincoln County on the west slope of the Meadow Valley Range, 29 miles southwest of Caliente.
Callaghan's (1937) report of the district is the chief reference for the summary of the history and geology that follows:
The first gold discovery was in 1891, and the district was organized the following year. The Delamar mine soon became the major mine in the district. Statewide, it outproduced all but a few mines at Goldfield and Tonopah. Despite primitive and costly means of transportation and lack of water in its early years, the district developed steadily until 1909, when the Delamar mine was closed. Only a few ounces of gold was produced in the ensuing two decades. In 1931, exploration at the Magnolia mine, the Jumbo claim, and several other leased properties was successful, and significant production began again in 1932.
Before 1902, district production of precious metals was reported in dollar values instead of ounces, but it may be assumed that most of this early production was in gold. According to Callaghan (1937, p. 38-40) the gross yield of the major mines from 1894 through 1901 was $9,407,555. From 1902 through 1957 the district produced 217,240 ounces of gold, all from lode mines.
A thick section of tilted and faulted Cambrian sedimentary rocks is exposed in the district. These rocks are overlain by Tertiary latites, andesites, tuff, and rhyolite that are also faulted but are less tilted than the Cambrian rocks. Small bodies of diorite and dikes and sills of basalt and rhyolite cut the sedimentary rocks. The rhyolite dikes are related to the flows. The mineral deposits were emplaced some time during the period of volcanic activity, as indicated by the fact that some dikes are older and some are younger than the ores.
The major deposits of the district were in the oldest rock, the Prospect Mountain Quartzite of Early Cambrian age. The deposits are of three types: (1) quartzite breccia, cemented and partly replaced by vuggy fine-grained quartz containing comb quartz and sulfides in the vugs, (2) small veins of fine-grained quartz containing free gold and sulfides, and (3) bedded quartzite with gold deposited in small fractures and along bedding planes. Other deposits are in volcanic breccia, intruded by rhyolite dikes, as at the Magnolia mine and in several of the limestone units where scattered prospect pits have revealed small amounts of silver.
The Pioche district is 19 miles west of the Utah-Nevada State line; the town of Pioche is the county seat of Lincoln County.
This is primarily a silver-lead-zinc copper district; gold is produced as a byproduct. Production began in 1869 but did not reach bonanza proportions until about 1870. Hostile Indians and poor transportation facilities prevented large-scale operations for the first few years. Westgate (in Westgate and Knopf, 1932, p. 5) reported two periods of accelerated production: from 1869 to 1875, and from 1911 through 1958. In the early days two companies - the Meadow Valley Co. and Raymond & Ely - dominated mining in the district, but they were mostly inactive after 1875. The second period of activity was accelerated by the entrance of Combined Metals Reduction Co. into the district in 1915.
Gold production was not recorded before 1906, but in view of the fact that $17 million in metals was produced in 6 years in the early days (Young, 1950, p. Ill), considerable gold must have been produced before 1906. From 1906 to 1959, a total of 104,583 ounces of gold was mined in the district.
A thickness of 17,000 to 18,000 feet of Paleozoic sedimentary rocks is exposed in the Pioche district. These are, according to Westgate (in Westgate and Knopf, 1932, p. 6-7), in ascending order: the Prospect Mountain Quartzite and Pioche Shale (Lower Cambrian); Lyndon Limestone, Chisholm Shale, and Highland Peak Limestone (Middle Cambrian) ; the Mendha Limestone (Upper Cambrian) ; Yellow Hill Limestone and Tank Hill Limestone (Lower Ordovician) ; Eureka Quartzite (Middle Ordovician) ; Ely Springs Dolomite (Upper Ordovician) ; dolomite of Silurian age; Silverhorn and West Range Dolomites (Devonian), Bristol Pass Limestone; Peers Spring Formation, Scotty Wash Quartzite (Mississippian) ; and Bailey Spring Limestone (Mississippian and Pennsylvanian). Unconformably overlying the sedimentary rocks is a thick series of Tertiary or late Mesozoic lava flows consisting of dacite, latite, andesite, and a little rhyolite and basalt.
Tuffs are interbedded with the flows. Locally, stocks and dikes of quartz monzonite cut the sedimentary rocks and lava flows which have been metamorphosed by the intrusions. The Paleozoic formations have been gently folded and the Paleozoic and pre-Pliocene(?) rocks have been shattered by block faults and thrusts.
Ore deposits, according to Knopf (in Westgate and Knopf, 1932, p. 45), are of three types: (1) silver-bearing fissure veins in Lower Cambrian quartzite, (2) argentiferous lenses and pods in granite porphyry dikes, and (3) replacement deposits of sulfides in limestone and dolomite units in the Pioche Shale, Lyndon Limestone, Mendha Limestone, and the Highland Peak Limestone. The deposit of the Combined Metals mine is in a limestone unit of the Pioche Shale. The deposits all seem to have been contemporaneous, having been formed between two periods of Tertiary dike injection (Knopf, in Westgate and Knopf, 1932, p. 51). The bonanza output of the initial years came from the fissure veins in the quartzite; more recently, interest has focused on the sulfide replacement ores.
Minerals of the fissure veins are argentite, cerargyrite, cerussite, and galena in quartz gangue. The podlike deposits in porphyry dikes contain the same minerals as the fissure veins. The replacement deposits are masses of argentiferous pyrite, sphalerite, and galena (Knopf, in Westgate and Knopf, 1932, p. 48-50).
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