The Northumberland district on on the east side of Toquima Range, 25 miles north of Belmont and 76 miles northeast of Tonopah. It is primarily a silver-producing district and was founded in 1866. By 1891 activity had ceased, and the district was dormant until 1936 when large deposits of low-grade gold ore, amenable to open pit mining, were discovered (Krai, 1951, p. 135-136). During 1939-42, gold totaling 32,756 ounces was mined. The War Production Board Order L-208 of 1942 caused operations to be recessed until after World War II. Total gold production from 1936 through 1959 was 35,353 ounces.
The rocks consist of dolomitic limestone and carbonaceous and calcareous shale (Krai, 1951, p. 136). A mass of monzonite and a younger porphyritic rhyolite or quartz latite intruded the sediments. The gold deposits occur in a carbonaceous shale bed, 60 to 70 feet thick, in the vicinity of the roof of the monzonite intrusion.
ROUND MOUNTAIN DISTRICT
The Round Mountain district is on the west flank of the Toquima Range, 45 miles north of Tonopah and about 8 miles north of Manhattan.
Rich gold ore was discovered in 1906, but no production was recorded until 1907. Both placer and lode mines were worked from the beginning. The lodes were worked until 1935, but the placers were still being worked, on a fairly large scale, in the early 1950's by the Round Mountain Gold Dredging Corp. Their operations were suspended during 1953-57, but were resumed in 1958; in 1958-59 this company was the largest gold producer in Nevada.
Ferguson and Cathcart (1954) estimated the total gold production of the district to be worth $8 million, of which about 15 or 20 percent was from placers and the remainder was from lodes. This would amount to about 329,000 ounces of lode gold and 58,200 ounces of placer gold. Most of the district's production during 1950-59 was combined with reports from other districts, but it was probably about 150,000 ounces. Total gold production for the district through 1959 was about 537,000 ounces.
The oldest rocks in the area are lower Paleozoic limestone, jasper, and dark slaty schist (Ferguson, 1922, p. 386-398). These rocks are intensely folded and were intruded during Cretaceous time by bodies of granitic magma. Tertiary rocks, chiefly porphyritic rhyolite (Oddie Rhyolite) and lake beds of the Siebert Formation, overlie the folded Paleozoic rocks.
There were two periods of mineralization. The first occurred just after the granitic intrusions and is characterized by huebnerite-bearing veins in the granite. The second period of mineralization is of Tertiary age and resulted in the formation of epithermal gold-bearing quartz veins in the rhyolite. The mineralogy of these veins is relatively simple and consists of gold, auriferous pyrite, and sparse realgar in a gangue of drusy and comb quartz, adularia, and alunite. After the primary gold mineralization, additional fissures were formed in which iron and manganese oxides and gold were deposited by supergene solutions. The gold in these later fractures was probably derived from the primary auriferous pyrite.
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