By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Mining began in Mono County in 1862, when silver ore was discovered at Blind Springs Hill (Sampson and Tucker, 1940, p. 117), but gold mining became commercially important some time later. The most important lode districts were Bodie and Masonic. Small amounts of placer gold have been mined near the headwaters of the Walker River, Virginia Creek, and Dog Creek, and at Bodie Digging, north of Mono Lake (Sampson and Tucker, 1940, p. 121). According to Rinehart and Ross (1956, p. 16), scattered mines in the Casa Diablo Mountains quadrangle have produced less than $1 million in gold and silver.
Total gold production for the county from 1880 through 1959 was 1,176,200 ounces, the bulk of which was lode gold. More than 90 percent of this came from the Bodie district. The following summary of the geology has been prepared from reports by Sampson and Tucker (1940, p. 118) and Rinehart and Ross (1956).
The western part of the county is underlain predominantly by granodioritic rocks of the Sierra Nevada batholith and a narrow belt of lower Paleozoic or Precambrian metasedimentary rocks. The Benton Range, in the southeastern part of the county, is also composed of these rocks. Most of the remainder of the county is underlain by bodies of granitic rocks, diorite, and gabbro, of Cretaceous (?) age, and by basalt and rhyolite flows and rhyolite tuff of Tertiary and Quaternary age. Granitic stocks of Cretaceous (?) age form the White Mountains, at the southeast boundary of the county, and Blind Springs Hill, east of Benton.
The Bodie district is in northeast Mono County, in T. 4 N., R. 27 E. Mining was started in 1860, and the district was active until 1955. Total gold production was 1,456,300 ounces, most of which came from the Standard mine.
Country rock in the district consists of a "complex of igneous rocks and breccias" overlain (perhaps along a fault) by Tertiary hornblende andesite (Brown, 1908, p. 343-344). Ore bodies occur in three sets of auriferous quartz veins in the andesite (Brown, 1908, p. 345-346).
The Masonic district is in northeast Mono County, in T. 6 N., R. 26 E. Gold occurs in quartz veins in metamorphic rocks and granite. The Pittsburg-Liberty mine, with a record of $470,000 in gold (about 34,000 ounces), has been the important producer (Sampson and Tucker, 1940, p. 121). The district was active on a small scale in 1959. No detailed descriptions of the geology were found.
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