By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Tuolumne County, one of the Mother Lode counties, is in central California between Calaveras County on the north and Mariposa County on the south.
In the 1850's the gold-rush prospectors and adventurers overran the entire Mother Lode country. They soon found the gold placers of Tuolumne County, which became the richest in California. During 1850-70 this county was one of the leading gold producers in the State. At least $151,175,000 (about 7,338,600 ounces) of placer gold was produced before 1899, mostly from the Tertiary and Quaternary gravels in the Columbia Basin and the Table Mountain channel in the Jamestown-Sonora area (Julihn and Horton, 1940, p. 69).
After 1890, as the placers were depleted, mining of quartz veins increased, and after 1903 lodes exceeded placers in production. Total estimated production for the county through 1959 was 10,131,000 ounces: 2,580,000 ounces from lodes and 7,551,000 ounces from placers.
Nearly all the lode mines lie in a zone about 14 miles wide that crosses the county from northwest to southeast. Deposits in the Mother Lode are along the southwest side of the zone near the contact between the Calaveras and Mariposa Formations. Deposits of the East Belt are on the northeast side, parallel to the Mother Lode. Between the two is a chain of small but rich deposits known as the Pocket Belt.
COLUMBIA BASIN-JAMESTOWN-SONORA DISTRICT
The Columbia Basin-Jamestown-Sonora district is in parts of Tps. 1 and 2 N., Rs. 14 and 15 E., in northwest Tuolumne County.
From 1853, when the placers were discovered, to 1870, an area less than 2 miles in diameter - the well-known Columbia Basin - produced more than $55 million in gold (Julihn and Horton, 1940, p. 71). Other rich deposits were at Sonora, Yankee Hill, and Jamestown. More than 95 percent of the placer gold of Tuolumne County was derived from Quaternary gravels, the gold of which was reworked from eroded Tertiary gravels (Julihn and Horton, 1940, p. 70). In the Columbia Basin most of the gold was extracted by hand from natural riffles and from between pinnacles on the limestone bedrock surface at the base of the gravels. In the vicinity of Jamestown and Sonora, Tertiary channel gravels were worked by drift mines. The richest placers were exhausted fairly early and by the late 1870's placer production dropped sharply. This trend was reversed for a short time in the late 1930's, but production decreased after World War II. Total gold production from this area was about $121 million, or about 5,874,000 ounces (Julihn and Horton, 1940, p. 69).
The Tertiary gravels, which yielded gold valued at between $5 and $6 million, were worked chiefly by drift mines in the Table Mountain channel and by surface mines at Chinese Camp and Montezuma (Julihn and Horton, 1940, p. 70).
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