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).
EAST BELT DISTRICT
The East Belt district includes a system of lodes parallel to the Mother Lode, about 10 miles to the east. The settlements of Pooleys Ranch, Soulsbyville, and Tuolumne are along the East Belt.
The first claims on the East Belt were located in the mid-1850's. The deposits proved to be extremely rich in gold, and before 1899 the yield of 38 East Belt mines was about $19,340,000 (about 938,800 ounces), a far greater output than that of the Mother Lode for this period (Julihn and Horton, 1940, p. 19). These veins, however, were for the most part shallow, and high production could not be maintained for any extended period. Nevertheless, the Soulsby mine has produced $6,750,000, and there are five other mines with a production of $1 million or more (Julihn and Horton, 1940, p. 52-53). Total production through 1959 of the East Belt in Tuolumne County was about 965,000 ounces.
The veins of this district are in and grouped around a stock of granodiorite that intruded the Calaveras Formation (Julihn and Horton, 1940, p. 51-52). Pegmatites cut both the granodiorite and Calaveras Formation, and the gold-bearing veins are later than the pegmatite. Most of the veins are in the granodiorite and along the granodiorite-country rock contact. The veins are narrow and they pinch and swell; nevertheless, they are remarkably persistent. Free gold is found in these veins with quartz and with sulfides which include pyrite, pyrrhotite, chalcopyrite, galena, sphalerite, and complex sulfides of lead and antimony.
The Groveland-Moccasin-Jacksonville area is in parts of T. 1 S., Rs. 14, 15 and 16 E., in southern Tuolumne County.
Before 1899, Quaternary gravels in this area yielded about $34 million in gold (Julihn and Horton, 1940, p. 69). There is no consistent record of later production, although Julihn and Horton (1940, p. 81, 82) reported dredging operations along Moccasin Creek in 1937 and 1938.
The Longfellow mine, the most productive of a group of lode mines near Groveland, produced an estimated $1/2 million in gold (about 24,200 ounces) before 1899. No record was found of any later lode mining in this district.
MOTHER LODE DISTRICT
The Mother Lode district is delineated by a chain of about 40 mines that crosses Tuolumne County from northwest to southeast from a point just west of Tuttletown in the north to the headwaters of Moccasin Creek in the south, where the lode enters Mariposa County.
Probably the first major property to be developed in this district was the Harvard mine, discovered in 1850 (Julihn and Horton, 1940, p. 30). In 1852 the Dutch claim was located. The claim was later consolidated with the Sweeney and App-Heslep mines, and this combination became the most productive property in the district with an output of about $9 million in gold to 1928 (Julihn and Horton, 1940, p. 20). Another important group of mines, first operated in the 1860's, was the Golden Rule (Julihn and Horton, 1940, p. 42).
The Mother Lode mines developed slowly, but as they were deepened, higher grade ore was found and their production increased. Probably the most active period was between 1890 and 1920 when the Rawhide, Harvard, Dutch-App, and Eagle-Shawmut were at their peaks of activity. After World War I, there was a long period of idleness which was ended by the increased price of gold in 1934. The mines were pumped dry and retimbered, and a short period of prosperity returned to the district.
During World War II the mines were closed again, and in the postwar period resumption of mining was discouraged by the low grade of the ore and the constantly increasing costs made even higher by the great depths of the mines.
The Mother Lode district in Tuolumne County is credited with $4,310,000 in gold before 1899 (Julihn and Horton, 1940, p. 18). The six largest mines produced a total of $29,750,000 in gold to 1928, and from 1933 through 1959 the Mother Lode produced 86,112 ounces of lode gold and 41,524 ounces of placer gold. The placer production is probably from Tertiary gravels near Jamestown and no doubt should have been credited to that district rather than the Mother Lode. The minimum total production for the district is about 1,550,000 ounces.
POCKET BELT DISTRICT
The Pocket Belt district, 5 to 6 miles wide, is between the Stanislaus River and the Jamestown-Sonora area.
This district is known for the extreme richness of small veins that produced minor fortunes in a very short time with small investments. Perhaps the best known of these mines was the Bonanza, located in 1851, which in a single week produced about $300,000 worth of gold. The Pocket Belt has been noted for spectacular short-term operations; thus it exerts a persistent lure, and sporadic activity will probably continue indefinitely.
Production of the district was about $5V2 million (267,000 ounces) (Julihn and Horton, 1940, p. 60). Bedrock is the Calaveras Formation, which has been fractured and laced with seams of quartz and calcite. Locally, where these seams swell, there are concentrations of coarse gold. Some of the gold is crystallized, and in some places it is accompanied by petzite, calaverite, and other tellurides. The seams or pockets are noted for their discontinuity (Julihn and Horton, 1940, p. 60).