This district is in west-central Calaveras County. It consists of that part of the Mother Lode gold belt that extends from the North Branch-Cottage Springs area southeast through San Andreas and Kentucky House to the vicinity of Fourth Crossing. It is both a lode- and placer-mining district.
The streams were mined during the early part of the gold rush. San Andreas Gulch was first settled by Mexicans in 1848 or 1849. The Tertiary channel deposits were mined by hydraulicking and drifting from the 1850s through the 1880s, and some drift mining has continued until the present time. Lode mining also began in the 1850s. Numerous lode mines were active from the 1870s until about 1900 and again during the 1930s. The Union gold mine was prospected for uranium in the middle 1950s.
The area is underlain by northwest-trending belts of greenstone, amphibolite schist, slate, and lenticular serpentine bodies. At Kentucky House, a large body of dolomitic limestone contains lenses of high-calcium limestone mined by the Calaveras Cement Company. To the east of San Andreas is a granodiorite stock. Overlying portions of the district are numerous deposits of auriferous Tertiary channel gravels.
The lode deposits consist of north-west-trending quartz veins with stringers that contain finely divided free gold and pyrite. The veins occur principally in amphibolite and greenstone. Most of the milling ore averages 1/3, ounce of gold per ton or less, but several of the ore shoots have been mined to depths of 700 feet. Considerable high-grade ore was taken from shallow workings during the early days. Small amounts of tellurides and uranium minerals have been found in this district.
Lode Commodore, Ema, Everlasting, Fellowcraft, Ford, Gottschalk, Golden Hill, Helen, Holland, Kate Hageman, Lookout Mtn., Mester, Pioneer Chief, Rathgeb, Thorpe, Union $200,000+
Placer:Benson, Central Hill, Lloyd, North Branch, Rising Star, Wheats.
Clark. W.B., and Lydon. P.A., 1962, Calaveras County, gold: California Div. Mines and Geology County Rept. 2, pp. 32-93.
Knopf, Adolph, 1929, The Mother lode system of California: U.S. Geol. Survey Prof. Paper 157, pp. 70-71.
Lindgren, Waldemar, 1911 Tertiary gravels of the Sierra Nevada: U.S. Geol. Survey Prof. Paper 73, pp. 209-210.
Logon, C.A., and Franke, H., 1936, Calaveras County, gold: California Div Mines Rept. 32, pp. 235-364.
Ransome, F.L., 1900, Mother Lode district folio: U.S. Geol. Survey Geol. Atlas of the U. S., folio 63, 11 pp.
Storms, W.H., 1894, Ancient chonnel system of Calaveras County: California Min. Bur. Rept. 12, pp. 482-492.
Turner, H.W., 1894, Jackson folio: U. S. Geol. Survey Geol. Atlas of the U.S., folio 11, 6 pp.