This district is in the southwest corner of Calaveras County about six miles south of Copperopolis. It was named by the author for the Alto mine, a major source of gold in the area. The region was first placer-mined during the gold rush, when the gravels underlying nearby Tuolumne Table Mountain were worked by drifting. The Alto mine was discovered in 1886 and operated on a large scale until 1907. It has an estimated total output of $1 million. The district is underlain by slate of the Mariposa Formation (Upper Jurassic) with some interbeds of massive greenstone. The gold occurs in thin quartz veins or with disseminated pyrite in the greenstone. Placer gold was recovered from Eocene quartzitic gravels overlying the slate. Some of the gravels are capped by latite porphyry of Tuolumne Table Mountain.
Clark, W. B., and Lydon, P. A" 1962, Calaveras Coonty, Alto mine: California Div. of Mines and Geology County Report 2, pp. 37 and 40. lowell, F. lo, 1919, Alto gold mine: California Min. Bur. Rept. 16, p. 629 (erroneously shown as being in Stanislaus County),
Taliaferro, N. lo, and Solari, A. J., 1948, Geologic map of the Copperopolis quadrangle: California Div. Mines colored quadrangle map (PI. I of Bull. 145).