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This is one of the famous pocket-mining districts of the Sierra Nevada cast gold belt. Sonora, the seat of Tuolumne County, was founded in 1848 soon after the discoverey of rich placer deposits here and at Shaws Flat to the north. It was named for the state of Sonora in Mexico. The placers were extremely rich; the Sonora placers were credited with an output of $11 million and those of Shaws Flat yielded $6 million. The famous Holden Chispa nugget, which weighed over 28 pounds, was taken from within the city limits of Sonora in Holden's Gardens. In 1879 the Bonanza mine, also in town, yielded a pocket that contained $300,000. Later, large amounts of beautifully crystallized gold with tellurides were recovered from the Sugarman and Negro mine. Pocket mining continued almost steadily until World War II, and there has been some prospecting and development work since.
The central part of the Sonora district is underlain by a belt of crystalline limestone, which extends south from Columbia. The limestone is associated with slate, schist, and quartzite all are part of the Calaveras Formation (Carboniferous to Permian). To the west is amphibolite and to the east is granodiorite.
The gold-quartz veins are largely confined to the slate, schist, and amphibolite. Much of the output from the lode mines has been from small but extremely rich pockets. The high-grade are commonly contains crystallized gold, and in a few places the telluride minerals petzite and sylvanite are present. The veins, usually only a few feet wide, are often associated with diorite and aplite dikes. The rich early-day placer deposits were in deep crevices and potholes in the limestone.
Aetna, Bonanza $1.5 million, Eureka, Fairview, Gerrymander, Golden Gate $100,000+, Hope $200,000+, Josephine, Lazar $100,000+, Lewis, Manzanita, O'Hara $100,000+, Rainbow, San Guiseppe, Sell, Stockton, Stuart and Morris, Sugarman $1 million, Tanzy, Vandelier.
Eric, J. H., Stromquist, A. A., and Swinney, C. M., 1955, Geology and mineral deposits of the Angels Camp and Sonora quadrangles: California Div. Mines Spec. Rept. 41, 55 pp.
Logan, C. A., 1949, Tuolumne County, gold: California Jour. Mines ond Geology, vol. 45, pp. 54-75.
Tucker, W. B., 1916. Tuolumne County, gold: California Min. Bur. Rept. 14, pp. 135-168.
Turner, F.L., and Ransome, F.L., 1897, Sonora folio: U. S. Geol. Survey Geol. Atlas of the U.S., folio 41, 7 pp.