Campo Seco and Valley Springs are in northwestern Calaveras County. Substantial amounts of gold have been produced in this district from a variety of mineral deposits. These deposits include Recent stream gravels, quartz-rich gravels of Eocene age, narrow quartz veins, and massive copper and zinc sulfide deposits of the noted Penn mine, from which gold was recovered as a by-product.
The older quartzitic gravels were mined by hydraulicking and ground sluicing in the early days and later by draft mining. The Recent gravels were worked by dragline dredgesThe Penn mine is credited with a production of 60,000 ounces of gold.
The principal rocks underlying the district are green schist, slate, serpentine and greenstone. The quartz-rich gravels which overlie these rocks occur in patches and are the remnants of an Eocene river channel system. Some of these gravels are capped by andesite. The massive sulfide deposits at the Penn mine and the gold-quartz veins are in greenstone and schist.
Clark, W. B., and Lydon, P. A., 1963, Calaveras County, placer gold: California Div. Mines and Geology County Report 2, pp. 76-84.
Heyl, G. R., et al., 1948, Copper in California, Penn zinc-copper mine: California Div. Mines Bull. 144, pp. 61-84.
Turner, H. W., 1894, Jackson folio: U. S. Geol. Survey Geal. Atlas of the U. S. folio 11,6 pp.