The Mammoth or Lake district is in southwestern Mono County about 50 miles south of Bridgeport. The district is on the east flank of the Sierra Nevada Mountains and is just east of Mammoth Lakes, a well-known resort area. The Devil's Postpile National Monument is about 10 miles to the west.
Gold and silver-bearing veins were discovered here in 1878, and a short-lived "rush" followed. Much of the production at that time was from the Mammoth mine, which yielded $200,000 in 1878-81. The district was organized in 1887. Several thousand people were in the area then, the principal settlements having been Mammoth City, Mill City, and Pine City. Some mining was done in the late 1890s, early 1900s, and again in the 1930s. The Beauregard mine was active from 1954 to 1958, and there has been minor prospecting and development work since. The value of the total production of the district is estimated at $1 million.
Geology and Ore Deposits
The district is underlain chiefly by northwest-trending beds of metamorphosed latite. Present in smaller amounts are schist, hornfels, marble, tactite, and quartzite. Granitic rocks are to the east and south. The ore deposits occur in a northwest-trending zone of alteration in the metamorphosed latite. This zone, 2.5, miles long and 1/2 mile wide, contains disseminated pyrite. The pyrite has been oxidized so that the zone is stained a bright reddish brown.
The ore deposits consist of northwest-striking and steeply-dipping quartz veins and stringers that contain free gold, auriferous pyrite, pyrrhotite, and smaller amounts of other sulfides. Silver commonly is abundant. Milling-grade ore usually averages 1/4 to 1/2 ounce of gold per ton. The veins range from a few to several tens of feet in thickness.
Argosy, Beauregard, Don Quixote, Lisbon, Mammoth $200,000, Mammoth Consolidated $100,000, Monte Cristo $100,000, Sierra Group.
DeGroot, Henry, 1890, Lake mining district: California Mining Bur. Rept. 10, pp. 340-342.
Mayo, E. B., 1934, Geology and mineral deposits of Laurel and Convict basins, southwestern Mono County: California Div. Mines Rept. 30, pp. 79-87.
Rinehart, C. D., and Ross, D.C., 1964, Geology and mineral deposits of the Mount Morrison quadrangle.
Sierra Nevada. Colifornia: U. S. Geol. Survey Prof. Paper 385, pp. 97-100.Sampson, R.J., and Tucker, W. B., 1940, Mineral resources of Mono County, gold: California Div. Mines Rept. 36, pp. 120-140.
Whiting, H. A., 1888, Lake mining district: California Min. Bur. Rept. 8, pp. 373-375.