United States.
—Magnesite in commercial quantity occurs in California, Nevada, and Washington. Reports of workable deposits in other states have not been verified.
In California there are magnesite deposits in many places throughout the Coast Range and on the west slope of the Sierras, from Mendocino and Placer counties on the north to Riverside County on the south. Before the war, mining was limited to a few localities and the annual output was about 10,000 tons, but the demand caused by large reduction in imports started active prospecting and development, with the result that in 1917 thirteen counties yielded a total of 211,663 tons, valued at $2,116,630. In nine counties the deposits are large and in four counties only small deposits have been found as yet. The most important deposits are in Napa, Santa Clara, San Benito, and Tulare counties. In 1917, 63 per cent. of the crude magnesite produced was mined in Tulare County. Practically all of the California magnesite deposits are irregular veins in serpentine, resulting from the alteration of magnesian igneous rocks.
In the state of Washington deposits of crystalline magnesite have been found at several places. The Washington magnesite differs greatly from the California deposits and occurs in larger masses. It is coarsely crystalline, like marble or coarse textured dolomite, and is red, pink, black, white, and gray. The Stevens County magnesite has been formed by the replacement of lenses of dolomite in sedimentary rocks. The recrystallization of the purer magnesian carbonate may have been secondary and influenced by the intrusion of basic magnesian rock which occurs above and below the magnesite in some places. The larger deposits are 200 or more feet thick and 1,000 or more feet long. Estimates of one million tons within 100 feet of the surface are reasonable for at least three of the deposits. Mining in Washington began in December, 1916, with a production of 715 tons. The output in 1917 was 105,175 tons, valued at $783,188.
The only known deposit of magnesite in Nevada is an extensive sedimentary bed in the valley of Muddy River, Clark County. The magnesite carries more than 5 per cent. lime and more than 11 per cent. silica. It has not been developed.
The total production of magnesite in the United States in 1915 was 30,499 short tons; in 1916, 154,974 short tons; in 1917, 316,838 short tons; and in 1918, 231,605 short tons.
Mexico.
—On the Island of Santa Margarita, in Magdalena Bay, Lower California, are extensive deposits of magnesite from which exports have been made to the United States. Walls of canyons in the mountains show masses of magnesite several feet thick, and magnesite boulders strew the stream beds. Large quantities can be obtained without mining and need only to be broken up for shipment. An analysis of calcined magnesite from Santa Margarita Island shows practically no silica, lime, or iron.
South America.
—The deposits on Margarita Island, Venezuela, are of the amorphous or California type and occur in veins and stockwork. No information is available regarding their extent, but 500 tons were exported to the United States in 1915.