Downieville Mining District

This is a lode and placer gold-mining district in west-central Sierra County in the general vicinity of the town of Downieville. It includes the Fir Cap Mountain, Craycroft, China Flat, and Slug Canyon areas and part of the Pliocene Ridge area. The Goodyear's Bar-Alleghany belt lies immediately to the west, the Sierra City district to the east, and the American Hill district to the south.

This area was prospected soon after the beginning of the gold rush. Major William Downie and his party arrived here in November, 1849. Soon a town was laid out, which was named for him early in 1850. The Downieville mining district was organized with "claims fixed at 30 feet per man". Many rich strikes were made; one claim 60 feet square yielded $80,000 in six months. At nearby Tin Cup Diggings, three men filled a tin cup with gold each day before quitting. A 25-pound nugget was found in the river upstream from the town in 1850. More than 5,000 persons lived here in 1851. After the surface placers were exhausted, the river was mined, and hydraulic and drift mining became important. Mining continued almost steadily until World War II, and intermittent prospecting and skin diving for gold continues.

The district is underlain predominantly by north-trending beds of phyllite, slate and quartzite of the Calaveras Formation (Carboniferous to Permian). To the west are greenstone, amphibolite, and serpentine. The higher ridges are capped by Tertiary andesite, which in places overlies rich Tertiary auriferous gravels. There are fairly extensive recent river and terrace gravels along the Yuba River and its branches.

A considerable number of gold quartz veins occur chiefly in greenstone and slate. The veins range from one to 10 feet in thickness. The ore contains free gold and varying amounts of sulfides. The milling-grade ore usually averages 0.25 to 0.33 ounce per ton. Some ore shoots had stoping lengths of as much as several hundred feet. Some high-grade ore pockets have been taken from some of the mines. The Tertiary channel gravels are quartzitic, often well-cemented, and in places contain extremely coarse gold.