San Juan County Colorado Gold Production
The Eureka district is in northeastern San Juan County, at the head of the Animas River drainage basin, and includes mines in the Animas Forks, Cement Creek, and Mineral Creek areas.
Silver, gold, lead, and zinc ores worth a total of $50 million have been mined from this district, chiefly from the Sunnyside and Gold King mines (W. S. Burbank, in Vanderwilt and others, 1947, p. 433). Total gold production of the district through 1959 was at least 500,000 ounces.
The district is on the north side of the Silverton caldera along a northeast-trending graben that extends from the Silverton caldera to the Lake City caldera in Hinsdale County. Part of the district extends into the central downfaulted block of the Silverton caldera, the major features of which are discussed under the description of the Sneffels-Red Mountain district in Ouray County (p. 107).
Most of the rock exposed in the district belongs to the Silverton Volcanic Series of Tertiary age; in addition a few small bodies of intrusive rhyolite and latite are present (W. S. Burbank, in Vanderwilt and others, 1947, p. 434).
The ore deposits are fissure fillings, most of which are in the southwest part of the graben within 1 mile of the crescentic faults that outline the central block of the Silverton caldera. Some of the larger fissures, however, are mineralized in places along a length of 6 miles. Veins in the Sunnyside mine display several stages of mineralization— quartz-pyrite, base-metal sulfides, and rhodonite. Gold-bearing quartz cuts the base-metal sulfides and rhodonite (W. S. Burbank, in Vanderwilt and others, 1947, p. 434). Ore deposits in the Cement Creek area are quartz-pyrite-gold veins characteristic of the Gold King mine, tungsten-bearing quartz veins, and chimney deposits rich in lead, silver, and a little gold (D. J. Varnes and W. S. Burbank, in Vanderwilt and others, 1947, p. 435, 436).