San Juan County Colorado Gold Production
The Animas (Silverton) district is in north-central San Juan County near Silverton.
The total gold production of the district is not recorded, but D. J. Varnes (written commun., 1960) estimated that the mineral output before 1901 was valued at $8,200,000, of which 65 percent (about 258,000 ounces) was in gold. Varnes (1963, table 6) credited the south Silverton mining area with a total of 616,000 ounces of gold from 1901 through 1957. The total output through 1959 was at least 874,000 ounces and it may have exceeded 1 million ounces.
The mineralized area of the district is along and south of the south rim of the Silverton caldera, the major features of which are discussed under the Sneffels-Red Mountain district in the Ouray County section (p. 107).
Overlying a dissected terrane composed of Pre-cambrian and Paleozoic sedimentary rocks is the Telluride Conglomerate, the lowermost unit of a thick section of dominantly volcanic Tertiary rocks. This conglomerate is succeeded, in ascending order, by the San Juan Tuff and the Silverton Volcanic Series. After an interval of faulting and subsidence in the caldera, the rocks in the zones of fractures were intruded by dikes of andesite and latite and small bodies of porphyritic quartz monzonite (Burbank, 1933, p. 138-155).
The ore deposits are in veins that fill fissures radial to the rim of the caldera and in veins that diverge from the radial fissures. Some of the fractures that intersect the radial system at high angles are filled with dikes and are mineralized in places.
The largest veins of the area are the Shenandoah-Dives, the Aspen, Silver Lake-Nevada, and the Highland Mary. They are located in Arrastre Basin, Silver Lake Basin, and Cunningham Gulch. These veins are valuable mainly for silver and lead, but some parts contain appreciable amounts of gold with pyrite and chalcopyrite. Locally these veins are rich in specularite and fluorite. The main veins in the upper part of Cunningham Gulch contain galena, sphalerite, and chalcopyrite in a gangue of quartz and some calcite. Near the mouth of Cunningham Gulch, however, in the vicinity of a quartz monzonite stock, the ores change character and consist predominantly of siliceous and pyritic gold-bearing ores with lesser amounts of base-metal sulfides (D. J. Varnes, in Vanderwilt and others, 1947, p. 432-433).