By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
San Juan County is in the western San Juan Mountains in southwestern Colorado, just south of Ouray County.
The county produced about 1,665,000 ounces of gold through 1959 and large quantities of silver, lead, copper, and zinc. The bulk of the gold came from lodes in the Animas (Silverton) and Eureka districts. There are no extensive placer deposits in the county.
Some prospecting in the county was carried on in the 1860's, and the first gold ore was mined in 1870 while the region was still part of the Ute Indian Reservation. The main rush to the region did not begin until 1874, after a treaty with the Ute Indians opened the region to settlement (Ransome, 1901b, p. 19-20). About 2,000 men came into the district during the summer of 1874, and at least that many claims were staked. San Juan County was formed in 1876 with the county seat at Silverton.
During the 1870's, the chief route into the region was by trail from Del Norte 125 miles away on the Rio Grande River and transportation was by pack train and wagon. Both transportation and ore treatment charges were high; consequently, only rich ore could be mined. The area, nevertheless, was actively prospected, and many lodes which subsequently proved valuable were located. In July 1882, the Denver and Rio Grande narrow-gage railroad was completed to Silverton, which reduced transportation rates and permitted the mining of lower grade ores.
There followed a brief but turbulent period marked by unscrupulous promoting, squandering of large sums of money on fruitless mining ventures, and repeated failures to treat successfully the complex San Juan ores (Ransome, 1901b, p. 21-24). In 1890, treatment of low-grade ores by concentration and amalgamation at the Sunnyside and Silver Lake mines near Silverton started a new period in mine development. The first large-scale selective flotation mill, which successfully separated lead and zinc from ores from the Sunnyside mine, began operation in 1917-18 (W. S. Burbank, in Vanderwilt and others, 1947, p. 403).
Silver and lead ores were mined almost continuously in San Juan County from 1873 through 1923 (Henderson, 1926, p. 216). Gold production, which began in 1873, increased steadily, and with few exceptions after 1894 the value of gold produced exceeded that of silver. Profitable operations continued until 1953, when the Shenandoah-Dives mill near Silverton closed. The metal output of the county was small from 1953 through 1959.
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).
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).