By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
San Miguel County is in southwestern Colorado and extends from the west boundary of Ouray County to the Utah-Colorado border.
From 1875 through 1959 the county produced 3,837,000 ounces of gold in addition to large quantities of silver, lead, zinc, and copper. It ranks third among the gold-producing counties of the State.
The first recorded discoveries of gold and silver in San Miguel County were in 1875 on the Smuggler vein (Purington, 1898, p. 755-756). After this discovery many additional mineral locations were made, but because of the inaccessibility of the area and distance from railroad transportation very little ore was mined or shipped before 1881. Prior to 1882 production was less than $50,000 annually (Henderson, 1926, p. 226), but by 1888, output had increased to $1 million annually.
Mining activity was stimulated further in 1890 when the Rio Grande Southern Railroad was completed from Ridgeway to Telluride. The panic of 1893, during which the price of silver dropped, caused a minor decline, but mining again increased in 1895 and gold production alone was valued at $1,421,159 (68,755 ounces), almost double that in 1894. This was the first year the value of gold production exceeded $1 million; from 1901 through 1919 it generally averaged more than $2 million annually (Henderson, 1926, p. 226), and from 1920 through 1926 more than $1 million annually.
The lowest gold production in the county since 1882 occurred during the depression years, 1929 through 1933, when the annual production dropped below $100,000 (4,838 ounces). From 1934 through 1959 the annual production for most years has exceeded $700,000 (20,000 ounces).
Placers in San Miguel County have been of minor importance and have yielded only about 9,700 ounces from 1878 through 1959.
Three districts in San Miguel County are important sources of gold the Ophir, the Telluride, and the Mount Wilson.
MOUNT WILSON DISTRICT
The Mount Wilson district is in southeastern San Miguel County on one of the western spurs of Mount Wilson.
The Silver Pick mine, which was the only property of importance in the district, was located in 1882 and was worked steadily until 1909. Its period of greatest productivity was from 1882 to 1906, when ore worth $750,000 was mined. Purington (1898, p. 847) noted that some ore was worth as much as $100 to $150 per ton, with gold as the major constituent and silver and lead as minor constituents.
The Silver Pick and most of the smaller mines were idle from 1909 through 1959, except for small-scale activity from 1932 through 1941 when about 520 ounces of gold was produced. Total gold production of the district through 1959 was roughly 24,800 ounces.
The oldest rock unit of the district is the Mancos Shale of Cretaceous age. This is overlain in turn by the Telluride Conglomerate, the San Juan Tuff, and the Silverton Volcanic Series, all of Tertiary age. The main mass of Mount Wilson is a stock which ranges in composition from granogabbro to quartz monzonite. The intrusive cuts rocks as young as the Silverton Volcanic Series, and thus is the youngest bedrock unit of the district (C. S. Bromfield, oral commun., 1963).
The ore deposits are in quartz veins containing pyrite, chalcopyrite, arsenopyrite, galena, sphalerite, tetrahedrite, stibnite, and calcite. Most of the gold is believed to be contained in chalcopyrite, galena, and arsenopyrite. The most productive veins, including the Silver Pick, are in the stock; however, a few veins of minor importance are in the invaded sedimentary rocks near the intrusive contact (C. S. Bromfield, oral commun., 1963).
The Ophir district, in eastern San Miguel County, includes the area south of San Miguel River, west of Bridal Veil Creek, and the Ophir Valley on the south. This district includes the Iron Springs, Ames, and South Telluride mining areas.
Mines in the Ophir Valley were operating as early as 1878, and the ore was shipped to the Silverton smelter (Henderson, 1926, p. 53). In 1879 two arrastres were built in the vicinity of Ophir. In 1883 a small smelter was built at the old town of Ames, but it was unsuccessful and was operated for only a year (Henderson, 1926, p. 216). According to H. C. Burchard, as quoted by Henderson (1926, p. 217-218), several mines including the Alta, the largest producer in the district, were developed as early as 1881. According to D. J. Varnes (in Vanderwilt and others, 1947, p. 427), the history of the district has been one of intermittent production except for a few large mines which operated fairly continuously. Most of the ore mined early from this district was rich in silver, and activity of the camp depended in part on the price of silver.
Some mines, which were idle most of the time, became substantial producers during times of favorable metal prices. The district was almost idle from 1930 through 1936, was substantially active from 1937 through 1948, and was idle again from 1949 through 1959.
Production records are fragmentary and the amount of gold is usually included with the value of other metals (D. J. Varnes, in Vanderwilt and others, 1947, p. 427). The total gold output of the district through 1959 was probably a minimum of 200,000 ounces.
The Ophir district is 5 to 7 miles west of the Silverton caldera, and its ore deposits are in structures related to the caldera. Paleozoic and Mesozoic sedimentary rocks are exposed in the bottoms of the deeper valleys and are overlain, in ascending order, by the Telluride Conglomerate, the San Juan Tuff, and the Silverton Volcanic Series. These are all intruded by small igneous bodies ranging in composition from quartz monzonite to diorite.
The rocks are cut by a network of fissures and veins. The most productive veins trend westerly and contain pyrite, galena, sphalerite, chalcopyrite, and freiburgite as the more common sulfide minerals. Hematite and magnetite also are present, and the gangue minerals are chiefly quartz, with some calcite, manganiferous iron carbonate, and barite. Some veins in the Ophir Valley trend north and northeast, and the minerals of these consist of quartz and pyrite carrying free gold and some silver.
Gold also occurs in the altered and pyrite-impregnated country rock adjacent to the veins. The veins in the San Juan Tuff and overlying andesite commonly are more sharply denned than those in overlying volcanic rocks. Veins also occur within the sedimentary rocks. Replacement deposits are rare and of small extent (Cross and Purington, 1899; D. J. Varnes, in Vanderwilt and others, 1947, p. 425-427).
The Telluride (upper San Miguel) district is along the east border of San Miguel County immediately southwest of the Sneffels-Red Mountain district in Ouray County. The geology is described on pages 107-108; only the history and production will be discussed in this section.
The first discoveries in the Telluride district were made on the Smuggler vein in 1875 (Purington, 1898, p. 752-754). There was only a small production through 1882, but in 1883 a shipment of 4 tons of high-grade ore from the Smuggler vein yielded 800 ounces of silver and 18 ounces of gold per ton and thereafter production increased rapidly.
Since 1898, the large output of the Telluride district has come chiefly from the mines of three large companies the Liberty Bell, Smuggler-Union, and the Tomboy (Henderson, 1926, p. 53, 224-225). The Liberty Bell mine, which produced only silver and gold, was operated from 1898 to 1921 and during that period had an output of 633,021 ounces of gold (Henderson, 1926, p. 225). The Tomboy group of mines closed in 1927 and the Smuggler-Union closed in 1928 after operating 52 years (W. S. Burbank, in Vanderwilt and others, 1947, p. 421).
In 1940 the Smuggler-Union group of mines and the Tomboy holdings were organized as the Telluride Mines, Inc., which was still active in 1959. Total production of gold through 1959 was at least 3 million ounces; thus Telluride is one of the 25 leading gold-producing districts in the United States. Large quantities of silver, lead, zinc, and copper have also been produced.