By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Ouray County is in southwestern Colorado in an area drained by the headwaters of the Uncompahgre River, a tributary of the Gunnison River. The mineralized areas are in the southern part of the county within the San Juan Mountains and include the Sneffels-Red Mountain and the Uncompahgre mining districts.
Ouray County was originally part of territory that was owned by the Ute Indian Tribe and that was ceded to the United States in 1873. Until 1873 the area had been little explored, but after the treaty was ratified, settlers and prospectors overÂ¬ran the county (Henderson, 1926, p. 24). By 1874 nearly four-fifths of the claims were on silver lodes; however, rich deposits of gold as well as silver were found in 1875 in the Mount Sneffels area, and the famous Camp Bird mine, the largest gold producer in the region, was located in 1877 (W. S. Burbank, in Vanderwilt and others, 1947, p. 403).
The area developed rapidly, and additional discoveries were made in the Red Mountain area. By 1896, however, increased mining costs, a drop in the price of silver, and depletion of the rich surface ores caused many of the mines to close, although the Camp Bird continued to be a significant producer until 1917. The mine was reopened in 1926 and was operated sporadically through 1956. Total gold production of Ouray County through 1959 was about 1,911,000 ounces, more than half of which came from the Camp Bird mine.
SNEFFELS-RED MOUNTAIN DISTRICT
The Sneffels-Red Mountain district is in southern and southwestern Ouray County, 8 to 12 miles from Ouray. The Sneffels camp is at the head of Canyon Creek in Imogene Basin west of Hayden Mountain, and the Red Mountain camp is at the head of Red Mountain Creek east of Hayden Mountain.
The district is the principal gold producer in Ouray County and one of the leading producers in the San Juan region, but little is known about its production in the early years. According to W. S. Burbank (in Vanderwilt and others, 1947, table 8, p. 404-405), deposits of late Tertiary age in Ouray County to 1945 produced about 1,693,000 ounces of gold, most of which came from the Sneffels area.
The Camp Bird mine, the principal producer of metals in Ouray County until it closed at the end of 1956, produced, from 1896 to 1916, gold, silver, lead, and copper valued at $27,269,768, of which at least $21,884,894 (1,058,774 ounces) was in gold (Henderson, 1926, p. 185). Production of other mines in the district has been small and sporadic. Total gold output of the district through 1959 was indicated to be about 1,723,000 ounces.
The Sneffels-Red Mountain district and the adjoining Telluride district to the southwest, in San Miguel County, are geologically contiguous, and therefore they are treated as a geologic entity in the following description.
The main geologic feature of the area is the Silver volcanic basin which Burbank (1941, p. 151) divided into two distinct parts or provinces: (1) an interior downfaulted circular block, the caldera, which may be subdivided into a hub of tilted and locally faulted rocks, surrounded by a ring of highly faulted rocks, and (2) an exterior unit of relatively undisturbed but fissured rocks. The Sneffels and Telluride districts are in the exterior unit, on the northwest flank of the caldera; the Red Mountain area is in the northern part of the highly faulted outer ring.
Most of the rocks exposed throughout the area are Tertiary volcanic rocks; however, the older underlying formations are exposed at lower elevations. In the Telluride district, sedimentary rocks ranging from the Cutler Formation of Permian age to the Dakota Sandstone of Cretaceous age are exposed, and in the Sneffels-Red Mountain district older Paleozoic as well as Mesozoic sedimentary rocks are found. The unconformable blanket of Tertiary rocks, which is 4,000 to 6,000 feet thick, consists of, from oldest to youngest, the Telluride Conglomerate, San Juan Tuff, Silverton Volcanic Series, and Potosi Volcanic Series.
During the outpouring of these volcanic rocks, subsidence occurred in the caldera, and a network of circular faults and radial fractures was formed. Bodies of rhyolite, andesite, quartz latite porphyry, diorite, and quartz monzonite porphyry were injected into the country rocks; some were guided by the preexisting fractures, and others domed and fractured the rocks still more. Mineralizing solutions followed the emplacement of intrusive rocks (W. S. Burbank, in Vanderwilt and others, 1947, p. 419-424).
The ore deposits are directly related to geologic structures and also to the forms of the intrusive bodies. Ores of the Red Mountain area are chimney deposits, which are vertical cylindrical bodies a few feet to a few tens of feet in diameter in and near volcanic pipes filled with breccia, quartz latite porphyry, and rhyolite. The common ore minerals are pyrite, enargite, chalcopyrite, tennantite, chalcocite, covellite, stromeyerite, bornite, sphalerite, and galena. Gold is associated with the copper minerals (Burbank, 1941, p. 178-209).
The ore deposits of the Telluride and Sneffels areas are in veins whose distribution is controlled by a zone of crustal sag extending northwestward from Red Mountain to Stony Mountain and Mount Sneffels. Many of the veins, such as the Smuggler-Union in the Telluride district, follow a swarm of dikes that trend northwest; a few, such as the Camp Bird in the Sneffels district, trend north or northeast.
The most productive veins are in the San Juan Tuff, and some are remarkably persistent; the Smuggler-Union, for example, has been mined for a horizontal distance of 8,000 feet. The veins contain variable amounts of pyrite, sphalerite, chalÂ¬copyrite, galena, tetrahedrite, tennantite, and pear-cite in a gangue of quartz, barite, sericite, ankerite, rhodochrosite, rhodonite, calcite, fluorite, adularia, and clay minerals. Silver is derived mainly from tetrahedrite, galena, and tennantite. Gold is found in the quartz (Burbank, 1941, p. 209-261).
The Uncompahgre district covers about 15 square miles near the town of Ouray, where most of the mine workings are in the canyon walls of the Uncompahgre River.
The headwaters of the Uncompahgre River and Poughkeepsie Gulch were prospected in 1874, and many claims near Ouray were located within the next few years (Henderson, 1926, p. 54). In 1875 gold-bearing lodes were discovered in the Permian and Pennsylvanian rocks in the canyon walls near Ouray, and a little later silver-lead deposits were found in the Leadville Limestone of Mississippian age. Early records of the district are incomplete, but according to Burchard, as quoted by Henderson (1926, p. 184), considerable development was done in 1884, but because of the low grade of ore and the lack of economical transportation, only a few mines shipped ore.
In 1889 phenomenally rich gold ore was discovered in the Dakota Quartzite at the American Nettie mine which, together with adjoining properties, accounted for most of the district's gold output (W. S. Burbank, in Vanderwilt and others, 1947, p. 409). Silver-lead ore bodies, which had been known for many years, were not worked until 1892 when the Bachelor ore body, the major silver producer, was discovered. For a few years production of silver and lead was high, but it declined after 1895. Since that time the main effort has been to treat lower grade ores by milling.
The total gold production of the district was about 200,000 ounces, most of which was mined before 1900.
The canyons of the Uncompahgre River and its tributaries expose a vertical section of rocks nearly 6,000 feet thick which reveals many features of Precambrian to late Tertiary geology.
The Precambrian rocks are exposed south of Ouray in the Uncompahgre gorge and consist of a compressed sequence of 3,000 feet of quartzite and shale called the Uncompahgre Formation. The Precambrian rocks are overlain with marked angular discordance by a thick section of sedimentary rocks that includes the Elbert Formation and Ouray Limestone of Devonian age, the Leadville Limestone of Mississippian age, the Molas and Hermosa Formations of Pennsylvanian age, the Dolores Formation of Late Triassic age, the Entrada Sandstone, Wanakah and Morrison Formations of Jurassic age, and the Dakota Sandstone and Mancos Shale of Cretaceous age.
The rocks were folded and faulted several times - in late Paleozoic and late Mesozoic or early Tertiary time - and then were covered by Tertiary rocks consisting of the Telluride Conglomerate, San Juan Tuff, Silverton Volcanic Series, and Potosi Volcanic Series. Late Cretaceous or early Tertiary intrusive rocks are found in a northeast-trending zone just north of Ouray. The most prominent of these are laccolithic masses of granodiorite porphyry that intruded the Dakota-Mancos contact; these intrusive bodies were probably connected at depth by a central conduit now filled with a small stock that is exposed about a mile north of Ouray and is known as the Blowout (Luedke and Burbank, 1962).
The ore deposits are of two ages - Late Cretaceous or early Tertiary and late Tertiary. Most of the production has come from the older deposits which are genetically associated with the granodiorite intrusions and consist of (1) low-grade contact-metamorphic deposits containing some gold, (2) pyritic base-metal deposits containing silver and gold tellurides and native gold, and (3) siliceous and baritic lead-zinc deposits containing silver and gold. These older deposits are fissure veins and flat-lying replacement deposits in the Paleozoic and Mesozoic sedimentary rocks.
The late Tertiary deposits are weakly mineralized gold-quartz and silver- and gold-bearing base-metal veins distributed around the margins of the district (Luedke and Burbank, 1962).