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).
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