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 timesâ€”in late Paleozoic and late Mesozoic or early Tertiary timeâ€”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 agesâ€”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).
Page 3 of 3