Geology and ore deposits of the Uncompahgre (Ouray) Mining District, Southwestern Colorado

The Uncompahgre mining district, part of the Ouray mining district, includes an area of about 15 mi² on the northwestern flank of the San Juan Mountains in southwestern Colorado from which ores of gold, silver, copper, lead, and zinc have had a gross value of $14 to 15 million.

Bedrock within the district ranges in age from Proterozoic to Cenozoic. At least three major uplifts of the ancestral San Juan Mountains occurred during the Proterozoic and at the close of the Paleozoic and Mesozoic, respectively. The last event, known as the Laramide orogeny, locally was accompanied by extensive intrusion of igneous rocks.

The principal ore deposits of the district were associated with crosscutting and laccolithic intrusions of porphyritic granodiorite formed during the Laramide orogeny. The ores were deposited chiefly in Paleozoic and Mesozoic sedimentary strata. Ore deposits range from low-grade, contact-metamorphic through pyritic base-metal bodies containing silver and gold tellurides and native gold to silver-bearing lead-zinc deposits.

Ore deposition was largely controlled by structural trends and axes of uplift established during Paleozoic deformation, and also in part by structural lines established during the Proterozoic. There are two main structural axes in the district: (1) a north-northwest-trending axis of uplift, called the Uncompahgre axis, and (2) an intrusive axis of northeastward trend. The two axes intersect near the center of eruptive activity and divide the district into four structural sectors.

Sources of the ore-forming fluids lie along the northeast-trending intrusive zone and appear to be related genetically to the igneous rocks. Contact-metamorphic deposits and most of the pyritic gold-bearing deposits were found in the central parts of the district. The silver-lead-zinc deposits were found chiefly in the northern and southern sectors of the district. Ore deposition was controlled by nearness to the intrusive contacts, nearness to zones of shallow tensional fractures, and other factors. The largest and highest grade deposits were either mostly developed or nearly exhausted at the time of our studies; they occurred in a zone near the Uncompahgre axis.