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The Uncompahgre mining district, covering about 15 square miles in Ouray County, southwestern Colorado, lies at the western margin of the San Juan Mountains. The geologic formations range from pre-Cambrian to Tertiary in age. They comprise a basement series of sedimentary rocks upturned and deformed along the margins, of ancestral?mountain ^uplifts of late^preVCambriah,latc Paleozoic, and late Cretaceous or early Eocene age. All the deformed sedimentary rocks were beveled by erosion in early Tertiary time, and middle to late Tertiary volcanic rocks were deposited on them. The pre-Cambrian rocks include about 8,000 feet of quartzites and slates, and the Paleozoic and Mesozoic rocks about 5,000 feet of limestones, sandstones, and shales. The basal volcanic rocks near the district are chiefly andesitic tuff-breccia, which is overlain by lava flows, and the entire volcanic group is nearly a mile thick. The volcanic formations were only slightly tilted and faulted in late Tertiary time. The principal ore deposits are associated with cross-cutting and laccolithic intrusions of granodiorite or quartz monzonite porphyry of late Upper Cretaceous or early Eocene age. The ores were deposited chiefly in the Paleozoic and Mesozoic sedimentary strata through a stratigraphic range of about 4,500 feet and lie beneath the early Tertiary unconformity, which in places truncates some of the uppermost early Eocene(?) deposits. There are also younger ore deposits of late Tertiary age mainly.in the overlying_,volc'am'c.formations but also to a lesser extent in the sedimentary rocks near the south margin of the Uncompahgre district. The deposits of the older Uncompahgre center of eruptive activity, which are considered in most detail in this report, have an estimated production of $10,000,000 to $12,000,000. The ore bodies of the Uncompahgre center range from contact-metamorphic deposits of low grade, through pyritic base-metal deposits containing silver and gold tellurides and native gold, to silver-bearing lead-zinc deposits. These are zoned about the center of eruptive activity. Ore deposition about the Uncompahgre center is largely governed by structural trends and axes of uplift established mainly in the late Paleozoic period of deformation, but also in part by structure lines established in late pre-Cambrian time. These older structural movements were renewed and accentuated during the late Mesozoic or early Tertiary deformation at the time of the igneous intrusions. There are two main structural axes in the district, a north-northwesterly axis of uplift,' called the Uncompahgre axis, which extends outward from the margin of the mountains, and an intrusive axis of northeasterly trend, which parallels that of the Mesozoic and Paleozoic mountain uplifts north of the district. The two axes cross near the center of eruptive activity and divide the district into four

Structural control of ore deposition in the Uncompahgre district, Ouray County, Colorado, with suggestions for prospecting