ABSTRACT

The Copper Mountain metamorphic complex lies within a westerly trending belt of Precambrian exposures known as the Owl Creek Mountains uplift. This mountain range lies along the southern edge of the Big Horn basin, separating that basin from the Wind River basin to the south. Rocks of the metamorphic complex are exposed at Copper Mountain, at Wind River Canyon, and apparently continue several miles to the west on the Wind River Indian Reservation.

The metamorphic complex at Copper Mountain is part of a larger complex known as the Owl Creek Mountains greenstone belt. Until more detailed mapping and petrographic studies can be completed, the Copper Mountain area is best referred to as a complex, even though it has some characteristics of a greenstone belt.

The metamorphic complex is amphibolite-grade metamorphosed supracrustal rocks that have been migmatized along the north and south margins of the complex by the intrusion of leucogranitic stocks and batholiths. The regional trend of the metamorphic complex is N50° to 80°E, and regional foliation dips steeply to the south. The dominant structure along the southeastern margin of the belt is a synform. The supracrustal rocks are quartz, hornblende, plagioclase gneisses and schists, quartzites, para-amphibolites, pelitic schists, cordierite schists, iron formation, quartz-mica schists and gneisses, and intercalated orthoamphibolites.

At least three episodes of Precambrian deformation have affected the supracrustals, and two have disturbed the granites. Prior to the intrusion of granite, the metamorphic complex experienced coaxial folding of the metasediments. Following the initial folding of the supracrustals, the Copper Mountain belt was intruded by leucogranite about 2.7 b.y. ago. This intrusive event is believed to be responsible for prograde metamorphism, as well as a second phase of deformation. Portions of the supracrustal belt were mineralized during the waning stages of the intrusive event. Some tungsten-bearing veins and calc-silicates were produced. Auriferous and cupriferous fracture-fill veins were formed, followed by emplacement of simple pegmatites.

A final Precambrian deformation event was preceded by a weak thermal event expressed by retrogressive metamorphism and restricted metasomatic alteration. During this event, a second phase of pegmatitization was accompanied by hydrothermal solutions.

During the Laramide orogeny, Copper Mountain was again modified by deformation. Laramide deformation produced complex gravity faults and keystone grabens. Uranium deposits were formed following major Laramide deformation. The genesis of these deposits is attributable to either the leaching of granites or the leaching of overlying tuffaceous sediments during the Tertiary.

Production of metals and industrial minerals has been limited, although some gold, copper, silver, tungsten, beryl, feldspar, and lithium ore have been shipped from Copper Mountain. A large amount of uranium was produced from the Copper Mountain district in the 1950s.

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