Abstract

The area described is about 25 miles southeast of Salt Lake City and about 2 miles west of Alta, Utah. Eastward-dipping Precambrian quartzites and argillites have been intruded by a Tertiary pluton called the Little Cottonwood quartz monzonite. Another intrusive body, recognized during this study, is exposed within the borders of the extensive Little Cottonwood pluton. The study has also resulted in the recognition of a widespread scheelite-mineralized zone and the compilation of previously reported molybdenite occurrences, which may be of commercial value. In general, the minerals present in the area are those associated with medium- and high-temperature hydrothermal deposits. Mineralized structures in the intrusive rocks consist of zones of jointing and fracturing which form a concentric pattern around an intensively fractured central zone. The more intense fracture zones are mainly within the exposures of the later and smaller intrusive body. Three stages of mineralization have been recognized. During stage I widespread scheelite, pyrite, and sericite were deposited on joint surfaces of the intrusive rocks. Sparsely distributed scheelite of stage I can be observed over an area of 6 square miles and a vertical range of 3000 feet. During stage II, molybdenite, pyrite, quartz, sericite, galena, sphalerite, and small amounts of scheelite and calcite were deposited in the more intensely fractured zones. Stage III minerals are fluorite and sericite in local zones.

A sample which is possibly representative of much of the 6-square-mile area contained 0.02 per cent WO3. Assays reported on the molybdenite-mineralized area indicated 0.24 per cent, 0.66 per cent, and 2.14 per cent MoS2. The tungsten minerals are relatively uniform in distribution, but the molybdenite distribution is erratic. An intensive sampling of the deposit is recommended to determine the economic value of known mineralized zones; the sampling project may also expose other minerals associated with high-temperature deposition. Prospecting for tungsten minerals in other igneous intrusive bodies of the region is also recommended on the basis of this investigation.

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