Abstract

The Big and Little Cottonwood districts have produced ~780,000 short tons of direct-smelting ore, plus ~65,000 short tons that were milled in the district. Typical ores (Ag-Pb-Zn with variable Cu and Au) came from veins and limestone-hosted mantos. Minor Au veins in amphibolites, and “granite” building stone were also extracted. The districts lie along a long-lived structural feature, a suture between crustal blocks, extending between the larger Bingham and Park City mining districts. A miles-thick weakly-metamorphosed Precambrian basement sedimentary sequence lacks sulfide mineralization. Phanerozoic sediments host mineralization only in favorable Cambrian through Triassic units. Multiple Tertiary plutons assumed a predominant role in generating the ores. The plutons and ore hosts have undergone deeper post-mineral erosion than in nearby districts.

Fractures and low angle (thrust) faults localized ores. “Fissures”, which often contain altered, pyritic dikes, guided magmatic fluids outward from the Alta stock, which crops out at the center of the districts. This pluton was localized by northeast and major north-trending structures. In carbonate rocks, the pluton is rimmed by magnetite- and boron-rich skarns that were locally mined for Cu-Au-Ag and W. The largest sediment-hosted orebodies, in the Emma Hill-Flagstaff Mountain-Cardiff Fork area, are along structures parallel the NNW side of the Alta stock. Three areas lying four to ten miles from the Alta stock are also mineralized. White Pine Gulch hosts molybdenum and tungsten zones around a siliceous intrusion within the Little Cotton-wood pluton. Paleozoic carbonates in the Argenta mining area of Big Cottonwood produced Ag-Pb-Au ores near dioritic sills, probably of Cretaceous age. Early Proterozoic schists host gold-quartz veins near the mouth of Little Cottonwood Canyon.

Only a few mines proved profitable, although some rich discoveries like the Emma claim received international recognition. Federal land was acquired for mining through pre-1920s patenting of Federal domain, and the Alta townsite was traded for Minnesota Sioux Indian scrip. These privatized lands later became the basis for resorts within the National Forest. The prize resource of the districts today is clean water, some flowing from old adits.