Big Cottonwood Mining District, Salt Lake County, Utah, USAi

Regional Level Types
Big Cottonwood Mining District	Mining District
Salt Lake County	County
Utah	State
USA	Country

The Big and Little Cottonwood mining district lies in easternmost Salt Lake County about 17 mi southeast of Salt Lake City. The district was a large Pb-Ag producer active intermittently between 1867 and 1976. Total district metal production at modern metal prices is estimated at $618 million. Although the Big Cottonwood and Little Cottonwood mining districts are sometimes reported separately, the division is completely arbitrary and the ore in both sub-districts is the result of a single, complex, magmatic-hydrothermal episode related to the intrusion of the Alta quartz monzonite porphyry stock along their common border. Approximately 60% of the past production has come from the Little Cottonwood subdistrict with the remainder from the Big Cottonwood subdistrict. The South Hecla and Cardiff underground mines are thought to be the most productive in the district, and the South Hecla is also the largest Bi producer in Utah.
The Big and Little Cottonwood district is geologically situated on the Bingham-Park City mineral belt in the Middle Rocky Mountain province. The ore deposits are clustered around the margins of the east-northeast-trending Oligocene stocks (35 Ma to 33 Ma), in particular the high-K, calc-alkaline, Alta quartz monzonite porphyry stock (~34 Ma). Dikes and veins parallel this east-northeast intrusive trend. The porphyritic phase of the Alta stock may be near the center of the district metal zoning pattern. Thrust faulting, domed by the intrusives, has played an important role in the localization of the replacement deposits, acting as a pathway for hydrothermal fluids. The Mississippian Fitchville and Humbug Formations are the preferred host rocks although the Cambrian Ophir Formation and Maxfield Limestone also acted as important hosts (Calkins and Butler, 1943; James, 1979).
Most of the major polymetallic replacement (USGS Model 19a) producers are clustered where all of these factors―intrusive trend, dikes, thrust faults, fissure veins, and favorable carbonate strata―come together and include the Flagstaff Mountain- Alta area Cardiff mine (Big Cottonwood), Emma, South Hecla, Michigan-Utah, and Flagstaff mines (Little Cottonwood). The primary ore/sulfide minerals are galena, sphalerite, tetrahedrite, pyrite, chalcopyrite, argentite, molybdenite, bornite, siderite, rhodochrosite, and enargite. Tetrahedrite is the main primary Cu ore mineral in the replacement deposits. Rare gold, tungstenite, scheelite, stolzite, bismuthinite, aikinite, jamesonite, dufrénoysite, dyscrasite, wulfenite, and huebnerite are found in some deposits (Calkins and Butler, 1943; James, 1979).
The porphyritic quartz monzonite phase of the Alta stock is potentially the bottom of a very deeply eroded porphyry Cu cupola (USGS Model 17), probably eroded down to a paleodepth of very roughly 4 mi, far below the Cu ore zone.