Iron Springs Mining District, Iron County, Utah, USAi

Regional Level Types
Iron Springs Mining District	Mining District
Iron County	County
Utah	State
USA	Country

The Iron Springs (Pinto) mining district is located about 15 mi west of Cedar City in south-central Iron County. The district was organized in 1871, is the most productive Fe district in the western U.S. (about 100 million long tons), and is estimated to be the fourth largest district in Utah by production value. Total district metal production at modern metal prices is estimated at $6.7 billion. The most productive mine in the district is the Comstock–Mountain Lion (CML) open pit on the east side of Iron Mountain.
The Iron Springs district covers a 20-mi-long, slightly arcuate, northeast-trending belt of laccoliths along the east-northeast- trending Miocene-age Iron axis of the Basin and Range Province of southwestern Utah. The Iron axis is a Sevier-age anticline intruded by Miocene quartz monzonites. The orebodies are associated with three oval-shaped Miocene (about 22 Ma) calc- alkaline, porphyritic quartz monzonite laccoliths, from southwest to northeast: Iron Mountain, Granite Mountain, and Three Peaks. The three intrusions have nearly identical composition. The porphyritic quartz monzonite is green-gray, fine-grained quartz and K-spar with plagioclase phenocrysts and subordinate hornblende, augite, and biotite with accessory magnetite, ilmenite, apatite, titanite, and zircon (Bullock, 1970).
Three basic types of orebodies are in the Iron Springs district: veins, breccia bodies, and skarn/replacement orebodies. The largest and most important orebodies are the stratabound skarn/replacement ores principally in the Middle Jurassic Homestake Limestone Member of the Carmel Formation draped over the laccoliths (USGS Model 18d). The Homestake Limestone is about 250 ft thick and near ore it is extensively recrystallized and bleached micritic limestone. The Homestake Limestone, equivalent to the Co-op Creek Member regionally, is often separated from the intrusive by a hornfelsed siltstone of the underlying Temple Cap Formation (30 to over 100 ft thick). Stratabound skarn/replacement mineralization begins at the base of the Homestake Limestone where it usually has the best grade (>50% Fe); however, the entire Homestake Limestone may ultimately be replaced. While some of the breccia orebodies constitute significant deposits, the veins are generally only small deposits (Bullock, 1970; Wray and Pedersen, 2009).
All Iron Springs ores are primarily magnetite and have minor hematite. Textures of the magnetite ore indicate cross-cutting veinlets and progressive replacement of the limestone along bedding planes leading to virtually massive magnetite containing only accessory minerals. The gangue minerals include calcite, quartz, dolomite, phlogopite, fluorapatite, quartz, siderite, ankerite, diopside, magnesite, gypsum, barite, epidote, andradite garnet, vesuvianite (idocrase), and scapolite (Bullock, 1970; Wray and Pedersen, 2009).

From 1923 to 1968, the district produced an estimated 87 million tons of iron ore. Most of this production was achieved through open-pit mining methods using steam shovels and diesel-powered equipment. A resource of as much as 500 million tons of ore is proven to still be available, most of it in the Iron Mountain area.

Parley P. Pratt discovered the iron deposits in 1849, while exploring southern Utah for the L.D.S. Church. His discovery led the L.D.S. Church to call for 120 men, 30 women, and 18 children to volunteer their services to produce iron from the ore. In 1852, this area became the first site west of the Mississippi River where iron was produced from native ores. After thousands of work hours, only 25 tons of iron were produced. After this venture ended in failure, several other unsuccessful attempts were made to process the iron ore. During the 1920s, new advancements in mining technology and iron production resulted in profitable mining operations. From 1923 to 1965, 72.1 million tons of ore were mined. In 1981, mining operations at the Comstock and Mountain Lion properties were suspended due to increased operating costs and foreign competition. A resurgence in the steel industry occurred in 1989 with the reopening of the Comstock mine. Mining was suspended in 1995 due to cheaper foreign sources of iron.