Iron Axis

Region West, Southwest
States
Mineral systems
Deposit types
Commodities
Other minerals

Information leading to the delineation of this focus area

Basis for focus area Area encompasses the Iron Springs (Pinto), Mineral Mountain, Antelope Range, Goldstrike, Iron Peak, and Bull Valley mining districts, which contain Miocene (~20 Ma) granitic laccoliths intruding Mesozoic sedimentary rocks and comprise the regional Iron Axis trend. Iron skarn mineralization is locally developed in Jurassic carbonates (Homestake Limestone member of Camel Formation) and in breccia zones and narrow magnetite-apatite veins (joint coatings) with bleached selvages in granitic rocks. Apatites in veins and iron skarn ore are enriched in REE except Eu relative to igneous apatite. Bleached selvages of veins are depleted in REE except Eu and Lu (data from Barker, 1995). The polygon was drawn to encircle the aeromagnetic signature of the iron deposits, and all the Miocene granitic laccoliths that are directly adjacent to the iron deposits. Outline also includes high sulfidation epithermal systems of the Antelope Range mining district.
Identified resources Historical production of iron.
Production Iron ore discovered in 1849. Major iron production in the Iron Springs and Pinto districts from 1923-1981; 1989-1995; 2008-2014. Estimated historical production of Iron Springs is about 100 million long tons, and is the most productive iron district in western U.S. No recorded production from Bull Valley district where iron grades are substantially lower (Bullock, 1970). 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 (Krahulec, 2018b).
Status Past mining. Active mining by Utah Iron LLC out of the Iron Springs district. Active geothermal exploration in the Antelope Range district (associated high sulfidation Ag mineralization), and an active project on carbon sequestration in the Navajo Formation north of Iron Springs.
Estimated resources Bullock (1970, 1973) estimated greater than 300-500 Mt of iron ore remained in skarn deposits at the Iron Springs district. A more recent estimate for Comstock-Mountain Lion skarn deposit was a resource of 28.44 million metric tons (31.35 million st) averaging 48.6% Fe (Mach and others, 2009). Phosphorus content of iron ores is not reported. Barker (1995) estimates that veins in the upper part of laccoliths in the Iron Springs district represent ~2 volume percent of the rock and contain about 5 volume percent apatite. Vein apatite in the Three Peaks laccolith contain up to 2.5% REE (mostly La, Cs, Pr, and Nd).
Geologic maps Rowley and others (2006), scale 1:100,000; Mackin and Rowley (1976), scale 1:24,000; Mackin and others (1976), scale 1:24,000; Biek and others (2010), scale 100,000.
Geophysical data Inadequate aeromagnetic and aeroradiometric coverage.
Favorable rocks and structures Thick limestone beds intruded by shallow quartz monzonites that form laccoliths. Iron-oxide-apatite mineralization in skarn deposits in sedimentary rocks and in veins (joint coatings) and breccia zones in quartz monzonite. Joints are relatively regularly spaced in upper parts of laccoliths.
Deposits Many iron mines in Iron Springs district (Bullock, 1973, lists 55); Fe skarn deposits with >10 Mt ore include A and B, Black Hawk, Blowout, Burke, Comstock, Desert Mound, Lindsay, McCahill, Mountain Lion, Pinto, Rex, Section 2, and Section 9. Bullock (1970) reports potential iron resources for the Pilot and Cove Mountain deposits in Bull Valley district.
Evidence from mineral occurrences MRDS; See Bullock (1973) and Barker (1995); UMOS (Utah Geological Survey, 2021).
Geochemical evidence REE analyses of apatite and whole rock samples and mass balance calculations are in Barker (1995). The gangue minerals include calcite, quartz, dolomite, phlogopite, fluorapatite, quartz, siderite, ankerite, diopside, magnesite, gypsum, barite, epidote, andradite garnet, vesuvianite (idocrase), and scapolite (Krahulec, 2018b).
Geophysical evidence Inadequate aeromagnetic and aeroradiometric coverage. The Utah aeromagnetic and gravity maps (Bankey and others, 1998) cover the area. The magnetic map is a compilation of data including the Aeromagnetic Map of Parts of the Delta and Richfield 1 ͦ x 2 ͦ quadrangles (U.S. Geological Survey, 1972d).
Evidence from other sources See Bain and others (2020).
Comments The focus area covers the northwest-southwest trending Miocene Iron Axis as well as barren Miocene laccoliths to the southeast. Focus area also includes the Antelope Range mining district, which is a minor Ag producer associated with high sulfidation system. Barton and others (2013) suggest that the Fe-oxide deposits are shallow (0-3 km) parts of IOCG deposits.
Cover thickness and description Variable.
Authors David A. John, Drew L. Siler.
New data needs Regional aeromagnetics survey to identify any other potential buried iron skarns or laccoliths (will respond well to geophysics); airborne hyperspectral data would be useful in this area.
Geologic mapping and modeling needs Digitization and synthesis of existing maps to identify gaps.
Geophysical survey and modeling needs High-resolution, Rank 1 aeromagnetic and radiometric surveys.
Digital elevation data needs Lidar in progress.