Central Laramie Range

Region West, Rocky Mountains
Mineral systems
Deposit types
Critical minerals
Other minerals

Information leading to the delineation of this focus area

Basis for focus area Focus area is based on the outline of the Laramie Anorthosite Complex as shown on the state geologic map compilation (Horton and others, 2017). More than 30 magnetite-ilmenite occurrences are reported for anorthosite in the Laramie Range. Three types of deposits are (1) massive magnetite-ilmenite with minor spinel; (2) magnetite-ilmenite with olivine and plagioclase; (3) magnetite-ilmenite with apatite. Ti-Fe-V tabular dikes with abundant apatite. Some Ti deposits have anomalous chromium and vanadium (Hausel, 1987). Elmers Rock and other similar terranes in the Central Laramie Range are underexplored. Paleoproterozoic greenstone belts and associated dike and sill complex are situated near the Cheyenne Belt and the northern margin of the Laramie Anorthosite. Reconnaissance work (Graff and others, 1982) suggests Au potential, as well as geology favorable for various types of magmatic deposits. This focus area includes Kennedy dikes and sills (Kennedy large igneous province (LIP) event) associated with the Elmers Rock Greenstone Belt-the peridotitic composition of the sills is a favorable indicator.
Identified resources Identified resources of titanium (magnetite, ilmenite); historical production of Ti-Fe oxides (for iron, vanadium, and titanium), aluminum (anorthosite), copper, gold, lead, and silver.
Production Tonnages unknown for Ti-Fe oxides; approximately 32,000 tons anorthosite mined for aluminum production (St. Clair and others, 1959).
Status Previous mining of Fe-Ti-V oxides for heavy aggregate at Iron Mountain Mine, Sybille Pit, and other Fe-Ti-V deposits; past assessments by U.S. Bureau of Mines. Small mine in anorthosite body for pilot plant for aluminum (St. Clair and others, 1959).
Estimated resources Laramie Range anorthosite (Woodruff and others, 2017, table 2); Strong Creek Complex hosts more than 100 million tons of titaniferous magnetite, while the Iron Mountain deposit hosts about 15 million tons (Frost and Simons, 1991; Sutherland and Cola 2015). Estimated 65 billion tons of anorthosite present (U.S. Bureau of Mines, 1967, p. 146).
Geologic maps Ver Ploeg and Boyd (2007), scale 1:100,00; Ver Ploeg and McLaughlin (2009, 2010), scale 1:24,000; Ver Ploeg (2009), scale 1:24,000; Carnes and others (2013), scale 1:24,000; Ver Ploeg and others (2012), scale 1:24,000; Newhouse and Hagner (1957), scale 1:63,360; Ramarathnam (1962), no scale; Kolker (1989), scale 1:24,000; Snyder (1984), scale 1:24,000; Snyder and others (1995), scale 1:24,000; Snyder, Peterman and others (1997), scale 1:24,000; Snyder, Siems and others (1997), scale 1:24,000; Snyder and others (1998), scale 1:24,000; Segerstrom and others (1977), scale 1:48,000.
Geophysical data Inadequate aeromagnetic and aeroradiometric coverage.
Favorable rocks and structures Lenticular or tabular ore bodies are in Laramie Anorthosite Complex. Lenses are usually parallel to layering and are within 2-3 km of the major anticlinal axis in the northern anorthosite mass, for the most part where the anticline begins to change direction. Mafic/ultramafic flows and intrusions, metasedimentary rocks in the greenstone belts; the state-scale map does not clearly show the density of sills and dikes in rocks north and adjacent to the greenstone belt.
Deposits Iron Mountain (Cr, V, Ti, Zr, MRDS dep_id: 10229543), Smaller Deposit #1-Iron Mtn Area (Ti, MRDS dep_id: 10205155), Smaller Deposit #3-Iron Mtn Area (Ti, MRDS dep_id: 10303144), Smaller Deposit #4-Iron Mtn Area (Ti, MRDS dep_id: 10303436), Smaller Deposit #5-Iron Mtn Area (Ti, MRDS dep_id: 10254878), Smaller Deposit #6-Iron Mtn Area (Ti, MRDS dep_id: 10232152), Smaller Deposit #7-Iron Mtn Area (Ti, MRDS dep_id: 10229564), Smaller Deposit #8-Iron Mtn Area (Ti, MRDS dep_id: 10229930), Smaller Deposit #9-Iron Mtn Area (Ti, MRDS dep_id: 10132924), Smaller Deposit #10-Iron Mtn Area (Ti, MRDS dep_id: 10281316), Smaller Deposit #11-Iron Mtn Area (Ti, MRDS dep_id: 10229238), Smaller Deposit #12-Iron Mtn Area (Ti, MRDS dep_id: 10205262), Smaller Deposit #13-Iron Mtn Area (Ti, MRDS dep_id: 10303256), Smaller Deposit #14-Iron Mtn Area (Ti, MRDS dep_id: 10303068), Smaller Deposit #15-Iron Mtn Area (Ti, MRDS dep_id: 10157256), Taylor deposit (Ti, MRDS dep_id: 10080922), Plaga-Judd deposit (Ni, MRDS dep_id: 10279317), Mill Creek chromite (MRDS dep_id: 10205206), Shanton prospect (MRDS dep_id: 10080933), Laramie Range Anorthosite area (Al, Ti, MRDS dep_id: 10279152).
Evidence from mineral occurrences MRDS.
Geochemical evidence Data from U.S .Bureau of Mines analyses for Shanton deposit ore: TiO2=19.566%, V2O5 = 0.4%. Iron Mountain mine: V2O5 ranges from 0.17 to 0.64%. Massive magnetite-ilmenite-estimated 30 Mt with an average grade of 45% Fe, 20% TiO2, 0.64% V2O5; disseminated magnetite-ilmenite-148 Mt with an average grade of 20.2% Fe, 9.7% TiO2, 0.17% V2O5 (Dow, 1961). Anorthosite analyses indicate 28% Al2O3 (U.S. Bureau of Mines, 1967).
Geophysical evidence Aeromagnetic survey completed in 1972 (Segerstrom and others, 1977).
Evidence from other sources No data.
Comments The focus area may host the following: 1) iron-titanium oxide (in the Laramie Anorthosite Complex); 2) conduit-type Cu-Ni-PGE sulfide (in the sill and dike complex); 3) komatiitic Ni (in the greenstone belt). Silicate-free dikes of iron ore are interpreted as residual liquids that evolved from a melt which crystallized anorthosite. Extreme enrichment in Fe, Ti, and P suggests the presence of late-stage residual liquids injected into surrounding, solidified anorthositic rock creating numerous dikes of magnetite-apatite ore (Goldberg, 1984). The concept of magmatic ores being concentrated in the feeders to greenstone belts and flood basalts was not recognized until the 1990s, after mapping was conducted in the area, suggesting unrealized potential.
Cover thickness and description Most of the region has exposed bedrock, or bedrock with shallow (<100 m) burial depth; however, Precambrian mapping would be greatly enhanced with geophysics; very high density of sill and dikes with complex outcrop patterns.
Authors Robert W. Gregory, Jane M. Hammarstrom, Patricia M. Webber.
New data needs 1:24,000 scale mapping, geochemical survey, modern geophysical data, lithogeochemistry of sills, dikes, and greenstones to determine if magma interacted with magmatic sulfide liquids.
Geologic mapping and modeling needs Geologic mapping of the igneous stratigraphy with precise ground control is essential. Requires outcrop-level mapping. Mapping with extensive geochemistry and with the help of geophysics.
Geophysical survey and modeling needs Rank 1 aeromagnetics and radiometrics, also additional gravity stations; Elmer's Rock greenstone belt might be a good place for airborne gravity gradient survey (i.e., small area, rugged terrain that may preclude ground gravity).
Digital elevation data needs Lidar complete or in progress.