Central Laramie Range

Region West, Rocky Mountains
Mineral systems
Deposit types
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). 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 gold 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 Historical production of copper, gold, lead, and silver.
Production More than 50 tons Pb-Ag ore shipped from Esterbrook mine prior to 1904, with additional unspecified tonnage shipped in 1945; 100 tons of Cu ore shipped from Saul's Camp (Hausel, 1989); 65 tons Cu ore produced in 1901 from Precious Honor No. 1 mine.
Status Past mining.
Estimated resources Unknown.
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 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. In the greenstone belts, mineralization typically occurs along contacts between schist and granite/granitic gneiss, shear- and pegmatite-hosted quartz veins, amphibolite sequences, and contacts between metasediment and felsic schist.
Deposits Esterbrook mine (MRDS dep_id: 10254756), Saul's Camp (MRDS dep_id: 10157279), Snowbird Group (MRDS dep_id: 10254738), Warbonnet District (MRDS dep_id: 10157751), Plaga Judd (Ni, MRDS dep_id: 10279317), Three Cripples mine (MRDS dep_ids: 10230065, 10096569), Maggie Murphy Group (MRDS dep_id: 10080868), Mc Ghee prospects (MRDS dep_ids: 10096583, 10157375), Big Five Prospect (MRDS dep_id: 10080875), Tenderfoot Group (MRDS dep_id: 10157639).
Evidence from mineral occurrences MRDS.
Geochemical evidence Assays are typically for base and precious metals: Esterbrook mine: 34.65% Pb, 1.3 oz/ton Ag, 0.035 oz/ton Au; Three Cripples prospect: 0.23% Cu with trace Co; Seller's Mountain: arsenopyrite-berthierite quartzite with 0.25 oz/ton Au; Whippoorwill mine: 0.32 oz/ton Au; maximum values from Laramie Peak Study Area: 7,000 ppm Cu, 100 ppm As, 0.015 Pt, 0.070 Pd (Hausel, 1989; Segerstrom and others, 1977).
Geophysical evidence Aeromagnetic survey completed in 1972 (Segerstrom and others, 1977).
Evidence from other sources Unknown.
Comments The focus area may host the following: Volcanogenic massive sulfide deposits in the Esterbrook-Braae greenstone belt, which were targeted largely for Cu, Au, and Ag. Other greenstone belts in the Laramie Range have similar lithology but very limited mineralization. Possibility of remobilization of metals during metamorphism should be explored. Uranium is elevated in surrounding granites with some coffinite noted in sample descriptions.
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 Patricia M. Webber.
New data needs 1:24,000 scale mapping, geochemical survey, high resolution geophysical data, lithogeochemistry of the sills, dikes, and greenstones to determine if any of the magma interacted with magmatic sulfide liquids or possibility of secondary remobilization.
Geologic mapping and modeling needs Geologic mapping of the metamorphic 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 aeroradiometrics, 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.