Bear Lodge District and Broader Region

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 outlines northern Black Hills alkalic province. Deposit type and basic geology: carbonatites and carbonatite dikes associated with a broader set of alkaline intrusions. Manganese-enriched porphyries and dikes are present in some areas (DeWitt and others, 1986; Hausel, 1987; Sutherland, Gregory and others, 2013). Veins related to the alkaline magmatism may include beryllium and fluorspar.
Identified resources Identified gold and REE resources; historical production of fluorite and gold.
Production Minor production.
Status Past mining with ongoing exploration for Au/Cu at Mineral Hill.
Estimated resources REE mineralization in oxidized Bear Lodge carbonatite constitutes a combined Measured and Indicated resource of 18 million tons averaging 3.05% ΣREO and an Inferred mineral resource of 31.8 million tons averaging 2.58% ΣREO, using a cut-off grade of 1.5% (Dahlberg and others, 2014). Inferred gold resource of 947,000 ounces (26,847 kg) contained in 76.4 million short tons (69.3 million tonnes) averaging 0.42 g/tonne, using a gold price of $1200/ounce and a 0.15 g/tonne cutoff grade. Gold resource outlined in an April 2012 NI 43-101 technical report does not include results from 2011 gold drilling program (Dahlberg and others, 2014).
Geologic maps Staatz (1983), scale 1:24,000; Sutherland (2007b, 2008), scale 1:100,000; Pillmore and Mapel (1963), scale 1:48,000; Mapel and Pillmore (1963), scale 1:48,000; Van Lieu (1969), scale 1:48,000; Robinson and others (1964), scale 1:96,000; Redden and DeWitt (2008), scale 1:100,000; Love and others (1990), scale 1:250,000; DeWitt and others (1989), scale 1:250,000; Reheis (2007), scale 1:100,000; Hallberg, Case and Kirkaldie (2001), scale 1:100,000; Lisenbee and Hargrave (2005), scale 1:24,000; Lisenbee and others (2007), scale 1:24,000; Lisenbee and others (2015), scale 1:24,000; Lisenbee, Redden, Fahrenbach and McCormick (2013), scale 1:24,000; Redden and others (2013a, b), scale 1:24,00; Fagnan and Lisenbee (2014, 2017), scale 1:24,000; Redden (2018), scale 1:24,000; Redden and Fahrenbach (2021), scale 1: 24,000.
Geophysical data Inadequate aeromagnetic and aeroradiometric coverage.
Favorable rocks and structures Numerous alkaline intrusions, including carbonatites; anastomosing carbonatite veins that range from several cm to ~15 m in thickness. Carbonatite laterite/weathered carbonatite; supergene monazite veins; unconformity-related? xenotime+anatase veins (Andersen and others, 2016; 2017).
Deposits Mineral Hill (MRDS dep_id: 10055664).
Evidence from mineral occurrences MRDS; Staatz (1983).
Geochemical evidence Anomalously high Ti in magnetite from the Mineral Hill complex. Scandium is either derived from pegmatites in the area, or from mafic rocks of the alkalic igneous province (DeWitt and others, 1986).
Geophysical evidence Often targeted magnetite destructive, potassium-ferric iron metasomatism (fenitization).
Evidence from other sources Unknown.
Comments Current gold exploration by Coeur Mining in Mineral Hill/Tinton area; no known carbonatites in South Dakota.
Cover thickness and description Mix of exposed and concealed alkaline intrusions; The carbonatite veins are divided into: (1) an oxide zone extending from the surface to ~120 m in which sulfide minerals were completely oxidized and matrix carbonate dissolved; (2) a transitional zone typically extending from 120 to 230 m depth in which sulfides were partially oxidized and matrix carbonate variably dissolved; and (3) an unoxidized zone (>230 m) in which sulfides and carbonates are preserved or minimally affected (Dahlberg and others, 2014; Moore and others, 2015; Andersen and others, in press).
Authors Allen K. Andersen, Robert W. Gregory, Benjamin J. Drenth, Laurel G. Woodruff, Joshua M. Rosera, Darren Johnson.
New data needs Additional geochemistry could be useful to confirm geophysics.
Geologic mapping and modeling needs 1:24,000 scale map available with USGS Professional Paper1049-D, plate-1 (Staatz, 1983). This map does not distinguish individual alkaline intrusions, simply refers to all as Tt (phonolite and trachyte). Newmont and RER (Rare Earth Resources) spent time compiling an ArcGIS database with mapping of individual intrusions, alteration, mineralization, structures, soil and rock geochemistry, geophysics (airborne magnetics, radiometrics, gravity, ground induced polarization/resistivity, and controlled-source audio-frequency magnetotellurics (CSAMT). Such data may be available from former consultants with permission from private mining companies.
Geophysical survey and modeling needs Rank 1 aeromagnetic and aeroradiometric data, also gravity.
Digital elevation data needs Needs lidar coverage, although expected to be covered by a FY20-funded 3DEP lidar survey.