Midcontinent Rift conduit-type magmatic sulfide Ni-Cu-PGE

Region Central, North Central
Mineral systems
Deposit types
Critical minerals
Other minerals

Information leading to the delineation of this focus area

Basis for focus area Conduit-type Ni-Cu-PGE-Co magmatic sulfide deposits and prospects are within the 1.1 Ga Midcontinent Rift System. Deposits are small chonoliths, sills, and dikes that encounter sulfur-bearing county rocks. Known occurrences in Michigan, Minnesota, and Ontario; possible occurrences in Wisconsin (Schulz, Woodruff, Nicholson and others, 2014).
Identified resources Identified resources for copper, nickel, and PGE; historical and current production of copper, nickel, and PGE.
Production Eagle mine (Michigan), opened in 2014; current production data are proprietary.
Status Current mining at the Eagle mine, with expansion to the nearby Eagle East deposit. Tamarack exploration (Minnesota) has achieved resource estimate status, with additional mineralized zones identified at Tamarack North. Mineralized intercepts and prospects have been encountered at Tamarack South (neck/bowl area). Ongoing active exploration in Minnesota and Ontario for this deposit type.
Estimated resources Eagle ~ 5 million tons with 2.87% Cu, 3.43% Ni, 0.73 g/t Pt, and 0.50 g/t Pd. Tamarack ~6.9 million tons with 0.92% Cu, 1.54% Ni, 0.34 g/t Pt, and 0.21 g/t Pd.
Geologic maps Eagle area, Michigan: Cannon (1977), scale 1:62,500; Tamarack area, Minnesota: Boerboom and others (1999), scale 1:100,000; Boerboom (2009), scale 1:100,000; Regional maps of Animike Group: Minnesota: Southwick and others (1988), scale 1:250,000; Wisconsin: Cannon and others (1996), scale 1:100,000; Michigan: Cannon (1986), scale 1:250,000.
Geophysical data Mix of adequate and inadequate aeromagnetic coverage in Minnesota, inadequate coverage in Wisconsin, 2022 acquisition of Rank 1 aeromagnetic coverage for much of Michigan; inadequate aeroradiometric coverage.
Favorable rocks and structures Sulfur-bearing country rocks intruded by small MRS mafic/ultramafic intrusions.
Deposits Eagle mine (MRDS dep_id: 10400290; USMIN Site_ID: MI00001), Tamarack deposit (USMIN Site_id: MN00008).
Evidence from mineral occurrences MRDS; USMIN.
Geochemical evidence Lithogeochemistry of the igneous rocks indicates potential. Glacial processes complicates interpretation of surficial geochemistry.
Geophysical evidence Deposits are quite small, so that the existing aeromagnetic data are not at a resolution to identify potential. The sulfidic nature of the Proterozoic sedimentary rocks that hosts the intrusions makes it difficult to use EM to target magmatic sulfide accumulations in intrusions.
Evidence from other sources No data.
Cover thickness and description Glacial cover. Intruded into Proterozoic sedimentary rocks at variable depths from surface. Glacial cover at Tamarack ~50 m.
Authors Michael L. Zientek.
New data needs High resolution geophysical surveys, geological mapping.
Geologic mapping and modeling needs Geological mapping at 1:24:000 scale.
Geophysical survey and modeling needs High resolution aeromagnetic data to identify dikes and small intrusions.
Digital elevation data needs Lidar inadequate for much of the area; adequate in small areas.