Midcontinent Rift large mafic intrusions

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 Contact-type Cu-Ni±PGE±Co magmatic sulfide deposits. The Duluth Complex (Minnesota) has proven reserves of Cu-Ni-PGE and Co: Mellen Complex (Wisconsin) has known Cu-Ni-PGE mineralization.
Identified resources Identified copper, nickel, and PGE resources.
Production Unknown.
Status One mine in permitting process (NorthMet). Five other sites have achieved reporting-code-compliant resource estimates. Exploration ongoing in the basal zone. Current exploration in the Duluth Complex includes: Twin Metals of the Maturi/Birch Lake deposi, Teck Resources of the Mesaba deposit, and Encampment of the Serpentine deposit. Past exploration of the Mellen Complex by Bear Creek (Kennicott).
Estimated resources For the Duluth Complex, 4.4 billion tons with 0.66% Cu and 0.2% Ni; Pd+Pt+Au estimated at 500-1000 ppb. Nearly 7 billion tonnes reported in modern resources as of November 2019 (see USMIN candidate entries for Minnesota). PolyMet NorthMet deposit: Total Proven and Probable: 2.55 Mst at 80 ppb Pd, 268 ppb Pd, 74.42 ppm Co. Twin Metals Maturi and Maturi SW deposits: Total Proven and Probable: 527 Mst at 0.154 ppm Pt, 0.35 ppm Pd.
Geologic maps Duluth Complex: Miller and others (2001), scale 1:200,000 and 1:500,000, with some quadrangle mapping by the Minnesota Geological Survey at 1:24,000 scale; Mellen Complex: Cannon and others (1996), scale 1:100,000.
Geophysical data Adequate aeromagnetic coverage in Minnesota, inadequate in Wisconsin; inadequate aeroradiometric coverage.
Favorable rocks and structures Mafic plutons in contact with sulfur-bearing country rock.
Deposits NorthMet deposit (USMIN Site_ID: MN00002), Birch Lake deposit, Maturi deposit (MRDS dep_id: 10096418; USMIN Site_ID: MN00003), Spruce Road deposit (MRDS dep_id: 10038935), Mesaba deposit, Serpentine deposit (MRDS dep_id: 10082823).
Evidence from mineral occurrences MRDS; USMIN.
Geochemical evidence Compilations of exploration borehole data available from the Minnesota Department of Natural Resources and in reports from the University of Minnesota-Duluth Natural Resources and Research Institute.
Geophysical evidence No data.
Evidence from other sources 23K feet of state-funded scientific drilling exists for the Duluth Complex outside the basal zone (85 sites, density <1 per township). Of 4.1M feet of industry drilling in the basal zone, 3.7M is deposit drilling, 400K is other exploratory drilling along the basal zone. A few thousand feet of industry drilling outsize the basal zone. Bear Creek drilled the Mellen Complex.
Comments Duluth Complex has achieved resource status at several locations along the basal zone. Interior intrusions and deeper basal zone are greatly under sampled. Mellen Complex has less information.
Cover thickness and description Duluth Complex, none to variable depending on local glacial features and irregular bedrock surface (0-10m +/-10m in the west; 35m +/-35m in interior; 0-10m at Lake Superior). Thin glacial cover over Mellen Comp lex.
Authors Laurel G. Woodruff.
New data needs New mapping, high resolution geophysics.
Geologic mapping and modeling needs 1:24,000 scale mapping around the Mellen Complex, Wisconsin. Rank 1 geologic mapping on Duluth Complex where practical.
Geophysical survey and modeling needs Rank 1 aeromagnetic data for Mellen Complex in Wisconsin. Rank 1 aeromagnetic data are also appropriate for southern Duluth Complex. Modeling of Duluth Complex footwall, footwall anomalies, and structural controls on Ti-Fe deposits.
Digital elevation data needs Lidar variable; adequate, inadequate, and in progress.