| 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. |
