MINERAL DEPOSITS OF THE MIDCONTINENT RIFT SYSTEM - A NEW TIME/SPACE CLASSIFICATION

The Midcontinent Rift System (MRS) hosts a diverse suite of magmatic and hydrothermal mineral deposits, largely know from rift rocks exposed at or near the surface in the western Lake Superior region. These deposits, which are significant past, present, and likely future providers of critical commodities, can be placed into a space/time metallogenic framework developed using metallic mineral resource data from the U.S. Geological Survey and Ontario Geological Survey. Deposits were classified by deposit type, host rock age and type, and estimated mineralization age. Deposition of MRS rocks followed an evolutionary path that began with an early plateau flood basalt and related intrusive stage (~1112 to ~1105 Ma) followed by an extrusive and intrusive rift stage (~1100 to ~1094 Ma). As rift stage magmatism ended, sedimentation during thermal collapse resulted in deposition of a thick section of clastic sedimentary rocks that overlie MRS basalt. Between ~1060 and 1040 Ma compressional tectonism created reversed faults that uplifted the margins of central rift basins. MRS mineralization began with intrusive magmatic deposits formed contemporaneously with basalt eruption, followed by an extended hydrothermal mineralizing era as magmatism waned. Plateau stage deposits include small high-grade Ni-Cu-PGE massive to semi-massive sulfide deposits, such the Eagle deposit (currently in production). World-class disseminated Cu-Ni-PGE sulfide deposits (some in pre-production stages) and small but potentially economic Ti-Fe-V oxide deposits along the basal section of the Duluth Complex formed during the rift stage. Younger MRS mineral deposits are hydrothermal. Notable hydrothermal deposit types (in decreasing age) include Ag-rich and five-element polymetallic veins cutting pre-MRS strata and MRS diabase dikes in Ontario, large stratiform Cu deposits, such as White Pine, hosted by post-rift stage black siltstone and shale, and Keweenawan native Cu deposits, contained within the rift stage Portage Lake Volcanics. This space/time classification of MRS mineral deposits is outlined in a USGS Story Map (https://wim.usgs.gov/geonarrative/MRS_mineral_deposits/). The metallogenic framework of western Lake Superior provides a guide to assessing the mineral potential of the extensive buried portions of the MRS.