MINERAL DEPOSITS OF THE 1.1 GA MIDCONTINENT RIFT SYSTEM (Invited Presentation)

The Midcontinent Rift System (MRS) is an approximately 2,200-km curvilinear continental rift stretching from Kansas northeast to the Lake Superior region, where it turns southeast and extends through lower Michigan. The MRS was created by a major thermal-tectonic rifting event ca. 1.1 Ga and is characterized by a huge volume of mafic intrusive and extrusive igneous rocks emplaced in a relatively short time span of ~30 million years, followed by deposition of a thick package (5 to 7 km) of mostly clastic sedimentary rocks. The overall distribution of a prolific and diverse suite of MRS-related mineral deposits reflects changing tectonic and magmatic development in time and space, as well as subsequent post-magmatic hydrothermal events. Among magmatic deposits are early conduit-type Ni-Cu-PGE sulfide deposits (e.g., Eagle nickel mine, Michigan) and contact-type Cu-Ni-PGE sulfide deposits (e.g., multiple, very large deposits of the Duluth Complex, Minnesota). Factors in forming these deposits include: 1) magma compositions that controlled metal types and concentrations; 2) exsolution of an immiscible sulfide liquid driven by assimilation of external sulfur into magma; 3) partitioning of metals from silicate magma into a sulfide phase; and 4) differing processes that concentrated sulfide liquids, creating localized massive to regional disseminated sulfide mineralization. Post-magmatic hydrothermal deposits include reduced-facies, stratiform Cu deposits (e.g., White Pine, Michigan), extensive native Cu mineralization in basalt and interflow clastic sedimentary rocks (e.g., Keweenaw Peninsula, Michigan), and polymetallic veins with varying settings, metal assemblages, and gangue mineralogy (e.g., Nipigon area, Ontario). Controlling factors for hydrothermal deposits include: 1) generation of metal-rich hydrothermal fluids in differing environments; 2) hydrothermal fluid movement driven by compaction, convection, or upward migration along faults and fractures; and 3) focusing of metal-rich fluids into favorable depositional areas with a chemical trap to precipitate metals. For the MRS, late tectonic events, likely related to Grenvillian compression, plausibly created extreme fluid permeability and fluid flow tens of millions of years after the end of rifting.