METALLOGENY OF THE 1.1 GA MIDCONTINENT RIFT (Invited Presentation)

The 1.1 Ga Midcontinent Rift (MCR), exposed in the Lake Superior region of North America, is one of the world’s major continental rifts. The interplay of tectonics and magmatism during a geologically brief interval produced a unique assemblage of magmatic and hydrothermal mineral deposits. Magmatic deposits are the primary focus of current exploration, and the Eagle mine is the only current producer. Magmatic deposit types related to the rift include: 1) small-volume, high-grade conduit-type Ni-Cu-PGE sulfide deposits (Tamarack, MN; Eagle, MI); 2) large-volume, low-grade contact-type Cu-Ni-PGE sulfide deposits (Duluth Complex, MN); 3) stratiform reef-type sulfide-bearing PGE occurrences (Sonju Lake, MN); 4) small plug-like Ti-Fe-oxide deposits (Longnose, MN; Round Lake, WI); and 5) U-REE-Nb±(Th, Zr) occurrences in pegmatites cutting alkalic intrusions (Coldwell Complex, ON).

Between about 1850 and 1997, MCR mineral production came largely from hydrothermal deposits. These include: 1) native Cu with minor Ag in basalt and interflow sedimentary rocks (Keweenaw Peninsula and Isle Royale, MI); 2) sediment-hosted stratiform Cu sulfide deposits (White Pine and Copperwood, MI); 3) basalt-hosted Cu sulfide deposits in flow tops (Keweenaw Peninsula, MI); and 4) polymetallic Ag veins (with Ni-Co-As±Bi,U) in Proterozoic and Archean country rocks (ON). A scarcity of sulfur in MCR volcanics resulted in deposition of native metals; where sulfide minerals are the principal ore mineral (e.g., sediment-hosted chalcocite at White Pine), the sulfur source was predominantly derived from the host rock.

Magma composition, rift structure and tectonism, and country rock contributions all played critical roles in the quantity, quality, and location of rift-related mineral deposits. Early conduit-type Ni-Cu-PGE deposits related to high-MgO picrites and later contact-type Cu-Ni-PGE deposits related to lower-MgO tholeiites formed when MCR magmas assimilated crustal sulfur. As the magmatic stage of the MCR waned (beginning about 1094 Ma), chalcocite was deposited in chemically reduced sediments that overlie the volcanic rocks. At about 1060 Ma, regional compressional stress related to the Grenville orogeny resulted in fluid expulsion from deep rift grabens, depositing native Cu and Ag higher in the volcanic section.