MINERAL DEPOSITS AND METALLOGENY OF THE MIDCONTINENT RIFT IN ONTARIO

In northwestern Ontario, Mesoproterozoic, Midcontinent Rift (MCR)-related, orthomagmatic deposits of copper, nickel and platinum group elements (PGE) are associated with early ultramafic to mafic intrusions. Stratabound, “reef-style” Cu-Ni-PGE mineralization occurs in peridotite (e.g. Seagull intrusion); within massive, cumulate Ti-Fe-oxide layers (e.g. border gabbro, Coldwell Complex (CC)); and with Cr-bearing spinel in cyclic anorthosite-gabbro units (e.g. Crystal Lake Gabbro (CLG)). Varied-textured to pegmatitic gabbro hosts the Great Lakes Nickel (CLG) and Marathon deposits (CC).

Late-stage, magmatic U-Nb deposits have been investigated in the Prairie Lake Carbonatite and in syenites of the CC (Nb-Zr-Th-U-Ce). Rare metal- (Be-Zr-U-Th-) mineralized structures crosscut the Dead Horse Creek carbonatitic diatremes.

Epigenetic deposits associated with MCR structures and hydrothermal activity are hosted in a variety of Neoarchean to Mesoproterozoic rocks. Native copper and Cu-sulphides occur in Keweenawan basalt and interflow sedimentary rocks at Mamainse Point, on Michipicoten Island and in Osler Group rocks in western Lake Superior. Hydrothermal fluids ascribed to MCR magmatism are believed to have produced Cu-Mo-Ag-Pb-mineralized breccia pipes in Archean country rocks (e.g. Tribag) and Cu-Mo-mineralization (e.g. Jogran). Copper-mineralized carbonate units of the Mesoproterozoic Sibley Group are found near the contacts with Nipigon diabase sills.

Silver-bearing, carbonate-quartz veins near Thunder Bay have been subdivided into two groups: the Mainland Belt (Ag + Zn-Cu-Pb-bearing veins in Paleoproterozoic Animikie Group sedimentary rocks and Logan diabase); and the Island Belt (Ag-Bi-Co-Ni-As-bearing veins in Keweenawan diabase/gabbro). Lead-zinc-barite veins occupy structures near the unconformity between Neoarchean basement and Mesoproterozoic Sibley Group sedimentary rocks. Some of these structures also host amethyst deposits. Silver-bearing, lead-zinc veins occur within and proximal to the Coldwell Complex. All of these vein types are likely contemporaneous but were generated by different fluid sources.

Uraniferous, hematite-rich veins and breccias that cut Neoarchean basement and locally contain Proterozoic xenoliths may also be related to MCR hydrothermal activity which remobilized uranium from granitic pegmatites in the basement.