THE MIDCONTINENT RIFT AND ITS MINERAL SYSTEMS: OVERVIEW AND TEMPORAL CONSTRAINTS OF NI-CU-PGE MINERALIZED INTRUSIONS

In an effort to better understand the spatial and temporal distributions of mineralized intrusions of the Midcontinent Rift (MCR), and what controls their style of magmatic Ni-Cu-PGE sulphide mineralization, we have compiled an overview of all known intrusions and their ages. We provide new U-Pb ages for more than ten such intrusions and/or their associated dyke systems. A number of these investigations are still in progress, and all ages should be treated as preliminary. Nevertheless, we discuss new results on the mineralized Current Lake, Sunday Lake, Tamarack, and Crystal Lake intrusions, as well as the Bovine Igneous Complex on the southern flank of the MCR (see Bleeker et al., 2020, Geological Survey of Canada Open File, in press).

These new results, as well as improved ages for a number of the associated major dyke swarms and sill complexes (e.g. the Logan Sills), favour a relatively sharp onset of high-volume mafic-ultramafic magmatism in the MCR at ca. 1110 to 1106 Ma, although a few of the older age “outliers” remain to be tested. Mineralized intrusions are not confined to any specific magmatic pulse but are distributed through time, correlating with the major magmatic pulses at 1110–1106 Ma (e.g., the Current Lake, Thunder, and Sunday Lake intrusions), 1104 Ma (Tamarack), of course at 1099 Ma (Duluth Complex), to as young as 1093 Ma (Crystal Lake Intrusion). All these intrusions were dynamic, multi-phase, feeder-type systems.

A major “post-Duluth Complex” reorganization in the magmatic plumbing system is identified starting at ca. 1097–1096 Ma, with magmatism contracting into a linear feeding zone along the northwestern shore of Lake Superior—a major magmatic fissure system we call the “north shore magmatic feeder zone” or NSMFZ. Across the Canada-USA border this zone is cored by the major Pigeon River dyke swarm of northeast-trending dykes, some of which are >100 m wide. Farther south this becomes the Beaver River Diabase complex. This major feeder zone likely fed the entire lava flow field of the Portage Lake Volcanic Group, which extends to both sides of Lake Superior. After an episode of high-volume and aerially extensive fissure eruptions (e.g., the giant Greenstone Flow), magmatism contracted further and the NSMFZ was overprinted by discrete, more localized, gabbroic complexes such as the Crystal Lake Intrusion.