Mineral potential and geology of the Duluth Complex

The Midcontinent Rift System developed in response to crustal-scale tectonic extension in the Middle Proterozoic, approximately 1.1 billion years ago. The western arm of the rift extends southwestward from Lake Superior - where rift-fill rocks are moderately well exposed - to the subsurface of the Twin Cities metropolitan area, and from there to the subsurface of northeastern Kansas. The fill associated with the active stages of rift development consists mainly of tholeiitic basalt that was erupted under subaerial conditions, together with petrologically related sills, dikes, and large layered intrusions that cooled beneath or within the cogenetic volcanic pile. The largest and most important of the layered intrusions is the Duluth Complex, a composite intrusion of troctolite and gabbro derived from periodic tapping of an evolving magma source. In the waning stages of rifting, the principal rock types deposited in the rift shifted gradually from magmatic to sedimentary; among the sedimentary sequences are those for which alluvial-fan, fluvial braid-plain, aeolian, and lacustrine depositional environments may be inferred.

Duluth Complex map

The Duluth Complex hosts four distinct types of magmatic mineral deposit, none of which is economic to mine at the present time. The deposit types include (1) large, low-grade, disseminated Ni-Cu concentrations, some of which contain local zones enriched in platinum-group elements (PGEs); (2) localized high-grade zones of massive Ni-Cu sulfides, some of which are moderately enriched in PGEs; (3) stratabound PGE-enriched "reefs" associated with specific types of phase-layer transitions; and (4) oxide-rich ultramafic plugs that in some instances are potential sources of Ti and V. Deposit types (1) and (2) occur only at or very near the basal contact of the Complex, whereas types (3) and (4) occur in the basal zone and also at higher levels.

Significant quantities of native copper, native silver, bornite, and other copper minerals were mined earlier this century from hydrothermal vein and stockwork deposits in basalts and interflow sediments of the Midcontinent Rift System on the Keweenaw Peninsula of Michigan. In addition, large amounts of finely dispersed native copper and other copper minerals were mined from a "kupferschiefer" type of deposit in lacustrine siltstone and shale at White Pine, Michigan. Although trace occurrences of native copper, native silver, and various other copper minerals have been found in basaltic rocks along the North Shore of Lake Superior in Minnesota, no mineable deposit of the Keweenaw or White Pine type has been discovered.

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