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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