Mineral potential and geology of the Archean in Minnesota

The Superior Province in Minnesota consists of three subprovinces defined and named in Canada (from north to south, the Wabigoon, the Quetico, and the Wawa); and a fourth, the Minnesota River Valley (MRV) subprovince, which lies south of the Wawa subprovince. The Wabigoon and Wawa subprovinces are volcanoplutonic belts that consist of deformed, relatively low-grade metavolcanic and metasedimentary rock sequences intruded by granitoid plutons. The volcanic-rich portions of both subprovinces possess lithologic and structural attributes broadly similar to those in mineralized greenstone belts in Ontario. The Quetico subprovince consists chiefly of metasedimentary schist, various migmatitic rocks derived primarily from sedimentary protoliths, and granitoid intrusions. The MRV subprovince consists dominantly of quartzofeldspathic gneiss and large granitoid intrusions. Radiometric ages for gneisses in the MRV terrane are as old as 3.6 Ga, substantially older than the 2.9 - 2.65 Ga ages for volcanic, sedimentary, and intrusive rocks in the volcanoplutonic and metasedimentary subprovinces to the north. The major MRV batholiths, on the other hand, yield radiometric ages in the 2.6 - 2.7 Ga range. These data, together with subhorizontal structural style, high metamorphic grade, and a sparsity of supracrustal protoliths, suggest that the MRV terrane probably was a pre-existing continental fragment that was accreted to the margin of the Superior craton by northward-verging (directed) subduction at the end of late Archean continental growth.

Subprovinces map

Based on the history of discoveries in Canada, the Archean greenstone belts of Minnesota are intrinsically prospective for diverse mineral-deposit types including shear-zone-hosted lode gold, iron-formation-hosted stratabound gold, volcanic-hosted base-metal sulfides (VMS), Ni-Cu-PGE-Cr deposits hosted by komatiitic volcanic sequences or associated sills, and Cu-Mo-Au deposits hosted by granitoid porphyry.

To date, the only commodity successfully mined from Archean rocks in Minnesota has been iron. Successful exploration in Canada has tended to focus on major faults and shear zones that are both marginal to and within the volcanoplutonic subprovinces (Wabigoon and Wawa). Similar fault structures have been identified through geologic and geophysical mapping in Minnesota, but relatively little systematic mineral exploration has been done along them.

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