Mineral potential and geology of the Penokean orogen

The Penokean orogen records an extended history of continental extension and convergence that affected the southern margin of the Superior craton in the time interval between 2.45 and 1.75 Ga. Several tectonic episodes took place, with the earliest activity in the Huronian belt of southern Ontario and the youngest in east-central Minnesota. The strongest collisional pulse apparently occurred at about 1.85 Ga, when intense deformation, metamorphism, and plutonism occurred along the entire strike length of the orogen. In Minnesota, the principal features of the Penokean belt are (1) an arcuate, northwest-verging fold and thrust terrane that involves supracrustal volcanic and sedimentary rocks as well as Archean basement; (2) a succession of tectonic foredeeps, the youngest, largest, and best-preserved of which is the Animikie basin; and (3) abundant syntectonic to post-tectonic granitoid plutons that range in age from about 1.85 to 1.77 Ga. The world-class iron deposits of the Mesabi iron range are localized in sedimentary iron-formation along the north, or cratonic, margin of the Animikie foredeep.

Penokean orogen map

Major structural breaks and tectonic elements in the Minnesota portion of the Penokean orogen appear to correspond with similar features on the eastern side of the Midcontinent Rift System in Wisconsin and Michigan. On this basis, the volcanic belts of northern Wisconsin that host significant deposits of massive-sulfide base-metal ore at Crandon, Ladysmith (Flambeau Mine), and other sites may have counterparts in Minnesota. Furthermore, the period of intense tropical weathering that affected Minnesota's Penokean rocks in the late Mesozoic may have produced supergene caps above massive sulfide deposits, as was the case at Ladysmith. Little systematic exploration has been done in Minnesota to follow up on these possibilities.

It should be noted also that the Penokean rocks of east-central Minnesota are tectonically and lithologically similar to rocks that host SEDEX Zn-Pb-Ag deposits in Australia (McArthur River, Mt. Isa, Broken Hill) and British Columbia (Sullivan). The details of this comparison cannot be fully described here, but they include the general inference of a rift-related depositional setting for iron- and manganese-rich strata and mineralogical indications of submarine hydrothermal contribution to the chemical systems from which the Fe-Mn sediments were precipitated. The formerly active iron-mining district known as the Cuyuna iron range is an inviting regional target for exploration based on the SEDEX model, as is the Glen Township area northeast of Lake Mille Lacs.

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