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Overview

Midcontinent Rift System
Left: The Midcontinent rift system cuts across terranes ranging in age from 3.6 to 1.65 Ga. The location of the rift south of Lake Superior in Minnesota, Michigan, Wisconsin, Iowa, Nebraska, and Kansas is inferred from gravity and magnetic data. The hachured areas denote volcanic basins and the stippled areas represent sedimentary basins (From Nicholson et al., 1992). Right: Regional aeromagnetic anomalies over the MRS region. White dots show locations of known Keweenawan Ni-Cu-PGE mineral deposits. Gray block pattern shows area of Phanerozoic sedimentary cover. Red polygon shows location of NE IIC. View individual images at full size: Left figure (500 KB) or Right figure (1.5 MB).

The Proterozoic Midcontinent Rift System extends for more than 2,000 km across the central North American craton, and is one of the world's major continental rifts. The Midcontinent Rift System hosts significant mineralization in its northern reaches: Michigan's Upper Peninsula hosts the largest accumulations of native copper in the world, and the Duluth Complex in northern Minnesota contains the largest undeveloped deposits of nickel, copper, and platinum-group metals (PGM). The geology and resources of the System are poorly understood in its middle and southern extensions, where the structure is buried by younger rocks and soils. This multidisciplinary study seeks to image and characterize the mineral resource potential of the Midcontinent Rift System. Combining various geophysical methods with detailed mapping and geochemical analyses will help to define the geographic extent of the Rift and clarify its geologic makeup and configuration. This, in turn, will enhance an understanding of the mineral-resource potential of this critically important metallogenic province and possibly lead to appropriate exploration guides for covered parts of the Rift.

Reference Cited

Nicholson, S.W., Cannon, W.F., and Schulz, K.J., 1992, Metallogeny of the midcontinent rift system of North America: Precambrian Research, 58(1-4), p. 355-386, http://dx.doi.org/10.1016/0301-9268(92)90125-8.

Project Activities

Geophysical Characterization of the Northeast Iowa Intrusive Complex

Contact: Ben Drenth, bdrenth@usgs.gov

In northeastern Iowa, a series of regional magnetic and gravity anomalies near the margin of the Midcontinent Rift System have historically been interpreted to reflect a buried igneous complex whose rocks are similar in composition and extent to the Duluth Complex in Minnesota. Dating of the surrounding rocks indicates a similar time of formation, as well. If these comparisons are valid, the Iowa complex may also have significant potential for Ni-Cu-PGM mineralization. Multi-method airborne geophysical surveys (magnetic, electromagnetic, and gravity) will allow qualitative interpretations of the complex's basement geology and structure, an important first step to assessing the area's mineral-resource potential.

Geology and Geochemistry of the St. Croix Horst in the context of the broader Midcontinent Rift System

Contact: Laurel Woodruff, woodruff@usgs.gov

The St. Croix Horst, in southeastern Minnesota, contains mineral deposits similar to those of the Lake Superior region, but smaller in scale and metal quantities, and the copper-sulfide mineralization exhibits substantial differences from its northern counterpart. At present, more detailed comparisons between the two areas are difficult due to the structural complexity of the Rift System. By combining known geological, geochemical, geophysical, rock age, and mineral-resource data for the two areas, researchers hope to provide significant insight into the mineralizing events and to define important constraints on the metallogeny and mineral-deposit potential of the different rift segments.

Products

Drenth, B.J., Anderson, R.R., Schulz, K.J., Feinberg, J.M., Chandler, V.W., and Cannon, W.F., 2015, What lies beneath: Geophysical mapping of a concealed Precambrian intrusive complex along the Iowa-Minnesota border: Canadian Journal of Earth Sciences, 52(5), p. 279-293, doi:10.1139/cjes-2014-0178.

Woodruff, L.G., Cannon, W.F., Nicholson, S.W., Schulz, K.J., and Wild, Robert, 2013, Geology of the Keweenawan Supergroup, Porcupine Mountains, Ontonagon and Gogebic Counties, Michigan: Institute on Lake Superior Geology, 59th Annual Meeting, Houghton, MI, v. 59, p. 1-28, https://pubs.er.usgs.gov/publication/70104183.

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