Region 1: Mineral Resources of the Superior Upland

In the discussions of each region to follow, the focus is on the following: currently mined and other significant minerals; where the minerals are most common (though they may occur in other places as well); and how the minerals formed in each particular area relate to the surrounding rocks and geologic history.

The Superior Upland portion of the Canadian Shield is composed mostly of igneous and metamorphosed Precambrian rock. Plumes of magma that welled up from the mantle (and then cooled) formed expanses of igneous rocks like granite, basalt, and gabbro. Sedimentary rock, including banded iron formations, formed as sediment slowly accumulated under the ancient ocean. Later, metamorphism converted much of the igneous rock into gneiss and the sedimentary rock into quartzite. Some nickel, copper, and platinum are produced in the region, but these are secondary products in the mining of the Superior Upland’s dominant mineral resource: iron. Minnesota and Michigan are the states that produce, respectively, the most and second most iron ore in the US. The ore, primarily hematite (Fe2O3) and magnetite (Fe3O4), is mined from banded iron formations (Figure 5.2).

Banded iron formations (BIFs) are rocks with regular, alternating thin layers of iron oxides (e.g., hematite and magnetite) and either shale or silicate minerals (e.g. chert, jasper, and agate). For example, BIFs in Michigan are composed of jasper between layers of hematite/magnetite and are found on Jasper Knob near Marquette. They are a primary source of iron ore, and their formation is discussed in Chapter 9: Climate.

In Minnesota, iron is principally produced from the aptly named Biwabik Iron Formation in Itasca and Saint Louis Counties. Two large open pit mines operate in Marquette County, in the Marquette Range found in the north central portion of Michigan’s Upper Peninsula. Most of the iron is used to make steel. The portion of the Superior Upland found in Wisconsin is not yet used for mineral resources. It is, however, quarried for stone used in construction and industry.

Figure 5.2: Principal mineral-producing localities in the Superior Upland, associated particularly with crystalline Precambrian basement rocks and sedimentary banded iron formations (see Chapter 1: Geologic History and Chapter 2: Rocks).

Figure 5.2: Principal mineral-producing localities in the Superior Upland, associated particularly with crystalline Precambrian basement rocks and sedimentary banded iron formations (see Chapter 1: Geologic History and Chapter 2: Rocks).