Lake Superior Iron-Mining Region

one of the largest iron-mining basins in the world, located in the USA at the western end of Lake Superior in the states of Minnesota, Wisconsin, and Michigan, providing over four-fifths of the iron ore output of the USA. Ore areas and large deposits are usually named after the range, such as Mesabi, Keeweenaw, Vermilion, Gogebic, Marquette, and Menominee. The ore beds contain Precambrian taconites (usually containing 20-33 percent iron) and rich ores (51-64 percent iron) that arose chiefly as a result of supergene leaching of the quartz of taconites.

The taconites are thin-layered, dense, fine-grained rocks, composed of magnetite, hematite (separately or together), and quartz and sometimes with an admixture of silicates, carbonates, or sulfides; in some places they are metamorphosed into ferruginous quartzites. The rich ores, incoherent and cloddy, or dense, are composed of martite, hematite, goethite, with admixtures of manganese and argillaceous minerals. The taconites are deposited in strata 15-300 m in thickness in Middle Proterozoic, and rarely in Lower Proterozoic, sedimentary and volcanogenic-sedimentary ranges. The rich iron ores on the one hand may form sloping deposits on taconites, on the other hand they may be discharged into them in pockets and deep wedges along tectonic faults. Ore-bearing formations are deposited in folds with varying angles of their limbs, broken by faults, pierced with dikes and sills of the principal intrusive rocks, and unevenly metamorphosed. The rich ores are to a considerable extent exhausted; extensive processing of taconites is being conducted, especially magnetic ones, with extraction from them of magnetic concentrates containing 63-65 percent iron. The yield of iron ore in the Lake Superior iron-mining region varies from 65 to 85 million tons per year. The production of pellets from concentrates is being intensively developed; these pellets, when used in a blast furnace, significantly increase its productivity. Output of pellets in plants of the Lake Superior iron-mining region reached 46 million tons by the end of 1967.