Brief History of High-Grade Iron Ore Mining in North America (1848–2008)
Philip E. Brown, 2008. "Brief History of High-Grade Iron Ore Mining in North America (1848–2008)", Banded Iron Formation-Related High-Grade Iron Ore, Steffen Hagemann, Carlos Alberto Rosière, Jens Gutzmer, Nicolas J. Beukes
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Approximately 3.4 billion tons (Gt) of iron ores containing >50 percent Fe were produced from U.S. mines in the Lake Superior region from 1848 until they were exhausted 20 to 30 years ago. The Vermilion Range in Minnesota produced nearly 100 million tons (Mt) of this ore from Archean greenstone belt-hosted iron formation. The remaining production has come from Proterozoic strata including 2.3 Gt from the Mesabi and 100 Mt from the Cuyuna Ranges in Minnesota while Michigan and Wisconsin contributed 230 Mt from the Marquette Range, 290 Mt from the Menominee Range, and 325 Mt from the Gogebic Range. The protore of these direct-shipping ores are carbonate- or oxide-facies banded iron formations that contained 25 to 35 percent Fe prior to undergoing leaching (desilicification), oxidation, and volume loss. The conventional model ascribing these changes to supergene processes has recently been challenged by research showing that hypogene fluids, channeled by faults into structurally favorable horizons and settings, have played a dominant role in producing some of the high-grade (>60% Fe) ores that are presently providing much of the world's iron ore. Descriptions of the North American iron ores, generally starting with the U.S. Geological Survey monographs published at the beginning of the 20th century provide many tantalizing clues, suggesting that hypogene fluids have indeed played an important role in the evolution of some of these districts. Application of modern geophysical techniques and structural and geochemical analyses may well guide the discovery of new high-grade ores either below or adjacent to the historic mining areas. The time seems to be ripe for exploration to return to the area that can claim to have begun geologists' understanding of this most important ore deposit type.