HISTORY OF HIGH-GRADE IRON ORE MINING IN NORTH AMERICA - IS THERE A FUTURE?

Approximately 3.4 billion tons (Gt) of high-grade (>50% Fe), iron ores were produced from U.S. mines in the Lake Superior region from 1848 until they were exhausted 20-30 years ago. The Vermilion range in Minnesota produced nearly 100 million tons (Mt) of 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 running (25-35% Fe) in dominantly Proterozoic rocks prior to their undergoing leaching (de-silicification), 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 has 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 Lake Superior iron ores, generally starting with the U.S.G.S. 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.

Historically in Canada high-grade iron ores were produced from the Labrador trough in northeastern Quebec (150 Mt) and the Steep Rock District in western Ontario (70 Mt). The immediate future of high-grade iron ore production appears to lie in the Archean rocks of Baffin Island where 3-800 Mt of proven, probable and indicated iron ores running 64 to 70+% are being actively prepared for mining in the Mary River district. Little is known about the enrichment origin of these massive specular hematite ores.

Modern application of 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 to return exploration interest to the area that can claim to have begun geologist’s understanding of this most important ore deposit type.