High-grade iron ore deposits of the Mesabi Range, Minnesota-product of a continental-scale proterozoic ground-water flow system

The Mesabi Range along the north edge of the Paleoproterozoic Penokean orogen in northern Minnesota has produced 3.6 billion metric tons of ore since its discovery in 1890. Of that amount, 2.3 billion metric tons were extracted from hematite-or goethite-rich deposits generally referred to as "high-grade" ores. The high-grade ores formed as the Biwabik Iron-Formation was oxidized, hydrated, and leached by solutions flowing along open faults and fractures. The source of the ore-forming solutions has been debated since it was first proposed that the ores were weathering products formed by descending meteoritic ground-water flowing in late Mesozoic time. Subsequently others believed that the ores were better explained by ascending solutions, possibly hydrothermal solutions of pre-Phanerzoic age. Neither Wolff nor Gruner could reconcile their observations with a reasonable source for the solutions. In this paper, I build on modern mapping of the Mesabi Range and mine-specific geologic observations summarized in the literature to propose a conceptual model in which the high-grade ores formed from ascending solutions that were part of a continent-scale topographic or gravity-driven ground-water system. I propose that the ground-water system was active during the later stages of the development of a coupled fold and thrust belt and foreland basin that formed during the Penokean orogen.