Iron stable isotope analysis to decipher hematite ore formation in the Lake Vermilion-Soudan Underground Mine State Park, Minnesota, USA

Iron (hematite) ore bodies that are enclosed within protore banded iron formations (BIFs) can form through supergene, hypogene, or combined supergene—hypogene processes. In the Soudan mine, located in the Lake Vermilion-Soudan Underground Mine State Park, high-grade hematite ores (>60 wt% Fe) occur as hard ore bodies in folded and faulted Neoarchean Algoma-type BIF within the Soudan Iron Formation member of the Ely Greenstone Formation. Upgrading of the BIF by hydrothermal fluids led to the genesis of the replacement-style iron ores, and hypogene alteration is coeval with distant Paleoproterozoic orogenic events (Mazatzal and Yavapai orogenies). Examples of other, globally significant, high-grade and hypogene hematite ore bodies include the ones within the Carajás Mineral Province, Brazil, and the Mount Tom Price of the Hamersley Province in Pilbara, Australia.

This study utilizes iron stable isotopes (δ⁵⁶Fe) to distinguish Fe sources necessary for the upgrade process. Hematite, magnetite, and coexisting silicates from surface and subsurface samples were analyzed to fingerprint Fe provenance and hydrothermal system(s). Samples include variably altered BIFs, hematite ore with different textures, and adjacent altered wall rocks. Results show δ⁵⁶Fe values ranging from approximately –0.4‰ to +1.3‰. Our results show that Fe in hematite is systematically lighter as a function of alteration; hematite in least altered BIF is heaviest and hematite formed as replacement in iron ore is lightest. Therefore, replacement hematite likely formed from fluid-derived Fe as opposed to residual Fe from BIF. We interpret hydrothermal fluid origin with heavy Fe sourced from BIF and light Fe sourced from deep-seated crustal hydrothermal fluids. We further explore whether a single hydrothermal system operated, or overprinting events produced composite ores by proposing a hydrothermal genetic model associated with iron ore formation at our research locality.