A hydrothermal model for metasomatism of neoarchean Algoma-Type banded iron formation to massive hematite ore at the Soudan Mine, NE Minnesota

Abstract

The genesis of banded-iron-formation (BIF) hosted massive hematite ore deposits has been debated extensively in the literature. Recent advances in exploration and analytical techniques have led to a better understanding of the tectonic setting and characteristic hydrothermal fluids responsible for the metasomatic upgrading of BIF to hematite ore and associated wall rock alteration. As late as the 1990's the general consensus was that the massive ore-bodies observed in the Carajas District, Brazil, and the Hammersley Province of Western Australia, among others, formed predominantly through supergene processes responsible for silica leaching and magnetite oxidation to hematite within BIF during the Mesozoic (Hagemann et al., 2007). The relatively young age for the genesis of massive hematite ore was brought into question by Martin et al. (1998) through SHRIMP U-Pb age dating of zircons from volcaniclastic breccias of the Wyloo Group, which provided an age of 2209 +/- 15 Ma for hematite detritus of the Hammersley province. Ohmoto (2003) provided an alternative mechanism for oxidation by the reaction:Fe3O4(mt) + 2H+ ↔ FeO(hm) + Fe2+ + H2O the leaching of Fe2+ from magnetite by an acidic hydrothermal fluid. The enrichment and conversion of magnetite-dominant BIF to massive hematite through an acid-base reaction is of particular significance for Algoma-type BIF-hosted massive hematite deposits such as that at the Soudan Mine in northeastern Minnesota. It has been generally accepted that the hematite ore at Soudan is a product of D2-shearing and subsequent hydrothermal activity active within the Mine Trend Shear Zone formed during the accretion of the Wawa-Abitibi terrain to the Superior Craton at ~2685-2690 Ma. The lack of a weathering profile (evidenced by supergene goethite) and the relatively low permeability and high specific gravity of Soudan ore relative to supergene ores precludes descending, oxygenated meteoric fluids driving alteration and mineralization of the massive hematite ores. Rather, a model of early calcic-sodic alteration during mature-arc rifting with progressive burial during subduction of the Wawa-Abitibi Terrane leading to potassic and Fe/Mg metasomatism is proposed here. Wall rock alteration assemblages, associated with the upgrading of magnetite-chert BIF through magnetite-siderite and hematite-ankerite precursor to massive hematite ore, is associated with early sericite (± paragonite), silica, and Mg-rich chlorite, with local secondary sericite, hematite, and Fe-enriched chlorite. Isocon analysis indicates a volume loss of 39% due to the leaching of silicate minerals from BIF, consistent with Fe-enrichment by silica subtraction, and breccia textures observed in massive hematite ore. LREE-enrichment of wall rock adjacent to ore is observed, perhaps resulting from monazite precipitation of elements released due to the breakdown of apatite within the BIF. Late stage alteration appears to be post-D2 and is associated with precipitation of microplaty hematite as breccia cement, as well as secondary Fe-enriched chlorite replacing earlier potassic alteration. This late alteration stage shows HREE-enrichment with a strong positive Eu anomaly for both the chlorite schists wall rock enveloping hematite ore, and the massive hematite ore.