Hematite double-dating defines Proterozoic mineralization and thermal history of Archean banded iron formations in northeastern Minnesota, USA

The age and origin of hematite deposits in the Vermilion District of Minnesota (USA), Lake Superior region, has been debated for over a century and inferred to be Neoarchean or Mesoproterozoic. Using a new geochronological approach combining U-Pb and (U-Th)/He double-dating of hematite, we present the first direct dates for hematite deposits at the Soudan iron mine, revealing a previously unknown Paleoproterozoic mineralization event and a thermal history recording the emplacement of the proximal Mesoproterozoic Midcontinent Rift System. Hematite phases yield U-Pb crystallization dates ranging between 1.8 Ga and 1.6 Ga and (U-Th)/He dates in the range of 1.63−0.53 Ga, with a distinct cluster at ca. 1.1 Ga. We propose that replacement-style hematite mineralization was generated during Paleoproterozoic orogenic events, including the Yavapai (1.71−1.68 Ga) and/or Mazatzal (1.65−1.60 Ga) accretionary orogenies and associated magmatism related to the assembly of Laurentia that reactivated shear zones and facilitated hydrothermal alteration deep into the Archean craton. (U-Th)/He data suggest that hematite ore experienced a thermal overprint that did not reset the U-Pb system, with the most consistent dates coinciding with the establishment of the Midcontinent Rift System at ca. 1.1 Ga. Double-dating of hematite is demonstrated to directly link iron mineralization to thermal and tectonic events in Precambrian cratons and to place constraints on genesis not available from coexisting accessory minerals.