The Vermilion District of northern Minnesota, USA, hosts hematite iron ore in the Lake Vermilion-Soudan State Park. The lens-shaped massive hematite ore bodies are enclosed within protore Algoma-type magnetite-chert banded iron formations (BIFs) (Hudak et al., 2016). The BIFs are folded (D1), and the ore is hypothesised to have formed syn-kinematically during regional shearing (D2), which activated fluid migration and resulted in hypogene hydrothermal alteration of BIFs to hematite ore (Klinger, 1960; Thompson, 2015). These ore units are nested in D2 shear zones, but field observations and hand sample analyses suggest the ore is devoid of both D1 and D2 deformation, indicating post-kinematic ore formation. The ore samples display distinctive textures suggestive of several stages of alteration, but the timing of ore mineralization is unknown, and the different stages of alteration processes are poorly understood. An early study (Symons, 1967) proposed a possible association between hematite ore mineralization and emplacement of the Mesoproterozoic Duluth Complex, a layered mafic intrusion within the Midcontinent Rift system. The hypothesis was based on magnetic paleopoles in the ore having similar remanence directions to the Duluth Complex and dissimilar to all adjacent and proximal units. This study aims to investigate if the intrusion could have initiated ore formation. The project utilizes standard petrography, electron microprobe analysis (EMPA), and a relatively new combined geo- and thermochronology technique to better understand ore formation and genesis. Petrographic analysis confirms at least two different ore textures: (1) fine-grained massive ore with microcrystalline specular hematite devoid of original banding of the protore, including chlorite- or quartz-filled vugs; (2) massive ore displaying more than one generation of hematite, comprising microcrystalline hematite that makes up the matrix with subhedral bladed specular hematite aggregates. Mineral separation did not yield datable accessory minerals (apatite or monazite), and other studies revealed trace amounts of monazite that are too small for radiometric dating (Thompson, 2015). In the absence of conventional accessory minerals, direct U-Pb geochronology coupled with (U-Th)/ He thermochronology of hematite (Courtney-Davies et al., 2022) is being applied to obtain hematite iron ore ages.

Reference(s)

Courtney-Davies, L., et al., 2022, Hematite geochronology reveals a tectonic trigger for iron ore mineralization during Nuna breakup: Geology, v. 50, p. 1318-1323,https://doi.org/10.1130/G50374.1. 

Hudak, G.J., et al., 2016, Bedrock geologic map of Lake Vermilion/Soudan Underground Mine State Park – Report to the Minnesota Department of Natural Resources: Natural Resources Research Institute, University of Minnesota Duluth, Technical Report NRRI/TR-2016/20, 23 p. 

Klinger, F.L., 1960, Geology and ore deposits of the Soudan mine, St. Louis County, Minnesota [thesis]. Symons, D. T., 1967, Paleomagnetic evidence on the genesis of the hard hematite ore deposits of the Vermilion Range, Minnesota. Canadian Journal of Earth Sciences, 4(3), 449–460. https://doi.org/10.1139/e67-023. 

Thompson, A., 2015, A hydrothermal model for metasomatism of Neoarchean Algoma-type banded iron formation to massive hematite ore at the Soudan Mine, Ne Minnesota [thesis].