The Biwabik Iron Formation (BIF), which is located along the Mesabi Range in NE
Minnesota, was deposited in the near shore environment of the Paleoproterozoic
Animikie Basin. Although mined for natural ore and taconite, it does contain measurable
amounts of sulfide minerals, as pyrite and pyrrhotite. This study is part of a larger study
to evaluate whether sulfur from waste rock piles and tailings basins along the Mesabi
Range are contributing to sulfate in the St. Louis River Watershed (SLRW).
The primary objective of this study is to characterize the mineralogic and lithologic
occurrence, spatial distribution, and sulfur isotope geochemistry of both primary and
secondary sulfide minerals in the BIF in order to better establish their variation and
understand their origin. Previous isotopic studies conducted on sulfides in Animikie
Basin sediments have focused largely on primary (syn-depositional) sulfides in order to
determine the chemistry of ocean water at the time of deposition. These studies
concluded that primary sulfides were the result of bacterial reduction of Paleoproterozoic
seawater sulfate. Consistent with previous studies, primary sulfides appear as small
anhedral “blebs” with δ34S values of -5.4‰ to +12.4‰. Secondary sulfides display a
wide range of morphologies (cubes, framboids, veins, and anhedral masses), geographic
and stratigraphic distribution, and δ34S values (+80.37‰ to -36.11‰). These secondary
occurrences are largely attributed to metamorphic effects of the mafic Duluth Complex or
to oxidation and desilicification processes attending the formation of natural iron ores.
A secondary objective of this study is to evaluate the source of sulfur to the SLRW.
Sulfur isotope values from sulfates collected in the SLRW near mining operations yielded
δ34S results of +4‰ to +9‰. This range is similar to the δ34S of primary sulfides in the
BIF. However, it was determined that the average δ34S value of all 72 sulfide occurrences
analyzed in this study is 8‰. Therefore, it is more probable that the entire range of
primary and secondary sulfide are contributing to sulfate in the SLRW, rather than one specific occurrence of sulfide.
University of Minnesota M.S. thesis. October 2011. Major: Geological sciences. Advisors: Dr. James Miller and Dr. Michael Berndt. 1 computer file (PDF); vi, 165 pages, appendices A-B.
Theriault, Stephanie Ann.
Mineralogy, spatial distribution, and isotope geochemistry of sulfide minerals in the Biwablk Iron Formation.
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