THE MINERALOGY, SPATIAL DISTRIBUTION, AND ISOTOPE GEOCHEMISTRY OF SULFIDE MINERALS IN THE BIWABIK IRON FORMATION

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 contains 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).

Our primary objective was to characterize the mineralogic and lithologic occurrences, spatial distribution, and sulfur isotope geochemistry of both primary and secondary sulfide minerals in the BIF to better establish and understand mineral variation and origin. Previous isotopic studies conducted on sulfides in Animikie Basin sediments have focused largely on primary (syn-depositional) sulfides 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 those studies, the primary sulfides collected appear as small anhedral “blebs” with δ³⁴S values of -5.4‰ to +12.4‰. Secondary sulfides, however, display a wide range of morphologies (cubes, framboids, veins, and anhedral masses), geographic and stratigraphic distribution, and δ³⁴S values (+80.37‰ to -36.11‰). The large range in δ³⁴S for secondary sulfides indicates they formed from the low-T reduction of sulfate. The sulfate, meanwhile, was possibly derived from oxidative weathering of the primary sulfides.

A secondary objective of this study is to evaluate the source of sulfur to the SLRW. Sulfur isotope values from sulfates collected in the watershed near mining operations yielded δ³⁴S results of +4‰ to +9‰. This range is similar to that of primary sulfides in the BIF, however the average δ³⁴S value for all 72 sulfide occurrences analyzed in this study is 8‰. Therefore, it is more probable that the entire range of primary and secondary sulfide is contributing to sulfate in the SLRW, rather than one specific occurrence of sulfide.