Mineralogy, spatial distribution, and isotope geochemistry of sulfide minerals in the Biwablk Iron Formation

Abstract

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.