PRE-MINING OXYGEN, HYDROGEN, AND SULFUR ISOTOPE CHARACTERISTICS OF SURFACE WATERS DRAINING THE MINERALIZED BASAL ZONE OF THE DULUTH COMPLEX, NORTHERN MINNESOTA

The basal zone of the Duluth Complex, northern Minnesota is the focus of new mine-development projects targeting disseminated Cu-Ni-PGM deposits. Three watersheds, Filson Creek, Keeley Creek, and the Saint Louis River, all of which transect the basal zone, were sampled for water-quality and stable isotope geochemistry over the course of a year focusing on peak-flow (June 2013) and base-flow (September 2012, 2013) conditions in an area dominated by wetlands and lakes. Water samples were analyzed for their oxygen and hydrogen isotopic compositions and the dissolved sulfate was analyzed for the sulfur and oxygen isotopic compositions. The stable isotopic compositions of water and dissolved sulfate provide important insights into the physical and chemical processes affecting baseline hydrological conditions prior to mining. The oxygen and hydrogen isotopic compositions of unevaporated meteoric water reflect seasonal variations (δ¹⁸O = -12.5 – -9.4 ‰; δD = -90.5 – -65.4 ‰). Isotopic analyses from samples from base-flow conditions isotopic compositions reflect the influence of evaporation (δ¹⁸O = -9.4 – -4.1 ‰; δD = -65.5 – -38.8 ‰). Modeling suggests that the amount of evaporation recorded for individual samples ranged from 0 to 55 percent, consistent with variations in specific conductance. In contrast, samples taken during peak flow show minimal deviation from the meteoric water line. The amount of evaporation also correlates with the size of the watersheds. The Saint Louis River watershed, the largest, shows the greatest isotopic evidence for evaporation. The sulfate-sulfur isotopic compositions cover a limited range (δ³⁴S = 5.6 – 8.6 ‰), show no significant differences among watersheds, and fall between published isotopic values for sulfide minerals in the basal zone of the Duluth Complex and in the Virginia Formation into which the Duluth Complex intruded. This observation suggests that sulfide oxidation in the near surface is the predominant source of sulfate in these watersheds. The sulfate-oxygen isotope values also cover a limited range (δ¹⁸O = -0.2 – 4.3 ‰). The lack of correlation between the sulfate-sulfur and sulfate-oxygen isotopic values at either the watershed or regional scale suggests that bacterial sulfate reduction is not an important process in these watersheds.