SULFUR ISOTOPE STUDIES OF NI-CU-(PGE) MINERALIZATION ASSOCIATED WITH THE TAMARACK INTRUSION, MINNESOTA

The Tamarack intrusion (1105.6 ± 1.2 Ma), located ~80 km west of Duluth, Minnesota, is part of the magmatic activity related to development of the Midcontinent Rift System. The intrusion occurs within black slates of the Paleoproterozoic Animikie Basin, and is host to potentially economic Ni-Cu-(PGE) mineralization. The intrusion is ~13km long and 1 to 4 km wide, as delineated using geophysical surveying and diamond drilling. A wide “bowl” area of the intrusion and narrow “tail” area are composed of mafic and ultramafic rocks with composition ranging from lherzolite to gabbronorite. Texturally distinct units described simply as coarse-grained and fine-grained host net-textured and disseminated sulfide mineralization characterized by pyrrhotite-pentlandite-chalcopyrite-magnetite assemblages. Texturally distinct host rocks also have different forsterite contents raging from Fo₈₉ to Fo₇₇ in coarse-grained lherzolite and from Fo₈₅ to Fo₇₉ in fine-grained gabbronorite. Sulfur isotope ratios of sulfide minerals in coarse-grained lherzolite units show subtle, but distinct, ranges. One coarse-grained unit has d³⁴S values that range from 1.2 to 1.5‰, and another shows values from 1.8 to 2.8‰. The fine-grained gabbronorite shows values from 0.4 to 2.2‰. Massive and semi-massive sulfide values range from 0.8 to 1.8‰. Immediately adjacent sedimentary rocks have d³⁴S values between 2.0 and 6.0‰. Chemical and isotopic data clearly support a model involving the input of multiple pulses of sulfide-saturated magmas. Because d³⁴S values of both igneous and sedimentary country rocks are relative low it is difficult to accurately assess the extent of sulfur assimilation from country rocks; however, external S contribution could exceed 50%.