The Eagle East intrusion, located near Marquette, Michigan, was emplaced into the Baraga Basin early in Midcontinent Rift System formation. This deposit is a conduit-style, high-grade Ni-Cu-[PGE] deposit hosted by feldspathic peridotite and olivine melagabbro. For most magmatic Ni-Cu-PGE deposits the assimilation of S from country rocks has been a key process in the generation of sulfide-rich ores. Ni grades of up to 8% present in the East Eagle deposit suggest that sulfide saturation was attained early in the magma crystallization sequence, before Ni was sequestered by olivine. This strongly indicates that some S originated from the country rocks of the Baraga Basin; either Paleoproterozoic sedimentary rock or Archean basement rock. Multiple sulfur isotope analyses of sulfide minerals, primarily pyrrhotite, chalcopyrite, and pentlandite, from Eagle East were conducted for all mineralization types. delta34S values (1.5‰ to 16‰) suggest a significant amount of S in the deposit originated from Paleoproterozoic rocks. However, Delta33S values (--0.07‰ to 0.05‰) are not suggestive of the involvement of S from Archean rocks. delta 34S and Delta33S values closer to zero at Eagle East may indicate that S isotopic exchange in the conduit with uncontaminated mantle-derived magma occurred, leading to lower isotopic ratios closer to those of pristine mantle S (near 0‰).

Sulfide ores were also analyzed from the Tamarack Intrusive Complex (TIC), located in NE Minnesota. The sulfide assemblages in the TIC are similar to those at Eagle East and represent another conduit-related deposit. Delta 33S values of sulfide minerals at Tamarack do not support involvement of S from Archean rocks. As at Eagle East, conduit exchange with uncontaminated mantle-derived magma may be partly responsible for the Delta33S values which are all near zero. Results from both deposits are consistent with the premise that the addition of external S is necessary for high-grade deposit formation, but also indicate that the source rock and depth at which S is contributed to the melt may vary substantially between deposits. Thus, exploration for small, high-grade Ni-Cu-[PGE] deposits should include S isotopic analyses from the entire stratigraphic section of country rock to evaluate the nature of potential magma-country rock interaction.