SOURCES OF SULFUR AND CONTAMINATION IN CONDUIT TYPE MAGMATIC NI-SULFIDE DEPOSITS: EVIDENCE FROM THE MIDCONTINENT RIFT SYSTEM-ASSOCIATED TAMARACK INTRUSION, MINNESOTA

Conduit type magmatic Ni-sulfide deposits reflect a complex formational history sometimes involving multiple batches of magma emplaced through different paths. The addition of crustal sulfur from both Archean and Proterozoic sources plays an important role in controlling sulfide saturation in the magmas during emplacement. The Tamarack intrusion (1105.6 ± 1.2 Ma) in NE Minnesota, related to the Midcontinent Rift System, occurs within the Paleoproterozoic Animikie Basin hosting potentially economic Ni-Cu-(PGE) mineralization. The two coarse-grained lherzolite units have δ³⁴S values ranging from 1.2-1.5‰ and 1.8-2.8‰; fine-grained olivine gabbronorite from 0.4-2.2‰; ores from 0.8-1.8‰; and country rocks from 2.0-6.0‰. Except for four samples, Re-Os isotopic data for all units fall on a 1161 Ma isochron with an initial ¹⁸⁷Os/¹⁸⁸Os of 0.1428±0.0099, higher than the primitive mantle at 1106 Ma. The remaining four samples have an even higher initial ¹⁸⁷Os/¹⁸⁸Os ratio. This suggests an addition of a crustal contaminant to the mantle-derived magma. Preliminary δ³³S values are between 0.83 ‰ and 3.7 ‰ with Δ³³S values between -0.41 ‰ and -0.11 ‰ for disseminated and 0.32 ‰ and 0.54 ‰ for semi-massive sulfides. Δ³³S values indicate that along the variable pathways through the system, magmas that produced the semi-massive ores incorporated an Archean sulfur source. The samples that show lesser degrees of crustal contamination may have formed by isotopic exchange between contaminated and pristine magmas within the conduit system. The addition of crustal sulfur was the principal cause of sulfide saturation in the magmas at Tamarack; isotopic data indicate that both Archean and Proterozoic sulfur were involved in the contamination of distinct magma batches.