Chemical and mineralogical heterogeneity in the basal zone of the Partridge River Intrusion: implications for the origin of Cu-Ni sulfide mineralization in the Duluth Complex, midcontinent rift system

Copper-nickel sulfide mineralization in the Partridge River Intrusion of the 1.1 Ga Duluth Complex is restricted primarily to a 100 m thick zone near the base of the intrusion, which is heterogeneous at meter scales in terms of both sulfide contents and rock types, which include dunite, melatroctolite, troctolite, leucotroctolite, gabbro, olivine gabbro, gabbronorite, and rare norite. Olivine-rich troctolites and melatroctolites appear to have required mineral accumulation on a substrate, whereas augite troctolite and gabbros are thought to have formed via in situ crystallization of magmas ranging in composition from high-Al olivine tholeiite to high-Ti tholeiite. delta O-18 values of orthopyroxene-poor rocks in the Partridge River Intrusion range from 5.2 to 6.7 parts per thousand. delta O-18 values of 6.7 parts per thousand are consistent with less than 20% contamination by high-O-18 metasedimentary country rock, either via devolatilization or local partial melting. Rocks with greater than 15% orthopyroxene, gabbronorites, and norites, are characterized by delta O-18 values in excess of 6.9 parts per thousand, and required the assimilation of larger amounts of siliceous country rocks. Sulfur isotopic values in leucotroctolitic rocks that contain less than similar to 400 ppm S and that overlie the basal zone range between -1.5 and 2 parts per thousand, values that are consistent with those of mantle-derived sulfur. In contrast, delta S-34 values in the basal zone range from -1.4 to 10.5 parts per thousand, where the S-34-enriched samples require an input of sulfur from metasedimentary country rocks. delta S-34 values of the rocks in the basal zone correlate with variations in olivine Fo content but not with S abundance. The wide range in delta S-34 values of rocks in the basal zone strongly suggests that magmas interacted with layers in the sedimentary country rocks that were themselves characterized by variable sulfide contents and delta S-34 values. The S isotopic data suggest that the heterogeneity observed in the basal zone results from the emplacement of relatively thin sheets of compositionally distinct magma. All rock types present in the basal zone can be produced as a result of variable degrees of fractionation of a parental high-Al olivine tholeiite, followed by varying degrees of contamination of derivative liquids by country rocks. The S-contamination process was essential for the development of Cu-Ni mineralization, and was restricted to the earliest stages in the development of the Duluth Complex at a time when volatile species such as S and H2O, and low-T partial melts of country rocks, were available to magmas.