Origin of Cu-Ni-PGE Sulfide Mineralization in thePartridge River Intrusion, Duluth Complex, Minnesota

Abstract

Four sub-economic Cu-Ni-PGE deposits occur near the base of the Partridge River mafic layered intrusion along the NE margin of the Duluth complex; the host troctolitic rocks are in contact with sulphide-bearing metasedimentary rocks of the Virginia formation. Three main types of sulphide mineralization are recognized: 1) PGE-poor disseminated sulphides, 2) PGE-rich disseminated sulphides and 3) semi-massive to massive sulphides. The PGE-poor disseminated sulphides occur in the lower 250 m of the intrusion; their mineralogy and geochemistry suggest that the magma had undergone substantial contamination. The PGE-rich disseminated sulphides occur directly beneath ultramafic layers and show few signs of contamination; they appear to have formed at high ratios of silicate magma to sulphide melt. The semi-massive and massive sulphides occur mainly as veins and lenses along the basal contact and within the country rocks; they are zoned with pyrrhotite-rich (Fe-rich) and chalcopyrite-cubanite-rich (Cu-rich) portions. It is concluded that three different processes operated in sequence from magma emplacement to complete crystallization of the sulphides: 1) country-rock assimilation, 2) interaction between sulphide melt and silicate magma and 3) fractional crystallization of the sulphide melt.