Origin of Cu-Ni-PGE sulfide mineralization in the Partridge River intrusion, Duluth Complex, Minnesota

Four subeconomic Cu-Ni-PGE sulfide deposits occur near the base of the Part ridge River intrusion, a mafic layered intrusion emplaced along die northwe stern margin of the Duluth Complex. The host troctolitic rocks are in conta ct with sulfide-bearing metasedimentary rocks of the Virginia Formation. Th e origin of the sulfide mineralization lias generally been linked to contam ination of the mafic magma through partial assimilation of the argillaceous country rocks. Three main types of sulfide mineralization have been recognized within the four deposits. These are (1) PGE-poor disseminated sulfides; (2) PGE-rich d isseminated sulfides; and (3) semimassive to massive sulfides. The PGE-poor disseminated sulfides typically occur within the lower 250 m of the intrus ion and are hosted mainly by heterogeneous norite and olivine gabbro, both of which contain abundant country-rock xenoliths. This type of mineralizati on shows numerous features, such as high proportions of pyrrhotite and arse nide minerals and high Cu/Pd, Ni/Pd, and Cu/Pt ratios, which suggest that t he magma had undergone substantial contamination. These sulfides appear to have formed at low to moderate ratios of silicate magma to sulfide melt (me an R factor = 600-2,400), as deduced from their metal-poor nature. The PGE- rich disseminated sulfides occur well within the intrusion directly beneath ultramafic layers and show little signs of contamination. They are compose d mainly of chalcopyrite and pentlandite, with lesser amounts of pyrrhotite and cubanite. They appear to have formed at high R factors (mean = 6,000-7 ,700), which explains their relatively high PGE and base metal contents. Th e semimassive to massive sulfides occur mainly as veins and lenses bed; alo ng the basal contact and within the underlying sedimentary country rocks. T hey are typically zoned, being composed of both pyrrhotite-rich (Fe-rich) a nd chalcopyrite-cubanite-rich (Cu rich) portions, the latter often forming along the base or top of massive sulfide bodies. The zonation in the massiv e sulfides is interpreted to be the product of fractional crystallization o f a sulfide melt, the pyrrhotite-rich sulfides representing the cumulate of this crystallization and the Cu-rich sulfides the fractionated liquid. Based on the above evidence, we interpret the compositional variations obse rved between the different types of sulfide mineralization to originate fro m the combined action of three different processes that operated in sequenc e from magma emplacement until complete crystallization of the sulfides. Th ese processes are (1) country-rock assimilation; (2) interaction between th e sulfide melt and the silicate magma (R factor); and (3) fractional crysta llization of the sulfide melt.