SULFUR ISOTOPE CONSTRAINTS FOR A DYNAMIC MAGMATIC SULFIDE ORE DEPOSITION MODEL IN THE SILL-LIKE SOUTH KAWISHIWI INTRUSION OF THE DULUTH COMPLEX, MINNESOTA, USA

The 1.1 Ga old Duluth Complex (DC) is the second largest mafic intrusion on Earth and probably contains the largest undeveloped magmatic Cu-Ni-(Co)-PGE ore resource. The troctolitic sill-like South Kawishiwi Intrusion (SKI) of the DC currently hosts three major magmatic sulfide deposits: the Spruce Road deposit (529 Mt, 0.43% Cu, 0.15% Ni); the Nokomis deposit (449 Mt, 0.63% Cu, 0.2% Ni, 0.6 g/t Pt+Pd+Au); and the Birch Lake deposit (195 Mt, 0.53% Cu, 0.16% Ni, 0.93 g/t Pt+Pd+Au). These deposits are located along the contact of the SKI and the monzonitic Giant Range Batholith (GRB) of Archean age in the footwall. The basal mineralised zone (BMZ) of the SKI is highly heterogeneous: rock compositions range from norite to melatroctolite-troctolite-anorthosite-olivine gabbro with fine- to coarse-grained ortho- and adcumulate and pegmatitic textures. The BMZ contains assimilated inclusions of GRB, BIF and metapelite. Charnokitisation of GRB is especially strong along the major conduits of the parent melts of SKI. The BMZ hosts fractionated ore in the forms of intercumulus patches of chalcopyrite, cubanite, pentlandite and pyrrhotite with local semi-massive, replacive, less-fractionated ore with pyrrhotite, pentlandite and chalcopyrite along the immediate contact. Disseminated ores in the footwall have replacive-infiltration texture with enrichment of chalcopyrite and bornite away from the contact. The highest sulfur isotope compositions (from +5 to +12‰ δ³⁴S) of sulfides and whole rock samples occur along the contact, with a systematic gradual decrease in δ³⁴S to around 0‰ upward within the BMZ. Ore in the footwall and in association with footwall inclusions is also characterized by the heavy sulfur isotope signature. Observations support a dynamic model for ore deposition consisting of early accumulation of magmatic sulfides from the mantle-derived and metal-laden mafic melts along the contact and in the footwall due to crustal sulfur assimilation, followed by superimposed sulfur isotope exchange between the early stage ores and later pulses of sulfur-undersaturated melts moving above the early stage sulfide mineralisation. This model offers keys for understanding the Cu- and PGE-enriched nature of ores in the BMZ, and suggests that re-mobilised volatile sulfur was included in the ore-forming process.