The Seagull and Kitto intrusions in the Nipigon Embayment of northwestern Ontario were studied to gain an understanding of the processes involved in sulfur saturation of the mafic to ultramafic magmas leading to the formation of platinum group element (PGE) concentrations. Profiles of sulfur, copper, nickel, gold, palladium, and platinum concentrations as a function of depth revealed that sulfur saturation occurred at the base of the Seagull intrusion. A higher grade horizon occurs well above the base of the intrusion, suggesting that a reef-type of process was significant here and possibly in the Kitto intrusion, as well. Olivine compositions indicate that, in both cases, the parental magmas were undersaturated with respect to sulfur. Sulfur, neodymium–samarium, and rubidium–strontium isotopic data suggest that assimilation of country rock and sulfide played a role, especially in the formation of basal concentrations of PGEs.