Ni-Cu-PGE sulfide mineralization in the Midcontinent rift system can be broadly classified into three types: (1) disseminated sulfide mineralization found within gabbroic to troctolitic sheet-like intrusions; (2) disseminated through massive sulfide occurrences associated with smaller, early rift, mafic to ultramafic intrusions that are conduit-related, and (3) relatively low sulfide, PGE enrichment found in well-differentiated tholeiite layered intrusions. The first two types are volumetrically dominant and are the subject of this review. Disseminated sulfide mineralization found within the Partridge River and South Kawishiwi intrusions of the Duluth Complex occurs in a variety of rock types, all produced from varying degrees of fractionation of a high-Al olivine tholeiite (HAOT) parental magma. Mineralization is characterized by high Cu/Ni ratios (bulk rock values averaging ~3–5) and variable, but locally elevated, PGE contents. Tenors of undepleted disseminated sulfide are not unlike those of the world-class ores at Noril’sk, but concentration of sulfide in the intrusive sheets did not occur. Massive, net-textured and disseminated sulfides are all part of the Eagle deposit in northern Michigan. The Eagle deposit is representative of the class of small, olivine-rich intrusions emplaced during early stages of rift development. Ni and Cu grades are considerably higher than those that characterize the disseminated mineralization in the Partridge River and South Kawishiwi intrusions; average Ni grade of the massive sulfide at the Eagle deposit is 3.57 wt %, making it one of the highest Ni grade orebodies in the world. The parental magma for the Eagle intrusion was a high-FeO picrite.
Isotopic and trace element studies indicate that contamination by country rocks, particularly the introduction of sulfur, has been important for sulfide genesis. Ni enrichment in the Eagle deposit appears to be related to the early attainment of sulfide saturation in relatively Ni rich, high MgO magma. Low Ni contents of sulfide mineralization in the sheet-style intrusions of the Duluth Complex are related in part to sequestration of Ni by early crystallizing olivine, low sulfide abundance, and relatively late attainment of sulfide saturation. However, these processes cannot account for the low bulk rock Ni/Cu ratios. Whether Cu enrichment is related to fractional crystallization of sulfide liquids and widespread incorporation of the fractionated liquids into the HAOT magmas or the introduction of country rock-derived Cu into the magmas remains an uncertainty. Other unanswered, but critical, questions pertaining to the genesis of sulfide mineralization in the Midcontinent rift system include how HAOT liquids are generated, and what the mechanisms are that permit the emplacement of sulfide-rich or sulfide-only magmas in subvertical to vertical conduits.