ORIGINS OF COUNTRY ROCK-HOSTED NI-CU-PGE MASSIVE SULFIDES NEAR MAFIC-ULTRAMAFIC INTRUSIONS OF THE NORTH AMERICAN MIDCONTINENT RIFT SYSTEM AND THE STILLWATER COMPLEX

Abstract

Igneous rock-hosted sulfides in mafic-ultramafic intrusions produce most of the world’s nickel and PGEs. These sulfides were produced by sulfide saturation in mafic-ultramafic magmas and accumulation of sulfide liquid in the magmatic systems. Some massive Ni-Cu-PGE sulfides also occur in local country rocks. Within the Midcontinent Rift System, sheet-style and conduit-style intrusions host disseminated to massive sulfides in igneous rocks. Country rockhosted massive Ni-Cu-PGE sulfides are also found near at least three Midcontinent Rift-related intrusions; the Partridge River intrusion, the Tamarack Intrusive Complex, and the Eagle Intrusion. These massive sulfides have no known physical connections to igneous rocks, and consequently, their genesis remains controversial. The Stillwater Complex in Montana also hosts disseminated to massive sulfides throughout the complex. Sulfide(-oxide) mineralization in the local country rocks also occurs below the Stillwater Complex as lenticular to laminated massive sulfide(-oxides) in the metamorphic aureole. Prevailing opinion is the sulfide(-oxides) are genetically related to igneous-hosted sulfides within the Stillwater Complex, but their genesis remains contentious. Trace element and isotopic analyses of country rock-hosted massive sulfides near Eagle and Tamarack indicate minor crustal contamination of mafic-ultramafic magmas. Trace element and isotopic compositions of samples near the Partridge River intrusion require substantial crustal contamination. Country rock-hosted sulfides near Tamarack were produced as immiscible sulfide liquids from the semi-massive sulfides from the underlying CGO unit were filter-pressed from the intrusion. Country rock-hosted massive sulfides at Eagle were produced via fractional crystallization of the massive sulfides in the igneous rocks. Massive sulfides below the Partridge River intrusion were produced when relatively low R-factor sulfide liquids leaked from the base of the intrusion. Trace element and isotopic analyses of massive sulfide(-oxides) below the Stillwater Complex suggest the sulfide was deposited as sedimentary or seafloor hydrothermal sulfides before emplacement of the Stillwater Complex. Contact metamorphism dehydrated the pelitic country rocks and the Stillwater iron formation to produce anhydrous silicates, Fe-Ti oxides, and a metamorphic-hydrothermal fluid. Increased temperatures and circulating metamorphic fluid caused desulfidation and produced a sulfide composition more enriched in Fe-Ni-Cu and other metals, which cooled to produce hexagonal and monoclinic pyrrhotite with trace pentlandite, chalcopyrite, and cubanite.