Mineralogic, petrological and isotopic studies of the Eagle nickel-copper sulfide deposit, Michigan

The Eagle and East Eagle intrusions are small, sub-vertical dike-like mafic-ultramafic bodies that cut Proterozoic sedimentary strata in the Baraga Basin in northern Michigan. The Eagle intrusion hosts a newly-discovered magmatic Ni-Cu-PGE deposit. The nearby East Eagle intrusion also contains sulfide mineralization, but the extent of this mineralization has yet to be determined by further drilling. Both intrusions contain olivine-bearing rocks such as feldspathic peridotite, melatroctolite and olivine melagabbro. Sulfide accumulations range from disseminated at both Eagle and East Eagle to semi-massive and massive at Eagle. U-Pb baddeleyite dating and compositions of parental magmas inferred from olivine and spinel compositions suggest that the Eagle and East Eagle intrusions are the intrusive equivalents of high-MgO basalts that erupted in the early stages of development of the Midcontinent Rift System. Variations in mineral compositions and incompatible trace element ratios suggest that at least four major pulses of magmas were involved in the formation of the Eagle intrusion. Accumulation of suspended olivine crystals and sulfide droplets from ascending magmas as they passed through wide parts of the conduits at Eagle and East Eagle played a critical role in the genesis of olivine-rich rocks and sulfide ores in the intrusions.;The trace element, Os, Nd, and O isotopic values of the igneous rocks are consistent with <5% of bulk contamination by Proterozoic and Archean country rocks, but could still be indicative of a contribution of S from country rocks of up to ∼50%. Delta33S values of the disseminated and massive sulfides range from -0.10 to 0.09‰ and are consistent with contaminant sulfide being derived from Proterozoic country rocks. In contrast, Delta 33S values of semi-massive sulfides show a broad range between -0.86 and 0.86‰. The Delta33S values are similar to those of pyrite in Archean meta-igneous rocks in the area of the Eagle deposit. Isotopic data from the Eagle deposit strongly indicate that multiple sources of sulfur were involved in the generation of the Cu-Ni-(PGE) mineralization, and that magmas which traversed variable paths through the mantle and crust utilized the same conduit at the level of the Eagle deposit.