PETROGENESIS OF THE EAST EAGLE INTRUSION GABBRO AND IT’S RELATIONSHIP TO THE EAGLE NI-CU-PGE MAGMATIC SULFIDE DEPOSIT, MARQUETTE COUNTY, MICHIGAN

The Eagle deposit is a high-grade, mafic to ultramafic Ni-Cu-bearing sulfide deposit located in Michigan’s Upper Peninsula in Marquette County. The Eagle deposit consists of two separate intrusions, Eagle and East Eagle, that are associated with the ~1.1 Ga Midcontinent Rift System and the east-west trending dikes of the Marquette-Baraga dike swarm. In 2016, delineation drilling encountered a vertical gabbroic intrusion in contact with the high-grade mineralization zone of the East Eagle intrusion. The intrusion is pipe-like with a diameter varying between 300-350 meters. The upper limit of the intrusion is located approximately 950 meters below the surface and have been traced down to depths of >2 km, however, the base of the intrusion has not been delineated. Surrounding the gabbro is a thin sleeve of fine-grained melagabbros and feldspathic pyroxenites containing minor blebs of chalcopyrite and pyrrhotite. In addition, a mineralized peridotite, like those of the East Eagle intrusion, can be found 300 meters below the top of the gabbroic intrusion.

Petrographic analysis of the intrusion shows high degrees of sericitic, propylitic, and carbonate alteration that decrease with depth away from the East Eagle intrusion. Upper portions of the intrusion experienced pervasive alteration, destroying almost all primary mineral phases. Less altered portions of the intrusion are dominated by a medium-grained magnetite gabbro with little to no sulfide mineralization. Initial geochemical analysis shows that the intrusion becomes more enriched in MgO and FeO with depth. Rare earth element (REE) diagrams of the intrusion depicts similar patterns to those of the Eagle and East Eagle peridotites. Age determination using the U-Pb zircon/baddeleyite method of the intrusion is currently underway. If the age of the intrusion is similar to that of the known age of the Eagle and East Eagle intrusions, then the gabbro likely represents a feeder dike into the East Eagle system.