Origin and Evolution of the Sulfide-Rich, Mafic Igneous Intrusion at Eagle Mine, Upper Peninsula, MI

Abstract

The Eagle Mine of the Upper Peninsula, Michigan is host to a productive copper-nickel, sulfide-rich ore deposit that is hosted within the Yellow Dog Intrusion, a mafic intrusive igneous rock. The origin of the ore and its relationship to its host intrusion are not known. To better understand the processes responsible for the formation of the deposit, we collected a series of rocks from the mine that represent distinct crystallization phases of the intrusion. Here, we apply a new model for differentiation within mafic magma chambers: The Sequential Extraction model. The model invokes the process of liquid immiscibility to generate complementary Si-rich and Fe-rich melts that, due to significant differences in density, rapidly segregate upon formation. We use whole-rock and trace-element geochemistry to follow chemical evolution within the magma chamber, and we apply petrographic techniques to observe characteristic textural features associated with segregated liquids within a crystallizing magma body. We show that the Cu, Ni deposits are co-magmatic, and that their origin is tied to the segregated Fe-rich liquid.