NI-CU-PGE DEPOSITS IN THE MIDCONTINENT RIFT SYSTEM: GENESIS AND TYPES

Ni-Cu-(PGE) sulfide mineralization in the Midcontinent Rift System (MRS) can broadly be classified into three types based on host rock, physical setting and platinum-group element content. Disseminated and locally massive sulfide mineralization occur in troctolitic and gabbroic rocks of several intrusions in the rift system. Mineralization is dominated by pyrrhotite, chalcopyrite and pentllandite; Cu/Ni ratios may vary from ~3:1-8:1. Isotopic data strongly indicate that contamination by crustal sulfur has been essential for the development of the mineralization. Deposits found in the Partridge River and South Kawishiwi intrusions of the Duluth Complex exemplify this style of mineralization. Deposits with high Ni/Cu ratios are associated with early, high MgO, magmatism in the system. The Eagle Deposit, located in the Baraga Basin of Michigan, is characterized by massive and net-textured sulfide mineralization hosted in olivine-rich (peridotite, mela-troctolite) rocks. The deposits and host rocks are part of conduit systems with geometries that vary from dike-like to chonolithic. Disseminated sulfide mineralization characterized by elevated PGE content occurs either in mela-troctolites at the base of individual layers in intrusions such as those found in the Duluth Complex, or as layers in well-differentiated tholeiitic intrusions such as the Sonju Lake intrusion. Mineralization is characterized by low sulfide content, but elevated PGE values over thicknesses of from 3 to 100 meters. Exploration activity in the MRS continues for all of these deposit types. The voluminous magmatism of the rift, driven largely by plume-mantle interaction, provided a favorable environment for the potential development of world class Ni-Cu-PGE sulfide-rich deposits.