Cu-Ni-PGE Mineralization in the South Filson Creek Area, South Kawishiwi Intrusion, Duluth Complex: Mineralization Styles and Magmatic and Hydrothermal Processes

The South Filson Creek mineralization occurs above the basal units of the South Kawishiwi intrusion and represents an uncommon geologic setting of Cu-Ni-platinum-group element (PGE) mineralization within the Duluth Complex. Unlike other mineralization of economic interest in the basal heterogeneous troctolite units along the lower contact, it is located at an approximate stratigraphic height of 1,000 in above the contact with the footwall granitoids in the usually unmineralized homogeneous troctolite unit. Magmatic sulfide mineralization forms disseminated fine-grained patches and pockets in an area of approximately 1/2 km(2) of homogeneous troctolitic rocks in the northeastern part of the South Kawishiwi intrusion. Pyrrhotite, chalcopyrite, pentlandite, and cubanite are the main sulfide minerals with subordinate amounts of other copper-bearing phases (bornite, covellite, talnakhite). Cu/Pd ratios for unaltered mineralization indicate that sulfides were formed from multiple injections of magma that segregated sulfides with different R factors or having different initial compositions. Variation in the halogen contents of apatite from pegmatoids within troctolitic rocks revealed that a fluid has segregated from the crystallizing melt into which fluid Cl and rare earth elements partitioned. Amphibole, chlorite, sericite, prehnite, pumpellyite, and carbonate are associated with alteration along brittle structures and isolated patches in the area. The hydrothermal fluid-rock interaction has affected the composition of primary magmatic sulfide mineralization and is also responsible fix mineralization in the anorthositic hanging wall of the South Kawishiwi intrusion. Hydrothermal alteration resulted in a slight Pd depletion in magmatic sulfide mineralization, which might indicate remobilization of precious metals to an unknown location. The hanging-wall mineralization is characterized by relative Cu enrichment in comparison to the magmatic sulfide mineralization with no PGE enrichment. Partial remobilization of platinum group elements (PGE) has also occurred as a result of a separate alteration event, on the evidence that platinum group minerals (PGM) are also associated with serpentinization of olivine and Ca enrichment of plagioclase along grain boundaries with sulfides. This alteration was probably effective on a very local scale.