The physical and petrologic setting and textural and compositional characteristics of sulfides from the South Kawishiwi intrusion, Duluth complex, Minnesota, USA.

Abstract

This intrusion in the N part of the Duluth complex is divisible into a sulphide-free upper zone and sulphide-bearing lower zone. The sulphide-free zone is mostly troctolite, forming cyclic units with anorthosite. These rocks occurred in a magma chamber that was being continuously replenished with compositionally similar liquids. In contrast, the sulphide-bearing zone consists of a heterogeneous mixture of troctolite, dunite, picrite, anorthosite, oxide cumulates, and hornfels in which sulphides are disseminated ubiquitously. The contact with the sulphide-free zone is sharp. The sulphides are predominantly pyrrhotite, chalcopyrite with cubanite, and pentlandite. The close association of sulphides with hydrous phases or with grains that show reverse compositional zoning indicates that volatiles derived from adjacent metasedimentary inclusions were important in sulphide formation.-R.A.H.US Geological Survey, Reston, VA 22092, USA.