Because of the enrichment of Re relative to Os in mantle-derived, basaltic partial melts, and the high Re/Os ratio of crustal rocks, the Re/Os isotopic system has proven to be a sensitive tracer in the evaluation of dynamic interaction processes between crust and mantle-derived mafic magmas. In the assessment of country rock contamination the application of the Re-Os isotopic system as a petrologic tracer is dependent upon the proper evaluation of open- or closed-system behavior in the contaminant rock, and immobility of Re and Os after crystallization of the igneous rock or sulfide-rich ore material. This project will evaluate the distribution and isotopic characteristics of Re and Os in organic-rich sedimentary rocks, changes in isotopic characteristics during contact metamorphism and conversion of these rocks to hornfels, and isotopic variation found in sulfide-bearing igneous rocks produced as a result of assimilation of country rocks or a released fluid phase. The geologic setting of the Duluth Complex and Proterozoic country rocks presents an excellent opportunity to trace the evolution of the Re-Os system through the processes of contact metamorphism and dehydration, and assimilation of country rocks by mafic magma. Both crustally-contaminated and uncontaminated gabbroic and troctolitic rocks are present in the area of the Babbitt Cu-Ni deposit, and systematic Re-Os analyses will permit an in-depth evaluation of the causes of isotopic variability that may originate as a result of metamorphic and hydrothermal processes in the country rocks, and magmatic processes related to changes in the ratio of silicate magma to sulfide magma.
