Causes of Re-Os Isotopic Variations in High-Temperature Sulfide Mineral Assemblages: Insights From the Duluth Complex-Virginia Formation System

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Intellectual Merit: This project will evaluate hypotheses to explain precipitation of sulfide minerals from crustally intruded magmas - that may explain the formation of certain important ore deposits associated with the Duluth intrusive complex. Country rock contamination has been proposed as a requirement to produce sulfide saturation in mantle-derived basaltic magmas, either through assimilation of crustal sulfur, or by assimilation of siliceous material that results in a decrease in the sulfide solubility of the mafic magma. The Re-Os isotopic system has proven particularly useful in evaluating the role of crustal contamination in magmatic ore deposits and in mantle-derived rocks in general. Because of the chalcophile nature of both Re and Os, the system is well-suited as an indicator of crustally derived metals in sulfide mineralization. However, recent studies have shown that the Re-Os isotopic system may be particularly prone to perturbations related to later interaction with fluids. Previous S isotopic studies of likely country rock contaminants suggest that these rocks were the principal source of sulfur for the magmatic sulfide mineralization associated with the Duluth complex. However, preliminary Re-Os isotopic studies indicate that sulfide minerals in the contact aureole are characterized by unradiogenic Os whereas the massive sulfide mineralization at Duluth is characterized by radiogenic Os - indicative of extensive Os contamination. Extreme Re-Os isotopic variations in the sulfide systems in these rocks are reflective of fundamental processes that must accompany the interaction of mafic magmas with crustal rocks. Veins in the contact aureole contain quartz and a sulfide assemblage (pyrrhotitecubainte-pentlandite) that is very similar to that found in the magmatic sulfide mineralization. In order to test the premise that the perturbation of the Re/Os isotopic system in the contact aureole is related to the reaction between Re-Os-bearing fluids and sulfide minerals, fluid inclusion, stable isotopic, and Re-Os isotopic studies of the vein material, as well as spatially associated sedimentary sulfides, are planned. These data will allow assessment of the origin and composition of the fluid, and its relationship to the magmas of the Duluth Complex. Because large xenoliths of country rocks that preserve low-grade mineral assemblages in their cores are present in the intrusive rocks of the Complex, we will also be able to systematically evaluate the mechanisms responsible for Re-Os and S isotopic variations from xenolith cores through metamorphosed margins and surrounding disseminated sulfide-bearing igneous rocks. The results of this study will shed light on the causes of Re-Os isotopic variations in sulfide systems related to mafic magma-crust interaction, and will be of particular value in the evaluation of Re-Os isotopic systematics associated with magmatic sulfide ore formation.Broader Impacts: Broader aspects of the proposed research include the training of both graduate and undergraduate students in systematic field sampling and in the use of state-of-theart analytical facilities for chemical and isotopic analyses at three NSF-funded laboratories. Cooperation with the Minnesota Natural Resources Research Institute will be essential for the project to succeed. Exploration for, and utilization of, metallic resources found in magmatic systems continues to grow as third-world economies strengthen. Knowledge of how an often utilized radiogenic isotopic system may respond to high-temperature events involving magmas and fluids is essential to the development of future exploration models.