Isotopic Studies of Hydrothermal Flow Systems Above and Below the Duluth Complex, Midcontinent Rift System, Minnesota

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9614165 Ripley In order to better understand the processes involved in the development of hydrothermal flow systems associated with continental rift-related magmas, a detailed oxygen and hydrogen isotopic study is planned, to be augmented by radiogenic isotopic analyses where competing processes may have produced similar stable isotopic signatures. Results of a one-year pilot study indicate that above the Duluth Complex hydrothermal fluid flow was channeled in the North Shore Volcanic Group within permeable flow tops and bottoms, and possibly along the contact of sills within the Proterozoic sedimentary rocks below the Complex. Additional O and H isotopic measurements of sills located both above and below the Complex are planned to better define the nature of the hydrothermal circulation system association with emplacement of the Duluth Complex and hypabyssal intrusives. Modeling of isotopic exchange and reactions will be pursued based on temperature constrains, flow parameters, and water/rock ratios, and both equilibrium and kinetic fractionation. Some Sr, Nd and Pb isotopic analyses are planned. Potential variations in radiogenic isotope values as a function of alteration and intensity of water-rock interaction will also be determined. Results will be of particular significance to the interpretation of possible origins of platinum-group element mineralization in the Duluth Complex and will lead to a better understanding of crust-magma interaction in rift environments worldwide.