OPEN-SYSTEM BEHAVIOR OF RE AND OS IN URANINITE, PYRITE, AND ORGANIC MATERIAL IN CONTINENTAL SEDIMENTARY ROCKS, GRANTS URANIUM REGION, NEW MEXICO

We apply the Re-Os isotopic system to continental sedimentary systems, focusing on the well-studied Grants Uranium Region because, like U, Re and Os are strongly controlled by redox processes. The figure below shows data for pyrite splits and organic material from four samples from the Mariano Lake U deposit, hosted by the Morrison Formation. Concentrations are variable but high (Re=220 to 4300 ppb; Os=3.5 to 28.6 ppb). In each sample, organic material is enriched in both Re and Os relative to co-existing pyrite. Among the pyrite samples, higher concentrations occur in finer size fractions, suggesting adsorption as a probable mechanism for fixing Re and Os. All analyses show high Re/Os relative to reference lines for proposed depositional and diagenetic ages, suggesting large increases in Re/Os relative to ¹⁸⁷Os/¹⁸⁸Os, and introduction of both Re and Os over an extended time. These observations suggest that repeated episodes of groundwater flow enriched both pyrite and organic material in Re and Os, disturbing the isotopic systematics. These processes fractionate Re and Os in crustal fluids. Preliminary analyses of uraninite from the nearby Todilto limestone U deposits also show variable, but high Re and Os concentrations (50 to 825 and 0.3 to 1.1 ppb, respectively), and highly variable Re/Os and ¹⁸⁷Os/¹⁸⁸Os, again suggesting open-system behavior. In contrast, pyrite from freshwater carbonate in the Morrison Formation, unrelated to U deposits, has Re and Os concentrations of only 6.45 +/- 0.07 and 0.053 +/- 0.006 ppb, respectively.