ISOTOPIC EVIDENCE FROM LATE-STAGE CALCITE IN BRECCIATED HOST ROCKS FROM ABANDONED URANIUM VANADIUM DEPOSITS IN MONTANA AND WYOMING SUGGESTS A POSSIBLE HYDROTHERMAL SOURCE

Hand samples within the upper paleokarst unit of the Mission Canyon Member of the Madison Group were collected from mineralized areas on Big Pryor Mountain and East Pryor Mountain, Montana and the Little Mountain area, Wyoming. Some rocks were cut and stained with alizarin red and potassium ferricyanide to identify the different carbonates within the sample. Calcite was stained red, ferroan calcite purple, ferroan dolomite turquoise or deep blue, and dolomite exhibited no color change. Vein fill material and brecciated host rock were prepared and sent to the stable isotope lab at the University of Michigan for carbon and oxygen stable isotope analyses. Thin sections made from the samples were examined using a Field Emission Scanning Electron Microscope with an EDS Energy-Dispersive x-ray Spectrometer, Cathodoluminescence microscope and a standard petrographic microscope. These techniques revealed several episodes of fluid migration. Comparing isotopic values of the samples with the value of carbonates that precipitated from Mississippian aged ocean waters gives an indication of the diagenetic processes that have affected δ¹⁸O and δ¹³C values of the carbonates in the mining districts. Results from the δ¹⁸O values of the late-stage calcite vein material sampled have the most negative δ¹⁸O values (-11.99 to -23.48 ‰) and a wide range of δ¹³C values (-0.40 to -6.20‰). The δ¹⁸O values for the late-stage calcite veins are depleted relative to the brecciated host rock, indicating that fluids were not in equilibrium with the host rocks and that the system was open to outside fluid flow. This isotopic depletion and the presence of disseminated sulfides as well as ytterbium (a rare earth element) detected by EDS, suggests that the fluids were hydrothermal in nature.