HYDROTHERMAL INFLUENCES FOR U-V MINERALIZATION IN THE MADISON LIMESTONE AND ADJACENT UNITS, PRYOR MOUNTAINS, MONTANA –WYOMING AND A POSSIBLE LINK TO ELEVATED PB AND HG IN THE BIG HORN RIVER IN MONTANA

An area spanning from the Big Pryor Mountain Mining District, in Montana to the Little Mountain Mining District in Wyoming has numerous abandoned, past productive U/V mines and prospects, as well as nearby documented thermal springs and caves within the Mississippian Madison Limestone. These deposits are found in collapse breccia pipe features formed on the paleokarst horizon, the top 200 feet of the Madison Limestone. These deposits are similar to the well documented U breccia pipes of northwestern Arizona, but much smaller in scale. The origin of these deposits has not been clearly established. Several theories on their origin have been proposed. One main school of thought for the U/V mineralization has been attributed to top-down groundwater leaching of Tertiary volcanic ash deposits that once covered the region prior to epeirogenic uplift and exhumation. Hydrothermal mineralization of the deposits was also proposed in earlier studies of nearby caves located where the present day Big Horn River cuts through the Little Sheep Mountain anticline in Wyoming. The structural relationship of the mineralized breccia pipes in the Red Pryor Mountain Quadrangle appears to be enhanced where NW-striking fractures intersect the crest of a large south-plunging anticline and alignment of the mines is spatially coincident with a reverse fault in the basement that subtends this anticline. Principal ore minerals are the calcium-uranium vanadates tyuyamunite and metatyuyamunite. Gangue mineral assemblages described in the mines include limonite, radioactive green calcite, barite, dark-purple fluorite, celestite, pyrite or marcasite, and a black crust of manganese mineral, possibly pyrolusite. The presence of fluorite and barite in the mines and the presence of geothermal waters at present day Big Horn River level may provide a link to elevated Pb and Hg in the Big Horn River in Montana, which was declared a 303d impaired waterway once it passes through the Crow Reservation, just downstream from the caves and mining districts.