FRANCEVILLITE [(BA,PB)(UO2)2(V2O8)•5H2O] IDENTIFIED IN THE URANIUM VANADIUM DEPOSITS IN THE PRYOR MOUNTAIN MINING DISTRICT, MONTANA AND THE LITTLE MOUNTAIN MINING DISTRICT, WYOMING MAY PROVIDE A LINK TO THE ELEVATED LEAD IN THE BIGHORN RIVER AND BE RELATED TO FLUID MIGRATION OF THE LOWER KANE CAVE, WYOMING

The Little Bighorn River is a declared 303d impaired waterway once it passes through the Crow Reservation in Montana. The 303d designation is due to elevated lead and mercury. Uranium vanadium deposits are present in the Pryor Mountains, Montana and the Little Mountain Mining District of Northern Wyoming. The districts border the Crow Reservation and are adjacent to the Bighorn River. The deposits are hosted in mineralized collapse breccia features within a paleokarst horizon of the Madison Limestone. Some of these features show multiple episodes of brecciation including hydrothermal breccias along the same fractures. Faults and fractures are critical factors in enhancing the porosity and permeability of otherwise tight carbonate reservoirs and have important implications for fluid migration. Both districts are located in Laramide structures. A similar structure located at the present day Bighorn River level near Lovell, Wyoming, the Little Sheep Mountain anticline, hosts the active Lower Kane Cave bearing hydrothermal fluids with radioactive muds and water. The cave is still forming through sulfuric acid speleogenesis, a mechanism in which limestone dissolves due to sulfuric acid instead of carbonic acid, aided by microbial processes. This structure may provide a modern day analogue to the depositional mode of mineralization for these districts. Rock samples were collected from several mines and water and mud samples from Lower Kane Cave for analytical work. Francevillite [(Ba,Pb)(UO2)2(V2O8)•5H2O] was identified using X-ray diffraction analysis (XRD) and scanning electron microscope (SEM) spot elemental analysis with energy dispersive x-ray spectrometry (EDS). The mineral was found with and closely resembles metatyuyamanite Ca (UO₂)₂V⁵⁺₂O₈•3(H₂O), the primary ore originally identified by XRD of the two districts. A couple variations of barite, deep purple fluorite, green radioactive calcite, dolomite and quartz, sometimes of the Herkimer style are common in the mineralized portions of the two districts. Lead was found in the muds and water in Lower Kane Cave. This study may show that structurally controlled migration of warm, metal enriched fluids, possibly mixed hydrothermal-meteoric waters may be the mode of mineralization for the two districts and the source of lead in the Bighorn River.