Paper No. 148-4
Presentation Time: 9:00 AM-6:30 PM
URANIUM VANADIUM MINERALIZATION IN MISSISSIPPIAN AGED PALEOKARST, NORTHERN BIGHORN BASIN, MONTANA AND WYOMING, INDICATES A HYDROTHERMAL PERMIAN PHOSPHORIA FORMATION SOURCE OF METALS INCLUDING REE AND TL
The origin of V, REE and other metals in the Permian Phosphoria Formation have been speculated and studied by numerous scientists. The exceptionally high concentrations of metals have been interpreted to be the result of a continuous strong upwelling marine environment reflecting fundamental transitions from anoxic to oxic marine conditions. Past productive U and V deposits hosted in Mississippian aged paleokarst of the northern Bighorn Basin MT and WY have a close association with hydrocarbons and contain anomalous high concentrations of many metals that are found in similar concentrations in the Phosphoria Formation. Original Phosphoria Formation sourced hydrocarbon accumulation in the Bighorn Basin was in stratigraphic traps created primarily by up dip facies change, pinch out and truncation of the reservoir carbonates, and by uneven Permo-Triassic Goose Egg Formation truncation of underlying Tensleep Sandstone. These stratigraphic traps were later released because of fracturing and faulting associated with Laramide folding and migrated into older Paleozoic reservoir rocks. Mineralizing fluids are interpreted to have migrated into collapse paleokarst by episodic tectonic brecciation related to hydrothermal activity due to crustal shortening during the Laramide orogeny. Isotopically depleted fluids with δ18O compositions between -24.8 and -11.99‰ VPDB indicate fluids with elevated temperatures cemented the breccia. Groups of minerals from different mines have similar 87Sr/86Sr compositions while within each mine site the 87Sr/86Sr composition of minerals vary. Some of the samples show enrichment of REE plus Y. Thallium (Tl) is found in anomalous concentrations in both the Phosphoria Formation and in all the mineralized samples from the UV deposits (10-490 ppm) of this study. Tl is not commonly concentrated in many rock types and Tl minerals are rare. Tl has been shown to be associated with hydrothermal sourced fluids in sediment hosted gold deposits. The average crustal abundance of Tl is 0.75 ppm. Bighorn Basin oilfield brines likely provided a transporting fluid and a source of metals for ore and gangue minerals in the UV mining districts in the northern Bighorn Basin.