Uranium hydrogeochemical and stream sediment reconnaissance of the Dillon NTMS Quadrangle, Montana/Idaho, including concentrations of forty-three additional elements

Surface waters were taken from flowing streams; ground waters were taken from springs and wells. All water samples were analyzed for uranium and 12 other elements. Sediment samples, collected from streams and springs, were analyzed for uranium, thorium, and 41 additional elements. The uranium contents of water samples range from below the detection limit of 0.02 ppB to 79.31 ppB. Most water samples having anomalously high uranium contents were collected in areas underlain by felsic rocks of the Boulder, Pioneer, and Idaho batholiths. Other groups of anomalous waters are associated with Belt metasediments in the Beaverhead and Salmon River Mountains and with Tertiary strata in intermontane basins. The uranium contents of sediment samples range between 0.95 and 130.50 ppM. Anomalous uranium values are associated with felsic rocks of Cretaceous batholiths and Belt quartzites near the Montana--Idaho border. Samples having uranium contents significantly above local background levels also occur in Tertiary strata adjacent to the anomalous areas in the crystalline terranes. Thorium concentrations in sediment samples range from 3.40 to 351.00 ppM. The average uranium/thorium (U/Th) ratio of samples with < 20 ppM uranium is 0.37, whereas samples with uranium concentrations > 20 ppM have an average ratio of 1.85. The association of high uranium values with relatively high U/Th ratios may be a useful tool for identifying uranium-enriched terranes.