Characterization of multiple fluid-flow events and rare-earth-element mobility associated with formation of unconformity-type uranium deposits in the Athabasca Basin, Saskatchewan

Three main stages of U mineralization have been identified in the Athabasca Basin on the basis of cross-cutting relationships, textures, O and U-Pb isotopic compositions and chemical composition. All three stages have been variably altered to becquerelite, calcio-uranoite and coffinite; their deposition occurred at approximately 1500 m.y. during peak diagenesis, 950 and as late as 300 m.y. Uraninite and pitchblende of the first two stages have the lowest Delta ^>18O values (-32 to -19.5 0/00) and uraninite of stage three has Delta ^>18O approximately -10 0/00; the three alteration minerals have Delta ^>18O near 0 0/00. The (super 87) Sr/ (super 86) Sr ratios of U minerals strongly reflect the proportion of basin and basement fluid involved in formation of these deposits: U minerals hosted by basement rocks have relatively low ratios, indicating the involvement of basinal brine, whereas the relatively high ratios of U minerals partially hosted by overlying sandstones show that a high proportion of the Sr (and by inference the fluid involved) was derived from basement rocks. High concentrations of REE-rich hydrothermal fluorapatite, crandallite-group minerals and xenotime are associated with the U mineralization. Also, the U minerals have total REE up to 12 000 ppm and are HREE-enriched. The REE and U were probably derived from detrital fluorapatite and zircon in the sandstone, garnet in the basement and also diagenetic clay minerals and zircon in the basement, and were probably transported as F-complexes. The high concentration of U (427 000 000 kg) and REE in this basin make unconformity-type deposits viable sources of REE as well as U.