Géochimie des eaux liées au gisement d'uranium de Cigar Lake (Saskatchewan, Canada) et apport des isotopes de l'uranium et du plomb comme guides de prospection

Abstract

The primary objective of this study is a better understanding of the chemical evolution of the waters crossing the Cigar Lake (Saskatchewan, Canada) uranium ore deposit and, once described, to investigate the behavior of the lead and uranium isotopes and to test their use as guidelines in exploration. The underground waters around the Cigar Lake ore deposit were sampled by pumping bore holes or by air lifting. Isotopic analyses of U and Pb and chemical analyses of the major elements and of some trace elements (Ba, Ra, U) were completed. Based on these results, a model is proposed to explain the genesis and evolution of the waters and also to account for the water–minerals equilibria. The saturation indices of uraninite and coffinite were determined by way of oxidation–reduction potentials calculated on the basis of measured Fe contents. The waters saturated with these minerals, which are also the most reduced, are systematically located directly above or across the ore deposit. These saturation indices, as well as the calculated oxidation–reduction potentials, are good guidelines for hydrogeochemical exploration in the Cigar Lake orebody setting; however, they must be used only in a relative fashion. Furthermore, the 234U/238U disequilibrium measurements and the U concentration in the waters permit the recognition of waters having percolated through the mineralized zones (higher U concentrations, lower disequilibria). The nature of the mineralization is probably responsible for the coexistence of the two evolutionary trends within the mineralized zones; in the high-grade zones, the disequilibria are low, whereas in the zones of disseminated mineralization or in halos surrounding the orebody, the disequilibria are greater, reflecting the prevailing influence of selective dissolution of 234U in the zones where exchange surfaces between the mineral carrying U and the solution are more important. The measurement of Pb isotopic ratios in the waters also permits of a clear characterization of waters that have percolated through the ore deposit: in this case, the Pb isotopic ratios are enriched in radiogenic Pb. [Journal Translation]