GEOCHEMISTRY OF WATERS LINKED TO THE CIGA R LAKE URANIUM DEPOSIT (SASKATCHEWAN, CANADA) AND USES OF URANIUM AND LEAD ISOTOPES IN PROSPECTING

The primary objective of this study is a better understanding of the c hemical evolution of the waters crossing the Cigar Lake (Saskatchewan, Canada) uranium ore deposit and, once described, to investigate the b ehavior of the lead and uranium isotopes and to test their use as guid elines in exploration. The underground waters around the Cigar Lake or e deposit were sampled by pumping bore holes or by air lifting. Isotop ic analyses of U and Pb and chemical analyses of the major elements an d of some trace elements (Ba, Ra, U) were completed. Based on these re sults, a model is proposed to explain the genesis and evolution of the waters and also to account for the water-minerals equilibria. The sat uration indices of uraninite and coffinite were determined by way of o xidation-reduction potentials calculated on the basis of measured Fe c ontents. 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 oxida tion-reduction potentials, are good guidelines for hydrogeochemical ex ploration in the Cigar Lake orebody setting; however, they must be use d only in a relative fashion. Furthermore, the U-234/U-238 disequilibr ium measurements and the U concentration in the waters permit the reco gnition of waters having percolated through the mineralized zones (hig her U concentrations, lower disequilibria). The nature of the minerali zation is probably responsible for the coexistence of the two evolutio nary trends within the mineralized zones; in the high-grade zones, the disequilibria are low, whereas in the zones of disseminated mineraliz ation or in halos surrounding the orebody, the disequilibria are great er, reflecting the prevailing influence of selective dissolution of U- 234 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 water s that have percolated through the ore deposit: in this case, the Pb i sotopic ratios are enriched in radiogenic Pb.