SM-ND, PB-PB, AND RB-SR SYSTEMATICS OF THE BASEMENT IN THE CIGAR LAKEAREA, SASKATCHEWAN, CANADA

The Sm-Nd, Pb-Pb, and Rb-Sr isotope geochemistry of graphitic metapeli tic gneisses and their altered equivalents from the Cigar Lake area (S askatchewan, Canada) has been investigated. Some granitic gneisses wer e also analyzed for Pb-Pb and Rb-Sr. Sm-Nd data show that the metapeli tic gneisses are composed of detritus from heterogeneous, mainly mantl e-derived Archean rocks (2.5-2.6 Ga) and that the Sm-Nd system has not been significantly perturbed during subsequent alteration arid metamo rphic events. The Pb - Pb age for samples of the less altered graphiti c metapelitic gneisses is 1. 77 +/- 0.03 Ga. The crustal common Pb is located on the Pb-Pb isochron, but there are different zones with high and variable U/Pb ratios (mu = 15-280). The Pb-Pb age for the graniti c gneisses is 1.79 +/- 0.11 Ma. The Pb isotope data show that there ha s been no major uranium redistribution in the basement after the Hudso nian orogeny. However, there has been a strong perturbation of the U - Pb system in the regolithic zone beneath the Athabasca cover. In some samples, uranium was added during the mineralizing event. The Rb-Sr s ystem in the graphitic metapelitic gneisses was also affected. The Sr- 87/Sr-87 ratio in pitchblende is 0.709. At 1.3 Ga, there is a strong c ontrast between the Sr-87/Sr-86 ratio in the Athabasca sandstones (0.7 06-0.710) and the Sr-87/Sr-86 ratio in the metapelitic gneisses from t he basement (0.725-0.775). The upper zone of the regolith is character ized by a low Sr-87/Sr-86 ratio (0.705-0.707). The Pb-Pb and Rb-Sr dat a are consistent with the circulation of a fluid with a low Sr-87/Sr-8 6 ratio, derived from the sedimentary cover; this fluid passed through the most permeable zones of the basement rocks, especially the regoli th. The mineralizing fluid had a Sr-87/Sr-86 value typical of a fluid in equilibrium with the Athabasca sandstones.