Sm-Nd, Pb-Pb, and Rb-Sr systematics of the basement in the Cigar Lake area, Saskatchewan, Canada

The Sm-Nd, Pb-Pb, and Rb-Sr isotope geochemistry of graphitic metapelitic gneisses and their altered equivalents from the Cigar Lake area (Saskatchewan, Canada) has been investigated. Some granitic gneisses were also analyzed for Pb-Pb and Rb-Sr. Sm-Nd data show that the metapelitic gneisses are composed of detritus from heterogeneous, mainly mantle-derived Archean rocks (2.5-2.6 Ga) and that the Sm-Nd system has not been significantly perturbed during subsequent alteration and metamorphic events. The Pb-Pb age for samples of the less altered graphitic 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 (μ 15-280). The Pb-Pb age for the granitic gneisses is 1.79 ± 0.11 Ga. The Pb isotope data show that there has been no major uranium redistribution in the basement after the Hudsonian 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 system in the graphitic metapelitic gneisses was also affected. The 87Sr/86Sr ratio in pitchblende is 0.709. At 1.3 Ga, there is a strong contrast between the 87Sr/86Sr ratio in the Athabasca sandstones (0.706-0.710) and the 87Sr/86Sr ratio in the metapelitic gneisses from the basement (0.725-0.775). The upper zone of the regolith is characterized by a low 87Sr/86Sr ratio (0.705-0.707). The Pb-Pb and Pb-Sr data are consistent with the circulation of a fluid with a low 87Sr/86Sr ratio, derived from sedimentary cover; this fluid passed through the most permeable zones of the basement rocks, especially the regolith. The mineralizing fluid had a 87Sr/86Sr value typical of a fluid in equilibrium with the Athabasca sandstones. 36 refs., 9 figs., 6 tabs