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 Ma. 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 Rb–Sr data are consistent with the circulation of a fluid with a low 87Sr/86Sr ratio, derived from the 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.