Abstract The Boomerang Lake unconformity-type uranium prospect is located in the Proterozoic western Thelon basin, Canada. Based on geological similarities to other uranium-producing Proterozoic basins, it represents a prospective target for uranium exploration. The potential of the western Thelon basin at Boomerang Lake to host high-grade, unconformity-type uranium deposits has been evaluated using alteration mineral paragenesis and chemistry, stable isotope geochemistry, 40 Ar/ 39 Ar geochronology, and a 2 percent HNO 3 leach method. Pre-Thelon basin basement rocks were subaerially weathered by low δ 18 O value meteoric waters at 1758 ± 7 Ma. Early diagenesis in the basin occurred at ca. 1667 Ma and is marked by a phosphate-dominated alteration mineral assemblage that formed from relatively reducing basinal fluids. Later peak diagenetic basinal fluids produced a widespread phyllosilicate-dominated mineral assemblage at temperatures of as much as 250°C, and had δ 18 O and δ 2 H values and chemical compositions consistent with those of oxidizing, saline basinal brines in other uranium-producing Proterozoic basins. Uranium mineralization is associated with hydrothermal alteration by 18 O- and 2 H-rich evolved basinal fluids at 200°C, but consists of minor amounts of the U +4 phosphate mineral tristramite. The distribution and stable isotope compositions of peak diagenetic and hydrothermal phyllosilicates indicate sandstones overlying the Boomerang Lake prospect were isolated from peak diagenetic basinal fluids that were capable of transporting uranium, resulting in the diminutive uranium phosphate mineralization. Radiogenic mobile Pb is present in sandstones and basement rocks at Boomerang Lake, but was predominantly produced in situ from U-bearing accessory and detrital minerals and probably not from an undiscovered uranium deposit at depth. The use of 238 U/ 206 Pb and (Zr+Th)/U ratios proved to be helpful in evaluating the prospectivity of anomalously radiogenic zones in the Thelon basin.