Bitumens collected in the east zone of the Cigar Lake deposit have been analyzed by 13C nuclear magnetic resonance, pyrolysis – gas chromatography, Rock-Eval, and C. H, and O methods. Carbon and hydrogen isotopic compositions and uranium contents have been determined. Results indicate that the bitumens consist mainly of aromatic structures (Fa ≈ 60%) and have undergone a late oxidation process. Similarities in carbon isotopic composition (−27.5‰ > δ13C > −28.2‰) between barren bitumens (U < 300 ppm) and graphite collected in the basement gneisses, as well as evidence for progressive disorganization of graphite along a basement profile, suggest a genetic relationship between graphite and bitumens from the mineralized areas. The presence of amorphous carbon in pit structures inside graphite flakes close to the unconformity supports this assessment.Significant differences in geochemical and isotopic characteristics of the Cigar Lake barren bitumens and the Cluff Lake mineralized bitumens suggest that there are two possible mechanisms for their formation. Barren 13C-enriched bitumens could have formed directly by hydrogenation of amorphous carbon, whereas mineralized, isotopically light bitumens from Cluff Lake (δ13C ≈ 48‰) may result from the polymerization of light hydrocarbons around uraninite grains.

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