Experimental evidence of uraninite formation from diagenesis of uranium-rich organic matter

Abstract

This study focuses on the experimental diagenetic evolution of an uranium-rich lignite from the Coutras uranium deposit (Gironde, France) located in Eocene sediments. Three samples containing 6 to 15.5 wt% uranium were analysed and compared to two barren samples located outside the ore deposit in a stratigraphically equivalent unit. Uranium in the mineralized samples is present as uranyl-carboxylic complexes. Dry thermal maturation was simulated in a cold-seal autoclave at 150, 200, 250, 300, and 350°C at 100 MPa for 24 h. With increasing temperature, uranyl-carboxylic complexes decompose while uraninite forms. The chemical composition of the experimentally matured barren samples, when plotted in a Van Krevelen diagram (H/C vs. O/C), parallels the natural trend of land-derived type-Ill organic matter evolution, while mineralized samples are characterized by a higher loss of hydrogen above 250°C. Reduction to form uraninite is directly related to the dehydrogenation of lignite. The resulting uraninite has a similar unit cell edge to that of natural uraninites observed in sedimentary environments. Uraninite not only forms in the pore space but also replaces the lignite constituents. The results demonstrate that uraninite may form during thermal evolution of uranium-rich organic matter during diagenesis.