Association of uranium with macerals in marine black shales: Insights from the Upper Devonian New Albany Shale, Illinois Basin

Marine organic-rich shales generally have high uranium content associated with sedimentary organic matter. However, organic matter in black shales is not homogeneous, but instead a mixture of different macerals. The uranium content and organic petrographic composition of 27 New Albany Shale samples indicate differential contributions of macerals to the uranium content in the Upper Devonian New Albany Shale of the Illinois Basin. Specifically, uranium is mainly associated with amorphous organic matter (microbially degraded phytoplankton, zooplankton, and bacterial biomass), where an average uranium content of approximately 550 ppm has been estimated. The amorphous organic matter's lack of structure, as well as admixed with mineral grains (<7.8 μm) suggest that it originated via microbial degradation in surficial sediments. Uranium may have been incorporated into amorphous organic matter during phytoplankton degradation and amorphous kerogen formation early in diagenesis. Pore-water redox conditions and pH control the reduction of uranium (VI) and its incorporation into amorphous organic matter. Alginite derived from Tasmanites algae (average uranium content <10 ppm) contributes minimally to uranium content. Terrestrial organic matter in the New Albany Shale contains on average ~ 20 ppm uranium, and because it accounts for <10% of total organic matter in the New Albany Shale, its contribution is equally negligible.