Palaeoenvironmental controls on the uranium distribution in an Upper Carboniferous black shale (Gastrioceras listeri Marine Band) and associated strata; England

The mudrocks associated with the Gastrioceras listeri (G. listeri) Marine Band contain 0 to 200 ppm authigenic uranium. Several geochemical (degree of pyritisation (DOP), C/S ratios and framboidal pyrite size distribution) and palaeontological indicators (oxygen-restricted biofacies (ORB) analysis) suggest that the highest authigenic uranium concentrations are within marine sediments associated with the most oxygen-restricted biofacies (ORBs 2 and 3). The uranium peaks tend to occur at the transition between biofacies rather than within the middle of more persistently anoxic intervals. These horizons may have been deposited when the oxygen minimum zone impinged on the seafloor. Brief oxygenation of an otherwise anoxic environment was conducive to francolite precipitation, which then scavenged dissolved uranium. Slow sedimentation rates are also important in concentrating uranium and francolite. Thus, enriched U values are only encountered under specific conditions of low, but fluctuating, oxygen regime and extremely slow sedimentation rates. Truly euxinic facies, lacking any fossils, and a uniformly small framboid population do not contain high concentrations of francolite and are not U-rich.