Barite-bearing cap dolostones of the Taoudéni Basin, northwest Africa: Sedimentary and isotopic evidence for methane seepage after a Neoproterozoic glaciation

Barite has been reported from identical stratigraphic levels overlying terrestrial glacial deposits throughout NW Africa and formed locally during fluid mixing on, and in cavities beneath the seafloor during late stages of the post-glacial marine transgression. Samples of barite were collected from two distant localities in the Taoudéni Basin, in Mauritania and Mali. 87Sr/86Sr ratios exhibit an unusually narrow range for barite that closely matches contemporaneous seawater 87Sr/86Sr (0.7077-8). Barite δ34S values range widely between 20‰ and 45‰ CDT, which indicates that sulphate derived from seawater and was subsequently modified by microbially mediated sulphate reduction. The consistent stratigraphic level and irregular distribution of barite deposits are consistent with a sedimentary exhalative origin for the barite, whereby Ba-rich fluids from shallow locations within the rock pile interacted with sulphate-bearing seawater; a hydrothermal origin for these fluids can be excluded. Isotopic constraints and the association of barite with terrestrial glacial deposits across the West African craton suggest that methane seepage from underlying permafrost may be one possible mechanism for Ba sequestration. The occurrence of seafloor barite precipitates at the contact between cap dolostones and overlying post-glacial limestones worldwide implies that changes in ocean composition, in particular increases in the sulphate content of ambient seawater provided an overriding control on barite mineralisation.