Role of organic matter and sulphate-reducing bacteria for metal sulphide precipitation in the Bahloul Formation at the Bou Grine Zn/Pb deposit (Tunisia)

The origin of the Bou Grine Zn/Pb deposit of Tunisia was investigated during a geochemical study of the Cretaceous Bahloul Formation. The inorganic and organic chemical compositions of the bituminous marls and limestones, as well as the light stable isotope ratios of organic carbon and carbonates were measured. Closely spaced samples were collected along two profiles, at different distances from the enrichments in Zn and Pb, which are best developed near the Bou Grine diapir. The organic geochemical data suggest sulphide precipitation at the Bou Grine deposit by sulphate-reducing bacteria (SRB). SRB utilized hydrocarbons derived both from the Bahloul sediments and from external source rocks. Production of S(-II) by the bacteria was most probably limited by the availability of sulphate, although, sulphate was available to the bacteria in high concentrations in close proximity to the Triassic salt dome. Ground waters evolved to hypersaline basinal brines by dissolution of salt and anhydrite at the contact of the diapir. These brines would have been capable of leaching metals from the sedimentary sequences adjacent to the salt dome. The Triassic series has often been advocated as the source of the metals, and the Cretaceous sediments themselves are carriers of metallic elements. Ore deposition apparently resulted when metal-bearing solutions that migrated upward into the roof zone of the diapir, mixed with S(-II)-bearing solutions. Those formed when SRB oxidized the high amounts of hydrocarbons within the Bahloul sediments and in the brines, using SO42− from sulphate-rich waters in the anhydrite caprock as an oxidant.