Characteristics and geological significance of Re-Os isotopic system of evaporites in Mboukoumassi deposit, the Republic of Congo

Evaporite dating has been an open problem. The study investigates the Re-Os isotopic system in the organic-rich sedimentary rocks to constrain the infilling of sedimentary basin and related geological events. In the Mboukoumassi potash deposit in the Republic of Congo (Congo-Brazzaville) in West Africa, several layers of organic-rich dark shale were found in the evaporite series. Through drilling core, the dark shale in the evaporite is found to satisfy the requirements of Re-Os isotope test. The result shows that the Re-Os isochron age of the dark shale in the study area ranges from 78.7 ± 1.1 to 96 ± 7 Ma, which is the first precise age of the Mboukoumassi potash deposit in the Republic of Congo (Congo-Brazzaville), West Africa. Therefore, the age of deposition of this set of evaporite may be Cenomanian-Turonian, which is younger than the age previously thought (around 113-125Ma, Aptian). The Re-Os isotopic dating technique used for the pioneering study on the precise dating of the Mboukoumassi potash deposit provides a new approach to the study of the sedimentary age of ancient evaporite deposits. The initial 187Os/188Os value decreasing from 2.02 ± 0.21 to 0.982 ± 0.03 for the core sample reflects the source rock chang along the core, and this is consistent with the geological evolution of the basin.