Paragenetic sequences of carbonate and sulphide minerals of the Mamfe Basin (Cameroon): Indicators of palaeo-fluids, palaeo-oxygen levels and diagenetic zones

The occurrence of evaporite minerals in the Mamfe Basin (Albian-Cenomanian) is considered to be an indicator of aridity and palaeo-saline waters. A detailed lithostratigraphic analysis of the Mamfe sedimentary basin-fill indicates the occurrence of carbonate and sulphide minerals in close association with halite and gypsum within the Manyu, Mamfe and Bagba Members of the Nfaitok Formation. The composition and textures of these carbonate and sulphide minerals were studied using the Scanning Electron Microscope (SEM), attached to Energy Dispersive Atomic X-ray analyser (EDAX) that generates Backscattered and Secondary Electron Imaging (BSEI). Two paragenetic suites were encountered in this study; calcite/dolomite/ankerite/siderite and pyrite/sphalerite/galena respectively. The compositional and textural variability of different suite members appear to be conditioned by the competition for CO32- and SO42- anions by metal cations with respect to redox conditions and diagenetic stages. Early formed authigenic minerals exhibit euhedral textures, while late diagenetic minerals exhibit pseudomorphic textures, as cations with higher ionic radii integrate into earlier structures. Calcite is a precursor for dolomite, ankerite and siderite at late diagenetic stages. Pyrite, sphalerite and galena indicate deposition in anoxic and sulphate reducing environments. Micron-sized euhedral pyrite framboids are syn-depositional, while Pb-Zn mineralization in voids and cavities are formed at late diagenetic stages below the sulphate reduction zone (SRZ), where siderite forms encrustations around skeletal pyrite. The pristine source of saline water, SO42- and Mg²⁺ could be related to sea water splay into a lagoon. The source of Fe²⁺ is detrital, while Zn²⁺ and Pb²⁺ indicate metalliferous fluids with high heat flux, probably of hydrothermal origin.