Mineralogy and geochemistry of the sediments of the Ouaka river, central-eastern Central African Republic: weathering, provenance and tectonic setting

Abstract

Textural, mineralogical and geochemical studies used to characterise the weathering context of the Ouaka River sediments, their provenance and the tectonic conditions of the source zone. All along the Ouaka, the textural data show almost the same granulometric distribution, ranging from very coarse to fine sands. Mineralogical data from X-ray diffraction dominated by quartz, followed by haematite, rutile and feldspar. The heavy minerals found in these sediments are opaque oxides, rutile, tourmaline, zircon, sillimanite, garnet, hypersthene and kyanite. The values of the alteration indices (CIA, PIA and CIW) and the A-CN-K and (A-K)-C-N diagrams indicate a high intensity of chemical alteration in the source zone. The sediments resulting from this alteration phenomenon are immature, as shown by the values of the SiO₂/Al₂O₃ ratio, the ICV and the ZTR index. The use of ternary V-Ni-Th*10 diagrams and binary diagrams (Zr versus TiO₂ and Th/Co versus La/Sc) provided evidence of the contribution of the felsic rocks from which these sediments originated. These felsic rocks are probably Archean to post-Archean in age, according to the Eu/Eu* binary diagram as a function of (Gd/Yb)cn. Based on major and trace elements, the discriminant function diagrams, DF1(Arc-Rift-Col)m1 versus DF (Arc-Rift-Col)m1, DF(A-P)M and DF(A-P)MT, of tectonics suggest a rift and passive margin environment of the watershed, consistent with the general geology of the CAR.