Mineralogical and Geochemical Characterization of the Unconsolidated Sands from the Mefou River Terrace, Yaoundé Area, Southern Cameroon

Nine sediment core samples from the right terrace of the Mefou River in the Yaounde area (southern Cameroon) were used to investigate the provenance and evolution of sediments. The mineralogical and geochemical data have been reported using XRD, XRF and ICP-MS instruments. The sediment core is 87 cm thick and is subdivided from the bottom to the top into sandy, clayey sandy and, silty sandy layers. The sandy layer is 70 cm thick with a decreasing grain size from the bottom to the top. The nine sediment core samples are mainly composed of quartz, and accessory feldspars, kaolinite, goethite, rutile and muscovite/Mite. This mineral assemblage can be explained by the combined effect of metamorphic source rocks and surrounding soils. The sandy texture as well as the abundance of quartz were confirmed by the high SiO2 contents particularly at the bottom of the sequence (71-85 wt%). The high SiO2/Al2O3, low alkali (Na2O + K2O) values, low REE contents (similar to 107-389 ppm; avg. = 247 ppm) and the negative correlation between SiO2 with most of major and trace elements confirm the siliciclastic nature of sediments. The SiO2/Al2O3 ratio justifies the high proportion of quartz relative to clay minerals and the coarse grain size of the sediments. The Mefou alluvial sediments are classified as Fe-shale and Fe-sand. The LREE/HREE ratio and (La/Yb)(N) values exhibit the light REE-abundance linked to the nature of REE mineral-bearers. The spider diagrams reveal the high depletion of K and Sr due to the high feldspar alteration and the quartz-abundance. The REE-normalization relative to chondrite portrays slightly positive Ce anomalies (Ce/Ce*similar to 1.11-1.15) and negative Eu anomalies (Eu/Eu*similar to 0.43-0.63). The mineralogical and geochemical features are in concordance with the textural parameters and portray the siliciclastic nature of sediments inherited from the gneiss basement. The predominance of sands and the low trace element contents in alluvial sequence from the Mefou River are linked to the intensity of the hydrodynamic forces. The rock sources have undergone high degree of weathering and there is no evidence of significant sediment recycling.