Mineralogy and geochemistry of sediments from Simbock Lake, Yaoundé area (southern Cameroon): provenance and environmental implications

Abstract

Two cores of sediments, named NR and EB, were collected in the Simbock Lake (Mefou watershed, Yaounde) to assess their provenance and the degree of heavy metal pollution based on mineralogical and geochemical data. The sediments are sandy, sand-clayey to clayey, and yellowish brown to greenish brown, and with high amounts of organic matter (average value of TOC is 1.95%). The sediments are mainly composed of quartz, kaolinite, accessory goethite, smectite, rutile, feldspars, illite, gibbsite, and interstratified illite-vermiculite. Fourier transform infrared (FT-IR) spectroscopy shows that kaolinite is less crystallized in the NR core than in the EB core. The Index of Compositional Variability (ICV), Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA), and the Rb/Sr and K2O/Rb ratios indicate a high weathering intensity in the source area. These sediments have low contents in Al2O3, Fe2O3, Na2O, K2O, MgO, and CaO as well as high values in SiO2, P2O5, TiO2, and MnO relative to the upper continental crust. The concentrations of Cr, V, Ba, and Zr are higher in the NR core than those in EB. The total rare earth element (REE) content varies between 78 and 405 ppm. The light REE are abundant (LREE/HREE ~ 18–59; avg. = 25.61). The chondrite-normalized REE patterns exhibit (i) negative Eu anomaly (Eu/Eu* ~ 0.38–0.62; avg. = 0.5), (ii) slight positive Ce anomaly (Ce/Ce* ~ 1.11–1.34; avg. = 1.11), and (iii) high REE fractionation ((La/Yb)N ~ 12.3–51.75; avg. = 25.61). The enrichment factor (EF) shows that the Mefou watershed through the Simbock Lake sediments is slightly polluted by the agricultural and urban activities.