Major, trace and REE geochemistry in contrasted chlorite schist weathering profiles from southern Cameroon: Influence of the Nyong and Dja Rivers water table fluctuations in geochemical evolution processes

• Quantification of chlorite schists weathering intensity downslope in an equatorial area.
• Document on Mo and W behaviour in saprolites.
• Explain the higher mobility of LREE than HREE in a lateritic environment.

Three weathering profiles developed on chlorite schists, formations on which little studies have been conducted, were chosen to understand the weathering processes prevailing downslope in Southern Cameroon. The materials nearest to Nyong River at Ayos weather under the influence of the fluctuations of groundwater table and acid rain, while those from Bengbis and Mbalmayo weather under the influence of acid rain. The result is the thickening of materials and weathering profiles, without formation of a nodular ferruginous horizon at Ayos. The Ayos weathered materials (CIA ∼ 92) are the most altered and the least lateritised (IOL ∼ 32). The most stable systems are Hf – U – Nb – Ti – Zr – Mo – W (Bengbis), Yb – U – Nb – Ti – Zr – Hf – Mo – W – Th (Mbalmayo) and Th – Nb – Zr – Hf – Mo – Ta (Ayos). Molybdenum accumulations are important in the studied materials. Uranium accumulations are found only in Mbalmayo. Coarse saprolitic materials at Ayos are the most depleted and fractionated in REE ((La/Yb)N = 0.07, Ce/Ce* = 2.24), while superficial clayey materials are less fractionated. This process is reversed at Bengbis and Mbalmayo. By contrast, weathered materials at Ayos do not show any Eu anomalies (as in Bengbis and Mbalmayo). Weathered materials from Bengbis, nearest to the Dja River, have (La/Yb)N < 1 ratios, indicating the relative immobility of HREE relative to LREE due to xenotime abnormally rich in HREE (HREE-PO4). Weak Ce anomalies (1.05–2.24) are ubiquitous in all the studied materials.