Geochemical and Petrographical Evolution of the Weathering Mantle Derived from Basalt in Bangam Locality (West-Cameroon): Implication in the Bauxitisation Process

A petrographical and geochemistry study of weathering mantle derived from the basaltic parent rock (plagioclase, olivine, pyroxene, zircon) has been conducted in the locality of Bangam (West-Cameroon). The weathered profile shows a vertical lithology succession of weathered parent rock, isalteritic clayed domain and superficial duricrust (alloterite). The weathering of basalt started by the formation of “pain d’epices” structure rich in gibbsite, metahalloysite, kaolinite. The geochemistry analysis of major elements indicate that SiO2 (46% -1.33%), K2O (0.84% - 0.01%), Na2O (3.6% -0.01%), MnO (0.3% - 0.04%), P2O5 (1.9% - 0.38%) and CaO (5% -0.02%) decrease from the bottom to the surface, however TiO2 (2.3% - 4.08%) remain constant, Fe2O3 (24.2% - 24,6%) and Al2O3 (14.5% - 45.2%) increase. The different weathering index such as, chemical index of alteration (55% - 99%), index of lateritization (41% - 103.5%) and Ruxton Ratio (0.12 - 3.21) just indicate an evolution of parent rock dominated by an alumina and iron phases under a control of hydrolysis phenomenon as bisialitisation, monosiallitisation and allitisation with the formation of minerals smectites group, kaolinite group gibbsite and iron oxides group. The fractionation patterns of rare earth elements (REE) show a positive and negative anomaly in Cerium and other rare earth elements, one more, the correlation between major, trace and REE prove a link of different pedological horizons developed on the basalt in redox condition.