Micromorphology, mineralogy and geochemistry of lateritic weathering over serpentinite in south-east Cameroon

摘要

A laterite profile developed on serpentinite in Lomié, south-east Cameroon, was studied to understand its structure and genesis. Micromorphological, mineralogical and geochemical characteristics were studied, using thin sections, SEM-EDS, X-ray diffraction and chemical analyses. In the lower saprolite, magnesite is dissolved and serpentine is gradually replaced by kaolinite, without loss of the lithic fabric (orientation of kaolinite, distribution of magnetite). This nearly iso-volumetric weathering, at the scale of the sample, can be considered as in situ replacement of serpentine by kaolinite. Iron oxide hypocoatings formed along planar voids, and also smectite formed from solution in the void system. Congruent dissolution of kaolinite starting in the mid saprolite eventually led to a collapse of the fabric and to a chaotic accumulation of fragments of the hypocoatings. In the overlying layer, this fabric is even more strongly expressed, and iron oxides are more concentrated. The weathering profile shows a severe loss of Mg and Si, and an accumulation of Al, Fe, Ti and some minor and trace elements. The overlying layers have fabrics and components different from those recognized for the saprolite, such as clastic quartz grains and iron oxide nodules with various lithic fabrics, pointing to an admixture of allochthonous materials. The appearance of mobile elements (e.g. Rb, Sr, Pb) and certain stable elements (Zr, Nb), as well as the removal of serpent-inite-related elements clearly show the mixing with materials derived from micaschists in the vicinity. The presence of a discontinuity in the profile is also confirmed by variations in iron oxide mineralogy. At least three lithological discontinuities were identified.