Published: 16-11-2018| Version 1 | DOI: 10.17632/3b7pjb6s46.1
Brunot NYECK


The petrological study of saprolite developed on gneisses in the Matomb region (South Cameroon) was carried out by petrographic, mineralogical and geochemical analyses. The gneisses have a granoblastic heterogranular texture and are made up of quartz, biotite, feldspars, kyanite, garnet, rutile and enstatite. Chemically, they show high contents in SiO2 (64.93%), Ba (723 ppm), Sr (347 ppm), V (135 ppm), Zr (126 ppm) and moderate contents in Al2O3 (15.24 %). The REE content is low (99.12%) with a slight positive Eu anomaly and no Ce anomaly. From bottom to top, the soil is composed of a coarse saprolite and fine saprolite. The coarse saprolite is mainly sandy, showing gray, pink, red phases as well as white loamy sand and yellow sandy loam phases. The main minerals are quartz, kaolinite, gibbsite, goethite, hematite, biotite, muscovite, kyanite and rutile. In addition, the coarse saprolite also contains chlorite, smectite, vermiculite and chlorite-vermiculite. The fine saprolite is composed of white, pink and red brown phases. They have a similar mineral assemblage, with the exception of the concomitant disappearance of biotite and 2/1 clay minerals which certainly represent the first fugacity steps of biotite weathering. The SiO2 content decreases from the coarse saprolite to the fine saprolite compared to the parent rock, but the white sample of the fine saprolite shows high SiO2 (70.31%) content. The Al2O3 content increases up the profile while the Fe2O3 content is variable. The white samples show low Fe2O3 values (4.07 to 0.19%). The red material is particularly rich in Fe2O3 (15.32%). Ba, Zr, Zn and V show high contents in the profile. Cr (967.00 ppm) shows the highest value in the profile and U (1982 ppm) content is very high in one of the pink materials of the fine saprolite. SiO2 has negative correlation with Al2O3, Fe2O3, TiO2, Zr, Ni and V. The correlations of Fe2O3 with Cr and V are positive. The correlations between Zr and Th, Nb, Hf are positive. The REE contents increase up the profile and fluctuate in the saprolite phases. The white samples are very poor in REE. The Matomb soil samples show positive Ce anomalies and negative Eu anomalies probably due to the depletion of Eu, the relative enrichment of other REE or to the absence of carrier minerals. Contrary to what has always been observed in the South Cameroon plateau, the white samples of the fine saprolite show positive Eu anomalies. This could be indicative of the probable presence of other carrier minerals apart from feldspars which always occur in this milieu or the leaching of some elements. The (La/Yb)N<1 ratio in some weathered phases indicates a high mobility of LREE in these materials. Mass balance shows that the transition from the rock to the weathered materials is accompanied by a strong depletion of several elements particularly HREE and a moderate LREE enrichment. Thus, the relative mobility of elements determines the colour differenciation of phases in the saprolite.