The influence of topography in duricrust development in the Ngaoundal-Dir section of central Cameroon: Understanding morphological, mineralogical and geochemical transformations

Abstract

Ngaoundal-Dir section was characterized in order to determine its duricrust potential and the role of topography in its mineral enrichment. The high altitude (Ngaoundal sector) and the low-altitude (Dir sector) were investigated using field methods and the collected samples were subjected to mineralogical and geochemical analysis. Six (6) facies of duricrust (scoriaceous, nodular, massive, pisolithic, conglomeratic and vacuolar) were distinguished. The scoriaceous and pisolithic facies were exclusively identified in Ngaoundal while the conglomeratic and vacuolar facies were found in Dir only. The scoriaceous facies was essentially made of gibbsite. The pisolithic facies is made up of kaolinite associated with gibbsite, goethite and hematite, while the other facies are mixture of gibbsite, hematite, quartz and kaolinite. Geochemically, the duricrusts display major oxide concentrations (in wt.%) for: Al2O3 (2.56 - 49.65), Fe2O3 (15.21 - 79.04) and SiO2 (0.89 - 34.46). A higher content of Al2O3 was recorded in scoriaceous facies (47.92 - 49.65%), while a higher content of Fe2O3 was observed in massive facies especially at Dir (79.04%). Trace elements were represented essentially by Cr (75 - 880 ppm); Zr (202 - 427 ppm); Ba (5.7 - 137.3 ppm) and Cu (5.1 - 134.2 ppm). Light rare earth elements were more concentrated than heavy rare earth elements in both sites (ƩLREE = 72.38 ppm and ƩHREE = 4.12 ppm in Ngaoundal; ƩLREE = 40.76 ppm and ƩHREE = 5.47 ppm in Dir). The low values of alkali metals and alkali earth metals combined with the high values of the Chemical Index of Alteration (˃ 99) supported an intense weathering origin of duricrusts. These findings suggested that the weathering resulted in the establishment of bauxitic formation in high-altitude and a concentration of iron-enriched formations in lower-altitude.