Geochemistry of major elements in soil-saprolite of different rocks in a microclimate of the Bambouto Mountains (Cameroon)

Abstract

The supergene alteration of rocks on the continental surface causes considerable chemical variations. Isovolumetric weathering, which is very valuable for analysing the differential behaviour of chemical elements, is very difficult to apply. The main objective of this work is to determine the distribution of major element concentrations during the processes that prevail in the formation of weathering blankets developed on different rocks in a sub-equatorial monsoonal microclimate attenuated by distance from the sea and tempered by altitude. The methodology is based on the careful selection of four fairly characteristic sites of alteration covers developed on orthogneiss, biotite-hornblende granitoids, anatexite and trachybasalt. Fluorescence (XRF) spectroscopy was carried out on 58 samples taken along each profile according to the variation of the soil layers, for the determination of major elements. The depth-to-surface variation in silica contents is 70–65 wt-%, 70–60 wt-%, 58–68 wt-% and 41–0.5 wt-% respectively on orthogneiss, biotite-hornblende granitoid, anatexite and trachybasalt alteration blankets respectively. Aluminium ranges on these covers between 15 and 20 wt-%, 13–20 wt-%, 22–18 wt-% and 17–44 wt-%. Iron varies widely in the bottom-to-top profiles: 0.02–5 wt-% on orthogneiss, 0.2–10 wt-% on granitoid, 0.8–10 wt-% on anatexite and 0.2–80 wt-% on trachybasalt. It varies respectively in these profiles from 0.09 to 2 wt-%, from 0.2 to 3 wt-%, from 0.01 to 0.7 wt-%, from 0.9 to 8 wt-%. The CIA is comprised between 58 and 91%, 43 and 100%, 87 and 100%, 56 and 100%, respectively in the profiles on biotite and hornblende granitoids, on orthogneiss, on anatexite and on trachybasalt. The SiO2/Al2 O3 ratio increases with depth and varies from 3 to 6%, 3–13475%, 0–5%, 0–3%, respectively in these profiles. The different soil-saprolite block-diagram profiles show the isovolumetric processes and movements of major elements during weathering and pedogenesis.