Major and trace elements behaviour in two weathering profiles developed on gabbroic rocks in Meka'a and Abiete (south Cameroon, central Africa)

Abstract

The present work deals with the behaviour of chemical elements in two weathering profiles developed on tholeiitic gabbro of Meka'a and alkali gabbro of Abiete, in southern Cameroon. The weathering profiles were characterized based on the colour, structure, texture, mineralogical and geochemical compositions. Elements mobility and redistribution during weathering processes are evaluated. The region is under an equatorial climate with four seasons strongly influenced by oceanic monsoon in the coastal zone. Geologically, the study area is located within the Ntem complex (Congo craton). Fresh rocks display a granular texture made up of plagioclase crystals, pyroxene, amphibole, biotite, apatite, chlorite, ilmenite and magnetite. The profile at Meka'a is made up of a coarse saprolite, a fine saprolite and a loose clayey horizon while that of Abiete comprises four horizons: coarse saprolite, fine saprolite, nodular, and loose clayey horizons. Common minerals are goethite, kaolinite, quartz and anatase. Halloysite is specific to Meka'a while gibbsite and hematite occur only at Abiete.In the weathered profile of Meka'a, organic matter contents decrease downwards while pH values increase. Fe2O3, Al2O3, TiO2, P2O5, Ba and Cu are partially removed from the top of the profile and accumulated in the saprolite layers whereas Cs, Th, HFSE and LREE (La, Ce, Pr) contents increase upwards. Transition elements and HREE are all leached out of the profile.At Abiete, OM content oscillates around 3.5 from the surface to the nodular horizon parallel with the accumulation of Al2O3 and Fe2O3. The nodular horizon shows a peculiar feature where abundant OM and an excellent drainage have induced the decrease of the pH value from 4.5 to 3 with concomitant leaching of Al2O3, Fe2O3, and many trace elements such as REE and HFSE. In the saprolite horizons, the OM content oscillates with concomitant accumulation of Al2O3 and Fe2O3. Positive Ce anomaly observed in the saprolitic set of Meka'a profile and in the coarse saprolite and the superficial clayey horizon at Abiete is assigned to oxidizing conditions in these levels.