Lateritic weathering of trachyte, and bauxite formation in West Cameroon: Morphological and geochemical evolution

Bauxites and Fe laterites were formed on Neogene volcanics from Fongo-Tongo region in the highlands of western Cameroon, and are distributed on plateaus, slopes and downslope surfaces. Bauxitic profiles result from intense in-situ weathering of trachytes that implied depletion of silica and labile elements from the saprolite, while alumina relatively accumulated from parent minerals pseudomorphosis by primary gibbsite formation. During ongoing lateritization and bauxite maturation, important leaching and illuviation processes resulted in secondary gibbsite crystallizations. Late incision and dissection of upper bauxitic plateau resulted in degradation and dismantling of bauxitic duricrusts with Fe-depletion and increasing silica. Compared to trachyte, bauxitic duricrusts are relatively enriched in Nb, Zr, Ga, Ni, Cu, Co, V, Cr, As, Pb, Th, Hf, U and Ta, while Y, Sr, Rb, Ba and Zn are depleted. Trace elements contents depend on relative proportions of gibbsite, kaolinite, iron oxides and anatase and their affinity with these minerals across the weathering sequence. The overall REE composition and C1-Chondrite normalized REE patterns highlight significant fractionations with enrichment in the upslope profile and labile behavior in other profiles of the sequence. REE behavior and Eu/Ce anomalies are controlled by intensity of weathering and lateritic processes during the evolution of profiles. Our results document two major bauxitic phases in the Fongo-Tongo area, i.e., mid-Miocene primary in situ bauxitic weathering of trachyte and late Miocene secondary bauxitization of previously formed bauxitic profiles that led to alumina enrichment up to 53.50 wt%. Combined together, the morphological distribution and geochemical composition of the studied bauxitic profiles constitute guides for bauxite exploration, and more generally document the dynamics and morphogenetic evolution of lateritic landforms in West Cameroon.