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