L'OR, LE ZIRCON ET LA TOURMALINE : TRACEURS DE L'ORIGINE DES MATERIAUX DES PROFILS LATERITIQUES D'AFRIQUE

The thick lateritic cover of tropical zones are often considered as old soils which have undergone several evolutionary cycles throughout time. Nevertheless, they contain some resistant minerals like gold, zircon and tourmaline which remain after surficial processes.These residual minerals, used as tracers of the genesis and evolution of laterites, have enabled us to identify the differents layers of the profile. They are also useful in demonstrating the presence or absence of filiation with the substratum. In an auriferous region of the soudano-sahelian zone of Burkina Faso, gold and zircon have been accumulated on non-mineralized granites, by physical transport. The chemical transfer of iron has cimented and protect this allochtonous material, forming an iron crust. In the ultrabasic region of the humid tropical zone of Cameroon, chemical transfers made up mainly of aluminium from acid country rocks, have favoured the individualization of manganesiferous phases bearing cobalt and nickel. Physical transport also occur and is revealed by the presence of residual tourmaline grains, mainly in the upper part of the profiles, thus reworked and semiautochtonous.