A EXPRESSÃO DA IDADE E DAS VARIÁVEIS AMBIENTAIS NOS PERFIS DE ALTERAÇÃO QUÍMICA PROFUNDA EM BOÉ (G. BISSAU) E MAYOMBÉ (BAIXO ZAIRE)
Abstract
A comparação da composlçao mineralógica e dos teores dos elementos vestigiais nas bauxites em Tsala (Zaire) e em FeIo Ore Suaibo (Guiné Bissau), formados pela alteração profunda dos doleritos, revelou diferenças importantes. As bauxites de Tsala compostas de gibsite, goethite e hematite, jazem directamente sobre rocha mãe e estão recobertas pela couraça ferruginosa formada de material altóctono. Em FeIo Ore Suaibo, entre o material parental e a bauxite, existem horizontes bissialítico e monossialítico de transição, com distintos modos de acumulação dos elementos vestigiais (Ni, Zn, Cu, Mn, ... ). As diferenças constatadas devem-se à constância das condições climáticas e morfológicas favoráveis a bauxitização que agiram em Zaire, provavelmente, durante o período limitado ao Quatemário, enquanto na Guiné Bissau, a formação do manto alterado data de Eocénico e abrangeu vários ciclos climáticos e/ou de erosão. As bauxites guineenses afIoram directamente na superfície e, para além dos minerais citados, contém boehmite característico para zonas sujeitas aos prolongados défices hídricos.Downloads
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