Bauxitas do nordeste do Amazonas

Abstract

This paper discuss the profiles with bauxitic crust formed from igneous and siliciclastic rocks of the northeastern part of the Amazonas. The structural, textural, mineralogic and chemical results show some differences among them, which reflect mainly in the parent rock nature. The bauxitic crust from profiles 1 (volcanic rocks) and 2 (granitic) is thicker than in the profile 3 (sedimentary), shows greater range of facies (massive, pronodular, nodular and pisolitic) and developed right over the saprolite. On the other hand, the profile 3 thick mottled horizon, ferruginous crust well developed and bauxitic crust not well formed. The principal component analysis (PCA) for elements and samples defined the following geochemical associations: (1) SiO2, Al2O3, K2O, Ba, Sr e LREE, where SiO2 predominate in the saprolite and mottled from profile 3 in the form of quartz, Al2O3 in the bauxitic crust (massive and pisolitic) and the fragments from the profiles 1 and 2 in the form of gibbsite and secondly in kaolinite, whereas Ba, Sr e LREE occur principally in the saprolite from profiles 1 e 3, associated to feldspar, illite and muscovite; (2) V, Pb, Fe2O3 and As are concentrated mainly in the ferruginous crust of the three profiles and their fragments rich in Fe oxy-hydroxides (hematite and goethite), whereas (3) Ga, Th, Ta, Sn, Nb, Hf, Zr, Y, TiO2 e HREE predominate in the matrix of the concretionary horizon and in the soil of profiles 1 and 2, assotiated with the resistant (zircon, columbite, thorite, xenotime, etc) or in the form of anatase (TiO2). The results indicate industrial quality for the bauxitic horizons of mine Pitinga (profiles 1 and 2) and are equivalent of the bauxites extracted from other Amazon regions. The three profiles were interpreted as the result of weathering in situ, and despite their resulting from poliphasic weathering acting in the whole Amazon region since Upper Cretaceous, the absence of mottled horizon in the profiles 1 and 2 in relation to the occurrence over the volcanic rocks further south suggests regional variations in the leaching conditions. This also justifies the absence of bauxitic horizon in other areas and the small development over Prosperanca Formation.