EVOLUÇÃO ATUAL DA COBERTURA LATERÍTICOBAUXÍTICA DA SERRA DE TRUCARÁ BAIXO RIO TOCANTINS PARÁ

Abstract (English)

Two distinct and antagonistic processes are affecting the weathering mantle of Serra de Trucará. At the base, ferralitization of the basalts and interbedded pelites of the Caraipé Formation is taking place with the consequent thickenning of the saprolitic zone, while, at the top, occurs chemical and physical degradation of the lateritic-bauxitic duricrust and of the colluvial deposits resulting from its dismantling. The study of solid products originated by superficial degradation of the duricrust and of waters sampled in springs and wells in four different periods of the year showed that: 1) at the base of the weathering profile, alkalis and alkali-earths are leached and silicon is partially removed, while quartz is preserved and neoformed goethite and poorly crystallized kaolinite periodically undergo dissolution and reprecipitation; 2) in the upper part of the saprolite, incongruent dissolution of kaolinite results in the formation of gibbsite, while quartz undergoes slow degradation and iron oxi-hydroxides are periodically dissolved and reprecipitated; 3) at the top of the profile, the lateriticbauxitic duricrust suffers chemical and physical degradation, in particular seasonal leaching of aluminum and iron, probably intensified by the presence of organic matter. Removal of aluminum at the top of the alteration mantle should result in absolute enrichment in mis element in the lower part of the duricrust and the upper part of the saprolite, and also in cementation of recent colluvium by gibbsite.