A lateritização na Amazônia Ocidental: sul de Roraima e norte e noroeste de Rondônia

Abstract

This paper approaches the laterite context based on textural, mineralogical, geochemical aspects. In the region of Vila Nova Colina south of the state of Roraima, the profiles have two types of lateritic crusts: (i) the ferruginous vermiform one with high amounts of kaolinite, goethite, hematite, magnetite and anatase, higher concentration of Al2O3, Sc, Th e Zr developed on granites and, (ii) the iron-titaniferous massive crust with high amounts of magnetite, hematite, maghemite, goethite, kaolinite, anatase, gibbsite and ilmenite, and consequently in higher amounts of Fe2O3, TiO2, V e Cu developed from quartz-hornblende gabbro. The higher topographic position of the massive crust (220 m) and its mineral and geochemical composition, indicate higher maturity than the vermiform crust on lower topographic position (140 m). The presence of gibbsite on the massive crust and its higher topographic position at the regional scale, indicates that it is relict of an erosive phase in the center-south of Roraima, probably associated with subsidence of the Tacutu basin in Miocene. In the north and northwest of Rondônia occur lateritic profiles developed from monzogranites of the Amazon craton (e.g. Serra da Providência, Santo Antônio, São Lourenço-Caripunas and Laje Intrusive Suite). The profiles are complete and incomplete according to the stage of evolution/truncation. The saprolite and mottled horizon consist of kaolinite, quartz, goethite, rutile/anatase, muscovite, illite, and albite marked by higher levels of Al2O3, PF, Nb, Pb, Th, U and Zr. The crusts lateritic show vermiform texture and columnar, pisolitic locally with higher content of goethite, hematite and gibbsite and Fe2O3, SiO2, Al2O3, PF, TiO2, Ga, Hf, Nb, Sc, Th, U, V and Zr. The soils on the top of the profiles are chemically similar to those crusts. As well as Roraima the profiles are remnants of an erosive phase, associated with the sculpting of Chapada dos Parecis and the formation of the current route of the Madeira River. The structure of profiles and the absence of bauxitic crust indicate typical structure of little evolved profiles developed from the Plio-Pleistocene, time of the second lateritization phase in the Amazon.