Bauxita, horizonte nodular e cobertura argilosa da região de Paragominas e Juruti, estado do Pará.

Abstract

The hot and tropical climate during the Cenozoic resulted in the formation mainly lateritebauxite in the Amazon with the frequent occurrence of nodular and/or concretionary horizon consisting of both bauxite and/or ferro-aluminous crusts. The relationship between the nodular and/or concretionary horizon with lateritic profile and origin of the clay cover in the Amazon were the objectives of the present study, so did the selection of two lateritic profiles, located in the region of Paragominas (plateau Jabuti and PA-256 km 17) derived from rocks siliciclastics Ipixuna Formation and/or Itapecuru Group the late Cretaceous, and Juruti generated from the rocks siliciclastics of the Alter do Chão Formation, all in the state of Pará. This study applied the techniques of X-ray diffraction, optical microscopy, scanning electron microscopy and energy dispersive system (SEM/EDS), addition to chemical analysis. The lateritic profiles in the region Paragominas consist from bottom to top: Saprolitic horizon with laminar structures composed mainly of kaolinite beyond quartz grains fractured and corroded; Horizon mottled variegated coloration composed of kaolinite, goethite and hematite; bauxite lilac, look massive, porous, and cut by structures of clay columnar aspect in the plateau Jabuti, while in the PA-256 the bauxite is rosy, columnar and porous; Iron-aluminous crust with nucleous digested or no in the aluminous matrix; Concretionary horizon, consisting of a kaolinitic clay matrix, formed the basis for suesferic ferruginous concretions, porous, zoned, while the top consists particles porcelained aspect that displays a diffuse zoning outline in the outside by white or pink, then a yellow core and a more nucleous red color that sometimes has brown color; Clay cover formed mainly of kaolinite, gibbsitic nodules, quartz grains and fragments scattered laterite. The trace elements show a relationship with iron oxyhidroxides (Pb, V, As e Mo), anatase (Nb, Ta, W, Sn and Sc), zircon (Zr, Hf, Y and U) and gibbsite (Ga), while the rare earth elements exhibit the same behaviour along the profile which indicates a great genetic relationship between the horizons of the profile. The lateritic profile Juruti consists from bottom to top: Mottled horizon variegated color, Bauxite saccharoidal aspect formed by granules of gibbsite and quartz grains fractured and corroded, further bauxite with rounded ferrugious plasm immersed in aluminous matrix generating a feature breccia; Ferruginous crust massive look massive, porous with cavities filled by goethitic cutas and gibbsite, besides the occurrence of quartz grains fractured and corroded; Ferruginous nodules decreasing grain size toward the topo, looks massive porous with cavities filled by goethitic cutas and gibbsite, quartz grains, being immersed in the aluminous matrix of kaolinite; Bauxitic nodules decreasimg grain size toward the top, irregular aspect, porous with cavities filled by gibbsite already in the portion top the nodules are digested by aluminous matrix; Yellowish clay cover with gibbsitic nodules. Since the trace elements in the profile Juruti show concentrations lower than Paragominas and same chemical relationship with iron oxyhidroxides (Pb, V, As e Mo), anatase (Nb, Ta, W, Sn and Sc), zircon (Zr, Hf, Y and U) and gibbsite (Ga), while the rare earth elements occur in the V-shape when normalized to chondrites by enrichment of light rare earth elements (La and Ce) as heavy rareearth elements (Yb and Lu), and marked depression in the range of Nd, Sm, Eu and Gd, and the parallelism of the distribution curves shows that the same genetic relationship between the horizons as Paragominas. The evolutionof lateritic profiles in the two regions are characterized by the following stages: 1 – formation of the crust from the rocks of the Alter do Chão Formation from Juruti, Paragominas derived of the Ipixuna Formation and/or Itapecuru Group; 2 – bauxitization crust; 3 – degradation and partial dismantling of the crust, possibly followed by erosion and deposition for more recessed in the case of PA-256; bauxitization of the nodules and/or concretions and , finally; 5 – formation of Clay cover, called Belterra Clay. The mineralogical and characteristics of the profile studied possibly indicate that these profiles are formed from an evolution in situ for the concretionary and/or horizon and clay cover in relation to lateritic crusts. Periodic climatic variations and tectonic activations are mainly responsible for this evolution.