MINERALOGIA E ASPECTOS GEOQUÍMICOS DOS LATERITOS FOSFÁTICOS DA ILHA PEDRA GRANDE DO GURUPI (PA)

Abstract

The Pedra Grande Island located at the mouth of the Gurupi River, is equivalent to a small plateau, which is currently partially drowned by the sea, where the impacts of the waves have already left longitudinal fractures and caves on the island. The island is entirely made up of lateritic rock, represented by the ferruginous lateritic crust, exhibiting a dominantly brownish red coloration. Texturally it consists of nodules, pisoliths and pseudo-fragments, which are surrounded by a whitish cream cement, giving the rock a general brechoid appearance, in addition to columnar and cavernous features. Two mineralogical associations are individualized in these laterites: a group formed by iron and calcium phosphates, widely developed in the ferruginous lateritic crust, together with iron oxy-hydroxides; and another formed by the aluminous phosphates, which constitute the whitish cream phosphate cement. The quantification of major, minor and trace elements revealed high contents of Fe2O3, P205 and A1203, and low of SiO2. In terms of traces, Mn and Zn display low contents, associated with ferruginous lateritic crust. On the other hand, rare earth elements are more concentrated in phosphate cement, mainly in Ce, Nd and La. The Ilha de Pedra Grande do Gurupi exhibits some mineralogical and chemical similarities with other occurrences of lateritic phosphates in the region, located on the continent. This correlation reveals that in the recent geological past, the ocean coastline presented a very different morphology from the current one. The geochemical and mineralogical analyzes of the Pedra Grande laterites allow us to suppose that this lateritic profile developed under a tropical climate and semi-flat topography, enriching itself in phosphorus from a phosfor-bearing parent rock, which could have the following characteristics: a clayey rock, with of shales, carbonates and phosphorites, or even a low-grade metamorphic rock, composed of schists and phyllonites containing phosphorite levels. Keywords: ferruginous crust; breccia like rocks; duphrenite; mitridatite; crandallite.