Geologia, mineralogia e geoquímica do perfil lateritobauxítico da lavra piloto Décio (Rondon do Pará)

Abstract

During the Eocene-Oligocene, extensive lateritic coverings were formed in the Amazon. In this context, the sedimentary sequences of the Parnaíba and the Amazon Basins were lateritized and generated four world-class bauxite deposits: Juruti, Trombetas, Paragominas and Rondon do Pará. The latter is the new bauxite exploration frontier of the Amazon and the Décio pilot mine was developed in it, which presents a 3 m thick laterite-bauxite profile, which extends laterally through vast plateaus with about 350 m of altitude. This work sought to deepen the study of bauxite and its relationships with lateritic landscape in the Amazon region. In the field, five lithological profiles were described in the Décio pilot mine. Then, 42 samples were collected for laboratory analysis involving mineralogical characterization (X-ray diffraction); textural (Optical Microscopy and Scanning Electronics); and chemical (Optical Emission Spectrometry and Mass Spectrometry with Inductively Coupled Plasma). The most probable precursor rocks are the clay facies of the Itapecuru Formation. These would have evolved to a bauxitic clay horizon and then to massive microcrystalline and microporous red bauxite. Bauxite, on the other hand, converges to spherolytic iron-rich crust, rich in iron oxide and partially dismantled. This set is covered by a horizon of ferroaluminous spherolites and a nodular bauxite horizon. The whole set is covered by unconsolidated yellow to red clay, equivalent to the Belterra Clay. The minerals that resisted laterite weathering were zircon, tourmaline, rutile, kyanite and quartz. The neoformed were hematite, goethite, gibbsite, part of the kaolinite, besides the anatase. The main chemical constituents are Al2O3, Fe2O3, SiO2 and TiO2, which together make up more than 99.5% of the profile. Its distribution suggests that the lateritic evolution developed in situ occurred until the formation of the spherolithic horizon. The TiO2 content is much higher in the bauxitic clay horizon than in the other overlying horizons, which is not common in a lateritic evolution from a single, homogeneous parent rock. In addition, Zr and ETR decrease towards the top of the profile, which would also not be expected. Therefore, although the precursor rocks of the entire profile were sedimentary, there was a compositional contrast between those that generated the bauxitic clay horizon and those that generated the other horizons. In addition, the pattern distribution of trace elements shows that even starting from slightly differentiated parent rocks, almost the entire profile followed a similar pattern of evolution, with the exception of the nodular bauxite horizon that behaves as separate unit. These characteristics differ the lateritic profile of the Décio and Ciríaco pilot mine, located in the same bauxite region, but formed from more homogeneous parent rocks.