Mineralogia e geoquímica do perfil laterítico do depósito de ferro da Serra Leste, Carajás-PA

Abstract

The Carajás Province hosts one of the largest high-grade iron ore deposits in the world, distributed in the districts Serra Norte, Serra Sul and Serra Leste. The process of Fe mineralization in the region is still a stage of discussion, due to the textural complexity typical of the ore, which induces different interpretations regarding its genetic model. In this context, in order to better understand its origin, the present work evaluates the contribution of lateritic weathering to the formation of the iron deposit of Serra Leste. In the field, a profile of weathering and its substrate were described, through two drill holes provided by the company VALE S.A, followed by sampling. After that, about 20 samples were described, photographed and prepared for mineralogical and chemical analysis. The mineralogical phases were identified by X-Ray Diffraction (XRD) and the micromorphological images obtained by Scanning Electron Microscopy (SEM), accompanied by semi-quantitative analyzes by Energy-Disperse Spectroscopy (EDS). The textural aspects also involved optical microscopy by reflected and transmitted light. Analysis by Mössbauer spectroscopy were used in order to identify the oxidation states of Fe ions present in the samples, complementing the information obtained by the other techniques. Chemical analyzes were performed by Inductively Coupled Plasma Atomic Emission Spectrometer (ICP-AES), and Inductively Coupled Plasma Mass Spectrometer (ICP-MS). The investigated profile comprises two successions of lateritic alteration, one derived from jaspilites and the other derived from mafic composition rocks. The profile comprises jaspilites and chloritites at the base, followed by saprolitic horizons (thick saprolite and fine saprolite) and ferroaluminous crusts. The chemical composition shows that the SiO2 contents (chert/quartz) decreased drastically during the formation of the horizons from the base of the profile, with an increase in the levels of Fe2O3 (mainly hematite and goethite) and its substantial concentration in the saprolitic horizon (mineralized zone). From the top of the fine saprolite there is an increase in the contents of Al2O3, TiO2 and P2O5, related to the presence of gibbsite, aluminous goethite and anatase, in the same way trace elements (Ga, V, Cr, Ta, Nb, W, Zr, ETR and others) present in the structure of newly formed minerals. The data obtained in the investigated profile, therefore, shows a lateritic evolution, and are similar to the mature lateritic profiles of the Amazon.