In the municipality of Barro Alto, Goiás, a bauxite deposit was developed, mostly, on Neoproterozoic anorthosites. According to recent published data, the general mode of occurrence reinforces the lateritic origin, since bauxite follows the current topographic surface. However, the succession of the horizons is not corresponding to the others of the lateritic model, at least partially. The absence or restricted size of the clay horizon with kaolinite, or the presence of kaolinite bodies sectioning the bauxitic body, which locally reaches large thicknesses, overturn the simple lateritic model. This is reinforced by the absence of the classic leopard skin-like pattern or similar iron-aluminous crust and the absence of other coverage. For various authors the wide occurrence of extensive macrocrystalline, botrioidal gibbsite, apparently occupying venules and fracture walls suggests, along with the diversity of the way of occurrence of clays, a contribution of another process, not only lateritization. From this information a study was elaborated to deepen the knowledge about the formation of the Barro Alto bauxite, detailing the generations of gibbsite and kaolinite, as well as to try to identify the lateritic and hydrothermal contributions with these minerals. In the field, 5 mines explored by the company EDEM - Mineração LTDA were studied (Mining 1, Mining 2, Mining 5, Mining 6 and the SELA trench). From these mines, a greater detail was made in mines 1, 2, 5 and in SELA trench. Forty-eight samples were collected from which 22 most representative samples were selected for laboratory analysis involving mineralogical (X-ray Diffractometry), textural (Optical and Scanning Electron Microscopy), and chemistry (Inductively Coupled Plasma Mass and Optical Emission Spectrometry) characterization. In general, the bauxitic profile of Barro Alto comprises the anorthosite as substrate and possible source rock, on which a porous bauxite horizon (HBP) with stockworks and flint-type veins of kaolin (CF), and locally with subcentimeter corundum crystals (HBPC) was established. They converge to a bauxitic clay zone (ZAB) and then to a massive bauxite horizon (HBM). This in turn is superimposed by a capping formed by centimetric to decametric blocks and nodules of massive bauxite (HBANB), whose occurrence suggests a colluvial formation. In the profiles the horizons HBP, HBPC, HBM and HBANB consist essentially of gibbsite and kaolinite, sometimes containing accessory minerals such as corundum (HBPC), followed by hematite and goethite. In CF and in the bauxite clay zone (ZAB) the dominant mineral is kaolinite and/or halloysite, followed by gibbsite. The chemical composition of the bauxitic profile is basically composed of Al2O3, SiO2 and Fe2O3. CaO, MgO, K2O, MnO, Na2O and P2O5 concentrations are very low (generally <0.09), even below the analytical detection limit. TiO2 contents are also relatively low, generally <0.2%. Trace element concentrations when compared to the average values of the Earth's Upper Crust (EUC) are generally very low. The concentrations of elements such as Cr, Co, Cu, Zn, Rb, Sr, Y, Zr, Nb, Cs, Ba, Hf, Pb, Th and U are below EUC in all horizons. Only the element Mo has more concentrated values in all horizons. On the other hand, V, Ni, Cu, Cr, Ga, and Pb may eventually have higher values especially in some parts of CF and ZAB. Rare Earth Element (REE) concentrations are below the crustal average in all samples studied and also in the anorthosite. The higher concentrations are typically related to the clayey units such as CF and ZAB, and also to the presence of Fe oxides-hydroxides (HBPC). When normalized to the chondrites, the curves exhibit a slightly similar distribution pattern except for porous bauxite with corundum. There is a slight LREE enrichment and a HREE tendency, but only from Tm to Lu. This behavior resembles that of anorthosite, but with much lower concentrations, except for Tm. It is clear the negative anomalies in Ce and positive in Tm, anomalies linked to LREE depletion and a slight enrichment of HREE. From these mineralogical and chemical data, it was possible to distinguish three distinct generations of gibbsite and kaolinite: Gibbsite (I): of the porous bauxitic horizon, which appears to be that formed directly from the plagioclase and almandine; Gibbsite (II): associated with flint kaolin, along with kaolinite, halloysite and even locally hematite; Gibbsita (III): Cohesive and macrocrystalline, sometimes drusic and overlapping the thinner and more compact botryoidal gibbsite. It forms isolated bodies, apparently disconnected from the larger set of bauxites, in pockets of tens of meters wide and depths greater than 40 m; Kaolinite (I) at the gibbsite - anorthosite interface. This kaolinite makes up the halos of anorthosite alteration, when it does not go straight to gibbsite; Kaolinite (II) corresponds to that of flint kaolin, associated with halloysite and sometimes gibbsite. It occurs in the form of venules, veins and pockets, and vertical fractured zones, the stockworks in general. Kaolinite (III) corresponds to that found mainly in the clayey bauxite horizon with nodules, in which this mineral is yellowish, earthy, invading the bauxite mass and involves the centimetric to decametric nodules. The generations of gibbsite, I, II and III, especially II and III, are not compatible with lateritic evolution, likewise the kaolinite, I, II, and III generations, where I and II are not clearly lateritic, and the III seems to be current pedogenetic. Therefore, it is likely that Barro Alto bauxites are the product of the intense subsurface hydrothermal activity of anorthosites, due to the strong structural deformation in which these rocks were subjected in their final post-emplacement stages.
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