On the geology, mineralogy and geochemistry of the bauxite-bearing regolith in the lower Amazon basin: Evidence of genetic relationships

摘要

The wide distribution of Cenozoic world-class bauxite deposits in the Amazon shows that this region was the scene of intense lateritization. The Juruti bauxites, currently being mined in the lower Amazon, are one of these deposits, where the bauxite-bearing profile is covered by thick yellow clay (Belterra Clay). The laterite profile sequence consists of a mottled horizon at the base, a massive to cavernous bauxite, an aluminous iron crust with ferruginous nodules and bauxitic nodules in contact with yellow clay on the top. Cryptocrystalline to microcrystalline gibbsite is the main mineral of bauxite; hematite, goethite, kaolinite and anatase are accessories. The same minerals constitute the upper horizons, but in distinct concentrations. The relict structures, minerals (zircon) and chemical composition suggest an affiliation to sedimentary rocks of the Paleo-Mesozoic Amazon basin (Alter do Chao formation). The mineralogy, geochemical associations and Ti mass balance show a continuous evolution from the mottled horizon to the aluminous iron crust and even to the ferruginous nodules. Kaolinite, hematite, goethite and anatase are the early minerals and gibbsite is the late mineral. Hematite, zircon and anatase are the potential carriers of trace elements. The dissolution of hematite and goethite and the reprecipitation of iron as Al-free goethite reached the bauxite horizon and strongly fixed the mobile elements (Ba, Ag, As, Bi, Hg, Mo, Se, V, Pb, Sb, Se and Zn). The increasing upward content of zircon, anatase and residual trace elements (HFSE, Sc, Ga, Sn, U, W and Y and REE) throughout the regolith, an inheritance of the bauxite-bearing laterites, enhances the affinities between the two units. The laterite was developed during the Eocene, and its cover may be a product of deep plant root activity (tropical soil forming) on the top of the profile, which formed kaolinite and Al-goethite that was transported by wash flooding during the Miocene. The bauxitic nodules may represent an incomplete second bauxitic event, as erosion had already established the actual plateau landscape during the Pliocene.