Abstract
Cenozoic world-class bauxite deposits developed on sedimentary sequences of the Parnaíba and Amazon Basins. The Décio pilot mine, excavated in the Rondon do Pará bauxite deposit, exposes a 3 m thick bauxite-bearing lateritic profile on the top of a 350 m high plateau. From the base to the top, this profile is composed of clayey bauxite; porous microcrystalline bauxite; spherulitic ferro-aluminous duricrust, fragmentary on the top; as well as loose ferro-aluminous spherulites and nodules, embedded in clay. The lower horizons represent a classic lateritic succession, while the top of the profile was produced through post-lateritic reworking. During this process, Ga, Cr, Mo, As, Pb, Hg, Sb, Zn, V and Sc were captured by the iron oxi-hydroxides; and REE, Hf, Nb, Ta, Y remained carried by relict zircon. Isocon diagram and mass balance demonstrate the affinity of the bauxite with claystone from Itapecuru Formation.
Highlights
From the bottom to the top, the Décio bauxite-bearing lateritic profile is composed of the following horizons: clayey bauxite (CBH); bauxite (BXH); ferro-aluminous duricrust (FAD); fragmentary ferro-aluminous duricrust (FFAD); ferro-aluminous spherulites (FASH); and nodular bauxite (NBH)
The high porosity of the spherulites of the BXH, FAD, FFAD and FASH was originated from the hydrolysis of clay minerals from the parent rock
The degradation of the FAD to form the FFAD indicates that after consolidation, the lateritic products started a new cycle of reorganization, marked by partial subjection to weathering, as demonstrated in the Paragominas and Juruti deposits (e.g., Kotschoubey et al 2005), indicating the intensification of warm and humid conditions and the development of a rainforest (Costa 1991)
Summary
Lateritization events affected the Amazon region during the Cenozoic, resulting in the formation of extensive bauxite-bearing lateritic covers (Costa 1991, Théveniaut and Freissynet 2002, Vasconcelos et al 2015), including the large deposits of Trombetas, Paragominas, Juruti and Rondon do Pará, which make up one of the largest bauxite provinces in the world (Boulangé and Carvalho 1997, Lucas 1997, Kotschoubey et al 2005, Costa et al 2014, Oliveira et al 2016, Lima 2018). The post-lateritic events include the formation of the covering material, e.g., the Belterra Clay unit, which comprises a friable yellow to red clay layer, with significant mineralogical and chemical affinity with the underlying bauxite-bearing laterites (Truckenbrodt and Kotschoubey 1981, Kotschoubey et al 2005, Cruz 2011, Negrão et al 2018)
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