Pétrologie du gisement latéritique manganésifère d'Azul (Brésil) |
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| Authors: | A Beauvais A Melfi D Nahon J J Trescases |
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| Institution: | (1) Laboratoire de Pétrologie de la Surface, U.A. au CNRS 721, Université de Poitiers, France;(2) Instituto de Astronomia e Geofisica, Universidade de São-Paulo, Brésil;(3) Laboratoire de Géologie dynamique et de Pétrologie de la Surface, U.A. au CNRS 132, Université d'Aix — Marseille III, Centre St Jérôme, 13397 Marseille Cédex 13, France |
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| Abstract: | At Azul (state of Para, Brazil) the lateritic weathering of organic shales with rhodochrosite and phyllosilicates (mainly muscovite) has led to one of the largest maganese deposits of Brazil (45 Mt of reserves with 42.65% Mn).The oxidation of the rhodochrosite-rich levels produces ore that typically breaks into plates preserving the original bedding of the rock. This oxidation corresponds to a sequence of transformation (rhodochrosite  cryptomelane nsutite pyrolusite) by pseudomorphic replacement of the initial textures. At the top of this platy ore layer the late weathering of the micas releases potassium, and the nsutite and pyrolusite plasmas undergo a true retromorphic evolution toward cryptomelane, which is subsequently epigenized by lithiophorite.The geochemical balance computation at each stage and the modeling of this evolution show that acid and reducing conditions of the superficial layer favor the recycling of the Mn to the bottom of the profiles. Cryptomelane and lithiophorite constitute the more stable forms of manganese in the superficial layers of the lateritic profiles. |
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