Anicet Beauvais

Formation and transformation processes of iron duricrust systems in tropical humid environment

Abstract Formation and transformation processes of iron duricrust developed under tropical humid conditions in the southeastern part of the Central African Republic have been investigated in terms of geomorphological, petrological and geochemical pathways. The study of five typical weathering profiles of the most extensive iron duricrust system now known in the world indicates that saprolitization and ferruginization are sequentially involved in such processes, as reflected by kaolinite-gibbsite and hematite-goethite replacements. Alkali earths, rare earths, metals and elements belonging to chemically inert minerals segregate during saprolite and iron duricrust formation and degradation. Mass-balance calculations performed on a typical weathering system enable us to quantify net mass element transfers deriving from lateritic weathering processes. Volumetric change, collapse or dilation, as well as Al and Ti clearly discriminate between the saprolitization and ferruginization processes. Saprolite development preserves the geochemical inheritance of parent rocks, whereas ferruginization depletes it and mostly reflects, through mineralogical pathways, pedoclimatic conditions and global climatic changes. The global iron duricrust system of the southern part of Central Africa derived from previous hematite-rich crust, and not from ancient bauxites as it is generally believed for iron crusts, formed on West African shields.

Chemical weathering of Mn-garnets under lateritic conditions in northwest Ivory Coast (West Africa)

Abstract Lateritic profiles developed on Mn 2+ -bearing metamorphic rocks in the Ziemougoula area (northwest Ivory Coast) permit the observation of chemical weathering of garnets, essentially spessartites. Two main stages are found in the process of weathering. In the lower part of profiles, spessartites alter principally to birnessite and the Al content of the parent garnet is leached out of the zone. Higher in the profile, garnets alter directly to lithiophorite; in this geochemical environment Al is no longer mobile. In such a layer, the earlier formed birnessite is transformed into nsutite along with minor cryptomelane. These latter minerals can locally evolve into ramsdellite or pyrolusite. The associated nsutite and lithiophorite are the principal phases of the hard manganese crust capping the hillrocks of the lateritic landscape of the Ziemougoula area.

Pétrologie du gisement latéritique manganésifère d'Azul (Brésil)

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.

Manganese Concentration Through Chemical Weathering of Metamorphic Rocks Under Lateritic Conditions

Laterite profiles developed on manganiferous metamorphic rocks in the Ziemougoula area (North West Ivory Coast) permit the observation of chemical weathering of tephroites, manganocalci tes, chlorites and spessartite garnets. Several stages are observed in the progress of weathering. In the lower part of prof files, tephroites and manganocalcites alter f first into manganite, followed by chlorite into todorokite then by spessartites which weather into birnessite; the Al content of the parent garnet is leached out of the zone. Higher in the profile, garnets alter directly into lithiophorite; in this geochemical environment Al is no longer mobile. In such a layer, early-formed birnessite and manganite are transformed into nsutite along with minor cryptomelane. These later minerals can locally evolve into ramsdellite or pyrolusite. The associated nsutite and lithiophorite are the principal phases of the hard manganese crust capping the hill-rocks of the lateritic landscape of the Ziemougoula area.

Manganite formation in the first stage of the lateritic manganese ores in Africa

Abstract Lateritic profiles on manganiferous rocks from two areas in central West Africa permit the observation of manganite formation on rocks of different primary nature. In an area of gondites and tephroitites (Ziemougoula, Ivory Coast) manganites are found to replace tephroites and manganocalcites, whereas in a weathered carbonate sequence (Moanda, Gabon) manganite replaces rhodochrosite. In both areas manganite occurrence is limited to a very narrow and basal portion of the altered profile, in conformity with the limited stability field established for this mineral. It is found that manganites of different mineral lineages may be distinguished chemically from one another by variation in the ratio MnO ( CaO + MgO ) .

Geochemical evolution and degeneration of ferricretes under a humid tropical climate in the East of Central African Republic

BEAUVAIS A. *, BOEGLIN J.L. **, COLIN F. ***, MAZALTARIM D. **** and J.C. MULLER J.C. **** * Centre ORSTOM, B.P. 893, Bangui, Republique Centrafricaine. Laboratoire de Petrologie de la Surface, U.A. au CNRS 721, Universite de Poitiers, 40, avenue du Recteur Pineau, 86022 Poitiers Cedex, France. ** Centre ORSTOM, B.P. 2528, Bamako, Republique du Mali. Centre de Sedimentologie et de Geochimie de la Surface, (CNRS), Universite L. Pasteur, 1 rue Blessig, 67084 Strasbourg Cedex, France. *** Laboratoire de Geosciences, U.R.A. au CNRS 132, Universite Aix-Marseille III, 13397 Marseille Cedex 13, France. **** Centre de Sedimentologie et de Geochimie de la Surface, (CNRS), Universite L. Pasteur, 1 rue Blessig, 67084 Strasbourg Cedex, France.