Bruno Boulangé

Rare earth element mobility during conversion of nepheline syenite into lateritic bauxite at Passa Quatro, Minais Gerais, Brazil

Abstract In a lateritic bauxite formed by weathering of nepheline syenite at Passa Quatro, Minas Gerais State, Brazil, bauxites on the hill-tops directly develop from the syenite bed-rock, while downslope, a kaolinitic layer occurs between bauxite and synetie. A petrological investigation was performed on undisturbed weathered rock samples collected from a representative upslope pit. The undisturbed weathered rocks were chemically analysed for major trace elements including REE and Zr. Mass balance calculations were applied, and the behaviour of the REE in the Passa Quatro weathering system was established compared to REE reference chondrite and to REE reference parent rock. In the lateritic bauxite, the results suggest that the first stages of weathering induce a volumetric change of 50%, i.e. collapse, with respect to the parent rock, and remove REE with a slightly larger loss of the LREE, except Ce, compared to the HREE. In the upper layers, where bauxite is more mature, a net mass gain in REE is observed relative to the underlying layers. This gain takes place during the reduction of the upper layer during the downward progression of the weathering front. Very significant REE losses occurs during the bauxitization processes throughout the upslope profile. In addition, the downslope kaolinitic system is demonstrated to be depleted in REE in the same proportions as the upslope bauxite. We proposed that the REE exported in solution from the whole weathering mantle have enriched neighbouring watershed sediments.

Mineralogical and geochemical characteristics of two bauxitic profiles, Fria, Guinea Republic

Bauxite deposits of the Fria district, Guinea, have been exploited since 1960. These lateritic bauxites, located on the upper parts of plateaus, result from weathering of paleozoic schists. The ores are composed of gibbsite associated with pyrophyllite, Al-substituted gœthite, and kaolinite. Pyrophyllite and Al-substituted gœthite may contain up to 9% of the total Al2O3 content of the bauxite; this cannot be recovered through the Bayer process because these phases are insoluble in the leaching solutions. Kaolinite is soluble under Bayer leaching but this dissolution induces precipitation of sodium aluminosilicates, which apart from loss of further alumina results in decreasing efficiency of the process through scale formation. Detailed knowledge of the distribution of the different ore types and their mineralogical composition is necessary for efficient processing.

Isotopic tracers of the origin of Ca in a carbonate crust from the Middle Atlas, Morocco

Abstract Carbonate crusts developed on basalt were studied at Tazouta in the Moroccan Middle Atlas. The chosen representative weathering profile is 2.8 m thick and is composed from base to top of a fresh basalt overlain by a coherent saprolite, a friable saprolite and a surface soil. Carbonate crust genesis occurs by replacement of primary and secondary clay minerals by low Mg calcite. The 14 C dates of the calcite range from 15 to 14 ka in the coherent saprolite, and from 12 to 8.5 ka in the friable saprolite. Smectitic weathering and replacement of Si-bearing minerals by carbonate are coupled processes leading to a strong calcium net gain (300 kg/m 3 ) within the friable saprolite. The low weathering rate of basalt compared to the calcrete genesis rate, and the high mobility of calcium in supergene weathering mantles, limit the basaltic Ca contribution to calcrete genesis. The contribution of the Triassic sediment is also limited by the mineralogical unavailability of Ca. The 87 Sr/ 86 Sr ratios confirm the low contribution of basalt and indicate an external origin of Ca. The Sr isotope ratios of calcrete are different from those of the adjacent Liassic carbonates and are close to rainfall values. The carbon isotope compositions of carbonates (−8‰ to −9‰ vs. PDB) imply a significant contribution of C3-dominated organic carbon during carbonate precipitation. Oxygen isotopic compositions of carbonates (average −5.24‰ vs. PDB), show that carbonates are 18 O-enriched compared to meteoric waters (−4‰ to −3.5‰ vs. SMOW).

Dissolution features of gold particles in a lateritic profile at Dondo Mobi, Gabon

Abstract This paper describes the morphological evolution of gold particles in a lateritic weathering profile under rain forest. Inherited from primary quartz veins associated with amphibolite, the gold particles can be recognized from the weathering front up through the saprolite and overlying regolith to the land surface in the central part of the supergene gold dispersion halo. Gold particles undergo some changes in the uppermost 10 m. Voids and corrosion pits appear on particle surfaces; their numbers and sizes increase toward the land surface. Edges of particles become progressively more blunt from the bottom to the top of the section. The morphological changes are attributed to dissolution of gold over time. Possible causes for such dissolution are discussed. Occurrence of gold particles in the soil could be helpful for gold explorations in the humid tropics.

Gold: a useful tracer in sub-Sahelian laterites

Sequential and connected lateritic sequences from the Tertiary to present day can be observed in sub-Sahelian conditions in northern Burkina Faso (Michel, 1973; Grandin, 1 975 ). Here, complex iron auriferous crust systems have developed on the stable West African Craton from Proterozoic auriferous greenstone belts and volcanic tuff. In order to distinguish between chemical and physical processes which have led to the development of the lateritic systems, we propose to use visible gold particles as pathfinders. In this paper, we focused on the lowland lateritic system (profile of pit P4) located downslope to the middle land lateritic system, where gold is associated with hydrothermally altered quartz veins and pebbles (Fig. 1 ) .

Laterites and Bauxites

The term «bauxite» was introduced by Berthier (1821) and refers to rock samples belonging to the laterite type. Lateritic bauxites have been studied by many, mainly by prospection and exploration geologists, and by geologists, soil scientists and geomorphologists interested in the mechanisms of bauxite formation and evolution. Many overviews have been published on the topic during this century (Lacroix 1913; Harrasowitz 1926; Harrison 1934; Patterson 1967; Valeton 1972; Bardossy and Aleva 1990). It is now well known that bauxites and laterites result from pedogenic processes, i.e. from weathering of a large variety of sedi-mentary, metamorphic and igneous rocks, under tropical and humid climates (Bocquier et al. 1984).

A comparison of properties of clay minerals in isalteritic and in degraded facies

Abstract The mineralogical, geochemical and micromorphological features of an isalteritic clay facies, which originated from weathering of an anorthosite, were compared to those of clay facies derived from the degradation of a bauxite developed from the same rock. The isalteritic clay was formed by the hydrolytic alteration of plagioclase, whereas the degraded clays were formed by decomposition of gibbsite and neoformation of kaolinite. This resilification process resulted from the reintroduction of silica via the oscillation of the phreatic level and/or the decomposition of organic matter on the surface. The degradation process was gradual and yielded two different facies: (a) degraded clays with almost total decomposition of gibbsite, and (b) degraded clays with gibbsite nodules. Morphologically, the isalteritic clays differ from the degraded clays because they contain larger hexagonal and pseudo-hexagonal crystals. The degraded clays have more irregular crystal shapes, ranging from laths to anhedral shapes.

Analyse de la porosité dans un profil d'encroûtement carbonaté sur les basaltes du Trias du Moyen Atlas (Maroc)

Abstract A carbonate crust profile has developed from Triassic basalt flows in the Moroccan Middle Atlas. The study of pore distribution of these supergene formations shows a good agreement between calculated (from density measurements) and directly measured values of porosity (mercury porosimetry). Weathering of basalt develops clayey layers (saprolite and soil) displaying increases in porosity (macroporosity essentially). In the Quaternary, these very porous and permeable layers permitted easy meteoric water circulation and a profile calcitization by per descensum. processes. Carbonate precipitation occurs by epigenesis replacement of silicate minerals. In the friable saprolite, carbonate replaces directly the basaltic primary minerals, while this replacement operates on the weathering products (smectites) and residual primary minerals in the coherent saprolite, indicating the very fast progression of the calcitization front. At the weathering front, within the coherent saprolite, both smectitic weathering and carbonatation compete as functions of time and fluid chemical composition.