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Two typical lateritic weathering profiles (Jacuba and Angiquinho) from the Niquelandia Ni deposits, Brazil, were studied in order to establish the petrological relationship between the supergene Ni products and the parental pyroxenes. From the base to the top of the profiles, pyroxenes are replaced by goethite and kaolinitc through a series of transitional Ni-bearing phyllosilicates. The mineralogy and the chemical composition (especially the Ni content) of these clay minerals depends on the degree of fracturing and serpentinization of the pyroxenite and the location of the pyroxenite with respect to neighboring dunitc. Within the Jacuba profile, smectite and pimelite pseudomorphs after pyroxene are especially Ni rich, and in fact, are the most Ni-enriched clay minerals now known in lateritic weathering profiles.

Many nickel deposits are known in Brazil, accounting for about 350.106 tons of ore with an average of 1.5% Ni. All are of the lateritic type. These deposits are scattered throughout the country, being rarer in the Northeastern Region and in the South, below 25~ latitude. They are mainly associated with mafic-ultramafic massifs of large dimensions and ultramafic alkaline complexes, and occur in climatic regions of contrasting seasons. The weathering profile developed over the fresh rock consists, from bottom to top, of the following horizons: altered rock, coarse saprolite, argillaceous saprolite, ferruginous saprolite and lateritic overburden. The thickness of each horizon varies from one deposit to another, the whole profile generally exceeding 20 m. The saprolitic horizons with inherited minerals (serpentine, chlorite) or neoformed minerals (smectites) constitute the silicated nickel ore and are thicker were climatic conditions are drier; the ferruginous upper horizons made up of iron oxide-hydroxides are more developed in more humid regions. In Brazil, the silicated ore generally prevails over the oxidized ore. The main Ni-bearing minerals are serpentine, smectite, garnierite and goethite. The latcritic nickel deposits of Brazil may be correlated with two erosion surfaces, corresponding to the Sul Americano (Lower Tertiary) and Velhas (Upper Tertiary) levelling cycles. The degree of dismantling of the higher and more ancient surface and the consequent development of the Velhas Surface control the position of the nickel accumulation in the landscape. Thus, the deposits may be found either in the lowlands or in the highlands, where they are always covered by a silcrete layer. The alteration profiles in the Brazilian lateritic nickel deposits are broadly similar to those described elsewhere in the world. However, they present two characteristic features: the silicated ore prevails over the oxidized ore, and a silicified layer covers the profies developed on the highlands.

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RESUME: L'alteration lateritique d~butante des orthopyroxenes et clinopyrox6nes de Niquelandia, Bresil, montre plusieurs stades. Le premier stade facultatif est la formation de produits 'amorphes" dont la composition chimique proche de celle des min~raux parentaux, montre d6j~i un enrichissement en nickel. Le deuxi6me stade est marqu6 par l'apparition de phyllosilicates dont la nature et la composition chimique varient scion le degr~ de porosit6 de la roche parentale. Cette porosit~ contr61e 6galement la distribution du nickel dans les diffbrents phyllosilicates, (saponite, talc, pimelite). Les pyrox6nes ne contenant pas ou tr6s peu de nickel, il est clair que le nickel qui s'accumule darts les produits de l'alt~ration est totalement import6 avec les eaux de percolation. L'origine de ce nickel est fi rechercher plus haut dans les profils ou plus en amont dans la sequence, l/l oti les dunites sont pr~sentes. A B S TR A C T:The initial weathering products under lateritic conditions of orthopyroxenes and clinopyroxenes in Jacuba, Niquelandia, Brasil, are amorphous and have chemical compositions very near those of the parent minerals but with enrichment of Ni. Later products are phyllosilicates such as talc, saponite and pimelite, the nature and composition of which vary with the development of porosity and fissuring of the parent rock. It appears that Ni distribution between these phyllosilicates is also controlled by fissure development. As the parent pyroxenes are Ni-depleted, it is clear that Ni accumulation in the structures of the phyllosilicate weathering products is a result of solution transfer from other parts of the profile.