Mineralogical and geochemical aspects of nickeliferous laterite of Vermelho region, Serra dos Carajas (State of Para)

Mineralogical and geochemical studies were carried out on the nickeliferous laterite of Vermelho region (Serra of Carajas, state of Para). A total of 64 samples were analysed by X-ray fluorescence analysis and atomic absorption, spectrophotometry, including bed-rock specimens. The laterite is derived from ultramafic rocks (serpentinites) and there horizons were recognized towards the top: saprolitic horizon with a thickness of more than 4 meters, clay horizon 6,1 meters thick and limonitic horizon with a thickness of 8,3 meters. The process of weathering affected both major and trace elements which were bleached, transported and reprecipitated. This caused variations in the concentration of the elements with depth, except for magnesium which was extensively bleached. Nickel concentrations in the clay horizon probably was a result of ion exchange between smectite and Ni-bearing solutions, the source of this Ni being goethite. Cobalt and zinc were also strongly enriched during the process of laterization, although maximum concentrations of these elements are found in different horizons. (author)