Abstract

Aluminous laterites north of the Amazon River have formed on poorly consolidated clayey terrestrial sediments of Pliocene or Plio-Pleistocene age. The essential processes affecting the major elements were leaching of the surficial portion of kaolinitic sedimentary rocks causing a reduction of nonquartz silica in the entire laterized zone, downward transport in solution of aluminum and iron, and concentration of aluminum and iron as hydroxides and hydrated oxides at the base of the laterized zone.Measurement of the vertical distribution of 20 elements through the laterized profile shows large variations in their concentrations. Leaching has predominated in the upper portion and strong enrichment of many elements is characteristic of the aluminum- and iron-rich lower parts of the zone. When compared with the subjacent material, Fe, V, P, Ga, and Mn are strongly enriched and Pb, Sr, and Ba are strongly depleted in the laterized zone. Enrichment of minor elements in the principal mineral phases is marked for Cr, Ga, K, P, Mg, and V (in iron oxides) and Ba, Pb, and Sr (in kaolin). None of the minor elements measured show a preference for gibbsite. The distribution of the elements in the laterized zone is not consistent with a geochemcial control related to their ionic potential.

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