Model for bauxite formation: Paragominas (Brazil)

Abstract

Samples from a profile of the Paragominas bauxite (Brazil) were analyzed extensively. Principal minerals were determined as well as the concentrations of 56 elements. The major-element chemistry can be described in terms of the oxides and hydroxides of Al, Si, Ti and Fe, bound mineralogically as gibbsite, kaolinite and hematite. 30 of the 45 minor and trace elements analyzed often maintain their average crustal concentrations. Depletions are most obvious for the alkali metals, the alkaline earths and the metals of the first-row transition series. Enrichment is observed for B, Sc, Zr, Nb, Sn, Sb, I, Hf and Th. Differences in concentration patterns for some refractory metals (Al, Ti, Zr, Hf, etc.) lead to the suggestion that some (e.g., Ti, Zr, Hf) of these metals may be participating in biological reactions. The economic bauxite zone is bracketed above and below by kaolinitic horizons, and this succession of alteration could be explained by extraordinarily high flow rates of meteoric waters for millions of years through highly permeable terrains. The formation and preservation of these highly leached profiles also require a quiescent tectonic setting for 10–100 Ma. Such a model is consistent with conditions which may have prevailed in eastern Amazonia for at least 100 Ma.