Control of the distribution of Mn, Co, Zn, Zr, Ti and REEs during the evolution of lateritic covers above ultramafic complexes

Abstract

Laterites above ultramafic complexes in Brazil and the Philippines consist of in-situ (autochthonous) and partly transported (semi-autochthonous) horizons. Enrichment and distribution of minor and trace elements in the laterites are controlled by weathering processes and the morphology of the peridotite massifs. Mn and Co migrate towards the base of doline-like depressions of the peridotite karst where they precipitate in the ferruginous horizon above the saprolite, due to rising pH and concentration of the solution. Mass balance calculations reveal absolute enrichment of Mn and Co and depletion of Zn and Cr. Semi-autochthonous laterites represent a mixture of Fe-Cr-rich limonitic material derived from ultramafic rocks and Al-Si-Ti-Zr-rich kaolinitic material derived from sialic rocks. They may reveal unusually high contents of Al2O3, SiO2, TiO2 and Zr, thus camouflaging the underlying ultramafic bedrock. During and after deposition of semi-autochthonous laterites, acidic leaching under humid (sub-) tropical climate leads to the mobilization of rees from the weathering products of sialic rocks. Subsequent migration and precipitation of rees together with Mn at the base of depressions result in concentrations of predominantly Ce which under favourable conditions exceed 0.5 wt. %. High proportions of sialic weathering material in laterites can be expected in geological settings where ultramafic and sialic rocks occur in close spatial association. Enrichment of rees in weathering profiles above ultramafics is favoured by the abundance of weathering material from ree-rich alkaline rocks like those of the Alkaline Complex of Jacupiranga, Brazil.