Some implications of lateritic weathering on geochemical prospecting — two Brazilian examples

Abstract

Geochemical and petrological studies were carried out in two Brazilian tropical areas. At the Cu prospect of ‘Chapada Grande’, under a wet savanna climate, the upper part of a lateritic profile with a partially eroded Fe crust was investigated. In the ‘Vale do Paramirim’, under a dry savanna climate, in an area of Pb-Zn-(Cu) anomalies, lateritic soils with an underlying stone-line change laterally into sandy leached soils.

In both cases, there exists a good lithodependence upon the fresh parent rock of all the weathered material. At Chapada, there is a strong bonding between Fe-oxyhydroxides and Cu. In soils above the crust, Fe-oxyhydroxides are mainly in the coarse granulometric fraction (>1 mm) in the form of Fe nodules whereas, in slopes soils without underlying crus, Fe-oxyhydroxides are mainly in the fine fraction (<105 μm). In the Vale do Paramirim, the lateral leaching of Fe and clays is accompanied by a strong diminution of the Pb-Zn-Cu content of the surface horizons and by an alteration of the correlations between these elements. In lateritic soils Cu and Zn are correlated but both elements present a very weak correlation with Pb; on the other hand, in sandy soils the correlation between the three elements are identically weak.

With respect to geochemical prospecting, the study shows that sampling and data interpretation methods might be adapted to the soil type in the prospected environment. In both cases, the grouping of the data by classes of soils defined according to surface features (such as colour or the presence of coarse particules) is highly recommended before any statistical treatment is undertaken.