The correlation between geochemical data and SPOT satellite imagery of lateritic terrain in southern Mali

Abstract

Abstract Detailed geochemical mapping of superficial lateritic formations is compared with recent high-resolution SPOT satellite images in the Dagadamou prospect in southern Mali. The two main landscape features standing out in SPOT images are expressed by thematic indices derived from multispectral data. They reflect the distribution of vegetation cover and the distinction between silty clay soils in the valleys and ferruginous duricrust exposed on the plateaus. Geochemical differentiation factors are closely related to the nature of sampled materials and reflect the relative amounts of major constituent minerals estimated by normative calculation. Duricrust samples with high Fe and Al content are also enriched in trace elements, P, V, Cr, As, Mo, Nb and Cu, immobilized in the weathering profile with secondary oxihydroxide minerals. Soils of flats are composed mainly of quartz and kaolinite; they also concentrate heavy minerals characterized by high contents of Zr, Ti, Ce and Y. Accumulation of detrital material at the periphery of duricrust plateaus is marked by a geochemical halo of higher contents in Zr and quartz at their periphery. This feature is clearly visible on SPOT imagery as a zone of high reflectance, devoid of vegetation. The density of vegetation on duricrust is related to its kaolinite content. Its distribution is characterized by a striped pattern, which is probably controlled by bedrock lithological structures preserved in the lateritic cover. This study shows the relationships, both direct and indirect, existing between spectral reflectance and geochemical composition of superficial lateritic formations. Many other landscape features identified with high resolution on SPOT images provide complementary information, which could be very helpful at various stages of geological and mineral exploration in lateritic terrain.