LITHODEPENDENCE OF PARTLY TRANSPORTED WEATHERING HORIZONS ABOVE A MIGMATITE DIABASE CONTACT IN CENTRAL BAHIA STATE, BRAZIL

In the semi-arid region of Central Bahia, the weathering cover above a concealed migmatite-diabase contact consisting of a loose sandy-claye y surface layer and an indurated gravelly laterite shows distinct chem ical and mineralogical relationships to the underlying parent rocks. H igh concentrations of Fe, V, Mn, Cu, Co and Ni characterize the weathe ring material above the diabase. In the overlying laterite layer, a ch emical dispersion of most of these elements towards the migmatite can be observed. On the other hand, the weathering material above the diab ase shows abundance of primary quartz, zircon and rutile derived from the migmatite. The evolution of the weathering cover is believed to be controlled by colluvial intermixing and vertical homogenization of pa rtly transported weathering material, being followed by lateral chemic al dispersion during laterite formation. Thus, the weathering horizons obliterate the chemical signal of the parent rocks increasingly from the bottom to the top of the profile. Similar profiles can be expected in most savannah-type regions and, therefore their identification is relevant for geochemical exploration.