Gossans in tropical areas : Comparison of the ferruginizations from central Togo and north Burkina in Neoproterozoic formations
Under tropical climates, especially in West Africa, weathering of sulfidic mineralizations displays, as for other rocks, successive stages of hydrolysis of the primary then secondary minerals and of mineral neogenesis. The end product is a quasi holoferruginous rock, i. e. the gossan which may be considered in fine as the unique indicator of a sulfidic mineralization. The contemporaneous outcropping or sub-outcropping gossans would be characterized by a structure. In contrast paleoweathered material of sulfidic mineralizations appear only in blocks separated from their weathering profile and emptied of their substance due to present but above all past climatic conditions. In West Africa, gossans are extremely mature, and display no other secondary minerals than ferriferous and manganiferous ones. They are principally composed of quartz and goethite the latter being the only support of residual geochemical information.
The Neoproterozoic formations, which compose the major part of the Volta and Taoudéni basins, have been neglected in the prospecting of metallic resources. However, the principle of elements redistribution cannot be dismissed. In fact, these two basins have collected along one billion years or more, thousands of meters of Birrimian formations from the West-African craton. In both areas, ferruginizations display an unquestionable picture of associations of elements typical of hydrothermal sulfides, principally low temperature sulfides and complex sulfides. The ferruginizations from Togo and North Burkina are roughly similar. The usual anomalies concern As, Au, Bi, Cd, Co, Fe, Mn, Mo, Pb, Pd, Re, Sb, Se, Te, U, Zn. The typical elements are Be, Pt, V at Tin Akoff and Ag, Ba, Hg, P, S at Pagala.
The principal mineralization of Pagala, which is zinciferous, has been rapidly considered as sedimentary and exhalative (Blot et Magat 1990 ; Deschamps et al. 1991), with at least 4 varieties of mineralization (Cortes et al., 2001). This Sedex type is considered as the probable form of the mineralizations of the Tin Akoff area (Castaing et al., 2003).
Key words : West Africa, weathering, gossans, sulfidic mineralizations, methodology for prospecting.