2 Ferruginous rocks of Pagala (Togo) and Tin Akoff (Burkina Faso)
2.8 Renewal of prospecting targets
Included in the Volta and Taoudeni basins, Pagala and Tin Akoff have been and keep at least partly excluded a priori from metallogenic studies and investigations without any justification but a formalistic postulate according which the more ancient formations have priority. This restriction was partially adjourned (Godonou et al., 1986 ; Castaing et al., 2003), but from the author’s point of view, the abundance and variety of surficial indexes are to related in a coherent ensemble of environments with polymetallic mineralizations. When the studies of geodynamic frameworks will be more advances – studies a posteriori as related to the study of indexes – whatever the economic interest, it will be possible to define their logics.
A first original aspect as well in Togo as in Burkina is that ferruginizations were defined as a mineralization signal, anteriorily to any mining prospection, with the purpose of academic investigations on the possibilities of application of weathering formations. In both cases, the polymetallic image of ferruginous rocks is clear together with the very certain occurrence of several mineralizations, but supergene history is still too indefinite to go further. It must be noted that the statistical geochemical hierarchization of the gossans of Pagala in 12 families, as proposed by Deschamps et al. (1991), did not lead to any diagnostic perspective, since founded on mixing samples with different weathering rates. The metallogenic hierarchization of Theron (2002) is more accurate since the latter little better adjusts gossans to the combination of occurrence and geochemistry. The only studied model which is that of formation of a gossan from a zinciferous siderite (Togbé, 1991) suffers of absence of comparison with the weathering of the other mineralization types.
It must be reminded that the nature of ferruginizations was defined out of any mining investigation in North Burkina as previously in Togo. Moreover, an identification of the mineralization signals preceded the knowledge and understanding of the geodynamic context, which should involve changes in prospecting methods and strategy.
The second original aspect is that both examples are situated outside of the zones usually considered as appropriate for metal mining in the Neoproterozoic cover formations. Those series represent one of the possible futures for metal mining in West Africa and there Lavoisiers’s principe may take another significance, as follows : “dans la nature rien ne se perd, rien ne se crée, tout se transforme” (in nature, nothing is lost, nothing is created, all is transformed) and the author would add “all is transported”. In fact, he thinks he has brought some examples on the eastern border of the Voltas’basin involved in the Panafrican chain of the Dahomeydes at once for zinc and for phosphore and that was not yet testified in North Burkina for the border of the Taoudéni basin involved in the Panafrican chain of the Gourma. In that region where the geodynamic context is still badly known, all remains possible, if the complexity of geochemical anomalies of ferruginous rocks for some forthy elements is considered.