2 Ferruginous rocks of Pagala (Togo) and Tin Akoff (Burkina Faso)
2.1 The geological context
Pagala in central Togo belongs to the structural domain of Dahomeyides as well as to the sedimentary domain of the Volta river basin (Affaton, 1990). The major part of rocks are of detrital and clayey sedimentary origin, and this context was considered as unfavorable to metallogenesis (Bessoles and Trompette, 1980). However, occurrences of carbonaceous, carbonated, phosphated sediments are numerous as well as magmatic events (Godonou et al., 1986). The incidence of Panafrican orogeny is determinant in the post-sedimentary history and permitted to define in the Proterozoic basement and on the border of the Volta basin some structured units having been affected by east-west strains (Affaton, 1990). However, the results of systematic prospecting over adjacent or adjoining sectors (Lasserre, 1980 ; Lasserre et al., 1980) had led to the conclusion of absence of mining perspectives and to the interpretation of the ferruginous outcrops as cataclastic breceias s.s.
The area of Tin Akoff, on the border of the Taoudéni basin is essentially identified by its sedimentary history generating arenaceous and clayey detrital formations as well as carbonated ones (Delfour, 1965). Lithology displays numerous affinities with the formations of the Volta basin (Delfour and Jeambrun, 1970), and can be considered as a metallotecte favourable to sulfidic mineralizations (UNDP, 1974). Its post-sedimentary history for the time being is very ill-known in the orogenic context of Gourma (Reichelt, 1972).
On another scale, that of the subarea as defined in gravimetry, the sector of the Taoudéni basin where ferruginisations are identified corresponds to a strong negative gravity anomaly centered on Tin Akoff-Fadar Fadar up to Gao in the northeast and Djibo to the west. This anomaly is oriented ENE-WSW, on a 600 km length and 100 to 300 km width and is bordered in the south by a positive anomaly from Ouagadougou to Niamey and in the north by a positive anomaly from Mopti to Tichit. The relation between the anomalies of Fadar Fadar and of Ouagadougou-Niamey is similar as for intensity, contrast and dimension to those typical of the Dahomeyides chain, i. e. positive anomaly of the benino-togolese basement and negative anomaly of the eastern border of the Volta basin oriented SSW-NNE (Crenn, 1957). The negative anomaly of Tin Akoff-Fadar Fadar should correspond partly to a sedimentary basin oriented east-west after interpretation of the aeromagnetic survey (Terra Surveys, 1976, 1977).
The present-day climatic conditions are strongly contrasted, i. e. sudano-guinese climate in central Togo with mean annual rainfall up to more 1300mm distributed over 8 to 9 months and Sahelian climate in North Burkina with mean annual rainfall of less than 500mm and a long dry season of 8 months. It is likely that the present-day or past climatic differences play an essential role in the conditions of outcropping. At Pagala, under a cover of tropical forest and of coffee and cacao plantations, there is a steep topography, with compact reticular network and slopes usually between 30 and 50°. By contrast at Tin Akoff cover is at the most shrubby, often bare, outcrops of some decimetres or metres represent “relieves”, general slopes are of less than 1% without real network. So, ferruginizations at Pagala are young and have developed in a context of active erosion of an ancient and deeply weathered plateau. At Tin Akoff erosion does not directly affect the outcropping ferruginizations since it is not incisive but works through water table without well defined axes and with indefinite limits of watersheds. This results in young ferruginizations and strong leaching and erosion at Pagala, ancient leaching and ferruginization at Tin Akoff and these two different states are not quantifiable in their respective dynamics.