2 Ferruginous rocks of Pagala (Togo) and Tin Akoff (Burkina Faso)
2.5 Chemical composition
At Pagala, there are numerous analyses of ferruginizations composition (more than 200 performed by ORSTOM) as well as of their environment (70 schists, micaschists, quartzites, breccias, carbonates …) which are sufficient to define and develop a “gossan” subject established from the filiations observed (Togbé, 1991). They did not fundamentally modify the simple picture proposed at a given time (Blot and Magat, 1989). Togbé showed also that the different facies were not characterized by significative composition differences. The analyses provided by prospectors, still more numerous, are semi-quantitative and are not complete enough to establish the composition firmly (around 1500 analyses performed for the BRGM, around 200 for Anglo American). In the composition peculiarities of the Pagala ferruginous rocks, the high contents in K2O are essentially related to the presence of white micas. The latter are maintained in the ferruginizations developed on schists in which they are abundant by definition as well as on carbonates in which aggressive conditions are weaker. Ti is in particular good correlation with the abundance of residual micas. This permits us to understand the high contents of K2O and TiO2 in analyses subsequent to our studies.
At Tin Akoff, there only about twenty compositional analyses of mainly goethitic ferruginous rocks. There is thus no statistical characterizations, which is frequent for the definition of gossans, in which only several useful elements of mineralizations are the most frequently analyzed. The other analyses (n=140) are semi-quantitative for major elements and incomplete.
Composition of Pagala and Tin Akoff gossans (median values)
These ferruginizations (table 18) have usual compositions of gossans developed on various sulfidic mineralizations (table 10). However, the rocky ensembles accounted for at Pagala and above all at Tin Akoff are not on the same scale as the others one. At Pagala, in an unique ensemble are represented weatherings developed on carbonates, on phosphates and on oxides at Tin Akoff where ascriptions are little defined, the formations developed on oxides (hematite-bearing jaspe and sandstone-hematitite) can already be discriminated from the others whose typology is non-defined.
The ferruginizations of Tin Akoff look silica-richer, whilst they are characterized by lower contents of alumina, manganese, potassium, titanium and hydroxyls. However, median and mean values are close to those of gossans known elsewhere.
Gossans s.s. (Kosakevitch, 1979) are by definition characterized by a predominance of iron accompanied by a most often free and abundant silica, whilst alumina usually displays very low content and may be substituted to iron in goethite and hematite. Manganese is rather frequent and often associated with iron. Ferruginizations of Togo and Burkina fit perfectly to this definition, whilst other composition references are very rare.