Gossans in tropical areas : Comparison of the ferruginizations from central Togo and north Burkina in Neoproterozoic formations


Before the approach I proposed in the study of the peculiar ferruginous outcrops of Pagala (Blot et Magat, 1989) and Tin Akoff (Blot, 2002 a,b), successively in Togo and in Burkina Faso, the denominations were different according, to the observers, between ferruginous incrustations, ferruginous cataclastic breccias, ferruginous lateritic breccias, even laterites, the genetic and descriptive notions, although very distinct, being often mixed. In the Sahelian zone of Burkina, high metallic contents in Zn, Cu and Mo have been pointed out as related to some ferruginous facies, but giving iron a role of collector without special justification or assumption of a direct economic value for these rocks (UNDP, 1974). In Togo, no interest was given either to these surficial ferruginous rocks or even to ferruginous breccias identified during prospection (Lasserre, 1980). Now, these outcropping formations can be considered as gossans for a major part. This early diagnosis before any mine exploration was confirmed by regional or local prospections and drillings having cut mineralizations of Zn sulfides and carbonates. In Burkina, validation remains an objective.


In a first part, will be defined the bases of the applications to climatic specificity of the gossans in tropical zone.


In a second part different examples of ferruginisations in the Neoproterozoic formations of Togo and Burkina will be compared. Interferences between these both countries are obvious.


We must remind that ironcrusts and gossans are relictual supergene ferruginous rocks and it is very important to specify that this secondarity means that primary rock does not exist more. The formations considered as parent rocks are indeed the equivalents of a disappeared rock.