Gossans in tropical areas 

1 Identification and interpretation of ferruginous rock in tropical environment

Ferruginous rocks are so common in West Africa, that they are considered, by geologists and prospectors, as “obstacles” to observation of the other rocks. This predominance of “climatic” ferruginous rocks makes difficult, in fact, the identification of other types of ferruginous rocks, particularly in the case of weathered sulfidic rocks, which are very often ferriferous.


The proportions of sulfidic mineralizations are tiny as compared to the whole of the rocks, since the latter are principally composed of silicates, especially magmatic rocks, almost all of the metamorphic rocks and 80% of sedimentary rocks and carbonated rocks represent 20% of sedimentary rocks and 1% of metamorphic ones. The others, for example phosphatic and sulfidic ones are very less represented, but as they provide incomparably higher perspectives of industrial applications, this gives those rocks a particular interest.


Therefore weathering of silicated rocks is well-known and generates the physiography of the principal landscapes on a global scale. The weathering of sulfidic rocks with its specific features and mechanisms is well-known too, due to its possible economic potential. However, these two domains are rarely confronted which results in fact that their respective teachings are not crossed. And Routhier (1963) emphazised the typical ambiguity of the tropical zones, where the discrimination between ironcrusts and gossans is impossible, whereas the stake is important in metal prospection.


The specific criteria of diagnosis for ironcrusts and gossans, which have changed since the pre-industrial era are discriminated and represented on table 1.


Table 1

Stakes of climatic ferruginous rocks for mining resources


The conjonction of tropical weathering and alteration of sulfidic rocks can be approached after different recalls upon these two types of processes. Last general conclusions will be inferred upon the possible consequences in the prospection of sulfidic mineralizations in tropical zone, at least in West Africa.