3.1 Identification of gossans in ironcrust-bearing environment
First, identification of gossans in such environment was attempted, which permitted to moderate the affirmation of Routhier (1963) relative to impossibility of discrimination between lateritic ironcrust and gossan. That mistrust aspect in regard to secondary iron-bearing rocks, constitutive minerals even metal itself must be pointed out, since it has been relayed by some concepts of allochtony of iron components whose interest keeps more local than general. For that purpose, the results obtained on tropical weatherings during the sixties have been included as well as those on alteration of sulfide mineralization.
The principles of lithodependence and autochtony of weathered formations (Blot and Leprun. 1973 ; Blot et al., 1976 ; Zeegers and Leprun, 1979) allow to propose schemes, which have direct applications to identify alteration of peculiar rocks, i.e., the mineralized ones.
Genesis of gossans was considered as resulting from the whole history of sulfidic mineralizations alteration, during which all the steps of mineral filiations generate secondary iron minerals.
Consequently what was true for Routhier in 1963, is no longer such, but much is still to do to make habits evolve in order to consider the possible occurrence of a gossan in presence of a ferruginous rock (Pouit, 1987 ; Chauris and Garreau, 1990 ; Blot, 2004), with a more or less well-defined meaning (Kosakevitch, 1979 ; Butt and Smith, 1980 ; Ryall and Taylor, 1981 ; Oliveira at al, 1993 ; Imbernon et al., 1999). The thorough of gossans’ use to improve prospection (Wilhelm and Kosakevitch, 1979) also depends for the major part on a precocious diagnosis, such as it has been proposed. It is important that at Pagala and Tin Akoff diagnosis was laid down before any mining prospection or posteriorly to prospections without prospect of success. In that case the abundance of ferruginous outcrops in Central Togo and in North Burkina has got a significance only as the latter were considered as markers of mineralizations with sulfidic or carbonated origin. The ore reserves in true and false gossans, in situ or not, with authochthonous or allochthonous iron are important only on second degree. Since there is no real model of mineralization transformation in that zone, a essential pedagogic purpose might be to undertake a study of a well-known mineralization in order to form prospectors.
It can be remember that in the West-African zone, supergene weathering is so intense that only iron oxides and hydroxides are the memory of primary mineralizations.