2 Ferruginous rocks of Pagala (Togo) and Tin Akoff (Burkina Faso)
2.7 Supergene metallogenic picture
The different geochemical associations, specific of the mineralization models (Andrew-Jones, 1968 in Levinson 1974 ; Butt and Smith, 1980 ; Ryall and Taylor 1981) cannot be identified precisely in gossans. These are neogenetic formations belonging to supergene domain and some important elements detected may have been redistributed by weathering up to complete removal.
In ferruginious rocks of Pagala and Tin Akoff, the enriched elements are numerous, which permits a polymetallic approach of the whole. But mineralizations likely originate from different and complex birth places, even if the main mineralization of Pagala has been rapidly considered as sedimentary and exhalative, displaying at least 4 mineralization varieties (Blot and Magat, 1990 ; Deschamps et al., 1991, Cortes et al., 2001). This Sedex type is considered as the probable type of mineralizations of the Tin Akoff area (Castaing et al., 2003). The major part of the ferruginizations, in both regions, testify to the association of hydrothermal sulfides, principally of low temperature and of complex sulfides. But as those are residual weathering formations, there are neither good models nor good examples, inasmuch the evolution rate in not the same everywhere and the selective mobility of element gives biased images of ground truth. If refering to the Clarke for the highest contents in a mean sampling of 10% of each population, a hierarchization of the significant assemblages for 25 standard mineralization types can be proposed (Andrew-Jones in Levinson, 1974). The only associations of hydrothermal or sedimentary sulfides give likely signals, excluding at once magmatic or metamorphic associations.
It can be noted that this approach by means of quite empirical calculation from the Clarke of 39 elements taken as markers, permits to seek back to genetic environments, well-known or not. The ferruginizations of Togo and Burkina display many similarities but also specific features. The most typical standard associations concern low-temperature sulfides and complex sulfides. More specifically, in the Sahel of Burkina, the best recognized associations are a hydrothermal association of “precious metals” type and another of “red beds” type, with more sedimentary than metamorphic aspect. At Pagala the associations of basic and precious metals, of hydrothermal type are noticeable.
Common to Pagala and Tin Akoff
- Low-temperature sulfides
- Complex sulfides
- Precious metals
- Basic metals
Specific Tin Akoff
- Continental red beds
If compared with same criteria, the gossan of Perkoa – ore-deposit of massive Zn sulfides appears first as representative of an association of low-temperature sulfides and second of a precious metals environment, whilst Bonga – lateritic Ni ore-deposit – is obviously typical of association of lateritic type.
At Tin Akoff, the study of the different silicification generations associated pp with ferruginization indicates genesis temperatures between 70 and 170°C, with fluids of diverse origins (Miningou, 2006). These results are consistent with the supposed mineralization types in North Burkina.