2 Ferruginous rocks of Pagala (Togo) and Tin Akoff (Burkina Faso)
2.2 The different objectives of investigation
Between both regions, the observation scales were very different, since the objectives are also different.
At Pagala, the ferruginous body as end weathering product was the centre of the first investigations using scales of 1/100 to 1/5,000. Regional extension remained rather marginal but however brought some new cases i.e. other types of ferruginization. But the sector denominated “of Pagala” considered as interesting is restricted to some tens square kilometers.
The objectives and methods used in further mining prospections were of course, very different since they did not take in account the results obtained, i. e. the occurrence of gossans in the Pagala sector. These prospections permitted the BRGM to show that over 5,000 km2 prospected on 1/200,000 scale, the major part of gossans extended up to 80 km2, centered on the sector previously discovered. Moreover, the prospections conducted afterwards on mineralizations by Coronation and Anglo American permitted to confirm the validity of ferruginizations in so far as gossans developed from sulfidic and sideritic rocks.
At Tin Akoff, where the objective was a tentative approach of gossans identification (Blot, 2000 a and b, 2004) the search of distribution of all the ferruginous bodies was privileged. More than 5000 km2 prospected on scales of 1/50,000 to 1/500,000 and especially conducted by the mapping of siliceous breccias (Delfour and Jeambrun, 1970) led to conclude to a regional extension of the ferruginous bodies occurrences.
From the different samplings it results that if parent rocks seem to be much more diverse in Burkina than in Pagala s.s. – in considering the diversity of high metals contents – this could be due to a scale effect.