Gossans in tropical areas : comparison of the ferruginizations from central Togo and North Burkina in Neoproterozoic formations.

1 Identification and interpretation of ferruginous rock in tropical environment.

1.1 Rock weathering

1.2 Weathering of sulfidic rocks

a Mechanisms and definition of alteration environments

b Transformations and mineralogical evolution

c Facies and microfacies

d From facies to outcrops

e Rich geochemical composition

f Storage of elements by iron oxides

1.3 Expression of sulfidic rocks in tropical zone

a Present – day gossans of Togo and Burkina

b Other occurrences of mineralizations weathering in tropical zones

c Consequences of mineralizations’weathering in the tropical zone of West Africa

1.4 Pertinence of geochemical prospection

a Definition of indexes

b Methodologies applied to indexes investigation

c Surficial formations

d Convergence between prospections and observations

In conclusion

2 Ferruginous rocks of Pagala (Togo) and Tin Akoff (Burkina Faso)

2.1 The geological context

2.2 The different objectives of investigation

2.3 Outcrops of ferruginous rocks

2.4 Mineralogical composition

2.5 Chemical composition

2.6 Geochemical study

2.7 Supergene metallogenic picture

2.8 Renewal of prospecting targets

3 Conclusions

3.1 Identification of gossans in ironcrust-bearing environment

3.2 Metallogenic potentialities of Neoproterozoic formations