Paleo-gossans within the lateritic iron crust: example of the nickeliferous prospect of Bonga, Burkina Faso

Abstract Bonga is a new example of nickel concentration developed by lateritic weathering of Paleoproterozoic ultramafic rocks. It is comparable with the residual nickel deposits of Cote d’Ivoire. However, within the lateritic iron crust at Bonga, massive to rubbly ferruginous nodules occur, most of which are highly nickeliferous (>7000 ppm). The lateritic iron crust itself however does not contain more than 300 to 400 ppm Ni. Their macroscopic facies (nodules), mineralogical and chemical compositions suggest that they could represent paleo-gossans. In the nodules, nickel is mainly associated with the poorly crystallized forms (plasma or amorphous phase) in goethite, maghemite and hematite. In the well-crystallized forms, nickel is mainly associated with maghemite and goethite, slightly more with the ferri-chromite, but not with hematite. With separation of goethite from the other lateritic phases (controlling by X-ray diffraction), and in regard to microprobe goethite analysis, a Ni-bearing pre-edge X-ray absorption spectrum of goethite sample shows an octahedral coordination for Ni, and the Mofissbauer experiment gives evidences of Ni incorporation in the structure of goethite.