The Archaean of West Africa

Summary

The nucleus of the West African craton is characterised by the typical Archaean granite-greenstone association in which the ‘granite’ component (basement + intrusive granites) is dominant in areal terms. Metavolcanic greenstone lithologies form the lower parts of supracrustal sequences, particularly in the west, but become less important in the east where the sequences are thinner, higher metamorphic grades are typical and banded iron formations are a major component.

Most of the greenstone belts are synformal in character, with steeply dipping isoclinal folds, but low-angle folding and thrusting is developed in the Marampa Group of southwestern Sierra Leone and Guinea. A belt of granulite facies rocks (the Kasila Group) forms the boundary of the craton in this region. It contains dismembered portions of large gabbro-anorthosite bodies and is separated from the gneiss-migmatite basement by a wide zone of sheared and mylonitised rocks.

Radiometric age determinations suggest that the major thermotectonic event responsible for the present distribution of rock types was the Liberian (c. 2750 Ma), though there is inconclusive evidence of earlier events. It is also possible that the Marampa and Kasila Groups may be younger than the bulk of the ‘typical’ greenstone belts.

Iron ores are the major mineral deposits presently being exploited in Archaean rocks, especially in Liberia. Bauxite and fluviatile deposits of rutile are mined in Sierra Leone. There is also gold and some low-grade chromite, but only traces of base metal mineralisation have been found.