STRATIFORM TOURMALINITES WITH DRAVITE OF COLLOID ORIGIN IN THE PALAEOPROTEROZOIC IN SENEGAL AND MALI

In the Palaeoproterozoic (2.1 Ga) rocks from Diale-Dalema, lenses of t ourmalinite (dravite) occur in felsic volcaniclastics. They characteri se a partially emerged N-S trending area. Dravite rich rocks were freq uently eroded and redeposited, forming intraformational conglomerates. They are associated with paralic metasediments, which are partly evap oritic: scapolite marbles and graphitic horizons. The study of the mic ro-and ultrastructures shows some of the dravite to be syngenetic, as it was originally deposited as a colloid. Boron is probably of hydroth ermal origin. Upon reaching the surface, the B-rich fluid probably col lected in depressions of palaeorelief near the shore. it was accompani ed by silica, alumina and ferrous Fe (anoxic atmosphere). From rare ea rth studies, and the palaeogeographical relations between the paralic deposits and the tourmalinite lenses, Na-Mg-rich waters of marine orig in met these solutions. This mixture formed the dravite: Mg2+ is more abundant than Fe2+. This characterises the borosilicates of both exhal ative and sedimentary origin. (C) 1997 Elsevier Science Limited.