NEW-TYPE OF GOLD ORE DEPOSIT IN THE BIRIM IAN GREENSTONE BELTS OF BURKINA-FASO - ALBITITES OF LARAFELLA

A new occurrence of a Au ore deposit (Larafella prospect, Poura distri ct) is defined within lenses of albitites (5-8 m thick) interbedded wi th metasediments and metavolcanites of the Birimian greenstone belts o f Burkina Faso (West Africa). Albitites have an homogeneous chemical c omposition close to albite and a well-developed microgranular texture. In contrast to the highly strained enclosing host metavolcanic format ions, which possess a strong penetrative cleavage, the unfoliated albi tite lenses exhibit only brittle structures, which are essentially ten sional gashes filled by quartz and calcite fibres, unorientated euhedr al albite crystals, quartz and calcite crystals and geared quartz crys tals. The strict delineation of the Au anomaly within the albitic lens es, together with the temporal relationships between Au precipitation and tensional gashes, suggest that albitites - due to their high visco sity and their brittle behaviour - constitute a potential reservoir fo r the concentration of CO2, S, H2O, As and Au fluids. (C) 1997 Elsevie r Science Limited.