Gold-telluride mineralisation in the Birimian at Diabatou, Burkina Faso: the role of CO2-N-2 fluids

Gold-telluride bearing quartz veins at Diabatou occur in rocks of the Palaeoproterozoic Birimian Supergroup of Burkina Faso. The main Au-telluride mineralisation and wallrock alteration occurred at temperatures between 300 and 380 degrees C and pressures of about 3-5 kbar. The Au-telluride-bearing quartz veins predominantly contain (>90%) CO2-N-2 fluid inclusions, some of which display densities of up to 0.99 g cm(-3), much in contrast to other lode-Au deposits of greenstone affiliation. The highest density isochores match the P-T conditions of the mineralisation. CO2-N-2 inclusions and Au-telluride mineralisation occur in all investigated samples, while aqueous inclusions are almost absent. Therefore, the CO2-N-2 inclusions at least represent residues of mineralising fluid, although the fluid composition calculated from thermodynamic modelling of wallrock mineral assemblages disagrees with the fluid inclusion composition. Post-entrapment modifications due to grain-boundary migration recrystallisation during deformation are suggested to be responsible for the formation of most CO2-N-2 inclusions. (C) 1997 Elsevier Science Limited.