CHARACTERIZATION AND PETROGENETIC SIGNIFICANCE OF CO2, H2O AND CH4 FLUID INCLUSIONS IN APATITE FROM THE SUKULU CARBONATITE, UGANDA

Microthermometry and laser Raman microprobe results show that aqueous inclusions in two generations of apatite from the Sukulu carbonatite, Uganda, consist essentially of three groups: CO2-bearing, moderate to high-salinity H2O-rich and CH4-bearing. The CO2- and CH4-bearing inclu sions, in general are never present together in individual apatite cry stals. It is considered that these compositionally discrete inclusions represent different fluids trapped during different stages of apatite crystallisation. The CO2-bearing fluid might have been formed from an originally H2O-rich fluid containing significant CO2 by immiscible se paration under high pressure and temperature. This precursor H2O-CO2 f luid was probably derived from a carbonatite melt, also by a possible process of liquid immiscibility. The CH4-bearing inclusions were proba bly formed by late-stage hydrothermal processes under different P-T co nditions. P-T-X isochores calculated for each type of fluid indicate t hat the evolution of the fluids in the system might be from a CO2-bear ing, through moderate to highly saline, to CH4-bearing stages, under p hysical conditions varying from high, through moderate P and T, to low P. This trend may be applied in interpreting the evolution of the car bonatite from a deep magmatic (carbonate melt) environment towards a s hallow level hydrothermal system. The three types of fluid inclusions, together with Mg-calcite inclusions in the apatite crystals, indicate that the compositions of the fluids associated with the Sukulu carbon atite complex appear to have evolved chemically from a Mg-bearing calc ite melt, through aqueous CO2-bearing and bicarbonate-rich melts (NaHC O3 daughters) to a final aqueous CH4-bearing fluid.