Petrogenesis of rare-element pegmatites in the Olary Block, South Australia, part 2. Fluid inclusion study

Abstract

Fluid inclusions were investigated in quartz, beryl, apatite and triplite from the border and intermediate zones and core of pegmatites within the Proterozoic Olary Block, South Australia. Three compositionally distinct types of inclusions were recognized including pure CO2 inclusions, mixed H2O-CO2 inclusions, and aqueous inclusions with some of them containing a solid phase. Three fluid events occurred during pegmatite formation and subsolidus alteration. Initial fluids are characterised by a low to intermediate salinity (4.1 to 23.4wt% NaCl equivalent), and a composition of about 10 mole% CO2, 4.2 mole% NaCl equivalent, and 85.6 mole% H2O. Fluids were trapped as homogeneous H2O-CO2 phases. The second pulse of fluids was of intermediate to high salinity at 11 to 33 wt% NaCl equivalent. These fluids were most likely trapped as separated CO2 and H2O phases. Finally, intermediate to high salinity fluids of post-pegmatite origin with approximately 15 to 30 wt % NaCl equivalent were introduced. The P-T regime for the three fluid events has been estimated at 520° to > 650 °C and 2 to 5 kbars, 400° to 650 °C and 1.8 to 3.3 kbars, and 380° to 480°C and 2.0 to 2.6 kbars, respectively. These conditions indicate a declining pressure path implying a tectonic uplift of the Olary Block during successive fluid emplacements.