A FLUID INCLUSION STUDY OF THE SINCENI RARE-ELEMENT PEGMATITES OF SWAZILAND

Fluid inclusions were studied from two groups of pegmatite minerals. O ne (schorl, garnet, quartz, beryl) represents the main stage of crysta llization and the second (cassiterite, schorl-dravite, elbaite) formed in late-stage, mineralized units. Three fluid types were recognized. Type 1 fluid is aqueous and moderately saline with very low CO2 conten ts. It forms secondary or pseudosecondary inclusions in the main-stage minerals and primary inclusions in late-stage elbaite, schorl-dravite and cassiterite. Type 2 fluid is carbonic, mixed H2O-CO2, and it form s secondary inclusions in main-stage quartz and beryl which were trapp ed at the solvus conditions (about 325 degrees C), well below the pegm atite solidus. Type 3 fluid is aqueous, highly saline, and contains cu bic daughter salts. It occurs as secondary or pseudosecondary inclusio ns in main-stage quartz. Isochore trajectories and independent P-T inf ormation show that inclusions of type 1 fluid in the main-stage minera ls cannot be primary although many look to be so. Type 1 fluids were e xsolved at a late stage of pegmatite consolidation, forming secondary inclusions in main-stage minerals and primary inclusions in tourmaline and cassiterite from mineralized units. Evidence is inconclusive whet her type 2 and 3 fluids represent evolved type 1 fluid or are external fluids.