GEOLOGICAL EVOLUTION OF SELECTED GRANITIC PEGMATITES IN MYANMAR (BURMA) - CONSTRAINTS FROM REGIONAL SETTING, LITHOLOGY, AND FLUID-INCLUSIONSTUDIES

Pegmatite deposits commonly occur in the 1500 km long, N-S-trending, t ungsten-tin-bearing granitoid belt in Myanmar. Pegmatites are emplaced as veins and dikes that cut granitoid, migmatite, granitoid gneiss, g neiss, and schist. The pegmatite veins and dikes are mostly 2 to 5 met ers wide and 30 to 150 meters long, and some are traceable over a dist ance of 300 meters. The pegmatites are composed of quartz, orthoclase, albite, microcline microperthite, and muscovite, with minor biotite, tourmaline, beryl, garnet, topaz, lepidolite, magnetite, wolframite, c assiterite, and rare columbite. They are commonly zoned, feldspars and muscovite being more abundant in the center and quartz more common at the margin. The zoning pattern is rather distinct in the pegmatite bo dy, where tourmaline is present. The light-colored felsic minerals are confined to the core zone and the dark-colored tourmaline crystals to the outer zone. Numerous fluid inclusions have been found in quartz, topaz, and beryl. Most of the inclusions are rounded to elliptical, wi th a variable degree of liquid filling. All inclusions are aqueous, tw o-phase (liquid and vapor) inclusions with no daughter minerals. Homog enization temperatures of 173 fluid inclusions were measured in this s tudy. Geothermometric studies indicate that the pegmatites were formed over a homogenization temperature range of 230 degrees to 410 degrees C. Salinities of fluid inclusions in pegmatite minerals yielded from 1.0 to 10.8 NaCl equiv, wt%. Topaz and quartz single crystals (several cm across) from the Sakangyi pegmatite provide an opportunity to extr act the fluids trapped in these minerals. The Na/K ratios of the fluid inclusions in two topaz samples were 3.0 to 4.9, and those of two qua rtz samples were 2.9 to 10.5, suggesting the presence of substantial p otassium in the pegmatite-forming fluids. In this study, evidence for phase separation of the pegmatite-forming fluids was not observed. The post-magmatic, hydrothermal fluids responsible for the pegmatite vein s evidently emanated from cooling S-type granitoids, with which they a re spatially associated.