THE EVOLUTION OF PEGMATITE-HOSTED SN-W MINERALIZATION AT NONG SUA, THAILAND - EVIDENCE FROM FLUID INCLUSIONS AND STABLE ISOTOPES

The Nong Sua aplite-pergmatite complex contains two dominant styles of Sn-W-Ta-Nb mineralization. Cassiterite +/- Nb-Ta-Ti oxide minerals ar e disseminated in the pegmatite, and cassiterite and wolframite are ho sted by quartz-tourmaline veins which are contained solely within apli te. The orthomagmatic fluid at Nong Sua is preserved as primary fluid inclusions in the cores of magmatic garnet crystals that have high tin concentrations (garnet cores without fluid inclusions do not contain elevated tin concentrations). These fluid inclusions have a compositio n of 3 wt% NaCl eq. The low salinity suggests that, at vapor saturatio n, tin was partitioned in favour of the melt, which allowed cassiterit e to initially crystallize directly from the melt. Primary, pseudoseco ndary, and secondary fluid inclusions in cassiterite, tourmaline, and quartz record three-component mixing of the orthomagmatic fluid with h igh salinity aqueous and with CO2-rich fluids. The orthomagmatic water is interpreted to have had a delta(18)O value of +8.7 to +9.9 per mil and a delta D value of -72 to -78 per mil from delta(18)O analyses of muscovite and quartz, and delta D of muscovite. The delta(18)O compos ition of muscovite decreased from 10.1 to 8.0 per mil and delta D incr eased from -106 to -85 per mil, from the magmatic to the hydrothermal stages of pegmatite evolution. These changes are consistent with an in flux of metamorphic fluids or evolved meteoric waters. We consider tha t the saturation of the melt with vapor caused the pressure in the peg matite to rise to approximately 3.8 kbar, at a temperature of 650 degr ees C. Fluid overpressure caused the aplite to fracture, and veins to form from fluids which migrated into the fracture-induced low pressure zones. This event can be modeled by an isothermal decompression to 2. 7 kbar. Cassiterite deposition was probably controlled by increasing f O(2), whereas wolframite deposition resulted from the mixing of W-rich with Fe-Mn-rich fluids. In both cases decompression, cooling, and add ition of volatiles may have also contributed to mineralization.