FLUID INCLUSION AND STABLE-ISOTOPE CONSTRAINTS ON THE GENESIS OF THE CLIGGA HEAD SN-W DEPOSIT, SW ENGLAND

Samples of veins from the Cligga Head granite-hosted Sn-W deposit, S.W . England, have been studied by microthermometry, and analysed for the oxygen-isotope composition of quartz, cassiterite and wolframite and the hydrogen-isotope composition of fluid inclusions. Samples of vein quartz were also analysed for fluid-inclusion chemistry using a crush- leach technique. Oxygen-isotope geothermometry using quartz-cassiterit e and quartz-wolframite pairs indicates temperatures of 350 to 400 deg rees C, although disequilibrium may be common. Coupled with fluid-incl usion homogenisation temperatures of between 300 to 350 degrees C the data suggests hydrothermal activity at 350 to 400 degrees C, and 0.7 t o 1.7 kbar fluid pressure. High Th in some fluid inclusions suggests t hat pressure may have dropped below 0.7 kbar during periods of vein di lation. The calculated oxygen- and measured hydrogen-isotopic composit ion of vein fluids (delta(18)O from 6.0 to 10.8%; delta D from -52 to -22%) is consistent with an origin as fluid that had equilibrated with granite at temperatures from approximately 500 to 300 degrees C. Crus h-leach analyses show the inclusion fluids to be Na-K-Fe-Ca chloride b rines with significant levels of B, F and SO4. The K/Na ratios are con sistent with equilibration with granite at progressively lower tempera tures. Log Br/Cl in the inclusion fluids ranges from -3.40 to -2.90, c omparable to the ratios of modem fumarole gases, and consistent with t he degassing of magmatic volatiles as the source of salinity.