Fluid inclusion and stable isotope constraints on the genesis of the Cligga Head Sn-W deposit, S.W. England
Smith, Martin; Banks, David A.; Yardley, Bruce W. D.; Boyce, Adrian
European Journal of Mineralogy Volume 8 Number 5 (1996), p. 961 - 974
published: Oct 30, 1996
manuscript accepted: May 2, 1996
manuscript received: Sep 22, 1995
ArtNo. ESP147050805007, Price: 29.00 €
Abstract 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-cassiterite and quartz-wolframite pairs indicates temperatures of 350 to 400°C, although disequilibrium may be common. Coupled with fluid-inclusion homogenisation temperatures of between 300 to 350°C the data suggests hydrothermal activity at 350 to 400°C, and 0.7 to 1.7 kbar fluid pressure. High Th in some fluid inclusions suggests that pressure may have dropped below 0.7 kbar during periods of vein dilation. The calculated oxygen- and measured hydrogen-isotopic composition of vein fluids (δ18O from 6.0 to 10.8‰; δD from -52 to -22‰) is consistent with an origin as fluid that had equilibrated with granite at temperatures from approximately 500 to 300°C. Crush-leach analyses show the inclusion fluids to be Na-K-Fe-Ca chloride brines with significant levels of B, F and SO4. The K/Na ratios are consistent with equilibration with granite at progressively lower temperatures. Log Br/Cl in the inclusion fluids ranges from -3.40 to -2.90, comparable to the ratios of modern fumarole gases, and consistent with the degassing of magmatic volatiles as the source of salinity.