CATION RATIOS OF FLUID INCLUSIONS IN A GOLD QUARTZ VEIN AT BRUSSON, VAL DAYAS, NORTHWESTERN ITALIAN ALPS - COMPARISON OF BULK CRUSH-LEACH RESULTS WITH SIMS ANALYSES OF INDIVIDUAL INCLUSIONS

This study tests the widely made assumption that, with favourable fluid inclusion samples, bulk crush-leach analyses accurately reflect the electrolyte compositions of individual fluid inclusions. Secondary-ion mass spectrometric (SIMS) analyses of individual inclusions, calibrated by synthetic fluid inclusion standards, are compared to crush-leach analyses of the same natural quartz sample. Quartz from the Bechaz mesothermal Au vein at Brusson, northwestern Italian Alps, is dominated by fluid inclusions of uniform, low-salinity, low-X(CO2) composition. These inclusions represent the liquid phase of an originally immiscible hydrothermal system, whereas much rarer high-X(CO2) inclusions, of even lower salinity, represent the conjugate vapour phase. SIMS analyses of the low-X(CO2) inclusions yield mean molar K/Na = 0.050 and Ca/Na = 0.016. These results are identical, well within experimental uncertainty, to replicate crush-leach analyses made in two different laboratories. The excellent agreement between the SIMS and crush-leach results confirms the accuracy of both analytical methods. Despite the low salinity of the Bechaz fluid inclusions, and the rare muscovite and dolomite impurities in the host quartz, the crush-leach analyses are both highly precise and accurate. Furthermore, the agreement between methods demonstrates that SIMS analyses of natural, multi-component fluid inclusions may be reliably calibrated by synthetic inclusions with simple compositions. A SIMS analysis of a high-X(CO2) "vapour" inclusion yields K/Na and Ca/Na ratios that are not significantly different from those of the coexisting, low-X(CO2) inclusions. This suggests that, under the P-T conditions of immiscibility in the Bechaz vein (approximately 230-degrees-C and 600 bar), liquid-vapour fractionation of K and Ca, relative to Na, was only very slight.