Fluid Inclusion Evidence for a Supercritical Magmatic Fluid, Modified by Wall-rock Interaction and Mixing with Meteoric Waters

Supercritical fluids derived from magmas may have different cooling paths depending on the confining pressures. Shallowly emplaced magmas generate supercritical fluids, which, during cooling form hypersaline fluid and vapor. Supercritical fluid derived from deeper emplaced magmas does not undergo phase separation and the fluids cool below the solvus curve to form a single aqueous-dominated phase. This paper presents an example of the latter in which a magmatically-derived fluid entered a metal deposit. The magmatic fluid was trapped as quartz-hosted fluid inclusions at ~8 km depth. Abundant primary fluid inclusions are trapped within vein and ore quartz. Fluid inclusion microthermometry, gas analysis using a quadrupole mass spectrometer, and bulk crush leach methods were applied to establish the fluid chemistry trapped within these inclusions. Microthermometry shows that the source fluids have salinities that range from 6-10 eq. wt.% NaCl. Conditions during trapping were around 300°C and 2 kbar pressure, corresponding to ~8 km depth under lithostatic conditions. Gas analysis results show that ore fluids have up to 8 mol. % CO2 for individual crushes whereas the average is 5-6 mol. % CO2. Methane is typically 0.5-1 mol. % and CO2/CH4 is generally 10. Ratios of N2/Ar range from 400-5000, which, are typical magmatic numbers. The H2S content averages 0.013 mol. % whereas some analyses gave values up to 0.03 mol. %. Fluids that have reacted with the carbonate host rocks exhibit changes to the fluid chemistry typified by the reaction: H2CO3 + CaCO3 = Ca + 2HCO3 Microthermometry measurements show an increase in salinity >20 eq. wt. % NaCl, driven by the addition of Ca. Ratios of Ca/Na measured show that Ca>Na and that a “missing” component (assumed to be bicarbonate) is highest in Ca-rich samples. Gas analysis shows that CO2 and H2S vary systematically as CO2 decreases one order of magnitude, the concentration of H2S decreases more than two orders of magnitude. This variation in CO2 and H2S is attributed to wall-rock reactions wherein CO2 dissolves limestone and H2S reacts with iron liberated from carbonate to form pyrite. Gold solubility in the ore fluid is calculated at ~200 ppb whereas fluids that have reacted with the wall rocks have a maximum gold solubility <2 ppb. Calcite was deposited after quartz. Fluid inclusion barometry indicates that subsequent to gold mineralization there was a change from lithostatic to hydrostatic conditions. Microthermometry shows that the calcite inclusion salinty is ~3 eq. wt. % NaCl. Gas analysis confirms that fluid gas content decreased by one order of magnitude and the N2/Ar ratio decreased to <100. The data indicate that the late calcite was deposited by an inflow of cooler, dilute meteoric waters presumably as a consequence of fracturing and pressure drop. Calcite deposition is consistent with fluid heating.