FLUID INCLUSION EVIDENCE FOR A SUPERCRITICAL MAGMATIC FLUID, MODIFIED BY WALL-ROCK INTERACTION AND MIXING WITH METEORIC WATERS

Abstract

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. %.