Interpretation of post-entrapment fluid-inclusion re-equilibration at the Three Mile Hill, Marvel Loch and Griffins Find high-temperature lode-gold deposits, Yilgarn Craton, Western Australia

Fluid inclusions that are interpreted to be related to vein filling of quartz veins at the Three Mile Hill and Marvel Loch gold deposits in amphibolite-facies rocks, and the Griffins Find deposits in granulite-facies rocks in the Yilgarn Craton of Western Australia are dominantly low-salinity H2O–CO2±CH4 mixtures or CO2–CH4 fluids. At each deposit, inclusions have a wide range of compositions with respect to both the aqueous:carbonic ratio and the CO2:CH4 ratio of the carbonic phase, and most fluids could not have been in equilibrium with the mineral assemblage in the vein and adjacent rock. Inclusion densities suggest a range of P–T conditions of entrapment, and are in general not consistent with the conditions of vein formation indicated by vein assemblages. Diffusional addition of H2 into inclusions, diffusional loss of H2O, and reduction of inclusion volume are possible during cooling and uplift along the inferred P–T path. The inclusion populations could have been derived from an originally uniform population of low-salinity, aqueous dominated H2O–CO2 inclusions by a combination of these processes. Inclusion modification as the cause of the complex inclusion populations is supported by relations of molar volume to composition, and, to an extent, by variations in fluid salinity. If inclusion re-equilibration is the cause of inclusion variability, it was of variable intensity within a single vein system, and within individual clusters of inclusions in some samples, and is suggested to have been a function of the local petrological and textural environment.