Methane-rich fluid inclusions in the Proterozoic Zn–Pb–Ag deposit at Dugald River, NW Queensland: potential as an exploration guide

Calculated isochores, determined from microthermometry and laser Raman microprobe analysis, for CH4 inclusions hosted in quartz grains from 3 different ore types in the Dugald River Zn–Pb–Ag deposit yield fluid trapping P–T conditions of 300–350°C and 0.8–2.3 kbar for post-depositional metamorphic conditions of sulfide mineralization. The consistent relationship between fluid composition and metamorphic grade of the host rocks suggests that CH4 has been formed by reaction of graphite with aqueous fluids during metamorphic retrogression. Even though fluids responsible for mineralization are generally similar to those for barren quartz veins in chemistry and microthermometry, the substantial contents of CH4 and CO2 in the Dugald River lode inclusions contrast sharply with CO2 alone in barren vein inclusions. The presence of CH4-rich inclusions in the mineralization at Dugald River and other similar deposits, and their absence in the barren veins in a regional context, can be regarded as a possible exploration guide for defining target areas hosting mineralization of this style.