Early, methane-rich fluids and their role in Archaean gold mineralisation at the Sand King and Missouri deposits, Eastern Goldfields Province, Western Australia

The Sand King and Missouri lode gold deposits in the Siberia district of the Eastern Goldfields Province are situated within an amphibolite-facies metamorphic aureole in deformed greenstones flanking a monzogranitic batholith, which crops out less than 1 km to the west. The Missouri deposit was mined from 1899 to 1912 and again in 1987-1988 and produced approximately 921 kg of gold. The Sand King mine has produced about 4000 kg of gold since 1980. Gold occurs in the margins of the quartz (-biotite-pyrite) veins and in the alteration assemblage adjacent to the vein systems. Fluid inclusions in quartz and carbonate veins from both deposits have been studied by microthermometry and Raman spectrometry and have been classified into six types according to their chemical composition. The vapour phase of Type I inclusions contains pure or nearly pure CH4 and the liquid-rich inclusions which appear least likely to have leaked contain up to 5 mole % CH4 and 95 mole % H20. Rare Type II inclusions are multiphase and contain approximately equal amounts of CO2 and CH4 in the vapour phase. They may also contain up to four solid phases, of which muscovite and carbonate have been identified by Raman spectroscopy. Type III inclusions may also contain a muscovite daughter crystal, but have highly variable vapour contents and CO2/CH4 ratios. Type IV inclusions are CO2-rich and may be either monophase or occur as liquid + vapour CO2 (±H2O) at room temperature . Type V inclusions are aqueous inclusions with an average salinity of 5.8 equiv. wt.% NaCI, and Type VI are high salinity inclusions (-27 equiv. wt. % CaCI2) that may contain halite and two other unidentified solids. The textural evidence suggests that Type I inclusions were trapped in the early phases of mineralisation at both the Sand King and Missouri deposits, around 500 to 600°C and 3-4 kbar. The fluid inclusions also provide evidence for mixing of the CH4-rich Type I fluid with a CO2-rich fluid. The laller may be related to a pervasive synmetamorphic fluid responsible for regional carbonation of the deformed metavolcanic rocks. Heterogeneous trapping is thought to be largely responsible for the variation in vapour content and CO2/CH4 ratio in inclusion Types II and III. The presence of muscovite as a daughter mineral in Types II and III links them to a potassic alteration event which is evident at both deposits. Type IV inclusions contain lillIe or no detectable CH4, which indicates that CO2-rich fluids continued to circulate after most of the CH4 and H2 in the system had been consumed. The aqueous Types V and VI inclusions are thought to represent, respectively, late influx of meteoric waters and connate brines. The early methane-bearing fluids indicate low fO2 conditions, which may have been generated during the serpentinisation of nearby ultramafic rocks or, alternatively, could have been derived from a deep source in the crust or lower mantle. The addition of methane to the CO2-rich ore-bearing fluid by fluid mixing greatly increases the field of fluid immiscibility over that of the conventional CO2-H2O-NaCI system. Thus, the fluid unmixes over a larger range of P-T conditions when methane is present and the sulphur co ntent of the fluid diminishes as H2S is strongly partitioned in the vapour phase during subsequent phase separation. These effects destabilise the auriferous sulphide ligands in solution and, hence, trigger gold precipitation. They may thus be important factors controlling gold precipitation under greenschist and amphibolite facies conditions.