ABSTRACT

The Junction orogenic gold deposit in the St Ives district of Western Australia is characterized by a syn-gold mineral assemblage dominated by quartz-calcite-albite-biotite-chlorite-pyrrhotite. The deposit had a light hydrocarbon,CO2, and O2 soil-gas signature above known mineralization prior to mining and it has been proposed that the source of the light hydrocarbon gases in the soil was gas-rich fluid inclusions trapped in gold-related alteration minerals, particularly calcite, albite, quartz, and pyrrhotite. Linking the soil gases with those in the deposit is extremely difficult. However, establishing that the gases in the soil are indeed present within the deposit and that those gases are related to the syn-Au alteration minerals is achievable through stable-isotope studies. Carbon and O stable-isotope compositions of pre-gold, syn-gold, and post-gold quartz veins; syn-gold and post-gold calcite; and CO2 and CH4 in the fluid inclusions that each of these minerals host were investigated to establish if the various mineral and fluid-gas species in the deposit are in isotopic equilibrium with each other, an important first step to relate syn-ore minerals with the relevant gases.

Pre-ore Mo-type quartz veins contain CO213Cgas = –1‰) and CH413Cgas = ca. –33‰) in fluid inclusions at a ratio of ca. 93:7. The paucity of Mo-type quartz veins in the deposit suggests that these veins were not the main source of the soil-gas signature. Syn-gold alteration post-dates the Mo-type quartz veins. Quartz and co-existing calcite in the Au-bearing Junction shear zone have δ18Omineral values around 12.0 and 10.5‰, respectively. Multiple co-existing quartz-calcite pairs indicate that gold deposition occurred at ∼400 °C. This temperature agrees with mineral equilibria temperature estimates, the entrapment temperatures of fluid inclusions, and temperature modelling of solid-solution mineral phases. The temperature dictates that the quartz and calcite are in isotopic equilibrium with each other.

The calcite in the Junction shear zone has δ13Cmineral values from –7.4 to –2.5‰, indicating that the CO2-rich ore fluid had a δ13Cfluid value of –3.7 ± 0.9‰. CO2 and CH4 released from quartz-hosted fluid inclusions have δ13Cgas values from –4.3 to +3.5‰ (mean = –1.5 ± 1.9‰) and –50.5 to –35.2‰, respectively. The isotopic composition of the fluid inclusion CO2 is in disequilibrium with co-existing CH4 that was co-released from the same quartz vein and the calculated δ13Cfluid value from co-existing calcite. Isotopic mass balance calculations using the two co-released gases show that the CO2 was initially in equilibrium with the syn-ore calcite but has since re-equilibrated with CH4 at temperatures below 200 °C. The abundance of CH4 in some quartz veins suggests that the syn-gold vein assemblage could be the source for the soil-gas anomaly.

Post-gold veins contain quartz and calcite that have δ18Omineral values of ca. 11.0 and 10.0‰, respectively. Individual mineral pairs indicate precipitation at ∼320 °C from a fluid with a δ18Ofluid value of 4.7 ± 0.9‰, distinct from that which formed the syn-gold quartz veins. The post-gold calcite has δ13Ccalcite values from –7.5 to –5.4‰, indicative of formation from a CO2-bearing fluid having a δ13Cfluid value of –4.6 ± 0.9‰. The δ13Cfluid values are indistinguishable from fluid inclusion CO2 values of –3.6 ± 0.9‰, indicating no post entrapment re-equilibration, which suggests that CH4 was at trace volumes or absent in the post-gold quartz veins.

These data lead to the conclusion that post-entrapment reequilibration between fluid inclusion CO2 and CH4 has occurred, but that the two gases were likely in equilibrium at the time of entrapment. This has implications for the interpretation of C isotope studies that focus on fluid inclusion CO2 measured from other gold and base-metal deposits, especially when the isotopic value of that CO2 is assumed to represent a specific source for the ore-forming fluids. The data also lead to a model that proposes that the syn-gold alteration assemblage could have produced the soil-gas anomalies observed above the mineralization.

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