Origin of CO2-rich fluid inclusions in synorogenic veins from the Eastern Mount Isa Fold Belt, NW Queensland, and their implications for mineralization

Synorogenic veins from the Proterozoic Eastern Mount Isa Fold Belt contain three different types of fluid inclusions: CO2-rich, aqueous two-phase and rare multiphase. Inclusions of CO2 without a visible H2O phase are particul arly common. The close association of CO2-rich inclusions with aqueous two- phase, and possibly multiphase inclusions suggests that phase separation of low- to -moderate salinity CO2-rich hydrothermal fluids led to the selecti ve entrapment of the CO2. Microthermometric results indicate that CO2-rich inclusions homogenize between -15.5 and +29.9 degrees C which corresponds t o densities of 0.99 to 0.60 g.cm(-3). The homogenization temperatures of th e associated aqueous two-phase inclusions are 127-397 degrees C, with salin ities of 0.5 to 18.1 wt.% NaCl equivalent. The rarely observed multiphase i nclusions homogenize between 250 and 350 degrees C, and have salinities ran ging from 34.6 to 41.5 wt.% NaCl equivalent. Evidence used to support the p resence of fluid immiscibility in this study is mainly derived from observa tions of coexisting H2O-rich and CO2-rich inclusions in groups and along th e same trail. In addition, these two presumably unmixed fluids are also fou nd on adjacent fractures where monophase CO2-rich inclusions are closely re lated to H2O-rich inclusions. Similar CO2-rich inclusions are widespread in mineral deposits in this region, which are simply metal-enriched synorogen ic veins. Therefore, we argue that fluid immiscibility caused volatile spec ies such as CO2 and H2S to be lost from liquid, thus triggering ore deposit ion by increasing the fluid pH and decreasing the availability of complexin g ligands.