Evidence for fluid phase separation in high-grade ore zones at the Porgeragold deposit, Papua New Guinea

Coexisting, liquid-rich and vapor-rich primary fluid inclusions in quartz p rovide direct evidence for fluid phase separation in high-grade quartz-rosc oelite-gold veins and breccias from the Porgera alkalic-type gold deposit. Vapor-rich fluid inclusions are CO2-rich, and sometimes contain liquid CO2 at room temperature. The close spatial and paragenetic relationship between these "boiling assemblage" fluid inclusions and gold suggests that gold wa s precipitated by phase separation, at least locally. Additionally, the occ urrence of carbonate and sulfate minerals in high-grade veins (reflecting p H increase and oxidation of the boiled fluid) and the appearance of hydroth ermal breccias, are consistent with the process of fluid phase separation. Liquid CO2-bearing fluid inclusions are rare in near-surface epithermal dep osits, and indicate that the Porgera vein system was formed at greater dept hs and pressures (our estimates suggest pressures between 250 and 340 bars) . It is suggested that alkalic-type gold deposits may be distinguished from other epithermal deposit types by the more gaseous nature of the ore-formi ng fluids, in addition to their association with alkalic magmas.