Fluid mixing versus unmixing as an ore-forming process in the Cloncurry Fe-Oxide-Cu-Au District, NW Queensland, Australia: evidence from fluid inclusions

Fluid mixing and/or unmixing (including boiling) are thought to be important mechanisms of mineralisation in copper-gold deposits. Detailed fluid-inclusion studies of regional sodic (-calcic) alteration and local mineralisation in the Cloncurry Fe-oxide-Cu_Au District, NW Queensland, suggest that both fluid mixing and unmixing occurred in these giant mineralised hydrothermal systems. In some cases, the primary character of coexisting multisolid, hypersaline brine inclusions and CO2- or vapour-rich inclusions, the latter crosscut by late Ca- and Na-rich fluid inclusions, indicate that fluid mixing probably occurred subsequent to fluid unmixing and finally resulted in Cu_Au mineralisation. However, the relationship between hypersaline brines and CO2, which was believed to result from an unmixing of a magma-derived H2O_CO2_NaCl ± CaCl2 fluid (see [Miner. Depos. 36 (2001) 93] and references therein), is rather complex as some hypersaline brine inclusions obviously predate CO2 inclusions