Fluid mixing versus unmixing as an ore-forming process in theCloncurry Fe-oxide-Cu [sbnd]Au District, NW Queensland, Australia: evidence from fluid inclusions

Abstract

Fluid mixing and/or unmixing (including boiling) are thought to be important mechanisms of mineralisation in copper-golddeposits. 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 CO 2- 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 CO 2, which was believed to result from an unmixing of a magma-derived H 2O CO 2 NaCl ± CaCl 2 fluid (see [Miner. Depos. 36 (2001) 93] and references therein), is rather complex as some hypersaline brine inclusions obviously predate CO 2 inclusions.