Fluid inclusions with NaCl–CaCl2–H2O composition from the Cloncurry hydrothermal system, NW Queensland, Australia

Fluid inclusions whose composition can be approximated by the ternary system NaCl–CaCl2–H2O are common in the Fe-oxide–Cu–Au deposits, and associated alteration, in the Cloncurry hydrothermal system, NW Queensland, Australia. Detailed microthermometric work indicates that a multi-staged CaCl2-bearing fluid trapping was involved during the formation of mineralization and alteration. The fluid evolution history is characterized by early high temperature and low salinity to late low temperature and high salinity, which is distinct from many intrusion-related hydrothermal systems in the world. The ubiquitous presence of low-temperature (Th<200°C) CaCl2-bearing inclusions is a remarkable feature of the mid-Proterozoic Cloncurry hydrothermal system and it is interpreted to be a result of the interaction between evaporites and downward penetration of meteoric water. The transition from early magmatic and/or metamorphic fluids to late meteoric water is ascribed to the shift of the crustal level during exhumation. Despite the controversies over the mechanisms of mineralization, fluid mixing, demonstrated by the rapid drop in temperature through addition of cooler meteoric water into the hydrothermal system, must have played a significant role with regard to the metal precipitation in some deposits of this type.