Noble gas and halogen constraints on regionally extensive mid-crustal Na–Ca metasomatism, the Proterozoic Eastern Mount Isa Block, Australia

Abstract

Fluid inclusions in late-Isan quartz veins associated with regional Na–Ca alteration (albitisation), in the Mary Kathleen Fold Belt and the Cloncurry District of the Eastern Mt Isa Block, have been analysed for naturally occurring and neutron produced isotopes of Ar, Kr and Xe. The noble gases have been extracted using a thermal decrepitation procedure that enables partial deconvolution of the different fluid inclusion types, including variably saline aqueous, liquid carbon-dioxide and mixed aqueous-carbonic varieties. The variably saline (<5–65 wt%) aqueous fluid inclusions dominate and have 40Ar/ 36Ar values of less than 2700 in most of the samples from across the region. These fluid inclusions have extremely variable molar Br/Cl values of 0.3–4 × 10 −3 and I/Cl values of 0.2–35 × 10 −6, and the fluids are interpreted to represent sedimentary formation waters derived from the upper crust that have dissolved variable quantities of halite (or scapolite) to achieve their ultra-high salinity. Fluid inclusions in a sample from the Snake Creek anticline in the Cloncurry District have the highest 40Ar/ 36Ar value of ∼25,000 demonstrating a deep magmatic or metamorphic fluid origin in this case. A magmatic origin is favoured because this fluid is very similar to a fluid involved in Iron-Oxide-Copper-Gold (IOCG) mineralization at Ernest Henry that was also interpreted to have a magmatic origin, and because the aqueous fluid inclusions have Br/Cl of ∼1–2 × 10 −3 and I/Cl of ∼10 × 10 −6 that are similar to mantle-derived igneous rocks. Carbon-dioxide fluid inclusions dominate samples from the Knobby Quarry in the Mary Kathleen Fold Belt and have a maximum 40Ar/ 36Ar value of 6000–7000. These fluid inclusions are estimated to have a 36Ar concentration of 1–4 ppb, that is similar to the range determined for aqueous fluid inclusions in all the other samples. In addition, aqueous and carbonic fluid inclusions in all the samples have similar 129Xe/ 36Ar plus 84Kr/ 36Ar values that are unfractionated, and close to the air—Air Saturated Water (ASW) range. These data are interpreted to indicate an independent and dominantly metamorphic origin for CO 2, and the presence of mixed aqueous-carbonic fluid inclusions are attributed to fluid mixing and/or mingling, rather than fluid unmixing. However, the data do not preclude the presence of a minor magmatic CO 2 component in the samples from the Knobby Quarry. Fluid inclusions in most of the samples from the Mary Kathleen Fold Belt have higher Br/Cl and I/Cl values, higher 36Ar concentrations and lower maximum 40Ar/ 36Ar values than fluid inclusions in samples from the Cloncurry District. This suggests Na–Ca alteration in these different parts of the Eastern Mt Isa Block occurred independently. However, fluid inclusions associated with Na–Ca alteration in the Cloncurry District have a very similar composition to fluid inclusions in IOCG mineralization-stage quartz veins from Ernest Henry. These data are therefore compatible with a genetic relationship between regional Na–Ca alteration and IOCG mineralization in the Cloncurry District. In both cases the ultra-saline hydrothermal fluids had a dominant origin from sedimentary formation water, but are interpreted to contain a magmatic component sourced from the late-Isan Williams-Naraku Batholiths which may have driven fluid convection.