Noble gas and halogen constraints on mineralizing fluids of metamorphic versus surficial origin: Mt Isa, Australia

Abstract

The origin of hydrothermal fluids responsible for silica–dolomite alteration associated with Cu ore in the giant Mt Isa deposit of Queensland, north east Australia has been investigated. Main-stage quartz and dolomite veins, and late- to post-ore quartz from the Buck Quartz Fault in the orebody footwall, plus the regional Mt Isa and Railway Faults contain two phase liquid–vapour fluid inclusions with variable salinity. Fluid inclusion molar Br/Cl values are mostly in the range 1.5–12 × 10 − 3 and molar I/Cl values are in the range 9–186 × 10 − 6 . Greater than seawater Br/Cl and I/Cl values can characterise either sedimentary formation waters with a bittern brine origin by the sub-aerial evaporation of surface water beyond the point of halite saturation or some metamorphic dehydration fluids with a deep crustal source. We propose that these alternative origins can be distinguished in our samples based on the relationship between 40Ar/ 36Ar and 36Ar concentration. Metamorphic fluids formed by dehydration of crystalline rock are predicted to have low 36Ar concentrations with the salinity, 40Ar/ 36Ar and 40Ar E/Cl values determined by the whole rock concentration of Cl, 36Ar, OH −, K and age. The main-stage quartz and Buck Quartz Fault samples host fluid inclusions with < 20 wt.% NaCl eq., 1–3 ppb 36Ar, 40Ar/ 36Ar in the range ∼ 10,000–28,000 and high 40Ar E/Cl values of ∼ 3–8 × 10 − 4 . These values are consistent with metamorphic fluids sourced by devolatilisation of either the underlying Eastern Creek Volcanics or the Barramundi Basement. In contrast, surface-derived fluids acquire elevated 40Ar/ 36Ar values (> 296) through fluid–rock interaction which leads to a simultaneous increase in 36Ar concentration. The main-stage Ca-rich dolomite fluid inclusions have high salinities close to the point of halite saturation (∼ 26 wt.%), 1.7–5.5 ppb 36Ar, 40Ar/ 36Ar in the range 1600–8400 and 40Ar E/Cl of ∼ 0.2–1 × 10 − 4 . These values are most similar to those expected in an evolved bittern brine; however, the 36Ar concentration and 40Ar/ 36Ar value exhibit a weakly negative correlation, which is the opposite to that expected in surface-derived fluids. Only fluid inclusions in samples from the Railway Fault and some fluid inclusions in samples from the Buck Quartz Fault have 36Ar concentrations of greater than Air Saturated Water (ASW = 1.3–2.7 ppb 36Ar), up to 27 ppb, and 40Ar/ 36Ar values of ∼ 1000–2500 that are expected for fluids with an exclusively sedimentary formation water origin. Fluid inclusions in the Mt Isa Fault samples have the highest 36Ar concentrations of up to ∼ 100 ppb, some of the lowest 40Ar/ 36Ar values and the highest Br/Cl value of up to ∼ 30 × 10 − 3 . These features distinguish it from all other sample groups in the mine area. Silica–dolomite alteration (and Cu mineralization) are interpreted to have resulted from mixing of the deeply derived metamorphic fluid, best preserved in quartz fluid inclusions, and a surface-derived bittern brine, partially preserved within the main-stage dolomite fluid inclusions. Inflow of a second surficial fluid, along regional faults and into the mine through the late- to post-D 3 Buck Quartz Fault may have terminated mineralization. The involvement of metamorphic fluids is compatible with the syn-D 3 (∼ 1530 Ma) timing of mineralization during the final stages of the Isan orogeny.