Metamorphic fluid origins in the Osborne Fe oxide–Cu–Au deposit, Australia: evidence from noble gases and halogens

Abstract

The Osborne iron oxide-copper-gold (IOCG) deposit is hosted by amphibolite facies metasedimentary rocks and associated with pegmatite sheets formed by anatexis during peak metamorphism. Eleven samples of ore-related hydrothermal quartz and two pegmatitic quartz- feldspar samples contain similarly complex fluid inclusion assemblages that include variably saline (<12-65 wt% salts) aqueous and liquid carbon dioxide varieties that are typical of IOCG mineralisation. The diverse fluid inclusion types present in each of these different samples have been investigated by neutron-activated noble gas analysis using a combination of semi-selective thermal and mechanical decrepitation techniques. Ore-related quartz contains aque- ous and carbonic fluid inclusions that have similar 40 Ar/ 36 Ar values of between 300 and 2,200. The highest- salinity fluid inclusions (47-65 wt% salts) have calculated 36 Ar concentrations of approximately 1-5 ppb, which are more variable than air-saturated water (ASW=1.3-2.7 ppb). These fluid inclusions have extremely variable Br/Cl values of between 3.8×10 −3 and 0.3×10 −3 , and I/Cl values of between 27×10 −6 and 2.4×10 −6 (all ratios are molar). Fluid inclusions in the two pegmatite samples have similar 40 Ar/ 36 Ar values of ≤1,700 and an overlapping range of Br/Cl and I/Cl values. High-salinity fluid inclusions in the pegmatite samples have 2.5-21 ppb 36 Ar, that overlap the range determined for ore-related samples in only one case. The fluid inclusions in both sample groups have 84 Kr/ 36 Ar and 129 Xe/ 36 Ar ratios that are mainly in the range of air and air-saturated water and are similar to mid-crustal rocks and fluids from other settings. The uniformly low 40 Ar/ 36 Ar values (<2,200) and extremely variable Br/Cl and I/Cl values do not favour a singular or dominant fluid origin from basement- or mantle-derived magmatic fluids related to A-type magmatism. Instead, the data are compatible with the involvement of metamorphic fluids that have interacted with anatectic melts to variable extents. The 'metamorphic' fluids probably represent a mixture of (1) inherited sedi- mentary pore fluids and (2) locally derived metamorphic volatilisation products. The lowest Br/Cl and I/Cl values and the ultra-high salinities are most easily explained by the dissolution of evaporites. The data demonstrate that externally derived magmatic fluids are not a ubiquitous component of IOCG ore-forming systems, but are com- patible with models in which IOCG mineralisation is localised at sites of mixing between fluids of different origin.