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New stable isotope data are consistent with a metasomatic origin for the controversial Mesoproterozoic ironstone-hosted Au-Cu deposit at Starra in the Cloncurry district of northwest Queensland. This supports textural and mineralogical evidence that the ore-bearing ironstones formed after the peak of metamorphism and were strongly controlled by brittle-ductile deformation. Three dominant paragenetic stages are recognized. These are: (1) early, widespread Na-Ca metasomatism (quartz-albite-scapolite-actinolite), (2) localized K-Fe metasomatism (biotite-magnetite-hematite-quartz-pyrite), and (3) mineralization (quartz-anhydrite-baritehematite-ca•cite-g••d-•yrite-cha•c••yrite-b•rnite-cha•c•cite-ch••rite-musc•vite).Crosscutting anhydrite veins associated with carbonate + hematite are rare in the upper parts of the system but more common at depths greater than 700 m. Stage 3 minerals demonstrate that the ore fluid was highly oxidized.A restricted range of Fe oxide •1sO implies isotopically similar fluids were responsible for the magnetite ironstones and later hematite alteration associated with gold and sulfides (between -0.2 to +3.3%0 except for one magnetite at 5.4%0). Sulfur isotope compositions suggest that pyrite (-0.2 to +3.9%0), chalcopyrite (-5.8 to -0.7 and -14.6%o), bornRe (-4.7%o), chalcocite (-0.1%o), and anhydrite (1.1-2.4%o) all formed from the same sulfur source. Anhydrite •a4S falls within the sulfide range, suggesting that the sulfate inherited its •a4S through hydrolytic processes similar to those of some porphyry environments. •i13C of ore-stage carbonates range from -7.3 to -2.2 per mil. Temperatures based on stage 2 quartz-magnetite pairs imply 400 ø to 500øC for ironstone formation and stage 3 calcite-hematite pairs indicate 180 ø to 330øC for mineralization. Stage 2 fluid inclusions have homogenization temperatures between 345 ø and 615øC and salinities of 34 to 52 wt percent NaC1 equiv. Stage 3 fluid inclusions homogenized between 225 ø and 360øC and have salinities of 30 to 42 wt percent NaC1 equiv. Calculated •ilsO fluid compositions for magnetite-quartz and hematite-calcite average 7.8 per mil and 9.5 per mil, respectively. Fluid compositions (for stages 2 and 3) fall within the range for a magmatic or metamorphic fluid. However, combined stable isotope and fluid inclusion data and thermodynamic considerations suggest a magmatic-metasomatic origin for both the magnetite ironstones and the highly oxidized mineral assemblage associated with high-grade gold deposition.

Studies of liquid-vapor-multisolid (L-V-nS) and liquid-vapor ± halite fluid inclusions suggest that at least three fundamentally different brines were involved in the genesis of the Proterozoic ironstone-hosted Starra Au-Cu deposit. Element concentrations determined from proton-induced X-ray emission (PIXE) analysis of groups of L-V-nS inclusions from the early barren magnetite ironstone stage (400°-550°C) and the younger mineralization stage (220°-360°C) covary over concentration ranges greater than the deduced analytical errors. The L-V-nS inclusions in the two stages are very different, with higher ratios of Fe and K to Ca, Mn, Cu, Zn, Sr, Ba, and Pb in the ironstone stage. Quite distinct, pseudosecondary NaCl-CaCl 2-H 2 O inclusions are abundant in mineralization-stage quartz, calcite, and anhydrite. High concentrations of Cu (ca. >1,000 ppm) are present in some L-V-nS inclusions of both stages, and the data suggest that large amounts of Cu, Mn, Zn, As, Ba, and Pb passed through the Starra system as it evolved. Cl/Br ratios in the inclusions are mostly less than 0.002 and clearly different from those of ore fluids of the giant sedimentary-hosted copper orebody at Mount Isa. This could reflect a major magmatic contribution to the salinity at Starra that is consistent with the preferred origin of the water in published stable isotope studies. Ba concentrations of at least several wt percent in some mineralization-stage L-V-nS inclusions are associated with oxidized sulfur-bearing mineral parageneses. Given the low solubility of barite this suggests that a S2-f O2 relationships varied during mineralization.

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