Stable isotope evidence for the origin of the Mesoproterozoic Starra Au-Cu deposit, Cloncurry district, northwest Queensland

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-barite-hematite-calcite-gold-pyrite-chalcopyrite-bornite-chalcocite-chlorite-muscovite). 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 delta(18)O 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 parts per thousand except for one magnetite at 5.4 parts per thousand). Sulfur isotope compositions suggest that pyrite (-0.2 to +3.9 parts per thousand), chalcopyrite (-5.8 to -0.7 and -14.6 parts per thousand), bornite (-4.7 parts per thousand, chalcocite (-0.1 parts per thousand), and anhydrite (1.1-2.4 parts per thousand) all formed from the same sulfur source. Anhydrite delta(34)S falls within the sulfide range, suggesting that the sulfate inherited its delta(34)S through hydrolytic processes similar to those of some porphyry environments. delta(13)C Of ore-stage carbonates range from -7.3 to -2.2 per mil. Temperatures based on stage 2 quartz-magnetite pairs imply 4000 to 500 degrees C for ironstone formation and stage 3 calcite-hematite pairs indicate 180 degrees to 330 degrees C for mineralization. Stage 2 fluid inclusions have homogenization temperatures between 345 degrees and 615 degrees C and salinities of 34 to 52 wt percent NaCl equiv. Stage 3 fluid inclusions homogenized between 225 degrees and 360 degrees C and have salinities of 30 to 42 wt percent NaCl equiv. Calculated delta(18)O 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.