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

New stable isotope data are consistent with a metasomatic origin for the co ntroversial Mesoproterozoic ironstone-hosted Au-Cu deposit at Starra in the Cloncurry district of northwest Queensland. This supports textural and min eralogical evidence that the ore-bearing ironstones formed after the peak o f metamorphism and were strongly controlled by brittle-ductile deformation. Three dominant paragenetic stages are recognized. These are: (1) early, wi despread Na-Ca metasomatism (quartz-albite-scapolite-actinolite), (2) local ized K-Fe metasomatism (biotite-magnetite-hematite-quartz-pyrite), and (3) mineralization (quartz-anhydrite-barite-hematite-calcite-gold-pyrite-chalco pyrite-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 demon strate that the ore fluid was highly oxidized. A restricted range of Fe oxide delta(18)O implies isotopically similar flui ds were responsible for the magnetite ironstones and later hematite alterat ion associated with gold and sulfides (between -0.2 to +3.3 parts per thous and except for one magnetite at 5.4 parts per thousand). Sulfur isotope com positions suggest that pyrite (-0.2 to +3.9 parts per thousand), chalcopyri te (-5.8 to -0.7 and -14.6 parts per thousand), bornite (-4.7 parts per tho usand, chalcocite (-0.1 parts per thousand), and anhydrite (1.1-2.4 parts p er thousand) all formed from the same sulfur source. Anhydrite delta(34)S f alls within the sulfide range, suggesting that the sulfate inherited its de lta(34)S through hydrolytic processes similar to those of some porphyry env ironments. delta(13)C Of ore-stage carbonates range from -7.3 to -2.2 per m il. Temperatures based on stage 2 quartz-magnetite pairs imply 4000 to 500 degrees C for ironstone formation and stage 3 calcite-hematite pairs indica te 180 degrees to 330 degrees C for mineralization. Stage 2 fluid inclusion s have homogenization temperatures between 345 degrees and 615 degrees C an d salinities of 34 to 52 wt percent NaCl equiv. Stage 3 fluid inclusions ho mogenized between 225 degrees and 360 degrees C and have salinities of 30 t o 42 wt percent NaCl equiv. Calculated delta(18)O fluid compositions for ma gnetite-quartz and hematite-calcite average 7.8 per mil and 9.5 per mil, re spectively. Fluid compositions (for stages 2 and 3) fall within the range f or a magmatic or metamorphic fluid. However, combined stable isotope and fl uid inclusion data and thermodynamic considerations suggest a magmatic-meta somatic origin for both the magnetite ironstones and the highly oxidized mi neral assemblage associated with high-grade gold deposition.