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

Abstract

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 + or - 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 +3.3ppm except for one magnetite at 5.4ppm). Sulfur isotope compositions suggest that pyrite (-0.2 to +3.9ppm), chalcopyrite (-5.8 to -0.7 and -14.6ppm), bornite (-4.7ppm), chalcocite (-0.1ppm), and anhydrite (1.1-2.4ppm) 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 400 degrees 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.