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

Jackie F. Rotherham, Kevin L. Blake, Ian Cartwright, Patrick J. Williams

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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-chalc ocite-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 δ 18O 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‰ except for one magnetite at 5.4‰). Sulfur isotope compositions suggest that pyrite (-0.2 to +3.9‰), chalcopyrite (-5.8 to -0.7 and -14.6‰), bornite (-4.7‰), chalcocite (-0.1‰), and anhydrite (1.1-2.4‰) all formed from the same sulfur source. Anhydrite δ 34S falls within the sulfide range, suggesting that the sulfide inherited its δ 34S through hydrolytic processes similar to those of some porphyry environments. δ 13C of ore-stage carbonates range from -7.3 to -2.2 per mil. Temperatures based on stage 2 quartz-magnetite pairs imply 400°to 500°C for ironstone formation and stage 3 calcite-hematite pairs indicate 180°to 330°C for mineralization. Stage 2 fluid inclusions have homogenization temperatures between 345°and 615°C and salinites of 34 to 52 wt percent NaCl equiv. Stage 3 fluid inclusions homogenized between 225°and 360°C and have salinities of 30 to 42 wt percent NaCl equiv. Calculated δ 18O 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.

Original languageEnglish
Pages (from-to)1435-1449
Number of pages15
JournalEconomic Geology
Volume93
Issue number8
DOIs
Publication statusPublished - 1 Dec 1998

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