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Oxidized and reduced mineral assemblages in greenstone belt rocks of the St. Ives gold camp, Western Australia: vectors to high-grade ore bodies in Archaean gold deposits?

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Abstract

Hydrothermal sulfide–oxide–gold mineral assemblages in gold deposits in the Archaean St. Ives gold camp in Western Australia indicate extremely variable redox conditions during hydrothermal alteration and gold mineralization in space and time. Reduced alteration assemblages (pyrrhotite–pyrite) occur in deposits in the southwest of the camp (e.g., Argo, Junction deposits) and moderately to strongly oxidized assemblages (magnetite–pyrite, hematite–pyrite) occur in deposits in the Central Corridor in the northeast (e.g., North Orchin, Revenge deposits). Reduced mineral assemblages flank the Central Corridor of oxidized deposits and, locally, cut across it along E–W trending faults. Oxidized mineral assemblages in the Central Corridor are focused on gravity lows which are interpreted to reflect abundant felsic porphyritic intrusions at about 1,000 m below present surface. Hydrothermal magnetite predates and is synchronous with early phases of gold-associated albite–carbonate–pyrite–biotite–chlorite hydrothermal alteration. Later-stage, gold-associated pyrite is in equilibrium with hematite. The spatial distribution and temporal sequence of iron sulfides and oxides with gold indicate the presence of at least two spatially restricted but broadly synchronous hydrothermal fluids with contrasting redox states. Sulfur isotope constraints support the argument that the different mineral assemblages reflect differences in redox conditions. The δ 34S values for pyrite for the St. Ives gold camp range between −8.4‰ and +5.1‰ with the negative values occurring in oxidized magnetite-rich domains and slightly negative or positive values occurring in reduced, pyrrhotitic domains. Preliminary spatial and paragenetic analysis of the distribution of iron sulfides and oxides in the St. Ives camp suggests that gold grades are highest where the redox state of the hydrothermal alteration assemblages switches from relatively reduced pyrrhotite–pyrite to relatively oxidized magnetite–pyrite and hematite–pyrite both in space and time. Gold deposition is inferred to have occurred where fluids of contrasting redox state mixed.

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Acknowledgements

This work forms part of MERIWA 358/377 and pmd*CRC Y3 and Y4 projects. The work on St. Ives gold camp is financially supported by St. Ives Gold Mining Company (Pty) Limited, and the authors wish to acknowledge their financial support and permission to present the data and interpretations. Current and former staff of St. Ives Gold Mining Company (Pty) Limited, most notably Ned Stolz, Janet Tunjic, Bob Morrison, Karen Connors, James Johnson, and Ed Ainscough are thanked for their support and countless discussions. The enthusiastic support of Greg Hall, formerly Chief Geologist of Placer Dome Ltd, for the development of conceptual exploration strategies is acknowledged.

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Authors and Affiliations

  1. Centre for Exploration Targeting, School of Earth and Geographical Sciences, University of Western Australia, Nedlands, WA, 6009, Australia

    Peter Neumayr, Steffen Hagemann & Klaus Petersen

  2. CSIRO-Exploration and Mining, 26 Dick Perry Ave, Kensington, Perth, WA, 6151, Australia

    John Walshe, Peter Frikken & Leo Horn

  3. Mineral Mapping Pty. Ltd., 24 Webb St., Rossmoyne, WA, 6148, Australia

    Scott Halley

  4. St. Ives Gold Mining Company Pty. Ltd., Kambalda-West, WA, 6444, Australia

    Anthony Roache

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  1. Peter Neumayr
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  3. Steffen Hagemann
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Correspondence to Peter Neumayr.

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Editorial handling: Adrian Boyce

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Neumayr, P., Walshe, J., Hagemann, S. et al. Oxidized and reduced mineral assemblages in greenstone belt rocks of the St. Ives gold camp, Western Australia: vectors to high-grade ore bodies in Archaean gold deposits?. Miner Deposita 43, 363–371 (2008). https://doi.org/10.1007/s00126-007-0170-2

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