Generation of porphyry copper deposits by gas–brine reaction in volcanic arcs

Abstract

Porphyry copper deposits, that is, copper ore associated with hydrothermal fluids rising from a magma chamber, supply 75% of the world’s copper. They are typically associated with intrusions of magma in the crust above subduction zones, indicating a primary role for magmatism in driving mineralization. However, it is not clear that a single, copper-rich magmatic fluid could trigger both copper enrichment and the subsequent precipitation of sulphide ore minerals within a zone of hydrothermally altered rock. Here we draw on observations of modern subduction zone volcanism to propose an alternative process for porphyry copper formation. We suggest that copper enrichment initially involves metalliferous, magmatic hyper-saline liquids, or brines, that exsolve from large, magmatic intrusions assembled in the shallow crust over tens to hundreds of thousands of years. In a subsequent step, sulphide ore precipitation is triggered by the interaction of the accumulated brines with sulphur-rich gases, liberated in short-lived bursts from the underlying mafic magmas. We use high-temperature and high-pressure laboratory experiments to simulate such gas–brine interactions. The experiments yield copper–iron sulphide minerals and hydrogen chloride gas at magmatic temperatures of 700–800 °C, with textural and chemical characteristics that resemble those in porphyry copper deposits. We therefore conclude that porphyry copper ore forms in a two-stage process of brine enrichment followed by gas-induced precipitation.

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Figure 1: Coexisting fluid inclusions trapped in fractured quartz from sulphur-free dacite experiment, M7.
Figure 2: Run product from ‘porphyry in a capsule’ experiment, M4.
Figure 3: Comparison of experimental and natural mineralization textures.
Figure 4: Gas–brine reaction model for PCD formation.

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Acknowledgements

We acknowledge research funding from BHP Billiton, a Benjamin Meaker Visiting Professorship to J.M. and a Royal Society Wolfson Research Merit Award and ERC Advanced Grant (CRITMAG) to J.B. This work has benefited from discussions with J. Dilles, D. Dolejs, J. Eiler, C. Ford, R. Gold, R. Henley, C. Heinrich, J. Price, D. Sherman, E. Stolper, A. Webb and G. Yeates, as well as members of the Caltech PRG. We are grateful to M. Pistone for synthesizing the starting materials and R. Brooker and B. Buse for technical assistance.

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J.B. developed the hypothesis in conjunction with B.T. J.M. performed the experiments and analysed the run products. B.T. characterized the synthetic fluid inclusions. A.G. made observations of the Don Manuel core. J.B. wrote the first draft of the manuscript; all authors assisted in producing the final version.

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Correspondence to J. Blundy.

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Blundy, J., Mavrogenes, J., Tattitch, B. et al. Generation of porphyry copper deposits by gas–brine reaction in volcanic arcs. Nature Geosci 8, 235–240 (2015). https://doi.org/10.1038/ngeo2351

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