A relationship between convergent margin magmas and copper–gold ore mineralization has long been recognized1,2,3,4,5,6. The nature of the genetic link is controversial, particularly whether the link is due to high-oxygen-fugacity () melts and fluids released from subducted slabs5,6,7 or to brine exsolution during magmatic evolution4. For submarine, subduction-related volcanic glasses from the eastern Manus basin, Papua New Guinea, we here report abrupt decreases in gold and copper abundances, coupled with a switch in the behaviour of titanium and iron from concentration increases to decreases as SiO2 rises. We propose that the abrupt depletion in gold and copper results from concurrent sulphur reduction as a result of buffering, causing enhanced formation of copper–gold hydrosulphide complexes that become scavenged from crystallizing melts into cogenetic magmatic aqueous fluids. This process is particularly efficient in oxidized arc magmas with substantial sulphate. We infer that subsequent migration and cooling of exsolved aqueous fluids create links between copper–gold mineralization and arc magmatism in the Manus basin8,9, and at convergent margins in general1,2,3,4,5,6.
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We thank S. M. Eggins, C. Allen and J. M. G. Shelley for technical support with the laser-ablation ICP-MS analyses; S.-S. Sun, A. W. Hofmann, J. E. Snow and S. J. Galer for discussions; and A. E. Williams-Jones for a review. This work was supported by a Alexander von Humboldt Research Fellowship to W.D.S. and by a Friedrich Wilhelm Bessel Award to V.S.K. R.J.A. was supported by the Australian Research Council, and R.A.B. by CSIRO and a mineral company consortium.
The authors declare that they have no competing financial interests.
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Sun, W., Arculus, R., Kamenetsky, V. et al. Release of gold-bearing fluids in convergent margin magmas prompted by magnetite crystallization. Nature 431, 975–978 (2004). https://doi.org/10.1038/nature02972
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