Abstract
Much of global gold production has come from deposits with uneconomic concentrations of base metals, such as copper, lead and zinc1. These ‘gold-only’ deposits are thought to have formed from hot, aqueous fluids rich in carbon dioxide2, but only minor significance has been attached to the role of the CO2 in the process of gold transport. This is because chemical bonding between gold ions and CO2 species is not strong3, and so it is unlikely that CO2 has a direct role in gold transport. An alternative indirect role for CO2 as a weak acid that buffers pH has also appeared unlikely, because previously inferred pH values for such gold-bearing fluids are variable2,4,5,6. Here we show that such calculated pH values are unlikely to record conditions of gold transport, and propose that CO2 may play a critical role during gold transport by buffering the fluid in a pH range where elevated gold concentration can be maintained by complexation with reduced sulphur. Our conclusions, which are supported by geochemical modelling, may provide a platform for new gold exploration methods.
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Acknowledgements
We thank J. Law and M. Hughes for critical comments; R. Smith for earlier discussions relating to the chemical behaviour of gold; and R. Phillips and S. Wood for comments and suggestions that helped to improve the manuscript. K.A.E. acknowledges honorary positions at Monash and Melbourne Universities.
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Supplementary information
Supplementary Table M1
Endmember apparent standard state Gibbs Free Energies for selected aqueous species (pressure = 200 MPa). (DOC 39 kb)
Supplementary Table M2
Bulk compositions used in modelling. (DOC 33 kb)
Supplementary Table M3
Modelled mineral assemblages. (DOC 30 kb)
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Phillips, G., Evans, K. Role of CO2 in the formation of gold deposits. Nature 429, 860–863 (2004). https://doi.org/10.1038/nature02644
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DOI: https://doi.org/10.1038/nature02644
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