Porphyry copper deposits—the primary source of the world’s copper—are a consequence of the degassing of intrusion complexes in magmatic arcs associated with ancient subduction zones1,2. They are characterized by copper and iron sulphides, commonly found with anhydrite (CaSO4), over scales of several kilometres through intensely altered and fractured rocks1. The magmatic source of the metals is broadly understood, but the processes that transport and deposit the metals at the megaton scale are unclear. The hydrogen sulphide necessary for metal deposition is commonly assumed to form by a reaction between sulphur dioxide and water, but this reaction is inefficient3 and cannot explain the formation of economic-grade deposits. Here we use high-temperature laboratory experiments to show that a very rapid chemisorption reaction occurs between sulphur dioxide gas, a principal component of magmatic gas mixtures, and calcic feldspar, an abundant mineral in the arc crust. The chemisorption reaction generates the mineral anhydrite and hydrogen sulphide gas, and triggers deposition of metal sulphides. We use thermodynamic calculations to show that as magmatic gas cools and expands the concentration of hydrogen sulphide gas increases exponentially to drive efficient deposition of metal sulphides and consequent formation of economic-grade porphyry copper deposits.
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This investigation was triggered by interactions with P. Delmelle and P. Ayris in relation to the reactivity of volcanic ashes. We thank R. Arculus, M. Goldhaber, R. King, R. Sillitoe, H. O’Neill, A. Putnis, J. Ward and T. Whan for valuable discussions. We also wish to acknowledge the foundations of porphyry copper analysis laid by L. Gustafson. We thank N. Bishop, D. Olson and K. Schroeder of Kennecott Exploration (Rio Tinto) for confirmation of the occurrence of anhydrite in deep drilling at Bingham Canyon, Utah. K. Friehauf very kindly provided the photograph of the Grasberg porphyry copper deposit for Fig. 1a. Graphic enhancement was provided by D. Henley, and D. Hill kindly provided his original cartoon for a generic volcano that we have used in Fig. 1c. Constructive comments on earlier versions of this work were received from J. Dilles and R. Herrington. Funding was provided by an Australian Research Council Future Fellowship to P.L.K. R.W.H. and P.L.K. wish to dedicate this paper to the late W. S. Fyfe, one of the founders of modern geochemistry, and a lifetime friend and mentor.
The authors declare no competing financial interests.
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Henley, R., King, P., Wykes, J. et al. Porphyry copper deposit formation by sub-volcanic sulphur dioxide flux and chemisorption. Nature Geosci 8, 210–215 (2015). https://doi.org/10.1038/ngeo2367
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