Transport of metals and sulphur in magmas by flotation of sulphide melt on vapour bubbles

Abstract

Emissions of sulphur1,2 and metals3,4 from magmas in Earth’s shallow crust can have global impacts on human society. Sulphur-bearing gases emitted into the atmosphere during volcanic eruptions affect climate5,6, and metals and sulphur can accumulate in the crust above a magma reservoir to form giant copper and gold ore deposits, as well as massive sulphur anomalies3,4,7,8. The volumes of sulphur and metals that accumulate in the crust over time exceed the amounts that could have been derived from an isolated magma reservoir2. They are instead thought to come from injections of multiple new batches of vapour- and sulphide-saturated magmas into the existing reservoirs1,4,9,10. However, the mechanism for the selective upward transfer of sulphur and metals is poorly understood because their main carrier phase, sulphide melt, is dense and is assumed to settle to the bottoms of magma reservoirs. Here we use laboratory experiments as well as gas-speciation and mass-balance models to show that droplets of sulphide melt can attach to vapour bubbles to form compound drops11 that float. We demonstrate the feasibility of this mechanism for the upward mobility of sulphide liquids to the shallow crust. Our work provides a mechanism for the atmospheric release of large amounts of sulphur, and contradicts the widely held assumption that dense sulphide liquids rich in sulphur, copper and gold will remain sequestered in the deep crust.

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Figure 1: Morphologies of compound drops.
Figure 2: Cross-sectional cartoon illustrating sulphide flotation in subvolcanic environments.
Figure 3: Cu versus Au for modelled and measured compositions of sulphides, fluids and ores at Alumbrera3,29.

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Acknowledgements

J.E.M. and J.M.B. were supported by Discovery Grants from the Natural Sciences and Engineering Research Council of Canada; B.G. and S.J.B. were funded by the CSIRO Mineral Resources Research Flagship, F.G. was supported by the European Research Council (ERC project #279790).

Author information

J.M.B. performed experiments; J.E.M. performed modelling of interfaces and metal mass balance and wrote the manuscript; B.G. performed CT scanning and related data reduction; F.G. performed gas-speciation modelling, S.J.B., J.M.B. and J.E.M. contributed to the original concept. All authors discussed the results and edited the manuscript.

Correspondence to J. E. Mungall.

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Mungall, J., Brenan, J., Godel, B. et al. Transport of metals and sulphur in magmas by flotation of sulphide melt on vapour bubbles. Nature Geosci 8, 216–219 (2015) doi:10.1038/ngeo2373

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