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Sulphide melt and aqueous fluid saturation in the PGE–Au mineralisation of the Skaergaard intrusion: evidence from melt inclusions

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Abstract

The Skaergaard intrusion in East Greenland hosts mineralisation of platinum group elements and gold that is characterised by a remarkably low amount of sulphides (about 0.02 wt% sulphide) and a very high metal/sulphur ratio. Here we present the first quantitative analyses of sulphur (S), copper (Cu), lithium (Li) and other trace elements from melt inclusions in plagioclase to trace their evolution in the magma and to constrain the formation of the mineralisation. The concentration of these elements in the melt inclusions varies considerably within each sample. The S content in melt inclusions ranges from 165 to 320 ppm in the lower part of the intrusion, but increases to 349–512 ppm immediately below the mineralisation interval. In the same interval, the Li content increases from 22–43 to 40–69 ppm and Cu ranges from 118 to 1586 ppm with no systematic variations. Across the mineralisation, the Li content drops an order of magnitude to 4–7 ppm and S drops to 250–326 ppm. We explain these compositional changes as reflecting the magmatic evolution as the result of fractional crystallisation below, and coinciding aqueous fluid exsolution and sulphide saturation across the mineralisation. Normally the S content of basaltic magmas is much higher at sulphide saturation. We propose that sulphide saturation is possible, nevertheless, mostly due to the high Cu content and high Cu/Fe in the evolved ferrobasaltic melt resulting in a lowering of the S concentration required for sulphide saturation. The relationship between exsolutions of aqueous fluids and sulphides remains unknown.

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Fig. 1

Modified from Wotzlaw et al. (2012) and Keays and Tegner (2016)

Fig. 2

Modified from Salmonsen and Tegner (2013) and Nielsen et al. (2005)

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Acknowledgements

This work was carried out as part of J. M. Pedersen’s Ph.D. study at Aarhus University funded by the Danish National Research Foundation and the Graduate School of Science and Technology, Aarhus University. At Aarhus University, a number of lab technicians including Birte Lindahl Eriksen, Kirsten Rosendal, Rikke Brok Jensen, and Charlotte Rasmussen are gratefully acknowledged for help during sample preparation. Special thanks to Lars Peter Salmonsen and Hans Dieter Zimmermann who helped with the vertical furnace for homogenisation of melt inclusion. This work has also greatly benefited from inspiring discussions and insightful comments from Ole Skursch and the Earth System Petrology Group, all at Aarhus University. We also would like to acknowledge the Geological Survey of Greenland and the Natural History Museum of Denmark for storing the samples used in this study. Troels Nielsen and an anonymous reviewer have provided many constructive comments to improve an earlier version of the manuscript. Chris Ballhaus is thanked for his valuable comments and editorial handling.

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Correspondence to Thomas Ulrich.

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Communicated by Chris Ballhaus.

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Pedersen, J.M., Ulrich, T., Nagel, T. et al. Sulphide melt and aqueous fluid saturation in the PGE–Au mineralisation of the Skaergaard intrusion: evidence from melt inclusions. Contrib Mineral Petrol 175, 14 (2020). https://doi.org/10.1007/s00410-020-1656-5

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  • DOI: https://doi.org/10.1007/s00410-020-1656-5

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