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Rapid crystallization of precious-metal-mineralized layers in mafic magmatic systems

Abstract

The solidified remnants of mafic magmatic systems host the greatest concentrations of platinum-group metals in the Earth’s crust. Our understanding of precious-metal mineralization in these intrusive bodies is underpinned by a traditional view of magma chamber processes and crystal mush solidification. However, considerable uncertainty remains regarding the physical and temporal controls on concentrating these critical metals, despite their importance to modern society. We present high-precision 87Sr/86Sr analyses of plagioclase and clinopyroxene from within centimetre-thick precious-metal-enriched layers in the Palaeogene open-system Rum layered intrusion (northwest Scotland). Isotopic heterogeneity is present between plagioclase crystals, between clinopyroxene and plagioclase and within plagioclase crystals throughout the studied section. On the basis of these observations, we demonstrate that platinum-group element mineralization formed by repeated small-volume reactive melt percolation events. The preservation of strontium isotope heterogeneities at 10–100 µm length scales implies cooling of the melts that formed the precious-metal-rich layers occurred at rates greater than 1 °C per year, and cooling to diffusive closure within tens to hundreds of years. Our data highlight the importance of cyclic dissolution–recrystallization events within the crystal mush and raise the prospect that precious-metal-bearing mafic intrusions may form by repeated self-intrusion during cooling and solidification.

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Fig. 1: Examples of intracrystalline 87Sr/86Sr variation in Unit 10 plagioclase and clinopyroxene.
Fig. 2: Comparison of new plagioclase and clinopyroxene 87Sr/86Sr data with a compilation of published data for the Rum intrusion.
Fig. 3: Schematic cartoon illustrating the reactive percolation model discussed in the text.
Fig. 4: Diffusive equilibration calculations.

Data availability

The data generated during this study are all accessible in the main article and accompanying supplementary data files, and have also been deposited with the National Geoscience Data Centre of the British Geological Survey (https://dx.doi.org/10.5285/093ddaf1-789a-4b8f-aaca-af4d2f070134). Source data are provided with this paper.

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Acknowledgements

L.N.H. acknowledges funding from a Natural Environmental Research Council (NERC) Studentship (grant no. 1361482) and Keele University. We thank Scottish National Heritage (SNH) for sampling permission on Rum during 2014 and 2015. We thank P. Greatbatch and D. Wilde (Keele University) for thin-section preparation and A. Kronz and G. Breedveld for assistance with EMPA analysis at the University of Göttingen. We also thank H. Horsch for her assistance generating the QEMSCAN images and M. Murphy (UCD) for technical assistance. Strontium isotopic analyses were carried out at the National Centre for Isotope Geochemistry (NCIG), Dublin, which is a joint venture of University College Dublin, Trinity College Dublin, University College Cork and National University of Ireland Galway, funded mainly by Science Foundation Ireland, including grant no. 04/BR/ES0007/EC07 awarded to J.S.D. J.S.D. was also supported in part by Science Foundation Ireland grant no. 13/RC/2092, which is co-funded under the European Regional Development Fund. We thank J. Day for helpful comments on an earlier version of the manuscript.

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B.O’D. conceived the study. B.O’D., L.N.H. and J.S.D. designed the programme of work. L.N.H. carried out fieldwork and EMPA analyses. L.N.H. and J.S.D. carried out the chemistry and mass spectrometry for strontium isotope analysis. B.O’D. and C.G.J. did the Sr diffusion-related calculations. All authors contributed to interpreting the results. B.O’D. wrote the manuscript. B.O’D., J.S.D., L.N.H., R.G. and C.G.J. contributed to editing the final version.

Corresponding author

Correspondence to Brian O’Driscoll.

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Extended data

Extended Data Fig. 1 Thin section photomicrographs of Unit 10 Cr-spinel seams.

Transmitted light (crossed polars) photomicrographs of microstructures associated with the lower peridotite Cr-spinel seams. a, Chain-textured arrangement of Cr-spinel around olivine primocrysts (and embedded in interstitial plagioclase) in a typical ~1-cm-thick seam. The white arrows point to embayments in olivine crystals that are occupied by Cr-spinel crystals. b, Example of a much less diffuse Cr-spinel seam than in (a) (black; this one may be chromitite proper). Note the olivine primocryst grainsize change across the seam. The yellow arrows point to the triple junctions between olivine crystals in the finer-grained peridotite, which suggest an approach to solid-state textural equilibrium. c, Oscillatory zoned interstitial plagioclase in a Cr-spinel seam. The brightly coloured crystals on all sides are olivine.

Extended Data Fig. 2 Element maps of plagioclase in the Unit 10 lower peridotite.

Element maps (Na) showing typical textures of intercumulus plagioclase in Cr-spinel seams from the Unit 10 lower peridotite. a, Oscillatory zoning of intercumulus plagioclase between olivine primocrysts. The smaller unlabelled black crystals are Cr-spinel. b, Chain-textured arrangement of Cr-spinel (small black-coloured) crystals around olivine crystals (labelled ol) in a Unit 10 Cr-spinel seam. Note the variation in intercumulus plagioclase composition and the tendency for Cr-spinel to occur in more anorthitic plagioclase. c, Compositional variation in intercumulus plagioclase from adjacent to a Unit 8 Cr-spinel seam. As for (b) above, note the occurrence of Cr-spinel in the more anorthitic plagioclase, supporting the argument that Cr-spinel crystallized by a peritectic reaction between picrite and relatively sodic plagioclase d, Complex compositional variation in intercumulus plagioclase (orange-green). The blue-coloured crystal is clinopyroxene.

Extended Data Fig. 3 QEMSCAN and back-scattered electron images of platinum-group mineral phases in a Unit 10 Cr-spinel seam.

a, QEMSCAN image of diffuse platinum-group mineral bearing Cr-spinel seam illustrating the general distribution and style of PGE mineralization in the Unit 10 lower peridotite. The solid black line illustrates the top boundary of the seam. The lower boundary is more diffuse and is mainly out of the image frame. b, Close up image of the area outlined in the red box in (a), illustrating the occurrence of sulfides at olivine three grain junctions. cf, Back-scattered electron micrograph images of platinum-group minerals from the seam in (a), associated with base-metal sulfides. The labelled red circles in (a) correspond to the locations in the sample of the images in c-f. Abbreviations as follows: Pn – pentlandite; Fe-ox – magnetite or ferrian Cr-spinel; ol – olivine.

Extended Data Fig. 4 Unit 10 plagioclase mineral chemical data plots.

Plagioclase compositional variations as measured by electron microprobe. ad, are K2O (wt.%), TiO2 (wt.%), FeO (wt.%) and Sr (ppm) versus An content, respectively. The black-filled symbols represent analyses from Cr-spinel seams in the Unit 10 lower peridotites not studied further (i.e., for 87Sr/86Sr) here. Note the strong anti-correlation of plagioclase TiO2 with anorthite content, linking the crystallization of Cr-spinel with that of relatively anorthitic plagioclase; with the onset of crystallization of Cr-spinel, Ti preferentially partitioned into it. Vertical bars show 2σ uncertainties.

Extended Data Fig. 5 Unit 10 clinopyroxene mineral chemical data plots.

Clinopyroxene compositional variations as measured by electron microprobe. a, Al2O3 (wt.%) versus Mg# [Mg# = Mg/(Mg+Fe2++Mn)*100]; b, TiO2 (wt.%) versus Mg#; c, Cr2O3 (wt.%) versus Al2O3 (wt.%); (d) Na2O (wt.%) versus Cr2O3 (wt.%). The black-filled symbols represent analyses from Cr-spinel seams in the Unit 10 lower peridotites not studied further (i.e., for 87Sr/86Sr).

Supplementary information

Supplementary Information

Supplementary Fig. 1 contains contextual field data for the samples analysed in this study, including geological maps and petrological logs of the sampled sequences.

Supplementary Data 1

Supplementary Tables 1–5 of EMPA and strontium isotope data, including additional back-scattered electron images of analysed crystals.

Supplementary Data 2

Source data for Supplementary Fig. 1.

Source data

Source Data Fig. 2

Source data for Fig. 2.

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Hepworth, L.N., Daly, J.S., Gertisser, R. et al. Rapid crystallization of precious-metal-mineralized layers in mafic magmatic systems. Nat. Geosci. 13, 375–381 (2020). https://doi.org/10.1038/s41561-020-0568-3

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