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Platinum-group element remobilization and concentration in the Cliff chromitites of the Shetland Ophiolite Complex, Scotland

Published online by Cambridge University Press:  12 April 2018

Brian O'Driscoll ,
Russell Garwood ,
James M. D. Day  and
Roy Wogelius
Brian O'Driscoll*
Affiliation:
School of Earth and Environmental Sciences, Oxford Road, University of Manchester M13 9PL, UK
Russell Garwood
Affiliation:
School of Earth and Environmental Sciences, Oxford Road, University of Manchester M13 9PL, UK Department of Earth Sciences, Natural History Museum, Cromwell Road, London SW7 5BD, UK
James M. D. Day
Affiliation:
Geosciences Research Division, Scripps Institution of Oceanography, UCSD, La Jolla, CA 92093-0244, USA
Roy Wogelius
Affiliation:
School of Earth and Environmental Sciences, Oxford Road, University of Manchester M13 9PL, UK
*
*E-mail: [email protected]
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Abstract

The ~492 Ma Shetland Ophiolite Complex contains an extensive mantle section, within which numerous podiform chromitite bodies formed during melt percolation in a supra-subduction zone setting. One of the Shetland ophiolite chromitite localities has an unusual style of platinum-group element (PGE) mineralization. Specifically, the Cliff chromitite suite has relatively high (>250 ppm) Pt plus Pd, compared to other chromitites in the Shetland Ophiolite Complex. In this study, we apply petrographic observation, mineral chemistry and novel X-ray microtomography data to elucidate the petrogenesis of PGE-bearing phases at Cliff. The combined datasets reveal that the PGE at Cliff have probably been fractionated by an As-rich fluid, concentrating Pt and Ir into visible (0.1–1 µm) platinum-group minerals (PGM) such as sperrylite and irarsite, respectively. The high (>1 ppm) bulk-rock concentrations of the other PGE (e.g. Os) in the Cliff chromitites suggests the presence of abundant fine-grained unidentified PGM in the serpentinized groundmass. The spatial association of arsenide phases and PGM with alteration rims on Cr-spinel grains suggests that the high Pt and Pd abundances at Cliff result from a late-stage low-temperature (e.g. 200–300°C) hydrothermal event. This conclusion highlights the potential effects that secondary alteration processes can have on modifying and upgrading the tenor of PGE deposits.

Keywords

platinum-group elementschromititesShetland Ophiolite Complex
Type
Article
Information
Mineralogical Magazine , Volume 82 , Special Issue 3: Special Issue dedicated to the work and memory of Professor Hazel M. Prichard (1954–2017) , June 2018 , pp. 471 - 490
Copyright
Copyright © Mineralogical Society of Great Britain and Ireland 2018 

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Footnotes

This paper is published as part of a thematic set in memory of Professor Hazel M. Prichard

Associate Editor: John Bowles

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