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Placer platinum-group minerals in the Shetland ophiolite complex derived from anomalously enriched podiform chromitites

Published online by Cambridge University Press:  16 April 2018

Hazel M. Prichard ,
Saioa Suárez ,
Peter C. Fisher ,
Robert D. Knight  and
John S. Watson
Hazel M. Prichard
Affiliation:
School of Earth and Ocean Sciences, Cardiff University, Main College, Park Place, Cardiff, CF10 3AT, UK
Saioa Suárez*
Affiliation:
School of Earth and Ocean Sciences, Cardiff University, Main College, Park Place, Cardiff, CF10 3AT, UK Department of Mineralogy and Petrology, UPV/EHU, 48940 Leioa & Ikerbasque, 48011 Bilbao, Spain
Peter C. Fisher
Affiliation:
School of Earth and Ocean Sciences, Cardiff University, Main College, Park Place, Cardiff, CF10 3AT, UK
Robert D. Knight
Affiliation:
School of Earth and Ocean Sciences, Cardiff University, Main College, Park Place, Cardiff, CF10 3AT, UK
John S. Watson
Affiliation:
Open University, Department of Environment, Earth and Ecosystems, Walton Hall, Milton Keynes, Buckinghamshire, MK7 6AA, UK
*
*Email: [email protected]
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Abstract

Highly anomalous platinum-group element (PGE) concentrations in the podiform chromitites at the Cliff and Harold's Grave localities in the Shetland ophiolite complex have been well documented previously. The focus of this study is alluvial platinum-group minerals (PGM) located in small streams that drain from the PGE-rich chromitites. The placer PGM assemblage at Cliff is dominated by Pt-arsenides (64%) and Pd-antimonides (17%), with less irarsite–hollingworthite (11%) and minor Pd-sulfides, Pt–Pd–Cu and Pt–Fe alloys and laurite. Gold also occurs with the PGM. Alluvial PGM have average sizes of 20 µm × 60 µm, with sperrylite the largest grain identified at 110 µm in diameter, matching the range reported for the primary PGM in the source rocks. The placer assemblage contains more Pt-bearing and less Pd-bearing PGM compared with the rocks. The more resistant sperrylite and irarsite–hollingworthite grains which are often euhedral become more rounded further downstream whereas the less resistant Pd-antimonides which are commonly subhedral may become striated and etched. Less stable phases such as Pt- and Pd-oxides and other Ni-Cu-bearing phases located in the rocks (i.e. Ru-pentlandite, PtCu, Pd–Cu alloy) are absent in the placer assemblage. Also the scarce PGM (PdHg, Rh- and Ir-Sb) and Os in the rocks are absent. At Harold's Grave only three alluvial PGM (laurite, Ir, Os) and Au were recovered reflecting the limited release of IPGM from chromite grains in the rocks. In this cold climate with high rainfall, where erosion dominates over weathering, the PGM appear to have been derived directly from the erosion of the adjacent PGE-rich source rocks and there is little evidence of in situ growth of any newly formed PGM. Only the presence of dendritic pure Au and Pd-, Cu-bearing Au covers on the surface of primary minerals may indicate some local reprecipitation of these metals in the surficial conditions.

Keywords

Shetland ophiolite complexCliffHarold's Gravealluvial concentratesplatinum-group mineralsplatinum-group elements
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. 491 - 514
Copyright
Copyright © Mineralogical Society of Great Britain and Ireland 2018 

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Footnotes

Associate Editor: Brian O'Driscoll

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

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