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

Citation:
O'Driscoll, B, Garwood R, Day JMD, Wogelius RA.  2018.  Platinum-group element remobilisation and concentration in the Cliff chromitites of the Shetland Ophiolite Complex, Scotland. Mineralogical Magazine. 82:471-490.

Date Published:

2018/06

Abstract:

The ~492 Ma Shetland Ophiolite Complex (SOC) contains an extensive mantle section, within which numerous podiform chromitite bodies formed during melt percolation in a supra-subduction zone setting. One of the SOC chromitite localities has an unusual style of platinum-group element (PGE) mineralisation. Specifically, the Cliff chromitite suite has relatively high (>250 ppm) Pt plus Pd, compared to other SOC chromitites. In this study, we use petrographic observation, mineral chemistry and X-ray microtomography to elucidate the petrogenesis of PGE-bearing phases at Cliff. The combined data reveal that the PGE at Cliff have likely 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 serpentinised 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.

DOI:

https://doi.org/10.1180/minmag.2017.081.108