Paragenesis of multiple platinum-group mineral populations in Shetland ophiolite chromitite: 3D X-ray tomography and in situ Os isotopes

Abstract

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

“…The PPGM are hollingworthite and scarce hongshiite (PtCu), Pt-alloy (Pt-Pd-Cu), genkinite and Pd-, Rh-and Ni-bearing antimonides totalling 9 grains only (<10%;Table 2). The survey byPrichard et al (2017) corroborates that PPGM, including minor sperrylite, Pt-Fe alloys and platarsite, typically occur in clusters with Ru-pentlandite and some IPGM interstitially to chromite grains.The placer PGM located at Harold's Grave (n=3 grains) consist of laurite, iridium alloy and osmium alloy, with sizes ranging from 20 to 60 µm. No PPGM or pentlandite were recovered(Table 2).…”

“…consists of laurite, osmium, irarsite and IrSbS. The size of the IPGM in the rocks ranges from a few microns to 500 µm, as found byBadanina et al (2013Badanina et al ( , 2016 from concentrates of the rocks produced using the hydroseparation technique, and byPrichard et al (2017) using 3D X-ray tomography. The PPGM are hollingworthite and scarce hongshiite (PtCu), Pt-alloy (Pt-Pd-Cu), genkinite and Pd-, Rh-and Ni-bearing antimonides totalling 9 grains only (<10%;Table 2).…”

“…Other minerals recorded include genkinite, hongshiite (PtCu), stibiopalladinite and unnamed Rh–Sb–S and Rh–Ni–Sb phases (Prichard and Tarkian, 1988). Recent studies reveal up to five generations of PGM in these chromitites (Prichard et al ., 2017). In contrast, the PGM in the rocks at Cliff are PPGM dominated.…”

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

“…The Os isotope data of O'Driscoll et al (2012) indicate that Cliff chromitites are more radiogenic ( 187 Os/ 188 Os initial of ~0.1292) than other SOC chromitites (0.125), potentially supporting the role of a hydrothermal fluid that had interacted with the metasediments in the provision of the As. In fact, Prichard et al (2017) recently reported variable Os isotope compositions for two PGM grains from Cliff ( 187 Os/ 188 Os between 0.1270-0.1300), suggesting the presence of >1 PGM paragenesis, i.e., that the more radiogenic Os isotope compositions observed at Cliff may be controlled by a specific PGM paragenesis. In this scenario, which is our preferred interpretation, we envisage that As-rich fluids altered the existing sulphide assemblage in the Cliff chromitites, such that the high PGE concentrations probably reflect the original chromitite PGE budgets, albeit with reworking and upgrading of the PPGE in particular.…”

“…Here, we combine X-ray microtomography (μCT) with petrography and mineral chemistry to visualise the 3D distribution and microstructure of the base-and precious metal-rich phases and link this to their chemical signatures. Establishing the microstructural relationships between Cr-spinel, base-metal phases and the PGM in chromitite is pivotal to understanding the petrogenesis of these deposits; distinguishing between these high density phases is an objective particularly well-addressed by μCT (e.g., Ketchum and Carlson 2001;Godel et al 2010;Godel et al 2013;Godel, 2015;Prichard et al 2017). The key goal in this study is to elucidate the distribution of the PGE between specific carrier phases and link this to the chromitite microstructure and mineral chemistry to formulate a comprehensive petrogenetic interpretation.…”