Abstract

The Shetland ophiolite, located to the northeast of the Scottish mainland, contains podiform chromitites. Two of these are known to host extremely anomalous PGE concentrations. The chromitite at Cliff is enriched in Pt, Pd, and Rh PGE (PPGE) and the Harold’s Grave chromitite is enriched in Ir, Ru, and Os PGE (IPGE). Analysis of multiple samples of chromitite from chromite quarry spoil tips and surrounding chromite-bearing dunite and chromite-bearing harzburgite has revealed that there are links between the chromite geochemistry and the PGE. Both Cliff and Harold’s Grave have a characteristic chromite geochemistry, which distinguishes them from the other PGE-poor chromitites.

The IPGE-rich Harold’s Grave chromite contains elevated trace elements (TiO2, V2O5, and Zn), lower Mg#, and a lower Fe3+/Fe2+ ratio than in the other chromitite deposits. This suggests formation from a more reduced magma that favors the concentration of IPGE in chromitite. It is proposed that the IPGE have been concentrated to such enriched and consistent concentrations due to large throughput of magma, leaving an unusually large dunite envelope that surrounds a rather small concentration of chromitite. The geochemistry of the Harold’s Grave deposit suggests formation from a mid-ocean ridge basalt (MORB)/island-arc tholeiite-type magma. There is a range of chromite compositions at Cliff that suggests formation of the chromitite from a variety of mantle melts from mantle of variable composition. PPGE-rich Cliff chromite has a lower Cr# combined with a higher TiO2 than the other chromite in chromitites at Cliff. It is suggested that a more island-arc tholeiite-type magma formed these PPGE-rich chromitites compared to a more boninitic magma that formed the other chromitites at Cliff. This island-arc tholeiite-type magma was closer to sulfur saturation than the boninitic magma and reached sulfur saturation, producing the very PPGE-rich chromite at Cliff.

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