Stratigraphical distribution of platinum-group minerals in the Eastern Layered Series, Rum, Scotland

Abundant and diverse platinum-group minerals (PGM) occur throughout the Ter tiary layered intrusion on Rum. Scotland. In this paper we document the dis tribution of PGM within the Eastern Layered Series (ELS) on Rum. which comp rises 16 alternating units of olivine-dominant feldspathic peridotite gradi ng to plagioclase-dominant allivalite. The PGM occur in six main chrome-spi nel layers in the Eastern Layered Series and are clearly associated with mi nor concentrations of interstitial sulphides. Common PGM phases include: Pd -Cu alloys, Pt-Fe alloys, native Pt, laurite, moncheite. sperrylite, isomer tiete, cooperite and braggite along with a large number of other less commo n arsenide, bismuthotelluride and sulphide phases. Analyses of the discrete chromitite layers yield up to Sigma PGE + Au 2618 ppb. Although present th roughout the 750-m-thick ELS. there are clear stratigraphical changes in th e PGM assemblage. The presence of PGM in the ELS on Rum are interpreted as being caused by mantle melting associated with the proto-Icelandic 'hot spo t', followed by localised concentration because of the combined effects of magma mixing, sulphide-silicate liquid immiscibility and fractional crystal lisation. Most of the PGM are magmatic in origin but some grains show evide nce of hydrothermal alteration.