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21.
Stratigraphical distribution of platinum-group minerals in the Eastern Layered Series, Rum, Scotland
Abundant and diverse platinum-group minerals (PGM) occur throughout the Tertiary layered intrusion on Rum, Scotland. In this
paper we document the distribution of PGM within the Eastern Layered Series (ELS) on Rum, which comprises 16 alternating units
of olivine-dominant feldspathic peridotite grading to plagioclase-dominant allivalite. The PGM occur in six main chrome-spinel
layers in the Eastern Layered Series and are clearly associated with minor concentrations of interstitial sulphides. Common
PGM phases include: Pd–Cu alloys, Pt–Fe alloys, native Pt, laurite, moncheite, sperrylite, isomertiete, cooperite and braggite
along with a large number of other less common arsenide, bismuthotelluride and sulphide phases. Analyses of the discrete chromitite
layers yield up to ΣPGE + Au 2618 ppb. Although present throughout the 750-m-thick ELS, there are clear stratigraphical changes
in the 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 spot', followed by localised concentration because of the combined effects of magma mixing,
sulphide-silicate liquid immiscibility and fractional crystallisation. Most of the PGM are magmatic in origin but some grains
show evidence of hydrothermal alteration.
Received: 27 November 1999 / Accepted: 27 April 2000 相似文献