Platinum-group element mineralization within ultramafic rocks at Corrycharmaig, Perthshire: implications for the origin of the complex

Alloys and complexes of the platinum-group elements (PGE) Pt, Pd, Ru, Ph, Ir and Os occur as discrete platinum-group minerals (PGM) throughout the chromite-dominant lithologies of the Corrycharmaig ultramafic body. PGM are either enclosed by chromite or occur in association with secondary Ni sulphide (heazlewoodite). The PGM enclosed by chromite are large (up to 40 mum), euhedral and interpreted to be primary magmatic in origin. Those that occur in association with heazlewoodite are small (m) and appear to have partitioned into the sulphide phase during alteration and metamorphism and subsequently exsolved. The dominance of Ru, Ir and Os (Ir PGE or IPGE) suggests that either the Corrycharmaig protolith was enriched in these elements relative to Rh, Pd and Pt (Pt PGE or PPGE) or that primary PPGE have been removed from the system. Previous interpretations for the genesis of the Corrycharmaig body have included ophiolite complexes or, more recently, komatiitic intrusions. The nature of the chromite mineralization and chemistry along with the presence of large laurite (IPGE-rich) grains enclosed by chromite are more indicative of an ophiolitic origin.