Re–Os geochronology and fluid inclusion microthermometry of molybdenite mineralisation in the late-Caledonian Omey Granite, western Ireland

Abstract

Re–Os geochronometry is combined with fluid inclusion microthermometry to investigate the granite molybdenite system associated with the late-Caledonian Omey Granite, Connemara, western Ireland. Molybdenite in the Omey pluton is hosted by thin vertical quartz veins (<5 cm wide) that trend in a NE–SW direction. The 187Re and 187Os systematics yield a model age of 422˙5 ± 1˙7 Ma for the vein molybdenite. Three fluid inclusion types occur in the molybdenite bearing quartz veins: Type 1(aqueous-carbonic fluid), Type 2 (carbonic fluid) and Type 3 (aqueous fluid), Type 1 and Type 3 also occur in granite quartz. The Type 1 and 2 fluid inclusions are interpreted as representing the molybdenite mineralising fluid as is the case elsewhere in the Connemara granites. The 422˙5 ± 1˙7 Ma age for molybdenite mineralisation is the oldest reported from this region and implies that the Omey Granite was emplaced before the main Galway Batholith and during activity on the major orogen parallel lineaments like the Great Glen and Southern Uplands Faults.