NATURE AND TIMING OF MINERALISING FLUIDS DURING THE APPALACHIAN-CALEDONIAN OROGENY: EVIDENCE FROM THE CONNEMARA GRANITES WESTERN IRELAND

Geofluid research is significantly enhanced if the age of fluids can be determined and linked with the physico-chemical role they play during orogenesis. Granite magma generation and emplacement occurs during orogenesis. Furthermore, the deposition of molybdenite in economic and sub-economic abundances is a common by-product of crystallizing granite magma. This genetic relationship provides the foundation for our ongoing studies of the timing and composition of granite related molybdenite-mineralising fluids along the length of the Appalachian-Caledonian Orogen (ACO). We report the results of these investigations in the Connemara sector of the ACO. Disseminated and quartz vein-hosted molybdenite occurs throughout the late-Caledonian Galway batholith and its satellite plutons e.g. the Omey Granite. Notable occurrences are at the western end of the Galway batholith at Mace Head and Murvey. At Mace Head, molybdenite-bearing quartz veins (~ 5 - 30 cm thick) trend NE-SW their orientation controlled by early jointing in the host granite. Additional vein minerals include chalcopyrite, pyrite, magnetite and muscovite. At Murvey, a leucogranite contains disseminated molybdenite intergrown with muscovite. Quartz veins in the area also contain molybdenite mineralisation. In the Omey Granite pluton molybdenite (2-4 mm disseminations and rosettes ~ 5 mm across) is hosted by thin, discontinuous quartz veins (< 5 cm across) that also trend in a NE-SW direction. Geochemical, fluid inclusion and limited stable isotope (O, H, S and C) studies indicate that the molybdenite mineralisation was produced by highly fractionated granite magma. An aqueous-carbonic fluid of moderate salinity (4-10 wt.% NaCl eq.) deposited the late-magmatic molybdenite mineralized quartz veins in the Galway batholith and the satellite Omey pluton.

Re-Os molybdenite age determinations however, indicate that in Connemara the granite related molybdenite mineralization took place at different times i.e. 407.3 ± 1.5 Ma (Mace Head), 410.5 ± 1.5 Ma and 410.8 ± 1.4 Ma (Murvey- two samples) and 422.5± 1.7 Ma (Omey). These results provide time constraints not only on granite emplacement but also on the timing of the aqueous-carbonic fluid activity responsible for the deposition of molybdenite in the Connemara sector of the ACO.