The Upper Palaeozoic Vein Hosted Copper Deposits of the Allihies Mining Area, Southwest Ireland – A New Structural and Chronological Evaluation

This study focusses on the vein-hosted copper sulphide deposits in the Upper Palaeozoic Munster and South Munster Basins of southwest Ireland. Detailed mapping of the Allihies mine area (Beara Peninsula), have led to a new interpretation of the timing and development of mineralised quartz veins. Macro- and microstructural investigations reveal that the copper sulphide-bearing, mainly E-W striking quartz veins are directly related to early extensional, basinal normal faults. Molybdenite Re-Os dating of the main-stage Cu lode yield ages from 367.3 ± 5.5 to 366.4 ± 1.9 Ma. Bi-phase (LV) aqueous fluid inclusions associated with the mineralised quartz veins range from moderate salinity with high homogenisation temperatures (>3.2 wt% NaCl_equiv, Th < 314°C) to high salinities with very low homogenisation temperatures (<28.5 wt% NaClequiv, Th >74°C) The extensional faults and associated quartz veins experienced subsequent late Carboniferous Variscan deformation, including cleavage development, sinistral SW-NE strike slip faulting, cataclastic deformation and recrystallization of vein fills. Later fluids with low to moderate salinities and Th values of about 200°C were trapped in syn-Variscan quartz-chlorite saddle reefs and en echelon tension gash arrays in semi brittle shear zones. The new timing of Cu mineralisation in SW Ireland has major implications for its relationship to the base metal deposits of the Irish Midlands.