Atypical gold mineralization in an orogenic setting : the Bohaun Deposit, western Irish Caledonides

The Bohaun gold deposit in County Galway, western Ireland is located in the Caledonian orogenic belt, proximal to a major accretionary boundary. The mineralization is hosted in a normal extensional fault, representing a zone of repeated extension and vein infilling of open cavities, within clastic sedimentary rocks, which have only reached the late diagenetic zone of low-grade metamorphism. The mineralization consists of undeformed veins, stockworks and breccias. Quartz dominates all the veins and commonly displays multiple growth stages and a variety of classic epithermal textures, including comb, plumose and banded, indicative of low confining pressures. The veins have a simple mineralogy comprising quartz–sericite–chlorite with minor sulfides, hematite, rare visible gold, with a high silver content (up to 41 wt %), and late barite and dolomite. Vein quartz is associated with two fluids compositions: Type I with moderate to high salinity (8.0–23.6 wt % NaCl equiv) and Type II with low to moderate salinity (0.0–7.8 wt % NaCl equiv). CO2 contents in both fluid types are