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 ladling of open cavities, within elastic sedimentary rocks that 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 shows 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 made up of 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 <3.5 wt percent. The majority of fluid inclusion homogenization temperatures range from 126 degrees to 257 degrees C and geologically constrained depth estimates (<= 5 km) indicate maximum fluid inclusion trapping temperatures between 175 degrees and 245 degrees C. The geodynamic setting of Bohaun, which is located in a deformed continental margin of an allochthonous terrane, is typical of orogenic gold deposits. However, an orogenic association for the Bohaun mineralization is discounted on the basis of fluid inclusion evidence, atypical vein textures, gold grain composition, and undeformed veins. While vein textures locally resemble those found in epithermal quartz-adularia mineralization, there is no obvious magmatic association and alteration is restricted to the veins and immediate host rocks. The origin of the gold at Bohaun remains enigmatic. However, we favor an association with widespread Carboniferous or later mineralizing fluids and a model which involves transport and deposition of gold from a basinal brine-type fluid, in a similar fashion to red-bed Au-Pd mineralization. Bohaun provides further evidence that low temperature brines may be capable of generating economic-grade gold mineralization, independent of other fluids.