The origin and nature of hydraulic fractures and veins within the Burren, County Clare, Ireland

Carboniferous (Mississippian) limestones of the Burren are cross-cut by sub-vertical veins, from 1µm up to 50cm thick, defining a strongly clustered and scale-indepen-dent system in which predominantly N-S veins are transected by longer NNE-trending veins. Vein infills mainly comprise of calcite, but with subordinate amounts of quartz, sulphide (mainly galena and sphalerite) and fluorite also occurring, particularly in the south-central part of the area. Thinner and shorter veins are planar and discontinuous in plan view, sometimes forming en-echelon arrays, with thicker veins forming better connected and more complex structures which extend for several kilometres across the Burren region. Veins with ‘exotic’ infills are generally both longer and thicker, and they appear to be spatially associated with, or up to 5km to the north of, a 5km wide zone of ENE-trending Variscan monoclinal folding. Individual veins are vertically persistent, and the same structures are seen throughout the exposed ca 1200m thick Carboniferous sequence, from Tournaisian limestones through to Serpukhovian-Bashkirian siliciclastics. The veins are mainly extensional, sometimes with a component of sinistral displacement particularly on NNE-trending veins, displaying fibrous growth through to hydraulic fracturing and brecciation. Their formation is attributed to the valving of overpressured fluids within Mississippian basins during N-S Variscan compression. Pb isotope analysis supports a model in which sulphide infills are scavenged from underlying basement rocks or hydrothermal Zn-Pb mineralisation during the tectonic inversion of post-rift sequences overlying Lower Carboniferous normal faults.