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Department of Geology, Museum Building, Trinity College, Dublin 2, Ireland mulhallc{at}tcd.ie
Dolomitized Tournaisian rocks occur widely in Ireland. They host the major Zn-Pb ore bodies, at Lisheen and Galmoy. Despite significant research and exploration into fluid flow related to both dolomitization and base-metal mineralization, the evolution of the porosity and permeability of the dolomites and precursor limestones remains poorly understood. Study areas, both within and outside the main mineralized zones, have been chosen in an effort to unravel the evolution of the porosity and permeability of the Tournaisian successions. In each area the earliest phase of dolomitization of the limestone lithosome occurred after all of the primary porosity had been occluded. Fluid inclusions from dolomite and authigenic quartz indicate relatively high temperatures (>105 °C) of formation of all phases of dolomite. The fluid-inclusion data demonstrate that there has been no re-equilibration due to later Variscan heating. Laterally restricted bodies of dolomite at Newcastle West and Subulter may have formed under hydrothermal or geothermal conditions. The geometry of the dolomite body at Newcastle West suggests that the dolomitizing fluids moved within a steeply dipping zone. The laterally extensive Regional Dolomite of SE Ireland, examined at Lisheen and in Co. Carlow, is likely to have had a geothermal origin and to have formed at depths of more than 2.5 km. The dolomites are moderately porous with the notable development of vugs; but their permeability is low. The vugs are not related to primary cavities within the limestone.
