Regional fluid flow and the genesis of Irish Carboniferous base metal deposits

The Irish carbonate-hosted base metal deposits have long been an enigmatic subclass of deposit. Some of the Irish deposits (Harberton Bridge, Allenwood) are clearly epigenetic, hosted in breccia pipes and have close affinities to Mississippi Valley-type deposits. Others, are characterised by stratabound and sometimes stratiform mineralisation (Abbeytown, Navan), while a third group is associated closely with concordant dolomitic breccias and cavity fill mineralisation at the base of carbonate mud mounds (Lisheen, Ballinalack). When the stratigraphic and textural evidence is reviewed, it is apparent that all of the base metal mineralisation occurred in already compacted sediments. Hence, mineralisation probably occurred at depths of at least several 100 m depth below the sea bed. The mineralisation occurred either during transtensional Lower Carboniferous basin development, or subsequently, during the onset of Variscan shortening. Fluids may have been derived from dewatering of Variscan-driven deformation to the south of the carbonate platform, with fluid flow through major fracture zones and basal elastic aquifers. Alternatively, high heat flow produced by Lower Carboniferous extension may have driven the mineralising system. Syn-genetic models are extremely difficult to sustain.