FAULT-ASSOCIATED DOLOMITIZATION AND BASE-METAL SULFIDE VEIN FORMATION IN CARBONIFEROUS ROCKS OF THE NORTH DUBLIN COAST, IRELAND: THE INTERPLAY OF BASINAL- AND BASEMENT-INVOLVED FLUIDS

Outcrops along the east coast of Ireland, on the northeastern edge of the Dublin Basin, comprise both shallow marine carbonate platform and deep-water turbidite sequences of Carboniferous age. Multiple generations of fault/fracture-related quartz and carbonate veins cutting these rocks are associated locally with dolomitization of adjacent limestone and deposition of base-metal sulfides. Fluid inclusions record the presence of two dominant types of fluids: a moderate to high-temperature (T_h ~ 150 to 300°C), low to moderate salinity brine and a low-temperature (T_h ~ 60 to 135°C), high-salinity brine. Isotope analyses of host limestone/dolomite and carbonate veins define trends of δ¹⁸O_(VPDB) values from ~ -3 toward -22‰, δ¹³C_(VPDB) values from +4 toward -4‰, and ⁸⁷Sr/⁸⁶Sr values from 0.7080 toward 0.7105. The δ¹⁸O and ⁸⁷Sr/⁸⁶Sr values of latest calcite veins are interpreted to reflect incursion of evolved waters that interacted with more radiogenic rocks along their flow paths. The geochemical and isotope data obtained in this study suggest that the northeastern edge of the Dublin basin was affected by a distal part of the regional Irish fluid flow system that interacted with local, structurally controlled hydrothermal fluids similar to those responsible for dolomitization and Cu-Zn-Pb sulfide mineralization in the Irish Midlands and the nearby Isle of Man. High-temperature fluid with basement rock involvement appears to be a prerequisite for the development of Cu-bearing mineralization. Basins that experienced deeper burial and/or thermal overprints imposed by localized hydrothermal cells are better analogues for the northeastern Dublin Basin’s fault-related hydrothermal mineralization.