Fluid migration and rock interactions during dolomitisation of the Dinantian Irish Midlands and Dublin Basin

Lower Carboniferous carbonate rocks of the southern Irish Midlands and the Dublin Basin record a complex history of dolomitisation involving multiple fluid sources. At least one regional migration of fluid in both the south M idlands and the Dublin Basin is interpreted based on regional dolomite ceme nt correlation in units as young as Asbian (Supra-Waulsortian). This cement paragenetically predates most main-stage Zn-Pb mineralisation. O isotope a nd fluid inclusion data appear to reflect stratigraphic and geographic cont rols on fluid type. delta(18)O values for the Sub-Waulsortian are the most negative (-10 PDB) in the Dublin Basin. Waulsortian delta(18)O values are a lso most negative (-10 PDB) in the Dublin Basin and in the south Midlands, near the Rathdowney Trend. The delta(18)O values for the Supra-Waulsortian are the most negative (-10 PDB) from the Rathdowney Trend southeast towards the Leinster granite. If the patterns of the increasing delta(18)O values reflect cooling along their respective flow paths, then the distinct patter ns noted in each stratigraphic interval require multiple fluids from multip le sources. In contrast, a single fluid and source affecting the entire str atigraphic section should yield a similar pattern throughout. Analysis of f luid inclusion data also suggests that three, chemically and thermally, dis tinct fluids precipitated dolomite, and those fluids in the Supra-Waulsorti an are at slightly higher temperature and are less saline than their counte rparts in the Waulsortian and Sub-Waulsortian. The patterns of the delta(18 )O values could reflect other processes besides cooling, such as difference s in fluid-rock interaction, or introduction of multiple chemically distinc t fluids at different times and loci throughout the diagenetic hydrothermal history. Stable isotopes alone cannot definitively answer questions regard ing the localised versus regional flow, or distinguish fluid sources from w ater-rock reactions. However, using these isotopes and other geochemical da ta we propose a hybrid model of fluid flow, incorporating both regional as well as localised fluid flow, occurring during the dolomitisation of the Di nantian rocks of Ireland. (C) 2000 Elsevier Science B.V. All rights reserve d.