GEOCHEMISTRY OF INCLUSION FLUIDS IN THE WAULSORTIAN OF THE IRISH MIDLANDS: RECONSTRUCTING MULTIPLE-FLUID INTERACTIONS DURING DOLOMITIZATION AND BASE-METAL ORE FORMATION

The primary host of large base-metal sulfide deposits in the Irish Midlands is the Lower Carboniferous Wauslortian limestone. Deposits are associated with southwest- to northeast-trending fault and fracture zones.  Base-metal ores are associated with large volumes of open-space-filling epigenetic dolomite and calcite cement. Fluid inclusion and geochemical analysis of carbonate cements was employed to determine the spatial and stratigraphic distribution of dolomitizing fluids in order to differentiate between regional-scale, stratigraphically controlled flow versus localized, fault-related flow. Fluid inclusion analysis indicates the presence of three chemically and thermally distinct fluid types (type 1, T_m -39.7 to -16.7° C, T_h 72 to 136° C; type 2, T_m -8.8 to -0.1° C, T_h 103 to 136° C; type 3, T_m -12.9 to -0.9° C, T_h 169 to 271° C). Halogen compositions of these fluids were examined using crush-leach-ICP analysis. Cl:Br ratios range from 92 to 354 and lie along the seawater evaporation/dilution trend, indicating fluid genesis from seawater evaporated beyond the point of halite precipitation (Cl:Br ~ 655). Cl:Br and Na:Br ratios define a linear trend and are enriched in Na relative to Cl, indicating fluid-rock reactions along flowpaths. Enrichment in chloride relative to bromide reflects a component of salinity derived from dissolution of halite. Laser ablation ICP-MS analysis of individual fluid inclusions is being performed to verify the crush-leach data. Initial results indicate that Na:Br ratios are the same as those derived via crush-leach analysis. The ability to examine individual fluid inclusions will enable us to analyze samples containing multiple fluid types. Combining geochemical and fluid inclusion analysis allows “fingerprinting” of the sources, flowpaths, and chemical evolution of dolomitizing fluids and will help constrain numerical models for fluid flow in the Irish Midlands.