THE ROLE OF EVAPORITES IN BRINE EVOLUTION IN LOWER CARBONIFEROUS CARBONATES OF IRELAND: IMPLICATIONS FOR DOLOMITIZATION AND ZN-PB ORE DEPOSITION

Zn-Pb ores hosted in Waulsortian Limestone of the Irish Midlands are associated with large volumes of open-space-filling epigenetic carbonate cements. Halogen chemistry of brine inclusions (15 to 25 equiv. wt. % NaCl) in these cements is consistent with genesis of hydrothermal fluids from evaporated seawater. Cl:Br ratios range from 92 to 354 and lie along the seawater evaporation-dilution trend, indicating genesis from seawater evaporated beyond the point of halite precipitation (Cl:Br ~ 655). Cl:Br ratios of these fluids are enriched dramatically in Cl, indicating either (1) dehydration of hydrothermal fluids at elevated temperatures, or (2) the dissolution of halite along flowpaths. Evaporites that may have influenced brine chemistries appropriate to ore transport and deposition include lower Tournaisian rocks in the NW Dublin Basin, upper Tournaisian rocks in Northern Ireland, and evaporite-bearing rocks of late Viséan age in NW Ireland. Additionally, we have recognized peritidal and sabkha deposits in Chadian rocks adjacent to the Leinster Massif in the eastern Irish Midlands that appear to be coeval with similar evaporite-bearing rocks in the southern Midlands. Fine-grained, planar-s dolomite associated with quartz pseudomorphs after evaporites, dolomite replacing gypsum laths, stromatolitic or algal laminated crusts and very fine, angular quartz grains, reflect dolomite precipitation in an evaporative tidal flat environment near a shoreline. δ¹³C and δ¹⁸O values (PDB) of micrite (0.4 to 1.6 per mil; -2.0 to 2.2 per mil) and early planar dolomite (3.0 per mil; -1.3 per mil) are indicative of rocks deposited in an evaporative environment. The presence of evaporites in the Irish Midlands is a plausible explanation of Cl enrichment in mineralizing brines. Since many metals are transported as chloride complexes, it is also likely that the presence of evaporites played a critical role in developing brine chemistries suitable for metal transport and deposition in the Irish ore field. To establish possible linkages between the presence of evaporites and the development of base-metal depositing fluids, future studies will focus on Lower Carboniferous sequences more distal to known ore deposits to assess their potential as sources/conduits for ore fluids and to reconstruct pathways of mineralizing fluids.