RECOGNIZING ORE-RELATED HYDROTHERMAL FLUIDS OUTSIDE OF MINERALIZED AREAS: A REGIONAL STUDY OF FLUID INCLUSION MICROTHERMOMETRY-HALOGEN GEOCHEMISTRY OF DOLOMITIZING FLUIDS IN THE IRISH ZN-PB ORE FIELD

Previous studies of dolomitization of Viséan rocks of the Irish Midlands focused on ore fluids in and near Zn-Pb ore zones. To provide a regional context within which to place studies of individual ore deposits/districts, we report results of a regional-scale study involving fluid inclusion microthermometry and halogen geochemistry of carbonate cements.

Fluid inclusion and halogen geochemical analyses indicate the presence of at least three distinct fluids after early dolomitization: 1) A low temperature, higher salinity fluid (T_h 72-136° C, 18.4-35.9 eq. wt. % NaCl; Cl:Br ratios of 114-2154) that is ubiquitous and is associated with saddle dolomite cement; 2) A low temperature, lower salinity fluid (T_h 103-136° C, 0.2-12.6 eq. wt. % NaCl; Cl:Br ratios of 246-499) found in SubWaulsortian and Waulsortian units; and 3) A higher temperature, lower salinity fluid (T_h 169-271° C, 1.6-16.8 eq. wt. % NaCl; Cl:Br ratios of 354-898) found in Waulsortian and SupraWaulsortian rocks. The halogen geochemistry of included fluids is consistent with modification of Lower Carboniferous seawater. Type 1 fluids are enriched in chloride relative to bromide indicating a component of salinity derived via dissolution of halite. Na and K concentrations indicate genesis of type 1 fluids from seawater evaporated to the point of epsomite precipitation whereas type 2 and 3 fluids were derived from seawater that did not evaporate to the point of halite precipitation. Low temperature saline fluids with halogen geochemistry similar to fluid type 1 have been reported at Tynagh, Lisheen, Silvermines, and in post-ore dolomite in the Rathdowney Trend. Warmer, less saline fluids with halogen compositions similar to fluid types 2 and 3 have been reported in ore-stage carbonates at Tynagh, Lisheen, Silvermines, Garrycam and Derrykearn, and Gortdrum.

The regional data are interpreted to indicate the presence of multiple fluids during dolomitization and mineralization in the Irish Midlands. Lower salinity fluids likely are ore-related fluids in non-mineralized zones. The high salinity fluid indicates a long-lived, regional fluid flow system that brackets in time the Zn-Pb mineralizing event. The presence of ore fluids in non-mineralized zones has important implications for mechanisms of ore deposition and for ore exploration in the Irish Midlands.