THE ROLE OF BITTERN BRINES AND FLUID MIXING IN THE GENESIS OF THE NAVAN ZN-PB DEPOSIT, IRELAND

The Navan deposit, located near the northern margin of the Irish Midlands basin, is the largest operating carbonate-hosted zinc deposit in Europe. It has long been established that most of the sulfur in the deposit is derived from bacterial reduction of Lower Carboniferous sea water sulfate, yet the mode of transport of this sulfur to sites of mineralization is unknown. Results from petrographic, isotopic and fluid inclusion studies reveal for the first time that low temperature (<130°C), high salinity (21-25wt%NaClequiv.), bromine-enriched brines transported bacteriogenic sulfur into the ore-forming environment. These dense, bittern brines probably migrated basinward into the Navan area from the shallow water shelves to the north via stratal aquifers. However, within the ore zones themselves, rapid salinity fluctuations (5 to 25wt%NaClequiv.) are recorded by fluid inclusions within strongly CL-zoned hydrothermal dolomite cements. Significant fluctuations in the d ³⁴S values of rhythmically-banded sulfides (~ -20 to +10) are also observed. These features are consistent with the injection of pulses of a second, lower salinity fluid transporting minor, isotopically heavier sulfur into the brine-filled aquifer system. Variably efficient fluid mixing at these sites is therefore considered to be the primary mechanism driving ore-related dolomitization and sulfide precipitation.