Blood from a stone: preliminary fluid inclusion results from Navan Zn-Pb deposit, Ireland

摘要

The Navan carbonate-hosted Zn-Pb deposit represents a remarkable base-metal anomaly even within the extensively mineralized carbonate basins of Ireland, containing approximately 54 percent of the orefield's known pre-mining reserves of Zn and 50 percent of the Pb. Despite its significance, however, very few fluid inclusion data exist on the properties of the hydrothermal fluids that were responsible for ore formation owing, at least in part, to the fine grain size and turbidity of potential inclusion-hosting phases.New fluid inclusion data obtained by high-resolution optical microthermometry show that Navan experienced a multi-stage fluid history, apparently dominated by low-temperature fluids (ca 120 +- 30 deg C) with a wide range in salinity (3-25 equiv. Wt percent NaCl). Although paragenetically earlier fluids generally fall at the lower-salinity and higher homogenization-temperature end of this distribution, evidence from detailed traverses across compositionally banded dolomite veinlets indicates that fluid salinities increased dramatically for short periods during vein growth. This is interpreted to reflect periods of low-temperature brine influx into a system dominated by lower-salinity, higher-temperature hydrothermal fluids, resulting in intermittent mixing episodes.