Abstract

The Harberton Bridge deposit is located 2 km NW of the Lower Palaeozoic Kildare inlier in east central Ireland and represents one of a number of small, breccia-hosted Fe–Zn–Pb deposits that occur in the Kildare district. Widely regarded as an example of Mississippi Valley-type mineralization, the Harberton Bridge deposit provides important clues regarding the origin of the carbonate-hosted Zn–Pb deposits of the Irish orefield as a whole. Mineralization was studied in core using cathodoluminescence, transmitted and reflected light microscopy and fluid inclusion microthermometry. Fluid inclusions from calcite associated with sphalerite–galena–marcasite mineralization have homogenization temperatures in the range 45–69°C and salinities mainly between 12 and 17wt% NaCl equivalent. The data obtained suggest that the Harberton Bridge deposit formed from a single, low temperature (<80°C), moderate salinity fluid, considered to be a cooler version of the higher temperature fluids involved in mineralization at all of the economic Irish deposits. The inferred common fluid origin with the major deposits confirms the position of the Kildare district as representative of an end-member of mineralization styles in the Irish orefield. The differences in style are attributed to a late timing of mineralization, absence of abundant biogenic H2S and the involvement of a relatively shallowly basement-circulated, low temperature orefluid.

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