A high-temperature hydrothermal origin for black dolomite matrix breccias in the Irish Zn-Pb orefield

We make the first report of fluid inclusion data from a black dolomite matrix breccia from the Irish orefield which confirm a relatively high-temperature hydrothermal origin. Breccia matrix dolomite formed from NaCl-dominated fluids with temperatures primarily in the range 150-220°C and salinities of 13-20 wt.% NaCl equivalent, comparable with inferred ore-stage fluids at the Silvermines, Tynagh, Lisheen and Navan deposits. The spread in salinity is considered to be due to mixing of a moderate salinity (~12 wt.%), higher temperature fluid, probably derived from the Lower Palaeozoic metasedimentary basement, with a low temperature brine. Non-stoichiometric, black dolomites are therefore considered to be a product of relatively high-temperature hydrothermal processes involving the principal orefluid in Ireland, and thus their occurrence is believed to be a prime exploration indicator. Earlier, coarse white dolomite, texturally similar to regional dolomite cements commonly observed in the Irish Midlands, precipitated from fluids in the temperature range 140-200°C and salinities of 10.3-13.5 wt.% NaCl equivalent. This is comparable with the higher temperature, moderate salinity end-member inferred for the black dolomite. These data call into question a uniquely low-temperature burial origin for coarse white dolomite cements in Ireland. Paragenetically late low-Mg calcites were formed as a result of an influx of relatively low salinity fluids.