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Mineralogical Magazine; December 2000; v. 64; no. 6; p. 1017-1036
© 2000 Mineralogical Society of Great Britain and Ireland
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A high-temperature hydrothermal origin for black dolomite matrix breccias in the Irish Zn-Pb orefield

J. J. Wilkinson1,* and G. Earls2

1 Fluid Processes and Mineralization Research Group, T.H. Huxley School of Environment, Earth Sciences and Engineering, Royal School of Mines, Imperial College, London SW7 2BP, UK
2 CSA Ltd., CSA House, 6-7 Dundrum Business Park, Windy Arbour, Dublin 14, Ireland

* E-mail: j.wilkinson{at}ic.ac.uk

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.

KEYWORDS: zinc mineralization, hydrothermal, dolomite, breccia, carbonate-hosted deposits, Silvermines, Tynagh, Lisheen, Navan, Ireland




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