REGIONAL DOLOMITE AT NEWCASTLE WEST, CO. LIMERICK, IRELAND AND ITS IMPLICATIONS FOR THE MODELS OF FLUID FLOW RELATED TO BASE-METAL MINERALISATION IN IRELAND

The Carboniferous carbonate succession in the south of Ireland has been pervasively replaced by dolomite on a regional scale. This so called ‘Regional Dolomite’ hosts the orebodies at two of the major Pb-Zn deposits currently being exploited in the south of Ireland, Lisheen and Galmoy.

Dolomite at Newcastle West, ~80km WSW from these deposits, is identical to Regional Dolomite found at Lisheen in terms of gross morphology in hand sample and its cathodoluminescence signature. In both cases it appears that the lithosome, Waulsortian Limestone (Tournaisian), was almost completely cemented prior to dolomitisation, although some local porosity may have been present in the centres of large original cavities. The earliest phase of dolomitisation in both areas preferentially replaces micrite and bryozoans. However, in contrast to the regional development of dolomite at Lisheen and Galmoy, the dolomite body at Newcastle West is restricted laterally and is apparently related to steep fractures. Limestone adjacent to the dolomite front at Newcastle West contains euhedral quartz crystals that increase in size and frequency towards the dolomite front. CL petrography shows that quartz precipitation predated dolomitisation. Homogenisation temperatures of fluid inclusions in quartz range between 160-190°C and in dolomite between 160-200°C. Fluid inclusions in dolomite contain brines with ~24 wt.% NaCl equivalent. d ¹⁸O of both quartz and dolomite are similar (19.0-19.5‰ and 17.2-18.0‰ respectively). Calculated values of fluid d ¹⁸O for both phases lie between 3 and 7‰, indicating that the quartz and dolomite probably precipitated from the same fluid, which had undergone significant water-rock reaction. d ¹³C of dolomite (3.6-3.8‰) is in equilibrium with the enclosing limestones, which is typical of Lisheen and other Irish-type deposits.

These new data indicate that dolomite at Newcastle West, indistinguishable petrographically and in hand sample from the Regional Dolomite that hosts base-metal deposits, was formed by hot, interacted saline fluids.