Diagenetic evidence for an epigenetic origin of the Courtbrown Zn-Pb deposit, Ireland

Mineralisation at the Courtbrown deposit in south-western Ireland is concen trated in the basal section of the Chadian Waulsortian Limestone, immediate ly above the Courceyan Ballysteen Limestone. Two episodes of sulphide depos ition have been identified: an early stage of minor pyrite precipitation, a nd a later base-metal-rich mineralisation event. Sphalerite, galena and pyr ite of the later mineralisation event occur predominantly as replacement ph ases along stylolites, dissolution seams, and within the micritic matrix of the host limestone. These sulphide minerals also occur as cements within l ate stage fractures. The following diagenetic phases are present in the Waulsortian and Ballyste en Limestones in the Courtbrown area (from oldest to youngest): non-lumines cent synsedimentary calcite cements, non-luminescent equant calcite cements , bright luminescent calcite cement, dull luminescent calcite cement, plana r dolomite cement and replacement dolomite (regional dolomite), saddle dolo mite cement, and fibrous dull luminescent calcite cement filling pressure-s hadows around the sulphide minerals. Homogenisation temperatures for primary fluid inclusions within dull lumine scent calcite cements (precipitated penecontemporaneously with base-metal m ineralisation) range from 160 to 200 degreesC, with a mode at 170-180 degre esC. These values are unlikely to be representative of mineralisation tempe ratures as the fluid inclusions may have been significantly affected by hea ting and/or deformation during late burial (maximum paleo temperatures from Ro and CAI data around 310 degreesC). The observed paragenetic sequence indicates that mineralisation is complete ly epigenetic. As the earliest mineralisation is hosted by macro-stylolites , the sequence must have obtained a minimum burial depth of around 800 in p rior to the onset of mineralisation. A burial depth of 800 m would correspo nd to an approximate early Chadian age for the Courtbrown area. Pressure-sh adows around sphalerite further indicate that mineralisation preceded the m ajor phase of Variscan deformation. Therefore, the base-metal mineralisatio n at Courtbrown is epigenetic, and the age of mineralisation is in the rang e of 350 to 307 Ma.