Ore petrology and metamorphic mobilization of copper ores in red beds, southwest County Cork, Ireland

Small copper deposits, many of which were mined during the nineteenth centu ry, are numerous in the West Carbery district of County Cork, Ireland. The mineralization is hosted by Old Red Sandstone, which was metamorphosed in t he Variscan orogeny. Petrographic, mineralogical, isotopic and reflectance studies indicate pressures of 2 +/-1 kbar and temperatures of 350 +/- 50 de grees C. Peak metamorphic conditions were reached during extension and pers isted during the early compression stage as pressure solution cleavage deve loped, well before major folding, faulting and thrusting. Sulphide minerali zation is present in three associations: (1) low-grade stratiform deposits of disseminated copper-iron and copper sulphides hosted in the Castlehaven Formation; (2) thin, minor quartz veinlets with the same mineralogy as the stratiform disseminated deposits at the same stratigraphic horizons; and (3 ) thick quartz veins (as at Ballycummisk and Dhurode) higher in the stratig raphic succession in the Toe Head and Old Head Sandstone Formations; these have a more varied mineralogy, including discrete Mo, Pb and Bi phases and significant amounts of sulpharsenides and antimony-bearing sulphosalts. Detailed petrography and ore-textural studies indicate a multistage deposit ion of the ores. The earliest mineral assemblage, now seen as disseminated bornite and djurleite/chalcocite with traces of wittichenite (Cu3BiS3), has resulted from the metamorphism of sulphide ores, themselves formed by the diagenetic bacteriogenic reduction of sulphate. As pressure solution cleava ge developed this disseminated ore was mobilized by metamorphic fluids from domains that developed cleavage and redeposited in adjacent rocks or in mi nor segregation veins. Later supergene fluids have altered this assemblage progressively to geerite, spionkopite, yarrowite and covelline together wit h copper-poor bornites that show a range of compositions. The major veins o riginated after peak metamorphism by remobilization of the stratiform disse minated deposits. Fluids scavenged Mo, Pb, Sb, As and Ri from large volumes of rock and deposited the major vein concentrations in faults at higher st ratigraphic levels than the stratiform deposits. Several stages of ore depo sition are recognizable in these thick quartz veins; the earliest sulphide is Fe-free (molybdenite) and thereafter the Fe content of the reduced miner als increases in the sequence tetrahedrite to bornite to chalcopyrite. An u pper limit on temperature at a late stage of the sulphide formation is set by emplectite, which is not stable above 318 degrees C. The final phase of ore deposition was associated with barite vein formation. Sulphur isotope data for sulphides from stratiform disseminated deposits, m inor veins and major veins show no distinction between them, offering no ev idence of any mixing with post-metamorphic meteoric fluids, pre-metamorphic basinal brines or any magmatic contribution. Thus, the transport of copper ores from stratiform disseminations by metamorphic fluids has been an impo rtant process of metal concentration, which emphasizes the importance of up per crustal fluid migration in ore remobilization in the Munster Basin.