THE GENESIS OF CU-BEARING QUARTZ VEINS BY METAMORPHIC REMOBILIZATION FROM STRATIFORM RED-BED DEPOSITS, SW COUNTY-CORK, IRELAND

Polymetallic major veins of the West Carbery district (County Cork) ar e compared with the nearby stratiform-disseminated copper mineralizati on in metasedimentary rocks, containing minor veins (metamorphic quart z veins and veinlets). These stratiform deposits are hosted by non-mar ine Devonian sediments (Old Red Sandstone), metamorphosed in the Hercy nian orogeny. In sulphides from the stratiform deposits and minor vein s, isotopic compositions of sulphur (delta(34)S) range from -21.00 to +5.14 parts per thousand, consistent with the vein sulphide being remo bilized stratiform-disseminated sulphide, and the latter being of diag enetic bacteriogenic origin. Sulphate (barite), found in veins separat e from the sulphides, has delta(34)S + 12.3 to 15.7 parts per thousand , consistent with groundwater origin. In minor-vein quartz, fluid incl usions have homogenization temperatures consistent with trapping under the estimated peak-metamorphic conditions (300-400 degrees C, 1-3 kba r). In the major veins, sulphide delta(34)S (-15.8 to -4.2 parts per t housand,) suggest remobilization of diagenetic sulphide. Oxygen and hy drogen isotopes suggest deposition from metamorphic fluids (calculated delta(18)O(H2O), approximately +8 to +13 parts per thousand, measured range of delta D -52.2 to -27.3 parts per thousand). Immiscible CO2-b earing fluids were trapped in the temperature range 280-350 degrees C with fluid pressure < ca. 600 bar. The inferred pressure-temperature h istory is attributed to late-metamorphic uplift, with fluid pressures falling below lithostatic. The sulphide-bearing veins are interpreted as a small-scale example of redistribution of mineral deposits by meta morphic fluids.