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) are compared with the nearby stratiform-disseminated copper mineralization in metasedimentary rocks, containing minor veins (metamorphic quartz veins and veinlets). These stratiform deposits are hosted by non-marine Devonian sediments (Old Red Sandstone), metamorphosed in the Hercynian orogeny. In sulphides from the stratiform deposits and minor veins, isotopic compositions of sulphur (delta(34)S) range from -21.00 to +5.14 parts per thousand, consistent with the vein sulphide being remobilized stratiform-disseminated sulphide, and the latter being of diagenetic bacteriogenic origin. Sulphate (barite), found in veins separate from the sulphides, has delta(34)S + 12.3 to 15.7 parts per thousand, consistent with groundwater origin. In minor-vein quartz, fluid inclusions have homogenization temperatures consistent with trapping under the estimated peak-metamorphic conditions (300-400 degrees C, 1-3 kbar). In the major veins, sulphide delta(34)S (-15.8 to -4.2 parts per thousand,) suggest remobilization of diagenetic sulphide. Oxygen and hydrogen 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-bearing fluids were trapped in the temperature range 280-350 degrees C with fluid pressure < ca. 600 bar. The inferred pressure-temperature history 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 metamorphic fluids.