THE GENESIS OF THE LLANRWST AND LLANFAIR VEINFIELDS, NORTH WALES - EVIDENCE FROM FLUID INCLUSIONS AND STABLE ISOTOPES

The Llanrwst Pb-Zn veinfield occupies fractures in an Ordovician volca no-sedimentary sequence, with three separate mineralizing events (M-1, M-2 and M-3). The Llanfair Pb-Zn-Cu veinfield consists of sporadic mi neralization in Silurian sediments. Fluid inclusion data from both vei nfields indicate mineralization took place from CaCl2-bearing brines, up to 26 wt% NaCl equivalent, between 125 and 190 degrees C with coole r (< 100 degrees C), more dilute fluids associated with later thrustin g at Llanrwst. Sulphur at both veinfields was derived during the main phases of sulphide precipitation from the host rocks, with paragenetic ally late sulphides and sulphates deriving sulphur from S-34-enriched surface waters. Carbon and oxygen isotopic data on vein carbonates sho ws that both deposits formed from waters that had undergone extensive water-rock interaction. Carbon was derived from at least two sources: marine carbonate dominated at Llanfair, whereas organic matter in the host rocks was more significant at Llanrwst. These data indicate that the veins formed from evolved brines which migrated along fault system s to the loci of deposition during a phase of extensional tectonics in late Devonian to Tournasian times.