The Llanrwst Pb–Zn veinfield occupies fractures in an Ordovician volcano-sedimentary sequence, with three separate mineralizing events (M1, M2 and M3). The Llanfair Pb–Zn–Cu veinfield consists of sporadic mineralization in Silurian sediments. Fluid inclusion data from both veinfields indicate mineralization took place from CaCl2-bearing brines, up to 26 wt % NaCl equivalent, between 125 and 190 °C with cooler (< 100 °C), more dilute fluids associated with later thrusting at Llanrwst. Sulphur at both veinfields was derived during the main phases of sulphide precipitation from the host rocks, with paragenetically late sulphides and sulphates deriving sulphur from 34S-enriched surface waters. Carbon and oxygen isotopic data on vein carbonates shows 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 systems to the loci of deposition during a phase of extensional tectonics in late Devonian to Tournasian times.