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The Llanrwst Pb-Zn veinfield occupies fractures in an Ordovician volcano-sedimentary sequence, with three separate mineralizing events ([M.sub.1], [M.sub.2] and [M.sub.3]). The Llanfair Pb-Zn-Cu veinfield consists of sporadic mineralization in Silurian sediments. Fluid inclusion data from both vein fields indicate mineralization took place from Ca[Cl.sub.2]-bearing brines, up to 26 wt% NaCl equivalent, between 125 and 190 [degrees] C with cooler (< 100 [degrees] C), more dilute fluids associated with later thrusting at Llanrwst. Sulphur at both vein fields 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.

Pb-isotope data on ore galenas from the West Shropshire orefield show a significant spread of 20Pb/24Pb values, with minor variation in 207Pb/204Pb and 208Pb/204Pb ratios. These indicate that lead was derived from multiple sources, some of them uranium-enriched and incompletely mixed prior to ore deposition. Four possible mineralizing agents are considered: circulating sea-water, metamorphic waters, basinal brines and convecting formation waters. Pb-isotope data exclude a circulating sea-water origin for the mineralization, and best support a convecting formation water mineralizing agent. A model involving a single fluid tapping multiple lead sources is proposed to explain the observed Pb-isotope variation.

The Llanrwst Pb-Zn and Llanfair-Talhaiarn Cu-Pb-Zn orefields contain vein mineralization similar to Lower Palaeozoic-hosted base metal deposits elsewhere in the British Isles, which are believed to have formed in the late Palaeozoic. K-Ar and Rb-Sr data on intensely sericitized wallrock material indicate that the main phase of mineralization at both orefields occurred between the mid-Devonian and the Tournasian (386-350 Ma). The latest veins at Llanrwst formed in Visean-Westphalian times. Gangue mineral Sr isotope compositions indicate a mixture of radiogenic wallrock-derived Sr with less radiogenic Sr, with a poorly-defined trend to more radiogenic compositions with time. The age of the main mineralization precludes dewatering of Carboniferous basins as a source of ore fluid. More generally Sr isotopes rule out unmodified basinal brines or metamorphic fluids as mineralizing agents, since their Sr content would be too radiogenic. Local sources of unradiogenic Sr are available at Llanrwst in metadolerites and calcareous sandstones and at Llanfair-Talhaiarn in thin Ludlow Limestone beds.