CONSTRAINTS ON THE AGE AND GENESIS OF THE LLANRWST AND LLANFAIR-TALHAIARN OREFIELDS, NORTH WALES FROM K-AR AND RB-SR STUDIES

The Llanrwst Pb-Zn and Llanfair-Talhaiarn Cu-Pb-Zn orefields contain v ein mineralization similar to Lower Palaeozoic-hosted base metal depos its elsewhere in the British Isles, which are believed to have formed in the late Palaeozoic. K-Ar and Rb-Sr data on intensely sericitized w allrock material indicate that the main phase of mineralization at bot h orefields occurred between the mid-Devonian and the Tournasian (386- 350 Ma). The latest veins at Llanrwst formed in Visean-Westphalian tim es. Gangue mineral Sr isotope compositions indicate a mixture of radio genic wallrock-derived Sr with less radiogenic Sr, with a poorly-defin ed trend to more radiogenic compositions with time. The age of the mai n mineralization precludes dewatering of Carboniferous basins as a sou rce of ore fluid. More generally Sr isotopes rule out unmodified basin al 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.