Basement controls on sediment-hosted Zn-Pb deposits; a Pb isotope study of Carboniferous mineralization in central Ireland

Abstract

Lead isotope ratios of galenas from carbonate-hosted Zn-Pb (+ or - Ba + or - Cd + or - Ag) deposits in central Ireland are homogeneous within individual deposits but vary between deposits. The variation correlates directly with geographic position, defining isoplumbs, and not with style of mineralization, age of host rock, or structural or physico-chemical factors. The least radiogenic Pb occurs in the northwest and the most radiogenic in the southeast. This pattern elegantly reflects Irish pre-Caledonian basement geology: Lewisian gneisses in the northwest and late Proterozoic, upper crustal rocks in the southeast. That the northwestern end member of the Irish Pb-mixing trend is ultimately Lewisian is demonstrated by the concordance of the Irish Pb isotope trend with the trends defined in Proterozoic to Tertiary rocks and ores of Scotland which contain Pb of demonstrably Lewisian derivation. Although geologic relations and Pb isotope ratios unambiguously implicate basement in the ore-forming process, we are unable to discriminate between Lewisian gneisses themselves and detritus derived from Lewisian rocks in the Caledonian metamorphic complex as the direct source of Pb.Comparison of the Irish deposits with McArthur River and Mississippi Valley deposits leads us to conclude that style of mineralization (syngenetic, syndiagenetic, epigenetic) is not a fundamental aspect of sediment-hosted Zn-Pb deposits but is a reflection of local stress regimes and physico-chemical conditions. The fundamental link between the types of sediment-hosted Zn-Pb deposits may be the onset of a condition, such as extension or compression, that provides a mechanism for driving fluids. In extensional regimes, seismic pumping processes may channel fluids toward the surface or into fractures or favorable pore spaces, resulting in a wide variety of styles of mineralization. The lack of dilatency in sediments in compressional regimes, however, may inhibit surface exhalation and favors epigenetic styles of mineralization.