NEW INSIGHTS INTO THE ROLE OF THE OLD RED SANDSTONE AS A METALS SOURCE FOR ZN-PB MINERALIZATION IN THE IRISH MIDLANDS

A distinguishing feature of the Irish Midlands Carboniferous Zn-Pb ore field is the homogeneous Pb isotope signatures of individual Zn-Pb occurrences. The signatures are interpreted as evidence of hydrothermal fluid sourcing of Pb at each deposit from immediately underlying Lower Palaeozoic basement driven by thermal convection. Intervening Upper Devonian Old Red Sandstone (ORS) is discounted as a Pb source despite a substantial constituent of the rocks are reworked Lower Palaeozoic detritus and sandstones transmitted fluids throughout ORS diagenesis.

We have undertaken a preliminary U/Pb isotope study of ORS rock components to examine its role as a source of Pb. Samples representing least-altered caliche horizons, or hydrothermal ferroan dolomite-ankerite altered sandstones were analysed. Acid leaching was avoided and total dissolution employed for two reasons. First, the acid type, its concentration and temperature-time conditions affects the extent to which different sample grains are leached in the laboratory. This has potential to bias Pb isotope compositions of leachates. Consequently, leaching techniques are unlikely to simulate realistic fluid-rock interaction in nature. Secondly, in order to avoid influence from robust phases, whole rock samples were processed using heavy liquids to separate major rock components from accessory heavy minerals. By employing this approach phases contributing to Pb isotope measurements better reflect those available to mineralising solutions transmitted through the sequence.

²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb data were age corrected to 360Ma for comparison with Pb isotope compositions of deposit galena. Hydrothermally altered samples exhibit ²⁰⁶Pb/²⁰⁴Pb_360Ma=16.385 to 19.061 and ²⁰⁷Pb/²⁰⁴Pb _360Ma=15.305 to 15.675. Values overlap the deposit galena field and are consistent with the observation that sandstones interacted with hydrothermal fluids. Two samples exhibit highly radiogenic signatures and derived Pb from an as yet unidentified source. Least-altered caliche horizons overlap the deposit galena field (²⁰⁶Pb/²⁰⁴Pb_360Ma=17.080 to 18.437; ²⁰⁷Pb/²⁰⁴Pb _360Ma=15.402 to 15.594). Thus, initial research has been successful on two fronts; the methodology has enabled isolation of mineralogical components in ORS lithofacies and identified a new potential source of deposit Pb.