Source or Sink? An Assessment of the Role of the Old Red Sandstone inthe Genesis of the Irish Zn-Pb Deposits

Abstract

The Zn-Pb (-Ba) deposits of central Ireland are hosted by Lower Carboniferous Waulsortian limestone and Navan Group carbonate sediments. The deposits are located near the margins of subbasins in close association with extensional faults. The source of metals and the flow pathways utilized by the hydrothermal fluids responsible for ore genesis have long been a matter of conjecture. The Devonian to Lower Carboniferous Old Red Sandstone has been suggested as a possible regional aquifer and metals source. Therefore, a petrographic and lead isotope study of this unit was conducted to assess its putative role in ore formation. Hydrothermally unaltered Old Red Sandstone has 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios ranging from 18.71 to 22.45, 15.52 to 15.86, and 38.57 to 42.71, respectively. The sedimentological characteristics of the red beds, coupled with results of isotopic modeling, support the conclusion that the lead isotope signature of the unit is predominantly controlled by radiogenic Caledonian granite detritus that has an Avalonian-like character. The modeled lead isotope composition of the Old Red Sandstone at the time of the Lower Carboniferous ore-forming event is too radiogenic for it to have acted as the main source of lead for the Irish ore field. Examination of drill core, outcrop, and underground exposures reveals that hydrothermal alteration within the Old Red Sandstone forms a halo around faults, indicating that fluid flow in the unit was predominantly structurally focused. These hydrothermally altered sediments have 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of 18.21 to 21.77, 15.54 to 15.91, and 38.02 to 41.57, respectively. Unlike the unaltered sediments, several altered samples have lead isotope compositions similar to galena from the Lower Carboniferous deposits. Petrographic examination reveals that these samples contain trace hydrothermal sulfides that are probably responsible for the orelike signature. Therefore, it appears that, at least locally, the Old Red Sandstone functioned as a sink for lead during the Lower Carboniferous mineralizing event. The conclusions from this study eliminate the Old Red Sandstone as the main source of lead in the ore field and suggest that deposit lead is derived primarily from lower Paleozoic rocks (± Precambrian crystalline basement). In view of these results, regional gravity-driven fluid flow models calling upon an Old Red Sandstone-focused flow regime and metals source can be discounted. Rather, deep circulation of fluids through the fractured basement underlying central Ireland plays an integral role in the evolution of the ore-forming system.