Tracing ancient fluid flow pathways: A study of the Lower Carboniferous base metal orefield in Ireland

The main objective of this study is to investigate whether fluid flow beneath sedimentary basins is an integral part of ancient ore-forming systems. The research focuses on the Lower Carboniferous Midlands Basin of Ireland, host to several significant Zn-Pb deposits and numerous sub-economic occurrences.; A petrographic, petrologic and wholerock Pb isotope study of the Old Red Sandstone (ORS) was conducted in order to test whether this unit acted as the sole regional aquifer and metal source for ore-forming solutions. The hydrothermal alteration features of the ORS are zoned around faults, consistent with structurally controlled (rather than aquifer-confined) fluid flow. Caledonian granite-derived detritus controls the lead isotopic signature of the ORS. The modeled lead isotopic composition of the sandstone at the time of mineralization suggests that it is unlikely to be the dominant source for Pb in the Irish orefield. Therefore, it is concluded that ore-forming fluids must have circulated within, and scavenged Pb from, the underlying Lower Paleozoic sequences.; A study of Lower Paleozoic-hosted vein systems in the Irish Midlands reveals the presence of fluids with identical properties (temperature, salinity, halogen chemistry, and presence of CO_2′) to those of the principal hydrothermal orefluid. The δD, δ 18O and major element chemistry of the fluids suggests equilibration with Lower Paleozoic metasediments at temperatures of 200–300°C. The fluid minor element chemistry is similar to that of contemporary hydrothermal vent fluids, and halogen data (Cl/Br = 682 ± 92) are indicative of an evaporated seawater origin. The strontium isotope values of Lower Paleozoic-hosted vein carbonate lie at the radiogenic end of deposit data arrays. The sulfur isotopic composition of vein-hosted sulfides matches that of the hydrothermal end-member recognized in the overlying deposits. Carbon and oxygen isotope modeling demonstrates that interaction of the Lower Paleozoic-circulated fluid with the Lower Carboniferous succession could account for the isotopic patterns observed within the hydrothermal carbonates of the Irish orefield.; It is proposed that the circulation of partially evaporated seawater within the Lower Paleozoic sequences was the fundamental process responsible for the genesis of ore-forming solutions. It is envisaged that hydrothermal activity was triggered by Lower Carboniferous crustal extension. Episodic tectonic activity promoted the expulsion of fluids from the Lower Paleozoic sequences via reactivated Caledonian structures. An additional fluid flux may have been provided by fault-controlled, topographically-driven flow through the Lower Paleozoic sequences.