Establishing a geochemical baseline for the Lower Carboniferous stratigraphy of the Rathdowney Trend, Irish Zn-Pb orefield

Abstract

Carbonate-hosted Zn-Pb mineralisation within strata of the Irish Midlands can be subdivided between deposits forming on either Lower Carboniferous basin margin. Those on the southern side, known as the Rathdowney or Lisheen trend deposits, are hosted within the Waulsortian Mudbank formation and have been exploited for over 35 Mt of massive sphalerite-galena-pyrite ore since the 1960s. Additional occurrences of Irish-style mineralisation along the Rathdowney trend are numerous; however, questions remain regarding the extent and geochemical signature of the accompanying alteration haloes as, until now, no geochemical baseline was established.Preliminary results from Rapla, an exploration prospect 15 km northeast of the decommissioned Lisheen mine, show that an unaltered stratigraphical geochemical baseline can be distinguished using bulk 4-acid ICP-AES/MS data, thus a chemostratigraphic sequence can be derived from select major elements (Ca, Mg, Al) and ΣREE concentrations. With Eu and Ce anomalies and redox sensitive elements (Mo-U-V) providing environmental conditions during formation.pXRF analysis on the sample pulps show >95% precision of 15 elements. Of these 5 elements display >90% accuracy (Al, Ti, Fe, Cu, Rb), 4 elements >80% accuracy (K, As, Cd, U), and finally 6 elements at >50% accuracy (S, Ca, Mn, Zn, Se, Sr). Therefore confidence in data collected from powdered samples from rocks of this type by pXRF is high.