The Irish Midlands Zn-Pb(-Ba-Ag) Orefield represents one of the best studied palaeo-hydrothermal
systems worldwide. Despite this, the detailed origins of ore and trace metals and the controls of their variability in
deposits across the Orefield, have not been considered in detail. In this contribution, I have used diverse data sources
in an attempt to track the metals from their source and assess how they were fractionated at various stages along
the reconstructed flow path. This includes: (1) circulation of fluids to depths of up to ~7 km in the Lower Palaeozoic
wedge of arc-derived detritus that underlies the Orefield and through a primary ‘reaction zone’; (2) fluid transit
within the wedge and then up through the redox boundary represented by the Old Red Sandstone; and (3) fluid
ingress into the receptive Lower Carboniferous host rocks in the ore deposits themselves, initially through focused
feeder pipes and then spreading out upwards and especially laterally into the surrounding carbonate units. The
results show that the Lower Palaeozoic package was a particularly fertile source from which metals were easily
extracted and that this exerted a primary control on metal tenor in the overlying deposits. However, additional
fractionation steps, including loss to and extraction of metals from, the Old Red Sandstone, and sequential deposition of metals during cooling, neutralisation and fluid mixing were also important. The presence beneath the Irish
Midlands Basin of a weakly metamorphosed rock package formed from arc-derived detritus is considered to be a
critical component of the mineral system.