Characterization of fluid-flow systems for Irish lead-zinc deposits - contributions from mass balance

The lead-zinc sulphide deposits of central Ireland are hosted primarily in Lower Carboniferous strata of the Midland Basin. The ores are thought to be approximately contemporaneous with their sedimentary hosts, but the mode of their formation is a subject of dispute. A range of fluid-flow systems was investigated by calculating, via a mass-balance approach: whether a candidate system can provide sufficient metals to create the known deposits; and how many pore volumes of fluid are needed to do this. Most of the systems that have been suggested for the deposits were found to be capable of producing adequate metals, but, in examples based on typical flow-system sizes, tens to several thousands of fluid pore volumes are required to deliver the metals to the sites of deposition. The only tested fluid-flow systems to survive both the metal-balance and fluid-balance tests are deep convection and (Midland and Munster) basinal flow driven by the Variscan orogeny.