CHARACTERIZATION OF FLUID-FLOW SYSTEMS FOR IRISH LEAD-ZINC DEPOSITS -CONTRIBUTIONS FROM MASS-BALANCE

The lead-zinc sulphide deposits of central Ireland are hosted primaril y in Lower Carboniferous strata of the Midland Basin. The ores are tho ught to be approximately contemporaneous with their sedimentary hosts, but the mode of their formation is a subject of dispute. The minerali zation is variously regarded as having either a predominantly sediment ary-exhalative or a predominantly Mississippi Valley type of genesis. The ores are hydrothermal, so the interpretation of mineral genesis is strongly linked to the interpretation of the fluid-how system that ge nerated the deposits. A range of fluid-flow systems was investigated b y calculating, via a mass-balance approach: whether a candidate system can provide sufficient metals to create the known deposits; and how m any pore volumes of fluid are needed to do this. Most of the systems t hat have been suggested for the deposits were found to be capable of p roducing adequate metals, but, in examples based on typical flow-syste m sizes, tens to several thousands of fluid pore volumes are required to deliver the metals to the sites of deposition. This requirement eli minates those proposed fluid-flow systems which are incapable of produ cing sufficient pore volumes. The only tested fluid-flow systems to su rvive both the metal-balance and fluid-balance tests are deep convecti on and (Midland and Munster) basinal flow driven by the Variscan oroge ny. When the driving mechanisms for fluid-flow and crustal heat distri butions are also considered the viability of tectonically driven flow in the Irish Midlands becomes questionable, leaving crustal-scale conv ection as the most likely transport system for the hydrothermal fluids .