Characterisation of fluid-flow systems for Irish lead-zinc deposits

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. The mineralisation is variously regarded as having either a predominantly sedimentary-exhalative, or a predominantly MVT-type of genesis. The ores are hydrothermal, so the interpretation of mineral genesis is strongly linked to the interpretation of the fluid-flow system that generated the deposits. This thesis investigates the structural setting of the deposits, particularly in relation to faults, on both the regional and more local scales, and the nature of the fluid-flow systems that could have provided the mineralised fluids. The structural setting is established by reconstructing the geometry of the Midland Basin region, and restoring it to several stages in its Early Carboniferous development. The reconstruction and restorations identify a style of basin development dominated by the movement (mostly subsidence) of fault-bounded blocks of basement, with superjacent sedimentary rocks. The restorations reveal a spatial relationship between ore deposits and fault-block corners. This spatial relationship is proposed to have significance for exploration. The flow systems which could have been active in the Midland Basin and its surrounds (including the Munster Basin) are investigated using both mass balance, and numerical simulations of heat- and fluid-flow. The mass balance study shows that most of the suggested flow systems could have produced adequate metals, but tens to several thousand fluid pore-volumes are required to deliver these metals to the sites of deposition, thus eliminating compaction as a driving mechanism, and severely limiting the applicability of some of the other putative causes of fluid flow. When the results of the mass-balance and the numerical models are combined, they show that: regional flow systems could not have developed in central Ireland during the Carboniferous; gravity-driven flow is cold, shallow and only very local; but local convective systems that penetrate deep into the fractured basement not only develop in this setting, but are very stable. Convective circulation of hydrothermal fluids is the most likely cause of the Irish lead-zinc mineralisation