Irish-type Zn-Pb deposits - What are they and can we find more?

Seven carbonate hosted Zn-Pb (+/-Ag, Ba) or Cu deposits have been mined in Lower Carboniferous limestones in Ireland since 1960 - representing a resource base of ca. 22 Mt of metal (Tynagh, Gortdrum, Magcobar, Silvermines, Navan, Galmoy and Lisheen). The orebodies are amenable to modern mining and metallurgical processes and exploration for further deposits continues. Several significant prospects remain unexploited with numerous smaller prospects and mineralized localities attesting to the metal endowment of the country (Pallas Green, Stonepark, Ballinalack, Kilbricken and Keel). These deposits share sufficient characteristics to be termed Irish-type Base Metal Deposits and define a metallogenic province of worldwide significance known as the Central Irish Ore-field. Early discoveries resulted from shallow soil geochemistry, Induced Polarization surveys, and follow-up of legacy mining or mineralized outcrops. More expensive, deeper penetration methods including deep drilling, airborne geophysics and seismic methods have been utilized extensively in more recent years - but thus far with variable success. Large amounts of ground remain unexplored at depth, but further discovery is extremely challenging. Following over 60 years of discovery and mining this contribution summarizes our knowledge of Irish-type deposits and their exploration with a view to generating new ideas for future discovery. The Tynagh, Silvermines, Lisheen and Galmoy deposits, all hosted in the Waulsortian Limestone, occur in a distinct area in the SW of the Orefield. Silvermines, Lisheen and Galmoy are remarkably similar, comprising several, tabular, stratabound, lenses containing locally massive and high-grade sphalerite-galena with substantial pyrite +/- barite. These deposits are hosted in the first clean limestone above basement in Lower Carboniferous limestones and dolomites and exhibit fine grained, replacive sulphides with significant breccia and crosscutting mineralization. Structurally all these deposits abut major syn-sedimentary faults usually trending NE or ENE. The Navan deposit occurs on the NE side of the Orefield and is hosted in stratigraphically lower, Pale Bed limestones, that are roughly age equivalent to the Waulsortian Limestone in the SW of the Orefield. Navan is much larger and more complex than the deposits in the SW Orefield but shares several of their key characteristics. A review of the characteristics of Irish-type deposits and exploration results suggests some important areas for future exploration and research. For example, most observers relate the location of Irish-type deposits to the regional distribution of host rocks and structures of a certain age. It is suggested that the spatial location of these deposits is more fundamentally controlled by deep structural heterogeneities related to the collisional zone between basement rock terranes termed the Iapetus Suture. Understanding the timing of fault development is likely key to further discovery at depth as the mineralizing feeder structures are located on faults that did not necessarily extend through post-Waulsortian aged strata. Several promising fields for research and development to help enable further discovery are discussed.