Geology
Ireland has a widely varied geological framework, with rocks ranging in age from Proterozoic (2500 million years ago) to the Present. The island can be divided into a number of mineral provinces that are endowed with a diverse suite of base and precious metals, as well as industrial minerals. These mineral provinces are described in brief below. For more details on the geology of Ireland, please visit the Geological Survey of Ireland website at www.gsi.ie.
Mineral Potential
In spite of its relatively small size (70,000 km2), Ireland has a notably diverse geology that is prospective for a range of mineral deposits. Large tracts of Ireland are underlain by metasediments and metavolcanics of Proterozoic and Lower Palaeozoic age. These lithologies are known to contain significant base metal mineralization (e.g. Avoca) and gold-bearing quartz veins. The latter style of mineralization has recently been the focus of extensive exploration efforts in Northern Ireland, and in 1999, an opencast gold mine was opened at Cavanacaw in Northern Ireland. The Lower Carboniferous limestones of the Irish Midlands are host to one of the great orefields of the world. Since 1960, fourteen significant zinc-lead deposits have been discovered, with six becoming producing mines. These include the world class mine at Navan, Co Meath. Ireland has been ranked first in the world in terms of zinc discovered per square kilometre, and second in the world with respect to lead. Ireland's three base metal mines account for some 40% of European zinc production, and 20% of its lead. Ireland's younger rocks contain significant deposits of industrial minerals, most notably the Kingscourt gypsum deposit that is found in Permian rocks in Co Monaghan. A better appreciation of Ireland's mineral potential can be obtained by looking at the different Mineral Provinces.
North-Western Basement
This province contains some of Ireland's oldest rocks, with Proterozoic gneisses overlain by metamorphosed sandstones, limestones and volcanics rocks. These are intruded by Palaeozoic granites. Demonstrated potential for base metals is shown by widespread 18th and 19th century workings, with many small mines and "trials" to be found. Quartz veins and shear zones are prime gold targets (e.g. Curraghinalt and Cavanacaw, both in Northern Ireland) and gold is also associated with massive sulphides (e.g. Glentogher, Co Donegal). Molybdenum-copper mineralization is associated with the Palaeozoic granites. Diamond and other gemstone targets have been identified in the far north of the province, in the Inishowen Peninsula. A number of types of dimension and ornamental stone have also been exploited from this province, including the famed green Connemara marble.
Longford-Down Massif
Three principal groups of metallic mineral deposits occur in the Lower Palaeozoic rocks of the Longford-Down Massif. Firstly, there are vein deposits, mainly containing lead and zinc, but also including antimony and gold (e.g. Clontibret, Co Monaghan). A number of these vein deposits have been exploited historically. Secondly, stratiform (bedded) iron-manganese deposits. Several of these deposits were worked during the late-19th century. Finally, there is minor copper-molybdenum mineralization associated with granites.
Leinster Massif
The Leinster Massif of Lower Palaeozoic metasediments and volcanics hosts the large low-grade copper-pyrite deposit at Avoca. There is sub-economic tungsten and lithium mineralization associated with the Leinster Granite, where there are also numerous minor vein-type copper and lead deposits. Also of note is the historically mined alluvial gold deposit at Gold Mines River.
Mayo-Curlew Basin
Significant gold deposits are known from Lower Palaeozoic rocks at Lecanvey and Cregganbaun in south Co Mayo. Other deposits in this province include talc-magnesite at Westport and the sub-economic Charlestown copper deposit.
Munster Basin
In the Munster Basin, Devonian sandstones with minor volcanics are overlain by Carboniferous marine sandstones, shales, and limestones. Major folding and faulting took place during the Variscan Orogeny. Vein-hosted copper at Allihies and barite at Clonakilty (both in Co Cork) are amongst a variety of mineral deposits worked in the past. The Devonian sequences also contain potential palaeoplacer gold targets, while shale-hosted zinc-lead mineralization has been discovered in the Carboniferous strata.
Central Ireland Basin
Thick Lower Carboniferous limestones in a number of sub-basins are the host in this important zinc-lead province. Known zinc-lead deposits contain some fourteen million tonnes of zinc metal. Target horizons are stratabound Irish-type deposits in the basal Carboniferous (e.g. Navan, Tatestown, Oldcastle, Keel, Moyvoughly), the overlying Waulsortian limestones (Tynagh, Silvermines, Ballinalack, Galmoy, Lisheen) and stratigraphically higher crosscutting deposits (Harberton Bridge). Vein and massive replacement copper-silver deposits (Gortdrum, Aherlow, Mallow) are associated with the structurally controlled southern margin of the Basin. Other significant deposits include barite (Ballynoe, Tynagh), gypsum (Glangevlin), calcite (Kilbreckan) and dolomite (Bennettsbridge).