The following account is designed to summarise the key events in Ireland’s geological history and relate these to mineralization. The simplified geological map of Ireland shows the distribution of the main geological units, along with the location of key mineral localities. The first event to have a significant influence on the economic geology of Ireland was the Caledonian Orogeny, some 470 Ma ago. This was related to the closure of the Iapetus Ocean, a large body of water that once separated the northwestern part of the island from the southeast. The closure of the Iapetus Ocean is marked by the Iapetus Suture,.
The Orogeny deformed the rocks that had been deposited in the northwestern part of the Iapetus Ocean, and played a key role in the formation of a number of gold deposits in northwest Ireland. The two largest deposits in Northern Ireland,Cavanacaw and Curraghinalt, both occur in close proximity to a lineament that marks the structural boundary between the Dalradian Supergroup and the Lower Palaeozoic rocks represented by the Tyrone Igneous Complex. This complex comprises an ophiolite sequence underlying a volcanic arc which stretched from Newfoundland to Scandinavia. In western Ireland, the gold prospects at Lecanvey (Croagh Patrick) and Cregganbaun lie along an extension of this lineament, in Lower Palaeozoic metasediments and volcaniclastics.
Before the Iapetus Ocean finally closed, a thick sequence of Lower Palaeozoic sediments was accreted to the edge of the palaeo-continent, just to the north of the present-day Iapetus Suture. A complex structural regime and igneous activity have resulted in precious (e.g. Au at Clontibret) and base-metal mineralization in these rocks. At the same time, on the southern side of the Suture, igneous activity was more pronounced, and large amounts of volcaniclastic material have been preserved, which can host polymetallic deposits (e.g. Avoca). The Dalradian and Lower Palaeozoic rocks of Ireland are also intruded by a number of Caledonian granites, and various different styles of mineralization have been found related to these intrusions.
The next major event was tectonic extension and the northward spread of tropical marine conditions, during the Lower Carboniferous. The resulting shallow-water limestones are host to the Irish Midlands zinc province, which has provided the deposits at Tynagh, Silvermines, Navan, Galmoy and Lisheen. The genesis of these deposits has been the subject of lively debate for decades, but it is now generally accepted that metals were leached from the basement and focused by extensional faults acting as conduits. The depositional history of the remainder of the Carboniferous is complex and resulted in a mosaic of limestones deposited in shallow and deeper water conditions.
Following the Variscan Orogeny, at the end of the Carboniferous, desert-like conditions dominated, with minor marine incursions leading to the formation of a number of Permo-Trias evaporate deposits. The best known of these support gypsum mines at Knocknacran and Drummond in Co Monaghan, and a salt mine at Kilroot, Co Antrim.
The final main event was related to the opening of the Atlantic Ocean and was responsible for the flood basalts of Northern Ireland and the three Palaeogene igneous complexes that intruded the Lower Palaeozoic rocks of northeast Ireland. Different mineralization styles have been identified in these rocks, and more work is warranted. Following this period of activity, non-marine basins formed on the top of the basalt plateau, and thick beds of lignite were deposited. This lignite represents Ireland’s largest onshore indigenous fuel resource.
Finally, Quaternary glaciations have deposited large amounts of unconsolidated material on top of the bedrock. While these deposits present challenges and opportunities for prospectors, and are amenable to a wide range of geochemical techniques they are also responsible for the lush green landscape that covers much of the island today.