The Geology of Northern Ireland

Northern Ireland represents one of the most complex and varied areas of geology in the world. The oldest rocks are Mesoproterozoic (c. 895Ma) and are succeeded by rocks representing every Phanerozoic system with the possible exception of the Cambrian. This diverse geological foundation has resulted in an equally diverse mineral heritage. Historical mining focussed on the production of iron ore, coal, lead and salt. However, since the 1970s economic deposits of gold and lignite, and a variety of other minerals, have been identified. The bedrock geology map of Northern Ireland can be divided into quadrants, each with unique geological characteristics and mineral prospectivity.

The Northwest

The northwest quadrant is underlain by the oldest rocks in Northern Ireland which belong predominantly to the Proterozoic Dalradian Supergroup and the early Ordovician Tyrone Igneous Complex. The Dalradian rocks are prospective for precious metals and host both the Curraghinalt gold deposit and Cavanacaw gold mine. The Middle Dalradian consists of metamorphic rocks which were originally deposited as sediments with minor submarine volcanics at the southern edge of the Laurentian continent in a tectonically unstable marine basin prior to the opening of the Iapetus Ocean (700–600Ma). The succeeding Upper Dalradian consists mainly of turbidites deposited in an unstable outer shelf environment. The Iapetus Ocean attained its maximum extent during the Cambrian when it would have resembled the present day Atlantic Ocean. Subsequent closure ensued during the Ordovician and early part of the Silurian and was accompanied, in the early stages, by the Grampian Orogeny (475–465Ma). All the sedimentary rocks deposited in the Iapetus Ocean were deformed in the late Silurian and early Devonian during the Caledonian Orogeny (425–400Ma). Much of the gold mineralisation is believed to be associated with tectonism and, in particular, thrusting. However, some mineralisation is also associated with the Variscan orogenic cycle. A number of long-lived lineaments may also have played a role in the localisation of the gold bearing fluids and studies of their wider implications are continuing. Other mineralised localities in the northwest quadrant require further exploration before their genesis is properly understood.

The Tyrone Igneous Complex lies southeast of the Dalradian outcrop and comprises the Tyrone Plutonic (TPG) and Tyrone Volcanic (TVG) Groups. The basal TPG represents the upper part of an early Ordovician ophiolite and has the potential to contain chromite and PGM deposits. It was obducted onto the southern edge of the Laurentian continent (during the Grampian Orogeny). The structurally overlying TVG is interpreted as an island arc and is the easterly extension of the system that hosts mineralisation in the Buchans District of Newfoundland. Acid intrusions in the TVG have also proved prospective for porphyry copper-style mineralisation.

The Southeast

The southeast quadrant is composed mainly of rocks of the Southern Uplands-Down-Longford terrane. The terrane is an allochthonous prism composed of an Ordovician and Silurian turbidite sequence comprising greywacke sandstone, siltstone and mudstone. The rocks consist of siliciclastic material that was deposited in the Iapetus Ocean. As northwestward subduction of the oceanic plate proceeded, packets of these sediments were accreted onto the Laurentian continental margin as a growing stack of underthrust slices. Each packet of sediment is bounded by major faults that define individual tracts. Large scale sinistral strike-slip movements occurred on these faults which are directly related to base metal mineralisation. The rocks host the lead mines in Counties Armagh and Down which operated in the 19th century.
Gold exploration programmes targeting these faults as conduits have identified potentially economic prospects in South Armagh.

The sedimentary rocks are intruded by granitoid bodies of which the oldest is the predominantly silicic late Caledonian Newry Igneous Complex (400Ma). This is itself intruded by the early Palaeogene Slieve Gullion Complex (c. 58Ma). In south Co. Down the Palaeogene Mourne Mountains Complex comprises five different granite units and is surrounded by a contact metamorphic aureole up to 1km wide. The mineral potential of this area (in particular REEs) remains to be fully explored and the source of alluvial gold found in streams draining the intrusive complexes is still elusive.

The Southwest

The southwest quadrant is underlain mainly by Upper Palaeozoic sedimentary rocks. The fault-bounded Fintona Block consists of Early Devonian and Lower and Upper Carboniferous red beds and volcanic rocks. Surrounding Carboniferous (350–300Ma) rocks commence with a thin continental sequence but the remainder were deposited in predominantly marine environments which varied from shallow water carbonate platform and deltas to deeper water basinal mudstone and carbonate facies. The Lower Carboniferous limestones are host to the Irish Midlands zinc province and targets have been identified in Co. Fermanagh that remain to be fully evaluated.

The Upper Carboniferous comprises a cyclical sequence of largely deltaic sediments which also include thin coal seams which were worked until 1967.

The Northeast

The northeast quadrant is mostly underlain by the early Palaeogene (60–55Ma) Antrim Lava Group. However, the earliest eruptions were violent and formed localised rhyolitic volcanoes in central parts of Co. Antrim. Within the Tardree Rhyolite Complex, the largest area of silicic volcanic rocks, is a significant deposit of perlite. The main basalt lavas were then erupted in two main cycles separated by a period of relative quiescence when deep weathering of the earlier basalts occurred in a sub-tropical climate and produced a conspicuous and thick layer of reddish lateritised basalt known as the Interbasaltic Formation. This represents the primary source of iron ore and bauxite in Northern Ireland which were mined mainly in the late 19th and early 20th Century.

The Antrim Lava Group has concealed and protected from erosion a Permian to Cretaceous sequence of softer sedimentary rock. These crop out at the margins of the Antrim Plateau but have been investigated more thoroughly in deep boreholes. In late Palaeogene (Oligocene) times (25Ma) localised non-marine basins which formed on top of the basalt plateau were filled mainly by deposits of lacustrine clay but also developed very thick beds of lignite at their margins. Exploration of the Oligocene lacustrine basins has revealed the presence of three large deposits containing about 1 billion tonnes of lignite.

In southeast Co. Antrim the Triassic (248–206Ma) rocks contain thick beds of halite (rock salt). This was historically mined at several locations but is now only worked by the room and pillar method at one mine producing about 500,000 tons per annum which is mainly used for gritting roads in the UK, Ireland and the USA. The suitability of deeper and thicker salt beds to host gas storage caverns is currently under investigation in the Larne area.