Base metals, mineral resources, Northern Ireland
|Mitchell, W I (ed.). 2004. The geology of Northern Ireland-our natural foundation. Geological Survey of Northern Ireland, Belfast.|
J W Arthurs and G Earls
Lead and zinc
Lead and zinc showings are scattered throughout the pre-Mesozoic formations in Northern Ireland. Most are epigenetic vein-type occurrences and are found mainly in the Ordovician and Silurian greywacke and slate of the Southern Uplands-Down-Longford Terrane and in Dalradian schists in Co. Tyrone. Two concentrations of vein-hosted lead are of particular historical significance. Three other areas have been examined more recently to investigate their potential for stratabound base metal deposits.
The South Armagh-Monaghan Mining District is centred on the town of Keady in south Armagh. In the GSNI records, 57 shafts and adits are recorded in Co. Armagh. A few of the veins were worked on what was a relatively large scale by the standards of the time. The Derrynoose Lead Mine, [H 796 316] abandoned in 1842, comprised 13 shafts and one adit, with one of the shafts reported to be 150 feet deep. Other workings of local significance, which had multiple shafts, were the Clay [H 828 310], Creggan [H 938 170], Carrickgallogy [H 984 289] and College mines [H 807 332]. The veins are vertical, and although they have variable strike directions, their trend is believed to be controlled by stress fields associated with the Orlock Bridge Fault. Mineralisation predominately comprises galena, sphalerite, pyrite and chalcopyrite. The gangue is mainly quartz with calcite and barite.
The Conlig-Whitespots Mines [J 492 765] between Bangor and Newtownards (P948056) in north Co. Down, consists of ten shafts which lie along a vertical north-south vein hosted by Silurian greywackes . The ore minerals are galena, sphalerite and chalcopyrite in a gangue mainly of quartz-cemented breccia with some barite and calcite. The vein has been re-brecciated and subsequently intruded by a Palaeogene dolerite dyke. Conlig was worked intermittently from at least 1780 until 1899, over which period an estimated 13 500t of lead metal was recovered, making it one of the major lead mines in the United Kingdom in the 19th Century.
Examples of stratabound base metal mineralisation occur in two geological environments in Northern Ireland. At Glenlark [H 595 901], in metasediments of the Dalradian Supergroup, significant grades of zinc and lead (up to 9.8% Zn and 2.9% Pb over 1m intervals) are associated with gold and silver mineralisation (see above). Massive, semi-massive and disseminated sulphide mineralisation occurs over a 21m section. Here, the Glengawna Formation has been interpreted as having suffered extensive hydrothermal alteration prior to metamorphism. The formation also contains numerous other base metal showings along strike. To the north of Glenlark other formations in the Dalradian Supergroup have the potential to host stratabound base metal mineralisation. Regional geochemical surveys indicate that anomalous base metal values are present in the Dungiven Limestone Formation and examples of disseminated zinc and lead (1.5% Zn+Pb) and massive pyrite mineralisation have been recorded around Park, Co. Londonderry [C 597 013].
The Tyrone Volcanic Group is host to copper, lead and zinc mineralisation at several localities. Occurrences of boulders of chloritic tuff analysing up to 7.6% Zn are recorded south of Greencastle [H 586 820]. In the vicinity of Cashel Rock, copper has been recorded in tonalite and rhyolite with grades of 90 m at 0.16% Cu intercepted in exploration drilling in the 1970s. The presence of jasperoid horizons of probable exhalative origin and the spatially close occurrence of graphitic pelites within the volcanic package indicates that the Ordovician arc is prospective for volcanogenic massive sulphide mineralisation.
Copper, as malachite and chalcopyrite, is ubiquitous in small amounts in the lead-zinc veins. While it may have been recovered in the 18th and 19th Century mines as a by-product, there is no record of primary copper mining in Northern Ireland. Very small amounts of native copper have also been recorded in amygdales in basalt lava of the Antrim Lava Group (see Mantle plumes , ocean spreading and the North Atlantic Igneous Province, Palaeogene extrusive igneous rocks article) as a mineralogical curiosity .
Other areas of prospectivity for base metals include the Clogher Valley along the trace of the Aghintain Fault where deep overburden geochemical surveys in the 1980s revealed anomalies of lead, barium and silver, together with galena and barite in heavy mineral samples . The geological environment of Lower Carboniferous shallow water sediments near a basin margin fault is analogous to sites of mineralisation in the major lead-zinc orefield of the Irish Midlands, (e.g. Navan, Co. Meath).
Tin as cassiterite was found in an outcrop of greisen at Pollaphuca in the Mourne Mountains in the 1980s. Channel sampling returned grades of 0.4% Sn over a 0.3 m interval. Given the current economics of tin mining and the location of the occurrence in an Area of Outstanding Natural Beauty, it is unlikely to be explored further.
- Griffith A E, and Wilson, H E. 1982. Geology of the country around Carrickfergus and Bangor. Memoir of the Geological Survey of Northern Ireland, Sheet 29 (Northern Ireland).
- Frances, P, and Preston, J. 1974. Native Copper in the Antrim Basalts. Irish Naturalists’ Journal.
- Smith, R, Smith, C G, and Legg, I C. 1996. Mineral Exploration in the Clogher Valley area, Co. Tyrone. Part1: Follow-up Investigations. Geological Survey of Northern Ireland Technical Report GSNI/96/3.