The Irish Midlands Orefield
The Irish Midlands Orfield is the world’s most fertile geological terrane in terms of tonnes of zinc / km2. The host lithologies are all Lower Carboniferous aged, Carbonate hosted, ‘Irish Type’ deposits of zinc / lead / silver +/- copper mineralisation. The location of deposits is controlled by regional scale mineralising trends in the underlying basement rocks.
The Orefield has hosted six large high-grade zinc mines, each producing a clean concentrate with few impurities that is sought after by smelters:
| Mine | Tonnage (Mt) | Zinc (Zn) | Lead (Pb) | Other Metals |
|---|---|---|---|---|
| Navan | >125 Mt | 8% | 2% | — |
| Lisheen | 22 Mt | 12% | 2% | — |
| Galmoy | 9 Mt | 13.5% | 2% | — |
| Silvermines | 18 Mt | 5% | 4% | — |
| Tynagh | 11 Mt | 5% | 6% | — |
| Gortdrum | 8 Mt | — | — | 1.19% Cu, 25.1 g/t Ag |
The Navan mine is still in production and a second, Galmoy (which is close to Unicorn’s Lisheen Block), is about to re-commence production. In addition, there is the 45.4m tonne undeveloped deposit at Pallas Green (8.4% Zn + Pb), which is close to Unicorn’s Kilmallock Block.
History of Exploration in the Irish Midlands Orefield
The exploration / mining history of the Irish Midlands Orefield has had a ‘stop/start’ nature due to a range of factors, however, the Orefield remains a premier destination for zinc with discoveries continuing to be made.
| Year / Period | Exploration Activities |
|---|---|
| 1950’s | Modern exploration commenced, with geologists comparing the Irish Midlands region to the Pine Point mining district in Canada. |
| 1959 | Discovery of the Tynagh Deposit. |
| 1960’s | Numerous deposits and significant occurrences discovered, most being shallow/sub-outcropping deposits amenable to soil geochemical sampling. |
| 1963 | Discovery of the Silvermines Deposit. |
| 1970 | Discovery of the massive Navan Deposit. |
| 1970s–80s | Exploration declined due to low metal prices and poor Government sentiment. |
| 1980’s–1990’s | Shift to geological-based targeting supported by ground geophysics, leading to discovery of deposits buried beneath cover rocks. |
| 1990’s | Use of airborne geophysics significantly enhanced structural and geological understanding of the Irish Midlands Orefield. |
| 1997 | Galmoy mine opens. |
| 1999 | Lisheen mine opens. |
| Late 1990’s | Noranda Exploration and Minco focused on the Limerick Basin. Geological/structural targeting led to the discovery of the large Pallas Green deposit, followed by Stonepark (Teck/Connemara). |
| 2010’s | Exploration declined due to low metal prices following the 2009–12 economic shock and Covid. Activity is now recommencing, with discoveries including Minco’s Rapla and Group 11’s Ballywire in Limerick. |
