A mineralogical tour of Ireland (Part 3): Connaught
Stephen Moreton (UK)
In the first two articles of this series, we looked at Leinster and Munster. Continuing in a clockwise fashion brings us to Connaught. Some of Ireland’s oldest rocks are to be found here, forming the Ox Mountains. The rugged and mountainous west is dominated by metamorphic rocks and a series of granite intrusions. Inland, Carboniferous limestone prevails.
Where the latter abuts Devonian sediments is found the jewel in the crown of Irish mineralogy – Tynagh Mine. This giant polymetallic deposit, near Loughrea in County Galway, was discovered in the 1960s and yielded close to a million tonnes of lead, zinc and copper. Much of this was as sulphides dispersed through black mud filling a huge depression in the limestone. This was formed by acid from rotting pyrite dissolving the country rock. Extensive oxidation and remobilisation of the primary ores produced hundreds of thousands of tonnes of smithsonite, cerussite, malachite and azurite. Scores of rarer species, such as linarite, anglesite, brochantite, native silver and numerous arsenates were also present.
Sadly, collectors were slow to learn of this treasure and most was sent to the crusher. By the time they did realise something glorious was going on, most was already turned into smelted metal. A fickle attitude on the part of management did not help either – sometimes, friendly, sometimes hostile. When the mine closed in 1982, the opencast swiftly flooded to the brim. Most of the tips had already been landscaped and covered in soil and grass, but the few patches that had not yielded a bonanza of specimens for the next two decades. In fact, it was during the early part of this time that a lot of the rarer species were found, many by a Swiss schoolboy.
One fellow lucky enough to have accessed the open pit before the pumps were turned off was the late Richard Barstow. The specimens of dundasite he rescued remain the best ever from the British Isles and, curiously, are virtually absent from the tips. Sadly, Tynagh is history as regards minerals. The best part of the tips was recently dug out and buried elsewhere on the site to make way for a power station.
Again, collectors knew nothing of this until it was too late, but access was banned by the contractors anyway. One can only guess at what has been lost and the saga is a sad reminder of the importance of collectors rescuing what they can when the opportunity permits. In situations like this, the “always leave some for others” attitude is a way of ensuring the permanent loss of whatever is left behind.
Moving northwards, but still in Co. Galway, we come to Connemara, a pretty landscape of loughs and mountains. At Oughterard, Ireland’s first show mine, Glengowla, beckons. The large, albeit colourless, octahedral fluorites it contains are remarkable. Being a show mine, visitors may look, but not collect. A rock shop provides opportunities for the silver pick collector.
Octahedral fluorite is widespread, if in small amounts, in the granites on the north side of Galway Bay, and probably every roadside quarry is worth a look. The classic location was Inveran mine, but this has been obliterated by the ravages of time. Specimens from here are rarer than hen’s teeth. Again, this is a salutary reminder of what happens if no one is on hand to collect minerals when they are available. The area is also noted for Connemara marble, a mottled green serpentinous rock used for ornamental purposes.
To the north of Connemara, we enter County Mayo. A little lead mine at Sheeffry offers rich mats of millerite needles, but it is gold that puts this district on the mineralogical map. In the 1980s, rich deposits of auriferous quartz were found along a more than 13km shear zone. The best occurrence was at Srahroosky, where visible gold can still be seen in the large quartz outcrop on the hillside. Even richer grades were found on Croagh Patrick, separate to the shear zone. Here, assays reached an extraordinary 0.1% Au, the gold being quite coarse in places.
The prospect of a gold rush did not please the locals. The shear zone straddled a rich salmon fishery, while Croagh Patrick is Ireland’s holy mountain where St Patrick is said to have fasted and prayed. Vigorous protests put a stop to any hopes of fortunes being wrested from the ground, and all prospecting and mining is now banned. Croagh Patrick has since been visited by every Tom, Dick and Harry trying their luck, but tiny specks in little cubic cavities left by the weathering out of pyrite can still be found with a little patience. They do, however, have the irritating habit of falling out when the specimens are trimmed. Most of the rivers yield a few grains of the metal, but perhaps not in as large amounts as might be expected from the richness of the bedrock sources nearby. Remember, they are also popular with anglers.
In the northwest of Mayo is Achill Island, connected to the mainland by a bridge. This gets very busy in the summer, as does the famous amethyst location by the roadside above Keem strand. Hardly a summer weekend passes by without tourists picking the surface for chips of purple quartz. It is only by back-breaking digging that large drusy pieces can be found. Even then, it is difficult and depends on a good deal of luck. The site has already been systematically excavated by mechanical digger and the proceeds sold to tourists. Much of what is on sale now looks suspiciously South American.
Heading eastwards, one passes by the Ox Mountains, a range of Precambrian gneiss and supposedly home to pink and green tourmaline. Unfortunately, Patrick Doran, the Victorian dealer who made the claim, neglected to say where exactly he found it. As some of his “Irish” aquamarines are of dubious provenance, it is hard not to be cynical about his coloured tourmaline. Large rutile crystals have certainly been found, but the locality information is almost as vague. Killala Bay, on the north coast of Counties Mayo and Sligo, has seen a series of basalt dykes intruded through the limestone. In places, contact metamorphism has produced very rare calcium silicate minerals, including killalaite, for which this is the type locality. These are definitely for the micro collector.
South of Sligo town are two limestone quarries. Abbeytown Quarry occupies the site of an old lead mine and iron/lead/zinc sulphide mineralisation turns up from time to time. The quarry is probably Ireland’s best calcite location, yielding pale yellowish crystals in a variety of habits. The best came from a vertical pipe-like structure exposed in the 1990s but now all blasted away. “Herkimer diamond” quartz crystals occasionally turn up and, for a short time in the late 1980s, occasional little fluorite cubes also. Aughamore Quarry contains a profusion of small dolomite lined vugs, sometimes with needle quartz, chalcopyrite crystals and orange calcite. Advance appointment is essential for visitors to this quarry.
Our tour of Connaught concludes at Benbulben, a flat topped, limestone mountain that dominates the local scenery. A large, but rather compact barite vein has been extensively mined on the plateau. A little quartz and calcite of little merit was still evident when the mine closed in about 1982, but crystallised barite is surprisingly scarce. The only examples known to the author were collected by abseiling into a stope and consisted of white platy crystals in a habit reminiscent of that from Force Crag in the north of England. It should be noted that the access track in Gleniff is strictly private property.
From here, one is well positioned to head into Ulster where we will conclude our tour of the Emerald Isle, not with emerald, but with its close relative – aquamarine.
Further reading
Introducing Mineralogy, by John Mason, Dunedin Academic Press, Edinburgh (2015), 118 pages (Paperback), ISBN: 978-17-80460-28-4.
Minerals of Britain and Ireland, by A G Tindle, Terra Publishing (2008), 624 pages (Hardback), ISBN: 978-19-03544-22-8.
Rocks and minerals: The definitive visual guide, by Ronald Louis Bonewitz, Dorling Kindersley (2008), 356 pages (Hardback), ISBN: 978-14-05328-31-9.