A mineralogical tour of Ireland (Part 4): Ulster
Stephen Moreton (UK)
Our journey around Ireland concludes in Ulster. This comprises Northern Ireland, which is part of the UK, and the counties of Cavan, Donegal and Monaghan, which are part of the Republic of Ireland. As geology is no respecter of politics, the national border is ignored here. I assure my gentle readers that this is not intended as a political statement! The geology consists of metamorphic rocks and granite intrusions in the west, a huge expanse of Tertiary basalt in the eastern half, and a series of Tertiary granite intrusions in the southeast corner. Carboniferous limestone makes an appearance in some places, but is not as well endowed with minerals as further south.
Donegal, occupying the northwest corner of the island, has such a varied geology that it has long been a favourite venue for university ﬁeld trips. In spite of this variety, there are few mining sites. Lead has been mined at Glenaboghil, Keeldrum and Glentogher, but these old mines are not noted for specimens. However, minor yellow powdery greenockite occurs at the ﬁrst location and green coatings of pyromorphite at the second. What it lacks in mines, the county makes up for in silicate minerals.
The beryl occurrence at Sheshkinnarone is probably the best known. Finger size green and blue-green prisms in a white quartz matrix occur at several spots here. The richest is just outside the garden wall of a relatively new house. The rock is so tough, however, that hammers simply bounce off it.
Skarns are widespread and yield a rather dull, euhedral, grossular garnet and lustrous brown vesuvianite. Small but gemmy red garnets turn up occasionally in the pegmatites that are commonplace in some of the many granite intrusions. Also present is black schorl. The shore north of Maghery has produced some very fine blades of dark green actinolite accompanying rose quartz.
Amethyst and smoky quartz are known from the Blue Stack Mountains, but precise locality details are not well recorded. A lack of records has also led to the loss of the legendary blue kyanite locality of Carrowtrasna. A few magnificent old specimens of sky blue blades, in length the span of a man’s hand, testify to the quality of the material. A lack of precise location data has meant that repeated visits by modern collectors have been in vain.
For the prospector, alluvial gold is widespread but in small amounts. Probably, most of the streams in a belt from the southwest peninsula, through the Blue Stack Mountains. and northwards to the tip of the Inishowen peninsula are auriferous to some degree. However, none stand out as being exceptional. Gold is more prominent in the neighbouring county of Tyrone. Curraghinalt saw the development of an underground trial level, now being re-evaluated, on an auriferous quartz/pyrite vein. Gold is being actively mined at Cavanacaw near Omagh and turned into jewellery. The local streams offer enticing prospects for panners.
The Tertiary basalts, occupying much of the counties of Londonderry and Antrim, have made Ulster famous for zeolites. While they rarely rival the treasures of Poona, India, they can compete in variety and rarity. The type locations of gmelinite, garronite and gobbinsite are all in these basalts. Of these, the ﬁrst is still readily found along the east coast, including the type locality at Little Deer Park, just south of Glenarm.
Look in boulders in the old roadside quarry opposite the sea arch called Madman’s Window. The most productive zeolite localities are the quarries. While many of the older ones, such as Magheramorne, are now closed, others have started up and taken their place as producers of good specimens. Almost any quarry in the basalts will yield zeolites, and other minerals, such as gyrolite, apophyllite, calcite and aragonite.
They do, however, tend to be unpredictable. A blast in the right place may create a bonanza, to be followed by months of nothing, as barren flows are worked through. Access is similarly variable, ranging from, “Take your car in, do you want tea or coffee?” to total refusal. It is advised that advance permission always be sought, and have personal accident insurance and the proper protective equipment at the ready. Coastal exposures and mountainside cliffs are also worthy of examination, although the latter are likely to have suffered from centuries of weathering.
Outside of the basalt, a group of old iron mines in Glenarrif, County Antrim has produced some good calcite. There is also an old lead mine at Newtownards near Belfast. Scawt Hill, north of Belfast, gave its name to scawtite, one of a variety of rare calcium silicates formed where a basalt intrusion reacted with chalk in a situation reminiscent of Killala Bay in Connaught. This is a protected site – collectors may look, but not take.
However , it is the granites of the Mourne Mountains that really stand out after the delights of the lava ﬂows further north. At least five intrusions make up the Mournes, and it is the northern ones that seem to be the better for minerals. However, to some extent, this may be a reflection of better exposures in this part of the range. In places, miarolytic cavities abound, sometimes in such profusion as to make the rock as porous as a Swiss cheese. Diamond Rocks is the traditional spot, but cavities are widespread and may be encountered almost anywhere in the hills.
Smoky quartz and feldspar are ubiquitous and, at Diamond Rocks little (less than 1cm) colourless topazes are common.
However, it is the beryls that are most sought after. Old time crystals, gemmy and bigger than a man’s thumb, are to be found in museum collections. Sadly, these have only vague locality information. Many came from the Victorian dealer, Patrick Doran, who worked in the local granite quarries. Unfortunately, modern trace element analyses of these has shown that they do not all match authenticated specimens.
Prisms on matrix and up to 2cm long, in shades of green and aquamarine blue, with internal colour zones, have been found at Diamond Rocks but are very rare. Elsewhere, beryl has been reported from Slieve Donard, Slieve Bignian, Chimney Rock Mountain, Rocky Mountain (of which there are two) and the intermittently worked quarry above Newcastle. An interesting recent ﬁnd is on Slievenamiskan, overlooking the Spelga Dam. Here, it occurs as sky blue prisms in matrix, and detached, gemmy blue corroded crystals up to 3cm long in smoky quartz lined cavities in a quartz vein stockwork in country rock at the margin of the granite. The latter crystals look like smaller versions of some of the detached old-time specimens.
And so we end our tour of the Emerald Isle. While it may not offer emeralds, it has plenty of other mineralogical delights. Being relatively untouched by collectors, and with few indigenous ones, there remains abundant scope for new discoveries. Indeed, around 30 of the approximately 50 minerals new to the island found since about 1980, have been found by amateur and commercial collectors.
The sad tales of past treasures consigned to the smelter, or otherwise lost because nobody was on hand to collect them – as at Tynagh, Silvermines, Inveran and many other sites – only serves to illustrate the essential role of the mineral collector in conserving our natural heritage for future generations.
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.