Economic geology, Palaeogene volcanic districts of Scotland
|Emeleus, C H, and Bell, B R. 2005. British regional geology: The Palaeogene volcanic districts of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
- 1 Introduction
- 1.1 Iron
- 1.2 Chromium, magnesium and precious metals
- 1.3 Vein deposits
- 1.4 Gemstones
- 1.5 Building stone
- 1.6 Aggregate
- 1.7 Sand and gravel
- 1.8 Silica sandstone and silica sand
- 1.9 Limestone and dolostone
- 1.10 Coal
- 1.11 Graphite
- 1.12 Oil
- 1.13 Peat
- 1.14 Water supply
- 1.15 Diatomite
- 1.16 Geological hazards
- 2 References
The exploitation pattern of the limited economic resources of the district has undergone notable changes since the first edition of this book. In place of a multiplicity of small-scale operations, the requirements for building materials and aggregate are now met by a few large concerns, and much of what was once obtained locally is now brought into the district, sometimes from considerable distances. Fuel requirements are met by imported coal, oil and (bottled) gas. Electricity is supplied to Mull, Skye and Arran by cable links to the mainland and the only local contribution of any significance comes from the hydroelectric scheme at Loch Fada on Skye.
Carbonate-rich ironstone was obtained from the Lower Jurassic Raasay Ironstone Formation on Raasay, where the reserves were estimated at about 10 million tonnes. The chamosite-oolite ore occurs in beds up to 2.5 m thick between Churchtown Bay and Hallaig (Lee, 1920). Both underground and opencast workings were employed and over 125 000 tonnes of ore were obtained during the active life of the operations, between 1916 and 1919 (Draper and Draper, 1990). Remains of the workings are visible at several places in the south of Raasay. The Raasay Ironstone Formation also occurs on Skye, Ardnamurchan, Mull and elsewhere in the Hebrides but the beds are thin, generally of low quality and have not been exploited.
Clay-ironstone of Carboniferous age was obtained from sedimentary rocks near Corrie, Arran, and is reputed to have been exported to the mainland. Elsewhere on Arran, there are signs of workings and smelting involving bog iron-ore. The magnetite-bearing skarns close to the margin of the Beinn an Dubhaich Granite on Skye were examined during the Second World War (1939—45) but the deposits, which are associated with small amounts of copper mineralisation, were not exploited.
Chromium, magnesium and precious metals
Substantial amounts of chromite and forsteritic olivine are present in shallow water marine sands off southern Rum (P914161) and in Loch Scavaig, Skye (Basham et al., 1989). Although the chromite tends to be iron-rich compared with ores in current commercial use, the abundant, associated magnesian olivine may be of value as an abrasive. The deposits in Loch Scavaig are mixed with glacial clays, which would make exploitation difficult, but those off Rum could easily be recovered. The sources of the chromite and olivine are the layered, olivine-rich ultrabasic rocks of the Rum and Skye central complexes, where chromite occurs in seams and layers generally less than 1 cm thick (P580479). It is commonly associated with sulphide minerals and the ultrabasic rocks on Rum, Skye and Mull show generally elevated levels of the platinum-group elements (Butcher et al., 1999; Pirrie et al., 2000). On Rum, electrum (an Au-Ag alloy; Dunham and Wilkinson, 1985), and platinum-group minerals are associated with the chromite seams (Butcher et al., 1999). The platinum-group minerals are also found as a minor constituent of small, chromite-rich placer deposits in streams on Rum. These primary and alluvial deposits are too small to be of direct commercial interest and, furthermore, they occur in environmentally sensitive localities; nonetheless, their paragenesis has proved to be of considerable interest (e.g. Power et al., 2003).
Good quality baryte has been obtained from a system of north-trending veins in Glen Sannox on Arran. The veins are up to 3 m wide and are hosted by Lower Old Red Sandstone strata near the margin of the Paleocene North Arran Granite Pluton, although it is unlikely that there is any genetic connection with the granite. Imprecise K-Ar dates suggest a Triassic age (Moore, 1979). Several hundred tonnes of ore were raised each year for over 20 years up to 1862 when working ceased. Extraction resumed in 1918, with annual production sometimes exceeding 8000 tonnes of barytes before the mine was abandoned in 1944.
Analyses of stream sediment samples close to the southern margin of the North Arran Granite Pluton show enhanced levels of tin, tungsten, uranium and thorium, and cassiterite has been recovered from sediments in the Glen Sannox River suggesting the presence of mineral veins in the granite or its host rock.
Small, impure sapphires have been obtained from hornfelses adjoining a major ring-intrusion at Glebe Hill, Kilchoan, Ardnamurchan (Richey and Thomas, 1930; p. 122). Sapphire has also been recovered from tholeiitic basalt sills at Loch Scridain and Carsaig, Mull, where the aluminous, sapphire-bearing accidental inclusions in the sills were probably derived from Moine pelitic rocks (Dempster et al., 1999; Chapter 8). However, the most significant find was made in 1992 at Loch Roag, Isle of Lewis, where large sapphires, some of gem quality and accompanied by corundum and zircon, were recovered from a lamprophye dyke (p. 83). An uncut, blue sapphire from this source was sold for several thousand pounds, the highest price paid for a gemstone originating in the British Isles. Bloodstone from lavas in north-west Rum has been used for ornamental purposes (and for weapons by early Man) and small, banded agates have been obtained from the same source.
Up to the beginning of the 20th century, local stone was employed extensively in houses and public buildings. Many examples remain, commonly roofed with Dalradian slate from Ballachulish and Easdale in Argyll. With improved transport, brick and stone from the Scottish mainland and slate from Wales came to dominate the market, giving way in recent years to concrete blocks, which are brought in from as far afield as Antrim (where the aggregate used is local basalt), and to ceramic tiles for roofing.
The only major granite quarries are those in the attractive pink porphyritic Caledonian granite of the Ross of Mull (Faithfull, 1995). The granite yields large blocks and takes a fine polish. It was quarried extensively in the 19th century, and sporadically since; quarried stone was generally sent for polishing to Aberdeen, Glasgow or Shap in Cumbria. It has been used locally in Iona Cathedral, together with flagstones of Moine strata from the Ross of Mull, and in many buildings on the Ross of Mull. Elsewhere, it has been used widely, for example in the construction of the Liverpool docks, in several Scottish lighthouses including Skerryvore and Ardnamurchan Point, and in London it may be found in Blackfriars and Westminster bridges and the foundations of the Albert Memorial;it was also exported to New York and other places in North America. Production largely ceased before the First World War (1914—18) but, since 1985, previously quarried blocks have been removed from the Tormore quarries and some new stone has also been won.
Despite an abundance of Paleocene granites, the only granitic rock of this age to have been quarried extensively is the distinctive blue-grey riebeckite-bearing microgranite of Ailsa Craig. This was used for paving sets, in Glasgow and elsewhere, and as an ornamental stone, as in the floor of the Scottish National War Memorial, Edinburgh Castle. However, it is best-known for the curling stones that were manufactured in their thousands towards the end of the 19th century. The roughed-out stones were finished on the mainland, with many exported to Canada. The running surfaces for curling stones currently used in all World Championship and Olympic events are from Ailsa Craig, and the polished microgranite continues to find ornamental uses (Nichol, 2001).
Very large amounts of excellent building stone have been obtained from the Carboniferous and New Red Sandstone strata of Arran. White freestone from the Lower Carboniferous at Corrie was used extensively in the late 18th and 19th centuries, for example in the construction of the Crinan Canal, but the most extensive exploitation was of the red-brown rocks of the New Red Sandstone. In addition to local use, the Lower Permian Corrie Sandstone was employed in many works outside Arran, for example at Troon Harbour. In southern Arran, the red sandstones on the south side of Lamlash Bay, at Cordon and Monamore Mill, were extensively quarried. Triassic sandstones in Morvern and elsewhere have been quarried for local building use, and around Broadford, Skye, the local Lower Jurassic rocks provided freestone suitable for building. The quartz porphyry and dolerite sills in the south of Arran have also been used locally.
Small roadside quarries are still used as sources of aggregate on several of the islands, particularly in the Small Isles where shipment facilities were poor until recently. Basalt rubble is applied to the tracks on Muck, Eigg and Canna, although in the past marble chips, probably obtained from Skye, were used to surface the tarred roads on Eigg and Muck. On Skye, the large, well-equipped quarry in Torridonian sandstone at Peinchorran, near Sconser, provides road aggregate for the island, although hornfelsed lava from a quarry near Sligachan and dolerite from a Paleocene sill at Invertote have in the recent past provided material for major road rebuilding programmes. On Mull, altered basalt from within the limit of pneumatolysis around the central complex is used as roadstone and is obtained from a quarry near Salen. Aggregate has also been obtained from a basalt quarry east of Bunessan and from a quarry in a granite ring-dyke south of Salen. On Arran, roadstone has been imported since the 1920s (Tyrrell, 1928). However, a quarry in felsite at Bennecarrigan in south-west Arran produces aggregate for roads and also some building stone. At Dereneneach, on the western margin of the Central Arran Ring-complex, a small quarry in granite supplies aggregate for local use. No large coastal quarries are projected for the district, although the granite of western Rum identified on geological grounds as a possible site. However, the exposed coast and the National Nature Reserve status of the island make its exploitation improbable.
Sand and gravel
Large amounts of sand and gravel are extracted from quarries in thick (over 10 m) gravel deposits on marine raised beaches at Allt Anavig and Lusa, west of Kyleakin, Skye, and local use has been made of the gravel deposits at the mouth of the River Brittle. On Mull, the extensive moraines north of Loch Don are a source of sand and gravel. Silica-rich beach sands are used locally in Ardnamurchan, Skye and Mull as sharp sand in cement and concrete; on Arran, sand from glacial outwash deposits near Brodick Castle was exported for building purposes.
Silica sandstone and silica sand
At Loch Aline, on Morvern, a deposit of white sandstone of exceptionally high purity (99.7 per cent quartz and generally 0.01 per cent Fe2O3 or less) is worked as a source of sand for glass manufacture (P580492). The sand is mined from a bed of Cretaceous sandstone that averages 12 m in thickness (of which approximately 5 m are worked), underlying the Paleocene lavas. Although the deposit has been known since the end of the 19th century, the mine was only opened in the 1940s when other European sources of pure sand were cut off during the war. Currently, production is about 120 000 tonnes per year. The deposit is the only United Kingdom source of sand suitable for high-grade domestic, scientific and commercial glass. The sand is crushed at the mine and all is shipped out. About 50 per cent of the production is currently exported, principally to the Republic of Ireland and to Scandinavia where cheap hydroelectricity makes it economic to mix the sand with graphite in the manufacture of 'carborundum' (silicon carbide) abrasive.
Limestone and dolostone
The principal carbonate rocks of the district are in the Cambro-Ordovician Durness Group on Skye, the Carboniferous succession of Arran, and the Mesozoic sequences throughout the Inner Hebrides. Durness Group dolostones are recrystallised to marble adjacent to the Paleocene granites of the Eastern Red Hills Centre and the Broadford Gabbro. The marble, which includes brucite- and serpentine-rich varieties, was formerly quarried extensively in the roof zone of the
Beinn an Dubhaich Granite for ornamental stone slabs and as chips for terrazzo flooring. A tramway was constructed to take products from Strath to Broadford, from where they were shipped. The quarries are long abandoned but nearby, at Torrin, the Skye Marble Company produces up to 20000 tonnes per year of crushed chips, used principally for harling and for roadstone.
The Lower Carboniferous Corrie Limestone in eastern Arran was extensively quarried and mined for lime. Calcining for agricultural lime has been the principal use for the Mesozoic limestone and the ruins of small lime kilns occur near many of the main outcrops, for example where Lias limestone crops out at Broadford, Skye and at Mingary and Swordle on Ardnamurchan. The Middle Jurassic limestones of the Duntulm and Kilmaluag formations of Skye and Eigg were calcined locally. Marble was obtained from a lens within the Lewisian of Iona (see Johnstone and Mykura, 1989). Shelly beach sands and the so-called 'coral sands' with a high calcium carbonate content occur in many places on the coast, but have been little used apart from local exploitation for agricultural lime.
In the 18th century, there was a small working for coal in the Carboniferous rocks south of the Cock of Arran, where seams about 1 m thick were reported (Tyrrell, 1928). Coal Measures crop out at Inninmore, Morvern, but do not contain any workable coal seams in the exposed 60 m of section. In northern Skye, lignitic coals occur in thin seams in the Great Estuarine Group, where they pass laterally into oil shale; locally, this poor quality coal was used as fuel. Thin seams and streaks of poor-quality coal between Paleocene lava flows occur sparsely throughout the district, for example close to the base of the lava sequence near Portree, Skye, in tuffs to the east of Fionchra, Rum, and at Carsaig on the south coast of the Ross of Mull. Lignites associated with the lavas of Ardtun, Mull, were investigated for the Duke of Argyll, but were found to have high ash and moisture contents. Similar lignitic coal was reported in a seam about 60 cm thick at Ardslignish, Ardnamurchan.
An unworked seam of graphite has been reported from Mesozoic strata on the north side of Loch Sligachan, Skye (Peach et al., 1910). Graphite also occurs in small blocks in the Loch Scridain sills, Mull, where attempts were made to extract it in the 19th century. Jet has been reported from the Jurassic sedimentary rocks of Holm, north Skye and from sedimentary rocks underlying basalt lavas on Ben Hiant, Ardnamurchan.
Although there is considerable interest in the oil-bearing potential of the Mesozoic and Paleocene sedimentary sequences off the west and north-west coast of Scotland, onshore possibilities are limited. However, there has been some exploratory drilling on Skye. In the north of Skye and the south of Raasay, oil-shale up to 3 m thick occurs in the Great Estuarine Group. The oil potential of these rocks has been compared favourably with that of the Carboniferous oil-shales in the Lothians, with up to 54 litres of crude oil per tonne of shale obtained in tests (Anderson and Dunham, 1966).
This was the principal fuel throughout the Highlands and Islands until well into the 20th century. It was gradually replaced by coal from the Midland Valley of Scotland, and more recently by oil and bottled gas. Small workings continue to the present, notably in northern Skye where consideable areas of blanket bog peat exist, up to 3 m thick in places. Nowhere has machinery been employed to win peat commercially.
Water power A small hydroelectric station on the Bearreraig River, Skye, makes use of the fall at sea cliffs near the outlet from Loch Leathan, north of Portree. The installation contributes power to National Grid. There are local hydroelectric schemes, for example on Rum, which has supplied Kinloch Castle since the early 20th century, and on Eigg.
The small communities scattered throughout the region rely extensively on surface water for supplies, which renders them vulnerable to drought and, less commonly, to frost. Water obtained from burns and wells is commonly piped short distances to isolated houses. The larger settlements are generally supplied from small reservoirs and from lochs in nearby hills. The largest groundwater system is on Arran where several boreholes into the Permian sandstones in the Brodick—Lamlash Bay area and in the Shiskine valley serve the southern half of the island.
Deposits of diatomite are common in the small lochs and hollows amongst the landslipped lavas resting on sills and Mesozoic strata in northern Trotternish, Skye (Haldane et al., 1940; Anderson and Dunham, 1966). Many of the occurrences have been worked on a small scale, but the Loch Cuithir deposit, at over 6 m, was the thickest, and was worked up to 1914; the diatomite was transported to the coast at Invertote by a 5 km-long tramway. It was formerly used in the manufacture of dynamite, but later the principal uses were as a filter and as an insulator. Thin diatomite deposits near Loch Bà, Mull have been used locally for whitewash.
The principal geological hazards in the district are rockfalls and landslips along the edges of the Paleocene lava outcrops. Landslipping has long been a problem in parts of northern Skye, where the roads are affected and movement of buildings has occurred as, for example at Flodigarry. These movements are but the latest manifestation of landslipping which has occurred in this area, on Raasay, and elsewhere, since the Late Devensian glaciation. Rockfalls are a particular problem along the edges of the basalt scarps, especially after frost and rain. On Eigg, a house was demolished at Cleadale and there are extensive, recent falls on the coast north of Kildonan.