Economic geology of Orkney and Shetland
|Mykura, W. 1976. British regional geology: Orkney and Shetland. Edinburgh, Her Majesty's Stationery Office.|
- 1 Ore Minerals
- 2 Talc
- 3 Baryte
- 4 Kaolin
- 5 Semi-precious and lapidary stones
- 6 Flagstones and building stones
- 7 Road metal and aggregate
- 8 Limestone
- 9 Sand and gravel
- 10 Peat
- 11 Bibliography
Copper ores and associated minerals
At Sand Lodge mine, south Mainland (438 248), copper and iron ores have been intermittently worked from 1789 till the early 1920s. The ore occurs in a gangue of ankerite, siderite and calcite, which forms two north-south trending veins, 3 to 5 m thick and converging at depth. The most important ore minerals were recorded as limonite, hematite, malachite and some chrysocalla above the 30 m level and chalcopyrite and siderite below. It is estimated that the mine has yielded over 12 000 tons of ore. Another, smaller mine, was in operation around 1880 at Setter (HU 435 256). This yielded small quantitites of pure chalcopyrite. Adits have also been driven at Levenwick (HU 409 217) and Hoswick (HU 418 237), where both iron and copper ores might be expected. It is not known, however, what minerals, if any, were obtained. In the extreme south of Mainland, at Quendale (HU 368 127), Wick of Shunni (HU 35 15) and Garths Ness (HU 364 113), adits have been driven for copper. Close to the last-named locality there is an ore body which forms a vertical lens up to 3-6 m wide and 30 m long, but consists largely of pyrrhotite with only small amounts of pyrite and chalcopyrite.
Small quantities of chalcopyrite, usually embedded in veins of ankerite and calcite, are to be found at the Bight of Vatsland (HU 467 462); at Muckle Hell (HU 526 400) and Rules Ness (HU 529 426), on the east coast of Bressay; in western Noss (HU 532 405); and at Aith Wick (445 297), Croo Taing (HU 429 269), Sand Wick (HU 437 237) and Levenwick Ness (HU 417 214). Chalcopyrite in quartz veins occurs at No Ness (HU 445 212). On Fair Isle the copper ores digenite, chalcopyrite, bornite, covellite, tetrahedrite, cuprite and malachite, together with pyrite and goethite have been recorded at Copper Geo (HU 203 728) where they occur in a gangue of scapolite and calcite. In 1912 over 15 tons of this ore was quarried and some of it exported. Smaller quantities of copper ores, also associated with scapolite, have been found in the North and South Reevas (HZ 200 709) and in other localities in south-west Fair Isle. Minute quantities of chalcopyrite, associated with ankerite, occur at Duttfield in east Fair Isle (HZ 222 722).
Other non-ferrous ores
Galena has been recorded near Hamnavoe in southern Yell (HU 500 799) and along the east coast of central Mainland at Vidlin Voe (HU 481 666), Dury Voe, Lax Firth (HU 446 486) and the Bight of Vatsland (HU 467 458). At Vidlin Voe the galena occurs within a 15 m wide band within the schists, which has a high concentration of pyrrhotite and some sphalerite and chalcopyrite. Sphalerite (zinc blende) is also found on the shores of Dales Voe (HU 456 455 and HU 455 450), and gersdorfifite, a nickel-arsenic sulphide, occurs on the west shore of Aith Voe (HU 422 291). Some of these ores are set in a gangue of ankerite and calcite. At Mo Geo (HU 443 283) in the south-west corner of the Helli Ness peninsula, at Blue Geo near Sand Lodge (HU 438 247) and at Honga Ness in Fetlar (HU 655 913) there are irregular dyke-like masses of ankeritic carbonate with specks of chalcopyrite. These are coloured bluish green by traces of fuclzsite, a chrome-mica. Chromite is a common accessory constituent of the serpentines of Unst where it is locally sufiiciently concentrated to be suitable for commercial exploitation. In the past it has been worked in quarries on the south slope of Nikka Vord (HP 625 103) just north of Balta Sound, at Hagdale (HP 638 102) and to a smaller extent near Hamaberg (HP 596 037). Between 1820 and 1944 some 50 000 tons of chromite ore were extracted in Unst, first for use in the chemical industry and, later, as a refractory and for the manufacture of chrome-magnesite bricks. In spite of extensive exploration in the early 1950s (See Rivington 1953) the deposits have not been worked in recent years. A number of relatively rare minerals are associated with the chromite of Unst. These include kämmererite (a chrome-chlorite), zaratite: Ni3(CO3)(OH)4 4H2O, uvarovite: Co3Cr2(SiO4)3 and pentlandite: (Fe, Ni)9S8. Chromite also occurs in appreciable quantities in the serpentine of Hesta Ness, north-east Fetlar (HU 663 927).
Hematite, limonite, goetizite and siderite are associated with the copper ores of Sand Lodge and Levenwick. The Sand Lodge Mine has yielded a considerable tonnage of hematite, but this was of secondary importance to the copper ore. At Clothister Hill (HU 342 729), Sullom, there is a lenticular ore body of magnetite which has a high degree of purity and an exceptionally low phosphorus content (60 to 67 per cent Fe and less than 0.006 per cent P). At the outcrop it has a north—south trend, a length of 53 m and an average width of 3 m. It has a steep westerly inclination and at a depth of 22 m it has an average width of 3.3 m, but thins out rapidly below that level. The estimated volume of the ore body is 4000 m3, giving a possible 20 000 tons of ore. The magnetite was mined between 1954 and 1957 and used in the manufacture of heavy mud for a coal ﬂotation process. Between 6000 and 10 000 tons were extracted at that time.
It has been known for more than four centuries that lead and copper ores are to be found in Orkney, and many unsuccessful attempts have been made to work them economically.
Galena is usually associated with the gangue minerals dolomite, calcite and strontianite, which fill veins and interstices in breccia and small faults. The largest lead-bearing deposit occurs on the coast near Warebeth (HY 224 087), 2 km W of Stromness, where a vertical north-east trending vein was worked about 1775. Good exposures of the vein-breccia with small specimens of galena are still seen on the coast. Other old lead mines are situated at Manse Bay (ND 477 921) on the east coast of South Ronaldsay (worked in the latter half of the 18th century), on Graemsay (exact location not known, closed early in 17th century), in western Rousay (HY 374 311) and in Sanday (HY 702 422) (opened about 1880). Other occurrences of galena are at Mill Bay in Stronsay (HY 656 267), on the shore of Mainland near Rennibister (HY 398 131), at Burnside (HY 259 104) 1-5 km N of Stromness, at the Bay of Navershaw (HY 265 088) and in Walliwall Quarry (HY 436 104) 1.5 km W of Kirkwall. There are also records and traditions of lead ore having been found in Deerness, St Andrews, Shapinsay, Fara and Yesnaby (HY 22 15).
The primary copper ores met with in Orkney are cltalcopyrite, chalcocite and some native copper. These occur in only small quantities. Secondary ores are malachite, azurite and c/trysocolla. Attempts to work these ores have been made at Wha Taing (ND 445 961) in the west of Burray (worked before 1774), and on Rousay (HY 387 285). Exposures of ore can still be seen at the old millstone quarry, Yesnaby (HY 219 154).
Hematite, limonite and goethite have been worked at the Bay of Creekland (HY 237 044) and at The Candle of the Sale (HY 273 017) in northern Hoy. In both localities the ore forms thin veins lining joints and fractures.
Manganese ore has been recorded and actually worked (prior to 1774) at Lead Geo on the western cliffs of Hoy (HY 186 032), in a position some 60 m below the top of the 275 m high cliff.
Disseminated uranium ore is present in the basal beds of the Stromness Flags in the Yesnaby and Stromness districts of West Mainland. Here dolomitic conglomerate abutting against the metamorphic inliers contains in places up to 300 ppm uranium and 500 ppm zinc, as well as some galena and baryte. The ﬂags immediately above the conglomerates also contain some uranium.
Considerable quantities of talc are present along the margins of, and in places within, the serpentine blocks of Unst and Fetlar. In Unst talc has been wrought at Queyhouse Quarry [HP 612 123], sited within the talc belt along the western margin of the Main Serpentine Block. This quarry has been in operation since 1945 and produces about 9000 tons per annum. The talc is used mainly in the manufacture of roofing felt. Talc is also present along the western margin of the Clibberswick Block, and the remains of ancient soapstone quarries, which formed the basis for an ancient cooking pot industry, can still be seen at Clibberswick (HP 652 121). On Fetlar near-vertical bands of high quality talc cross the serpentine of Hesta Ness (HU 662 925). These were worked for a short time around 1914.
An extensive outcrop of talc-magnesite-schist, associated with serpentine and metamorphosed basic igneous rocks, crops out in the vicinity of Cunningsburgh, Shetland south Mainland (Bain and others 1971). This contains several areas of good quality talc-magnesite rock. One of the largest and best exposed masses lies close to the Burn of Catpund, some 250 m W of the main road (HU 425 271). Here the rock appears to be almost homogeneous and covers an area of about 550 000 m2. The remains of ancient (?Viking) quarries can be seen in this area. Talc and magnesite are present in roughly equal amounts in these deposits. They also contain up to 10 per cent magnetite, chromite and chlorite. Investigations have been carried out into methods of separating the talc from the other components. It should be possible to use the whole rock directly as a raw material for various industrial products, such as a filler for rubber, plastics etc.
Baryte does not occur in commercial quantities in either Orkney or Shetland. Along the north-east coast and on parts of the south coast of Papa Stour it forms narrow veins emplaced along joints in the rhyolite cliffs. Small zones with thin baryte veins are also present in Lunning, at Muckle Hell on Bressay, and in western Noss. In Orkney baryte forms part of the gangue mineral of the galena veins at Warebeth, Rousay and Stronsay (Orkney, lead ores).
A vertical belt of soft kaolinitised metamorphic rock crops out along the course of the Tactigill (HU 373 516) and Dale (HU 372 482) burns, east and south of Tresta, Shetland Mainland. The belt varies in width from 9 to 30 m and contains up to 40 per cent kaolin. A much smaller deposit of relatively pure kaolin occurs close to the shore at Moo Wick (HU 622 877) in the Lamb Hoga peninsula, Fetlar (May and Phemister 1968).
Semi-precious and lapidary stones
Though none of the minerals of Shetland can be classed as precious stones, there are many rocks and minerals suitable for cutting and polishing into ornaments, brooches etc. Some fine mineral specimens can also be obtained. Serpentine is abundant in Unst and Fetlar, but not all of it is of gemstone quality. On Mainland serpentine of good colour has been found on the north shore of Colla Firth (HU 357 844). There is also a suitable outcrop on Vementry Island near the north-west corner of Maa Loch (HU 297 604). Calcite of a fine pink to reddish colour forms thick irregular veins at the Nabb of Lerwick (HU 479 403), on the north-east shore of Brei Wick (HU 477 405) and the west coast of the Ness of Beosetter, Bressay (HU 492 443). Dyke-like masses of carbonate rock (mainly ankerite) coloured green by an admixture of chrome minerals crop out on the shore at Aith Voe (HU 443 283), Sand Lodge (HU 439 248) and eastern Fetlar (HU 656 913). Magnetite octahedra up to 1 cm in diameter, set in chlorite, crop out in a bay in the north-east of North Roe (386 936); smaller octahedra occur in eastern Fetlar (HU 670 914). Scapolite, a pale bluish to purplish white mineral, which takes a good polish, forms a thick vein at Skelda Ness, at the south end of the Walls Peninsula (HU 303 404) and gives rise to thinner veins on the eastern cliffs of Wester Wick (HU 285 423), in the roadside quarry at Mavis Grind (HU 341 682) and in south Fair Isle. Rather poor quality agates and small amounts of ﬂuorite are present in the basalt lavas on the west (HU 149 609) and south (HU 176 596) shores of Papa Stout. Agates and rare amethysts are found in the lavas of Stenness (HY 214 772) and Esha Ness (HU 203 780).
One of the Shetland rocks suitable for lapidary work is the riebeckite-felsite. Dykes of this are particularly abundant at the Beorgs of Uyea, North Roe (HU 33 90). Many of them contain bluish green spherulites. One of the dykes (HU 327 901) was quarried in Neolithic times and the stone was used for the production of stone knives (Ritchie 1968). Another attractive hard green rock is rodingite, a jade-like rock composed of diopside, garnet and chlorite which forms lenses and veins in the serpentine of Fetlar. The best outcrops of rodingite are at Tressa Ness (HU 617 948) and Swart Houll (HU 644 918).
There are many other Shetland rocks and minerals suitable for polishing and tumbling. A comprehensive account of Shetland minerals is given by Heddle (1878) and an excellent collection of both Shetland minerals and rocks is to be seen in Lerwick Museum.
Flagstones and building stones
Most Shetland rocks have been used for the building of crofts and houses sited near their outcrop. Many buildings in Lerwick are constructed of Lerwick Sandstone, probably obtained mainly from quarries on South Stony Hill (HU 461 417). Flagstones have been quarried on Aith Ness, northern Bressay (HU 512 444) and in south-east Mousa (HU 465 233). The Mousa quarry supplied the original paving ﬂags for Commercial Street, Lerwick. The condition of Mousa Broch (HU 458 237), which is still virtually in its original state, is a fine tribute to the suitability as building stone and the durability of the ﬂaggy sandstones of south-east Shetland.
The ﬂagstones of Orkney have in the past been quarried for use as building and paving stones. The thinly bedded calcareous siltstones and shales of the Sandwick Fish Bed have been extensively quarried for roofing slates, but at present the bed is only worked at Cruady Quarry, Quoyloo (HY 247 217). Thinly bedded ﬂagstones in the Rousay Flags have also been extensively quarried on the hillsides north and south of the valley leading west from Finstown. The Orkney Flags are not of the same quality as those of Caithness and they have never been exported. They were used in Neolithic and later times for building the chambered Cairns, brochs and monuments, and an ancient quarry from which slabs similar to those forming the Standing Stones of Brogar and Stenness were obtained is still to be seen on the south slope of Vestra Fiold, West Mainland (HY 239 217).
The red and yellow sandstones of the Eday Beds have been extensively used as a building stone suitable for decorative work. A quarry in the Lower Eday Sandstone at Fersness, Eday (HY 536 336) produced a fine yellow freestone which has been employed for the construction of many buildings in Kirkwall. The quarries from which the stone used for the building of St Magnus Cathedral was obtained were probably sited on the Head of Holland, 4 km ENE of Kirkwall. Much of the Upper Old Red Sandstone of Hoy appears to be a good quality building stone, but has only been used locally.
Road metal and aggregate
Most of the igneous and many metamorphic rocks of Shetland have in the past been used for road metal and aggregate. At present most of the material is obtained from quarries at Scord, Scalloway (HY 412 400) (phyllitic schist); Sullom (‘granite’); Houlland, South Yell (HY 50 80) (‘gneiss’); Setter, Unst (HY 636 115) (serpentine) and Whalsay (HY 546 623) (gneiss). In the past beach gravels have also been locally used as a source for concrete aggregate.
The calcareous ﬂagstones of the Rousay and Stromness Flags are at present extensively used for road metal and aggregate. The principal quarries are at Cursiter (HY 376 125), Walliwall (HY 2436 104) and Quoyloo (HY 2247 217) on Mainland, and there are smaller quarries on most of the other islands. The granite-gneiss of Brinkie’s Brae, Stromness (HY 2252 096) has been quarried on a small scale in the past and should be suitable for both road metal and high quality aggregate. The basic dykes of Orkney are generally too thin for quarrying.
The thick crystalline limestones of east Mainland, Unst and the Out Skerries are all suitable for the production of lime for building and agricultural purposes. At present, however, the only quarry in operation is that working the Girlsta Limestone at Girlsta (HU 429 505), 10 km N of Lerwick. The limestone contains nearly 44 per cent CaO and 5 per cent MgO. It has up to 10 per cent of quartz as well as accessory chlorite and muscovite (Muir and others 1956). The limestone is used for the production of agricultural lime. The lime workings at Girlsta and Ukensetter have been described by O’Dell (1939).
Some of the calcareous, fish-bearing, shales in the Rousay Flags have a high lime content. Though they are less than 1 m thick they have been quarried in the past and used for the production of building lime. Examples of such quarries are to be found at Gallowhall in Evie (HY 374 240) and Skel Wick in Westray (HY 493 453).
Sand and gravel
Both Orkney and Shetland are devoid of ﬂuvioglacial sand and gravel, and there is very little river alluvium. The principal source of sand in the islands is from the accumulations of blown sand and from the sea beaches. In Orkney the blown sand is composed largely of comminuted shells and contains between 60 and 90 per cent CaCO3. The principal deposits on Mainland are at Sandside Bay, Skaill, Deerness (HY 589 067) where they are being extensively worked at present, as well as at Bay of Skaill (HY 235 195), Birsay (HY 248 270) and Aikerness (HY 380 264). Considerably larger deposits of blown sand cover nearly one third of Sanday and large parts of North Ronaldsay, Westray and Eday. Small areas of silica-sand consisting essentially of quartz grains have been recorded on beaches at Westray (HY 479 462 and HY 472 447), Eday (HY 564 343), Stronsay (Mill Bay) and Hoy (HY 205 986). The largest area of blown sand in Shetland is found at Quendale (HU 380 130), but all the sand at present used commercially comes from sea beaches such as those at St Ninian’s Isle and Gulber Wick.
Peat is extensively dug as a domestic fuel in both Shetland and Orkney, but there has as yet been no successful exploitation on a commercial scale. Peat briquettes were manufactured at one time at the Loch of Brindister (HU 431 368), 5.5 km SW of Lerwick, but the venture was not financially viable. The greatest resources of peat are on the Island of Yell, which contains an estimated 200 million tons of raw peat or 16 million tons of peat solids. Considerable reserves are also available in parts of Shetland Mainland.
- RIVINGTON, J. B. 1953. Recent Chromite Exploration in Shetland. Min. Mag. Lond., 89, 329-37.
- BAIN, J A, BRIGGS, D A, and MAY, F. 1971. Geology and mineralogical appraisal of an extensive talc-magnesite deposit in the Shetlands. Trans. Instn. Min. Metall., 80, B, 77–84.
- MAY, F, and PHEMISTER, J. 1968. Kaolin deposits in the Shetland Islands, U.K. Rep. XXIII Int. Geol. Congr., Prague, 14, 23-9.
- RITCHIE, P R. 1968. The stone implement trade in third-millennium Scotland, in COLES, J M, and SIMPSON, D D A. (editors). Studies in Ancient Europe, pp. 117–36, Leicester.
- HEDDLE, M F. 1878. The County Geognosy and Mineralogy of Scotland, Orkney and Shetland. Truro.
- MUIR, A, HARDIE, H G M, MITCHELL, R L, and PHEMISTER, J. 1956. The Limestones of Scotland: Chemical Analyses and Petrography. Mem. geol. Surv. Gt Br. Min. Resources, 37.
- O’DELL, A C. 1939. The historical geography of the Shetland Islands. Lerwick.