Metalliferous mineralization, Southwest Scotland

From Earthwise
Jump to: navigation, search

By M J Gallagher. From: Stone, P (editor). 1996. Geology in south-west Scotland: an excursion guide. Keyworth, Nottingham: British Geological Survey.

Metalliferous mineralization of south-west Scotland

Introduction

Most of the mineral locations in south-west Scotland are veins, many of which were trialled or mined for lead, copper and zinc in the 18th and 19th centuries, with limited work continuing in places until about 1920 (Wilson, 1921). Production of haryte from Barlocco probably continued until 1954.

Location of significant metalliferous mineralisation in south-west Scotland. Full details are listed in the table below.

Subsequently, uranium veins and both disseminated and vein occurrences of minerals of arsenic, molybdenum, lead, copper and zinc were discovered using modern techniques, but none has proved economic. Minor quantities of gold have also been discovered recently at several localities.

The known mineral veins of south-west Scotland are concentrated at the southern and western margins of the granitic complexes of Cairnsmore of Fleet in the west, and Criffell in the east. The composition of the veins changes in a general way from dominantly lead-zinc in the west to copper, barium and uranium in the east. The veins are highly variable in trend (Table 5), cutting sedimentary rocks of Ordovician to Lower Carboniferous age and intrusions of diorite, granodiorite and granite dated isotopically at late Silurian to early Devonian (Stephens and Halliday, 1984; Stephens et al., 1985). The lead-zinc vein at Blackcraig may postdate a dyke of Permo-Carboniferous type (Gallagher, 1964) and pitchblende from one of the Dalbeattie veins has yielded a Mesozoic age (Miller and Taylor, 1966).

Eight types of mineralisation (A–H) are described in this account, with reference to 29 of the more significant mineral locations in the region (located on Figure 67). Much further information can be found in the geochemical atlas Regional geochemistry of southern Scotland and part of northern England (British Geological Survey, 1993c) which contains coloured distribution maps for many elements.

A Associated with ultrabasic rocks

The oldest mineralisation known in southwest Scotland is of chrome-spinel, as concentrations in the early Ordovician Ballantrae ophiolite complex. Accessory quantities of chrome spine' occur widely in serpentinised harzburgite and dunite, the main rock types of the complex, and at Poundland Burn nodular chrome spine' is visible in outcrop (location 7, Table 5 and Figure 67). At Pinbain (6), true chromite forms 30-90 per cent of a small unit 4-5 m wide, most probably a chromitite pod (Stone et al., 1986).

B Associated with Ordovician or Silurian sedimentary rocks

Two mineral occurrences very unusual to Scotland fall into this group. Ordovician siliceous mudstone and siltstone contain finely disseminated pyrite and sphalerite some 2 km SW of the Loch Doon granitic pluton in the vicinity of Penkiln Burn (9); thin quartz veinlets containing galena, sphalerite and pyrite cut across the strata (Stone and Leake, 1984; Stone et al., 1984). A third style of mineralisation in the same area contains lead and arsenic, in the form of plumbogummite–beudantite assemblages, both in the altered margins of dykes and in a gossan occupying a N–S fault zone.


Stratabound arsenopyrite and pyrite were intersected by drilling in Silurian greywackes on the north side of Glenshanna Burn close to the old Glendinning antimony mine (3) (Gallagher et al., 1983). The arsenopyrite is enriched in antimony and the pyrite in arsenic, thus providing a suitable source of metals for the later vein mined for stibnite (see Type F).

C Associated with diorite and granodiorite

Base and precious metal mineralisation is located in or near the margins of several diorite–granodiorite intrusion in south-west Scotland. Three varieties can be recognised.

A specimen of arsenopyrite, an iron arsenic sulphide from Talnotry about six miles south-west of Newton Stewart. British Geological Survey Petrology Collection sample number MC 7339. An old abandoned mine is located on the west side of Palnure Burn 250 yards south-east of Talnotry Cottage. The deposit is said to have been discovered in about 1885 and was first opened up by a small syndicate. It was mined for its pyrrhotite and niccolite ore. Arsenopyrite is a hard, very heavy, fragile mineral with good cleavage and is regarded as the principal ore of arsenic with tin, gold, silver and cobalt as by-products. P527559.

i The Talnotry deposit (11) is the only instance of a magmatic copper-nickel ore-body in the Southern Uplands. The host rock is a late Caledonian diorite sill in which a basal lens of sulphide, 4 X 20 m in size, contains pyrrhotite, pentlandite, chalcopyrite, nickeline and gersdorffite, plus numerous minor constituents including gold (Stanley et al., 1987).

ii Volcanic porphyry Cu-Mo-Au-Ag mineralisation. This is exemplified by disseminations and veinlets of mineral in diorite complexes. The Fore Burn complex (2) is a Lower Devonian igneous assemblage lying immediately north of the Southern Upland Fault. Arsenopyrite, pyrite, chalcopyrite, gold, silver and tetrahedritetennantite occur in quartz-carbonate veins and in small intrusion breccias which are intensely tourmalinised. Gold is also present locally in quartz-arsenopyrite-chalcopyrite veins. Grades varying up to about 50 gm per tonne are reported by Charley et al. (1989). Porphyry-style copper mineralisation is weakly developed in a late Caledonian complex of intersecting porphyrite dykes, gran odiorite intrusions and breccia pipes at Black Stockarton Moor (22) on the western margin of the Criffell granodiorite pluton (Leake and Cooper, 1983). Breccia veins are enriched in As, Sb and gold (maximum 0.06 ppm Au in samples) in association with molybdenite (350 ppm Mo) and with chalcopyrite and bornite (4400 ppm Cu). In one borehole, an average of 0.05 per cent Cu was maintained over 34 m (Leake and Brown, 1979).

iii Arsenic-antimony-gold mineralisation is a third variey, represented by mineral locations at Glenhead Burn, Hare Hill and Moorbrock Hill. These, together with the Fore Burn occurrences, are interpreted as mesothermal gold systems spatially related with regional strike-slip shears as well as with late-tectonic Caledonian intrusives (Boast et al., 1990). Quartz veins containing gold (8.8 ppm Au maximum), arsenopyrite (up to 3.5 per cent As) and pyrite cut Ordovician turbidites at Glenhead Burn (8) at the southern margin of the Loch Doon granitic pluton. The hornsfelsed wallrocks are strongly sericitised and contain fine-grained, disseminated arsenopyrite. The mineralisation may be related, in part at least, to a swarm of dioritic dykes which are older than the pluton. At Hare Hill (1) (Excursion 6), gold is associated with a zoned As-Sb-Cu-Pb-Zn assemblage in fractured and sericitised granodiorite (Boast et al., 1990) adjacent to late-stage antimony veins (see Type F). At the margin of the Carsphairn intrusion at Moorbrock Hill (29), gold values of 1-3 ppm occur mainly in quartz-pyrite-arsenopyrite veins in a NE-trending zone of intense brecciation and hydrothermal alteration. Native gold forms isolated grains, 5-10 p m in size, adjacent to chalcopyrite-pyrite intergrowths and inclusions of graphitic Moffat Shale Group wall rock (Naden and Caulfield, 1989).

D Haematite veins

Haematite from Auchenleck, Kirkudbrightshire. At Auchenleck, about five miles east of the town of Kirkcudbright a haematite vein occurs in the Dalbeattie granite mass. British Geological Survey Petrology Collection sample number MC 7439. The trend of the vein is west 18 degrees north and it is the most important vein of iron ore in the district. Hay Cunningham's 'Geognostical Account of the Stewartry of Kirkcudbright' (Highland Society's Transactions v. viii, 1843 p. 730) describes the vein as being mined by a horizontal shaft. 'The ore is of red botryoidal haematite and, in drusy cavities fine specimens of this beautiful mineral may be found. The minerals which accompany the iron-ore are ferruginous quartz and sulphate of barytes.'

Fracture-bound haematite deposits are located at the margins of the Criffell granodiorite pluton (Phillips, 1956) and a weakly radioactive deposit occurs within the granodiorite (see Type H). The Auchenleck vein (25) was the most important economically, consisting of botryoidal haematite, quartz and baryte. The NE margin of the Loch Doon granitic pluton is transected by a haematite breccia vein, 0.5 m to at least 3 m in width, which has been prospected and/or mined over some 3 km of strike length. The occurrence lies about 7 km west of Carsphairn (5) and has yielded around 400 tonnes of haematite (Excursion 7).

E Lead-zinc veins

Numerous metalliferous veins were exploited in the past around Newton Stewart and lead mining near Carsphairn was important for a time (Excursion 7). In a little known review of the mines and trial workings in south-west Scotland (Foster-Smith, 1967), minimum productions of lead, zinc and copper in ore concentrates are given as 25.6, 1.3 and 0.24 thousand tonnes respectively.

Of this production, more than half the lead, almost all of the zinc and a proportion of the copper were recovered from the Blackcraig mines (14), some 5 km from the SW margin of the Cairnsmore of Fleet granodiorite pluton. Little can now be seen of this deposit, the most important in southwest Scotland, and the extensive dumps are mainly landscaped. Wilson (1921) recorded the presence of galena, sphalerite and chalcopyrite, set in a gangue of calcite, dolomite, baryte and some quartz. Mineralisation extended for 0.8 km along a fault zone trending ESE and up to 18 m wide in Silurian greywackes. The fault zone was evidently intruded by a Permo-Carboniferous dolerite dyke prior to mineralisation (Gallagher, 1964). The Cairnsmore lead-zinc vein (15) probably represents an extension of the vein at Blackcraig, as may also the copper-lead vein at Pibble (see Type F). The Wood of Cree mine (12) was sited on a wide line of fracture trending SSE and containing numerous stringers of sphalerite and galena, mingled with chalcopyrite and pyrite.

The Woodhead mines (4) near Carsphairn (Excursion 7) produced 6700 tonnes of lead ore from veins cutting Ordovician greywackes. Sphalerite and chalcopyrite are associated with the galena in a gangue of calcite, dolomite and quartz. The location is midway between the Loch Doon and Carsphairn granitic bodies which are probably continuous at depth, providing a favourable structural and geochemical environment for mineralisation.

F Copper, antimony and arsenic veins

Pibble Mine engine house, 5 km. east-north-east of Creetown, on the north-west slope of Pibble Hill, Kirkudbrightshire. A former lead-zinc mine that worked a four-feet wide vein trending 20 degrees north of west and hading to the west at from 75 to 80 degrees. The vein comprised broken country rock with quartz and barytes and carrying a little galena (lead ore), zinc-blende and chalcopyrite and other minor minerals. The hade is the angle at which the vein dips. P001528.

Foster-Smith (1967) refers to Pibble Mine (18) as the dominant 19th century copper producer in south-west Scotland, although no production figures are given by Wilson (1921). The vein, about 1 m thick, cuts Silurian greywackes interbedded with black shales some 3 km south of the southern contact of the Cairnsmore of Fleet granitic pluton. It contains lenses of galena, sphalerite and chalcopyrite and, unusually for the veins of south-west Scotland, a variety of secondary minerals. These include linarite, pyromorphite, hemimorphite and malachite, all recorded from the upper levels of the mine. A variety of ruined mine buildings and spoil dumps remain at the site. Also to the south of the Cairnsmore of Fleet pluton, chalcopyrite-bearing veins have been trialled at Drumruck (16), Dromore (17) and Kings's Laggan (19).

Copper veins at Colvend (24) and Tonderghie (28) postdate Caledonian felsite intrusions in Silurian rocks. Chalcopyrite, in a calcite-quartz gangue, is accompanied by malachite and azurite at Colvend whereas at Tonderghie the gangue is baryte-quartz and the associate minerals are pyrite and malachite.

The Louisa mine at Glendinning (3) produced nealy 200 tonnes of antimony from narrow quartz-carbonate veins cutting Silurian greywackes. The remains of mine buildings, shafts, adits and crushing floors are still clearly visible. Stibnite and other antimony minerals (semsyite, bournonite and tetrahedrite) are accompanied by pyrite, arsenopyrite, galena, sphalerite and chalcopyrite. There is no evidence of Caledonian intrusive igneous activity in the area and the vein contents probably derive from the stratabound mineralisation nearby (see Type B) (Gallagher et al., 1983). Similarly, at Hare Hill (1), enrichments of metal in the granodiorite (see Type C) are probably the source of metals in later stibnite-galena veins, one of which has been trialled for antimony.

Modern investigations have demonstrated that arsenopyrite and geochemical enrichments of arsenic are much more widespread in the rocks of South-west Scotland than had previously been reported (Stone et al., 1995). Nevertheless only near Talnotry (10), at the west edge of the Cairnsmore of Fleet granodiorite, is there any record of recovery. A few tonnes of arsenopyrite were raised from a small shaft on a quartz vein about 1 m thick.

G Baryte veins

A group of baryte veins south of Auchencairn underwent exploitation in the late 19th century and until just after World War 2 (cf. MacGregor et al., 1944). Some 2800 tons of baryte were produced from Barlocco Mine (27), mainly from the more northerly of two approximately east—west subvertical veins. The principal vein is 0.5-2.1 m thick and occupies a fault breccia traceable for at least 300 m in altered Silurian shale. The high-quality white baryte remaining on the dumps is accompanied by calcite and traces of chalcopyrite, malachite and bornite. At Auchencairn mine (26) '700 tons of baryte are said to have been taken out of one pocket' in the latter part of the 19th century, and a little more in 1916 (Wilson, 1921). The Auchencairn vein, which is 0.5 m thick at surface and reputedly widens at depth, cuts Lower Carboniferous conglomeratic sandstone and is accompanied by multiple thin stringers of baryte and a 0.5 m thick quartz vein seen in the foreshore exposures.

H Uranium veins

Pitchblende is present in thin veins cutting hornfelsed Silurian turbidites at the southern margin of the Criffell pluton and cutting Lower Carboniferous strata at Needle's Eye (21), on the south side of the ENE-trending North Solway Fault. Over 30 anomalously radioactive veins are concentrated in a 400 m coastline section between Marbruie Cove and Powbrade Burn (23) which is one of the principal localities. Pitchblende is accompanied by black vitreous hydrocarbon, specular haematite, chalcopyrite, pyrite and native bismuth. Quartz and dolomite are the main gangue minerals with smaller amounts of calcite and baryte. Pitchblende from a vein at Steps gave a U-Pb isotopic age of 185 ± 20 Ma (reported in Miller and Taylor, 1966), that is Upper Triassic or Lower Jurassic and therefore the youngest vein mineralisation in Britain.

A uraniferous structure of similar NW trend also occurs at the northern edge of the Criffell pluton, at Beeswing (13), where joints in the greywackes adjacent to thin, impersistent quartz-haematite veins are coated with uraninite and chalcopyrite. Within the granodiorite mass itself, a haematite-impregnated shatter zone at Ironhash Hill (20) is anomalously radioactive.

The presence of anomalous radioactivity in association with copper veins of Type F at King's Laggan (19) and Pibble (18) was observed by Miller and Taylor (1966).

Table 5. Significant metalliferous mineral locations in south-west Scotland.

Location no Deposit name Exposure ‘type' Commodity Mineralisation type3 Trend of vein NGR 1:50000 geol. map Key reference
1 Hare Hill T Sb Au C, F 010 NS 658 104 15W Dewey et al., 1920
2 Fore Burn I As Cu Au C NX 420 996 8E Allen et al., 1982
3 Glendinning M Sb As Zn Pb Cu B, F 045 NY 313 966 10E Gallagher et al., 1983
4 Woodhead M Pb Zn Cu E 110 NX 531 936 8E Wilson, 1921
S Carsphairn (near) M Fe D 000 NX 504 929 8E Macgregor et al., 1920
6 Pinbain O Cr A NX 138 917 7 Stone and Smellie, 1988
7 Poundland Burn O Cr A NX 170 882 7 Stone and Smellie, 1988
8 Glenhead Burn I As Pb Zn Au C, F 000 NX 449 780 8E Leake et al., 1981
9 Penkiln Burn I Pb Zn As Cu B 000 NX 446 767 8E Stone et al., 1984
10 Talnotry (near) T As F 045 NX 480 702 4E Dewey et al., 1920
11 Talnotry T Ni Cu C NX 477 704 4E Wilson, 1921
12 Wood of Cree M Pb Zn Cu E 155 NX 386 695 4E Wilson, 1921
13 Beeswing I U Cu H 135 NX 885 681 5E Gallagher et al., 1971
14 Blackcraig M Pb Zn Cu Ba E 110 NX 435 650 4E Wilson, 1921
15 Cairnsmore M Pb Zn Ba E 105 NX 463 636 4E Wilson, 1921
16 Drumruck T Cu F 110 NX 583 637 4E Wilson, 1921
17 Dromore T Cu Zn F 155 NX 537 622 4E Wilson, 1921
18 Pibble M Cu Pb Zn Ba E 110 NX 525 607 4E Foster-Smith, 1967
19 Kings Laggan T Cu Pb Zn F 105 NX 562 578 4E Wilson, 1921
20 Ironhash Hill I Fe U D 080 NX 858 563 5E Gallagher et al., 1971
21 Needle's Eye I U Cu Bi Fe HC H 135-170 NX 915 562 5E Miller and Taylor, 1966
22 Black Stockarron Moor I Cu Mo C NX 725 555 5W Brown et at, 1979
23 Powbrade Burn I U H 045; 135 NX 902 554 5E Miller and Taylor, 1966
24 Colvend M Cu F 045 NX 869 538 5E Wilson, 1921
25 Auchenleck M Fe D 108 NX 773 525 5W Macgregor et al., 1920
26 Auchencairn M Ba Cu G 070 NX 821 485 5E Wilson et al., 1922
27 Barlocco M Ba Cu G 098 NX 788 475 5E Wilson et al., 1922
28 Tonderghie O Cu Ba F 080 NX 438 350 2 Wilson, 1921
29 Moorbrock Hill I As Au Cu Zn C 045 NX 620 980 9W Naden and Caulfield, 1989

1 Exposure type, M — old mine; T — old trial; O — outcrop; I — detected by modern investigation

2 Commodity (mined where bold): As — Arsenopyrite; Au — gold; Ba — baryte; Bi — native bismuth and secondary Bi — Cu minerals; Cr —chromite; Cu — chalcopyrite; Fe — haematite; HC —hydrocarbon; Ni — niccolite, nickeliferous pyrrhotice; Mo — molybdenite; Pb — galena; Sb — stibnite; bournonite; U — uraninite, secondary uranium minerals; Zn — sphaleritc

3 Mineralisation type: A— associated with ultrabasic rocks; B — associated with Ordovician or Silurian sedimentary rocks; C — associated with diorite and granodiorite; D — haematite vein; E — lead-zinc vein; F — copper, antimony and/or arsenic veins; G — baryte vein; H — uranium vein.