The Channel Islands, shown in Figure 1, from north to south are Alderney, Guernsey, Herm, Sark and Jersey. These, together with a number of smaller islands, reefs and shoals, lie in the Gulf of St. Malo. Geologically they are part of the North Armorican Massif of northwest France; they have been largely unaffected by the two major British orogenies - the Caledonian and Hercynian - and so, despite rock types and geographical position, the islands are not part of Cornubia. However, mining in terms of people, methods, exploration rationale and techiniques (but not money) were all Cornish and the history of mineral exploration in the islands is a watered-down reflection of the events in Cornwall during the eighteenth and nineteenth centuries - complete with smugglers' coves, short-lived boom times and lot of bust.
Mining, and this only applies to metalliferous mining, (non-metalliferous mining for building stone, sands, gravels and water was, and is, of greater significance) assumed different degrees of importance within the islands; from Alderney where there was no mining, to Herm and Guernsey where short-lived exploration took place, to Jersey where a little ore was exported and Sark where, for a time mining was the major influence on the island and is directly responsible for the present ownership. The peak of mining activity was 1830 - 1845: the "Channel Islands Mining Boom".
Geology of the Islands
Essentially the islands consist of the Pentevrian, old crystalline metamorphic basement, older than 2,000Ma, that was deformed during the Icartian at about 2,200Ma and crops out in the south of Guernsey (including the eponymous Icart Gneiss), western Alderney and Sark. This is overlain by metasediments of Brioverian (late Precambrian) to Lower Palaeozoic in age and exposed on Jersey and Alderney. In the late Precambrian, during the Cadomian orogeny, acidic to basic igneous rocks intruded the basement and metasediments of the islands to form a number of igneous complexes seen on all of the main islands (Bishop and Bisson 1989).
The igneous and volcanic rocks of Jersey are calc-alkaline and, in theory, therefore, the geology is suitable for the wide range of metalliferous ore deposits that are associated with orogenic deformation and igneous emplacement. However, in practice the islands are only sparsely mineralized.
The island can be divided into three based on the mapping of Laffoley (Figure 2). To the west is the Western Granodiorite (2,200Ma); in the centre of the island basement rocks have been intruded by the Central Diorite Complex, a series of Cadomian basic to intermediate rocks dated between 600-500Ma, whilst to the east lies the Alderney Sandstone, a feldspathic sandstone of Lower Palaeozoic age. Despite a variety of suitable lithologies, no mineral veins nor metalliferous mineralization have been recorded. (Laffoley 1985b).
Guernsey is geologically complex but can be simplified into two halves (Figure 3). A larger southern half consists of Pentevrian metamorphic rocks separated by metasedimentary screens and with adamellites and granodiorites to the southwest. In the northern half, the Northern Igneous Complex is a series of five major plutonic bodies ranging in composition from gabbro through diorite (the most voluminous) to granodiorite and adamellite (Roach 1966).
The only record of mining is from the Parish of St Martin on the south coast where between 1842 - 1843 exploration and a little mining took place (as the Blanchelande Mines) before a successful anti-mining campaign caused the mines to disband in April 1843.
Four veins/lodes were described and promoted by William Row: "the district is rich in minerals and able to give a good return on investment", all within a four kilometre stretch of coastline. Amongst these veins were: a 1.5m wide silver-lead lode at Petit Bot showing gossan and soft calcite but "promising for silver and lead"; a copper lode lying between Icart Point and Petit Bot with gossan and pyrite and another silver-lead lode at Moulin Huet Bay which was 10 - 60cm wide with soft pyrite, gossan, silver and lead (Mourant & Warren 1934). These veins are now lost or are extremely difficult to reach; there is no associated spoil and only limited material in the local museum collections. One specimen from the Lukis collection (an early to mid nineteenth century rock and mineral collection) is labelled "argentic galena Guernsey" and comprises galena, iron-poor sphalerite, pyrite and traces of chalcopyrite, plus a little tetrahedrite in a calcite-quartz gangue. This assemblage is common in the islands and if the tetrahedrite is argentiferous (as in Jersey and Sark) then this specimen may well represent one of the silver-lead lodes.
Within the Northern Igneous Complex sulphide veinlets cut diorite at Baubigny near St Sampson and carry magnetite, ilmenite and abundant pyrite with small monoclinic pyrrhotite, mixed chalcopyrite-pyrrhotite and chalcopyrite with cubanite exsolution lamellae inclusions. Minor iron-rich sphalerite and molybdenite are also presnt. Similar assemblages from elsewhere in the Northern Igneous Complex have trace amounts of pentlandite, or as at Bouette Quarry (St Peter Port) consist of galena-sphalerite, pyrite and chalcopyrite, pyrrhotite and magnetite.
The island of Herm and nearby Jethou are made of Cadomian granite overlain by Recent sediments. Exposures of granite are confined to the coastline, especially to the south of Herm. Two lodes were discovered /rediscovered on Herm by John Hunt in 1837. These were:
1) the Great Copper Lode, a NNW - SSE trending vein cutting across the south of the island and visible on both the west and eastern coastlines and described as being 10 - 12m wide (30 - 40 feet) containing copper ore, pyrite, gossan and spar. The lode was variously described as being "regular and kindly" and "of flattering appearance". Two shafts were sunk and a few hundredweight of ore raised.
2) a silver-lead vein at the extreme southeast of the island, trending almost due east-west. The vein comprised "kindly gossan and good stones of silver-lead ore". Mining stopped in 1839 and soon after work ceased on the Great Copper Lode (Laffoley 1985a).
Alongside these ventures large amounts of granite were quarried and shipped to the mainland.
Today little is visible; material from trials close to the Great Copper Lode consists of vein quartz carrying haematite, minor magnetite and trace amounts of chalcopyrite and cubanite.
Although the island has produced very little ore the quarrying industry has always been important. Metalliferous mining ventures have included iron ore at La Corbière, molybdenite at L' Etacq, copper at La Moye and, most seriously, lead-silver at Le Pulec.
The geology of Jersey is shown in Figure 4. The Cambro-Ordovician Rozel Conglomerate unconformably overlies the Jersey Volcanic Group, a series of andesites overlain by rhyolites. Beneath the Jersey Volcanic Group is the Brioverian Jersey Shale Formation. These rocks have been intruded by Cadomian igneous rocks, namely the Northwest Complex. The rocks are calc-alkaline and range from granite to diorite. Mineralization has been reported in all of the main divisions except for the Rozel Conglomerate (Bishop and Bisson 1989).
The Jersey Shale Formation, comprising mudstones and conglomerates but mainly siltstones and sandstones, has suffered chlorite grade metamorphism. Locally the formation is host to minor mineralization. At Meadow Banks, in the German Hospital Tunnels, calcareous mudstones are cut by galena, sphalerite, pyrite, marcasite, chalcopyrite and calcite veinlets or by quartz veins carrying chalcopyrite, pyrite, sphalerite, galena and traces of bournonite. Nearby lenses of disseminated, banded pyrite are present in a highly altered acidic dyke that cuts the metasediments; in this assemblage pyrite, chalcopyrite, galena and sphalerite are accompanied by trace amounts of molybdenite and pyrrhotite (Ixer 1980, 1990). Other lenses in the dyke are composed of quartz and chalcopyrite (Mourant pers comm 1988).
More substantial mineralization is hosted by the Jersey Shale Formation at Le Pulec, a small bay on the northwest coast, close to the junction of the shales and the Northwest Granite. In 1871 three sphalerite-galena-carbonate veins were discovered on the foreshore. The veins cut the shales and were of variable length; the longest (Lode No. 1) had a trend of 300o and was approximately 180m long, whilst the other two were less than 100m long with trends of 300o (Lode No. 2) and 275o (Lode No. 3). The width of all three was between 0.5 - 2.0m. Good silver values for the galena were reported by Williams (1871) and independently by Ogier (1871). Intermittent attempts were made to mine the veins (between high tides) from 1872 to 1876 when mining was abandoned (Mourant & Warren 1934, Howell 1977).
Two of the lodes are exposed (Nos. 1 & 3) and their mineralogy and petrography have been described by Ixer & Stanley (1980) and Stanley & Ixer (1982). At Le Pulec the Brioverian shales close to the granite contact have been bleached and silicified and carry sulphide-arsenide mineralization, namely early pyrrhotite (now altered to tabular pyrite and marcasite), pyrite and finally arsenopyrite with minor chalcopyrite, galena, sphalerite and molybdenite. Cross-cutting this mineralization are the three veins, at least one of which is associated with a highly altered basic dyke (Howell 1977). Within the limited sampling allowed, lodes 1 and 3 are the same and are composed of polyphase mineralization. The sequence is:-
1) ferroan dolomite and iron-poor sphalerite
2) a number of generations of quartz-minor chlorite plus galena, sphalerite, chalcopyrite, cubanite, tetrahedrite, bournonite, native antimony and traces of silver sulphosalts.
3) ferroan calcite, stibnite and other antimony sulphosalts.
Analyses of the tetrahedrite show it to have been the main silver carrier with 6 - 20 wt % Ag and rarely up to 40 wt %. Potassium-Argon dating of the altered basalt next to lode No. 1 has given 424±4 Ma, a Silurian age (Ineson per comm. 1982).
Very minor amounts of mineralization are present in the overlying Jersey Volcanic Group. Sometime in the middle of the nineteenth century at West Mount on the outskirts of St Helier, an audit was driven into green-stained tuffs and agglomerates, close to a dolerite intrusion (Mourant & Warren 1934). Similar mineralization has been noted from Bouley Bay where the green-stained Middle Bouley Ignimbrite has very fine quartz net-veining with chalcopyrite, bornite-chalcopyrite intergrowths ("idaite") plus minor pyrite and enargite. Secondary alteration to blaubleibender-covelline, covelline, malachite, limonite and cuprite is responsible for the colouration of the outcrops.
The Southwest Granite (dated 553 ± 12Ma, Adams 1976, Bishop & Bisson 1989) carries minor quartz-sulphide joint infills and was host to nineteenth century trial diggings at La Corbière for iron ore, whilst at La Moye a copper vein was discovered in 1843 but not exploited (Mourant & Warren 1934).
At La Corbière quartz-chlorite veins carry "massive" magnetite-haematite-rutile followed by later haematite, pyrrhotite, pyrite, chalcopyrite, galena and sphalerite. At L'Ouaisné Bay thin joint infills are composed of quartz-calcite-fluorite-chlorite with haematite, sphalerite, galena and minor magnetite, rutile, pyrite and a little chalcopyrite.
At La Moye the granite/aplogranite is net-veined by bornite, bornite-chalcopyrite intergrowths ("idaite") and tetrahedrite-bearing quartz veinlets. Extensive alteration is responsible for the malachite and azurite staining (Ixer 1988).
The Northwest Granite of the Northwest Igneous Complex is the largest and youngest of the three granites and is dated at 480Ma. In joints at L'Etacq Quarry the granite carries quartz-calcite veins with molybdenite (which may have been collected and sold to collectors), pyrite, chalcopyrite, sphalerite, marcasite, arsenopyrite, galena, pyrrhotite, bournonite, tetrahedrite and stannite (Ixer 1980) plus silver-rich cosalite and boulangerite (Mourant 1985). Molybdenite was collected from the china-stone quarries at Handois. Elsewhere in the granite, quartz veins have pyrite, marcasite, chalcopyrite, sphalerite, molybdenite, bournonite and native antimony as at Gigoulande Quarry, or carbonate veins carrying arsenopyrite, pyrite, sphalerite, galena plus minor chalcopyrite, marcasite, haematite, pyrrhotite, tetrahedrite, magnetite and bornite as at Douet de la Mer.
The Southwest Igneous Complex has no recorded mineralization, but carbonate veinlets cutting the Fort Regent Granophyre carry haematite, chalcopyrite and marcasite, whereas in Queens Valley quartz-carbonate veins enclose chalcopyrite-bornite-tetrahedrite intergrowths with minor quantities of galena, sphalerite and pyrite.
An island-wide synthesis of these mineral occurrences has been given by Ixer (1980) who suggested a number of ore associations, namely:
1) porphyry-style copper mineralization in the Jersey Volcanic Group (Bouley Bay)
2) quartz-molybdenite-base metal sulphide pegmatitic veins followed by quartz-carbonate-base metal sulphide hydrothermal veins, both cutting granites
3) unclassified carbonate-sulphide veinlets in the Jersey Shale Formation
4) Le Pulec
With the advantage of additional material, both from Jersey and the other islands, the mineralization can be simplified.
Polymetallic mineralization associated with granites has the following sequence: magnetite/haematite, pyrrhotite, pyrite, arsenopyrite, base metal sulphides and finally antimony-bearing phases. Tin and tungsten minerals are noticeably scarse on the island, only trace amounts of stannite have been positively identified. The youngest Northwest Granite appears to be more evolved than the other granites, with greater amounts of molybdenum, antimony, silver, tin and bismuth in its associated mineralization.
All of the mineralization in the Jersey Shale formation (Le Pulec and the German Underground Tunnels) is similar. Briden et al (19892) have suggested, on geophysical grounds, that the Northwest and Southwest granites are joined at depth and that the Jersey Shale Formation is only 0.25km thick. If this is so then the base metal, minor antimony, with or without silver mineralization, could also be a manifestation of the granites.
However, the presence of post-Brioverian dykes at both sites of mineralization is worth noting (dykes are also recorded close to the mineralization at West Mount and at Douet de la Mer) as an alternative/additional factor in the ore formation process. The dykes are too small to be the direct cause of mineralization and may just have used the same fractures as the mineralising fluids. The chemistry of the dykes, which is very different from that of the metasediments, may have promoted mineralization. The regional heat source responsible for the dykes may also be responsible for localized mineralising systems.
The geology of Sark is given in Figure 5. The island consists of Pentevrian folded pelitic and hornblendic gneisses intruded by foliated granitic sheets, the latter forming Little Sark to the south and the extreme north of Great Sark (Sutton & Watson 1957).
The island has a number of mineral veins and the majority were explored between 1833 - 1845. These include the copper and silver lodes at Le Pot, copper-silver lodes at Port es Saies and a 5m wide chalcopyrite-calcite vein on the island of Brecqhou. However, the most important vein was Sark's Hope lead-silver lode on Little Sark (Mourant & Warren 1934, Laffoley 1985a).
The lodes at Le Pot were first discovered by John Hunt in 1833 (although Lukis claims to have collected material in 1817) and both were explored from 1834 - 1836 before being abandoned. The Le Pot Copper Lode was followed for more than 150m whilst the Le Pot Silver Lode, only 60cm wide, was explored for a much shorter distance, although it was said to carry good indications of quartz, galena and antimony-rich silver (Henwood 1871). Material from the Lukis Collection labelled 'Le Port (sic) Sark galena and ?silver' only showed galena, pyrite and altered pyrrhotite. Material from the copper lode (also from the Lukis Collection) consists of blue copper sulphides including chalcocite and djurleite and a little bornite.
At Port es Saies three parallel lodes, the largest of which was explored at two levels, were said to carry quartz, gossan, chalcopyrite and argentiferous pyrite (300 - 450ppm Ag). No ore was recovered (Henwood 1871). No material from Port es Saies has been seen but elsewhere on the island small veins carry some or all of the following assemblage: magnetite, haematite, pyrite, marcasite, pyrrhotite, galena, sphalerite, chalcopyrite and bornite.
In 1836 a chance find during a rabbit hunt led to the discovery of lead-silver ore, a find that was to become Sark's Hope Mine. By 1839 ore was being exported, approximately 250 Cornishmen had been imported (doubling the population), a small gauge railway and steam pumps installed and a rich vein of silver exploited. A silver tea and coffee service was on display in St Peter Port to encourage investment and day trips were organised. But leaner times were to come: as the vein narrowed, the ore became poorer and the mining depth increased. It has been estimated that the mine had a strike length of 600m, went to a depth of 200m and that £34,000 was invested for a £4,000 return from "25,000oz of fine silver and many tons of lead" (Laffoley 1985a). One of the main casualties of the mine was the Le Pelley family, the seigneurs of Sark, one of whom, Ernest Le Pelley, mortgaged the fiefdom for £4,000 to a Guernseyman, Jean Allaire, an ex-British privateer and freelance "French" pirate who owned the island of Jethou. In 1852 the mortgage was foreclosed and Allaire's daughter became seigneur (Gibbons 1975, Bell 1988).
Mining ceased in 1845. In addition to the geological reasons for failure, the running costs were too high. Fuel in terms of coal for the steam pumps had to be imported (the islands have no indigenous fuels) and the costs of shipping the ore for smelting, initially to Hull but later to Devon, also were too great.
The silver-lead mineralization occurred as a brecciated vein, trending 037o and between 0.5 and 3m wide, that cut foliated hornblende granite. An associated copper lode between 0.7 and 1.5m is composed of pyrite and thin stringers of chalcopyrite (Henwood 1871).
Contemporaneous accounts stressed the vertical and lateral zoning of the ore with lead-silver mineralization to the southwest of the lode, passing into silver without lead to the northeast, and that oxidized secondary ores overlay primary sulphides and sulphosalts (Henwood 1871). The mineralization has been described by Bishop et al (1977) and Ixer & Stanley (1983).
The primary ore (collected from the tips on the north-eastern end of the lode) is composed of pyrite and quartz in a galena or, locally, chalcopyrite and tetrahedrite cement with subordinate amounts of marcasite and sphalerite. Minor amounts of bravoite, acanthite and pyrrhotite occur. Gangue phases are haematitic, quartz, calcite and illite.
Silver is present in the antimony sulphosalts, polybasite, pyrargyrite and tetrahedrite (up to 23 wt % Ag) and to a lesser extent as the arsenic sulphosalts, pearceite and tennantite (up to 20wt % Ag).
Other than at Sark's Hope Mine the mineralization as described and seen on Sark is like that of the other islands. Sark's Hope Mine differs, however, both in scale and in metal and mineral assemblage and is closer in style to some of the economic lead-zinc-silver deposits of mainland France than to the poor mineral occurrences of the Channel Islands.
The Channel Islands belong to the Normano-Breton metallogenic-structural zone of the Amorican Massif, a zone that is partially characterized by the presence of poorly mineralized Late Cadomian granites (Chauris 1980). Ixer & Stanley (1983) following Chauris (1977, 1979) have classified the mineralization of Le Pulec and Sark's Hope Mine and the other lead-zinc-silver occurrences (the "lead-silver lodes") as belonging to the uneconomic lead-zinc-silver vein deposits, present in the north of the Massif, that are aligned along WSW-ENE-trending Cadomian lineaments but of uncertain age. Little can be added to this, except that dating the ores and ?associated dykes may yet provide surprising results.
Although the Channel Islands were not a mining area much of the rest of Amorica was, with northwest France being a major metal producer in the eighteenth and nineteenth centuries. Indeed the lead-zinc-silver mines at Poullaouen-Huelgoat (1750 - 1868) were of national importance (Chauris 1989).
The 400 tons of ore exported from Sark were trivial in comparison with the amount of available French ore and are less than the 700 tons of lead exported to Britain from the Channel Islands between 1836 - 1846 (Laffoley 1985a). This, together with 70 tons of pig/sheet lead (that was not produced on the islands), traces of manganese ore (11cwt) and copper ores, suggest that smuggling of French ore into the islands and its re-export was taking place. Later, between 1870 and 1880, the customs' records show that only 4 tons of lead ore but 170 tons of pig/sheet lead were imported into England from the islands (Hill pers comm. 1982) indicating that the concept of "place value" was now understood, but at the cost of several smuggling convictions.
The Société Jérsiaise and Guernsey Museum are both thanked for material from their collections and for travel and subsistence monies. Drs. A. E. Mourant, A. Hill and N. d'A Laffoley are thanked for providing many of the data.
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First published in Symposium Edition, 20th National Symposium "Roches sans Frontieres" Exeter.
The Journal of the Open University Geological Society, 1992, 13(2), 16-23.