Silver Mineralization at Sark's Hope Mine, Sark, Channel Islands

MINERALOGICAL MAGAZINE, DECEMBER 1983, VOL. 47, PP. 539-45 Silver mineralization at Sark's Hope mine, Sark, Channel Islands R. A. IXER Department of Geological Sciences, University of Aston in Birmingham, Gosta Green, Birmingham B4 7ET AND C. J. STANLEY Department of Mineralogy, British Museum (Natural History), Cromwell Road, South Kensington, London SW7 5BD ABSTRACT. Sark's Hope silver lead lode, which was The lead-silver-bearingvein, 'Sark's Hope Silver- mined during the 1830s and 1840s cuts a Late Precambrian Lead lode', cuts a foliated hornblende granite of granite at the southernmost point of the island of Sark. Late Precambrian age (Sutton and Watson, 1957). The primary ore assemblageis pyrite, galena,chalcopyrite, Although there is little trace of the vein today, tennantite, tetrahedrite, sphalerite, marcasite, arseno- with the outcrop workings being mostly obscured pyrite, pyrrhotine, bravoite, enargite, and the silver minerals pyrargyrite, pearceite, polybasite, and acanthite. by a thick cover of vegetation, the early accounts Gangue minerals are hematitic quartz, calcite, and illite. describe it as 0.4 to 2.5 m in width, more than 600 m Alteration products include chalcosine, covelline, blau- in length, oriented 028-037 ~ and dipping at 65-85 ~ bleibender covelline,limonite, malachite, azurite, cerussite, to the northwest. and anglesite. The generalized paragenesis is of early Fe, Small amounts of in situ ore were obtained from Co, Ni, As, and S species and later minerals of Pb, Cu, Ag, the poorly exposed and weathered parts of the vein Zn, Fe, As, Sb, and S. The earliest alteration products at the southwestern end, but most of our specimens are copper sulphides; these are followed by lead and were collected from the spoil tips, in particular from copper carbonates and sulphates, and hydrated iron and Le Pelley's shaft tip at the northeastern end of the manganese oxides. Growth zoning is a common feature of many of the ore minerals, and electron microprobe lode. Additional material was obtained from the analysis shows that this is sometimes related to composi- Guernsey Museum collection, and the specimen tional differences. Tetrahedrite and tennantite,particularly, in the British Museum (Natural History) collection exhibit a wide range of compositions. BM 1964R, 440 was re-examined. Forty-three polished sections and five polished thin sections THE small island of Sark, about 10 km east were investigated by reflected and transmitted of Guernsey, was a site of active mining and light microscope techniques. Selected sulphides exploration during the middle of the nineteenth and sulphosalts were analysed by electron micro- century. Of all the mineral prospects on the island probe, and some secondary minerals were identified Sark's Hope mine on Little Sark was the most by X-ray diffraction. important and, indeed, was the only metalliferous The present study forms part of a general mine in the Channel Islands to produce significant investigation of the mineralization of the Channel amounts of ore. The most comprehensive account Islands. of the mineralization and mining history is that of Mourant and Warren (1934), which includes Mineralogy sections and" maps of Sark's Hope mine and an extensive list of mineral species. Their work is The host-rock is a coarse-grained granitic rock largely a review of early accounts by Prince (1846) with major hornblende, plagioclase and orthoclase and Henwood (1871a, b), supplemented by reports feldspars, and quartz, and it is cut by numerous of the mining from the local press. A more recent quartz-epidote veinlets. Adjacent to the vein the systematic mineralogical and paragenetic study is rock is intensely altered to quartz, sericite, chlorite, that of Bishop et at. (1977) on one specimen, BM and calcite. Brecciated fragments of this altered 1964R, 440 from the Russell Collection in the granite are contained within the ore, cemented by British Museum (Natural History). rhythmically banded pyrite, hematitic quartz, and Copyright the Mineralogical Society 540 R. A. IXER AND C. J. STANLEY galena, or by mixed grains of galena, euhedral bravoite, enargite, pyrargyrite, pearceite, poly- pyrite, and quartz. basite, acanthite, pyrrhotine, and arsenopyrite The primary ore consists predominantly of pyrite occur. Gangue minerals are hematitic quartz, calcite, and galena. In some specimens, chalcopyrite and and illite. Chalcosine, covelline, blaubleibender members of the tennantite-tetrahedrite group are covelline, limonite, malachite, azurite, cerussite, common, and there are subordinate quantities and anglesite have formed as alteration products of of sphalerite and marcasite. Minor amounts of the primary ore minerals. FIG. 1. Reflected light photomicrographs; plane polarized light with oil immersion. A. Intergrowths of polybasite (grey) in galena (light grey). E. 598. B. As for A but polybasite shows light-etching (after 10 minutes exposure to the quartz-iodide microscope lamp). C. Skeletal polybasite intergrowth in galena. Polybasite may have nucleated on euhedral pyrite (white). E.598. D. Oriented aggregates of polybasite grains form the cores of galena grains. E.598. E. Zoning, demonstrated by reflectance differences, in 'plumbian tennantite'. BM 1964R, 440. F. Pyrargyrite (grey) with minor tetrahedrite (dark grey inclusions in black gangue) and chalcopyrite (white). SH.1. SILVER MINERALIZATION 541 T~LE I Electron microprobe analyse~ - Sark's Hope Lode. Mineral species Cu Ag Fe Zn Hg As Sb 8 Total Spec. No. 1 Polybasite 3.0 73.2 O.1 0.3 10.3 12.8 99.7 E.598 2 Pearceite 1.9 76.2 0.2 O.1 - 5.7 14.1 98.2 86 5/80 3 Pyrsmgyrite 1.6 61.3 - 22.0 15.6 100.51 SH 1 4 1.4 60.0 - 22.6 15.7 99.7J 5 0.6 58.7 2A.2 16.1 99.6 6 Aeauthite 1.5 85.3 - 3.0 0.2 10.1 100.1 86 5/8 0 7 Ensrgite ~A.5 3.9 ~8.3 1.3 31.4 99-4 86 5/80 8 M+.4 t.5 2.6 18.7 ?.4 3o.6 99.2 Cu Ag Fe Zn Cd Hg Pb Hi As Sb S Total Spec. No. 9 Tennantite/ 42.7 0.4 2.2 4.6 0.3 O.1 10.9 12.6 26.6 100.4 7 BM 196~,R~0 margin 10 tetrahedrite 43.2 0.6 1.5 4.2 - 1.8 0.9 15.1 7.2 26.9 101.4 1 11 /*3.2 1.8 1.2 2.0 - 4.4 - 11.5 9.7 25.7 99.5] 12 40.O 6.5 1-3 1.9 - 5.5 11.7 9.2 25.4 101.51 core E.600 13 42.4 1.5 2.4 3.1 0.7 11.7 10.6 26.2 98.6] incl.in gn 14 41.8 1.4 2.0 3-7 0.4 2.0 14.2 6.8 26.6 98.9J 15 35.7 6.4 1.9 4.9 2.9 16.0 6.1 26.4 100.3] incl.in gn 16 35.7 7.9 2.6 3.9 O.2 - 1.3 17.5 3-3 26.8 99.21 17 38.7 1.4 0.9 7.1 3.7 23.7 25.2 100.7] 18 38.4 0.9 1.2 6.5 - - 2.7 25.0 25.3 100.O{ incl. in cpy 19 38.4 3-5 2.3 4.2 0.1 2.0 9.6 14.1 25.6 99.8 J 20 38.6 1.4 2.4 4.4 - 4.5 23.3 25.4 100.0] incl. in cpy 21 41.1 1.1 2.0 5.0 8.9 15.3 26.2 99.6 22 40.9 0.7 1.7 5.5 - - 9.0 16.3 26.5 100.63 $6 5/80 incl. in cpy 23 40.1 1.3 2.3 2.7 2.9 1.1 0.7 9.3 12.4 26.0 98.8J 2/+ 3/+.4 9.5 1.9 3.7 0.6 2.3 - 6.8 17.3 2L,.2 100.71 25 34.0 10.0 2.0 3.1 3.0 1.3 0.2 9.2 12.8 24.4 100.01 incl. in cpy 26 30.8 13.1 2.7 &.1 1.2 3.0 1.4 - 7.5 15. 2 22.6 101.6 J 27 27.2 17.9 2.2 4.3 "1.2 0.3 0.4 7.1 14.6 24.1 99.3"~ incl. in cpy 28 24.9 20.2 1.4 4-5 0.5 1.1 O.1 10.O 12.5 24.1 99.3J 29 22.9 21.5 0.8 6.2 - 4.5 20.5 23.2 99.6] incl. in cpy 30 21.9 23.2 1.O 5.0 - - 0.5 25.8 21.9 99.3J 31 38.4 0.9 o.7 6.8 - - 3.o 2J+.) 25.4 99.5"[ incl. in cpy 32 38.3 1.0 1.7 6.2 - - - 2.4 25.6 25.1 IO0.3J 33 6.2 45.1 1.4 4.8 0.9 - 1.8 - 8.5 12.o 20. 5 101.2 SH1 17/80 incl. in gn 3~ 6.5 44.9 1.4 3.6 1.6 1.9 2.5 - 11.1 6.5 2o.& 100. 4 35 16.0 30.7 5.1 1.0 27.6 19.9 100.3 GM I 36 Galena 0.2 - 86.1 0.2 12.7 99.21 E. 598 ' normal ' 37 0.5 - 86.8 - 0.4 12.6 lO0.3 / lower reflectance 38 Sphalerit e - 4.0 62.8 - - - 33.1 99-9 s6 5/80 39 0.4 0.3 0.9 66.3 - 0.5 32.5 100.9 E.6oo ~0 Chalc opyrit e 33.6 30.5 - - Q.4 0.3 3~.7 99-5 E.

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