Plumbian tennantite from Sark, Channel Islands

TI~E island of Sark comprises two unequal parts; the larger northern portion (Great Sark) is composed dominantly of Precambrian metamorphic rocks. I t is joined by the narrow ridge of La Coup6e to Little Sark in the south. Little Sark is composed of foliated granitic rocks of Late Precambrian age (Sutton and Watson, I957), which, like all the rocks of the island, are cut by numerous dykes and faults, some of which afforded access to mineralizing fluids. Ores of lead and silver were mined on Little Sark as early as 16o9 but it was between I83O and 1845 that mining operations were most intensive. An account of the mining activity of Sark is given by Mourant and Warren (I934), and there are several earlier accounts, of which that of Henwood (I87I) is among the best. There is in the Mineral Collection at the British Museum (Natural History) an unprepossessing specimen, B.M. 1964, R44o, of mineralized breccia (14 • 8 • 5 cm in size) from Sark's Hope Mine in Little Sark. The specimen belonged to the late Sir Arthur Russell and was collected, according to the label, by 'Captain John Prince, manager of the mine, circa 1843'. It consists of an aggregate of fragments up to 2 cm across of quartz and fine-grained granitic material loosely cemented by anglesite and scorodite. A grey metallic mineral in the breccia, said to be argentite, had given an X-ray powder pattern indicative of tetrahedrite, a mineral hitherto unrecorded in the Channel Islands. The specimen was examined during the course of an optical, chemical, and X-ray study of fahlerz and found to be an unusual variety of tennantite in that it contained a significant concentration of lead. It was for this reason that the occurrence was studied in more detail. The presence of major lead in minerals of the tennantite-tetrahedrite series has not previously been established with any degree of certainty. Hoffmann 0895) reported 9"38 % Pb in a tetrahedrite from West Kootenai district, British Columbia, claiming that the particular specimen analysed contained the pure mineral associated only with white opaline quartz. However, in other specimens from the same locality, the tetrahedrite was reported to I~e associated with galena. Guimar~es (I934) found I4'76 % Pb in a tetrahedrite from Ouro Preto, Brazil, but admits that the mineral could well be associated with jamesonite. In both cases, therefore, part or all of the lead may have been concentrated in minerals associated with the tetrahedrite, and only by modern microscopical and microanalytical techniques can the problem be settled. Specimen preparation. Part of the hand specimen was sectioned with a diamond disc saw and then serially sectioned with a Lastec diamond-impregnated wire saw. The polishing procedure employed was that described in Clark et al. 0974). A thin section was also prepared from one of the slices.