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FELBERTALITE AND RELATED BISMUTH SULFOSALTS FROM THE FUNIUSHAN COPPER SKARN DEPOSIT, NANJING, CHINA

Gu Xiang-Ping^1,, Makoto Watanabe¹, Makio Ohkawa¹, Kenichi Hoshino¹, Yasuhiro Shibata¹ and Chen Desong²

¹ Department of Earth and Planetary Sciences, Hiroshima University, Hiroshima 739-8526, Japan
² Guilin Research Institute of Geology for Mineral Resources, Guangxi 541004, China

E-mail address: guxp{at}geol.sci.hiroshima-u.ac.jp

Electron-microprobe and single-crystal X-ray-diffraction analyses have confirmed the second occurrence of felbertalite in the Funiushan copper skarn deposit, near Nanjing, in eastern China. It can be chemically described as Cu₂Ag_xPb_6–2xBi_8+x(S,Se)₁₉, with a wide range of substitution of Bi + Ag for 2Pb, with x ranging from 0.64 to 1.03 (probably up to 1.33), suggesting the presence of a solid-solution series in felbertalite. The measured unit-cell parameters of a felbertalite grain with a composition of Cu_2.20Ag_0.70Pb_4.65Bi_8.45S_19.12 are: a 27.68(4), b 4.046(1), c 20.67(3) Å, ß 131.06(8)°, space group C2/m. A set of phases in the bismuthinite–aikinite series, mostly free of Ag and characterized by intermediate compositions between ideal formulae, were identified with electron-microprobe data. Compositional variations from Cu_1.80Pb_1.80Bi_2.20S₆, through Cu_1.53Pb_1.53Bi_2.47S₆, Cu_1.07Pb_1.07Bi_2.93S₆, Cu_0.73Pb_0.73Bi_3.27S₆, to Cu_0.58Pb_0.58Bi_3.42S₆ within a single grain were observed, indicating that an intergrowth of three structural units, i.e., bismuthinite ribbons, krupkaite ribbons and aikinite ribbons, could occur in the progressive exsolution or precipitation of a single phase in the bismuthinite–aikinite series. The exceptionally high contents of Ag (0.76–1.27 wt%) in the phase Cu_0.83Ag_0.10Pb_0.90Bi_3.13S₆, presumably corresponding to krupkaite, are noted. Electron-microprobe analyses have also revealed the occurrence of a number of rare minerals, including junoite, unknown Cu₂Ag_xPb_10–2xBi_12+xS₂₉ (1.23 < x < 1.49), berryite, benjaminite?, schirmerite?, Bi- and Ag-bearing galena, wittichenite and tetradymite. Within the intergrowths of Cu–Ag–Pb–Bi–S(Se,Te) minerals, Te (especially) and Se are preferably partitioned into Ag-bearing phases.

Keywords: felbertalite, bismuthinite—aikinite series, bismuth sulfosalts, unknown Cu₂Ag_xPb_10–2xBi_12+xS₂₉, copper skarn, Funiushan, Nanjing, China.

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