Bismuthinite

Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Bismuth sulphide
Chemical Formula: Bi2S3
Method(s) of Verification: all recently described occurrences of bismuthinite were identified by EMPA.

Chemical Group:

  • Sulphides

Geological Context:

  • Hydrothermal: mesothermal polymetallic veins
  • Hydrothermal: volcanogenic massive sulphides
Introduction: bismuthinite typically occurs in medium-temperature hydrothermal mineral veins, where it is often associated with a wide variety of other sulphides, tellurides, native bismuth and native gold. Unless well-crystallized, it requires analytical work to confirm its presence.
Occurrence in Wales: the first reports of bismuthinite from Wales were by Readwin (1888), with reference to the Dolgellau Gold-belt, and by Greenly (1919), who described a foliated to scaly, steel-grey metallic mineral associated with pyrite from Rhos-mynach Fawr on Anglesey (specimens in the National Museum of Wales and the Natural History Museum, London), which was identified by H.H. Thomas of the Geological Survey as bismuthinite. These occurrences remain to be confirmed. However, in recent years, occurrences have been confirmed at Parys Mountain (Pointon & Ixer, 1980), from two mines in the Dolgellau Gold-belt (Naden, 1988; Mason et al., 2002), from the Snowdon Caldera copper-bearing veins (Mason et al., in press) and from the Castell Carn Dochan Mine near Bala.

Key Localities:

  • Braich-yr-oen Mine, Snowdon, Gwynedd: small amounts of unidentified bismuth-bearing minerals were noted by Reedman et al. (1985) from two localities within the copper-mining area of the Snowdon Caldera. During the 1997 Gwynedd phase of the MINESCAN project, bismuthinite was confirmed as extremely small (1 µm) grains occurring along cosalite-galena grain boundaries from this mine (Mason et al., in press).
  • Castell Carn Dochan Mine, Llanuwchllyn, Bala, Gwynedd: a few specimens, typically showing 2-3 mm nests of acicular bismuthinite crystals associated with pyrite and quartz, have been collected on the deep adit dumps (Richard Bell, unpublished data). In specimen terms, these are the best examples, in aesthetic terms, of bismuthinite collected from Wales to date.
  • Clogau Mine, Bontddu, Gwynedd: small (50 µm) inclusions of bismuthinite were identified by Naden (1988) in galena, associated with tetradymite and native bismuth.
  • Ffridd-Goch Mine, Llanfachreth, Gwynedd: bismuthinite was identified in association with electrum and minor telluride minerals, forming extremely fine-grained intergrowths in polished sections taken from ore samples (J.W.G. Gilbey Collection) dominated by chalcopyrite and arsenopyrite (Mason et al., 2002).
  • Parys Mountain, Anglesey: Pointon and Ixer (1980) identified very minor microscopic bismuthinite, containing laths of native bismuth, in polished sections of the complex ore from this site.

References:

  1. Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
  2. Mason, J.S., Bevins, R.E. & Alderton, D.H.M., 2002. Ore Mineralogy of the mesothermal gold lodes of the Dolgellau Gold Belt, North Wales. Transactions of the Institution of Mining and Metallurgy (Section B, Applied earth science), 111, B203-B214.
  3. Mason, J.S., Cotterell, T.F. & Bevins, R.E., (in prep.). Cosalite, the first Welsh Occurrence. UK Journal of Mines & Minerals.
  4. Naden, J., 1988. Gold mineralisation in the Caledonides of the British Isles with reference to the Dolgellau Gold Belt and the Southern Uplands of Scotland. Unpublished Ph.D thesis, University of Aston, UK.
  5. Pointon, C.R. & Ixer, R.A., 1980. Parys Mountain mineral deposit, Anglesey, Wales: geology and ore mineralogy. Transactions of the Institution of Mining and Metallurgy (Section B: Applied earth science), 89, B143-B155.
  6. Readwin, T.A., 1888. Gold in Wales. London, 12pp.
  7. Reedman, A.J., Colman, T.B., Campbell, S.D.G. & Howells, M.F., 1985. Volcanogenic mineralization related to the Snowdon Volcanic Group (Ordovician), Gwynedd, North Wales. Journal of the Geological Society, London, 142, 875-888.