Crystal System: Hexagonal
Status of Occurrence: Confirmed Occurrence
Chemical Composition: Native bismuth
Chemical Formula: Bi
Method(s) of Verification: recent identifications EMPA, all others by optical microscope.
- Elements & Alloys
- Hydrothermal: mesothermal polymetallic veins
- Hydrothermal: volcanogenic massive sulphides
Introduction: native bismuth typically occurs in medium-temperature hydrothermal mineral veins, where it is often associated with a wide variety of other sulphides, tellurides and sometimes native gold. A spatial, if not genetic, connection between bismuth-bearing mineral deposits and igneous rocks is not infrequent. Unless well-crystallized, it requires analytical work to confirm its presence.
Occurrence in Wales: bismuth is of limited occurrence in Wales, being restricted to three areas in which medium-temperature hydrothermal mineralization is associated, directly or indirectly, with igneous rocks. The first (unconfirmed) claim of its presence was by Readwin (noted by Andrew, 1910), but the three confirmed occurrences, from the Dolgellau Gold-belt (Naden, 1988), from the Snowdon Caldera (Mason et al., in press) and from Parys Mountain (Sivaprakash, 1977; Pointon & Ixer, 1980) all owe their discovery to the advent of high-resolution ore petrology and electron microprobe analyses. In all recent identifications analyses show bismuth to be the only element present in the grains analysed.
- 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, bismuth was confirmed as extremely small (1 µm) grains occurring (with bismuthinite) along cosalite-galena grain boundaries from this mine (Mason et al., in press).
- Clogau Mine, Bontddu, Gwynedd: inclusions (10-40 µm in size) of bismuth in galena in sulphide-dominated telluride-bearing assemblages were identified by Naden (1988).
- Parys Mountain, Anglesey: small laths of native bismuth, only 5-10 µm in size, were noted by Sivaprakash (1977) and Pointon & Ixer (1980). They occur enclosed within small aggregates of bismuthinite and bismuth sulphosalts.
- Andrew, A.R., 1910. The geology of the Dolgelley gold-belt, North Wales. Geological Magazine, 47, 159-171, 201-221, 261-271.
- Mason, J.S., Cotterell, T.F. & Bevins, R.E., (in prep.). Cosalite, the first Welsh Occurrence. UK Journal of Mines & Minerals.
- 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.
- 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.
- 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.
- Sivaprakash, C., 1977. Geochemistry of some sulphides and sulphosalts from Parys Mountain, Anglesey. Unpublished M.Phil. thesis, University of Aston in Birmingham.