Tetrahedrite

Crystal System: Cubic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Copper antimony sulphide with highly variable amounts of iron, zinc, silver and other metals
Chemical Formula: (Cu,Ag)10(Fe,Zn)2Sb4S13
Method(s) of Verification: Central Wales - XRD (J.S. Mason, University of Wales, Aberystwyth).

Chemical Group:

  • Sulphosalts

Geological Context:

  • Hydrothermal: mesothermal polymetallic veins
Introduction: tetrahedrite occurs in many types of ore deposits globally but in Wales it mainly occurs in mesothermal polymetallic vein associations and in the highly modified exhalative deposit at Parys Mountain. Capable of carrying significant amounts of silver, it is an important source of that metal. It requires a certain amount of experience to identify it visually or in polished section. Tetrahedrite forms two series: the silver-rich end-member is freibergite while arsenic takes the place of antimony in tennantite. Neither are easy to distinguish from tetrahedrite on a visual basis alone.
Occurrence in Wales: The first Welsh occurrence of tetrahedrite was described by Lentin (1800) at Parys Mountain, Anglesey. Subsequently, it was described by Gilbey (1968) in samples from the Dolgellau Gold-belt. In the 1980s and 1990s, richly argentiferous tetrahedrite was found at numerous formerly important silver mines in Central Wales (Mason, 1997; 1998). Here, it occurs as numerous microscopic inclusions in galena and also, at some mines, as larger masses visible to the naked eye. The quantities present constitute its most significant Welsh occurrence, which, coincident with high silver grades in lead ore concentrates (in the order of 900-1000ppm Ag), suggests that tetrahedrite was the principal silver carrier in this area. All occurrences in Central Wales are within a specific assemblage ('A1-c') within the overall 12-stage regional paragenesis of Mason (1994). Meanwhile, a further discovery of significant tetrahedrite at the old St Elvis silver mine in Pembrokeshire led Mason & Bevins (2002) to speculate that it may have been recognised by German silver-miners in the 1600s.

Key Localities:

  • Alltycrib Mine, Tal-y-bont, Ceredigion: present in polished sections of galena where it forms small inclusions associated with bournonite, boulangerite and ullmannite (Mason, 1994, 1998).
  • Bwlch Mine, Goginan, Ceredigion: inclusions in galena with bournonite and ullmannite (Mason, 1994, 1998).
  • Ceunant Mine, Cwmerfin, Ceredigion: frequent as inclusions in galena, with bournonite and ullmannite. Rarely as 1-2cm masses in quartz (Mason, 1994, 1998).
  • Clogau Mine, Bontddu, Gwynedd: reported by Naden (1988) on the basis of an analysis of a specimen in the Kingsbury Collection at the Natural History Museum, London.
  • Cwmerfin Mine, Cwmerfin, Ceredigion: tetrahedrite was formerly abundant as inclusions in galena with chalcopyrite, bournonite and ullmannite, and occasionally found as small (<1cm) masses in hand specimens (Mason, 1994, 1998). The site has now been landscaped.
  • Cwmheisian Isaf Mine, Ganllwyd, Dolgellau, Gwynedd: inclusions in galena visible in polished sections (J.W.G. Gilbey Research Collection, National Museum of Wales).
  • Cwmsymlog Mine, Cwmsymlog, Ceredigion: abundant as inclusions in galena with chalcopyrite, bournonite and ullmannite (Mason & Hughes, 1990; Mason, 1994; 1997; 1998).
  • Darren Mine, Pen-bont Rhydybeddau, Ceredigion: common as inclusions in galena with chalcopyrite, bournonite and ullmannite and also present in visible masses to 40mm in quartz-cemented breccia with galena, bournonite and chalcopyrite (Mason & Hughes, 1990; Mason, 1994; 1997; 1998). Occasional part-crystals to 15mm have been found in quartz-lined cavities.
  • Esgairhir Mine, Tal-y-bont, Ceredigion: common as microscopic inclusions in galena with ullmannite, siegenite, millerite, tucekite, bournonite and boulangerite (Rust & Mason, 1988; Mason, 1994, 1998).
  • Foel Ispri Mine, Llanelltyd, Dolgellau, Gwynedd: very rarely as microscopic inclusions in galena (Gilbey, 1968; Mason et al., 2002).
  • Goginan Mine, Goginan, Ceredigion: prior to the landscaping of this site, tetrahedrite occurred as common microscopic inclusions in galena, associated with bournonite and ullmannite (Mason, 1994, 1998).
  • Graigddu Mine, Llangynog, Powys: as inclusions in galena close to chalcopyrite grain boundaries, associated with bournonite. In polished sections of ore samples collected by J.S. Mason during MINESCAN fieldwork in 1996.
  • Gwynfynydd Mine, Ganllwyd, Dolgellau, Gwynedd: as microscopic inclusions in sphalerite (Gilbey, 1968; Mason et al., 2002).
  • Leri Valley Mine, Talybont, Ceredigion: inclusions in galena with bournonite and ullmannite (Mason, 1994, 1998).
  • Llechweddhelyg Mine, Penrhyncoch, Ceredigion: uncommon as inclusions in galena with bournonite and ullmannite - its scarcity due in part to the fact that most of the ore-zone has been oxidized at this locality (Mason, 1994, 1998).
  • Llettyhen Mine, Bontgoch, Ceredigion: present as inclusions in galena with bournonite and ullmannite (Mason, 1994, 1998).
  • Parys Mountain, Anglesey: with chalcopyrite and occasionally sphalerite and galena as inter-grain rims, inclusions and veinlets. Generally microscopic (Greenly, 1919; Wheatley, 1971; Sivaprakash, 1977; Pointon & Ixer, 1980).
  • Pengraigddu Mine, Cwmerfin, Ceredigion: frequent as inclusions in galena with bournonite and ullmannite (Mason, 1994, 1998).
  • Penpompren Mine, Talybont, Ceredigion: rare as inclusions in galena with bournonite and ullmannite (Mason, 1994, 1998).
  • South Darren Mine, Pen-bont Rhydybeddau, Ceredigion: formerly abundant as inclusions in galena with as inclusions in galena with bournonite and ullmannite and also as visible masses in quartz, exceptionally to 100mm (Mason & Hughes, 1990; Mason, 1994; 1997; 1998); the site is now largely obliterated.
  • St. Elvis Mine, Solva, Pembrokeshire: at the isolated St Elvis Mine, near Solva in W Pembrokeshire, tetrahedrite was discovered in 1996 and was later shown to significantly argentiferous (Mason & Bevins, 2002). It occurs in quartz, in close association with bournonite, in masses to 3 cm. Other sulphides present are galena, chalcopyrite and minor sphalerite. St Elvis was a silver mine in the 16th and 17th centuries: again the coincidence, as in the Darren area of Central Wales, of visible tetrahedrite, 16th-17th century silver mining and German mining technologists is an interesting one, with Mason & Bevins (2002) suggesting that the German miners recognised tetrahedrite (or 'fahl-ore', as it was known then) as a silver-ore.
  • Tyddyn Gwladys Mine, Ganllwyd, Dolgellau, Gwynedd: in an important occurrence by the standards of the Dolgellau Gold-belt, tetrahedrite was first identified here by Forbes (1868) but called by him 'polytelite'. Confirmed by Gilbey (1968), tetrahedrite is locally common in polished sections of galena, where it occurs as small (<1 mm) masses intergrown with pyrargyrite, chalcopyrite and rare low-fineness electrum (Mason et al., 2002).

References:

  1. Forbes, D., 1868. Researches in British Mineralogy. The London, Edinburgh and Dublin Philosophical Magazine and Journal of Science, 35, 171-184.
  2. Gilbey, J.W., 1968. The mineralogy, paragenesis and structure of the ores of the Dolgellau Gold Belt, Merionethshire, and associated wall rock alteration. Unpublished Ph.D thesis, University of London, UK.
  3. Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
  4. Lentin, A.G.L., 1800. Briefe über die Insel Anglesea, vorzuglich über die dasigen Kupferbergwerke und die dazu gehorigen Schmelzwerke und Fabriken. Leipzig.
  5. Mason, J.S., 1994. A Regional Paragenesis for the Central Wales Orefield. Unpublished M.Phil thesis, University of Wales (Aberystwyth).
  6. Mason, J.S., 1998. Tucekite, a mineral new to Britain, and other rare ore minerals from the Central Wales Orefield. UK Journal of Mines and Minerals, 19, 30-36.
  7. Mason, J.S., 1997. Regional polyphase and polymetallic vein mineralisation in the Caledonides of the Central Wales Orefield. Transactions of the Institution of Mining and Metallurgy (Section B: Applied Earth Science), 106, B135-B144.
  8. Mason, J.S. & Bevins, R.E., 2002. St Elvis Mine, Solva, Pembrokeshire: Another Elizabethan tetrahedrite occurrence? British Mining 71, 5-12, Northern Mines Research Society.
  9. Mason, J.S. & Hughes, S.J.S., 1990. Geology of the Darren District. In: Hughes, S.J.S. The Darren Mines. British Mining, Northern Mine Research Society, 40, 131-141.
  10. 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.
  11. 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.
  12. 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.
  13. Sivaprakash, C., 1977. Geochemistry of some sulphides and sulphosalts from Parys Mountain, Anglesey. Unpublished M.Phil. thesis, University of Aston in Birmingham.
  14. Wheatley, C.J.V., 1971. Economic geology of the Avoca mineralised belt, S.E. Ireland, and Parys Mountain, Anglesey. Unpublished Ph.D. thesis, Imperial College, University of London.