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Cymraeg

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:

Geological Context:

Tetrahedrite in polished section, forming brownish-grey inclusions a fraction of a millimetre across, associated with a little ullmannite (white) in galena (grey-white). Yellow is chalcopyrite. © J.S. Mason.
Tetrahedrite (steel-grey, no cleavage, top right) adjacent to galena (blue-grey, obvious cleavage, bottom left) in quartz. Darren mine, Central Wales. Specimen National Museum of Wales (NMW 86.100G.M8), ex J.S.Mason, © National Museum of Wales.
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:

There are no key localities for this specimen.

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.

There are no references for this specimen.