Bornite

Crystal System: Orthorhombic
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Copper iron sulphide
Chemical Formula: Cu5FeS4
Method(s) of Verification: all localities optical identification; Dinorwic Quarry - XRD (NMW).

Chemical Group:

  • Sulphides

Geological Context:

  • Supergene: in situ natural oxidation & weathering deposits
  • Hydrothermal: alpine type veins
Bornite on Precambrian sandstone matrix from Dolyhir Quarry. The metallic pinkish colour is well-displayed. A little malachite and azurite are also present. Sample 4 cm across.
Introduction: bornite is an important ore of copper. It can occur as a primary mineral, for example in porphyry-copper deposits, or commonly as a supergene species occurring in the secondary enriched zones of copper ore bodies. Bornite is a distinctive mineral when fresh and unweathered and may be identified in hand specimen by its reddish-pink colour. However, like many sulphides it readily tarnishes to iridescent blues and greens. The commonplace name for bornite, 'peacock ore', reflects this property but has also caused confusion: chalcopyrite also tarnishes in this manner and many specimens labelled bornite often turn out to simply be tarnished chalcopyrite. The colour of the fresh sulphide is critical in identification.
Occurrence in Wales: a scan through the literature may give the impression that bornite is apparently widespread in Wales, but many old reports of bornite are in fact tarnished chalcopyrite. In fact, bornite is confirmed from five Welsh localities only. These are all of relatively recent discovery and are described below. The unconfirmed bornite localities where tarnished chalcopyrite is suspected are Cwm Cynfal Mine (Dewey & Eastwood, 1925); Clogau Mine (NMW specimens) and Glasdir Mine (Andrew, 1910), all in the Dolgellau Gold-belt, where recent sampling of the mineralization and examination of cited specimens has failed to reveal a trace of bornite. In addition, NMW specimens labelled bornite, from Dylife Mine and a site near to Devil's Bridge in Central Wales, are both tarnished chalcopyrite. Likewise, 'peacock ore' recorded from Drws-y-Coed Mine (Dewey & Eastwood, 1925) and Tal-y-sarn Quarry, Llanllyfni (NMW specimen) are again tarnished chalcopyrite.

Key Localities:

  • Capel Hermon, Coed y Brenin, Gwynedd: bornite was recorded from borehole cores during the drilling of the Coed-y-Brenin porphyry-copper deposit (Rice & Sharp, 1976). It is present as intergrowths, occasionally large enough to be visible to the naked eye, with chalcopyrite and tennantite in irregular veins, stringers and pods cutting altered porphyritic microtonalite in a forest road cutting exposure of the "ore-zone" at Capel Hermon (Bevins & Mason, 1998; Armstrong et al., 2003). The exposure, which is a SSSI, has been cleared of talus and is part of a geological trail covering aspects of the porphyry-copper deposit.
  • Dinorwic Quarry, Llanberis, Gwynedd: as a very minor component of the copper-sulphide (chalcocite-dominated) assemblage occurring in boudin necks of altered and deformed dykes hosted by Cambrian slate (Bevins, 1994).
  • Dolyhir Quarry, Old Radnor, Powys: massive bornite was common in a temporary exposure present in 1996, where it was present as intergrowths with chalcopyrite and minor chalcocite associated with minor quartz, lining narrow fractures in Precambrian sandstone. Areas of bornite to several square centimetres are present on specimens collected at that time (Bevins & Mason, 1997). Alteration to malachite and azurite was frequent. Available data indicate that this occurrence is probably of supergene origin.
  • Llechweddhelyg Mine, Penrhyncoch, Ceredigion: intense supergene alteration of primary chalcopyrite is preserved at this site and the chalcocite-dominated secondary sulphide assemblage occasionally reveals bornite in polished section, occurring as an intermediate alteration phase intergrown with relict chalcopyrite (Mason, 1994).
  • Parys Mountain, Anglesey: Pointon & Ixer (1980) recorded bornite as an oxidation product of chalcopyrite in polished sections made from shallow borehole cores, associated with covellite, chalcocite and cuprite.

References:

  1. Andrew, A.R., 1910. The geology of the Dolgelley gold-belt, North Wales. Geological Magazine, 47, 159-171, 201-221, 261-271.
  2. Armstrong, R., Herrington, R.J. & Savage, M.A., 2003. Tennantite and tyrolite from the Coed y Brenin forest, North Wales. Journal of the Russell Society, 8(1), 18.
  3. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  4. Bevins, R.E. & Mason, J.S., 1998. Welsh Metallophyte and metallogenic evaluation project: Results of a Minesite Survey of Gwynedd. National Museums of Wales, Cardiff.
  5. Bevins, R.E. & Mason, J.S., 1997. Welsh metallophyte and metallogenic evaluation project: Results of a minesite survey of Dyfed and Powys. CCW Contract Science Report No. 156. National Museums & Galleries of Wales.
  6. Dewey, H. & Eastwood, T., 1925. Copper ores of the Midlands, Wales, the Lake District and the Isle of Man. Memoirs of the Geological Survey. Special Report on the Mineral Resources of Great Britain, 30.
  7. Mason, J.S., 1994. A Regional Paragenesis for the Central Wales Orefield. Unpublished M.Phil thesis, University of Wales (Aberystwyth).
  8. 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.
  9. Rice, R. & Sharp, G.J., 1976. Copper mineralization in the forest of Coed-y-Brenin, North Wales. Transactions of the Institution of Mining and Metallurgy, (Section B: Applied earth science), 85, B1-B13