Copper

Crystal System: Cubic
Status of Occurrence: Confirmed Occurrence
Distribution: Uncommon
Chemical Composition: Native copper
Chemical Formula: Cu
Method(s) of Verification: all occurrences cited - visual and microscopic identification.

Chemical Group:

  • Elements & Alloys

Geological Context:

  • Hydrothermal: amygdale infill & veins in volcanic rocks
  • Supergene: in situ natural oxidation & weathering deposits
Dendritic copper from Rhos-mynach Mine, Anglesey. Field of view 14.5 mm wide. National Museum of Wales specimen NMW 83.41G.M.14, ex R.J. King Collection. © National Museum of Wales.
Dendritic native copper from Rhos-mynach Mine, Anglesey. Field of view 5 cm across. National Museum of Wales Collection (NMW 83.41G.M.14) ex R.J. King Collection. Photo T.F. Cotterell, © National Museum of Wales.
Introduction: native copper can occur as a primary mineral in a variety of settings, especially in altered basic lavas: however in Wales the occurrences are all supergene in nature, in association with cuprite, malachite and other copper minerals. Where fresh, native copper is unmistakeable, but specimens may develop a black tarnish on exposure to moist air.
Occurrence in Wales: native copper has long been known from Wales, for example at Drws-y-coed Mine (Pennant, 1781) and at Parys Mountain where the presence of masses of the metal to 30 lbs in weight was noted by Lentin (1800). In 1856, W.J. Henwood, describing the famous and highly profitable Turf Copper Mine, near Dolgellau, wrote: 'Some of the lowest portions of the upper peat-bed were so rich in copper, that they were carried to the Swansea smelting works in the condition in which they were extracted; some of the leaves are said to have been covered with a thin pellicle of bright metallic copper; nuts were coated in like manner, and on being broken afforded also a kernel of the same; and I was informed that the copper was in some cases deposited between the fibres of the wood; so that on being cut it exhibited alternate layers of vegetable matter and metal'. Other 19th Century accounts were provided by Vivian (1859) and Readwin (1888). More recently, native copper has been found at many localities as a component of predominantly microcrystalline supergene assemblages, with some particularly noteworthy occurrences in Central Wales.

Key Localities:

  • Afon Wen, Gwynedd: very rare 'nuggets' of intergrown copper and cuprite have been occasionally recorded from the alluvial heavy mineral suite in this river: the largest known, found in 1995, was 30 mm across (J.S. Mason Collection).
  • Benallt Mine, Llŷn, Gwynedd: very rare copper forms thin veinlets in banded chert with chalcocite and cuprite (Bevins & Mason, 1998).
  • Central Wales Orefield: native copper, associated with cuprite and supergene copper sulphides, occurs in Central Wales in both natural and post-mining assemblages. Native copper of purely natural origin has been recorded at Eaglebrook, Camdwrbach, Llechweddhelyg, Dolwen and Geufron mines, forming intergrowths to 10 mm with massive cuprite, copper sulphides and malachite (Mason, 1994; Bevins & Mason, 1997). Post-mining native copper is more widespread, but significant specimens are rare: the best to date have come from Lodge Park copper trial near Tre'r-ddol, Ceredigion, where thin dendrites to several mm along shale partings, associated with cuprite, were collected in the 1990s (Mason & Green, 1996). Other localities include Ystrad Einion (Mason & Rust, 1997) and Frongoch (Green et al., 1996).
  • Coed-y-Brenin, Dolgellau, Gwynedd: copper occurred at the Turf Copper Mine, impregnating/replacing decayed vegetable matter in a peat bog fed by groundwaters percolating through the Coed-y-Brenin porphyry-copper deposit. It is not believed that any of the interesting examples described by Henwood (1856) have been preserved.
  • Dolgellau Gold-belt, Gwynedd: native copper occurs as a supergene (and often post-mining) mineral at several localities as thin plates and wires (up to 2 mm) in quartz. In this mode it has been found at Clogau, Prince Edward and Nantgoch mines (Cambrian Goldfields Ltd., unpublished data), but in most cases specimens rapidly develop a black tarnish.
  • Dolyhir Quarry, Old Radnor, Powys: rare copper dendrites embedded in colourless to white calcite and associated with azurite and tyrolite were noted in a temporary vein exposure in 2001.
  • Great Orme Copper Mines, Llandudno, Gwynedd: native copper was reported by Vivian (1859) as arborescent growths. The petrology of the supergene mineralization at Great Orme was described by Ixer & Davies (1996); native copper was noted to form microscopic (to 20 µm) grains intergrown with cuprite and malachite.
  • Parys Mountain, Anglesey: significant occurrences were reported by Lentin (1800) in the near-surface zone of the orebody, while Greenly (1919) noted the presence of leaflike or mosslike aggregates in gossan in the Great Lode. Specimens are present in the National Museum of Wales collections (e.g. NMW 27.111.GR.358 & NMW 87.43G.M.8).
  • Rhos-mynach Mine, Anglesey: arborescent masses up to 60 mm across are held by the National Museum of Wales (NMW 83.41G.M.14 & 15, ex R.J. King Collection).
  • Snowdon, Gwynedd: within the Snowdon Caldera area, 'very thin laminae of the native metal' were mentioned as occurring in the wallrocks at Drws-y-coed Mine by Pennant (1781) and further examples have been found since at this and other mines such as Lliwedd (Bevins & Mason, 1998). The occurrence, involving thin wires and plates to a few millimetres in size, is similar to that described above in the Dolgellau Gold-belt and, similarly, the specimens tend to develop a black tarnish on exposure to air.

References:

  1. 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.
  2. 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.
  3. Green, D.I., Rust, S.A. & Mason, J.S., 1996. Classic British mineral localities: Frongoch Mine, Dyfed. UK Journal of Mines & Minerals, 17, 29-38.
  4. Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
  5. Henwood, W.J., 1856. Notice of the Copper Turf of Merioneth. Reports of the Royal Institution of Cornwall, 41-43.
  6. Ixer, R.A. & Davies, J., 1996. Mineralisation at the Great Orme Copper Mines, Llandudno, North Wales. U.K. Journal of Mines and Minerals, 17, 7-14.
  7. Lentin, A.G.L., 1800. Briefe über die Insel Anglesea, vorzuglich über die dasigen Kupferbergwerke und die dazu gehorigen Schmelzwerke und Fabriken. Leipzig.
  8. Mason, J.S., 1994. A Regional Paragenesis for the Central Wales Orefield. Unpublished M.Phil thesis, University of Wales (Aberystwyth).
  9. Mason, J.S. & Green, D.I., 1996. Supergene copper mineralisation in situ at Lodge Park Copper trial, Dyfed. UK Journal of Mines and Minerals. 17. 19-23.
  10. Mason, J.S. & Rust, S.A., 1997. The Mineralogy of Ystrad Einion Mine, Dyfed, Wales. U.K. Journal of Mines and Minerals, 18, 33-36.
  11. Pennant, T., 1781. A Journey to Snowdon (part l). London, Henry Hughes, 197pp.
  12. Readwin, T.A., 1888. Gold in Wales. London, 12pp.
  13. Vivian, W., 1859. On arborescent native copper in the Great Ormeshead, North Wales. Quarterly Journal of the Geological Society of London, 15, 109-110.