Gold

Crystal System: Cubic
Status of Occurrence: Confirmed Occurrence
Distribution: Widespread
Chemical Composition: Native gold
Chemical Formula: Au
Method(s) of Verification: all occurrences verified by its physical properties.

Chemical Group:

  • Elements & Alloys

Geological Context:

  • Hydrothermal: mesothermal polymetallic veins
  • Hydrothermal: epithermal polymetallic veins & pipes
  • Hydrothermal: volcanogenic massive sulphides
  • Sedimentary
  • Hydrothermal: porphyry-type mineralization
Leaf gold, from Gwynfynydd Mine, in the Dolgellau Gold-belt. The specimen measures 40 mm x 20 mm. National Museum of Wales Collection (NMW 72.33G.M.7). © National Museum of Wales.
Native gold associated with sphalerite, Prince of Wales Mine, Dolgellau Gold-belt, Gwynedd. National Museum of Wales Collection (NMW 70.19G.M.171). © National Museum of Wales.
Waterworn gold nugget 20 mm in length and 6.5 g. Found in 2001 following severe flash-floods on the Afon Wen, Gwynedd. Nuggets of this size are now exceptionally rare in North Wales rivers following centuries of prospecting. © J.S. Mason.
Blebs of bright yellow gold in galena (white), both partially replaced by sphalerite (dark grey). Black is quartz. Sample from Gwynfynydd Mine, field of view 1 mm. © J.S. Mason.
Introduction: gold can occur in a variety of deposits including low-to-high temperature veins, pegmatites, porphyry-type ores and volcanogenic exhalative ores. It may be associated with a wide range of minerals amongst which pyrite, arsenopyrite, galena, tellurides and enargite are frequent. Because of its high density, gold eroded from primary deposits tends to become concentrated, with other heavy minerals, in placer deposits in adjacent river systems. Gold is a distinctive mineral that is readily identified by its colour, density and malleability. In polished section its relative softness, colour and high reflectivity are diagnostic, although it is notoriously difficult to polish well.
Occurrence in Wales: gold is still an economically-important Welsh mineral despite the decline of the other metal-mining industries, being mined today in small amounts exclusively for the jewellery industry. It is not accurately known when the first discoveries of gold in North Wales were made: there can be little doubt that they predated the announcement by Dean (1844) or the counterclaim by Robert Roberts that it had been found in 1836. A painting in the NMW collection depicting gold-panners is entitled 'Panning gold: Mawddach' and is by John Glover (1767-1849) and dated ca. 1795 which implies that the above 19th Century claims were misplaced. However, to the south at Dolaucothi Mine, between Lampeter and Llandovery, gold was certainly known much longer ago, as the mine was worked extensively by the Romans and recent evidence has come to light implying a potentially pre-Roman age. Gold formed part of the mid-19th Century mining boom in Wales and many lodes outcropping in the Dolgellau Gold-belt were opened up. Some proved to be highly productive while others produced only small quantities and many more were unsuccessful. The most important mines in the Gold-belt were Clogau and Gwynfynydd, with the greatest annual production being 18,417 ounces of gold from Clogau in 1904 (Hall, 1990). Many of the mines declined in activity into the 20th Century but there was a revival, both in the Gold-belt and at Dolaucothi, when Britain abandoned the Gold Standard in the early 1930s (Hall, 1993). Dolaucothi finally closed at the outbreak of the Second World War but intermittent activity has continued in the Gold-belt to the present day, with occasional significant finds being made, particularly at Gwynfynydd in the 1980s (Hall, 1990).

Key Localities:

  • Anglesey: gold occurs as a trace metal in the Rhos-mynach Fawr Mine and also at the Parys Mountain deposit (Greenly, 1919). It is also present in minor quantities in Cu-Pb-Zn bearing quartz veins in the Carmel Head area (Cooper et al., 1990).
  • Berwyn Hills, Clwyd: small grains of gold have been found in streams draining the NW part of the range, but no convincing bedrock source has been identified (Smith, 1993).
  • Castell Carn Dochan Mine, Llanuwchllyn, Bala, Gwynedd: gold was discovered here in the 1860s (Readwin, 1864) and was mined intermittently until about 1905 (Hall, 1990). It occurs in a quartz-sulphide vein cutting Middle Ordovician volcanic rocks and its relationship to the mineralization in the nearby Dolgellau Gold-belt is the subject of continuing research.
  • Central Wales Orefield: gold was said to have been discovered in the 19th Century at a number of localities, such as Caegynon Mine, although nothing subsequently came of it (Bick, 1983). More recently, microscopic gold has been noted in polished sections from Erglodd Mine (Mason, 1994); however the chief gold-bearing mineral in Central Wales is electrum and this is only present in trace quantities (Mason, 1998).
  • Clwydian Hills: a series of small trial workings on irregular quartz veins in Silurian turbidites (e.g. at Moel Arthur) have long been rumoured to have been for gold. Calvert (1853) mentioned a high-grade specimen, but he was notorious for citing similar 'finds' in almost every county of the UK. Foster-Smith (1973, 1974) lists five supposed gold trials. However, fieldwork in recent years has failed to find a trace of gold (Bevins & Mason, 1999; D.H.M. Alderton, pers. comm.).
  • Dolaucothi Gold Mine, Pumpsaint, Carmarthenshire: gold has been worked over two millennia from a large mass of pyritized and arsenopyritized Upper Ordovician to Lower Silurian shale shot through with low-angle to steep quartz veins (also carrying pyrite and arsenopyrite) at this site. The older workings comprise a large opencast and it has been conjectured (e.g. Hall, 1993) that these workings exploited the oxidized zone, in which the gold could be removed from gossanous clays simply by washing and sluicing with water. The deeper workings produced auriferous sulphide concentrates, the treatment of which was so difficult that they had to be shipped overseas, firstly to Hamburg and, by the late 1930s when the political situation in Europe made this impossible, to Seattle. The outbreak of war put pay to the shipping and without a way to treat the ore the mine closed for the last time. Today it is owned by the National Trust and used as a teaching facility by the Department of Geology at Cardiff University, in parallel with its other use as a tourist attraction. Visible gold is rare at Dolaucothi although there are a limited number of specimens in the National Museum of Wales Collection. Generally, it occurs as small (up to 200 µm but more typically 15-30 µm) inclusions in pyrite or arsenopyrite. The deposit has been interpreted (e.g. Annels & Roberts, 1989) as a syn-orogenic saddle-reef, but it also exhibits features consistent with an alternative model of a part-remobilized sedimentary-exhalative deposit (J.S. Mason, unpublished data). The mineralization continues beyond the immediate mine area, as has been demonstrated by drilling undertaken by students at Cardiff University and by alluvial gold occurrences in streams to the NE (Brown, 1993).
  • Dolgellau Gold-belt, Gwynedd: this highly-important gold-producing area stretches from the Cardigan Bay coast near Barmouth in the SW around the south and east flanks of the Harlech Horst up to Cwm Prysor near Trawsfynydd in the NE. A great deal of information has been published on the area and its gold deposits, including works by Calvert (1853), Ramsay (1854), Readwin (1860, 1862), Smyth (1862), Forbes (1867, 1868), Readwin (1888) and Andrew (1910). More recent investigations into the mineralization include an extensive paragenetic study (Gilbey, 1968), investigations into isotopic and fluid-inclusion properties of the mineralization (Bottrell, 1986; Bottrell & Spiro, 1988; Bottrell et al., 1990) and major revisions of the age of the mineralization (Mason et al., 1999; Platten & Dominy, 1999) and its ore mineralogy (Mason et al, 2002).The Dolgellau Gold-belt mineralization is of the mesothermal gold-lode type and comprises mainly steep, ENE-trending ribboned quartz-sulphide-carbonate veins hosted by Middle to Upper Cambrian sedimentary rocks, including black carbonaceous shales, and associated intrusive sills referred to as greenstones. The lodes are folded and boudinaged in places, a feature indicative of a pre-orogenic age with respect to the late Caledonian (Acadian) regional compressive deformation. Gold is of localized occurrence within the lodes, but the 'gold pockets' that have been found have proved to be of exceptionally high grade. Many fine specimens of visible gold have been recovered from these 'gold pockets' during the last 150 years and a comprehensive sample suite is preserved in the National Museum of Wales. In addition, the collection includes some large nuggets of alluvial gold found in the rivers of the area. Alluvial prospecting and mining formerly went on hand-in-hand with hardrock mining, although the accessible placers have long since been worked out. An unsuccessful application was made to work the less-accessible placer gold in the 1970s by dredging the sediments of the Mawddach estuary. It was rejected on environmental terms.The chief associates of gold in the Dolgellau Gold-belt are bismuth, silver and lead tellurides and galena (e.g. Clogau Mine) or galena and sphalerite (e.g. Gwynfynydd Mine). At some mines, the abundance of pyrite and arsenopyrite in the lodes made gold separation by amalgamation difficult. Within the same area weakly auriferous porphyry-type copper mineralization occurs in Coed-y-Brenin and a number of mineralized breccia-pipes are also present, some of which carry modest gold levels. Glasdir Copper Mine worked one such occurrence: although the overall grade of the ore was unspectacular the concentrates carried several grams of gold to the ton (Hall, 1990).
  • Rhiwnant Dome, SW of the Elan Valley, Central Wales: an area with a similar geological setting to Dolaucothi, small (<0.5 mm) gold grains occur in the stream sediments (Brown, 1993), although the bedrock source for the gold has not been identified.
  • Snowdonia, Gwynedd: in the Snowdon Caldera low-level geochemical gold anomalies are a feature of the copper-dominated vein mineralization (T.B. Colman, pers. comm.) and it is recorded (Bick, 1985) that at the Drws-y-Coed Mine sufficient gold was obtained during ore-processing to make a tiepin for the mine chemist.
  • South Wales Coalfield: collomorphic gold, 2-10 µm across, has been recorded with other ore-minerals on bedding-normal microfractures in Coal Measures strata from the Western Anthracite Zone of the coalfield (Gayer & Rickard, 1993).
  • Treffgarne, Pembrokeshire: a zone of highly altered Lower Ordovician volcanic and sedimentary rocks includes highly pyritiferous dark mudstones with geochemically enhanced gold levels detected in drillcore (Brown et al., 1987).

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. Annels, A.E. & Roberts, D.E., 1989. Turbidite-hosted gold mineralisation at the Dolaucothi Gold Mines, Dyfed, Wales. Econonic Geology, 84, 1293-1314.
  3. Bevins, R.E. & Mason, J.S., 1999. Welsh Metallophyte and metallogenic evaluation project: Results of a Minesite Survey of Clwyd. National Museums & Galleries of Wales, Cardiff.
  4. Bick, D.E., 1982. The Old Copper Mines of Snowdonia. Second Edition. Pound House, Newent.
  5. Bick, D.E., 1975. The Old Metal Mines of Mid-Wales. Part 2: The Rheidol to Goginan (Revised Edition). Pound House, Newent, Glos.
  6. Bottrell, S.H., 1986. The origin of the gold mineralization of the Dolgellau district, North Wales: the chemistry and role of the fluids. Unpublished Ph. D. thesis, University of East Anglia.
  7. Bottrell, S.H. & Spiro, B., 1988. A stable isotope study of black shale-hosted gold mineralization in the Dolgellau Gold Belt, North Wales. Journal of the Geological Society, London, 145, 941-949.
  8. Bottrell, S.H., Greenwood, P.B., Yardley, B.W.D., Shepherd, T.J. & Spiro, B., 1990. Metamorphic and post-metamorphic fluid flow in low grade rocks of the Harlech Dome, N. Wales. Journal of Metamorphic Geology, 8, 131-43.
  9. Brown, M.J., 1993. Exploration for gold in Central Wales. Mineral Reconnaisance Programme Open File Report 13, British Geological Survey.
  10. Brown, M.J. and others, 1987. Volcanogenic mineralisation in the Treffgarne area, south-west Dyfed, Wales. Mineral Reconnaissance Programme Report, British Geological Survey, No. 86.
  11. Calvert, J. F., 1853. The Gold Rocks of Great Britain and Ireland. Reproduced by Mining Facsimilies, 1988.
  12. Cooper, D.C. et al., 1990. Base-metal and gold mineralisation in north-west Anglesey, North Wales. Mineral Reconnaisance Programme Report 99, British Geological Survey
  13. Dean, A., 1845. Notice on the discovery of gold ores in Merionethshire, North Wales. Report of the British Association for the Advancement of Science (for 1844), 56
  14. Forbes, D., 1868. Researches in British Mineralogy. The London, Edinburgh and Dublin Philosophical Magazine and Journal of Science, 35, 171-184.
  15. Forbes, D., 1868. Researches on British Mineralogy. The London, Edinburgh and Dublin Philosophical Magazine and Journal of Science, 34, 329-354.
  16. Foster-Smith, J.R., 1973. The Non-Ferrous Mines of Denbighshire. Occasional Publication, 5.
  17. Foster-Smith, J.R., 1974. The Non-Ferrous Mines of Flintshire. Occasional Publication, 7.
  18. Gayer, R.A. & Rickard, D., 1993. Gold in South Wales coal. Nature, 364, 395.
  19. 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.
  20. Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
  21. Hall, G.W., 1993. Metal Mines of Southern Wales. 2nd Ed. Griffin Pubs., Herefordshire.
  22. Mason, J.S., 1994. A Regional Paragenesis for the Central Wales Orefield. Unpublished M.Phil thesis, University of Wales (Aberystwyth).
  23. 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.
  24. 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.
  25. Mason, J.S., Fitches, W.R & Bevins, R.E., 1999. Evidence for a pre-tectonic origin for the auriferous vein-type mineralisation in the Dolgellau Gold-belt, North Wales. Transactions of the Institution of Mining and Metallurgy (Section B, Applied earth science), 108, B45-B52.
  26. Platten, I.M. & Dominy, S.C., 1999. A re-evaluation of quartz vein history in the Dolgellau Gold-belt, North Wales, U.K. Geological Journal, 34, 369-391.
  27. Ramsay, A.C., 1854. On the geology of the gold-bearing district of Merionethshire, North Wales. Quarterly Journal of the Geological Society of London, 10, 242-247
  28. Readwin, T.A., 1888. Gold in Wales. London, 12pp.
  29. Readwin, T.A., 1860. The gold discoveries in Merionethshire. Transactions of the Manchester Geological and Mining Society, 2, 97-101.
  30. Readwin, T.A., 1862. The Gold-Bearing Strata of Merionethshire.
  31. Readwin, T.A., 1864. On the recent discovery of gold near Bala Lake, Merionethshire. Report of the British Association for the Advancement of Science (for 1862), 86-87.
  32. Smith, R.T., 1983. Geochemical surveys for gold in the Berwyn Hills. Mineral Reconnaisance Programme Open File Report 14, British Geological Survey.
  33. Smyth, W.W., 1862. Gold mining at Clogau, North Wales. Mining and Smelting Magazine, 1, 359-366.