Crystal System: Cubic
Status of Occurrence: Confirmed Occurrence - 1st UK recording
Chemical Composition: Gold-silver alloy with in excess of 20% Ag.
Chemical Formula: (Au,Ag)
Method(s) of Verification: all identifications optical means; Eaglebrook (Nantycagl) Mine - EMPA (Mason, 1998).
- Elements & Alloys
- Hydrothermal: mesothermal polymetallic veins
- Sedimentary: allogenic (detrital)
Thin veinlet (0.02 mm thick) of electrum (bright yellow) forming along a crack in arsenopyrite (slightly bluish-white). Deeper yellow, slightly tarnished chalcopyrite. Sample from No. 2 Shaft, Tyn Y Cornel Section, Clogau Mine. © J.S. Mason.
Electrum (bright yellow) forming a 50 µm long grain enclosed by galena (grey, very scratched) and tucekite (buff). The large deeper yellow area is chalcopyrite. Polished section of a sample from Eaglebrook Mine, Central Wales. © J.S. Mason.
Introduction: electrum occurs in gold deposits of various types, being particularly prevalent in epithermal assemblages in association with tellurides, quartz, carbonates and sulphides. It also occurs as a minor mineral in polymetallic low to medium-temperature hydrothermal veins, in association with quartz, carbonates, galena and other base-metal sulphides. In all occurrences, electrum is relatively easy to distinguish from gold due by its bright colour (lemon yellow when macroscopic, bright yellow-white in polished section). Polished sections of low-fineness electrum (i.e. with very high silver content) may tarnish black in time due to the reaction of the silver with atmospheric sulphur.
Occurrence in Wales: electrum is an important ore of gold in some mining districts worldwide but is relatively uncommon in Wales, where the gold deposits are dominated by high-fineness native gold with only small amounts of silver. However it has been recorded from the Dolgellau Gold-belt (Forbes, 1867; Readwin, 1888; Andrew, 1810) although these earlier references are borderline if the Au/Ag ratio of 80/20 is accepted as the defining minimum silver level. Gilbey (1968) recorded electrum from a number of Gold-belt mines, particularly in association with a Pb-Sb-Ag dominated assemblage. Recent work has confirmed the presence of electrum at a number of mines, notably Clogau, Ffridd-goch and Tyddyn Gwladys, in samples acquired by the National Museum of Wales from the J.W. Gilbey PhD Collection and from recent mineral exploration in the area, during which very rare grains of electrum have also been noted in heavy-mineral concentrates obtained by stream sediment sampling. Electrum has also been recently found to occur in the Central Wales orefield, in trace amounts, as an inclusion-forming mineral (Mason, 1994, 1997).
- Central Wales Orefield: electrum has been noted at Brynyrarian and Eaglebrook mines. It typically forms microscopic inclusions in galena associated with a phase of polymetallic mineralization and it is accompanied by an inclusion-forming assemblage including ullmannite and tucekite (Mason, 1994, 1997, 1998).
- Dolgellau Gold-belt, Gwynedd: widespread in small amounts although the bulk of gold production came from high-grade shoots of high-fineness native gold. At Clogau, and more rarely at Ffridd-goch Mine, electrum forms thin veins within cracks in shattered arsenopyrite, with associated galena, tellurides and chalcopyrite (Mason et al., 2002). Gilbey (1968) also notes minor occurrences at the Borth Valley and Foel Ispri mines. At Tyddyn-Gwladys Mine, very low fineness electrum (which readily tarnishes on exposure to light) forms microscopic inclusions in galena associated with tetrahedrite and pyrargyrite (Gilbey, 1968; Mason et al., 2002).
- Andrew, A.R., 1910. The geology of the Dolgelley gold-belt, North Wales. Geological Magazine, 47, 159-171, 201-221, 261-271.
- Forbes, D., 1868. Researches in British Mineralogy. The London, Edinburgh and Dublin Philosophical Magazine and Journal of Science, 35, 171-184.
- 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.
- Mason, J.S., 1994. A Regional Paragenesis for the Central Wales Orefield. Unpublished M.Phil thesis, University of Wales (Aberystwyth).
- 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.
- 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.
- 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.