Silver

Crystal System: Cubic
Status of Occurrence: Confirmed Occurrence
Distribution: Rare
Chemical Composition: Silver
Chemical Formula: Ag
Method(s) of Verification: Frongoch Mine - visual, the rapid development of a black tarnish is diagnostic

Geological Context:

  • Supergene: in situ natural oxidation & weathering deposits
Introduction: silver is a very common associate of lead in a variety of base-metal deposits. Many lead-mines around the world have produced significant silver as a by-product. However, in a great number of such cases the silver occurs as a chemical impurity in galena, or the galena carries microscopic inclusions of silver-bearing minerals such as tetrahedrite. Native silver is more restricted in its occurrence. The classic occurrence is in high-grade 'bonanza' deposits in which it abundantly occurs in association with other silver minerals, cobalt and nickel arsenides and uranium and bismuth-bearing minerals. The Alva Mine in the Ochil Hills, Central Scotland is a celebrated UK example of such an association. Silver also occurs, with native copper, in mineralization associated with basaltic lavas and it also accompanies gold in vitrually all types of gold deposits. Finally, it may occur in supergene mineral assemblages derived from the weathering of argentiferous base-metal deposits. Native silver, where sufficiently coarse grained, is readily recognisable by its malleability and its colour when fresh: however it readily tarnishes black, due to the reaction between the metal and atmospheric sulphur: the black tarnish is the silver sulphide, acanthite.
Occurrence in Wales: although the mineral native silver itself is rare in Wales, the metal has arguably had a greater role in the economic and political development of Wales than the country's other important precious metal, gold. During the 16th and 17th centuries it was extracted in great quantity from the richly argentiferous lead ores of Central Wales and a mint was established at Aberystwyth Castle for striking coins made from Welsh silver. The most celebrated silver-producers were the mines in the Darren-Goginan area, where tetrahedrite is a common associate of galena and silver grades frequently exceed 1 kg/tonne of concentrated ore (Hughes, 1990; Mason, 1998). However, it was extracted at lower grades from numerous lead-mines in Central, S, N and NE Wales as evidenced from post-1845 mineral production figures (Burt et al., 1986, 1990, 1992). The total produce remains unknown because detailed figures are not available for much of the time during which silver was obtained. However the fact that the above-referenced mineral statistics show that, in Cardiganshire and Montgomeryshire alone, 2.81 million troy ounces were declared in the post-1845 period. This suggests that the total Welsh production of silver runs into hundreds of tonnes. Silver also occurs associated with gold in the Dolgellau Gold-belt and at Dolaucothi, but the classic silver-bonanza (Ag-Co-Ni-As-Bi) association is unknown from Wales and native silver has only been reported from a single locality where it occurs as part of a supergene assemblage.

Key Localities:

  • Frongoch Mine, Devil's Bridge, Ceredigion: a single specimen of native silver forming minute (millimetre-scale) dendrites in cerussite was found at this site in 1992 (Green et al., 1996). This is the only known specimen from Wales.

References:

  1. Burt, R., Waite, P. & Burnley, R., 1992. The Mineral Statistics of the United Kingdom 1845-1913, he Mines of Flintshire and Denbighshire Volume 10, University of Exeter Press, 167pp.
  2. Burt, R., Waite, P. & Burnley, R., 1986. The Mines of Cardiganshire: Metalliferous and Associated Minerals 1845-1913. Volume 7, University of Exeter Press.
  3. Burt, R., Waite, P. & Burnley, R., 1990. The Mines of Shropshire & Montgomeryshire with Cheshire and Staffordshire: Metalliferous and Associated Minerals 1845-1913. Volume 9, University of Exeter Press.
  4. 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.
  5. Hughes, S.J.S., 1990. The Darren Mines. British Mining No. 40. Northern Mine Research Society, 131-141.
  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.