Silver - Lead - Zinc
Silver-mining was formerly an important part of the Welsh economy, particularly in the 1600s, when the mint at Aberystwyth produced coinage made from locally mined silver. Many millions of ounces of silver have been produced in Wales over the last four hundred years. But silver is very different in its occurrence to gold.
Unlike gold, silver in Wales does not occur as the native metal in anything except academic quantities, although in some parts of the world native silver has been mined in large amounts. In Wales, silver occurs associated with lead-ores, and was produced as a by-product of lead mining, at the smelting stage. So when you think that typical Welsh lead-ores contain 5-10 ounces of silver to the ton, and then think how many tons of lead-ores have been mined - nearly half a million in Central Wales alone - you can see how much silver has been produced! Because of this connection, and the fact that zinc-ores accompany lead-ores, the three metals can be treated as one topic. The silver-lead-zinc mining areas of Wales are numerous. There are mines in the Caerphilly-Llantrisant area of South Wales. Further north we have the major Central Wales ore field with Llywernog being the only silver mine open for visitors in the whole of Wales; N and E from this are the Llangynog mines while to the W of Llangynog we are back in the Dolgellau Gold-belt which produced some silver-lead-zinc ores. Further north again there is the important Llanwrst orefield and to the E of this in Clwyd the Halkyn-Minera orefield was a major producer. Then there are isolated but interesting localities such as the Llanengan mines on the Llyn Peninsula and Llanfyrnach and St Elvis mines down in Pembrokeshire.
All of these ore fields have one thing in common: they produced metals from mineral-veins. But the veins are of various types and occur in rocks of various ages. Also, the way in which the silver occurs varies from area to area and from vein-type to vein-type. Silver can occur in two ways in galena, the lead sulphide: either in "solution", as a chemical constituent of the galena, or as included grains within the galena of silver-bearing minerals. In Halkyn-Minera, Llanwrst and South Wales, the first mode of occurrence is the case. But in other areas, for example parts of Central Wales, it is the second mode of occurrence that is important. In Central Wales, an important and globally frequent association occurs which is of a mineral called tetrahedrite occurring in the galena. Tetrahedrite is a widespread mineral worldwide and is a sulphide of copper and antimony, but it can carry a whole range of other metals - including silver. In Central Wales tetrahedrite contains up to 20 % silver, so that it is easy to see why some of the ores were so rich here, compared to other areas of Wales.
Zinc mining only became important from the mid-19th century onwards, when technology had advanced enough to smelt efficiently the hitherto rather stubborn zinc-ore, sphalerite, and uses for zinc metal were becoming more widespread. Old underground workings, where sphalerite had been left in place, and spoil-heaps, where it had been thrown away, were subsequently reworked. From then on until the 1920s, when the industry started to peter out, mines produced all three metals.
The veins in which the three metals occur, sometimes accompanied by copper, are mineralised faults and fractures cutting the host-rocks. Faults are able to tap reservoirs of fluid deep in the Earth's crust: as such fluids rise up them, they eventually become unstable and lose their ability to carry dissolved metals and other substances. Then, the dissolved components precipitate out, crystallising as minerals.
The Central Wales veins, along with those in Llanwrst, Llangynog, Pembroke and the Llyn Peninsula, are hosted by Lower Palaeozoic (mainly Ordovician and Silurian) sedimentary (and sometimes volcanic) rocks, which were folded during the Caledonian Orogeny. The veins are thought to have been formed after the folding, in a broad period of time from the Devonian through the Carboniferous to the Permian Period.
Rocks, mainly limestones, of Carboniferous to Jurassic age, host the Halkyn-Minera and South Wales veins. At Ogmore, on the S Wales coast, evidence for mineralising fluids having reached surface in Lower Jurassic times is well exposed in a coastal section that is a Site of Special Scientific Interest.top