Geology and Mineralisation
Central Wales consists of a large area of sedimentary rocks of Upper Ordovician to Middle-Silurian age. At Llywernog the rocks are Lower Silurian, which makes them somewhat less than 430 million years old! The rocks vary from slates and shales through to siltstones and grits. They were deposited underwater in an extensive sea-basin that lay on the southern edge of the great ocean known by geologists as Iapetus, on whose opposite shores lay Scotland and Northern Ireland!
At the end of the Silurian Period, continental collision occurred when the two opposite sides of Iapetus came together. Over millions of years, the old seabed was squashed and forced upwards into a range of mountains. The Cambrian Mountains of Mid-Wales are the eroded remnants of part of this mountain range. So when you walk through the underground tunnels at Llywernog, you are entering a seabed over 400 million years old!The mineral deposits of Central Wales are grouped together and termed the Central Wales Orefield. Within this area, over 450,000 tons of lead-ore, 140,000 tons of zinc ore and 2,500,000 ounces of silver have been produced, since records began in the 1840s.
Since we now know that mining went back as far as the early Bronze Age, it is fair to say that the above figures are merely an unknown percentage of the true total!In Central Wales, lead and particularly silver attracted much interest in the 17th century, and there was a mint at Aberystwyth that made coins from locally produced silver. Later on, in the mid-19th century, there was an unprecedented surge of mining activity with literally dozens of mines at work. By then, zinc had also become important and previously discarded zinc ores were reworked. The area has also produced a modest quantity of copper plus some barytes and iron sulphides - the latter sold for sulphuric acid manufacture.
Mining finally died out in the early 20th century, and since then, Nature has worked to reclaim the sites back to wilderness. However, some excellent remains still survive - and at Llywernog, one of the finest collections of mine-buildings anywhere in the area was restored in a project that has grown steadily from its beginnings in January 1974, when the first working parties descended on the site to begin their task.Llywernog mine included two principal groups of workings. These were at times worked separately and at times in conjunction. On the southern side of, and close to, the main A44 road lies the group of workings known as Powell’s mine, while to the north of the A44 lie the more extensive remains of the Poole’s mine that in now the Llywernog Silver-Lead Mining Mine and Museum.
Poole’s mine consists of one main shaft, the Engine Shaft, through which the ores and waste-rock were drawn and water pumped. It was sunk to a depth of 72 fathoms, or well over 400ft. Although this sounds deep, it is very much less than the depths to which some Cornish mines were finally sunk (3000 feet and more). Central Wales’s mines were generally quite shallow with the deepest shafts approaching 1050 feet in depth.In the mine there were two principal lodes. A lode is a steeply inclined (or often vertical) vein or seam of minerals that may or may not include valuable ores. The position of one lode may be clearly seen in the course of the underground tour that you can take during your visit. The vertical, slot-like cavern hewn out of the rock that you will walk through is the area where one of the lodes was found to be ore bearing and was mined away. Such slot-like excavations are called stopes and on the walls of this stope, remnants of the mineralisation may still be seen. The lodes at Llywernog mine are mineralised geological faults - literally cracks in the upper part of the Earth’s crust through which mineralising fluids have passed.
Mineralising fluids can be very variable but in general are hot, briny waters laden with dissolved metals and other substances that are forced upwards through the Earth’s crust under pressure. When they enter a favourable chemical and physical environment, in which drops in pressure and temperature play an important role, the dissolved metals can no longer stay in solution and precipitate out as ores and other minerals.It has been estimated, by a combination of geological, mineralogical and geochemical research, that the lodes of Llywernog were formed between the early Carboniferous and Permian periods.
As at many Central Wales mines, there is evidence that the faults hosting the mineral lodes were active repeatedly, so that new pulses of mineralising fluid would come up and deposit new bands of minerals to add to those already there. It is fair to say that each such episode would have been noticeable at surface because of the violent earthquakes that would have accompanied the fault movement.The chief ore sought by the Llywernog miners was galena, lead sulphide. Galena was a very valuable mineral, as it contains 80% lead and, as at Llywernog, several ounces of silver in each ton of ore. Galena is a very distinctive mineral. The most striking features are its heavy weight and its beautiful, glittering metallic silvery-grey colour when freshly broken.
The other common ore at Llywernog is known by several names. Its mineral name is sphalerite, but to the miners it was variously known as black-jack or zinc-blende. Sphalerite is zinc sulphide, with varying amounts of iron. Pure zinc sulphide is white, but naturally occurring sphalerite is most commonly brown in colour. Iron-rich sphalerite is black. A freshly broken piece reveals the sparkly brown interior, but when it weathers the iron turns to a rusty colour.
Some other ores have been recorded at Llywernog, particularly during the work done to open the underground part of the mine. The iron sulphide, marcasite, has been found in places. It is yellowish-white and metallic in appearance when freshly broken, but soon weathers and decays in damp air. There was a lot of marcasite in the veins worked at Ystumtuen, to the south of the Llywernog mines, and the product of marcasite decay, yellow-ochre, was at one time collected there for use as a pigment. The golden-yellow ore of copper, chalcopyrite, a sulphide of iron and copper, also occurs at Llywernog but not in commercial quantities. Finally, the very rare mineral ullmannite, a sulphide of nickel and antimony, has been found as very bright metallic grains, but only a few specimens are known.
Gold occurs at Llywernog, but only in what we would call academic amounts - less than a quarter of a part per million - or a fraction of one gramme per ton of ore. Gold levels are low, like this, throughout Central Wales. The gold-mining areas of Wales lie to the north, in the Dolgellau Gold-belt, and to the south, at Dolaucothi.With the ore-minerals, there are accompanying waste or gangue (pronounced “gang") minerals. Quartz (silica) is the commonest, and may be seen underground. It forms crusts of small, colourless, pyramidal crystals. Calcite, calcium carbonate, also occurs in places. The ores were either picked from the gangue by hand, or, where they were mixed together, the entire content of the lode was crushed and milled to separate them.
Llywernog was a modest producer of lead , silver and zinc. The group of mines worked under this name yielded, since 1845, 3813 tons of galena, 560 tons of sphalerite and 4260 ounces of silver, according to the official returns. However, silver returns were only filled in certain years and it has been estimated that the galena produced actually contained nearly 15000 ounces of silver!