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Esgair Hir & Esgair Fraith mines (Cambrian Mine; Cardigan Consolidated Mine; Welsh Potosi Mine; Kylon Potosi Mine), Nant-y-Moch Reservoir area, Talybont, Ceulanymaesmawr, Ceredigion (Dyfed; Cardiganshire), Wales, UK
Extensive history is given in British Mining No. 22, Northern Mines Resarch Society and in UKJMM No. 5, 1988.
History:
Esgair Hir and Esgair Fraith mines was usually worked in conjunction as they where both on the same vein. Esgair Fraith is where most of the copper production came from in the output figures. The first record of activity was in 1691 ( though the vein was known of before this date) when it was developed by Sir Carbery Pryse (died 1704), the mine continued to be worked until 1708, with a recorded production of over 2000 tons of lead ore 1702-08. There is circumstantial evidence that Esgair Fraith was being worked about this time. Later some work was undertaken in 1760 when a level was driven under the old workings, and a little work was done in 1788. It was not until 1839 that the mine was reopened, this lasted to 1849, with only 321 tons of lead ore recorded. About 1850 the mine was reopend and although 1353 tons of lead ore was produced for no profit, it closed again in 1857. The mine was back at work the following year, which lasted untill 1868 and produced 700 tons of copper ore and 72 tons of lead ore. The mine was to be worked on at least 6 more occasions, which finally ended in 1904. Records would appear to indicate that the ore shoots were vertical pipe shaped, maybe only producing a few hundred tons each. The rest of the ore was probably as a dissemination through the vein fault system, if the dump material is any thing to go by. The lead ore was also said to be rich in silver, but recent tests (dump material?) have only indecated 3.06 ppm; while sphalerite was recorded with 163.2 ppm and copper ore with 57.8 ppm.
Recorded production (the figures from 1702-08 have not been included):
2792 tons of lead ore, and 2670 tons of copper ore, which is belived to fall short of the actual output. There are no production figures for silver extracted from the ore other than that the average yield was 7oz per ton.
The mine is also of some significance as it was possibly used to break the monopoly of the crown on Mines Royal in 1692. The mines came under the crown ownership if the Ag/Au content of the ore was more than the cost of mining and smelting. Sir Cadbury Pryse was in dispute with the Mines Royal agent over the silver content, the agent saying that the ore from Esgair Hir contained 60lbs per ton of lead ore, whilst Sir Cadburys assayer claimed that the ore only contained a few pounds.
Geology: Ref UKJMM No. 5, 1988
Esgair Hir and Esgair Fraith developed on a 2km stretch of an east-west trending mineralised fracture system, which has been partially traced over a lenth of 4km. The vein system intersects a number of north-south trending anticlines and synclines in mudstone and siltstone of lower and middle Llandovery age, (440 million years). Most of this is in bedded mudstone of the Devil's Bridge & Cwmere formation between the Plynlimon and Machynlleth inliers (Ordovician). In the Devil's Bridge formation the vein is quartz dominant whilst further to the east, at Esgair Fraith mine in the Cwmere formation, ferroan dolomite predominates. An increase in copper content and corresponding decrease in lead is noted at Esgair Fraith.
The Esgair Hir-Esgair Fraith fault is associated with other east-west fault systems in the Mid-Wales ore field. The Faults are believed to be related to a period of crustal tension in early Carboniferous times some 350 million years ago.
Early researchers were of the opinion that the mineralisation was associated with deep seated magmatic phenomena. More recently research attributes the detailed mechanism to interstitial brines. Fractures were propagated by the hydraulic effect of the mineralising fluids, each period of fracture extension being followed by a sudden drop in pressure. The wall rocks under cosiderable lithostatic and pore water pressure imploded into the fluid. Minerals were deposited around the resultant angular rock clasts giving rise to the breccias which are a conspicuous feature of many mines in Mid-Wales.
Fracturing followed the lines of mechanical weakness, including post-folding relaxation joints. At Esgair Hir it is recorded that the ore-bodies were pipe shaped reaching up to 120m deep, 25m long and 2m wide within a brecciated zone up to 18m wide; and tended to occur in areas where the fracture zone changed direction; this may have reflected an element of transverse movement on the fracture zone, which would form open spaces suitable for ore deposition at bends in the fault system.
Mineral List
57 entries listed. 54 valid minerals.
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