Castle-an-Dinas Wolfram Mine, Cornwall

Castle-an-Dinas Mine, Cornwall

 

Castle-an-Dinas Mine, near St Austell in Cornwall, it was the only mine in the country where wolfram was the sole ore produced. Wolfram and scheelite are the chief ores of tungsten.

The value of tungsten for hardening steel was not realised until the end of the nineteenth century. From then, wolfram, the ore of tungsten, was sought after. Some existing Cornish tin mines produced wolfram as a by-product and many new mining operations were set up. A lode at Castle-an-Dinas in mid-Cornwall was mined from 1917 until 1957 and the mine became Cornwall's premier wolfram producer.

The demand for tungsten peaked during war-time, but restrictions on capital and supplies meant that 'Castle' mine used hand-drilling and steam-powered machinery for much of its life.

This book tells how wolfram was mined and processed at Castle-an-Dinas including reminiscences of men who worked there. The book also reviews many other wolfram mining operations in Cornwall and west Devon.

A soil survey over Devonian slates to the south of Castle-an-Dinas wolfram mine produced anomalies indicative of at least two sub-parallel zones of W veining and a broad area of anomalously high Sn values. Percussive drilling confirmed widespread Sn mineralisation beneath the soil anomaly, but the in situ W mineralisation was confined almost entirely to one zone, which can be correlated with the Wolfram Lode in the mine. To the north of the former workings three sets of traverses were also sampled from percussive drill-holes. Two zones of W-Sn mineralisation—sometimes with Cu—were located, one correlatable with the Wolfram Lode and the other sub-parallel and some 90 m to the west. Close to the surface these lode extensions are sub-economic, but it appears that viable ore grades are located in the metamorphosed slates within about 200 m of the contact with the small granite outcrop at Castle-an-Dinas. The ore potential south of the old workings can be estimated at about 1000 tonnes of recoverable tungsten. To the north the strike length of possible mineralisation is less predictable, but there is little doubt that this area offers the better target for exploration.

Tungsten ores were formerly regarded as an impurity in tin concentrates and, until the middle of the 19th Century were treated as waste. After the recognition of the properties of tungsten metal as an additive to alloys and steel, there was a demand for tungsten mineral concentrates and production from a number of Cornish mines. Dines records a fluctuating output between 1910 and 1934, and a steady production of about 200 tons of concentrates per annum thereafter, up to the closure of the main producer, Castle-an-Dinas Mine in 1958. There is no production of tungsten ores in Cornwall at the present day.

ungsten ores were formerly regarded as an impurity in tin concentrates and, until the middle of the 19th Century were treated as waste. After the recognition of the properties of tungsten metal as an additive to alloys and steel, there was a demand for tungsten mineral concentrates and production from a number of Cornish mines. Dines records a fluctuating output between 1910 and 1934, and a steady production of about 200 tons of concentrates per annum thereafter, up to the closure of the main producer, Castle-an-Dinas Mine in 1958. There is no production of tungsten ores in Cornwall at the present day.

Tungsten ores occur at many localities in Cornwall, normally in vein deposits, and commonly in association with tin. The tungsten-bearing ore bodies are generally within, or very close to, outcrops of granite, and are often enclosed by a distinctive alteration product of granite "greisen", in which original granite is replaced by a dark grey mass of mica and quartz. At several localities, as, for example Cligga Head or St Michael's Mount, tungsten ores occur in sheeted (i.e. multiple and parallel) vein complexes, while elsewhere, as in the Gunnislake district, the ores may be found in classical fissure veins.

Tungsten finds use alloyed with steel and other metals to impart hardiness and as tungsten wire in the manufacture of electric light filaments. Tungsten carbide is widely used in the manufacturing of cutting tools and dies, and in powdered form as an abrasive. Tungsten compounds find various minor uses in the chemicals industry and in the manufacture of pigments.

The principal ore of tungsten is the iron/manganese tungstate, wolframite. Calcium tungsten, or scheelite, also occurs but is not important as an ore in Cornwall. Wolframite occurs in veins together with quartz, some cassiterite may also be present in variable proportions. Small amounts of sulphides and arsenic minerals also occur in tungsten veins, but are usually minor.

Wolframite was formerly separated from tin concentrates using the Oxland process, which involved roasting with sodium hydroxide, and subsequently leaching sodium tungstate. Later, use was made of the magnetic susceptibility of wolframite, and magnetic separation became the standard technique for the production of concentrates.

Castle-an-Dinas 1916-1957: Cornwall's Premier Wolfram Mine with Brief Histories of Other Tungsten Ore Mines in Cornwall and West Devon
Tony Brooks

 

Mines on the North Cornwall Coast