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. A lode at Castle-an-Dinas was mined from 1917 until 1958 and the mine became Cornwall's premier wolfram producer.
Castle-an-Dinas Mine records a fluctuating output until 1934, and a steady production of about 200 tons of concentrates per annum thereafter, up to the closure of the mine in 1958.
The value of tungsten for hardening steel was not understood until the late 1800's. Tungsten ores were up till then regarded as an impurity in tin concentrates and 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.
The demand for tungsten peaked during war-time, but restrictions on capital and supplies meant that Castle-an-Dinas Mine used hand-drilling and steam-powered machinery for much of its life. Today tungsten wire is used 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 other uses in the chemicals industry and in pigments.
The tungsten-bearing lodes are within outcrops of granite, and are often enclosed by a distinctive product of granite "greisen", in which original granite is replaced by a dark grey mass of mica and quartz. At several places, as, for example Cligga Head or St Michael's Mount, tungsten ores occur in sheeted vein complexes, while elsewhere, as in the Gunnislake district, the ores may be found in classical fissure veins.
The principal ore of tungsten is the iron/manganese tungstate, wolframite. 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.
Wolframite was difficult to from tin concentrates. Originally it was separated using the Oxland process, which involved roasting with sodium hydroxide, and subsequently leaching sodium tungstate. Later magnetic separation became the standard technique for the production of concentrates of Wolfram.
The ore potential south of the old workings has been estimated at about 1000 tonnes of recoverable tungsten. To the north the strike length of possible mineralisation is less predictable, but there is a belief that this area offers the better target for exploration.
Castle-an-Dinas
1916-1957: Cornwall's Premier Wolfram Mine
Tony Brooks