Hemerdon Mine
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Hemerdon Mine, alternatively known as the Hemerdon Ball or Hemerdon Bal Mine[1], is a historic tungsten and tin mine. It is located 7 miles NE of Plymouth, near Plympton, in Devon, UK. It lies to the north of the villages of Sparkwell and Hemerdon and adjacent to the large china clay pits near Lee Moor. The mine, out of operation since 1944, hosts one of the largest tungsten and tin deposits in the world.[2][3]
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[edit] Geology
The Hemerdon Ball granite is an outlying cupola intrusion surrounded by Devonian slates, known regionally as killas. Fractures in the granite and killas have been penetrated by mineralising fluids containing metallic in the area around the Hemerdon mine, forming.[4] Two types of vein are discernible with three different orientations. Quartz and quartz-feldspar veins form a stockwork with minor mineralisation, whilst greisen bordered veins are found in a sheeted vein system with wolframite and minor cassiterite mineralisation.[5]
The mineralisation begins at surface and extends to depths of at least 400 metres. The vein system is hosted in a dyke like granite body, extending from the Hemerdon Ball towards the Crownhill Down granite. It is flanked by killas, formed by contact metamorphism, which also contains veins although wolframite and cassiterite is found as a lower percentage of the rock bulk. Kaolinisation occurs to depths of up to 50 metres in the granitic body.[6]
The locality is renowned for its high quality scorodite specimens, which are among the best in Europe. Pharmacosiderite, Cassiterite and Wolframite of specimen quality have also been recovered from the mine. Scorodite and Pharmacosiderite are secondary arsenate minerals, that form in the upper oxidation zones of ore bodies. They are formed from alteration of arsenopyrite, and are found in the weathered zone of the deposit. At depths beneath the existing pit it is likely they will become scarce.[7]
[edit] History
[edit] 1867-1959
The Hemerdon tungsten-tin deposit was discovered in 1867.[8] In 1916, due to war associated tungsten shortages, an exploration and development program was initiated, which outlined a tin-tungsten stockwork suitable for opencast extraction. In 1917, Hemerdon Mines Ltd decided to construct a 140,000 tonne per year mill, and shortly afterwards opencast ore mining operations began. The mine was operated in 1918-1919, during which time it processed 16,000 tonnes of ore. When the British government stopped accepting tungsten ores under the war pricing scheme the mine was forced to suspend mining operations.[9]
Several attempts were made to establish a higher and stable price for tungsten from the government, including an application supported by Winston Churchill for recognition of wolfram mining as a key industry. [10] However after further price decreases, milling operations were suspended and the mill components were sold off.[11] In 1934 increased tungsten prices resulted in renewed prospecting of the deposit, along with metallurgical testwork. In 1939 further shortages of tungsten due to WW2 led to Hemerdon Wolfram Ltd constructing a 90,000 tonne per year mill with 55% wolfram recovery, which began operation in 1941.[12]
The Ministry of Supply carried out extensive evaluation of tungsten deposits in the UK, and it was concluded by 1942 that Hemerdon offered the most potential for producing tungsten on a large scale.[13] The government took over the mine from Hemerdon Wolfram Ltd. A resource of 2.5 million tonnes of 0.14% tungsten trioxide in addition to tin was outlined, and a new plant was hastily constructed.
The new plant took over operation from the old plant in 1943, and theoretically should have been able to treat over 1 million tonnes per year; however labour shortages and mechanical faults resulted in a much lower production. Ore output from a mixture of underground and opencast mining methods was documented as over 200,000 tonnes, with a resulting 180 tonnes of tin/tungsten concentrate during the period of government operation.[14] Operations ceased in June 1944 due to access to overseas supplies being restored.[15]
The plant was kept in place after the war, and the government was rumoured to have planned restarting production during the tungsten shortages associated with the Korean War.[16] However, nothing came of this and following the Westwood Report in 1956, the government decided to seek a private partner to move the mine’s development forwards. After further decreases in the tungsten price, resulting in the closure of the Castle-an-Dinas Mine tungsten mine in Cornwall, the government sold off all the plant in 1959.
[edit] 1960-2006
However in the mid-1960s work on the prospect was recommenced by British Tungsten Ltd, owned by Canadian entrepreneur W.A.Richardson. In 1969 a planning application for opencast working of tin, tungsten and china clay was submitted, but it was withdrawn before a decision could be made. Further work commencing in 1970 by British Tungsten Ltd increased the resource to 5.6 million tonnes of ore.[17][18]
The leases were transferred to Hemerdon Mining and Smelting Ltd in 1976. They initiated a drilling program shortly before they entered a joint venture to develop the project with international mining firm AMAX in 1977.[19] An extensive exploration programme costing in excess of $10 million was completed between 1978 and 1980.[20] By the end of 1978, deeper drilling enlarged the resource size to 20 million tonnes of ore. In 1979 this was expanded to 45 million tonnes.[21] At the end of the exploration programme in 1980, over 14,000 metres of diamond drilling had been undertaken, outlining a resource of 0.17% tungsten trioxide and 0.025% tin over 49.6 million tonnes.[22]
Bulk sampling of the deposit using an underground drift for ore, and a pilot HMS and Gravity plant for processing, was undertaken in 1980.[23] On average recoveries of around 65% were made, although in excess of 70% was achieved.[24] The final revision of the mining feasibility study concluded in 1982 that a within a global resource of 73 million tonnes of ore, at grades of 0.143% tungsten trioxide and 0.026% tin, there was an in pit reserve of 38 million tonnes, at grades of 0.183% tungsten trioxide and 0.029% tin. [25]
The venture was joined by Billiton Minerals Ltd, providing further finance and expertise, and forming a consortium that planned to commence production in 1986. [26] The initial planning application was made in 1981, but a public enquiry and ‘calling in’ of the application by the Secretary of State resulting in an initial refusing of the application in 1984.[27] This resulted in Billiton Minerals Ltd pulling out of the consortium.[28] Hemerdon Mining and Smelting Ltd also sold their 50% stake in the project to AMAX.[29] After making a revised application, permission was finally obtained in 1986. [30] By then a collapse in both the tin and tungsten prices had damaged the economic feasibility of making an investment in opening the mine. Its tungsten assets were passed onto to a newly formed holding company; Canada Tungsten Ltd, in 1986. [31]
Canada Tungsten implemented the planning permission that was obtained in 1986, and kept the project in its portfolio of prospects for many years. Before AMAX was sold to Phelps Dodge, it gradually transferred Canada Tungsten into the ownership of Aur Resources. In 1997, a new company, North American Tungsten plc, purchased all the tungsten assets from Aur Resources, and was listed with the aim of reopening the Cantung mine, and developing the Hemerdon and Mactung prospects.[32]
However during a review of peripheral assets in 1999, it decided that with the depressed prices of tungsten, the Hemerdon prospect was not central to its future. With upkeep costs of in excess of C$150,000 per annum, almost a third of the company’s annual costs, attempts were made with the mineral rights holders to reduce fees. The negotiations were unsuccessful and therefore during 2000, two of the three mineral rights were surrendered.[33] To further reduce costs, it disposed the remaining assets of the Hemerdon prospect in 2003. [34]
[edit] 2007-present
Sustained tungsten metal price rises resulted in an increase of the Ammonium paratungstate (an intermediate product of tungsten) price by five times, from around $60 per STU in 2003, to in excess of $240 per STU from 2006.[35] This has resulted in increased tungsten mining exploration and development activities globally since 2005.[36]
In June 2007, ASX listed specialty metal exploration and development company, Wolf Minerals Limited, suspended trading of shares pending the acquiring of mineral leases.[37] On December 5, 2007 trading recommenced following the public announcement of acquiring the mineral leases for the Hemerdon Mine project. The mineral leases were made for a period of 40 years, with the Hemerdon Mineral Trust and the Olver Trust. An agreement with Imerys to purchase remaining mineral rights and freehold land was also made.[38]
SRK Consulting were commissioned to produce a JORC-compliant resource using previous drilling data.[39] This was released in March 2008, and is shown in the table below. At the lowest cut-off grade of 0.1% WO3 the resource of over 259,000 tonnes of tungsten makes Hemerdon the 4th largest tungsten deposit in the world.[40] A feasibility study is underway, and completion is planned for December 2008. It is anticipated that mining operations could commence in 2010. [41]
| Hemerdon Ball Maiden JORC Resource estimate completed by SRK Consulting in March 2008 | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| W Cut-Off (%) | WO3 Cut-Off (%) | Resource Category | Ore Tonnage (Mt) | Sn grade (%) | W grade (%) | WO3 grade (%) | Contained Sn (tonnes) | Contained W (tonnes) | Contained WO3 (tonnes) |
| 0.08 | 0.101 | Inferred | 205.674 | 0.021 | 0.126 | 0.159 | 43192 | 259149 | 327022 |
| 0.10 | 0.126 | Inferred | 126.093 | 0.022 | 0.150 | 0.189 | 27740 | 189139 | 238316 |
| 0.12 | 0.151 | Inferred | 81.800 | 0.022 | 0.172 | 0.216 | 17996 | 140696 | 176688 |
| Source: [42] | |||||||||
[edit] References
- ^ Mindat online database
- ^ Mineweb Article
- ^ Tungsten and tin mine to reopen, BBC news, Tuesday, 4 December 2007, 09:12 GMT
- ^ Devon County Council Register of Geological Sites
- ^ CSM Virtual Museum Field Excursion
- ^ Wolf Minerals
- ^ Crystal Classics Article
- ^ Devon CMLP – IP38,Devon County Council
- ^ , Cornwall’s Premier Tungsten Mine with brief comparative histories of other Wolfram Mines in Cornwall & West Devon, Cornish Hillside Publications, 2001, p128
- ^ Statement by WSC, Churchill Archives Centre
- ^ Terrell E. The Hemerdon Wolfram-Tin Mine, Mining Magazine February 1920, p75-87
- ^ Dines HG, The Metalliferous Mining Region of SW England, HMSO, 1956, p689
- ^ Cameron J, The Geology of Hemerdon Wolfram Mine, Devon, IMM, Oct 1951, p1
- ^ Cameron J, The Geology of Hemerdon Wolfram Mine, Devon, IMM, Oct 1951, p121
- ^ Tungsten Mineral Resources Consultative Committee, HMSO, 1973, p8.
- ^ Mining Journal, November 24, 1950, p504
- ^ Perkins JW, Geology Explained: Dartmoor and the Tamar Valley, p71, 1972
- ^ US Bureau of Mines, Minerals yearbook metals, minerals, and fuels 1970
- ^ US Bureau of Mines, Minerals yearbook area reports: international 1977
- ^ Amax-Hemerdon Venture Evaluation Major Tin-Tungsten Property – Skillings Mining Review, vol.69, No.23, June 7, 1980
- ^ US Bureau of Mines, Minerals yearbook area reports: international 1979
- ^ US Bureau of Mines, yearbook metals and minerals 1980
- ^ US Bureau of Mines, Minerals yearbook metals and minerals 1980
- ^ US Bureau of Mines, yearbook metals and minerals 1980
- ^ Feasibility Study for a Mine and Concentrator Complex at Hemerdon Nr Plymouth, England. Produced for AMAX/HMSL Joint Venture, February 1982
- ^ US Bureau of Mines, Minerals yearbook area reports: international 1983
- ^ US Bureau of Mines, Minerals yearbook area reports: international 1984
- ^ US Bureau of Mines, Minerals yearbook area reports: international 1985
- ^ Mining Journal, April 19, 1985
- ^ Devon CMLP – IP38,Devon County Council
- ^ US Bureau of Mines, Minerals yearbook metals and minerals 1986
- ^ Press Release October 16, 1997, North American Tungsten Corporation Ltd
- ^ Long Form Prospectus, North American Tungsten Corporation Ltd, May 14, 2002
- ^ North American Tungsten Financial Statement 2005
- ^ Tungsten Price Graph
- ^ Tungsten AIMR Report 2007
- ^ Wolf Minerals Press Release
- ^ Wolf Minerals Press Release
- ^ Wolf Minerals Press Release
- ^ , ITIA Newsletter Dececember 2006, p9
- ^ Wolf Minerals Presentation
- ^ Wolf Minerals ASX Press Release
