Tungsten

Tungsten's many alloys have numerous applications, most notably in incandescent light bulb filaments, X-ray tubes (as both the filament and target), electrodes in TIG welding, and superalloys. Tungsten's hardness and high density give it military applications in penetrating projectiles. Tungsten compounds are most often used industrially as catalysts.

Wolframite - (Fe,Mn)WO₄

Photo: Parent Géry

Tungsten News

Tungsten price: 12 months

PRICING

In recent years, trade in concentrates has diminished and the market has relied more and more upon the APT quotation as a price guide since APT is the product traded in the largest quantity. Industry prices are mainly based on the quotations published twice a week by London’s "Metal Bulletin", although other trade journals also publish quotations or indicative prices.

This graph is based on the quotations published by "Metal Bulletin" to which acknowledgement is due.  The US APT quotation (stu) and the FeW quotation have been converted to mtu of WO₃ to facilitate price comparisons and the annual averages have been calculated by ITIA.

Notes:

• A metric ton unit (mtu) is 10kg
• A metric ton unit of tungsten trioxide (WO₃) contains 7.93kgs of tungsten
• A short ton unit (stu) is 20 pounds

SUPPLY AND DEMAND

World tungsten demand is growing at an annual rate of approximately 4-5 percent. World supply declined from 90 800 metric tonnes in 2006 to 72 000 tonnes in 2013.Tungsten is one of few metals to have withstood the recent metals bear market China produces approximately 85 percent of world tungsten supply but has greatly restricted tungsten exports to keep pace with its own industrial demand.

That there is plenty of tungsten in the ground is not in doubt but some of the biggest deposits are in the areas where access is difficult, or have a low ore grade, making the long-term view of tungsten prices the governing factor in determining their economic viability.

 List of base metal stocks with latest financial data

Rank	Country	World Concentrate Production, By Country (Metric tons, tungsten content)
1	China	51,000
2	Russian Federation	2,500
3	Canada	1,964
4	Bolivia	1,023
5	Austria	900
6	Portugal	900
7	Thailand	600
8	Peru	502
9	Brazil	500
10	Rwanda	499
11	Spain	260
12	Burundi	190
13	Congo, The Democratic Republic Of The	170
14	Korea, Democratic People's Republic Of	100
15	Myanmar	87
16	Mongolia	39
17	Australia	33
18	Uganda	20

Source: United States Geological Survey (USGS) Minerals Resources Program

Year of Estimate: 2009

Production in 2010

Not only have the sources of supply altered but so have the tungsten compounds traded, as fluctuating price differentials between concentrate and upgraded products and governmental restrictions played their part in the market.  Intermediate products include tungstates, tungsten oxides and hydroxides, W and WC powders, and ferrotungsten.

Supply and Demand

This graph compares the ITIA estimates for the production of concentrate over the last 10 years with demand, calculated by the addition of imports of concentrates (ex world) as well as imports of intermediate products from China and Russia.  As supply and demand have largely been in balance over this period, production has been supplemented by releases from stocks.  Actual consumption, including recycled material , is much more difficult to assess.

GEOLOgy

Tungsten ranks number 56 of the elements in terms of earth crust abundance and number 18 among the metallic elements.

A main factor determining the location of tungsten deposits is the proximity to orogenic belts because of a marked association between tungsten deposits and those mountain belts as, for example, in the Alpine-Himalayan and the Circumpacific Belts.

In particular, major tungsten deposits occur in the fold belts of the Far East, in South China, Thailand, Burma, South Korea and Japan. Rich ore deposits are situated in the Chinese provinces of Jiangxi, Guangdong  and Hunan, where all important types of ore deposits can be found (amongst others, the largest scheelite mine in the world: Shizhuyuan).  In these areas even basaltic rocks contain higher concentrations of tungsten, which has been interpreted by Chinese researchers as an anomaly of the underlying mantle. Similar belts extend throughout the Asiatic part of the former Soviet Union.  Tungsten is also present in the eastern costal fold belt in Australia and the Alpine belt from France to Turkey. The Rockies and Andes as part of the Circumpacific belt contain a number of important deposits in Canada, USA, Bolivia, and Peru.  Ore deposits in Africa (Rwanda, Uganda and the Republic of Congo) and Brasil (Rio Grande del Norte, Rhodonia) indicate a common ore formation history of Sn/W deposits before the opening of the Atlantic Ocean (ie separation of Africa and South America) about 100 million years ago.

Minerals and Deposits

Tungsten occurs in nature only in the form of chemical compounds.  Although more than thirty tungsten-bearing minerals are known, only two of them are important for industrial use, namely wolframite and scheelite.

Pure scheelite has blue-white fluorescence in ultraviolet light, a property which is utilised in prospecting.  Wolframite is a general term for iron and manganese tungstates where the iron/manganese ratio can vary.  A mineral with more than 80% FeWO₄ is called ferberite and a mineral with more than 80% MnWO₄ is called hübnerite.

Wolframite - (Fe,Mn)WO₄

Photo: Parent Géry

Scheelite - Ca(WO₄)

All tungsten deposits are of magmatic or hydrothermal origin. During cooling of the magma, differential crystallisation occurs, and scheelite and wolframite are often found in veins where the magma has penetrated cracks in the earth's crust. Most of the tungsten deposits are in younger mountain belts, ie the Alps, the Himalayas and the circum-Pacific belt.

The concentration of workable ores is usually between 0.3 and 1.0% WO₃.

Tungsten Reserves

Reserves are defined by the USGS, as "That part of the reserve base which could be  economically extracted or produced at the time of determination.  The term reserves need not signify that extraction facilities are in place and operative.  Reserves include only recoverable materials". But different countries use different definitions and, furthermore, what is deemed "could be economically extracted" will also differ between countries. So the estimated tonnages should be treated with caution. 

Tungsten Resources

China is the major producer of primary tungsten. The other principal producing countries are Austria, Bolivia, Canada, Peru, Portugal, Russia, Thailand and several countries in Africa. Some mines which have closed in recent decades in Australia, South Korea and the USA are now considering re-opening. In addition, several new projects were started recently for exploration and exploitation worldwide, although the economic crisis towards the end of 2008 brought many to a stop.

Cantung Tungsten Mine, Yukon, Canada

Source: http://www.natungsten.com/s/Home.asp

Tungsten mining and beneficiation

Tungsten is usually mined underground. Scheelite and/or wolframite are frequently located in narrow veins which are slightly inclined and often widen with the depth. Open pit mines exist but are rare.

Tungsten mines are relatively small and rarely produce more than 2000t of ore per day.  Mining methods for tungsten ore are not at all exceptional and usually are adapted to the geology of the ore deposit.

Most tungsten ores contain less than 1.5% WO₃ and frequently only a few tenths of a percent. On the other hand, ore concentrates traded internationally require 65-75% WO₃. Therefore, a very high amount of gangue material must be separated.  This is why ore dressing plants are always located in close proximity to the mine to save transportation costs.

The ore is first crushed and milled to liberate the tungsten mineral crystals.  Scheelite ore can be concentrated by gravimetric methods, often combined with froth flotation, whilst wolframite ore can be concentrated by gravity (spirals, cones, tables), sometimes in combination with magnetic separation. 

Tungsten concentrates

Marketable ore concentrates of scheelite and wolframite (hübnerite, ferberite) contain typically 65 – 70 % WO₃.  Off-grade concentrates having lower WO₃ concentrations are seldom on the market and if so, only for a lower price.  In fully integrated companies lower grades (6-40% WO₃) are often preferred, because upgrading to high concentrations is mostly combined with a lower yield.

Concentrates are packaged either in large polyethylene “big bags” (1 to 2 tons) or, alternatively, in steel drums and individual packaging (40 to 200kg).

Tungsten processing

Modern processing methods dissolve scheelite and wolframite concentrates by an alkaline pressure digestion, using either a soda or a concentrated NaOH solution. The sodium tungstate solution obtained is purified by precipitation and filtration, before it is converted into an ammonium tungstate solution. This stage is carried out exclusively by solvent extraction or ion exchange resins. Finally, high purity Ammonium-Paratungstate (APT) is obtained by crystallisation, with the formula (NH₄)₁₀(H₂W₁₂O₄₂) ·4H₂O.

Wolframite concentrates can also be smelted directly with charcoal or coke in an electric arc furnace to produce ferrotungsten (FeW) which is used as alloying material in steel production. Pure scheelite concentrate may also be added directly to molten steel.

INTERMEDIATES

Ammonium Paratungstate (APT)

APT [(NH₄)₁₀[H₂W₁₂O₄₂] · 4 H₂O] is the main intermediate and also the main tungsten raw material traded in the market. APT is usually calcined to yellow (WO₃) or blue oxide (WO_3-X; a slightly substoichimetric trioxide with varying oxygen content). 

Tungsten Metal Powder (W)

Yellow or blue oxide is reduced to tungsten metal powder by hydrogen. The reduction is carried out either in pusher furnaces, in which the powder passes through the furnace in boats, or in rotary furnaces, at 700-1,000°C.

Tungsten Carbide (WC)

Most of the tungsten metal powder is converted to tungsten carbide (WC) by reaction with pure carbon powder, e.g. carbon black, at 900 - 2,200°C in pusher or batch furnaces, a process called carburization.

Tungsten carbide is, quantitatively, the most important tungsten compound. Because of its hardness, it is the main constituent in cemented carbide (hardmetal).

Cast Carbide

By melting tungsten metal and tungsten monocarbide (WC) together, a eutectic composition of WC and W₂C is formed. This melt is cast and rapidly quenched to form extremely hard solid particles having a fine crystal structure. A tough, feather-like structure is preferred over the brittle, blocky structure obtained by insufficient quenching. The solids are crushed and classified to various mesh sizes.

Namibia

(Source: Mineral Resources of Namibia, Geological Survey of Namibia)

Most of the tungsten mineralisation in Namibia is confined to the central portion of the country but minor deposits do occur in the south along the Namibian - South African border. Wolframite mineralisation is mainly associated with late Pan African pegmatites and hydrothermal quartz veins which occur in the central part of Namibia and are hosted by rocks of the Damara Sequence. In southern Namibia, wolframite occurs in early Damara pegmatites and quartz veins intrusive into rocks of the Namaqualand Metamorphic Complex.The Krantzberg wolframite deposit and possibly the Brandberg West deposit are both of Mesozoic age and are genetically related to anorogenic magmatism. Scheelite occurrences are associated with some granitic pegmatites at Natas, but most of the mineralisation occurs as scheelite bearing skarns which are located either proximal to acid intrusives or as stratabound scheelite skarns within calc-silicates and marbles of the Karibib and Kuiseb Formations in central Namibia.

Tungsten mineralisation hosted by rocks of the Namaqualand Metamorphic Complex

Grunau Area

Wolframite and scheelite mineralised quartz veins are reported from the Grunau Area, Karasburg District (Schwellnus, 1940). On Grabwasser 261, Schwellnus (1940) has reported the occurrence of a tungsten-bearing quartz vein, up to 0.5 m in width and dipping 25 degrees west-northwest. The vein shows scattered nests of wolframite, varying in grade from 0.18% to 3.4% tungsten oxide, accompanied by subordinate copper mineralisation and traces of scheelite. In the surrounding area, many other quartz veins occur in porphyroblastic gneiss of the Namaqualand Metamorphic Complex. Two smaller deposits of this type are located within light-grey megacrystic granite-gneiss on the farms Kanebis 5 and Geiaus 6. The wolframite and scheelite mineralisation seems to be structurally controlled by a major fault system trending towards the farm Karios 8. Geochemical stream sediment sampling and ultraviolet light surveys conducted on the three farms by Falconbridge Exploration Company in 1971 (Carr, 1972) failed to indicate additional tungsten concentrations.

Warmbad - Orange River Area 

Numerous small scheelite deposits occur in the area between Warmbad and the Orange River. The tungsten mineralisation is associated with quartz and pegmatite veins that are hosted by amphibolites and biotite schists. Four deposits are known from an area about 2 to 4 km south of the homestead on the farm Umeis 110 (Beukes, 1973; Kartun, 1979). Scheelite occurs within andradite-bearing pegmatitic dykes and sheets that crosscut amphibolitised olivine picrites and norites in a mixed gneiss zone. Mineralisation has probably been redistributed into plagioclase-rich, scheelite-bearing veins which form offshoots from the pegmatite bodies (Kartun, 1979). Along the contacts between the pegmatite veins and the amphibolitised olivine picrites and norites, coarse-grained, phlogopite-rich hornblende metasomatites have developed. Although unevenly disseminated, anhedral greenish-grey scheelite crystals can often be observed in hand specimen, from the alteration zones. Kartun (1979) speculated that there is a genetic relation between pegmatite, metasomatites, the ultramafic host rock and the formation of scheelite mineralisation.

A total of 6.3 t of scheelite concentrate was produced from such pegmatite veins in the Tantalite Valley area. Similar but smaller deposits occur on the farms Keimas 99, Bankwasser 138, Soekwater 139, Houms River 133, Witputs 258, Garub 266, Gaidip 146, Oranje Fall 101 and Kumkum 413. (Beukes, 1973).

On the farms Kinderzitt 132, Umeis 110 and Schwarzeck 130, scheelite mineralisation occurs in small, biotite-rich veinlets within amphibolite lenses together with chalcosite, chalcopyrite,

bornite and malachite. These scheelite-rich lenses and veins are enveloped by epidotisised

country rock (Beukes, 1973). On the farms Sperlingsputs 259, Ramansdrift 135, Staatsgrond west of Haakiedoorn 137 and Goabis 138, wolframite mineralisation is associated with quartz veins and tourmaline-bearing quartz veins (Beukes, 1973) Swanson Enterprises have reported the occurrence of tungsten mineralisation on the farms Plattrand 154, Border 155 and Komsberg 156. The wolframite mineralisation is associated with quartz veins that strike in a southerly direction and cut across the Orange River (Burdett, 1983)

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Tungsten mineralisation hosted by rocks of the Damara Sequence

Hydrothermal vein-type wolframite scheelite mineralisation

Brandberg West Mine

The Brandberg West deposit is located in the lower Ugab River area, about 45 k west-northwest of the Brandberg complex. Tintungsten mineralised quartz veins occur in

northeast-trending, foliated, turbiditic metasedimentary rocks of the Zebraputz and Brak River Formations, Lower Ugab Group, Damara Sequence. The mineralisation is associated with a

sheeted vein system which covers an area of about 900 by 300 m, mainly in quartz-biotite

schist of the Zebraputz Formation. From 1946 to 1980, the tin and tungsten-bearing vein deposit was mined in an opencast operation. During this period 14 374 t of concentrate grading 32 to 56% tin oxide and 14.5 to 19% tungsten oxide were produced. (The production figures of tin-tungsten concentrate from Brandberg West are given in the tin chapter). For detailed geological descriptions of the Brandberg West deposit the reader is referred to the Mineral Resource Series chapter on tin, Jeppe (1952), Petzel (1986) and Pirajno et al. (1987).

Paukuab

Wolframite-scheelite mineralised veins are reported from the Paukuab area situated about 2.8 km north of the Omaruru-Uis road and a few kilometres north of the Okombahe settlement in Damaraland. The quartz veins occur together with cassiterite-bearing rare metal pegmatites (see the chapter on tin) hosted by biotite schist of the Kuiseb Formation (Haughton et al., 1939). The Paukuab quartz veins form lenticular, highly irregular veins that consist of quartz, feldspar, tourmaline together with accessory, patchily distributed woframite, scheelite and copper and iron sulphides. A total of 11.3 t of wolframite-scheelite was produced during 1937 and 1938 (Source: Directorate of Mines)

Okarundu Nord West

On the farm Okarundu Nord West 118, south of the Kohero pegmatite swarm, a number of narrow (up to 4 cm wide) tourmalinised quartz veins carry accessory wolframite and scheelite. The tourmalinisation occurs preferentially along the selvages of the veins. Tungsten ore occurs

partly in tourmaline and partly in quartz-rich portions of the veins. Accessory minerals are iron and copper sulphides together with native bismuth and its oxidation products (Haughton et

al., 1939). A total of 0.82 t of wolframite-scheelite concentrate was produced in 1936 (Source:

Directorate of Mines).

Jan Jonker Mine

Quartz veins, up to 2 cm wide and some hundred metres long, occur along the western extension of the Natas anticline (Reuning, 1925). Some of the quartz veins contain scheelite, pyrite, chalcopyrite and locally free gold. Copper-rich portions of the quartz veins were mined on a very small scale by Namas (Jan Jonker) during the period from 1850 to 1860

(Reuning, 1925).

Pot Mine

Gurich (1890) and Reuning (1925) have described scheelite-bearing quartz veins and lenses one kilometre north of the Pot Mine on the farm Palmental 86 (see also the chapters on gold and copper).

Wolframite-scheelite mineralisation associated with late Pan African pegmatites and granites

Woframite and scheelite mineralisation occurs only rarely within Damaran granites and pegmatites. However, a number of smaller occurrences are described from the Karibib and Omaruru Districts by Frommurze et al. (1942) and Haughton et al. (1939). A larger tungsten mineralised pegmatite swarm is known from the Natas area, Windhoek District (Reuning, 1925).

Natas Mine

The Natas mine is located about 150 km southwest of Windhoek on the farm Natas 220. A number of narrow pegmatites have intruded biotite schist of Gaub Valley Formation and Pan African granite . The major pegmatite (Natas Mine) is about 80 m long and has a maximum width of 1 m. The upper portions of the dyke dip 40-50 degrees north, but at depth, the pegmatite plunges very irregularly (Reuning, 1925). The pegmatite comprises quartz, flesh-red orthoclase, oligoclase-albite, biotite, muscovite and subhedral, green crystals of scheelite (up to 20 cm in diameter), together with subordinate molybdenite, chalcopyrite, bornite, chalcosite,

ilmenite, native gold, apatite and tourmaline. The oxidation zone carries in addition secondary

copper ores, chrysocolla, cuprite, and tenorite together with hematite (see also the gold chapter and Reuning, 1925). Scheelite is the major ore of the Natas pegmatite and varies in grain size from 0.1 to 20 cm. Unaltered scheelite crystals are colourless and transparent, whereas scheelite from the upper portion of the pegmatite is green due to the development of a secondary crust of coppertungsten minerals. 

Scheelite crystals from the Natas Mine contain characteristic inclusions of bornite, chalcopyrite, chalcosite and free gold. In the upper portions of the pegmatite, up to 3 m from surface, the gold values vary from 0.20 to 4.8 g/ t. The native gold contains on average 18.3% silver. Quartz veins in the vicinity of the Natas pegmatite yielded copper concentrations of about 4% (Reuning, 1925).The Natas pegmatite was mined very sporadically for the gold-bearing copper ore, scheelite and molybdenite from about 1911. Mining operations ceased in 1919, were later resumed, and finally stopped in 1950. An attempt to reopen the mine during 1978 and 1979 failed, mainly due to technical problems. No records on the scheelite production are available. Reuning (1925) however stated that the scheelite concentration of the Natas pegmatite and neighbouring veins averaged 2 to 3%, with locally high-grade scheelite ore yielding 10 to 30%. (See also the gold chapter).

Natas West pegmatite

Tungsten anomalies are known from an area west of the Natas Mine on the boundary between the farms Chaibis 29 and Tantus 30. Seven scheelite mineralised quartz veins occur in micaceous quartzite, marble and schist of the Swakop Group. The veins have a total length of

765 m and values of up to 2.4% tungsten oxide were obtained from channel sampling (Bertram, undated)

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Wolfsbank pegmatites

At Wolfsbank, about 10 km south of the Okombahe village in Damaraland, quartzose pegmatites were worked for woframite and scheelite (Haughton et al., 1939). The quartzofeldspathic pegmatites are highly tourmalinised and contain muscovite and biotite. Woframite which often has overgrown scheelite, occurs in the central, quartz-rich portion of the pegmatites or in offshoots in schist. Haughton et al. (1939) described the Wolfsbank tungsten-bearing pegmatites as very sporadically mineralised.

Okarundu Nord West pegmatite

Wolframite and scheelite mineralisation is reported from a greisenised portion of a biotitemuscovite- albite pegmatite on the farm Okarundu Nord West 118, south of Kohero

(Haughton et al., 1939) 

Gross Okandjou - Kompaneno pegmatites

A wolframite-cassiterite mineralised pegmatite was reported from the farm Gross Okandjou 187. Wolframite occurs within the greisenised quartz core of the pegmatite and presumably crystallised prior to cassiterite which is associated with narrow bands of quartzmuscovite-

albite greisen (Haughton et al., 1939).

A very similar occurrence of a wolframitecassiterite- bearing pegmatite is known from the western boundary of the neighbouring farm Kompaneno 104 (Haughton et al., 1939). Approximately 0.38 t of wolframite concentrate were produced from the pegmatite during 1939

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Otjompaue Nord pegmatites

Wolframite-bearing pegmatites are found on the farm Otjompaue Nord 125 about 23 km west of Omaruru. Wolframite occurs as disseminated crystals throughout the intrusion and in quartz-tourmaline veinlets that cut the pegmatite. In addition, small quartz-wolframite veinlets which crosscut the core zone of the pegmatite have been reported by Haughton et al. (1939). During 1939, 0.27 t of wolframite concentrate were produced.

Ubib, Goas, Stink Bank, Tsawisis occurrences

Radial aggregates of woframite are found as an accessory constituent within Pan African granite on the farms Ubib 76, Goas 79, Stink Bank 62 and Tsawisis 16 in the Karibib District. At the first two localities, wolframite is associated with bismuth minerals in an auriferous quartz vein (Burg, 1942)

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Kudubis, Goabeb, Davib Ost, Ameib pegmatites

In the area south of the Erongo complex (Sandamap-Erongo tin belt), wolframite was recovered from quartz-rich pegmatites on the farm Kudubis 19, Goabeb 63, Davib Ost 61 and Ameib 60 (Frommurze et al., 1942), and about 2.t of wolframite concentrate was locally

recovered.

Rossing pegmatites

Small quantities of wolframite occur within pegmatites and quartz veins a few kilometres north of the Rossing siding.

Okahandja pegmatites

The occurrence of wolframite-bearing pegmatites is known from an area about one kilometre northwest of Okahandja, but no detailed investigations of the occurrence have been carried out (Blaine, 1975)

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Stratabound tungsten occurrences associated with tourmalinites of the Damara Sequence

Gross Okandjou 187 - Kompaneno 104 tourmalinite

Close to the eastern boundary of the farm Okandjou Nord 105, wolframite-scheelite mineralisation, which is genetically related to exhalative, stratabound tourmalinites, is known from two tourmaline-quartz-bearing “veins” (Haughton et al., 1939) Exploration activities carried out by Rossing Exploration (Louwrens, 1986) revealed anomalous metal contents associated with the tourmalinite horizons. Whole rock concentration of up to 4.59 wt per cent tungsten oxide, up to 8.06% copper and up to 0.16 g/t gold were obtained.

Skarn-type scheelite mineralisation

Skarn-type scheelite mineralisation in the Damara Province occurs either as stratiform mineralisation associated with calc-silicates and marbles of the lower marble unit of the Karibib Formation or as scheelite-skarns peripheral to acid intrusives. Scoon (1987) proposed a twofold classification: a stratabound scheelite association and a scheelite-skarn type proximal to Damara-age granites and pegmatites. At Otjua, Steven (1987) has firstly identified scheelite mineralisation hosted by calc-silicate hornfels (metamorphosed marls) and secondly scheelite-fluorite skarns which occur as replacement bodies within carbonate units of the Rossing Formation [Okawayo Formation of Badenhorst (1987)].

Skarn-type scheelite occurrences associated with calc-silicates and marbles of the Damara Sequence

Otjua skarn

The Otjua scheelite occurrence is situated about 30 km north of Omaruru on the farms Otjua 37 and Schonfeld 92 (Fig. 2) along the southern side of a major anticlinal structure (Schonfeld dome). The metasedimentary sequence at Otjua comprises biotite schists, calcsilicates and marbles of the Khan, Rossing, Oberwasser and Karibib Formations, which are overturned, strike 103 degrees with a steep northerly dip, and are intruded by Damara granites and pegmatites (Steven, 1987). Steven (1987) has reported two contrasting styles of scheelite mineralisation: Scheelite mineralisation associated with calc-silicate hornfels and metasomatic scheelite-skarns replacing marble.

At Otjua, scheelite-bearing hornfelses comprise 2 to 3% of the Khan and Oberwasser

metasediments (Lower Marble Unit, Badenhorst, 1987) and occur as intercalations within the metasediments, but scheelite mineralisation is not associated with marble horizons (Steven, 1987) indicating a synsedimentary origin. The dense hornfelses vary in thickness from a few centimeters to 0.5 m and are composed of quartz, plagioclase, scapolite, hornblende, clinozoisite, carbonate, scheelite and accessory pyrite and chalcopyrite. According to Steven (1987) metasomatic skarns are developed within several marble units, but occur more continuously in marbles of the Rossing (Okawayo) Formation (Fig. 3), and can be subdivided into three facies:

1) Idocrase-skarn with minor scheelite mineralisation

2) Garnet-skarn which hosts the majority of the scheelite mineralisation

3) Pyroxene-skarn 

Intensive exploration of the Otjua skarn occurrence started in 1981 followed by a drilling programme and geophysical surveys (Comline, 1983b) until 1985. Drilling indicates that the  low grade (0 to 1% scheelite) mineralised idocrase facies skarn extends at least 370 m below surface and is well developed towards the footwall of the carbonate (Steven, 1987). The economically most important garnet facies skarn has been intersected in drillcore at a depth of 350 m and is composed of garnet, hedenbergitic pyroxene, scapolite, fluorite, scheelite (0 to 3%), plagioclase, apatite, epidote, hornblende, chlorite, carbonate, pyrite, pyrrhotite and chalcopyrite. According to Steven (1987), 92% of all samples from garnet facies skarn have tungsten oxide contents of less than 1% reflecting the relatively low grade of the Otjua deposit with indicated reserves of 5.4 million t of skarn. As indicated by whole rock geochemistry and stable isotope studies (Steven, 1987), the Otjua skarn mineralisation was derived from the Otjua granite (infiltration metasomatism), whereas the hornfels-hosted scheelite mineralisation is considered to result from syn-sedimentary tungsten concentrations (Steven, 1987).

Tjirundo-Schonfeld-Roidina scheelite skarns

Towards the west and the east of the Otjuadeposit, similar scheelite-skarns occur on the farms Tjirundo 91, Schonfeld 92 and Roidina 44 (Steven, 1987). The garnet facies skarns are characterised by an inequigranular assemblage ranging from hundreds of microns to 30 cm and are often vuggy containing average  WO3 concentrations of 600 ppm (Tjirundo). A geochemical investigation of the Tjirundo scheelite occurrence yielded an average grade of 580 g/t tungsten oxide. Some samples contained up to 0.84% tungsten oxide (Comline, 1983a). At Schonfeld, two garnet facies skarns are present and at Roidina only one. At all three occurrences, the tungsten concentrations are significantly lower compared with the Otjua skarns (Steven, 1987). In contrast to Steven (1987), who postulated a metasomatic granite-related origin for these skarns, Scoon (1987) has interpreted the Schonfeld occurrence as

stratabound synsedimentary in origin, as the tungsten mineralisation is strictly limited to one calc-silicate bed, but absent within the marbles.

Ais dome skarn

The Ais domal structure occurs approximately 35 km north of Uis and consists of meta-rhyolite and ignimbrites of the Naaupoort Formation (Nosib Group) which are overlain by biotite schists, calc-silicates and marbles of the Okotjize and Orusewa Formations (Miller, 1980). The periphery of the Ais Dome is intruded by a suite of Damaran granites which contain mega xenoliths of country rock. According to Scoon (1987), in these xenoliths, many of which pinch out at a shallow depth, both skarn and calc-silicatehosted stratabound scheelite mineralisation

occurs. Single calc-silicate beds are interbedded with schist and contain locally disseminated

scheelite mineralisation over a strike length of 200-300 m, up to 5 m in thickness. At specific localities, relict pockets of partially recrystallised marble occur in the core of the skarns, or directly along strike from unaltered marble beds having selectively replaced the marble beds within the xenoliths (Scoon, 1987). Trekkopje Exploration and Mining Company (Le Roux, 1981) investigated the Ais Dome scheelite occurrence in 1981. Skarn-type scheelite mineralised portions that consist of vesuvianite, scapolite, diopside, actinolite, fluorite, grossularite and plagioclase with accessory scheelite, muscovite, sericite, clinozoisite, quartz, apatite, sphene, zircon, wollastonite and limonite show tungsten oxide concentrations ranging from 0.02 to 2.5% (Le Roux, 1981).

Omaruru scheelite skarn

An occurrence of scheelite mineralisation has been reported from the Omaruru Townlands near the Omaruru River. A rock sample yielded 0.13% tungsten oxide and 50 ppm bismuth (Coxall, 1981a).

Pforte and Black Range skarns

In an area west of the Erongo complex on the farms Pforte 37 and Black Range 72 scheelite-bearing skarns similar to those of the Omaruru area have been reported by Scoon (1987).

Schwarze Spitzkoppe 69 - Ketelbank 66 skarn

A scheelite-bearing skarn occurrence is located in the southeastern portion of the farm Schwarze Spitzkuppe 69. The skarn is hosted by a narrow marble-calc-silicate horizon of the

Kuiseb Formation and consists of garnet, pyroxene, idocrase and scapolite, which occurs in pods that are preferentially developed along the contact between calc-silicate and calcitic marble (Holman, 1988). Tungsten oxide concentrations obtained from 33 samples range from 0.004 to 9.28% (Holman, 1988). Within a marble horizon on the neighboring farm Ketelbank 66, cupriferous gossan veins, stringers and pods of scheelite mineralised calcsilicate hornfels and skarn occur along the footwall and hanging wall contacts. On surface the mineralisation can be followed over a distance of 30 m. 

Gamikaub - Ukuib - Tsaobismund skarns

Scheelite mineralisation within calc-silicates of the Karibib Formation, similar to that found in the Omaruru area, as well as quartz veinhosted scheelite mineralisation was reported from the farms Gamikaub 78, Ukuib 84, and Tsaobismund 85 south of Karibib (Gurich, 1890; Reuning, 1925; Burg,1942; Scoon, 1987)

.

Rudenau Nord - Gross Barmen skarns

Occurrences of a scheelite-bearing skarn within marble of the Karibib Formation on the farms Rudenau Nord 6 and Gross Barmen 7 have been reported (Marsh, 1980).

Aus skarn 
A calc-silicate hosted-scheelite skarn is known from an area north of Aus, Luderitz District. The scheelite mineralisation occurs discontinuously along the strike of the calcsilicates and covers an area of about 6.5 km2. Tungsten oxide concentrations range from a few g/t to about 20 000 g/t (Anonymous, 1981)

Skarn-type scheelite mineralisation proximal to acid intrusions of Namibian age

Kunibes skarn

Garnet-bearing scheelite skarn has been reported from the farm Kunibes 88. The mineralisation is located about 400 m south of the Swakop River close to the contact of Pan African granite with calc-silicates and biotite schist of the Karibib Formation.

Kwabab skarn

Skarn-type scheelite mineralisation was found on the farm Kwabab 117. The mineralisation occurs along a marble-granite contact as well as in thin calc-silicate intercalations in both the Karibib and Kuiseb Formations (Coxall, 1981b).

Pforte - Husab skarns

Occurrences of scheelite mineralisation along the contact between marbles and schists with granite are described from the farms Pforte 37 and from Husab on the southern bank of the Swakop River, about 75 km east of Swakopmund (Reuning, 1925; Burg, 1942).

Occurrences associated with Mesozoic granites

Krantzberg deposit

Introduction

In the Omaruru area, tin-tungsten mineralised veins and the greisen-type ore deposit at the

Krantzberg, northeast of the Erongo complex were first reported by Haughton et al.(1939). Subsequently Krantzberg Mines (Pty) acquired mining rights in 1940 and worked the deposit until 1956. In 1968, Nord Mining and Exploration started an exploration programme and reopened the mine in 1979. An intensive exploration and drilling programme was carried out in early 1980 by Anglo American Prospecting Services Namibia (Pty), but no additional ore reserves could be outlined and production stopped in 1980. Descriptions of the Krantzberg deposit are given by Haughton et al. (1939) and Schlogl (1984).

Geology and mineralisation

The Krantzberg deposit is situated northeast of the Erongo complex on the farms Pistelwitz 128 and Omaruru Townlands 85 . The prominent Krantzberg Mountain consists of biotite schist of the Kuiseb Formation and Pan African granite overlain by a tourmalinised basal breccia of the Erongo complex and basaltic lava flows. Wolframite together with subordinate cassiterite mineralisation is associated with greisen bodies, greisen veins and breccia bodies (Schlogl, 1984) Two major zones of greisen deposits, the Koppie Zone and the C-Zone are situated along the southern and the northeastern slopes respectively. Most of the mineralisation is confined to irregularly distributed alteration zones along the contact of the biotite schist and Pan African granite. According to Schlogl (1984) the Koppie greisen zones are almost mined out and only some crown pillars are left at the 1210 and 1160 m levels. The bulk of the tungsten ore was mined from two larger greisen bodies which are situated on the granite side of the schist-granite contact. The largest ore body extended over an area of about 45 by 30 m and has been mined out to a depth of 90 m. Diamond drilling results indicate that the ore body diminishes at a depth of 170 m. A smaller ore body covers a surface outcrop of about 27 by 8 m and, in addition to greisen-type mineralisation, carried vein-type wolframite mineralisation. The prevailing strike of all Koppie greisen veins is about 80 degrees northwest with a northerly dip (Schlogl, 1984). The C-Zone is situated on the northeastern slope of the Krantzberg along the contact between schist and granite. Various greisen stockworks are developed on the granite side of the contact, about 450 m along strike. Small, greisen-type ore bodies within the Erongo breccia are developed only on the northern slope of the Krantzberg and are found from the unconformable contact upwards to 1579 m elevation. Narrow greisen veins, locally up to 6 cm wide, occur in the biotite schist and were targets for mining operations during the 1950s. The greisen at the Krantzberg can be described as a greyish-white to light brown, fine- to medium-grained rock consisting of mainly quartz and topaz with fluorite, tourmaline, muscovite and sericite. The accessory minerals are biotite, chlorite, zircon, sphene, calcite, limonite, wolframite (ferberite) and cassiterite, together with minor amounts of chalcopyrite, pyrite, arsenopyrite, bismuth, molybdenite, scheelite and powellite (Schlogl, 1984). 

The narrow greisen veins are grey to blackish-grey and are composed of quartz, topaz, fluorite, tourmaline, beryl, limonite and calcite, together with accessory apatite, mica, scheelite, serpentine and ferberite (Schlogl, 1984). No production figures prior to 1979 are

available, but it is estimated that approximately 1 Mt of ore were extracted from a few greisen

bodies including about 550 000 t from the Koppie Zone, 200 000 t from narrow greisen veins, and about 250 000 t from the C-Zone (Anonymous, 1980). In November 1979, Anglo American had an option to explore for new tungsten mineralisation on the Nord Mining and Exploration owned grant. In December 1979 the first ore (old tailings) was treated in the plant and stoping subsequently began. Ore reserves for the C-Zone were estimated at 129 000 t at a grade of 0.5% WO3. For the Koppie-Zone the planned production was 160 t/day grading 0.4% WO3, whereas the total production target planned was 6240 t/month at a grade averaging 0.35% WO3. In July 1980, the lack of ore reserves caused the closure of the mine and the suspension of exploration activities (Anonymous, 1980).

References

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