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The Sangdong scheelite-molybdenum deposit is located in Kangwon-do province, some 170 km SE of Seoul in South Korea (#Location: 37° 08' N, 128° 50' E).
Tungsten has been mined in the Sangdong area from as early as 1916. During the first world war and over the next two decades wolframite was extracted from the quartz veins of the nearby Ungbongsan Mine while minor scheelite was mined from the Sungyong-san Mine which was developed on the Sangdong body. Annual production during this period averaged around 20 tonnes of contained WO3, never exceeding 100 tonnes. In 1939 more intensive exploration revealed the extent of the main Sangdong body and in 1940 large scale mining was commenced by Japanese interests, reaching a peak production of 11 500 tonnes of concentrate (7700 tonnes of contained metal) in 1944. In 1949 the Korean Tungsten, a government agency assumed control to mine Sangdong. A peak production of around 4000 tonnes of contained metal per year was achieved by 1952. During the Korean war production fell drastically, as did the tungsten price and the mine did not recover fully until the mid 1960s. The mine operated until 1992, with annual rates of production of up to 0.6 Mt of ore at an average grade of 0.65% WO3 (cut off grade of 0.3% WO3 and metallurgical recovery of 75%) before finally closing in early 1994 due to low tungsten prices. Between 1952 to 1987 total production was 74 911 t of scheelite concentrate.
The Korean Peninsula includes elements of both the North China Craton and Yangtse Craton section of the South China Block. The North China Craton occurs to the north, represented by the Archaean basement Nangrim-Pyeongnam Block. This is separated from the the Yangtse Craton Gyeonggi Block to the south by the narrow ENE-trending Imjingang mobile belt just to the north of Seoul (Carter et al., 2010). The Gyeonggi Block is composed of Archaean to Proterozoic crystalline basment, intruded by a series of NE-SW to ENE-WSW trending Triassic to Jurassic granite batholiths that extend across the peninsula (Kihm and Hwang, 2011). The southeastern half of South Korea is occupied by the Yeongnam Block that is interpreted to be part of the North China Craton, separated from the Gyeonggi Block by the NNE-trending Okcheon mobile belt, a fold-and-thrust belt which may have been the locus of dextral movement that displaced the Gyeonggi Block section of the north China Craton to the south (Lee and Walker, 2006). The Yeongnam Block is composed of Archaean to Proterozoic crystalline basement intruded by Late Palaeozoic (Late Devonian to Middle Permian) and Late Cretaceous to Tertiary granites, and covered by a thick sequence of Cretaceous terrestrial, terriginous rocks (Lee et al., 1999).
In summary, the Yangtse Craton Gyeonggi Block is sandwiched between the Nangrim-Pyeongnam and Yeongnam blocks of North China Craton to the NW and SE respectively, bounded by the Imjingang and Okcheon mobile belts to the NW and the SE.
The Sangdong deposit lies within the Okcheon mobile belt, which comprises low to medium-grade metasedimentary and metavolcanic rocks of Cambrian to Ordovician age to the SW. The Taebaeksan zone segment of the Okcheon mobile belt, on its NE extremity, contains weakly-metamorphosed, shallow-marine, Palaeozoic sedimentary rocks and marginal-marine to non-marine Early Mesozoic sedimentary rocks that contain economically important coal measures. These rocks rest unconformably on Archaean to Proterozoic gneisses and metasedimentary rocks of the Yongnam Block.
In the Sangdong area, the Cambro-Ordovician succession includes the Joseon System that is divided into the lower Yangdeok and overlying Great Limestone Series, which have been further sub-divided as detailed below. Plutonism occurred primarily during the Jurassic and Cretaceous, and most intrusions are biotite granites.
The Sangdong Scheelite deposit occurs within a Cambro-Ordovician carbonate-quartzite-shale suite which is represented by an arcuate synclinorial structure trending from E-W to NE-SW over a length of 120 km and width of 30 km. This belt is the remnant of a slightly larger basin which was developed on an extensive Precambrian basement. The Cambro-Ordovician is overlain by mid to late Palaeozoic sediments and extensively intruded by Mesozoic granites. The stratigraphic succession is as follows, from the top:
Cretaceous
Mica tourmaline granite, occurring as 2 x 3 km to 30 x 20 km bodies.
Jurassic
• Gobsansang Group - Milky white quartzose sandstone, light coarse sandstone, blackish, brownish, greyish, purplish and greenish shale, coal and coaly shale.
Triassic
Granites - These are extensive, forming batholiths which are up to 400 km in length.
Unconformity
Permian
• Sangdong Group - Dark to black sandstone, shales, black limestone, coal and coaly shale.
Unconformity
Carboniferous
• Hong Jaeom Group - Purplish and/or greenish grey shale and sandstone, white limestone and purple conglomerate.
Unconformity
Cambrian to Ordovician
Joseon Supergroup
• Great Limestone Group, > 1000 m thick.
- Makdong Limestone - the uppermost member of the Great Limestone Series, composed of interbedded grey limestone, light-brown calcareous, and black shale.
- Dumudong Limestone - interbedded light-brown calcareous shale, dark-grey and light-grey limestone.
- Tongjeom Quartzite - varicoloured quartzite and basal, interbedded black shale.
- Hwajeol Limestone - principally composed of limestone with a basal, interbedded grey to dark-grey shale.
- The Sesong Shale - thinly-interbedded grey to dark grey shale, arenaceous shale and white to pink limestone.
- Pungchon Limestone - the basal unit of the Great Limestone Series, composed of white, light pink and dark grey limestone and dolomite, black, grey, greyish-white shale, mudstone and "worm eaten" limestone. A number (~9) of cherty and finely banded calc-silicate layers with thin diopside and hornblende laminae are developed, each being from 1 to 2 m thick;
• Yangdeog Group
- Myobong Slate, 150 to 200 m thick - Well laminated black, dark greenish-grey and brownish-grey shale, sandy shale and phyllite with 8 interbedded horizons of calcareous shale to white limestone that, within the deposit area, have been altered to skarn. This formation is the host to all significant tungsten mineralisation.
- Jangsang (Changsan) Quartzite, ~250 m thick - Highly siliceous, massive, clean milky-grey, milky-white and light-yellow quartzite with a grain size of ~0.5 mm.
Unconformity
Proterozoic
• Sobaegsa Gneiss Complex - Migmatites and metatectic gneiss.
• Gyeonggi Group - Granite gneiss, equivalent to the Sobaegsa Gneiss Complex.
Unconformity
Archaean to Proterozoic
• Weonman Group - Augen gneiss, banded gneiss, mica-schist, schist and injection gneiss, grey fine grained gneiss, white quartzite,quartzite, white to light yellow quartz-sericite-schist, crystalline limestone, black shale, light green and black hornblende schist.
In the immediate deposit area at Sangdong, the Jangsang, Myobong, Pungchon and several overlying formations have been affected by thermal metamorphism above the concealed Cretaceous (85 Ma; Le, 2001) Sangdong Biotite Granite that was intersected at 700 m below surface during exploration drilling. Several Cretaceous-age granitic intrusive bodies also outcrop within several kilometres of the Sangdong deposit. The lowermost formation is the Jangsang (Changsan) Quartzite. Above lies the Myobong shale unit, a phyllite with calcareous horizons for a total 150 to 200 m. Completing the succession is the Great Limestone series of Cambrian-Ordovician age comprising six formations extending over 1000 m of limestones and dolomites and interbedded with grey and black shales, and quartzites. These rocks are situated on the south limb of a syncline that plunges gently to the SE, strikes at ~110° and dips at 20 to 30°NNE.
The tungsten mineralisation of the Sangdong deposit occurs as a series of tabular, bedding-parallel skarn horizons within a skarn altered zone that is ~150 m thick within the Myobong Shale, the protolith of which are interpreted to have been carbonate-bearing horizons that were altered and mineralised by fluids ascending from the underlying Sangdong Biotite Granite. The mineralised zone strikes east-west and dips to the north at between 15 to 30° for maximum strike and dip extensions of 1.3 and 1.5 km respectively.
From top to bottom, these horizons are termed the Hanging wall, Main and Footwall horizons. Calc-silicate layers from 50 cm to 1 m in thickness have developed on the upper and lower contacts of the Main and Footwall Horizons.
The Hangingwall Horizon was formed near the upper contact of the Myobong shale, varying from ~5 to 30 m in thickness due to the irregular boundary of the shale with the overlying Pungchon Limestone. This zone extends over a strike length of ~600 m, and down-dip for ~800 m. Above the most strongly-altered section of the Main Horizon, the Hanging wall Horizon is not tabular, but extends steeply into the overlying limestone. The Hanging wall Horizon has an alteration assemblage of diopside, garnet, fluorite, zoisite, quartz, hornblende, wollastonite and up to 50% calcite, and although there is some zonal variation in mineral assemblages (diopside- to hornblende- and to quartz-rich zones) the zonation is less well-developed in comparison with the underlying Main Horizon. It carries around 0.4 to 0.5% WO3, with scheelite that is finer than in the Main Horizon below. The base of the Hanging wall and Main horizons are separated by 10 to 20 m (averaging 14 m) of shales and carbonatic shales.
The Main Horizon varies from 5 to 6 m in thickness over a strike length of >1300 m, trending at ~100° and dipping at between 15 and 30°N. Skarn alteration within the Main Horizon occurs as three, concentric, elongate zones with diffuse boundaries, comprising the:
Quartz-mica zone - central quartz-rich (up to 40%) core, accompanied by an assemblage of fine to medium muscovite, biotite and minor phlogopite, chlorite, sericite and disseminated pyrite. The zone is up to 350 m wide and plunges down the Main Horizon at 355°W for up to 1 km. It is coincident with the higher-grade part of the deposit, carrying 0.5 to 3% WO3 as evenly disseminated scheelite grains of from 0.5 to 1 mm across.
Quartz-hornblende-diopside zone - outward from the core is a hornblende-rich zone with an assemblage that includes hornblende or tremolite, diopside, chlorite, fluorite and calcite. It comprises a dark mass of fine hornblende cut by white quartz veins and clear to white quartz segregations. The quartz comprises up to 5% of the rock with the veins being from 0.5 to 20 mm thick. Veins of diopside cut the hornblende. Scheelite is present both as fine disseminations throughout and occasionally as coarse segregations up to 20 x 5 mm. This zone carries from 0.3 to 0.8% WO3.
Garnet-diopside zone - This zone is found laterally outwards from, and in part above, the preceding zone, and contains garnet, diopside, quartz, fluorite, zoisite and plagioclase. It is a fine, pale-green diopside rock with pink andradite garnets 1 to 3 mm across spotted through it, comprising ~20% of the rock. In places it is banded with 3 to 10 mm thick bands of more garnet rich material. Scheelite is present on fracture faces and disseminated through the rock as 0.5 mm diameter grains. This zone carries around 0.1 to 0.3% WO3.
Garnet-wollastonite zone - This zone is not well developed and occurs laterally outwards from the garnet diopside zone. It has only low grade scheelite.
Cherty Calc-silicate - which is closely associated with the Main Horizon, being found in its immediate footwall, to a lesser extent the hanging wall and is laterally equivalent. It is a cherty, siliceous, diopside and/or hornblende rich calc-silicate. Locally in the orebody area, it has up to 40% plagioclase and is brecciated with associated diopside rich zones, cut by a network of dark hornblende veinlets from <1, up to 3 or 4 mm thick. Away from the orebody it is a finely laminated, extremely fine grained, grey siliceous calc-silicate with 0.5 to 1 mm thick dark hornblende and diopside laminae separated by 1 to 2 mm of the paler calc-silicates. It occurs over an up to ~4 m thickness, which may also include siliceous, quartz veined slate/shale, and outcrops over a strike length of some 8 km with the orebody near its centre, and carries elevated tungsten levels for several kilometres along strike.
Within the Main Horizon, molybdenum and bismuth abundance are concentrically zoned in a similar pattern to tungsten.
The Footwall Horizons comprise three separate layers: Footwall I, which is ~2 m thick, is 1 to 4 m below the Main Horizon. Footwall II and III are each <1 m thick and are 35 to 40 m below the Main Horizon. The intervening wall rock comprises slates/shales with thin localised calc-silicate lenses. The areal extent of these horizons and zonal distribution of calc-silicate assemblages are similar to that of the Main Horizon.
The three footwall beds have up to 1% WO3 or more but apart from Footwall I are unworkable because of their restricted thickness. The interval of siliceous and quartz veined slate between the Main Horizon and Footwall I carries from 0.2 to 0.3% WO3 such that when bulked with the two sandwiching ore beds it can also be extracted giving as a composite ore zone with >0.65% WO3.
Below the Footwall III mineralised horizon there are a series of concordant 1 to 3 cm thick, semi-massive pyrite lenses, each being up to 3 m long, some of which carry patchy scheelite.
The 9 mineralised horizons within the Pungchon Limestone are each from 50 cm to 1 or 2 m in thickness and in places carry 0.1 to 0.2% WO3, with occasional levels of up to 0.3% WO3. The better mineralisation in these is over an area of some 300 m in diameter, directly above the Main Ore Zone of the Myobong Slate.
At the top of the Jangsang Quartzite thin quartz veinlets carry cassiterite and wolframite. These are developed immediately below the main orebody and are largely parallel to bedding, but are also in part cross cutting.
Age determinations of alteration in the metapelites beneath the footwall of the Main Horizon gave K-Ar ages of 81.2 and 84.0 Ma, consistent with the age of the underlying Sangdong Granite, implying that this intrusive was responsible for the alteration and mineralisation (Le, 2001)
The main tungsten mineral is scheelite (calcium tungstate, CaWO4), accounting for >95% of the contained tungsten, with minor wolframite. Molybdenite is present throughout in substitution with scheelite mineralisation, more abundantly occurring below the skarn mineralisation near the underlying granite and mainly hosted in the Jangsang quartzite. Gold and silver occur in association with bismuthinite and native bismuth and were recovered from the bismuth concentrate. Tellurides, arsenopyrite, pyrite, chalcopyrite and sphalerite also are found within the deposit.
Mineralisation is largely associated with quartz veins within the ore horizons, with the exception of the central portion of the Hanging wall Horizon. Le (2001) notes that the mineralisation is hydrothermal in nature and that there were two stages of mineral deposition; i). related to skarn alteration and; i). to quartz vein emplacement. Molybdenum-poor scheelite was deposited during the first event, while scheelite, molybdenite and bismuthenite were deposited during the second.
The Jangsang Quartzite hosts a molybdenum-rich zone, immediately underlying the skarn footwall mineralised zone. This zone is hosted within the quartzite, and comprises a quartz vein stockwork zone of predominantly molybdenum mineralisation. This zone is located above the Sangdong Biotite Granite intrusion and extends into the base of the skarn altered units.
The historic resource estimate by Korea Tungsten mine staff in 1985 was 20 Mt @ 0.5% WO3.
The remaining resource estimate based on a cut-off grade of 0.10% WO3 (Carter et al., 2010 for Woulfe Mining Corporation) were:
Inferred resource - Hanging wall Zone, 45.8 Mt @ 0.32% WO3, 0.05 MoS2;
Inferred resource - Main Zone, 57.4 Mt @ 0.37% WO3, 0.04 MoS2;
Inferred resource - Total, 103.2 Mt @ 0.35% WO3, 0.04 MoS2;
At a cut-off grade of 0.20% WO3 (Carter et al., 2010 for Woulfe Mining Corporation) the resources are:
Total indicated resource - 16.43 Mt @ 0.45% WO3, 0.04 MoS2; plus
Total Inferred resource - 19.67 Mt @ 0.44% WO3, 0.05 MoS2;
A resource estimate of the Jangsang Molybdenum Stockwork at a 0.16% MoS2 cut-off (reported in Carter et al., 2010 for Woulfe Mining Corporation) was:
7.1 Mt @ 0.18% MoS2.
This summary is based on Carter et al., 2010 and from observations and data collected during a mine visit in 1977.
The most recent source geological information used to prepare this decription was dated: 2010.
This description is a summary from published sources, the chief of which are listed below.
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