Geomdeok, Komdok, Geumdeok, Keomdeok
Hamgyeongnam, North Korea
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The Geomdeok, Geumdeok, Keomdeok or Komdok (alternate spellings within the literature) zinc-lead deposit is located in the northern part of Hamgyeongnam Province of the Democratic People's Republic of Korea (DPRK, or North Korea), ~50 km north of the port of Tanchŏn on the Sea of Japan (#Location: 40° 55' 4"N, 128° 48' 53"E).
The Geomdeok mine has been in operation since 1932 and has a capacity to produce 10 Mt of ore per annum from seven mining blocks, known as West Jungtojang, East Jungtojang, Footwall Jungtojang, Takgol, Geomdeoksan, Muhakdong and Bonsan. These are exploited via 10 mines, Geumgol Mine, 7.1 Gang, Geomdeok Gang, Chengyeun Mine, Nampung Mine, Roeun Mine, West Budgol, Geomdeoksan, Muhak and Bonsan Mine. Zinc concentrates are hauled 68 km by rail to the Dancheon Zinc Smelter, whilst lead concentrates are also hauled by rail to the Muncheon Smelter, which is located at Gangwon-do.
The Geomdeok deposit lies within the Mid to Upper Palaeoproterozoic Hyesan-Rivong Rift Basin/Trough (aulacogene) located immediately to the east of the Nangrim (or Langrim) Block of the Eastern North China Craton. The trough corresponds to the southeastern extension of the Jiao-Liao-Ji Belt that is characterised by deformed bimodal volcano-sedimentary and intrusive rocks that have undergone high temperature and high pressure metamorphism. The sequence within the Hyesan-Rivong Rift is predominantly found unconformably overlying Archaean basement, with some intervening Lower to Mid Palaeoproterozoic metasedimentary and metasomatic granites of the Jungsan Group in the north. The Mid to Upper Palaeoproterozoic sequence is known as the Machenren Series (or Machollyong System), which has been correlated with the Liaohe Group in China (e.g., at the Qingchengzi Ore Field Zn-Pb deposits). The base of the Machenren Series comprises the Sonjin Formation, representing a sequence of predominantly volcano-sedimentary protoliths that have been intensely metamorphosed to crystalline schists, leucocratic granitic, garnet-plagioclase, biotite-muscovite and biotite-plagioclase gneisses, quartzite, migmatite, amphibolite and amphibole schists, with occasional marble interbeds. These are conformably overlain by a predominantly carbonate sequence of mica and calcareous-mica schist, tremolite marble and banded calcareous-dolostone rocks of the Puktechon Formation. The section is capped by the mainly clastic sedimentary protoliths of the Namdechon Formation. This unit comprises a lower sequence of siliceous, calcareous and mica schists with amphibolite interbeds and predominantly micaceous, sillimanite-feldspar, tourmaline-sillimanite, tourmaline-quartz-feldspar and other augen schists, with quartzite interbeds in its upper part (Ratkin et al., 2014).
Apart from extensive Archaean granitic complexes that form much of the basement Nangrim Block to the west, the principal intrusions within the Hyesan-Rivong Rift Basin include significant Palaeoproterozoic granitoids and very extensive Jurassic to Early Cretaceous granitoids that occupy large areas of the southern half of the basin, as well as the flanking Nangrim and Longgang blocks to the east and west respectively.
The Hyesan-Rivong Rift Basin/Trough has been subdivided into three north-south elongated zones. In the western of these, pre-Machenren Series rocks are unconformably overlain, or structurally separated across the Hochongang Fault Zone, by a well developed, 1500 to 2000 m thick volcano-sedimentary sequence of the Sonjin Formation. These are followed by westward thinning, 0 to 200 m of carbonates and pelitic facies of the Puktechon Formation and thicker developments, 500 to >1200 m of psammitic and pebbly facies of the Namdechon Formation. The Sonjin Formation, although assumed present, is not exposed in the central zone, where dolomitic and calcareous facies and overlying pelitic rocks of the of the Puktechon Formation are well developed, totalling ~3000 m in thickness; overlain by psammites of the Namdechon Formation which may total >2000 m in thickness. In the eastern zone, the succession comprises well developed volcanic rocks of the Sonjin Formation, totalling >600 m in thickness, overlain by >3000 m of dolostones with lesser interbeds and rare schists after shales of the Puktechon Formation, with <300 m of schist without psammitic rocks in the Namdechon Formation. The three zones are separated by the Jangphari and Puktaechon Fault Zones to the west and east respectively (Paek Ryong Jun, 1993).
The host to ore at the Geomdeok deposit comprises alternating dolostone, pistomesite (magnesian siderite) and micaceous schist of the Puktechon Formation that are overlain by schists of the Namdechon Formation.
In the Geomdeok area, the host Puktechon Formation comprises the following, from the base:
• Tremolitic dolostone, ~100 m thick;
• Greyish white massive dolostone, with alternating thin intercalated bands of amphibole schist and limestone, ~100 m thick;
• Chloritic limestone with thin schist intercalations, ~120 m thick;
• Massive yellowish white dolostone and dirty bedded, 60 to 80 m thick.
• Calcareous and mica schist, with thin rhythmic intercalations of amphibole schist, ~150 to 200 m thick;
• Two-mica schist and gneiss that is 200 to 300 m thick, characterised by feldspar or sillimanite augen. This unot has been subjected to migmatisation as a result of the injection of the leucocratic gneissic granite of the Riwon Complex (aged between 1847 and 1,330 Ma; Paek Ryong Jun, 1993).
• Thin rhythmic bands of calcareous schist, mica schist and amphibole schist, totalling ~50 m in thickness.
• Suchigi Subunit - mainly white to greyish white crystalline limestone that is 5 to 20 m thick, with intercalated dolostone low the unit;
• Komaidok Subunit - calcareous dolostone with tremolite metacrysts, ~220 to 280 m thick;
• Taebokri Subunit - white to greyish white crystalline limestone, with thin interbands of limestone and schist, alternating with siliceous dolostone, quartzite and dolomitic limestone, ~80 to 100 m thick;
• Tongjom Subunit - mainly white crystalline dolostone, with thin intercalated schists in the upper, middle and lower sections, 80 to 100 m thick;
• Phyonghwagol Subunit - dark bedded calcareous dolostone, 40 to 80 m thick;
• Kongukri Subunit - white to greyish white, fine-grained bedded dolostone, with interbedded laminae of limestone and schist, 80 to 120 m thick;
• Sungrigol Subunit - alternating calcareous schist and pink bedded limestone, 100 to 150 m thick;
• Mulbang-a-gol Subunit - alternating white to yellowish white dolostone and limestone, 100-130 m thick.
Fourth Unit, which is primarily composed of tremolitic dolostone, and is 300 to 500 m thick. The protolith is interpreted to have been alternating dolostome and chert with individual beds that were as much as several cm thick. It is the product of metasomatic replacement whereby tremolite developed along the contact between the two lithologies, depositing a radial assemblage crystals that range from several mm to several cm in size. Tremolitic dolostone intervals alternate with tremolite free dolostones that were each several to 30 m in thickness.
Fifth Unit, or Komdok 'Formation', comprises:
• First Subunit - massive white to grey dolostone, 150 to 200 m thick;
• Second Subunit - rhythmically and finely banded dolostone and schist, 200 to 300 m thick;
• Third Subunit - grey to greyish black stromatolitic dolostone, 100 to 150 m thick;
• Fourth Subunit - flesh or white calcitic marble, with phlogopite and chlorite marble in the uppermost sections, ~50 m thick;
• Fifth Subunit - rhythmically and finely banded grey and white dolostone, with intercalated fine chert bands locally in the sections, 50 to 80 m thick;
• Sixth Subunit - banded white marble and phlogopite-chlorite marble, 5 to 10 m thick;
Namdaechon Formation - The basal schist of this formation rests conformably on the Sixth Subunit.
Diagrams in Ratkin et al. (2014) appear to indicate mineralisation is localised to the uppermost 50 to 100 m of the Puktechon Formation, below the conformably overlying schists of the Namdaechon Formation, presumably within the Komdok 'Formation'.
The sedimentary rocks of the Puktechon Formation have been deformed by isoclinal folding with north-south trending axes, hosting ore that occurs as bedding parallel bands and lenses concordantly following the banded dolostone stratigraphy. The multiple orebodies and lenses are each 0.5 to 10 m thick, locally bulging to as much as 30.0 m. Individual orebodies may persist over strike lengths of 1 to 2 km, although the host unit, which is no more than 100 m thick, can be traced for >10 km in a near north-south direction. The orebodies are generally steeply dipping and have been traced to a depth of ~1 km.
The dominant sulphide mineral is sphalerite which contains 6 to 7.75 wt.% Fe, intergrown with minor galena. The galena contains inclusions of tetrahedrite and boulangerite, as well as rounded framboidal inclusions of pyrite. This may be taken to imply original framboidal syndiagenetic pyrite was replaced by the main base metal sulphide mineralisation. Massive sulphides dominate, occurring as alternating sulphide, dolostone and pistomesite layers which range in thickness from 3 to 5 mm, increasing to 5 to 10 mm close to the thinning lateral marginal zones of ore bodies. Boudinage of carbonate interbands results in brecciated textures, where striated sulphide aggregates wrap around dolostone fragments. Sulphide interbands are rarely boudinaged, with sphalerite bands anastomosing around more competent boudins, whilst galena forms thin bands within the massive sulphide. In thin section, deformation twins are evident in sphalerite, which can also be seen to be recrystallised along the periphery of galena bands, occurring as crystalline aggregates with grains elongated parallel to the foliation (Ratkin et al., 2014).
A large Mesozoic (Jurassic to Lower Cretaceous) granitoid intrusion occurs on the southern flank of the deposit, with exocontact zones of skarn alternation of the clastic and carbonate rocks. Sulphide-bearing skarn alteration is confined to the contact zone between the granitoids and the ore bearing dolostone unit. The silicate paragenesis of the skarns is predominantly grossular and andradite garnet. The skarn hosts small nests and interbeds filled with galena and low-Fe sphalerite. Compared to galena from the banded ores, that from skarns is anomalously enriched in Bi (up to 0.43%). Where close to the intrusive contact, the metamorphic recrystalisation textures in the banded mineralisation, as described above, are preserved. However, in polished section, galena from tis section of the deposit can be seen to have polygonal-grain microstructure and triple junction angles, whilst In galena-sphalerite aggregates, sphalerite occurs as droplet shaped segregations, interpreted to be typical of thermally transformed ores. This implies folded sulphide mineralisation was subsequently locally affected by the thermal field of a granite intrusion. The conformable, folded nature of the massive sulphides is taken to imply their deposition was pre-folding although no data has been seen that precludes replacement of already folded metasediments. The heavier sulphur isotope composition in sphalerite (δ34S = +6.0 to +14.0‰) is interpreted as evidence that sulphates from marine water was involved in the deposition of sulphides. The calculated Pb-Pb age of the polymetallic ore is ~1.94 Ga (Ratkin et al., 2014), whilst Paek Ryong Jun (1993) quotes model age of lead ore of the Komdok Ore Deposit at 1940±10 Ma (Pb-Pb; ten samples) which they take to represent an early metamorphic age of the Puktechon Formation, rather than peak metamorphism (Paek Ryong Jun, 1993).
The estimated resources at Komdok (after Nautilus Institute, Special Report 'The Mining Industry of North Korea', August 2011) was:
226 Mt @ 0.88% Pb, 4.21% Zn (James Park and Burt in Meinert, 2005 also list 18 g/t Ag, 0.17 g/t Au).
The most recent source geological information used to prepare this summary was dated: 2014.
This description is a summary from published sources, the chief of which are listed below.
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Ratkin, V.V., Gvozdev, V.I. and Karas, O.A., 2014 - Boron-Polymetallic Metallogeny of the North and Northeast of the Sino-Korean Craton: in Russian Journal of Pacific Geology, Pleiades Publishing, Ltd., (Original Russian text published in Tikhookeanskaya Geologiya, 2014, Vol. 33, No. 5, pp. 66-76), v.8, No.5, pp. 372-380.|
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