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Mongolia's gold potential.

Mongolia is a large, sparsely inhabited country sandwiched between Russia and China in central Asia. The country has deposits of fluorite, copper-molybdenum, coal, gold, tin, tungsten, rare earths, and phosphorite as well as numerous occurrences of these and other commodities[1]. Mongolia is the world's largest producer of acid- and metallurgical-grade fluorite. Copper and molybdenum are derived from the Erdenet mine, the country's principal earner of foreign exchange. Fifteen coal mines are active, whereas the last of the small lode and placer tin and tungsten operations closed recently.

Placer gold reportedly was exploited in western Mongolia as early as the 11th and 12th centuries, before Ghengis Khan and his horsemen began their famed conquests of large tracts of Eurasia. Gold production, 4.7 tonnes in 1995, is still from placer deposits, mainly in the Zaamar and Eroogol districts of Khentei province[2]. Nevertheless, hardrock gold also was exploited, from the Boroo-Zuunmod and Eroogol districts by a multinational company during the first two decades of this century.

Mongolia's geology and mineral resources have been documented comprehensively since World War II by a series of collaborative projects carried out with the former Soviet Union and other COMECON countries. Hardrock gold mineralization seems to have been paid less attention than some other commodities, although about 400 occurrences in 40 districts are known. Most of these are of vein type, and many contain metals besides gold[8].

The market for Mongolia's mineral products contracted markedly with the demise of the Soviet system, and gold now is the most readily salable commodity given Mongolia's remote, landlocked position. Bulk-mineable deposits are particularly attractive exploration objectives, which were not targeted specifically by the joint Mongolian-COMECON projects. Only one known gold deposit, Boroo, possesses obvious bulk potential, as determined during exploration by teams from the former East Germany[4].

Geological evolution

Mongolia's complex geology is made up of a series of arcuate, southward-concave terranes, most of Paleozoic age and divided by sub-latitudinal fault zones. These units comprise the eastern part of the Altaids, a tectonic collage of subduction-accretion complexes and magmatic arcs along the southern margin of the Angara craton in Siberia[5].

Following Sengor et al.'s synthesis of the Mongolian portion of Altaid geology[6], six principal tectonic units are distinguishable [ILLUSTRATION FOR FIGURE 1 OMITTED]. The Tuva-Mongol and South Gobi units possess Precambrian basements, whereas the Ozernaya, South Mongolian, Khangai-Khentei and composite Kharkhirin-West Sayan units comprise accretionary wedges and, commonly superimposed upon them, magmatic arcs no older than Vendian. Arc magmatism spans the Vendian to Permian interval, except in the South Mongolian unit where arc construction did not commence until late Silurian.

Ophiolite fragments are found in association with subduction-accretion [TABULAR DATA FOR TABLE 1 OMITTED] complexes in several parts of the country, but do not define coherent collisional sutures, except for the Solonker suture of Late Carboniferous-Permian age in the extreme south[6]. The Main Mongolian Lineament[7] is the country's principal latitudinal fault zone, and is interpreted by Sengor et al to have originated in the Late Carboniferous as an intra-arc strike-slip fault. The subduction-accretion complexes display penetrative deformation, including tight folding and cleavage development, which contrasts with the complete absence of ductile fabrics in many of the arc terranes. The latter seem to have acted as buttresses during collisional events.

The Triassic through Cenozoic interval was a time of post-accretion extensional tectonism and accompanying calc-alkaline and alkaline magmatism, especially in northeastern Mongolia and adjoining Russia. This activity is assigned traditionally by the Soviet school to tectono-magmatic "activization" (reactivation) in the Mongol-Okhotsk fold belt.

Gold deposits and occurrences

The Paleozoic accretionary wedges and magmatic arcs as well as the Mesozoic extensional terranes all possess evidence for gold mineralization of various types.

The arcs are believed to host the main porphyry copper deposits: Tsagaan Suvarga and Kharmagtai in the South Mongolian unit, Erdenetiin-ovoo in the Tuva-Mongol unit, and Bayan-uul in the Khangai part of the Khangai-Khentei unit. Nevertheless, many investigators assign Erdenetiin-ovoo and Bayan-uul to the Triassic initiation of the extensional stage[7,8,9] but the few supporting radiometric ages are considered of low reliability. These four porphyry copper deposits display different characteristics (Table 1) and erosion levels, but all are associated with magmatic complexes, including trachyandesitic volcanics of elevated alkalinity. Although the gold contents of these porphyry copper deposits are low, they contain the largest proportion of Mongolia's known gold resources. The low-grade Kharmagtai prospect is the only one with a high Au/Cu ratio in accord with the high content of hydrothermal magnetite in the metal-bearing K-silicate alteration zone.

The Bayan-uul porphyry copper prospect is dominated by pyrite-rich sericitic and quartz-tourmaline alteration representing a shallow erosion level[8,10], and Erdenetlin-ovoo is also considered to be exposed relatively shallowly. The even shallower, epithermal parts of porphyry copper systems are also preserved in the South Mongolian unit, where lithocaps[11] characterized by pyrite-rich quartz-alunite and quartz-pyrophyllite alteration of trachyandesitic volcanic rocks occupy extensive areas at Shuteen and in the Ikh Shanhai range[12]. At both localities, panned concentrates obtained from surrounding drainages contain gold. Minor gold also is reported in panned concentrates from the vicinity of copper-bearing, tourmaline- and specularite-cemented breccia pipes at Oyut Ulaan Tolgoi and from quartz-tourmaline veins around Bayan-uul porphyry copper prospect.

Along the southern edge of the South Gobi unit, at Zuun Baruun Togoo-uul, abutting the Solonker suture, silicified Permian limestones cut by intrusive rocks have yielded up to 1% and more of arsenic and antimony besides weakly anomalous gold. A sediment-hosted (Carlin-type) gold affiliation may be suggested.

Deeper erosion affected the arc environment elsewhere in Mongolia, especially in the Khangai-Khentei unit where three of the best known gold districts are located. These are dominated by placer gold, which seems to have been derived from pluton-related mesothermal quartz veins, albeit possibly recycled via Mesozoic and Cenozoic paleo-conglomerates[2]. Equigranular granodiorite-granite plutons of early Paleozoic age appear to be the source of the gold-bearing veins in the Zaamar district, whereas Late Permian or Triassic granodiorite is most likely related to the veins in the Boroo-Zuunmod district; intrusions of both ages may be involved farther north in the Eroogol district. The mesothermal veins are composed of massive, sulphide-poor quartz and are bordered by sericite-carbonate alteration. At Boroo, where arsenopyrite is a prominent sulfide, the largely intrusion-hosted vein and associated quartz stringers occupy a very shallowly dipping fault zone[4] and hence offer bulk-mineable potential.

The subduction-accretion complexes are sites of metamorphogenic (turbidite-hosted) gold mineralization associated with sulphide-poor quartz bodies in cleaved terrigenous sedimentary rocks. The Siluro-Devonian flysch, in the northern part of the South Mongolian unit, hosts auriferous quartz veins and saddle reefs at Olon Ovoot, whereas cleavage-parallel quartz veins and boudins that contain gold at Taulag-uul are present in the east-striking Manlai fault zone, characterized by ultramafic slivers and iron formation and subsidiary to the Main Mongolian Lineament. Riphean meta-sedimentary rocks are the host for the gold mineralization at Urgamal.

Many minor gold-bearing quartz veins, some containing elevated contents of lithophile elements, occur along with fluorite deposits in the Mesozoic extensional province of northeastern Mongolia[3]. One of the most important occurrences is Tsagaan, where Early Cretaceous conglomerates in a north-east-trending rift zone characterized by bimodal volcanic rocks host quartz-(fluorite) veins and stockworks of low-sulphidation epithermal character.

Gold potential and exploration

The Paleozoic magmatic arcs and subduction-accretion complexes as well as the Mesozoic rift zones are believed to constitute exploration objectives for bulk-mineable gold in Mongolia. The arc terranes, especially those in the South Mongolian unit, have demonstrated potential for gold-rich porphyry copper deposits and high-sulphidation epithermal gold in lithocaps; possibilities for sediment-hosted gold exist; and, given the local preservation of the epithermal environment, low-sulphidation epithermal gold deposits, possibly alongside "lithocaps, also deserve consideration. Mesothermal vein deposits are also widespread in the more deeply eroded arc segments but, notwithstanding the potentially open-pittable Boroo deposit, are less likely to constitute bulk targets.

Metamorphogenic gold deposits in accretion-subduction complexes are highlighted because of their importance farther west in the Altaids. Although many metamorphogenic gold deposits are small and high-grade, the Muruntau deposit in Uzbekistan, hosted by a deformed accretionary wedge of Paleozoic age, is the world's largest open-pit gold deposit[13].

The potential for low-sulphidation epithermal gold-(silver) deposits in the Mesozoic rift zones of northeastern Mongolia is shown by the existence of the Balei district only 180 km from the Mongolian frontier in Transbaikalia, southern Russia. Balei, which reportedly contained 200 tonnes of gold, comprises crustiform chalcedonic quartz-adularia-carbonate veins and stockworks in Late Jurassic volcanics and Early Cretaceous conglomerates filling a northeast-striking graben[14]. The post-accretion extensional setting in the Mongol-Okhotsk belt possesses some similarities to that of the late Paleozoic through Jurassic Gondwana margin[15], which is characterized by major gold concentrations such as the Cerro Vanguardia low-sulphidation epithermal deposit, Argentina, and the Kidston orthomagmatic breccia pipe, Queensland, Australia.

Gold exploration in Mongolia is at a lower level than in virtually any other prospective region of the world. Government agencies and companies are only beginning to organize gold programmes, and foreign company activity is minimal. This situation is surprising given that a mining code, albeit not ideal, is in place; the geological-geochemical-metallogenic data base is extensive; known gold occurrences are numerous and widespread; and jeep access from May to November is straightforward in much of the partly forested northern mountains through the central grassy steppe to the Gobi Desert in the south. Reconnaissance field visits by foreign geologists to inspect selected prospects in any of the three main geotectonic settings described above may be organized easily. TM imagery and the BLEG geochemical technique should prove particularly effective tools in both the steppe and Gobi environments.

Acknowledgements

Much of Mongolia's geological and mineral resource data base is in Russian. Only a few of the many key papers are cited here, that by Tcherbakov et al.[2] being the most complete summary of Mongolia's gold metallogeny. The map of gold occurrences in Tcherbakov et al. has been updated by one of the writers (G. Dejidma) and published in English[18] with an accompanying list of prospects.

This article stems from a multi-organizational reconnaissance of gold-bearing geological environments in Mongolia. The sponsoring organizations, the EREL Company and those listed under the Mongolian authors' affiliations, are thanked for financial and logistical support of the field studies.

References

1. Kovalenko, V. I., Zaitzev, N. S., Yarmoluk, V. V., Bakhteev, R. H., Bold, D., Jumsran, M., Koval, P. V., Sotnikov, V. I., Tcherbakov, Yu. G., and Yanshin, A. L., 1984, Geodynamic environments in major geological epochs of Mongolia and their metallogenic characteristics: Joint Soviet-Mongolian Scientific-Research Geological Expedition Transactions, v. 38, p. 7-41 (in Russian).

2. Jamsrandorj, G., and Diatchkov, S. A., 1996, Placer deposits of Mongolia: SEG Newsletter, no. 24, p. 1,10-14.

3. Tcherbakov, Yu. G., Dejidma, G., Kalinin, Y. A., Osintzev, C. P., and Roslyakov, N. A., 1986, Metallogeny of the Mongolian People's Republic (gold): Joint Soviet-Mongolian Scientific-Research Geological Expedition, Preprint no. 12, Novosibirsk, Institute of Geology and Geophysics of the Siberian Branch of USSR Academy of Sciences, 49p. (in Russian).

4. Dornfeld, G., and Kraft, M., 1964, Uber die primaren Goldlagerstatten im Chentejgebirge nordwestlich von Ulan-bator: Zeitschrift fur angewandte Geologie, v. 10, p. 523-530.

5. Sengor, A. M. C., Natal'in, B. A., and Burtman, V. S., 1993, Evolution of the Altaid tectonic collage and Palaeozoic crustal growth in Eurasia: Nature, v. 364, p. 299-307.

6. Byamba, J., 1985, Tectonic map of MPR. 1:2 500 000: Ulaanbaatar, State Geodetic and Cartographic Agency of Mongolian People's Republic (in Russian).

7. Volchanskaya, I. K., Sapozhnikova, E. N., and Baskina, V. A., 1983, Morphostructural regularities in the distribution of endogenous mineralization in Mongolia: Global Tectonics and Metallogeny, v. 2, p. 95-110.

8. Koval, P. V., Gotovsuren, A., Ariunbileg, S., and Libatorov, Yu. I., 1989, On prospecting for porphyry copper mineralization in intracontinental mobile zones (Mongol-Okhotsk belt, Mongolian People's Republic): Journal of Geochemical Exploration, v. 32, p. 369-380.

9. Gerel, O., 1995, Mineral resources of the western part of the Mongol-Okhotsk foldbelt: Resource Geology Special Issue, no. 18, p. 151-157.

10. Koval, P. V., Ariunbileg, S., Libatorov, Yu. I., and Maksimyuk, I. Ye., 1988, The Bayanula molybdenum-copper porphyry prospect, central Mongolia, and its relation to magmatism: International Geology Review, v. 30, p. 900-911.

11. Sillitoe, R. H., 1995, Exploration of porphyry copper lithocaps: Pacrim Congress 1995, Auckland, New Zealand, Proceedings, p. 527-532.

12. Chovan, M., Chubldikova, N. I., and Moravek, R., 1990, Secondary alunite quartzites of southern Mongolia, in Geology and raw materials of Mongol National Republic: Moscow, Nedra Press, p. 130-134 (in Russian).

13. Berger, B. R., Drew, L. J., Goldfarb, R. J., and Snee, L. W., 1994, An epoch of gold riches: The Late Paleozoic in Uzbekistan, central Asia: SEG Newsletter, no. 16, p. 1, 7-11.

14. Borodaevskaya, M. B., and Rozhkov, I. S., 1977, Deposits of gold, in Stairnov, V. I., ed., Ore deposits of the USSR, v. 3: London, Pitman Publishing, p. 3-81.

15. Kay, S. M., Ramos, V. A., Mpodozis, C., and Sruoga, P., 1989, Late Paleozoic to Jurassic silicic magmatism at the Gondwana margin: Analogy to the Middle Proterozoic in North America?: Geology, v. 17, p. 324-328.

16. Gavrilova, S. P., Maksimyuk, I.E., and Orolma, D., 1989, The molybdenum-copper porphyry deposit Erdenetiin-ovoo (MPR): Moscow, Institute of Mineralogy, Geochemistry and Crystal Chemistry of Rare Elements, Preprint, 40p. (in Russian).

17. Kuznetsov, V. A. (editor), 1985, Copper-bearing formations of Mongolia: Joint Soviet-Mongolian Scientific-Research Geological Expedition Transactions, v. 43, 217p. (in Russian).

18. Jargalsaikhan, D., and Oyunbat, S. (compilers), 1995, Gold resources map, Mongolia. 1:3 400 000: Ulaanbaatar, State Geological Fund, National Center of Geoinformation and Remote Sensing.

The authors

Richard H. Sillitoe, Consultant, 27 West Hill Park, Highgate Village, London N6 6ND, England

Oehir Gerel, Department of Geology and Mineralogy, Mongolian Technical University, P.O. Box 46, Ulaanbaatar 46, Mongolia

Gunchingiin Dejidma, Institute of Geology and Mineral Resources, Uldverchnii evleliin gudamj, Ulaanbaatar 210537, Mongolia

Ayurzana Gotovsuren Geochemical Bureau, Undsen Khuliin Gudamj, Ulaanbaatar 37, Mongolia

Dambiin Sanjaadorj and Shirbazariin Baasandorj, Geological Exploration Consulting and Services Company Ltd, Khuvsgelchdyn Avenue, Ulaanbaatar 38, Mongolia

Dashiin Bat-Erdene, Department of Geology, Ministry of Energy, Geology and Mining, Baga toiruu 6, Ulaanbaatar 46, Mongolia
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Title Annotation:gold deposits
Author:Sillitoe, Richard H.
Publication:Mining Magazine
Date:Jul 1, 1996
Words:2359
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