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Muruntau

Uzbekistan

Main commodities: Au
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The Muruntau gold deposit, which is located within the Kyzyl Kum desert in central-western Uzbekistan is the largest gold resource in Eurasia. It is some 400 km to the west of the capital, Tashkent, and is served by the adjacent town of Zarafshan (#Location: 41° 29' 45"N, 64° 34' 36"E).

The deposit was discovered in 1953, although ancient gold mines have since also been revealed in the vicinity of the ore deposit. Detailed exploration took place between 1960 and 1965. Construction commenced in 1964, and mining in 1967, with the gold plant beginning operation two years later.

The deposit originally contained more than 5400 tonnes (175 Moz) of gold at an open pit recovered grade of 3.4 g/t Au.

Setting

The deposit occurs within a pile of imbricated thrusts that was deformed into west-east trending synforms and antiforms exposed in the Tamdy mountains near the western extremity of the South Tien Shan tectonic province (Drew et al., 1996). It lies to the south west of a major regional suture zone (marked by the occurrence of mafic rocks of an ophiolitic association in a zone of intense deformation) which separates the Middle Tien Shan tectonic province to the north east (represented by the Carboniferous Valerianov-Beltau-Kurama magmatic arc), from the South Tien Shan fold and thrust deformed accretionary complex overlying older Meso- to Neoproterozoic basement of the Karakum micro-continent. The South Tien Shan has been interpretted to comprise four gross nappe units, each of which is composed of more than one interleaved thrust slices.

In the Muruntau area, the South Tien Shan consists of tectonically superimposed lithologies (Savchuk et al., 1991; Drew et al., 1996), which represent early-middle Palaeozoic oceanic to accretionary and fore-arc complex rocks thrust onto Meso- to Neoproterozoic to middle Palaeozoic passive margin sedimentary rocks whose late Neoproterozoic (Vendian) to lower Paleozoic section was metamorphosed to amphibolite to greenschist facies.

Geology

The host sequence to the Muruntau mineralisation is the Cambrian to Silurian Besapan Formation. This sequence is overlain by a structurally emplaced unit of Devono-Carboniferous carbonates. The Besapan Formation lies on the northeastern flank of a granite gneiss dome, in whose axial core highly metamorphosed Proterozoic rocks are exposed outside of the ore field. These comprise retrograde biotite-garnet gneisses, garnet amphibolites and migmatites. Metamorphism in the region increases towards the south west, from the almost un-metamorphosed upper member of the Besapan Formation, to meta-siltstones of the middle sequence, to the grey schists of the basal unit, which are underlain by more strongly metamorphosed biotite-muscovite schists further to the south-west. On the whole however, the metamorphism of the Besapan Formation is relatively slight, expressed mainly in the pelitic matrix of siltstones and sandstones. The matrix has become schistose with the development of chlorite and sericite in the less affected rocks, to muscovite and biotite in the most strongly metamorphosed bands, with associated albite and quartz. This pattern is overlain by local zones of brecciation, banding and phyllonites, and the superposition of alteraton characterised by biotite, plagioclase, K-feldspar, quartz, graphite and carbonaceous, accompanied by magnetite and sulphides. This alteration has led to the development of an envelope of carbonaceous biotite rocks around the Muruntau deposit that are often brecciated, either conformably or transgressively, with anomalous gold levels (Marakushev and Khokhlov, 1992).

The stratigraphy within the immediate Muruntau district can be summarised as follows, from the base:
Meso- to Neoproterozoic - Taskazgan Suite
Retrograde Gneiss - exposed at the base of the succession to the west of the Muruntau ore field, comprising Mesoproterozoic retrograde gneisses, dated at 1750 ±80 Ma.
Two Mica Schists - predominantly biotite-muscovite schists in the single small exposure found in the Muruntau area, where they are unevenly enriched in carbonaceous material. They contain beds of meta-pelite, siliceous rocks (cherts), mafic volcanics, dolomites and meta-sandstones, and grade upward into the phyllites of the lower sections of the Grey Besapan unit (Marakushev and Khokhlov, 1992).
Late Neoproterozoic to Silurian - Besapan Formation
Grey Besapan - The rocks of the Grey Besapan occupy vast areas of the western and central sections of the ore field. They are uniform, carbonaceous phyllites, consisting of chlorite, muscovite, biotite, albite and quartz. The lower section, sometimes referred to as bS1, is a meta-siltstone, while bS2, the upper part of the unit, is a mixed meta-siltstone and meta-sandstone. The fragmental material in the sediments is composed of quartz and feldspar, with clasts of intermediate and silicic volcanics in isolated horizons. The bedding is especially marked by gritty meta-sandstones. Late Upper Proterozoic to lower Cambrian and Ordovician fauna have been described from this unit. The phyllitic rocks of the Grey Besapan grade gradually into the underlying phyllites, which differ only in their greater abundance of biotite.
Variegated Besapan - This unit hosts the largest of the Muruntau orebodies, and is predominantly composed of carbonaceous, meta-siltstone, meta-sandstone, meta-volcanic rock and minor radiolarian chert. The volcanics are predominantly intermediate and siliceous tuffs. The unit is both carbonaceous and pyritic and is referred to as 'variegated' because of its variable red and green colouration in outcrop (Berger, et al., 1994). It is referred to as bS3, and has been subdivided into three members, as follows (Marakushev & Khokhlov, 1992):
Lower Member - is the most heterogeneous in composition, including quartz-mica meta-siltstones, sometimes containing carbonaceous material, meta-sandstones, phyllites and isolated beds of gritty and calcareous meta-sandstones and meta-siltstones
Middle Member - is characterised by thin beds and lesser lenses of metatuffs, within a mixture of the lithologies described for the 'Lower Member'.
Upper Member - which consists of greenish-grey and varicoloured carbonaceous-micaceous phyllites, polymict meta-siltstone and isolated beds of meta-sandstone.
The Variegated Besapan has been tentatively assigned to the Ordovician to lower Silurian period (Marakushev & Khokhlov, 1992).
Green Besapan - approximately 1000 m thick - This is the uppermost and least metamorphosed unit of the Besapan Formation. It is predominantly a sandstone, composed mainly of quartz, with clay material replaced by chlorite and sericite, giving it a greenish colouration. The base is slightly metamorphosed, or epigenetically altered, but bears the trace of submarine hiatuses and ripple marks. Greenish grey sandstones and siltstones with green argillite are the dominant lithologies with beds and lenses of grits up to 1.5 m thick. Graptolites indicate a Silurian age (Marakushev and Khokhlov, 1992).
Devono-Carboniferous
Carbonates - up to 3000 m thick - composed predominantly of sandstones and dolomitised limestones. The contact is sometimes an angular unconformity and at other places a structural plane (Marakushev and Khokhlov, 1992).
Permo-Carboniferous
Granitoids - The closest exposure of a significant granitoid body is some 7 km to the south-east of Muruntau. Here it is largely concealed by Cenozoic cover, but is seen to comprise a medium grained, slightly porphyritic granodiorite-adamellite (Kotov & Poritskaya, 1992). Other igneous rocks have been developed, mainly on the periphery of the ore field. They comprise leucocratic dykes which have been concentrated in several differently oriented zones, in different parts of the field. In the northern section of the field, a 7 km long, east-west striking belt of 34 dykes is recorded, while to the east 44 dykes strike to the north-east. To the south-east a further east-west striking set of 50 dykes are found with two associated stock-like bodies which are around 120x300 m across. These dykes are plagiogranite porphyries, syenite porphyries and spherulitic syenite porphyries (Smirnov, 1981). They have been largely emplaced along regional shear zones (Berger, et al., 1994) and also include lamprophyres, quartz-diorite, syenite-diorite and granosyenite (Kotov and Poritskaya, 1992). Granitoids have also been intersected in deep drilling 4000 m below the deposit.

Kotov and Poritskaya (1992) summarise the tectonic evolution of the South Tien Shan Belt in the Muruntau area into the following stages:
i). The development of regional nappes which dip northwards in the north, and to the south in the south. This thrusting has led to duplication of the sequence in the district, with four major thrust slices being represented;
ii). Formation of conjugate, sub-latitudinal regional fold structures to form a series of parallel antiform and synforms. According to Berger, et al., (1994), following the compression that formed the nappes, subsequent oblique convergence resulted in transpressional deformation and the formation of a broad, sinistral shear zone, the Sangruntau-Tamdytau Shear zone which accompaniedthe folding. This shear zone, which is believed to represent a splay of the major suture zone to the north east, in turn bifurcates and splits into a number of splays to the north west and west of the mine;
iii). Compression of these folds and the Sangruntau-Tamdytau Shear zone to form a number of regional 'Z-shaped' structures, especially at Muruntau where the deposit is near the elbow of one such structures;
iv). Formation of deep seated faults as the regional deformation surface was over-printed by brittle fracture which also influenced the location of Permo-Carboniferous granitoids. The Muruntau deposit lies near the intersection of the brittle north-east trending Muruntau-Daugyztausk fault zone, and the coincident antiform and Sangruntau-Tamdytau Shear zone, where the latter structures are deformed into a regional 'Z-shaped' structure (Kotov and Poritskaya, 1992).

Alteration and Mineralisation

The orebodies at Muruntau essentially constitute a megastockwork (Kurbanov, 1999) of quartz-dominant veins and associated quartz-albite-phlogopite and sericite-chlorite-(K feldspar)-carbonate alteration of two generations.

The ore grade mineralisation at Muruntau is developed within a characteristic massive, light pink to yellow, biotite-plagioclase-quartz-orthoclase rock. The compositional range of these rocks is generally: 25 to 50% orthoclase, 25 to 40% quartz, 15 to 25% plagioclase (albite and albite-oligoclase), and 20 to 40% biotite, representing an enrichment in alkali metals. The gold content of this alteration type is typically 1 to 3 g/t, locally increasing to 20 to 30 g/t Au (Marakushev and Khokhlov, 1992).

These more highly mineralised, two feldspar-biotite-quartz rocks replace, and are found within the core of a larger envelope of black banded rocks rich in carbon and biotite, which contain low grade disseminated gold mineralisation, commonly of around 1 ppm. The mineralised carbon-biotite rocks replace carbonaceous meta-siltstones of the Besapan Formation, preserving their original banded nature and surrounding relicts of un-altered siltstone. In general the replacement is bedding controlled above the base of the Variegated Besapan and is predominantly manifested in a north-east plunging synclinal structure (Marakushev and Khokhlov, 1992).

Prior to the alteration, and the emplacement of both the ore and the regional granitic bodies, the host siltstones had been metamorphosed to greenschist facies to produce siltstones composed of 40 to 50% detrital quartz, with the remainder being chlorite, sericite, oligoclase and K-feldspar in variable amounts (Kotov and Poritskaya, 1992).

Deformation prior to the emplacement of the gold mineralisation is considered important in the development of a high permeability within the Besapan Formation. Schistosity and axial plane cleavage were developed during regional metamorphism, folding and shearing, while brittle fracturing and brecciation accompanied later faulting. The brecciation is commonly sufficiently developed to classify the rocks as phyllonites which exhibit shattering, mylonitisation and crumpling, with the development of films of biotite, muscovite, chlorite and graphite or carbonaceous material along directions that often cut the bedding planes. The schistosity and cleavage are the controls on the largest proportion of the mineralisation which is interpreted to have taken place during the late Carboniferous to early Permian, coincident with a change from compressional to transpressional tectonics (Berger, et al., 1994; Marakushev and Khokhlov, 1992).

Drew et al., (1996), recorded the following paragenesis of alteration and gold mineralisation:
i). Quartz + albite + biotite + chlorite + oligoclase alteration forming linear, subparallel zones of quartz veins and veinlets. Oligoclase was an early phase, overprinted by quartz, albite and later K feldspar. This alteration phase in general has weak associated gold and overprinted the original spotted schists which resulted from regional and contact metamorphism;
ii). The second stage is characterised phlogopite and some pyrite (±arsenopyrite), in en echelon veins with selvages of muscovite, magnesian chlorite, quartz, phlogopite, K feldspar and minor iron-magnesium carbonate, with associated weak gold mineralisation to at least several hundred ppb;
iii). Quartz + K feldspar + muscovite veinlets with ankeritic carbonate and sulphide cut all of the above. This phase is associated with the Central Veins which contain the highest grade gold with grades averaging 3.5 to 11 g/t Au. Siliceous dykes intrude all of thepreceding and follow the emplacement of the Central Veins;
iv). Quartz + K feldspar + dolomitic carbonate + tourmaline (dravite) ± sulphide (pyrite) veinlet set which cuts the siliceous dykes and adjacent wall rocks, being apparently related to brittle fracture and brecciation; and
v). Calcite veinlets and pervasive calcite with rare pyrite (brookite?) phase which is the last hydrothermal event and destroys all earlier phases. Rare earth minerals, including monazite and bastnaesite occur in clusters in some calcite veins.
vi). A late quartz-sericite alteration has been observed, which may be intense, followingbrittle faults that offset the Central Veins.
Gold has apparently been introduced with each of the alteration assemblages detailed above.

Four principal types of veins are recognised at Muruntau, namely:
i). Flat Q1 veins which are mostly gently dipping, controlled by the foliation-parallel Tamdytau-Sangruntau shear zone (Drew et al., 1996). The host rocks carry grades of 0.03 to 0.3 g/t Au with higher concentrations of 1.5 to 2.5 g/t Au in the alteration halo (Kurbanov, 1999). The Q1 veins also host subeconomic scheelite in stratabound zones that formed before the introduction of gold mineralisation (Uspenskiy and Aleshin, 1993) and revealed Sm-Nd ages of 279 ± 18 Ma (Kempe et al., 2001).
ii). Stockwork-type Q2 veins which form an extensive, gently dipping, overall concordant zone of mainly small to microscopic veinlets that have been mined to a depth of 300 m (Berger, et al., 1994). The zone was generated by several stages of mineralisation formed under different rheological conditions, ranging from plastic to brittle (Smirnov, 1981; Sokolov, 1995; Berger, et al., 1994). The zone exhibits an irregular distribution of mineralisation, although Kurbanov (1999) mentioned grades of 3.5 to 5 g/t Au.
iii). Steep Central Veins (Q3), which generally cross-cut the sedimentary layering and schistosity and trend near east-west. They may reach thicknesses of 15 to 20 m in the bulges, are lensoid and traceable for up to 160 m (Graupner et al., 2001), with average grades of 15 to 20 g/t Au (Kurbanov, 1999).
iv). Silver-rich steep veins (Q4) which dip at 60 to 70° and are discordant and also strike near east-west. They are poorly auriferous, quartz-sulphide and sulphide veins which host silver and lead sulphides (Zairi and Kurbanov, 1992).

The discordant portions of the vein systems are the richest in gold. The conformable veins generally contain lesser, and more complexly distributed gold and are only significantly auriferous where discordant veinlets of gold bearing quartz-sulphide have been developed along with the concordant veining (Smirnov, 1981). However, gold is generally found in each of the vein types, namely quartz, quartz-sulphide and sulphide veins, especially where they intersect (Sokolov, 1995).

The principal ore mineral is native gold which occurs in the megascopic to microscopic quartz veins. The gold forms unevenly disseminated fine inclusions and veinlets, and sometimes small nests confined to places where sulphides have accumulated, or to rock fragments, and to the boundaries of quartz grains. The main vein mineral is quartz, with minor amounts of K-feldspar, biotite, ankeritic carbonate or calcite, tourmaline and albite.

The principal sulphide is pyrite with significant arsenopyrite, some marcasite and pyrrhotite. Other minerals include scheelite, gold and bismuth tellurides and selenides, galena, sphalerite, chalcopyrite, molybdenite, wolframite, magnetite and ilmenite. The sulphide minerals are generally associated with quartz, ankeritic carbonate, phlogopite, K feldspar and muscovite, with accessory apatite (turquoise is reported in the Muruntau area), monazite and TiO2 (as brookite?) (Berger, et al., 1994).

In addition to the free native gold, veins and veinlets contain gold in association with pyrite, arsenopyrite, chalcopyrite, sphalerite, bismuthinite, native Bi, ankeritic carbonate and sulpho-salts of silver. In sulphides there are fine (1 to 100 µm) segregations and thin short veinlets of gold from 5 to 10 µm thick. The gold segregations are localised in the cataclased portions of the sulphides or on boundaries of grains, particularly on the mutual contact of pyrite and arsenopyrite. On the whole the gold is fine and partially dispersed. The fineness averages 880 to 910, with traces of Ag, Cu, Bi, Pb, As, and Fe all having been identified (Smirnov, 1981).

Visible gold is rare and its presence may generally only be determined by sampling. The grade of the deposit apparently continues down dip un-diminished to at least 1000 m. The ore is oxidised to an average depth of 300 m below the surface. Silver values are generally low and comparable to those of gold.

The most recent source geological information used to prepare this summary was dated: 2002.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


Muruntau

  References & Additional Information
   Selected References:
Berger B R,  1998 - The Muruntau gold deposit, Tamdy Mountains, Uzbekistan: in Porter T M, (Ed.), 1998 Porphyry and Hydrothermal Copper and Gold Deposits: A Global Perspective, PGC Publishing, Adelaide,    pp. 213-221
Dolgopolova, A., Seltmann, R., Konopelko, D., Biske, Yu. S., Shatov, V., Armstrong, R., Belousova, E., Pankhurst, R., Koneev, R. and Divaev, F.,  2017 - Geodynamic evolution of the western Tien Shan, Uzbekistan: Insights from U-Pb SHRIMP geochronology and Sr-Nd-Pb-Hf isotope mapping of granitoids: in    Gondwana Research   v.47, pp. 76-109.
Drew L J, Berger B R and Kurbanov N K,  1996 - Geology and structural evolution of the Muruntau gold deposit, Kyzylkum desert, Uzbekistan: in    Ore Geology Reviews   v11 pp 175-196
Dunin-Barkovskaya, E.A., Aripov, U.K., Tsoy, L.A. and Kim, M.A.,  2005 - Mineralogical features and ore-forming conditions of goldbearing deposits of Uzbekistan: in   IGCP Project 486, 2005 Field Workshop, Kiten, Bulgaria, 14-19 September 2005 Geochemistry, Mineralogy and Petrology, Bulgarian Academy of Sciences,   v.43, pp. 69-74.
Graupner T, Kempe U, Spooner E T C, Bray C J, Kremenetsky A A, Irmer G   2001 - Microthermometric, Laser Raman Spectroscopic, and Volatile-ion Chromatographic analysis of hydrothermal fluids in the Paleozoic Muruntau Au-bearing Quartz vein ore field, Uzbekistan: in    Econ. Geol.   v96 pp 1-23
Graupner, T., Niedermann, S., Kempe, U., Klemd, R. and Bechtel, A.,  2006 - Origin of ore fluids in the Muruntau gold system: Constraints from noble gas, carbon isotope and halogen data: in    Geochimica et Cosmochimica Acta   v.70, pp. 5356-5370.
Kempe U, Belyatsky B, Krymsky R, Kremenetsky A and Ivanov P  2001 - Sm-Nd and Sr isotope systematics of scheelite from the giant Au(-W) deposit Muruntau (Uzbekistan): implications for the age and sources of Au mineralization: in    Mineralium Deposita   v36 pp 379-392
Kempe, U., Graupner, T., Seltmann, R., De Boorder, H., Dolgopolova, A. and Zeylmans Van Emmichoven, M.,  2016 - The Muruntau gold deposit (Uzbekistan) - A unique ancient hydrothermal system in the southern Tien Shan: in    Geoscience Frontiers   v.7, pp. 495-528.
Kotov, N.V. and Poritskaya, L.G.  1992 - The Muruntau gold deposit: Its geological structure, metasomatic mineral association and origin: in    International Geology Review   v34 pp 77-87
Marakushev, A.A. and Khokhlov, V. A.  1992 - A petrological model for the genesis of the Muruntau gold deposit: in    International Geology Review   v34 pp 59-76
Morelli R, Creaser R A, Seltmann R, Stuart F M, Selby D and Graupner T,  2007 - Age and source constraints for the giant Muruntau gold deposit, Uzbekistan, from coupled Re-Os-He isotopes in arsenopyrite: in    Geology   v35 pp 795-798
Uspenskiy, Ye.I. and Aleshin, A.P.   1993 - Patterns of scheelite mineralization in the Muruntau gold deposit, Uzbekistan: in    International Geology Review   v35 pp 1037-1051
Wan, B., Xiao, W., Windley, B.F., Gao, J., Zhang, L. and Cai, K.,  2017 - Contrasting ore styles and their role in understanding the evolution of the Altaids: in    Ore Geology Reviews   v.80,  pp. 910-922.
Wilde A R, Layer P, Mernagh T, Foster J  2001 - The giant Muruntau Gold deposit: geologic, geochronologic, and fluid inclusion constraints on ore genesis: in    Econ. Geol.   v96 pp 633-644
Yakubchuk, A.S., Cole, A., Seltmann, R. and Shatov, V.,  2002 - Tectonic setting, characteristics and regional exploration criteria for gold mineralization in central Eurasia: The southern Tien Shan province as a key example: in Goldfarb, R. and Nielsen, R., (Eds.)  Integrated Methods for Discovery: Global Exploration in Twenty-First Century; Econ. Geol.   Special Publication No. 9 pp 177-201
Zairi N M and Kurbanov N K,  1992 - Isotopic-geochemical model of ore genesis in the Muruntau ore field: in    International Geology Review   v34 pp 88-94

   References in PGC Publishing Books: Want any of our books ? Pricelist
Berger B, 1998 - The Muruntau gold deposit, Uzbekistan,   in  Porter T M, (Ed.),  Porphyry and Hydrothermal Copper and Gold Deposits: A Global Perspective,    pp 213-221
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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