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Kumtor

Kyrgyzstan

Main commodities: Au
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The Kumtor gold deposit is located in the Tian Shan Mountains at an altitude of 4000 m asl., 350 km SE of Bishkek in eastern Kyrgyzstan (Kyrgyz Republic) and about 60 km north of the border with the Peoples Republic of China (#Location: 41° 51' 35"N, 78° 12' 6"E).

The deposit was estimated to contain a pre-mining resource of >550 tonnes (18 Moz) of gold, and lies within the Hercynian Tien Shan orogenic belt which extends for some 3500 km, westward from the Gobi Desert of Mongolia in the east, to the Caspian Sea in the west.

The Tien Shan formed during the late Palaeozoic collision between the Karakum-Tarim continent, presumably a rifted fragment of Gondwana, and the Paleo-Kazakhstan continent, a Caledonian component of the huge central Asian orogenic belt and is one of the largest gold provinces on Earth. The gold deposits are controlled by the southern Tien Shan suture zone, formed after the closure of the Paleo-Turkestan ocean.

The eastern Tien Shan in Kyrgyzstan is composed of three major structural terranes, namely:  i). the North Tien Shan, representing the deformed margin of the Palaeo-Kazakhstan continent;  ii). the Middle Tien Shan, a late Palaeozoic volcanoplutonic arc with older thrust slices; and  iii). the South Tien Shan, an intensely deformed fold and thrust belt, dominated by Early Palaeozoic sediments with subordinate mafic volcanics and some felsic intrusive bodies and ophiolites.

In Kyrgyzstan, the Middle Tien Shan is cored by Meso- or Neo-Proterozoic granitic and granodioritic intrusive rocks which contain remnants of even older gneisses. The intrusive rocks are thrust NW over Meso- to NeoProterozoic sedimentary and volcanic rocks, which are in turn in fault and thrust contact with (Vendian) late Neoproterozoic to early Palaeozoic clastic sedimentary rocks. Further to the NW, additional thrust and fault slices including the Kumtor Fault Zone (KFZ) complete a section of thrusting and faulting several km wide, with the youngest rocks exposed in the footwall of the KFZ (Redmond et al., 2011).

The Kumtor gold deposit lies within the eastern part of the Middle Tien Shan, where the whole terrane is just 20 km wide. It is hosted in Vendian metasediments in the immediate hanging wall of a SW-trending moderately southeast dipping structure known as the Kumtor fault zone). In the vicinity of the deposit Cambro-Ordovician limestones in the footwall of the fault are juxtaposed with Neoproterozoic metasediments.

The Kumtor area has been subjected to four main deformation events, denoted D1 to D4. D1 and D2 are pre-Carboniferous. D1, which peaked at low to mid-greenschist facies regional metamorphism, is related to early burial metamorphism event. D2 is of Caledonian age, and folded S1 schistosity into a series of open, asymmetric, NE-SW trending F2 folds. D3, which is Hercynian (i.e., late Carboniferous to early Permian), resulted in both S1 and S2 being deformed by an south to north compressional event, producing east-west trending D3 fore-thrusts (south-dipping ), back-thrusts (north-dipping) and a series of roughly north-south trending strike-slip faults, lateral ramps and small-scale kink bands. D3 encompasses the Kumtor mineralisation episode with pre-, syn- and post-mineralisation D3 structures. Mao et al. (2004) report a late Carboniferous to early Permian 285.5±1.2 and 288.4±0.6 Ma age for the Kumtor alteration and mineralisation respectively, which overlap with the age of D3 at Jilau (Cole et al., 2000) and Muruntau, where the age of the mineralisation, however, is Triassic (Wilde et al., 2001). D4 which is of Alpine or Himalayan age, from Tertiary to the present, re-activated many pre-existing structures, and has imparted a NE-SW striking fabric, with an overall SE dip to the main faults and S1 foliation. Many of the D2 cohesive structures were re-activated during D4 resulting in unconsolidated fault breccias and gouges. The D4 tectonic axis trends SE-NW. (Redmond et al., 2011).

These multiple deformation events have resulted in the generation of several major thrust slices at Kumtor, with an inverted age relationship. The dominant structural direction is NE-SW (D
4), with moderate SE dips. Each thrust sheet contains rocks that are older than the sheet it structurally overlies. The four major stacked structural slices, which are bounded by long-lived faults that were repeatedly reactivated, comprise:
Slice 0 - Cambro-Ordovician limestone and phyllite, thrust over Tertiary sediments of possible continental derivation that in turn unconformably rest on Carboniferous clastic sediments;
Slice 1, which constitutes the Kumtor Fault Zone (KFZ), whose upper limit is the actual Upper Kumtor Fault. The KFZ is generally an up to 600 m wide, dark-grey to black, graphitic gouge zone that strikes NE-SW and dips moderately to the SE. The adjacent hanging wall rocks are strongly sheared and faulted for a distance of up to several hundred metres;
Slice 2, which includes the mineralisation, is hosted by Vendian meta-sediments, mainly grey carbonaceous quartz-sericite-chlorite schists or phyllites that are strongly faulted, folded and schistose sheared rocks. Slice 2 is delimited in the footwall and hanging wall by the Upper Kumtor and Lysii fault respectively. The mineralising event appears to be multi-phase event that has healed some of the earlier brittle features within Slice 2; and
Slice 3 composed of Vendian phyllites that exhibit several phases of folding. The schistosity has a shallow to steep to the NW or shallow to the SE, and is subdivided into three units based on the orientation of the foliation. Subsequent brittle deformation is less strongly developed as in Slices 1 and 2.

Gold mineralisation extends over a strike distance of >12 km, with ore in three main locations. The Central Deposit is the most important accumulation identified to date, developed over a strike length of 2.4 km, a vertical extent of 1 km and width of up to 300 m. The other known significant occurrences are the contiguous Southwest and Sarytor deposits that are ~1 km to the SW of the Central Deposit, distributed over sections of the next 2.5 km of the mineralised trend. The three deposits are separated by two glaciers - see the descriptions of each below.

Gold mineralisation within Slice 2, is hosted in low-grade metasediments of the 800 to 900 m thick Dzhetymtau Formation which consists of two units. The lower of the two comprises dark grey and black carbonaceous phyllites with carbonaceous chert and abundant disseminated pyrite. The upper unit consists of foliated diamictite.

According to Ivanov et al., (2000) gold mineralisation occurred in four stages:
Stage 1 - represented by weakly auriferous (<1.2 g/t Au) pervasive quartz-carbonate-albite-chlorite-sericite-pyrite alteration with minor veinlets. This early pulse, while not significantly contributing gold, may have made the host rocks more brittle, make them susceptible to the intense fracturing, veining, stockwork and hydrothermal breccia development during the next two pulses that deposited the economic gold mineralisation.
Stage 2 - is characterised by intense veining, stockwork and hydrothermal breccia formation, and was formed at a temperature of 310±15°C (Ivanov and Ansdell, 2002). The mineral assemblage during this phase is made up of variable quantities of carbonates (dolomite, ankerite, siderite), quartz, pyrite, K feldspar, sericite, and chlorite. Chalcopyrite, hematite, barite, and strontianite comprise <10 vol %, while magnetite, scheelite, ferberite, rutile, cassiterite, sphalerite, galena, native gold, calaverite, petzite, sylvanite, altaite, melonite and tetrahedrite are accessories. Stage two is characterised by K feldspar.
Stage 3 is accompanied by similar vein, stockwork, hydrothermal breccia mineralisation and associated pervasive alteration, although the mineral assemblage is dominated by carbonates and pyrite, and K feldspar is absent. Other minerals include albite, quartz, sericite, chlorite, chalcopyrite, barite and hematite, with accessory minerals scheelite, rutile, Au-Ag tellurides, and native gold. The combined mineralogy of the two main stages (2 and 3) results in an assemblage in which the feldspars combine to comprise nearly 20% of the ore, the carbonates collectively 25 to 30%, pyrite 15 to 20%, quartz 5 to 10%, with the remainder being host rock inclusions. Mineralisation is most intense, and the gold grade is the highest, where metasomatic activity was continuous through phases 2 and 3.
Stage 4 this last pulse created planar carbonate-pyrite metasomatic rocks that are associated with zones of intense deformation of previously altered phyllites and hydrothermal rocks, crosscutting earlier formed stockwork and hydrothermal breccias.

Native gold and the gold-silver tellurides are intimately associated with pyrite as inclusions, on grain boundaries and in fractures, with an average grain size of 9 µm, with grades commonly correlating with the pyrite content. Most of the mineralisation occurs as veins, veinlets and breccia bodies in which the gold occurs within the matrix. The gold fineness is 920 to 960.

Within the Central Deposit, a number of zones of gold mineralisation have been delineated. The two parallel South and North zones strike NE, dip at 45 to 60° SE, and are separated by 30 to 50 m of barren or poorly mineralised rock. The ore in both zones consist of pyrite-K feldspar-carbonate altered phyllite, but with albite dominant over K-feldspar in the North Zone. The South Zone, with a length of 700 to 1000 m and a horizontal width of 40 to 80 m, is reasonably well mineralised throughout its entire length, with an average grade of 3 to 4 g/t Au. The North Zone, which is somewhat more extensive along strike but has a similar width, has a poorer grade continuity, splitting into a number of individual lenses that have average 2 to 3.5 g/t Au. At their NE ends, the North and South Zones coalesce into the Stockwork Zone, which has the highest gold grades and the best grade continuity, consists of pyrite-K feldspar-carbonate altered phyllite. Its dimensions in the upper part of the deposit are 400 to 500 m long by 50 to 200 m wide, with an average grade of 5 to 6 g/t Au. It plunges NE at 40 to 50°, and diminishes in size below the 3700 m elevation. The Stockwork Zone extends down dip with a higher grade core beneath the bottom of the planned open pit. In the SW of the Central Deposit, the SB Zone (structurally a part of the South Zone) tops out at elevation 3900 metres, below which it widens significantly, with a total strike length to the SW of >800 m, a vertical extent of 650 m, and a width that ranges from 6 to 75 m, with grades of ~5 g/t Au.

The Southwest Deposit, is located three km along the KFZ to the SW of the centre of the Central Deposit, across the Davidov glacier. Due to a lack of drilling below the glacier, little is known about continuity of mineralisation between the two deposits. To the SW, the Southwest Zone is covered by the Sarytor glacier, beyond which additional mineralisation is known as the Sarytor Deposit. The structural/lithological framework of the Southwest and Sarytor Deposits is identical to those of the Central Deposit, with the structural dips generally at angles ranging from 20 to 50°, somewhat shallower than at the Central Deposit. A number of individual zones of mineralisation have been outlined at the Southwest Deposit within an overall envelope that is around 100 m thick and can be traced along strike for in excess of 1 km. Individual zones tend to be relatively narrow and of varying mineralisation intensity, with contacts often marked by tectonic crush zones with black fault gouge. While the footwall contacts are generally sharp and clearly defined, the hanging wall is more gradational. Gold enrichment can be observed along both contacts on many sections.

The Sarytor Deposit, is located to the SW of the Southwest Deposit, with which it is probably continuous below the Sarytor glacier. The main geological structures of the two deposits are very similar. The Sarytor Deposit strikes east-west and dips at 20 to 30°S. The thickness of the mineralised envelope is relatively consistent, varying from 80 to 120 m, with a known strike length of ~800 m, and down dip extent of 200 to 300 m. The envelope hosts three mineralised zones separated by intervals of strongly faulted host rocks, with the alteration intensity and thickness of the zones increasing southward. Metasomatism is represented by banded albite-carbonate-quartz alteration with 3 to 5% pyrite. Barite and siderite are well developed in the southern part of Sarytor. The host rocks are structurally disturbed slates and phyllites with lenses of till-like conglomerates and dolomitic slates and do not carry any elevated gold values. Background alteration is weak and represented mainly by vein-type silicification.

Production in 2011 was 18.12 t (0.583 Moz) of gold.

Remaining ore reserves and mineral resources at December 31, 2011 (Centerra Gold Inc., 2012) were:
    Proven + probable reserves - 59.694 Mt @ 3.3 g/t Au; (reserves are in addition to resources)
    Measured + indicated resources - 65.949 Mt @ 2.3 g/t Au;
    Inferred resources - 9.195 Mt @ 2.4 g/t Au.

This description is sourced from Mao et al., 2004 and Redmond et al., 2011.

The most recent source geological information used to prepare this summary was dated: 2011.     Record last updated: 23/11/2012
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.


Kumtor

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Jenchuraeva, R.J., Nikonorov, V.V. and Litvinov, P.   2001 - The Kumtor gold deposit: in Seltmann, R. and Jenchuraeva, R., (Eds.),  Palaeozoic Geodynamics and Gold Deposits in the Kyrgyz Tien Shan; GCP-373 Field Conference in Bishkek and the Kyrgyz Tien Shan; 16-25 August, 2001 & Pre-meeting Field Trip A5 of the Joint 6th Biennial SGA Meeting in Krakow, Poland,    Excursion Guidebook, pp 139-152
Mao J, Konopelko D, Seltmann R, Lehmann B, Wen Chen, Yitian Wang, Eklund O, and Toorat Usubaliev  2004 - Postcollisional Age of the Kumtor Gold Deposit and Timing of Hercynian Events in the Tien Shan, Kyrgyzstan: in    Econ. Geol.   v99 pp 1771-1780
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


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