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Kochbulak, Kairagach
Uzbekistan
Main commodities: Au Ag


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The Kochbulak meso- to epithermal Au-Ag deposit is located in Uzbekistan, 30 km northeast of the Kalmakyr-Dalnee porphyry copper deposits and 55 km southeast of the capital, Tashkent. It was emplaced within the Carboniferous Valerianov-Beltau-Kurama magmatic arc at approximately 290 to 280 Ma, and prior to mining contained approximately 135 tonnes of gold at an average grade of 12 g/t Au, 120 g/t Ag.

Kochbulak is hosted by the same magmatic arc that has produced the giant Kal'makyr and Dalnee deposits a few tens of kilometres to the southwest in the Almalyk district, and is less than 20 M.y. younger than the 315 to 290 Ma age of mineralisation at Kal'makyr (Seltmann et al., 2004, Golovanov et al., 2005).

The Kairagach deposit (see below) is hosted by similar rocks within the same caldera, some 3.5 km to the northeast of Kochbulak

Geology

The Kochbulak gold deposit is located within the Karatash caldera at the intersection of the South Angren and Lashkerek-Dukent fault zones. The caldera is filled by:
i). The Middle to Upper Carboniferous Akcha Formation which comprises more than 1000 m of andesitic and dacitic lavas, and pyroclastic rocks.
ii). The unconformably overlying Nadak Formation, which has been divided into ten units and commences with a basal volcani-mictic conglomerate and sandstone, overlain by andesitic and dacitic lavas and tuffs. The relatively thick units of lava and tuff are separated by thin interlayers of tuffite, sandstone and siltstone.
iii). The Upper Carboniferous Oyasai and Upper Permian to Lower Triassic Kyzylnura formations which comprise rhyolitic lava and pyroclastics confined to the southern part of the caldera (Islamov et al., 1999).

The volcanic succession of the caldera, which represents a calc-alkaline to sub-alkaline, high potassic latite series, is cut by dykes, sub-volcanic intrusions and associated extrusives. The sequence is also cut by Middle Carboniferous pre-mineral granodiorite and monzodiorite porphyry which are comagmatic with the Akcha Formation at the base of the caldera, and by minor rhyolite intrusions related to the Oyasai Formation. Pre-mineral basic dykes of Early Permian age are widespread, while rhyolite, granosyenite, syenite, monzodiorite porphyry and late basic dykes are post-mineral (Islamov et al., 1999).

The deposit area is cut by four large, near north-south trending faults which dip steeply to the west and southwest. Further sets of intervening fractures parallel to the main trend are found in the deposit area, as are intra-formational detachments along the contacts between massive lava units (Islamov et al., 1999).

The Kochbulak mineralisation is restricted to volcanics of the Middle to Upper Carboniferous Nadak Formation on the northern flank of the caldera, close to the Shaugaz Fault. The setting corresponds to the near vent facies of a strato-volcano which was rimmed by sub-volcanic intrusives. Approximately 120 orebodies have been tested, controlled by 32 mineralised structures within a volume of some 4500 x 3000 x 550 m (Kovalenker et al., 1997; Islamov et al., 1999; Yakubchuk et al., 2002).

Alteration and Mineralisation

Three types of orebody are recognised, as follows:
i). Steeply dipping, north to northeast aligned veins (40% of the reserve) controlled by the major and intervening faults described above. Some 45 of these steep veins are recognised;
ii). Moderately dipping, (20 to 40°) near east-west veins (20% of reserves) which are concentrated where the north-south fault set intersects the intraformational detachments, also mentioned above, and
iii). Pipe-like orebodies (40% of the reserves), which are composed of mineralised explosion breccia and which terminate the steeply dipping vein set. There are some 14 pipes, each with a small diameter, but high grade (Islamov et al., 1999).

Mineralisation occurs as massive, banded, brecciated and breccia like textures, with festoon and incrustate structures. Quartz is the dominant gangue mineral, varying from coarse-grained to meta-colloidal to drusy, chalcedonic and amethyst, accompanied by subordinate carbonates and barite. The sulphide content of the two vein types is generally <10%, while in the breccia pipes it may reach 20%. Gold is mainly present as microscopic inclusions, occurring as sheeted, dendritic and cloddy grains in the upper levels and as spongy and drusy gold lower in the deposit. The finest gold is within meta-colloidal quartz, calaverite, sylvanite and altaite, while that in goldfieldite, chalcopyrite and galena is of lower fineness. Electrum accompanies sulphosalts and sulphostannites (Islamov et al., 1999).

The gold mineralisation is present in three associations, namely:
i). Gold-telluride, which occurs as calaverite, petzite, sylvanite, hessite, stutzite, empessite, goldfieldite and a wide range of other tellurides, and is particularly well developed in the upper level veins and in shallow-formed breccia pipes.
ii). Gold polysulphide comprises the association of native gold with sulphides of Cu, Pb, Zn, Bi and Sb, and is most frequently found in the upper levels of both the steep and flat veins.
iii). Gold-pyrite, which is found to varying degrees throughout the system, but is best developed and mineralised with increasing depth. It predominantly occurs as disseminated, uneconomic mineralisation with finely dispersed gold in pyrite, generally only averaging 4 g/t Au (Islamov et al., 1999). In general, the explosive breccia pipes are found in the upper levels of the deposit, passing through a transition zone to steeply dipping mesothermal veins at depth. Mineralisation is known to extend a depth of more than 2000 m.

The pattern of development of the three gold mineralisation associations is zoned both vertically (as described above), and laterally, with the gold-telluride association being the most proximal, within and immediately adjacent to the veins, flanked by the gold-polysulphides, passing out into the lower grade quartz-sulphide association. The distribution is also complicated by the telescoping and resultant superposition of the three zones from different episodes of mineralisation as the deposit evolved (Islamov et al., 1999).

The host volcanics underwent a mild propylitic alteration forming chlorite-carbonate and epidote prior to mineralisation. Alteration related to mineralisation within both the steeply dipping and shallow veins is evident as a regular zonation, with a progressive outward gradation from the ore vein to: i). hydrosericite; ii). adularia-sericite; and iii). chlorite-carbonate, to iv). the 'unaltered' country rock. All of the altered rock contains pyrite, which decreases from around 30% in the hydrosericite to 10% in the chlorite-carbonate zone. Pervasive sericite-hydromica dominates in the exploited parts of the deposit, while the chlorite facies was only penetrated in drilling at depths of >1200 m. The breccia-pipe bodies are accompanied by an intense silicification of the hosts, accompanied by variable amounts of sericite, alunite and diaspore (Islamov et al., 1999).


Kairagach

The Kairagach high-sulphidation (acid-sulphate) gold deposit is hosted by similar rocks within the same caldera, some 3.5 km to the northeast of Kochbulak. It is confined to a volcanic andesite-dacite sequence in the northeastern section of the 15 km diameter Karatash caldera. In the central part of the caldera, the volcanogenic sequences are intruded by a 1.2 x 3 km stock-shaped subvolcanic body of porphyritic trachyandesite, the northern endocontact part of which hosts the ore-bearing zones of the Kairagach deposit. The main host volcanic suite at Kairagach is composed of lithoclastic andesite and andesite-dacite tuffs alternating with porphyritic andesite lavas. These are intruded by subvolcanic bodies of dacite porphyry (with large feldspar phenocrysts) and diorite porphyry intrusions, as well as granodiorite porphyry and NE striking porphyritic dolerite dykes. The gold-sulphide-selenide-telluride mineralization of the Diabazovaya zone, which encloses the main gold resources, is associated with these dykes (Kovalenker et al., 2003).

The trachyandesite-porphyry subvolcanic body is rimmed by 5 to 500 m wide zones of intense alteration. The most extensive of these are early propylitic albite-chlorite, chlorite-carbonate, sericite-chlorite and sericite-carbonate assemblages which are overprinted by pre-ore and ore related wall rock alteration. Preore rocks are mostly quartzites (silicification), frequently containing diaspore (similarly to the Kochbulak deposit), usually developed along the contacts of dolerite dykes, and advanced argillic pyrophyllite-diaspore-kaolinite-alunite assemblages. The latter are broadly zoned with a kaolinite-alunite association at upper levels, supplemented by diaspore and pyrophyllite in the lower sections. The quartzites grade downward into more extensive quartz-carbonate-sericite-pyrite alteration with with narrow zones of feldspar alteration. Four elongated 3 to 5 km long zones of silicification hoating ore mineralisation have been defined (Diabazovaya, Pervaya, Chukurkotanskaya and Bedrengetskaya), controlled by NE-oriented faults. Economic mineralisation had beenoutlined in the first two of these in 2003, that together represent the Kairagach deposit. Both are located close together in the northern contact zone of the subvolcanic trachyandesite porphyry stock in the immediate proximity of dolerite-porphyry dykes. The deposit is characterised by a distinct Au-Sn-Bi-Se-Te mineralised assemblage (Kovalenker et al., 2003).

The Diabazovaya Zone is the richest and best-studied. It strikes at 50° and dips at from 10 to 80°. The axial sections of the zone are represented by an intricate system of upward-expanding quartz, quartz-barite, and barite veins and lenticular stringers and breccia bodies with disseminated, and stringer-disseminated ore mineralisation that are closely associated with 0.5 to 15 m thick dolerite-porphyry dykes. Both the dykes and enclosing volcanic rocks are intensely silicified, sericitised, and pyritic (Kovalenker et al., 2003).

Two mineral types are distinguished within the Kairagach deposit (Kovalenker et al., 2003).
Gold-quartz type, substantially composed of quartz with ≤3 to 5 wt.% sulphide. predominantly pyrite. These ores are spatially associated with 'quartzite' silicified zones with a characteristic massive fabric and low concentrations economic minerals, although the Au content can be as high as several hundred grams per tonne in some intersections. Quartz is fine-grained to amorphous, with abundant caverns and pores, and relicts of the enclosing volcanics. Cavities and caverns are often filled with younger white transparent quartz and barite with sphalerite and different sulphide-selenide-telluride mineralisation and significantly increased Se, Te, Bi and Sn.
Gold-sulfide-selenide-telluride type, only found in the Diabazovaya zone. It is represented by vein and lenticular bodies, as well as by stringer-disseminated and nest-shaped quartz, quartz-barite and barite accumulations with sulphides, sulphosalts, selenides and tellurides, irregularly distributed both within 'quartzite' and within quartz-sericite-carbonate-pyrite alteratio assemblages. These ores represent the bulk of the ressource and have variable contents of gold, silver.

The Kairagach deposit has a potential resource of 50 t of Au and 150 t of Ag at a comparable grade to Kochbulak and is similar in many aspects, but with variations in detail (Islamov et al., 1999).

The most recent source geological information used to prepare this decription was dated: 2004.    
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.


  References & Additional Information
   Selected References:
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.
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.
Islamov, F., Kremenetsky, A., Minzer, E. and Koneev, R.  1999 - The Kochbulak - Kairagach ore field: in Shayakubov, T., Islamov, F., Krementsky, A. and Seltmann, R., (Eds.),  Au, Ag, and Cu deposits of Uzbekistan International Field Conference of IGCP-373, Excursion B6 of the Joint SGA-IAGOD symposium, London/Tashkent, 27/28 August - 4 September 1999   Excursion guidebook pp 91-106
Kovalenker, V.A., Plotinskaya, O.Yu., Prokofev, V.Yu., Gertman, Yu.L., Koneev, R.I. and Pomortsev, V.V.,  2003 - Mineralogy, Geochemistry, and Genesis of Gold-Sulfide-Selenide-Telluride Ores from the Kairagach Deposit (Uzbekistan): in   Translated from Geologiya Rudnykh Mestorozhdenii, Vol. 45, No. 3, 2003, pp. 195-227. Geology of Ore Deposits (Pleiades Publishing)   v.45, pp. 171-200.
Plotinskaya, O.Yu., Kovalenker, V.A., Seltmann, R. and Stanley, C.J.,  2006 - Te and Se mineralogy of the high-sulfidation Kochbulak and Kairagach epithermal gold telluride deposits (Kurama Ridge, Middle Tien Shan, Uzbekistan): in    Mineralogy & Petrology,   v.87, pp. 187-207.
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
Zu, B., Seltmann, R., Xue, C., Wang, T., Dolgopolova, A., Li, C., Zhou, L., Pak, N., Ivleva, E., Chai, M. and Zhao, X.,  2019 - Multiple episodes of Late Paleozoic Cu-Au mineralization in the Chatkal-Kurama terrane: New constraints from the Kuru-Tegerek and Bozymchak skarn deposits, Kyrgyzstan: in    Ore Geology Reviews   v.113, https://doi.org/10.1016/j.oregeorev.2019.103077, 17p.


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, its employees and servants:   i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and   ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.

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