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Kochbulak

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 Kalmakyr 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 Kalımakyr (Seltmann et al., 2004, Golovanov et al.,2005).

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).

The Kairagach gold deposit is hosted by similar rocks within the same caldera, some 3.5 km to the northeast of Kochbulak. It 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 summary 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
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   v87 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.

   References in PGC Publishing Books: Want any of our books ? Pricelist
Seltmann R and Porter T M, 2005 - The Porphyry Cu-Au/Mo Deposits of Central Eurasia: 1. Tectonic, Geologic & Metallogenic Setting and Significant Deposits,   in  Porter T M, (Ed),  Super Porphyry Copper and Gold Deposits: A Global Perspective,  v2  pp 467-512
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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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