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 Kal¹makyr (Seltmann et al., 2004, Golovanov et al.,2005).
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).
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2006)