Taldy Bulak (or Taldy Bulak-Talus and Andash are members of a group of gold rich, Ordovician age, porphyry related copper deposits which define a 30 km long mineralised corridor in northern Kyrgyzstan, approximately 120 km to the southwest of the capital, Bishkek.
The principal deposits include porphyry style mineralisation at Taldy Bulak, Andash and Tokhtonysai and skarn ores at Aktash (Nikonorov et al., 2000; Jenchuraeva et al., 2005). All are developed within rocks formed in the Lower Palaeozoic Kipchak magmatic arc, and are associated with Middle Ordovician diorite to monzodiorite porphyries which intrude Late Cambrian to Middle Ordovician island arc terriginous volcanogenic sequences (Seltmann et al., 2004; Jenchuraeva, 1997).
Taldy Bulak, is the largest of the group. Mineralisation is predominantly hosted by a stock of Middle Ordovician diorite porphyry developed on the margin of a more extensive Late-Ordovician to Early-Silurian granite-granodiorite-leucogranite complex which intrudes a Late Cambrian to Middle Ordovician island arc sequence that locally comprises shale, siltstone, sandstone, conglomerates, tuffs and Middle Ordovician porphyritic andesite and basaltic-andesite. This latter sequence overlies a basement of Neoproterozoic platformal carbonate, sandstone, phyllite and shale, and is respectively overlain and intruded by Devonian basalt, andesite, rhyolite, tuffs and volcaniclastics and an intrusive complex of quartz diorite, monzodiorite, granodiorite and granite of a similar Devonian age.
The mineralised stock covers an area of approximately 1200x800 m at surface and comprises 50 to 70% plagioclase, 15% mafic minerals and 15% quartz. It has a complex shape, with an irregular contact characterised by numerous embayments and large xenoliths of the surrounding country-rock. The dense xenolithic zone near the outer contact resembles a breccia with a diorite porphyry matrix. The diorite becomes finer grained and more porphyritic in the vicinity of the xenolith zone where it also tends to have a higher gold grade. The stock includes an early phase, dark-grey gabbro-diorite and is cut by a series of late acid and basic dykes.
The main mineralisation is almost completely confined to within the diorite porphyry stock, occupying approximately 75% of its surface exposure, and is accompanied by dense fracturing and intense alteration. The fracturing, which reaches a density of as much as 150 to 200 per metre, is healed by grey quartz which forms five main, steeply dipping (70-90°) mineralised veins sets, each with a differing trend direction, namely, 32°, 70°, 90°, 140° and 170°. The best mineralised set trends east-west. The veins vary from 0.5 to 25 mm in thickness.
The orebody has a general elliptical shape, defining an overall mineralised envelope with dimensions of 1100x700 m, enclosing three east-west trending lensoid developments of higher grade ore, comprising a northern 200x60 m, central 1200x25 to 170 m and southern 700x140 m zone. Mineralisation has been traced to a depth of 500 m below the surface.
An early resource estimate quoted by Mutschler et al., (2000) comprised 540 Mt at 0.27% Cu, 0.5 g/t Au, 0.008% Mo, based on data sourced from the Metal Mining Agency of Japan (Kamitani and Naito, 1998; Hedenquist and Daneshfar, 2003). However, following a program of close spaced drilling by Gold Fields of South Africa, a SAMREC compliant, open pit optimisation resource of 423 Mt at 0.17% Cu, 0.46 g/t Au, 0.01% Mo was calculated in 2010.
At Andash, 15 km to the east of Taldy Bulak, similar mineralisation is present as a flat dipping, 400 x 200 m stockwork within a granodiorite to diorite porphyry host, associated with pipe- to lens-like explosive breccia zones which have been silicified to form 'quartzose metasomatites'. The principal alteration types recorded include quartz-feldspar, quartz-tourmaline, propylitic, phyllic and argillic. Copper mineralisation is generally related to zones of quartz-sericite-chlorite alteration, while gold is associated with quartz-tourmaline veining and argillic altered zones. The principal metallic minerals include pyrite, chalcopyrite, magnetite and hematite with minor sphalerite, tetrahedrite, galena and pyrrhotite. Gold is closely associated with pyrite, and occurs at grades of around 2.6 g/t Au with 0.4% Cu (Seltmann, et al., 2004). Kentor Gold Limited announced JORC compliant ore reserves and mineral resources in 2010, as follows at Andash, with potential for expansion:
Indicated + inferred resources - 19.2 Mt @ 0.40% Cu, 1.10 g/t Au;
Proven + probable reserves - 16.0 Mt @ 0.40% Cu, 1.05 g/t Au.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2010)