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Sayak I, III and IV, Tastau |
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Kazakhstan |
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Cu Au Mo
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Sayak group of skarn deposits are located in the 'Balkhash-Ili' zone of the Upper Palaeozoic Kazakh-Mongol magmatic arc in eastern Kazakhstan, which also embraces the large Kounrad, Aktogai Group and other porphyry Cu-Au-Mo deposits (Bespaev and Miroshnichenko, 2004). Sayak is approximately 140 km east of the Kounrad deposit, a few tens of kilometres north of Lake Balkhash, and 450 km NNW of Almaty. The deposits of the district are, with respect to Sayak 1: Sayak 2 - 5 km to the WSW; Tastau - 10 km to the west; Sayak 3 - 10.5 km to the west (and 1.7 km NNW of Tastau); Sayak 4 - 14 km to the WSW (#Location: Sayak 1 - 46° 59' 50"N, 77° 24' 15"E; Sayak 4 - 47° 0' 7"N, 77° 13' 26"E; 24' 15"E; Tastau - 46° 59' 18"N, 77° 16' 32"E).
The skarns of the district are medium sized Cu-Au-Mo deposits hosted by a Middle Carboniferous carbonate unit, and related to Carboniferous (330 Ma) granodiorite intrusives, similar to those hosting the large porphyry copper deposits of the same magmatic arc. The deposits contain more than 1 Mt of Cu and 30 tonnes of Au (Seltmann et al., 2004; Kudryavtsev, 1996).
The total resource quoted comprises 55 Mt @ 1.1 to 3.2% Cu and 0.2 to 2.1 g/t Au.
Geology
The Sayak deposits are hosted within the Sayak Group, a sequence of marine volcanogenic-carbonate-terrigenous molasse sediments of Middle Carboniferous age (from middle Visean to middle Moscovian). The base of this sequence, is separated from the underlying Siluro-Devonian terrigenous sediments and pre-Silurian ophiolites by an angular unconformity, and the intervening Late Devonian Alabiin Formation. The Alabiin Formation comprises 350 to 500 m of alternating greywacke-sandstone, siltstone, gritstone and tuffite, with thin layers of limestone which pass into calcareous sandstone. The succeeding basal unit of the Sayak Group, the Buruntas Formation, is made up of 650 m of intercalated greywacke, polymict and arkosic sandstone, conglomerate, tuffaceous sandstone and tuffite. The conformably overlying Late Carboniferous Tastyquduk Formation totals 1300 to 1650 m in thickness and commences with a sequence of clastic sediments which progressively fine upwards from a basal conglomerate. The uppermost 200 m of this formation is a carbonate unit that hosts all of the skarns of economic significance within the district. It commences in the west with 1 to 2 limestone beds, increasing to 5 to 7 in the centre of the basin, coinciding with a proportionate increase in the thickness of the unit. The carbonates of the Tastyquduk Formation are conformably followed by the lower Moscovian Kungheisayak Formation, composed of a 150 m thick basal conglomerate, overlain by 1000 m of rhythmically intercalated greenish-grey and buff sandstone, siliceous siltstone, thin limestone and tuffite beds (Kudryavtsev, 1996).
The Sayak Group is more than 4000 m thick and is intruded by numerous sub-volcanic bodies of basic, intermediate and felsic composition. The sedimentary basin in which it was deposited, the Sayak Basin, is an asymmetric graben with steep southern and southwestern, and gentle northern slopes. The structure of the basin is complicated by several anticlinal flexures and abundant faulting. All of the known skarn deposits are associated with local anticlinal ore controlling structures (Kudryavtsev, 1996).
Five main composite plutons cut the Sayak Basin sequence, localised in the central axis and the southern and northern margins of the basin, namely, the Aqshoqy, Zhambas, Kungheisayak, Lebai and Umit plutons. The latter two are accompanied by Cu-Mo mineralisation, both in skarns and as disseminated, low grade porphyry-style. Two extended dyke swarms are evident, a northeast trending swarm associated with the Kungheisayak intrusive, while a northwest aligned set that cuts the northeast faults is associated with the Umit pluton. The Umit and Lebai plutons are predominantly composed of granodiorite, although quartz diorite represents the initial phase of their intrusion. The other three plutons are tonalitic in composition, cutting initial phases of plagiogranite and hornblende-biotite granite. The tonalites of the Kungheisayak pluton fall within the age range of 347 to 307 Ma, while its plagiogranite is 314 to 310 Ma. In comparison, the granodiorite of the Umit pluton are 335 to 304 Ma (Kudryavtsev, 1996).
Alteration and Mineralisation
Mineralisation within the Sayak district is represented by i). hydrothermal skarn, ii). porphyry Cu and iii). veins. All of the mineralisation of economic significance however, is skarn-type, containing arseniferous Co-Au-Mo-Cu. The most important are in 'near intrusion' locations on the northern contact of the Umit and western margin of the Lebai plutons, within a 2 to 3 km wide belt, hosted by Tastyquduk Formation carbonates. All are characterised by the presence of dyke swarms of diverse composition, systems of cross-cutting faults and fracture zones in anticlinal crests capped by impermeable tuffaceous siltstones. Individual orebodies have strike lengths of from 0.5 to 12 km, although the distribution of grade is very patchy, occurring as veins, or as sheet-, ribbon-, pipe- or pocket-like masses, or as irregular shapes (Kudryavtsev, 1996).
The deposits exhibit a zonation outwards from the intrusive contact along the host carbonate bed, as follows:
i). An 'inner zone' of magnetite, immediately outboard of the granodiorite contact, with Mo-Cu ores, native gold, and associated late garnet and pyroxene-garnet skarn alteration. Pre-skarn potassic and sodic alteration of alumino-silicate rocks are rare. Poorly mineralised endo-skarn alteration of the granodiorite plutons and dyke rocks was coeval with the skarn development.
ii). The 'middle part' of the ore-metasomatic zonation is represented by Au-Bi-Cu mineralisation associated with epidote- and actinolite-altered carbonates. The actinolite rich segment marks the outer rim of the ore deposit.
iii). The 'outer' propylitic transition to unaltered marble and limestone is characterised by quartz-calcite-chlorite alteration (Kudryavtsev, 1996).
The largest of the deposits of the Sayak district is Sayak-I (Sayak), developed on the western exo-contact of the Lebai pluton, and concentrated in a north to northwest trending anticlinal axis which plunges to the south. The fold is asymmetric, with a gently dipping (<15°) western limb, opposite the pluton margin. In contrast, the eastern limb dips into the pluton at 25 to 30°, steepening to 60 to 90° at depth. The area also corresponds to the maximum development of the host carbonate of the upper Tastyquduk Formation which at Sayak-I totals 200 m in thickness. The host carbonate is composed of two beds which are 30 to 50 m and 130 to 150 m thick respectively, separated by a thin impersistent layer of tuffaceous siltstone and sandstone, and overlain by a thick bed of tuffaceous siltstone to siltstone. The axis of the mineralised fold is almost parallel to the pluton contact in the centre of the deposit, where the contact is near vertical, although, it shallows to the north. The pluton is composed of granodioritic adjacent to Sayak-I, while the dense array of dykes that cut the ore deposit are predominantly diorite porphyry and dolerite, with lesser granite and plagiogranite. These dykes are generally 3 to 5 m thick and of different relative ages, including pre-, intra- and post-ore (Kudryavtsev, 1996).
Contact metamorphism has hornfelsed the silicate lithologies and produced marbles from the carbonates, while metasomatic processes have formed the skarns over a width of 500 to 1000 m outboard from the intrusive contact, over a length of 3 km parallel to the pluton margin. Skarn alteration has also locally affected dykes of diorite and granodiorite. At the immediate intrusive contact, the limestone host is completely altered to skarn over a 40 to 100 m thickness parallel to the contact, forming a steeply dipping, generally tabular body. On its western margin, this skarn body splits into isolated sheets parallel to the host bedding, which finally give way to marble and recrystallised limestone at the rim of the skarn zone, down dip on the western limb of the fold (Kudryavtsev, 1996).
The main skarn is composed of pyroxene-garnet and pyroxene-feldspar. Post-skarn alteration is widespread, with quartz-actinolite-calcite-chlorite and epidote rich propylitic rocks well developed at the transition from skarn to marble and limestone. A major magnetite body formed on the northern part of the deposit almost simultaneously with the epidote- and actinolite-bearing alteration. Au-Mo-Cu mineralisation occurs in both skarn and magnetite hosts, as well as in actinolite and quartz-calcite-chlorite calc-silicate altered rocks, while a steeply dipping 10 to 15 m thick zone of poorly disseminated mineralisation occurs within the granodiorite intrusion. The bulk of the ore is concentrated as a complex 'near contact' lode, dipping roughly parallel to the host carbonates and to the intrusive contact to a depth of 500 m. The Cu grades are highest in the centre of the lode, decreasing down dip and towards the margins. Mo mineralisation is best developed on the gently dipping west limb of the fold where it is hosted by the basal sections of the skarn and by altered sandstone in the footwall of the carbonate unit. These bodies extend for 350 to 750 m in a northeast direction, parallel to the fold axis, with widths of 25 to 60 m in the northeast and up to 250 m in the southeast. They average 0.35% Mo with 0.2 to 0.3% Cu (Kudryavtsev, 1996).
There is a well developed zonation within the orebody. Pyroxene-feldspar and pyroxene-garnet skarns contain magnetite rich ore and limited bornite-chalcopyrite, while the garnet skarn hosts molybdenite-bornite-chalcopyrite to the north. Arsenopyrite-cobalt mineralisation predominates in the outer actinolite and quartz-calcite-chlorite altered rocks. The intensity of mineralisation is related to the Fe/(Fe+Mn) ratio of the alteration minerals. The highest grades accompany skarns with 70-80 mol% andradite and 20-30 mol% grossular and high Fe pyroxene (>60 mol% hedenbergite). In contrast, the barren skarn has >60 mol% grossular and 5 to 25% hedenbergite. Similarly, rich Cu mineralisation accompanies Fe-rich epidote and chlorite of the thuringite series. The primary ores contain chalcopyrite, magnetite, bornite, pyrrhotite, arsenopyrite and cobaltite, with rare pyrite, molybdenite, marcasite, melnikovite-pyrite, native gold, galena, sphalerite, electrum, calaverite, petzite, sylvanite and nessite. Oxidation is only poorly developed, limited to depths of 10 to 15 m, accompanied by the formation of azurite, cuprite, chalcocite and scorodite (Kudryavtsev, 1996).
There is strong evidence to confirm that the metallic mineralisation post dates skarn formation. Fluid inclusion studies indicate a reverse temperature zoning of skarn and ore minerals. The earliest and highest temperature (590 to 540°C) gersdorffite-arsenopyrite-cobalt assemblage is found in the relatively low temperature (430 to 210°C) outer quartz-calcite-chlorite rocks. The medium temperature (540 to 470°C) emplectite-chalcopyrite-pyrrhotite assemblage is typically found in the middle epidote-actinolite (630 to 440°C) zone, while the relatively cool wittichenite-molybdenite-chalcopyrite mineralisation occurs in the high temperature (670 to 600°C) proximal skarn (Kudryavtsev, 1996).
In addition to Sayak-I, there are a number of other similar skarns with significant economic mineralisation, including Sayak-III and IV, and Tastau.
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.
Sayak 1 Sayak 2 Sayak 3 Sayak 4 Tastau
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Kudryavtsev Yu K 1996 - The Cu-Mo deposits of Central Kazakhstan: in Shatov, Seltmann, Kremenetsky, Lehmann, Popov and Ermolov (Eds.) Granite-Related Ore Deposits of Central Kazakhstan and Adjacent Areas INTAS-93-1783 Project, St. Petersburg, 1996 pp 119-145
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Li, G.-M., Cao, M.-J., Qin, K.-Z., Hollings, P., Evans N.J. and Seitmuratova, E.Y., 2016 - Petrogenesis of ore-forming and pre/post-ore granitoids from the Kounrad, Borly and Sayak porphyry/skarn Cu deposits, Central Kazakhstan: in Gondwana Research v.37, pp. 408-425.
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