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Bozshakol, Boshchekul

Kazakhstan

Main commodities: Cu Au
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The Bozshakol or Boshchekul porphyry Cu-Au deposit is located near the town of Eqibastuz in northeastern Kazakhstan, approximately 1000 km north of Almatyand 220 km northeast of Astana (#Location: 51° 51' 23"N, 74° 18' 59"E).

The deposit was emplaced in an ensimatic segment of the Kipchak magmatic arc during the Late Cambrian to Early Ordovician at approximately 481 Ma.

Geology

The Bozshakol district straddles an east to ENE trending anticline, which is on the limb of a larger regional anticlinorium that deforms a sequence of predominantly Lower to Middle Cambrian volcanogenic rocks of calc-alkaline to sub-alkaline composition, typical of an island arc setting. Some 5 to 15 km to the south of the ore deposit, the core of the anticlinorium exposes underlying late Neoproterozoic (Vendian) to Cambrian rocks of an ophiolite association and Proterozoic metamorphics. The Cambrian volcanogenic rocks are cut by a progression of Cambrian to Ordovician intrusives, ranging from gabbros through tonalite, granite and syenite and finally gabbro again. Mineralisation is associated with Middle Cambrian granitoids of this suite, mainly tonalites, with the bulk of the ore being hosted by the volcanic wallrocks to the intrusives. The volcanogenic and intrusive rocks are overlain by post-ore Ordovician sediments (Kudryavtsev, 1996).

Locally, the lowest sections of the Cambrian volcanogenic sequence are exposed in the core of the Bozshakol anticline, from the western flank of the orebody, to approximately 4 km to the northeast of the deposit. The sequence commences with greenish-grey sandstone, siliceous siltstone, tuff and dacitic lavas with thin interlayers of pale jasper and andesitic flows, overlain by a unit of predominantly andesitic lavas and tuffs which host the northern section of the East Bozshakol deposit. The succeeding unit, also of Lower to Middle Cambrian age, comprises a basaltic rocks that outcrop within the district as folded inliers (Kudryavtsev, 1996).

On the northern limb of the anticline the sequence is dominated by a sedimentary-volcanogenic package that overlies the units described above. The volcanic suite of this package is composed predominantly of amygdaloidal and massive hornblende-pyroxene basaltic rocks. These are intercalated with compositionally similar layers of litho- and crystallo-clastic tuffs and thin bands of gritstone, sandstone, siltstone and siliceous rocks. The rocks of this limb have been simply folded, with dips averaging 40 to 45° (Kudryavtsev, 1996).

Intrusive rocks are areally less extensive than the volcano-sedimentary intruded sequence and have been subdivided into four groupings, as follows:
i). an initial Middle Cambrian complex of gabbro and diorite occurring as small bodies in the central and eastern part of the district;
ii). a spatially separated, closely succeeding Middle Cambrian Bozshakol granitic complex, exposed at surface as quartz-diorite and tonalite stocks, including the main Bozshakol intrusion and others east and south of the deposit, as well as apophyses of porphyritic tonalite, dykes of tonalite porphyry and the more deep seated Dalnezapadny tonalite intersected in drilling at depths of 300 to 400 m;
iii). a Middle Ordovician syenite complex, which includes the Ashchikol intrusion and a series of associated scattered dykes to its west and east;
iv). an Upper Ordovician sub-alkaline gabbro complex, known as the Southwestern Intrusive Sheet. The youngest magmatic activity in the district is represented by rhyolite sills within Middle Ordovician sediments (Kudryavtsev, 1996).

The core intrusive within the district is the Bozshakol Intrusion. Gravity data indicates it to be a sheet-like mass that can be traced over a distance of 7 km long by 0.5 to 0.7 km wide, underlying a corridor of outcropping, isolated granitoid stocks, dykes and apophyses, the largest of which separates the main Bozshakol and the East Bozshakol orebodies. The early phases of the Bozshakol Intrusion are represented by fine-grained hornblende diorite and diorite porphyry which cut Lower to Middle Cambrian volcanic rocks in the western part of the Bozshakol deposit. These early phases are cut in turn by medium- to coarse-grained quartz diorite and tonalite, and subsequently by porphyritic tonalite, the first of the ore related porphyry bodies (Kudryavtsev, 1996).

The porphyritic tonalite of the first mineralising phase is composed of up to 80% phenocrysts, most of plagioclase, set within a cryptic groundmass. In the western part of the Bozshakol deposit, it occurs as a steeply plunging stock some 300 m in diameter, with numerous faulted offsets across a northeast trending fracture set. In the East Bozshakol deposit it occurs as several lenticular dyke-like bodies (Kudryavtsev, 1996).

The second phase of ore-related granitic rocks are represented as tonalite porphyry dykes in the eastern part of the deposit. It differs from the earlier phase in that it has lesser phenocrysts - 30 to 40%, of which idiomorphic quartz is particularly prominent - in a fine-grained to crypto-felsic groundmass. This phase intrudes Lower to Middle Cambrian country rocks, but has only rarely been observed cutting Middle Cambrian lithologies. In the southeastern parts of the district, granitic rocks of the Bozshakol Intrusive are overlain by sediments containing Late Cambrian to Early Ordovician fossil fragments. In addition, grit bands within Middle Ordovician terrigenous sediments in the district contain mineralised pebbles of the ore bearing granitoids. Rb-Sr dating of the phaneritic tonalite yielded an age of 481 ± 23 Ma. These observations all suggest a latest Cambrian to Early Ordovician age of intrusion and mineralisation (Kudryavtsev, 1996).

The unconformably overlying Middle Ordovician sequence is largely restricted to the southeast of the deposit and is composed of grey carbonateŠterrigenous sediments with lenses and horizons of gritstone, siliceous siltstone, and shale at the base. It partially covers the ore on the southern margin of the deposit and has only been affected by post ore hydrothermal alteration (Kudryavtsev, 1996).

Alteration and Mineralisation

The emplacement of the mineralised intrusives and the distribution of ore are controlled by a northeast trending fault set. A second, northwest striking set offset the mineralised system and divide it into four blocks Š the eastern, central, western and far-western blocks. Ore grade mineralisation is restricted to the central block, which includes the main Bozshakol and South Bozshakol deposits. The eastern block only contains un-economic mineralisation within Lower Cambrian andesite and andesitic tuff. Minor lenses of ore grade intersected by drilling in the western block are pinched out, fault dislocated extremities of the main orebody in the central block. The far-western block contains disseminated Zn-Cu mineralisation within pyritic, propylitic altered hosts, principally Lower to Middle Cambrian mafic volcanic rocks. Overall, the Cu-Mo mineralisation is localised in altered volcanics adjacent to dyke-like apophyses from deeper intrusions of porphyritic tonalite and tonalite porphyry. It passes outwards into zones characterised by base metal enrichment and siderophile geochemical associations. The tonalite porphyry dykes are mineralised to a much lesser extent, while at depth some ore has also been found within phaneritic granite (Kudryavtsev, 1996).

The mineralisation in both the main Bozshakol orebody and at East Bozshakol represent an elongate stockwork zone with a steep, mostly northerly dip, and a northeast plunge. It lenses out into a series of fingers on the lateral (northeast and southwest) extremities and down dip, related to both faulting and the distribution of dykes. The maximum length of the main orebody is 3 km, with a thickness of near 285 m at its widest point, and a down-dip extent of over 600 m. Bozshakol South has been traced for 425 m along strike, has a thickness of 33 m and persists down dip for 120 m. The mineralisation at East Bozshakol extends over a length of from 1.2 to 1.5 km, is 400 to 500 m wide and has a grade of 0.3 to 0.35% Cu (Kudryavtsev, 1996; Seltmann et al., 2004 and sources quoted therein).

Ore grade mineralisation is located within the inner sections of the alteration system, largely associated with a phyllic assemblage of quartz-sericite-carbonate-chlorite in the volcanic hosts and by quartz-sericite within granitic rocks. Dykes of tonalite porphyry and adjacent narrow zones near their contacts have been affected by quartz-hydromica argillic alteration. However, while phyllic alteration dominates, all of the stockworks, particularly on their northern margins, show evidence of a biotite (potassic) phase. Potassic alteration is most marked in the central portions of the system, focused on the granitoid intrusions, where K feldspar pseudomorphically replaces plagioclase within the granitoids, and forms veins and veinlets accompanied by quartz and less frequently by biotite. Fragments of this style of alteration are found within intra-ore hydrothermal breccias (Kudryavtsev, 1996).

These central zones, which form the elongate core of the mineralised system, are surrounded by shells of biotite enrichment and by a pyritised propylitic halo. Weak biotite alteration has been observed in Cambrian volcanics as much as 1 km distant from the orebody. Quartz-chlorite altered volcanics with abundant veinlets and disseminations of pyrite are developed closer to the orebody, while actinolite bearing varieties are localised closest to the intrusive contacts. These manifestation of propylitisation form a halo that is 6.5 km long and ranges from 0.4 to 1 km wide on the western part of the mineralised system, is up to 700 m wide in the east and 2.2 km wide in the central sections (Kudryavtsev, 1996).

The principal hypogene mineral assemblage comprises pyrite and chalcopyrite with accessory magnetite, molybdenite and sphalerite, and rare galena, marcasite, maghemite, mushketovite, martite, bornite, hematite, tetrahedrite, pyrrhotite, pentlandite, cubanite and other metallic minerals. The initial potassic alteration phase resulted in assemblages, listed in order of formation, of hematite-quartz, biotite-magnetite, pyrrhotite-chalcopyrite-pyrite, chalcopyrite with prehnite and molybdenite-chalcopyrite. The subsequent phyllic leaching produced pyrite, pyrite-molybdenite, pyrite-chalcopyrite-molybdenite, chalcopyrite-sphalerite and chalcopyrite-galena. In the western part of the deposit, cobalt, nickel and platinoid minerals and early gold are widely associated with the pyrrhotite-chalcopyrite assemblage. In addition, the chalcopyrite-sphalerite and chalcopyrite-galena associations contain electrum, tellurides and silver minerals. Post ore alteration minerals comprises quartz, zeolite and quartz-calcite veinlets (Kudryavtsev, 1996, and sources quoted therein).

The lower grade East Bozshakol mineralisation has a simpler suite of minerals, predominantly composed of pyrite, chalcopyrite, magnetite and molybdenite, with accessory hematite, martite, mushketovite, sphalerite, galena, tetrahedrite and pyrrhotite, and rare maghemite, barite, marcasite, and native gold and silver veinlets (Kudryavtsev, 1996, and sources quoted therein).

The upper 5 to 54 m of the orebody have been subjected to oxidation producing malachite-kaolinite-atacamite and goethite-malachite-clay assemblages. Over sections of the orebody, the zone of oxidation has been subjected to leaching to depths of as much as 35 m, to produce underlying supergene enrichment with a chalcanthite-chalcopyrite-covellite mineralogy over vertical thicknesses of from 5 to 60 m. This supergene enrichment was most intensely developed in the upper sections of the main Bozshakol ore deposit. Palaeo-supergene enrichment from the Cambrian is also recognised, preserved below the Late Cambrian to Ordovician cover on the south of the ore deposit, typified by the presence of barite. The next phase of supergene enrichment, which is only poorly developed, is of Mesozoic age and is characterised by the development of chalcocite and covellite veinlets (Kudryavtsev, 1996, and sources quoted therein).

Reserves and Resources

Kudryavtsev (1996) quoted a proven reserve of 176.2 Mt @ 0.72% Cu, 0.014% Mo, 0.28 g/t Au, which indicates a higher grade core to the deposit. The grade of the reserve may have influenced the estimated resource published in Mutschler et al. (2000) of >1 Gt at 0.67% Cu, 0.05g/t Au. However, following further testing, Khazakhmys (2012), the current mine developer, has quoted a total mineral resource of 1.173 Gt @ 0.35% Cu, 0.14 g/t Au, 0.88 g/t Ag, 0.004% Mo.
Reserve and resource estimates published in KAZ Minerals Annual Report 2014 (resources are inclusive of reserves) were:
    Proved+probable reserves - 0.573 Gt @ 0.38% Cu, 1.13 g/t Ag, 0.18 g/t Au,
    Measured+indicated resources - 0.829 Gt @ 0.37% Cu, 0.93 g/t Ag, 0.16 g/t Au, 0.01% Mo.
    Inferred resource - 0.341 Gt @ 0.31% Cu, 0.71 g/t Ag, 0.16 g/t Au, 0.01% Mo.

The most recent source geological information used to prepare this summary was dated: 2014.     Record last updated: 3/2/2016
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.


Bozshakol

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Gao, J., Klemd, R., Zhu, M., Wang, X., Li, J., Wan, B., Xiao, W., Zeng, Q., Shen, PO., Sun J., Qin, K. and Campos, E.,   2017 - Large-scale porphyry-type mineralization in the Central Asian metallogenic domain: A review: in    J. of Asian Earth Sciences   Available on-line from October 18, 2017, 30p.
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
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), 2005 Super Porphyry Copper & Gold Deposits - A Global Perspective, PGC Publishing, Adelaide,   v.2 pp. 467-512
Seltmann, R., Porter, T.M. and Pirajno, F.,  2014 - Geodynamics and metallogeny of the central Eurasian porphyry and related epithermal mineral systems: A review: in    J. of Asian Earth Sciences,   v.79, pp. 810-841.
Yakubchuk, A., Degtyarev, K., Maslennikov, V., Wurst, A., Stekhin, A. and Lobanov, K.,  2012 - Tectonomagmatic Settings, Architecture, and Metallogeny of the Central Asian Copper Province: in Hedenquist J W, Harris M and Camus F, 2012 Geology and Genesis of Major Copper Deposits and Districts of the World - A tribute to Richard H Sillitoe, Society of Economic Geologists   Special Publication 16, pp. 403-432

 References to this deposit in PGC Publications: 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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