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The Sokolov'sk iron deposit is located in the Turgay (Turgai) iron ore province of Kazakhstan, ~30 km SW of Kustanay and ~300 km ESE of Magnitogorsk (#Location: 52° 59' 39"N, 63° 10' 31"E).

The Sarbai, Kachar and Sokolovsk iron ore deposits, which are hosted within the Carboniferous Valerianovskoe arc of northwest Kazakhstan, contain an aggregate of more than 3 billion tonnes of mineable massive magnetite. The Valerianovskoe arc lies within the broader Uralides, a 2500 km long, north-south trending mountain belt that extends from the steppes of northern Kazakhstan to the Arctic ocean, and were formed as a result of the collision of the Baltica (largely the East European craton) and Siberia-Kazakh plates during the Late Carboniferous to Early Permian period. The region hosting these giant iron deposits is located on the eastern margin of the Southern Uralides, within the tectonic domain known as the Trans-Uralian zone (Herrington et al., 2005). The Valerianovskoe arc is the possible westward extension to the South Tien Shan arc that is host to the giant Almalyk Cu-Au porphyry system in Uzbekistan. The magnetite bodies of the Turgai belt replace limestone and tuffs, and are distal to locally proximal to the contacts of gabbro-diorite-granodiorite intrusive complexes (Hawkins et al., 2010).

Sokolov'sk occurs on the eastern limb of the NNE-trending Sokolovsk-Sarbai anticline, while Sarbai (see separate record) is found 8 km to the NNW on the western limb.

The deposit is hosted by carbonates with lesser intercalated tuffaceous sediments, and by intermediate volcanics, in the middle unit of the Valerianovo supergroup. Within the Sokolovsk-Sarbai anticline, the host is described as comprising, from the base:
Sarbai Group, up to 1500 m thick - porphyritic andesites, volcanic breccias and tuffs;
Sokolovsk Group, up to 800 m thick - limestone, micro-porphyritic basalts and associated tuffs, and calcareous tuffs;
Kurzhunkul Group, up to 1500 m thick - porphyritic andesite-basalts, tuffs and tuff breccias;
Upper Palaeozoic, up to 400 m thick - red beds;
Mesozoic to Cenozoic, 50 to 120 m thick - cover sequence (Smirnov, 1977).

The host sequence is intruded by the northeast elongated, 15 x 3.5 km intrusive mass of the Sarbai-Sokolovsk gabbro-diorite-granodiorite suite of the Valerianovskoe volcano-plutonic complex. Mineralisation varies from being locally proximal to distal to the Sarbai-Sokolovsk suite which abuts the deposit to the south. In the southern part of the ore-zone, there are sill-like and discordant bodies of pre-ore porphyritic dolerite, plagiogranites and diorite porphyries, while on its other margin there are both pre- and post-ore vein-like developments of plagiogranite porphyries and micro-dolerites (Sokolov and Grigor’ev 1977).

All of the primary rocks of the deposit have been subjected to alteration, as follows:
i). alkaline alteration products, comprising plagioclase-biotite, pyroxene-K feldspar, albite-quartz-K feldspar, pyroxene-albite and epidote-actinolite-albite, after various intrusive and volcanic rocks;
ii). pyroxene-scapolite products, commonly found in the hanging wall, after alumino-silicates;
iii). pyroxene and garnet-pyroxene skarns formed from all rock types, although skarns derived from limestones and calcareous tuffaceous sediments contain more ferruginous garnets with 80% andradite, 20% grossular;
iv). a hydro-silicate or propylitic suite, comprising albite-actinolite, epidote, prehnite, calcite, chlorite and zeolite, formed from skarns and direct from the host sequence (Smirnov, 1977).

As a result of i). the facies variations of the host volcanics, ii). the influence of the long-lived Sokolovsk Fault which follows the trend of the deposit, iii). the series of faults developed in a number of directions and iv). the complex of pre- and post-ore dykes, the skarn mineralisation at Sokolovsk consists of alternating, layer like, partly discordant, orebodies of different composition, with a predominantly eastern dip. The thickness and lateral dimensions of these orebodies vary considerably.

There are five main mineralised layers carrying massive lenses of ore within the host limestone unit, surrounded by large envelopes of disseminated magnetite. As in all of the skarns in the area, magnetite is the dominant ore mineral, although there are significant accessory apatite and sulphides, mainly pyrite, pyrrhotite and chalcopyrite. Thin sulphide layers are formed in the footwall of the magnetite bodies, with the highest concentrations in the central portions of the deposit, but are not currently of commercial interest. Overall the sulphur content of the ores varies from 2.4 to 3.3% (Sokolov and Grigor'ev 1977). Textures observed in the ore include massive, disseminated, banded, brecciated and streaky.

The orebodies usually have a central core of rich, massive magnetite ore, which towards the peripheries passes gradually into segregated ores. These ores are characterised by the relict stratigraphic and shearing textures of the replaced volcanics and sediments. Mineralised crush breccias are common. Statistically the richest ores have been formed by replacement of skarns, which have themselves formed after limestone or calcareous tuffaceous sediments (Smirnov, 1977).

Extensive scapolite alteration (10 to 28 vol.% meionite) occurs on the margins of the mineralisation and forms a hanging-wall blanket to the ore. Skarn type alteration of calcareous rocks is dominated by garnet (80% andradite and 20% grossular) and diopsidic clinopyroxene, with lesser associated albite, chlorite, actinolite, epidote and calcite alteration accompanying the magnetite mineralisation. The magnetite ore has also been hematised, and oxidised to martite. The ore zones of both the Sarbai and Sokolovsk deposits lie in close proximity to intermediate to mafic intrusives of the Sarbai-Sokolovsk intrusives. These intrusive bodies form part of the larger Sarbai-Sokolovsk batholith that is modelled to extend many kilometres beneath both deposits and the surrounding country rock

Most of the iron ore is found in a series of five stacked, stratabound, magnetite lenses distributed over a strike length of 5.6 km, stratigraphic thickness of 500 m, and extending down a 50°E dip for ~1 km. The orientation changes to a westerly dip towards the southern end of the deposit.

The richer ores contain 55.5% Fe with 2.9% S, 0.07% P, while the poorer mineralisation with 39% Fe has 2.5% S, 0.11% P. The highest S is usually in the central portion of the deposit (Smirnov, 1977).

The higher grade resource quoted by Smirnov (1977) was 967 Mt @ 41% Fe, with 0.07 to 0.11% P (after Sokolov and Grigor’ev, 1977), which was being exploited by open pit.

Remaining ore reserves and mineral resources at December 31, 2012 (ENRC Annual Report, 2012), were:
    Underground - proved + probable reserves - 221.8 Mt @ 32% Fe;
    Open-pit - proved + probable reserves - 48.2 Mt @ 34.9% Fe;
    Underground - measured + indicated + inferred resources - 1330.3 Mt @ 40.1% Fe;
    Open-pit - measured + indicated + inferred resources - 66.9 Mt @ 36.7% Fe.

The most recent source geological information used to prepare this summary was dated: 2010.     Record last updated: 7/9/2013
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
 References to this deposit in the PGC Literature Collection:
Hawkins T, Herrington R, Smith M, Maslenikov V and Boyce A,   2010 - The Iron Skarns of the Turgai Belt, Northwestern Kazakhstan: in Porter T M, (Ed), 2010 Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective PGC Publishing, Adelaide   v.4 pp. 461-474
Herrington R, Smith M, Maslennikov V, Belogub E and Armstrong R,  2002 - A Short Review of Palaeozoic Hydrothermal Magnetite Iron-Oxide Deposits of the South and Central Urals, and their Geological Setting: in Porter T M (Ed.), 2002 Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective, PGC Publishing, Adelaide   v.2 pp. 343-353

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