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The Iron Skarns of the Turgai Belt, Northwestern Kazakhstan
by
Thomas Hawkins, Richard Herrington,  The Natural History Museum, London, UK,  Martin Smith, School of the Environment, University of Brighton, UK.,  Valery Maslenikov,  Institute of Mineralogy, Miass, Russia  and  Adrian Boyce,  SUERC, East Kilbride, UK

in - Porter, T.M. (ed.), 2010 - Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective, v. 4,  Advances
      in the Understanding of IOCG Deposits; PGC Publishing, Adelaide.  
pp. 461-474.

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ABSTRACT

The world-class Sarbai, Kachar and Sokolovsk iron ore deposits of the Turgai belt, in the Carboniferous Valerianovskoe arc of northwest Kazakhstan, contain an aggregate of more than 3 billion tonnes of mineable massive magnetite. 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. Three main stages of alteration and mineralisation can be recognised at these deposits, namely: (1) pre-ore; (2) the main magnetite forming; and (3) post ore phases. The pre-ore stage is characterised by high temperature, metamorphic/metasomatic calc- and alumino-silicates. The main magnetite ore phase formed when hot, sulphur poor, acidic, iron-, silica- and aluminium-rich fluids were structurally focused to dissolve and replace the dominantly limestone hosts. This was accompanied by a skarn assemblage gangue of epidote, calcic-pyroxenes, calcic-garnet and calcic-amphiboles, minor sulphide minerals and high field strength element (HFSE)-bearing accessory minerals such as titanite and apatite. This magnetite-skarn mineralisation was followed by a late sulphide phase, when comparatively cooler fluids, which produced distinctive and extensive alteration assemblages of sodium-rich scapolite, albite, chlorite and K feldspar, accompanied by chalcopyrite, pyrite and minor sphelarite and galena. The post-ore phase, is characterised by cross cutting barren veins composed of calcite, lesser albite and K feldspar, and minor quartz, and by widespread alteration comprising scapolite, albite and silica, which surrounds the deposit, and extends for several kilometers into the host rock. Many of the geological and mineralogical features of these deposits closely resemble those of IOCG deposits and provinces around the world. However, as the copper sulphide mineralisation is sub economic, they may only be classified as either IOCG-style or IOCG-related deposits. Stable isotope (C, O, S) studies have been carried out on a range of sulphides, carbonates and silicates related to the mineralisation. Preliminary results from sulphides intergrown with magnetite support a magmatic source for the sulphur. Oxygen isotope data from associated silicates and iron oxides also support an igneous, or igneous-rock equilibrated source for the mineralising fluids. Carbon and oxygen isotope data from gangue carbonates suggest carbonate is derived from the interaction of igneous-derived or igneous-equilibrated fluids with host limestones.

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