The genesis and tectonic setting of the magnetite skarn deposits of the Turgai belt, north-western Kazakhstan

The world-class Sarbai, Kachar and Sokolovskoe ore deposits of the Turgai belt are part of the Carboniferous Valerianovka arc of northwest Kazakhstan. They contain an aggregate of more than 3 billion tonnes of mineable massive magnetite. The Valerianovka arc is part of the Transuralian zone that forms the eastern most boundary of the Uralide collisional zone. The Uralides, a 2500 km long, north-south trending mountain belt that extends northwards from the steppes of northern Kazakhstan to the Arctic Ocean were formed as a result of the collision of the Laurussia and Siberia-Kazakh plates during the Late Carboniferous to Early Permian period. Geochronological work completed during this study has shown that intrusive rocks within the Transuralian zone were formed along tow sub-parallel north-south trending volcanic arcs; the first with ages from 370 to 350 Ma near to the current surface expression of the Troitsk fault and the second approximately 100km East with ages from 340 to 310 Ma within the centre of the zone. Late sulphide mineralisation at the Sarbai deposit has a measured Re-Os age of 336.2 ± 1.3 Ma, within the estimatde range of intrusive age for the spatially associated Sarbai-Sokolov intrusive series. This age is also within error of the similar Magnitogorsk giant magnetite skarn deposit in the West of the Uralides, which was previously thought to be unrelated to the deposits in the Turgai belt. Younger granitic plutons of the Dzhabyk-Karagandski intrusive series and the Pridorogny granitic dykes that clearly cross cut mineralisation have measured U -Pb SHRIMP ages of between 300Ma and 280 Ma that correspond to an igneous event that affected all of the Southern Urals. The magnetite bodies of the iron skarn deposits replace limestone and tuffs, and are either distal or locally proximal to the contacts of gabbro-diorite-granodoirite intrusive complexes. Four main stages of alteration and mineralisation can be recognised at these deposits, namely: (l-)pre-ore; (2) the main magnetite forming; (3) late sulphide and (4) post- ore phases. The pre-ore stage is characterised by high temperature, metamorphic/metasomatic calc- and alurnino-silicates. The main magnetite ore phase formed when hot, sulphur poor, acidic-, iron and 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. The magnetite-skarn mineralisation was followed by a late sulphide phase, when comparatively cooler fluids, produced distinctive and extensive alteration assemblages of sod-rich scapolite, albite, chlorite and K-feldspar, accompanied by chalcopyrite, pyrite and minor sphalerite and galena. The post-ore phase, is characterised by cross-cutting barren veins composed of calcite, lesser albite, 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 the Iron Oxide-Copper-Gold (IOCG) deposits and provinces around the world though it is here proposed that these be classified as iron skarn deposits. Stable isotope (C, 0, S) studies carried out on a range of sulphides, carbonates and silicates, support an igneous, or igneous rock-equilibrated source for the mineralising fluids. Carbon and oxygen isotope data from gangue carbonates suggest that carbonate is derived from the interaction of igneous derived or igneous equilibrated fluids with host limestones. It is proposed that the source of the magmatic fluid is the diorites of the Sarbai-Sokolov intrusive series which underwent considerable fractionation and degassing after they were emplaced. This study aims to establish the link between the formation of the giant magnetite skarns and the tectonic history of the Uralides. Establishing a genetic model for these deposits to will lead to a better understanding of similar deposits worldwide.EThOS - Electronic Theses Online ServiceGBUnited Kingdo