The geology and genesis of the iron skarns of the Turgai belt, northwestern Kazakhstan

The magnetite deposits of the Turgai belt (Kachar, Sarbai and Sokolov), in the Valerianovskoe zone of the southern Urals, Kazakhstan, contain a combined resource of over 3 Gt of iron oxide ore. The deposits are hosted by carbonate sediments and volcaniclastic rocks of the Carboniferous Valerianovka Supergroup, and are spatially related to the gabbroic to granitoid composition intrusive rocks of the Sarbai deposits are developed dominantly asmetasomatic replacement of limestone, but also, to a lesser extent, of volcanic rocks. Pre-mineralisation metamorphism and alteration resulted in the formation of wollastonite and the silici skarn assemblage of diopside, grossular sulphidemineralisation overprints the magnetitemineralisation and is associatedwith deposition of hydrothermal calcite and the formation of an extensive sodium alteration halo dominated by albite and scapolite. Chlorite formation accompanies this stage and further later stage hydrothermal overprints. The replacement has in places resulted in preservation of primary features of the limestone, including fossils and sedimentary structures in magnetite, skarn calc-silicates and sulphides. Analysis of Re 336.2±1.3 Ma, whilst U at 326.6 ± 4.5 Ma with re-equilibration of isotope systematics down to ~270 Ma. Analyses of mineral assemblages,–Sokolov intrusive series. The magnetitefication of limestone. Magnetitemineralisation is associatedwith the development of a high temperature–andradite garnet, actinolite, epidote and apatite. Sub-economic copperbearing–Os isotopes in molybdenite indicates formation of the sulphide mineral assemblage at–Pb analyses of titanite from the skarn alteration assemblage suggests skarn alteration fl of 600 °C fromhypersaline brines (45 150 °C and 20 wt.% NaCl eq. by the time of calcite deposition in late stage sulphide-bearing veins. ( ( to+12.9 magmatic source for S. All these data imply skarn formation and mineralisation in a magmatic system that maintained high salinity to relatively late stages resulting in the formation of the large Na-alteration halo. Despite the reported presence of evaporites in the area there is no evidence for evaporitic sulphur in the mineralising system. These skarns show similarities to some members of the iron oxide classes and the model presented here may have implications for their genesis. The similarity in age between the Turgai deposits and the deposits of the Magnitogorsk zone in the western Urals suggests that they may be linked to similar magmatism, developed during post-orogenic collapse and extension following the continentuid inclusion microthermometry, O and S isotopes suggest initial mineralisation temperatures in excess–50wt.% NaCl eq.),with subsequent cooling and dilution of fluids to aroundδ18O inmagnetite−1.5 to +3.5‰) and skarn forming silicates (+5 to +9‰), δ18O and δ13C in limestone and skarn calciteδ18O +5.4 to +26.2‰; δ13C −12.1 to +0.9‰) and δ34S in sulphides (−3.3 to +6.6‰) and sulphates (+4.9‰) are all consistent with the interaction of a magmatic-equilibrated fluid with limestone, and a dominantly–hydrothermal–apatite and iron oxide–copper gold deposit– continent collision, which has resulted in the assembly of Laurussian terranes with the Uralide orogen and the Kazakh collage of the Altaids or Central Asian Orogenic Belt. This model is preferred to themodel of simultaneous formation of very similar deposits in arc settings at either side of an open tract of oceanic crust forming part of the Uralian ocean.