Elsevier

Ore Geology Reviews

Volume 85, May 2017, Pages 216-246
Ore Geology Reviews

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

https://doi.org/10.1016/j.oregeorev.2015.10.016Get rights and content

Highlights

  • The Turgai deposits, Kazakhstan, host a resource of ~ 3 billion tonnes of iron ore.

  • The deposits are unequivocally limestone replacement skarn bodies.

  • They have affinities to Kiruna-type deposits (scapolite and albite alteration).

  • The deposits were formed by igneous equilibrated fluids between ~ 600 and ~ 150 °C.

  • The age of skarns (336 ± 1 Ma) has implications for the closure of the Uralian ocean.

Abstract

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–Sokolov intrusive series. The magnetite deposits are developed dominantly as metasomatic 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 silicification of limestone. Magnetite mineralisation is associated with the development of a high temperature skarn assemblage of diopside, grossular–andradite garnet, actinolite, epidote and apatite. Sub-economic copper-bearing sulphide mineralisation overprints the magnetite mineralisation and is associated with 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–Os isotopes in molybdenite indicates formation of the sulphide mineral assemblage at 336.2 ± 1.3 Ma, whilst U–Pb analyses of titanite from the skarn alteration assemblage suggests skarn alteration at 326.6 ± 4.5 Ma with re-equilibration of isotope systematics down to ~ 270 Ma. Analyses of mineral assemblages, fluid inclusion microthermometry, O and S isotopes suggest initial mineralisation temperatures in excess of 600 °C from hypersaline brines (45–50 wt.% NaCl eq.), with subsequent cooling and dilution of fluids to around 150 °C and 20 wt.% NaCl eq. by the time of calcite deposition in late stage sulphide-bearing veins. δ18O in magnetite (− 1.5 to + 3.5‰) and skarn forming silicates (+ 5 to + 9‰), δ18O and δ13C in limestone and skarn calcite (δ18O + 5.4 to + 26.2‰; δ13C − 12.1 to + 0.9‰) and δ34S in sulphides (− 3.3 to + 6.6‰) and sulphates (+ 4.9 to + 12.9‰) are all consistent with the interaction of a magmatic-equilibrated fluid with limestone, and a dominantly magmatic source for S. All these data imply skarn formation and mineralisation in a magmatic–hydrothermal 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–apatite and iron oxide–copper gold deposit 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 continent–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 the model 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.

Introduction

The Kachar, Sokolov and Sarbai magnetite iron deposits are located in the north-west of Kazakhstan near the border with Russia, in a belt which extends into Russian territory (Fig. 1). These stratabound, massive, magnetite deposits contained at least 3000 Mt of high grade iron ore (Fig. 2) and have been mined for more than 60 years. These deposits, together with other smaller satellite deposits and occurrences, form a long NNE-SSW trending magnetite rich belt, called the Turgai belt, which extends from the Sarbai deposit in the south to the Glubochensk deposit in the north (Fig. 3). The Turgai belt is hosted within the larger Valerianovka Arc, which is part of the larger Transuralian tectonic terrane that forms the easternmost edge of the Southern Uralides (Brown et al. 2006). The Uralides were formed during the collision of the Laurussia and Kazakhstan plates between the Carboniferous and Triassic (Brown, D, et al., 2002, Brown, D, et al., 2006).

Since their discovery by aeromagnetic geophysics in 1949 (Porotov et al. 1987) the massive magnetite skarns of the Turgai belt have been subject to several detailed studies (e.g. Bekmuhametov, AE, 2004, Smirnov, VI and Dymkin, AM, 1989, Sokolov, GA and Grigorev, VM, 1977, Porotov, GS, et al., 1987, Zakharov, AM, et al., 1987). These have revealed some distinctive characteristics that are shared with a number of different deposit types. Most recently a number of workers have noted the similarities of these deposits to Kiruna-type deposits (Iron oxide–apatite or IOA) and related iron oxide–copper–gold (IOCG) deposits (Barton, MD and Johnson, DA, 1996, Herrington, R, et al., 2002, Williams, PJ, et al., 2005). Whilst the Turgai magnetite-rich ore deposits are predominantly carbonate-hosted skarns, the genesis and relative classification of the iron deposits of the IOA class in general is still a matter of controversy, with arguments favouring either direct magmatic crystallisation (e.g. Nyström 1985), hydrothermal mineralisation involving magmatically derived fluids (e.g. Pollard 2001), or hydrothermal mineralisation involving brines derived from interaction with evaporites (e.g. Barton and Johnson 1996).

In this paper we review the available literature on the geology and mineralogy of the Turgai deposits, and present new geological, geochemical and isotopic observations of the ores. These are used to help constrain the formation of these deposits and to further develop a tectonic model for the formation of the Transuralian zone. These data are then compared with skarn, IOA and IOCG deposits worldwide to investigate implications for the development of this deposits class. The data are also compared with data from major skarn-type magnetite deposits in other tectonics zones of the Urals, notably at Magnitogorsk and Maliy Kuibas (Herrington et al. 2002), to provide constraints on the tectonic setting of major iron skarn development, and to provide further understanding of the genetic relationships to other deposit types.

Section snippets

Local geology

The Turgai deposits are hosted within the volcanic and sedimentary rocks of the Transuralian zone. The Uralides are a 2500 km long, north-south trending mountain belt that extend from northern Kazakhstan to the Arctic Ocean, and were formed as a result of the collision of the Laurussia and the Siberia–Kazakh plates during a period spanning the Late Carboniferous to Early Permian. On a regional scale, the Southern Uralides can be divided into geologically distinct sub-zones, bounded by large,

Deposits

The Valerianovka arc rocks host the giant, and currently producing, Sarbai, Kachar and Sokolov iron deposits along with many prospects, such as Glubochensk, in a district known collectively as the ‘Turgai belt’ (Bekmuhametov 2004). Sarbai, Kachar and Sokolov together represent over 3 billion tonnes of iron ore, comparable in scale to some of the largest hydrothermal iron oxide–apatite deposits, notably the Kirunavaara deposit, Sweden (Fig. 2). The Sarbai and Sokolov deposits are located near the

Electron microscopy and probe analyses

Back scattered electron (BSE) images of the microscopic features of the samples were taken using a JEOL 5900 V Scanning Electron Microscope (SEM) and mineral analyses were made using a Cameca SX100 electron microprobe. Scanning electron microscopy was carried out at 20 kV accelerating voltage and 2 nA beam current. Energy dispersive spectra were acquired using an Oxford Instruments INCA X-sight Si (Li) energy dispersive X-ray micro-analyser, and data analysed using the INCA software. For electron

Alteration and mineralisation

A simplified and generalised paragenesis of each of the deposits is presented in Fig. 9. Mineralisation is developed replacing limestone and volcaniclastic rocks, both of which preserve well defined primary features away from the main ore zones (Fig. 10a, b). Macro-scale field relations (Fig. 8) where magnetite can clearly be seen to be developed along limestone bedding, previous mapping (Fig. 5, Fig. 6, Fig. 7) and the occurrence of clear limestone replacement textures, all indicate that

Timing of mineralisation

SHRIMP dating of zircon from agranite-porphyry dyke (Hawkins 2012) which clearly cross-cuts mineralisation at the Sarbai deposit yielded an age of 322.2 ± 4 Ma. The altered rock that hosts the mineralisation at both the Sarbai and Sokolov deposits corresponds to the Sokolov and Sarbai suites of the Valerianovka Supergroup (Porotov et al. 1987). These have been interpreted as being middle Visean to late Visean in age (345.3 to 326.4 Ma) on the basis of fossil evidence by Porotov et al. (1987). These

Conclusions

The magnetite deposits of the Turgai zone, Kazakhstan, are unequivocally Fe skarn-type deposits, with characteristics including late stage sulphide mineralisation and an extensive sodic (albite + scapolite) alteration halo that suggest they may have genetic links to the IOCG deposit class. The characteristics of mineral assemblages and fluid inclusions, and stable isotope geothermometry using oxygen and sulphur isotopes suggest initial formation of pre-skarn alteration at > 600 °C, skarn alteration

Acknowledgements

We would like to thank the CERCAMS project and Reimar Seltmann in particular at the Natural History Museum, London and Jim Coppard, formerly of Anglo American Plc for financially supporting this project. Sokolovsko-Sarbaiskiy Gorno-Obogatitelnoe Objedineniye (The Sokolov–Sarbai Mining Production Association, SSMPA) allowed access to the open pits for fieldwork and sampling. The work has benefitted from discussion and fieldwork with J. Coppard, L. Mordberg, J. Alitaalo, and G. Fershtater. Stable

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