Field Workshop
Iron oxide deposits of the Turgai region, Kazakhstan, and Southern Urals, Russia
Dates: September 12-23, 2003
Sponsored by:
CERCAMS, Department of Mineralogy, Natural History Museum, London, UK
Institute of Mineralogy, Urals Branch, Russian Academy of Sciences, Miass, Russia
In cooperation with:
Institute of Geology & Geochemistry Urals Branch RAS, Ekaterinburg, Russia
Ekaterinburg Mining Institute, Russia
Kazakhstan Iron (+-Cu-Au) deposits - Sokolovskoe, Sarbayskoe, Kachar, Kuzhunkul ‘skarn’ magnetite deposits
Magnetite deposits of the Valerianov trend (extracted from Herrington et al. 2002)
The 700km long, NNE striking Valerianov trend (see Figure 1) is host to a major group of magnetite bodies which are described as skarns (Koroteev et al. 1997).
This trend contains the major iron-ore producing area of Turgai which includes the Sarbai, Sokolovsk and Kachar deposits. Kachar has reported reserves of some two billion tonnes of ore, probably in the order of 45% contained iron in magnetite. The trend contains two major regions of mineralisation in the south and the north, respectively the Turgai and Glubochensk regions.
Turgai region
The Turgai region (Valerianovsky belt) contains the Sarbai, Solovsk and Kachar magnetite deposits which are major iron ore producers, supplying magnetite ore to the Magnitogorsk iron and steel complex since the Magnitogorsk deposits are practically exhausted with the exception of Kuibas.
The region comprises a NNE trending fault-bounded linear corridor of Tournasian to Namurian volcano-sedimentary rocks developed between large sedimentary basins (Figure 1). The Carboniferous units in the corridor are entirely covered by Mesozoic to Cainozoic sediments which are sub-horizontal ranging from 40 to 180m thick. The magnetite bodies were discovered by airborne and ground geophysics in the 1940s, a magnetic compasses being reportedly deflected when the area was overflown.
Sarbai
This deposit lies in the south of the belt at the contact of the Sarbai dioritic intrusion. This intrusion comprises a pyroxene-quartz-diorite accompanied by various dyke phases culminating in post-ore granite porphyries (Dymkin 1966). The deposits lie above the Valerianov subzone of rocks within andesitic porphyries and diverse volcaniclastic rocks, some of which show extensive early hematite alteration. This unit defines the Sokolovsk-Sarbai anticline and the deposits occur in the western limb. Pre, syn and post ore structure is important in the location of the various intrusive phases and the mineralisation.
Alteration comprises pre-ore hornfelsing and development of biotite-k-spar and albite alteration facies. Following this is development of pyroxene-scapolite, pyroxene-garnet and epidote-actinolite alteration directly associated with the ore formation. Post ore alteration comprises chlorite-prehnite, calcite-silica and zeolite assemblages. The pervasive formation of scapolite is a noted feature of the Sarbai deposit and of the Turgai province as a whole (Smirnov 1977). Alteration appears zoned away from the Sarbai diorite pluton as follows: a) biotite-albite-scapolite, b) garnet-pyroxene ‘skarn’, c)’skarn’ ore (magnetite and scapolite dominantly), d) scapolite-pyroxene alteration, e) pyroxene skarns passing out into hornfels, f) hornfels and albitised host rocks.
Orebodies at Sarbai are interpreted as having replaced bituminous limestones, calcareous tuffs and tuffites (Chuguevskaya 1969). The ore layers are conformable and appear bedded, passing laterally into less altered tuffaceous units and calcareous sediments. Post-ore dykes complicate the present geometry. The dimensions of the ore lenses at Sarbai are impressive, three main lenses each measure between 1000m and 1700m long, 800 to 1700m down-dip and are up to 170 to 185m thick. Around 50% of these bodies has a grade of around 50% Fe, whilst the remainder is lower grade between 20-50% Fe. The dominant ore mineral is magnetite but there is significant accessory sulphide, mainly pyrite, pyrrhotite and chalcopyrite. Sulphides can form layers in the footwall of the magnetite bodies but are currently not of commercial interest. Other gangue minerals are scapolite, pyroxene, garnet, wollastonite, albite, epidote, actinolite, apatite, calcite and quartz. Quoted reserves in 1970 were 725 million tonnes of ore at a grade of 45.6% Fe, 4.05% S and 0.13% P.
Sokolovsk
This deposit is adjacent to Sarbai and was also discovered in the late 1940s. The association of Sokolovsk is similar to Sarbai, located at the margin of a dioritic pluton. Again extensive scapolite alteration accompanies the margins of the mineralisation, forming a hanging-wall blanket to the ore. The deposit has reserves of 967 million at a grade of 41% Fe. Again the deposit carries significant sulphide with contents of between 2.5 and 3.3% S in the bulk ore (Sokolov & Grigor’ev 1977).
Kachar
Kachar is the largest of the Turgai region deposits with published ore reserves of 1 billion tonnes of ore @ 44.9% Fe (Figure 2). More recent press reports indicate that the tonnages may be at least double that figure. The deposit was discovered in 1943 by aeromagnetics.
West East
Two supergroups have been recognised in the Carboniferous volcano-sedimentary series in the Kachar region. The Lower Carboniferous Valerianovo group underlies a Middle-Late Carboniferous Kachar group. The Valerianovo group comprises 1km of andesitic volcanics and pyroclastics with layers of sediments and carbonates. Anhydrite layers with clay are intercalated in the limestones. The overlying Kachar group contains over 800m of polymict conglomerates, tuffs and sediments and andesitic volcanics basalt and andesite flows and tuff equivalents.
Granite porphyry bodies cut the sequence and these are scapolite-altered. The ore is associated with gabbro-diorite bodies. Deep geophysics suggests a buried gabbro could be present 2 to 2.5 km below the deposit.
Kachar is distinctive in the region due to the absence of proximal intrusive bodies. The high-temperature alteration appears to have developed in the absence of a proximal intrusion. Scapolite alteration around the deposit extends for several hundreds of metres. In addition there are broad zones of sulphide alteration accompanied by anhydrite within the deposit and alunite alteration peripheral to the deposit (Sledzyuk & Shiryaev 1958).
Extensive scapolite alteration, associated with pyroxene post-dates all the intrusive phases. Associated phases are actinolite, tourmaline, apatite, chlorite, albite, zeolite and calcite. Pyroxene-albite and pyroxene-garnet alteration replaces the scapolite alteration in places. Anhydrite occurs as bodies in the limestone and as replacement phase in the intrusives, common in the magnetite ores. Belyashov & Plekhova (1965) consider the anhydrite syngenetic, remobilised into the ore although it only occurs close to ore and appears to be an epigentic feature.
The ores at Kachar are closely associated with scapolite and albite in addition to other phases (see Table 1). Sulphides are common accessories although lower than at other deposits. Ores vary from 0.5 up to 3%S and 0.15-0.33% P and 0.02-0.03% Zn and significant vanadium.
Dacite porphyry, interpreted as the extrusive facies of the igneous complex of the Kachar ore field are dated by Rb/Sr as 31 5±24 Ma (Sokolov & Grigor’ev 1977).
Glubochensk region
These deposits are located at the northeast end of the Valerianov trend as it enters Russia. The four main deposits of Glubochensk, Berezovsk, Medvezh’yeozersk and Petrovo are currently undeveloped but are similar to the deposits in production in the Turgai region although higher grade ores (>50% Fe) are only recorded in the latter two deposits mentioned.
Again mineralisation relates to a northeast trending structural zone which takes the form of a fault-bounded syncline. Volcanic structures of Lower Carboniferous age are an acknowledged feature of the zone and the association of mineralisation with the early Carboniferous volcanism was recognised by Galkin (1963) and Dymkin et al. (1982). The Lower Carboniferous volcano-sedimentary host package has been dated as middle Visean and in the region is subdivided into a lower Valerianovo and upper Kachar supergroup as at Kachar. In the Glubochensk region, mineralisation is largely confined to the Valerianovo supergroup which comprises mafic to intermediate lavas and tuffs with associated sediments which include limestones. Limestones are erratically distributed through the supergroup but generally form less than 10% of the sequence.
The Kachar supergroup, which to the southwest host magnetite bodies farther southwest, overlies this and comprises dominantly mafic to intermediate volcanics. In the Glubochensk region this supergroup only contains minor magnetite bodies. The volcanics of the Kachar supergroup are alkalic basalts to andesites with trachytes. Flow facies are common and there is a widespread development of mafic tuffs, commonly hematite altered, evidence for an early, probably subaerial, oxidation.
The two supergroups are considered to part of the same mega-volcanic event but it is recognised that the volcanics of the Kachar supergroup were erupted in a largely subaerial environment (Pumpyanskiy et al. 1985). These volcanics are considered to be co-magmatic with the Sokolovsk-Sarbay intrusive complex, dated as Early to Middle Carboniferous (Ksenofontov & Ivlev 1971).
The chemistry of the igneous suites ranges from basalt to dacite in a continuous series showing a common igneous source and the Glubochensk rocks are directly comparable with the rocks of Turgai. The suites have been compared to continental alkaline tholeiites formed in a rifted platform environment (Samarkin & Pumpyanskiy 1983).
The orebodies are located at the intersections of the main NNE trending structures with easterly trending faults, with the main mineralisation centres seemingly regularly spaced every 30 to 35km, a similar feature noted in the Turgai district (Teterev 1970). All the orebodies are generally conformable to sub-conformable layers and differ mainly by those hosted in volcano-sedimentary packages (Glubochensk and Berezovsk) and those in dominantly volcanic host rocks (Medvezh’yeozersk and Petrovo).
Glubochensk takes the form of three layers of magnetite mineralisation hosted within a volcano-sedimentary package. The mineralisation extends for around 4.5 km along strike and the lenses of magnetite reach 1300m by 750m in extent up to 300m in thickness. In its southern part, sulphides are common, dominated by pyrite, pyrrhotite and chalcopyrite.
The Berezovsk body extends over 2.8km of strike and up to 1.3 km down-dip. The mineralisation takes the form of up to 10 lenses of magnetite, often disseminated with abundant pyrite.
Medvezh’yeozersk is poorly defined, locted beneath 400m of Mesozoic cover, but forms a lens of disseminated magnetite some 200m thick within pyroxene-scapolite and garnet alteration. Petrovo is almost entirely hosted in volcanics, highly altered mafic to intermediate tuffs. The rocks are highly altered to albite, amphibole, chlorite ‘skarns’ with common garnet-pyroxene skarn. Scapolitisation is common. Magnetite zones are up to 40m thick and pyrite, chalcopyrite, pyrrhotite, galena and sphalerite are common accessories and the sulphur content of the mineralisation is between 1.5 and 5 weight %.
In review, the orebodies are characterised by alternating layers of magnetite-bearing and magnetite-poor material. The magnetite-poor zones are generally altered whilst ore horizons can be nmassive, disseminated, patchy or veinlet-type. Main ore minerals are magnetite, martite, maghemite, hematite, mushketovite with minor Ti-bearing spinels. Sulphide mineralisation is widespread, with copper elevated at Glubchensk and Berezovsk whilst zinc and lead are enhanced at Petrovo. Minor molybdenite is recorded.
Alteration is ubiquitous in the host rocks (Pumyanskiy et al. 1985). An early scapolitisation is recorded, accompanied by either pyroxene or epidote-albite. Albite alteration forms a ‘halo’ of alteration around the scapolite zones. This occurs in both volcanic and intrusive rocks indicating the timing to be post volcanism and intrusion. Scapolitisation may reach thicknesses of several hundreds of metres and these zones often have small pods of mineralisation within them. Skarn assemblages in limestone, volcano-sediment or volcanic units are common, associated directly with magnetite bodies. They comprise calc-silicate assemblages of garnet, pyroxene-garnet and epidote garnet in the case of Glubochensk. These are also developed in intrusive rocks at Berezovsk. Overprinting the scapolite alteration are hydrous silicate assemblages of epidote, actinolite and chlorite; often associated with albite and carbonate. The latest stage alteration identified is carbonate associated with silicification accompanied by sulphides and gypsum or anhydrite.
Discussion
On closer inspection, the classification of the Turgai region magnetite deposits as simple contact skarns is not so evident and many of these deposits have poorly defined relationships with intrusive rocks. Less than 20% of the magnetite bodies actually form in contact with igneous bodies. The largest body in the Turgai district, Kachar, lies some 18km laterally and probably more than 2 km vertically (based on geophysical modelling) from the contact zone of any prospective intrusive body of sufficient size to have provided sufficient heat-flow. Furthermore, the presence of uniform zones of pyroxene-scapolite alteration many kilometres from the intrusive contact has also been pointed out as incompatible with a simple skarn origin (Belevtsev et al. 1982), although these authors propose a metamorphosed syngenetic origin for the ores. There is also a general lack of spatial association between the extensive zones of scapolitisation and the intrusive bodies, suggesting that the scapolite alteration may not be directly controlled by the presence of igneous bodies. Structure is a key component to deposit formation, both in controlling the large intrusive complexes and for focusing hydrothermal systems.
Table 1 summarises the alteration and mineralisation for key magnetite deposits of the region. Much of the alteration is of a regional nature, developed well beyond the aureole of any of the related intrusive bodies. Evidence for the high chloride activity in the alteration fluids is manifested by the large regional scapolite alteration halos. The fluids were highly oxidised from the bulk mineralogical evidence (magnetite, often anhydrite). Apatite is a ubiquitous associated phase in the deposits.
In the Turgai district and at Goroblagodat, scapolite metasomatism is developed on a regional scale, evidence for major regional fluid flow linked to favourable structural trends rather than simple contact alteration skarn development. Many of the deposits have base metal sulphides, dominated by copper, as a late phase associated with hydrous silicates and carbonate, similar to many of the Cu-Au bearing iron-oxide camps.
Exploration in the Urals for major Cu-Au bodies related to the iron-oxide systems has not been carried out systematically, as these belts have been targeted simply for magnetic iron-ore deposits. There must be potential for large tonnage base metal discoveries being discovered given the scale of alteration shown by the systems. Existing bodies show significant copper and gold values and currently these are not being recovered.
A further feature to note is the dominance of subaerial mafic volcanic suites in the volcanic sequences of the Urals belts. Previous Russian authors have also noted the presence of early hematisation of these volcanics, another positive indicator for the generation of oxidised, copper and gold bearing fluids in regional hydrothermal systems (Hitzman 2000).
References
Belevtsev, Ya. N. et al. 1982, Volcanogenic origin for magnetite ores of the Urals, International Geology Review.
Belyashov, M.M. & Plekhova, K.R., 1965, The effect of sedimentary anhydrites on the metasomatic processes during the formation of the Kachar magnetite deposit (Turgai downwarp), Geol. Rudn. Mestorozh. 7, pp. 38-49 (in Russian).
Brown, D., Juhlin, C., Alvarez-Marron, A., Perez-Estaun, A. & Olianski, A., 1998, Crustal-scale structure and evolution of an arc-continent collision zone in the southern Urals,Russia, Tectonics, 17, 158-171.
Chuguevskaya, O.M., 1969, The genetic features of the Sarbai and Yrltai magnetite deposits in Turgai, Autoref. Diss. Kand. Geol. Miner. Nauk, Alma Ata (in Russian).
Dymkin, , A.M., 1966, The petrology and origin ot the magnetite deposits of Turgai, Nauka Press, Novisibirsk (in Russian).
Dymkin, A.M., Poltavets, Yu. A. & Nechkin, G.S., 1982, The geological-petrological features of the iron-bearing volcanic-plutonic associations, Sverdlovsk (in Russian).
Ferstater, G.B., Montero, P., Borodina, N.S., Pushkarev, E.V., Smirnov, V.N. & Bea, F., 1997, Uralian magmatism: an overview, Tectonophysics, 276, pp.87-102.
Galkin, P.S., 1963, Some problems of the geology, volcanism and metallogeny of western Turgai, Trudy I Ural’sk. Petrogr. Soveshch., Vol. 2, pp. 137-141, Sverdlovsk (in Russian).
Hitzman, M.W., 2000, Iron Oxide-Cu-Au Deposits: What, Where, When and Why, in: Porter, M. (ed.), Hydrothermal Iron Oxide Copper-Gold & Related Deposits A Global Perspective, Australian Mineral Foundation Inc.
Koroteev V.A., de Boorder H., Netcheukhin V.M., Sazonov V.N. 1997. Geodynamic setting of the mineral deposits of the Urals. Tectonophysics 276, pp. 291-300.
Ksenofontov, O.K., & Ivlev, A.I., 1971, The magmatism and metamorphism of the Turgai trough, in: The Geology of the USSR, Vol 34 Book 2, Nedra Press Moscow, pp. 9-141 (in Russian).
Puchkov, V.N., 1997, Structure and Geodynamics of the Uralian Orogen, in: Burg, J-P. & Ford, M. (eds.), Orogeny through time, Geol. Soc. Spec. Publ., 121, pp. 201-236.
Pumpyanskiy, A.M., Viryuchev, S.I. & Samarkin, G.N., 1985, The Glubochensk magnetite deposits, Intwernational Geological Review, pp. 93-101.
Ronkin, Yu. L., 1989, Sr isotopes as indicator of the magmatic evolution of the Urals, Yearbook-1988, Inst. Geol. Geochem. Uralian Branch of RAS, Sverdlovsk, pp. 107-109 (in Russian).
Samarkin, G.I., & Pumyanskiy, A.M., 1983, The evolution of the Early Carboniferous magmatism of the Valer’yanovo volcanic belt and its metallogeny, in: The geology and mineral raw-resources of the West Siberian Plate and its folded surroundings, Tyumen, pp. 141-144 (in Russian).
Sidorenko, A.V., 1973, Geology of the USSR, V. XII, Moscow.
Sledzyuk, P.E., & Shiryaev, P.A., 1958, The magnetite ores of the Kustanai district and the means of utilising them, Acad. Nauk SSSR Press, Moscow (in Russian).
Smirnov, V.I. & Dymkin, A.M., (eds.), 1989, Magnetite skarn deposits of the Urals: Central and southern Urals), Sverdlovsk: Uralian Branch Acad. Sci. USSR, 212 p. (in Russian).
Sokolov, G.A. & Grigor’ev, V.M., 1977, Deposits of Iron, in: Smirnov, V.I. (ed.), Ore Deposits of the USSR Volume 1, Pitman Publishing, London, ISBN 0 273 01034 4. pp. 7-113.
Teterev, G.M., 1970, The principal patterns of distribution and formation of the skarn-magnetite deposits of Turgai, in: The magmatism and endogenic metallogeny of the Transurals, Kustanay, pp.108-109.
Table 1 - Summary of key ore and alteration mineralogies for key deposits
| Deposit | Ore Mineralogy | Alteration Mineralogy |
|---|---|---|
| SW Valerianovsk Turgai region (Kazakhstan) |
||
| Sarbai |
Mag, pyx, scap, gar, woll, alb, epi, act, apat, py, calc, qtz Sulphide ore: py, po, cpy, sphal |
Pre-ore: Biotite, kspar, albite Syn-ore: Pyroxene-scapolite, pyroxene, garnet, pyroxene-scapolite-garnet Post-ore: Chlorite-prehnite, calcite-quartz, zeolite Regional: Scapolite |
| Sokolovsk |
Mag, scap, alb, pyx, gar, act, epi, ido, hem, chl, apat, preh, calc, py, po, ms, cpy, sphal |
Pre-ore: Plagiclase-biotite, pyroxene-kspar Syn-ore: Pyroxene-scapolite, pyroxene-garnet Post-ore: Albite-actinolite, epidote, prehnite, calcite, quartz |
| Kachar |
Mag, mat, scap, alb, pyx, gar, epi, act, chl, anhy, apat, sph, zois, preh, ser, calc, py, po, sphal, cpy, gal, bn, cc |
Regional: Scapolite, pyroxene, actinolite, tourmaline, apatite, chlorite, albite, zeolite, calcite Marginal to ore: Pyroxene-albite, pyroxene-garnet, garnet Syngenetic/replacement: Anhydrite |
| NE Valerianovsk (Russia) |
||
Glubochensk |
Mag, mart, magh, hem, mush, titano, il (cpy, sphal, gal, py, po, cc, moly, cov, bn, val |
Early pervasive: Scapolite, pyroxene-scapolite Skarn: Calcic-garnet, pyroxene-scapolite, epidote-garnet Flanking skarn: Epidote, actinolite, chlorite Late: Calcite, silica, sulphides, anhydrite, gypsum |