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(1)AGH University of Science and Technology Faculty of Geology, Geophysics and Environmental Protection Department of Economic Geology. Doctoral Dissertation. FLUID TYPES AND THEIR GENETIC MEANING FOR THE BIF BELT FORMATION, KRIVOY ROG, UKRAINE. Marta Sośnicka. Supervisor: Prof. dr hab. inż. Adam Piestrzyński. Kraków 2014.

(2) Abstract The Krivoy Rog banded iron formation (BIF), situated in the eastern part of Ukrainian Shield (Ukraine), hosts world class Fe deposits with documented Fe ore reserves exceeding 21,8 Gt (Shcherbak and Bobrov, 2005). This Doctoral Thesis presents new data from the Krivoy Rog Fe deposits and it is focused on their genesis. It examines generation of iron ores in terms of fluid systems, which gives new ways of understanding the genetic model of iron ore deposits. The research includes microthermometry, Raman spectroscopy, decrepitometry and stable carbon and nitrogen isotope analyses of fluid inclusions and it relies on inclusions preserved within ore as well as gangue minerals present within various types of iron ores. Based on acquired data describing the fluid systems in terms of fluids compositions and their sources, the genetic model was re-interpreted. Keywords: banded iron formation, BIF, iron deposit, iron ore, fluid inclusions, ore genesis. Streszczenie W obrębie krzyworoskiej warstwowanej formacji żelazistej (BIF), usytuowanej we wschodniej części Tarczy Ukraińskiej (Ukraina), znajdują się światowej klasy złoża Fe z udokumentowanymi zasobami wynoszącymi ponad 21,8 mld ton rudy Fe (Szczerbak i Bobrov, 2005). Niniejsza Praca Doktorska prezentuje nowe dane z krzyworoskich złóż Fe, przyczyniające się do pogłębienia wiedzy na temat genezy BIF-u z Krzywego Rogu. Analiza systemów fluidów związanych ze złożami żelaza w obrębie BIF ma na celu poszukiwanie nowych rozwiązań prowadzących do doskonalszego zrozumienia modelu genetycznego tego typu złóż. Badania uwzględniają: mikrotermometrię, spektroskopię Ramana, dekrepitometrię oraz analizy izotopów węgla oraz azotu w inkluzjach ciekło-gazowych obecnych zarówno w minerałach rudnych, jak i minerałach płonnych rud Fe. Uzyskane wyniki badań posłużyły re-interpretacji modelu genetycznego złóż Fe w warstwowanej formacji żelazistej w Krzywym Rogu. Słowa kluczowe: warstwowane formacje żelaziste, BIF, złoża żelaza, ruda żelaza, inkluzje ciekłogazowe, geneza rud.

(3) Niniejszą Pracę Doktorską dedykuję Mojej Kochanej Mamie. I dedicate this Doctoral Dissertation to my Loving Mother..

(4) Acknowledgments. I. am. very. greatful. to. supervisor. Prof.. Adam. Piestrzyński,. Prof. Igor S. Paranko, Assoc Prof. Ronald J. Bakker, Dr. Iain Pitcairn and Curt Broman for their kind support. I am very glad that all they appeared on my scientific way and contributed to my scientific as well as personal growth. I am also truly thankful to Mr. Kingsley Burlinson and Dr. Volker Lueders for advices and fruitful collaboration. International collaboration, dissemination and presentation of the research results at Conferences to the international scientific community would not have been possible without strong financial support of international societies: SGA (Society for Geology Applied to Mineral Deposits), SEG (Society of Economic Geologists), Swedish Institute SI and Stockholm University. Thank you! I would like to say thank you to Dr Ewa Szewczyk for standing by my side during my scientific endeavors. It is not possible to mention individually all of those who have encouraged me, have had a faith in me and have provided strong support for this research. I warmly thank them all. The Doctoral Dissertation benefited from Erasmus Scholarship at the University of Leoben in Austria (2010/2011), the Dean Grant no. 15.11.140.205 at the Faculty of Geology, Geophysics and Environmental Protection (AGH, 2012), SEG Student Research Grant – Grant Hugo Dummett Mineral Discovery Fund (2012) and the Swedish Institute Scholarship (Visby Program for PhD studies in Sweden 2013) at Stockholm University in Sweden..

(5) “Gold is for the mistress Silver for the maid Copper for the craftsman, cunning at his trade ‘Good’ said the Baron, sitting in his hall, BUT IRON, COLD IRON, IS MASTER OF THEM ALL” (Rudyard Kipling).

(6) Contents Abstract ............................................................................................................................. 2 Acknowledgments ............................................................................................................. 4 1.. Introduction ................................................................................................................ 9. 2.. Literature review ...................................................................................................... 11. 3.. Regional geology...................................................................................................... 14. 4.. 3.1. Lithostratigraphy ................................................................................................ 18. 3.2. Tectonic framework ........................................................................................... 25. 3.3. Metamorphism ................................................................................................... 29. 3.4. Metasomatism .................................................................................................... 32. Ore mineralization .................................................................................................... 33 4.1. Ore minerals ....................................................................................................... 33. 4.2. Classification of Fe ores ..................................................................................... 34. 4.3. Iron ore genesis .................................................................................................. 35. 5.. Origin of quartz from the Krivoy Rog BIF ................................................................ 40. 6.. Previous studies of fluid inclusions within the Krivoy Rog BIF ................................ 43. 7.. Research area and rock material................................................................................ 50. 8.. Methodology ............................................................................................................ 64. 9.. Fluid inclusion petrography ...................................................................................... 72 9.1. Low-grade iron ore ............................................................................................. 72. 9.1.1. Quartz bands of the low-grade iron ore ........................................................ 72. 9.1.2. Petrography of the veins .............................................................................. 72. 9.1.3. Fluid inclusion petrography – vein 1 and vein 2 .......................................... 75. 9.2. Iron-rich quartzites ............................................................................................. 87. 9.3. Thrust zone ........................................................................................................ 88. 9.4. Conclusions ........................................................................................................ 94. 10. Microthermometry.................................................................................................... 96 10.1 Low-grade iron ore ............................................................................................. 96 10.2 Iron-rich quartzites ........................................................................................... 101 10.3 Thrust zone ...................................................................................................... 107 10.4 Conclusions ...................................................................................................... 116.

(7) 11. Raman spectroscopy ............................................................................................... 119 11.1 Low-grade iron ore ........................................................................................... 120 11.2 Iron-rich quartzites ........................................................................................... 122 11.3 Thrust zone ...................................................................................................... 122 11.4 Conclusions ...................................................................................................... 126 12. Stable C and N isotope analyses of gases in fluid inclusions ................................... 127 12.1 Discussion ........................................................................................................ 131 12.2 Conclusions ...................................................................................................... 134 13. Baro-acoustic decrepitation .................................................................................... 135 13.1 Low-grade iron ore ........................................................................................... 135 Quartz veins ............................................................................................................ 135 Magnetite bands ...................................................................................................... 136 Quartz boudins from the MOPR outcrop ................................................................. 137 13.2 Iron-rich quartzites ........................................................................................... 138 13.3 Quartz bands from schists................................................................................. 138 13.4 Thrust zone ...................................................................................................... 139 13.5 High-grade iron ores......................................................................................... 141 13.6 Quartz veins from the Saksaganskiy granite ..................................................... 142 13.7 Discussion and conclusions .............................................................................. 144 14. Vacuum decrepitation ............................................................................................. 146 14.1 Low-grade iron ore ........................................................................................... 146 14.2 Thrust zone ...................................................................................................... 148 14.3 High-grade iron ore .......................................................................................... 148 14.4 Quartz from schists .......................................................................................... 149 14.5 Quartz veins from the Saksaganskiy granite ..................................................... 150 14.6 Conclusions ...................................................................................................... 151 15. Discussion and Conclusions ................................................................................... 152 15.1 Fluid types ....................................................................................................... 152 15.2 Trapping mechanisms....................................................................................... 153 15.3 Trapping conditions.......................................................................................... 156 15.4 Fluid sources .................................................................................................... 159 15.5 Implications for the genetic model .................................................................... 162.

(8) References ..................................................................................................................... 169 List of Figures ............................................................................................................... 180 List of Tables ................................................................................................................ 190 List of Appendices ........................................................................................................ 191 APPENDIX A ............................................................................................................... 192 APPENDIX B ............................................................................................................... 202.

(9) 1.. Introduction This PhD Thesis contributes to understanding of genesis of BIF-hosted iron deposits from. Krivoy Rog in Ukraine. The deposits within Precambrian banded iron formations (BIFs) are the most profitable sources of iron what is making them very attractive exploration targets (Duuring et al., 2012). At the same time, they still remain the most exciting enigma of economic geology because their genesis and evolution are controversial and not fully understood (Hagemann et al., 2006). Especially, mechanisms of Fe ore enrichment have recently been the most intensively discussed issue (Lascelles, 2006; Spier et al., 2008; Thorne et al., 2009; Angerer et al., 2012), which has significant impact on the world’s iron exploration. The Krivoy Rog BIF is unique within Europe as it hosts Fe ores being actively exploited in numerous open-pits as well as underground mines even at depths exceeding 1,3 km. The generation of iron ores in this region has been attributed to metamorphic and hydrothermal activity (Belevtsev et al., 1991), and therefore unraveling the evolution of fluid systems within these deposits is crucial to understanding of origin of these ore bodies. The aim of this study is characterization of fluid flow history within the Krivoy Rog BIF to provide constraints for the formation of iron ores. Facing this challenging task required applying a fluid inclusion analysis, which is a key to understanding of origin of fluids present during different stages of ore formation as well as the whole thermal evolution of ore deposits (Roedder, 1984; Wilkinson, 2001; Touret, 2001; González-Partida et al., 2003). The fluid inclusion analysis is not a common method in dealing with genesis of BIF-hosted iron ores, however it has been successfully applied recently (Figueiredo e Silva et al., 2013). The Krivoy Rog Belt underwent very complex evolution beginning from deposition through metamorphism, extensive tectonic activity, metasomatism and supergene alteration (Bobrov et al. 2002). Careful investigation of fluids present in these evolutionary stages provides means for construction of improved genetic model. Application of the correct genetic model, in turn, leads to successful exploration of iron ore deposits. This research is focussed on fluid inclusions hosted by quartz and magnetite within different types of iron ore as they have significant genetic meaning for the Krivoy Rog BIF belt formation. Combined evidence from fluid inclusions and known geological data lead to critical constraints on ore-forming conditions and history. Application of the fluid inclusion analysis to metamorphic rocks must deal with difficulties including decrepitation, re-equilibration and other post-entrapment changes (Bakker and Mamtani, 2000). Fluids are both the leaching and transporting medium and were not only responsible for precipitation of ore minerals in veins, cavities and pores of surrounding rocks, but also facilitated the tectonogenesis and deformation 9.

(10) events (Hollister and Crawford, 2012). Modeling and determination of origin of complex fluid systems aims to provide answers to important questions: what are the sources of fluids, what types of fluids were involved in the Krivoy Rog Belt (KRB) evolution and which events influenced their compositions? In order to find out genetic meaning of fluids within the Kirivoy Rog BIF as stated in the topic of the PhD Thesis: “Fluid types and their genetic meaning for the BIF belt formation, Krivoy Rog, Ukraine”, four testable hypotheses were designed: 1.. Fluid inclusions, which are preserved in minerals associated with different types of iron ores, provide information about fluid systems responsible for formation of BIF-hosted Fe deposits of the Krivoy Rog Belt.. 2.. Fluid inclusion compositions vary depending on the iron ore type.. 3.. Massive, magnetite, high-grade ores and quartz veins from the thrust zone contain fluid inclusions representing fluids, which led to the iron ore upgrade.. 4.. Fluids responsible for iron ore upgrade to massive, high-grade iron ore were derived from more than one source. The PhD Thesis is divided into 15 chapters. The first 6 chapters are dedicated to the. geological background and relevant context to this study. They include regional geology, ore mineralization and critical analysis of previous studies concerning iron ore genesis and fluid inclusions in rocks of the Krivoy Rog BIF. Chapters 7 and 8 present the research area, material and methodology. Chapters from 9 to 14 present the research results including: fluid inclusion petrography, microthermometry, Raman spectroscopy, stable C and N isotope analyses of gaseous fluid inclusions, baro-acoustic decrepitation and vacuum decrepitation. Combined information gathering actual knowledge and achieved results enabled re-interpretation of the evolutionary stages of the Krivoy Rog BIF, which is presented in Chapter 15. This Chapter is also dedicated to conclusions regarding the fluid inclusion trapping mechanisms, trapping P-T conditions, fluid types and fluid sources. This PhD Thesis may be also useful in construction of genetic models of other BIF-hosted Fe deposits worldwide.. 10.

(11) 2.. Literature review Precambrian banded iron formations are of great interest due to economic as well as. scientific purposes. They not only provide iron ores for a steel industry but they also hide a mystery behind the emergence of life (Garrels et al., 1973; Goodwin et al., 1976; Konhauser et al., 2011). BIFs were recognized as host rocks for many gold deposits e.g. Cuiabá Au deposit hosted by Archean BIF in the Quadrilátero Ferrífero district, in Brazil (Ribeiro-Rodrigues et al., 2007), the Nevoria Au deposit within Archean BIF in Southern Cross Greenstone Belt, in Western Australia (Fan et al., 2000) or the Bar 20, Vubachikwe, Blanket and Lima Au deposits hosted by Archean BIF in the Gwanda Greenstone Belt, in Zimbabwe (Saager et al., 1987). Recently, BIFs have been linked to Volcanogenic Massive Sulphide (VMS) deposits as well (Shanks et al., 2012; Lascelles, 2007). Numerous papers, which are focused on origin and evolution of Superior, Algoma and Rapitan type BIFs have been published worldwide. They deal with the most famous BIFs, which occur within almost all continents. The Algoma type BIFs, hosted by greenstone belts, are usually older and of relatively smaller sizes compared to the Superior type (Klein, 2005). It is also assumed that these BIF types were formed in various tectonic settings. The Algome-type BIFs are believed to result from volcanism and hydrothermal activity associated with greenstone belts, whereas the Superior type is related to deposition on continental shelves (Bekker et al., 2010). In Australia the research is primarily focused on the giant Superior type BIFs from the Hamersley Province e.g. the Dales Gorge Member of the Brockman Iron Formation, which hosts e.g. the huge Mount Tom Price iron deposit (Barley et al., 1999; Ewers and Morris, 1981; Ayres, 1972). Australian BIFs of the Algoma type occur within the Pilbara and Yilgarn cratons e.g. BIF hosted by the Koolyanobbing greenstone belt (Angerer et al., 2012; Angerer and Hagemann, 2010) or the Mount Gibson BIF, hosted by the Retaliation greenstone belt (Lascelles, 2006). BIFs crop also out in Southern Australia e.g. the Holowilena and Braemar Iron Formations (Trendall, 1973; Bekker et al., 2010). In Southern America BIF-hosted iron deposits predominantly occur in Brazil. The biggest deposits are located within the Proterozoic Cauê Formation, Itabira Group (The Minas Supergroup) in the Quadrilátero Ferrífero district in Minas Gerais State (Spier et al., 2008; Klein and Ladeira, 2000). Archean, Algoma-type BIFs, located in Brazil, include the Carajás Formation situated in the state of Pará (Klein and Ladeira, 2002) and the Cuiabá-BIF within the Nova Lima Group, which belongs to the Rio das Velhas Supergroup in Quadrilátero Ferrífero (Ribeiro-Rodrigues et al., 2007). Iron ore deposits are also hosted by Neoproterozoic BIFs of the Jacadigo Group (Rapitan type) in the Urucum district in the Brazilian state of Mato Grosso do Sul (Klein and Ladeira, 2004). 11.

(12) Besides Brazil, BIFs in Southern America are also well-known from Venezuela e.g. iron deposits in the Cuadrilatero Ferrifero de San Isidro hosted by BIFs of the Imataca belt (Ferenčić, 1969). BIFs were recently discovered in Argentina e.g. BIF hosted by the Nogolí Metamorphic Complex of western Sierra de San Luis, Eastern Sierras Pampeanas of Argentina (González et al., 2009). In northern Uruguay BIFs outcrop in the Isla Cristalina Belt, which is a future exploitation target with ore resources of 69.4Mt (Zapucay Project) (www.gladiatorresources.com.au/). Within the African continent BIFs have been discovered in Nigeria e.g. BIFs hosted by the Maru, Birnin Gwari, Kushaka and Malumfash schist belts (Mücke and Annor, 1993; Mücke et al., 1996), South Africa e.g. the Superior type Griqualand Iron Formation and the Kuruman Iron Formation in the Transvaal Supergroup (Horstmann and Hälbich, 1995), Zimbabwe e.g. Archean BIFs within the Zimbabwe craton (Oberthür et al., 1990), Mauritania e.g. BIFs hosted by the Archean Tiris Group and Proterozoic Ijil Group (Bronner and Chauvel, 1979), Namibia e.g. Neoproterozoic BIF of the Jakkalsberg Member of Numees Formation (Macdonald et al., 2010) or the Neoproterozoic Chuos Formation from the Damara Supergroup (Beukes, 1973), Botswana e.g. Proterozoic the Shoshong Formation (Mapeo et al., 2004; Ermanovics et al., 1987; Beukes, 1973) and Egypt e.g. the Umm Nar BIF (El Aref et al., 1993). The oldest BIF (3.8 Ga) is hosted by the Isua Supracrustal Belt in Greenland, in Northern America (Zuilen et al., 2003; Appel, 1979). Numerous BIFs have been discovered in USA and Canada. The most famous is the Lake Superior region extending in Ontario (Canada), Michigan, Minnesota and Wisconsin states (USA). BIFs in this region are represented by five large, Proterozoic formations: the Gunflint Iron Formation in Canada and the Negaunee, Biwabik, Ironwood, and Riverton Iron-Formations in USA (Bayley and James, 1973). Smaller Proterozoic as well as Archean BIFs also outcrop in the Northern Rocky Mountains and Southwestern United States (Bayley and James, 1973). The largest Canadian, Proterozoic BIF is the Sokoman Iron Formation, situated in the Labrador Trough (Klein and Fink, 1976). Archean BIFs occur within Canadian Shield e.g. in the Slave Province (Central Slave Cover Group), whereas Neoproterozoic, Rapitan type BIFs are situated in eastern Canada e.g. the Rapitan Formation (Yukon), the Upper Tindir Group or the Aok Formation (Bekker et al., 2010). Banded iron formations of Asia are represented by well-known occurrences in India e.g. BIF in the Kustagi and Bababudan schist belts (Khan and Naqvi, 1996; Arora et al., 1995). Besides India, BIFs were also reported in China (Fulu Formation) and eastern Russia (Maly Khinghan Formation) (Bekker et al., 2010). In Europe, BIF occurrences of the greatest economic value are limited to three localities: the Kursk Group (Belykh et al., 2007) and the Kola Peninsula in Russia e.g. actively mined Olenegorsk, Bauman, Yuzhno-Kakhozerskiy, Komsomolskiy, Oktabrskiy, Kurkenpakhskiy, 12.

(13) Kirovogradskiy, Zhelezna-Varaka or Pechegubskiy deposits hosted by Archean, Algoma type BIF in the Imandra region (Sośnicka, 2009; Sośnicka, 2010; Goryainov, 1976), and Ukraine e.g. Fe deposits hosted by the Krivoy Rog BIF of Superior or transitional type (Alexandrov, 1973). BIF also outcrops in Finland e.g. the Pääkkö Iron Formation (Bekker et al., 2010).. 13.

(14) 3.. Regional geology The Krivoy Rog Belt is situated within the Ukrainian Shield between two terranes of. different ages, the Paleoproterozoic Kirovogradskiy terrane and the Archean Middle Dniprean terrane (Bobrov et al., 2002) (Fig. 1). The Kirovogradskiy terrane is composed of metavolcanosedimentary and granitoid complexes intruded by younger Mesoproterozoic rapakivi granites, anorthosites, gabbro-anorthosites and monzonites (Fig. 1). The Middle Dniprean terrane is built predominantly of Mesoarchean granitoids and Neoarchean metavolcano-sedimentary and granitoid rock complexes (Fig. 1). The Krivoy Rog Belt forms an elongated structure extending longitudinally. It is constrained by the deep fault zone called the Krivoy Rog-Kremenchug megashear to the west and by metavolcanic rocks and granitoid bodies to the east (Fig. 2). In the northern part the Krivoy Rog Belt borders with the Demurinskiy granitic massive, which is the result of metasomatic alteration of granitoids of the Dnyepropetrovskiy Complex (grey gneisses complex), which formed the crust (Fig. 2) (Shcherbakov, 2005). The microcline bearing, pinkish granites of age 2.75-2.85 Ga contain low amounts of silica and are enriched in alkali, Zr, Mo, V and REE (Shcherbakov, 2005). The eastern border of the Krivoy Rog Belt contacts with the Saksaganskiy igneous complex (Fig. 2). The age of these granitoids is 3.067±0.081 Ga (Stepanyuk et al., 2010), however the time span of their formation is unknown. They are represented by grey tonalities, plagiogranites, granodiorites, diorites, which are composed of quartz, biotite, plagioclase, sercite, chlorite, clinozoisite, muscovite, epidote, garnet, carbonate, apatite, zircon, titanite and monacite, and they almost completely lack alkali feldspar (Shcherbakov, 2005).. 14.

(15) Fig. 1. Tectonic provinces of the Ukrainian Shield (Paranko et al., 2011). Legend: 1.. Paleoarchean rock complexes: a - metavolcano-sedimentary, b– granitoids. 2.. Mesoarchean rock complexes: a - metavolcano-sedimentary, b - granitoids. 3.. Neoarchaean rock complexes: a - metavolcano-sedimentary, b - granitoids. 4.. Paleoproterozoic rock complexes: a - metavolcano-sedimentary, b – granitoids. 5.. Mesoproterozoic, plutonic (intrusive) rock complexes: a - diorite, gabbro-diorite, b - rapakivi granites, anorthosite, gabbro-anorthosite, monzonite. 6.. Neoproterozoic sedimentary-volcanogenic complexes. 7.. borders of the terranes (megablocks). 8.. tectonic zones. 9.. borders of the Ukrainian Shield. Terranes (in circles): I - Volynskiy, II - Dnistrovsko-Bugskiy, III - Rosinsko-Tikichskiy, IV - Іngulskiy (Kirovogradskiy), V – Srednyepridneprovkiy (Middle Dniprean), VI – Priazovskiy Suture zones (in squares): 1-Golovanovskaya, 2 - Inguletsko-Krivorozhskaya, 3 - Orekhovo-Pavlogradskaya. 15.

(16) Fig. 2. Geological map of crystalline basement of the Ukrainian Shield 1:50 000 (Paranko et al., 1992). 16.

(17) Legend: 1-graphite-biotite, garnet-pyroxene gneisses and schists of the Spasovska and Chechelevska Suites of the Paleoproterozoic Ingulo-Inguletskaya Series; 2-carbonate-schists complex (marbles, dolomites, graphite schists) of the Mesoproterozoic: Rodionovskaya (Ingulo-Inguletskaya Series) and Gdantsivskaya Suites (Krivorozhskaya Series); 3-metaconglomerates, metasandstones, quartz-biotite schists of the Neoproterozoic Gleyevatskaya Suite; 4-iron quartzites and silicate schists of the Paleoproterozoic Saksaganskaya Suite of the Krivorozhskaya Series; 5-metaconglomerates, metasandstones, talc schists of the Skelevatskaya Suite, Krivorozhskaya Series, Paleoproterozoic; 6-chlorite-amphibole,. biotite-chlorite schists and metabasites of the Novokrivorozhskaya. Suite,. Krivorozhskaya Series, Paleoproterozoic; 7- amphibole-biotite schists of the Aulskaya Series, Paleoarchean; 8- granites and migmatites of the Kirovogradskiy Paleoproterozoic Complex; 9-plagiogranites and migmatites of the Dnyepropetrovskiy Paleoarchean Complex; 10-plagiogranites, granodiorites and tonalites of the Saksaganskiy Mesoarchean Complex; 11- microcline granites of the Tocovskiy Mesoarchean Complex; 12-metabasites (dacites, andesites, tholeites) of the Mesoarchean Konkskaya Series; 13-intrusive ultramafic rocks (dunites, pyroxenites, peridotites) of the Neoarchean (dykes); 14-microcline granites of the Mesoarchean Demurinskiy Complex; 15-plagiogranites, granodiorites and migmatites, Paleoarchean; 16-fault zones reaching mantle: a-Krivoy Rog-Kremenchug, b-Devladovskiy, B- crustal faults of higher orders. 17.

(18) 3.1. Lithostratigraphy. The development of the Krivoy Rog Belt started in the Neoarchaean by a rifting stage, which resulted in formation of greenstone belts, marked currently by the metavolcano-sedimentary complex (Konkskaya Series) (Fig. 2, 3) (Bobrov et al., 2002). During this stage the Archaean plagiogranite (trondhjemite)-amphibolite protocrust was split into two terranes: the Middle Dniprean and the Kirovograd (Bobrov et al., 2002). The Konkskaya Series (600-1100 m) is composed of amphibolites, amphibole-biotite and chlorite-biotite-amphibole schists representing metamorphosed volcano-sedimentary rocks (Paranko and Mikhnitskaya, 1991). In the Paleoproterozoic, the metaterrigenous-ferruginous complex (Krivoy Rog Series) was deposited (Fig. 3). This complex includes the Novokrivorozhska, Skelevatska, Saksaganska Suites and metakomatiite rock association (Bobrov et al., 2002). The rift basin was initially filled with products of ocean floor weathering (metabasites), which comprise the metaconglomerate-schist rock association (Novokrivorozhska Suite). The climate change reflected by increased temperatures and humidity, increased chemical weathering of the continental crust, which led to formation of silica-rich rocks represented by the metaconglomerate-sandstone-schist rock association (Skelevatska Suite) (Paranko and Mikhnitskaya, 1991; Paranko 1993) (Fig. 2, 3). The Novokrivorozhska Suite, the thickness of which varies between 20-30 m and 150-300 m, is primarily composed of quartz-chlorite, quartz-sericite-chlorite, amphibole-chlorite and biotitechlorite-quartz schists and less commonly of metasandstones and metagravelites (Paranko and Mikhnitskaya, 1991). The Skelevatska Suite (320 m) is comprised of quartz metasandstones, metagravelites, metaconglomerates, quartz-biotite, quartz-sericite-biotite and sericite-biotite schists (Paranko and Mikhnitskaya, 1991). Stabilized tectonic conditions favoured deposition of the banded iron formation (jaspilite-silicate-schist rock association=Saksaganska Suite=Krivoy Rog BIF) overlaying the Skelevatska Suite (Bobrov et al., 2002) (Figs. 2, 3).. 18.

(19) Fig. 3. Geological sketch of the Krivoy Rog Belt (Bobrov et al., 2002). 19.

(20) The Krivoy Rog BIF has a thickness of up to 1300 meters (Bobrov et al., 2002) or according to Shcherbak and Bobrov (2005) even up to 1500 m and comprises seven suites of alternating schist and ore horizons (Fig. 4) (Bobrov et al., 2002). These horizons represent two distinct paragenerations: first one is comprised of schists and barren iron quartzites, and the second one consists of jaspilites, iron quartzites, high-grade iron ores and schists (Paranko and Mikhnitskaya, 1991). The lithology of the horizons change depending on the part of the Krivoy Rog Belt. The 1st schists horizon, which do not exceed 300 m, is primarily comprised of amphibolechlorite-biotite schists (bands thickness: 0,5-30 cm) and barren quartzites (bands thickness: 0,2-15 cm) (Paranko and Mikhnitskaya, 1991). Less abundant are amphibole-chlorite, chlorite-biotite, garnet-amphibole-chlorite schists and carbonates (Paranko and Mikhnitskaya, 1991). The 1st schist horizon, within the Skelevatske-Magnetitove deposit is comprised of: quartz-sericite-chlorite, quartz-amphibole-chlorite, quartz-biotite-chlorite schists, barren quartzites and rarely carbonate and silicate-carbonate quartzites (Bobrov et al., 2002). The thickness of the horizon reaches 160 m (83 m on average) and it contains up to 33% of Fe (Semenenko, 1978). The 1st iron ore horizon (50-450 m) consists generally of: carbonate-silicate-magnetite, magnetite and silicate-magnetite quartzites, and a carbonate-magnetite variety, which is less common (Paranko and Mikhnitskaya, 1991). Within the Skelevatske-Magnetitove deposit this horizon reaches up to 50 m and is composed of amphibole(cummingtonite)-magnetite and magnetite-amphibole quartzites and less commonly magnetite-carbonate-silicate quartzites (Semenenko, 1978; Bobrov et al., 2002). The total Fe content of the horizon within this deposit is 25-32% (Fe comes dominantly from magnetite: 15-25%) (Bobrov et al., 2002). All rocks of the horizon typically contain low quantities of carbonates, up to 1-3% (Semenenko, 1978). In the Skelevatske-Magnetitove deposit the 1 st Fe ore horizon is additionally divided into 3 subhorizons. The first subhorizon (8-20 m), directly overlying the 1st schist horizon, is composed of low-grade magnetite-silicate (mainly cummingtonite), rarely magnetite-carbonate-silicate quartzites of Fe content up to 23,7% (Semenenko, 1978). The second subhorizon (10-100 m) is composed of rich amphibole-magnetite quartzites with an Fe content reaching 32% (Semenenko, 1978). The third subhorizon (2-25 m) is composed of low-grade magnetite-silicate, rarely magnetite-carbonatesilicate quartzites of Fe content up to 25,3% (Semenenko, 1978). The 2nd schist horizon is composed of biotite-chlorite-amphibole, quartz-chlorite-amphibole schists and barren quartzites (Paranko and Mikhnitskaya, 1991), and also quartz-chlorite-amphibole schists (Bobrov et al., 2002). This parageneration is well developed mostly in the southern part of the Krivoy Rog Belt and its average thickness is between 10 and 20 m (Paranko and Mikhnitskaya, 1991). 20.

(21) The 2nd iron ore horizon is 30-150 m thick and is comprised of silicate-carbonate-magnetite and silicate-magnetite quartzites (Paranko and Mikhnitskaya, 1991). In the SkelevatskeMagnetitove deposit these varieties of iron quartzites form three alternating subhorizons of lower Fe content (up to 23%) and higher Fe content (up to 34%) (Semenenko, 1978; Bobrov et al., 2002). The first subhorizon (10-40 m, Fe content = 22,8%), overlaying the 2nd schist horizon, is composed of low-grade magnetite-silicate(dominantly cummingtonite) quartzites and less commonly of magnetite-carbonate-silicate quartzites (Semenenko, 1978). The thickness of the second subhorizon varies between 15 and 110 m and it is composed of rich silicate-magnetite quartzites (Fe content = 34,4%) (Semenenko, 1978). The third subhorizon (8-25 m, Fe content = 27,5%) contains low-grade magnetite-silicate and magnetite-carbonate-silicate quartzites (Semenenko, 1978). Among the iron quartzites sometimes there are beds (up to 10 m) of amphibole-chlorite-biotite schists and barren quartzites (Semenenko, 1978). The 3rd schist horizon is well preserved in central part of the Krivoy Rog BIF and reaches thickness from 70 up to 140 m (Paranko and Mikhnitskaya, 1991). It is composed predominantly of graphite-chlorite-biotite, graphite-biotite-chlorite schists and barren quartzites (Paranko and Mikhnitskaya, 1991). In the Skelevatske-Magnetitove deposit, 3rd and 4th horizons are usually undivided with rare exceptions in southern and western parts of this deposit (Bobrov et al., 2002). In the undivided section they are composed of prevailing biotite-chlorite-amphibole schists, other schist varieties: biotite-chlorite, garnet-chlorite-biotite, garnet-amphibole-chlorite-biotite and barren quartzites (Bobrov et al., 2002). The 3rd iron ore horizon is composed generally of magnetite-chlorite and magnetiteamphibole quartzites of low Fe content and not exceeding thicknesses of 40-50 m (Paranko and Mikhnitskaya, 1991). In the Skelevatske-Magnetitove deposit it is divided into three parts (subhorizons) of different iron content: lower part built of magnetite-amphibole-biotite quartzites (thickness of 1-40 m, Fe content = 27%), middle part consisting of silicate-carbonate-magnetite, silicate-magnetite and magnetite quartzites (thickness of 5-35 m, Fe content = 33%) and the upper part, which is comprised of magnetite-biotite-amphibole quartzites (thickness of 4-45 m, Fe content = 27%) (Semenenko, 1978; Bobrov et al., 2002). The 4th schist horizon is similar in composition to the 3rd schist horizon. The main difference is additional occurrence of sericite-biotite-chlorite, biotite-chlorite and quartz-carbonatechlorite schists lacking graphite (Paranko and Mikhnitskaya, 1991). The thickness of this horizon depends on the locality and varies between 80-120 m (Komintern and Frunze underground mines) and 200-300 m (Libknekhta and Dzerzhinskovo deposits) (Paranko and Mikhnitskaya, 1991). The 4th iron ore horizon reaches maximum thickness of 400-700 m in the southern part of the Krivoy Rog Belt, where it is an exploitation target e.g. in Skelevatske-Magnetitove deposit 21.

(22) (Yugok open-pit) (Bobrov et al., 2002). The central part of lithological profile of the horizon is comprised of magnetite, hematite-magnetite and magnetite-silicate quartzites, whereas above and below this part, these varieties are replaced by silicate-carbonate-magnetite, magnetite-silicatecarbonate quartzites (Paranko and Mikhnitskaya, 1991). Within the Skelevatske-Magnetitove deposit the same trends and mineral zonation are observed: prevailing hematite-magnetite quartzites in the central part, which are replaced by magnetite, carbonate-magnetite, chloritecarbonate-magnetite and magnetite-amphibole-chlorite-siderite quartzites close to the margins of the lithological profile (Bobrov et al., 2002). Semenenko (1978) described 7 subhorizons within the deposit: low-grade magnetite-carbonate and carbonate-magnetite-silicate quartzites (1st: 29,7% Fe), silicate-carbonate-magnetite quartzites (2nd: 35% Fe), grey, rich magnetite quartzites (3rd: 37,9% Fe), rich hematite-magnetite quartzites (4th: 37,4% Fe), reddish, rich magnetite quartzites (5th: 35,9% Fe), silicate-carbonate-magnetite quartzites (6th: 33,6% Fe) and low-grade amphibolemagnetite-carbonate and carbonate-magnetite quartzites (7th: 30% Fe). The average Fe content in the deposit reaches 35,4% (Semenenko, 1978; Bobrov et al., 2002). The oxidation zone (from 3 up to 370 m thick) of the deposit is enriched additionally in martite and limonite, which are the products of weathering (Bobrov et al., 2002). The 5th schist horizon, of thickness between 20 and 120 m, is comprised of schists (70-75%) and lenses or bands of barren quartzites (25-30%) (Paranko and Mikhnitskaya, 1991). Varieties with graphite (graphite-chlorite-biotite and graphite-amphibole-chlorite) prevail among schists, however chlorite-amphibole and garnet-chlorite-amphibole schists also occur (Paranko and Mikhnitskaya, 1991). In the Skelevatske-Magnetitove deposit this horizon attains only 25-50 m and is composed of barren quartzites and quartz-chlorite-biotite, graphite-biotite, amphibole-biotite, goethite-hematite and quartz sericite schists (Bobrov et al., 2002). The 5th iron ore horizon (100-300 m) is dominantly composed of hematite (iron mica)martite quartzites hosting beds of less abundant silicate-martite and silicate-magnetite-martite quartzites (Paranko and Mikhnitskaya, 1991). In the southern part of the Saksaganskiy tectonic block rocks of this horizon were oxidized and replaced by hematite (iron mica)-hematite-martite and martite quartzites (Paranko and Mikhnitskaya, 1991). Oxidation of this horizon was also noticed in the Skelevatske-Magnetitowe deposit and is manifested by red and grey, fine-layered martite and hematite-martite quartzites (Fe content = 46-55%), which reach thickness of 50-140 m (Bobrov et al., 2002). Within the deposit the horizon is divided into 5 subhorizons, four of which have undergone oxidation processes (excluding the 1st subhorizon) (Semenenko, 1978). Thickness of the 6th schist horizon varies depending on locality and it increases generally towards the south (100-150 m in the Dzerzhinskovo deposit and 30-60 m in the Frunze underground mine) (Paranko and Mikhnitskaya, 1991). The horizon is divided into two parts: 22.

(23) oxidized, which is composed of a homogeneous layer of goethite-hematite rocks with martite and lower, unoxidized part, which is composed of cummingtonite-chlorite, biotite-chlorite schists with carbonates and magnetite (Paranko and Mikhnitskaya, 1991). In the Skelevatske-Magnetitove deposit this horizon contains 20-27% of Fe, reaches thickness up to 40 m and is composed of barren quartzites replaced by mashalite (a very fine-grained variety of quartz: 0,01-0,06 mm) and quartz-kaoline rocks (Semenenko, 1978; Bobrov et al., 2002). The 6th iron ore horizon hosts high-grade iron ores and also consists of oxidized and unoxidized zones (Paranko and Mikhnitskaya, 1991). The oxidized zone includes martite, hematitemartite, chlorite-magnetite-martite quartzites, whereas the unoxidized zone is comprised of magnetite, silicate-magnetite quartzites and their varieties: carbonate-biotite-magnetite, chloritemagnetite and chlorite-biotite-magnetite quartzites (Paranko and Mikhnitskaya, 1991). Thickness of the 6th horizon varies between 30-40 m (Kirov deposit) and 220-300 m (Frunze underground mine) (Paranko and Mikhnitskaya, 1991). In the Skelevatske-Magnetitove deposit it reaches up to 300 m and ends the lithological profile of the Saksaganskaya Suite (the Krivoy Rog BIF) (Bobrov et al., 2002). The horizon is divided into 4 subhorizons and consists of oxidized rocks of average iron content of 36% (Semenenko, 1978). The 7th schist horizon is well developed within the Saksaganskiy tectonic block and its thickness increase from southern (30 m) towards northern (350 m) part of the Krivoy Rog Belt (Paranko and Mikhnitskaya, 1991). It is composed of magnetite-carbonate-amphibole and magnetite-chlorite-amphibole schists and barren quartzites (Paranko and Mikhnitskaya, 1991). This horizon is gradually replaced by the iron rocks of the 7th iron ore horizon (Paranko and Mikhnitskaya, 1991). The thickness of the 7th Fe horizon increases from southern part (250 m) towards northern part (640 m) of the Krivoy Rog Belt (Paranko and Mikhnitskaya, 1991). It is composed of: hematite (iron mica)-magnetite, amphibole-chlorite-magnetite, amphibole-carbonate-magnetite quartzites and less commonly occurring: amphibole-biotite-magnetite, magnetite-carbonateamphibole and riebeckite-magnetite schists (Paranko and Mikhnitskaya, 1991). The oxidized zone is comprised of hematite (iron mica)-martite, martite, goethite-hematite, martite-hematite, goethitehematite-martite quartzites and rarely of goethite-hematite schists (Paranko and Mikhnitskaya, 1991).. 23.

(24) Fig. 4. Lithological profile of the Krivoy Rog BIF (Saksaganskaya Suite) (Bobrov et al., 2002). 24.

(25) The metakomatiite rock association (Fig. 3) is related to tectonic activity and represents rocks of ultramafic lava flows, which were metamorphosed to talc-carbonate schists (Paranko and Mikhnitskaya, 1991; Pieczonka et al., 2011). However, the origin of this horizon, which is enriched in Ni, Cr and PGEs, is still not clear, despite many studies (Paranko and Mikhnitskaya, 1991). The association is composed of talc, chlorite-carbonate-talc, carbonate-talc-actinolite, carbonate-talctremolite, chlorite-tremolite, chlorite-actinolite schists and actinolitites (Paranko and Mikhnitskaya, 1991). Its thickness varies from several meters up to 150 m (Frunze underground mine) and it extends throughout the entire length of the Krivoy Rog Belt (Paranko and Mikhnitskaya, 1991). The talc-carbonate horizon has a gradational contact with the Skelevatskaya Suite, whereas its upper boundary is associated with thrust zones (Paranko and Mikhnitskaya, 1991). During the rift closure intensive tectonic activity took place and led to folding, formation of faulting, thrusting, metamorphism and metasomatism of the Krivoy Rog rocks (Bobrov et al., 2002). Belevtsev (1991) distinguished within this period episodes of granitization and formation of igneous bodies within both terranes surrounding the Krivoy Rog Belt. Those events took place 2.02±0.02 Ga (Belevtsev, 1991). The stage is also associated with formation of low-grade and high-grade iron ore deposits between 2.0 and 1.8 Ga (Bobrov et al., 2002). These Fe deposits are confined lithologically to the Krivoy Rog BIF (Belevtsev, 1991). Low-grade iron ores are exploited in the southern part of the Krivoy Rog Belt, whereas the high-grade Fe deposits are restricted to the 5th and 6th iron ore horizons of the Saksaganskiy tectonic block (Belevtsev, 1991). In the Mesoproterozoic a new, closed paleobasin opened and was filled with lagoon-like sediments (Paranko and Mikhnitskaya, 1991). They are represented by the carbonatecarbonaceous-metaterrigenous complex (Gdantsivskaya Suite) (Figs. 2, 3), which is dominantly comprised of carbonate-graphite schists (Paranko and Mikhnitskaya, 1991; Bobrov et al., 2002). This Suite, with a thickness of around 1400 m, unconformably overlies the Krivoy Rog BIF and it was intruded by mafic dikes (Kalyayev, 1965). The youngest metaterrigenous complex (Gleyuvatskaya Suite) (Figs. 2, 3) with a thickness ranging between 1500 and 2000 m was formed in the Neoproterozoic and is represented by molasse sediments deposited at the orogen front (Paranko and Mikhnitskaya, 1991). They include metaconglomerates, quartz-feldspar metasandstones, biotite, quartz-biotite, feldspar-quartz-biotite, garnet-quartz-biotite, amphibole-biotite and garnet-amphibole-biotite schists unconformably overlaying the Gdantsivskaya Suite (Paranko and Mikhnitskaya, 1991).. 3.2. Tectonic framework. The structural evolution of the Krivoy Rog Belt was previously attributed to numerous theories including the formation of geosynclines (Semenenko, 1946; Kalyayev, 1964), rifts 25.

(26) (Kalyayev et al., 1984) or marginal basins (Kalyayev, 1991). According to the most recent hypothesis, the crystalline basement of the Krivoy Rog Belt was formed during 4 tectonic cycles: Archean, Paleoproterozoic, Mesoproterozoic and Neoproterozoic (Paranko, 1993). Each cycle was associated with deposition, which was interrupted by tectonic activity, magmatism and metasomatic processes (Khudur, 2006). In the Archean the plagiogranite (trondhjemite)-amphibolite basement (protocrust) of the Ukrainian Shield was disrupted by proto-rifting that led to formation of greenstone belts including the Konkskaya Series (Malyuk and Paranko, 1992; Paranko, 1997; Bobrov et al. 2002). During this stage the ultramafic metagabbro-dunite-piroksenite massifs and tonalite-plagiogranite massifs (including Saksaganskiy massif) were also formed (Kirilyuk et al., 1991) (Fig. 2). In the Paleoproterozoic, the graben-like structure inherited from the former proto-rift basin, was filled with sediments (Krivoy Rog Series) during the stabilized tectonic conditions (Bobrov et al. 2002). However, the deposition was interrupted by the effusive, fissure type, ultramafic volcanism (Khudur, 2006). Late Paleoproterozoic rifting stage occurred 2.0-1.8 Ga and was associated with extensive tectonic activity including folding, faulting, thrusting, metamorphism and metasomatism of the Krivoy Rog Series (Bobrov et al., 2002). Those events also affected and altered the Archean plagiogranite basement (Belevtsev et al., 1992). During that time the rocks were intensively altered by silicification, carbonatisation, chloritisation, sulphidation and aegirinisation processes (Bobrov et al., 2002). In the Mesoproterozoic, 1.81-1.76 Ga, in a new lagoon-type, closed basin a new transgressive-regressive cycle was started and resulted in deposition of carbonaceous sediments (Paranko and Mikhnitskaya, 1991). The deposition was interrupted by diabase dyke intrusions (Kalyayev, 1965). Development of the Krivoy Rog Belt was completed after the orogenic stage in the Neoproterozoic (Belevtsev et al., 1992; Paranko, 1997), which included vertical faulting, deposition of terrigenous sediments and intrusions of pegmatitic veins (Khudur, 2006). Archaean Konkskaya Series (metavolcano-sedimentary complex) and Paleoproterozoic Krivoy Rog Series (metaterrigenous-ferruginous complex) dip to the west (Belevtsev et al., 1992) and form a monoclinal structure (Khudur, 2006), which is crosscut by thrust zones (Bobrov et al., 2002) (Fig. 5). The younger, Mesoproterozoic Gdantsivska Suite (carbonate-carbonaceousmetaterrigenous complex) forms a syncline, which is overlain by the Neoproterozoic Gleyuvatskaya Suite (metaterrigenous complex) (Bobrov et al., 2002) (Fig. 5). The Krivoy Rog Belt is built of 7 tectonic blocks: Vostochno-Annovskiy, Piervomayskiy, Saksaganskiy, Novokrivorozhskiy, Osnovnoy, Yekaterininskiy, Likhmanovsko-Tarapakovskiy (Fig. 6) (Khudur, 2006). They are constrained by mantle and crustal faults: Vostochno-Annovskiy, Annovskiy, Vostochniy, Novokrivorozhskiy, Skelevatskiy, Yekaterininskiy, Likhmanovskiy, Visokopolskiy, Krivorozhsko-Kremenchugskiy (Fig. 6) (Khudur, 2006). The Devladovskiy fault 26.

(27) zone, which is oriented perpendicular to the strike of the Krivoy Rog Belt, constrains the northern border of the Saksaganskiy tectonic block. This tectonic unit, shown in red in Fig. 6, is cut by three thrust zones: Vostochniy, Saksaganskiy and Dalniy-Zapadniy (Khudur, 2006). The Saksaganskiy (Saksagan) thrust zone was studied and described in detail by Khudur (Khudur, 2006). The zone is developed within rocks of the Saksaganskaya Suite and extends the entire length of the Saksaganskiy tectonic block (40 km) (Fig. 6) (Khudur, 2006). Its surface dips westward and the dip angle decreases with the depth (Khudur, 2006). Close to the surface, it reaches up to 80°, whereas at the depth of around 2 km it decreases to 35° (Khudur, 2006). Vertical offset along the thrust zone did not exceed 2 km according to Plotnikov (Plotnikov, 1995). The inner structure of the zone is imbricated and consists of smaller units (thrust slices), which link with each other in a fan-like manner (Khudur, 2006). The character and mineralization of the thrust zone depends on the location (Khudur, 2006). Khudur, 2006 divided the Saksaganskiy thrust zone into two parts: southern and northern. The boundary between them is situated close to the Fruzne underground mine. The southern part of the zone is comprised of two major thrust slices and is built of talc, talc-carbonate and chlorite-carbonate rocks, tectonic breccias, metaconglomerate-breccias and mylonites (Khudur, 2006). The northern part of the thrust zone contains 2-4 main thrust slices and is more variable along the strike and dip compared to the southern part (Khudur, 2006). It is marked by intensively fractured rocks and exhibits a non-uniform, discontinuous surface dipping steeply, reaching even 75° (Khudur, 2006). The thrust zone in the southern part is much thicker compared to the northern part, where the dip angles are greater (Khudur, 2006). In the northern part the thrust zone is developed within: 6th and 7th ore and schists horizons, whereas in the southern part in lower horizons: 1st, 2nd, 4th ore and schist horizons, and 6th ore and 7th schist horizons (Khudur, 2006). In the vicinity of the Fruzne mine the thrust zone is developed along the boundary between hematite-martite iron quartzites of the 6th iron ore horizon in the footwall and the talc-schist horizon in the hanging wall (Khudur, 2006). In this area the zone is 100 m wide and consists of 4 slices thrusted one on another and is comprised of brecciated iron quartzites, fractured and brecciated schists of the 1st schist horizon, talc-schists and intensively fractured rocks of the 1st iron ore horizon (Khudur, 2006). This part of the thrust zone also contains increased amount of quartz and quartz-carbonate veins (Khudur, 2006). Close to the Devladovskiy tectonic zone, in the Lenin mine area, the thrust zone (200-250 m) is composed of 2 thrust slices. The first one is developed within rocks of the 6th iron ore horizon, which are fractured and contain quartz and carbonate-quartz veins of thickness up to 5 cm (Khudur, 2006). The footwall of this slice contains high-grade iron ores (Khudur, 2006). The second thrust slice is developed in 7th schist and ore horizons (Khudur, 2006). Both the southern and northern parts of the thrust zone are controlled by the talc-schist horizon, however it outcrops only in the southern part, whereas in the northern part it was found in boreholes 27.

(28) at a depth of 0,8-1,0 km (Khudur, 2006). Nevertheless, in the northern part, the near surface thrust slices, link up at depth with the main thrust surface associated with the talc-schist horizon (Khudur, 2006). Rocks within the thrust zone in all localities were altered by intensive silicification, carbonatisation, chloritisation and sulphidation processes (Khudur, 2006). Due to hydrothermalmetasomatic activity numerous quartz and quartz-carbonate veins have been recognized within this zone (Khudur, 2006). Their thickness ranges from 1-2 cm up to 1 m and they are attributed to two generations (Khudur, 2006). The older generation, which is oriented parallel to the banding and resulted from healing of shear fractures, and the younger generation, crosscutting the old one (Khudur, 2006).. Fig. 5. Geological cross-section through the central part of the Krivoy Rog Belt, SDBH-8 – location of the Krivoy Rog Super-Deep Borehole (5432 m) (Bobrov et al., 2002). 28.

(29) Fig. 6. Tectonic sketch of the Krivoy Rog Belt (Khudur, 2006). Faults (red numbers): 1- Vostochno-Annovskiy, 2-Annovskiy, 3-Vostochniy, 4-Novokrivorozhskiy, 5Skelevatskiy, 6-Yekaterininskiy, 7-Likhmanovskiy, 8-Visokopolskiy, 9-Krivorozhsko-Kremenchugskiy (Krivoy Rog-Kremenchug). 3.3. Metamorphism. Proterozoic rocks of the Krivoy Rog BIF underwent regional metamorphism in greenschist and epidote-amphibolite facies (Belevtsev et al., 1991) during the Early Proterozoic orogeny, which took place 2,0±0,3 Ga (Lazarenko et al., 1977; Belevtsev et al., 1983). The degree of metamorphism is varying depending on the part of the Krivoy Rog BIF (Belevtsev et al., 1983; Belevtsev et al., 1991). The central part was metamorphosed in greenschist facies, whereas the southern and northern parts in amphibolite facies (Belevtsev et al., 1983; Belevtsev et al., 1991). During 29.

(30) progressive metamorphism rocks of the Krivoy Rog BIF were buried, whereas during regressive metamorphism, in conditions of decreasing temperatures and pressures, they released metamorphic fluids, underwent foliation, deformation and thrusting accompanied by metasomatic alteration along tectonic zones (Belevtsev et al., 1991; Belevtsev et al., 1992). The granitoids adjacent to the Krivoy Rog BIF and metabasites overlain by the BIF repeatedly underwent metamorphism at the same time (Belevtsev et al., 1983; Belevtsev et al., 1992). The granitoids were affected by hydration (e. g. sericitisation of plagioclase), whereas the metabasites were altered by biotitisation and hydration (Belevtsev et al., 1992). Besides regional metamorphism, within the Krivoy Rog Belt there was also contact metamorphism related to intrusions of diabase dykes and microcline granites, which is manifested by formation of riebeckite, crossite, rhodusite and aegirine (Khudur, 2006). Metamorphic pressure within the Krivoy Rog Belt was calculated using staurolite-garnet and clinopyroxene-garnet geobarometers and it varies from 300 to 600 MPa, most frequently between 400 and 500 MPa (Belevtsev et al., 1983). It was concluded that the metamorphism of rocks of the Krivoy Rog Belt was isobaric (Belevtsev et al., 1983). Metamorphic zonation is spread laterally (Lazarenko et al., 1977; Belevtsev et al., 1983), whereas vertically it is not observed to great depths (Belevtsev et al., 1983). Regional metamorphism of pelites produced wider range of minerals compared to metamorphism of iron quartzites, which generated iron mica, magnetite and quartz (Lazarenko et al., 1977). Belevtsev and others (1983) distinguished 3 metamorphic zones in rocks of the Krivoy Rog BIF: garnet (almandine), staurolite and sillimanite-muscovite. Minerals formed in the garnet zone (greenschist facies), in temperatures between 400 and 500°C, were grunierite, calcite, magnetite, siderite, ankerite, actinolite, almandine, riebeckite, chlorite, biotite, chloritoid, pyrophyllite and muscovite (Belevtsev et al., 1991). The staurolite zone (500-600°C, epidote-amphibolite facies) is characterized by presence of grunierite, magnetite, actinolite, almandine, calcite, riebeckite, aegirine, biotite, staurolite, andalusite, kyanite and muscovite (Belevtsev et al., 1991). Typical minerals occurring in the sillimanite-muscovite zone (600-630°C, epidote-amphibolite facies) are grunierite, magnetite, actinolite, almandine, calcite, fayalite, hedenbergite, biotite, sillimanite, orthoclase and muscovite (Belevtsev et al., 1991). The almandine zone is developed within the Saksaganskiy tectonic block and is typically comprised largely of schists with chlorite, chloritoid, sericite, biotite, almandine and chlorite-cummingtonite iron quartzites, siderite-magnetite rocks and actinolite-epidote amphibolites (Belevtsev et al., 1991). Temperatures of metamorphism in this area, based on garnet-biotite geothermometry results vary between 430 and 550°C and most frequently they fall in the interval of 450-520°C (Belevtsev et al., 1983). Marginal areas of the Krivoy Rog Belt are built of rocks metamorphosed in epidoteamphibolite facies, in staurolite zone, which dominantly consists of schists with andalusite, staurolite, biotite, muscovite, garnet, cordierite, garnet-biotite schists, tremolite-calcite marbles, 30.

(31) garnet-cummingtonite schists and gneisses (Belevtsev et al., 1991). According to the garnet-biotite geothermometer temperatures of metamorphism in these parts of the Krivoy Rog Belt vary between 510 and 600°C and most frequently they fall in the interval of 530-590°C (Belevtsev et al., 1983). The sillimanite-muscovite zone is developed solely in the Annovskaya range (Belevtsev et al., 1991). Metamorphic reactions, which occurred within rocks of the Krivoy Rog BIF, include dehydration and decarbonation (Belevtsev et al., 1992). Dehydration of clay and water-bearing minerals, present in metapelites, caused release of aqueous fluids into the system (Belevtsev et al., 1992). Due to hydraulic overpressure these metamorphic fluids were expelled from the rock through more permeable zones or sets of fractures developed e. g. along the schistosity (Belevtsev et al., 1991; Belevtsev et al., 1992). Apart from dehydration, one of the most important metamorphic reactions occurring within banded iron formations composed of oxides and carbonates was low temperature decarbonation, releasing carbon dioxide into the system (Melnik, 1973). To proceed, these reactions required CO2 removal from the rock through dislocations or foliation, due to the overpressure build-up in temperatures around 400-600°C (Belevtsev et al., 1992). One of the reactions producing CO2 was decarbonation of siderite or ankerite to form magnetite (siderite+hematite=magnetite+CO2) in greenschist facies at fixed temperature of 425°C (Belevtsev et al., 1991). Metamorphic redox reactions in the BIF rocks do not occur due to the low oxygen fugacity of the metamorphic fluid, however these reactions may be triggered by fluids in equilibrium with graphite as well as mantle derived fluids (Melnik, 1973). The regressive metamorphism is associated with partial reduction of magnetite in the iron-bearing rocks of the Krivoy Rog BIF (Belevtsev et al., 1991). In metapelites and metabasites of the Krivoy Rog Belt this type of metamorphism is manifested by chloritisation of biotite, cummingtonite and garnet, and sericitisation of albite and kyanite (Lazarenko et al., 1977). Metamorphic fluids, which circulated within rocks of the Krivoy Rog BIF, are composed in general of an aqueous solution and carbon dioxide (Belevtsev et al., 1992). It is claimed that the ratio of these components largely depends on the temperature of metamorphism e. g. fluids released in greenschist facies contain predominantly pure H2O in majority, whereas fluids liberated in epidote-amphibolite and amphibolite facies are enriched in CO2 (Belevtsev et al., 1992). However, the variations of the H2O/CO2 ratios within individual facies are unknown and are assumed to originate from introduction of e. g. magmatic fluids (Belevtsev et al., 1992). The solubility of components in aqueous solution increases with increasing temperature and pressure, and also increasing chloride content (Belevtsev et al., 1992). The increasing amounts of chlorides in fluids results in higher solubility of alkali, FeO, MgO, CaO and lower solubility of SiO2 and Al2O3 (Belevtsev et al., 1992) The decrease in silica solubility may be enhanced by presence of CO2, 31.

(32) which may ultimately trigger silicification of rocks (Belevtsev et al., 1992). Higher amounts of chlorides in an aqueous solution also shifts the H2O-CO2 immiscibility field to higher temperatures (Belevtsev et al., 1992). Metamorphic fluids, which were released from iron quartzites and schists of the BIF, migrated upwards to areas of lower pressures and temperatures (Belevtsev et al., 1991). They formed metamorphic veins composed of quartz, hematite, carbonates and sometimes magnetite (Yevtekhov et al., 1999). This vein type is the most common within iron-bearing rocks of the Krivoy Rog BIF and the most abundant component in veins is quartz, with lesser amounts of iron mica and hematite (Lazarenko et al., 1977). Sulphides are rare or not present in metamorphic veins because they are usually associated with distant sources other then metamorphic fluids (Lazarenko et al., 1977). According to Lazarenko and others (1977) formation temperatures of hypogene veins (metamorphic and hydrothermal together) based on paragenetic associations range between 100-160°C and 350-400°C. Regional metamorphism and fluid flow was strongly associated with metasomatic processes and hypogene removal of quartz from iron quartzites to form high-grade iron ores (Lazarenko et al., 1977).. 3.4. Metasomatism. The Krivoy Rog Belt was locally altered by metasomatic fluids (Lazarenko et al., 1977). The most widespread and known is alkaline metasomatism, which took place 1.8±0.1 Ga (Belevtsev et al., 1991; Yevtekhov and Paranko, 1996) mainly in the northern part of the Belt in areas of the Piervomayskiye and Annovskiye deposits (Lazarenko et al., 1977). The rocks were metasomatized by fluids enriched in Na, CO2, U, V, Zr, REE (lanthanides, Y, Sc) to form aegirine and riebeckite metasomatites (Yevtekhov et al., 1999). The alkaline metasomatism involved aqueous fluids with dissolved Na2CO3, which interacted with metapelitic and iron-bearing rocks usually producing fluids containing H2O, K2CO3, CO2 and H2 (Lazarenko et al., 1977). Lazarenko and others (1977) also mentioned other types of metasomatic processes: Mg-Fe, Fe, carbonate and silicate. Mg-Fe and Fe metasomatism used to be associated with formation of high-grade iron ores (Lazarenko et al., 1977), however nowadays this hypothesis has been rejected (Belevtsev et al., 1991). The carbonate metasomatism was described by Lazarenko and others (1977) as an independent process, however it usually occurred in a strict association with alkaline metasomatism and is noticed by replacement of alkaline minerals, magnetite, aegirine by calcite, dolomite, siderite or ankerite. The silicate metasomatism was present dominantly in Annovskiy, Zheltoryechenskiy areas and less commonly in Inguletskiy and Saksaganskiy areas (Lazarenko et al., 1977). It might have been a multiple process, which led to formation of quartz breccias and secondary quartzites (Lazarenko et al., 1977). In the Saksaganskiy area silicification is related to occurrence of high-grade Fe ore deposits (Lazarenko et al., 1977). 32.

(33) 4.. Ore mineralization. 4.1. Ore minerals. The Krivoy Rog BIF hosts iron ore deposits composed of following minerals: magnetite, martite, hematite and goethite. Other less common ore minerals occur in minority and include pyrite, phyrrotite and chalcopyrite. Magnetite grain sizes are usually between 0,01 and 3 mm in size and exhibit a big range of textures from irregular to euhedral. This mineral comprises the lamines within banded iron ores or occurs as inclusions within silicate lamines. The grain shapes depend on the degree of metamorphism and overlapping, later processes. The higher the metamorphic degree the coarser, cleaner and more euhedral are the magnetite grains. Overlapping processes are frequently expressed by visible reaction rims surrounding the magnetite grains (Shcherbak and Bobrov, 2005). Hematite belongs to 5 different genetic types: sedimentary, metamorphogenic, metasomatic or hydrothermal, supergene or forms aggregates in presence of decomposed silicates and iron sulphides (Shcherbak and Bobrov, 2005). Sedimentary hematite occurs as inclusions in quartz, the metamorphogenic type forms hematite and magnetite-hematite ores and resulted from metamorphism of primary iron-bearing sediments, the metasomatic-hydrothermal hematite was formed during the recrystallization processes and the supergene type comprises porous martite ores and resulted from martitisation (Shcherbak and Bobrov, 2005). Goethite is a secondary mineral coexisting frequently with quartz, chalcedony, gibbsite and kaolinite (Shcherbak and Bobrov, 2005). It is the major component of oxidized Fe ores and soft Fe ores, and it resulted from supergene replacement of magnetite, hematite, Fe-rich silicates and sulphides (Shcherbak and Bobrov, 2005). The most common sulphides are pyrite and phyrrotite, wheras the rarest is chalcopyrite. The earlier generation of sulphides occurs within the bands of the iron ore and forms usually euhedral or spherical grains and/or aggregates (Shcherbak and Bobrov, 2005). The youngest generation is associated with hydrothermal activity and it fills fractures or forms parallel and/or crosscutting veins (Shcherbak and Bobrov, 2005). Pyrite resulted from replacement of phyrritite, chalcopyrite and magnetite and it is usually replaced by goethite in the supergene zone (Shcherbak and Bobrov, 2005). Pyrite may be enriched in Cu, Ni, Co, Zn and As, whereas phyrrotite shows higher content of Cu and Ni (Shcherbak and Bobrov, 2005).. 33.

(34) 4.2. Classification of Fe ores. The Krivoy Rog BIF hosts different types of Fe ores. Shcherbak and Bobrov (2005) classified them into four main groups: iron quartzites, high-grade ores, oxidized iron quartzites and soft iron ores. According to another classification, based on the Fe content, 5 types of hard iron ores can be distinguished: iron quartzites corresponding to low-grade ores (31-37% Fe), iron-rich quartzites (38-45% Fe) and high-grade ores: massive, magnetite containing 52-56% of Fe, massive martite of nearly the same iron content as the magnetite type and porous martite ore containing up to 70% of Fe (Belevtsev et al., 1991). The unoxidized iron quartzites (30-45% Fe) include magnetite, martite, hematite-martite, iron. mica-magnetite,. hematite-magnetite,. silicate-magnetite. and. cummingtonite-magnetite. quartzites, however the magnetite varieties are definitely more prevalent (Shcherbak and Bobrov, 2005). Within the Skelevatske-Magnetitove deposit there are 4 technological varities of unoxidized iron quartzites: magnetite dominant quartzites (18-32% Fe), hematite-magnetite quartzites (18-34% Fe), silicate-carbonate-magnetite quartzites (18-23% Fe) and silicate quartzites (18-26% Fe) (Pieczonka et al., 2011). Deposits of unoxidized iron quartzites are usually associated with hinges of folded iron ore horizons (Shcherbak and Bobrov, 2005). The oxidized iron quartzites are primarily comprised of hematite, martite and goethite which comprise goethite-iron mica-hematite-martite and goethite-iron mica quartzites and other similar varieties (Shcherbak and Bobrov, 2005). The high-grade iron ores (46-70% Fe) are developed within the iron (ore) quartzite horizons and include different mineralogical varities i. e. magnetite, hematite-magnetite, martite, hematitemartite, goethite-iron mica-martite and goethite-iron mica with domination of martite ores (Shcherbak and Bobrov, 2005). The classification by Shcherbak and Bobrov (2005) includes ironrich quartzites as the high-grade ores, whereas they do not belong to this group according to Belevtsev and others (1991). In general, the high-grade Fe ores (52-70%) were divided into two main groups: massive (compacted) and porous (not compacted). The unoxidized, massive magnetite high-grade ores are typically banded, however they do not contain quartz bands and they form lenslike ore bodies within 4th, 5th and 6th iron ore horizons (Belevtsev et al. 1991). The massive, martite ores (52,5-56,3% Fe) are composed of martite and iron mica (54-60%) and quartz (40-45%) and they occur in contact zones between porous iron ore and iron quartzites (Belevtsev et al. 1991). Other classification divides the high-grade Fe ores into “Saksaganskiy” and “Pyervomayskiy” types named after exploitation ore fields (Belevtsev et al., 1991). The Saksaganskiy Fe ore type is situated within the Saksaganskiy tectonic block and comprises deposits of unoxidized as well as oxidized Fe ores associated with great variety of structures. In the southern part of the block they form steep deeping bodies occurring in zones of intensive folding and faulting, whereas in the northern part of 34.

(35) the block they are associated with fold hinges and flexures (Belevtsev et al., 1991). The high-grade ores form bed-like, lenses or columnar ore bodies and they occur only in footwalls of thrust zones controlled by talc-carbonate schists (Paranko 1993; Shcherbak and Bobrov, 2005). The Pyervomayskiy Fe ore type is located within the Pyervomayskiy tectonic block. This block hosts unoxidized iron ores covered with thick metasomatic aureole (alkaline metasomatism overprint), which prevented the ore from being deeply oxidized (Belevtsev et al. 1991). The soft iron ores were formed by weathering processes and they comprise primarily iron oxide-hydroxides (Shcherbak and Bobrov, 2005). Estimated reserves of unoxidized iron quartzites are 13.4 Gt (cat. A+B+C 1+C2, up to depth of 500-800 m), of high-grade ores: 0.97 Gt (cat. A+B+C), 0.35 Gt (cat. C 2, up to depth of 1500 m), of oxidized iron quartzites: 3.67 Gt (cat. A+B+C 1), 0.58 Gt (cat. C2) and of soft iron ores: 0.01 Gt (cat. A+B+C), 0.0013 Gt (cat. C2) (Shcherbak and Bobrov, 2005). The most profitable, among all of the ore types, are high-grade iron ores, which usually do not require processing, and unoxidized iron quartzites, which need to be enriched to become a profitable product (Shcherbak and Bobrov, 2005). In contrast, the least beneficial are oxidized iron quartzites, requiring complicated processing due to presence of only non-magnetic minerals (Shcherbak and Bobrov, 2005).. 4.3. Iron ore genesis. Each type of Krivoy Rog iron ore is attributed to different genesis, which determined its quality and mineralogy. Unoxidized iron quartzites are metasedimentary rocks, which underwent regional metamorphism in greenschists and epidote-amphibolite facies (Belevtsev et al., 1991; Shcherbak and Bobrov, 2005). Different varieties of this iron ore type were partly influenced by overlapping younger processes e. g. alkaline metasomatism (Shcherbak and Bobrov, 2005). Oxidized iron quartzites resulted from oxidation of unoxidized iron quartzites in supergene conditions (Shcherbak and Bobrov, 2005). Soft iron ores formed weathering caps developed within all types of iron ores, during the Phanerozoic (Shcherbak and Bobrov, 2005). The genesis of high-grade iron ores is more complex and is still under debate (Belevtsev et al., 1991; Shcherbak and Bobrov, 2005; Khudur, 2006). It is an undeniable fact that all high-grade ores are controlled by and restricted only to iron ore horizons of the Krivoy Rog BIF (Belevtsev et al., 1991). Massive, magnetite and martite ores are the result of hypogene processes, whereas porous martite ores were formed by involvement of supergene processes (Belevtsev et al., 1991). One of the earliest hypotheses assumed that schists and iron quartzites were replaced by magnetite and hematite during Mg-Fe and Fe metasomatism (Lazarenko et al., 1977). According to this point of view, initially during the first stage, the Mg-Fe metasomatism caused crystallization of Mg-Fe35.

(36) rich amphiboles and therefore replacement of various schists and iron quartzites by cummingtonite schists and magnetite-cummingtonite quartzites, respectively (Lazarenko et al., 1977). The second, final stage involved the Fe metasomatism, which induced precipitation of iron (magnetite and hematite) in conditions of increased redox potential (Lazarenko et al., 1977). The involvement of various schists in high-grade ores genesis was later definitely denied and excluded by other authors (Belevtsev et al., 1991). Moreover, the cummingtonite schists and quartzites are currently believed to be a result of metamorphism instead of metasomatic processes (Belevtsev et al., 1991). According to an idea by Belevtsev and others (1991) there is a strict relationship between low-grade iron ores, iron-rich quartzites, high-grade magnetite and martite ores. The low-grade Fe ores and iron-rich quartzites are claimed to be precursors for massive, magnetite ores and massive, martite ores, which in turn were parental rocks for the oxidized, porous martite ores (Belevtsev et al. 1991). Based on structural analyses of contacts between different Fe ore types it was concluded that highgrade Fe ores show epigenetic character (Belevtsev et al. 1991). The relationship between hematite veins within quartz veins and the surrounding high-grade ore also served as a good evidence of epigenetic origin of high-grade Fe ores (Belevtsev et al. 1991). The hematite veins, originally perpendicular to the iron quartzite banding and the quartz vein orientation, resisted the influence of ductile deformation and were preserved within high-grade Fe ores but were re-oriented parallel to the banding (Belevtsev et al. 1991). This fact also suggests that compressional forces were involved in ore-forming processes. Massive, high-grade Fe ores. The origin of high-grade, massive ores (52-56% Fe) is poorly understood. The processes responsible for their formation included a volume reduction of iron quartzites (around 30% on average) and metasomatic removal of silica (hypogene metasomatic contraction in Russian terminology) (Belevtsev et al. 1991). These processes have not been investigated enough yet and require more research (Belevtsev et al. 1991). The hypothesis concerning hypogene metasomatic contraction is supported by structural data i. e. tectonic pinching out of quartz bands close to the contacts between iron quartzites and high-grade ores, swarms of boudines of iron quartzites surrounded by zones of massive Fe ores, and sharp, short, transverse contacts between e. g. martite iron quartzites, massive high-grade martite ore and porous high-grade martite ore (Belevtsev et al. 1991). Another clue is the Fe content of the massive, high-grade ores, which is similar to the Fe content of individual ore bands within iron quartzites (Belevtsev et al. 1991). This resulted from the fact that ore bands are thought to have been chemically inert and have undergone ductile deformation during the metasomatic removal of quartz (Belevtsev et al. 1991). Belevtsev and others (1991) also considered a possibility of replacement of banded quartz by ore minerals, however they did not find convincing evidence and assumed that there had not been additional input of Fe. 36.