Oligocene boundary

Ore Geology Reviews 20 (2002) 65 – 82 www.elsevier.com/locate/oregeorev

Genesis of the Eastern Paratethys manganese ore giants: impact of events at the Eocene/Oligocene boundary Igor M. Varentsov Geological Institute, Russian Academy of Sciences, Pyzhevskii pereulok 7, Moscow, ZH-17, 109017, Russia Received 10 August 2001; accepted 20 February 2002

Abstract The giant Mn ore deposits of the Paratethys are located on the slopes and/or margins of deep basins. The Eocene ocean and associated basins were strongly influenced by large-scale submarine hydrothermal activity which supplied prodigious amounts of Ca, CO2, SiO2, Mn, and other components to the oceans. This caused a substantial shift in seawater composition and resulted in the deposition of carbonates as well huge amounts of Mn. The giant Mn ore deposits of the Eastern Paratethys were formed as a consequence of the Early Oligocene global transgression in shelf environments of marginal and epicontinental seas, frequently with anoxic regime of the bottom water. D 2002 Published by Elsevier Science B.V. Keywords: Mn ore giant deposits; Eastern Paratethys; Ocean hydrothermal activity; Black shale basins; Early Oligocene; Global transgression; Recycling; Diagenesis

1. Introduction (formulation of the problem) The largest of the Phanerozoic Mn ore deposits of the South Ukraine (Nikopol and others), Georgia (Chiatura and others), the Mangyshlak Peninsula, northeast Bulgaria and northwest Turkey, as well as the small deposits of Hungary and Slovakia, are synchronous accumulations. They occurred at the base of the Lower Oligocene over a large area stretching from Central Europe to Middle Asia. They have a common structural position (southern margin of the Eurasian plate), geological setting (tectonic rigid subplatform substrate), facies relationship, and sedimen-

E-mail address: [email protected] (I.M. Varentsov).

tary environment (shelf of inland, marginal and islandarc basins). The deposits contain over 30% of the Phanerozoic world manganese ore resources and more than 70% of the Cenozoic resources (Varentsov, 1996). The majority of geologists accepted a sedimentary – diagenetic origin of the Early Oligocene Mn deposits, with variations of the source of ore components (Varentsov, 1963; Betekhtin, 1964a,b; Strakhov et al., 1967, 1968; Gryaznov, 1971; Aleksiev and Bogdanova, 1974; Orlovskii, 1981; Shnyukov et al., 1993; ¨ ztu¨rk and Frakes, 1995). However, in some works, O the derivation of the ores is considered as immediately associated with deep-seated, or hypogene processes (Aleksiev, 1959; Chukhrov, 1974; Mstislavskii, 1980, 1984, 1985; Knyazev and Shevchenko, 1986). The empirical fact of accumulation of a great amount of dissolved Mn(II) in basin water with an anoxic regime is reflected in sedimentological and

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geochemical models of formation of the giant shallowmarine Mn deposits (Sapozhnikov, 1971; Cannon and Force, 1983; Frakes and Bolton, 1984, 1992; Force and Cannon, 1988; Stolyarov, 1993). The model was applied for elucidation of the genesis of such appreciable deposits as Chiatura, Georgia, (Bolton and Frakes, 1985), Groote Eyland, Australia (Frakes and Bolton, 1984), and Molongo, Mexico (Okita, 1992). The problem under review was clearly stated in the beginning of the last century. A close look at the Mn ore formation within the Oligocene basin of the Paratethys (see reviews: Obruchev, 1934; Vernadskii, 1954a,b) revealed that Mn deposits of the South Ukraine (the Nikopol et al.), West Georgia (the Chiatura et al.), the east Caspian Region (the Mangyshlak), and of Asia Minor occurred at closely approximating stratigraphic levels. Obruchev (1934, p. 436) emphasized the most essential point of the problem: «The confinement of the manganese ores only to a certain age, and the lack of the ore in overlying and underlying beds can hardly be explicable. These beds were deposited in the same sea which for a short interval of time became rich in manganese compounds». Vernadskii (1954b, p. 521) noted the giant scale of the Mn ore accumulation: «. . .the Chiatura and other deposits show that they are relics of even lager ancient concentrations of manganese which occurred over a great space in sediments of the Eurasian Oligocene sea about 40 Ma ago. . . A major part of the manganese accumulated by this way

was again mobilized and disseminated, the remnants survived in the regions of Dnieper, Laba, Transcaucasus, the North Urals, Georgia, and in Asia Minor». The main subject of this paper is to propose a genetic model which unambiguously describes the known information, establishes a possible correlation between Mn ore formation and the geological events which occurred at the Eocene/Oligocene boundary, and reveals the relationships of the Mn ore forming processes with phenomena on a regional and/or global scale. In order to do this, it has been necessary to make an initial assessment of the association of Mn ore formation and the global reorganization of the plate system. This reorganization involved a considerable increase in the rates of production of new oceanic crust at the Eocene/Oligocene boundary as well as profound changes in ocean chemistry, climate, sedimentation, ocean productivity, and features of biosphere development which occurred at the time.

2. Materials and methods The work is based on material accumulated by the author during previous studies in different regions of the Paratethys. This has been augmented by studies of geochemical aspects of sedimentation processes at the Eocene/Oligocene boundary in some regions of the World Ocean which have been carried out under the aegis of DSDP and ODP.

Fig. 1. Cross-section through the ore stratum and mineral composition of manganese ores. The Nikopol deposit, western ore field, Ordzhonikidze area, Shevchenko quarry (Varentsov et al., 1997). 1—Miocene clays, limestone, marls, and sands, thickness up to 30 m; 2 and 3, clays above ores, thickness from 2 to 8 m (Lower Oligocene); 2—grayish-green with ocherous patches, sandy; 3—clays green, sandy – silty, separate from Mn ore stratum by layer with Fe-oxyhydroxide aggregates; 4—ore stratum represented by sandy – clayey sediments with aggregates, pisolites, oolites, and earthy irregular – lenticular accumulations of Mn oxyhydroxides developed after Mn carbonate ores altered by supergene processes; Thickness of the ore stratum varies from 0 to 4.5 m, averaging 1.8 m; 5—weathering crust after Precambrian granite – gneiss rocks; 6—cryptomelane; 7—pyrolusite; 8—manganite; 9—manganocalcite; 10—goethite; 11—lewistonite (Ca, K, Na-apatite); 12— quartz; 13—sample location. Mineral composition of samples: 1—weathering crust after Precambrian granite – gneiss: quartz (40 – 50%), kaolinite (40%), hematite (10%); 2—sand subore, green, clayey, oxidized: clastic quartz (40 – 50%), feldspar (20 – 30%), heulandite – clinoptilolite (10%), hydromica (5 – 10%), finely dispersed montmorillonite (5 – 10%); 3—Sample n-91-24. Oxidized Mn ore with rare relics of host rocks (sandy – clayey material) and Mn carbonate: pyrolusite (60 – 70%), manganocalcite (20 – 30%), birnessite (<5%); 4—Sample n-91-23. Mn-oxidized ore with rare relics of host rocks (sandy – clayey material and Mn carbonate: pyrolusite (60 – 70%), manganite II (20 – 30%), quartz (10 – 20%), mixed-layered smectitemica phase (<5%); 5—Sample n-91-21. Mn-oxidized ore with rare relics of host rocks (sandy – clayey material) and Mn carbonate: cryptomelane (60%), lewistonite (Ca, K, Na-apatite) (20 – 30%), goethite (5 – 10%), and quartz (5 – 10%); 6—Sample n-91-20. Oxidized Mn ore with rare relics of host rocks (sandy – clayey material) and Mn carbonate: cryptomelane (90%), and quartz (10%); 7—clay, sandy, olive-green: finely dispersed montmorillonite (60%), clastic quartz (20%), hydromica (5 – 10%), feldspar (5%), kaolinite (5%); 8—clay grayish-green, with ochreous patches and jarosite sandy, close in mineral composition to Sample 7.

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3. Manganese ore basins Zonal stratigraphic trends based on the studies of planktonic foraminifera and nannoplankton fossils have enabled us to correlate sedimentary events, particularly those associated with Mn ore accumula-

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tion with the highest possible accuracy. Calcareous nannoplankton respond to changes in environmental parameters to a lesser extent than planktonic foraminifera. The zonal trends are therefore mainly based on studies of nannoplankton. The boundary of the Eocene/Oligocene is established at the base of the

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Fig. 2. The S(La
Sm)/S(Eu
Lu) ratio distribution in the main types of manganese ores of the Nikopol deposit. Dot-and-dash line, the North American shale, average (Haskin et al., 1966). Sample description: 1—footwall clay, subore (Lower Oligocene) average, gray, sandy: quartz (60 – 70%), feldspar (10%), zeolite of the heulandite – clinoptilolite type (10 – 15%). Clay fraction (<0.001 mm): hydromica (70%), dispersed montmorillonite (up to 20%), chlorite (up to 20%), zeolite of the heulandite – clinoptilolite type; 2—Sample n-91-13. Carbonate – manganite ore, partly oxidized. Aggregates of primary manganite I with an admixture of Mn carbonate (see Sample n-91-13a): manganite (30%), goethite (20 – 30%), manganocalcite (20 – 30%), kutnahorite (20%), todorokite (10%), quartz (<5%). Aleksandrovskii quarry, eastern face; 3—Sample 2/3. Manganese carbonate ore with an admixture of sandy material: kutnahorite (60 – 70%), ankerite (20%), heulandite – clinoptilolite (10 – 15%), quartz (10%). Aleksandrovskii quarry, eastern face; 4—Sample 2/8. Oxyhydroxide ore: manganite II (90%), pyrolusite (5 – 10%), quartz (to 5%). Aleksandrovskii quarry, eastern face; 5—Sample n-91-23. Mn-oxidized ore with rare relics of host rocks (sandy – clayey material and Mn carbonate: pyrolusite (60 – 70%), manganite II (20 – 30%), quartz (10 – 20%), mixed-layered smectite-mica phase (<5%). Shevchenko quarry; 6—Sample n-91-13a. Mn carbonate – manganite ore, oxidized, with aggregates of primary manganite I (see Sample n-91-13): manganocalcite (30 – 40%), todorokite (30 – 40%), kutnahorite (5 – 10%), heulandite – clinoptilolite (5 – 10%), quartz (5%). Aleksandrovskii quarry, eastern face; 7—Sample 2/7. Almost completely altered, oxidized Mn carbonate ore: todorokite (30 – 40%), cryptomelane (30 – 640%), manganite II (15 – 20%), lewistonite (Ca, K, Na-apatite) (5 – 10%), heulandite – clinoptilolite (5 – 10%), Mn calcite (<5%), kutnahorite (<5%), Ca-rhodochrosite (<5%). Aleksandrovskii quarry, eastern face; 8—Sample n-91-20. Mn-oxidized ore with rare relics of host rocks (sandy – clayey material) and Mn carbonate: cryptomelane (90%), and quartz (10%). Shevchenko quarry; 9—overlying clay, supraore (Lower Oligocene) average, lightgreenish-gray, sandy.

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Globigerina tapuriensis zone (foraminifera) and the foot of the CP16 Helicosphaera reticulata zone (nannoplankton), as well as at the transition from the Beloglinsk Suite to the Maikop Series (Krasheninnikov and Muzylev, 1977; Muzylev and Tabachnikova, 1987; Krasheninnikov and Akhmetiev, 1998). These relationships show that the sediments underlying the manganese ore deposits correlate with zone CP16. The upper boundary of the manganese ore sequence is determined by the age of the Polbinsk (Ostracod) Bed overlying the Mn ore sediments. The Polbinsk Bed is correlated with a narrow stratigraphic interval between zones CP16 and CP17 (Muzylev et al., 1992; Muzylev, 1998). According to modern determinations, the following absolute ages of the main stratigraphic units are taken: the lower boundary of the Upper Eocene—37.0 Ma; the Eocene/Oligocene—34.0 Ma; the Lower Oligo-

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cene/Upper Oligocene—29.0 Ma (Montanari et al., 1985; Odin and Luterbacher, 1992; Golovin and Krasheninnikov, 1998). The manganese ore sequence of most, if not all, of the deposits of the Paratethys is associated with clear evidence of an unconformity and transgression on underlying rocks. The Nikopol and other Mn ore deposits of the South Ukrainian basin occur as a wide belt (20 –25 km) which extended along the southern slope of the Ukrainian Shield for 250 km (Varentsov, 1963; Betekhtin, 1964a). Three ore subzones (oxide, oxidecarbonate, and carbonate ores) followed in succession as the crystalline basement of the shield subsided to the Black Sea basin. The carbonate ores comprise minerals of the manganocalcite – Ca-rhodochrosite series with occasional admixture of kutnahorite – ankerite (Fig. 1) (Strakhov et al., 1967; Varentsov et al., 1997). The

Fig. 3. Paleogeographic sketch map of the Early Oligocene, Georgia, and localization of the major Mn deposits and ore occurrences (after Laliev, 1964; Varentsov, 1966; Avaliani, 1982). 1—Land; 2—sea; 3—isopachs of the Oligocene sediments; 4 – 5—Mn deposits and ore occurrences: 4—stratiform (sedimentary – diagenetic); 5—ore occurrences; 6—vein; 7—vein polymetallic. Index number: 1 – 7—Lower Oligocene Mn deposits and ore occurrences, sedimentary – diagenetic: 1—Chiatura; 2—Kvirila; 3—Shkmeri; 4—Chkhari – Adzhameti group; 5—Vani – Zestafoni group; 6—Racha group; 7—Surami – Kareli group; 8 – 10—fields of polymetallic mineralization: 8—Tsedisi – Kvaisi low temperature hydrothermal, Upper – Middle Jurassic (Mn, Fe, Pb, Zn, Cu, Ba, Hg, As); 9—Bol’nisi – Tetritskaro hydrothermal, Upper Cretaceous (Mn, Fe, Ba, Cu, Pb, Zn); 10 – 11—vein Mn ore occurrences: 10—Kodman group (hydrothermal accumulations in the fracture zone crosscutting Upper Cretaceous limestones; 11—Gegechkari – Tskhakaya group (hydrothermal accumulations in the Upper Cretaceous volcanogenic sequences).

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Fig. 4. Geologic – lithological profiles of the Lower Oligocene Mn deposits, Georgia (Avaliani, 1982). (I) Chiatura deposit, Perevisi Highland. 1—Karaganian – Sarmatian limestones, sandy clays; 2—Tchokrakian quartz sands and clays; 3— Oligocene (a—spongolite sandstones, b—Mn ores); 4—Upper Eocene – Oligocene (subore sands and sandstones ); 5—Upper Cretaceous (limestones); 6—Lower and Middle Paleozoic (crystalline rocks of the Dzirul massif); 7—Pliocene (basalts); 8—major fault. (II) Shkmeri deposit. 1—Oligocene (a—clays of the Maikop Series, b—Mn ores); 2—Upper Eocene – Oligocene (subore sands and sandstones); 3—Touronian (volcanogenic sediments and limestones); 4—Senomanian (glauconitic sandstones and sandy limestones). (III) Chkhari-Adzhameti deposit. 1—Lower Sarmatian (clays and sandy limestones); 2—Middle Miocene (limestones, marls, calcareous sandstones); 3—Oligocene – Lower Miocene (a—Maikop clays and spongolite sandstones; b—Mn ores). 4—Middle and Upper Eocene (marls and calcareous sandstones). (IV) Meleshuri deposit (after D.V. Tabagari). 1—Oligocene (a—Maikop clays; b—Mn ores); 2—Upper Eocene – Oligocene (spongolite sandstones); 3—Bajocian (Porphyrite Suite); 4—fault.

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geological setting, structure, and oxygen and carbon isotopic compositions are indicative of diagenetic formation of the ore. The carbonate – manganite ores, in addition to the carbonates mentioned above, have a relatively early (sedimentary – diagenetic) nature. The distribution and association of major components, trace elements, and REEs are not significantly different from the average composition of the lithosphere (Fig. 2) (Varentsov, 1963; Strakhov et al., 1967; Varentsov et al., 1997). The products of supergene oxidation (in succession of their formation) are: manganite II!pyrolusite!todorokite!cryptomelane!birnessibirnessite!rancieite (Figs. 1 and 2). Annual production in 1995 to 1999: 3200– 1986 thousand metric tons, with 30 –35% Mn (Jones, 2000). The Chiatura and other Mn deposits of the West Georgia basin occur unconformably at the base of the Oligocene on the tectonically rigid substrate of the Dzirul median massif (Figs. 3 and 4) (Betekhtin, 1964b; Avaliani, 1982). A salient feature of the orebearing and enclosing sandy – clay sediments is the widespread occurrence of spongioliths, opokas, and gaizes. The ores are present mainly as oxide and/or carbonate varieties, sometimes with the admixture of neotocite. The oxidation products depend on the depth of basement subsidence. The data on mineralogy, geochemistry of major components, trace elements, oxygen and carbon isotopes (Kuleshov and Dombrovskaya, 1997a,b) suggest that the main ore types are genetically similar to the South Ukrainian varieties. Annual production in 1991: 1321 thousand metric tons, with 29– 30% Mn (Jones, 1996). Mn accumulations of the South Mangyshlak are rather modest. They occur on a tectonically rigid substrate and comprise primary carbonate ores (f50 metric tons of ore) in sandy – clayey and carbonate sediments (the Kuyuluss Suite) on the northern slope of the Chakyrygan syncline zone which borders the Mangyshlak Island Rise (Stolyarov, 1991; Stolyarov and Kochenov, 1995). The Mn ore sediments were

Fig. 5. Cross-section through the ore stratum, Mine No. 1, the Obrochishte deposit, northeastern Bulgaria (Aleksiev and Bogdanova, 1974). 1—Clays and marls; 2—silty clays, silts and sandstones; 3—Mn carbonate pisolites; 4—Mn carbonate – silicate pisolites; 5—Mn silicate and clay pisolites; 6—concretions of Mn carbonate and calcite; 7—Mn carbonate interbeds; 8—Mn silicate interbeds; 9—clay admixture.

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deposited in littoral environments on the flanks of a deep basin in depths greater than 500 m. The Mn deposits of northeastern Bulgaria (the Obrochishte and other, the Varna region) developed as a rather narrow belt along the Black Sea coast (Aleksiev and Bogdanova, 1974). The region belongs to the Misian Platform. Glauconite silt occurred at the base of the ore bed (2– 24 m). However, the Mn ore frequently overlies Eocene marls (Fig. 5). Acid pyroclastic accumulations and their alteration products are characteristic of the enclosing sandy –silty –clay sediments. The ores (pisolitic, nodular, and layered) are composed of Mn hydrosilicates (neotocite) and carbonates (rhodochrosite, manganocalcite). The Mn deposits Binkilicß and others in northwestern Turkey are located within the Thrace basin, a small structural basin ringed by ancient massifs on the ¨ ztu¨rk and northern coast of the Sea of Marmara (O Frakes, 1995). The Lower Oligocene basal beds are characterized by lateral transition from shallow water limestones to conglomerates, and occur unconformably on Eocene micritic limestones showing evidence of transgression. The major ores (carbonate, mostly concretionary and pisolitic varieties, together with oxidation products) developed in the basal parts of Lower Oligocene (Congeria Series), with slight mineralization in the uppermost layers of the Eocene ¨ ztu¨rk and Frakes, 1995). Textural features rocks (O of the ores, as well as the distribution of major components, trace elements, REEs, oxygen and carbon isotopes suggest a sedimentary –diagenetic origin (Varentsov, 1963; Varentsov et al., 1997; Kuleshov ¨ ztu¨rk and Frakes, and Dombrovskaya, 1997a,b; O 1995). Eu/Sm ratios=1.33 – 1.50 are common in the REE patterns of the ores, and could indicate a hydrothermal origin of the primary ore components.

4. Paleostructure–paleogeographic features of the Mn ore basins At the termination of Eocene—beginning of Early Oligocene, eustatic rise of the World Ocean level and the elevation of regions of the Alpine fold system caused a separation of the basins north of the main Tethys sea. Three subbasins were formed as a result: the Western, Central, and Eastern Paratethys (Baldi, 1986; Popov et al., 1993). Mn ore sediments accu-

mulated in littoral-upper shelf environments of relatively deep basins, troughs, and/or deeps adjacent to the paleo-Black Sea margins: northern Black Sea (Prichernomorskaya) basin, Kvirila depression, as a part of the Kolkhida subbasin of the northern Black Sea basin, Middle Caspian basin (trough), eastern margin of the Misian Platform, as a slope of the Black Sea basin, Thrace basin on the southwestern margin of the Black Sea. The bottom waters of these basins were characterized by anoxic regimes in which hydrogen sulfide was commonly present.

5. Geochemical features of the World Ocean at the Eocene/Oligocene boundary Sandberg (1983) has subdivided the Phanerozoic Eon into five major global cycles on the basis of his study of the world data on primary marine nonskeletal carbonate mineralogy. Two cycles were dominated by conditions that were favorable for the deposition of calcite carbonates in the World Ocean: from Cambrian to Namurian, and from Late Triassic (Early Jurassic) to Eocene. The ‘‘calcite’’ episodes alternated with three cycles which were favorable for the deposition of aragonite – Mg calcite: from Late Precambrian to Early Cambrian, from Late Carboniferous to Early Jurassic, and from Oligocene to recent. These intervals correlate with eustatic, climatic, and magmatic cycles established during the course of previous studies (Hallam, 1977; Vail et al., 1977). A key part in recognition of carbonate mineralogical cycles in the Phanerozoic and their meaning for atmospheric CO2 levels is due to the work by Mackenzie and Pigott (1981). These trends have subsequently been confirmed (Wilkinson and Algeo, 1989; Hardie, 1996; Stanley and Hardie, 1998), and it has been shown that the periods of ‘‘aragonite’’ seas are synchronous with intervals of accumulation of MgSO4 evaporites in continental margin regions, whereas periods of ‘‘calcite’’ seas correlate with KCl evaporite deposition. The problem of fluctuation of ocean chemistry is not so easily determined, particularly in the light of the analyses of distribution of elements in evaporite fluid inclusions through geologic time (Lazar and Holland, 1988; Stein and Krumhansl, 1988; Das et al., 1990; Horita et al., 1991; Ayora et al., 1994; Land

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et al., 1995; Land and Lynch, 1998). In this context, it is often assumed that changes in ocean water chemistry, carbonate deposits, and evaporite accumulations can be interpreted as the result of secular variations (100 –200 Ma) in submarine hydrothermal activity in the World Ocean.

6. The role of submarine hydrothermal activity in the geochemistry of sedimentation in the World Ocean This is essentially predetermined by the worldscale interaction of seawater and the basalt lithosphere produced in the oceanic spreading zones. As a consequence, magnesium and sulfate ions are removed from seawater, and an extremely acid fluid (pH=2– 4) considerably enriched in calcium, potassium, silica, iron, manganese, and some trace elements is generated (Bischoff and Dickson, 1975; Wolery and Sleep, 1976; Mottl and Holland, 1978; Edmond et al., 1979, 1982; Staudigel and Hart, 1983; Lavelle et al., 1992; Lilley et al., 1995). The uptake of Mg2+ ions from seawater is associated with the release of an equivalent amount of Ca2+ from the basalt.

7. Geochemical effects of submarine hydrothermal activity as exemplified by Ca, CO2, and Mn In the Holocene – Pleistocene, hydrothermal alteration of tholeiitic basalt supplied to the World Ocean: Ca 181013 g year
1, CO2 9.31013 g year
1 and Mn (0.14 – 0.8) 1013 g year
1 (Wolery and Sleep, 1976; Wedepohl, 1981, 1988), or Mn 6851013 g year
1 (German and Angel, 1995). This compares with the flux of Mn from the rivers of 271013 g year
1 (Glasby, 1988, 2000; Elderfield and Schulz, 1996). Most of the hydrothermal Ca and Mn passed into seawater (to 95– 98%); 123.41013 CaCO3 g year
1 was deposited from HCO3
and Ca2+ derived from continental runoff. In addition, transformations of the basalt rocks (high-temperature alteration, palagonitization, and halmyrolysis) in the ocean supplied 7.3 –40.11013 g year
1 of Ca2+ which led to the deposition of 18.25 – 100.01013 CaCO3 g year
1, or 12.9 – 44.8% of the total amount of accumulated CaCO3. The same proportion of the hydrothermal

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CO2 introduced into the World Ocean was released to the atmosphere. An estimated CO2 budget shows that the ocean share of the total global input of carbon dioxide into the atmosphere is 75% (with 33.6% of hydrothermal origin). The annual input of dissolved Mn as a consequence of basalt alteration (6851013 g year
1; German and Angel, 1995) exceeds by several orders of magnitude the value of the continental dissolved runoff of Mn. The most notable geodynamic event at the Eocene/ Oligocene boundary: 34.0 Ma (Montanari et al., 1985; Odin and Luterbacher, 1992; Golovin and Krasheninnikov, 1998) was the collision of India and Eurasia (F38 Ma, and somewhat later, in the Late Eocene) which caused considerable rearrangement of the lithospheric plates (Zonenshain and Savostin, 1980; Lisitzin, 1980; Schwan, 1985; Lomize, 1986). This resulted in a reduction and narrowing (in some regions a closing) of the sea strait which separated Eurasia from the continent of Gondwana. This is clearly seen in active belts of the continental lithosphere (folded regions of the Alpine type) and in the spreading zones of oceans and seas. Studies of the rates of the ocean crust production for the last 150 Ma show that these parameters exceeded the modern World Ocean values in the Eocene by a factor of 1.5, with the highest average rate of crust generation in the Cenozoic in the Pacific Ocean during this epoch (Figs. 6– 8) (Rich et al., 1986; Zonenshain and Khain, 1989; Baker and Hammond, 1992). Accumulation rates of ores and metalliferous sediments were higher by an order of magnitude in Eocene than in Oligocene and Pleistocene – Holocene (Figs. 9 and 10) (Owen and Rea, 1985; Leinen et al., 1986; Gurvich, 1998).

8. Features of sedimentation, climate changes, and eustatic oscillations of the World Ocean level at the Eocene/Oligocene boundary The global reorganization of the plate system and the tectonic events which took place in the Eocene (the Pyrenean orogenic episode: 45 and 37 Ma) were accompanied by intensive volcanism and hydrothermal activity, large-scale climate change, and transformation of the ocean circulation system. A drastic increase of volcanogenic – hydrothermal CO2 input

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Fig. 6. Diagram of the Earth’s tectonic activity for the last 155 Ma. Variations of oceanic crust (km2) formation rates for Atlantic (1), Indian (2), Pacific Oceans (3), and a cumulative diagram for the World Ocean (4) as a whole are shown. A—Curve of the World transgressions and regressions (Vail et al., 1977) (after Zonenshain and Khain, 1989).

into the atmosphere (80.03 – 440.001013 g year
1) caused a global temperature rise (the mean annual atmospheric temperature of the Middle Eocene in Europe was 27 jC) (Savin, 1977; Owen and Rea, 1985). At the end of the Eocene/Oligocene, strong global cooling and the increased density of oceanic surface waters resulted in a system of vertical circulation and activated strong bottom currents (Lisitzin, 1980; Muzylev et al., 1992; Popov et al., 1993). These currents have prevailed to the present, transporting considerable amounts of nutrients and lead-

ing to a pronounced erosion of the sea floor (Lisitzin, 1980). In the Eocene, the increased spreading rates at the axial zones resulted in an elevation of the World Ocean level followed by transgression (Haq et al., 1988). This was accompanied by significant and prodigious release of hydrothermal components as well as an increase in biogenic sedimentation which was a function of higher primary biologic productivity resulting from the increased supply of nutrients. The components (Ca, Corg, CO2, Si, as well as Mn, Fe, and other heavy metals) must have undergone

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Fig. 7. Fluctuation of mean seafloor spreading rates in the northern part of the Pacific Ocean (at the top) and the northern part of the Atlantic Ocean (at the bottom) from post-Jurassic to present time (Rich et al., 1986).

repeated recycling and biochemical transformations but were initially endogenous in origin (Lisitzin, 1981). As an example, the Eocene/Oligocene boundary clearly shows the relationship between the midocean spreading rate and global ocean transgressions (Hays and Pitman, 1973; Gaffin, 1987).

9. Mn ore formation in the basins of the Eastern Paratethys The geological rearrangements and related changes in the oceanic environment at the Eocene/Oligocene boundary made themselves evident in Eastern Paratethys configuration and, particularly, in drastic

changes in the hydrological and geochemical regimes which were a consequence of the deteriorating links with the World Ocean. At the beginning of the Oligocene, relatively limited basins with restricted water exchange, high biological productivity, and distinct oxygen deficiency as shown by stagnation and/or anoxia in the bottom waters were formed. Accumulation of sediments enriched in organic matter (the black shale type) was common in many regions, e.g., the Maikop Suite in southern Russia, Caucasus, Caspian Region, and others. Mn ore sediments were deposited in littoral environments as a result of the upwelling of deep waters rich in Mn(II) and dissolved Corg. from deep basins. The deep water was displaced due to the invasion of high salinity (dense) oceanic

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Fig. 8. Correlation of hydrothermal activity occurrences (venting probability) and oceanic spreading rates (Baker and Hammond, 1992).

water supplied by repeated global transgression (Fig. 11). Among the modern seas, where processes of Mn ore formation occur, no basins of a similar nature with the discussed Early Oligocene Paratethys are known. However, some features of resemblance to these basins are observed in the Baltic Sea (Varentsov, 1973, 1975; Varentsov and Blazhchishin, 1974, 1976; Varentsov et al., 1975; Blazhchishin, 1995; Glasby et al., 1997). The most intensive Mn ore formation in this shelf basin (average depth: 65 –68 m) occurs in the Gulfs of Riga, Finland, and Bothnia, as well as in the Central Baltic. The clearly displayed stratification of the water column is the distinctive characteristic of the regions: the lower and/or bottom water layers are characterized by higher density (i.e., salinity) and concentrations of Mn, Fe, and associated metals against the surface water. Some relatively deep basins are the salient features of the open part of the Baltic Sea: the Arkona Basin (55 M), the Bornholm Basin (105 M), the Gdansk Basin (116 M), the Gotland Basin (249 M), the Landsort Basin (459 M), and the Faro Basin (208 M). The basins as a rule exhibit a

restricted water exchange with water masses of the open sea with the distinct evidence of anoxic regime and hydrogen sulfide contamination. The effect of these basins on the geochemistry of heavy metals in the bottom water is of key significance despite the fact that they occupy only 5% of the total area of the sea. In open parts of the Baltic, where there are no distinctly localized sources, the bottom currents play a major part in distribution of ore-forming components. These currents essentially control behaviour of Mn and associated metals supplied by river runoff, and particularly introduced from anoxic deep basins. Concentrations of dissolved Mn in surface and bottom waters of the open sea (Gdansk – Klaipeda region) are, respectively, 0.2 and 4.1 ppb; Fe: 14.8 and 28.9 ppb (Varentsov, 1975). Concentrations of dissolved Mn and Fe in waters of the above-mentioned anoxic basins distinct to open sea values are over 700 and 120 ppb, respectively (Bostro¨m et al., 1988; Dyrssen and Kremling, 1990; Ingri et al., 1991). Moreover, Mn and the associated metals are fluxed by diffusion from the reduced muds (of black shale type). They are also involved into the general system of currents and

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higher density with a salinity of 20 – 36x, as compared to 7xin the Baltic. This difference between salinity values causes underflow of the more dense North Sea water and development of a water column stratification (i.e., halocline). The displaced basin water is involved into the general bottom circulation system. The accumulations of Mn –Fe oxyhydroxide gel-like sediments, crusts, and nodules form in areas with normal aeration and favorable hydrological conditions.

Fig. 9. Variations of accumulation rates of hydrothermal Fe (mg cm
2 103 year) in sediments of the northern Pacific Ocean section, along a submeridional belt of 10 – 15j width, between 0j and 16jN, East Pacific Rise region. The straight lines at 45j correspond the deep sea drilling holes (DSDP-IPOD, ODP), with numbers indicated at the left of vertical scale (after Gurvich, 1998).

transported to the sites with normal aeration, where they are deposited as oxyhydroxides. Mn carbonates (Ca-rhodochrosite) commonly develop after oxyhydroxide accumulations in local depressions of the sea floor and on slopes of the deep-like basins. The water of anoxic basins with high concentrations of Mn and the associated metals are displaced as a result of periodical invasions of North Sea water of

Fig. 10. Variations of accumulation rates of hydrothermal Fe (mg cm
2 103 year) in sediments of the southern Atlantic Ocean along a submeridional belt of 10 – 15j width, located east and west of the axis of the Mid-Atlantic Ridge. The straight lines at 45j correspond to deep sea drilling holes (DSDP-IPOD, ODP), with numbers indicated at the left of vertical scale (after Gurvich, 1998).

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Fig. 11. Paleogeographic sketch map showing localization of Mn ore deposits within the Early Oligocene basins of the Eastern Paratethys with additions and modifications (after Stolyarov, 1991; Popov et al., 1993). (1) Shelf regions; (2) bathyal deep basins; (3) coastal plains flooded at times by sea; (4) land; (5) Mn ore deposits: (I) South Ukraine (Nikopol, Bol’shoi Tokmak, and others); (II) Georgia (Chiatura, ChkhariAdzhameti, and others); (III) Mangyshlak; (IV) Northeastern Bulgaria, the Varna region (Obrochishte and others); (V) northwestern Turkey, the Thrace Basin, and others (Binkilicß and others).

The modern Black Sea features a water stratification essentially distinguished by density (salinity) and caused by river runoff (346 km3/year, Cl=0.016x) and waters of relatively high salinity from the Sea of Marmara (340 km3/year, Cl=995x) (Skopintsev, 1979). The density stratification causes restricted vertical circulation within the Black Sea and a wide distribution of sulfide hydrogen below a depth of about 200 m, as well as the associated total suppression of phytoplankton, zooplankton, and aerobic bacteria. A major mass of H2S in water column is formed by of reduction of the seawater sulfates. The vertical distribution of Mn in the water is controlled by the redox determining parameters (H2S, O2, and Eh). Minimal concentrations are recorded in the surface zone (depth from 0 to 50 m, Mntot.=25 ppb), where this element is mainly in particulate, oxyhydroxide form (Mokievskaya, 1961; Skopintsev and Popova, 1963; Spencer and Brewer, 1971; Brewer and Spencer, 1974). The concentrations of Mntot. in the hydrogen sulfide zone exceed 250– 300 ppb, with a substantial predominance of the dissolved form: Mn(II).

The total mass of Mn in the anaerobic zone of the Black Sea is approximately 100 metric tons (Skopintsev and Popova, 1963) The average residence time of Mn in this sea, assuming modern geologic conditions, is around 1700 years (Skopintsev, 1979). In other words, the periodical renewal of Mn in the Black Sea water may occur after 1000 –2000 years. Renewal of the Mn masses maybe by replacement caused by transgression, and a pulsating invasion of denser oceanic water. A schematic model of replacement and subsequent transportation of the Mn-enriched anoxic waters to the shallow well aerated shelf areas with accumulation of oxyhydroxide, carbonate-, and hydrosilicate compounds was considered above using the Baltic Sea as an example. There is good reason to believe that the Early Rupelian Paratethys was supplied for a geologically short time (about 0.3 Ma) by a giant amount of Mn, Fe, and other heavy metals, as well as Ca, Corg., CO2, and SiO2 transported by global oceanic transgression. Some stagnant and/or anoxic basins of the Paratethys, like the paleo-Black Sea, Kvirila and Transcaucasian Depressions, Middle Caspian Basins, and the Panno-

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nian Basin, served as the systems pumping over the introduced oceanic water which was converted into Mn-rich reduced solutions. Mn ore deposition occurred in shelf environments with normal aeration for as long as the capacity of the metal supplying system of the Paratethys retained its ore forming potential, other factors being the same.

10. Conclusions The highest rates of ocean lithosphere production in the Cenozoic are recorded in the Eocene and resulted in intensive hydrothermal activity. This was accompanied by a prodigious input of Ca, Si, CO2, Mn, and associated components into the World Ocean, which resulted in climate warming (greenhouse effect), an increase in biological productivity, rates of sedimentation, and eustatic elevation of the World Ocean (transgressions). This global episode of endogenous activity and the Pyrenean orogenic phase at the Eocene/Oligocene boundary (F38 Ma) were followed by the Rupelian transgression. At the beginning of the Early Oligocene, oceanic water rich in Mn and associated components and nutrients invaded the Paratethys basins. The giant manganese ore deposits were formed in the shelf environments of these basins as a result of the repeated recycling of Mn, organic matter, and associated components.

Acknowledgements I am grateful to my colleagues, Dr. Nikita Muzylev, Geological Institute, Russian Academy of Sciences, and Professor Larry A. Frakes, Adelaide University, Australia, for broad discussions on the Eocene/Oligocene boundary problems, Academician Alexander P. Lisitzin, P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences, for helpful comments on oceanic hydrothermal sedimentation, and Academician Dmitrii V. Rundquist, V.I. Vernadskii State Geological Museum, Russian Academy of Sciences, for close consideration of aspects of ore formation. Dr. Geoff Glasby is thanked for his careful review of the manuscript. Special thanks are due to the reviewers (particularly Dr. W. Pohl) and editors for useful suggestions and help.

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