Ultramafic–mafic igneous complexes of the Precambrian East Siberian metallogenic province (southern framing of the Siberian craton): age, composition, origin, and ore potential

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Ultramafic–mafic igneous complexes of the Precambrian East Siberian metallogenic province (southern framing of the Siberian craton): age, composition, origin, and ore potential G.V. Polyakov a, N.D. Tolstykh a,*, A.S. Mekhonoshin b,c, A.E. Izokh a,e, M.Yu. Podlipskii a, D.A. Orsoev d, T.B. Kolotilina b,c a

V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia b Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia c Irkutsk State Technical University, ul. Lermontova 83, Irkutsk, 664074, Russia d Geological Institute, Siberian Branch of the Russian Academy of Sciences, ul. Sakh’yanovoi 6a, Ulan Ude, 670047, Russia e Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia Received 11 March 2013; accepted 10 April 2013

Abstract Based on new data on the age, mineralogy, and geochemistry of ultramafic–mafic complexes in the Precambrian structures of the southern periphery of the Siberian Platform, the East Siberian metallogenic (PGE–Cu–Ni) province is recognized. It includes the Yenisei Ridge, Precambrian Kan uplift, Alkhadyr terrane with the adjacent structures of the Biryusa block, and northern Baikal region (Yoko-Dovyren and other massifs of the Baikal–Patom basin). We have established that the U–Pb and Ar–Ar ages of ore-bearing complexes of dunite–peridotite– pyroxenite–gabbro association correspond to the Late Riphean (728–710 Ma). The mineralogical and geochemical similarity of ore-bearing complexes in different areas testifies to their genetic entity. All parental melts were similar in composition to picrites. The calculation results and the PGE enrichment of rocks and ores show high degrees of melting of the mantle source, which agrees with the plume model of formation of the ore-magmatic system. The recognized province is similar in the type of magmatism and time of its occurrence to the Franklin LIP in northern Canada. It is one of the highly promising ore districts of East Siberia. © 2013, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: PGE–Cu–Ni deposits; dunite–peridotite–pyroxenite–gabbro association; Late Riphean; mineralogy; PGE–Cu–Ni geochemistry; metallogeny; parental picrite melts; East Siberian province

Introduction Based on the results of investigations performed in the Precambrian structures of the southern framing of the Siberian craton in recent decade, the East Siberian metallogenic province (ESMP) has been recognized and is actively studied today. It abounds in various, including commercial, Pt–Cu–Ni deposits and ore occurrences localized in Precambrian ultramafic–mafic igneous complexes (Gertner et al., 2005; Glazunov et al., 2003; Mekhonoshin et al., 2011; Polyakov et al., 2006). The province (Fig. 1) includes the Yenisei Ridge and Kingash group deposits (Kan superterrane) in the west and deposits of the Barbitai and Gutara–Uda ore clusters * Corresponding author. E-mail address: [email protected] (N.D. Tolstykh)

(Alkhadyr terrane (Mekhonoshin et al., 2012)) in the east. The results of recent studies have given the grounds to assign the Yoko-Dovyren group deposits (Baikal–Patom pericratonal trough), localized east of the above ore clusters, to the ESMP, since they are also associated with Precambrian ultramafic– mafic complexes (Ernst et al., 2012; Polyakov and Izokh, 2011; Polyakov et al., 2011; Yurichev et al., 2013). Despite the long-term investigations of ore-bearing complexes in some ore districts of the ESMP, their genesis and mutual relations within the province are still unclear for two reasons: (1) insufficient data on their radiological age, which, in turn, is due to the specific composition and high degree of metamorphism of their rocks, and (2) different degrees of their petrological, mineralogical, and geochemical research and the lack of similar models of their formation. The models constructed for particular massifs (Yoko-Dovyren) are difficult

1068-7971/$ - see front matter D 201 3, V.S. So bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.201 + 3.10.008

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Fig. 1. Schematic occurrence of ore-bearing complexes in the East Siberian metallogenic province. 1, continents and microcontinents; 2, exposures of the Precambrian basement of the Siberian craton; 3–7, folded areas: 3, Riphean; 4, Riphean–Vendian; 5, Early Caledonian; 6, Late Caledonian; 7, cover of the West Siberian Plate; 8, suture zones; 9, ultramafic–mafic massifs hosting PGE– Cu–Ni deposits and ore occurrences: 1, Shumikha; 2, Kingash; 3, Verkhnii Kingash; 4, Golumbei; 5, Tartai; 6, Ognit; 7, Zhelos; 8, Tokty-Oi; 9, Malyi Zadoi; 10, Yoko-Dovyren. I, Yenisei Ridge; II, Sayan zone (Kan and Alkhadyr terranes); III, Baikal–Patom pericratonal trough.

to use for the solution of the above problem for the entire province because of the lack of data on the age and, correspondingly, the genetic entity of ore-magmatic systems separated from each other. New geochronological, mineralogical, and geochemical data obtained by now permit correlation between ore-bearing complexes of different areas. In this paper we compare ore-producing rock complexes of all parts of the ESMP and consider their ages and mineralogic, geochemical, and genetic features.

The age of ore-bearing complexes of the ESMP Geological data on the age of ore-bearing ultramafic–mafic complexes in the ESMP are fragmentary and have been insufficient for the correlation of these complexes until recently. Reliable radiological data have also been scarce. The available dates show a wide spread in values and are inconsistent both with each other and with geological data. The ore-bearing dunite–peridotite–pyroxenite–gabbro massifs of the Kingash group of PGE–Cu–Ni deposits in the western ESMP (Kan block) were initially dated at the Proterozoic, 2.1 Ga, according to geological data (Glazunov et al., 2003). Later on, the same authors (Glazunov and Radomskaya, 2010)

estimated the time of formation of the Kingash ore-magmatic system at 1460–1410 Ma. Gertner et al. (2005) recognized three stages (with a long gap between them) in the formation of the Kingash massif: (1) Early Riphean (1410 ± 49 Ma) ultrabasic intrusions, which are considered by O.M. Glazunov as the main hosts of mineralization; (2) Late Riphean (874 ± 70 Ma and 873 ± 38 Ma) gabbroid intrusions; and (3) much later Early Paleozoic (491 ± 45 Ma), close in time to the Late Ordovician metamorphism and the intrusion of Early Paleozoic granitoids (500–480 Ma). The approximately estimated ages of metamorphic and igneous complexes of the Gutara–Uda and Barbitai ore districts in the central ESMP were published at the same time (Mekhonoshin and Kolotilina, 2006). The Ar–Ar age of metamorphism superposed on the rocks of the ore-bearing dunite–peridotite–pyroxenite complex of the Alkhadyr terrane is 600 Ma, and the age of the younger granitoids is 450 Ma. Thus, the upper age boundary of the ore-producing regional magmatism was roughly estimated. More consisting dates were obtained in study of the ore-bearing ultrabasic–basic complex of the Baikal–Patom pericratonal trough on the eastern flank of the ESMP. The complex includes the well-studied Yoko-Dovyren and other layered plutons of the northern Baikal region, assigned to dunite–peridotite–troctolite–gabbro association. The plutons are located in the Synnyr–Patom rift structure and are associated with Riphean picrite-basalts and basanites of the Synnyr and Inyaptuk Formations composing it. Sulfide Cu–Ni mineralization was revealed in the lower, essentially ultrabasic part of the Yoko-Dovyren pluton, and low-sulfide PGE ore occurrences were found in the middle, predominantly gabbroid part. The radiological age of the complex is within 740– 700 Ma; in particular, the U–Pb age of the volcanic rocks of the Synnyr and Inyaptuk Formations is 700 ± 20 Ma (Neimark et al., 1991). Close ages were obtained for the rocks of the Yoko-Dovyren massif: 700 ± 35 Ma (Sm–Nd dating) (Neimark et al., 1989) and 740 ± 35 Ma (Rb–Sr dating) (Kislov et al., 1989). Comparison of the earlier obtained radiological dates for the ore-bearing complexes of the East Siberian province shows a wide range of their values and their difficult correlation. Therefore, the recently obtained geochronological data on the three main districts of ore occurrence are worthy of special attention. The U–Pb age of the Tartai massif of the ore-bearing complex of the Alkhadyr terrane in the central ESMP was obtained by SHRIMP zircon dating of weakly altered olivine melanogabbro. We studied several zircon grains and calculated a concordia, which corresponds to an age of 712 Ma, i.e., the Late Riphean (Fig. 2). The studied zircons are characterized by Th/U = 0.9–1.9, typical of basic systems. Dating at some points of zoned zircon grains yielded younger ages, 670– 630 Ma, corresponding to later superposed metamorphism. Similar age was obtained by Ar–Ar biotite dating of peridotites that underwent superposed metamorphism nearly at the same time.

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Fig. 2. U–Pb age concordia for the Tartai massif.

Recent studies (Ernst et al., 2012) have yielded close Late Riphean ages for the gabbroids of the Verkhnii Kingash (Kan block) and Yoko-Dovyren (Baikal–Patom trough) plutons, which are ore-bearing complexes on the western and eastern flanks of the East Siberian province, respectively. In both areas, these are the first U–Pb dates for baddeleyite from the relatively fresh gabbroids of the middle and upper parts of layered intrusions bearing PGE–Cu–Ni mineralization and being reference for the ore-bearing complexes of the ESMP. The baddeleyite age of the gabbroids of the Verknii Kingash intrusion is 726 ± 18 Ma (four fractions), and that of the gabbroids of the Yoko-Dovyren pluton is 724 ± 2.5 Ma (five fractions). Based on these data, Ernst et al. (2012) recognized a large igneous province (Dovyren–Kingash LIP) in southern Siberia, which is similar to the Franklin LIP formed in northern Canada and in western Greenland at 725–710 Ma. The above ages agree with our date for the Tartai massif. The age of olivine-free gabbronorite from the near-roof zone of the Yoko-Dovyren pluton has been recently determined by U–Pb dating of zircons with their local laser ablation analysis (Ariskin et al., 2012), 730 ± 6 Ma (33 grains from three samples). It agrees with the age of peridotite sill located below the pluton base, 731 ± 4 Ma (56 grains from five samples), and overlaps with the age of hornfels hosted in the intrusion, 723 ± 7 Ma (ten grains), and gabbronorite from the near-base dike, 725 ± 8 Ma (15 grains). According to these data, the most likely age of the Yoko-Dovyren massif is 728.4 ± 3.4 Ma, which agrees with the U–Pb dates obtained

after metaporphyrites from the volcanic Inyaptuk Formation, 729 ± 14 Ma (eight grains). The obtained dates for the volcanic and intrusive rocks of the Dovyren complex argue for the synchronous manifestation of the Inyaptuk–Synnyr phase of volcanism and intrusion processes (Yoko-Dovyren pluton), which accompanied the opening of the Synnyr–Patom rift. Similar geochronological dates were obtained for the Neoproterozoic basic dikes of the Nersa complex in the southern Siberian craton (Gladkochub et al., 2006, 2007; Sklyarov et al., 2003). In chemical composition these dikes correspond to basalts of normal or moderate alkalinity. In geochemical features they are similar to mantle melts generated in intraplate dilatation settings. Gladkochub et al. (2007) suggest that the Yoko-Dovyren massif formed at the active stage of rifting (after the mass intrusion of dikes) nearly at the time of the Franklin rifting widely proceeding in northern Laurentia. In general, the age correlation based on the new geochronological data suggests that the ore-bearing complexes of the three main areas of the ESMP formed synchronously in the same ore-magmatic system. This is confirmed by our mineralogical and geochemical studies of the ore-bearing igneous complexes and associated PGE–Cu–Ni ore occurrences of the Alkhadyr terrane and by comparison of these data with the earlier determined composition of the ore-bearing complexes of the Kan (Kingash area) and Baikal–Patom (Yoko-Dovyren pluton) blocks.

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Fig. 3. Schematic location of ore districts in the central ESMP. Circles mark ore districts: 1, Biryusa–Tagul; 2, Uda–Biryusa (Ognit, Tartai, Medvezhii Log, and El’gedak massifs); 3, Barbitai (Zhelos and Tokty-Oi massifs).

Composition of ore-bearing complexes The composition of ore-bearing complexes is considered based on our mineralogical and geochemical data on the ore districts of the Alkhadyr terrane in the central ESMP (Fig. 3). These data are juxtaposed with those on the ore-producing complexes on the western (Kingash group of PGE-Cu-Ni deposits) and eastern (Yoko-Dovyren pluton) flanks of the province. Three large areas of ore-producing (PGE-Cu-Ni) ultrabasic–basic magmatism are recognized within the Alkhadyr terrain (Mekhonoshin et al., 2013): Barbitai (Zhelos, Tokty-Oi, and other massifs), Uda–Biryusa (Ognit, Tartai, Medvezhii Log, El’gedek, and Malaya Shita massifs), and Biryusa–Tagul (Fig. 3). The ore-bearing massifs belong to dunite–peridotite– pyroxenite–gabbro association. These are small lenticular intrusive bodies significantly deformed and metamorphosed, often subjected to plicate dislocations and boudinage, with tectonically disturbed contacts with the enclosing rocks. The largest and best studied intrusions are Zhelos and Tokty-Oi in the Barbitai area and Tartai and Ognit (Medek) in the Uda–Biryusa area. At the Zhelos site there are outcrops of about ten boudin-like bodies of different shapes (Fig. 4) formed by ultrabasic rocks varying in composition from lherzolites to olivine pyroxenites. The rocks are differently amphibolized but almost nonserpentinous, with several ore zones with Cu–Ni–PGE mineralization, up to 100 m long and 50 m wide. The Tokty-Oi massif is made up of the most highly altered rocks, mainly serpentinous peridotities and dunites grading into apoperidotite and apodunite serpentinites with zones of densely disseminated Cu–Ni sulfide and Pt–Pd mineralization.

The massifs in the Uda–Biryusa area are larger than the intrusive bodies in the Barbitai area; they are nonboudinaged and have preserved many elements of the internal structure and morphology of the bodies. They are composed of a differentiated series of rocks varying in composition from dunites and peridotites to melanocratic olivine gabbro. At each denudation level the massifs are dominated by particular rocks. The better studied Tartai massif (Fig. 5) is a lenticular body of N–S strike, 3.0 × 0.5 km in size. Most of it is composed of serpentinous dunites and wehrlites; the latter are often amphibolized near the massif contacts. The Mg# value of predominant olivine is within 83–90%, and the NiO content is 0.18–0.36% (Fig. 6). Clinopyroxenes correspond in composition to subcalcic augite. Their Mg# varies from 75.7 to 89.7%, Al2O3 = 2.0–4.7%, and Cr2O3 = 0.27–1.7%. Orthopyroxenes occur mainly as coronite rims at the boundaries between olivine and plagioclase grains and seldom as large oikocrysts. Plagioclase crystals are rare and are highly basic (up to 77–86% An). Primary magmatic highly magnesian hornblende and secondary amphiboles (edenite and tremolite) are scarce. Cr-spinels, as will be shown below, fall in the composition field of typical chromites of Pt-bearing layered ultramafic–mafic massifs, thus differing seriously from chromites of ophiolite complexes. Comparison of the petrochemical and mineralogical characteristics of ore-bearing ultramafic–mafic complexes of different areas shows their similarity. On the MgO–Al2O3 diagram, the composition points of rocks of these complexes form a distinct trend related to olivine fractionation (Fig. 7). The compositions of main rock-forming minerals are also similar, especially those of Cr-spinels (Fig. 8). In all studied massifs these minerals have elevated contents of TiO2 (3–4%) and contain exsolution structures with ilmenite phase. Their Ti contents are in positive correlation with Fe contents. These elements accumulate, most likely, in residual melt during the

Fig. 4. Schematic geologic structure of the Zhelos site. 1, ultrabasic rock bodies: peridotites and pyroxenites; 2, amphibolites; 3, garnet–biotite gneisses; 4, marbles; 5, biotite gneisses.

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evolution of the ore-forming system. High Ti contents are also revealed in the chromites of the Kingash (Kornev et al., 2001) and Yoko-Dovyren (Kislov, 1998) massifs. This is due to the interaction of cumulus chromite with Ti-enriched intercumulus liquid, from which ilmenite then crystallizes (Barnes and Kunilov, 2000). According to the experimental data, it is most likely that ferrochromites are produced in the course of subliquidus reactions (Ewers et al., 1976). The studied chromites are similar in composition to chromites from mafic rocks of continental ultrabasic–basic complexes (Barnes and Roeder, 2001), including Pt-bearing layered and zoned plutons (Fig. 8). Moreover, the rocks and ores of the studied complexes have close REE and PGE contents and patterns. The rocks of the ore-bearing massifs of the Alkhadyr terrane are twice to ten-fold enriched in LREE. Their REE patterns are of weak negative slope. In the Gutara–Uda area, the REE patterns of rocks are more gently sloped and lack Eu anomalies. The

Fig. 5. Schematic geologic structure of the Tartai massif. 1, dunites; 2, wehrlites; 3, plagioperidotites and olivine gabbro; 4, granites of the Ognit complex; 5, Alkhadyr Formation; 6, localities of Pt–Pd mineralization.

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Fig. 6. Contents of Ni and f values of olivine from rocks of the Tartai massif.

dunites and peridotites of the Yoko-Dovyren pluton are less enriched in REE. Nevertheless, the REE patterns of the ore-bearing complexes of the Alkhadyr terrane are similar in general (Fig. 9). The ore-bearing dunite–peridotite–pyroxenite–gabbro massifs of the terrane and other ESMP areas are characterized by elevated PGE contents, with a distinct domination of Pd over Pt. The (Pd + Pt)/(Os + Ir + Ru + Rh) ratio is within 10–15. The ore occurrences in the Barbitai area have elevated contents of high-refractory PGE (IPGE), especially Ru. The ores contain minerals of Os, Ir, and Ru, briefly considered below. In general, the PGE ratios in the ore-bearing complexes of the Alkhadyr terrane are similar to those in the rocks and ores of the Kingash and Yoko-Dovyren plutons (Fig. 10). Sulfide ores in the massifs of the Alkhadyr terrane are of Ni type, which is also specific for ore occurrences in other ESMP areas, including the Kingash and Yoko-Dovyren massifs (Table 1). In disseminated ores pentlandite is usually subordinate to pyrrhotite, whereas in massive and densely disseminated ores it is more abundant and is often predominant among sulfides. Chalcopyrite is a subordinate mineral. The ore minerals show some common compositional features. The pyrrhotite group minerals are characterized by a wide variation in Ni contents (0.04–1.4%). The minimum contents of Ni in the massifs of the Alkhadyr terrane, as in all world deposits, are revealed in troilites, and the maximum ones are observed in monoclinic pyrrhotite. Correspondingly, the composition of coexisting pentlandite also varies. The concentration of iron in pentlandite decreases in the series troilite–hexagonal pyrrhotite–clinopyroxene as the Ni content increases. In general, Fe/Ni in the pentlandites varies from 1.0 to 1.71 in the Zhelos, Tokty-Oi, and Ognit (Medek) massifs and from 0.68 to 1.66 in the Tartai massif. The pentlandites of the Kingash and Yoko-Dovyren plutons are of similar composition (e.g., elevated S and Fe contents) (Fig. 11). We revealed similar PGE minerals in all studied massifs of the ESMP (Fig. 12). At the same time, some massifs of the Alkhadyr terrane have specific features. The main minerals of the Zhelos intrusion are arsenides, with sulfoarsenides of IPGE prevailing, whereas the predominant mineral of the Ognit and Tartai massifs is sperrylite. All massifs include Pd-Bi and Pd-Sb compounds, but in the Tartai massif they are replaced

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Fig. 7. Compositions of rocks of dunite–peridotite–pyroxenite–gabbro intrusions of the Alkhadyr terrane (1) and Kingash (2) (Glazunov et al., 2003) and Yoko-Dovyren (3) (Tolstykh et al., 2008) plutons. Fig. 8. Compositions of Cr-spinels from rocks and ores of massifs of the Alkhadyr terrane (1) and Kingash (2) (Glazunov et al., 2003) and YokoDovyren (3) (Tolstykh et al., 2008) plutons.

by secondary Pd–Cu–Sb phases. The similar PGE mineralization within the ESMP is expressed as sperrylite in variable amounts in all ore districts. A specific feature of this mineral is the presence of IPGE (Os, Ir, Ru) and elevated contents of Fe and Ni (Fig. 13). The type of ore mineralization in the massifs of the Alkhadyr terrane suggests the formation of immiscible oxide–sulfide liquid enriched in specific ore components at the magmatic stage of evolution of the ore-magmatic system. This liquid was in equilibrium with silicate melt. The composition of the equilibrated phases, pyrrhotite and pentlandite, changed, most likely, by the following scheme: Pentlandite in assemblage with early high-temperature troilite was rich in Fe, whereas in assemblage with late hexagonal clinopyroxene it was enriched in Ni. In this case, the evolution of the ore-forming system runs through the successive enrichment of produced parageneses with Ni. This process is accompanied by an increase in sulfur activity. The Ni/(Ni + Fe) ratio in pentlandite reflects the conditions of its formation. In the Tokty-Oi massif it varies from 0.41 to 0.50, which indicates that pentlandite was produced in the range of sulfur fugacity (lg fS2) from –12 to –10 at ~600 ºC (Vaughan and Craig, 1978). The Fe-pentlandite of the massifs of the Barbitai ore cluster (Zhelos and Tokty-Oi) is similar in composition to the pentlandites of the ore occurrences of the Gutara–Uda area and of the Kingash (Glazunov et al., 2003) and Yoko-Dovyren (Konnikov, 1990) plutons. At the same time, it differs from the Ni-richer pentlandites of the Noril’sk and Pechenga ore complexes. The PGE mineralization of the above-considered ore-bearing massifs of the Alkhadyr terrane shows some specific features. In particular, some sulfide ores of the Zhelos massif bear PGE mineralization with elevated contents of IPGE and Rh, which form their own mineral phases, including omeiite

(Os,Ru)As2, irarsite IrAsS, and hollingworthite RhAsS. Note that the Zhelos massif is similar in enrichment in IPGE to ophiolites but differs from them in the type of ore mineralization, since in ophiolites these elements occur as native Os–Ir–Ru alloys or Os and Ru sulfides. The high activity of As in the formation of sulfide ores of the Zhelos massif led to the abundance of PGE arsenides and sulfoarsenides. The proportions of minerals evidence that irarsite (together with pyrrhotite) was the first to crystallize, the next was hollingworthite, which, in turn, was replaced by cobaltite. All Pd minerals, namely, compounds of Pd with Ni, As, Sb, Te, and Bi, were produced, most likely, at the final stage of evolution of the ore-forming system. The Ognit and Tartai massifs of the Uda–Biryusa area are rather similar in mineralogic and geochemical features, including PGE mineralization (Mekhonoshin et al., 2013). In both massifs, the PGE mineral assemblages are diverse in composition. They lack IPGE as well as Ir and Rh sulfoarsenides, specific for the Zhelos massif. This is probably due to the more highly fractionated composition of melt in the intermediate chamber, which formed the massifs of the Uda–Biryusa area. Speryllite, often in paragenesis with secondary orcelite and reduced intermetallic alloys of Pt and Cu, is a prevailing PGE mineral in the ore-bearing massifs of the Alkhadyr terrane, as in the other ESMP areas (Kingash, Yoko-Dovyren, etc.). The PGE minerals of the Uda–Biryusa area contain Cu and Ni as minor elements, up to 9.13 and 13.86 wt.%, respectively. Speryllite of the Kingash deposit on the western flank of the ESMP has high contents of Ir, like speryllite of the Tartai massif (Tolstykh and Podlipskii, 2010) (Fig. 13).

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Fig. 9. REE patterns of rocks and ores of massifs of the Alkhadyr terrane and other ESMP areas. a, Barbitai ore cluster; b, Uda–Biryusa zone; c, Kingash (1) (Glazunov et al., 2003) and Yoko-Dovyren (2) (Tolstykh et al., 2008) plutons.

Comparison of PGE minerals from basic and ultrabasic rocks of the Alkhadyr terrane and those from the other ESMP areas (Kingash, Yoko-Dovyren) showed their similarity. The presence of minerals of IPGE in the Zhelos massif and the presence of Ir in speryllites of the Ognit, Tartai, and Kingash massifs might be due to the high degree of melting of mantle substratum during the generation of parental magmas.

et al. (2005) recognized two stages in the formation of ultramafic–mafic series of intrusions in the Kingash area. The first, Early Riphean, stage was the time of formation of mainly ultrabasic intrusions, which include the lower ore-hosting horizons of the Kingash massif. At the second, Late Riphean, stage, gabbroid intrusions appeared, as well as a contact-reac-

The conditions of formation of ore-bearing complexes in the ESMP and the composition and evolution of parental magmas Since the above complexes belong to the same ore-magmatic system, their genesis, in particular, the composition of parental magmas, can be considered based on a set of data on different areas, including those remote from each other. Comparison of these data shows that the parental melts of rocks of all ore-bearing complexes were highly magnesian and corresponded in composition to picrites. The Kingash group of deposits, which was the first in the ESMP to be studied, is juxtaposed (Glazunov et al., 2003; Glazunov and Radomskaya, 2010) with the Pechenga type of essentially nickel sulfide deposits localized in layered ultramafic–mafic intrusions of gabbro–clinopyroxenite–wehrlite association. It is believed that intrusions of this type formed from ultrabasic melts of picrite composition. Later on, Gertner

Fig. 10. Chondrite-normalized PGE patterns of massifs of the Alkhadyr terrane (1) (Zhelos, Tokty-Oi, and Ognit massifs (Mekhonoshin and Kolotilina, 1999)) and Yoko-Dovyren (2) (Tolstykh et al., 2008) and Kingash (3) (Glotov et al., 2004) plutons.

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Table 1. Compositions of ores in massifs of different ESMP areas, wt.% Massif

Ni

Co

Cu

S

Ni/Co

Ni/Cu

Zhelos

2.15

0.08

0.50

14.15

26.9

4.3

2.78

0.18

2.77

31.65

15.44

1.00

2.00

0.08

0.27

–

24.39

7.41

0.93

0.03

0.29

1.76

36.61

3.21

1.01

0.02

0.27

2.38

43.91

3.74

1.12

0.02

0.24

1.69

48.70

4.67

3.84

0.13

0.03

28.02

29.31

147.69

Kingash

6.2

0.10

0.38

26.94

62

16.32

0.72

0.02

0.28

1.30

36

2.57

Yoko-Dovyren

1.78

0.09

0.41

15.3

19.34

4.34

0.23

0.02

0.053

1.70

11.5

4.34

Tokty-Oi

Note. Kingash massif (Glotov et al., 2004), Yoko-Dovyren pluton (Tolstykh et al., 2008).

tion series of wehrlite–clinopyroxenite composition. The first (essentially ultrabasic) association was produced from picrite magma, and the second, from basic one. Picrite composition of ore-bearing magmas was also established for the ultramafic–mafic complexes of the Alkhadyr terrane in the central ESMP. Mineralogical and geochemical studies (Mekhonoshin et al., 2006, 2011) showed that the

ore-bearing intrusions of the Barbitai area of the terrane, composed of dunite–wehrlite–clinopyroxenite–gabbro series, are products of picritic magma differentiated to various degrees. It was established that the compositionally different rocks of this association were produced from picritic magma evolving during the crystallization and cumulation of olivine. This is evidenced from the SiO2/Al2O3–(MgO + FeO)/Al2O3

Fig. 11. Compositions of pentlandite from massifs of the Alkhadyr terrane (a) and Yoko-Dovyren and Kingash plutons (b). a: 1, Tokty-Oi, 2, Zhelos, 3, Ognit, 4, Tartai; b: 1, Yoko-Dovyren, 2, Kingash.

G.V. Polyakov et al. / Russian Geology and Geophysics 54 (2013) 1319–1331

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Fig. 12. Correlation of PGE minerals in ores from massifs of the Alkhadyr terrane (a–c) and Yoko-Dovyren (d, e) (Tolstykh and Podlipsky, 2010) and Kingash (f) (Shvedov et al., 1997) plutons.

diagram (Fig. 14) with a distinct composition trend of ultramafic–mafic rocks of the Barbitai area, which is parallel to the fractionation vector of olivine. Analysis of the coefficients of the Fe–Mg exchange between olivine and melt yielded the composition of parental melt, which corresponds to picrites. Moreover, the above researchers revealed drop-like inclusions of silicate minerals of near-picrite composition (data of probe microanalysis) in Cr-spinels from ultrabasic rocks of the Barbitai area. The picritic composition of parental melt is also confirmed by the composition of quenched-facies rocks from intrusions of the Barbitai ore cluster (Table 2). For the well-studied Yoko-Dovyren pluton, the picritic composition of parental magma was determined based on the composition of quenched facies and the weighted average

composition of the main pluton body and near-base sills (Kislov, 1998; Konnikov, 1986). In relation to the studies of the phase layering of ultramafic–mafic massifs, a model of the formation of the Yoko-Dovyren massif was elaborated (Ariskin et al., 2003, 2009). The model calculations showed that by the time of the magma chamber filling and the pluton formation, the parental magma (intrusion temperature 1185 ºC) underwent a serious supracritical crystallization and consisted of solid phase (mainly olivine) and near-boninite melt to the extent of 40–50%. This composition corresponds to the average composition of ultrabasic magmas similar to picrites. We studied the formation conditions of the Tartai intrusion, which was considered above on the description of the

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Fig. 13. Impurities of refractory PGE, Fe, and Ni in sperrylites from ores of the Alkhadyr terrane (1) and Kingash (2) and Yoko-Dovyren (3) plutons.

ore-bearing complexes of the Alkhadyr terrane. This massif is a typical representative of dunite–peridotite–pyroxenite–gabbro association in the ESMP and bears low-sulfide Pt–Pd mineralization. Mineralogical and geochemical studies showed that the massif rock association resulted from the fractionation of early olivine in ultrabasic melt. By model calculations, we estimated the composition of melt being in equilibrium with the true composition of olivine from the most melanocratic rocks. The composition of olivine varies from forsterite (≤10% Fa) in dunites and wehrlites to chrysolite (~17% Fa) in melanogabbro, which is accompanied by a significant decrease in NiO contents in it (Fig. 6). At the same time, the liquidus olivine very quickly comes to equilibrium with more ferruginous residual melt; therefore, its Mg# value is often underestimated, which leads to the erroneous evaluation of the parental-melt composition. To prevent errors in calculation of the composition of liquidus olivine, we made its independent evaluation by the

technique of A.A. Ariskin. As seen from the diagram (Fig. 15), the variations in rock compositions are due only to the fractionation of olivine present in the intruded magma. The f value of intratelluric olivine from the ultrabasic rocks of the Tartai massif was calculated from the equation of linear regression (Fig. 15); it is 10–12% Fa, which corresponds to the true composition of olivine in dunites and peridotites. Judging from the composition of early olivine and calculated FeO/MgO ratio in olivine and melt, the MgO content in the melt equilibrated with olivine varied from 17 to 11%. We took its maximum value as basic because the MgO content in the rocks of the Tartai massif is >15%. Taking this into account, we calculated the rest parameters of the parental melt by the equations of linear regression for MgO and other rock-forming components (Table 3). Thus, here, as in the case of ore-bearing complexes in other considered ESMP areas, the parental magmas correspond in composition to picritoids, which agrees with the ubiquitous elevated Pt contents in the main types of rocks of the studied association. Conclusions

Fig. 14. Composition variation diagram for ultrabasic and basic rocks from ore-bearing complexes of the Barbitai ore cluster.

1. The reported data show similarity and, correspondingly, genetic entity of ore-bearing (PGE-Cu-Ni) complexes of different areas throughout the East Siberian metallogenic province: Kingash (Precambrian Kan uplift) and Yenisei Ridge in the west of the ESMP (Nozhkin et al., 2013), Alkhadyr terrane and neighboring structures of the Biryusa block in the central zone, and Yoko-Dovyren pluton (Baikal–Patom pericratonal trough) in the east. 2. The radiological (U–Pb and Ar–Ar) age of ore-bearing complexes in all ESMP areas is within 728–710 Ma, i.e., Late Riphean. These dates agree with the age of the reef dike belts of the Nersa complexes and show the time correlation of formation of the studied ore-bearing complexes with the formation of the Franklin LIP during the breakdown of Rodinia in the Late Riphean.

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Table 2. Calculated composition (wt.%) of parental melt for massifs of the Barbitai area in comparison with compositions of rocks from the quenched facies and of silicate inclusions in Cr-spinels No. 1

SiO2

Cr2O3

FeOtot

MnO

MgO

CaO

Na2O

K2O

Mg/(Mg + Fe)

0.56

7.85

–

11.62

–

27.49

6.71

–

–

0.808

7.53

–

11.2

–

27.8

6.74

–

–

0.815

4.58

0.11

25.94

8.13

1.19

0.34

0.910

Quenched sample (262) 43.42

3

Al2O3

Calculated composition of melt 47.53

2

TiO2

0.52

Composition of silicate inclusion Cr-spinel 51.24

0.35

6.71

1.32

Table 3. Estimated compositions (wt.%) of parental melts for the Tartai massif No.

SiO2

TiO2

Al2O3

FeO

MnO

MgO

CaO

Na2O

K2O

P2O5

1

49.5

0.50

9.3

9.14

0.5

17.00

12.86

0.66

0.22

0.11

2

47.83

0.68

11.68

11.25

0.50

16.56

10.34

0.89

0.16

0.09

3

50.19

0.85

14.58

8.34

0.62

11.08

12.92

1.11

0.19

0.11

Note. 1, composition of parental melt calculated from the equations of linear regressions for MgO = 17%. 2, 3, compositions of parental melt calculated from the fractionation trends of the most magnesian (2) and most ferruginous (3) olivines, controlled using the COMAGMAT program.

3. The studied ore-bearing intrusions belong to dunite–peridotite–pyroxenite–gabbro association with predominant ultrabasic rocks and subordinate gabbroids. The only exclusion is the Yoko-Dovyren pluton in the North Baikal area, with a predominance of gabbroids. The mineralogical and geochemical trends of the differentiated rocks series composing the ore-bearing massifs are similar in all studied areas, which indicates similar compositions of their parental magmas. In addition, the ore minerals, including PGE ones, of these rocks have similar characteristics.

4. The parental melts of all studied rocks are close in composition to picrites or picrite basalts. This is evidenced from the presented geological, mineralogical, geochemical, and calculated data as well as the rock and ore enrichment in PGE, which, in turn, points to high degrees of melting of the mantle source. 5. The results obtained show that the ESMP can be regarded as a Late Riphean LIP highly promising for PGE– Cu–Ni deposits. East of it, on the southern folded framing of the Siberian craton and Aldan Shield, a still older (Proterozoic)

Fig. 15. Calculation of the composition of early olivine from rocks of the Tartai massif. 1, analysis points; 2, olivine line with estimated-composition points; 3, linear trend; 4, 95% confidence interval of the linear trend.

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Udokan–Chinei metallogenic province with a wide spectrum of ore deposits, including PGE–Cu–Ni ones, is recognized (Gongal’skii, 2012). Thus, this vast near-platform area of Precambrian structures in southern Siberia can be considered a large ore district of commercial importance. This work was supported by Program ONZ-2 from the Department of Geosciences of the Russian Academy of Sciences and by grants 12-05-00435 and 12-05-00112 from the Russian Foundation for Basic Research.

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