Relationship between platinum-bearing ultramafic-mafic intrusions and large igneous provinces (exemplified by the Siberian Craton)

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Relationship between platinum-bearing ultramafic–mafic intrusions and large igneous provinces (exemplified by the Siberian Craton) A.S. Mekhonoshin a,b,*, R. Ernst c,d, U. Söderlund e, M.A. Hamilton f, T.B. Kolotilina a,b, A.E. Izokh g,h, G.V. Polyakov g, N.D. Tolstykh g a

Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1a, Irkutsk, 664033, Russia b Irkutsk Research Technical University, ul. Lermontova 83, Irkutsk, 664074, Russia c Department of Earth Sciences, Carleton University, Ottawa, ON K1S 5B6, Canada d Tomsk State University, ul. Lenina 36, Tomsk, 634050, Russia e Lund University, 12 Sulvegatan, Lund, 223 62, Sweden f J. Sutterlay Geochronology Laboratory, Toronto University, Toronto, ON N5S 3B1, Canada g V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia h Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia Received 8 July 2015; accepted 24 September 2015

Abstract This study aims at summarizing available geological and geochemical data on known Proterozoic platinum-bearing ultramafic-mafic massifs in the south of Siberia. Considering new data on geochemistry and geochronology of some intrusions, it was feasible to compare ore-bearing complexes of different time spans and areas and to follow their relationships with the recognized large igneous provinces. In the south of Siberia, the platinum-bearing massifs might be united into three age groups: Late Paleoproterozoic (e.g., Chiney complex, Malozadoisky massif), Late Mesoproterozoic (e.g., Srednecheremshansky massif), and Neoproterozoic (e.g., Kingash complex, Yoko-Dovyren massif, and massifs in the center of the East Sayan Mts.). In most massifs but Chiney the initial magmas are magnesium-rich. On paleogeodynamic reconstructions, the position of the studied massifs is the evidence that three most precisely dated events in North Canada continued into southern Siberia: In the period 1880–1865 Ma, it was the Ghost–Mara River–Morel LIP; at 1270–1260 Ma, the Mackenzie LIP; and at 725–720 Ma, Franklin LIP. In Siberia, the mostly productive massifs with respect to PGE–Ni–Cu mineralization are those linked with the Franklin LIP: Verkhny Kingash, Yoko-Dovyren, and central part of the Eastern Sayan Mountains, e.g., Tartay, Zhelos, and Tokty-Oy. © 2016, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: PGE–Ni–Cu deposits; mafic-ultramafic intrusions; large igneous provinces

Introduction As regards the economic value, magmatic PGE–Ni–Cu deposits typify: (1) sulfide PGE–Ni–Cu and (2) low-sulfide Pt–Pd (Ernst, 2014; Maier, 2005; Naldrett, 1981). Formation of sulfide PGE–Ni–Cu deposits is largely due to separation of sulfide melt rich in chalcophyle elements and PGE from basic or ultrabasic magma. In the works (Naldrett, 2010a,b) Naldrett recognized seven types of PGE–Ni–Cu deposits, and in four deposits magma formation is linked with LIPs (Ernst and Jowitt, 2013). They are referred to magmatic sulfide deposits associated with Archean and Proterozoic komatiites, trap

* Corresponding author. E-mail address: [email protected] (A.S. Mekhonoshin)

magmatism, derivatives of ferropicritic magmas and diverse magmas of picritic and tholeiitic composition. The best instances are unique deposits of Norilsk group (250 Ma Siberian trap LIP) (Lightfoot and Keays, 2005; Mitrofanov et al., 2013; etc.), PGE–Ni–Cu deposits in China (Permian Eimeshan plume) (Borisenko et al., 2006). The Pt–Pd-bearing deposits commonly sit within layered mafic-ultramafic intrusions (Eckstrand and Hulbert, 2007; Naldrett, 2003), their formation associated with two types of magmas (Naldrett, 2010a). High abundances of Ni, Cu, Co, Cr, V, PGE in basic and ultrabasic magmas result from enrichment with these elements of magma-generating mantle substrates, which is related to deep mantle plumes emplaced on the core-mantle boundary (Dobretsov et al., 2010; Kuzmin and Yarmolyuk, 2014; Pirajno and Santosh, 2014). It is

1068-7971/$ - see front matter D 201 6, 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 + 5.09.020

A.S. Mekhonoshin et al. / Russian Geology and Geophysics 57 (2016) 822–833

noteworthy, that all mantle magmas of high melting degree may potentially be Ni-bearing ones. However, as to productivity for PGEs, their formation is not possible without deep mantle plumes involved (Ernst and Jowitt, 2013; Maier, 2005; Naldrett, 2010a). In Russia, the Pt-bearing ultramafic-mafic intrusions have been discovered on the Kola Peninsula, in Karelia, Siberia, Russian Far East, and Kamchatka (Bychkova et al., 2009; Konnikov et al., 2009; Mitrofanov et al., 2013; Semenov et al., 2004). They occurred within a broad time span from 2500 to 75 Ma. For part of them the relation to LIP is the established fact, some others require additional research. In southern Siberia, irrespective of the fact, that platinum potential of basic-ultrabasic complexes has been studied through some decades (Glotov et al., 2004; Konnikov et al., 2000; Mekhonoshin and Kolotilina, 1997; Platinum-bearing…, 1995), it was only recently found out that new available data point to the relationship of PGE–Cu–Ni deposits and ore occurrences to LIPs (Ariskin et al., 2013; Ernst et al., 2012; Podlipsky et al., 2015; Polyakov et al., 2013). This paper offers new data on geochemistry and geochronology of some basic-ultrabasic intrusions, as well as the summary of acquired geological and geochemical data on the recognized Proterozoic Pt-bearing ultramafic-mafic massifs in South Siberia to compare ore-bearing complexes of various time spans and areas, and to identify their relation to the established LIPs.

Samples and analytical methods In the explored massifs, the rock and ore samples have been collected from outcrops, open mine workings and borehole cores. The analyses were performed at IGC SB RAS (Irkutsk) and at the Baikal Analytical Center for Collective Use, ISC SB RAS. The isotope compositions of Pb and U in baddeleyites from Malozadoisky and Srednecheremshansky massifs were measured by mass spectrometer Finningan TRITON at the Swedish Museum of Nature History (Stockholm, Sweden). All errors in isotope ratios and measured ages are given at confidence interval α = 95%. The contents of micro elements were determined in samples by mass spectrometry with ionization in inductively coupled plasma (ICP-MS). The measurements were conducted by mass spectrometer with magnetic sector ELEMENT 2 (Finnigan MAT, Germany) with double-focusing and registration of signal in three resolutions: low (LR)-300, middle (MR)-4000 and high (HR)-10000 M/∆M. The analyses were performed at standard operation conditions. The accuracy to determine the trace element concentrations and drift of device were controlled following international standards of dunite SDU-1 and peridotite JP-1. The contents of PGE (Ru, Rh, Pd, Pt, Ir, and Os), Au, and Re were measured by ICP-MS method with high-resolution mass spectrometer Element 2 (Finnigan MAT) using open acid decomposition and separation of matrix elements by cationite

823

KU-2-8 after the technique described in the work (Vlasova et al., 2010). The accuracy of concentration determination and drift of device were controlled with reference samples Zh-3, RP-1 (Ru, Rh, Pd, Pt, Ir, Au), and ESO-2 (Pd, Pt), as well as samples Jp-1 (Japan), OZE-1 (China)—(Ru, Rh, Pd, Pt, Ir, and Au). The relative errors received in measuring standards were <10%. Proterozoic platinum-bearing ultramafic–mafic intrusions in South Siberia Chiney pyroxenite-gabbro-anorthosite massif is located in the southeastern part of the Chara–Olekma geoblock of the Aldan shield (Fig. 1). Chiney intrusion is a lopolite-like irregularly shaped and slightly elongated magmatic body with outcropped area 150 km2. The nearest margin of Chiney massif comprises widespread intrusive formations—platy irregularly shaped bodies of basic rocks and numerous dikes to 200 m thick. These are Mailavsky, Luktur massifs and major dike of Udokan (Gongalsky et al., 2008). The Chiney massif is composed of the rocks of basic composition and layered structure. These are predominantly gabbronorites and gabbro with varying ratio of the amount of plagioclase and pyroxenes, anorthosites, pyroxenites, as well as gabbroids with increased content of titanium magnetite. Considering composition of rocks, this massif is compared with the upper part of the Bushveld pluton (Gongalsky et al., 2008). The Luktur massif and Major dike of Udokan are composed of gabbronorites. The Chiney massif involves three petrochemical rock series characterized by the increased contents of Ti and Fe, as well as Na and K, which resulted from crystallization from initial melts of ferrobasalt composition (Gongalsky et al., 2008). The gabbronorites of the Luktur massif and Major dike of Udokan show enrichment with light rare earths (Gongalsky et al., 2008). Impregnated chalcopyrite-pyrrhotite mineralization is conjugate to olivine gabbronorite sill located at the foot of the main phase of intrusion. Besides, there are exocontact essentially chalcopyrite ores with much higher concentrations of Pd (Rudnyi site) (Tolstykh et al., 2008). Both ore types contain palladium, however the endocontact ores contain Pd with Sn, As and Te compounds, while exocontact ones comprise Sb and Bi. The age of gabbroids in the center of Chiney massif was identified by Sm–Nd method to be 1850 ± 90 Ma (Gongalsky, 2013). The dates were measured as 1867 ± 3 Ma for rocks of marginal facies of the Chiney massif by studying the U–Pb system in zircons (Popov et al., 2009). The age dates1880 ± 16 and 1890 ± 6 Ma were acquired by Ar–Ar method for rocks of Chiney and Luktur massifs (Tolstykh et al., 2008), accordingly. Malozadoisky and Srednecheremshansky massifs (Fig. 1) are located in the Irkut–Kitoy block of the Sharyzhalgay uplift of the Siberian Craton basement composed of Archean and Proterozoic magmatic and metamorphic complexes. This uplift is elongated nearly for 300 km from the southwestern shore

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A.S. Mekhonoshin et al. / Russian Geology and Geophysics 57 (2016) 822–833

Fig. 1. Position of Proterozoic platiniferous mafic-ultramafic massifs in the structures of the Siberian Cration and its folded framework. 1, sedimentary cover; 2, uplifts of basement; 3, collision structures. Numbers denote massifs: 1, Chiney; 2, Malozadoisky; 3, Srednecheremshansky; 4, Yoko-Dovyren; 5, Kingash and Verkhny Kingash; 6, central part of East Sayan.

of Lake Baikal reaching the Oka River midstream. The southwestern boundary is the Major Sayan fault, and the northeastern one consists of sediments of the Siberian Craton cover. The metamorphosed sequence is strongly dislocated and arranged into NW-striking steep folds. The internal structure of Irkut–Kitoy block is largely composed of widespread granite-gneiss cupolas. Like in many other similar structures referred to granite greenstone belts, the general tectonic setting of the block is dominated by oval granite-gneiss cupolas located approximately at equal distances from each other and divided by star-like, primarily synform zones. Malozadoisky massif occurs in the SW part of Irkut–Kitoy block. It is dike-shaped, 120–150 m thick, and extends for nearly a kilometer. The massif encompasses a series of rocks varying in composition from plagioperidodites to leucoratic gabbronorites and ore anorthosites of cumulative or poikilitic structure. Macroscopically, layering appears in the massif as alternated melanocratic gabbronorites and plagioperidotites. It displays rhythmic change of contents, both petrogenic and rare elements: Ni, Cr, Co and Cu. Considering the ratio of SiO2 contents and the sum of alkalis, the rocks are referred to low-alkaline series of basic-ultrabasic rocks. However, they are marked by abundant Rb, Cs and Ba and essential enrichment in light REE relative to middle and heavy ones (Table 1, Figs. 2, 3). The cross-section exhibits six ore horizons enclosing abundant impregnated sulfide ores; their sulfide minerals pyrrhotite and pentlandite occur in the ratio 3:1. Chalcopyrite is not as abundant, and it is found in the margins of pentlandite–pyrrhotite aggregates. Sulfide parageneses are closely associated with phlogopite aggregates. The segregation nature of sulfide clusters, shape of grains, absence of veinlets indicates their early magmatic nature. The summary content of PGE (Table 2) in disseminated ores reaches 100 ppb, and in thickly disseminated it is 550 ppb. The discovered PGE minerals represent palladium telluride.

Srednecheremshansky massif is also dike-shaped, but more extensive as compared to Malozadoisky, that largely defines its internal structure. The massif, varying in thickness from 100 to 200 m, is elongated for 6 km, and dips towards west under angles 75º–80º. The host rocks are biotite and garnet-biotite gneisses. The massif is composed of gabbro, gabbronorites, olivine gabbro, olivine norites and plagioperidotites. In contrast to Malozadoisky massif, the rocks replace each other monotonously without producing rhythms. The petrochemical and rare element compositions of rocks of this massif are basically close to those of Malozadoisky . In contrast, it has a smaller scatter of TiO2, Cr, and Ni contents (Fig. 2) and lower HREE (Fig. 3). The ore mineralization includes nodules of disseminated pyrrhotite-pentlandite ores. Thickness of zone is about 0.5 m, the nodules are sized 5 × 5 cm. Pentlandite displays the emulsion dissemination of chalcopyrite partly replaced by native copper. In some grains copper is not associated with sulfides, and in cleavage fractures it sits in plagioclase. In Srednecheremshansky massif the heightened contents (total 380 ppb) of platinoids are common for the sulfide horizon located in the hanging wall. The U–Pb age of formation of Malozadoisky and Srednecheremshansky massif rocks was obtained through dating some baddeleyite fractions. For purpose of dating it was feasible to recover three and three grains of baddeleyite of 80 µm size from samples of gabbro (90–22) of Srednecheremshansky massif and olivine gabbronorite (93–85) of Malozadoisky massif, accordingly with mass about 200 g applying the technique described in (Söderlund and Johansson, 2002). The obtained data are provided in Table 3 and in Figs. 4 and 5. The upper intersection with Concordia may be interpreted as the age of intruding of Malozadoisky and Srednecheremshansky massifs 1863 ± 1 and 1258 ± 5 Ma, accordingly. Yoko-Dovyren dunite-troctolite-gabbro layered massif is located within the Olokit paleorifting zone (Fig. 1). The lens-shaped massif (sized 26.0 × 3.5 km) extends in NE direction. It is composed of differentiated rock series varying in composition from bottom to top from dunites to vehrlites and troctolites. The bottom and hosting layer of siltstones comprise crossing plagiolherzolite bodies, which in contrast to rocks of layered series show increased contents of TiO2, Na, K, REE, Nb and Ta. Ni–Cu sulphide mineralization and associated minerals of platinum group are primarily concentrated in plagiolherzolite zone of Yoko-Dovyren massif and accompanying sills of the same composition. Besides, they contain low-sulfide type of Pt-metal mineralization confined to lens-like seams of anorthosites developed on the contact of troctolites with olivine gabbro (Tolstykh et al., 2008). Sulfides represent association of charlcopyrite-pentlandite-pyrrhotite. The principal PGE mineral is sperrilite (PtAs2), tellurides and bismuth tellurides of Pt and Pd are fairly widespread (Tolstykh et al., 2008). The age of the Dovyren intrusive complex obtained by local analysis of zircons in samples by laser ablation (LA-ICP-MS) was measured as 728.4 ± 3.4 Ma (Ariskin et al., 2013) and by

825

A.S. Mekhonoshin et al. / Russian Geology and Geophysics 57 (2016) 822–833 Table 1. Rare-element composition of representative varieties of rocks of Malozadoisky (1–5) and Srednecheremshansky (6–10) massifs Component, ppm

89-385

89-386

89-387

93-85

93-86

90-22

93-130

90-71

90-75

93-128

1

2

3

4

5

6

7

8

9

10

Cs

0.18

0.77

0.17

0.22

0.22

1.21

0.14

2.61

0.25

0.22

Rb

9.71

10.50

10.51

14.81

14.21

26

9.3

29

14.2

11.25

Ba

212

160

165

327

311

–

–

184

172

164

Th

1.50

1.33

1.10

2.23

2.22

4.23

1.80

2.50

1.86

1.90

W

0.18

0.20

0.11

0.16

0.21

0.58

1.57

0.24

0.21

0.23

U

0.22

0.21

0.16

0.36

0.29

0.45

0.24

0.58

0.33

0.25

Nb

1.6

1.3

1.9

2.2

2.1

9.8

5.6

9.7

6.7

4.87

Ta

0.10

0.08

0.12

0.13

0.12

0.72

0.93

0.61

0.43

0.34

La

10.9

8.5

10.7

18.7

17.4

19.4

9.6

15.0

10.2

9.6

Ce

23

18

24

37

38

42

20

32

21

20

Sb

0.06

0.01

0.01

0.04

0.01

0.026

0.034

0.15

0.045

0.05

Pb

8.65

4.27

3.78

5.18

4.52

3.52

2.25

4.21

2.97

2.57

Mo

0.55

0.29

0.19

0.45

0.42

–

–

0.48

0.34

0.47

Pr

2.75

2.20

3.06

4.40

4.36

4.60

2.23

3.5

2.5

2.38

Nd

11.27

9.01

13.35

17.18

17.21

18

8.9

14.4

10.2

9.86

Sr

158

87

174

164

165

185

160

156

163

106

Sm

2.0

1.7

2.7

2.8

2.9

3.9

1.9

2.7

1.9

2.0

Sn

0.63

0.39

0.40

0.40

0.29

0.71

0.46

0.55

0.46

0.37

Hf

1.04

0.90

1.35

1.39

1.23

2.74

1.31

1.99

1.28

1.33

Zr

37

33

45

53

46

101

49

74

47

45

Yb

0.83

0.71

1.1

0.91

0.91

2.09

0.99

1.5

1.0

1.0

Ti

1350

1065

1861

1316

1349

3682

2034

3692

2369

2306

Eu

0.62

0.46

0.74

0.78

0.80

0.80

0.46

0.92

0.65

0.62

Eu

0.52

0.41

0.70

0.66

0.68

–

–

0.82

0.60

0.55

Gd

1.6

1.3

2.3

1.9

2.0

2.9

1.48

2.7

1.9

1.6

Gd

1.8

1.4

2.2

2.0

2.0

3.1

1.60

2.6

1.8

1.9

Tb

0.26

0.20

0.35

0.31

0.32

0.48

0.25

0.46

0.29

0.31

Tb

0.21

0.17

0.31

0.23

0.23

0.29

0.16

0.36

0.24

0.25

Dy

1.3

1.1

2.0

1.5

1.5

3.6

1.7

2.5

1.7

1.8

Ho

0.27

0.22

0.40

0.30

0.32

0.75

0.35

0.55

0.37

0.37

Y

6.47

5.25

9.32

7.16

7.42

20

9.5

13.1

8.8

8.61

Er

0.79

0.66

1.14

0.84

0.93

2.20

1.03

1.6

1.1

1.06

Tm

0.12

0.10

0.17

0.14

0.14

0.31

0.15

0.23

0.16

0.16

Lu

0.14

0.11

0.17

0.15

0.15

0.35

0.17

0.25

0.16

0.17

Sc

25

24

30

19

20

29

20

27

19

17

Co

100

114

78

117

97

–

–

66

98

103

Ni

2161

1406

708

1609

1022

882

1914

636

1281

1431

Cu

220

135

44

85

40

48

0.23

40

39

36

Note. Analyzed by E.V. Smirnova and N.N. Pakhomova (IGC SB RAS, Irkutsk).

U–Pb method after baddeleyite as 724.7 ± 2.5 Ma (Ernst and Hamilton, 2009; Ernst et al., 2012). Kingash instrusive complex is located in the NW part of East Sayan in the Idar block of Kansk super terrane. The most studied and productive massifs for PGE–Ni–Cu mineralization are Kingash and Verkhny Kingash dunite-peridotite-pyro-

xenite-gabbro massifs (Glazunov, 2003). Massifs share similar structure. They represent drop-shaped lopolite-like intrusions with outcrops onto the surface up to 1.5 km2. Both massifs show similar properties of rock chemistry. The ultrabasic component of massifs (dunites and vehrlites) are distinguished by high Mg, intermediate Ti and low Na, K

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A.S. Mekhonoshin et al. / Russian Geology and Geophysics 57 (2016) 822–833

Fig. 2. Plot of MgO–Al2O3, Ni, TiO2, Cr correlation in rocks of Malozadoisky (1) and Srednecheremshansky massifs (2).

and REE contents. Gabbroids are featured by higher abundances of Ti and rare earths. In these massifs the disseminated sulfide ores are primarily located in dunites, vehrlites, their maximum accumulations occurring in the near-bottom part of massif (Platinum-bearing…, 1995). Chalcopyrite-pentlandite-pyrrhotite sulfide is impregnated irregularly. Platinoids are primarily associated with sulfide ores (Glazunov et al., 2003; Shvedov et al., 1997). Platinum group minerals basically occur as sperrilite, tellurides, bismuth tellurides Pd, and irarsite.

Fig. 3. Plot to show distribution of PGE normalized after C1 chondrite in the rocks of Malozadoisky and Srednecheremshansky massifs. Designations see in Fig. 2.

The rocks of Kingash massif were dated by Sm–Nd and Rb–Sr methods; the acquired dates cover the range from 1410 ± 50 to 495 ± 45 Ma (Gertner et al., 2009). The U–Pb dates obtained from baddeleyite for gabbroids of the Verkhny Kingash massif (Ernst et al., 2012) amount to 726 ± 18 Ma. In the central part of East Sayan the occurrences of PGE–Cu–Ni sulfide ores are discovered within spread of large areas of ultramafic to mafic massifs (Mekhonoshin and Kolotilina, 1997; Mekhonoshin et al., 2013). This territory encompasses three ore zones, e.g., Barbitai, Uda–Biryusa and Biryusa–Tagul. The ore-bearing massifs belong to dunite-peridotite-pyroxene-gabbro formation. In the Barbitai ore zone (massifs Zhelos and Tokty-Oy) they produce lenticular bodies essentially deformed and metamorphosed, in places experiencing plicative dislocations and boudinage. The massifs are composed of ultrabasic rocks varying in composition from lherzolites to olivine pyroxenites; they are differently amphibolized and serpentinized. Besides, the massifs contain some ore zones with Cu–Ni–PGE mineralization with dimensions from 100 to 200 m long and up to 50 m wide. The massifs of the Uda–Biryusa ore zone (Ognitsky and Tartaysky) are large-sized against the bodies of the Barbitai region. They are composed of differentiated rock series varying in composition from dunites and peridotites to melanocratic olivine gabbro.

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A.S. Mekhonoshin et al. / Russian Geology and Geophysics 57 (2016) 822–833 Table 2. Abundances of noble metals and Re (ppb) in rocks of Malozadoisky (1–8) and Srednecheremshansky (9–10) massifs Component

89-44

89-35à

89-93

89-380-3

89-389

89-380-7

89-423

89-424

92-1p

93-125

1

2

3

4

5

6

7

8

9

10

Au

163

67

43

255

132

19.7

7.0

3.9

347

290

Pt

34

18

173

76

67

46.4

14.7

8.7

104

70

Pd

17

15

252

28

14

296

9

16

253

197

Ir

1.1

0.8

8.1

2.5

2.2

1.0

0.7

0.2

3.5

2.5

Ru

4.8

0.9

6.3

7.7

8.1

4.1

0.9

0.4

5.8

4.9

Rh

1.5

1.2

1.8

2.9

2.9

3.0

1.1

1.2

5.1

4.1

Os

0.1

0.1

9.5

1.9

0.1

3.7

0.4

0.1

0.7

1.0

Re

2.9

4.5

98

6.2

5.3

3.7

6.2

3.5

5.1

3.2

ΣPGE

61.4

40.1

548.7

125.3

99.3

357.5

32.5

30.2

377.2

282.7

Note. Analyzed by V.I. Menshikov and V.N. Vlasova (IGC SB RAS).

The PGE mineralization in all massifs is closely associated with chalcopyrite-pentlandite-pyrrhotite associations of sulfides. The main Pt mineral, the same as in Yoko-Dovyren and Kingash massifs, is sperrilite; in addition, fairly widespread are tellurides, bismuth tellurides of palladium, and the massifs of the Barbitai ore zone contain arsenides and sulfur arsenides of Os, Ir, and Ru. Geochronological studies of ore productive ultrabasic–basic intrusions of this region identified their formation in Late Proterozoic (712 ± 6 Ma, U–Pb method after zircons) (Podlipsky et al., 2015; Polyakov et al., 2013). Thus, in the south of Siberia the platiniferous massifs might be united into three age groups: Late Paleoproterozoic (Chiney complex, Malozadoisky massif), Late Mesoproterozoic (Srednecheremshansky massif) and Neoproterozoic (Kingash complex, Yoko-Dovyren massif and those in the center of East Sayan). For majority of massifs, but Chiney massif, the parent magmas had high-Mg composition (picritic or picrobasaltic) (Ariskin et al., 2015; Gongalsky et al., 2008; Polyakov et al., 2013). In Neoproterozoic massifs the economically significant Pt mineralization is basically associated with Ni–Cu-sulfide ores.

Discussion The date yielded for Malozadoisky massif is not rare for the south of Siberia. Close dates exist for some gabbro-dolerite dikes in the Sharyzhalgay uplift of the Siberian Craton basement (Gladkochub et al., 2012). The same age range 1860–1880 Ma comprises the dates for the Chiney and Luktur massifs. This data agrees with time of formation of KalarNimnyr dike swarm (1865 Ma), which by some authors (Ernst, 2014; Ernst et al., 2008, 2014; Gladkochub et al., 2010) is believed to be a large igneous province. Moreover, the dike swarms (Ghost swarm) and sills (Morel sills, Mara River) close in age (1880–1870 Ma) are found in North Canada in Slave Craton (Buchan et al., 2010; Ernst et al., 2014). Considering that paleomagnetic reconstructions imply close spatial position of the Siberia south and Laurentia north in the Late Paleoproterozoic (Didenko el al., 2009), one might suspect that Malozadoisky intrusion, Kalar-Nimnyr dike swarm, Chiney massifs, and dike swarms in the Slave Craton represent fragments of a single LIP (Fig. 6). The age obtained for olivine gabbronorite of Srednecheremshansky massif (1258 ± 5 Ma) is the unique magmatic age,

Table 3. Results of U–Pb-isotope studies of baddeleyite from gabbro of Malozadoisky (sample 93-85) and olivine gabbronorite of Srednecheremshansky (sample 90-22) massifs Sample

Fraction

U/Th

Pbc/Pbtot

206

Pb/204Pb

Isotope ratios (1) 207

Pb/

93-85

90-22

235

U

Age, Ma

±2s % err

206

Pb/

238

U

±2s % err

207

Pb/

206

Concordance, %

Pb

Bd-1

5.4

0.027

2158.7

5.1677

0.40

0.32905

0.32

1862.6 ± 4.2

98.5

Bd-2

4.6

0.011

5507.5

5.2008

0.21

0.33122

0.19

1862.2 ± 1.7

99.0

Bd-3

4.4

0.007

8864.2

5.2007

0.17

0.33093

0.15

1842.9 ± 1.5

98.9

Bd-1

17.7

0.045

1480.2

2.4432

0.75

0.21483

0.68

1257.1 ± 3.8

99.8

Bd-2

14.1

0.115

532.4

2.3961

1.14

0.21169

0.86

1247.8 ± 14.2

99.2

Bd-3

16.7

0.039

1713.3

2.4433

0.69

0.21422

0.55

1262.7 ± 8.0

99.1

Note. The U/Th ratios are calculated from 208Pb/206Pb ratio and age of sample. Pbc and Pbtot are common and total (radiogenic + blank + initial) Pb, accordingly. (1) isotopic ratios corrected for fractionation, spike contribution, blank (1 pg Pb and <0.1 pg U), and initial common Pb. Initial common Pb corrected with isotopic compositions from the model of Stacey and Kramers (1975) at the age of the sample.

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Fig. 4. Diagram with Concordia for olivine gabbronorite of Malozadoisky massif.

Fig. 5. Diagram with Concordia for gabbro of Srednecheremshansky massif.

which was formerly not known in Siberia. However, this age is close to the well-dated magmatic events in North Canada: 1267 ± 2 Ma—the time for formation of radial dike swarm in the Mackenzie LIP, 3 mln km2 (Baragar et al., 1996; Buchan and Ernst, 2004). Although the age of Srednecheremshansky massif is about 10 Ma less, it is quite close to this mark and above that, all three fractions of baddeleyite lie along the Concordia, and the ellipses of uncertainty reach 1265 Ma. On the reconstruction of dike swarm distribution within Mackenzie LIP (Ernst et al., 2015) depicted on the united map of

Nuna supercontinent (Evans and Mitchell, 2011) the Srednecheremshansky intrusion is oriented approximately towards the center of plume (Fig. 7). That is why, one might assume with due caution that the Srednecheremshansky intrusion could be the fragment of this LIP. The range 728–716 Ma is marked by formation of the largest number of Pt-bearing complexes in the south of Siberia. Besides, available data of Ar–Ar dating indicate that in the Sharyzhalgay uplift of the Siberian Craton basement part of N–E-striking dolerite dikes in the Prebaikal block have close

Fig. 6. Reconstructed distribution of mafic dikes dated as 1865–1880 Ma in Siberia and Laurentia (Ernst et al., 2015, 2016). 1, Malozadoisky intrusion; 2, Chiney massif; 3, Kalar-Nimnyr dike swarm; 4, Ghost dike swarm. Position of continents taken from (Evans and Mitchell, 2011) with minor modification (Siberia turned by 10° counter clockwise relative to Laurentia).

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829

Fig. 7. Position of Srednecheremshansky intrusion (1) on the reconstructed plot of distribution of mafic dikes dated as 1270 Ma in Laurentia (Ernst et al., 2015, 2016). Position of continents with minor modification (Siberia turned by 10° counter clockwise relative to Laurentia), center of plume (star) and dike swarm of Mackenzie (2), borrowed from (Evans and Mitchell, 2011).

Fig. 8. Reconstructed distribution of mafic dikes of Franklin LIP in Canada and position of massifs dated as 728–726 Ma in Siberia, borrowed from (Ernst et al., 2015, 2016). 1, Yoko-Dovyren massif; 2, Verkhny Kingash massif; 3, massifs in the center of East Sayan. Position of continents, as suggested by (Evans and Mitchell, 2011), plume center (star), borrowed from (Ernst et al., 2015, 2016). Dashed line displays position of the dikes dated by Ar–Ar method as 700–800 Ma (Gladkochub et al., 2007).

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Fig. 9. Variations of (Th/Yb)PM and (Nb/Yb)PM ratios in Malozadoisky massif (1), Major dike of Udokan (2) (data from (Gongalsky et al., 2008)), in Srednecheremshansky massif (3) and Mackenzie dike swarm, taken from (Jowitt and Ernst, 2013) (4). Element contents in the primitive mantle, borrowed from (McDonough and Sun, 1995), composition of upper continental crust (UCC), taken from (Taylor and McLennan, 1985), data on compositions of enriched (EMI, EMII) and high-U (HIMU) mantle, taken from (Condie, 2001).

ages (Gladkochub et al., 2007; Sklyarov et al., 2003). This fact and position of massifs on the reconstructed pattern of the Franklin dike swarm distribution (Ernst et al., 2015) let us extend the Franklin LIP from North Laurentia to South Siberia (Fig. 8). The parameters like composition and volume of the derived magma (Naldrett, 2003; Zhang et al., 2008), composition of mantle source and mode of interaction with ancient lithosphere mantle (Zhang et al., 2008) are required to assess the LIP potential, so that PGE–Ni–Cu mineralized intrusions could be identified. This is common for the distribution pattern and the ratio of rare earth and rare elements. In Canada, authors Jowitt and Ernst (Jowitt and Ernst, 2013) found out, that in Proterozoic mostly productive are Chukotat, Mackenzie, and Matachevan LIPs. In the Franklin

Fig. 10. Comparison of REE distribution pattern of Malozadoisky massif (circle) and Major dike of Udokan (filled field), taken from (Gongalsky et al., 2008).

LIPs no significant PGE–Ni–Cu mineralization was discovered over the Canadian territory. We tried to compare some geochemical parameters of platinum-bearing massifs of the southern Siberia massifs. Though, as was marked above, the rocks of Malozadoisky and Srednecheremshansky massifs show a typical distribution of REE with essential enrichment of light elements; on diagram (Th/Yb)PM and (Nb/Yb)PM (Fig. 9) they demonstrate quite opposite tendencies. Malozadoisky massif shows characteristically gradual increase of values (Th/Yb)PM at insignificant variations of (Nb/Yb)PM within 1.2–1.9. While in rocks of Srednecheremshansky the value of (Nb/Yb)PM varies from 2 to 9 at relatively constant value of (Th/Yb)PM 8 to10, that is comparable with the data on rocks of the dike swarm and gabbronorite dikes of the Muskox complex of Mackenzie LIP (Day et al., 2008). In addition, the same as for Malozadoisky massif the ratio between (Th/Yb)PM and (Nb/Yb)PM values at a close level of REE contents is common for the rocks of the Major dike of Udokan (Figs. 9 and 10) referred, as suspected, to the fragments of the same LIP.

Fig. 11. Spidergram of rare element distribution in impregnated ores from Neoproterozoic massifs: Yoko-Dovyren (1), Zhelos (2) and Tokty-Oy (3) (our data). Composition of primitive mantle, taken from (McDonough and Sun, 1995).

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Such features demonstrate control of the composition of rocks of Kalar–Nimnyr LIP primarily with crust contamination, in contrast to the Mackenzie LIP rocks, their formation affected both the crust contamination and the mantle source composition. The rocks and ores of Neoproterozoic complexes show similar contents and the pattern of REE and PGE distribution. The REE concentrations in the rocks of ore-bearing massifs in the center of East Sayan Mts. are specified by two-, ten-fold enrichment with light elements. The curves of their distribution have weak negative sloping. Dunites and peridotites of Yoko-Dovyren and Verkhny Kingash massifs are least enriched in rare earths. On the multielement diagram (Fig. 11) disseminated ores of Yoko-Dovyren, Zhelos, and Tokty-Oy massifs (Barbitai ore zone) are featured by close profiles of element distribution. The most significant variations are observed for Rb, Ba, and Sr, all samples having Th and Pb positive anomalies. It was found out, that in Phanerozoic mineralization was spread over the area due to plume magmatism (Dobretsov et al., 2010). Zonation is noted in spreading of diverse mineralization relative to LIP centers, and large and unique PGE–Ni– Cu deposits sit in their central zones (Borisenko et al., 2006; Dobretsov et al., 2010). However, in contrast to the Phanerozoic LIPs, in the Proterozoic ones such relationship can be identified only due to paleogeodynamic reconstructions. Analyzing position of platinum-bearing intrusions in the Siberia south on appropriate time reconstructions (Figs. 6–8) it was marked that Neoproterozoic massifs are specified by essential Pd mineralization; whereas close to the plume center the role of refractory PGE increases. At first, it occurs as Ir admixture in sperrilite (Tartai massif), and then as formation of own mineral phases, e.g., Os, Ir, and Ru. This can be associated with involvement of deep mantle horizons into the process of magma formation next to plume origination.

Conclusions Thus, the analysis of published data and acquired geochronological results suggest, that three precisely located events taking place in North Canada continued into South Siberia: 1880–1865 Ma—Ghost–Mara River–Morel LIP, 1270– 1260 Ma—Mackenzie LIP, and 725–720 Ma—Franklin LIP. Mostly productive relative to PGE–Cu–Ni mineralization in Siberia are the massifs related to the Franklin LIP, which could be determined by their spatial affinity to the plume center. Though the Srednecheremshansky intrusion contains only disseminated sulfide ores with increased Pt and Pd contents, the relationship to a huge magmatic system referred to as the Mackenzie LIP being economically potential, increases its perspectives for discovering commercial ores. The work has been performed under RFBR support (grants 13-05-12026-ofi-m, 14-05-00747 and 15-05-08843).

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