The Shakhtama porphyry Mo ore-magmatic system (eastern Transbaikalia): age, sources, and genetic features

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Russian Geology and Geophysics 54 (2013) 587–605 www.elsevier.com/locate/rgg

The Shakhtama porphyry Mo ore-magmatic system (eastern Transbaikalia): age, sources, and genetic features A.P. Berzina a,*, A.N. Berzina a, V.O. Gimon a, R.Sh. Krymskii b, A.N. Larionov b, I.V. Nikolaeva a, P.A. Serov 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 Karpinsky All-Russian Research Geological Institute, Srednii pr. 74, St. Petersburg, 199106, Russia c Geological Institute, Kola Research Center of the Russian Academy of Sciences, ul. Fersmana 14, Apatity, Murmansk Region, 184209, Russia Received 9 July 2012; accepted 18 October 2012

Abstract Two intrusive complexes are recognized at the Shakhtama deposit: Shakhtama and ore-bearing porphyry. The U–Pb zircon dates (SHRIMP II) are 161.7 ± 1.4 and 161.0 ± 1.7 Ma for the monzonites and granites of the Shakhtama complex and 159.3 ± 0.9 and 155.0 ± 1.7 Ma for the monzonite- and granite-porphyry of the ore-bearing complex. The igneous complexes formed in a complex geodynamic setting in the late Middle Jurassic and early Late Jurassic, respectively. The setting combined the collision of continents during the closure of the Mongol-Okhotsk ocean and the influence of mantle plume on the lithosphere of the Central Asian orogenic belt. The intrusion of the Shakhtama granitoids took place at the end of the collision, and the intrusion of porphyry of the ore-bearing complex, during the change of the geodynamic setting by a postcollisional (rifting) one. The complexes are composed of monzonite–granite series with similar geochemical characteristics of rocks. The performed geological, geochemical, and isotope-geochemical studies suggest that the sources of magmas were juvenile crust and Precambrian metaintrusive bodies. The juvenile mafic crust is considered to be the predominant source of fluid components and metals of the Shakhtama ore-magmatic system. The granitoids of both complexes include calc-alkalic high-K rocks with typical geochemical characteristics and with characteristics of K-adakites. These geochemical features indicate that the parental melts of the former rocks were generated at depths shallower than 55 km, and the melts of the latter, at depths of 55–66 km. K-adakite melts resulted from the melting of crust submerged into the mantle during the lithosphere delamination, which was caused by the crust thickening as a result of the repeated inflow of basic magma into the basement of the crust and tectonic deformations in its upper horizons. The high-Mg monzonitic magma produced under these conditions ascended and was mixed with melts generated in the upper horizons, which accounts for the high Mg contents of the Shakhtama granitoids. The similar compositions and petrogeochemical characteristics of the granitoids of the Shakhtama and porphyry complexes point to the same sources, transport paths, and evolution trend of their parental melts. This indicates that the igneous rocks of both complexes are products of the same long-living magmatic system, which produced Mo mineralization at the final stage. The favorable conditions for the ore production in the magmatic system during the formation of the porphyry complex appeared as early as the preceding stage—during the formation of the Shakhtama complex, which we regard as a preparatory stage in the evolution of the ore-magmatic system. © 2013, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: porphyry Mo deposits; ore-bearing magmatism; granitoid magmatism; K-adakites; sources of magmatism; Pb and Nd isotopes; geochemistry of granitoid magmatism; mantle plume; Shakhtama deposit; eastern Transbaikalia

Introduction The Shakhtama deposit belongs to essentially molybdenum deposits of a porphyry Cu–Mo association. Such deposits are spatially and temporally associated with small porphyry bodies (stocks, dikes) formed at the final stage of magmatism in areas

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

of repeated intrusive and/or effusive activity. Such areas of ore mineralization are recognized as magmatic centers of long-term activity (Berzina and Sotnikov, 1999). Investigation of magmatism and the interrelation of its stages significantly helps to elucidate the conditions of formation and evolution of porphyry Cu–Mo ore-magmatic systems. But this relation is still unclear in many respects. The ore-generating role of particular magmatic objects is treated in different ways. Some researchers (Khomichev, 2010; Pokalov, 1992) think that ores

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.2013.04.009 +

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Fig. 1. Geologic localization of the Shakhtama deposit, simplified after Rutshtein (1989) (a), occurrence of dikes of the ore-bearing porphyry complex in the deposit region (b), and geographic location of the deposit (c). a: 1, volcanosedimentary rocks, MZ; 2, 3, intrusive rocks: 2, J2–3; 3, P; 4, PZ1; 5, R3–PZ1. 6, Precambrian rocks; 7, faults; 8, Shakhtama deposit. b: 1, 2, ore-bearing porphyry complex: dikes of intermediate (1) and felsic (2) compositions; 3, 4, granitoids of: Shakhtama (3) and Unda (4) complexes; 5, Jurassic sandstones, siltstones, and conglomerates; 6, faults; 7, Shakhtama deposit.

were produced in plutons or batholiths hosting porphyry complexes and mineralization. Others (Sotnikov et al., 1988) suppose that the porphyry magma chambers formed later and at a greater depth and regard magmatic objects of different ages as products of a long-living chamber localized at the crust–mantle level. Porphyry Cu–Mo ore-magmatic systems are considered to be of mantle–crust origin. Most of researchers relate them to the impact of mantle melts on continental crust, resulting in its melting and the formation of granitoid magma, which is then mixed with basaltoid magma (Richards, 2011). The role of the mantle and crust in the formation of magmatic systems and their ore potential is treated differently. Continental crust is assumed to play a significant role in porphyry Mo systems. However, some researchers regard the metasomatized mantle as a source of metals for porphyry Mo deposits (Pettke et al., 2010). The problems of magmatism of the Shakhtama deposit were earlier considered elsewhere (Kuznetsov, 1977; Sidorenko, 1961). The development of modern analytical methods favored progress in the solution of some problems concerned with the genesis of ore-magmatic systems. As shown in recent years, porphyry Cu–Mo deposits are present not only on active continental margins of the Andean type. New data on the geology and geodynamics of eastern Transbaikalia (Yarmolyuk et al., 1995; Zorin et al., 2001) and the geochronology of igneous rocks of the Shakhtama deposit suggest the existence and activity of an ore-magmatic system at the collision and postcollisional (rift formation) stages of the regional evolution. In this paper we discuss the results of U–Pb

isotope study of zircons and the geochemical and isotopic (Nd, Pb) compositions of these igneous rocks. Our investigations were aimed at the elucidation of the sources and conditions of evolution of the long-living magmatic system that produced porphyry Mo mineralization at the final stage.

Geological essay The Shakhtama deposit is located in the southeast of eastern Transbaikalia, within the Aga–Borzya structure-formational zone of the Mongol-Okhotsk orogenic belt (Zorin et al., 1998, 2001). Paleozoic and Mesozoic magmatism was of wide occurrence in the region (Fig. 1a). Paleozoic granitoids, which formed as a result of the subduction of the lithospheric plate of the Mongol-Okhotsk ocean beneath the Mongol-China continent, are predominant (Zorin et al., 1998). The widespread ore (Mo, Cu, Au, Ag, base metals) mineralization is spatially and temporally associated with the Mesozoic magmatism. The evolution of Mesozoic magmatism in the southeast of eastern Transbaikalia proceeded in an intricate geodynamic setting combining collision and the hot-spot impact on lithosphere (Yarmolyuk et al., 2000; Zonenshain et al., 1989, 1990). The Mongol-Okhotsk ocean was closed at the Early– Middle Jurassic boundary. In the Middle Jurassic, the main deformations took place, which were caused by the collision between the Siberian and Mongol-China continents (Zorin et al., 1998). Numerous granitoid intrusions emplaced in this period. In the Late Jurassic, postcollisional troughs began to

A.P. Berzina et al. / Russian Geology and Geophysics 54 (2013) 587–605

form (Parfenov et al., 2003; Ruzhentsev et al., 2008), in which thick effusive and terrigenous strata accumulated. The lower beds (J2) of the volcanic series are composed of andesites, trachyandesites, latites, andesite-basalts, trachyandesite-basalts, shoshonites, and tuffaceous-sedimentary rocks up to 800 m thick (Rutshtein and Chaban, 1997). The middle part (J2) of the series (up to 1000 m thick) is made up of moderately felsic volcanics, predominantly trachydacites, andesite-dacites, trachyandesite-dacites, quartz latites, latites, and trachyrhyodacites. The upper part (J2–3) of the series (up to 1000 m thick) is formed mainly by basaltoids. The volcanics are of drastically discordant occurrence; they rest upon deposits of different ages. All rocks have high contents of alkalies, often with K dominating over Na. The coeval intrusive rocks are much similar in petrogeochemical characteristics to these volcanics (Koval’, 1998). The Middle–Late Jurassic magmatic area of eastern Transbaikalia is spatially confined to the northeastern margin of the East Mongolian area of Late Mesozoic intraplate magmatism (Yarmolyuk et al., 1995). The Shakhtama deposit (Fig. 1b) occurs in the Shakhtama massif located among the Permian granitoids of the Unda complex (Kozlov et al., 2003) and Lower Jurassic terrigenous deposits. The massif belongs to the Shakhtama granitoid complex (Kozlov, 2011) and is composed predominantly of rocks varying in composition from monzonites to granites. The massif and the host rocks abound in small bodies and dikes of porphyry complex. Some researchers assign the dikes to the final phase of the Shakhtama complex (Sidorenko, 1961). V.I. Sotnikov and his colleagues recognized them as an individual porphyry complex (Kuznetsov, 1977). Jurassic granitoid massifs in the south of eastern Transbaikalia are of collision origin (Zorin et al., 2001). They mark tectonic structures of NE strike. The ore-bearing porphyry complex formed at the postcollisional (rifting) stage of the regional evolution (Berzina et al., 1996). Porphyry dikes traceable for tens of kilometers (Sidorenko, 1961) are localized in dilatation structures of NW strike. In geodynamic position the ore-bearing complex of the Shakhtama deposit is similar to the Miocene porphyry of the Gangdese Cu–Mo ore-bearing belt, Tibet (Hou et al., 2009), and the Middle Jurassic porphyry of the Dexing deposit, South China (Wang et al., 2006). At these deposits, in contrast to the porphyry Cu–Mo deposits of the Andean (Pacific) type characteristic for continental margins, ore-bearing porphyry intruded under dilatation (rifting) or at the collision–rifting intermediate stage. The Shakhtama granitoids are accompanied by molybdenite dissemination in pegmatoids. The major Mo mineralization, expressed as a series of quartz veins of nearly E–W strike and veinlet stockwork mineralization, resulted from the intrusion of dikes and stocks of the porphyry complex. Methods of analysis The contents of major elements were determined by the X-ray fluorescence method on a VRA-20R X-ray analyzer at the Analytical Center of the Institute of Geology and Miner-

589

alogy, Novosibirsk. The determination error was no more than 5%. The concentrations of trace and rare-earth elements were measured by ICP MS on an ELEMENT (Finnigan Mat) high-resolution mass spectrometer with a U-5000AT+ ultrasound sprayer at the same Analytical Center. The chemical preparation of samples included their fusion with lithium metaborate of special-purity grade, stabilization of the solution after leaching, and up to 60,000-fold dilution. The concentrations were calculated using the calibration plots and the internal standard in order to take into account the influence of variations in plasma parameters on the analytical signal. The detection limits of trace elements were 0.005 to 0.1 µg/g. The average standard deviation was 2–7% depending on elements and their contents. The validity of the analytical technique was confirmed by comparison of the results obtained for the most reliably attested international standard samples (BHVO-1, BCR-1, G-2) with the accepted values. For geochronological studies we used monomineral fractions of zircon from the freshest samples of representative rocks of the Shakhtama and porphyry complexes. The mineral was separated from rocks at the Analytical Center of the Institute of Geology and Mineralogy, Novosibirsk, following the standard technique with the use of electromagnetic separation and separation in bromoform. U–Pb analyses were carried out on individual grains on a SHRIMP-II high-resolution secondary-ion mass spectrometer at the Center of Isotope Studies of the All-Russian Research Geological Institute, St. Petersburg, following the standard technique (Larionov et al., 2004; Williams, 1998). Zircon grains were implanted into epoxy resin together with TEMORA (Black et al., 2003) and 91500 (Wiedenbeck et al., 1995) standard grains. The current of a primary beam of molecular negative oxygen ions was 4 nA. The obtained data were processed using the SQUID and ISOPLOT/EX programs (Ludwig, 1999, 2000). The errors of singular analyses were at the 1σ level, and the errors of calculated concordant and weighted average 206Pb/238U ages were at the 2σ level. Cathodoluminescent images were obtained on a CamScan MX2500S scanning electron microscope. The concentrations and isotopic compositions of Sm and Nd were determined on a Finnigan-MAT-262 (RPQ) sevenchannel mass spectrometer in the static regime in the Laboratory of Geochronology and Geochemistry of the Geological Institute of the Kola Scientific Center, Apatity, following the technique described by Bayanova (2004). The blank sample contained 0.06 ng Sm and 0.3 ng Nd. The accuracy of determination of Sm and Nd concentrations and 147Sm/144Nd ratio was ±0.2% (2σ) and that of 143Nd/144Nd is ±0.003% (2σ). The measured 143Nd/144Nd values were normalized to 146 Nd/144Nd = 0.7219 and then recalculated to 143Nd/144Nd = 0.511860 in the La Jolla standard. During the studies, the weighted average values of 143Nd/144Nd in the standards were as follows: 0.511833 ± 6 (2σ) for La Jolla (N = 11) and 0.512968 ± 15 (2σ) for JiNdil (N = 100). On calculation of εNd and T (DM-2st), we used the following isotope ratios: 143 Nd/144Nd = 0.512638 and 147Sm/144Nd = 0.1967 (Jacobsen and Wasserburg, 1984) for a chondrite uniform reservoir

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(CHUR) and 143Nd/144Nd = 0.513151 and 147Sm/144Nd = 0.21365 (Goldstein and Jacobsen, 1988) for depleted mantle (DM). The isotopic composition of Pb was determined at the Center of Isotope Studies of the All-Russian Geological Institute, St. Petersburg, following the technique proposed by Krymsky et al. (2007). The Pb content and 206Pb/204Pb, 207 Pb/204Pb, and 208Pb/204Pb ratios were measured on a Triton mass spectrometer in the static multichannel one-tape regime. Each measurement included 50 blocks of 10 scanned images and was performed at an evaporator current of 2.2–2.3 A and a temperature of 1300 ºC. Before analyses of sample batches, measurements for the NIST 981 standard sample (50 ng) were made. The average accuracy of analyses was 0.05% (2σ) for 206 Pb/204Pb. Correction for the device mass fractionation was made after the average determined values for NIST 981 (206Pb/204Pb = 16.9374, 207Pb/204Pb = 15.4916, 208Pb/204Pb = 36.7219) at the same temperature. The measured Pb isotope ratios were corrected for mass fractionation of 0.120% a.m.u. for 206Pb/204Pb and 207Pb/204Pb and 0.135% a.m.u. for 208 Pb/204Pb. The blank sample contained no more than 0.2 ng Pb, with 206Pb/204Pb = 18.120, 207Pb/204Pb = 15.542, and 208 Pb/204Pb = 37.354. The ratio of the Pb content of the blank sample to the Pb content of the sample did not exceed 1/200,000; therefore, no correction for Pb of the blank sample was made for the measured ratios.

Petrogeochemical features of rocks Petrography. The Shakhtama and porphyry complexes are formed by series of compositionally similar rocks. Below, we call the rocks according to the classification diagram (Fig. 2). The Shakhtama massif is composed of diorites, monzonites, and quartz monzonites of phase I and granodiorites, quartz

monzonites, and granites of phase II. These phases are in intrusive relationship, with gradual mutual transitions of rocks within each of them. The rocks are composed of plagioclase, K-feldspar, amphibole, and biotite. The accessory minerals are apatite, sphene, zircon, and magnetite. Amphibole is a predominant mafic mineral in the diorites and monzonites and often replaces pyroxene. In the quartz monzonites and granites, biotite dominates over hornblende. The porphyry complex is a series of rocks varying in composition from monzonite-porphyry to granite-porphyry. They are dominated by quartz monzonite- and granite-porphyry, which often have mutual transitions. Most of dikes are of homogeneous structure. There are also dikes of complex composition, from monzonite-porphyry in the selvages via intermediate varieties to granite-porphyry in the core. The dikes of complex composition show both gradual and drastic transitions between rocks of different compositions. In mineral composition the porphyry are close to the similar rocks of the Shakhtama massif. Phenocrysts amount to 10–60 vol.%. They are composed of plagioclase, hornblende, biotite, K-feldspar, and quartz in different proportions, with hornblende and biotite being predominant in monzonite-porphyry and plagioclase and K-feldspar prevailing in quartz monzonite- and granite-porphyry. Rock-forming elements. The contents of major oxides in the most representative igneous rocks of the Shakhtama deposit are listed in Table 1. Hereafter, these contents are converted to 100% dry substance. The SiO2 contents of rocks of the Shakhtama and porphyry complexes are 57.90–70.22 and 57.38–71.34 wt.%, respectively (Fig. 2). In the Shakhtama complex, the SiO2 contents show a gap in the range 64– 67 wt.%. The igneous rocks of the deposit are generally rich in alkalies (K2O + Na2O), whose content increases in passing from the Shakhtama (5.44–8.69%) to the porphyry (6.11–

Fig. 2. (Na2O + K2O)–SiO2 classification diagram for rocks from the Shakhtama deposit, after Middlemost (1994). Contents of oxides are converted to 100% dry substance. 1, 2, igneous complexes: 1, Shakhtama; 2, porphyry (ore-bearing).

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A.P. Berzina et al. / Russian Geology and Geophysics 54 (2013) 587–605 Table 1. Contents of oxides (wt.%) and trace elements (ppm) in rocks of the Shakhtama and porphyry complexes Component

Shakhtama complex

Porphyry complex

1

2

3

4

5

6

7

8

9

10

11

12

SiO2

56.40

58.27

61.92

63.40

67.50

69.47

55.50

59.80

62.57

65.20

67.70

69.20

TiO2

0.74

0.84

0.56

0.62

0.40

0.43

1.24

0.77

0.73

0.67

0.44

0.41

Al2O3

13.50

15.70

17.30

14.00

14.00

15.33

13.7

13.25

13.85

14.87

15.4

15.33

FeOtot

6.85

6.07

3.69

4.95

3.42

2.79

6.70

5.09

4.49

3.60

2.80

2.00

MnO

0.09

0.12

0.08

0.09

9.06

0.06

0.09

0.06

0.03

0.02

0.02

0.04

MgO

8.40

5.30

2.66

4.50

2.80

1.21

7.77

7.12

5.83

2.23

1.56

1.81

CaO

6.06

5.00

4.75

4.30

2.85

2.67

6.37

4.42

2.85

2.63

2.38

2.2

Na2O

2.90

4.00

4.27

3.90

3.60

3.77

3.58

3.50

3.50

4.00

4.12

4.5

K2O

2.40

3.15

3.70

3.40

4.80

3.59

3.23

4.18

3.69

4.18

3.81

4.00

P2O5

0.07

0.07

0.17

0.01

0.01

0.08

0.22

0.09

0.08

0.13

0.07

0.06

LOI

2.40

1.42

0.46

0.90

0.50

0.95

1.68

1.57

1.79

1.65

0.96

0.71

Total

99.81

99.94

99.56

100.07

99.94

100.34

100.08

99.85

99.41

99.18

99.26

100.26

Sc

22

21

5.8

11.9

4.2

5.2

14.1

11.3

10.0

4.9

4.2

4.0

V

147

139

40

82

36

44

125

91

59

45

34

33

Cr

768

308

108

228

88

15

427

429

312

107

74

83

Co

30

24

5.3

13.7

6.1

5.0

24

16.5

10.8

8.7

4.6

5.5

Ni

197

93

39

60

11.6

5

143

260

149

43

<2

24

Rb

117

144

237

192

144

139

117

214

256

138

118

128

Sr

481

514

302

425

763

485

962

562

492

959

852

927

Y

14.3

17.3

13.6

20

10.1

15

19.7

14.5

12.3

12.4

11.2

8.7

Zr

152

233

73

231

141

110

153

121

142

140

137

108

Nb

6.9

10.0

14.4

14.0

11.1

10

12.0

9.1

8.3

9.8

11.6

8.8

Cs

17.6

10.6

9.9

10.7

5.1

4.6

3.9

11.0

10.4

7.1

2.4

2.2

Ba

534

910

609

599

829

678

1 082

844

748

1 186

1 030

1 178

La

26

31

21

33

34

23

67

34

29

42

39

29

Ce

51

59

48

64

63

49

128

66

55

85

78

60

Pr

6.2

7.4

6.4

7.8

7.7

6.0

16.6

8.4

7.1

11.0

9.6

7.3

Nd

23

26

23

28

27

22

62

31

26

39

33

26

Sm

4.2

4.5

3.8

4.8

4.2

3.5

9.9

5.4

4.6

6.0

5.1

4.0

Eu

0.99

0.94

0.61

0.85

0.67

0.87

2.2

0.94

1.06

1.09

0.98

0.74

Gd

3.4

3.5

3.2

3.6

3.1

3.3

7.6

4.2

3.6

4.3

3.7

2.8

Tb

0.44

0.44

0.44

0.51

0.38

0.41

0.81

0.50

0.44

0.44

0.44

0.32

Dy

2.5

2.6

2.3

2.8

1.71

2.1

3.8

2.6

2.3

2.1

1.90

1.60

Ho

0.45

0.51

0.44

0.58

0.32

0.44

0.57

0.44

0.38

0.38

0.38

0.26

Er

1.22

1.47

1.33

1.73

0.89

1.2

1.51

1.27

1.08

1.02

0.89

0.70

Tm

0.19

0.21

0.22

0.26

0.13

0.15

0.19

0.19

0.15

0.15

0.13

0.10

Yb

1.15

1.41

1.27

1.75

0.76

1.10

1.13

1.08

0.95

0.96

0.83

0.64

Lu

0.19

0.20

0.18

0.25

0.11

0.15

0.15

0.16

0.14

0.13

0.11

0.097

Hf

4.1

5.7

2.5

5.7

4.4

2.9

4.2

3.9

4.4

4.3

4.3

3.6

Ta

0.63

0.91

1.66

0.98

1.18

0.79

0.90

0.69

0.76

0.77

1.46

0.84

Pb

36

15.3

23

12.8

22

19

16.9

14.9

65

24

19.5

21

Th

10.0

11.3

19.4

12.5

13.5

2.9

12.6

10.2

10.5

10.1

12.6

10.3

U

3.0

3.2

6.7

4.3

3.1

1.4

4.2

3.4

4.2

5.6

3.5

2.7

Note. FeOtot, total iron as Fe2+. Shakhtama complex, 1–6: 1, diorite; 2, 3, monzonite; 4, 5, quartz monzonite; 6, granite. Porphyry (ore-bearing) complex, 7–12: 7–9, monzonite-porphyry; 10, 11, quartz monzonite-porphyry; 12, granite-porphyry.

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Fig. 3. SiO2–major oxide diagrams (wt.%) for igneous rocks from the Shakhtama deposit. Fields of igneous series on the SiO2–K2O diagram, after Rickwood (1989): I, tholeiitic, II, calc-alkalic, III, high-K calc-alkalic, IV, shoshonitic. Designations follow Fig. 2.

9.47%) complex. The K2O content is 1.07–5.77 wt.% in the Shakhtama rocks and 2.65–5.99 wt.% in the porphyry. On the K2O–SiO2 diagram (Fig. 3), the points of the Shakhtama rocks are localized mainly in the field of high-K calc-alkalic series, and those of the porphyry, in the field of high-K calc-alkalic and shohosnite series. The K2O/Na2O values are commensurate in the rocks of these complexes and increase from 0.7 to 1.9 as the SiO2 content becomes higher. The rocks are rich in MgO (8.62–1.21 and 8.53–1.32% in the Shakhtama and porphyry complexes, respectively). As the SiO2 content in the rocks increases, the contents of TiO2, FeOtot, MgO (Fig. 3), and CaO decrease. The porphyry with SiO2 < 65% are richer in TiO2 and MgO than the Shakhtama granitoids, and those with SiO2 > 65% are poorer. The contents of FeOtot (Fig. 3) and CaO in all porphyry are lower than those in the Shakhtama rocks. In general, the igneous rocks are metaluminous: The A/NK [Al2O3/(Na2O + K2O), molar quantity] and A/CNK [Al2O3/(CaO + Na2O + K2O), molar quantity] ratios are 1.2–

1.9 and 0.7–1.0, respectively, in the Shakhtama complex and 1.2–1.8 and 0.6–1.1 in the porphyry complex. The Mg# = 100Mg/(Mg + Fe2+) value varies within 44–69 in the Shakhtama complex and within 50–71 in the porphyry complex. Trace elements. Results of analyses of trace elements are presented in Table 1 and Fig. 4. The contents of trace elements in the rocks of the Shakhtama and porphyry complexes are commensurate though show some difference. The rocks are relatively enriched in compatible elements: The diorites of the Shakhtama complex and monzonites of the porphyry complex contain up to 226 and 265 ppm Ni, 768 and 498 ppm Cr, and 147 and 145 ppm V, respectively. The contents of LILE are high; in the porphyry they are higher than those in the Shakhtama rocks (ppm): Rb = 115–407 and 107–237, Ba = 687–1186 and 454–910, and Sr = 225–962 and 302–763, respectively. The contents of HFSE in the Shakhtama and porphyry complexes vary (ppm): Zr = 73–233 and 108–294 and Nb = 6.6–14.4 and 8.2–14.7, respectively. The magmatites are in general rich in LREE, depleted in MREE, and poor in

A.P. Berzina et al. / Russian Geology and Geophysics 54 (2013) 587–605

593

Fig. 4. SiO2–trace-element (ppm) diagrams for igneous rocks from the Shakhtama deposit. Designations follow Fig. 2.

HREE. The REE contents in the Shakhtama and porphyry complexes vary in the following ranges (ppm): La = 21–33 and 29–61, Sm = 3.5–4.8 and 4.0–7.7, Yb = 0.76–1.79 and 0.64–1.40, Y = 10.1–20.0 and 8.7–19.7. The REE patterns for the rocks of both complexes are similar (Fig. 5). They are of negative dip, rather steep in the MREE region and gentle in the LREE and HREE regions, and show a slight negative Eu anomaly (Eu/Eu* = 0.52–0.78 and 0.58–0.81 in the Shakhtama and porphyry complexes, respectively). The chondrite-normalized REE ratios are as follows: (La/Yb)n = 11–30 and 20–47, (Dy/Yb)n = 1.0–1.5 and 1.5–2.1,

and (Sm/Dy)n = 2.5–4.1 and 3.3–4.7, respectively. That is, the porphyry show higher REE ratios and more steeply dipping REE patterns than the Shakhtama rocks. Among the high-K calc-alkalic magmatites of the Shakhtama deposit, there are adakite-like and ordinary rocks with low Sr and high Yb and Y contents and low (La/Yb)n values (Fig. 6). According to Martin et al. (2005), adakites contain (ppm): Sr > 400, Yb ≤ 1.8, and Y ≤ 18; (La/Yb)n > 10. The REE patterns usually lack a negative Eu anomaly. All this indicates an equilibrium of melts with amphibole and garnet and the absence of plagioclase from the melt source. In

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A.P. Berzina et al. / Russian Geology and Geophysics 54 (2013) 587–605

Fig. 5. Chondrite-normalized (McDonough and Sun, 1995) REE spectra of the rocks from the Shakhtama (a) and porphyry (b) complexes of the Shakhtama deposit.

Fig. 6. Sr/Y–Y (a) and (La/Yb)n–Ybn (b) diagrams for igneous rocks from the Shakhtama deposit. Designations follow Fig. 2. Adakite fields: a, after Martin (1999); b, after Drummond and Defant (1990).

contrast to typical adakites of continental margins, the adakitelike rocks of the deposit have high K2O contents and high K2O/Na2O values (~1). In these features they are similar to the K-adakites widespread, in particular, in the Tibet orogenic structures (Xiao et al., 2007). As in the Late Triassic adakites in the eastern Tibet, all REE patterns of the adakite-like rocks of the Shakhtama deposit show a negative Eu anomaly. According to the experimental data on adakites (Xiao and Clemens, 2007), plagioclase is stable at P < 17 kbar, but when the melt composition and fluid regime were changed, this mineral was also detected at higher pressures (up to 20– 25 kbar). Following Xiao et al. (2007), we admit that the parental melts of rocks with Eu/Eu* = 0.5 formed at P < 17 kbar, which corresponds to the crust thickness of <55 km, and the melts of rocks with Eu/Eu* = 0.6–0.8 formed at 17 < P < 20 kbar, i.e., at a depth of 55–66 km.

Results of study of the isotopic composition of rocks U–Pb isotope geochronology. The results of in situ SIMS (SHRIMP-II) analysis of zircons are given in Table 2 and Fig. 7. In addition, cathodoluminescent images are presented

in Fig. 8. The isotope dates for the Shakhtama monzonites and granites vary over the ranges of 163.7–157.7 and 162.8– 156.3 Ma, respectively, and those for the monzonite-porphyry and granite-porphyry are within 161.5–156.4 and 159.3– 153.4 Ma. The intervals of dates for the rocks of the two complexes partly or totally overlap. The weighted average ages obtained for the above rocks from the 206Pb/238U ratio are 160.0 ± 0.9, 159.0 ± 1.1, 159.3 ± 0.9, and 155.0 ± 1.7 Ma, respectively. The available U–Pb geochronological dates for the Shakhtama granitoids and early porphyry overlap. However, the breaking-through contacts between the granitoids of the two complexes evidence that the porphyry formed after the Shakhtama granitoids. The U–Pb dates for the porphyry agree (within the error of determination) with the Ar–Ar dates after amphibole of granodiorite-porphyry (159.5 ± 1.5 Ma) and amphibole of granite-porphyry (157.5 ± 1.5 Ma) (Sotnikov et al., 1998), and the U–Pb dates for the Shakhtama granitoids differ slightly from the Ar–Ar dates (167 ± 1.6 Ma) (Sotnikov et al., 1998). The coincidence of U–Pb dates for rocks that are of different ages according to geological data is, most likely, due to the short temporal gap (smaller than the error of the applied in situ SIMS method) between the formation of granitoids of both complexes and to the presence

595

A.P. Berzina et al. / Russian Geology and Geophysics 54 (2013) 587–605 Table 2. Results of U–Pb zircon isotope dating of igneous rocks of the Shakhtama deposit Grain, point

206

Pbc, U

%

Th

232

Th

238

U

ppm

206

Pb*, ppm

206

Pb

238

U

238

age, Ma

U

±%

206

Pb∗

207 206

Pb

±%

∗

Pb∗

207

±%

235

Pb

Pb∗

206

±%

Rho

0.299

238

U

U

Shakhtama complex Monzonite (S-881/15) 1.1

0.00

176

133

0.78

3.89

163.7

2.1

0.31

488

307

0.65

10.7

162.5

3.1

0.32

772

788

1.05

16.7

159.8

4.1

0.30

353

250

0.73

7.64

160.0

5.1

0.45

355

215

0.62

7.78

161.4

6.1

0.14

1442

638

0.46

30.9

158.7

7.1

0.00

1131

743

0.68

24.4

159.8

8.1

0.37

1048

960

0.95

22.7

159.8

9.1

0.00

529

324

0.63

11.6

161.7

9.2

1.70

4664

1900

0.42

101

157.7

10.1

0.19

598

454

0.78

13.1

161.6

±2.4 ±1.6 ±1.5 ±1.8 ±1.9 ±1.2 ±1.3 ±1.4 ±1.8 ±1.1 ±1.5

38.88

1.5

0.05

4.6

0.177

4.9

0.02572

1.5

39.17

1.0

0.049

4.3

0.173

4.4

0.02553

1

0.231

39.84

0.9

0.049

4.7

0.17

4.7

0.02510

0.92

0.195

39.79

1.1

0.05

4.4

0.173

4.5

0.02513

1.1

0.251

39.44

1.2

0.05

5.4

0.175

5.5

0.02535

1.2

0.212

40.13

0.8

0.049

2.3

0.169

2.4

0.02492

0.78

0.320

39.85

0.8

0.049

2.8

0.171

2.9

0.02509

0.82

0.285

39.84

0.9

0.049

3.8

0.168

3.8

0.02510

0.86

0.222

39.36

1.1

0.048

3.2

0.17

3.4

0.02541

1.1

0.332

40.38

0.7

0.049

3.9

0.167

4

0.02476

0.72

0.182

39.38

0.9

0.049

3.1

0.17

3.3

0.02539

0.94

0.288

Granite (S-869) 1.1

0.28

341

339

1.03

7.44

161.2

2.1

0.00

179

112

0.65

3.92

162.8

3.1

0.12

1739

2390

1.42

37.8

160.7

3.2

0.30

473

364

0.80

10.2

159.9

4.2

0.48

245

158

0.67

5.19

156.3

5.1

1.08

246

166

0.70

5.26

157.0

6.1

0.34

442

237

0.55

9.41

157.3

7.1

0.13

1113

1231

1.14

23.9

158.6

8.1

0.45

333

223

0.69

7.23

160.1

9.1

0.39

633

495

0.81

13.5

158.0

10.1

0.47

426

256

0.62

9.07

156.9

±1.8 ±2.3 ±1.3 ±1.6 ±2 ±2.2 ±1.7 ±1.5 ±1.9 ±1.6 ±1.8

39.48

1.1

0.049

5

0.171

5.1

0.02533

1.1

0.222

39.09

1.4

0.05

4.5

0.177

4.7

0.02558

1.4

0.304

39.62

0.9

0.049

1.8

0.172

2

0.02524

0.85

0.423

39.81

1.0

0.048

5

0.168

5.1

0.02512

1

0.200

40.75

1.3

0.05

7.7

0.169

7.8

0.02454

1.3

0.170

40.55

1.4

0.049

9.9

0.168

10

0.02466

1.4

0.139

40.49

1.1

0.047

4.7

0.16

4.9

0.02470

1.1

0.220

40.14

1.0

0.05

2.3

0.17

2.5

0.02491

0.96

0.391

39.77

1.2

0.047

5.9

0.164

6

0.02514

1.2

0.199

40.3

1.0

0.047

5.7

0.161

5.7

0.02482

1

0.178

40.57

1.2

0.051

6.2

0.172

6.3

0.02464

1.2

0.186

Porphyry complex Monzonite-porphyry (S-883a) 1.1

0.00

434

282

0.67

9.36

160.0

2.1

0.17

2003

1357

0.70

43.5

160.7

2.2

0.34

1157

542

0.48

24.7

157.7

3.1

0.58

184

115

0.65

3.99

159.8

3.2

0.32

851

434

0.53

18.4

159.4

4.1

0.59

492

431

0.91

10.8

161.5

4.2

0.18

2067

1035

0.52

44.7

160.1

4.3

0.32

710

346

0.50

15.4

160.1

5.1

0.00

412

246

0.62

8.94

160.7

6.1

0.13

905

482

0.55

19.3

158.1

6.2

0.06

1771

888

0.52

37.4

156.4

±1.7 ±1.2 ±1.3 ±2.3 ±1.4 ±1.7 ±1.2 ±1.5 ±1.7 ±1.4 ±1.2

39.78

1.1

0.049

3

0.171

3.2

0.02514

1.1

0.331

39.62

0.7

0.049

2.1

0.17

2.2

0.02524

0.73

0.327

40.38

0.8

0.049

3.3

0.168

3.4

0.02476

0.82

0.238

39.83

1.5

0.052

7

0.18

7.2

0.02510

1.5

0.208

39.93

0.9

0.05

3.4

0.172

3.5

0.02504

0.88

0.252

39.4

1.1

0.049

6.3

0.172

6.3

0.02538

1.1

0.167 0.340

39.78

0.7

0.049

2

0.169

2.2

0.02514

0.73

39.77

0.9

0.05

4.6

0.172

4.7

0.02514

0.94

0.201

39.61

1.1

0.05

3.1

0.173

3.3

0.02525

1.1

0.332

40.27

0.9

0.051

2.4

0.173

2.6

0.02483

0.87

0.339

40.71

0.8

0.049

1.6

0.167

1.8

0.02456

0.76

0.430

41

1.1

0.0479 7.6

0.16

7.7

0.024

1.1

0.149

41

1.9

0.0467 5.3

0.16

5.7

0.024

1.9

0.329

41

2.4

0.0522 3.2

0.18

4.0

0.024

2.4

0.599

41

4.2

0.0361 45.2

0.12

45.4

0.025

4.2

0.092

40

2.1

0.0493 3.2

0.17

3.9

0.025

2.1

0.546

40

2.6

0.0500 5.5

0.17

6.0

0.025

2.6

0.424

42

1.2

0.0498 4.0

0.17

4.2

0.024

1.2

0.285

25

2.6

0.0554 4.9

0.31

5.5

0.041

2.6

0.477

41

1.7

0.0490 3.3

0.16

3.7

0.024

1.7

0.467

41

2.2

0.0500 2.8

0.17

3.5

0.025

2.2

0.625

41

2.0

0.0428 11.9

0.14

12.1

0.024

2.0

0.166

Granite-porphyry (S-883b) 1.1

0.00

304

205

0.70

6.32

154

2.1

0.26

389

234

0.62

8.12

155

3.1

0.00

431

288

0.69

9.06

156

4.1

5.70

175

186

1.10

3.68

156

5.1

–

674

195

0.30

14.4

158

6.1

0.83

724

403

0.57

15.6

159

7.1

–

635

349

0.57

13.1

153

8.1

0.00

74

34

0.47

2.59

257

9.1

0.00

293

136

0.48

6.13

155

10.1

–

768

386

0.52

16.3

157

11.1

0.83

242

139

0.59

5.04

155

±2 ±3 ±4 ±6 ±3 ±4 ±2 ±7 ±3 ±3 ±3

Note. Pbc and Pb*, Common and radiogenic lead, respectively. Error for common lead was introduced after measured 204Pb. Calibration error for the TEMORA standard does not exceed 0.42. All errors are at the 1σ level. Rho, Coefficient of correlation between 207Pb*/235U and 206Pb*/238U.

596

A.P. Berzina et al. / Russian Geology and Geophysics 54 (2013) 587–605

Fig. 7. Diagrams with concordia for zircons from the igneous rocks of the Shakhtama deposit.

of zircons of different genesis in the rocks. The latter is observed first of all in rocks of the Shakhtama complex hosting the small stocks and dikes of the porphyry complex. Some specific features of the internal structure of the zircons and the U and Th distribution in the mineral permit us to divide them into two genetic types. Zircons of two populations were recognized in monzonite (S-881/15). Grains 1, 2, 4, 5, 9 (core), and 10 are assigned to population 1. These zircons with a contrasting thin concentric growth zoning (see cathodoluminescent images) show medium contents of U and Th (176–598 and 133–454, respectively), typical of magmatic zircons (Hoskin and Schaltegger, 2003). Their dating yields an age of 163.7–160.0 Ma. The weighted average age determined over seven points (2σ) is 161.7 ± 1.4 Ma, MSWD = 0.37, p = 0.87. Grains 3, 6–8, and the outer rim of grain 9 belong to zircons of population 2. They have high U and Th contents (772–4700 and 538–1900 ppm, respectively) and are dark gray to black on cathodoluminescent images. Their age is within 159.8– 157.7 Ma. The weighted average age determined over five points (2σ) is 159.0 ± 1.1 Ma, MSWD = 0.61, p = 0.65. Note that the Shakhtama granitoids are intruded by thin porphyry dikes. All this permits us to regard zircons of population 1 as

magmatic and zircons of population 2 as superposed (metasomatic), under the action of porphyry-generated fluids on the granitoids. According to the above data, the U–Pb age of monzonites is 161.7 ± 1.4 Ma, and the probable age of the porphyry that caused their transformation is 159.0 ± 1.1 Ma. In granite (S-869), one population of zircons includes grains 1, 2, 3 (core), and 8. The oldest age (162.8 ± 2.3 Ma) was determined for the periphery of grain 2 with a sector zoning, an unclear internal structure, and low contents of U (179 ppm) and Th (112 ppm). Dates of 161.2–160.1 Ma were obtained for the cores of grains 1, 3, and 8. The core of grain 3 is dark gray (on cathodoluminescent images) prismatic homogeneous zircon rich in U (1739 ppm) and Th (2390 ppm). It yielded an age of 160.7 ± 1.3 Ma. The cores of grains 1 and 8 are isometric crystals with sector zoning, U = 341 and 333 ppm, and Th = 339 and 223 ppm, respectively, which is typical of magmatic zircons. The internal structure of zircon and the distribution of U and Th in it are obviously related to its crystallization under unstable conditions during the melt ascent and localization in the intracrustal chamber. The weighted average age (±2σ) of zircons of this type determined over four points is 161.0 ± 1.7 Ma (MSWD = 0.30, p = 0.82); it corresponds to the time of the granite crystallization.

597

A.P. Berzina et al. / Russian Geology and Geophysics 54 (2013) 587–605

Fig. 8. Cathodoluminescent images of zircons from igneous rocks from the Shakhtama deposit. Dating points and ages (Ma) are shown. The grain numbers follow Table 2. In the case of two or three analyses, the first numeral is the grain number, and the second is the point number.

Zircons of population 2 are mainly short-prismatic crystals with a well-pronounced rhythmic zoning. This population includes grains 3 (periphery), 4–7, 9, and 10 with dates from 159.9 to 156.3 Ma. Zircon with a wide-band zoning (grain 7) has high contents of U and Th (1113 and 1231 ppm, respectively). Zircon with a narrow-band zoning has low contents of U (245–633 ppm) and Th (158–495 ppm). All these zircons might have been produced as a result of the intrusion of porphyry. The weighted average age (±2σ) of the zircons determined over seven points is 157.9 ± 1.3 Ma (MSWD = 0.50, p = 0.81); it corresponds to the time of the action of porphyry on the Shakhtama granitoids. In general, the U–Pb dates for the superposed zircon in these granitoids

agree with the Ar–Ar dates for the porphyry and with the Re–Os dates for the molybdenites (158 ± 1 and 159 ± 1 Ma) (Berzina et al., 2003). The geochronological age of magmatic zircons from the Shakhtama granitoids (161.7 ± 1.4 and 161.0 ± 1.7 Ma) agrees with the U–Pb age of zircons from the Shakhtama granitoids of the ajacent areas (161.6 ± 1.3 Ma) (Shatkov, 2009). The obtained dates evidence that the granitoids formed in the late Middle Jurassic. The intrusion of porphyry took place in the period from 159.3 ± 0.9 to 155.0 ± 1.7 Ma, i.e., in the early Late Jurassic. In general, the few U–Pb zircon dates for the magmatites of the Shakhtama and porphyry complexes vary from the

Table 3. Sm and Nd isotope compositions of igneous rocks of the Shakhtama deposit Sm, ppm

Nd, ppm

147

143

T, Ma

εNd(T)

T(DM-2st), Ma

Monzonite

4.41

24.65

0.108147

0.512489 ± 16

160

–1.1

1057

Granite

5.59

38.10

0.088737

0.512386 ± 14

159

–2.7

1189

Rock

Sm/144Nd

Nd/144Nd

Shakhtama complex

Porphyry complex Monzonite-porphyry

5.97

35.49

0.101661

0.512643 ± 17

159

+2.0

795

Granite-porphyry

5.23

32.38

0.097655

0.512645 ± 16

155

+2.1

787

598

A.P. Berzina et al. / Russian Geology and Geophysics 54 (2013) 587–605

Fig. 9. εNd–87Sr/86Sr isotopic composition diagrams for igneous rocks from the Shakhtama deposit. 1, 2, Shakhtama complex: 1, monzonite, 2, granite; 3, 4, porphyry complex: 3, monzonite-porphyry; 4, granite-porphyry. Mantle: depleted (DM), Nd-enriched (EM I), 86Sr-enriched (EM II), with high µ = 238 204 U/ Pb (HIMU) (Hofmann, 2007), and metasomatized (MM) (from data on the average composition of island-arc basalts (Kelemen et al., 2007)). CC, Precambrian continental crust (Kovalenko et al., 2004), CHUR, chondrite uniform reservoir. The Sr isotopic compositions of apatites (Sotnikov et al., 2000) are also used.

maximum value (163.7 ± 2.4 Ma) in monzonites to the minimum one (153.4 ± 2.0 Ma) in granite-porphyry. These data indicate that the endogenic activity in the deposit region lasted about 10 myr. The Sm–Nd isotopic composition of rocks. The Sm and Nd isotopic compositions were determined in the monzonite and granite of the Shakhtama complex and the monzonite- and granite-porphyry of the ore-bearing complex. The results are given in Table 3 and in Fig. 9. In general, the rocks are characterized by high εNd(T) values close to those of CHUR. Among the Shakhtama rocks, the estimated εNd(T) values

increase in passing from granite (–2.7) to monzonite (–1.1) and from the Shakhtama rocks to monzonite-porphyry (+2.0) and granite-porphyry (+2.1). The Nd model ages T(DM-2st), equal to >1000 Ma for the Shakhtama complex rocks and ~800 Ma for the porphyry complex rocks, point to the participation of ancient protolith in the magma formation. In Fig. 9, the isotopic compositions of rocks are located between the compositions of metasomatized mantle, mantle EM II, and ancient continental crust. The isotopic compositions close to those of CHUR point to the significant role of mantle component in their formation. The participation of EM II component in the magmatic process is not confirmed by the pair ratios of incompatible elements. In the trace-element ratios (Fig. 10) the igneous rocks of the deposit are similar to MM and CC and differ from EM II. The Pb isotopic composition of rocks and ore minerals. The results of analyses of the Pb isotopic composition in feldspars from the igneous rocks and in sulfides from orebodies are given in Table 4 and in Fig. 11. We studied feldspars from the monzonite, quartz monzonite, and granite of the Shakhtama complex and from the monzonite-porphyry and granite-porphyry of the ore-bearing complex. The investigated sulfides were molybdenite, chalcopyrite, pyrite, and galena. The ratios of Pb isotopes in feldspars from the rocks of the Shakhtama and porphyry complexes are close to each other and vary in narrow ranges: 206Pb/204Pb = 18.508–18.738 and 18.577–18.587, 207Pb/204Pb = 15.565–15.600 and 15.613– 15.574, and 208Pb/204Pb = 38.273–38.440 and 38.347–38.497, respectively. These data testify to the homogeneous Pb isotopic composition of the rock sources or common source during the formation of igneous complexes. On the 206Pb/204Pb–207Pb/204Pb diagram (Fig. 11), the isotopic compositions of minerals lie right of the geochron, which points to their enrichment in radiogenic Pb. The isotopic

Fig. 10. Incompatible-element ratio–ratio diagrams for igneous rocks from the Shakhtama deposit. Designations follow Fig. 2. The compositions of MM, MORB (Kelemen et al., 2007), PM (McDonough and Sun, 1995), OIB (Sun and McDonough, 1989), and CC (Rudnick and Gao, 2003) are shown.

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A.P. Berzina et al. / Russian Geology and Geophysics 54 (2013) 587–605 Table 4. Pb isotope composition of igneous rocks and ore minerals of the Shakhtama deposit Mineral

206

207

208

Pl

18.5083 ± 0.0004

15.5654 ± 0.0004

38.2731 ± 0.0014

Quartz monzonite

Pl + Kfs

18.5618 ± 0.0009

15.5936 ± 0.0008

38.3980 ± 0.0028

Granite

Pl

18.7384 ± 0.0003

15.6001 ± 0.003

38.4403 ± 0.0011

Monzonite-porphyry

Pl + Kfs

18.5770 ± 0.0011

15.6134 ± 0.0009

38.4971 ± 0.0032

Granite-porphyry

Pl + Kfs

18.5875 ± 0.0003

15.5739 ± 0.0003

38.3473 ± 0.0028

Molybdenite

19.3946 ± 0.0002

15.6164 ± 0.0001

38.3274 ± 0.0004

Rock

Pb/204Pb

Pb/204Pb

Pb/204Pb

Shakhtama complex Monzonite

Porphyry (ore-bearing) complex

Sulfides Quartz vein Quartz vein

Molybdenite

19.1304 ± 0.0003

15.6008 ± 0.0003

38.3131 ± 0.0009

Carbonate nest

Chalcopyrite

18.4946 ± 0.0002

15.5808 ± 0.0003

38.2791 ± 0.0008

Dissemination in granodiorite

Pyrite

19.0936 ± 0.0003

15.6091 ± 0.0003

38.7085 ± 0.0009

Carbonate nest with polymetallic mineralization

Galena

18.4902 ± 0.0003

15.5713 ± 0.0.0003

38.2509 ± 0.0009

Note. Pl, Plagioclase; Kfs, K-feldspar.

compositions form two linear sequences. One of them, with a steep, almost vertical dip, corresponds to feldspars from the monzonite and quartz monzonite of the Shakhtama complex and from the monzonite-porphyry and granite-porphyry of the ore-bearing complex and to ore minerals — chalcopyrite and galena. The second sequence with a gentle dip includes feldspar from the Shakhtama granite, pyrite, and molybdenite. The isotopic compositions of the first group are localized between those of metasomatized mantle and continental crust. The compositions of the second group deviate from the first group toward the HIMU-type mantle. This distribution of Pb compositions suggests the participation of at least three sources in the ore-magmatic process.

Fig. 11. 207Pb/204Pb–206Pb/204Pb isotopic composition diagram for minerals from the Shakhtama deposit. Feldspars from: 1, monzonite; 2, quartz monzonite; 3, granite; 4, monzonite-porphyry; 5, granite-porphyry. Ore minerals: 6, molybdenite; 7, chalcopyrite; 8, pyrite; 9, galena. DM, EM I, EM II, and HIMU are after Hofmann (2007), MM is after Kelemen et al. (2007), and CC is after Gao et al. (1998).

Discussion Mixing and crystallization differentiation as predominant mechanisms of magmatic process. The negative correlation between the contents of TiO2, MgO, FeOtot (Fig. 3), and trace elements Ni, Cr, V, and Zr (Fig. 4) and the contents of SiO2 as well as the change in the mineral composition of rocks in the series from monzonites to granites testify to the significant role of crystallization differentiation. However, the distribution of Rb, Ba, and Sr is inconsistent with crystallization differentiation. This fact, as well as the wide scatter of composition points on all diagrams, suggests that the crystallization differentiation was complicated by the superposition of other processes, in particular, the mixing of melts in magma chambers and during their ascent to the upper horizons. Four-component binary diagrams are used to illustrate the mixing of melts (sources). On mixing, the rock compositions form a hyperbolic sequence on the diagram of pair ratios of four components and a linear sequence on the diagram with pair ratios of three components (Barnes et al., 2001; Hollanda et al., 2003). The distribution of the compositions of the Shakhtama and porphyry complex rocks on the FeOtot/SiO2– K2O/CaO and FeOtot/SiO2–CaO/SiO2 diagrams (Fig. 12) shows the important role of the melt mixing during the magma formation. The melt mixing was, most likely, a repeated process, which took place in the deep-seated and intracrustal chambers of the magmatic system and at the level of the massif and dike emplacement. However, there is a hypothesis that the melt mixing at depth (in the mantle–crust transition zone) significantly governed the composition and ore potential of the magmatic system. The sources of ore-magmatic system. The Shakhtama and porphyry complexes are formed mainly by rocks of high-K calc-alkalic and scarcer shoshonitic series (Fig. 3). Type I

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Fig. 12. FeOtot/SiO2–K2O/CaO and FeOtot/SiO2–CaO/SiO2 diagrams for rocks from the Shakhtama deposit. Designations follow Fig. 2.

metaluminous rocks are predominant, which suggests the participation of mantle component in the magma formation. This is confirmed by the high contents of MgO (up to 8.62 and 7.91%), Ni (up to 226 and 265 ppm), Cr (up to 768 and 498 ppm), and V (up to 147 and 145 ppm) in the rocks of the Shakhtama and porphyry complexes, respectively (Fig. 4). The trace-element patterns of the rocks from both complexes show positive anomalies of Rb, Th, U, K, Pb, and Sr, negative anomalies of Nb, Ta, and Ti, high contents of LREE, and low contents of HREE (Fig. 13), which point to the significant role of subduction component in the magmatic system. The Middle–Late Jurassic intrusive complexes in eastern Transbaikalia formed after the termination of subduction (Zonenshain et al., 1990; Zorin et al., 2001). This suggests that the mantle metasomatized during the subduction and/or mafic juvenile crust formed during the supply of basic melts (mantle derivates) (Li et al., 2011; Richards, 2011) were the primary source of the ore-magmatic system. On the composition diagrams of incompatible elements (Fig. 10), used as indicators of model reservoirs — sources of magmas (Kovalenko et al., 2009), the rocks of the Shakhtama deposit are localized both near metasomatized mantle and near continental crust, which suggests the participation of both of them in the magma formation. The arrangement of the isotopic composition points of rocks between the MM and CC (Figs. 9 and 11) indicates that the magma composition was controlled by the proportion of mantle and crustal substances. According to the εNd increase, the contribution of crustal component to the magma formation in the porphyry complex is smaller than that for the Shakhtama

complex. Within the latter complex, it became weaker in the series from granites to monzonites. Among the deposit magmatites (high-K calc-alkalic type I rocks), there are granitoids with usual geochemical characteristics and with characteristics of K-adakites. Experimental studies (Xiao and Clemens, 2007) showed that the formation of melts corresponding in composition to K-adakites is possible at >20 kbar (crust thickness of > 66 km), 1075 ºC, and melting of high-acid protolith (of TTG type). According to Xiao et al. (2007), the melts of the Tibet K-adakites formed during the melting of the lower crust that was transferred into the mantle as a result of lithosphere delamination. A similar situation was, most likely, in eastern Transbaikalia. In its southeast, the crust thickening was caused by the collision between the Siberian and the Mongol-China continents (Zorin et al., 2001) and the transition of mantle melts into the crust basement. These melts formed juvenile mafic crust at the subduction, collision, and postcollisional (rifting) stages of the regional evolution. The above facts suggest that the parental melts of the granitoids geochemically similar to K-adakites were produced in the mantle as a result of the melting of juvenile and mature crust during the lithosphere delamination. The produced melts interacted with the mantle, thus making them enriched in Mg. The magma of high-K granitoids with usual geochemical characteristics was generated during the melting of juvenile and mature crust at higher levels corresponding to P < 17 kbar and was mixed with melts arrived from lower horizons. This explains the high MgO contents in all igneous rocks of the deposit.

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601

Fig. 13. PM-normalized (McDonough and Sun, 1995) multielemental patterns of igneous rocks from the Shakhtama (a) and porphyry (b) complexes of the Shakhtama deposit.

Genetic peculiarities of the ore-magmatic system. The development of porphyry Cu–Mo magmatic systems is related to the activity of mantle processes. The produced basaltoid magmas are regarded as a source of substance (including metals) and heat sustaining the long run of magmatic process in the depth range from the lower crust to the level of the porphyry complex formation. Favorable conditions for the origin of ore-magmatic systems appear during subduction, when the depleted mantle wedge becomes enriched in water, volatiles, and metals. Magma generated during the melting of the metasomatized mantle at the subduction and subsequent stages of geologic evolution inherits many of its geochemical characteristics (Richards, 2011). The subduction setting in the study region existed from Permian to Jurassic (Zorin et al., 2001). This is a period when magmatism took place and large Andean-type batholiths formed. The Shakhtama massif is confined to one of such batholiths. In the subduction setting, granitoid magmatism was accompanied by the formation of juvenile crust in the upper horizons (the crust–mantle zone), which was composed mainly of basites, products of the metasomatized mantle (Richards, 2011). The Shakhtama deposit lacks basic rocks, which implies that during the formation of the Shakhtama and porphyry complexes, basaltoid magma stayed at the mantle– crust boundary for a long time, thus thickening the crust from below and initiating the melting of the above-lying crust. Each stage of magmatism was accompanied by the supply of fluid components and metals from the mantle. Therefore, the formed juvenile crust can be regarded as a source potentially favorable for the ore production in the Shakhtama magmatic system.

Melting of the earlier formed juvenile crust as a result of the intrusion of basaltoid magma at the late stages of endogenic activity caused the redistribution of volatiles and metals into the melt, which could then have migrated to the upper horizons. Zones composed of amphibole–restite resulted from the melting of mafic crust were favorable for the concentration of fluids and metals. These zones, formed at the collision stage, were later involved in the endogenic process related to porphyry magmatism, which increased the ore potential of the latter. The different scales of occurrence of the Shakhtama and porphyry magmatism in the region suggest the significant role of the former in the thickening of the juvenile crust from below and the transformation of the above-lying crust and, hence, in the ore potential of the magmatic system. The mature crust, being the source of the ore-magmatic system, significantly determined the specific composition of magma—its high acidity and alkalinity favorable for the concentration of Mo in melt and its subsequent redistribution into fluid. According to the geochemical and isotope-geochemical data, the parental magma for the most primitive rocks of the system (high-Mg monzonites of the Shakhtama and porphyry complexes with MgO = 7–8% and with geochemical characteristics of K-adakites) resulted from the melting of crust (subsided during the delamination to depth) and the interaction of melts with the mantle. Ascending high-Mg monzonitic magma mixed with the melts of upper horizons. The produced melts underwent crystallization differentiation on ascent and in magma chambers. The close εNd values of monzonite- and granite-porphyry indicate that they are products of the same melt of rather homogeneous isotopic composition and, hence,

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with close proportions of components of juvenile and mature crusts, whereas the εNd increase in the series from granites to monzonites of the Shakhtama complex means that the contribution of mature crust to the formation of melts was reduced in the same series. The melts that produced the porphyry complex were transported from the intracrustal chamber localized below the Shakhtama massif and functioning after the crystallization of the Shakhtama granitoids. The melts that formed dikes of homogeneous and complex compositions were supplied from different horizons of the magma chamber or, possibly, from chambers of different compositions. Within the ore field, magmatism took place in the late Middle Jurassic–middle Late Jurassic, according to U–Pb dates. In the south of eastern Transbaikalia, collision processes terminated in the Middle Jurassic. In the Late Jurassic, postcollisional depressions began to form (Parfenov et al., 2003; Ruzhentsev et al., 2008). The geochronological age (Middle Jurassic) of the Shakhtama granitoids corresponds to the final stage of collision — collision between two continents in the setting of the closure of the Mongol-Okhotsk ocean (within its Transbaikalian sector). Porphyry magmatism in the region proceeded during the change of the geodynamic regime by the postcollisional one. The change in the geodynamic/tectonic regime is considered a favorable factor for the formation of porphyry Cu–Mo systems (Cooke and Hollings, 2005; Yang et al., 2009). The spatial coexistence of granitoids from different depths (no less than 3 km for the Shakhtama granitoids and about 1 km for the porphyry (Kuznetsov, 1977)) suggests that the porphyry complex formed after the rapid ascent and erosion of the Shakhtama granitoids as a result of the change of the collision setting by the postcollisonal (rifting) one. A similar situation was observed during the formation of the largest Qulong porphyry Cu–Mo deposit, Tibet, which is referred to as postcollisional. Before the intrusion of ore-bearing magma, the asthenosphere ascent led to the transfer of earlier formed granitoids (deeper-seated than ore-bearing porphyry) for no less than 2 km to the surface and their partial erosion. The ascent and erosion of granitoids proceeded for ~2 Myr. During these processes, porphyritic magma that melted out under the action of the asthenospheric heat ascended to the depth level of the transferred granitoids, among which dikes of ore-bearing porphyry formed. The different geodynamic settings of formation of the studied complexes determined their specific features. In the dilatation setting, magma was generated in deeper horizons, with the greater contribution of mantle component and, most likely, with the involvement of plume (HIMU) component, as evidenced from the high contents of radioactive Pb in sulfides and feldspars. The different scales of occurrence of igneous complexes in the region suggest that the degree of melting of the protolith (juvenile and mature crusts) controlled their composition and geochemical features. The degree of melting during the magma formation at the stage of the porphyry complex evolution was lower as compared with that for the Shakhtama complex. This might be the cause of the high alkalinity of the porphyry and their enrichment in incompatible elements

(including Mo). At the same time, we cannot rule out the influence of plume during the porphyry complex formation. The mineral assemblages of the Shakhtama and porphyry complexes have similar compositions and close petrogeochemical characteristics indicating the same (or compositionally similar) sources of the rocks and the same transport paths and evolution trends of melts. All this permits us to regard the igneous rocks of the complexes as products of the same magmatic system of long activity, which produced Mo mineralization at the final stage. Favorable conditions for the ore production in the magmatic system during the porphyry complex formation appeared mainly at the preceding stage, during the Shakhtama complex formation, which we recognize as a preparation stage in the evolution of the ore-magmatic system. The plume effect. Magmatism evolution in the southeast of eastern Transbaikalia in the Late Mesozoic took place in the intricate conditions of continent collision during the closure of the Mongol-Okhotsk ocean and under the effect of mantle hot spot on the regional lithosphere (Yarmolyuk et al., 2000; Zonenshain et al., 1990). The obtained U–Pb geochronological dates for the massif granitoids and porphyry dikes evidence that the magmatism of the Shakhtama deposit took place in the setting of collision and the hot-spot activity, which permits such settings to be considered a favorable factor for the evolution of the Shakhtama ore-magmatic system. The Shakhtama deposit is spatially associated with the East Mongolian area of Late Mesozoic intraplate magmatism (Yarmolyuk et al., 1995), where a series of rare-metal deposits is localized, including the Yugodzyr W–Mo deposit, eastern Mongolia. The age of intrusive magmatism in the deposit region, 151–165 Ma (Rb–Sr dating) (Kovalenko et al., 1999), overlaps with the age of the magmatism in the Shakhtama deposit. These deposits also have some similar geochemical characteristics, e.g., Sr and Nd isotopic compositions (Kovalenko et al., 1999), which admits the participation of compositionally similar sources in the formation of their intrusive complexes. On the ratio–ratio diagrams of incompatible elements (Fig. 10) used as indicators of model reservoirs, sources of magmas (Kovalenko et al., 2009), the Shakhtama deposit rocks differ significantly in composition from products of plume magmatism. In accordance with these geochemical characteristics, the contribution of plume component to the formation of the igneous rocks of the deposit was insignificant. However, the Pb isotopic composition suggests the participation of plume HIMU component in the evolution of the ore-magmatic system. During the formation of the Shakhtama ore-magmatic system, mantle plume exerted mainly a thermal effect on lithosphere, which caused its melting and favored the origin of an ore-magmatic system and its prolonged activity. A similar situation was observed in other porphyry Cu–Mo deposits in the areas of plume magmatism (Berzina and Borisenko, 2008; Berzina et al., 2011). The relationship of calc-alkalic magmatism and mineralization with mantle plumes is considered elsewhere (Dobretsov, 2003; Dobretsov and Buslov, 2011; Dobretsov et al., 2001, 2010).

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Conclusions Two intrusive complexes are recognized in the Shakhtama deposit: Shakhtama and ore-bearing porphyry. The U–Pb zircon dates (SIMS) coincide within the determination error and are 161.7 ± 1.4 and 161.0 ± 1.7 Ma for the monzonites and granites of the Shakhtama complex, respectively, and 159.3 ± 0.9 and 155.0 ± 1.7 Ma for the monzonite-porphyry and granite-porphyry of the ore-bearing complex. Magmatism took place there in the late Middle Jurassic and in the early Late Jurassic. It proceeded in an intricate geodynamic setting combining the influence of mantle plume on the lithosphere of the Central Asian orogenic belt and the collision of continents during the closure of the Mongol-Okhotsk ocean. The Shakhtama complex formed during the collision termination, and the porphyry complex originated in the period when the geodynamic setting gave way to a postcollisional (rifting) one. The complexes are composed of monzonite–granite series with similar geochemical characteristics of rocks. Taking into account the geological data, subduction geochemical characteristics of granitoids, and their εNd values close to zero, we suggest that magmas were generated from the juvenile crust resulted from the intrusion of basic magma and its localization at the mantle–crust boundary. The geochemical features of the juvenile mafic crust that were inherited from the metasomatized mantle permit it to be considered the main source of fluids and metals for the Shakhtama ore-magmatic system. The Nd model ages (~1000 Ma) and geochemical characteristics of granitoids testify to the participation of Precambrian metaintrusive rocks in the magma formation. Among the granitoids of both complexes, there are high-K calc-alkalic rocks with usual geochemical characteristics and those of K-adakites. These characteristics suggest the generation of parental melts at a depth of <55 km for the former rocks and at a depth of 55–66 km for the latter. Magma with geochemical features of K-adakites was produced as a result of the melting of crust submerged into the mantle during the lithosphere delamination, which was caused by the crust thickening as a result of the repeated inflow of basic magma into the basement of the crust and tectonic deformations in its upper horizons. The high-Mg monzonitic magma produced under these conditions ascended and was mixed with melts generated in the upper horizons, which accounts for the high Mg contents of the Shakhtama granitoids. The similar compositions and petrogeochemical characteristics of the granitoids of the Shakhtama and porphyry complexes point to the same sources, transport paths, and evolution trend of their parental melts. This indicates that the igneous rocks of both complexes are products of the same long-living magmatic system, which produced Mo mineralization at the final stage. The favorable conditions for the ore production in the magmatic system during the formation of the porphyry complex appeared as early as the preceding stage—the formation of the Shakhtama complex, which we

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regard as a preparatory stage in the evolution of the ore-magmatic system. We thank A.E. Izokh and A.A. Sorokin for recension and critical remarks on the paper. This work was supported by grant 11-05-00323 from the Russian Foundation for Basic Research.

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