Ongonite–elvan magmas of the Kalguty ore-magmatic system (Gorny Altai): composition, fluid regime, and genesis

Russian Geology and Geophysics 52 (2011) 1378–1400 www.elsevier.com/locate/rgg

Ongonite–elvan magmas of the Kalguty ore-magmatic system (Gorny Altai): composition, fluid regime, and genesis E.N. Sokolova a,b, S.Z. Smirnov a,b,*, E.I. Astrelina b, I.Yu. Annikova a, A.G. Vladimirov a,b,c, P.D. Kotler b 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 Novosibirsk State University, ul. Pirogova 2, Novosibirsk, 630090, Russia c Tomsk State University, pr. Lenina 36, Tomsk, 634050, Russia Received 17 February 2011; accepted 5 April 2011

Abstract The Kalguty ore-magmatic system (OMS) is a complex combination of a granite pluton, a hydrothermal Mo–W deposit, pegmatites, greisens, and a belt of rare-metal (RM) and ultra-rare-metal (URM) elvan and ongonite dikes. Studies of melt inclusions (MI) in quartz phenocrysts in the dike rocks have demonstrated that quenched glass has major element contents close to those of the dike rocks but rare elements (Li, Rb, Be, Cs) and P contents. This suggests that the MI represent magma at the stage preceding the dike emplacement. The MI in quartz from the URM rocks are poorer in Si, Fe, Mg, and REE than those in quartz from the RM rocks but richer in Cs, Rb, Nb, and Ta, like the URM rocks themselves. This indicates that the melts had segregated into RM and URM ones before the studied quartz phenocrysts began to crystallize. The composition of MI glass corresponds to “the albite trend” of differentiation, suggesting that the initial melt compositions were ongonitic, while their K enrichment and formation of elvan magma followed the crystallization of the quartz phenocrysts. According to our estimates, the melt contained 6–7 wt.% H2O. The quartz phenocrysts crystallized in a heterogeneous medium consisting of a silicate melt and an aqueous fluid. The latter was a high-density supercritical fluid with 3–12 wt.% NaCl equiv. Variations in the gas and salt compositions of the fluid inclusions (FI) are attributed to the interaction between fluids of magmatic and hydrothermal systems. This possibility is confirmed by ample evidence of their coeval formation. Quartz crystallization from the RM melts took place at 630–650 °C, whereas quartz from the URM melts formed at 20–30 °C lower temperatures. Quartz phenocrysts crystallized at 4.5–5.5 kbar. Additional estimates with regard to the mineral composition and quartz compressibility yielded values of 3–6.5 kbar. A petrogenetic model of some crystallization stages of the dike rocks within the Kalguty OMS was constructed on the basis of the results obtained in this study. The melts which formed the dikes of the East Kalguty belt are derivatives of the same magma which formed the major-stage granite pluton. Quartz is present as intratelluric phenocrysts, which crystallized at considerably greater depths than those of the dike emplacement. Differentiation of the parental magma was accompanied by rare element and P accumulation. The compositions of the FI and MI confirm that the magma and hydrothermal system of the Kalguty OMS exchanged their substances. It is associated with the increasing K content of the melts and the subsequent elvan crystallization as well as considerable variations in the salt and gas compositions of the magmatic fluid inclusions. © 2011, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: ongonites; elvans; rare-metal granites; melt inclusions; fluid inclusions; Kalguty deposit

Introduction The East Kalguty dike belt is part of the Kalguty ore-magmatic system (OMS), which comprises a granite–leucogranite

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

pluton, a dike belt, hydrothermal veins, and greisens (Annikova, 2003; Potseluev et al., 2008). Ongonites and elvans play a special role in reconstructing mineral and ore formation in the Earth’s crust. They form at the end of the granitic-magma differentiation and are very often localized within the areas of Mo, W, Sn, and other rare-metal deposits or occurrences (Antipin et al., 1999, 2002; Kovalenko and Kovalenko, 1976). The major features of the typical ongonites and elvans, which

1068-7971/$ - see front matter D 201 1, V . S. S o bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.rgg.2011.10+.017

E.N. Sokolova et al. / Russian Geology and Geophysics 52 (2011) 1378–1400

are part of OMS, are elevated contents of trace elements and volatiles (Kovalenko and Kovalenko, 1976). The Li, Rb, Cs, Ta, Nb, and Be contents of these rocks are tens of times higher than those of typical granites and reach the level of commercial rare-metal (RM) pegmatites (Dergachev, 1988; Kovalenko, 1977; Solodov, 1969). The key problem of the petrology of granite-related RM OMS is the determination of the physicochemical factors responsible for trace element accumulation in residual granitic melts during magma differentiation at various crustal levels. On the basis of natural and experimental data on RM granitic melts of sodic (ongonitic) compositions, a fairly reliable proof was obtained that, with the decreasing temperature and increasing pressure of volatiles (H2O, F), the temperature minimum of the quartz–feldspar cotectic curve shifted toward the albite apex of the haplogranitic quartz–albite–orthoclase triangle (“albite trend”) (Kovalenko and Kovalenko, 1976). For the potassic (elvan) compositions, most researchers presume a more complex evolution of RM magmas, presupposing the participation of intratelluric potassic fluids of magmatic origin (Annikova et al., 2006; Antipin et al., 2002; Hall, 1970; Henley, 1972, 1974). In Central Asia the best studied ongonites are associated with the RM granite plutons and deposits of central Mongolia (Kovalenko and Kovalenko, 1976; Naumov et al., 1982; Odgerel and Antipin, 2009), the Ary-Bulak ongonite stock (eastern Transbaikalia) (Antipin et al., 2009; Kovalenko et al., 1975; Naumov et al., 1982; Pretyazhko and Savina, 2010; Peretyazhko et al., 2007), and the elvans and ongonites of the Urugudei–Utulik intrusion–dike series (Cisbaikalia) (Antipin et al., 1999, 2006; Perelyaev et al., 1987, 1988). Also, ongonites occur in other parts of Central Asia (Begmagambetov et al., 1985; Dovgal et al., 1995; Trifonov and Solomovich, 1982, 1984; Vladimirov et al., 1991). The geologic and petrologic features of the elvans and ongonites in the East Kalguty dike belt (Gorny Altai) are widely studied (Annikova, 2003; Annikova et al., 2006; Dergachev, 1988, 1990; Titov et al., 2001; Vladimirov et al., 1997, 1998). Analyzing the composition and structure of the above complexes, it is noticeable that dikes of ongonites and elvans seldom coexist in the same OMS. Therefore, this belt is of particular interest, because here, according to detailed field observations, ongonites and elvans make up a system of closely aged or coeval subvolcanic dikes, which are synchronous with the ore veins of the Kalguty Mo–W deposit (Potseluev et al., 2008; Seltmann et al., 2007). This study aims to construct a petrogenetic model for the formation of the ongonite and elvan dikes in the East Kalguty belt, which is to explain the chemical heterogeneity and fluid regime of RM melt crystallization, and to explain the formation dynamics of the Kalguty OMS on this basis. The major attention was paid to studying melt and fluid inclusions responsible for the magmatic stage in the formation of ongonite and elvan dikes. This paper presents the first data on magmatic-fluid inclusions (FI) from the Kalguty OMS and the trace element compositions of glass in homogenized melt

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inclusions (MI) in quartz from the dike rocks of the East Kalguty belt.

The Kalguty ore-magmatic system: tectonic setting and age Tectonic setting and relationship with Mo–W mineralization. The Kalguty OMS formed at the intraplate stage, associated with the tectonomagmatic activation of the Altai accretionary–collisional system under the effect of the Siberian superplume (Dobretsov et al., 2005; Vladimirov et al., 2005). Under these geodynamic conditions, several large OMS formed (Dobretsov et al., 2010; Kuz’min and Yarmolyuk, 2011): Mo–W (Kalguty OMS, Gorny Altai), Sn–W (Rudny Altai, eastern Kazakhstan), and Li–Ta–Nb deposits (spodumene–pegmatite deposits Koktogai, China and Asubulak, eastern Kazakhstan); Alakha spodumene–granite-porphyry deposit, Gorny Altai) (Fig. 1). The Kalguty OMS belongs to the western sector of the Central Asian Fold Belt and is located in southern Gorny Altai (Annikova et al., 2006; Seltmann et al., 2007). The East Kalguty dike belt intrudes the older RM granites of the Kalguty pluton and coexists with the coeval hydrothermal veins and greisens of the Kalguty Mo–W deposit (Fig. 2). The belt (total length 10–15 km, ~3 km wide) consists of more than a hundred dikes from tens of centimeters to the first meters thick. Owing to the poor exposure, the dikes seldom occur in bedrock outcrops. They usually occur in talus, occupying plateau slopes and tops as linearly elongated bodies, which are lighter in color than the host rocks. It is noteworthy that the richest part of the Kalguty deposit is spatially related to the axial part of the dike belt. The ore zone at the Kalguty Mo-W deposit is 2 km long and 0.5 km wide. The major ore minerals are wolframite, molybdenite, chalcopyrite, beryl, and bismuthinite (Potseluev et al., 2008; Volochkovich and Leont’ev, 1964). Along with quartz veins, rich Mo mineralization is contained in the greisens and greisenized microgranites of the stockwork, which is also located at the dike belt center and has the local name Molybdenum Stock (Fig. 2). Field observations revealed evidence of greisenization in some elvan and ongonitic dikes. On the other hand, some other dikes contain xenoliths of greisenized rocks with ore minerals. Both observations suggest that at least some dikes formed simultaneously with the ore formation. U-Pb and Ar-Ar isotopic age. The Kalguty OMS comprises two intrusive complexes: the Kalguty granite–leucogranite complex and the East Kalguty complex of dikes and minor intrusions. The U-Pb isotope dating of single zircon grains from the major-stage granites (Kalguty granite–leucogranite complex) showed that the age of small crystals and the rims of large crystals was subconcordant (216 ± 3 Ma). The Ar–Ar isotope dating of biotites from the major-stage granites sampled within the Kalguty ore field (202 ± 1 Ma) revealed isotopic disequilibrium in the K–Ar radiogenic system (loss of radiogenic Ar). The Ar–Ar dating of musco-

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Fig. 1. Sketch map of large and unique rare-metal (RM) deposits in the Altai accretionary–collisional system (a) (Vladimirov et al., 2005); geological sketch map of the Kalguty RM ore-magmatic system (OMS) (b). a, 1, undivided Neoproterozoic–Early Paleozoic structural–compositional complexes of the Siberian and Kazakhstan continents; 2, Altai–Mongol microcontinent; 3–8, Middle–Late Paleozoic continental-margin and oceanic terranes; 9, Kuznetsk sedimentary basin; 10, traps (T); 11, Cenozoic sediments; 12, granitoids (C–J1); 13, faults: actual (a), presumed (b); 14, largest RM deposits of the Altai accretionary–collisional system: 1, Kalguty OMS, Gorny Altai (Mo, W); 2, spodumene granite-porphyry of the Alakha stock, Gorny Altai (Li, Ta, Nb); 3, spodumene RM pegmatites of the Koktogai deposit, Mongolian Altay, China (Li, Rb, Cs, Ta, Nb); 4, spodumene RM pegmatites of the Asubulak deposit, eastern Kazakhstan (Li, Rb, Cs, Ta, Nb); 5, spodumene pegmatites of the Lake Teletskoe deposit (Li); 6, spodumene pegmatites of the Tashelga deposit (Li). Inset shows the geographic location of the Altai accretionary– collisional system; b, 1, undivided Devonian volcanosedimentary rocks; 2, major-stage porphyritic biotite granites; 3, leucogranites of the additional-intrusion phase; 4, ongonite and elvan dikes of the East Kalguty complex; 5, area with the richest Mo–W mineralization. Rectangle shows the studied part of the East Kalguty dike belt. Inset shows the geographic location of the Kalguty RM OMS (Annikova et al., 2006).

vites from the dikes of the East Kalguty complex (both coeval with and postdating ore bodies), with no evidence of postmagmatic recrystallization or greisenization, yielded a similar result (205–201 Ma). This age estimate is interpreted as the real age of the dikes and associated W–Mo–Bi–Be minerali-

zation (Annikova et al., 2006). Note that postmagmatic muscovite from the core of a Mo-bearing pegmatite body (socalled “quartz–Mo core”) has an Ar–Ar age of 202 ± 2.3 Ma. This age is interpreted as a result of the intense thermal alteration of ores and rocks at the deposit. Thus, the age

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Fig. 2. Geological sketch map of the East Kalguty dike belt after (Dergachev et al., 1988), supplemented by the authors. 1, volcanic rocks (D); 2, major-stage granites; 3, leucogranites of the additional-intrusion phases; 4, elvans; 5, ongonites; 6, URM ongonites; 7, URM elvans; 8, elvan plutons; 9, Molybdenum Stock greisen stockwork; 10, Quaternary sediments; 11, sampling sites; 12, C, N, E, S, the Central, Northern, Eastern, and Southern areas of dike distribution.

estimates of the Kalguty OMS suggest two pulses of magmatism: first, the formation of the RM granite–leucogranite pluton at 216 ± 3 Ma; second, the formation of the East Kalguty dike belt and associated Mo–W–greisen and quartz vein mineralization, which is spatially and temporally related to the dikes. The dike emplacement age coincides with that of some other Mo–W orebodies at the deposit (204–200 Ma) (Seltmann et al., 2007).

Materials and methods A representative set of samples was collected from granites, ongonite and elvan dikes, and ores during the International Geological Trip (2007) (Seltmann et al., 2007) and the field trip (2009). Also, samples were used from the personal collections of A.V. Titov, S.A. Vystavnoi, and V.B. Dergachev. The sketch map of the East Kalguty dike belt shows sampling sites, which are located all along the belt and permit describing the major petrographic varieties among the rocks of the granite pluton and dike belt (Fig. 2).

The major-element contents of the rocks were determined by X-ray fluorescence analysis on a SRM-25 spectrometer at the Analytical Center of the Sobolev Institute of Geology and Mineralogy. The trace element (Y, Zr, Nb, Ta, Hf, Th, U) and REE contents were determined by ICP MS on an ELEMENT (Finnigan) at the same center by a standard technique. The contents of F, B, Sr, Ba, and ore elements (Cu, Zn, Ge, Mo, Ag, Sn, Tl, Pb, W) were determined by quantitative atomicemission analysis. Alkali metal contents (K, Na, Li, Rb, Cs) were determined by flame photometry at the Analytical Center of the Vinogradov Institute of Geochemistry (Irkutsk). Inclusions of mineral-forming media were studied by optical microscopy. Small (<5 mm) MI were heated under atmospheric pressure by the quenching technique in a SUOL0.15.2/12M-I3 electric furnace. Larger inclusions were heated in an autoclave under confining water pressures of 1 and 2.5 kbar by the procedure described in (Smirnov et al., 2003, 2011). Autoclave heating under water pressure allowed the remelting and homogenization of MI up to 20–30 µm in size without decrepitation. Inclusions 1–5 µm in size were heated in a furnace for 2.5–3 h; in the autoclave—for 1–2 days.

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Fragments of doubly polished plates (with quartz phenocrysts containing MI) and quartz grains 0.25–1 mm in size were used for autoclave heating with incremental increase of temperature from 550 to 710 °C. After the quenching the grains were embedded in an epoxy pellet and polished to reveal the inclusions. Fluid inclusion microthermometry was conducted in a Linkam THMSG600 heating and freezing stage at –180 to +300 °C, the measurement error being 0.1 °C. Note that the temperature measurement error might have reached several degrees because of the small size of the FI (5–15 µm). Daughter minerals in unheated MI were identified under a LEO 1430 VP SEM equipped with an Oxford energy-dispersive spectrometer (Analytical Center of the Sobolev Institute of Geology and Mineralogy). Chemical analysis was done with the INCAEnergy 300 system. Quartz cathodoluminescence was studied under a LEO 1430 VP to determine the growth history (Perny et al., 1992; Rusk and Reed, 2002; Smith et al., 2010). The accelerating voltage was 20 kV; the current, 1.9–2.5 nA. Homogeneous and remolten MI, as well as partially molten crystalline phases in the heated MI, were studied on a Camebax-Micro electron microprobe (Analytical Center of the Sobolev Institute of geology and mineralogy). The analyses were done at an accelerating voltage of 20 kV, a probe current of 30–50 nA, and a beam diameter of 8–10 µm. Well-characterized natural minerals (albite, orthoclase, pyrope, fluorophlogopite, diopside) were used as primary standards. Orthoclase, albite, and fluorophlogopite were used as secondary standards. If glass contains water, Na, as a light alkali element, migrates easily from the electron beam and, therefore, Na contents can be underestimated (Bazarova et al., 1975; Lineweaver, 1962; Morgan and London, 1996, 2005; Nielsen and Sigurdsson, 1981). To restore the actual Na content, the decrease in the NaKα line intensity with exposure time was measured on a JEOL JSM 8100 electron microprobe. The correction for the Na “loss” was calculated as a cumulative

average from the intensity decrease over the first 10 s. It ranges from 8 to 30 rel.% (18 rel.% on average). The minor- and trace element, H2O, and F contents of glass in the homogeneous MI were determined by SIMS on a Cameca IMS-4f ion microprobe (Instutute of Microelectronics, Yaroslavl). The gas phase composition was determined and some crystalline phases of the MI were identified by Raman spectroscopy on a Jobin Yvon U-1000 single-channel spectrometer. The excitation of a Spectra Physics solid-state laser with a wavelength of 514 nm and a power of 1.8 W was used for this study.

The dike belt rocks: petrography, major and trace element geochemistry, and nomenclature According to the accepted petrographical nomenclature, the rocks of the East Kalguty dike belt belong to ongonites and elvans (ongonite group) and are subvolcanic equivalents of RM Li–F granites (Bogatikov et al., 1987, 2009; Kovalenko and Kovalenko, 1976). These ongonites and elvans have an abnormally high P content; along with F, this element is among the major fluxing agents in the differentiation of RM granitic magma (Annikova et al., 2006). The dike rocks of the East Kalguty belt have a porphyritic texture. The phenocrysts are 0.1–1 mm to 1–2 cm in size and make up 5–50% of the rock. The texture changes within a single dike from miniphyric and aphyric near the contacts to magnophyric, with a more crystallized groundmass in the center. Dike endocontacts often show flow structures. The major minerals of the dike rocks are quartz, feldspars, and mica. Mica is represented by muscovite or, occasionally, biotite. Muscovite, along with the other listed minerals, occurs as phenocrysts and, therefore, also formed in the course of magmatic crystallization. The groundmass also consists of quartz, feldspars, and muscovite. The main accessory minerals

Fig. 3. Histogram of the Na2O/K2O ratio (a), (b) and rare metals (c) in the rocks of the East Kalguty belt (a, c) and the reference RM granitoids (b). 1, elvans and ongonites of the East Kalguty belt (Annikova, 2003) and the samples used in this study; 2, Cornwall elvans and the granites of the Cornubian batholith (Antipin et al., 2002), 3, ongonites of the Amazonite Dike (Mongolia) and Baga-Gazriin dikes (Transbaikalia) (Kovalenko, 1977). N, Number of samples.

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E.N. Sokolova et al. / Russian Geology and Geophysics 52 (2011) 1378–1400 Table 1. Average chemical compositions of the rocks from the Kalguty pluton and East Kalguty dike belt Component

Major-stage granites

Ongonites

Elvans

URM ongonites

URM elvans

Greisenized rocks

X

s

X

s

X

s

X

s

X

s

X

s

SiO2

71.90

1.48

73.02

0.53

TiO2

0.44

0.12

0.05

0.18

73.02

0.85

71.98

0.85

72.97

0.88

74.32

1.99

0.10

0.06

0.03

0.03

0.08

0.03

0.12

Al2O3

13.86

0.53

14.80

0.87

0.07

14.56

0.52

15.95

0.21

14.93

0.55

14.04

0.46

FeOtot

2.54

0.62

1.11

0.26

1.53

0.61

0.95

0.12

1.09

0.13

2.72

0.73

MnO

0.07

0.02

0.09

0.03

0.07

0.03

0.18

0.07

0.08

0.03

0.06

0.01

MgO

0.75

0.25

0.11

0.02

0.23

0.10

0.10

0.01

0.20

0.09

0.27

0.18

CaO

1.59

0.46

0.62

0.45

0.59

0.15

0.52

0.26

0.55

0.16

0.28

0.29

Na2O

3.00

0.27

4.73

0.63

3.52

0.44

4.23

0.09

3.72

0.37

0.42

0.19

K2O

4.64

0.44

3.75

0.54

4.90

0.59

3.51

0.23

4.15

0.35

6.95

0.71

P2O5

0.19

0.07

0.42

0.04

0.31

0.44

0.71

0.09

0.39

0.08

0.26

0.09

LOI

0.76

0.33

0.80

0.40

1.20

0.36

1.67

0.38

1.08

0.29

2.05

0.73

Total

99.74

F

0.07

0.04

0.40

0.20

0.26

0.13

0.56

0.29

0.34

0.08

Li

145

36

277

133

232

131

2269

461

618

322

240

55

Be

7.0

3

68.2

10.5

15.20

15.9

135.6

43.7

33.1

32.8

4.1

2.1

B

–

48.2

19.1

61.9

42.3

140.0

31.1

–

–

–

99.49

100.03

99.84

99.23

101.49 0.36

0.03

Cu

–

93.5

5.2

30.8

19.3

56.2

70.3

–

1375

1025

Zn

–

44.4

4.8

36.8

7.7

110.6

9.6

–

387.0

461.0

Rb

318

75

694

192

579

172

1530

176

719

161

795

79

Sr

134

26

54

14

53

45

77

49

180

70

112

12

Y

44.8

14.8

8.2

3.2

13.6

7.9

5.2

4.4

–

8.6

11.5

Zr

175

71

27

9

46

27

18

4

–

47

41

Nb

66.4

41.3

67.6

6.5

42.4

16.0

80.9

2.6

51.1

26.9

31.0

14.3

Ta

2.40

0.6

43.26

7

12.07

7.2

80.00

34.30

6.1

7.75

Mo

7.9

8.6

2.3

2

1.7

1.3

0.5

0.1

–

Sn

8.3

3.2

0.9

0.7

2.6

0.8

1.1

0.2

–

Cs

41.9

26.6

104.6

53.0

76.2

67.7

839.7

207.3

275.5

Ba

347

61

66

39

98

85

42

19

Hf

5.2

1.3

1.8

0.1

2.1

0.8

1.8

W

6.1

5.3

24.0

12

46.6

50.6

30.8

Pb

38.3

8.1

9.0

6.1

33.5

12.7

–

74.0

60.2

7.2

4.4

55.0

19.4

–

133

59

0.1

–

2.1

0.8

3.9

–

81.0

60.6

–

–

171.0

Th

26.0

3.4

3.0

1.6

10.4

8.6

1.1

0.6

–

15.8

20.3

U

8.29

3.31

25.37

8.19

20.59

12.8

21.85

4.14

–

24.27

20.7

REE

166

24

63

7

–

165

Na2O/K2O

0.65

1.26

0.72

1.20

0.89

0.06

n

50

8

25

26

30

4

Note. Hereafter, X, averaged values; s, standard deviations; n, number of analyses. Major-element and F contents are given in wt.%; other components, in ppm. Dash, not analyzed.

are apatite, pyrite, magnetite, and fluorite. The bulk chemical composition of the rocks of the Kalguty OMS is shown in Table 1. Now some authors divide the rocks of the East Kalguty dike belt, on the basis of their petrography, into mini- and magnophyric felsite-, microgranite-, and granite-porphyry (Annikova, 2003; Potseluev et al., 2008; Titov et al., 2001). Rare-metal felsic dike rocks are divided into elvans (K2O predominates) and ongonites (Na2O predominates) (Annikova,

2003; Antipin et al., 2002; Dergachev, 1992; Vladimirov et al., 2007). On the Na2O/K2O histogram (Fig. 3, a), the rocks of the East Kalguty belt show a unimodal distribution with a peak at 0.8–1; that is, a vast majority of the dikes in the belt consist of elvans. At the same time, the Mongolian typical ongonites have a maximum at Na2O/K2O = 1.4–1.8 and the Cornwall typical elvans have a maximum at 0.4–0.6 (Fig. 3, b). Although the Na2O/K2O ratios for the dike rocks of the East Kalguty belt differ from those for the typical

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Fig. 4. Accessory minerals of the dike rocks in the East Kalguty belt. a, b, Ongonite, sample kl-14; c, ongonite, sample ku-6; d, elvan, sample l-765; e, elvan, sample 1-259; f, ongonite, sample kl-15; g, kalgutite, sample kl-142. Ab, Albite; Ap, apatite; Bt, biotite; Col, columbite-tantalite; KFsp, K-feldspar; Mon, monazite; Ms, muscovite; Mtb, montebrasite; Pcs, polycrase-euxenite; Pyr, pyrite; Qu, quartz; Rt, rutile; Ti-Mgt, titanomagnetite; Trp, triplite; Wt, wolframite; Xen, xenotime; Zrn, zircon. a, g, Transmitted-light images; b–f, BSE images.

ongonites and elvans, these terms will be used in this paper to specify the Na or K predominance in the rock composition. Notably, some samples contain 10–20 times more K2O than Na2O (Na2O/K2O = 0.01–0.1). These rocks are located near

the Mo-bearing stockwork; interestingly, they are also rich in Zn, W, and Mo (Table 1). Also, they contain resorbed feldspar phenocrysts replaced by quartz and muscovite along with large muscovite rosettes in the groundmass. On the basis of these

E.N. Sokolova et al. / Russian Geology and Geophysics 52 (2011) 1378–1400

features, we assigned these rocks to “greisenized felsite-pophyry.” The vast majority of the rocks in the East Kalguty belt are rich in rare metals and are called rare-metal (RM) rocks. Some of the dikes are composed of rocks much richer in rare metals than the RM ones. They are called ultra-rare-metal (URM) rocks (Annikova, 2003; Dergachev, 1988, 1992). The LILE contents show a bimodal distribution (Fig. 3, c). The first maximum corresponds to the RM rocks. The second one corresponds to the URM rocks, which are up to ten times richer in Li, Rb, Cs, and Be than the RM ones. Dike bodies consisting of URM rocks are traced discontinuously along a NE-striking line in the axial part of the belt for ~5 km (Fig. 2). Part of the URM dike consists of elvans in the center of the belt, while the southern and northern parts are composed of ongonites. The sodic URM rocks in the East Kalguty belt are also abnormally rich in P (0.65–0.96 wt.% P2O5) (Table 1); this permits considering them a separate rock series, which was named “kalgutites” by V.B. Dergachev (1988). Their P content is 5–10 times higher than the average P contents of the typical ongonites. Together with the relatively low F content, this causes F in the dikes of the East Kalguty belt to concentrate in fluorapatite rather than in topaz, as is the case with the typical ongonites (Dergachev, 1988; Kovalenko and Kovalenko, 1976). The P enrichment is also manifested in the presence of 0.5–0.8 wt.% apatite in the CIPW norm compositions of the dike rocks. After the calculation of the apatite norm, “free” P2O5 often constitutes up to 1.2 wt.% of the RM rocks and up to 3.5 wt.% of the kalgutites. Along with apatite, the dikes contain phosphate phases such as monazite (Ce,Th)PO4, triplite (Mn,Fe,Mg,Ca)2(PO4)(F,OH), xenotime YPO4, and montebrasite LiAl(PO4)(OH,F). The presence of columbite-tantalite (Fe,Mn)(Nb,Ta)2O6, polycrase-euxenite (Y,Ca,Ce,U,Th)(Ti,Nb,Ta)2O6, tourmaline, and montebrasite highlights the RM enrichment of the melts. Zircon, ilmenite, rutile, pyrophanite MnTiO3, chalcopyrite, sphalerite, and wolframite were also identified among the accessory minerals in the dikes (Fig. 4).

Melt and fluid inclusion studies Microscopy. Melt and fluid inclusions were studied in quartz phenocrysts from the ongonites and elvans of the East Kalguty belt and from the Kalguty pluton granites. In addition, FI in greisen quartz were studied. Melt inclusions occur both in the cores and rims of the phenocrysts (Figs. 5, 6). In the cores the inclusions form nonzonal groups and are larger than those in the rims. SEM cathodoluminescence (SEM CL) studies of the phenocrysts showed that MI sometimes violated the linearity of the growth zones (Fig. 5, c). Large MI are often surrounded by a halo of radial cracks associated with numerous FI (Fig. 5, a). Undoubtedly, all these features confirm that the MI are primary. The FI in the quartz phenocrysts are represented by single or nonzonal inclusions, groups located along the healed cracks,

1385

or in halos around the MI (Fig. 5, a, b). Single inclusions usually do not change the morphology of the quartz growth zones, while those located along the cracks are marked by dislocation patterns in the SEM CL images (Fig. 5, c). The MI vary in size from the first to 20 µm (less often, 50 µm); the FI seldom reach 15–20 µm. Phase composition of the inclusions at room temperature. The MI are completely crystallized. Separate crystalline phases can be distinguished in some well-crystallized ones. Sometimes a gas bubble is observed among the daughter mineral crystals. Almost all the MI have indented edges owing to daughter quartz deposited on the inclusion walls (Fig. 6, a, c). The crystalline aggregate of the MI is dominated by quartz, muscovite, and feldspars, with apatite or, rarely, monazite. The quartz phenocrysts contain crystalline inclusions of biotite, muscovite, apatite, monazite, triplite, rutile, zircon, ilmenite, wolframite, magnetite, and chalcopyrite. The FI found in the quartz phenocrysts from the ongonites and elvans comprise two or three phases at room temperature. Two-phase inclusions predominate; they contain a gas bubble and an aqueous–saline solution (Fig. 7, b). Sometimes these inclusions are attached to magmatic crystalline phases (as a rule, apatite or zircon) (Fig. 7, d). Three-phase inclusions are scarce and, in contrast to the two-phase gas–liquid inclusions, contain liquid carbon dioxide (Fig. 7, a, c). Since aqueous– carbon dioxide inclusions are rare, their origin is not clear enough and needs further study. The primary FI in the quartz phenocrysts are single or form nonzonal groups. They contain a gas bubble and an aqueous solution. Similarly to the primary MI, they can be attached to inclusions of magmatic crystalline phases. The secondary inclusions are associated with healed cracks. At room temperature they consist of two phases: a gas bubble and an aqueous solution, or, less often, of three phases: an aqueous solution, gaseous carbon dioxide, and liquid carbon dioxide. Additionally, the studied phenocrysts contain FI in halos of radial cracks around the MI. As a rule, these inclusions consist of two phases and contain a gas bubble and an aqueous solution. The FI in greisen quartz are similar to those in the dike rock phenocrysts. At room temperature inclusions with liquid carbon dioxide occur only in the dikes and greisens and are absent from granites or leucogranites. Special emphasis in this study was placed on FI that are coeval with the melt ones and, therefore, reflect the stage of magmatic crystallization. These two-phase gas–liquid inclusions are similar to the primary ones, described above. Melt inclusion thermometry. When heated in a heating stage at atmospheric pressure, most of the MI decrepitated. This behavior is evidence for high fluid pressure during their entrapment. Afterward only small inclusions (no larger than 5 µm) were heated. At atmospheric pressure melting began at 640 °C. The first homogeneous inclusions appeared at 680– 685 °C. After heating to 710 °C, shrinkage gas bubbles were often preserved in glass. To prevent larger MI from decrepitation, autoclave heating was conducted under the confining

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Fig. 5. Melt and fluid inclusions in the quartz phenocrysts from the rocks of the East Kalguty dike belt. a, c, Cathodoluminescence image; b, d, the same phenocrysts in polarized transmitted light. a, b, Large melt inclusions (MI) in the phenocryst core (C), with a negative crystal shape, and small zonal MI in the phenocryst rim (R). The large inclusions are surrounded by cracks filled with fluid inclusions (FI). A square shows the zone in which the cracks are interrupted at the boundary with the next zone; c, d, zonal inclusions in the phenocryst core; c: arrow (ben) shows a bending of the growth zones near the MI; arrow (dis) shows the location of the FI marked by dislocation patterns.

pressure of water. At 1 kbar, the crystal aggregate of the MI in some samples began to melt at 600 °C. The first homogeneous inclusions appeared after heating at 635–650 °C. Most of the MI in the studied samples homogenized at 660–695 °C. At 2.5 kbar, the smallest MI in a quartz sample from URM ongonite homogenized at 600 °C. At the same pressure and 640 °C, the MI in quartz samples from the RM rocks are close to homogenization and contain partially molten crystalline phases. The MI become considerably larger after the heating, and their boundaries become smooth owing to the melting of quartz from the cavity walls. After a considerable part of the crystal aggregate of the inclusions remelts, only gas bubbles occupying no more than 5–10% of the cavity or single fine crystals often remain in glass (Fig. 6, b, d). The latter consist of feldspars (with almost equal Na2O and K2O ratios) or apatite. Fluid inclusion thermometry. Data on the FI are presented in Table 2. Almost all the groups of inclusions in the granites, ongonites, elvans, and greisens start melting at –24 to –19 °C. Consequently, the major components of the inclusion solutions should be NaCl and KCl. Quartz in one elvan sample from

the central area contains primary inclusions with a eutectic temperature (Teut) of –33.6 °C along with inclusions similar to those reported above. The salt composition of these inclusions might be dominated by MgCl2, because the Teut of the MgCl2–H2O system is from –33.6 to –33.8 °C, according to various authors (Kirgintsev et al., 1972; Pel’sh, 1973; Zdanovskii et al., 1961). This conclusion is confirmed by studies of inclusion solutions with a low Teut by the acid crush-leach method (ICP AES). The ice melting temperatures (Tm) in most of the inclusions from the quartz phenocrysts in the dike rocks are observed at –8 to –2 °C. The Tm of ice in the FI coeval with the melt ones are also within this range (–8 to –5 °C). Some quartz phenocrysts from an elvan dike at the eastern locality contain rare primary inclusions with melting points at –2 to –1 °C. In the inclusions with Teut = –33.6 °C, ice melts at –23 to –10 °C. The Tm of ice decreases: it is from –6 to –3 °C in the quartz of major-stage granite and from –3 to –2 °C in leucogranite quartz. In the greisen quartz inclusions, the typical Tm of ice are also from –5 to –2 °C. The salinity of the solutions can be estimated from the Tm of ice. Most of the inclusions in ongonite and elvan quartz

E.N. Sokolova et al. / Russian Geology and Geophysics 52 (2011) 1378–1400

1387

Fig. 6. Zonal MI in the quartz phenocrysts from the rocks of the East Kalguty dike belt before and after the heating. a, Finely crystallized MI with indented outlines; b, the same inclusions with gas bubbles in glass (g.b.) after the heating; c, crystallized MI; d, the same after the heating. Some of the MI homogenized (hom.); glass in the others contained crystalline phases (cr.) or a gas bubble surrounded by fine crystalline phases (cr. + g.b.).

contain solutions with 3–12 wt.% NaCl equiv. The salinity of the solutions in the FI coeval with the melt ones is within the same range (Tables 2, 3). Inclusions with high ice melting temperatures are filled with solutions containing 1.5–3 wt.% NaCl equiv, and those which have supposedly chloride–magnesium compositions are richer in salt (~11–18 wt.% MgCl2 equiv). The solution salinity in the inclusions in quartz from the granites and greisens is close to that in the vast majority of the FI in quartz from the dike rocks and equals 3–9 wt.% NaCl equiv.

In the three-phase FI, carbon dioxide melts at –57.3 to –56.8 °C. The lower temperature, –57.3 °C, might suggest the presence of a nitrogen admixture, whose traces were detected by Raman spectroscopy. The carbon dioxide bubble homogenizes at 27–29 °C. The FI in all the studied samples homogenize to liquid. The inclusions from quartz in various dikes and greisens homogenize over a wide temperature range. Inclusions without liquid carbon dioxide homogenize at 140–200 °C. In quartz from the dike rocks, two-phase FI, coeval with the melt ones,

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Fig. 7. Fluid and melt inclusions in the quartz phenocrysts from the rocks of the East Kalguty dike belt. a, Accompanying aqueous and aqueous–carbon dioxide FI at room temperature; inset b, two-phase aqueous inclusion; inset c, threephase aqueous–carbon dioxide inclusion; d, FI coeval with a MI and combined with an apatite crystal; e, FI coeval with the MI. a.s., Aqueous–saline solution; CO2l, liquid carbon dioxide. Dashed arrows show small two-phase gas–liquid inclusions. See Fig. 6 for the rest of the legend.

homogenize at 200 °C. Inclusions with liquid carbon dioxide have the highest homogenization temperature (Thom) (230– 250 °C). In different phenocrysts from one sample, the Thom also vary widely (Fig. 8). The FI in granite quartz homogenize over a narrower temperature range (150–190 °C). Homogeneous fluid density was estimated from the equation of the state of the system NaCl–KCl–CaCl2–H2O (Zhang and Frantz, 1987) at 0.88–0.94 g/cm3. Composition of glass from the melt inclusions. To obtain homogeneous MI larger than 10–15 µm, suitable for analyzing the melt composition, quartz samples with large MI were heated at 30–50 °C above the specified Thom, at a water pressure of 1 kbar. Glass from the homogeneous MI was analyzed for major elements, trace elements, and REE (Tables 4, 5). In general, the composition of glass from the MI corresponds to that of the dike rocks. At the same time, deviations were observed in some components. For example, glass from the inclusions has more variable SiO2 contents than the bulk rock compositions (Fig. 9, a, b), the average values

being the same (72–72.5 wt.%). The Al2O3 contents behave in the same way (Fig. 9, a, b). The MI in quartz from the URM rocks are also the most aluminous, similarly to the bulk rock compositions. Their average FeO and MgO contents are lower than that of the rocks (Fig. 9, c, d). Glass in the MI from the phenocrysts in the URM rocks is the most depleted in FeO and MgO, similarly to the dike rocks. The K2O contents of MI glass vary less widely than those of the rocks. The maximum K2O content of glass in the inclusions does not exceed 4 wt.%, and that of the rocks reaches 8 wt.% (Table 4, Fig. 9, e, f). In most of the samples, the MI in quartz from the RM dike rocks and granites are poorer in K2O than the corresponding rocks (Fig. 10). Like the dike rocks, the MI in granite quartz are rich in K; in most cases, with Na2O/K2O = 0.4–0.5. For the MI in quartz from the dike rocks, this ratio is 0.8–1 (Fig. 11). The P2O5 content in the bulk rock composition increases regularly from 0.1–0.2 wt.% in the granites to 0.7 wt.% in the URM ongonites. However, the MI do not follow this trend. Note that the inclusions are poorer in P2O5 than the corresponding rocks (Fig. 12). The F contents of the rocks and the MI vary irregularly from 0 to 1 wt.% (Table 4). In the norm composition of the analyzed inclusion glasses, the apatite content is 0.2–0.8 wt.%. It is important that, after the calculation of the apatite norm, MI glass contains up to 2.1 wt.% P2O5. The same was observed for the bulk composition of the rocks in the East Kalguty dike belt. The compositions of the MI from the phenocrysts in the rocks of the East Kalguty belt correspond precisely to “the albite trend” of fractionation, without division into the elvan and ongonitic branches (Fig. 13, a). In the orthoclase–quartz– albite–H2O diagram, the ongonitic field is almost filled with compositional points for the MI. However, part of them plot within the elvan field, and the other one lies outside the typical compositional fields of these rocks (Antipin et al., 1999, 2002; Kovalenko and Kovalenko, 1976). The points for the bulk compositions of the dike belt rocks plot more closely. The elvan composition points plot within the elvan field, and the

Fig. 8. Homogenization temperatures of the MI and FI in quartz from the rocks of the East Kalguty dike belt. The Thom of the MI at 1 kbar is given. 1, majorstage granites; 2, ongonites; 3, elvans; 4, URM ongonites and elvans.

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E.N. Sokolova et al. / Russian Geology and Geophysics 52 (2011) 1378–1400 Table 2. Microthermometry of the fluid inclusions Sample

Rock

Thermometry Teut, °C

RS Tm ice, °C

C, wt.% NaCl equiv

Thom, °C

Tm CO2, °C

Thom CO2, °C

CCO2, wt.%

n

CO2

N2 –

5-699

Granite

–24 to –19

–2 to –5

4–8

170–190

–

–

–

–

1-448

Granite

–23 to –19

–2 to –3

3–5

150–170

–

–

–

N.a.

22

25

1-173

Granite

<–19

–3.8 to –6.1 6.5–10

178–198

–

–

–

N.a.

10

kl-17k

Granite

–19 to –24

–3.3 to –4.3 5

180

–

–

–

N.a.

5

kl-14

Ongonite

–20 to –27

–3.3 to –3.9 5.9–6.5

175–180

–

–

–

N.a.

15

<–17

–

–

>200 with CO2

–57 to –56.8

27–29

21–22

+

+

kl-15

Ongonite

–20 to –24

–3.1

5

190–212

–

–

–

N.a.

12

5-698

Ongonite

–17.8 to –25.8

–4.2

7

160

–

–

–

N.a.

7

1-262-3

Ongonite

<–9

–3 to –4

5–6.8

–

–

–

–

N.a.

3

kl-17

Elvan

–20 to –21

–3.6 to –4

5.9–7

150–157

–

–

–

N.a.

9

5-692

Elvan

<–19

–3 to –5

5–8

200

–

–

–

N.a.

7

5-446

Elvan

–21.8

–3 to –4

5–6.8

>200

–

–

–

N.a.

l-765

Elvan

–23 to –19 –29 to –34

–2 to –3 –23 to –10

3–5 11–18 (MgCl2 equiv)

130–157

–

–

–

+

ku-8

Elvan

–19 to –24

–4 to –4.5– 1.1 to –1.7

6.8–7 1.7–3

180–190

–

–

–

N.a.

–

–

–

>200 with CO2

–56.7 to –56.8

25–28

9–10

1-270

3 +

Greisenized elvan

<–13 to –18

–3 to –8

5–12

180–210

–

–

–

N.a.

–

–

–

230–250 with CO2

–57.3 to –57.1

27.7–30.8

9–10

+

kl-209

URM elvan

<–13 to –17

–2 to –3

3–5

130–150

–

–

–

N.a.

kl-142

URM ongonite

<–10

–1.5

2

150–167

–

–

–

N.a.

kl-28

Greisen

–19 to –24

–2 to –5

3–8

140–160 170–190

–

–

–

N.a.

–

–

230–250 with CO2

–56.6 to –56.8

27.4–29.2

8

+

18

11

9 – 4

49 –

Note. Teut, Eutectic temperature; Tm, melting temperature; Thom, homogenization temperature; C, content; RS, Raman spectroscopy (+, present; –, absent); n, number of studied fluid inclusions. Dash, No data available. N.a., Not analyzed. Table 3. Microthermometry of the fluid inclusions coeval with the melt ones and estimates of the inclusion entrapment pressure Sample

Rock

C, wt.% NaCl equiv

Thom, °C

Pressure estimate, kbar

FI

MI

1

2

3

1-448

Leucogranite

5

170

640

5.4

8.7

8.1

1-270

Greisenized elvan

8–12

200

640

4.5

7.5

7–7.8

kl-17

Elvan

3-6

145

650

5.6

9.8-9.9

9-9.5

kl-209

URM elvan

5

150

600

5.3

8.6

8.2

l-765

Elvan

3

157

610

5.6

8.7

7.9

Note. Pressure, according to various authors: 1, (Roedder, 1984); 2, (Zhang and Frantz, 1987); 3, (Bodnar and Vityk, 1994).

ongonitic ones plot within the ongonitic field (Fig. 13, b). Most of the glass composition points of the MI plot within the quartz stability field if we accept the position of the cotectic curve for 5 kbar (Huang and Wyllie, 1975). The trace element contents of MI glass are shown in Table 5. The REE contents of the MI are extremely low (up

to 3–6 ppm). Their average REE and trace element contents are lower than those of the dike rocks. Similarly to the bulk URM rocks, the MI from their phenocrysts have the lowest REE contents, whereas MI glass is enriched in rare metals (Cs, Rb, Nb, Ta). Their content reaches a maximum in inclusions from the URM rock phenocrysts (kl-209, Table 5).

0.06

0.05

12.78

0.69

0.06

0.13

0.10

3.14

3.46

0.09

0.18

0.01

–

0.34

93.04

8

TiO2

Al2O3

FeO

MnO

MgO

CaO

Na2O

K2O

P2O5

BaO

Cs2O

Rb2O

F

Total

n

4

93.88

0.29

–

0.00

0.04

0.13

4.05

4.55

0.14

0.12

0.06

0.69

12.10

0.05

72.08

0.08

–

0.00

0.04

0.14

0.70

1.30

0.11

0.10

0.01

0.32

2.29

0.04

3.80

s

4

92.13

0.47

–

0.01

0.00

0.37

3.37

2.51

0.25

0.09

0.23

0.42

11.96

0.01

72.89

X

l-765

0.14

–

0.01

0.00

0.20

0.28

0.88

0.17

0.09

0.05

0.06

2.90

0.02

1.31

s

Note. n, Number of melt inclusions analyzed. Dash, Not analyzed.

0.05

–

0.02

0.14

0.06

0.21

1.65

0.05

0.19

0.02

0.38

1.59

1.94

X

X

72.78

kl-17

kl-17k

s

Elvans

Granites

SiO2, wt.%

Component

2

90.00

0.31

0.025

0.01

–

0.14

3.53

0.76

0.10

0.24

0.02

0.66

11.65

0.03

72.71

X

5-692

1

93.27

0.41

–

0.04

–

0.48

2.94

2.01

0.13

0.07

0.24

0.37

12.93

0.01

73.94

X

ku-7

3

90.04

0.57

–

0.00

0.00

0.23

2.93

2.15

0.17

0.10

0.12

0.13

11.06

0.08

72.88

X

ku-8

5

93.50

0.35

0.03

0.00

0.04

0.21

3.69

2.77

0.21

0.27

0.03

0.79

12.16

0.14

73.14

X

1-270

0.12

0.04

0.00

0.03

0.20

0.51

1.39

0.12

0.31

0.02

0.48

1.45

0.12

3.19

s

Greisenized elvan

2

93.08

0.43

–

0.02

–

0.52

2.98

2.52

0.07

0.06

0.37

0.46

11.90

0.01

74.21

X

ku-6

9

90.97

0.58

–

0.00

0.06

0.38

3.37

2.28

0.47

0.27

0.17

0.79

13.64

0.08

69.43

X

1-262-3

Ongonites

0.22

–

0.01

0.06

0.31

0.33

1.47

0.36

0.36

0.07

0.59

0.73

0.08

3.26

s

2

91.37

0.42

–

0.01

0.00

0.07

3.80

2.78

0.10

0.34

0.12

1.28

12.19

0.16

70.43

X

kl-15

Table 4. Compositions of quenched glass in homogenized melt inclusions in the quartz phenocrysts from the rocks of the East Kalguty dike belt

2

89.89

0.53

–

0.02

0.00

0.15

3.76

3.72

0.21

0.23

0.12

0.89

12.70

0.05

68.10

X

kl-16

6

93.90

0.26

–

0.01

0.07

0.21

3.75

3.33

0.23

0.07

0.08

0.44

13.84

0.05

72.24

X

5-447

0.10

–

0.01

0.10

0.09

0.30

1.30

0.10

0.03

0.05

0.09

0.50

0.01

1.06

s

6

90.43

0.37

–

0.04

–

0.36

3.15

2.34

0.31

0.10

0.19

0.55

11.76

0.04

71.65

X

kl-209

0.13

–

0.01

–

0.09

0.42

0.86

0.17

0.06

0.03

0.18

1.01

0.01

2.58

s

URM ongonites URM elvans

0.16

0.025

0.032

0.11

0.007

0.01

0.024

0.032

0.015

0.023

0.024

0.013

0.041

0.008

Detection limit, wt.%

1390 E.N. Sokolova et al. / Russian Geology and Geophysics 52 (2011) 1378–1400

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E.N. Sokolova et al. / Russian Geology and Geophysics 52 (2011) 1378–1400

Table 5. Water (wt.%) and trace element (ppm) contents of quenched glass in homogenized melt inclusions in the quartz phenocrysts from the rocks of the East Kalguty dike belt Component

1-270

kl-17

kl-209

5-447

S, rel.%

H2O

2.14

0.93

3.44

2.88

4–11

Cs

13.29

35.73

123.61

28.54

7–39

Rb

153.60

190.54

334.79

168.49

10–34

Ba

56.72

1.10

4.56

3.89

11–39

Th

1.65

0.22

0.56

0.10

12–56

Ta

0.63

2.43

5.76

0.77

10–37

Nb

11.50

21.58

29.35

4.43

5–17

Sr

6.87

1.25

3.32

5.31

18–30

Hf

0.69

0.66

0.32

0.06

52–142

Zr

14.02

1.76

7.84

0.98

7–46

Y

2.00

1.07

0.99

0.24

9–51

La

0.75

0.38

0.33

0.15

26–47

Ce

1.69

0.76

0.74

0.24

14–60

Nd

0.78

0.58

0.21

0.12

20–130

Sm

1.22

0.30

0.63

0.27

110–156

Eu

0.03

0.19

0.09

0.17

120–200

Gd

0.28

0.06

0.25

0.21

80–230

Dy

0.36

0.28

0.22

0.34

26–136

Er

0.58

0.31

0.25

0.20

102–228

Yb

0.32

0.14

0.12

0.02

62–223

Note. Sample kl-17, elvan; 1-270, greisenized elvan; kl-209, URM elvan; 5-447, URM ongonite. S, Error of element determination.

The water content, according to ion microprobe analysis, is 0.93–3.44 wt.% (Table 5), whereas the light- and volatileelement contents, which can be estimated from the analytical total deficiency, are 7–10 wt.% (Table 4). The use of external water pressure in MI heating experiments prevents significant volatile loss. Therefore, the total deficiency of the electron probe analysis should be attributed to the high volatile content. Apart from H2O, this deficiency might be caused by CO2. However, the absence of carbon dioxide or carbonate daughter phases from the MI and coeval FI suggests that the entire deficiency depends on the water content, which is not determined by electron probe analysis. In this case the water content determined by ion microprobe analysis might be underestimated. On the basis of the deficiency, the water content of most of the analyzed inclusions (Table 4) is estimated at 6–7 wt.% (sometimes up to 10 wt.%).

Discussion The experimental data on inclusions of the mineral-forming medium obtained in this study provide an insight into the composition of the medium as well as the temperature and pressure of the crystallization of the magma which gave rise to the East Kalguty ongonite–elvan dike belt and the Kalguty Mo–W deposit.

PT-conditions for quartz crystallization in the dike rocks. First, note that the dike rocks show evidence for crystallization at high fluid pressures: muscovite phenocrysts; inclusions of a relatively high-density magmatic aqueous fluid; MI with decrepitation halos. In the last case, the cracks resulting from the decrepitation are healed and contain inclusions of the fluid phase segregated from the trapped melt. The MI are completely crystallized; this is not typical of subvolcanic intrusive rocks (Roedder, 1984; Sobolev and Kostyuk, 1975). According to the microthermometry, pressure was estimated from the isochores of aqueous solutions, which belong to the NaCl–H2O system, with 3–12 wt.% NaCl, at 600– 650 °C. These parameters correspond to the accompanying FI and MI, or those combined with the magmatic crystalline phases trapped by quartz. After the MI entrapment temperatures had been plotted to the calculated isochores, the fluid pressure during the phenocryst crystallization was estimated at 4.5–5.5 kbar (Potter and Brown, 1977; Roedder, 1984) (Table 3). Pressure estimates on the basis of the data from (Bodnar and Vityk, 1994; Zhang and Frantz, 1987) yielded unreal values (Table 3). The coexistence of muscovite phenocrysts with 1–2 wt.% F and quartz in the rock with ~0.5 wt.% F limits the minimum possible pressures of the magma crystallization to 3 kbar (Berman, 1988; Johannes and Holtz, 1996; Wyllie and Tuttle, 1961; Wyllie et al., 1996). The presence of decrepitation halos

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Fig. 9. Variation diagrams of the major-element contents of the East Kalguty belt rocks (a, c, e) and MI glass (b, d, f). 1, major-stage granites with MI glass; 2, ongonites with MI glass; 3, elvans with MI glass; 4, URM ongonites with MI glass. The bulk rock compositions are after (Annikova, 2003) and this study.

E.N. Sokolova et al. / Russian Geology and Geophysics 52 (2011) 1378–1400

Fig. 10. Averaged K2O contents of glass in the homogenized MI and in the bulk compositions of the corresponding rocks. See legend in Fig. 9. Dash-and-dot line shows the equal K2O contents of the rocks and MI. The bulk rock compositions are after (Annikova, 2003) and this study.

around the MI suggests that the internal pressure in the inclusions exceeded the external pressure by a value larger than the tensile strength of the host mineral. These conditions might have accompanied the entrapment of inclusions at depth and the transport of mineral grains to the surface. Since the decrepitated inclusions might reach 20–30 µm in size, the calculated ultimate stress σi, at which brittle deformations begin in quartz, is 2–2.5 kbar (Tait, 1992). Cracks in quartz develop around an inclusion larger than 20–30 µm when the pressure difference within the inclusion and outside the crystal ∆P = 2σi is up to 4–5 kbar. To determine the initial entrapment pressure, which is approximately equal to that within the inclusion at the moment of decrepitation (Tait, 1992), assume that the pressure during the formation of the dikes was 1–1.5 kbar. In this case the entrapment pressure is estimated at 5–6.5 kbar. The crystallization pressure of RM magmas in other regions is estimated at lower values. For example, the pressures were 1.4–4.2 kbar for the Ongon Hayrhan ongonites and Li–F granites (Naumov et al., 1982), at least 3–4 kbar for the Ary-Bulak stock (Erokhin et al., 1989), 3.8–4.2 kbar for the early leucogranites, and 3.2–3.6 kbar for the late-rhythm spodumene aplites of the Kungurdzhara pluton (Gorny Altai) (Vladimirov et al., 1998). The correlation between the composition of glass in quartz from the dike rocks of the East Kalguty belt and the position of the cotectic curve in the water-saturated haplogranitic system yielded pressure estimates of 2–4 kbar (Titov et al., 2001). These data contradict the estimates in the present paper, because the position of the cotectic curve might change greatly under the joint effect of F, P, and trace elements, which are important components of RM granitic systems (Glyuk and Trufanova, 1977; London et al., 1993). The relatively high pressure estimates for the subvolcanic rocks of the East Kalguty belt are explained by the fact that quartz is represented by intratelluric phenocrysts, which formed at considerably greater depths than those of the dike

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Fig. 11. Histogram of the Na2O/K2O ratio in MI glass. 1, major-stage granites of the Kalguty pluton; 2, ongonites and elvans of the East Kalguty dike belt.

emplacement. This agrees with the MI compositions that plot in the diagram for the haplogranitic system, within the field where quartz is the first mineral crystallizing from the melts (Fig. 13). Also, summarized geological, geophysical, isotopic, and geochemical data show that the source of the ongonite– elvan melts might have been located at the root of the Kalguty granite–leucogranite pluton at a depth no shallower than 10–11 km from the present-day erosion surface (Annikova, 2001, 2003). Comparison of the Thom of the MI obtained in the autoclave heating at different confining pressures shows that Thom at atmospheric pressure regularly exceeds Thom at 1 kbar by an average of 20–40 °C and is 10–25 °C higher at 1 kbar than at 2.5 kbar in the same samples. This is because an inclusion heated at atmospheric pressure is larger than at confining pressures of 1 or 2.5 kbar. The tensile modulus of a silicate melt is known to be smaller than that of crystals by an order

Fig. 12. Averaged P2O5 contents of glass in the homogenized MI and in the bulk compositions of the corresponding rocks. See legend in Fig. 9. Dash-and-dot line shows the equal P2O5 contents of the rocks and MI. The bulk rock compositions are after (Annikova, 2003) and this study.

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Fig. 13. Compositions of MI glass (a) and the East Kalguty belt rocks (b) in the orthoclase–quartz–albite–H2O diagram. 1, compositions of glass in the MI from granite-, elvan-, and ongonite-hosted quartz (Titov et al., 2001); 2, compositional field of the elvans; 3, compositional field of the ongonites (Antipin et al., 1999, 2002; Kovalenko and Kovalenko, 1976). See Fig. 9 for the rest of the legend. Lines show cotectics at an elevated H2O pressure (Huang and Wyllie, 1975).

of magnitude (Tait, 1992). Therefore, to estimate numerically the stretching of the inclusion cavity, we considered the stretching and compression of host quartz in thermometrical experiments. The change in the volume of the inclusion cavity under normal conditions was taken to be zero. For the experimental conditions (atmospheric pressure, 1 kbar, 2.5 kbar; average temperature 650 °C), the volume effect (volume increase or decrease ∆V) was calculated as a difference between the thermal expansion (∆VT) and compression under pressure (∆VP) of quartz, with regard to the polymorphic-transition temperatures as functions of pressure (Clarke, 1966; Deer et al., 1966). The thermal expansion coefficient of quartz at temperatures above 600 °C varies only slightly; this permits calculating volume effects for an average temperature of 650 °C. At all the pressures considered, the stretching exceeds the compression by several percent (Table 6). Evidently, the determination of the real entrapment temperature of the MI requires a temperature correction for the host mineral stretching corresponding to the temperature rise required for the melt to fill additional volume. The temperature correction was found by V.P. Chupin and O.N. Kosukhin (1982) from experiments on the temperature difference between complete homogenization and heterogenization as well as from the size of the shrinkage bubble. The temperature correction is 30–50 °C at 5–7 kbar with respect to atmospheric pressure. Additional stretching might be due to the internal pressure in the fluids within the inclusions during heating, which “stretches” the inclusion cavity (Chupin et al., 1975; Firsov, 1988). With the correction for the entrapment pressure, the crystallization temperature of the quartz phenocrysts in various RM dikes of the East Kalguty belt might have been 600– 650 °C. In the URM rock samples, Thom and, correspondingly, the entrapment temperature of the MI, is 20–30 °C below that

of the MI from the phenocrysts in the RM rocks; this confirms and supplements the previous thermometrical data on a kalgutite sample (Titov et al., 2001; Vladimirov et al., 1998). A 30 °C decrease in the estimates of the crystallization temperature will reduce the pressure estimate by 0.1–0.2 kbar, which is within the calculation accuracy. In general, the obtained crystallization temperatures correspond to those determined previously for similar RM systems in Mongolia, Transbaikalia, and Cisbaikalia (Antipin et al., 2009; Chupin et al., 1994; Kuznetsov et al., 2004; Naumov et al., 1982; Peretyazhko et al., 2007). They all have low crystallization temperatures as compared with the water-containing haplogranitic system (Kovalenko, 1979), which is due to the high content of volatile and fluxing components in the RM granitic melts. On the other hand, it was pointed out (Vladimirov et al., 1998) that the kalgutites and elvans of the East Kalguty belt had significantly higher crystallization temperatures than the major-stage granites of the Kalguty pluton, the spodumene granites of the Chindagatui pluton, Kungurdzhara pluton, and Alakha stock (southern Altai) as well as individual Ongon Hayrhan dikes (Naumov et al., 1982; Titov et al., 2001). The overheating of the melt under the effect of high-temperature magmas, probably of mantle origin, is regarded by A.V. Titov et al. (2001) as a possible cause of the magma crystallization at higher temperature. However, the crystallization temperature of magma is known to depend on its composition and pressure. Thus, the overheating of the melt by an external heat source will slow down the crystal growth in it or dissolve the crystals, but not increase the crystallization temperatures. We explain the crystallization of the quartz phenocrysts at higher temperatures than that of the other Li–F granites in the region by the low F content and elevated P content of magma. Phosphorus is known to be less efficient than F in decreasing

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Table 6. Volume effect for quartz depending on pressure and temperature as well as the homogenization temperatures of the melt inclusions during thermometry at corresponding pressures Parameter

Heating stage

Autoclave

Presumed natural environment

Pressure

1 atm

1 kbar

2.5 kbar

5 kbar

Temperature, °C

650

650

650

650

Quartz modification

β-quartz

β-quartz

β-quartz

α-quartz

Expansion ∆VT, vol.%

4.54

4.54

4.54

–

Compression ∆VP, vol.%

0

0.18

0.44

–

Volume effect (∆VT – ∆VP), vol.%

4.54

4.36

4.1

2.75

Thom, °C (kl-14)

685

665

645

–

Thom, °C (5-692)

700

660

635

–

Thom, °C (1-270)

710

660

650

–

Note. The calculation was done on the basis of expansion and compression constants (Clarke, 1966) and with regard to the quartz α-β transition temperature at various pressures (Deer et al., 1966). α-quartz, Low-temperature, high-pressure modification; β-quartz, high-temperature, low-pressure modification. Homogenization temperatures are given for the smallest melt inclusions (1–5 µm). Dash, No data available.

the solidus temperature of aluminosilicate melts (London et al., 1993). The crystallization temperatures of the quartz phenocrysts in the rocks of the East Kalguty belt were previously estimated (Titov et al., 2001) at 670–700 °C for minute inclusions in the RM rocks by the quenching technique without confining pressure and at 630–650 °C in the URM rocks; this is higher than the temperatures determined in the present paper. As was shown, this might be because an increase in the entrapment pressure of the inclusions by several kilobars increases Thom in the experiment by several tens of degrees. Magmatic melt compositions. The results show that the glass obtained from the complete remelting or homogenization of the MI has the same major-element content as the dike rocks. This permits regarding the studied inclusions as representative of the melt. At the same time, the minor- and trace element contents are considerably, sometimes by orders of magnitude, lower than those of the dike rocks. The causes of this difference might be the overheating of the inclusions, which results in the dissolution of excess silica from the cavity walls in the melt or compositional difference between the melts from which quartz crystallized and the dikes were produced. To test the first possibility, consider that the content of excess SiO2 with respect to that of the rocks is 10–20 wt.% when the composition of quenched glass is recalculated for the “quartz,” “albite,” and “orthoclase” norm compositions. When this amount of SiO2 is added to the bulk rock composition, the minor and trace element contents decrease by ~10 rel.%, which is close to (or, sometimes, less than) the error of the determination of their contents. Thus, the inclusion overheating cannot give the deviations observed, and they should be attributed to the characteristics of the trapped melts. Considering these calculations, the resulting minor-element contents can be used for further studies. For example, the analyzed glass from the MI is enriched in Cs, Rb, Nb, and Ta, similarly to the dike rocks, and the contents of these

elements exceed the clarkes by one or two orders of magnitude (Rudnik and Gao, 2003). The contents of these elements in MI glass are close to those in the dike rocks. Also, the accessory minerals include Ta, Nb, and Li minerals: columbite-tantalite, polycrase-euxenite, tourmaline, and montebrasite (Fig. 4). So, the melts trapped by the quartz phenocrysts are compositionally close to the RM granites. However, apparently, these melts correspond to an earlier stage in the magma chamber evolution than the dike rocks. This determines lower RM contents than those of the rocks. The low REE contents of the inclusion melts can be explained by the preliminary crystallization of monazite, which is the main REE concentrator in the dike rocks. This is confirmed by the presence of MI attached to monazite inclusions in quartz. Apparently, monazite and the other REE minerals began to crystallize earlier than the studied quartz phenocrysts. The melts trapped in the inclusions are rich in P2O5 (up to 0.5 wt.%), similarly to the dike rocks. Thus, the parental melt was already rich in P at the moment of the quartz phenocryst crystallization. However, the P2O5 content of glass in the inclusions is, as a rule, lower than that of the corresponding rocks (Fig. 12). As pointed out previously, the elevated P content of the rocks is reflected in the abundance of phosphate minerals. However, the Ca and REE contents of the melts were certainly not enough to bond P completely into apatite and REE phosphates. This explains the excess of P2O5 after calculating the apatite norm. Thus, despite the crystallization of the phosphate minerals, the melt must have been enriched in P during the differentiation, and this is observed in the bulk rock composition (Annikova, 2003; Vladimirov et al., 2007). The lower P and rare-metal contents as compared with the dike rocks support the conclusion that the inclusion melts represent magma in the earlier evolutionary stages, which preceded the dike belt formation. The Nd isotope compositions of the dikes are intermediate between those of the metamorphic rocks in the Altai–Mongol terrane, which are the likeliest substrate for granitoid magmas,

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and the lamprophyre dikes of the Chuya complex, consisting of mantle magmas (Annikova et al., 2006). This suggests that the supply of mantle matter played a substantial role in the formation of the RM melts in the Kalguty OMS. Thus, the high P content of the dikes in the East Kalguty belt might be due to the influence of a mantle source (fluid syntexis). One of the most important problems in the genesis of granite-related OMS is the source of the ore elements. In studies of granitoid-related OMS, magma itself is believed to be the source of these elements (Barnes, 1979). However, the geochemical and genetic diversity of ore-bearing mineral assemblages requires special consideration of the ore-generating potential of granitic magmas in each case. The trace element contents of MI glass show that the melts were already enriched in these elements during the crystallization of the quartz phenocrysts. Note that glass in the quartz-hosted inclusions in the URM rocks can be substantially richer in rare metals (Cs, Rb, Ta, Nb) than that in the quartz-hosted inclusions in the RM rocks. Thus, the magma during the quartz crystallization was already divided into RM and URM. The MI in quartz from the URM rocks are poorer in SiO2, FeO, MgO, and REE than the inclusions from the RM rocks; this is the evidence that the URM rocks are more differentiated than the RM ones. The above comparison of the MI and bulk rock compositions shows that the parental melts of the quartz phenocrysts were different from those which formed the dike bodies. The quartz phenocrysts crystallized not at the dike emplacement level but deeper: in the RM magma chamber or in intermediate chambers located at greater depths. As pointed out, the coexistence of ongonites and elvans in the same igneous complexes is quite rare, and the East Kalguty belt is an example of this. It was shown that the elvans differed considerably from the ongonites in the elevated K2O content, whereas the K2O contents of the MI from elvan and ongonite quartz overlapped (Fig. 9, e, f). This suggests that the phenocrysts in the elvans and ongonites crystallized from melts with similar Na2O/K2O ratios. Moreover, the K2O contents of MI glass vary over quite a narrow range and are lower in most of the studied MI than in the corresponding rocks (Figs. 9, e, f, 10). The orthoclase–quartz–albite–H2O diagram shows “an albite trend,” with a decrease in SiO2 and the accumulation of Na2O, the K2O content being constant (Fig. 13). This evolution is typical of most RM granitic systems, including typical ongonites (Kovalenko, 1977). The rocks of the East Kalguty dike belt show a considerably smaller difference in the Na2O/K2O ratio than between typical elvans and ongonites (Fig. 3, a, b). The maximum K2O enrichment among the studied samples was observed in the dike rocks supposedly subjected to greisenization. These rocks have a lower Na content and secondary muscovite in the groundmass and show extreme enrichment in Zn, Cu, W, and Mo, which are typical of greisenized orebodies. Note that the elvan portion of the URM dike is confined to the central area, with well-developed orebodies and greisens (Dergachev, 1992). Moreover, the central area does not contain RM rocks which could be assigned to ongonites. The reported similarity

in Na2O/K2O between MI glass both from the elvans and ongonites suggests that the magma which formed the dikes of the belt was, most probably, similar to ongonitic magma (Kovalenko, 1979) and the increase in the K2O content should be attributed to later processes of uncertain nature. One of them might have been the supply of K to the magma by the ore-forming fluids. The water content of magma is one of the most important problems in studying hydrothermal magmatic mineralization. Studies of the MI in the quartz phenocrysts suggest that the parental melts of quartz contained ~6–7 wt.% water. The presence of aqueous FI coeval with the MI suggests that the water content of the melt corresponded to saturation. Thus, the quartz phenocrysts in the East Kalguty belt dikes crystallized in a heterogeneous medium consisting of crystals, represented by mineral inclusions, a silicate melt, represented by MI, and an aqueous fluid, represented by FI. The P–T–Xparameters of this medium were close to a water-saturated granitic solidus. Fluid phase composition. The presence of the fluid phase in the heterogeneous system during the crystallization of the quartz phenocrysts is reflected in the abundance of FI. However, the presence of hydrothermal products closely aged with the dikes requires caution in interpreting the data on the FI. Therefore, we interpret as magmatic FI only those accompanying the MI or magmatic crystalline phases. According to our data, the magmatic-fluid phase accompanying the crystallization of the quartz phenocrysts was a high-density homogeneous phase of the supercritical fluid. The microthermometry (Table 2) suggests NaCl as the likeliest solute in this fluid. The occasional appearance of CO2- and MgCl2-rich primary FI suggests that the fluid composition was not constant in the course of the quartz crystallization. However, we found no inclusions of this composition which were undoubtedly coeval with the MI. Thus, it is not clear whether these variations in the fluid composition belong directly to the magma chamber evolution or they are of different origin. In any case, it seems impossible to explain them, considering that the fluid segregated and evolved in a closed system. We think that fluids rich in carbon dioxide can enter the magma chamber from an outside source during the evolution of the RM granitic system, which formed the dike belt. This model seems quite reasonable, assuming that the dike belt formed simultaneously with the greisens and hydrothermal ore veins. The presence of a free fluid phase during the development of the East Kalguty dike belt poses the question about its possible genetic relationship with the fluids which formed W–Mo mineralization. According to our data, the inclusions in quartz from ore-bearing greisens do not differ from those in the dike phenocrysts in their microthermometrical parameters and phase composition. The inclusions in quartz from the ore veins of the Kalguty deposit are also close to them. The FI in quartz from the ore-bearing bodies contain aqueous or, less often, potassium–sodium chloride aqueous solutions with 5–13 wt.% NaCl equiv enriched in carbon dioxide (Ivanova et al., 2006; Potseluev et al., 2006, 2008). These solutions also

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contain calcium and magnesium hydrocarbonates, chlorides, and sulfates. Our observations revealed the similarity of the inclusions in quartz from the granites and dike rocks (Table 2). The only difference between them was the absence of liquid carbon dioxide in the inclusions from the granite quartz. The similar characteristics of the FI in quartz from the rocks of the Kalguty pluton, dike belt, and orebodies suggest that the fluid subsystem of the Kalguty magma chamber might have functioned for a long time and is related to the genesis of the Kalguty deposit. The relatively long history of hydrothermal-system development and compositional variation can be explained by its open character and the introduction of substance from outside sources. Petrogenetic model for the formation of the rare-metal rocks of the Kalguty ore-magmatic system. The similar major- and trace element geochemistry of the Kalguty pluton granites and the East Kalguty belt ongonites and elvans, as well as the compositional similarity of the MI and FI in the granite and dike minerals, suggest that the parental melts evolved in a single system. According to U–Pb and Ar–Ar dating, these two intrusive complexes are 10–15 myr apart. The parental magma which produced the dikes was already rich in P. The high P contents might be due to the supply of this element from a mantle source (presumably by fluid syntexis). In the early stage, P-containing accessory minerals (apatite, monazite, and others) crystallized from this melt; this determined the abnormally low REE contents of the melt. The melts were divided into RM and URM ones before the formation of the studied quartz phenocrysts. After the REE had been depleted and the melts had segregated into RM and URM ones, intratelluric phenocrysts appeared and trapped the inclusions at up to 5.5 kbar and 650–600 °C from magma consisting of a melt with 6–7 wt.% water and an aqueous fluid phase. The estimated pressure and the more primitive compositions of the melts than the bulk rock compositions (RM contents of the MI lower than those of the rocks) at the moment of the phenocryst crystallization suggest that the crystallization began in a deep-seated chamber located in the lower part of the Kalguty pluton. Judging by the MI composition, the melts were ongonitic (Na dominated over K). The evolution of the melt was controlled by “the albite trend” of fractionation, typical of ongonitic magma. At the same time, the predominance of elvan (not ongonite) dikes in the present-day erosion surface shows that a considerable part of K might have entered crystallizing granitic magma after the formation of the studied quartz phenocrysts, but before the end of the dike crystallization. The variations in the salt and gas compositions of the primary FI in the quartz phenocrysts are evidence for an opened RM magmatic system. The change in the compositions of the melts and fluid phase which formed the East Kalguty dike belt can be explained by substance exchange owing to the close spatial and temporal relationship between the magmatic and hydrothermal systems of the Kalguty OMS.

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Conclusions The studies of the inclusions in the quartz phenocrysts from the rocks of the East Kalguty dike belt have shown that the phenocrysts crystallized in a heterogeneous medium consisting of a silicate melt, crystals, and, predominantly, an aqueous fluid. The P–T–X-parameters of the mineral-forming medium might have been close to a water-saturated granitic solidus. The melts trapped in the quartz phenocrysts are rich in RM and P. However, the low contents of some LILE and P with respect to those in the rocks suggest that these melts correspond to an earlier stage in the magma chamber evolution than the dike belt rocks. During the phenocryst formation, the division into the RM and URM melts already existed. Note that the elevated contents of some LILE and the low Si, Fe, Mg, and REE contents of the MI in the phenocrysts from the URM rocks suggest that they correspond to deeper magma differentiation. The parental melts of the ongonite and elvan phenocrysts were not divided into well-defined potassic or sodic ones. “The albite trend” of evolution and similar Na2O/K2O ratios in the MI suggest that the melts that formed quartz in both rock types were most probably ongonitic. The increase in the K2O content probably occurred in a later process. This process was not reflected in the MI compositions but manifests itself in the compositions of the RM and URM dike rocks of the East Kalguty belt. It most probably involved the introduction of additional substance by the fluid phase. The first studies of the magmatic fluids in the Kalguty OMS have shown that the quartz phenocrysts crystallized in the presence of the high-density homogeneous phase of the supercritical fluid, represented by weak aqueous solutions. Variations in the volatile and salt compositions of the fluid can be explained by mixing with fluids from external sources during the magma crystallization. The estimates showed that the quartz phenocrysts might have formed at 3 to 4.5–5.5 kbar and 600–650 °C. The quartz phenocrysts in the URM rocks crystallized at a 20–30 °C lower temperature than those in the RM rocks. On the one hand, the relatively high pressures for hypabyssal rocks and less differentiated compositions of the MI than those of the rocks are evidence that the crystallization of the quartz phenocrysts occurred at deeper levels than the dike emplacement and from more primitive melts, undifferentiated into elvan and ongonitic ones. On the other hand, in the course of quartz crystallization, the melts already differ in their RM contents and crystallization temperatures. Also, the system was open to new matter, as is reflected in the FI and rock compositions in comparison with the MI. Geological observations and some compositional features of the rocks show that the Kalguty OMS is a complex and multistage system. Studies of the inclusions of mineral-forming media confirm that the late-magmatic fluid was related to the genesis of the Kalguty deposit, being related to the hydrothermal system, which might have determined the fairly complex history of its development and compositional variation.

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Acknowlegements. We thank Profs. V.N. Sharapov (Sobolev Institute of Geology and Mineralogy) and V.S. Antipin (Vinogradov Institute of Geochemistry) for detailed reviews, which helped us improve the manuscript, and Dr. N.N. Kruk (Sobolev Institute of Geology and Mineralogy), for useful discussion of the main ideas of the paper. Also, our thanks go to Dr. E.I. Petrushin (Sobolev Institute of Geology and Mineralogy) for heating the inclusions at atmospheric pressure and to Dr. V.G. Tomas (Sobolev Institute of Geology and Mineralogy) for the autoclave heating of the MI. The electron microprobe analyses of glass from the MI were done by Drs. L.N. Pospelova and E.N. Nigmatulina (Sobolev Institute of Geology and Mineralogy). The SIMS analysis of glass from the inclusions was done by Drs. S.G. Simakin and E.V. Potapov (Institute of Microelectronics and Informatics, Yaroslavl). The study was supported by the Russian Foundation for Basic Research (grant no. 10-05-00913) and the Presidium of the Russian Academy of Sciences (Siberian Branch of the Russian Academy of Sciences) (project ONZ-9.3).

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