ISSN 10757015, Geology of Ore Deposits, 2013, Vol. 55, No. 7, pp. 515–524. © Pleiades Publishing, Ltd., 2013. Original Russian Text © S.L. Votyakov, V.V. Khiller, Yu.V. Shchapova, Yu.V. Erokhin, 2012, published in Zapiski Rossiiskogo Mineralogicheskogo Obshchestva, 2012, No. 3, pp. 16–28.

Composition and Chemical Microprobe Dating of U–ThBearing Minerals. Part 2. Uraninite, Thorite, Thorianite, Coffinite, and Monazite from the Urals and Siberia S. L. Votyakov1, V. V. Khiller, Yu. V. Shchapova, and Yu. V. Erokhin Zavaritsky Institute of Geology and Geochemistry, Ural Division, Russian Academy of Sciences, Pochtovyi per. 7, Yekaterinburg, 620075 Russia Received July 19, 2011

Abstract—To develop further chemical microprobe timing of U–Thbearing minerals on the basis of upgraded measurement techniques and special age calculation, uraninite, thorite, thorianite, coffinite, mon azite from several localities in the Urals and Siberia have been dated. The samples were taken from granitic rocks of the Pervomaisky pluton in the Central Urals; the preJurassic basement of western Siberia and Yamal Peninsula; carbonatitelike dolomite from the Karabash ultramafic massif in the Southern Urals; granitic pegmatites of the Lipovsky vein field; and quartz–sulfide veins of the Pyshma–Klyuchevsky Cu–Co–Au deposit in the Central Urals. Scrutiny of the composition and chemical heterogeneity of mineral grains is a necessary stage of chemical dating aimed at the estimation of the degree of closeness of the U–Th–Pb system and unbiased screening of analytical data. The condition (Si + Ca)/(U + Th + Pb + S) ~ 1 was used as evi dence for significant secondary alteration of monazite; the negative correlation between Pb and Th or U +Th in uranitite was used for the same purpose. The positive correlation between Pb and U, along with low con centrations of Ca, Si, and Fe admixtures, implies that the stoichiometric composition of thorite is close to 100%. The reliability and accuracy of the chemical dating of minerals with high contents of radioactive ele ments can be enhanced by using bimineralic or multimineralic isochrons, e.g., monazite–uraninite, ura ninite–coffinite, etc. The results obtained have been compared with the available isotopic ages of the studied minerals; the compared data are satisfactorily consistent. DOI: 10.1134/S107570151307012X 1

INTRODUCTION

This publication continues the reports on the results of our study concerned with chemical micro probe dating of U and Thbearing minerals from the Urals and Siberia on the basis of upgraded measure ment techniques and special age calculation. The first part of the paper (Votyakov et al., 2012) was concerned with monazite, which is used most widely in chemical microprobe timing. This mineral is enriched in Th and U (up to 3–15 and 5 wt %, respectively). Radiogenic lead is gained in monazite rather fast, and over less than 100 million years reaches a level at which it can be measured on microprobe with high locality and accu racy. The content of nonradiogenic Pb is minimal in comparison with radiogenic Pb. Postcrystallization losses of U, Th, and Pb are low because of rather good retention of the mineral structure under effect of self radiation. As a result, the measured U–Pb and Th–Pb ages are in many cases concordant. Thus, the tech niques used for the chemical dating of monazite are sufficiently substantiated. The total amount of Pb in the mineral is determined by U and Th concentrations and by the time of the system closure. 1 Corresponding

[email protected]

author

S.

L.

Votyakov.

Email:

direc

On the contrary, the chemical dating of minerals with high contents of radioactive elements (uraninite, thorite, thorianite, coffinite) meets problems related to the significant autoradiation destruction, the occur rence of anionic and cathionic vacancies, hydration, and the incomplete closeness of the U–Th–Pb sys tem. Publications on the dating of thorite and coffinite are not numerous (Parslow et al., 1985; Enami et al., 1993; Forster et al., 2000; Jercinovic et al., 2002; Tracy, 2002; Cocherie and Legendre, 2007). As a rule, these minerals are not stoichiometric; adsorbed molecular water or hydroxyl groups can occur; grains are small in size, and zones of homogenic composition therein are limited; nanosized microfratures and inclusions cannot be identified in BSE images; oxi dized U6+ is present. Because of the above circum stances, microprobing of these minerals and their dat ing present a number of problems (Pointer et al., 1998; Hansley and Fitzpatric, 1989; Forster, 2006). Publica tions on the chemical dating of uraninite are more numerous; however, a high (3–10 to 20 wt % in older samples) content of radiogenic lead in uraninite gives rise to an accumulation of significant structural stresses (Janeczek and Ewing, 1992c, 1995) and auto oxidation of uranium (Frondel, 1958) from U4+ to U6+

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with increase in U6+/U4+ ratio from 0.02 to 0.75 in natural uraninites and reduction of lead to Pb2+. As a result, the structure of uraninite is destabilized; min eral is recrystallized and loses geochronological infor mation (Janeczek and Ewing, 1992a, 1995; Kotzer and Kyzer, 1993). The content of impurities in the partly altered uraninite is rather high; a partial replacement of uraninite with coffinite is possible (Janeczek and Ewing, 1992b, 1992c). Kotzer and Kyzer (1993), Fayek et al. (1997), and Kempe (2003) proposed to use a negative correlation between Pb and U or U + Th and a positive correlation of Pb and U as chemical criteria of closure of the U–Th–Pb system. The elevated Ca, Si, and Fe concentrations, con versely, can be an indicator of secondary alteration of the mineral. The enhanced reliability and precision of chemical datings of minerals with high contents of radioactive elements can be ensured by using bi or multiminer alic isochrons with calculation of the age on the basis of analytical data for two, three, or more minerals (monazite–uraninite, uraninite–coffinite, etc.). Due to the significant dispersion of U, Th, and Pb contents in the bimineralic system, the isochron age is calcu lated with much less uncertainty than when calculated separately for each mineral. Note that this approach suggests similar formation conditions of all minerals, a comparable temperature of their U–Th–Pb system closure, etc. The objective of this work is to study the composi tion and chemical dating of U–Thbearing minerals, in particular, coexisting uraninite, thorite, thorianite, coffinite, and monazite from the Urals and Siberia and their chemical timing, including bimineralic isochron dating. Objects of research. Accessory monazite and uran itite were studied from granitic rocks of the Pervo maisky pluton in the Central Urals (sample of T.A. Osipov); uraninite, thorite, and monazite from the preJurassic basement of western Siberia and Yamal Peninsula (Hole 28r in the East Shebur area, Hole 10486, in the Okunevo area, and Hole 1 in the Upper Rechka area; the samples are from K.S. Ivanov); thorianite from carbonatitelike dolo mite of the Karabash ultramafic massif in the South ern Urals (sample of Yu.Y. Erokhin); uraninite and coffinite from granitic pegmatites of the Lipovsky vein field in the Central Urals (sample of Yu.Y. Erokhin); and uraninite from quartz–sulfide veins of the Pyshma–Klyuchevsky Cu–Co–Au deposit in the Central Urals (sample of V.V. Murzin). Analytical technique and calculation of age. The search for and identification of minerals, elemental mapping of grains, and quantitative analysis of their composition were performed on a Cameca SX 100 microprobe (Khiller, 2010; Khiller et al. 2011). The age of minerals was calculated using an original and a modified Isoplot 3.66 program in the frames of three

alternative approaches described in detail by Votyakov et al. (2012): (1) single determinations of the U, Th, and Pb contents and the calculation of N point values of the U–Th–Pb age and a weighted average value; (2) the Th/Pb–U/Pb isochron with calculated U/Pb and Th/Pb ages and weighted average; and (3) ThO* – 2

PbO isochron for highTh minerals or UO*2 –PbO isochron for highU minerals with the estimation of the Th*/Pb (U*/Pb age) and the fixed nonradiogenic Pb content at all points of the crystal. Here, U* is the modified U content equal to U + Thequiv; Thequiv is the Th content equivalent to the U content, which is able to produce the same amount of Pb during the lifetime of the mineral at equal U/Pb and Th/Pb ages (Khiller, 2010). RESULTS AND DISCUSSION (1) Monazite and uraninite from leucogranite of the Pervomaisky pluton, the Central Urals. The coexisting monazite and uraninite up to 100 and 20 μm in size, respectively (Fig. 1a), occur as accessory dissemina tions in the leucogranite, which is close to raremetal granites in its petrologic and geochemical attributes. Both minerals are identified throughout the rock matrix without forming intergrowths. The variable compositions based on 4 and 6 point analyses of mon azite and uraninite, respectively, are shown in the table. Following the published data (Kotzer and Kyzer, 1993; Fayek et al., 1997; Kempe, 2003), the composi tion of uraninite indicates an extremely low degree of secondary alteration. According to the microprobe results, the total oxide contents are higher than 97%. The positive PbO–UO2 correlation is combined with the negative ThO2 and UO2 correlation (Fig. 2); Ca and Fe admixtures have not been detected; the SiO2 content is intermediate (up to 0.82%). Taking these characteristics into account, the high degree of close ness of the Th–U–Pb system, as well as the correct ness of chemical datings, is suggested. The point determinations of the chemical U–Th– Pb age of monazite and uraninite grains yield 266–284 and 273–275 Ma, respectively; the bar charts of age distribution show that weighted average ages are 275 ± 23 Ma (MSWD = 0.04) and 274 ± 6 Ma (MSWD = 0.03), respectively. The isochron dating would be incorrect because of the small U and Th dispersion in mineral grains. Synchronism in the formation of mon azite and uraninite in leucogranite of the Pervomaisky pluton and the close weighted averages of point age determinations allows us to use this bimineralic system for the calculation of composite isochrons. The results presented in the PbO– UO* and Th/Pb–U/Pb plots 2

(Fig. 3) corroborate the correctness of the proposed approach: the regression lines obtained on the basis of all data points for monazite and uraninite are close to GEOLOGY OF ORE DEPOSITS

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COMPOSITION AND CHEMICAL MICROPROBE DATING Q

Mnz Ur

Ur Mnz

Mnz Q

Bi

100 µm

(a)

517

Zr

50 µm

(b)

50 µm

(c)

(e)

Zr

Ur Mnz 20 µm

25 µm

(d)

20 µm

YLa

Q

Bi

Th Ma

200 µm

(f)

Q Tr

Ur

Th

Ti (g)

50 µm

25 µm

(h)

Dol 25 µm

(i)

Q Xt

Ur

Kf

Zr Zr

(j)

50 µm

10 µm

(k)

Q

Ur Py (l)

Ur 25 µm

Fig. 1. BSE images (a–d, g–l), map of element distribution (e), and photomicrograph in transmitted light (f) of monazite, ura ninite, thorite, and coffinite from the Urals and Siberia: (a) monazite and uraninite in leucogranite of the Pervomaisky massif; (b) monazite; (c) monazite and uraninite in granite from the basement of Yamal Peninsula; (d, e) monazite, (f, g) uraninite, and (h) thorite in granitic rocks from the basement of western Siberia; (i) thorianite from the Karabash ultramafic massif, (j, k) ura ninite and coffinite from granite pegmatite of the Lipovsky vein field; (l) uraninite from quartz–sulfide vein at the Pyshma–Kly uchevsky deposit.

the theoretical isochrons. The U*/Pb age is 274 ± 4 Ma (MSWD = 0.07) and Th/Rb age is 277 ± 17 Ma and U/Pb age is 274 ± 4 Ma (MSWD = 0.08). Notice that when the age is calculated from bimineralic isochrons, uncertainties of their determinations are lesser in comparison with those of monomineralic isochrons. Isotopic datings of minerals from the Pervomaisky pluton are unknown for us. (2) Monazite and uranitite from granite of basement in the Yamal Peninsula (Hole 1, Upper Rechka area). Monazite in granite (Hole 1, 1748–2034 m) is com monly associated with biotite clusters and occurs as wellfaceted, short, prismatic individual crystals up to 100 μm long (Fig. 1b). Variable monazite composi tions (30 microprobe analyses) are given in the table. It is seen that the grains are heterogeneous and reveal distinctly zonal distribution of Th contents. Parameter β = (Si + Ca)/(Th + U + Pb) introduced by Suzuki GEOLOGY OF ORE DEPOSITS

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and Kato (2008) is 0.97–1.03 and thus indicates close ness of Th–U–Pb system in this mineral. The point determinations of monazite age cover a wide interval of 237 to 286 Ma, but their statistical processing shows that the mineral is chronologically homogeneous with a weighted average of 258 ± 11 Ma and MSWD = 0.13 (Fig. 3). Significant dispersion of Th contents in monazite grain makes it possible to cal culate an isochron. Being plotted in the ThO*2 –PbO and U/Pb–Th/Pb diagrams, the data points satisfac tory fall on the regression lines close to isochrons. The Th*/Pb and Th/Pb ages are 249 ± 28 (MSWD = 0.24) and 270 ± 20 Ma (MSWD = 0.28), respectively; the U/Pb age is estimated only qualitatively, with an uncertainty of ±118 Ma quite expectable for a highTh mineral. The Th*–Pb isochron practically goes through the point of origin. This implies that the content of non

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4.5 PbO, wt %

radiogenic Pb is negligibly small ( 1.05 have been screened out. The point age determinations of the given monazite range from 242 to 274 Ma (weighted average is 255 ± 16 Ma, MSWD = 0.30). Despite sig nificant dispersion of U, Th, and Pb throughout a monazite grain, the isochron age remains rather ambiguous because of elevated uncertainties in U, Th, and Pb determinations. This is due to timedelay inte gration of the signal, caused by the substantial burning of the grain surface under the electron beam. The Th*–Pb age is 275 ± 21 Ma (MSWD = 0.52); the weighted average age from the Th/U–U/Pb regression is 257 ± 10 Ma (Th/Pb and U/Pb ages are estimated only in qualitative terms). The obtained results do not come in conflict with concordant isotopic U–Pb zir con age of 277.5 ± 2.0 Ma estimated with SHRIMP II (Ivanov et al., 2010b). Accessory uraninite was found in granosyenite of the Okunevo area (Hole 10486, depth of 1734 and 1744 m) (Khiller and Erokhin, 2009), along with thor ite; compositions of these minerals are given in the

(a) PbO, wt % 3.5 3.0 2.5

(b) Th/Pb

T = 259 ± 4 Ma MSWD = 0.29

TU/Pb = 259 ± 4 Ma TTh/Pb = 260 ± 8Ma MSWD = 0.33

100

0.20

80

0.16

1.5

0.12

2.0

519

1.0 Uraninite

0.08 0.04

60

0.5

3.4 2

1.5

3

4

5

0 26

6

40

3.2

1.0

Xaver = 23.17 Yaver = 16.68

Monazite 3.0

0.5

20

27

Yaver

28

29

Uraninite

Isochron

Xaver

2.8

Monazite

84 86 88 90 92

0

10

20

30

40 50 60 UO2* , wt %

70

80

90 100

0

4

8

12

16 U/Pb

20

24

28

Fig. 5. Bimineral (a) PbO– UO *2 and (b) Th/Pb–U/Pb plots for uraninite and monazite from granites in basement of Yamal Pen insula. GEOLOGY OF ORE DEPOSITS

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1.12

Standard deviation

GEOLOGY OF ORE DEPOSITS

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14.02

2.03

Standard deviation

11.79–16.93

2.52

27.19

Average

La2O3 Range

Standard deviation

Average

24.89–30.35

29.13

Average

0.03

Standard deviation

27.31–30.40

0.12

Range

0.09–0.18

0.60

Standard deviation

Average

0.64

Average

Range

0.10–1.74

Range

1.08

Standard deviation

Ce2O3 Range

P2O5

PbO

UO2

8.41

7.56–9.84

monazite N=6

Average

ThO2 Range

Composition

2.62

9.65

5.37–16.31

monazite N = 40

0.88

2.87

2.05–4.94

uraninite N = 11

Basement of Yamal

No. 7

0.07

0.08

0.00–0.14

0.06

0.06

0.00–0.12

–

–

n.d.

0.04

3.34

3.30–3.38

1.11

87.87

1.04

12.79

11.20– 13.83

1.27

27.03

23.85– 26.82

1.36

27.44

24.39– 29.89

0.03

0.13

0.08–0.19

0.12

0.56

–

–

n.d.

0.15

0.19

0.00–0.41

–

–

n.d.

0.10

3.10

2.98–3.26

2.10

87.02

uraninite N=9

2.49

14.13

2.39

13.83

10.25– 17.24

1.89

26.08

22.82– 28.16

1.07

25.97

24.84– 28.22

0.04

0.16

0.09–0.20

0.19

0.31

0.15

0.45

0.20–0.67

0.76

3.25

1.89–3.77

–

–

n.d.

0.12

3.13

3.01–3.31

4.04

72.18

67.51– 78.73

3.30

11.86

8.27–17.77 7.38–15.32

monazite N = 11

–

–

n.d.

0.06

0.12

0.03–0.20

0.01

0.11

0.09–0.12

0.03

1.24

1.20–1.29

0.12

8.03

7.90–8.28

0.57

73.55

72.84– 74.32

thorite N=9

Basement of western Siberia

86.78–88.91 0.40–0.81 83.18–89.85 0.15–0.62

0.08

4.98

4.89–5.08

uraninite N=4

Pervomaisky pluton

Variations of local chemical compositions of U–Thbearing minerals, wt %

0.04

0.16

0.11–0.19

0.30

1.57

1.17–1.87

–

–

n.d.

0.18

1.87

1.61–2.04

3.15

23.82

18.34– 26.58

4.62

62.52

54.11– 67.54

thorianite N=9

Karabash massif

0.02

0.05

0.03–0.06

0.08

0.14

0.01–0.23

0.01

0.04

0.03–0.05

0.02

3.14

3.11–3.16

0.04

85.16

85.09– 85.20

0.05

5.55

5.46–5.59

uraninite N=6

–

–

n.d.

0.04

0.05

0.05–0.08

–

–

n.d.

0.11

2.49

2.37–2.60

2.36

67.47

65.12– 69.88

0.13

2.16

2.03–2.33

coffinite N=4

Lipovsky vein field

–

–

n.d.

0.14

0.20

0.00–0.51

–

–

n.d.

0.20

4.49

3.90–4.74

2.19

88.41

80.37– 91.31

0.41

0.49

0.02–1.13

uraninite N = 20

Pyshma– Klyuchev sky deposit

520 VOTYAKOV et al.

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Average

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1.41

0.03

Average

Standard deviation

99.84– 100.61

0.70–2.15

0.76

Standard deviation

Range

1.03

Average

0.95

Standard deviation

0.38–2.46

1.40

Average

Range

0.22–2.53

0.53

1.55

Range

Note. n.d., not detected.

Total

CaO

SiO2

Y2O3

Standard deviation

Average

0.85–2.27

2.58–3.02

Range

0.52

Standard deviation

Sm2O3 Range

Pr2O3

9.91

9.00–10.64

monazite N=6

96.71– 98.89

–

–

n.d.

0.07

0.73

0.65–0.82

0.02

0.74

0.71–0.75

–

–

n.d.

–

n.d.

–

–

n.d.

uraninite N=4

Pervomaisky pluton

Average

Nd2O3 Range

Composition

Table (Contd.)

98.49– 100.21

0.29

0.59

0.37–1.27

0.75

1.91

0.57–3.50

0.43

1.35

0.91–2.60

0.26

1.72

1.36–2.46

2.95

2.76–3.26

0.47

11.26

10.88– 12.57

monazite N = 40

95.12– 97.23

–

–

n.d.

0.20

0.21

0.09–0.68

0.90

1.95

0.40–3.67

–

–

n.d.

–

n.d.

0.12

0.22

0.00–0.46

uraninite N = 11

Basement of Yamal

98.13– 100.44

0.50

1.54

1.10–2.58

0.40

1.97

1.11–2.34

0.61

0.51

0.13–2.03

0.58

1.39

0.85–2.34

2.71

2.42–3.04

0.72

9.65

9.09–11.11

monazite N = 11

93.69– 95.83

0.09

0.36

0.30–0.59

0.04

0.02

0.00–0.11

0.04

0.97

0.92–1.04

–

–

n.d.

0.39

0.28–0.49

0.41

1.60

0.86–1.90

uraninite N=9

99.83– 100.90

–

–

n.d.

0.07

17.64

17.55– 17.75

0.06

0.20

0.06–0.25

–

–

n.d.

–

n.d.

–

–

n.d.

thorite N=9

Basement of western Siberia

94.08– 100.51

5.52

6.44

2.31–18.05

0.12

0.38

0.29–0.62

0.02

0.17

0.15–0.21

–

–

n.d.

–

n.d.

0.24

1.33

0.90–1.53

thorianite N=9

Karabash massif

96.47– 96.81

0.02

0.75

0.73–0.77

0.03

0.66

0.61–0.71

0.01

1.06

1.04–1.08

–

–

n.d.

–

n.d.

0.03

0.19

0.16–0.24

uraninite N=6

91.78– 97.66

–

–

n.d.

1.62

15.85

13.78– 17.62

2.00

7.25

5.33–9.84

–

–

n.d.

–

n.d.

–

–

n.d.

coffinite N=4

Lipovsky vein field

99.88– 100.42

0.33

0.83

0.51–1.49

0.34

0.27

0.02–1.18

0.67

1.59

0.24–2.64

–

–

n.d.

–

n.d.

–

–

n.d.

uraninite N = 20

Pyshma– Klyuchev sky deposit

COMPOSITION AND CHEMICAL MICROPROBE DATING 521

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VOTYAKOV et al.

sample. Uraninite occurs as round grains (Figs. 1f, 1g) up to 20 μm in diameter, incorporated into biotite flakes. These inclusions are surrounded by distinct pleochroic halos clearly seen in the micaceous matrix. Large grains are zonal: the ThO2 content in the core is increased up to 14–15 wt %, while at the margins it decreases to 7–9 wt %. The pronounced positive PbO–UO2 correlation is combined with negative ThO2–UO2 correlation (Fig. 2) as evidence for good preservation of the U–Th–Pb system of uraninite, which makes it possible to measure the correct age determinations on its basis. This statement is con firmed by extremely low SiO2 content (