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Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

   

A Method for Petrochemical Differentiation of Kimberlites from Other Igneous Carbonatite Associations

V.B. Vasilenko and L.G. Kuznetsova

Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
e-mail: vasilenko@ igm.nsc.ru

 

The tenuous view of carbonatites as rocks whose rock-forming minerals include nepheline is common even in experts. An example is the attribution of the Seligdar apatite deposit to the gneissmarble association (Mineralogical Map of the USSR, 1985). Our studies have proven that this deposit belongs to carbonatites (Vasilenko et al., 1982, 1994). The conclusion of the carbonatite nature of Seligdar apatites agrees with the data reported by Kukharenko (1966), who has shown that carbonatites are typical of platforms and regions of completed orogeny joined to platforms. There are two subfacies of platform carbonatites in central-type complexes: kimberlite (presumably deeper) and alkalineultrabasic (less deep). The carbonatites of completed orogeny regions are associated with rocks of the alkalinegabbro association. The depth of associated rocks is of great importance for carbonatite identification. We emphasize this statement because alkalineultrabasic magmas formed at depths of tens of kilometers, and kimberlites, at hundreds of kilometers.

An important feature of the rocks of these associations is that carbonate and silicate rocks are complementary. An excess of alumina and alkalis in rocks of alkalineultrabasic associations is compensated by the deficiency of calcium in silicate rocks. The excess of calcium in kimberlites is compensated by its deficiency in silicate rocks. It is pertinent to consider the classification of peridotites and ultrabasic foidolites (Bogatikov et al., 1981). The former are classified mainly according to CaO content, and the latter, according to Al2O3. In both cases, the high chemical affinity of calcium to carbonate is reflected. Uniform calcium leaching reactions occurred in peridotites and alkaline basaltoids saturated with aqueouscarbonate fluids of mantle plumes. At moderate pressures, the immiscibility of silicate and carbonate melts was apparent, but at high pressures, this phenomenon was absent. Nevertheless, all rock types contained silicate rocks, rocks transitional to carbonate, and carbonate rocks.

 

Table 1. Mean rock compositions.

Region, deposit

Rock association

n

SiO2

TiO2

Al2O3

Fe2O3

MgO

CaO

Na2O

K2O

P2O5

LOI

Carbonatite associations

Tomtor1

U/b with Al2O3

726

30.25

4.13

12.75

17.52

5.45

6.68

0.41

4.41

3.71

14.57

U/b with CaO

664

10.48

1.16

2.70

13.85

4.21

30.41

0.21

1.41

8.16

25.51

East Sayan2 etc

K-feldspar-

74

18.09

/

3.78

7.27

5.57

31.92

3.23

1.30

2.21

19.73

Albite-

63

13.79

/

3.11

10.74

3.96

36.43

1.50

0.75

3.43

23.32

Amph.dol.

30

6.41

/

1.27

4.97

3.53

43.88

0.80

0.70

2.83

33.74

Seligdar1

-rocks

752

10.69

/

1.39

3.82

11.64

23.22

0.17

0.34

4.81

24.60

Oshurkovskoe1

-rocks

19

39.91

3.71

10.25

11.01

7.24

18.17

2.00

2.06

7.31

2.35

Al-rocks

129

42.75

2.93

14.92

10.80

6.39

10.47

3.10

2.79

3.91

1.70

Unaltered kimberlites and picrites

 

Pipes

Kimberlite type

Yakutia1

Maiskaya

7

20.00

0.35

4.05

2.33

15.42

22.91

0.01

2.02

0.48

32.02

Nyurbinskaya

136

22.68

0.42

3.78

4.98

21.40

17.69

0.00

1.16

0.50

27.39

Botuobinskaya

108

26.63

0.51

3.39

5.69

25.68

13.87

0.01

1.16

0.54

22.40

Internatsional'naya

239

29.03

0.42

2.40

6.02

30.64

6.52

1.56

0.99

0.42

21.99

Mir

49

27.68

1.40

2.21

7.50

28.04

11.94

0.24

1.05

0.56

19.39

Aikhal

136

23.53

0.43

2.28

4.43

24.37

16.34

1.17

0.78

0.65

27.04

Yubileinaya

395

27.11

1.08

1.67

7.57

31.35

9.05

0.10

0.28

0.41

20.60

Sytykanskaya

112

28.38

1.82

1.83

8.07

31.54

7.13

0.09

0.27

0.27

19.12

Udachnaya-West

480

25.11

0.89

2.26

5.84

25.84

15.52

0.20

0.68

0.31

23.52

Udachnaya-East

534

25.56

1.12

2.09

6.87

27.93

13.76

0.28

0.62

0.31

21.58

Other picrite types

Picrites I

71

27.61

3.03

4.04

12.24

20.58

14.99

0.21

1.22

0.74

16.00

Picrites II

74

27.15

3.65

4.08

12.47

21.13

14.48

0.19

1.17

0.84

15.11

Picrites III

47

27.26

4.14

4.03

12.56

21.11

12.85

0.19

1.39

0.75

14.45

Picrites IV

41

28.82

4.90

4.61

13.42

22.25

11.32

0.22

1.60

0.62

11.86

* Here and in the following tables: n, number of analyses; U/b, ultrabasic; 1 authors' collection; 2 V. S. Samoilov (1977).

 

The approach to the petrographic identification of rocks from different depths reduces itself to the skill of discrimination of kimberlite rocks from other rock types. Mean compositions of various carbonatites and kimberlites are shown in Table 1. At the first glance, carbonatites are characterized by elevated phosphorus contents, but it is not true in some cases. Comparison of correlations of CaO with Al2O3 and MgO (Table 2) is less ambiguous. Carbonatites generally demonstrate a negative correlation between CaO and Al2O3, which illustrates the degradations of feldspars of alkaline gabbroids. Kimberlites and other alkaline picrite types are universally characterized by a negative correlation between CaO and MgO, resulting from clinopyroxene cotectics melting. Our recommendations can be tried out. If we find a negative CaOMgO correlation in a new pipe with the composition close to the compositions of other pipes, it will bring us to the conclusion that these rocks belong to the alkaline picrite family, which, in addition to kimberlites, includes other types of alkaline picrites. By comparison of TiO2 contents in rocks under study with the limits of TiO2 contents in picrites of other types, we determine whether the rocks belong to kimberlites.

Let us apply this approach to the hypothesis that diamondiferous lamproites belong to kimberlites. This hypothesis must be rejected for both leucite and olivine lamproites, because they are characterized by a highly negative correlation between Al2O3 and MgO. Such correlations are most typical of oceanic submelapicritoids (Vasilenko et al., 1994). It is reasonable to suggest that lamproites arose in magma formation zones containing oceanic crust fragments enriched in high-potassium sedimentary matter.

Table 2. Correlation profiles of major oxides. Correlation coefficients ( = 0.99) between the contents of CaO and other rock-forming oxides

Region, deposit

Rock association

n

r01

SiO2

TiO2

Al2O3

Fe2O3

MgO

Na2O

K2O

P2O5

LOI

Carbonatite associations

Tomtor

U/b with Al2O3

15

0.64

 

 

 

 

0.80

0.83

 

 

 

U/b with CaO

14

0.66

 

-0.83

-0.65

 

 

 

 

 

 

E. Sayan2 and others

K-feldspar-

74

0.30

-0.61

 

-0.54

 

-0.35

-0.63

-0.5

 

0.93

Albite-

63

0.33

-0.84

 

-0.75

-0.61

-0.44

-0.56

0.41

-0.39

-0.60

Amph.dol.

30

0.46

-0.94

 

-0.80

-0.70

-0.49

 

 

 

 

Seligdar

-rocks

12

0.71

-0.88

 

-0.81

-0.86

-0.54

 

-0.8

 

0.74

Oshurkovo

Apatitic

9

0.79

-0.97

0.90

-0.99

 

 

-0.95

-0.9

0.99

0.94

Unaltered kimberlites and picrites

 

Pipe

Kimberlite type

Yakutia

Maiskaya

7

0.75

-0.85

 

 

-0.74

-0.92

 

 

 

 

Nyurbinskaya

136

0.22

-0.87

-0.27

 

-0.38

-0.91

 

 

 

0.78

Botuobinskaya

108

0.25

-0.96

-0.65

0.36

-0.47

-0.95

 

0.41

-0.43

0.90

Internatsional'naya

239

0.17

-0.79

 

-0.36

 

-0.82

 

-0.3

0.34

0.65

Mir

49

0.36

-0.83

 

 

 

-0.91

 

 

0.71

 

Aikhal

136

0.22

-0.92

 

 

-0.31

-0.92

 

 

 

0.55

Yubileinaya

395

0.13

-0.90

-0.18

0.29

-0.37

-0.97

 

 

 

0.81

Sytykanskaya

112

0.25

-0.91

-0.47

 

-0.30

-0.96

 

 

-0.35

0.86

Udachnaya-West

480

0.19

-0.94

 

 

-0.64

-0.95

 

0.25

 

0.82

Udachnaya-East

534

0.11

-0.90

-0.33

0.25

-0.67

-0.93

 

-0.4

 

0.67

Other picrite types

15

0.60

-0.61

-0.84

 

-0.74

-0.81

 

 

0.74

0.83

 

Table 3. Examples of petrochemical reconstructions

Mean compositions

Region

Rock association

n

SiO2

TiO2

Al2O3

Fe2O3

MgO

CaO

Na2O

K2O

P2O5

LOI

Yakutia

Rocks of a new pipe

21

24.89

1.53

2.32

6.61

26.34

15.48

0.25

0.36

0.42

22.13

Australia3

Olivine lamproites

44

41.86

3.53

4.44

8.23

20.92

4.78

0.58

3.98

1.27

8.08

Leucite lamproites

76

50.59

5.95

7.79

7.01

7.73

3.24

0.48

9.30

0.14

4.98

Correlation coefficients ( = 0.99) between rock-forming oxides and CaO

Region

Rock association

r01

SiO2

TiO2

Al2O3

Fe2O3

MgO

CaO

Na2O

K2O

P2O5

LOI

Yakutia

Rocks of a new pipe

0.55

-0.97

 

 

-0.72

-0.98

1.00

0.97

 

 

0.97

Correlation coefficients ( = 0.99) between rock-forming oxides and MgO

Australia3

Olivine lamproites

0.38

 

0.69

0.83

 

1.00

 

 

0.4

 

 

Leucite lamproites

0.29

0.57

 

0.82

 

1.00

0.47

 

0.5

 

0.34

3 Jaques A.L. et al. (1986).

References

Vasilenko V.B., Kuznetsova L.G., and Kholodova L.D. Apatitic rocks of Seligdar. Novosibirsk: Nauka. 1982 [in Russian].

Vasilenko V.B., Zinchuk N.N., Kuznetsova L.G., and Serenko V.P. Petrochemistry of subalkaline carbonatite-containing associations in Siberia. Novosibirsk: Nauka. 1997 [in Russian].

Jaques A. L., Lewis J. D. and Smith C. B. The kimberlites and lamproites of Western Australia: Geological Survey of Western Australia Bulletin. 1986.

Bogatikov O.A., Gon'shakova V.I., Efremova S.V., et al. Classification and nomenclature of igneous rocks. Moscow: Nedra. 1981 [in Russian].

Kukharenko A.A. On the nature of carbonatite origin, in: Proceedings of the 2nd conference on wall-rock metasomatism. Leningrad: Leningrad State University. 1966, pp. 3447 [in Russian].

Note on the Mineragenic map of the USSR. Phosphate materials. Leningrad: VSEGEI. 1985 [in Russian].

Samoilov V.S. Carbonatites. Moscow: Nauka. 1977 [in Russian].