Siderophile and chalcophile metals as tracers of the evolution of the Siberian Trap in the Noril'sk region, Russia

Gdrimica

et Cosmnchimica ACIQ Vol. 57, pp. 200 I-20 I8

Copyright0 1993 Pergamonhss

0016-7037/93/'56.00 + .oO

Ltd. Printed in U.S.A.

Siderophile and chalcophile metals as tracers of the evolution of the Siberian Trap in the Norif’sk region, Russia G. E. BR~GMANN,“* A. J. NALDREIT,’ M. ASIF,’ P. C. LIGHTFOOT,’ N. S. GORBACHEV,~and V. A. FEDORENKO~

‘mpartment of Geology, Earth Science Center, University of Toronto, Toronto, Canada, MSS 3Bl %eoscience Laboratories, Ontario Geological Survey, 77 Grenville Street, Toronto, Ontario, Canada, M7A 1W4 ‘Institute of Experimental Petrology9Russia Academy of Science, 142432 Chemogolovka, Moscow District, Russia ‘Central Geological Institute for Exploration and Research ( TsNIGRI ) , Russian Ministry of Geology, Warshauskoye 128b, 113545 Moscow, Russia (Received April 7, 1992; accepted in revised form October 19, I 992)

Abstract--In this study Cu, Ni, and platinum-group elements (PGE) were determined in a sequence of basaltic and picritic lavas from the Siberian Trap in the Noril’sk area of Russia to constrain genetic relationships between the basalts and the petrogenesis of Ni-Cu-PGE sulfide deposits associated with the Talnakh and Noril’sk intrusions. In the most primitive basalts ( S- 19 wt% MgO ) of the Tuklonsky ( Tk ) suite, Pt and Pd concentrations range from 4-l 3 ppb, increasing with decreasing MgO content; whereas Ir contents decrease with MgO from 0.8-0.05 ppb. The contrasting behavior of these elements, which all have very high sult5dasilicate partition coefficients, as well as the primitive mantle-like ratios of Cu/Y and Pd/Y, suggests that these magmas were not sulfide-saturated. The high PGE abundances imply that their parental magmas were also not sulfide saturated during partial melting in the mantle. Due to sulfide segregation, the overlying basalts of the Nadezhdinsky (Nd) series are low in Cu and Ni (52 and 38 ppm, respectively); highly depleted in all PGE; and have very low Cu/Y, PdfY, and Pd/Cu ratios. However, in s~ti~p~~lly higher levels, Cu, Ni, and PGE concentrations increase systematically through the Morongovsky (Mr) suite to reach a concentration plateau in the uppermost Mokulaevsky (Mk) suite (Pt 8 ppb, Pd: 9 ppb, Ir: O-12 ppb, Rh: 0.4 ppb). At the same time, ratios such as Cu/Y increase and approach primitive mantle values. However, ratios involving PGE, such as W/Y, remain low, suggesting the removal of small amounts of sulfide (0.0 l-0.03% ) . The compositional variations in the basalts and the sulfide liquids can be quantitatively described by fractional segregation of a sulfide liquid in an open- or closed-system magma chamber. The latter model suggests that the basalts represent the eruption products of a zoned magma chamber in which light magma, with crustal com~nents con~minat~, overlies less con~minat~, denser magma. Crustal contamination caused sulfide saturation, and the resulting sulfide liquids settled through a magma column and accumulated at the bottom of the chamber. In this model, the sulfide liquid is not in equilibrium with the whole magma mass, and sulfide segregation is compared with the zone-refining process of metalfurgy. The sulfides become more enriched as they move through the magma; and although the magma left behind is depleted in PGE, Cu, and Ni, their #ncent~tions also incrm with depth. Eventually, the magma chamber is emptied from the top to the bottom, producing the flood basalt sequence and the associated intrusions and ore deposits. In the open-system model, sulfide saturation was initially caused by assimilation of crustal material by the Tuklonsky magma. Continuous and ~rn~~neous ~pl~nishment, assimifation, and ~~~alIization processes formed the lower Nd iavas. The concurrent removal of OS- I % sulfide strongly depleted these magmas in chalcophile and siderophile metals. Due to the continuous replenishment of the magma chamber with uncontaminated PGE-rich magma, succeeding lavas (Mr, Mk) show diminishing signs of crnstal con~mination and become less suIfide-~turated, as indicated by the increasing Ni, Cu, and PGE abundances. During the evolution of the chamber, the magma remained sulfur-saturated, and sulfides accumulated at the base. The composition of the sulfide ores could be regarded as a mixture consisting of low Ni-, Cu-, and PGE-sulfides derived with a low silicate/sulfide ratio ( 100) from the Tk-Nd magma and high Ni-, Cu-, and PGE-sulfides formed with a high ratio ( 10,000) from the Mr-Mk magma. ore deposits are associated with the Talnakh and Noril’sk intrusions, the emplacement of which appears to be controlled by the Noril’sk-Kharayelakh fault ( Fig. 1) .One striking feature of the sulfide deposits is their very high p~tinum-coup element (PGE) content; massive and disseminated sulfides contain up to 100 ppm Pt + Pd + Au (NALDRETT et al., 199 1; ZIENTEK et al. 199 1) .In order to concentrate the PGE to this extent by separating an immiscible sulfide liquid from a silicate magma which contains less than 20 ppb of Pt or Pd, a large silicate magma/sulfide liquid ratio (> 1000) has to be achieved. However, at Talnakh, this ratio is very low

INTRODUCTION THE NORIL’SKREGION AT THE northwest

margin of the Siberian Platform (Fig. 1) hosts several world-class magmatic Ni-Cu sulfide deposits. They occur in differentiated maficf ultramafic sills which represent intrusive equivalents of the Siberian Trap-continental flood basalts (CFB) which erupted about 248 Ma ( FZENNEand BASU, 199 1) .The most important * Presenf Address: Max-Planck Institut Wr Chemie, Abteilung Geochemie, SaarstraBe 23, D-6500 Maim, Germany. 2001

2002

G. E. Briigmann et al.

Distribution of Lithophile Elements in the Flood Basalts of the Noril’sk Region: Previous Studies

FIG. I. Location and geology from NALDRETT et al., 1993).

of the Noril’sk region (modified

because the s&fide ore comprises 10% of the mass of the intrusion ( NALDRETTet al., 1992). One possible solution to the problem is that the parental magmas were very PGErich. Do such magma exist in the Siberian Trap? In this study, siderophile and chalcophile elements in the Siberian flood basalts near Noril’sk in Russia have been determined to answer this question. In addition. their dist~bution in different magma types is used to develop criteria distinguishing between sulfide-saturated and -undersaturated magmas and to unravel the history of sulfide saturation into which the formation of the sulfide deposits is bound. Finally, by using the distribution of chalcophiie and lithophile elements, the evolution of the volcanic sequence is quantitatively described in terms of open and closed magma chambers from their initiation until their emptying.

The geology of the Noril’sk area has recently been described by LIGHTFWT et al. ( 1990) and NALDRETT et al. ( 1992). Details of the geologic setting are discussed in both papers, which also reference Russian publications on the Noril’sk area. In addition, they described the petrography and geochemistry of the flood basalts based on samples from drill hole SG-9 and surface outcrop 1F (Fig. 1). As the geochemicai features and affinity of these two sample sets are further characterized in the present study, the conclusions of these two studies relevant to the discussion of our data will be described briefy. In the Noril’sk region, eleven volcanic suites exceeding 3700 m in thickness have been distinguished using petrographic and geochemical criteria. From base to top, these are the Ivakinsky (Iv), Syverminsky (Sv), Gudchichinsky (Gd), Kkakanchansky, Tuklonsky (Tk), Nadezhdinsky (Nd), Morongovsky (Mr), Mokulaevsky (Mk), Kkaraelakhsky, Kumginsky, and Samoedsky suites. Drill hole SG-9 intersected 2250 m of basaltic and picritic lava flows near the center of the Noril’sk basin (Fig. I). These included subalkalic to alkalic basalts of the Iv and Sv suites; subalkalic basalts and picrites of the Gd suite; and tholeiitic basalts of the Nd, Mr, and Mk suites. Tholeiitic basalts and pi&es of the Tk suite are not present in drill hole SG-9. They have only been identified in the eastern part of the area and were sampled as part of outcrop I F (Fig. 1) . Samples of the four remaining suites were not available. Two fundamentally different groups were distinguished by LIGHTFOOT et al. ( 19901, based on REE patterns. One group is comprised of the Iv, Sv, and Gd suites which have fractionated middle to HREE abundances as indicated by high (Sm/Yb)pM ratios of 2.3-3.6. LIGHTFOO’Iet al. ( 1990) suggested that these features indicate the involvement of garnet during magma formation for these volcanic suites. However, the large variation of the La/ Sm ratio (2.0-4.6; Fig. 2) indicates that the lava flows belonging to this group are not derived from one parental magma. The differences are likely caused by variable degrees of partial melting, assimilation of crustal material, and even heterogeneous source compositions, although the relative contributions of each of these processes are not well constrained ( LIGI~TFOOT et al., 1990). The second group is comprised of the Tk, Nd, Mr, and Mk suites and is not comagmatic with the first group. These volcanic rocks are characterized by flat middle to HREE patterns and (Sm/Yb),, ratios of 1.3-2, indicating no significant garnet control. This group also displays a large range in the La/Sm ratio, which systematically decreases with stratigraphic height from the lower part of the Nd suite through the Mr and Mk suites (Fig. 2). A decrease in Si02 content (54-49 wt%; (LIGHTFOOT et al., 1990) matches the La/Sm trend despite little change in the Mg-number (0.5-0.6; Fig. 2). Additional isotopic studies by P. C. Lightfoot et al. (unpubl. data) indicate that tSr and cNdalso decrease and increase, respectively, with decreasing SiOZ and La/Sm. LIGHTFO~I et al. ( 1990) suggest that these variations are due to assimilation of upper continental crust by the Mk magma. They also calculate that the average composition of the Nd volcanic

Siderophiles and chalcophiles in the Siberian Trap

2003 (4

2400 20

30

40

SO

60

70

801.5

2

2.5

3

3.5

4

4.5

50

30

La/%

MG#

60

90

cu

120

150

180

10

100 Ni

ppm

1000

ppm

O-4

l.,l,,l.6l..l..

20

30

40

50 MG#

60

70

801.5

2

2.5

3

3.5

4

4.5

LalSm

50

30

60

90 cu

120

150

180

10

ppm

100 Ni

1000

ppm

FIG. 2. Chemical variation of selected iithophile and ch~~phiie elements along (a) drill core SG-9 and (b) outcrop IF. Data are from LIGHTFOOT et al. ( 1990) and NALDRETT etal.(1992), except Ni and Cu data for SG-9, which are from this study. Abbreviations for lava suites are described in Table 1. The vertical scale in profilesfrom drill hole SG9 represents the depth of the drill hole; in profiles of outcrop IF, the distance to the base of the volcanic suite.

suite can be achieved by contaminating the Mk magma, with 23% of material having a tonalitic composition. Rocks of the Sv to Nd suites are also exposed in outcrop IF. However, tholeiitic basalts and pierites of the Tk suite occur between the Cd and Nd suites. The lavas of the Tk suite are characterized by low La/Sm (2.2-3.0; Fig. 2) and SiOz contents ((50 wt%) similar to those of the Mk and upper Mr lavas. However, some impo~ant geochemical differences exist and will be discussed at a later stage, Overlying flows have significantly higher SiOl contents (>52 wt%) and high La/Sm ratios (4.2-4.6; Fig. 2). Their chemical composition is similar to that of the ~n~minated Nd lavas from core SG-9. If the profiles from SG-9 and IF are combined, one observes a continuous increase of the I_.a/Sm ratio from the Tk basalts until the middle of the Nd suite, above which it systematicafiy decreases toward the top of the volcanic pile (Fig. 2). NALDRETT et al. ( 1992) suggest that the basalts of the Mk to Nd suites are related by shallow level crystallization and contamination processes in a zoned magma chamber.

ANALYTICAL PROCEDURES The samples from drill hole SG-9 analyzed in this study were collected at the same stratigraphic levels as those of LIGHTFOOT et al. ( 1990). However, in order to have samples of sufficient size to give representative results, it was necessary to take different splits of the core. Major element and S, Cu, and Ni concentrations of the new sample powders were determined again and compared with previous results of LIGHTFOOTet al. ( 1990). Comparison of the two data sets indicated no significant differences, and the average con~n~tions within individual volcanic suites are identical within analytical error. The analyses were performed by X-ray Assay Laboratories in Don Mills, Ontario, Canada, by means of conventional XRF-analysis; the data are shown in Table 1.Sample powders from outcrop 1F are the same as those described by NALDRETT et al. ( 1993). Platinum-group Element and Gold Analysis Platinum-group element and Au were determined by neutron activation analysis after p~n~ntration in a Ni-sulfide bead. The conventional Ni-sulfide fire assay technique uses about 10 g of Ni and 7 g of S in order to collect the PGE out of 20-25 g of rock sample (ROBERT et al., 1971;HOFFMAN etal., 1978). The sulfide bead is

2004

G. E. Briigmann et al. Tabie 1: Trace element data for basaltic and picritic ktvss from (he Siberian Trap in the Norifsk Region Sample

S

Ni

CU

Rh

Pd

tr

4

AU

Y

-

WY

CLV‘Y

*mm ppb_____.____-___.. wb .___.___ ppm ppm ppb ._..- ppb mb ,._____... Pw _...__.._.____... I_pm . . . . _I..._----_-..-.-.__ ___________. .____.._..

__...._._

ha-1

3

6

85.6

DL

123

162

0.40

9.0

0.120

8.0

1.75

24.9

0.36

6.51

295.5

63

11

87

0.33

8.3

0.074

7.0

1.26

23.9

0.35

3.64

367

195

119

144

0.36

7.2

0.073

7.5

2.00

21.7

0.33

6.64

414

DL

119

140

0.31

4.1

6.0

1.44

22.2

O.lR

6.31

449

DL

79

13s

0.36

4.1

5.5

1.t5

23.9

0.17

6.49

502

203

87

131

0.38

6.9

6.0

1.52

23.1

0.30

5.67

557

DL

106

122

0.26

7.4

4.0

1.07

21.8

0.34

5.m

8

636.5

DL

108

130

0.20

5.2

6.0

2.m

22.5

0.23

5.78

9

683

221

100

133

0.35

5.3

6.5

2.12

22.8

0.23

5.83

10

788

100

104

124

0.25

4.5

6.0

1.64

21.9

0.21

5.66

cl56

96

133

0.32

6.2

6.3

1.60

22.9

0.27

5.81

819

DL

120

110

0.25

3.6

0.100

5.5

1.14

21.4

0.17

5.14

12

864

81

110

134

0.36

5.4

0.110

4.5

2.20

23.4

0.23

5.73

13

947

148

104

t2s

0.29

5.0

0.090

6.0

t.82

22.5

0.22

5.69

14

1011.5

to3

114

147

0.25

7.0

0.139

X.0

2.30

22.6

0.31

6.30

15

1036

DL

iIf

loo

0.21

3.6

0.063

5.4

1.56

23.7

0.15

4.22

16

1118

DL

51

105

0.25

2.7

0.045

5.0

1.a5

25.4

0.11

4.13

i7

1129

Dt

91

77

0.26

3.3

0.039

4.0

0.95

20.4

0.16

3.77

18

1150.5

DL

91

123

0.19

4.1

0.046

3.0

1.63

23.4

0.18

5.26

19

1160

DL

93

Xl

0.10

2.0

0.040

3.5

1.69

23 1

0.09

3.51

20

1271

240

X0

82

0.17

3.1

0.029

2.0

0.93

24.0

0.13

3.42


97

tO9

0.23

4.0

0.070

41

1.61

23.0

0.17

4.74 5.1 t

l&-Average m-11

Mr.Average Nd-21

1300

DL

X5

I 14

0.20

DL

0.046

3.5

1.10

22.3

co.09

22

1311.5

DL

47

89

0.10

DL

0.023

2.5

0.69

25.3

<0.08

3.52

23

1337

86

40

75

DL

DL

0.020

1.0

0.58

25.5


2.94

24

1475.5

60

40

59

DL

DL

DL

DL

0.20

22.0


2.68

25

1490.5

SR

22

27

DL

DL

DL

DL

0.33

18.5

ao.1

1.46

26

1498

274

15

31

DL

DL

DL

DL

0.36

21.3

<0.09

1.46

27

1546

DL

25

27

DL

DL

DL

DL

0.44

20.9


1.29

28

1590.5

DL

30

26

DL

DL

DL

DL

0.50

23.0

<0.09

1.13

29

1644

56

25

22

DL

DL

Ix.

DL

0.49

20. I


I.09

30

1708.5

DL

39

37

DL

DL

DL

DL

0.39

20.9


1.77

1F3s

-664

NA

12

9

DL

DL

DL

DL

0x2

20.2

co.1

0.45

I F40

-736

NA

41

54

DL

DL

III.

DL

0.52

NA

1F44

-820

NA

79

10X

DL

DL

0.056

DL

1.55

NA

IF34

-565

NA

35

42

DL

DL

0.019

DL

0.56

19.6


2.14

IF36

.623

NA

19

21

DL

DL

DL

DL

0.76

21.2

<0.09

0.99

Cl07

37

49


c3

co.03

<2

0.62

21.6

*).09

2.00

-512

NA

29X

61

0.30

YO

0.205

11.0

1.31

I1 3

0.80

5.40

-520

NA

292

77

0.28

9.0

0.176

10.0

I.6X

Il.5

0.7X

6.70

295

6Y

0.29

9.0

0.190

10.5

1.50

11.4

0.79

6.05

-364

NA

100

122

0.45

10.0

0.052

9.0

2.35

16.4

0.61

7.44

IF24

-414

NA

136

104

0.30

12.3

0.069

11.0

t.85

15.2

0.81

6.84

IF27

472

NA

103

9X

0.45

13.0

0.139

13.0

1.98

14.9

0.87

6.58

IF29

-510

NA

148

85

0.40

11.0

0.200

13.0

2.27

14.0

0.79

6.07

122

102

0.40

11.6

0.110

11.5

2.11

IS.1

0.77

6.73

1760.7

DL

855

54

0.29

4.0

OS90

5.0

1.07

12.4

0.32

4.35

Nd-Average n-1F30 lF32 ‘lk-Pie Avc TklF21

Tk Bas Ave Gd-31 32

1790

DL

528

81

0.24

6.9

0.345

X.0

1.15

14.X

0.47

5.47

33

1821

DL

477

48

0.17

6.2

0.270

6.0

4.49

16.7

0.37

2.87

lF18

-329

NA

X83

134

0.30

DL

0.734

6.0

1.01

NA

tii6

79

0.25

e5.7

0.4x5

6.3

I.93

4.23

Cd-Pit Ave

14.6

0.39

1X41

X0

127

12X

0.10

4.5

0.083

3.5

1.43

21.‘)

0.21

5.84

35

1862

DL

37

14

0.10

DL

DL

DL

0.97

24.4

<0.08

0.57

36

18X6.5

Gd-34

DL

X4

4X

0.10

RI..

DL

DL

0.76

17.5


2.74

~80

83

63

o.to

<4

CO.08

<3

1.05

21.3


3.05

1930.5

DL

43

26

DL

DL

DL

DL

0.29

25.2

so.08

1.03

1991

DL

75

23

DL

DL

DL

DL

0.56

25.1


0.92

Gd-3as Ave Su-37 38 iF6

-190

NA

64

35

DL

DL

0.043

3.0

0.87

IF10

-242

NA

61

32

DL

DL

0.036

2.0

0.X7

IF15

-290

NA

33

22

DL

DL

0.020

1.5

0.4x

Siderophiles and chalcophiles in the Siberian Trap

2005

Table 1. (Continued) sample

L?epth s

Ni

Cu

Rh

Pd

k

__._............_~_.__~_..._~~____~~__.__~~______~~___~ Sv-Average

PI

Au

Y

wpr

CWY

.___._ ppb.._ppb_..._~...l~________

55

28


<3

0.033

2.2

0.61

25.2

co.08

0.97

Iv - 39

u115

DL

20

21

DL

DL

DL

DL

0.49

33.2

<0.06

0.63

40

2059

343

1

15

DL

DL

DL

DL

1.06

54.3

co.04

0.28

41

2117

70

8

17

DL

4.9

DL

DL

0.58

50.1

0.10

0.34

42

2136

DL

17

16

DL

0.0

DL

DL

0.92

42.1

0.00

0.38

43

2203

970

16

43

DL

4.6

0.022

1.5

O.%

43.8

0.10

0.98

12

22

-xi.1

<3



0.80

44.7

co.07

0.52

2110

28

1.6

4.4

4.4

8.3

1.2

3.9

1.13

7.18

Iv-Average

<460

Rim.hffmtle

NA: Not Analyzed; Y. Ni and Cu Dali for 1F from Nald~~ et al. ( 1992 ); Y dala for SC-9 fmm Lightfootet al. (1990); DL: Below DetectionLimit Samples with labels 1F are fmm outcropIF. remainingsamplesare from drill mm SG-9 Mk=Mokulaevskybasal& Mr=Momngovskybasal& Nd=Naduhdinsky basalts:Tk=Tuklonskybsalts and picriles; Gd=Gudchichinskybasal&and picriles:Iv=Ivankinskylxx&s.; Sv=Siverminsky&al&.

then dissolved in HCI, and the insoluble PGE-rich residuum irradiated and counted with germanium detectors. The present study applied a modified technique developed by ASIF and PARRY ( 1989, 1990). These authors quantitatively preconcentrated the noble metals in a small sulfide bead by using only 0.5 g of Ni-powder. The advantages of this new technique are as follows:

1) lower cost due to the use of lesser amounts of chemicals and faster chemical procedures; 2) improvement of sensitivity and detection limits because. the amount of the PGE residuum is smaller and cleaner containing less Ni, Cu and other chalcophile elements; and 3) the problem of reagent blanks is minimized. This is demonstrated in Table 2, which summarizes parameters describing the quality of the PGE data. The calculated recovery rate from twenty-two analyses of the PGE standard Sarm-7 is greater than 90% for all PGE. This is higher than that reported by ASIF and PARRY ( 1989), probably due to the use of filter paper with a smaller pore size ( ~0.45 pm). The recovery rate of Au is significantly lower (80%), which is due to Au losses during the dissolution of the Nisulfide bead. This is also reflected in the low reproducibility of our Au data, as the average Au concentration of Sarm-7 varies by more than 30% (Table 2). The results for the PGE analyses are significantly better, with variations of I; 10%. This includes variations caused by fluctuations of the neutron flux, which introduces an error of ~4%. Detection limits (defined as background + 3 standard deviations) for all PGE and Au are 52 ppb. An improvement of the detection limits for Pd and Rh was achieved by irradiating the samples with an epithermal neutron flux (2 * 10 ” neutrons cm-* s-l) for 5 min, followed by a counting period of 5 min at the McMaster Nuclear Reactor, Hamilton, Ontario, Canada. The remaining PGE and Au were determined after a 3 h irradiation with a thermal neutron flux of 7 * lo’* neutrons cm-* SK’ at the same reactor and counting periods lasting from 4- I2 h. Except for Au and Ir, reagent blanks of the PGE are below detection limits. However, for 20 g of rock powder, 0.04 ppb of Au and 0.01 ppb Ir are introduced by the chemical reagents. Considering the counting errors associated with blank and sample determinations, samples with Au and Ir concentrations of less than ten times the reagent blank value have been corrected; sam-

Table 2: Paramctm demibing tbe quality of the FGE analyses SARM-7 Rh Fd ---~~-------------------~----~---~-----Number Analyses 22 22 Average 221 1365 std. Dcv. 16 101 SWLDCV.% 7 7 RmXnmClKICd 240 1530 92 89 Recovuy % Detection Limit # 0.10 2.00

AU

R

Ir

22 253 86 34 310 82 0.04

22 3488 251 7 3740 93 1.00

22 71 3 4 74 96 0.02

Concena-ationsare ppb: *: Backgroundplus 3 StandardDeviations

pies with concentrations less than two times the blank value are recorded as below detection limit in Table 1. Samples from drill hole SG-9 were analyzed in duplicate using 20 and 40 g of sample powder. In order to concentrate the PGE out of 40 g samples, two fire assays each with 20 g of sample powder were made, and the two sulfide beads were then dissolved together. For samples from outcrop lF, only 10 g of powder were available, and no duplicates were made. The results of the PGE determinations are shown in Table 1; 0s and Ru data are not reported because their concentrations are below detection limits for most samples. The reproducibility of the Au, Ir, Pd, Pt, and Rh data improves with increasing PGE and Au concentrations. This indicates that the error of the analyses is controlled mainly by counting statistics and ranges from 50% near the detection limits to 10% at concentration levels of 0.1-10 ppb, depending on the element. RESULTS Platinum-group Element Distribution in Volcanic Suites of the Siberian Trap

The PGE concentrations in profiles of drill hole SG-9 and outcrop 1F are shown in Fig. 3. The lowermost basalts of the Iv and Sv suites are very depleted in these metals, and most samples do not contain detectable concentrations of PGE (4 ppb Pd, <2 ppb Pt, < 1 ppb Au, ~0.05 ppb Ir, and ~0.1 ppb Rh). The very low concentrations of compatible elements like Ni and Ir (Table 1) and the very high concentrations of lithophile, incompatible elements (e.g., La > 20 ppm, Th > 3 ppm; LIGHTFOOT et al., 1990) are consistent with their fractionated nature (Mg-number: 0.31-0.58; Fig. 2). However, the low concentrations of elements like Pd, Pt, and Cu could indicate that they also suffered the removal of an immiscible sulfide liquid. These elements behave incompatibly during the crystallization of S-undersaturated magmas (HAMLYN et al., 1985; CROCKETand MACRAE, 1986; BRUGMANNet al., 1987). Therefore, fractionated, sulfidefree magmas should contain elevated concentrations of these elements. The lowermost basalts of the Gd suite also have very low Cu, Au, and F’GE concentrations similar to those of the Iv and Sv basal& However, their concentrations tend to increase with stratigraphic height as the picritic basalts have higher Cu and noble metal contents than the underlying Cd basalts (Table 1; Fig. 3). The overlying basalts and picrites of the Tk suite display a PGE distribution similar to the Gd pi&es;

G. I%Briigmann et al

7006

_ ._I 0

2

4

6

8

10

12

140

2

4

DfXtxtion Limit

4---

8

10

12

140

0.1

0.2

0.3

0.4

Rh wb

Pt wb

Pd wb

-700

6

Deteclion Limit

-600

-100 0

2

4

6

8

Pd ppb

10

12

140

2

4

6

8

10

PC Ppb

12

140

0.1

0.2

0.3

Rh wb

0.4

01

0.1

i

Ir wb

FIG. 3. Variation of the PGE concentrations in the Siberian Trap from (a) drill core X3-9 and (b) outcrop IF in the Noril’sk region. Abbreviations for lava suites are described in Table 1.

however, the Tk lavas appear to be more enriched in Pd, Pt, and Au and lower in Ir than Gd picrites. The basalts of the Tk suite contain the highest PGE concentrations observed in this study approaching 13 ppb Pd and Pt and 2 ppb Au. In contrast, the PGE contents of the lowermost Nd iavas are below the detection limits f Table 1; Fig. 3). They also have very Iow Ni and Cu concentrations, but these elements increase with stratigraphic height, as do the PGE and Au contents, and the uppermost lavas contain detectable although low amounts of noble metals (Pt and Pd I 4 ppb; Au < 1 ppb; Ir I 0.05 ppb; Rh r. 0.25 ppb; Fig. 3). The increase of chalcophiIe and siderophile elements continues through the Mr suite and reaches a maximum at the top of the Mk basalts with about 8 ppb Pt, 9 ppb Pd, 2 ppb Au, 0.35 ppb Rh, and 0.1 ppb Ir (Fig. 3). Palladium and Pt display a reasonable correlation (Fig. 4), indicating a Pt/Pd of 1. This is significantly lower than the chondritic ratio of I .9 f NALDRETT and DUKE, 1980). The least PGEdepleted magmas, like the Tk basal&, contain Pd and Cu in primitive mantle proportions relative to im-

mobile. lithophile elements such as Y (Table I). As the overall PGE concentrations decrease, so do the Pd/Y and Cu/Y ratios, but without changing the Pd/Pt ratio. This suggests the relative Pd, Cu, and Y abundances in the primitive magmas were inherited from a chondritic source. The lower than chondritic Pt/Pd ratio reelects a Pt depletion relative to Pd, which is either a source feature or was caused by fractionation processes during or subsequent to the removal of the magma from the source. Because it is unlikely that a constant Pt/Pd ratio can be retained during progressive partial melting and fractional crystallization involving silicates, sulfides, and Ptrich phases such as Pt-alloys (FLEET et ai., 199 f ), the low Pt/Pd ratio in the Siberian Trap is likely a source feature. Models describing the origin of continental flood basalts involve the whole mantle, ranging from the mantle-core boundary to the subcontinenta lithosphere (DUNCAN and RITHARDS, 1991: HAWKESWORTHet al.. 1990; WHITE and MCKENZIE, 1989}. Estimates based on mass balance calculations, as well as noble metal determinations of mantle materials such as spine1 and garnet ihenolites, have estab-

Siderophiles and chalcophiles in the Siberian Trap

2007

Pd/Pt ratio because they seem to p~fe~n~y transport Pd, either due to its higher solubility or due to the stability of Ptrich residual phases such as Pt-Fe alloys. Such fluids could have been formed during the dehydration of subducted oceanic lithosphere and then stored in the lithospheric mantle. A comprehensive discussion of these processes is beyond the scope of this paper but will be addressed elsewhere.

% E

Platinum-group Element Abundances in the Siberian Trap and Other Mantle-derived Magmas 1.0

3.0

5.0

7.0 pt

9.0

11.0

13.0

ppb

F‘lG. 4. Pd-Pt correlation. Note the low Pt/Pd ratio of 1in ah lavas from the Noril’sk area, which is less than the primitive mantle (PM) value of I .9. Symbols as in Fig. 3.

lished that the upper mantle is depleted in precious metals relative to refractory lithop~e elements but has retained the precious metals in chondritic proportions ( JAGOUTZ et al., 1979; MORGAN et al., 1981; MITCHEL and UAYS, 1981; CHOU et al., 1983; MORGAN, 1986). Therefore, if one is looking for sources with a nonchondritic Pt/Pd ratio, one should call upon other mantle re8ions, such as the subcontinental lithosphere or plumes o~~nating in the lower mantle. Such a source could have developed during the formation of a magma ocean or the interaction of the outer core and lower mantle. Fluids could also be responsible for the nonchondritic

A detailed examination of the variation of the PGE with the MgO content reveals irn~~nt differences between the picrites and the basal& The basalts (MgO < 8 w-t%) are low in all PGE and Au and become progressively more depleted in these metals with decreasing MgO content (Fig. S ) . However, in the high-MgO basalts (MgO > 8 wt%), Ir and Ni concentrations decrease, whereas Pt, Pd, Au, and Cu concentrations tend to increase with decreasing MgO content. This behavior is similar to that displayed in other mafic and ultramafic magmas like komatiites and komatiitic basalts or boninites (HAMLYN et al., 1985; CROCKET and MACRAE, 1986; BRUGMANNet al., 1987). It suggests that the lavas with more than 8 wt% MgO were S-undersaturated, whereas the common decrease of all PGE in the low-MgO basalts ( MgO < 8 wt%) implies the fractionation of a sulfide liquid. In mantle-normalized diagrams such as Fig. 6, the picrites of the Tk and Gd suite display steeper patterns than komatiitic

>O,.,.,...,...,.Ol

2

4

6

8

10

MgO

12

14

16

18

wt.%

FrG. 5. Variation of& (a), Ni (b), Pt (c), and Ir (d) with MgO. In basaits with MgO > 8 wt%, Ir concentrations tend to decrease; whereas Pt and Cu contents tend to increase with decreasing MgO content, suggesting a S-undersaturated parental magma. Basahs with MgO <8 wt% show progressive depletion of noble metals and ehalcophile elements, indicating their compatible behavior and implying fractional segregation of a sulfide-oxide liquid. Sulfide segregation has been modeled using the concentrations in the top Mk lava and the partition coefficients of Table 3. Note that in order to explain the Ni and Cu depletion in the Mk and Mr suites, about 0.1% of sulfide segregation are necessary; whereas Pt and Ir indicate less than 0.02%.

G. E. Briigmann et al.

2008

POEplot(Piie/pfimitkefrw&) ‘“3

Komaliile . Gd-Picrite

n

x Tk-Picrite

’

0.01

I

I

I

I

I

I

,

I

I

I

Ni

OS

t

Ru

Rh

Pt

Pd

Au

Cu

Y

PGE

10-3

plot(BasaWptimitive mantle)

1

0.1

0.01 ;

1 0.001

’

I

/

I

,

,

,

,

,

,

Ni

OS

r

Ru

Rh

PI

Pd

Au

Cu

7Y

FIG. 6. Mantle-normalized distribution patterns of average PGE, Au, Ni, Cu, and Y in basaltic and picritic lava flowsfrom the Noril’sk region. Normalization factors are from BARNESet al. (1987), except for Y, which is from HOFMANN( 1988); Komatiite data (MgO = 19 wt”/,)are from BROGMANNet al. (1987).

rocks with similar MgO content ( 19- 13 wt% ). They are lower in all PGE, Au, Cu, and Ni; this is especially obvious for the compatible elements Ni and Ir. The compatible behavior of Ir has been the subject of many discussions and is attributed to preferred solution in olivine (BRUGMANN et al., 1987) and chromite (MORGAN et al., 1976) or to the formation of Ir-0s alloys, which coprecipitate with olivine (KEAYS et al., 198 1; CROCKET and MACRAE, 1986; BARNES and NALDRETT, 1987). The picritic basalts of the Cd and Tk suites are not regarded as parental magmas as their more primitive composition appears to be due to accumulation of olivine crystals in a magma chamber as opposed to a high degree of partial melting in the mantle ( LIGHTFOOT et al., 1990; NAI.. DRETTet al., 1992). These authors interpreted the Tk-picrites as mixtures of basaltic Tk-magma and olivine crystals, which were in equilibrium with the fractionated magma. Chromite is not a liquidus phase in the basalts of the Tk suite; therefore, it cannot account for the low Ir concentrations. In order to evaluate the role of olivine as a host of Ir, the Ir partition coefficient for olivine/liquid and the amount of olivine added to the fractionated magma have to be estimated. Comparison of the average Y concentration in Tk basalts and picrites

(Table 1) suggests that the picrites represent mixtures of about 24% olivine and 76% Tk basalt. On average, the incompatible noble metals (Au, Pd, and Pt) imply similar proportions of 23% and 77%, respectively. Calculations based on these data suggest that olivine contains 835 ppm Ni and 0.44 ppb Ir. Given that this olivine is in equilibrium with Tk basal&, olivine-liquid partition cdefficients of 7.6 and 4 are calculated. The Ni data are consistent with the estimates and observations of NALDRETT et al. ( 1979, 1992). Although the Ir concentration of the olivine is not known, the calculated partition coefficient can be compared with published data. BROGMANN et al. ( 1987) determined a lower coefficient of about 2, but this was observed in ultramafic high-MgO magmas. Based on the Ir distribution in sulfide deposits associated with basaltic magmas, NALDRETT and BARNES( 1986) proposed an olivine-liquid partition coefficient of 7-l 0 and suggested that it depends on the composition of the magma, in a way similar to that of Ni. Preliminary experimental data by MALVIN et al. ( 1986 ) also suggest partition coefficients of at least 7. Considering the uncertainties involved, a partition coefficient of 4 appears to be consistent with these data and would suggest that olivine is the host phase of Ir. CRICKET ( 198 I) summarized the poor PGE data base for CFB, which has not been significantly improved since then. Data are restricted to Pd and Ir in fifty-four samples of tholeiitic basalts from the Parana, Karoo, De.ccan, and Columbia River flood basalts; their average is 8.3 ppb Pd, with a range of about 0.3-40 ppb, and 0.092 ppb Ir, with a range of O.Ol0.5 ppb ( CROCKET, 198 I ). The concentration range of these elements is similar to the volcanic suites in the Noril’sk area, although the high Pd concentrations (> 15 ppb) reported by CROCKET ( I98 1) have not been observed in this study. The average concentrations agree very well with the PGE concentrations of the Mk suite of the Siberian Trap (Table 1). Mafic dikes and dyke swarms provide additional constraints to the petrogenesis of tholeiitic, continental magmas (HALLS and FAHRICI, 1987 ). The studies by GOTTFRIED and GREENLAND ( 1972) on diabase dikes in Carolina and by GREENLAND ( 197 1) on the Great Lake dolerite in Tasmania offered only Ir data, and the range (0.06-0.5) overlaps with that of CFB. A more complete data set has recently been provided by BRUGMANNet al. ( 1990), who studied the PGE and Au distribution in ten different dyke systems from the Superior Province in Ontario. These dikes contain on average 7 ppb Pd, 9 ppb Pt, 0.09 ppb Ir, and 2.1 ppb Au, which also is within the range observed in this study of the Siberian Traps and other CFB ( CROC‘KFT, I 98 1). The PGE data base for oceanic basalts is more comprehensive (e.g., CROCKFI‘, I98 1; HAMLYN et al., 1985; HERTOC;EN et al.. 1980 ). These studies established the large concentration range of PGE over 2-3 orders of magnitude in these basal& The average Pd and Ir concentrations (~0.7 and co.06 ppb, respectively) are significantly lower than those of CFB, which may reflect the large contribution from oceanic basalts having PGE concentrations below the detection limits of available analytical techniques. The basalts of the Iv, Sv, Cd, and Nd suites from the Siberian Trap also do not contain measurable amounts of PGE; and it may be that, as the data set for CFB becomes more representative, the average values will decrease. The large range of the PGE abundances in

Siderophiles and chalcophiles in the Siberian Trap oceanic basalts is likely the result of a combination of silicate and sulfide fractionation, although it is not clear as to when it occurs, during partial melting or subsequent shallow level fractional crystallization and magma mixing ( HAMLYN et al., 1985; PEACH et al., 1990; CROCKET, 1981; BARNES et al., 1985). It is interesting that the range and upper limit of PGE concentrations in oceanic and continental basalts agree very well, which suggests that the variation of these metal concentrations is controlled by similar fractionation processes, especially by the segregation of sulfide liquids. However, in the case of the Siberian Trap, the high PGE contents in the Tk and Mk basalts rule out that significant amounts of sulfides were left behind during the partial melting event, giving rise. to the parental magmas of these basalts. Noble Metal Abundances as Indicators of Sulfur Saturation The primary S content of the magmas giving rise to the Noril’sk basalts would provide important information about the processes leading to S saturation in these magmas. However, the lavas represent subaerial or shallow water eruptions, and the very low S concentrations (<300 ppm; Table 1) suggest that most of the S escaped during degassing of the melt at the time of eruption. The siderophile PGE exhibit strong chalcophile affinities, as indicated by their extremely high sulfide/silicate liquid partition coefficients. Recent data suggest partition coefficients in the order of lo3 to lo5 for Pd, Ir, Rh, and Pt ( BEZMEN et al., 199 1; FLEET et al., 199 1; STONE et al., 1990;PEACH et al., 1990). This large variation of the partition coefficients of the PGE reflects their strong dependency on fo,, fs,, and probably also on the composition of the silicate and sulfide liquids (FLEET et al., 1991). An estimate of these parameters for the Noril’sk magmas is currently not possible; nevertheless, the assumption of an average sulfide/silicate liquid partition coefficient of lo4 appears to be reasonable. Chalcophile elements like Ni and Cu have significantly lower partition coefficients of about 250- 1400, depending on the composition of the sulfide and silicate liquids and their fo, (RAJAMANIand NALDRETT, 1978; PEACH et al., 1990). Based on the Fe0 content ofthe Siberian flood basalts (8-12 wt%) and the data of PEACH et al. ( 1990), the sulfide-silicate partition coefficients for Ni and Cu are believed to be about 500 and 1000, respectively. In order to describe the behavior of the individual PGE and Au during fractionation of silicate and sulfide liquids, NALDRETTet al. ( 1979) introduced the concept of displaying the chondrite-normalized data in the sequence of decreasing melting points of the metals. BROGMANNet al. ( 1987) modified the diagram by adding the chalcophile elements Ni and Cu, which provide additional information about sulfide fmctionation processes. However, in order to obtain smooth distribution patterns, these authors normalized the data to mantle values. BARNESet al. ( 1987) discussed in detail the philosophy and merits of this approach and calculated normalization factors for all PGE. The comparison of the abundances of chalcophile, siderophile, and lithophile elements identifies a magma which has suffered even a small amount of segregation of a sulfide liquid. Yttrium has been chosen as a representative lithophile ele-

2009

ment and has been added to the mantle-normalized diagrams of Fig. 6. This element is regarded as immobile during alteration and behaves moderately incompatibly in S-undersatmated magmas, like Pd, Pt, and Cu. Thus, given a primitive mantle source composition and a similar degree of incompatibility in sulfide-free systems, metals such as Pd, Pt, Cu, and Y are expected to have similar enrichment factors in mantle-normalized diagrams. This is the case in the Tk basalts, where Y, Cu, and Pd have enrichment factors of 2.53.5 (Fig. 6). They appear, however, to be depleted in Au and Pt relative to Cu and Y. Negative Au anomalies could be due to losses during the analytical determination and remobilization during the alteration of the lava flows. The low enrichment factors for Pt have to be expected given the lower than chondritic Pt/Pd ratio of the rocks that has been pointed out above. The Gd pi&es are depleted in Cu, Pt, and Pd relative to Y (Fig. 6). The explanation could be that they represent mixtures of olivine and PGE- and Cu-depleted basalt. Indeed, the basalts of the Gd suite have very low PGE and Cu contents and are depleted in these metals relative to Y. In basalts having less than 8 wt% MgO, chalcophile and siderophile element concentrations drop suddenly below detection limits over a very restricted MgO range ( < 1 wt%; Fig. 5 ). This is very suggestive that sulfide fractionation may have been important. Additional evidence for this suggestion is derived from a comparison of the enrichment/depletion patterns of Y, Cu, and PGE in Mr and Mk lavas relative to Tk basalts. The higher Y and Cu concentrations in the Mk and Mr suites are consistent with a fractional crystallization model involving the Tk basalt as the parental magma. In a sulfide-free system, Pd, Pt, and Au should become similarly enriched. Instead, Mr and Mk rocks have lower concentrations of these metals than the Tk basalts, which implies that sulfide saturation was achieved in a MgO range between 7 and 8 wt%. The low PGE abundances in Mr and Mk lavas also are obvious in the mantle-normalized diagram of Fig. 6. Despite having near chondritic Cu/Y ratios, the Mk lavas are depleted in Pt, Au, and Pd relative to these elements. Because of its much lower sulfide-silicate partition coefficient, fractional segregation of a very small amount of a sulfide liquid would not significantly affect the Cu abundances in the silicate magma. Thus, the chondritic Cu / Y ratio would be preserved, despite a noticeable depletion in PGE. The increasing depletion in Cu, Ni, and PGE from the Mk, Mr, to the Nd suite (Fig. 6) suggests increasing amounts of sulfide removal. However, in detail, the Ni, Cu, and PGE variation is not entirely consistent with such a model. The Ni and Cu depletion in the lower Nd lavas suggests fractional crystallization of 0.4-0.5% sulfide (Fig. 5). This amount of sulfide would remove all PGE from the silicate magma, which is consistent with the below detection limits data for these lavas. However, the Ni and Cu variation in the upper Nd, Mk, and Mr lavas suggest segregation of 0. I % of sulfide; whereas the amount of sulfide necessary to explain the PGE depletion is one order of magnitude lower. Thus, an additional process is needed to explain the enhanced Ni and Cu depletion relative to that of the PGE. As noted earlier, the composition of the Nd basalts suggests that their parental magma assimilated continental crust, but the amount of contamination decreases with stratigraphic height as

G. E. Briigmann et al.

2010

portents probably decreases the capacity of a mafic-ultramafic magma to dissolve S, which eventually results in the formation of an immiscible sulfide liquid (IRVINE, 1975). It is believed that crustal contamination leads to the accumulation of large masses of Fe-Ni sulfide ores like those of the Sudbury Igneous Complex in Canada or at Kambalda in western Australia ( IRVINE, 1975; NALDRETT et al., 1986; LESHER, 1989). Therefore, for the Noril’sk case, it is proposed that during the building stage of a magma chamber beneath the Noril’sk basin, the liquids assimilated su~oundi~ crustal rocks, became S-saturated, and accumulated sulfides at the bottom of the chamber. The Nd lavas represent this episode. With time, the magmas become less contaminated and less Ssaturated; and progressively smaller amounts of sulfide segregated. During this time, the Mt and Mk lavas erupted. The Iv, Sv, and Gd lavas from the base of the volcanic suite show a similar fractionation of Y, Cu, and Pt (Table 1), which suggests that a similar process controlled the abundances of their siderophile and chalcophile elements. However, a quantitative description of the fractionation processes in the lower volcanic suites is not possible due to the limited number of samples available and because of the lack of constraints defining the genetic relationship among them.

indicated, for example, by the decreasing L.a/Sm ratio (Fig 2 ) . The diminishing influence of contamination should also have a pronounced effect on the Ni, Cu, and PGE concentrations depending on the relative metal contents in magma and contaminant. This is demonstrated in Fig. 7, assuming a simple mixing model which uses the average upper continental crust and the least contaminated Mk samples at the top of the basalt sequence as endmembers. As the PGE concentrations in crustal materials are not well constrained but known to be very low, a Pd concentmtion of zero has been assumed to perform the calculations. The lavas from the lower part of the Nd suite have La/Sm ratios greater than 4; and their PGE, Ni, and Cu contents plot below the mixing line (Fig. 7). This is consistent with the removal of a relatively large amount of sulfide (0.3-0.5%) from the lower Nd magma. The Ni and Cu contents of the overlying basalts follow more closely the mixing line, which implies that their variation is controlled mainly by the dilution of the components introduced during contamination. However, most samples of the upper Nd, Mr, and Mk lavas have lower PGE concentrations than is predicted by the mixing line (Fig. 7). This suggests that their abundances in the basalts are controlled by both processes, the segregation of small amounts of sulfides (eO.170) and the diminishing influence of crustal contamination. One question which has not been addressed so far is the cause of sulfide saturation. Incorporation of the contamination process into the model also provides a mechanism which can cause S saturation. Assimilation of crustal com-

Quantitative Description of Sulfide and Silicate Fractionation in the Thoteiites This is more stmightfo~ard, though still limited, for the tholeiitic Tk, Nd, Mr, and Mk suites. The sequence of events 15

120

*

No Suiiide

9 90

.

60

‘L

60

.

6 ‘.p

3

E

04

90

.& f 60 : 30 ~ 1

2

3

4 La/Sm

5

6

7

FIG. 7. Cu (a), Ni (b), Pd (c), and Y (d) variation in the tholeiitic suites of the’siberian Trap as a function of the La/Sm ratio. Data for the upper crust (UC) are from TAYLORand MCLENNAN ( 1985), with the exception of the Pd content, which is assumed to be zero. The cont~nuo~ and Iong dashed lines with arrows indicate the sequence of Iavas which have been ernpted from an open-system magma chamber, which becomes s~~de-~t~t~ due to the assimiiation of UC by the Tk magma. This yields the chalcophile element-depteted Nd magma, which becomes replenished with Mk magma, forming the Nd-Mr-Mk suites. The calculations are shown in Table 4. The short dashed line represents a mixing line between UC and Mk basalts. For details, see text.

-a i (4

Siderophiles and chalcophiles in the Siberian Trap outlined above can be quantitatively described in terms of two endmember models, conveniently labeled as static and dynamic.

magma in the whole chamber was constant and given by the Mk-basalts at the top of the volcanic suite. Then the evolution of the sulfide liquid can be described by using the following equation for zone refining (e.g., COX et al., 1979):

The static approach

Such a model has recently been suggested by NALDRETT et al. ( 1992) in order to explain the petrogenesis of the intrusions and the associated ore deposits in the Noril’sk area. It proposes that the tholeiitic lavas of the Nd, Mr, and Mk suites represent a sequence of eruptions which emptied a stratified magma chamber from the top to bottom. The zonation was achieved after a large chamber was formed and lighter, contaminated magma collected near the top of the chamber, with progressively less contaminated, denser layers beneath it. The sulfides which were formed in the most contaminated magma at the top of the chamber settled through the progressively less contaminated liquid, saturating it, and scavenging chalcophile elements from it until they are collected at the bottom. The basalts of the Nd, Mr, and Mk suites show little variation in the Mg-number (Fig. 2), which implies that crystallization of silicates plays a minor role during the evolution of this series of basalts. Calculations by LIGHTFOOT et al. ( 1990) indicate only 10% of fractionation of a gabbro assemblage could have occurred. If silicate fractionation can be ignored, then the chemical variation of a zoned magma chamber can be described by a simple two endmember mixing model, involving the least contaminated Mk basalts on top of the volcanic pile and a contaminant. The actual rock type which has been assimilated by the parental magma is not known and is approximated with the average composition of the upper crust (TAYLOR and MCLENNAN, 1985). Such a model has been discussed in the previous section (Fig. 7) and has been shown to be qualitatively consistent with the Ni, Cu, and PGE distribution, assuming a gradually decreasing amount of sulfide segregation downward as a function of the decreasing amount of contamination. As noted earlier, the Mk basalts at the top of the lava sequence are also Ssaturated. If this magma initially was not S-saturated and had a primitive mantle-like Cu/Pd or Y/Pd ratio, it could have had a Pt or Pd content anywhere between 8 and 24 ppb. In this case, the lower Mr lavas (La/Sm = 3) have segregated up to 0.03% of sulfide, which decreases to about 0.01% for the Mk lavas at the top of the volcanic sequence. These amounts are too small to cause significant variations in the Ni and Cu content of the magmas. A more precise determination of the concentrations of chalcophile and siderophile metals in the magmas is achieved by describing the evolution of the sulfide liquid as it settles through the magma column. This process can be compared to zone refining, where a melting zone proceeds through a solid matrix picking up the incompatible elements ( BRUGMANN et al., 1989). Translated to our problem, the static model of contamination at the top of a magma chamber and settling of sulfide droplets can be represented in a simplified way as a sheet of sulfide, which formed from the Nd magma at the top of the magma chamber, then settles through a column of silicate magma, thereby processing more and more silicate magma. The simplest case would assume a constant mass of sulfide and that originally the composition of the

2011

CSUf,C$!

=

DSUl

_

(DSUi

_

I)ee-"/Dd.N),

where CsU’and C$’ represent the concentration in the sulfide and original silicate liquid, Dsu’ is the sulfide/silicate partition coefficient ( Table 4)) and N is the ratio of the mass of magma processed to the mass of sulfide liquid. The metal concentration in the silicate liquid after the sulfide liquid passed (C’“) can be calculated at any time by assuming equilibrium between sulfide and silicate liquids, thus Cri’ = Csu’/DSu’. The results of the calculations are summarized in Table 3. Because the original PGE content of the Mk magma is not well constrained, the calculations assume that it contained between 8 and 24 ppb Pd, as suggested previously. The segregation of OS-l% sulfide (N = 199 to 99) from the Mktype magma would leave a magma behind with very low PGE, Cu, and Ni concentrations, similar to those of the Nd basalts (Table 1). As the sulfides continue to settle through the magma column, i.e., N increases, they scavenge more and more chalcophile and siderophile metals, becoming very rich in these metals. However, in the magma left behind, metal concentrations also increase (Table 3). At N values between 1000 and 2500, calculated concentrations agree well with those observed in the Mr lavas; and N values between 2500 and 5000 simulate the PGE, Ni, and Cu variation in the Mk suite. Figure 8 compares the predicted and observed Ni, Cu, and Pd contents in the silicate magmas. In this diagram, the calculated metal variation is shown as a function of depth by converting N into units which correspond to the thickness of the Nd, Mk, and Mr basalts in drill hole SG-9. At N = 5000, the calculated composition of the silicate liquid (Table 3) agrees well with that of the uppermost Mk basalts (85 m, in Table 1); whereas the composition of the Nd basalts at about 1500 m (Table 1) is similar to those calculated with N = 99 (Table 3). The results of this conversion are shown in Fig. 8. The observed and predicted Pd variation agrees very well if the original magma in the chamber had 24 ppb Pd. As discussed previously, this is the maximum Pd content Table 3: Sulftie composition of ore deposits in the Noril’skregion and Pd, Ni and Cu variations in sultide and silicate liquids after zone Wining Pd

Ni

Cu

wm 18.5 19.3 47

WI.%

wt.%

4.14 5.5 9

8.99 8.72 10.8

OreType TaInti

D&em. Dissem. Noril’sk Dissem.

______!iY!!___42____1!___!!?________------_---____ N Pd Pd Pd Pd Ni C(sulf) C(si1) C!(sulfj C(sil) C(sulf) wt.% ppm ppb wm wb ___ 0 24 --8 99 0.80 0.08 2.39 0.24 1.09 249 1.98 0.20 5.93 0.59 2.36 3.91 0.39 500 11.7 1.20 3.80 7.61 999 0.76 22.8 2.30 5.19 2500 17.7 1.8 53.1 5.30 5.96 5000 31.5 3.1 94.4 9.40 6.00

Ni C(sil) ppm 120 22 47 76 104 119 120

cu C(sulf) wt.% --1.52 3.54 6.30 10.10 14.70 15.90

cu C(sil) PPm 160 15 35 63 101 147 159

C(sul) and C(sil) rue the metal concentrationsin the sulfide and silicate liquids; N is the ratio of the mass of silicate professed to the mass of sulfiidc:N-0 represents composition of the silicate liquid before zone Aning stated. C(sul) has been calculated

according

to: C(sul)/C(sil)=

D(sul)-(D(sul)-l)‘exp-(l/D(sul)‘N

SultideBilicste PartitionGxfficients @(sul)) an listed in Tab. 4. Data for on dqnsita are from Nakben et al. (1991) and Naldrett(1989) and arc recalcubed to 100% sulfide

2012

G. E. Brtigmann et al.

the Y contents of the lower Nd basalts are also lower than those of the Mk and Mr basahs and lower than the Mk-upper crust mixing line predicts. Although this can be accommodated by the static model by using a contaminant with a lower Y content (about 15 ppm), it is also consistent with a dynamic model which involves the Tk-type magma in the petrogenesis of the Nd basalts.

600 E E 900 1

The dynamic approach

1200

1500

1800 I

1 10

m

I 100

1 500

Metal Cwcentration

FIG. 8. Comparison of the Ni, Cu, and Pd contents in the Nd, Mr,

and Mk basalts from SC-9 (data points) with those predicted by the zone-refining model of Table 3 (continuous lines). The calculations of the metal contents in the magma assume that the original magma was of Mk-type containing 8 or 24 ppb Pd, 120 ppm Ni, and 160 ppm Cu. The location of the calculated silicate magmas has been determined by assuming that magmas formed at N = 5000 represent the uppermost Mk basalts (depth = 85 m); whereas the melt at N = 99 represents the Nd basalts (depth = 1500 m). For details, see text and Table 3.

a Mk magma could have had before becoming S-saturated. The zone-refining model also explains the Ni and Cu variation in the Nd, Mk, and Mr basalts, although the observed metal concentrations tend to be lower than the calculated ones. This, however, has to be expected because the simplified model neglected the dilution effect on these metals caused by the contamination of the magmas. Thus, the PGE, Ni, and Cu variation of the Nd, Mr, and Mk basalts is consistent with a closed magma chamber model, in which assimilation of crustal components at the top of the chamber caused the development of a zoned magma and triggered the formation of a sulfide liquid, which then settled through the magma column. On geological grounds, a completely closed magma chamber is unlikely to exist. Although zonation is an important feature of magma chambers, especially those of felsic magmas (e.g., BAKER and MCBIRNEY, 1985), there is ample evidence that tapping and replenishment also are essential processes. For example, current observations of magma chambers on Hawaii show the importance of such processes within a time frame of only one decade ( HELIKER and WRIGHT, 199 1). The static model also does not consider an involvement of the Tk magma at the base of the Nd suite. A characteristic feature of the Tk magma is its low Y content (Fig. 7 ), which cannot be accommodated by the model. In order to produce Mk-type magmas from Tk basal& about 45% ofgabbro fractionation is necessary, based on the Y abundances. This amount is not reflected in the major element composition;

for example, the MgO contents of Tk and Mk basalts differ by just 1% (Fig. 5 ), suggesting less than 10% of gabbro fractionation. Differences in trace-element ratios such as the Th/ U (Mk, Mr: ~3; Tk: >3; NALDRETT et al., 1992; WOODEN et al., 199 la) also suggest that Mk and Tk basalts are derived from different parental magmas. Yet, the Tk magma could have taken part in the evolution of at least the Nd lavas because there are gradual compositional variations from the Tk basalts to the Nd basalts (Fig. 2). As indicated in Fig. 7,

The dynamic model suggests that the basalts are the product of an open-system magma chamber, which experiences continuous and simultaneous replenishment, assimilation, and crystallization. It comprises two phases. During the building stage of the magma chamber, continuous input of Tk-type magma accompanied by assimilation of crustal material, fractional crystallization, and tapping, formed the lower Nd suite. Then a new magma, the Mk-type, replenished the magma chamber and mixed with the residual magma, giving rise to the basalts of the upper Nd, Mr, and Mk suites. Sulfide liquids segregated throughout the evolution of the magma chamber and have been collected at its bottom. Models of open-system magma chambers have been invoked to explain the geochemistry of many CFB (e.g., COX, 1980, 1988) and have also been proposed for the evolution of the Siberian Trap ( FEDORENKO, 1981; SHARMA et al., 1991). O’HARA and MATTHEWS ( 1981) presented a mathematical method which describes the chemical evolution of such a system. However, in their model, the different processes of replenishment, assimilation, and crystallization are independent of each other in the sense that they are separated in time. During the initiation of a magma chamber, however, these processes are likely to occur simultaneously. This is partly accounted for in the assimilation and fractional crystallization ( AFC) model of DEPAOLO ( 198 1 ), but it overlooks the replenishment process. An algorithm accounting for the concurrent operation of all processes has been provided by HAGEN and NEUMANN ( 1990) in their continuous replenishment, assimilation, and fractional crystallization (RAFC) model. The concentration of an element at any time during the evolution of a magma chamber can be described by the differential equation. dC,,ldt = R,Ikf,(C’, ~- C,) + R,IM,(Ca - C,) - RJMAD

- 1)G,

where subscript n indicates the cycle number; subscripts r, a, c mark the processes of replenishment, assimilation, crystallization; R describes the rate of mass change caused by these processes; C is the concentration of an element with a bulk partition coefficient D; and M refers to the total mass at time 1 (Eqn. 10 of HAGEN and NEUMANN, 1990). This equation can be solved by replacing t with an independent variable such as the fractionation index (F), which is the ratio of the mass in the new cycle to that of the last cycle ( DEPAOLO, I98 1; HAGEN and NEUMANN, 1990). Obviously, such solutions are restricted to magma chambers which lose mass during their evolution because F > 1 is geologically not meaningful. However, during the initiation stage of a magma chamber, the volume of magma has to increase. In order to describe this situation, HAGEN and NEUMANN( 1990) solved the equation directly as a function of arbitrary time (t) by using an approximation technique.

Siderophiles and chalcophiles in the Siberian Trap

Such a model is suggested to describe the initiation stage of the magma chamber, thus the formation of the lower Nd lavas from the Tk magma; it is presented in Table 4 and shown graphically in Fig. 7. The average upper crust is assumed to be the contaminant (TAYLOR and MCLENNAN, 1985). The composition of the original and replenishing magma is given by the least contaminated Tk basalts. The crystallizing assemblage consists of 10% olivine, 30% clinopyroxene, and 60% plagioclase, f sulfide, as suggested by LIGHTFOOT et al. ( 1990). The partition coefficients of the elements and phases involved are given in Table 4. The rates of assimilation, crystallization, and replenishment have been optimized by using the variation of lithophile elements (Fig. 7). This initially neglects sulfide segregation, but the amount of sulfide fractionation is small (< 1%); therefore, it has no significant influence on the lithophile element variation. The rates of assimilation, crystallization, and replenishment which achieved the best agreement between observed and calculated data are Ra = 0.2, Rc = 0.25, and R, = 0.2 (Table 4). Other combinations are possible; however, the relative contribution of each process appears to be well constrained as changes in the ratio of Ra/ Rc or Rr/ Ra notably change the enrichment in incompatible elements or ratios such as La/Sm. The value for arbitrary time was chosen to be 1. Variation of this factor changes the degree of enrichment of incompatible elements, but it does not have a significant effect on element ratios. Thus, it changes only the speed of the process. The path of erupted lavas is indicated in Fig. 7 and agrees well with the La, Y, and La/Sm variation observed in the lower Nd lavas. Better fits can easily be achieved by decreasing the Y content of either the Tk parent or the contaminant by 2 ppm. However, such a correction is insignificant, if one appreciates the uncertainties involved with the analysis of Y and the selection of the parental magma and the contaminant. As expected, without sulfide segregation, Pd and Cu contents would increase and Ni contents decrease (Fig. 7); but, clearly, such models do not agree with the chalcophile element contents of the Nd suite. At which point in time or at which degree of crustal contamination during the formation of the Nd lavas sulfide saturation is achieved is not known. This would influence the amount of sulfide necessary to explain the extent of depletion of chalcophile elements in the Nd basalts; however, the errors introduced due to the uncertainties in the

Table 4: Compositional variations in a RAFC magma chambet as a function of time Assimilation Rate: RteO.2

Rcpknishmcnt Rate: Rt=O.Z

Crystallization Rate: Rc=O.ZS

La

Sm

Y

La/Stll

Ni

Cu

Pd

--

PPm 30

PPm 4.5

PPm 22

6.61

PPm 20

PPm 25

PPb 0

4.9

2.1

15

2.33

110

110

13

I

0.0

10.7

2.97

18.6

3.60

76.5

112

13.3

2

0.0

15.6

3.71

21.5

4.20

59.1

114

13.5

2

0.5

36.2

39.7

0.2

2

1.0

23.1

lb.6

0.1

3

0.0

50

115

13.8

3

0.5

24.8

23.2

0.2

3

1.0

15.1

10.3

0.1

4

0.0

45.2

116

13.9

4

0.5

21.7

19.4

0.2

4

1.0

13.9

9.9

0.1

Sulfide 96 uppucmst I)t=O

19.9

23.6

4.32

4.84

23.8

25.6

4.61

4.88

D (Silicate)

_ _

0.01

0.1

0.3

2.0

0.3

D (Sulfh)

_ _

0

0

0

5cKl

loo0

Y: “t” npnsenu arbiuaty rim;

0.1 LOOal

t=O gives trace clement cancenwtion of the original

magma and of that replenishing the magma chamber

2013

sulfide-silicate partition coefficient are of far greater magnitude. A model which assumes that saturation occurred in the initial, least contaminated Nd lavas (at t = 2 in Table 4) suggests that the removal of OS- 1.O% of sulfide explains the low Cu and Ni abundances in the most depleted Nd lavas (Fig. 7 ) . The calculated PGE concentrations are far below the detection limits as soon as sulfides are removed (Table 4), which is also in agreement with the measured values. The model as described to this stage cannot explain the composition of the upper Nd, Mr and Mk basalts (Fig. 7). They represent a sequence of liquids erupted from a magma chamber in which residual Nd-like magma becomes continuously replenished with Mk-type magma. Their decreasing La/Sm ratio implies that assimilation ofcrustal material was minimal at this stage and can be ignored. Similarly, the constant Mg-numbers in the Nd, Mr and Mk basalts indicate insignificant amounts of silicate crystallization (
to the Formation of the

The Noril’sk region hosts numerous intrusive bodies which have been correlated with specific lava suites of the Siberian Trap by using geologic and geochemical criteria (see, for example, NALDRETT et al., 1992). The emplacement of those bodies, which contain the important PGE and Ni-sulfide mineralization, show a close relationship to the Noril’skKharayelakh fault and intrude the volcanic sequence to the stratigraphic level of the lower Nd suite. The Noril’sk and Talnakh intrusions are the most important representatives of this mineralized type, and their cumulates are derived from Mr-type magmas as suggested by similar La/ Sm and Sm/ Yb ratios and Pb-isotopic compositions ( NALDRE’Met al., 1992; WOODEN et al., 199 1a). The sulfide ores of the Noril’sk and

2014

G. E. Briigmann et al.

Talnakh intrusions are mineralogically and chemically very heterogeneous. Recent studies have distinguished three main ore types: disseminated ore, massive ore, and Cu-rich ore (DISTLER et al., 1988; NALDRETTet al., 1992). Disseminated ore is predominant in association with the Noril’sk intrusion, whereas the ore types are more equally represented in association with the Talnakh intrusion. The disseminated ore occurs in the lower parts of the intrusions and is associated with taxitic and picritic gabbro-dolerite. The massive ore commonly occurs as a thick layer (5-25 m) beneath the intrusions. It can be in direct contact with the intrusion, or separated from it by l-5 m thick horizons of sediments. Commonly, veins of massive ore penetrate the basal parts of the intrusions. Therefore, this ore type intruded after the emplacement and solidification of the intrusion ( DUZHIKOV et al., 1988; NALDRETT et al., 1993). Nevertheless, the close spatial relationship and the similar Pb isotopic composition of the massive and disseminated ore types suggest a common origin (WOODEN et al., 199 1b). Another important feature of the massive ore is its pronounced zonation, giving rise to Cu-rich ore types which contain up to 25 wt% of Cu (NALDRE~T et al., 1992). The compositional variations are believed to be caused by the fractional crystallization of the sulfide liquid as well as by the presence of batches of sulfide with variable compositions ( NALDRET~et al., 199 1; ZIENTEK et al., 199 1). Because of the complexity, it is difficult to provide representative chemical data for the different ore types. The original composition of the zoned massive ore, especially, has not yet been established. Average data for disseminated ores from the Noril’sk and Talnakh intrusions were given by NALDRET~ et al. ( 199 1) and NALDRETT ( 1989) and are shown in Table 3, recalculated to 100% sulfide. The sulfide composition in the closed magma chamber model has been calculated previously in order to predict the PGE, Ni, and Cu variation in the Nd, Mr, and Mk basalts (Table 3). It was also suggested that the sulfide evolution can be described by a zone refining process in which the Ni, Cu, and Pd contents increase with N, as shown in Fig. 9. Comparison of the metal contents in the calculated and observed sulfide liquids suggests that the sulfide liquid has to settle through a magma column about 800-1500 times its own mass in order to achieve the metal enrichments seen in the natural sulfides (Table 3; Fig. 9). Because the composition of the Nd basalts is similar to the silicate liquids estimated with N = 99, the total magma volume which has to be processed by the sulfide liquid is about 8- 15 times the volume the Nd-type magma occupied in the chamber. The boundaries of the Nd-Mr and Mr-Mk suites (Fig. 8) would occur at N = 790 and 25 10, respectively, if one expresses their location in drill hole SG-9 in units of N. This implies that at the time the sulfide liquid has concentrated the metals to the extent seen in the deposits, it is in equilibrium with the Mr-type magma. This close association has also been established by lead isotope and REE data. For example, WOODEN et al. ( 199la,b) showed that the lead isotope composition of massive and disseminated ore types, their host intrusions, and the Mr basalts are the same. Similarly, NALDRETT et al. ( 1992), found that the Talnakh and Nor&k intrusions have La/Sm ratios like those observed in the Mr basalts. In the case of the open magma chamber model, the sulfide liquid is in equilibrium with a series of silicate magmas (Tk-

to Mk-type), which were formed during the mixing of the new replenishing magma with magma residing in the chamber. The Ni, Cu, and PGE contents of a sulfide liquid (C’“‘) which is in equilibrium with one of the magma batches depend on their sulfide/silicate partition coefficient (D”‘), their concentration in the silicate liquid (D’“), and most importantly, on the mass ratio of the sulfide and silicate liquids or the R-factor (R; CAMPBELLand NALDRETT, 1979): C”“’ = C’““*DS”‘*(R + l)/(R

+ 0”“‘).

All these parameters have been estimated in previous sections and may be used to calculate the composition of the sulfides that collected at the base of the magma chamber. To simplify the calculations, it is assumed that the sulfide liquids accumulating at the chamber bottom mix completely. Additionally, the ore composition is regarded as a mixture of two sulfide liquids, one that has reached equilibrium with the Tktype magma containing 13 ppb Pd and 110 ppm Cu and Ni; the other with the Mk-type magma containing 8 ppb Pd, 120 ppm Ni, and 160 ppm Cu. The Ni, Cu, and Pd contents of these two endmembers are shown as a function of the sulfide/ silicate ratio (R = 20-100000) by the dotted lines in Fig. 9. The composition of sulfides which are in equilibrium with a Mk magma containing 24 ppb Pd are not shown because they would follow a path which would lie within the area defined by the Tk and Mk sulfides shown in the diagram. The actual composition of the ore is constrained by the amount and composition of the sulfide that segregated from each magma, which is governed by the R-factor that operates in each case. It has been demonstrated in previous text that not more than 1% sulfide segregated from the Tk-magma (R = 99) and about 0.0 1% from the Mk-magma (R = 9999). If the proposed model is applicable, the ore deposits comprised of disseminated sulfides should plot close to the mixing line defined by these two endmember compositions (Fig. 9). Indeed, the Cu and Pd contents of the disseminated sulfide ores plot within the area defined by the calculated sulfide compositions which are in equilibrium with Tk- and Mktype magmas (Fig. 9). They also lie close to the mixing line defined by the low and high R-factor sulfides which segregated from the Tk-magma and the Mk-type magma. However, only the Ni content ofthe disseminated sulfides from the Talnakh intrusion ( NALDRETT et al., 199 1 ) lies close to the mixing line; whereas the disseminated sulfides from the Noril’sk intrusion have far higher Ni contents than predicted by the calculations. This is also the case in the closed magma chamber model, which predicted about 6% of Ni in the sulfide liquid compared to 5-9% actually observed in the ores (Fig. 9). A sulfide/silicate partition coefficient for Ni higher than that applied in the calculations could correct the discrepancy. However, it is believed that the chosen value of 500 is already at the high side of the possible range. Thus, it is more reasonable to assume that the parental magma of the Mk basalts was more primitive and contained more than 120 ppm Ni. The disseminated sulfides from Noril’sk have higher Cu and PGE concentrations than the Talnakh sulfides (Fig. 9 ). This can be explained by a higher proportion of Mk-type sulfide in the Noril’sk ore; thus, it was formed with a higher silicate/ sulfide ratio. As discussed above, the Noril’sk intrusion appears to contain less massive sulfides than the Talnakh intrusion, which could be the manifestation of the higher R-

Siderophiles and chalcophiles in the Siberian Trap

-

l

Norll'sk Sulfide

-

2015

80

Talmkb

Sulfide Noril'sk SulIide

0

60 -

0

0

2

4

6

10

12

14

16

FIG. 9. Comparison of Ni, Cu, and Pd contents in disseminated sulfides of the Talnakh and Noril’sk intrusions with those calculated for sulfide liquids. (a) As predicted by the static model, which describes the evolution of the sulfide liquid (continuous line) as a consequence of the zone-refining model of Table 3. The metal concentrations in the sulfide increase with N, the ratio of the mass of processed silicate magma, and the mass of sulfide and assume that the original magma was of Mk-type containing 24 ppb Pd, 120 ppm Ni, and 160 ppm Cu. (b) As predicted by the dynamic model. The dashed lines delimit the range of compositions of sulfide liquids in equilibrium with Mk- and Tk-type magmas; their Ni, Cu, and Pd contents increase as a function of the silicate/sulfide ratio (R-factor = 20 to 105). The continuous line represents the mixing of sulfide liquids formed at the beginning (low R-factor; Tk-type) and at the end (high R-factor; Mk-type) of the evolution of the tholeiitic basahs. Data for the disseminated ore types are from NALDRETT et al. ( 1991) and NALDRETT( 1989) and are recalculated to 100%sulfide.

factor. The dynamic model also suggests that the metal content of the ores is controlled by sulfides derived from the Mk-type magmas, as they comprise 60-80% of the mixture (Fig. 9). However, the actual fraction of this component could be even higher because the simplified mixing model outlined above tends to overestimate the contribution of the initial sulfide composition, which was in equilibrium with the TkNd magma. In reality, the ore consists of a series of sulfide liquids which are in equilibrium with a series of magma compositions which are the result of the continuous mixing of residual and replenishing magma. Each of these sulfide liquids was formed with a higher R-factor than that of the initial Tksulfides because the replenishment process increased the capacity of the magmas to dissolve sulfur, which resulted in the segregation of smaller amounts of sulfide, by diluting the components introduced during the contamination process. In addition, it is unlikely that the sulfide liquid initially collected at the bottom of the chamber’can retain its original composition during the refilling of the chamber. If one visualizes a magma chamber which is fed from the bottom, every replenishment event would stir up the previously accumulated sulfide liquids, and during the mixing process, the entrapped sulfides would have to equilibrate with the new hybrid composition of the silicate magma. How much of the original sulfide liquid survived would depend on the mixing mechanism and the aspect ratio of the chamber. For example,

in a magma chamber spread out horizontally, less sulfides would be stirred up during the replenishment than in the case of a vertically large chamber. Thus, the dynamic model suggests that the chemistry of the sulfide ore is dominated by the composition of sulfide liquids which were derived from Mr- and Mk-type magmas, which is therefore consistent with the observed trace-element and isotope signatures described previously ( WARDENet al., 199 1a,b; NALDRETT et al., 1992 ) . One problem, which has not been discussed so far, concerns the source of S. Several Russian studies, summarized by GRINENKO ( 1985 ), established that the 6 34Sof the sulfides from the Talnakh and Noril’sk intrusions is very high, with an average value of 10 and values ranging from 6- 16. This implies that a significant fraction of the S is nonmagmatic and has been provided by external sources. GRINENKO ( 1985) suggested that the assimilation of sulfate-bearing evaporites could explain the high S34S values of the sulfides and determined that 20-36% of the S could have been of nonmagmatic origin. The static and the dynamic model discussed above both suggest that sulfide saturation was achieved in the Nd magma and that the fractionation of about OS-l% sulfide ( 1700-3300 ppm S) is necessary to explain their depletion of chalcophile and siderophile elements. The question is whether the parental magma could have had enough S to account for this amount of sulfide segregation. If this is not the case, the assimilation of S-bearing country rocks during

G. E. Brtigmann et al

2016

the initiation of the magma chamber has to be inferred. Alternatively, if the primary S content of the magma was sufficient, other processes have to be invoked, such as assimilation of S during the rise of the sulfide and silicate liquids from the magma chamber to their resting places or during their emplacement, in order to explain the sulfur isotope composition of the sulfides. The primary S content of flood basalts is not known but tholeiitic, submarine basalts with similar compositions commonly contain 800- 1800 ppm S ( MATHEZ, 1976; NALDRE?T et al., I978 ). The S concentration in the magma would depend on the S content in its source, the degree of partial melting and fractional crystallization, and its S solubility. All these parameters are difficult to assess. However, assuming a source with a primitive mantle-like composition (200 ppm; MORGAN, 1986), the magma could have had 2000 ppm of S if it was derived by 20% of partial melting and total consumption of the sulfide component in the source and experienced additional 50% of fractional crystallization involving no segregation of sulfides. The chosen parameters for the fractionation processes tend to maximize the S content in the magma; therefore, significantly higher S content in the magma can only be expected if its source contained more S. The solubility of S in a magma is controlled mainly by its Fe content, temperature, and pressure. The Nd basalts contain on average 10 wt% FeO. CARROL and RUTHERFORD ( 1985) determined that under low pressure (~2 kb) and reducing conditions, which correspond to the quartz-fayalite-magnetite buffer, such melts dissolve about 700 ppm S. This is in good agreement with the experimental data of HAUGHTON et al. ( 1974) and slightly lower than the S concentrations observed in most unfractionated, submarine basalts (900 ppm; MATHEZ, 1976 ). The effect of pressure on the S solubility is not well understood. At low pressure (<2 kb), it tends to increase with increasing pressure (CARROL and RUTHERFORD, 1985); but between 10 and 30 kb, WENDLANDT( 1982) observed the opposite behavicr. However, even in these latter experiments, the liquids which contain 10 wt% Fe0 always dissolve more than 1000 ppm S. Thus, even under the least favorable conditions, where the magma can only dissolve 700 ppm S and only 0.5% sulfide segregate, about 400 ppm S has to be supplied from external sources. Hence, a significant portion of the S required for the estimated 0.5-2 wt% sulfide fraction would have to be of external origin. This implies that the Tk and Nd magmas assimilated S-bearing rocks during the initiation of the magma chamber, regardless of whether it was formed at upper or lower crustal levels. CONCLUSIONS The continental flood basalts in the Noril’sk region consist of eleven volcanic suites, each having a distinct chemical fingerprint ( FEDORENKOand DUZHIKOV, 198 1). This study concentrated on describing the relationship among the basalts in terms of magma chamber processes. Features inherited from the source and during partial melting will be discussed in future studies using additional isotope data (P. C. Lightfoot et al., unpubl. data). However, one feature common to all basalts from the Noril’sk area and which appears to reflect a source property is the nonchondritic, low Pt/Pd ratio of 1. Several studies established the chondritic relative proportion

of all PGE in the upper mantle (e.g., MORGAN, 1986). Therefore, other sources, like subcontinental lithosphere or plumes originating in the lower mantle, could contain the PGE in nonchondritic proportions. Based on trace-element data, seven of the eleven basalt suites are derived from at least three different parental magmas, whose differences are not related to fractional crystallization and crustal contamination processes, as follows: the basal basalts with alkaline affinities, and the tholeiitic basalts of the Tk and Mk suites. The relationships among the basaltic suites of the initial volcanic phase have not yet been studied in detail. However, the variation of trace elements, for example, La/Sm and PGE, suggest that they experienced similar shallow level magma chamber processes as the overlying tholeiitic basalts. The base of the tholeiitic basal& the Tk suite, is derived from a distinct magma type characterized by relatively low Ni, Y, and SC contents and high PGE contents. The basalts with the highest PGE concentrations have chondritic Pd/Cu and Cu/Y ratios. If these are source properties, then the basalts are S-undersaturated. All the basalts overlying the Tk suite have lower PGE contents and lower than chondritic Pd/Cu and Pd/Y ratios, which indicate sulfide segregation. The emplacement of the Tk magma into the continental crust built a magma chamber, the evolution of which is reflected in the erupted flood basalt. It can be described with a model of simultaneous replenishment, assimilation, and fractional crystallization. Initially, as the chamber was formed and growing in size, the magma became progressively contaminated leading to a sequence of eruptions forming the Tk and lower Nd basalts. The assimilation of continental crust triggered sulfide immiscibility. This resulted in the segregation of up to 1% of sulfide, which depleted the residual magma in chalcophile elements and accumulated at the bottom of the magma chamber; at this stage, the basalts of the lower Nd suite erupted. During this period, a new magma type, the parental magma of the Mk basahs, continuously or periodically intruded the chamber and mixed with the residual lower Nd magma or formed a zoned magma chamber. This led to the formation of progressively less-contaminated and sulfide-saturated magma, which represents the upper Nd, Mr, and Mk lavas. At the top of the volcanic pile, just 0.0 1% of sulfide segregation is necessary to explain the distribution of the chalcophile elements. It is appreciated that this model does not represent reality. Continuous replenishment over a period of hundreds of thousands of years is unlikely. If the chamber was not refilled for an extensive period of time, the magma chamber could become chemically stratified, which represents the other extreme of possible models. Whereas the dynamic model describes the geochemical evolution of the volcanic sequence in general terms, it can be improved by incorporating the zoned magma chamber concept as it could explain the sequence of eruptions of individual or of groups of lavas. Furthermore, with time, the importance of a single process probably changed. For example, during the building stage of the magma chamber replenishment, assimilation and crystallization were dominant. However, it is possible that the system eventually reached an equilibrium state in terms of replenishment and tapping. Then assimilation of wall rock would have become insignificant and both models, tapping of a continuously replenished chamber and of a stratified magma,

Side~phi~~ and ch~~phil~ would have been able to produce an indistinguishable sequence of basal& The high PGE content of the sulfide ores associated with the Noril’sk and Talnakh intrusions is not the result of abnormally high FGE contents in the parental magmas but rather the result of magma chamber processes. During the initiation of the chamber, the magma became S-saturated due to the ~imilation of crustal material. As long as replenishm~t and tapping processes could sustain the chamber, the components of contamination became diluted in the residual magma, and sulfides segregated and accumulated at the bottom. If the magma chamber at the time of sulfide saturation behaved as a closed system, magma and sulfide compositions can be determined by adapting a zone refining model, in which sulfides, formed at the top of the chamber during assimilation of crustal material, successively equilibrate with segments of the magma column as they settle to the bottom of the chamber. In the open-system magma chamber, the ore composition can be interpreted as a mixture of sulfide liquids which were in equilibrium with a series of silicate magmas formed during the mixing of residual magma in the chamber with new inputs. During a phase of low magmatic activity, the chamber was emptied, and eventually magmas with sulfides in suspension escaped along deep fault zones forming the intrusions and the disseminated ore types. Following this, the sulfide liquid was tapped and followed the path of its precursor magma. This sulfide liquid is represented by the massive ore type beneath the Talnakh intrusion. Acknowledgments-The

PGE analyses were done at the Research Reactor of the McMaster University in Hamilton, Ontario, and we

wish to thank its staff, especially Alice Pidrucnzy and Mike Butler, for handling our samples. Thanks to Mrs. Wenan Li, who assisted in the PGE analysis; Mr. J. Charters, who assisted in the implementation of the computer algorithm for the magma chamber model; and Mr. S. Shanbhag, who drafted Fig. 1.We appreciate the thoughtful comments of the reviewers A. R. Basu, M. E. Fleet, J. M. Motgan, and an anonymous reviewer; their constructive criticism improved the manuscript. Financial support provided the Natural Science and Engineering Council Grant A4244 to A. J. N. Editorial handling: R. A. Schmitt REFERENCES ASIFM. and PARRYS. J. ( 1989) Elimination of reagent blank problems in the fire assay preconcentration of the platinum-group elements and gold with a nickel sulfide bead weighing less than one gram. Analyst 114, 1057-1059. ASIFM. and PARRYS. J. ( 1990) Nickel sulfide assayfor the collection of the p~atinum~oup elements and gold from chromitites using reduced bead size. Mineral. Petrol. 42,321-326. BAKERB. H. and MCBIRNEYA. R. ( 1985) Liquid fractionation. Part III: Geochemistry of zoned magmas and the compositional effect of liquid fractionation. J. Volcanol. Geotherm. Res. 24,5581. BARNESS.-J. and NALDRE-I~A. J. ( 1987) Fractionation of the platinum-group elements and gold in some komatiites of the Abitibi greenstone belt, northern Ontario. Econ. Geof, 82, 165-183. BARN= S.-J., NALDRE~ A. J., and GOR-~X?N M. P. ( 1985) The origin of the ~~onation of ~atinum~up elements in terrestrial magmas. Chem. Geol. 53,303-323. BARNEY S.-J., BOYDR., KORNELIUSSEN A., NILSSONL.-P., OREN M., PEDERSEN R. B., and ROBINSB. ( 198’7)The use of mantle normalization and metal ratios in discriminating between effects of partial melting, crystal fractionation, and sulfide segregation on platinum-group elements gold, nickel, and copper: Examples from Norway. In GeoPlatinum 87 (ed. H. M. PR~CWARD et al.), pp. 1I3- 141. Elsevier.

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