©2009 Society of Economic Geologists, Inc.
Economic Geology, v. 104, pp. 291–301
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A NEW GENETIC MODEL FOR THE GIANT Ni-Cu-PGE SULFIDE DEPOSITS
ASSOCIATED WITH THE SIBERIAN FLOOD BASALTS
CHUSI LI,† EDWARD M. RIPLEY,
Department of Geological Sciences, Indiana University, Bloomington, Indiana 47401
AND
ANTHONY J. NALDRETT
Department of Geology, University of Toronto, Ontario, Canada M5S 3B1
Abstract
The Kharaelakh intrusion is one of several sill-like, multiphase gabbroic intrusions that host world-class NiCu-PGE deposits in the Noril’sk-Talnakh region. The sulfide ores of the Kharaelakh intrusion are characterized by elevated δ34S values (10–12‰) and high PGE concentrations (e.g., up to10 ppm Pt). The δ34S values
require addition of crustal S with elevated isotope ratios such as evaporite-bearing country rocks (δ34S, ~20‰)
and R factors (magma/sulfide mass ratios) of <400 during sulfide segregation, whereas the Pt concentration requires a much higher R factor (>2,000) if Pt content in the magma is similar to the values in the coeval, most
undepleted lavas (~10 ppb). Such a discrepancy can be explained by sulfide resorption by a new flux of mantle-derived magma into a deep staging chamber to form PGE-enriched magma. Interaction of the PGE-enriched magma with evaporite-bearing country rocks in the plumbing system at higher crustal levels could have
produced sulfide liquids with high PGE contents as well as elevated δ34S values. Mass-balance calculations indicate that <0.9 wt percent anhydrite assimilation from evaporite-bearing country rocks is required to explain
the elevated δ34S values of the sulfide ores. This model, although developed purely based on data from the
Kharaelakh deposit, can also be applied to other coeval Ni-Cu-PGE deposits. For the coeval Noril’sk-I and Talnakh deposits different depths of anhydrite assimilation in the respective plumbing systems are required. This
model differs from the previously proposed models in detail but reenforces the idea that chalcophile element
depletion in continental flood basalts is a useful exploration tool for Ni-Cu-PGE deposits associated with coeval subvolcanic intrusions.
Introduction
The focus of current debate on the genesis of Ni-Cu-PGE
sulfide ores in several subvolcanic intrusions, including the
Kharaelakh intrusion in the Noril’sk-Talnakh region, Siberia,
is on the contributions of chalcophile elements from the coeval flood basalts. A research group led by Naldrett (Naldrett,
1992; Naldrett et al., 1992, 1996; Naldrett and Lightfoot,
1999) proposed a conduit model that emphasizes a direct link
between the ore-bearing intrusions and a specific suite of the
coeval lavas (i.e., the Nadezhdinsky suite) that is depleted in
chalcophile elements including Ni, Cu, Au, and platinumgroup elements (PGE; Brügmann et al., 1993; Lightfoot and
Keays, 2005). The various versions of the conduit model have
been criticized by other researchers because of differences
between the intrusive and extrusive rocks shown by liquidus
phase relationships (Latypov, 2002), S isotopes (Ripley et al.,
2003), and Sr-Nd isotopes plus trace elements (Wooden et al.,
1993; Czamanske et al., 1995; Arndt, 2005; Arndt et al., 2003,
2005). To explain some of these differences Arndt and
coworkers (Arndt, 2005; Arndt et al., 2003, 2005) proposed
that the ore-bearing intrusions were not the conduits of the
chalcophile element-depleted lavas and that the depleted
lavas lost their chalcophile elements to sulfide liquid that segregated and remained in a deep staging chamber, not to the
† Corresponding
author: e-mail, [email protected]
0361-0128/09/3812/291-11
sulfide ores in the ore-bearing intrusions. The different genetic models render different implications for global mineral
exploration. In this communication we propose a new genetic
model that can explain the combination of elevated δ34S values and high PGE concentrations in the sulfide ores that have
not been explained by the previous models, and the occurrence of magmatic anhydrite-sulfide assemblages in the
Kharaelakh intrusion (Li et al., 2009). To avoid confusion that
may arise from the presence of anhydrite of different origins
in the intrusion, a detailed description of anhydrite textures
will be given first.
Geologic Background
U-Pb zircon dating has indicated that the ore-bearing intrusions in the Noril’sk-Talnakh region, Siberia, are contemporaneous with the voluminous Late Permian continental
flood basalts in the region (Kamo et al., 1996, 2003). The
most important ore-bearing intrusions are the Noril’sk-1, Talnakh, and Kharaelakh intrusions (Fig. 1a). The Kharaelakh intrusion was also called the northwestern Talnakh intrusion by
some researchers (Naldrett, 1992; Naldrett et al., 1992, 1996;
Li et al., 2003). All mine geologists now refer to it as the
Kharaelakh intrusion. In both the Noril’sk and Talnakh areas,
weakly mineralized intrusions are also present. Although the
ore-bearing (e.g., Kharaelakh) and weakly mineralized (e.g.,
Lower Talnakh) intrusions are spatially close to each other
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292
Noril'sk
RUSSIA
Fau
lt
88oE
SCIENTIFIC COMMUNICATIONS
a kh
Basalt
Soil
69.5oN
rael
a
Kha
n
sio e
tru ac
In rf
h su
lak to
ae d
ar cte
e
Kh oje
dyk
pr
roic
bb
Ga
A
A'
5 km
b
km
0
N
A'
A
Tun
g
uss
kay
Basalt
as
De
erie
ss
ian
e
sed dimen
ime
ts
nts
von
-0.5
Gabbro
-1
Picritic / taxitic gabbro
with disseminated sulfide
Kharaelakh
Talnakh
Lower
Talnakh
1000 m
Horizontal
Massive sulfide
FIG. 1. (a). Simplified geologic map of the Talnakh region. (b). Cross section of the Kharaelakh ore-bearing intrusion.
(Fig. 1a), their S isotope compositions as well as their radiogenic isotopes and trace elements are different. The mineralized intrusions have δ34S values from 6 to 12 per mil,
whereas the weakly mineralized intrusions have δ34S values
that are generally <6 per mil (Grinenko, 1985). In the Talnakh area the ore-bearing intrusions occur in both the Devonian evaporate-bearing sedimentary strata and the overlying coal-bearing sedimentary strata (Fig. 1b). In the Noril’sk
area all ore-bearing intrusions are above the Devonian evaporate-bearing sedimentary strata. The internal structures of
the ore-bearing intrusions, including the Kharaelakh intrusion, have been described by Czamanske et al. (1995). These
intrusions all comprise multiple rock units that are suggestive
of multiple magma injections (Czamanske et al., 1995; Li et
al., 2003). The studies of the overlying basaltic sequence by
Brügmann et al. (1993) and Lightfoot and Keays (2005) indicated that the lower and middle parts of the Nadezhdinsky
suite (Nd1-2) are significantly depleted in chalcophile elements compared to the overlying Morongovsky suite. The
Nd1-2 lavas and the weakly mineralized intrusions, which are
considered to have predated emplacement of the ore-bearing
intrusions (Zen’ko and Czamanske, 1994; Czamanske et al.,
1995), are characterized by high Th/Nb and γOs, and low εNd,
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suggesting that the assimilation of old granitoid crust or sedimentary rocks derived from old granitoid crust (Lightfoot et
al., 1990, 1993; Wooden et al., 1993; Walker et al., 1994;
Hawkesworth et al., 1995; Horan et al., 1995; Fedorenko et
al., 1996, Arndt et al., 2003). The trace element ratios of the
ore-bearing intrusions are significantly different from those of
the Nd1-2 lavas but broadly similar to those of the overlying
Morongovsky lavas (Naldrett et al., 1992, 1996). However, the
Morongovsky lavas and ore-bearing intrusions are different in
estimated parental magma compositions (Latypov, 2002),
87Sr/86Sr, ε
Nd (Arndt, 2005), and S isotopes (Ripley et al.,
2003).
Samples and Analytical Methods
Two (K53, K58) of the samples used in this study are from
drill core P5536 which is located in the eastern part of the
Kharaelakh intrusion. The mineralogical and S isotope variations in this drill core are given in Li et al. (2003). Another
sample (KH-2) is from drill cores M690 that is located in the
western part of the intrusion. Sample K53 is a taxitic gabbro
(Fig. 2a) and samples K58 and KH-2 (Fig. 2c) are picritic gabbros. Taxitic and picritic gabbros are Russian terms; they correspond to variable-textured and olivine gabbros in western
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a
c
1 cm
Anh
1
Anh
K53
b
1 cm
Anh
Cp
KH-2
KM-1
FIG. 2. Photographs of hand specimens showing hydrothermal anhydrite patches in altered gabbro from the Kharaelakh
intrusion (a), hydrothermal intergrowth of anhydrite and chalcopyrite in the aureole of the Kharaelakh intrusion (b), and sulfide bleb-bearing picritic gabbro (olivine gabbro) from the Kharaelakh intrusion (c). Anh = anhydrite, Cp = chalcopyrite.
literature (e.g., Li et al., 2003). Sample KM-1 is from the aureole of the Kharaelakh intrusion at the Komsomolsky mine.
Anhydrite in the samples was identified using transmitted and
reflected light microscopy, followed by backscattered electron imagery and wavelength dispersive analysis using a
CAMECA SX50 electron microprobe.
Textures of Anhydrite
There are three types of anhydrite in the Kharaelakh intrusion: (1) xenoliths, (2) hydrothermal, and (3) magmatic.
The first two types were reported previously by Gorbachev
and Grinenko (1973). Anhydrite-bearing evaporite xenoliths
up to 0.5 m in thickness are most common in the western
part of the intrusion. The second type is also called metasomatic anhydrite by many Russian geologists (e.g., Gorbachev
and Grinenko, 1973). This type of anhydrite occurs within
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the intrusion as well as in the aureole of the intrusion (Fig.
2b). It is not known whether or not the different occurrences
are related to the same hydrothermal activity. Magmatic anhydrite was not reported in the western literature until recently by Li et al. (2009).
Hydrothermal anhydrite in the Kharaelakh intrusion occurs
as irregular patches up to ~1 cm in diameter, commonly
bounded by microfractures, and surrounded by secondary hydrous silicates (Fig. 2a). The hydrous silicates include hydrogrossular, pectolite, prehnite, thomsonite, and xonotlite.
Their compositions determined by electron microprobe
analysis are listed in Table 1. Minor calcite, pyrite, chalcopyrite, and magnetite are present within some of the secondary
hydrous silicate assemblages.
Magmatic anhydrite is hard to recognize in hand specimens
due to its small size and similar color to plagioclase which is
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TABLE 1. Compositions of Secondary Silicates in the Kharaelakh Intrusion
Mineral
Hydrograssular
Pectolite
Prehnite
Thomsonite
Xonotlite
SiO2
TiO2
Al2O3
MgO
CaO
MnO
FeO
Na2O
K2O
H2O1
Total
31.37
0.04
25.92
0.27
37.58
0.03
1.29
0.04
0.01
not calc.
96.55
53.92
0.00
0.18
0.00
33.20
0.26
0.06
9.16
0.00
2.09
98.88
43.67
0.02
25.79
0.00
26.82
0.00
0.00
0.00
0.00
4.41
100.72
37.51
0.02
31.30
0.04
13.10
0.06
0.92
3.78
0.03
13.40
100.16
50.13
0.01
0.10
0.00
46.03
0.00
0.00
0.00
0.01
4.08
100.35
Cation
Si
Ti
Al
Mg
Ca
Mn
Fe
Na
K
Total
24O
4.58
0.00
4.47
0.06
5.89
0.00
0.02
0.01
0.00
15.03
24O
7.76
0.00
0.03
0.00
5.12
0.03
0.01
2.56
0.00
15.50
24O
5.94
0.00
4.14
0.00
3.91
0.00
0.00
0.00
0.00
13.99
80O
26.18
0.01
25.75
0.04
9.80
0.03
0.54
5.11
0.03
67.49
24O
7.36
0.00
0.02
0.00
7.25
0.00
0.00
0.00
0.00
14.63
1 Calculated
by stoichiometry
ubiquitous in the rocks. For example, in a picritic gabbro
(sample KH-2) from the Kharaelakh intrusion, rare magmatic
anhydrite crystals are found in thin sections under microscopic observation but are not visible in split drill core specimens (Fig. 2c). Li et al. (2009) reported the typical textures of
magmatic anhydrite in the Kharaelakh intrusion such as planar grain boundaries between anhydrite, olivine, and augite,
and inclusions of anhydrite in augite and vice versa. Here we
report the textures of magmatic anhydrite in association with
magmatic sulfides in the Kharaelakh intrusion. Figure 3a and
b illustrates a typical magmatic anhydrite-sulfide assemblage
in a thin section under both reflected and transmitted light.
In this assemblage anhydrite crystals occur as randomly orientated crystals and form a framework; sulfides (pyrrhotite,
pentlandite, and chalcopyrite) occur in the interstices. Elongated anhydrite crystals at the margins of the assemblage
have sharp contacts with granular augite crystals. In some
cases sulfides in the assemblage are completely isolated from
silicate minerals by anhydrite crystals (Fig. 3c).
Figure 3d through f illustrates the textures of hydrothermal
anhydrite assemblages in the Kharaelakh intrusion. In relatively large alteration patches anhydrite commonly occurs as
needles with other secondary hydrous silicates and calcite
(Fig. 3d). In relatively small alteration patches finer grained
anhydrite crystals occur intergrown with secondary hydrous
silicates (Fig. 3e). In this type of assemblage backscattered
electron image (Fig. 3f) and energy dispersive X-ray spectrum
are needed for mineral identification.
Origin of High δ34S Values
With the exception of two sulfide-poor gabbroic samples
(<500 ppm S) which have δ34S values between 1.5 and 3.6 per
mil, all sulfide-mineralized samples from the Kharaelakh intrusion analyzed to date yield δ34S values from 10 to 12 per
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mil (Grinenko, 1985; Li et al., 2003, 2009). The positive correlation between S content and δ34S value indicates a link between sulfide saturation-segregation and contamination with
34S-enriched S. However, some Russian geologists (e.g.,
Godlevsky and Likachev, 1986) have proposed that the high
δ34S values are of mantle origin rather than being related to
crustal contamination. Anomalously high δ34S values have not
been found in the coeval basalts to date (Ripley et al., 2003).
The δ34S values in the basalts are similar to those of the two
sulfide-poor gabbroic samples from the Kharaelakh intrusion.
The significant difference between the basalts and the sulfide-mineralized intrusive rocks is inconsistent with the hypothesis of anomalously high δ34S values in the mantle beneath the Siberian platform.
Degassing may affect S isotope compositions in volcanic
and subvolcanic systems (e.g., Mandeville et al., 2009). Ripley
et al. (2003) discussed the possible effects of degassing on the
S isotope compositions of volcanic rocks in the Noril’sk area.
During degassing the S isotope compositions of magma is
critically dependent on the temperature, pressure, the mass
of the vapor lost, the composition of the vapor, and the fO2
condition which controls speciation in the melt and vapor. At
fO2 conditions where sulfate is the predominant species in the
magma the loss of a vapor can lead to 34S enrichment in the
magma. However, Ripley et al. (2003) showed that the vast
majority of volcanic rocks in the Noril’sk region are characterized by δ34S values less than 5 per mil. Abundant sulfide
ores in the Kharaelakh intrusion indicate that the ore-bearing
intrusive rocks have certainly not degassed to the extent of
the relatively low δ34S coeval lavas. It is therefore extremely
unlikely that the elevated δ34S values found in the sulfide ores
of the Kharaelakh intrusion are a result of degassing.
Grinenko (1985) proposed assimilation of sulfur in sour gas
as the cause of elevated δ34S values in the sulfide ore-bearing
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Reflected light (-)
Sample K58
c
Transmitted light (+)
Sample K58
Transmitted light (+) d
Sample K58
Anh
Xonotlite
Hgs
Calcite
Transmitted light (+)
Sample K53
1 mm
e
f Back-scattered electron image
Pl
Hgs
Cpx
Prehnite
Anh
Anh
Anh
Anh
Pectolite
Hgs
0.5 mm
Cpx
0.2 mm
Transmitted light (+)
Sample K53
FIG. 3. Microphotographs of magmatic and hydrothermal anhydrites in the Kharaelakh intrusion. (a), (b), and (c). Magmatic anhydrite-sulfide assemblages. (d), (e), and (f). Hydrothermal anhydrite. Anh = anhydrite, Cp = chalcopyrite, Cpx =
clinopyroxene, Hgs = hydrograssular, Pn = pentlandite, Po = pyrrhotite.
intrusions in the Noril’sk region. However, sulfur in sour gas
commonly occurs as reduced species such as H2S. It is extremely unlikely that addition of reduced S species into a
mantle-derived magma can drive magma to sulfate saturation, as indicated by the presence of magmatic anhydrite in
the Kharaelakh intrusion (Li et al., 2009). Therefore, we support the hypothesis made previously by many other authors
(e.g., Naldrett et al., 1992, 1996; Li et al., 2003, 2009) that
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assimilation of anhydrite-bearing evaporite country rocks is responsible for the elevated δ34S values of sulfide ores in the intrusion. Because the melting temperature of anhydrite
(>1,400ºC) is significantly higher than the temperature of a
basaltic magma (1,200º–1,300ºC), it is unlikely that the magma
of the Kharaelakh intrusion could have assimilated anhydrite
by melting. Chemical dissolution by magma that was originally
unsaturated with anhydrite is a more plausible mechanism.
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CaSO4solid ⇔ CaSO4melt,
(1)
CaSO4melt + 9FeOmelt ⇔
FeSmelt + 4Fe2O3melt + CaOmelt.
(2)
and
Equation (2) can be used to calculate the increase of
Fe2O3/FeO in the magma due to anhydrite assimilation. The
increase of fO2 in the contaminated magma can be calculated
using the MELTS program of Ghiorso and Sack (1995) in
which the fO2-(Fe2O3/FeO)-T relationship is implemented.
Figure 5 illustrates the change of fO2 due to anhydrite assimilation by a magma with composition similar to the average
composition of the Noril’sk-type sills (Zen’ko and Czamanske,
1994) and initial fO2 at 2 log units below the buffer of fayalitemagnetite-quartz (i.e., FMQ-2). The assumed original oxidation state is within the range of midocean ridge basalts
1.0
‰
34
δ S = 12
0.9
0.8
(δ34S = 20‰)
wt% anhydrite assimilated
1.1
ma
inated mag
Contam
34 = 10‰
δ S
0.7
0.6
0.5
δ34 S = 8‰
0.4
0.3
0.2
0.13
0.14
0.15
0.16
wt% S in initial magma
(δ34S = 0‰)
FIG. 4. Two-component mixing calculation showing the produced δ34S
value, the amount of anhydrite assimilation, and the initial content of S in
magma.
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2.0
Oxidation state of contaminated
magma (∆FMQ)
Mass-Balance Constraints
The
value of sulfide ores in the Kharaelakh intrusion
vary mostly between 10 and 12 per mil (Li et al., 2003). The
δ34S values of anhydrite country rocks in the region are ~20
per mil (Gorbachev and Grinenko, 1973). The content of sulfur in mantle-derived magmas in continental rift settings such
as the Deccan magmas is estimated to be ~0.14 wt percent
based on the analysis of melt inclusions in olivine phenocrysts
(Self et al., 2008). To produce a contaminated magma with a
δ34S value of 12 per mil, the magma needs to assimilate 0.89
wt percent anhydrite if the content of S in the uncontaminated magma is 0.14 wt percent and the δ34S value of the uncontaminated magma is 0 per mil. The amount of anhydrite
assimilated increases to 0.96 wt percent if the content of S in
the uncontaminated magma is 0.15 wt percent. The relationship between δ34S values in contaminated magmas and the
amounts of anhydrite assimilated by the magmas are illustrated in Figure 4.
Anhydrite assimilation by magma will increase fO2 in the
magma through the following equilibria:
δ34S
1.5
1.0
0.5
0.0
-0.5
Oxidation state of initial magma: FMQ-2
-1.0
0.4
0.5
0.6
0.7
0.8
0.9
1.0
wt% anhydrite assimilated
FIG. 5. Relationship between change in oxidation state of a basaltic
magma initially at FMQ-2 and the amount of anhydrite assimilated by the
magma based on reaction (2) and the MELTS program of Ghiorso and Sack
(1995) in which the fO2-(FeO/Fe2O3)-T relationship is implemented.
(Christie et al., 1986) and the Hawaiian ocean island basalts
(Rhodes and Vollinger, 2005). Our calculations indicate that
fO2 in the magma will increase from FMQ-2 to FMQ+1.5
after 0.9 wt percent anhydrite assimilation (Fig. 5). The redox
changes due to anhydrite assimilation are likely maximum values because in addition to FeO, potential organic matter from
country rocks (Arndt et al., 2005; Jugo and Lesher, 2005) may
also serve as reduction agents.
In addition to total pressure, temperature, and composition, the content of S dissolved in silicate melts at S saturation
depends on the oxidation state because that controls sulfur
speciation. Results from measured wavelength shifts of sulfur
Kα X-rays (Carroll and Rutherford, 1988; Wallace and
Carmichael, 1994) indicate that under reducing conditions
(<∆FMQ+1.5) S is dissolved predominantly as S2– (>90% of
total S), under oxidizing condition (>∆FMQ+2) S is dissolved
predominantly as S6+ (>90% of total S), and under transitional
oxidation states (between ~∆FMQ+1.5 and ~∆FMQ+2) both
S2– and S6+ species are important in silicate melts. At S saturation, the average concentration of sulfur dissolved in a
basaltic magma at 1,300ºC and 10 kbars total pressure is 0.14
wt percent under reducing conditions (line a-b, Fig. 6) and
1.45 wt percent under oxidizing conditions (line c-d, Fig. 6;
Jugo et al., 2005). These values do not vary with fO2 if other
parameters remain unchanged (Jugo et al., 2005). Incidentally, the former is the same as the value estimated for the
parental magma of the Deccan Traps (Self et al., 2008) as well
as a magma with composition similar to the average composition of the Noril’sk-type sills (Zen’ko and Czamanske, 1994)
under the conditions of 1 kb and FMQ+1.2, calculated using
the empirical equation of Li and Ripley (2005) in conjunction
with the MELTS program of Ghiorso and Sack (1995) for the
estimation of liquid temperature. Due to lack of experiments,
little is known about the contents of S in magma at S saturation under the transitional oxidation states. The dashed line bc in Figure 6 illustrates a situation where the content of S in
magma at S saturation under the transitional oxidation states
is assumed to increase with increasing fO2. Line e-h in the
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10
500
S content in hydrous felsic magma at S-saturation
927-1027oC, 2 kb (Carroll and Rutherford, 1987)
S content in anhydrous mafic magma at S-saturation
1300oC, 10 kb (Jugo et al., 2005)
d
1
wt% S
Ope
n
syst
em
300
R-factor
c
400
f g h
200
Closed
sy
a
stem
e
b
0.1
0.01
-4
Reducing
0
-2
Transitional
100
8
10
11
12
δ S (‰) in contaminated magma
Oxidizing
2
4
6
Oxidation state, ∆FMQ
FIG. 6. Consequences of increasing anhydrite assimilation by a basaltic
magma initially at FMQ-2 and with 0.14 wt percent S. The increase of total
S (in both dissolved and immiscible forms) in the magma due to anhydrite assimilation is indicated by curve e-h. Line a-b represents the average content
of S in anhydrous mafic magma at sulfide saturation determined by Jugo et
al. (2005); line c-d represents the average content of S in anhydrous mafic
magma at anhydrite saturation (Jugo et al., 2005); and curve b-c represents
the estimated content of S in magma saturated with both sulfide and anhydrite. Compared to anhydrous mafic magma, the contents of S in hydrous felsic magma at S saturation determined by Carroll and Rutherford (1987) are
significantly lower mainly due to lower FeO content and temperature. Both
systems indicate that the content of S in magma is drastically higher at sulfate saturation than at sulfide saturation.
figure represents the increase of total S with increasing
amounts of anhydrite assimilation by a magma with initial S
content of 0.14 wt percent and oxidation state of FMQ-2.
During the early stages of anhydrite assimilation (from point
e-g), the contaminated magma becomes sulfide saturated,
and immiscible sulfides develop within it. However, further
assimilation of anhydrite (after point g) drives the contaminated magma below S saturation. The S saturation history of
the contaminated magma can be further divided into two
parts: the first part from point e to f during which the R factor decreases with increasing anhydrite assimilation, and the
second part from point f to g during which the R factor increases with increasing anhydrite assimilation due to decreasing amount of immiscible sulfide liquid. The results of massbalance calculations (see Figs. 4, 5) indicate that after point f
the δ34S values of the contaminated magmas are higher than
12 per mil. Since the majority of the sulfide ores in the
Kharaelakh intrusion and other ore-bearing intrusions in the
region have δ34S values below this value (Grinenko, 1985; Li
et al., 2003), we envision that the process from point f to g was
not important for the formation of the deposits. The R factors
from point e to f for both open and closed systems have been
calculated and are illustrated in Figure 7. In the open system
case the contaminated magma is withdrawn from the system
after segregation of immiscible sulfide liquid due to anhydrite
assimilation. In the closed system case no magma leaves the
system after anhydrite assimilation. The R factors for these
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9
34
FIG. 7. Relationships between R factor (magma/sulfide mass ratio) and
δ34S value of contaminated magma after anhydrite assimilation. The solubility of S at sulfide saturation in the magma is assumed to be 0.14 wt percent.
In the open system case the magma was completely withdrawn from the system after sulfide segregation. In the closed system case no magma leaves the
system after anhydrite assimilation.
two different systems vary from 100 to 400. These values are
likely the maximum values because the content of S in the
magma before anhydrite assimilation may be higher than the
assumed value of 0.14 wt percent due to higher FeO content.
For example, the sulfur content at sulfide saturation in a
magma with composition similar to the average composition
of the Noril’sk-type sills (Zen’ko and Czamanske, 1994) under
the conditions of 4 kbars and FMQ-2, calculated using the
empirical equation of Li and Ripley (2005) in conjunction
with the MELTS program of Ghiorso and Sack (1995) for the
estimation of liquid temperature, is 0.16 wt percent. The R
factors of 100 to 400 (Fig. 7) are one order of magnitude
lower than R factors estimated based on the Pt tenors of the
sulfide ores in the Kharaelakh intrusion and initial magma Pt
content similar to the value in the coeval, most undepleted
lavas (~10 ppb Pt; Naldrett et al., 1996). The Pt tenors in the
sulfide ores in the Kharaelakh intrusion vary between 2 and 8
ppm (Naldrett et al., 1996). To form the sulfide liquids with
such high Pt concentrations and R factors of <400 the
parental magmas must contain Pt >60 ppb (Fig. 8), which is
six times the value of the coeval, most undepleted lavas that
have been analyzed to date (Brügmann et al., 1993; Lightfoot
and Keays, 2005).
Sulfide Resorption
The most effective way to form a PGE-enriched magma is
by resorption of PGE-rich sulfide liquid (Kerr and Leitch,
2005). Among the lavas that are coeval with the Kharaelakh
intrusion, the Nd1-2 lavas are most depleted in PGE, an indication of sulfide segregation at depth (Brügmann et al., 1993;
Lightfoot and Keays, 2005). The Nd1-2 lavas are also characterized by high La/Sm, high SiO2, and low εNd, which indicate contamination with granitoid crust (Lightfoot et al.,
1993; Lightfoot and Hawkesworth, 1997). Sulfide saturation
in these lavas at depth was thought to be due to contamination with granitic crustal materials in a deep staging chamber
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1000
Parameters
Batch #1
Batch #4
Batch #8
Pt = 11 ppb
Pt = 16 ppb
Pt = 42 ppb
61.5% sulf-liq.
remaining
Pt = 323 ppm
23% sulf-liq.
remaining
Pt = 831 ppm
ppb Pt in initial magma
Pt
D of sulf-liq/magma
20,000
Sulf
ide
liq
8p
pm uid
6 p Pt
pm
Pt
4p
pm
Pt
2p
pm
Pt
100
Pt in initial sulf-liq.
200 ppm
Mass of initial sulf-liq.
100 g
Pt in new magma
10 ppb
Each batch of magma
10,000 g
S in new magma
is 350 ppm below
S content at
sulfide saturation
in the magma
10
100
300
1000
R-factor
FIG. 8. Relationships between Pt content in sulfide liquid, R factor, and
Pt content in initial magma.
perhaps in the lower parts of the upper crust (Lightfoot et al.,
1993; Lightfoot and Hawkesworth, 1997; Arndt et al., 2005).
The results of numerical modeling by Lightfoot and Keays
(2005) suggest that the overlying Morongovsky lavas may
have equilibrated with sulfide liquid with Pt content in excess
of 200 ppm. When a new, S-unsaturated magma from the
mantle entered the staging chamber, it would have dissolved
FeS from the sulfide liquids. Nickel, Cu, and especially PGE
would have stayed with the remaining sulfide liquid because
of their much higher sulfide liquid/magma partition coefficients relative to Fe. As magma replenishment continued, the
sulfide liquid would have become progressively more PGE
(and Ni and Cu) enriched. Since the magma flowing through
the chamber remained in equilibrium with the sulfide liquid
present there, it would also have become PGE enriched,
eventually acquiring a much higher PGE content than the
original magma. As an example, Figure 9 illustrates the results of such a process based on a two-component mass balance on Pt and the parameters given in the figure. In our
mass-balance calculations we used the maximum sulfide/
magma DPt given by Stone et al. (1990) and Fleet et al.
(1999). The initial Pt content in the sulfide liquid is within the
range of values for the sulfide liquids in equilibrium with the
Morongovsky magma given by Lightfoot and Keays (2005).
The Pt content in the mantle-derived, S-unsaturated magma
was fixed at 10 ppb and is similar to the values in the relatively
PGE undepleted lavas above the Nadezhdinsky suite (Lightfoot and Keays, 2005). Our calculations indicate that Pt content in the magma will exceed 60 ppb after nearly complete
resorption of sulfide liquid in the staging chamber.
A New Model
In light of the mass-balance constraints given above, we
propose a new model for the formation of Ni-Cu-PGE sulfide
ores in the Kharaelakh intrusion. The key aspects of the
model are illustrated in Figure 10 and summarized here.
Early sulfide segregation took place in a deep staging chamber due to contamination with granitic crustal materials in the
lower parts of the upper crust, as suggested previously by
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90.4% sulf-liq.
remaining
Pt = 221 ppm
FIG. 9. S-unsaturated magma interacting with sulfide liquids in a conduit
or sill would dissolve FeS from the sulfide liquids. Nickel, Cu, and especially
PGE would stay with the remaining sulfide liquids because of their much
higher sulfide liquid/magma partition coefficients relative to Fe. Repetitive
batches of magma passing through a system, dissolving out FeS, would
steadily upgrade the Ni, Cu, and PGE contents of the residual sulfide liquid.
As the sulfide liquids became more PGE enriched, the flowing magma itself
would also become PGE enriched, eventually having a much higher PGE
content than the original magma.
other researchers (Naldrett et al., 1992, 1996; Naldrett and
Lightfoot, 1999; Arndt et al., 2003, 2005). The magma then
rose to form the weakly mineralized intrusions and erupted to
the surface to form the Nd1-2 lavas, leaving a sulfide liquid
with relatively low tenors of Ni, Cu, and PGE in the staging
chamber. The PGE-poor sulfide liquid in the chamber was
then upgraded in chalcophile elements (PGE, Ni, and Cu) by
the Morongovsky magma from the mantle to form a PGErich sulfide liquid. The PGE-rich sulfide liquid remained in
the staging chamber while the Morongovsky magma erupted
to form the Morongovsky lavas (Fig. 10a). New, S-unsaturated magma from the mantle continued to enter the chamber, progressively dissolved the PGE-rich sulfide liquid in the
chamber to form a PGE-enriched magma. The PGE-enriched magma then rose to the upper parts of the upper crust
where it reacted with anhydrite-bearing evaporite country
rocks and became sulfide saturated, thereby producing immiscible sulfide liquid with high PGE concentrations as well
as high δ34S values (Fig. 10b). The sulfide liquid became
lodged in the hydraulic traps of the plumbing system at
Kharaelakh (Fig. 10c).
According to Zen’ko and Czamanske (1994), the weighted
average sulfide content (calculated as FeS) in the Noril’sk
ore-bearing intrusion and its peripheral sills intercepted by
drilling to date is ~0.17 wt percent, which is within the values
for the closed system case described above. The weighted average sulfide content in the Kharaelakh-Talnakh ore-bearing
intrusions and their peripheral sills intercepted by drilling to
date is ~1.6 wt percent, which is 60 percent higher than the
maximum value for the closed system case (~1 wt %) described above. In other words, ~40 percent of the magma
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a
b
Mr lavas (slightly depleted in PGE)
Nd1-2 lavas
(highly depleted in PGE)
aring
rite-be
Anhyd tary rocks
en
sedim
Ore-barren intrusion
(e.g., Lower Talnakh)
Magma
Magma
Ore-bearing intrusion
(e.g., Kharaelakh)
c
PGE-enriched
magma
Immiscible sulfide liquid
segregation due to
assimilation of granitoid crust
Magma
Magma
PGE-depleted
magma
Sulfide resorption
by S unsaturated magma
Sulfide saturation
due to assimilation of
anhydrite-bearing evaporites
Exit to
peripheral sills
PGE-enriched
magma
Sulfide liquid
injection into footwall
Kharaelakh
subvolcanic chamber
FIG. 10. A new model for the formation of Ni-Cu-PGE sulfide ores in the Kharaelakh intrusion. The sulfide liquid, segregated in a deep staging chamber from the Nd1-2 magmas, was upgraded by the Morongovsky magma (a), and then dissolved
by a new, S-unsaturated magma from the mantle to form a PGE-enriched magma (b). Reaction of the PGE-enriched magma
with anhydrite-bearing evaporite country rocks at a higher level produced immiscible sulfide liquids with high PGE tenors
as well as elevated δ34S values. The sulfide liquids became lodged in the hydraulic traps of the plumbing system at Kharaelakh to form the deposit (c).
involved in the formation of the Kharaelakh-Talnakh deposits
has not been accounted for by the host intrusions and their
peripheral sills intercepted by drilling to date. It is possible
that more peripheral sills in the Kharaelakh-Talnakh oreforming system may have not been found due to a lack of
drilling beyond the orebodies. Alternatively, the missing
magma with elevated δ34S values may have erupted to the
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surface but have not been identified due to insufficient sampling. However, the difference in the estimated parental
magma compositions between the intrusive and extrusive
rocks (Latypov, 2002) makes the second scenario less likely.
Although the above mismatch may be explained by shallowlevel processes such as contamination and fractional crystallization that took place after the magma left the intrusions
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and flowed onward toward the surface, the shallow-level
processes cannot explain lower Rb/Sr, 87Sr/86Sr, and εNd in the
erupted lavas than in the ore-bearing intrusions (Arndt,
2005).
With slight modification our model can also be applied to
other Ni-Cu-PGE deposits in the region, including the Noril’sk-I deposit which occurs several hundred meters above
the evaporite strata. In this case anhydrite assimilation
needed to take place before the magma reached the hydraulic
trap of the plumbing system at Noril’sk-I. The sulfide ores of
the Noril’sk-I deposit generally have higher Pt tenor (up to 12
ppm: Naldrett et al., 1996) and lower δ34S values (8–10‰: Li
et al., 2003) than the Kharaelakh deposit, which is consistent
with less amounts of anhydrite assimilation and thereby
higher R factors for the resultant sulfide liquids.
Concluding Remarks
Our new model adequately explains why the Nd1-2 lavas,
which provided much of the PGE to the sulfide ores in the
ore-bearing intrusions, have S isotopes, Sr-Nd isotopes, and
trace element ratios different from the ore-bearing intrusions
(Lightfoot et al., 1990, 1993; Wooden et al., 1993; Arndt et al.,
2003, 2005; Ripley et al., 2003). This is because the Nd1-2
lavas and the ore-bearing intrusions were not directly linked
(Arndt, 2005; Arndt et al., 2003, 2005). Chalcophile element
depletion in flood basalts as an exploration tool for important
Ni-Cu-PGE deposits in coeval subvolcanic intrusions, which
was proposed previously by Naldrett and coworkers (Naldrett, 1992; Naldrett and Lightfoot, 1999) based on different
approaches, remains valid according to our new model.
Acknowledgments
We thank Viktor Radko of the Noril’sk Nickel Company and
the chief geologist of the Komsomolsky mine for supplying
some of the samples used in this study, and Nick Arndt for
thoughtful review of the manuscript. This research was supported by the Natural Science Foundation of China
(40534020), Project-111 from the Ministry of Education of
China (B07011), and the Natural Science Foundation of the
United States (EAR 0710910).
December 3, 2008; February 13, 2009
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