Applications of stable and radiogenic isotopes to magmatic cu ni-pge deposits examples and cautions

EARTH SCIENCE FRONTIERS
Volume 14, Issue 5, September 2007
Online English edition of the Chinese language journal

Cite this article as: Earth Science Frontiers, 2007, 14(5):124–132. RESEARCH PAPER

Applications of Stable and Radiogenic Isotopes to
Magmatic Cu-Ni-PGE Deposits: Examples and Cautions
Edward M. RIPLEY1,2,∗, Chusi LI1,2
1 Department of Geological Sciences, Indiana University, Bloomington, IN 47401, USA
2 State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China

Abstract: Measurements of S, O, and radiogenic isotope ratios are all potentially powerful tracers of magma interaction with
country rocks and the importance of assimilation processes in the genesis of magmatic Ni-Cu-PGE deposits. Sulfur isotope
measurements of deposits such as those in the 1.1 Ga Duluth Complex, the Permo-Triassic intrusions of the Noril’sk area, and the
1.4 Ga Kabanga intrusions provide evidence for the derivation of S from both sulfide-and sulfate-bearing country rocks. The 1.3 Ga
Voisey’s Bay deposit provides an example where δ34S values of the ores commonly fall within the accepted mantle range of 0 ± 2‰,
but detailed studies of Proterozoic metasedimentary country rocks show that their weighted average δ34S value is also within this
range. A thorough knowledge of the isotopic compositions of potential contaminants is essential for a proper evaluation of the role
of country rock derived S in ore formation. When O and radiogenic isotopic measurements are employed as tracers to evaluate
magma-country rock interaction, it is essential to provide evidence that open system processes have not perturbed the isotopic
systematics. Low-temperature hydrothermal processes can mask evidence of high-temperature processes in the oxygen isotope
system and in radiogenic systems such as Re/Os and Pb where involved elements may be mobile under hydrothermal conditions, or
host phases may close to exchange and uptake at different temperatures. Careful petrographic observation and analyses of
individual minerals may be required before the models involving the contamination of magmas by country rocks can be
meaningfully applied.

Key Words: S isotopes; O isotopes; radiogenic isotopes; magma-country rock interaction

importance of S assimilation in promoting sulfide saturation in
1 Introduction
a mafic magma[6,7].
This review is intended to highlight some of the
Isotopic analyses are a powerful tool for evaluating the
fundamental applications of isotopic measurements to the
potential interaction between mantle-derived magmas and
assessment of magma contamination as applied to the studies
crustal rocks. Both stable and radiogenic isotopic systems
of magmatic ore genesis and cautionary notes that need to be
have been applied to the studies of magma contamination and
origin. It has become apparent that the contamination of mafic considered to avoid potential pitfalls in the interpretation of
magma by crust plays a significant role in the generation of isotopic data. Examples from well-known Cu-Ni-PGE
large, sulfide-rich magmatic Cu-Ni-PGE deposits[1,2]. deposits will be utilized to illustrate key concepts.
Interaction between siliceous country rocks and mafic magma
has also been suggested as an important prerequisite for the 2 Sulfur isotopes
development of chromite deposits in layered intrusions[3,4] and
sulfide deposits in some intrusions[5]. Experimental studies Owing to the relatively large difference in δ34S values
that emphasize the role of pressure as a control of the between mantle-derived magma (MORB as the principal
solubility of sulfur in mafic magmas have also stressed the example) and either sulfide or sulfate minerals in sedimentary

Received date: 2007-08-20; Accepted date: 2007-09-11.
* Corresponding author. E-mail: ripley@indiana.edu
Foundation item: Supported by the Ministry of Education of China project (111-B07011); the National Natural Science Foundation of China (40534020); the
National Science Foundation of the United States (EAR-0710910).
Copyright © 2007, China University of Geosciences (Beijing) and Peking University, Published by Elsevier B.V. All rights reserved.

Edward M. RIPLEY et al. / Earth Science Frontiers, 2007, 14(5):124 –132

rocks, sulfur isotopic studies have provided important insights Tanzania[17]. Mica schists and graphitic schists contain
into the processes of magma contamination. Mantle sulfur is pyrrhotite with δ34S values ranging from -2‰ to 24‰. Sulfide
commonly taken as having δ34S values between -2‰ and 2‰. mineralization in mafic to ultramafic intrusive rocks is
This range is defined largely from the analyses of sulfide characterized by a range of δ34S values between 10‰ and
minerals in meteorites and sulfide-bearing basalts (see 24‰, again lending strong evidence to a genetic model that
Ripley[8] for a review of early S isotope studies). It should involves magma contamination by country rocks.
always be kept in mind, however, that sulfide minerals in
diamonds from kimberlites are characterized by a wide range
of δ34S values from -11‰ to +14‰[9,10], suggesting that the
deep mantle may be far more heterogeneous in terms of δ34S
values than the shallow mantle appears to be. Seawater δ34S
has varied between ~10‰ and 30‰ from the Neoproterozoic
to the present[11,12]. Sulfide minerals are produced from
seawater sulfate owing to reduction that is often related to the
life cycles of sulfate-reducing bacteria. Depending on the
variables related primarily to the rate of sulfate reduction, the
sulfide produced may be characterized by positive or negative
δ34S values. Pyrite that forms owing to the reaction of
produced sulfide and reactive iron may show a wide range in
δ34S values, typically from values close to the source sulfate
(e.g. ~ 20‰), to values as low as -50‰, or occasionally even
less. Sulfate minerals that form via evaporative processes
show little isotopic fractionation relative to the source sulfate,
and hence are generally characterized by δ34S values between
~10‰ and 30‰.
It is clear that because of the potentially large S isotopic
difference between sedimentary S-bearing minerals and
mantle-derived magmas, assimilation processes may be
elucidated using S isotope measurements. Three examples will
serve to illustrate the basic principles.

2.1 Duluth Complex, Minnesota

Cu-Ni sulfide mineralization in the 1.1 Ga Midcontinent
Rift-related Duluth Complex represents one of the best-known
examples of major sulfur derivation from country rocks. The
Proterozoic (~1.85 Ga) Virginia Formation is a pelitic
sequence that contains layers that are sulfidic and
carbonaceous, and are characterized by pyrite δ34S values
ranging from 0 to 30‰ (Fig. 1)[13–15]. Metamorphism of the
sulfide-bearing sedimentary rocks has led to the production of
pyrrhotite and the generation of an H2S-bearing, carbonic fluid.
Magma that formed the Duluth Complex interacted with
–15]
various levels of the sedimentary sequence, assimilated the Fig. 1 Sulfur isotope data[13 from sulfide mineralization in the
sedimentary-derived H2S, and crystallized the sulfide minerals Duluth Complex and Virginia Formation country rocks
that were characterized by elevated δ34S values ranging from
~0 to 20‰ (Fig. 1). Various mixing models involving external
2.2 Noril’sk, Siberia
S and mantle S can be derived[16], but the importance of
externally derived S is highlighted by the δ34S values and the Another well-known example where δ34S values of sulfide
internally consistent models of fluid production in the ores strongly suggest that assimilation has been operative is
metamorphic rocks and localized partial melting. the world famous mineralization at Noril’sk, in the
A similar geologic setting in terms of mafic intrusions Permo-Triassic Siberian flood basalt province. Early sulfur
found hosted by pelitic and graphitic metasedimentary rocks isotope studies[18–20] suggested that assimilation of evaporites
has been described for the large 1.4 Ga Kabanga Ni-deposit in that are common in the stratigraphic sequence occurred, or


that sulfur may have been derived from pyritic shales and envisioned, or hydrothermal leaching of sulfate followed by
coals, or H2S derived from the coals. Li and his partial reduction, or a process involving the dissolution, rather
co-workers[21,22] developed slightly different models to explain than melting, of sulfate in mafic magma. The observation of
the anomalous S isotopic signatures (Fig. 2); however, the abundant wollastonite in contact aureole rocks suggests that
probable involvement of sulfate as a contaminant is reactions similar to CaSO4 + SiO2 + H2O = CaSiO3 + H2S +
noteworthy. The assimilation of sulfate via partial melting by a 2O2 occurred, and that sulfate may have been reduced to
basatic magma is difficult to conceive owing to the high sulfide prior to incorporation into the magma. The important
melting points of sulfate minerals (typically ranging between point here is that sulfate minerals may serve as S sources, but
1360 and 1450°C). Therefore, either a fluxing mechanism is the mechanism of S assimilation must be carefully evaluated.

Fig. 2 Sulfur isotope data[21,22] from the Noril’sk area

process, and positive values indicating a relatively rapid rate
2.3 Voisey’s Bay
of sulfate reduction, possibly under closed system conditions.
A third example where the interpretation of δ34S values is The weighted average δ34S value of the sulfide assemblages in
critical is the 1.3 Ga Voisey’s Bay Ni-Co-Cu deposit in the Tasiuyak Gneiss is -1.3‰. This value is very similar to the
Labrador, Canada. The deposit is associated with the Nain average value of the sulfide ores and illustrates a very
Plutonic Suite, which was emplaced along a boundary important principle. It is important to precisely know the δ34S
between crustal blocks, one comprised predominantly of values of the proposed contaminant before concluding. In the
Archean rocks and the other dominated by Proterozoic rocks. case of Voisey’s Bay, even though the δ34S values are close to
Sulfide mineralization is found in troctolitic to gabbroic rocks. 0‰, the additional study clearly showed that S could have
Country rocks include the Proterozoic Tasiuyak Gneiss, which been derived from the country rocks, in agreement with the
is locally sulfidic and graphitic. Several investigators[23,24] widespread geological evidence, which suggests that S
proposed that large amounts of S in the mineralization may assimilation occurred.
have been derived from the Tasiuyak Gneiss. Initial S isotopic Voisey’s Bay is also an excellent example where supporting
studies[25] determined that the bulk of the sulfide radiogenic isotopic data was extremely valuable. The Re/Os
mineralization was characterized by δ34S values ranging isotopic data (see below)[27] clearly indicated that the magma
between 2‰ and -4‰ (Fig. 3). The δ34S values in the sulfide from which the sulfides were derived assimilated crustal
ores were significantly depleted only locally near the contact material. Carbon isotopic data also indicated that assimilation
with the Tasiuyak Gneiss, which contained pyrrhotite with of sedimentary country rocks had occurred[26], as did the S/Se
negative δ34S values. However, additional studies[26] ratios of the sulfide mineralization. The supporting data
determined that pyrrhotite in the Tasiuyak Gneiss was strengthens the promise that even though the δ34S values of
characterized by a wide range in δ34S values, including both the ores are close to those of mantle sulfides, a large
negative values suggesting an open system sulfate reduction component of the S was derived from country rocks.


derived in part from underlying sedimentary rocks. The
anomalous isotopic signature illustrated in the δ34S-δ33S plot
strongly suggests that the interpretation was correct.

Fig. 4 δ33S versus δ34S for sulfide samples from the
Stillwater Complex, Voisey’s Bay, and Alexo
IAEA S-1 is an international standard. Note the anomalous
[26]
Fig. 3 Sulfur isotope data from sulfide mineralization at character of the Alexo samples, strongly supporting the premise
Voisey’s Bay and sulfide minerals in the Tasiuyak Gneiss that sulfur from Archean-aged metasedimentary country rocks has
country rocks been involved in ore genesis.

A final point with respect to the use of S isotopes as 3 Oxygen isotopes
indicators of crustal contamination regards Archean-aged
sulfide deposits. Ripley[8] discussed that during the Archean Mantle-derived mafic magmas are generally characterized
bacterial sulfate reduction, then if operative, appears to have by δ18O values in the range of 5‰ to 7‰[31–33]. Sedimentary
generated considerably smaller ranges in δ34S values of rocks and crustally derived granitic rocks are normally
reduced sulfur species, and values close to 0‰. For this characterized by considerably higher δ18O values, typically in
reason, the determination of externally derived S in Archean the range of 8‰ to 30‰. Owing to these large differences,
deposits, or in younger deposits that may have interacted with oxygen isotopes may also be sensitive indications of country
Archean rocks, may be difficult using δ34S values. However, a rock assimilation. General features of how oxygen isotopes
process known as mass-independent fractionation (MIF) of S may be applied in the study of magmatic ore deposits have
isotopes has led to variations between δ33S and δ34S, which are been discussed by Ripley[8] and will not be repeated here.
distinct from most terrestrial processes that fractionate S However, common difficulties with a straight-forward
isotopes and are referred to as mass-dependent fractionation application of δ18O values to problems of assimilation by
processes. Farguhar and Wing[28] discussed the observation mafic magmas must be highlighted.
that sedimentary pyrite of Archean age is characterized by It is often assumed that elevated δ18O values in mafic
δ33S-δ34S relations that are indicative of MIF. The process has igneous rocks (e.g. >7‰) are indicative of contamination and
been attributed to low oxygen levels in the Archean assimilation of high-18O country rocks. Although this may
atmosphere and photochemical reactions involving SO2, but indeed be the case, careful petrographic examination of the
the controversy with respect to the origin of the process rocks, along with an evaluation of the magnitude of the δ18O
continues. The record of MIF may be present in magmas that shift, should be undertaken. For example, consider a mafic
have assimilated S of the Archean age[29,30], and future studies rock with bulk δ18O value of 9‰, a contaminant with a δ18O
using the technique may be able to distinguish the importance value of 16‰, and a presumed magma initial value of 5.5‰.
of S derived from Archean rocks in ore genesis. Figure 4 is a Assuming that the concentrations of O in the contaminant and
δ33S-δ34S plot showing sulfide samples from Voisey’s Bag (1.3 the magma are similar, a value of 9‰ in the mafic rock will be
Ga) and Alexo (a komatiite-associated, ~2.7 Ga deposit). suggested in ~35‰ bulk contamination. This degree of
Naldrett[1] proposed that S in the Alexo deposit may have been contamination is unreasonable in most situations because of


the amount of heat required to achieve such a large geosciences. Several excellent text books and review articles
contaminant/magma ratio[34]. Unless xenoliths provide address these methodologies[41–44]. Here we wish to emphasize
evidence for such high degrees of contamination (the presence that care must be taken to assure that closed-system conditions
of refractory oxides such as corundum and spinel), other prevailed before conclusions regarding contamination history
methods that can explain the 18O-elevation must be explored. can be made. For example, the Re/Os isotopic data from
Obviously knowledge of the δ18O values of potential sulfide minerals in the Voisey’s Bay deposit[27] mentioned
contaminants is extremely important. The assimilation of very above produces an isochron, which is consistent with the age
high-18O siliceous rocks, carbonates, or hydrothermally altered determined via U-Pb geochronology Fig. 5. The elevated
felsic rocks can be responsible for elevated δ18O values in initial 187Os/188Os ratio clearly indicates that crustal
mafic igneous rocks and can satisfy reasonable thermal contamination occurred, the sulfide system was well-mixed,
constraints. and that closed system conditions have generally prevailed. In
The most common cause of elevated δ18O values in mafic contrast, in situations where a reasonable isochron is not
rocks that is often overlooked is low-temperature produced, an interpretation involving crustal contamination
hydrothermal alteration and isotopic exchange. Low- must proceed with caution. Figure 6 illustrates a case where an
temperature isotopic exchange involving a fluid has been initially chondritic system interacts with a fluid after
discussed by several investigators[35–37]; one pertinent example crystallization. Loss of rhenium during the interaction may
will be presented here. At temperatures below ~200°C, the significantly lower the 187Re/188Os ratio. Continued decay may
oxygen isotopic fractionation between plagioclase and water is then produce samples with 187Os/188Os ratios that will generate
greater than 6‰[38]. For a situation where the fluid-rock anomalous γOs values, assuming that an age for the rocks is
interaction occurs below 200°C with a large excess of fluid known from a system such as U/Pb. The elevated γOs values
with a δ18O value of, say 0‰, a plagioclase δ18O value in in this case will not be a signal of crustal contamination, but
excess of 6‰ can result. Alteration minerals that may form at will have been produced as a result of the interaction with a
low temperatures (clays, zeolites, oxides, carbonates) are also hydrothermal fluid. A scattered population of Re/Os data
characterized by large mineral-water fractionation factors, and points may offer little insight into the contamination processes
therefore, elevated whole rock δ18O values will be expected, if an age is unavailable from other isotopic systems. Even if
even if the fluid involved in alteration and isotopic exchange an age is available, the potential effects of hydrothermal
was relatively low-18O meteoric water. Elevated δ18O values processes must be carefully evaluated.
should be evaluated in concert with trace elements and other
isotopic systems, but often careful petrographic examination is
sufficient to detect the involvement of hydrothermal fluids.
The measurement of mineral separates is also encouraged.
Where contaminated magma is envisioned, all primary
minerals should be characterized by anomalous δ18O values.
Owing to the kinetic exchanged effects at low temperature, all
minerals may not illustrate the same extent of exchange with
the hydrothermal fluid.
Large degrees of fractional melting of country rocks may
lead to the development of small xenoliths in mafic rocks that
contain refractory minerals such as corundum and spinel[39,40].
In this case, elevated and widespread δ18O values in mafic
rocks may be related to unusually high degrees of assimilation.
However, such degrees of assimilation are frequently related
to repeated magma passage in a conduct system, and no single
magma pulse attains an anomalously high δ18O value.
Elevated δ18O values may also be related to diffusive oxygen
isotopic exchange near high-18O xenoliths, but such exchange Fig. 5 Re-Os isotopic data[27] from sulfide minerals in the
zones tend to be confined to contacts with xenoliths and are Voisey’s Bay deposit
The sulfide samples define a very reasonable isochron and the
not spatially widespread[8].
initial ratio strongly indicates that assimilation of radiogenic crust
has occurred.
4 Radiogenic isotope systems
Recent studies of the Bushveld Complex by Mathez and
Radiogenic isotopes and trace elements have been the coworkers[45,46] have shown the importance of analyzing
standard methods of assessing contamination problems in the individual minerals. These suggest that the variability in Pb


isotopic ratios result from the introduction of at least one other minerals are open. When whole rocks are analyzed, the
fluid characterized by a different Pb isotopic composition at a isotopic signal is that of the mixed populations and
time when feldspars are closed to Pb addition but sulfide meaningful interpretations of the data may be impossible.

Fig. 6 Theoretical example of the open system behavior for a sulfide deposit formed at
1.1 Ga with a chondritic Os isotopic ratio
Hydrothermal perturbation to the system during decay since 1.1 Ga may lead to a lowering of
187
Re/188Os ratios. In this example, hydrothermal alteration at 0.25 Ga has led to a low 187
Re/188Os
ratio and a γOs value, if computed for 1.1 Ga, which will be highly anomalous. An interpretation
of crustal assimilation at 1.1 Ga will be erroneous; isochronous samples similar to those at Voisey’s
Bay (Fig. 5) will be required to substantiate such an interpretation.

A similar situation has been described by Griffin and his evaluation of possible perturbations to isotopic systems as a
co-workers[47,48], who noted the presence of several distinct result of hydrothermal alteration and exchange should be
Re/Os populations associated with different generations of undertaken before an interpretation involving magma
sulfide minerals in mantle peridotites. A whole rock approach contamination by country rocks is invoked.
in this case renders data that are not only difficult to interpret,
but essentially meaningless. Technological advances now Acknowledgements
permit the determination of isotopic ratios with high precision
in individual minerals using either laser ablation or ion This invited review paper is a contribution to the Ministry
microprobe methods. When spot analyses are not practical, of Education of China Project 111-B07011 awarded to China
mineral separates must be utilized if possible. University of Geosciences (Beijing) where the junior author is
appointed as a Concurrent Professor. The senior author is
5 Conclusions proud to be a Guest Professor at Lanzhou University.
Research in magmatic deposits at Indiana University is
Both stable and radiogenic isotopic measurements can currently funded by grants from the National Science
provide significant information in the evaluation of country Foundation of China (40534020) and from the National
rock contamination in the genesis of magmatic Cu-Ni-PGE Science Foundation of the United States (EAR-0710910).
deposits. Sulfur isotopes provide direct evidence for the
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