1. 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): 177–185. RESEARCH PAPER
Controls on the Fo and Ni Contents of Olivine in
Sulfide-bearing Mafic/Ultramafic Intrusions: Principles,
Modeling, and Examples from Voisey’s Bay
Chusi LI1,2,∗, Anthony J. NALDRETT3, Edward M. RIPLEY1
1 Department of Earth Sciences, Indiana University, Bloomington, IN 47405, USA
2 State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China
3 Department of Geology, University of Toronto, Toronto M5S 3B1, Canada
Abstract: Both Ni and Mg are compatible in mafic minerals that form early during the fractional crystallization of
mafic/ultramafic magma; thus, both decrease in abundance in the silicate magma, and hence in later-forming silicates as
fractionation proceeds. The concentration of Ni in silicates such as olivine and the MgO/FeO ratio of the silicates are related to
values in the magma from which the olivines are crystallized by coefficients, which have been experimentally determined and
therefore can be used to infer information about the magma. If the magma is saturated in sulfide so that sulfide droplets are
removed along with mafic silicates during fractionation, additional Ni will be removed in comparison with the sulfide-absent
situation. This will be reflected in a more rapid decrease of Ni with Fo than if sulfides were not separated. Variations of Ni with Fo
are examined in the light of model curves for the Voisey’s Bay Intrusion that hosts a world-class Ni-Cu-Co sulfide deposit in
Labrador, Canada. In the past, it has been a practice to compare the Ni and Fo contents of olivines from a given intrusion with the
field determined by Simkin and Smith (1970) for a wide variety of igneous olivines to identify those that are Ni-depleted. The
objective is that these are presumed to have come from sulfide-saturated, and therefore, economically-interesting magma. This
study shows that this simple comparison can lead to errors. It is important to compare natural data with model curves that have
been generated for, and reflect the cumulus mineralogy of each intrusion in question. Using this approach, the natural data can be
closely duplicated by model curves, which, in some cases, place additional constraints on possible petrologic interpretations. For
example, at Voisey’s Bay, a period of sulfide-unsaturated fractionation can be shown to have been succeeded by the removal of a
sulfide liquid plus silicate minerals, followed by a period of silicate crystallization.
Key Words: olivine; Ni; mafic-ultramafic magma; sulphide deposit
1 Introduction of the first studies of the Ni content of olivine from
mafic/ultramafic intrusions in the world. They showed that as
Olivine is an important phase of mafic and ultramafic will be expected from the relative compatibilities of Ni, Mg,
magmas that is parental to several important Ni-Cu sulfide and Fe in the olivine structure, the Ni content of olivine
deposits in the world. This review article is concerned with the decreases with decreasing Fo content, with Ni showing a very
use of olivine in mafic/ultramafic intrusions as an indicator of sharp initial decrease that becomes more gentle at lower Fo
processes that have affected the intrusions, particularly contents. The data from this study have commonly been used
whether the magmas forming the intrusions have become as a yardstick with which to evaluate whether olivine from a
saturated in sulfide at any stage during their emplacement or given suite of rocks crystallized from chalcophile-depleted
subsequent crystallization. Simkin and Smith[1] conducted one magma or not. The recent studies of some sulfide-bearing
Received date: 2007-09-10; Accepted date: 2007-10-10.
* Corresponding author. E-mail: cli@indiana.edu
Foundation item: Supported by the Ministry of Education of China Project (111-B07011); the National 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.

2. Chusi LI et al. / Earth Science Frontiers, 2007, 14(5): 177–185
intrusions in the world[2,3] show that this simple comparison K = KD (γNiS/γFeS)sulfide / (γNiO/γFeO)olivine (3)
can lead to errors. It is important to compare natural data with where, γ refers to the activity coefficient. If Ni and Fe
model curves that have been generated for and reflect the substitutions into olivine are ideal, as suggested by Campbell
cumulus mineralogy of each intrusion in question. Using this and Roeder[6], i.e., both γNiO and γFeO are equal to 1, the
approach, the natural data can be closely duplicated by model exchange partition coefficient for the reaction will be equal to
curves, which in some cases, place additional constraints on the equilibrium constant multiplied by the ratio of the activity
possible petrologic interpretations. The importance of this coefficients of the two sulfide components in the sulfide liquid.
aspect in evaluating any specific intrusion as a host for Brenan[7] found that although it is relatively insensitive to
magmatic sulfide ore is self-evident. temperature, KD varies with ƒO2, and the Ni content of the
sulfide liquid decreases as NiS decreases. Thus the ratio of
2 Controls on olivine composition NiS/FeS is not constant, but is considerably greater than unity.
However, at constant ƒO2 and sulfide liquid composition, KD is
Controls on olivine composition include parental magma constant[7], and equation (2) predicts that when a suite of
composition, fractional crystallization, reaction with olivine crystals with varying FeO contents are immersed in a
interstitial silicate and sulfide liquids, and hydrothermal sulfide liquid, after reaction with the sulfide liquid, the olivine
alteration at elevated temperatures. Nickel and magnesium are with higher FeO content will contain more Ni than the olivine
compatible in early crystallizing olivine and thus decrease in with lower FeO content, producing an inverse Ni-Fo relation
of olivine. This relation can be used to evaluate the sulfide
abundance as crystallization proceeds. In contrast, Fe is less
liquid composition using olivine from sulfide-bearing rock
compatible in comparison with Mg; thus, the FeO/MgO ratios
samples. An example of such application to the Jinchuan
of a magma and of olivine crystallizing from it increase and
Ni-Cu sulfide deposit in western China is given in Li et al[8].
provide an index to the fractional crystallization experienced
Finally, metamorphism/alteration may also affect the
by the magma. Since nickel is present as a trace element in
olivine composition. Olivine in mafic and ultramafic rocks is
magmas, its behavior during crystallization of olivine tends to
commonly partially altered to serpentine along grain margins
follow Henry’s law, and its concentration in olivine provides a
and micro-fractures. The available data indicate that
guide to the concentration in the magma. The same is not true
serpentinization does not affect the composition of residual
for major elements such as Mg and Fe, but Roeder and
olivine. However, olivine may be converted to a more Fe-rich
Emslie[4] have shown that in the case of olivine crystallizing
variant during actinolite alteration, as noted in the Jinchuan
from basaltic magma, the ratio (FeO/MgO)olivine/(FeO/MgO)magma
intrusion[8]. Re-crystallized olivine during metamorphism/
is constant with a value of 0.3±0.03. Thus, the Fo (forsterite)
alteration should be avoided in attempts to interpret the
content of olivine indicates the FeO/MgO ratio of the magma
composition of parental magma.
from which it crystallized.
The compositions of early cumulus olivine can be modified
3 Approach to modeling
by subsolidus re-equilibration with trapped silicate liquid in
intrusive rocks. Olivine crystallizing from the trapped silicate
The crystallization of solid phases in magma chambers and
liquid, nucleating on a core of original cumulus olivine, will sub-volcanic conduits can occur in a number of ways. For
become poorer in the Fo component as the trapped silicate example, solids can form in a magma, remain in suspension,
liquid fractionates further. Diffusion of Fe and Mg within the and thus remain in equilibrium with the magma as
olivine structure will result in olivine with a composition that crystallization proceeds. This results in solid phases of
is poorer in Fo than the original cumulus material. Barnes[5] uniform composition, and is referred to as equilibrium
referred to this effect as “trapped silicate liquid shift”. Discrete crystallization. Alternatively, the above process can continue,
and poikilitic olivine crystals are inevitably subjected to such until, at some stage, all the existing solids are removed at once,
compositional modification, resulting in lower Fo contents and the magma then continues crystallizing out of contact with
than their original compositions. the early formed material. This is referred to as bulk
When olivine is immersed in a sulfide liquid, the crystal and equilibration. Phenocrysts that are presented in a magma as it
liquid will exchange Ni and Fe according to the following intrudes, and which are then settled out once flow stops before
reaction: reaching the surface, are also an example of bulk
NiOolivine + FeSsulfide = NiSsulfide + FeOolivine (1) equilibration.
The exchange partition coefficient KD for this reaction is Perfect fractional crystallization occurs if crystals are
defined as: removed from a magma as soon as they form. This gives rise
KD = (NiS/FeS)sulfide / (NiO/FeO)olivine (2) to the maximum change in the composition of the remaining
where, the concentrations are expressed in mole fractions. magma. In intrusive rocks, some magma will remain trapped
KD is related to the equilibrium constant, K, by the expression: between accumulating crystal phases. This has two results (1)

3. Chusi LI et al. / Earth Science Frontiers, 2007, 14(5): 177–185
the fractional crystallization of the main body of magma is initial liquids, respectively, F is the proportion of remaining
less extreme than that due to perfect fractional crystallization, liquid, and D is the bulk partition coefficient of Ni between all
and (2) the trapped liquid reacts with early forming phases, solid phases and the coexisting magma. When the proportions
modifying their compositions. In some cases, a liquid that is of crystallizing phases change, new values of CO, F, and bulk
out of equilibrium with the cumulus phases percolates through D must be computed and used in the new steps of
the cumulus pile, perhaps because it is denser than the trapped fractionation. The proportions of crystallizing phases and the
intercumulus liquid, reacting and modifying the composition partition coefficient of Ni between each of the crystallizing
of the cumulates[9]. phases and the magma are required for the calculation of the
Crystallization of magma is clearly not a simple process. bulk DNi value. The DNi value between a solid and magma,
Since this review article is concerned with identifying particularly between olivine and magma (see a recent review
magmatic systems in which chalcophile element depletion due by Bédard 2005[11]), is related to the melt composition and
to reaction with sulfide liquid has occurred, we have chosen to temperature. Li and his co-workers[2,3] suggested an
model perfect fractional crystallization of silicates, since this olivine/melt DNi value of 7 for S-bearing basaltic magmas
will produce the maximum compositional change. If the based on the results of MORB samples[12]. In komatiitic
observed change is greater than this, then one must look to magma, the olivine/melt DNi is between 3 and 5[13]. Based on
other factors such as segregation of immiscible sulfide liquid. the available experimental data, we believe that the DNi values
There are two ways to model the variation of Fo in olivine of 1 and zero for clinopyroxene and plagioclase, respectively,
during fractional crystallization, depending on whether or not are reasonable for basaltic systems.
the full composition (i.e., all major element concentrations) of In situations where in the segregation of liquid sulfide
the parental magma is known. The compositions of associated occurs during silicate crystallization, the DNi value of 500
lavas, dykes, and chilled marginal rocks may be used to between the sulfide liquid and magma from Peach and
represent the parental magmas of associated intrusions. If only Mathez[14] has been used by several people. The Ni content of
the FeO/MgO ratio of the parental magma is known, then one the sulfide can be calculated by applying this coefficient to the
can use a numerical approach in which a very small fraction Ni content of the magma. The Fe removed as sulfide is then
(generally 0.1 wt%) of magma is removed as olivine, subtracted as FeO from the FeO content of the magma, along
olivine+plagioclase, olivine+plagioclase+clinopyroxene, and with the FeO involved in the silicate phases removed at each
the composition of the magma remaining after the subtraction step.
is recalculated. The proportions of the silicate phases that are The “trapped liquid shift” can be modeled using the
removed at each step should be based on the cumulus proportions of silicate phases by equilibrium crystallization.
mineralogy of the igneous bodies in question. Crystallization The proportions of silicate phases crystallized from the
of plagioclase does not change the FeO/MgO ratio in the trapped liquid can be calculated using the MELTS program of
magma, and therefore, does not affect the content of Fo in Ghiorso and Sack[10] if all major components of the liquid are
olivine. However, it affects the bulk solid/magma DNi value known. If only FeO and MgO in the liquid are known,
that is required for modeling the fractionation of trace assumption has to be made for the proportions of silicate
elements such as Ni. Crystallization of clinopyroxene affects phases in the liquid based on the available experimental
both the bulk DNi value and the FeO/MgO ratio in the magma. results of similar systems. The amounts of “trapped liquid” in
In the case of the modeling of olivine and clinopyroxene cumulate rocks can be estimated using the concentrations of
fractionation, the (FeO/MgO)olivine/(FeO/MgO)magma ratio of incompatible trace elements from whole rock analyses.
0.3 from Roeder and Emslie (1970) and the
(FeO/MgO)clinopyroxene/(FeO/MgO)magma ratio of 0.26 are most 4 Examples from Voisey’s Bay
appropriate for basaltic systems based on the available
experimental data. The Voisey’s Bay deposit was discovered in 1994 by
If all major components of the parental magma are known, prospectors working for Diamond Fields Resources Ltd. It
fractional crystallization of the magma can be simulated using was acquired by INCO Ltd. in August 1996 through their
the MELTS program of Ghiorso and Sack[10]. The updated subsidiary, Voisey’s Bay Nickel Company. Numerous articles
version of the MELTS program is currently available online at describing the geology of the Voisey’s Bay deposit have been
http://melts.of mresearch.org. published[3,15–21]. The deposit occurs in and at the base of the
In either case, after the relation between the Fo content of bodies of 1.334 Ga troctolite (Fig. 1) that occur close to the
olivine and the degree of crystallization is obtained from the 1.85 Ga collisional boundary between Archean Nain Province
above calculations, the fractionation of Ni can be calculated to the east and the Proterozoic Churchill Province to the west.
using the following equation for trace elements: The most current understanding of the genesis[3] is that mafic
CL = CO F(D-1), (4) magma rose to intrude sulfide- and graphite-bearing pelitic
where, CL and CO are the concentrations in the residual and gneisses. During its ascent, it reacted with mid-crustal rocks

4. Chusi LI et al. / Earth Science Frontiers, 2007, 14(5): 177–185
and developed a mid-crustal trace element and isotopic km-long sheet-like conduit to spread out as a higher level
signature. Further fractionation and reaction occurred at the intrusion. The fresh, undepleted magma upgraded the sulfides
level of the sulfidic-graphitic gneisses, peridotitic and that had lodged in the conduit and at the mouth of the conduit
mela-troctolitic (olivine+intercumulus plagioclase) cumulates with the base of the higher level intrusion. It also disrupted
developed, an additional trace element signature related to the cumulates within the lower intrusion, giving rise to the
enclosing gneisses was imposed on the magma, and sulfides peridotitic and mela-troctolitic inclusions, which are the
segregated, depleting the magma in chalcophile elements. A record of this early phase of crystallization. The bulk of the
second wave of fresh magma entered the system, forcing the troctolite that has resulted from the second wave of magma is
depleted magma, along with its contained sulfides, upward a 1 a plagioclase-olivine cumulate.
Fig. 1 A 3D view of the Voisey’s Bay Ni-Cu-Co sulfide deposit in Labrador, Canada
Plots of Ni versus Fo content in olivine are shown in Fig. Deeps chamber and the feeder do not follow the same model
2A and 3B. Olivines in the melatroctolite inclusions observed fractionation curve but fall below it. Their behavior is
in the feeder show a steep trend of decreasing Ni content with explicable if the magma underwent a period fractionation
decreasing Fo, in which Ni decreases from 3100 ppm at about involving the removal of sulfide and silicate in a ratio of 1:133
Fo 80 to 1500 ppm at Fo 72 (Fig. 2A). Olivines in the (red line in Fig. 2A), followed by further sulfide free
Leucotroctolite of the Reid Brook chamber, and some fractionation of the Ni-depleted magma (purple line in Fig.
Troctolites and Olivine Gabbro of the Eastern Deeps chamber 2A). Li and Naldrett[16] proposed that the interaction between
and Feeder Olivine Gabbro show a more gentle trend with Ni the troctolite magma and the surrounding sulfide-bearing
decreasing from 1500 ppm at Fo 75 to as low as 200 ppm at Tasiuyak gneiss had caused this sulfide saturation. The
Fo 41 (Fig. 2A). The model curves shown in the figure have sulfide/silicate ratio used in the model (red line in Fig. 2A) is
been derived using the MELTS program[10] for the fractional similar to the cotectic ratio calculated based on the results of
crystallization of a mafic magma with composition similar to crystallization simulation and the empirical equation of sulfur
the average compositions of two high-MgO and low-TiO2 solubility at sulfide saturation by Li and Ripley[22]. An
chilled marginal rock samples from the feeder dyke[18]. example of calculating the cotectic ratio of sulfide/silicate
The olivines of the melatroctolite inclusions follow the during sulfide segregation from sulfide-saturated magma is
model curve for fractional crystallization of the assumed given in Barnes[23].
initial magma. Olivines from sulfide-free troctolites from the Olivines in the Normal Troctolite and Varied-textured
Eastern Deeps chamber and Leucotroctolites of the Reid Troctolite (Fig. 2B), both of these rock types contain minor to
Brook chamber, along with olivine gabbros from the Eastern significant amounts of sulfide and show no systematic trend

5. Chusi LI et al. / Earth Science Frontiers, 2007, 14(5): 177–185
Fig. 2 Plot of nickel (Ni) content of olivine versus forsterite (Fo) content for (A) rocks related to the
initial introduction of magma; and for (B) rocks related to the later introduction(s) at Voisey’s Bay
The field of Simkin and Smith[1] is shown in (B) for comparison.

6. Chusi LI et al. / Earth Science Frontiers, 2007, 14(5): 177–185
but form a “cloud” with Fo varying between 73 and 52 and Ni Smith[1] is not a satisfactory method of identifying chalcophile
varying between 2000 and 700 ppm. The cloud extends from depletion (Fig. 2B). Olivines from a single rock unit such as
the model curve for depleted magma up to and above the the troctolite of the Eastern Deeps chamber or the
curve for undepleted magma. Li and Naldrett[16] showed that leucotroctolite of the Reid Brook chamber plot within and
olivines that are in contact with sulfide react with the sulfide, below the field of Simkin and Smith[1]. A simple comparison
exchanging Ni and Fe. Figure 3 illustrates the difference in the can lead to the conclusion that both Ni-undepleted and
Ni-Fo co-variation in olivines from the Normal and Varied- Ni-depleted magmas were involved in the formation of these
textured Troctolites at Voisey’s Bay between those in contact rock units. However, the results of our numerical modeling
with sulfide and those that do not show such contact. Olivines have shown that only an Ni-depleted magma was involved.
in contact with sulfide are distinctly poorer in Ni than those
that occur as inclusions in plagioclase and are insulated from Acknowledgements
sulfide. This is evident since at Voisey’s Bay, the reaction
between sulfide and olivine has depleted the olivine in Ni, and It is our great pleasure to contribute this invited review
therefore, the Ni contents shown for the Normal and Varied-textured paper to Earch Science Frontiers, an important geological
Troctolites at Voisey’s Bay are minimum values, whereas the journal in China. Professor Yu-sheng Zhai, Editor-in-Chief of
original contents were likely higher. Geologic relationships this journal, is a former supervisor of the senior author and a
indicate that the Normal and Varied-textured Troctolites close friend of the second author since 1992. This is a
postdate the other intrusive phases, thus providing evidence contribution to the Ministry of Education of China Project
for the passage of a later, undepleted magma through the 111-B07011. Research in magmatic sulfide deposits at Indiana
system. University is currently funded by grants from the National
Science Foundation of China (40534020) and from the
National Science Foundation of the United States
(EAR-0710910).
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