JOURNAL OF PETROLOGY
VOLUME 54
NUMBER 7
PAGES 1455^1481
2013
doi:10.1093/petrology/egt018
Deposition Mechanisms of Magmatic Sulphide
Liquids: Evidence from High-Resolution X-Ray
Computed Tomography and Trace Element
Chemistry of Komatiite-hosted Disseminated
Sulphides
BE¤LINDA M. GODEL1*, STEPHEN J. BARNES1 AND
SARAH-JANE BARNES2
1
CSIRO EARTH SCIENCE AND RESOURCE ENGINEERING, AUSTRALIAN RESOURCES RESEARCH CENTRE, 26 DICK
PERRY AVENUE, KENSINGTON, WA 6151, AUSTRALIA
2
UNITE DES SCIENCES DE LA TERRE, UNIVERSITE DU QUEBEC A CHICOUTIMI, CHICOUTIMI, QC, G7H 2B1, CANADA
RECEIVED DECEMBER 7, 2011; ACCEPTED MARCH 5, 2013
ADVANCE ACCESS PUBLICATION APRIL 23, 2013
Magmatic sulphides are a widespread component in mafic and
ultramafic rocks and contain variable concentrations of nickel,
copper and platinum-group elements. Previous literature has been
concerned with the whole-rock geochemistry of magmatic sulphide
ores and their host-rocks and relatively little attention has been paid
to the physical nature of magmatic sulphide transport and accumulation. Our high-resolution X-ray computed tomography study quantifies for the first time the 2D and 3D size, shape and textural
relationships, and distribution of disseminated magmatic sulphides
and olivine in adcumulates from komatiites. These new data are
combined with analysis of trace-element concentrations within sulphides to provide important information about the mechanisms of
transport, deposition and post-accumulation migration of sulphide
liquid in dynamic magmatic systems. Olivine shows evidence of textural maturation, with larger crystals growing at the expense of
small ones to different degrees depending on the sulphide content of
the rock. The olivine texture and the presence of poikilitic chromite
provide evidence of in situ nucleation of olivine and chromite at the
interface between a flowing magma and a basal pile of crystals.
Disseminated to strongly interconnected base-metal sulphides are
located at contacts between olivine crystals or in some cases can be entirely or partially enclosed within chromite. Based on their 3D
morphologies, their size distribution and their Pd concentrations,
the sulphides are divided into four main categories: finely disseminated sulphides; disseminated to slightly interconnected sulphides;
disseminated to globular sulphides; disseminated to strongly interconnected sulphides. All samples contain a population of sub-spherical
sulphide blebs (51000 mm equivalent sphere diameter; ESD),
which are observed in the olivine^sulphide cotectic proportion and
which contain the lowest Pd concentrations. These small droplets
are interpreted to have formed by segregation of immiscible sulphide
liquid upon cooling of a komatiitic magma flowing in a magma conduit or channel.These newly formed droplets were trapped in situ by
the crystallizing framework of olivine and/or chromite. Larger sulphide blebs (up to 10 mm ESD) are present where the sulphide
abundance is 43 wt % and the sulphide bleb size population is
multi-modal. The Pd content of the sulphide blebs is variable and
positively correlated with the sulphide bleb size.The overall sulphide
abundance, sulphide bleb size and Pd concentrations indicate that
these sulphides have been transported in a flowing sulphur-saturated
magma over some distance and accumulated at their present site by
mechanical processes. Strongly interconnected network to matrix sulphides are observed in samples containing more than 5 wt % sulphide with small variability in Pd concentrations within and
between blebs. These sulphides are interpreted to reflect the accumulation and coalescence (by film drainage) of small sulphide blebs.
*Corresponding author. Telephone: þ61 8 6436 8908. Fax: þ61 8 6436
8586. E-mail: [email protected]
ß The Author 2013. Published by Oxford University Press. All
rights reserved. For Permissions, please e-mail: journals.permissions@
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JOURNAL OF PETROLOGY
VOLUME 54
Overall our results show that komatiite-hosted disseminated sulphides form by a mechanical accumulation process that takes place
against a background of steady-state in situ nucleation of small
blebs along the olivine^sulphide liquid cotectic.
KEY WORDS: X-ray computed tomography; komatiites; disseminated
sulphides; adcumulate
I N T RO D U C T I O N
Magmatic sulphides are a widespread trace component in
mafic and ultramafic rocks, and are universally agreed to
have formed by accumulations of immiscible Fe^S^O liquids with variable contents of Ni, Cu, Co and platinumgroup elements (PGE including Pt, Pd, Rh, Ru, Ir and
Os). These sulphides account for much of the world’s
known reserves of economically exploitable Ni and PGE.
A very large body of literature is concerned with the
whole-rock geochemistry of magmatic sulphide ores and
their host-rocks (Naldrett, 2004; Barnes & Lightfoot,
2005; and references therein) but relatively little attention
has been paid to the physical nature of magmatic sulphide
transport and accumulation. Textural data potentially contain important information about the mechanisms of
transport, deposition and post-accumulation migration of
sulphide liquid droplets (de Bremond d’Ars et al., 2001;
Mungall & Su, 2005; Godel et al., 2006; Barnes et al.,
2008b; Chung & Mungall, 2009). However, data describing
the relationship between the texture of the sulphides (size,
shape and distribution) and the silicate minerals in the
host-rocks are largely lacking. This information is important not only for the understanding of the genesis of ore
deposits, but also for the understanding of the distribution
of highly chalcophile and siderophile elements such as Ni
and the PGE in the crust and the mantle.
Komatiites are ultramafic lavas predominantly restricted to the Archean and inferred to be formed by a
large degree (430%) of partial melting of the mantle
(Arndt et al., 2008). Magmatic Ni^Cu^PGE deposits typically form by segregation of immiscible sulphide liquid
from mafic to ultramafic magmas (Naldrett, 2004, and references therein). Since the discovery in the mid-1960s of
nickel sulphide mineralization at Kambalda in Western
Australia, komatiites have been recognized as significant
hosts of magmatic Ni ores, with deposits located mostly in
the Yilgarn Craton of Australia, the Zimbabwe Craton
and the Superior Province of Canada. Komatiite-hosted
sulphide deposits are well suited to the study of the physical
processes of sulphide accumulation, for two main reasons:
(1) they form in volcanic or subvolcanic environments,
and hence there is a strong probability of preserving magmatic textures; (2) most komatiite ores are essentially
simple three-component mixtures of silicate magma, sulphide liquid and olivine (with or without trace amounts
NUMBER 7
JULY 2013
of a fourth component, chromite). Sulphide liquid crystallizes as monosulphide solid solution (MSS) and intermediate solid solution (ISS). These subsequently unmix to form
polymineralic aggregates, typically of pentlandite [(Fe,
Ni)9S8], pyrrhotite [Fe(1^x)S] and chalcopyrite (CuFeS2)
with variable but minor amounts of pyrite (FeS2). These
sulphide aggregates are commonly referred to as ‘blebs’.
Based on host-rock lithologies and the distribution and
abundance of the mineralization, komatiite-hosted nickel
deposits have been divided into two main types (Lesher,
1989; Hill & Gole, 1990; Lesher & Keays, 2002): Type I
ores consisting of massive, net-textured or matrix ores
where sulphides represent 40^60% of the rock and are
located at the base of komatiite lava flows; Type II ores
consisting of disseminated sulphides (typically 1^5 vol.
%) hosted within thick olivine cumulate bodies that form
large but low-grade nickel deposits. The Type II ores are
further divided into Type IIa and Type IIb ores. In the
Type IIa ores, the sulphides occur as accumulations of
spherical blebs forming disseminated mineralized halos
around Type I mineralization. In the Type IIb ores, the sulphides occur within large lenticular olivine cumulate
bodies as interstitial sulphide blebs or networks surrounding olivine grains.
The genesis of Type I and Type II deposits has been
debated over the years and the question remains as to
whether Type I and II have different origins or whether
they represent a continuum between two end-members
(Duke, 1986; Hill & Gole, 1990; Barnes, 2006a, 2006b,
2007; Grguric et al., 2006). Based on the consistency of
grades observed in Type II deposits, Duke (1986) and Hill
& Gole (1990) argued that the disseminated sulphide
formed by cotectic precipitation of sulphide liquid and olivine from komatiite magma undergoing fractional crystallization. According to this model, the sulphides are
formed in situ and are not transported and deposited to a
particular location as proposed for Type I deposits, suggesting that Type I and II are genetically distinct. More
recent results, based on the analysis of sulphur distribution
within various deposits (Barnes, 2006a; Grguric et al.,
2006) and the modelling of the proportion of cotectic precipitation of sulphide liquid and olivine (Barnes, 2007), suggest that Type II ores may represent a mixture of sulphide
formed by in situ liquation and sulphide droplets transported and mechanically deposited. Hence a genetic continuum may exist between Type I and II ores. The
abundance of sulphides in Type II ores tends to vary at
the centimetre scale (Grguric et al., 2006) and potentially
reflects the balance between physical and chemical processes leading to ore genesis. However, the distinction between in situ and transported sulphides is far from trivial.
The present study focuses on the analysis of the threedimensional (3D) abundance, size and shape distribution
of sulphide and chromite determined by high-resolution
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GODEL et al.
MAGMATIC SULPHIDE DEPOSITION
X-ray computed tomography, the 2D crystal-size distribution of olivine and the trace element chemistry [determined by laser ablation inductively coupled plasma mass
spectrometry (LA-ICP-MS)] of sulphides observed in samples from disseminated sulphides hosted in dunites from
the Mount Keith deposit (Western Australia), considered
to be the type example of Type II ores (Barnes et al., 2011,
and references therein). The combined results provide new
insights into the relative roles of in situ liquation and mechanical transport and deposition of sulphide liquid droplets
in natural systems.
G EOLO GY
Regional setting
The Mount Keith Ni-sulphide deposit (referred to as
MKD5 deposit, Fig. 1) is located in the northern part of
the Agnew^Wiluna Greenstone Belt within the Archean
Yilgarn Craton of Western Australia (Hill & Gole, 1990).
The Agnew^Wiluna domain (c. 2·7 Ga in age) consists of
felsic to intermediate volcanic and volcaniclastic rocks and
sedimentary rocks (mainly sulphidic cherts and carbonaceous shales) with komatiites and komatiitic basalts. This
domain is considered to be one of the most Ni-endowed regions in the world (Hronsky & Schodde, 2006) with several world-class Ni sulphide deposits (Fig. 1). Among
them, the Mount Keith deposit represents the world’s largest accumulation of komatiite-associated magmatic sulphide and contains an estimated 2·77 million tonnes of Ni
(Barnes et al., 2011). In the Mount Keith area, the ultramafic rocks have been divided into three horizons from
stratigraphic bottom to top; the Mount Keith Ultramafic
unit, the Cliffs Ultramafic unit and the Monument
Ultramafic unit, which are separated by felsic and mafic
rocks (Fiorentini et al., 2012). The Cliffs and Monument
Ultramafic units contain spinifex-textured komatiites
(Grguric et al., 2006; Fiorentini et al., 2010) with massive
sulphide accumulation typical of basal mineralization
within channelized compound komatiite flows (Type I
described above) that occur locally at the base of the
Cliffs Ultramafic unit. Rosengren et al. (2005, 2008) suggested that the Mount Keith Ultramafic unit represents a
subhorizontal shallow-level intrusive sill and that the
Cliffs Ultramafic and Monument units represent subaqueous extrusive komatiites. The origin of the Mount Keith
Ultramafic unit has been challenged by recent work by
Gole et al. (in press), on the basis of a detailed study of the
upper (western) contact of the complex that indicates extensive structural juxtaposition, and an absence of thermal
effects on the immediate hanging wall dacites. Gole et al.
(in press) favoured the model of Hill et al. (1995) for an extrusive origin within a high-flux feeder channel to a major
flow field.
Geology of the Mount Keith deposit
The MKD5 deposit (Fig. 2) represents the type example of
low-grade, large tonnage Type II nickel sulphide hosted in
komatiite (Grguric et al., 2006). Primary igneous silicate
minerals have been completely replaced by secondary minerals, with excellent pseudomorphic preservation of igneous
textures. The descriptions here are based on the inferred primary igneous lithologies and are expressed in igneous
terms; the prefix ‘meta-’ should be assumed throughout.
The MKD5 deposit is hosted in the Mount Keith
Ultramafic Unit consisting mainly of pods or lenses of olivine adcumulates (dunites) flanked laterally by olivine
meso- to ortho-cumulates, which have been divided into
seven primary units (Fig. 2) based on the texture of the cumulates and the presence of base-metal sulphides
(Rosengren et al., 2005, 2008; Grguric et al., 2006, and references therein). Most of the economic disseminated Ni-sulphide mineralization is hosted in the central Unit 104 of
the Mount Keith Ultramafic unit, a sequence of olivine
adcumulates to mesocumulates (Fig. 2), with mineralization up to 300 m in thickness intersected in the southern
and central parts of the complex (e.g. MKD153 drill core,
sampled in the present study). In the northern part of the
complex, additional mineralization is found within adcumulate to mesocumulate dunite (unit 106, Fig. 2), which
overlies ortho- to meso-cumulate peridotite (unit 105,
Fig. 2). The ultramafic rocks have been completely serpentinized (lizardite^brucite^chlorite assemblages) pseudomorphing original olivine crystals. Veinlets of various
mixed-layer hydroxycarbonate minerals such as tochilinite
and pyroaurite occur (Grguric, 2003) locally within
51m wide talc^carbonate veins formed by infiltration of
H2O^CO2-rich fluids (Barrett et al., 1977; Ro«dsjo«, 1999)
that exploited preferential paths along contacts between
lithologies, faults and/or shear zones (Fig. 2).
The ore zone has been divided into two main zones,
referred to as the Core zone and the Selvage zone
(Grguric, 2003; Grguric et al., 2006, and references
therein). Most of the economic mineralization is hosted by
the Core zone, which is characterized by whole-rock S/Ni
ratios 41 and sulphide abundance between 1 and 5 vol. %
(Grguric et al., 2006). The sulphide assemblage consists of
intergrowths of pentlandite and pyrrhotite forming sulphide blebs averaging 0·5 mm in size (up to 4 mm for
the largest blebs). In this zone, pyrrhotite may be partially
replaced by a corona formed by a fine intergrowth of magnetite (Fe3O4), pyrite (FeS2) and marcasite (FeS2). In contrast, the Selvage Zone is characterized by S/Ni ratios
lesser than unity, corresponding to modal sulphide contents of 51%, and is restricted to a thin 10^40 m wide
zone located at the contacts between units 102 and 105
(Grguric et al., 2006, and references therein). In that zone,
the sulphide assemblage is dominated by pentlandite with
hypogene cobaltian violarite and pyrite (Grguric, 2002).
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JOURNAL OF PETROLOGY
VOLUME 54
NUMBER 7
JULY 2013
Fig. 1. Simplified regional geology of the Agnew^Wiluna Domain and the Mount Keith area, Western Australia. Modified after Barnes (2006a).
The original magmatic morphology of the sulphide blebs is
typically preserved faithfully during the pseudomorphic
replacement in both zones.
A N A LY T I C A L M E T H O D S
Sampling
Representative samples were selected from Unit 104 based
on petrography, apparent sulphide morphology in two dimensions and a continuous whole-rock geochemical profile
through the entire unit from Barnes et al. (2011). A total of
25 samples, 20^50 cm in length, were cut from quarter or
halved MKD153 core after detailed logging of the ore zone
(Fig. 2). From these, six samples were selected to represent
the range of observed sulphide morphology and analysed
in further detail in three dimensions using high-resolution
X-ray computed tomography, LA-ICP-MS, 2D and 3D crystal-size distribution and whole-rock geochemistry.
Whole-rock geochemistry
Slabs of 0·5 cm thickness and 10 cm length were cut
through the samples to match as far as possible the geochemistry of polished blocks drilled from corresponding
rocks. The slabs were crushed and powdered in an aluminium mill at the University of Quebec at Chicoutimi
(UQAC). Major elements, Co, Cr concentrations and loss
on ignition (LOI) analyses were carried out at Geosciences
Laboratory (Sudbury, Canada). The major element, Co
and Cr concentrations were determined by X-ray fluorescence (XRF) analysis. Nickel and Cu were determined at
UQAC (LabMaTer) by atomic absorption spectrometry
(AAS) using a Thermo Scientific system. Sulphur and CO2
were determined using a HORIBA EMIA-220 V induction
furnace at UQAC following the method described by
Be¤dard et al. (2008). Platinum-group elements and Au were
determined at UQAC by nickel^sulphur-fire-assay Te-coprecipitation followed by ICP-MS analysis at UQAC using
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GODEL et al.
MAGMATIC SULPHIDE DEPOSITION
quantification of the internal structure of rocks. Over the
past few years, high-resolution XRCT (HRXCT) has
been successfully applied in various fields of the geosciences including paleontology (Balanoff et al., 2010), metamorphic geology (Carlson, 2010), igneous petrology
(Philpotts & Dickson, 2000), volcanology (Zandomeneghi
et al., 2010), and ore deposits including nickel and PGE
(Godel et al., 2006, 2010, 2012, 2013; Barnes et al., 2008b)
and gold (Kyle & Ketcham, 2003). The image obtained is
a 3D volume grey-scale representation of the variation in
attenuation of X-rays throughout the sample. The X-rays
are attenuated through the sample following the Beer^
Lambert Law as described by the equation
I ¼ I emh
where I is the intensity measured by the detectors, I8 is the
initial intensity of the incident X-ray beam, m is the attenuation coefficient of the material and h is the thickness of
the sample. The attenuation coefficient largely depends on
the average atomic number and the density of the material
and hence reflects variation in mineral chemistry and/or
mineralogy (Fig. 3).
Scanning and data reconstruction
Fig. 2. Simplified plan view of the Mount Keith Ultramafic Complex
(modified after Grguric et al., 2006).
the method described by Savard et al. (2010). Based on the
analysis of reference materials and duplicates, the precision
and the accuracy are better than 10%.
High-resolution X-ray computed
tomography
Principle
X-ray computed tomography (XRCT) is an entirely nondestructive technique that allows the 3D visualization and
Cylinders of around 25 mm diameter were drilled from the
selected samples and the circular surface was polished for
further optical microscope and mineral chemistry analysis
(Table 1). The cylinders were scanned using a Skyscan 1172
desktop high-resolution scanner at CSIRO Process Science
and Engineering (Perth, Australia). The scanner was set
up to 100 kV, 100 mA electron source, an Al^Cu filter and
pixel size optimized to have the maximum coverage of
the sample (Table 1). A projection of the sample was recorded at each step (i.e. 0·38 rotation of the sample) over
3608. Image (slice) reconstruction was carried out using
NRecon software. Beam hardening and ring artefacts
were corrected and minimized during the data reconstruction (Table 1). The reconstruction was optimized to differentiate between the phases of interest (sulphide, magnetite
and chromite). Serpentine after original olivine grains has
uniform attenuation coefficients and hence appears as
grey groundmass without variation at former olivine
grain boundaries.
Data processing and quantitative analysis
In some of the Mount Keith samples considered, pyrrhotite is pseudomorphically (up to 20 vol. %) replaced by
a corona of magnetite (Fig. 3). In these cases, to access the
original sulphide distribution in these samples (e.g.
MKD153-578·2 Part A and Part B) we considered the sulphide and magnetite assemblage as a single phase referred
to as sulphide. Chromite and sulphide were delineated
using a combination of 3D gradient watershed and 3D
region growing using AvizoFireÕ and/or in-house
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JULY 2013
Fig. 3. Representative slices reconstructed from HRXCT data, photomicrographs and backscattered-electron images showing representative
2D morphology of base-metal sulphide and their textural association with magnetite and/or chromite in the sample from Mount Keith.
(a) Slice reconstructed from HRXCT (sample MKD153-591·5) showing the 2D relationship between disseminated sulphide (e.g. white arrowhead) and poikilitic chromite (e.g. white rectangle). Lz, lizardite. (b) Photomicrograph (reflected light) showing the 2D textural relationship
between chromite (Chr) partially replaced by magnetite (Mt) and the base-metal sulphides (Sul). (c) Slice reconstructed from HRXCT
(sample MKD153-649·5) showing the relationship between sulphide (Sul) and magnetite (Mt). (d) Backscattered-electron image (sample
MKD153-669·5) illustrating the relationship between pentlandite (Pn), pyrrhotite (Po) and magnetite (Mt). (e) Slice reconstructed from
HRXCT (sample MKD153-578·2 Part A) showing the relationship between sulphide (Sul) and magnetite (Mt) surrounding the sulphide
blebs. (f) Backscattered-electron image (sample MKD153-578·2) illustrating the relationship between pentlandite (Pn), pyrrhotite (Po), magnetite (Mt) and accessory arsenide (white arrowheads).
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GODEL et al.
MAGMATIC SULPHIDE DEPOSITION
Table 1: Summary of samples and high-resolution X-ray computed tomography parameters during acquisition and
reconstruction
Sample
Pixel size
Beam Hardening
(mm)
correction (%)
MKD153-514·6
12·40
51
MKD153-578·2 Part A*
11·72
65
MKD153-578·2 Part B*
12·40
MKD153-591·5
Sample volume (mm3)
Volume of
Number of
sulphide (%)
sulphide blebs
2·08E þ 12
3·57
3027
1·99E þ 12
9·09
4266
51
1·82E þ 12
3·30
1486
11·89
80
2·32E þ 12
0·51
3761
MKD153-649·5
14·43
86
2·41E þ 12
4·45
2036
MKD153-669·5
11·89
86
2·33E þ 12
0·74
3583
*Two cylinders were drilled 5 cm apart to taken into account sulphide layering observed from two dimensions (Fig. 6).
algorithms following a method similar to that used by
Godel et al. (2010, 2012, 2013). The accuracy of phase segmentation was verified using either optical or scanning
electron microscopy (SEM). Isosurfaces were created for
the sulphides and the chromites using AvizoFireÕ and
were used to quantify the volume, size and shape characteristics of each sulphide bleb or chromite grain. Statistics
on sulphide bleb-size distribution and shapes were then
calculated and displayed using CSDToolbox1.0 software
(Ricard et al., 2012) and in-house algorithms.
Varying degrees of replacement by magnetite, and more
rarely by carbonate, accounts for the major source of uncertainty in measuring the sizes of sulphide blebs from the
HRXCT scans. If the magnetite rims on the sulphide
blebs formed by accretionary overgrowth rather than replacement, which is thought unlikely, then the sizes of
blebs may be overestimated by up to 10%. Because the frequency of bleb sizes is logarithmically distributed, this is
not a significant error, and the assumption is made that
the measured sizes of partially altered blebs are good approximations of original pre-alteration dimensions. There
is also a very small component of entirely secondary sulphide minerals developed within olivine pseudomorphs
during alteration, probably as a result of sulphidation of
magnetite during or post serpentinization (Donaldson
et al., 1986); however, these grains fall within the size
range close to the resolution of the HRCXT scans (i.e.
550 mm) and do not affect any of the conclusions.
Trace element mineral chemistry
Quantitative trace element concentrations in sulphides
were determined by LA-ICP-MS at the University of
Quebec at Chicoutimi (UQAC, Canada) using a Thermo
X7 ICP-MS system coupled with a New Wave Research
213 nm Nd:YAG UV laser ablation system. The analyses
were carried out in raster mode by using a 55 mm laser
beam, a laser speed of 5 mm s1, a laser frequency of
10 Hz, and a power of 0·3 mJ cm3. A large range of
isotopes were analysed but only a small number of them
are relevant to the present study (29Si, 33S, 34S, 57Fe,
59
Co,60Ni, 61Ni, 63Cu, 65Cu, 66Zn, 68Zn, 75As, 77Se, 82Se,
105
Pd, 106Pd, 108Pd). 34S was used as an internal standard
to calculate the concentration of trace elements in the sulphides with stoichiometric values used for each of the sulphide minerals. The ICP-MS calibration and quality
control were carried out using Laflamme-Po727, Mass-1,
MSS-5, and Po62 reference materials, as described by
Godel et al. (2007, 2012) and Godel & Barnes (2008b).
Trace element concentrations and interference corrections were calculated using a method similar to that
described by Godel et al. (2007, 2012) and Godel & Barnes
(2008b).
P E T RO G R A P H Y A N D
W H O L E - RO C K G E O C H E M I S T RY
The detailed petrography and whole-rock geochemistry of
the Mount Keith MKD5 deposit has been the topic of several studies (Rosengren et al., 2005, 2008; Grguric et al.,
2006; Fiorentini et al., 2007; Barnes et al., 2011). The following description focuses only on the characteristics of each
of the samples analysed in the present work.
All the samples were taken at various stratigraphic intervals within unit 104 and are interpreted as olivine adcumulates on the basis of their high Mg-numbers (Fig. 4a) and
low (50·5 wt %) Al2O3 concentrations (Fig. 4b and
Table 2). Disseminated sulphides are present in all of the
samples, with sulphur concentrations ranging from 0·95
to 2·22 wt % and S/Ni ratios varying from 1·2 to 3·2
(Table 3 and Fig. 4). Based on the 2D apparent morphology
and proportions of the sulphides, our samples can be classified into four groups: (1) disseminated sulphides containing
poikilitic chromite (i.e. MKD153-669·5 and MKD153591·5), whose 3D morphology and origin were described
by Godel et al. (2013); (2) disseminated sulphides with
extensive talc^carbonate alteration (MKD153-514·6);
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Fig. 4. Stratigraphic variations of major and trace elements across the Mount Keith deposit (MKD153 hole). The dashed lines represent 2 m aggregate samples reported by Fiorentini et al. (2007) and Barnes et al. (2012). The open diamonds represent the samples analysed in the present
study. Samples are named from their depth down hole MKD153.
(3) disseminated to globular sulphides; (4) disseminated to
matrix sulphides (MKD153-578·5 Part A and B). In all of
the samples, sulphide blebs consist mostly of intergrowths
of pyrrhotite and pentlandite with trace chalcopyrite.
Minor hydrothermal pyrite is observed in samples from
the talc^carbonate alteration zone (i.e. MKD153-514·6).
Pyrrhotite either fills ramifying fractures within pentlandite or occurs as coarse aggregates associated and intergrown with pentlandite. Magnetite partially replaces
sulphide around fractures or more commonly forms selvages around sulphide blebs, in some cases in association
with ferroan magnesite. Lobate and apparently interstitial
chromite is common in the disseminated sulphides (e.g.
MKD153-669·5 and MKD153-591·5), forming complex
branching interconnected networks around original olivine
crystals and in some cases around sulphides.
The whole-rock compositions of our samples match the
trends observed using 2 m aggregate samples (Figs 4 and
5), with some discrepancies observed for S and Cr concentrations (Fig. 4c and f). These variations are interpreted to
be caused by heterogeneity of sulphide distribution (e.g.
MKD153-578·2 Part A and Part B) at the decimetre scale
and by the random and sparse distribution of chromite.
Mantle-normalized PGE concentration patterns of various
samples are parallel; relative metal abundances are independent of sulphide abundances and the degree of talc^
carbonate alteration (Fig. 5).
2D OLI V I NE SI ZE A N D SU LPH I DE
A B U N DA N C E
Methods
Serpentinized olivines from the Mount Keith samples exhibit constant X-ray attenuation coefficients, and have been
subjected to intense micro-veining. As a result, the differentiation and segmentation of original olivine pseudomorphs
was impossible using the 3D dataset obtained by high-resolution X-ray computed tomography. Consequently, analysis
of the original grain size and shape was carried out on mosaics of 2D images of polished rock slabs, and polished surfaces of the blocks used for HRXCT and mineral analysis.
Original olivine, sulphide and in some cases chromite grain
boundaries were traced manually from photographs using
vector graphic software. One layer was created for each
mineral type (Fig. 6) and then converted to a binary image
(mask). Each mask was then analysed using ImageJÕ following a method similar to that described by Godel &
Barnes (2008a) to extract mineral proportion, size and
shape of each of the mineral species considered. To avoid
the problem of edge effects, crystals partially visible and
located at the border of the images were excluded and the
border and area of the samples were modified accordingly.
Statistics on mineral size distribution and shape were then
calculated and displayed using 2D/3D CSD software
(Ricard et al., 2012) and in-house MatlabÕ codes.
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MAGMATIC SULPHIDE DEPOSITION
Table 2: Whole-rock major element chemistry (in wt %) recalculated on a volatile- and
sulphide-free basis
Sample:
MKD153-514·6
MKD153-578·2
MKD153-591·5
MKD153-649·5
MKD153-669·5
Depth (m):
514·6
578·2
591·5
649·5
669·5
SiO2
45·11
45·16
44·45
44·78
44·02
TiO2
0·03
0·04
0·03
0·01
0·01
Al2O3
0·42
0·14
0·23
0·20
0·21
Fe2O3
0·08
0·04
0·13
0·10
0·00
FeO
5·43
4·03
6·48
5·53
2·28
MnO
0·10
0·08
0·06
0·09
0·13
MgO
48·15
50·00
48·00
48·04
51·67
CaO
0·36
0·05
0·04
0·90
0·96
Na2O
0·00
0·00
0·00
0·00
0·00
K2O
0·01
0·00
0·00
0·01
0·01
Cr2O3
0·20
0·17
0·25
0·18
0·20
P2O5
0·00
0·00
0·00
0·00
0·00
Total
99·90
99·71
99·66
99·83
99·50
Mg#
93·9
95·6
92·8
93·8
97·4
Table 3: Whole-rock trace element abundances
Sample:
MKD153-514·6
MKD153-578·2
MKD153-591·5
MKD153-649·5
MKD153-669·5
Depth (m):
514·6
578·2
591·5
649·5
669·5
wt %
S
CO2
1·00
1·65
2·22
1·45
0·95
22·02
5·79
3·81
2·66
8·96
ppm
Se
0·44
1·04
0·65
0·28
0·33
Ni
4940
9056
6912
7299
7781
Cu
187
528
164
496
136
Co
137
141
134
133
165
ppb
Os
12·56
25·53
16·38
16·53
24·03
Ir
9·55
20·40
12·02
12·47
19·80
Ru
17·26
39·69
23·44
24·10
30·05
Rh
5·62
10·88
6·76
7·12
8·02
Pd
46·92
79·42
53·07
56·23
64·12
Pt
18·63
46·26
25·68
27·81
32·84
Au
50·68
3·33
1·38
1·65
3·76
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Fig. 5. Base-metal and PGE concentrations in whole-rocks. (a) Variation of Pd/Pt ratios as a function of sulphur concentration and CO2 content. The filled diamonds represent 2 m aggregate samples along the MKD153 hole. The open diamonds are the sample described in the present
study with open diamond size linearly proportional to the CO2 content of the rocks. (b) Mantle-normalized base-metal and PGE concentrations as a function of corresponding sulphur concentrations (linearly proportional to circle size).
Results
Relationship between olivine size and sulphide abundance
A total of 2873 olivine crystals were analysed on both rock
slab and polished blocks (Fig. 6a). More than 80% of the
olivine crystals (Fig. 7a) analysed have sizes 55 mm equivalent circle diameter (ECD; corresponding to the diameter
of a circle having an area equivalent to the crystal area),
with 50% of the crystals smaller than 2 mm ECD. Olivine
crystals are usually round with aspect ratios (AR; defined
as AR ¼ major axis length/minor axis length of the best
fitted ellipse) mostly varying between one and two (Fig. 7b).
Empirical data indicate that, in drill cores, the overall
sulphide abundance tends to decrease with increasing average olivine size (Grguric et al., 2006). This relationship between the olivine size and the modal abundance of
interstitial sulphide seems to be observed in our samples
from Mount Keith (Fig. 6a). To quantify this relationship,
we analysed statistics of olivine size (reported as ECD)
and sulphide abundance by considering successive 25 mm
length intervals along the core, from 0 to 200 mm
(Fig. 6a). Olivine size distributions (histogram not shown)
are similar for each interval and follow a lognormal distribution. Figure 7c illustrates the variation in olivine ECD
(reported as average and median values) as a function of
sulphide abundance by interval. These trends fit secondorder polynomial models with correlation coefficient between the observed and the filled values of 0·956 and
0·942 for the average and median, respectively (Fig 7c
and Table 4). Overall, at sulphide abundance less than 7
area %, olivine ECD decreases as sulphide abundance increases, whereas at higher sulphide abundance the olivine
size tends to remain constant (Fig. 7c)
Crystal-size distribution of olivine
Crystal-size distribution (CSD) analysis was introduced to
the study of igneous rocks by Marsh and co-workers
(Marsh & Maxey, 1985; Marsh, 1988). The distribution of
crystal sizes in a rock potentially provides information on
rates and mechanisms of crystal nucleation and growth
(Eberl et al., 1998; Higgins, 2006, and references therein).
Figure 8 shows the CSD of olivine from Mount Keith
(MKD153-578 rock slab, 25 mm interval) using the natural
logarithm of olivine size population density (ln D) per
unit bin size as a function of olivine ECD, as commonly
used for quantification of crystal-size distribution (Marsh
& Maxey, 1985; Marsh, 1988; Higgins, 2006, and references
therein) with symbol size scaled to sulphide abundance
within a given interval. On such plots, linear CSD are
interpreted to be the results of linear crystal growth
with homogeneous nucleation increasing exponentially with time in an open system, as observed in some
igneous mafic rocks. Change in the curvature of the CSD
curves may reflect different processes such as magma
mixing, mechanical sorting or Ostwald ripening (Marsh,
1998, and references therein; Jerram et al., 2003; Higgins,
2006).
Calculation of olivine CSD by sample interval
(defined by steps of 25 mm along core length; Fig. 6a)
indicates that olivine from Mount Keith exhibits a
concave CSD (Fig. 8a) at size smaller than 5 mm, reflecting the deficiency of olivine crystals of 0·5^2 mm size. In
that case, the overall shapes of the CSD curves can be
modelled using third-degree polynomials (Fig. 8b) of the
form
Ln D ¼ aOl3ECD þ bOl2ECD þ cOlECD þ d
where a, b, c and d are constants (Table 4), OlECD is the
olivine ECD and ln D is the natural logarithm of olivine
size population density. The fit between the calculated
polynomial curves and the observed CSD data was estimated by calculating, for each observed data point, the
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MAGMATIC SULPHIDE DEPOSITION
Fig. 6. Examples of 2D olivine crystals, sulphide blebs and poikilitic chromite manual segmentation highlighting textural relationships between
the phases. (a) Mask of olivine and sulphide from a slab (20 cm length, 4 cm wide) cut through drill core MKD153 at 578·1m depth. (b^g)
Masks of olivine, sulphide chromite from polished blocks (25 mm diameter).
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Table 4: Results of fitting of olivine size vs sulphide
abundance models
Type
a
b
c
R2
Average
0·0169
–0·3247
3·4298
0·956
Median
0·0143
–0·2963
3·1667
0·924
a, b and c are constants of the second-order polynomial
(aX2 þ bX þ c ¼ Y) fitting the olivine ECD (Y) vs sulphide
abundance (X) models presented in Fig. 7c. R2 is the correlation coefficient between the measured and fitted data.
error between the observed and measured values
[Err(OlECD)] as follows:
ErrðOlECD Þ ¼ abs In DOlECD obs In DOlECD Calc
100=In DOlECD obs
where ln DOlECD obs and ln DOlECD Calc are the observed and
calculated natural logarithm of olivine size population density, respectively and abs represents absolute values. The results are represented in Fig. 8b. In addition, correlation
coefficients between the observed and fitted values were calculated for each curve (Table 5). Overall, the error on each
point is less than 5% with correlation coefficients for the
curves varying from 0·890 to 0·986 (average 0·9653, n ¼ 8
curves). The curvature of each curve (g) with respect to
olivine ESD is calculated for each point as follows:
gðOlESD Þ ¼ 3aOl2ECD þ 2bOlECD þ c =
3=2
1 þ ð6aOlECD þ 2bÞ2
and the average curvature (gAverage) is given by
gAverage ¼ gMax gMin =2
Fig. 7. Statistics of olivine and sulphide size and abundance extracted
from 2D sections. (a) Histogram of 2D olivine size distribution from
rock slabs and polished blocks. Olivine sizes are reported as equivalent circle diameter (ECD). (b) Plot of olivine crystal aspect ratio
(AR ¼ length3D/width3D) as a function of olivine crystal area. The
points represent each crystal and the shaded surface their abundance.
(c) Variation of olivine ECD as a function of sulphide abundance at
different sampling intervals (sample MKD153-578·1; Fig. 6a). The diamonds represent the average olivine size with error bars representing
the mean absolute deviation of the olivine size distribution (within a
given interval). The circles represent the median of the olivine size
distribution with error bars representing the interquartile range (also
referred to as mispread). The dashed lines are the fitted curves and
R2 is, for each curve, the correlation coefficient between the observed
and fitted values.
where gMin and gMax are the minimal and maximal curvature, other ranges of OlECD considered.
The shape of the fitted curves at olivine size smaller than
5 mm (ECD) can be divided into two groups based on sulphide abundance (Fig. 8b). Sample intervals containing
less than 3·5 area % sulphide have a greater deficiency in
smaller olivine crystal sizes and skewnesses tending
toward larger crystal sizes than the sample intervals containing 43·5 area % sulphide (Fig. 8b). In addition, at
abundance less than 3·5 area %, the average curvature of
the CSD curves is smaller (0·52) than the average curvature (0·71) observed at larger sulphide abundances.
3 D MOR P HOLO GY A N D B L E B
SI Z E DI ST R I BU T ION OF
SU LPH I DE
Based on the sulphide abundance, their 3D size and morphologies and the 3D sulphide distribution, the samples
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GODEL et al.
MAGMATIC SULPHIDE DEPOSITION
Table 5: Results of fitting of olivine crystal-size distribution
Interval
Sulphide
(mm)
(area %)
a
b
c
d
R2
0–25
3·46
0·40986
2·19201
6·5871
0·971
25–50
3·03
0·126575
1·43076
4·46932
7·7246
0·976
50–75
5·56
0·1507
1·62207
4·47895
6·75
0·95
75–100
10·51
0·117691
1·34365
3·78111
6·0121
0·981
100–125
7·94
0·10872
1·28394
3·48834
5·4831
0·982
125–150
8·96
0·116452
1·45361
4·20341
6·1575
0·987
150–175
6·51
0·109104
1·31969
4·01954
6·7338
0·986
175–200
7
0·149645
1·56712
4·10222
6·0680
0·89
0·00368
a, b, c and d are constants of the third-order polynomial
(aX3 þ bX2 þ cX þ d ¼ Y) fitting the ln D (Y) vs olivine ECD
(X) models presented in Fig. 8b. R2 is the correlation coefficient between the measured and fitted data.
and 11a) are observed in all of the Mount Keith samples.
The finely disseminated sulphides (e.g. MKD153-591·5,
Fig. 12) are characterized by the presence of a single
sulphide size population (Fig. 13a) where sulphides are
small (mostly 5800 mm ESD). Sulphide morphology
ranges from subspherical blebs (Fig. 9a^e) to aggregates
(Fig. 9f^h) formed by up to 10 blebs (100^200 mm equivalent sphere diameter; ESD). Sulphides occur mostly at the
triple junction of olivine crystals with only a few subspherical blebs entirely enclosed within original olivine crystals.
The sulphide CSD, on a plot of ln D vs size (Fig. 14a), is
straight with a characteristic sulphide ESD [referred to as
LC and corresponding to mean ESD for a CSD that extends to all bleb sizes (Marsh, 1988), defined by ^1/slope of
the CSD curves] averaging 100 mm (Table 6).
Fig. 8. Crystal-size distribution (CSD) of olivine based on 2D olivine
crystal measurements. (a) CSD of olivine plotted as a function of sulphide abundance (scaled circle sizes) and calculated for sampling
interval (sample MKD153-578·1; Fig. 6a). (b) Third-degree polynomials fitting the CSD curves of olivine at olivine ECD size varying
from 0·5 to 5 mm. The error bars represent the relative error between
observed and calculated values. The shaded areas represent the range
of values obtained for two groups defined (i.e. 53·5 area% and 43·5
area% of sulphide). The dashed lines represent average values within
a group. (See text for further details.)
were divided into four main categories: (1) finely disseminated sulphides; (2) disseminated to slightly interconnected
sulphides; (3) disseminated to globular sulphides; (4) disseminated to strongly interconnected sulphides.
Finely disseminated sulphides
Subspherical (sphericity 0·98 and aspect ratio 1) sulphide blebs of 100^500 mm diameter (Figs 9a^c, 10
Disseminated to slightly interconnected
sulphides
Overall the sulphide blebs in samples from the disseminated to slightly interconnected sulphide group (e.g.
MKD153-669·5, MKD153-578·5 Part B and MKD153514·6, Fig. 15) have ESD 52000 mm (Fig. 13b^d). Several
populations of sulphide sizes are present in the samples
and overall the sulphide bleb sizes are smaller than
2000 mm ESD (Fig. 15a^c). The sulphides occur in the following forms: (1) subspherical blebs (Fig. 9a^e); (2) aggregates of small sulphide blebs forming larger blebs (up to
1500 mm ESD), resulting in overall complex sulphide
shapes (Fig. 9f^h) and large variability in sphericity
(Fig. 10); (3) elongated (Fig. 9k) linear blebs (1^2 mm
long) oriented along olivine crystal faces, which usually
terminate at olivine grain boundaries; (4) small interconnected sulphide networks (Fig. 9l^n), connected on lengths
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Fig. 9. Types of morphology of the sulphides determined from high-resolution X-ray computed tomography results.
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GODEL et al.
MAGMATIC SULPHIDE DEPOSITION
(Feret length; i.e. longest distance between two points
along the object boundaries) up to 15 mm (Fig. 11). In this
case, the sulphides form channels along serpentinized olivine crystal boundaries similar to those described by
Barnes et al. (2008b), and in many cases partially cast original olivine crystals. The shapes of the CSD curves vary
depending on the sample considered (Fig. 14b^d). For samples containing only a few sulphide blebs forming a network (e.g. MKD153-669·5, Fig. 15b), the CSD curve for
ESD size 5900 mm is identical to that observed in the
finely disseminated sulphide sample described above
(Table 6). In the other samples (i.e. MKD153-578·5 Part B
and MKD153-514·6), where small sulphide networks are
more abundant, the CSD curves are strongly kinked and
can be split into distinctive straight line segments (Fig. 14c
and d). Sulphides in sample MKD153-578·5 Part B exhibit
a particular CSD curve shape with a turn-down at the
small (300 mm) sulphide ESD and are subsequently slightly
curved (Fig. 14c), with characteristic ESD of 160 and
240 mm, respectively (Table 6). In the talc^carbonate
sample (i.e. MKD153-514·6), the sulphide CSD is separated
into two sublinear trends (Fig. 14d) with characteristic
ESD of 120 and 250 mm, respectively (Table 6).
Disseminated to globular sulphides
Fig. 10. Plot of average sphericity of sulphide blebs as a function of
their volume or equivalent sphere diameter. The sphericity is defined
as the ratio of surface area of a sphere with the same volume as the
sulphide blebs to the surface area of the particle. Hence, perfect
spheres have a sphericity of unity and a tetrahedron has a sphericity
0·67). ESD, equivalent sphere diameter.
The sulphide distribution in the sample containing large
globular sulphides (e.g. MKD153-649·5, Fig. 16) can be
divided into two groups (Fig. 13e): (1) a ‘homogeneous’
population of sulphides where ESD are 51500 mm, similar
to the one present in the disseminated to slightly interconnected sulphide samples; (2) multiple populations of
Fig. 11. Length and width of sulphide blebs from various samples from Mount Keith. The length3D and width3D are calculated by fitting a
box around each sulphide bleb. The length represents the largest intersection (Feret length in three dimensions), whereas the width represents
the smallest.
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Fig. 12. Three-dimensional distribution of sulphides in sample
MKD153-591·5 containing finely disseminated sulphides.The sulphide
blebs are colored by ESD size (online).
large sulphide blebs (up to 4700 mm in ESD). The large
(42 mm ESD) sulphide blebs form complex globular
structures moulding one or several serpentinized-olivine
crystals (Fig. 9i and j) and can be up to 10 mm in size,
with aspect ratios varying between 2 and 3·5 (Fig. 11e).
Owing to the heterogeneous sulphide distribution at large
ESD values, the CSD curve (Fig. 14e) was considered
only for an ESD 51600 mm (Fig. 14e). In that case, the
sulphide CSD is separated into two sublinear trends
(Fig. 14e) with characteristic ESD of 109 and 305 mm,
respectively (Table 6).
Disseminated to strongly interconnected
sulphides
The distribution of sulphide in the samples containing
‘matrix’ sulphide can be divided into two groups (Fig. 17):
(1) a ‘homogeneous’ population of blebs with ESD
52000 mm and (2) multiple populations of large blebs up
to 4700 mm in ESD reflecting the extent of strongly interconnected sulphide networks (Fig. 9n and o). In this case,
the sulphides form complex shapes around olivine crystals
on scales greater than the sample size (i.e. 25 mm length),
leading to a sulphide net- or matrix-texture (Fig. 11f and
17b). Owing to the heterogeneous sulphide distribution at
a bleb size 42000 mm, the regressions were calculated only
for the population of small (i.e. 52000 mm) blebs. The
population of blebs 5500 mm is characterized by a characteristic ESD of 85 mm. At larger bleb size, the characteristic ESD increases by a factor of 3 to 255 mm (Fig. 14f).
T R AC E E L E M E N T S I N S U L P H I D E
MINERALS
Previous LA-ICP-MS studies on the distribution of PGE
and other trace elements (including Se, As, Bi, Te, Co)
within magmatic base-metal sulphides have focused on
NUMBER 7
JULY 2013
sulphides from PGE and Ni^PGE sulphide deposits hosted
in mafic intrusions (e.g. Barnes et al., 2008a; Godel &
Barnes, 2008b; Holwell & McDonald, 2010; Dare et al.,
2011; Pina et al., 2012) with, to date, only one study carried
out on sulphides associated with komatiites (Godel et al.,
2012). The detailed behaviour of PGE and other trace elements within base-metal sulphides from the Mount Keith deposit will be reported elsewhere and is only briefly
summarized here.
Selenium and Pd are the only two elements (of those
considered) whose concentrations remain relatively constant within pentlandite and pyrrhotite at the sulphide
bleb scale. Selenium concentrations in pentlandite and
pyrrhotite show little variation within or between samples,
with values averaging 13·1ppm (1·6 ppm, n ¼111 analyses)
and 5·7 ppm (1·1ppm, n ¼ 44 analyses) for pentlandite
and pyrrhotite, respectively. These concentrations are
within the range observed in the adjacent Yakabindie deposits (CSIRO, unpublished data) and are about five
times lower than those recorded in base-metal sulphides
hosted in rocks of komatiitic affinity in the Duketon belt,
Yilgarn craton (Godel et al., 2012).
In contrast to Se concentrations, Pd concentrations in
pentlandite and pyrrhotite exhibit a larger degree of scatter within and between samples (Fig. 18). Pentlandite is
the main host of Pd with concentrations ranging from 1·23
to 7·18 ppm (average 3·14 1·41ppm, n ¼111 analyses),
with greater concentrations observed in large sulphide
blebs (e.g. MKD153-649·5) or interconnected sulphide networks (e.g. MKD153-578·2 Part A). Palladium concentrations in pyrrhotite are an order of magnitude lower, with
most values below 0·5 ppm.
C O T E C T I C P RO P O RT I O N O F
OLI V I NE A N D SU LPH I DES
Principles
The abundance of sulphide formed at equilibrium in an
adcumulate rock can be predicted by considering the
cotectic ratios between crystallizing phases and immiscible
sulphide liquid (Cawthorn, 2005b; Barnes, 2007), assuming
that migration of sulphide through the cumulus pile has
not occurred. Immiscible sulphide liquid segregates from
a magma when the concentration of sulphur reaches a critical value referred to as the sulphur content at sulphide saturation (SCSS). Empirical (Li & Ripley, 2005, 2009) and
experimental (Liu et al., 2007) equations have been developed to predict the S content of a mafic magma at the
time of sulphide saturation (SCSS), which is mainly dependent on magma composition and intensive parameters
such as temperature, pressure or fO2 (Mavrogenes &
O’Neill, 1999). The Li & Ripley (2005) empirical equation
for SCSS was used by Barnes (2007) to calculate the cotectic ratios between crystallizing phases and immiscible
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GODEL et al.
MAGMATIC SULPHIDE DEPOSITION
Fig. 13. Crystal-size distribution of sulphide blebs from various samples from Mount Keith presented as histogram of normalized frequency
weighted by sample volume. The sulphide blebs are represented by their equivalent sphere diameter (ESD) calculated from the volume of
each sulphide bleb.
sulphide liquid in the case of a komatiite magma. This calculation is repeated here using the Li & Ripley (2009)
updated empirical equation combined with PELE software
(Boudreau, 1999) to re-evaluate the evolution of SCSS
upon cooling (58C at each step) and fractional crystallization of a komatiitic magma [starting composition from
Barnes (2007)] at a pressure of 0·2 kbar and fO2 of QFM
^ 1, where QFM is the quartz^fayalite^magnetite buffer.
The results obtained are compared with those obtained recently by Ariskin et al. (2009) using COMAGMAT software with similar starting composition and with sulphide
abundances inferred from the analysis of X-ray computed
tomography data described above.
Results
Based on the Li & Ripley (2009) equation, the SCSS decreases from 3650 ppm for olivine of Fo95 to 2850 ppm
for olivine of Fo92 during fractional crystallization of the
Mount Keith komatiite magma (Fig.19a).This would imply
that a resulting cotectic olivine^sulphide adcumulate would
contain 1·6 to 1·9 wt % sulphide (Fig.19b) for olivine ranging in composition from Fo94·5 to Fo92, in agreement with
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Fig. 14. Crystal-size distribution of sulphide blebs from various samples from Mount Keith. The data are presented for each sample with symbols and continuous lines. The dashed lines represent linear regression curves for different parts of the CSD curves. The numbers next to the
arrows are the characteristic equivalent-sphere diameter of sulphide blebs calculated from each given regression curve.
the range of sulphide abundances (averaging 1·5 wt %
sulphide) calculated by Ariskin et al. (2009) with a similar
starting composition and following the COMAGMATcrystallization model (Ariskin et al.,1993). The values are higher
than the 0·78^0·92 wt % sulphide calculated using Li &
Ripley (2005) and reported by Barnes (2007).
DISCUSSION
In the light of the results presented above and literature
surveys, any model for the formation of disseminated
sulphide hosted in the Mount Keith komatiite needs to
consider the following: (1) the 3D textural association of
olivine, disseminated sulphides and poikilitic chromite;
(2) the decrease of olivine size as sulphide abundance increases to 7 area % and the broadly constant size of olivine
as sulphide abundance increases above this value; (3) the
various 3D morphologies and sulphide-bleb size distribution of sulphide observed in the samples; (4) the variability
of trace-element characteristics between sulphide bleb size
populations; (5) the relationship between observed sulphide abundances and predicted cotectic ratios.
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GODEL et al.
MAGMATIC SULPHIDE DEPOSITION
Table 6: Summary of 3D crystal-size distribution of sulphides
Sample
ESD (mm)
Sulphide*
Proportion
Number
of total
of blebs
Slope
Intercept
R2
LC (mm)
N0 (bleb
NT (drop
Nuclei/mm4)
Nuclei/mm3)
1·8
0·21
4·7
0·40
1·1
0·17
sulphide (%)
From
MKD153-514·6
MKD153-578·2 Part A
MKD153-578·2 Part B
To
(vol. %)
0
400
0·30
8
2662
0·00845
0·561
0·99
118
400
1400
3·01
84
363
0·00405
0·872
0·99
247
0
400
0·28
3
4106
0·01179
1·539
0·98
85
400
1600
1·86
21
156
0·00392
1·682
0·94
255
200
500
0·54
16
515
0·00632
0·101
0·99
158
500
1500
2·69
82
210
0·0042
0·797
0·96
238
MKD153-591·5
0
800
0·51
100
3761
0·01043
1·181
0·99
96
3·3
0·31
MKD153-649·5
0
600
0·27
6
1983
0·00915
0·194
0·99
109
1·2
0·13
600
1600
0·67
15
49
0·00327
2·918
0·80
305
0
1100
0·68
95
3582
0·00883
0·764
0·98
113
2·1
0·24
MKD153-669·5
*Volume (per cent of sample volume) represented by suphide blebs within the size range.
LC, characteristic length; N0, nucleation density; NT, total number density of drop nuclei, calculated as NT ¼ N0LC.
Origin of olivine texture
Quantitative analysis of the olivine texture in komatiites is
to date restricted to olivine cumulates where olivine forms
clusters of touching crystals that accumulated to form
high-porosity frameworks (Jerram et al., 2003). In adcumulate rocks, crystals are densely packed with virtually no remaining porosity or evidence of trapped melt. The origin
of adcumulate crystallization has been debated over the
years (Wager & Deer, 1939; Campbell, 1968; Jakobsen
et al., 2005; Arndt et al., 2008, and references therein;
Holness et al., 2011; Godel et al., 2013).
Our quantitative analysis of olivine crystal-size distribution in an olivine adcumulate from Mount Keith shows
that olivine exhibits distinctive concave CSD curves
(Fig. 8). The smaller size fraction of olivine crystals contains a low proportion of highly elongate grains, which is
missing in the larger fraction (Fig. 7b). Concave CSD patterns can be interpreted in two ways: as the result of mechanical sorting and accumulation of crystals by a
mechanism such as crystal settling, whereby larger crystals
accumulate preferentially over smaller ones, or as the
result of a process of textural coarsening (Higgins, 2006,
and references therein), whereby larger crystals grow at
the expense of smaller ones.
A mechanical accumulation model for both olivine and
sulphide would predict hydraulic equivalence, whereby
coarser olivine grains would be expected to be associated
with coarser sulphide blebs, having equivalent Stoke’s Law
settling velocities. No correlation between olivine and
sulphide grain size is observed, suggesting that the concave-up CSD patterns are not due to the mechanical accumulation of both olivine and sulphide.
The degree of modification of the olivine CSD pattern
in the Mt Keith rocks is greatest for those samples with sulphide content less than 3·5 area % (Fig. 8b). This coarsening is accompanied by increasing textural maturity as
measured by the decreasing abundance of highly elongate
grains. These observations favour a mechanism of in situ
coarsening whereby growing olivine crystals compete for
space with sulphide liquid droplets. Coarsening can occur
only when the nucleation rate of new crystals is low, and
when the growth rate of existing crystals is high. These
conditions are best achieved when the temperature is
close to the liquidus throughout the solidification history
of the rock. These textural observations therefore imply
that the adcumulate rocks formed by near-isothermal solidification of a crystalline crust, at a low degree of undercooling, at a temperature close to the liquidus, rather than
by a mechanism whereby trapped liquid is expelled from
a thick mush zone having a steep thermal gradient. This is
consistent with a mechanism whereby olivine accumulates
as a solid adcumulate crust at the top of the crystal pile,
in contact with turbulently flowing komatiite magma and
in the absence of a mush layer more than a few crystals
thick, as suggested by Hill et al. (1995) and Barnes et al.
(2006a). This conclusion is supported by the presence of
dendritic chromite in the olivine^sulphide adcumulates as
described and discussed by Godel et al. (2013).
Sulphide bleb morphologies
The morphology of sulphide blebs in cumulate rocks is
mainly controlled by the interfacial surface tension between phases, which governs the wetting relationships.
Experimental work by Rose & Brenan (2001) concluded
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Fig. 16. Three-dimensional distribution of sulphides in sample
MKD153-649·5 containing finely disseminated to globular sulphides.
The sulphide blebs are colored by ESD size (online).
Fig. 15. Three-dimensional distribution of sulphides in samples containing finely disseminated to network sulphides. The sulphide blebs
are colored by ESD size (online).
that the wetting ability of a sulphide melt against olivine
depends on the concentration of Ni, Cu and Co in the sulphide melt and oxygen fugacity, with the behaviour of the
sulphide melt ranging from wetting to non-wetting depending on compositions and conditions. Based on experiments (Mungall & Su, 2005) and the analysis of natural
samples (Godel et al., 2006; Barnes et al., 2008b), the
morphology and behaviour of magmatic sulphides is
mainly controlled by the wetting properties of sulphides
against silicates or oxides and the amount of trapped interstitial silicate melt present. Mungall & Su (2005) found
that the presence of even trace amounts of silicate melt
Fig. 17. Three-dimensional distribution of sulphides in samples containing finely disseminated to matrix sulphides. (a) Distribution of
sulphide blebs in sample MKD153-578·2, considering finely disseminated to network sulphide only. (b) Distribution of sulphide blebs in
sample MKD153-578·2, considering all sulphide types.
along silicate grain boundaries will inhibit the wetting
ability of the sulphide, and hence sulphide liquid will tend
to remain trapped as droplets. Barnes et al. (2008b) found
that sulphide blebs form interconnected networks along
triple olivine grain boundaries in olivine adcumulates, but
unconnected spherical droplets in orthocumulate rocks,
consistent with these predictions.
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GODEL et al.
MAGMATIC SULPHIDE DEPOSITION
Fig. 18. Box and whiskers plots showing the variability in Pd and Se concentrations within pentlandite (left column) and pyrrhotite (right
column) from Mount Keith. The boxes are defined by the lower and upper quartiles calculated from each sample (n analyses per samples).
The caps at the end of each whisker indicate the minimum and maximum values, respectively. The dashed line represents the median calculated
from the entire population.
Our detailed high-resolution X-ray computed tomography study of disseminated sulphides from Mount Keith
has revealed a broad range in 3D morphologies of sulphides within and between samples (Fig. 9), more complex
than that reported by Barnes et al. (2008b). Based on the
low and similar Al2O3 concentrations of the whole-rocks,
the samples studied are inferred to contain very little if
any intercumulus trapped melt (Table 2) and hence sulphide blebs are expected to be interconnected with low dihedral angles between sulphide and olivine. The Mount
Keith sulphide blebs actually have a range of morphologies
including small subspherical blebs, large ‘amoeboid’ globular blebs and strongly interconnected networks, in olivine
cumulates containing apparently similar amounts of
trapped melt. The presence or absence of trapped melt
cannot explain all of this variability. The strong variability
of sulphide abundance at the decimetre scale, the similarity in 3D distribution of small sulphide blebs within the
samples and the variability in 3D sulphide morphologies
are interpreted to reflect changes in nucleation density,
local variability in the fluid dynamics of the system, and
the changing balance of gravitational and surface tension
forces with droplet size. The detail of computational fluid
dynamics describing the physical behaviour of sulphide
droplets in natural magmatic systems is beyond the scope
of the present paper and is the topic of continuing study.
Nucleation of sulphide droplets
Magmatic Ni^Cu^PGE sulphide deposits typically form
by segregation of immiscible sulphide liquid from mafic to
ultramafic magmas (Naldrett, 2004). In komatiitic systems
and in the Mount Keith ores, sulphur is interpreted to be
primarily derived from crustal sources as inferred from
lithophile trace elements (Fiorentini et al., 2007), sulphur
isotopes (Bekker et al., 2009) and whole-rock S/Se ratios
(Groves & Keays, 1979; Lesher & Keays, 2002, and references therein). The nucleation and growth of immiscible
sulphide liquid droplets in a silicate magma requires some
degree of supersaturation; in other words, the magma
must exceed the SCSS described above. The kinetics of nucleation of sulphide droplets has been discussed by
Mungall & Su (2005) in the light of Gibb’s thermodynamic
theory of heterogeneous systems (Dowty, 1980). Sulphide
droplet nucleii, like crystals, must grow past a critical
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JOURNAL OF PETROLOGY
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radius beyond which the Gibbs free energy of the new
phase assemblage overcomes the surface energy barrier of
the interface.
Our high-resolution X-ray computed tomography results
indicate that in the Mount Keith samples small sulphide
blebs (typically5800 mm ESD) are mostly near-spherical, accumulated in proportions similar to those predicted
for the segregation of immiscible sulphide liquid from a komatiitic magma crystallizing olivine in cotectic proportion. In addition, the analysis of the crystal-size
distribution of the sulphide blebs indicates that, regardless
of the complexity and variability in sulphide distribution
and morphology, the CSD curves for the smallest sulphide
blebs are linear and almost identical, with broadly similar
characteristic ESD (113 25 mm). Based on CSD theory
(Marsh, 1988, 1998; Higgins, 2006), log^linear trends on
CSD plots may be interpreted as a result of homogeneous
nucleation increasing exponentially with time. In such a
case the intercept of the regression curves can be used to
calculate the nucleation density of crystals in the magma
(intercept ¼ ln N0, where N0 is the nucleation density).
The nucleation density can be combined with the characteristic length (LC) to calculate the total number density
(NT) of nucleii in a sample volume (Castro et al., 2003)
defined as NT ¼ LCN0. Application of this concept to our
samples from Mount Keith suggests that sulphide droplets
in a komatiitic system nucleated at a density varying
from 1·1 to 4·7 droplets mm4 magma (average 2·4 1·4;
Table 6) with total number density varying from 0·13 to
0·40 droplets mm3 (0·24 0·9; Table 6).
Based on nucleation theory and experimental studies
(Mungall & Su, 2005), the nucleation density of the droplets depends on the degree of supersaturation of the
system. At small degrees of supersaturation, the sulphide
droplets are expected to nucleate rarely and at widely
spaced intervals. In contrast, at larger degree of supersaturation, the activation energy to form sulphide droplets
decreases and hence their nucleation density increases.
Comparison of calculated nucleation densities with overall
sulphide morphology and distribution indicates that the
highest nucleation density (4·7 droplets mm4) is observed
in the sample containing disseminated to matrix sulphide
(i.e. MKD153-578·2 Part A), whereas the lowest nucleation
density (1·1 droplets mm4) is found in the adjacent
sample (i.e. MKD153-578·2 Part B) containing disseminated to slightly interconnected network sulphide.
The sudden drop in apparent nucleation density (by a
factor of about four) on a 5 cm scale may be interpreted
in two ways: by a sudden change in the local environment
leading to large variability in local degrees of sulphide
supersaturation, or by the mechanical deposition or percolation and accumulation of a component of transported
sulphide liquid droplets. In the latter case, the nucleation
density indicated by the CSD curve is apparent, as the
NUMBER 7
JULY 2013
shape of the curve is influenced by superimposed effects of
mechanical sorting.
Several factors may affect the degree of saturation of sulphide in a silicate magma, including changes in the major
elemental composition of the silicate magma, a change in
temperature or total pressure, a change in fO2 and fS2 or
a combination of these (Mavrogenes & O’Neill, 1999;
Holzheid & Grove, 2002; Cawthorn, 2005a; Li & Ripley,
2009). In komatiitic systems, sulphide saturation is inferred
to be mainly triggered by the assimilation of sulphur-bearing crustal rocks (Lesher & Keays, 2002). Addition of S
can cause the bulk composition of the magma to evolve to
meet the sulphide saturation surface, and subsequently
fractionate along it; or, where large amounts of external
sulphide are added, for the magma to be brought to sulphide saturation instantaneously, and for excess sulphide
to melt directly to form droplets. At Mount Keith, olivine
compositions in the adcumulate samples vary between
Fo92 and Fo94·6 (at 2 m intervals) with numerous reversals
suggesting multiple injections of more primitive magma in
an open magma conduit or channel (Barnes et al., 2011).
No correlation is observed between calculated nucleation
densities and Mg-number (taken as a proxy for parental
magma composition), suggesting that magma composition
had little effect on sulphide nucleation. If the observed
variability in apparent nucleation density observed in our
samples is related to variability in local degrees of sulphide
supersaturation, this may be explained by localized erosion
or assimilation of sulphur-rich zones in the substrate along
the komatiitic magma channel (or flow). In that case, the
magma may locally be brought to sulphide saturation instantaneously with formation of a large number of sulphide
droplets. This population of sulphide liquid droplets suspended within the flowing magma in the conduit may accumulate to form the observed sulphide texture as a
results of instabilities (e.g. earthquakes, or different floor
topology resulting in a change of flow dynamics). Once
the droplets have accumulated the system returns to a
‘normal’ state where the SCSS is controlled by the fractionation of the komatiitic magma (Fig. 19), leading to cotectic
sulphide saturation and a decrease in the nucleation density of the sulphide as observed. The above hypothesis can
further be tested by reference to the following trace element data.
Relationship between sulphide bleb sizes
and trace-element concentrations
Platinum-group elements and other metals have large
partition coefficients with respect to sulphide liquid (see
review by Barnes & Lightfoot, 2005) and hence once sulphide liquid droplets form they collect PGE from the silicate magma. The PGE concentrations within the sulphide
liquid at equilibrium depend on the relative masses of the
two liquids, as expressed by the silicate to sulphide liquid
ratio, referred to as the R-factor (Campbell & Naldrett,
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GODEL et al.
MAGMATIC SULPHIDE DEPOSITION
Fig. 19. Modelling of sulphur content at sulphide saturation and proportion of sulphide in olivine cumulate during the fractionation of
Mount Keith komatiitic magma. (a) Evolution of sulphur content at
sulphide saturation (SCSS) calculated using the Li & Ripley (2009)
equation and by considering fractional crystallization of the komatiite
composition given by Barnes (2007). The black arrows delineate the
range of olivine composition from Mount Keith. (b) Comparison of
modelled sulphide concentrations (small filled circles and curve) in
cumulates from Mount Keith with drill core geochemistry (MKD153
hole) and abundance of 51000 mm equivalent sphere diameter sulphide inferred from high-resolution X-ray computed tomography.
1979). Platinum-group element concentrations will also
depend on kinetic factors (Mungall, 2002), specifically the
ability of PGE to diffuse through depleted boundary
layers around growing sulphide liquid droplets. Where partition coefficients are extremely high, as they are for PGE,
this becomes a very significant factor. As a result, PGE
concentrations in sulphide blebs may be used as a fingerprint of silicate magma^sulphide liquid interactions.
Barnes & Liu (2012) showed that Pd and Pt behave as
immobile elements during alteration or metamorphism of
disseminated sulphides hosted in carbonate-bearing komatiites, as observed for our centimetre-scale samples
(Fig. 5), based on thermodynamic modelling and analysis
of komatiite samples including Mount Keith. Our analysis
of trace elements within pentlandite and pyrrhotite from
Mount Keith indicates that Pd and Se are the only elements that have homogeneous concentrations at the mineral
scale and hence these elements can potentially be used to
draw inferences about sulphide liquid compositions at the
scale of single samples.
Selenium concentrations within pentlandite and pyrrhotite in all the samples are identical within error. In contrast, Pd concentrations in pentlandite vary from 1 to
8 ppm (Fig. 18). This is consistent with a component of diffusion control or the difference in R-factor, owing to the
smaller partition coefficient of Se relative to Pd into sulphide liquid. Comparison of Pd concentrations in pentlandite from the samples reveals a relationship between
Pd concentration and sulphide bleb size, with two endmembers (Fig. 18). Large globular sulphides (e.g.
MKD153-649·5; Fig. 16) contain higher Pd concentrations
(by 2^4 times) than samples containing disseminated and
network sulphides (Fig. 18). The sulphide size distribution
in the sample (Fig. 14) and the supra-cotectic sulphide
abundance are interpreted to be a result of mechanical accumulation of sulphide droplets transported for some distance within the flowing magma. During transport the
sulphide blebs grow and interact with a larger volume of
magma, and also experience mechanical stirring that
breaks down compositional boundary layers around the
blebs, allowing more efficient equilibration, thereby
increasing the apparent R-factor for these blebs. Hence
transported blebs collect more Pd than the fine ‘cotectic’
sulphide droplets that have been trapped rapidly after
their formation during the crystallization of adcumulate.
The similarity in Pd concentrations within small sulphide blebs and interconnected networks in the sample
containing matrix sulphide (i.e. MKD153-578·5 Part A)
are interpreted to reflect the coalescence of many droplets
to form an interconnected sulphide network at olivine crystal boundaries. The presence of a large number of droplets
may be interpreted in two ways as suggested above: (1) by
a sudden change in the local environment leading to sulphide supersaturation and formation of a large number of
sulphide droplets that coalesce; or (2) by the mechanical
deposition and accumulation of a component of transported sulphide liquid droplets. In the latter case, it is expected that transported sulphide droplets would have
trace element signatures (i.e. Pd concentration) different
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JOURNAL OF PETROLOGY
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NUMBER 7
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from those that form in situ, with large variability depending on the droplet sizes and their origins. This is not in
agreement with our observations, which show that sulphides forming an interconnected network have Pd concentrations similar to those observed in the finely
disseminated sulphides (Fig. 18a). In contrast, mechanical
accumulation of large numbers of droplets that formed
mainly as a result of sulphide supersaturation are expected
to have a relatively homogeneous Pd concentration, as
observed.
around 3 wt %, with a size distribution reflecting mainly
homogeneous nucleation and growth. Nucleation density
of the sulphide droplets is of the order of 2 droplets mm4.
Palladium concentrations in pentlandite show a small to
moderate degree of variability, with concentrations averaging 3 ppm. The presence of small droplets and interconnected sulphide networks and the variability in Pd
concentration are interpreted to reflect the combination of
in situ nucleation with a component of mechanically accumulated sulphide droplets, originally formed upstream in
the channel and transported to their present site.
S U M M A RY A N D C O N C L U S I O N S
Formation of sulphide in situ with a large
proportion of transported sulphides
Quantitative 3D and 2D textural observations of olivine,
chromite and sulphide blebs in the Mount Keith ore body
constrain models for deposition and accumulation of adcumulate rocks in dynamic magma conduits or channels.
Olivine populations show evidence for textural maturation, with growth of larger crystals at the expense of
smaller ones. This process is inhibited by the presence of
sulphides on a scale of 10^100 times the typical grain size.
This observation, combined with the presence of dendritic
poikilitic chromite, provides evidence for in situ nucleation
and growth of olivine at the interface between flowing
magma and a basal bed of crystals. Sulphide blebs define
a number of characteristic assemblages, based on size
distribution, 3D morphology and PGE concentrations as
estimated from Pd concentration of pentlandite.
Formation of sulphide in situ
A unimodal population of small sub-spherical sulphide
blebs (usually 51000 mm ESD) forms in cotectic proportions with olivine during fractional crystallization at the
base of the flowing komatiite magma. The size distribution
of the sulphide blebs is interpreted as a result of homogeneous in situ nucleation and growth of the sulphide with a
nucleation density of 2 sulphide droplets mm4. Sulphides are finely disseminated within olivine adcumulate,
which exhibits a larger degree of textural maturation. The
sulphides are commonly associated with poikilitic chromite, which in some cases encloses the sulphides. The
newly formed sulphide droplets are rapidly trapped by
olivine and/or chromite and interact with a limited
volume of magma leading to low Pd concentration
(2 ppm) in the droplets. This assemblage is considered to
represent the ‘normal’ evolution of sulphide-saturated
magma in a dynamic conduit or channel.
Formation of sulphide in situ with a small
component of transported sulphides
Sulphide morphologies range from sub-spherical blebs to
small interconnected networks, having a bimodal size
population usually 51600 mm. Blebs are located at olivine
triple junctions and may be in some cases associated with
poikilitic chromite. The sulphide abundance is usually
In the case of samples with a high proportion of transported sulphides, the sulphides are represented by a multimodal size population in which most of the sulphide
occurs as larger globules and small networks with bleb
sizes up of to 10 mm. A large variability in Pd concentration within pentlandite is observed, with values up to
8 ppm for the larger sulphide blebs. The globular shape of
the large sulphide blebs and their high Pd contents are interpreted to reflect their transport in the flowing sulphursaturated komatiitic magma over a large distance relative
to the other sulphide blebs. The relatively high Pd content
arises from a combination of two factors: (1) interaction
with a large volume of magma (i.e. high R-factor); (2) turbulent stirring, which minimizes the development of
PGE-depleted boundary layers around the blebs that
would otherwise inhibit equilibration. In this case, most of
the sulphide volume has been transported within the
magma conduit or channel and accumulated at its present
site.
Formation of strongly interconnected
sulphides
Strongly interconnected to matrix sulphides represent a
minor and localized (a few centimetres wide) proportion
of the disseminated ore. The sulphide abundance in this
ore type is 45 wt % and shows moderate variability in Pd
concentration within pentlandite. This high sulphide abundance is interpreted to be the result of temporary bursts of
mechanical accumulation of sulphide droplets. Large networks form through impingement and coalescence where
the abundance of accumulated blebs is high enough that
most of them are in physical contact and can coalesce
through the process of film drainage (Manga & Stone,
1994). The sudden accumulation of the sulphide droplets
may be the result of abrupt drops in magma flow rate, formation of vortices above topographic irregularities in the
floor, instabilities owing to earthquakes or arrival at the
deposition site of a batch of abnormally sulphide-charged
magma.
The results of this study have a number of broader
implications for the origin of magmatic sulphide
1478
GODEL et al.
MAGMATIC SULPHIDE DEPOSITION
accumulations. Our results highlight that the most favourable environments for deposition of sulphide droplets require a combination of factors: access to sulphide-bearing
country rocks; formation of sulphide liquid droplets; episodic stagnation of the flow to allow settling and accumulation. The accumulation process takes place against a
background of steady-state in situ nucleation of small
blebs along the olivine^sulphide liquid cotectic. Sulphide
bleb populations in disseminated deposits in more mafic
settings would be expected to have different characteristics
reflecting different rates of change of SCSS with fractionation and different coalescence behaviour of blebs in more
viscous magmas. High-resolution X-ray computed tomography provides the means for the kind of rigorous measurements needed to test this model.
AC K N O W L E D G E M E N T S
BHP Billiton is acknowledged for its co-operation in providing access to drill core and permission to collect samples. Dany Savard (UQAC) is thanked for carrying out
the whole-rock PGE analysis. Accessibility to supercomputing facilities were provided by iVEC at the Australian
Resources Research Centre (Perth). This paper is an
output from the CSIRO Minerals Down Under National
Research Flagship. Andy Tomkins and two anonymous reviewers are acknowledged for their reviews. Gerhard
Wo«rner is thanked for additional comments and his editorial handling.
FUNDING
B.M.G. is funded by the CSIRO Office of the Chief
Executive Post-Doctoral Fellowship scheme. This project is
funded by the CSIRO Minerals Down Under National
Research Flagship. S.-J.B. is funded by the Canadian
Research Chair in Magmatic Metallogeny and NSERC
Discovery Grant.
S U P P L E M E N TA RY DATA
3D movies of selected samples are available for download
on the CSIRO Data Access Portal (DOI:10.4225/08/
50863D7E652C5).
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