Trevorite: Ni-rich spinel formed by metasomatism and
desulfurization processes at Bon Accord, South Africa?
B. O’DRISCOLL1,*, P. L. CLAY
2, R. G. CAWTHORN3, D. LENAZ
4, J. ADETUNJI5AND A. KRONZ
6
1 School of Physical and Geographical Sciences, Keele University, Keele ST5 5BG, UK2 School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester M13 9IL, UK3 School of Geosciences, University of the Witwatersrand, PO Wits, 2050 South Africa4 Dipartimento di Geoscienze, Trieste University, Via Weiss 8, 34127-Trieste, Italy5 Geographical, Earth and Environmental Sciences, School of Science, University of Derby, Kedleston Road,
Derby DE22 1GB, UK6 Geowissenschaftliches Zentrum der Universitat Gottingen, Gottingen, D-37077, Germany
[Received 21 March 2013; Accepted 16 December 2013; Associate Editor: C. Storey]
ABSTRACT
The 3.5 Ga Bon Accord Ni deposit occurs within the lowest serpentinized mafic�ultramafic lavas of
the Barberton Greenstone Belt (South Africa). Though now completely mined out, it comprised a suite
of rare Ni-rich minerals that led to its interpretation as either an extraterrestrial body or as an oxidized
fragment of Fe-Ni alloy originating from the terrestrial core. In this study, we draw on detailed
petrographic observation and mineral chemical data, as well as previous work, to re-evaluate these
ideas. The balance of evidence, from thin section (<1 mm) to regional (~10s of km) scales, appears to
support an alternative origin for Bon Accord, possibly as an oxidized Ni-sulfide deposit formed in
association with ocean floor komatiite eruptions.
KEYWORDS: trevorite, spinel, metasomatism, desulfurization, Bon Accord, South Africa.
Introduction
TREVORITE is a rare Ni-rich spinel (NiFe3+2 O4)
endmember more commonly reported in meteor-
ites than from terrestrial settings (e.g. Robin et al.,
1992; Pierrard et al., 1998; Hart et al., 2002).
There are reports of isolated terrestrial trevorite
inclusions in olivine megacrysts in some mid-
ocean ridge basalts (Pandey et al., 2008), and
further reports of disseminated occurrences in
association with secondary sulfides (e.g. heazle-
woodite) and in association with native metals
and alloys (Ni-Fe alloys) in disseminated and
massive sulfide deposits (e.g. Eckstrand, 1975;
Hudson and Travis, 1981). Trevorite has also been
reported in impact melt veins associated with the
~144 Ma Morokweng crater (South Africa), in a
paragenesis that also contains unusual Ni-rich
silicates (liebenbergite and willemseite) and
sulfides (millerite and bornite). However, the
type locality for trevorite is the ~3.5 Ga Bon
Accord Ni deposit in the Barberton Greenstone
Belt of the Kaapvaal craton, South Africa
(Fig. 1a; Trevor, 1920; De Waal, 1978; Keenan,
1986; Tredoux et al., 1989). Although completely
mined out now, it has been estimated that the
deposit originally comprised a core of Ni-rich
spinels, oxides and silicates of ~6.3 m3,
surrounded by a serpentinite-schist envelope
2�3 m wide, also dominated by Ni-rich minerals.
There is no clear consensus on the origin of the
Bon Accord deposit. Two dominant competing
hypotheses emerged in the 1970s and 1980s. De
Waal (1978, 1979) originally suggested that the
Ni-Fe rich mineralogy might represent an
oxidized form of a metallic Fe-Ni occurrence* E-mail: b.o’[email protected]: 10.1180/minmag.2014.078.1.11
Mineralogical Magazine, February 2014, Vol. 78(1), pp. 145–163
# 2014 The Mineralogical Society
and speculated that an iron meteorite was the
origin. In the most comprehensive petrographic
and geochemical study carried out on the Bon
Accord rocks to date, Tredoux et al. (1989)
argued that the deposit might comprise a fragment
of siderophile-rich material left over in the lower
mantle from inefficient core formation, and
subsequently oxidized during ascent through the
mantle via a thermal plume. Other plausible
scenarios such as oxidation of either (a) podiform
chromitite (a common feature of the upper
oceanic mantle portions of ophiolites) or
FIG. 1. (a) Principal elements of the geology and regional setting of the Barberton Greenstone Belt, South Africa,
showing the position of the Bon Accord ore deposit and the Bien Venue massive sulfide deposit. (b) Schematic cross
section of the Bon Accord deposit and host rocks. (c) Local geology of the Bon Accord trevorite deposit. a and b are
adapted and redrawn from Tredoux et al. (1989); c is redrawn from Keenan (1986). The red star, green diamond and
blue circle mark the localities of the Bon Accord trevorite body, the hematite-magnetite body reported by Tredoux et
al. (1989) and the Scotia Talc Mine, respectively.
146
B. O’DRISCOLL ET AL.
(b) oxidized awaruite (Ni-Fe alloy stabilized
during the highly reducing conditions associated
with serpentinization) were dismissed (Keenan,
1986; Tredoux et al., 1989). The formation of Bon
Accord as an oxidized volcanic massive sulfide
(VMS) deposit was also ruled out by these
workers (Tredoux et al., 1989).
There is considerable value in furthering our
understanding of this trevorite ore body. Although
Bon Accord is possibly unique on Earth, the
preservation of Archaean environments in which
extreme redox (Eh) variation may have existed
offers a valuable window into early ocean basin-
forming processes as well as lithosphere-hydro-
sphere interactions on primitive Earth (e.g.
Seyfried et al., 2004; Reinhard et al., 2013). The
Bon Accord body contains an array of Ni-rich
oxides, sulfides and silicates that have not been
reported in such abundance or variety anywhere
else on Earth. In addition, trevorite is of significant
interest to the materials sciences both for its
magnetic properties and for its numerous applica-
tions in battery electrodes (Liebermann, 1972;
Bousquet-Berthelin et al., 2008). However, the
studies of De Waal (1978, 1979) and Tredoux et
al. (1989) seem to have largely overlooked or
misinterpreted some key details of the lithological
associations of Bon Accord, which indicate
significant evidence for hydrothermal alteration
and mineralization along strike from the deposit
(Keenan, 1986; De Ronde et al., 1994). In
particular, extensive zones of Ni-sulfide miner-
alization occur within 3 km along strike of Bon
Accord (Scotia Talc Mine) as well as massive
sulfide deposits at Bien Venue (Fig. 1a) and
abundant ironstone pods at similar stratigraphic
levels throughout the sequence. These features are
suggestive of seafloor-related hydrothermal
activity and VMS formation (De Ronde et al.,
1994). Under these circumstances, the unusually
high Ni content in the deposit might be explained
by derivation from olivine in the host ultramafic
(komatiitic), rather than mafic, lavas. It is therefore
the aim of the present contribution to shed new
light on this intriguing and potentially important
problem by re-evaluating the evidence for the
formation of the Bon Accord deposit. In particular,
its origin as a ‘siderophile-rich heterogeneity’ in
the deep mantle (Tredoux et al., 1989) is
evaluated. New mineralogical and quantitative
textural data are presented and are discussed in the
context of previous work to suggest an alternative
origin for the Bon Accord trevorite body as an
altered (oxidized) Ni-sulfide deposit.
Geological setting
It has been argued that the Archaean (3.4�3.6 Ga)Barberton Greenstone Belt represents one of the
oldest documented examples of terrestrial oceanic
crust (i.e. the Jamestown Ophiolite Complex; de
Wit et al., 1987). However, although an ophiolite
origin has been proposed for these rocks
(Hoffman et al., 1986; de Wit et al., 1987;
Tredoux et al., 1989), this interpretation is not
universally accepted (e.g. Lowe, 1999;
Anhaeusser, 2001). One specific objection is the
apparent complete lack of a sheeted dyke
component (Hamilton, 1998; McCall, 2003).
The Bon Accord Ni deposit is contained within
a sequence of mafic and felsic schistose rocks,
together with cherts and ultramafic lithologies,
that are collectively assigned to the lowest
Tjakastad Subgroup of the Onverwacht Group in
the Barberton Greenstone Belt (Keenan, 1986;
Tredoux et al., 1989; De Ronde et al. 1994; de
Wit et al., 2011; Fig. 1). Two other groups, the
Fig Tree and Moodies Groups, respectively,
complete the overlying stratigraphic succession
of the Barberton Greenstone Belt. A considerable
body of work has been carried out on elucidating
the tectonostratigraphy of the southern and
western portions of the Barberton Greenstone
Belt (e.g. de Wit et al., 1987; Lowe and Byerly,
1999; de Wit et al., 2011; Furnes et al., 2011,
2013), but comparatively less published literature
exists for its northern region where Bon Accord is
located. There is evidence throughout the
Barberton Greenstone Belt for multiple episodes
of metamorphism: following ocean floor serpenti-
nization, the rocks were subjected to amphibolite-
grade thermal metamorphism associated with a
granitoid terrane to the north. U-Pb zircon ages
for this event are 3347+67–60 Ma and 3250 � 30 Ma
(Tegtmeyer and Kroner, 1987).
The Bon Accord Ni-deposit lies immediately to
the south of the tectonic boundary between
granitic gneisses of the Stentor pluton and the
ultramafic rocks of the Barberton Greenstone Belt
(Keenan, 1986; Tredoux et al., 1989). The
complexly deformed lithological sequence to the
south of this boundary comprises serpentinite,
talc-schist, banded chert, chlorite-magnetite schist
and other mafic schists and metasediments
(Fig. 1; Keenan, 1986). The tectonic boundary
itself occurs within a larger thrust complex and
constitutes a low-angle southward dipping
(20�40º) zone of thrust-related deformation.
Tredoux et al. (1989) suggest that the Bon
METASOMATIC FORMATION OF TREVORITE
147
Accord deposit comprised a boudin-like structure
(Fig. 1b,c), contained within pervasively serpen-
tinized ultramafic rocks. According to the latter
authors, the core of the body behaved like a rigid
‘augen’ structure during deformation, so that the
effects of strain are observed in a pervasive
tectonic foliation around the margins and in the
serpentinite envelope, but not in the centre of the
ore body itself. The host serpentinites to Bon
Accord have upper and lower contacts that are
also tectonic, suggesting that they are contained
within an exotic thrust sheet. Keenan (1986)
stated that ‘‘intense alteration and shearing’’ haveaffected these serpentinites such that ‘‘primary
textures have generally been obliterated and their
origin is unclear’’. In contrast, Tredoux et al.
(1989) recorded relict silicate textures in the
serpentinites and invoked the ophiolite model of
de Wit et al. (1987) to suggest that these rocks
may have originated as mantle-derived perido-
tites. Approximately 1 km along strike to the east,
an elongate zone of Ni-sulfide mineralization,
stratabound within similar serpentinites and
chlorite-magnetite schists to those enclosing the
Bon Accord rocks, is recognized (Fig. 1c).
Tredoux et al. (1989) also note the presence of
a hematite-magnetite rich body ~1 km along
strike from the Bon Accord deposit, close to the
westernmost mapped outcrop of the Ni-sulfide
mineralization (Fig. 1c). The Bon Accord deposit,
the hematite-magnetite body and the zone of Ni-
sulfide mineralization all occur within the
metamorphic aureole of the Stentor pluton. The
ore deposit itself consists mainly of trevorite
(60�80 vol.%), together with Co-rich spinels and
Ni-silicate phases (e.g. Ni serpentine, Ni olivine).
Cr spinel and cochromite (CoCr2O4) occur
sporadically as relict inclusions in trevorite
crystals.
Petrography
The petrographic description of Tredoux et al.
(1989) included a subdivision of their observed
trevorite textures into two (A and B) miner-
alogical domains. Type A consists of coarse-
grained trevorite crystal aggregates, within the
interstices of which occur subordinate silicates
(e.g. nepouite (Ni,Mg)3[(OH)4Si2O5], willemseite
( N i , M g ) 3 [ ( O H ) 2 S i 4 O 1 0 ] , n i m i t e
(Ni,Mg,Al)6[(OH)8(Si,Al)4O10]), bonaccordite
(Ni2FeBO5) and gaspeite ([NiMg]CO3). Tredoux
et al. (1989) noted complex zoning in some
type A trevorite crystals (their fig. 5b), with cores
composed of bunsenite (NiO) or cochromite and
trevorite mantles/rims. The type B domains
consist of hydrated silicates (e.g. nepouite,
willemseite and nimite) with scarce relict
liebenbergite (Ni2SiO4) in close association with
the nepouite. The schistose outer envelope of the
Bon Accord deposit is dominated by nimite and
willemseite, with bonaccordite also present
aligned parallel to the foliation planes. The
accessory trevorite is rarely observed. Based on
FIG. 2 (facing page). Petrography of the Bon Accord trevorite. (a) Thin-section scan of the Bon Accord sample. The
black grains are mainly trevorite, whilst the green-coloured groundmass is Ni serpentine (nepouite) and Ni chlorite
(nimite). The width (left to right) of the thin section is 5 cm. The area mapped and digitized for CSD analysis in
Fig. 3a is bounded (approximately) by the red outline. The white arrows mark the position of two micro-shear bands
that can be traced through the thin section. (b) Reflected light photomicrograph illustrating a typical coarse-grained
trevorite aggregate. Note the presence of the relatively highly reflective sulfide inclusion close to the image centre
and planar exsolution(?) bands of more brightly reflective trevorite (both marked by a red arrow) which are oriented
parallel to the groundmass foliation. (c) Backscattered electron micrograph of a trevorite aggregate. Note the sieve-
textured zone around the outer part of the aggregate, the inner margin of which is highlighted in red. Nickel-sulfide
and Ni-arsenide inclusions are arrowed. Close examination of the interior of the aggregate reveals different electron
reflectance in different domains which is due to channeling effects and hence different crystallographic orientations.
This is interpreted as reflecting the construction of the aggregate from multiple coalesced individual trevorites. The
black coloured material is dominantly Ni-rich silicate such as nepouite and/or nimite. (d) Backscattered electron
micrograph of the trevorite (bright crystals) textures within a micro-shear band. Note the idioblastic shape of crystals
and their NW–SE alignment across the image. The grey material is dominantly an Ni-rich silicate such as nepouite
and nimite. (e) Backscattered electron micrograph of typical idioblastic trevorite crystal from within a micro-shear
band. Note the sieve-textured core of the crystal. The crystal is elongate along the NW–SE direction, parallel to the
foliation in the serpentinite groundmass. (f) False-coloured backscattered electron micrograph of a sulfide inclusion
(green) in trevorite (blue). Note there is compositional variation within the inclusion. Note also the presence of
another small (red) inclusion, probably arsenide, to the right of the sulfide.
148
B. O’DRISCOLL ET AL.
these and other observations, Tredoux et al.
(1989) suggested an early paragenesis of lieben-
bergite + bunsenite + Ni-Cr-rich spinel � mill-
erite, secondary trevorite + nepouite and a final
assemblage comprising bonaccordite + willem-
seite + nimite, approximately contemporaneous
with the development of schistosity. They
suggested that within the core of the body, a
coarse-grained peridotite texture was preserved
(pseudomorphed). Tredoux et al. (1989) did not
document platinum-group minerals or an exten-
sive sulfide mineralogy, but did note rare grains of
millerite (NiS), stibnite (Sb2S3) and breithauptite
(NiSb).
The Bon Accord ore sample studied here is
composed of opaque trevorite crystals
METASOMATIC FORMATION OF TREVORITE
149
(~60 vol.%), hosted in a dominantly fibrous
silicate groundmass (Fig. 2a). The texture is
evidently metamorphic. Two broad textural
occurrences of trevorite are observed which in
some respects match the A and B domains of
Tredoux et al. (1989). Alternating layers define a
crude anastomosing planar fabric (Fig. 2a).
Coarse-grained (0.5�1 cm) clustered aggregates
of polygonal trevorite crystals occur together with
minor amounts of silicate mineral (Fig. 2a,b,c).
These aggregates are wrapped by anastomosing
planar bands, up to 1 cm thick, that are interpreted
as micro-shears. These bands have a much smaller
trevorite/silicate volume ratio, and are dominantly
composed of fine-grained fibrous silicate minerals
(see Fig. 2a and the digitized texture map in
Fig. 3a). The trevorite crystals that do occur
within the bands are small (<0.1 mm long) and
markedly idioblastic compared to trevorite
contained within the aggregates (Fig. 2d,e).
They also typically define a planar foliation,
broadly parallel to the outer margins of the
anastomosing bands (Fig. 2d). Backscattered
electron microprobe imaging confirms that the
coarse-grained trevorite aggregates are composed
of multiple texturally equilibrated and coalesced
trevorite crystals (Fig. 2c). This ‘coarsening’
effect is commonly observed in oxide mineral
ore bodies from other environments, such as
chromitite seams in layered mafic intrusions or in
ophiolites, and is a solid-state effect that can be
enhanced by the presence of certain silicates, e.g.
olivine (Waters and Boudreau, 1996; O’Driscoll
et al., 2010). The presence of sieve-textured zones
demarcating internal boundaries within the
aggregates, rather than individual trevorite crys-
tals (Fig. 2c), suggests metasomatism or a
reactive metamorphic event after coalescence.
Inclusions in the sieve textures are typically
silicate in composition, and range in size from
<0.1 to 30 mm. However, highly reflective
inclusions are also abundant in many of these
sieve-textured zones (Figs 2b,c,f, and 4a), and are
often rounded in form suggesting that they have
texturally equilibrated with their trevorite host. At
high magnification (660) under the petrographic
microscope, the number of brightly reflective
inclusions that are visible increases dramatically,
suggesting that the small size fraction of these
inclusions is dominant. These brightly reflective
inclusions are not observed in the serpentinized
groundmass or in the finer-grained idioblastic
trevorite that occupies the micro-shears. At high
resolution, complex exsolution textures are
observed in many trevorite crystals, with rela-
tively reflective finely spaced lamellae abundant
in most trevorite crystal aggregates (Fig. 2b). A
degree of brittle deformation is also preserved, as
small micro-fractures (offsets of up to several mm
are observed) that cut through the coarse trevorite
aggregates.
Analytical techniques
Quantitative textural analysis: crystal sizedistributions (CSDs)
Crystal size distribution (CSD) analysis is a
quantitative means of analysing the number of
crystals of a mineral per unit volume within a
series of defined size intervals (for a review, see
Higgins, 2006). Standard CSDs (such as those
presented here) therefore plot the crystal size of a
given mineral phase against the natural logarithm
of population density (Marsh, 1998; Higgins,
2006). It has been suggested that simple magmatic
crystallization leads to straight, or log-linear,
CSDs on such plots, representing kinetic textures
developed solely through nucleation and growth
(Marsh, 1998; Higgins, 2006). The deviation of
the CSD plot shape from a straight line has the
potential to yield valuable information on textural
evolution of crystalline rocks during postcumulus
and metamorphic processes (Cashman and Ferry,
1988; Waters and Boudreau, 1996; Higgins, 2006;
Jerram et al., 2003; O’Driscoll et al., 2010). In
particular, crystal coarsening caused by annealing
or Ostwald ripening is a process that is often
readily discerned from CSD analysis (Boorman et
al., 2004; Higgins, 2006). In this study, crystal
coarsening (the growth of larger crystals at the
expense of smaller ones), driven by textural
equilibration, is considered to be the extent to
which the rock, and the mineral phase in
particular, has evolved toward a polygonal
aggregate exhibiting apparent (two-dimensional)
dihedral angles that approach 120º.
The CSDs were determined from thin sections
following the methodology of Higgins (2000) and
the program CSDCorrections 1.37, to comple-
ment the petrographic observations in investi-
gating the petrogenesis of the trevorite ore; in
particular to quantify the variation between the
coarsened trevorite aggregates and the idioblastic
crystals in the micro-shear bands. The CSDs were
calculated by distinguishing individual trevorite
crystal boundaries in digitized photomicrographs
captured in reflected light (using the image
analysis software ImageJ and Image Tool; cf.
150
B. O’DRISCOLL ET AL.
FIG.3.(a)Digitized
texture
map
oftheBonAccord
trevoriteore.Purpledashed
lines
boundthepresence
ofmicro-shearsthatseparatecoarse-grained
trevoriteaggregate
domains.(b)Deconstructed
CSDplotsfortrevorite,calculatedusingan
aspectratioof1:1:1
(see
textfordetails)andcrystalarea
asthemeasuredcrystal-sizeparam
eter.
TheCSD
plotsarepresentedforallcrystalsin
thetexture
map
in(a),as
wellas
forasubsetofthetrevorite
crystalsfrom
theaggregates
andthemicro-shearbands.
(c)Deconstructed
CSD
plotsfortrevorite,calculatedusingan
aspectratioof1:1:2
andcrystal
length
asthemeasuredcrystal-sizeparam
eter
(see
textfordetails).
METASOMATIC FORMATION OF TREVORITE
151
O’Driscoll et al. 2010; Fig. 3a). The CSD data are
typically plotted as population density (loga-
rithmic number of crystals per unit volume)
against crystal size (maximum length). In this
study, the length of a square with an area equal to
that of the analysed crystal is also adopted as a
measured crystal-size parameter, so that two sets
of CSDs are produced (Fig. 3b,c). The reason for
this is that CSDs calculated based on crystal
length may in some circumstances overestimate
the stereological (volumetric) 2D to 3D correction
(Boorman et al., 2004). The former approach
means that an aspect ratio of 1:1:1 and roundness
value of zero are input into the CSDCorrections
software. Where CSDs are plotted using crystal
length, an aspect ratio of 1:1:2 was calculated
using the CSDSlice program of Morgan and
Jerram (2006). The localized alignments of
trevorite crystals in some parts of the sample
were taken into account utilizing the fabric option
in CSDCorrections; otherwise a ‘massive’ fabric
was input. The smallest crystals measured (at a
magnification of625) were ~0.01 mm in size. As
all of the sample area measured is holocrystalline,
and the smallest trevorite crystals are easily
visible in reflected light in thin section (Fig. 3a),
it is assumed that all crystals have been measured
for each sample. We infer, therefore, that the
smallest grain size reported for each sample is the
lower limit for that sample.
Mineral composition
Backscattered electron imaging and quantitative
analyses of trevorite were carried out using the
JEOL 8900 RL electron microprobe at the
D e p a r t m e n t o f G e o c h e m i s t r y ,
Geowissenschafliches Zentrum der Universitat
Gottingen (Germany). Mineral compositions and
backscattered electron images were obtained with
an acceleration voltage of 20 kV and a beam
current of ~20 nA. Count times on peak and on
background for Mg, Al, Cr, Fe, Si and Ti were
15 s and 5 s, respectively, and 30 s and 15 s
respectively for V, Mn, Ni, Co and Zn. The
proportion of ferric to ferrous iron was calculated
assuming perfect stoichiometry, using the equa-
tion of Droop (1987). Ferric iron contents
calculated in this way should be treated with
caution, as the assumption of an ideal formula
AB2O4 has been shown in some cases to be
erroneous (see below for discussion) .
Approximately 200 spot analyses were carried
out on fresh trevorite crystals from different
textural domains in the Bon Accord sample,
including measurements of crystal cores and rims
(away from cracks, veins and other evidence of
alteration). This dataset includes several traverses
consisting of 10�20 spot analyses carried out
across the zoned trevorite aggregates to evaluate
the textural observation that they represent
multiple coalesced crystals (see Figs. 2c, 4a,b).
Approximately 15 sulfide and arsenide inclu-
sions were also analysed using the electron
microprobe at Universitat Gottingen, with an
acceleration voltage of 20 kV and a beam
current of ~20 nA. Count times were 15�60 s
and 5�30 s for peak and background, respec-
tively, for S, Te, Cu, Ni, Pb, Se, Sb, Fe, Co and
Zn. Standards analysed during study of the
inclusions were PbS, ZnS, CuFeS2, GaAs and a
FIG. 4. (a) Backscattered electron micrograph of a
trevorite aggregate, with the position of a compositional
traverse highlighted. (b) Compositional traverse as
demarcated in (a) above, left to right in traverse is
bottom to top on the image. The plot serves to highlight
the magnitude of intra-aggregate NiO variation that
typically exists, which matches the positions of trevorite
crystal boundaries where visible.
152
B. O’DRISCOLL ET AL.
variety (Fe, Ni, Cu, Zn, Se, Sb, Te and Co) of
native metals. An additional ten inclusion
analyses were carried out using the Cameca
SX100 electron microprobe at the Open
University (UK). Na, Mg, Al, Si, Cl, K, Ca, Cr,
Mn, Fe, Co, Ni, Cu, Zn, As and Zr, were
measured using a 20 nA current, 20 kV
acceleration voltage, and 2 mm beam size. Peak
count times on the Cameca ranged from 10 to 45 s
and background count times from 5 to 22.5 s.
Various standards were used for calibration on the
Cameca, including: jadeite (Na), forsterite (Mg),
K-feldspar (K, Al, Si), sylvite (Cl), pyrite (Fe, S),
bustamite (Ca), synthetic Cr2O3 (Cr), Fe-metal
(Fe), Ni-metal (Ni), Cu-metal (Cu), willemite
(Zn), cobalite (Co, As) and zirconia (Zr).
Trevorite crystal structure
The spinel structure is based on a nearly ideal
cubic close-packed array of oxygen atoms with
tetrahedral (T) and octahedral (M) cavities. In
common 2�3 spinels, one eighth of the T sites
and one half of the M sites are occupied by
heterovalent cations A and B, in the ratio AB2O4,
where A = (Mg, Fe2+, Ni, Mn2+) and B = (Al,
Fe3+, Cr3+). However, spinels do not generally
show the idealized configuration, with A cations
in T sites and B cations in M sites. A common
octahedral-tetrahedral disorder of A and B cations
exis ts , depending on thermal his tory.
Consequently, the crystal chemistry of spinels is
d e s c r i b e d b y t h e g e n e r a l f o rm u l aIV(A1�iBi)
VI(AiB2�i)O4, where i refers to the
inversion parameter. Righter et al. (2006a,b) and
Barnes (1998) noticed that the unit-cell sizes of
Ni-rich spinels will be larger than most natural
spinels, with a consequent effect on the partition
coefficients; partition coefficients for Ni in spinel
[D(Ni)] are slightly larger in Fe3+-bearing spinels.
Ni shows a strong octahedral preference so that,
coupled with high-Fe3+ contents it moves the
spinel structure towards an inverse configuration
with divalent cations in octahedral sites and
trivalent cations in the tetrahedral sites.
Moreover, large amounts of Fe3+ and Ni generate
modifications in the octahedral site causing an
increase in the tetrahedral distance with respect to
the size of the cations entering it (cf. Lavina et al.,
2002).
Trevorite is an inverse spinel with Fe3+ in both
tetrahedral and octahedral sites and Ni in the
octahedral sites. Previous studies performed by
means of a Debye-Scherrer camera and a
diffractometer reported a cell edge equal to
8.339 (1) A for a nearly pure trevorite (De
Waal, 1972) and 8.367 (3) A for a ferroan
trevorite (De Waal, 1969) with ~0.4 Fe2+ atoms
per formula units (a.p.f.u.). The former value is
identical to that published in the JCPDS file cards
for a synthetic trevorite (PDF # 10-325).
Subsequently, Hill et al. (1979) reported a
theoretical value of 8.325 A and an oxygen
positional parameter, i.e. the location of the
oxygen position along the cube diagonal, equal
to 0.2573. The crystal data reported here were
recorded on an automated KUMA-KM4
(K-geometry) single-crystal diffractometer, using
MoKa radiation, monochromatized by a flat
graphite crystal. Data were collected up to 50º2yin the o-2y scan mode, scan width 1.8º2y,counting times from 20 to 50 s, depending on
the peak standard deviation. Twenty four
equivalent reflections of (8 4 0) peaks at ~50º2y(it was not possible to centre reflections at higher
angles due to the size of the crystal) were
accurately centred at both sides of 2y, and the
a1 peak baricentre was used for cell-parameter
determination. Structural refinement using the
SHELX-97 program (Sheldrick, 2008) was
carried out against Fo2hkl in the Fd3m space
group (with an origin at 3m), as no evidence of
different symmetries appeared.
Mossbauer spectroscopy
Mossbauer measurement was made on 48 mg of
trevorite absorber with a 25 mCi 57Co source in a
Rh matrix, which was driven at a constant
acceleration in a triangular mode at room
temperature. The spectrum was recorded in 1024
channels and the Lorentzian lines of the folded
data were fitted, using the least-square RECOIL
1.04 computer program developed by Lagarec and
Rancourt (1998). All centroid shifts values are
relative to a-iron.
Results
Crystal-size distribution measurements
The CSD output raw data have been deposited
with the Principal Editor of Mineralogical
Magazine and are available at http://www.
m i n e r s o c . o r g / p a g e s / e _ j o u r n a l s /
dep_mat_mm.html. CSD plots of trevorite texture
are presented in Fig. 3b,c. In general, the errors
associated with the population density values are
not significant, particularly at the small size
METASOMATIC FORMATION OF TREVORITE
153
fractions, suggesting that the large numbers of the
smallest crystal sizes revealed is not an artifact.
The CSDs are strongly curved, irrespective of
whether crystal area or crystal length is used as
the size parameter. In order to assess quantita-
tively textural variation in the trevorites of both
the coarse-grained aggregates and the intervening
micro-shears, both sets of CSDs, each comprising
~4900 crystals, are deconstructed into a CSD for
each textural domain (Fig. 3b,c). It is clear that
the CSD (~2450 crystals) for the coarse-grained
aggregates dominates the overall plot slope in
each case, with only very slight differences
observed between the overall CSD and that for
the coarse-grained aggregates at the largest crystal
size fractions. In contrast, the CSD (~1100
crystals) for the relatively fine-grained idioblastic
trevorites in the micro-shear bands highlights that
the coarse-grained clusters are absent from these
portions of the sample (Fig. 3b). The curvature of
the CSDs makes standard (least squares) regres-
sion of some or all of the CSD plot difficult (the
CSDs are not log linear) so this is not performed
here. However, the CSDs for the coarse-grained
trevorite aggregates exhibit profiles that extend to
shallower slopes at the large size fractions,
emphasizing that these crystals have undergone
different processes to those in the micro-shear
bands. Despite the curvature observed in both
plots, the CSD data also (qualitatively) suggest
similar size characteristics (and hence origins) for
trevorite crystals from both textural domains in
the small size fraction range. The CSDs
calculated based on crystal length are similar in
some respects to those calculated using crystal
area, but some interesting differences are also
observed. For example, an anomaly is observed in
the CSD plotted using crystal length as the size
parameter in the small size fraction range, more
pronounced for those crystals from the micro-
shear bands, where there is a deficiency of crystals
of ~0.05 mm in length (Fig. 3c). A second
difference is that the CSD for idioblastic trevorite
crystals in the micro-shear bands is straighter than
that measured for the coarse-grained trevorite
crystals (Fig. 3c).
Trevorite crystal structure and chemistry
The full suite of mineral (trevorite and inclusion)
compositional data are available at http://www.
m i n e r s o c . o r g / p a g e s / e _ j o u r n a l s /
dep_mat_mm.html, and important aspects of the
data are summarized in Figs 4b and 5a.
Correcting the Bon Accord trevorite data for the
ferric iron component using the method of Droop
(1987) results in NiO and Fe2O3 (ferric iron)
displaying an approximately stoichiometric rela-
t i onsh ip [ (N i ,Fe 2+ ,Mn ,Mg,Co) 0 . 9 9�1 . 0 2(Fe3+,Al,Cr)1.95�2.04O4] over a fairly restricted
compositional range (typically 14.7�18.5 wt.%
NiO and 68.4�71.8 wt.% Fe2O3). Several
interesting observations are revealed in the data.
Firstly, a strong positive correlation is observed
between Fe2+ and Ni content (Fig. 5a), over a
relatively small range in Ni content (~4 wt.%),
that is likely to be an effect of the stoichiometric
correction carried out on the data. No correlation
is evident between NiO and Fe2O3. The formula
calculation of the trevorite–magnetite solid
solution series reveals a nearly 1:1 ratio of both
endmembers (trevorite: 45.0�55.7 mole%;
magnetite 39.8�51.0 mole%). The ulvite compo-
nent (Fe2TiO4) reaches up to 2.42 mole%,
chromite (FeCr2O4): 5.7 mole%; hercynite
(FeAl2O4): 0.95 mole% and a hypothetical
CoFe2O4 endmember ranges between 1.0 and
1.3 mole%. All other spinel endmembers are
mostly below the detection limits or do not exceed
1 mole%. The traverses carried out across the
coarse-grained aggregates reveal subtle composi-
tional variations that correspond to individual
trevorite crystals whose boundaries can be
identified in backscatter mode on the electron
microprobe, suggesting that the constituent
trevorite sub-grains retain subtle compositional
heterogeneity (see Fig. 4b). This supports the
petrographic observation that the coarse-grained
trevorite aggregates are clusters of annealed
trevorites (see also Fig. 2c). No apparent spatial
correlation is observed between the abundances of
NiO wt.% and Fe3+/Fe2+ at the outer edges of
aggregates vs. their interiors, or between indivi-
dual trevorite crystal edges and cores. A final
observation concerns Cr contents in some
trevorite crystals. The trevorite crystals in the
micro-shear bands display consistently high Cr
(~0.08�0.63 wt.%) when compared to trevorites
in the aggregates, in which Cr is typically lower
than the detection limits of the instrument
(~0.035 wt.%). This apparent dichotomy in
trevorite Cr concentrations between different
textural domains is not matched by Ti, Al and
Co contents (maximum concentrations of
~0.5 wt.%, ~0.14 wt.% and ~0.33 wt.%, respec-
tively) which are variable in both domains and
often below detection limits (0.03, 0.018 and
0.015 wt.%, respectively). However, where
154
B. O’DRISCOLL ET AL.
measurable, the concentrations of all of the minor
elements in trevorite generally correlate positively
with NiO content. The vast majority of the intra-
trevorite inclusions are too small for quantitative
compositional determination (<5 mm). However,
those measured show considerable compositional
heterogeneity, with Ni sulfides (heazlewoodite
[Ni3S2] and godlevskite [(Ni,Fe)9S8]), Ni
arsenides (maucherite [Ni11As8] and orcelite
[Ni5�xAs2]) and Ni antimonides (breithauptite
[NiSb]), all recorded.
Crystal structure data are reported in Table 1.
The measured trevorite cell edge, equal to
8.3626(5) A, is comparable to that reported by
De Waal (1969) for ferroan trevorite. The
observed oxygen positional parameter for the
analysed crystal is equal to 0.2555 (2) A and this
is, to our knowledge, the only available datum
obtained on a single crystal for a natural trevorite.
On a diagram of oxygen positional parameter, u,
vs. the cell edge of the magnetite and trevorite
endmembers (Fig. 5b) where an Fe2+ $ Ni
FIG. 5. (a) Plot of Fe2+ (a.p.f.u.) vs. Ni (a.p.f.u.), calculated on the basis of four oxygens. Note that different textural
varieties of trevorite are not really distinguishable on the basis of Fe2+ or Ni contents. Rather, it is their textural
context and Cr contents that highlight them as being a different population (see text for details). An estimation of the
error associated with each parameter is illustrated and the dashed line shows the position of an ideal AB2O4
composition. (b) Oxygen positional parameter, u, vs. the cell-edge length of magnetite and trevorite spinel
endmembers. The analysed spinel falls close to the tie line connecting the two phases in an intermediate position as
would be expected from its composition of ~0.52 a.p.f.u. Fe2+.
METASOMATIC FORMATION OF TREVORITE
155
substitution is present, the analysed spinel falls
close to the tie line connecting the two phases in
an almost central position, as might be expected
from its composition of ~0.52 a.p.f.u. Fe2+.
Mossbauer spectroscopy data
The best fits to the Mossbauer data were obtained
by reduced w2. The uncertainties were calculated
using the covariance matrix. Errors were esti-
mated at ~�0.002 mm/s and 0.020 mm/s for
centroid shift and quadrupole splitting (DEQ)
respectively. In this study the Mossbauer spec-
trum, (Fig. 6) was fitted with one doublet and
three sextets. A correction for the difference
between the recoil-free fractions for Fe2+ and Fe3+
at room temperature was applied (Quintiliani,
2005). The Mossbauer parameters and the
corrected (Fe3+/SFe) ratio are shown in Table 2.
C e n t r o i d s h i f t ( C S ) , i n t h e r a n g e
0.235�0.266 mm/s and quadrupole splitting
(QS), �0.13 to 0.507 mm/s, are indicative of
Fe3+ in octahedral sites; CS and QS values of
0.838 and QS of 0.089 mm/s, respectively,
indicate Fe2+ ion distribution into octahedral
sites. Thus, the fitting model for the distribution
of Fe ions in the sample is of the form, Fe2+ (tet)
+Fe2+ (oct) +2Fe3+(oct).
Discussion
Conditions and timing of trevorite crystallizationin the Bon Accord Ni deposit
Petrographic evidence indicates the presence of
two textural domains in the trevorite ore sample
studied here. The petrographic observations are
supported in this respect by quantitative CSD data
(Fig. 3). Additional evidence is provided by the
TABLE 1. Results of structure refinement.
a0 8.3626 (5)u 0.2555 (2)m.a.n.T 25.0 (5)m.a.n.M 26.0 (6)Nrefl 117U (T) 0.0041 (2)U (M) 0.0054 (2)U (O) 0.0067 (5)R1 2.54wR2 4.87GooF 1.188Diff. peaks 2.09; �1.34
a0: cell parameter (A)u: oxygen positional parameterm.a.n. T and M: mean atomic number for T sites and Msites, respectivelyU(M), U(T), U(O): displacement parameters for M site,T site and O, respectivelyNrefl.: number of unique reflectionsR1 all (%), wR2 (%), GooF as defined by Sheldrick(2008)Diff. peaks: maximum and minimum residual electrondensity (� e/A3)Space group: Fd3m
FIG. 6. Room-temperature Mossbauer spectrum of Bon Accord trevorite.
156
B. O’DRISCOLL ET AL.
compositional data: the Cr contents of the
trevorites are sufficiently different to suggest
two distinct crystal populations. The coarse-
grained aggregates are the dominant textural
type. These aggregates resemble ‘augen’-type
microstuctures. They are coalesced, rounded
clusters of equilibrated trevorite that contain
abundant rounded Ni-sulfide, Ni-arsenide,
Ni-antimonide and silicate inclusions that are
predominantly spatially restricted to specific
sieve-textured zones. Relatively fine-grained
trevorite crystals occupy micro-shear bands and
are set in a foliated Ni-serpentine groundmass.
They do not contain visible sulfide or arsenide
inclusions. Their idioblastic shapes and parallel
alignment along the serpentine foliation suggest
that they grew during deformation and in the
presence of a fluid phase, plausibly during
formation of the micro-shears. It is interesting to
note that some of the largest trevorite crystals in
the micro-shear bands have sieve-textured cores
(Fig. 2e), perhaps suggesting that their early
growth coincided with that of sieve-textured
zones in the coarse-grained aggregates. The lack
of sulfide inclusions in the micro-shear bands may
also imply that the cores of the coarse-grained
aggregates preserve an earlier stage in the
sequence of texture-forming event(s).
Together with observations from Tredoux et al.
(1989) that record the presence of rare trevorite
grains preserving NiO (bunsenite), Cr-spinel and
cochromite cores, there is convincing evidence
that the coarse-grained trevorite aggregates have
experienced a more protracted annealing and
alteration history than those in the groundmass.
The shapes of the CSDs support this conclusion;
typical magmatic oxide ore bodies (e.g. chromi-
tites) exhibit CSDs that are generally straight for
the large size-fraction crystals (Waters and
Boudreau, 1996; O’Driscoll et al., 2010;
Vukmanovic et al., 2013), whereas those for
Bon Accord are markedly curved, with additional
complexity superimposed at the small size
fractions (Fig. 3b,c). It is quite likely that the
abundance of the smallest crystals reflects
trevorite subgrain development, a notion that is
supported by petrographic observation. However,
recrystallization has also led to a coarsening
effect, whereby the CSDs are extended to flattish
shapes at the large size fraction. The curved
convex nature of the Bon Accord CSDs is
therefore interpreted as reflecting a complex
history of metamorphic recrystallization. It is
possible that significant coarsening in some partsTABLE2.Mossbauer
param
etersanddistributionofFe2
+andFe3
+at
differentsitesandtheFe3
+/SFeratiodetermined
from
thespectrum.
Sites
CS
(mm/s)
QS
(mm/s)
Oxidation
Area
(%)
Linewidth
(mm/s)
X2�0.126
Fe3þ
Fe2þ
Fe3þ
Fe t
ota
l
�� co
rrHF
(kOe)
Tetrahedral
0.566
2.425
Fe2
+3.5
0.172
1.57
–Octahedral
0.235
0.507
Fe3
+3.4
0.112
2.22
0.632
281.7
Octahedral
0.838
0.089
Fe2
+27.6
0.340
472.0
Octahedral
0.266
�0.013
Fe3
+65.5
0.231
487.7
METASOMATIC FORMATION OF TREVORITE
157
of the sample has served to reduce the effects of
subgrain development, so that the CSDs may
underestimate the numbers of the smallest crystals
and would be driven to greater degrees of
curvature. In this instance, the EBSD technique
might serve to track to an earlier stage of textural
evolution, by mapping out crystallographic
misorientations, in the manner outlined by
Vukmanovic et al. (2013). The observation that
the sieve-textured zones (with attendant inclu-
sions) have developed surrounding aggregates
that themselves contain multiple annealed
trevorite grains also suggests that incorporation
of the inclusions occurred during a metamorphic
event. It seems plausible from the abundance and
the relatively fine grain size of the sulfides and
other inclusions that this event was associated
with dispersion and removal of sulfide, as none
has been preserved outside of the aggregates. The
(relatively) higher Cr contents of trevorite crystals
in the groundmass probably reflects redistribution
of Cr via fluids that migrated through the micro-
shear bands.
The assumption of stoichiometry in spinel-
group minerals has been the topic of considerable
discussion (see Dyar et al., 1989; Ballhaus et al.,
1991; Ghiorso and Sack, 1995; Quintiliani et al.,
2006; Rollinson and Adetunji, 2013). A common
observa t ion is tha t where Mossbauer
Spectroscopy is carried out to determine the true
Fe3+/Fe2+ (or Fe3+/SFe) ratio, the ratio calculated
utilizing the electron microprobe data under-
estimates the true ratio. A comparison of the
true Fe3+/SFe ratio calculated from the
Mossbauer data (0.69) with that from the micro-
probe data (0.86; n = 128) shows that in trevorite,
the true ratio is actually overestimated by the
assumption of stoichiometry. The implication of
this is that the Fe3+/Fe2+ ratios calculated using
the equation of Droop (1987) are not correct. One
explanation is that excess Fe3+ has been assigned
to the chemical formula because there is an
unaccounted for trivalent cation present. If this is
the case, the most likely possibility is Ni, which
can exist in Ni2+ and Ni3+ valence states in
geological materials, including bunsenite (NiO;
Pointon et al., 1971; Wells, 1984). For example,
Pointon et al. (1971) produced trevorite that
contained both divalent and trivalent Ni in
experiments, with all of the Ni located on the
octahedral sites. Further study is required to
confirm this hypothesis for the Bon Accord
natural example, potentially via Mossbauer
spectroscopy measurements using a 61Ni source.
Local and regional lithological constraints on theorigin of the Bon Accord Ni deposit
The arguments of Tredoux et al. (1989) for the
origin of the Bon Accord trevorite body as a
siderophile-rich mantle heterogeneity rest on two
key points: (1) that there is little (or no) textural
and mineralogical evidence for a sulfide-rich
protolith to the Bon Accord ore body and
(2) that the ultramafic cumulates in which the
ore body is hosted comprise the lower (mantle)
portion of a dismembered ophiolite. On the basis
of evidence presented here as well as observations
reported by previous workers, it would seem that
neither of these contentions is unassailable.
Firstly, numerous albeit fine-grained, Ni-rich
sulfides, Ni-arsenides and Ni-antimonides occur
as inclusions in restricted zones of the coarse-
grained trevorite aggregates (Fig. 2c). The popu-
lation density of these inclusions appears to
increase at increasingly smaller grain sizes,
suggesting that many more of these may be
present than are visible. No sulfides are present in
the groundmass, suggesting that the inclusions are
a relatively early paragenesis. Secondly, there is
reasonable evidence to suggest that the ultramafic
rocks that host the Bon Accord ore body may not
be derived from the oceanic mantle. There are
few, if any, occurrences of ophiolite mantle
peridotites in the Barberton Greenstone Belt for
which another origin has not been convincingly
argued. For example, the Stolzburg Complex
peridotites treated in detail by de Wit et al.
(1987) and argued to be ophiolite ‘mantle
tectonites’ have also been interpreted convin-
cingly as a highly deformed layered intrusion
(Anhaeusser, 2001). At Bon Accord, the close
association of banded cherts and magnetite-
bearing metasediments with the ultramafic rocks
does not support the origin of the latter at the base
of an ophiolite, especially given the complex
interleaved lithological relationships exhibited
(Fig. 1c). Instead, a sea-floor environment,
where the serpentinized ultramafics have an
intrusive or extrusive komatiitic precursor seems
much more plausible. Indeed, komatiite flows
erupted and intruded in such settings are an
abundant and ubiquitous feature of the
Onverwacht Group of the Barberton Greenstone
Belt (Lowe and Byerly, 1999; de Wit et al., 2011).
One key feature of the argument of Tredoux et
al. (1989) for a deep mantle origin for Bon Accord
was their presentation of a suite of platinum-group
element (PGE) data for nine ore body samples,
158
B. O’DRISCOLL ET AL.
including three samples from the margins of the
body that preserved evidence for deformation.
Their data revealed that whole-rock samples of the
Bon Accord trevorite deposit are characterized by
slightly supra-chondritic PGE abundances. The
chondrite-normalized PGE patterns that they
produced indicated that the undeformed ore
showed relatively flattish patterns, whilst the
deformed samples had lower (sub-chondritic) Os
and Ir concentrations. It is worthwhile to
reconsider these PGE data briefly in the light of
an additional 25 years of PGE published literature
(Naldrett, 2004; Carlson, 2005; Walker, 2009).
The data presented in table 3b of Tredoux et al.
(1989) are replotted in Fig. 7 for ease of reference,
normalized to the chondritic values reported by
Horan et al. (2003). At the outset, the relatively
high abundances of the PGE are useful in that they
rule out ore formation via skarn-related processes,
a plausible scenario given the close proximity of
the Bon Accord ore body to the southern margin of
the Stentor Granitoids. However, a number of
other possible origins are permitted by these PGE
patterns and abundances. In particular, podiform
chromitite formation and Ni-sulfide mineralization
are terrestrial processes that have the potential to
generate the PGE characteristics observed in the
Bon Accord deposit (Fig. 7), although these were
discussed and discounted by Tredoux et al. (1989).
In any case, it should be noted that a podiform
chromitite origin is only likely to be feasible if the
protoliths to the host ultramafics are mantle
peridotites.
Indirect evidence of the Bon Accord ore body’s
origin can be derived from consideration of the
lithologies exposed locally along strike from the
deposit. For example, extensive Ni-sulfide miner-
alization occurs several km along strike from the
ore body (100�200 m north of the Scotia Talc
Mine; Fig. 1c), as well as a hematite-magnetite
ore body ~1 km along strike from the trevorite ore
body that was described by Tredoux et al. (1989).
The presence of the thin units of chlorite-
magnetite schist in close proximity to the trevorite
ore body, the hematite-magnetite body and the
Ni-sulfide mineralization described by Keenan
(1986) suggests that all of these lithologies
occupy a similar stratigraphic level (Fig. 1).
Keenan (1986) provided a detailed lithological
description of the Ni-sulfide mineralized zone and
identified the major Ni-bearing sulfide phase as
pentlandite (noting the presence of pyrite�pyr-
rhotite as well). The latter author also highlighted
the position of this Ni-sulfide mineralization at the
boundary between siliceous (cherty) sediments
and the overlying ultramafic lavas and used this
evidence to infer a primary magmatic origin for
the concentration of the Ni-sulfides at the seafloor.
FIG. 7. Chondrite-normalized (CI-type Orgueil; Horan et al., 2003) platinum-group element (PGE) patterns for the
Bon Accord trevorite deposit. The data are replotted from those reported by Tredoux et al. (1989). The green
diamonds represent material from the deposit core and the blue circles, two samples from the serpentinized margin.
The primitive mantle (PM) estimate (yellow stars) is from Becker et al. (2006). The range of Archaean Ni-sulfide
deposit compositional data (in purple) included for comparison are from Naldrett (2004) and the data for podiform
chromitites (in red) are from the Shetland Ophiolite Complex (O’Driscoll et al., 2012) and the Troodos Ophiolite
(Buchl et al., 2004).
METASOMATIC FORMATION OF TREVORITE
159
Many large Archaean Ni-sulfide deposits are sited
at the boundary between seafloor sediments and
overlying komatiites, so this is a common and
unsurprising geological setting for such miner-
alization. Keenan (1986) did not have a firm
explanation for the trevorite ore body, besides
hypothesizing some form of genetic link between
it and the Ni-sulfide mineralization. Further afield,
regional evidence that supports massive Ni-sulfide
formation, as well as hydrothermal activity, in an
ocean-floor setting is widespread. For example,
De Ronde et al. (1994) documented in detail
abundant field and petrographic evidence for
hydrothermal discharge structures in rocks occur-
ring at similar stratigraphic levels to Bon Accord
in the Barberton sequence. In particular, the latter
authors studied ironstone pods that they inter-
preted as Fe-oxide precipitates, distinguishing
them from massive sulfide deposits such as at
Bien Venue (see Fig. 1a) on the basis of major
and trace-element geochemistry. This difference
was attributed in part to host rock composition;
such differences in the compositions of hydro-
thermal activity are observed in modern day
equivalents (e.g. the Trans-Atlantic Geotraverse
on the Mid-Atlantic ridge near 26ºN). Widespread
sea-floor metasomatism of these Barberton rocks
preceded the thermal metamorphism that accom-
panied intrusion of younger granite bodies,
according to De Ronde et al. (1994; and
references therein).
On balance, therefore, the evidence seems to
indicate that the Bon Accord trevorite body under
study here formed in a sea-floor setting, closely
associated with Archaean komatiite eruptions and
Ni-sulfide mineralization. Although these litholo-
gical associations make the sulfide mineralization
at Bon Accord area a typical komatiite-hosted
Ni-sulfide deposit (e.g. Kambalda, Western
Australia) in some respects, the presence of the
trevorite ore body means that there are interesting
differences. The Marriotts Ni-sulfide occurrence
at Mount Clifford (Hudson and Travis, 1981) has
previously been shown to contain trevorite as a
result of alteration of the primary sulfide
assemblage, but only in minor quantities as a
disseminated mineral phase. The comparatively
massive trevorite ore body at Bon Accord may
have formed via oxidation and desulfurization of
Ni-sulfide ore, but it is difficult to speculate on the
nature and timing of the purported oxidation
event. The alteration of pentlandite to trevorite
could occur via a two-stage oxidation process as
follows:
2(NiS.2FeS) + 9O2 = 4FeO +2NiO + 6SO2
4FeO + 2NiO + O2 = 2NiFe2O4
It is interesting to recall that Tredoux et al.
(1989) describe bunsenite at the cores of coarse-
grained trevorite crystals, lending support to the
sequence of reactions suggested above.
Furthermore, Hall (1924) estimated proportions
of 71% NiS and 25% NiO for ovoid lumps of ore
material in the Barberton region, suggesting that
the occurrence of NiO is widespread, if not
abundant. Furthermore, trevorite and bunsenite
are typical and commonly reported oxidation
products of pentlandite in the metallurgical
sciences, cf. Zhu et al. (2012), who reported
similar textures to those observed here and by
Tredoux et al. (1989) in their experimental
observations. Another plausible explanation is
that the trevorite body may have an exhalative
origin as an Archaean ‘black smoker’. However, its
high Ni content (without any evidence of
accompanying Cu enrichment in any of the sulfides
or in the bulk rock composition) does not
immediately seem to support this. In addition,
much of the groundmass to the trevorite ore is Ni-
serpentine, indicating an olivine-rich precursor and
therefore suggesting the intimate involvement of
the ultramafic lava flows in the formation of the
body. It is possible that the location of the Bon
Accord body adjacent to the magnetite-bearing
metasediments is not a coincidence, and that a
contribution of a magnetite component from these
rocks, perhaps via assimilation in the komatiite,
may have facilitated Bon Accord trevorite forma-
tion in some way. The mineral chemical data
indicate the involvement of other base metals, with
consistent levels of Co present in trevorite
(ppm6103) and extremely uniform Co/Ni too.
This may imply consistent Co/Ni in the precursor
sulfides. Indeed, of the inclusion phases, godlevs-
kite shows similar Co/Ni to the trevorites, whereas
other sulfides, arsenides and antimonides do not.
Cobalt is quite soluble during low-temperature
metasomatism (Liu et al., 2011), but given the
consistent Co/Ni and chondritic PGE abundances,
it would seem that some aspects of Bon Accord’s
original compositional profile have been preserved.
The Ni-rich composition of the ore can be
attributed to the fact that much of the sulfide was
pentlandite, in turn a result of the surrounding
ocean-floor lavas being ultramafic (komatiite) and
much richer in olivine (and therefore Ni) than
typical MORB pillow basalts.
160
B. O’DRISCOLL ET AL.
Conclusions
(1) Petrographic observations and mineral
chemical data indicate that trevorite crystals in
the Bon Accord Ni deposit have undergone a
complex history involving multiple growth and
annealing events. The presence of abundant, albeit
small (<10 mm) inclusions of Ni arsenides,
Ni sulfides and Ni antimonides in restricted
‘zones’ of coarse-grained trevorite aggregates
shows that these minerals existed at a compara-
tively early stage in the ore deposit’s history. Their
presence raises the possibility that desulfurization
of a massive sulfide deposit was responsible for
Bon Accord trevorite ore formation.
(2) Observations on the regional geology of the
lowest Barberton sequence also support links
made between Bon Accord and a massive Ni-
sulfide deposit. A stratigraphically constrained
massive sulfide formation is well developed
several hundred metres along strike from Bon
Accord, as well as in numerous localities further
afield, most notably the Bien Venue deposit (De
Ronde et al., 1994). Indeed the stratigraphic
position of Bon Accord, above marine sediments
and below ultramafic extrusive rocks is typical of
komatiite-hosted massive Ni-sulfide ore forma-
tion. These observations, together with a reconsi-
deration of the PGE data of Tredoux et al. (1989)
are in agreement in this respect and appear to
favour oxidation of such a Ni-sulfide deposit,
instead of viewing the origin of Bon Accord as a
mantle-derived entity.
Acknowledgements
Cian O’Driscoll is gratefully acknowledged for
excellent technical support. Andy Tindle is
thanked for his assistance in the Open
University (UK) electron microprobe laboratory.
The authors also thank Dr Tom Molyneux, who
donated the sample used in this study to RGC.
Hugh Rollinson and Steve Barnes provided
review comments that improved an earlier draft
of this manuscript.
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