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’driscoll@keele.ac.ukDOI: 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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