JOURNAL OF PETROLOGY
VOLUME 55
NUMBER 3
PAGES 643^670
2014
doi:10.1093/petrology/egu002
Prograde Sulfide Metamorphism in Blueschist
and Eclogite, New Caledonia
JULIE L. BROWN1*, ANDREW G. CHRISTY2, DAVID J. ELLIS3 AND
RICHARD J. ARCULUS3
1
CANADIAN NUCLEAR SAFETY COMMISSION, OTTAWA, ON K1P 5S9, CANADA
2
CENTRE FOR ADVANCED MICROSCOPY, AUSTRALIAN NATIONAL UNIVERSITY, CANBERRA, ACT 0200, AUSTRALIA
3
RESEARCH SCHOOL OF EARTH SCIENCES, AUSTRALIAN NATIONAL UNIVERSITY, CANBERRA, ACT 0200, AUSTRALIA
RECEIVED DECEMBER 22, 2012; ACCEPTED DECEMBER 31, 2013
ADVANCE ACCESS PUBLICATION JANUARY 29, 2014
In New Caledonia, blueschist and eclogite preserve, as inclusions in
porphyroblastic minerals, a record of sulfide present during prograde
subduction processes. Sulfide inclusions in prograde garnet and lawsonite became chemically isolated from the matrix whereas sulfide
minerals in the matrix continued to equilibrate with matrix fluids,
or grew later, during retrogression. Cu^Fe sulfide mineral inclusions
have been found across metamorphic grade within silicate-defined
metamorphic mineral zones spanning a crustal profile of 30 km.
Bulk area scans of sulfide inclusions provide compositions that represent mixtures of the solid sulfide that were included as the host silicate minerals grew. In general, single sulfide inclusion compositions
and aggregate sulfide assemblages are distinct from those of matrix
phases. High Cu contents in sulfide inclusions are interpreted to be
a consequence of Fe lost from sulfide to growing garnet, rather than
the result of intrinsically high Cu in the bulk-rock. The distribution
of sulfide inclusion compositions across metamorphic grade, considered together with the available thermodynamic data, suggests
that covellite/nukundamite-bearing inclusions in lawsonite, high in
both Cu and S, disappear at higher grades as these sulfide minerals
are no longer stable. Similarly, clustering near the ratio Fe:Cu ¼ 1:1
may cease with increasing grade owing to the replacement of chalcopyrite/intermediate solid solution (iss) by the denser assemblage
pyrite þ pyrrhotite þ bornite/digenite.
I N T RO D U C T I O N
New Caledonia; sulfide metamorphism; blueschist;
eclogite; subduction
Most studies of regional metamorphism ignore sulfide
mineralogy because sulfide minerals re-equilibrate much
more quickly than silicate minerals (Barton, 1974), and
therefore do not represent the conditions of peak metamorphism of the host-rock. Furthermore, although there
are abundant experimental data concerning the stabilities
of common low-pressure sulfide minerals of Cu and Fe
over wide ranges in temperatures, it is currently difficult
or impossible to incorporate such phases with confidence
in thermodynamic models of high-pressure metamorphic
reactions because little work has been done on the system
Cu^Fe^S at pressures above 1atm. As a result, the geochemical behaviour of Cu in metamorphic rocks remains
very poorly understood. However, Kawakami et al. (2006)
showed that in amphibolite- and granulite-facies rocks, sulfide inclusions within porphyroblasts preserved compositions consistent with equilibration during prograde
metamorphism, whereas matrix sulfide was retrogressed.
The study of sulfide inclusions trapped under metamorphic
conditions therefore affords a unique window into the
high-pressure phase relations of natural Cu-bearing sulfide
minerals. This study extends this approach by investigating
whether sulfide inclusions in silicate minerals that formed
during subduction show evidence for preserving high-pressure, low-temperature assemblages and compositions.
By examining blueschist- and eclogite-facies rocks from
*Corresponding author. Telephone: þ1 (613) 944-1984. Fax: þ1 (613)
995-5086. E-mail: [email protected]
ß The Author 2014. Published by Oxford University Press. All
rights reserved. For Permissions, please e-mail: journals.permissions@
oup.com
KEY WORDS:
JOURNAL OF PETROLOGY
VOLUME 55
New Caledonia, and in particular sulfide inclusions within
garnet and lawsonite porphyroblasts, we present evidence
for sulfide processes characteristic of the Earth’s deep
sulfur cycle.
Sulfide inclusions in porphyroblasts should retain their
overall bulk chemical composition from the time of enclosure, reflecting the solid sulfide phases that were stable
under subduction conditions. In the absence of cracks or
fractures in host silicate minerals, sulfide compositions
remain isolated from, and unmodified by, subsequent
retrograde fluid^mineral processes that characterize much
of blueschist^eclogite excavation metamorphism. A sulfide
included during prograde porphyroblast growth is nearly
a closed system. Major chalcophile elements such as Ni,
Cu and S are not accommodated in silicate phases above
trace levels, and are trapped in the inclusion. The only
element that can act as a major component in both sulfide
inclusions and the silicate host, and thus may participate
in exchange reactions between them, is Fe. Iron present in
sulfide trapped at low pressure can change its behaviour
from chalcophile to lithophile at high pressures, principally
owing to garnet-producing reactions. Thus, sulfide trapped
within garnet may become enriched in those elements
that remain chalcophile (e.g. Cu) and do not partake in
blueschist^eclogite silicate-forming reactions. At temperatures of 7008C, rates of intra-granular diffusion in
garnet are high enough to eliminate zoning at a scale
larger than a single crystal; at such temperatures, an
entire porphyroblast may be in chemical equilibrium with
the matrix (Yardley, 1977). At lower temperatures, such as
the conditions in this study, diffusion would occur on a
scale that is smaller than the grain size of the enclosing
garnet host (Marmo et al., 2002).
The range of processes that can be experienced by a
trapped inclusion is therefore limited to the following.
(1) The loss or gain of Fe from the surrounding silicate,
with a concomitant change in the cation:sulfur ratio
of the inclusion, and the ratio of Fe to non-Fe cations.
The associated need to change the cation:oxygen
ratio in the host, or to diffuse other components
through it, may be difficult to accommodate, which
would inhibit such processes.
(2) Even in the absence of Fe exchange with the host, the
sulfide assemblage will still re-equilibrate on both the
heating and cooling legs of the P^T path, but such
changes will be isochemical.
Possible isochemical changes to sulfide include the
following.
(1) The polymorphic transformation of a phase, with no
reaction between phases.
(2) The change in composition of a solid-solution phase in
response to P^T changes. If the overall inclusion
NUMBER 3
MARCH 2014
composition remains fixed, then at least one other
phase must be exsolved or resorbed.
(3) The homogenization of a multiphase assemblage to a
single solid-solution phase.
(4) The breakdown of a phase to a multiphase assemblage
of the same overall mean composition.
Based on these possible isochemical changes, we anticipate
that the current phase assemblages occurring as inclusions
may be different from either those present at peak metamorphism or those originally incorporated into the host
porphyroblast, as discussed by Vernon et al. (2008).
However, it is likely that the bulk composition of a given
inclusion remains constant.
At present, there is very little published work on highpressure phase equilibria in petrologically important sulfide systems, or on sulfide^silicate reactions, although
Kullerud (1967) presented some early data for both. This
study is a reconnaissance of sulfide minerals from a suite
of rocks from the high-pressure belt of New Caledonia,
focusing on inclusions trapped in prograde porphyroblasts
such as garnet. We examined these inclusions for systematic
differences from matrix sulfide minerals, which are vulnerable to retrogression, and also for evidence of the phases
that were trapped during porphyroblast growth, and the
processes that they have undergone.
The Fe^Cu^Ni^S system at 1 bar
Many phases are known in the Fe^Cu^Ni^S system: their
phase relations at 1bar and high temperature have been reviewed by Fleet (2006). The Fe^S subsystem contains, in
addition to pyrite (FeS2) and pyrrhotite (Fe1^xS), the minerals greigite, smythite, mackinawite and marcasite,
which are known either to never be stable or at best to be
stable only at very low temperatures (Kullerud & Yoder,
1959; Lennie & Vaughan, 1996; Fleet, 2006). ‘Pyrrhotite’ is
actually a group of closely related monoclinic, orthorhombic or hexagonal minerals with differently ordered cations
and vacancies. Troilite, the stoichiometric hexagonal relative of pyrrhotite, is stable only below 1478C at 1bar
(Fleet, 2006).
A distinctive feature of the Fe^Cu^Ni^S system is that
the large number of ordered, stoichiometric phases known
at low temperature tend to coalesce into a small number
of variable-composition phases with broad solid solution
fields at higher temperature.
The very large number of minerals such as yarrowite,
anilite and djurleite that exist between Cu2S and CuS
(Potter & Evans, 1976) no longer exist above 2008C,
where the only phase between high-chalcocite (Cu2S)
and covellite (CuS) is an increasingly broad digenite solid
solution, which eventually replaces chalcocite as well,
above 4358C (Barton, 1973). This solid solution spans the
range Cu2S^Cu1·75S at 6008C, and also extends well into
the Cu^Fe^S ternary system, beyond the ideal composition
644
BROWN et al.
PROGRADE SULFIDE METAMORPHISM
of bornite (Cu5FeS4) to about Cu4·1Fe1·4S4 (Cabri, 1973).
This is the metal-rich ‘bornite^digenite solid solution’
(bss). Bornite in various structural states exists as a separate phase of narrow compositional range up to 2658C,
where it is subsumed by this solid solution (Grguric et al.,
2000).
Although the nickel-rich part of the system is of only
marginal relevance to the current study, in which Ni concentrations were found to be small, we note that pyrrhotite
(Fe1^xS), an important phase in our rocks, forms a ‘monosulfide solid solution’ (‘mss’) towards Ni1^xS that becomes
complete at a temperature between 300 and 4008C
(Misra & Fleet, 1973). Solid solution in pyrrhotite towards
CuS is much more restricted (0·3 at. % Cu at 3008C and
still only 0·6% at 3508C; Sugaki et al., 1975).
Covellite, with a different structure type from that of
pyrrhotite, exhibits no significant solid solution, being
close to pure CuS up to its incongruent melting at 5078C
(Fleet, 2006). There are two known Cu-rich phases with
1:1 metal:sulfur ratios in the Cu^Fe^S system: idaite
(‘orange
bornite’,
Cu3FeS4)
and
nukundamite
(Cu3·38Fe0·62S4). Nukundamite is a rare fixed-composition
phase occurring with covellite and sulfur, structurally
related to covellite (Rice et al., 1979; Sugaki et al., 1981).
Idaite is poorly characterized: the original report of a
supergene alteration product of bornite by Frenzel (1959)
assumed a formula identical to that of nukundamite, and
the two phases have occasionally been confused in the literature, but the true composition of this mineral appears
to be nearer Cu3FeS4 (Constantinou, 1975; Sugaki et al.,
1981). This idaite may be metastable, and in any case decomposes to chalcopyrite þ nukundamite above 2608C
(Wang, 1984), which itself appears to be a metastable assemblage. Seal et al. (2001) re-evaluated earlier thermodynamic data for nukundamite and concluded that its
rarity is due to a stability field that spans a very narrow
range (maximum 0·4 log units) of fS2, sulfidizing to covellite þ pyrite above that range, and desulfidizing to bornite þ pyrite below. It is not stable at all below 2248C, and
is stable with excess sulfur only in the interval 434^5018C.
The stability of pyrite þ bornite over that whole temperature range implies that nukundamite þ chalcopyrite is
never stable (Seal et al., 2001; Inan & Einaudi, 2002).
The third major solid solution is the ‘intermediate solid
solution’ (iss), lying in the middle of the Cu^Fe^S triangle.
At 6008C, this spans a range of Fe/(Fe þ Cu) ¼ 0·4^0·7,
(Fe þ Cu)/S ¼1·00^1·15 (Cabri, 1973). The compositional
field shrinks and breaks up at lower temperature, differentiating into ordered phases of restricted composition range
such as chalcopyrite (CuFeS2), cubanite (CuFe2S3), and a
range of low-temperature metal-rich phases such as mooihoekite, Cu9Fe9S16 (see Cabri & Hall, 1972). The mineral
isocubanite is a rare instance of cubic iss solid solution persisting at low temperature in nature. It is formed by
quenching of black smoker fluids in near-freezing ocean
water (Caye et al., 1988), but becomes the stable polymorph
of CuFe2S3 only above 2008C. Chalcopyrite is thermally
the most stable of the ordered iss derivatives: it persists as
an ordered tetragonal phase with a distinct composition
field until 5578C, when it breaks down to iss þ pyrite
(Barton, 1973).
The common association of covellite with native sulfur
in nature and sulfur vapour in experiments suggests that
CuS2 is unstable at near-ambient conditions, and indeed
the natural Cu-dominant pyrite mineral, villamaninite,
always contains substantial amounts of other components
such as Fe, Co and Ni, and is probably metastable (Fleet,
2006). ‘Fukuchilite’, nominally Cu3FeS8, appears to be
Fe-rich villamaninite, implying the occurrence of substantial solid solution towards pyrite (Bayliss, 1989). However,
the high-P/high-T synthetic study of Munson (1966)
showed that pure pyrite-structure CuS2 formed from covellite þ sulfur at pressures above 15 kbar.
Many of the sulfide minerals discussed above could have
been present in seafloor basalts and sediments, either as
primary precipitates or secondarily through sulfidation/
desulfidation reactions. Some sulfides such as pyrite possess
strong, well-directed bonding, as evidenced by their high
hardness. Therefore, they are kinetically inert, and can
persist metastably and retain minor element zonation
through geological time. However, most sulfide minerals
are considerably more reactive. The small copper cation
often shows high diffusivity in sulfide, which allows fixedcomposition phases to merge into broad solid solutions
through cation disorder at geologically low temperatures
as described above, and is manifest to an extreme degree
in Cu2S, where the Cu sublattice ‘melts’ above 1058C to
form the solid-electrolyte high-chalcocite phase (Wang,
2012). Given the resulting fast kinetics of sulfide reactions,
Cu-bearing phases would have re-equilibrated at an early
stage of subduction to produce intermediate and bornite^
digenite solid solutions. Thus, it is unlikely that Cu-bearing
sulfide incorporated into garnet growing at depth would
reflect the earlier unsubducted seafloor mineralogy.
Instead, some such inclusions would be expected to
sample single-phase solid-solution phases that were stable
under the P^Tconditions of incorporation. Given the paucity of experimental data for sulfide systems in situ at high
pressure, it is uncertain at present how increasing pressure
modifies the extent of solid solutions, or the compatibility
between phases. However, inclusions give us access to the
bulk sulfide compositions that were in existence at the
time of their isolation in growing porphyroblasts. Some inclusions, such as those of single-phase pyrite, are stable
over the whole P^T path inferred for New Caledonian
subduction, and may have undergone little change. Other
sulfide inclusions will have resorbed or exsolved phases
owing to a change in composition range of a solid solution
645
JOURNAL OF PETROLOGY
VOLUME 55
or its complete breakdown, or will have undergone crossed
tie-line reactions, at various points during subduction and
exhumation. In all cases, we expect that high-P/high-T
phases will have been replaced during uplift by those stable
at or near ambient conditions by the time they reach the surface. However, because the inclusions are diffusionally isolated from the rock matrix, the bulk composition of each
inclusion should remain unchanged. Bulk compositions
should correspond to either single phases or assemblages
that were stable at metamorphic conditions. Therefore, the
analysis of populations of such compositions will give an indication of high-P phase relations.
There are cases where sulfide inclusions within a porphyroblast are in fact connected to the matrix through
fractures, and have been exposed to retrogressive fluids.
These sulfide minerals were termed ‘pseudo-inclusions’ by
Kawakami et al. (2006). Because we are primarily interested in inclusions that were trapped on the prograde
path, we aim to distinguish genuine prograde inclusions
from pseudo-inclusions, as well as from inclusions that are
hosted in minerals that formed during retrograde metamorphism. Nevertheless, we record pseudo-inclusion and
matrix sulfide, because systematic differences between the
phases present in these microenvironments and in true inclusions provide evidence that the entrapment of inclusions
protects them from reacting with fluids during retrogression.
It should be noted that such fluids may have reintroduced
sulfur into the system (e.g. Itaya et al., 1985), resulting in the
precipitation of new sulfide minerals or the replacement of
old ones. Alternatively, sulfur may have been leached from
the system, resulting in the absence of matrix sulfide minerals. Any new sulfide mineral may have sourced Fe either
from existing metamorphic minerals or from the fluid, and
would have incorporated Cu that was present in the matrix
at the time of fluid influx.
Here, we examine the sulfide phases present in rocks
sampled across a high-pressure, low-temperature metamorphic belt, and determine the following: (1) whether sulfide assemblages in the matrix of a host-rock are different
from those trapped as inclusions in porphyroblasts; (2)
whether there are systematic differences in these assemblages as a function of lithology or metamorphic grade;
for example, pyrite is generally common in metamorphosed basalt, whereas pyrrhotite is common in metasedimentary rocks (Frost, 1991); (3) what the identities of
single-phase sulfide inclusion assemblages and the mean
compositions of multiphase sulfide assemblages reveal
about the sulfide compositions that were trapped at high
pressure.
Area scan analyses of polymineralic inclusions were
performed to estimate original sulfide compositions.
The area scan technique and data collection method
have been described by Brown (2007), and are described
below.
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MARCH 2014
GEOLOGIC A L S ET T I NG
New Caledonia is a 400 km long island in the SW Pacific
that is the largest emergent portion of the Norfolk Ridge
(Sdrolias et al., 2004). It is surrounded by submerged continental ridges, volcanic-arc ridges and both extinct and
active back-arc basins. The Norfolk Ridge is considered to
be a piece of continental crust rifted from the margin of
Gondwana in the late Cretaceous (Sdrolias et al., 2004).
New Caledonia is a mosaic of diverse terranes, including
a Mesozoic basement complex overlain by a series of
Cretaceous to Eocene sedimentary sequences and a variety
of mafic and ultramafic ophiolitic nappes (Fig. 1). The basement rocks of New Caledonia comprise three Mesozoic,
arc-derived terranes: the Permian to Jurassic Teremba
island arc terrane (Paris, 1981); the schistose metasediments
of the Boghen terrane (Aitchison et al., 1995); and the late
Carboniferous, fore-arc related Koh ophiolite (Aitchison
et al., 1998; Cluzel et al., 2001). These terranes were amalgamated to Australian Gondwana by the Early Cretaceous.
This basement is unconformably overlain by a sequence of
Late Cretaceous to Eocene sandstones, siltstones, shales
and carbonates, the lowermost units marking the onset of
rifting during the break-up of Gondwana (see Brothers,
1974; Paris, 1981; Aitchison et al., 1995; Cluzel et al., 2001).
A more complete account of the New Caledonian terranes
has been given by Brown (2007).
The high-P/low-T metamorphic rocks, which are the
focus of this study, are located in the northeastern portion
of the island. This high-pressure metamorphic belt is one
of the most extensive blueschist- to eclogite-facies terranes
in the world, covering an area of more than 2000 km2
(Lillie, 1975). From SW to NE, there is a progressive
increase in metamorphic grade (Black, 1977). In many
places, true isograds have been removed by normal faulting related to late-stage exhumation and extension
(Rawling & Lister, 2002), accounting for the rapid increase
in metamorphic grade to the north.
The high-P/low-T rocks have been divided into the
Diahot and Poue¤bo terranes on the basis of differences in
protolith lithology, geochemistry, metamorphic grade,
and structural features (Brothers, 1974; Black & Brothers,
1977; Briggs et al., 1977; Brothers & Yokoyama, 1982; Cluzel
et al., 1994; Clarke et al., 1997; Carson et al., 1999; Rawling
& Lister, 2002). Early work established that the highest
stratigraphic units retain the highest-pressure mineral
assemblages.
The Diahot terrane is thought to be tectonically underlain by the Poue¤bo terrane (Carson et al., 1999, 2000). It
consists of a range of lawsonite- to epidote^omphacitebearing blueschists, whose dominant protoliths were
Cretaceous sandstones and siltstones. It also contains subordinate lenses of basalt, rhyolite, chert, conglomerate and
carbonate (Briggs et al., 1977). The petrology of the Diahot
terrane metabasites between the villages of Pam and
646
BROWN et al.
PROGRADE SULFIDE METAMORPHISM
Fig. 1. Regional geology of New Caledonia after Aitchison et al. (1995) and Clarke et al. (1997). The box outlines the study area, depicted at a
larger scale in Fig. 2.
Oue¤goa indicates two distinct prograde metamorphic
events (Fitzherbert et al., 2003). M1 assemblages are
patchily preserved; typically, metamorphic minerals
(omphacite, chlorite, lawsonite, glaucophane) completely
pseudomorph igneous plagioclase and augite. S2 deformation fabrics envelop M1 assemblages, increasing in intensity toward the NE, and preserve M2 transitional
blueschist- to lower eclogite-facies assemblages. Peak metamorphic conditions indicated from the Diahot metabasites
are P ¼1·7 GPa and T ¼ 6008C (Fitzherbert et al, 2003).
The lithology of the Poue¤bo terrane is dominated
by metamorphosed basaltic units, referred to as eclogite
and garnet glaucophanite (Clarke et al., 1997; Carson
et al., 1999). Black & Brothers (1977) suggested that the
mafic eclogites of the Poue¤bo terrane represent a metamorphosed back-arc basin sequence. However, Spandler
et al. (2005) pointed out that there is a wide variety of rock
types in this terrane, including pelitic and ultramafic material, and referred to it as the Poue¤bo Eclogitic Me¤lange.
Peak metamorphic conditions of P 1·9 GPa and
T 6008C have been deduced for the eclogite (Carson
et al., 1999). Semi-pervasive fluid influx during isothermal
decompression formed garnet glaucophanite at
P 1·6 GPa (Carson et al., 2000).
The entire belt experienced high-P subduction metamorphism in the Eocene, and has been interpreted as a
metamorphic core complex (Aitchison et al., 1995; Clark
et al., 1997). However, the structural studies of Rawling &
Lister (1999, 2002) show that the eclogites form a sheet
that lies along the northern limb of a regional antiform.
A link has been proposed between the obduction of the
New Caledonian Ophiolite Nappe and the eclogite-forming event (Cluzel et al., 2001; Rawling & Lister, 2002).
However, Spandler et al. (2005) constrained the timing of
peak metamorphism to 44 Ma, 10 Myr prior to the obduction of the New Caledonian Ophiolite Nappe, and suggested that the metamorphic peak was related to the
large-scale reorganization of the plate-tectonic configuration in the SW Pacific at this time.
ST U D I E D SA M P L E S
The high-pressure belt has been divided into metamorphic
zones defined on the basis of silicate equilibria by
647
JOURNAL OF PETROLOGY
VOLUME 55
Black (1977), Carson et al. (1999) and Fitzherbert et al. (2003).
In order of increasing pressure, they identified lawsonite,
epidote, omphacite and hornblende zones, spanning a
range of conditions from c. 0·7 GPa, 3508C up to 1·9 GPa,
6008C. A traverse across these zones thus allows us to
access sulfide inclusions from a wide range of high-P conditions. Sample locations are shown in Fig. 2, and their lithologies and non-sulfide mineralogy are given in Table 1. P^T
estimates for the zones (above) define similar trajectories to
those shown in Fig. 3, but with rocks of the different zones
reaching their metamorphic peak at different points along
the path prior to exhumation. Inclusions were found in
almost all zones. Assignment of a rock to a metamorphic
zone was used to constrain the pressure and temperature
conditions of the sulfide inclusions, given it is the growth of
the distinctive silicate porphyroblasts of garnet and lawsonite that isolated sulfide inclusions with compositions reflecting the high-pressure sulfide mineralogy.
No prograde sulfide inclusions were found in samples of
the epidote zone, although numerous garnet porphyroblasts were studied from samples collected for this study.
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MARCH 2014
We propose that this exception does not simply reflect insufficient sampling, although that remains a possibility.
The composition and textural associations of matrix and
inclusion sulfide within specified metamorphic zones are
described below, in order of increasing P. P^T estimates
from silicate assemblages are based on the previous studies
of Clarke et al. (1997), Carson et al. (1999), Marmo et al.
(2002) and Fitzherbert et al. (2003), and are summarized
in Fig. 3.
A full account of analytical methods and interpretation
of analyses is presented in the Appendix.
S U L F I D E M I N E R A L C H E M I S T RY
A N D P E T RO G R A P H Y
Lawsonite-zone sulfide
The lawsonite zone is characterized by the appearance of
lawsonite in metamorphosed sedimentary and basalt
units. P^T conditions for the lawsonite zone of 0·7^
1·0 GPa and 350^4008C are based on the metabasalt silicate mineral assemblages (Fitzherbert et al., 2003). No Co
Fig. 2. Simplified geological map of study area, showing metamorphic isograds. Geology and isograds compiled from Black (1977), Maurizot
et al. (1989), Rawling & Lister (1999) and Fitzherbert et al. (2003). The apparent epidote-in isograd is actually a fault. Samples from locality numbers are: (1) J36, (2) J15, (3) J35, (4) J16, (5) 23956, (6) 23903, (7) 23897, (8) 73004, (9) 23874, (10) J17, (11) E803, (12) J22, (13) 729xx series, (14)
J40, (15) 96312k, (16) 72811, (17) NC2, (18) 99xx series, (19) J37, (20) 73101, (21) 73102, (22) J43, (23) J38, (24) 73112. The locations of the Balade
and Me¤re¤trice mines are shown by crossed hammers; Balade is located with (17) in the omphacite zone whereas Me¤re¤trice lies further to the
south along the edge of the high-pressure belt.
648
BROWN et al.
PROGRADE SULFIDE METAMORPHISM
Table 1: Lithologies and major silicate or oxide minerals present in rocks of this study
Sample no.
Lithology
Lws
Czo/Ep
Om
Gln
Hbl
Phg
Chl
x
x
Prg
Grt (Alm)
Qtz
Ab
x
x
x
x
Others
Lawsonite zone
J15a
mafic lawsonite blueschist
J15
metasedimentary schist
J16b
metasedimentary lawsonite schist
J16c
metasedimentary schist
J36
schist
x
x
x
x
x
x
Ttn, Stp
x
x
x
Sps
x
Ttn
x
x
x
Sps
x
Ttn
x
x
x
Epidote zone
73004a
glaucophanite
x
x
23874
mica schist
x
x
23897
mica schist
x
x
23903
quartzite
23956
mineralized metasedimentary schist
x
x
x
x
x
x
x
x
x
x
x
x
x
Ttn
x
Ilm
x
x
Ilm
x
x
Ilm
Omphacite zone
96312k
aluminous glaucophanite
x
x
9908
glaucophanite
x
x
x
x
9918
aluminous eclogite
x
x
x
9930b
eclogite boudin in schist
x
x
x
9931a
eclogite
x
9939
eclogite
x
x
9941b
glaucophanite
x
x
9946
eclogite
x
x
9949
eclogite
x
x
72811
glaucophanite
x
73101
schistose aluminous eclogite
x
x
x
x
x
x
x
x
x
x
x
x
x
Rtl
Rtl
x
x
x
x
x
x
x
Rtl
Rtl
x
x
x
Ttn, Rtl
x
x
x
x
x
x
x
x
x
x
Ttn
Rtl
x
x
Ttn
x
Ttn
x
Ttn
Ttn
(metasedimentary?)
J17
eclogite
x
J40
aluminous glaucophanite (metagabbro)
x
73102
felsic rock
x
9919a
metasedimentary schist
9919t
metasedimentary schist
9923
metasedimentary mylonite
x
x
x
x
x
x
x
9924a
metasedimentary schist
x
x
x
x
x
x
x
9931b
metasedimentary schist
x
x
9931c
metasedimentary schist
x
x
x
x
NC2
mineralized schist from Balade mine
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
x
Ttn
Ttn
x
Gr, Mt
x
Rtl
Hornblende zone
J22
glaucophanite
J37
amphibolite
J38
amphibolite
x
x
x
x
x
x
Brs
x
x
x
xSps core
Rtl, Ilm in Grt
and rim
72908
amphibolite
x
x
x
x
x
72909
glaucophanite
x
x
Brs
x
x
x
x
72915
amphibolite
x
x
x
x
x
x
x
73112
amphibolite
x
Brs
x
x
x
x
E803
glaucophanite
x
J37a
metasedimentary schist
x
J43
metasedimentary schist
x
x
x
x
x
Act
x
x
Rtl
Rtl
Rtl
Rtl
x
Rtl, Stp
x
Rtl
Ab, albite; Act, actinolite; Alm, almandine; Brs, barroisite; Chl, chlorite; Czo, clinozoisite; Ep, epidote; Gln, glaucophane;
Gr, graphite; Grt, garnet group; Hbl, hornblende or other Ca-rich amphibole; Ilm, ilmenite; Lws, lawsonite; Om, omphacite; Qtz, quartz; Phg, phengite; Prg, paragonite; Rtl, rutile; Sps, spessartine; Stp, stilpnomelane; Ttn, titanite.
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Fig. 3. P^T path for a New Caledonian eclogite reaching hornblende-zone conditions. Mineral assemblages and phase relationships are from
Schmidt & Poli (1998) based on experiments for water-saturated mid-ocean ridge basalt. The lawsonite to anorthite transition is from Liou
(1971). grt, garnet; ep, epidote; zo, zoisite; amph, amphibole. Other abbreviations are given in Table 2. P^Testimates are correlated to mineral
zones based on studies by Clarke et al. (1997), Carson et al. (1999, 2000) and Fitzherbert et al. (2003). P^Tregions: Lws, lawsonite zone; Ep, epidote
zone; Om, omphacite zone; Hbl, hornblende zone.
or Ni was detected in sulfide inclusions from the lawsonite
zone. The phases observed or deduced by composition to
occur in inclusions and pseudo-inclusions of prograde porphyroblasts are summarized in Table 2, distinct from those
that were observed in the matrix or included in silicates
that are believed to be retrograde (also given in Table 2).
Lawsonite-zone metamafic rocks
The only lawsonite-zone mafic rock studied is metabasalt
sample J15a. The metabasalt forms a boudinaged sheet
intruding the metasediments from which sample J15 was
taken (see below). It contains abundant, fracture-free lawsonite grains. Sulfide minerals were not observed in
thin section, but grain mounts made from lawsonite
mineral separates revealed rare sulfide inclusions.
The dense mineral separates also contained admixed
pyrite, which probably originated from the matrix. Most
sulfide inclusions found in lawsonite were of the order
of 1 mm in size, smaller than the microprobe interaction volume. Consequently, many compositions were
extrapolated from area scans. The compositions obtained
cluster around the chalcopyrite^iss region of the Cu^Fe^S
triangle (Table 3, Fig. 4). Point analyses of single phases
(with some cross-contamination) were possible only for
a single much larger inclusion that contained
pyrite þ covellite þ chalcopyrite, and a nearby separate
grain of digenite with an in situ weathering rim implying
that it was a pseudo-inclusion (Table 3, Fig. 5a and b).
Covellite was not observed to be in contact with chalcopyrite at the surface of the section through the three-phase inclusion; this is a case where three-dimensional (3D)
mapping of the inclusion would be required to verify such
a contact relationship. It should be noted that the above
three-phase assemblage is not stable in the 1atm phase diagrams of Fig. 6 (see Discussion).
Apart from the Cu-rich grain discussed above, a few
pseudo-inclusions were found along the edges of lawsonite
grains, which are easily recognized by the presence of
Fe^O^OH weathering rims and/or by visible fractures
connecting the sulfide to the edge of the host lawsonite.
Pseudo-inclusion point analyses gave compositions close to
the FeS^FeS2 line.
Sulfide grains in the matrix are exclusively pyrite. These
are generally 50^100 mm, much larger than sulfide inclusions, and comparable in size with the lawsonite and with
matrix baryte grains. Baryte grains are found both as inclusions in lawsonite and as a matrix phase in this sample.
Previously in the study area, baryte was reported only
from the now-defunct Me¤re¤trice mine (Briggs et al., 1977),
located in the lawsonite zone.
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BROWN et al.
PROGRADE SULFIDE METAMORPHISM
Table 2: Sulfide mineral inclusions and pseudo-inclusions in prograde porpohyroblasts in lawsonite-zone rocks
Sample no.
Host
Point analyses of
inclusions
Observed
Phases deduced
contacts in
from area scans
Pseudo-
Matrix sulfide
inclusions
Contacts
observed
inclusions
among matrix
sulfide
J15a
mafic lawsonite
Lws
blueschist
Py( Cu), Cp,
Cv(Fe), MP,
Py þ Cp,
Cp, MP, MR
Py þ Cv
metasedimentary
Alm
Py, Bn
Py(Cu)
Cc(Fe),
[Po þ Py]
J15
Po, Py(Cu),
[Py þ Po]
Py þ Cv
[Po þ Py],
schist
Po, [Py þ Po] Py
Py( Cu),
Cv(Fe), MP
J16b
metasedimentary
Sps
Py, Cp, Bn, MR
Py þ Cp
MP, MR, Py( Cu)
Py, Cp, Gln
lawsonite schist
J16c
metasedimentary
Py þ Cp, Py þ Gln
incl. in Py
Sps
[Po þ Py], Py(
schist
Py, Cp, MR,
Cu), Cp, MP
Py þ Cp, Py þ Gln
Gln incl. in
Py
J36
schist
Ab
Po(Cu)R
Py
Also shown are sulfide inclusions in a retrograde silicate host (in this case albite; retrograde sulfide inclusion is denoted R)
and matrix sulfide. Bn, bornite; Cp, chalcopyrite; Cv, covellite; MP, bulk composition similar to metal-poor iss with metal/
sulfur50·95; MR, bulk composition similar to metal-rich iss with metal/sulfur41·05; Po, pyrrhotite; Py, pyrite; Lws,
lawsonite; Alm, almandine; Sps, spessartine; Ab, albite; Gln, galena. Element(s) in parentheses indicate that the phase
contains significant quantities of the element in solid solution or unresolved intergrown phase. Square brackets indicate
unresolved intergrowth of two phases.
Lawsonite-zone metasedimentary rocks
Samples J15, J16b and J16c are metasedimentary rocks from
the lawsonite zone, where sample J15 was collected from
the same outcrop as the lawsonite metabasalt J15a. Garnet
is the host for inclusion sulfide in all cases, and was in general recovered from mineral separates as few grains were
found in thin section. The garnets of sample J15 are almandine-rich with XFe ¼ 0·61^0·64 and XMn ¼ 0·02^0·09, in
contrast to the garnets of J16b and J16c, which are spessartine with XFe ¼ 0·29^0·55 and XMn ¼ 0·21^0·61 (Brown,
2007). Spessartine garnet (samples J16b and J16c) has been
documented from the upper portions of the lawsonite zone
(Black, 1977), whereas Fe dominance had not been recorded
in lawsonite-zone garnets prior to Brown (2007); it probably
reflects the greater Fe content of the host-rock. Spessartine
cores in samples J16b and J16c are often zoned to Fe-rich
rims, and this accounts for the spread of analyses.
Sulfide inclusions in these lawsonite-zone garnets are
normally very small (1^3 mm), with few exceptions.
Larger 15^20 mm inclusions appear to consist of homogeneous single phases (Fig. 5c). Most sulfide inclusions are
found in garnet rims that are generally enriched in the
almandine component relative to garnet cores. Pseudoinclusions lie along narrow quartz-filled fractures within
garnets. Matrix sulfide is abundant, comprising much
larger euhedral pyrite crystals (up to 1mm), which
commonly overgrow silicate deformation fabrics evidenced
by corresponding inclusion trails. Sulfide inclusion backscattered electron (BSE) images are shown in Fig. 5c^e.
Representative analyses of inclusion sulfide are reported
in Table 3, and the overall distribution of compositions for
inclusion and matrix sulfide in Fig. 4.
Sulfide inclusion compositions from the almandine-bearing metasediment (J15) differ from those in the lawsonite
metabasalt (J15a) at the same location. Whereas mafic
rock inclusions tend to be Cu-rich,18 out of 20 sulfide inclusions in metasediment J15 are pyrite (Fig. 5c), with only
two Cu-bearing inclusions. One is an isolated, homogeneous bornite inclusion. The bulk area scan of the other
plots along the covellite^chalcopyrite line (Fig. 4), implying intergrowth of covellite with Fe-bearing phases, probably nukundamite or pyrite þ bornite. Pyrite and
pyrrhotite pseudo-inclusions are hosted in fractures in almandine; the pseudo-inclusions range in average composition from Fe0·75S to Fe4S as a result of oxidation.
Samples J16b and J16c were collected from the same
location (Fig. 2). Recent blasting in the area provided
particularly fresh samples from locality J16, including
metasedimentary and ultramafic rock types, although the
ultramafic rocks contain no sulfide minerals. The mineral
assemblage in the metasediments (spessartine^chlorite^
paragonite^phengite^quartz) corresponds to the upper
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Table 3: Representative lawsonite-zone sulfide mineral compositions extrapolated from area scans and from point analyses
Extrapolated from area scans (normalized to 100%) for lawsonite-zone sulfide minerals
Sample:
J15a
J15a
J15a
J15
J15
J15
J16b
J16b
J16b
J16c
J16c
J16c
Location:
J15a-a2
J15a-a5
J15a-a6
J15-a1
J15-a2
J15-a3
J16b-a9
J16b-a10
J16b-a11
J16c-i5
J16c-a2
J6c-i3
Silicate host:
Lws
Lws
Lws
Grt
Grt
Grt
Grt
Grt
Grt
Grt
Grt
Grt
at. % Fe
22·22
24·81
25·67
37·12
33·43
13·55
30·05
31·78
25·39
24·05
38·27
4·87
at. % Co
–
–
–
–
–
–
–
–
–
–
–
–
at. % Ni
–
–
–
–
–
–
–
–
–
–
–
–
at. % Cu
25·77
30·48
25·16
45·01
0·00
0·00
41·31
11·37
1·15
23·94
23·42
0·00
at. % S
52·01
44·71
49·17
50·12
62·88
66·57
45·14
58·58
67·07
50·67
52·52
61·73
Fe/M
0·46
0·45
0·50
0·10
1·00
1·00
0·25
0·73
0·97
0·51
0·51
1·00
Co/M
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
Ni/M
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
Cu/M
0·54
0·55
0·50
0·90
0·00
0·00
0·75
0·27
0·03
0·49
0·49
0·00
M/S
0·92
1·24
1·03
1·00
0·59
0·50
1·22
0·71
0·49
0·97
0·90
0·62
MP
MR
Cp
Cv(Fe)
Py
MR
Cp
MP
Identity
[Py þ Po]
MP
Py(Cu)
[Py þ Po]
Representative point analyses for lawsonite-zone sulfide minerals
Sample:
J15a
J15a
J15a
J15a
J15a
J15a
J15
J15
J16b
J16b
J16b
J16b
Location:
J15a-i10
J15a-i11
J15a-i13
J15a-i12
J15a-i16
J15a-i17
J15-i1
J15-i2
J16b-i1
J16b-i2
J16b-i5
J16b-i6
Silicate host:
Lws
Lws
Lws
Lws*
Lws
Lws
Grt
Grt
Grt
Grt
Grt
Grt
wt % Fe
46·40
30·70
0·33
1·40
29·17
30·48
14·10
46·04
15·09
12·60
32·25
30·64
wt % Cu
0·23
34·62
69·72
77·75
33·59
30·75
62·03
0·00
56·51
61·58
35·14
34·81
wt % S
52·99
34·65
30·04
21·04
36·84
38·99
26·35
52·94
26·59
25·12
32·75
34·82
Total
99·62
99·97
100·09
100·19
99·59
100·23
102·48
98·98
98·19
99·30
100·14
100·27
at. % Fe
33·25
25·29
23·55
24·34
12·32
33·23
11·41
11·28
26·84
25·14
wt % Co
wt % Ni
0·30
2·80
at. % Co
–
–
–
–
–
–
–
–
–
–
–
at. % Ni
–
–
–
–
–
–
–
–
–
–
–
at. % Cu
0·15
25·04
53·86
63·42
24·05
21·44
47·60
0·00
48·99
48·71
25·70
25·10
at. % S
–
–
66·59
49·67
45·84
33·78
52·40
54·21
40·08
66·57
39·60
40·01
47·46
49·75
Fe/M
1·00
0·50
0·01
0·04
0·49
0·53
0·21
1·00
0·19
0·19
0·51
0·50
Co/M
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
Ni/M
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
Cu/M
0·00
0·50
0·99
0·96
0·51
0·47
0·79
0·00
0·81
0·81
0·49
0·50
M/S
0·50
1·01
1·18
1·96
0·91
0·84
1·50
0·50
1·52
1·50
1·11
1·01
Py
Cp
Cv(Fe)
Dg(Fe)
MP
MP
Bn
Py
Bn
Bn
MR
Cp
Identity
*Pseudo-inclusion.
portion of the lawsonite zone (1·0 GPa, 400^4508C;
Fitzherbert et al., 2003), with no glaucophane present.
J16b inclusion compositions are almost all chalcopyrite
(Fig. 5d), pyrite (Fig. 5e), bornite or metal-rich ‘MR’
compositions (Table 3). Inclusions appear homogeneous at
the resolution of analysis, implying that any intergrowth
is on a very fine (submicron) scale. Although the majority
of area scan compositions for J16b inclusions lie between
652
BROWN et al.
PROGRADE SULFIDE METAMORPHISM
Fig. 4. Composition distribution for lawsonite-zone sulfide, distinguished by host lithology. Continuous tie lines indicate observed contacts.
pyrite and chalcopyrite, some approach bornite (Table 3,
Fig. 4). Area scans for J16c inclusions indicate chalcopyrite,
pyrite and, in one case, a mixture of pyrite þ pyrrhotite
(Table 3). Pseudo-inclusions of pyrite in garnet occur
along quartz-filled fractures.
Matrix sulfide in both J16b and J16c is dominated by
200^500 mm euhedral pyrite and 50^100 mm subhedral
chalcopyrite aggregates. Matrix pyrite grains contain silicate inclusion trails contiguous with the deformation
fabric in the silicate matrix, suggesting that the pyrite is
post-tectonic. Matrix pyrite grains in sample J16c show
Co-rich zones in X-ray maps (Brown, 2007) and contain
small galena inclusions, which form elongate blebs at a
high angle to folded silicate inclusion trails (Brown, 2007).
The chalcopyrite is free of such inclusions.
Pyrrhotite was found hosted within albite in other metasedimentary rocks in the lawsonite zone (sample J36,
Table 2). Albite is interpreted to have grown subsequent to
peak metamorphism, as suggested by Fitzherbert et al.
(2005), which would imply that the included sulfide is also
retrograde.
In summary, the lawsonite-zone rocks demonstrate
differences between matrix and inclusion assemblages,
and also between the inclusion assemblages of different
host lithologies. Inclusion hosts also differ between lithologies, being lawsonite in metabasalt but garnet in the
metasediments. Inclusions contain a variety of Fe^Cu^S
phases, and are relatively Cu-rich in the metabasalt and
spessartine-bearing metasediments in comparison with
the almandine-bearing metasediments. Matrix sulfide
minerals are dominated by pyrite, with textural evidence
of late development in at least J16c. The latter sample also
shows evidence of incursion by late mineralizing fluids, in
the form of matrix chalcopyrite, the high Co content of
pyrite, and galena inclusions in pyrite. Pyrrhotite grains
are infrequently observed as visibly altered pseudo-inclusions, and as two inclusions in albite, which is likely to be
retrograde.
Epidote-zone sulfide
A large regional fault divides the lawsonite from the epidote zone, cutting out the isograd corresponding to the
appearance of epidote and disappearance of lawsonite
(Fig. 2). Fitzherbert et al. (2003) constrained P^T conditions within the epidote zone to 1·4^1·5 GPa, 450^5008C
(Fig. 3). The epidote zone is marked by the occurrence of
almandine garnet and absence of spessartine garnet in all
rock types.
No sulfide inclusions were found within garnet from this
zone. However, sulfide minerals were observed in the
matrix, and as inclusions in albite and quartz in metasedimentary rock types. Pyrite and chalcopyrite were found
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Fig. 5. Lawsonite-zone sulfide. (a) Reflected light micrograph of lawsonite-zone metabasalt J15a (lawsonite grain mount); the box indicates
the area shown in (b). (b) Backscattered electron (BSE) image showing covellite, pyrite, chalcopyrite, and digenite inclusions in lawsonite.
The bright edge effect around part of the pyrite grain boundary extending from the left of the triangular-shaped covellite grain should be
noted. The darker gap between part of the covellite and the host pyrite is a void caused by plucking during polishing. (c) BSE image of lawsonite-zone metasedimentary sample J15, from the same outcrop as J15a, showing pyrite inclusions in almandine. (d, e) Garnet grain mounts
from lawsonite-zone metasedimentary sample J16 showing pyrite and chalcopyrite inclusions in spessartine.
included within quartz in quartzite sample 23903, whereas
chalcopyrite was found in quartz, and pyrrhotite was
found in albite in 23897, and both chalcopyrite and pyrrhotite were found in albite in 23874. The matrix contains
pyrite þ chalcopyrite in those host-rocks, schist 29356 and
also quartzite 23903, where sulfide is both entrained
within and apparently replacing quartz that may itself
have been retrograde. Albite formed subsequent to peak
metamorphism, during decompression from 1·5 GPa to
0·8 GPa. Therefore, the albite inclusions are interpreted
to be retrograde. The same is likely to be true of the
quartz, which recrystallizes readily.
In glaucophanite sample 73004a, matrix sulfide minerals are pyrite, with minor chalcopyrite and pyrrhotite.
These minerals are texturally different: pyrite forms large
and commonly euhedral crystals, whereas chalcopyrite
forms much smaller, sub- to anhedral grain masses.
Pyrrhotite contains inclusions of the other two sulfide
minerals.
The epidote-zone sulfide minerals found in the matrix
and hosted by albite or quartz are summarized in Table 4,
and their compositions are plotted in Fig. 7.
Omphacite-zone sulfide
Similar to the lawsonite^epidote transition, the boundary
between the epidote and omphacite zones in New
Caledonia is also defined by a fault that was probably
modified by late extension (Rawling & Lister, 2002). The
omphacite mineral zone is characterized by the appearance of omphacite in metasedimentary rocks (Black, 1977).
Omphacite-zone rocks formed at P^T conditions of
1·4^1·6 GPa, 550^6008C (Fitzherbert et al., 2003). The
rocks collected for this study are likely to represent a
spread of temperatures, as 96312k was found close to the
epidote zone whereas J17 and J40 were obtained near the
hornblende zone, with the other omphacite-zone samples
structurally between them. Host-rocks studied are summarized in Table 1, and include metabasaltic eclogites
(‘type I’ of Clarke et al., 1997), and more aluminous metagabbros with abundant large clinozoisite porphyroblasts
(‘type II’ of Clarke et al., 1997). These are relatively fresh
rocks occurring as boudins or sheets within omphacitebearing schists. Glaucophanites and the felsic clinozoisite^
phengite^garnet rock 73102 show fracturing of garnet and
abundant chlorite alteration. The glaucophanites are
mainly inferred to be metabasaltic, but two correspond to
metagabbroic cumulates on the basis of their clinozoisiterich petrology and resulting Al-rich composition (and
relict gabbroic texture in the case of J40). Some metasedimentary schists were also collected.
Sulfide inclusions are found almost exclusively in garnet,
although a few were also found in clinozoisite and paragonite. Even in highly altered glaucophanite garnets, where
the garnets were fractured and extensively replaced by
654
BROWN et al.
PROGRADE SULFIDE METAMORPHISM
Fig. 6. Experimental phase relationships in the Cu^Fe^S system at atmospheric pressure from Craig & Scott (1974), modified to include the
nukundamite stability data of Seal et al. (2001), shown for temperatures relevant to the New Caledonian belt. Tie lines intersecting the bases of
the diagrams point to metallic Cu and Fe. These diagrams illustrate the extent of ternary solid solution as a function of temperature. Pure
end-member compositions such as that of Cc are shown as reference marks, even though the phases may be subsumed into growing solid solution
fields (for instance, Cc is subsumed into the bss solid solution above 4358C).
chlorite during retrogression, sulfide mineral inclusions are
preserved. Overall, sulfides enclosed within garnet are
much larger in this metamorphic zone (410 mm on average) than sulfide inclusions in lawsonite and garnet at
lower pressures. Most of the samples with sulfide inclusions
are located on the south side of the Pam Peninsula
(Fig. 2), in well-preserved, interlayered metamafic and
medasedimentary rocks.
Sulfide phases observed or deduced to be present in
omphacite-zone rocks are summarized in Table 5. Representative analyses of inclusions are given in Table 6.
Omphacite-zone metamafic rocks
Sample 96312k was collected near the boundary with the
epidote zone (Fig. 2). Although it is dominated by almandine, clinozoisite and glaucophane and contains no
omphacite, its silicate compositions are consistent with
omphacite-zone conditions. Figure 8 includes a BSE
image illustrating the variety of sulfide inclusions found in
one garnet from this rock. The 5 mm pyrrhotite inclusion
appears to be single-phase; however, whereas a point analysis contained no Cu, an area scan shows that it contains
almost 7 wt % (5 at. %) Cu (Table 6). Sample 96312k
marks the first appearance of unequivocal single-phase
pyrrhotite inclusions trapped in garnet. The two larger inclusions in Fig. 8 are more complex. The lower of the two
contains a phase that is close to covellite in composition
but contains 1wt % Fe, possibly owing to unresolved intergrown species, coexisting with pyrite (Table 6). The uppermost inclusion in Fig. 8 contains pyrite and digenite.
Some point analyses also indicate intimate mixtures of
these minerals covellite. It should be noted that the
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Table 4: Sulfide minerals present as inclusions in retrograde silicates and in the matrix of epidote-zone rocks
Sample no.
Host
Inclusions
Matrix sulfide
Contacts observed among
matrix sulfide
73004a
glaucophanite
23874
mica schist
Ab
Po(Ni), Py( Co, Ni, Cu), Cp
Po þ Py þ Cp
Po, Cp
Py, Cp
Py þ Cp
23897
mica schist
Ab
Po, Cp; in contact
[Py þ Cp]
23903
quartzite
Qtz
Py, Cp; in contact
Py, Cp
Py þ Cp
23956
mineralized schist
Py(Ni), Cp
Py þ Cp
Fig. 7. Composition distribution for epidote-zone sulfide, distinguished by host lithology.
stability of pyrite^digenite^covellite is inconsistent with
the tie lines in Fig. 6; the possibility that this is a disequilibrium assemblage is discussed below. An area scan of the
upper inclusion yielded an average composition that is
S-poor relative to the pyrite^digenite tie line (Table 6),
consistent with the presence of a metal-rich third phase
such as pyrrhotite or chalcopyrite. Although the data for
each inclusion were obtained only from a 2D section
through a 3D object, which results in bulk compositions
scattered along a tie line for genuinely two-phase inclusions, departure of the average composition from that tie
line cannot occur without the presence of a third phase.
In all of the low-P experimental data, pyrrhotite never coexists with digenite (Fig. 6), so it is likely that the S-poor
phase is chalcopyrite. This is the highest-pressure rock in
which covellite was observed as an inclusion phase. No
matrix sulfide was observed in 96312k.
Pyrrhotite is an inclusion phase in garnet in every other
omphacite-zone metamafic rock in which sulfide inclusions
were observed (9918, 9930b, 9931a, 9939, 9941b, 9946, 9949,
73102, J17, J40). Figure 9 shows examples of pyrrhotite inclusions, some of which are in contact with chalcopyrite.
The range of sulfide analyses is illustrated in Fig. 10 for
rocks other than the atypical sample 96312k (discussed
656
schistose aluminous eclogite
73101
657
metasedimentary schist
metasedimentary mylonite
metasedimentary schist
metasedimentary schist
glaucophanite
glaucophanite
glaucophanite
schistose aluminous eclogite
9919t
9923a
9924a
9931c
9908
9941b
72811
73101
Czo
Alm
Alm
Alm
Alm
Alm
Alm
Alm
Alm
Alm
Alm, Czo
Alm, Prg
Alm
Alm
Alm
Alm
PoR
Po
Po( Ni)
Po( Ni)
Po( Ni)
Po
MP, MM, MR
Po(Ni), Py( Ni), Cp,
Po( Co, Ni), Tsp
MP(Ni), PoR
Po þ Cp
Py þ Cp
Py(Ni), Cp, Po(Ni)R
Po( Co, Ni, Cu),
Po þ MP
Po( Co, Ni, Cu)
Po(Ni)
Po
Po( Ni,Cu)
Po(Ni)
Cv(Fe), MP
Py þ Cv, Py þ Dg
in inclusions
Po, Py, Cp, Dg,
Observed contacts
Point analyses of
inclusions
Po( Cu)
[Po þ Py]
Po( Ni, Cu), Py,
Po(Cu)
Po, [Po þ Py]
Po(Cu)
Po( Ni, Cu)
MP( Co, Ni, Cu)
[Po þ Py], Po( Ni, Cu),
MP( Ni), MM, MR
Po( Co, Ni, Cu),
[Po þ Py], Tsp
Po( Co, Ni, Cu),
Po( Ni, Cu), MP(Ni)
Po( Cu), MP
Po, MR
Po( Ni, Cu)
Po( Cu)
[Dg þ Cv](Fe)
Po(Cu), MP,
from area scans
Phases deduced from
Po, Py, MR
Po, Py
Py
Py, Po( Cu)
Pseudo-inclusions
Cp, Py, Po
Po( Co, Ni)
Po, Cp
[Py þ Cp]
Po, Py, Cp
Cp
Po( Ni), Py, Cp
Po, Py, Cp
Po( Co)
Po(Ni), Cp, rare Py
Po(Ni)
Matrix sulfide
Po þ Py þ Cp
Po þ Cp
Po þ Py þ Cp
Po þ Py þ Cp
Po þ Py þ Cp
Po þ Cp, Py þ Cp
among matrix sulfide
Contacts observed
in glaucophanites 9908, 9941b, 72811, aluminous eclogite 73101, metasedimentary schist 9931b, mineralized schist NC2. No matrix
9931a, 9939, J40, 73102, 9923 or 9931c.
documented in prograde (Alm) and retrograde (Prg, Czo) silicate mineral hosts. Abbreviations as for Table 2, plus Tsp, thiospinel
MM, bulk composition close to iss with metal/sulfur ¼ 0·95–1·05 and Fe4Cu.
mineralized schist, Balade mine
NC2
*No prograde inclusion sulfides
sulfide in 96312k, 9918, 9930b,
Sulfide mineral inclusions were
(polydymite–linnaeite–violarite);
metasedimentary schist
9931b
(metasedimentary?)
Felsic
metasedimentary schist
73102
9919a
aluminous glaucophanite
Eclogite
J40
J17
Eclogite
9949
(metasedimentary?)
Glaucophanite
Eclogite
9941b
Eclogite
9939
9946
eclogite boudin in schist
Eclogite
9930b
aluminous glaucophanite
9918
9931a
Alm
aluminous glaucophanite
96312k
Alm
Host
Sample no.
Table 5: Sulfide mineral inclusions, pseudo-inclusions and matrix sulfide in omphacite-zone rocks*
BROWN et al.
PROGRADE SULFIDE METAMORPHISM
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Table 6: Representative analyses for omphacite-zone sulfide mineral compositions extrapolated from area scans and from
point analyses
Extrapolated from area scans (normalized to 100%) for omphacite-zone sulfide minerals
Sample:
96312k
96312k
9930b
9941b
9939
J40
J17
9949
9919t
9924a
9924a
Location:
S1
S2
S1
S9
S6
S3
S17
S6
S2
S18
S11
S3
Silicate host:
Grt
Grt
Grt
Grt
Grt
Grt
Grt
Grt
Grt
Grt
Grt
Grt
at. % Fe
43·55
24·14
41·81
42·65
38·13
34·36
44·22
44·61
45·38
46·80
38·85
29·14
at. % Co
0·00
0·00
0·00
0·00
0·00
1·38
2·20
0·26
0·00
0·00
0·00
2·69
at. % Ni
0·00
0·88
0·00
0·00
0·00
3·16
2·69
1·32
0·00
0·00
0·00
8·85
at. % Cu
4·47
20·90
3·80
3·06
15·99
7·86
0·00
1·02
0·00
0·00
0·00
0·99
at. % S
51·98
54·09
54·39
54·29
45·88
53·24
50·89
52·79
54·62
53·20
61·15
58·33
Fe/M
0·91
0·53
0·92
0·93
0·70
0·73
0·90
0·94
1·00
1·00
1·00
0·70
Co/M
0·00
0·00
0·00
0·00
0·00
0·03
0·04
0·01
0·00
0·00
0·00
0·06
Ni/M
0·00
0·02
0·00
0·00
0·00
0·07
0·05
0·03
0·00
0·00
0·00
0·21
Cu/M
0·09
0·46
0·08
0·07
0·30
0·17
0·00
0·02
0·00
0·00
0·00
0·02
M/S
0·92
0·85
0·84
0·84
1·18
0·88
0·97
0·89
0·83
0·88
0·64
0·71
Po(Cu)
MP
Po(Cu)
Po(Cu)
MR
Po
Po
Identity
MP(CoNiCu)
Po(CoNi)
Po(NiCu)
73101
Po þ Py
Tsp
Representative point analyses for omphacite-zone sulfide inclusions
Sample:
96312k
96312k
96312k
9946
9949
J17
9941b
9919t
9924a
73101
73101
Location:
P4
P5
P10
P5
P8
P1
P4
P4
P12
P4
P7
73101
P8
Silicate host:
Grt
Grt
Grt
Grt
Grt
Grt
Czo
Grt
Grt
Grt
Grt
Grt
wt % Fe
46·11
2·22
1·14
41·92
60·74
59·15
56·26
60·72
60·43
3·08
13·25
60·19
wt % Co
0·32
0·00
0·00
0·23
0·00
0·00
0·82
0·00
0·00
17·76
7·46
0·49
wt % Ni
0·82
0·00
0·00
4·09
0·46
1·32
3·59
0·27
0·68
33·87
28·30
0·80
wt % Cu
0·86
70·03
66·93
0·00
0·00
0·29
0·35
0·00
0·00
2·37
4·21
0·00
wt % S
53·02
29·99
30·95
52·14
39·48
39·72
37·42
39·90
38·75
40·58
36·56
40·11
101·13
102·24
99·01
98·38
100·68
100·48
98·44
100·89
99·86
97·66
89·78
101·59
at. % Fe
32·87
1·91
1·00
30·63
46·75
45·56
44·67
46·54
47·00
2·47
11·56
45·85
at. % Co
0·22
0·00
0·00
0·16
0·00
0·00
0·62
0·00
0·00
13·47
6·17
0·35
at. % Ni
0·56
0·00
0·00
2·84
0·34
0·97
2·71
0·20
0·50
25·80
23·49
0·58
at. % Cu
0·54
53·06
47·34
0·00
0·00
0·20
0·24
0·00
0·00
1·67
3·23
0·00
at. % S
Total
65·82
45·03
51·66
66·36
52·92
53·28
51·75
53·26
52·49
56·59
55·55
53·22
Fe/M
0·96
0·03
0·02
0·91
0·99
0·98
0·93
1·00
0·99
0·06
0·26
0·98
Co/M
0·01
0·00
0·00
0·00
0·00
0·00
0·01
0·00
0·00
0·31
0·14
0·01
Ni/M
0·02
0·00
0·00
0·08
0·01
0·02
0·06
0·00
0·01
0·59
0·53
0·01
Cu/M
0·02
0·97
0·98
0·00
0·00
0·00
0·01
0·00
0·00
0·04
0·07
0·00
M/S
0·52
1·22
0·94
0·51
0·89
0·88
0·93
0·88
0·90
0·77
0·80
0·88
Identity
Cp
Cv(Fe)
Py(Ni)
Po(Ni)
Po(Ni)
Po(Ni)
Tsp
Tsp
Po(CoNi)
[Dg þ Cv] (Fe)
658
Po(CoNiCu)
Po
BROWN et al.
PROGRADE SULFIDE METAMORPHISM
Fig. 8. Compositions of inclusion sulfide for omphacite-zone sample 96312k. Arrows show correspondence between analysis points and the BSE
image (right); continuous tie lines show observed contacts.
Fig. 9. Omphacite-zone photomicrographs. (a) BSE image of metabasaltic eclogite 9931a showing pyrrhotite and plagioclase in almandine. (b)
Sample J17; BSE image of intergrown pyrrhotite þ chalcopyrite inclusion in almandine. Bright area is Cu-rich. (c) Reflected light photomicrograph of pyrrhotite and chalcopyrite in garnet in sample J17. (d) Reflected light photomicrograph of pyrrhotite þ chalcopyrite in the matrix
of J17.
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Fig. 10. Compositions of inclusion sulfide from the omphacite zone, excluding atypical samples 96312k, 73101 and NC2. (a) Eclogites 9918, 9931a,
9930b, 9939, 9946 and 9949. (b) Relatively heavily altered glaucophanites 9908, 9941b, 72811 and felsic rock 73102. (c) Metabasalt J17 and metagabbro J40 from near the hornblende zone. (d) Metasediments 9919a, 9919t, 9923, 9924a and 9931c. It should be noted that average compositions
are confined below the Py^Dg line (dashed).
above; see Fig. 8), thiospinel-bearing 73101 and the heavily
mineralized, but sulfide inclusion free, Balade Mine
sample NC2 (discussed below).
Cu was commonly detected in apparently homogeneous
pyrrhotite inclusions, which may imply intergrowth with
an unresolved Cu sulfide, probably chalcopyrite. Such
compositions were observed in 9930b, 9941b, 9946 and J17
(Table 6). A metal-rich area analysis in 9939 can be interpreted as pyrrhotite þ chalcopyrite þ bornite/digenite.
Pyrrhotite ( chalcopyrite) inclusions in three omphacitezone metamafic samples, 9946, J17 and J40, contain a
small amount of Ni (up to 3 at. % in J40); the most Nirich inclusion in J40 also has 1·4 at. % Co. One point analysis from sample 9946 also corresponds to Ni-rich pyrite,
and one pyrite analysis with no detectable Ni was obtained
from J17 (Table 6).
Pyrrhotite inclusions were observed not just in garnet
but also in paragonite (9946) and clinozoisite (9941b,
9949). Nickel was detected in inclusions in paragonite and
clinozoisite from 9941b (good analyses were not obtained
for those from 9949). It was also detected in matrix
pyrrhotite for 9946, but matrix chalcopyrite and pyrrhotite
of 9941b were Ni-free, and there was no matrix sulfide
in 9949. Therefore, it seems likely that clinozoisite, like
garnet, is effective at isolating inclusions from the matrix,
whereas paragonite may not be. Definite pyrite and
mixed pyrite^pyrrhotite pseudo-inclusions in J17 and
9931a are associated with chloritic alteration in garnet.
Matrix sulfide is diverse and abundant in glaucophanites;
the phases observed are chalcopyrite (9908), pyrrhotite
(9946), pyrite þ chalcopyrite (72811) or all three phases
(9941b). Some matrix pyrrhotite was also observed in
metabasaltic eclogite 9946, whereas matrix sulfide in J17
is almost exclusively pyrrhotite þ chalcopyrite, except for
one pyrite grain that contains patches of chalcopyrite aggregate. No matrix sulfide was seen in other eclogite samples 9918, 9930b, 9931a, 9939, 9949, 73102, and J40.
Omphacite-zone metasedimentary rocks
These are well-layered quartz þ albite þ phengite þ almandine schists, usually preserving omphacite (Fitzherbert
et al., 2003; Brown, 2007). Metasedimentary rock samples
660
BROWN et al.
PROGRADE SULFIDE METAMORPHISM
from the omphacite zone along the south side of the Pam
Peninsula contained abundant sulfide inclusions in garnet.
Bulk area scans yielded compositions that trend from
close to troilite in composition (M:S ¼1:1) to pyrite
(M:S ¼1:2). However, point analyses indicated that pyrrhotite was usually near Fe0·88S in composition (Table 6).
The trend towards low M:S ratios probably indicates intergrowth of pyrite and pyrrhotite: some rims of pyrite
around pyrrhotite were observed, even when inclusions
were not obviously connected to the matrix by fractures.
Variation towards high M:S may indicate the occurrence
of a range of pyrrhotite compositions, or, when associated
with high Co or Ni, intergrowth with a metal-rich phase
that concentrates those elements, such as pentlandite.
Matrix sulfide in metasedimentary rocks is usually pyrrhotite, and is abundant in some samples (9924a, 9931b,
9919). Chalcopyrite and pyrite are occasionally present.
Nickel was variably detectable in matrix pyrrhotite in 9924a.
Sample 73101 is interpreted as a strongly foliated eclogite,
which may be of sedimentary origin. It is discussed separately because of the large amounts of Co and Ni in the sulfide inclusions. In addition to pyrrhotite and pyrite, which
usually contain these elements above the detection limit,
there are some inclusions whose compositions correspond
to violarite^polydymite^greigite thiospinels (Fe,Co,Ni)3S4
with Ni up to 25·8 wt % and Co up to 13·5 wt %
(Table 6). The matrix in this rock contains abundant
pyrrhotite, usually with detectable Ni (up to 1·1wt %)
and minor Ni-free chalcopyrite, although one ‘MP’ point
analysis was both Ni- and Cu-rich, corresponding to
(Fe0·48Co0·01Ni0·10Cu0·37)¼0·96S, which may represent a
pyrrhotite^pentlandite^chalcopyrite intergrowth.
Balade mine
The Balade Mine is located within the Diahot terrane,
hosted by the metasedimentary rocks within the omphacite zone. The dominant ore metal is Cu, with lesser Pb
and Zn. The host-rocks are variably altered, with abundant
chloritoid and chlorite. According to Briggs et al. (1977),
the Balade sulfide deposits were sourced from volcanic intrusions, cogenetic with basin sedimentation prior to subduction, and are hosted along stratigraphic horizons
within the Diahot terrane. Briggs et al. (1977) stated that although these chalcopyrite-rich deposits did experience subduction, they do not preserve a record of subduction
metamorphism.
The massive sulfide sample NC2 from Balade Mine has
470% chalcopyrite, with pyrite more common than pyrrhotite. Pyrrhotite is rare, present as small, discrete grains
within the chalcopyrite mass. The massive sulfide is interstitial to large quartz crystals. Graphite, where present, is
rimmed by magnetite, and is never in direct contact with
chalcopyrite. The presence of magnetite, the nearby chloritoid-bearing alteration zone and related normal faulting,
and the fact that none of the sulfide is armoured against
any changes in external fluid composition during decompression and exhumation, all imply that these rocks do
not record prograde sulfide metamorphism. Sulfide minerals will in general have been re-equilibrated or are the
products of retrogression; Rawling & Lister (2002) reported the existence of normal faults nearby that would
have facilitated fluid influx and sulfide retrogression. We
emphasize that prograde relationships can be examined
only where sulfide has been protected from retrogression
and fluid influx.
Hornblende-zone sulfide
Previous studies have hypothesized a gap in P^T conditions between the Diahot and Poue¤bo terranes, given the
faulted contact between them. However, in this study, we
assume that both terranes were metamorphosed together
to high pressure, as the faulted contact predates subduction
metamorphism.
This zone includes rocks corresponding to zone 4 of
Fitzherbert et al. (2003) within the Diahot terrane, and all
of the Poue¤bo terrane. Peak P^T estimates for this zone
are approximately 1·7^1·9 GPa, 600^6508C (Carson et al.,
1999; Fitzherbert et al., 2003). All rocks collected for this
study are from the Diahot terrane, except for 73112 and
J38, which are from the Poue¤bo terrane (Fig. 2). The metamafic rocks of this zone are referred to as ‘amphibolite’ in
Table 1, as omphacite is very rare and largely confined to
inclusions in garnet (Brown, 2007). The omphacite þ garnet þ clinozoisite of unaltered eclogite is represented by a
hydrous equivalent, barroisitic hornblende.
Sulfide minerals observed in hornblende-zone rocks are
summarized in Table 7, and representative analyses shown
in Table 8. The distribution of point and area scan analyses
are shown in Fig. 11.
Metamafic amphibolites
Sulfide inclusions occur in almandine. In contrast to the
omphacite-zone rocks, inclusions in garnet of the hornblende zone all contain pyrite as the dominant Fe-sulfide,
except for 73112 (Tables 7 and 8, Fig. 11). Inclusions are typically single-phase pyrite.
The Poue¤bo terrane samples show distinct differences
from those of the Diahot terrane. Sample J38 point analyses indicate chalcopyrite, bornite and digenite inclusions,
and contacts between pyrite and bornite were observed
(Tables 7 and 8). Polyphase inclusions in sample 73112
gave analyses corresponding to pyrrhotite, chalcopyrite
and MR intergrowths but not pyrite. One pyrrhotite analysis showed Ni above detection limit. Area analyses were
consistent with expectations from the point analyses, corresponding to pyrite and bornite compositions (J38), or
Fe^Cu sulfide intergrowths with (Fe þ Cu):S greater than,
equal to or less than 1:1 (J38 and 73112).
Matrix sulfide minerals observed are pyrite in J22 and J37
and chalcopyrite in E803. Samples 72908, 72909 and 72915,
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Table 7: Sulfide mineral inclusions in prograde porphyroblasts and matrix sulfide in hornblende-zone rocks*
Sample no.
Host
Point analyses
Observed
Phases deduced
of inclusions
contacts in
from area scans
Matrix sulfide
among matrix sulfide
inclusions
J22
glaucophanite
Alm
Py(Ni), Cp
J37
amphibolite
Alm
Py
J38
amphibolite
Alm
Py( Cu), Cp,
Contacts observed
minerals
Py(Cu), MP
Py
Py
Py þ Bn
MP, MR, MN, Bn,
Bn, Dg(Fe)
Py( Cu),
[Cv þ Bn þ Dg]
73112
amphibolite
Alm
Po( Ni), Cp, MR
E803
glaucophanite
Alm
Py
MP, MM, MR
J37a
metasedimentary schist
Alm
Py( Co, Ni), Po
J43
metasedimentary schist
Alm
72908
amphibolite
Py, Po(Ni þ Co),Cp
Py þ Po
72909
glaucophanite
Py( Co), Po, Cp,
Py þ Po þ Cp
72915
amphibolite
Po( Co, Ni), Py( Co,
Cp
MP, MR
Py
MN
Cp
MP, Bn, Dg, Cv( Fe)
Py þ Po þ Cp þ Ttd
Ni, Cu), Cp, Bn, Ttd
*No sulfide inclusions in amphibolite 72908, glaucophanite 72909. No matrix sulfide in amphibolites J38, 73112, 72915.
MN, bulk composition close to iss with metal/sulfur ¼ 0·95–1·05 and Fe 5 Cu. Ttd, tetradymite.
(1) Sulfide inclusion assemblages and the overall distribution of inclusion compositions are significantly different from those of matrix sulfide.
(2) Inclusion compositions tend to be richer in Cu than
matrix sulfides. This may imply that Fe has been lost
to a growing silicate phase (garnet) during prograde
metamorphism. In evidence, we note that it is possible
to write balanced reactions such as
which did not yield any sulfide inclusions, have rather diverse matrix sulfide assemblages including pyrite and pyrrhotite (both of which sometimes contain Ni and/or Co),
bornite, covellite, digenite, chalcopyrite and, in 72915,
tetradymite (Bi2Te2S), which is intergrown with Py, Po
and Cp.
Metasedimentary rocks
Samples J37a and J43 are glaucophane-rich metasedimentary schists collected along the boundary between the
Diahot and Poue¤bo terranes (Fig. 2). The only garnet inclusions large enough for point analysis were found in sample
J37a and are again of pyrite, sometimes with small
amounts of Co or Ni (50·4 wt %). Area scans were also obtained of chalcopyrite or cubanite composition in J43, but
departing in either direction from 1:1 cation:sulfur ratio in
J37a, implying the presence of pyrrhotite þ chalcopyrite þ pyrite and/or a metal-rich sulfide such as bornite
(Fig. 11d). Only pyrite forms grains large enough for
single-phase point analysis. Matrix sulfide is pyrite in
J37a, and chalcopyrite in J43 (Table 7).
DISCUSSION
In general, two major features show that sulfide inclusions
were captured by porphyroblasts as the slab subducted, as
follows.
25CuFeS2 þ 8Ca2 Al3 Si3 O12 ðOHÞ þ 12SiO2 þ14O2 !
Cp
Czo
Qtz
5Cu5 FeS4 þ 12 Ca4=3 Fe5=3 Al2 Si3 O12 þ15S2 þ 4H2 O:
Bn
Grt
ð1Þ
Such sulfide^silicate reactions are analogues of those discussed for very different bulk compositions and P^Tconditions by Thompson (1972) and Tracy & Robinson (1988).
Lawsonite zone
Tables 2, 5 and 7 show that whereas chalcopyrite frequently occurs as a matrix phase in all zones, the Cu-rich
phases covellite, bornite and digenite do not occur as
matrix phases. The latter minerals were observed in the
matrix in only two hornblende-zone rocks from one locality, which also show other unusual mineralization.
Many inclusions cluster around chalcopyrite-like compositions in metabasalt J15a and spessartine-bearing metasediments J16b and c. The scatter of analyses in Fig. 4 is
662
BROWN et al.
PROGRADE SULFIDE METAMORPHISM
Table 8: Representative analyses from area scans and sulfide inclusions for hornblende-zone sulfides
Representative analyses extrapolated from area scans (normalized to 100%) for hornblende-zone sulfide
Sample:
J22
J22
J38
J38
J38
73112
73112
73112
J37a
J37a
J37a
Location:
S2
S3
S21
S17
S8
S4
S1
S2
S1
S2
S3
J43
S1
Silicate host:
Grt
Grt
Grt
Grt
Grt
Grt
Grt
Grt
Grt
Grt
Grt
Grt
at. % Fe
32·85
25·97
31·73
12·64
14·63
28·90
31·89
31·29
34·70
32·78
37·14
14·86
at. % Co
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
at. % Ni
0·00
0·00
0·00
0·00
0·00
5·15
0·00
0·00
0·00
0·00
0·00
0·00
at. % Cu
1·58
22·99
2·71
47·85
34·40
15·23
12·09
20·48
17·97
4·18
10·03
35·50
at. % S
65·57
51·04
65·56
39·51
50·97
50·72
56·02
48·24
47·33
63·03
52·84
49·64
Fe/M
0·95
0·53
0·92
0·21
0·30
0·59
0·73
0·60
0·66
0·89
0·79
0·30
Co/M
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
Ni/M
0·00
0·00
0·00
0·00
0·00
0·10
0·00
0·00
0·00
0·00
0·00
0·00
Cu/M
0·05
0·47
0·08
0·79
0·70
0·31
0·27
0·40
0·34
0·11
0·21
0·70
M/S
0·53
0·96
0·53
1·53
0·96
0·97
0·79
1·07
1·11
0·59
0·89
1·01
Identity
Py(Cu)
MP
Py(Cu)
Bn
MM
MM
MP
MR
MR
MP
MP
MM
J37a
Representative point analyses for hornblende-zone sulfide inclusions
Sample:
J22
J22
E803
J38
J38
J38
J38
J38
73112
73112
J37a
Location:
P1
P2
P1
P17
P16
P9
P12
P14
P2
P1
P1
P2
Silicate host:
Grt
Grt
Grt
Grt
Grt
Grt
Grt
Grt
Grt
Grt
Grt
Grt
wt % Fe
45·55
30·53
45·89
46·18
46·45
13·17
13·68
3·84
60·38
31·11
46·72
46·42
wt % Co
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·33
0·00
wt % Ni
0·36
0·00
0·23
0·00
0·00
0·00
0·00
0·00
0·39
0·31
0·00
0·32
wt % Cu
0·26
34·13
0·21
0·00
0·68
60·93
60·30
74·52
0·00
34·29
0·00
0·35
wt % S
53·14
34·07
52·46
53·61
52·79
25·68
25·15
21·53
37·39
34·24
53·46
53·19
Total
99·31
98·73
98·79
99·79
99·92
99·77
99·13
99·89
98·16
99·95
100·51
100·28
at. % Fe
32·85
25·47
33·34
33·08
33·41
11·82
12·38
3·60
47·97
25·67
33·34
33·24
at. % Co
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·22
0·00
at. % Ni
0·25
0·00
0·16
0·00
0·00
0·00
0·00
0·00
0·29
0·24
0·00
0·22
at. % Cu
0·16
25·02
0·13
0·00
0·43
48·05
47·97
61·30
0·00
24·87
0·00
0·22
at. % S
66·74
49·51
66·37
66·92
66·15
40·13
39·65
35·10
51·74
49·21
66·44
66·33
Fe/M
0·99
0·50
0·99
1·00
0·99
0·20
0·21
0·06
0·99
0·51
0·99
0·99
Co/M
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·01
0·00
Ni/M
0·01
0·00
0·00
0·00
0·00
0·00
0·00
0·00
0·01
0·00
0·00
0·01
Cu/M
0·00
0·50
0·98
0·00
0·01
0·80
0·79
0·94
0·00
0·49
0·00
0·01
M/S
0·50
1·02
0·94
0·49
0·51
1·49
1·52
1·85
0·93
1·03
0·51
0·51
Py(Ni)
Cp
Cv(Fe)
Py
Py(Cu)
Bn
Bn
Dg(Fe)
Po(Ni)
Cp
Py(Co)
Py(Ni)
Identity
dispersed over a range of Cu:Fe and metal:sulfur ratios
around the chalcopyrite composition and does not concentrate along trend lines to pyrrhotite or pyrite. We interpret
this scatter to represent multiphase intergrowths derived
by breakdown of original single-phase intermediate solid
solution. In contrast, sulfide inclusions in almandine-
bearing metasediment J15 are dominated by pyrite, except
for one area analysis that is interpreted as Fe-contaminated
covellite (maybe covellite þ nukundamite) and one point
analysis that is close to bornite. The metabasalt J15a contains covellite as the most Cu-rich inclusion sulfide.
Inclusions in J15a show Py þ Cp and Py þ Cv contacts
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JOURNAL OF PETROLOGY
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NUMBER 3
MARCH 2014
Fig. 11. Compositions of inclusion sulfide from the hornblende zone. (a) Diahot terrane metamafic rocks J22, J37 and E803. Dashed lines enclose bulk composition field that could correspond to original pyrite þ iss. (b) Poue¤bo terrane metamafic rock J38. Inset photomicrograph
shows coexisting pyrite and bornite. Inclusion at highest cation/sulfur ratio is probably an oxidized bornite pseudo-inclusion. (c) Pue¤bo terrane
metamafic rock 73112. Inset photomicrograph shows pyrrhotite surrounding chalcopyrite with mottled texture in backscattered electron contrast, implying multiphase intergrowth at submicron scale owing to iss breakdown. (d) Diahot terrane metasedimentary samples J37a and J43.
It should be noted that average compositions are confined below the Py^Dg line (dashed).
(Fig. 4). However, the stability of a Py þ Bn tie line and of
nukundamite over the temperature range from ambient to
the peak of metamorphism would imply that Cp þ Cv are
not mutually stable at atmospheric pressure (Fig. 6), and
that the current assemblage in the inclusion of Fig. 5b is a
disequilibrium one, preserved by rapid cooling or uplift.
The assemblage may have been stable at peak pressure, or
result from the breakdown of a different peak assemblage.
Mutual stability of Py þ Cp þ Cv would require the following vapour-absent reactions to be favoured with
increasing P:
destabilization of nukundamite,
Nk
!
Py
þ
Bn
þ
Cu338 Fe062 S4 ! 0 4133FeS2 þ 0 2067Cu5 FeS4 þ
:
Cv
2 3467CuS
ð2Þ
and
Py
2FeS2
þ
þ
Bn
Cu5 FeS4
!
!
Cp
2CuFeS2
þ
þ
Cv
: ð3Þ
3CuS
The molar volume data in Table 9 show that V for reaction (2) is 5·20 cm3 mol1, a 6·2% decrease. Therefore,
we propose that nukundamite is strongly destabilized relative to the dense Py þ Bn assemblage with increasing pressure, and is unlikely to be a stable phase under
subduction-zone conditions. However, the V for reaction
(3) is þ2·12 cm3 mol1, a 1·4% increase, so Cp þ Cv, already unstable relative to Py þ Bn at 1atm, will not
become any more stable at depth. The shift of bornite to
more Cu-rich compositions discussed by Seal et al. (2001)
will act to enhance the stoichiometric ratio of bornite relative to Nk/Cv in the reactions, increasing the magnitude
of these volume changes. Although high-P crystallographic
data for covellite have been collected at 30 kbar by
664
BROWN et al.
PROGRADE SULFIDE METAMORPHISM
Table 9: Unit cell and molar volume data for some Cu^Fe^S phases
Phase
Formula unit
Hf (kJ mol1)
S (J mol1 K1)
Crystal
a (Å)
b (Å)
c (Å)
Z
Molar volume
Reference
(cm3 mol1)
system
Bornite
Cu5FeS4
–371·6
398·5
Orth.
10·95
16
98·66
1
Chalcopyrite
CuFeS2
–194·9
116·2
Tetr.
5·281
10·401
4
43·67
2
Covellite
CuS
–
–
Hex.
3·7938
16·341
6
20·44
3
Nukundamite
Cu3·38Fe0·62S4
–
–
Trig.
3·782
11·187
1
83·45
4
171·5
52·9
Cubic
5·4179
4
23·94
5
97·5
60·7
Hex.
6·8673
17·062
24
17·48
6
102·6
60·3
Trig.
3·452
5·762
2
17·90
7
Pyrite
FeS2
Pyrrhotite
Fe0·875S
Troilite
FeS
10·95
21·862
Standard enthalpies of formation and entropies are from Robie & Hemingway (1995); unit cell and molar volume references are as indicated. References: 1, Koto & Morimoto (1975); 2, Hall & Stewart (1973); 3, Evans & Konnert (1976); 4,
Sugaki et al. (1981); 5, Brostigen & Kjekshus (1969); 6, Fleet (1971); 7, Skála et al. (2006).
Take¤uchi et al. (1985) and Peiris et al. (1995), the phase may
have persisted only because of slow decomposition kinetics
at low temperature.
It should be noted that that very (Cu,S)-rich overall inclusion compositions dominated by Cv Py, such as analysis J15-a1 in Table 3 and illustrated in Fig. 4 for J15,
cannot be achieved using any of the Cu-rich sulfide phases
considered in Figs 4 and 6, other than covellite. Once a
Cv þ Py inclusion becomes a closed system, its S-rich bulk
composition must include either (1) covellite, (2) digenite
(or bornite) þ S2, to which covellite decomposes at low P
and high T (5078C at 1atm: Kullerud, 1965; Roseboom,
1966; Fleet, 2006), or (3) villamaninite, if P becomes high
enough for this phase to be stable. If both covellite and villamaninite are unstable at the time of enclosure, such
(Cu,S)-rich bulk compositions cannot be represented by
all-solid assemblages and hence cannot be incorporated as
inclusions in growing silicates.
The pseudoinclusion identified optically as ‘digenite’ in
sample J15a (Fig. 5a) has an unusually Cu-rich composition, close to djurleite/chalcocite; it is likely to have
formed by additional Fe loss owing to interaction with
metamorphic fluids.
Matrix sulfide in the lawsonite zone does not include the
Cu-rich phases seen in inclusions, but is dominated by
pyrite with some chalcopyrite. The predominance of
pyrite and absence of pyrrhotite in the matrix is distinct
from the inclusion sulfide suite, implying a lower cation/S
ratio and higher temperature-adjusted fS2 in the matrix
fluids in comparison with any fluid present at the time of
inclusion entrapment. Matrix pyrite in J16 metasediments
also contains inclusions of galena, implying that the
matrix experienced an input of Pb from which the inclusions were isolated. We infer that matrix and inclusion
sulfide were essentially different systems during
metamorphism, and experienced different changes in
their bulk chemistry.
Epidote zone
Pyrite and chalcopyrite are ubiquitous as matrix sulfide
minerals in the epidote-zone metasediments, as they are
in the lawsonite zone, again implying the presence of late
fluids with low Fe/S ratios. Inclusions in quartz are of the
same species, consistent with the hypothesis that these inclusions were vulnerable to re-equilibration with the
matrix. However, the observation of Po þ Cp in albite
implies that an earlier sulfide suite with a lower cation/
sulfur ratio has been preserved here, as in the lawsonite
zone. The matrix of glaucophanite 73004a also contains
pyrrhotite and pyrite, recording a different fluid composition from that of the metasediments. Matrix sulfide
minerals in samples 73004a and 23956 show detectable Ni
( Co), potentially derived from the breakdown of peridotite olivine under conditions where it was not incorporated
into phyllosilicates such as serpentine minerals or talc.
Omphacite zone
A major difference between the omphacite zone and the
lower-grade rocks is that pyrrhotite rather than pyrite becomes the dominant Fe sulfide in the matrix, irrespective
of host-rock lithology. In the matrix, pyrrhotite is associated with pyrite and chalcopyrite, but not with the more
diverse range of sulfides observed in inclusions, discussed
below. Garnet served to isolate inclusions from matrix
fluids during retrogression, as in the lower-grade rocks,
but the matrix sulfides themselves are different from those
of the lawsonite and epidote zones, owing to either differing fluid compositions at the time of crystallization (lower
fS2) or a greater degree of subsequent sulfur loss.
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JOURNAL OF PETROLOGY
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Pyrrhotite is also by far the most abundant inclusion
mineral, observed as both an inclusion and a matrix
phase in all lithologies. Nickel ( Co) was detected much
more frequently than in the lower grades in both inclusion
and matrix pyrrhotite, suggesting a greater input of Ni to
the system, presumably from fluids derived from serpentinized mantle olivine. A broad range of metal-rich ‘MR’
(with M/S 1·05; probably containing digenite or bornite),
medium-metal ‘MM’ (with 0·955M/S51·05 and Fe4Cu;
probable breakdown products from intermediate solid solution) and metal-poor ‘MP’ (with M/S 0·95; probably
containing pyrite) analyses imply frequent intergrowth
with other phases in both inclusion and matrix sulfide in
metamafic rocks; this variety was not observed in metasediment sulfide. Metamafic sample 96312k contains pyrrhotite, pyrite, chalcopyrite and the cation-rich Cu sulfide
minerals digenite and bornite. The only sulfide minerals
clearly identified in other metamafic samples are chalcopyrite and (in 73101 only) thiospinels of the polydymite^
violarite^greigite series (Ni3S4^NiFe2S4^Fe3S4). However,
the wide spread of Cu and Ni contents and cation:sulfur
ratios in the analyses may also imply intergrowth with
cation-rich sulfide minerals such as pentlandite and bornite or digenite: the latter were observed in 96312k.
Inclusions in sample 96312k show contacts between
Py þ Dg and Py þ Cv (Fig. 8). The latter association
implies that a (Cu,S)-rich solid phase, either covellite or
villamaninite, was stable at the time of incorporation. It is
noteworthy that this sample, whose peak temperature is
near the 1atm thermal stability limit for covellite, is the
highest-grade rock in which covellite was observed in
inclusions.
It should be noted that the overall envelope of analysis
points in Fig. 10 lies below the line from Py to Bn and/or
Dg. This suggests that (Cu,S)-rich phases were not stable,
either because fluid fS2 was intrinsically too low for them,
or because the pressure at the time of garnet growth was
too high for covellite/nukundamite stability and too low
for villamaninite, so pyrite þ bornite/digenite was the
most S-rich sulfide assemblage possible. The fresh eclogites
of Fig. 10a and more altered rocks of Fig. 10b show a similar
pattern in which the vast majority of inclusions are close
to pyrrhotite in composition, with subordinate pyrite and
chalcopyrite, as discussed above. There is no cluster of analyses around chalcopyrite/iss compositions, as seen in the
lawsonite zone. However, the possibly higher-T metamafic
rocks of Fig. 10c do show a more even scatter of analyses
across the Py^Po^Cp triangle, suggesting that these inclusions tend to be more Cu-rich. Conversely, the metasediment inclusions of Fig. 10d are dominated by Py þ Po and
almost devoid of Cu.
Temperature estimates for the omphacite zone are
higher than the maximum temperature of stability of
ordered tetragonal chalcopyrite at 1atm (Kullerud, 1965).
NUMBER 3
MARCH 2014
At T45478C, 1atm, the disordered isometric intermediate
solid solution (iss) extends far enough to include the composition of chalcopyrite. Because this transformation is
order^disorder driven and has a large S/V, the temperature is likely to be similar at high P, in which case
any chalcopyrite observed in inclusions of this zone is a
breakdown product of a variable-composition iss phase.
The broad scatter of analyses from Py þ Po to Cp compositions in Fig. 10c, with no extension towards Bn, suggests
that the most Cu-rich phase at peak conditions in these
rocks was the intermediate solid solution.
Hornblende zone
Rocks of the hornblende zone differ again in that the dominant inclusion sulfide is pyrite rather than pyrrhotite.
Given the positive slope of the Py^Po equilibrium in fS2^
T space (Toulmin & Barton, 1964), this is most probably a
consequence of much higher fS2 in the associated fluids
relative to the omphacite zone, although differences in
bulk-rock composition may also be a factor. Small amounts
of Co and Ni were sometimes detected in both pyrite and
pyrrhotite, but not as frequently as in the omphacite zone.
Bulk inclusion compositions again indicate crystallization
of prograde sulfide from different fluids in different suites
of rocks (Fig. 11): the analyses of sulfide contained in
garnet from the Diahot terrane metamafics and Poue¤bo
terrane metamafic 73112 lie in the Py^Po^iss triangle,
with a tendency to higher Cu content for the inclusions in
73112. The tendency to Cu-rich compositions is even more
marked for the Diahot terrane metasediments and Poue¤bo
terrane amphibolite J38, where compositions tend towards
bornite or the pyrite^digenite line. There is no obvious difference between matrix and inclusion populations for the
Diahot terrane samples (Fig. 11a and d), which is inconsistent with the hypothesis of copper enrichment of inclusions
owing to garnet growth. The matrix sulfide minerals Cv,
Bn and Dg (Table 7; Poue¤bo terrane samples 72909, 72915)
presumably reflect the presence of a late, Cu-rich fluid
during retrogression. Enrichment of the fluid in other chalcophile elemenys is suggested by the presence of tetradymite (Bi2Te2S) in the matrix. It should be noted that
covellite probably requires P considerably below conditions of peak metamorphism for stability.
Cu sulfide phase equilibria at high
pressure
Although thermodynamically consistent data are available
for pyrite, pyrrhotite^troilite and major S-bearing fluid
phases (Evans et al., 2010), this is not the case for Cu-bearing sulfides, so phase equilibria cannot be calculated reliably. As discussed above, molar volume data (Table 9)
suggest that covellite and nukundamite will destabilize by
volatile-absent reactions with increasing P, making it impossible to produce sulfide inclusions that are rich in both
Cu and S, in line with our observations. Such compositions
666
BROWN et al.
PROGRADE SULFIDE METAMORPHISM
will again be accessible if villamaninite becomes stable at
higher P. The role of chalcopyrite/iss and its significance
under subducting slab conditions remains in question.
From the data in Table 9, it is possible to estimate approximate conditions for the volatile-absent breakdown of Cp:
Cp
5CuFeS2
!
Py
! 2FeS2
þ
þ
Tr þ
2FeS þ
Bn
Cu5 FeS4
ð4Þ
or, with a Po composition (Fe0·875S ¼ Fe7S8) that is richer
in S:
Cp
!
Py
þ
Po
þ
Bn
:
5CuFeS2 ! 1 667FeS2 þ 2 667FeS þ Cu5 FeS4
ð5Þ
Equations (4) and (5) have, respectively, H ¼ þ54·7
and 56·98 kJ mol1, S ¼ þ43·9 and 67·57 kJ mol1 K1,
and V ¼ 36·00 and 33·16 cm3 mol1 (data of Robie
& Hemingway, 1995). Therefore, Cp would be expected to
decompose by equation (4) at P ¼11·4 kbar at 258C,
6·9 kbar at 4008C, or by equation (5) at 11·0 kbar (258C)
to 3·4 kbar (4008C). Cation-disordered, higher-entropy iss
would persist to slightly higher P and T, but it is possible
that Cp/iss would be replaced by Py þ Po þ Bn/Dg under
blueschist- or eclogite-facies conditions. Although the clustering of analyses in Figs 4, 10c and 11c suggests that a chalcopyrite/iss phase was incorporated in the inclusions, the
breakdown of such a phase may explain the absence of
such clustering in Fig. 11b and d.
CONC LUSIONS
We have surveyed the sulfide minerals present in a suite of
high-pressure rocks from New Caledonia that experienced
metamorphic conditions across a wide range of P^T
space. Sulfide minerals included in porphyroblasts such as
garnet, lawsonite and clinozoisite differ from matrix sulfide, demonstrating that enclosure can protect the inclusions from reacting with fluids during retrogression. The
composition ranges of sulfide inclusions vary with both
metamorphic grade and bulk composition of the hostrock. We interpret the inclusion compositions to correspond to solid sulfide assemblages that were stable at the
time of enclosure. At atmospheric pressure, the range of
possible compositions extends to high contents of Cu and
S, given the stability of pyrite þ nukundamite/covellite.
However, those Cu sulfide minerals are low-density
phases and are expected to destabilize at moderate pressures; overall inclusion compositions would then be poorer
than the Py^Dg line. This pattern was observed in the
rocks we studied, as no covellite was observed in sulfide inclusions for peak P conditions above the lower omphacite
zone. We predict that (Cu,S)-rich inclusion compositions
might return at pressures higher than those of this study,
if the Cu-rich pyrite mineral villamaninite becomes
stable. Furthermore, there is some evidence that at high P
and T inclusion compositions may cease to cluster near
Fe:Cu ¼1:1 owing to the replacement of Cp/iss by the
denser Py þ Po þ Bn assemblage.
AC K N O W L E D G E M E N T S
C. Spandler, J. Fitzherbert, G. Clarke and P. Black provided samples that greatly helped to complete this study.
We thank K. Evans, T. Kawakami, A. Tomkins and an anonymous reviewer for their incisive and thorough reviews.
J.L.B. acknowledges the support of an Australian
International Postgraduate Research Scholarship and an
Australian Postgraduate Award from the Australian
National University.
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A P P E N D I X : A N A LY T I C A L
M ET HODS
Major element analyses of all minerals were obtained using
a JEOL 6400 scanning electron microscope equipped with
an energy-dispersive spectrometer and Oxford Link-ISIS
quantification software, in the Electron Microscopy Unit
(now the Centre for Advanced Microscopy) of the
Australian National University. An accelerating voltage of
15 kV, a beam current of 1nA, and a counting time of 180
live seconds were used for each analysis. Elements analysed
that were above detection limit in sulfide or silicate minerals were Mg, Al, Si, S, K, Ca, Ti, Mn, Fe, Co, Ni and Cu.
Quantification was conducted in all-elements mode,
which also returned a weight percentage for oxygen.
However, this was not regarded as accurate, given sensitivity to coating quality and overlap from transition metal L
peaks.
Data were collected for sulfide inclusions, along with
data for pseudo-inclusions and matrix sulfide. Where
grains were large enough (45 mm), point analyses were
obtained. Analyses of smaller grains were necessarily contaminated by contributions from the host silicate. This
was also true for area scans used to obtain an estimate of
the average compositions of polyphase inclusions. In these
cases, the sulfide composition was derived as follows. A
set of area scans were obtained, beginning with a relatively
large area containing the inclusion and host silicate, and
then successively spanning smaller areas. The overall compositions of such areas are weighted averages of the sulfide
and silicate contributions. In the simplest case, where sulfide is in contact with only one silicate phase (usually
almandine garnet), linear regression of all weight percentages plotted against a component that is present in silicate
but not sulfide (such as Si or Ca) allowed calculation of
the true sulfide composition, as percentages in the sulfide
were given by the y-intercepts of the regression lines.
Where more than one silicate phase was present, it was
necessary to extract the contributions owing to minor silicate phases first, using data for components that were significant in one silicate but not another. For instance, the
titanite contribution to a titanite þ garnet mixture could
be estimated from the overall content of Ti, which is an
essential major component in titanite but negligible in the
garnets and sulfides of this study.
In any given analysis, we collected data from only a
planar section of a 3D inclusion. In the case of a sulfide
inclusion that is a polyphase intergrowth, the proportions
of phases that are visible at the polished surface may not
be exactly representative of the true proportions in three
dimensions, and, furthermore, phases that appear isolated
from one another in one cross-section may actually be in
contact in the third dimension. To obtain the truest overview of inclusion makeup, we analysed numerous inclusions where possible. Repolishing inclusions to reveal
progressively deeper surfaces was not successful owing to
the small grain sizes.
The Cu^Fe^S system at low temperature is known to
contain a very large number of CuxS phases with
x ¼ 0·50^2·00, and also CuxFeyS phases with x þ y 1
(see Fleet, 2006). In the absence of diffraction data, we
were reliant for this study on optical appearance and compositions alone to identify the major phase or phases present, and have erred on the conservative side in our
assignment of likely mineral species identifications.
Atomic ratios of Fe:Co:Ni:Cu and M/S (where
M ¼ Fe þ Co þ Ni þ Cu) were examined, and where
appropriate, were attributed to phases that are known to
persist at T42008C. The corresponding phases were generally pyrrhotite, pyrite, chalcopyrite, bornite, digenite
and covellite. Pyrrhotite and pyrite occasionally showed
minor Co or Ni above detection limit, interpreted to be
solid solution components, and Cu, which may have been
in solid solution or may have been contamination from an
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VOLUME 55
adjacent sulfide phase. Similarly, minor Fe was frequently
observed in the very Cu-rich phases covellite and digenite.
These were readily recognized by their blue colours in
reflected light microscopy (and strong anisotropy in the
case of covellite), but the (Cu þ Fe):S ratio was frequently
far from ideal, suggesting unresolved intergrowth with
phases of different metal:sulfur ratio. Rare instances of
analyses with Ni as the dominant cation and M/S close to
0·75 were identified as thiospinels in the polydymite^linnaeite^violarite series (Ni3S4^CoNi2S4^FeNi2S4).
Many analyses lay in the Cu^Fe^S subsystem, but at
Cu:Fe:S ratios removed from the known or likely solid solution ranges of the phases listed above. These analyses are
interpreted as unresolved fine intergrowths, which may
result from breakdown of high-P/T solid solutions. Such
compositions were classified as ‘MP’ ¼ ‘metal-poor’
(M/S 0·95; probably containing pyrite), ‘MR’ ¼ ‘metalrich’ (M/S 1·05; probably containing digenite or bornite),
‘MM’ ¼ ‘medium-metal’ (0·955M/S51·05 and Fe4Cu;
probable breakdown products from intermediate solid solution, dominated by pyrrhotite þ chalcopyrite or maybe
cubanite) and ‘MN’ ¼ ‘nukundamite-like’ (0·955M/
S51·05 and Cu4Fe; probable breakdown products of a
solid solution, dominated by chalcopyrite þ covellite or
maybe nukundamite). Some ‘MM’ analyses corresponded
closely to the CuFe2S3 composition of cubanite, but these
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were rare enough for this to appear coincidental. The
‘MN’ analyses clustered at Fe/(Cu þ Fe) ¼ 0·2^0·3, which
is an Fe content appreciably higher than that expected for
nukundamite [Fe/(Cu þ Fe) ¼ 0·17].
We endeavour to maintain a distinction between the
phases and compositions that are detected in the specimens
under ambient conditions and those that were likely to
have been present at peak conditions. For instance, an
area scan that is within analytical error of the ideal chalcopyrite composition is referred to as ‘Cp’ in the results, as it
is extremely likely to correspond to an area that is singlephase Cp or close to it, Cp being a phase that is stable at
ambient conditions. Conversely, there is much evidence
that Cp was not a distinct phase under peak conditions,
where the same composition was merely part of the iss
field, so in the discussion, the same analysis may be
inferred to imply the former presence of iss.
The interpretation of matrix and pseudo-inclusion sulfide compositions was in some cases further complicated
by the effect of weathering. Oxidation causes loss of sulfur,
and an increase in overall M/S ratio of the resulting sulfide^oxide mixture. However, obvious oxide contamination was rare, and the majority of sulfide inclusions do not
have in situ weathering rims. Samples with highly weathered sulfide minerals were rejected from further
consideration.
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