©2005 Society of Economic Geologists, Inc.
Economic Geology, v. 100, pp. 773–779
SULFIDE MELT INCLUSIONS AS EVIDENCE FOR THE EXISTENCE OF
A SULFIDE PARTIAL MELT AT BROKEN HILL, AUSTRALIA
HEATHER A. SPARKS † AND JOHN A. MAVROGENES
Research School of Earth Sciences, Australian National University, Canberra, ACT, 0200, Australia
and
Department of Earth and Marine Sciences, Australian National University, Canberra, ACT, 0200, Australia
Abstract
Polyphase sulfide melt inclusions are hosted within garnetite rocks and quartz veins in garnetite surrounding droppers and large masses of the orebody at Broken Hill, Australia, and record the presence of a former
sulfide melt. Sulfide melt inclusions are either primary or occur along healed fractures in both garnets and
quartz veins. Common daughter minerals in the inclusions are galena, sphalerite, arsenopyrite, chalcopyrite,
tetrahedrite-tennantite, and minor amounts of argentite, bornite, dyscrasite (Ag3Sb), and gudmundite (FeSbS).
The inclusions exhibit a strong enrichment in low-melting-point chalcophile elements compared to the main
orebody. Experimental reequilibration of sulfide melt inclusions shows homogenous melt at temperatures as
low as 720° ± 10°C and 5 kbars, well below that of peak metamorphism at Broken Hill (800° ± 10°C and 5
kbars). Thus, these inclusions are interpreted to represent a trapped sulfide melt formed during peak metamorphism at Broken Hill, Australia.
Introduction
Broken Hill, New South Wales, Australia, is the world’s
largest Pb-Zn-Ag deposits. Despite over a century of study,
aspects of ore genesis remain elusive (for review see Stevens,
1975). White et al. (1995) postulated that emplacement of the
orebody occurred after peak metamorphism. Others
(Gustafson and Williams, 1981; Phillips et al., 1985; Stevens
et al., 1988) suggested that the orebody is premetamorphic
and syngenetic in origin (i.e., exhalative). Broken Hill is
hosted within a suite of complexly folded and metamorphosed Proterozoic metasediments and metavolcanic rocks
(Stevens et al., 1988) and reached peak metamorphic conditions of at least 800°C and 5 kbars at 1600 ± 5 Ma (Phillips
and Wall, 1981; Page and Laing, 1992; Cartwright, 1999).
The effects of metamorphism on the Broken Hill orebody
are not entirely clear, although some studies (Brett and
Kullerud, 1966, 1967; Lawrence, 1967) have suggested that
syngenetic Pb-Zn-Ag ores may have melted during peak
metamorphism. Partial melting of silicates is well documented from textural and chemical criteria (Phillips, 1980;
Phillips and Wall, 1981). In contrast to silicate melts, sulfide
melts quench to complex intergrowths of sulfide minerals
that tend to reequilibrate at very low temperatures (Frost et
al., 2002). As a result, textures of sulfide melts are rarely
preserved.
Recent experimental work (Mavrogenes et al., 2001) has
demonstrated that eutectic melting in the system PbSFe0.96S-ZnS-(1% Ag2S) begins at 795°C at 5 kbars. This temperature is well within independently derived estimates for
peak metamorphic conditions at Broken Hill. Frost et al.
(2002) has shown that the addition of low-melting-point chalcophile elements (Ag, As, Au, Bi, Hg, Sb, Se, Sn, Tl, Te, Cu,
Pb, Fe, Mn) depresses the onset of sulfide partial melting.
Wykes and Mavrogenes (2005) show that the addition of H2O
lowers sulfide eutectics. Frost et al. (2002) also suggest that
with progressive melting a polymetallic melt will become
† Corresponding
author: e-mail, [email protected]
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enriched in low-melting-point chalcophile elements to the
point where remobilized ore may form discrete high-grade
pockets. Based on these experimental and empirical observations, Mavrogenes et al. (2001) and Frost et al. (2002) concluded that at least some of the Pb-Zn-Ag ore at Broken Hill
must have melted.
Hofmann (1994) and Hofmann and Knill (1996) unambiguously established that the ores of Lengenbach, Switzerland, partially melted during metamorphism. They described
polyphase sulfide melt inclusions trapped in quartz. These
sulfide melt inclusions exhibit strong enrichment in low-melting-point chalcophile elements, including Pb, Tl, As, Sb and
Bi, and fully homogenize at reasonable temperatures
(<500°C). They proposed that sulfide melts formed during
metamorphism were trapped as sulfide melt inclusions. The
Challenger Au mine in South Australia was shown to be a
metamorphosed Au deposit (Tomkins and Mavrogenes, 2002)
by the recognition of polyphase melt inclusions in peak metamorphic mineral assemblages. Frost et al. (2002) established
that the massive sulfide ores of Snow Lake, Manitoba. melted
during metamorphism. More recently, partial melting has
been used to explain the distribution of sulfide and sulfosalt
mineral at Hemlo, Ontario (Tomkins et al., 2004). Until now,
however, no direct evidence for the existence of a sulfide melt
at Broken Hill has been documented.
Materials Studied
Sulfide melts can migrate and concentrate into pockets,
known as “droppers,” and these were the focus of the present
study. Droppers were first identified and described as sulfide
dikes by King and O’Driscoll (1953), Mackenzie (1968) and
Maiden (1975, 1976). They are interpreted as piercement
structures of remobilized ore extending out from the main
orebodies (1–50 m) into the country rocks. They typically
crosscut foliations and igneous rocks (Fig. 1). Droppers are
surrounded by an alteration package of rocks primarily comprised of garnetite. Garnetite is composed of ~95 to 98 percent (by volume) equidimensional (~100 µm), orange-brown
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FIG. 1. Cross section through the Broken Hill orebody (coordinate 1101S)
showing 1 lens (Zn lode) overlying 2 lens (Pb lode) from which a dropper extends into the country rock. This clearly illustrates the remobilized nature of
the dropper (after Mackenzie, 1968). Drill hole N4689 passed through this
dropper at 26–51.2 m.
spessartine garnets. Interstitial material in the garnetite comprises minor galena and rare quartz veins. We sampled garnetite and one quartz vein in four drill holes (3028, N4689,
Z3031, and C144) from Perilya Ltd.’s Broken Hill mine.
Methodology
Garnet grains were separated from the garnetite and
mounted in epoxy approximately one garnet layer deep, and
polished to expose sulfide melt inclusions. Quartz chips were
mounted separately and carefully polished until sulfide melt
inclusions were exposed along healed fractures. All samples
were then inspected using reflected light microscopy and
imaging with a scanning electron microprobe (SEM), using
backscattered electrons. The compositions of the sulfide inclusions were estimated using a JEOL 6400 SEM equipped
with an Oxford Link ISIS energy dispersive (EDS) detector
located at the Research School of Biological Sciences, Australian National University (ANU). A 15-kV accelerating voltage, a 1-nA beam current, and 120-s counting times were
employed. Bulk compositions were obtained from a ~10-µm
area scan. Individual phases were analyzed by spot analysis
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wherever possible. As with previous experimental studies
(Mavrogenes et al., 2001) the presence of quenched molten
sulfide (in experimentally reequilibrated samples) was indicated by characteristic myrmekitic intergrowth of sulfides
(Fig. 2A). Where these textures were present, numerous area
scans were performed over the entire exposed sulfide melt inclusion to acquire an average composition.
High-pressure homogenization experiments were performed using a 12.7 mm end-loaded piston cylinder apparatus located at the Research School of Earth Sciences, Australian National University (RSES, ANU). A series of
experiments at 620°, 720°, and 840°C, and 5 kbars was performed. For the 620°C run, a volume of garnet separates was
loaded into a drilled-out MgO rod, and heated for 4 hours.
For the 720° and 840°C runs, approximately 0.16 g of garnet
grains and one quartz sample (two chips, each ~2 mm diam)
were loaded into silver-palladium capsules (3 garnets, one
quartz) and welded shut. All four capsules were loaded into
machined MgO rod and run simultaneously. The experiment
durations were four hours for the 720 °C run and one hour for
840 °C. Runs were quenched by cutting the power to the apparatus. For further details regarding sample assembly and
run procedures see Hermann and Green (2001).
Inclusions were analyzed using laser ablation-inductively
coupled plasma-mass spectrometry (LA-ICP-MS) at the
RSES. Si, S, Ca, Mn, Fe, Co, Ni, Cu, Zn, As, Se, Sr, Mo, Ag,
Cd, In, Sn, Sb, Ba, Re, Os, Pt, Te, Au, Hg, Tl, Pb and Bi were
investigated. Laser output energy was set at 100 mJ with a
repetition rate of 10 Hz. Relatively slow ablation rates
smoothed out the signal, allowing qualitative estimates of the
compositions of the analyzed phases. To minimize overlap between isotopes, two isotopes of the same element were measured simultaneously. Analyses for major and trace element
concentrations of entire unexposed sulfide melt inclusions
were acquired in real time, ensuring maximum control of the
ablation procedure (Fig. 3). Each acquisition started with a
collection of carrier gas for approximately 30 to 40 s (carrier
gas in Fig. 3). The laser was then turned on (On in Fig. 3) initially ablating pure host (garnet host in Fig. 3). Upon intersection of a sulfide melt inclusion the signals include material
ablated from both host and inclusion, with the contribution
from each component evolving as the ablation pit deepens
(sulfide melt inclusion = SMINC in Fig. 3). Once the entire
sulfide melt inclusion was ablated, pure host was again intersected and the analysis was stopped (Off in Fig. 3). Spot sizes
were selected based on the diameter of individual SMINCs.
Data was acquired in blocks of 10 with external standards analyzed at the beginning and end of each block.
A similar method to that described by Halter et al. (2004)
was employed to calculate sulfide melt inclusion compositions obtained by LA-ICP-MS. First, the background count
rate determined from ablation of the host (garnet) was subtracted from the sulfide melt inclusion to give background
corrected count rates. If inclusions were close to the surface,
representative host signal from the same sample was used.
Second, element ratios were calculated by referencing to
NIST 610, which was analyzed as the external standard. Halter et al. (2004) assumed their sulfide melt inclusions to be
stoichiometric (Fe, Cu)S, and element concentrations were
normalized accordingly by assuming 50 mol percent S. This
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FIG. 2. Broken Hill sulfide melt inclusions. A. Backscattered electron image of a fully homogenized melt inclusion
quenched from 840°C and 5 kbars. Note the myrmekitic texture typical of rapidly quenched sulfide melts. B. Reflected light
photomicrograph of polished garnetite, showing the distribution of inclusions. Note the abundance of randomly distributed
melt inclusions within garnet grains, and the monomineralic character of interstitial sulfides. C. Transmitted light photomicrograph of a sulfide inclusion trail in a healed fracture in quartz. Within one inclusion array, all inclusions are compositionally similar. D. Backscattered electron image of a negative crystal-shaped polyphase melt inclusion containing 8 daughter
phases (as labelled). This inclusion graphically illustrates the high levels of Ag (tetrahedrite and dyscrasite), As (arsenopyrite)
and Sb (tetrahedrite and gudmundite) present in Broken Hill melt inclusions. E. Reflected light photomicrograph of a
rounded polyphase melt inclusion containing four daughter phases (as labelled). F. Backscattered electron image of a partially homogenized melt inclusion quenched from 620°C and 5 kbars. Note that roughly 10 percent of the inclusion shows
quenched melt textures.
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FIG. 3. Ablation depth (time) vs. Mn, Cu, Zn, As, Ag, Sn, Sb, and Pb counts for an individual garnet-hosted sulfide melt
inclusion (M337, sample Z3331-33.8). This profile first ablates through garnet, then a complete (unhomogenized) sulfide
melt inclusion, followed by garnet. Note that individual daughter phases can be recognized in the ablation profile. Initially,
galena (Pb) is ablated, followed by tetrahedrite (Sb, As, and Ag), and finally chalcopyrite (Cu). In fully homogenized inclusion profiles, no individual phases are seen. This inclusion contains approximately 50 wt percent Pb, 4 wt percent As, 1 wt
percent Zn, 1 wt percent Cu, and 800 ppm Ag.
method is inappropriate for the current study as the bulk
compositions of the inclusions are not stoichiometric metalsulfide compositions, as demonstrated by EDS analyses of
quenched melt from homogenized inclusions. Therefore, element concentrations were normalized to 100 percent (S +
Mn + Fe), where S, Fe and Mn values were collected by
EDS, an approach that also accounts for the host element
contribution.
A total of 134 individual ICP-MS analyses were processed
and plotted. To ensure analysis of entire inclusions, only submerged (unexposed) inclusions were analyzed. Analyses are
considered representative of the melt only if their composition was polyphase in nature and not monomineralic. Mineral
inclusions were easily identified as such and rejected.
Results
The presence of sulfide melt inclusions in all garnetite
samples studied and in one quartz vein sampled within the
garnetite supports the existence of a former sulfide melt.
Sulfide melt inclusions are randomly distributed inside garnet grains (Fig. 2B) or along planes in healed fractures in
garnet or quartz (Fig. 2C). Extensive petrographic study revealed that sulfide melt inclusions generally show the negative crystal shape of their host (Fig. 2D) or are spherical
(Fig. 2E), with several daughter minerals present inside. At
least eight discrete daughter minerals have been observed
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within one single sulfide melt inclusion (e.g., Fig. 2D).
Many of the phases have euhedral to subhedral morphology,
suggesting slow cooling and crystallization from a homogenous melt during post-entrapment cooling. Common
daughter minerals are galena, sphalerite, arsenopyrite, chalcopyrite, tetrahedrite-tennantite, and minor amounts of argentite, bornite, dyscrasite (Ag3Sb), and gudmundite
(FeSbS). Other inclusions are monomineralic (e.g., arsenopyrite and lollingite) and exhibit their own crystal habit.
In all garnetite samples studied, all interstitial sulfides were
monomineralic galena or pyrrhotite (Fig. 2B). None of the
low-melting-point chalcophile element-rich phases recognized as daughter minerals in sulfide melt inclusions (e.g.,
tetrahedrite, gudmundite, argentite, or dyscrasite) have
been found in garnet interstices.
Partial homogenization of sulfide melt inclusions was recognized at 620ºC and 5 kbars (Fig. 2F), and total homogenization was observed at 720ºC and 5 kbars (Fig. 2A). EDS
analyses of quenched melt from 30 garnet-hosted and 7
quartz-hosted inclusions from both 1 atm (not reported; see
Sparks, 2003) and high-pressure heating experiments in
which complete homogenization occurred are compiled in
Table 1. Individual sulfide melt inclusions within a single population (e.g., a single healed fracture) homogenized at the
same temperature, further suggesting that these inclusions
represent trapped melt.
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PbS-FeS-ZnS-Ag2S would result in an increase in Ag concentration in the melt, whereas in the present study Ag increased
(Fig. 4B) with increasing Cu/Pb. Previous work suggests that
Ag increased in the melt until tetrahedrite saturation occurred. This is supported by our observation that tetrahedrite
is the major Ag host in sulfide melt inclusions. Thus, Ag behaved incompatibly during galena fractionation.
Garnet-hosted sulfide melt inclusions reveal the same correlation between Cu and Pb as those hosted by quartz (Fig.
4A), and different samples record a similar phenomenon. For
example, sample N4689-51.2 (solid circles; Fig. 4C) and sample Z3031-107.8 (open triangle; Fig. 4C) plot at the opposite
ends of the fractionation trend shown by Cu/Pb. In contrast
to Ag, Co behaves compatibly during fractionation. This is illustrated by sample Z3031-107.8 (open triangle; Fig. 4D),
which has a much higher Co content than sample N4689-51.2
(solid circles; Fig. 4D) and may have been trapped earlier.
The similar incompatible behavior of other elements may
explain why droppers are enriched in Ag, Sn and Sb. Extreme
sulfide melt fractionation may eventually lead to melts that
are very rich in these elements. This might also explain the
anecdotal correlation at Broken Hill between high Au grades
and garnetite.
Average Ag grades at Broken Hill are hard to determine
owing to the variable nature of the lodes, but the average
composition of the entire deposit has been estimated to be
148 ppm Ag (Parr and Plimer, 1993), and grades higher than
400 ppm are not reported. Our estimates of sulfide melt inclusion bulk compositions yield extreme enrichment of Ag, as
well Pb and Cu, compared to the Broken Hill main lodes
(Table 2 and Table 3). The Ag/Pb ratio of sulfide melt inclusions is one order of a magnitude higher than the average ore
grades of the main lodes (Table 2). It is likely that a sulfide
melt formed during the waning stages of metamorphism at
Broken Hill, based on the textural evidence in the form of sulfide melt inclusions, and melt likely persisted to temperatures
as low as 720°C, and potentially even lower.
TABLE 1. Concentrations of S, Fe and Mn (wt %) in Homogenized Sulfide
Melt Inclusions (used in data reduction from both garnet and quartz hosts)
Drill hole
S
Mn
Fe
Total
18.30
19.76
21.04
21.02
17.71
20.66
19.05
17.23
21.04
19.99
20.62
19.06
17.30
20.58
22.30
18.90
20.18
24.28
21.43
21.79
21.54
21.26
21.18
20.64
20.95
18.53
19.72
22.16
22.69
19.02
19.00
21.50
21.03
1.56
1.19
1.01
1.87
1.62
1.49
1.05
2.92
1.37
2.19
0.94
1.86
1.59
1.41
1.31
1.35
2.58
1.33
1.71
1.89
1.89
1.67
1.06
0.74
1.00
1.93
2.29
0.59
0.23
1.14
0.72
0.55
0.72
10.45
11.35
12.73
11.64
7.74
10.91
10.94
10.19
10.12
11.82
11.09
13.09
10.85
12.20
14.49
11.15
10.52
18.29
12.40
11.81
12.33
11.21
10.78
13.25
13.11
12.37
10.34
16.97
16.45
13.02
9.82
12.91
13.86
30.31
32.30
34.78
34.53
27.07
33.06
31.04
30.34
32.53
34.00
32.65
34.01
29.74
34.19
38.10
31.40
33.28
43.90
35.54
35.49
35.76
34.14
33.02
34.63
35.06
32.83
32.35
39.72
39.37
33.18
29.54
34.96
35.61
22.27
22.78
22.78
23.25
22.41
22.49
21.71
22.28
20.35
1.60
22.50
6.28
0.00
0.28
0.53
0.05
0.27
0.00
0.00
0.00
1.42
0.60
0.14
0.43
11.18
13.26
13.94
15.52
12.02
11.97
9.12
13.42
12.13
2.13
12.55
3.53
33.45
36.32
37.25
38.82
34.70
34.46
30.83
35.70
33.89
4.33
35.19
10.24
Garnet host
3028-60.8
3028-60.8
3028-60.8
3028-60.8
3028-60.8
3028-60.8
3028-60.8
3028-60.8
3028-60.8
3028-60.8
3028-60.8
3028-60.8
3028-60.8
3028-60.8
3028-60.8
3028-60.8
3028-60.8
3028-60.8
3028-60.8
N4689-51.2
N4689-51.2
N4689-51.2
N4689-51.2
N4689-51.2
N4689-51.2
N4689-51.2
N4689-51.2
Z3031-107.8
Z3031-107.8
Z3031-107.8
Z3031-107.8
Z3031-107.8
Z3031-107.8
Quartz host
C144
C144
C144
C144
C144
C144
C144
C144
Average garnet
σ
Average quartz
σ
Discussion and Conclusions
The presence of sulfide melt inclusions within garnets associated with the ores of Broken Hill conclusively establishes
that the ore partially melted during metamorphism. That
these inclusions homogenize at temperatures reasonable for
Determined by EDS
TABLE 2. Comparision of Average Compositions of
Sulfide Melt Inclusions and Ore Grade
The compositions of 44 quartz-hosted and 45 garnet-hosted
sulfide melt inclusions are plotted in Figure 4. Only S, Ca,
Mn, Fe, Co, Ni, Cu, Zn, As, Ag, Sn, Sb, Au, Hg, Pb and Bi
were present above background. The negative correlation between Cu and Pb shown in Figure 4A (closed symbols) is
likely due to fractionation of chalcopyrite and galena which
are two major phases revealed by reflected light microscopy
and SEM analyses of exposed sulfide melt inclusions in these
samples. However, we had no prior knowledge of the behavior of these phases during sulfide crystal fractionation. Mavrogenes et al. (2001) suggested that fractionation in the system
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SMINC
Pb (wt %)
Zn (wt %)
Ag (ppm)
Ag (ppm)/Pb (wt%)
51.1
0.8
6,005
118
7.8
16.4
9.6
4.3
4.3
3.2
11.9
12.4
22.4
10.4
12.4
6.4
169
118
53
31
33
34
21.7
7.2
5.5
7.2
7.7
10.6
Orebody1
No. 3 lens
No. 2 lens
No. 1 lens
A lode
B lode
C lode
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SMINC = sulfide melt inclusion
1 From Haydon and McConachy, 1987
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FIG. 4. Chemical trends of LA-ICP-MS analyses of individual sulfide melt inclusions. Each point represents an individual
inclusion. A. Pb vs. Cu in sulfide inclusions in garnet (open squares) and quartz (solid squares) form a linear array that must
have resulted from a chemical process such as fractionation. B. Ag vs. Cu/Pb in inclusions in quartz showing Ag increasing
with Cu/Pb due to the incompatibility of Ag during galena fractionation. C. Pb vs. Cu in inclusions in garnet from four separate garnetite samples. Note that these four separate samples define a linear trend toward the high Pb, low Cu end of the
spectrum. Inclusions in sample N4689-51.2 (filled circles) plot at the extreme other end of the trend. Thus, the different
samples appear to have trapped melt at different stages of the melting history. D. Co vs. Cu/Pb in inclusions in garnet from
four separate garnetite samples, again define a chemical trend with Co behaving compatibly during fractionation. Note that
the most evolved sample from C (N4689-51.2; filled circles) plots at the lowest Co (highest Cu/Pb) end, whereas the least
fractionated sample from C (Z3031-107.8; open triangles) plots at the highest Co, (lowest Cu/Pb) end of the trend.
TABLE 3. Average Sulfide Melt Inclusion Compositions Determined by LA-ICP-MS
Garnet host
Quartz host
All inclusions
Pb (wt%)
Cu (wt%)
Zn (wt%)
As (ppm)
Ag (ppm)
Sb (ppm)
Sn (ppm)
Co (ppm)
Ni (ppm)
Au (ppb)
54.6
48.2
51.1
7.2
14.7
11.3
1.3
0.3
0.8
9,559
2,250
5,542
7,078
5,125
6,005
6,685
7,378
7,066
217.5
188.6
201.6
45.5
36.1
40.3
18.1
256.8
149.3
4
4
4
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peak metamorphic conditions at Broken Hill and display systematic chemical trends, establishes that they are trapped
melts. Furthermore, their enrichment in metals well established as constituents of the Broken Hill lodes suggests that
sulfide melt inclusions formed by the melting of ores.
The abundance of sulfide melt inclusions in garnets associated with droppers confirms the previous suggestions that
droppers represent solidified sulfide dikes. Dropper ores as
well as associated sulfide melt inclusions are enriched in Cu,
Sb, As, Ag, Ni and Au compared to the main lodes, consistent
with their derivation from the main lodes.
Acknowledgments
Heather Sparks received financial support through a student research grant from the SEG Foundation. John Mavrogenes received support from the Australian Research Council. John Ridley in particular is thanked for his constructive
criticism of an earlier version of this manuscript. We also
thank Perilya Broken Hill Ltd for access to drill core, in particular, Ian Groves, Jane Murray, and Noel Carol. Mike Shelly
and Charlotte Allan assisted with LA-ICP-MS analyses. Discussions with Joerg Hermann, Richard Arculus, Ron Frost,
Carl Spandler, Jeremy Wykes, and Chris McFarlane were
helpful. Finally, John Vickers’ help with sample preparation
was invaluable.
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