957
The Canadian Mineralogist
Vol. 39, pp. 957-978 (2001)

GEOCHEMISTRY AND MINERALOGY OF GOLD-RICH HYDROTHERMALPRECIPITATES FROM THE EASTERN MANUS BASIN, PAPUA NEW GUINEAROGER MOSS§ AND STEVEN D. SCOTT
Scotiabank Marine Geology Research Laboratory, Department of Geology, University of Toronto, 22 Russell Street,Toronto, Ontario, Canada M5S 3B1

ABSTRACT
Three of the five hydrothermal vent fields currently known in the Manus back-arc basin of Papua New Guinea are enrichedin gold. Samples of precipitates from the PACMANUS and Susu Knolls hydrothermal fields in eastern Manus Basin contain up
to 56.8 ppm gold, and average 11.3 ± 13 (1) ppm (n = 103). Chimney samples dominated by sphalerite, barite and chalcopyrite
have significantly higher contents of gold than sulfide-poor samples. Gold in the sulfide chimneys occurs as micrometric grainsof silver-poor (1.4 to 4.2 wt.% Ag) native gold in three distinct associations. In copper-rich chimney samples, gold occurs as
inclusions in chalcopyrite associated with tennantite, bornite and covellite. In zinc-rich chimney samples, gold is present mainly
as inclusions in tennantite associated with chalcopyrite and sphalerite. A high activity of sulfur is indicated for the gold-richeastern Manus Basin precipitates by the sulfide assemblages, the low iron content of sphalerite (average = 2.3 ± 3 mole % FeS),
and the silver-poor nature of gold grains. Evaluation of possible complexes of gold indicates that AuHS° is the dominant complex, which is consistent with the high activity of sulfur in the hydrothermal system. According to available experimental data,
AuHS° is the dominant complex up to 325°C. This complex thus is the only one required for gold enrichment in both the Zn–Au
and the Cu–Au associations. Gold transport as the AuHS° complex is likely to be equally important in other seafloor hydrothermal systems with acidic end-member hydrothermal fluids.
Keywords: hydrothermal, gold, vent, seafloor, massive sulfide, PACMANUS vent field, Papua New Guinea.

SOMMAIRE
Trois des cinq champs d’évents hydrothermaux connus dans l’arrière-arc du bassin de Manus, en Papouasie Nouvelle Guinée,montrent un enrichissement en or. Les échantillons des précipités provenant des champs hydrothermaux de PACMANUS et deSusu Knolls, dans le secteur oriental du bassin de Manus, contiennent jusqu’à 56.8 ppm d’or, et atteignent en moyenne 11.3 ± 13(1) ppm (n = 103). Les échantillons de cheminées à prédominance de sphalérite, barite et chalcopyrite contiennent des teneursen or nettement plus élevées que les échantillons à faible teneur en sulfures. L’or des cheminées sulfurées se présente sous forme
de grains micrométriques d’or natif à faible teneur en argent (de 1.4 à 4.2% Ag, poids), en trois associations distinctes. Dans leséchantillons de cheminées riches en cuivre, l’or se présente sous forme d’inclusions dans la chalcopyrite associée à la tennantite,la bornite et la covellite. Dans les échantillons de cheminées riches en zinc, l’or se présente surtout sous forme d’inclusions dansla tennantite associée à la chalcopyrite et la sphalérite. Dans le secteur oriental du bassin de Manus, une activité élevée en soufre
serait indiquée pour expliquer ces précipités aurifères associés à des assemblages riches en sulfures, la faible teneur en fer de lasphalérite (en moyenne 2.3 ± 3% FeS, base molaire), et la faible teneur en argent des grains d’or. Une évaluation des complexes
potentiels pour transporter l’or montre que AuHS° est le plus important, ce qui concorde avec l’activité accrue du soufre dans lesystème hydrothermal. Selon les données expérimentales disponibles sur la stabilité de ces complexes, AuHS° serait prédominantjusqu’à 325°C. Il serait donc le seul requis pour expliquer l’enrichissement en or à la fois dans les associations Zn–Au et Cu–Au.
Le transfert de l’or sous forme du complexe AuHS° serait également important dans d’autres milieux d’évents hydrothermaux
dont les fluides s’approchent du pôle acide.
(Traduit par la Rédaction)
Mots-clés: hydrothermal, or, évent, fonds marins, sulfures massifs, champ d’évents PACMANUS, Papouasie Nouvelle Guinée.

§E-mail address: roger_moss@sympatico.ca

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THE CANADIAN MINERALOGISTINTRODUCTION
Exploration of the modern seafloor over the past two
decades has resulted in the discovery of many areas ofhydrothermal activity in geographically and tectonicallydiverse settings. Initial work between 1977 and 1985
concentrated on the investigation of vent sites on the
mid-ocean ridges of the East Pacific, and on the MidAtlantic Ridge. Hydrothermal precipitates from severalof these ocean-ridge deposits were found to be enrichedin gold (Hannington & Scott 1988, 1989a, Hannington
et al. 1986, 1991), although such gold-rich precipitates
are not typical of the majority of ocean-ridge deposits.
More recently, attention has turned to the submarine
arcs and marginal basins of the western Pacific. This
exploration has resulted in the identification of morethan 27 hydrothermal sites (Ishibashi & Urabe 1995)
ranging in water depth from the shallow-water(~ 200 m) Whakatane vent site south of White Island,
New Zealand (Hannington et al. 1999) to the 3,700meter-deep Alice Spring site in the central Mariana
Trough (Craig et al. 1987a). Many of the precipitates
associated with these vent sites are extremely gold-rich,with values up to 71 ppm Au (average 28 ppm, n = 11)
reported for sulfides from the Suiyo Seamount on the
Izu–Bonin arc (Watanabe & Kajimura 1994), and up to
49 ppm Au (average 20 ppm, n = 37) from the Sunrise
Deposit, 400 km to the south (Iizasa et al. 1999).
Clearly, the prolific gold mineralization found on landin the volcanic arcs of the region (Sillitoe 1989) continues into the submarine arcs, forearcs and back-arc basins associated with these subaerial environments.
Our understanding of the processes involved in theenrichment of gold in volcanogenic massive sulfide
deposits has been shaped by complementary studies onboth the modern seafloor (e.g., Hannington & Scott
1989a, b, Hannington et al. 1991, 1995, Herzig et al.
1993) and ancient deposits on land (e.g., Huston &Large 1989, Hutchinson 1990, Large 1990, Huston etal. 1992). In this study of gold-rich precipitates fromthe eastern Manus Basin, Papua New Guinea, we describe the geochemistry and mineralogy of gold in hydrothermal precipitates from the PACMANUS and Susu
Knolls vent fields, two of the most gold-rich back-arc
basin deposits of the western Pacific.

145oE

150oE
Inactive subduction
zone
Active subduction
zone

0o

Seafloor spreading
2000 m Depth
Contour

Transform Fault

Manus Trench

PAC
I

FIC

Willaumez Transfer Zone

0o

PLA
TE

Central Manus Basin

Manus I.
Ne

w

Ire

lan

d

Eastern Manus Basin
SOUTH BISMARCK
PLATE

5oS

NEW
GUINEA

5oS
New Britain
ch

New

145oE
FIG. 1.

ren
in T

a
Brit

SOLOMON
PLATE

150oE

Main tectonic elements of the Manus Basin region (modified from Benes et al. 1994).

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GOLD-RICH PRECIPITATES, EASTERN MANUS BASINGEOLOGICAL SETTING
The Manus Basin is the back-arc of the active New
Britain volcanic arc, situated to the east of the island of
New Guinea (Fig. 1). Subduction began under New
Britain after earlier subduction along the Manus trench
stopped in the Oligocene owing to the effect of theOntong Java Plateau docking against the Manus trench(Martinez & Taylor 1996). Back-arc spreading occurs
along the central Manus spreading center, decreasingfrom 92 mm/year (full rate) in the southwestern end of
the spreading center to 0 mm/year in the northeast(Martinez & Taylor 1996). Eastern Manus Basin is apull-apart structure that consists of a series of en échelon
volcanic ridges between two transform faults withinthinned arc crust (Binns & Scott 1993). Developmentof this structure is believed to have been contemporaneous with that of the Bismarck microplate (Martinez &Taylor 1996).
Four active vent fields and one inactive field havebeen discovered in Manus Basin, three of which occupy
an area of approximately 4,000 km2 in the eastern
Manus Basin (Binns et al. 1997). The inactive deposit
is located on the Willaumez transfer zone, and consists
primarily of iron oxide mounds (Binns 1994). The
Vienna Woods hydrothermal field occurs at a depth of
2,500 meters on the central Manus spreading center. The
first reports of hydrothermal activity in the region resulted from deep camera tows, which photographed ventfauna and inactive chimneys (Both et al. 1986). Activehydrothermal venting was discovered on the central
Manus spreading center during the OLGA II researchcruise of the RV Sonne (Tufar 1990). Both black andwhite smoker chimneys with abundant vent fauna were
found. The vent field consists predominantly of zincrich chimneys, venting 285° to 300°C fluid, sitting atop959

a sulfide mound, with fewer anhydrite-rich chimneys
sitting directly on the seafloor (Auzende et al. 1996a,b). Hydrothermal precipitates are enriched in zinc sulfide (sphalerite and wurtzite) and silica with an averagebase-metal content of 29.7% Zn, 2.2% Cu and 0.6% Pb(n = 25, Tufar 1990). These sulfides also are enriched ingold (average 30 ppm) and silver, which ranges from25 ppm to 1036 ppm (Binns 1994).
The discovery of CH4 and 3He anomalies in thewater column (Craig & Poreda 1987, Craig et al. 1987b)
was the first indication of hydrothermal activity in theeastern part of Manus Basin. Subsequently, three main
vent fields, DESMOS, Susu Knolls and PACMANUS,
were found in the region. DESMOS is a cauldron withinbasaltic andesite over which a large methane and manganese plume was discovered in 1990 (Sakai 1991,
Gamo et al. 1993). Low-temperature (118°C), acidic(pH = 2.1), vigorous venting is associated with a fault
in the northwest wall of the cauldron (Auzende et al.
1996a, b). This vent site, named Onsen (Fig. 2), is characterized by a high sulfate content in the vent fluid andthe deposition of abundant native sulfur (Gamo et al.
1997). Mineralization consists of enargite and pyrite
associated with alunite (Gena et al. 1998), suggesting
similarities with the subaerial to shallow submarine
high-sulfidation deposits described by Hannington et al.
(1999).
The Susu Knolls vent field, discovered during the1996 PACMANUS III expedition in the eastern part ofthe eastern Manus Basin, is situated on a northwest–
southeast-trending andesitic ridge named Tumai Ridge.
The vent field consists of clusters of vigorously active
chimneys on the tops of two coalesced porphyritic dacite
domes (North Su and South Su), and an adjacent site ofchimneys and massive sulfides known as Suzette(Fig. 2). The mineralization at Suzette is particularly

FIG. 2. Simplified bathymetric map of eastern Manus Basin (after Sakai 1991) showing location of vent sites (stars). Abbreviations: R Rogers Ruins, RR Roman Ruins, SC Snowcap, SM Satanic Mills, O Onsen, S Suzette, NS North Su, SS South Su.

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THE CANADIAN MINERALOGIST

copper-rich, with an average of 19% Cu reported fromthree chimneys. The three samples reported to date also
have high contents of zinc (average 22%), silver (average 125 ppm) and gold (average 23 ppm) (Binns et al.
1997).
The PACMANUS vent field was discovered in 1991
(Binns & Wheller 1991), underlying a strong Mn
anomaly in the water column (Sakai 1991). It is situatednear the bathymetric minimum of a northeast–southwest-trending ridge named Pual Ridge, which is composed of andesite, dacite and rhyolite. Andesite
dominates the lower reaches of the 500-meter-highridge, whereas the crest is predominantly dacite andrhyolite (Binns & Scott 1993, Scott & Binns 1995).
Detailed investigations of the area by deep-tow cameraand video (Binns & Wheller 1991, Binns & Scott 1993)
and submersible (Auzende et al. 1996a, b) revealed four
discrete vent-sites (Fig. 2), separated by areas of intermittent hydrothermal activity in the form of iron–manganese oxides and low-temperature shimmering vents.
This hydrothermal activity covers an area of approximately 1100  600 meters on the crest of Pual Ridge.
The Rogers Ruins vent site, located at the northernedge of the ridge crest, consists of actively venting blacksmokers up to 7.5 meters high, together with inactive
chimneys. Active chimneys have complex shapes, manywith peripheral spires growing around a dominant central structure. Samples from the vent field are dominantly zinc-rich with iron oxide coatings. Immediately
to the east of the vent field, numerous small (approximately one meter diameter) mounds of iron and manganese oxides are venting clear, low-temperature (40°C)
fluid.
The Roman Ruins vent site lies approximately 250
meters to the south of Rogers Ruins, and consists ofnumerous large (up to 20 meters high and 3 meters indiameter) predominantly inactive chimneys. Many suchchimneys appear to have coalesced to form a chimney“wall”, with active venting of clear hydrothermal fluid
from small chimneys and fissures on ledges in this“wall”.
Coalesced chimney structures venting high temperature (up to 268°C) fluid through small “chimneylets”
are characteristic of the Satanic Mills site. In contrast to
samples collected from Roman Ruins and Rogers Ruins, those from Satanic Mills tend to be copper-rich.
The fourth vent site, Snowcap, covers a knoll of altered dacite–rhyolite and is characterized by low-temperature diffuse venting and abundant white bacterial
mat associated with altered hyaloclastite (Binns et al.
1997). Active chimneys with beehive-type diffuser tops
(cf. Fouquet et al. 1993) are located immediately southof this site.

ANALYTICAL TECHNIQUES
A total of 103 chimney samples and sub-samples
were analyzed during this study. Samples were recov-

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ered by dredging during three RV Franklin cruises(PACMANUS II, III and IV) and by Shinkai 6500 submersible during the 1995 Manusflux cruise. Concentrations of copper, lead, silica and calcium were obtainedby X-ray fluorescence (XRF), of sulfur by the LECO
infrared spectrometry technique, and of bismuth, tellurium and selenium by hydride generation followed byinductively coupled plasma – atomic emission spectrometry (ICP–AES), all at Activation Laboratories,
Ancaster, Ontario. Concentrations of the remaining elements reported here, including gold, were establishedby instrumental neutron activation analysis (INAA) at
the University of Toronto. Details of the analytical procedure are given in Moss & Scott (1996).
Analyses of gold and sphalerite were carried out using a Cameca SX–50 electron microprobe. A synthetic
gold–silver alloy (“electrum”) standard with 20 at.% Ag
was used to analyze for gold and a synthetic sphalerite
standard containing 20 at.% Fe was used for the sphalerite analyses. Repeat analyses of these standards indicate
relative precisions [(standard deviation/mean)  100]
of 2% for Au, 3% for Ag, 0.3% for Zn, 0.7% for Fe and0.1% for S. X-ray powder diffraction was performed at
the Department of Chemistry, University of Toronto,with a Siemens D5000 diffractometer operating at
50 kV, 35 mA.

BULK COMPOSITION OF HYDROTHERMAL PRECIPITATESThe samples analyzed during this study include hydrothermal precipitates from the four PACMANUS ventsites and the three sites in the Susu Knolls area (Table 1).
The gold content of the precipitates ranges from below
detection (approximately 4 ppb) to 56.8 ppm and averages 11.3 ppm (Table 2). Precipitates from Roman Ruins (average 17.2 ± 13 ppm Au, n = 37), Susu Knolls(average 14.8 ± 18 ppm Au, n = 9) and Satanic Mills(average 13.3 ± 11 ppm Au, n = 25) are clearly moregold-rich than those from Snowcap (average 2.3 ± 3
ppm Au, n = 13) and Rogers Ruins (average 1.3 ± 4
ppm Au, n = 18) (Fig. 3a). This difference in gold content is reflected in the mineralogical composition of the
precipitates, which is controlled by the physical andchemical characteristics of the hydrothermal fluid. Temperatures of the vent fluid measured during theManusFlux Cruise averaged 245°C at the Satanic Mills
vent site, and 160°C at Roman Ruins (Auzende et al.
1996b, Gamo et al. 1997). Higher temperatures, up to
285°C, were measured for the Susu Knolls vent sites
during the 2000 Manaute Cruise (Y. Fouquet, pers.
commun., 2000), consistent with the more copper-richnature of samples from the Suzette vent site than thosefrom the PACMANUS area.
Samples to be analyzed were selected to represent
the range of mineralogical types occurring in the region,
and were distinguished according to their dominant
mineral assemblage. Oxidized chimneys rich in ironoxide and silica from the Rogers Ruins vent site at

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GOLD-RICH PRECIPITATES, EASTERN MANUS BASIN

TABLE 1. DESCRIPTION OF SULFIDE SAMPLES FROM VARIOUS VENT-SITES IN THE EASTERN MANUUS BASIN

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THE CANADIAN MINERALOGISTPACMANUS typically contain little gold (Fig. 3b).
Similarly, iron–manganese oxide crusts and mounds andiron oxide chimneys are gold-poor. The sphalerite –barite-rich, predominantly inactive chimneys from the
Roman Ruins vent sites are enriched in gold, as are thezoned chalcopyrite–sphalerite chimneys from the Satanic Mills and Susu Knolls areas. Massive chalcopyrite-rich chimneys, or chalcopyrite-rich portions, arevariably enriched in gold, with one sample from theSusu Knolls area having the highest gold content encountered, 56.8 ppm (Fig. 3b). The concentration ofgold shows statistically significant (at the 95% confidence level) positive correlations with that of Zn, Cu, S,
Sb and Cd, (Figs. 4a–e). A significant negative correlation of Au with SiO2 (Fig. 4g) is consistent with the low
gold content of sulfide-poor samples in the area.
There is no unique position of gold enrichment withinzoned chimneys from eastern Manus Basin, as both interior (Cu-rich) and exterior (Zn-rich) chimney portions
contain high (>1 ppm) values of gold (Fig. 5). However, zinc-rich portions generally have the higher gold
contents. One complexly zoned chimney selected for
detailed analysis has a broad zone of gold enrichment
in the Zn-rich middle chimney wall (Fig. 6). Analysis
of sub-samples from a zoned chimney from Lau Basinproduced a similar result (Herzig et al. 1993).

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MINERALOGY OF HYDROTHERMAL PRECIPITATES
Sphalerite + barite samples
These samples consist predominantly of porous aggregates of sphalerite and prismatic crystals of barite
that fill cavities in the sphalerite. Rosettes of barite arecommonly intergrown with granular sphalerite, which
may also be present as dendrites. In some places, there
has been small-scale replacement of barite by sphalerite. Sphalerite contains most of the other sulfides encountered as inclusions or as cavity fill. Dendritic
sphalerite contains predominantly pyrite inclusions,whereas granular sphalerite contains mostly chalcopyrite and tennantite inclusions.
Chalcopyrite forms fine (1–5 m thick) “ribbons”
outlining growth zones in sphalerite aggregates. Suchzones are also outlined in places by tennantite and a Pb–
As sulfosalt with the composition of dufrénoysite(Pb2As2S5). Euhedral to subhedral grains of tennantite
are associated with sphalerite, commonly forming a cavity-lining on sphalerite aggregates, and in some places
replacing sphalerite inward from the cavities (Fig. 7a).
Euhedral, triangular-shaped grains of tennantite up to
40 m in diameter occur as inclusions in sphalerite.

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GOLD-RICH PRECIPITATES, EASTERN MANUS BASIN

30

10tation on this dendritic framework. Galena is present as
subhedral to anhedral grains of 10–50 m diameter that
rim and replace sphalerite, and as minor dendritic grains
generally interstitial to sphalerite. Chalcopyrite,
dufrénoysite, and tennantite occur as fine-grained (1–5m) inclusions in sphalerite. Tennantite may replacegalena in places (Fig. 7c). Rare grains of pyrite are interstitial to sphalerite.

5(Sphalerite + chalcopyrite)-rich samples

N=103

Frequency

25

Snowcap
Roman Ruins
Rogers Ruins
Susu Knolls
Satanic Mills

20
15

0

a

0 1 5 10 15 20 25 30 35 40 45 50 >50
Au (ppm)

N=103
35

Sphalerite-barite
Sphalerite-galena
Sphalerite-chalcopyrite
Chalcopyrite
Sulfide-poor

Frequency

30
25
20
15
10
5
0

b

963

0 1 5 10 15 20 25 30 35 40 45 50 >50
Au (ppm)

FIG. 3. Frequency distributions of gold content of hydrothermal precipitates. a. Distribution arranged by vent site. b.
Distribution arranged by mineralogical composition of the
precipitates. See text for description of mineralogical types.

Dufrénoysite is found predominantly as micrometricdisseminations within sphalerite (Fig. 7b).
Spheroidal grains of pyrite up to 50 m across oraggregates of spheroidal pyrite are associated with dendritic sphalerite. In some cases, marcasite replaces pyrite, as grains radially oriented toward the core of thespheroidal pyrite. Rare bornite and covellite are associated with chalcopyrite in the chimney interior.
(Sphalerite + galena)-enriched samples
This mineralogical type is represented by only two
samples, recovered from the Snowcap and Roman Ruins vent sites at PACMANUS. Sphalerite occurs bothas dendritic aggregates and as porous granular aggregates that appear to have formed by continuous precipi-

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Mineralogically zoned chimneys, typically with achalcopyrite-rich interior and sphalerite-rich exterior,comprise this type of sample. Chalcopyrite in the chimney interior occurs as both dendritic grains and massive
aggregates that are porous in places. Exsolution lamellae of chalcopyrite, showing a typical basket-weave texture, occur in bornite in some samples (Fig. 7d). Later
chalcopyrite replaces this bornite–chalcopyrite assemblage in places. Covellite is always associated withbornite, generally forming a rim around bornite grains,
and replacing bornite along fractures and grain boundaries. Sphalerite lines cavities in the chalcopyrite andforms a thin (1–5 m) rim around chalcopyrite aggregates. In some places, sphalerite also occurs as ribbonlike aggregates in chalcopyrite outlining growth zones
or grain boundaries. Marcasite is common in chalcopyrite-rich zones, and occurs together with pyrite as veryfine-grained intergrowths rimmed by chalcopyrite, andas grains and granular aggregates included in, and locally replaced by, chalcopyrite. Pyrite also occurs withinchalcopyrite as euhedral to subhedral inclusions andinterstitial grains. Minor barite may also occur in chimney interiors as large (up to 0.5 mm) bladed grains in
the cavities of chalcopyrite aggregates.
The mineralogy in the middle to outer chimney wallis dominated by massive aggregates of sphalerite thatare commonly colloform and banded. The growth zones
are outlined by tennantite, dufrénoysite and, more rarely,
chalcopyrite. The sphalerite typically becomes moredendritic toward the outer wall of the chimneys. Chalcopyrite occurs as inclusions in sphalerite, or lines cavities in sphalerite aggregates. Tennantite and a later
generation of sphalerite commonly form rims on thischalcopyrite which, in some places, replaces the earlier
sphalerite. Rosettes of barite are found only in the outerportions of chimneys, typically concentrated right at theexterior wall.
Chalcopyrite-rich samples
Although some samples from the Satanic Mills ventsite are chalcopyrite-rich, most samples of this typecome from the Suzette vent site in the Susu Knolls area.
Massive aggregates of chalcopyrite dominate, and commonly have a thin (20 m) rim of sphalerite, which also
outlines grain boundaries within the chalcopyrite. Isolated grains of chalcopyrite (up to 20 m) occur around

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964

THE CANADIAN MINERALOGIST60

r=0.36N=103

50

Cu (%)

Zn (%)

40
30
20
10
0
0

a

10

20

30

40

4000

60

r=0.22N=1020b

Sb (ppm)

0

c

40

50

60

Au (ppm)60

Ag ( ppm)

S (%)

20

0

20

30

40

50

60

50

60

r=0.16
N=103

1500
1000

r=-0.32
N=83

80
60
40

0

f

0

10

20

30

40

50

60

Au (ppm)
30

sphalerite-barite
sphalerite-galena
sphalerite-chalcopyrite
chalcopyrite
sulfide-poor

r=-0.04
N=103

25
20

Ba (%)

100

SiO2 (%)

10

Au (ppm)

15
10
5

20

FIG. 4.

40

Au (ppm)

0

g

30

500

10

0

20

2000

30

e

10

2500

40

60

r=0.48N=1030dr=0.46
N=73

50

50

2000

0
30

40

3000

1000

20

30
Au (ppm)

1000

10

20

4000

2000

0

10

5000

r=0.50N=103

3000
Cd (ppm)50

Au (ppm)

40
35
30
25
20
15
10
5
0

0

0

10

20

30

Au (ppm)

40

50

60

h

0

10

20

30

Au (ppm)

Scatterplots of gold versus a) Zn, b) Cu, c) Cd, d) Sb, e) S, f) Ag, g) SiO2, h) Ba.

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40

50

60

GOLD-RICH PRECIPITATES, EASTERN MANUS BASIN

I

965

118556EI132616

M
E
I

Chalcopyrite-rich
Sphalerite-rich
Sphalerite-barite rich
Sphalerite-Fe-Oxide rich

134067

E

0

5

10

15

20

25

30

35

40

45

50

55

Au (ppm)aI132651E

Mn-Oxide rich
Fe-Oxide rich
0

20

b

40

60

80

100

120

140

Au (ppb)

FIG. 5. Gold content of selected zoned chimneys from the PACMANUS vent field. a.
Sulfide-rich chimneys. b. Oxide-rich chimneys. Note change of units (to ppb) in b.
Numbers to the right of the bars are sample numbers. Abbreviations: I interior, M middle, E exterior chimney wall.

sphalerite rims in places, suggesting rapidly changing
conditions in the fluid, with alternating precipitation ofchalcopyrite and sphalerite. Tennantite is associatedwith the chalcopyrite–sphalerite assemblage as rare
grains that separate chalcopyrite and sphalerite. Marcasite and pyrite are also associated with the massive chalcopyrite, mostly as pyrite–marcasite aggregates inwhich pyrite has been variably replaced by marcasite.
Bornite fills cracks in the chalcopyrite aggregates, andmay replace chalcopyrite inward from grain boundaries
and cavities (Fig. 7e). Covellite replaces bornite along
fractures in places (Fig. 7f), but also occurs as euhedralgrains rimming chalcopyrite. The exterior walls of copper-rich chimneys contain minor barite, iron oxides and,rarely, atacamite [Cu2Cl(OH)3].

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Sulfide-poor samples
Sulfide-poor samples from the eastern Manus Basinfall into three groups: 1) iron–manganese oxides predominantly composed of goethite, ferrihydrite, hematite and Mn-oxide hydrate (Mn 7 O 13 •5H 2 O), 2)
sulfate-rich samples dominated by barite or, more rarely,
anhydrite, with lesser gypsum, and 3) silicate-richsamples that are composed mostly of amorphous silica
and rare kaolinite.
Occurrences of gold in sulfides at Manus
A total of 303 grains of native gold were observed infour samples of hydrothermal precipitates from Eastern

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F IG . 6. Cross-section of zoned chimney sample 132620 from the Roman Ruins vent site,
PACMANUS vent field. a. Photograph. b. Dominant mineralogical zones. c. Sub-samples with Au
content in ppm. d. Contours of gold concentration. Abbreviations: sp sphalerite, ccp chalcopyrite,ba barite. Mineral in parentheses is a minor constituent of the zone.

966

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THE CANADIAN MINERALOGIST

966

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GOLD-RICH PRECIPITATES, EASTERN MANUS BASIN

967

FIG. 7. Reflected-light photomicrographs of hydrothermal precipitates from the eastern Manus Basin. a. Sphalerite + barite
sample. Tennantite (ten) rims massive sphalerite (sp). b. Sphalerite + barite sample. Dufrénoysite inclusions (duf) in massive
sphalerite (sp). c. Sphalerite + galena ± barite sample. Replacement of galena (gn) by tennantite (ten) in massive sphalerite(sp). d. Sphalerite – chalcopyrite sample. Exsolution lamellae of chalcopyrite (ccp) in bornite and replacement of this assemblage by chalcopyrite. e. Chalcopyrite-rich sample. Replacement of chalcopyrite (cpp) by bornite (bn) inward from a cavity
located at the top of the photo. Bornite is replaced along grain boundaries by covellite (cv). f. Chalcopyrite-rich sample.
Replacement of bornite (bn) by covellite (cv) along fractures and grain boundaries. Chalcopyite (cpp) rims bornite grains andlines cavities in the bornite. It also replaces bornite inward from cavities. Scales are in micrometers.

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968

THE CANADIAN MINERALOGIST160
n = 284

140
120

Frequency

100
80
60
40
20
0

0

1

2

3

5

4

6

7

8 >8

Grain Size (Micrometers)

FIG. 8. Grain-size distribution of grains of native gold in
sulfide chimneys from the PACMANUS and Susu Knolls
vent sites.

associated with bornite (Table 3). The association ofgold primarily with chalcopyrite indicates that it formed
prior to replacement of chalcopyrite by bornite andcovellite.
The interior chimney wall of zoned (sphalerite +chalcopyrite)-rich sample 132376 contains grains ofgold associated with a bornite – chalcopyrite – tennantite
assemblage (Fig. 9d). Gold occurs mainly as inclusions
in, or as grains interstitial to, tennantite and chalcopyrite that is later than the bornite – chalcopyrite – covellite
assemblage.
All of the grains of gold are silver-poor. Electronmicroprobe analyses of several grains from samples
118554–6 [(sphalerite + barite)-rich] and 132376 (chalcopyrite-rich) average 4 ± 0.15 wt.% and 1.9 ± 0.3 wt.%
Ag, respectively (Table 4). Gold grains of similar low
content of Ag are associated with Fe-poor sphalerite insulfides in the Lau Basin (Herzig et al. 1993). Although
there is a slight decrease in silver content of gold in the
Cu-rich samples over that in the Zn-rich samples, this isvery much less than that observed by Huston et al.
(1992) for gold in Australian volcanogenic massive sulfide deposits.

Manus Basin. The grains are typically roundish andsmall; 75% of the grains are less than 3 m in diameter(Fig. 8). The main occurrence of gold grains is as inclusions in tennantite (77% of grains found), followed byinclusions in chalcopyrite (16%). The remainder occur
as inclusions in sphalerite or bornite, fracture fill in various sulfides, and are found as isolated grains along grainboundaries and in cavities (Table 3).
Gold grains were found in a (sphalerite + barite)rich sample, two (sphalerite + chalcopyrite)-richsamples and in a chalcopyrite-rich sample. In the(sphalerite + barite)-rich sample, the grains are associated with a chalcopyrite – sphalerite – tennantite assemblage and occur mostly as inclusions in tennantite or
chalcopyrite aggregates (Fig. 9a). Gold grains in chalcopyrite tend to occur close to grain boundaries withtennantite. Inclusions of tennantite also occur in sphalerite, but these inclusions do not contain grains of gold.
Gold therefore occupies a very specific position in themineral paragenesis, occurring as part of late-stage cavity-filling episodes, together with tennantite and sphalerite that lines the walls of cavities in chalcopyrite(Fig. 9b). All of the gold in this assemblage was precipitated prior to sphalerite, however, as no gold grainsare visible in the sphalerite, nor in the tennantite inclusions in sphalerite.
Gold grains in chalcopyrite-rich sample 133012 fromthe Suzette vent site are mostly present as inclusions inchalcopyrite (Fig. 9c). Although bornite and rarecovellite also form part of this assemblage, primarily
replacing chalcopyrite, only one grain of gold was found957 39#4-août-01-2256-02

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GOLD-RICH PRECIPITATES, EASTERN MANUS BASIN

969

FIG. 9. Photomicrographs of native gold and associated assemblages. a. (Sphalerite + barite)-rich sample. Several inclusions of
native gold (Au) in tennantite (ten) are associated with sphalerite (sp) in a cavity lining. b. Gold – tennantite – chalcopyrite –sphalerite association. Box indicates position of photomicrograph in a. The native gold occurs mainly in the tennantite (ten)that forms a rim on the chalcopyrite (cpp), but a few grains also are present in the chalcopyrite. Triangular inclusions oftennantite in sphalerite (sp) do not contain visible grains of gold. c. Zoned sphalerite–chalcopyrite-rich sample. Gold –chalcopyrite – bornite association. Gold (Au) occurs mainly in the massive chalcopyrite (cpp) aggregates, and only rarely as
inclusions in bornite (bn). d. Chalcopyrite-rich sample. Gold is typically associated with chalcopyrite (cpp), which is later
than the chalcopyrite–bornite exsolution assemblage. Scales are in micrometers.

Gold is known to occur as submicroscopic grains,
so-called “invisible” gold (Cook & Chryssoulis 1990).
This may be structurally bound gold in common sulfides such as pyrite and chalcopyrite, fine-grained gold
adsorbed onto mineral surfaces, or very fine-grained(colloidal) gold in sulfide grains. The presence of such
submicroscopic gold in the PACMANUS sulfides
would explain the absence of grains of native gold fromsome samples having high bulk-gold contents, as suggested by Hannington et al. (1995) for gold in the TAG
precipitates. The significant positive correlation of goldwith Zn, Cd, and S in the PACMANUS sulfides (Fig. 4)
and the lack of such a correlation with Ag or As suggestthat although the microscopically visible gold occurs inassociation with tennantite and chalcopyrite, a signifi-

957 39#4-août-01-2256-02

969

cant proportion of the gold at PACMANUS is present
as “invisible” gold in sphalerite. Attempts to determine
the amount of gold in the sphalerite by secondary-ion
mass spectroscopy were unsuccessful owing to the veryfine-grained nature of the sphalerite.

DISCUSSION
Mineralogical associations of native gold
There are three mineralogical associations of goldin eastern Manus Basin sulfides: a chalcopyrite –sphalerite – tennantite association, a chalcopyrite –bornite ± covellite association and a chalcopyrite –tennantite ± bornite – covellite association. Native gold

26/10/01, 14:40

970

THE CANADIAN MINERALOGIST

occurs predominantly as micrometric inclusions intennantite and chalcopyrite. The chalcopyrite – sphalerite – tennantite association is present in the (sphalerite+ barite)-rich chimneys. Similar occurrences of native
gold have been described from other seafloor vent-sites(e.g., Hannington & Scott 1989a, b, Herzig et al. 1993),
but in most of these cases the host is Fe-poor sphaleritewith associated tetrahedrite or tennantite. Native gold
and a gold–silver alloy (“electrum”) have also been reported from zinc-rich assemblages in ancient deposits.
Australian volcanogenic massive sulfide deposits of theZn–Pb–Ag–Au type typically contain “electrum” associated with galena, sphalerite, chalcopyrite and tetrahedrite–tennantite (Huston et al. 1992), as do deposits of
the Rio Tinto – Aznalcollar–La Zarza type of the Iberian Pyrite Belt (Leistel et al. 1998). The association ofnative gold with low-temperature (<300°C) Zn-rich precipitates is commonly believed to be due to the transport of gold predominantly as the Au(HS)2– complex(e.g., Hannington & Scott 1989a, b, Huston & Large1989).
Gold–copper associations, such as the gold – chalcopyrite – bornite ± covellite assemblage found in copper-rich chimneys at Suzette, are rare in seafloor sulfide
deposits. Gossans developed on the TAG and MIR hydrothermal mounds on the Mid-Atlantic Ridge have an
average gold content of 11.4 ppm and contain abundantgrains of native gold associated with covellite, digenite,
native copper and atacamite (Hannington et al. 1988).
However, these copper minerals at TAG are secondary,
resulting from oxidation processes on the seafloor(Hannington et al. 1988, Herzig et al. 1993), and are
not comparable to the primary association found in eastern Manus Basin. A gold–copper association occurs in
the ultramafic-rock-hosted Logatchev hydrothermal
field at 14°45’ N on the Mid-Atlantic Ridge (Murphy &Meyer 1998). Here, gold mineralization is found bothas primary grains of native gold included in chalcopyrite associated with sphalerite and isocubanite, and as
secondary gold associated with a chalcopyrite – bornite– covellite assemblage. Occurrences of gold–copper
associations have been well documented in ancient
volcanogenic massive sulfide deposits (Huston & Large
1989, Large 1990, Huston et al. 1992, Leistel et al.
1998). Such associations have been interpreted as resulting from chloride complexing of gold in high-temperature (>300°C) fluids with precipitation due, in mostpart, to decreasing temperature (e.g., Huston & Large1989).
Effect of sulfur activity on gold concentration
The assemblages of sulfide minerals observed in
Manus Basin, such as tennantite, chalcopyrite, bornite
and pyrite, indicate a high activity of sulfur for sample
types containing native gold. Also, electron-microprobe
analyses of 68 sphalerite grains and aggregates from 13
PACMANUS samples have a low average iron content,

957 39#4-août-01-2256-02970

2.3 ± 3.1 mole % FeS (Fig. 10a), implying a high activity of sulfur. The low silver content of the gold grains ineastern Manus Basin also indicates a high activity ofsulfur. The silver content of “electrum” in equilibriumwith Ag2S (or another Ag-bearing phase) is inversely
proportional to the activity of sulfur (Barton & Toulmin1964), as shown in equation 1.
Ag(s) + ¼S2(g) = ½Ag2S(s)aAg = aAg2S1/2  aS2–1/4  k–1(1)

The high activity of sulfur is consistent with the transport of gold as a reduced sulfur complex. There is nocorrelation of mole % FeS in sphalerite with the bulkgold content of the PACMANUS samples (Fig. 10b), as
was found to be the case with Tasmanian volcanogenic
massive sulfide deposits (Zaw & Large 1996). However,when these parameters are plotted together with resultsfrom other seafloor vent-sites, the eastern Manus Basindata lie on a trend toward higher gold values with lower
iron content of sphalerite (Fig. 11).
The concentration of gold in hydrothermal precipitates has been shown to be inversely related to the FeS
content of sphalerite (Hannington & Scott 1989b). The
FeS content of sphalerite coexisting with pyrite or pyrrhotite is known to be inversely related to the activity ofsulfur (Barton & Toulmin 1966, Scott & Barnes 1971).
Thus the gold content of seafloor sulfide deposits is related to the activity of sulfur, being generally highest
where the activity of sulfur is highest (Hannington &Scott 1989b). This correlation is primarily due to thetransport of gold as a sulfide complex in low-temperature (<300°C), reduced, sulfur-rich fluids that typicallyform gold-rich precipitates (Hannington & Scott 1989b).
The Cambrian massive sulfide deposits in Tasmania also
show a trend of increasing gold values with decreasing
mole % FeS in sphalerite (Zaw & Large 1996). Although the correlation within an individual deposit ispoor, it improves significantly if the values for eachdeposit are averaged and plotted as an individual point.
Several reasons are put forth by Zaw & Large (1996)
for this lack of correlation within a deposit, including
later metamorphic effects, and factors other than sulfur
activity and temperature controlling gold transport. Although it is clear that factors such as pH and oxygen
activity play a role in transporting gold, inspection of
relevant equations presented below indicates that theactivity of reduced sulfur (in this case as H2S) is themost important control on gold transport as a reduced
sulfur complex.
Transport of gold in seafloor hydrothermal systems
Experimental studies of gold solubility have focused
mainly on the ability of chloride and reduced sulfur
ligands to complex gold, owing to their availability inhydrothermal systems (Seward 1991). There is a general consensus that gold is transported as the bisulfide

26/10/01, 14:40

GOLD-RICH PRECIPITATES, EASTERN MANUS BASIN45

N = 68Average = 2.3
Std. Dev. = 3.1

40

Sample Type
Sphalerite - barite

3530Frequency

971

Sphalerite - galena
Sphalerite - chalcopyrite

25

Chalcopyrite

20
15
10
5
0

1

a

2

3
4
5
6
Mole % FeS in Sphalerite

8

Sample Typesphalerite - barite

7
Mole % FeS in Sphalerite

8 >8

7

sphalerite - galena
sphalerite - chalcopyrite
chalcopyrite

6
5
4
3
2
1
0
0

10

b

20

30

40

50

60

Bulk Au (ppm)

FIG. 10. a. Frequency distribution of mole % FeS in sphalerite in samples from the
PACMANUS and Susu Knolls vent fields. b. Plot of average mole % FeS versus average gold content of the samples. See text for description of sample types.

Au(HS)2– complex in fluids of low salinity, near-neutral pH, temperatures less than about 350°C and moderately reducing conditions (e.g., Seward & Barnes 1997).
For gold transport in fluids of low pH, low salinity andtemperatures up to 350°C, Hayashi & Ohmoto (1991)
proposed the HAu(HS)2° complex. However, recent

957 39#4-août-01-2256-02

971

experimental work has shown that the AuHS° complex
is dominant under these conditions (Benning & Seward1996).
Chloride complexing of gold has also been thesource of some debate, as Cole & Drummond (1986)
have suggested that the stability of the AuCl2– complex

26/10/01, 14:40

972

THE CANADIAN MINERALOGISTMole % FeS in Sphalerite

50

10

GB
ERMV
21N
11N
SJFR

ER
GR

TAG
AS
EMB

1

LB

0.1
0.1

1

100

10

Bulk Au (ppm)
FIG. 11. Plot of average mole % FeS versus average gold
content for seafloor VMS deposits. Abbreviations: GB
Guaymas Basin, MV Middle Valley, ENR Endeavour
Ridge, 21N 21°N East Pacific Rise, 11N 11°N East PacificRise, SJFR Southern Juan de Fuca Ridge, GR Galapagos
Ridge, ER Explorer Ridge, AS Axial Seamount, EMB eastern Manus Basin, LB Lau Basin. Sources of data: Lau Basin: Herzig et al. (1993); eastern Manus Basin: this study;
all other data: Hannington & Scott (1989b).

is far greater than that predicted by Helgeson (1969).
However, Seward (1991) demonstrated that the data ofHelgeson comprise part of a consistent dataset over thetemperature range 25°C to 374°C. In addition, the experimental results of Zotov et al. (1991) and Gammons
et al. (1994) have confirmed Helgeson’s predicted solubilities. The solubility predicted by Sverjensky et al.
(1997) and determined experimentally by Gammons &Williams-Jones (1995) is somewhat higher (Fig. 12).
Sverjensky et al. (1997) also presented data for the AuCl
and AuCl32– complexes, which are used in this study
together with the solubility for the AuCl2– complex predicted by Sverjensky et al. (1997) to evaluate gold transport as chloride complexes. The Au(OH)o complex may
also play a role in gold transport at temperatures between 300° and 500°C (Gammons & Williams-Jones
1997).
Six complexes are therefore evaluated as possible
carriers of gold in the hydrothermal systems of the eastern Manus Basin. Gold transport as a reduced sulfur
complex may be represented by:
Au(s) + 2H2S(aq) + ¼O2 = Au(HS)2–+ H+ + ½H2O

(2)

Au(s) + H2S(aq) + ¼O2 = AuHS° + ½H2O

(3)

or

25

20

Gammons etal.
(1994)
Gammons and
W illiams-Jones(1995)X Helgeson(1969)
Zotovetal.
(1991)(1,000atm.)

logK

Sverjensky
etal.
(1997)

15
X
X

10

X

X

5
150

200

250

300

350

400

450

o

T ( C)
FIG. 12. Comparison of experimentally determined and calculated stabilities of the AuCl2–
complex (Equation 5).

957 39#4-août-01-2256-02

972

26/10/01, 14:40

GOLD-RICH PRECIPITATES, EASTERN MANUS BASIN

Au(s) + 2Cl– + H+ + ¼O2 = AuCl2– + ½H2O

(5)

Au(s) + 3Cl– + H+ + ¼O2 = AuCl32– + ½H2O

(6)and hydroxide complexing of gold by:
Au(s) + ½H2O + ¼O2 = Au(OH)°(7)

From equation 2, the solubility of gold as the Au(HS)2–
complex is primarily dependent on the activity of H2S
and pH and, to a lesser extent, on oxygen activity. Equation 3 indicates that the solubility of gold as the AuHS°
complex is also primarily dependent on the activity of
H2S, but is independent of pH where H2S is the stablesulfur species. From equations 4, 5 and 6, the stabilityof the gold chloride complexes is primarily dependent
on the activity of chloride and pH and, to a lesser extent, on oxygen activity. Equation 7 indicates that gold
carried as the Au(OH)° complex is predominantly dependent on oxygen activity.
The effect of pH on the chloride and sulfide complexes is shown for a PACMANUS-type vent fluid at
250°C in Figure 13a. The AuHS° complex is clearly themost stable in the acid pH region, but Au(HS)2– becomes the most stable complex at pH greater than about
4.5. The gold chloride species AuCl2– is more stable
than the bisulfide species Au(HS)2– (but much lessstable than the AuHS° species) only in the very acidregions up to a pH of approximately 2. Thereafter, thebisulfide complex rapidly becomes the more stable ofthe two.
Temperature is also a key control on the complexingof gold, and is evaluated, for the PACMANUS-type
fluid, at pH = 3, in Figure 13b. The sulfide complexes
are clearly more stable than the chloride complexes up
to close to 300°C, at which point the AuCl2– species
becomes more stable than the Au(HS)2– species. However, even at this temperature, the concentration of the
AuHS° species is two orders of magnitude greater thanthe chloride complex. Therefore, the bisulfide complex
Au(HS)2– is the most stable of the three complexes up
to about 140°C, after which AuHS° becomes the stablecomplex up to 325°C. Calculations of gold solubility inthe temperature range 300° to 400°C at 1 kbar indicatethat the AuHSo complex remains the most stable of thecomplexes to a temperature of about 322°C. At this temperature, the neutral Au(OH)° complex becomes themost stable, and continues to be the dominant complex
beyond 400°C (Fig. 13c).
The availability of suitable ligands to complex thegold plays a significant role in the transport of gold. A
predominance-area diagram illustrating the fields of thecomplexes as a function of chloride and H2S activity isshown in Figure 14. At very high activities of H2S, the957 39#4-août-01-2256-02

973

-4

T=250oC aCl=0.154
ΣS=0.007 P = SVP

-5
-6
-7
-8

-

AuCl2
o
Au(HS)
ΣAu

AuCl
2AuCl3Au(HS)2

-9
-10
1

2

3

4

a

5

6

7

8

250

300

350

pH

-5

Gold solubility (log molality)

(4)

-6

aCl=0.154
pH=3ΣS=0.007 P = SVP

-7
-8
2-

AuCl
AuCl2

-9

AuCl3
o
Au(HS)-ΣAu

Au(HS)2

-10
50

100

150

b
Gold solubility (log molality)

Au(s) + Cl– + H+ + ¼O2 = AuCl° + ½H2O

Gold solubility (log molality)

chloride complexing of gold by:

973

c

-5

200

T (oC)

aCl=0.154
pH=3ΣS=0.007 P = 1 kbar

-6
-7
-8
-9
-10
300

Au(HS)2AuCl2-ΣAu

Au(OH)o350

T (oC)

Au(HS)o400

FIG. 13. Solubility of gold in a PACMANUS-like fluid a) as
a function of pH, b) as a function of temperature, and c) forT between 300 and 400°C and P = 1 kbar. Gold acetate
complexes have solubilities of < –10 log molal. Fluid composition is based on analyses of PACMANUS vent fluids(T. Gamo, pers. commun., 1995) extrapolated to 0 Mg, withactivity of species and high-temperature pH calculated using SOLMINEQ.88 (Kharaka et al. 1988). Solubility wascalculated using the EQBRM code (Anderson & Crerar1993) with thermodynamic data from Benning & Seward(1996), Sverjensky et al. (1997), Suleimenov & Seward(1997), Johnson et al. (1992), and Zotov et al. (1985, 1991).
SVP: saturated vapor pressure.

26/10/01, 14:40

974

THE CANADIAN MINERALOGIST

Au(HS)2– complex is favored, and gold solubility canreach very high values. At lower activities of H2S and
low activity of chloride, gold is transported predominantly as the AuHS° complex, with a maximum solubility of 1.2 ppm Au. At sufficiently low values of H2S
activity and relatively high chloride activity, gold istransported predominantly as a chloride complex. The
AuCl complex is only important at very low activitiesof H2S [log a(H2S) < –6] and low chloride activity. As
the chloride activity increases, AuCl2– becomes the predominant chloride complex, and finally at very highactivity of chloride, AuCl32– is predominant. Averagevalues of a(H2S) and a(Cl–) for the PACMANUS vent
field indicate that gold in the PACMANUS system ismost likely carried as the AuHS° complex, and has amaximum solubility of about 100 ppb (Fig. 14). Of
course, the actual concentration of gold in the
PACMANUS end-member fluid could be less than thisvalue indicative of saturation.
Deposition of gold carried as the AuHS° complex
can be caused by 1) a decrease in a(H2S) (see Fig. 14),which may result from dilution by ambient seawater,
precipitation of sulfides, or phase separation with lossof H2S to the vapor phase, 2) a change in pH (seeFig. 13a), and 3) sufficient oxidation to convert reduced

1

sulfide to sulfate (Fig. 15). The effects of temperatureon the solubility of gold are variable. At temperatures
between approximately 322° and 200°C, gold solubility actually increases as temperature decreases owing to
the predominance of bisulfide complexes (Fig. 13b). At
lower temperatures, gold solubility decreases with theconcentration of the gold bisulfide complexes. In thecase of seafloor hydrothermal systems, such as
PACMANUS, precipitation of gold from the hydrothermal fluid can be accomplished efficiently by mixing
with seawater (Hannington & Scott 1989a). Such mixing causes a decrease in a(H2S) and an increase in pH
and a(O2), all of which can result in precipitation ofgold. Boiling may also be important in shallow submarine hydrothermal systems (Hannington et al. 1999). If
boiling does occur, it will cause loss of H2S to the gas
phase prior to, or together with, mixing with seawater.
This would result in more efficient precipitation of gold
carried as a bisulfide complex than would mixing withseawater alone.

CONCLUSIONS
Gold-rich hydrothermal precipitates from the eastern Manus Basin are associated with both Zn- and Cu-

AuCl32100 ppb

AuCl2-

1 ppb

-1.5

Au(HS)2-

0.1 ppb-1SM

10 ppm-0.5

100 ppm

10 ppb0

1 ppm

log aCl-

0.5

o

AuHS

AuCl
-

-2
-8

-7

-6

5

-4

-3

log aH2S

-2

-1

0

FIG. 14. Predominance-field diagram for selected complexes of gold, showing the position of the Satanic Mills (SM) vent fluid. T = 250°C, pH = 3, P = saturated vapor
pressure. Contours are for gold solubility in parts per billion. Oxygen fugacity is fixedby a(H2S) = a(SO4)2– compatible with the common occurrence of gold in sulfide–sulfate
assemblages. Thermodynamic data as for Figure 13.

957 39#4-août-01-2256-02

974

26/10/01, 14:40

GOLD-RICH PRECIPITATES, EASTERN MANUS BASIN-20

ΣS = 0.007
P = SVP

1 ppb

py-hem10 ppb-30

log f(O2)

al. 1994). Precipitation of gold from such a hydrothermal fluid is mainly accomplished by a decrease ina(H2S) that may occur by mixing with seawater but, inshallow seafloor hydrothermal systems, may also result
from loss of H2S to a gas phase during boiling.

Σ

-25

0.1

-35

ppb

H2S - HSO4-

pb

p
10

pb

1p

-40

0.1

ACKNOWLEDGEMENTS

ppb

py po

-45
-50
200

(2.75)

250(2.88)T (oC)

300

350(3.14)(4.03)

FIG. 15. Gold solubility as the AuHS° complex as a functionof oxygen fugacity and temperature. Also shown areboundaries for selected iron sulfide and iron oxide minerals and the sulfate–sulfide boundary (heavy lines). The pH
is given in parentheses below each temperature and reflects
the decrease in pH accompanying conductive cooling of
the PACMANUS vent fluid. The total reduced sulfur andpressure are the same as in Figure 13, as are the thermodynamic data.

rich mineral assemblages. In Zn-rich assemblages,
grains of native gold occur predominantly as inclusionsin tennantite that occupies an intermediate paragenetic
position between early-formed chalcopyrite and later
cavity-filling sphalerite. Gold in Cu-rich assemblages
is present as micrometric grains included in massive
chalcopyrite. This gold is primary, and is not the resultof seafloor oxidation, as has been reported from the
TAG (Hannington et al. 1988) and Logatchev (Murphy& Meyer 1998) hydrothermal fields.
The neutral AuHS° complex is considered the mostimportant carrier of gold in the PACMANUS hydrothermal system. Such transport as a sulfide complex, rather
than a chloride complex, is consistent with the high activity of sulfur indicated by the mineral assemblages
associated with the gold mineralization, the coexistingiron-poor sphalerite and the Ag-poor nature of gold
grains. Because the AuHS° complex is more stable thanthe AuCl2– complex under all the conditions considered
here, gold is precipitated from the sulfide complex inboth Cu- and Zn-rich assemblages. Therefore, there isno need to rely on a low-temperature Au(HS)2– complex and a high-temperature AuCl2– complex for therespective Zn-rich and Cu-rich assemblages, as was proposed by Huston & Large (1989) for ancient massive
sulfide deposits. Indeed, for the conditions considered
here, Au(OH)° and not AuCl2– is the most stable complex at temperatures greater than 325°C. This situation
at Manus is probably common in all hydrothermal systems where fluids have a low pH, such as at Lau Basin(Fouquet et al. 1993) and Suiyo Seamount (Ishibashi et957 39#4-août-01-2256-02

975

975

We are grateful to Anthony E. Williams-Jones forsuggestions that improved an earlier version of thismanuscript. Referees Ian R. Jonasson and J. Bruce
Gemmell are thanked for their constructive comments
and suggestions, which led to further improvements inthe manuscript. The suggestions and careful editing ofRobert F. Martin are gratefully acknowledged. This
work was funded by a Natural Sciences and Engineering Research Council grant to SDS and by Ontario
Graduate Student Scholarships to RM.

REFERENCES
ANDERSON, G.M. & CRERAR, D.A. (1993): Thermodynamics inGeochemistry: the Equilibrium Model. Oxford University
Press, Oxford, U.K.
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978________, ________, ________, ________ & KOLOTOV, V.P.
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Received November 4, 2000, revised manuscript accepted
July 8, 2001.

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