©2014 Society of Economic Geologists, Inc.
Economic Geology, v. 109, pp. 1705–1733
High-Sulfidation Epithermal Pyrite-Hosted Au (Ag-Cu) Ore Formation
by Condensed Magmatic Vapors on Sangihe Island, Indonesia*
Julia King,1,† A.E. Williams-Jones,1 Vincent van Hinsberg,1 and Glyn Williams-Jones2
1Department
of Earth and Planetary Sciences, McGill University, 3450 University Street, Montréal, Québec, Canada H3A 0E8
2Department
of Earth Sciences, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia, Canada V5A 1S6
Abstract
Although gold in high-sulfidation epithermal deposits generally occurs as the native metal or electrum, in
some deposits, a significant proportion of the gold is hosted in pyrite. Here we use a combination of petrography, whole-rock geochemistry, pyrite chemistry, crystallography, and phase stability relationships to determine how gold was transported and incorporated into pyrite in two relatively young high-sulfidation epithermal
deposits, where the gold occurs almost exclusively in solid solution or as nanoparticles in pyrite.
The genetically related Bawone and Binebase Au (Cu-Ag) deposits, located 1 km apart on the volcanic island
of Sangihe, northeastern Indonesia, are hosted by andesitic volcaniclastic rocks that were altered to a proximal
advanced argillic association of quartz + pyrite (py I) + pyrophyllite + natroalunite + alunite + dickite + kaolinite and a more distal intermediate argillic association of quartz + pyrite (py I) + kaolinite + dickite + illite. The
economic mineralization takes the form of multiple generations of auriferous pyrite, the first of which, pyrite
I (py I), developed during advanced argillic alteration. Mass balance calculations show that all elements were
mobile with the exception of Nb, Ti, some rare earth elements, and possibly Al.
The highest gold concentration is in pyrite II (py II), which occurs in veins that cut pyrite I. This drusy variety of pyrite is characterized by complex growth and sector zoning, and contains as much as 6.0 wt % Cu. The
elevated Cu concentrations correlate positively with Au and As concentrations, whereas the Ag concentration
correlates strongly with Au but not Cu. Later barite-enargite mineralization exploited py II veins and vugs, and
significant concentrations of Ag and Au are hosted by enargite, although the Au concentration in enargite is
lower than in py II or py I.
A model is presented in which the fluid responsible for advanced argillic and intermediate argillic alteration
and associated stage 1 gold mineralization was a condensed magmatic vapor derived from an oxidized magma.
The gold and other metals were transported as hydrated species that ascended through the volcanic pile via
fractures and zones of enhanced permeability to a depth between 900 and 1300 m, where the vapor condensed
at a temperature between 250 and 340°C to form an acidic liquid with a pH of ~2.5; fO2 ranged up to four log
units above the hematite-magnetite buffer. Interaction of this liquid with the host andesites caused advanced
argillic and intermediate argillic alteration, including sulfidation of mafic minerals to form py I. During crystallization of py I, Au, Cu and Ag were adsorbed onto the surface of the pyrite and deposited as nanoparticles, or
were incorporated in the pyrite structure. Adsorption of Au, Cu, and Ag from the condensed vapor reached a
peak during the crystallization of vein-hosted py II, and the uptake of Ag and minor Au continued during later
crystallization of enargite. From the distribution of metals among growth and sector zones in py II, incorporation of gold and other metals appears to have been maximized when physicochemical conditions were relatively
stable. This is in contrast to the requirement for native gold precipitation, namely that physicochemical gradients be steep to ensure supersaturation of gold in the ore fluid.
Introduction
Although considerable progress has been made in understanding the formation of high-sulfidation epithermal precious metal deposits, opinion is still divided over the nature
of the ore fluid. There is general agreement that the characteristic residual (vuggy) silica and advanced argillic alteration are the result of interaction of rocks with the condensate
of highly acidic and oxidizing vapors (e.g., HCl, and H2S and
H2SO4 produced by reaction of H2O with SO2) from a proximal magma source (e.g., Hemley and Jones, 1964; Stoffregen
1987; Rye, 1993; Arribas 1995). However, until recently, most
researchers, noting that the ore minerals commonly fill vugs
and therefore postdate the alteration, have concluded that the
fluid responsible for the alteration did not transport the ore
† Corresponding
author: e-mail, julia.j.king@gmail.com
*A digital supplement to this paper, containing four Appendices with raw
data, is available at http://economicgeology.org/ and at http://econgeol.geoscienceworld.org/.
metals (Stoffregen, 1987; White and Hedenquist, 1990; Arribas, 1995; Hedenquist et al., 1998). Instead, these researchers
have attributed the mineralization to collapse of the vapordominated system and transport of the metals by a magmatichydrothermal liquid with a large meteoric water component.
The discovery that gold mineralization in the Pascua deposit,
Chile, was contemporaneous with advanced argillic alteration
(Chouinard et al., 2005a), indicates that for some high-sulfidation epithermal deposits (cf. Voudouris, 2010) the two-stage
hydrothermal model does not apply. This, and a combination of fluid inclusion (Heinrich et al., 1999; Landtwing et
al., 2010) and experimental evidence (Archibald et al., 2001,
2002; Williams-Jones et al., 2002; Zezin et al., 2011b; Migdisov
and Williams-Jones, 2013; Hurtig and Williams-Jones, 2014)
showing that gold, silver, and copper may be considerably
more soluble in aqueous vapors than previously suspected,
supports a model for these deposits in which hydrothermal
alteration and economic mineralization were both products
of a magmatic hydrothermal vapor (cf. Williams-Jones and
0361-0128/14/4246/1705-291705
Submitted: March 31, 2013
Accepted: December 9, 2013
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KING ET AL.
Heinrich, 2005; Mavrogenes et al., 2010; Berger and Henley,
2011; Henley and Berger, 2011; Scher et al., 2013).
Most studies of high-sulfidation epithermal systems have
focused on deposits in which the gold occurs as a discrete
mineral or minerals (e.g., native gold, electrum, and/or a telluride phase, such as calaverite; Kesler et al., 1981; Stoffregen,
1987; Moritz et al., 2004; Deditius et al., 2008). However, in
some high-sulfidation deposits, notably the Pascua deposit in
Chile, a high proportion of the gold is hosted in sulfide minerals, particularly pyrite (e.g., Chouinard et al., 2005b; Deditius
et al., 2009). Here, we report results of a study of two highsulfidation epithermal deposits, Bawone (9.5 Mt of ore with
an average grade of 1.32 g/t Au and 3.97 g/t Ag) and Binebase (17.9 Mt of ore with an average grade of 0.76 g/t Au and
18.7 g/t Ag), located on Sangihe Island, Indonesia, in which
virtually all the hypogene gold is hosted in pyrite. Most significantly, greater than 50% of this gold is in pyrite that forms part
of the early advanced argillic alteration mineral association;
the rest is contained mainly in pyrite veins that cut the altered
rocks. Thus, gold mineralization was both contemporaneous
with and postdated alteration. Based on this and other observations, we develop a model designed to explain the genesis of
the Bawone and Binebase deposits involving transport of the
metals in a highly acidic vapor and sorption of the gold onto
the surfaces of growing pyrite crystals. Given the early timing of the Au-Ag mineralization and the observation that it is
hosted almost exclusively by pyrite, we also propose that these
deposits are representatives of a subclass of high-sulfidation
epithermal precious metal deposits, in which the bulk of the
metal is hosted in pyrite and the ore fluid was a condensed
magmatic vapor.
Regional Geologic Setting
The Bawone and Binebase Au (Ag-Cu) deposits are located
in the southern part of Sangihe Island, the largest of the
islands in the 500 km long Sangihe arc, which runs northsouth from southern Mindanao, Philippines, to the north arm
of Sulawesi, and separates the Celebes and Molucca Seas
(Figs. 1, 2; Hall, 2000). The Sangihe arc formed as a result of
the westerly subduction of the Molucca Sea plate under the
Eurasian plate, whereas the facing Halmahera arc was produced by the easterly subduction of the Molucca Sea plate
under the Philippine Sea plate (Morrice et al., 1983; Morrice
and Gill, 1986; Garwin, 1990; Hall, 1996, 2000). Twenty-five
Quaternary stratovolcanoes are located along the length of the
arc; eight of these are active, including Awu at the northern
tip of Sangihe Island (Morrice and Gill, 1986; Fig. 1). Previous studies of volcanism along the Sangihe Arc have shown
that the arc is composed mainly of two-pyroxene and hornblende andesites. Calc-alkaline suites dominate and vary from
low to high K, depending on their distance from the volcanic
front (Morrice and Gill, 1986).
Sangihe Island has a lobate form defined by volcanic centers, the active stratovolcano, Awu, at the northern end and
the dormant/extinct stratovolcanoes, Tahuna and Kakiraeng,
and strongly weathered centers of Taware and Malisang to the
south (Fig. 1). The oldest part of the island is in the southeast
and the youngest in the northwest; new volcanoes are forming
off the northwestern tip and western shores of Sangihe Island
(Beaulieu, 2010). Given the heavy rainfall (greater than 3 m
per year), dense vegetation and inferred rapid uplift due to
the compressive tectonic regime, even the oldest volcanoes, in
the southern part of the island where the Bawone and Binebase deposits are located, may be only tens of thousands of
years in age.
The southern part of Sangihe Island is dominated by clinopyroxene andesite flows, breccias, lahars, and tuffs of the
Tamako Group, except in the southern and eastern parts
where rocks of the Taware Group, Malisang Group, Binebase Group, and Pinterang Formation are exposed (Fig. 2).
The Binebase deposit is 1 km to the north of Bawone, and
both deposits are hosted by rocks of the Binebase Group. At
Bawone, the Binebase Group is overlain unconformably by
the Pinterang Formation, which covers the deposit, whereas
at Binebase, the deposit crops out and is strongly oxidized to
a maximum depth of 70 m (Fig. 3). The Binebase Group has a
northeasterly strike, moderate to steep southeasterly dip, and
comprises andesitic ash and crystal tuffs, hornblende-pyroxene andesite flows, biotite-hornblende-magnetite diorite and
minor dacite to rhyolite flows, which are interpreted to be
volcanic and subvolcanic facies of the extinct and eroded
Taware volcano (Garwin, 1990). To the east of the Binebase
deposit, Binebase Group rocks are overlain by rocks of the
Tamako Group (consisting of hornblende andesite flows, sills,
and dikes), the proximal facies of the dormant and/or extinct
Kakiraeng volcano, and to the south by the Pinterang Formation. East of the Bawone deposit, the Pinterang Formation overlies the Tamako Group rocks unconformably. This
unit is thickest (<100 m) in topographic lows, and consists of
reworked cross-bedded volcanic silts and sands, carbonates
and organic-rich sediments, which record a marine incursion
in the southeast part of Sangihe Island (Garwin, 1990). The
Pinterang Formation also contains slightly rounded, unoxidized aggregates of pyrite fragments, similar to pyrite II (py
II) from the Bawone and Binebase hypogene ores. The Pinterang Formation and Tamako Group overlie the Malisang
Group, which in turn unconformably overlies the Binebase
Group in the southeast part of the island. Stegodon fossils
found in conglomeritic channels in the Pinterang Formation indicate that the unit is between 2 million years and
60,000 years in age (de Vos et al., 2007). The Malisang Group
consists of hornblende andesite flows, sills, dikes, and diorite
intrusions that form local highs and is related to the Malisang
volcanic center (Garwin, 1990).
Local Geologic Setting
Recent oxidation has enriched both Au and Ag, and consequently, much of the potentially economic mineralization
of the Binebase deposit and a part of the Bawone deposit are
supergene in origin. However, both deposits have appreciable
reserves of hypogene sulfide mineralization. The inferred
resources, using a 0.25 g/t Au cutoff, are summarized in Table
1.
The Bawone and Binebase deposits are hosted by andesites
and dacites of the Binebase Group. The Binebase deposit is
exposed at the erosional surface and has been oxidized up to
a depth of 70 m (Fig. 3B). The host andesite is a crystal-rich
tuff containing 10 to 15 vol % of plagioclase laths and round
quartz crystals (1–2 mm in length and diameter, respectively) and occasional lapilli, and lithic fragments (0.5–3 cm
HIGH-SULFIDATION EPITHERMAL PYRITE-HOSTED Au ORE FORMATION, SANGIHE ISLAND, INDONESIA
1707
3°40'0"N
Sangihe I.
Awu (1327m)
Kakiraeng (1006m)
Binebase (65m)
Bawone (86m)
Malisang (295m)
3°20'0"N
Taware (306m)
0
3°25'0"N
3°30'0"N
3°35'0"N
Tahuna (697m)
10
Km
125°30'0"E
125°40'0"E
Fig. 1. Inset: Location of Sangihe Island. Digitial elevation model of Sangihe Island showing the locations and elevations
of the active volcano, Awu, dormant and/or extinct volcanic centers (Tahuna, Kakiraeng, Malisang) and the Bawone and Binebase deposits. Bathymetry contours are at 50-m intervals.
in diameter). Rare breccia zones containing a wide variety of
rock fragments are likely epiclastic deposits.
At Bawone, a thick andesite unit overlies flow-banded plagioclase-quartz-phyric dacite (Figs. 3A, 4A). In least-altered
Table 1. Inferred Resources for the Bawone and
Binebase Deposits (Stone, 2010)
Deposit Tonnes
Bawone Oxide 3,475,000
Sulfide 5,999,000
Au (g/t) Ag (g/t)
1.66 9.16
1.12 0.97
Au (oz)
Ag (oz)
185,464 205,933
216,020 187,089
Binebase
Oxide 7,851,0001.10 25.13277,6616,343,299
Sulfide 10,002,000
0.49
13.60 157, 573 4,373,443
samples, the andesite contains 5 to 30 vol % white plagioclase laths (2 mm long) and rare, rounded quartz phenocrysts
(2 mm in diameter) in an aphanitic matrix. Conformable layers of welded crystal tuff are locally present in the andesite.
These contain 10 vol % of megascopic crystals of <3-mm-long
plagioclase laths and rare, rounded <3-mm-diameter quartz
grains. This unit is intruded by hypabyssal andesitic porphyry (Fig. 3A). The hypabyssal porphyry contains 20 vol %,
5 mm long, subhedral to euhedral hornblende, magnetite,
biotite and plagioclase phenocrysts, and rare quartz eyes in
an aphanitic matrix. An intrusive breccia with subangular
to subrounded, 1- to 10-cm-diameter fragments of the host
andesite and dacite generally defines the margins of the hypabyssal porphyry (Fig. 4B). At Bawone, the locally overlying
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KING ET AL.
125°40'0"E
125°35'0"E
Extinct Volcano
Intrusions
3°30'0"N
3°30'0"N
Quaternary Alluvium
Pinterang Formation
Tamako Group
Binebase (65 m)
Batunderang Group
Malisang Group
Bawone (86 m)
Binebase Group
Taware Group
Malisang (295 m)
3°25'0"N
3°25'0"N
Taware (306 m)
0
4
Km
125°35'0"E
125°40'0"E
Fig. 2. The geology of south Sangihe Island and the location of the Bawone and Binebase deposits, modified from Garwin
(1990).
A
Soil
Pinterang Formation
Oxide zone
Binebase Group
Andesitic crystal tuff
BID15
BID11
BID63
BID57
BID16
BID27
BID72
BOD1
B
BOD42
BOD3
BID69
BID56
BID60
BID66
Soil
Pinterang Formation
Oxide zone/weathered
Binebase Group
Porphyritic Intrusion
Binebase Group
Andesite porphyry
Andesitic tuff
Dacite
100 m
25 m
100 m
25 m
Fig. 3. Representative lithological cross sections (A) through the Binebase deposit (NW-SE) and (B) through the Bawone
deposit (SW-NE) based on drill holes. The black lines show the locations of the drill holes. Most primary features have been
destroyed in the oxide zone but the original nature of the rock is assumed to have been the same as that of the underlying
bedrock.
HIGH-SULFIDATION EPITHERMAL PYRITE-HOSTED Au ORE FORMATION, SANGIHE ISLAND, INDONESIA
A
1709
B
Mag
Pl
Bt
Hbl
D
D
C
Kao + Qtz + Al
Py I
Py I + Qtz
Py II
Fig. 4. Photographs of the host rocks of the Bawone and Binebase deposits (cut slabs and core). (A) Argillically altered
flow-banded dacite, the basal unit of the Binebase Group at Bawone, showing phenocrysts and a matrix replaced by kaolinite
and quartz. (B) Plagioclase (Pl)-biotite (Bt), hornblende (Hbl)-magnetite (Mag) andesite porphyry. (C) Vein of py II (brassy)
with fragments of py I (black) in a matrix of vuggy py I with py II occasionally lining vugs. (D) Advanced argillic (alunite =
Al, pyrite = Py, quartz = Qtz) alteration of fragments (light gray) in a matrix of fine-grained quartz and pyrite (dark gray). The
scale bar is 1 cm.
Pinterang Formation (poorly consolidated volcaniclastic sandstones and siltstones, organic-rich siltstones, and calcareous
mudstones) ranges up to 100 m thick and is separated from
the Binebase Group rocks by a thin (several-cm) oxide layer,
possibly a paleosol. The Pinterang Formation is overlain by an
oxidized soil that is up to 4 m thick.
Hydrothermal Alteration
In hand sample, the altered rocks vary from dark gray to
white in color, depending on the pyrite content, and from very
hard (quartz dominated; Fig. 4C) to powdery (clay dominated;
Fig. 4D) in drill core; the latter variation commonly occurs
over intervals of tens of centimeters to tens of meters. Alteration intensity is variable. In some samples, the primary minerals have been completely replaced by secondary minerals,
destroying all primary textures, whereas in other samples, volcanic textures are preserved (Figs. 4D, 5A). The pervasively
altered samples comprise very fine-grained, clay particles
intergrown with fine-grained, generally anhedral, equigranular quartz and pyrite (Fig. 5A, B, D). Less intensely altered
samples commonly retain porphyritic textures, although the
phenocrysts have generally been replaced by kaolinite or
dickite. The most intensely altered samples consist of a finegrained mosaic of intergrown anhedral quartz, clay minerals
(kaolinite, dickite, pyrophyllite), sulfate minerals (alunite,
natroalunite) and pyrite. Hereafter, the fine-grained, anhedral pyrite intergrown with clay minerals and quartz will be
referred to as pyrite I (py I).
Alteration created secondary porosity in the form of mmscale vugs (formed by the dissolution of phenocrysts) and
irregular, cm-scale cavities. The latter are commonly infilled
by alunite, and/or pyrophyllite, and/or drusy pyrite, and/or
chalcocite, and/or barite crystals (Figs. 4C, 5C, D).
Alteration is most intense (few primary textures preserved)
along subvertical fluid conduits that fan out horizontally at
lithologic contacts (Fig. 6A). Distal to the mineralized zones,
alteration is generally less intense, primary volcanic textures
and minerals are preserved, and the rock is more competent.
The matrix of the rock is preferentially altered and has a grey
to light-brown color.
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KING ET AL.
A
B
Qtz
Py I
Al
Py I
C
D
Py II
Py II
Cc
Brt
E
Py I + Qtz
F
Py II
Sp
El
Ccp
Apy
En
Brt
Brt
Fig. 5. Photomicrographs showing the textures of the altered and mineralized rocks in reflected light (10 μm scale bar).
(A) Amphibole phenocrysts replaced by py I in a groundmass replaced by fine-grained quartz, py I, kaolinite ± dickite.
(B) Fine-grained association of quartz, alunite and py I, typical of argillic alteration. (C) Massive py II and a vug containing
rare chalcocite (Cc) and barite (Brt) crystals. (D) Growth zoned drusy crystals of py II infilling a vug in andesite altered to
fine-grained py I and quartz (blue-grey is epoxy). (E) Fractured and zoned crystals of massive py II cut by a barite-enargite
(En) vein. (F) Sphalerite (Sp), arsenopyrite (Apy), chalcopyrite (Ccp) and electrum (El) crystals in a coarse-grained barite
matrix. The earliest stage of gold-silver mineralization is manifest by fine-grained py I associated with equally fine-grained
quartz and clay minerals (A, B). This was followed by a second generation of Au-Ag mineralization in the form of coarsegrained py II (C, D, E). The third generation of Au-Ag mineralization is represented by barite-enargite (En) veins that commonly contain brecciated py II crystals (E). Rare cm-scale veins of barite containing electrum, sphalerite, arsenopyrite and
chalcopyrite that cut py I and py II mineralization constitute a fourth generation of Au-Ag mineralization observed only in
the Binebase deposit.
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HIGH-SULFIDATION EPITHERMAL PYRITE-HOSTED Au ORE FORMATION, SANGIHE ISLAND, INDONESIA
A
Supergene zone
IA I (qtz + py + kao ± dck)
IA II (qtz + ill ± py)
BID 63
BID 15
BID 11
BID 16
BID 57
BID 27
BID 72
BID 69
BID 56
BID 60
Overburden and
Pinterang Fm.
AA I (qtz+py+prl±nal)
AA II (qtz+py±nal±dck±kao)
AA III (qtz+py+kao±al)
IA I (qtz+kao±py)
BOD 1
B
BOD 42
BOD 3
BID 66
100 m
25 m
100 m
25 m
Fig. 6. Representative cross section (A) NW-SE through the Binebase (BID) deposit and (B) SW-NE through the Bawone
(BOD) deposit showing the distribution of hypogene alteration facies established using X-ray diffraction analysis and core logging. The locations of drill holes are indicated by the black lines. Abbreviations: alunite = al, dickite = dck, illite = ill, kaolinite
= kao, natroalunite = nal, pyrite = py, pyrophyllite = prl, quartz = qtz.
Two main alteration facies have been recognized, advanced
argillic and intermediate argillic, based on the mineral associations of quartz + pyrite + pyrophyllite + natroalunite +
alunite + dickite + kaolinite and quartz + pyrite + kaolinite
+ dickite + illite, respectively. Quartz and pyrite are nearly
ubiquitous alteration phases, but the proportions of these
minerals and those of the accompanying phases, which were
used to define the alteration facies, vary significantly. In principle, the advanced argillic and intermediate argillic facies
can be subdivided into subfacies with fewer minerals that
reflect different physicochemical conditions. For example,
at quartz saturation (the case here), pyrophyllite is stable
at higher temperature than dickite, and dickite is stable at
higher temperature than kaolinite (Hemley et al., 1969; Stoffregen and Alpers, 1987; Stoffregen and Cygan, 1990). Similarly, natroalunite is stable at higher temperature than alunite
provided that the K/Na ratio of the fluid is relatively constant (Stoffregen and Cygan, 1990). Finally, illite is stable at
higher pH than kaolinite, dickite, and pyrophyllite. In view of
these relationships, mapping, X-ray diffraction and infrared
spectroscopy (TerraSpec) evidence were used to subdivide
the advanced argillic association into a high temperature,
lower pH subfacies (advanced argillic I) of quartz + pyrite
+ pyrophyllite ± natroalunite, an intermediate temperature,
low pH, subfacies (advanced argillic II) of quartz + pyrite
± natroalunite ± dickite ± kaolinite, and a low temperature,
low pH subfacies (advanced argillic III) of quartz + pyrite +
kaolinite + alunite (see Discussion; Temperature and pH) .
The intermediate argillic association was subdivided into a
low pH subfacies (intermediate argillic I) of quartz + kaolinite ± pyrite ± dickite and a higher pH subfacies of quartz +
illite ± pyrite (intermediate argillic II).
The alteration facies are zoned, both vertically and horizontally, and the intensity of alteration decreases with depth and
laterally away from apparent fluid conduits. At Bawone, the
upper part of the deposit is dominated by advanced argillic
alteration. However, at Binebase the upper part of the deposit
is intensely oxidized due to weathering and supergene processes, and advanced argillic alteration is not observed (Fig.
6B). The Bawone deposit was protected from the effects of
weathering by the overlying Pinterang Formation (Figs. 3A,
6A). Advanced argillic alteration in the Bawone deposit terminates abruptly at the contact with the overlying Pinterang
Formation, although a thin (cm-scale) oxide layer is present
at this contact. This layer indicates that the uppermost part
of the deposit was previously oxidized, and perhaps partially
removed by erosion, prior to deposition of the Pinterang
Formation.
Advanced argillic alteration at Bawone forms subhorizontal zones just below the Pinterang Formation and about 75 m
lower stratigraphically, at the contact with a zone of andesitic
tuff (Figs. 3A, 6A). Both zones appear to flare outward from
a steeply inclined advanced argillic alteration zone that likely
represents the conduit for the altering fluids. The upper parts
of the subhorizontal alteration zones and the central part of
the inclined alteration zone are dominated by the high temperature, advanced argillic I alteration subfacies. Below the
subhorizontal advanced argillic I alteration zones and adjacent
to the inclined advanced argillic I alteration zone, this subfacies changes to advanced argillic II alteration and distally to
intermediate argillic I alteration. Low temperature advanced
argillic III alteration is present only at depth and locally may
develop directly below advanced argillic I alteration without
intervening advanced argillic II alteration (Fig. 6A). Given its
distribution, the advanced argillic III alteration may be related
to the unmineralized hypabyssal porphyry intrusion (Fig. 3A).
At Binebase, the zone of supergene oxidation passes directly
into intermediate argillic alteration. The latter comprises a
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KING ET AL.
blanket of intermediate argillic I alteration with several subvertical roots that pass downward and laterally into intermediate argillic II alteration (Fig. 6B).
Hypogene Mineralization
Neither free gold nor electrum is observed in the hypogene zone of the Bawone deposit and very rarely in the Binebase deposit. Instead, the hypogene gold in both deposits is
“invisible,” i.e., hosted almost exclusively by pyrite. Silver is
also invisible, hosted by both pyrite and enargite, and copper occurs mainly in pyrite and enargite (volumetrically much
less abundant than pyrite), as well as rare grains of chalcocite
in vugs (Fig. 5C). Gold (-silver) mineralization occurred in
three stages: (1) an early stage of disseminated Au-Ag–bearing pyrite during advanced argillic and intermediate argillic
(minor) alteration (pyrite I; Figs. 5A, B, D, 7B), (2) an intermediate stage characterized by multiple generations of lenses,
veins and breccias containing subhedral to euhedral, drusy,
and massive to semimassive Au-Ag-Cu-bearing pyrite II (py
II; Figs. 5C-E, 7A, C), and (3) late-stage barite-enargitepyrite II veins (Figs. 5C, E, 7C). At Binebase, there is a fourth
A
stage in the form of rare barite, base-metal sulfide, and electrum veins (Figs. 5F, 7D).
The earliest gold occurs in py I, which is an ubiquitous
alteration product intergrown with quartz and hydrous alumina-rich phases and occurs as small (2–30 μm diameter),
isotropic, anhedral grains, accounting for 2 to 20 vol % of the
rock; the gold grade corresponds to the proportion of pyrite
in the rock. As part of the advanced argillic alteration and to
a much lesser extent the intermediate argillic alteration, this
stage of mineralization was focused along vertical structures
and spread out laterally near the top of the deposit where
fluids were confined by less-permeable strata or paleo-water
tables. At the Bawone deposit, advanced argillic I alteration is
also concentrated in a subhorizontal zone below an impermeable andesitic tuff about 75 m below the base of the Pinterang
formation (Figs. 3A, 6A).
The second stage of gold mineralization is characterized
by veins, veinlets, blebs, breccias, lenses, and massive bodies
of coarse-grained, brassy py II, which overprinted advanced
argillic alteration and intermediate argillic I alteration,
exploited the secondary porosity created during alteration,
B
Qtz + Al
Py II
Py I
Brt
C
D
Brt
Py II
En
Brt
Ccp
El
Fig. 7. BSE images of auriferous pyrite, py I and py II, and later generations of mineralization. The scale bar is 100 μm
unless otherwise indicated. (A) Fragment of drusy py II exhibiting strong growth zoning in a barite (Brt) vein. (B) Disseminated PyI in a matrix of quartz (Qtz) and alunite (Al). (C) Barite-enargite (En) vein with μm-scale, homogeneous subhedraleuhedral enargite crystals intergrown with mm-scale subhedral barite surrounding fragments of py II. (D) Electrum (El)
grain surrounded by subhedral chalcopyrite (Ccp), barite (Brt), and sphalerite (Sp) (10 μm scale bar).
1713
HIGH-SULFIDATION EPITHERMAL PYRITE-HOSTED Au ORE FORMATION, SANGIHE ISLAND, INDONESIA
Ag Cu
As
Sb
40
60
250
50
1000
5000
3000
2000
200
80
1000
Distribution of gold-silver mineralization
Gold and silver grades in the Bawone deposit are highest
just below the contact with the Pinterang Formation, decrease
with depth and then increase again just below a thin tuffaceous andesite unit in a zone of advanced argillic I alteration
(Figs. 3A, 6A, 8). Metal concentrations correlate with the proportion of py II, pyrite breccias and/or barite-enargite veins. A
similar geometry is inferred for the hypogene mineralization
at Binebase. However, because the upper part of the Binebase deposit was subjected to supergene oxidation, we infer
Au
20
10
and crosscut fine-grained py I. As is the case with py I, the gold
grade is controlled by the abundance of py II, which forms 3
to 90 vol % of the rock, although py II is variably enriched
in gold. In thin section, py II is coarse grained (0.1–2 mm
long), massive or net-textured, and ranges in morphology
from anhedral to euhedral (Figs. 5A, D, 7A, C). Botryoidal
and drusy textures also are common, and many crystals are
complexly zoned (Fig. 7A). In reflected light, some pyrite II
crystals have an anhedral blue anisotropic core, which chemically is indistinguishable from the rest of the crystal (Fig. 5E).
Rare, isolated anhedral chalcocite crystals (3 mm in diameter)
are observed locally in vugs rimmed by drusy py II (Fig. 5C).
Growth zoning of py II crystals on the μm-scale is common,
particularly in drusy crystals, which show a relatively stable
growth history recorded by continuous growth surfaces. It is
also observed in massive pyrite composed of crystals that grew
separately and were later annealed, resorbed, brecciated and
overgrown (Fig. 7A).
Pyrite I and II are commonly crosscut by barite-enargite
veins (0.2–5 cm wide). These veins appear to have re-opened
earlier py II veins, as they are commonly lined by fractured
py II crystals, and py II commonly occurs as brecciated fragments in the veins, indicating that they were incorporated
mechanically during barite-enargite vein formation (Figs. 5E,
7C). In the centers of these veins, intact, blue-purple, isotropic subhedral to euhedral (10–60 μm long) rectangular or
bladed enargite crystals and fractured py II fragments are suspended in a matrix of coarse-grained, subhedral to drusy barite crystals. Texturally, the enargite is homogeneous, unzoned,
and appears to have coprecipitated with barite (Figs. 5E, 7C).
Barite and enargite also commonly occur in vugs. The density of barite-enargite veins (the proportion of veins relative to
host-rock) is variable but generally correlates with that of py
II veins, suggesting that both vein stages exploited the same
structures.
Although electrum is volumetrically insignificant, the gold
grade of these rare base-metal sulfide-electrum-barite veins is
20 times higher than that of the auriferous-pyrite dominated
zones of the deposit. In the Binebase deposit, these veins
range from 0.5 to 8 cm in width and are observed in the center
of the hypogene mineralization. This rare vein association was
only observed at Binebase, where it crosscuts all other mineralization stages, except for the enargite-barite veins. The
relative timing of these two late vein generations therefore
cannot be established. Mineralogically, the base metal sulfideelectrum-barite veins comprise small subhedral to euhedral
crystals of spalerite, arsenopyrite, chalcopyrite, galena, pyrite,
electrum and Sb-sulfides that occur as aggregates in a coarsegrained matrix of euhedral barite crystals (Figs. 5F, 7D).
Depth (m)
Concentration (ppm)
Fig. 8. The concentrations of Au, Ag, Cu, As, and Sb (ppm) in a drill hole
from the Bawone deposit (BOD1); Pb and Zn concentrations were below the
detection limit and Mo and Ba were not analyzed.
that any high-grade hypogene mineralization in this deposit
was at the level of the supergene zone. This interpretation is
consistent with the observation that the remaining hypogene
mineralization has a much lower concentration of gold and
silver than the Bawone deposit (Table 1).
Assay data for 1m intervals of drill core for Bawone and
Binebase, made available by East Asia Minerals Corp., show
that there is a strong spatial correlation between Au and Ag
concentrations, and also of As with Cu, and a weaker correlation of Cu and As with Sb (and Pb and Zn, not shown) concentrations (Fig. 8). Molybdenum concentrations are below the
2 ppm detection limit. The distribution of Au, Ag, As, and Cu
is vertically zoned, with the highest concentrations of these
elements occurring at the top of the hypogene zone. The
highest Au value coincides with high Cu, As, and Sb values
reflecting the presence of barite-enargite veins. This relationship, however, does not hold for the second highest Au value
(Fig. 8). Indeed, most of the higher Cu, As, and Sb values are
accompanied by only minor to negligible enrichment in Au.
We interpret these metal distributions to indicate that bariteenargite veins contributed only minor amounts of gold and
locally re-opened veins of py II to produce the coincident Au,
Cu, As, and Sb peaks.
Mass Changes During Alteration
Fifty-four drill core samples of the different lithological
units and alteration types were analyzed for major, trace, and
rare earth elements (REE) by inductively coupled plasmamass spectrometry (ICP-MS), and for gold by instrumental
neutron activation analysis (INAA), by Actlabs in Vancouver
(Table 2). The compositions of the altered rocks were compared to those of the least-altered rocks to evaluate the gains
and losses of elements during alteration. In order to assess
these mass changes, it was first necessary to identify potentially immobile elements that could be used to normalize
1714
KING ET AL.
Table 2. Major Element Compositions from Drill Core and Hand
d.l.
IA I
IA I
IA I
IA I
IA I
IA I
IA I/BaEn
IA I/BaEn
BOD1-134.5BOD3-158.8BOD3-85.4 BOD3-74.1BOD3-106.4BOD3-83.3 BOD3-99.2 BOD3-96.8
SiO2
(%)0.01 55.4 64.68 8.2 58.66 51.41 48.19 19.69 85.78
TiO2
(%)
0.001 0.5460.5470.0360.5280.4840.5410.1850.501
Al2O3
(%)
0.0119.814.360.339.132.912.850.3 2.34
Fe2O3
(%)0.01 4.26 8.38 35.08 13.45 27.98 30.09 40.04 6.14
MnO
(%)0.001 0.008 0.009 0.005 0.006 0.007 0.006 0.006 0.006
MgO
(%)
0.01 0.750.020.020.010.020.020.020.02
CaO
(%)
0.01 0.78 0.03 0.04 0.06 0.05 0.04 0.03 0.06
Na2O
(%)
0.01 0.35 0.05 0.06 0.56 0.06 0.07 0.04 0.04
K2O
(%)
0.01 2.17 0.08 0.02 0.69 0.02 0.04 0.05 0.04
P2O5
(%)
0.01 0.1 0.08 b.d. 0.06 0.04 0.05 b.d. 0.08
Total
(%)0.01 98.53100.1 66.75 99.83100.3 100.3 82.61 99.49
LOI
(%)
14.36 11.82 23.02 16.69 17.33 18.38 22.25 4.49
La
(ppm)
0.05 15.8 8.9111
Ce
(ppm)
0.05 32.1 16.5 7.29
Pr
(ppm)
0.01 3.83 1.94 0.49
Nd
(ppm)
0.05 14
8
1.21
Sm
(ppm)
0.01 2.66 1.64 0.34
Eu
(ppm)
0.005 0.791 0.596 b.d.
Gd
(ppm)
0.01 2.18 1.2 1.51
Tb
(ppm)
0.01 0.36 0.14 0.04
Dy
(ppm)
0.01 2.19 0.72 0.09
Ho
(ppm)
0.01 0.45 0.15 0.02
Er
(ppm)
0.01 1.41 0.52 0.09
Tm
(ppm)
0.005 0.237 0.103 0.014
Y
(ppm)
0.5 12.8 4 1.3
Yb
(ppm)
0.01 1.78 0.72 0.08
Lu
(ppm)
0.002 0.31 0.119 0.012
5.52
8.67
0.96
3.95
0.81
0.197
0.61
0.13
0.76
0.19
0.75
0.157
5.6
1.02
0.173
5.38 6.03
8.7710
1.17 1.33
4.15 4.67
0.78 0.84
0.155 0.185
0.57 0.58
0.08 0.09
0.42 0.44
0.09 0.08
0.43 0.39
0.08 0.067
3 2.8
0.61 0.52
0.101 0.091
8.3813.9
8.0326
0.79 2.97
2.5512.1
0.61 2.31
0.472 0.649
1.02 1.49
0.1 0.16
0.28 0.89
0.07 0.16
0.23 0.73
0.042 0.146
2.5 5.2
0.31 1.03
0.053 0.174
Ag
(ppm)
0.5 b.d. 0.5 11.5 b.d. 0.8 0.7 3.9 b.d.
As
(ppm)
5 487 64
1,350124 66 22
1,180 19
Au
(ppb)
2
b.d.
3061,500 7511,1101,1902,000 723
Ba
(ppm)
3
327 129
178,900 6,822 3,050 4,09498,830 212
Bi
(ppm)
0.1 b.d.8.426.7 0.6 1 1.4 3.6 5
Co
(ppm)
1 13 10 47 23108112104 18
Cr
(ppm)
20
b.d. b.d. b.d. b.d.30 30 60 b.d.
Cs
(ppm)
0.1 0.5 b.d. 0.2 b.d. 0.2 b.d. 0.3 b.d.
Cu
(ppm)
10
40 170 4,550 1,780 490 2,590 4,540 270
Ga
(ppm)
1 18 28 4 7 10 7 3 8
Ge
(ppm)
0.5 1.8 2.8 0.6 b.d. b.d. b.d. 0.5 1.2
Hf
(ppm)
0.1 2.2 1.8 0.2 2.4 2 1.6 0.8 3.1
In
(ppm)
0.1 b.d. b.d. 0.4 b.d. b.d. b.d. 0.2 0.1
Mo
(ppm)
2 b.d.
528 412 61413
Nb
(ppm)
0.2 6.42.21.12.72.62.71.32.3
Ni
(ppm)
20
b.d. b.d. b.d. b.d.30 30 40 b.d.
Pb
(ppm)
5 8 49174 12 38 25 57 18
Rb
(ppm)
1 36 b.d. b.d. 1 3 b.d. 2 b.d.
Sb
(ppm)
0.2 b.d.2.8
112 10 2.5 b.d.70.5 4.8
Sc
(ppm)
1 11 13 b.d.9 7 5 3 10
Sn
(ppm)
1
b.d. b.d.15 b.d. 1 b.d. 5 b.d.
Sr
(ppm)
2 248 7021,484 477 298 310 772 515
Ta
(ppm)
0.01 0.35 0.18 0.06 0.18 0.2 0.15 0.08 0.21
Th
(ppm)
0.05 2.79 1.53 0.5 1.51 1.19 1.96 0.5 2.05
Tl
(ppm)
0.05 2.28 1.34 4.56 0.68 1.31 0.48 0.16 0.36
U
(ppm)
0.01 1.02 0.59 0.14 0.47 0.87 0.55 0.38 1.68
V
(ppm)
5 128158 8 80 46 39 11 33
W
(ppm)
0.5 b.d.4.119.2 4 4.4 9.9 2.6 6.4
Zn
(ppm)
301,060 140 b.d.
1,030 360 100 b.d.
320
Zr
(ppm)
1 81 78 6 86 75 58 30144
HIGH-SULFIDATION EPITHERMAL PYRITE-HOSTED Au ORE FORMATION, SANGIHE ISLAND, INDONESIA
1715
Samples from the Bawone (BOD) and Binebase (BID) Deposits
Porphyry
AA I
AA I
AA I
AA I
AA I
AA I
AA I/BaEn
AA II
AA II
AA II
AA II
BOD3-49.8 BOD3-55 BOD3-85.5BOD3-48.6BOD3-100BOD3-78.7BOD3-113.7
BOD42-62.3
BOD3-125.4BOD3-46.9BOD3-34.4
56.2943.2842.6548.8 29.11 8.7463.8449.7951.3753.6520.65
0.53 0.3730.4920.4920.3770.22 0.43 0.46 0.4740.6030.684
13.226.173.28
10.380.261.257.59
10.59
11.21
12.29
19.21
8.2315.1732.3817.7646.6459.4410.0616.7312.0612.7230.85
0.0090.01 0.0070.0070.0090.0140.0050.01 0.0030.0080.012
0.010.010.02b.d.0.020.030.010.02b.d.0.020.03
0.080.050.040.060.040.050.060.060.070.120.07
0.760.420.150.480.050.060.840.380.920.710.17
1.350.470.150.710.030.020.810.361.210.710.18
0.120.040.040.1 0.020.020.080.120.080.2 0.22
99.5884.1 99.8299.86100.1100.8 99.1597.6697.92100 99.21
18.9818.1120.6121.0623.5831.0115.4219.1320.5219 27.12
8.799.014.957.792.932.586.538.767.6210.8 8.29
16.314.8 8.4214 4.483.3611.517 13.920.515.5
1.921.961.141.620.570.411.632.081.612.432.06
7.947.173.916.712 1.355.768.386.7910.4 7.02
1.491.560.751.360.370.261.131.8 1.432.331.25
0.4420.3490.1760.3770.0860.0580.3180.4530.4250.6890.319
1.131.550.530.980.270.2 0.821.231.152.150.87
0.180.160.070.160.040.030.140.150.150.310.13
1.040.690.390.820.260.160.750.770.851.690.71
0.250.150.080.190.080.030.150.160.170.360.16
0.860.560.350.730.3 0.140.660.560.621.210.59
0.1740.1010.0650.1480.0570.0260.1340.1040.1220.2370.115
6.65.22.55.52.11.14.54 5.210.44
1.160.780.451.040.460.170.9 0.840.841.640.83
0.1890.13 0.0850.1770.0750.0290.1540.1620.1370.2740.138
b.d.0.6 b.d.b.d.2.3 2.2 0.6 1 b.d.b.d.1.9
38314142016736 8742856141
166666
1,470580
1,710
1,690287487300396
1,390
77995,9701,9901,3441,473 511 3701,865 6881,4581,736
b.d.b.d.0.3b.d.5.61.70.13.21.10.20.7
10221151015718825 9281228
20 50 b.d.b.d.20 20 30 b.d.b.d.b.d.30
b.d.0.1 b.d.b.d.0.2 b.d.b.d.0.3 b.d.b.d.0.2
2301,5501,4501,0102,8702,010 490 410 350 7101,460
1819 732 1 31110104858
0.8 b.d.b.d.b.d.b.d.b.d.b.d.0.9 0.6 b.d.b.d.
2.51.91.52 2.40.62.22 1.72.83.3
b.d.0.2 b.d.b.d.0.1 b.d.0.1 b.d.0.2 b.d.b.d.
b.d.
34b.d.
212932249
2.62 2.62.21.91 2.12.12 3 2.9
b.d.b.d.30 b.d.40 60 b.d.b.d.b.d.b.d.30
23 b.d.
4762841514354143046
b.d.2 1 2 b.d.1 b.d.2 b.d.1 1
1 6.5b.d.b.d.
11.59.30.82.11.31.98.7
13 8 613 5 2 811101328
b.d.b.d.b.d.b.d.2 b.d.b.d.5 1 b.d.4
669564310592 60 83455641532931
1,717
0.2 0.140.140.120.140.060.140.150.150.190.21
1.611.3 1.081.380.720.311.051.241.251.782.41
0.720.330.754.030.610.820.6 2.7 1.152.524.41
0.510.450.790.680.770.350.560.570.540.631.25
132734214821328894109174259
3.31.620.30.81.76.146 0.56.47.92.2
430260360460 80 50270120500
1,070290
9568597087186965759899
1716
KING ET AL.
Table 2.
Porphyry
AA II
AA II
AA II
AA II
BOD3-27.3 BOD3-79.5 BOD3-132.9 BOD3-41
AA II
AA II/BaEn
AA III
AA III
AA III
BOD3-65.4 BOD3-65.8 BOD1-115 BOD1-124.9 BOD3-122.6
SiO2
(%) 7.08 65.52 59.09 39.59 56.91 17.64 59.72 61.62 39.9
TiO2
(%) 1.029 0.485 0.624 0.567 0.431 0.082 0.61 0.773 0.298
Al2O3
(%)24.05 6.24 16.11 3.97 5.67 0.81 16.49 16.96 3.6
Fe2O3
(%)
19.8614.38 7.8832.6418.1731.27 7.98 5.7332.09
MnO
(%) 0.005 0.006 0.016 0.017 0.005 0.003 0.014 0.022 0.006
MgO
(%)0.03 b.d. 0.02 0.02 0.02 0.02 0.02 0.03 0.02
CaO
(%)0.12 0.05 0.08 0.07 0.08 0.04 0.11 0.1 0.04
Na2O
(%)1.22 0.05 0.23 0.16 0.43 0.06 0.07 0.14 0.31
K2O
(%)3.49 0.01 0.17 0.21 0.42 b.d. 0.03 0.03 0.29
P2O5
(%)0.43 0.1 0.18 0.13 0.11 b.d. 0.21 0.11 0.04
Total
(%)98.86 98.17 98.46 99.66100
77.25 98.39 99.98 98.21
LOI
(%)41.55 11.31 14.06 22.28 17.79 22.94 13.13 14.47 21.62
La
(ppm)
16.8 8.9711.4 10.4 8.48 6.8412.1 9.83 3.87
Ce
(ppm)
31.1 15.8 22.1 19.4 15.4 5.7624.7 17.9 5.81
Pr
(ppm)
3.44 2.12 2.59 2.68 2.08 0.53 2.84 2.1 0.71
Nd
(ppm)
13.5 7.4811.2 9.94 7.45 1.4812
8.17 2.53
Sm
(ppm)
2.4 1.34 2.45 2.19 1.4 0.35 2.44 1.56 0.53
Eu
(ppm)0.496 0.368 0.854 0.68 0.415 0.18 0.616 0.784 0.181
Gd
(ppm)
1.53 0.88 1.96 1.77 1
0.76 1.59 1.19 0.49
Tb
(ppm)
0.19 0.12 0.24 0.22 0.15 0.05 0.2 0.2 0.06
Dy
(ppm)
0.95 0.65 1.23 1.14 0.77 0.17 1.04 1.25 0.28
Ho
(ppm)
0.2 0.13 0.26 0.22 0.18 0.03 0.23 0.28 0.06
Er
(ppm)
0.71 0.6 0.99 0.83 0.74 0.13 0.78 0.99 0.24
Tm
(ppm)0.14 0.123 0.184 0.144 0.151 0.022 0.142 0.18 0.044
Y
(ppm)
5.34.26.96.65.11.76.27.11.8
Yb
(ppm)
1.06 0.9 1.35 1.02 1.07 0.16 1.17 1.42 0.31
Lu
(ppm)0.183 0.15 0.218 0.164 0.174 0.028 0.23 0.28 0.055
Ag
(ppm)
3.2 b.d. b.d. 1.6 2.8 3.7 b.d. 1.1 4.5
As
(ppm)
55106104442311
2,500 60125242
Au
(ppm)
1,240717345
1,310
1,080
1,280273130
1,760
Ba
(ppm)
1,5462,3541,5172,5194,145
106,900 8021,609
13,280
Bi
(ppm)
1.2 b.d. 1.5 0.7 0.3 0.3 0.4 1 0.6
Co
(ppm)
17 53 11 25 26 54 8 10111
Cr
(ppm)
403020303030302030
Cs
(ppm)
0.1 b.d.b.d.b.d.b.d.b.d.b.d.0.3 0.3
Cu
(ppm)
640
1,440180
3,500
2,720
15,000 70 90
2,470
Ga
(ppm)
184 13 18 24 19 6 13 19 8
Ge
(ppm)
b.d.1.7 0.7 0.6 1.2 1.2 3.7 2.1 0.6
Hf
(ppm)
4 3.6 2.1 2.1 2.2 0.3 2.4 2.8 1.1
In
(ppm)
0.2 b.d. b.d. b.d. 0.1 0.2 b.d. b.d. 0.1
Mo
(ppm)
12181511 521584938
Nb
(ppm)
4.2 2.4 2.6 2.2 2.2 0.6 4.2 2.9 1.5
Ni
(ppm)
20 20 b.d.30 b.d.20 b.d. b.d.50
Pb
(ppm)
149 b.d.
17 11181 b.d.
74120 28
Rb
(ppm)
2 b.d.b.d.1 1 1 b.d.b.d.2
Sb
(ppm)
8.834.4 2.135.624.6120 4.510.427.7
Sc
(ppm)
35 10 14 10 9 2 14 23 5
Sn
(ppm)
53b.d.
646423
Sr
(ppm)
2,840682
1,060655571
1,109685295411
Ta
(ppm)
0.3 0.21 0.2 0.17 0.16 0.04 0.19 0.2 0.1
Th
(ppm)
3.8 1.66 1.85 1.52 1.33 0.32 1.83 1.93 0.52
Tl
(ppm)
3.81 0.93 1.53 0.49 4.59 0.07 1.21 3.13 0.4
U
(ppm)
5.45 1.91 1.45 0.84 0.51 0.25 0.6712.6 0.4
V
(ppm)
410 69 157 91 59 17 120 199 36
W
(ppm)
5.3 2.1 0.8 3.412.9 1.6 b.d.b.d.2.5
Zn
(ppm)
310 40 120 70 300 190 360 960 b.d.
Zr
(ppm)
141109 95 55 73 9 89101 36
Note: The sample name indicates the drill hole number and the depth; samples are organized by lithology (porphyry or crystal tuff) and alteration-type
(IA = intermediate argillic, BaEn = barite-enargite veins, AA = advanced argillic; see text for description), identified based on petrographic observations,
bulk rock geochemistry and/or X-ray diffraction analyses
HIGH-SULFIDATION EPITHERMAL PYRITE-HOSTED Au ORE FORMATION, SANGIHE ISLAND, INDONESIA
1717
(Cont.)
Crystal Tuff
Least altered
IA I
IA I
IA I
IA II
IA II
IA II
IA II
IA II
IA II
IA II
PANTAI BID70-61.7 BID24-101 BID39-41.2 BID43-39.5BID26-102.5BID15-84.2BID18-144.5BID18-183.2BID23-68.6 BID26-58
58.3763.8965.1 29.5866.1860.5858.0256.5456.8356.0247.52
0.5460.6270.7061.4230.5870.68 0.7710.7960.8520.6670.902
15.2 11.6212.7126.7312.6 14.9616.2417.8418.2115.5 19.49
5.599.898.1318.125.456.7 7.156.926.796.866.34
0.1320.0050.0050.0040.0170.0230.0180.0150.0270.0190.025
2.860.040.030.060.991.040.961.860.841.150.68
7.020.060.070.110.330.380.140.450.8 0.570.62
2.790.040.040.070.040.030.070.060.060.060.09
1.1 0.260.020.053.553.794.292.122.414.176.09
0.2 0.150.160.430.160.170.150.230.2 0.140.23
99.5598.2798.7798.6199.8899.0799.2598.3199.5100.5 98.29
5.7411.6911.8122.04 9.9710.7211.4411.4712.4915.3816.31
12.5 7.539.0510.5 6.126.048.0410.310.511.912.7
24.614.618.616.111.713.518.321.521.723.325.6
3.131.522.131.351.461.692.522.832.933.043.33
12.8 6.088.823.576.057.9411.112.212.412.614.1
2.961.051.880.391.982.512.983.373.362.913.51
1.03 0.3520.5950.2 0.5810.84 0.8580.9471.09 0.8911.1
2.890.731.360.322.463.123.113.493.582.973.74
0.480.120.170.050.460.570.550.580.630.520.65
2.870.810.860.322.943.593.393.583.9 3.284.01
0.6 0.210.190.090.610.730.690.770.810.680.82
1.810.750.710.371.792.192.042.382.362.042.42
0.2830.1360.1350.0790.2770.3440.3240.37 0.3790.3190.382
16.66.15.52.4
17.7
21.7
19.2
21.6
22.2
19.2
22.4
1.971.091.1 0.751.922.4 2.392.622.672.252.64
0.3310.2050.2090.1730.3220.4110.4350.4450.4660.3740.452
b.d.0.6 b.d.1.912.7 6.5 3.2 b.d.b.d.b.d.b.d.
b.d.
10 19 25204 43 15 9 10 94118
b.d.
576639131727542113 b.d.
b.d.
181664107
1,837
8,105
2,045
1,381130139749744
b.d.0.4 0.5 4.2 b.d.b.d.b.d.b.d.b.d.b.d.b.d.
14 8218016141421211618
20b.d.
b.d.
20020b.d.
b.d.
20b.d.
b.d.
b.d.
1.5b.d.b.d.0.31.12 1.71.21.33 4.4
30200100490160405060806070
14 8121614141516171519
1.51.41.72 0.80.60.51.51.70.70.8
2.32.12.24.21.92.22.42.62.42 2.7
b.d.b.d.b.d.b.d.b.d.b.d.b.d.b.d.b.d.b.d.b.d.
b.d.
b.d.
101916 3b.d.
3b.d.
b.d.
b.d.
2.72.72.44.92.12.32.82.73.12.33.3
b.d.b.d.b.d.30 b.d.b.d.b.d.b.d.b.d.b.d.b.d.
b.d.
152555
1,0804454102330 b.d.
21 4 b.d.
3798293425280105
b.d.1.7b.d.4.112.7 9.7 1.8b.d.6.711.5 9.7
1814141821262925322532
b.d.
33
112b.d.
b.d.
b.d.
b.d.
b.d.
b.d.
367
1,040688
3,854184 73 59 23 47 54 61
0.170.150.170.3 0.130.140.160.150.160.120.18
1.831.221.031.731.291.231.631.595.431.311.85
0.1 0.380.750.394.040.9 1.070.350.283.852.83
0.770.380.910.595.010.740.610.521.840.6 0.65
171120123222178201248245285221283
2.40.80.82.43.65.68.72 1.3b.d.b.d.
60 b.d.
80 b.d.
3,450320300 70320350470
897280130677682928570
100
1718
KING ET AL.
the compositions of the altered rocks to those of their leastaltered equivalents. This was done by making binary plots of
the data, and determining which element pairs were linearly
distributed, as such a distribution would correspond to a relatively constant mass ratio for the elements and could indicate
that they were immobile (e.g., MacLean and Kranidiotis,
1987; Warren et al., 2007; Agrawal et al., 2008). These plots
show that concentrations of pairs of the following elements,
Zr, Ti, Nb, Hf, and Ta, form linear arrays. However, because
Ti, Nb, and Ta are not incorporated into the same minerals as
Zr and Hf (Ti, Nb, and Ta occur in rutile and Zr and Hf in zircon), the linear variation of Zr with Ti was taken as evidence of
immobility (Fig. 9). Assuming that Zr and Ti were immobile,
the concentration of these elements in each altered sample
was normalized to their concentration in a least altered sample for each rock type using the method of Grant (1986). To
scale the changes for each element proportionally so that the
mass change for an unaltered rock is zero, the concentration
of each element was multiplied by the ratio of the immobile
element concentrations (Zr and Ti) in the fresh rock to that in
the altered rock. Values less than unity indicate losses of elements relative to their concentrations in the unaltered rock,
and values greater than unity indicate gains (Fig. 10).
Three distinct patterns of relative gains and losses are
observed for advanced argillic alteration (the small size
of the data set precluded separate treatment of advanced
argillic I, II, and III alteration) and intermediate argillic I
(Bawone and Binebase) and II (Binebase) alteration (Fig.
10). As expected from the mineralogy, the advanced argillic
and intermediate argillic I samples were strongly depleted
in the major elements, Mn, Mg, Ca, and Na, moderately
depleted or unchanged in K, weakly depleted in Al and P,
and relatively undepleted in Si. Somewhat unexpectedly,
however, these samples appear to be enriched in Fe. As py II
1.6
BOD
BID
Least Altered
TiO2 (wt. %)
1.2
0.8
0.4
0
40
80
120
160
Zr (ppm)
Fig. 9. Plot showing the distribution of TiO2 and Zr in variably altered
rocks of the Bawone (BOD) and Binebase (BID) deposits. See the text for an
interpretation of the significance of these data. The Pearson correlation coefficient for both deposits (r) = 0.80.
commonly occurs in the form of blebs that filled pores created by advanced argillic and intermediate argillic I alteration (and thus postdated this alteration), we suspect that most
if not all of the apparent enrichment in Fe is an artifact due
to unavoidable inclusion of py II in the samples analyzed to
represent this alteration. The intermediate argillic alteration II samples were also depleted in Mn, Mg, Ca, P, and
particularly in Na, but appear to have conserved Al, Si, and
Fe. In contrast to advanced argillic and intermediate argillic
I alteration, these samples are strongly enriched in K. The
overall distribution of trace metals and semimetals (Au, Cu,
As, Ag, Cr, Co, Ni, Zn, Mo, Sn, Sb, Tl, W, and Pb) indicates
that they were added during all stages of alteration, albeit in
variable quantities (it should be noted that because of the py
II problem referred to above, the additions of Cu and Au for
advanced argillic and intermediate argillic I alteration may be
overestimated). However, whereas Rb and Cs were depleted
(or unchanged) and Sr added during advanced argillic and
intermediate argillic I alteration, the opposite occurred during intermediate argillic II alteration, i.e., Sr was depleted
and Rb and Cs enriched (Fig. 10).
Both the advanced argillic and intermediate argillic I
altered rocks underwent substantial mass losses in all REEs,
with the degree of depletion increasing progressively with
atomic number from La to Dy and then decreasing progressively to Lu (Fig. 10). In contrast to advanced argillic and
intermediate argillic I alteration, the depletion of REEs during intermediate argillic II alteration was greatest for La and
decreased with atomic number to Gd with the exception of
Eu. We consider that the very small additions of the heavy
rare earth elements (HREEs: Tb, Dy, Ho, Er, Tm, Yb, Lu)
reflect uncertainties in the mass transfer calculations and that
these elements were immobile during alteration. The preferential leaching of the light rare earth elements (LREEs: La,
Ce, Pr, Nd) during intermediate argillic II alteration is consistent with results of experiments showing that the LREEs
are more mobile in chloride-bearing hydrothermal fluids than
the HREEs (Migdisov et al., 2009). The reason for the preferential depletion of Eu, Gd, Tb, Dy, and Ho during advanced
argillic and intermediate argillic I alteration is unclear. A possible explanation for this anomalous behavior is that depletion
of the LREEs was inhibited because of their incorporation
by sulfates and clay minerals (e.g., Miller et al., 1982; Hopf,
1993; Fulignati et al., 1999; Karakaya, 2009), and that without this uptake they would have been more depleted, consistent with the experimental predictions (Migdisov et al.,
2009). The differential behavior of the REEs reported above
supports observations from active hydrothermal systems that
REE chemistry offers a useful tool for distinguishing alteration types (e.g., Michard, 1989; Fulignati et al., 1998, 1999;
Lottermoser, 1992; Lewis et al., 1997; Salaün et al., 2011).
Ore mineral composition
The compositions of pyrite and enargite were analyzed
using a combination of electron microprobe (EMP) and laser
ablation-inductively coupled plasma-mass spectrometry (LAICP-MS). Quantitative electron microprobe analyses for Fe, S,
Cu, As, Sb, Co, Ni, Zn, Se, and Te were conducted on carboncoated samples at McGill University using a JEOL 8900 instrument equipped with five wavelength dispersive spectrometers
HIGH-SULFIDATION EPITHERMAL PYRITE-HOSTED Au ORE FORMATION, SANGIHE ISLAND, INDONESIA
1719
Fe 2O 3
100
1
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Cs
Rb
Sr
Bi
Ba
Pb
Ni
Zn
Mo
Sb
Co
0.1
Sn
1
Cu
As
10
Au
Ag
P2O 5
Na2O
MgO
0.01
CaO
MnO
0.1
SiO 2
1
1000
K2O
10
Al2O 3
Advanced Argillic
10
10
Fe 2O 3
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Cs
Sr
Rb
Ba
Bi
Pb
Zn
Ni
Mo
Co
Sn
Sb
0.1
Cu
As
1
Au
Ag
P2O 5
Na2O
10
1
CaO
MgO
0.01
K2O
100
MnO
0.1
SiO 2
10
1
1000
Al2O 3
Intermediate Argillic I
0.1
10
10
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Sr
Cs
Rb
Ba
Zn
Pb
Bi
Ni
Mo
Co
Sb
Cu
As
Sn
0.1
1
Au
Ag
P2O 5
CaO
MgO
100
1
Na2O
0.01
MnO
0.1
Al2O 3
1
Fe 2O 3
10
K2O
1000
SiO 2
Intermediate Argillic II
0.1
0.1
Fig. 10. Relative gains and losses of elements for advanced argillically altered rocks from Bawone (red), intermediate
argillically altered I porphyry and crystal tuffs from Bawone and Binebase (yellow) and intermediate argillically II altered
(purple) crystal tuffs from Binebase, calculated using the method of Grant (1986). A value of 1 indicates immobility, less than
1 indicates a relative mass loss and greater than 1 indicates a relative mass gain.
(WDS). The operating conditions were an excitation potential
of 20 kV, a beam current of 50 nA and a spot-size of 2 μm.
Analyses were standardized using pyrite (Fe, S), chalcopyrite
(Cu), AsNiCo (As, Ni, Co), CdTe (Cd, Te), sphalerite (Zn),
stibnite (Sb), and AgSe (Se) supplied by CANMET. In addition to quantitative spot analyses, the EMP was also used to
produce element maps for Cu, Fe, Se, and As. The operating
conditions were an excitation voltage of 20 kV, an operating
current of 90 nA, and a beam diameter of 2 μm. The counting
time was 30 ms and the pixel size between 0.20 and 1.28 μm.
The LA-ICP-MS analyses were conducted at the Geological Survey of Canada (pyrite) using a Photon Machines Nalyte
193 mm Excimer laser coupled to an Agilent Technologies
7700 Series ICP-MS, and at Université de Chicoutimi (enargite) using an Excimer Resolution M-50 (Resonetics) Laser
coupled to an Agilent 7700x ICP-MS. The concentration of
Fe57, S34, Cu65, Au197, Se77, Te125, Ag107, As75, and Sb121 was
measured in both pyrite and enargite. Pyrite was also analyzed for Co59, Ni60, Pb208, Zn66, and Bi209. The ablation pits
ranged from 14 to 54 μm in diameter, and the counting time
was 100 s (30 s of background, 70 s of ablation) for pyrite
and 90 s (30 s for background, 60 s of ablation) for enargite.
The concentrations of Fe and S determined by EMP analysis
were used as an internal standard for the LA-ICP-MS analyses of pyrite and enargite, respectively; GSE-IG, NIST 610
and Po689 were used as external standards for the analysis of
pyrite and GSE-1G, PS1 and JB5 for enargite.
Pyrite: The following trace elements were detected in pyrite
using the EMP: As, Co, Cu, Ni, Sb, Se, Te, and Zn (Table AI).
LA-ICP-MS analyses yielded results for these elements similar to those obtained with the EMP, and also detected Ag, Au,
Bi, and Pb (Table A2). Copper is the principal trace element
in both generations of pyrite followed by Co, As, and Pb in py
I or by Co, Pb, and As in py II. Significantly, both generations
of pyrite contain ppm levels of Au and Ag.
Pyrite I crystals are compositionally unzoned (Fig. 7B) and
have a wide inter- and intra-crystal range of Cu concentrations
(0.02–1.5 wt %); the median Cu concentration is 0.44 wt %
and the interquartile range (IQR) is 0.24 to 0.72 wt %. Cobalt
concentration is significantly lower, with a median of 62.5 ppm
(IQR = 15.6–206 ppm). The median As concentration is
43.9 ppm (IQR = 15.6–83 ppm) and that of Pb is 59.6 ppm
(IQR = 9.5–79.7 ppm). As mentioned above, py I is auriferous and argentiferous. The median concentrations of Au and
1720
KING ET AL.
Ag are 1.0 ppm (IQR = 0.6–1.5 ppm) and 9.1 ppm (IQR =
1.1–29.6 ppm), respectively. Somewhat surprisingly, the concentrations of Se and Te, elements that with As are commonly
elevated in auriferous pyrite (e.g., Fleet et al., 1993; Reich et
al., 2005), are relatively low with median values of 9.6 ppm
(IQR = 6.1–41.1 ppm) and 3.8 ppm (IQR = 2.2–10 ppm),
respectively (Table 3).
The concentrations of all trace elements detected in py I
are higher in py II. The median concentration of Cu in py II
is 1.1 wt % (IQR = 0.42–3.4 wt %) and the maximum concentration 6.0 wt % (EMP Analysis, Table A1). The median concentration of Co in py II is 148 ppm (IQR = 48.4–533 ppm),
that of Pb is 69.9 ppm (IQR = 2.7–557 ppm) and that of As is
118 ppm (IQR = 16.1–417 ppm). Unlike py I, the concentration of Te is relatively high, with a median value of 31.3 ppm
(IQR = 11–143 ppm) and that of Se is significant, with a
median of 25.9 ppm (IQR = 12.8–102 ppm). The maximum Au
concentration in py II is 13.6 ppm, and the median, 2.4 ppm
(IQR = 0.9–3.2 ppm), and the Ag maximum concentration is
1,273 ppm, and the median, 3.7 ppm (IQR = 1.4–19 ppm).
Although the concentrations of the trace elements, on average, are lower in py I than in py II, the interelement associations are similar. Binary plots (Fig. 11) and element maps
(Fig. 12) show that high Au concentrations in py I and II are
generally associated with elevated Cu, As, Ag, and Te, but Au
concentration may be high even if concentrations of these
elements are relatively low; this is especially true for Ag and
Te. Gold concentration is almost entirely independent of Se
concentration and shows no correlation with Co (or Ni, Sb,
Zn, Pb, and Bi, which are not shown) concentration. The
inter-element correlations for Ag are similar to those for gold,
except that Ag and Te are negatively correlated.
As mentioned above, py II is complexly compositionally
zoned and py I is unzoned. The backscatter electron images
indicate that this zonation reflects differences in concentrations of trace elements, mainly of Cu (Fig. 7A). Compositional
zoning was confirmed by quantitative spot analyses of py II,
which show that there are appreciable compositional variations within single crystals.
Copper concentrations show the largest variation between
growth zones, the distribution of which reveals a complex
history of growth, overgrowth, and resorption (Fig. 12). In
addition to this growth zoning, sector zoning is evident in the
distribution of Cu (i.e., within single growth zones there are
sharply defined sectors of higher and lower Cu concentration
corresponding to differences in the crystal faces presented).
The nature of the sector zoning varies considerably with the
crystal habit and is most easily observed in drusy crystals (Fig.
12).
Based on electron microprobe analyses, the Cu concentration of individual zones can range from as low as 0.12 to as
high as 6.0 wt %, and vary inversely with the concentration
of Fe. In some zones, high Cu concentration is matched by
an elevated concentration of As, but in others, the concentrations of the two elements do not correlate positively (Fig.
12D, F). In addition to As, the concentrations of Ni, Se, and
Te were measured across the transects. All three elements
show increases in concentration with elevated Cu concentration. Where the concentrations are consistently above the
detection limits, Ni, Se, and Te covary. In some pyrite crystals,
they covary with Cu, in others with As, and in some crystals,
with both.
Further insight into how trace elements in py II covary
is provided by LA-ICP-MS depth profiles through crystal
growth zones. Overall, the profiles show that zones enriched
in Cu are enriched in all trace elements, although the concentration of individual elements covaries with some, but not
all, of the elements analyzed. This is illustrated in Figure 13,
which shows the signal intensity (counts) for a suite of elements through three growth zones. From this figure, it is evident that the concentrations of Au, As, and Te covary (they all
decrease strongly in the central zone), that Cu, Co, Ni, Zn, Bi,
Table 3. Average Composition of Py I, Py II, and Enargite Based on LA-ICP-MS Analyses
AgAs AuBiCoCuFeNiPb S SbSeTeZn
(ppm)(ppm) (ppm)(ppm)(ppm)(wt %)(wt %)(ppm)(ppm)(wt %)(ppm)(ppm)(ppm)(ppm)
Py I
Mean
33.095.41.110.1117.1
0.459.743.1 55.3
53.3 3.7
31.8 10.15.8
Median
9.143.91.0 3.9 62.50.358.4 33.7 59.6
53.4 3.79.6 3.82.8
Min
0.3 2.3d.l. 0.1 0.1
d.l.52.3 0.5 0.5
51.7 0.31.2 0.70.6
Max
145.8467.3 2.5 47.9 404.31.5 71.7 182.9 199.854.6 11.0118.6 35.522.9
1st quartile 1.115.60.6 1.0 15.60.253.1 14.2 9.5
53.4 0.86.1 2.21.0
3rd quartile 29.683.01.513.3206.1
0.662.549.9 79.7
53.4 4.3
41.1 10.06.1
Py II
Mean
80.6329.7 3.0 9.43,015.61.1 48.6 138.3 511.852.3 10.7121.0 218.391.7
Median
3.7117.6 2.4 5.9 148.00.6 49.0 40.3 69.952.6
3.725.9 31.3 6.8
Min
d.l. 0.8d.l. d.l. d.l.
d.l. 30.9 0.1 0.1
48.3 0.10.7 0.10.5
Max
1,273.21,636.6 13.6 43.221,417.7 4.4 76.1 800.27,205.3 55.2 169.6640.11,430.9
1,936.3
1st quartile
1.4 16.1 0.9 3.0 48.40.3 40.7 11.8 2.752.1
2.212.8 11.0 1.1
3rd quartile 19.0417.2 3.2 13.9 533.61.6 54.5 124.2 557.253.6
8.7102.1 142.740.4
Enargite
Mean
Median
Min
Max
1st quartile
3rd quartile
101.6 13.50.533.8
108.5 13.40.633.2
31.111.80.124.9
214.5 15.50.845.8
52.712.50.428.8
114.9 14.20.737.2
Note: See digital Appendices 1 to 4 for raw data
0.0 32.76,410.8 3.1141.0
0.0 32.75,527.7 3.2135.9
0.032.7
4,273.71.8 79.5
0.1 32.79,543.1 5.5211.3
0.032.7
4,876.62.6 95.1
0.1 32.78,167.5 3.4188.9
HIGH-SULFIDATION EPITHERMAL PYRITE-HOSTED Au ORE FORMATION, SANGIHE ISLAND, INDONESIA
10-4
10-4
10-6
10-1
10-1
10-3
10-3
Te (afu)
Cu (afu)
10-6
10-5
10-5
Se (afu)
As (afu)
10-2
Pyrite I
Pyrite II
10-3
10-3
10-5
10-5
10-7
10-3
10-7
Te (afu)
Cu (afu)
Aa (afu)
10-2
1721
10-5
10-5
10-5
Sb (afu)
10-7
10-7
10-4
10
10-3
-5
10-5
10-6
Co (afu)
Ag (afu)
10-1
10-7
10-7
10-9
10-8
10-7
10-6
Au (afu)
10-8
10-7
10-6
10-5
10-4
Ag (afu)
Fig. 11. Binary plots of trace element concentration in pyrite analyzed by LA-ICP-MS and shown in atoms per formula
unit (afu).
and Se covary and do so independently of Au, As, and Te, and
that Ag and Pb covary independently of the other two groups
of elements. It should be noted, however, that these groupings are not observed in all crystals. For example, Te behaves
independently of Au and As in some crystals and covaries with
Se in others. Nonetheless, the “Cu group” of Cu, Ni, and Co
generally covaries independently of the “Au group” of Au, As,
and Te (Se). Silver concentration is usually decoupled from
those of both Au and Cu, and commonly covaries with Pb, Bi
and, rarely, Se concentration.
Enargite: On the basis of quantitative EMP analyses
(Table A3), enargite has close to an end member composition with a median As concentration of 18.5 wt % (IQR
= 18.1–18.8 wt %), a median S concentration of 32.8 wt %
(IQR = 32.7–33.0 wt %) and a median Cu concentration of
48.6 wt % (IQR = 48.1–48.8 wt %). The trace element concentrations were analyzed by a combination of EMP and LAICP-MS methods, and predictably, Sb was the trace element
with the highest concentration with a median of 5527 ppm
(IQR = 4876–8167 ppm; Tables 3, A4). This is well below its
concentration in stibioenargite (Springer, 1969; Maske and
Skinner, 1971; Posfai and Buseck, 1998). Iron is the next
most important trace element with a median concentration of
316 ppm (IQR = 117–649 ppm), followed by Te and Se, with
median concentrations of 135 ppm (IQR = 95.1–189 ppm)
and 3.2 ppm (IQR = 2.6–3.4 ppm), respectively (Table 3). The
1722
KING ET AL.
A
BSE Images
B
D
E
F
G
H
Composite element maps
Arsenic
C
Cu map
Copper
Iron
Arsenic
Copper
Iron
Selenium
Copper
Iron
HIGH-SULFIDATION EPITHERMAL PYRITE-HOSTED Au ORE FORMATION, SANGIHE ISLAND, INDONESIA
Au content is significantly lower than that of py I or py II, with
a median of 0.6 ppm (IQR = 0.4–0.7 ppm), but the median
silver content is significantly higher than that of either generation of Py (108 ppm, IQR = 52.7–114.9 ppm). From BSE
images, the enargite appears to be unzoned (Fig. 8C), but LAICP-MS depth profiles show small variations in the signals of
some trace elements and Cu (Fig. 13). Despite the presence
of a number of trace elements in significant concentration in
the enargite, none of them display a systematic correlation
with another trace element.
Sulfur Isotopes
Isotopic compositions of pyrite and sulfate (alunite and barite) were measured to better understand the physiocochemical conditions of ore formation. In particular, they were used
to estimate temperature and assist in the interpretation of fO2
and pH conditions.
Method
Because it is impossible to physically separate the finegrained, intergrown pyrite and alunite, separation was
achieved chemically in two stages. In the first stage, finely
ground altered rock was reacted with a Cr-reducing solution to liberate H2S from the sulfides (Canfield et al., 1986).
This procedure stripped the sample of sulfide, allowing the
solid residue to be filtered and reacted with Thode’s solution
(Thode et al., 1961) to convert the remaining sulfur (sulfate)
to H2S. In both cases, the resulting H2S was trapped using a
zinc-acetate solution and reacted with a solution of AgNO3
to produce solid Ag2S that was purified and reacted with SF6
(Thode and Rees, 1979) prior to introduction into a Thermo
Finnigan MAT 253 dual-inlet gas-source mass spectrometer
at McGill University for measurement of sulfur isotope ratios,
with IAEA-S-1 as an internal standard (Sharman, 2011). The
ratios are reported relative to V-CDT in δ notation.
Results
Two samples of argillically and advanced argillically altered
material and one sample of pyrite and barite from the late
barite-enargite veins were selected for sulfur isotope analysis
(Table 4). The sulfate concentration in the preliminary analysis was below the detection limit but consistent δ34S values
were obtained for pyrite. The δ33S, δ34S, and δ36S per mil values for pyrite average –2.65 (± 1s = 0.25), –5.17 (± 1s = 0.48),
and –9.93 (± 1s = 0.77), respectively. The standard deviations
are small considering that the pyrite represents different generations of mineralization.
1723
Table 4. Sulfur Isotope Compositions of Py I and Py II
from the Bawone and Binebase Deposits
Samples
δ33S
δ34S
δ36S
BID16-66.3PyI
BOD3-81.5
PyII
BOD1-99.4
PyI
–2.94
–2.54
–2.48
–5.73 –10.82
–4.93 –9.59
–4.86 –9.39
Note: The full analytical uncertainty (1s) for sulfur isotope analysis is
estimated to be ±0.13‰ for δ34S values, based on the long-term standard
deviation for repeat analyses of in-house standards
Discussion
Geologic setting
The Bawone and Binebase deposits occur at the southeastern end of a young volcanic island with an active volcano to
the northwest and a chain of extinct/dormant volcanic edifices
to the south. Both deposits are hosted by rocks of the Binebase Group, a thick andesitic sequence of alternating, interbedded porphyritic lava flows or epiclastic deposits and crystal
tuffs that were probably deposited in close proximity to a volcanic center. If, as seems likely, a volcano was located above
the Binebase and Bawone deposits (and was thus close to the
coast), its erosion would have been very rapid due to a combination of rising sea level (as indicated by the marine deposits
of the overlying Pinterang Formation), thermal subsidence as
the magmatic center moved northwest, and the hot, wet tropical climate. Consequently, all traces of the volcanic edifice
could have disappeared in tens of thousands of years.
Paleodepth and pressure
A very crude, approximate estimate of the depth and pressure of emplacement of the Bawone and Binebase deposits
can be made by comparing their current elevation to that of
the base of the crater of Awu volcano, the only active volcano
on Sangihe Island. This assumes that the volcano overlying
these deposits reached an elevation similar to that of Awu volcano, and although this cannot be known, it is suggested by the
crater elevation of the somewhat larger dormant/extinct Kakiraeng volcano located immediately to the west of the deposits.
The elevation of the crater of this volcano is 1,006 m (it has
likely been reduced in height due to erosion), whereas that
of the crater of Awu volcano is 1,327 m (Fig. 1). By contrast,
the Bawone and Binebase deposits are exposed at elevations
of 86 and 65 m, respectively. Thus, if the crater of the volcano
above the deposits was at an elevation similar to those of Awu
Fig. 12. BSE images (A, C, E, G) and corresponding copper map (B) or composite Cu (green), Fe (blue) and As/Se (red)
maps (D, F, H) showing strong sector and growth zoning in py II. Brighter colors correspond to higher concentrations of the
given element. (A) BSE image of complex, μm-scale, strongly growth zoned drusy pyrite overgrowing massive, porous pyrite,
also zoned. (B) Cu map of the same crystal, light blue-green represents higher Cu concentration; the darker the blue, the
lower is the Cu concentration. The growth zoning is perpendicular to the crystal growth direction and records the primary
growth history (50 μm scale). (C, D) Drusy, py II with growth and sector zones and very high Cu concentrations (green)
and variably As-rich (red) bands transitioning outward into massive, porous py II with lower Cu concentration and isolated
zones of As enrichment (scale bar is 50 μm). (E, F) Cross section is through a euhedral py II crystal displaying growth zoning
with alternating As (red) and Cu (green) enriched bands. Sector zoning most strongly evident as a Cu enrichment on three
equivalent faces about a three-fold axis (scale bar is 500 μm). (G, H) Drusy py II crystals displaying complex growth zoning in
either Cu (green) or Se (red) bands. Sector zoning in Cu is also present in the form of a cross at the center of a cubic pyrite
grain, top-center (scale bar is 500 μm).
1724
KING ET AL.
Growth Zone 1
Growth Zone 3
Co59
1 000 000
Fe57
Cu65
100 000
Signal Intensity (counts)
Growth Zone 2
S34
Ni60
10 000
As75
Se77
1000
Te125
Bi209
Sb121
100
Au197
Zn66
Ag107
10
Pb208
1
50
100
150
200
250
Time (s)
Fig. 13. An example of a depth profile through three growth zones of a py II crystal produced by LA-ICP-MS, in counts vs.
time (depth) for major and trace elements. Arsenic, Te, and Au covary through the three growth zones and are independent
of Co, Cu, Ni, Zn, Se, and Bi, whereas Ag, Pb, and to some extent Sb covary independently of the other two groups.
and Kakiraeng volcanoes, it follows that the deposits formed
at depths of approximately 900 to 1,300 m below the paleovent. These depths fall within the range of depths estimated
for other high-sulfidation epithermal deposits (Jannas et al.,
1990; Arribas, 1995; Cooke and Simmons, 2000). The depths
inferred for Bawone and Binebase (900–1,300 m) correspond
to lithostatic pressures of 250 to 350 bars or hydrostatic pressures of 90 to 125 bars (assuming the water table was near the
paleosurface), respectively. As much of the mineralization is
disseminated and there is no evidence of large through-going
fractures that might have intersected the paleosurface, pressure was probably intermediate between the two extremes. In
the discussion that follows, we therefore assume that the pressure accompanying hydrothermal alteration and gold mineralization was ~200 bars.
Temperature and pH
Earlier, we showed that advanced argillic (AA) and intermediate argillic (IA) alteration in the Bawone and Binebase
deposits are characterized by the following mineral associations: AA I (quartz + pyrite + pyrophyllite ± natroalunite), AA
II (quartz + pyrite ± natroalunite ± dickite ± kaolinite), AA III
(quartz + pyrite + kaolinite + alunite), IA I (quartz + kaolinite
± pyrite ± dickite), and IA II (quartz + illite ± pyrite). Here, we
use stability relationships among these minerals in the system
Al-Si-O-K-Na-Fe-H-S to make inferences about the temperature and pH of the fluids responsible for the different types of
alteration (Figs. 14, 15). The total S concentration was assumed
to be 0.01 m, consistent with that inferred for other high-sulfidation epithermal systems (e.g., Muntean et al., 1990). At a
pH below 2, aluminum is mobile, an observation confirmed by
our mass balance calculations that provides evidence for local
Al mobility. For these conditions, Al activity was assumed to be
0.1 (Knight, 1977; Fulignati et al., 1999). At higher pH, Al was
assumed to be immobile and its activity buffered by the aluminum silicate minerals (Stoffregen, 1987; Salvi et al., 1998).The
Na/K ratio was taken to be 10, assuming equilibrium between
natroalunite and alunite (natroalunite and alunite commonly
occur together but their textural relationships could not be
determined because of the very fine-grained nature of the
rock), and the conclusion that alunite transformed to natroalunite at a temperature of ~275°C (see below). Oxygen fugacity was inferred from the δ34S values for pyrite to be that of
the hematite-magnetite buffer (see below). The activities of
Fe and Si were assumed to have been buffered by pyrite and
quartz, respectively, which are ubiquitous in the deposits.
At the conditions for which Figures 14 and 15 were constructed, pyrite is stable above 250°C (hematite is stable
below 250°C), making this the lower temperature limit for all
alteration facies. Because kaolinite and quartz react to form
pyrophyllite at temperatures greater than 280°C (Fig. 15), the
coexistence of quartz and kaolinite in AA II, AA III, and IA
I alteration zones constrains their temperatures to have been
less than 280°C. Conversely, the presence of pyrophyllite in
HIGH-SULFIDATION EPITHERMAL PYRITE-HOSTED Au ORE FORMATION, SANGIHE ISLAND, INDONESIA
HSO
4
-
400
And
350
Py
Na-Al
Mag
Pyr
1
300
2
3
Albite
Musc
T (°C)
1725
SO 24
5
4
250
Microcline
Hem
Fe2+
Al
Kao
Range of alteration
conditions
200
AA I fields 1 & 2
AA II field 1
AA III fields 4 & 5
150
IA I field 5
IA II field 3
100
0
2
4
6
pH
8
10
12
14
Fig. 14. Stability relationships among minerals in the K-Na-Al-Si-O-H and Fe-S-O systems as a function of temperature
and pH. Also shown are the fields of stability of the five main alteration facies (shaded areas), advanced argillic I (AA I),
advanced argillic II (AA II), advanced argillic III (AA III), intermediate argillic I (IA I), and intermediate argillic II (IA II).
The diagram was calculated for 200 bar (SVP), log fO2 = –30, quartz saturation, Na+/K+ = 10, ΣaS = 0.01 and aAl3+ = 0.1 (see
text for details). Silicate and sulfate species are shown by solid lines, Fe species by dashed lines and sulfur species by dotted
lines. The model was calculated using SULF34 (Zhang, 1993) and The Geochemist’s Workbench (GW; Bethke, 2005) with
thermodynamic data from Holland and Powell (1998), Stoffregen et al. (2000), Helgeson (1978), Robie and Hemingway
(1995), and Johnson et al. (1992).
the AA I alteration zone indicates that this alteration occurred
at a temperature above 275°C (Holland, 1998). Natroalunite
is the stable form of the alunite group minerals in the AA I
alteration zone, and alunite is the stable form in the AA III
alteration zone. Unexpectedly, however, given the coexistence
of quartz and kaolinite (implying a temperature less than
280°C), natroalunite is also the stable member of the alunite
family in the AA II alteration zone. We interpret this to indicate that AA II alteration occurred at ~280°C, an interpretation that is supported by the presence of dickite in the AA II
alteration zone (kaolinite is widely inferred to convert to dickite above 150°C: Hemley et al., 1969; Stoffregen and Alpers,
1987; Stoffregen and Cygan, 1990). In view of the distribution
of alunite group minerals among the AAI, AAII, and AAIII
zones, we conclude that alunite was replaced by natroalunite
at ~275°C.
An upper temperature limit for AA I alteration is provided
by the absence of andalusite, which forms from the reaction of
pyrophyllite and quartz at ~340°C (Figs. 14, 15). In summary,
we conclude that AA I alteration occurred at a temperature
between 275° and 340°C, AA II alteration at ~275°C, and AA
III and IA I alteration at between about 250° and 275°C. As
the IA II alteration association includes pyrite, this alteration
occurred at >250°C and, although we cannot constrain the
upper temperature from its mineralogy, it is reasonable to
conclude that this limit was less than or similar to that for IA I
alteration, i.e., ≤275°C.
On the basis of the stability relationships illustrated in Figure 14, specifically the presence of both natroalunite and
pyrophyllite in the AA I association, natroalunite and kaolinite/dickite in the AA II association, and alunite and kaolinite in
the AA III association, we interpret the pH of advanced argillic alteration to have been ~2.5. In principle, the pH could
have been significantly lower, if natroalunite and pyrophyllite
or alunite and kaolinite were not in equilibrium (Hemley and
Jones, 1964; Knight, 1977). If, however, the pH was below 2,
aluminum would have been highly mobile (Stoffregen, 1987),
which based on our analysis of mass changes (Fig. 10) was not
the case; aluminum experienced only minor depletion. This
supports a conclusion that the pH was at least 2 and more
likely ≥2.5. The presence of kaolinite in the IA I alteration
association constrains its pH to between 2.5 and 4, and the
presence of illite, but absence of K-feldspar and albite in the
IA II association, constrains its pH to between ~4 and 6.
1726
KING ET AL.
log fO2
-25
Al
Kao
-30
10
Microcline
Musc
5
Hem
δ 34H
2
-35
Mag
S= -1
0
δ 34H
2
0
S = -1
Fe
2+
2
0
δ 34H
2
HS-
H 2S
Py
-40
250°C 15
∆ log fO2 (HM)
AlSO4+
HSO4-
-20
SO42-
A
Pr
6
4
S= -5
8
AlSO4+
Na-Al
-25
Pyr
300°C
Albite
Musc
5
SO2
log fO2
10
∆ log fO2 (HM)
-20
SO42-
B
HSO4-
pH
Hem
-30
Py
δ 34H
δ 34H
2
2
Mag
0
S= -1
S= -1
0
δ 34H
2
-35
Fe+2
H 2S
-5
Pr
2
4
pH
6
HS-
-40
0
S= -5
8
Fig. 15. Log fO2-pH diagrams showing stability relationships for minerals in the K-Na-Al-Si-O-H and Fe-S-O systems (A)
at 250°C, and (B) at 300°C. The diagrams were created assuming a pressure of 200 bar (SVP), quartz saturation, Na+/K+ = 10,
ΣaS = 0.01 and aAl3+ = 0.1 (see text for details). Silicates and sulfate species are shown by solid lines, Fe-species by dashed lines
and sulfur species by dotted lines. The red lines are δ34SH2S contours at –1, –5 (the measured δ34S value for Sangihe pyrite)
and –10, and constrain the range of log fO2 values for Au-Ag mineralization to approximately –32 (Fig 16A) and –27 (Fig 16B)
assuming a pH of 2.5 (see text for details). The term Dlog fO2 (HM) refers to the number of log units of fO2 above or below that
of the hematite-magnetite buffer (calculated using The Geochemist’s Workbench).
fO2
Ore-forming high-sulfidation epithermal systems are characteristically oxidizing, with oxygen fugacity straddling or just
below that of the hematite-magnetite buffer. For example, values for ∆log fO2 (HM) reported for some other high-sulfidation
deposits have ranges of 4.24 to –1.32 (Thiersch et al., 1997),
2.4 to 0.07 (Muntean et al., 1990) and <2.5 (Voudouris, 2011).
If the sulfur source were dominantly magmatic, which is considered to be the case for most high-sulfidation deposits (e.g.,
Muntean et al., 1990; Rye et al., 1992; Rye, 1993; Arribas, 1995;
Hedenquist et al., 1998b; Bethke et al., 2005; Deyell et al.,
2005a, b; Fifarek and Rye, 2005; Taylor, 2007), the Sδ34S value
of the fluid would have been approximately zero. Using this
assumption, it is possible to calculate the δ34S of the fluid from
the measured pyrite δ34S values and, in turn, calculate log fO2
for given pH values (Zhang, 1993). The negative δ34S values
calculated for reduced sulfur (H2S) at Bawone and Binebase,
assuming a temperature between 250° and 340°C, are similar to those of other high-sulfidation deposits (Muntean et
al., 1990; Hedenquist et al., 1994; Voudouris, 2010), and are
consistent with sulfur isotope fractionation between sulfides/
HIGH-SULFIDATION EPITHERMAL PYRITE-HOSTED Au ORE FORMATION, SANGIHE ISLAND, INDONESIA
sulfosalts and sulfates in an oxidizing environment (Arribas,
1995; Cooke and Simmons, 2000). The ∆log fO2 (HM) values
corresponding to the δ34SH2S values for advanced and intermediate argillic alteration are between 4 and –1 (Fig. 15; Table 5;
Rye, 1993, 2005; Cooke and Simmons, 2000).
Metal transport
According to many researchers, there is a hyper-acidic
“ground preparatory” or “pre-ore” stage of hydrothermal alteration in high-sulfidation epithermal systems that precedes gold
mineralization and results in advanced argillic alteration (Stoffregen, 1987; Hedenquist et al., 1994, 1998, 2000; Arribas,
1995). There is also a consensus among these researchers that
the extreme acidity required to produce the advanced argillic
alteration that characterizes this ground preparation stage can
only be explained by interaction of the rocks with a condensed
acidic vapor (e.g., Hedenquist et al., 1994, 2000). At Bawone
and Binebase, the alteration does not reflect interaction with
fluids having the extreme acidity of some high-sulfidation
epithermal systems (residual “vuggy” silica is not observed).
Nonetheless, the very low pH of ~2.5 estimated for the fluid
requires that it was a liquid condensed from gas, as other fluids
that might have been present in the system, such as meteoric
water/groundwater, seawater and magmatic liquid would have
had higher pH (Stoffregen and Alpers, 1987; Meyer and Hemley, 1967; Hemley et al., 1969; Stoffregen, 1987; White, 1957).
Because advanced argillic alteration involved crystallization of
auriferous and argentiferous pyrite (pyrite I) and this pyrite
accounts for between 30 and 50% of the hypogene Au (Ag-Cu)
mineralization in the Bawone and Binebase deposits, it therefore follows that much of the gold and the other metals were
introduced into the deposits by this condensed gas. Whether
or not pyrite II and enargite, which were introduced later, also
crystallized from this condensed gas is less clear but given the
similar trace element and isotopic compositions of pyrite I and
II, it is a strong possibility. There were also likely multiple iterations of this process as porosity was created and subsequently
filled by alteration products (Berger and Henley, 2011).
Evidence that metals can be transported in high concentrations in magmatic vapors has been provided by analyses of the
compositions of vapor inclusions, which have yielded potentially ore-forming concentrations of some metals, including
ppm levels of Au and Ag (Audétat et al., 1998, 2008; Ulrich et
al., 1999; Williams-Jones and Heinrich, 2005; Seo et al., 2009).
In addition to the fluid inclusion data, there is also a growing
body of evidence from experiments that Au, Ag and Cu can be
transported in appreciable concentrations in aqueous vapor
(Migdisov et al., 1999; Archibald et al., 2001, 2002; Migdisov
and Williams-Jones, 2013; Hurtig and Williams-Jones, 2014).
Table 5. Physicochemical Conditions for Each Alteration Facies Based
on Stability Relationships Among Minerals and δ34S Values for Py I and Py
II
IA I
IA II
AA I
AA II
AA III
T
pHlog fO2
260–275 3–4
>265 4–6
275–330
0.5–4
270–275
0.5–4
250–270
0.5–4
–34 to –31
–35 to –33
–28 to –27
–28 to –27
–33 to –32
∆log fO2 (HM buffer)
+1 to +3
–1 to +2
+2 to +4
+1 to +4
+1 to +3
1727
For example, Migdisov and Williams-Jones (2013) and Hurtig and Williams-Jones (2014) have shown that HCl-bearing
aqueous gases can dissolve ppm levels of Ag and Au, respectively, at temperatures as low as 400°C. They have shown,
moreover, that the solubility of these metals increases exponentially with increasing f H2O. At the temperature and pressure predicted for the formation of the Bawone and Binbase
deposits, i.e., close to those of the H2O vapor-liquid boundary,
conditions would have been optimal for the transport of gold
and silver by magmatic vapor.
Direct evidence that condensed gases in environments
analogous to those of high-sulfidation epithermal deposits
can transport significant concentrations of metals is provided
by Kawah Ijen, an active volcano in eastern Java, Indonesia.
There, advanced argillic alteration (and residual silica alteration) is ongoing and accompanied by the crystallization of
pyrite enriched in Cu, Ag, and Au (Scher et al., 2013). The
fluid responsible for this alteration is a condensed magmatic
gas with a pH <1 containing ppm concentrations of Cu and
As (the concentrations of Au and Ag are below the level of
detection). Although the concentrations of Cu, Ag, and Au
in the Kawah Ijen pyrite are much lower than those in the
pyrite at Bawone and Binebase (because of the much lower
pressure and hence density of the gas), the metal ratios, e.g.,
Cu/Au and Ag/Au are very similar to those of the two Sangihe
Island deposits (Fig. 16). We can therefore conclude that the
two pyrite occurrences likely formed by the same process and
that the fluid which transported the ore metals in the Bawone
and Binebase deposits was therefore a magmatic gas. Several
other ore deposit studies have also documented gold mineralization associated with early advanced argillic alteration (e.g.,
Muntean et al., 1990; Voudouris, 2011).
Ore deposition
For the reasons presented above, we propose that the fluid
responsible for the formation of the Bawone and Binebase
deposits was a condensed magmatic gas. However, we consider it likely that this gas encountered a physicochemical
barrier, such as the water table or an impermeable cap rock.
This seems to have been the case for the deeper mineralization at Bawone (Figs. 3A, 6A), as the gas condensed to a lowpH liquid from which the ore mineral (pyrite) precipitated
or formed by replacement of primary iron-bearing minerals.
According to this interpretation, the ore metals were transported in the vapor as hydrated species, e.g., MeCl-H2O or
MeHS-H2O (Migdisov et al., 1999; Williams-Jones and Heinrich, 2005; Zezin et al., 2011b; Migdisov and Williams-Jones,
2013; Hurtig and Williams-Jones, 2014), whereas after condensation of the vapor to liquid, they would have been transported as charged or neutral aqueous species like MeCl2– or
MeHSo (Crerar and Barnes, 1976; Gammons and Barnes,
1989; Zotov et al., 1990; Gammons and Williams-Jones, 1997;
Mountain and Seward, 2003; Stefánsson and Seward, 2004;
Williams-Jones et al., 2009).
If saturation of the fluid with the metal had occurred, ore
deposition would have been controlled by reactions such as
the following:
Au(HS)o + ½ H2O(l) = Au(s) + H2S + ¼ O2
AuCl2–
–
+
(1)
+ ½ H2O(l) = Au(s) + 2Cl + H + ¼ O2(g)(2)
1728
KING ET AL.
Pyrite (Scher et al., 2013)
Pyrite I
Pyrite II
As (ppm)
1000
100
10
1
Cu (ppm)
1000
100
10
1
Ag (ppm)
1000
100
10
1
0.1
0.01
0.0001
0.001
0.01
0.1
1
10
Au (ppm)
Fig. 16. Log-log plots comparing the trace element composition of py I
and py II with that of pyrite from an active high-sulfidation epithermal system
at Kawah Ijen volcano (Scher et al., 2013). From these plots, it is evident that
the ratios, Cu/Au and Ag/Au of py I and py II, and particularly the Ag/Au ratio
of py I, are broadly similar to those of Kawah Ijen pyrite.
However, as native gold does not occur in the deposits,
instead, the gold occurs with other metals as nanoparticles in
pyrite or within the structure of this mineral, it follows that
gold was undersaturated in the liquid.
Deposition of gold and other metals is interpreted to have
occurred as a result of their adsorption onto the faces of pyrite
during crystal growth. This deposition began at the onset of
advanced argillic alteration with the precipitation of pyrite I
and continued with the precipitation of pyrite II. Adsorption
of gold onto pyrite has been proposed previously as a mechanism to explain the occurrence of “invisible gold” in other epithermal deposits (e.g., Simon et al., 1999; Wilder and Seward,
2002; Pals et al., 2003). It also has been investigated experimentally at ambient pressure and temperature for pH values
from 2 to 10. Results of these experiments for pyrite having
negatively charged crystal faces show that over 90% of the
gold in solution (undersaturated with respect to native gold)
is adsorbed at a pH <5 (Widler and Seward, 2002). Pyrite can
also have positively charged faces if arsenic bearing (Favorov
et al., 1974), but the effects of Cu on the surface properties of
pyrite are unknown.
Pyrite I is an integral part of advanced and intermediate
argillic alteration, and accounts for 30 to 50% of the gold in
the Bawone deposit and in the hypogene part of the Binebase deposit with an median concentration of 1.0 ppm Au;
the median concentrations of Ag and Cu are 9.1 ppm and
0.44 wt %, respectively. The texturally complex pyrite II contains median values of 2.4 ppm Au, 3.7 ppm Ag, and 1.1 wt %
Cu, and accounts for most of the remaining gold. Some gold
and even more silver are present in enargite (median values of
0.6 ppm Au and 108 ppm Ag).
Insights into the conditions favorable for the uptake of gold
by pyrite in the Bawone and Binebase deposits are afforded
by the presence of both growth and sector zoning in pyrite
II. The latter zoning reflects differences in the concentrations
of elements on two nonequivalent faces within single growth
zones, and enables partition coefficients to be calculated
for these faces, which record the relative changes in physicochemical conditions during crystal growth (van Hinsberg
and Schumacher, 2007). We have used analyses of sector and
growth zones in pyrite II to determine the conditions favorable for the incorporation of gold and other trace elements
into pyrite. The element concentrations in two adjacent
nonequivalent faces were measured for a series of growth
zones and the partition coefficients (Kd) were calculated. We
observed that the highest gold concentrations coincided with
periods of growth marked by relatively constant Kd, and lower
gold values coincided with sharp increases and decreases in
the Kd (Fig. 17). Thus, in contrast to mineralization produced
by saturation of an ore mineral, which is favored by rapid
changes in the physicochemical conditions due to processes
like mixing and boiling, concentration of gold in the Bawone
and Binebase deposits was favored when physicochemical
conditions were relatively stable.
Incorporation of trace elements in pyrite
Unlike many examples of deposits of invisible gold, the
pyrite in the Bawone and Binebase deposits contains only
trace quantities of As (Tables 3, A1, A2). Despite this low
concentration of As relative to that in auriferous pyrite from
other deposits (e.g., Reich et al., 2005), Au concentration correlates positively with the concentration of As (Figs. 11, 13,
16); the latter is a hundred times greater than the concentration of Au in py I and py II. This correlation suggests that
Au + As substituted for Fe, as has been proposed previously
for other deposits containing gold hosted in pyrite (Cook and
Chryssoulis, 1990; Fleet et al., 1993; Huston et al., 1995). It
is also possible, however, that some or all of the gold occurs
as nanoparticles within the pyrite. Although our data do not
allow us to determine whether the gold was incorporated into
the pyrite structure or is present as nanoparticles, Reich et al.
(2005) have observed that in pyrite with Au/As > 0.02, gold
occurs as microinclusions, whereas if the ratio is less than this,
it is substituted into the pyrite structure. The average Au/As
for pyrite in the Bawone and Binebase deposits is 0.019, suggesting that much of the gold may have been incorporated in
the structure of this mineral.
HIGH-SULFIDATION EPITHERMAL PYRITE-HOSTED Au ORE FORMATION, SANGIHE ISLAND, INDONESIA
16.79
1729
Kd
Cu light
Au (ppm)
4
3
12.22
3
Kd
2
2
Cu (wt %)
11.46
1
1
2.65
2.23
0
0
0
20
40
Distance
60
80
100
Fig. 17. A plot showing the variation in the partition coefficient (Kd; black line) and the concentration of Cu (red line)
as a function of the distance (in percent) from the core to the rim of a single pyrite crystal (BID34-84). In this plot, the Kd
corresponds to the ratio of the Cu concentration in one sector zone (the one with the higher overall Cu concentration, i.e.,
the “light” zone in BSE images, e.g., Fig. 13) to the Cu concentration in the adjacent sector zone (the “dark” zone), within
the same growth zone. The Kd is inferred to vary inversely with temperature following van Hinsberg and Schumacher (2007).
The yellow squares represent gold concentrations analyzed by LA-ICP-MS, with the width of the square corresponding to
the diameter of the pit in relation to the growth zones. The vertical scale on the right hand side of the diagram indicates the
Cu concentration. The highest gold concentrations correspond to regions where the Kd is stable, whereas lower gold concentrations correspond to intervals over which there is a rapid change in Kd values; Cu concentrations show the same behavior.
Incorporation of Au in the pyrite structure is also favored
by the presence of Te, which substitutes for the smaller S
ion, thereby expanding the lattice and providing space for the
large Au ion (Chouinard et al., 2005a; Bi et al., 2011). The
presence of appreciable Te (concentrations of Te are tens (py
I) to a hundred (py II) times greater than that of Au) and its
positive correlation with Au (Fig. 14) suggests that this substitution was also important in the uptake of Au.
Although the As concentration in pyrite II is low compared
to that of auriferous pyrite elsewhere, the Cu concentration is
anomalously high, up to 6 wt % Cu (Abraitis et al., 2004). Significantly, most of the other examples of auriferous pyrite with
high copper contents are also from high-sulfidation epithermal
deposits, e.g., Pascua Lama (up to 1.5 wt % Cu; Chouinard
et al., 2005a), Pueblo Viejo (up to 3.0 wt % Cu; Deditius et
al., 2011) and Chelopech (up to 4.5 wt % Cu; Pacevski et al.,
2008). Moreover, the exceptions, such as pyrite from the Coka
Marin polymetallic deposit (up to 8 wt % Cu; Pacevski et al.,
2008) have many characteristics of high-sulfidation deposits.
It has been shown that copper can be incorporated in
the pyrite structure both as nanoparticles and by substitution for Fe, even within a single ore deposit such as Peublo
Viejo (Huston et al., 1995; Oberthür et al., 1997; Deditius et
al., 2011). However, there is no agreed upon mechanism to
explain the rare and anomalous Cu concentrations in pyrite.
Some studies have shown that Cu-rich pyrite is the result of
recrystallization of Cu-phases (Pacevski et al., 2008). This was
not the case at Bawone and Binebase because of the welldocumented zoning that records the growth history of the
crystals (Fig. 12). Other researchers have suggested oxidation
of aqueous Cu+ and direct substitution of Cu2+ for Fe2+ (e.g.,
Chouinard et al., 2005a; Pacevski et al., 2008) accompanied by
the incorporation of other trace elements that cause distortion
of the lattice (e.g., Radcliff and McSween, 1970). Whatever
the mechanism, it seems likely that the highly oxidizing conditions that are characteristic of high-sulfidation deposits play a
role in the incorporation of Cu in the pyrite structure.
Genetic model
We propose that the metals forming the Bawone and Binebase deposits originated from a relatively oxidized magma that
was emplaced at high crustal levels and exsolved a low density supercritical fluid (that subsequently evolved to vapor)
into which metals (including Au, Ag, and Cu) partitioned
1730
KING ET AL.
preferentially. The gold and other metals were transported
as hydrated species that ascended through the volcanic pile
via fractures and zones of enhanced permeability to depths
between 900 and 1,300 m, where the vapor condensed at temperatures between 250° and 340°C to form an acidic liquid
with a pH of ~2.5; fO2 ranged up to four log units above the
hematite-magnetite buffer.
The acidic condensate altered the host andesite, replacing primary mafic minerals, such as hornblende, biotite, and
magnetite, with auriferous py I (sulfidation), and converting
feldspars to an advanced argillic alteration association that
includes kaolinite, pyrophyllite, alunite, natroalunite, and
dickite. If fluids had been previously neutralized by reactions
with rocks or fluid/rock ratios were lower, the alteration led
to an intermediate argillic I alteration association of kaolinite, quartz, and pyrite, and a distal intermediate argillic II
alteration association of illite, quartz, and pyrite. The alteration also created porosity through the dissolution of minerals.
During alteration, all major elements, except for Zr and Ti
were depleted proximal to the main alteration/mineralization
conduits (Al and particularly Si experienced only minor depletion; Fig. 10). Iron and many trace elements, except for the
REE, Rb and Cs, were enriched, in some cases by orders of
magnitude, relative to their original abundance.
Auriferous py II was deposited in veins and in open space
created by mineral dissolution. These same veins were fractured and the fractures filled by barite and enargite. Rare, late
auriferous (electrum) barite-base metal (sphalerite and chalcopyrite) veins formed at Binebase.
Py I, which is associated with advanced and intermediate
argillic alteration, and py II, precipitated under conditions
for which metals, including gold, were undersaturated in
the fluid. The metals, i.e., Au, Ag, and Cu, were adsorbed
onto the surfaces of the growing pyrite crystals and incorporated in the pyrite either as a solid solution or as nanoparticles. Some gold and silver were incorporated in enargite by
the same mechanisms (Fig. 18). The Bawone and Binbase
deposits were subsequently exhumed and partially oxidized,
creating an upper zone of supergene-enriched oxide ore and
deeper lower-grade hypogene ores.
AuHS . nH2O
Impermeable
H2 SO42-
AuCl2-
AuHSo
H 2O
Impermeable
H2 SO42Cu
AuHS . nH2O
CuHS . nH2O
AuCl2 nH2O
.
Advanced Argillic
Py II ± Ba-En veins
AuCl2-
Cu
Cu
Pyrite
Au
AgCl . nH2O
H 2S
SO2
H 2O
CO2
A
CuHS. nH2O
Argillic
B
AuHSo
C
Fig. 18. A cartoon cross section through the stratovolcano that may have hosted the Bawone and Binebase gold deposits.
Moderately dipping flows and volcaniclastic units with variable porosity and permeability are crosscut by structures that
permitted transfer of heat and fluids (red) upward. The latter caused hydrothermal alteration of the adjacent rocks (pink).
Some of the conduits vented at the surface, producing solfataras, whereas others terminated at impermeable barriers, such as
the welded andesite tuff in the Bawone deposit, or at perched water tables corresponding to permeable and less permeable/
impermeable layers. The small circle in the cross section indicates the location of the large circles, labeled A to C. Circle A
illustrates the transport of the metals as hydrated species in magmatic vapor rising through the volcanic pile and condensing at
an impermeable boundary. Circle B shows the distribution of rocks altered by highly acidic condensed vapor, and the accompanying pyrite-hosted gold (copper, silver) mineralization. Finally, Circle C depicts a pyrite crystal with two types of faces
onto which copper and gold adsorbed preferentially from a gold-undersaturated condensed vapor to produce gold (copper,
silver)-rich sector- and growth-zoned pyrite II. Multiple iterations of this process are likely required to produce an economic
high-sulfidation deposit of this style.
HIGH-SULFIDATION EPITHERMAL PYRITE-HOSTED Au ORE FORMATION, SANGIHE ISLAND, INDONESIA
Conclusions
Textural and geochemical observations provide compelling evidence that the Bawone and Binebase gold deposits
were the products of a condensed acidic magmatic vapor.
This fluid produced the advanced argillic and intermediate
argillic alteration that is characteristic of high-sulfidation epithermal systems and deposited gold, as well as copper and
silver, either within the structure of pyrite or as nanoparticles
within this mineral. The fluid did not deposit native gold or
electrum (except in very late veins at Binebase) and so we
conclude that it was undersaturated in respect to these metals. Instead of precipitating directly from the fluid, gold and
silver were concentrated by adsorption onto the surfaces of
growing pyrite (and to a much lesser extent enargite) crystals,
where they either formed nanoparticles or substituted for iron
in the structure of the mineral. Much of the auriferous pyrite
formed during advanced and intermediate argillic alteration
as a result of the sulfidation of primary mafic minerals. Auriferous pyrite (py II) crystallized later, filling fractures in the
earlier formed pyrite, and was joined even later by Au-Ag–
bearing enargite that precipitated in reopened pyrite II veins.
Although the later pyrite II is richer in Au than pyrite I, and
enargite is richer in Ag than both pyrite generations, the precious metals in both minerals are interpreted to have concentrated from a condensed magmatic vapor by adsorption
during crystal growth. The Bawone and Binebase deposits are
part of the continuum of high-sulfidation deposits, formed
from a condensed magmatic vapor, and record a mineralizing
process that likely has contributed to the formation of many
other high-sulfidation deposits.
Acknowledgments
The authors thank East Asia Minerals for their financial and
logistical support, in particular Tom Mulja for suggesting the
project and for his assistance and comments. The field component of the research would not have been possible without
the help of Arodji Wisanggono, Johnnedy Situmorang, Mardy
Posumah, Grace Kapal, and the staff on Sangihe Island and
in Jakarta. Simon Jackson and Jeanne Percival at the GSC,
Boswell Wing and Libby Sharman of the Sulfur Isotope laboratory at McGill University, Lang Shi and Jeanne Paquette
provided invaluable assistance with the analytical aspects of
the research. The project was funded by East Asia Minerals
and an NSERC-CRD grant to A.E. Williams-Jones and G.
Williams-Jones with additional support by DIVEX. Constructive reviews by R. Fifarek, R.W. Henley, and associate editor
D. John improved the paper considerably.
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