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Economic Geology
BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS
VOL. 101
May 2006
Geology and Reconnaissance Stable Isotope Study of the
Oyu Tolgoi Porphyry Cu-Au System, South Gobi, Mongolia
BAT-ERDENE KHASHGEREL,†,*
Mongolian University of Science and Technology, School of Geology, Ulaan Baatar, Mongolia
ROBERT O. RYE,
U.S. Geological Survey, Box 25046, Mail Stop 963, Denver, Colorado 80225
JEFFREY W. HEDENQUIST,
Colorado School of Mines, Golden, Colorado 80401
AND IMANTS
KAVALIERIS
Ivanhoe Mines Mongolia Inc., Zaluuchuud Avenue 26, Ulaan Baatar 210349, Mongolia
Abstract
The Oyu Tolgoi porphyry Cu-Au system in the South Gobi desert, Mongolia, comprises five deposits that extend over 6 km in a north-northeast–oriented zone. They occur in a middle to late Paleozoic arc terrane and
are related to Late Devonian quartz monzodiorite intrusions. The Hugo Dummett deposits are the northernmost and deepest, with up to 1,000 m of premineral sedimentary and volcanic cover rock remaining. They are
the largest deposits discovered to date and characterized by high-grade copper (>2.5% Cu) and gold (0.5–2 g/t)
mineralization associated with intense quartz veining and several phases of quartz monzodiorite intruded into
basaltic volcanic host rocks. Sulfide minerals in these deposits are zoned outward from a bornite-dominated
core to chalcopyrite, upward to pyrite ± enargite and covellite at shallower depth. The latter high-sulfidation–state sulfides are hosted by advanced argillic alteration mineral associations. This alteration is restricted
mainly to dacitic ash-flow tuff that overlies the basaltic volcanic rock and includes ubiquitous quartz and pyrophyllite, kaolinite, plus late dickite veins, as well as K alunite, Al phosphate-sulfate minerals, zunyite, diaspore,
topaz, corundum, and andalusite.
A reconnaissance oxygen-hydrogen and sulfur isotope study was undertaken to investigate the origin of several characteristic alteration minerals in the Oyu Tolgoi system, with particular emphasis on the Hugo Dummett
deposits. Based on the isotopic composition of O, H, and S (δ18O(SO4) = 8.8–20.1‰, δD = –73 to –43‰, δ34S =
9.8–17.9‰), the alunite formed from condensation of magmatic vapor that ascended to the upper parts of the
porphyry hydrothermal system, without involvement of significant amounts of meteoric water. The isotopic data
indicate that pyrophyllite (δ18O = 6.5–10.9‰, δD = –90 to –106‰) formed from a magmatic fluid with a component of meteoric water. Muscovite associated with quartz monzodiorite intrusions occurs in the core of the
Hugo Dummett deposits, and isotopic data (δ18O = 3.0–9.0‰, δD = –101 to –116‰) show it formed from a
magmatic fluid with water similar in composition to that which formed the pyrophyllite. Mg chlorite (δ18O =
5.5‰, δD = –126‰) is a widespread mineral retrograde after hydrothermal biotite and may have formed from
fluids similar to those related to the muscovite during cooling of the porphyry system. By contrast, paragenetically later and postmineralization alteration fluid, which produced dickite (δ18O = –4.1 to +3.3‰, δD = –130 to
–140‰), shows clear evidence for mixing with substantial amounts of meteoric water. Relatively low δD values
(–140‰) for this meteoric water component may indicate that its source was at high elevations.
The geologic structure, nature of alteration, styles of mineralization, and stable isotope data indicate that the
Oyu Tolgoi deposits constitute a typical porphyry system formed in an island-arc setting. The outward zonation
of sulfide minerals for the Hugo Dummett deposits, from a bornite-dominated core to chalcopyrite and pyriteenargite, can be interpreted to be related to a cooling magmatic hydrothermal system which transgressed outward over enclosing advanced argillic alteration. This resulted in some unusual alteration and sulfide parageneses, such as topaz, or pyrite, enargite, and tennantite, entrained by high-grade bornite.
† Corresponding
* Present
author: e-mail, [email protected]
address: Ivanhoe Mines Mongolia Inc., Zaluuchuud Avenue 26, Ulaan Baatar 210349, Mongolia.
©2006 by Economic Geology, Vol. 101, pp. 503–522
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KHASHGEREL ET AL.
Introduction
THE OYU TOLGOI porphyry Cu-Au system is located in the
South Gobi desert of Mongolia, 80 km north of the Mongolia-China border (Fig. 1). Since 2001, Ivanhoe Mines Mongolia Inc. has extensively explored this system with over 1,400
diamond drill holes totaling over 630 km, as of May, 2006.
The drilling has defined a north-northeast–trending zone, >6
km long, with five porphyry Cu ± Au deposits referred to as
South, Southwest, and Central Oyu Tolgoi, and Hugo Dummett South and North (Fig. 2). Deep diamond drilling is continuing at the time of writing at Hugo Dummett North and
farther to the north, concurrent with sinking of a 1,200-mdeep exploration shaft to gain access to the Hugo Dummett
North and South deposits. Published measured and indicated
resources are 1.15 billion metric tons (Bt) at 1.30 wt percent
Cu and 0.47 g/t Au at a cutoff of 0.69 wt percent Cu equiv
(Ivanhoe Mines Mongolia Inc., press release, May 2005); the
large majority of these resources are hosted by the two Hugo
Dummett deposits.
The Oyu Tolgoi system is characterized by extensive advanced argillic alteration zones caused by extreme hydrolytic
alteration, similar to those observed in many porphyry Cu-Au
districts worldwide, and these zones provide useful first-pass
exploration guides in the region. A leached outcrop with supergene alunite and Cu oxide minerals occurs as a prominent
hill at Central Oyu Tolgoi. This outcrop was noted by Magma
Copper Company geologists in September 1996 (Perelló et
al., 2001) and ultimately led to the extensive exploration undertaken by BHP Minerals and subsequently by Ivanhoe
Mines Mongolia Inc., which culminated in the discovery of
the high-grade primary mineralization of the Hugo Dummett
deposits in 2003. Each of the five deposits of this large system
has variable features and appears related to separate intrusive
centers. All deposits are related to Late Devonian quartz
monzodiorite intrusions with similar field characteristics.
Central Oyu Tolgoi, however, is different from the other four
chalcopyrite-bornite–dominant deposits, in that hypogene
covellite is the main copper ore mineral.
The term “porphyry Cu-Au deposit” used here refers to a
large tonnage of disseminated mineralization related to intermediate composition intrusions. “System” refers to connected
entities, including intrusions related to each deposit, as well
as alteration, mineralization, and their parent fluids.
This paper provides an outline of the geological and mineralogical characteristics of the system and then focuses on
a reconnaissance stable isotope (O, H, and S) study, with
most samples collected from the hypogene alteration that is
FIG.1. Location map and regional geologic setting showing the middle to late Paleozoic Gurvansayhan terrane, distribution of granitoids, major faults, and Cu-Au-Mo and coal occurrences.
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OYU TOLGOI PORPHYRY Cu-Au SYSTEM, SOUTH GOBI, MONGOLIA
FIG. 2. Geologic structure and main features of the Oyu Tolgoi porphyry Cu-Au system based largely on drill hole information and aeromagnetic results (Ivanhoe Mines Mongolia Inc., unpub. data). Representative sections through each deposit
are shown, simplified at this scale.
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KHASHGEREL ET AL.
associated with the Hugo Dummett deposits. This study is a
first attempt to elucidate the nature, source, and evolutionary
history of fluids responsible for alteration and mineralization
of the Oyu Tolgoi porphyry Cu-Au system, with particular
emphasis on the origin of advanced argillic alteration assemblages and associations, as this alteration can provide an exploration guide.
Geologic Setting and Host Rocks
The Oyu Tolgoi exploration block is located in the middle
to late Paleozoic Gurvansayhan terrane (Badarch et al., 2002;
Fig. 1), which comprises basaltic to dacitic volcanic and sedimentary rocks of island-arc affinity (Helo et al., 2006), intruded by Late Devonian and Early Carboniferous granitoids.
The large plutons (Fig. 1), which are also visible on satellite
imagery, are believed to be mainly of Early Carboniferous age
based on two unpublished U-Pb zircon ages (Wainwright et
al., 2005), whereas Late Devonian intrusions are only definitely known from the Oyu Tolgoi exploration block and at
Tsagaan Suvarga, 140 km northeast. Other major intrusions
include the 35-km-diameter Hanbogd Na alkalic granite complex of Early Permian age, located 5 km east of the Hugo
Dummett deposits. Major tectonic features include N 110° E
and N 70° E faults (Figs. 1, 2). The Gurvansayhan terrane
hosts several late Paleozoic Cu-Au-Mo porphyry prospects, as
well as exploited Early Permian coal at Tavan Tolgoi (Fig. 1).
The local geology (Fig. 2) comprises Late Devonian
basaltic to dacitic volcanic and sedimentary rocks belonging
to the Alagbayan Group overlain unconformably by Early
Carboniferous basaltic volcanic rocks, with minor sedimentary units, belonging to the Sainshandhudag Formation (Ch.
Minjin, writ. commun., 2004). A simplified stratigraphy of the
exploration block (Fig. 3) shows the main geologic units that
host the Oyu Tolgoi porphyry Cu-Au system, U-Pb zircon
ages of intrusions (Wainwright et al., 2005), and cover rocks.
Alteration and mineralization occur in the lower part of the
Late Devonian stratigraphic sequence; principal units are
augite basalt flows (Fig. 4a-c), overlying dacitic ash flow (Fig.
4d), and block-ash tuff (Fig. 4e). The dacitic ash-flow tuff,
based on U-Pb zircon dating, is 365 ± 4 Ma (Wainwright et
al., 2005). The major part of this porphyry system (Fig. 2), including an extensive hypogene zone (at least 6 × 0.5 km) of
extreme hydrolytic alteration, is concealed beneath younger
late Paleozoic formations. In addition, Cretaceous red soil, up
FIG. 3. Stratigraphy in the Oyu Tolgoi exploration area and U-Pb zircon dates on intrusions and host rocks (Wainwright
et al., 2005; G. Brimhall, writ. commun., 2003).
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OYU TOLGOI PORPHYRY Cu-Au SYSTEM, SOUTH GOBI, MONGOLIA
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FIG. 4. Altered volcanic host rocks and mineralized quartz monzodiorite intrusions from the Hugo Dummett deposit
(sample location indicated by drill hole and depth, e.g., OTD976D-1140). (a). Augite basalt, with green alteration due to
clinochlore-illite, cut by a chalcopyrite vein with muscovite selvage (OTD976D-1140). (b). Augite basalt with yellowish alteration due to pervasive siderite-illite-kaolinite (OTD976D-1120). (c). Augite basalt, characterized by augite phenocrysts,
with brown alteration due to relict biotite (?) and hematite, cut by a yellow pyrophyllite-topaz vein (OTD976D-1088). (d).
Dacitic ash-flow tuff, with strong quartz-alunite alteration; the arrow indicates alunite isotope sample (OTD385-165). (e).
Dacitic block ash tuff, typically relatively unaltered (OTD514-950.7). (f). Quartz monzodiorite with intense quartz veining
(OTD576D-1103). (g). Au-rich quartz monzodiorite, typically red, with moderate intensity quartz veining and fine bornite
on hairline fractures, and (OTD514-1493.5) (h). Quartz monzodiorite with intense muscovite alteration and fine disseminated bornite (OTD576C-1165.8).
to 45 m thick (not shown in Fig. 2), and Quaternary gravel
and sand cover most of the mineralized area. Elevation is
about 1,100 m and the relief in the district is minimal (<20–30
m). The geologic map (Fig. 2) is mainly constructed from drill
hole data and a detailed ground magnetic survey.
General Characteristics of the Oyu Tolgoi Deposits
The main features of the Oyu Tolgoi porphyry Cu-Au system are summarized in Table 1, and the overall structure,
with type cross sections, is illustrated in Figure 2.
Intrusions
In general, the porphyry system is genetically related to a
series of small, multiply intruded quartz monzodiorite intrusions, which are similar at all deposits and appear to be of
similar age. The small intrusions are developed along the
eastern margin of a large essentially unmineralized quartz
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monzodiorite. Airborne gravity interpretation, limited outcrop, and rotary percussion drilling in a grid indicate the presence of an elliptical quartz monzodiorite intrusion, oriented
north-northeast over a 5- × 8-km area.
The quartz monzodiorite intrusions are typically phenocryst rich (35–50 vol % feldspar phenocrysts, mainly 2–5
mm long). Ion probe U-Pb dating on zircon undertaken at
Stanford University indicates a 362 ± 2 Ma Late Devonian
age for the quartz monzodiorite intrusion associated with the
Southwest Oyu Tolgoi deposit (Wainwright et al., 2005). Two
other ion probe U-Pb zircon ages for syn- and postmineralization quartz monzodiorite intrusions from this deposit, undertaken at the Australian National University, returned 378
± 3 and 371 ± 3 Ma ages, respectively (G. Brimhall, writ.
commun., 2003). Two Re-Os ages of molybdenite (372 ± 1.2
and 373 ± 1.2 Ma) from the Southwest and Central Oyu Tolgoi deposits (H. Stein, writ. commun., 2003) show that the
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Quartz monzodiorite +
>20 vol % quartz veins;
abundant postmineral
andesite, basalt, and
rhyolite dikes
Subcircular zone 600 × 400 m (~0.3% Cu), with quartz veins
oriented N 110° E and small irregular quartz monzodiorite dikes
intruding basaltic host rocks; small area (300 × 50 m) of malachite
in oxide zone (40 m thick), overlies hypogene sulfide zone;
bounded to N and S by postmineralization N 70° E faults
Pipelike high-grade zone (>1% Cu) ~250 m diameter, 700 m high,
centered on small quartz monzodiorite intrusions (up to 10s m
wide), intruding basaltic host rocks, >20 vol % quartz veining;
large low-grade (0.3% Cu) envelope 600 m to N and SW of
high-grade core; weakly altered quartz monzodiorite intrusions
flank or encircle high-grade core
Subcircular zone 600-m diam, an inverted cone comprising intense
muscovite and advanced argillic alteration, hosted by >80 vol %
quartz monzodiorite dikes, minor dacitic ash flow tuff, and basaltic
volcanic rocks; principal mineralization is pyrite-covellite to depths
>600 m associated with mainly muscovite; some important zones
of hypogene chalcocite; minor porphyry Cu-Au style mineralization
(chalcopyrite-bornite) at depth flanks the advanced argillic zone
4) Cretaceous supergene chalcocite blanket up to 40 m thick
overlies highest grade pyrite-covellite
Elongate, NNE zone >2.6 km, related to subvertical quartz
monzodiorite intrusions; high-grade (>2.5% Cu) mineralization
is centered on a zone of intense quartz veining (>90% by vol)
associated with small quartz monzodiorite intrusions; at Hugo
Dummett North Au content increases and occurs in upper parts
of high-grade bornite-dominant Cu zone and with a quartz
monzodiorite intrusion characterized by red feldspar alteration,
and fine bornite, but only moderate (25%) quartz veining;
general sulfide zonation, from bornite-chalcopyrite to pyrite
with decreasing Cu grade; advanced argillic alteration in
dacitic ash-flow tuff, forms E hanging wall to mineralized zone,
and capped by sedimentary and basaltic volcanic rocks
South
Oyu
Tolgoi
Southwest
Oyu
Tolgoi
Central
Oyu
Tolgoi
Hugo
Dummett
North
and South
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Early quartz veins +
biotite alteration in
basaltic host rocks;
advanced argillic
alteration in dacitic
ash-flow tuff; late
muscovite-illite
overprints core of
porphyry system
Early quartz veins with
small quartz monzodiorite
intrusions; quartz
monzodiorite principally
altered to intense
muscovite, and locally to
advanced argillic alteration
along fracture zones and
breccias; basaltic host rocks
to quartz monzodiorite
intrusions may have thin
(m to 10 m) zones of
pyrophyllite alteration,
zoned out to chlorite
Early quartz veins, +
biotite, late magnetitealbite, muscovite-illite,
chlorite; intense muscovite
-illite overprints biotite
and probable K-feldspar
alteration in quartz
monzodiorite; outward
zonation to weak epidotechlorite-illite
Similar to SW (see below),
except outward zonation
is not apparent due to
bounding faults
Alteration
Bornite-chalcopyritechalcocite,
minor enargite,
tennantite,
<100 ppm Mo
Pyrite-covellite,
hypogene chalcocite,
chalcopyrite, bornite
at depth, <100 ppm
Mo (supergene
chalcocite)
Chalcopyrite, minor
bornite, <5% pyrite,
<100 ppm Mo
Bornite-chalcopyrite,
<5% pyrite, <100 ppm
Mo
Sulfides
High (~1) for
Hugo Dummett
North (Au-rich
quartz
monzodiorite)
Very low for
Hugo Dummett
South
Very low in
pyrite-covellite
zone but deep
chalcopyritebornite zone has
similiar Au:Cu
ratios to Southwest
Oyu Tolgoi
High: 1.2
Low: 0.2–0.4
Au:Cu (ppm:wt%)
1 The alteration mineralogy has been mainly determined by detailed SWIR spectrometer analyses on drill core plus polished and thin section petrography, all on site, and limited X-ray diffraction
analyses
Several quartz
monzodiorite intrusions;
large quartz monzodiorite,
1.5 × 3 km in area, on
west side; major late to
postmineralization
biotite granodiorite dike
intrudes axially along
the system
Quartz monzodiorite
dikes in several
generations; minor,
andesite, rhyolite,
and basalt dikes
Quartz monzodiorite +
>20 vol % quartz veins;
postmineral andesite,
rhyolite, and basalt dikes
Intrusions
Geometry and main features
Deposit
TABLE 1. Summary of the Oyu Tolgoi Porphyry Cu-Au System1
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KHASHGEREL ET AL.
OYU TOLGOI PORPHYRY Cu-Au SYSTEM, SOUTH GOBI, MONGOLIA
mineralization is similar in age to the postmineralization
quartz monzodiorite intrusion. Consequently, the age of
quartz monzodiorite intrusions and mineralization is believed
to be Late Devonian, albeit not precisely dated.
The quartz monzodiorite intrusions are high K calc-alkaline
in composition and of island-arc affinity, based on whole-rock
compositions (Kavalieris and Wainwright, 2005); they are Itype (Chappell and White, 1974) and hence relatively oxidized. This oxidized nature is supported by the occurrence of
abundant hydrothermal magnetite at all deposits and the occurrence of hydrothermal anhydrite at Southwest Oyu Tolgoi.
In addition to the quartz monzodiorite intrusions, all deposits
share a similar suite of postmineralization intrusions, which
range in composition from basalt to rhyolite. However, most
late intrusions are believed to be Early Carboniferous and not
comagmatic with the porphyry system.
At all deposits there is a generation of small multiple dikelike intrusions (a few meters to 10 m wide) associated with
intense quartz veining (Fig. 4f). At the Central Oyu Tolgoi deposit, hydrothermal breccias composed of quartz monzodiorite and quartz vein clasts are intruded by later and less quartzveined quartz monzodiorite phases. In addition, at Hugo
Dummett North, there is a conspicuous red quartz monzodiorite phase (Fig. 4g), with about 25 vol percent quartz veining, referred to as the Au-rich quartz monzodiorite. A general
feature of the porphyry system is that late quartz monzodiorite dikes cutting zones of intense quartz veining are rare, but
small dikes (meters in width) do occur at Hugo Dummett
North and the Southwest Oyu Tolgoi deposits. At the latter,
these dikes entrain both copper sulfide and quartz vein fragments, but at the former, sulfide clasts are not present and the
quartz vein clasts may be presulfide in timing.
Veins
Quartz veins at Oyu Tolgoi are characteristically highly
contorted (Fig. 4f) and anastomosing, and individual veins
branch and split. The contorted textures suggest that the
veins formed at high pressure and temperature (i.e., were
deformed under ductile conditions; Fournier, 1999). All deposits are characterized by similar quartz vein textures. The
geometry of the intensely quartz veined zones varies from
pipelike at the Southwest Oyu Tolgoi deposit to subvertical
and tabular at the Hugo Dummett deposits. Much of the
quartz veining, based on a reconnaissance study of fluid inclusion morphology (T.J. Reynolds, pers. commun., 2005),
appears to have formed at high temperatures (>450°C).
However, intense recrystallization of the quartz veins, possibly due to later thermal effects from the plutons in the district, has deformed and obliterated many of the early fluid
inclusions, rendering the quartz veins milky. Nevertheless,
sparse quartz-hosted fluid inclusions in the Hugo Dummett
North deposit were protected from this recrystallization
where they were sheltered by sulfide minerals. These inclusions consist of critical behavior as well as hypersaline and
vapor-rich types (T. J. Reynolds, pers. commun., 2005). Thus,
the fluid was close to, and at times intersected, its solvus, as
did that recorded by the inclusions at the base of the Bingham Canyon porphyry deposit, Utah, which Redmond et al.
(2004) concluded were formed at 2.5- to 3-km paleodepth
(assuming lithostatic pressure).
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A possible exception to the generally contorted nature of
the quartz vein arrays associated with most quartz monzodiorite intrusions is the quartz veining related to the Au-rich
quartz monzodiorite (Fig. 4g). This intrusive phase has typically about 25 vol percent quartz veins, which are relatively
planar and usually ≤1 cm wide. Intense hairline fracturing is
an additional characteristic (Fig. 4g). In other quartz monzodiorites, such intense hairline fracturing is not evident, perhaps due to the strong muscovite overprint.
Quartz vein xenoliths entrained in quartz monzodiorite intrusions are widespread and are good evidence for multiple
and closely timed intrusions of quartz monzodiorite, where
each intrusive phase may have been accompanied by early
quartz veining.
Advanced argillic alteration
The term “advanced argillic” may be used in a general
sense to encompass the following hypogene minerals present
at Oyu Tolgoi: K alunite, Al phosphate-sulfate minerals, pyrophyllite, diaspore, zunyite, topaz, corundum, andalusite,
kaolinite, and dickite (Meyer and Hemley, 1967). Advanced
argillic alteration is widespread at Oyu Tolgoi, proximal to
Cu-Au mineralization, and is mainly hosted by chemically receptive host rocks: dacitic ash flow tuff (80–>400 m in thickness) and to a lesser extent quartz monzodiorite and basaltic
volcanic rocks. Despite detailed mapping of this alteration
from over 300 drill holes at 5- to 10-m intervals using a visible to shortwave infrared (350–2,500 nm) reflectance spectrometer (SWIR spectrometer), as well as limited X-ray diffraction and thin-section petrography, advanced argillic
alteration is generally undifferentiated, and subdivision into
alteration assemblages (cf. Seedorff et al., 2005) has not been
possible, apart from recognition of a discrete quartz-alunite
zone at the Hugo Dummett deposits. In addition, there is no
evidence of mineral zoning with depth or toward the quartz
monzodiorite intrusions, except that kaolinite and dickite become more abundant in the upper 50 m of the advanced
argillic zone.
Pyrophyllite is the dominant component of advanced
argillic alteration at Oyu Tolgoi, irrespective of protolith, but
it always occurs with kaolinite, suggesting that pyrophyllite
may have converted to kaolinite with either decreasing temperature or silica content of the fluid (Hemley et al., 1980).
The upper limit of kaolinite stability in Philippine geothermal
systems is about 200°C (Reyes, 1990, 1991), whereas pyrophyllite has a typical temperature range of 250° to 350°C
(Hemley et al., 1980; Reyes, 1990, 1991).
In field terms, advanced argillic alteration is recognized by
its buff color. An important characteristic for Oyu Tolgoi is
the ubiquitous occurrence of dickite veins, which provide a
reliable field guide to presence of this alteration type. Dickite
veins occur to the deepest levels of the advanced argillic zone
and are virtually absent in rocks outside the advanced argillic
zone. White to purple anhydrite and hypogene orange-red
gypsum occur in fractures over narrow zones of several meters at the margins of the advanced argillic zone irrespective
of depth. Extreme base leaching of rock components to the
extent that only residual quartz remains is rare, except locally
in Central Oyu Tolgoi as 1- to 2-m-wide zones controlled by
structures.
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KHASHGEREL ET AL.
Muscovite is not a primary component of the advanced
argillic alteration as the term is used here, but muscovite occurs with pyrophyllite in less intensely altered rocks, such as
quartz monzodiorite dikes that intruded advanced argillic alteration or in basalt below the dacitic tuff, where reactive
fluid responsible for hydrolytic alteration was rapidly neutralized. Muscovite or illite may be relict from earlier phyllosilicate alteration or remain stable with incipient pyrophyllitedominant alteration.
Advanced argillic alteration at Oyu Tolgoi is capped by sedimentary and mafic volcanic rocks which were present at the
time of porphyry intrusion and alteration; the distribution of
the advanced argillic alteration (Fig. 2) suggests that it may
have originally existed above most of the Oyu Tolgoi porphyry
system where favorable lithology was present. At Hugo Dummett North, assuming the present surface is close to the Late
Devonian paleosurface, the top of advanced argillic alteration
formed at paleodepths of >1,000 m. The southern deposits,
Southwest, South, and Central Oyu Tolgoi, are eroded to
deeper levels (Fig. 2) compared to Hugo Dummett, and the
nature of the overlying rocks is unknown. At Southwest Oyu
Tolgoi there is no evidence for advanced argillic alteration.
Supergene alunite occurs at Central Oyu Tolgoi and is easily recognized as late buff to pink veins, usually at depths of
<100 m below the present surface, and from its stable isotope
geochemistry and mineralogy, but this alunite is not the focus
of the present study. The supergene alunite, based on K-Ar
dating (Perelló et al., 2001), formed as the result of deep
weathering during the Late Cretaceous.
Early K silicate alteration
At deeper levels, basaltic host rocks underwent biotite (K
silicate) alteration in conjunction with quartz veining. Biotite
is a widespread and characteristic alteration mineral in the
basaltic host rocks at Oyu Tolgoi and typifies all deposits.
However, the hydrothermal biotite is partly or completely retrograded to Mg chlorite. Augite phenocrysts up to 1 cm in diameter (Fig. 4a, c) are invariably altered to actinolite or actinolite + biotite. This feature indicates that there may have
been an early Na alteration stage before the biotite alteration.
At the Southwest Oyu Tolgoi deposit, biotite alteration of
basaltic volcanic rocks is zoned outward to a weakly developed epidote-chlorite-illite-pyrite assemblage at a distance of
about 600 m from the high-grade Cu-Au mineralized core.
This outer zone is equivalent to propylitic alteration.
In quartz monzodiorite, relicts of early biotite alteration replacing ferromagnesian phases occur at the Southwest Oyu
Tolgoi deposit. In addition, K-feldspar alteration may have
been widespread in strongly mineralized quartz monzodiorite, but intense overprinting by muscovite has destroyed most
K-feldspar alteration, except for relicts in the Au-rich quartz
monzodiorite. However, pink K-feldspar vein selvages occur
in deep drill holes beneath the Hugo Dummett and Central
Oyu Tolgoi deposits.
Magnetite alteration
Hydrothermal magnetite occurs in two paragenetic stages:
early as ≤2-cm-wide dismembered veins, which predate
quartz veins at Hugo Dummett North, and late as disseminated grains and in millimeter-wide veins and open-space
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fillings, which postdate the quartz veins. The first stage may
be related to early Na or K silicate alteration, whereas the second stage postdates biotite alteration. The second stage appears to be ubiquitous in the Oyu Tolgoi porphyry system and
is commonly associated with pink albite, which forms intergrowths or selvages to the magnetite veins.
Muscovite alteration
Quartz monzodiorite intrusions at all deposits are extensively overprinted by muscovite alteration (Fig. 4h). Finegrained white mica alteration was identified by SWIR spectrometer analysis as muscovite but mineralogically also
encompasses well-crystallized illite. Muscovite postdates
early biotite, magnetite, and albite alteration but generally
predates chlorite and late carbonate alteration. In addition,
muscovite alteration may in general predate the advanced
argillic alteration, since where advanced argillic alteration becomes weaker, muscovite alteration becomes predominant.
This relationship occurs irrespective of depth or host rock.
Therefore, muscovite alteration may be regarded as the final
stage of early K silicate alteration, as temperatures wane to
below 350°C.
During muscovite alteration, all magmatic or hydrothermal
ferromagnesian mineral phases were converted to hematite
and Ti oxides (leucoxene). The muscovite alteration affects
mainly the quartz monzodiorite but also basaltic host rocks
and dacitic ash-flow tuff. In basaltic rocks it is less apparent
and is associated with late chlorite. At deeper levels in the porphyry system, the overprint by muscovite alteration decreases
and the quartz monzodiorite is notably red (see below).
Albite alteration
Salmon-pink albite is common in the Oyu Tolgoi porphyry
system and is attributed to fine hematite dusting of the altered feldspar phenocrysts. Albite alteration occurs in vein
selvages or with magnetite and sulfides, including chalcopyrite at Southwest Oyu Tolgoi, and also characterizes the Aurich quartz monzodiorite at Hugo Dummett North. Albite alteration at Oyu Tolgoi is paragenetically late with respect to
the early high-temperature quartz veins and biotite and Kfeldspar alteration but predates muscovite.
Chlorite alteration
Late chlorite alteration of quartz monzodiorite and basaltic
wall rocks is a widespread feature, especially in the Southwest
Oyu Tolgoi deposit and in the deeper parts of the Hugo Dummett deposits. Chlorite occurs in two forms: brown Mg chlorite (clinochlore based on SWIR spectrometer analysis) as retrograde alteration of hydrothermal biotite, and dark-green
chlorite that may be intergrown with chalcopyrite. The latter
is a late filling of quartz veins and fractures and also occurs
pervasively in the quartz monzodiorite and basaltic host rocks.
Carbonate alteration
Carbonate alteration occurs as pale-brown siderite with
minor fluorite, pervasive or in fractures in a yellow-green–colored alteration zone in basaltic rocks on the margin of advanced argillic alteration. In contrast, calcite is ubiquitous as
a fracture filling in chlorite-altered basaltic host rocks.
Dolomite also occurs in high-grade mineralized zones from
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OYU TOLGOI PORPHYRY Cu-Au SYSTEM, SOUTH GOBI, MONGOLIA
the Hugo Dummett deposit, where it lines submillimetersized vugs or openings in quartz veins and is intergrown with
bornite.
Copper-gold mineralization
Chalcopyrite and bornite are the dominant copper minerals and, along with gold, appear to be paragenetically late in
all deposits, based on extensive alteration logging and mineralogical studies. The sulfide minerals are intergrown with
muscovite, as well as with late chlorite and even dolomite alteration. However, there is a close spatial relationship between
sulfide mineralization and quartz veining (and therefore earlier biotite and K-feldspar alteration), possibly dependent
upon development of permeability. Although there may have
been earlier stages of sulfide mineralization, this is difficult to
establish due to the intense later alteration.
The high-sulfidation–state sulfide minerals, enargite and
covellite, accompanied by pyrite and tennantite, account for a
small proportion of the total copper content, except in the Central Oyu Tolgoi deposit. The high-sulfidation–state sulfide minerals are largely hosted by the dacitic ash-flow tuff or quartz
monzodiorite, in zones of advanced argillic alteration (i.e., the
typical association of such sulfides; Einaudi et al., 2003). Disseminated sulfides, mainly pyrite with variable amounts of
enargite, tennantite, bornite, chalcopyrite, covellite, and chalcocite (moderate copper grades of 0.3–0.6 wt % Cu), are widespread in advanced argillic-altered dacitic ash-flow tuff in a
zone beneath the sedimentary-basaltic volcanic cover and extending from South to Central Oyu Tolgoi (Fig. 2).
Central Oyu Tolgoi is atypical in being dominated by quartz
monzodiorite with intense muscovite alteration, with minor
deep zones of alunite, pyrophyllite, topaz, zunyite, diaspore,
and kaolinite-dickite alteration associated with fractures,
breccias, or contacts. Pyrite-covellite mineralization at Central Oyu Tolgoi is cone shaped, mainly hosted by quartz monzodiorite in a zone about 600 m in diameter, and extends to
depths of >600 m. The pyrite-covellite mineralization appears
to be related to muscovite, whereas minor enargite on structures is related to alunite, pyrophyllite, topaz, zunyite, diaspore, and kaolinite-dickite alteration.
Chalcocite is common in the Hugo Dummett South and
Central Oyu Tolgoi deposits, either finely disseminated or in
thin veins, and is paragenetically the latest sulfide mineral in
zones of advanced argillic alteration but is not exclusive to
these zones. In general, chalcocite appears to be a late alteration product of earlier sulfide minerals, particularly bornite,
and accounts for significant copper in moderately mineralized
zones.
Individual deposits vary according to their dominant sulfide
mineralogy and Au (ppm)/Cu (wt %) ratio. Southwest Oyu
Tolgoi is characterized by high gold content and chalcopyritedominant mineralogy, with an overall Au/Cu ratio of about 1
that increases to 2 or 3 in the highest grade core. This Au/Cu
signature occurs over a wide zone, extending 600 m to the
southwest of the high-grade core of Southwest Oyu Tolgoi,
and 800 m to the north, to a position flanking Central Oyu
Tolgoi. In contrast, South Oyu Tolgoi has a low Au/Cu ratio
(0.4) and a bornite-chalcopyrite– or bornite-dominated mineralogy. At Central Oyu Tolgoi, Au/Cu ratios are generally low
(<0.2), but at depth in basaltic host rocks, high Au/Cu ratios,
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511
similar to Southwest Oyu Tolgoi, are present. The Hugo
Dummett deposits are both similar and dominated by bornite
(±chalcocite) in areas of highest grades, with Hugo Dummett
North being the most strongly mineralized and increasingly
Au rich at the deeper northern end. In general, the Au/Cu
ratio for Hugo Dummett South is low (0.1) but increases to 1
or more at Hugo Dummett North.
Semiductile deformation of sulfides
As well as highly contorted quartz vein textures, a significant feature of the Oyu Tolgoi deposits is syn- to postmineral
semiductile deformation of sulfides. Locally chalcopyrite and
bornite occur as intercalated bands compressed into submillimeter-wide laminated layers against quartz. This deformation is also manifest as foliated fabrics in minerals such as pyrophyllite in the advanced argillic zone which overlies and
partly encloses the Hugo Dummett deposits.
Hugo Dummett Deposits
The geology, alteration, and mineralization are similar at
Hugo Dummett South and North. The main sulfide minerals
associated with Cu grades >1 wt percent are bornite and chalcopyrite, with subordinate tennantite and chalcocite. Highgrade zones in the Hugo Dummett deposits are defined as
>2.5 wt percent Cu and are bornite dominated. Gold mineralization occurs mainly as electrum (8–28 wt % Ag), which occurs as micron-sized grains occluded by or on the margins of
sulfide minerals, mainly bornite but also tennantite. In addition, micron-sized inclusions of hessite (Ag2Te) and
clausthalite (PbSe) are also present in bornite.
The main differences between Hugo Dummett South and
North are that at the former, the Au-rich quartz monzodiorite is absent, and advanced argillic alteration is more extensive
and overprints the quartz monzodiorite intrusions and
basaltic host rocks to a relatively deeper level; in addition, the
zone of high-grade mineralization (>2.5 wt % Cu) is about
one-quarter the size of that at Hugo Dummett North (Fig. 2).
Another important difference is that the upper parts of the
Hugo Dummett North deposit are extensively cut by postmineral biotite granodiorite intrusions (Fig. 2), which widen
upward and obliterate the apex of the mineralization, whereas
at Hugo Dummett South the apex of the porphyry system is
preserved and defined by the upward termination of synmineral intrusions and quartz veins. In addition, a major fault
zone truncates the western side of the Hugo Dummett North
deposit (Fig. 2).
Due to the relatively deeper advanced argillic alteration
(with respect to the quartz monzodiorite intrusions) at Hugo
Dummett South compared to Hugo Dummett North, and the
main mineralized zone at Hugo Dummett South being several hundred meters higher in elevation at present (Fig. 2), it
is possible that Hugo Dummett South may have formed at a
shallower paleodepth than Hugo Dummett North.
Quartz veining and quartz monzodiorite intrusions
The Hugo Dummett deposits are centered on a remarkable
zone of intense quartz veining, termed the Qv90 zone (for
>90 vol % quartz veins), and several phases of quartz monzodiorite intrusion (Fig. 5). The Qv90 zone, which hosts much
of the mineralization, is related to a small zone of quartz
511
512
KHASHGEREL ET AL.
FIG. 5. Alteration zones in the Hugo Dummett deposit, shown for two type sections, oriented approximately east-west
(A-B and C-D, respectively, Fig. 2).
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OYU TOLGOI PORPHYRY Cu-Au SYSTEM, SOUTH GOBI, MONGOLIA
monzodiorite dikes and is best developed at Hugo Dummett
North, where it forms a lens up to 90 m wide in cross section,
about 600 m in vertical extent, and with a strike length >1.5
km (Fig. 5). The Qv90 zone is discontinuous between the
Hugo Dummett South and North deposits.
At Hugo Dummett North, a significant part of the highgrade mineralization occurs in the Au-rich quartz monzodiorite (Figs. 2, 5). In addition, fine disseminated bornite mineralization (1–2 wt % Cu) associated with intense muscovite
alteration and sparse quartz veins occurs in the top few hundred meters and on the margins of a large bell-shaped quartz
monzodiorite intrusion, with dimensions exceeding 1.5 × 3
km in plan at depths >1,000 m below the present surface
(Fig. 2).
Alteration zoning
The alteration zones at Hugo Dummett North are illustrated in Figure 5. The advanced argillic alteration is not well
differentiated, perhaps due to the effects of retrograde alteration and overprinting, particularly by the ubiquitous pyrophyllite and kaolinite, which partly obliterate earlier and
higher temperature mineral stages. However, although not
spatially separated, high- (andalusite) versus low-temperature
(kaolinite) stages of advanced argillic alteration are recognized, and a general paragenesis of the advanced argillic minerals can be inferred from field relationships and mineral stability temperatures, from early to late: (1) andalusite, (2)
diaspore, (3) residual quartz in the advanced argillic zone, (4)
K alunite with Al phosphate-sulfate (APS) minerals, (5) zunyite, (6) topaz, (7) pyrophyllite, (8) kaolinite, and (9) dickite
veins.
Pyrophyllite-dominant buff-gray alteration, hosted mainly
by dacitic ash-flow tuff, comprising quartz, pyrophyllite, dickite, kaolinite, diaspore, zunyite, topaz, and andalusite (Fig. 5),
may be regarded as typical of advanced argillic alteration at
Oyu Tolgoi. At Hugo Dummett, this alteration appears to be
similar from relatively shallow depths (100 m) to the deepest
levels drilled (1,400 m). Enclosed within this zone are lenses
of quartz, K-alunite, kaolinite, dickite, and APS minerals,
from a few meters to 150 m thick (Fig. 5). Alunite (Fig. 4d)
occurs in discrete zones, partly controlled by favorable horizons within dacitic ash-flow tuff, and proximal to the hightemperature porphyry system, as defined by the limit of the
early quartz veins. At Hugo Dummett South, alunite forms a
discontinuous carapace over high-grade mineralization (Fig.
5).
At the northern part of the Hugo Dummett North deposits
(north of the type section illustrated in Fig. 5), the alunite
zone becomes thin (meters thick) or disappears on some drill
sections, and the advanced argillic alteration in general weakens. Instead of advanced argillic alteration extending right to
the top of the dacitic ash flow-tuff, muscovite alteration dominates the top 20 or 30 m. Although there is a mixed zone of
both muscovite and pyrophyllite over a few meters, in general
field relationships suggest that the advanced argillic alteration
is overprinting earlier muscovite alteration.
The paragenetic relationship between alunite and pyrophyllite is difficult to establish; alternating zones (on a meter
scale) of pyrophyllite and alunite are found, but pyrophyllite
crosscutting alunite along structures is not observed at hand
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513
specimen scale. However, at thin section scale, pyrophyllite in
contact with alunite is commonly foliated and encloses small
domains of alunite, suggesting that the pyrophyllite is paragenetically later than alunite.
Diaspore is most common in the upper parts of the pyrophyllite-dominant zone and usually above zones of alunite.
Diaspore is recognized macroscopically by a distinctive finely
mottled alteration effect, and it may also occur in diffuse
veins. Petrographic studies suggest that the diaspore may be
a paragenetically early mineral, locally associated with andalusite and predating or at least intergrown with alunite.
Andalusite occurs in small aggregates (<0.5 mm), possibly
replacing feldspar phenocrysts, and is enclosed and corroded
by pyrophyllite. The occurrence of andalusite suggests temperatures of at least 260°C (Reyes, 1991) and perhaps
>350°C (Hemley et al., 1980). Over 20 occurrences of andalusite have been noted in thin section petrography from the
Hugo Dummett deposits, and their distribution suggests that
andalusite may occur throughout the advanced argillic zone.
In general the upward and outward limit of the advanced
argillic zone commonly shows a sharp transition over a few
meters to relatively unaltered dacitic block-ash tuff (Figs. 2,
4e). The block-ash tuff is characterized by illite or chlorite-illite alteration and generally acted as a lithologic barrier to extreme hydrolytic alteration. Conversely, the inward limit of
the buff-gray alteration zone is marked by a yellow mottled
rock, either dacitic ash-flow tuff or basalt, consisting of illite,
muscovite, pyrophyllite, kaolinite, siderite, and specularite
alteration (Figs. 4b-c, 5). This latter group of minerals forms
a transitional zone to chlorite-dominated alteration. This
transition is interpreted to result from the neutralization of
relatively reactive fluid by feldspar and mafic minerals. Alteration mapping and thin section petrography suggest that
the hematite (specularite in this zone, but with depth increasing amounts of hematite occur as granular grains) derives mainly from the breakdown of early magnetite and possibly to a lesser extent from hydrothermal biotite and
chlorite. The yellow mottled alteration zone grades to chlorite, illite, and hematite (green rock), mainly in underlying
basaltic volcanic units (Figs. 4a, 5), and the latter alteration
can be seen clearly to be a remnant of hydrothermal magnetite and biotite.
Specularite also extends upward into the advanced argillic
alteration to about the base of the alunite zone, and the occurrence of hematite may therefore mark the extent of former biotite alteration with magnetite.
Locally at contacts between dacitic ash-flow tuff and basalt
or quartz monzodiorite and basalt, the more mafic volcanic
rocks can be intensely altered to pyrophyllite over widths of
less than several meters (not shown in Fig. 5).
A distinctive mineralogic feature of the extreme hydrolytic
alteration at Hugo Dummett, and for the Oyu Tolgoi porphyry system generally, is the widespread occurrence of
topaz. Topaz alteration zones are not shown in Figure 5 because discrete topaz zones are generally <10 m wide. Petrographic study indicates that topaz occurs as 1- to 20-µm-sized
disseminated grains as a minor component of the pyrophyllite-dominant alteration, and as later relatively coarser, 5- to
50-µm-sized grains along fracture zones (typically 1–5 m
wide) that cut other advanced argillic assemblages, except the
513
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KHASHGEREL ET AL.
late dickite. The topaz forms pseudohexagonal-shaped crystals, usually as short prisms but locally forms long radiating
needles and invariably contains micrometer-sized inclusions
of rutile and small liquid-vapor fluid inclusions. The abundance of topaz reflects aluminous host rocks and presumably
the availability of fluorine in the reactive fluid. A moderate
formation temperature is inferred (<250°C) for the coarser
topaz due to its late paragenetic relationship with pyrophyllite
and alunite.
In general, quartz monzodiorite intrusions in the intensely
mineralized zone are altered to quartz and muscovite (Figs.
4f, h, 5), representing complete feldspar destruction. The
intense muscovite alteration of these rocks converts primary
texture to an aggregate of granular or corroded quartz and
fine muscovite (<5 µm in size), accompanied by complete destruction of all iron-bearing mineral phases. Relicts of ferromagnesian minerals are indicated by the presence of Ti oxides
(leucoxene). The muscovite-dominant alteration in the core
of the Hugo Dummett deposits is generally not associated
with significant amounts of pyrite, except where advanced
argillic alteration occurs.
At Hugo Dummett South, the top of the large quartz monzodiorite body is characterized by quartz, muscovite, pyrophyllite, dickite, kaolinite, diaspore, zunyite, topaz, andalusite, and corundum (Fig. 5). This group of minerals is
mainly a product of pyrophyllite-dominant alteration overprinting muscovite-altered quartz monzodiorite. However, in
strongly copper mineralized zones, there is also evidence that
muscovite may locally overprint advanced argillic alteration,
as discussed below.
Sulfide paragenesis and zoning
Paragenetic relationships between sulfide minerals show
similar patterns throughout both the Hugo Dummett deposits. On a large scale there is a general zonal pattern outward and upward from bornite-chalcopyrite to chalcopyrite,
followed by pyrite-enargite. High-grade bornite or bornitechalcopyrite mineralization is intergrown with muscovite alteration (Fig. 6a-b) and in some areas with late, dark-green
chlorite. This chlorite may be part of the chlorite, illite, and
hematite zone (Fig. 5). Bornite and chalcocite are also commonly intergrown with dolomite (Fig. 6c).
Enargite typically occurs with advanced argillic alteration
within dacitic ash-flow tuff and mainly occurs with pyrite in
veins. At deeper levels, tennantite rather than enargite occurs
in veins and may cut high-grade bornite-dominated mineralization. Tennantite is particularly common at Hugo Dummett
South and possibly is related to deep overprinting by pyrophyllite-dominant alteration, which also includes zunyite,
topaz, diaspore, kaolinite, and dickite. Tennantite (and very
rarely enargite) also occurs as early inclusions in bornite and
chalcopyrite in the high-grade core of the deposits.
Pyrite, where present, appears to be an early sulfide mineral in any particular assemblage or sulfide zone. In the highgrade, bornite-dominant zone, pyrite is absent, but if host
rocks contain pyrophyllite, zunyite, topaz, diaspore, kaolinite,
and dickite alteration, then pyrite is present. However, in
these situations, the pyrite does not overprint the main sulfide
minerals present but occurs as early, relatively coarse (several
millimeter diameter) subhedral to anhedral grains, enclosed
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by a paragenetic sequence of sulfide minerals. This starts with
intermediate (e.g., tennantite) to high-sulfidation–state
(rarely enargite) sulfides, with paragenetically later sulfides
indicating a progressively lower sulfidation state (chalcopyrite
and bornite), and ending with chalcocite. Moreover, the bornite and chalcopyrite are typical of high-grade zones at Hugo
Dummett, where there is no advanced argillic alteration.
These alteration-sulfide relationships suggest two stages of
alteration and sulfide precipitation, i.e., early alteration, usually typified by pyrophyllite, topaz, zunyite, diaspore, kaolinite, dickite, and accompanied by pyrite-enargite-tennantite,
followed by the later high-grade bornite-chalcopyrite mineralization associated with muscovite-dominant alteration. Alteration and sulfide mineral textures indicate that the highgrade sulfide assemblage may postdate advanced argillic
alteration and includes bornite occluding tennantite (Fig. 6d),
bornite-chalcocite occluding specularite hematite (Fig. 6e),
and bornite occluding topaz (Fig. 6f).
However, in addition to the above observations, early-stage
sulfide parageneses related to early K silicate alteration and
quartz veining cannot be excluded, but their significance currently is impossible to quantify. Possible residual chalcopyrite
and bornite are found trapped as tiny inclusions in quartz
veins and occur as rounded blebs up to several micrometers
in diameter in pyrite.
Pyrite is relatively abundant in the outer parts of the chalcopyrite zone, as subhedral grains intergrown with or enclosed by chalcopyrite. Finally, pyrite and enargite occur in
veins beyond the chalcopyrite zone, typically with enargite
filling the vein center. Minor covellite rimming pyrite occurs
in advanced argillically altered ash-flow tuff at Hugo Dummett North, whereas in the Central Oyu Tolgoi deposit,
pyrite-covellite comprises the bulk of the deposit.
Stable Isotopes
In this study, 19 samples of hydrous minerals were analyzed
for oxygen and hydrogen isotopes, and 27 samples of sulfide
(enargite, covellite, chalcopyrite, bornite, chalcocite, and
pyrite) and sulfate minerals (alunite, anhydrite, and gypsum)
were analyzed for sulfur and, where appropriate, oxygen isotopes (Table 2). For the hydrous minerals, one sample (161297, referring to drill hole followed by meter depth), a chlorite-altered biotite, is from Southwest Oyu Tolgoi and two
samples (881-306 and 881-306 vein), pyrophyllite replacing
basalt and a green pyrophyllite vein from the same location,
are from Central Oyu Tolgoi; the rest are from the Hugo
Dummett deposits. For the sulfide minerals, most samples
are from the Hugo Dummett deposits, with a few samples
from Central and Southwest Oyu Tolgoi. Two samples (15945 and 556-84) of supergene alunite are from Central Oyu
Tolgoi.
Alteration mineralogy and field relationships
of analyzed samples
Alunite forms a discrete zone within pyrophyllite-dominant
alteration at the Hugo Dummett deposits; it is invariably fine
grained (50–200 µm) and intergrown with quartz (Fig. 6g)
and unidentified APS minerals. SWIR spectrometer analyses
suggest that alunite is generally well crystallized K alunite, although Na and NH4 alunites also have been identified by
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OYU TOLGOI PORPHYRY Cu-Au SYSTEM, SOUTH GOBI, MONGOLIA
515
FIG. 6. Aspects of alteration and mineralization in the Hugo Dummett deposit. (a). High-grade bornite mineralization impregnating quartz (OTD918-1127.2). (b). Bornite intergrown with muscovite (OTD576C-1165.8; Fig. 4h shows same sample in hand specimen). (c). Bornite-chalcocite intergrown with dolomite (OTD918-1002.4). (d). Tennantite inclusions in
chalcopyrite-bornite (OTD918H-1618.5). (e). Plates of specularite occluded in bornite-chalcocite in basaltic volcanic rock
(OTD623-570.3). (f). Topaz occluded in pyrite and bornite (OTD313-593.5). (g). Fine-grained alunite sampled for isotope
study (OTD385-165). (h). Dickite vein sampled for isotope study (OTD364-550).
spectrometer analysis. All alunite samples are from dacitic
ash-flow tuff; samples 385-165, 623-233, and 367D-1129
(Table 2) represent typical pervasive alunite alteration at the
Hugo Dummett deposits, whereas sample 385-199 represents a less common alunite occurrence as fracture fillings,
and sample 470-271 is an alunite vein with pyrite and enargite
mineralization.
The pyrophyllite samples are varied in terms of their host
rock and location. Sample 470-145 is from dacitic ash-flow
tuff at relatively shallow depth in the alteration system, above
the alunite zone and outboard to strong Cu-Au mineralization. By contrast sample 514I-1235.5 is hosted by quartz
monzodiorite in the core of the high-grade bornite-dominant
copper mineralization at Hugo Dummett North. The four
other samples are hosted by augite basalt and are generally
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deep with respect to the alteration and mineralization. Sample 967C-1148.5 is notable as a 5-cm-wide, almost pure pyrophyllite vein, <1 m from a quartz monzodiorite intrusion.
Zunyite is a widespread but relatively minor component of
the advanced argillic alteration and also occurs with quartzalunite as well cutting these minerals. Zunyite invariably
forms euhedral crystals, with grain sizes up to 0.2 mm, and is
zoned. Small (<5 µm) liquid-vapor fluid inclusions are present in zunyite. Sample 448-310 comes from a 2-cm-wide
zone of brown, relatively coarse zunyite (0.1–0.2 mm) in pyrophyllite-dominant alteration from the Hugo Dummett
South deposit. Sulfide minerals associated with this zunyite
include pyrite and enargite.
All samples of muscovite analyzed for isotopic composition
come from strongly altered quartz monzodiorite and from
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KHASHGEREL ET AL.
TABLE 2. Oxygen, Hydrogen, and Sulfur Isotope Data from This Study
Sample
Location
Mineral
385-199
385-165
623-233
367D-1192.3
470-271
Hugo D South
Hugo D South
Hugo D South
Hugo D North
Hugo D South
Alunite
Alunite
Alunite
Alunite
Alunite
Pyrite
Enargite
Alunite
Alunite
Pyrite
Alunite
Pyrite
Muscovite
Muscovite
Muscovite
Muscovite
Muscovite
Pyrophyllite
Pyrophyllite
Pyrophyllite
Pyrophyllite
Pyrophyllite
Pyrophyllite
Dickite
Dickite
Chlorite
Zunyite
Covellite
Enargite
Enargite
Enargite
Pyrite
Pyrite
Pyrite
Chalcopyrite
Pyrite
Anhydrite
Pyrite
Anhydrite
Anhydrite
Gypsum
Gypsum
Gypsum
Pyrite
Chalcopyrite
Bornite
Chalcopyrite
Chalcopyrite
Chalcocite
Bornite
Bornite
159-45
515-158.9
Central
Central
556-84
Central
377-923.9
963A-1305
340-843
576C-1165.8
576C-1073
514I-1235.5
881-306
881-306_vn
340-633
967C-1148.5
470-145
572-558.3
364-550
161-297
448-310.5
514I-1045.7
576D-1028
451-513.2
921-546.2
Hugo D South
Hugo D North
Hugo D South
Hugo D North
Hugo D South
Hugo D North
Central
Central
Hugo D South
Hugo D North
Hugo D South
Hugo D South
Hugo D South
Southwest
Hugo D South
Hugo D North
Hugo D North
Hugo D South
North of South
319-412
451-513.2
976B-1202.6
Hugo D South
Hugo D South
Hugo D North
EGD008-1239.4
Hugo D North
168-344
976A-1385
340-691
173-309
963B-938.7
Southwest
Hugo D North
Central
Southwest
Hugo D North
918C-1187.9
Hugo D North
841A-900.3
770A-1052
401-592
576C-1165.8
918C-1127.5
Hugo D North
Hugo D North
Hugo D South
Hugo D North
Hugo D North
δ18O(SO4)
(‰)
17.5
14.2
20.1
11.8
8.8
δ34S
(‰)
δD
(‰)
17.9
13.5
13.7
16.3
10.4
–11.3
–9.7
–8.2
9.8
–12.2
–9.4
–11.8
–51
–43
–44
–55
–98
2.0
12.9
5.2
3.6
3.8
T (°C) Fluid
calculation
T (°C)
Mineral pairs
260
260
260
260
–73
260
263
260
260
259
δ18OH2O
(‰)
δDH2O
(‰)
9.6
6.3
12.2
3.9
–45
–37
–38
–49
0.9
–67
5.6
-0.5
5.2
3.9
4.0
3.7
5.5
6.0
6.0
3.1
1.6
–12.8
–5.4
4.2
–68
–53
–63
–56
–53
–56
–56
–58
–43
–53
–42
–120
–110
–88
260
–116
–101
–111
–104
–101
–104
–104
–106
–91
–101
–90
–140
–130
–126
–111
6.1
δ18O
(‰)
–16
–12.6
–6.1
–11.4
–22.8
–8.9
6.0
–8.3
–6.5
16.2
–8.5
6.1
10.4
16.7
4.2
15.4
–5.4
–4.6
–4.2
–5.6
–2.0
–8.4
–2.4
–5.7
9.0
3.0
8.7
7.4
7.5
8.6
10.4
10.9
10.9
8.0
6.5
–4.1
3.3
5.5
5.4
300
300
300
300
300
300
300
300
300
300
300
150
150
250
227
223
269
Notes: Mineral grains were obtained by handpicking from drill core and then crushing a small sample by mortar and pestle, and sieving off the –100-µm
fraction; sieve fractions were cleaned by ultrasound in water and dried; in some cases the mineral grains were removed from the rock sample using a small
hand drill, before sieving and cleaning; mineral separates were handpicked under low-power binocular microscope and analyzed by SWIR spectrometer and
X-ray diffraction for purity; the X-ray diffraction analyses show that all separates are generally >90 percent pure; analytical work was undertaken at the U.S.
Geological Survey Isotope Laboratory in Denver; alunite samples were treated using a 1:1 HF-H2O solution to remove silicate contamination (Wasserman et
al., 1992); sulfur isotope ratios were determined by an online method using an elemental analyzer coupled to a Micromass Optima mass spectrometer (Gieseman et al., 1994); oxygen isotope data for alunite were collected for both sulfate and hydroxyl oxygen; for analysis of δ18OSO4, alunite was dissolved in a hot
NaOH solution, and sulfate was precipitated as BaSO4 (Wasserman et al., 1992); the BaSO4 precipitate was then analyzed by fluorination with BrF5 at 580°C
in accordance with the standard analytical procedure of Clayton and Mayeda (1963) and as used by Pickthorn and O’Neil (1985) and Wasserman et al. (1992)
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OYU TOLGOI PORPHYRY Cu-Au SYSTEM, SOUTH GOBI, MONGOLIA
relatively deep parts of the porphyry system (>800-m depth).
Despite this, muscovite may be proximal to zones of advanced argillic alteration locally. Sample 576C-1073 comes
from the high-grade, bornite-dominant zone at Hugo Dummett North, within 1 to 2 cm of relatively late patches of
pyrophyllite-topaz-kaolinite alteration; sample 377-923.9
comes from a 1-cm-wide muscovite selvage to a thin chalcopyrite vein cutting altered quartz monzodiorite (this sample appears typical of D veins; Gustafson and Hunt, 1975),
and sample 963-1305 comes from a thin muscovite vein cutting quartz monzodiorite (i.e., possible evidence for postmineral muscovite alteration).
The chlorite sample 161-297 comprises brown, coarsegrained platelets (up to 1 cm in size) intergrown with a hightemperature quartz vein. This sample comes from the core of
the Southwest Oyu Tolgoi deposit, in the highest grade CuAu zone. In general, clinochlore appears to be a ubiquitous
retrograde alteration product of hydrothermal biotite in the
Oyu Tolgoi porphyry system.
Dickite is widespread as pale-green to white, translucent to
milky-white veins up to 1 cm wide (Fig. 6h) and is the latest
advanced argillic alteration mineral. The two samples of dickite (Table 2) are from the Hugo Dummett South deposit and
from moderate depths in the alteration system.
Anhydrite-gypsum veins sampled from the Hugo Dummett
deposit all come from the advanced argillic zone or close to its
margin; sample 963B-938.7 is from the outer margin, sample
EGD008-1239.4 is from the inner margin, and samples 976A1385 and 340-691 are from relatively deep levels hosted by
quartz monzodiorite and augite basalt, respectively. Samples
168-344 and 173-309 from the Southwest Oyu Tolgoi deposit
come from biotite (retrograded to chlorite)-altered basaltic
host rocks adjacent to high-grade chalcopyrite mineralization
(i.e., on the flank of the porphyry system).
Isotope equilibrium calculations for OH-bearing minerals
Oxygen and H isotope composition of water in parent fluids for OH-bearing minerals was calculated in the same manner as by Hedenquist et al. (1998) for the study of the Far
Southeast porphyry deposit, Philippines, with isotope equilibrium fractionations and equations given in Table 3. Because
there are no applicable fluid inclusion data on alteration
minerals at Oyu Tolgoi, temperature estimates of alteration
minerals are mainly based on mineral stabilities from experimental studies (Hemley et al., 1980) and from Philippine geothermal systems (Reyes, 1990, 1991), for example, as applied
by Watanabe and Hedenquist (2001) in their study of the El
Salvador porphyry deposit, Chile. Typical temperatures of
formation of minerals in the active Philippine systems are
200° to 350°C for pyrophyllite, >260° to as high as 500+°C for
andalusite, 225° to 300+°C for muscovite, 235° to 300°C for
zunyite, 200° to 300°C for diaspore, 280° to 400°C for topaz,
and 125° to 280°C for dickite.
An estimate of 260°C for alunite is based on sulfur isotope
fractionation of two samples of pyrite and alunite (Table 3),
and this is used here, consistent with typical data from other
published sources (Rye et al., 1992; Rye, 2005). Since pyrophyllite is found spatially with alunite (Fig. 5), and possibly
postdating alunite, we infer that the pyrophyllite may have
formed at a similar or slightly lower temperature. However,
for fluid calculation we consider a temperature range for pyrophyllite from 250° to 350°C and similar to muscovite. However, the lower end of the temperature range for the calculated fluid is preferred for pyrophyllite, whereas for
muscovite the higher end of the temperature range is suggested, since it forms the core of the porphyry system. For
clinochlore, a temperature range of 200° to 300°C is used.
In general, the advanced argillic minerals form at temperatures below 350° to 400°C (Giggenbach, 1997), under conditions shallower than the brittle-ductile transition in a porphyry environment (Rye, 1993, 2005; Fournier, 1999).
The H and O isotope data on minerals and the water in
their parent fluids are summarized in Table 2 and Figure 7,
the latter showing the classic meteoric water line (Craig,
1961) as well as the compositions for felsic magmatic water
(FMW; Taylor, 1992) and water from condensed volcanic
vapor (Giggenbach, 1992). Aqueous liquid exsolved from a
melt is somewhat enriched in deuterium over the water dissolved in the melt, due to fractionation on exsolution (summarized by Hedenquist and Richards, 1998). In turn, volcanic
vapor (Giggenbach, 1992) is about 10 to 20 per mil enriched
in deuterium over the hypersaline liquid from which it separates at porphyry depths, the vapor then ascending either to
condense near the surface and create acidic fluid and alteration related to the extreme hydrolytic base leaching or to
vent as high-temperature fumaroles (Hedenquist et al.,
TABLE 3. Isotope Fractionation Equations Used in This Study
Element-fractionation equation
Isotope
Temperature range (°C)
Reference
103lnαkaolinite-H2O = 2.76 × 106T–2–6.75
103lnαpyrophyllite-H2O = 2.76 × 106T–2 + 1.08 × 103T–1–5.37
103lnαillite-muscovite-H2O = 2.39 × 106T–2–3.76
103lnαalunite (SO4) =3.09 × 106T–2–2.94
103lnαchlorite-H2O = 2.69 × 109T–3–6.34 × 106T–2 +2.97 × 103T–1
103lnαkaolinite-H2O =–2.2 × 106T–2–7.7
103lnαpyrophyllite-H2O = –20 ± 5
103lnαillite-muscovite-H2O = –20 ± 5
103lnαalunite-H2O = –6
103lnαchlorite-H2O = –3.7 × 106T–2 –24
Oxygen
Oxygen
Oxygen
Oxygen
Oxygen
Hydrogen
Hydrogen
Hydrogen
Hydrogen
Hydrogen
0–350
0–700
0–700
250–450
170–350
0–300
120–400
120–400
250
500–700
Sheppard and Gilg (1996)
Savin and Lee (1988)
Sheppard and Gilg (1996)
Stoffregen et al. (1994)
Cole and Ripley (1999)
Sheppard and Gilg (1996)
Marumo et al. (1980)
Marumo et al. (1980)
Stoffregen et al. (1994)
Graham et al. (1984)
Note: The hydrogen isotope fractionation factor of muscovite (Marumo et al., 1980) was used for pyrophyllite because of the lack of experimental data
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KHASHGEREL ET AL.
FIG. 7. Isotopic compositions of alteration minerals from the Oyu Tolgoi porphyry Cu-Au system. Solid symbols show
measured oxygen and hydrogen isotopes, open symbols show calculated fluids using fractionation equations listed in Table 3
at temperatures discussed in the text. Bars show a range of temperature and change in the calculated fluid water composition for alunite, muscovite, pyrophyllite, chlorite, and dickite. FMW = felsic magmatic water, dissolved in melt (after Taylor,
1992) and andesitic volcanic vapor (after Giggenbach, 1992).
1998). The magmatic water referred to above tends to be typical of magmatic fluids derived from felsic melts in the Circum-Pacific, where subducted seawater may be a significant
contributor to the starting isotope composition for water originally dissolved in the magmas. Care must be exercised in
using these reference boxes in interpretations, as the composition of magmatic water in a given system is dependent on
the starting value for water in magmas; in addition, the water
in fluids may have changed its primary isotopic composition
as a result of equilibration with lower temperature igneous
rocks (e.g., Ohmoto and Rye, 1974; Bethke et al., 2005).
Results and discussion
The δ18O and δD values (Table 2, Fig. 7) of alunite minerals and the calculated isotopic composition of water in parent
fluids, and corresponding δ34S values (Table 2), indicate that
alunite from the Oyu Tolgoi deposits formed from magmatichydrothermal fluids. The calculated δD values of water in the
condensed vapor that formed the alunite, based on a depositional temperature of 260°C, are slightly larger than the alunite (Stoffregen et al., 1994). The high δ34S values of the alunite (8.4–17.9‰) are consistent with formation from aqueous
sulfate derived from the disproportionation of magmatic SO2.
Assuming covellite, enargite, and pyrite were close to isotopic
equilibrium with alunite (Fig. 8), the low δ34S values (–6.0 to
–16.0‰) for the sulfides, as well as the wide range for pyrite
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and enargite, suggest relatively oxidized or sulfate-rich fluids,
somewhat more oxidized than is typically indicated based on
alunite-sulfide pairs from more shallow-formed high-sulfidation epithermal deposits (Arribas, 1995; Rye, 2005). Sample
921-546.2 (Table 2) is late botryoidal pyrite coating enargite
and not spatially associated with alunite. Its low δ34S value
(–22.8‰) is typical of pyrite formed during the final collapse
of hydrothermal systems, when residual acidic ground waters
dissolved early-formed sulfide minerals (Plumlee and Rye,
1989, 1992).
The δD and δ18O isotope values for water in parent fluids
in equilibrium with muscovite and pyrophyllite are similar,
using a similar formation temperature, but their field relationships are different. Pyrophyllite-dominant alteration occurs mainly in dacitic ash-flow tuff overlying the porphyry
system, where it may be closely related to alunite, whereas
muscovite occurs deeper in the porphyry system, where it
occurs mainly as an alteration of quartz monzodiorite. It is
clear that the magmatic-derived fluids in equilibrium with
muscovite and pyrophyllite had a significant meteoric water
component. However, some of the meteoric water in the
calculated fluid compositions may have resulted from exchange between the hydrous minerals and meteoric water
during system collapse. Resolution of this problem will require detailed isotope measurements of well-documented
fluid inclusions.
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OYU TOLGOI PORPHYRY Cu-Au SYSTEM, SOUTH GOBI, MONGOLIA
FIG. 8. Sulfur isotope data for sulfide and sulfate mineral species. Note:
(1) Late pyrite botryoidal coating enargite and (2) supergene alunite from the
Central deposit.
All six of the pyrophyllite samples, and four in particular
from deep levels at Hugo Dummett North (967C-1148.5 and
514I-1235.6) and from deep portions of Central Oyu Tolgoi
(881-306 and 881-306 vein), have similar isotopic compositions, suggesting that there is not a significant difference in
their parent fluid compositions. Thus, common hydrothermal
fluid processes may have operated across the whole Oyu Tolgoi porphyry system. The sample of pyrophyllite (470-145)
with a parent fluid whose water composition is most discordant is the shallowest pyrophyllite sample and is close to the
overlying sedimentary contact (90 m below unaltered premineral rocks). Of all the pyrophyllite samples, the fluid that
formed this sample may be expected to have the greatest
involvement with meteoric water.
The δ18OH2O values calculated at 300°C (Table 2) for the
muscovite samples also show a tight cluster (3.9–5.5‰) for
four of the five samples, similar to the minor variation in values noted for muscovite that formed from magmatic fluid
over a broad area at El Salvador (Watanabe and Hedenquist,
2001). One sample (963A-1305) has low δ18OH2O value
(–0.5‰), but its position deep in the system suggests that
perhaps a 350°C temperature (resulting in a larger δ18OH2O
value) may be more appropriate for calculation.
One sample of zunyite was analyzed (δ18O = 5.4‰ and δD
= –111‰), yielding values similar to those for muscovite and
pyrophyllite. Experimental or theoretical zunyite-water O
and H isotope fractionation factors do not exist but the δ18O
and δD values obtained are consistent with the phyllosilicate
minerals, suggesting perhaps similar fractionation factors.
Zunyite is an integral component of the pyrophyllite-dominant alteration and therefore may be inferred to have formed
from similar magmatic fluid (cf. Reyes, 1991). Similarly, the
calculated parent fluid for chlorite, assuming a temperature
of 200° to 300°C, appears to be isotopically similar to the
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519
muscovite parental fluid (Fig. 7) and suggests that the widespread retrograde alteration from biotite to chlorite at Oyu
Tolgoi occurred during cooling of the magmatic-hydrothermal system, without major influx of external fluids.
In contrast, late dickite on fractures has δ18O and δD values for a parent fluid that is close to the meteoric water line
at 150°C (Fig. 7). This is a likely temperature for dickite,
similar to that found at Far Southeast, El Salvador, and elsewhere (Hedenquist et al., 1998; Watanabe and Hedenquist,
2001; Bethke et al., 2005; Fifarek and Rye, 2005), where it
also is found to have formed from meteoric-dominated
water. Dickite may result from the breakdown of pyrophyllite
at Oyu Tolgoi, since the two minerals are closely spatially
associated.
The low δD values (–140‰) for the meteoric water component are compatible with recharge at high elevation, since
paleogeographic reconstructions suggest that Mongolia was
close to its present position during the late Paleozoic (Gradstein et al., 2004). The high elevation could have been related
to a stratovolcano (e.g., Frank, 1995), given the tectonic setting of Mongolia at that time (Badarch et al., 2002).
The sulfur isotope data on sulfide minerals (Fig. 8) shows
that the range of values for each mineral is generally similar
for all of the deposits throughout the Oyu Tolgoi system. All
sulfate minerals are isotopically heavy and all sulfide minerals
are isotopically light, as observed in numerous examples of
magmatic-hydrothermal systems (e.g., Rye, 1993, 2005; Arribas, 1995). In addition, there is a consistent difference in
sulfur isotope values for different sulfide minerals that reflects sulfur isotope equilibrium in the hydrothermal fluids,
typical of such systems. As noted above, sulfur isotope data
for two samples of coexisting alunite and pyrite from Hugo
Dummett South (470-271 and 515-158.9) give equilibrium
isotopic fractionation temperatures averaging about 260°C
(Table 2). Coexisting pyrite and anhydrite from Hugo Dummett North give equilibrium isotopic fractionation temperatures of 225° to 270°C, consistent with the late paragenetic
position of the anhydrite. The calculated temperature for a
pyrite-chalcopyrite pair from Hugo Dummett North is about
230°C, the significance of which is questionable as chalcopyrite is susceptible to postmineralization recrystallization
(Barton and Skinner, 1979; Field et al., 2005). However, the
temperature may be realistic considering that copper mineralization is relatively late in the Oyu Tolgoi system.
Supergene alunite from the Central deposit has δ34S values
(–9.4 to –8.2‰) within the range of δ34S values of pyrite
(Table 2, Fig. 8), the most abundant sulfide in the Central
Oyu Tolgoi deposit, and from which aqueous sulfate may have
been derived by oxidation during deep weathering in the Cretaceous. Supergene and hypogene alunite can clearly be distinguished at Oyu Tolgoi by their δ34S values (Table 2, Fig. 8).
A single hypogene chalcocite sample (401-592) from the
Hugo Dummett South deposit has a δ34S value of –8.4 per
mil, in the range of other sulfide minerals (Table 2, Fig. 8) but
isotopically lighter than bornite samples (δ34S, –2.4 to
–5.7‰) from the main high-grade mineralization at Hugo
Dummett. The chalcocite sample is intergrown with or replaces bornite and may have formed from late, low-temperature fluid, consistent with other samples where bornite and
chalcocite are intergrown with dolomite (Fig. 6c).
519
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KHASHGEREL ET AL.
The S and O isotope values of sulfate minerals are summarized in Figure 9. The alunite samples from Hugo Dummett
North have high δ18OSO4 and δ34S values that reflect the predominant magmatic origin of their parent aqueous sulfate.
The one alunite sample from the Central Oyu Tolgoi deposit
has lower δ34S and δ18OSO4 values; this may reflect the fact
that the magmatic vapor that transported the SO2 condensed
into meteoric water. Such 18O-depleted values occur in magmatic-hydrothermal alunites that form at shallow depths in
areas of wet climate (Rye et al., 1992). The later anhydrite
and gypsum from Central and Hugo Dummett North also
have high δ34S and δ18O values, and all of the data in Figure
9 show a trend to lower δ18O values below the general δ18S
and δ18O isotope trend. Gypsum and anhydrite veins from the
Southwest deposit have the lowest δ18S and δ18O values.
These stable isotope systematics are nearly identical to those
observed for alunite and barite at the Summitville high-sulfidation deposit, Colorado (Bethke et al., 2005). As discussed in
detail by Rye (2005), this trend of S and O isotope values reflects the mixing of sulfate from magmatic sources with sulfate derived from the oxidation of H2S at shallow levels. The
mixing is particularly obvious in the parent fluids of anhydrite
and gypsum samples from the Southwest deposit. The scatter
to lower δ18OSO4 values in the anhydrite at Hugo Dummett
North may reflect the oxygen isotope exchange of aqueous
sulfate with 18O-depleted meteoric water in some of the parent fluids.
Summary and Conclusions
The alunite in the advanced argillic alteration at the Hugo
Dummett North and South deposits formed largely from condensed magmatic vapor, consistent with the origin of similar
alunite found in numerous studies of other intrusion-centered deposits (e.g., Rye et al., 1992; Arribas, 1995; Rye,
FIG.9. Sulfur and oxygen isotope data on sulfate mineral species.
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2005). There appears to have been little mixing of the condensed vapor with meteoric water during alunite deposition
at the Hugo Dummett deposits.
Magmatic-hydrothermal fluids responsible for muscovite
and pyrophyllite, based on δ18O isotope data, are similar and
involve a component of meteoric water. Pyrophyllite is a
widespread component of the advanced argillic alteration
above the porphyry system (although there is also deep
ingress of pyrophyllite-dominant alteration along lithologic
contacts and fractures), whereas muscovite dominates the
core of the porphyry system. Early high-temperature advanced argillic alteration (represented by andalusite) and alunite-bearing advanced argillic alteration underwent retrograde alteration and overprinting by pyrophyllite, kaolinite,
and dickite, as is typically observed in porphyry-related alteration of other deposits (Einaudi et al., 2003). The precursor
alteration to advanced argillic alteration was most likely muscovite in the upper parts as well as deep parts of the porphyry
system, where dacitic ash-flow tuff and quartz monzodiorite,
respectively, are the host rocks.
Despite these relationships it is unresolved whether muscovite and pyrophyllite are related to the same evolving fluid,
since in terms of spatial distribution and paragenesis, pyrophyllite appears to be closely related to evolution of the advanced argillic system and postdates the formation of condensate fluids which formed alunite. In addition, the
muscovite alteration hosted by quartz monzodiorite intrusions in the core of the porphyry system is generally separated
by a screen of basaltic host rocks (with early biotite and retrograde chlorite as well as muscovite alteration) from overlying advanced argillic alteration in dacitic ash-flow tuff (Fig.
5).
The distribution and paragenesis of the muscovite-dominant alteration indicate that it was related to the thermal collapse of the early biotite-stable porphyry system (Hedenquist
et al., 1998; Heinrich, 2005); i.e., it occurred as isotherms
contracted downward. Muscovite alteration at deep levels
may also have been synchronous with early advanced argillic
alteration in the upper parts of the porphyry system.
Copper-gold mineralization at Hugo Dummett is paragenetically late, despite the general spatial relationships to
early-formed quartz veins. The bulk of the mineralization is
closely linked by silicate-sulfide textures to the pervasive
muscovite-dominant stage (Fig. 6b), as well as partially to
dark-green chlorite (<300°C) and even later dolomite
(<250°C) alteration (Fig. 6c).
A systematic outward zonation of sulfides from bornitechalcopyrite in the core of the porphyry system, to chalcopyrite, and then pyrite-enargite on the fringe may be related
partly to an evolving hydrothermal system, where magmatic
fluids moved outward and cooled, coupled with a synchronous enveloping outer zone of advanced argillic alteration in
which the high-sulfidation–state sulfide minerals precipitated. This alteration accompanies pyrite, tennantite, and
rarely enargite and preceded the high-grade bornite-chalcopyrite mineralization.
The high-grade bornite-chalcopyrite mineralization is apparently related to late muscovite- and chlorite-stable fluid
which overprinted earlier alteration stages, including the advanced argillic alteration. This has resulted in some unusual
520
OYU TOLGOI PORPHYRY Cu-Au SYSTEM, SOUTH GOBI, MONGOLIA
sulfide-silicate textures, such as topaz occluded by bornite
(Fig. 6f).
Fluid related to postsulfide, low-temperature alteration,
such as fracture-filling dickite, shows evidence for a significant
component of meteoric water deep into the Oyu Tolgoi porphyry Cu-Au system late in its history. This meteoric water is
isotopically very light, and thus was derived from a high-elevation setting, since paleogeographic reconstructions indicate a
position of Mongolia during the late Paleozoic (Gradstein et
al., 2004) similar to the present. The meteoric water invasion
that formed the late veins is also recorded in the sulfur and
oxygen isotope data for late anhydrite and gypsum, whose parent fluid in some areas acquired a component of aqueous sulfate from the shallow vadose zone oxidation of H2S as the magmatic system waned. This is especially the case for the purple
anhydrite veins found at depth surrounding the high-grade
chalcopyrite mineralization in the Southwest deposit.
Despite a geologic environment where extreme hydrolytic
alteration appears to have formed in a deep environment, at
least 1,000 m below the paleosurface, current geologic reconstructions of the Oyu Tolgoi porphyry Cu-Au system, as well
as this reconnaissance stable isotope study, suggest that the
Hugo Dummett deposits may be interpreted in terms of a
typical porphyry model, albeit with some atypical alteration
and sulfide characteristics.
Acknowledgments
We thank Ivanhoe Mines Mongolia Inc. for financial support and permission to publish this study, and in particular
Charles Forster, Douglas Kirwin, Paul Chare, and all our geologic colleagues at the Oyu Tolgoi field site for their contributions. BK acknowledges the assistance from Ivanhoe Mines
Mongolia Inc. to undertake an M.Sc. study at the Mongolian
University of Science and Technology, School of Geology, on
the Hugo Dummett deposit, and wishes to thank all her close
colleagues and teachers, and in particular, Dash Bat-Erdene,
S. Jargalan, O. Gerel, J. Lhamsuren, D. Garamjav, Peter
Terry, G. Niislelkhuu, G. Jargaljav, R. Oyunchimeg, Tosi
Rindra, S. Zultsetseg, Ch. Oyunomin, B. Bayarmaa, and D.
Davaa-Ochir. We also thank Cyndi Kester, Cayce Gulbransen,
and Pamela Gemery-Hill for performing many of the isotope
analyses. Review comments by Phil Bethke, Richard Fifarek,
and Brian Rusk helped to clarify the initial presentation, and
we are particularly indebted to Richard Sillitoe and Albert
Gilg for detailed and careful reviews, which led to considerable improvements in scientific content.
January 3, June 7, 2006
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