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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

AbstractThe Oyu Tolgoi porphyry Cu-Au system in the South Gobi desert, Mongolia, comprises five deposits that ex-

tend over 6 km in a north-northeast–oriented zone. They occur in a middle to late Paleozoic arc terrane andare related to Late Devonian quartz monzodiorite intrusions. The Hugo Dummett deposits are the northern-most and deepest, with up to 1,000 m of premineral sedimentary and volcanic cover rock remaining. They arethe 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 intobasaltic volcanic host rocks. Sulfide minerals in these deposits are zoned outward from a bornite-dominatedcore to chalcopyrite, upward to pyrite ± enargite and covellite at shallower depth. The latter high-sulfida-tion–state sulfides are hosted by advanced argillic alteration mineral associations. This alteration is restrictedmainly to dacitic ash-flow tuff that overlies the basaltic volcanic rock and includes ubiquitous quartz and pyro-phyllite, 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 sev-eral characteristic alteration minerals in the Oyu Tolgoi system, with particular emphasis on the Hugo Dummettdeposits. 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 theporphyry hydrothermal system, without involvement of significant amounts of meteoric water. The isotopic dataindicate that pyrophyllite (δ18O = 6.5–10.9‰, δD = –90 to –106‰) formed from a magmatic fluid with a com-ponent of meteoric water. Muscovite associated with quartz monzodiorite intrusions occurs in the core of theHugo Dummett deposits, and isotopic data (δ18O = 3.0–9.0‰, δD = –101 to –116‰) show it formed from amagmatic 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 fromfluids similar to those related to the muscovite during cooling of the porphyry system. By contrast, parageneti-cally 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 theOyu Tolgoi deposits constitute a typical porphyry system formed in an island-arc setting. The outward zonationof sulfide minerals for the Hugo Dummett deposits, from a bornite-dominated core to chalcopyrite and pyrite-enargite, can be interpreted to be related to a cooling magmatic hydrothermal system which transgressed out-ward over enclosing advanced argillic alteration. This resulted in some unusual alteration and sulfide parage-neses, such as topaz, or pyrite, enargite, and tennantite, entrained by high-grade bornite.

† Corresponding author: e-mail, hashgerel@yahoo.com*Present address: Ivanhoe Mines Mongolia Inc., Zaluuchuud Avenue 26, Ulaan Baatar 210349, Mongolia.

©2006 by Economic Geology, Vol. 101, pp. 503–522

Economic GeologyBULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS

VOL. 101 May 2006 NO. 3

Introduction

THE OYU TOLGOI porphyry Cu-Au system is located in theSouth Gobi desert of Mongolia, 80 km north of the Mongo-lia-China border (Fig. 1). Since 2001, Ivanhoe Mines Mongo-lia Inc. has extensively explored this system with over 1,400diamond drill holes totaling over 630 km, as of May, 2006.The drilling has defined a north-northeast–trending zone, >6km long, with five porphyry Cu ± Au deposits referred to asSouth, Southwest, and Central Oyu Tolgoi, and Hugo Dum-mett South and North (Fig. 2). Deep diamond drilling is con-tinuing at the time of writing at Hugo Dummett North andfarther to the north, concurrent with sinking of a 1,200-m-deep exploration shaft to gain access to the Hugo DummettNorth and South deposits. Published measured and indicatedresources are 1.15 billion metric tons (Bt) at 1.30 wt percentCu and 0.47 g/t Au at a cutoff of 0.69 wt percent Cu equiv(Ivanhoe Mines Mongolia Inc., press release, May 2005); thelarge majority of these resources are hosted by the two HugoDummett deposits.

The Oyu Tolgoi system is characterized by extensive ad-vanced argillic alteration zones caused by extreme hydrolyticalteration, similar to those observed in many porphyry Cu-Audistricts worldwide, and these zones provide useful first-pass

exploration guides in the region. A leached outcrop with su-pergene alunite and Cu oxide minerals occurs as a prominenthill at Central Oyu Tolgoi. This outcrop was noted by MagmaCopper Company geologists in September 1996 (Perelló etal., 2001) and ultimately led to the extensive exploration un-dertaken by BHP Minerals and subsequently by IvanhoeMines Mongolia Inc., which culminated in the discovery ofthe high-grade primary mineralization of the Hugo Dummettdeposits in 2003. Each of the five deposits of this large systemhas variable features and appears related to separate intrusivecenters. All deposits are related to Late Devonian quartzmonzodiorite intrusions with similar field characteristics.Central Oyu Tolgoi, however, is different from the other fourchalcopyrite-bornite–dominant deposits, in that hypogenecovellite is the main copper ore mineral.

The term “porphyry Cu-Au deposit” used here refers to alarge tonnage of disseminated mineralization related to inter-mediate composition intrusions. “System” refers to connectedentities, including intrusions related to each deposit, as wellas alteration, mineralization, and their parent fluids.

This paper provides an outline of the geological and min-eralogical characteristics of the system and then focuses ona reconnaissance stable isotope (O, H, and S) study, withmost samples collected from the hypogene alteration that is

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FIG.1. Location map and regional geologic setting showing the middle to late Paleozoic Gurvansayhan terrane, distribu-tion of granitoids, major faults, and Cu-Au-Mo and coal occurrences.

OYU TOLGOI PORPHYRY Cu-Au SYSTEM, SOUTH GOBI, MONGOLIA 505

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FIG. 2. Geologic structure and main features of the Oyu Tolgoi porphyry Cu-Au system based largely on drill hole infor-mation and aeromagnetic results (Ivanhoe Mines Mongolia Inc., unpub. data). Representative sections through each depositare shown, simplified at this scale.

associated with the Hugo Dummett deposits. This study is afirst attempt to elucidate the nature, source, and evolutionaryhistory of fluids responsible for alteration and mineralizationof the Oyu Tolgoi porphyry Cu-Au system, with particularemphasis on the origin of advanced argillic alteration assem-blages and associations, as this alteration can provide an ex-ploration guide.

Geologic Setting and Host RocksThe 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 sedi-mentary rocks of island-arc affinity (Helo et al., 2006), in-truded by Late Devonian and Early Carboniferous granitoids.The large plutons (Fig. 1), which are also visible on satelliteimagery, are believed to be mainly of Early Carboniferous agebased on two unpublished U-Pb zircon ages (Wainwright etal., 2005), whereas Late Devonian intrusions are only defi-nitely known from the Oyu Tolgoi exploration block and atTsagaan Suvarga, 140 km northeast. Other major intrusionsinclude the 35-km-diameter Hanbogd Na alkalic granite com-plex of Early Permian age, located 5 km east of the Hugo

Dummett deposits. Major tectonic features include N 110° Eand N 70° E faults (Figs. 1, 2). The Gurvansayhan terranehosts several late Paleozoic Cu-Au-Mo porphyry prospects, aswell as exploited Early Permian coal at Tavan Tolgoi (Fig. 1).

The local geology (Fig. 2) comprises Late Devonianbasaltic to dacitic volcanic and sedimentary rocks belongingto the Alagbayan Group overlain unconformably by EarlyCarboniferous basaltic volcanic rocks, with minor sedimen-tary units, belonging to the Sainshandhudag Formation (Ch.Minjin, writ. commun., 2004). A simplified stratigraphy of theexploration block (Fig. 3) shows the main geologic units thathost the Oyu Tolgoi porphyry Cu-Au system, U-Pb zirconages of intrusions (Wainwright et al., 2005), and cover rocks.Alteration and mineralization occur in the lower part of theLate Devonian stratigraphic sequence; principal units areaugite 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 etal., 2005). The major part of this porphyry system (Fig. 2), in-cluding an extensive hypogene zone (at least 6 × 0.5 km) ofextreme hydrolytic alteration, is concealed beneath youngerlate Paleozoic formations. In addition, Cretaceous red soil, up

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FIG. 3. Stratigraphy in the Oyu Tolgoi exploration area and U-Pb zircon dates on intrusions and host rocks (Wainwrightet al., 2005; G. Brimhall, writ. commun., 2003).

to 45 m thick (not shown in Fig. 2), and Quaternary graveland sand cover most of the mineralized area. Elevation isabout 1,100 m and the relief in the district is minimal (<20–30m). The geologic map (Fig. 2) is mainly constructed from drillhole data and a detailed ground magnetic survey.

General Characteristics of the Oyu Tolgoi DepositsThe main features of the Oyu Tolgoi porphyry Cu-Au sys-

tem 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 aseries of small, multiply intruded quartz monzodiorite intru-sions, which are similar at all deposits and appear to be ofsimilar age. The small intrusions are developed along theeastern margin of a large essentially unmineralized quartz

monzodiorite. Airborne gravity interpretation, limited out-crop, and rotary percussion drilling in a grid indicate the pres-ence of an elliptical quartz monzodiorite intrusion, orientednorth-northeast over a 5- × 8-km area.

The quartz monzodiorite intrusions are typically phe-nocryst rich (35–50 vol % feldspar phenocrysts, mainly 2–5mm long). Ion probe U-Pb dating on zircon undertaken atStanford University indicates a 362 ± 2 Ma Late Devonianage for the quartz monzodiorite intrusion associated with theSouthwest Oyu Tolgoi deposit (Wainwright et al., 2005). Twoother ion probe U-Pb zircon ages for syn- and postmineral-ization quartz monzodiorite intrusions from this deposit, un-dertaken 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 ± 1.2and 373 ± 1.2 Ma) from the Southwest and Central Oyu Tol-goi deposits (H. Stein, writ. commun., 2003) show that the

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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 toclinochlore-illite, cut by a chalcopyrite vein with muscovite selvage (OTD976D-1140). (b). Augite basalt with yellowish al-teration 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 borniteon hairline fractures, and (OTD514-1493.5) (h). Quartz monzodiorite with intense muscovite alteration and fine dissemi-nated bornite (OTD576C-1165.8).

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TAB

LE

1. S

umm

ary

of th

e O

yu T

olgo

i Por

phyr

y C

u-A

u Sy

stem

1

Dep

osit

Geo

met

ry a

nd m

ain

feat

ures

Intr

usio

nsA

ltera

tion

Sulfi

des

Au:

Cu

(ppm

:wt%

)

Sout

hSu

bcir

cula

r zo

ne 6

00 ×

400

m (

~0.3

% C

u), w

ith q

uart

z ve

ins

Qua

rtz

mon

zodi

orite

+

Sim

ilar

to S

W (

see

belo

w),

Bor

nite

-cha

lcop

yrite

, L

ow: 0

.2–0

.4O

yu

orie

nted

N 1

10°

E a

nd s

mal

l irr

egul

ar q

uart

z m

onzo

dior

ite d

ikes

>2

0 vo

l % q

uart

z ve

ins;

ex

cept

out

war

d zo

natio

n <5

% p

yrite

, <10

0 pp

m

Tolg

oiin

trud

ing

basa

ltic

host

roc

ks; s

mal

l are

a (3

00 ×

50 m

) of

mal

achi

te

abun

dant

pos

tmin

eral

is

not

app

aren

t due

to

Mo

in o

xide

zon

e (4

0 m

thic

k), o

verl

ies

hypo

gene

sul

fide

zone

; an

desi

te, b

asal

t, an

d bo

undi

ng fa

ults

boun

ded

to N

and

S b

y po

stm

iner

aliz

atio

n N

70°

E fa

ults

rhyo

lite

dike

s

Sout

hwes

tPi

pelik

e hi

gh-g

rade

zon

e (>

1% C

u) ~

250

m d

iam

eter

, 700

m h

igh,

Q

uart

z m

onzo

dior

ite +

E

arly

qua

rtz

vein

s, +

C

halc

opyr

ite, m

inor

H

igh:

1.2

Oyu

ce

nter

ed o

n sm

all q

uart

z m

onzo

dior

ite in

trus

ions

(up

to 1

0s m

>2

0 vo

l % q

uart

z ve

ins;

bi

otite

, lat

e m

agne

tite-

born

ite, <

5% p

yrite

, To

lgoi

wid

e), i

ntru

ding

bas

altic

hos

t roc

ks, >

20 v

ol %

qua

rtz

vein

ing;

po

stm

iner

al a

ndes

ite,

albi

te, m

usco

vite

-illit

e,

<100

ppm

Mo

larg

e lo

w-g

rade

(0.

3% C

u) e

nvel

ope

600

m to

N a

nd S

W o

f rh

yolit

e, a

nd b

asal

t dik

esch

lori

te; i

nten

se m

usco

vite

high

-gra

de c

ore;

wea

kly

alte

red

quar

tz m

onzo

dior

ite in

trus

ions

-il

lite

over

prin

ts b

iotit

e fla

nk o

r en

circ

le h

igh-

grad

e co

rean

d pr

obab

le K

-fel

dspa

r al

tera

tion

in q

uart

z m

onzo

dior

ite; o

utw

ard

zona

tion

to w

eak

epid

ote-

chlo

rite

-illit

e

Cen

tral

Subc

ircu

lar

zone

600

-m d

iam

, an

inve

rted

con

e co

mpr

isin

g in

tens

e Q

uart

z m

onzo

dior

ite

Ear

ly q

uart

z ve

ins

with

Py

rite

-cov

ellit

e,

Very

low

in

Oyu

m

usco

vite

and

adv

ance

d ar

gilli

c al

tera

tion,

hos

ted

by >

80 v

ol %

di

kes

in s

ever

al

smal

l qua

rtz

mon

zodi

orite

hy

poge

ne c

halc

ocite

, py

rite

-cov

ellit

e To

lgoi

quar

tz m

onzo

dior

ite d

ikes

, min

or d

aciti

c as

h flo

w tu

ff, a

nd b

asal

tic

gene

ratio

ns; m

inor

, in

trus

ions

; qua

rtz

chal

copy

rite

, bor

nite

zo

ne b

ut d

eep

volc

anic

roc

ks; p

rinc

ipal

min

eral

izat

ion

is p

yrite

-cov

ellit

e to

dep

ths

ande

site

, rhy

olite

, m

onzo

dior

ite p

rinc

ipal

ly

at d

epth

, <10

0 pp

m

chal

copy

rite

->6

00 m

ass

ocia

ted

with

mai

nly

mus

covi

te; s

ome

impo

rtan

t zon

es

and

basa

lt di

kes

alte

red

to in

tens

e M

o (s

uper

gene

bo

rnite

zon

e ha

s of

hyp

ogen

e ch

alco

cite

; min

or p

orph

yry

Cu-

Au

styl

e m

iner

aliz

atio

n m

usco

vite

, and

loca

lly to

ch

alco

cite

)si

mili

ar A

u:C

u (c

halc

opyr

ite-b

orni

te)

at d

epth

flan

ks th

e ad

vanc

ed a

rgill

ic z

one

adva

nced

arg

illic

alte

ratio

n ra

tios

to S

outh

wes

t 4)

Cre

tace

ous

supe

rgen

e ch

alco

cite

bla

nket

up

to 4

0 m

thic

k al

ong

frac

ture

zon

es a

nd

Oyu

Tol

goi

over

lies

high

est g

rade

pyr

ite-c

ovel

lite

brec

cias

; bas

altic

hos

t roc

ks

to q

uart

z m

onzo

dior

ite

intr

usio

ns m

ay h

ave

thin

(m

to 1

0 m

) zo

nes

of

pyro

phyl

lite

alte

ratio

n,

zone

d ou

t to

chlo

rite

Hug

oE

long

ate,

NN

E z

one

>2.6

km

, rel

ated

to s

ubve

rtic

al q

uart

z Se

vera

l qua

rtz

Ear

ly q

uart

z ve

ins

+ B

orni

te-c

halc

opyr

ite-

Very

low

for

Dum

met

tm

onzo

dior

ite in

trus

ions

; hig

h-gr

ade

(>2.

5% C

u) m

iner

aliz

atio

n m

onzo

dior

ite in

trus

ions

; bi

otite

alte

ratio

n in

ch

alco

cite

, H

ugo

Dum

met

t N

orth

is c

ente

red

on a

zon

e of

inte

nse

quar

tz v

eini

ng (

>90%

by

vol)

larg

e qu

artz

mon

zodi

orite

, ba

salti

c ho

st r

ocks

; m

inor

ena

rgite

, So

uth

and

Sout

h as

soci

ated

with

sm

all q

uart

z m

onzo

dior

ite in

trus

ions

; at H

ugo

1.5

×3

km in

are

a, o

n ad

vanc

ed a

rgill

ic

tenn

antit

e,

Dum

met

t Nor

th A

u co

nten

t inc

reas

es a

nd o

ccur

s in

upp

er p

arts

w

est s

ide;

maj

or la

te to

al

tera

tion

in d

aciti

c <1

00 p

pm M

oof

hig

h-gr

ade

born

ite-d

omin

ant C

u zo

ne a

nd w

ith a

qua

rtz

post

min

eral

izat

ion

ash-

flow

tuff

; lat

e m

onzo

dior

ite in

trus

ion

char

acte

rize

d by

red

feld

spar

alte

ratio

n,

biot

ite g

rano

dior

ite d

ike

mus

covi

te-il

lite

and

fine

born

ite, b

ut o

nly

mod

erat

e (2

5%)

quar

tz v

eini

ng;

intr

udes

axi

ally

alo

ng

over

prin

ts c

ore

of

gene

ral s

ulfid

e zo

natio

n, fr

om b

orni

te-c

halc

opyr

ite to

pyr

ite

the

syst

empo

rphy

ry s

yste

mH

igh

(~1)

for

with

dec

reas

ing

Cu

grad

e; a

dvan

ced

argi

llic

alte

ratio

n in

H

ugo

Dum

met

t da

citic

ash

-flo

w tu

ff, f

orm

s E

han

ging

wal

l to

min

eral

ized

zon

e,

Nor

th (

Au-

rich

an

d ca

pped

by

sedi

men

tary

and

bas

altic

vol

cani

c ro

cks

quar

tz

mon

zodi

orite

)

1 T

he a

ltera

tion

min

eral

ogy

has

been

mai

nly

dete

rmin

ed b

y de

taile

d SW

IR s

pect

rom

eter

ana

lyse

s on

dri

ll co

re p

lus

polis

hed

and

thin

sec

tion

petr

ogra

phy,

all

on s

ite, a

nd li

mite

d X

-ray

diff

ract

ion

anal

yses

mineralization is similar in age to the postmineralizationquartz monzodiorite intrusion. Consequently, the age ofquartz monzodiorite intrusions and mineralization is believedto be Late Devonian, albeit not precisely dated.

The quartz monzodiorite intrusions are high K calc-alkalinein composition and of island-arc affinity, based on whole-rockcompositions (Kavalieris and Wainwright, 2005); they are I-type (Chappell and White, 1974) and hence relatively oxi-dized. This oxidized nature is supported by the occurrence ofabundant hydrothermal magnetite at all deposits and the oc-currence of hydrothermal anhydrite at Southwest Oyu Tolgoi.In addition to the quartz monzodiorite intrusions, all depositsshare a similar suite of postmineralization intrusions, whichrange in composition from basalt to rhyolite. However, mostlate intrusions are believed to be Early Carboniferous and notcomagmatic with the porphyry system.

At all deposits there is a generation of small multiple dike-like intrusions (a few meters to 10 m wide) associated withintense quartz veining (Fig. 4f). At the Central Oyu Tolgoi de-posit, hydrothermal breccias composed of quartz monzodior-ite and quartz vein clasts are intruded by later and less quartz-veined quartz monzodiorite phases. In addition, at HugoDummett North, there is a conspicuous red quartz monzodi-orite phase (Fig. 4g), with about 25 vol percent quartz vein-ing, referred to as the Au-rich quartz monzodiorite. A generalfeature of the porphyry system is that late quartz monzodior-ite dikes cutting zones of intense quartz veining are rare, butsmall dikes (meters in width) do occur at Hugo DummettNorth and the Southwest Oyu Tolgoi deposits. At the latter,these dikes entrain both copper sulfide and quartz vein frag-ments, but at the former, sulfide clasts are not present and thequartz vein clasts may be presulfide in timing.

Veins

Quartz veins at Oyu Tolgoi are characteristically highlycontorted (Fig. 4f) and anastomosing, and individual veinsbranch and split. The contorted textures suggest that theveins formed at high pressure and temperature (i.e., weredeformed under ductile conditions; Fournier, 1999). All de-posits are characterized by similar quartz vein textures. Thegeometry of the intensely quartz veined zones varies frompipelike at the Southwest Oyu Tolgoi deposit to subverticaland tabular at the Hugo Dummett deposits. Much of thequartz veining, based on a reconnaissance study of fluid in-clusion morphology (T.J. Reynolds, pers. commun., 2005),appears to have formed at high temperatures (>450°C).However, intense recrystallization of the quartz veins, possi-bly due to later thermal effects from the plutons in the dis-trict, has deformed and obliterated many of the early fluidinclusions, rendering the quartz veins milky. Nevertheless,sparse quartz-hosted fluid inclusions in the Hugo DummettNorth deposit were protected from this recrystallizationwhere they were sheltered by sulfide minerals. These inclu-sions consist of critical behavior as well as hypersaline andvapor-rich types (T. J. Reynolds, pers. commun., 2005). Thus,the fluid was close to, and at times intersected, its solvus, asdid that recorded by the inclusions at the base of the Bing-ham Canyon porphyry deposit, Utah, which Redmond et al.(2004) concluded were formed at 2.5- to 3-km paleodepth(assuming lithostatic pressure).

A possible exception to the generally contorted nature ofthe quartz vein arrays associated with most quartz monzodi-orite intrusions is the quartz veining related to the Au-richquartz monzodiorite (Fig. 4g). This intrusive phase has typi-cally about 25 vol percent quartz veins, which are relativelyplanar and usually ≤1 cm wide. Intense hairline fracturing isan additional characteristic (Fig. 4g). In other quartz monzo-diorites, such intense hairline fracturing is not evident, per-haps due to the strong muscovite overprint.

Quartz vein xenoliths entrained in quartz monzodiorite in-trusions are widespread and are good evidence for multipleand closely timed intrusions of quartz monzodiorite, whereeach intrusive phase may have been accompanied by earlyquartz veining.

Advanced argillic alteration

The term “advanced argillic” may be used in a generalsense to encompass the following hypogene minerals presentat Oyu Tolgoi: K alunite, Al phosphate-sulfate minerals, pyro-phyllite, diaspore, zunyite, topaz, corundum, andalusite,kaolinite, and dickite (Meyer and Hemley, 1967). Advancedargillic alteration is widespread at Oyu Tolgoi, proximal toCu-Au mineralization, and is mainly hosted by chemically re-ceptive host rocks: dacitic ash flow tuff (80–>400 m in thick-ness) and to a lesser extent quartz monzodiorite and basalticvolcanic rocks. Despite detailed mapping of this alterationfrom over 300 drill holes at 5- to 10-m intervals using a visi-ble to shortwave infrared (350–2,500 nm) reflectance spec-trometer (SWIR spectrometer), as well as limited X-ray dif-fraction and thin-section petrography, advanced argillicalteration is generally undifferentiated, and subdivision intoalteration assemblages (cf. Seedorff et al., 2005) has not beenpossible, apart from recognition of a discrete quartz-alunitezone at the Hugo Dummett deposits. In addition, there is noevidence of mineral zoning with depth or toward the quartzmonzodiorite intrusions, except that kaolinite and dickite be-come more abundant in the upper 50 m of the advancedargillic zone.

Pyrophyllite is the dominant component of advancedargillic alteration at Oyu Tolgoi, irrespective of protolith, butit always occurs with kaolinite, suggesting that pyrophyllitemay have converted to kaolinite with either decreasing tem-perature or silica content of the fluid (Hemley et al., 1980).The upper limit of kaolinite stability in Philippine geothermalsystems is about 200°C (Reyes, 1990, 1991), whereas pyro-phyllite 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 byits buff color. An important characteristic for Oyu Tolgoi isthe ubiquitous occurrence of dickite veins, which provide areliable field guide to presence of this alteration type. Dickiteveins occur to the deepest levels of the advanced argillic zoneand are virtually absent in rocks outside the advanced argilliczone. White to purple anhydrite and hypogene orange-redgypsum occur in fractures over narrow zones of several me-ters at the margins of the advanced argillic zone irrespectiveof depth. Extreme base leaching of rock components to theextent that only residual quartz remains is rare, except locallyin Central Oyu Tolgoi as 1- to 2-m-wide zones controlled bystructures.

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Muscovite is not a primary component of the advancedargillic alteration as the term is used here, but muscovite oc-curs with pyrophyllite in less intensely altered rocks, such asquartz monzodiorite dikes that intruded advanced argillic al-teration or in basalt below the dacitic tuff, where reactivefluid responsible for hydrolytic alteration was rapidly neutral-ized. Muscovite or illite may be relict from earlier phyllosili-cate alteration or remain stable with incipient pyrophyllite-dominant alteration.

Advanced argillic alteration at Oyu Tolgoi is capped by sed-imentary and mafic volcanic rocks which were present at thetime of porphyry intrusion and alteration; the distribution ofthe advanced argillic alteration (Fig. 2) suggests that it mayhave originally existed above most of the Oyu Tolgoi porphyrysystem where favorable lithology was present. At Hugo Dum-mett North, assuming the present surface is close to the LateDevonian paleosurface, the top of advanced argillic alterationformed at paleodepths of >1,000 m. The southern deposits,Southwest, South, and Central Oyu Tolgoi, are eroded todeeper levels (Fig. 2) compared to Hugo Dummett, and thenature of the overlying rocks is unknown. At Southwest OyuTolgoi there is no evidence for advanced argillic alteration.

Supergene alunite occurs at Central Oyu Tolgoi and is eas-ily recognized as late buff to pink veins, usually at depths of<100 m below the present surface, and from its stable isotopegeochemistry and mineralogy, but this alunite is not the focusof the present study. The supergene alunite, based on K-Ardating (Perelló et al., 2001), formed as the result of deepweathering during the Late Cretaceous.

Early K silicate alteration

At deeper levels, basaltic host rocks underwent biotite (Ksilicate) alteration in conjunction with quartz veining. Biotiteis a widespread and characteristic alteration mineral in thebasaltic host rocks at Oyu Tolgoi and typifies all deposits.However, the hydrothermal biotite is partly or completely ret-rograded to Mg chlorite. Augite phenocrysts up to 1 cm in di-ameter (Fig. 4a, c) are invariably altered to actinolite or acti-nolite + biotite. This feature indicates that there may havebeen an early Na alteration stage before the biotite alteration.At the Southwest Oyu Tolgoi deposit, biotite alteration ofbasaltic volcanic rocks is zoned outward to a weakly devel-oped epidote-chlorite-illite-pyrite assemblage at a distance ofabout 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 re-placing ferromagnesian phases occur at the Southwest OyuTolgoi deposit. In addition, K-feldspar alteration may havebeen widespread in strongly mineralized quartz monzodior-ite, but intense overprinting by muscovite has destroyed mostK-feldspar alteration, except for relicts in the Au-rich quartzmonzodiorite. However, pink K-feldspar vein selvages occurin deep drill holes beneath the Hugo Dummett and CentralOyu Tolgoi deposits.

Magnetite alteration

Hydrothermal magnetite occurs in two paragenetic stages:early as ≤2-cm-wide dismembered veins, which predatequartz veins at Hugo Dummett North, and late as dissemi-nated grains and in millimeter-wide veins and open-space

fillings, which postdate the quartz veins. The first stage maybe related to early Na or K silicate alteration, whereas the sec-ond stage postdates biotite alteration. The second stage ap-pears to be ubiquitous in the Oyu Tolgoi porphyry system andis commonly associated with pink albite, which forms inter-growths or selvages to the magnetite veins.

Muscovite alteration

Quartz monzodiorite intrusions at all deposits are exten-sively overprinted by muscovite alteration (Fig. 4h). Fine-grained white mica alteration was identified by SWIR spec-trometer analysis as muscovite but mineralogically alsoencompasses well-crystallized illite. Muscovite postdatesearly biotite, magnetite, and albite alteration but generallypredates chlorite and late carbonate alteration. In addition,muscovite alteration may in general predate the advancedargillic alteration, since where advanced argillic alteration be-comes weaker, muscovite alteration becomes predominant.This relationship occurs irrespective of depth or host rock.Therefore, muscovite alteration may be regarded as the finalstage of early K silicate alteration, as temperatures wane tobelow 350°C.

During muscovite alteration, all magmatic or hydrothermalferromagnesian mineral phases were converted to hematiteand Ti oxides (leucoxene). The muscovite alteration affectsmainly the quartz monzodiorite but also basaltic host rocksand dacitic ash-flow tuff. In basaltic rocks it is less apparentand is associated with late chlorite. At deeper levels in the por-phyry system, the overprint by muscovite alteration decreasesand the quartz monzodiorite is notably red (see below).

Albite alteration

Salmon-pink albite is common in the Oyu Tolgoi porphyrysystem and is attributed to fine hematite dusting of the al-tered feldspar phenocrysts. Albite alteration occurs in veinselvages or with magnetite and sulfides, including chalcopy-rite at Southwest Oyu Tolgoi, and also characterizes the Au-rich quartz monzodiorite at Hugo Dummett North. Albite al-teration at Oyu Tolgoi is paragenetically late with respect tothe early high-temperature quartz veins and biotite and K-feldspar alteration but predates muscovite.

Chlorite alteration

Late chlorite alteration of quartz monzodiorite and basalticwall rocks is a widespread feature, especially in the SouthwestOyu Tolgoi deposit and in the deeper parts of the Hugo Dum-mett deposits. Chlorite occurs in two forms: brown Mg chlo-rite (clinochlore based on SWIR spectrometer analysis) as ret-rograde alteration of hydrothermal biotite, and dark-greenchlorite that may be intergrown with chalcopyrite. The latteris a late filling of quartz veins and fractures and also occurspervasively in the quartz monzodiorite and basaltic host rocks.

Carbonate alteration

Carbonate alteration occurs as pale-brown siderite withminor fluorite, pervasive or in fractures in a yellow-green–col-ored alteration zone in basaltic rocks on the margin of ad-vanced argillic alteration. In contrast, calcite is ubiquitous asa fracture filling in chlorite-altered basaltic host rocks.Dolomite also occurs in high-grade mineralized zones from

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the Hugo Dummett deposit, where it lines submillimeter-sized vugs or openings in quartz veins and is intergrown withbornite.

Copper-gold mineralization

Chalcopyrite and bornite are the dominant copper miner-als and, along with gold, appear to be paragenetically late inall deposits, based on extensive alteration logging and miner-alogical studies. The sulfide minerals are intergrown withmuscovite, as well as with late chlorite and even dolomite al-teration. However, there is a close spatial relationship betweensulfide mineralization and quartz veining (and therefore ear-lier biotite and K-feldspar alteration), possibly dependentupon development of permeability. Although there may havebeen earlier stages of sulfide mineralization, this is difficult toestablish due to the intense later alteration.

The high-sulfidation–state sulfide minerals, enargite andcovellite, accompanied by pyrite and tennantite, account for asmall proportion of the total copper content, except in the Cen-tral Oyu Tolgoi deposit. The high-sulfidation–state sulfide min-erals are largely hosted by the dacitic ash-flow tuff or quartzmonzodiorite, in zones of advanced argillic alteration (i.e., thetypical association of such sulfides; Einaudi et al., 2003). Dis-seminated sulfides, mainly pyrite with variable amounts ofenargite, tennantite, bornite, chalcopyrite, covellite, and chal-cocite (moderate copper grades of 0.3–0.6 wt % Cu), are wide-spread in advanced argillic-altered dacitic ash-flow tuff in azone beneath the sedimentary-basaltic volcanic cover and ex-tending from South to Central Oyu Tolgoi (Fig. 2).

Central Oyu Tolgoi is atypical in being dominated by quartzmonzodiorite with intense muscovite alteration, with minordeep zones of alunite, pyrophyllite, topaz, zunyite, diaspore,and kaolinite-dickite alteration associated with fractures,breccias, or contacts. Pyrite-covellite mineralization at Cen-tral Oyu Tolgoi is cone shaped, mainly hosted by quartz mon-zodiorite in a zone about 600 m in diameter, and extends todepths of >600 m. The pyrite-covellite mineralization appearsto be related to muscovite, whereas minor enargite on struc-tures is related to alunite, pyrophyllite, topaz, zunyite, dias-pore, and kaolinite-dickite alteration.

Chalcocite is common in the Hugo Dummett South andCentral Oyu Tolgoi deposits, either finely disseminated or inthin veins, and is paragenetically the latest sulfide mineral inzones of advanced argillic alteration but is not exclusive tothese zones. In general, chalcocite appears to be a late alter-ation product of earlier sulfide minerals, particularly bornite,and accounts for significant copper in moderately mineralizedzones.

Individual deposits vary according to their dominant sulfidemineralogy and Au (ppm)/Cu (wt %) ratio. Southwest OyuTolgoi is characterized by high gold content and chalcopyrite-dominant mineralogy, with an overall Au/Cu ratio of about 1that increases to 2 or 3 in the highest grade core. This Au/Cusignature occurs over a wide zone, extending 600 m to thesouthwest of the high-grade core of Southwest Oyu Tolgoi,and 800 m to the north, to a position flanking Central OyuTolgoi. In contrast, South Oyu Tolgoi has a low Au/Cu ratio(0.4) and a bornite-chalcopyrite– or bornite-dominated min-eralogy. At Central Oyu Tolgoi, Au/Cu ratios are generally low(<0.2), but at depth in basaltic host rocks, high Au/Cu ratios,

similar to Southwest Oyu Tolgoi, are present. The HugoDummett deposits are both similar and dominated by bornite(±chalcocite) in areas of highest grades, with Hugo DummettNorth being the most strongly mineralized and increasinglyAu rich at the deeper northern end. In general, the Au/Curatio for Hugo Dummett South is low (0.1) but increases to 1or more at Hugo Dummett North.

Semiductile deformation of sulfides

As well as highly contorted quartz vein textures, a signifi-cant feature of the Oyu Tolgoi deposits is syn- to postmineralsemiductile deformation of sulfides. Locally chalcopyrite andbornite occur as intercalated bands compressed into submil-limeter-wide laminated layers against quartz. This deforma-tion is also manifest as foliated fabrics in minerals such as py-rophyllite in the advanced argillic zone which overlies andpartly encloses the Hugo Dummett deposits.

Hugo Dummett DepositsThe geology, alteration, and mineralization are similar at

Hugo Dummett South and North. The main sulfide mineralsassociated with Cu grades >1 wt percent are bornite and chal-copyrite, with subordinate tennantite and chalcocite. High-grade zones in the Hugo Dummett deposits are defined as>2.5 wt percent Cu and are bornite dominated. Gold miner-alization occurs mainly as electrum (8–28 wt % Ag), which oc-curs as micron-sized grains occluded by or on the margins ofsulfide minerals, mainly bornite but also tennantite. In addi-tion, micron-sized inclusions of hessite (Ag2Te) andclausthalite (PbSe) are also present in bornite.

The main differences between Hugo Dummett South andNorth are that at the former, the Au-rich quartz monzodior-ite is absent, and advanced argillic alteration is more extensiveand overprints the quartz monzodiorite intrusions andbasaltic host rocks to a relatively deeper level; in addition, thezone of high-grade mineralization (>2.5 wt % Cu) is aboutone-quarter the size of that at Hugo Dummett North (Fig. 2).Another important difference is that the upper parts of theHugo Dummett North deposit are extensively cut by post-mineral biotite granodiorite intrusions (Fig. 2), which widenupward and obliterate the apex of the mineralization, whereasat Hugo Dummett South the apex of the porphyry system ispreserved and defined by the upward termination of synmin-eral intrusions and quartz veins. In addition, a major faultzone truncates the western side of the Hugo Dummett Northdeposit (Fig. 2).

Due to the relatively deeper advanced argillic alteration(with respect to the quartz monzodiorite intrusions) at HugoDummett South compared to Hugo Dummett North, and themain mineralized zone at Hugo Dummett South being sev-eral hundred meters higher in elevation at present (Fig. 2), itis possible that Hugo Dummett South may have formed at ashallower paleodepth than Hugo Dummett North.

Quartz veining and quartz monzodiorite intrusions

The Hugo Dummett deposits are centered on a remarkablezone of intense quartz veining, termed the Qv90 zone (for>90 vol % quartz veins), and several phases of quartz monzo-diorite intrusion (Fig. 5). The Qv90 zone, which hosts muchof the mineralization, is related to a small zone of quartz

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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).

monzodiorite dikes and is best developed at Hugo DummettNorth, where it forms a lens up to 90 m wide in cross section,about 600 m in vertical extent, and with a strike length >1.5km (Fig. 5). The Qv90 zone is discontinuous between theHugo Dummett South and North deposits.

At Hugo Dummett North, a significant part of the high-grade mineralization occurs in the Au-rich quartz monzodior-ite (Figs. 2, 5). In addition, fine disseminated bornite miner-alization (1–2 wt % Cu) associated with intense muscovitealteration and sparse quartz veins occurs in the top few hun-dred meters and on the margins of a large bell-shaped quartzmonzodiorite intrusion, with dimensions exceeding 1.5 × 3km in plan at depths >1,000 m below the present surface(Fig. 2).

Alteration zoning

The alteration zones at Hugo Dummett North are illus-trated in Figure 5. The advanced argillic alteration is not welldifferentiated, perhaps due to the effects of retrograde alter-ation and overprinting, particularly by the ubiquitous pyro-phyllite and kaolinite, which partly obliterate earlier andhigher temperature mineral stages. However, although notspatially separated, high- (andalusite) versus low-temperature(kaolinite) stages of advanced argillic alteration are recog-nized, and a general paragenesis of the advanced argillic min-erals can be inferred from field relationships and mineral sta-bility 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) zun-yite, (6) topaz, (7) pyrophyllite, (8) kaolinite, and (9) dickiteveins.

Pyrophyllite-dominant buff-gray alteration, hosted mainlyby dacitic ash-flow tuff, comprising quartz, pyrophyllite, dick-ite, kaolinite, diaspore, zunyite, topaz, and andalusite (Fig. 5),may be regarded as typical of advanced argillic alteration atOyu Tolgoi. At Hugo Dummett, this alteration appears to besimilar from relatively shallow depths (100 m) to the deepestlevels drilled (1,400 m). Enclosed within this zone are lensesof 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 hori-zons within dacitic ash-flow tuff, and proximal to the high-temperature porphyry system, as defined by the limit of theearly quartz veins. At Hugo Dummett South, alunite forms adiscontinuous 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 alunitezone becomes thin (meters thick) or disappears on some drillsections, and the advanced argillic alteration in general weak-ens. Instead of advanced argillic alteration extending right tothe top of the dacitic ash flow-tuff, muscovite alteration dom-inates the top 20 or 30 m. Although there is a mixed zone ofboth muscovite and pyrophyllite over a few meters, in generalfield relationships suggest that the advanced argillic alterationis overprinting earlier muscovite alteration.

The paragenetic relationship between alunite and pyro-phyllite is difficult to establish; alternating zones (on a meterscale) of pyrophyllite and alunite are found, but pyrophyllitecrosscutting alunite along structures is not observed at hand

specimen scale. However, at thin section scale, pyrophyllite incontact with alunite is commonly foliated and encloses smalldomains of alunite, suggesting that the pyrophyllite is parage-netically later than alunite.

Diaspore is most common in the upper parts of the pyro-phyllite-dominant zone and usually above zones of alunite.Diaspore is recognized macroscopically by a distinctive finelymottled alteration effect, and it may also occur in diffuseveins. Petrographic studies suggest that the diaspore may bea paragenetically early mineral, locally associated with an-dalusite and predating or at least intergrown with alunite.

Andalusite occurs in small aggregates (<0.5 mm), possiblyreplacing feldspar phenocrysts, and is enclosed and corrodedby pyrophyllite. The occurrence of andalusite suggests tem-peratures of at least 260°C (Reyes, 1991) and perhaps>350°C (Hemley et al., 1980). Over 20 occurrences of an-dalusite have been noted in thin section petrography from theHugo Dummett deposits, and their distribution suggests thatandalusite may occur throughout the advanced argillic zone.

In general the upward and outward limit of the advancedargillic zone commonly shows a sharp transition over a fewmeters to relatively unaltered dacitic block-ash tuff (Figs. 2,4e). The block-ash tuff is characterized by illite or chlorite-il-lite alteration and generally acted as a lithologic barrier to ex-treme hydrolytic alteration. Conversely, the inward limit ofthe buff-gray alteration zone is marked by a yellow mottledrock, either dacitic ash-flow tuff or basalt, consisting of illite,muscovite, pyrophyllite, kaolinite, siderite, and specularitealteration (Figs. 4b-c, 5). This latter group of minerals formsa transitional zone to chlorite-dominated alteration. Thistransition is interpreted to result from the neutralization ofrelatively reactive fluid by feldspar and mafic minerals. Al-teration mapping and thin section petrography suggest thatthe hematite (specularite in this zone, but with depth in-creasing amounts of hematite occur as granular grains) de-rives mainly from the breakdown of early magnetite and pos-sibly to a lesser extent from hydrothermal biotite andchlorite. The yellow mottled alteration zone grades to chlo-rite, illite, and hematite (green rock), mainly in underlyingbasaltic volcanic units (Figs. 4a, 5), and the latter alterationcan be seen clearly to be a remnant of hydrothermal mag-netite and biotite.

Specularite also extends upward into the advanced argillicalteration to about the base of the alunite zone, and the oc-currence of hematite may therefore mark the extent of for-mer biotite alteration with magnetite.

Locally at contacts between dacitic ash-flow tuff and basaltor quartz monzodiorite and basalt, the more mafic volcanicrocks can be intensely altered to pyrophyllite over widths ofless than several meters (not shown in Fig. 5).

A distinctive mineralogic feature of the extreme hydrolyticalteration at Hugo Dummett, and for the Oyu Tolgoi por-phyry system generally, is the widespread occurrence oftopaz. Topaz alteration zones are not shown in Figure 5 be-cause discrete topaz zones are generally <10 m wide. Petro-graphic study indicates that topaz occurs as 1- to 20-µm-sizeddisseminated grains as a minor component of the pyrophyl-lite-dominant alteration, and as later relatively coarser, 5- to50-µm-sized grains along fracture zones (typically 1–5 mwide) that cut other advanced argillic assemblages, except the

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late dickite. The topaz forms pseudohexagonal-shaped crys-tals, usually as short prisms but locally forms long radiatingneedles and invariably contains micrometer-sized inclusionsof rutile and small liquid-vapor fluid inclusions. The abun-dance of topaz reflects aluminous host rocks and presumablythe availability of fluorine in the reactive fluid. A moderateformation temperature is inferred (<250°C) for the coarsertopaz due to its late paragenetic relationship with pyrophylliteand alunite.

In general, quartz monzodiorite intrusions in the intenselymineralized zone are altered to quartz and muscovite (Figs.4f, h, 5), representing complete feldspar destruction. Theintense muscovite alteration of these rocks converts primarytexture to an aggregate of granular or corroded quartz andfine muscovite (<5 µm in size), accompanied by complete de-struction of all iron-bearing mineral phases. Relicts of ferro-magnesian minerals are indicated by the presence of Ti oxides(leucoxene). The muscovite-dominant alteration in the coreof the Hugo Dummett deposits is generally not associatedwith significant amounts of pyrite, except where advancedargillic alteration occurs.

At Hugo Dummett South, the top of the large quartz mon-zodiorite body is characterized by quartz, muscovite, pyro-phyllite, dickite, kaolinite, diaspore, zunyite, topaz, an-dalusite, and corundum (Fig. 5). This group of minerals ismainly a product of pyrophyllite-dominant alteration over-printing muscovite-altered quartz monzodiorite. However, instrongly copper mineralized zones, there is also evidence thatmuscovite may locally overprint advanced argillic alteration,as discussed below.

Sulfide paragenesis and zoning

Paragenetic relationships between sulfide minerals showsimilar patterns throughout both the Hugo Dummett de-posits. On a large scale there is a general zonal pattern out-ward and upward from bornite-chalcopyrite to chalcopyrite,followed by pyrite-enargite. High-grade bornite or bornite-chalcopyrite mineralization is intergrown with muscovite al-teration (Fig. 6a-b) and in some areas with late, dark-greenchlorite. This chlorite may be part of the chlorite, illite, andhematite zone (Fig. 5). Bornite and chalcocite are also com-monly intergrown with dolomite (Fig. 6c).

Enargite typically occurs with advanced argillic alterationwithin dacitic ash-flow tuff and mainly occurs with pyrite inveins. At deeper levels, tennantite rather than enargite occursin veins and may cut high-grade bornite-dominated mineral-ization. Tennantite is particularly common at Hugo DummettSouth and possibly is related to deep overprinting by pyro-phyllite-dominant alteration, which also includes zunyite,topaz, diaspore, kaolinite, and dickite. Tennantite (and veryrarely enargite) also occurs as early inclusions in bornite andchalcopyrite in the high-grade core of the deposits.

Pyrite, where present, appears to be an early sulfide min-eral in any particular assemblage or sulfide zone. In the high-grade, bornite-dominant zone, pyrite is absent, but if hostrocks contain pyrophyllite, zunyite, topaz, diaspore, kaolinite,and dickite alteration, then pyrite is present. However, inthese situations, the pyrite does not overprint the main sulfideminerals present but occurs as early, relatively coarse (severalmillimeter diameter) subhedral to anhedral grains, enclosed

by a paragenetic sequence of sulfide minerals. This starts withintermediate (e.g., tennantite) to high-sulfidation–state(rarely enargite) sulfides, with paragenetically later sulfidesindicating a progressively lower sulfidation state (chalcopyriteand bornite), and ending with chalcocite. Moreover, the bor-nite and chalcopyrite are typical of high-grade zones at HugoDummett, where there is no advanced argillic alteration.

These alteration-sulfide relationships suggest two stages ofalteration and sulfide precipitation, i.e., early alteration, usu-ally typified by pyrophyllite, topaz, zunyite, diaspore, kaolin-ite, dickite, and accompanied by pyrite-enargite-tennantite,followed by the later high-grade bornite-chalcopyrite miner-alization associated with muscovite-dominant alteration. Al-teration and sulfide mineral textures indicate that the high-grade sulfide assemblage may postdate advanced argillicalteration 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-stagesulfide parageneses related to early K silicate alteration andquartz veining cannot be excluded, but their significance cur-rently is impossible to quantify. Possible residual chalcopyriteand bornite are found trapped as tiny inclusions in quartzveins and occur as rounded blebs up to several micrometersin diameter in pyrite.

Pyrite is relatively abundant in the outer parts of the chal-copyrite zone, as subhedral grains intergrown with or en-closed by chalcopyrite. Finally, pyrite and enargite occur inveins beyond the chalcopyrite zone, typically with enargitefilling the vein center. Minor covellite rimming pyrite occursin advanced argillically altered ash-flow tuff at Hugo Dum-mett North, whereas in the Central Oyu Tolgoi deposit,pyrite-covellite comprises the bulk of the deposit.

Stable IsotopesIn this study, 19 samples of hydrous minerals were analyzed

for oxygen and hydrogen isotopes, and 27 samples of sulfide(enargite, covellite, chalcopyrite, bornite, chalcocite, andpyrite) and sulfate minerals (alunite, anhydrite, and gypsum)were analyzed for sulfur and, where appropriate, oxygen iso-topes (Table 2). For the hydrous minerals, one sample (161-297, referring to drill hole followed by meter depth), a chlo-rite-altered biotite, is from Southwest Oyu Tolgoi and twosamples (881-306 and 881-306 vein), pyrophyllite replacingbasalt and a green pyrophyllite vein from the same location,are from Central Oyu Tolgoi; the rest are from the HugoDummett deposits. For the sulfide minerals, most samplesare from the Hugo Dummett deposits, with a few samplesfrom Central and Southwest Oyu Tolgoi. Two samples (159-45 and 556-84) of supergene alunite are from Central OyuTolgoi.

Alteration mineralogy and field relationships of analyzed samples

Alunite forms a discrete zone within pyrophyllite-dominantalteration at the Hugo Dummett deposits; it is invariably finegrained (50–200 µm) and intergrown with quartz (Fig. 6g)and unidentified APS minerals. SWIR spectrometer analysessuggest that alunite is generally well crystallized K alunite, al-though Na and NH4 alunites also have been identified by

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spectrometer analysis. All alunite samples are from daciticash-flow tuff; samples 385-165, 623-233, and 367D-1129(Table 2) represent typical pervasive alunite alteration at theHugo Dummett deposits, whereas sample 385-199 repre-sents a less common alunite occurrence as fracture fillings,and sample 470-271 is an alunite vein with pyrite and enargitemineralization.

The pyrophyllite samples are varied in terms of their hostrock and location. Sample 470-145 is from dacitic ash-flowtuff at relatively shallow depth in the alteration system, abovethe alunite zone and outboard to strong Cu-Au mineraliza-tion. By contrast sample 514I-1235.5 is hosted by quartzmonzodiorite in the core of the high-grade bornite-dominantcopper mineralization at Hugo Dummett North. The fourother samples are hosted by augite basalt and are generally

deep with respect to the alteration and mineralization. Sam-ple 967C-1148.5 is notable as a 5-cm-wide, almost pure pyro-phyllite vein, <1 m from a quartz monzodiorite intrusion.

Zunyite is a widespread but relatively minor component ofthe advanced argillic alteration and also occurs with quartz-alunite as well cutting these minerals. Zunyite invariablyforms euhedral crystals, with grain sizes up to 0.2 mm, and iszoned. Small (<5 µm) liquid-vapor fluid inclusions are pre-sent in zunyite. Sample 448-310 comes from a 2-cm-widezone of brown, relatively coarse zunyite (0.1–0.2 mm) in py-rophyllite-dominant alteration from the Hugo DummettSouth deposit. Sulfide minerals associated with this zunyiteinclude pyrite and enargite.

All samples of muscovite analyzed for isotopic compositioncome from strongly altered quartz monzodiorite and from

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FIG. 6. Aspects of alteration and mineralization in the Hugo Dummett deposit. (a). High-grade bornite mineralization im-pregnating quartz (OTD918-1127.2). (b). Bornite intergrown with muscovite (OTD576C-1165.8; Fig. 4h shows same sam-ple in hand specimen). (c). Bornite-chalcocite intergrown with dolomite (OTD918-1002.4). (d). Tennantite inclusions inchalcopyrite-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 isotopestudy (OTD385-165). (h). Dickite vein sampled for isotope study (OTD364-550).

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TABLE 2. Oxygen, Hydrogen, and Sulfur Isotope Data from This Study

δ18O(SO4) δ34S δD δ18O T (°C) Fluid T (°C) δ18OH2O δDH2O

Sample Location Mineral (‰) (‰) (‰) (‰) calculation Mineral pairs (‰) (‰)

385-199 Hugo D South Alunite 17.5 17.9 –51 260 9.6 –45385-165 Hugo D South Alunite 14.2 13.5 –43 260 6.3 –37623-233 Hugo D South Alunite 20.1 13.7 –44 260 12.2 –38367D-1192.3 Hugo D North Alunite 11.8 16.3 –55 260 3.9 –49470-271 Hugo D South Alunite 10.4 –98

Pyrite –11.3 260 263Enargite –9.7

159-45 Central Alunite –8.2 260515-158.9 Central Alunite 8.8 9.8 –73 260 259 0.9 –67

Pyrite –12.2556-84 Central Alunite –9.4 260

Pyrite –11.8377-923.9 Hugo D South Muscovite –116 9.0 300 5.6 –68963A-1305 Hugo D North Muscovite –101 3.0 300 -0.5 –53340-843 Hugo D South Muscovite –111 8.7 300 5.2 –63576C-1165.8 Hugo D North Muscovite –104 7.4 300 3.9 –56576C-1073 Hugo D South Muscovite –101 7.5 300 4.0 –53514I-1235.5 Hugo D North Pyrophyllite –104 8.6 300 3.7 –56881-306 Central Pyrophyllite –104 10.4 300 5.5 –56881-306_vn Central Pyrophyllite –106 10.9 300 6.0 –58340-633 Hugo D South Pyrophyllite –91 10.9 300 6.0 –43967C-1148.5 Hugo D North Pyrophyllite –101 8.0 300 3.1 –53470-145 Hugo D South Pyrophyllite –90 6.5 300 1.6 –42572-558.3 Hugo D South Dickite –140 –4.1 150 –12.8 –120364-550 Hugo D South Dickite –130 3.3 150 –5.4 –110161-297 Southwest Chlorite –126 5.5 250 4.2 –88448-310.5 Hugo D South Zunyite –111 5.4514I-1045.7 Hugo D North Covellite –16576D-1028 Hugo D North Enargite –12.6451-513.2 Hugo D South Enargite –6.1921-546.2 North of South Enargite –11.4

Pyrite –22.8319-412 Hugo D South Pyrite –8.9451-513.2 Hugo D South Pyrite 6.0976B-1202.6 Hugo D North Chalcopyrite –8.3 227

Pyrite –6.5EGD008-1239.4 Hugo D North Anhydrite 6.1 16.2 223

Pyrite –8.5168-344 Southwest Anhydrite 2.0 6.1976A-1385 Hugo D North Anhydrite 12.9 10.4340-691 Central Gypsum 5.2 16.7173-309 Southwest Gypsum 3.6 4.2963B-938.7 Hugo D North Gypsum 3.8 15.4 269

Pyrite –5.4918C-1187.9 Hugo D North Chalcopyrite –4.6

Bornite –4.2841A-900.3 Hugo D North Chalcopyrite –5.6770A-1052 Hugo D North Chalcopyrite –2.0401-592 Hugo D South Chalcocite –8.4576C-1165.8 Hugo D North Bornite –2.4918C-1127.5 Hugo D North Bornite –5.7

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-µmfraction; sieve fractions were cleaned by ultrasound in water and dried; in some cases the mineral grains were removed from the rock sample using a smallhand drill, before sieving and cleaning; mineral separates were handpicked under low-power binocular microscope and analyzed by SWIR spectrometer andX-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 etal., 1992); sulfur isotope ratios were determined by an online method using an elemental analyzer coupled to a Micromass Optima mass spectrometer (Giese-man 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 hotNaOH solution, and sulfate was precipitated as BaSO4 (Wasserman et al., 1992); the BaSO4 precipitate was then analyzed by fluorination with BrF5 at 580°Cin 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)

relatively deep parts of the porphyry system (>800-m depth).Despite this, muscovite may be proximal to zones of ad-vanced argillic alteration locally. Sample 576C-1073 comesfrom the high-grade, bornite-dominant zone at Hugo Dum-mett North, within 1 to 2 cm of relatively late patches ofpyrophyllite-topaz-kaolinite alteration; sample 377-923.9comes from a 1-cm-wide muscovite selvage to a thin chal-copyrite vein cutting altered quartz monzodiorite (this sam-ple appears typical of D veins; Gustafson and Hunt, 1975),and sample 963-1305 comes from a thin muscovite vein cut-ting quartz monzodiorite (i.e., possible evidence for post-mineral muscovite alteration).

The chlorite sample 161-297 comprises brown, coarse-grained platelets (up to 1 cm in size) intergrown with a high-temperature quartz vein. This sample comes from the core ofthe Southwest Oyu Tolgoi deposit, in the highest grade Cu-Au zone. In general, clinochlore appears to be a ubiquitousretrograde alteration product of hydrothermal biotite in theOyu Tolgoi porphyry system.

Dickite is widespread as pale-green to white, translucent tomilky-white veins up to 1 cm wide (Fig. 6h) and is the latestadvanced argillic alteration mineral. The two samples of dick-ite (Table 2) are from the Hugo Dummett South deposit andfrom moderate depths in the alteration system.

Anhydrite-gypsum veins sampled from the Hugo Dummettdeposit all come from the advanced argillic zone or close to itsmargin; sample 963B-938.7 is from the outer margin, sampleEGD008-1239.4 is from the inner margin, and samples 976A-1385 and 340-691 are from relatively deep levels hosted byquartz monzodiorite and augite basalt, respectively. Samples168-344 and 173-309 from the Southwest Oyu Tolgoi depositcome from biotite (retrograded to chlorite)-altered basaltichost 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 flu-ids for OH-bearing minerals was calculated in the same man-ner as by Hedenquist et al. (1998) for the study of the FarSoutheast porphyry deposit, Philippines, with isotope equilib-rium fractionations and equations given in Table 3. Becausethere are no applicable fluid inclusion data on alterationminerals at Oyu Tolgoi, temperature estimates of alteration

minerals are mainly based on mineral stabilities from experi-mental studies (Hemley et al., 1980) and from Philippine ge-othermal systems (Reyes, 1990, 1991), for example, as appliedby Watanabe and Hedenquist (2001) in their study of the ElSalvador porphyry deposit, Chile. Typical temperatures offormation of minerals in the active Philippine systems are200° to 350°C for pyrophyllite, >260° to as high as 500+°C forandalusite, 225° to 300+°C for muscovite, 235° to 300°C forzunyite, 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 isotopefractionation of two samples of pyrite and alunite (Table 3),and this is used here, consistent with typical data from otherpublished sources (Rye et al., 1992; Rye, 2005). Since pyro-phyllite is found spatially with alunite (Fig. 5), and possiblypostdating alunite, we infer that the pyrophyllite may haveformed at a similar or slightly lower temperature. However,for fluid calculation we consider a temperature range for py-rophyllite from 250° to 350°C and similar to muscovite. How-ever, the lower end of the temperature range for the calcu-lated fluid is preferred for pyrophyllite, whereas formuscovite the higher end of the temperature range is sug-gested, since it forms the core of the porphyry system. Forclinochlore, a temperature range of 200° to 300°C is used.

In general, the advanced argillic minerals form at tempera-tures below 350° to 400°C (Giggenbach, 1997), under condi-tions shallower than the brittle-ductile transition in a por-phyry environment (Rye, 1993, 2005; Fournier, 1999).

The H and O isotope data on minerals and the water intheir 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 volcanicvapor (Giggenbach, 1992). Aqueous liquid exsolved from amelt is somewhat enriched in deuterium over the water dis-solved in the melt, due to fractionation on exsolution (sum-marized by Hedenquist and Richards, 1998). In turn, volcanicvapor (Giggenbach, 1992) is about 10 to 20 per mil enrichedin deuterium over the hypersaline liquid from which it sepa-rates at porphyry depths, the vapor then ascending either tocondense near the surface and create acidic fluid and alter-ation related to the extreme hydrolytic base leaching or tovent as high-temperature fumaroles (Hedenquist et al.,

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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 Oxygen 0–350 Sheppard and Gilg (1996)103lnαpyrophyllite-H2O = 2.76 × 106T–2 + 1.08 × 103T–1–5.37 Oxygen 0–700 Savin and Lee (1988)103lnαillite-muscovite-H2O = 2.39 × 106T–2–3.76 Oxygen 0–700 Sheppard and Gilg (1996)103lnαalunite (SO4) =3.09 × 106T–2–2.94 Oxygen 250–450 Stoffregen et al. (1994)103lnαchlorite-H2O = 2.69 × 109T–3–6.34 × 106T–2 +2.97 × 103T–1 Oxygen 170–350 Cole and Ripley (1999)103lnαkaolinite-H2O =–2.2 × 106T–2–7.7 Hydrogen 0–300 Sheppard and Gilg (1996)103lnαpyrophyllite-H2O = –20 ± 5 Hydrogen 120–400 Marumo et al. (1980)103lnαillite-muscovite-H2O = –20 ± 5 Hydrogen 120–400 Marumo et al. (1980)103lnαalunite-H2O = –6 Hydrogen 250 Stoffregen et al. (1994)103lnαchlorite-H2O = –3.7 × 106T–2 –24 Hydrogen 500–700 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

1998). The magmatic water referred to above tends to be typ-ical of magmatic fluids derived from felsic melts in the Cir-cum-Pacific, where subducted seawater may be a significantcontributor to the starting isotope composition for water orig-inally dissolved in the magmas. Care must be exercised inusing these reference boxes in interpretations, as the compo-sition of magmatic water in a given system is dependent onthe starting value for water in magmas; in addition, the waterin fluids may have changed its primary isotopic compositionas a result of equilibration with lower temperature igneousrocks (e.g., Ohmoto and Rye, 1974; Bethke et al., 2005).

Results and discussion

The δ18O and δD values (Table 2, Fig. 7) of alunite miner-als and the calculated isotopic composition of water in parentfluids, and corresponding δ34S values (Table 2), indicate thatalunite from the Oyu Tolgoi deposits formed from magmatic-hydrothermal fluids. The calculated δD values of water in thecondensed vapor that formed the alunite, based on a deposi-tional temperature of 260°C, are slightly larger than the alu-nite (Stoffregen et al., 1994). The high δ34S values of the alu-nite (8.4–17.9‰) are consistent with formation from aqueoussulfate derived from the disproportionation of magmatic SO2.Assuming covellite, enargite, and pyrite were close to isotopicequilibrium with alunite (Fig. 8), the low δ34S values (–6.0 to–16.0‰) for the sulfides, as well as the wide range for pyrite

and enargite, suggest relatively oxidized or sulfate-rich fluids,somewhat more oxidized than is typically indicated based onalunite-sulfide pairs from more shallow-formed high-sulfida-tion epithermal deposits (Arribas, 1995; Rye, 2005). Sample921-546.2 (Table 2) is late botryoidal pyrite coating enargiteand not spatially associated with alunite. Its low δ34S value(–22.8‰) is typical of pyrite formed during the final collapseof hydrothermal systems, when residual acidic ground watersdissolved early-formed sulfide minerals (Plumlee and Rye,1989, 1992).

The δD and δ18O isotope values for water in parent fluidsin equilibrium with muscovite and pyrophyllite are similar,using a similar formation temperature, but their field rela-tionships are different. Pyrophyllite-dominant alteration oc-curs mainly in dacitic ash-flow tuff overlying the porphyrysystem, where it may be closely related to alunite, whereasmuscovite occurs deeper in the porphyry system, where itoccurs mainly as an alteration of quartz monzodiorite. It isclear that the magmatic-derived fluids in equilibrium withmuscovite and pyrophyllite had a significant meteoric watercomponent. However, some of the meteoric water in thecalculated fluid compositions may have resulted from ex-change between the hydrous minerals and meteoric waterduring system collapse. Resolution of this problem will re-quire detailed isotope measurements of well-documentedfluid inclusions.

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FIG. 7. Isotopic compositions of alteration minerals from the Oyu Tolgoi porphyry Cu-Au system. Solid symbols showmeasured oxygen and hydrogen isotopes, open symbols show calculated fluids using fractionation equations listed in Table 3at temperatures discussed in the text. Bars show a range of temperature and change in the calculated fluid water composi-tion for alunite, muscovite, pyrophyllite, chlorite, and dickite. FMW = felsic magmatic water, dissolved in melt (after Taylor,1992) and andesitic volcanic vapor (after Giggenbach, 1992).

All six of the pyrophyllite samples, and four in particularfrom deep levels at Hugo Dummett North (967C-1148.5 and514I-1235.6) and from deep portions of Central Oyu Tolgoi(881-306 and 881-306 vein), have similar isotopic composi-tions, suggesting that there is not a significant difference intheir parent fluid compositions. Thus, common hydrothermalfluid processes may have operated across the whole Oyu Tol-goi porphyry system. The sample of pyrophyllite (470-145)with a parent fluid whose water composition is most discor-dant is the shallowest pyrophyllite sample and is close to theoverlying sedimentary contact (90 m below unaltered prem-ineral rocks). Of all the pyrophyllite samples, the fluid thatformed this sample may be expected to have the greatestinvolvement with meteoric water.

The δ18OH2O values calculated at 300°C (Table 2) for themuscovite samples also show a tight cluster (3.9–5.5‰) forfour of the five samples, similar to the minor variation in val-ues noted for muscovite that formed from magmatic fluidover 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 thatperhaps 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 andpyrophyllite. Experimental or theoretical zunyite-water Oand H isotope fractionation factors do not exist but the δ18Oand δD values obtained are consistent with the phyllosilicateminerals, suggesting perhaps similar fractionation factors.Zunyite is an integral component of the pyrophyllite-domi-nant alteration and therefore may be inferred to have formedfrom similar magmatic fluid (cf. Reyes, 1991). Similarly, thecalculated parent fluid for chlorite, assuming a temperatureof 200° to 300°C, appears to be isotopically similar to the

muscovite parental fluid (Fig. 7) and suggests that the wide-spread retrograde alteration from biotite to chlorite at OyuTolgoi occurred during cooling of the magmatic-hydrother-mal system, without major influx of external fluids.

In contrast, late dickite on fractures has δ18O and δD val-ues for a parent fluid that is close to the meteoric water lineat 150°C (Fig. 7). This is a likely temperature for dickite,similar to that found at Far Southeast, El Salvador, and else-where (Hedenquist et al., 1998; Watanabe and Hedenquist,2001; Bethke et al., 2005; Fifarek and Rye, 2005), where italso is found to have formed from meteoric-dominatedwater. Dickite may result from the breakdown of pyrophylliteat Oyu Tolgoi, since the two minerals are closely spatiallyassociated.

The low δD values (–140‰) for the meteoric water com-ponent are compatible with recharge at high elevation, sincepaleogeographic reconstructions suggest that Mongolia wasclose to its present position during the late Paleozoic (Grad-stein et al., 2004). The high elevation could have been relatedto a stratovolcano (e.g., Frank, 1995), given the tectonic set-ting of Mongolia at that time (Badarch et al., 2002).

The sulfur isotope data on sulfide minerals (Fig. 8) showsthat the range of values for each mineral is generally similarfor all of the deposits throughout the Oyu Tolgoi system. Allsulfate minerals are isotopically heavy and all sulfide mineralsare isotopically light, as observed in numerous examples ofmagmatic-hydrothermal systems (e.g., Rye, 1993, 2005; Ar-ribas, 1995). In addition, there is a consistent difference insulfur isotope values for different sulfide minerals that re-flects sulfur isotope equilibrium in the hydrothermal fluids,typical of such systems. As noted above, sulfur isotope datafor two samples of coexisting alunite and pyrite from HugoDummett South (470-271 and 515-158.9) give equilibriumisotopic fractionation temperatures averaging about 260°C(Table 2). Coexisting pyrite and anhydrite from Hugo Dum-mett North give equilibrium isotopic fractionation tempera-tures of 225° to 270°C, consistent with the late parageneticposition of the anhydrite. The calculated temperature for apyrite-chalcopyrite pair from Hugo Dummett North is about230°C, the significance of which is questionable as chalcopy-rite is susceptible to postmineralization recrystallization(Barton and Skinner, 1979; Field et al., 2005). However, thetemperature may be realistic considering that copper miner-alization 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 CentralOyu Tolgoi deposit, and from which aqueous sulfate may havebeen derived by oxidation during deep weathering in the Cre-taceous. Supergene and hypogene alunite can clearly be dis-tinguished at Oyu Tolgoi by their δ34S values (Table 2, Fig. 8).

A single hypogene chalcocite sample (401-592) from theHugo Dummett South deposit has a δ34S value of –8.4 permil, in the range of other sulfide minerals (Table 2, Fig. 8) butisotopically lighter than bornite samples (δ34S, –2.4 to–5.7‰) from the main high-grade mineralization at HugoDummett. The chalcocite sample is intergrown with or re-places bornite and may have formed from late, low-tempera-ture fluid, consistent with other samples where bornite andchalcocite are intergrown with dolomite (Fig. 6c).

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FIG. 8. Sulfur isotope data for sulfide and sulfate mineral species. Note:(1) Late pyrite botryoidal coating enargite and (2) supergene alunite from theCentral deposit.

The S and O isotope values of sulfate minerals are summa-rized in Figure 9. The alunite samples from Hugo DummettNorth have high δ18OSO4 and δ34S values that reflect the pre-dominant magmatic origin of their parent aqueous sulfate.The one alunite sample from the Central Oyu Tolgoi deposithas lower δ34S and δ18OSO4 values; this may reflect the factthat the magmatic vapor that transported the SO2 condensedinto meteoric water. Such 18O-depleted values occur in mag-matic-hydrothermal alunites that form at shallow depths inareas of wet climate (Rye et al., 1992). The later anhydriteand gypsum from Central and Hugo Dummett North alsohave high δ34S and δ18O values, and all of the data in Figure9 show a trend to lower δ18O values below the general δ18Sand δ18O isotope trend. Gypsum and anhydrite veins from theSouthwest deposit have the lowest δ18S and δ18O values.These stable isotope systematics are nearly identical to thoseobserved for alunite and barite at the Summitville high-sulfi-dation deposit, Colorado (Bethke et al., 2005). As discussed indetail by Rye (2005), this trend of S and O isotope values re-flects the mixing of sulfate from magmatic sources with sul-fate derived from the oxidation of H2S at shallow levels. Themixing is particularly obvious in the parent fluids of anhydriteand gypsum samples from the Southwest deposit. The scatterto lower δ18OSO4 values in the anhydrite at Hugo DummettNorth may reflect the oxygen isotope exchange of aqueoussulfate with 18O-depleted meteoric water in some of the par-ent fluids.

Summary and ConclusionsThe alunite in the advanced argillic alteration at the Hugo

Dummett North and South deposits formed largely from con-densed magmatic vapor, consistent with the origin of similaralunite found in numerous studies of other intrusion-cen-tered deposits (e.g., Rye et al., 1992; Arribas, 1995; Rye,

2005). There appears to have been little mixing of the con-densed vapor with meteoric water during alunite depositionat the Hugo Dummett deposits.

Magmatic-hydrothermal fluids responsible for muscoviteand pyrophyllite, based on δ18O isotope data, are similar andinvolve a component of meteoric water. Pyrophyllite is awidespread component of the advanced argillic alterationabove the porphyry system (although there is also deepingress of pyrophyllite-dominant alteration along lithologiccontacts and fractures), whereas muscovite dominates thecore of the porphyry system. Early high-temperature ad-vanced argillic alteration (represented by andalusite) and alu-nite-bearing advanced argillic alteration underwent retro-grade alteration and overprinting by pyrophyllite, kaolinite,and dickite, as is typically observed in porphyry-related alter-ation of other deposits (Einaudi et al., 2003). The precursoralteration to advanced argillic alteration was most likely mus-covite in the upper parts as well as deep parts of the porphyrysystem, where dacitic ash-flow tuff and quartz monzodiorite,respectively, are the host rocks.

Despite these relationships it is unresolved whether mus-covite and pyrophyllite are related to the same evolving fluid,since in terms of spatial distribution and paragenesis, pyro-phyllite appears to be closely related to evolution of the ad-vanced argillic system and postdates the formation of con-densate fluids which formed alunite. In addition, themuscovite alteration hosted by quartz monzodiorite intru-sions in the core of the porphyry system is generally separatedby a screen of basaltic host rocks (with early biotite and ret-rograde chlorite as well as muscovite alteration) from overly-ing advanced argillic alteration in dacitic ash-flow tuff (Fig.5).

The distribution and paragenesis of the muscovite-domi-nant alteration indicate that it was related to the thermal col-lapse of the early biotite-stable porphyry system (Hedenquistet al., 1998; Heinrich, 2005); i.e., it occurred as isothermscontracted downward. Muscovite alteration at deep levelsmay also have been synchronous with early advanced argillicalteration in the upper parts of the porphyry system.

Copper-gold mineralization at Hugo Dummett is parage-netically late, despite the general spatial relationships toearly-formed quartz veins. The bulk of the mineralization isclosely linked by silicate-sulfide textures to the pervasivemuscovite-dominant stage (Fig. 6b), as well as partially todark-green chlorite (<300°C) and even later dolomite(<250°C) alteration (Fig. 6c).

A systematic outward zonation of sulfides from bornite-chalcopyrite in the core of the porphyry system, to chalcopy-rite, and then pyrite-enargite on the fringe may be relatedpartly to an evolving hydrothermal system, where magmaticfluids moved outward and cooled, coupled with a synchro-nous enveloping outer zone of advanced argillic alteration inwhich the high-sulfidation–state sulfide minerals precipi-tated. This alteration accompanies pyrite, tennantite, andrarely enargite and preceded the high-grade bornite-chal-copyrite mineralization.

The high-grade bornite-chalcopyrite mineralization is ap-parently related to late muscovite- and chlorite-stable fluidwhich overprinted earlier alteration stages, including the ad-vanced argillic alteration. This has resulted in some unusual

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FIG.9. Sulfur and oxygen isotope data on sulfate mineral species.

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 significantcomponent of meteoric water deep into the Oyu Tolgoi por-phyry Cu-Au system late in its history. This meteoric water isisotopically very light, and thus was derived from a high-eleva-tion setting, since paleogeographic reconstructions indicate aposition of Mongolia during the late Paleozoic (Gradstein etal., 2004) similar to the present. The meteoric water invasionthat formed the late veins is also recorded in the sulfur andoxygen isotope data for late anhydrite and gypsum, whose par-ent fluid in some areas acquired a component of aqueous sul-fate from the shallow vadose zone oxidation of H2S as the mag-matic system waned. This is especially the case for the purpleanhydrite veins found at depth surrounding the high-gradechalcopyrite mineralization in the Southwest deposit.

Despite a geologic environment where extreme hydrolyticalteration appears to have formed in a deep environment, atleast 1,000 m below the paleosurface, current geologic recon-structions of the Oyu Tolgoi porphyry Cu-Au system, as wellas this reconnaissance stable isotope study, suggest that theHugo Dummett deposits may be interpreted in terms of atypical porphyry model, albeit with some atypical alterationand sulfide characteristics.

AcknowledgmentsWe thank Ivanhoe Mines Mongolia Inc. for financial sup-

port and permission to publish this study, and in particularCharles Forster, Douglas Kirwin, Paul Chare, and all our ge-ologic colleagues at the Oyu Tolgoi field site for their contri-butions. BK acknowledges the assistance from Ivanhoe MinesMongolia Inc. to undertake an M.Sc. study at the MongolianUniversity of Science and Technology, School of Geology, onthe Hugo Dummett deposit, and wishes to thank all her closecolleagues and teachers, and in particular, Dash Bat-Erdene,S. Jargalan, O. Gerel, J. Lhamsuren, D. Garamjav, PeterTerry, G. Niislelkhuu, G. Jargaljav, R. Oyunchimeg, TosiRindra, 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 isotopeanalyses. Review comments by Phil Bethke, Richard Fifarek,and Brian Rusk helped to clarify the initial presentation, andwe are particularly indebted to Richard Sillitoe and AlbertGilg for detailed and careful reviews, which led to consider-able improvements in scientific content. January 3, June 7, 2006

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