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Characteristics and controls of the largest porphyry copper gold and

epithermal gold deposits in the circum Pacific region

R. H. Sillitoe

To cite this ArticleSillitoe, R. H.'Characteristics and controls of the largest porphyry copper-gold and epithermal golddeposits in the circum-Pacific region', Australian Journal of Earth Sciences, 44: 3, 373 388

To link to this Article: DOI: 10.1080/08120099708728318

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Australian Journal ofEarthSciences

(1997) 44, 37 3-388

Characteristics and controls of the largest porphyry

copper-gold and epithermal gold deposits in the

circum-Pacific region

R. H. SILLITOE

27 West Hill Park Highgate Village London N6 6ND England.

Eleven gold-rich porphyry copper and 14 epithermal gold deposits around the Pacific rim contain > 200 t

(-7 million oz) of gold. These large porphyry-type deposits conform to a single overall model, whereas the large

epithermal gold deposits are varied in both genetic type and mineralisation style. Most regional and local

characteristics of the largest porphyry and epithermal deposits fail to explain convincingly their extremely high

gold contents. Nevertheless, a number of hypothetical processes operative alone, or in combination, in the mantle,

in upper crustal magma chambers and at the sites of gold accumulation are believed to maximise the likelihood of

exceptional gold concentrations. Partial melting of the upper parts of stalled lithospheric slabs in the mantle,

immediately following collision or arc migration, promotes oxidation of mantle sulfides and the release of gold.

These tectonic scenarios may also result in rapid cooling and uplift-induced depressurisation of upper crustal

magma chambers, thereby accelerating the release of gold-bearing magmatic fluids. Upper crustal magma mixing

and passive degassing of the resultant SO

2

are also considered to favour gold availability. Rheological and

permeability contrasts at sites of gold deposition are important controls on the size and tenor of gold deposits. The

summation of these mechanisms tends to result in short-lived, areally restricted gold-forming events, commonly as

an end-stage of arc development. These hypothetical processes translate into several practical criteria of potential

use to explorationists. Both large gold-rich porphyry and epithermal deposits seem to be more common in atypical

arc settings and in association with unusual, especially highly potassic, igneous rocks. During the search for gold-

rich porphyry deposits, high hydrothermal magnetite contents, very young arcs in the tropical environment and

impermeable host rocks, especially limestones, deserve emphasis. In contrast, large epithermal gold deposits are

commonly controlled by marked lithologic differences and associated with flow-dome and/or maar-diatreme

systems.

Key words: circum-Pacific region epithermal deposits exploration giant deposits gold magm atic arcs

porphyry copper deposits.

I N T R O D U C T I O N

Exceptionally large gold deposits, often designated as

giant, world-class or super-large, are the principal

exploration objectives in the circum-Pacific region and

elsewhere. Gold contents of either > 1001 (Singer 1995)

or> 2001 (applying the formula of Laznicka 1983) have

been used to define such superior gold deposits. The

latter size limitation, equivalent to about 7 million oz of

gold, is employed here in order to restrict the number of

examples to 25 (Figure 1; Tables 1, 2).

This short review deals with the largest porphyry and

epithermal gold deposits around the Pacific rim, but

excludes other genetic types of deposits containing

> 200 1 gold, namely sediment-hosted and m esothermal

(including slate belt) types. All the porphyry deposits

included, except Far Southeast, and most of the

epithermal deposits, except McDonald, Round Mountain,

Ladolam and Hishikari, have undergone various degrees

of erosional removal and hence originally were larger

than their reserves ( production) imply.

Most of the largest gold-rich porphyry deposits contain

appreciable copper, as either

a

principal or co-product,

whereas several of the large epithermal deposits, most

notably Pachuca-Real del Monte, have metal budgets

dominated by silver rather than gold. Several huge

porphyry copper deposits characterised by relatively modest

gold grades also possess > 20 01 of gold (e.g. Escondida,

Chile; 4501 Au), but are excluded from consideration

because they cannot be considered as gold deposits.

The review commences with 'thumb nail' geological

sketches of the large gold-rich porphyry and epithermal

deposits. It continues with a brief assessment of

a

number

of regional (metallogenic) and local (deposit-scale)

factors that neither singly nor in combination explain

adequately the exceptional gold contents of the deposits

considered. Three mechanisms are then proposed for

enhancing gold availability and accumulation in porphyry

and epithermal systems, followed by a set of geological

criteria that seem to be useful indicators of large gold-

rich systems (Table 3).

GEOLOGICAL CHARACTERISTICS

Gold-rich porphyry deposits

Gold-rich porphyry copper deposits dominate this cate-

gory, although gold-rich, relatively copper-poor (Fish

Lake, Cadia Hill) and essentially gold-only (Refugio)

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374

R . H . S ILLITOE

Figure 1 Location of the largest

gold-rich porphyry and epithermal

gold deposits of the circum-Pacific

region.

Fish Lake

McDonald

,

x

Bingham

ff

Comstock Lode O o O Cripple Creek

lound

fountain

Pachuca-Real del Moi

Spreading ridge

, Subduction zone

Gold-rich porphyry

deposit

o

Epithermal gold

deposit

Table 1 Selected geological characteristics of large gold-rich porphyry deposits .

Deposit Au Tectonic Regional Porphyry Age Coeval Ore- Abundant Quartz Litho- Associated Recent

co nte nt setting structural stock (Ma)* volcan- related magnetite stock- cap mineral- reference

control ics alteration work isation

Fish Lake

Canada

Bingham

USA

Bajode La

Alumbrera

Argentina

Refugio

Chile

Cadia Hill

Australia

Panguna

PNG

OkTedi

PNG

Grasberg

Indonesia

Batu Hijau

Indonesia

Santo

TomasII

Philippines

Far Southeast

Philippines

471

937

489

259

224

766

368

1598*

353

230

441

Cont

Cont

(BA)

Cont

(BA)

Cont

I A ( ? )

IA

Cont

Cont

IA

IA

IA

No

Lin

Lin

No

Lin

No

No

No

Lin

Fault

Fault

QdiCA

QmonKCA

DaeKCA

Qdi-DiCA

Qmon

KCA

Di-QdiCA

MonKCA

Md iKCA

QdiCA

D i C A

QdiCA

80

39

8

23

- 4 4 0

3.4

1.2

3.3-3.0

5 . 1^ . 9

1.0

1.5-1.2

No

Yes

Yes

Yes

Yes

Yes

No

Yes

Yes

No

Yes

K

K

K

IA-P-K

P- K

K

K

K

K-IA

K

K-IA

Yes

N o

Yes

Yes

Yes

Yes

No

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

No

No

No

Yes

No

No

No

No

Yes

No

Yes

No

Sk,CR , SH

LS

No

Sk

No

Sk

Sk,CR

LS

LS

HS ,LS

Cairaet al.

1995

Babcock

et al.

1995

Guilbert1995

Flores1993

Newcrest

Min-

ing Staff1996

Clark1990

Rush&Seegers

1990

MacDonald&

Arnold1994

Irianto

&

Clark1995

Serafica&

Baluda1977

Garcia1991

*Age data supplemented by: Arribas et al. 1995; McDowell et al. 1996; Perkins et al. 1995; Sillitoe et al. 1991; E. H. McKee

and R. H. Sillitoe unpubl. data; S. J. Turner pers. comtn. 1996.

f

Mineable reserve; geological reserve is -25001 Au

Tectonic setting: BA, back-arc; Cont, continental margin; IA, island arc. Regional structural control: Lin, lineament. Porphyry stock:

Dae, dacite; Di, diorite; Mdi, monzodiorite; Mon, monzonite; Qdi, quartz diorite; Qmon, quartz monzonite; CA, calc-alkaline; KCA,

high-K calc-alkaline. Ore-related alteration: IA, intermediate argillic; K, K-silicate; P, propylitic. Associated mineralisation: CR,

carbonate replacement; H S, high-sulfidation ep ithermal; LS , low-sulfidation epithermal; SH , sediment-hosted gold; Sk, skarn.

deposits are also included (Figure 2). Refugio is con- Grasberg, Bingham and Panguna, all containing > 700 1

sidered as a porphyry gold deposit (Vila & Sillitoe 1991). of gold (Figure 3). However, Grasberg and Far Southeast

The pre-eminent gold-rich porphyry deposits are clearly possess the highest gold grades, with both containing

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LARGEST CIRCUM-PACIFIC GOLD DEPOSITS 375

1.2-

1.0-

0 .8 -

o

0.6-

0 . 2 -

0.0

Grasberg

Bingham Ok Ted

Batu Hljau

I Far Southeast

Panguna

Fish Lake

Ba)o de la Alumbrera

Santo Tomas II

Cadia Hill

Refuglo

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Au

g/t)

Figure

2 Gold and copper contents of thelargest gold-rich

porphyry deposits of the circum-Pacfic region. All deposits

contain

>

200

t of

gold. Note

the

pre-eminence

of

Grasberg

and

Far Southeast grades.

8

10 100

million tonnes

1000

1 Fish Lake

2 Bingham

3 Bajode aA lumbrera

Refugio

Cadia Hil l

6 Panguna

7 Ok Tedi

8 Grasberg

9 Batu Hijau

10 S anto Tomas II

11 Far Southeast

12 McDonald

13 Comstock

14 C r ipp le Creek

15 Round Mounta in

16 P achuca-Real de l Monte

17 Pueblo Vie jo

18 Yanacocha

19 E l l nd l o

20 Waih i

21 Ladolam

22 Porgera

23 Kelian

24 Baguio

25 Hishikari

* Bonanza part only

+

pproximate

Figure

3 Gold grade-tonnage plot for thelargest gold-rich

porphyry and epithermal gold deposits of the circum-Pacific

region.

All

deposits contain

>

200

t of

gold. Note that

the

bulk

low-grade epithermal depositsatMcDonald, Round Mountain and

Yanacocha plot with the gold-rich porphyry deposits. ,gold-rich

porphyry deposit;O,epithermal gold deposit.

appreciable tonnages

at > 2 g/t Au as

well

as

high copper

contents (Figure

2).

Molybdenum contents

are low in

many

of the

deposits,

but

average 0.025

at

Bingham

(Babcock

et al.1995) and

0.01

at Ok

Tedi (Rush

&

Seegers 1990), which

are the

deposits hosted

by the

most

potassic stocks (Table

1).

Several

of the

deposits

(e.g.

Bajo

de La

Alumbrera, Batu Hijau) display molybdenum-

enriched haloes that overlap

the

outer parts

of the

copper-gold orebodies. Silver contents in all thelarge

gold-rich porphyry deposits

are

low, generally

< 3

ppm.

The large gold-rich porphyry deposits

are

located

in

magmatic arcs along both sides

of the

Pacific Ocean ,

although they

are

somewhat m ore abundant

on the

west-

ern side (Figure

1). The

deposits

are

distributed about

equally

in

continental-margin

and

island-arc terranes

(Table

1).

Bingham

and

Bajo

de La

Alumbrera, both

underlain

by

continental crust, occupy back-arc positions

that were mildly extensional

at

the time

of

mineralisation.

In marked contrast,

Ok

Tedi

and

Grasberg,

at the

leading

edge

of

the Australian craton, w ere emplaced through

an

active fold-thrust belt linked

to

continent-island-arc

col-

lision. The Late Ordovician age

of

the C adia H ill deposit

precludes proper assessment

of its

geotectonic setting,

although a late extensional stageof island-arc develop-

ment

is

proposed

on the

basis

of

selected modern

ana-

logues (Walshe

etal .

1995).

A multitude

of

faults

is

present

in and

around most

ore

deposits

and the

large gold-rich porphyry deposits

analysed here

are no

exception. However,

it is not

clear

that major faults

or

lineaments were responsible

for

localising mostof thedeposits, exceptfor FarSoutheast

by

the

Lepanto fault (Garcia 1991).

No

truly major faults

or lineaments have been recognised

in the

vicinities

of

five of the deposits, although major pre-mineral faults

or

lineaments

are

present

in

proximity

to the

rest (Table

1).

Arc-transverse structures, such

as the

Uinta axis

at

Bingham (Babcock

et al.

1985),

the

Hualfin

and

Aconquija lineaments

at

Bajo

de La

Alumbrera (Guilbert

1995) and theLachlan River lineament at Cadia Hill

(Walsheet

al .

1995), are common.

The stocks that host

the

large gold-rich porphyry

deposits

are all of

I-type

and

belong

to the

magnetite

series,

thereby indicating contributions from oxidised

subcrustal melts (Ishihara 1981). The mineralised porphyry

stocks

are

either calc-alkaline

or

high-K calc-alkaline

in

petrochemical affiliation (Table1),with thoseatBajode

La Alumbrera

and

Cadia H ill also qualifying

as

members

of

the

shoshonite suite. Moreover, volumetrically minor

mafic alkaline

and

shoshonitic igneous rocks

in the

Bingham district

are

modelled

as

parental

to the

porphyry

copper-gold stock (Keith

et al.

1995).

The

m ineralised

porphyries span

a

broad compositional range: diorite

through quartz diorite

and

dacite

to

monzodiorite, monzo-

nite and quartz monzonite (monzogranite).

The large gold-rich porphyry deposits range

in age

from Ordovician

to

Pleistocene, although deposits along

the western side

of

the Pacific Ocean,

all

=

5 Ma

except

for Cadia Hill,

are

generally younger than those

in the

western Americas (Table

1).

This relationship

is

attri-

buted

to

more rapid erosion

and,

consequently, quicker

unroofing of depositsin thewestern Pacific island arcs

than

in

much

of

the western Americas (Sillitoe 1993a).

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376 R. H. SILLITO E

All the large gold-rich porphyry deposits conform to a

single unified model (Sillitoe 1993b; Figure 4), which

differs little from that for porphyry copper deposits in

general. The gold-copper mineralisation is centred on

composite porphyry stocks, which are circular to ovoid in

cross-section and essentially vertical over intervals of at

least 2 km . The stocks generally include inter- and late-

mineral phases, which contain lesser amounts of gold and

copper because they were emplaced during or late,

respectively, in the alteration-mineralisation event. These

later porphyry pulses were commonly intruded up the

axial zones of the pre-existing stocks (Figure 4). Less

common are barren or low-grade, pre-mineral intrusions,

either equigranular like those at Bingham (Babcock et al.

1995) and Panguna (Clark 1990) or porphyritic as at Bajo

de La Alumbrera (Guilbert 1995) and Batu Hijau (Irianto

& Clark 1995). Diatreme vents post-dated porphyry

deposit formation at Batu Hijau, Santo Tomas II and Far

Southeast (Table 1; Figure 4).

The mineralisation may be confined to the stocks (e.g.

Grasberg) or extend appreciably into surrounding wall-

rocks (e.g. Fish Lake, Santo Tomas II). The wallrocks are

varied in both age with respect to the stock and

composition. Eight of the large deposits were emplaced

into volcanic rocks that are broadly co-temporal with the

stocks themselves, whereas the rest intruded older

'basement' units (Table 1).

Gold and copper in all the deposits are components of

K-silicate alteration, the product of magmatic brines

(Burnham 1979), although propylitic assemblages are

noted within the copper-poor Refugio and Cadia Hill

deposits. Each deposit displays a close, although gener-

ally non-linear, correlation between copper and gold

contents, especially where bornite is present as a signifi-

cant copper mineral. Biotite is typically the ubiquitous

K-silicate alteration mineral, and may be accompanied by

K-feldspar and/or actinolite. Quartz-veinlet stockworks,

including subparallel veinlet arrays, characterise all the

deposits (Table 1) and host much of the copper and gold.

Hydrotherm al magn etite comp rises 3= 5 vol. of ore in

nine of the 11 deposits (Table 1), as pre- and/or syn-metal

veinlets and disseminated grains.

K-silicate alteration grades outwards to propylitic

alteration, with several deposits (e.g. Fish Lake, Bingham,

Bajo de La Alumbrera, Grasberg) containing a complete

or partial annulus of intervening sericitic alteration

(Figure 4). Sericitic and/or innermost propylitic alteration

commonly coincide with pyrite haloes. Intermediate

argillic assemblages, typified by illite/sericite and chlo-

rite,

partially overprinted the K-silicate alteration (Figure 4)

at Refugio, Batu Hijau and Far Southeast, but do not

seem to have been accompanied by appreciable metal

introduction. Remnants of an advanced argillic lithocap

are preserved in proximity to the uppermost parts of only

three of the deposits (Table 1; Figure 4), which implies

that the rest were eroded relatively deeply. This observa-

tion accords well with the observed downward increases

of gold contents in several deposits, although Ok Tedi,

with a gold-rich cap (Rush & Seegers 1990), is an

exception.

Eight of the large gold-rich porphyry deposits are the

foci of zoned mineral districts (Table 1). Copper-gold

Intermediate argillic overprinted

on K-silicate alteration

Remnant of advanced argillic

lithocap

p

Co-temporal

volcanic

.

sequence

f

Low-sulphidation

veins, Zn-Pb-Ag-Au

p

Propylitic-altered

wallrocks

k-silicate alteration

with magnetite, Cu+Au

Intermineral porphyry phase

with less Cu+Au

Figure 4 Model for large gold-rich porphyry deposits in the

circum-Pacific region. Copper and gold are present in K-silicate

alteration and overprinted intermediate argillic alteration from the

present surface to a depth of at least 2 km.

skarns abut the porphyry deposits at Bingham, Cadia

Hill, Ok Tedi and the deeper portions ( > -100 0 m) of

Grasberg, while carbonate-replacement zinc-lead deposits

and sediment-hosted gold deposits are also present more

distally at Bingham (Babcocket al.1995). The top of the

Far Southeast deposit is characterised by a high-

sulfidation epithermal copper (enargite)-gold deposit

(Garcia 1991) generated at the base of the lithocap,

whereas low-sulfidation epithermal veins are located

distally with respect to four of the deposits (Table 1;

Figure 4).

Epithermal gold deposits

The pre-eminent epithermal gold deposits, with 3= 6001

of gold, are Cripple Creek, Pueblo Viejo, Ladolam,

Porgera, and Baguio (Table 2; Figure 3). Most of the

large gold-rich epithermal systems in the circum-Pacific

region are economically gold-only deposits. The

exceptions are Pachuca-Real del Monte and Comstock

Lode, which were primarily rich silver deposits, with

Ag/Au ratios of roughly 200 (Geyne et al. 1963) and

> 2 0 ,

respectively. Copper, as enargite, is an important

by-product of the early-stage veins at El Indio.

The large epithermal gold deposits are split almost

equally between the eastern and western Pacific

regions (Figure 1). Ten of the 14 deposits are located in

continental-margin magmatic arcs (Table 2), with only

Cripple Creek unequivocally occupying an extensional

back-arc position. Four of the deposits are parts of island

arcs, which in the case of Pueblo Viejo and Ladolam are

built directly on oceanic lithosphere.

Faults are recognised in all the deposits, although their

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LARGEST CIRCUM-PACIFIC GOLD DEPOSITS

377

Table 2 Selected geological characteristics of large epithermal gold deposits.

Deposit

Au

content

McDonald

U SA

Comstock

Lode USA

Cripple

Creek USA

Round

(t)

25 1

- 2 6 0

- 7 5 5

413

Mountain USA

Pachuca-Real

del Monte,

Mexico

Pueblo V iejo

Dominican

Republic

Yanacocha

Peru

El Indio

Chile

Waihi

New Zealand

Ladolam

PN G

Porgera

PN G

Kelian

Indonesia

Baguio

Philippines

Hishikari

Japan

23 5

- 7 0 0

29 2

f

29 5

230

595

- 6 0 0

> 2 0 0

> 7 0 0

25 0

Tectonic

setting

Cont

Cont

Cont

(BA)

Cont

Cont

IA

Cont

Cont

Cont

IA

Cont

Cont

IA

IA

Related

volcanic

rocks

Felsic

ig

CA

A nd

CA

Phon-Trach

A

Rhyodac

ig

C A

And-rhy

C A

Ker-Spil

IA T

A nd

C A

Rhy?

K CA

A nd

C A

Trachyb

+ t r achA

Bas(int)

A

Rh y

C A

Dae

K CA

D ae

C A

A ge

(Ma)*

39-37

13.7

32-31

26

21-20

- 1 3 0

10.9

_ 7

7

0 . 3 5 -

0.1

6.0-5.6

E M i o ?

0. 6

1.25-0.6

Volcanic

setting

Caldera?

Flow dome

Diatreme

Caldera

Uncertain

Diatreme

Flow dome?

Flow dome

Strato-

volcano?

Strato-

volcano

No volcanics

Diatreme

Diatreme

Flow dome

Form

of

orebody

Disseminated

+ stockwork

Vein

Vein

+

disseminated

Disseminated

+ vein

Vein

Stockwork

Disseminated

Vein

Vein

Breccia

Stockwork+

vein breccia

Vein-

stockwork

Vein

+

breccia

Vein

A t

litho-

logic

contact

Ye s

N o

Yes

Yes

N o

Ye s

N o

Ye s

N o

N o

Yes

N o

N o

Yes

Local

fault

control

Normal

Normal

(45)

Normal

+

ring fault

Normal

Normal

Ring fault

Not known

Normal +

reverse

Normal

Normal?

Normal

Normal

Strike slip

+

ring fault

Normal

Ore-

related

alteration

LS

Qu

+ Ad

LS

Q u

LS

Ad

LS

Ad

LS

Q u+ Ad

H S

Q u

H S

Q u

H S

Q u

LS

Q u

+ Ad

LS

Q u+ Ad

LS

Q u

LS

Ad

LS

Q u+ Ad

LS

Q u+ Ad

Abun-

dant

base-

metals

N o

Ye s

N o

N o

Yes

Yes

N o

Ye s

Ye s

N o

Yes

Yes

N o

N o

Palaeo-

surface

evidence

Yes

N o

N o

Ye s

N o

N o

Yes(?)

Yes

Ye s

Ye s

N o

N o

N o

Ye s

Asso-

ciated

mineral

-isation

N o

N o

N o

No

N o

N o

N o

LS

N o

Pp y

Cu

N o

N o

Pp y

Cu

N o

Recent

reference

Bartlett

etal.1995

Vikre

1989

Thompson

1992

Tingley

&

Berger

1985

Geyne

etal.1963

Russell&

Kesler

1991

Harris

etal.1994

Jannas

etal.

1990

Brathwaite

&

Blattner

1995

Moyle

et al.

1990

Richards&

Kerrich1993

Van Leeuwen

etal.1990

Cooke

etal.1996

Izawa

et al.

1990

*Age data supplemented by: Aokiet al. 1993; Henryet al. 1995; Izawaet al. 1993a; McKeeet al. 1992; R. Jannas pers. comm. 1995;

D .

E. Noble pers. comm. 1995.

f

173 t Au production + reserves (July 1996).

Tectonic setting: BA, back-arc; Cont, continental margin; IA, island arc. Related volcanic rocks: And, andesite; Bas, basalt; Dae, dacite;

Ig , ignimbrite; Int, intrusions; Ker, keratophyre; Phon, phonolite; Rhy, rhyolite; Rhyodac, rhyodacite; Spil, spilite; Trach, trachyte;

Trachyb, trachybasalt; A, alkaline; CA, calc-alkaline; KCA, high-K calc-alkaline; IAT, island-arc tholeiite. Ore-related alteration: Ad,

adularia; HS, high sulfidation; LS, low sulfidation; Qu, quartz. Associated mineralisation: LS, low-sulfidation epithermal; Ppy Cu,

porphyry copper.

role

as ore

localisers appears

to be

minimal

at

Pueblo Viejo

and Ladolam. Major fault zones controlled vein emplace-

mentatComstock Lodeand ElIndio, whereas more local

faults localisedthegoldore in theother deposits. Mostof

the faults that controlled the large gold-bearing vein

systems

and

acted

as

feeders

for the

bulk-tonnage gold

mineralisation

are

reported

to

have undergone normal

displacements (Table2), insome cases withacomponent

of oblique slip being recognised. Fault jogs within strike-

slip fault zones have been proposed for Waihi (Sibson

1987)

and

Baguio (Ringenbach 1992). Diatreme-bounding

ring faults imposed additional structural controls

on

some

of the gold mineralisation at Cripple Creek (Thompson

1992), Pueblo Viejo (Russell&Kesler1991) andBaguio

(Damasco& deGuzman 1977).

The large epithermal gold deposits show

a

spread

in

ages,

from Early Cretaceous (Pueblo Viejo)

to

Pleistocene,

although 13 of the deposits are younger than 40 Ma

(Table 2). As in the case of the gold-rich porphyry

deposits,

the

epithermal gold deposits along

the

western

side

of the

Pacific Ocean

are, on

average, notably younger

than thoseon theeastern side.

A spectrumof volcanic settings hoststhe large gold-

rich epithermal deposits (Table2). Atleast three , possibly

four, deposits occur either

in or

around diatremes; three,

possibly four,

of the

deposits

are

associated with flow-

dome complexes; one, possibly two,arepartsofash-flow

calderas;andLadolamispresentin asmall stratovolcano

that underwent sector collapse duringthemineralisation

event (Sillitoe 1994).

The

remainder

of the

deposits

occupy uncertain volcanic settings, which

at

Porgera

is

dueto acomplete absenceofvolcanic roc ks.

Eightofthe large epithermal gold deposits are judged ,on

the basisofavailable data,to berelated geneticallytocalc-

alkaline volcanic rocks ranging

in

composition from

rhyolite

to

andesite (Table

2).

Two deposits

may be

related

to high-K calc-alkaline volcanics (Table2) andone, Pueblo

Viejo,to theisland-arc (low-K) tholeiite series (Lebron&

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378

R. H. SILLITOE

Perfit 1993). The volcanic rocks are bimodal at Pueblo

Viejo and Hishikari, thereby suggesting extensional set-

tings.

Felsic ignimbrites are spatially and temporally related

to the McDonald and Round Mountain deposits, whereas

the host ignimbrite at El Indio is unrelated temporally to the

gold mineralisation. Three deposits are associated with

alkaline magmatism, which is basic and sodic at Porgera

(Richards 1990), but potassic and, in part, more felsic at

Cripple Creek and Ladolam (Table 2). I-type, magnetite-

series volcanic rocks appear to be ubiquitous.

In contrast to the large gold-rich porphyry deposits, the

large epithermal gold deposits are extremely varied in

form and mineralisation style. They may be subdivided

broadly into vein systems and large dispersed deposits of

either predominantly disseminated, stockwork or breccia-

hosted mineralisation (Figure 5). The vein systems con-

stitute both bonanza deposits (Comstock Lode, Cripple

Creek, El Indio, Zone VII at Porgera, Hishikari) as well

as more extensive, but lower grade deposits (Pachuca-

Real del Monte, Waihi, Baguio). Average gold grades

were as low as 2.4 g/t at Pachuca-Real del Monte. Veins

are also present in several of the other deposits (Table 2),

either subsidiary to disseminated ore (Round Mountain),

dominant over disseminated ore (Cripple Creek) or

of approximately equal importance to stockwork ore

(Porgera). The stockwork ore at Porgera is intrusion-

related and formed relatively deeply, but is juxtaposed

with the epithermal gold veins and breccias of Zone VII

(Richards & Kerrich 1993).

In common with epithermal deposits in most parts of

the circum-Pacific region, large low-sulfidation deposits

are more abundant than those of high-sulfidation type.

Table 2 demonstrates that the large low-sulfidation deposits

are nearly five times as abundant. The various styles of

mineralisation include examples of both epithermal types.

Half of the low-sulfidation deposits contain appreciable

amounts of base metals, mainly zinc and lead, whereas

the other half are poor in sulfides (Sillitoe 1993c). The

sulfide-poor low-sulfidation vein deposits, with the excep-

tion of Baguio, tend to possess greater concentrations of

adularia as a vein and alteration component, although

adularia is also abundant in several of the bulk-tonnage

deposits (McDonald, Round Mountain, Ladolam ). Two of

the large low-sulfidation deposits (Round Mountain,

Kelian) contain relatively minor quantities of quartz.

Kelian and the intrusion-related stockwork gold ore at

Porgera are rich in a variety of carbonate minerals, and are

classified as carbonate-base-metal deposits by Leach and

Corbett (1994).

Two of the three large high-sulfidation deposits do not

adhere to the typical model for this type of epithermal

gold deposit (Hedenquist

et al.

1994) because residual

vuggy quartz does not host the gold. Ore is associated

with quartz-alunite and quartz-pyrophyllite alteration at

Pueblo Viejo (Muntean

et al.

1990), whereas at El Indio

it is present in m assive ena rgite-pyrite and bonanza-grade

quartz veins (Jannas

et al.

1990).

Six, probably seven, of the large epithermal gold deposits

were formed at shallow depths because features denoting

the interval between the palaeo-surface and palaeo-water

table (Figure 5; Sillitoe 1993c) are partly preserved.

Features include hot-spring sinter of palaeo-surface origin

Lacustrine

sediment

Rem nant Minor quartz-

sinter adularia veins + Au

Welded

ignimbrite

Non-welded

I

ignimbrite

Smectite-chlorite

alteration

W e l d e d - * -

A

ignimbrite A A

Illite-adularia

alteration + Au

Quartz-adularia veins + Au

b)

Acid-leached rock formed

in steam-heated zone

0.5

km

_ 0 . 5

Quartz-carbonate veins + Au

(minor illite/sericite-adularia

selvages)

Tuff sequence

Bonanza Au

beneath

unconformity

Basement

rocks

Figure 5

Selected end-member models for large epithermal gold

deposits in the circum-Pacific region, (a) Large-tonnage, low-

grade deposit hosted by an aquifer below an aquitard. (b) Major

vein system containing bonanza shoots immediately beneath an

unconformity separating tight basement rocks from an overlying

porous sequence. Both models are for low-sulfidation deposits

poor in base metals and sulfides.

at McDonald (Bartlett

et al.

1995); acid-leached rock

generated in the steam-heated environment above the

palaeo-water table at or near El Indio (Sillitoe 1991), Waihi

(Brathwaite & Blattner 1995), Ladolam (Moyle

et al.

1990), Hishikari (Izawa et al. 1990) and, possibly,

Yanacocha; a hydrothermal eruption crater filled with

lacustrine mudstone and interbedded chert at Hishikari

(Izawa et al. 1993b); and remnants of lacustrine sedi-

mentary rocks at El Indio (Sillitoe 1991) and Round

Mountain (Tingley & Berger 1985; Henry

et al.

1995). The

remaining deposits were formed at substantially deeper

levels,

as deep as 900 m at Kelian (Van Leeuwen

et al.

1990) and even - 2 km at Porgera (Richards & Kerrich

1993).

Moreover, the vertical extents of at least seven of the

deposits exceed 600 m, and attain at least 1 km at

Comstock Lode and Cripple Creek.

Most of the large epithermal gold deposits lack asso-

ciated mineralisation types (Table 2) although, as noted

above, Porgera is a combination of intrusion-related and

epithermal mineralisation. The exceptions are the low-

sulfidation gold deposits at Ladolam and Baguio, which

are related to low-grade porphyry copper-gold minerali-

sation, and El Indio, which is near a low-sulfidation

epithermal gold deposit.

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L A R G E S T C I R C U M - P A C I F I C G O L D D E P O S I T S

3 7 9

FACTORS NOT RELATED DIRECTLY TO

LARGE GOLD CONTENTS

Perusal of these brief overviews of the characteristics of

large gold-rich porphyry and epitherraal gold deposits in

the circum-Pacific region and reference to Tables 1 and 2

reveal a series of factors that, either singly or in com-

bination, appear unable to explain the exceptional gold

contents of the 25 deposits under consideration. These

factors are explained further in this section.

Geotectonic setting

Neither the large gold-rich porphyry deposits nor the

large epithermal gold deposits seem to be influenced

unduly by the nature of the underlying crust. Both

deposit types were generated in the shallow parts of

either cratonic or island-arc crust (Figure 1). The crustal

extremes are represented by Bingham and Cripple Creek

atop thick crust and Pueblo Viejo and Ladolam underlain

by essentially nothing but oceanic lithosphere. Nor does

the distance of the large deposits from the trenches or

underlying subduction zones active during or just before

their emplacement seem to have been influential.

No unique stress regime in the upper crust at the time

of mineralisation appears to account for the localisation

of unusually large gold deposits. Cripple Creek and

Hishikari were probably formed during incipient rifting,

whereas other deposits, especially most of those of

epithermal type in the Great Basin of the western USA

(Seedorff 1991), were emplaced under mildly extensional

conditions. In contrast, regional compression charac-

terised the upper crust during emplacement of the

Grasberg and Ok Tedi porphyry copper-gold and Porgera

gold deposits in New Guinea.

The composition and redox state of the concealed arc

crust and subjacent mantle in the vicinities of these large

gold deposits are unknown except at Ladolam, where a

study of xenoliths from nearby young volcanic rocks

reveals a highly oxidised mantle assemblage (B. I. A.

Mclnnes, R. A. Binns, P. M. Herzig & M. D. Hannington

unpubl. data). Such oxidised mantle is believed to be the

source of the I-type igneous rocks related genetically to

these large gold deposits (e.g. Richards 1990). Even

where shallow crustal rocks comprise organic carbon-

bearing sedimentary sequences, as at Porgera and

Hishikari, the igneous rocks did not undergo appreciable

reduction.

Igneous

rocks

A broad spectrum of intrusive and/or volcanic rocks is

observed to be related genetically to the large gold

deposits reviewed here. Island-arc tholeiite, calc-alkaline,

high-K calc-alkaline (including shoshonitic) and alkaline

magma suites were all capable of generating large gold

concentrations. Moreover, the degree of fractionation and

composition of the associated igneous rocks span a broad

range, from hawaiite and mugearite (at Porgera: Richards

1990) to rhyolite.

Notwithstanding the varied petrochemistry of the asso-

ciated igneous rocks, a surprisingly large percentage,

approximately 2 0 , of the large gold deposits accom-

pany shoshonitic or alkaline suites (Mutschleret al.1991;

Miiller & Groves 1993). Shoshonites are estimated to

constitute only about 2.5 by volume of igneous rocks in

circum-Pacific arc terranes (Baker 1982). Alkaline rocks

are even less abundant in circum-Pacific arcs so, in total,

these suites probably do not exceed 3 by volume of

circum-Pacific igneous rocks.

Structural setting

The structural controls noted in Tables 1 and 2 and the

comments above suggest that no specific structural

setting is responsible for the formation of very large gold

deposits in arc terranes. Major faults and lineaments

appear to be associated with some, but not all, of the

porphyry deposits and, therefore, cannot be considered

as a prerequisite for the formation of large gold con-

centrations. Similarly, the district-scale structures that

localised the large epithermal gold deposits, although

dominated by normal faults, do not seem to differ

substantially from the profusion of other faults in arc

terranes. Provision of dilatant sites for the passage of

magma and fluids is the only basic requirement and this

may be accomplished in a variety of structural settings

which may or may not involve regional faults or

lineaments. For example, in the context of the regional

compressive setting for the Grasberg porphyry copper-

gold deposit, dilatancy for stock emplacement and metal

introduction was facilitated by a restricted pull-apart

connecting district-scale strike-slip faults (Sapiie & Cloos

1994).

Age of deposits

The range of deposit ages clearly precludes formation of

the large gold deposits at one or more specific times

during the Phanerozoic. Nevertheless, all but three of the

25 deposits were generated after 40 Ma (Figure 6) because

average depths of erosion are shallower and hence the

preservation potential for epithermal and subvolcanic

deposits is greater.

Surprising, however, are the extremely young ages

(^5 Ma: Table 1; Figure 6) for the five large porphyry

copper-gold deposits in the western Pacific island arcs.

In fact, only three of the region's seven large epi-

thermal gold deposits, two of which (Ladolam and

Baguio) are associated with porphyry copper-gold for-

mation, are as young (Table 2). This observation

confirms that erosion and exhumation rates were excep-

tionally rapid in the vicinities of these large gold-rich

porphyry systems.

Volcanic setting

Most volcanic settings, including stratovolcanoes, ash-

flow calderas, flow-dome complexes and maar-diatreme

systems, are represented by the 25 gold deposits under

consideration. In contrast to the epithermal gold deposits,

the gold-rich porphyry deposits are generally eroded too

deeply to ascertain the nature of volcanic landforms that

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