1399

0361-0128/11/3996/1399-26 1399

IntroductionCLOSE SPATIAL and temporal relationships between porphyryand high sulfidation epithermal deposits have been demon-strated in several mineral districts (e.g., Mankayan, Philip-pines: Arribas et al., 1995; Hedenquist et al., 1998; Chang etal., 2011; Deyell and Hedenquist, 2011; Nevados del Famatina,Argentina: Losada-Calderon and McPhail, 1996; Pudack et al.,2009; Maricunga, Chile: Muntean and Einaudi, 2001). Thesegeochronological, stable isotope, and fluid inclusion studieshave provided compelling evidence for a genetic associationbetween porphyry deposits, high sulfidation epithermal

Cu-Au-Ag mineralization, and zones of hypogene advancedargillic and silicic alteration (e.g., Sillitoe, 2010). In contrast,it can be difficult to demonstrate a genetic association be-tween porphyry deposits and epithermal deposits dominatedby quartz-carbonate-pyrite-illite-(adularia-sphalerite-galena)veins and breccias (aka carbonate-base metal sulfide veins:Corbett and Leach, 1998, or intermediate sulfidation veins:Hedenquist et al., 2000; Sillitoe and Hedenquist, 2003). Thespatial association of these deposit types has been well-estab-lished in several districts, including Mankayan, Philippines(Sajona et al., 2002), Collahuasi, Chile (Masterman et al.,2005), and Rosia Montana, Romania (Wallier et al., 2006).Stable isotopic results give strong indications of magmaticcontributions to the hydrothermal fluids forming epithermalmineralization in some districts (e.g., Rosia Montana: Wallieret al., 2006), but equivocal results in others (e.g., Victoria,

Evidence for Magmatic-Hydrothermal Fluids and Ore-Forming Processes in Epithermal and Porphyry Deposits of the Baguio District, Philippines

DAVID R. COOKE,1,† CARI L. DEYELL,1,* PATRICK J. WATERS,2 RENE I. GONZALES,2,** AND KHIN ZAW1

1 CODES ARC Centre of Excellence in Ore Deposits, University of Tasmania, Private Bag 126, Hobart, Tasmania, 7001, Australia2 Anglo American Exploration Philippines Inc., 1101 Pearlbank Centre Building, 146 Valero Street, Salcedo Village,

Makati City, Metro Manila, Philippines

AbstractThe Baguio district contains a diverse array of epithermal, porphyry and skarn deposits, together with a large,

broadly strata bound, advanced argillic lithocap. Magmatism, mineralization, and alteration occurred in response to subduction of the South China Sea plate and the Scarborough Ridge beneath northern Luzon overthe past 3 m.y. Rapid uplift and exhumation resulted in epithermal veins overprinting several porphyry Cu-Audeposits. Most of the epithermal Au-Ag deposits of the Baguio district (including Antamok and Acupan, thetwo largest Au deposits) are intermediate sulfidation state quartz-carbonate-adularia-illite-base metal sulfideveins that contain electrum and minor Au-Ag tellurides. In contrast, high sulfidation mineralization at Kelly includes enargite, tennantite, electrum, and precious metal tellurides and is associated with advanced argillicalteration.

Although the mineralizing fluids that formed the porphyry and epithermal deposits had distinct tempera-tures and salinities, stable and radiogenic data provide evidence for direct magmatic contributions into eachdeposit type. The epithermal mineralizing fluids were dilute (generally, <2 wt % NaCl equiv) and had moder-ate temperatures (<300°C). Porphyry-style mineralization was associated with high temperature (300° to>600°C) hypersaline brines (30 to >70 wt % NaCl equiv) and low-density vapor. Sulfur isotope compositionsof sulfides in the porphyry, skarn and intermediate sulfidation epithermal veins of the southern and centralBaguio district are mostly between +1 and +6 per mil, consistent with a predominance of H2S in the mineral-izing fluids (i.e., reducing conditions). In contrast, sulfides from the high sulfidation, porphyry, and intermedi-ate sulfidation deposits located adjacent to the Baguio lithocap mostly have negative sulfur isotope values (–6.9to +0.8‰), consistent with oxidizing (SO42–-predominant) mineralizing fluids.

Intermediate sulfidation epithermal veins at Acupan have crosscut a well-mineralized porphyry Cu-Au stock-work at Ampucao. The two deposits cannot be distinguished on the basis of radiometric age determinations(Ampucao: 0.51 ± 0.26 Ma; Acupan: 0.65 ± 0.07 Ma), and are interpreted to be cogenetic, with telescoping ofthe two environments caused by the rapid uplift and exhumation associated with ridge subduction. Measuredδ34Ssulfide (+1.1 to +6.6‰), δ34Ssulfate (+10.4 to +31.8‰) values and initial strontium ratios of anhydrite(0.70378–0.70385) are consistent with identical and predominantly magmatic sources of these components forthe Ampucao porphyry and Acupan epithermal veins. Helium isotopes provide further evidence of mantle- derived components in the epithermal veins (R/Ra values of 6.0 and 6.7). Oxygen, deuterium, and carbon iso-topes provide evidence for predominantly magmatic water at Ampucao and for hybrid magmatic-meteoric waters at Acupan that precipitated precious metals due to boiling. The proportion of magmatic water relativeto meteoric water and precious metal grades both decreased with time during epithermal vein formation atAcupan. The common observation of cross-cutting relationships between porphyry and epithermal veins observed throughout the Baguio district imply that the evolution of porphyry-style to intermediate sulfidation-style mineralization was a common phenomenon in this region, and contributed significantly to its rich metalendowment.

† Corresponding author: e-mail, [email protected]*Present address: Spectral International Inc., Arvada, Colorado.**Present address: PT. Sorikmas Mining, Jl. Torsiojo, Kotanopan, Mandail-

ing Natal, North Sumatra, Indonesia.

©2011 Society of Economic Geologists, Inc.Economic Geology, v. 106, pp. 1399–1424

Philippines: Sajona et al., 2002; Madjarova, Bulgaria: Rice etal., 2007; Cowal, Australia: Zukowski et al., submitted).

The Baguio district, Philippines (Fig. 1), contains two giantand numerous smaller intermediate sulfidation state epither-mal vein systems. Mineralized veins at the giant Antamok andAcupan deposits extend from surface to depths of almost akilometer below surface (Sawkins et al., 1979; Cooke andBloom, 1990; Cooke et al., 1996), and mining of these veinsby Benguet Corporation produced more than 535 t Au duringthe 20th century (Waters et al., 2011). The district also con-tains porphyry Cu-Au deposits and prospects (e.g., SantoTomas II, Ampucao, Nugget Hill), in addition to skarns(Thanksgiving, Mexico) and a high sulfidation state epither-mal vein system (Kelly: Comsti et al., 1990; Aoki et al., 1993;Deyell and Cooke, 2003; Waters et al., 2011; Fig. 2). The por-phyry deposits of the Baguio district were emplaced between3 and 0.5 Ma, whereas epithermal veins appear to haveformed in the last million years (Aoki et al., 1993; Imai, 2001;Waters et al., 2011; Table 1).

This manuscript presents new stable and radiogenic iso-topic data for epithermal and porphyry deposits of the Baguiodistrict. These data are used to assess the relative importanceof magmatic and meteoric components in the mineralizingfluids, evaluate likely depositional processes, and, in combi-nation with new geochronological data from Waters et al.

(2011), to establish whether the porphyry and intermediatesulfidation epithermal deposits are cogenetic.

Geologic Setting of the Baguio DistrictThe geology of the Baguio district is described in Waters et

al. (2011) and major tectonic elements of the Philippines areshown in Figure 1. Table 1 summarizes the characteristics ofthe main volcanosedimentary and intrusive rock units, andtheir distribution is shown on Figure 2. Key elements in thegeologic evolution of the district are as follows: (1) sedimen-tation and volcanism in a submarine basin setting (Pugo For-mation: Eocene-Oligocene); (2) shallow marine clastic andcarbonate sedimentation and minor andesitic volcanism (Zig-Zag and Kennon Formations: late Oligocene-early to midMiocene); (3) middle Miocene arc magmatism related to sub-duction of the South China Sea plate beneath northernLuzon along the Manila trench (Central Cordillera DioriteComplex; Hollings et al., 2011 a); (4) onset of mountain build-ing— uplift and terrestrial sedimentation (Klondyke Forma-tion: middle to late Miocene); (5) commencement of subduc-tion of the Scarborough ridge during the Pliocene, shallowingof subduction angle, transpressional displacement on the left-lateral Philippine fault system, rapid uplift and erosion, intru-sion of mafic dikes, quartz diorite porphyries and dacitedomes, diatreme volcanism, widespread Pliocene-Pleistoceneporphyry and epithermal mineralization (Hollings et al.,2011b; Waters et al., 2011; Tables 1, 2).

Mineral DepositsThe mineral deposits of the Baguio district are described in

Waters et al. (2011). Their characteristics are summarized inTable 2 and locations are shown on Figure 2. We provide de-tailed summaries of the Acupan and Kelly epithermal Au-Agdeposits and the Ampucao porphyry Cu-Au prospect, below,as most of our new analytical data are derived from these deposits.

Acupan

Pleistocene epithermal Au-Ag veins at the Acupan and Ito-gon mines (hereafter referred to collectively as Acupan) weredescribed by Callow and Worley (1965), Sawkins et al. (1979),Cooke and Bloom (1990), Cooke et al. (1996) and Cooke andMcPhail (2001). Acupan is the second-largest epithermal de-posit in the Baguio district in terms of gold production (Table2), and is also one of the largest epithermal vein systems inthe world, in terms of contained gold (Hedenquist et al.,2000). Since the commencement of underground mining op-erations in 1931 until mine closure in 1993, the Acupan andItogon mines exploited the western and eastern halves of theAcupan vein system (Fig. 3). More than 225 t Au was ex-tracted during underground mining of the composite bandedquartz-carbonate veins and hydrothermal breccias. In the late1980s, average mining grades were approx. 6.1 g/t Au, with acutoff grade of ~4.2 g/t (United Nations Development Pro-gramme-UNDP, 1987). A remaining bulk mining resourcethat contains about 99 t Au was defined by Placer Dome Inc.in the mid-1990s (Waters et al., 2011).

The principal host rocks to mineralization at Acupan are theVirac granodiorite, Balatoc diatreme, and andesites of the Zig-Zag Formation (Fig. 3). The epithermal veins have intensely

1400 COOKE ET AL.

0361-0128/98/000/000-00 $6.00 1400

200 km

EurasianPlate

PhilippinePlate

20°N

16°N

8°N

12°N

122°E118°E 126°E

NMankayan district(3.5 - 1.4 Ma)

Baguio district(3.1 - 0.5 Ma)

Scarborough

Ridge

Sulu

Trench

Philip

pine

Fault

Man

ilaTr

ench

Phi

lippi

neTr

ench

FIG. 1. Major tectonic elements of the Philippines. The locations of theBaguio and Mankayan districts in northern Luzon are highlighted by whitediamonds. Active volcanoes are denoted by black triangles. The approximateposition of the Scarborough Ridge has been interpreted from bathymetrydata provided by the National Geophysical Data Centre (unpub. data, 2004).Active and inactive subduction zones are highlighted by black and white-filled ticks, respectively. Diagram modified from Cooke et al. (2005).

EPITHERMAL AND PORPHYRY DEPOSITS OF THE BAGUIO DISTRICT, PHILIPPINES 1401

0361-0128/98/000/000-00 $6.00 1401

FIG. 2. Geology and mineral deposits of the Baguio district, Philippines (modified from Waters et al., 2011).

1402 COOKE ET AL.

0361-0128/98/000/000-00 $6.00 1402

TAB

LE

1. G

eolo

gy o

f the

Bag

uio

Dis

tric

t with

Geo

logi

cal U

nits

Sor

ted

by F

orm

atio

n A

ge (

youn

gest

at t

he to

p of

the

tabl

e)

Lith

ofac

ies

and

Uni

tA

geco

ntac

t rel

atio

nshi

psA

ssoc

iatio

n w

ith m

iner

aliz

atio

nC

omm

ents

Ref

eren

ces

Am

puca

o-H

artw

ell-

Har

twel

l dac

ite:

Har

twel

l: da

cite

por

phyr

yM

argi

nal c

last

-sup

port

ed fa

cies

of

Dac

ite in

trus

ions

hav

e ad

akiti

c de

Guz

man

(pe

rs. c

omm

un.,

Bal

atoc

clu

ster

1.09

± 0

.10

Ma

Am

puca

o: q

uart

z di

orite

to

Bal

atoc

dia

trem

e ho

sts

sign

ifica

nt

com

posi

tions

1986

), C

ooke

and

Blo

om

Am

puca

o po

rphy

ry:

quar

tz-p

hyri

c ho

rnbl

ende

A

u m

iner

aliz

atio

n at

Acu

pan

(199

0), D

efan

t and

0.

51 ±

0.2

6 M

ada

cite

por

phyr

yL

ate-

stag

e B

alat

oc d

acite

pip

e D

rum

mon

d (1

990)

, B

alat

oc d

acite

plu

g:

Bal

atoc

: pol

ymic

t mat

rix-

rich

cu

ts e

pith

erm

al v

eins

Wat

ers

et a

l. (2

011)

, 0.

8 M

a (D

efan

t and

br

ecci

a (d

iatr

eme)

and

late

-A

mpu

cao

porp

hyry

rel

ated

to h

igh

Hol

lings

et a

l. (2

011b

)D

rum

mon

d, 1

990)

stag

e B

alat

oc d

acite

por

phyr

ygr

ade

porp

hyry

Au-

Cu

min

eral

iza-

tion

at A

mpu

cao

Sant

o To

mas

II-

Sant

o To

mas

II:

Q

uart

z di

orite

por

phyr

ySa

nto

Tom

as I

I po

rphy

ry

Mos

t pro

duct

ive

cent

er o

f Im

ai (

2001

), W

ater

s et

al.

Bum

olo-

Clif

ton

1.48

± 0

.05

Ma;

C

u-A

u de

posi

tpo

rphy

ry C

u-A

u m

iner

aliz

a-(2

011)

clus

ter

1.47

± 0

.05

Ma

Bum

olo

porp

hyry

Cu-

Au

pros

pect

tion

in th

e B

agui

o di

stri

ct

Bla

ck M

ount

ain

3.09

± 0

.15

Ma

to

Qua

rtz

dior

ite p

orph

yry,

B

lack

Mou

ntai

n po

rphy

ry C

u-A

u O

ldes

t kno

wn

porp

hyry

cen

ter

Cal

low

(19

67),

Swee

t et a

l. In

trus

ive

Com

plex

2.81

± 0

.24

Ma

plag

iocl

ase

horn

blen

de

depo

sits

, Tha

nksg

ivin

g A

u-ba

se

in th

e di

stri

ct(2

008)

, Wat

ers

et a

l. an

desi

te p

orph

yry

met

al s

karn

, Mex

ico

Cu-

Au

skar

n(2

011)

Kel

ly d

iori

te3.

1 ±

1.1

Ma

(Tog

ashi

, N

orth

-tre

ndin

g 1

km+

diam

eter

H

osts

hig

h su

lfida

tion

stat

e K

elly

dio

rite

dat

ed a

t Tud

ing

Com

sti e

t al.

(199

0),

in A

oki e

t al.,

199

3)m

icro

dior

ite p

luto

n ep

ither

mal

gol

d ve

ins

at K

elly

si

lica

pit;

the

smal

l dio

rite

at

Aok

i et a

l. (1

993)

< 2.

3 M

a at

Bag

uio

Gol

d H

as in

trud

ed th

e Zi

gzag

and

A

u-A

g m

ine

Bag

uio

Gol

d yi

elde

d a

(Aok

i et a

l., 1

993)

Klo

ndyk

e F

orm

atio

nsSm

all d

iori

te b

ody

at B

agui

o G

old

max

imum

K-A

r ag

e of

2.3

Ma

corr

elat

ed w

ith K

elly

dio

rite

by

(Aok

i et a

l., 1

993)

Com

sti e

t al (

1990

)

Bag

uio

For

mat

ion

3.59

4 ±

0.07

0 M

a Te

rres

tria

l and

esiti

c tu

ffac

eous

Pr

inci

pal h

ost t

o B

agui

o lit

hoca

p—M

alat

erre

(19

89)

repo

rted

an

Smith

and

Edd

ingf

ield

ro

cks,

bre

ccia

s an

d flo

ws,

inte

r-sy

nchr

onou

s w

ith B

lack

Mou

ntai

n ag

e of

3.5

7 M

a fr

om a

ndes

ite

(191

1), D

e lo

s Sa

ntos

ca

late

d w

ith c

ongl

omer

ates

po

rphy

ry c

ompl

ex?

brec

cia;

(1

982)

, Mal

ater

re (

1989

), an

d m

inor

san

dsto

nes

Wat

ers

et a

l. (2

011)

Cam

p 4

Intr

usiv

e Pr

e-m

iner

aliz

atio

n m

afic

H

ornb

lend

e di

orite

, qua

rtz

Kid

ao a

nd U

bola

n po

rphy

ry C

u-A

u M

axim

um a

ge o

nly

prov

ided

by

Wat

ers

et a

l. (2

011)

Com

plex

dike

s: 4

.55

± 0.

15 M

a;

dior

ite p

orph

yry

pros

pect

spr

e-m

iner

aliz

atio

n m

afic

4.

028

± 0.

074

Ma

dike

s. L

ikel

y to

be

youn

ger.

Hor

nble

nde-

phyr

ic

Plio

cene

(4.

56 ±

0.2

8 M

a,

Dia

base

, lam

prop

hyre

and

Pr

ecur

sors

to m

iner

aliz

atio

n in

the

Hor

nble

nde

meg

acry

sts

up to

U

ND

P (1

987)

, Wat

ers

et a

l. di

ke c

ompl

ex

4.11

± 0

.42

Ma,

ap

pini

te d

ike

swar

mB

agui

o di

stri

ct10

cm

in d

iam

eter

in in

di-

(201

1), H

ollin

gs e

t al.

4.03

± 0

.14

Ma)

Dis

tinct

ive

horn

blen

de

vidu

al d

ikes

(201

1b)

meg

acry

sts

Mir

ador

Lim

esto

neM

ioce

ne-P

lioce

ne

Lim

esto

neSo

me

deba

te e

xist

s as

to

Lei

th (

1938

), M

alat

erre

(f

auna

l age

)w

heth

er th

is is

par

t of t

he

(198

9), P

eña

(199

8),

Ken

non

For

mat

ion

or a

W

ater

s et

al.

(201

1)se

para

te u

nit

Klo

ndyk

e F

orm

atio

nM

iddl

e-la

te M

ioce

ne

Con

glom

erat

es, g

rits

, san

d-H

osts

par

ts o

f Ant

amok

and

H

igh

ener

gy d

epos

ition

al e

nvi-

Dur

kee

and

Pede

rson

m

icro

foss

il as

sem

blag

esst

ones

and

mud

ston

es w

ith

Bag

uio

Gol

d ve

ins

ronm

ent—

detr

itus

gene

rate

d (1

961)

; UN

DP

(198

7),

inte

rbed

ded

ande

sitic

lava

s,

May

hav

e or

igin

ally

hos

ted

surf

ace

by u

plift

and

exh

umat

ion

of

Mitc

hell

and

Lea

ch

brec

cias

and

tuff

aceo

us b

eds

expr

essi

on o

f epi

ther

mal

th

e L

uzon

Cen

tral

Cor

dille

ra(1

991)

, Peñ

a (1

998)

, m

iner

aliz

atio

n at

Acu

pan

Wat

ers

et a

l. (2

011)

Ken

non

For

mat

ion

Ear

ly-m

iddl

e M

ioce

ne

Gre

y bi

oher

mal

lim

esto

ne,

Skar

n al

tera

tion

and

min

eral

izat

ion

Shal

low

mar

ine

sett

ing—

shoa

ling

Pena

(19

98),

Wat

ers

et a

l. fa

unal

age

sca

lc-a

reni

tes,

cal

c-ru

dite

spr

oxim

al to

Plio

cene

intr

usiv

e of

the

Mio

cene

arc

?(2

011)

com

plex

es

developed sericitic (muscovite ± illite) alteration halos thatpass outward to propylitic alteration assemblages. Five para-genetic stages have been defined in the crustiform bandedveins: type A chalcedony (restricted to higher mine levels,commonly occurring as clasts in later-formed bands), mar-ginal type B fine-grained gray quartz bands that contain abun-dant hydrothermal breccia textures, intermediate type Cwhite quartz with moderately abundant hydrothermal brec-cias, central type D calcite with minor breccia textures, andbarren late-stage type E anhydrite bands and veins (Cooke etal., 1996; Fig. 4). More than 1,000 fluid inclusions analyzedfrom two vein systems (381-No Name fault vein system and209-Odd vein system) had homogenization temperatures be-tween 300° and 200°C, with a gradual decline in tempera-tures from type B to type D bands. Some anomalously hightemperatures (>300°C) were recorded, but these were attrib-uted to mixed entrapment of liquid and vapor in boiling fluidinclusion populations. Calculated salinities were between 0.0and 5.1 wt percent NaCl equiv, but mostly below 1 wt percent(Cooke and Bloom, 1990; Fig. 5).

Over 440 veins have been mined underground. Most ofthe northeast-trending veins dip steeply to the southeast. Asubordinate set of structures have easterly strikes and dip tothe south (Fig. 3). High-grade ore shoots occur at the inter-sections of these two structural orientations. The veins aregrouped into five main series. These are, from north tosouth, the 100, 200, 300, 400, and 500 series veins (Fig. 3).Both the direction and amount of displacement along thesestructures prior to epithermal mineralization is uncertain.Minor gouge is locally developed on the margins of someveins, suggesting minor postmineralization strike-slip move-ment. Displacements along the Star and Pine faults may bein excess of 150 m (Worley, 1967). Most of the veins are 0.5to 1 m wide. The 364 vein has an average width of 4 m andattains a maximum of 12 m (Worley, 1967). The Acupan veinsare exposed at surface, implying that the top of the deposithas been removed by erosion. The currently exposed verticalextent of vein mineralization is almost 1 km, considerablylarger than many epithermal veins (cf. Cooke and Simmons,2000; Simmons et al., 2005), and similar to the spectacularepithermal ore shoots at Pachuca Real-del Monte, in Mexico(Geyne et al., 1963).

At the southern end of the Acupan mine, the 400 and 500series epithermal veins have crosscut porphyry-style quartz-magnetite-chalcopyrite veins associated with the Ampucaoporphyry Au-Cu prospect (Cooke and Bloom, 1990). TheAcupan epithermal veins formed at 0.65 ± 0.07 Ma, based ona K-Ar age determination from an illitic alteration halo to the409 vein (Aoki et al., 1993).

Ampucao

The Ampucao porphyry Au-Cu prospect (also known asAcupan South) has been described by Cooke and Bloom(1990), Cooke (1991), and Waters et al. (2011). It was firstdiscovered by underground drilling on the southern fringe ofthe Acupan mine (Figs. 3, 6). Subsequent field mapping byAngloAmerican in the late 1990s identified outcrops of potas-sic alteration and mineralizing intrusions. Porphyry-style min-eralization is associated with the Ampucao porphyry complex,which comprises early- and intramineralization quartz diorite


0361-0128/98/000/000-00 $6.00 1403

Cen

tral

Cor

dille

ra

Mid

dle

mio

cene

Dio

rite

, qua

rtz

dior

ite, g

rano

-T

he V

irac

gra

nodi

orite

is th

e m

ain

Mid

dle

Mio

cene

arc

mag

mat

ism

—Sh

anno

n (1

979)

, JIC

A

Dio

rite

Com

plex

(V

irac

gra

nodi

orite

; 20.

23

dior

ite, g

abbr

o, tr

ondj

hem

iteho

st r

ock

to e

pith

erm

al v

eins

at

calc

-alk

alin

e in

trus

ive

suite

(1

983)

, Kni

ttel

and

Def

ant

(Agn

o B

atho

lith)

± 0.

38, 2

0.2

± 0.

7 M

a;

Has

intr

uded

the

Zig-

Zag

and

Acu

pan

char

acte

rize

d by

low

K a

nd

(198

8), C

ooke

and

Blo

om

Luc

buba

n ga

bbro

; Pu

go fo

rmat

ions

Acu

pan

epith

erm

al v

eins

pin

ch

larg

e io

n lit

hoph

ile a

bund

ance

s(1

990)

, Coo

ke e

t al.

22.6

± 0

.5 M

a)U

ncon

form

ably

ove

rlai

n by

the

out t

o th

e ea

st in

the

Luc

buba

n T

he V

irac

gra

nodi

orite

is a

hig

h (1

996)

, Peñ

a (1

998)

, K

lond

yke

For

mat

ion

gabb

roK

cal

c-al

kalin

e in

trus

ion

pre-

Wat

ers

et a

l. (2

011)

, K

elly

dio

rite

hos

ts v

ein

min

eral

izat

ion

viou

sly

thou

ght t

o be

Plio

cene

H

ollin

gs e

t al.

(201

1a)

at K

elly

in a

ge

Epi

ther

mal

vei

ns a

t Ant

amok

pin

ch

out t

o th

e ea

st in

the

Ant

amok

dio

rite

Zig-

Zag

For

mat

ion

Lat

e O

ligoc

ene

to m

iddl

e M

arin

e se

dim

enta

ry r

ocks

, H

osts

por

phyr

y an

d ep

ither

mal

Sh

allo

w m

arin

e se

ttin

g—lo

cal

Wor

ley

(196

7), P

ena

(197

0),

Mio

cene

faun

al a

ges

min

or b

asal

tic to

and

esiti

c m

iner

aliz

atio

n at

sev

eral

dep

osits

, he

mat

itic

detr

itus

may

be

of

Bal

ce e

t al.

(198

0),

lava

s an

d re

late

d br

ecci

asin

clud

ing

Acu

pan

and

Ant

amok

terr

estr

ial o

rigi

nM

itche

ll an

d L

each

U

ncon

form

ably

ove

rlai

n by

the

Lim

esto

ne h

oriz

on h

osts

ska

rn

(199

1), C

ooke

et a

l. K

lond

yke

For

mat

ion

min

eral

izat

ion

at T

hank

sgiv

ing

(199

6), W

ater

s et

al.

(201

1)

Pugo

For

mat

ion

/ C

reta

ceou

s to

Eoc

ene

Bas

altic

and

and

esiti

c vo

lcan

ic

Subm

arin

e ba

sin—

inte

rpre

ted

Scha

ffer

(19

54),

Bal

ce e

t al.

Dal

upir

ip S

chis

tro

cks,

san

dsto

nes,

arg

illite

s to

be

the

sedi

men

tary

cov

er

(198

0), P

eña

(199

8),

and

cher

tsto

an

ophi

olite

by

Mal

ater

re

Wat

ers

et a

l. (2

011)

(198

9)

Not

e: 40

Ar–

39A

r ag

e de

term

inat

ions

from

Wat

ers

et a

l. (2

011)

, unl

ess

othe

rwis

e in

dica

tedTA

BL

E1.

(C

ont.)

Lith

ofac

ies

and

Uni

tA

geco

ntac

t rel

atio

nshi

psA

ssoc

iatio

n w

ith m

iner

aliz

atio

nC

omm

ents

Ref

eren

ces

1404 COOKE ET AL.

0361-0128/98/000/000-00 $6.00 1404

TAB

LE

2. M

iner

al D

epos

its o

f the

Bag

uio

Dis

tric

t Sor

ted

by A

ppro

xim

ate

Age

of F

orm

atio

n

Dep

osit

Age

Min

eral

izat

ion

styl

eR

ock

chip

ass

ays

/ gra

de a

nd to

nnag

e da

taR

efer

ence

s

Ant

amok

Plei

stoc

ene

?In

term

edia

te s

ulfid

atio

nE

stim

ated

pro

duct

ion

of a

ppro

xim

atel

y 31

0 t A

u; a

vera

ge g

rade

Sa

wki

ns e

t al.

(197

9); D

amas

co, (

1979

); ~

4 g/

t Au

UN

DP

(198

7); W

ater

s et

al.

(201

1)St

rata

-bou

nd W

ildca

t Cu

oreb

ody

had

rese

rves

of 0

.65

Mt

@ 3

.7%

Cu

in 1

971

Acu

pan-

Itog

onK

-Ar

illite

age

: 0.6

5 ±

Inte

rmed

iate

sul

fidat

ion

Com

bine

d pr

oduc

tion

of >

225

t Au

from

193

1 to

199

3, w

ith a

Sa

wki

ns e

t al.

(197

9); U

ND

P (1

987)

; 0.

07 M

a (A

oki e

t al.,

re

mai

ning

bul

k m

inea

ble

reso

urce

~99

t A

uC

ooke

and

Blo

om (

1990

); C

ooke

et

1993

)A

vera

ge m

inin

g gr

ade

at A

cupa

n of

6.1

g/t

(4.2

g/t

cuto

ff)

duri

ng th

e al

. (19

96);

Coo

ke a

nd M

cPha

il 19

80s;

ave

rage

gra

de a

t Ito

gon

was

10

g/t d

urin

g th

e sa

me

peri

od(2

001)

; Wat

ers

et a

l. (2

011)

Kel

ly-B

aco

K-A

r ill

ite m

axim

um

Hig

h an

d in

term

edia

te

Und

ergr

ound

min

ing

oper

atio

n du

ring

the

1980

sC

omst

i et a

l. (1

990)

; Aok

i et a

l. (1

993)

; ag

e: <

0.6

Ma

sulfi

datio

n ve

ins

with

Tw

o st

ages

of m

iner

aliz

atio

n: S

tage

I in

term

edia

te s

ulfid

atio

n st

ate

Dey

ell a

nd C

ooke

(20

03);

Wat

ers

et

(Aok

i et a

l., 1

993)

adva

nced

arg

illic

m

iner

aliz

atio

n av

erag

ed 2

–3 g

/t A

u (lo

cally

up

to 1

0 g/

t); s

tage

II

high

al

. (20

11)

alte

ratio

nsu

lfida

tion

stat

e m

iner

aliz

atio

n ty

pica

lly h

ad g

rade

s of

up

to

11–1

7 g/

t Au

Ato

k-B

ig W

edge

Pl

eist

ocen

e?In

term

edia

te s

ulfid

atio

nTw

o le

vels

of u

nder

grou

nd p

rodu

ctio

n in

the

1980

sU

ND

P (1

987)

; Mitc

hell

and

Lea

ch

The

upp

er d

rive

at 9

20 m

ele

vatio

n ha

d an

ave

rage

gra

de o

f 17

(199

1); W

ater

s et

al.

(201

1)g/

t Au,

whe

reas

the

low

er le

vel h

ad a

n av

erag

e gr

ade

of 7

g/t

Au

Bag

uio

Gol

dPl

eist

ocen

e?In

term

edia

te s

ulfid

atio

nT

he L

ittle

Cor

pora

l vei

n ha

d gr

ades

of u

p to

29

g/t A

u; 1

00 t/

day

UN

DP

(198

7); L

ivin

gsto

n (1

939a

, b);

of o

re w

as p

rodu

ced

from

the

Lon

esom

e ve

in in

the

mid

-198

0sM

itche

ll an

d L

each

(19

91)

Cal

Hor

rPl

eist

ocen

e?In

term

edia

te s

ulfid

atio

nU

nder

grou

nd m

ine

activ

e pr

ior

to W

orld

War

II

UN

DP

(198

7); M

itche

ll an

d L

each

L

ow-g

rade

(2.

5 g/

t Au)

ope

n cu

t and

hea

p le

ach

oper

atio

n fr

om

(199

1); W

ater

s et

al.

(201

1)19

84 to

198

9

Chi

co M

ine

Plei

stoc

ene?

Inte

rmed

iate

sul

fidat

ion

Cam

aso

Vein

had

ave

rage

gra

des

of a

ppro

xim

atel

y 5

g/t A

uW

ater

s et

al.

(201

1)

Sier

ra O

roPl

eist

ocen

e?In

term

edia

te s

ulfid

atio

nU

nder

grou

nd m

ine

prio

r to

Wor

ld W

ar I

I; r

eope

ned

1984

–198

7U

ND

P (1

987)

; Wat

ers

et a

l. (2

011)

Eps

ilon,

Chi

, and

Bay

atin

g A

+ B

vei

ns a

vera

ged

6–13

g/t

Au

Am

puca

o0.

51 ±

0.2

6 M

aC

u-A

u po

rphy

ryD

rill

hole

inte

rcep

t of 7

0 m

@ 0

.18%

Cu,

0.9

5 g/

t Au

Coo

ke a

nd B

loom

(19

90);

Wat

ers

et a

l. (2

011)

Sant

o To

mas

II

1.48

± 0

.05

Ma;

C

u-A

u po

rphy

ryU

nder

grou

nd m

inin

g st

arte

d in

195

7Im

ai (

2001

); W

ater

s et

al.

(201

1)1.

47 ±

0.0

5 M

aTo

tal o

re r

eser

ves

are

estim

ated

at 3

70 M

t with

ave

rage

gra

des

of 0

.3%

Cu

and

0.6

g/t A

u

Clif

ton

K-A

r ag

e: 1

.7 ±

0.6

Ma

Cu-

Au

porp

hyry

No

data

ava

ilabl

eIm

ai (

2001

); W

ater

s et

al.

(201

1)(I

mai

; 200

1)

Bum

olo

2.06

± 0

.70

Ma

Cu-

Au

porp

hyry

Ave

rage

gra

des

of 0

.2%

Cu

and

0.5

g/t A

u de

tect

ed in

ear

ly m

iner

al

Wat

ers

et a

l. (2

011)

quar

tz d

iori

te p

orph

yry

Mex

ico

3.09

± 0

.15

Ma

to

Cu-

Au

skar

n B

est i

nter

cept

s: 3

7m o

f por

phyr

y m

iner

aliz

atio

n @

0.1

1% C

u,

Wat

ers

et a

l. (2

011)

; Hol

lings

et a

l. 2.

90 ±

0.1

5 M

a0.

03 g

/t A

u; 3

m o

f ska

rn @

0.1

4% C

u, 0

.41

g/t A

u(2

011b

)

Bla

ck M

ount

ain

3.09

± 0

.15

Ma

to

Cu-

Au

porp

hyry

M

ain

(Ken

non)

ore

body

: pre

-pro

duct

ion

rese

rve

of 4

7 M

t ~

Cal

low

(19

67);

Bur

eau

of M

ines

and

(K

enno

n an

d 2.

81 ±

0.2

4 M

a0.

38%

Cu,

0.3

5 g/

t Au,

0.0

1 %

Mo

Geo

scie

nces

(19

86);

Swee

t et a

l. So

uthe

ast)

Sout

heas

t ore

body

—pr

e-pr

oduc

tion

rese

rve

of 1

5 M

t ~ 0

.37%

(2

008)

; Wat

ers

et a

l. (2

011)

Cu,

0.2

6 g/

t Au

Tha

nksg

ivin

g~

3 M

a?A

u-Zn

ska

rn (

adja

cent

Pr

oduc

tion

of 1

.1 M

t ~12

.8 g

/t A

u up

to 1

987;

ave

rage

gra

des

Cal

low

(19

67);

Bur

eau

of M

ines

and

to

Bla

ck M

ount

ain

of 3

.8%

Zn,

0.2

8% C

u, 3

6.9

g/t A

g, 5

.7 g

/t A

uG

eosc

ienc

es (

1986

); U

ND

P (1

987)

; po

rphy

ry)

Wat

ers

et a

l. (2

011)

Chi

co p

rosp

ect

Plio

cene

?C

u-A

u po

rphy

rySu

rfac

e ro

ck c

hip

assa

ys o

f up

to 0

.97%

Cu,

1.8

3 g/

t Au

Wat

ers

et a

l. (2

011)

Nug

get H

illPl

ioce

ne?

Cu-

Au

porp

hyry

Surf

ace

rock

chi

p as

says

of u

p to

1.1

7% C

u, 4

.58

g/t A

uW

ater

s et

al.

(201

1)

Cam

p 4

(Kid

ao

<4.0

28 ±

0.0

74 M

aC

u-A

u po

rphy

ryR

ock

chip

s fr

om K

idao

dio

rite

ave

rage

0.2

7% C

u, 0

.25

g/t A

u, w

ith

Wat

ers

et a

l. (2

011)

and

Ubo

lan)

loca

l hig

hs o

f 1.1

8% C

u, 0

.64

g/t A

u; U

bola

n re

port

ed to

be

“low

gra

de”

Not

e: 40

Ar-

39A

r ag

e de

term

inat

ions

from

Wat

ers

et a

l. (2

011)

, unl

ess

othe

rwis

e st

ated

porphyries and a late-mineralization dacite porphyry withadakitic affinities (Waters et al., 2011). A40Ar-39Ar age deter-mination of 0.51 ± 0.26 Ma has been obtained from the Am-pucao porphyry complex, making it the youngest known por-phyry deposit in the Baguio district (Waters et al., 2011).

Figure 6 shows the geology and assay data from discoverydrill hole 741-1FJ. One-meter composite samples mostly as-sayed between 0.2 and 5 g/t Au, although free gold was onlyobserved from one vein intersection in the ~300 m long drillhole (de Guzman, pers. commun., 1986; Cooke and Bloom,

1990). Average grades in the bottom 70 m of this drill holewere ~0.18% Cu and 0.95 g/t Au.

Potassic alteration has produced a secondary biotite-mag-netite-quartz-anhydrite assemblage in the andesites of theZig-Zag Formation, and orthoclase-quartz-anhydrite assem-blages in the Ampucao porphyry complex and Virac granodi-orite (Cooke and Bloom, 1990). Intermediate argillic andphyllic alteration assemblages overprinted the early-formedpotassic assemblages. Early quartz-magnetite-anhydrite-chal-copyrite veins were deposited from high temperature (300°C


0361-0128/98/000/000-00 $6.00 1405

Lithological boundary (definite)

Lithological boundary (inferred)

Fault / vein

209–Odd Vein System

381 Vein–No Name Fault Vein System

Klondkye Formation

Zig-Zag Formation

Sample locations

Structures

Collapse breccia

Andesite dikes

Balatoc diatreme(dacite plug, young plug)

Hartwell plug

Ampucao dacite

Virac granodiorite

Lucbuban gabbro

LEGENDAcupan Itogon

0 500 m

Acupan Itogon

501 Vein

410 Vein

14 Vein

Star Fault

Pine Vein

210 Vein

403 Vein

381 VeinNo Name Fault

237 Vein

286 Vein

364 Vein

90 Vein

99Ve

in

B Vein

3000 W16000 S

19000 S

20500 S

22000 S

23500 S

25000 S

17500 S

3000 E1500 W 1500 EZERO

Cur

t Fau

lt

Frog

Vein

Hawkeye Fault

422 Vein

412 Vein

364 Vein

36 ShaftGolden Gate

Fault

LodeShaft

FogShaft

vv

vv

v

v v

v

vv

vv

vv

v

FIG. 3. Geology of the Acupan intermediate sulfidation epithermal vein system, as exposed on underground mine level1500. Also shown is the location of the Balatoc diatreme and the property boundary between the Acupan and Itogon mines(modified after Cooke et al., 1996).

D

D

C

C

BB

A B

~1 m

~1 km

E

Ser

icit

e-al

tere

d w

allr

ock

s Sericite-altered

wallro

cks

BA

FIG. 4. Schematic space-time diagram for mineralization hosted by com-posite banded epithermal veins at Acupan (modified after Cooke et al.,1996). The granodiorite and andesite wallrocks are typically altered toquartz-sericite-pyrite (K-mica) assemblages adjacent to the veins. Earliest-formed gray quartz-cemented breccia bands (type B) occur on the marginsof the crustiform banded veins, and contain high grade gold-silver mineral-ization and abundant altered wallrock clasts. Locally brecciated bands domi-nated by coarse-grained white quartz (type C) are typically the most abun-dant component of the veins volumetrically, and contain some high-gradeAu-Ag mineralization, particularly near contacts with type B bands. Centralcavities of the composite veins have typically been infilled by calcite (type D)and/or anhydrite (type E). Anhydrite only occurs in the epithermal veins atdepth in the southern part of the deposit. Type A chalcedony occurs as clasts(A) within type B and C bands at high elevations within the mine. Chal-cedony is only a minor clast component within the breccia veins, which aredominated by clasts of locally-derived sericite-altered wallrocks and earlier-formed vein material. Gold occurs primarily in type B and C bands.

1406 COOKE ET AL.

0361-0128/98/000/000-00 $6.00 1406

200100 100 100 500200 200 200300 300 300400 400 400

1600

Ele

vatio

n (m

)

Homogenization temperature ( C)°

Elevation (m

)

1400

1200

1000

800

600

400

200

1600

1400

1200

1000

800

600

400

Type Bgray quartz

Type Cwhite quartz

Type Dcalcite

FIG. 5. Fluid inclusion homogenization temperatures for type B, C, and D bands from the 381-No Name fault system atAcupan, plotted with respect to elevation (modified after Cooke and Bloom, 1990). Symbols: white diamonds = vapor-rich two-phase fluid inclusions that homogenize to vapor, gray circles = liquid-rich two-phase fluid inclusions that homogenize to liq-uid, black squares = liquid-rich, three-phase fluid inclusions that homogenize to liquid and that contain an insoluble non-ionicdaughter mineral. Also shown are boiling point-depth curves for pure H2O liquid (after Haas, 1971) that have been fitted tothe data sets visually in order to estimate paleodepths below surface (Cooke and Bloom, 1990). The gradual decrease in tem-peratures with elevation is consistent with boiling-induced cooling of the mineralizing fluids. The decrease in paleodepths isconsistent with landsurface degredation during evolution of the hydrothermal system (Cooke and Bloom, 1990). Anomalouslyhigh temperatures (above 300°C) were obtained from samples that contain both liquid-rich and gas-rich fluid inclusions, andare interpreted to be spurious data, caused by mixed entrapment of water and steam during boiling (Cooke and Bloom, 1990).

0

0.2

0.4

Cu

(%)

0

5

10

15

Au

(g/t

)

Zig-Zag Formation

Ampucao dacite

Virac granodiorite

Stable isotope sample location

FogShaft

0 50 m

L-2300

9695

1

9695

2

9695

496

955

9695

3

9695

6

9695

7

9695

9

L-2600

9696

2

9696

4

9696

9

FIG. 6. Drill hole log and assay results for diamond drill hole 741-1FJ, modified after Cooke and Bloom (1990). Also shownare stable isotope sample locations from the Ampucao porphyry deposit. Drill hole 741-1FJ was collared at Fog Shaft on the2300L of the underground mine at Acupan, and was drilled at an angle of 15° due south of Fog Shaft, where it intersected theAmpucao porphyry system. The last 70 m of the drill hole had an average grade of 0.95 g/t Au and 0.17 wt percent Cu.

to >600°C) hypersaline (30 to > 70 wt % NaCl equiv) mag-matic-hydrothermal brines and low density vapors (Cookeand Bloom, 1990). Later stage phyllic alteration and relatedquartz–pyrite veins formed from low salinity, moderate tem-perature fluids (1–2 wt % NaCl equiv; 260°–345°C; Cookeand Bloom, 1990).

Kelly

The Kelly-Baco gold deposit (hereafter referred to as Kelly)was described by Comsti et al. (1990), Aoki et al. (1993), andDeyell and Cooke (2003). Kelly is located in the northern partof the Baguio district, immediately to the east of, and 200 to300 m below the Baguio lithocap, a large, strata-bound do-main of silicic and advanced argillic alteration that crops outat high elevations on the eastern side of Baguio city (Waterset al., 2011; Fig. 2). Kelly is atypical of the Baguio epithermalveins, in that it contains tennantite, enargite, and bornite min-eralization, and hypogene advanced argillic alteration assem-blages (pyrophyllite-diaspore-alunite; Comsti et al., 1990;Aoki et al., 1993), in addition to intermediate sulfidation illite+ base metal ± calcite veins that are more typical of theBaguio district. The deposit is distinguished by the presenceof high sulfidation state sulfides (e.g., enargite, tennantite;Fig. 7A) and the presence of tellurium-rich minerals(calaverite and goldfieldite; Fig. 7B). An average grade of 4g/t was reported by Mitchell and Leach (1991) for Kelly, al-though resource figures are not available. Hypogene ad-vanced argillic alteration (pyrophyllite-illite) along the Kellyvein has an approximate age of <0.6 Ma, whereas alunite fromthe nearby Baguio lithocap has been dated at 1.4 ± 0.3 to 0.9± 0.1 Ma (Aoki et al., 1993). Mineralization at Kelly occurs insteeply north-dipping, east-striking quartz veins hosted by theOligocene-Miocene Zig-Zag Formation and Pliocene Kellydiorite (K-Ar age of 3.1 ± 1.1 Ma; Aoki et al., 1993; Table 1).The major vein systems include the Kelly, Bungalow, Kelly 35south split, Maptung, Comasom, Frank, Lost Herd, and Da-pung veins (Comsti et al., 1990). These veins are oriented or-thogonal to the major northeast- and northwest-trending veinsystems at Acupan, Baguio Gold, and Antamok (Fig. 2). Com-sti et al. (1990) noted that the Kelly veins had cut relict biotiteand actinolite alteration at surface in the southern (Baco) areaof the Kelly vein system, and also on level 170 of the Kellymine, which suggests that an as-yet undiscovered porphyrysystem may be located nearby.

Fluid and Mineral ChemistryStable and radiogenic isotope data can be used to trace

fluid sources in hydrothermal ore deposits, and to help con-strain likely ore-forming processes. We have used a variety ofisotopic techniques in order to assess fluid compositions, de-positional processes, and genetic relationships between theporphyry and epithermal styles of mineralization.

Sulfur isotopes

We have completed detailed sulfur isotope studies of theAcupan, Ampucao, and Kelly deposits, and reconnaissancesurveys of other systems in the Baguio district. Imai (2001)undertook sulfur isotope analyses of porphyry-style mineral-ization from Santo Tomas II, and his data are used for com-parative purposes.

Sulfur isotope data for sulfides from Acupan, Ampucao, andKelly were determined at the University of Tasmania, by use ofboth conventional and laser ablation sulfur isotope techniques.Powdered mineral separates (~30 mg) were analyzed accord-ing to Robinson and Kusakabe (1975) with measurements per-formed on a VG Sira Series II mass spectrometer. Laser analy-ses on doubly polished mineral chips were completed using aQuantronix model 117 Nd:YAG laser coupled with a VG SiraSeries II mass spectrometer using the technique of Huston etal. (1995). Conventional sulfate analyses were completed on~100 mg powdered mineral separates, according to the meth-ods of Fritz et al. (1974), on the VG Sira Series II mass spec-trometer. The results are expressed in standard δ per mil (‰)notation relative to the Canyon Diablo Troilite (CDT). Internalstandards used are homogeneous galena samples from BrokenHill (δ34S = 3.2‰) and Rosebery (δ34S = 12.4‰), and baritefrom Madame Howard (δ34S = 31.2‰) and Tasul I (δ34S =20.9‰), together with an SO2 reference gas (δ34S ≈ CDT).


0361-0128/98/000/000-00 $6.00 1407

B

A

0.25 mm

0.1 mm

Au

calav

Au

calav

en

qz

qz

gf

tn

en

qz

qz

gf

tn

cp

cp

cp

cp

cp

cp

FIG. 7. Reflected light photomicrographs of high sulfidation state miner-alization from the Kelly gold mine. (A) Native gold (Au) associated withcalaverite (calav) and surrounded by goldfieldite (gf) and chalcopyrite (cp).(B) Enargite (en) overgrown by tennantite (tn) and chalcopyrite (cp), infill-ing void space between quartz crystals (qz).

Approximately 8 to 10 samples were analyzed per day, to-gether with one standard. Analytical uncertainties for sulfideanalyses are estimated at ± 0.1 per mil and for sulfate analy-ses at ± 0.4 per mil.

Acupan: Sulfur isotope samples were selected from severalof the major epithermal veins, primarily the 381 vein-NoName fault (NNF) and the 209-Odd vein systems (Fig. 3;Table 3). Samples include sulfides from type B, C, and Dbands (Fig. 4) and type E vein anhydrite and selenite. A chal-copyrite clast was analyzed from the Balatoc diatreme. All re-sults are listed in Table 3 and are summarized in Figure 8A.

The δ34S values of 23 pyrite and 4 sphalerite grains fromAcupan range from +1.1 to +5.9 per mil with a median valueof +3.2 per mil. Sulfates from type E bands (16 anhydrite and2 selenite analyses) have δ34S values ranging from +13.4 to

+31.8 per mil, with most data between +14 and +18 per miland a median value of +15.9 per mil. The median δ34S valuesfor pyrite from individual paragenetic stages are constant(type B: 3.1‰, type C: 3.2‰, type D: 3.1‰) and are lowerthan the median value for pyrite from the altered wall rocks(4.9‰). Sphalerite has lower median δ34S values than pyritefor each paragenetic stage analyzed (type B: 2.8‰, type C:1.0‰). The chalcopyrite clast from the Balatoc diatremeyielded a distinctive δ34S value of –1.4 per mil.

The δ34S values of pyrite and sphalerite from type C and Dbands in the 381 vein – NNF vein system increase with in-creasing elevation (Fig. 9A). No such trend was observed fortype C samples collected at lower elevations from the 20-Oddvein system (Fig. 9A), or from type E anhydrite from eithervein system (Fig. 9B). The lowest δ34Ssulfide values from the

1408 COOKE ET AL.

0361-0128/98/000/000-00 $6.00 1408

TABLE 3. Sulfur Isotope Data as Per Mil Values Relative to CDT for Samples of Epithermal Vein and Breccia-Hosted Mineralization from the Acupan Deposit

Mine Mine Elevation1 δ34Spy δ34Ssp δ34San δ34Ssel

Sample no. Host rock Vein / breccia easting northing (m) Stage (‰) (‰) (‰) (‰)

96811 Virac 403 vein 2000 W 22700 S 1371.6 B: gray qz 1.996635 Virac 381 vein 2664 W 22594 S 980.9 B: gray qz 4.496664* Virac 381 vein 3045 W 23210 S 819.0 C: white qz 2.296713 Virac No name fault 2460 W 22800 S 766.8 C: white qz 2.396649 Virac No name fault 2062 W 22620 S 801.1 C: white qz 2.896782 Virac 403/Hawkeye 167 W 21687S 862.8 C: white qz 3.196866 Virac 381 vein 3012 W 23249 S 760.8 C: white qz 3.196664* Virac 381 vein 3045 W 23210 S 819.0 C: white qz 3.296664 Virac 381 vein 3045 W 23210 S 819.0 C: white qz 3.796664* Virac 381 vein 3045 W 23210 S 819.0 C: white qz 3.896939 Virac Odd vein 2041 W 21071 S 461.0 C: white qz 3.996887 Virac 210 vein 1730 W 21150 S 495.7 C: white qz 4.5 3.196892 Virac 210 vein 1695 W 21035 S 450.9 C: white qz 4.8 2.696877 Virac No name fault 1871 W 22762 S 672.9 D: calcite 1.2 1.096697 Virac No name fault 2185 W 22637 S 762.8 D: calcite 2.8 1.196642 Virac 403 vein 1028 W 22118 S 801.1 D: calcite 3.496616 Virac 381 vein 2341 W 22143 S 1072.3 D: calcite 5.296877 Virac No name fault 1871 W 22762 S 672.9 E: anhydrite 13.496877 Virac No name fault 1871 W 22762 S 672.9 E: anhydrite 13.896797 Virac No name fault 1870 W 22000 S 618.0 E: anhydrite 14.096730 Virac No name fault 2140 W 22830 S 709.5 E: anhydrite 14.196894 Virac 210 vein 1695 W 21035 S 450.9 E: anhydrite 14.296800 Virac No name fault 1943 W 22935 S 633.9 E: anhydrite 15.297062 Balatoc GW#5 528.9 E: anhydrite 15.396726 Virac No name fault 2538 W 23000 S 709.5 E: anhydrite 15.596731 Virac No name fault 2140 W 22830 S 709.5 E: anhydrite 15.696801 Virac No name fault 1943 W 22935 S 633.9 E: anhydrite 16.196876 Virac No name fault 1871 W 22762 S 672.9 E: anhydrite 16.296802 Virac No name fault 1923 W 22935 S 620.0 E: anhydrite 16.696805 Virac No name fault 2090 W 22945 S 629.9 E: anhydrite 16.696745 Virac No name fault 2200 W 22960 S 661.0 E: anhydrite 3.5 16.996914 Zig-Zag 209 vein 2590 W 21150 S 572.2 E: anhydrite 17.696677 Virac No name fault 2485 W 22887 S 833.0 E: anhydrite 17.896666 Virac 381 vein 3308 W 23292 S 801.1 E: anhydrite 18.396732 Virac No name fault 2140 W 22830 S 709.5 E: anhydrite 31.896664* Virac 381 vein 3045 W 23210 S 819.0 Sericite alt 4.396664* Virac 381 vein 3045 W 23210 S 819.0 Sericite alt 4.996646 Virac No name fault 1984 W 22576 S 801.1 Sericite alt 5.196664* Virac 381 vein 3045 W 23210 S 819.0 Sericite alt 5.996744 Virac No name fault 2200 W 22960 S 661.0 Ser-chl alt 2.8

Notes: Vein stages defined by Cooke et al. (1996) are illustrated schematically in Figure 4.Abbreviations: 403/Hawkeye = intersection of 403 vein and Hawkeye fault; alt = alteration, an = anhydrite, Balatoc = Balatoc diatreme, GW#3 = GW brec-

cia-hosted orebody no. 3, GW#5 = GW breccia-hosted orebody no. 5, py = pyrite, qz = quartz, sel = selenite, ser-chl = sericite-chlorite, sp = sphalerite, Zig-Zag = Zig-Zag Formation

1 Elevation relative to sea level*indicates sample analyzed by laser ablation

381 vein-NNF vein system were from the NNF, 403 vein andthe margin of the Hartwell Plug (Fig. 10).

Ampucao: Chalcopyrite, pyrite, and anhydrite were ana-lyzed from porphyry-style quartz–magnetite-sulfide and an-hydrite-chlorite-sulfide veins from the Ampucao porphyryCu-Au prospect. The samples were collected from drill hole741-1FJ, on the southern margin of the Acupan mine (Figs.3, 6). All results are listed in Table 4 and are illustrated onFigure 8B.

The δ34S values of 19 pyrite and 5 chalcopyrite grains fromAmpucao range from +2.3 to +6.6 per mil, with a medianvalue of +4.0 per mil. These results are similar to δ34S valuesof pyrite from the Acupan veins (Figs. 8A, B, 11). Six anhy-drite samples collected over a 140-m downhole interval atAmpucao yielded δ34S values from +10.4 to +19.2 per mil anda median of +14.5 per mil (Table 4; Fig. 8B), similar to mostof the anhydrite data obtained from a much broader samplingarea at Acupan (Table 3).

Kelly: Sulfur isotope data for the Kelly deposit were first re-ported by Deyell and Cooke (2003). All sulfide δ34S values arecompiled in Table 5, which includes data for pyrite, chalcopy-rite, enargite, and tennantite. Sulfide δ34S values range from–3.2 to +0.8 per mil (n = 24; Figs. 8C, 11) and are lower thanvalues obtained from the Acupan and Ampucao sulfides. Aokiet al. (1993) analyzed four alunite samples from the nearbyBaguio lithocap, obtaining δ34S values from +15 to +24 permil.

Other deposits and prospects: Reconnaissance sulfur iso-tope analyses of sulfides from other deposits and prospects inthe Baguio district were completed in order to examine the


0361-0128/98/000/000-00 $6.00 1409

-10 0 10 20 30

D: SantoTomas II(Imai, 2001)

chalcopyritecp – bornite

cp pyrite–anhydritegypsum

0

5

10

15

20

-10 0 10 20 30

C: Kelly pyritechalcopyriteenargitetennantite

0

2

4

6

8

10

12

δ34S (‰, CDT)

E: All Baguiodata

PCD sulfidesHS sulfides

IS sulfidesskarn sulfidesPCD sulfates

IS sulfates

-10 0 10 20 300

5

10

15

20

25

30

35

0

2

4

6

8

10B: Ampucao I: chalcopyrite

I: anhydriteIIA: chalcopyriteIIA: pyriteIIA: anhydriteIIB: pyriteIIB: anhydriteser-chl alt: py

-10 0 10 20 30

0

2

4

6

8

10

-10 0 10 20 30

A: Acupan C: pyriteC: sphaleriteD: pyriteD: sphaleriteE: pyriteE: anhydriteE: selenite

bx clast: pyriteB: pyriteser alt: pyrite

FIG. 8. Histograms showing range of sulfur isotope data (‰, CDT) for oredeposits of the Baguio district. (A) Acupan intermediate sulfidation epither-mal deposit (Table 3). (B) Ampucao porphyry Cu-Au prospect (Table 4). (C)Kelly high sulfidation epithermal deposit (Table 5). (D) Santo Tomas II por-phyry Cu-Au deposit (data from Imai, 2001; includes mixtures of chalcopy-rite–bornite and chalcopyrite–pyrite). (E) Compilation of all data from theBaguio district (Tables 3–6; Imai, 2001). Abbreviations: bx clast = chalcopy-rite clast from Balatoc diatreme, cp = chalcopyrite, HS = high sulfidation de-posit, IS = intermediate sulfidation deposit, PCD = porphyry Cu deposit, py= pyrite, ser alt = sericite alteration, ser-chl alt = sericite-chlorite alteration.

B

A

400

600

800

1000

1200

1400

400

600

800

1000

1200

1400

12

0

δ34S (‰, CDT)

Ele

vatio

n (m

)E

leva

tion

(m)

14 16 18 20

E: anhydriteE: selenite

B: pyriteC: pyriteC: sphaleriteD: pyrite

E: pyriteD: sphalerite

δ34S (‰, CDT)2 4

209 - Odd Vein

6 8

381 Vein - No Name Fault

FIG. 9. Relationships between δ34S values (‰, CDT) of epithermal veinsamples and elevation at the Acupan gold mine. All data listed in Table 3. (A)δ34Ssulfide values for samples of type B (pyrite), C (pyrite and sphalerite), D(pyrite and sphalerite) and E (pyrite) vein material from the 381 vein-NoName fault vein system, and from the 209-Odd vein system, plotted againstelevation. Note that pyrite samples from altered wall rocks have not beenplotted on this diagram, in order to highlight the positive correlation betweenδ34S values and elevation for type C and D sulfides in the 381 vein-No Namefault vein system. (B) δ34Ssulfate values for samples of type E (anhydrite) andselenite from the Acupan epithermal veins plotted against elevation.

variability of sulfur isotope signatures as a function of spatial lo-cation and mineralization style. Samples were selected from arange of porphyry prospects, epithermal veins, skarn mineral-ization, and one massive sulfide occurrence (Wildcat; Table 6).

The sulfur isotope data compiled in Figure 8 and Tables 3to 6 show that there are two distinct modes of δ34S values forsulfides from the Baguio district (–7 to +1‰ and +1 to+6‰). The sulfide isotope compositions appear to be influ-enced by geographic location, specifically with regards prox-imity to the Baguio lithocap. Most of the intermediate sulfi-dation epithermal veins from the central, eastern, andsouthern Baguio district, distal to the lithocap, have δ34S val-ues of +2 to +6 per mil. In contrast, δ34S values obtained fromthe intermediate sulfidation veins at Baguio Gold, locatedproximal to the northern end of the Baguio lithocap (Fig. 2),have δ34S values of –6.8 to +0.5 per mil, and two sulfide sam-ples from intermediate sulfidation veins at Chico mines haveδ34S values of –2.9 and –2.8 per mil. These results are com-parable to the data from the Kelly high sulfidation veins,which are also located adjacent to the lithocap (Fig. 2).

The porphyry deposits from the central and southern Baguiodistrict mostly have sulfides with δ34S values between +4 and+6 per mil. This includes the sulfides analyzed by Imai (2001)from the Santo Tomas II porphyry deposit, which have simi-lar sulfur isotope values to the Ampucao porphyry samples(compare Fig. 8B and D). Lower δ34S values were obtained

1410 COOKE ET AL.

0361-0128/98/000/000-00 $6.00 1410

Lithologicalboundary Fault / vein

Sulfur isotope sample(circle size correspondsto S values)δ34

Zig-Zag Formation

Structures

Hartwell plug

Virac granodiorite

0 250 m

Pine Vein

210 Vein

403 Vein

No Name Fault

237 Vein

286 Vein

364 Vein

20500 S

22000 S

23500 S

3000 W 1500 W ZERO

Cur

t Fau

lt

Frog

Vein

Hawkeye

Fault

422 Vein

412 Vein

LodeShaft

381 Vein

FIG. 10. Spatial distribution of δ34S data (‰, CDT) obtained from pyritesamples from the Acupan epithermal vein system with respect to the localhost rocks. Data are grouped according to the host vein and are projectedonto the 1500L geologic map modified from Cooke et al. (1996). Circle sizecorresponds to δ34Spyrite value (small circle = low δ34Spyrite). All δ34S data arelisted in Table 3.

TABLE 4. Sulfur Isotope Data as Per Mil Values Relative to CDT for Samples of Porphyry-Style Mineralization from the Ampucao Porphyry Cu-Au Prospect

Drill hole Mine Mine Elevation1 Vein δ34Spy 34Scp 34San

Sample no. Host rock depth (m) easting northing (m) stage (‰) (‰) (‰)

96954 Zig-Zag Formation 132.0 1660 W 24640 S 673.3 I 6.696955 Zig-Zag Formation 147.5 1660 W 24690 S 669.5 I 2.596955 Zig-Zag Formation 147.5 1660 W 24690 S 669.5 I 4.196962 Ampucao dacite 252.0 1660 W 25023 S 641.1 I 3.496954 Zig-Zag Formation 132.0 1660 W 24640 S 673.3 IIa 3.996954 Zig-Zag Formation 132.0 1660 W 24640 S 673.3 IIa 4.896954 Zig-Zag Formation 132.0 1660 W 24640 S 673.3 IIa 4.996954 Zig-Zag Formation 132.0 1660 W 24640 S 673.3 IIa 10.496955 Zig-Zag Formation 147.5 1660 W 24690 S 669.5 IIa 2.396955 Zig-Zag Formation 147.5 1660 W 24690 S 669.5 IIa 2.996955 Zig-Zag Formation 147.5 1660 W 24690 S 669.5 IIa 3.496957 Zig-Zag Formation 152.5 1660 W 24706 S 668.2 IIa 19.296959 Zig-Zag Formation 179.5 1660 W 24792 S 660.9 IIa 12.996956B Zig-Zag Formation 152.5 1660 W 24706 S 668.2 IIa 4.296956B Zig-Zag Formation 152.5 1660 W 24706 S 668.2 IIa 4.596956B Zig-Zag Formation 152.5 1660 W 24706 S 668.2 IIa 5.096956B Zig-Zag Formation 152.5 1660 W 24706 S 668.2 IIa 5.996956B Zig-Zag Formation 152.5 1660 W 24706 S 668.2 IIa 4.896956C Zig-Zag Formation 152.5 1660 W 24706 S 668.2 IIa 16.796953 Zig-Zag Formation 120.0 1660 W 24602 S 676.6 IIb 16.096962 Ampucao dacite 252.0 1660 W 25023 S 641.1 IIb 2.896962 Ampucao dacite 252.0 1660 W 25023 S 641.1 IIb 5.796964 Ampucao dacite 270.5 1660 W 25082 S 636.1 IIb 3.496964 Ampucao dacite 270.5 1660 W 25082 S 636.1 IIb 3.896964 Ampucao dacite 270.5 1660 W 25082 S 636.1 IIb 4.096964 Ampucao dacite 270.5 1660 W 25082 S 636.1 IIb 5.396964 Ampucao dacite 270.5 1660 W 25082 S 636.1 IIb 5.696959 Zig-Zag Formation 179.5 1660 W 24792 S 660.9 IIc 3.396954 Zig-Zag Formation 132.0 1660 W 24640 S 673.3 ser-chl alt 2.7

Note: All samples were collected from drill hole 741-1FJ (locations shown on Figure 6). Vein stages defined by Cooke and Bloom (1990)Abbreviations: an = anhydrite, cp = chalcopyrite, py = pyrite, ser-chl alt = sericite-chlorite alteration1 Elevation relative to sea level

from porphyry-style mineralization at Black Mountain (–1.5and +0.8‰; Table 6), located at the southern end of theBaguio lithocap (Fig. 2).

Skarn sulfides have δ34S values that range from +1.0 to +4.6per mil. They overlap with the porphyry and intermediate sul-fidation epithermal data from the central and southernBaguio district (Figs. 8, 11; Table 6).

Strontium isotopes

The results of strontium isotope analyses of five anhydritevein and breccia samples from the Acupan epithermal andAmpucao porphyry deposits are listed in Table 7. Analyseswere conducted at La Trobe University by Roland Maas.Samples were hand-drilled, yielding powders that were thenleached in cold 1M HCl, with the resulting supernatant solu-tion extracted. The residue was washed with water once, andthen leached and partially dissolved in 5M HCl. The HCl wasthen evaporated, and the residue picked up in 1M HCl andcentrifuged. Part of each solution was loaded as phosphateonto single Ta filaments and analyzed in static multicollectionmode on a Finnigan-MAT 262 mass spectrometer. Mass frac-tionation was corrected by normalizing to 86Sr/88Sr = 0.1194.The initial strontium isotopic compositions of anhydrite fromAcupan and Ampucao range between 0.70378 and 0.70385,and are essentially indistinguishable within analytical error(Fig. 12; Table 7).

Oxygen-deuterium isotopes

We analyzed the oxygen isotope values of quartz (n = 31)and chalcedony (n = 1) from crustiform banded veins at Acu-pan, and also quartz veins from the Ampucao porphyryprospect (n = 4). Quartz was separated by handpicking ofcrushed samples, and the purity of mineral separates was assessed by optical examination to be greater than 95 percent.Quartz was sonically cleaned in a water bath and then baked


0361-0128/98/000/000-00 $6.00 1411

-12 -10 -8 -6 -4 -2 0 42 6 8

Bag

uio

dis

tric

t

IS epithermal

Porphyry

IS epithermal (Acupan)

IS epithermal (others)

Porphyry (Ampucao)

Porphyry (others)

Skarn

HS epithermal

HS epithermal

δ34S (‰)

Man

kaya

nd

istr

ict

FIG. 11. Comparison of sulfur isotope ranges (‰, CDT) for sulfides from the Baguio and Mankayan districts. Baguio dis-trict high sulfidation (HS) epithermal data are from the Kelly deposit (Table 5). Intermediate sulfidation (IS) epithermal in-cludes data from the Acupan deposit (Table 3) and other IS deposits and prospects in the district (Table 6). Porphyry dataincludes results from Ampucao (Table 4), Santo Tomas II (Imai, 2001) and other prospects from this study (Table 6). Skarndata are compiled from all samples in this study (Table 6). Mankayan district data taken from the Far Southeast porphyryCu-Au deposit (Imai, 2000; Hedenquist and Sasaki, unpub. data), Lepanto high sulfidation Cu-Au deposit (Hedenquist andGarcia, 1990; Hedenquist and Mancano, unpub. data; Hedenquist and Sasaki, unpub. data) and Victoria intermediate sulfi-dation Au-Ag deposit (Sajona et al., 2002).

TABLE 5. Sulfur Isotope Data as Per Mil Values Relative to CDT for Sulfides and Sulfosalts Collected from Underground Exposures and Surface

Outcrops at Kelly (data originally reported by Deyell and Cooke, 2003)

Sample Mine δ34Spy δ34Scp δ34Sen δ34Stn

no. Mine Vein level (‰) (‰) (‰) (‰)

B-11 Kelly Kelly vein 170 –3.2B-18 Kelly Kelly vein 45 –2.6B-14a Kelly K 35 SS vein 45 –2.2B-17 Kelly Kelly vein 45 –2.1B-18 Kelly Kelly vein 45 –1.7B-20a Kelly Kelly vein 45 –1.B-6a Kelly K 35 SS vein 270 –1.3KR-4a Baco K 35 SS vein 45 –1.3B-4 Baco Kelly vein 100 –1.0B-2 Baco K 35 SS vein 170 –0.9KR-1 Kelly Pines vein 45 –0.8KR-2 Baco Frank vein 100 –0.7B-2 Baco K 35 SS vein 170 –0.6B-2 Baco K 35 SS vein 170 –0.6KR-2 Baco Frank vein 100 –0.6B-8 Kelly K 35 SS vein 170 –0.4B-8a Kelly K 35 SS vein 170 –0.4KR-6a Baco K 35 SS vein 170 –0.2B-15a Kelly K 35 SS vein 100 –0.1B-7b Kelly K 35 SS vein 270 0.2KR-3 Baco Pines vein 45 0.2B-13 Kelly K 35 SS vein 45 0.6B-12a Kelly Kelly vein Surface 0.7KR-6b Baco K 35 SS vein 170 0.8

Abbreviations: cp = chalcopyrite, en = enargite, K = Kelly, py = pyrite, SS= south split, tn = tennantite

in an oven for 12 h at 100°C. A total of 26 samples were ana-lyzed at the University of Tasmania, where oxygen was ex-tracted from quartz and chalcedony by reaction for 12 h withBrF5 at 520°C in evacuated nickel reaction vessels (Claytonand Mayeda, 1963). The liberated oxygen was converted to

CO2 when the gas was reacted with heated graphite (Taylorand Epstein, 1962). Isotope ratios were measured using theVG Micromass 602D mass spectrometer in the Central Sci-ence Laboratory. One internal standard, UT quartz, was runwith every five quartz samples, and was calibrated against the

1412 COOKE ET AL.

0361-0128/98/000/000-00 $6.00 1412

TABLE 6. Sulfur Isotope Data as Per Mil Values Relative to CDT for Sulfides Collected from Mines and Prospects Around the Baguio District

Deposit Easting Northing δ34Spy δ34Scp δ34Ssp δ34Sgl

Sample no. type Location (UTM) (UTM) (‰) (‰) (‰) (‰)

804971 IS Antamok (Wildcat orebody) 249774 1814310 –2.5804962 IS Antamok (440 vein) 249398 1815114 1.5804969 IS Baguio Gold (Lonesome vein) 249111 1816010 –6.8804970 IS Baguio Gold (Lonesome vein) 249111 1816010 –5.5804968 IS Baguio Gold (Lonesome vein) 249111 1816010 0.5811643 PCD Black Mountain 243826 1809792 –1.5811645 PCD Black Mountain 245591 1808396 0.8804965 PCD Camp 4 246823 1805641 –0.7804964 PCD Camp 4 247061 1805645 2.2804966 IS Chico mines 246346 1813120 –2.9804967 IS Chico mines 246346 1813120 –2.8811549 PCD Chico prospect 247216 1811963 1.0811548 PCD Chico prospect 247054 1811778 2.9804961 Skarn Mexico 244956 1813536 1.9806119 Skarn Mexico 243091 1809000 3.2806121 Skarn Mexico 243023 1809080 3.4806122b Skarn Mexico 242964 1809038 3.7806122a Skarn Mexico 242964 1809038 3.8806117a PCD Nugget Hill 246441 1809771 2.0806116 PCD Nugget Hill 246439 1809835 2.2806118a PCD Nugget Hill 246441 1809771 3.3806118b PCD Nugget Hill 246441 1809771 3.3806117b PCD Nugget Hill 246441 1809771 5.0806113b IS Sierra Oro 246275 1809277 0.1806113a IS Sierra Oro 246275 1809277 1.0806112b IS Sierra Oro 246205 1809249 1.1806115b IS Sierra Oro 246331 1809306 1.5806112a IS Sierra Oro 246205 1809249 2.4806113c IS Sierra Oro 246275 1809277 3.5806115a IS Sierra Oro 246331 1809306 3.5811642b Skarn Thanksgiving 244513 1810000 1.0811641a Skarn Thanksgiving 244513 1810000 1.9811641b Skarn Thanksgiving 244513 1810000 3.9811642a Skarn Thanksgiving 244513 1810000 4.6

Notes: Deposit locations are shown on Figure 2; deposit descriptions are provided in Table 2Abbreviations: cp = chalcopyrite, gl = galena, IS = intermediate sulfidation deposit, PCD = porphyry Cu deposit, py = pyrite, sp = sphalerite

TABLE 7. Strontium Isotope Data for Anhydrite from the Acupan Epithermal Gold Mine and the Ampucao Porphyry Cu-Au Prospect

Mine Mine Elevation1

Sample no. Location Host rock easting northing (m) 87Sr/86Sr Error (±2 sigma mean)

96914 Acupan: 209 Vein Zig-Zag Formation 2590 W 21150 S 572.2 0.70378 ±0.00001796894 Acupan: 210 Vein Virac granodiorite 1695 W 21035 S 450.9 0.70384 ±0.00001897062 (leachate) Acupan: GW#5 orebody Balatoc diatreme 0.70380 ±0.00001397062 Acupan: GW#5 orebody Balatoc diatreme 0.70385 ±0.00001696956C Ampucao Zig-Zag Formation 1640 W 24601 S 668.2 0.70380 ±0.00001896957 Ampucao Zig-Zag Formation 1660 W 24706 S 664.2 0.70382 ±0.000018

Notes: The isotopic compositions of Sr standards used in this study are as follows: SRM987: 0.71023 ± 4, BCR-1: 0.70500 ± 4, BHVO-1: 0.70348 ± 5, sea-water: 0.70916 ± 2; all errors reported are external reproducibilities at 2σ population; note that during sample preparation, sample 97062 dissolved morerapidly that the other four samples in the 1M HCl leach, and there was very little undissolved material left after that stage; consequently, two aliquots of thissample were analyzed: the first aliquot analyzed (97062 leachate) was the 1M HCl leachate, and the second aliquot was the 5M HCl digest after the mild HClleach had been removed

1 Elevation relative to sea level

international isotope standard NBS-28, which was run occa-sionally. Duplicate analyses showed a precision of ±0.2 permil. An additional six quartz samples were analyzed atMonash University, where isotopic ratios were measured on aFinnigan MAT Delta-E mass spectrometer, and ClF3 used asthe oxidizing reagent. A single δ18O analysis of the NBS-28quartz standard during the sample run at Monash Universityyielded a value of 9.59 per mil; the accepted value is 9.62 permil. δ18O values of water in equilibrium with quartz were cal-culated using the fractionation curve of Matsuhisa et al.(1979) combined with fluid inclusion homogenization tem-peratures from Cooke and Bloom (1990). All δ18O values arereported in standard per mil notation relative to V-SMOW.

The results of our silicate oxygen isotope analyses are listedin Table 8 and illustrated on Figures 13 and 14. The range ofδ18O values from Acupan are as follows: type A (chalcedony):6.4 per mil (n = 1); type B (gray quartz): 6.3 to 10.6 per mil (n= 7); type C (white quartz): –1.7 to 10.7 per mil (n = 24). Theanalyzed δ18O values of quartz veins from Ampucao are: stageI: 8.2 and 9.0 per mil (n = 2); stage IIa: 8.1 and 9.5 per mil (n= 2).

Previous oxygen-deuterium isotope studies in the Baguiodistrict focused primarily on determining the isotopic compo-sitions of modern geothermal waters (Sawkins et al., 1979;Japan International Cooperation Agency-JICA, 1984; Table9). The waters were found to have values that range from–11.5 to –2.2 per mil (δ18O) and –75 to –55.4 per mil (δD).They plot on or close to the meteoric water line (Fig. 15). Sixoxygen and four deuterium isotopic analyses of type C whitequartz from the Acupan and Antamok epithermal vein sys-tems were completed by Sawkins et al. (1979; Table 9; Fig.15). They found that the ore-forming solutions at both de-posits could contain no more than a few percent by volume of

magmatic water, with meteoric waters predominant in bothsystems. Sawkins et al. (1979) also analyzed five alteredwhole-rock samples from the Baguio district (Table 9). Theyfound that 18O depletion in the altered rock samples had beencaused by partial isotopic equilibration with meteoric waters.

Carbon-oxygen isotopes

We analyzed C-O isotope values from 15 calcite samplesfrom Acupan. These are the first C-O isotope data obtainedfor any epithermal veins of the Baguio district. All calcitesamples were hand drilled to yield ~10 mg of fine powders.CO2 was liberated by reaction with H3PO4 at 25°C (Walterset al., 1972; Swart et al., 1991) and analyzed using the methodof McCrea (1950). All δ18O values are reported in standardper mil (‰) notation relative to SMOW, and δ13C values arereported relative to the Cretaceous Peedee Belemnite(PDB). The δ18O and δ13O values of water in equilibriumwith calcite have been calculated using the fractionationequations derived by Field and Fiferak (1985) from the dataof Friedman and O’Neil (1977), combined with fluid inclu-sion homogenization temperatures from Cooke and Bloom(1990). Analytical uncertainties for both δ18O and δ13C are es-timated at ± 0.03 per mil. Approximately 8 to 10 sampleswere analyzed per day, together with one internal standard(ANU–M1: δ13C = 1.45‰; δ18C = –5.90‰). The δ18Ocalcite

values range from –0.8 to 11.1 per mil, whereas δ13Ccalcite val-ues range from –8.5 to –3.6 per mil (Figs. 13, 16; Table 8).

Helium isotopes

Two fluid inclusion samples from Acupan and Antamokthat had previously been analyzed for their deuterium isotopecompositions by Sawkins et al. (1979) were subjected to he-lium isotope analyses by Stuart Simmons (pers. commun.,


0361-0128/98/000/000-00 $6.00 1413

0.703 0.704 0.705 0.706 0.707 0.708 0.709 0.710 0.711

Initial Sr

Seaw

ater

Central & Northern Chile

Cadia, NSW

North Parkes, NSW

Philippines & PNG

Mt Polley, BC

Bajo de la Alumbrera

Laurani

SW Arizona

Crustally contaminated magmasSub-crustal magmas

Ampucao

Acupan

FIG. 12. Strontium isotope results for anhydrite from Acupan and Ampucao (Table 7), compared to Sr isotope analysesof intrusions from calc-alkalic porphyries from island arc and continental arc settings (data from Sillitoe, 1987), and from al-kalic porphyries at Mt. Polley, British Columbia (data from Pass et al., submitted), Cadia and North Parkes, Australia (datafrom Cooke et al., 2007).

1414 COOKE ET AL.

0361-0128/98/000/000-00 $6.00 1414

TABLE 8. Oxygen Isotope Analyses, as Per Mil Values Relative to VSMOW, of Type B and C Quartz, Type D Calcite, and Type A Chalcedony from the Acupan Epithermal Vein System, and Stage I and IIa Quartz from the Ampucao Porphyry Deposit

(also listed are carbon isotope analyses of type D calcite, as per mil values relative to PDB, from Acupan)

Mine Mine Elevation1 Temperature δ18Omineral δ18Owater δ13CPDB

Sample no. Host vein / Rock easting northing (m) Vein stage (ºC) (‰) (‰) (‰)

Ampucao

96959M Zig-Zag Formation 1640 W 24792 S 660 Stage I quartz 515 8.2 5.696951M Zig-Zag Formation 1640 W 24389 S 687 Stage I quartz 511 9.0 6.396956BM Zig-Zag Formation 1640 W 24601 S 668 Stage IIa quartz 513 8.1 5.596969M Ampucao dacite 1640 W 24792 S 632 Stage IIa quartz 499 9.5 6.7

Acupan

96817 Santoy vein 3440 E 19460 S 1033.9 A: chalcedony 220* 6.4 –4.096811 403 vein 2000 W 22700 S 1371.6 B: gray quartz 250* 7.7 –1.296711 381 vein 1564 W 22220 S 754.9 B: gray quartz 268 7.2 –0.996616 381 vein 2341 W 22143 S 1072.3 B: gray quartz 300 6.3 –0.696796 409 vein 785 W 22285 S 709.5 B: gray quartz 300* 6.6 –0.396630 381 vein 2727 W 22569 S 1016.7 B: gray quartz 273 8.1 0.296756 403-Hawkeye vein 2004 W 21687 S 916.8 B: gray quartz 279 9.4 1.796814 Santoy vein 3220 E 19633 S 1033.9 B: gray quartz 270* 10.6 2.696732 No Name fault 2140 W 22830 S 709.5 C: white quartz 216 –1.7 –12.496926 Odd vein 2805 W 21310 S 528.9 C: white quartz 231 2.0 –7.896753 403 vein 2736 W 22834 S 916.8 C: white quartz 212 4.4 –6.596635 381 vein 2664 W 22594 S 980.9 C: white quartz 228 4.5 –5.596896 209 vein 2960 W 21295 S 620 C: white quartz 256 3.2 –5.496942 Odd vein 2316 W 21169 S 461 C: white quartz 276 2.6 –5.296882 210 vein 1780 W 21165 S 489.4 C: white quartz 269 3.1 –5.096913 209 vein 2965 W 21260 S 572.2 C: white quartz 262 3.4 –5.096931 Odd vein 2304 W 21158 S 502 C: white quartz 279 2.8 –4.996679 No Name fault 2460 W 22484 S 862.8 C: white quartz 209 6.3 –4.896737 No Name fault 1590 W 22580 S 709.5 C: white quartz 248 4.3 –4.796807M No Name fault 2859 W 23198 S 711 C: white quartz 281 4.0 –4.296724 No Name fault 2497 W 22801 S 746.9 C: white quartz 277 3.6 –4.196807M,D No Name fault 2859 W 23198 S 711 C: white quartz 281 4.3 –3.996864 381 vein 3012 W 23249 S 761 C: white quartz 291 3.4 –3.896947 206 vein 180 W 20420 S 450.9 C: white quartz 265 4.7 –3.596836 Taka hanging wall split 1250 E 20735 S 1061.6 C: white quartz 220* 7.4 –3.096782 403-Hawkeye vein 167 W 21687 S 862.8 C: white quartz 264 5.9 –2.496664 381 vein 3045 W 23210 S 819 C: white quartz 237 8.2 –1.397105 Taka HWS 1686 E 20371 S 1184 C: white quartz 270 6.8 –1.296608 381 vein 2468 W 22381 S 1028.5 C: white quartz 217 9.6 –1.096802 No Name fault 1923 W 22935 S 620 C: white quartz 293 6.4 –0.796713 No Name fault 2460 W 22800 S 766.8 C: white quartz 245 9.4 0.396649 No Name fault 2062 W 22620 S 801.1 C: white quartz 289 10.7 3.496808 418 vein 1500 W 22000 S 1341.1 D: calcite 140* 1.0 –12.4 –7.896642 403 vein 1028 W 22118 S 801.1 D: calcite 147 4.0 –8.9 –6.696920 209 vein 3269 W 21311 S 537.1 D: calcite 251 –0.8 –8.1 –8.196912 209 vein 3125 W 21300 S 572.2 D: calcite 257 0.0 –7.0 –8.296942 Odd vein 2316 W 21169 S 461 D: calcite 264 –0.3 –7.0 –8.596775 409 vein 936 E 21416 S 980.9 D: calcite 166 4.6 –6.9 –6.696674 No Name fault 2272 W 22652 S 858.6 D: calcite 225 1.7 –6.6 –7.996814 Santoy vein 3220 E 19633 S 1033.9 D: calcite 160* 5.3 –6.6 –5.896646 No Name fault 1984 W 22576 S 801.1 D: calcite 233 1.5 –6.5 –7.596926 Odd vein 2805 W 21310 S 528.9 D: calcite 238 2.1 –5.7 –8.096711 381 vein 1564 W 22220 S 754.9 D: calcite 247 2.0 –5.4 –6.896814D Santoy vein 3220 E 19633 S 1033.9 D: calcite 160* 7.0 –4.9 –3.696724 No Name fault 2497 W 22801 S 746.9 D: calcite 240* 4.4 –3.3 –6.396616 381 vein 2341 W 22143 S 1072.3 D: calcite 140* 11.1 –2.3 –7.596737 No Name fault 1590 W 22580 S 709.5 D: calcite 223 6.2 –2.2 –5.5

1 Elevation relative to sea level*Temperature estimated from boiling-point depth curve on Figure 5 (no fluid inclusion data available); all other temperatures are average homogenization

temperatures calculated for each sample from the fluid inclusion data of Cooke and Bloom (1990); δ18Owater values were calculated using the fractionationequations of Matsuhisa et al. (1979)

M Samples analyzed at Monash University; all other samples analyzed at the University of TasmaniaD Duplicate analysis

1989; Table 9). These samples yielded R/Ra values of 6.7 and6.0 respectively, where R is the 3He/4He ratio of the inclu-sion fluid and Ra is the atmospheric ratio of 3He/4He. Thehigh R/Ra values provide evidence for mantle-derived mag-matic helium with minimal crustal contamination in the ep-ithermal fluids of the Baguio district (Simmons et al., 1987).

DiscussionBaguio contains all of the classic elements of a porphyry-re-

lated mineral district (cf. Sillitoe, 1989, 2010), with severalporphyry Cu-Au and intermediate sulfidation epithermal Au-Ag deposits, two skarn deposits, one high sulfidation epither-mal Au-Ag deposit, and a large, broadly strata-bound lithocap(Waters et al., 2011). Magmatism and hydrothermal activityoccurred episodically in the Baguio district over the past 3m.y., based on the geochronological data of Aoki et al. (1993),Imai (2001), and Waters et al. (2011). Our new stable and ra-diogenic isotopic data now provide an opportunity to assessgenetic relationships between the porphyry and epithermaldeposits, particularly at Acupan and Ampucao. However,given that most of our isotopic data come from the Acupanepithermal Au-Ag veins, we can first use this more detaileddata set to assess likely epithermal ore-forming processes, be-fore moving on to the bigger picture and assessing possiblelinks between deposit styles.

Isotopic modeling of epithermal ore-forming processes

Mineral deposition in intermediate sulfidation epithermalenvironments can result from a range of processes, includingdecompressional boiling, gas condensation, fluid mixing,water-rock interaction, and conductive cooling (Cooke andSimmons, 2000). Each process produces distinctive mineralassemblages, but only the first three are likely to be importantfor gold deposition (Henley, 1984; Drummond and Ohmoto,1985; Reed and Spycher, 1985; Spycher and Reed, 1989; Se-ward, 1989; Cooke and McPhail, 2001; Rae et al., 2011). De-tailed geochemical modeling of epithermal ore formation forthe Acupan vein system was undertaken by Cooke andMcPhail (2001). They concluded that boiling was the pre-dominant ore-forming mechanism, but that certain mineralassemblages were better explained by the processes of fluidmixing, gas condensation or water-rock interaction.

Stable isotope analyses of quartz and calcite have the po-tential to discriminate between cooling, boiling, and mixing-induced mineral deposition in the epithermal environment.In general, the temperature-dependent fractionation be-tween quartz and water (Truesdell et al., 1977; Larson andTaylor, 1987) results in isotopically heavier quartz being pre-cipitated at lower temperatures. Boiling aqueous solutionswill preferentially fractionate 18O into the residual liquidphase (Truesdell et al., 1977; Lynch et al., 1990; Bowers,1991), and any quartz precipitated from a boiling solutionshould therefore be enriched in 18O compared to quartz de-posited by conductive cooling from non-boiling waters.Open-system boiling of a fluid ascending along an open con-duit should result in a systematic and distinctive increase inδ18O isotope values with respect to elevation for minerals pre-cipitated from that boiling solution. Fluid mixing with coolnear-surface groundwaters also has the potential to cause goldprecipitation in the epithermal environment (e.g., Spycher


0361-0128/98/000/000-00 $6.00 1415

Ampucao – Stage Iquartz veins

Acupan – Type Dcalcite bands

0

1

2

3

4

5

6

7

8

-4 -2 0 2 4 6 8 10 12 14

δ18O (‰, VSMOW)mineral

Acupan Type A–chalcedony clasts

Acupan – Type Cwhite quartz bands

Acupan – Type Bgray quartz bands

Ampucao – Stage IIaquartz veins

FIG. 13. Histogram showing all measured δ18Omineral values (‰,VSMOW) measured from stage I and IIa quartz veins from Ampucao andtype A, B, C, and D gangue minerals from Acupan. All data listed in Table 8.

400

600

800

1000

1200

1400

Ele

vatio

n (m

)

-12 -8 -4 0 4 8 12

Stage IStage IIaType AType BType CType D

A


400

600

800

1000

1200

1400

-12 -8 -4 0 4 8 12

Ele

vatio

n (m

) B

δ18O (‰ )water , VSMOW

FIG. 14. Plots of (A) measured δ18Omineral values (‰, VSMOW) versussample elevation, and (B) calculated δ18Owater values versus sample eleva-tion for quartz vein samples from Ampucao (stages I and IIa) and quartz-cal-cite vein samples and chalcedony breccia clasts from Acupan (types A, B, C,and D). All data listed in Table 8. Note that for samples listed in Table 8 thatlack fluid inclusion data, depositional temperatures have been estimatedfrom the boiling point-depth curves on Figure 5. Type A = chalcedony, typeB = gray quartz, type C = white quartz, type D = calcite.

and Reed, 1989; Cooke and McPhail, 2001), and could resultin shifts towards more negative δ18Owater values, if the localmeteoric groundwaters have lower δ18O values than the as-cending hydrothermal fluids.

The δ18Omineral and calculated δ18Owater values fromAcupan broadly increase with elevation (Fig. 14), consistentwith epithermal quartz and calcite deposition from lowertemperature fluids at higher elevations. We have undertakenisotopic modeling of δ18O values for quartz and calcite fromAcupan in order to further evaluate the relative importance ofdifferent types of ore-forming processes in the Acupan ep-ithermal veins. Cooke and McPhail (2001) calculated the pro-portions of water and vapor produced during incrementalopen system boiling of an Acupan-type water (5°C steps)using CHILLER (Reed and Spycher, 1985; Spycher andReed, 1989). These proportions were used to calculate thetheoretical isotopic compositions of quartz and calcite pro-duced by incremental (5°C) open-system boiling from 300º to100ºC (Fig. 17). Average δ18O values of mineralizing fluidswere assumed to be -0.5 per mil for type B quartz, –4.0 per

mil for type C quartz and –6.5 per mil for type D calcite(Table 8; Figs. 16, 18). The theoretical δ18Omineral values com-positions of quartz and calcite precipitated by cooling, boilingand mixing from 300° to 100°C have been plotted on Figure19A-C as a function of elevation using paleowater table ele-vations inferred for type B, C, and D mineralization from Fig-ure 5. Also plotted are the measured δ18O isotope composi-tions of type B and C quartz and type D calcite (Fig. 19 A, B,and C, respectively).

The measured δ13C and δ18O values of type D calcitesmostly vary systematically and are consistent with depositionfrom a cooling hydrothermal fluid with a constant isotopevalue (δ13CH2CO3 = –7.0; δ18Owater = –6.1; Fig. 16). How-ever, spatial variations of δ18Ocalcite values are not as system-atic, with a broad scatter in data between 700 and 1,000 m(Fig. 19C) corresponding to locations where high grade oreshoots occur and where empirical evidence for boiling hasbeen detected from fluid inclusion petrography (observationsof coexisting primary liquid- and vapor-rich fluid inclusions ingrowth zones; Cooke and Bloom, 1990) and textural studies

1416 COOKE ET AL.

0361-0128/98/000/000-00 $6.00 1416

TABLE 9. Oxygen and Deuterium Data, as Per Mil Values Relative to VSMOW, from Sawkins et al. (1979) and JICA (1984), for Whole-Rock Samples, Quartz Veins, Fluid Inclusions and Geothermal Waters from the Baguio District

Mine Elevation1 Temperature δ18Omineral δ18Owater δDSample no. Material Location level (m) (°C) (‰) (‰) (‰) R/Ra Reference

76–B–1 Geothermal water –8.12 –65 176–B–3 Geothermal water Acupan 3150 451 63.7 –6.54 –57 176–B–4 Geothermal water Acupan 2600 618 46.1 –10.71 –71 176–B–5 Geothermal water Acupan 2600 618 46.1 –10.45 –71 176–B–11 Geothermal water Itogon surface 87.9 –2.17 –67 176–B–12 Geothermal water Antamok 1850 51.7 –5.82 –61 176–B–14 Geothermal water Antamok 1850 40.6 –10.23 –58 176–B–15 Geothermal water Antamok 1550 46.1 –9.3 –66 176–B–26 Geothermal water Acupan 3150 451 61.1 –8.12 –68 176–B–27 Geothermal water Acupan 1500 981 20 –11.48 –75 1DA Geothermal water Dalupirip surface 42.5 –9.8 –67.1 2IT–1 Geothermal water Itogon town surface 89.5 –9.8 –67.2 2IT–2 Geothermal water Itogon town surface 62.2 –9.6 –67.6 2KL Geothermal water Klondyke surface 49.5 –9.2 –64.9 2AS Geothermal water Asin surface 73.8 –8.5 –59.9 2PU Geothermal water Pugo surface 36.3 –8.1 –55.4 2LA–1 Geothermal water Laboy surface 47.5 –10.1 –69.2 2LA–2 Geothermal water Laboy surface 47.5 –10.5 –70.7 2BA–1 Geothermal water Acupan 3300 ~4002 81 –7.7 –62.3 2BA–2 Geothermal water Acupan 3150 451 62.1 –7.6 –62.5 2DA–ST Geothermal water East of Dalupirip surface –10.1 –71 2PRE Geothermal water Baguio surface –9.9 –66.9 275–B–25 Pyroxene andesite 1.6 175–B–88 Balatoc diatreme Acupan 2.6 175–B–43 Volcaniclastic

conglomerate 4.2 175–B–35 Hornblende diorite 4.6 175–B–23 Graywacke 2.4 175–B–22 C: white quartz Antamok 400 261 5.3 –3.55 176–B–16 C: white quartz Antamok 700 251.5 3.2 –6.2 –79 176–B–21 C: white quartz 4.4 –4 –82 176–B–22 C: white quartz qz Antamok 700 313.5 6.1 –0.85 –72 6.0 1, 375–B–1 C: white quartz Acupan 2450 661 278 5.0 –3.25 176–B–2 C: white quartz Acupan 3300 ~4002 272.5 5.2 –3.3 176–B–17 C: white quartz Acupan –76 6.7 1, 3

Notes: Helium isotope ratios from Simmons (unpub. data) for inclusion fluids; location of prospects and deposits shown on Figure 2; references: 1 =Sawkins et al. (1979), 2 = JICA (1984), 3 = Simmons (unpub. data)

1 Elevation relative to sea level2 Precise elevation uncertain

(presence of bladed calcite; Cooke et al., 1996). Cooling-in-duced spatial variations in δ18O values have also been de-tected from type B and C quartz bands at Acupan, but withconsiderable scatter in calculated δ18Owater values at higherelevations (Fig. 19A, B).

Conductive cooling could explain some of the quartz iso-topic data, but this mechanism is not a viable alternative forcalcite deposition due to calcite’s retrograde solubility (Rim-stidt, 1997). Conductive cooling is sluggish and will only occurin areas of low flow rates and intimate fluid-rock contacts

(Drummond and Ohmoto, 1985). It is therefore consideredunlikely to have been important for quartz or calcite vein for-mation at Acupan. The principal cooling mechanism respon-sible for calcite deposition at Acupan is likely to have beenboiling, based on the common observed occurrence of bladedcalcite. However, the scatter in δ18Ocalcite values at higherelevations (Fig. 19C) may imply that mixing with local


0361-0128/98/000/000-00 $6.00 1417

McLaughlin

Broadlands-Ohaaki

Comstock

Antamok-Acupan

δD (‰

, VS

MO

W)

(‰, VSMOW)18δ O

0

-20

-40

-60

-80

-100

-120

-140

-20 -15 -10 -5 0 5 10 15 20M

eteo

ric w

ater

Volcanic vapors

Felsic magmasWhite Island

Great Valleysequences

Comstock

Tayoltita

Broadlands–Ohaaki

McLaughlin

Antamok–Acupan

FIG. 15. Plot of δ18O vs. δD (‰, VSMOW) for mineralizing waters and modern geothermal waters of deposits of theBaguio district (modified after Cooke and Simmons, 2000). Baguio district data from Sawkins et al. (1979) are listed in Table9. The range of compositions of water discharged from high temperature fumaroles on andesitic volcanoes is outlined by thebox labeled “volcanic vapors” (Giggenbach, 1992), and the range of compositions of water dissolved in felsic melts is outlinedby the box labeled “felsic magmas” (Taylor, 1992). Other data sources from Cooke and Simmons (2000).

-10-8 -6 -2

-2

-4

-4

0 4 6

-6

2 128

-8

10

150 C°

200 C°

100 C°

250 C°300 C°350 C°400C°

δ18O (‰, VSMOW)calcite

δ13C

(‰, P

DB

)ca

lcite

FIG. 16. Plot of measured δ18Ocalcite (‰, VSMOW) and δ13Ccalcite val-ues (‰, PDB) from type D calcite bands at Acupan. All data listed in Table8. Also shown is the theoretical compositions of calcite precipitated from aH2CO3-predominant cooling water with a bulk isotopic composition of –6.5‰ (δ18O) and –7.0‰ (δ13C), calculated using the fractionation equa-tions derived by Field and Fiferak (1985) from the data of Friedman andO’Neil (1977). The calcite data from Acupan are broadly coincident with thetheoretical cooling trend, indicating that calcite deposition could have oc-curred from a cooling, predominantly meteoric, water over a temperaturerange of approximately 300° to 150°C.

8 18 226 16 204 142 120

A: Boil

A: Boil

B: Cool

-2 10

300

100

150

200

250

(a) multistage boiling (300 100 C)° – °(b) conductive cooling (300 100 C)

(c) mix 250 C chloride water with20°C cold water ( O = -10‰)

° – °°

δ18

Cooling mechanisms


Tem

per

atur

e (

C)

°

B: Cool

C: Mix

C: M

ix

Type Bquart z

δ 18O

=-0.5

water

TypeD

calcite

δ 18O=

-6.5

water

FIG. 17. Calculated isotopic compositions of modeled fluids for conduc-tive cooling, boiling, and fluid mixing simulations, plotted as a function oftemperature. Irrespective of the depositional process (boiling, cooling, ormixing), the 18O isotope compositions of quartz and calcite are predicted toincrease with decreasing temperature. Initial 18Owater compositions are as-sumed to be –0.5 per mil (type B) and –6.5 per mil (type D). Calculations ofcooling, boiling, and mixing increments are based on the numerical simula-tions of Cooke and McPhail (2001).

groundwaters occurred intermittently during vein formation.This would help to explain the observed occurrence of coarserhombohedral calcite crystals in some type D bands, andcould also help to explain the unusual transition at Acupanfrom alkaline pH conditions at low elevations associated withquartz-adularia-carbonate gangue to weakly acidic pH condi-tions at high elevations associated with quartz-muscovite-pyrite alteration that was highlighted by Cooke and McPhail

1418 COOKE ET AL.

0361-0128/98/000/000-00 $6.00 1418

0

1

2

3

4

Type A(chalcedony)

Type B(gray quartz)

B

-14 -12 -10 -8 -6 -4 -2 0 2 4 6 8

0

1

2

3

Stage I

Stage II

A

-14 -12 -10 -8 -6 -4 -2 0 2 4 6 8

0

1

2

3

4

5

6

Type C(white quartz)

C

-14 -12 -10 -8 -6 -4 -2 0 2 4 6 8

0

1

2

3

4

Type D(calcite)

D

-14 -12 -10 -8 -6 -4 -2 0 2 4 6 8

0

1

2

3

4

5

6

Modern thermalwaters

E

-14 -12 -10 -8 -6 -4 -2 0 2 4 6 8

δ18O (‰ )water , VSMOW

FIG. 18. Histograms showing calculated δ18Owater values (‰, VSMOW) forveins from Ampucao and Acupan. All data are listed in Table 8. (A) Ampucaoporphyry-style quartz veins: stage I (black bars), stage IIa (pattern fill). (B)Acupan epithermal veins: early-stage type A chalcedony (stippled bar) andtype B gray quartz (gray bars). (C) Acupan epithermal veins: main-stage typeC white quartz. (D) Acupan epithermal veins: late-stage type D calcite. (E)Compositions of modern geothermal waters from the Baguio district (Table9), shown for comparative purposes. Note that Ampucao quartz has valuesconsistent with magmatic water compositions. At Acupan, calculated δ18Owater

values are highest for the gold-rich type B gray quartz bands that occur on thevein margins (Fig. 4). Calculated δ18Owater values decrease to near-meteoricvalues in the central type D calcite bands, indicating an increase in the pro-portion of meteoric to magmatic water with time, and/or decreasing amountsof isotopic exchange between meteoric waters and igneous wall rocks.

IIIa: MixII: Boil

400

600

800

1000

1200

1400

Ele

vatio

n (m

)

-2 0 2

200 C°

200 C°

200 C°200 C°

200 C°

150 C°

150 C°

150 C°

100 C°

100 C°

100 C°

150 C°

150 C°

275 C°

275 C°

300 C°

250 C°

250 C°

250 C°

IIIa: Mix

I: Cool

I: Cool

I: Cool

II: Boil

IIIb:

Mix

IIIb: Mix

IIIb: Mix

4 6 8 10 12

400

600

800

1000

1200

1400

Ele

vatio

n (m

)

-2 0 2 4 6 8 10 12

δ18O (‰ )mineral , VSMOW

400

600

800

1000

1200

1400

Ele

vatio

n (m

)

-2 0 2 4 6 8 10 12

δ18O = -0.5water ‰A: Type B gray quartz

δ18O = -6.5water ‰C: Type D calcite

δ18O = -4.0water ‰B: Type C white quartz

IIIa: Mix

II: Boil

FIG. 19. Acupan epithermal veins: measured δ18Oquartz values (‰,VSMOW) from (A) type B and (B) type C bands, and (C) δ18Ocalcite valuesfrom type D bands, plotted as a function of elevation. Also shown are curvesillustrating the calculated δ18Omineral values for quartz (panels A and B) andcalcite (panel C) that would be precipitated from the boiling, cooling andmixing conditions outlined in Figure 17, assuming initial 18Owater values of(A) –0.5‰, (B) -4.0‰ and (A) –6.5 per mil. Modeling conditions: curvesshow calculated δ18Omineral values for quartz or calcite precipitated fromwaters that undergo (I) conductive cooling from 300° to 100°C; (II) isoen-thalpic boiling from 300° to 100°C, and (III) cooling to 250°C and then mix-ing with a shallow, cool (20°C) meteoric groundwater that has an estimatedδ18Owater value of – 10 per mil. Two extreme end-members are shown forcase III. In the first (IIIa), mixing occurs entirely at the elevation where thetwo waters first meet. In the second, (IIIb) mixing occurs incrementally upto the elevations that correspond to the estimated depth of the paleowa-tertable for each vein stage as estimated from Figure 5. Calculations of cool-ing, boiling, and mixing increments are based on the numerical simulationsof Cooke and McPhail (2001). For the white quartz data (B), excursions tohigh δ18Oquartz values at high elevations can be explained by mixing withmeteoric groundwaters, whereas type C bands precipitated in the deepermine levels have compositions consistent with deposition from boiling hy-drothermal fluids. Most of the calcite data (C) are consistent with depositionfrom a boiling hydrothermal fluid (consistent with the occurrence of bladedcalcite textures). Excursions away from the boiling curve may be explained bysubsurface fluid mixing with cool meteoric groundwaters.

(2001). Excursions above the theoretical boiling-relatedδ18Oquartz values for type B and C bands could also be ex-plained by intermittent fluid mixing, in a similar fashion totype D calcite. Fluid mixing with cool meteoric groundwateris therefore inferred to have occurred in the upper levels ofthe Acupan vein system, causing the broad scatter in isotopicdata at higher mine levels (Fig. 19C). We speculate that mix-ing may have occurred due to episodic descent of the paleo -groundwater table, possibly in response to periods of veryheavy rainfall that can occur in a monsoonal climate, as is thecase in the modern-day setting of the Baguio district.

Sulfur isotope geothermometry and bulk sulfur isotopic compositions

Epithermal mineralizing fluids at Acupan were dilute (gen-erally < 2 wt % NaCl equiv) and had moderate temperatures(< 300°C; Fig. 5), whereas porphyry-style mineralization atAmpucao was associated with high temperature (300° to>600°C) hypersaline brines (30 to >70 wt % NaCl equiv) andlow-density vapor (Cooke and Bloom, 1990). In order to fur-ther constrain the temperatures of ore formation at these twodeposits, sulfur isotope thermometry was attempted for min-eral pairs using the fractionation equations of Ohmoto andRye (1979). The δ34S values from sulfate-sulfide pairs areused to help constrain the oxidation state and bulk sulfurcompositions (i.e., ∑δ34S values) of the mineralizing fluids.

Two sulfide-sulfate pairs analyzed from stage IIa veins atAmpucao yielded sulfur isotope thermometry results of 860°and 530°C. These results could be explained by equilibriumprecipitation of anhydrite and pyrite from a fluid with a ∑δ34Svalue of ~5 per mil and a H2S(aq)/SO4

2–(aq) ratio of about 10

(i.e., reducing conditions). Analysis of primary brine inclusionsfrom one of these samples yielded an average homogeniza-tion temperature of 450°C, although the overall temperaturerange recorded by the individual inclusions was large (400° to> 600°C; Cooke and Bloom, 1990). Isotopic disequilibrium issuspected, and so the results of this attempt to constrain thetemperature and bulk sulfur isotopic composition of the Am-pucao mineralizing fluids must be treated with caution.

Four pyrite–sphalerite pairs from Acupan yielded temper-atures from 96° to 192°C, with an outlier at >800°C. Some ofthese results are consistent with the fluid inclusion data ofCooke and Bloom (1990; Fig. 5), whereas others are not. Onepyrite-anhydrite pair analyzed from a type E anhydrite bandat Acupan yielded a sulfur isotopic temperature of 480°C,much higher than the fluid inclusion temperatures recordedfrom these epithermal veins (Cooke and Bloom, 1990; Fig. 5).The isotopic compositions of the coexisting pyrite and anhy-drite could be explained by co-deposition from a fluid with asimilar ∑δ34S value to that estimated for Ampucao (~5‰),but with a less reducing composition (H2S(aq)/SO4

2–(aq) ~ 5).

However, this result is equivocal due to suspected isotopicdisequilibrium, as implied by the discrepancies between tem-perature estimates by fluid inclusion and sulfur isotopicanalyses. More detailed analyses and microsampling of coex-isting sulfate and sulfide grains are required to better con-strain the bulk sulfur isotope compositions of mineralizingfluids at Acupan and Ampucao.

The absence of coexisting sulfide-sulfate mineral pairs atSanto Tomas II and Kelly prevents the bulk sulfur values of

the magma and mineralizing fluids being determined directly.The ∑δ34S must be between the δ34S value of the sulfates andsulfides, i.e., between +4 and +13 per mil at Santo Tomas II(Imai, 2001), and between +0.8 and +15 per mil at Kelly, as-suming that the alunite from the nearby lithocap is represen-tative of Kelly sulfate. Enrichment in ∑δ34S for porphyry de-posits and related magmas is common in the western Luzonarc (Imai et al., 1996; Imai, 2000, 2001) and elsewhere (Ar-ribas, 1995), and ∑δ34S estimates of +5 per mil for the Baguiodistrict probably implies contamination of magmatic sulfur bysedimentary sulfate, possibly due to assimilation (Ohmotoand Rye, 1979).

Isotopic tracing

Our radiogenic isotope analyses provide evidence for man-tle-derived magmatic components in the mineralizing fluidsthat formed the porphyry and epithermal deposits of theBaguio district. Early anhydrite veins from the Ampucao por-phyry Cu-Au deposit and late anhydrite veins from the Acu-pan epithermal Au-Ag deposit have identical initial strontiumratios (0.70378–0.70385; Table 7). The data overlap with themain range of initial strontium ratios for Pliocene and Pleis-tocene igneous rocks from the Baguio district (0.70366–0.70388; Hollings et al., 2011b) and are well within thebroader range of data reported for the Philippines by Mc-Dermott and Hawkesworth (1991; 0.70356–0.70476). Theanhydrite initial Sr data also overlap with results obtainedfrom porphyry deposits that formed in oceanic island arc ter-rains elsewhere (Fig. 12). The initial Sr ratios of anhydritefrom Acupan and Ampucao are consistent with a primitivemantle-derived magmatic Sr source for both deposits, andpreclude any significant crustal contamination of Sr duringanhydrite deposition (Fig. 12). Similarly, the helium isotopiccompositions of fluid inclusions from Acupan and Antamokare indicative of a mantle-derived (magmatic) noble gas com-ponent in the epithermal mineralizing fluids, and precludethe possibility of contamination by atmospheric He duringhydrothermal activity (Table 9; Simmons et al., 1987).

The oxygen and deuterium isotopic data listed in Tables 8and 9 provide evidence for the following: (1) a predominanceof magmatic water in the Ampucao porphyry system, (2) in-volvement of both magmatic and meteoric water in the Acu-pan epithermal veins, and (3) a predominance of meteoricwater in the present-day geothermal systems of the Baguiodistrict. The calculated δ18O values of water that precipitatedstage I and IIa quartz veins from Ampucao are +5.6 to +6.3per mil and +5.5 to +6.7 per mil, respectively (Fig. 18A;Table 8). These are the highest δ18Owater values detectedfrom the Baguio district, and are consistent with a predomi-nance of magmatic water at Ampucao. Calculated isotopevalues of hydrothermal fluids for the major vein stages atAcupan are: type B gray quartz-δ18Owater from –1.2 to +2.6per mil (median = –0.3‰; Fig. 18B); type C white quartz -δ18Owater from –12.4 to +3.4 per mil (median = –4.2‰; Fig.18C); type D calcite-δ18Owater from –12.4 to –2.2 per mil (me-dian = –6.6‰; Fig. 18D; Tables 8, 9). There is a notable de-crease in δ18Owater values with time at Acupan (Fig. 18). Theearly-formed gray quartz bands (Fig. 4) have the highestδ18Owater values, whereas the central type D calcite bands(Fig. 18D) have δ18Owater values similar to the modern-day


0361-0128/98/000/000-00 $6.00 1419

meteoric geothermal waters (Fig. 18E). This is interpreted toindicate a significant magmatic component in the mineraliz-ing waters during the deposition of gold-rich type B mineral-ization, and a progressively smaller magmatic component andincreasing meteoric component during deposition of gold-rich type C and gold-poor type D bands. Alternatively, itcould be argued that the waters that precipitated type B grayquartz underwent the greatest amount of isotopic exchangewith the surrounding igneous wall rocks, shifting their δ18Ovalues away from meteoric compositions. Mantling of themajor banded veins by early-formed quartz could then haveisolated late-stage waters from the surrounding wallrocks, al-lowing preservation of near-meteoric δ18O values in the cen-tral calcite bands. However, this alternative model does notadequately explain the mantle-derived radiogenic isotopiccompositions of Sr and He obtained from the epithermalveins, and so our preferred model is for a significant mag-matic water component in the earliest-formed epithermalveins at Acupan. Based on gold, silver and tellurium solubil-ity relationships, Cooke and McPhail (2001) argued that thepresence of tellurides at Acupan indicates direct input ofmagmatic H2Te(g), and a magmatic source of gold and silverremains the simplest explanation for the huge endowment ofthese metals in the Acupan vein system, with boiling the prin-cipal ore-forming mechanism.

Our carbon isotope analyses of calcite from Acupan cannotdiscriminate between a magmatic or atmospheric source ofcarbon. The δ18O and δ13C values for type D calcite plot on atheoretical cooling curve for a H2CO3-predominant hydrother-mal water with a δ13C value of –6.5 per mil (Fig. 16), meaningthat the carbon could be sourced from atmospheric or mag-matic CO2 (Field and Fifarek, 1985). The calculated δ18O valueof –7.0 per mil for this cooling array is consistent with a pre-dominance of meteoric water during calcite deposition.

A predominance of magmatic sulfur (i.e., values near 0‰;Ohmoto and Rye, 1979; Field and Fifarek, 1985) is evidentfrom our sulfur isotope dataset (Tables 3–6), but individualdeposits appear to have been associated either with oxidized(SO42–-predominant) or reduced (H2S-predominant) hydro -thermal fluids. Predominantly negative δ34Ssulfide values (–6.8to +0.8‰) were obtained from sulfides and sulfosalts from ep-ithermal and porphyry deposits located adjacent to the Baguiolithocap, specifically Kelly, Baguio Gold, Black Mountain andChico prospect (Figs. 2, 8C, E; Tables 5, 6). The sulfur iso-topic compositions obtained from these deposits are consis-tent with sulfide deposition from an oxidized (sulfate-pre-dominant) magmatic fluid (cf. Rye, 1993; Wilson et al., 2007;Wolfe and Cooke, 2011). Their δ34S values are similar to theisotopic compositions of sulfides and sulfosalts from the Lep-anto high sulfidation epithermal Cu-Au and Far Southeastporphyry Cu-Au deposits in the nearby Mankayan district(Fig. 11), where detailed investigations have demonstrated agenetic link between the Mankayan lithocap, Lepanto highsulfidation Cu-Au deposit and Far Southeast porphyry Cu-Audeposit (Arribas et al., 1995; Hedenquist et al., 1998; Imai,2000; Chang et al., 2011; Deyell and Hedenquist, 2011).

Evidence for reduced magmatic-hydrothermal fluidscomes from the δ34S values of sulfides from Acupan, Ampu-cao and Santo Tomas II. Their δ34S values range from +1.1 to+6.6 per mil, with one exception—a chalcopyrite clast from

the Balatoc diatreme that yielded a value of -1.4 per mil (Fig.8A, B, D; Tables 3, 4; Imai, 2001). The more limited δ34Sdatasets obtained from Nugget Hill, Sierra Oro, Chico Mine,Camp 4, and Antamok overlap with this restricted range ofpositive δ34Ssulfide values, with the exception of a chalcopyritegrain from the Wildcat Cu replacement orebody at Antamokthat yielded a value of –2.5 per mil, and a chalcopyrite veinfrom Camp 4 (–0.7‰; Table 6). The δ34S values obtainedfrom sulfate minerals from Acupan, Ampucao, and SantoTomas II range from +13.4 to +18.3 per mil, apart from oneoutlier of +31.8 per mil (Tables 3, 4; Imai, 2001). The sulfurisotope results from these porphyry and epithermal depositsare consistent with a common, homogenized sulfur sourcethat was predominantly magmatic with a ∑δ34S value of ~5per mil, and mineralizing fluids that were most likely reduced(H2S-predominant with H2S(aq)/SO42–(aq) ~ 5 to 10; cf. Rye,1993; Wilson et al., 2007). The δ34Ssulfide values obtained fromskarns of the Baguio district are similar, ranging from +1.0 to+4.6 per mil (Table 6; Fig. 8E). It appears likely that reduced(H2S-predominant) magmatic-hydrothermal fluids prevailedin the porphyry, epithermal, and skarn deposits of the centraland southern Baguio district.

Genetic links between deposit styles

Absolute age determinations imply that the Acupan ep-ithermal veins (0.65 ± 0.07 Ma; Aoki et al., 1993) are the sameage as the Ampucao porphyry deposit within analytical error(0.51 ± 0.26 Ma; Waters et al., 2011), although crosscuttingrelationships show that the porphyry vein stockwork formedfirst (Cooke and Bloom, 1990; Cooke, 1991; Fig. 20A). Theoxygen isotope data from Ampucao and Acupan shows anevolution from magmatic to meteoric water-dominated fluidcompositions with time (Fig. 18). The sulfur isotope compo-sitions of sulfides and sulfates from both deposits are similar(Fig. 8A, B) and the strontium isotope compositions of earlyporphyry and late epithermal anhydrite are identical withinanalytical error (Table 7). All of these features are consistentwith a genetic link between these two deposits. We thereforeconclude that the Acupan veins are a late-stage manifestationof hydrothermal activity associated with the Ampucao por-phyry deposit, with rapid uplift and exhumation promotingtelescoping of the epithermal veins into the early-formed por-phyry stockwork (Fig. 20A). Evidence for uplift rates of ~1km per million years were obtained from fluid inclusion stud-ies (Cooke and Bloom, 1990; Fig. 5), and this rapid uplift re-sulted in superposition of the shallow-crustal epithermal en-vironment into the earlier-formed, deeper-level porphyrysystem.

The Kelly high sulfidation epithermal veins (illite K-Ar <0.6Ma; Aoki et al., 1993) are apparently younger than the Baguiolithocap (alunite K-Ar ages of 1.4 to 0.9 Ma; Aoki et al., 1993),implying that there were at least two major phases of acidicmagmatic-hydrothermal fluid activity in the northern parts ofthe Baguio district (Fig. 20B). These episodes of intenselyacidic hydrothermal alteration and mineralization were asso-ciated with oxidized magmatic-hydrothermal fluids, whichprecipitated sulfides with negative δ34S values that are dis-tinct from the sulfides associated with the porphyry, epither-mal, and skarn deposits of the southern and central Baguiodistrict. This suggests that the high sulfidation state veins at

1420 COOKE ET AL.

0361-0128/98/000/000-00 $6.00 1420

Kelly are likely to have formed proximal to a very young, andtherefore possibly more deeply buried, magmatic-hydrother-mal (porphyry?) center in the northern part of the Baguio dis-trict (Fig. 20B). Detailed geochronological investigations arerequired to establish whether there are genetic links betweenKelly and the nearby Antamok, Baguio Gold, or Atok-BigWedge intermediate sulfidation vein systems.

The fluid source(s) for the Baguio lithocap remains un-known. Potential candidates include the Nugget Hill andChico porphyry prospects, for which no age determinationsare currently available, or another, as yet undiscovered, por-phyry center located somewhere beneath or to one side of thelithocap (Fig. 20B).

ConclusionsThe porphyry, epithermal, and skarn deposits of the Baguio

district formed over the past 3 m.y. when dioritic to daciticmagmas intruded the shallow crust in response to ridge sub-duction beneath northern Luzon. This geodynamic settingtriggered shallow crustal hydrous magmatism, promotedrapid uplift and erosion, and juxtaposed porphyry, skarn, highand intermediate sulfidation deposit styles. Individual deposittypes and locations were influenced by local structural, strati-graphic, magmatic and hydrothermal phenomena. Magmaticfluid sources were fundamental to ore formation. Thestrongest isotopic evidence for direct magmatic contributionsto the mineralizing fluids are provided by the δ18O values ofquartz from Ampucao (Fig. 18A; Table 8), δ34S values and ini-tial strontium ratios of anhydrite from Ampucao and Acupan(Figs. 8A, B, 12), δ34S values of sulfides and sulfosalts (Fig. 8)and helium isotope compositions of fluid inclusions fromAcupan and Antamok (Table 9). The oxygen isotope data col-lected for quartz and calcite samples from the Acupan veins(Fig. 18A-D), combined with the oxygen and hydrogen iso-tope data of Sawkins et al. (1979; Fig. 15) and JICA (1983;Fig. 18E), and fluid inclusion data of Cooke and Bloom(1990) suggest epithermal vein deposition from boiling hy-brid magmatic-meteoric waters, with the magmatic compo-nent decreasing to minimal amounts in later, weakly mineral-ized vein stages (Fig. 18). Isotope values of quartz and calcitevary markedly in the upper mine levels at Acupan, possibly in-dicating some fluid mixing with cool meteoric groundwaters.

The Baguio district has been underexplored for bulk ton-nage porphyry Cu-Au-style targets, despite almost a centuryof underground mining of high-grade epithermal Au-Ag


0361-0128/98/000/000-00 $6.00 1421

S N

Acupan IS epithermal veins0.65 0.07 Ma±

S = +1.1 to +5.9δ34sulfides ‰

not to scale

Ampucao porphyry0.56 0.21 Ma±

‰S = +2.3 to +6.6δ34sulfides

Balatoc diatreme> 0.8 Ma

S = -1.4δ34chalcopyrite ‰

vv

v

v

vv

vv v

v

vvv

v

v

A

LEGEND

High sulfidation vein(advanced argillic halo)

diorite porphyry

Intermediate sulfidation vein(illite alteration halo)

potassic alteration

Balatoc diatreme(> 0.8 Ma)

Baguio lithocap(1.4 to 0.9 Ma)

pyrite halo

propylitic alteration

skarnoo

o

oo

vv v

SSW

NNE

????

?

Black Mt porphyry3.09 0.15 Ma±

‰S = -1.5 to +0.8δ34sulfides

Chico porphyryunknown age

δ34S = +1.0 to +2.9sulfides ‰

Nugget Hill porphyryunknown age

δ34S = +2.0 to +5.0sulfides ‰ not to scale

Kelly HS epithermal veins< 0.6 Ma

S = –3.2 to +0.8δ34sulfides ‰

Chico IS epithermal veinsunknown age

S = -2.9 to -2.8δ34sulfides ‰

Thanksgiving skarn ( 3 Ma?)~δ34S = +1.0 to +4.6sulfides ‰

Baguio GoldIS epithermal veins

unknown ageS = -6.8 to +0.5δ34

sulfides ‰

oo

o

oo

o

o

oo

oo o o

oo o

ooooo o

o

oooo

o

o

o

oo

o

o

o

B

FIG. 20. Schematic cross sections through the Baguio district showing keyspatial relationships between ore deposits, prospects and alteration zones, to-gether with geochronological data and δ34Ssulfide values. Note that these sec-tions are schematic cartoons—they are not drawn precisely to scale, and in-clude speculative elements indicated by “?” symbols. (A) N-S section (~2.5km long) through the Acupan intermediate sulfidation epithermal veins, Bal-atoc diatreme, and Ampucao porphyry. Potassic alteration (biotite-mag-netite-quartz-chalcopyrite-anhydrite and orthoclase-quartz-chalcopyrite-an-hydrite assemblages) occurs around the Ampucao intrusive complex and alsoin the deeper levels of the Acupan mine adjacent to the Balatoc diatreme.The intermediate sulfidation veins dip towards the Ampucao porphyry, andhave cut the porphyry and diatreme, but cannot be discriminated from theporphyry using radiometric age determinations. (B) NNE-SSW section be-tween Baguio Gold and Black Mountain (~ 8 km long), with the Kelly highsulfidation veins, Chico porphyry, Chico intermediate sulfidation veins, andNugget Hill porphyry projected onto the section from locations up to 1 to 2km to the east. Mineralization in the Baguio district commenced with the for-mation of the Black Mountain porphyry Cu-Au deposit and Thanksgivingskarn at about 3 Ma, and continued intermittently through to <0.6 Ma withthe formation of the Kelly high sulfidation veins. The 1.4 to 0.9 Ma Baguiolithocap is a single contiguous body of advanced argillic and silicic alterationup to 7.5 km long and 4 km wide that has a crescent-shaped outcrop pattern(Fig. 2). It is shown here in section as two discrete bodies because the crosssection only passes through the southern and northeastern edges of the litho-cap. The fluid source(s) for the Baguio lithocap remain unknown, but basedon the outcrop patterns in Figure 3, we speculate that Nugget Hill may havebeen a source for the southern part of the lithocap, and that one or more as-yet undiscovered porphyries may have been the source(s) of fluids for thelithocap, and also for the high and intermediate sulfidation state epithermalveins at the northern end. Abbreviations: HS = high sulfidation, IS = inter-mediate sulfidation.

veins. This may relate in part to a lack of appreciation of thepotential links between porphyry deposits and intermediatesulfidation state veins, despite the best efforts of Sillitoe(1989, 2010) and others to promote this potential genetic re-lationship. We believe that several porphyry-style targets inthe Baguio district currently warrant drill testing. They in-clude the Ampucao, Chico, and Nugget Hill prospects, wheresignificant rock chip and drill core assay results indicate thepotential for discovery of high-grade Cu-Au resources (Table2; Waters et al., 2011). The high sulfidation-state mineraliza-tion at Kelly is a favorable sign of a magmatic-hydrothermal(porphyry?) fluid source nearby, with supporting evidenceprovided by the early biotite and actinolite alteration that hasbeen cut by the high sulfidation veins (Comsti et al., 1990).The geology and sulfur isotope data of Kelly compares favor-ably with the Lepanto deposit in the Mankayan district (Fig.11), where deep drilling resulted in the discovery of the FarSoutheast porphyry deposit (Hedenquist et al., 1998; Changet al., 2011). High temperature quartz-magnetite-sulfideveins are cut by intermediate sulfidation calcite-quartz veinsat the Baguio Gold deposit, similar to the observed crosscut-ting relationships between Acupan and Ampucao, and againsuggesting proximity to a porphyry-style centre. We believethat the Wildcat massive chalcopyrite replacement-style ore-body, which is cut by the Antamok intermediate sulfidationveins, probably lies close to a magmatic-hydrothermal source,given the high copper grades of 1.4 to 3.7 wt percent thatwere mined from Wildcat during the 1970s (Damasco, 1979;Table 2). All of these empirical observations imply that one ormore buried porphyry-style Cu-Au deposits remain to be dis-covered in the Baguio district.

AcknowledgmentsWe thank Christine Cook and Mike Power for their assis-

tance in performing C-O and S isotope analyses. IanCartwright is thanked for undertaking O isotope analyses, andRoland Mass for Sr isotope analyses. Stuart Simmons isthanked for providing access to his unpublished helium iso-tope data. Mike Baker and Norman Tamayo are thanked forassistance with figure preparation. The manuscript was im-proved markedly by the thoughtful reviews of Antonio Arribas, Kevin Faure, Stuart Simmons, and Pete Hollings.CODES is the Australian Research Council’s Centre for Ex-cellence in Ore Deposits.

REFERENCESAoki, M., Comsti, E.C., Lazo, F.B., and Matsuhisha, Y., 1993, Advanced

argillic alteration and geochemistry of alunite in an evolving hydrothermalsystem at Baguio, northern Luzon, Philippines: Resource Geology, v. 43, p.155–164.

Arribas, A., Jr., 1995, Characteristics of high-sulfidation epithermal deposits,and their relation to magmatic fluid: Mineralogical Association of CanadaShort Course, v. 23, p. 419–454.

Arribas, A., Jr., Hedenquist, J.W., Itaya, T., Okada, T., Concepcion, R.A., andGarcia, J.S., Jr,, 1995, Contemporaneous formation of adjacent porphyryand epithermal Cu-Au deposits over 300 ka in northern Luzon, Philippines:Geology, v. 23, p. 337–340.

Balce, G.R., Encina, R.Y., Momongan, A. and Lara, E., 1980, Geology of theBaguio district and its implication on the tectonic development of theLuzon Central Cordillera: Geology and Palaeontology of Southeast Asia, v.21, p. 265–287.

Bowers, T.S., 1991, The deposition of gold and other metals: pressure-in-duced fluid immiscibility and associated stable isotope signatures:Geochimica et Cosmochimica Acta, v. 55, p. 2417–2434.

Bureau of Mines and Geosciences, 1986, Geology and mineral resources ofthe Philippines. Volume 2. Mineral resources: Metro Manila, Philippines,Bureau of Mines and Geosciences, Ministry of Natural Resources, 446 p.

Callow, K.J., 1967, The geology of the Thanksgiving mine, Baguio district,Mountain Province, Philippines: ECONOMIC GEOLOGY, v. 62, p. 472–481.

Callow, K.J., and Worley, B.W., Jr., 1965, The occurrence of telluride miner-als at the Acupan gold mine, Mountain Province, Philippines: ECONOMICGEOLOGY, v. 60, p. 251–268.

Chang, Z., Hedenquist, J.W., White, N.C., Cooke, D.R., Roach, M., Deyell,C.L., Garcia, Jr., J., Gemmell, J.B., McKnight, S., and Cuison A.L., 2011,Exploration tools for linked porphyry and epithermal deposits: Examplefrom the Mankayan intrusion-centered Cu-Au district, Luzon, Philippines:ECONOMIC GEOLOGY, v. 106, p. 1365–1398.

Clayton, R.N., and Mayeda, T.K., 1963, The use of bromine pentafluoride inthe extraction of oxygen from oxides and silicates for isotopic analysis:Geochimica et Cosmochimica Acta, v. 27, p. 43–52.

Comsti, M.E.C., Villones, R.I., de Jesus, C.V., Natividad, A.R., Rollan, L.A.,and Duroy, A.C., 1990, Mineralization at the Kelly gold mine, Baguio dis-trict, Philippines: fluid-inclusion and wall-rock alteration studies: Journalof Geochemical Exploration, v. 35, p. 297–340.

Cooke, D.R., 1991, Styles and controls of mineralization, Acupan gold mine,Baguio district, Philippines: Unpublished Ph.D. thesis, Clayton, Victoria,Monash University, 396 p.

Cooke, D.R., and Bloom, M.S., 1990, Epithermal and subjacent porphyrymineralization, Acupan, Baguio district, Philippines: A fluid inclusion andparagenetic study: Journal of Geochemical Exploration, v. 35, p. 297–340.

Cooke, D.R., and McPhail, D.C., 2001, Epithermal Au-Ag-Te mineraliza-tion, Acupan, Baguio district, Philippines: numerical simulations of mineraldeposition, ECONOMIC GEOLOGY, v. 96, p. 109–131.

Cooke, D.R. and Simmons, S.F., 2000, Characteristics and genesis of ep-ithermal gold deposits: Reviews in Economic Geology, v. 13, p. 221–244.

Cooke, D.R., Hollings, P., and Walshe, J.L., 2005, Giant porphyry deposits:Characteristics, distribution and tectonic controls: ECONOMIC GEOLOGY, v.100, p. 801–818.

Cooke, D.R., McPhail, D.C., and Bloom, M.S., 1996, Epithermal gold min-eralization, Acupan, Baguio district, Philippines: geology, mineralization,alteration and the thermochemical environment of ore deposition: ECO-NOMIC GEOLOGY, v. 91, p. 243–272.

Cooke, D.R., Wilson, A.J., House, M.J., Wolfe, R.C., Walshe, J.L., Lickfold,V., and Crawford, A.J., 2007, Alkalic porphyry Au-Cu and associated min-eral deposits of the Ordovician to Early Silurian Macquarie Arc, New SouthWales: Australian Journal of Earth Sciences, v. 54, p. 445–463.

Corbett, G.J., and Leach, T.M., 1998, Southwest Pacific rim gold-copper sys-tems: Structure, alteration and mineralisation: Society of Economic Geolo-gists Special Publication no. 6, 236 p.

Damasco, F.V., 1979, Geology of the Wildcat copper deposit in the Antamokmine of Benguet Consolidated, Inc.: Journal of the Geological Society ofthe Philippines, v. 33, p. 1–16.

De los Santos, R.B., 1982, Geology of the Southwestern Baguio district: Bu-reau of Mines and Geosciences, Metro Manila, Philippines, Technical In-formation Series, Geology, v. 53, 17 p.

Defant, M.J. and Drummond, M.S. 1990, Derivation of some modern arc mag-mas by melting of young subducted lithosphere: Nature, v. 347, p. 662–665.

Deyell, C.L., and Cooke, D.R., 2003, Mineralogical and isotopic evidence forthe genesis of the Kelly gold-silver deposit, Baguio district, Philippines: Di-verse mineral assemblages in a hybrid epithermal system, in Eliopoulos,D., et al., Mineral Exploration and Sustainable Development–Proceedingsof the Seventh Biennial SGA Meeting: Millpress, Rotterdam, v. 1, p.469–472.

Deyell, C.L., and Hedenquist, J.W., 2011, Trace element geochemistry ofenargite in the Mankayan district, Philippines: ECONOMIC GEOLOGY, v. 106,p. 1465–1478.

Drummond, S.E., and Ohmoto, H., 1985, Chemical evolution and mineraldeposition in boiling hydrothermal system: ECONOMIC GEOLOGY, v. 80, p.126–147.

Durkee, E.F., and Pederson, S.L., 1961, Geology of Northern Luzon, Philip-pines: Bulletin of the American Association of Petroleum Geologists, v. 45,p. 137–168.

Field, C.W., and Fifarek, R.H., 1985, Light stable-isotope systematics in theepithermal environment: Reviews in Economic Geology, v. 2, p. 99–128.

Friedman, I., and O’Neil, J.R., 1977, Data of geochemistry. Compilation ofstable isotope fractionation factors of geochemical interest: U.S. GeologicalSurvey Professional Paper 440-KK, p. 1–12.

1422 COOKE ET AL.

0361-0128/98/000/000-00 $6.00 1422

Fritz, P., Drimmie, R.J., and Nowicki, V.K., 1974, Preparation of sulfur diox-ide for mass spectrometer analyses by combustion of sulfides with copperoxide: Analytical Chemistry, v. 46, p. 164–166.

Geyne, A.R., Fries, C., Jr., Segerstrom, K, Black, R.F., and Wilson, I.F., 1963,Geology and mineral deposits of the Pachuca-Real del Monte district, Stateof Hidalgo, Mexico: Mexico, Consejo de Recursos Naturales no Renova-bales Publication 5E, 203 p. (in English and Spanish).

Giggenbach, W.F., 1992, Magma degassing and mineral deposition in hy-drothermal systems along convergent plate boundaries: ECONOMIC GEOL-OGY, v. 87, p. 1927–1944.

Haas, J.L., Jr., 1971, The effect of salinity on the maximum thermal gradientof a hydrothermal system at hydrostatic pressure: ECONOMIC GEOLOGY, v.66, p. 940–946.

Hedenquist, J.W., and Garcia, J.S., Jr., 1990, Sulfur isotope systematics in theLepanto mining district, northern Luzon, Phillippines: Mining Geology, v.40. p. 67.

Hedenquist, J.W., Arribas, A., Jr. and Reynolds, T.J., 1998, Evolution of an in-trusion-centered hydrothermal system: Far Southeast-Lepanto porphyryand epithermal Cu-Au deposits, Philippines: ECONOMIC GEOLOGY, v. 93, p.373–404.

Hedenquist, J.W., Arribas, A., Jr., and Gonzalez-Urien, E., 2000, Explorationfor epithermal gold deposits: Reviews in Economic Geology, v. 13, p.245−277.

Henley, R.W., 1984, Chemical structure of geothermal systems: Reviews inEconomic Geology, v. 1, p. 9–28.

Hollings, P., Wolfe, R., Cooke, D.R., and Waters, P.J., 2011a, Geochemistryof Tertiary igneous rocks of northern Luzon, Philippines: Evidence for aback-arc setting for alkalic porphyry copper-gold deposits and a case forslab roll-back?: ECONOMIC GEOLOGY, v. 106, p. 1257–1277.

Hollings, P., Cooke, D.R., Waters, P.J., and Cousens, B., 2011b, Igneous geo-chemistry of mineralized rocks of the Baguio district, Philippines: Implica-tions for tectonic evolution and the genesis of porphyry-style mineraliza-tion: ECONOMIC GEOLOGY, v. 106, p. 1317–1333.

Huston, D.L., Power, M., Gemmell, J.B., and Large, R.R., 1995, Design, cal-ibration and geological application of the first operational Australian laserablation sulphur isotope microbrobe: Australian Journal of Earth Sciences,v. 42, p. 549–555.

Imai, A., 2000, Mineral paragenesis, fluid inclusions, and sulfur isotope sys-tematics of the Lepanto Far Southeast porphyry Cu-Au deposit, Mankayan,Benguet, Philippines: Resource Geology, v. 50, p. 151–168.

——2001, Generation and evolution of ore fluids for porphyry Cu-Au min-eralization of the Santo Tomas II (Philex) deposit, Philippines: ResourceGeology, v. 51, p. 71–96.

Imai, A., Listanco, E.L., and Fujii, T., 1996, Highly oxidized and sulfur-richmagma of Mount Pinatubo, Philippines: Implication for metallogenesis ofporphyry copper mineralization in the western Luzon arc, in Newell, C.G.and Punongbayan, R.S., eds., Fire and mud: Eruptions and lahars of MountPinatubo, Philippines: Quezon City, Philippine Institute of Volcanology andSeismology, and Seattle, University of Washington Press, p. 865–874.

Japan International Cooperation Agency (JICA), 1983, Report on Acupan-Itogon Geothermal Development. First Phase Survey: Tokyo, Japan Inter-national Cooperation Agency, 94 p.

——1984, Report on Acupan-Itogon geothermal development. SecondPhase Survey: Tokyo, Japan International Cooperation Agency, 123 p.

Knittel, U., and Defant, M.J., 1988, Sr isotopic and trace element variationsin Oligocene to Recent igneous rocks from the Philippine island arc: Evi-dence for recent enrichment in the sub-Philippine mantle: Earth and Plan-etary Science Letters, v. 87, p. 87–99.

Larson, P.B., and Taylor, H.P., Jr., 1987, Solfataric alteration in the San JuanMountains, Colorado: Oxygen isotope variations in a boiling hydrothermalenvironment: ECONOMIC GEOLOGY, v. 82, p. 1019–1036.

Leith, A., 1938, Geology of the Baguio gold district: Philippine Departmentof Agriculture and Commerce, Technical Bulletin No. 5, 55 p.

Livingston, C.W., 1939a, Mechanics of vein formation in the northern half ofthe Baguio District: Engineering and Mining Journal, v. 140, no. 9, p.38–42.

Livingston, C.W., 1939b, Mechanics of vein formation in the northern half ofthe Baguio district Part II: Engineering and Mining Journal, v. 140, no. 10,p. 49–51.

Losada-Calderon, A.J., and McPhail, D.C., 1996, Porphyry and high-sulfida-tion epithermal mineralization in the Nevados del Famatina mining dis-trict, Argentina: Society of Economic Geologists Special Publication no. 5,p. 91–118.

Lynch, J.V.G., Longstaffe, F.J., and Nesbitt, B.E., 1990, Stable isotopic andfluid inclusion indications of large-scale hydrothermal paleoflow, boiling,and fluid mixing in the Keno Hill Ag-Pb-Zn district, Yukon Territory,Canada: Geochimica et Cosmochimica Acta, v. 54, p. 1045–1059.

Maleterre, P., 1989, Histoire, sedimentation, magmatique, tectonique et me-tallogenique d’un arc oceanique deforme en regime de transpression: Un-published Ph.D. thesis, Brest, France, Universite de Bretagne Occidentale,304 p.

Masterman, G.J., Cooke, D.R., Berry, R.F., Walshe, J.L., Lee, A.W., andClark, A.H., 2005, Fluid chemistry, structural setting, and emplacementhistory of the Rosario Cu-Mo porphyry and Cu-Ag-Au epithermal veins,Collahuasi district, northern Chile: ECONOMIC GEOLOGY, v. 100, p.835–862.

Matsuhisa, Y., Goldsmith, J.R., and Clayton, R.N., 1979, Oxygen isotope frac-tionation in the systems quartz-albite-anorthite-water: Geochimica et Cos-mochimica Acta, v. 43, p. 1131–1140.

McCrea, J.M., 1950, On the isotopic chemistry of carbonates and palaeotem-perature scale: Journal of Chemical Physics, v. 18, p, 849–857.

McDermott, F., and Hawkesworth, C.J., 1991, Th, Pb and Sr variations inyoung island arc volcanics and oceanic sediments: Earth and Planetary Sci-ence Letters, v. 104, p. 1-15.

Mitchell, A.H.G., and Leach, T.M., 1991, Epithermal gold in the Philip-pines—island arc metallogenesis, geothermal systems and geology: Lon-don, Academic Press Geology Series, 457 p.

Muntean, J.L., and Einaudi, M.T., 2001, Porphyry–epithermal transition:Maricunga belt, northern Chile: ECONOMIC GEOLOGY, v. 96, p. 743–772.

Ohmoto, H., and Rye, R.O., 1979, Isotopes of sulfur and carbon, in Barnes,H.L., ed., Geochemistry of hydrothermal ore deposits, 2nd edition: NewYork, Wiley, p. 491–559.

Peña, R., 1970, Brief geology of a portion of the Baguio mineral district: Jour-nal of the Geological Society of the Philippines, v. 23, p. 42–43.

——1998, Further notes on the stratigraphy of the Baguio district: Journal ofthe Geological Society of the Philippines, v. 3–4, p. 141–157.

Pudack, C., Halter, W.E., Heinrich, C.A., and Pettke, T., 2009, Evolution ofmagmatic vapor to gold-rich epithermal liquid: The porphyry to epithermaltransition at Nevados de Famatina, northwest Argentina: ECONOMIC GE-OLOGY, v. 104, p. 449−477.

Rae, A.J., Cooke, D.R., and Brown, K.L., 2011, The trace metal chemistry ofdeep geothermal water, Palinpinon geothermal field, Negros island, Philip-pines: Implications for precious metal deposition in epithermal gold de-posits: ECONOMIC GEOLOGY, v. 106, p. 1425–1446.

Reed, M.H., and Spycher, N.F., 1985, Boiling, cooling and oxidation in ep-ithermal systems: a numerical approach: Reviews in Economic Geology, v.2, p. 249–272.

Rice, C.M., McCoyd, R.J., Boyce, A.J., and Marchev, P., 2007, Stable isotopestudy of the mineralization and alteration in the Madjarovo Pb-Zn district,south-east Bulgaria: Mineralium Deposita, v. 42, 691–713.

Rimstidt, 1997, Gangue mineral transport and deposition: in Barnes, H.L.(ed.), Geochemistry of Hydrothermal Ore Deposits, 3rd edition: New York,Wiley, p. 487–515.

Robinson, B.W, and Kusakabe, M., 1975, Quantitative preparation of sulfurdioxide, for 34S/32S analyses, from sulfides by combustion with cuprousoxide: Analytical Chemistry, v. 47, p. 1179–1181.

Rye, R.O., 1993, The evolution of magmatic fluids in the epithermal envi-ronment: The stable isotope perspective: ECONOMIC GEOLOGY, v. 88, p.733–752.

Sajona, F.G., Izawa, E., Motomura, Y., Imai, A., Sakakibara, H., and Watan-abe, K., 2002, Victoria carbonate–base metal gold deposit and its signifi-cance in the Mankayan mineral district, Luzon, Philippines: Resource Ge-ology, v. 52, p. 325–328.

Sawkins, F.J., O’Neill, J.R. and Thompson, J.M., 1979, Fluid inclusion andgeochemical studies of vein gold deposits, Baguio district, Philippines:ECONOMIC GEOLOGY, v. 74, p. 1420–1434.

Schaffer, P.A., 1954, Progress report on the geology of the Baguio district,Philippines: Philippines Society of Mining, Metallurgical, and GeologicalEngineers Mining Newsletter, v. 5, p. 5.

Seward, T.M., 1989, The hydrothermal chemistry of gold and its implicationsfor ore formation: Boiling and conductive cooling as examples: EconomicGeology Monograph 6, p. 398–404.

Shannon, J.R., 1979, Igneous petrology, geochemistry and fission trackages of a portion of the Baguio district, Northern Luzon, Philippines:Unpublished M.Sc. thesis, Golden, Colorado, Colorado School of Mines,173 p.


0361-0128/98/000/000-00 $6.00 1423

Sillitoe, R.H., 1987, Copper, gold and subduction: a trans-Pacific perspective:Pacific Rim Congress, Gold Coast, Australia, 1987, Proceedings: Mel-bourne, Australasian Institute of Mining and Metallurgy, p. 399–403.

——1989, Gold deposits in Western Pacific island arcs: The magmatic con-nection: Economic Geology Monograph 6, p. 274–291.

——2010, Porphyry copper systems: ECONOMIC GEOLOGY, v. 105, p. 3–41.Sillitoe, R.H., and Hedenquist, J.W., 2003, Linkages between volcanotec-

tonic settings, ore-fluid compositions, and epithermal precious metal de-posits: Society of Economic Geologists Special Publication 10, p. 315−343.

Simmons, S.F., Sawkins, F.J. and Schlutter, D.J., 1987, Mantle-derived he-lium in two Peruvian hydrothermal ore deposits: Nature, v. 329, p.429–432.

Simmons, S.F., White, N.C., and John, D.A., 2005, Geological characteristicsof epithermal precious and base metal deposits: ECONOMIC GEOLOGY100TH ANNIVERSARY VOLUME, p. 485–522.

Smith, W.D. and Eddingfield, F.T., 1911, Additional notes on the economicgeology of the Baguio district: Philippine Journal of Science, v. 6, p.429–445.

Spycher, N.F. and Reed, M.H., 1989, Evolution of a Broadlands-type ep-ithermal ore fluid along alternative P-T paths: Implications for the trans-port and deposition of base, precious and volatile metals: ECONOMIC GE-OLOGY, v. 84, p. 328–359.

Swart, P.K., Burns, S.J. and Leder, J.J., 1991, Fractionation of the stable iso-topes of oxygen and carbon in carbon dioxide during the reaction of calcitewith phosphoric acid as a function of temperature and technique: Chemi-cal Geology, v. 86, p. 89–96.

Sweet, G., Waters, P., Baker, M., Hollings, P., and Cooke, D.R., 2008, TheBlack Mountain porphyry Cu-Au deposit, Baguio, Philippines: AustralasianInstitute of Mining and Metallurgy, PACRIM Congress 2008, The PacificRim: Mineral Endowment, Discoveries & Exploration Frontiers, p.451–456.

Taylor, B.E., 1992, Degassing of H2O from rhyolite magma during eruptionand shallow intrusion, and the isotopic composition of magmatic water inhydrothermal systems: Japan Geological Survey Report, v. 279, p. 190–194.

Taylor, H.P., Jr., and Epstein, S., 1962, Relationships between 18O/16O ratiosin coexisting minerals of igneous and metamorphic rocks, Part 1: Geologi-cal Society of America Bulletin, v. 73, p. 461–480.

Truesdell, A.H., Nathenson, M., and Rye, R.O., 1977, The effects of sub-sur-face boiling and dilution on the isotopic composition of Yellowstone ther-mal waters: Journal of Geophysical Research, v. 82, p. 3694–3703.

United Nations Development Programme (UNDP), 1987, Geology and min-eralization in the Baguio area, northern Luzon: United Nations Develop-ment Programme, Bureau of Mines and Geo-Sciences, Department of En-vironment and Natural Resources, Technical Report no. 5, 82 p.

Wallier, S., Rey, R., Kouzmanov, K., Pettke, T., Heinrich, C.H., Leary, S., O’Connor, G., Tamas, C.G., Vennemann, T. and Ullrich, T., 2006, Mag-matic fluids in the breccia-hosted epithermal Au-Ag deposit of Rosia Mon-tana, Romania: ECONOMIC GEOLOGY, v. 101, p. 923–954.

Walters, L.J., Claypool, G.E., and Choquette, P.W., 1972, Reaction rates andδ18O variation for the carbonate-phosphoric acid preparation method:Geochimica Cosmochimica Acta, v. 36, p. 129–140.

Waters, P.J., Cooke, D.R., Gonzales, R.I., and Phillips, D., 2011, Porphyryand epithermal deposits and 40Ar/39Ar geochronology of the Baguio district,Philippines: ECONOMIC GEOLOGY, v. 106, p. 1335–1363.

Wilson, A.J., Cooke, D.R., Harper, B.J., and Deyell, C.L., 2007, Sulfur iso-topic zonation in the Cadia district, southeastern Australia: exploration sig-nificance and implication for the genesis of alkalic porphyry gold–copperdeposits: Mineralium Deposita, v. 42, p. 465–487.

Wolfe, R.C., and Cooke, D.R., 2011, Geology of the Didipio region and gen-esis of the Dinkidi alkalic porphyry Cu-Au deposit and related pegmatites,northern Luzon, Philippines: ECONOMIC GEOLOGY, v. 106, p. 1279–1315.

Worley, B.W., Jr., 1967, Gold mines of Benguet Consolidated, Inc.: MineralEngineering Magazine, v. 18, p. 12–22.

1424 COOKE ET AL.

0361-0128/98/000/000-00 $6.00 1424

1399

Documents

santo tomas

initial strontium

major tectonic

advanced argillic

sulfur isotope

fluid inclusion

radiometric

intermediate