0361-0128/11/3994/1335-29 1335

IntroductionTHE BAGUIO DISTRICT of Northern Luzon, Philippines, is oneof the world’s premier mineral provinces. It has been themost prolific gold-producing district in the Philippines interms of economic value and abundance and diversity of hy-drothermal ore deposits. Current resources are estimated tobe approximately 12 Moz of gold and 1.2 Mt of copper withcombined historical production and remaining resources ofover 35 Moz of gold and 2.7 Moz of copper. This metal en-dowment is contained in a variety of mineral deposit types, in-cluding giant epithermal gold-base metal veins (e.g., Anta-mok: Sawkins et al., 1979; Acupan: Cooke et al., 1996) andlarge copper-gold porphyry deposits (e.g., Santo Tomas II,Imai: 2001). Proximal copper-gold, distal gold-base metal skarnand sediment-hosted gold deposits also occur (e.g., Thanks-giving Au-Zn skarn: Callow, 1967), along with an unusual style

of sediment-hosted copper mineralization (Wildcat: Dam-asco, 1979). Mine closures in the 1990s led to a significant re-duction of production, with Santo Tomas II currently the onlyoperating mine in the district, apart from small-scale miners.

Although Baguio is renowned for its gold-rich epithermalveins, the porphyry potential of the district has remained un-derevaluated, despite a protracted mining history. Explo-ration by AngloAmerican Exploration Philippines Ltd. in thelate 1990s led to several new discoveries of porphyry andskarn deposits within the Baguio district, including the high-grade Nugget Hill porphyry Cu-Au deposit, Chico porphyryCu-Au prospect, and Mexico Cu-Au skarn.

This paper provides an update of the geology and mineraldeposits of the Baguio district, with an emphasis on the por-phyry systems. Despite its wealth of mineral resources, variousaspects of the geology of the Baguio district remain poorly un-derstood. We have therefore remapped the district in order toresolve some long-standing issues with regards to its geology.

Porphyry and Epithermal Deposits and 40Ar/39Ar Geochronology of the Baguio District, Philippines

PATRICK J. WATERS,1 DAVID R. COOKE,2,† RENE I. GONZALES,1 AND DAVID PHILLIPS3

1 Anglo American Exploration Philippines Inc., 1101 Pearlbank Centre Building, 146 Valero Street, Salcedo Village, Makati City, Metro Manila, Philippines

2 CODES, the Australian Research Council’s Centre of Excellence in Ore Deposits, University of Tasmania, Private Bag 126, Hobart, Tasmania, 7001, Australia

3 Geology Department, University of Melbourne, Australia

AbstractThe Baguio district is located in the Central Cordillera of northern Luzon, Philippines. It contains numer-

ous mineralized porphyry copper-gold, epithermal gold-silver and skarn gold-lead-zinc deposits. The district isfloored by Cretaceous-Eocene metavolcanic and metasedimentary rocks, which are overlain by marine to ter-restrial sedimentary and volcanic rocks of early Miocene to Pliocene ages. Tertiary arc magmatism related toeast-directed subduction of the South China Sea plate along the Manila Trench beneath northern Luzon pro-duced a major batholith, the Central Cordillera Intrusive Complex, which defines the eastern boundary of theBaguio district. The intrusive complex was emplaced in the early Miocene based on 40Ar/39Ar age determina-tions for the Lucbuban gabbro (22.6 ± 0.5 Ma) and Virac granodiorite (20.23 ± 0.38 and 20.2 ± 0.7 Ma).

Recent exploration has led to the discovery of several new porphyry copper-gold and skarn prospects in thewestern Baguio district. Mineralization was preceded by the intrusion of a suite of hornblende megacrystic andesite dikes in the central part of the district from 4.55 ± 0.15 to 3.45 ± 0.19 Ma. Porphyry copper-gold andskarn deposits at Black Mountain and Mexico, on the western side of the district, formed between 3.09 ± 0.15and 2.81 ± 0.24 Ma. The Santo Tomas II Cu-Au-(Pd) porphyry deposit was emplaced into the central south-ern part of the district at about 1.5 Ma, based on secondary biotite 40Ar/39Ar ages of 1.48 ± 0.05 Ma and 1.47± 0.05 Ma. On the eastern side of the district, porphyry-style mineralization at the Hartwell and Ampucao por-phyry Cu-Au prospects returned 40Ar/39Ar ages of 1.09 ± 0.10 and 0.51 ± 0.26 Ma, respectively.

Quartz-carbonate-base metal sulfide style epithermal gold-silver veins are well-developed on the easternside of the Baguio district, and crosscut porphyry-style mineralization at the Acupan, Baguio gold and NuggetHill deposits. Epithermal veining is inferred to have occurred in the past million years, based on a reported K-Ar age determination of 0.65 ± 0.07 Ma from the Acupan gold mine. A large advanced argillic alteration zone(the Baguio lithocap) crops out in the northwest part of the district, and formed between 1.4 and 0.9 Ma, basedon previous K-Ar dating.

Pliocene-Pleistocene mineralization in the Baguio district was triggered by the east-directed subduction ofthe Scarborough Ridge. Ridge subduction caused the subduction angle along the Manila Trench beneathnorthern Luzon to decrease, which facilitated crustal thickening and exhumation. Ridge subduction also pro-moted the development of northwest-trending faults in the upper plate that interacted with major arc-parallelnorth- and arc-normal northeast-trending fault sets associated with the Philippine fault system. Fertile magmaswere emplaced into transtensional strike-slip relay basins, resulting in the accumulation of more than 35 Mozof gold and several million tonnes of copper over approximately 3 m.y.

† Corresponding author: e-mail, d.cooke@utas.edu.au

©2011 Society of Economic Geologists, Inc.Economic Geology, v. 106, pp. 1335–1363

We also provide results from 18 new 40Ar/39Ar age determi-nations and demonstrate that intrusive complexes emplacedinto the district over the last 3 m.y. were intimately related toporphyry Cu-Au and epithermal Au-Ag mineralization.

Tectonic SettingThe Baguio district is located on the island of Luzon, at the

southern end of the Luzon Central Cordillera. It is part ofthe magmatic arc associated with present-day eastward-di-rected subduction of the South China Sea plate beneath thePhilippines and Taiwan (Fig. 1). The region remains seismi-cally active.

The Plio-Pleistocene arc is underlain by Tertiary basement.There are two competing models for the Tertiary evolution ofNorthern Luzon. Several workers infer that northern Luzonexperienced Tertiary arc-polarity reversal (e.g., Wolfe, 1981;Maleterre, 1989; Solomon, 1990; Florendo, 1994; Bellon andYumul, 2000; Yumul et al., 2003), and that this was an impor-tant element in porphyry copper metallogeny (Solomon,1990). Others have argued that Tertiary arc reversal did notoccur (e.g., Bautista et al., 2001; Queano et al., 2007; Hollingset al., 2011a).What is clear is that the Eocene-Oligocene pe-riod was characterized by marginal marine basin sedimenta-tion in northwestern Luzon, and arc-type magmatism in east-ern Luzon. Initiation of east-directed subduction of the SouthChina Sea plate probably occurred due to the northeastward

propagation of the North Borneo-Manila Trench, which wasactive at its southwest end by approximately 40 Ma (Hall,2002). It appears that this trench reached Luzon during thelate Oligocene to early Miocene, and some proponents of thearc reversal model argue that its arrival would have extin-guished west-directed subduction and triggered arc reversal.Conversely, Hollings et al. (2011a) argue that slab rollback inthe late Oligocene caused the migration of the magmatic arcfrom eastern to western Luzon, and synchronous opening ofthe Cagayan back-arc basin during a protracted history ofeast-directed subduction.

Sea-floor spreading in the South China Sea slowed by theearly Miocene and gave way to central vent eruptions of al-kali basalts on the spreading ridge, forming large seamounts(Briais et al., 1993). The extinct spreading axis defines a lin-ear bathymetric swell, along which are distributed alkali-basalt seamounts which are 30 to 40 km in diameter and havea relief of 1,500 to 3,000 m, constituting the ScarboroughRidge (Briais et al., 1993; Fig. 1). This ridge is currentlybeing subducted eastward beneath northern Luzon. Overthe past 4 m.y., the intersection of the Scarborough Ridgewith the Manila Trench has propagated southward. A con-comitant warp of the subducting slab has also migratedsouthward under the Luzon Central Cordillera, as evidencedby the eastward flaring of slab-depth contours (Bautista etal., 2001). Above the shoaling slab, the Luzon CentralCordillera has undergone rapid Pliocene-Quaternary upliftwhich postdated deposition of the early to mid-Miocenelimestones of the Kennon Formation. Porphyry copper-golddeposits that formed between ~3.5 and 1.0 Ma have been ex-humed from depths of 3 to 1 km and eroded since their em-placement (Cooke and Bloom, 1990; Cooke and Berry,1996). Uplift of the Luzon Central Cordillera has been ac-companied by a virtual cessation of volcanism in the last 0.5m.y., following a 3 m.y. period of discrete eruptions and hy-pabyssal intrusions of highly differentiated andesites anddacites that were extraordinarily fertile for copper and goldmineralization.

The Luzon Central Cordillera is cut by a series of north-,northwest-, and northeast-trending faults, some of which re-late to the Philippine fault system. This major, north-north-west–trending, left-lateral strike-slip fault system transectsthe Philippine archipelago (Fig. 1) but has been re-orientedinto an arc-parallel north-trending fault system after it passesthrough a restraining bend at the southern end of the CentralCordillera, immediately south of the intersection with thesubducting Scarborough Ridge (Fig. 1). The Baguio district issituated just to the north of the major bend in the fault sys-tem, and at least some of the extreme uplift and exhumationrates in this region may relate to transpressional uplift withinthis restraining bend (Cooke and Berry, 1996).

StratigraphyThe Baguio district is part of a volcanosedimentary fold

belt that defines the 300-km-long, 50-km-wide, and up to2,900-m-high northerly trending Central Cordillera mountainrange (Figs. 2–4a). The district has evolved from an Eocene-Oligocene marginal basin through a Miocene magmatic arc toa Pliocene to Pleistocene fold belt, and this evolution is re-flected in the rock units exposed within it.

1336 WATERS ET AL.

0361-0128/98/000/000-00 $6.00 1336

200 km

EurasianPlate

PhilippinePlate

20°N

16°N

8°N

12°N

122°E118°E 126°E

N

Lepanto - Far SouthEast (1.5 Ma )

Baguio district(3.5 - 1 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. Active vol-canoes are denoted by black triangles. The approximate position of the Scar-borough Ridge has been interpreted from bathymetry data provided by theNational Geophysical Data Centre (unpub. data, 2004). Diagram modifiedfrom Cooke et al. (2005).

The stratigraphy of the Baguio district is summarized inTable 1. It has been the subject of significant debate since thework of Pena and Reyes (1970). Subsequent refinementswere suggested by Balce et al. (1980), de los Santos (1982)and Maleterre (1989). The United Nations DevelopmentProgram (1987) and Mitchell and Leach (1991) reinterpretedthe stratigraphy of the district, introducing some controver-sial new nomenclature. The stratigraphic column presentedin Figure 2 is a hybrid between that erected by Maleterre(1989), who correlated the Baguio district with the Mankayandistrict and the interpretation of Pena (1998). The vol-canosedimentary formations exposed in the Baguio districtare correlative with rock units found elsewhere in the CentralCordillera (Pena, 1998; Fig. 2).

The basal sequence exposed in the Baguio district is theEocene to Oligocene Pugo Formation (Fig. 3; Table 1). Thispackage of mafic to intermediate volcanic and related sedi-mentary rocks has undergone greenschist facies metamor-phism locally, and is interpreted to have been deposited in amarginal marine basin. The Pugo Formation is overlain un-conformably by the Oligocene to middle Miocene Zig-ZagFormation, a shallow marine sequence of conglomerates,sandstones, shales, andesitic lavas and tuffs, and minor lime-stones. Shallow marine sedimentation in the early to middleMiocene produced massive gray biothermal limestone (Ken-non Formation). Limestone deposition preceded middleMiocene arc magmatism in northern Luzon, during whichplutons and batholiths of the Central Cordillera Diorite Com-plex were emplaced (Tables 1, 2). Mountain building during

the middle Miocene led to the deposition of the KlondykeFormation, a thick sequence of terrestrial to shallow marinepolymict conglomerates with interbedded finer grained sedi-mentary and volcaniclastic rocks. Shallow marine sedimenta-tion in the Miocene to Pliocene resulted in deposition of theMirador Limestone.

Deformation, uplift, and magmatism during the Plioceneand Pleistocene resulted in deposition of the Baguio Forma-tion, a poorly exposed sequence of tuff, tuff breccia, andesiteflows, volcanic breccia, conglomerates, and minor sandstonesthat crops out at high altitudes on the western flank of theBaguio district (Fig. 3). Dating of andesitic breccia yielded anage of 3.57 Ma (Maleterre, 1989), and an andesitic pyroclas-tic rock from Mt Santo Tomas yielded a 40Ar/39Ar plateau ageof 3.594 ± 0.070 Ma (Table 2). To the southeast of BaguioCity, andesite of the Baguio Formation is overlain by pyro-clastics and conglomerates. These rocks are extensively al-tered to stratabound domains of silicic, advanced argillic(quartz-alunite, pyrophyllite) and argillic alteration assem-blages, which together define the Baguio “lithocap” (Fig. 3;terminology of Sillitoe, 1995). Deposition of the Baguio For-mation in the northwestern part of the Baguio district wasbroadly synchronous with porphyry copper-gold ore forma-tion and related intrusive activity in the central and southernBaguio district (Fig. 3; Table 3).

StructureThe Philippine fault zone in the Luzon Central Cordillera

is characterized by a complex braided system of sinistral strike

40Ar/39Ar GEOCHRONOLOGY OF PORPHYRY-EPITHERMAL DEPOSITS IN THE BAGUIO DISTRICT, PHILIPPINES 1337

0361-0128/98/000/000-00 $6.00 1337

Zig-ZagFormation3000 m

Balili Sequence2,000 m

Apaoan Sequence300 m~

Imbanguila dacite00 m7

Malaya Formation00 m7

Klondyke Formation00 m1,2

Klondyke Formation00 m3,5

Kennon Formation0 m24

Pugo Formationm1,100 - 1,600

LepantoMetavolcanics

Mirador Limestone00 m3

Baguio Formation00 m4

Sagada Limestone00 m3

Malitep Formation00 m2,5

Ophiolitic substratum

Lapangan Tuff0 m4

3: Mankayan 4: Baguio2: Cervantes1: Bontoc

Zig-Zag Formation1,500 m

Basement complex

Maximumthickness CompositionStratigraphy

Klondyke Formation

Kennon and Segada Formation

Pugo Formation

Mirador Limestone

Lapangan Tuff

Baguio and Malaya Formations

Zig-Zag Formation

3,500 m

300 m

1,600 m

300 m

40 m

1,500 m

3,000 m

Andesitic lahars, polymict conglomerates, wackes, lenses of limestone

Reefal coralline limestones

Regionally metamorphosed basalt, andesitic lavas, tuffs and siltstones

Coralline limestone

Dacitic to andesitic tuffs

Andesitic to dacitic tuffs and breccias, sandstones, shales, limestone

Upper Zig-Zag:Andesitic to basaltic conglomerate, wackes and limestones

Basement complex - schists and amphibolite, sheeted dikes

Lower Zig-Zag: Interbedded sandstones, wackes, siltstones and limestones

12

3

4

Section Locations

Upper

Lower

FIG. 2. Stratigraphy of the Luzon Central Cordillera, highlighting correlations and thickness variations of the major strati-graphic units. Modified from Maleterre (1989), Pena (1998) and Chang et al., this volume. Inset: locality map – NorthernLuzon, Philippines, showing the districts used to compile the four stratigraphic sections. 1 = Bontoc, 2 – Cervantes, 3 =Mamkayan, 4 – Baguio district.

1338 WATERS ET AL.

0361-0128/98/000/000-00 $6.00 1338

FIG. 3. Geologic map of the Baguio district, based on new mapping by R.I. Gonzales. 40Ar/39Ar sample locations are shownas black filled circles.

40Ar/39Ar GEOCHRONOLOGY OF PORPHYRY-EPITHERMAL DEPOSITS IN THE BAGUIO DISTRICT, PHILIPPINES 1339

0361-0128/98/000/000-00 $6.00 1339

A

C

E

2 cm

G

B

D

F

H

2 cm

FIG. 4. Styles of mineralization in the Baguio district. A) View east from the Mexico skarn prospect to the Nugget Hillporphyry copper-gold prospect. The Acupan gold veins crop out on the ridge line behind Nugget Hill. B) Intense quartz-an-hydrite-sulfide vein stockwork in biotite-orthoclase altered and iron-stained early-mineralization quartz diorite porphyry,Southeast orebody, Black Mountain porphyry copper-gold mine. C) Contact between intensely veined early-mineralizationquartz diorite porphyry and less intensely veined but strongly biotite-altered intra-mineralization quartz diorite porphyry,Southeast orebody, Black Mountain porphyry copper-gold mine. D) Diopside-wollastonite-garnet-calcite skarn, Mexico cop-per-gold prospect. E) Laminated quartz-magnetite-chalcopyrite-bornite-anhydrite veins, Santo Tomas II porphyry Cu-Au-(Pd) mine. F) Quartz-bornite-chalcopyrite stockwork in magnetite-chlorite-altered Zig-Zag Formation andesite, Nugget Hillporphyry Cu-Au prospect. G) Underground exposure (looking overhead) of epithermal quartz-rhodochrosite-sphalerite-galena-gold vein, Acupan gold mine (252 vein, 2000 level). G) Underground exposure of anhydrite-quartz-cemented ep-ithermal vein breccia, with large angular clasts of sericite-quartz-pyrite-altered Virac granodiorite, Acupan gold mine (422vein, 2000 level).

1340 WATERS ET AL.

0361-0128/98/000/000-00 $6.00 1340

TAB

LE

1. B

agui

o D

istr

ict S

trat

igra

phy,

Bas

ed o

n th

e O

bser

vatio

ns a

nd I

nter

pret

atio

ns o

f Pre

viou

s W

orke

rs C

ombi

ned

with

the

Res

ults

of O

ur N

ew M

appi

ng

Thi

ckne

ss

Dis

trib

utio

n an

d F

orm

atio

nR

ock

type

s(m

)co

ntac

t rel

atio

nshi

psA

geC

orre

latio

nsIn

terp

reta

tion

Ref

eren

ces

Bag

uio

Tuff

, tuf

f bre

ccia

, 20

0–40

0C

rops

out

at h

igh

altit

udes

A

ndes

itic

brec

cia:

M

alay

a F

orm

atio

n Su

baer

ial a

ndes

itic

Smith

and

F

orm

atio

nan

desi

te fl

ows,

vol

cani

c ar

ound

Bag

uio

city

on

the

3.57

Ma

(Mal

ater

re,

(Bon

toc-

Cer

vant

es

volc

anis

m a

nd a

ssoc

iate

d E

ddin

gfie

ld

brec

cia,

con

glom

erat

es

nort

hwes

t sid

e of

the

dist

rict

; 19

89),

ande

sitic

ar

ea);

Imba

ngui

la

volc

anic

last

ic s

edim

enta

tion

(191

1); d

e lo

s an

d m

inor

san

dsto

nes

part

ly o

bscu

red

by a

larg

e (6

-br

ecci

a: 3

.594

±

daci

te a

nd p

yroc

last

ics

Sant

os (

1982

); km

-long

) do

mai

n of

inte

nse

0.07

0 M

a (T

able

2)

(Man

kaya

n di

stri

ct)

Mal

eter

re (

1989

); si

licifi

catio

n, a

dvan

ced

Pena

(19

98 )

argi

llic

and

argi

llic

alte

ratio

n kn

own

as th

e B

agui

o “l

ithoc

ap”

Mir

ador

M

assi

ve li

mes

tone

300

Con

tact

rel

atio

nshi

ps w

ith

Mio

cene

–Plio

cene

M

ay b

e a

loca

l cal

care

ous

Lei

th (

1938

); L

imes

tone

over

lyin

g B

agui

o F

orm

atio

n (f

auna

l age

)fa

cies

with

in th

e B

agui

o M

alet

erre

(19

89);

unce

rtai

n; m

ay b

e a

calc

areo

us

For

mat

ion;

loca

l sha

llow

Ta

n (1

995)

; fa

cies

with

in th

at fo

rmat

ion,

or

mar

ine

sett

ing?

Pena

(19

98)

may

hav

e an

unc

onfo

rmab

le

cont

act r

elat

ions

hip

with

it

Klo

ndyk

e Po

lym

ict c

ongl

omer

ates

1,

800–

Cro

ps o

ut e

xten

sive

ly o

n M

iddl

e to

late

A

bund

ant c

obbl

es a

nd

Dur

kee

and

For

mat

ion

with

inte

rbed

ded

3,50

0th

e w

este

rn s

ide

of th

e M

ioce

ne m

icro

foss

il bo

ulde

rs o

f int

rusi

ve r

ocks

Pe

ders

en (

1961

); sa

ndst

ones

, silt

ston

es,

Bag

uio

dist

rict

; wel

l-exp

osed

as

sem

blag

es; a

dac

ite

deri

ved

from

the

Cen

tral

D

e L

eon

et a

l. sh

ales

and

loca

lized

on

the

flank

s of

Mt S

anto

bo

ulde

r yi

elde

d an

C

ordi

llera

n D

iori

te C

ompl

ex;

(199

1); P

ena

volc

anic

flow

s, p

yro-

To

mas

; ove

rlai

n, a

ppar

ently

ag

e of

20

± 1.

0 M

a ve

ry h

igh

ener

gy, f

luvi

atile

(1

998)

; Bel

lon

clas

tics,

limes

tone

and

co

ncor

dant

ly, b

y th

e (B

ello

n an

d Yu

mul

, de

posi

tiona

l env

iron

men

t, an

d Yu

mul

(20

00)

calc

irud

ites

beds

Mir

ador

Lim

esto

ne20

00)

prox

imal

to th

e up

lifte

d C

entr

al C

ordi

llera

Cen

tral

K

adan

g tr

ondh

jem

iteN

ot

Has

intr

uded

the

Pugo

, L

ucbu

ban

gabb

ro:

Def

ines

the

axis

of t

he

Dee

ply

erod

ed p

luto

ns

Shan

non

(197

9);

Cor

dille

ra

appl

icab

leZi

g-Za

g an

d K

enno

n 22

.6 ±

0.5

Ma

Cen

tral

Cor

dille

ra o

f an

d ba

thol

iths

that

B

alce

et a

l. (1

980)

;D

iori

te

Itog

on q

uart

z di

orite

For

mat

ions

(Tab

le 2

)N

orth

ern

Luz

onfo

rmed

a m

iddl

e M

alet

erre

(19

89);

Com

plex

A

ntam

ok d

iori

teM

ioce

ne m

agm

atic

arc

Coo

ke a

nd

Vir

ac g

rano

dior

ite:

Blo

om (

1990

); (A

gno

Lia

ng g

abbr

o20

.23

± 0.

38, 2

0.2

Pena

(19

98);

Bat

holit

h)L

ucbu

ban

gabb

ro±

0.7

Ma

(Tab

le 2

)B

ello

n an

d Yu

mul

(20

00);

Vir

ac g

rano

dior

iteH

ollin

gs e

t al.

(201

1a)

Ken

non

Mas

sive

, gra

y bi

o-19

0–24

0O

verl

ain

unco

nfor

mab

ly b

y F

auna

l age

s of

ear

ly

Rec

ogni

zed

in th

e D

epos

ited

in a

sha

llow

M

alet

erre

(19

89);

For

mat

ion

ther

mal

lim

esto

ne w

ith

the

Klo

ndyk

e F

orm

atio

nto

mid

dle

Mio

cene

Cer

vant

es-B

onto

c m

arin

e (r

eefa

l) se

ttin

g Ta

n (1

995)

; as

soci

ated

cal

care

nite

s an

d Il

loco

s ar

eas

to

duri

ng a

tect

onic

ally

Pe

na (

1998

)an

d ca

lcir

udite

sth

e no

rth

quie

scen

t per

iod

prio

r to

m

iddl

e M

ioce

ne u

plift

Zig-

Zag

Con

glom

erat

es, s

and-

1,70

0–O

verl

ain

unco

nfor

mab

ly b

y F

auna

l age

s of

late

C

ontin

ues

nort

hwar

d Sh

allo

w m

arin

e se

ttin

gPe

na a

nd

For

mat

ion

ston

es, s

hale

s, a

ndes

itic

1,80

0th

e K

enno

n L

imes

tone

Olig

ocen

e to

mid

dle

to th

e w

este

rn p

art o

f R

eyes

(19

70);

lava

s an

d tu

ffs,

and

M

ioce

ne

the

Bon

toc-

Cer

vant

es

Mal

eter

re (

1989

); m

inor

lim

esto

nes

area

Pena

(19

98)

Pugo

B

asal

tic a

nd a

ndes

itic

1,10

0–U

nder

lies

muc

h of

the

east

ern

Cre

tace

ous

to E

ocen

eL

epan

to M

etav

olca

nics

D

epos

ition

in a

sub

duct

ion

Scha

ffer

(19

54);

For

mat

ion

volc

anic

roc

ks w

ith m

inor

1,

600

part

of t

he d

istr

ict;

intr

uded

by

(Man

kaya

n di

stri

ct);

rela

ted-

mar

gina

l mar

ine

Pena

(19

98);

inte

rbed

ded

sand

ston

es,

the

Cen

tral

Cor

dille

ra D

iori

te

Mal

itep

For

mat

ion

basi

n; in

terp

rete

d to

be

the

Mal

eter

re (

1989

)ar

gilli

tes,

che

rt, a

nd m

inor

C

ompl

ex, o

verl

ain

unco

nfor

mab

ly

(Cer

vant

es-B

onto

c ar

ea)

sedi

men

tary

cov

er o

f an

pyro

clas

tic r

ocks

by th

e Zi

g-Za

g F

orm

atio

nop

hiol

ite

slip faults that facilitated the development of a series oftranstensional strike-slip relay basins in releasing bends, andareas of transpressional uplift in restraining bends. The cen-tral part of the Baguio district comprises steeply dissectedtopography of 900 to 1,500 m elevation (Fig. 4a) surroundedby mountains of up to 2,200 m elevation. The Baguio districtis therefore interpreted to occupy a dilational fault jog due tothe interaction of two segmented left-lateral sinistral shearsplays of the Philippine fault zone known as the Tuba andTebbo faults (Maleterre, 1989; Fig. 3). Pliocene-Pleistocenemagmatism and copper and gold mineralization has been lo-calized within this relay basin.

Folds with north-northwest– to nothwest-trending axesoccur in marine clastic and carbonate rocks in the westernhalf of the district, including a major syncline defined byreefal limestones of the Kennon Formation (Fig. 3). Majordislocations include prominent east-northeast– to northeast-trending extensional faults, including the Bued River fault(Fig. 3), which form distinctive topographic features that areclearly visible on aeromagnetic imagery. West-northwest– tonorthwest-trending faults are interpreted to be sinistral (An-gloAmerican Pty. Ltd., unpub. data). North-northwest–ori-ented faults are inferred to be reverse faults and are probablylocal splays of the Philippine fault system. Air photo interpre-tation suggests easterly dipping fault planes on portions of thenorth-northwest–trending fault zone are indicative of west-erly vergence and a dominantly reverse sense of movement orthrusting.

There is a marked parallelism between the Bued River faulttrace and a series of high-level porphyry stocks, includingthose at Camp 4 and Ampucao (Fig. 3). The Albian fault cor-relates with the Santo Tomas II and Bumolo Cu-Au porphyrydeposits (Fig. 3). The Nugget Hill Cu-Au porphyry and

Mexico skarn prospects appear to be located within dilationalrhombic structures bounded by tensional northeast- andnorthwest-trending fault sets. Zones of closely spaced east-northeast– to northeast-trending faults host epithermalquartz-gold-base metal sulfide veins and clay gouge at theAcupan gold mine (Fig. 3). These structures appear to extendfrom, and are on the same trend as, the Bued River faultzone. In contrast the northwest-trending Antamok vein sys-tem (Fernandez et al., 1979) appears to have been emplacedalong splays from the Tebbo fault (Fig. 3).

A stress field with northeast-oriented principal compres-sion can explain most of the observed structural features inthe Baguio district. West-verging thrusting was caused byeast-west compressional tectonics during westward motionof North Luzon against the South China Sea Plate. This de-veloped in the late Miocene to Pleistocene. Contemporane-ous sinistral motion on the transcurrent Philippine faultzone in the Pliocene resulted in the development of a trans-pressive stress regime, rapid uplift throughout the LuzonCentral Cordillera and rapid erosion resulting in the super-position of epithermal mineralization onto copper-gold por-phyry deposits in the Baguio district (e.g., Cooke and Berry,1996).

40Ar/39Ar analysesWe have completed 18 new 40Ar/39Ar ages of hornblende

and biotite separates from Miocene and younger intrusiverocks of the Baguio district. Minerals separates were pre-pared from whole-rock samples using standard crushing,sieving, desliming, and magnetic separation methods. Theseparates were hand-picked to greater than 99 percent purityand washed in dilute nitric acid, deionized water, and ace-tone prior to being shipped for irradiation. Mineral separates

40Ar/39Ar GEOCHRONOLOGY OF PORPHYRY-EPITHERMAL DEPOSITS IN THE BAGUIO DISTRICT, PHILIPPINES 1341

0361-0128/98/000/000-00 $6.00 1341

TABLE 2. Summary of Preferred 40Ar-39Ar Plateau, Maximum, and Mean Ages for Amphibole and Biotite Samples from the Baguio District (spectra and analytical details are provided in App. 1)

Apparent age Sample no. Min Locality Rock type Size (#) Pur (%) (Ma) ± 2σ % of 39Ar Type of age

353202 A Bumulo Creek Diorite 60–150 99 1.90 ± 0.20 61.1 Plateau353216 A Jaime Creek Diabase 60–120 98 4.028 ± 0.074 98.5 Plateau353217 A Jaime Creek Diabase 60–120 99 4.55 ± 0.15 69.7 Plateau353218 A Liw-Liw Creek Diabase 36–120 99 4.12 ± 0.22 100.0 Plateau354206 B Santo Tomas II Andesite porphyry 150–200 ~95 1.47 ± 0.05 42.0 Average

maximum354207 B Santo Tomas II Diorite 120–170 99 1.48 ± 0.05 9.0 Maximum354212 A Samoyao Virac granodiorite 85–150 99 20.23 ± 0.38 45.6 Mean354216 A Mt Santo Thomas Diabase 60–120 99 3.594 ± 0.070 69.9 Plateau806101 A Ampucao Inter-mineral diorite 36–85 99 0.51 ± 0.26 2.3 Maximum806102 A Nugget Hill Hornblende andesite porphyry 36–85 99 3.45 ± 0.19 100.0 Plateau806103 A Mexico Hornblende andesite porphyry 36–85 98 2.90 ± 0.15 82.5 Plateau806104 A Mexico Hornblende andesite porphyry 36–85 99 3.09 ± 0.15 63.7 Plateau806106 A Black Mountain Late mineral hornblende andesite 36–85 99 2.81 ± 0.24 65.7 Plateau

porphyry806107 B Black Mountain Intra-mineral quartz-diorite porphyry 36–85 99 2.95 ± 0.05 25.7 Maximum806108 A Black Mountain Pre-mineral hornblende-quartz 36–85 98 2.98 ± 0.30 97.8 Plateau

diorite porphyry814526 A Hartwell Plug Dacite 36–85 99 1.09 ± 0.10 69.2 Plateau814527 A Lucbuban Gabbro 36–85 99 22.6 ± 0.5 25.8 Maximum814528 A Acupan Virac granodiorite 36–85 99 20.2 ± 0.7 100.0 Total gas

Notes: % of 39Ar = fraction of 39Ar released used to calculate the apparent 40Ar-39Ar age; Min = mineral dated, either B for biotite or A for amphibole; Pur(%) = sample purity

1342 WATERS ET AL.

0361-0128/98/000/000-00 $6.00 1342

TAB

LE

3. M

iner

aliz

ed I

ntru

sive

Com

plex

es o

f the

Bag

uio

Dis

tric

t

Gra

des

and

tonn

ages

/ M

ine

/ pro

spec

tW

all r

ocks

assa

y da

ta

Geo

chro

nolo

gyA

ltera

tion

and

min

eral

izat

ion

Flu

id c

hem

istr

yR

efer

ence

s

Bla

ck M

ount

ain

intr

usiv

e co

mpl

ex

Bla

ck M

ount

ain

Pugo

Fm

47 M

t @ 0

.38%

Cu,

Pr

emin

eral

izat

ion

Subc

ircu

lar

min

eral

ized

pip

e di

ps 8

0°E

and

ext

ends

T

h~4

30°C

Bal

ce (

1979

); Si

llito

e (K

enno

n)

Zig-

Zag

Fm

0.35

g/t

Au,

0.0

1%

hb-p

hyri

c di

abas

e di

ke

300

m b

elow

sur

face

P ~2

75 b

ars

and

Gap

pe (

1984

); C

u-A

u po

rphy

ry

Mo

(pre

prod

uctio

n fr

om L

iw-L

iw C

reek

: T

he p

ipe

is d

efin

ed b

y qz

vei

ning

and

ass

ocia

ted

(1.1

km

dep

th)

Uni

ted

Nat

ions

de

posi

tre

serv

e)4.

12 ±

0.2

2 M

a1se

cond

ary

bi a

ltera

tion

Salin

ity ~

42 w

t %

Dev

elop

men

t Pr

emin

eral

izat

ion

qz

Min

eral

izat

ion

cons

ists

mai

nly

of c

p an

d py

with

N

aCl e

quiv

Prog

ram

me

(198

7);

dior

ite p

orph

yry:

m

inor

bn,

mo

and

Ag

Swee

t et a

l. (2

008)

; 2.

98 ±

0.3

0 M

a1Pe

riph

eral

py-

chl-e

pi-g

yp a

ltera

tion

Ang

loA

mer

ican

Ltd

. B

i-alte

red

intr

amin

eral

-L

arge

sel

enite

cry

stal

s oc

cur

at th

e ba

se o

f oxi

datio

n(u

npub

. dat

a)iz

atio

n qz

dio

rite

por

-ph

yry:

2.9

5 ±

0.05

Ma1

Post

min

eral

izat

ion

pl h

b an

desi

te p

orph

yry:

2.

81 ±

0.2

4 M

a1

Bla

ck M

ount

ain

Pugo

Fm

15 M

t @ 0

.37%

Cu,

N

o da

ta a

vaila

ble

Surf

ace

expr

essi

on a

ppro

x. 1

50 m

wid

e an

d 60

0 m

long

, N

o da

ta a

vaila

ble

Bal

ce (

1979

); Si

llito

e (s

outh

east

)Zi

g-Za

g F

m0.

26 g

/t A

u (p

re-

with

a s

imila

r su

lfide

min

eral

ogy

to K

enno

nan

d G

appe

(19

84);

Cu-

Au

porp

hyry

pr

oduc

tion

rese

rve)

Tabu

lar,

200-

m-w

ide

oreb

ody

dips

70°

WU

nite

d N

atio

ns

depo

sit

The

pyr

itic

alte

ratio

n ha

los

to th

e tw

o B

lack

Mou

ntai

n D

evel

opm

ent

oreb

odie

s ha

ve c

oale

sced

into

an

elon

gate

NW

-tre

ndin

g Pr

ogra

mm

e (1

987)

; zo

ne, a

ppro

x. 2

.75

km lo

ng b

y 1

km w

ide

Swee

t et a

l. (2

008)

Tha

nksg

ivin

gK

enno

n F

m

1.1

Mt @

12.

8 g/

t Au

No

data

ava

ilabl

eA

u-B

MS-

rich

ska

rn o

res

wer

e ri

ch in

sp

with

min

or c

p,

No

data

ava

ilabl

eC

allo

w (

1967

); A

u-Zn

ska

rn a

nd

limes

tone

pr

oduc

ed p

rior

to

occu

rrin

g w

ith c

hl-c

c-gt

-cz-

qz a

long

the

limes

tone

-dik

e B

urea

u of

Min

es

mas

sive

sul

fide

roof

19

87co

ntac

t, w

ith o

re g

rade

s ty

pica

lly 1

0% Z

n, 8

g/t

Au

and

Geo

scie

nces

de

posi

tpe

ndan

t M

assi

ve s

ulfid

e or

es e

xten

ded

beyo

nd th

e sk

arns

into

the

(198

6); U

nite

d lim

esto

ne a

nd c

onsi

sted

mai

nly

of s

p, p

y, m

inor

gl,

asp,

N

atio

ns D

evel

op-

mt,

hm, h

ess,

pet

z, s

ylv,

and

alt,

with

avg

gra

des

of

men

t Pro

gram

me

abou

t 30

g/t A

u(1

987)

Rar

e m

inut

e gr

ains

of n

ativ

e A

u in

py

and

in te

lluri

des

Mex

ico

Zig-

Zag

Fm

Up

to 5

% C

u, 6

g/t

Lat

e-m

iner

aliz

atio

n hb

-Sk

arns

con

tain

and

radi

tic g

t, m

t, qz

± d

iop,

wol

l, co

rd,

Coe

xist

ing

aque

ous

Ang

loA

mer

ican

Ltd

. C

u-A

u sk

arn

Ken

non

Fm

Au

in s

urfa

ce r

ock

ande

site

por

phyr

y di

kes

bi, r

ut, s

pn, e

pi, a

nth,

ms,

ap

liqui

d- a

nd v

apor

-(u

npub

. dat

a); t

his

and

porp

hyry

K

lond

yke

Fm

chip

s re

turn

ed a

ges

of 3

.09

± R

etro

grad

e sk

arn

asse

mbl

ages

hav

e be

en c

ut b

y m

assi

ve

rich

flui

d in

clu-

stud

ypr

ospe

ctIn

200

5, A

nglo

0.

15 M

a an

d 2.

90 ±

py

inte

rgro

wn

with

var

iabl

e qz

, cc,

chl

, and

spe

csi

ons

in q

z an

d gt

Am

eric

an L

td in

ter-

0.15

Ma1

Mas

sive

py

zone

s co

nsis

tent

ly h

ave

low

Cu

valu

es b

ut

Th

from

220

° to

se

cted

37

m o

f lo

cally

ele

vate

d A

u26

0°C

porp

hyry

-sty

le

Hb

dior

ite p

orph

yry

dike

s ar

e cu

t by

wid

ely

spac

ed

Salin

ities

up

to

min

eral

izat

ion

@

qz-p

y-cp

-mo-

chl-e

pi v

einl

ets

that

hav

e in

cipi

ent b

i 18

wt %

NaC

l 0.

11%

Cu,

0.0

3 g/

t al

tera

tion

halo

s an

d ar

e cu

t by

shee

ted

py v

einl

ets

equi

vA

u in

dri

ll ho

le

Dri

ll ho

le X

SD-1

inte

rsec

ted

a zo

ne o

f por

phyr

y-st

yle

XSD

-1 (

221–

258

m)

bi a

ltera

tion

and

a qz

-mt-

cp s

tock

wor

k in

med

ium

-A

lso

3 m

of s

karn

@

grai

ned

dior

ite p

orph

yry

and

basa

lt co

untr

y ro

cks

0.14

% C

u, 0

.41

g/t A

u

40Ar/39Ar GEOCHRONOLOGY OF PORPHYRY-EPITHERMAL DEPOSITS IN THE BAGUIO DISTRICT, PHILIPPINES 1343

0361-0128/98/000/000-00 $6.00 1343

Cam

p 4

Intr

usiv

e C

ompl

ex

Kid

ao

Zig-

Zag

Fm

Roc

k ch

ip s

ampl

es

Pre-

min

eral

izat

ion

hb-

Min

or e

xpos

ures

of i

nter

- an

d in

tram

iner

aliz

atio

n di

orite

N

o da

ta a

vaila

ble

M. d

e G

uzm

an,

Cu-

Au

porp

hyry

av

erag

e 0.

27%

Cu

phyr

ic d

iaba

se d

ikes

po

rphy

ry p

hase

s ar

e ch

arac

teri

zed

by w

eak

to in

tens

e un

pub.

rep

ort t

o pr

ospe

ctan

d 0.

25 g

/t A

ufr

om J

aim

e C

reek

: qz

vei

n st

ockw

orks

, bi-a

ct-m

t, lo

cal d

isse

min

ated

mo

Ben

guet

A

ssay

val

ues

of u

p to

4.

55 ±

0.1

5 an

d 4.

028

and

cp-b

n ve

inle

tsC

orpo

ratio

n (1

986)

; 1.

18%

Cu

and

0.64

±

0.07

4 M

a1H

ydro

ther

mal

bre

ccia

s an

d a

silic

eous

dac

itic

brec

cia

this

stu

dyg/

t Au

obta

ined

from

pi

pe in

ters

ecte

d in

the

Pugo

Wes

t cou

nter

dri

ve

dior

ite-h

oste

d qz

–(A

cupa

n A

u m

ine)

hav

e zo

nes

of p

erva

sive

bi a

ltera

tion

mt-

mal

sto

ckw

ork

host

ing

qz-c

p-m

t-m

o-py

sto

ckw

orks

ove

r ap

prox

. 250

mM

iner

aliz

atio

n in

the

Loc

al il

l-py

alte

ratio

n–up

to 5

vol

% p

ysi

liceo

us d

acite

W

ides

prea

d, w

eak

prop

yliti

c (c

hlor

ite-e

pido

te)

alte

ratio

nav

erag

e 0.

15%

Cu

Low

, per

sist

ent a

nd e

xten

sive

Au

and

Cu

geoc

hem

ical

ov

er a

dis

tanc

e of

an

omal

y ov

er 2

km

2

appr

ox. 2

50 m

in

Pug

o W

est

coun

terd

rive

Ubo

lan

Zig-

Zag

Fm

Dri

lled

by F

ilim

ag in

N

o da

ta a

vaila

ble

Wea

k C

u-A

u m

iner

aliz

atio

n as

soci

ated

with

the

Ubo

lan

No

data

ava

ilabl

eT

his

stud

yC

u-A

u po

rphy

ry

the

1960

s, w

ho o

nly

horn

blen

de d

iori

te p

orph

yry

pros

pect

enco

unte

red

low

-gr

ade

min

eral

izat

ion

Sant

o To

mas

II–

Bum

olo–

Clif

ton

clus

ter

Sant

o To

mas

II

Pugo

U

nder

grou

nd b

lock

B

iotit

e-al

tere

d in

tra-

Cp-

bn-g

old

(-pa

lladi

um)

ore

is h

oste

d in

anh

-qz

vein

s N

o da

ta a

vaila

ble

Silli

toe

and

Gap

pe

Cu-

Au-

(Pd)

F

orm

atio

nca

ve p

rodu

ced

min

eral

izat

ion

dior

ite:

that

hav

e cr

ossc

ut e

arlie

r-fo

rmed

qz-

mt-

bn-c

p-an

h (1

984)

; Bur

eau

of

porp

hyry

dep

osit

0.80

9 M

t of f

ine

1.48

± 0

.05

Ma2

vein

s (F

ig. 4

e)M

ines

and

co

pper

and

4.7

0 M

oz

Bio

tite-

alte

red

intr

a-G

eosc

ienc

es (

1986

); of

gol

d fr

om 2

88 M

t m

iner

aliz

atio

n an

desi

te

Bel

lon

and

Yum

ul

of o

re b

etw

een

porp

hyry

: 1.4

7 ±

(200

0); I

mai

(20

01)

1958

and

200

30.

05 M

a2L

east

-alte

red

and

biot

ite

alte

red

dior

ites

and

ande

site

s: 2

.1 to

1.2

Ma3

Alte

red

dior

ite: 3

.71

to

2.31

Ma4

TAB

LE

3. (

Con

t.)

Gra

des

and

tonn

ages

/ M

ine

/ pro

spec

tW

all r

ocks

assa

y da

ta

Geo

chro

nolo

gyA

ltera

tion

and

min

eral

izat

ion

Flu

id c

hem

istr

yR

efer

ence

s

1344 WATERS ET AL.

0361-0128/98/000/000-00 $6.00 1344

Sant

o To

mas

II–

Bum

olo–

Clif

ton

clus

ter

(con

t.)

Bum

olo

Pugo

Fm

G

rade

s fr

om r

ock

Min

eral

ized

hb-

qz d

iori

te

Ear

ly-m

iner

aliz

atio

n qz

dio

rite

por

phyr

y ha

s un

derg

one

No

data

ava

ilabl

eT

his

stud

y;C

u-A

u po

rphy

ry

(min

eral

iza-

chip

sam

ples

ave

rage

po

rphy

ry:1

.8 ±

0.6

Ma3

text

ural

ly d

estr

uctiv

e bi

, act

, and

mt a

ltera

tion

Imai

(20

01)

pros

pect

tion

exte

nds

0.2%

Cu

and

0.5

g/t

Una

ltere

d, p

ost-

min

eral

-M

t att

ains

con

cent

ratio

ns o

f 10

vol %

loca

lly, o

ccur

ring

on

ly a

lim

ited

Au

in th

e qz

-mt

izat

ion

pl-h

b-qz

dio

rite

as

irre

gula

r pa

tche

s an

d ve

inle

ts w

ith a

ct

dist

ance

into

st

ockw

ork

in th

e po

rphy

ry: 1

.90

± 0.

20 M

a1D

isse

min

ated

cp

is c

lose

ly a

ssoc

iate

d w

ith m

t, an

d al

so

the

coun

try

earl

y in

tram

iner

al-

occu

rs a

s co

arse

r gr

ains

in a

den

se s

tock

wor

k of

qz

and

rock

s)iz

atio

n di

orite

qz-m

t vei

nlet

s th

at o

verp

rint

ed b

i and

act

alte

ratio

nT

he in

tra-

and

ear

ly-

Inte

rmed

iate

arg

illic

alte

ratio

n pr

oduc

ed a

bund

ant

min

eral

izat

ion

por-

diss

emin

ated

py

but f

ew q

z ve

inle

ts, a

nd h

as o

ver

phyr

ies

wer

e ov

er-

prin

ted

bi-a

ct-m

t alte

ratio

n, d

efin

ing

a pa

rtia

l py

halo

prin

ted

by q

z-m

t L

euco

crat

ic c

oars

e-gr

aine

d hb

-qz

dior

ite p

orph

yry

has

vein

lets

con

tain

ing

unde

rgon

e w

ides

prea

d ch

l alte

ratio

n of

hb

phen

ocry

sts

low

-gra

de c

p, a

vg

Wea

k ch

l alte

ratio

n of

late

-min

eral

izat

ion

hb-q

z di

orite

0.

1% C

u, 0

.1 g

/t A

upo

rphy

ry w

ith o

nly

min

or p

yrite

; sto

ckw

ork

vein

s ar

e ab

sent

in th

is p

hase

A

n un

alte

red

mic

rodi

orite

por

phyr

y an

d a

late

-min

eral

iza-

tion

qz d

iori

te p

orph

yry

form

the

barr

en c

ore

to B

umol

o

Clif

ton

Ken

non

Fm

No

data

ava

ilabl

eB

iotit

e-al

tere

d C

lifto

n N

arro

w h

ornf

else

d an

d py

ritiz

ed z

ones

, tog

ethe

r w

ith

No

data

ava

ilabl

eIm

ai (

2001

)C

u-A

u po

rphy

ry

Klo

ndyk

e F

mqz

-dio

rite

por

phyr

y lo

caliz

ed s

karn

adj

acen

t to

biot

ite-a

ltere

d C

lifto

n qz

pr

ospe

ct1.

8 ±

0.7

Ma3

dior

ite p

orph

yry

Am

puca

o–H

artw

ell–

Bal

atoc

clu

ster

Am

puca

o Zi

g-Za

g D

rill

hole

FJ1

In

tram

iner

aliz

atio

n pl

-ric

h E

arly

-min

eral

izat

ion

daci

te p

orph

yry

has

unde

rgon

e T

h~

350°

C to

M

. de

Guz

man

, (A

cupa

n So

uth)

For

mat

ion

retu

rned

70

m @

hb

mic

rodi

orite

: 0.5

1 ±

text

ural

ly d

estr

uctiv

e bi

alte

ratio

n th

at h

as b

een

over

->

600°

Cun

pub.

rep

ort t

o C

u-A

u po

rphy

ry

~0.1

8% C

u an

d 0.

26 M

a1pr

inte

d by

chl

-ser

alte

ratio

nP

~600

bar

s (2

.5

Ben

guet

pr

ospe

ct0.

95 g

/t A

uA

-typ

e qz

vei

nlet

s co

nstit

ute

a pr

omin

ent s

tock

wor

k km

dep

th)

Cor

pora

tion

(198

6);

Surf

ace

rock

chi

ps

and

are

asso

ciat

ed w

ith 5

–10%

hyd

roth

erm

al m

t; cp

Sa

linity

up

to 7

0 C

ooke

and

Blo

om

typi

cally

1% C

u an

d an

d bn

occ

ur in

qz

vein

lets

, on

frac

ture

s, a

nd a

s w

t % N

aCl e

quiv

(199

0); C

ooke

4

g/t A

u (e

arly

min

-di

ssem

inat

ions

(199

1); t

his

stud

yer

aliz

atio

n da

cite

po

rphy

ry),

0.3%

Cu

and

0.6

g/t A

u (in

tra-

min

eral

izat

ion

mic

ro-

dior

ite),

and

<0.1

%

Cu

and

<0.2

g/t

Au

(late

min

eral

izat

ion

daci

te)

Har

twel

lV

irac

N

o da

ta a

vaila

ble

Wea

kly

min

eral

ized

hb-

bi

Wea

k po

rphy

ry-s

tyle

alte

ratio

n an

d m

iner

aliz

atio

nN

o da

ta a

vaila

ble

Thi

s st

udy

Cu-

Au

porp

hyry

gr

anod

iori

teda

cite

por

phyr

y:1.

09 ±

pr

ospe

ct0.

10 M

a1

TAB

LE

3. (

Con

t.)

Gra

des

and

tonn

ages

/ M

ine

/ pro

spec

tW

all r

ocks

assa

y da

ta

Geo

chro

nolo

gyA

ltera

tion

and

min

eral

izat

ion

Flu

id c

hem

istr

yR

efer

ence

s

40Ar/39Ar GEOCHRONOLOGY OF PORPHYRY-EPITHERMAL DEPOSITS IN THE BAGUIO DISTRICT, PHILIPPINES 1345

0361-0128/98/000/000-00 $6.00 1345

Am

puca

o–H

artw

ell–

Bal

atoc

clu

ster

(con

t.)

Bal

atoc

Zig-

Zag

Fm

No

data

ava

ilabl

eL

ate-

stag

e da

cite

“pl

ug”:

C

last

s of

pot

assi

c-al

tere

d di

orite

por

phyr

y w

ith c

p-A

u N

o da

ta a

vaila

ble

Scha

ffer

(19

56);

Dia

trem

e w

ith

Vir

ac

0.8

Ma5

min

eral

izat

ion

repo

rted

from

the

cent

ral s

ectio

n of

the

Saw

kins

et a

l. cl

asts

of C

u-A

u gr

anod

iori

teIl

lite

from

epi

ther

mal

vei

ns

brec

cia

pipe

(1

979)

; M. d

e po

rphy

ry

that

cro

sscu

t the

“yo

ung

Porp

hyry

-sty

le p

otas

sic

alte

ratio

n oc

curs

aro

und

the

Guz

man

, unp

ub.

min

eral

izat

ion

brec

cia”

pro

vide

a m

ini-

brec

cia

pipe

at d

epth

s of

1 k

m b

elow

the

pres

ent-

day

repo

rt to

Ben

guet

m

um K

-Ar

age

of 0

.65

± su

rfac

eC

orpo

ratio

n (1

986)

;0.

07 M

a6fo

r th

e di

atre

me

Acu

pan

epith

erm

al A

u-A

g ve

ins

cut t

he b

recc

ia p

ipe

Coo

ke a

nd B

loom

St

eepl

y pi

tchi

ng b

recc

ia-h

oste

d ep

ither

mal

Au-

Ag

ore

(199

0); A

oki e

t al.

zone

s (“

GW

” or

ebod

ies)

occ

ur w

here

the

Acu

pan

(199

3); C

ooke

et a

l. ep

ither

mal

vei

ns c

ut th

e m

argi

n of

the

brec

cia

pipe

(199

6); t

his

stud

y

Oth

er p

orph

yry

pros

pect

s

Nug

get H

illZi

g-Za

g F

mSu

rfac

e ro

ck c

hip

Pre-

min

eral

izat

ion

hb-

Ear

ly a

nd lo

cally

str

ong

Kf-

mt-

bi a

ltera

tion

asso

ciat

ed

Th

~345

° to

425

°C

Ang

loA

mer

ican

Ltd

. C

u-A

u po

rphy

ry

sam

ples

ret

urne

d ph

yric

dia

base

dyk

e w

ith q

z-bi

-mt-

cp-b

n ve

inle

tsP

~600

bar

s (2

.5

(unp

ub. d

ata)

; pr

ospe

ctas

say

valu

es o

f up

from

Liw

-Liw

Cre

ek:

Hig

h te

mpe

ratu

re p

ropy

litic

alte

ratio

n (d

iop

+ ab

) ha

s km

dep

th)

this

stu

dyto

1.1

7% C

u an

d 3.

45 ±

0.1

9 M

a1ov

erpr

inte

d th

e K

f ass

embl

age

Salin

ity ~

33 to

35

4.58

g/t

Au

Nug

get H

ill is

infe

rred

to

Ill-p

y-qz

alte

ratio

n as

soci

ated

with

late

sta

ge p

y-ri

ch v

eins

eq. w

t. %

NaC

lbe

sim

ilar

in a

ge to

the

Epi

ther

mal

-sty

le A

u an

d ba

se m

etal

-bea

ring

qz

vein

s Sa

nto

Tom

as I

I cl

uste

r ha

ve o

verp

rint

ed m

t vei

ns a

t the

top

of N

ugge

t Hill

(~1.

5 M

a) b

ased

on

thei

r si

mila

r po

sitio

ns in

the

cent

ral p

art o

f the

dis

tric

t

Chi

coZi

g-Za

g F

m

50-m

-long

exp

osur

e N

o da

ta a

vaila

ble

Two

zone

s of

por

phyr

y-st

yle

qz-m

t-cp

-bn

stoc

kwor

k N

o da

ta a

vaila

ble

Thi

s st

udy

Cu-

Au

porp

hyry

re

turn

ed r

ock

chip

ve

ins

spat

ially

rel

ated

to e

pith

erm

al g

old-

base

met

al

pros

pect

assa

ys o

f up

to 0

.97%

ve

ins

at th

e C

hico

and

Ato

k-B

ig W

edge

min

esC

u an

d 1.

83 g

/t A

u

Abb

revi

atio

ns: A

g =

silv

er, a

lt =

alta

ite, a

nd =

and

esite

, anh

= a

nhyd

rite

, ant

h =

anth

ophy

llite

, ap

= ap

atite

, asp

= a

rsen

opyr

ite, A

u =

gold

, bi =

bio

tite,

BM

S =

base

met

al s

ulfid

es, b

n =

born

ite, c

c =

calc

ite, c

hl =

chl

orite

, cor

d =

cord

ieri

te, c

p =

chal

copy

rite

, cz

= cl

inzo

isite

, di =

dio

rite

, dio

p =

diop

side

, epi

= e

pido

te, F

m =

For

mat

ion,

gt =

gar

net,

gl =

gal

ena,

gyp

= g

ypsu

m, h

b =

horn

blen

de, h

ess

= he

ssite

, hm

= h

emat

ite, i

ll =

illite

, mo

= m

olyb

deni

te, m

t = m

agne

tite,

ms

= m

usco

vite

, pet

z =

pet

zite

, po

= po

rphy

ry, p

y =

pyri

te, q

z =

quar

tz, r

ut =

rut

ile, s

p =

spha

leri

te, s

pn =

sph

ene,

syl

v =

syl-

vani

te, T

h =

hom

ogen

izat

ion

tem

pera

ture

, wol

= w

olla

ston

ite, w

r =

who

le r

ock

1 40

Ar/

39A

r (h

ornb

lend

e) a

ge d

eter

min

atio

n (t

his

stud

y)2

40A

r/39

Ar

(bio

tite)

age

det

erm

inat

ion

(thi

s st

udy)

3 K

-Ar

age

dete

rmin

atio

n (I

mai

, 200

1)4

K-A

r (w

hole

roc

k) a

ge d

eter

min

atio

n (B

ello

n an

d Yu

mul

, 200

0)5

K-A

r (w

hole

roc

k) a

ge d

eter

min

atio

n (D

rum

mon

d an

d D

efan

t, 19

90)

6 K

-Ar

(illit

e) a

ge d

eter

min

atio

n (A

oki e

t al.,

199

3)

TAB

LE

3. (

Con

t.)

Gra

des

and

tonn

ages

/ M

ine

/ pro

spec

tW

all r

ocks

assa

y da

ta

Geo

chro

nolo

gyA

ltera

tion

and

min

eral

izat

ion

Flu

id c

hem

istr

yR

efer

ence

s

were wrapped in aluminium foil packets and irradiated in acadmium-lined aluminium vial, together with interspersedaliquots of the fluence monitor GA1550 (age = 98.8 ± 0.5Ma; Renne et al., 1998). The irradiation canister was irradi-ated in position X33 or X34 of the ANSTO, HIFAR reactor,Lucas Heights. The canister was inverted three times duringthe irradiation, which reduced neutron flux gradients to <2percent along the length of the canister. 40Ar/39Ar analyseswere carried out at the Research School of Earth Sciences,The Australian National University, using procedures simi-lar to those described by Fergusson and Phillips (2001).After irradiation, aliquots of each sample were loaded intotin-foil packets for analysis and step-heated in a tantalum re-sistance furnace. 40Ar/39Ar stepheating analyses were carriedout on a VG MM12 mass spectrometer using an electronmultiplier detector. Sensitivity was approximately 7 × 10–17

mol/mv. Mass discrimination was monitored by analyses ofstandard air volumes. 40Ar production from potassium wasdetermined from analyses of degassed potassium glass. Cor-rection factors for interfering reactions are as follows:(36Ar/37Ar)Ca = 3.50 (±0.01) × 10-4; (39Ar/37Ar)Ca = 7.86(±0.01) × 10–4; (40Ar/39Ar)K = 0.030 (±0.010). The reporteddata have been corrected for mass spectrometer back-grounds, mass discrimination, and radioactive decay. Errorsassociated with the age determinations are 1 sigma uncer-tainties and exclude errors in the J-value estimates. Decayconstants are those recommended in Steiger and Jäger(1977).

Our results are summarized in Table 2, Figures 5, 6, andAppendix 1. Ages were calculated using Isoplot (Ludwig,2003) and are described in detail below, together with briefdescriptions of the related mineral deposits. All sampleswere analyzed in duplicate to assess possible heterogeneitieswithin individual samples. Apparent age spectra are pre-sented in Appendix 1. Age plateaus have been defined for 11age spectra as flat portions consisting of at least three suc-cessive steps that comprise a significant proportion of the39Ar released, and have ages that are concordant at the 95percent confidence level, based on internal errors. For theseven spectra that did not yield plateau ages, we report

maximum (n = 4), average maximum (n = 1), mean (n = 1),or total gas ages (n = 1; Table 2; App. 1). Our new ages canbe interpreted to provide evidence for eastward-migration ofmagmatism and mineralization over ~3 m.y., as outlinedbelow.

Magmatism and MineralizationWidespread intrusive activity occurred in the Luzon Cen-

tral Cordillera during the early Miocene (Shannon, 1979).Dioritic to gabbroic plutons were emplaced synchronous withcalc-alkaline volcanic activity related to subduction of theSouth China Sea Plate at the Manila Trench (Hollings et al.,2011a). After magmatic quiescence in the late Miocene, aresurgence of intrusive activity occurred in the Pliocene andPleistocene, with the emplacement of small stocks, dikes, andplutons of mafic to intermediate composition (Fig. 3; Hollingset al., 2011b). This second phase of magmatism was inti-mately associated with porphyry Cu-Au and epithermal Au-Ag mineralization (e.g., Cooke and Bloom, 1990), althoughthe limited amount and poor quality of geochronological datapreviously available for the district caused some confusionover the timing of magmatism and porphyry ore formation(e.g., Wolfe, 1981). We interpret our new 40Ar-39Ar data to in-dicate that several discrete clusters of Pliocene to Pleistoceneintrusions were emplaced in the Baguio district, namely theBlack Mountain Intrusive Complex, Camp 4 Intrusive Com-plex, Santo Tomas II-Bumolo-Clifton cluster and the Ampu-cao-Hartwell-Balatoc cluster (Figs. 3, 5). These intermediateto felsic intrusions were spatially and temporally related toporphyry copper mineralization, based on crosscutting andoverprinting relationships.

Central Cordillera Diorite Complex

A multiphase dioritic intrusive complex occurs on the east-ern side of the Baguio district (Fig. 3). Formerly known as the“Agno Batholith” (e.g., Balce et al., 1980), the Miocene intru-sive complex was renamed the Central Cordillera DioriteComplex by Pena (1998). It contains at least six intrusivephases, namely the Kadang trondhjemite, Itogon quartz dior-ite, Antamok diorite, Liang gabbro, Lucbuban gabbro, and

1346 WATERS ET AL.

0361-0128/98/000/000-00 $6.00 1346

Lucbuban gabbro2

Itogon gabbro1

40 39Ar- Ar (hornblende)

K-Ar (hornblende)Virac granodiorite2Virac granodiorite2

Itogon quartz diorite3Itogon quartz diorite3

gabbro (Sto Nino)4diorite (Ansagan) 4

granodiorite (Antamok Rd)4granodiorite (Sto Tomas tailings dam)4

diorite (Antamok Rd)4 Thermally resetK-Ar ages

Central Cordilleradiorite complex

Virac granodiorite (Acupan mine)3

0510152030 25

Age (Ma)

K-Ar (whole rock)

U-Pb (zircon)

FIG. 5. Geochronology of the Central Cordillera diorite complex. The preferred age of the diorite complex (grey shad-ing) is constrained between 27.3 and 20.0 Ma by the available U-Pb zircon and 40Ar/39Ar (hornblende) data. The K-Ar dataare inferred to have been thermally reset by Plio-Pleistocene intrusive activity. Data sources: (1) Encarnación et al. (1993);(2) this study; (3) Japan International Cooperation Agency, unpub. report (1983); (4) Bellon and Yumul (2000).

Virac granodiorite (Shannon, 1979). The plutonic rocks arepredominantly medium- to coarse-grained, equigranularhornblende quartz diorites with subordinate pyroxene-bear-ing diorites, hornblende diorites, tonalites, granodiorites, andminor gabbro. The Central Cordillera Diorite Complex hasintruded the Pugo and Zig-Zag formations, and has a narrowhornfelsed contact aureole.

Shannon (1979) reported a fission track (zircon) age of 7.6± 0.6 Ma for the Lucbuban gabbro, and K-Ar (whole-rock)

dating of the Virac granodiorite returned an age of 5.2 ± 0.3Ma (Japan International Cooperation Agency, unpub. report,1983). Both of these ages have been superceded by our new40Ar/39Ar age determinations for hornblende from theLucbuban gabbro (maximum age: 22.6 ± 0.5 Ma) and fromtwo samples of Virac granodiorite (total gas age: 20.2 ± 0.7Ma; mean age: 20.23 ± 0.38 Ma; Table 2; Figs. 3, 5). Our re-sults are bracketed by other age determinations from thediorite complex, specifically by a U-Pbzircon age determination

40Ar/39Ar GEOCHRONOLOGY OF PORPHYRY-EPITHERMAL DEPOSITS IN THE BAGUIO DISTRICT, PHILIPPINES 1347

0361-0128/98/000/000-00 $6.00 1347

Mafic dyke swarm

Baguio Formation

Black Mountainintrusive complex

Santo Tomas II - Bumolo - Clifton cluster

Ampucao - Hartwell - Balatoc cluster

diabase (Jaime Creek, Camp 4, Kidao)1diabase (Liw-Liw Creek, W of Nugget Hill)1

diabase (Jaime Creek, Camp 4, Kidao)1hb andesite porphyry (Nugget Hill)1

appinite breccia (SE of Acupan mine)2andesite dyke (Philex Rd)3andesite dyke (Philex Rd)3

diabase dyke1

pre-min hb-qz diorite porphyry (Black Mt)1late min hb andesite porphyry (Mexico)1late min hb andesite porphyry (Mexico)1

intra-min qz diorite porphyry (Black Mt)1late min hb andesite porphyry (Black Mt)1

granodiorite (Clifton)3cpx-hb andesite porphyry (Clifton)4

post-min pl-hb-qz diorite porphyry (Bumolo)1bi-alt hb-qz diorite porphyry (Bumolo)4

cpx-hb andesite porphyry (Ligay)4diorite (Sto Tomas II)3

dark diorite (Sto Tomas II)3diorite (Sto Tomas II)3

andesite porphyry (Sto Tomas II)3late-min porphyritic hb qz diorite (Sto Tomas II)4

cpx andesite porphyry (Sto Tomas II)4hb-qz diorite porphyry (Sto Tomas II)4

bi-alt intra-min diorite porphyry (Sto Tomas II)1bi-alt intra-min andesite porphyry (Sto Tomas II)1

bi-alt late-min porphyritic hb qz diorite (Sto Tomas II)4syn-min andesite porphyry (Sto Tomas II)4

bi-alt hb-qz diorite porphyry (Sto Tomas II)4bi-alt late-min porphyritic hb qz diorite (Sto Tomas II)4

post-min cpx andesite porphyry (Sto Tomas II)4bi-alt syn-min andesite porphyry (Sto Tomas II)4

hb-bi dacite porphyry (Hartwell)1intra-min pl-hb microdiorite porphyry (Ampucao)1

alunite (Tuding silica pit)5andesite dyke cutting AA alteration (John Hay)2

alunite (Bua ridge)5

illite (Acupan - 409 vein)5

012345Age (Ma)

Lithocap

Acupan epithermal Au-Ag veins

40Ar-39Ar (hornblende)

K-Ar (alunite)

40Ar-39Ar (biotite)

K-Ar (illite)

K-Ar (whole rock)

FIG. 6. Pliocene-Pleistocene geochronology for the Baguio district. Preferred ages of the mineralized intrusive complexesare highlighted using grey shading. These have been inferred from our 40Ar/39Ar (hornblende) data. Data sources: (1) thisstudy; (2) United Nations Development Program (1987); (3) Bellon and Yumul (2000); (4) Imai (2001); (5) Aoki et al. (1993)

of 26.8 ± 0.5 for the Itogon gabbro (Encarnación et al., 1993);K-Ar ages of 18.8 ± 0.9 and 16.1 ± 1 Ma for a quartz dioriteintrusion (Maleterre, 1989) and with K-Ar ages of 16.9 ± 0.4to 11.9 ± 0.7 Ma for samples of the dioritic phases (Bellon andYumul, 2000; Fig. 5). Based on these results, we infer an earlyto middle Miocene age of intrusive activity associated withthe Central Cordillera Diorite Complex.

The Virac granodiorite is the main host rock for the epi -thermal veins at Acupan (Cooke and Bloom, 1990; Cooke etal., 1996; Fig. 3). It comprises a series of intermediate to acidphaneritic intrusions with plagioclase + hornblende + quartzphenocrysts and crops out immediately to the southwest ofthe Acupan mine (Schaffer, 1956). The Virac granodioritehas intruded the Pugo and the Zig-Zag formations, but its re-lationship with the Agno Batholith was uncertain prior to thecurrent study. It was previously considered to be part of theyounger phase of magmatism that is associated with mineral-ization (e.g., Cooke and Bloom, 1990), but our geochrono-logical data show that it is part of the early Miocene dioritecomplex.

Mafic dike complex

The central part of the Baguio district has been intrudedby a hornblende-megacrystic mafic dike complex of Plioceneage (Fig. 3; Table 3). The dike swarm includes diabases, lam-prophyres ,and appinites, and is characterized by prominenthornblende phenocrysts, up to 10 cm in diameter. The dikeshave intruded the Zig-Zag Formation, Virac granodiorite,and Pugo Formation in the central Baguio district (Fig. 3).The hornblende-megacrystic dikes contain amphibolitexenoliths, which appear in various stages of disaggregation,suggesting that the xenoliths are not cognate features. Thearc basement appears to be the most likely source of thexenoliths.

Hornblende phenocrysts from a diabase dike and a horn-blende andesite porphyry from Liw-Liw Creek returnedPliocene 40Ar/39Ar plateau ages of 4.12 ± 0.22 and 3.45 ± 0.19Ma, respectively (Figs. 3, 6). Petrochemically similar diabasedikes from Jaime Creek, to the northeast of Camp 4, yielded40Ar/39Ar plateau ages of 4.55 ± 0.15 and 4.028 ± 0.074 Ma forhornblende phenocrysts (Table 2; Figs. 3, 6). These unminer-alized dikes are the oldest Pliocene intrusive rocks in theBaguio district. Their emplacement immediately precededdeposition of the Baguio Formation, and the onset of por-phyry Cu-Au mineralization at Black Mountain.

Black Mountain intrusive complex

The Black Mountain intrusive complex is located approxi-mately 6 km southwest of Baguio City (Fig. 3). It consists ofthe Black Mountain quartz diorite porphyry (Fig. 4b, c), theMexico diorite, and numerous andesite porphyry dikes. Thecomplex has intruded the Pugo, Zig-Zag, and Klondyke for-mations. It is associated with several porphyry Cu-Au andskarn deposits (Table 3), and is the oldest and westernmostmineralized intrusive complex in the Baguio district.

The Black Mountain porphyry Cu-Au deposit contains twoorebodies (Kennon and Southeast) that are hosted primarilywithin the Black Mountain intrusive complex (Table 3; Fig. 3).The Kennon (or Main) porphyry Cu-Au-Mo) orebody at BlackMountain is a 425-m-diameter mineralized pipe centered on

diorite porphyry intrusions (Balce, 1979). It was mined byblock caving methods from 1969 to 1983 (United Nations Development Program, 1987). The elongate, north-trendingSoutheast orebody is located southeast of the Kennon ore-body (Fig. 3). Southeast is a tabular, 200-m-wide orebody thatdips 70° W and was block caved from 1977 to 1983 (Balce,1979; United Nations Development Program, 1987). Miner-alization at Kennon and Southeast was associated with em-placement of several phases of plagioclase-hornblende-quartzdiorite porphyries (Figs. 4b, c).

Premineralization hornblende quartz diorite porphyry atKennon yielded a hornblende 40Ar/39Ar plateau age of 2.98 ±0.30 Ma (Table 2; Fig. 6). Secondary biotite from a potassic-altered intramineralization quartz-diorite porphyry returneda maximum 40Ar/39Ar age of 2.95 ± 0.05 Ma (Table 2; Fig. 6).An unaltered (postmineralization) specimen of a plagioclasehornblende andesite porphyry dike that has cross-cut theintra-mineralization quartz diorite porphyry returned a40Ar/39Ar plateau age of 2.81 ± 0.24 Ma (Table 2; Fig. 6).Some hornblende-megacrystic dikes have cross-cut the min-eralized intrusions at Black Mountain, indicating that mag-matism related to the mafic dike swarm continued after min-eralization at Black Mountain.

The Thanksgiving mine lies 200 m east of the Kennon por-phyry Cu-Au deposit (Fig. 3). Thanksgiving was mined un-derground from 1957 until 1987, initially for gold and later forzinc, copper and pyrite. The main host to mineralization is a120-m-thick limestone, dipping steeply to the west, which isinterpreted as a Kennon Formation roof pendant within theBlack Mountain intrusive complex (Fig. 3). Gold telluride-rich copper-zinc mineralization at Thanksgiving producedskarn and massive sulfide ores (Table 3).

The Mexico prospect was discovered by AngloAmericanLtd. in the late 1990s, approximately 1 km southwest of theKennon porphyry deposit (Fig. 3). The Klondike, Kennon,and Zig-Zag Formations at Mexico have been intruded byhornblende diorite and plagioglase-phyric hornblende an-desite dikes and stocks of the Black Mountain intrusive com-plex. Prograde massive garnet and wollastonite skarn (e.g.,Fig. 4d) formed by metasomatic replacement of limestonesat dike contacts. Copper–gold mineralization occurs in mas-sive retrograde magnetite–chalcopyrite skarn and as minorreplacements of garnet skarn by bornite. The magnetiteskarns have been cut by locally auriferous massive pyrite do-mains. Diamond drilling in 2005 by AngloAmerican Ltd.yielded a shallow, 3-m intercept of skarn in drill hole XSD-1,with ~ 0.14 percent Cu and 0.41 g/t Au. A 37-m intercept ofporphyry-style mineralization was encountered at greaterdepths, from 221 to 258 m. This zone had average grades of0.11 percent Cu and 0.03 g/t Au within K silicate-altereddiorite and basalt, associated with a weak quartz-magnetite-chalcopyrite stockwork. Late-mineralization hornblende an-desite porphyry dikes from the Mexico prospect returned40Ar/39Ar hornblende plateau ages of 3.09 ± 0.15 and 2.90 ±0.15 Ma, indicating formation synchronous with the BlackMountain porphyry deposits (Tables 2, 3).

Camp 4 intrusive complex

The Camp 4 intrusive complex (Fig. 3; Table 3) has in-truded the Zig-Zag Formation, about 5 km north of the Santo

1348 WATERS ET AL.

0361-0128/98/000/000-00 $6.00 1348

Tomas II porphyry Cu-Au deposit (Fig. 3). At least seven dis-crete intrusive phases have been recognized during fieldmapping of the complex. These range from plagioclase-horn-blende-quartz-phyric diorites and andesite dikes, through tohornblende-megacrystic mafic dikes. The mineralized intru-sions have not been dated, but two premineralization horn-blende megacrystic mafic dikes from Jaime Creek yielded40Ar/39Ar hornblende plateau ages of 4.55 ± 0.15 and 4.028 ±0.074 Ma (Fig. 3), providing a maximum age for the intrusivecomplex (Tables 2, 3).

Two discrete Cu-Au porphyry prospects have been delin-eated near Camp 4, namely Kidao and Ubolan. Ubolan wasdrilled by Filmag in the 1960s but only contains low-grademineralization associated with the Ubolan hornblende dior-ite porphyry. An unaltered pyroxene hornblende lampro-phyre dike has crosscut the Ubolan porphyry. The KidaoCu-Au prospect covers an area of approx. 2 km2 (Fig. 3), in-cluding numerous weakly veined, late-mineralization dioriteporphyry dikes. Most of the prospect has been subjected toweak propylitic (chlorite-epidote) alteration, resulting in lowand persistent copper and gold geochemical assay valuesand an extensive geochemical anomaly. On the eastern mar-gin of the geochemical anomaly, quartz-molybdenite vein-lets and magnetite veinlets were intersected in the PugoWest counter drive on the 1500 level of the Acupan ep-ithermal gold mine. Localized hydrothermal breccias and asiliceous dacitic breccia pipe are associated with zones ofpervasive biotite alteration that host quartz-chalcopyrite-magnetite-molybdenite-pyrite stockworks, assaying 0.15percent Cu over a distance of approx. 250 m along thecounter drive (M. de Guzman, unpub. report to BenguetCorporation, 1986). Elsewhere, minor exposures of inter-and intramineralization diorite porphyries at Kidao are char-acterized by weak to intensely developed quartz vein stock-works and have undergone biotite-actinolite-magnetite andlocal illite-pyrite alteration. These diorite porphyries con-tain up to 5 percent pyrite by volume, plus disseminatedmolybdenite, and localized chalcopyrite-bornite veinlets(Table 3). A weakly altered late-mineralization phase hastruncated earlier-formed veins.

Santo Tomas II-Bumolo-Clifton cluster

This cluster of intrusions is related to a major porphyry Cu-Au deposit at Santo Tomas II, and two other porphyry Cu-Auprospects (Bumolo and Clifton; Fig. 3). The porphyry intru-sions have similar compositions and ages (Tables 2, 3).

The Santo Tomas II porphyry deposit occurs at the south-ern end of the Baguio district (Fig. 3) and is the only por-phyry copper deposit being mined in the Philippines today.The underground block cave mining operation produced0.809 Mt of fine copper and 4.70 Moz of gold from 288 Mtof ore from 1958 to 2003, and is currently exploiting thePadcal orebody. The geology and mineralization has beendescribed by Imai (2001). Three phases of intermediate in-trusions are discernable in drill core samples. All phasescontain variable amounts of primary plagioclase and horn-blende phenocrysts, and all have been altered partially tosecondary biotite. The pipe-like intrusions are localizedwithin a braided section of the Albian fault and were em-placed into the Pugo Formation. There are also several

phases of hornblende plagioclase quartz diorite porphyryand quartz andesite porphyry dikes. Chalcopyrite-bornite-gold-palladium ore is hosted in anhydrite-quartz veins thathave crosscut earlier-formed quartz-magnetite-bornite-chal-copyrite-anhydrite veins (Fig. 4e).

Bellon and Yumul (2000) completed seven K-Ar age deter-minations on whole-rock samples of altered diorite fromSanto Tomas II, which yielded ages of 3.71 to 2.31 Ma. Imai(2001) determined eight K-Ar whole-rock ages for least-al-tered and biotite altered diorites and andesites, which yieldedresults of 2.1 to 1.2 Ma. We have completed two 40Ar/39Aranalyses of secondary biotite from Santo Tomas II. Secondarybiotite from the intramineralization diorite yielded a maxi-mum 40Ar/39Ar age of 1.48 ± 0.05 Ma, and secondary biotitefrom the intra-mineralization andesite porphyry returned anaverage maximum age of 1.47 ± 0.05 Ma (Table 2). Theseages are consistent with the results of Imai (2001).

The Bumolo porphyry Cu-Au prospect is located 1.5 km tothe northeast of Santo Tomas II (Fig. 3). It is hosted by asmall, multiphase quartz diorite porphyry intrusion that cropsout over a 0.25 km2 area. Mineralization extends only a lim-ited distance into the country rocks. The oldest intrusivephase, a plagioclase hornblende diorite porphyry, has under-gone texturally destructive biotite, actinolite, and magnetite alteration, which has in turn been overprinted by an interme-diate argillic alteration assemblage. Magnetite attains concen-trations of 10 vol percent locally, occurring as irregularpatches and veinlets with actinolite. Disseminated chalcopy-rite is closely associated with the magnetite as well as occur-ring as coarser grains in a dense stockwork of quartz andquartz-magnetite veinlets that overprinted the biotite andactinolite alteration. Grades average 0.2 percent Cu and 0.5g/t Au in this early intramineralization phase. Imai (2001) re-ported a K-Ar age for the hornblende quartz diorite porphyryof 1.8 ± 0.2 Ma (Fig. 6). The early-mineralization porphyry iscut and enveloped by a coarser-grained hornblende quartzdiorite porphyry that is more leucocratic and less altered. Sec-ondary biotite is absent, and chlorite alteration of hornblendephenocrysts is widespread. Magnetite and actinolite alter-ation is limited. This intramineralization porphyry and theearly-mineralization intrusion were overprinted by quartz-magnetite veinlets containing chalcopyrite, albeit of lowgrade, averaging 0.1 percent Cu and 0.1 g/t Au. A late-miner-alization hornblende quartz diorite porphyry is texturally sim-ilar to the intramineralization porphyry. It has undergoneweak chloritization, contains only minor pyrite and stockworkveins are absent. An unaltered microdiorite porphyry and thelate-mineralization quartz diorite porphyry form a barrencore to the system. It contains xenoliths of the earlier dioritesand returned a 40Ar/39Ar plateau age of 1.90 ± 0.20 Ma (Table2; Fig. 6).

The Clifton quartz diorite porphyry has intruded the Ken-non Formation and the Klondike Formation at the Cliftonporphyry Cu-Au prospect (Fig. 3), producing narrow horn-felsed and pyritized zones, together with localized skarn. Anearly-mineralization hornblende andesite porphyry intru-sion has undergone intense biotite alteration (Imai, 2001). Alate-mineralization clinopyroxene hornblende andesite por-phyry yielded a whole-rock K-Ar age of 1.7 ± 0.6 Ma (Imai,2001).

40Ar/39Ar GEOCHRONOLOGY OF PORPHYRY-EPITHERMAL DEPOSITS IN THE BAGUIO DISTRICT, PHILIPPINES 1349

0361-0128/98/000/000-00 $6.00 1349

Ampucao-Hartwell-Balatoc cluster

The Ampucao porphyry (M. de Guzman, unpub. report toBenguet Corporation, 1986; Cooke and Bloom, 1990) cropsout 1 km south of the Balatoc diatreme (Fig. 3; Table 3). It hasintruded the Zig-Zag Formation and the Virac granodioritebut is cut by epithermal quartz veins associated with the Acu-pan gold mine. It is associated with high-grade porphyry cop-per-gold mineralization at the Ampucao prospect (previouslydescribed as Acupan South by Cooke and Bloom, 1990). Atsurface, the Ampucao complex is approximately 200 m in di-ameter and comprises at least three porphyritic phases, whichare early, intramineralization and late mineralization in theirtiming with respect to the alteration and copper-gold miner-alization. A single diamond drill hole tested the Ampucaoporphyry (Cooke and Bloom, 1990; Cooke et al., 2011). Thisunderground drill hole, which was drilled southward from theAcupan epithermal gold mine, encountered biotite-mag-netite-orthoclase-chalcopyrite-bornite and chlorite-quartz-sericite-pyrite-chalcopyrite alteration assemblages in its lower70 m, which returned an intercept of ~0.18 percent Cu and0.95 g/t Au (M. de Guzman, unpub. report to Benguet Cor-poration, 1986). The drill hole terminated in the late-miner-alization dacite (Cooke and Bloom, 1990). The interpretedvertical extent of the Ampucao porphyry stock is greater than800 m from surface, based on the alteration zonation and thedrill hole intercept. The cross-sectional area of the Ampucaostock may also increase significantly at depth as evidenced byour unpublished reconnaissance mapping of alteration zona-tion in the Acupan underground workings.

We have dated hornblende from an intramineralization,plagioclase-rich hornblende microdiorite from Ampucao,which returned a 40Ar/39Ar maximum age of 0.51 ± 0.26 Ma(Table 2). This supersedes an inferred age of 2.4 ± 0.5 Ma(Cooke and Bloom, 1990), which was based on a fission track(zircon) age determination for secondary orthoclase alterationin the Virac granodiorite (Shannon, 1979).

The Hartwell dacite (formerly the Hartwell “plug”; M. deGuzman, unpub. report to Benguet Corporation, 1986) is asmall hornblende biotite dacite porphyry that has intrudedthe Virac granodiorite some 600 m south of the Balatoc dia-treme (Fig. 3; Table 3). It is semicircular in plan with a brec-ciated margin and hosts weak porphyry-style alteration andmineralization. We have obtained a 40Ar/39Ar plateau age of1.09 ± 0.10 Ma for hornblende from the Hartwell dacite(Table 2).

The Balatoc diatreme crops out at the northern end of theAcupan gold mine, ~1.5 km north of the Ampucao porphyryCu-Au prospect (Fig. 3; Table 3). Balatoc is a subverticalpolymict breccia pipe, ovoid in section and approx. 1 km in di-ameter at surface, tapering downward to a maximum knowndepth of 2,000 m. The diatreme has intruded the Zig-ZagFormation and the Virac granodiorite but is cut by later horn-blende andesite dikes and younger epithermal quartz veins.Three main facies were recognized by M. de Guzman(unpub. report to Benguet Corporation, 1986). The polymic-tic “old breccias” contains poorly sorted, polymictic, angularto rounded clasts of the country rocks set in a clastic matrix ofandesitic composition. The young breccias are also polymic-tic. It contains clasts of country rock, the “old breccias,” and

carbonized wood, suggesting considerable vertical movementof clasts, given that charcoal fragments were reported from650 m below the present-day erosional surface by Sawkins etal. (1979). The young breccias are supported by an andesiticto dacitic sand- to mud-sized matrix that has been intenselysericite-chlorite–altered and carbonatized. The diatreme hasbeen intruded by the “dacite plug,” which truncates some ep-ithermal veins and has been dated at 0.8 Ma by K-Ar meth-ods (Defant and Drummond, 1990). Epithermal veins thatcrosscut the young breccia returned a K-Ar age of 0.65 ± 0.07Ma (Aoki et al., 1993). Samples of carbonized wood withinthe young breccia provided an estimated 14C age of 0.65 Ma(M. de Guzman, unpub. report to Benguet Corporation,1986).

Clasts of potassic-altered diorite porphyry with chalcopy-rite-gold mineralization have been reported from the centralsection of the Balatoc diatreme (Schaffer, 1956). Early potas-sic (orthoclase) alteration has been recognized at several lo-calities in the deepest underground workings at Acupan,such as in the vicinity of 210 vein (M. de Guzman, unpub. report to Benguet Corporation, 1986; Cooke et al., 1990;Cooke, 1991). These occurrences are more than a kilometernorth of the northern extent of the potassic alteration haloaround the Ampucao porphyry, suggesting that another por-phyry copper-gold system may be located at the roots of theBalatoc diatreme.

Other porphyry prospects

Several porphyry Cu-Au prospects were discovered byAnglo American Ltd. in the Baguio district during the pastdecade. Some of these prospects have not been dated due toa lack of suitable material. They are inferred to have formedbetween 3 and 1 Ma, based on the ages of other porphyry sys-tems in the district.

Nugget Hill was discovered by AngloAmerican in the late1990s. It is located approximately 2.5 km east of the BlackMountain mine (Figs. 3, 4a). Porphyry-style alteration and as-sociated mineralization encompass a 0.5 km2 area and arehosted by hornfelsed andesitic breccias of the Zig-Zag For-mation and also within diabase dikes of the Dike Complex inthe headwaters of Liw-Liw Creek and on the flanks of NuggetHill. Strongly mineralized andesitic breccia float and outcropscontain early, sinuous magnetite veinlets, crosscut by sheetedand stockwork quartz-magnetite-chalcopyrite-bornite veins(Fig. 4f). Grains of chalcopyrite and bornite are intergrownwith and interstitial to quartz, and also occur as inclusionswithin quartz. Gold occurs as inclusions in bornite and chal-copyrite. Early potassic alteration (K feldspar-magnetite-bi-otite) is associated with the mineralized veins, and rock chipsamples have returned assay values of up to 1.17 percent Cuand 4.58 g/t Au. Reconnaissance fluid inclusion analyses ofhypersaline fluid inclusions from the porphyry-style veins in-dicate homogenization temperatures of 345° to 425°C andsalinities of 33 to 35 wt percent NaCl equiv (AngloAmericanLtd., unpub. data; Table 3).

The potassic assemblage at Nugget Hill has been overprintedby a high-temperature propylitic alteration assemblage (diop-side-albite), and then by illite-pyrite-quartz alteration associ-ated with late-stage pyrite-rich veins. Fault-controlled epi -thermal gold-base metal veins have overprinted the porphyry

1350 WATERS ET AL.

0361-0128/98/000/000-00 $6.00 1350

system a 100-m vertical interval near the summit of NuggetHill. The epithermal quartz-adularia-sphalerite-galena-goldveins contain comb, vuggy, and breccia textures, and are en-veloped by intense, laterally extensive, magnetite-destructivealteration halos of quartz, adularia, chlorite and pyrite. Fluidinclusion analyses of the epithermal quartz veins indicate thathydrothermal alteration and gold mineralization took place attemperatures of about 210° to 240°C with fluid salinities ofabout 1.4 to 4.5 wt percent NaCl equiv (AngloAmerican Ltd.,unpub. data).

The Chico Cu-Au porphyry prospect is located immedi-ately to the east of an advanced argillic lithocap that cropsout in the northwestern part of the Baguio district (Fig. 3).Two zones of porphyry-style quartz-magnetite-chalcopy-rite-bornite stockwork veins hosted by potassic-altered epi-clastic rocks of the Zig-Zag Formation have been delin-eated by Anglo American Ltd. The northernmost zoneincludes a 50-m-long exposure that returned rock chip as-says of up to 0.97 percent Cu and 1.83 g/t Au. The Chicoporphyry prospect is spatially related to epithermal gold-base metal sulfide veins at the Chico and Atok-Big Wedgemines (Fig. 3).

Epithermal gold mineralization

There are at least eight bonanza-style epithermal vein de-posits in the Baguio district, including the giant Antamok andAcupan-Sangilo deposits and the smaller Chico-Kelly, AtokBig Wedge, Cal Horr, Baguio Gold, and Sierra Oro deposits.Antamok and Acupan were the major gold producers in thePhilippines during the 20th century, with a combined produc-tion of more than 19 Moz of gold. They were described byWorley (1967), Sawkins et al. (1979), Cooke et al. (1996),Cooke and McPhail (2001) and Cooke et al. (2011). Key fea-tures are summarized in Table 4.

Although geochronological data for the epithermal vein sys-tems are mostly lacking, we have observed evidence for ep-ithermal veins crosscutting porphyry-style veins and alter-ation assemblages in the vicinity of the Acupan, Chico,Atok-Big Wedge, Sierra Oro, and Baguio gold mines. Datingof illite at the Acupan gold mine by Aoki et al. (1993) yieldedan age of 0.65 ± 0.07 Ma, suggesting that epithermal miner-alization is associated with the youngest porphyry suite in theBaguio district (the Ampucao-Hartwell-Balatoc cluster; 1.1 to0.5 Ma). Cooke et al. (2011) discuss isotopic evidence formagmatic-hydrothermal fluid contributions to individual ep-ithermal vein systems of the Baguio district.

Lithocap

A zone of intense advanced argillic alteration crops out atelevations above 1,200 to 1,300 m in the northwestern Baguiodistrict, and defines the Baguio “lithocap” (Fig. 3). Aoki et al.(1993) reported alunite K-Ar dates of 1.4 ± 0.2 Ma from theTuding silica pit and 0.9 ± 0.1 Ma from Bua Ridge, which cor-relate with the youngest phases of porphyry activity in the dis-trict (Fig. 6).

ConclusionsPorphyry and epithermal mineralization occurred in the

Baguio district during the last 3.5 m.y., associated with sub-duction of the Scarborough Ridge and the South China Sea

plate beneath northern Luzon. Initiation of ridge subductionwas marked by the intrusion of the mafic dike swarm (4.7–3.2Ma; Figs. 6, 7; Hollings et al., 2011b). Copper-gold mineral-ization commenced during the waning stages of mafic mag-matism, between 3.2 and 2.6 Ma, with the formation of theBlack Mountain-Mexico-Thanksgiving cluster of porphyryand skarn deposits on the western side of the district (Figs. 3,6, 7; Table 3). Emplacement of the Santo Tomas II-Bumolo-Clifton cluster occurred in the central (southern) part of thedistrict between 2.1 and 1.4 Ma (Figs. 3, 6, and 7; Table 3).The youngest porphyry prospects (Ampucao-Hartwell-Bala-toc cluster; 1.2–0.5 Ma) occur on the eastern side of the dis-trict (Figs. 3, 6 and 7) and are most closely associated with thelithocap and the giant epithermal gold deposits at Acupan andAntamok, both spatially and temporally. The epithermal veinshave not been subjected to detailed geochronological analy-ses (cf. Lepanto-Far Southeast; Arribas et al., 1995), and fur-ther work is required to determine the precise nature of ge-netic relationships between individual porphyry andepithermal deposits, and also between these deposits and theextensively developed Baguio lithocap. What is apparent,however, is that the gold endowment of the district increasedmarkedly with time, based on the available resource figures(Tables 3, 4).

Overall, our geochronological data provide evidence for aneastward migration of magmatism and mineralization acrossthe Baguio district during the Pliocene and Pleistocene (Fig.7). We infer that this magmatic migration and, also, the in-creasing fertility of the intrusive complexes with time relate tothe combined effects of ongoing slab flattening and associatedcompressional deformation, coupled with episodic movementson splays of the left-lateral Philippine fault system, allowingmagmas to be emplaced into transtensional strike-slip relaybasins. Transpression on the major restraining bend in thePhilippine fault to the south of Baguio (Fig. 1), coupled withcompressional tectonism during shallowing of the subductionzone, caused extreme uplift and exhumation during this pe-riod, allowing the common superposition of epithermal veinsinto porphyry systems (Fig. 7). Overall, the geodynamic set-ting and crustal architecture proved highly effective for goldmineralization, with over 35 Moz deposited in porphyry andepithermal systems since 3.5 Ma.

The Baguio district contains several examples of complexinterrelationships between porphyry and epithermal styles ofmineralization. Numerous mineralized centers formed in anenvironment that was undergoing rapid uplift and exhuma-tion. The close spatial associations of high-grade porphyrycopper-gold and epithermal gold-silver ore zones within theBaguio district provide encouragement for brownfields explo-ration in other porphyry-epithermal mineral districts locatedin the circum-Pacific.

AcknowledgmentsThis update on the geology of the Baguio district would

not have been possible without the careful field observa-tions AngloAmerican’s team of exploration geologists.Thanks also to Anglo’s support staff and to Benguet Corpo-ration for providing access to their epithermal gold mines.Thank you also to Mike Baker (CODES) and NormanTamayo (AngloAmerican) for their assistance in compiling

40Ar/39Ar GEOCHRONOLOGY OF PORPHYRY-EPITHERMAL DEPOSITS IN THE BAGUIO DISTRICT, PHILIPPINES 1351

0361-0128/98/000/000-00 $6.00 1351

and revising some of the figures. Jim Dunlap and Marc Nor-man at the Australian National University are thanked forradiometric age determinations and whole-rock geochemi-cal analyses, respectively. Pete Hollings, John Muntean, JimLawless, and Stuart Simmons providing review commentson earlier versions of this manuscript, which have helped tosubstantially improve it. DRC thanks the Australian Re-search Council for support through the Linkage and Centreof Excellence grant schemes, and AMIRA International andthe industry sponsors of AMIRA project P765, Transitionsand zoning in porphyry-epithermal mineral districts, fortheir support.

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

Balce, G.R., 1979, Geology and ore genesis of the porphyry copper depositsin the Baguio district, Luzon Island, Philippines: Journal of the GeologicalSociety of the Philippines, v. 33, p. 1–43.

Balce, G.R., Encina, R.Y., Momongan, A. and Lara, E., 1980, Geology of theBaguio district and its implication on the tectonic development of the

1352 WATERS ET AL.

0361-0128/98/000/000-00 $6.00 1352

Stratigraphy

Mexico

0 7 km

BlackMountain

NuggetHill

Acupan /Balatoc diatreme

Rosario Formation

Mirador Limestone

Klondyke Formation

Kennon Formation

Zig-Zag Formation

Pugo Formation

Baguio Formation

Intrusive RocksDacite porphyry

Balatoc diatreme

Andesite dike

Andesite porphyry

Hornblende diorite porphyry

Diabase dike

Virac granodiorite

Lucbuban gabbro

Central Cordillera Diorite Complex

MineralizationFault

Epithermal vein

Mineralized breccia

Skarn

Carbonate-hostedAu

Porphyry Cu-Au

FIG. 7. Conceptual model for the formation of porphyry and epithermal mineralization in the Baguio district. Porphyrymineralization is centered on composite quartz diorite porphyry complexes that have intruded the broadly folded Tertiaryand Quaternary volcano-sedimentary units. The porphyry systems young eastwards, from Black Mountain (approximately 3Ma) to Ampucao (0.95 Ma). Epithermal veins cross-cut the porphyry systems, but have been juxtaposed into the porphyrydeposits due to extreme rates of uplift and exhumation during the Pleistocene.

40Ar/39Ar GEOCHRONOLOGY OF PORPHYRY-EPITHERMAL DEPOSITS IN THE BAGUIO DISTRICT, PHILIPPINES 1353

0361-0128/98/000/000-00 $6.00 1353

TAB

LE

4. E

pith

erm

al G

old-

Silv

er D

epos

its o

f the

Bag

uio

Dis

tric

t

Dep

osit

Age

Min

eral

izat

ion

styl

eM

inin

g hi

stor

y an

d as

say

data

Ref

eren

ces

Ant

amok

Plei

stoc

ene

?Q

uart

z-ca

rbon

ate-

Ben

guet

Cor

pora

tion

carr

ied

out u

nder

grou

nd o

pera

tions

at A

ntam

ok fr

om 1

907

until

W

orle

y (1

967,

196

8);

BM

S ep

ither

mal

vei

ns19

90, a

nd e

xtra

cted

res

idua

l sha

llow

ore

by

open

cut

from

198

6 un

til 1

998;

the

depo

sit i

s Sa

wki

ns e

t al.

(197

9);

dom

inat

ed b

y th

ree

maj

or N

W-t

rend

ing,

SW

-dip

ping

qua

rtz

vein

s: K

eyst

one,

440

, and

F

erna

ndez

and

Dam

asco

(19

79);

Am

igo-

Cam

ote;

Key

ston

e an

d 44

0 ex

tend

ing

for

abou

t 3 k

m a

long

str

ike,

mor

e th

an

Fer

nand

ez e

t al.

(197

9);

700

m d

ownd

ip a

nd a

re u

p to

6 m

wid

e; a

sto

ckw

ork

zone

sur

roun

ds th

e 44

0 ve

in a

nd

Dam

asco

(19

79);

Mitc

hell

and

loca

lly a

ttai

ns a

wid

th o

f 50

m b

ut ta

pers

with

dep

th; A

ntam

ok h

ad a

n es

timat

ed

Lea

ch (

1991

)pr

oduc

tion

of a

ppro

x. 1

1 M

oz a

t an

avg

grad

e of

5 g

/t A

u; s

mal

l-sca

le m

iner

s ar

e st

ill

expl

oitin

g th

e de

posi

t; go

ld-s

ilver

rat

io is

1 to

1.1

Acu

pan-

Itog

onA

oki e

t al.

(199

3)

Qua

rtz-

carb

onat

e-T

he A

cupa

n-Sa

ngilo

vei

n sy

stem

was

min

ed u

nder

grou

nd b

y B

engu

et C

orpo

ratio

n an

d C

allo

w a

nd W

orle

y (1

965)

; re

port

ed a

K-A

r ag

e B

MS

epith

erm

al v

eins

Itog

on-S

uyoc

Min

es, I

nc. f

rom

192

9 un

til 1

993;

the

two

min

es h

ad a

com

bine

d W

orle

y (1

967,

196

8); S

awki

ns e

t of

0.6

5 ±

0.07

Ma

pr

oduc

tion

of >

8 M

oz g

old,

with

rem

aini

ng r

esou

rces

est

imat

ed to

be

abou

t 3.5

Moz

al

. (19

79);

de G

uzm

an (

unpu

b.

gold

; the

avg

min

ing

grad

e w

as 7

g/t

(4.5

g/t

cuto

ff)

duri

ng th

e 19

80s;

mor

e th

an 4

60

repo

rt to

Ben

guet

Cor

pora

tion,

in

divi

dual

vei

ns h

ave

been

min

ed w

ith a

vg w

idth

s of

30

cm to

1 m

; vei

ns h

ave

a 19

86);

Coo

ke a

nd B

loom

(19

90);

com

posi

te s

trik

e le

ngth

>4

km; s

ever

al b

recc

ia-h

oste

d or

ebod

ies

wer

e m

ined

aro

und

the

Coo

ke (

1991

); C

ooke

et a

l. m

argi

ns o

f the

Bal

atoc

dia

trem

e; s

mal

l-sca

le m

inin

g co

ntin

ues

toda

y(1

996,

201

1)

Kel

ly<0

.6 M

a? (

Aok

i et a

l.,

HS

and

quar

tz-

Und

ergr

ound

min

ing

oper

atio

n ac

tive

duri

ng th

e 19

80s;

two

stag

es o

f min

eral

izat

ion:

C

omst

i et a

l. (1

990)

; Dey

ell a

nd

1993

)ca

rbon

ate-

BM

S ep

i-st

age

I in

term

edia

te s

ulfid

atio

n st

ate

min

eral

izat

ion

aver

aged

2–3

g/t

Au

(loca

lly u

p to

C

ooke

(20

03);

Coo

ke e

t al.

ther

mal

vei

ns a

ssoc

i-10

g/t)

; sta

ge I

I hi

gh s

ulfid

atio

n st

ate

min

eral

izat

ion

typi

cally

had

gra

des

as h

igh

as

(201

1)at

ed w

ith a

dvan

ced

11–1

7 g/

t Au

and

incl

uded

ena

rgite

-ten

nant

ite m

iner

aliz

atio

nar

gilli

c al

tera

tion

Ato

k-B

ig W

edge

Pl

eist

ocen

e ?

HS

and

quar

tz-

Two

leve

ls o

f und

ergr

ound

pro

duct

ion

in th

e 19

80s;

the

uppe

r dr

ive

at 9

20-m

ele

vatio

n C

omst

i et a

l. (1

990)

; Mitc

hell

(Bac

o)ca

rbon

ate-

BM

S ep

i-re

port

edly

gra

ded

17 g

/t A

u, w

here

as th

e lo

wer

leve

l gra

ded

7 g/

t Au;

the

base

of

and

Lea

ch (

1991

); D

eyel

l and

th

erm

al v

eins

ass

oci-

min

eral

izat

ion

was

mar

ked

by a

bund

ant a

nhyd

rite

bel

ow 6

00-m

ele

vatio

n, a

lthou

gh th

e C

ooke

(20

03)

ated

with

adv

ance

d de

pth

of th

e ba

se o

f min

eral

izat

ion

incr

ease

d ea

stw

ard

tow

ard

Ant

amok

argi

llic

alte

ratio

n

Bag

uio

Gol

dPl

eist

ocen

e ?

Qua

rtz-

carb

onat

e-N

orth

ernm

ost e

pith

erm

al v

eins

in th

e B

agui

o di

stri

ct; m

inin

g fr

om th

e 19

30s

up to

the

Mitc

hell

and

Lea

ch (

1991

)B

MS

epith

erm

al v

eins

1980

s fo

cuse

d pr

imar

ily o

n 1-

m-w

ide

quar

tz-c

arbo

nate

-sul

fide

vein

s: th

e N

E-t

rend

ing

Tree

vei

n, a

nd th

e N

W-t

rend

ing,

NE

-dip

ping

Litt

le C

orpo

ral a

nd L

ones

ome

vein

s, th

e fo

rmer

hav

ing

grad

es a

s hi

gh a

s 29

g/t

Au;

ear

ly m

agne

tite

vein

s ob

serv

ed d

urin

g th

e cu

rren

t stu

dy

Cal

Hor

rPl

eist

ocen

e ?

Qua

rtz-

carb

onat

e-U

nder

grou

nd m

ine

activ

e pr

ior

to W

orld

War

II

expl

oite

d th

e E

-tre

ndin

g R

idge

and

M

itche

ll an

d L

each

(19

91)

BM

S ep

ither

mal

vei

nsM

anch

este

r ve

ins

at e

leva

tions

of 1

,314

to 9

30 m

; low

-gra

de (

2.5

g/t A

u) o

pen-

cut a

nd

heap

leac

h op

erat

ion

from

198

4 to

198

9; g

old-

silv

er r

atio

is 1

to 1

.1

Chi

co M

ine

Plei

stoc

ene

?Q

uart

z-ca

rbon

ate-

Vein

s oc

cur

over

a 2

-km

str

ike

leng

th; m

inin

g fo

cuse

d on

the

Cam

aso

vein

, whi

ch h

ad

Mitc

hell

and

Lea

ch (

1991

)B

MS

epith

erm

al v

eins

wid

ths

of u

p to

5 m

and

avg

gra

des

of a

ppro

x. 5

g/t

Au

Sier

ra O

roPl

eist

ocen

e ?

Qua

rtz-

carb

onat

e-U

nder

grou

nd m

ine

prio

r to

Wor

ld W

ar I

I an

d re

open

ed fr

om 1

984

to 1

987;

exp

loite

d M

itche

ll an

d L

each

(19

91)

BM

S ep

ither

mal

vei

nsfo

ur E

-tre

ndin

g, S

-dip

ping

vei

ns, E

psilo

n, C

hi, a

nd B

ayat

ing

A +

B, w

hich

had

avg

gr

ades

of 6

–13

g/t A

u

Abb

revi

atio

ns: B

MS

= ba

se m

etal

sul

fides

(pr

imar

ily s

phal

erite

and

gal

ena)

; HS

= hi

gh s

ulfid

atio

n st

ate;

WW

II =

Wor

ld W

ar I

I

Luzon Central Cordillera: Geology and Palaeontology of Southeast Asia, v.21, p. 265–287.

Bautista, B.C., Bautista, M.L., Oike, K., Wu, F.T., and Punongbayan, R.S.,2001, A new insight on the geometry of subducting slabs in northernLuzon, Philippines: Tectonophysics, v. 339, p. 279–310.

Bellon, H., and Yumul Jr., G.P., 2000, Mio-Pliocene magmatism in theBaguio mining district (Luzon, Philippines): Age clues to its geodynamicsetting: Earth and Planetary Sciences, v. 331, p. 295–302.

Briais, A., Patriat, P., and Tapponier, P., 1993, Updated interpretation of mag-netic anomalies and seafloor spreading stages in the South China Sea: Im-plications for the Tertiary tectonics of southeast Asia: Journal of Geophysi-cal Research, v. 98, p. 6299–6328.

Bureau of Mines and Geosciences, 1986, Geology and mineral resourcesof the Philippines. Volume 2—mineral resources: Bureau of Mines andGeosciences, Ministry of Natural Resources, Metro Manila, Philippines,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.

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: Journal ofGeochemical 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 Berry, R.F., 1996, Porphyry-epithermal relationships in theBaguio district, Philippines—telescoping of low sulfidation mineralizationby transpressional uplift and erosion: Geological Society of America, Ab-stracts with Programs, v. 28, no. 7, p. A155.

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., 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., 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., Deyell, C.L., Waters, P.J., Gonzales, R.I., and Zaw, K., 2011,Evidence for magmatic-hydrothermal fluids and ore-forming processes inepithermal and porphyry deposits of the Baguio district, Philippines: ECO-NOMIC GEOLOGY, v. 106, p. 1399–1424.

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 Leon, M.M., Tamesis, E.V., and Militante-Matias, P.J., 1991, Calcareousnannofossil study of the Klondyke Formation section along km posts278–251, Baguio City-Pugo, La Union Province: Journal of the GeologicalSociety of the Philippines, v. 46, p. 35–49.

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 arcmagmas 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: Rotterdam, Millpress, v. 1, p.469–472.

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.

Encarnación, J.P., Mukasa, S.B. and Obille, E.J., 1993, Zircon U-Pbgeochronology of the Zambales and Angat ophiolites, Luzon, Philippines:Evidence for an Eocene arc-back arc pair: Journal of Geophysical Re-search, v. 98, p. 19,991–20,004.

Fergusson, C.L., and Phillips, D., 2001, 40Ar/39Ar and K-Ar age constraints onthe timing of regional deformation, south coast of New South Wales, Lach-lan fold belt: Problems and implications: Australian Journal of Earth Sci-ences, v. 48, p. 395–408.

Fernandez, H.E., and Damasco, F.V., 1979, Gold deposition in the Baguiodistrict and its relationship to regional geology: ECONOMIC GEOLOGY, v. 74,p. 1852–1868.

Fernandez, H.E., Damasco, F.V., and Sangalang, L.A., 1979, Ore shoot de-velopment in the Antamok mines, Philippines: ECONOMIC GEOLOGY, v. 74,p. 606–627.

Florendo, F.F., 1994, Tertiary arc rifting in northern Luzon, Philippines: Tec-tonics, v. 13, p. 623–640.

Hall, R., 2002, Cenozoic geological and plate tectonic evolution of SE Asiaand the SW Pacific: Computer Based Reconstructions, Models and Ani-mations: Journal of Asian Earth Sciences, v. 20 (4), 431 p.

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.

Imai, A., 2001, Generation and evolution of ore fluids for porphyry Cu-Aumineralization of the Santo Tomas II (Philex) deposit, Philippines: Re-source Geology, v. 51, p. 71–96.

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

Ludwig, K.R., 2003, Isoplot 3.00, A geochronological toolkit for MicrosoftExcel: Berkeley Geochronology Center Special Publication 4, 70 p.

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.

Mitchell, A.H.G., and Leach, T.M., 1991, Epithermal Gold in the Philip-pines–Island Arc Metallogenesis, Geothermal Systems and Geology: Acad-emic Press Geology Series, London, 457 p.

Pena, R.E., 1998, Further notes on the stratigraphy of the Baguio district:Journal of the Geological Society of the Philippines, LIII, v. 3–4, p.141–157.

Pena, R.E., and Reyes, M.C., 1970, Sedimentological study of a section of theUpper Zig-Zag Formation along Bued River, Tuba, Benguet: GeologicalSociety of the Philippines, v. 24, p. 1–19.

Queano, K.L., Ali, J.R., Milsom, J., Aitchison, J.C., and Pubellier, M., 2007,North Luzon and the Philippine Sea Plate motion model: Insights follow-ing paleomagnetic, structural and age-dating investigations: Journal of Geo-physical Research, 112, B05101, doi:10.1029/2006JB004506.

Renne, P.R., Swisher, C.C., Deino, A.L., Karner, D.B., Owens, T.L., and De-Paolo, D.J., 1998, Intercalibration of standards, absolute ages and uncer-tainties in 40Ar/39Ar dating: Chemical Geology, v. 145, p. 117–152.

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.

——1956, Some phases of the geology of the Baguio district: National Re-search Council of the Philippines: 8th Pacific Science Congress, v. IIa, p.347–355.

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

Sillitoe, R.H., 1995, Exploration of porphyry copper lithocaps: PublicationSeries - Australasian Institute of Mining and Metallurgy, v. 9/95, p. 527-532.

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.

1354 WATERS ET AL.

0361-0128/98/000/000-00 $6.00 1354

Solomon, M, 1990, Subduction, arc reversal, and the origin of porphyry cop-per-gold deposits in island arcs: Geology, v. 18, p. 630–633.

Steiger, R.H., and Jager, E., 1977, Subcommission on geochronology: Con-vention on the use of decay constants in geo- and cosmochronology: Earthand Planetary Science Letters, v. 36, p. 359–362.

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.

Tan, M.N., 1995, Kennon and Mirador: Early Middle Miocene carbonates inthe Baguio District, Northern Luzon, Philippines: Research DevelopmentCooperation ITIT Projects No. 8813, p. 48–71.

United Nations Development Programme, 1987, Geology and mineralizationin the Baguio area, Northern Luzon: United Nations Development Pro-gramme, v. 5, 82 p.

Wolfe, J.A., 1981, Philippine geochronology: Journal of the Geological Soci-ety of the Philippines, v. 35, p. 1–30.

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

Worley, B.W., Jr., 1968, Geology of the Acupan and Antamok gold mines ofBenguet Consolidated, Inc.: Philippine Mining Journal, v. 10, p. 22–28.

Yumul, G.P., Jr., Dimalanta, C., Tamayo, R.A. and Maury, R.C., 2003, Colli-sion, subduction and accretion events in the Philippines: A synthesis: IslandArc, v. 12, p. 77–91.

40Ar/39Ar GEOCHRONOLOGY OF PORPHYRY-EPITHERMAL DEPOSITS IN THE BAGUIO DISTRICT, PHILIPPINES 1355

0361-0128/98/000/000-00 $6.00 1355

1356 WATERS ET AL.

0361-0128/98/000/000-00 $6.00 1356

APPENDIX 140Ar/39Ar Geochronology

Cumulate Temp (°C) 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar * 10-2 40Ar* (%) 40Ar*/39Ar(K) 39Ar (%) Age (Ma) ± 1σ Ca/K

Sample 353202 – amphibole900 595.2 9.321 197.200 2.3 13.670 0.8 8.33 7.2 17.8960 539.3 19.06 181.800 0.8 4.164 1.5 2.54 11.04 36.81000 502.5 29.72 167.500 2.1 10.610 2.6 6.47 3.24 57.81030 227.7 23.4 76.020 2.4 5.571 4.9 3.4 1.54 45.31050 100.4 20.24 33.070 4.7 4.846 9 2.96 0.98 39.11080 38.98 18.94 12.930 7 2.754 39.8 1.68 0.13 36.51100 34.17 18.81 11.010 10.4 3.606 61.1 2.2 0.17 36.31130 47.65 18.89 15.360 8.8 4.256 77.5 2.6 0.24 36.41170 48.86 18.78 15.790 8.4 4.184 96.8 2.56 0.25 36.21220 56.35 19.85 17.940 9.5 5.420 99.1 3.31 0.95 38.31450 482.2 21.4 160.700 2 9.710 100 5.92 7.04 41.4Total 3.91 2.39 0.49Lambda 40K = 5.543E-10 J = 0.0003387 ± 0.78% Mass = 281 mg

Sample 353216 – amphibole800 93.05 2.772 29.860 5.4 5.044 1.5 3.11 0.71 5.3920 181.5 2.007 59.070 3.9 7.176 3 4.42 1.45 3.8960 207.9 5.481 69.710 1.2 2.434 3.8 1.5 2.11 10.51000 76.09 9.482 24.010 8 6.104 6.9 3.76 0.54 18.21030 38.2 10.5 11.150 16.5 6.371 15.8 3.93 0.23 20.11050 29.13 10.69 8.146 21.1 6.187 25.2 3.81 0.13 20.51070 18.85 10.66 4.525 34.8 6.613 40.1 4.08 0.09 20.41090 15.03 10.67 3.314 42 6.362 60.4 3.92 0.07 20041110 16.52 10.62 3.707 40.2 6.695 75.9 4.13 0.09 20.31140 18.85 10.72 4.508 35.1 6.665 87.6 4.11 0.11 20.51170 16.69 10.91 3.789 39.5 6.658 97.3 4.1 0.08 20.91240 33.85 12.54 9.584 20.1 6.875 99.3 4.24 0.43 24.11450 80.99 12.72 25.700 7.8 6.42 100 3.96 1.21 24.4Total 6.467 3.99 0.18Lambda 40K = 5.543E-10 J = 0.0003421 ± 0.18 Mass = 290 mg

Sample 353217 – amphibole750 474.9 25.4 150.300 7 34.13 0.4 20.91 8.33 49.2850 410.6 3.395 130.900 5.9 24.11 1 14.79 4.61 6.5950 407.9 10.57 124.300 10.2 42.06 1.9 25.73 6.1 20.2970 169.2 12.89 52.970 8.3 14.18 2.3 8.72 4.65 24.71000 96.44 13.46 28.270 14.8 14.43 3.7 8.87 5.72 25.81030 44.76 13.34 12.790 18.6 8,417 9.9 5.18 0.32 25.61050 31.55 12.98 8.474 24.8 7.904 23.5 4.86 0.2 24.91070 23.91 12.98 6.057 30.7 7,416 55 4.56 0.09 24.91090 23.27 13.44 5.946 30.4 7.14 79.6 4.39 0.14 25.81110 37.28 14.08 9.824 26 9.797 88.2 6.03 0.31 27.11130 35.29 13.75 9.181 27.1 9.671 93.4 5.95 0.2 26.41150 49.82 13.66 13.450 23 11.56 94.9 7.11 0.65 26.21200 47.06 13.44 12.450 24.7 11.76 98.2 7.23 0.4 25.81260 110 13.04 33.450 11.3 12.56 99.8 7.72 3.18 251320 1058 16.68 340.500 5 53.63 99.9 32.75 37.1 32.11450 1862 16.4 615.100 2.5 46.62 100 28.5 72.74 31.6Total 8.81 5.42 0.58Lambda 40K = 5.543E-10 J = 0.0003415 ± 0.45% Mass = 257 mg

Sample 353218 – amphibole1025 758.3 14.43 253.100 1.5 11.600 0.8 7.17 8.02 27.71040 410.3 12.13 135.900 2.4 9.978 2.8 6.17 3.37 23.31055 211.6 9.66 67.910 5.6 11.940 6.7 7.38 3.37 18.51070 87.55 10.32 27.740 7.6 6.671 15.5 4.13 0.41 19.81090 33.31 9.949 9.307 20.5 6.872 38.1 4.25 0.22 19.11110 27.32 10.09 7.321 24.5 6.752 72.5 4.18 0.15 19.31130 39.67 10.68 12.040 13 5.220 86.6 3.23 0.65 20.51130 58.16 10.38 17.810 11.3 6.648 88.6 4.11 0.67 19.91170 56.75 10.26 17.390 11.3 6.472 94.7 4.01 0.47 19.71270 76.62 10.42 24.300 7.6 5.896 99.8 3.65 0.37 201400 597.7 9.938 199.700 1.4 8.586 100 5.31 18.42 19Total 6.801 4.21 0.59Lambda 40K = 5.543E-10 J = 0.0003434 ± 0.94% Mass = 268 mg

40Ar/39Ar GEOCHRONOLOGY OF PORPHYRY-EPITHERMAL DEPOSITS IN THE BAGUIO DISTRICT, PHILIPPINES 1357

0361-0128/98/000/000-00 $6.00 1357

40Ar/39Ar incremental heating data and laser-heated 40Ar/39Ar age spectra for samples of primary hornblende and sec-ondary biotite from intrusions of the Baguio district, calculated using Isoplot (Ludwig, 2003). Plateau steps are filled black,rejected steps are open. Box heights are 1σ.

1358 WATERS ET AL.

0361-0128/98/000/000-00 $6.00 1358

Sample 354206 – biotite

720 62.14 19.46 7.6 4.748 1.1 3.15 1.16760 35.98 11.21 7.9 2.851 3.7 1.89 0.38800 23.48 6.652 16.2 3.81 7.7 2.53 0.21840 25.01 7.219 14.7 3.667 12.8 2.43 0.22880 31.63 9.414 12 3.798 17.8 2.52 0.25920 19.26 5.475 15.9 3.067 23.9 2.03 0.11960 17.53 4.901 17.4 3.046 32.6 2.02 0.111000 10.83 2.826 23 2.495 46.4 1.65 0.091040 5.125 0.9863 43 2.206 69.3 1.46 0.031070 4.927 0.9063 45.3 2.234 87.9 1.48 0.041120 6.361 1.275 40.8 2.594 98.5 1.72 0.061350 72.28 22.01 10.3 7.492 100 4.96 0.91Total 2.762 1.83 0.12

Lambda 40K = 5.543E-10 J = 0.0003678 ± 0.61% Mass = 28 mg

Sample 354207 – biotite

720 55.47 16.43 12.4 6.885 1.3 4.56 0.74760 20.63 5.782 17 3.51 5.2 2.32 0.14800 9.856 2.39 28 2.762 12.9 1.83 0.08840 9.401 2.141 32.4 3.048 19.4 2.02 0.24880 9.574 2.163 33 3.156 25.1 2.09 0.09920 10.03 2.43 28.1 2.818 30 1.87 0.06960 9.974 2.337 30.5 3.039 37.7 2.01 0.091000 6.25 1.233 41.3 2.58 51.1 1.71 0.041040 4.58 0.7417 51.5 2.36 71.6 1.56 0.031070 4.492 0.7552 49.7 2.23 90.6 1.48 0.021120 4.86 0.7725 52.5 2.55 99.6 1.69 0.051350 112.7 33.38 12.4 13.95 100 9.22 1.92Total 2.733 1.81 0.08

Lambda 40K = 5.543E-10 J = 0.0003673 ± 0.61% Mass = 29 mg

Sample 354212 – amphibole

800 621.1 2.822 193.400 8.1 50.13 0.5 30.53 13.27 5.4880 264.8 3.399 85.860 4.3 11.36 1.1 6.96 11.58 6.5920 172.3 6.132 52.800 9.8 16.97 1.5 10.39 2.74 11.7970 119.1 12.93 29.500 27.9 33.56 3.3 20.5 0.92 24.81010 76.59 11.01 15.240 42.7 32.96 12.5 20.13 0.32 21.11030 63.43 10.37 10.730 51.7 33 22.6 20.18 0.3 19.91050 52.64 9.983 6.899 63.2 33.52 33.5 20.47 0.23 19.11070 45.77 9.991 4.748 71.5 33 47.1 20.15 0.16 19.11090 45.79 10042 4.487 73.3 33.87 54.7 20.68 0.14 201150 43.07 10040 3.358 79.4 34.48 83.8 21.05 0.16 19.91200 47.43 10.31 4.729 72.7 34.78 90.8 21.24 0.17 19.71300 46.03 10.27 4.316 74.5 34.6 99.6 21.13 0.1 19.71450 142.6 9.345 37.010 23.9 34.35 100 20.98 3.87 17.9Total 33.73 20.6 0.36

Lambda 40K = 5.543E-10 J = 0.0003405 ± 0.75% Mass = 290 mg

Sample 354216 – amphibole

800 95.5 1.564 30.740 5 4.786 0.4 2.95 1.31 3950 179.6 6.296 59.820 1.9 3.504 1 2.16 2.01 121000 45.65 6.382 13.790 12.1 5.552 4.4 3.42 0.23 12.21030 24.91 6.284 6.740 22.6 5.647 11.8 3.47 0.14 121050 19.74 6.202 4.990 28.4 5.641 22.2 3.47 0.14 11.81070 16.34 5.837 3.772 35.3 6 38.8 3.57 0.05 11.11090 13.01 5.47 2.603 45 5.883 55.6 3.62 0.08 10.41110 13.76 5.435 2.827 43.2 6 70.9 3.67 0.06 10.41140 14.33 5.615 2.946 43.1 6.204 85.9 3.82 0.07 10.71170 16.02 5.972 3.462 39.8 6.406 95.2 3.94 0.1 11.41240 17.96 7.132 4.217 34.6 6.252 99.4 3.85 0.18 13.61450 57.87 7.788 17.410 12.5 7.258 100 4.46 0.9 14.9Total 5.931 3.65 0.12

Lambda 40K = 5.543E-10 J = 0.0003413 ± 0.36% Mass = 284 mg

APPENDIX (Cont.)

Cumulate Temp (°C) 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar * 10-2 40Ar* (%) 40Ar*/39Ar(K) 39Ar (%) Age (Ma) ± 1σ Ca/K

40Ar/39Ar GEOCHRONOLOGY OF PORPHYRY-EPITHERMAL DEPOSITS IN THE BAGUIO DISTRICT, PHILIPPINES 1359

0361-0128/98/000/000-00 $6.00 1359

Series 35XXXX samples processed on 2/03/2002 by Dr. William James Dunlap.

1360 WATERS ET AL.

0361-0128/98/000/000-00 $6.00 1360

Sample 806101 – amphibole

850 176.15 3.01 56.066 6.1 10.78 4.5 5.26 1.00 5.72E+00950 105.17 1.22 33.515 5.9 6.25 7.1 3.05 0.89 2.31E+001020 97.04 6.46 30.912 6.5 6.37 10.5 3.11 0.85 1.23E+011080 32.22 14.00 10.509 8.0 2.61 46.3 1.28 0.16 2.69E+011110 12.24 7.96 3.791 15.0 1.84 80.8 0.90 0.06 1.52E+011160 24.28 12.51 8.296 4.3 1.05 96.3 0.51 0.13 2.40E+011220 59.52 15.48 20.022 3.2 1.93 98.6 0.94 0.63 2.98E+011320 137.53 17.58 42.928 9.1 12.64 99.6 6.17 1.42 3.39E+011450 577.17 14.33 191.942 2.0 11.72 100.0 5.72 7.67 2.75E+01Total 2.81 1.37 0.25

Lambda 40K = 5.543E-10 J = 0.000271 ± 0.37% Mass = 169.4 mg

Sample 806102 – amphibole

850 1022.49 2.58 336.195 2.9 29.36 2.3 14.51 9.16 4.92E+00950 199.46 4.25 66.858 1.2 2.32 5.2 1.15 1.24 8.10E+001020 158.34 12.00 50.878 5.8 9.31 9.8 4.62 5.62 2.30E+011060 90.01 9.95 29.013 5.8 5.30 15.0 2.63 0.55 1.90E+011090 42.93 10.55 12.433 16.9 7.31 33.1 3.62 0.21 2.02E+011110 30.11 11.00 8.157 23.6 7.18 62.1 3.56 0.15 2.11E+011130 39.91 11.12 11.618 16.8 6.75 76.5 3.35 0.20 2.13E+011160 44.86 11.68 13.284 15.1 6.84 89.3 3.39 0.39 2.24E+011200 43.20 11.65 12.948 14.1 6.16 97.0 3.05 0.51 2.23E+011280 104.30 12.49 33.529 6.2 6.55 99.3 3.25 0.79 2.40E+011450 356.40 12.32 119.550 1.2 4.49 100.0 2.23 2.85 2.36E+01Total 7.34 3.64 0.77

Lambda 40K = 5.543E-10 J = 0.00027513 ± 0.38% Mass = 169.4 mg

Sample 806103 – amphibole

720 278.59 25.03 91.421 3.9 11.16 2.0 5.65 5.37 4.85E+01770 240.44 26.40 80.620 2.1 5.13 3.1 2.60 2.04 5.12E+01840 160.48 2.33 53.307 2.0 3.19 4.1 1.62 1.27 4.43E+00920 95.28 2.57 31.672 2.0 1.94 5.4 0.99 0.89 4.90E+001000 65.75 6.07 21.377 4.9 3.21 7.7 1.63 0.57 1.16E+011070 23.10 9.85 6.032 27.1 6.31 66.1 3.20 0.19 1.89E+011100 17.90 9.00 4.497 30.8 5.55 78.1 2.82 0.12 1.72E+011140 22.11 10.09 5.924 25.4 5.67 90.2 2.87 0.12 1.93E+011180 22.39 9.82 5.773 28.2 6.37 98.8 3.23 0.13 1.88E+011240 105.06 11.74 35.274 2.0 2.09 99.6 1.06 1.08 2.25E+011450 514.35 10.29 173.026 0.8 4.08 100.0 2.07 6.24 1.97E+01Total 6.03 3.06 0.35

Lambda 40K = 5.543E-10 J = 0.00028119 ± 0.2% Mass = 180.1 mg

Sample 806104 – amphibole

800 103.92 4.89 31.743 10.2 10.63 2.9 5.51 0.97 9.32E+00860 165.22 1.19 55.462 0.9 1.44 3.8 0.75 1.18 2.26E+00930 124.48 2.35 42.043 0.4 0.48 4.6 0.25 1.27 4.48E+00990 82.61 5.17 27.350 2.8 2.33 5.8 1.21 2.73 9.86E+001040 40.23 10.37 12.094 13.8 5.59 15.0 2.90 0.29 1.99E+011080 17.68 10.41 4.328 33.6 6.00 68.3 3.11 0.08 1.99E+011110 24.19 9.63 6.826 20.6 5.03 75.7 2.61 0.12 1.84E+011160 19.24 10.25 4.865 30.6 5.94 97.6 3.08 0.08 1.96E+011200 47.78 10.30 15.981 3.4 1.62 99.1 0.84 0.43 1.97E+011280 91.97 11.14 31.474 0.0 0.08 99.9 0.04 1.22 2.14E+011450 902.65 10.42 300.192 1.8 16.83 100.0 8.71 16.44 2.00E+01Total 5.79 3.00 0.21

Lambda 40K = 5.543E-10 J = 0.00028759 ± 0.2% Mass = 204.8 mg

APPENDIX (Cont.)

Cumulate Temp (°C) 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar * 10-2 40Ar* (%) 40Ar*/39Ar(K) 39Ar (%) Age (Ma) ± 1σ Ca/K

40Ar/39Ar GEOCHRONOLOGY OF PORPHYRY-EPITHERMAL DEPOSITS IN THE BAGUIO DISTRICT, PHILIPPINES 1361

0361-0128/98/000/000-00 $6.00 1361

Series 8XXXXX samples processed on 1/07/2004 by Dr. William James Dunlap.

1362 WATERS ET AL.

0361-0128/98/000/000-00 $6.00 1362

Sample 806106 – amphibole800 539.90 1.73 175.525 4.0 21.51 0.6 11.40 34.39 3.29E+00900 327.98 1.19 107.576 3.1 10.33 1.3 5.48 16.08 2.26E+00970 187.23 2.10 57.643 9.1 17.11 1.8 9.07 16.36 4.00E+001030 85.75 7.49 27.966 4.5 3.90 5.9 2.07 0.65 1.43E+011070 32.60 10.85 9.486 17.4 5.72 45.4 3.04 0.15 2.08E+011090 21.50 10.08 6.016 22.0 4.77 65.7 2.53 0.19 1.93E+011120 32.16 10.14 9.896 12.2 3.96 75.1 2.11 0.21 1.94E+011170 24.68 10.40 6.877 21.9 5.45 98.1 2.89 0.17 1.99E+011210 98.28 9.99 33.014 1.8 1.77 98.9 0.94 1.30 1.91E+011300 91.31 10.05 30.803 1.4 1.32 99.9 0.70 0.99 1.92E+011450 1571.43 10.09 518.318 2.6 41.02 100.0 21.67 24.81 1.93E+01Total 5.37 2.85 0.64Lambda 40K = 5.543E-10 J = 0.0002946 ± 0.57% Mass = 208.2 mg

Sample 806107 – biotite720 55.40 0.06 18.142 3.2 1.78 2.0 0.97 0.29 1.07E-01750 69.45 0.08 22.757 3.2 2.20 4.3 1.20 0.37 1.53E-01770 43.54 0.07 13.262 9.9 4.33 5.9 2.37 0.27 1.34E-01800 27.52 0.02 7.565 18.7 5.15 8.1 2.82 0.14 4.33E-02840 13.55 0.01 2.794 38.9 5.27 12.8 2.89 0.06 1.94E-02880 12.16 0.01 2.270 44.6 5.42 17.5 2.97 0.06 1.67E-02920 12.75 0.01 2.352 45.3 5.77 21.6 3.16 0.06 1.57E-02970 13.51 0.01 2.714 40.4 5.47 26.7 2.99 0.06 1.62E-021010 13.44 0.01 2.534 44.1 5.93 30.2 3.24 0.07 1.84E-021060 11.56 0.01 1.894 51.3 5.94 36.7 3.25 0.04 1.72E-021100 8.62 0.01 0.978 66.2 5.70 49.3 3.12 0.02 1.16E-021150 6.96 0.00 0.482 79.2 5.51 72.2 3.02 0.01 9.17E-031220 6.76 0.00 0.453 79.8 5.39 97.9 2.95 0.01 8.35E-031350 26.51 0.10 6.976 22.2 5.87 100.0 3.22 0.17 1.87E-01Total 5.37 2.94 0.05Lambda 40K = 5.543E-10 J = 0.0003037

Sample 806108 – amphibole750 316.72 6.61 102.422 4.6 14.80 1.8 8.20 2.58 1.26E+01800 113.86 2.10 36.811 4.6 5.27 3.4 2.92 0.72 4.00E+00870 102.85 0.75 32.204 7.5 7.75 5.6 4.30 0.69 1.43E+00940 46.36 0.75 13.898 11.5 5.33 8.6 2.96 0.48 1.42E+001000 32.79 1.80 10.009 10.2 3.36 13.4 1.87 0.62 3.43E+001060 28.50 5.69 7.996 19.0 5.45 30.3 3.02 0.52 1.09E+011080 20.90 6.71 5.657 23.2 4.88 46.8 2.71 0.39 1.28E+011110 19.39 7.14 4.635 33.1 6.45 72.1 3.58 0.58 1.36E+011135 22.19 7.46 5.858 25.4 5.67 85.8 3.14 0.33 1.43E+011160 23.47 8.04 6.464 22.0 5.21 95.3 2.89 0.28 1.54E+011185 31.60 8.06 9.958 9.4 2.99 97.5 1.66 1.61 1.54E+011230 44.84 8.71 13.473 13.2 5.94 99.6 3.30 1.21 1.67E+011280 136.27 8.78 29.014 37.7 51.78 99.9 28.53 3.67 1.68E+011450 681.73 8.48 187.292 18.9 130.02 100.0 70.79 27.75 1.62E+01Total 5.99 3.32 0.60Lambda 40K = 5.543E-10 J = 0.00030778 ± 0.25% Mass = 213 mg

Sample 814526 – amphibole850 22.11 0.31 6.868 8.2 1.818 16.2 1.07 0.08 5.88E–01920 20.33 0.92 6.649 3.6 0.741 22.4 0.43 0.14 1.75E+001000 25.83 4.03 8.428 5.1 1.319 30.8 0.77 0.11 7.68E+001060 34.59 11.03 11.450 5.4 1.878 47.4 1.10 0.12 2.11E+011100 22.52 12.25 7.384 8.6 1.958 74.8 1.15 0.07 2.35E+011130 18.16 9.89 5.922 9.1 1.671 90.3 0.98 0.09 1.89E+011180 38.50 13.46 12.767 5.6 2.170 98.4 1.27 0.20 2.58E+011230 97.78 15.28 33.447 0.5 0.499 99.3 0.29 1.25 2.94E+011290 255.78 14.84 85.243 2.1 5.407 99.8 3.17 2.67 2.85E+011340 1997.63 14.45 665.017 1.7 34.19 99.9 19.9 54.66 2.78E+011450 2730.98 14.96 905.187 2.1 58.41 100.0 33.9 70.64 2.88E+01Total 1.881 1.10 0.28Lambda 40K = 5.543E-10 J = 0.00032477 ± 0.67%

APPENDIX (Cont.)

Cumulate Temp (°C) 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar * 10-2 40Ar* (%) 40Ar*/39Ar(K) 39Ar (%) Age (Ma) ± 1σ Ca/K

40Ar/39Ar GEOCHRONOLOGY OF PORPHYRY-EPITHERMAL DEPOSITS IN THE BAGUIO DISTRICT, PHILIPPINES 1363

0361-0128/98/000/000-00 $6.00 1363

Sample 814527 – amphibole

850 1990.09 22.03 635.518 5.7 116.3 2.3 67.1 23.5 4.26E+01950 196.77 16.51 51.590 23.4 46.61 10.7 27.2 1.4 3.18E+011000 89.25 11.76 15.862 48.8 43.97 33.4 25.7 0.2 2.25E+011040 66.03 9.25 9.665 58.2 38.69 59.2 22.6 0.2 1.77E+011080 78.08 10.01 12.870 52.6 41.36 70.1 24.1 0.6 1.92E+011120 90.90 11.20 16.350 48.1 44.13 78.2 25.8 0.4 2.15E+011170 108.23 12.33 20.794 44.4 48.50 91.1 28.3 0.4 2.36E+011220 199.82 58.06 41.312 41.9 87.74 98.7 50.8 0.8 1.16E+021270 655.94 362.53 149.598 38.3 351.3 99.9 195.5 3.9 9.63E+021350 3490.03 208.91 1171.543 1.4 58.10 99.9 33.8 287.5 4.75E+021450 9238.16 211.39 3070.007 2.0 223.0 100.0 126.5 812.6 4.82E+02Total 51.77 30.2 1.7

Lambda 40K = 5.543E-10 J = 0.00032573

Sample 814528 – amphibole

950 519.88 12.26 164.526 6.7 35.33 2.9 20.5 2.9 2.35E+011000 88.58 11.31 18.819 38.5 34.43 9.3 20.0 0.4 2.17E+011040 56.23 8.86 7.439 62.5 35.39 30.0 20.5 0.1 1.70E+011070 44.19 8.42 3.544 78.2 34.78 57.4 20.2 0.1 1.61E+011110 45.12 8.78 4.112 75.0 34.08 74.0 19.8 0.1 1.68E+011150 44.16 9.37 3.693 77.4 34.44 89.2 20.0 0.1 1.79E+011190 50.11 9.51 5.937 66.9 33.77 95.3 19.6 0.2 1.82E+011250 57.67 17.03 7.910 62.5 36.51 99.5 21.2 0.3 3.28E+011310 319.61 20.82 98.822 9.3 30.16 99.8 17.5 3.5 4.02E+011450 2060.70 19.85 687.543 1.5 31.68 100.0 18.4 46.0 3.83E+01Total 34.72 20.2 0.3

Lambda 40K = 5.543E-10 J = 0.00032349

APPENDIX (Cont.)

Cumulate Temp (°C) 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar * 10-2 40Ar* (%) 40Ar*/39Ar(K) 39Ar (%) Age (Ma) ± 1σ Ca/K