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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, [email protected]
©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.
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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
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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.
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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
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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.
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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
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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).
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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
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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
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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.
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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
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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.
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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
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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.
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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
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Series 35XXXX samples processed on 2/03/2002 by Dr. William James Dunlap.
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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
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Series 8XXXXX samples processed on 1/07/2004 by Dr. William James Dunlap.
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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
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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