metallogenesis of porphyry cu deposits of the western luzon arc, k-ar ags, so3 apatite

1. Introduction

Cenozoic arc magmatism along converging plateboundaries of the western Pacific rim has generatednumerous porphyry-type Cu deposits. A chain of por-phyry Cu deposits from the Philippines through PapuaNew Guinea to the Solomon Islands defines a significantmetallogenic province. The purpose of this paper is todocument the characteristics of the magmatic arc associ-ated with porphyry Cu deposits, in the western Luzon arcas an example. In this paper, whole-rock K-Ar ages ofpotassically altered intrusive rocks of some porphyry Cudeposits of the western Luzon arc are reported to repre-sent the age of hydrothermal biotitization associated withporphyry Cu mineralizations.

The intermediate to silicic intrusive rocks which aregenetically associated with porphyry Cu mineralizationgenerally belong to the magnetite-series, a relatively oxi-dized granitoid type (e.g., Ishihara, 1975, 1977, 1981,1998). Magmatic water saturation and highly oxidizingnature have been suggested for the intrusive rocks relatedto porphyry Cu deposits (Mason, 1978; Chivas, 1981;Imai, 2000a, 2001). Imai et al. (1993, 1996) and Imai(2001) documented the petrologic similarities between the

ore-generating intrusion at the Santo Tomas II (Philex)deposit and the pumices erupted from Mount Pinatubo in1991 in terms of magmatic water saturation and high mag-matic fO2. Microphenocrystic apatite in Mount Pinatubodacitic pumices and intrusive rocks associated with theSanto Tomas II deposits and in the vicinity exhibit signifi-cant SO3 contents (Imai et al., 1993, 1996; Imai, 2001).Such high SO3 contents in microphenocrystic apatite sug-gest that sulfur is dominantly accommodated as oxidizedspecies in oxidizing hydrous magma. Chemical composi-tions of microphenocrystic apatite in intrusions associatedwith porphyry Cu deposits and other late Cenozoic inter-mediate to silicic intrusive rocks throughout the westernLuzon arc are further examined in this paper.

Relationships between the adakitic rocks and Cu-Aumineralization have been argued (e.g., Sajona and Maury,1998). Oyarzun et al. (2001) argued that giant porphyryCu deposits in Chilean Andes are associated with adakiticrocks. Some recent papers reported the occurrence ofadakitic rocks in the western Luzon arc and discussedtheir geneses (e.g., Yumul et al., 2000). In this paper,whole-rock major and trace element compositions of inter-mediate to silicic rocks of the late Cenozoic westernLuzon arc including those associated with porphyry Cudeposits will be documented. Then, the significance to

147

RESOURCE GEOLOGY, Vol. 52, no. 2, 147161, 2002

Metallogenesis of Porphyry Cu Deposits of the Western Luzon Arc, Philippines: K-Ar ages, SO3 Contents of Microphenocrystic

Apatite and Significance of Intrusive Rocks

Akira IMAI

Department of Earth and Planetary Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan[e-mail: [email protected]]Received on September 19, 2001; accepted on February 20, 2002

Abstract: K-Ar ages of the following porphyry Cu deposits in the western Luzon arc are determined: Lobo-Boneng (10.50.4Ma), Santo Nio (9.50.3 Ma), Black Mountain (2.10.1 Ma), Dizon (2.50.2 Ma) and Taysan (7.30.2 Ma). Microphenocrys-tic apatite in the late Cenozoic intermediate to silicic intrusions associated with porphyry Cu deposits in the western Luzon arccontains sulfur as SO3 detectable by electron probe microanalyzer. Sulfur is supposed to have been accommodated dominantlyas oxidized species in oxidizing hydrous magmas that generated porphyry Cu deposits. Likewise, such high SO3 contents inmicrophenocrystic apatite are common characteristics of the intermediate to silicic magmatism of the western Luzon arc, fromtonalitic rocks of the Luzon Central Cordillera of about 15 Ma to an active magmatism at Mount Pinatubo. Thus, the westernLuzon arc has been generating porphyry Cu mineralization associated with oxidizing hydrous intermediate to silicic magmatismrelated to eastward subduction, since Miocene to the present day. Intermediate to silicic rocks since 15 Ma to present-day west-ern Luzon arc generally show high whole-rock Sr/Y ratio ranging from 20 to 184. However, porphyry Cu deposit is not neces-sarily related to the rocks that show higher Sr/Y ratios compared to the other barren rocks in the western Luzon arc. The charac-teristics of the intermediate to silicic magma associated with porphyry Cu deposit are not attributed to the composition of thesource material of the magma, but to the properties defined by the high activity of oxidized species of sulfur in the fluid phasethat is encountered during the generation of intermediate to silicic magmas.

Keywords: porphyry Cu deposit, western Luzon arc, Philippines, K-Ar age, apatite, sulfur, metallogeny, adakite

the metallogenesis of the porphyry Cudeposits in the late Cenozoic westernLuzon arc will be discussed.

2. Geologic Background

The Philippine archipelago is situated inthe western Pacific rim along the plate con-verging margin, being subducted both at theManila Trench in the west and at the Philip-pine trench in the east (e.g., Yang et al.,1996). A number of porphyry Cu depositshave been formed through the Cenozoic inthe island arc setting (Sillitoe and Gappe,1984; Zanoria et al., 1984; Bureau of Minesand Geosciences, 1986). A chain of por-phyry Cu deposits of the western Luzon arc(Fig. 1) delineates late Cenozoic metallo-genic province typical of the western Pacificmargin. The chain of porphyry Cu depositsincludes the Lepanto-Far Southeast (FSE)(Concepcon and Cinco, 1989; Hedenquistet al., 1998; Imai, 2000b), the Guinaoang(Tirad) (Sillitoe and Angeles, 1985), theLobo and Boneng (Jacinto, 1977), the SantoNio (Bryner, 1970), the Black Mountain(Kennon) (Togonan, 1977), and the SantoTomas II (Serafica and Baluda, 1977;Piestrzynski et al., 1994; Tarkian and Koop-mann, 1995; Imai, 2001) deposits of thewestern flank of the Philippine CentralCordillera in northern Luzon, the Dizondeposit (Malihan, 1982, 1987) in the centralwestern Luzon, and the Taysan deposit(Wolfe et al., 1978) in the southwesternLuzon. These porphyry type deposits aregenetically associated with complexes ofintermediate to silicic intrusions of calc-alkaline affinity, that comprise the ancient western Luzonmagmatic arc initiated by eastward subduction ofEurasian plate (e.g., Mitchell and Leach, 1991).

The Philippine archipelago is composed of aggregatedallochtonous crustal fragments and overlappingautochtonous magmatic arcs. The Philippine Fault, agroup of sinistral strike slip faults, traverses along theaxis of the archipelago from Luzon to Mindanao, and isinterpreted as a collision suture due to the NW-wardoblique convergence of the Philippine Sea Plate againstthe eastern margin of the Eurasian Plate (e.g., Rangin,1991). Miocene (15 to 9 Ma; Wolfe, 1972, 1981) inter-mediate to silicic, mainly dioritic (e.g., Yumul et al.,1995) intrusive complexes named as the Agno Batholithin the Baguio mineral district, comprise the backbone ofthe Luzon Central Cordillera and extend N-S to the

northern edge of Luzon Island (e.g., Bureau of Minesand Geosciences, 1982) (Fig. 1).

A number of small intermediate to silicic intrusions ofupper Miocene to Plio-Pleistocene occur in the Baguiomineral district. Some of these intrusions are accompa-nied with porphyry-type mineralization (Balce, 1979;Balce et al., 1980; Serafica et al., 1977; Mitchell andBalce, 1990). However, volcanic landform is scarcely pre-served except for some domes and plugs/necks presum-ably due to rapid erosion rate as well as rapid uplifting.These magmatisms are supposed to have stimulatedhydrothermal systems at shallow levels as presentlyobserved as Plio-Pleistocene epithermal systems and asso-ciated argillic and advanced argillic alterations in the dis-trict (e.g., Mitchell and Balce, 1990; Aoki et al., 1993). Inaddition to the common calc-alkaline series rocks, recent

A. IMAI148 RESOURCE GEOLOGY :

Fig. 1 Map showing location, geologic setting and K-Ar age (Ma) of majorporphyry Cu deposits, western Luzon arc, Philippines. Modified afterBureau of Mines and Geosciences (1982, 1986) and Sillitoe and Gappe(1984). Sources of K-Ar ages; Lepanto-Far Southeast: Arribas et al.(1995), Guinaoang: Sillitoe and Angeles (1985), Lobo: this study, SantoNio: this study, Santo Tomas II: (Imai, 2001), Black Mountain: thisstudy, Dizon: this study, Taysan: this study.

petrochemical investigations suggested the presence ofPliocene-Quaternary lavas and intrusive rocks showingadakitic characteristics (Prouteau et al., 2000; Yumul etal., 2000).

The western Luzon arc magmatism is presently appar-ently active in its southern segment (De Boer et al., 1980;Datuin, 1982; Defant et al., 1988, 1989, 1991;PHIVOLCS, 1991; Knittel et al., 1997). Mount Pinatubocalc alkaline dacitic pumices erupted in June, 1991 arecharacterized by high fO2 and by high sulfur contentsdemonstrated by the presence of microphenocrysticanhydrite (Bernard et al., 1991, 1996; Pallister et al.,1992; Imai et al., 1993, 1996; Hattori, 1993, 1996;Kress, 1997). Sulfur is considered to have existed domi-nantly as oxidized species in the magma (e.g., Imai etal., 1993, 1996; Rutherford and Devine 1996). Petrolog-ic evidences suggest the magmatic vapor saturation pri-or to the eruption (Westrich and Gerlach, 1992; Gerlachet al., 1996; Imai et al., 1993, 1996).

3. Reported Ages of Porphyry Cu Deposits and Intru-sive Rocks

Ages of intrusive rocks and of porphyry Cu depositshave been reported from the western Luzon arc.

The Lepanto high sulfidation epithermal enargite/luzonite Cu-Au deposit (Gonzalez, 1956; Garcia, 1991;Imai, 1999; Claveria, 2001), Victoria low sulfidationepithermal Au deposit (Claveria, 2001) and the underly-ing Lepanto-FSE Au-rich porphyry Cu deposit as well asthe Guinaoang porphyry Cu deposit are located in thenorth of Baguio mineral district. Arribas et al. (1995)reported that the ages of the high-sulfidation epithermalLepanto enargite/luzonite Cu-Au deposit and the por-phyry Cu-Au mineralization of the FSE deposit are almostcontemporaneous within 0.3 m.y., at ca. 1.4 Ma. A K-Arage on sericite from the Guinaoang (Tirad) deposit wasdated as 3.50.9 Ma (Sillitoe and Angeles, 1985).

Serafica and Baluda (1977) reported the intrusive suc-cession at the Santo Tomas II deposit. Fission track agesdetermined on microphenocrystic apatite yields 2.10.5Ma for the "clear diorite" and 1.40.4 Ma for the "ande-site porphyry" (Philex Mining Corporation unpublisheddata). Togashi et al. (1990) reported the K-Ar ages of the"dark diorite" as 5.91.6 and 3.81.1 Ma for the leastaltered quartz diorite and hydrothermally altered quartz

diorite porphyry, respectively. The K-Ar age of dioriteporphyry was reported as 1.90.5 Ma, and the age ofquartz diorite as < 2.8 Ma. Bellon and Yumul (2000)reported whole rock K-Ar ages of diorite from the San-to Tomas II deposit as 3.70.4 and 3.40.8 Ma and a K-Ar age of andesite porphyry as 2.30.2 Ma. Imai (2001)reported the age of mineralization at the Santo Tomas IIdeposit as 1.50.4 Ma.

Within the vicinity of the Santo Tomas II deposit in anarea of several kilometers across, intrusive complexesthat have broadly similar petrography, composition andages are known (Serafica et al., 1977). Bellon and Yumul(2000) reported a K-Ar age of diorite in the Cliftonarea as 2.30.2 Ma. Imai (2001) reported the K-Ar agesof the intrusive rocks in the vicinity of the Santo TomasII deposit as follows, andesite porphyry at Clifton(1.70.6 Ma), andesite porphyry at Ligay (Binang)(1.80.4 Ma), biotitized quartz diorite porphyry at Bumo-lo (Waterhole) (1.80.2 Ma) and andesite porphyry atPhilex Main Camp (1.80.6 Ma).

A K-Ar age on sericite from the Dizon deposit wasreported as 2.7 Ma (Malihan, 1987).

Wolfe (1972) reported K-Ar ages of coarse-grainedquartz diorite/tonalite, a part of the Agno batholith collect-ed from east of the Santo Tomas II deposit as about 15Ma (14.80.8 and 15.01.5 Ma). Another intrusive rockcollected from south of Baguio yielded a K-Ar age of9.71.0 Ma (Wolfe, 1972). Japan International Coopera-tion Agency (JICA) (1983) reported K-Ar ages of the Ito-gon granodiorite collected from east of Baguio as21.70.9 and 17.20.9 Ma, whereas Shannon (1979)reported a fission track age of 132 Ma. A K-Ar age ofthe Virac granodiorite, the major host to the Acupanepithermal vein system in the Baguio mineral district, wasreported as 5.20.3 Ma (JICA, 1983), while a fission trackage of 2.40.5 Ma was reported by Shannon (1979). Inaddition, a fission track age of 81 Ma was obtained forthe Antamok diorite in the Baguio mineral district (Shan-non, 1979). Bellon and Yumul (2000) reported K-Ar agesof intrusive rocks near the Santo Tomas II deposit as13.10.3 and 15.50.5 Ma, near the Antamok deposit as11.90.7, 13.40.3, 14.71.6, 18.12.0 and 19.91.2 Ma,and near the Santo Nio deposit as 16.90.4 Ma.

The K-Ar ages of intrusive rocks distributed aroundthe Taysan deposit, Batangas, southern Luzon, werereported by Wolfe (1972) and Wolfe et al. (1978) as

Porphyry Cu Metallogeny, Western Luzon Arc, Philippines 149vol. 52, no. 2, 2002

Table 1 Whole-rock K-Ar age determination of porphyry Cu deposits.

SampleID Locality K2O (wt%) Rad 40Ar K-Ar Age (Ma) Non Rad Ar (%)

08413 Lobo 0.794 0.016 32.40 0.95 10.49 0.37 58.206328 Santo Nio 1.465 0.029 53.95 0.92 9.47 0.25 34.801417 Camp 6 (Black Mountain) 1.843 0.037 15.14 0.83 2.12 0.12 72.010324 Dizon 0.864 0.017 8.39 0.78 2.50 0.23 84.7CT-3-4 Taysan 1.353 0.027 38.53 0.80 7.32 0.21 43.8

9.20.2 and 14.90.9 Ma, respectively.

4. Geologic Outline of the Deposits and K-Ar Ages

K-Ar age determination was carried out convention-ally at the Hiruzen Institute of Okayama Science Uni-versity. Results of whole-rock K-Ar age determinationof the potassically altered intrusive rocks of the Lobo-Boneng, Santo Nio, Black Mountain, Dizon, Taysandeposits are presented in Table 1 and in Figure 1.

4.1. Lobo and Boneng deposits

The Lobo and Boneng deposits are located north ofthe Baguio mineral district. Two discrete orebodies arelocated in a few hundred meters apart. The porphyry Cumineralizations are associated with a quartz diorite por-phyry intrusion, which was intruded by post-ore daciteporphyry and hornblende diorite porphyry dikes (Jacin-to, 1977). The mineralization is accompanied with thequartz veinlet stockwork, associated with silicificationand hydrothermal biotitization. Hydrothermal tremolite-actinolite and chlorite alterations envelop the fringezone. A K-Ar age determined for a sample of biotitizedquartz diorite porphyry from the Lobo deposit, whichrepresents the biotitization, yielded 10.50.4 Ma.

4.2. Santo Nio (Southwest) deposit

The Santo Nio deposits, situated north of the Baguiomineral district, consist of two orebodies, the Ullmanand Southwest deposits (Bryner, 1970). A coarse-grained, equigranular intrusion of dioritic composition,exposed widely in this area, was intruded by porphyriesof intermediate to silicic composition. Among them, adacite porphyry intrusion has been considered to begenetically related to the porphyry Cu mineralization(Bryner, 1970). Potassic alteration characterized byhydrothermal biotitization is extensive along the intru-sive contact between the older coarse-grained dioriteand the dacite porphyry intrusion.

A sample of potassically altered coarse-grained dioritewas subjected to K-Ar age determination. Primary maficminerals such as hornblende are totally replaced by poly-crystalline aggregates of hydrothermal biotite. In addition,primary feldspars are also replaced by an aggregate ofmuscovite. Thus, the K-Ar age of 9.50.3 Ma suggeststhe age of biotitization associated with mineralization.

4.3. Black Mountain (Kennon) deposit

The Black Mountain deposits, consisting of two ore-bodies, the Kennon and Southeast deposits (Togonan,1977), are located at Camp 6, southern Baguio mineraldistrict. Stocks and swarm of dikes of quartz dioriticintrusions are exposed in this area. A relatively coarse-grained hornblende quartz diorite exposed to the south

is intruded by swarms of hornblende quartz diorite por-phyries having corroded phenocrystic biotite. Thesequartz diorite porphyry intrusions are associated withporphyry Cu mineralizations at the Kennon and South-east deposits in addition to a Au-rich skarn deposit atthe Thanksgiving mine (Callow, 1967).

The Cu mineralization at the Kennon deposit is associ-ated with quartz veinlet stockworks in the zone of potassicalteration characterized by polycrystalline aggregates ofhydrothermal biotite that replaced phenocrystic horn-blende in the quartz diorite porphyry. A K-Ar age deter-mined for the sample, that represents the intense biotitiza-tion of the quartz diorite porphyry, yielded 2.10.1 Ma.

4.4. Dizon deposit

The Dizon deposit is located in the western centralLuzon, Zambales, along the present-day Zambales-Bataan-Batangas volcanic chain. The host rocks to theintrusions at the Dizon deposit are Neogene subaerialvolcaniclastics overlying the Zambales ophiolite com-plex (Malihan, 1982, 1987).

A K-Ar age was determined on the potassicallyaltered fine-grained diorite porphyry from the orebodycenter, which is exposed at the bottom of the open pit.Hornblende phenocrysts are totally replaced by aggre-gates of fine-grained hydrothermal biotite, and the K-Arage yielded 2.50.2 Ma. This is consistent with the ageof 2.7 Ma previously reported on hydrothermal sericite(Malihan, 1987).

4.5. Taysan deposit

The Taysan deposit is located in Batangas, southernpart of western Luzon. The Cu mineralization at theTaysan deposit occurred at the western edge of the Tolosbatholith (Wolfe et al., 1978). The Tolos batholith con-sists mainly of biotite quartz diorite in the eastern part,which grades gradually westward into hornblende quartzdiorite and hornblende diorite. Younger multiple intru-sions occurred at the western and northwestern margin ofthe batholith. The younger intrusions consist mainly ofdacite porphyry. A quartz diorite porphyry dike was datedas 14.8 Ma (Wolfe et al., 1978).

Diorite and diorite porphyry are the main host rocksto mineralization. The K-Ar age of the biotitized dioriteporphyry was determined as 7.30.2 Ma. Since primaryhornblende is totally replaced by polycrystalline aggre-gates of hydrothermal biotite, the obtained K-Ar agesuggests the age of biotitization associated with miner-alization.

5. Chemical Composition of Microphenocrystic Apatite

Mineral chemistry of accessory microphenocrysticapatite in the intrusive rocks associated with porphyry Cu


mineralizations at Lepanto-FSE,Lobo-Boneng, Black Mountain,Dizon, and Taysan deposits, wereexamined. In addition, the miner-alizing and barren intrusive rockswith least-altered and biotitizedsamples from the Santo Tomas IIdeposit and other intrusions in theClifton, Binang (Ligay) andBumolo (Waterhole) areas arequoted from Imai (2001). In addi-tion, compositions of accessoryapatite of the Miocene batholithsof the Luzon Central Cordillerawere examined. Samples werecollected from outcrops at Agno,Virac, and Natubleng. As well asmicrophenocrystic apatite indacitic rocks from Mount Pinatubopumices (Imai et al., 1993, 1996),shallow-depth intrusions at Camp4 and New CT areas, and thoseoutcropping along the PhilexRoad and Naguilian Road werealso studied.

Chemical composition of apa-tite was determined by a wavelength dispersive electron probemicroanalyzer, JEOL JCMA 733mkII at the Department of Earthand Planetary Science, Universityof Tokyo, with acquisition time of20 seconds for each elements attheir characteristic X-ray, withbackground directly counted for10 seconds each. Determinationswere made at 15 kV and 1.210-8

A, and computed by conventionalZAF calculation using factors andprogram supplied by JEOL.Localities of samples studied are shown in Figure 2. TheCl, F and SO3 contents in accessory microphenocrysticapatite are presented in Table 2 and the average Cl andSO3 contents are plotted in Figure 3.

Significant Cl contents in apatite in intrusive rocks atthe Santo Tomas II deposit and other intrusions in theClifton, Binang (Ligay) and Bumolo (Waterhole) areas(Imai, 2001) are clearly shown in Figure 3. Whereaslower than those of the Santo Tomas II deposit, the Clcontents in apatite in mineralizing intrusive rocks at theLepanto FSE, Lobo-Boneng, Black Mountain, Dizonand Taysan deposits are higher than those of theMiocene batholiths of the Luzon Central Cordillera.The importance of Cl in hydrothermal fluids has been

argued in the genesis of ore deposits associated withgranitic intrusion (Holland, 1972; Ishihara and Imai,2000; Imai and Anan, 2000; Imai, 2000a, 2000b, 2001).Elevated Cl contents in apatite in mineralizing intrusiverocks at the Santo Tomas II deposit (Imai, 2001) indi-cate whether hypersaline brine existed during crystal-lization or exchange reaction of Cl between initial low-Cl apatite and hypersaline brine after crystallization.The hypersaline brine is present under two-fluid immis-cible region (e.g., Sourirajan and Kennedy, 1962; Clineand Bodnar, 1991), encountered at shallow levels (Bod-nar et al., 1985) during crystallization of phenocrysticphases, or subsequent exchange reaction of Cl betweenthe initially low Cl apatite and hypersaline brine.


Fig. 2 Map showing locality of intermediate to silicic rock samples in whichchemical composition of accessory microphenocrystic apatite was determined.Modified after Bureau of Mines and Geosciences (1982, 1986) and Sillitoe andGappe (1984).


Table 2 Cl, F and SO3 contents of accessory microphenocrystic apatite.

Locality Sample ID Rock type n Cl (wt%) F (wt%) SO3 (wt%)ave 1 ave 1 ave 1 max

Intrusive Rocks Associated with Porphyry Cu DepositsMankayan/Lepanto-Far 81-26

quartz diorite porphyry (alt) 44 1.27 0.51 3.14 0.50 0.16 0.10 0.53Southeast 2015-2152Mankayan/Imbanguila 041114/4 dacite porphyry 81 1.29 0.44 3.89 0.65 0.20 0.08 0.52Mankayan/Bato 051115/5 dacite porphyry 129 1.70 0.12 1.98 0.26 0.18 0.09 0.57Mankayan/Bato 011209 dacite porphyry 100 1.67 0.39 2.24 0.53 0.19 0.09 0.62Lobo-Boneng 01413A porphyritic quartz diorite 57 1.69 0.53 2.08 0.32 0.20 0.08 0.37Lobo-Boneng 02414 quartz diorite porphyry 29 1.85 0.75 2.25 0.61 0.21 0.10 0.43Lobo-Boneng 13414 dacite porphyry 0 ----- ----- ----- ----- ----- ----- -----Santo Nio 11412 quartz diorite porphyry (alt) 46 0.67 0.32 3.02 0.39 0.07 0.05 0.26Santo Tomas II (Philex) 05401 cpx andesite porphyry 73 2.52 0.33 1.82 0.26 0.24 0.05 0.40Santo Tomas II (Philex) 11405 andesite porphyry 68 3.09 0.39 1.50 0.29 0.30 0.07 0.45Santo Tomas II (Philex) 04401 andesite porphyry 75 2.39 0.11 1.78 0.24 0.25 0.07 0.62Santo Tomas II (Philex) 06405A andesite porphyry (alt) 61 4.26 0.55 0.98 0.25 0.25 0.08 0.69Santo Tomas II (Philex) 15331* andesite porphyry (alt) 46 3.81 1.14 1.15 0.48 0.29 0.08 0.46Santo Tomas II (Philex) 08401 quartz diorite porphyry 59 2.26 0.42 1.79 0.38 0.26 0.08 0.43Santo Tomas II (Philex) 08331 quartz diorite porphyry (alt) 84 2.33 0.40 1.89 0.36 0.26 0.10 0.84Santo Tomas II (Philex) 11331 quartz diorite porphyry (alt) 86 3.14 0.24 1.42 0.20 0.28 0.08 0.62Santo Tomas II (Philex) 04410 porphyritic quartz diorite 32 2.17 0.67 1.84 0.77 0.20 0.09 0.46Santo Tomas II (Philex) 34Q-1 porphyritic quartz diorite (alt) 38 1.92 0.49 2.02 0.52 0.22 0.11 0.54Clifton CLF1-4 andesite porphyry 41 3.20 0.93 1.26 0.30 0.29 0.14 0.76Clifton 02402 cpx andesite porphyry 51 2.83 0.12 1.56 0.21 0.27 0.08 0.48Clifton X6403* cpx andesite porphyry (alt) 96 3.07 0.19 1.40 0.24 0.30 0.06 0.46Clifton 08402 quartz diorite porphyry 51 3.32 1.26 1.27 0.30 0.23 0.10 0.48Clifton CLF1-10 porphyritic quartz diorite 44 2.67 0.88 1.83 0.58 0.22 0.13 0.69Clifton CLF1-12 porphyritic quartz diorite 49 2.01 0.94 2.34 0.59 0.15 0.12 0.43Ligay (Binang) BIN-3 cpx andesite porphyry 24 2.72 0.71 1.29 0.25 0.34 0.21 0.70Bumolo (Waterhole) WH9 quartz diorite porphyry 31 2.54 0.86 1.77 0.44 0.21 0.15 0.59Bumolo (Waterhole) WH8 cpx-porphyritic quartz diorite 29 2.14 0.59 1.86 0.34 0.10 0.06 0.24Bumolo (Waterhole) WH6 porphyritic quartz diorite 33 2.67 0.81 1.63 0.41 0.15 0.10 0.33Philex/Main Camp 10404 cpx-andesite porphyry 0 ----- ----- ----- ----- ----- ----- -----Camp 6 (Black Mountain) X3415 andesite poprhyry 50 0.98 0.20 2.69 0.30 0.12 0.09 0.48Camp 6 (Black Mountain) 01417* andesite porphyry (alt) 51 2.41 0.58 1.98 0.31 0.18 0.08 0.56Camp 6 (Black Mountain) 01415 porphyritic quartz diorite 51 0.81 0.21 2.36 0.27 0.19 0.11 0.74Dizon 011220 quartz diorite porphyry 45 0.86 0.54 2.74 0.41 0.09 0.09 0.31

inclusions 12 1.52 0.66 2.48 0.45 0.22 0.06 0.31discrete 33 0.63 0.16 2.83 0.35 0.05 0.05 0.18

Dizon 031103A quartz diorite 40 1.14 0.51 2.24 0.30 0.05 0.03 0.11Dizon 011107 dacite porphyry 106 1.33 0.29 2.18 0.46 0.17 0.08 0.57Taysan CT2-13 quartz diorite porphyry 40 1.47 0.61 2.73 0.42 0.20 0.11 0.55Taysan BT58-1 quartz diorite 77 1.05 0.38 2.48 0.37 0.14 0.09 0.33

Late Miocene-Pliocene-Pleistocene Shallow IntrusionsNaguilian Road 03328B dacite porphyry 27 1.00 0.12 3.29 0.33 0.52 0.23 0.88Philex Road 06410 andesite porphyry 13 1.27 0.35 2.24 0.28 0.50 0.10 0.65New CT 10407B dacite porphyry 71 0.64 0.12 2.60 0.27 0.20 0.11 0.49Camp 4 03416 andesite porphyry 52 1.31 0.56 1.96 0.47 0.41 0.20 0.98Camp 4 01416B porphyritic quartz diorite 9 0.96 0.16 2.52 0.20 0.03 0.02 0.06

Miocene Intrusive RocksNatubleng 011111 porphyritic quartz diorite 22 0.71 0.77 3.34 0.81 0.20 0.15 0.55Natubleng 021111 porphyritic quartz diorite 37 0.63 0.50 2.99 0.40 0.07 0.08 0.47Near Santo Nio 20412 quartz diorite 83 0.77 0.35 3.13 0.39 0.03 0.03 0.09Virac (Balatoc-Acupan) 741-27 quartz diorite 85 1.15 0.89 2.58 0.62 0.05 0.04 0.21Agno (East of Philex) 07404 quartz diorite 98 0.42 0.16 3.30 0.39 0.13 0.06 0.49Agno (East of Philex) 09404 quartz diorite 58 0.86 0.13 2.74 0.25 0.15 0.07 0.27Agno (East of Philex/New CT) 13407 quartz diorite 71 0.76 0.26 3.07 0.31 0.10 0.05 0.27

Rocks Effused from Mount PinatuboPinatubo (pumice) fall-1 crystal-rich dacite 53 1.17 0.14 1.77 0.30 0.17 0.13 0.78Pinatubo (pumice) inclusions 21 1.26 0.09 1.83 0.38 0.24 0.19 0.78Pinatubo (pumice) discrete 32 1.11 0.14 1.74 0.23 0.13 0.05 0.31Pinatubo (pumice) fall-2 crystal-poor dacite 9 1.23 0.14 1.66 0.43 0.10 0.02 0.13Pinatubo (flow) flow-1 crystal-rich dacite 21 1.28 0.11 2.06 0.22 0.35 0.21 0.83Pinatubo (dome?)Sacobina river 011117A1 dacite 75 0.98 0.21 3.03 0.53 0.13 0.12 0.91Pinatubo (dome?)Sacobina river 011117A2 cpx dacite 13 1.36 0.15 2.10 0.23 0.19 0.14 0.49Pinatubo (dome?)Sacobina river 011117A3 cpx dacite 22 1.16 0.07 2.29 0.27 0.17 0.07 0.32

n denotes the number of analysis. * indicates no whole rock analysis in Table 3. (alt) indicates hydrothermally altered. cpx indicates clinopyroxene-bearing.

Significant SO3 contents in microphenocrystic apatitewere reported from Mount Pinatubo dacitic pumices(e.g., Imai et al., 1993, 1996), with the highest SO3 con-tents being about 0.8 wt% (Table 2). Imai (2001) docu-mented that the average SO3 contents in microphe-nocrystic apatite in the intrusive rocks at the SantoTomas II deposit and in the vicinity are generally >0.2wt% with the highest being >0.6 wt%. ConsiderableSO3 contents in accessory apatite in the intrusive rocksare recognized at the Lepanto-FSE, Lobo-Boneng,Black Mountain, Dizon and Taysan deposits, with aver-age SO3 generally >0.1 wt% and the highest >0.3 wt%,whereas the average SO3 content at the Santo Niodeposit is not significant (Fig. 3). The average SO3 con-tents of accessory apatite in the intrusive rocks of thebatholiths (samples from Agno, Virac and Natubleng)range from 0.05 to 0.2 wt% with the highest varyingfrom 0.1 to 0.55 wt%. On the other hand, the averageSO3 contents of microphenocrystic apatite in quenchedshallow intrusions (samples from Camp 4, New CT,Naguilian Road and Philex Road) range from 0.2 to >0.5 wt% with the highest of 0.98 wt% (Table 2, Fig. 3).

The elevated SO3 contents in microphenocrysticapatite indicate that sulfur has been accommodated inmagmas dominantly as oxidized species (e.g., Imai etal. 1993, 1996), whereas the partitioning of SO3between apatite and the coexisting melt with respect tofO2 and temperature has not been quantitatively estab-lished (Peng et al., 1997).

6. Whole-rock Chemistry

Whole-rock compositions were determined by meansof X-ray fluorescence spectroscopy using a PhilipsPW1480 at the Department of Earth and Planetary Sci-ence, University of Tokyo. Major element oxides andtrace element concentrations were determined on afused glass bead, according to the procedure describedby Tanaka and Orihashi (1997).

Whole-rock compositions of the least-altered intru-sive rocks associated with porphyry Cu mineralizationsat Lepanto-FSE, Lobo-Boneng, Black Mountain, Dizonand Taysan deposits, were determined, in addition tothe mineralizing and barren intrusive rocks of the SantoTomas II deposit and other intrusions in the Clifton,Binang (Ligay), Bumolo (Waterhole) and Main Campareas (Imai, 2001). In addition, whole-rock composi-tions of the Miocene batholiths of the Luzon CentralCordillera were analyzed. Samples were collected fromoutcrops at Agno, Virac and Natubleng. In addition towhole-rock compositions of dacitic rocks from MountPinatubo, those of shallow-depth intrusions at Camp 4and New CT areas, and those outcropping along thePhilex Road and Naguilian Road were also determined.The whole-rock SiO2 contents range from 50.5 to 67.1wt%, while the majority varies from 54 to 63 wt%(Table 3). Some rocks exhibited by low SiO2 contents,such as the andesite porphyry from an outcrop along thePhilex Road (#06410, Table 3) are characterized by anaccumulation of significant abundance (occasionally>30 % in volume) of coarse-grained (sometimes >2 cmlong) phenocrystic hornblende.

The studied rocks are characterized by relatively highSr contents, ranging from 417 to 1428 ppm. The pres-ence of Miocene-Quaternary rocks in the western Luzonarc showing characteristics of adakitic rocks has beenpointed out (Prouteau et al., 2000; Yumul et al., 2000).The high Sr/Y ratio is one of the characteristics of theadakitic rocks, as displayed by Y versus Sr/Y diagram(Defant and Drummond, 1990; Fig. 4) and the Sr/Yratios of the studied rocks range from 15.5 to 182.5(Table 3). The Sr/Y ratio of the andesite porphyry froman outcrop along the Philex Road characterized by anaccumulation hornblende is 31.7. On the other hand, thehigh Sr/Y ratios were exhibited by dacite at Naguilian(Sr/Y ratio=182.5, Sr=1416.6 ppm), andesite porphyryat Camp 4 (Sr/Y=105.8 and 99.5, Sr=1182.4 and 1427.8ppm, respectively), dacite porphyry at Santo Nio(Sr/Y=134.2, Sr=982.3 ppm) and post-ore dacite por-phyry at Boneng (Sr/Y=68.7, Sr=1010.0 ppm). Therange of Sr/Y ratio and Sr contents of these rocks aresimilar to the adakites reported from the western Luzonarc by Yumul et al. (2000). These rocks are character-


Fig. 3 Cl and SO3 contents in accessory microphe-nocrystic apatite in intermediate to silicic rocks, west-ern Luzon arc. Average contents are plotted. :Mankayan (Lepanto-Far Southeast, Imbanguila, Bato),: Lobo-Boneng, : Santo Nio, : Santo Tomas II(Philex), Clifton, Ligay (Binang), Bumolo (Water-hole), Philex Main Camp, : Camp 6 (Black Moun-tain), : Dizon, : Taysan, : Naguilian, : PhilexRoad, : New CT, : Camp 4, : Miocene intrusiverocks (Natubleng, near Santo Nio, Virac (Balatoc-Acupan), Agno), : Pinatubo.


Table 3 Whole-rock major and trace element composition.Sample ID 81-26 2015 041114/4 051115/5 011209 01413A 10414 13414 11412' 05401 11405 04401 06405 08401 08331Locality Mankayan Mankayan Mankayan Mankayan Lobo Boneng Boneng Santo Santo Santo Santo Santo Santo Santo

Lepanto Imbanguila Bato Bato Nio Tomas II Tomas II Tomas II Tomas II Tomas II TomasIIFSE Philex Philex Philex Philex Philex Philex

Rock type quartz dacite dacite dacite porphyritic quartz dacite dacite cpx- andesite andesite andesite quartz quartz diorite quartz diorite porphyry porphyry andesite porphyry porphyry porphyry diorite dioriteporphyry diorite porphyry porphyry porphyry porphyry(alt) (alt) (alt) (alt)

SiO2(wt%) 61.6 61.4 65.0 64.4 63.3 60.2 61.5 64.0 60.5 60.1 58.5 60.4 61.5 57.6TiO2 0.5 0.5 0.4 0.4 0.4 0.4 0.5 0.4 0.5 0.6 0.5 0.6 0.5 0.5Al2O3 16.6 17.8 18.3 17.3 17.5 18.4 17.8 17.9 17.7 17.4 16.8 17.9 17.2 16.6Fe2O3 7.4 5.9 4.3 4.0 5.4 6.3 5.5 5.0 6.3 6.6 8.5 6.9 7.0 9.6MnO 0.0 0.1 0.1 0.1 0.2 0.2 0.2 0.0 0.1 0.1 0.1 0.1 0.1 0.1MgO 2.2 2.8 1.7 1.6 1.9 2.0 2.1 1.5 2.4 2.8 2.3 2.4 2.1 2.9CaO 5.2 7.0 5.6 6.0 6.9 7.4 7.2 3.6 7.2 7.0 6.7 5.2 6.5 6.2Na2O 5.2 3.2 3.9 4.0 3.9 3.6 3.8 5.4 4.5 4.7 5.4 4.7 4.5 4.8K2O 1.2 1.1 1.1 0.9 0.4 1.1 1.1 1.4 0.6 0.5 1.2 0.9 0.5 0.4P2O5 0.1 0.2 0.1 0.1 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.1 0.2total 100.0 100.0 100.6 98.9 100.1 100.0 99.8 99.2 100.0 100.0 100.0 99.1 100.0 98.9Ba(ppm) 112.8 382.6 348.9 288.9 194.1 197.1 198.4 424.6 341.4 311.3 268.7 223.4 260.9 87.7Co 7.5 11.8 7.0 5.7 7.6 9.0 8.4 4.9 12.1 12.4 10.1 10.5 7.8 14.2Cr 16.4 10.1 5.7 7.8 4.7 7.0 4.8 8.1 13.0 38.7 20.3 13.0 7.1 43.4Ga 21.7 18.0 18.5 19.6 18.3 17.4 18.2 20.3 18.1 19.0 17.8 18.4 18.4 18.6Nb 1.8 0.0 0.2 2.0 1.6 2.3 2.6 4.5 3.3 1.4 2.8 2.4 1.8 3.7Ni 6.3 5.5 4.5 4.5 3.3 3.8 2.7 5.5 6.2 14.1 6.1 6.1 6.5 10.4Rb 36.1 25.8 27.6 22.3 7.5 21.5 20.9 21.7 7.8 7.2 16.8 28.0 9.3 7.7Sr 633.1 762.3 778.2 774.7 772.6 676.9 1010.0 982.3 671.9 643.8 546.6 597.8 601.1 479.4V 92.1 138.1 79.3 75.5 106.1 77.3 106.0 83.4 134.3 137.8 117.2 135.9 119.8 169.7Y 9.8 11.9 15.4 10.5 14.8 20.5 14.7 7.3 19.6 18.1 18.0 19.2 15.2 20.6Zr 68.8 67.3 70.2 70.1 79.6 65.2 76.9 98.6 96.3 96.6 87.0 96.6 88.9 89.2Ce 13.2 22.0 12.0 24.0 21.6 21.1 22.2 21.4 27.3 7.1 9.3 23.3 20.1 12.4La 6.0 9.2 5.4 9.0 7.4 9.9 14.2 9.9 11.0 5.8 7.1 6.5 7.8 6.2Pb 7.0 4.8 7.6 9.0 1.3 4.7 2.8 3.0 3.5 1.5 2.3 4.8 1.4 2.8Sc 9.5 16.0 8.6 9.4 11.2 9.7 10.6 5.4 14.3 15.4 13.1 11.5 11.7 17.8Zn 92.0 62.3 60.2 54.4 73.0 86.0 60.5 24.0 43.7 55.3 48.1 50.5 33.2 36.5Th 2.3 9.3 5.8 2.7 1.6 2.9 3.1 3.6 2.4 2.6 2.2 3.7 1.4 7.0Sr/Y 64.7 64.1 50.5 74.0 52.4 33.0 68.7 134.2 34.4 35.7 30.4 31.2 39.4 23.3cpx- indicates clinopyroxene-bearing. (alt) indicates hydrothermally altered.

Sample ID 11331 04410 34Q-1 CLF1-4 02402 08402 CLF1-10 CLF1-12 BIN-3 WH9 WH8 WH6 10404 X3415Locality Santo Santo Santo Clifton Clifton Clifton Clifton Clifton Ligay Bumolo Bumolo Bumolo Main Camp6

TomasII TomasII TomasII (Binang) (Waterhole) (Waterhole) (Waterhole) Camp (BlackPhilex Philex Philex Philex Mountain)

Rock type quartz porphyritic porphyritic andesite cpx- quartz porphyritic porphyritic cpx- quartz cpx- porphyritic cpx- andesitediorite quartz quartz porphyry andesite diorite quartz quartz andesite diorite porphyritic quartz andesite porphyryporphyry diorite diorite porphyry porphyry diorite diorite porphyry porphyry quartz diorite porphyry

diorite(alt) (alt)

SiO2(wt%) 60.4 60.7 61.1 59.5 60.1 60.2 61.4 60.3 56.3 61.3 61.0 60.6 53.9 59.2TiO2 0.5 0.6 0.5 0.6 0.6 0.6 0.5 0.6 0.8 0.6 0.6 0.6 0.7 0.7Al2O3 16.9 18.1 16.5 18.4 18.2 18.1 17.8 17.7 17.9 18.0 17.7 17.7 18.9 16.6Fe2O3 6.8 6.3 7.2 6.8 6.7 6.4 6.3 6.3 8.1 6.1 6.0 6.1 9.2 6.1MnO 0.1 0.1 0.1 0.2 0.1 0.2 0.1 0.1 0.2 0.1 0.1 0.2 0.3 0.1MgO 2.3 2.2 2.0 2.4 2.4 2.3 2.4 2.5 3.2 2.1 2.1 2.2 3.9 3.9CaO 5.9 6.6 6.0 7.8 7.1 7.5 6.0 6.9 8.3 6.9 7.2 7.4 10.1 6.4Na2O 4.5 4.5 4.5 4.4 4.3 4.3 4.4 4.3 4.2 3.9 3.9 4.0 3.2 4.0K2O 0.8 0.6 0.6 0.4 0.5 0.6 0.7 0.6 0.5 1.1 1.1 1.0 0.3 1.7P2O5 0.2 0.1 0.1 0.2 0.2 0.2 0.1 0.1 0.2 0.2 0.2 0.2 0.1 0.2total 98.4 99.8 98.8 100.7 100.2 100.3 99.7 99.5 99.5 100.3 100.0 99.8 100.5 99.0Ba(ppm) 229.5 215.3 202.7 258.8 237.5 309.5 232.1 228.0 184.9 302.1 326.8 361.0 132.7 332.3Co 10.7 8.1 7.7 10.2 12.2 11.2 8.5 11.4 12.5 10.8 10.4 11.6 21.4 14.9Cr 9.8 12.4 11.0 10.0 7.6 6.9 14.0 13.9 16.7 4.8 7.6 5.3 6.1 97.9Ga 18.4 18.3 18.8 17.4 17.9 18.6 17.3 19.0 19.1 18.6 18.7 18.4 18.4 18.6Nb 2.5 2.1 2.6 1.2 1.7 2.1 3.3 4.3 2.0 1.8 3.8 3.0 0.6 5.1Ni 6.2 6.3 4.5 7.4 5.6 5.9 6.2 7.8 7.9 3.2 6.6 4.2 7.0 37.3Rb 21.9 14.3 15.1 7.6 10.9 9.0 19.9 13.1 12.2 25.5 23.5 20.6 4.9 37.6Sr 611.4 626.9 527.2 555.7 532.7 552.9 511.9 542.6 492.7 543.2 547.8 545.3 440.5 822.1V 129.7 131.2 121.5 142.6 135.6 130.4 125.4 133.5 184.7 124.6 123.1 124.1 216.0 144.0Y 18.0 19.6 17.9 22.8 22.0 28.8 19.4 20.9 29.5 22.4 22.9 23.0 21.7 16.6Zr 94.5 94.2 96.1 101.9 100.0 106.4 91.8 89.8 98.2 107.2 106.7 106.3 73.7 118.1Ce 11.9 17.4 13.5 11.7 18.0 11.4 18.5 19.2 12.5 20.0 19.9 14.0 13.1 40.5La 9.1 4.7 8.6 4.9 6.8 10.9 8.3 12.8 7.9 5.6 10.4 11.9 2.1 11.5Pb 4.0 2.7 3.8 2.8 0.7 2.3 1.2 3.0 4.0 4.9 6.5 5.9 8.4 8.0Sc 12.5 13.6 13.7 14.9 14.6 13.8 12.1 15.6 23.1 12.3 11.4 13.3 22.8 18.1Zn 40.9 28.3 38.7 78.3 54.3 92.3 36.9 42.1 74.5 48.8 51.3 60.3 89.3 67.8Th 2.3 0.7 0.5 1.2 1.6 1.6 2.7 1.2 1.8 1.0 1.6 5.8 0.9 5.1Sr/Y 33.9 31.9 29.4 24.4 24.3 19.2 26.3 26.0 16.7 24.3 23.9 23.7 20.3 49.4

ized by the scarcity of phenocrysticminerals. Thus, these rocks appearto be classified as adakites, whereastheir heavy rare earth elements con-tents were not determined in thiswork.

7. Discussion

The position of the chain of por-phyry Cu deposits is almost identicalto that of the present western Luzonvolcanic arc related to the eastwardsubduction at the Manila Trench. Thetemporal and spatial distribution ofporphyry Cu deposits in the westernLuzon arc shows no systematicmigration with respect to the present-day Manila Trench. The ancient mag-matic arc which is presently recog-nized as the belt of intermediate tosilicic plutonic rocks of the LuzonCentral Cordillera initiated at least 15Ma is overlapped by the present-daywestern Luzon volcanic arc. Thehydrous intermediate to silicic mag-


Table 3 (continued)Sample ID 01415 011220 031103A 011107 CT2-13 BT58-1 03328B 06410 10407B 03416 01416B 011111 021111 20412 741-27Locality Camp6 Dizon Dizon Dizon Taysan Taysan Naguilian Philex New Camp4 Camp4 Natubleng Natubleng Sto Virac

(Black Road Road CT dike Nio AcupanMountain)

Rock type porphyritic quartz diorite dacite quartz quartz dacite andesite dacite andesite porphyritic porphyritic porphyritic diorite quartzquartz diorite porphyry diorite diorite porphyry porphyry porphyry porphyry quartz quartz quartz dioritediorite porphyry porphyry diorite diorite diorite

SiO2(wt%) 61.5 61.9 53.6 63.9 64.9 62.6 62.5 50.5 64.8 55.0 61.5 63.3 60.9 56.8 59.0TiO2 0.6 0.5 0.8 0.5 0.4 0.4 0.6 1.0 0.4 0.7 0.4 0.5 0.6 0.7 0.6Al2O3 17.3 15.9 18.2 16.8 17.6 17.7 16.3 17.5 17.9 18.0 18.0 16.4 16.6 16.8 17.5Fe2O3 5.1 6.5 9.7 4.4 4.5 4.9 4.0 9.8 4.2 7.3 4.6 5.8 6.6 9.1. 6.8MnO 0.1 0.1 0.4 0.1 0.1 0.1 0.1 0.2 0.1 0.2 0.1 0.1 0.1 0.2 0.1MgO 2.6 2.0 4.6 1.4 1.6 2.1 3.3 6.1 1.4 3.0 2.3 2.4 3.0 3.8 2.8CaO 6.6 5.4 8.1 6.4 3.7 5.8 5.0 9.9 5.9 6.0 5.1 5.9 6.5 8.1 6.6Na2O 3.9 5.0 3.7 3.8 4.1 4.1 4.0 3.3 4.4 4.9 6.8 3.4 3.2 3.1 3.9K2O 1.6 0.6 0.2 1.4 2.0 1.5 2.0 1.0 0.6 2.7 0.3 1.9 2.1 0.9 1.8P2O5 0.2 0.2 0.2 0.2 0.2 0.1 0.2 0.2 0.2 0.3 0.2 0.1 0.1 0.1 0.2total 99.4 98.1 99.5 98.9 99.0 99.3 98.0 99.3 99.9 98.2 99.3 99.9 99.7 99.5 99.4Ba(ppm) 327.6 168.4 111.2 302.9 395.0 320.0 341.8 190.7 292.2 644.1 233.1 281.5 326.6 197.4 442.0Co 9.9 12.7 9.7 8.3 5.6 8.1 13.0 32.2 6.8 13.5 7.8 11.4 15.0 25.5 10.4Cr 33.3 20.3 31.3 15.8 7.7 5.2 53.5 74.9 4.6 15.3 25.5 10.6 10.8 10.7 7.9Ga 18.3 17.1 19.9 17.7 19.3 18.5 21.6 16.7 18.3 22.0 17.2 17.2 17.1 15.3 16.0Nb 7.1 2.8 2.3 4.3 3.8 3.6 3.6 2.7 4.0 4.4 2.1 3.4 3.1 2.2 7.6Ni 13.9 10.8 17.5 10.1 3.8 5.7 47.7 27.1 4.1 11.0 11.8 6.8 9.1 8.7 8.7Rb 36.9 12.3 4.6 31.6 30.1 28.1 25.1 16.6 12.8 36.6 5.5 54.2 46.0 14.9 37.7Sr 769.1 474.6 712.2 607.2 641.8 685.5 1416.6 635.8 804.4 1427.8 1182.4 545.5 570.3 462.4 687.7V 115.6 92.1 255.8 95.1 75.2 104.0 110.3 295.6 75.7 197.8 112.3 127.5 174.7 244.2 178.1Y 16.4 15.1 19.0 13.5 12.0 16.4 7.8 20.0 12.4 14.3 11.2 21.2 22.9 25.2 20.5Zr 109.5 84.4 63.9 93.8 95.4 89.1 106.7 63.5 84.5 95.4 77.8 129.5 183.6 66.8 142.9Ce 29.2 10.0 15.4 22.1 26.0 15.4 26.1 20.1 14.8 24.4 31.3 23.5 29.8 17.3 35.5La 15.0 7.0 7.8 13.1 8.3 14.1 13.5 7.0 16.7 19.3 7.1 22.8 12.4 6.8 18.6Pb 5.3 8.8 5.8 6.8 3.8 4.8 6.3 6.0 3.5 10.1 3.2 5.9 3.7 3.1 6.1Sc 13.2 14.1 28.4 14.1 7.5 14.6 10.6 37.7 8.3 13.6 10.5 14.6 17.1 31.3 18.6Zn 28.1 94.6 95.7 52.4 47.5 46.0 48.3 105.3 232.0 259.9 27.3 48.2 33.3 75.8 54.8Th 3.9 3.9 2.7 4.7 2.6 1.6 6.8 3.9 1.9 3.8 1.1 5.0 6.9 2.0 5.8Sr/Y 47.0 31.3 37.6 44.9 53.6 41.7 182.5 31.7 64.7 99.5 105.8 25.8 24.9 18.3 33.5

Sample ID 07404 09404 13407 fall-1 fall-2 flow-1 011117A1 011117A2 011117A3Locality Agno Agno Agno Pinatubo Pinatubo Pinatubo Pinatubo Pinatubo Pinatubo

New CT Mabalacat Mabalacat SanFernando Sacobina Sacobina SacobinaRock type quartz quartz quartz dacite dacite dacite dacite cpx-dacite cpx-dacite

diorite diorite diorite

SiO2(wt%) 66.9 64.5 67.1 64.5 64.0 64.7 65.1 62.0 62.1TiO2 0.3 0.4 0.4 0.5 0.5 0.5 0.5 0.6 0.6Al2O3 17.2 17.5 16.5 15.9 16.6 16.0 16.6 16.8 16.5Fe2O3 3.0 4.7 3.8 4.4 4.3 4.2 4.2 5.3 5.3MnO 0.1 0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.1MgO 1.2 1.7 1.4 2.3 2.4 2.2 2.2 3.0 3.5CaO 5.0 6.3 5.1 4.8 5.3 5.0 5.0 6.1 6.0Na2O 4.4 4.0 4.1 4.5 4.5 4.5 4.5 4.3 4.3K2O 1.4 0.9 0.6 1.6 1.5 1.5 1.5 1.5 1.3P2O5 0.1 0.2 0.1 0.2 0.2 0.2 0.2 0.2 0.2total 99.7 100.3 99.3 98.9 99.4 98.9 100.0 100.0 100.0Ba(ppm) 354.2 304.9 303.3 485.2 438.3 449.0 465.6 436.6 417.6Co 5.4 8.6 5.3 8.3 10.2 8.8 8.9 13.3 14.3Cr 2.3 3.9 5.2 38.7 46.5 35.2 39.6 48.1 121.0Ga 18.0 17.5 17.8 18.7 18.5 17.9 17.9 17.3 17.5Nb 4.3 3.9 6.1 5.5 4.9 4.5 3.6 3.1 2.5Ni 2.3 4.0 3.6 17.6 21.1 17.1 16.3 19.2 38.2Rb 27.7 18.8 12.7 43.8 39.1 41.3 45.1 43.5 37.8Sr 753.5 747.6 652.6 727.2 798.6 752.1 815.3 843.6 803.1V 60.5 88.0 59.3 91.5 91.3 83.7 88.6 127.2 119.8Y 11.6 13.1 12.5 14.5 14.4 15.1 14.1 16.7 15.3Zr 58.3 82.9 79.6 111.6 100.3 107.8 101.2 103.3 100.0Ce 3.7 25.0 15.4 31.6 26.7 18.9 26.4 36.7 21.3La 13.9 10.9 13.0 17.5 19.7 14.9 13.0 13.2 15.3Pb 2.7 2.7 2.9 9.8 7.6 10.1 9.5 7.3 6.4Sc 5.8 11.4 8.4 12.5 11.4 10.2 12.0 14.5 14.4Zn 42.4 33.4 207.3 60.8 57.8 56.5 64.3 61.3 65.2Th 1.6 1.5 1.2 5.3 4.2 4.3 5.9 6.7 4.9Sr/Y 65.2 57.1 52.2 50.3 55.6 49.9 57.8 50.5 52.5

matism has generated porphyry Cu mineralization since atleast 11 to 9 Ma at the Lobo-Boneng and Santo Niodeposits through 3 to 1 Ma at the Lepanto-FSE,Guinaoang, Santo Tomas II, Black Mountain and Dizondeposits to the present-day analogue at Mount Pinatubo.

Oyarzun et al. (2001) demonstrated that Late Eocene toEarly Oligocene giant porphyry Cu deposits (e.g.,Chuquicamata deposit) in Chilean Andes are associatedwith adakitic rocks. Relationships between the adakiticrocks and Cu-Au mineralization in the Philippines havebeen argued (Sajona and Maury, 1998). The rocks show-ing adakitic characteristics are present in the westernLuzon arc (Prouteau et al., 2000; Yumul et al., 2000) andtheir existences are confirmed in this work, in terms of thepresence of rocks showing high whole rock Sr/Y ratios.

The SO3 contents in microphenocrystic apatite in somerocks in the western Luzon arc having high whole-rockSr/Y ratios are high, such as the dacite at Naguilian(0.520.23 wt% SO3 in apatite, 182.5 whole-rock Sr/Y)and at Camp 4 (0.410.21 wt% SO3 in apatite, 99.5whole-rock Sr/Y). On the other hand, the SO3 content inmicrophenocrystic apatite in the dacite porphyry at SantoNio is low (0.070.05 wt%) whereas its whole-rockSr/Y ratio is high (134.19). The whole-rock Sr/Y ratios ofthe dacitic rocks in the Mankayan district range from50.5 to 74.0. Likewise, the whole rock Sr/Y ratios of theintrusive rocks of the Agno batholith range from 52.2 to65.2, and those from Mount Pinatubo range from 49.9 to

57.8. Thus, a general tendency of high whole-rock Sr/Yratios is recognized in the intermediate to silicic rocksfrom 15 Ma to present-day western Luzon arc. On theother hand, the whole-rock Sr/Y ratios of the intrusiverocks at the Santo Tomas II deposit and the vicinity rangefrom 16.7 to 39.5. Thus, porphyry Cu mineralization isnot necessarily related to the adakitic rocks.

Mason and MacDonald (1978) reported major andtrace element composition of intrusive rocks related toporphyry Cu deposits in the Papua New Guinea andSolomon Island region. They showed that the Sr con-tents range from 163 (at Lemau area) to 1517 ppm (atMount Fubilan - Ok Tedi area). They pointed out thatthe Sr contents were variable (300-900 ppm) in theisland arc suites, moderate (400-600 ppm) in the conti-nental margin suites, and high (>700 ppm) in the cra-tonic suites. On the other hand, they suggested that theY contents increase from low- to normal-K, normal-Kto high-K suites rocks, from island arc through conti-nental margin to continental block areas. According totheir data, the Sr/Y ratios of the intrusive rocks associat-ed with porphyry Cu deposits range from 6.8 to 68.5,while the Sr/Y ratios of the barren intrusive rocks rangefrom 7.8 to 73.7. Thus, a similarity is found throughoutwestern Luzon through Papua New Guinea to SolomonIsland, in terms of variable whole rock Sr/Y ratios,though SO3 contents of microphenocrystic apatites inthe intrusive rocks in the Papua New Guinea-SolomonIsland region have not been examined yet. The Sr and Ycontents in the intermediate to silicic magmas reflectthe chemical characteristics of source materials as wellas assimilation and fractionation during magmatic dif-ferentiation processes. The Sr/Y ratio of the intermedi-ate to silicic rocks is not necessarily related to porphyryCu mineralization. Thus, the source material does notappear to be the major and critical factor that defineswhether the magma can generate porphyry Cu deposit.

Intermediate to silicic intrusive rocks that are geneti-cally related to porphyry Cu mineralization are character-ized by tremolite rim on hornblende phenocryst whichimplies water saturation in the magma at the time ofemplacement to shallow crustal levels, and high magmat-ic fO2 as documented at the Santo Tomas II deposit (Imai,2001). In this regard, petrologic similarity between theMount Pinatubo dacitic pumices and the intrusions relat-ed to porphyry Cu mineralization suggests that MountPinatubo dacitic magmatism can be regarded as a modernanalogue of porphyry Cu mineralizing magmatic system(Imai et al., 1993, 1996). Sulfur is supposed to exist inoxidizing magmas dominantly as oxidized species assuggested by elevated SO3 contents in microphenocrysticapatite. Kress (1997) argued that dacitic magma was sat-urated with anhydrite prior to eruption. However, a bulkrelease of magmatic volatiles during the volcanic erup-


Fig. 4 Whole-rock Y (ppm) versus Sr/Y diagram forintermediate to silicic rocks, western Luzon arc. Sym-bols are same as Figure 3. Fields of adakites andtypical arc are after Defant and Drummond (1990).

tion seems to prevent the formation of porphyry Cudeposit (e.g., Pasteris, 1996).

The SO3 contents of apatite inclusions in other phe-nocrystic minerals are higher than discrete accessorymicrophenocrystic apatite in Pinatubo pumices (Imai etal., 1993, 1996). The lower SO3 contents in discrete apa-tite in matrix of Pinatubo pumices suggest a decrease inthe activity of oxidized sulfur in the magma probably dueto SO2 degassing. This is supported by partial decomposi-tion of microphenocrystic anhydrite. Likewise, the SO3contents of apatite inclusions in other phenocrystic miner-als (0.220.06 wt%, number of analysis (n)=12) are high-er than discrete accessory microphenocrystic apatite(0.050.05 wt%, n=33) in the quartz diorite porphyry atthe Dizon deposit. In addition, the SO3 contents of acces-sory microphenocrystic apatite of shallow-depth intru-sions at Camp 4, Binang (Ligay) and New CT areas, aswell as those from outcrops along the Philex Road andNaguilian Road, characterized by quenched groundmass,are significantly higher than those of coarse-grainedequigranular plutonic rocks. These suggest a decrease inthe activity of oxidized sulfur in the magma by removal ofSO2 due to degassing during solidification. Thus, the highactivities of oxidized sulfur in the magmas of westernLuzon arc are supposed to have been attained since earlymagmatic processes.

Porphyry type Cu deposits are genetically associatedwith intermediate to silicic intrusive rocks that belong tothe magnetite-series, the oxidized granitoid type (e.g.,Ishihara, 1975, 1977, 1981, 1998). Takagi and Tsukimura(1997) discussed that it plays an important role in buffer-ing the redox condition and thus in defining the granitoidtype whether the oxidized sulfur species or the reducedspecies is dominated in the granitic magma. The highactivity of oxidized sulfur in the intermediate to silicicmagmas in the western Luzon arc since the early stage ofmagmatic differentiation is suggested by the high SO3contents in microphenocrystic apatite. Since MiddleMiocene when the Philippine Fault was formed, thus,since the tectonic environments became those of the pre-sent-day, the SO3 contents of accessory apatite are signif-icant throughout the intrusive and extrusive rocks in thewestern Luzon arc. Thus, the significant activity of oxi-dized sulfur is considered to be a character of the regionwhere hydrous intermediate to silicic magmas in thewestern Luzon arc are generated.

Carroll and Rutherford (1988) demonstrated experi-mentally that the solubility of oxidized species of sulfurinto the magma increases as the fO2 increases. Under theoxidizing condition where sulfur is accommodated inthe magma dominantly as oxidized species, precipita-tion of sulfide minerals from the magma may not be amajor process that may define the behavior of sulfur.Fractionation of sulfide from the magma (e.g., Ueda and

Sakai, 1984) does not seem to be encountered in theoxidized magma where sulfur is accommodated domi-nantly as oxidized species. Thus, the concentration ofsulfur as oxidized species seems to become increasedthrough differentiation of the oxidizing magma.

Considering that chalcophile elements tend to be parti-tioned into sulfide phases from the coexisting silicatemelt, the unlikeness of sulfide fractionation in the oxidiz-ing magma where sulfur is present dominantly as oxi-dized species may result in the enrichment of the chal-cophile elements in the magma during differentiation.Therefore, the highly oxidizing condition of the magmaappears favorable for the concentration of sulfur (as oxi-dized species) and chalcophile elements. Then metalsenriched in the oxidizing magma are partitioned into thefluid phase exsolved from the magma, to form porphyryCu deposits (e.g., Burnham, 1979; Larocque et al., 2001).

The 34S of the porphyry Cu deposits of westernLuzon including the Lepanto-FSE deposit (ca. +6 ;Imai, 2000b) and the Santo Tomas II deposit (between+4.4 and +13.7 ; Imai, 2001), as well as the hydrochlo-ric acid leached sulfur of Mount Pinatubo daciticpumices (+8 to +9 ; Imai et al., 1993, 1996) areenriched in 34S compared to those of mid-oceanic ridgebasalts (Moore and Fabbi, 1971). This is likewise com-mon in Quaternary volcanic rocks and high temperaturevolcanic gases from subduction-related island arc settings(Ueda and Sakai, 1984; Rye et al., 1984) and associatedhydrothermal ore deposits (e.g., Hamada and Imai,2000). In addition, granitoids and associated ore depositsformed in ancient magmatic arc and/or continental mar-gin settings are also enriched in 34S (Sasaki and Ishihara,1979; Ishihara et al., 2000; Imai and Anan, 2000).Enrichment of 34S in hydrous magmas in arc setting isthus a common feature in plutonic and volcanic rocksboth in barren and mineralized arcs with respect to por-phyry Cu metallogenesis. This suggests a common heavysulfur source beneath the arc mantle wedge such as seawater-derived sulfur in the subducted slab (Sasaki andIshihara, 1979). Introduction of aqueous fluid which isderived from the subducted slab plays an important rolein the genesis of arc magmas (Tatsumi et al., 1986; Tat-sumi, 1989; Imai and Ozawa, 1991; Luhr, 1992;Iwamori, 1998). For instance, Prouteau et al. (1999)experimentally demonstrated that water contents of themelt were at least 10 wt% and temperatures were similarto 900C during the dacite magma of Mount Pinatuboascent from the slab. In addition, Yoshino and Satish-Kumar (2001) reported the occurrence of sulfate-bearingscapolite in metagabbro of lower crustal level of theKohistan arc, formed by fluid derived from the subductedslab.

The activity of oxidized sulfur in the western Luzon arcmagmas as demonstrated by the SO3 contents of accesso-


ry microphenocrystic apatite, is ultimately a function offluid phase originated from the subducted slab. Incorpora-tion of oxidized sulfur into the source region of arc mag-mas is supposed to be significant in the western Luzonarc. The subducting oceanic lithosphere is pervasivelyaltered chiefly due to seawater circulation as well ashydrothermal activity near spreading axis. Through thesealteration processes, sulfur is accommodated in the ocean-ic floor as sulfates mostly anhydrite, in addition to sul-fides deposited by hydrothermal activities near spreadingaxis. Such sulfur fixed in the oceanic floor is supposed tobe incorporated to the source region of arc magmas.

8. Summary and Conclusions

The western Luzon arc has been generating porphyryCu mineralization associated with oxidizing hydrousintermediate to silicic intrusions related to eastward sub-duction, since about 10 Ma at the Lobo-Boneng (10.50.4Ma), and Santo Nio (9.50.3 Ma), deposits throughTaysan (7.30.2 Ma), Dizon (2.50.2 Ma), Black Moun-tain (2.10.1 Ma) and Santo Tomas II (1.50.4 Ma)deposits, to the possible present-day analogue at MountPinatubo.

Microphenocrystic apatite in shallow intrusions associ-ated with porphyry Cu deposits throughout the westernLuzon arc contains sulfur detectable by electron probemicroanalyzer. Such high SO3 contents in accessoryapatite are common characteristics of the intermediate tosilicic rocks of the western Luzon arc, from tonaliticrocks of the Luzon Central Cordillera of 15 Ma to anactive magmatism at Mount Pinatubo. Sulfur is supposedto have been accommodated as oxidized species in oxi-dizing hydrous magmas since the early stage of magmat-ic differentiation. Oxidizing hydrous magmas are capableto form porphyry Cu deposits. Intermediate to silicicrocks from Miocene to present-day western Luzon arcgenerally show high whole-rock Sr/Y ratio ranging from20 to 184. However, porphyry Cu deposit is not necessar-ily related to the rocks that show significantly high Sr/Yratios compared to the other barren rocks in the westernLuzon arc. The characteristics of the intermediate to sili-cic magma associated with porphyry Cu deposit are notattributed to the composition of the source material of themagma, but to the properties defined by the contents ofoxidized species of sulfur in the fluid phase that isencountered in the region where intermediate to silicicmagmas are generated.Acknowledgments: I thank the Mines and GeosiencesBureau, Philex Mining Corp., Benguet Corp. and LepantoConsolidated Mining. R. L. Almeda, J. S. Aquino, A.Arribas, Jr., B. S. Austria, C. Baguilat, R. T. Balboa, G. R.Balce, R. P. Baluda, J. C. Cinco, Jr., E. C. Comsti, R. A.Concepcon, F. V. Damasco, R. T. Datuin, M. de los San-

tos, Jr., E. G. Domingo, M. T. Einaudi, H. E. A. Franco, J.C. Garcia, Jr., J. W. Hedenquist, S. Ishihara, E. Izawa, Y.Kajitani, F. B. Lazo, E. Lillon, E. L. Listanco, A. Madera,H. S. R. Magdamit, T. D. Malihan, J. Manipon, H. Mat-sueda, J. D. Muyco, A. Sajona, D. Sajona, R. A. Santos,S. S. Shimada, H. Shimazaki, Y. Shimazaki, R. H. Silli-toe, F. Tejada, T. Suzuki, Y. Togashi, Y. Urashima, R. I.Villones, Jr. and G. P. Yumul, Jr. are appreciated forwarm support and encouragement, constructive sugges-tions and discussions, valuable instructions and assistance.M. de los Santos, Jr. kindly provided samples from theTaysan deposit. A part of field surveys were supported bythe Arai Foundation of the Society of Mining Geologistsof Japan (Society of Resource Geology presently).

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(Editorial handling: Masaaki SHIMIZU)


metallogenesis of porphyry cu deposits of the western luzon arc, k-ar ags, so3 apatite

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