FIRE and MUD Contents

Highly Oxidized and Sulfur-Rich Dacitic Magma of Mount Pinatubo: Implication for Metallogenesis of Porphyry Copper Mineralization in the Western Luzon Arc

By Akira Imai,¹ Eddie L. Listanco,^{2 3} and Toshitsugu Fujii²

¹ Geological Institute, University of Tokyo.

² Earthquake Research Institute, University of Tokyo.

³ Present address: Philippine Institute of Volcanology and Seismology.

ABSTRACT

Dacitic pumices from pyroclastic-flow deposits and tephra fall of the June 15, 1991, eruption of Mount Pinatubo, Philippines, are rich in sulfur, and the presence of microphenocrystic anhydrite suggests that sulfur existed dominantly as oxidized species in the magma. A high sulfur content in the magma is corroborated by unusually high sulfur contents (up to 0.78 weight percent as SO₃) in apatite microphenocrysts and apatite inclusions in other phenocrystic minerals; the oxidized state of that sulfur is consistent with highly oxidized magma, which, by extrapolation from the two-oxide method, was close to the manganosite-hausmanite buffer. The highly oxidized state of the magma may have caused the extraordinarily high sulfur content of that magma by prohibiting sulfide fractionation and by increasing solubility of sulfur as oxygen fugacity increased. Hornblende geobarometry indicates pressure of about 2 kilobars for phenocryst formation. The mineralogical similarity of pumice from Mount Pinatubo to intrusives that are genetically related to porphyry copper deposits suggests magmatic water saturation and high oxygen fugacity at the time of emplacement at shallow crustal levels.

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INTRODUCTION

Calc-alkaline dacitic pumices from Mount Pinatubo have unusually high sulfur contents (Bernard and others, 1991) compared with ordinary calc-alkaline magmas in subduction-related arcs; other unusually high sulfur contents have been noted in subalkaline rocks such as the trachyandesite at El Chichón volcano, Mexico (Luhr and others, 1984) and the trachybasalt at Mount Lamington, Papua New Guinea (Arculus and others, 1983). Primary magmatic anhydrite microphenocrysts are a notable feature of pumice from Mount Pinatubo (hereafter referred to as Pinatubo pumice, for simplicity). We report mineral compositions and sulfur isotopic determinations and their implications for the probable cause of unusually high sulfur contents in Mount Pinatubo's dacitic magma. Samples were collected from the equivalent of tephra layer C (Koyaguchi and Tokuno, 1993) (tephra layer C₁ of Koyaguchi, this volume) at Mabalacat, Pampanga (fig. 1). The strong similarity of the dacite of Mount Pinatubo (hereafter referred to as Pinatubo dacite) to intrusives genetically related to porphyry copper deposits is noted. Characterization of the Pinatubo pumice will, we hope, contribute to an understanding of the mechanism by which magmas become enriched in sulfur, one of the most important elements in magmatic-hydrothermal metallic deposits such as porphyry copper deposits.

Figure 1. Distribution of Quaternary volcanoes, major porphyry copper deposits, Tertiary intermediate to silicic intrusives, and ophiolitic rocks, Luzon and vicinity, Philippines. Samples for the present study were collected in Mabalacat, Pampanga.

GEOLOGIC BACKGROUND

Mount Pinatubo is one of the active volcanoes in the western Luzon volcanic chain, which is associated with eastward subduction of the Eurasian plate at the Manila trench (PHIVOLCS, 1991) (fig. 1). Porphyry copper-gold deposits of Miocene to Pliocene age are genetically associated with hydrous intermediate to silicic magmatism along this same arc. This metallogenic province extends from the Lepanto-FSE deposit in the northern Luzon, past the Dizon deposit in west-central Luzon, only 19 km south of Mount Pinatubo, to the Taysan deposit in southern Luzon. The basement underlying Mount Pinatubo includes the Zambales ophiolite complex.

PETROGRAPHY

The June 1991 eruption of Mount Pinatubo produced voluminous dacitic pyroclastic-flow and tephra deposits. Essential ejecta can be classified into two types, namely type 1 (white and phenocryst rich) and type 2 (yellowish and phenocryst poor) (Listanco, 1991; Pallister and others, 1992). Both have broadly similar mineralogic constituents. Phenocrysts in type 2 pumice are mostly shattered. Major phenocrystic phases in both types include plagioclase and hornblende with subordinate quartz, biotite, titanomagnetite, ferrian ilmenite, and anhydrite. Cummingtonite-rimmed hornblende phenocrysts commonly occur in type 1 pumice (fig. 2A); in type 2 pumice, rare, discrete, shattered hypersthene occurs instead of cummingtonite. Olivine occurs in type 2 pumice (Pallister and others, 1992); in addition, hornblende-rimmed olivine is also found in type 1 pumice. In both instances, olivine is inferred to be a xenocryst.

Figure 2. Photomicrographs of pumice from the June 15, 1991, eruption of Mount Pinatubo. A, Cummingtonite (cm) -rimmed hornblende (hb) phenocryst in type 1 pumice with silica-poor compositional domain (dom). B, Apatite (ap) inclusion (with the highest SO₃ contents, table 1, ap *h) and titanomagnetite (mt) inclusion in hornblende (hb) phenocryst in type 1 pumice. C, Coexisting titanomagnetite (mt) and ferrian ilmenite (il) microphenocrysts in type 1 pumice. D, Biotite (bt) microphenocryst attached to hornblende (hb) phenocryst in type 1 pumice.

Apatite, titanomagnetite, and ferrian ilmenite occur as discrete microphenocrysts and as inclusions in various phenocrysts (fig. 2B, C). Trace amounts of biotite also occur as discrete microphenocrysts and as inclusions in and (or) attached to other phenocrysts (fig. 2D).

TEMPERATURE, OXYGEN FUGACITY, AND SULFUR SPECIES OF MOUNT PINATUBO DACITIC MAGMAS

The presence of primary, magmatic anhydrite indicates that sulfur existed dominantly as oxidized species under highly oxidized conditions at least since the phenocrystic minerals crystallized. This is corroborated by unusually high SO₃ contents (up to 0.78 weight percent) of apatite microphenocrysts and apatite inclusions in phenocrystic minerals such as hornblende (table 1; fig. 2B). In type 1 pumice, the sulfur content in apatite is generally higher in the inclusions in other phenocrysts than in the discrete microphenocrysts (table 1). No apatite inclusion was analyzed in type 2 pumice due to scarcity of phenocrysts. Apparently, the activity of sulfate in the magma decreased during phenocryst formation prior to eruption, probably due to degassing of SO₂.

Considering the miscibility gap for the ilmenite-hematite solid-solution series, unusually high hematite contents of ferrian ilmenite (about 43 mole percent; table 1) indicate a primary magmatic origin at a temperature higher than about 800°C (Lindsley, 1976). Extrapolation of the two-oxide method (Buddington and Lindsley, 1964; Spencer and Lindsley, 1981; Andersen and Lindsley, 1988) by utilizing ferrian ilmenite and titanium-poor titanomagnetite compositions gives a temperature of about 800°C and an oxygen fugacity of about 10^-11 bar (Andersen and Lindsley, 1988), close to the manganosite-hausmanite buffer. The fact that discrete microphenocrystic iron-titanium oxides are identical in composition to inclusions in hornblende and plagioclase (fig. 2C, table 1) indicates that the high oxygen fugacity existed even before crystallization of those phenocrystic minerals.

Assuming a temperature of about 800°C extrapolated from the two-oxide method, the atomic ratio of Mg/(Mg+Fe) (abbreviated as X_Mg) of microphenocrystic biotite (fig. 2D) (about 0.7), high for a dacite, suggests a high oxygen fugacity (Wones and Eugster, 1965). However, an assumption of the presence of K-feldspar can result in an overestimate of the oxygen fugacity (Imai and others, 1993). The X_Mg values of calcium-poor mafic silicates (biotite, cummingtonite, and hypersthene) are almost identical (around 0.7), except for that of olivine (X_Mg = 0.82) (table 1). The high X_Mg of olivine and existence of rimming hornblende suggest that olivine was not in equilibrium with dacitic melt and thus was probably xenocrystic.

Geschwind and Rutherford (1992) experimentally demonstrated that cummingtonite is stable only below 790-800°C. Because cummingtonite occurs only as a rim on hornblende phenocrysts in Pinatubo pumice, the temperature at which cummingtonite formed might be lower than those temperatures at which the other phenocrysts formed.

Estimated temperature and oxygen fugacity for dacitic magma erupted on June 15 is plotted along with other previously published estimates for andesites and rhyolites (Whitney, 1984) (fig. 3). Dacites and andesites from the other areas contain microphenocrystic pyrrhotite and plot near or below the SO₂=H₂S boundary curve, whereas Mount Pinatubo dacite is more highly oxidized, with sulfur existing dominantly as oxidized species such as SO₂. This high oxygen fugacity, because it has prohibited sulfide fractionation (Ueda and Sakai, 1984) and because sulfate solubility is high in oxidized silicate melt (Carroll and Rutherford, 1987, 1988; Luhr, 1990), is the possible cause of extraordinarily high sulfur content.

Figure 3. Estimated temperature and oxygen fugacity (f_O2) for Pinatubo pumices of June 15, 1991, and some other volcanoes in the world. After Whitney (1984) and Ueda and Itaya (1981). Abbreviations: fay, fayalite; hm, hematite; mt, magnetite; po, pyrrhotite; py, pyrite; qtz, quartz.

Table 1. Microprobe analyses of phenocrystic minerals of Pinatubo pumice of June 15, 1991.

[All compositions are mean values, except for apatite inclusions in phenocrystic hornblende and plagioclase in type 1 pumice (indicated by *h and *p, respectively) which have high SO₃ contents. Numbers in parentheses after mean values are standard deviations; dom indicates a compositional domain that was found within a phenocryst. Dashes indicate that the oxide was below or around detection limit. Abbreviations: ol, olivine; pl, plagioclase; hb, hornblende; cm, cummingtonite; opx, orthopyroxene (hypersthene); bt, biotite; ap, apatite; mt, titanomagnetite; il, ferrian ilmenite. X_Mg denotes the atomic Mg/(Mg + Fe) of mafic silicates. The Fe₂O₃* contents of magnetite and ferrian ilmenite were calculated assuming stoichiometry. The mole fractions of ulvospinel of titanomagnetite (X_usp(mt)) in type 1 pumice and type 2 pumice are 0.121±0.002 and 0.120±0.002, respectively, while the mole fractions of hematite in ferrian ilmenite (X_hm(il)) in type 1 and type 2 pumice are 0.434±0.015 and 0.428±0.012, respectively. Microprobe analyses were done with the JEOL 733 MKII of the Geological Institute, University of Tokyo, using 15keV, 12nA, and a beam diameter of 5 m]

Table 1. Microprobe analyses of phenocrystic minerals of Pinatubo pumice of June 15, 1991--Continued.

Table 1. Microprobe analyses of phenocrystic minerals of Pinatubo pumice of June 15, 1991--Continued.

DEPTH AND WATER FUGACITY OF MOUNT PINATUBO DACITIC MAGMAS

Phenocrystic minerals were formed at pressures of around 2 kbar, on the basis of igneous hornblende geobarometry (Hammarstrom and Zen, 1986) (fig. 4). These depth estimates seem consistent with an earthquake-poor, low-velocity zone (a magma chamber?) between 6 and 11 km in depth (Mori, Eberhardt-Phillips, and Harlow, this volume). Silica-poor hornblende is found as compositional domains in ordinary hornblende phenocrysts in both pumice types (table 1) (fig. 2A), and anorthite-rich plagioclase domains are found in plagioclase phenocrysts (table 1). These compositional domains might be remnants of the early history of magmatic differentiation, suggestive of the probable existence of less differentiated magma at depth.

Figure 4. Hornblende geobarometry (Hammarstrom and Zen, 1986) applied to Mount Pinatubo pumice. Regression lines suggested by Hammarstrom and Zen (1986) are indicated for reference. The total atomic aluminum contents per half unit cell of amphibole formula of phenocrystic hornblende in type 1 and type 2 pumice are 1.28±0.15 and 1.31±0.15, respectively.

Considering the solubility of water in dacitic magma with respect to pressure and the igneous-hornblende stability field with respect to magmatic water content, the Mount Pinatubo dacitic magma was apparently close to or at water saturation prior to eruption (Naney and Swanson, 1980). In addition to the occurrence of cummingtonite, which indicates high magmatic water fugacity, near or at water saturation (Wood and Carmichael, 1973; Geschwind and Rutherford, 1992), studies on glass inclusions and matrix glasses of the Mount Pinatubo pumice (Westrich and Gerlach, 1991; Gerlach and others, this volume) suggest magmatic vapor saturation prior to eruption.

IMPLICATION FOR POSSIBLE METALLOGENY OF PORPHYRY COPPER MINERALIZATION

A chain of late Miocene to Pliocene porphyry copper-gold deposits, including the Lepanto-FSE, the Santo Tomas II, and the Dizon deposits, delineates a metallogenic province extending from northern Luzon to west-central Luzon (fig. 1). The Dizon deposit of 2.7 Ma (Malihan, 1987) is located only 19 km south of Mount Pinatubo, and several prospects of porphyry-type deposits are known in the vicinity. Shallow-depth intermediate to slicic intrusives that are genetically related to porphyry copper deposits in the western Luzon arc are usually corroded quartz-phenocryst-bearing hornblende-quartz-diorite porphyry to hornblende-andesite porphyry (Balce, 1979; Imai, unpub. data, 1991). Hornblende-andesite porphyry at the Santo Tomas II deposit in northern Luzon (Imai, unpub. data, 1993) is characterized by tremolite-rimmed hornblende phenocrysts which, at the low pressure (<1 kbar) that is implied by fluid-inclusion compositions and amphibole geobarometry, suggests water saturation in the magma at the time of its emplacement. The porphyry of Santo Tomas II also has coexisting magnetite and titanohematite (X_ilm = 42 mole percent) in high temperature sulfide-rare quartz stockworks (T>700°C on the basis of Lindsley, 1976), and high X_Mg of tremolite and biotite. Again, high oxygen fugacity, at the hematite-magnetite buffer, is suggested. Lastly, anhydrite is a common gangue and alteration constituent.

These characteristics of the porphyry copper-related hydrous magmatic system of northern Luzon are also common in other porphyry copper systems, including several in Papua New Guinea and vicinity (Mason, 1978; Chivas, 1981). Petrologic similarity between Mount Pinatubo dacitic magma and such intrusives is notable.

Oblique subduction is thought to be favorable to porphyry copper metallogeny in the Chilean Andean cordillera (see Davidson and Mpodozis, 1991). The Philippine archipelago is regarded as the product of the late Cenozoic oblique convergence between the Philippine Sea plate and the Eurasian margin (for example, Rangin, 1991). With a tectonic stress field that produces shallow reservoirs of hydrous magma, the Western Luzon arc of both the present and ancient (late Miocene to Pliocene) times has been favorable for porphyry copper mineralization.

Because sulfur in the dacitic magma of June 15 existed dominantly as oxidized species (see above), the sulfur isotopic composition of bulk hydrochloric-acid leachate from pumice (+7.8 to +9 per mil) (table 2) can be regarded as the sulfur isotopic value of the magma within ±2 per mil (see McKibben and others, this volume, for detailed sulfur isotopic data). That sulfur isotopic ratios of Pinatubo dacite are similar to those of the aforementioned porphyry copper-gold deposits of northern Luzon (+9 to +10 per mil, estimated for copper-iron sulfide-bearing anhydrite veins at the Santo Tomas II and Lepanto-FSE deposit, Imai, unpub. data, 1991) indicates the same origin of sulfur. The sulfur isotopic compositions of these two deposits are almost identical; copper-iron sulfides with sulfur isotopes of +3 per mil (Lepanto-FSE) to +4 per mil (Santo Tomas II) coexist with gangue anhydrite with +13.5 per mil at both deposits. In addition, the sulfide minerals from the Dizon deposit also yield similar sulfur isotopic compositions (+3 to +5 per mil), but no hypogene anhydrite has been analyzed. These sulfur isotopes, heavier than those of midoceanic ridge basalts that derived directly from the mantle (Moore and Fabbi, 1971), are likewise a common feature of Quaternary volcanic rocks and high-temperature volcanic gases from subduction-related arc settings (Ueda and Sakai, 1984; Rye and others, 1984). Furthermore, magnetite-series granitoids, formed in an ancient island-arc and (or) continental-margin setting in the present Japanese islands, are also enriched in heavy sulfur (Sasaki and Ishihara, 1979). Enrichment of heavy sulfur in hydrous magmas in an arc setting is thus a common feature of both plutonic and volcanic rocks and suggests a heavy sulfur source (1) beneath the arc's mantle wedge, such as sea-water-derived sulfur in the subducted slab (Sasaki and Ishihara, 1979), and (or) (2) possible assimilation from hydrothermally altered sulfate-bearing country rocks.

Table 2. Whole rock sulfur isotopic compositions of Mount Pinatubo pumices of June 15, 1991.

[Sulfur isotopic ratios were determined for hydrochloric acid leachate from pumice]

CONCLUSIONS

The dacitic magma erupted by Mount Pinatubo June 15, 1991, was highly oxidized, close to the manganosite-hausmanite buffer. This condition was attained prior to formation of the phenocrystic minerals. Sulfur has existed dominantly as oxidized species in the magma since the crystallization of the phenocrysts.

The bulk sulfur isotopic composition of the Mount Pinatubo magma is almost identical with the isotopic composition of the late Cenozoic porphyry copper-gold deposits in northern Luzon. Cummingtonite rims on phenocrystic hornblende and high X_Mg of biotite and hornblende are also common to the Pinatubo magma and to the intrusive rocks associated with northern Luzon porphyry copper deposits. We infer that in both cases, magma was saturated with water at the time of emplacement at shallow crustal levels.

ACKNOWLEDGMENTS

We thank R.S. Punongbayan, E.G. Domingo, G.P. Yumul, Jr., J.W. Hedenquist, S. Aramaki, T. Koyaguchi, H. Shimazaki, I. Kushiro, N. Shikazono, and M. Toriumi for their encouragement and discussions. S. Yui helped in ZAF correction of Fe-Ti oxide microprobe analyses as part of a Grant-in-Aid for Scientific Research (No. 02302032) to him. Constructive review comments by T.M. Gerlach, J. Luhr, and C.G. Newhall are appreciated.

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