PORPHYRY COPPER-GOLD MINERALIZATION AT YULONG, CHINA

©2009 Society of Economic Geologists, Inc. Economic Geology, v. 104, pp. 587–596

SCIENTIFIC COMMUNICATIONS PORPHYRY COPPER-GOLD MINERALIZATION AT YULONG, CHINA, PROMOTED BY DECREASING REDOX POTENTIAL DURING MAGNETITE ALTERATION HUA-YING LIANG,1,† WEIDONG SUN,2,† WEN-CHAO SU,1 AND ROBERT E. ZARTMAN3 1 Key

Laboratory for Metallogenic Dynamics, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China

2 CAS Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China; Research Center for Mineral Resources, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China 3 Department

of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts

Abstract The Yulong porphyry Cu (Au) deposit occurs on the eastern margin of the Tibetan plateau and is one of the largest porphyry Cu (Au) deposits in China, containing more than 6 million tons of Cu metal. Similar to other porphyry Cu deposits in the world, the Yulong porphyry is highly oxidized. Sulfate is the dominant sulfur species in fluid inclusions hosted by magmatic quartz phenocrysts, and no sulfide is observed, indicating an oxygen fugacity above the SSO buffer during magmatic processes. The sulfur species changed from sulfate dominant during magmatic processes to sulfide dominant during the main mineralization processes, with sulfate and sulfide mineral assemblages observed in fluid inclusions in mineralized quartz veinlets and only sulfides in the orebodies, corresponding to a decrease in redox potential from the porphyry to the hydrothermal fluid. Magnetite crystallization was coincident with the onset of major sulfate reduction, indicating that magnetite isolated trivalent iron, reducing sulfate, and consequently leading to the formation of porphyry Cu-Au mineralization. Most porphyry Cu-Au ore deposits are spatially associated with, and genetically related to, oxidized felsic magmas. Our results indicate that sulfate reduction promoted by magnetite crystallization is essential for the final precipitation of Cu-Au–bearing sulfides, i.e., decreasing redox potential of the fluid is the key to, and direct cause of, ore formation in Yulong. Initial high redox potential is a prerequisite for and may be an indirect indicator of most mineralization of this kind, because it enables efficient transportation of Cu-Au. Redox potential fluctuations can be well preserved in zircon, a resistant accessory mineral commonly found in porphyries, which might be developed as a handy and reliable exploration tool for porphyry Cu deposits associated with magnetite.

Introduction Most porphyry Cu-Au ore deposits are spatially associated with and genetically related to oxidized felsic magmas (Gustafson and Hunt, 1975; Hedenquist and Lowenstern, 1994; Sillitoe, 1997; Mungall, 2002; Audetat et al., 2004; Sun et al., 2004). Some authors have proposed that high oxidizing conditions may promote the oxidation of residual sulfide in the magma source and liberate chalcophile elements (Sillitoe, 1997) and/or that under oxidizing conditions, more sulfur is present in the form of SO42– and thus sulfides stay undersaturated during fractional crystallization (Wyborn and Sun, 1994; Mungall, 2002; Sun et al., 2004; Mungall et al., 2006). Given that both Cu and Au are moderately incompatible elements in sulfide-undersaturated magmas (Sun et al., 2003a, b) their concentrations in silicate melts increase during the early stage of magma evolution (Sun et al., 2004). Remarkably, Cu and Au concentrations can drop dramatically in the late stages (e.g., at ~55–58 wt % SiO2) of magma evolution (Togashi and Terashima, 1997; Moss et al., 2001; Sun et al., 2004). These sudden decreases in Cu and Au concentrations during the late-stage magmatism coincide with magnetite †Corresponding

authors: e-mail, [email protected], [email protected]

0361-0128/09/3827/587-10

crystallization. The crystallization of magnetite, particularly in oxidizing felsic magmas, can help to reduce SO42– to H2S (12[FeO] + SO42– + 2H1+ = 4Fe3O4 + H2S) and consequently promote Cu-Au mineralization (Sun et al., 2004). Here we show that redox changes linked to magnetite crystallization were important for the precipitation of Cu, Au-bearing sulfides at Yulong porphyry Cu deposit. Geology The Yulong porphyry Cu belt is located on the eastern margin of the Tibetan Plateau (Fig. 1). It is characterized by pyrite-tourmaline-quartz cemented breccias (Tang and Luo, 1995), associated with complicated multiphase intrusions that are typically dominated by two major intrusive stages: early quartz monzonite and late syenogranite (Zhang et al., 1998). The Yulong porphyry belt contains one giant, two large, and two medium-sized deposits (Tang and Luo, 1995; Bai et al., 1996; Gu et al., 2003; Hou et al., 2003; Liang et al., 2006, 2008b). There are also numerous mineralized porphyry bodies distributed along the northwestern extension of the Red River-Ailao Shan fault system. The belt hosts a total of about 9 million tons (Mt) of Cu metal, 0.2 Mt of Mo and 40 t of Au

587

Submitted: July 11, 2008 Accepted: May 27, 2009

588

SCIENTIFIC COMMUNICATIONS

FIG. 1. Sketches of geologic maps for (a) the Yulong ore deposits, and (b) porphyry bodies. Modified after Tang and Luo (1995).

(Tang and Luo, 1995; Bai et al., 1996; Hou et al., 2003, 2006, 2007; Liang et al., 2006, 2009). The ore-related intrusions have porphyritic textures, defined by plagioclase, K feldspar, amphibole, mica, and quartz phenocrysts in a mediumgrained phanerocrystalline groundmass of similar mineralogy. The accessory minerals include magnetite, titanite, apatite, and zircon. These porphyries, varying from mafic to felsic in composition (Jiang et al., 2006), were emplaced into Triassic volcanic and sedimentary rocks. Zircon U-Pb ages of porphyries from the Yulong porphyry deposit belt range from 41.2 to 36.9 Ma (Liang et al., 2006). These porphyries are inferred to have been related to movement along the Red River-Ailao Shan shear zone (Scharer et al., 1994; Wang et al., 2001; Liang et al., 2007, 2008a, b). The Yulong porphyry is associated with the largest intrusive complex in the Yulong belt (Fig. 1). It crops out as small stocks over a surface area of about 0.64 km2. The Yulong porphyry deposits consist of a pipe-like orebody hosted in the porphyry intrusion and a stratiform orebody hosted in the 0361-0128/98/000/000-00 $6.00

thermally metamorphosed strata surrounding the intrusion (Tang and Luo, 1995), with 6.5+ Mt of Cu, 0.15 Mt of Mo, 28 t of Au, and more than 86 Mt of Fe ore reserves as magnetite (Bai et al., 1996). The pipe-like orebody, characterized by veinlet and disseminated mineralization, is called the upper portion of the Yulong porphyry intrusion (>4,300 m a.s.l.). The lenticular orebody is 1,000 m long, 600 m wide and 300 m thick, with an average Cu grade of 0.52 percent and molybdenum grade of 0.028 percent. The lower portion of the porphyry intrusion (4,300 m a.s.l.) and is characterized by the formation of secondary quartz, potassium feldspar, and biotite associated with quartz-sulfide (chalcopyrite + bornite + pyrite)-magnetite mineralization; 3. A quartz-sericite alteration stage, which formed fine replacement grains of quartz and sericite, together with abundant quartz-sulfide-sericite veinlets. These veinlets have reopened earlier-formed, magnetite-bearing veins, causing partial replacement of magnetite by pyrite. They formed peripheral to and partly superimposed on the second K silicate alteration zone; 4. A propylitic alteration stage, which formed chlorite, epidote, albite, pyrite, and carbonate minerals. This alteration assemblage partly overprinted the hornfels and also the early potassic alteration zone locally; and 5. An argillic to advanced alteration stage, which formed hydromica, montmorillonite, kaolinite, halloysite, and vuggy quartz associated with chalcocite, covellite, and molybdenite mineralization (Tang and Luo, 1995), which occurs mainly in the contact zone and in the surrounding Late Triassic sedimentary strata. The average Cu and Mo contents of 23 samples from the second K silicate alteration are 0.34 and 0.028 percent, respectively (Tang and Luo, 1995). The quartz-sericite alteration zone has average Cu and Mo contents (34 samples) of 0.54 and 0.017 percent, respectively. Argillic altered rocks have average Cu and Mo contents (39 samples) of 0.52 and 0.039 percent, respectively, and the advanced argillic altered rocks, which are mostly restricted to halos around fractures, have Cu and Mo contents (7 samples) of 1.08 and 0.039 percent, respectively (Tang and Luo, 1995). The orebody straddles the zone where sericite alteration has overprinted the second K silicate zone (Tang and Luo, 1995; Hou et al., 2003), which is similar to the scenario of Seedorff et al.’s figure 18C (Seedorff and Einaudi, 2004), suggesting that Cu was precipitated during the second K silicate and quartz-sericite alteration stages. The Yulong porphyry Cu deposit is rich in magnetite. In addition to the skarn iron orebodies, the porphyry has magnetite contents ranging from 0.27 to 0.5 percent, with an average of 0.4 percent (Tang and Luo, 1995). Magnetite occurred mainly in the potassic alteration zone as disseminated grains or as quartz-sulfide-magnetite veinlets (Fig. 2a-j). Its abundance decreases with increasing strength of quartz-sericite alteration and advanced argillic alteration (Tang and Luo, 1995). Fluid Inclusions Fluid inclusions hosted in quartz veinlets and quartz phenocrysts of the Yulong porphyry have been analyzed using microthermometry and Raman microspectroscopy. Fluid inclusion microthermometry was carried out at Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, using a Linkam MDSG600 heating-freezing stage with Olympus 0361-0128/98/000/000-00 $6.00

589

microscope optics. Most measurements were made at heating rates of 1° to 10°C/min. Raman analyses of fluid inclusions were performed at Institute of Geochemistry, Chinese Academy of Sciences and Sun Yatsen University, China, using a Renishaw inVia Raman spectrometer equipped with Olympus microfocusing optics. The quartz veinlets are from the second K silicate alteration zone and are composed of quartz, potassic feldspar, some mica, minor chalcopyrite, pyrite, and magnetite. The quartz veinlets are inferred to have formed during the second K silicate alteration stage. They contain secondary and primary fluid inclusions. The secondary fluid inclusions occur in trails that crosscut mineral grain boundaries and have features similar to those described by Roedder (1984). Primary fluid inclusions are characterized by regular and locally negative shapes. They are aligned along growth zones within individual quartz crystals or occur as scattered and isolated inclusions in quartz. Primary fluid inclusions can be divided into three types based on the observed phases at room temperature: (1) two-phase fluid inclusions with vapor/liquid ratios of less than 30 to ~40 percent, (2) gas-rich, two-phase fluid inclusions with vapor/liquid ratios of more than 50 percent, and (3) daughter mineral-bearing multiphase fluid inclusions with vapor/liquid ratios between 20 to ~40 percent. The multiphase fluid inclusions contain daughter minerals such as chalcopyrite and anhydrite, and are enriched in CO2 (Fig. 3). Measured homogenization temperatures range from 390° to 560°C, with a peak at 440° to 510°C (Fig. 4a). The quartz phenocrysts in the Yulong porphyry are smoky gray in color. They have irregular to rounded shapes, with grain sizes varying from 1 to more than 8 mm. Some quartz phenocrysts suffered from resorption and recrystallization. The resorption has led to the formation of embayed quartz phenocrysts. Recrystallization has formed overgrowths around the phenocrysts consisting of quartz grains with different optical orientations. Quartz phenocrysts from the Yulong porphyry also contain both primary and secondary fluid inclusions. The primary fluid inclusions occur mainly as isolated individuals, along growth zones of quartz crystals. The primary fluid inclusions contain mainly anhydrite and halite as daughter minerals (Fig. 5). The primary fluid inclusions can be further divided into three types on the basis of the phases and petrographic features at room temperature: (1) gas-rich, two-phase fluid inclusions with vapor/liquid ratios of more than 60%; (2) gasrich, two-phase fluid inclusions with vapor/liquid ratios of more than 50 percent that contain a daughter mineral; and (3) multiphase fluid inclusions with daughter minerals and vapor/liquid ratios of less than 30 percent. The homogenization temperatures measured from 32 primary fluid inclusions hosted by quartz phenocrysts are all more than 580°C. Secondary fluid inclusions in quartz phenocrysts are aligned mainly along microfractures or occur in secondary overgrowths in the recrystallized embayments of some quartz phenocrysts. The secondary fluid inclusions contain halite and sulfide daughter minerals, similar to those observed in the mineralized quartz veinlets. The homogenization temperatures of the secondary fluid inclusions range from 220° to 500°C, with two peaks of 300° to 340°C and 450° to 480°C, respectively (Fig. 4b).

589

590


FIG. 2. Photomicrographs of magnetite occurrences in the Yulong porphyry. (a) Amphibole grain partially replaced by magnetite, biotite, and K feldspar (plane-polarized light); (b) amphibole partially altered to magnetite and K feldspar (plane-polarized light); (c) disseminated magnetite and quartzchalcopyrite-magnetite veinlet from the second K silicate alteration zone (reflected light); (d) disseminated magnetite and chalcopyrite and quartz-magnetite veinlet from the second K silicate alteration zone (reflected light); (e) association of chalcopyrite and magnetite in the second K silicate alteration zone overprinted by quartz-sericite alteration (reflected light); (f) same as e (cross-polarized light); (g) pyrite and magnetite association from the second K silicate alteration zone, where magnetite has been partially replaced by pyrite (reflected light); (h) same as g (cross-polarized light); (i) association of pyrite, chalcopyrite, and magnetite from the quartz-sericite alteration zone, where the pyrite grain contains remnants of earlier-formed magnetite (reflected light); (j) closeup of part of (i) (reflected light). Abbreviations: Amph = amphibole, Bio = biotite, Chl = chlorite, Cp = chalcopyrite, Ksp = K feldspar, Mt = magnetite, Py = pyrite, Q = quartz, Se = sericite. 0361-0128/98/000/000-00 $6.00

590

591


FIG. 3. Photos of mineral assemblage of sulfate and sulfide (anhydrite and chalcopyrite) in a CO2-rich fluid inclusion hosted by mineralized quartz veinlets and laser Raman spectra of (a) anhydrite, (b) chalcopyrite, and (c) CO2.

The primary and secondary fluid inclusions contain different daughter minerals. The primary fluid inclusions contain mainly anhydrite and halite as daughter minerals (Fig. 5). Previous studies found that some quartz phenocrysts also contain melt inclusions (Tang and Luo, 1995). Consistently, some fluid inclusions have higher homogenization temperatures, ranging from 900° to 1,000°C, similar to the temperature of melt inclusions (Tang and Luo, 1995), indicating that these quartz phenocrysts are of magmatic origin. Discussion

FIG. 4. Frequency diagrams of homogenization temperature of (a) fluid inclusions of quartz veinlet in potassic alteration zone, and (b) secondary fluid inclusions of quartz phenocrysts. 0361-0128/98/000/000-00 $6.00

Oxidizing conditions and redox fluctuations The Yulong porphyry is highly oxidized, similar to many other porphyry Cu deposits (Gustafson and Hunt, 1975; Hedenquist and Lowenstern, 1994; Sillitoe, 1997; Mungall, 2002; Audetat et al., 2004; Sun et al., 2004). The most remarkable phenomenon found in the Yulong porphyry is that the changes in sulfur species occurred roughly concurrently with the commencement of magnetite alteration (Fig. 2a, b, c). Sulfate (e.g., anhydrite) is abundant in primary fluid inclusions (with homogeneous temperatures of more than 580°C) hosted in quartz phenocrysts (Fig. 5). No magmatic sulfide has been found yet in the Yulong porphyry and the oxygen fugacity of the Yulong porphyry is therefore interpreted to be above the sulfide-sulfur oxide buffer. In other words, most of the sulfur in the Yulong porphyry magmas was oxidized, occurs in the form of sulfate, consistent with previous observations from other porphyry Cu deposits (e.g., Streck and

591

592


FIG. 5. Photos of sulfate (anhydrite) in magmatic fluid inclusions hosted by quartz phenocrysts, and laser Raman spectra of, a: water, b: anhydrite, c: anhydrite, d: water+ CO2.

Dilles, 1998). In contrast, sulfate and sulfide (e.g., anhydrite and chalcopyrite) coexist in fluid inclusions hosted by mineralized quartz veinlets in the K silicate alteration zone (Fig. 3), indicating that sulfate reduction started slightly before the main mineralization stage. Sulfide is the predominant sulfur species in the main stage of mineralization. No sulfate has been reported in the orebody as yet. Hydrogen loss from fluid inclusions some time after trapping can change the redox state of the fluid inclusions significantly (Mavrogenes and Bodnar, 1994). This is not likely to be the case for the Yulong deposit. For fluid inclusions that 0361-0128/98/000/000-00 $6.00

undergo hydrogen loss, the solid phases in the fluid inclusions will not dissolve during heating (Mavrogenes and Bodnar, 1994). The homogenization temperatures of two-phase fluid inclusions and multiphase fluid inclusions with daughter minerals (including sulfate daughter minerals in primary fluid inclusions) of the Yulong porphyry are less than 680°C (Tang and Luo, 1995), suggesting that the fluid inclusions of the Yulong porphyry did not undergo major hydrogen loss. Moreover, no sulfide daughter minerals have been observed in primary fluid inclusions, but are common in secondary fluid inclusions hosted in quartz phenocrysts and these primary

592


and secondary inclusions coexist in the same quartz crystal. This would be difficult to explain if the high redox potential in primary inclusions was due to hydrogen loss, because the primary fluid inclusions are generally better conserved than the secondary fluid inclusions. The anhydrite found in the high-temperature fluid inclusions (>580°C) suggests that the Yulong porphyry was oxidized, with a high sulfate/sulfide ratio prior to the onset of mineralization. This is consistent with the observed mineral assemblages from the Yulong porphyry. The Yulong porphyry has a mineral assemblage of biotite + amphibole + titanite + magnetite, which requires strongly oxidized conditions, as proposed for other locations (Dilles, 1987; Wones, 1989). The oxygen fugacity of the Yulong porphyry is inferred to have been more than ~2 to 3 log units higher than to the fayalite-magnetite-quartz buffer under which sulfate would predominate over sulfide as the melt sulfur-species (Kiyosu and Kurahashi, 1983; Whitney, 1984; Ohmoto, 1986; Burnham, 1997). This is similar to redox potentials estimated for the Yerington batholith (Dilles, 1987), the Butte Cu-Mo deposit (Field et al., 2005), and Santa Rita (Audetat et al., 2004). All these suggest that redox fluctuations were essential for ore formation at Yulong and, by analogy, at other porphyry Cu deposits. Redox changes can be well preserved in zircon, a resistant accessory mineral commonly found in porphyries. This mineral provides a promising exploration tool (Ballard et al., 2002). The Ce4+/Ce3+ ratio in zircon from ore-forming porphyries in the Yulong region shows high and varied redox, whereas zircon from barren porphyries nearby have low and fairly constant Ce4+/Ce3+ ratios (Liang et al., 2006). Sulfur isotope compositions of different sulfides, including pyrite, chalcopyrite, and molybdenum from the Yulong porphyry Cu deposit, exhibit a narrow range of δ34S values (–0.5 to 3.8‰: Rui et al., 1984; Ma, 1990). These are interpreted to indicate a magmatic sulfur source without major contribution from the sedimentary rocks. Sulfate is typically the dominant species of sulfur in highly oxidized porphyry magmas (Kiyosu and Kurahashi, 1983; Whitney, 1984; Ohmoto, 1986; Burnham, 1997; Jugo et al., 2005), whereas the associated Yulong deposit contains abundant sulfides, such as chalcopyrite, molybdenite, pyrite, ± bornite, ± tetrahedrite, ± cubanite. Therefore, the Yulong porphyry ore-forming system evolved from a sulfate-dominated (oxidizing) environment during the magmatic stage to a sulfide-dominated (reducing) environment during the main mineralization stage. The reduction of sulfate to sulfide was obviously crucial to the formation of the Yulong porphyry deposit. Magnetite crystallization in the Yulong porphyries began before the commencement of mineralization (Fig. 2a-j), probably coincident with the onset of major sulfate reduction. Sulfur speciation in magmatic and hydrothermal systems is influenced strongly by redox potential (Fig. 6). In a magmatic system, redox potential is usually buffered by redox-sensitive elements, e.g., C, H, S and Fe (Mungall, 2002; Evans, 2006; Sun et al., 2007b), through reaction involving mafic minerals (Carmichael, 1991) and oxides (Sun et al., 2004), which are the major hosts for Fe. Given that Fe is the most effective and abundant redox agent in magmas (Mungall, 2002), redox potential can therefore be estimated by considering the mineral equilibration between ferromagnesian silicates and oxides 0361-0128/98/000/000-00 $6.00

593

FIG. 6. Sulfur speciation versus oxygen fugacity for Yulong samples. The fO2 of Yulong porphyries is close to the transition between oxidizing and reducing conditions in the sulfur speciation diagram. The implication is that minor decreases in fO2 can produce a large amount of reduced sulfur. This provides a feasible explanation for the close relationship between oxidized magmas and porphyry ore deposits. (a) Magmatic stage; (b) pre-ore magnetite alteration stage; and (c) main mineralization stage. Modified after Jugo et al. (2005).

(Ghiorso and Sack, 1991; Xirouchakis et al., 2001). The log fO2 of the Yulong porphyry magmas is estimated to be ∆FMQ + 2 or higher, where FMQ is the fayalite-magnetite-quartz oxygen buffer (Baker and Rutherford, 1996; Ducea et al., 1999; Jugo et al., 2005). The proportion of trivalent iron (Fe3+) in the Yulong porphyry magma is estimated to be about 30 percent of the total Fe at fO2 of ∆FMQ + 2 and a temperature of 850°C (Kress and Carmichael, 1991). Ferrous/ferric ratios of biotite and amphibole in the Yulong porphyry have been determined using the Moessbauer method and calculated on the basis of the charge balance of microprobe analyses, respectively (Ma, 1990). Biotite from the porphyry has a negligible amount of Fe3+, whereas Fe3+ in amphibole accounts for about 5 to 28 percent of the total Fe. Given the lower Fe3+ proportion in biotite and amphibole compared to the magma, the crystallization of biotite and amphibole increases the residual proportion of Fe3+ in the magmatic system. Significant amounts of ferrous ion (Fe2+) in silicates can be transported by supercritical fluids and aqueous vapor in magmatic-hydrothermal systems (Simon et al., 2004). These could increase the proportion of Fe3+ in the magma. All these promote magnetite crystallization. Two-thirds of the iron in magnetite is Fe3+. Therefore, the formation of magnetite is likely to be controlled by the amount of Fe3+ in the system. When amphibole and biotite are replaced by secondary magnetite (Fig 2a, b, d), considerable amounts of Fe2+ needs to be oxidized to Fe3+, such that Fe2+ can act as a reducing agent that transforms the oxidized

593

594


sulfur in the system to reduced sulfur during magnetite formation (Carmichael and Ghiorso, 1986; Sun et al., 2004): 12[FeO] + H2SO4 = 4Fe3O4 + H2S.

(1)

In the Yulong porphyry, magnetite deposition was closely associated with K feldspar alteration. This is likely due to oxidation of iron in biotite and hornblende by aqueous sulfate, producing reduced sulfur and magnetite. The reaction can be well illustrated by the following equation: 8KFe3AlSi3O10(OH)2 + 2H2SO4 = 8KAlSi3O8 + 8Fe3O4 + 8H2O + 2H2S.

(2)

Iron may be transported by low-density aqueous vapor at high pressures in the magmatic-hydrothermal environment (Simon et al., 2004), or fluids. Magnetite may also be formed by the oxidation of iron transported by magmatic aqueous liquid by the following equation: 12FeCl2 + 12H2O + H2SO4 = 4Fe3O4 + 24HCl + H2S (Field et al., 2005).

(3)

According to these reactions, the formation of magnetite supplies reduced sulfur to the ore-forming system, promoting the precipitation of sulfide minerals. This is a viable mechanism for ore formation at the Yulong porphyry Cu deposits. The precipitation of Cu-Fe-sulfides can be influenced by many factors, such as cooling, fluid mixing, boiling, and waterrock interaction (Hemley and Hunt, 1992; Woitsekhowskaya and Hemley, 1995; Hedenquist et al., 1998; Herzarkhani et al., 1999; Ulrich et al., 2002a; Liu and McPhail, 2005). Our focus is on redox changes (Sun et al., 2004) owing to the obvious mineralogical and fluid inclusion evidence at Yulong. Large amounts of H2S were required for the main stage of mineralization in the Yulong porphyries. This implies that considerable amounts of Cu and, possibly also, some Au, were complexed by agent(s) other than sulfur in the hydrothermal fluid. In addition to bisulfide species, chloride is a potentially important anion for Cu and Au transport (Heinrich et al., 1999, 2004; Phillips and Evans, 2004). Exsolved Cl-bearing volatile phases and brines could scavenge metals from the porphyry silicate melt during crystallization. Cu and Au can be transported as chloride complexes in high-temperature aqueous liquids (Henley, 1973; Hedenquist and Lowenstern, 1994; Heinrich et al., 1999; Frank et al., 2002; Harris et al., 2003; Liu and McPhail, 2005; Simon et al., 2005, 2006), depending on pressure (Sun et al., 2007a). CO2 could act as a pH buffer to maintain the fluid in a pH range favorable for maintaining elevated gold concentrations by complexation with reduced sulfur (Phillips and Evans, 2004) or more acid conditions could promote gold-chloride complexing (Cooke and Simmons, 2000). The close relationship between oxidized magmas and porphyry Cu-Au deposits can be explained plausibly through three major steps. First of all, a high redox potential in the mantle source region can promote the oxidation of residual sulfide in mantle sources, liberating chalcophile elements into magmas (Sillitoe, 1997; Mungall, 2002). Secondly, given that most sulfur is present in the form of sulfate in oxidized magmas, sulfide remains undersaturated during fractional crystallization (Wyborn and Sun, 1994; Mungall, 2002), and thus moderately incompatible elements such as Au and Cu (Sun et al., 2003a, b, 2004) become increasingly 0361-0128/98/000/000-00 $6.00

enriched during magmatic evolution (Wyborn and Sun, 1994). Thirdly, magnetite crystallization during the late stage of the magmatic process reduces some SO42– into H2S, allowing chalcopyrite precipitation. Magnetite alteration In general, magnetite-rich alteration is commonly associated with Cu-Au mineralization in porphyry deposits (Hedenquist and Lowenstern, 1994; Arancibia and Clark, 1996; Sillitoe, 2000; Ulrich and Heinrich, 2001; Sillitoe, 2003). Gustafson and Hunt (1975) found that quartz-magnetite veinlets are common in the core of El Salvador porphyry Cu deposit, Chile. Magnetite associated with porphyry Cu-Au systems is inferred to have formed either prior to potassic alteration, or during the main stage potassic alteration associated with sulfide formation (Arancibia and Clark, 1996). The formation temperature of intense quartz-magnetite alteration that preceded potassic alteration at Bajo de la Alumbrera is estimated to have been 750°C (Ulrich and Heinrich, 2001, 2002; Ulrich et al., 2002a). Magnetite alteration can be plausibly interpreted by oxidation of high Fe minerals, e.g., amphibole. This is supported by the close association of magnetite with amphibole in Yulong (Fig. 2a, b). In addition to oxidizing iron, Reaction (2) also produces secondary K feldspar. The separation between magnetite and K feldspar alteration is probably controlled by different precipitation temperatures. The Yulong porphyry Cu deposit is rich in magnetite. In addition to more than 86 million tons of skarn iron ore reserves, the porphyry itself contains magnetite varying from 0.27 to 0.5 wt percent with an average content of about 0.4 wt percent (Ma, 1990; Tang and Luo, 1995). Most of the magnetite crystals occur around and within amphibole and biotite grains (Fig. 2a, b, d), as disseminated grains (Fig. 2c, d) or quartz-sulfide- magnetite veinlets (Fig. 2c, d), suggesting that most of the magnetite formed by secondary alteration. The Yulong orebodies contain >6.5 Mt of Cu. The Cu concentration of felsic magmas is assumed to be in the range of 50 to