Journal of Asian Earth Sciences 41 (2011) 525–536

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Journal of Asian Earth Sciences

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Magmatic-hydrothermal evolution of the Cretaceous Duolong gold-richporphyry copper deposit in the Bangongco metallogenic belt, Tibet: Evidencefrom U-Pb and 40Ar/39Ar geochronology

Jinxiang Li ⇑, Kezhang Qin ⇑, Guangming Li ⇑, Bo Xiao, Junxing Zhao, Lei ChenKey Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, PR China

a r t i c l e i n f o

Article history:Received 5 May 2010Received in revised form 25 February 2011Accepted 2 March 2011Available online 13 March 2011

Keywords:Gold-rich porphyry copper depositZircon U–Pb geochronology40Ar/39Ar geochronologyDuolongBangongco metallogenic beltTibet

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⇑ Corresponding authors. Address: Beitucheng WestBeijing 100029, PR China. Tel.: +86 10 82998187; fax

E-mail addresses: ljx@mail.iggcas.ac.cn (J.-X. Li), kzlgm@mail.igcas.ac.cn (G.-M. Li).

a b s t r a c t

The Duolong gold-rich porphyry copper deposit was recently discovered and represents a giant prospect(inferred resources of 4–5 Mt fine-Cu with a grade of 0.72% Cu; 30–50 t fine-gold with a grade of 0.23 g/tAu) in the Bangongco metallogenic belt, Tibet. Zircon SHRIMP and LA-ICP-MS U–Pb geochronology showsthat the multiple porphyritic intrusions were emplaced during two episodes, the first at about 121 Ma(Bolong mineralized granodiorite porphyry (BMGP) and barren granodiorite porphyry (BGP)) and the sec-ond about 116 Ma (Duobuza mineralized granodiorite porphyry (DMGP)). Moreover, the basaltic ande-sites also have two episodes at about 118 Ma and 106 Ma, respectively. One andesite yields an U–Pbzircon age of 111.9 ± 1.9 Ma, indicating it formed after the multiple granodiorite porphyries. By contrast,the 40Ar/39Ar age of 115.2 ± 1.1 Ma (hydrothermal K-feldspar vein hosted in DMGP) reveals the close tem-poral relationship of ore-bearing potassic alteration to the emplacement of the DMGP. The sericite fromquartz-sericite vein (hosted in DMGP) yields a 40Ar/39Ar age of 115.2 ± 1.2 Ma. Therefore, the ore-formingmagmatic-hydrothermal evolution probably persisted for 6 m.y. Additionally, the zircon U–Pb ages (106–121 Ma) of the volcanic rocks and the porphyries suggest that the Neo-Tethys Ocean was still subductingnorthward during the Early Cretaceous.

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1. Introduction

The Cretaceous Duolong (Duobuza-Bolong, Duobuza is thenortheastern ore section; Bolong is the southwestern ore section)gold-rich porphyry copper deposit was recently discovered bythe No.5 Geological Team (at the Bureau of Tibetan Geology andExploration) in 2000, together with the super-large prospect inthe Bangongco metallogenic belt (Fig. 1a), Tibet (inferred resourcesof 4–5 Mt fine-Cu, with a grade of 0.72% Cu; 30–50 t-fine gold, witha grade of 0.23 g/t Au; Li et al., 2008). Due to the discovery of theDuolong deposit, the Bangongco metallogenic belt may be the thirdporphyry copper belt following the Yulong and Gangdese (Fig. 1a).All of the deposits belong to the Tethyan-Himalaya metallogenicprovince. Moreover, the porphyry deposits of the Bangongco met-allogenic belt are a Cretaceous, Cu–Au mineralization assemblageand formed in a magmatic active arc setting (Li et al., 2008), differ-ing from those of the Yulong and Gangdese. The porphyry depositsof the Yulong and Gangdese metallogenetic belt respectivelyformed in the Eocene and Miocene, and both of them belong to

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Road 19#, Chaoyang District,: +86 10 62010846.q@mail.igcas.ac.cn (K.-Z. Qin),

Cu–Mo mineralization assemblage and formed in a post-collisiongeotectonic setting (Hou et al., 2004, 2007, 2009; Qu et al., 2004,2007, 2009; Qin et al., 2005, 2006; Liang et al., 2006, 2009). In addi-tion, the Bangongco metallogenic belt is analogous to the Panag-yurishte ore district of the Srednogorie zone in Bulgaria, whichformed in the Late Cretaceous (Von Quadt et al., 2002, 2005;Tarkian et al., 2003; Kouzmanov et al., 2009) and is present inthe western segment of the Tethyan-Himalaya metallogeneticprovince. The Late Cretaceous Elatsite porphyry deposit (locatedabout 55–60 km east of Sofia, Bulgaria) in the Panagyurishte oredistrict has a Cu–Au–PGE mineralization assemblage and wasformed in an arc setting (Von Quadt et al., 2002, 2005; Tarkianet al., 2003), resembling the Duolong mineralization. However,the study of the Bangongco metallogenic belt is in its initial stage,and preliminary studies have assessed only the geochronology (Quand Xin, 2006; Li et al., 2008), fluid inclusions (She et al., 2006; Liet al., 2007) and geochemical characteristics of the Duolong depos-it (Li et al., 2008). In this paper, we systematically sample the Duo-buza and Bolong mineralized granodiorite porphyry, barrengranodiorite porphyry, and mafic – intermediate volcanic rocks(Fig. 1b) from the Duolong gold-rich porphyry copper deposit,and present a new work on their formation age using the precisezircon SHRIMP and LA-ICP-MS U–Pb geochronology methods. Also,we date the hydrothermal K-feldspar, biotite and sericite using

a

b

Fig. 1. The sketch tectonic and location map (a) (after Hou et al., 2004) and generalized geologic map (b) of the Duolong gold-rich porphyry copper deposit (Li et al., 2008).

526 J. Li et al. / Journal of Asian Earth Sciences 41 (2011) 525–536

40Ar/39Ar geochronology. Finally, we discuss the duration of themagmatic-hydrothermal evolution of the Duolong deposit as wellas the implications on the metallotectonic setting of the Bangongcometallogenic belt.

2. Geologic setting of the Duolong gold-rich porphyry copperdeposit

The Duolong gold-rich porphyry copper deposit is located ca.100 km northwest of Gerze city, and north of the Bangongco-Nujiang suture zone (Fig. 1a). It is a typical volcanic type in the por-phyry copper deposit classifications (Mcmillan and Panteleyev,1980), which formed in the Early Cretaceous and the magmaticarc resulted from the Neo-Tethys Ocean subduction (Li et al.,2008). The stratigraphy is mainly made up of the Middle JurassicYanshiping and the Late Cretaceous Meiriqie groups (Fig. 1b). TheMiddle Jurassic Yanshiping group is a clastic-interbeded volcanic

sequence of littoral facies, with an EW strike and WNW dip of50–80�. It is composed of arkosic sandstone, siltstone-interbededsiliceous rock, basalt and dacite. The Late Cretaceous Meiriqiegroup contains basaltic andesite, dacite, volcanic–clastic rocks,andesite porphyry and andesite (Fig. 1b). The Paleogene Kangtuogroup is composed of the brown–red clay and sandy gravel.

2.1. Magmatic activity

The hypabyssal intrusive rocks are mainly composed by themultiple granodiorite porphyries, which intruded into the MiddleJurassic Yanshiping group (Fig. 1b). These porphyries mostlyappear as stock and dyke. Duobuza mineralized granodioriteporphyry (DMGP; Fig. 2A and B) has a kidney shape with anorth–south extension of 200 m and an east–west length of1000 m (Fig. 1b) in the northeastern section of Duolong deposit.This porphyry mainly contains plagioclase, quartz, chloritized

Fig. 2. Thin section photomicrographs of the main lithologic units in the Duolong gold-rich copper deposit. A. Duobuza mineralized granodiorite porphyry (DMGP) in theDuobuza ore section, which developed potassic and argillic alteration (DbzJ2-1); B. DMGP that developed an intensive argillic alteration with plagioclase altered to sericite,illite-hydromuscovite and kaolinite (DbzTC-6); C. Bolong mineralized granodiorite porphyry (BMGP) in the Bolong ore section, which developed mainly an argillic alteration(DW2-8); D. barren granodiorite porphyry (BGP), and chlorite altered hornblende (Dbz-crp); E. basaltic andesite from the western partition of the Duolong deposit (Nd-1); F.basaltic andesite (Dw2-1); G. basaltic andesite from the central partition of the Duolong deposit (Dbz-183); H. andesite in the northeastern part of the Duolong deposit (DM-1). Hb: hornblende, Pl: plagioclase, Chl: chlorite, Q: quartz, IL-Hm: illite-hydromuscovite, Kao: kaolinite; Kf: K-feldspar.

J. Li et al. / Journal of Asian Earth Sciences 41 (2011) 525–536 527

Table 1Description of the lithology of the main magmatic rocks from the Duolong gold-rich porphyry copper deposit, Tibet.

Sample Lithology Texture Mineral composition Alteration

Phenocryst Matrix

DW2-8 Granodioriteporphyry

Porphyritic texture,phenocryst (40–45%)

Pl (10–15%, 0.6–3 mm, Carlsbad-Albite compound twin and oscillatory zoning), Q(25%, rounded shape, 0.5–2.5 mm), chloritized Hb and Bio (5–10%)

Q, Pl, Kf; 0.1–0.2 mm

Argillic-potassic

Dbz-cdp

Granodioriteporphyry

Porphyritic texture,phenocryst (40–50%)

Pl (15–20%, 0.5–2.5 mm, Carlsbad-Albite compound twin and oscillatory zoning),Q (20%, rounded shape, 0.4–2 mm), Hb and Bio (6–10%)

Q, Pl, Kf; 0.1–0.3 mm

Weaklypotassic

DbzJ2-1

Granodioriteporphyry

Porphyritic texture,phenocryst (43–50%)

Pl (15–20%, 0.5–4 mm, Carlsbad-Albite compound twin and oscillatory zoning), Q(20%, rounded shape, 0.5–2 mm), chloritized Hb and Bio (8–10%)

Q, Pl, Kf; 0.1–0.2 mm

Argillic-potassic

DbzTC-6

Granodioriteporphyry

Porphyritic texture,phenocryst (44–55%)

Pl (18–25%, 0.5–3 mm, Carlsbad-Albite compound twin and oscillatory zoning), Q(20–22%, rounded shape, 0.4–2.5 mm), chloritized Hb and Bio (6–8%)

Q, Pl, Kf; 0.1–0.2 mm

Argillic-potassic

Zk001-78

Granodioriteporphyry

Porphyritic texture,phenocryst (46–55%)

Pl (20–23%, 0.5–4 mm, Carlsbad-Albite compound twin and oscillatory zoning), Q(18–22%, rounded shape, 0.6–3 mm), chloritized Hb and Bio (8–10%)

Q, Pl, Kf; 0.1–0.15 mm

Argillic-potassic

Zk001-91

Granodioriteporphyry

Porphyritic texture,phenocryst (41–50%)

Pl (16–20%, 0.5–3.5 mm, Carlsbad-Albite compound twin and oscillatory zoning),Q (20–22%, rounded shape, 0.4–3 mm), chloritized Hb and Bio (5–8%)

Q, Pl, Kf; 0.1–0.2 mm

Argillic-potassic

Zk001-140

Granodioriteporphyry

Porphyritic texture,phenocryst (41–48%)

Pl (18–21%, 0.5–4 mm, Carlsbad-Albite compound twin and oscillatory zoning), Q(17–19%, rounded shape, 0.6–3 mm), chloritized Hb and Bio (6–8%)

Q, Pl, Kf; 0.1–0.2 mm

Argillic-potassic

Dbz-183

Basalticandesite

Porphyritic texture,phenocryst (7–9%)

Hb (2–4%), Pl (5%), Py (<1%), 0.3–3 mm Pl (strip shape),Hb; 0.05–0.1 mm

Fresh

Nd-1 Basalticandesite

Porphyritic texture,phenocryst (7–11%)

Hb (1–5%), Pl (6%), Py (<1%), 0.3–2 mm Pl, Hb; 0.05–0.15 mm

Fresh

DW2-1 Basalticandesite

Porphyritic texture,phenocryst (10–13%)

Hb (2–5%), Pl (8%), Py (<1.5%), 0.3–2.5 mm Pl (strip shape),Hb; 0.05–0.1 mm

Fresh

DM-1 Andesite Porphyritic texture,phenocryst (15–25%)

Pl (10–15%), Hb (5–10%); 1–2 mm Pl, Hb, Q; 0.05–0.1 mm

Fresh

Abbreviations: Pl, plagioclase; Q, quartz; Hb, hornblende; Bio, biotite; Kf, K-feldspar.

528 J. Li et al. / Journal of Asian Earth Sciences 41 (2011) 525–536

hornblende and biotite, and shows argillic-potassic alterationassemblages (Table 1). Bolong mineralized granodiorite porphyry(BMGP; Fig. 2C) exposes as an elliptical shape of the width of200 m and length of 300 m (Fig. 1b) in the southwestern section.This porphyry is similar to the DMGP on the mineral composition(Table 1), and dominantly shows argillic alteration. In addition,the restricted appearance of the barren granodiorite porphyry(BGP, Fig. 2D) occurs in the central section of Duolong deposit.The texture and mineral assemblage are similar to the DMGP andBMGP. But it is weakly altered by the hydrothermal fluid (Table 1).

Based on the alteration characteristics, volcanic rocks (Fig. 2)can be divided into pre- or syn-mineralization basaltic andesite,post-mineralization basaltic andesite and andesite. The pre- orsyn-mineralization basaltic andesites occur in the northeasternand southwestern section, and show weak and intensive propyliticalteration caused by the mineralized porphyries. The basaltic ande-sites (Fig. 2E) have porphyritic texture and are mainly composed ofplagioclase, hornblende and minor pyroxene (Table 1). The post –mineralization (Fig. 2F and G) basaltic andesites are distributedin the central section of Duolong deposit. The post-mineralizationfresh andesite (Fig. 2H) occurs in the northeastern section of Duo-long deposit, and mainly contains plagioclase and hornblende (Ta-ble 1).

Geochemical characteristics show that the porphyries and vol-canic rocks have the same arc magma characteristics (Li et al.,2008): a high-K calc-alkaline series, a depletion of HFSE (such asNb, Ta, Zr and Hf), and an enrichment in large ion lithophile ele-ments (such as Rb and Ba) of basaltic andesite (SiO2 of 49–53%),andesite (SiO2 of 58%) and granodiorite porphyry (SiO2 of 65–68%; Li et al., 2008).

2.2. Hydrothermal alterations

At Duolong deposit, a wide range of hydrothermal alteration isdeveloped, comprising mainly albitization, biotitization, K-felds-pathization, sericitization, silicification, epidotization, chloritiza-tion, illitization, and kaolinization. In addition, these alterationsoccur in an area of more than 10 km2. The alteration zone can bedivided into the potassic alteration, intermediate argillic and

propylitic alteration zone from the ore-bearing porphyry centeroutwards and upwards. However, the phyllic alteration is not welldeveloped and quartz–sericite veins occur only locally.

2.2.1. Potassic alteration zoneDispersive K-feldspathization displays a dominant develop-

ment. The secondary K-feldspar altered mainly the plagioclasephenocryst and the dispersive matrix. The K-feldspar alterationhalo also occurred at the edge of quartz–chalcopyrite–magnetiteveins (A-type, Gustafson and Hunt, 1975). Additionally, quartz –K-feldspar veinlet occurred. Moreover, secondary biotite replaceshornblende, primary biotite, and other Mg- and Fe-minerals.Quartz–biotite–chalcopyrite veins and biotite veinlets (EB type,Gustafson and Quiroga, 1995) were recognized. Hydrothermalmagnetites developed dominantly in the potassic alteration zone,while chalcopyrite coexists closely with magnetite. Petrographicobservations suggest that magnetite may be formed at the sametime or slightly earlier than the chalcopyrite. Moreover, the potas-sic alteration zone is mostly developed in the mineralized granodi-orite porphyry and at depth of the mineralized body. It wassuperposed by an intermediate argillic alteration.

2.2.2. Intermediate argillic zoneThis zone is superposed on the potassic alteration zone. It is

characterized mainly by kaolinization and illitization-hydromus-covitation of plagioclase and chloritization of biotite. The quartz–chalcopyrite vein (chalcopyrite present in the center of the vein,with argillic alteration halo; B type, Gustafson and Hunt, 1975),and chalcopyrite veinlets occur in the intermediate argillic alter-ation zone.

2.2.3. Propylitic zoneThis zone occurs mainly in the pre-mineralization basaltic

andesite (in the Duobuza section). The main alteration mineralscontain epidote, chlorite and carbonate. Moreover, the chlorite re-places biotite phenocryst rims, cleavages and centers. The carbon-ate, quartz, epidote and other minerals fill the amygdalas of thebasaltic andesite.

J. Li et al. / Journal of Asian Earth Sciences 41 (2011) 525–536 529

2.2.4. Phyllic alterationOnly quartz–sericite veinlets occur in the local area, consisting

with a typical alteration model of the gold-rich porphyry copperdeposit (Sillitoe, 2000).

2.3. Mineralization characteristics

At present, the Duobuza mineralization (the northeastern orebody of the Duolong copper deposit) occurs in the granodioriteporphyry and within the contact zone of the wall rock of feldsparquartz sandstone from the Middle Jurassic Yanshiping group. Fur-thermore, it is closely associated with the potassic alteration zone,superposing by the intermediate argillic alteration. Moreover, Duo-buza ore body has been controlled with a width (north–south) ofabout 100–400 m, a length (east–west) of about 1400 m, a 200�(SW) dip and an angle of 65–80�. The mineralization displays a cer-tain degree of vertical variation, which possesses a stockwork ofdisseminated veinlets in the upper part of the ore body with agradual transition to sparsely disseminated ore in the lower part,accordingly, a reduced copper content. A preliminary analysis re-veals a positive correlation between the gold and copper (Liet al., 2007). The measured amount of the resource is 2.7 Mt fine-Cu with a grade of 0.94% and 13 t fine Au with a grade of 0.21 g/tin the Duobuza ore body (Li et al., 2008). The Bolong ore sectionof the Duolong copper deposit is of about 900 m width and ofabout 1000 m length with a lateral vergence towards the south-east. The estimated copper resources of Bolong ore section arearound 2.68 Mt with a mean grade of 0.55%, while the estimatedgold resources are around 28 t with a mean grade of 0.24 g/t (Liet al., 2008).

The hypogene ore minerals are composed mainly of chalcopy-rite and magnetite, followed by pyrite, hematite, rutile, and min-or chalcocite, bornite and native gold. In general, hydrothermalmagnetite closely coexists with chalcopyrite in the Duolong de-posit. The intimate relationship between the copper–gold andhydrothermal magnetite is consistent with the mineralizationcharacteristics of a typical gold-rich porphyry copper deposit (Sil-litoe, 2000; Li et al., 2006; Imai and Nagai, 2009). Quartz –molybdenite veins are sparsely present. The amount of chalcopy-rite is greater than that of bornite and far more than pyrite.Chalcopyrite occurs mainly as disseminated stockwork veinlets,while bornite occurs mainly as blebs exsolution from chalcopy-rite. Furthermore, K-feldspar, albite, quartz, sericite, chlorite, car-bonate, illite and gypsum are associated with the hypogeneminerals.

3. Analytical methods and results of the zircon U–Pb and40Ar/39Ar geochronology

3.1. Zircon U–Pb geochronology

3.1.1. Zircon SHRIMP U–Pb geochronologyBased on detailed field work, the DMGP (DbzJ2-1), and BMGP

(DW2-8) and basaltic rocks (Nd-1, Dbz-183; Fig. 1b) were sam-pled for SHRIMP zircon U–Pb geochronology. Zircons were ex-tracted using conventional separation techniques and thenhandpicked under a binocular microscope. Together with theTEMORA zircon standard (Black et al., 2003), they were mountedin epoxy and polished to expose the cores of the grains. Pictureswere taken with reflection and transmission light microscopyand under cathodoluminescence (CL) using a scanning electronmicroscope. U–Pb dating was performed on the SHRIMP II atthe Beijing SHRIMP Center at the Chinese Academy of GeologicalSciences. The spot size of the ion beam is between 25 and30 lm. The SL13 (572 ± 1.2 Ma, U = 238 ppm) and the TEMORA

(417 ± 1.1 Ma) standards were used in the analyses, as discussedby Black et al. (2003). The ablation spot sites for analysis wereselected on the basis of the cathodoluminescence and micro-scope images. In order to maintain precision, one TEMORA anal-ysis was performed after every three or four spots on the samplezircons during data collection. The ages and concordia diagramswere produced with the SQUID 1.03 (Ludwig, 2001) and ISO-PLOT/Ex 2.06 (Ludwig, 1999) programs. The uncertainties forindividual analyses (ratios and ages) in tables are quoted atthe 1d level, whereas the errors on weighted mean ages arequoted at the 2d level in figures. The zircon SHRIMP U–Pb dataare presented in Supplementary Table A1.

3.1.1.1. Mineralized porphyries. The size of the zircon from the Duo-buza and Bolong mineralized granodiorite porphyry (DbzJ2-1,DW2-8) changes from 150 to 400 lm. Their Th/U ratios range from0.45 to 0.9 and are higher than 0.1 (the value of magmatic zircon;Fernando et al., 2003). The CL images indicate that the majority ofthe zircons have a euhedral crystal form and an obvious oscillatoryzone (Fig. 3a and c), which indicates that these zircons have a mag-matic origin and that their age can represent the crystallization ageof the granodiorite porphyry (Song et al., 2002; Fernando et al.,2003; Samuel and Mark, 2003). The zircon 207Pb/235U–206Pb/238Uconcordia age of the DMGP (DbzJ2-1) from the northeastern oresection of the Duolong deposit is 116.8 ± 1.7 Ma (n = 12,MSWD = 1.2; Fig. 3b). The zircon 207Pb/235U–206Pb/238U concordiaage of the BMGP (DW2-8) from the southwestern ore section is121.1 ± 1.7 Ma (n = 9, MSWD = 0.8; Fig. 3d). One zircon with theU–Pb age of 115.2 ± 2.7 Ma may be contaminated.

3.1.1.2. Basaltic andesites. The zircon size, which ranges between100 and 350 lm (Fig. 3e and g), of the basaltic andesite (Nd-1,Dbz-183) is slightly smaller than that of the mineralized granodio-rite porphyries. These zircons have a magmatic origin that wasidentified by their euhedral crystal form, obvious oscillatory zone(Fig. 3e and g), and their Th/U ratios (0.38–1.77) higher than 0.1(Song et al., 2002; Fernando et al., 2003; Samuel and Mark,2003). Zircon U–Pb age of basaltic andesite (Dbz-183) show twogroups as follows. One group yields a 207Pb/235U–206Pb/238Uconcordia age (Fig. 3f) of 105.7 ± 1.7 Ma (n = 7, MSWD = 1.0), repre-senting the true crystallization age of basaltic andesite. The othergroup yields an older 207Pb/235U–206Pb/238U concordia age(Fig. 3f) of 121.5 ± 1.2 Ma (n = 4, MSWD = 1.3), possibly represent-ing the age of inherited zircon. Zircon 207Pb/235U–206Pb/238U con-cordia age of the basaltic andesite (Nd-1) from the western partof the Duolong deposit (Fig. 3h) is 117.9 ± 1.5 Ma (n = 11,MSWD = 1.1), which indicates the basaltic andesite formed afterthe BMGP.

3.1.2. Zircon LA-ICP-MS U–Pb geochronologyZircon LA-ICP-MS U–Pb dating of intensive argillic DMGP

(DbzTC-6), BGP (Dbz-cdp), basaltic andesite (Nd-1, DW2-1) andandesite (DM-1) was performed. The analyses were conducted ona Neptune MC-ICP-MS equipped with a 193-nm laser at the Insti-tute of Geology and Geophysics, Chinese Academy of Sciences inBeijing, China. During the analyses, a laser repetition rate of 6–8 Hz at 100 mJ was used. The spot sizes are 40–60 lm. Each spotanalysis consisted approximately of 30 s background acquisitionand 40 s sample data acquisition. The detailed analytical techniquehas been described by Yuan et al. (2004) and Xie et al. (2008).207Pb/206Pb, 206Pb/238U, 207Pb/235U (235U = 238U/137.88), and208Pb/232Th ratios are corrected by the Harvard zircon 91,500(Wiedenbeck et al., 1995) as the external calibrant. Common Pbcontents were evaluated using the method described by Anderson(2002). The age calculations and concordia diagrams were gener-ated using ISOPLOT (ver 3.0) (Ludwig, 2003). The uncertainties

(a)

(c)

(e)

(g)

(b)

(d)

(f)

(h)

Fig. 3. Cathodoluminescence (CL) images (a, c, e, g) and 207Pb/235U–206Pb/238U concordia plots (b, d, f, h) of SHRIMP U–Pb dating of zircons from DMGP, BMGP and basalticandesite (Dbz-183, Nd-1) in the Duolong gold-rich porphyry copper deposit, Tibet. Errors are quoted at the 2d level.

530 J. Li et al. / Journal of Asian Earth Sciences 41 (2011) 525–536

for individual analyses (ratios and ages; in tables) are quoted at the1d level, whereas the errors on concordia and weighted mean ages

are quoted at the 2d level in figures. The zircon LA-ICP-MS U–Pbdata are presented in Supplementary Table A2.

J. Li et al. / Journal of Asian Earth Sciences 41 (2011) 525–536 531

The size of the zircons from the intensive argillic DMGP (DbzTC-6), BGP (Dbz-cdp), andesite (DM-1) and basaltic andesite (Nd-1,DW2-1) changes from 150 to 500 lm. CL images indicate thatthe majority of the zircons have a euhedral crystal form and anobvious oscillatory zone. Their Th/U ratios range from 0.39 to0.98 and are all higher than 0.1. These zircon characters indicatethat they have a magmatic origin (Song et al., 2002; Fernandoet al., 2003; Samuel and Mark, 2003).

3.1.2.1. Mineralized and barren porphyries. The zircon207Pb/235U–206Pb/238U concordia age of the intensive argillic DMGP(DbzTC-6) is 116.4 ± 2.5 Ma (n = 15, MSWD = 1.1; Fig. 4a), and theweighted mean age is 116.1 ± 1.9 Ma (n = 15, MSWD = 1.5;Fig. 4b). This age is in good concordance with the age of DMGP(DbzJ2-1). The zircon 207Pb/235U–206Pb/238U concordia age of theBGP (Dbz-cdp) is 120.7 ± 1.9 Ma (n = 15, MSWD = 1.0; Fig. 4c),while the weighted mean age is 122.4 ± 1.9 Ma (n = 15, MSWD =1.4; Fig. 4d), which is concordant within their errors with the crys-tallization age of the BMGP (DW2-8).

3.1.2.2. Basaltic andesites and andesite. The zircon 207Pb/235U–206P-b/238U concordia age of the andesite (DM-1) is 111.9 ± 1.9 Ma(n = 17, MSWD = 0.9; Fig. 4e), and the weighted mean age is110.9 ± 1.8 Ma (n = 17, MSWD = 1.2; Fig. 4f). The zircon207Pb/235U–206Pb/238U concordia age of the basaltic andesite(Nd-1) is 118.1 ± 1.6 Ma (n = 11, MSWD = 0.3; Fig. 4g), while theweighted mean age is 118.5 ± 1.4 Ma (n = 11, MSWD = 0.3;Fig. 4h), which is concordant within their errors with the SHRIMPzircon U–Pb age of the same sample. The zircon 207Pb/235U–206P-b/238U concordia age of the basaltic andesite (DW2-1) is 106.4 ±1.4 Ma (n = 14, MSWD = 1.1; Fig. 4i), while the weighted meanage is 105.7 ± 1.6 Ma (n = 14, MSWD = 1.5; Fig. 4j), which is concor-dant within the error range of the SHRIMP zircon U–Pb age of thebasaltic andesite (Dbz-183), both north of the mineralized Bolongarea.

3.2. 40Ar/39Ar geochronology

The secondary K-feldspar (Zk001-91), biotite (Zk001-78) andsericite (Zk001-140) were respectively separated for 40Ar/39Ar dat-ing from the depth of 91 m, 78 m, and 140 m belonging to the drillhole No. Zk001 of Duobuza area (Fig. 1b). They all belong to theintermediate argillic alteration superposed on the potassic alter-ation zone, and hosted in the DMGP. The secondary K-feldspar isseparated from the quartz – K-feldspar – chalcopyrite veinlet(width of 6–8 mm). The secondary biotite typically occurs asfine-ragged clots that have replaced the igneous groundmass, andmicro-scale chlorite intergrowths occur within it. The secondarysericite is derived from the quartz – sericite – minor pyrite veinlet(width of 6–7 mm).

The 40Ar/39Ar step-heating analysis was performed at the Labo-ratory of Paleomagnetism and Geochronology (SKL-LE) at the Chi-nese Academy of Sciences, Beijing, on a MM5400 massspectrometer that operated in static mode. The detailed experi-mental process has been previously reported (Wang et al., 2004,2006). The secondary K-feldspar, biotite, sericite and Bern-4 Mstandards were irradiated in vacuo within a cadmium-coatedquartz vial for 47.5 h in position H8 of the reactor at the facilityof Beijing Atomic Energy Research Institute (49–2). The Bern-4 Mbiotite standard age is 18.7 ± 0.1 Ma, recalculating by more appro-priate Ga1550 biotite standard age of 98.8 Ma (Renne et al., 1998)than of 97.8 Ma (Baksi et al., 1996). The data were corrected by thesystem blanks, mass discrimination, interfering Ca, K-derived ar-gon isotopes and the decay of 37Ar and 39Ar. The plateau and iso-chron ages were calculated using ArArCALC (Koppers, 2002).Plateau ages were defined using the criteria of Dalrymple and

Lanphere (1971) and Fleck et al. (1977), specifying the presenceof at least three contiguous gas fractions that together representmore than 50% of the total 39Ar released from the sample and forwhich no age difference can be detected between any two fractionsat the 95% confidence level. Furthermore, inverse isochron dia-grams test the assumption made in the plateau ages where anytrapped nonradiogenic Ar has an atmospheric composition(40Ar/36Ar = 295.5). Both the plateau and inverse isochron ageuncertainties are given at a 2r confidence level.

The age results from the step-heating experiments are pre-sented in Supplementary Table A3. The plateau age of secondaryK-feldspar (Zk001-91) from the DMGP is 115.2 ± 1.1 Ma (Fig. 5a),which accounts for about 75% of the released 39Ar. The inverse iso-chron age is 114.5 ± 1.5 Ma with an elevated (40Ar/36Ar)i ratio of325.6 ± 5.2 (Fig. 5b).

The age spectrum of secondary biotite from the altered granodi-orite porphyry (Zk001-78) is highly disturbed. The high apparentage spectrum (Fig. 5c) may be caused by 39Ar recoil due to the pres-ence of minor chlorite (Snee, 2002), which is consistent with thechloritization of biotite along rims and fractures. The age calcula-tions on three steps gives an age of 119.2 ± 1.1 Ma (Fig. 5c), andthe inverse isochron age of 119.9 ± 2.3 Ma with the (40Ar/36Ar)i ra-tio of 154.5 ± 246.7, which deviates from the atmospheric ratio of295.5 (Fig. 5d). This biotite alteration age is older than zircon U–Pb age of 116.8 ± 1.7 Ma or partly overlapped with it. So, we con-clude that this biotite 40Ar/39Ar age do not have any geologicalsignificance.

The age spectrum of secondary sericite from the Zk001-140quartz–sericite vein is highly disturbed and L-shaped, and showsexcess argon (40Ar) at least in the first three steps. This suggeststhat excess argon trapped in the sericite could have affected theplateau age. Therefore, the age spectrum of five steps gives an inte-grated age of 115.2 ± 1.2 Ma, including 32% of the released 39Ar(Fig. 5e). The inverse isochron yields an good age of 115.5 ±1.5 Ma with the (40Ar/36Ar)i ratio of 290.8 ± 11.7 (Fig. 5f), whichis in good concordance within their errors with the integratedage. This indicates that the integrated age of 115.2 ± 1.2 Ma mightbe reliable.

4. Discussion

4.1. Magmatic-hydrothermal evolution

4.1.1. Magma evolutionZircon U–Pb geochronology of porphyries and volcanic rocks

can reveal the complex history of magmatic evolution (Harriset al., 2004; Deckart et al., 2005). At Duolong deposit, Zircon U–Pb age (120.7 ± 1.9 Ma) of the BGP (Dbz-cdp) is concordant withintheir errors with the crystallization age (121.1 ± 1.7 Ma) of theBMGP (DW2-8). This shows that BMGP (DW2-8) and BGP (Dbz-cdp) were nearly simultaneously emplaced at about 121 Ma. Sub-sequently, the emplacement of DMGP occurred at about 116 Ma.Therefore, these ages show that there may have been at leasttwo episodes of porphyritic intrusions in this deposit, which gener-ally possesses a consistent model of multi-pulse ore-bearing mag-matic activities in other porphyry copper deposits (Sillitoe, 2000;Ballard et al., 2001; Maksaev et al., 2004; Deckart et al., 2005).

Basaltic andesite (Nd-1) from the western part of the Duolongdeposit were erupted between BMGP and DMGP, as evidenced bythe SHRIMP zircon U–Pb age of 117.9 ± 1.5 Ma and the LA-ICP-MS zircon U–Pb age of 118.1 ± 1.6 Ma. The SHRIMP zircon U–Pbage of the fresh basaltic andesite (Dbz-183) is 105.7 ± 1.7 Ma,which is concordant within their uncertainties with the LA-ICP-MS zircon U–Pb age of 106.4 ± 1.4 Ma for the basaltic andesite(DW2-1) in the Bolong ore section. This indicates that they formed

(a)

(c)

(e)

(g)

(i) (j)

(h)

(f)

(d)

(b)

Fig. 4. 207Pb/235U–206Pb/238U concordia (a, c, e, g, i) and weighted mean (b, d, f, h, j) ages of LA-ICP-MS zircon U–Pb dating of DMGP (DbzTC-6), BGP (Dbz-cdp), andesite(DM-1), and basaltic andesite (Nd-1, DW2-1) from Duolong gold-rich porphyry copper deposit in Tibet. Errors are quoted at the 2d level.

532 J. Li et al. / Journal of Asian Earth Sciences 41 (2011) 525–536

clearly later than the mineralized porphyries with an age of 121and 116 Ma, and are post-mineralization volcanic rocks. In addi-tion, the LA-ICP-MS zircon U–Pb age of the andesite (DM-1) is111.9 ± 1.9 Ma, which is also significantly later than the formation

age of the mineralized porphyries and the mineralization age. Thisshows that this andesite is also post-mineralization volcanic rock.

In conclusion, the magmatic evolution sequence of the Duolonggold-rich porphyry copper deposit is as follows (Fig. 6): the earliest

K (a)

(c)

(e) (f)

(d)

(b)

Fig. 5. The 40Ar/39Ar ages (a, c, e) and their corresponding inverse isochon ages (b, d, f) of the hydrothermal K-feldspar, biotite, and sericite from the Duolong porphyry copperdeposit, Tibet. WMPA: weighted mean plateau age.

J. Li et al. / Journal of Asian Earth Sciences 41 (2011) 525–536 533

BMGP (DW2-8) and BGP (Dbz-cdp) ? basaltic andesite (Nd-1) ?DMGP (DbzJ2-1, DbzTC-6) ? andesite (DM-1) ? basaltic andesite(Dbz-183, DW2-1). The magma composition show an evolutiontrend from the intermediate-acid to basic.

4.1.2. Duration of magmatic-hydrothermal evolutionEstimates of the longevity of igneous-related hydrothermal ore

deposits vary significantly among deposits. Short-lived hydrother-mal systems ranging from 100,000 to 300,000 yr were confirmed insome porphyry – related deposits (e.g., Far Southeast-Lepanto,Arribas et al., 1995; Round Mountain, Henry et al., 1995;Potrerillos, Marsh et al., 1997). By contrast, long-lived hydrother-mal systems can last for several million years and consist ofnumerous short-lived hydrothermal pulses, such as La Escondida,

Chile (Padilla-Garza et al., 2004), Chuquicamata, Chile (Ballard etal., 2001), El Teniente (Maksaev et al., 2004), Río Blanco (Deckartet al., 2005), and Bajo de la Alumbrera, Argentina (Harris et al.,2008).

At Duolong deposit, two episodes of magmatic-hydrothermalactivity have been identified. Firstly, the published molybdenite(quartz – molybdenite veins, hosted in the BMGP and wall rock)Re–Os age of 118.0 ± 1.5 Ma (2d, She et al., 2006), indicating theCu–Au mineralization is closely related to the emplacement ofthe BMGP (Fig. 6). Secondly, the secondary K-feldspar from thepotassic zone yields a good 40Ar/39Ar plateau age of 115.2 ±1.1 Ma, which overlaps within their uncertainties with the U–Pbage of the DMGP. The 40Ar/39Ar age of 115.2 ± 1.2 Ma for secondarysericite (from quartz–sericite veinlet hosted in the DMGP) is

Fig. 6. Duration of ore-forming magmatic-hydrothermal evolution in Duolong gold-rich porphyry copper deposit, Tibet.

534 J. Li et al. / Journal of Asian Earth Sciences 41 (2011) 525–536

consistent with the 40Ar/39Ar plateau age of hydrothermal K-feld-spar. The Cu–Au mineralization is dominantly associated withpotassic alteration assemblages. Therefore, the two 40Ar/39Ar agesconsistency suggests that the hypogene Cu–Au mineralizationand potassic alteration probably was produced during a short per-iod. These 40Ar/39Ar ages reveal the close temporal relationshipwith the emplacement of DMGP. Finally, combined with the U–Pb ages of BMGP and DMGP mineralized porphyries, the durationof magmatic-hydrothermal evolution probably persisted for6 m.y. (Fig. 6).

4.2. Implications for the tectonic setting

The Bangongco-Nujiang suture zone is an important plateboundary in the northern part of the Qinghai-Tibet Plateau (Yinand Harrison, 2000), but detailed studies are lacking. The openingand closure time of the Bangongco-Nujiang Neo-Tethys Ocean hasbeen on debate. The latest research results of the 1:25 million re-gional geological survey show that the Neo-Tethys Ocean formedduring the Late Permian – Early Triassic (Ren and Xiao, 2004).Whole-rock Sm-Nd dating of the Shemalagou gabbro in the centralsection of the Bangongco-Nujiang suture zone indicated that theNeo-Tethys Ocean may have opened in the Early Jurassic (Qiuet al., 2004). Based on the geological phenomena of the UpperJurassic stratigraphy unconformity on the ophiolite, the Neo-Tethys Ocean may have closed in the Early Cretaceous (Guoet al., 1991). However, the Duoma pillow basalt and the TarenbenOIB-type basalt of the Late Cretaceous (around 110 Ma) indicatethat the Bangongco-Nujiang Ocean had not yet completely de-mised around 110 Ma (Zhu et al., 2006) and may imply that theclosure time of the Neo-Tethys Ocean was later than the previousopinion of Late Jurassic – Early Cretaceous (Huang and Chen, 1987;Yin and Harrison, 2000; Kapp et al., 2003).

The boninitic (basaltic andesite and andesite; SiO2 > 53%,MgO > 7% and TiO2 < 0.6; Ti/V ratio of 5–13, Ti/Sc of 40–80) rocks

that form in the forearc of the island arc setting were discoveredin the ophiolite melange, which suggests that intra-ocean subduc-tion occurred (Shi et al., 2004). Moreover, the gabbro of the SSZ(Super subduction zone)-type ophiolite yield a zircon SHRIMP U–Pb age of 167.0 ± 1.4 Ma, which indicates the Neo-Tethys Oceansubducted at least during the Middle Jurassic (Shi, 2007). Accord-ing to a regional tectonic and sedimentary facies analysis, theNeo-Tethys Ocean subducted northward under the Qiangtangblock in the Late Jurassic (Huang and Chen, 1987; Kapp et al.,2003). Significantly, the subduction polarity of the Neo-TethysOcean has been debated to be as follows: subduction northwardduring the Late Triassic – Early Cretaceous (Murphy et al., 1997;Kapp et al., 2003; Ding et al., 2003; Zhang et al., 2004; Li et al.,2008) or subduction southward during the Late Jurassic – EarlyCretaceous (Pan et al., 1997, 2004; Mo et al., 2005; Zhu et al.,2009). In summary, the opening and closure times of the Bang-ongco-Nujiang Neo-Tethys Ocean and its evolution process havebeen a topic of debate.

Trace element geochemistry characteristics and tectonic dia-grams of volcanic rocks from the Duolong gold-rich porphyry cop-per deposit confirm that this deposit formed in a continentalmargin arc setting (Li et al., 2008). The zircon U–Pb ages of the vol-canic rocks and the porphyries fall in the range from 106 Ma to121 Ma, and the Duolong porphyry copper deposit is located northof the Bangongco-Nujiang suture zone. These evidences suggestthat the Neo-Tethys Ocean still subducted northward during theEarly Cretaceous. And its closure time should be later than 106 Ma.

5. Conclusions

Zircon SHRIMP and LA-ICP-MS U–Pb geochronology shows thatthe multiple porphyritic intrusions were emplaced during two epi-sodes at Duolong deposit, the first at about 121 Ma (BMGP andBGP) and the second about 116 Ma (DMGP). The 40Ar/39Ar age of115.2 ± 1.1 Ma (hydrothermal K-feldspar vein hosted in DMGP)

J. Li et al. / Journal of Asian Earth Sciences 41 (2011) 525–536 535

and 40Ar/39Ar age of 115.2 ± 1.2 Ma (quartz–sericite vein hosted inDMGP) reveals the close temporal relationship with the secondporphyritic intrusions. The ore-forming magmatic – hydrothermalevolution probably persisted for 6 m.y. Moreover, the basalticandesites also have two episodes, the first (about 118 Ma) formedbetween the two episodes of porphyritic intrusions, and the second(about 106 Ma) and andesite (about 112 Ma) formed after the mul-tiple granodiorite porphyries. Additionally, the zircon U–Pb ages(106–121 Ma) of the volcanic rocks and the porphyries formed ina continent margin arc setting, suggest that the Neo-Tethys Oceanwas still subducting northward during the Early Cretaceous.

Acknowledgments

This article was funded by the Natural Science Foundation Pro-ject (40902027, 40672068, 40772066) and the China PostdoctoralScience Foundation (20090450567). We obtained support and helpfrom Mr. Tianping Zhang (a senior geologist) and other geologistsin the No.5 Geological Team at the Tibet Bureau of Geology andExploration. We also obtained specific guidance and assistancefrom Prof. Quanren Yan concerning the SHRIMP zircon U–Pb datingprocess at the Beijing SHRIMP Center, Chinese Academy of Geolog-ical Sciences; and from Assistant Prof. Liewen Xie and YuehengYang concerning the LA-ICP-MS zircon U–Pb dating process; andfrom Prof. Fei Wang and Huaiyu He concerning the 40Ar/39Ar datingprocess at the Institute of Geology and Geophysics, Chinese Acad-emy of Sciences. Constructive comments by Bor-ming Jahn, Juhn G.Liou, and Katja Deckart improved the manuscript.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.jseaes.2011.03.008.

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