geochronologic constraints on magmatic intrusions and mineralization of the zhunuo porphyry copper...

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www.scichina.com www.springerlink.com Chinese Science Bulletin | November 2007 | vol. 52 | no. 22 | 3139-3147

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Geochronologic constraints on magmatic intrusions and mineralization of the Zhunuo porphyry copper deposit in Gangdese, Tibet

ZHENG YouYe1,2, ZHANG GangYang1†, XU RongKe1 ,GAO ShunBao3, PANG YingChun1, CAO Liang1, DU AnDao4 & SHI YuRuo5 1 Facuty of Earth Resources, China University of Geosciences, Wuhan 430074, China; 2 State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China; 3 No. 2 Geological Team, Tibet Bureau of Geology and Exploration, Lhasa 850003, China; 4 National Geological Analytical and Testing Center, Chinese Academy of Geological Sciences, Beijing 100037, China; 5 Beijing Ion Microprobe Center of the Chinese Academy of Geological Sciences, Beijing 100037, China

In situ zircon U-Pb ages for the recently discovered Zhunuo porphyry copper deposit in the western part of the Gangdese metallogenic belt in Tibet were determined by sensitive high-resolution ion mi-croprobe (SHRIMP). The ages can be divided into two separate groups, reflecting more than four major tectono-magmatic events in the area. The 62.5±2.5 Ma age of inherited zircons may be related to the volcanic eruption of the Linzizong Group formed shortly after the India-Asia continental collision. The 50.1±3.6 Ma age most likely corresponds to the time of underplating of mantle-derived mafic magma in Gangdese. The 15.6±0.6 Ma age obtained from magmatic zircons is interpreted as the age of crystalli-zation of the Zhunuo ore-forming porphyry. Finally, a molybdenite Re-Os isochron age of 13.72±0.62 Ma is consistent with another zircon U-Pb age of 13.3 ±0.2 Ma, representing the time of copper mineraliza-tion. These ages, in combination with available literature data, indicate that magmatic crystallization and copper mineralization in the Gangdese metallogenic belt became gradually younger westward, and further suggest that the Zhunuo porphyry copper deposit was formed in the same tectonic stage as other porphyry copper deposits in the eastern and central Gangdese belt. This conclusion provides critical information for future exploration of porphyry copper deposits in western Gangdese.

western Gangdese, Zhunuo, porphyry copper deposit, ages of magmatic crystallization and mineralization

The Zhunuo porphyry copper deposit, 300 km western away from the Xigazê City, is located at the Yamo Township of Angren County, Tibet. Preliminary work indicates that it is a high quality copper project with sig-nificant exploration potential. The discovery of the Zhunuo deposit actually extended the prospecting area of the Gangdese metallogenic belt for several hundreds of kilometers westwards and the belt is likely to become a giant porphyry copper province[1]. The Gangdese met-allogenic belt can be subdivided into two blocks by the large Dangxiong-Bailang strike-slip fault. There are sig-nificant differences in mineralization character, ore de-

posit and commodity types as well as ore-forming age between the eastern and the western parts of the belt. The recently discovered porphyry copper deposits in the eastern and central parts of Gangdese, such as Qulong, Chongjiang, Tinggong, Bairong, Dabu and Chuibaizi, are genetically related to the high level magmatic com-plexes that are distributed some 35―65 km from the Received January 16, 2007; accepted August 15, 2007 doi: 10.1007/s11434-007-0406-7 †Corresponding author (email: [email protected]) Supported by the National Key Project for Basic Research of China (Grant No. 2002CB412610), National Key Project for New-round National Resource Investiga-tions (Grant No. 200210200001) and pre-National 973 Project (Grant No. 2005CCA05600)

3140 ZHENG YouYe et al. Chinese Science Bulletin | November 2007 | vol. 52 | no. 22 | 3139-3147

northern side of the Yarlung Zangbo River. These igne-ous rocks were mainly formed at 17.8―15.6 Ma, whereas the copper deposits were mainly formed at 15.99―14.85 Ma; both time periods are correlated with the transitional period of the Gangdese orogenic belt from continental collision to strike-slip extension[2,3]. The question of how far the eastern and central Gang-dese porphyry belt extends to the west remains unre-solved. It is also unclear if the Zhunuo porphyry copper deposit in the western part formed at the same time as those deposits in the eastern and central Gangdese. Clarification of these questions will not only help im-prove the understanding of the Gangdese metallogeneis, but also has practical implications for exploration of porphyry copper deposits further west along the Gang-dese metallogenetic belt.

Together with field observations and petrography, this study provides geochronologic constraints on the mag-matism and mineralization of the Zhunuo porphyry copper deposit using SHRIMP zircon U-Pb dating and molybdenite Re-Os dating methods. In addition, the Cenozoic tectonic evolution and mineralization of the Qinghai-Tibet Plateau and the evolution of porphyry copper mineralization ages in the Gangdese metal-logenic belt are discussed.

1 Geology of the ore deposit

The local stratigraphic sequence is dominated by a suite of dacite, andesite, rhyolitic pyroclastics, sandstone and conglomerate of the Paleocene-Eocene Nianbo and Pana Formations of the Linzizong Group. The sedimentary sequence generally dips towards the north but is locally overturned by fault structures (Figure 1). The absolute age of the Nianbo Formation is 56.5 Ma while the age of the Bona Formation is 53.52―43.93 Ma[4,5] (plagioclase 40Ar/39Ar dating method). The fault structures exhibit two main orientations, i.e. NE and NW, of which the NE striking faults control the emplacement of porphyritic intrusions and copper orebodies. The intrusive rocks are mainly present in the northwest and south of the deposit. They mainly comprise the Baiwengpuqu and Nongshang intrusions and lithologically are characterized by por-phyritic hornblende adamellite and fine-medium-grained biotite granite porphyry. These intrusive rocks were formed during the Late Cretaceous and occur as batho-liths. They are part of the Andes-type granites in the

Gangdese belt that were produced by subduction of the Neotethys oceanic plate northwards under the Eurasia continental plate. Younger granitic porphyries, quartz porphyries and dioritic porphyries normally occur as small stocks and dykes intruding the granitic batholiths and the Linzizong Group volcanics to form a complex volcano-magmatic mineralization system.

There are three porphyries and three copper orebodies in Zhunuo, of which the No.1 porphyry is most impor-tant in terms of mineralization. This porphyry is granitic in composition and grey to light grey in colour. The phenocrysts are dominated by quartz and plagioclase with a minor amount of biotite. Quartz and plagioclase phenocrysts account for about 25% of the rock and the matrix comprises micro to felsitic quartz and feldspar. The major accessory minerals are apatite, magnetite, zircon, titanite and rutile. The orebodies are mainly hosted within the porphyries and the outer contact zone of the porphyritic hornblende adamellite and biotite granite. Ore minerals include malachite, azurite, cuprite, native copper, chalcopyrite, pyrite and molybdenite. The wallrock alteration is primarily potassic, phyllic, pro-pylitic, silicic, argillic and carbonate. From the center to the distal zone of porphyries, alteration types vary from potassic, phyllic, calcite and kaolinitic alteration to pro-pylitic and ferroan carbonate alteration. Mineralization is mainly related to phyllic alteration and silicification.

2 Samples and analytical method

Zircons used in this study were collected from different localities in the No.1 adit driven through the No.1 cop-per orebody within the No.1 porphyry. The host rock is granitic porphyry that has experienced pervasive hydro-thermal activity. As a result, the samples have varying degrees of alteration, particularly weak kaolination and sericitization but without veining overprint.

Selection of zircon grains was carried out at the met-allurgical laboratory in China University of Geosciences (Wuhan). Microphotographs of the selected zircon were taken under transmitted and reflected light modes at the Chinese Academy of Geological Sciences (CAGS), and Cathodoluminescene (CL) images were obtained at the Electron Microscope Laboratory of the College of Physical Sciences in Peking University. The SHRIMP U-Pb analyses of zircon were made with the SHRIMP II in Beijing Ion Microprobe Center, CAGS. For the dating procedure and details of the technique, please refer to

ZHENG YouYe et al. Chinese Science Bulletin | November 2007 | vol. 52 | no. 22 | 3139-3147 3141

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Figure 1 Simplified geologic map of the Zhuonuo porphyry copper deposit in Tibet. 1, Quaternary; 2, biotite granitic porphyry; 3, quartz porphyry and granitic porphyry; 4, dioritic porphyry; 5, rhyolitic porphyry; 6, porphyritic hornblende adamellite; 7, Linzizong Group volcanics; 8, copper orebody and number; 9, kaolinization; 10, phyllic alteration. refs. [6―8]. The Ludwig SQUID 1.0 and ISOPLOT softwares[9] were used for data processing. As the abun-dance of 204Pb in young zircon is very low, it is not cor-rect to use the actual value of 204Pb to calibrate common Pb to counter significant analytical errors. Therefore, we used the actual value of 208Pb to calibrate both the common Pb values and the 206Pb/238U ages[10,11]. In order to monitor the quality of analyses, 1 TEM standard was analyzed for every 3 measurements and a total of 8 standards were analyzed during this study.

Four molybdenite samples were collected at 153.5, 158.6, 165.8, and 195.8 m from the opening of the No.1 adit in the Zhunuo No. 1 copper orebody. The host rocks are pyrite, chalcopyrite and molybdenite mineralized biotite adamellite, within which molybdenite is present as molybdenite-quartz veins. The veins are about 0.5―4.5 cm wide and composed of loose quartz and euhedral tabular molybdenite. Molybdenite grains were hand-

picked under a binocular microscope to ensure they are free of oxidation and contamination with purity >98%. The molybdenite Re-Os ages were determined by Prof. Du Andao in the China National Geological Analytical and Testing Center. For the details of this analytical method, please refer to refs. [12, 13]. The model age is calculated using the following formula:

187

1871 Osln 1

Ret

λ⎡ ⎤⎛ ⎞

= +⎢ ⎥⎜ ⎟⎢ ⎥⎝ ⎠⎣ ⎦

,

λ(187Re decay constant) = 1.666×10−11 a−1.

3 Analytical results

3.1 Zircon in ore-bearing porphyry

Most zircons in ore-bearing porphyries (granitic por-phyry) are colourless or light yellow, euhedral to semi- euhedral crystals with a length/width ratio of 1.5―2 and


grain size of 100―200 μm. The clear oscillation zoning of most zircons in CL

images indicates a magmatic origin (Figure 2)[14―16]. Some zircon grains are very complex in texture, dis-playing inherited grains overgrown by new ones. Taking No.8 zircon as an example, the ages of core, mantle and rim are 60.3, 23.5 and 13.4 Ma, respectively. No.7 zir-con with patchy texture also yielded three ages: 48.1 Ma and 22.3 Ma for the core and 12.6 Ma for the rim. The different ages in a single zircon grain record several tec-tono-magmatic events in the area, and provide informa-tion about the tectonic evolution of the Gangdese orogeny. There are some zircons showing oscillation zoning, with cores enveloped by narrow irregular non-zoning or weak zoning rims. Rarely, zircon grains demonstrate sector zoning feature. The textures of inher-ited zircons from Zhunuo are partly correlated with those from other porphyries in the Gangdese porphyry copper metallogenic belt[17].

The SHRIMP zircon U-Pb ages obtained are listed in Table 1. Figure 3 displays the zircon U-Pb concordia curve, by which the ages can be subdivided into two groups, reflecting at least 4 stages of tectono-magmatic events. The detailed description of these events is as follows:

Nine analyses are from the inherited or relict zircons. Their Th and U contents are 169―1266 μg·g−1 and 272―2229 μg·g−1, respectively, and ratios of Th/U are 0.3951― 1.3976. The 206Pb/238U ages of those analyses

span continuously from 68.5±2.4 to 48.1±0.9 Ma. Such a wide continuous distribution in age may reflect a pro-tracted magmatic event. Considering the textural differ-ence and frequency of age values, two main tectono- magmatic events can be identified: the first happened at 62.5±2.5 Ma (MSWD = 2.1, n = 5) and is characterized by the inherited zircon with irregular angular shape, patchy texture, varying colour and similar ages; the second happened at 50.1±3.6 Ma (MSWD = 4.7, n = 4) and is characterized by the inherited zircon with rounded shape, indicating post-crystallization corrosion by mag-matic melt.

The Th and U contents of five zircons with clear zon-ing are 333―1479 μg·g−1 and 490―1095 μg·g−1, respectively, and the ratios of Th/U are between 0.5168 and 1.7745. Those magmatic zircons have higher Th/U values than those of the inherited ones, but their chang-ing trend is not as obvious as the latter. The 206Pb/238U ages of magmatic zircon are between 16.6±0.8 Ma and 14.4±0.6 Ma with a weighted average of 15.6±0.6 Ma (MSWD = 1.8, n = 6), recording the third tectono- magmatic event.

The young zircons outside the residual magmatic core (grain Nos. 4 and 8) or the inherited core (grain Nos.7, 13 and 14) have ages from 68.5―48.1 Ma for the core and 13.4±1.0―12.4±0.1 Ma for the rim. Apart from the Th/U ratio of 0.1910 and maximum U content of 5301×10−6 obtained from the probing point 7.2, all the other probing points yielded Th and U contents of 332―

Figure 2 Cathodoluminescene images of zircon from the Zhunuo deposit in Tibet.


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Figure 3 U-Pb concordia diagram for zircons from the Zhunuo ore-bearing porphyries in Tibet.

Table 1 SHRIMP U-Th-Pb analytical data of zircon from the Zhunuo mineralized porphyriesa)

Sample No. 206Pbc (%) U (μg/g) Th (μg/g) 232Th/238U 206Pb* (μg/g)206Pb/238U age (Ma)

206Pb*/238U Error correlation

ZL01-2.1 0.00 1776 789 0.4591 16.30 68.5±2.4 0.0107 0.81 ZL01-2.2 0.00 1250 614 0.5078 10.70 63.5±2.2 0.0099 0.77 ZL01-2.3 0.00 359 190 0.5461 3.10 63.5±2.4 0.0099 0.53 ZL01-2.4 2.13 442 169 0.3951 3.75 61.9±0.8 0.0097 0.25 ZL01-8.1 0.00 2229 1266 0.5870 17.70 60.3±2.1 0.0094 0.90 ZL01-4.1 0.00 2058 981 0.4927 15.40 55.5±1.9 0.0087 0.83 ZL01-7.3 1.74 272 233 0.8867 1.78 48.1±0.9 0.0075 0.19 ZL01-13.2 1.62 332 365 1.1371 2.30 51.0±0.9 0.0079 0.24 ZL01-14.1 1.47 286 387 1.3976 1.94 50.0±1.1 0.0078 0.23 ZL01-1.1 0.00 827 844 1.0542 1.88 16.6±0.8 0.0026 0.44 ZL01-1.2 0.00 581 444 0.7905 1.24 15.2±0.7 0.0024 0.20 ZL01-3.1 0.00 1095 550 0.5186 2.17 14.4±0.6 0.0022 0.33 ZL01-5.1 0.00 861 1479 1.7745 1.83 15.0±0.9 0.0023 0.27 ZL01-15.1 1.36 775 471 0.6285 1.67 15.9±0.3 0.0025 0.23 ZL01-17.2 1.95 490 333 0.7029 1.02 15.3±0.4 0.0024 0.26 ZL01-4.2 0.00 754 383 0.5248 1.33 12.8±0.6 0.0020 0.38 ZL01-7.2 0.94 5301 980 0.1910 8.97 12.6±0.1 0.0020 0.31 ZL01-8.3 2.03 2110 1019 0.4991 3.85 13.4±0.2 0.0021 0.23 ZL01-11.1 1.51 2517 773 0.3175 4.51 13.4±1.0 0.0021 0.47 ZL01-12.1 2.19 1865 1621 0.8980 3.24 12.7±0.3 0.0020 0.36 ZL01-13.1 1.61 2430 1013 0.4308 4.08 12.4±0.1 0.0019 0.24 ZL01-16.2 1.35 1735 584 0.3481 3.09 13.2±0.2 0.0020 0.24 ZL01-18.1 1.12 878 332 0.3912 1.61 13.6±0.2 0.0021 0.25 ZL01-17.1 0.50 733 853 1.2022 3.31 33.6±0.5 0.0052 0.29 ZL01-8.2 0.00 802 588 0.7566 2.40 23.5±1.0 0.0037 0.37 ZL01-7.1 0.00 288 262 0.9419 0.86 22.3±7.1 0.0035 0.882 ZL01-14.2 6.50 553 83 0.1548 0.77 9.7±0.4 0.0015 0.06 a) Pbc=common Pb; Pb* = radiogenic Pb; standard errors are given at 2σ; correction for common Pb is based on measured 208Pb.


1621 μg·g−1 and 754－2517 μg·g−1, respectively. Their corresponding Th/U ratios vary within the range of 0.3175 to 0.8980. The 206Pb/238U ages of zircon rim are 13.6±0.2―12.4±0.1 Ma with a weighted average of 13.3±0.2 Ma (MSWD = 1.3, n = 6), reflecting the fourth tectono-magmatic event. Probing point 11.1 is located within the area of patchy texture, which is influenced by hydrothermal activity. Its CL image is deeper in color and irregular in texture without pronounced zoning, all of which are attributed to the U, Th, and HREE con-tents[18―21]. Probing points 4.2, 7.2, 8.3 and 13.1 are lo-cated on the margin of magmatic zircons. Their CL im-ages are inhomogeneous in color and free of zoning, implying that they were either modified by hydrothermal fluids or precipitated directly from the hydrothermal solutions. The 206Pb/238U apparent ages of these patch-textured and poorly zoned zircons are concen-trated and consistent. Their values of 207Pb/235U- 206Pb/238U ratios are close to those on the concordia curve. This demonstrates that the hydrothermal activity happened during the same geological event. Otherwise, the 206Pb/238U apparent ages will be affected by the sig-nificant loss of Pb after crystallization or by the obvious influence of inherited radiogenic Pb[22, 23].

Within the suite of the samples, there is also an age of 33.6 Ma for the core and an age of 15.3 Ma for the rim of a zircon grain. This age is close to the Concordia curve and may represent the regional magmatic activity that took place around 33 Ma. In addition, 23.5 Ma and 22.3 Ma ages may indicate another magmatic event. Furthermore, the 9.7 Ma age is the youngest and falls on the Concordia curve. The CL image shows that this zir-con is irregular in shape and dark in color, and crosscuts the inherited zircon core. This young age could refer to a post-mineralization geologic event. 3.2 Re-Os ages of molybdenite The Re-Os analytical data of molybdenite are listed in Table 2 and the Re-Os isotopic isochrones are shown in Figure 4. Four analyses of molybdenite gave Re concen-trations from 227148±1728 ng·g−1 to 312113±2471

ng·g−1. The concentrations of 187Re are correlated well with those of 187Os. Four model ages of molybdenite are from 13.99±0.17 Ma to 13.82±0.16 Ma with variations less than 0.2 Ma. The weighted average age of the four molybdenite analyses is 13.92±0.08 Ma, with MSWD equal to 0.79 (Figure 4(a)). The isochron age obtained is 13.72±0.62 Ma, with MSWD equal to 1.14 (Figure 4(b)) and initial 187Os equal to 0.6±1.8 ng/g. The variations between isochron age and single model age are less than 0.5 Ma.

4 Discussion and conclusions

Numerous evidence implies that[24―32] initiation of the India-Asia collision occurred no later than 65 Ma and the completion of collision is between 45 Ma and 40 Ma. Both magma underplating and magma mixing took place approximately at 50 Ma during collision. Mo et al.[4] considered that the age of the bottom unit from the Lin-zizong Group volcanic rocks widely distributed in Gangdese represents the start of the India-Asia conti-nental collision, i.e. ca. 65 Ma. The SHRIMP zircon age of 62.5±2.5 Ma obtained from this study is very similar to the timing of initiation of major India-Asia collision. This might be related also to the origin of the Linzizong Group volcanics. However, further geochemical re-search is required to justify such a conclusion. Zircons with 50.1±3.6 Ma ages obtained from the study area are rounded in shape, and their age is consistent with an event where large-scale matle-derived mafic magma was underplated during the Gangdese continental collision between 52.5 and 47.0 Ma (about the Eocene)[5,32]. There are some other similar inherited or rudimental zircons in the porphyries of the Gangdese porphyry copper belt[17]. Based on their widespread occurrence in regional porphyritic rocks and textural characteristics, this group of zircons most likely represents melting products in the source region. If this is the case, it is likely to prove that partial melting of the lower crust thickened by large scale magmatic underplating formed the ore-bearing magma[33].

Table 2 Re-Os isotopic data of molybdenite from the Zhunuo deposit a)

Sample No. Weight (g) Re (μg/g) 187Re (μg/g) 187Os (μg/g) Model age (Ma) ZLY01 0.00225 227148±1728 142776±1086 33.27±0.30 13.99±0.17 ZLY02 0.0022 294950±2214 185394±1392 43.03±0.36 13.93±0.17 ZLY03 0.0024 312113±2471 196182±1553 45.68±0.41 13.98±0.18 ZLY04 0.00521 292988±2153 184160±1353 42.42±0.35 13.82±0.16

a) Analyses were made by Du Andao at the National Geological Analytical and Testing Center. Analytical errors in the table are 2σ.


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Figure 4 The Re-Os ages of molybdenite from the Zhunuo deposit. (a) Weighted average diagram of Re-Os isotopic model age of molybdenite; (b) Re-Os isochron diagram of molybdenite.

Previous research[34―36] indicated that zircon SHRIMP U-Pb isotopic ages from the ore-bearing porphyries in the Qulong, Chongjiang and Tinggong deposits in the middle and eastern parts of the Gangdese porphyry cop-per belt are 15.99, 14.85 and 15.49 Ma, respectively. The molybdenite Re-Os isotopic ages from these three deposits are 15.99, 14.85 and 15.49 Ma. The zircon SHRIMP U-Pb isotopic age of 15.6 Ma represents the age of the ore-bearing porphyry in the Zhunuo deposit. This age is not only very close to the average age of 16.14 Ma of the ore-bearing porphyries in the middle of the Gangdese porphyry copper belt (such as the Ting-gong and Chongjiang deposits), but also close to the average age of 17.8 Ma of the ore-bearing porphyries in the eastern part of the Gangdese porphyry copper belt (such as the Qulong and Demingding deposits). There is also a decrease in the ages of ore-bearing porphyries from east to west. The Re-Os isotopic dating of molyb-denite from the Zhunuo deposit yielded an isochron age of 13.72±0.62 Ma, representing the actual ore-forming age. The zircon SHRIMP U-Pb isotopic age of 13.3±0.2 Ma is slightly younger than the molybdenite Re-Os iso-topic age, which may reflect the impact of hydrothermal

alteration on zircon. This is also consistent with the av-erage ore-forming age of 14.56 Ma for the deposits in the middle part (e.g. Tinggong and Chongjiang) and the average ore-forming age of 15.68 Ma for the deposits in the eastern part of the Gangdese porphyry copper belt. The ore-forming ages of these deposits also become in-creasingly younger from east to west, consistent with the decreasing closing time of the Yarlung Zangbo suture zone from east to west.

The zircon ages of 23.5 Ma and 22.3 Ma in the Zhunuo deposit were also recognized in the Chongjiang and Nanmu deposits[17,37]. This indicates that the geo-logic event taking place around 22 Ma is of regional significance. It is also close to the second temporal peak of magmatic intrusions and the time of thrusting in the Gangdese belt[25,27,28]. Qu et al.[17] suggest that some 21 Ma ago asthenospheric upwelling resulted in partial melting of the underplated mafic rocks leading to the formation of the source of ore-bearing porphyries and rapid uplifting of the southern Tibet Plateau. The impli-cation of zircon ages of 23.5 Ma and 22.3 Ma is worth further investigations. It should be noticed that the 13th zircon CL image with a 13 Ma age for the rim is directly attached to the age of 50―62 Ma in the core. Compared with the 8th zircon, the age of white middle area of the 13th zircon is likely to be ca. 22 Ma. The core of the 7th zircon gave 48 Ma coexisting with 22.3 Ma. Therefore, the zircon age of 13 Ma does not seem to be directly attached to the zircon age of 50―62 Ma. Most zircons may have three stages of growth history: 48―60 Ma, 22 Ma and 13 Ma though other interpretations cannot be excluded.

As mentioned above, the zircon SHRIMP U-Pb iso-topic ages from ore-bearing porphyries in the Zhunuo deposit record more than four major tectono-magmatic events. The ages of magmatic intrusions and mineraliza-tion in Zhunuo are generally consistent with those for the porphyries and related copper mineralization in the Gangdese orogenic belt. They all formed during the transitional period from intracontinental post-collision to strike slip extension orogenesis. The emplacement of porphyries is restricted to a short period of time (the longevity of the porphyritic crystallization and copper mineralization in the region is about 2－3 Ma), which is in sharp contrast to the island arc and active continental margin environments typical of the eastern Pacific por-phyry copper belt (the longevity of porphyritic crystalli-


zation and mineralization could be up to 25 Ma). The Zhunuo deposit was formed in the same tectonic setting as other deposits located in the eastern and middle Gangdese belt. These deposits are the products of the same stage of tectonic evolution, and form an interre-lated geologic continuum. On the other hand, it also in-dicates that the western Gangdese belt has great poten-tial for exploration of porphyry copper deposits, which provides critical information for strategic planning of

mineral exploration further west in the Gangdese metal-logenic belt.

The authors appreciate the constructive comments made by the reviewers of this paper. Profs. Song Biao and Yan Quanren are thanked for their instruction in SHRIMP dating. Many thanks are due to Prof. Wu Yuanbao for interpretation of the data, to Prof. Xu Guojian for translation of this paper. Prof. Sue Golding from Department of Earth Sciences in the Uni-versity of Queensland, Australia are specially thanked for her critical reading of this paper and improvement in English.

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