Control on the size of porphyry copper reserves in the North Balkhash–West Junggar Metallogenic Belt

Accepted Manuscript Control on the size of porphyry copper reserves in the North Balkhash–West Junggar Metallogenic Belt

Changhao Li, Ping Shen, Hongdi Pan, Eleonora Seitmuratova PII: DOI: Reference:

S0024-4937(19)30050-7 https://doi.org/10.1016/j.lithos.2019.01.030 LITHOS 4960

To appear in:

LITHOS

Received date: Accepted date:

15 June 2018 26 January 2019

Please cite this article as: C. Li, P. Shen, H. Pan, et al., Control on the size of porphyry copper reserves in the North Balkhash–West Junggar Metallogenic Belt, LITHOS, https://doi.org/10.1016/j.lithos.2019.01.030

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ACCEPTED MANUSCRIPT

Control on the size of porphyry copper reserves in the North Balkhash–West Junggar Metallogenic Belt Changhao Li a,b,c, Ping Shen a,b,c*, Hongdi Pan d, Eleonora Seitmuratova e Key Laboratory of Mineral Resources，Institute of Geology and Geophysics, Chinese

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a

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Academy of Sciences, Beijing 100029, China;

Institutions of Earth Science, Chinese Academy of Sciences, Beijing 100029, China;

c

University of Chinese Academy of Sciences, Beijing 100049, China;

d

College of Earth Sciences, Chang’an University, Xi’an 710054, China;

e

Laboratory of Geological Formations, K. Satpaev Institute of Geological Sciences,

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b

Name: Ping Shen

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Corresponding author:

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Almaty 050010, Kazakhstan

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E-mail: [email protected]

Abstract

The North Balkhash–West Junggar Metallogenic Belt is one of the most important areas within the Central Asian Metallogenic Domain. The belt comprises the North Balkhash and West Junggar regions, which have significantly different Cu reserves. We present mineral geochemistry, whole-rock major and trace element chemistry, zircon Hf and O isotope compositions, and zircon trace element chemistry, on the

ACCEPTED MANUSCRIPT basis of which we conclude that porphyry copper deposits within the belt formed by different processes. We interpret ore-related magmas in the West Junggar region to have formed in an immature arc setting by the partial melting of lower crust (30–40 km), with some mantle input and sediments contamination. In contrast, ore-related

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magmas within the North Balkhash region are interpreted to have formed in a mature

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arc setting by the partial melting of lower crust (~40 km), with mantle input and

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possible upper-crustal contamination. The oxygen fugacities of ore-related intrusions within the North Balkhash region are typically higher than those within the West

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Junggar region. A high magmatic water content is considered to be important, but not

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sufficient, for the formation of large porphyry copper deposits. Thicker crust and high oxygen fugacity are the two main reasons for the higher Cu reserves in the North

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Balkhash region compared with the West Junggar region.

Keywords

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Cu reserves difference, crustal thickness, oxygen fugacity, North Balkhash–West

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Junggar Metallogenic Belt

1. Introduction Porphyry copper deposits (PCDs) are important because they account for >80% of global copper resources. However, the formation mechanisms of and controls on PCDs remain unclear. Richards (2003, 2011) suggested that the size of PCDs is

ACCEPTED MANUSCRIPT controlled by magmatic water, oxygen fugacity, and the volume of magma within the shallow magma chambers. Wilkinson (2013) identified four triggers for PCD formation and proposed that sulfide saturation of the magma is the most important factor. Sun et al. (2013) suggested that high oxygen fugacity (fO2) and sulfate

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reduction (initially via magnetite crystallization) are also essential. Shen et al. (2015a)

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concluded that Ce4+/Ce3+=120 of zircon can be used to discriminate large and

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intermediate PCDs from smaller deposits.

The Central Asian Metallogenic Domain comprises significant Cu, Mo, W, and

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Fe reserves, and it is one of the most well known metallogenic reserves in the world

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(Seltmann et al., 2014). The North Balkhash–West Junggar Metallogenic Belt contains many PCDs (e.g., Aktogai and Baogutu) and is located within the North

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Balkhash and West Junggar regions in Kazakhstan, and Xinjiang, China, respectively.

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Several studies have investigated the differences between the metal reserves in these two regions (e.g., Shen et al., 2013, 2015). The North Balkhash region contains

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several large to giant PCDs, such as the Aktogai and Kounrad deposits. Reserves in

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the West Junggar region have yielded less Cu (>0.7 Mt) than those in the North Balkhash region, but the reason for this difference remains unclear. We present mineral geochemistry, whole-rock major and trace element results, zircon Hf and O isotope compositions, and zircon trace element data for the Shiwu deposit in the Xinjiang, China. We also present mineral geochemical data for the Kounrad deposit and mineral geochemistry and zircon trace element data for the barren Koldar Complex in the North Balkhash region. The results of this study are

ACCEPTED MANUSCRIPT combined with those of previous work (Cao et al., 2014, 2016a, 2016b; Li GM et al., 2016; Li CH et al., 2017; Shen et al., 2013, 2017, 2018) to investigate the differences between the North Balkhash and West Junggar regions, including their magma sources, petrogenesis, and tectonic settings. We present several reasons to explain the

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observed difference in the size of Cu reserves from deposits within these two regions.

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2. Regional geology

The Central Asian Orogenic Belt records an 800 Myr geological history and is

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one of the largest accretionary orogenic belts in the world (Fig. 1a; Sengör et al., 1993;

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Kröner et al., 2007). The belt extends from the Ural Mountains in Russia to the Pacific Ocean (west–east) and from Siberia to northern China and Tarim (north–

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south).

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The North Balkhash region is located in central and eastern Kazakhstan and is an important ore reserve within the North Balkhash–West Junggar Metallogenic Belt

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(Fig. 1b; Chen et al., 2014；Li et al., 2017; Shen et al., 2013, 2015, 2017). The strata

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within the North Balkhash region comprise Precambrian metamorphic basement rocks, deformed Paleozoic sedimentary and metamorphic rocks, and Mesozoic–Cenozoic sandstones and mudstones (Heinhorst et al., 2000). The Precambrian metamorphic basement rocks are exposed in the western and northern parts of central Kazakhstan and comprise amphibolites, gneisses, and schists. The Paleozoic rocks consist of Cambrian–Ordovician felsic volcanics, carbonates, and terrigenous clastic rocks, Silurian–Devonian

terrigenous

clastic

and

shallow-marine

clastic

rocks,

ACCEPTED MANUSCRIPT Carboniferous volcanic rocks, and Permian basalts–rhyolites and terrestrial volcanic tuffs. Relic early Paleozoic oceanic crust is preserved in central Kazakhstan. These ophiolitic rocks are not interpreted to represent suture zones (Sengör et al., 1993) as they exhibit features consistent with back-arc oceanic crust (Kröner et al., 2007). The

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intrusions in this region were emplaced between the Proterozoic and Permian, but

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PCDs have closely relationship with the Carboniferous intrusions (Heinhorst et al.,

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2000).

The West Junggar region is located in the Xinjiang Province and can be

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subdivided into northern and southern parts (Fig. 1c). This study focuses on the

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southern West Junggar region, in which Paleozoic volcanic rocks are widespread. The early Paleozoic strata occur primarily within the Mayile area and crop out along the

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Mayile Fault. The Devonian and Carboniferous strata crop out to the west and east of

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the Mayile Fault, respectively (Fig. 1c). Several NE-trending faults, including (from east to west) the Darbut, Mayile, and Barluk faults, as well as ophiolitic rocks, also

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occur in the southern West Junggar region. The granitoids comprise a large batholith

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(e.g., the Miaoergou Pluton) that was emplaced during the Variscan (ca. 300 Ma) and small-scale, randomly distributed intermediate–acidic intrusions. The PCDs in this region (e.g., the Baogutu and Shiwu deposits; Fig. 1c) are associated with the latter granitoid type.

ACCEPTED MANUSCRIPT 3. Ore geology 3.1 Deposits in the southern West Junggar region 3.1.1 The Shiwu Cu deposit

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The Shiwu porphyry Cu deposit was discovered by the No. 1 Regional

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Geological Survey Party of the Xinjiang Bureau of Geology and Mineral Exploration

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and Development. The deposit is located to the southeast of the Barluk Fault (Fig. 1c)

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and is still under exploration. The strata in the Shiwu region comprise Devonian– Carboniferous volcano-sedimentary rocks, including the Middle Devonian Barluk and

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Lower Carboniferous Baogutu groups (Fig. 2a). The Barluk Group comprises tuffs, tuffaceous siltstones, crystal-lithic tuffs, feldspar lithic sandstones, andesites, and

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andesitic breccias. The Baogutu Group comprises tuffaceous siltstones, feldspar lithic sandstones, tuffs, and limestone lenses. Abundant NE-trending faults are observed in the Shiwu region (Fig. 2a). Diorite and quartz-diorite intrusions within this region

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were emplaced at ca. 322 Ma (Hu et al., 2018) into the Middle Devonian Barluk

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Group (Fig. 2a). Disseminated and vein-hosted mineralization within the Shiwu deposit is associated primarily with quartz-diorite and tonalite porphyries that were emplaced at ca. 310 Ma (Li et al., 2017). The quartz-diorite porphyries (SA figure 1a, c) are closely associated with hydrothermal alteration and mineralization, whereas the tonalite porphyries (SA figure 1b, d) are associated with propylitic alteration and local disseminated mineralization within outcrop. Regardless, we consider the tonalite porphyries as ore-related intrusions because of their association with disseminated

ACCEPTED MANUSCRIPT mineralization, as revealed by previous limited ore exploration projects. The hydrothermal alteration includes potassic, sericitic, and propylitic alteration. Potassic alteration occurs only within the lower sections of the drill core and is characterized by K-feldspar, quartz, and biotite. Sericite alteration is ubiquitous and is characterized

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by quartz, sericite, and chlorite. Propylitic alteration is characterized by quartz,

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chlorite, epidote, and actinolite and is observed within outcrop and within the drill

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core. The hydrothermal fluids correspond to the H2O–NaCl–CO2(–C2H6–CH4) system, and their homogenization temperatures and salinities are 130°C–458°C and 0.2–54.5

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3.1.2 The Jiamantieliek Cu deposit

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wt.% NaCleq, respectively (Li et al., 2017).

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The Jiamantieliek porphyry Cu deposit was first discovered by the Bureau of

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Geology and Mineral Resources of the Xinjiang Uygur Autonomous Region and is located to the northwest of the Barluk Fault (Fig. 1c). The deposit is hosted within

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Middle Devonian volcano-sedimentary sequences of the Tielieketi and Barluk groups

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(Fig. 2b). The Tielieketi Group comprises conglomerates, calcareous sandstones, tuffs, and tuffaceous siltstones. The Barluk Group comprises sandstones, glutenite, tuffaceous sandstones, coal beds, calcareous tuffs, and argillaceous siltstones. NE- and NW-trending faults occur within this area (Fig. 2b). The Jiamantieliek intrusion comprises diorite and quartz-diorite and was emplaced at ca. 313 Ma (Shen et al., 2013) within the Middle Devonian Barluk Group (Fig. 2b). Several diorite and quartz-diorite porphyries were emplaced at ca. 310 Ma, cross-cutting the main

ACCEPTED MANUSCRIPT intrusive bodies and volcano-sedimentary sequences (Shen et al., 2013). Mineralization was associated with emplacement of these diorites and quartz-diorites. Hydrothermal alteration includes propylitic, silicatic, and calcitic alteration (Shen et

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al., 2013).

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3.1.3 The Baogutu Cu deposit

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The Baogutu porphyry Cu deposit is located to the southwest of the Darbut Fault (Fig. 1c) and was the first PCD to be discovered in the West Junggar region. This

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deposit contains 0.63 Mt Cu, 0.018 Mt Mo, and 14 t Au (Shen et al., 2010). The

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deposit is hosted within Lower Carboniferous volcano-sedimentary sequences of the Xibeikulasi and Baogutu groups (Fig. 2c). The Xibeikulasi Group comprises

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greywackes that exhibit graded bedding, tuffaceous mudstones, and tuffaceous

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siltstones showing soft-sediment deformation textures. The Baogutu Group includes tuffaceous siltstones, tuffs, feldspar lithic sandstones, and limestone lenses.

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NE-trending faults are observed in this region (Fig. 2c). The Baogutu Complex

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contains diorite, quartz-diorite, and late diorite porphyries and was emplaced at ca. 313 Ma (Shen et al., 2013) within the Lower Carboniferous Baogutu Group (Fig. 2c). Mineralization is associated with the diorites and late diorite porphyries (Shen et al., 2010). Hydrothermal alteration includes potassic, propylitic, and sericitic alteration. Potassic alteration was responsible for disseminated Cu mineralization and is characterized by biotite, magnetite, K-feldspar, and quartz. The propylitic alteration is characterized by quartz, chlorite, epidote, and calcite and occurs predominantly within

ACCEPTED MANUSCRIPT the wall-rocks. The sericite alteration is characterized by quartz and sericite, overprints zones of potassic and propylitic alteration, and is associated with the highest ore grades. Various sulfide minerals occur, including pyrrhotite, pyrite, chalcopyrite, and molybdenite (Fig. 2c). The hydrothermal fluids correspond to the

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H2O–NaCl–CH4(–CO2) system and have homogenization temperatures and salinities

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of 151°C–530°C, and 0.2–63.9 wt.% NaCleq, respectively. The Baogutu deposit

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contains large amounts of pyrrhotite and CH4 gas and is regarded as a reduced PCD

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(Shen and Pan, 2013, 2015; Cao et al., 2014, 2016b).

3.2.1 The Kounrad Cu deposit

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3.2 Deposits in the North Balkhash region

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The Kounrad Cu deposit is located 15 km north of Balkhash township, was exploited in 1934, and contains >5 Mt Cu and >600 t Au (Seltmann et al., 2014). The deposit is hosted within Upper Devonian sandstones, shales, and minor interbedded

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felsic tuffs, and Early Carboniferous andesitic basalts, andesites, and dacites.

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NW-trending faults are observed in this region (Fig. 3a–1). The main intrusions comprise the Toktay Complex, which was emplaced prior to mineralization, ore-related granodiorite porphyries (SA figure 1i, j), and dikes that were emplaced post-mineralization

(Fig.

3a–1).

The

Toktay

Complex

comprises

diorites,

granodiorites, and porphyritic granodiorites intruded into volcano-sedimentary rocks, with the diorites and granodiorites being emplaced between 361 to 339 Ma (Li et al., 2016). The ore-related granodiorite porphyry being emplaced between 331 to 325 Ma

ACCEPTED MANUSCRIPT (Chen et al., 2014; Li et al., 2016; Shen et al., 2017). Hydrothermal alteration and mineralization comprise three stages (Zvezdov et al., 1993; Seltmann et al., 2014). The earliest hydrothermal alteration phase involved quartz–sericite and quartz– sericite–diaspore alteration, occurred during the final stages of volcanism, and was

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not associated with mineralization. The second hydrothermal alteration phase was

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caused by emplacement of the granodiorite porphyry and involved silica, sericite,

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propylitic, and late argillic alteration (Figs 2 and 3a). Compared with typical PCDs, the Kounrad deposit is associated with a higher amount of silicic alteration than

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potassic alteration. The final hydrothermal alteration phase was associated with

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emplacement of the late intrusions and dikes. This phase of alteration was characterized by mica–quartz–tourmaline alteration in the porphyritic granodiorites

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and felsic dikes and by the formation of hydrothermal albite, K-feldspar, and biotite

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within the dolerite dikes. The sulfide minerals comprise pyrite, chalcopyrite, bornite, molybdenite, enargite, and chalcocite (Fig. 3a–3). The hydrothermal fluids correspond

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to the H2O–NaCl–CO2 system and have homogenization temperatures and salinities

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of 151°C–490°C and 0.4–58.4 wt.% NaCleq, respectively (unpublished data).

3.2.2 The Aktogai Cu deposit

The Aktogai deposit is located 22 km from the Aktogai Railway Station, within the Balkhash–Ili Terrane, and contains >5.8 Mt Cu and 68 t Au (Seltmann et al., 2014). The deposit is hosted within the Upper Carboniferous–Lower Permian Koldarskaya and Middle–Upper Carboniferous Keregetasskaya groups (Fig. 3b–2). The

ACCEPTED MANUSCRIPT Koldarskaya Group comprises sedimentary rocks, volcano-sedimentary sequences, and minor felsic tuffs. The Keregetasskaya Group contains andesites and rare siltstones and rhyolites. NE- and NW-trending faults are observed in this area. The main intrusions are the Koldar Pluton (SA figure 1f, h), granodiorite porphyries, and

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dikes, which were emplaced before, during, and after mineralization, respectively (Fig.

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3b–1). The Koldar Pluton is compositionally variable, comprises primarily

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quartz-diorite and diorite (Zvezdov et al., 1993), and was emplaced at ca. 344.7 Ma (Cao et al., 2016). Ore-related tonalite porphyry (SA figure 1e, g) intruded the Koldar

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Pluton between 331 to 327 Ma (Chen et al., 2014; Cao et al., 2016; Shen et al., 2018),

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and several dikes and intrusive bodies were emplaced after mineralization. The hydrothermal alteration includes potassic, propylitic, and sericite–chlorite alteration

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(Fig. 3b–3; Li et al., 2018a). The potassic alteration is characterized by hydrothermal

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K-feldspar, quartz, biotite, and magnetite and is associated with disseminated and vein-hosted mineralization. The sericite–chlorite alteration forms high-grade ore zone,

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overprints the zones of potassic alteration, is characterized by sericite, quartz, and

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chlorite, and is associated with disseminated and vein-hosted mineralization. The propylitic alteration is characterized by chlorite and epidote, and is associated with disseminated and vein-hosted mineralization, but to a lesser extent than the other two alteration stages. The sulfide minerals include pyrite, chalcopyrite, bornite, and molybdenite (Fig. 3b–3). The hydrothermal fluids correspond to the H2O–NaCl–CO2 system and have homogenization temperatures and salinities of 162°C–466°C and 0.4–51.1 wt.% NaCleq, respectively (Li et al., 2018b).

ACCEPTED MANUSCRIPT 4. Samples and methods We analyzed samples of the Shiwu Cu deposit, Kounrad Cu deposit and barren Koldar Complex in this study. Samples selected from outcrop and drill holes. Five

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samples of the Shiwu deposit, three samples of the Kounrad deposit, and two samples of the barren Koldar Complex are used to electron microprobe analysis (EMPA),

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respectively. Two samples of the Shiwu deposit are used to whole rock major and

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trace elements and zircon Hf-O isotopes analysis, respectively. Two samples of the

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Shiwu deposit and one sample of the barren Koldar Complex are used to zircon trace elements analysis. Considering some data have large variation, we use geometric

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mean (geomean) in this study to decrease the effect of extreme values. All samples are examined in the Institute of Geology and Geophysics, Chinese Academy of

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Sciences (IGGCAS).

Major and trace element components of mineral are examined using a JXA 8100 electron microprobe with a voltage of 15kV, a beam current of 20nA, a spot size

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of 5μm and 20s peak counting time. The synthetic oxides and natural minerals are

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used calibration: apatite (P), diopside (Ca and Si), rutile (Ti), jadeite (Na and Al), garnet (Fe), bustamite (Mn), pyrope (Mg), K-feldspar (K), fluorite (F), tugtupite (Cl) and barite (S). The spectral line adopted of each element is P (66.764), Si (77.371), Ti (88.751), Al (90.551), Fe (134.625), Mn (146.186), Mg (107.399), Ca (107.907), Na (129.393), K (120.157), F (84.249), Cl (151.416) and S (171.68), respectively. Major elements of igneous rocks are analyzed using Shimadzu XRF-1700/1500 X-ray fluorescence spectrometer after fusion with lithium tetraborate. Duplicate

ACCEPTED MANUSCRIPT analysis of Chinese national reference GBW07101-07114 reveals the precision is 1% of quoted values for elements greater than 5 wt%, and 10% of quoted values for elements less than 5 wt% in the reported values. Loss on ignition (LOI) is measured as weight loss of samples after 1h baking at temperature of 1000℃. Trace element

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abundance is analyzed using a Thermo-Finnigan ELEMENT inductively coupled

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plasma mass spectrometry (ICP-MS) after HNO3 + HF digestion of ~40 mg of sample

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powder in a Teflon vessel at 150℃. Analytical precision and accuracy are monitored by Chinese national reference GSR1 (granite), GSR2 (rhyolite) and GSR3 (basalt).

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The precision is 5% of the quoted values for elements present at ＞1ppm, and about

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10% for elements present less than 1ppm. Accuracy is estimate to be superior to 5% in the reported values.

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Oxygen isotope components are analyzed on zircon grains used for SIMS U-Pb

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dating. Samples are re-polished to ensure any oxygen implanted in the zircon surface from the O2- beam used for U-Pb analysis was removed. Zircon oxygen isotopes are

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examined using the same Cameca IMS-1280 SIMS. Analytical procedures are similar

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with the description provided by Li et al (2010) and Tang et al. (2015). The Cs+ primary ion beam is accelerated at 10 kV to sputter zircon for O-isotope analysis. Primary beam size is ~10μm in diameter, and ~2nA in intensity. Oxygen isotopes are measured in multi-collector static mode using two off-axis Faraday cups. Normal electron gun is used to compensate charging effect in the bombarding area. The NMR (Nuclear Magnetic Resonance) probe is used for magnetic field control. One analysis takes ~3 min composing of pre-sputtering (20 s), automatic beam centering

ACCEPTED MANUSCRIPT (~90 s) and integration of oxygen isotopes (16 cycles × 4 s, total 64 s). The instrumental mass fractionation factor (IMF) was corrected using Penglai zircon standard with δ18O = 5.25±0.10‰ (2σ; Li et al., 2010). Qinghu zircon is a second zircon standard analyzed as an unknown to ascertain the veracity of the IMF.

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Repeated analyses of them during the course of analysis got mean δ18O value of

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5.49±0.24‰ (2σ; n=4), which is consistent with recommended value for Qinghu

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zircon (5.39±0.10‰ (2σ)) (Li et al., 2013).

In situ zircon Lu-Hf isotopic analyses are examined using a Neptune

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multi-collector ICP-MS interfaced with a Geolas-193 laser-ablation system. Lu-Hf

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isotopic analyses are measured on same zircon grains. Analyses parameters are 44μm of spot size, 26s of ablation time, 8Hz of repetition rate and 10J/cm2 of laser beam

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Hf/177Hf ratio for the standard zircon Mud Tank is

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al. (2006). The

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energy density. Instrumental conditions and data acquisition are as described by Wu et

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0.282500±0.000030 for the standard. Measured 176

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Lu/177Hf ratios are

Hf/177Hf ratios, taking the decay constant for

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Lu as

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used to calculate initial

Hf/177Hf and

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1.867×10-11a-1 (Söderlund et al., 2004). The present-day chondritic ratios of Hf/177Hf = 0.282785 and

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Lu/177Hf = 0.0336 (Bouvier et al., 2008) are used to

calculate εHf(t) values. The depleted mantle Hf model ages (TDM2) are calculated using the measured

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Lu/177Hf ratios of zircon. The mantle extraction model age

(TDM2) is calculated by projecting initial depleted mantle model growth line using continental crust (Griffin et al., 2002).

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Lu/177Hf ratios of the zircon to the

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Lu/177Hf value (0.015) for average

ACCEPTED MANUSCRIPT Zircon trace elements analyses are examined using an ArF excimer laser ablation system, attached to a Neptune Plasma multi-collector ICP-MS with a Geolas-193 laser-ablation system (LA-MC-ICP-MS). Analyses parameters are 40μm of ablation pit in diameter, 26s of ablation time and 6 to 8Hz of a laser pulse frequency. The

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detailed analytical procedures are same as those provided by Yuan et al (2004).

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Helium was used as a carrier gas to transport the ablated sample from the

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laser-ablation cell to the ICP-MS torch through a mixing chamber where it is mixed with argon. Reference material and internal calibrant are NIST SRM 610 and

Si,

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respectively.

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5. Results

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5.1.1 Plagioclase

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5.1 Mineral geochemistry of the intrusive rocks

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Representative EMPA chemical compositions (wt.%) and the calculated formulae for plagioclase are provided in Supplementary Material A1. Plagioclase from the

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Shiwu deposit exhibits significant compositional variation (Fig. 4a). The FeO and CaO contents and An and Al*(Al* = (Al/(Ca + K + Na)-1)/(An/100)) ratios of plagioclase crystals from the quartz-diorite porphyries are 0.17–0.41 wt.%, 7.66– 11.95 wt.%, 37–58, and 0.88–0.99, respectively (Fig. 4b). These values for plagioclase from the tonalite porphyries are 0.12–0.88 wt.%, 6.14–12.98 wt.%, 30–63, and 0.86–1.18, respectively, and for plagioclase from the quartz-diorite and diorite

ACCEPTED MANUSCRIPT intrusions that were emplaced after mineralization within the Shiwu deposit are 0.18– 0.73 wt.%, 6.12–13.58 wt.%, 30–68, and 0.80–0.96, respectively (Fig. 4b). Plagioclase crystals from granodiorite porphyries within the Kounrad deposit exhibit little compositional variation (Fig. 4a), with FeO and CaO contents and An and Al*

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ratios of 0.14–0.20 wt.%, 5.75–7.85 wt.%, 28–38, and 0.91–1.03, respectively (Fig.

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4b). These values for plagioclase crystals from granodiorites within the barren Koldar

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Complex are 0.13–0.35 wt.%, 5.92–11.01 wt.%, 29–54, and 0.66–1.56, respectively (Fig. 4b). The compositions of plagioclase from the Aktogai and Baogutu deposits

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have been investigated by previous studies (Shen and Pan, 2013; Cao et al., 2014; Li

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et al., 2018a, 2018b). The FeO and CaO contents and An and Al* ratios of plagioclase crystals from tonalite porphyries within the Aktogai deposit are 0.12–0.33 wt.%,

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5.58–7.21 wt.%, 27–35, and 0.80–1.05, respectively (Fig. 4b). These values for

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plagioclase from ore-related diorites within the Baogutu deposit are 0.10–0.53 wt.%, 5.75–11.33 wt.%, 28–55, and 0.77–1.37, respectively (Fig. 4b). The FeO and CaO

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contents and An and Al* values of plagioclase crystals from gabbros and tonalite

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porphyries within the Baogutu deposit that were emplaced prior to and after mineralization, respectively, are 0.06–0.62 wt.%, 5.19–11.54 wt.%, 24–57, and 0.76– 1.16, respectively (Fig. 4b).

5.1.2 Biotite

Representative EMPA chemical compositions (wt.%) and calculated formulae for biotite are provided in Supplementary Material A2. Biotite crystals from granodiorite

ACCEPTED MANUSCRIPT porphyries within the Kounrad deposit are Mg rich, show little compositional variation (Fig. 4c), and have TiO2 contents and Mg/(Mg + Fetotal) and Fe3+/(Fe2+ + Fe3+) ratios of 1.15–2.66 wt.%, 0.48–0.60, and 0.12–0.28, respectively. The Mg–Fe3+– Fe2+ diagram in Figure 4d reveals that the majority of samples have high oxygen

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fugacities (>NNO). Biotites from granodiorites within the barren Koldar Complex are

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also Mg rich, have high oxygen fugacities (>NNO; Fig. 4c and d), and have TiO2

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contents and Mg/(Mg + TFe) and Fe3+/(Fe2+ + Fe3+) ratios of 3.78–5.39 wt.%, 0.48– 0.51, and 0.18–0.23, respectively. The compositions of biotite from ore-related

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diorites and from post-mineralization tonalite porphyries within the Baogutu deposit

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have been investigated previously (Shen and Pan, 2013, 2015). Biotite crystals from the diorites are Mg rich but have variable compositions and oxygen fugacities (Fig. 4c

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and d). These crystals have TiO2 contents and Mg/(Mg + TFe) and Fe3+/(Fe2+ + Fe3+)

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ratios of 0.93–4.61 wt.%, 0.48–0.77, and 0.00–0.35, respectively. Biotites from the tonalite porphyries are Mg rich, have high oxygen fugacities (>NNO; Fig. 4c and d),

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and have TiO2 contents and Mg/(Mg + TFe) and Fe3+/(Fe2+ + Fe3+) values of 0.91–

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4.72 wt.%, 0.52–0.65, and 0.11–0.23, respectively.

5.1.3 Amphibole

Representative EMPA chemical compositions (wt.%) and calculated formulae for amphibole are provided in Supplementary Material A3. Amphiboles from the granodiorite porphyries within the Kounrad deposit show little compositional variation (Si = 7.03–7.42 a.p.f.u.) and classify as Mg-rich hornblende (Fig. 4e).

ACCEPTED MANUSCRIPT According to the calculations of Ridolfi et al. (2010), the formation temperatures and pressures, △NNO and logfO2 values, and H2O contents of the melt are 732°C–804°C, 58–102 MPa, 1.18–2.00, −13.43 to −12.49, and 4.03–4.83 wt.%, respectively (Fig. 4f). These values are 659°C–786°C, 56–116 MPa, 0.46–1.48, −14.61 to −13.66, and 4.70–

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5.69 wt.%, respectively, for granodiorites within the barren Koldar Complex. The

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compositions of amphiboles from ore-related intrusions within the Shiwu and

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Baogutu deposits have been investigated previously (Li et al., 2017; Shen and Pan, 2013, 2015). The formation temperatures and pressures, △NNO and logfO2 values,

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and H2O contents of quartz-diorite porphyries within the Shiwu deposit are 717°C–

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797°C, 51–107 MPa, 1.56–2.09, −13.89 to −12.31, and 3.90–4.87 wt.%, respectively (Fig. 4f). These values are 771°C–828°C, 79–129 MPa, 1.14–1.78, −12.93 to −11.87,

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and 3.39–4.88 wt.%, respectively (Fig. 4f), for diorites within the Baogutu deposit.

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These values for the gabbros (pre-mineralization) and tonalite porphyries (post-mineralization) within the Baogutu deposit are 640°C–877°C, 25–136 MPa,

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1.12–3.24, −15.13 to −12.01, and 2.95–4.99 wt.%, respectively.

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5.2 Whole-rock major and trace element chemistry Whole-rock geochemical data from this study and previous studies (e.g., Cao et al., 2016a, b; Li et al., 2016, 2017; Hu et al, 2018; Shen et al., 2013, 2015, 2017, 2018) indicate that the ore-related intrusions within the Shiwu, Jiamantieliek, Baogutu, Kounrad, and Aktogai deposits are compositionally variable (Supplementary Material B). The Kounrad and Aktogai deposits have similar SiO2 contents (65.51–76.17 wt.%),

ACCEPTED MANUSCRIPT and the Shiwu and Jiamantieliek deposits have similar SiO2 contents (57.27–69.12 wt.%). Rocks within the Baogutu deposit have lower SiO2 contents (53.76–62.88 wt.%). In the TAS diagram in Figure 5a, the compositions of samples from the Shiwu, Jiamantieliek, and Kounrad deposits fall within the granodiorite field. Samples from

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the Baogutu and Aktogai deposits are compositionally variable and are classified as

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gabbro-diorite to diorite and granodiorite to granite, respectively (Fig. 5a). In the

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SiO2–K2O diagram of Figure 5b, samples from the West Junggar region fall within the low-K tholeiite and medium-K calc-alkaline fields. In contrast, samples from the

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North Balkhash region fall within the medium-K calc-alkaline and high-K

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calc-alkaline fields. Except for the Baogutu deposit (MgO >3.60 wt.%), the MgO contents of the deposits are consistent (1.15–2.87 wt.%). The Mg-number (Mg#) and

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Fe2O3T/(Fe2O3T + MgO) ratios of the samples range between 0.46 and 0.87 (Fig. 5c)

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and 0.46 and 0.75 (excluding two samples, Fig. 5d), respectively, indicating that they are Mg rich. The chondrite-normalized rare earth element (REE) patterns of the

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analyzed samples are enriched in light REEs (LREEs) and show slightly negative to

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flat Eu anomalies (0.85–1.23, Fig. 6a, c, e, g, and i). The primitive-mantle-normalized trace element spider diagrams reveal consistent patterns across all samples, pronounced negative Nb, Ta, and Zr anomalies, and positive Sr and K anomalies (Fig. 6b, d, f, h, and j).

5.3 In situ zircon O isotope compositions Zircon O isotope compositions are provided in Supplementary Material C. We

ACCEPTED MANUSCRIPT calculated the δ18O values of melt by using the following equation (Valley et al., 2005): △18O(zircon − whole rock) = −0.0612(SiO2 wt.%) + 2.5. Eighteen zircon grains from a quartz-diorite porphyry within the Shiwu deposit yielded δ18O values of +7.06‰ to +8.02‰ (geomean = +7.54‰; Fig. 7a) and calculated melt δ18O values of

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+8.59‰ to +9.55‰ (geomean = +9.07‰; Fig. 7e). Twenty-one zircon grains from a

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tonalite porphyry within the Shiwu deposit yielded δ18O values of +5.36‰ to +6.38‰

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(geomean = +5.79‰; Fig. 7a) and calculated melt δ18O values of +6.83‰ to +7.85‰ (geomean = +7.27‰; Fig. 7e). The δ18O values of the samples analyzed in this study

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are higher than those of depleted mantle (+5.3 ± 0.3‰; Valley et al., 2005).

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The zircon δ18O values of diorites within the Baogutu deposit range between +5.75‰ and +6.47‰ (geomean = +6.08‰, n = 32; Fig. 7b; Cao et al., 2016b) and

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calculated melt δ18O values of +6.81‰ to +7.53‰ (geomean = +7.14‰; Fig. 7f).

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These values are also higher than those of depleted mantle. The zircon δ18O values of granodiorite porphyries within the Kounrad deposit

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range between +5.01‰ and +6.90‰ (geomean = +6.03‰, n = 41; Fig. 7c; Li et al.,

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2016; Shen et al., 2017), and their calculated melt δ18O values range between +6.64‰ and +8.65 ‰ (geomean = +7.72‰; Fig. 7g). These values are consistent with or slightly higher than those of depleted mantle. The zircon δ18O values of tonalite porphyries within the Aktogai deposit range between +4.3‰ and +5.9‰ (geomean = +5.15‰, n = 103; Fig. 7d; Cao et al., 2016a; Shen et al., 2018), and their calculated melt δ18O values range between +6.0‰ and +7.6‰ (geomean = +6.84‰; Fig. 7h). These values are consistent with or slightly

ACCEPTED MANUSCRIPT lower than those of depleted mantle.

5.4 In situ zircon Hf isotope compositions We used same parameters ratios (such as

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Hf/177Hf and

176

Lu/177Hf etc), which

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have little difference in previous works, when calculating isotope values. The

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parameters used in this study are listed in Section 4.

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Zircon Hf isotope compositions for the Shiwu deposit are provided in Supplementary Material C. Zircon grains from quartz-diorite porphyries within the 176

Hf/177Hf ratios of 0.282764–0.283007, calculated

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Shiwu deposit yielded

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(176Hf/177Hf)i values of 0.282757–0.283001, εHf(t) values of +5.92 to +14.56 (Fig. 8a), and TDM2(Hf) ages of 921–368 Ma (mainly <669 Ma; Fig. 8e).

Hf/177Hf ratios of 0.282770–0.283041, calculated (176Hf/177Hf)i values of 0.282763–

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176

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Zircon grains from tonalite porphyries within the Shiwu deposit yielded

0.283034, εHf(t) values of +6.12 to +15.72 (Fig. 8a), and TDM2(Hf) ages of 909–294

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Ma (mainly 784–408 Ma; Fig. 8e).

Zircon grains from diorites within the Baogutu deposit have yielded

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Hf/177Hf

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ratios of 0.282906–0.283034 (Cao et al., 2016b), calculated (176Hf/177Hf)i values of 0.282899–0.283018, εHf(t) values of +11.01 to +15.20 (Fig. 8b), and TDM2(Hf) ages of 598–330 Ma (Fig. 8f). Zircon grains from granodiorite porphyries within the Kounrad deposit have yielded

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Hf/177Hf ratios of 0.282730–0.282934 (Li et al., 2016; Shen et al., 2017),

calculated (176Hf/177Hf)i values of 0.282726–0.282916, εHf(t) values of +5.29 to

ACCEPTED MANUSCRIPT +11.88 (Fig. 8c), and TDM2(Hf) ages of 978–553 Ma (mainly >684 Ma; Fig. 8g). Zircon grains from tonalite porphyries within the Aktogai deposit have yielded 176

Hf/177Hf ratios of 0.282846–0.283074 (Cao et al., 2016a; Shen et al., 2018),

calculated (176Hf/177Hf)i values of 0.282842–0.283068, εHf(t) values of +9.33 to

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+17.33 (Fig. 8d), and TDM2(Hf) ages of 719–205 Ma (mainly 595–318 Ma; Fig. 8h).

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5.5 Zircon trace element

The concentrations of REEs, U, Th, Hf, and Ti in zircon and the

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chondrite-normalized REE patterns are presented in Supplementary Material D. The

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traditional method for calculating the Ce anomaly is not accurate because La and Pr concentrations in zircon are often close to the detection limit of these elements. We

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therefore used the method of Ballard et al. (2002) to calculate the Ce4+/Ce3+ ratios and

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the method of Ferry and Watson (2007) to estimate zircon crystallization temperatures. The Eu/Eu* values (Eu* = EuN/(SmN × GdN)1/2) were calculated by normalizing the

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Sm and Gd concentrations. The zircon crystallization temperatures and the Eu/Eu*

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and Ce4+/Ce3+ values of the quartz-diorite porphyries in the Shiwu deposit (n = 7) are 786°C–1004°C (geomean = 84°C), 0.25–0.39 (geomean = 0.31), and 24–38 (geomean = 29), respectively (Fig. 9a and b). These values for the tonalite porphyries within the Shiwu deposit (n = 14) are 689°C–781°C (geomean = 723°C), 0.48–0.74 (geomean = 0.61), and 39–122 (geomean = 74), respectively (Fig. 9a and b). These values for granites within the barren Koldar Complex (n = 8) are 720°C–1284°C (geomean = 840°C), 0.10–0.23 (geomean = 0.16), and 11–69 (geomean = 26), respectively (Fig.

ACCEPTED MANUSCRIPT 9a and b).

6. Discussion 6.1 Magma sources

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Previous studies have suggested that the ore-related intrusions within the Aktogai

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and Kounrad deposits were sourced predominantly from juvenile lower crust with

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little mantle contribution (Shen et al., 2018) and with 5%–15% ancient crust (Li et al.,

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2016; Shen et al., 2017), respectively. The intrusions in both deposits are interpreted to have formed by the partial melting of thick crust (~40 km) and subsequent

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assimilation (Shen et al., 2018). Accordingly, Sections 6.1 and 6.2 focus on the West Junggar region.

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The ore-related intrusive bodies within the Shiwu, Jiamantieliek, and Baogutu

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deposits have high SiO2, Sr, and Ba contents, moderately fractionated REE patterns, and low Eu contents (Figs 5 and 6a–f). These observations suggest that their parent

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magmas were sourced from lower crust that contained abundant amphibole and lesser

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garnet (Sen and Dunn, 1994). The average Mg# values of the three deposits are 0.58 (n = 8), 0.67 (n = 11), and 0.63 (n = 41), respectively, reflecting the involvement of mantle material (Rapp and Watson, 1995). The concentrations of some mobile elements (e.g., Sr) within the deposits were affected by hydrothermal alteration, as evidenced by their variable LOI contents (0.48–7.03 wt.%). Some high-field-strength elements such as Nb, Ta, and Ti, and some robust igneous minerals such as zircon, are capable of recording the original geochemical characteristics of the parent magma

ACCEPTED MANUSCRIPT (Trail et al., 2012). The Nb/Ta ratios of the intrusions can reflect their magma source because these elements are unaffected by magmatic differentiation, crustal assimilation, and hydrothermal alteration (Hawkesworth et al., 1993). Previous studies (e.g., Hofmann, 1988; Plank and Langmuir, 1998; Rudnick and Gao, 2003)

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have calculated the Nb/Ta ratios of primitive mantle, N-MORB, lower crust, upper

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crust, and marine sediments to be 17.59, 18.27, 8.33, 13.33, and 14.19, respectively.

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The Nb/Ta ratios of rocks from the Shiwu, Jiamantieliek, and Baogutu deposits are 11.40–13.53 (geomean = 12.21, n = 14), 12.27–15.63 (geomean = 13.66, n = 11), and

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11.87–19.80 (geomean = 15.27, n = 41), respectively. The Nb/Ta ratios of rocks from

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the Shiwu and Jiamantieliek deposits fall between those of the primitive mantle and lower crust. Rocks from the Baogutu deposit have variable Nb/Ta values that fall

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between those of N-MORB and the lower crust.

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Zircons preserve reliable records of magmatic isotope ratios and can therefore be used to trace magma sources. The average εHf(t) values of rocks from the Shiwu and

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Baogutu deposits are +10.64 (n = 14; tonalite porphyry in the Shiwu deposit), +11.29

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(n = 16; quartz-diorite porphyry in the Shiwu deposit), and +13.42 (n = 68; diorite in the Baogutu deposit). These values are similar to the εHf(t) values of depleted mantle (εHf(t) = +15.8; Salters and Stracke, 2004). The average δ18O value of rocks from the Baogutu deposit is +6.08 (n = 32), which is slightly higher than mantle δ18O values. Rocks from the Shiwu deposit yield two peaks in δ18O. A tonalite porphyry from the Shiwu deposit yielded an average δ18O value of +5.79, which is similar with mantle δ18O values. A quartz-diorite porphyry, also from the Shiwu deposit, yielded an

ACCEPTED MANUSCRIPT average δ18O value of +7.54, which is substantially higher than mantle δ18O values (Fig. 7a). The high δ18O values (between +5.3‰ and +7.5‰) may reflect the recycling of supracrustal material into the parent magma. Even higher δ18O values (> +7.5‰) from the quartz-diorite porphyry may reflect the intra-crustal recycling of

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high δ18O material and/or maturation of the crust (Valley et al., 2005). The zircon

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TDM2(Hf) ages of rocks from the Baogutu deposit (average TDM2(Hf) = 436 Ma, n = 68)

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are suggestive of a juvenile lower-crustal source. The zircon TDM2(Hf) ages of rocks

juvenile lower-crustal source (Fig. 8e).

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from the Shiwu deposit occur over a wide range (921–294 Ma) but similarly imply a

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The whole-rock trace element chemistry and zircon Hf and O isotope data collectively suggest that the parent magmas of rocks within the Shiwu, Jiamantieliek,

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and Baogutu deposits were sourced from juvenile lower crust with minor

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mantle-derived components. The quartz-diorite porphyry within the Shiwu deposit

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was also contaminated by marine sediments (Fig. 10).

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6.2 Petrogenesis and tectonic setting In the (Al2O3 + TFe2O3 + MgO + TiO2)–Al2O3/(TFe2O3 + MgO + TiO2) diagram in Fig. 11a, the majority of the samples fall between the “high pressure” and “low pressure” fields, reflecting crust–mantle interaction (Patiňo Douce, 1999). Previous studies (e.g., Rapp and Waston, 1995; Van Westrenen et al., 2001; Barth et al., 2002) have suggested that a clinopyroxene-rich residue will lead to low La/Yb (<20), La/Sm (<4), and Sm/Yb (<3) ratios within the melt, whereas an amphibole-rich residue will

ACCEPTED MANUSCRIPT lead to higher La/Yb (>20–30), La/Sm (>4), and Sm/Yb (3–5) ratios, and a garnet-rich residue will lead to high La/Yb (>30) and Sm/Yb (>5) ratios. As pressure increases, the source transitions from being rich in clinopyroxene to being rich in amphibole and subsequently in garnet (Rapp and Waston, 1995; Shen et al., 2018).

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The PCDs within the West Junggar region were therefore derived from a shallower

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source than that of the PCDs within the North Balkhash region, and their source vary

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from 30–40km to ~40 km (Fig. 12). The PCDs within the West Junggar region have lower Th and LREE contents and Ce/Yb ratios but higher TiO2 contents

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(Supplementary Material B) compared with the PCDs within the North Balkhash

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region, reflecting a smaller formation pressure (e.g., Hildreth and Moorbath, 1988). The North Balkhash region has higher K2O contents (geomean = 3.11) and lower

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Na2O/K2O ratios (geomean = 1.30) compared with the West Junggar region (geomean

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K2O = 1.36, geomean Na2O/K2O = 3.04). Rapp et al. (2002) suggested that different magma sources, degrees of partial melting, and the pressure at which melting occurs

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can affect the K2O contents of melts. High K2O contents may reflect very small

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amounts of melting (<10%) or increasing pressure. As discussed in Section 6.1 and in previous studies (Shen et al., 2017, 2018), the parent magmas of rocks within the PCDs in the North Balkhash–West Junggar Belt are inferred to have been generated from similar sources (i.e., juvenile lower crust with a mantle-derived component). The high K2O contents cannot be associated with low degrees of partial melting (<10%) because K and LILEs behave as highly incompatible elements during melting, and therefore the LILE contents (e.g., Ba, Th, U, and Sr) are expected to be high if the

ACCEPTED MANUSCRIPT K2O contents were also high (Fig. 6). Therefore, the high K2O contents of the PCDs within the North Balkhash region more likely reflect high-pressure conditions during melting. It is also possible that wall-rock assimilation may have affected the K2O contents of the ascending magmas. At present, this possibility cannot be excluded as

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there are limited geochemical studies on this region.

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In the Nb + Y–Rb diagram in Figure 13a, the samples show little compositional

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variation and fall within the “VAG” field. In the Nb/Yb–Th/Yb diagram in Figure 13b, samples from the Kounrad and Aktogai deposits fall within the “continental-arc” field,

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whereas samples from the Shiwu, Jiamantieliek, and Baogutu deposits fall within the

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“overlapping region” field. In Figure 13c and d, samples from the Shiwu, Jiamantieliek, and Baogutu deposits fall mainly within the “overlapping region” field,

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whereas samples from the Kounrad and Aktogai deposits fall within the “adakite”

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field. True adakites are characterized by low Sc (<10 ppm) and high Zr/Sm (>50 ppm) contents (Defant and Drummond, 1990, 1993). The geochemical characteristics of

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rocks from the Shiwu (Zr/Sm = 13–54; geomean = 35), Jiamantieliek (Zr/Sm = 14–49;

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geomean = 25), and Baogutu (Sc = 14–25, geomean = 19; Zr/Sm = 7–56, geomean = 18) deposits are inconsistent with these criteria, indicating that these rocks are not true adakites. Following evidences suggest that rocks from the Shiwu, Jiamantieliek, and Baogutu deposits may have undergone melting–assimilation–storage–homogenization (MASH) processes. First, the samples do not exhibit negative Eu, Ba, and Sr anomalies (Fig. 6a–f), and they fall along the “partial melting” line in Figure 11b and c, indicating that they did not undergo significant fractional crystallization and

ACCEPTED MANUSCRIPT therefore cannot reflect assimilation–fractional–crystallization (AFC) processes (Defant and Drummond, 1993; Richards and Kerrich, 2007). Second, Elliott et al (1997) indicates addition of aqueous fluids to the mantle wedge can significantly affect Ba/La, Ba/Nb and Pb/Ce ratios. In the West Junggar region, the Ba/La, Ba/Nb

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and Pb/Ce ratios of the PCDs are 6–82 (mainly >25, geomean=34), 22–334

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(mainly >110, geomean=132) and 0.05–1.39 (mainly >0.13, geomean=0.19), which

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are higher and more variable than those from MORB and OIB. Furthermore, hydrous mafic minerals such as amphibole and biotite occur within these deposits. These

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features suggest that the magma source interacted with hydrous fluids from the

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subducting slab and that the parent magma has high water contents. Water-rich magmas suppress the growth of plagioclase but promote amphibole crystallization,

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resulting in magmas with high Sr/Y and low Y contents (Richards, 2011). Third, rocks

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from the Shiwu, Jiamantieliek, and Baogutu deposits have variable Sr–Nd contents (Supplementary Material E) and O (Fig. 7a–g) and Hf (Fig. 8a–g) isotope

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compositions, suggesting that these magmas formed by crustal processes, which can

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result in variable isotope compositions (e.g., Brandon et al., 1993). Owing to their low densities, parent magmas that form within MASH zones will ascend towards the upper crust via dikes to form intrusive bodies and batholiths (Richards, 2003). Changes in the An and FeO contents of the plagioclase crystals may reflect different magmatic processes. Plagioclase crystals with little compositional variation (△An = 1–10) may be indicative of a closed system, whereas increased compositional variation (△An = 10–25) may reflect open-system behavior (Singer et

ACCEPTED MANUSCRIPT al., 1995). Furthermore, correlation between the An and FeO contents of the plagioclase may reflect recharging within an open system or magma mixing (Ruprecht and Worner, 2007). Plagioclase from the West Junggar region shows significant variation in An content and a positive relationship between An and FeO

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content (Fig. 4b), suggesting that the parent magmas formed by magma mixing and/or

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involved wall-rock contamination.

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6.3 Reasons for the difference in Cu reserve size

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The formation of porphyry deposits is a complex process that depends on many

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factors (Cooke et al., 2005; Wilkinson, 2013). Significant variation in the whole-rock trace element and zircon Hf and O isotope compositions of ore-related intrusions

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within the North Balkhash–West Junggar Metallogenic Belt suggests that the PCDs

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formed by different processes. Three significant differences, namely, inferred tectonic setting (Fig. 13b), crustal thickness (Fig. 12), and oxygen fugacity (Fig. 9), are

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identified between the North Balkhash and West Junggar regions. Previous studies

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have suggested that large–giant PCDs can form in both island-arc and continental-arc settings (e.g., Cooke et al., 2005, and references therein). Therefore, the tectonic setting cannot be the main reason for the observed difference in Cu reserves between the two regions. Previous work has suggested that thickened arcs may be conducive environments for the formation of large PCDs (Sillitoe, 2010; Cooke et al., 2005). Thickening of the crust (to >30 km) may facilitate sulfide accumulation in the lower crust or at the

ACCEPTED MANUSCRIPT crust–mantle boundary (Chiaradia, 2013). The accumulated copper sulfides will release the metals when they interact with oxidized magmas, subsequently forming large PCDs (Lee et al., 2012; Chiaradia, 2013). Geophysical studies (e.g., Li et al., 2014; Xu et al., 2016; Zhao et al., 2003) have investigated the region between the

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Tacheng Basin and Karamay–Urho Fault (Fig. 1c). Geophysical imaging has

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suggested that the Moho depth in this region is depressed from 40 to 45 km. We

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consider these geophysical data to be robust, but the inferred depth of the Moho at ca. 310 Ma is speculative because the region underwent vertical growth (crustal

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thickening) after subduction (Han et al., 2006). The Sm/Yb–La/Sm and Yb–La/Yb

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diagrams in Figure 12 were used to determine whether the North Balkhash and West Junggar regions can accumulate sulfides, and therefore whether these regions have the

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potential to host large PCDs. The majority of samples from the North Balkhash region

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formed at a higher pressure (Fig. 12) and higher K contents (Fig. 5b) compared with those from the West Junggar region. These features suggest that the North Balkhash

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region has greater potential than the West Junggar region to form large PCDs.

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Furthermore, thicker crust, resulting in higher temperatures in the lower crust, is conducive for the formation of long-lived MASH zones. Within these MASH zones, lower-crustal rocks are assimilated into mantle-derived magmas, causing the MASH zone to undergo high degrees of fractionation and voluminous acidic and volatile-rich magmatism (Anthony and Titley, 1988; Hildreth and Moorbath, 1988). Richards (2003) proposed that the magma supply rate is a crucial factor for sustained upper-crustal magmatism. MASH zones beneath thicker crust facilitate the formation

ACCEPTED MANUSCRIPT of shallow crust chambers because acidic and volatile-rich magmas have low viscosity and density, causing them to ascend. We therefore interpret the PCDs within the North Balkhash region to have been associated with more evolved and volatile-rich magmas than those in the West Junggar region, which was beneficial to

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the formation of large–giant PCDs.

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Previous studies have suggested that the saturation/elimination of sulfides during

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magmatism is closely related to the chalcophile elements contents (Lee et al., 2012; Sun et al., 2015) and that the transition from a sulfide- to sulfate-dominated

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environment occurs between FMQ and FMQ+2 (Jugo et al., 2005). Figure 4d and f

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reveal that the hydrous mafic minerals (e.g., biotite and amphibole) within the Shiwu, Baogutu, and Kounrad deposits have high oxygen fugacities that are higher than those

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of the intrusive bodies emplaced before and after mineralization. These observations

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suggest that the parent magmas associated with ore formation were oxidized (> NNO). However, some of mafic minerals have undergone variable degrees of hydrothermal

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alteration (SA figure 2) and therefore do not record their original/accurate

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compositions from the parent magma. Meanwhile, the Mg/Fe and Fe3+/Fe2+ of biotite in deposits are 0.93 to 3.35 (geomean=1.48) and 0.00 to 0.53 (geomean=0.21), respectively, also indicating they experienced different degrees of hydrothermal alteration (Beane, 1974). This is particularly the case for biotite and amphibole from the Baogutu (Fig. 4c) and Shiwu (Fig. 4e) deposits. Nonetheless, the majority of the data are considered to be reliable because they come from samples affected by only minor alteration. Zircon can provide a record of the composition of the parent magma

ACCEPTED MANUSCRIPT because it is relatively resistant to weathering and alteration (Ballard et al., 2002; Ferry and Watson, 2007). The zircon trace element compositions for samples from the North Balkhash region reveal that they have a higher oxygen fugacity than those from the West Junggar region (Fig. 9b). Samples from the barren Koldar Complex yielded

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the lowest oxygen fugacity, which is consistent with the lowest Cu reserves (actually

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no Cu reserves; Fig. 9c).

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Richards (2011) suggested that high magmatic water content is a prerequisite for the formation of magmatic–hydrothermal ore deposits and that oxygen fugacity also

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influences metal endowment. The ore-related intrusions within the PCDs investigated

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in this study contain amphibole and/or biotite and had magmatic water contents of >4 wt.% (refer to Section 5.1.3 and Fig. 14a), indicating that their parent magmas were

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water rich; however, the barren Koldar Complex and intrusions that were emplaced

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before and after mineralization in the Baogutu deposit exhibit these same characteristics (Fig. 4c and e). These observations suggest that although a high

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magmatic water content is an essential requirement, it is not the only factor

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controlling the formation of PCDs. The Koldar Complex has the highest water contents (Fig. 14a) but lowest oxygen fugacity (Fig. 9) comparing with other studied deposits, and therefore hydrothermal fluids were unable to transport metals and form a PCD. Williamson et al. (2016) suggested that excess Al (Al* > 1, Al* = (Al/(Ca + Na + K)–1)/0.01An) in plagioclase can distinguish fertile calc-alkaline systems from barren ones; however, this criterion could not be used in the present study, as many of the plagioclase phenocrysts from the ore-related intrusions yielded low Al* contents

ACCEPTED MANUSCRIPT (<1; Fig. 14b). This observation may reflect that PCDs in the North Balkhash region formed from high-K calc-alkaline magmas where K occupies the M-sites instead of Al (Williamson et al., 2016). The same observation for PCDs in the West Junggar region may relate to deposit size. Fertile systems in Williamson et al. (2016) are typically

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large–giant porphyry deposits. However, the Baogutu deposit, which is the largest

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PCD in the West Junggar region, contains only 0.63 Mt Cu and 14 t Au. It is possible

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that small PCDs such as the Baogutu deposit may form in normal systems and do not require a pre-concentration process, such as the presence of excess Al. However,

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further work is required to verify this hypothesis.

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Despite being a complex process, several conditions are necessary for the formation of large–giant PCDs. The first requirement is thicker crust. In thick crust,

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arc-related magmas will become depleted in copper as a result of early sulfide and/or

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magnetite crystallization, potentially forming a fertile metal source (Chiaradia, 2013; Sun et al., 2013). The second, and more important, requirement is high oxygen

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fugacity. Only a magma that is oxidized can remove metal from magmatic sulfides in

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the lower crust and then transport these sulfides from the deep mantle towards the upper crust. A high magmatic water content is also important, but it was not essential for the formation of PCDs in the North Balkhash–West Junggar region.

7. Conclusion Porphyry copper deposits (PCDs) within the North Balkhash–West Junggar Metallogenic Belt formed by different mechanisms. The parent magmas of intrusive

ACCEPTED MANUSCRIPT rocks within the PCDs were derived mainly from a juvenile lower-crustal source with mantle input. The parent magmas of intrusive rocks within the Shiwu deposit were also contaminated by sediments. Ore-related intrusions within the West Junggar region formed by MASH processes in an immature arc setting. This interpretation

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differs from that for PCDs within the North Balkhash region, which is inferred to

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have formed in a mature arc setting. A high magmatic water content is interpreted to

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be important, but not essential, for the formation of large PCDs. Thicker crust and oxidized magmas are crucial, resulting in the formation of enormous Cu reserves.

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These factors may account for the observed difference in Cu reserves between the

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Acknowledgements

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North Balkhash and West Junggar regions.

We are very grateful to Editor-in-chief Xian-Hua Li and two anonymous

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reviewers for their constructive comments and assistance in improving manuscript.

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We thank Di Zhang for assistance with EMPA, Yue-Heng Yang for assistance with zircon Hf and trace elements analysis, Guo-Qiang Tang for assistance with zircon O isotope analysis. This work was granted by National Key R&D Program of China and the National Natural Science Foundation of China (2017YFC0601206, 41390442, 41772089, 2018YFC0604004, U1303293), and Open foundation of KLMR of Chinese Academy of Sciences (KLMR2017-16).

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Figure Captions Figure 1. (a) Simplified map of Central Asian Orogenic Belt (CAOB) and locations of giant PCDs (after Shen et al., 2015). (b) Simplified geological map of the Central and Eastern Kazakhstan and

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the West Junggar (after Shen et al., 2017). (c) Simplified geological map of the West Junggar

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region (after Li et al., 2017).

Figure 2. Simplified geological map of the (a) Shiwu Cu-Au deposit (after Li et al., 2017), (b)

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Jiamantieliek Cu deposit (after shen et al., 2013), and (c) Baogutu Cu-Au deposit (after Shen et al.,

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2010).

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Figure 3. Simplified geological map of the (a1-3) Kounrad Cu, and (b1-3) Aktogai Cu deposits.

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(a-1) Geological, (a-2) hydrothermal alteration, and (a-3) hypogene mineral zoning map of the Kounrad Cu deposit modified after Zvezdov et al (1993) and Seltmann et al (2014). (b-1)

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Geological, (b-2) geological of NE-SW oriented section, and (b-3) hydrothermal alteration and

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hypogene mineral zoning map of the Aktogai Cu deposit modified after Zvezdov et al (1993) and Seltmann et al (2014).

Figure 4. (a) Plagioclase plot in Or-An-Ab diagram. (b) An-FeO diagram of plagioclase. (c) (Fe2++Mn)-Mg-(AlVI+Fe3++Ti) diagram of biotite. (d) Mg-Fe3+-Fe2+ diagram of biotite. (e) Si-Mg/(Mg+Fe2+) diagram of amphibole. (f) T-(-logfO2) diagram. Note: Green circles stand for quartz diorite porphyry from the Shiwu deposit, yellow-green circles stand for tonalite porphyry

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Figure 5. Whole rocks major elements diagram. (a) SiO2-(Na2O+K2O) (TAS) diagram (Middlemost, 1994). (b) SiO2-K2O diagram (Peccerillo and Taylor, 1976). (c) SiO2-Mg# diagram.

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(d) SiO2-Fe2O3T/(Fe2O3T+MgO) diagram (Frost et al., 2001). Note: Green circles stand for quartz

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diorite porphyry from the Shiwu deposit, yellow-green circles stand for tonalite porphyry from the

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Shiwu deposit.

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Figure 6. Whole rocks trace elements diagram. (a, c, e, g, i) Chondrite normalized REE patterns

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diagram, chondrite REE values from McDonough and Sun (1995). (b, d, f, h, j) Primitive mantle normalized trace element spider diagram, primitive mantle trace elements values from Sun and

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McDonough (1989).

Figure 7. Zircon δ18O (a-d) and calculated melt δ18O (e-h) of the Shiwu, Baogutu, Kounrad, and

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Aktogai deposit. Melt δ18O calculated by δ18Owhole rock ≈ δ18Ozircon+0.0612×SiO2(wt%)-2.5 (Valley

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et al., 2005), SiO2 used average values of whole rocks major elements mentioned in section 5.2.

Figure 8. Calculated εHf(t) (a-d) and T2DM(Hf) (e-h) of the Shiwu, Baogutu, Kounrad, and Aktogai deposit. Formation ages of quartz diorite porphyry and tonalite porphyry from the Shiwu deposit used 310.1Ma and 310.4Ma, respectively (Li et al., 2017). Formation age of diorite from the Baogutu deposit used 313Ma (Shen et al., 2013). Other deposits’ formation ages are consistent with data source.

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Figure 9. Zircon trace elements diagram. (a) Eu/Eu*-Ce4+/Ce3+ diagram. (b) 104/T(K)-Ce4+/Ce3+ diagram (Trail et al., 2012). (c) Cu(Mt)-Ce4+/Ce3+ diagram. Note: Green circles stand for quartz diorite porphyry from the Shiwu deposit, yellow-green circles stand for tonalite porphyry from the

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Shiwu deposit.

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Figure 10. Zircon Hf-O isotopes two components mixing model. Two components include deleted mantle (Salters and Stracke, 2004) and subducted oceanic sediment (Chauvel et al., 2008; Hoefs,

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2009). Note: Green circles stand for quartz diorite porphyry from the Shiwu deposit, yellow-green

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circles stand for tonalite porphyry from the Shiwu deposit.

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Figure 11. (a) Whole rock (Al2O3+TFe2O3+MgO+TiO2-Al2O3)/(TFe2O3+MgO+TiO2) diagram

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(Patiňo Douce, 1999). (b) Whole rock La-La/Sm diagram. (c) Whole rock Zr-Nb/Zr diagram.

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Figure 12. (a) Whole rock Sm/Yb-La/Sm diagram (Haschke et al., 2006). (b) Whole rock

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Yb-La/Yb diagram (after Shen et al., 2018).

Figure 13. (a) Whole rock Nb+Y-Rb diagram (Pearce et al., 1984). (b) Whole rock Nb/Yb-Th/Yb diagram (Pearce and Peate, 1995). (c) Whole rock Y-Sr/Y diagram (Defant and Drummond, 1990, 1993). (d) Whole rock Ybn-(La/Yb)n diagram (Drummond and Defant, 1990).

Figure 14. (a) H2Omagmatic-Al*plagioclase diagram. The magmatic H2O content calculated by

ACCEPTED MANUSCRIPT amphibole through Ridolfi et al (2010), different symbols stand for average values of rocks, and lines stand for values variations. (b) An-Al/(Na+K+Ca) of plagioclase, “Barren field” and “Fertile field” according to Williamson et al (2016). Note: Green circles stand for quartz diorite porphyry

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from the Shiwu deposit, yellow-green circles stand for tonalite porphyry from the Shiwu deposit.

ACCEPTED MANUSCRIPT Highlights (1) Sources of PCDs in the West Junggar region are principally juvenile lower crust and form in the normal crust. (2) Thicken crust and higher oxygen fugacity are crucial factors resulting in

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significant reserves difference.

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