4.1 Analysis Method

Analysis of major element, rare earth element, and microelement is completed in Southwest China Supervision and Inspection Center of Mineral Resourced, Ministry of Land and Resources (Chengdu Center, China Geological Survey, Chengdu Institute of Geology and Mineral Resources). According to GB/T 14506.28-93 Methods for Chemical Analysis of Silicate Rocks, the detected temperature is 18°C to 25°C, and the humidity is 35% to 65%. The major element is tested using AXIOS-X fluorescence spectrophotometer, whose analytical precision is higher than 5%; microelement is tested by ICP-MS method, using plasma mass spectrometry (X-series), whose precision is greater than 5%. The separation of zircon is finished in Lang fang Cheng xin Geo-service Co., Ltd. Backscattered electronic phase, image analysis of zircon cathodoluminescence and zircon U-Pb dating are all conducted in China University of Geosciences (Wuhan) State Key Laboratory of Geological Process and Mineral Resources (GPMR). Backscattered electronic phase and cathodoluminescence image analysis mainly aim at confirming zircon particle's internal structure, determining the test point and avoiding cracks and inclusions. Zircon U-Pb isotope in situ analysis is conducted by use of Agilent7500a inductively coupled plasma mass spectrometry of Agilent Company and GeoLas2005 excimer laser ablation on-line system of MicroLas Company (^{Ridzuan, Zahar, & Noor, 2017}). Helium is used as a carrier gas of ablation material. The spot diameter of laser beam is 32um. Revision is implemented with standard zircon 91500 used as the external standard and NIST610 as the internal standard. Data obtained from experiments is processed by ICPMDataCal software. After common Pb correction, isoplot 4.15 is used for the calculation of the weighted average age of zircon and the drawing of concordia diagram. For method and process, see document (Ludwig, 2013). The obtained isotope ratio and age error are on the 1σ level.

4.2 Geochemistry

For sample analysis result of quartz diorite and granodiorite, see List 1. SiO₂ content of tested granide is 53.73% to 61.58%, among which the SiO₂ content of quartz diorite sample is between 53.73% and 56.16% (averagely 55.28%) and that of granodiorite is 60.48% to 61.58% (averagely 61.16%). The Na₂O content is 0.79% to 5.19% (averagely 3.18%), the K₂O content is 2.08% to 2.92% (averagely 2.52%), and total alkali content w (Na₂O+K₂O) is 7.13% to 7.84% with an average of 7.41% which is alkali-rich. The ratio of w (Na₂O/K₂O) is between 1.56 and 2.51 with an average of 1.98%. The Rittman index (σ) is 1.80 to 4.49, which belongs to calc-alkaline rock and falls in high-K calc-alkaline series rock area in SiO₂-k₂O diagram (Fig.3a); Al₂O₃ content is between 15.10% and 17.39% with an average of 16.26%. Aluminum saturation index A/CNK is between 0.67 and 0.80 (averagely 0.76). A/NK is between 1.42 and 1.64 (averagely 1.51). In CIPW standard mineral calculation, diopside (DI) appeared, and corundum (C) didn't, which belongs to quasi-aluminous series (Fig.3b). The overall characteristics of samples suggest that granide in this area mainly belongs to quasi-aluminouscalc-alkaline granite series rock (^{Rahim & Usli, 2017}).

Figure 3 Diagrams of SiO₂ -K₂O and A/CNK-A/NK (a: Base map from Pecerillo & Taylor, 1976; ^{Middlemost, 1985}; b: Base map from ^{Maniar, & Piccoli, 1989}.) 

Table.1 The major and trace elements characteristic values of granitoids in Ga'erqiong area 

The total amount of rare earth element of granodiorite and quartz diorite in this area is between 100 and 155x10^-6 with an average of 127x10^-6. The rare earth element content of quartz diorite is 100 to 115x10^-6(averagely 105><10^-6), which is obviously lower than that of granodiorite (ΣREE: 140~155x10^-6, averagely 150>10^-6). LERR/HERR is 5.8 to 1.1 (averagely 8.6) and (La/Yb)_N is 6.2 to 16.8 (averagely 11.4), which indicates that LREE is enriched while HREE is depleted, and the fractionation is clear. δEu is averagely 0.9 without obvious anomaly. δCe is averagely 1.0, which suggests that the samples have no Ce anomaly. The distribution mode pattern of rare earth of granodiorite and quartz diorite is in mild right deviation. The differentiation of two lithologies is evident. The rare earth distribution of quartz diorite is above that of granodiorite (Fig. 4a). There is no obvious Eu anomaly. Tilt level of light rare earth's curve of all samples is higher than that of the heavy rare earth, which indicates that fractionation degree of light rare earth element is remarkably greater than that of the heavy rare earth element, reflecting the clear differentiation of light and heavy rare earth during magmatism.

Figure 4 Chondrite-normalized REE (a) and primitive-mantle-normalized trace element patterns (b) or the magmatic rocks in Qingcaoshan region(Chonodrite values from [21]; Primitivem antle from[22] ) 

On first mantle-normalized spider diagram (Fig. 4b), the overall characteristics of granodiorite and quartz diorite of Ga'erqiong area are similar. They are both "multi-peak" fluctuant curves inclining to the right. There is an obvious differentiation between high field -strength element (HFSE) and large ion lithophile element (LILE) (Fig. 4b), which manifests as enriched in large ion lithophile element and depleted in high field -strength element, among which Ta and Nb are strongly depleted.

4.3 Zircon U-Pb Age

Zircons picked from granodiorite, and quartz diorite samples of Ga'erqiong area have been tested. Check points are all in parts with clear oscillatory zoning structure. For testing position and result, see Fig. 5 and List 2. All tested zircons are transparent, colorless and with good crystalline form and typical magmatic zircon dense oscillation girdle. Zircon Th/U ratio (0.8 to 2.6) is within the magmatic zircon scope (normally metamorphic and hydrothermal origin zircon's Th/U<0.1, magmatic zircon Th/U>0.1. ^{Zhao, 2010}), which matches typical characteristics of magmatic zircon. Therefore, zircon analyzed in this paper is magmatic zircon which crystallized with Ga'erqiong granodiorite at the same time. For young zircon, ²⁰⁶Pb/²³⁸U age can provide more reliable test result (^{Compston, Williams, & Kirschvink, 1992}). The ²⁰⁶Pb/²³⁸U surface ages of 14 analysis points of this test are between 87 and 89 Ma, and the weighted average is 88±2Ma (MSWD=3.8) (Fig.5), which represent the crystallization age of granodiorite. The weighted average U-Pb age of granite porphyry zircon is 83±1Ma, which is about 4 Ma later than quartz diorite (^{Yao et al., 2012}).

Figure 5 Cathodolum inescence (CL) images of representative zircon (a) and concordia diagrams (b) for zircons form the magmatic rocks 

Table 2 LA-ICP-MS zircon U-Pb dating results in Ga'erqiong region 

5.1 Petrogenesis

Geochemical compositions of granitic magmas without remarkable fractional crystallization can reflect the source rock characteristics of the magma. Quartz diorite and granodiorite are both enriched in large ion lithophile elements. Compared with depleted high-field-strength elements, they show notable characteristics of depleted Nb and Ta, which is similar to that of island arc magmatite in subduction zone; both of their mineral compositions are generally consistent with those of amphibole calc-alkaline granitoid (ACG) under the mechanism of subduction (^{Lei et. al, 2012}). In the diagram for discriminating granite Ta-Yb tectonic setting (Fig.6a), all the analyzed samples are classified into the scope of volcanic arc granite. These evidence all indicate that granite is formed in the island arc environment (Qu, Wang, Xin, H. B., Zhao, Y. Y., & Fan, 2009; You & ^{Rahim, 2017}). However, the lateral intraplate magmatism of the crust can also generate characteristics similar to those of island arc granite (^{Plank, 2005}). Thus, only by combining the regional tectonic evolution can their tectonic settings of the formation be effectively discriminated. Current research shows that the Bangong Lake-Nujiang River section of Meso-Tethys started bidirectional subduction in Late Jurassic, and after collision and closure had occurred in middle stage of Early Cretaceous, it got into the intracontinental environment (^{Zhu et al., 2009}; Wang, Zeng, Liu, Xiao, & Gao, 2013). The emplacement of quartz diorite and granite porphyry both happened in Late Cretaceous and notably after a collision between Lhasa plate and Qiangtang plate. This indicates that the Ga'erqiong Cu-Au Deposit is formed in Bangong Lake-Nujiang River metallogenic belt crash environment, instead of island arc environment.

Figure 6 Tectonic environment (a) and Magma mixing trend diagrams (b) of the magmatic rocks in the Gaerqiong area (a: Base map from [30], b: Base map from [31]) 

At the earliest, Adakite refers to the intermediate-acidic igneous rock with unique geochemical characteristics, which is formed by young subducting oceanic crust melting under eclogite facies condition. Its geochemical characteristics often manifest as containing SiO₂≥56%, high Al content (Al₂O₃≥15%), MgO content generally <3% (seldomly>6%), Y lower than normal island arc (generally≤18x10^-6) and HREE (e.g. Yb≤1.9x10^-6), Sr higher than normal island arc (seldomly<400x10^-6), without Eu anomaly or slight negative Eu anomaly, and with low initial ratio of ⁸⁷Sr/⁸⁶Sr (<0.704) (^{Defant & Drummond, 1990}). Seeing from the geochemical characteristics, quartz diorite and granodiorite both have geochemical characteristics similar to those of adakite. The research suggests that formation of adakite has various reasons including partial melting of subducted oceanic crust, crystallization differentiation of mantle-derived basaltic magma, migmatization of magma, partial melting of delaminated lower crust and partial melting of thickened lower crust. That Lhasa plate and Qiangtang plate collided in middle stage of the Early Cretaceous, which caused shortening, thickening and rapid lifting of the crust, and the low K₂O/N₂O ratio formed by partial melting of Ga'erqiong adakite and subducted oceanic crust does not conform to reality indicates that the Ga'erqiong intermediate-acid rocks can't be formed by partial melting of subducted oceanic crust. No same-period basalt is found in the mining area. The rocks are a lack of notable negative anomaly of Sr and Eu. In the diagram for FeO^T~MgO (Fig.6a), the sample also showed no evolution tendency of crystallization differentiation (You & ^{Rahim, 2017}). All these indicate the Ga'erqiong rock is not generated from crystallization differentiation of mantle-derived basaltic magma. In addition, the Ga'erqiong adakite also can't be the result of partial melting of the thickened lower crust, as Ga'erqiong intermediate-acid and acid rocks notably contain more Mg# than melt formed by partial melting of thickened lower crust or underplating basalt. There is higher content of Cr and Ni; zirconHf isotopes of the rocks show notable characteristics of mantle-derived or mantle-crust mixed source (^{Yao et al., 2012}). These indicate that there is mantle-derived material taking part in the diagenetic process.

Dark micro granular inclusions are generally considered as a product of halfway migmatization of mantle-derived basic magma, which is the most direct and powerful evidence for migmatization of magma. Irregular dark micro granular inclusions are found in quartz diorite. The inclusions are dioritic type with distinctive boundary with its host rock and chilled border, which indicates the inclusions are not the residual after partial melting, but the product of magma crystallization. Also, the zircon Hf isotope system has very high closure temperature which won't be changed along with partial melting or fractional crystallization. This can be used for identifying magma migmatization between different magma end members. Zircon Hf isotopes of quartz diorite have significant inhomogeneity (^{Yao et al., 2012}), which indicates the magma source of quartz diorite has occurred magma migmatization. In the SiO2-K2O and SiO2-La second variant diagram, quartz diorite showed both good linear variation trends. All these evidence show that, during formation of quartz diorite in Ga'erqiong, the second end-member magma migmatization occurs. Therefore, quartz diorite might be formed in the post-collision extension environment after the collision of Lhasa block and Qiangtang block; lower crust delamination caused uplifting of asthenosphere material, which further caused partial melting of lithospheric mantle and let basic melt mix with the acid melt.

Emplacement of granite porphyry is significantly later than quartz diorite, and there are no dark micro granular inclusions found in the rock, which implies that there might be different reasons for the formation of granite porphyry and quartz diorite. The granite porphyry is rich in K, which indicates it comes from thickened lower crust. The Qinghai-Tibet Plateau started crust shortening and thickening in middle stage of the Early Cretaceous. Thickening of the crust caused the formation of eclogite at the bottom of the lower crust. Due to gravity differences, the eclogite-facies lower crust is easier to have delamination; what's more, bimodal volcanic rocks appeared in Nyixung region indicates that in the early stage of Late Cretaceous, the north-central area of Lhasa block was in post-collision extension environment (^{Qu, Xin, Xu, Yang, & Li, 2006}). Hence, partial melting of crust-mantle material caused by delamination of lower crust might be a major reason for the formation of Ga'erqiong granite porphyry.

5.2 Geodynamic background of mineralization

There are still disputes about the geodynamic mechanism of the Cretaceous volcanic rock in north and middle part of Lhasa block. So far, the mainstream opinions include: it's generated from Neo-Tethys crust's northward subduction. It's related to re-melting of thickened (delaminated) lower crust during or after collision of the Qiangtang block and Lhasa block. It's related to southwards subduction of the Meso-Tethys oceanic crust and slab breaking off (^{Zhu et al., 2009}; ^{Harris, Inger, & Ronghua, 1990}; ^{Coulon, Maluski, & Bollinger, 1986}). In recent years, many Late Cretaceous magmatite are found in south part of the Lhasa block. These geological phenomena can't be explained by plate subduction model of the Neo-Tethys crust. Thus, quartz diorite and granite porphyry in Ga'erqiong can't be the product of Neo-Tethys crust's plate subduction. In the early stage ofthe Early Cretaceous, Neo-Tethys had occurred oceanic-ridge subduction (Zhu et al., 2009). Both quartz diorite and granite porphyry in Ga'erqiong have characteristics similar to those of island arc magma, and they are probably the product of Neo-Tethys' oceanic-ridge subduction. In the second place, Bangong Lake-Nujiang River Meso-Tethys had been closed at that time, and the magmatism of crust-mantle material caused by slab break-off during its southwards subduction also has the features of island arc magma. So, some scholars hold the opinion that Late Cretaceous magmatite in middle Gangdese is the common outcome of the tectonic evolution of Meso-Tethys and Neo-Tethys (^{Wang et al., 2013}). As Ga'erqiong is about 300km away from the Yarlung-Zangbo Suture Zone, it's still lack of enough evidence so far to prove that the formation of intermediate-acid rock in Ga'erqiong is related to oceanic-ridge subduction of Neo-Tethys in early stage of Late Cretaceous. On the contrary, Ga'erqiong is not so far from the Bangong Lake-Nujiang River Suture Zone. Therefore, we think the formation of intermediate-acid rock in Ga'erqiong is more likely to be the magmatic response of Meso-Tethys' tectonic evolution.

Judging from the geologic features, the formation of ore body I and II in Ga'erqiong Cu-Au Deposit is closely related to magmatic emplacement of quartz diorite. There has been testing data to support the result of the field inspection. Zircon U-Pb age of the quartz diorite is 88±2Ma, which is consistent with the Re-Os isotope age (86.89 Ma) of molybdenite in skarn (^{Li et al., 2011}). It indicates that 1Ma after the quartz diorite emplacement, the hydrothermal activity of magma formed the Cu, Au ore body. Zircon U-Pb age of the granite porphyry is 83±1Ma, which is significantly later than that of quartz diorite, and its mineralization is likely to be related to the subsequent mineralization of porphyry molybdenum (^{Yao et al., 2013}). Research suggests that, the Bangong Lake-Nujiang River Meso-Tethys ocean basin opened in Late Triassic, and after ephemeral expansion, it started subduction in Late Jurassic and resulted in formation of Shiquan River - Gêrzê Subduction Zone and Rutog Subduction Zone separately in the south and north, and cultivation of abundant island arc magmatite on both sides of the suture zones at the same time (^{Qu et al., 2009}). In the middle stage of Early Cretaceous (around 113Ma), the Lhasa block and Qiangtang block had a collision, and after that, they entered into the arc-arc (continent) collisional evolution stage. The Meso-Tethys oceanic crust subducted southwards underneath the Lhasa block. Slab break-off caused upwelling of mantle-derived material and further resulted in upwelling of mantle material, which had mantle-derived magma underplating in different scales on both sides of the suture zones (^{Zhu et al., 2009}). After that, crust in north and middle part of Lhasa block kept thickening under the compression of north-south stress and finished shift from intracontinental compression to a partial extension in the early stage of Late Cretaceous (^{Rahim & Usli, 2017}; ^{You & Rahim, 2017}). The thickened lower crust reached eclogite facies and started delamination, forming a series of magmation along with mineralization. It thus appears that the formation of Ga'erqiong Cu-Au Deposit is due to the metallogenic response of collision orogeny during the closing of Meso-Tethys.