Using Whole Rock and Zircon Geochemistry to Assess Porphyry Copper Potential of the Tonggou Copper Deposit, Eastern Tianshan

: Eastern Tianshan hosts a number of porphyry Cu deposits. However, these mainly formed in the Jueluotage Belt, in the middle part of Eastern Tianshan. The Tonggou porphyry Cu mineralization is an exception to this, since it is located in the Bogda Orogenic Belt, north of Eastern Tianshan. We obtained new zircon U-Pb ages, whole-rock geochemical data, zircon Hf isotope data, and zircon trace element compositions. LA-ICP-MS zircon U-Pb dating indicates a crystallization age of 302.2–303.0 Ma for the Tonggou mineralized granodiorite (TMG), which suggests that the Tonggou porphyry Cu mineralization formed in the Late Carboniferous period. ε Hf (t) data (1.8–14.1) for TMG suggests it was sourced from juvenile crustal melts, mixed with some mantle materials. TMG displays low Σ REE, compatible elements (Ba, Sr, Zr, and Hf), Zr / Hf and Nb / Ta ratios, as well as clearly negative Eu anomalies in whole rocks analyses. In addition, TMG is enriched in P, Hf and Th / U ratios in zircon, and has lower crystallization temperatures (734 to 735 ◦ C) than the Daheyan barren granodiorite (DBG) (753 to 802 ◦ C). Whole rock and zircon geochemical analyses show that the TMG was formed by fractional crystallization to a greater extent than the DBG in the Bogda Orogenic Belt. Moreover, zircon grains of the TMG show high Ce 4 + / Ce 3 + ratios (159–286), which are consistent with related values from large porphyry deposits of the Central Asian Orogenic Belt (CAOB). High Ce 4 +/ Ce 3 + ratios reﬂect oxidizing magmas as a result of fractional crystallization, which indicates that the Tonggou deposit has potential to host a large porphyry Cu deposit. zircon U-Pb ages, whole-rock geochemical analyses, zircon Hf isotopes and trace element data for the Tonggou mineralized granodiorite (TMG) and Daheyan barren intrusions (DBG) of the Bogda Orogenic Belt. We use the to Cu mineralization at we examine and compare geochemistry


Introduction
Porphyry Cu systems provide~75% of global copper production and a set amount of co-or by-product Mo, Au, Pb and Zn [1,2]. Porphyry Cu is one of the most important metallogenic systems, and has attracted the attention of many economic geologists [3][4][5][6]. These systems always form in association with magmatism at subduction and collision setting [6][7][8]. The high water content and the oxidation state of magmas in these settings lead to mobilizing metals such as Cu partitioned into exsolved hydrothermal fluids and reprecipitated upon cooling to form porphyry Cu deposits [9,10].  [38]; (c) geological map of the mineralized district in the Tonggou-Miao'ergou area [57].

Ore Deposit
The Tonggou deposit is located 44 km southeast of Urumqi, and belongs to the Bogda Orogenic Belt (Figure 1c). Both porphyry Cu and vein Cu-Zn mineralization occurs at the Tonggou deposit. Porphyry Cu mineralization is associated with TMG that was emplaced into the Early Carboniferous Qijiaojng Formation. The disseminated and stockwork veins of Cu mineralization occur at the top or contact zone of TMG (Figure 2a,b). The porphyry Cu mineralization is accompanied by potassic and propylitic alteration (Figure 2c). The mineral assemblages of the ore bodies mainly include pyrite, magnetite, hematite, and chalcopyrite (Figure 2d-f). Hematite-magnetite intergrowths were found in Tonggou porphyry mineralization, which suggests that Tonggou deposit formed under oxidizing conditions. The aforementioned TMG was sampled for the present study.
Vein Cu-Zn mineralization occurs in the Qijiaojing Formation as veinlet, stratiform, lenticular, and vein-shaped ore bodies, the shape and size are fault-controlled (Figure 2g,h). Vein mineralization contains 36 economic ore bodies, which are currently sources of Cu and Zn [55]. Vein  [38]; (c) geological map of the mineralized district in the Tonggou-Miao'ergou area [57].

Ore Deposit
The Tonggou deposit is located 44 km southeast of Urumqi, and belongs to the Bogda Orogenic Belt (Figure 1c). Both porphyry Cu and vein Cu-Zn mineralization occurs at the Tonggou deposit. Porphyry Cu mineralization is associated with TMG that was emplaced into the Early Carboniferous Qijiaojng Formation. The disseminated and stockwork veins of Cu mineralization occur at the top or contact zone of TMG (Figure 2a,b). The porphyry Cu mineralization is accompanied by potassic and propylitic alteration (Figure 2c). The mineral assemblages of the ore bodies mainly include pyrite, magnetite, hematite, and chalcopyrite (Figure 2d-f). Hematite-magnetite intergrowths were found in Tonggou porphyry mineralization, which suggests that Tonggou deposit formed under oxidizing conditions. The aforementioned TMG was sampled for the present study. Cu-Zn ore is associated with propylitic alteration (Figure 2i). The mineral assemblages of the ore bodies mainly include pyrite, magnetite, chalcopyrite, and sphalerite (Figure 2j-l), with minor amounts of hematite, galena, and bornite.  Vein Cu-Zn mineralization occurs in the Qijiaojing Formation as veinlet, stratiform, lenticular, and vein-shaped ore bodies, the shape and size are fault-controlled (Figure 2g,h). Vein mineralization contains 36 economic ore bodies, which are currently sources of Cu and Zn [55]. Vein Cu-Zn ore is associated with propylitic alteration (Figure 2i). The mineral assemblages of the ore bodies mainly include pyrite, magnetite, chalcopyrite, and sphalerite (Figure 2j-l), with minor amounts of hematite, galena, and bornite.

Analytical Methods
Zircon U-Pb ages were determined for granodiorite sample from the Tonggou deposit. The samples were crushed, zircon grains were extracted, and cathodoluminescence (CL) images were generated at the Faith Geological Service Company, Langfang, Hebei province, China. U-Pb-Hf isotopic and trace element analyses of the zircons were performed at Wuhan Samplesolution Analytical Technology Co., Ltd., Wuhan, China. Zircon grains were mounted on epoxy blocks and polished to expose grain centers. CL images are shown in Figure 4. U-Pb dating, Hf isotopic and trace element analyses of zircons were obtained using an Agilent 7700e inductively coupled plasma (ICP) mass spectrometer (MS) and a GeolasPro laser ablation system. 206 Pb/ 238 U ratios were calculated by ICPMSDataCal [58,59], while concordia diagrams and weighted mean calculations were made using Isoplot 3.0 [60]. In order to better control analytical uncertainty and to assure instrument stability, an internal standard zircon GJ-1 (603 ± 3 Ma) and external standard zircon 91500 (1064 ± 4 Ma) were carried out for twice and once before and after every six analyses of unknown samples, respectively. Uncertainties are quoted at 1σ for individual U-Pb dating and trace element analyses and at 2σ (with 95% confidence level) for Hf isotopic analyses, respectively.

Analytical Methods
Zircon U-Pb ages were determined for granodiorite sample from the Tonggou deposit. The samples were crushed, zircon grains were extracted, and cathodoluminescence (CL) images were generated at the Faith Geological Service Company, Langfang, Hebei province, China. U-Pb-Hf isotopic and trace element analyses of the zircons were performed at Wuhan Samplesolution Analytical Technology Co., Ltd., Wuhan, China. Zircon grains were mounted on epoxy blocks and polished to expose grain centers. CL images are shown in Figure 4. U-Pb dating, Hf isotopic and trace element analyses of zircons were obtained using an Agilent 7700e inductively coupled plasma (ICP) mass spectrometer (MS) and a GeolasPro laser ablation system. 206 Pb/ 238 U ratios were calculated by ICPMSDataCal [58,59], while concordia diagrams and weighted mean calculations were made using Isoplot 3.0 [60]. In order to better control analytical uncertainty and to assure instrument stability, an internal standard zircon GJ-1 (603 ± 3 Ma) and external standard zircon 91500 (1064 ± 4 Ma) were carried out for twice and once before and after every six analyses of unknown samples, respectively. Uncertainties are quoted at 1σ for individual U-Pb dating and trace element analyses and at 2σ (with 95% confidence level) for Hf isotopic analyses, respectively.  The major and rare earth element compositions of four TMG samples were measured and analyzed at ALS Minerals-ALS Chemex, Guangzhou, China. The samples were crushed in a steel jaw crusher and then powdered to 200 mesh in an agate mill. Major-element compositions were analyzed using ME-XRF06 X. Trace element compositions were analyzed using ICP-AES and ICP-MS (ME-MS61 and ME-MS81), respectively. The analytical results for the Chinese standard GDW07104 indicated that the analytical precision was better than 5% for major elements and 10% for trace elements and REEs [61].

Zircon LA-ICP-MS U-Pb Ages
Zircons from the TMG exhibit oscillatory or planar zoning under CL and have Th/U ratios ranging from 0.39 to 2.00, consistent with a magmatic origin [62]. U-Pb dating results for 23 zircons, collected from TMG (sample TG1), are listed in Table 1. The 206 Pb/ 238 U ages can be divided into two sets. One group has 206 Pb/ 238 U ages ranging from 298 to 305 Ma, and ten analyses plots of Tera-Wasserburg concordia intercept ages at 303.0 ± 3.3 Ma (MSWD = 0.53; Figure 5a), with a mean of 302.2 ± 2.5 Ma (MSWD = 0.38; Figure 5b); this represents the crystallization age (302.2-303.0 Ma) of the TMG. In addition, other samples have 206 Pb/ 238 U ages ranging from 309 to 372 Ma, which represent the crystallization age of zircons captured or entrained from the wall rock during magma ascent. Moreover, the U-Pb age of the TMG is consistent with that of the DBG [51], and both are associated with Late Carboniferous magmatism. The major and rare earth element compositions of four TMG samples were measured and analyzed at ALS Minerals-ALS Chemex, Guangzhou, China. The samples were crushed in a steel jaw crusher and then powdered to 200 mesh in an agate mill. Major-element compositions were analyzed using ME-XRF06 X. Trace element compositions were analyzed using ICP-AES and ICP-MS (ME-MS61 and ME-MS81), respectively. The analytical results for the Chinese standard GDW07104 indicated that the analytical precision was better than 5% for major elements and 10% for trace elements and REEs [61].

Zircon LA-ICP-MS U-Pb Ages
Zircons from the TMG exhibit oscillatory or planar zoning under CL and have Th/U ratios ranging from 0.39 to 2.00, consistent with a magmatic origin [62]. U-Pb dating results for 23 zircons, collected from TMG (sample TG1), are listed in Table 1. The 206 Pb/ 238 U ages can be divided into two sets. One group has 206 Pb/ 238 U ages ranging from 298 to 305 Ma, and ten analyses plots of Tera-Wasserburg concordia intercept ages at 303.0 ± 3.3 Ma (MSWD = 0.53; Figure 5a), with a mean of 302.2 ± 2.5 Ma (MSWD = 0.38; Figure 5b); this represents the crystallization age (302.2-303.0 Ma) of the TMG. In addition, other samples have 206 Pb/ 238 U ages ranging from 309 to 372 Ma, which represent the crystallization age of zircons captured or entrained from the wall rock during magma ascent. Moreover, the U-Pb age of the TMG is consistent with that of the DBG [51], and both are associated with Late Carboniferous magmatism.    [63,64]; this is consistent with geological evidences of Tonggou porphyry Cu mineralization (Figure 3c). The Zr/TiO2 vs. Nb/Y content of most samples in the fields of granodiorite is shown on a discrimination diagram for the TMG (  Table 2). The primitive-mantle-normalized trace-element diagram indicates that the TMG is enriched in incompatible elements (e.g., Th, U, and Zr) and some LILEs (e.g., Sr), as well as depleted in HFSEs (e.g., Ta, Nb and Ti) (Figure 7b).   [63,64]; this is consistent with geological evidences of Tonggou porphyry Cu mineralization (Figure 3c). The Zr/TiO 2 vs. Nb/Y content of most samples in the fields of granodiorite is shown on a discrimination diagram for the TMG (Figure 6).   Table 2). The primitive-mantle-normalized trace-element diagram indicates that the TMG is enriched in incompatible elements (e.g., Th, U, and Zr) and some LILEs (e.g., Sr), as well as depleted in HFSEs (e.g., Ta, Nb and Ti) (Figure 7b).  [67] and [68], respectively. Partial data for Daheyan granodiorite cited from [51].

Hf Isotopes
In situ Hf isotopic compositions of zircon from the TMG are listed in

Hf Isotopes
In situ Hf isotopic compositions of zircon from the TMG are listed in Table 3. The nine analyses obtained from samples of the TMG show 176 Hf/ 177 Hf ratios of 0.2826-0.2830, and have ε Hf (t) values varying from +1.9 to +14.1 (with an average of 8.6). The corresponding T DM1 is ca. 375-870 Ma, whereas the T DM2 is ca.414-1189 Ma.

Trace Elements in Zircon
Minor and trace-element concentrations in zircon from TMG are listed in Table 4, and chondritenormalized REE patterns are shown in Figure 8. Zircon grains commonly contain minute inclusions of REE-bearing minerals (e.g., apatite, titanite, monazite, and allanite), which may yield higher LREE and lower Ce 4+ /Ce 3+ values than their true concentrations in zircon. For this reason, zircon grains for LA-ICP-MS analysis were carefully examined with CL to make sure that they contained no inclusions. In addition, the possible presence of such inclusions was monitored by measuring Sr, Th, Al, and P content during LA-ICP-MS analysis; when spikes of these elements were encountered, the analysis was discarded. In this study, we select valid data from zircon analyses which have very low P, Ca, Sr, and Al (Table 4). The calculation of Ce 4+/ Ce 3+ ratios requires an estimation of the concentration of Ce 3+ and Ce 4+ . The concentration of Ce 3+ is commonly based on La and Pr values with the assumption that the normalized values of La, Ce 3+ , and Pr plot along a smooth curve. However, the concentrations of La and Pr are very low in zircon, close to the detection limits of LA-ICP-MS (Table 3), we employed the calculation method proposed by [26], which used the concentrations of REE from Nd to Lu. The concentration of Ce 4+ is commonly based on the Hf and U values with the assumption that the normalized values of Hf, Ce 4+ , and U plot along a smooth curve as well [69]. Due to the fact that P contents of the TMG and DBG are low, <0.2 wt % (  (Table  4), respectively. In addition, since La and Pr are very low in zircon, the classic method to quantify a  Table 4).
The calculation of Ce 4+/ Ce 3+ ratios requires an estimation of the concentration of Ce 3+ and Ce 4+ . The concentration of Ce 3+ is commonly based on La and Pr values with the assumption that the normalized values of La, Ce 3+ , and Pr plot along a smooth curve. However, the concentrations of La and Pr are very low in zircon, close to the detection limits of LA-ICP-MS (Table 3), we employed the calculation method proposed by [26], which used the concentrations of REE from Nd to Lu. The concentration of Ce 4+ is commonly based on the Hf and U values with the assumption that the normalized values of Hf, Ce 4+ , and U plot along a smooth curve as well [69]. Due to the fact that P contents of the TMG and DBG are low, <0.2 wt% (Table 2, [51]), the effects of apatite and monazite on whole-rock compositions should be small. Thus, the compositions of whole-rock samples are assumed to represent those of the parental melt for calculation of Ce 4+/ Ce 3+ ratios. The calculated Ce 4+/ Ce 3+ ratios for Tonggou and Daheyan intrusions vary from 159 to 286, and from 72 to 138 (Table 4), respectively. In addition, since La and Pr are very low in zircon, the classic method to quantify a Ce anomaly incurs criticism. Thus, we employed the calculation method proposed by [25]; the calculated Ce/Ce* values for TMG and DBG vary from 227 to 234, and from 397 to 544 (Table 4), respectively.

Timing of Tonggou Porphyry Cu Mineralization
The LA-ICP-MS zircon U-Pb age for TMG varies from 302.2 to 303.0 Ma, which represents Tonggou porphyry mineralization. Thus, Tonggou porphyry Cu mineralized age is consistent with vein Cu-Zn mineralized age [55], the similarity in ages of porphyry and vein mineralization in the Tonggou deposit indicate that vein Cu polymetallic mineralization is closely associated with porphyry Cu mineralization in temporal and spatial terms. Furthermore, similar U-Pb ages have been reported for Daheyan intrusions in Bogda Orogenic Belt [43]. Whereas, the time of Tonggou porphyry Cu mineralization differs from porphyry Cu mineralization in the Jueluotage Belt (e.g., Tuwu, Yandong, and Fuxing deposits [35,70,71]). Thus, the Late Carboniferous period was a time of significant magmatism and porphyry Cu mineralization in Eastern Tianshan.

Source of Magmas and Petrogenesis
Zircon ε Hf (t) values from TMG range from 1.8 to 14.1 with a wide range of values (Table 4), indicating that the primary magma of the TMG was derived from multiple sources. On the ε Hf (t) vs. age diagram, all samples from TMG and DBG plot between the depleted mantle and CHUR reference line (Figure 9), suggesting that the TMG and DBG were mainly derived from crustal melting with some contribution from mantle-derived materials. Early Carboniferous-Devonian zircon xenocrysts in the TMG and DBG both show positive Hf (t) values, suggesting that Devonian juvenile crustal rocks possibly existed in the basement of the Bogda Orogenic Belt. Moreover, two-stage Hf model ages (T DM 2) of the TMG and DBG correspond to 427−872 Ma and 414−1189 Ma [51], respectively. These values are younger than those of basement rocks in the Cathaysia Block (1.8-2.2 Ga) [72], also indicating that these granodiorites were both mainly derived from a juvenile crustal source and some mixed mantle materials.
The TMG is characteristically enriched in SiO 2 and Sr, but the values of Y (19.8−21.0) and Yb (1.98−2.15) are obviously higher than the related values of adakitic rocks [73]. The Sr/Y vs. Y of samples plot in the fields of non-adakite on a discrimination diagram for the TMG (Figure 10a). However, the DBG is enriched in SiO 2 (≥68.14) and Al 2 O 3 (≥15.54) and depleted in Y (≤12.9) and Yb (≤1.64). The Sr/Y vs. Y of samples are plot in the fields of adakite on a discrimination diagram for the DBG (Figure 10a), which suggests that DBG is adakite. Generally, adakitic rocks have higher initial contents of Cu, which is essential for porphyry Cu mineralization [74,75]. Porphyry Cu mineralization has only been discovered in the top of TMG, it was not discovered in DBG. Thus, there are two possible reasons for the fact that porphyry Cu mineralization has not been discovered in DBG. 1) The porphyry Cu mineralization may have formed at the top of the Daheyan granodiorite, but was later eroded away, being consistent with the large area of DBG. 2) Although the DBG had a high initial Cu value, porphyry Cu mineralization may have not formed due to other factors (such as water content, fractional crystallization, and the magma oxidation state).

Fractional Crystallization of Magmas
Due to alteration of epidote, chlorite, and kaolinite, some major elements in samples from the TMG have suffered large compositional variations. However, fractional crystallization of granitic magma can be identified by its REE and trace elements compositions in whole rocks and zircon. Compared to the DBG, the TMG has lower ΣREE values, LREE/HREE ratios (Table 2), and clearly negative Eu anomalies (Table 2) in whole rocks. With decreasing ΣREE values, LREE/HREE ratios typically decrease (Figure 10b), indicating that fractional crystallization was more evident in the TMG [16,77,78]. Furthermore, TMG is more depleted in compatible elements (e.g., Ba ≤ 49.8 ppm, Sr ≤ 774 ppm, Zr ≤ 137 ppm, and Hf ≤ 3.7 ppm) compared to granodiorite from DBG in whole rocks (Figure 7; [51]), which indicates that TMG is of a more fractionated nature [16,79]. In addition, Zr/Hf and Nb/Ta ratios in whole rocks decrease significantly in the TMG (Table 4, Figure 10c), which is also regarded as a sign of fractional crystallization [15,80,81].
According to Zr/Hf ratios in zircon, granites can be classified into three types: common granites (Zr/Hf > 55), moderately evolved granites (25 < Zr/Hf < 55), and highly evolved granites (Zr/Hf < 25) [82]. Zircon Zr/Hf ratios from the TMG and DBG vary from 45.8 to 47.9 and 43.7 to 54.3 (Table 4), respectively. The TMG and DBG both belong to moderately evolved granites. Furthermore, highly fractionated granites are characteristically enriched in P and Hf in zircon [17,83,84], and the TMG has higher P and Hf values in zircon than DBG. On the Zr/Hf vs. P diagram for zircon, P values typically increase with decreasing Zr/Hf ratios (Figure 10d), and the TMG has a more obviously fractionated trend compared with Daheyan granodiorite. A more important issue is that the crystallization temperature of granites decreases with fractional crystallization [85]. We evaluated the crystallization temperatures of the TMG and DBG based on the Ti-in-zircon thermometer; the calculated temperatures for TMG and DBG vary from 734 to 735 • C and 753 to 802 • C, respectively [73]. With decreasing Zr/Hf ratios, crystallization temperatures typically decrease (Figure 10e), and the TMG becomes a more highly evolved unit (Figure 10e). Moreover, the zircon Th/U ratio may increase along with temperature due to fractionation (Figure 10f; [25]). TMG experienced a higher degree of fractional crystallization than the DBG. [82]. Zircon Zr/Hf ratios from the TMG and DBG vary from 45.8 to 47.9 and 43.7 to 54.3 (Table 4), respectively. The TMG and DBG both belong to moderately evolved granites. Furthermore, highly fractionated granites are characteristically enriched in P and Hf in zircon [17,83,84], and the TMG has higher P and Hf values in zircon than DBG. On the Zr/Hf vs. P diagram for zircon, P values typically increase with decreasing Zr/Hf ratios (Figure 10d), and the TMG has a more obviously fractionated trend compared with Daheyan granodiorite.
A more important issue is that the crystallization temperature of granites decreases with fractional crystallization [85]. We evaluated the crystallization temperatures of the TMG and DBG based on the Ti-in-zircon thermometer; the calculated temperatures for TMG and DBG vary from 734 to 735 °C and 753 to 802 °C, respectively [73]. With decreasing Zr/Hf ratios, crystallization temperatures typically decrease (Figure 10e), and the TMG becomes a more highly evolved unit (Figure 10e). Moreover, the zircon Th/U ratio may increase along with temperature due to fractionation (Figure 10f; [25]). TMG experienced a higher degree of fractional crystallization than the DBG. Th/U in zircon. Partial data for Daheyan granodiorite cited from [51]. Partial melting curves are calculated for accumulated fractional melting of lower crust, with starting values from [87]. ADR = island andesite-dacite-rhyolite.  [51]. Partial melting curves are calculated for accumulated fractional melting of lower crust, with starting values from [87]. ADR = island andesite-dacite-rhyolite.

Oxidation Conditions of Magmas
Eu and Ce anomalies in zircons have been proposed to reflect oxidation conditions in magmas that form porphyry deposits [23][24][25]. Zircons from TMG and DBG both have high Ce/Ce N * values ( Table 4), indicating that they both show an elevated oxidation state [25]. The high oxygen fugacity of Tonggou granodiorite is also verified by the widespread presence of magnetite and hematite.
However, Eu and Ce anomalies in zircons from TMG are lower than related values of DBG (Table 4), which indicates that DBG may form in a higher oxidation condition than TMG. In addition, Eu anomalies (Eu/Eu N * < 0.22) in zircon from TMG are similar to tonalite porphyry from Tuwu-Yandong deposit in Eastern Tianshan, these zircons of tonalite porphyry have low Eu/Eu N * ratios, <0.4 [30]. Low calculated Eu/Eu N * values in zircons from TMG and Tuwu-Yandong tonalite porphyry may be explained by removal Eu by plagioclase crystallization, resulting in larger negative Eu anomalies in zircon with time [30,88].

Implications for Porphyry Cu Mineralization
Ce 4+/ Ce 3+ ratios in zircons have been used to indicate the potential of porphyry Cu mineralization; zircons in ore-bearing intrusions always show high Ce 4+/ Ce 3+ ratios [26,30,[89][90][91]. Shen [30] considers that mineralized intrusions associated with the large-to-intermediate porphyry deposits have higher Ce 4+/ Ce 3+ ratios than 120 in zircons. There is therefore a correlation between the Ce 4+/ Ce 3+ ratio in zircon and the potential of an intrusion to produce a large porphyry deposit.
In this study, the calculated Ce 4+/ Ce 3+ ratios for TMG are similar to related values of large-to-intermediate porphyry deposits at the CAOB, indicating that TMG has a potential to form large-to-intermediate porphyry Cu deposits. To further evaluate the relationship between the Ce 4+/ Ce 3+ ratios and porphyry Cu mineralization, we also examined the Ce 4+/ Ce 3+ ratios in zircon from DBG. However, Ce 4+/ Ce 3+ ratios of DBG are lower than the related values of TMG and intermediate-to-large porphyry deposits at the CAOB. Zircon Ce 4+/ Ce 3+ ratios are affected by not only oxygen fugacity but also the melt compositions and the temperature at which zircon crystallized [30]. There is a possible reason for low Ce 4+/ Ce 3+ ratios in zircons from DBG samples: although DBG formed at a higher magmatic oxidation state than TMG, DBG has higher crystallization temperature, experienced a lower degree of fractional crystallization than TMG.
In addition, we plotted the Ce 4+/ Ce 3+ values of our samples on the Ce 4+/ Ce 3+ vs. 10 4 /T diagram for granitoids, TMG and DBG both plot between the magnetite-hematite (MH) buffer and Ni-NiO (NNO) buffer ( Figure 11). The oxygen fugacity of the magmas lower than the hematite-magnetite buffer, which is favorable for porphyry copper deposits since it can provide ferrous iron in the porphyry Cu system [75]. Moreover, sample of TMG plots in to corresponding range for large porphyry deposit, this suggests that the Tonggou deposit is likely to host a large porphyry Cu deposit.
Ce 4+/ Ce 3+ ratios in zircons have been used to indicate the potential of porphyry Cu mineralization; zircons in ore-bearing intrusions always show high Ce 4+/ Ce 3+ ratios [26,30,[89][90][91]. Shen [30] considers that mineralized intrusions associated with the large-to-intermediate porphyry deposits have higher Ce 4+/ Ce 3+ ratios than 120 in zircons. There is therefore a correlation between the Ce 4+/ Ce 3+ ratio in zircon and the potential of an intrusion to produce a large porphyry deposit.
In this study, the calculated Ce 4+/ Ce 3+ ratios for TMG are similar to related values of large-to-intermediate porphyry deposits at the CAOB, indicating that TMG has a potential to form large-to-intermediate porphyry Cu deposits. To further evaluate the relationship between the Ce 4+/ Ce 3+ ratios and porphyry Cu mineralization, we also examined the Ce 4+/ Ce 3+ ratios in zircon from DBG. However, Ce 4+/ Ce 3+ ratios of DBG are lower than the related values of TMG and intermediateto-large porphyry deposits at the CAOB. Zircon Ce 4+/ Ce 3+ ratios are affected by not only oxygen fugacity but also the melt compositions and the temperature at which zircon crystallized [30]. There is a possible reason for low Ce 4+/ Ce 3+ ratios in zircons from DBG samples: although DBG formed at a higher magmatic oxidation state than TMG, DBG has higher crystallization temperature, experienced a lower degree of fractional crystallization than TMG.
In addition, we plotted the Ce 4+/ Ce 3+ values of our samples on the Ce 4+/ Ce 3+ vs 10 4 /T diagram for granitoids, TMG and DBG both plot between the magnetite-hematite (MH) buffer and Ni-NiO (NNO) buffer ( Figure 11). The oxygen fugacity of the magmas lower than the hematite-magnetite buffer, which is favorable for porphyry copper deposits since it can provide ferrous iron in the porphyry Cu system [76]. Moreover, sample of TMG plots in to corresponding range for large porphyry deposit, this suggests that the Tonggou deposit is likely to host a large porphyry Cu deposit. Figure 11. (a) Average Ce 4+ /Ce 3+ vs. 10 4 /T for porphyry Cu deposits of the CAOB; (b) amplifying average Ce 4+ /Ce 3+ vs. 10 4 /T diagram. Temperatures were calculated using the Ti-in-zircon thermometer [73]. Partial data for porphyry Cu deposits of the CAOB cited from [30]. Curves for Figure 11. (a) Average Ce 4+ /Ce 3+ vs. 10 4 /T for porphyry Cu deposits of the CAOB; (b) amplifying average Ce 4+ /Ce 3+ vs. 10 4 /T diagram. Temperatures were calculated using the Ti-in-zircon thermometer [73]. Partial data for porphyry Cu deposits of the CAOB cited from [30]. Curves for oxygen fugacity buffers include magnetite-hematite (MH), Ni-Ni oxide (NNO), and Fe-wustite (IW) buffers.

Conclusions
(1) The zircon U-Pb age of the Tonggou granodiorite is 302.2 ± 2.5 Ma, which indicates that porphyry Cu mineralization of the Tonggou deposit was associated with Late Carboniferous magmatism.
(3) Ce/Ce N * values (>227) in zircons from TMG show a high magmatic oxidation state, whereas, they are lower than related values (397-544) of DBG. Low Eu/Eu N * (<2.2) in zircon grains of TMG can be explained by removal Eu by plagioclase crystallization.
(4) Ce 4+/ Ce 3+ ratios (159-286) for the TMG approach those of large-to-intermediate porphyry deposits at the CAOB. The high Ce 4+/ Ce 3+ ratios reflect oxidizing magmas produced by fractional crystallization, indicating that TMG has the potential to form a large porphyry Cu deposit.