Tectonomagmatic Setting and Cu-Ni Mineralization Potential of the Gayahedonggou Complex, Northern Qinghai–Tibetan Plateau, China

The Gayahedonggou magmatic Cu-Ni sulfide deposit was recently discovered in the East Kunlun orogenic belt (Northern Tibetan Plateau, China). The mineralization in this region is associated with mafic–ultramafic intrusions. To date, the formation age and metallogenic model of these ore-bearing intrusions have not been studied systematically. In this paper, the petrology, zircon U-Pb chronology, and geochemistry of ore-bearing wehrlite and quartz diorite are investigated. The results show that the zircon U-Pb isotopic age of wehrlite is 419.9 ± 1.5 Ma with an average εHf(t) value of 3.0, indicating that wehrlite originated from a depleted mantle or the asthenosphere. The (La/Yb)N, (La/Sm)N, (Gd/Yb)N, Nb/U, and Ce/Pb ratios of wehrlite are between 3.01–7.14, 1.69–3.91, 1.36–1.51, 2.07–2.93, and 0.55–1.42, respectively, indicating that the parent magma of the wehrlite had been contaminated by the upper crust. The zircon U-Pb isotopic age of quartz diorite is 410.2 ± 3.5 Ma with an average εHf(t) value of 8.0, and the A/CNK and A/NK ratio of quartz diorites ranges from 1.02 to 1.04 and from 2.13 to 2.23, respectively. These features are similar to those of the type I granite, and the quartz diorite was likely derived from the lower crust. Combined with the regional geological evolution, the Gayahedonggou complex formed in a post-collision extensional environment. The pyroxene in the Gayahedonggou complex is mainly clinopyroxene, which is enriched in the CaO content, indicating that the CaO content of the parent magma of the Gayahedonggou complex is high or that the complex has been contaminated by Ca-rich surrounding rocks, which hinders Cu-Ni mineralization.


Introduction
Magmatic Cu-Ni sulfide deposits are an important source of copper and nickel metal. Although most of the world's large magmatic Cu-Ni sulfide deposits are known to have formed in large igneous provinces [1][2][3], those that occur in orogenic environments also have considerable metallogenic potential [4][5][6][7][8][9][10][11][12][13]. The East Kunlun orogenic belt (EKOB), located in Northwest China (Figure 1), is considered one of the important Cu-Ni mineralization regions in all of Asia in which mafic-ultramafic intrusions are widely distributed. In the last 10 years, some magmatic Cu-Ni sulfide deposits (such as Xiarihamu, Shitoukengde, Akechukesai, and Binggounan) that formed in orogenic environments have been discovered, and the associated Ni resources reached a total of 1.

million
The Gayahedonggou Cu-Ni deposit was discovered by the Sichuan Nuclear Industry Geology Bureau in 2018. Preliminary geological studies suggest that the formation of the deposit is related to ultramafic intrusions at the end of the Early Paleozoic. Presently, the Ni resource reaches a total of 4 million tons, which may have the potential to be a large deposit. However, to date, no dating and isotope research has been performed on this deposit. Thus, geological samples of wehrlite (ore-bearing rock) and quartz diorite were collected from the Gayahedonggou magmatic Cu-Ni sulfide deposit. The zircon U-Pb isotopic ages of the magmatic intrusions and the petrochemical characteristics of rocks were investigated, and zircon Lu-Hf isotopic determinations were performed. The results provide new evidence for the metallogenic model of the Cu-Ni sulfide deposits in the EKOB.

Regional Geology
The EKOB belongs to the Tethys tectonic domain, which has an extremely complicated geological history [16][17][18][19]. The evolution of the Proto-Tethys began in the Precambrian and ended in the Devonian, during which the ocean basin was subducted in both directions, creating the basic tectonic framework of the EKOB [20,21]. The Paleo-Tethys evolved between the late Paleozoic and Mesozoic eras, and voluminous magmatic activity associated with the subducted oceanic crust has been recorded [22][23][24][25]. The evolution of the Neo-Tethys had little influence on the EKOB, and only a few magmatic rocks developed during this period. The north, middle, and south Kunlun faults divide the East Kunlun into the North East Kunlun orogenic belt, the Middle East Kunlun orogenic belt, and the South East Kunlun orogenic belt [26][27][28][29]; the secondary faults in the region generally develop towards the northeast (Figure 1). Regional stratigraphic outcropping is relatively complete The Gayahedonggou Cu-Ni deposit was discovered by the Sichuan Nuclear Industry Geology Bureau in 2018. Preliminary geological studies suggest that the formation of the deposit is related to ultramafic intrusions at the end of the Early Paleozoic. Presently, the Ni resource reaches a total of 4 million tons, which may have the potential to be a large deposit. However, to date, no dating and isotope research has been performed on this deposit. Thus, geological samples of wehrlite (ore-bearing rock) and quartz diorite were collected from the Gayahedonggou magmatic Cu-Ni sulfide deposit. The zircon U-Pb isotopic ages of the magmatic intrusions and the petrochemical characteristics of rocks were investigated, and zircon Lu-Hf isotopic determinations were performed. The results provide new evidence for the metallogenic model of the Cu-Ni sulfide deposits in the EKOB.

Regional Geology
The EKOB belongs to the Tethys tectonic domain, which has an extremely complicated geological history [16][17][18][19]. The evolution of the Proto-Tethys began in the Precambrian and ended in the Devonian, during which the ocean basin was subducted in both directions, creating the basic tectonic framework of the EKOB [20,21]. The Paleo-Tethys evolved between the late Paleozoic and Mesozoic eras, and voluminous magmatic activity associated with the subducted oceanic crust has been recorded [22][23][24][25]. The evolution of the Neo-Tethys had little influence on the EKOB, and only a few magmatic rocks developed during this period. The north, middle, and south Kunlun faults divide the East Kunlun into the North East Kunlun orogenic belt, the Middle East Kunlun orogenic belt, and the South East Kunlun orogenic belt [26][27][28][29]; the secondary faults in the region generally develop towards the northeast (Figure 1). Regional stratigraphic outcropping is relatively complete and includes the Paleoproterozoic Jinshuikou Group, the Mesoproterozoic Wanbaogou Group, the Cambrian-Ordovician Nachitai Group, the Qimantage Group, the Upper Devonian Maoniushan Formation, the Carboniferous Dagangou Formation, the Siyangjiao Formation, and the Upper Triassic Elashan Formation [30].
The mafic-ultramafic intrusions are mainly distributed north of the Middle Kunlun fault. The Xiarihamu deposit is the largest magmatic Cu-Ni-Co sulfide deposit in the region and the second-largest Ni deposit in China [4,5,12]. The Shitoukengde is a large-scale Cu-Ni deposit [9,16]. The ore-bearing lithologies in the belt are orthopyroxenite, websterite, harzburgite, lherzolite, and dunite, and the area is between 0.012 and 5.8 km 2 [4,5,9,13,16]. The surrounding rocks include the Jinshuikou Group and the Tanjianshan Group. Pentlandite, pyrrhotite, and chalcopyrite are the principal ore minerals in the intrusion-hosted sulfide mineralization.

Geology of the Gayahedonggou Cu-Ni Deposit
The Gayahedonggou Cu-Ni deposit is located in the Middle East Kunlun orogenic belt. The surrounding rock is the Paleoproterozoic Jinshuikou Group (Figure 2), which is the oldest metamorphic rock series in the EKOB and is composed of the Baishahe Formation and the Xiaomiao Formation [31,32]. The Baishahe Formation includes gneiss, amphibolite, and schist, and the Xiaomiao Formation is comprised of schist, gneiss, and marble. The Gayahedonggou complex consists of three mafic-ultramafic intrusions and quartz diorite. Mafic-ultramafic intrusions are comprised of wehrlite, olivine-bearing clinopyroxenite, and gabbro. The Cu-Ni ore is hosted in the wehrlite. Four Cu-Ni ore bodies have been identified. The ore bodies are 50-280 m in length and 9-34 m in thickness, with an average Ni grade of 0.415 wt%.  [30]. The mafic-ultramafic intrusions are mainly distributed north of the Middle Kunlun fault. The Xiarihamu deposit is the largest magmatic Cu-Ni-Co sulfide deposit in the region and the second-largest Ni deposit in China [4,5,12]. The Shitoukengde is a large-scale Cu-Ni deposit [9,16]. The ore-bearing lithologies in the belt are orthopyroxenite, websterite, harzburgite, lherzolite, and dunite, and the area is between 0.012 and 5.8 km 2 [4,5,9,13,16]. The surrounding rocks include the Jinshuikou Group and the Tanjianshan Group. Pentlandite, pyrrhotite, and chalcopyrite are the principal ore minerals in the intrusion-hosted sulfide mineralization.

Geology of the Gayahedonggou Cu-Ni Deposit
The Gayahedonggou Cu-Ni deposit is located in the Middle East Kunlun orogenic belt. The surrounding rock is the Paleoproterozoic Jinshuikou Group (Figure 2), which is the oldest metamorphic rock series in the EKOB and is composed of the Baishahe Formation and the Xiaomiao Formation [31,32]. The Baishahe Formation includes gneiss, amphibolite, and schist, and the Xiaomiao Formation is comprised of schist, gneiss, and marble. The Gayahedonggou complex consists of three mafic-ultramafic intrusions and quartz diorite. Mafic-ultramafic intrusions are comprised of wehrlite, olivine-bearing clinopyroxenite, and gabbro. The Cu-Ni ore is hosted in the wehrlite. Four Cu-Ni ore bodies have been identified. The ore bodies are 50-280 m in length and 9-34 m in thickness, with an average Ni grade of 0.415 wt%. The typical olivine-bearing clinopyroxenite contains 80% clinopyroxene and 20% olivine. The clinopyroxene and olivine crystals are 1-2 mm and 1-1.5 mm in diameter, respectively. Some pyroxenes have been altered to tremolite, and some olivine has been serpentinized ( Figure 3).

Zircon U-Pb and IN SITU Hf Isotope Analyses
Zircon grains were separated from bulk samples by conventional heavy-liquid and magnetic techniques and then purified by hand-picking under a binocular microscope. Representative zircon grains were mounted in epoxy resin and then polished so that the crystals were approximately sectioned in half at the Langfang Regional Geological Survey, Hebei Province, China. Zircon target and cathodoluminescence (CL) images were obtained at the State Key Laboratory of Continental Dynamics of Northwest University, Xi'an, China; zircon U-Pb determinations analysis was carried out by LA-ICP-MS at the Key Laboratory for the Study of Focused Magmatism and Giant Ore Deposits, Xi'an Center of Geological Survey, using a Geolas 2005 excimer ArF laser-ablation system coupled with an Agilent 7700 ICP-MS. Zircon isotopic determinations were obtained with a spot size of 32 μm. The detailed parameters and operating methods of the instrument have been published previously [33]. The content of common lead is low, so no correction for this parameter has been considered [34], and the zircon isotope ratio and age data were analyzed using the Glitter (ver 4.0) program for calculations and processing [35]. Isoplot 3.0 was used for age calculations and concordia plot drawings [36].

Zircon U-Pb and IN SITU Hf Isotope Analyses
Zircon grains were separated from bulk samples by conventional heavy-liquid and magnetic techniques and then purified by hand-picking under a binocular microscope. Representative zircon grains were mounted in epoxy resin and then polished so that the crystals were approximately sectioned in half at the Langfang Regional Geological Survey, Hebei Province, China. Zircon target and cathodoluminescence (CL) images were obtained at the State Key Laboratory of Continental Dynamics of Northwest University, Xi'an, China; zircon U-Pb determinations analysis was carried out by LA-ICP-MS at the Key Laboratory for the Study of Focused Magmatism and Giant Ore Deposits, Xi'an Center of Geological Survey, using a Geolas 2005 excimer ArF laser-ablation system coupled with an Agilent 7700 ICP-MS. Zircon isotopic determinations were obtained with a spot size of 32 µm. The detailed parameters and operating methods of the instrument have been published previously [33]. The content of common lead is low, so no correction for this parameter has been considered [34], and the zircon isotope ratio and age data were analyzed using the Glitter (ver 4.0) program for calculations and processing [35]. Isoplot 3.0 was used for age calculations and concordia plot drawings [36].
Hf isotopes were analyzed over a spot size of 44 µm, and instrumental conditions and data-acquisition procedures were similar to those described by [37]. All of the Hf analyses were carried out on the same spots that were used for U-Pb laser ablation. Zircon GJ-1 was used as the reference standard and yielded a weighted mean 176 Lu/ 177 Hf ratio of 0.282030 ± 40 (2SE) [33].

Major and Trace Element Compositions
Major and trace elements were analyzed at the Western Mineral Resources and Geological Engineering Key Laboratory of the Ministry of Education. The major elements were identified by using X-ray fluorescence spectroscopy (XRF), following the procedure of [38]. The analytical error is less than 1%. Trace elements were identified using a PQ2 Turbo ICP-MS following the technique of [39]. The precision was generally <5% for trace elements.

Zircon U-Pb Ages
Representative CL images of zircons from the Gayahedonggou samples are shown in Figure 4. The U-Pb dating results are listed in Table 1. The zircon Th/U ratio of wehrlite is between 0.1 and 0.74, with an average of 0.33 and that of quartz diorite is between 0.27 and 0.70, with an average of 0.51. All the grains exhibit concentric zoning, indicating a magmatic zircon feature. All analyses report concordant U-Pb ages within analytical errors (Figure 4), yielding a concordia age of 419.9 ± 0.8 Ma (mean standard weighted deviation(MSWD) = 0.2) and 410.2 ± 3.5 Ma for the wehrlite and quartz diorite, respectively. These results indicate that wehrlite and quartz diorite formed during the Later Silurian and Early Devonian, respectively. Hf isotopes were analyzed over a spot size of 44 μm, and instrumental conditions and data-acquisition procedures were similar to those described by [37]. All of the Hf analyses were carried out on the same spots that were used for U-Pb laser ablation. Zircon GJ-1 was used as the reference standard and yielded a weighted mean 176 Lu/ 177 Hf ratio of 0.282030 ± 40 (2SE) [33].

Major and Trace Element Compositions
Major and trace elements were analyzed at the Western Mineral Resources and Geological Engineering Key Laboratory of the Ministry of Education. The major elements were identified by using X-ray fluorescence spectroscopy (XRF), following the procedure of [38]. The analytical error is less than 1%. Trace elements were identified using a PQ2 Turbo ICP-MS following the technique of [39]. The precision was generally <5% for trace elements.

Zircon U-Pb Ages
Representative CL images of zircons from the Gayahedonggou samples are shown in Figure 4. The U-Pb dating results are listed in Table 1. The zircon Th/U ratio of wehrlite is between 0.1 and 0.74, with an average of 0.33 and that of quartz diorite is between 0.27 and 0.70, with an average of 0.51. All the grains exhibit concentric zoning, indicating a magmatic zircon feature. All analyses report concordant U-Pb ages within analytical errors (Figure 4), yielding a concordia age of 419.9 ± 0.8 Ma (mean standard weighted deviation(MSWD) = 0.2) and 410.2 ± 3.5 Ma for the wehrlite and quartz diorite, respectively. These results indicate that wehrlite and quartz diorite formed during the Later Silurian and Early Devonian, respectively.

Major and Trace Elements
The major and trace element compositions of eight samples from Gayahedonggou are listed in

Major and Trace Elements
The major and trace element compositions of eight samples from Gayahedonggou are listed in Chondrite-normalized rare-earth element (REE) patterns and primitive mantle-normalized spidergrams for wehrlite and quartz diorite are illustrated in Figure 5. Quartz diorite and wehrlite show nearly the same pattern. They are strongly enriched in large ion lithophile element(LILEs) (Rb, Th, and K) and depleted in Nb, Ta, and Ti. The total rare-earth element contents (ΣREE) of wehrlite are low, ranging from 14.79 to 22.39 ppm. The (La/Yb) N , (La/Sm) N , (Gd/Yb) N , Nb/U, and Ce/Pb ratios of wehrlite are 3.01-7.14, 1. 69 Chondrite-normalized rare-earth element (REE) patterns and primitive mantle-normalized spidergrams for wehrlite and quartz diorite are illustrated in Figure 5. Quartz diorite and wehrlite show nearly the same pattern. They are strongly enriched in large ion lithophile element(LILEs) (Rb, Th, and K) and depleted in Nb, Ta, and Ti. The total rare-earth element contents (ΣREE) of wehrlite are low, ranging from 14.

Zircon Lu-Hf Isotopes
The Lu-Hf isotopes of the selected zircon crystals from the wehrlite and quartz diorite are listed in Table 3. The εHf(t) values for wehrlite range from −0.8 to 4.2, with an average of 3.0. The two-stage model age (TDM2) of peridotite zircons ranges from 1.14 to 1.46 Ga, with an average of 1.24 Ga. One zircon grain from quartz diorite shows an εHf (t) value of −0.8, and the εHf(t) value of the other four grains from quartz diorite is between 5.1 and 9.9, with an average of 8.0. The two-stage model age (TDM2) of quartz diorite zircons ranges from 0.76 to 1.45 Ga, with an average of 1.01 Ga. In the εHf(t) vs. t(Ma) diagram, the data are plotted between the Hf isotopic evolution lines of chondrite and depleted mantle ( Figure 6).

Zircon Lu-Hf Isotopes
The Lu-Hf isotopes of the selected zircon crystals from the wehrlite and quartz diorite are listed in Table 3. The εHf(t) values for wehrlite range from −0.8 to 4.2, with an average of 3.0. The two-stage model age (TDM2) of peridotite zircons ranges from 1.14 to 1.46 Ga, with an average of 1.24 Ga. One zircon grain from quartz diorite shows an εHf (t) value of −0.8, and the εHf(t) value of the other four grains from quartz diorite is between 5.1 and 9.9, with an average of 8.0. The two-stage model age (TDM2) of quartz diorite zircons ranges from 0.76 to 1.45 Ga, with an average of 1.01 Ga. In the εHf(t) vs. t(Ma) diagram, the data are plotted between the Hf isotopic evolution lines of chondrite and depleted mantle ( Figure 6).

Zircon Lu-Hf Isotopes
The Lu-Hf isotopes of the selected zircon crystals from the wehrlite and quartz diorite are listed in Table 3. The εHf(t) values for wehrlite range from −0.8 to 4.2, with an average of 3.0. The two-stage model age (TDM2) of peridotite zircons ranges from 1.14 to 1.46 Ga, with an average of 1.24 Ga. One zircon grain from quartz diorite shows an εHf (t) value of −0.8, and the εHf(t) value of the other four grains from quartz diorite is between 5.1 and 9.9, with an average of 8.0. The two-stage model age (TDM2) of quartz diorite zircons ranges from 0.76 to 1.45 Ga, with an average of 1.01 Ga. In the εHf(t) vs. t(Ma) diagram, the data are plotted between the Hf isotopic evolution lines of chondrite and depleted mantle ( Figure 6).

Wehrlite Source and Crustal Contamination
The average εHf(t) value of the wehrlite from the Gayahedonggou complex is 3.0. Such high εHf(t) values indicate that the Gayahedonggou wehrlite is unlikely to have been derived from the continental lithospheric mantle, which is characterized by εHf(t) values of less than 0 [44]. Consequently, this study concludes that the Gayahedonggou wehrlite was likely derived from a depleted mantle or the asthenosphere. The ratios of elements with similar atomic numbers are not easily affected by the evolution of magmatic rocks; therefore, Th/Yb, Zr/Yb, Nb/U, Ce/Yb, etc., are useful proxies for evaluating the role of upper/lower crustal contamination vs. mantle source enrichment [45][46][47]. In Figure 7a,b, Zr is positively correlated with Th, and the ratios of Th/Yb and Nb/La show a weak positive correlation. Mid-Oceanic Ridge Basalts(MORB) and Oceanic-Island Basalt(OIB)have high, uniform Nb/U ratios of 47 ± 10 [48]. In contrast, the Nb/U ratio of the Gayahedonggou wehrlite is 2.51-2.06, which is similar to the mean values of the continental crust (~9.7) [49]. The average Ce/Pb ratio of the mantle is 25 ± 5, whereas that of the continental crust is <15 [50]. The Ce/Pb ratios of the Gayahedonggou intrusion range from 0.55 to 1.42, which differ from mantle values but are relatively similar to those of the continental crust, suggesting significant crustal contamination.

Wehrlite Source and Crustal Contamination
The average εHf(t) value of the wehrlite from the Gayahedonggou complex is 3.0. Such high εHf(t) values indicate that the Gayahedonggou wehrlite is unlikely to have been derived from the continental lithospheric mantle, which is characterized by εHf(t) values of less than 0 [44]. Consequently, this study concludes that the Gayahedonggou wehrlite was likely derived from a depleted mantle or the asthenosphere. The ratios of elements with similar atomic numbers are not easily affected by the evolution of magmatic rocks; therefore, Th/Yb, Zr/Yb, Nb/U, Ce/Yb, etc., are useful proxies for evaluating the role of upper/lower crustal contamination vs. mantle source enrichment [45][46][47]. In Figure 7a,b, Zr is positively correlated with Th, and the ratios of Th/Yb and Nb/La show a weak positive correlation. Mid-Oceanic Ridge Basalts(MORB) and Oceanic-Island Basalt(OIB)have high, uniform Nb/U ratios of 47 ± 10 [48]. In contrast, the Nb/U ratio of the Gayahedonggou wehrlite is 2.51-2.06, which is similar to the mean values of the continental crust (~9.7) [49]. The average Ce/Pb ratio of the mantle is 25 ± 5, whereas that of the continental crust is <15 [50]. The Ce/Pb ratios of the Gayahedonggou intrusion range from 0.55 to 1.42, which differ from mantle values but are relatively similar to those of the continental crust, suggesting significant crustal contamination. The values of (La/Nb)PM and (Th/Ta)PM of the upper and lower crusts are markedly different and can be used to identify crustal contamination materials [51]. In Figure 7c, which considers these parameters, the wehrlite data plot towards the upper crust field. In summary, the Gayahedonggou wehrlite originated from a depleted mantle, and the material was added to the original magma through upper-crustal contamination.

The Source of Quartz Diorite
Generally, there are four types of granite: S-, I-, A-, and M-type granite. A-type granite is generally anhydrous and is characterized by low CaO, Sr, Ti, P, and Ba contents and high Na2O + K2O, Rb, Zr, Ga, and SiO2 contents and Ga/Al ratios [52,53]. I-type granites commonly contain amphibole, apatite, and titanite, with (A/CNK) ratios < 1.1 and Na2O > K2O. Typical S-type granites often contain minerals such as muscovite, cordierite, and garnet, with A/CNK ratios > 1.1 [54]. The MgO and Na2O contents of quartz diorite are relatively high, and the A/CNK and A/NK ratios range from 1.02 to 1.04 and from 2.13 to 2.23, respectively. In the A/CNK-A/NK figure, the data for the quartz diorite samples are plotted within the I-type granite area (Figure 8a). Quartz diorite is composed of plagioclase, amphibole, quartz, and biotite. In Figure 8b, the quartz diorite samples fall in the peraluminous region and the tholeiite to calc-alkaline series region. The aforementioned The values of (La/Nb) PM and (Th/Ta) PM of the upper and lower crusts are markedly different and can be used to identify crustal contamination materials [51]. In Figure 7c, which considers these parameters, the wehrlite data plot towards the upper crust field. In summary, the Gayahedonggou wehrlite originated from a depleted mantle, and the material was added to the original magma through upper-crustal contamination.

The Source of Quartz Diorite
Generally, there are four types of granite: S-, I-, A-, and M-type granite. A-type granite is generally anhydrous and is characterized by low CaO, Sr, Ti, P, and Ba contents and high Na 2 O + K 2 O, Rb, Zr, Ga, and SiO 2 contents and Ga/Al ratios [52,53]. I-type granites commonly contain amphibole, apatite, and titanite, with (A/CNK) ratios < 1.1 and Na 2 O > K 2 O. Typical S-type granites often contain minerals such as muscovite, cordierite, and garnet, with A/CNK ratios > 1.1 [54]. The MgO and Na 2 O contents of quartz diorite are relatively high, and the A/CNK and A/NK ratios range from 1.02 to 1.04 and from 2.13 to 2.23, respectively. In the A/CNK-A/NK figure, the data for the quartz diorite samples are plotted within the I-type granite area (Figure 8a). Quartz diorite is composed of plagioclase, amphibole, quartz, and biotite. In Figure 8b, the quartz diorite samples fall in the peraluminous region and the tholeiite to calc-alkaline series region. The aforementioned features correspond to the I-type granite in the EKOB during the same period. Furthermore, a geological field investigation reveals that mafic microgranular enclaves (MMEs), which are considered to have originated from the mixing of lower-crust-and enriched mantle-derived magmas, exist in the quartz diorite samples [55,56]. The Rb/Sr values of quartz diorite are between 0.04 and 0.07, which are significantly lower than the crustal ratio (5.36~6.55) [40]. Moreover, the average εHf(t) of quartz diorite is 8.0; such high εHf(t) values generally reflect the typical characteristics of the mantle. This does not display the features expected of A-type granite based on the major elements; thus, it is unlikely that the quartz diorite was derived from the mantle. Therefore, the quartz diorite was likely derived from the lower crust, which is a mixture of the mantle and the crust magma region.
Minerals 2020, 6, x FOR PEER REVIEW 12 of 19 features correspond to the I-type granite in the EKOB during the same period. Furthermore, a geological field investigation reveals that mafic microgranular enclaves (MMEs), which are considered to have originated from the mixing of lower-crust-and enriched mantle-derived magmas, exist in the quartz diorite samples [55,56]. The Rb/Sr values of quartz diorite are between 0.04 and 0.07, which are significantly lower than the crustal ratio (5.36~6.55) [40]. Moreover, the average εHf(t) of quartz diorite is 8.0; such high εHf(t) values generally reflect the typical characteristics of the mantle. This does not display the features expected of A-type granite based on the major elements; thus, it is unlikely that the quartz diorite was derived from the mantle. Therefore, the quartz diorite was likely derived from the lower crust, which is a mixture of the mantle and the crust magma region.

Tectonic Setting
It has been confirmed that the EKOB entered the stage of Proto-Tethys evolution in the Cambrian [60][61][62]. Magmatic rocks with ages of 515-436 Ma generally have island arc characteristics, and the lithologies include diorite, andesite, and gabbro. After 428 Ma, A-type and peraluminous granites appeared in large quantities, and the nature of magmatic rocks changed from island arc to intracontinental [63,64]. Moreover, eclogite, which is formed under ultra-high pressure, also provides evidence to support these findings. The formation age of eclogite in the Xiarihamu is 437 ± 3.6 Ma [65], while that in the Wenquan is 451 ± 2 Ma [66]. This indicates that there was a deep subduction event in the EKOB at the end of the early Paleozoic; the collision was completed at approximately 428 Ma (Figure 9a), and the region entered the post-collision extension stage [5,60].
There are two views on the metallogenic model for the late Paleozoic magmatic Cu-Ni sulfide deposits in the EKOB. One theory is the subduction island arc environment metallogenic model [10,12,13], which posits that the genesis of ore-bearing intrusions was controlled by the Proto-Tethys subduction process. The deep subduction of oceanic crust resulted in the partial melting of the lithospheric mantle and Cu-Ni enrichment and mineralization [13]. However, the Fe 3+ /∑Fe ratio of Cr-spinel for the Alaskan-type ultramafic intrusions, which is in the arc setting, is >0.3; in contrast, this ratio for the ultramafic in the post-collision, or rift, setting is <0.3 [5]. The Fe 3+ /∑Fe ratio of Cr-spinel from the Xiarihamu giant Ni-Co deposit in the EKOB is <0.3 [5], indicating that it was not in the arc environment. The other theory is the post-collision extensional environment metallogenic model [4,5,14,15], which is supported by reginal tectonic evolution. The age of the wehrlite of the Gayahedonggou complex is 419.9 ± 0.8 Ma. A high εHf(t) value indicates that the Gayahedonggou complex was likely derived from a depleted mantle or the asthenospheric mantle. In the Th/Yb-Nb/Yb diagram (Figure 10a), wehrlite samples are located at the MORB-OIB evolution line and tend to approach the volcanic arc region, reflecting the influence of subduction components [67]. At the same time, the Th/Nb and Th/Yb ratios are relatively small, which suggests the presence of a large amount of subduction-related fluid in the source region. In the Nb/Zr vs.

Tectonic Setting
It has been confirmed that the EKOB entered the stage of Proto-Tethys evolution in the Cambrian [60][61][62]. Magmatic rocks with ages of 515-436 Ma generally have island arc characteristics, and the lithologies include diorite, andesite, and gabbro. After 428 Ma, A-type and peraluminous granites appeared in large quantities, and the nature of magmatic rocks changed from island arc to intracontinental [63,64]. Moreover, eclogite, which is formed under ultra-high pressure, also provides evidence to support these findings. The formation age of eclogite in the Xiarihamu is 437 ± 3.6 Ma [65], while that in the Wenquan is 451 ± 2 Ma [66]. This indicates that there was a deep subduction event in the EKOB at the end of the early Paleozoic; the collision was completed at approximately 428 Ma (Figure 9a), and the region entered the post-collision extension stage [5,60].
There are two views on the metallogenic model for the late Paleozoic magmatic Cu-Ni sulfide deposits in the EKOB. One theory is the subduction island arc environment metallogenic model [10,12,13], which posits that the genesis of ore-bearing intrusions was controlled by the Proto-Tethys subduction process. The deep subduction of oceanic crust resulted in the partial melting of the lithospheric mantle and Cu-Ni enrichment and mineralization [13]. However, the Fe 3+ / Fe ratio of Cr-spinel for the Alaskan-type ultramafic intrusions, which is in the arc setting, is >0.3; in contrast, this ratio for the ultramafic in the post-collision, or rift, setting is <0.3 [5]. The Fe 3+ / Fe ratio of Cr-spinel from the Xiarihamu giant Ni-Co deposit in the EKOB is <0.3 [5], indicating that it was not in the arc environment. The other theory is the post-collision extensional environment metallogenic model [4,5,14,15], which is supported by reginal tectonic evolution. The age of the wehrlite of the Gayahedonggou complex is 419.9 ± 0.8 Ma. A high εHf(t) value indicates that the Gayahedonggou complex was likely derived from a depleted mantle or the asthenospheric mantle. In the Th/Yb-Nb/Yb diagram (Figure 10a), wehrlite samples are located at the MORB-OIB evolution line and tend to approach the volcanic arc region, reflecting the influence of subduction components [67]. At the same time, the Th/Nb and Th/Yb ratios are relatively small, which suggests the presence of a large amount of subduction-related fluid in the source region. In the Nb/Zr vs. Th/Zr diagram, the trends of the samples are consistent with that of subduction-fluid metasomatism (Figure 10b). The addition of this subduction-related fluid resulted in the geochemical characteristics of island arc magmatic rocks in the source region, but this does not mean that these intrusions formed in the arc environment [18]. At the end of the late Paleozoic, the Proto-Tethys Ocean completed subduction at approximately 428 Ma, it entered the post-collision extension stage, and slab break-off occurred [5,60].
Th/Zr diagram, the trends of the samples are consistent with that of subduction-fluid metasomatism (Figure 10b). The addition of this subduction-related fluid resulted in the geochemical characteristics of island arc magmatic rocks in the source region, but this does not mean that these intrusions formed in the arc environment [18]. At the end of the late Paleozoic, the Proto-Tethys Ocean completed subduction at approximately 428 Ma, it entered the post-collision extension stage, and slab break-off occurred [5,60].   Figure 10a is modified from [47] and Figure 10b is modified from [68].
Some A-type granites with low CaO, MgO, and Sr contents in the EKOB are considered to have formed in the post-collision environment, such as the Nniantang A-type syenogranite (403 ± 2 Ma) [69]. To date, the latest Paleozoic A-type granite reported in the East Kunlun area is the Binggou granite with an age of 391 ± 3 Ma [70]. Thus, the post-collision extension setting appears to have lasted up to approximately 390 Ma. These mafic-ultramafic rocks and those of the Gayahedonggou Th/Zr diagram, the trends of the samples are consistent with that of subduction-fluid metasomatism (Figure 10b). The addition of this subduction-related fluid resulted in the geochemical characteristics of island arc magmatic rocks in the source region, but this does not mean that these intrusions formed in the arc environment [18]. At the end of the late Paleozoic, the Proto-Tethys Ocean completed subduction at approximately 428 Ma, it entered the post-collision extension stage, and slab break-off occurred [5,60].   Figure 10a is modified from [47] and Figure 10b is modified from [68].
Some A-type granites with low CaO, MgO, and Sr contents in the EKOB are considered to have formed in the post-collision environment, such as the Nniantang A-type syenogranite (403 ± 2 Ma) [69]. To date, the latest Paleozoic A-type granite reported in the East Kunlun area is the Binggou granite with an age of 391 ± 3 Ma [70]. Thus, the post-collision extension setting appears to have lasted up to approximately 390 Ma. These mafic-ultramafic rocks and those of the Gayahedonggou  Figure 10a is modified from [47] and Figure 10b is modified from [68].
Some A-type granites with low CaO, MgO, and Sr contents in the EKOB are considered to have formed in the post-collision environment, such as the Nniantang A-type syenogranite (403 ± 2 Ma) [69]. To date, the latest Paleozoic A-type granite reported in the East Kunlun area is the Binggou granite with an age of 391 ± 3 Ma [70]. Thus, the post-collision extension setting appears to have lasted up to approximately 390 Ma. These mafic-ultramafic rocks and those of the Gayahedonggou complex formed at 420 Ma and 410 Ma, respectively. Thus, it is reasonable to posit that the mafic-ultramafic rocks and quartz diorite in the Gayahedonggou complex formed in the post-collision extension setting (Figure 9b).

Cu-Ni Mineralization Potential
The Cu-Ni deposit in the EKOB formed between the late Early Paleozoic and the early Late Paleozoic. Some Cu-Ni deposits have been discovered in the eastern and western parts of the EKOB. However, no Cu-Ni deposits exist in the central area of the EKOB before the discovery of the Gayahedonggou deposit. Thus, the discovery of the Gayahedonggou deposit indicates that a Cu-Ni metallogenic belt spans 800 km in the EKOB. Liu et al. (2019a) summarized that the large Cu-Ni deposit in the EKOB shares the following characteristics: (1) the mafic-ultramafic complexes have a broader range of m/f values (m/f max − m/f min > 2.5) and higher m/f values (m/f max > 5.5); (2) the pyroxene in the intrusion is mainly orthopyroxene; and (3) the olivine and the clinopyroxene in mineralized intrusions contain low contents of FeO and CaO [17]. The highest m/f ratio is 3.75 in the Gayahedonggou complex, which is much lower than 5.5. Typically, only the m/f ratio of dunite in the EKOB could reach 5.5. This is consistent with the lack of dunite found in the Gayahedonggou deposit. This reflects the fact that the ultramafic magma in the Gayahedonggou complex is not much stronger than that in the large Cu-Ni deposits in the EKOB. The pyroxene in the Gayahedonggou complex is mainly clinopyroxene, which is enriched in the CaO content, indicating that the CaO content of the parent magma of the Gayahedonggou complex is high or that the complex has been contaminated by Ca-rich surrounding rocks. During the crustal contamination process, the crustal components that promote sulfide saturation were defined as "beneficial crustal contamination"; in contrast, the crustal components that hinder sulfide saturation were defined as "harmful crustal contamination" [71]. The introduction of calcite marble is regarded as typical "harmful crustal contamination", because it increases the CaO content of the magma, which hinders sulfide saturation [9,72]. High-temperature and high-pressure experiments also show that the contamination of CaCO 3 inhibits sulfide saturation of the mafic-ultramafic magma [73]. Meanwhile, the Gayahedonggou complex is characterized by a high CaO feature, which hinders Cu-Ni mineralization.

Conclusions
Important conclusions from this study are as follows: 1.
The zircon U-Pb ages of ore-bearing wehrlite and quartz diorite are 419.9 ± 1.5 and 410.2 ± 3.5 Ma, respectively. According to Lu-Hf isotope and geochemical analyses, wehrlite was likely derived from a depleted mantle or the asthenosphere, and the source region of quartz diorite is likely the lower crust.

2.
The parental magma of the wehrlite was modified by subduction-related fluids.

3.
The Gayahedonggou complex formed in a post-collision extensional environment.

4.
The pyroxene in the Gayahedonggou complex is mainly clinopyroxene, which is enriched in the CaO content, indicating that the CaO content of the parent magma of the Gayahedonggou complex is high or that the complex has been contaminated by Ca-rich surrounding rocks, which hinders Cu-Ni mineralization.