Ore Genesis and the Magmatism of the Yuhaixi Mo(Cu) Deposit in Eastern Tianshan, NW China: Constraints from Geology, Geochemistry, Zircon U-Pb and Molybdenite Re-Os Dating

: The Yuhaixi Mo(Cu) deposit is a new discovery in the eastern section of the Dananhu-Tousuquan island arc, Eastern Tianshan. However, the genesis of the Yuhaixi Mo(Cu) deposit is still not fully understood. The Yuhaixi intrusion is composed of monzonitic granites, diorites, granites, and gabbro dikes, among which disseminated or veinlet Mo and Cu mineralization is mainly hosted by the monzonitic granites. The LA-ICP-MS zircon U-Pb dating yields emplacement ages of 359.4 ± 1.6 Ma for the monzonitic granite, 298.8 ± 1.8 Ma for the diorite, and 307.0 ± 2.3 Ma for the granite. The Re-Os dating of molybdenite hosted by monzonitic granite yields a well-constrained 187 Re-187 Os isochron age of 354.1 ± 6.8 Ma (MSWD = 1.7) with a weighted average age of 344.5 ± 3.1 Ma. The Mo mineralization is closely associated with the Yuhaixi monzonitic granite. The Yuhaixi monzonitic granite rocks are characterized by high silica (SiO 2 > 70 wt.%), low MgO (0.23–0.36), Ni, Cr contents, and they are enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs: e.g., K, Ba, Pb and Sr), and depleted in heavy rare earth elements (HREEs) and high ﬁeld-strength elements (HFSEs: e.g., Nb, Ta and Ti). They are weak peraluminous and have high ε Hf(t) (11.37–17.59) and ε Nd(t) (1.36–7.75) values, and varied initial 87 Sr/ 86 Sr (0.7037–0.7128) values. The Yuhaixi post-ore granites exhibit similar geochemical and isotopic signatures to the Yuhaixi monzonitic granite. These characteristics suggest that the Yuhaixi felsic rocks are likely sourced from the partial melting of the juvenile lower crust. The Yuhaixi diorite has low SiO 2 , and K 2 O contents, relatively high Na 2 O, MgO (Mg # = 45–53) contents, and depletions in HFSE (e.g., Nb, Ta, and Ti). These geochemical features, coupled with isotopic data such as low initial 87 Sr/ 86 Sr ( ≤ 0.7043), high ε Nd(t) (2.5 to 3.0) and ε Hf(t) ( ≥ 11.6) values, and young Hf model ages, suggest that their parental magmas possibly originated from the partial melting of the depleted lithospheric mantle that was metasomatized by hydrous melts or ﬂuids from the subducting oceanic plate. Integrating our new results with previous works on the Dananhu-Tousuquan island arc belt, we suggest that the Yuhaixi Mo(Cu)deposit is likely sourced from the juvenile lower crust, which was formed in an arc setting, where the bipolar subduction of the North Tianshan oceanic slab forms the Dananhu Tousuquan belt to the north and the Aqishan-Yamansu belt to the south. The eastern section of the Dananhu-Tousuquan island arc is a promising target for late Paleozoic porphyry Mo(Cu) deposits.


Ore Deposit Geology
The Yuhaixi Mo(Cu) deposit is located in the eastern section of the Dananhu-Tousuquan arc belt, approximately 110 km southeast of Hami City, Xinjiang (Figure 1b).The main lithostratigraphic units in the area include granulite, hornblende schist of the
The zircon grains were separated by conventional density and magnetic techniques and then carefully hand-picked under a binocular microscope.Subsequently, they were mounted on epoxy resin discs and polished to expose the crystal cross-sections at the Langfang Regional Geological Survey.The selection of potential target sites for the U-Pb dating of all the mounted zircons were based on photomicrographs of both transmitted and reflected light, as well as cathodoluminescence (CL) images.Zircon U-Pb dating and trace element analyses were conducted using an Agilent 7500 a inductively coupled plasma mass spectrometer (ICP-MS) simultaneously coupled with a GeoLas 2005 at the Tianjin Institute of Geology and Mineral Resources.The analytical procedures were described in [38].Laser ablation was operated at a constant energy of 60 mJ, with a repetition rate of 4 Hz and a spot diameter of 32 µm.Zircons 91500 and GJ-1, NIST SRM 610 [30] were used as external standards.Zircon 91500 was analyzed twice for every six analyses in order to calibrate the isotope fractionation.NIST SRM 610 was analyzed once every eight analyses to make the instrumental drift and mass discrimination correction of the trace element analysis.Individual errors in analyses were cited at the 1σ level, and the weighted mean 206 Pb/ 238 U ages were quoted at the 95% confidence level.The ICPMSDataCal software GeoLas 2005 were used for the adjustment of background and ablation signals, time drift correction [38].Concordia diagrams and weighted mean calculations were determined using Isoplot 3.0 [39].Zircon Ce anomalies were calculated using the method of the lattice strain model [17].

Zircon Hf Isotopes
In situ Hf isotope analyses were conducted using a Neptune MC-ICP-MS and New Wave UP 213 ultraviolet LA-MC-ICP-MS at the National Research Center for Geoanalysis, Beijing, China.The analysis was undertaken on the adjacent spots used for the LA-ICP-MS zircon U-Pb dating to match the Hf isotope data with the U-Pb ages.The ablated samples wrapped in helium were transported from the laser ablation chamber to the ICP-MS torch via a mixing chamber of Argon.Based on the zircon size, the stationary beam spot size was set to either 40 or 50 µm.During testing, GJ-1 international standard [30] zircon samples were used as a reference for the instrumental mass bias correction.The weighted average of the 176 Hf/ 177 Hf of the GJ-1 zircon samples was 0.281017 ± 0.000007 (2 SD, n = 10), which is consistent with the values reported by [18].More detailed operating conditions for the MC-ICP-MS instrument, the laser ablation system, and the analytical method are given in [39,40].

Whole-Rock Major and Trace Elements
Whole-rock major and trace elements analyses were performed at the National Research Center for Geoanalysis, Beijing, China.The samples were powdered to approximately 200 mesh before testing.The major elements were determined using a Philips PW 2404 X-ray fluorescence (XRF) spectrometer with a rhodium X-ray source.The trace elements and the rare earth elements were determined using an Element-I plasma mass spectrometer (Finnigan-MAT Ltd., Bremen, German); two national geological standard reference samples, GSR-3 and GSR-15, were used for the analytical quality control purpose.The analytical precision for the major elements is better than 1% and for the trace elements is better than 5%, and the analytical procedures were described by [19].

Whole-Rock Sr-Nd Isotopes
Sr-Nd isotopic analyses were carried out using a Neptune multi-collector ICP-MS at the National Research Center for Geoanalysis, Beijing, China, using analytical procedures described by [40].The Separation of the Sr and REE were performed using cation columns, and the Nd fractions were further separated using HDEHP-coated Kef columns.The measured 87 Sr/ 86 Sr and 143 Nd/ 144 Nd ratios were normalized to 86 Sr/ 88 Sr = 0.1193 and 146 Nd/ 144 Nd = 0.7217, respectively.The reported 87 Sr/ 86 Sr and 143 Nd/ 144 Nd ratios were adjusted to the NBS SRM 987 standard 87 Sr/ 86 Sr = 0.71027 and the Shin Etsu JNdi-1 standard 143 Nd/ 144 Nd = 0.512116 [11], respectively.

Re-Os Isotopic Analyses
Nine molybdenite samples were collected from the Yuhaixi porphyry Mo(Cu) deposits for Re-Os isotope analyses.Among them, five molybdenite samples came from drill holes ZK3601.Four molybdenite samples came from drill hole ZK3201.The photographs and photomicrographs of the representative samples are shown in Figure 4. Molybdenite occurs as disseminations in monzonitic granite or quartz veinlets and molybdenite is cogenetic with chalcopyrite (Figure 4H).The molybdenite was magnetically separated and then handpicked under a binocular microscope at the Langfang Regional Geological Survey.Fresh, nonoxidized molybdenite powders (<0.1 mm in size and purity >99%) were used for Re-Os isotope analyses. 187Re and 187 Os concentrations of molybdenite were measured using a TJA PQ ExCell ICP-MS at the Re-Os Laboratory of National Research Center of Geoanalysis, Chinese Academy of Geological Sciences, Beijing.The chemical separation of the Re and Os and the analytical procedure were in accordance with [18].The weighed molybdenite samples were loaded into a Carius tube through a thin-neck long funnel.The mixed 190 Os and 185 Re spike solutions and 2 mL HCl and 4 mL HNO 4 were loaded, while the bottom part of the tube was frozen at −50 • C to −80 • C in an ethanol-liquid nitrogen slush, and the top was sealed using an oxygen-propane torch.The tube was then placed in a stainless-steel jacket and heated for 24 h at 230 • C.After 24 h and upon cooling, the sample-bearing tubes were opened to transfer the supernatants out and the Os was separated using the method of direct distillation from the Carius tube for 50 min and trapped in 2 mL of water that was used for the ICP-MS (X-Series) determination of the Os isotope ratio.After that, the residual Re-bearing solution was saved in a 150 mL Teflon beaker for Re separation.The residual Re-bearing solution was heated to near-dryness.The acetone phase was transferred to a 150 mL Teflon beaker that contained 1 mL of water.Finally, the solution was picked up in 2% HNO 3 , which was used for the ICP-MS(X-Series) determination of the Re isotope ratio.
The average blanks for the method were ca. 3 pg Re and ca.0.5 pg Os.The working conditions of the instrument were controlled by the reference material JDC, which produced a measured value of 139.8 ± 2.0 Ma, which is consistent with the recommended value of 139.6 ± 3.8 Ma [41].The analytical uncertainty in the Re-Os model ages includes 1.02% uncertainty (at 95% confidence level) for the 187 Re decay constant.The Re-Os model ages were calculated following the equation: t = [ln(1 + 187 Os/ 187 Re)]/λ, where λ is the decay constant of 187 Re (λ 187 Re = 1.666 × 10 −11 year −1 ; [41]) and denotes the age.The Re-Os isochron ages were calculated using the least-squares method [42], employing the program ISOPLOT 3.0 [23].

Zircon Hf Isotopes
Zircon Hf isotope analysis results are listed in

Zircon Hf Isotopes
Zircon Hf isotope analysis results are listed in Table 4 4).

Re-Os Isotopic Ages
The Re-Os isotopic data for nine molybdenite samples from the Yuhaixi porphyry Mo deposit are listed in Table 6 and are plotted in an isochron diagram in Figure 10.The concentrations of 187 Re and 187 Os ranged from 93.9 to 322.9 µg/g and from 539.5 to 1889.2 ng/g, respectively.Nine samples yielded model ages ranging from 337.9 ± 6.7 to 350.2 ± 6.1 Ma and a well-constrained 187 Re-187 Os isochron age of 354.1 ± 6.8 Ma, with MSWD = 1.7 and an initial 187 Os of 24 ± 17ng/g (Figure 10A).The data also yields a weighted average age of 344.5 ± 3.1 Ma (MSWD = 2.1) (Figure 10B).These ages are concordant within the errors, indicating that the Yuhaixi Mo(Cu) deposit was formed in the Carboniferous.

Re-Os Isotopic Ages
The Re-Os isotopic data for nine molybdenite samples from the Yuhaixi porphyry Mo deposit are listed in Table 6 and are plotted in an isochron diagram in Figure 10.The concentrations of 187 Re and 187 Os ranged from 93.9 to 322.9 µg/g and from 539.5 to 1889.2 ng/g, respectively.Nine samples yielded model ages ranging from 337.9 ± 6.7 to 350.2 ± 6.1 Ma and a well-constrained 187 Re-187 Os isochron age of 354.1 ± 6.8 Ma, with MSWD = 1.7 and an initial 187 Os of 24 ± 17ng/g (Figure 10A).The data also yields a weighted average age of 344.5 ± 3.1 Ma (MSWD = 2.1) (Figure 10B).These ages are concordant within the errors, indicating that the Yuhaixi Mo(Cu) deposit was formed in the Carboniferous.Multiple magmatic activities were documented in the eastern segment of the Dananhu-Tousuquan island arc belt [25][26][27][28][29].Our study reveals that at least two stages (~359 Ma and ~307-299 Ma) of magmatic activities occurred in the Yuhaixi area.Zircon U-Pb dating showed that Yuhaixi monzonitic granite, granite, and diorite were emplaced at ~359 Ma, ~307 Ma, and ~299 Ma, respectively.However, other intrusive rocks near the Yuhaixi orefield were reported to form at ~443-430 Ma, earlier than that of Yuhaixi intrusive rocks.For example, Wang et al. (2016) obtained the age of the rocks in the Yuhai Mo(Cu) deposit to be 441.6 ± 2.5 Ma, 430.3 ± 2.6 Ma for the diorite and granodiorite, respectively [5]; Wang et al. (2015) obtained the age of the Sanchakou pluton to be 443 Ma; Wang et al. (2016a) obtained the age of the felsic intrusion in the Sanchakou mining area to be 440-426 Ma [30]; and Wang et al. (2018) obtained the age of the Yuhai quartz diorite to be 443.5 ± 4.1 Ma [12].Overall, at least three stages of magmatic activities were identified and recorded in the eastern segment of the Dananhu-Tousuquan island arc belt, namely ~430-443 Ma, ~359 Ma, and ~307-299 Ma.
The molybdenite Re-Os dating shows that the Yuhaixi Mo(Cu) deposit was formed at 354 ± 6.8 Ma (Figure 10), which is approximately coeval with the emplaced ages of the Yuhaixi monzonitic granite (359.4 ± 1.6 Ma).Previous studies have shown that the Yuhai molybdenite age [5] and the Sanchakou molybdenite age are concentrated in 370-350 Ma (Figure 11 [30]), which are consistent with the molybdenite Re-Os age of Yuhaixi Mo(Cu) deposit.So, the Yuhaixi Mo(Cu) deposit is suggested to form at ~354-360 Ma.The Yuhaixi granite and diorite rocks are post-ore intrusive plutons.Field investigations revealed that Yuhaixi Mo(Cu) deposits are characterized by disseminated or veinlet ores, and Mo mineralization mainly occurs in the potassic alteration zone (Figure 4I).It is highly likely that the monzonitic granite contributed to the generation of the Yuhaixi deposit.

Petrogenesis
The Yuhaixi monzonitic granite is characterized by high SiO 2 and K 2 O + Na 2 O contents, low Al 2 O 3 content, and the depletion of aluminum-rich minerals (e.g., muscovite, tourmaline, and garnet).They have a weak peraluminous character (1 < A/CNK < 1.1), and they exhibit low Sr (<188 ppm) and Y contents with relatively low Sr/Y ratios (<25).These features suggest that the Yuhaixi intrusion can be classified as the I-or A-type granite [21].In addition, the Yuhaixi monzonitic granite shows low FeOt/(FeOt + MgO) ratios, Zr content, and 10 4 × Ga/Al ratios (2.1-2.4),excluding the possibility of A-type granite (Figure 8d).Therefore, the Yuhaixi monzonitic granite likely belongs to I-type granites [59].The Yuhaixi post-ore granite is a high silica granite and exhibits similar geochemical signatures to the causative monzonitic granite.The Yuhaixi post-ore granite also has relatively low A/CNK (<1.1) and Ga/Al ratios, which is consistent with the features of I-type granites.
Generally, the Mg # values of magmatic rocks formed by the partial melting of the basaltic lower crust are less than 40 regardless of the degree of melting [43,44,60].If mantle material was involved in the origin of ore-forming felsic rocks, Mg # values of these rocks will be higher than 40 [60].Thus, the Mg # values can be used as an important indicator to track the addition of mantle-derived magma.Overall, the Yuhaixi felsic rocks both show high silica and low MgO, Cr, Ni contents, which is in favor of the crust origin.They both are enriched in LREE and LILE and depleted in HFSE elements (e.g., Nb, Ta) in the primitive mantle-normalized diagram (Figure 9).The Yuhaixi mozonitic granite samples have significant variable 87 Sr/ 86 Sr ratios, ranging from 0.7041-0.7127,and high εHf(t) (11.37-17.59)and εNd(t) (1.36-3.4)values, suggesting that it may have formed by melting the juvenile crust with the involvement of crustal components [43][44][45]60].In addition, the Yuhaixi monzonitic granite is high silica granite and is lithologically homogeneous, arguing against the significant upper crustal contamination and fractional crystallization of basaltic magmas [40].The studied monzonitic granite is characterized by high K 2 O (3.8-5.0 wt.%) and K 2 O/Na 2 O (1.1-1.6)ratios and shows relatively low Rb/Sr (0.34-0.47) and high K/Rb (396-615) ratios without obvious Eu-negative anomaly, indicating that it is less evolved than high-K granites.Thus, we proposed that Yuhaixi monzonitic granite likely originated from the partial melting of the juvenile lower crust with the involvement of hydrous melts or fluids sourced from continental crustal components (e.g., subducting sediments).However, Yuhaixi post-ore granite shows high Na 2 O (4.5-4.6 wt.%) contents and low K 2 O/Na 2 O (~0.6) ratios and it has low initial 87 Sr/ 86 Sr ratios (0.7042-0.7043) but high εHf(t) (11.7-14.8)and εNd(t) (4.6-6.4)values, which suggests that Yuhaixi granite likely originated from the partial melting of the juvenile crust with the addition of ocean crustal components (e.g., subducting oceanic slab) [45].

Implication for Tectonic Evolution and Porphyry Mineralization in Eastern Tianshan
The tectonic evolution of the Dananhu-Tousuquan arc belt has been widely addressed in previous studies [5,12,[27][28][29], and a growing number of Paleozoic arc-related magmatic rocks have been reported in the Dananhu-Tousuquan island arc belt [11][12][13][14][15][16][25][26][27][28][29][30].As reflected by the tectonic discrimination diagrams (Figure 13), Yuhaixi intrusive rocks including felsic and In a word, the Yuhaixi felsic intrusive rocks are high silica (SiO 2 > 70 wt.%)and are characterized by high total alkali and low MgO, Cr, Ni contents and high εNd(t) and εHf(t) values.These characteristics suggest that their melts are mainly derived from the partial melting of the juvenile lower crust.However, the post-ore diorite has a feature of high Al 2 O 3 , Na 2 O contents with depleted isotopic signatures, indicating that diorite melts likely originated from the partial melting of depleted lithospheric mantle metasomatized by fluids or hydrous melts from the subducting slabs [5,9,12,21,23].

Conclusions
(1) Zircon LA-ICP-MS U-Pb dating suggests that the emplaced ages of the Yuhaixi monzonitic granite, diorite, and granite are 359.4 ± 1.6 Ma, 298.8 ± 1.8 Ma, and 307.0 ± 2.3 Ma, respectively.(2) The Re-Os dating of molybdenite hosted by Yuhaixi monzonitic granite yields a well-constrained 187 Re-187 Os isochron age of 354.1 ± 6.8 Ma (MSWD = 1.7).(3) Whole-rock geochemical characteristics and Sr-Nd-Hf isotopic compositions indicate that the Yuhaixi monzonitic granite and granite were formed via the partial melting of the juvenile crust.The post-ore diorite was formed via the partial melting of the metasomatized mantle wedge.(4) The eastern section of the Dananhu-Tousuquan island arc is a promising target for late Paleozoic porphyry Mo(Cu) deposits.

Figure 2 .
Figure 2. (A) Geological map of the Yuhaixi region showing the distribution of Cu and Cu-Ni deposits.(B) Simplified geological map of the Yuhaixi Mo(Cu) deposit (modified from [30]).

Figure 3 .
Figure 3. Geologic cross section of the ZK3601 and ZK3201 exploration lines across the Yuhaixi porphyry Mo deposit with lithology and mineralization (A) and alteration (B).

Figure 2 .
Figure 2. (A) Geological map of the Yuhaixi region showing the distribution of Cu and Cu-Ni deposits.(B) Simplified geological map of the Yuhaixi Mo(Cu) deposit (modified from [30]).

3 , 26 Figure 2 .
Figure 2. (A) Geological map of the Yuhaixi region showing the distribution of Cu and Cu-Ni deposits.(B) Simplified geological map of the Yuhaixi Mo(Cu) deposit (modified from [30]).

Figure 3 .
Figure 3. Geologic cross section of the ZK3601 and ZK3201 exploration lines across the Yuhaixi porphyry Mo deposit with lithology and mineralization (A) and alteration (B).

Figure 3 .
Figure 3. Geologic cross section of the ZK3601 and ZK3201 exploration lines across the Yuhaixi porphyry Mo deposit with lithology and mineralization (A) and alteration (B).

Figure 5 .
Figure 5. Cathodoluminescence images of representative zircon grains of the Yuhaixi showing the inner structures and analyzed spots.

Figure 5 .
Figure 5. Cathodoluminescence images of representative zircon grains of the Yuhaixi showing the inner structures and analyzed spots.

Figure 6 .
Figure 6.U-Pb concordia diagrams and weighted average ages of the zircons from monzonitic granite, diorite, and granite in the Yuhaixi porphyry Mo(Cu) deposit.

Figure 6 .
Figure 6.U-Pb concordia diagrams and weighted average ages of the zircons from monzonitic granite, diorite, and granite in the Yuhaixi porphyry Mo(Cu) deposit.

Figure 10 .
Figure 10.Re-Os isochron diagram and weighted average model age diagram for four molybdenite samples in the Yuhaixi porphyry Mo(Cu) deposit.

Figure 10 .
Figure 10.Re-Os isochron diagram and weighted average model age diagram for four molybdenite samples in the Yuhaixi porphyry Mo(Cu) deposit.

Figure 14 .Figure 14 .
Figure 14.Schematic cartoons illustrating the tectono-magmatic-metallogenic evolution model of the Yuhaixi porphyry Mo(Cu) deposit in eastern Tianshan.(A) N-dipping subduction of the North Tianshan ocean plate gave rise to the Bogeda-Haerlike and Dananhu-Tousuquan island arcs in theFigure 14.Schematic cartoons illustrating the tectono-magmatic-metallogenic evolution model of the Yuhaixi porphyry Mo(Cu) deposit in eastern Tianshan.(A) N-dipping subduction of the North Tianshan ocean plate gave rise to the Bogeda-Haerlike and Dananhu-Tousuquan island arcs in the early Paleozoic period.(B) Bipolar subduction of the North Tianshan ocean plate gave rise to the Dananhu-Tousuquan and Aqishan-Yamansu arcs in the Carboniferous period, forming Yuhaixi monzonitic granite and granite.(C) The Permian period post-collisional extension forming Yuhaixi diorite.

Table 1 .
Whole-rock geochemical data of the studied intrusive rocks in the Yuhaixi Mo(Cu) deposit (major elements: wt.%; trace elements: ppm).

Table 2 .
LA-ICP-MS zircon U-Pb data for the studied intrusive rocks in the Yuhaixi Mo(Cu) deposit.

Table 4 .
In situ zircon Hf isotopic data on the studied intrusive rocks in the Yuhaixi Mo(Cu) deposit.

Table 6 .
[46].Abbreviation: MG, monzonitic granite.Decay constant: λ 187 Re = 1.666 × 10 −11 year −1[46].Uncertainty in the Re and Os concentrations includes errors associated with the weighing of the sample and diluent, the calibration error of the diluent, the mass spectrometry analytical error, and the measurement error of the isotope ratios for the test sample; the confidence level is 95%.Uncertainty in the Re-Os model ages includes the uncertainty of the 187 Re decay constant, with a confidence level of 95%.Uncertainties for ages are absolute (2σ).