The Metallogeny of the Tieling Cu-Mo Porphyry Deposit in Eastern Tianshan, NW China: New Insights from Zircon U-Pb, Fluid Inclusion, and H-O-S Stable Isotope Analyses

The eastern Tianshan metallogenic belt is an important molybdenum resource base in Xinjiang and is characterized by large-scale porphyry Mo deposits formed during the Triassic. The Tieling Cu-Mo porphyry deposit, which is situated in the western part of the eastern Tianshan metallogenic belt, was recently recognized as being related to Carboniferous granite porphyry. Three stages of hydrothermal mineralization were identi ﬁ ed, including quartz+K-feldspar+pyrite ± molybdenite ± magnetite (stage I), quartz +molybdenite+pyrite+chalcopyrite (stage II), and quartz+pyrite ± molybdenite ± epidote (stage III). Fluid inclusion petrography and microthermometry analyses indicate the presence of gas-liquid inclusions with a H 2 O-NaCl composition. The ore-forming ﬂ uids have a characteristic temperature ranging from 157 to 262 ° C under stage II and 135 to 173 ° C under stage III, which correspond to salinities of 7.2-17.2wt% NaCl equiv. and 5.9 to 9.6wt% NaCl equiv., respectively. The hydrogen and oxygen isotope data indicate that the ore-forming ﬂ uids of the Tieling deposit were originally derived from magmatic hydrothermal ﬂ uids and then mixed with meteoric water. The sulfur isotope compositions indicate that the ore-forming materials were mainly derived from the Late Carboniferous felsic magma. Furthermore, zircon U-Pb analysis of ore-bearing granite porphyry yields a concordant age of 298 : 4 ± 0 : 7 Ma , indicating that the Tieling Cu-Mo deposit formed during the Late Carboniferous and di ﬀ ered from that processed under pre-Early Carboniferous and Triassic mineralization in the eastern Tianshan metallogenic belt. These results also indicate that the Tieling porphyry deposit was formed in the transition condition between subduction-related accretion and postcollisional orogeny, and it should be given more attention in prospect evaluations.


Introduction
Porphyry deposits, as one of the most important types of Cu-Mo-Au deposits, have attracted extensive interest from the geologic community [1,2]. Subduction-related magmatic arcs are considered to be closely related to porphyry deposits worldwide, especially that of porphyry Cu-Mo in the Circum-Pacific metallogenic belt [3][4][5]. Recent studies of metallogenic systems in China have shown that a postcol-lisional setting is favourable for the development of porphyry deposits [6][7][8][9][10], especially for porphyry Mo-only or Mo-dominated polymetallic deposits in the Dabie orogen [11][12][13].
The postcollision-type porphyry Mo deposits in eastern Tianshan are found within a Triassic formation [14,15], and the subduction-type porphyry Cu-Mo deposits are characterized by a formation preceding the Early Carboniferous [16,17]. However, the Tieling Cu-Mo deposit, which was discovered by the Xinjiang Geological Survey Academy during a geochemical element anomaly survey, may be closely related to Late Carboniferous magmatism [18]. As an unexploited blind mineral system, the geological characteristics and metallogenic processes supporting its formation are still unclear. In this contribution, we first report new data, including the zircon U-Pb isotopic age of ore-bearing granite porphyry rocks, fluid inclusions, and H-O-S isotopes, to constrain the age of mineralization, determine the origin of the ore-forming fluids and material, and establish a metallogenic model of the Tieling deposit, which will be beneficial to enhance metallogenic theory and provide insights for the exploration of porphyry Cu-Mo mineralization in eastern Tianshan.
Eastern Tianshan, as an important part of the CAOB, has been subdivided into four tectonic units from north to south, namely, the Dananhu-Tousuquan island arc, the Kanggur-Huangshan shear zone, the Aqishan-Yamansu belt, and the Central Tianshan massif, with a series of approximately east-west-trending faults defining the boundaries, including the Dacaotan, Kanggur, Yamansu, and Aqikekuduke faults (Figure 1(c)). The Dananhu-Tousuquan belt comprises Ordovician to Carboniferous volcanic and intrusive rocks that host a series of large porphyry Cu-(Mo) deposits, such as the Yudai [30], Tuwu, Yandong, Linglong [31], Chihu [32], Fuxing [33,34], Yuhai [17], and Sanchakou deposits [35]. The Kanggur-Huangshan shear zone, located between the Yamansu and Kanggur faults, contains a set of tectonic slices, disordered strata, and intrusive rocks. Most strata-originating rocks with strong deformation and metamorphism host gold deposits in the west, including the Kangguer, Matoutan, and Shiyingtan deposits [28]. Most mafic-ultramafic rocks in the east host nickelcopper deposits, such as Huangshan, Huangshandong, Huangshannan, Hulu, and Tulargen deposits [26,27]. Most notably, the Baishan and Donggebi superlarge porphyry Mo deposits formed in the Triassic are also found in the Kanggur-Huangshan shear zone [25,36]. The Aqishan-Yamansu belt, located between the Aqikekuduke and Yamansu faults, consists of lavas, volcaniclastic rocks, and terrigenous clastic sedimentary rocks interbedded with bioclastic limestones. Carboniferous granitic intrusions are widely distributed and intrusively bedded by diabase walls [37]. A series of iron deposits related to volcanism have been recognized in this belt, including the Hongyuntan, Bailingshan, Duotoushan, Heijianshan, Chilongfeng, and Yamansu deposits [18,29,38,39]. The Central Tianshan massif, bounded by the Aqikekuduke fault in the north, is an ancient block composed of calc-alkaline basaltic to andesitic volcanic and volcaniclastic rocks, slightly altered granites and granodiorites, and Precambrian basement rocks [34]. In addition to iron and nickel-copper deposits, there is also a skarn-type tungstenmolybdenum deposit (Xiaobaishitou, Li et al. [40]) in the eastern part of the belt.

Geology of the Tieling Ore District
The Tieling Cu-Mo deposit is situated southwest of the Bailingshan intrusion in the Aqishan-Yamansu arc belt ( Figure 1(c)). The lithostratigraphic unit in the ore district is dominated by the Late Carboniferous Tugutubulake Formation, which consists of tuffaceous dacitic lava with an age of 324 Ma [41]. The intrusive rocks at Tieling mainly include granite porphyry, monzogranite, and granodiorite, with minor gabbroic and dioritic dikes ( Figure 2). The monzogranite and granodiorite of the Bailingshan complex were emplaced early into the strata at 317-307 Ma [41], and the gabbroic and dioritic dikes were both emplaced in the Late Carboniferous (311 to 315 Ma, Long et al. [37]). The concealed granite porphyry, as an important ore-bearing rock, was emplaced into the granodiorite (Figure 3). The granite porphyry is characterized by a medium-or fine-grained porphyritic texture (Figure 4(a)) and mainly consists of Kfeldspar (65%) and quartz (30%), with minor accessory minerals such as molybdenite, zircon, apatite, pyrite, and magnetite. The phenocryst content is approximately 15% and mainly includes K-feldspar with a particle size of approximately 0.5 mm (Figure 4(b)). Muscovite is distributed among the K-feldspar and quartz aggregates in sheet form ( Figure 4(c)).
The iron orebodies in the NE direction on the surface, which were regarded as target ores, are mainly distributed in the northeastern ore district ( Figure 2). Newly identified molybdenum mineralization is only found in the deep part of granite porphyry, forming a combination of "upper iron and lower copper-molybdenum" with the previously mined iron ores. The iron ores are composed of magnetite, haematite, and pyrite [42]. The metal sulfide minerals of Cu-Mo mineralization mainly include molybdenite, chalcopyrite, and pyrite, with disseminated, massive, and vein structures, while gangue minerals mainly include K-feldspar, quartz, epidote, and chlorite. Molybdenite occurs as coarse-grained clusters in quartz veins (Figures 4(d) and 4(e)) and as a disseminated mineral (Figure 4

Geofluids
surrounding rock shows spatial zonation. From the granite porphyry to the surrounding rock, the alteration varies from a potassium silicate zone to a sericitization zone and finally a propylitization zone.
Based on field and microscopic observation of the mineralogy and the textural and paragenetic relationships of various hydrothermal minerals, three paragenetic stages of mineralization were identified in the Tieling deposit ( Figure 5). These sequences are quartz+K-feldspar+pyrite +molybdenite±magnetite (stage I), quartz+molybdenite +pyrite+chalcopyrite (stage II), and quartz+pyrite+epidote ±molybdenite±chlorite (stage III). were separated using heavy liquid and magnetic techniques, hand-picked under a binocular microscope, and mounted in epoxy resin. The internal texture of the zircons (including zoning, structures, and fractures) was characterized via cathodoluminescence (CL) imaging using a CAMECA electron microprobe at Yujin Technology Co., Ltd. U-Pb isotope analysis was performed at the State Key Laboratory of Mineral Deposits Research, Nanjing University, using an Agilent 7500a laser ablation system coupled with an iCAP RQ ICP-MS. The analytical spot size, laser frequency, and energy density were 32 mm, 5 Hz, and 6.5 J/cm 2 , respectively. The analytical drift of the U-Th-Pb isotopic ratios was corrected using linear interpolation (with time) for every twelve analyses based on the signal variations of the zircon standard GJ1. Weighted mean age calculations and concordia diagrams were processed using ISOPLOT software [43].

Fluid Inclusion Microthermometry and Laser Raman
Spectroscopy. The mineral assemblage and its paragenetic sequence were observed in the field and under the microscope for dividing three metallogenic stages. However, due to the low degree of exploration, we did not collect representative stage I samples for fluid inclusion study. Therefore, Fluid inclusion analyses were carried out at the Laboratory of Mineralization and Dynamics, Chang'an University, Xi'an, China, using the fluid inclusion assemblage (FIA). Microthermometric measurements were conducted using a Linkam THMSG 600 heating-freezing stage mounted on a Leica DMR microscope. The estimated accuracies of the freezing and heating measurements were ±0.1°C from -100°C to 25°C, ±1°C from 25°C to 400°C, and ±2°C above 400°C. Heating and freezing rates were generally 0.2-5°C/min and were reduced to 0.2°C/min near the temperatures of phase change. The salinity of aqueous fluid inclusions was estimated using equations for the NaCl-H 2 O system provided by Bodnar [44]. The density of the oreforming fluid was estimated using data provided by Liu and Shen [45]. Volatile components of representative fluid inclusions were identified using a HORIBA HR Evolution 800 mm laser Raman spectrometer at the same institution. The laser wavelength was 532 nm, and the single spectrum collection time was 2 s. The Raman shift ranged from 100 cm -1 to 4000 cm -1 . The spectrum resolution was ±2 cm -1 with a beam size of 1 μm. The instrumental setting was kept constant during all analyses. The δD values of water from fluid inclusions and the δ 18 O values of their host minerals were measured at the Beijing Research Institute of Uranium Geology, China. The water contained in the fluid inclusions of the host minerals was released via thermal decrepitation at 400°C and then collected, frozen, and purified. Then, using reductive zinc, we replaced and released the hydrogen in the water to perform mass spectrometry. The analytical precision of the δD values was within ±2‰ (1σ) and that of the δ 18 O values was within ±0.2‰ (2σ). The δ 18 O values in quartz water (α quartz water ) were calculated from the δ 18 O quartz values of the analysed quartz by using the fractionation equation 1000Inα quartz water = ð3:38 × 10 6 ÞT −2 − 3:40, where T is the temperature in Kelvin [46], and the average corrected temperature of fluid inclusion was used to calculate the δ 18 O water value.
We separated the pyrite and chalcopyrite samples from the disseminations, veins, and crumb ores for sulfur isotope analysis. Sulfur isotopic analyses were carried out on a MAT253 mass spectrometer at the Beijing Research Institute of Uranium Geology, China. Seven pyrite samples and one chalcopyrite sample were converted to SO 2 under hightemperature vacuum conditions. The δ 34 S values were measured on a DELTA V plus gas isotope mass spectrometer. Sulfur isotope values are reported in per mil relative following the Vienna Canyon Diablo Troilite (V-CDT) standard, and the analytical uncertainty was within ±0.2‰ for δ 34 S. The equilibrium temperature of sulfur isotopes was calculated from symbiotic sulfides by using the fractionation equation 1000Inα Py−Ccp = 0:45 × 10 6 /T −2 , where T is the temperature in Kelvin [47].

Zircon U-Pb
Isotopes. The granite porphyry (TL1) was dated by zircon U-Pb analysis. LA-ICP-MS zircon U-Pb dating results are summarized in Table 1, and representative CL images of zircon grains from these rocks are shown in Figure 6. All the analysed zircons are prismatic, euhedral, and colourless, and most of them show oscillatory zoning patterns, which imply a magmatic origin. The analysed zircons have variable U (37-433 ppm) and Th (164-547 ppm) contents, with Th/U ratios ranging from 0.22 to 0.81 (mainly >0.1). Twenty-four zircon grains from the granite porphyry samples define a narrow range with 206 Pb/ 238 U ages of 296 to 298 Ma, yielding a concordant age of 298:4 ± 0:7 Ma (MSWD = 28; Figure 7(a)), with a weighted mean age of 297:0 ± 1:6 Ma (MSWD = 0:027; Figure 7(b)).

Fluid
Inclusions. Based on phase characteristics at room temperature, phase transitions during heating and cooling, and the results of laser Raman spectroscopy, fluid inclusions in the Tieling deposit mainly include liquid-rich inclusions (WL type). Fluid inclusions occurring as isolated inclusions, random distributions, or clusters are interpreted as primary features at various stages ( Figure 8). Each cluster or group of fluid inclusions along growth zones was considered to represent an FIA. Fluid inclusions occurring in linear arrays along fractures or grain boundaries were considered to be secondary fluid inclusions, which were not analysed by microthermometry because they formed later with respect to mineralization. The inclusions are ovular, polygonal, and irregular, with diameters in the range of 3-18 μm, mainly 5-10 μm, and contain gas bubbles that account for 10-40% of the total volume. These inclusions homogenize to the liquid phase when heated.
The microthermometric results and fluid inclusion parameters are shown in Table 2. For the fluid inclusions in quartz of stage II, the final ice melting temperatures of WL-type fluid inclusions range from -13.8 to -4.5°C (with an average of -9.1°C), with corresponding salinities of 7.2 to 17.2 wt% NaCl equiv. (with an average of 12.9 wt% NaCl equiv.). The homogenization temperatures of fluid For the fluid inclusions in quartz (samples 20TL-3-83) of stage III, the final ice melting temperature of WL-type fluid inclusions is -6.3 to -3.6°C (with an average of -5.0°C), the salinity is 5.9 to 9.6 wt% NaCl equiv. (with an average of 7.9 wt% NaCl equiv.), the homogenization temperatures of fluid inclusions range from 135 to 173°C (with an average of 154.2°C), and the fluid densities are 0.959 to 0.995 g/cm 3 . Previous studies indicate that the emplacement depths for porphyry deposits are generally around 3-5 km [48]. Due to lack of pressure data for this deposit, we used the lithostatic pressure at 5 km to calculate the entrapment temperatures of the analysed fluid inclusions. The fluid lithostatic pressure of the Tieling deposit can be estimated to be 1350 bar. Then, the average ore-forming temperatures of stage II and stage III were corrected to range from 268°C to 225°C (Figure 9). We conducted a laser Raman spectroscopic speak scan of the gas-phase components in the fluid inclusions of quartz from the Tieling deposit. The results of representative laser Raman spectroscopic analyses are shown in Figure 10. The         show that the ore-forming fluid has the characteristics of a relatively medium-low temperature, medium-high salinity, and medium-high density and represents a NaCl-H 2 O system. The average corrected mineralization temperatures of stage II and stage III range from 268°C to 225°C, which is consistent with the equilibrium temperature of sulfur isotopes (275°C) and indicates that the corrected mineralization temperature is within the acceptable error range. The δ 18 O values of quartz in the Tieling deposit range from 8.8‰ to 9.5‰, with an average value of 9.1‰ (Table 2), which is consistent with those of crustal remelting granite (10.0-12.0‰ [49]), indicating that the formation of hydrothermal quartz was associated with Carboniferous felsic intrusive rocks and was derived from magmatic fluids. The δD values of the fluids in the Tieling deposit mostly range from −79.5‰ to −63.3‰ (Table 3) and approach those of magmatic water and meteoric water [50,51]. The H-O isotope data of stages II and III plot between the primary magmatic water field and the meteoric water line (Figure 11), which is similar to other deposits in the same metallogenic belt [25,[52][53][54][55][56][57], indicating that the ore-forming fluids of the Tieling Cu-Mo deposit were derived from Carboniferous magmatic hydrothermal fluids and then mixed with meteoric water.
6.1.2. Sources of Sulfur. The sulfide assemblage in the Tieling deposit is dominated by molybdenite and pyrite with minor chalcopyrite, and no sulfate minerals have been detected. Therefore, the hydrothermal system during the ore-forming process in the Tieling deposit was dominated by H 2 S. Molybdenite, pyrite, and chalcopyrite were formed under low-f O2 and low-pH conditions [58]. According to Table 3, the average δ 34 S V-CDT value for the seven pyrite samples collected from the Tieling deposit is 0.1‰, and  Figure 11: δD fluid vs. δ 18 O fluid values from various stages in the Tieling Cu-Mo deposit. Fields for magmatic and metamorphic water are from Taylor [50] and Sheppard [51]. The data of previous studies were referenced from Zhang et al. [54], Zhang et al. [25], Liu et al. [55], Wang et al. [56], and Li et al. [57].
the δ 34 S V-CDT value for one chalcopyrite sample is -0.1‰. The basic sequence is consistent with the δ 34 S enrichment condition of δ 34 S Py > δ 34 S Ccp at isotopic equilibrium, indicating that the mineral S isotopes had reached equilibrium [59]. Therefore, the S isotopic compositions of pyrite are close to the total S isotopic compositions of the hydrothermal system and can be used to trace S sources. The overall range of the δ 34 S V-CDT values of sulfides in the Tieling deposit is narrow, indicating that the S sources of sulfides deposited in the ore-forming hydrothermal fluids were undiversified. The δ 34 S V-CDT values are consistent with those of felsic magma [60] and other representative deposits related to magmatism [14,16,25,39,54,56,61,62], indicating that the sulfur in the Tieling deposit was mainly derived from Late Carboniferous felsic magma ( Figure 12).   [64], Sanchakouxi [35], and Yudai [30]. During the Early Carboniferous, porphyry Cu-Mo deposits in the northern Kanggur belt, such as Yuhai and Tuwu [65,66], formed under the north-south bidirectional subduction of the Kanggur ocean basin. At the same time, a series of iron ores related to magmatism were formed in the southern Kanggur belt, including those of the Bailingshan [67], Hongyuntan [39], and Duotoushan [68]. The ductile shear gold deposits were formed in Late Carboniferous [69], followed by abundant magmatic sulfide Cu-Ni under the postcollisional extensional environment of the Early Permian [70,71]. After approximately ca. 240 Ma, porphyry Mo deposits related to Triassic granite porphyry in a postcollisional environment were formed, including the Baishan and Donggebi deposits [72,73]. The zircon U-Pb age from ore-bearing Previous studies have shown that the iron orebodies taking tabular or lensoid shapes of the Tieling deposit were hosted in monzogranite and granodiorite along fractures [42]. The formation of the iron orebodies was considered related to the Bailingshan complex generated by the southward subduction of the Kanggur oceanic plate beneath the Yili-Central Tianshan block during 324 to 303 Ma [42,67], and Cu-Mo mineralization was supposed to have formed later than Fe mineralization. Therefore, the mineralization process of the Tieling deposit has been described as two distinct metallogenic periods, including Fe and Cu-Mo periods. The iron orebodies related to volcanic rocks were formed first in the strata during the subduction stage of the early Late Carboniferous [42]. With the emplacement of the Bailingshan complex, iron orebodies were formed in monzogranite and granodiorite and partially in the contact zone with skarnization ( Figure 13(a)). Magnetite was found in the later basic dikes [37]. At the end of the Late Carboniferous, the hydrothermal fluids released from the ore-bearing granite porphyry in the late crystallization stage were mixed with meteoric water, resulting in precipitation mineralization in favourable structural positions (Figure 13(b)).

Geofluids
(3) The H-O-S isotopes indicate that the ore-forming fluids were derived from a mixture of magmatic hydrothermal water and meteoric water and the ore-forming materials were derived from Late Carboniferous felsic magma (4) The zircon U-Pb of ore-bearing granite porphyry yields a concordant age of 298:4 ± 0:7 Ma, indicating that the Tieling Cu-Mo deposit was a rare metallogenic event in the Late Carboniferous of the eastern Tianshan, which should be given more attention for prospecting

Data Availability
The data used to support the findings of this study are available from the corresponding authors upon request.

Conflicts of Interest
The authors declare that there is no conflict of interest regarding the publication of this paper.