The Metallogenic Setting of the Jiangjiatun Mo Deposit, North China: Constraints from a Combined Zircon U–Pb and Molybdenite Re–Os Isotopic Study

: The Jiangjiatun Mo deposit is a recently discovered molybdenum deposit in the easternmost area of the Yan-Liao metallogenic belt, North China Craton. Quartz vein-type Mo mineralization at Jiangjiatun is associated with the granitic porphyry stock. In this study, we performed a combined zircon U–Pb and molybdenite Re-Os dating study on the Jiangjiatun Mo deposit to constrain its mineralization age and metallogenic setting. Laser ablation inductively coupled mass spectrometry (LA-ICP-MS) zircon U–Pb analyses suggest that the granitic porphyry was formed during the Late Jurassic, with a weighted mean 206 Pb/ 238 U age of 154 ± 1 Ma (2 σ ). Seven molybdenite samples from the Jiangjiatun deposit yield a 187 Re– 187 Os isochron age of 157.5 ± 0.5 Ma (2 σ ). The discrepancy between the U–Pb and Re–Os ages may be explained (1) by the “2 sigma” measurement uncertainty, or (2) by the different closure temperature of the Re–Os isotopic system of molybdenite and the U–Pb isotopic system of zircon. Even though there is a small difference between the zircon U–Pb and molybdenite Re–Os ages, we can clearly identify a Late Jurassic Mo mineralization event at Jiangjiatun in the easternmost area of the Yan-Liao metallogenic belt. The moderate Re concentrations (13 to 73 ppm) in molybdenite from the Jiangjiatun Mo deposit are indicative of the involvement of the mantle materials into the Mo mineralization. The Jiangjiatun Mo deposit is likely the result of the subduction of the paleo-Pacific plate beneath the North China Craton during the Late Jurassic. Combined with the available published regional robust geochronological data, we proposed that the Mo mineralization in the Yan-Liao belt is in good agreement with the tectonic transition from Late Triassic post-collision extensional setting due to the closure of the paleo-Asian ocean to the Yanshanian (J–K 1 ) continental arc setting in response to the subduction of the paleo-Pacific Plate. The study highlights that regional mineralization may provide an excellent constraint on tectonic change. The Mo mineralization at Jiangjiatun is likely affected by the subduction of the paleo-Pacific Plate. Combining previously published Re–Os isotopic data of molybdenite, we propose a three-stage evolution model that can explain well the regional magmatism, Mo mineralization, and the remarkable change of Re concentrations in molybdenites. A tectonic change from the paleo-Asian Ocean regime to paleo-Pacific regime is revealed by the Mo mineralization in the Yan-Liao metallogenic belt. Hence, the regional mineralization is a potential indicator of tectonic setting.


Introduction
China contains the most abundant molybdenum resources in the world, with a proven Mo metal reserve of >25 Mt [1]. The North China Craton (NCC) contains two important molybdenum metallogenic belts, including the Yan-Liao molybdenum metallogenic belt (YLMB) near the northern margin and the Eastern Qinling-Dabie metallogenic belt (EQL-DB) near the southern margin [1][2][3] ( Figure 1A). The Yan-Liao metallogenic belt near the north margin of the NCC contains about 2. 34 Mt of Mo reserves [4,5]. The Mo mineralization in the YLMB is believed to experience long-term magmatism during the Mesozoic Period, lasting nearly 100 million years [6,7]. The large-scale Mesozoic Mo mineralization in the YLMB was interpreted to be associated with the closure of the Paleo-Asian ocean [5], the subduction of the paleo-Pacific plate [8,9], or post-orogenic extension after the closure of the Mongolia-Okhotsk Ocean [10,11]. Thus, the viewpoints on the tectonic settings of the YLMB remain unclear.
The Jiangjiatun deposit is a recently discovered Mo deposit in the easternmost area of the YLMB in NCC. To date, no geochronological studies have been performed on the Jiangjiatun Mo deposit. In this study, the laser ablation inductively coupled mass spectrometry (LA-ICP-MS) zircon U-Pb dating of the granitic porphyries and Re-Os isotope analysis of molybdenite in the Jiangjiatun molybdenum deposit were performed. The molybdenum mineralization age of the Jiangjiatun deposit is accurately constrained, and then its tectonic setting is discussed. Moreover, the Re contents of the molybdenite are used to explore the mantle-crust interaction process. Furthermore, to compile previously published reliable geochronological data, we tentatively propose the tectonic genetic model for the YLMB.

Geological Setting
The basement of the NCC has been divided into the eastern and western blocks by the ca. 1.85 Ga Trans-North China Orogen ( Figure 1A) [12][13][14]. The basement of the NCC was covered by thick sedimentary sequences of Meso-to Neo-Proterozoic and Paleozoic periods [15][16][17]. The north margin of the NCC was tectonically active, and large-scale magmatism occurred from Triassic to Early Cretaceous periods [4,[18][19][20][21] The YLMB in the north margin of the NCC consists of the Archaean basement rocks and the Proterozoic, Paleozoic, and Mesozoic cover sequences from the bottom up [22]. The Paleozoic strata consist of the Cambrian, Ordovician, and Carboniferous limestone, dolomitic limestone, muddy limestone, and shale [23]. The Late Mesozoic strata are distributed in basin, and the simultaneous granites intrude into the regional basement and cover strata [24][25][26]. This metallogenic belt is featured by the ubiquitous presence of the Late Triassic to Early Cretaceous granites and a group of porphyryskarn Mo deposits, occurrence and prospects that related to these Late Mesozoic felsic intrusions ( Figure 1B) [4,27].

Deposit Geology
The Jiangjiatun molybdenum deposit is located in the easternmost part of the YLMB. The ore reserve is 160,000 tons, where the average ore grade is 0.09%. The ore-related granite porphyry is the primary magmatic rocks in the deposit, and it hosts the quartz vein-type Mo ore bodies ( Figure 2). The essential minerals of the granite porphyry are feldspar, quartz, biotite, and the accessory minerals consist of zircon, apatite, magnetite, and titanite ( Figure 3A-F). The strata of the Mesoproterozoic Changcheng and Jixian formations are present in the nearby region, such as the Yangjiazhangzi skarn deposit, but are not exposed at Jiangjiatun. The faults in this deposit are mainly NE-, EW-, and SNtrending, and they are the ore-hosting structures of the Mo ore bodies ( Figure 2). In addition, all of these ore bodies are nearly upright. There are 6 distinct quartz-molybdenite veins in the Jiangjiatun molybdenum deposit, of which 4 veins strike east-west and northeast in the north part, and 2 veins strike north-south in the south part. The ore bodies are featured by the wide quartz veins, with the width up to 1 m. Molybdenite occurs mostly in the form of thin film and attached to the wall of quartz veins ( Figure 3G,H). The wall rock alteration of the Jiangjiatun molybdenum deposit includes potassification, silicification, chloritization, carbonation, and fluoritization. Three alteration zones are recognized from deep to shallow: (1) In the deep level of the ore deposit, potassic alteration is strong and associated with significant mineralization; (2) fluoritization and carbonation mainly occur in the middle level; and (3) carbonation and chloritization are common in the upper level. The principal ore mineral is molybdenite, with minor pyrite, specularite, and chalcopyrite. The gangue minerals are dominated by quartz, K-feldspar, calcite, sericite, chlorite, and fluorite.

Zircon U-Pb Dating
Two granite samples were collected from the drill hole. Representative zircons were picked using a binocular optical microscope (United Scope LLC, Irvine, CA, USA) and mounted in epoxy resin disks. The disks were polished and then coated with gold. The microphotographs of zircons were taken in transmitted and reflected light, and cathodoluminescence (CL) was used to examine the internal structure of the analyzed zircons.
LA-ICP-MS zircon isotope analyses were performed using an Agilent 7700x ICP-MS (Agilent Technologies, Santa Clara, CA, USA) equipped with a 193 nm ArF excimer laser ablation system (ASI RESOnetics, Australian Scientific Instruments Pty Ltd, Canberra, Australia), at the FocuMS Technology company, Nanjing, China. Detailed analytical procedures and the data reduction method were described by Zong et al. [29]. Here, we give a brief summary. A laser beam size of 33 μm and an energy density of 3.5 J/cm 2 was chosen with a repetition rate of 6 Hz. The make-up gas, argon (Ar), and the carrier gas, the mixture of argon (Ar) and helium (He), were used by a T-connector before entering the ICP. Nitrogen (N2) was added into the central gas flow (Ar + He) of the Ar plasma to significantly improve precision by declining the detection limit and [30]. During each analysis run, a background acquisition of 20-30 s (gas blank) and a following 50 s data acquisition were performed. The mass discrimination and U-Th-Pb isotope fractionation was calibrated by the zircon standard 91500. The recommended U-Th-Pb isotopic ratios used for 91500 following Wiedenbeck et al. (2004) [31]. The precision and accuracy of U-Th-Pb dating were also evaluated by comparison with another zircon standard GJ-1 [32]. An external standard of NIST SRM 610 glass [33] was further analyzed to normalize U, Th, and Pb. Data reduction was performed by the software package ICPMSDataCal (Version 9.0) [34]. Uncertainties of recommended values for the external zircon standard 91500 were propagated into the reduction of the U-Pb isotopic results of the zircon samples. The weighted mean values were calculated, and the Concordia diagrams were plotted via Isoplot/Ex (Version 3.0) [35].

Molybdenite Re-Os Dating
Seven molybdenite samples were selected from the ore-bearing quartz vein in the Jiangjiatun Mo deposit, Xincheng, China. Gravity and magnetic separation were first used for crushing the ore samples, and molybdenite grains were handpicked under a binocular optical microscope, with the purity better than 99%). Re-Os isotope analyses were performed at the Re-Os Laboratory, National Research Center of Geoanalysis, Chinese Academy of Geological Sciences (CAGS), Beijing, China. Detailed chemical procedures of Re-Os analysis performed in this research are described by Du et al. 2004 andLi et al., 2010 [36,37]. The selected pure molybdenite grains were dissolved in HCl-HNO3-H2O solutions using the Carius tube. The Re-Os isotope was analyzed via an inductively coupled plasma source mass spectrometer TJA X-series ICP-MS manufactured by TJA, Waltham, MA, USA. The analytical reliability was tested by the reference material GBW04436 certified by the JDC standard [36]. The blank during these analyses was approximately 2.9 ± 0.9 pg for Re and 0.1 pg for Os. The model age of molybdenite was calculated using the following equation and parameters: t = (ln (1 + 187 Os/ 187 Re))/λ, where λ (1.666 × 10 −11 yr −1 ) is the decay constant of 187 Re [38]. The isochron age was calculated, and the iso-chronological age diagram was drawn using Isoplot/Ex (Version 3.0) [35]. Absolute uncertainties of the Re-Os isotopic results were present at the 2σ level.

Zircon U-Pb Dating
The euhedral zircon grains were 100-300 μm in length, and the CL images indicated that most of the grains showed oscillatory magmatic zoning ( Figure 4). The LA-ICP-MS U-Pb ages data for the two granite porphyry samples are listed in Table 1 and are plotted in Figure 5. The zircon grains showed variable U (46-602 ppm) and Th (179-1146 ppm) concentrations, and Th/U ratios ranged from 0.3 to 1.2 (Table 1).

Molybdenite Re-Os Isotopic Dating
The Re-Os systems of molybdenite had high closure temperature, which avoided the overprint and later hydrothermal or metamorphic effects [39], thus Re-Os dating of molybdenite had become a powerful tool for directly determining the mineralization age. Molybdenite, as the most important economic mineral in the Jiangjiatun deposit, contained high Re and low common Os concentrations (Table 2), which means the method of Re-Os dating of molybdenites can be used to directly and precisely constrain the ore-forming age.  [3,4]. The uncertainty in each analytical procedure was approximate 1.5%, including the uncertainty of 187 Re, uncertainty in isotope ratios, and spike calibration. The unit of weight is "g", the unit of concentration of Re, 187 Re, and 187 Os is "ng/g"; and the unit of model age is "Ma".
The concentrations of common Re, 187 Re, and 187 Os of molybdenites from the Jiangjiatun deposit are shown in Table 2. The Re contents ranged from 13.3 to 73.3 μg/g. The 187 Os concentrations ranged from 22.2 to 121.8 ng/g ( Table 2). The Re-Os model ages of molybdenite in the Mo-bearing quartz vein ranged from 159 to 157 Ma ( Figure 6). These model ages of molybdenites showed a narrow variation range, indicating the consistent age within the measurement uncertainty. In addition, the 187 Re and 187 Os data for molybdenite yield, with a well-constrained isochron age of 157.5 ± 0.5Ma (MSWD = 0.72, 2σ), which was identical to the weighted mean age of 158.1 ± 1.0 Ma (MSWD = 0.4, 2σ, n = 7)  Figure 6A and pillar in Figure 6B represents analytical sample of molybdenite; Green line in Figure 6B represents the weight mean of the individual molybdenite model ages).

Timing of Mo Mineralization at Jiangjiatun
The LA-ICP-Ms zircon U-Pb dating results demonstrate that the granite porphyry was emplaced at 154 ± 1 Ma ( Figure 5), suggesting that the ore-related magmatism in the Jiangjiatun deposit occurred during the Late Jurassic. Seven selected molybdenite samples from the Jiangjiatun deposit obtained Re-Os ages of 159 to 157 Ma (Table 2). In addition, these molybdenite samples from the Jiangjiatun deposit show a well-constrained 187 Re-187 Os isochron age of 158 Ma, which was suggested to represent the age of Mo mineralization ( Figure 6). The crystallization age of the granite porphyry is consistent with the mineralization ages constrained using the molybdenite Re-Os method, indicating that the Mo mineralization and coeval granitic magmatism at Jiangjiatun took place during late Jurassic.
The YLMB has undergone multi-stage Late Mesozoic magmatic activities [40,41]. Many scholars have carried out precisely single zircon U-Pb and molybdenite Re-Os isotopic dating in the metallogenic belt. The available published age data of the Mo deposits in the YLMB were collected, and 3 peaks of Mo mineralization were identified, including the Late Triassic (T3), Early Jurassic (J1), and Late Middle Jurassic and Early Cretaceous (J2-K1) ( Table 3 and Figure 7). The first stage of Mo mineralization and related magmatism (237 Ma to 224 Ma) occurred along the E-W trending boundary between the Central Asian Ocean Belt (CAOB) and NCC, e.g., Dasuji, Sadaogoumen, and Hekanzi [39,[42][43][44]. The secondary stage of Mo mineralization is featured by a short period ranging from 188 to 183 Ma and a limited area of the easternmost region of the YLMB, e.g., Lanjiagou and Yangjiazhangzi [45][46][47][48][49]. The last stage of Mo mineralization was widely developed in the nearly entire Yan-Liao metallogenic belt and lasted a relatively long time from 166 Ma to 140 Ma [40,[50][51][52][53][54][55].
This study provides the easternmost example of the last stage Mo mineralization, implying that the potential to search for Late Jurassic Mo deposits in the nearby area of the Jiangjiatun Mo deposit.

Crust-Mantle Interaction Process Indicated by Re Concentrations in Molybdenite
The wide variation of Re concentrations in molybdenite from the porphyry mineral systems likely provides a critical clue for the origin of ore deposits [57,58]. The higher concentrations of Re in molybdnites may be explained by the involvement of the mantle contributions such as mantle underplating or mixing of mafic rocks as part of their genesis. By contrast, the molybdenites from ore deposits that are derived from partial melts of pure crustal rocks or organic-poor sedimentary rocks are featured by lower concentrations of Re [57]. For instance, the mantle materials involved porphyry Cu-Mo ore systems generally contain high concentrations of Re (up to thousands of ppm), whereas crustal derived porphyry Mo ore systems typically contain low to single digit ppm of Re.
The concentration of Re in molybdenites is suggested to be the key information to the relative contributions of mantle and crust for the Mo mineralization. Mao et al. (1999) [58] concluded that Re concentrations in molybdenite evidently decline from mantle source through the mixture of mantle and crust, to pure crustal source. The Re concentrations in molybdenites derived from a mantle source is the highest (commonly >250 ppm), and decrease in those produced by the mixtures between mantle (most between 10 and 200 ppm) and crust, and become very low in those sourced from the pure crust (most <10 ppm) [59]. In this study, the Reconcentraions in the molybdenites from the Jiangjiatun Mo deposit range from 13 ppm to 73 ppm. As show in the Figure 8, all of the molybdenite samples are plotted in the field associated of the mixture of mantle and crust. Thus, we tentatively proposed that the molybdenum in the Jiangjiatun deposit comes from the mixture of crust and mantle.

Tectonic Evolution and Metallogenic Setting of the YLMB
The concentration of Re in molybdenites may provide a crucial constraint on the regional tectonic evolution related to Mo mineralization. We compiled all the available published Re-Os isotopes of molybdenites in the YLMB, and a clear spatial and temporal framework was established ( Figure 9 and Table 4). The Re concentration in molybdenites displays a gradually increasing trend from early to late, with the relatively low Re concentration during Late Triassic, the moderate Re concentration in Early Jurassic, and the relatively high Re concentration during Late Jurassic-Early Cretaceous ( Figure 9A). The early stage Mo mineralization during Late Triassic featured by low Re concentration took place near the suture zone between CAOB and NCC [40,44,45]. The second stage Mo mineralization during Early Jurassic with moderate Re Early Jurassic occurred locally, only in the east part of the YLMB [8,20]. The last stage Mo mineralization during Late Jurassic-Early Cretaceous is widely present in the whole metallogenic belt [52,53] (Figure 9B). The systematical change of Re concentration in molybdenites and the spatial-temporal distribution of the Mo mineralization may outline a tectonic transition history in the YLMB.  The late Triassic Mo mineralization along the boundary between CAOB and the NCC may be explained by the post-extensional setting in response to the closure of the Paleo-Asian ocean [11,60]. There is a broad consensus that the final closure of the Paleo-Asian Ocean marked by the Solonker Suture Zone occurred during the Late Permian and Early Triassic [61][62][63]. During the Late Triassic, the widespread occurrences of alkaline and mafic-ultramafic complexes, rift basins, and metamorphic core complexes indicate that the northern NCC was likely under a post-orogenic extension setting [64][65][66][67][68][69][70]. Thus, the Triassic Mo mineralization in the Yan-Liao metallogenic belt occurred along the boundary between CAOB and NCC is inferred to be generated in the postorogenic extensional setting following the final closure of the Paleo-Asian ocean ( Figure 10A). A short magmatic hiatus (ca. 220 Ma to 190 Ma) took place (Figure 7) in response to the tectonic switch from the CAOB tectonic environment to the paleo-Pacific tectonic setting. The subduction of the paleo-Pacific Plate beneath the NCC begun with the widespread formation of the Early Jurassic accretionary complexes along the eastern margin of the Eurasian continent, such as the Heilongjiang Group [71][72][73]. In addition, the NE-SW striking main tectonic lines of the Late Mesozoic age along the Taihangshan area in NCC are consistent with the spatial and temporal distribution of magmatic belts along the eastern margin of the Eurasian continent, which further supports that the tectonic evolution of the north margin of NCC may be affected by the subduction of the Paleo-pacific Plate [74,75]. Thus, the subduction of the Paleo-Pacific Plate beneath the eastern margin of the NCC initiated during Early Jurassic, and narrow arc-related magmatism was formed at that time ( Figure  10B).
The evolution of the Paleo-Pacific Plate has a great influence on the evolution of regional geodynamics ( Figure 10C). The paleo-Pacific Plate gradually subducted westward beneath the NCC, and likely reached its climax during the Late Jurassic-Early Cretaceous periods [71,[76][77][78]. The wide subduction zone results in widespread partial melting of the mantle wedge, and then the rising mantle-derived hot mafic magmas likely promote the large-scale melting of the lower to middle continental crust. The large-scale magmatism caused by the subduction of the paleo-Pacific Plate resulted in the widespread Mo deposits in the Yan-Liao metallogenic belt.

Conclusions
The Jiangjiatun ore-forming granitic porphyry was formed during the Late Jurassic (154 ± 1 Ma), together with the molybdenite 187 Re-187 Os isochron age of 157.5 ± 0.5 Ma, indicating that the Jiangjiatun Mo deposit was formed during Late Jurassic. The intermediate Re concentrations of molybdenite from the Jiangjiatun deposit indicate that the metal Mo is derived from the mixture of continental crust and mantle. The Mo mineralization at Jiangjiatun is likely affected by the subduction of the paleo-Pacific Plate. Combining previously published Re-Os isotopic data of molybdenite, we propose a three-stage evolution model that can explain well the regional magmatism, Mo mineralization, and the remarkable change of Re concentrations in molybdenites. A tectonic change from the paleo-Asian Ocean regime to paleo-Pacific regime is revealed by the Mo mineralization in the Yan-Liao metallogenic belt. Hence, the regional mineralization is a potential indicator of tectonic setting.