Two Periods of Porphyry Cu Mineralization and Metallogenic Implications in the Tuwu–Yandong Belt (NW China), Based on Re–Os Systematics of Molybdenite

: The Tuwu–Yandong belt contains ﬁve porphyry Cu deposits (Fuxing, Yandong, Tuwu, Linglong, and Chihu), constituting the largest Cu metallogenic belt in Northwest China. However, the metallogenic framework for porphyry Cu deposits in the belt remains controversial. Rhenium-osmium dating of molybdenite from the Tuwu, Linglong, and Chihu deposits and comparisons with previous geochronological data of ﬁve deposits suggest that two episodes (335–330 Ma and 323–315 Ma) of porphyry Cu–Mo mineralization occurred in the Tuwu–Yandong belt, and the metals were mainly sourced from the mantle. Moreover, combined with the geodynamic framework of this belt, the compressional environment may be more favorable for porphyry Cu mineralization, and further exploration into the Early Carboniferous porphyry Cu deposits in this belt is expected.


Tuwu Deposit Geology
The Tuwu porphyry Cu deposit was discovered in 1997, with proven reserves of Cu estimated to be 0.9Mt@0.62% [11]. It is located about 4 km north of the Kanggur fault ( Figure 2a) [17]. The outcropping strata mainly consist of Units 1 and 2 of the Qi'eshan Group and Jurassic Xishanyao Formation, with the orebody being mainly located in Unit 1 of the Qi'eshan Group and tonalitic porphyry (Figure 3a-c) [11,18]. The tonalitic porphyry yields zircon U-Pb ages of 332.8 ± 2.5 Ma (SIMS; [17]), 334.7 ± 3 Ma (SIMS; [46]), and 332.3 ± 5.9 Ma (SHRIMP; [44]). Three mineralized zones, from east to west, have been recognized as I, II, and III ore zones ( Figure 3a) [11,47,48]. Ore zone I is an approximate length of 1300 m and an average width of 39 m, with relatively low average grades (0.35 wt%) and the highest grade in the thickest, easternmost orebody [47]. Ore zone II is between 130 and 1400 m long and 4 and 125 m thick. It has grades of 0.2 to 2.8% Cu, is the most economically important orebody, and accounts for 96% of the total metal reserves of the deposit [47]. Ore zone III is 230-400 m long, 1-53 m thick, and dips 60° to 75° to the south [11]. This is an oxidized ore body, with Cu grades of 0.17-0.53 wt% [48]. The Tuwu porphyry Cu deposit includes veins, veinlets, and disseminated mineralization [44]. The ore minerals are dominated by chalcopyrite and pyrite, with minor bornite, magnetite, molybdenite, sphalerite, and galena [48]. The gangue minerals are composed of quartz, sericite, muscovite, chlorite, epidote, biotite, calcite, plagioclase, and albite [48].
The Tuwu deposit has experienced early potassic alteration, overprinted by phyllic and pervasive propylitic alteration (Figure 3c) [47]. The ore-forming processes may be divided into four paragenetic stages [48]. Stage I is composed of quartz-magnetite-pyrite ± biotite ± K-feldspar ± albite veins and quartz-magnetite veins, associated with weak and local potassic alteration (K-feldspar, quartz, and biotite). Stage II includes assemblages of quartz-chalcopyrite ± pyrite and quartz-chalcopyrite-bornite ± pyrite ± epidote, associated with phyllic alteration (sericite, quartz, muscovite, pyrite, chlorite, epidote, or rutile), and is considered the main mineralization stage. Stage III is characterized by Mo-bearing quartz veins, including quartz-molybdenite-chalcopyrite veins and quartz-calcite-molybdenite veins, associated with pervasive intense propylitic alteration (chlorite, epidote, and calcite). Stage IV is composed of pyrite-quartz-calcite and chlorite-epidote-anhydrite, associated with weak chlorite-epidote alteration. Three mineralized zones, from east to west, have been recognized as I, II, and III ore zones ( Figure 3a) [11,47,48]. Ore zone I is an approximate length of 1300 m and an average width of 39 m, with relatively low average grades (0.35 wt%) and the highest grade in the thickest, easternmost orebody [47]. Ore zone II is between 130 and 1400 m long and 4 and 125 m thick. It has grades of 0.2 to 2.8% Cu, is the most economically important orebody, and accounts for 96% of the total metal reserves of the deposit [47]. Ore zone III is 230-400 m long, 1-53 m thick, and dips 60 • to 75 • to the south [11]. This is an oxidized ore body, with Cu grades of 0.17-0.53 wt% [48]. The Tuwu porphyry Cu deposit includes veins, veinlets, and disseminated mineralization [44]. The ore minerals are dominated by chalcopyrite and pyrite, with minor bornite, magnetite, molybdenite, sphalerite, and galena [48]. The gangue minerals are composed of quartz, sericite, muscovite, chlorite, epidote, biotite, calcite, plagioclase, and albite [48].

Linglong Deposit Geology
The Linglong porphyry Cu deposit was discovered in 1999 and is located about 14 km east of Tuwu and about 3.5 km north of the Kanggur fault ( Figure 2a) [14]. The outcropping strata mainly consist of Units 2 and 3 of the Qi'eshan Group and Jurassic Xishanyao Formation, with the orebody mainly occurring near the contact zone between quartz albite porphyry and Unit 2 of the Carboniferous Qi'eshan group (Figure 4a).

Linglong Deposit Geology
The Linglong porphyry Cu deposit was discovered in 1999 and is located about 14 km east of Tuwu and about 3.5 km north of the Kanggur fault ( Figure 2a) [14]. The outcropping strata mainly consist of Units 2 and 3 of the Qi'eshan Group and Jurassic Xishanyao Formation, with the orebody mainly occurring near the contact zone between quartz albite porphyry and Unit 2 of the Carboniferous Qi'eshan group (Figure 4a).  [14]).
The orebodies are elongated in the E-W direction and characterized by banding on the surface, and tabular, vein, and bedded textures in sections ( Figure 4b) [14,49]. The main orebodies are between 300 and 1000 m long and 47 and 132 m thick and dip to the south at 40° to 65°. The ore styles in the Linglong porphyry Cu deposit mainly include disseminations, veins, or stockworks [14]. The ore minerals are dominated by chalcopyrite, pyrite with minor molybdenite, magnetite, and limonite, and the gangue minerals are composed of quartz, plagioclase, sericite, chlorite, and epidote, with minor biotite and calcite [14,49].

Chihu Deposit Geology
The Chihu porphyry Cu deposit has proven reserves of Cu (9129.6t) and Mo (7998.5t) and is located about 35 km northeast of the Tuwu and about 7 km north of the Kanggur fault ( Figure 2a). The Chihu granodiorite and porphyritic granodiorite intrude the intermediate to mafic volcanic rocks of the Qi'eshan Group (Figure 5a).
The Chihu molybdenum-copper orebodies have gradual relationships with the surrounding rocks. The orebody has a tabular surface morphology and is elongated in the E-W direction. The main orebodies are about 400-700 m long and 7-15 m thick (Figure 5b), and dip to the south at 30° to 60°, with relatively low average grades (0.22%-0.36%). The ore styles in the Chihu porphyry Cu deposit mainly include disseminations and veinlets. The ore minerals are dominated by chalcopyrite and pyrite, with minor molybdenite, bornite, and chalcocite. The gangue minerals are quartz, sericite, plagioclase, chlorite, and epidote [43]. The Chihu deposit shows a zonation of alteration, as is the case for the majority of porphyry deposits. From the center outward, quartz-sericitization, discontinuous The orebodies are elongated in the E-W direction and characterized by banding on the surface, and tabular, vein, and bedded textures in sections ( Figure 4b) [14,49]. The main orebodies are between 300 and 1000 m long and 47 and 132 m thick and dip to the south at 40 • to 65 • . The ore styles in the Linglong porphyry Cu deposit mainly include disseminations, veins, or stockworks [14]. The ore minerals are dominated by chalcopyrite, pyrite with minor molybdenite, magnetite, and limonite, and the gangue minerals are composed of quartz, plagioclase, sericite, chlorite, and epidote, with minor biotite and calcite [14,49].

Chihu Deposit Geology
The Chihu porphyry Cu deposit has proven reserves of Cu (9129.

Sampling and Analytical Methods
Eighteen molybdenite samples were collected from drill holes of the Tuwu, Linglong, and Chihu porphyry Cu deposits for Re-Os isotope analyses. Among them, five molybdenite samples came from drill holes ZK2401 and ZK2403 in the Tuwu deposit. Four molybdenite samples came from drill hole ZK801 in the Linglong deposit. Nine molybdenite samples came from drill hole ZK001 in the Chihu deposit. The sample (or sampling drill holes) positions are shown in Figures 3a, 4a, and 5b, respectively. The photographs and photomicrographs of the representative samples are shown in Figure 6. Molybdenite occurs as disseminations in granite or quartz veinlets and is cogenetic with chalcopyrite ( Figure 6a,c,f,i).
The molybdenite was magnetically separated and then handpicked under a binocular microscope. Fresh, nonoxidized molybdenite powders (<0.1 mm in size and purity >99%) were used for Re-Os isotope analyses. 187 Re 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 [51][52][53]. The chemical separation procedure is described here briefly [54][55][56][57]: The weighed molybdenite samples were loaded in 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 HNO4 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. Upon cooling, and keeping the bottom part of the tube frozen, the neck of the tube was broken, and the Os was separated using the method of direct distillation from the Carius tube for 50 min and trapped in 3 mL of water that was used for the ICP-MS (X-Series) determination of the Os isotope ratio. 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. Then, 5 mL of 30% NaOH was added to the residue, followed by Re extraction with 5 mL of acetone in a 50 mL centrifuge tube. The acetone phase was transferred to a 150 mL Teflon beaker that contained 1 mL of water. It was evaporated to dryness, and picked up in 2% HNO3, 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 comparable to the recommended value The Chihu molybdenum-copper orebodies have gradual relationships with the surrounding rocks. The orebody has a tabular surface morphology and is elongated in the E-W direction. The main orebodies are about 400-700 m long and 7-15 m thick (Figure 5b), and dip to the south at 30 • to 60 • , with relatively low average grades (0.22%-0.36%). The ore styles in the Chihu porphyry Cu deposit mainly include disseminations and veinlets. The ore minerals are dominated by chalcopyrite and pyrite, with minor molybdenite, bornite, and chalcocite. The gangue minerals are quartz, sericite, plagioclase, chlorite, and epidote [43]. The Chihu deposit shows a zonation of alteration, as is the case for the majority of porphyry deposits. From the center outward, quartz-sericitization, discontinuous argillization, and propylitic alteration zones are recognized.

Sampling and Analytical Methods
Eighteen molybdenite samples were collected from drill holes of the Tuwu, Linglong, and Chihu porphyry Cu deposits for Re-Os isotope analyses. Among them, five molybdenite samples came from drill holes ZK2401 and ZK2403 in the Tuwu deposit. Four molybdenite samples came from drill hole ZK801 in the Linglong deposit. Nine molybdenite samples came from drill hole ZK001 in the Chihu deposit. The sample (or sampling drill holes) positions are shown in Figures 3a, 4a and 5b, respectively. The photographs and photomicrographs of the representative samples are shown in Figure 6. Molybdenite occurs as disseminations in granite or quartz veinlets and is cogenetic with chalcopyrite ( Figure 6a,c,f,i).
The molybdenite was magnetically separated and then handpicked under a binocular microscope. Fresh, nonoxidized molybdenite powders (<0.1 mm in size and purity >99%) were used for Re-Os isotope analyses. 187 Re 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 [51][52][53]. The chemical separation procedure is described here briefly [54][55][56][57]: The weighed molybdenite samples were loaded in 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. Upon cooling, and keeping the bottom part of the tube frozen, the neck of the tube was broken, and the Os was separated using the method of direct distillation from the Carius tube for 50 min and trapped in 3 mL of water that was used for the ICP-MS (X-Series) determination of the Os isotope ratio. The residual Re-bearing solution was saved in a 150 mL Teflon beaker for Re separation.

Results
The rhenium-osmium abundances and the isotopic data for eighteen molybdenite samples are given in Table 2. The rhenium, common Os, and 187 Re and 187 Os concentrations of the five molybdenite samples from the Tuwu deposit vary from 267.0 to 1656.1 ppm, 1.003 to 61.32 ppb, 167.8 to 1040.9 ppm, and 940 to 17,022 ppb, respectively ( Table 2). The calculated model ages are all within error of each other, ranging from 333.0 ± 4.8 to 335.6 ± 5.5 Ma (2σ). These results yield a well-constrained 187 Re-187 Os isochron age of 334.1 ± 3.3 Ma (2σ, MSWD = 0.56, n = 5; Figure 7a) and a weighted average age of 334.5 ± 2.2 Ma (95% confidence level with MSWD = 0.24, n = 5; Figure 7b). These two ages are consistent within The residual Re-bearing solution was heated to near-dryness. Then, 5 mL of 30% NaOH was added to the residue, followed by Re extraction with 5 mL of acetone in a 50 mL centrifuge tube. The acetone phase was transferred to a 150 mL Teflon beaker that contained 1 mL of water. It was evaporated to dryness, and 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 comparable to the recommended value of 139.6 ± 3.8 Ma [53]. 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 ; [58]) and denotes the age. The Re-Os isochron ages were calculated using the least-squares method [59], employing the program ISOPLOT 3.0 [60].

Results
The rhenium-osmium abundances and the isotopic data for eighteen molybdenite samples are given in Table 2. The rhenium, common Os, and 187 Re and 187 Os concentrations of the five molybdenite samples from the Tuwu deposit vary from 267.0 to 1656.1 ppm, 1.003 to 61.32 ppb, 167.8 to 1040.9 ppm, and 940 to 17,022 ppb, respectively ( Table 2). The calculated model ages are all within error of each other, ranging from 333.0 ± 4.8 to 335.6 ± 5.5 Ma (2σ). These results yield a well-constrained 187 Re-187 Os isochron age of 334.1 ± 3.3 Ma (2σ, MSWD = 0.56, n = 5; Figure 7a) and a weighted average age of 334.5 ± 2.2 Ma (95% confidence level with MSWD = 0.24, n = 5; Figure 7b). These two ages are consistent within the error limits and indicate that the Tuwu porphyry Cu deposit was mineralized in the Early Carboniferous.
The rhenium, common Os, 187 Re, and 187 Os concentrations of the four molybdenite samples from the Linglong deposit vary from 2225.0 to 9018.2 ppm, 1.845 to 9.761 ppb, 1398.4 to 6009.5 ppm, and 7364 to 31,862 ppb, respectively ( Table 2). The calculated model ages are all within error of each other, ranging from 314.6 ± 5.3 Ma to 317.4 ± 4.9 Ma (2σ). These results yield a well-constrained 187 Re-187 Os isochron age of 316.8 ± 3.7 Ma (2σ, MSWD = 0.36, n = 4; Figure 7c) and a weighted average age of 315.7 ± 2.4 Ma (95% confidence level with MSWD = 0.23, n = 4; Figure 7d). These two ages are consistent within the error limits and indicate that the Linglong porphyry Cu deposit was formed in the Late Carboniferous.
The rhenium, common Os, 187 Re, and 187 Os concentrations of the nine molybdenite samples from the Chihu deposit vary from 4370.9 to 18,040.0 ppm, 3.257 to 19.54 ppb, 2747.2 to 11,338.5 ppm, and 14,493 to 60,129 ppb, respectively ( Table 2). The calculated model ages are all within error of each other, ranging from 315.3 ± 4.9 Ma to 317.5 ± 5.3 Ma (2σ). These results yield a well-constrained 187 Re-187 Os isochron age of 317.0 ± 3.6 Ma (2σ, MSWD = 0.20, n = 9; Figure 7e) and a weighted average age of 316.5 ± 1.6 Ma (95% confidence level with MSWD = 0.113, n = 9; Figure 7f). These two ages are consistent within the error limits and indicate that the Chihu porphyry Cu deposit was mineralized in the Late Carboniferous.  [58]. 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%. The model age and isochron age were calculated using ISOPLOT 3.0 [60]. Uncertainties for ages are absolute (2σ).  Table 2). The calculated model ages are all within error of each other, ranging from 314.6 ± 5.3 Ma to 317.4 ± 4.9 Ma (2σ). These results yield a well-constrained 187 Re-187 Os isochron age of 316.8 ± 3.7 Ma (2σ, MSWD = 0.36, n = 4; Figure 7c) and a weighted average age of 315.7 ± 2.4 Ma (95% confidence level with MSWD = 0.23, n = 4; Figure 7d). These two ages are consistent within the error limits and indicate that the Linglong porphyry Cu deposit was formed in the Late Carboniferous.
The rhenium, common Os, 187 Re, and 187 Os concentrations of the nine molybdenite samples from the Chihu deposit vary from 4370.9 to 18,040.0 ppm, 3.257 to 19.54 ppb, 2747.2 to 11,338.5 ppm, and 14,493 to 60,129 ppb, respectively ( Table 2). The calculated model ages are all within error of each other, ranging from 315.3 ± 4.9 Ma to 317.5 ± 5.3 Ma (2σ). These results yield a well-constrained 187 Re-187 Os isochron age of 317.0 ± 3.6 Ma (2σ, MSWD = 0.20, n = 9; Figure 7e) and a weighted average age of 316.5 ± 1.6 Ma (95% confidence level with MSWD = 0.113, n = 9; Figure 7f). These two ages are consistent within the error limits and indicate that the Chihu porphyry Cu deposit was mineralized in the Late Carboniferous. The five molybdenite samples from ore-bearing quartz veins and Mo-mineralized tonalite at ore zone I of Tuwu yielded Re-Os model ages of 333.0 ± 4.8 Ma to 335.6 ± 5.5 Ma (Table 2), with an isochron age of 334.1 ± 3.3 Ma (Figure 7a) and a weighted average age of 334.5 ± 2.2 Ma (Figure 7b). This Re-Os age for molybdenite (334 Ma) is older than the 40 Ar/ 39 Ar age (328.1 ± 1.4 Ma) of sericite, although molybdenite is intimately associated with sericite alteration. A possible explanation is that the sericite age was disturbed due to Ar loss from grain margins [61]. Our Re-Os age also overlaps with published molybdenite Re-Os ages (335.6 ± 4.1 Ma; ore zone II of Tuwu; [11]), both of which fall within the zircon U-Pb ages for the Tuwu tonalite porphyry (335-332 Ma; Table 3). It is reasonable to speculate that porphyry Cu-Mo mineralization at Tuwu (ore zones I and II) is genetically related to the Early Carboniferous tonalite porphyry. However, Rui et al. (2002) reported a molybdenite Re-Os age of 322.7 ± 2.3 Ma for Tuwu and Yandong [26], but the sampling location and mineral paragenesis were absent. The younger molybdenite Re-Os age was attributed to the late superimposed mineralization event at Yandong or Tuwu [12]. The four molybdenite samples from the Linglong deposit yielded Re-Os model ages of 314.6 ± 5.3 Ma to 317.4 ± 4.9 Ma (Table 2) and an isochron age of 316.8 ± 3.7 Ma (Figure 7c), which overlaps within the error of the SIMS zircon U-Pb age of the quartz albite porphyry (318.6 ± 3 Ma; [14]). The nine Re-Os molybdenite model ages range from 315.3 ± 4.9 Ma to 317.5 ± 5.3 Ma (Table 2) and yielded an isochron age of 317.0 ± 3.6 Ma for Chihu (Figure 7e), comparable with the SIMS zircon U-Pb ages of granodiorite (320.2 ± 2.4 Ma) and porphyritic granodiorite (314.5 ± 2.5 Ma) in the Chihu area [22,43]. Thus, the porphyry Cu mineralizations at Linglong and Chihu were contemporaneous, and both were related to the emplacement of Late Carboniferous granitoid intrusions (e.g., quartz albite porphyry, granodiorite, and porphyritic granodiorite).

Molybdenite Re Contents and Origin of Ore Metals
Molybdenite Re-Os isotopic approaches can provide evidence of the timing of mineralization and may also be used to track the sources of rhenium [67]. The rhenium content of molybdenite decreases gradually from mantle sources to mixtures of mantle and crust, with the lowest values in crustal rocks because Re is moderately incompatible during mantle melting [68]. It has been widely suggested that the molybdenite Re contents progressively decrease from >100 ppm for a mantle source through tens of ppm for mixed mantle/crust sources to <10 ppm for crustal sources [69][70][71][72]. In this study, the Re contents of molybdenite from Tuwu, Linglong, and Chihu were all higher than 100 ppm (Table 2), comparable with the previously published Re contents of molybdenite from Tuwu and Yandong (Table 4) [11,19]. Thus, we tentatively propose that the Re and, by inference, other metals (e.g., Cu and Mo) of these deposits originated from the mantle. This explanation is also supported by other evidence: sulfur compositions of sulfides (e.g., pyrite, chalcopyrite, and molybdenite) exhibit a near-zero δ 34 S range (−1.9 to +2.3‰ for Fuxing [20]; − 3.3 to +0.8‰ for Yandong [64]; −0.8 to +0.6‰ [18], −3.0 to +1.7‰ [44], and −3.9 to

Molybdenite Re Contents and Origin of Ore Metals
Molybdenite Re-Os isotopic approaches can provide evidence of the timing of mineralization and may also be used to track the sources of rhenium [67]. The rhenium content of molybdenite decreases gradually from mantle sources to mixtures of mantle and crust, with the lowest values in crustal rocks because Re is moderately incompatible during mantle melting [68]. It has been widely suggested that the molybdenite Re contents progressively decrease from >100 ppm for a mantle source through tens of ppm for mixed mantle/crust sources to <10 ppm for crustal sources [69][70][71][72]. In this study, the Re contents of molybdenite from Tuwu, Linglong, and Chihu were all higher than 100 ppm (Table 2), comparable with the previously published Re contents of molybdenite from Tuwu and Yandong (Table 4) [11,19]. Thus, we tentatively propose that the Re and, by inference, other metals (e.g., Cu and Mo) of these deposits originated from the mantle. This explanation is also supported by other evidence: sulfur compositions of sulfides (e.g., pyrite, chalcopyrite, and molybdenite) exhibit a near-zero δ 34 S range (−1.9 to +2.3‰ for Fuxing [20]; − 3.3 to +0.8‰ for Yandong [64]; −0.8 to +0.6‰ [18], −3.0 to +1.7‰ [44], and −3.9 to +4.4‰ [11] for Tuwu), which are consistent with the magmatic source (−3 to + 7‰ [73]); lead isotopic compositions of the Yandong deposit plot along the mantle growth curve [19]; fluid inclusions and C-H-O isotope data from Fuxing, Yandong and Tuwu indicate that the ore-forming fluids are dominated by a magmatic signature [18,20,47]; finally, the mineralization-related porphyries have positive εNd(t) and εHf(t) values [12,14,19,21,22], which indicates the partial melting of a subducted oceanic crust or subduction-modified mantle [12,19,21,22].
Rhenium in porphyry Cu deposits is concentrated primarily as ReS 2 in solid solution in molybdenite at concentrations ranging from about 100 to 3000 ppm [75]. Molybdenite with >1 wt% (10000 ppm) Re is rarely observed in porphyry deposits [71,72,75], although Re concentrations in molybdenite from porphyry systems of up to 4.2% have been reported in the Kirki prospect and 4.7% in the Pagoni Rachi prospect in northern Greece [69,76]. Extremely Re-rich molybdenite from porphyry Cu-Mo-Au prospects and deposits may be affected by a complex interplay of several factors, such as the nature and source of the host rock and the physicochemical conditions of the ore formation (fO 2 , Cl activity, P, T) [71,72]. The molybdenite from Linglong and Chihu contains extremely high Re content ( Table 2; 2225.0-9018.2 ppm and 4370.9-18,040.0 ppm), much higher than that from the Tuwu and Yandong ( Table 4). Considering that these porphyry Cu deposits were generated in different geodynamic settings (flat subduction started ca. 340 Ma, and slab rollback at around 323 Ma) [12], the increased molybdenite Re contents from Tuwu-Yandong to Linglong-Chihu (Table 4) may have been controlled by the tectonic transition from the Early Carboniferous to the Late Carboniferous [77]. Moreover, considering the significant variation in the ore reserves and Cu-Mo grades between the deposits (Table 1), highly variable Re contents of molybdenite from these deposits may also be simply interpreted as a mass-balance mechanism that less abundant molybdenite in porphyry Cu deposits has higher Re concentrations and vice versa [78].

Exploration Implications
Porphyry Cu deposits provide a significant proportion of global copper production [1][2][3]. Most porphyry Cu deposits form in magmatic arcs at convergent plate margins [2,3]. The compressional regimes are ideal for the formation of porphyry Cu deposits [79][80][81], whereas the convergent plate margin settings, characterized by slab rollback and consequent crustal extension, are not conducive to its formation [82]. Compressional environments may effectively prevent magma from passing through the upper crust to form volcanic rocks, thereby forming a shallow magma chamber that is larger than the extensional environment. The shallow magma chamber in the compressional environment is difficult to erupt, which promotes crystallization-differentiation of the magma and may lead to the saturation of hydrothermal fluids. It is difficult to develop a steep extensional fault under the compressional environment, which effectively limits the number of conduits on top of the magma chamber and, therefore, is beneficial to the accumulation of metals [81][82][83][84]. To date, Tuwu contains proven ore reserves of 145 million tons (Mt) at a grade of 0.62% Cu, and Yandong contains 372 Mt at an average grade of 0.58% Cu, together with significant amounts of Mo and Au [11], of which the main mineralization period formed in the compressional environment in a flat subduction environment [12], whereas Linglong and Chihu are small porphyry Cu deposits (Table 1), which formed in the extensional environment of subduction slab rollback [12]. Obviously, the Early Carboniferous compressional regimes were favorable for porphyry Cu mineralization in the Tuwu-Yandong belt.
However, the coeval Fuxing porphyry Cu occurrence in the Tuwu-Yandong belt is sulfide barren (Figures 2a and 8), suggesting that the compressional environment is not a unique factor responsible for the formation of large porphyry Cu-Mo deposits. The formation of the large porphyry Cu-Mo deposits may result from the favorable convergence of many factors, from the crust to the deposit scale (e.g., geodynamics, lithological, permeability architecture, and supergene enrichment blankets) [1][2][3].