Re-Os Geochronology, Whole-Rock and Radiogenic Isotope Geochemistry of the Wulandele Porphyry Molybdenum Deposit in Inner Mongolia, China, and Their Geological Signiﬁcance

: The Wulandele molybdenum deposit is a porphyry-type Mo deposit in the Dalaimiao area of northern Inner Mongolia, China. Molybdenite Re-Os dating yields a model age of 134.8 ± 1.9 Ma, with the ﬁne-grained monzogranite most closely related to the mineralization. The lithogeochemical data show that the monzogranite is weakly peraluminous, high-K calc-alkaline series, with reduced to slightly oxidized, highly fractionated I-type granite characteristics. The relatively low initial 87 Sr / 86 Sr (range from 0.705347 to 0.705771), weakly negative ε Nd ( t ) (range from − 2.0 to − 1.3), and crust-mantle mixing of Pb isotopes suggest that the monzogranite originated from the partial melting of maﬁc juvenile lower continental crust derived from the depleted mantle, with a minor component of ancient continental crust. Combined with the regional tectonic evolution, we argue that the partial melting, then injection, of the monzogranite melt was probably triggered by collapse or delamination of the thickened lithosphere, which was mainly in response to the post-orogenic extensional setting of the Mongol–Okhotsk belt; this is possibly coupled with a back-arc extension related to Paleo-Paciﬁc plate subduction. The extensively fractional crystallization of the monzogranite melt is the crucial enrichment process, resulting in magmatic hydrothermal Mo mineralization in the Wulandele deposit, and the Cretaceous granitoids are generally favorable to form Mo mineralization in the Dalaimiao area. in the MG with the accessory mineral of ilmenite (CPL).


Introduction
In the past two decades, several molybdenum deposits, such as the Wulandele, Wurnitu, Zhunsujihua, Wuhua'aobao, and Dalaiaobao molybdenum deposits, have been found in the Dalaimiao area, which is located in the southwest of Xing'an-Mongolia Orogenic Belt (XMOB) [1][2][3][4][5][6] (Figure 1). These molybdenum deposits have close temporal-spatial relationships with intermediate-felsic intrusive rocks, and most of them formed in late Jurassic to early Cretaceous, except the Zhunsujihua Mo deposit, which formed during the late Carboniferous to early Permian [1][2][3][4][5][6]. Among these molybdenum deposits, the Wulandele is a porphyry-type deposit formed during the Mesozoic in the Erlian-Dongwuqi metallogenic belt, which was discovered in 2006 by the Inner Mongolia Institute Geological Survey [1,7]. The molybdenite in the Wulandele Mo deposit mainly occurs within the inner contact zone of the fine-grained monzogranite (MG) and its wall rocks, with minor development as veins in the wall rocks,  [10,12,13]). The insert is a simplified map showing the position of Xing'an-Mongolia Orogenic Belt in the eastern Central Asian Orogenic Belt (b after [14]).   [10,12,13]). The insert is a simplified map showing the position of Xing'an-Mongolia Orogenic Belt in the eastern Central Asian Orogenic Belt (b after [14]).  [14]).

Samples and Analytical Methods
The Re-Os model age of molybdenite, bulk rock major and trace elements of the MG, Rb-Sr, Sm-Nd, and Pb isotopes of the MG were carried out in this research. All samples of the MG and

Samples and Analytical Methods
The Re-Os model age of molybdenite, bulk rock major and trace elements of the MG, Rb-Sr, Sm-Nd, and Pb isotopes of the MG were carried out in this research. All samples of the MG and molybdenite were collected from the drill cores in the Wulandele Mo deposit. All the bulk rock samples for these analyses were fresh or least-altered rocks.

Re-Os Model Age of Molybdenite
One sample of the Re-Os model age of molybdenite was collected from the drill (XA10-607). We first selected the fine-grained, fresh (without oxidation), and non-contaminated molybdenite assemblages, and crushed and separated to obtain monomineralic molybdenite with purity over 99% for molybdenite Re-Os dating. The Re-Os isotope analyses were carried out in the Re-Os laboratory at the National Research Center of Geoanalysis, Chinese Academy of Geological Sciences, Beijing. The details of the Re-Os chemical separation procedures have been described by Du et al. [15]. Rhenium and Os isotope ratios were determined by a TJA X-series inductively coupled plasma-mass spectrometer (ICP-MS). During this study, Re blanks were < 0.0067 ng and Os blanks were <0.0008 ng. The uncertainty for each individual age determination was 1.4%, including the uncertainties for the 187 Re decay constant, isotope ratio measurements, and spike calibrations. The model ages were calculated using the following formula: t = ln( 187 Os/ 187 Re + 1)/λ. A 187 Re decay constant of 1.666 × 10 −11 y −1 (±1.02%) was used [16].

Major and Trace Element Analyses
Six samples of the MG were prepared for bulk rock major and trace element analysis. The major elements were analyzed by X-ray fluorescence spectrometry (XRF) at the ALS Chemex Laboratory, and the accuracy and precision of the XRF analyses were estimated to be within 5%. The ferrous oxide (FeO) was analyzed for all samples by acid digestion and potassium dichromate titration. Trace element analyses were determined by ICP-MS (Perkin Elmer Elan 9000 and Agilent 7700× with a shielded torch) after fusion and then multi-acid digestion of the samples, and the accuracy and precision were estimated to be better than 10%.

Sr-Nd-Pb Isotopes
Four samples were selected for Sr, Nd, and Pb isotope ratio analysis. The bulk rock Sr-Nd isotopes were determined by Phoenix thermal ionization mass spectrometry, and the bulk rock Pb isotopes were determined by Isoprobe-T thermal ionization mass spectrometry at the Beijing Research Institute of Uranium Geology (BRIUG). The 100 mg sample powders were dissolved in HF + HNO 3 + HClO 4 mixture and separated using the conventional cation-exchange technique with HCl as eluent for Sr and Nd, and using strong alkali anion exchange resin with HBr and HCl as eluents for Pb.

Molybdenite Re-Os Geochronology
The Re-Os isotope results are listed in Table S1. The total Re and normal Os are 31.82 ppm with an uncertainty of 0.24 and 0.2757 ppb with an uncertainty of 0.02, respectively. The 187 Re and 187 Os concentrations are 20 ppm with an uncertainty of 0.15 and 44.98 ppb with an uncertainty of 0.40, respectively. The Re-Os model age is 134.8 ± 1.9 Ma, which is consistent with the isochron age of 134.1 ± 3.3 Ma reported by Tao et al. [1]. Thus, the Mo mineralization age of the Wulandele Mo deposit can be considered as early Cretaceous.

Major and Trace Element Compositions
The major element compositions are presented in Table S2. The MG has high SiO 2 (71.  (Figure 6), most compatible major elements show a decrease in content coincident with increasing SiO 2 content, except K 2 O content, which increases with increasing SiO 2 content. Using the bulk rock Zr composition to represent the melt composition in the model [17], zircon saturation temperatures (T Zr ) were calculated for the MG (Table S2). The calculated results show the MG has a mean T Zr of 778 • C (760 to 786 • C).

Bulk Rock Sr-Nd-Pb Isotopes
Bulk rock Sr-Nd-Pb isotopic compositions of the MG and the calculation parameters are listed in Table S4. Initial 143 Nd/ 144 Nd ratios and ε Nd (t) were calculated using the average crystallization age 131.3 Ma [1]. The Rb/Sr ratios for all samples are below or near 3.0, suggesting that they can be used in the calculation of ( 87 Sr/ 86 Sr) i values and petrogenetic discussion [23,24]

Mo Mineralization Age and Ore-Related Instrusion
The molybdenite Re-Os isotopic model age of 134.8 ± 1.9 Ma from the Wulandele deposit is consistent with the Re-Os isotopic isochron age of 134.0 ± 4.2 Ma and weight mean age of 134.1 ± 3.3 Ma reported by Tao et al. [1], within uncertainties. Thus, we suggested that the Mo mineralization age of the Wulandele Mo deposit can be considered as early Cretaceous.
Tao et al. [1,6] reported the zircon U-Pb ages of the QD and GD are 292.6 ± 0.5 Ma and 299.3 ± 2.4 Ma, respectively, indicating that the QD and GD are the wall rocks for the Mo mineralization. The MG has the zircon U-Pb age of 131.3 ± 1.6 Ma, suggesting its intimately temporal relationship with Mo mineralization. Furthermore, the geological investigations show that the disseminated Mo mineralization mainly occurs inner to the MG or along the contact zones between the MG and its wall rocks (QD and GD), which implies that the MG has a closer spatial relationship with Mo mineralization.
Thus, we argue that the Mo mineralization of Wulandele deposit occurred during the early Cretaceous and is genetically associated with the MG intruding the QD and GD, which are just wall rocks hosting parts of Mo-mineralized veins.

Petrogenesis of the MG
The contents of CaO, Al 2 O 3 , MgO, and total Fe 2 O 3 decrease with increasing SiO 2 contents in the MG (Figure 6), indicating that fractional crystallization has occurred in the magmatic process.  (Figure 8a,b), which suggests that it experienced relatively high degrees of fractional crystallization. All samples have low A/CNK of 1.03-1.05 (all < 1.1, Figure 5c), and fall in the I-type granite field in both the A/NK vs. A/CNK diagram and the TiO 2 vs. Zr diagram (Figure 5d) indicating its I-type granitic affinity, which is consistent with the variation trend of P 2 O 5 vs. SiO 2 (Figure 6e). Although slightly higher, the 10,000×Ga/Al values are very close to 2.6, which is the boundary of S-, I-, and A-type granites (Figure 8c,d) [26]. Thus, we argued that the MG is a highly fractionated I-type granite. The features of high-silica, high-K2O, and low-CaO, high-K calc-alkaline series, weakly peraluminous, suggest that the MG can be classified as K-rich and alkali feldspar porphyritic calc-alkaline granitoids (KCG), as defined by Barbarin [27,28]. This is also supported by its characteristics of enriched Rb, U, Th and K, depleted Ba, Sr, Nb, P and Ti, and right-sloping chondrite-normalized REE pattern. These features implied the MG might originate from a mixed source that involved both the mafic juvenile lower continental crust (mantle-derived materials) and older continental-crustal materials, which are consistent with its mixed Sr and Nd isotope features.
The MG has the characteristics of lower initial 87 Sr/ 86 Sr ratios (0.705347 to 0.705771), weak negative εNd(t) (−2.0 to −1.3). All samples of MG fall between the "depleted mantle" and "lower continental crust" fields in the εNd(t) vs.  In the plot of the Pb isotope evolution curves (Figure 10), the samples fall in the field between the typical mantle and orogenic belts in the 207 Pb/ 204 Pb vs. 206 Pb/ 204 Pb diagram (Figure 10a), and the The features of high-silica, high-K 2 O, and low-CaO, high-K calc-alkaline series, weakly peraluminous, suggest that the MG can be classified as K-rich and alkali feldspar porphyritic calc-alkaline granitoids (KCG), as defined by Barbarin [27,28]. This is also supported by its characteristics of enriched Rb, U, Th and K, depleted Ba, Sr, Nb, P and Ti, and right-sloping chondrite-normalized REE pattern. These features implied the MG might originate from a mixed source that involved both the mafic juvenile lower continental crust (mantle-derived materials) and older continental-crustal materials, which are consistent with its mixed Sr and Nd isotope features.
The MG has the characteristics of lower initial 87 Sr/ 86 Sr ratios (0.705347 to 0.705771), weak negative ε Nd (t) (−2.0 to −1.3). All samples of MG fall between the "depleted mantle" and "lower continental crust" fields in the ε Nd (t) vs. The features of high-silica, high-K2O, and low-CaO, high-K calc-alkaline series, weakly peraluminous, suggest that the MG can be classified as K-rich and alkali feldspar porphyritic calc-alkaline granitoids (KCG), as defined by Barbarin [27,28]. This is also supported by its characteristics of enriched Rb, U, Th and K, depleted Ba, Sr, Nb, P and Ti, and right-sloping chondrite-normalized REE pattern. These features implied the MG might originate from a mixed source that involved both the mafic juvenile lower continental crust (mantle-derived materials) and older continental-crustal materials, which are consistent with its mixed Sr and Nd isotope features.
The MG has the characteristics of lower initial 87 Sr/ 86 Sr ratios (0.705347 to 0.705771), weak negative εNd(t) (−2.0 to −1.3). All samples of MG fall between the "depleted mantle" and "lower continental crust" fields in the εNd(t) vs. ( 87 Sr/ 86 Sr)i diagram (Figure 9a, [23]), indicating multiple contributions from juvenile mafic lower continental crustal components and ancient metamorphic basement with possible slight contamination of upper continental crust. All samples plot within the field of basaltic magma in the ( 87 Sr/ 86 Sr)i vs. t diagram, further supporting this model (Figure 9b). Furthermore, according to the mixing calculation using different end members (Figure 9a, [23]), the source of MG includes mainly juvenile mafic lower continental crust (or depleted mantle) components (60-70%) with minor ancient continental crust (30-40%). In the plot of the Pb isotope evolution curves (Figure 10), the samples fall in the field between the typical mantle and orogenic belts in the 207 Pb/ 204 Pb vs. 206 Pb/ 204 Pb diagram (Figure 10a), and the In the plot of the Pb isotope evolution curves (Figure 10), the samples fall in the field between the typical mantle and orogenic belts in the 207 Pb/ 204 Pb vs. 206 Pb/ 204 Pb diagram (Figure 10a), and the vicinity of the oceanic island volcanic and orogenic belts in the 208 Pb/ 204 Pb vs. 206 Pb/ 204 Pb diagram (Figure 10b), indicating that the source of MG melt is closely related to the mixed origin of the mantle and continental crustal contribution. Combined with the above discussion, we argue that the MG melt originated from a contribution of the predominantly mafic juvenile lower continental crust partial melts (or depleted mantle) with the possible minor ancient continental crust, and possibly contaminated by upper continental crust during melt ascent.
Minerals 2020, 10, x FOR PEER REVIEW 11 of 19 vicinity of the oceanic island volcanic and orogenic belts in the 208 Pb/ 204 Pb vs. 206 Pb/ 204 Pb diagram (Figure 10b), indicating that the source of MG melt is closely related to the mixed origin of the mantle and continental crustal contribution. Combined with the above discussion, we argue that the MG melt originated from a contribution of the predominantly mafic juvenile lower continental crust partial melts (or depleted mantle) with the possible minor ancient continental crust, and possibly contaminated by upper continental crust during melt ascent.

Tectonic Implications
The genesis of the different types of granitoid is strongly constrained by the geodynamic environment. KCG could present in various geodynamic environments, such as the periods of relaxation that separate periods of culmination within a collision event, or transition from a compressional regime to an extensional regime [27,28].
In the Rb vs. Y + Nb diagrams (Figure 11a; [30,31]), the MG occurred in the domain of syn-collisional granite and post-collisional granite. In the Rb/30-Hf-3Ta ternary plot ( [32]) ( Figure  11b,c), the MG also falls in the vicinity of the syn-collisional domain to the late-and post-collisional domain. Meanwhile, in the R1-R2 ( [32]) diagrams, the MG samples show a closer relation to a late orogenic affinity. All these geochemical affinities suggest that the MG probably formed in an extensional environment, which is related to collapse or delamination of the thickened lithosphere post-collisional tectonic setting that was probably affected by local stretching within the syn-collisional setting.

Tectonic Implications
The genesis of the different types of granitoid is strongly constrained by the geodynamic environment. KCG could present in various geodynamic environments, such as the periods of relaxation that separate periods of culmination within a collision event, or transition from a compressional regime to an extensional regime [27,28].
In the Rb vs. Y + Nb diagrams (Figure 11a; [30,31]), the MG occurred in the domain of syn-collisional granite and post-collisional granite. In the Rb/30-Hf-3Ta ternary plot ( [32]) (Figure 11b,c), the MG also falls in the vicinity of the syn-collisional domain to the late-and post-collisional domain. Meanwhile, in the R1-R2 ( [32]) diagrams, the MG samples show a closer relation to a late orogenic affinity. All these geochemical affinities suggest that the MG probably formed in an extensional environment, which is related to collapse or delamination of the thickened lithosphere post-collisional tectonic setting that was probably affected by local stretching within the syn-collisional setting. Figure 11. Tectonic discrimination diagrams for granitoid rocks with MG samples plotted from the Wulandele Mo deposit (a after [30,31]; b after [32]; c after [33]); VAG, Volcanic arc granites; Syn-COLG, Syn-collisional granites; WPG, Within-plate granites; ORG, Ocean ridge granites; post-COLG, post-collisional granites.
Two geodynamic processes play important roles in the formation of granites in XMOB during Jurassic to Cretaceous; one is the closure of the Mongol-Okhotsk ocean, and another is the western subduction of the Paleo-Pacific plate. Although there is controversy concerning the closure time and style of the Mongol-Okhotsk ocean, an increasing number of researchers have come to a consensus Figure 11. Tectonic discrimination diagrams for granitoid rocks with MG samples plotted from the Wulandele Mo deposit (a after [30,31]; b after [32]; c after [33]); VAG, Volcanic arc granites; Syn-COLG, Syn-collisional granites; WPG, Within-plate granites; ORG, Ocean ridge granites; post-COLG, post-collisional granites.
Two geodynamic processes play important roles in the formation of granites in XMOB during Jurassic to Cretaceous; one is the closure of the Mongol-Okhotsk ocean, and another is the western subduction of the Paleo-Pacific plate. Although there is controversy concerning the closure time and style of the Mongol-Okhotsk ocean, an increasing number of researchers have come to a consensus that the closure of the Mongol-Okhotsk ocean was a scissor-like closure (diachronous) from the west (Mongolia) in the Triassic or Jurassic to the east (Amur) in the Early Cretaceous [34][35][36][37][38][39][40][41], which is consistent with the post-collision setting suggested by MG located near the west Mongol-Okhotsk tectonic belt during the early Cretaceous (131 Ma). However, the affinity of the syn-collision setting indicated the MG was also likely related to local extension associated with western subduction of the Paleo-Pacific plate. Therefore, we suggested that the MG formed mainly in response to post-orogenic extensional collapse or delamination of the thickened lithosphere of the Mongol-Okhotsk belt, weakly coupled with local extension related to Paleo-Pacific plate subduction.

Formation of Mo Mineralization
Redox condition seems to be a crucial factor that affects the behavior of Mo as a highly incompatible element, with highly oxidizing magma conditions considered to be beneficial to the formation of porphyry Mo deposits ( [42,43]). The ∆Ox values (∆Ox = log 10 (Fe 2 O 3 /FeO) + 0.3 = 0.03TFeO) of the MG (−0.11 to 0.04) are near 0, which is considered a critical value of oxidized and reduced granites [44]. Half the samples fall into the moderately oxidized field, and the other half of the samples fall into the moderately reduced field (Figure 12a; [44]), indicating that the MG mainly shows weakly oxidized to weakly reduced features. Furthermore, the values of Fe 2 O 3 /FeO for all samples are in the range of 0.35 to 0.49, which is below 0.5, suggesting that the MG can be classified into the ilmenite series rather than the magnetite series (Figure 12b; [45]). These features imply that the MG does not seem to have the characteristics of high oxidization, although analysis of ferrous iron from disseminated pyrite would skew the interpretation of the abundance of FeO. However, it may appear that the highly oxidative magma may not be necessary for the formation of porphyry Mo deposits, and weakly oxidizing magma can also form Mo deposits; caution should be used in interpretation of Fe 2 O 3 /FeO alone.
Minerals 2020, 10, x FOR PEER REVIEW 13 of 19 that the closure of the Mongol-Okhotsk ocean was a scissor-like closure (diachronous) from the west (Mongolia) in the Triassic or Jurassic to the east (Amur) in the Early Cretaceous [34][35][36][37][38][39][40][41], which is consistent with the post-collision setting suggested by MG located near the west Mongol-Okhotsk tectonic belt during the early Cretaceous (131 Ma). However, the affinity of the syn-collision setting indicated the MG was also likely related to local extension associated with western subduction of the Paleo-Pacific plate. Therefore, we suggested that the MG formed mainly in response to post-orogenic extensional collapse or delamination of the thickened lithosphere of the Mongol-Okhotsk belt, weakly coupled with local extension related to Paleo-Pacific plate subduction.

Formation of Mo Mineralization
Redox condition seems to be a crucial factor that affects the behavior of Mo as a highly incompatible element, with highly oxidizing magma conditions considered to be beneficial to the formation of porphyry Mo deposits ( [42,43]). The ΔOx values (ΔOx = log10(Fe2O3/FeO) + 0.3 = 0.03TFeO) of the MG (−0.11 to 0.04) are near 0, which is considered a critical value of oxidized and reduced granites [44]. Half the samples fall into the moderately oxidized field, and the other half of the samples fall into the moderately reduced field (Figure 12a; [44]), indicating that the MG mainly shows weakly oxidized to weakly reduced features. Furthermore, the values of Fe2O3/FeO for all samples are in the range of 0.35 to 0.49, which is below 0.5, suggesting that the MG can be classified into the ilmenite series rather than the magnetite series (Figure 12b; [45]). These features imply that the MG does not seem to have the characteristics of high oxidization, although analysis of ferrous iron from disseminated pyrite would skew the interpretation of the abundance of FeO. However, it may appear that the highly oxidative magma may not be necessary for the formation of porphyry Mo deposits, and weakly oxidizing magma can also form Mo deposits; caution should be used in interpretation of Fe2O3/FeO alone. Figure 12. Redox classification scheme for the MG from Wulandele Mo deposit (a after [44]; b after [45]). FeOT refers to the total Fe in the sample reported as FeO (total). VSO-very strongly oxidized, SO-strongly oxidized, MO-moderately oxidized, MR-moderately reduced, SR-strongly reduced.
Crystal fractionation is an important evolved process for examining the association of ore with felsic magmatism. The role of magmatic fractionation can be seen in association of Cu porphyry deposits with the lower fractionated and most oxidized rocks, the association of W deposits with moderately fractionated rocks, and of Mo and Sn with the most highly fractionated rocks [31,46,47]. The Mo to Cu ratio of the Wulandele molybdenum deposit is remarkably high (up to 39.2 [1]). This copper-poor character is compatible with the ore mineral assemblage of the Wulandele molybdenum deposit, which has very limited chalcopyrite in the deposit according to field and microscopic observations (Figure 4). As a copper-poor stock, the MG represents an obviously highly Figure 12. Redox classification scheme for the MG from Wulandele Mo deposit (a after [44]; b after [45]). FeO T refers to the total Fe in the sample reported as FeO (total). VSO-very strongly oxidized, SO-strongly oxidized, MO-moderately oxidized, MR-moderately reduced, SR-strongly reduced.
Crystal fractionation is an important evolved process for examining the association of ore with felsic magmatism. The role of magmatic fractionation can be seen in association of Cu porphyry deposits with the lower fractionated and most oxidized rocks, the association of W deposits with moderately fractionated rocks, and of Mo and Sn with the most highly fractionated rocks [31,46,47]. The Mo to Cu ratio of the Wulandele molybdenum deposit is remarkably high (up to 39.2 [1]). This copper-poor character is compatible with the ore mineral assemblage of the Wulandele molybdenum deposit, which has very limited chalcopyrite in the deposit according to field and microscopic observations ( Figure 4). As a copper-poor stock, the MG represents an obviously highly fractionated feature, defined by very low abundance of ferromagnesian and Ca-rich minerals. Biotite is the major mafic mineral of the MG, and it comprises less than 3% of the total volume of the rock (Figure 3g,h). Copper and Au tend to be depleted during fractional crystallization of mafic minerals [48], therefore extensive magma (crystal) fractionation resulted in copper depletion compared with molybdenum in the Wulandele deposit. This relationship is also evident in the fractionation diagram with an evolved granitic composition ( Figure 13).
Blevin [44] have suggested that K/Rb is an effective proxy to discriminate the evolved degree for the felsic melt, and the magma can be considered to experience a strong fractionation process when its K/Rb ratio is below 140. The MG had a K/Rb value of 82.5 to 125.2, with an average of 96. 19, and all samples fall in the strongly evolved field in the K/Rb vs. SiO 2 diagram (Figure 13a), which indicates that the MG was undergoing a strong process of fractionation, favorable to Mo mineralization. Barium and Sr are more compatible, so also appear to be specifically sensitive indicators for tracing possible differentiation trends [49]. Within the Rb-Ba-Sr ternary diagram (Figure 13b), the samples are near the field of strongly differentiated granites, which suggest that the MG has experienced a strong differentiation. In the Fe 2 O 3 /FeO vs. Rb/Sr diagram (Figure 13c, [50]), the samples of MG fall in the Mo-W mineralization area, also indicating that the MG is a Mo-favourable intrusion.
In the XMOB, most Cretaceous intermediate-felsic intrusions are cooper-infertile and Mo-favorable. The granitoids related to Mo mineralization mainly occurred during the late Jurassic to early Cretaceous, whereas the granitoids related to Cu (Mo) or Mo (Cu) mineralization mainly occurred in earlier ages, such as Duobaoshan granodiorite porphyry (477 Ma, Ordovician), Wunugetushan monzogranite porphyry deposit (Cu-Mo mineralization, 188 Ma, Early Jurassic), and Zhunsujihua granodiorite and leucogranite (Mo-Cu mineralization, 299-300 Ma, late Carboniferous) [4,6,51,52]. In the Dalaimiao area, besides 39.2 for the Wulandele deposit, the Mo/Cu value is 48.6 for the Wurinitu deposit, as another large Mo deposit formed during the early Cretaceous. As for the Zhunsujihua Mo deposit formed in the late Carboniferous to early Permian, the value is as low as 6.7 [5]. Thus, we deduce that the older intermediate-felsic intrusions (such as Ordovician to early Jurassic) are more favorable to form Cu mineralization, while the younger intrusions are more favorable to form Mo mineralization (such as early Cretaceous) in the Dalaimiao area.

Conclusions
From the above discussion, conclusions are summarized as follows: (1) Both the temporal and spatial relationship between the Mo mineralization and the intrusive rocks suggest that the MG is the ore-related intrusion, and the QD and GD are just ore-hosted wall rocks that host parts of those Mo-mineralized veins. (2) The MG is classified as a high-K, calc-alkaline granitoid, with A/CNK < 1.1, quite low I Sr , and relatively high ε Nd (t) values. The geochemical features constrain the MG to be highly evolved I-type granite, which indicates that its source magma originated by partial melting of a mixed source, including depleted mantle and a lower continental crustal source, possibly contaminated by assimilation of upper continental crust during ascent. (3) The geological and geochemical features imply that the MG and Mo mineralization formed mainly in response to post-orogenic extensional collapse or delamination of the thickened lithosphere of the Mongol-Okhotsk belt, weakly coupled with local extension related to Paleo-Pacific plate subduction. (4) The MG shows weakly oxidized to weakly reduced features, indicating that the highly oxidative magma maybe not be absolutely necessary for formation of a porphyry Mo deposit, although it is important to be aware of the effect of iron sulfides affecting the ferric/ferrous analyses. The process of extensive fractional crystallization is critical to Mo fertility in this evolved magmatic-hydrothermal system.  [25,53]. Funding: This research was funded by the program of China Geological Survey (12120113089300, 12120113088800, 12120113079400).