Petrogenesis and Tectonic Implications of the Ore-Associated Intrusions in Bayanbaolege Ag Polymetallic Deposit, Inner Mongolia, NE China

: The large Bayanbaolege Ag polymetallic ore deposit is located in the Tuquan-Linxi Fe (Sn)-Cu-Pb-Zn-Ag-Nb (Ta) integrating geological background work and previous research work, implying the mineralization age of the Bayanbaolege deposit should have been formed in the 125–130 Ma. Conceptualization, Y.-S.R.; ﬁeld investigation, Q.Y. Y.-S.R.; experimental X.W.; software, validation, X.W., Q.Y. resources, Y.-S.R. data curation, X.W.;


Introduction
The Great Xing'an Range (GXR) polymetallic metallogenic belt is located in the eastern segment of the Central Asian Orogenic Belt (CAOB), northeast (NE) China (Figure 1a,b), which hosts numerous important metal deposits, such as porphyry Sn-Cu-Ag, Cu (Mo) and Cu, and skarn polymetallic and hydrothermal vein types (Figure 1c) [1]. These deposits mainly formed in the Late Jurassic and Early Cretaceous, with a minor amount in the Triassic-Permian [2]. The formation of these deposits in this area were closely related to the contemporary tectono-magmatic events, which are considered to be the results of the evolution of Paleo-Asian, Mongol-Okhotsk, and Paleo-Pacific tectonic regimes ( [3] and references therein).
The east slope of the southern GXR hosts a series of large Cu polymetallic deposits, always considered a Cu polymetallic metallogenic belt. However, several large Ag-Pb-Zn deposits have been discovered in recent years (Figure 1c), showing a considerable Ag-Pb-Zn polymetallic mineralization potential in this area [2].
The large Bayanbaolege Ag polymetallic deposit occurs in the northeastern Tuquan-Linxi Fe (Sn)-Cu-Pb-Zn-Ag-Nb (Ta) metallogenic belt in the east slope of the southern GXR, which contains proven reserves including 1440.41 t Ag, 381,826.1 t Zn and 51,493.27 t Pb, with average grades of 187.8 g/t Ag, 2.75% Zn and 1.89% Pb. A series of studies about the Bayanbaolege deposit have been reported, mainly regarding deposit geology, ore-forming conditions, Rb-Sr isotopic dating, fluid evolution, and S-Pb stable isotopes of the sulfides (pyrite and sphalerite) [4,5]. Although Wang et al. [4] have reported that the granodiorite porphyry had relationships with the polymetallic mineralization spatially, temporally, and genetically. However, the sources of the material of the granodiorite porphyry and the post-ore diorite porphyrite, as well as the relationship between mineralization and magmatism, have not been well-constrained; especially how and to what extent the mixture of the crustal or mantle materials have been involved in the forming of the deposit are still not clear, which restricts the better understanding of the metallogenetic mechanism and regularity of the deposit.
In this study, the major trace elements (including REEs), and Sr-Nd-Hf isotopic data of the ore-associated intrusive rocks and the zircon U-Pb dating of the diorite porphyrite in the Bayanbaolege deposit are combined with the Sr-Nd isotopic data of the Early Cretaceous granitoids and the age of the diagenetic and metallogenic in the adjacent regions. In combination with previous results, we present these study data and aim to discuss the petrogenesis and tectonic setting of the ore-associated intrusions in the Bayanbaolege deposit, which have important implications for better understanding the geological evolution history and metallogenic process of this region.  [1]; (b) simplified geotectonic division of northeastern China (modified after [6]); (c) regional geologic map of the southern Great Xing'an Range (modified after [1]).

Geological Setting
The large Bayanbaolege Ag polymetallic deposit is located in the Songliao block, which is considered to belong to the Xing'an-Mongolian Orogenic Belt (XMOB) in a tectonic unit of the eastern segment of the CAOB (Figure 1a,b). The XMOB is separated by the Xinlin-Xiguitu fault, Hegenshan-Heihe fault, and Solonker-Xar Moron-Changchun fault, dividing the XMOB into several elongated terranes including the Erguna, Xing'an, Songliao, and Jiamusi blocks (Figure 1b). The GXR is located in the eastern segment of the XMOB, NE China, which at least experienced the subduction closure of the Paleo-Asian Ocean, the closure of Mongol-Okhotsk Ocean, and the subduction of the Paleo-Pacific Ocean Plate, and resulted in widespread associated magmatism and metamorphism [7][8][9].
The regional strata are dominated by Permian and Mesozoic volcano-sedimentary rocks. The Permian strata include, from bottom to top, the Shoushangou Formation (P 1 s), comprising thick-bedded limestone. The Dashizhai Formation (P 2 d) is composed of slightly metamorphic intermediate-to-acidic lava, rhyolite, and volcaniclastics intercalated with some sandstone. The Zhesi Formation (P 2 zs) consists of silty mudstone, tuffaceous sandstone, and tuffaceous silty sandstone intercalated with oolitic crystalline limestone lens. The Linxi Formation (P 3 l) is composed of sandy conglomerate and siltstone intercalated. These formations are unconformably overlain by Mesozoic volcano-sedimentary sequences consisting of rhyolite, andesite, and felsic pyroclastic units. The Mesozoic strata, from oldest to youngest, consist of four formations, namely the Manketouebo, Manitu, Baiyingaolao, and Meiletu. The Manketouebo Formation is mainly composed of rhyoitic tuff with minor rhyolite and dacite. The Manitu Formation consists of andesite, trachy dacite, and volcaniclastic rocks. The Baiyingaolao Formation consists of rhyolite and rhyolitic tuff with intercalated sedimentary rocks. The Meiletu Formation is composed of basalt, basaltic andesite, andesite, and trachyte [10].

Deposit Geology
The Bayanbaolege deposit (44 • The strata in this ore district consist of the Zhesi Formation, the Linxi Formation, and the Manitu Formation ( Figure 2). The Manitu Formation unconformably overlies the Zhesi Formation in the ore district and is composed of andesitic breccia clastic tuff, andesite, and dacite. The Linxi Formation consists of silty slate, sandy slate, and metamorphic fine sandstone. The Zhesi Formation consists of silty slate interbedded with a less argillaceous slate.
The NE-SW-trending F1 and F3 faults, and the NW-SE-trending F2 fault form a major tectonic framework in the ore district. The F1 and F3 faults control the distribution of intrusive and volcano-sedimentary rocks and are cut across by the F2 fault ( Figure 2).
Two periods of magmatic activity have been identified in the ore district (Figures 2 and 3). The earlier period is represented by the granodiorite porphyry, which is spatial-temporal and genetically closely related to the polymetallic mineralization (Figure 4a,b) [4], and the latter is represented by diorite porphyrite (Figure 4c,d), which broke down the integrity of the ore body ( Figure 4c).
The deposit consists of 86 sulfide-quartz vein-type polymetallic ore bodies, which show the characteristics of upper Ag bodies and lower Zn bodies as a whole, and distribute in the contact zone between the Late Permian strata and the Early Cretaceous granodiorite porphyry ( Figure 3). The trend of these orebodies extends NE (16-67 • ) and dips along the NW with steep inclination (58-80 • ). Orebodies are 0.86-5.73 m thick, 38-1060 m in horizontal length, and 22-815 m in vertical depth. Ore structures consist of vein-type, and minor banded-type, brecciated-type and disseminated-type structures. The metallic minerals include pyrite, sphalerite, galena and minor chalcopyrite, cubanite, pyrrhotite, arsenopyrite, and loellingite. Gangue minerals include the quartz and calcite. The alterations developed such as limonitization, carbonatization, silicification and weak chloritization and epidotization.

Ore-Forming Elements, Major and Trace Elements
Whole-rock major and trace elements of intrusive rock were analyzed at Tianjin Geological Mineral Testing Center, Tianjin, China. First, these samples were crushed in a steel jaw crusher and then powdered to 200 mesh in an agate mill, and then, trace element compositions were analyzed using the X-Series II ICP-MS (Thermo Electron Corporation, Waltham, MA, USA), following Qi et al. [21] and yielding an analytical precision better than 5%, and major element compositions were analyzed using X-ray fluorescence spectrometer (XRF), with a precision better than 2%. The selected samples were first characterized in terms of chemical composition (major oxides wt.% and trace elements ppm) as listed in Table 1.

Sr-Nd Isotopes
Whole-rock Sr-Nd isotopes of the granodiorite porphyry were analyzed at the Isotopic Laboratory, Analytical Laboratory Beijing Research Institute of Uranium Geology, China National Nuclear Corporation, Beijing, China. Sr-Nd isotopic analyses were carried out at an ISOPROBE-T thermal ionization mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA), and with the 86 Sr/ 88 Sr ratio of 0.1194 and 146 Nd/ 144 Nd ratio of 0.7219 for mass fractionation corrections. The analytical NBS987 Sr standard yields 87 Sr/ 86 Sr = 0.710250 ± 0.000007 (2σ) and the SHINESTU Nd standard yields 143 Nd/ 144 Nd = 0.512118 ± 0.000003(2σ).

Zircon U-Pb Dating and Hf Isotope
The zircon grain extraction and cathodoluminescence (CL) images were completed at the Institute of Regional Geology and Mineral Resources Survey, Langfang, Hebei Province, China. Zircons were extracted from the diorite porphyrite sample of No. ZK1704 and granodiorite porphyry sample of No. ZK7 using standard density and magnetic separation techniques, and cathodoluminescence (CL) images were obtained using a JSM-IT300 scanning electron microscope (JEOL, Tokyo, Japan). Zircon U-Pb isotopes were analyzed by a NEW WAVE 193 nm-FX ArF Excimer laser-ablation system (ESI Ltd., Beaverton, OR, USA) at the Isotopic Laboratory, Tianjin Institute of Geology and Mineral Resources of China Geological Survey, Tianjin, China. Samples analyses were carried out at an energy density of 11 J/cm 2 , with a beam diameter of 30 µm and a repetition rate of 8 Hz. In situ zircon Hf isotopic analyses were measured on the same spot zones where U-Pb age determinations were previously analyzed, with a 50 µm laser beam diameter and a repetition rate of 11 Hz. Details of the operating conditions for the laser ablation system and the MC-ICP-MS instrument are described by Geng et al. [22]. The values of the isotopic ratios were calculated using ICPMS Data Cal 8.4 [23]. Concordia diagrams and weighted mean U-Pb ages were processed using ISOPLOT 3 [24]. Age data and Concordia plots were reported at one error and the uncertainties for weighted mean ages are given at a 95% confidence level.

Major and Trace Elements
The On the rare earth element (REE) chondrite-normalized spider diagram (Figure 6a), the granodiorite porphyry and diorite porphyrite rocks are characterized by the significant enrichment of light rare earth elements (LREEs) and by the variable depletion in heavy rare earth elements (HREEs). In the primitive mantle-normalized trace element spider diagrams (Figure 6b), all samples are relative enrichments in K, Rb, Th, and U, and depletions in Ba, Nb, Sr, P, and Ti.

Sr-Nd Isotopes
Five samples of the granodiorite porphyry from the Bayanbaolege deposit were analyzed for Sr-Nd isotopic compositions, the aged-corrected (130 Ma) initial Sr and Nb isotope data for whole-rock samples, and the results are shown in Table 2. Initial ε Nd (t) values of the samples range from −0.51 to 0.69 and ( 87 Sr/ 86 Sr) i isotope ratios vary from 0.708576 to 0.710536; all samples of the granodiorite porphyry have relatively homogeneous Nd-Sr isotopic compositions, which are consistent with the values of Mesozoic granites in NE China [28,29], and plotted within the overlapped field of the Mesozoic granites from NE China and the Mesozoic granitoids from CAOB in the ε Nd (t) vs. ( 87 Sr/ 86 Sr) i diagram and the Phanerozoic granites of juvenile origin in NE China (Figure 7).  [29]). The area for Mesozoic granitoids in NE China is from [28,29]); Mesozoic granitoids in CAOB are from [30][31][32]; Early Cretaceous granitoids related to mineralization (data from Table 2); (b) ε Nd (t) vs. T DM2 diagram [28], and the curve in this diagram approximates the proportions of mixing between juvenile and ancient crustal components. Data for Phanerozoic granites of juvenile origin in NE China are from [28].

Magmatic Material Source of the Ore-Associated Intrusions
There are three possible interpretations for the source of I-type granites: (1) partial melting of metamorphic intermediate-basic igneous rocks from the crust [47,48]; (2) frac-tional crystallization from mantle magma and assimilation of the crust material in the late stage of magma [49,50] and (3) magma mixing of basaltic magma with felsic magmas [51]. Table 1 showed that samples of granodiorite porphyry from the Bayanbaolege deposit contain low MgO (0.31-0.43 wt.%), Ni (1.71-2.45 ppm), and Cr (10.91-16.90 ppm) element, indicating a few involvements of mantle-derived magma [52,53]. The Nb/Ta ratios for the Bayanbaolege intrusion vary between 8.47 and 10.36, significantly lower than the mantle values (17.5 ± 2) [54,55] and close to the crust values (~11) [55,56]. Combined with the high SiO 2 , K 2 O, low Al 2 O 3 and Sr, Nb, Ta, Ti and P, and Figure 6 showed the positive Rb, Th, and K anomalies, with enrichment in LREEs and LILEs. These geochemical features are commonly matched to crust-derived rocks [57]. Hence, the granodiorite porphyry from the Bayanbaolege deposit is likely to have come from a crustal source or a mixing of the mantle-and crustal-derived magmas. The granodiorite porphyry yielded the ε Nd (t) values ranging from −0.51 to 0.69 and a young two-stage Nd model age (T DM2 =870-967 Ma), indicating that the primary magmas were extracted from a mixed juvenile-crust and oldcrust source (Figure 7).
The  (16.24-16.41 wt.%) contents, suggesting that these rocks are metaluminousperaluminous. The trace elements diagrams in Figure 6 show that the diorite porphyrite and the granodiorite porphyry have a similar tendency. The Nb/Ta values (14.48-14.56) are higher than the crust values (11.0) and lower than those of the primitive mantle (17.5 ± 2) [55], implying these rocks are from the mixing of the mantle-and crustalderived magmas.
These rocks of the diorite porphyrite and granodiorite porphyry have positive ε Hf (t) (+5.7 to +8.8) and (+3.1 to +11.2), and T 2DM ages (827~1108 Ma) and (615~1341 Ma), respectively ( Table 4). All analyses plot above the Chondrite uniform reservoir (CHUR) evolutionary line in a diagram of ε Hf (t) vs. age (Figure 9a) located in the region of Early Cretaceous magmatic rocks in the southern GXR (Figure 9b). The similar εHf(t) values of these samples and the wider ranges and more mantle-like values suggest the possible involvement of heterogeneous juvenile sources, including moderately depleted mantle and newly underplated lower crust.

Mineralization Age of Polymetallic Mineralization
There are two main methods for determining the mineralization age of hydrothermal deposits. One is indirect, using the isotopic ages of rocks (strata or intrusions) associated with mineralization; the other directly measures the ages of ore minerals or coexistence alteration minerals with ore minerals [58]. The indirect mineralization age of the granodiorite porphyry associated genetically with mineralization reported is 130 ± 1 Ma [4] and the direct isochron age of sphalerite is 129.9 ± 2.9 Ma [5]. Hence, the mineralization of the Bayanbaolege deposit occurred in the Early Cretaceous. However, the Bayanbaolege is a large-scale polymetallic deposit including Ag, Zn, and Pb. The different metallogenic temperatures and precipitation phases of these metals have been verified by Wang et al. [4]. The mineralization of these metals should last quite a long time. However, the indirect mineralization age (130 ± 1 Ma) and the direct mineralization age (129.9 ± 2.9 Ma) are too close. To constrain the mineralization duration of the Bayanbaolege polymetallic deposit, the diorite porphyrite was measured and weighted with a mean 206 Pb/ 238 U age of 124.8 ± 1.1 Ma (Figure 8b). These results of the magmatism and mineralization mentioned above give a range of 125-130 Ma, and this result is consistent with the mineralization age (126-132 Ma) of the nearby Aolunhua porphyry Mo-Cu deposit [59]. Meanwhile, the mineralization age of the lager-scale Baiyinnuoer skarn Pb-Zn deposit (129-136 Ma) [37] and the lager-scale Dajing Sn-Cu-Pb-Zn-Ag (133-136 Ma) [60] also show that the hydrothermal mineralization is probably the product of the large-scale tectono-magmatic-mineralization events in the southern GXR in the Early Cretaceous.
The extensional environment during the 138-145 Ma was the common result of the closure of the Mongolia-Okhotsk Ocean and the subduction of the Paleo-Pacific Ocean Plate [83][84][85], and the active continental margin setting of the 106-133 Ma mainly related to the subduction of the Paleo-Pacific Ocean Plate [83,86]. Furthermore, there is a large crossing angle between the tendency (NE) of the Mongol-Okhotsk Ocean suture zone and the tendency (NNE) of the GXR, implying that the Mongol-Okhotsk Ocean tectonic domain cannot efficaciously control the Early Cretaceous magmatism in the GXR area [87,88].
The diorite porphyrite and granodiorite porphyry magmatism of the Bayanbaolege took place in the Early Cretaceous (125 Ma and 130 Ma), and seven samples plotted in the VAG field (Figure 11a-c) and the subduction zone field (Figure 11d) in the geochemical discrimination diagrams, indicating that the granitoids in the Bayanbaolege area have closely related to the subduction tectonic setting instead of the post Mongol-Okhotsk Ocean tectonic regime. This result is consistent with the point that the influence scope of the Paleo-Pacific plate subduction extended to the GXR during the period of the Early Cretaceous [89]. So, it can be concluded that the affected region of the westward subduction of the Paleo-Pacific Ocean plate reached the east slope of the Great Xing'an Range during the early Cretaceous. This viewpoint is also consistent with the thoughts of Zhu [90] and Tian et al. [91], who thought that the gravity gradient zones began to form in the early cretaceous and did not cross the GXR, in other words, the leading edge of the subduction Paleo-Pacific Ocean plate stagnates in the east segment of the GXR. Figure 11. (a) Y vs. Nb diagram [92]; (b)Y + Nb vs. Rb diagram [92]; (c) Rb/30-Hf-3Ta discrimination diagram [93]; (d) Zr versus (Nb/Zr) diagram [94]. Abbreviations: VAG-volcanic arc granite; ORGocean ridge granite; WPG-within-plate granite; Syn-COLG-syncollision granite.

Conclusions
The magmatism in the Bayanbaolege area occurred during the Early Cretaceous (diorite porphyrite: 124.8 ± 1.1 Ma), and the mineralization age should be formed in the 125-130 Ma. The whole-rock elemental compositions of the intrusions implied that the intrusive rocks are the I-type granite. The Sr-Nd-Hf isotopes indicated that the primary magmas of the granodiorite porphyry and the diorite porphyrite were the possible involvement of heterogeneous juvenile sources including moderately depleted mantle and newly underplated lower crust. The Early Cretaceous intrusive rocks in the Bayanbaolege area were formed in an extensional tectonic setting and compactly related to the retreat of the Paleo-Pacific Ocean subducted plate rather than the closure of the Mongol-Okhotsk Ocean.