Petrogenesis and tectonic setting of igneous rocks from the Dongbulage porphyry Mo deposit, Great Hinggan Range, NE China: Constraints from geology, geochronology, and isotope geochemistry

Journal Pre-proofs Petrogenesis and tectonic setting of igneous rocks from the Dongbulage porphyry Mo deposit, Great Hinggan Range, NE China: Constraints from geology, geochronology, and isotope geochemistry Xiang-Guo Guo, Jin-Wen Li, De-Hui Zhang, Fei Xue, Han-biao Xian, ShuaiJie Wang, Tian-Long Jiao PII: DOI: Reference:

S0169-1368(19)30604-3 https://doi.org/10.1016/j.oregeorev.2020.103326 OREGEO 103326

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Ore Geology Reviews

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3 July 2019 8 January 2020 9 January 2020

Please cite this article as: X-G. Guo, J-W. Li, D-H. Zhang, F. Xue, H-b. Xian, S-J. Wang, T-L. Jiao, Petrogenesis and tectonic setting of igneous rocks from the Dongbulage porphyry Mo deposit, Great Hinggan Range, NE China: Constraints from geology, geochronology, and isotope geochemistry, Ore Geology Reviews (2020), doi: https:// doi.org/10.1016/j.oregeorev.2020.103326

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Petrogenesis and tectonic setting of igneous rocks from the Dongbulage porphyry Mo deposit, Great Hinggan Range, NE China: Constraints from geology, geochronology, and isotope geochemistry

Xiang-Guo Guo a,b, Jin-Wen Li b*, De-Hui Zhang a, Fei Xue ac, Han-biao Xiana ， Shuai-Jie Wang a, Tian-Long Jiao b

a

School of Earth Sciences and Resources, China University of Geosciences Beijing,

Beijing 100083, China b

MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral

Resources, Chinese Academy of Geological Sciences, Beijing 100037, China c

Graduate School of Life and Environmental Sciences, University of Tsukuba, Ibaraki

305-8572, Japan

First author, E-mail address: [email protected]; *Corresponding author at: MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China. E-mail address: [email protected]

Abstract The Dongbulage porphyry Mo deposit is a recently discovered deposit located in the Huanggang–Ganzhuermiao polymetallic metallogenic belt of Inner Mongolia, NE China. Here, we present zircon U–Pb ages and Hf isotopic compositions, and whole-rock geochemical and Sr–Nd–Pb isotopic data, for magmatic rocks associated with Mo mineralisation to constrain the age and petrogenesis of these rocks. The rocks are dominated by mineralised granite porphyries, quartz-monzonites, and rhyolite. Zircon U–Pb dating shows that the ore-bearing granite porphyries have ages of 154.4 ± 3.5, 155.4 ± 1.1, and 158.7 ± 0.6 Ma, the quartz-monzonites have ages of 157.8 ± 1.6 and 166.5 ± 1.3 Ma, and the rhyolite has an age of 172.9 ± 3.0 Ma. The granite porphyries and rhyolites are characterised by high K2O and SiO2 contents, enrichment in light rare-earth elements, strong negative Eu anomalies, and pronounced depletion in Ba, Nb, Ta, Sr, P, and Ti. The quartz-monzonites show enrichment in large-ion lithophile elements (Rb and K), are depleted in heavy rare-earth elements, Nb, Ta, Sr, P, and Ti, and exhibit weak negative Eu anomalies. All of the rocks have low initial 87Sr/86Sr (0.7022–0.7064) and εNd(t) values (−3.62 to +3.99), positive εHf(t) values (+1.1 to +13.8), and young two-stage Nd and Hf model ages (TC DM(Nd) = 623–1240 Ma and TC DM(Hf) = 305–1108 Ma, respectively). Whole-rock Pb isotopic compositions show a narrow range of values, with 206Pb/204Pb = 18.314–19.116,

207Pb/204Pb

= 15.573–15.595, and

208Pb/204Pb

= 38.731–39.296,

which, together with their Sr–Nd–Hf isotopic compositions, indicate the dominance of a mantle source component. The isotopic data suggest that the Dongbulage magmatic

rocks were derived from partial melting of juvenile lower crust. The granite porphyries are highly evolved I-type magmas with geochemical characteristics similar to those of porphyry granitoids associated with Mo mineralisation in the Great Hinggan Range. On the basis of the regional geology and geochemistry, we suggest that the Dongbulage porphyry Mo deposit formed in a subduction setting associated with southward subduction of the Mongol–Okhotsk oceanic plate. Keywords: Porphyry Mo deposit, Dongbulage, Zircon U–Pb geochronology, Sr– Nd–Pb–Hf isotopes, Great Hinggan Range, NE China

1. Introduction The Central Asian Orogenic Belt (CAOB), located between the Siberian Craton and the North China Craton (Fig. 1), is regarded as the largest accretionary orogen on Earth, preserving evidence for significant crustal growth and metallogenesis during the Phanerozoic (Goldfarb et al., 2014; Mao et al., 2014; Pirajno, 2010; Şengör et al., 1993; Xiao et al., 2004). The Great Hinggan Range (GHR) in NE China, located in the eastern segment of the CAOB (Figs 1, 2a), underwent a complex tectonic evolution and extensive magmatism during the Palaeozoic and Mesozoic (Bai et al., 2014; Ouyang et al., 2013; She et al., 2012). The GHR hosts a series of porphyry Mo (Cu), skarn Fe (Sn), epithermal Au–Ag, and vein-type Ag–Pb–Zn deposits (Shu et al., 2016; Zeng et al., 2011; Zhai et al., 2017), and is the most important polymetallic metallogenic belt in NE China. Over the past decade, a number of Mesozoic porphyry Mo deposits have been discovered in the GHR, including at Dongbulage, Wunugetushan, Diyanqin'amu, Xing'a, and Chalukou, making the GHR a new focus

for Mo exploration and research investigation. On the basis of geological, geochemical, and chronological data, the formation of these porphyry Mo deposits has been considered to be closely associated with Mesozoic magmatism (Chen et al., 2012; Li et al., 2012; Li et al., 2014; Shu et al., 2015; Wang et al., 2017; Wang et al., 2015b; Zeng et al., 2015; Zhou et al., 2018). These ore-bearing intrusions and deposits provide a unique opportunity to decipher the geodynamic setting and metallogenetic characteristics of the GHR during the Mesozoic. Zeng et al. (2011, 2012, 2015) and Chen et al. (2017) carried out a systematic study of Mo deposits in NE China (including the GHR), and defined the Mesozoic geodynamic setting of the GHR based on these data. Wu et al. (2003a, 2003b, 2011), She et al. (2012), and Tang et al. (2016) studied the petrogenetic types, geochemical characteristics, and magma source of granitoids in NE China, including the GHR, and concluded that the Mesozoic geodynamic setting of the area was related to closure of the Mongol–Okhotsk ocean and subduction of the Paleo-Pacific plate. Zhang et al. (2008, 2010) studied the age and geochemical characteristics of Mesozoic volcanic rocks in the GHR, and concluded that the geodynamic setting was related to subduction of the Paleo-Pacific plate. Ren et al. (2018) conducted paleomagnetic studies on the Jurassic to Early Cretaceous strata in southern Mongolia. Combined with the distribution of magmatic rocks, they suggested that that the late Mesozoic tectonic evolution in the GHR was related to closure of the Mongol–Okhotsk ocean. These results show the value of research on the temporal and spatial distribution of porphyry Mo deposits with respect to the Mesozoic geodynamic setting of the GHR.

The newly discovered Dongbulage porphyry Mo–Pb–Zn polymetallic deposit, in the southern part of the GHR, lies within the Huanggang–Ganzhuermiao polymetallic belt. It is located approximately 55 km east of Xiujimqin and is a small- to medium-sized deposit. Recent drilling and mining in the area have identified new Mo–Pb–Zn-bearing veins and magmatic rocks, including granite porphyry, rhyolite, and quartz-monzonite. The Mo–Pb–Zn mineralisation is closely related to magmatism. As there is no systematic research on the Dongbulage deposit (Li et al., 2017; Li, 2015; Wang et al., 2019; Zhou et al., 2018), the source of the magmatic rocks, and the geodynamic setting in which they formed, are poorly understood. We present new zircon U–Pb ages and Hf isotopic compositions, and whole-rock geochemical and Sr–Nd–Pb isotopic data from igneous rocks in the Dongbulage ore deposit. We use these new data, together with a compilation of data from other porphyry Mo deposits in the GHR, to help understand the timing of magmatism in this area and to elucidate the petrogenesis and tectonic setting of the intrusions and associated Mo mineralisation.

2. Geological setting Northeastern China has traditionally been considered as the eastern part of the CAOB, located between the Siberian and North China cratons (Li, 2006; Zhou et al., 2011; Fig. 1). The region is generally regarded as consisting of a series of micro-continental fragments (Li, 2006; Zhou et al., 2009) comprising, from west to east, the Erguna, Xing'an, Songliao, and Jiamusi–Khanka blocks, which are separated by terrane-bounding faults (Figs 1, 2a).

The GHR generally refers to the southern CAOB in NE China (Zeng et al., 2015). This range contains widespread Mesozoic I- and A-type granitoids (Wu et al., 2011; Xiao et al., 2004). Most of the granitoids were formed during the Late Jurassic to Early Cretaceous and constitute one of the largest plutonic provinces in the world, and thus provide important information regarding the formation of juvenile crust during the Phanerozoic (Wu et al., 2011). The Mesozoic granitoids were emplaced diachronously throughout NE China and can be roughly divided into two stages according to their crystallisation ages (Zhai et al., 2017). The early-stage I- and A-type granitoids were emplaced into the Xing’an and Songliao blocks during the Late Triassic to Middle Jurassic and are considered to record subduction of the Paleo-Pacific plate (Wu et al., 2011). However, contemporaneous granitoids in the northwest might be related to closure of the Mongol–Okhotsk Ocean (Meng, 2003; Tang et al., 2016). The late-stage I- and A-type granitoids were derived from the lower crust, were emplaced along NNE- to NE-trending extensional fault zones during the Late Jurassic to Early Cretaceous, and young towards the northeast (Shu et al., 2016; Wei et al., 2008; Zhai et al., 2017). These late-stage granitoids may be related to subduction of the Paleo-Pacific plate and formed in a post-orogenic extensional setting (Sun et al., 2013; Zhang et al., 2010). Extensive Late Triassic to Middle Jurassic mineralisation developed contemporaneously with the early-stage magmatism. Many large to giant ore deposits formed in the GHR, including numerous porphyry, skarn, epithermal, and vein-type deposits, which together constitute the GHR metallogenic province (GHRP) (Zeng et al., 2011).

The GHRP can be divided into three polymetallic sub-belts on the basis of geological setting, including the Erguna Cu–Pb–Zn–Ag–Mo–Au belt in the northwestern part, the Cu–Mo–Fe–Pb–Zn–Au belt in the northern part, and the Pb– Zn–Ag–Cu–Sn–Fe–Mo belt in the southern part of the GHRP (Zeng et al., 2011). The Huanggang–Ganzhuermiao polymetallic district (HGD) (Fig. 2b) lies in the southern part of the GHRP. It is characterised by numerous porphyry Mo–(Cu) deposits (e.g., at Dongbulage, Chamuhan, Hashitu, Haisugou, Banlashan, Haolibao, Laojiagou, and Aolunhua; Fig. 2b), as well as similar porphyry deposits in neighbouring areas (e.g., at Diyanqin’amu, Caosiyao, Xing’a, Xiaodonggou, Jiguanshan, Chehugou, and Chalukou).

3. Geology of the Dongbulage ore deposit Several porphyry Mo deposits have been discovered in the HGD during the past several years. The Dongbulage deposit, located in the central HGD (Fig. 2b), is a recently discovered small to medium-sized porphyry Mo–Pb–Zn polymetallic deposit. Sedimentary strata in the area include the Permian Zhesi Formation, the Jurassic Manketou’ebo Formation, and Quaternary alluvial sediments (Fig. 3). The Zhesi Formation is composed of metamorphic and sedimentary rocks, including silty slate, argillaceous slate, and pelitic siltstone. The Jurassic Manketou’ebo Formation, which unconformably overlies the Zhesi Formation, comprises volcanic and volcaniclastic rocks, including lithic tuffs, crystal tuffs, and minor ignimbrite. The structural pattern in the ore district is dominated by a series of NE-striking faults and fractures (Fig. 3) that provided the conduits for the passage of hydrothermal fluids and Mo–Pb–Zn

mineralisation. Magmatism was most intensive during the Middle to Late Jurassic. The main igneous rocks exposed in the Dongbulage ore district are quartz-monzonite, rhyolite, and tuff (Fig. 3). The quartz-monzonite and rhyolite occur as stocks and dykes in the northeastern and southeastern parts of the mining area. According to detailed field and thin-section observations, porphyry-type alteration and mineralisation are not associated with the quartz-monzonite and rhyolite and their surrounding rocks, which indicates that these rocks are not related to mineralisation. The concealed granite porphyry is ubiquitous in drill cores (Fig. 4). It occurs as stocks and dykes with widths ranging from 40 to 270 m that intrude the Manketou’ebo and Zhesi formations. The Mo–Pb–Zn mineralisation shows a close spatial relationship with the concealed granite porphyry (Fig. 4). Typical porphyry alteration and mineralisation are well developed in the granite porphyry and its contact zone with the surrounding rocks, indicating a close relationship between the granite porphyry and mineralisation. Although different types of hydrothermal alteration are evident in the Dongbulage deposit, three major alteration zones can be distinguished (Fig. 4): (i) a quartz–potassic zone, (ii) a quartz–sericite zone, and (iii) a propylitic zone. Quartz– potassic alteration is weakly developed at the contact between the granite porphyry and wall rocks. It is present mainly as veins and patches of quartz and K-feldspar, with minor sericite, fluorite, molybdenite, and magnetite. This alteration zone shows a close relationship with Mo mineralisation (Fig. 5a, b, g). Quartz–sericite alteration is relatively intense and overprints the quartz–potassic alteration, occurring mainly in

the lithic tuff, crystal tuff, silty slate, and pelitic siltstone. It is characterised by quartz, sericite, and minor carbonate, and is closely associated with Mo–Pb–Zn mineralisation (Fig. 5c, d, h, j). Most quartz–molybdenite veins are surrounded by quartz–sericite alteration zones. Propylitic alteration is intensely developed in the Manketou’ebo and Zhesi formations (Fig. 5e, f, i, k, l). It is characterised by pervasive development of disseminated chlorite and calcite in tuff, silty slate, and pelitic siltstone. Plagioclase has been partially to completely replaced by chlorite and/or calcite in the propylitic alteration zone. The Pb–Zn ore bodies occur predominantly in this alteration zone. The majority of the Mo–Pb–Zn mineralisation is found in the Zhesi and Manketou’ebo formations (Fig. 4) and is hosted in silty slate, pelitic siltstone, and lithic crystalline tuff, with minor mineralisation in granite porphyry. Core drilling in the Dongbulage ore district has identified 90 porphyry-type Mo orebodies in the northern ore zone and 25 hydrothermal vein-type Pb–Zn orebodies in the southern ore zone. Molybdenum orebodies in the northern ore zone occur as veinlet-disseminated and veins ranging from ~1 to 22 m in thickness and 50 to 180 m in length. Pb–Zn orebodies in the southern ore zone occur as veinlet-disseminated and vein-type deposits with an average thickness of 3.6 m and an average length of 70 m. The total resource has not been fully quantified, as exploration is ongoing. However, the proven resource is estimated at 28.50 Mt of ore grading 0.11 wt% Mo and 1.18 Mt of ore grading 6.06 wt% Pb and Zn (Inner Mongolia Zhongxing Exploration Technology Co., 2010). In addition, recent drilling has discovered new Mo–Pb–Zn mineralised

veins in the deposit. The major ore minerals in the Dongbulage deposit are sphalerite and galena, with lesser quantities of pyrite, pyrrhotite, and chalcopyrite, and minor magnetite. Gangue minerals include quartz, K-feldspar, calcite, sericite, chlorite, and fluorite.

4. Sampling and analytical methods Samples, which are variably altered, were collected from both surface exposures and drill cores, then examined using standard optical microscopy. Twenty-nine fresh samples were selected for geochemical and Sr–Nd–Pb isotopic analyses. Six additional samples were selected for zircon U–Pb dating and Hf isotopic analysis, including three samples of granite porphyry (from drill cores), two samples of quartz-monzonite (from surface exposure), and one sample of rhyolite (from surface exposure). Petrographic characteristics of the samples are presented in Fig. 6, and their mineral compositions are listed in Table 1.

4.1. Zircon U–Pb dating, trace elements, and Hf isotopes Zircon grains were separated from six samples using conventional heavy liquid and magnetic techniques then hand-picked under a binocular microscope. Internal textures in zircons were examined using cathodoluminescence (CL) images at the Electronic Probe and Electron Microscope Laboratory of the Beijing Gaonian Pilot Technology Co. Ltd., Beijing, China. The CL images were then used to select targets for U–Pb isotopic analysis. In situ zircon U–Pb analyses were carried out by inductively coupled plasma–mass spectrometry (ICP–MS) using a Finnigan Neptune-type ICP–MS instrument equipped with a New Wave UP213 laser ablation

system at the Institute of Mineral Resources, Chinese Academy of Geological Sciences, using the analytical procedures outlined by Hou et al. (2009). Analyses used a laser repetition rate of 10 Hz and a spot size of 25 μm. Helium gas was used as a carrier gas to enhance the transport efficiency of the ablated materials. The external standard GJ-1 was used to monitor zircon ages, and Plešovice zircon to ensure measurement precision and accuracy. Zircon U–Pb age concordia and weighted mean age diagrams were plotted using Isoplot/Ex_ver3 (Ludwig, 2003). Analysis of the Plesovice standard yielded an age of 336.8 ± 1.6 Ma (n = 3, 2σ), within uncertainty of the recommended age of 337.13 ± 0.37 Ma (2σ; Sláma et al., 2008). Hf isotopic data for zircons were collected from similar domains previously analysed for U–Pb. Analyses were performed using a Neptune plus multi-collector (MC)–ICP–MS (Thermo Fisher Scientific, Germany) in combination with a Geolas 2005 excimer ArF laser ablation system (Lambda Physik, Göttingen, Germany) at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan, China. High-purity helium was used as a carrier gas to transport the ablated sample, and the laser spot size was 44 μm. Zircon standard 91500 was used for calibration. Instrumental conditions for MC–ICP–MS and further details of procedure are provided by Hu et al. (2012).

4.2. Major and trace elements For whole-rock geochemical analysis, fresh rock sample chips were crushed to 200 mesh after removing weathered surfaces. Whole-rock major- and trace-element analyses were conducted at the Sanhe Research Center for Metallurgical Geology,

Hebei, China. Whole-rock major-element compositions were determined by X-ray fluorescence (XRF), for which analytical uncertainties for all oxides were <5%. Trace-element contents were determined by ICP–MS (Finnigan MAT), for which analytical uncertainties were better than 10% for most elements.

4.3. Whole-rock Sr–Nd–Pb isotopes Whole-rock Rb–Sr, Sm–Nd, and Pb isotopic compositions were measured using a multi-collector Finnigan MAT-262 mass spectrometer at the Laboratory for Radiogenic Isotope Geochemistry, University of Science and Technology of China (USTC), Hefei, China. Each sample powder was weighed and mixed with 87Rb–84Sr and

147Sm–150Nd

spikes prior to dissolution in concentrated HF–HNO3 for one week.

Chemical separation was performed using conventional ion exchange resin columns. The Sr and Nd isotopic ratios were normalised to 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219, respectively, for mass fractionation correction. Repeated analyses of NBS987 Sr standard solution yielded a mean 87Sr/86Sr value of 0.710239 ± 0.000012 (2σ), and the La Jolla Nd standard solution yielded a mean

143Nd/144Nd

value of 0.511870 ±

0.00008 (2σ). The estimated analytical uncertainties for Sm and Rb isotopic ratios are less than 0.5% and 1.0%, respectively. Mass-dependent fractionation for Pb isotope ratios was 0.1% per atomic mass unit based on repeated analyses of the NBS 981 standard. Detailed descriptions of the analytical procedures are given in Chen et al. (2000, 2007).

5. Results 5.1. Zircon U–Pb ages Zircon grains in the different igneous rocks in the Dongbulage ore district are mostly light brown or colourless, prismatic, transparent to translucent, and euhedral, with aspect ratios of 5:1. In CL images, most crystals are characterised by oscillatory zoning (Fig. 7), typical of a magmatic origin. The results of zircon U–Pb and Hf analyses are given in Supplementary Tables S1 and S2, respectively. 5.1.1 Granite porphyry According to their morphology, zircon grains from the granite porphyry samples (DB1, DB2, and DB3) comprise two populations. Some zircon grains in samples DB1 and DB2 have a light grey colour, show faint zoning, and are subhedral, indicating that they are xenocrysts (Miller et al., 2007). Some zircon grains have inherited cores, indicating that they are inherited zircons (Miller et al., 2007). The spot ages of this type of zircon in sample DB1 range from 212 to 284 Ma and in DB2 from 278 to 450 Ma. One zircon in sample DB3, with a spot age of 193 Ma, is also interpreted as a xenocryst. Of the three granite porphyry samples (DB1, DB2, and DB3), seven, nine, and five analysed spots, respectively, were discarded owing to low concordance. The remaining zircon grains yield consistent 206Pb/238U weighted mean ages of 155.4 ± 1.1 Ma (MSWD = 1.3, n = 13), 154.4 ± 3.5 Ma (MSWD = 1.3, n = 12), and 158.7 ± 0.6 Ma (MSWD = 1.1, n = 15), respectively (Fig. 8a–c), which are interpreted as the

crystallisation ages of the granite porphyry. Zircon crystals in sample DB1 have Th contents ranging from 72 to 1670 ppm, U contents from 93 to 882 ppm, and high Th/U ratios ranging between 0.74 and 1.94 (with one outlier at 2.88). Thorium contents of sample DB2 range from 164 to 732 ppm, and U contents from 320 to 1685 ppm, with a narrow range in Th/U ratios of 0.31–0.67. Zircon crystals in sample DB3 have Th contents of 82 to 1243 ppm (with one outlier at 1970 ppm), and U contents from 104 to 707 ppm, with a wide range of Th/U ratios of 0.61–2.79. 5.1.2. Quartz-monzonite For quartz-monzonite sample MD001, seven analysed spots were discarded owing to low concordance. The remaining zircon grains give spot ages ranging from 150 to 163 Ma (except one xenocryst with an age of 223 Ma). These analyses yield a 206Pb/238U

weighted mean age of 157.8 ± 1.6 Ma (MSWD = 1.9, n = 13) (Fig. 8d),

interpreted as the crystallisation age of the quartz-monzonite. Zircon grains in this sample have Th contents ranging from 18 to 1910 ppm (with one outlier at 2723 ppm), U contents from 38 to 1233 ppm (with one outlier at 1849 ppm), and Th/U values ranging between 0.22 and 2.11 (with one outlier at 0.02). On the basis of the different textures, zircon grains from quartz-monzonite sample QD1 can be subdivided into two groups. One group with well-developed oscillatory zoning has spot ages ranging from 168 to 174 Ma and yields a zircon 206Pb/238U

weighted mean age of 170.0 ± 1.1 Ma (MSWD = 0.49, n = 12) (Fig. 8e).

Uranium contents range between 59 and 198 ppm. In contrast, zircons from the other group show weak oscillatory zoning and have spot ages ranging from 161 to 167 Ma

that define a weighted mean 206Pb/238U age of 164.0 ± 0.9 Ma (MSWD = 1.01, n = 16) (Fig. 8e). Thorium contents in this group range from 75 to 349 ppm. Among these two groups, the older zircon crystals are interpreted as antecrysts or xenocrysts from an earlier magmatic event, whereas the younger grains are regarded as autocrysts within the quartz-monzonite magma (Miller et al., 2007). The 164 Ma age is interpreted as the crystallisation age of the quartz-monzonite. 5.1.3. Rhyolite For rhyolite sample MD002, 12 analysed spots were discarded owing to low concordance. The remaining zircon grains define a

206Pb/238U

weighted mean age of

172.9 ± 3.0 Ma (MSWD = 0.36, n = 12) (Fig. 8f), which is considered to represent the crystallisation age of the rhyolite. Zircon crystals in sample MD002 have Th contents ranging from 71 to 1378 ppm, U contents from 449 to 2383 ppm (with one outlier at 3016 ppm), and Th/U values ranging between 0.16 and 0.64.

5.2. Zircon Hf isotopes Hafnium

isotopic

compositions

of

the

three

granite

porphyry,

two

quartz-monzonite, and one rhyolite sample are listed in Supplementary Table S2. With the exception of two grains in samples DB2 and DB1 with

176Hf/177Hf

ratios of

0.282722 and 0.283210, εHf(t) values of 18.1 and 1.1, and TC DM values of 20 and 1108 Ma, respectively, zircon grains in the granite porphyry show highly variable Hf isotopic compositions (176Hf/177Hf = 0.282777–0.283072; εHf(t) = 2.9 to 13.3), and two-stage model ages (TC DM = 322–989 Ma). In contrast, zircon grains from the two quartz-monzonite samples exhibit relatively consistent 176Hf/177Hf ratios between

0.282831 and 0.283061, with εHf(t) and TC DM varying from 5.0 to 13.1 and 349 to 861 Ma, respectively. With the exception of one spot with a

176Hf/177Hf

ratio of

0.282744, εHf(t) of 2.3, and TC DM of 1041 Ma, most zircon grains in the rhyolite exhibit a narrow range in Hf isotopic compositions (176Hf/177Hf = 0.282872– 0.283075; εHf(t) = 6.7–13.8; TC DM = 305–762 Ma).

5.3. Whole-rock Sr, Nd, and Pb isotopes Whole-rock Sr, Nd, and Pb isotopic compositions of the granite porphyry, quartz-monzonite, and rhyolite associated with the Dongbulage ore deposit are given in Supplementary Tables S3 and S4. Initial

87Sr/86Sr

ratios ((87Sr/86Sr)i) and εNd(t)

values were calculated in accordance with their measured zircon U–Pb ages. The granite porphyry shows (87Sr/86Sr)i values of 0.7030 to 0.7054, εNd(t) values of −3.6 to 4.0, and two-stage Nd model ages (TC DM) of 623 to 1240 Ma. The quartz-monzonite has εNd(t), (87Sr/86Sr)i, and TC DM values of −1.1 to 3.9, 0.7042 to 0.7064 and 635 to 1038 Ma, respectively, whereas the rhyolite has values of −2.3 to 2.0, 0.7022–0.7033, and 797 to 1142 Ma, respectively. All samples from the granite porphyry, quartz-monzonite and rhyolite exhibit a wide range of radiogenic

206Pb/204Pb

(18.314–19.493),

207Pb/204Pb

(15.566–15.711),

and 208Pb/204Pb (38.364–39.482) values, in common with other igneous rocks from the GHRP (206Pb/204Pb = 17.295–19.167,

207Pb/204Pb

= 15.406–15.666, and

208Pb/204Pb

=

37.605–39.185; Guo et al., 2010). All of the rocks plot well above the Northern Hemisphere Reference Line (NHRL; Fig. 16).

5.4. Whole-rock element geochemistry Whole-rock compositions of 13 granite porphyry samples, 10 quartz-monzonite samples, and 6 rhyolite samples from the Dongbulage ore district are reported in Supplementary Table S5. In general, samples with Ce/Ce* ratios significantly different from 1 and loss on ignition (LOI) values of >4.5 wt% were considered to be strongly altered, and are not considered further (Polat and Hofmann, 2003; Spitz and Darling, 1978). The remaining samples have Ce/Ce* ratios between 0.95 and 1.43, and LOI values of 0.19 to 2.74 wt% (with the exception of one sample, MD002-1, with 3.93 wt%), suggesting that they are unaltered or weakly altered. Most samples of granite porphyry have high SiO2 (75.22–80.26 wt%) and K2O (5.06–6.76 wt%), low MgO (0.08–0.22 wt%) and CaO (0.20–1.28 wt%), and very low P2O5 (0.01–0.02 wt%) contents. In comparison, the rhyolite samples show widely varying K2O (2.86–7.03 wt%) and Na2O (0.20–4.97 wt%), low CaO (0.12–0.68 wt%), and widely varying SiO2 (70.04–77.39 wt%) contents. The quartz-monzonite samples have high K2O contents (3.18–6.81 wt%), relatively low K2O/Na2O ratios (0.71– 1.78), and a wide range of SiO2 contents (58.33–67.10 wt%). In a total alkali versus silica (TAS) diagram, the granite porphyry and rhyolite are classified as subalkaline, whereas the quartz-monzonite is subalkaline to alkaline (Fig. 9a). The samples straddle the high-K calc-alkaline to shoshonitic series in a K2O vs SiO2 diagram (Fig. 9b). In a (Na2O + K2O − CaO) vs SiO2 diagram (Fig. 9c), the granite porphyry and rhyolite samples plot in the fields of calcic to alkali–calcic, whereas the quartz-monzonite samples plot in the fields of alkali–calcic to alkali. The

granite porphyry samples fall in the peralkaline and peraluminous fields, with A/CNK values (molar Al2O3/[CaO + Na2O + K2O]) ranging between 0.80 and 1.20. The rhyolite is peraluminous, with A/CNK values from 1.34 to 1.73, and the quartz-monzonite is weakly metaluminous to peraluminous, with A/CNK values of 0.95 to 1.17 (Fig. 9d). The granite porphyry, rhyolite, and quartz-monzonite have mean differentiation index (DI) values of 94.41, 89.52 and 80.11, respectively. In addition, the geochemical features of the granite porphyry are consistent with those of granitic rocks related to Mo mineralisation elsewhere in the GHR (e.g., at Caosiyao, Diyanqin'amu, Chalukou, and Xing'a; Fig. 9), and their uniformly high SiO2 and K2O contents and high DI values confirm that they were derived from highly evolved magmas (Sun et al., 2014; Wang et al., 2017; Wu et al., 2017). The total rare-earth-element contents (ΣREE) of the granite porphyry, quartz-monzonite, and rhyolite samples are 97 to 135 ppm, 46 to 195 ppm, and 179 to 223 ppm, respectively. All samples exhibit significant light REE (LREE) enrichment relative to heavy REEs (HREEs), with chondrite-normalised La/Yb ratios ((La/Yb)N) of 3.3 to 12.0. The granite porphyry samples show strong negative Eu anomalies, with Eu/Eu* values of 0.12 to 0.21 (Fig. 10a), whereas the rhyolites exhibit strong to moderate negative Eu anomalies (Eu/Eu* = 0.44–0.87; Fig. 10b), and the quartz-monzonites show weak to moderate negative Eu anomalies (Eu/Eu* = 0.68– 0.93; one sample with Eu/Eu* = 1.03; Fig. 10c). These Eu anomalies indicate variable fractionation of plagioclase. In primitive-mantle-normalised element diagrams, the most significant features

of the granite porphyry and rhyolite samples are marked depletion in Ba, Nb, Ta, Sr, P, and Ti, and enrichment in Rb, Th, U, K, Nd, and Hf (Fig. 10d, e). Similarly, the quartz-monzonites show positive Rb, U, K, and Zr anomalies, and weak depletions in Th, Nb, Ta, Sr, P, and Ti (Fig. 10f). The trace-element characteristics of the granite porphyry are similar to those of many of the granitoids associated with Mo mineralisation in the GHR (Fig. 10a, d). They are generally enriched in LREEs and depleted in HREEs, Ba, Nb, Ta, Sr, P, and Ti, and exhibit strong to moderate negative Eu anomalies.

6. Discussion 6.1. Timing of magmatism and mineralisation Previous studies have reported two stages of zircon U–Pb ages for ore-related magmatism in the Dongbulage deposit: 164.2 ± 2.7 to 160.9 ± 2.9 Ma and 155.6 ± 3.2 to 158.5 ± 2.9 Ma (Li, 2015; Zhou et al., 2018). The ages of mineralisation based on molybdenite Re–Os data are 165.5 ± 1.1 to 162.2 ± 2.2 Ma and 151.8 ± 2.8 Ma (Li et al., 2017; Li, 2015; Zhou et al., 2018). Li (2015) argued that these two stages of magmatism correspond to the two stages of mineralisation. In

this

study,

magmatic

zircon

grains

from

the

granite

porphyry,

quartz-monzonite, and rhyolite in the Dongbulage deposit yielded crystallisation ages of 158.7 ± 0.6 to 154.4 ± 3.5 Ma, 166.5 ± 1.3 to 157.8 ± 1.6 Ma and 172.9 ± 3.0 Ma, respectively (Fig. 8). Our ages for the granite porphyries are slightly younger than those of previous studies. Field observations show that disseminated molybdenite occurs within the granite porphyry (Fig. 6b, c), indicating a close genetic relationship.

In addition, the Mo–Pb–Zn mineralisation shows a close spatial relationship to the granite porphyry. The typical porphyry alteration and mineralisation are well developed in the granite porphyry and its contact zone with the surrounding rocks (Fig. 4), further supporting a genetic relationship. According to field and thin-section observations, there may be two stages of mineralisation in the Dongbulage deposit, for which the mineral assemblage is clearly different. The early mineral assemblage was quartz, molybdenite, and minor pyrite (Fig. 5b, g), whereas the later was sphalerite, galena, pyrite, and scarce molybdenite (Fig. 5c, h, i). Magmatic activity and related Mo–Pb–Zn mineralisation in the study area occurred at 158.7–154.4 Ma, which may correspond to the latter magmatic–mineralisation event.

6.2. Sources and petrogenesis of the magmatic rocks Many porphyry granitoids and associated Mo mineralisation (180–125 Ma) in the GHR have been studied with respect to their whole-rock geochemistry. Most rocks have high SiO2 contents, are classified on TAS diagrams as subalkaline (Fig. 9a), and plot in the high-K calc-alkaline to shoshonitic fields (Fig. 9b). Most are calc-alkalic to alkali–calcic (Fig. 9c) and are metaluminous to peraluminous (Fig. 9d). The mineralised granitoids are highly evolved, with high mean DI values. The granitoids have high SiO2 and K2O contents (Fig. 9b), show marked depletion in Ba, Nb, Ta, Sr, P, and Ti (Fig. 10d), and display moderate to strong negative Eu anomalies (Fig. 10a). Many of the ore-forming granitoids are highly fractionated I-type granitoids, such as the intrusions at Caosiyao, Diyanqin'amu, Chalukou and Xing'a (Chen et al., 2017; Li et al., 2014; Sun et al., 2014; Wu et al., 2017).

Ascertaining the petrogenesis of the magmatic rocks is key to understanding their magmatic source, magmatic processes, and tectonic setting (Barbarin, 1999; Pearce et al., 1984; Wang et al., 2015b). Consequently, it is crucial to identify the genetic type of the Dongbulage magmatic rocks. Jurassic to Early Cretaceous I- and A-type granites are widely distributed in the GHR, whereas S-type granites are rare (Wu et al., 2011; Wu et al., 2002). Various studies have shown that the P2O5 contents of S-type granites increase or remain unchanged with increasing SiO2 content, whereas the P2O5 and SiO2 contents of I-type granites are negatively correlated (Chappell and White, 1992; Li et al., 2007). For the granitoids in the GHR, P2O5 contents decrease with increasing SiO2, whereas Th and Y contents increase with increasing Rb in the quartz-monzonites and rhyolites, respectively (Fig. 11g–i). P2O5 contents in the granite porphyries are very low, and there is no clear linear trend with increasing SiO2 content, whereas Th and Y contents increase with increasing Rb (Fig. 11h, i). These characteristics are consistent with I-type granites (Chappell and White, 1992; Wu et al., 2003a; Zhu et al., 2009). In addition, the rocks do not contain characteristic minerals of S-type granites, such as cordierite and garnet. It should be noted that the distinction between different granite types is not always clear. This is particularly true for distinguishing A-type from highly fractionated I-type granites (Chappell and White, 2001; Chappell et al., 2000; King et al., 2001; King et al., 1997; Wu et al., 2003a). Li (2015) concluded that the granite porphyries in the Dongbulage deposit have geochemical characteristics similar to those of A-type granites. However, Wang et al. (2019) argued that the magmatic rocks

in the Dongbulage deposit are not A-type granites but fractionated I-type granites. Although the Dongbulage granite porphyries have geochemical characteristics similar to those of A-type granites, such as strong depletion in Ba, Sr, P, Ti, and Eu, we do not consider them to be A-type intrusions. Generally, A-type granites contain mafic peralkaline minerals (such as arfvedsonite and riebeckite), display enrichment in Zr, Nb, Y, Ga, and REEs, and show marked depletion in Ba, Sr, P, Ti, and Eu. However, the granite porphyries in the Dongbulage deposit lack mafic peralkaline minerals and have relatively low Zr, Ce, Nb, Y, and La contents, which suggests that they are I-type granites. The marked depletion in Ba, Sr, P, Ti, and Eu in the Dongbulage granite porphyries is most likely the result of strong fractionation. In the Ce, Zr, and K2O + Na2O vs. Ga/Al discrimination diagrams (Fig. 12a–c), the data plot in the highly-fractionated I-type granite field of Wu et al. (2003a). This classification is supported by the (K2O + Na2O)/CaO vs. (Zr + Nb + Y + Ce) diagram (Whalen et al., 1987), which is effective in discriminating fractionated I-type and A-type granites. Zircon saturation thermometry provides a simple and reliable method for estimating the temperature of granitic magma (Miller et al., 2003; Watson and Harrison, 1983). Zircon saturation temperatures (TZr) provide a useful estimate of initial magma temperatures for granites that are rich in inherited zircon, but likely underestimate initial temperatures for granites containing little or no inherited zircon (Miller et al., 2003). The granite porphyries in the Dongbulage deposit contain inherited zircons with older U–Pb ages (Fig. 7; Supplementary Table S1). The calculated TZr values for these granite porphyries are 735–761°C, with a mean of

754°C (Supplementary Table S5), much lower than the typical formation temperatures of A-type granite (~900°C; Clemens et al., 1986; Patiño Douce, 1997). The Jurassic I-type granites in NE China yield TZr values of 700 to 800°C (average of 742°C; Wu et al., 2007a). Furthermore, the granite porphyries from the Dongbulage deposit show geochemical features that are similar to those of highly fractionated I-type granitoids from the GHR, including enrichment in LILEs (e.g., Rb and K) and LREEs, depletion in HFSEs (e.g., Nb, Ta, P, and Ti), and negative Eu anomalies (Fig. 10a, d; Jiang et al., 2016; Li et al., 2014; Wang et al., 2017; Wu et al., 2003a). Therefore, we suggest that the Dongbulage magmatic rocks belong to highly fractionated I-type granites.

6.2.1. Fractional crystallisation The role of fractional crystallisation/accumulation and its influence on the content of incompatible elements needs to be considered before attempting to constrain the petrogenesis of the Dongbulage magmatic rocks. As shown on Harker diagrams (Fig. 11), the majority of the mineralised granitoids throughout the region (e.g., at Caosiyao, Aolunhua, Diyanqin'amu, Chalukou, Wunugetushan, and Dongbulage) show clear fractionation trends. Fractionation of Ti-bearing phases, such as ilmenite and titanite, will result in depletion in Nb and Ta and in Ti and P, respectively. Negative P anomalies indicate fractionation of apatite. Pronounced depletion in Eu requires extensive fractionation of plagioclase and/or K-feldspar, and negative Ba and Sr anomalies reflect fractionation of K-feldspar and plagioclase, respectively.

The negative correlations of Al2O3, CaO, TFe2O3, TiO2, and P2O5 with SiO2 for the quartz-monzonite samples (Fig. 11) indicate fractionation of biotite, plagioclase, apatite, and ilmenite. Linear negative correlations of Al2O3, CaO, Na2O, TiO2, and P2O5 with SiO2 for the rhyolite samples suggest significant degrees of fractionation of plagioclase and accessory minerals, including apatite and Fe–Ti oxides. The very low TFe2O3, TiO2, and P2O5 contents and the clear negative linear correlations of Al2O3, K2O, and Na2O with SiO2 for the granite porphyries indicate fractional crystallisation of K-feldspar, plagioclase, apatite, and Fe–Ti oxides (Fig. 11), as supported by depletions in P and Ti, and negative Eu anomalies (Fig. 10). Plots of Ba vs Sr and Rb/Sr vs Sr (Fig. 13a, b) indicate a role for fractional crystallisation of plagioclase in the evolution of the quartz-monzonite and rhyolite, whereas fractional crystallisation of both K-feldspar and plagioclase occurred during the evolution of the source magma of the granite porphyry. The negative correlation between Al2O3 and SiO2 contents, and the weak negative Eu anomalies, indicating that only limited fractionation of plagioclase in the quartz-monzonites. Compared with the granite porphyries and rhyolites, the quartz-monzonites have lower DI values, weak depletions in Th, Nb, Ta, Sr, P, and Ti (Fig. 10f), and weak to moderate negative Eu anomalies (Fig. 10c). This indicates that the quartz-monzonites underwent lower degrees of fractional crystallisation compared with the granite porphyries and rhyolites.

6.2.2. Source region characteristics Radiogenic isotopic ratios, which are unaffected by processes such as fractional

crystallisation, can be more useful than major and trace elements in constraining the origin of magma (Li et al., 2015; Shu et al., 2015). Most of the late Mesozoic granitoids in NE China, including those of the southern GHR, are characterised by low initial

87Sr/86Sr

ratios, positive εNd(t) values, and young Sm–Nd model ages of

300 to 1200 Ma (Jahn et al., 2000; Jiang et al., 2016; Wu et al., 2003b; Wu et al., 2002). The results of this study combined with previously reported data give relatively uniform Sr–Nd–Pb–Hf isotopic compositions for the mineralised porphyry granitoids and magmatic rocks in the GHR (Supplementary Tables S2–S4 and S6; Figs. 14–16). The majority of the granite porphyries, rhyolites, and quartz-monzonites from Dongbulage have low initial 87Sr/86Sr ratios, positive εNd(t) values, and young Sm–Nd model ages. In a plot of εNd(t) versus (87Sr/86Sr)i (Fig. 14), most rocks from Dongbulage plot on the high (87Sr/86Sr)i side of the mantle array, consistent with Jurassic granitoids in the GHR, Mesozoic granitoids in NE China, and granitic rocks related to porphyry Mo deposits (e.g., at Aolunhua, Diyanqin'amu, Wunugetushan, and Chalukou), suggesting that they are products of partial melting of juvenile lower crust. Therefore, the mineralised porphyry granitoids in the GHR likely share a common juvenile source. The slight difference in Sr–Nd isotopic compositions among these granitoids reflects either heterogeneity of the magma source and/or crustal contamination (Shu et al., 2014; Shu et al., 2015). According to the Sr–Nd isotopic method of Wu et al. (2002), mixing calculations indicate that the parental magma of the Dongbulage magmatic rocks contained 70%–90% juvenile crustal material with

minor contributions from ancient crustal material. Continuous variation of zircon εHf(t) and large numbers of zircon grains with positive εHf(t) values in the porphyry Mo deposits also suggest a dominance of juvenile material (Jahn et al., 2004; Wang et al., 2009). εHf(t) values of zircon grains from the Dongbulage magmatic rocks range from +1.1 to +18.1 (n = 67; most grains between +5 and +13.8; Fig. 15a), and show a frequency peak at around +7 (Fig. 15b, c). The zircons yield TC DM ages of 305–1108 Ma, consistent with other mineralised granitoids in the GHR, indicating that these rocks had a similar magma source. Furthermore, most of the εHf(t) values in zircon grains from these rocks plot between the chondrite uniform reservoir reference line and the depleted mantle evolution line (Fig. 15a), similar to the porphyry Mo mineralisation-related granitoids in the GHR (e.g., Daheishan, Diyanqin'amu, Aolunhua, Banlashan, Wunugetushan, and Chalukou; except Jiguanshan). These data suggest that these rocks were derived from primary magmas that probably originated from partial melting of juvenile lower crust. The Pb isotopic compositions of the Dongbulage magmatic rocks are within the range of those of the Mesozoic igneous rocks and define a clear linear trend (Fig. 16) between the orogenic and mantle Pb evolution lines, with most falling close to the orogenic evolution line (Fig. 16a), consistent with a common source. In summary, on the basis of the geochemical and isotopic data for the Dongbulage magmatic rocks, we propose that the contribution by the juvenile lower crust component was dominant during the generation of these rocks, with the magma undergoing various degrees of fractional crystallisation and limited crustal contamination during emplacement.

6.3. Geodynamic setting Several proposals have been advanced to explain the driving force for Mesozoic magmatism in the GHR. These include: (1) a mantle plume or other intraplate processes (Ge et al., 1999; Lin et al., 1998; Shao et al., 2001; Shao et al., 1994), (2) lithospheric delamination or slab rollback related to subduction of the Paleo-Pacific plate (Wang et al., 2006; Wu et al., 2011; Zhang et al., 2010; Zhang et al., 2008), and (3) subduction of the Mongol–Okhotsk oceanic plate (Fan et al., 2003; Meng, 2003; Xu et al., 2013; Ying et al., 2010). The mantle plume model cannot reasonably account for the fact that mantle-derived mafic rocks (such as basalt and gabbro) are only rarely observed in the GHR (Wang et al., 2006). Geochronological data show that Mesozoic volcanic activity in the GHR was sustained for at least 40 Ma (Ying et al., 2010), much longer than volcanism typically associated with a mantle plume (<8 Ma; Campbell and Griffiths, 1990; Morgan, 1971; Richards et al., 1989). The spatial distribution of Mesozoic volcanic rocks in the region is linear rather than clustered, which also argues against a mantle plume origin (Zhang et al., 2011). Lithospheric delamination or slab rollback related to subduction of the Paleo-Pacific plate is proposed to explain the gradually diminishing westward trend of Mesozoic volcanism along the entire East Asian continental margin (Wang et al., 2006; Wu et al., 2011; Zhang et al., 2010). It should be noted that the Late Jurassic to Cretaceous granitoids in NE China include both I- and A-type intrusions, which show an eastward-younging trend (Wu et al., 2002). This younging trend is interpreted to be

related to northwest-directed flat-slab subduction of the Paleo-Pacific plate (Shu et al., 2016; Wang et al., 2006; Zhang et al., 2010). Furthermore, this model fits well with the occurrence of a Jurassic–Cretaceous accretionary complex along the East Asian continental margin (Isozaki et al., 2010; Wu et al., 2007b; Zhou et al., 2009). However, this model cannot explain the occurrence of Middle–Late Jurassic (155–166 Ma) and Early Cretaceous (138–145 Ma) volcanic rocks that are confined mainly to the GHR and surrounding areas, but which also occur rarely in the Lesser Xing'an and Zhangguangcai ranges, and in the Heilongjiang and Jilin provinces in eastern China (Xu et al., 2013). Importantly, most of the late Mesozoic volcanic rocks in NE China, especially in central and southern Mongolia, lie far (>1000 km) from the position of the subduction zone of the Paleo-Pacific plate during the Mesozoic, and the Japan Sea had not even opened at that time (Wang et al., 2006). Paleogenetic data, Palaeocene adakitic andesites, and Tertiary–Quaternary basalts all indicate that subduction of the Paleo-Pacific plate began during the Late Cretaceous (Engebretson, 1985; Kimura et al., 1990; Maruyama and Send, 1986), much later than the formation of the Dongbulage deposit at ca. 158.7–154.4 Ma. Therefore, it is difficult to explain the tectonic setting of the Dongbulage deposit within the context of subduction of the Paleo-Pacific plate. Subduction of the Mongol–Okhotsk oceanic plate most easily explains the petrogenesis of the igneous rocks in the Dongbulage deposit. The Mongol–Okhotsk Ocean closed gradually eastwards, from central Mongolia, through Transbaikalia, to Amur province. Various ideas have been advanced regarding the timing of closure of

this ocean and can be categorised into three principal viewpoints: Early–Middle Jurassic (Chen et al., 2011; Sorokin et al., 2007; Tomurtogoo et al., 2005; Zorin, 1999), Middle–Late Jurassic (Kuzmin et al., 2002; Xu et al., 2013; Ying et al., 2010), and Early Cretaceous (Cogné et al., 2005; Enkin et al., 1992; Metelkin et al., 2007). The most recent palaeomagnetic data indicate that complete closure of the Mongol– Okhotsk Ocean occurred during the Early Cretaceous (ca 130 Ma) (Ren et al., 2018). Closure triggered the formation of voluminous calc-alkaline to peralkaline plutons and lavas, with associated metallogenesis in adjacent areas. The Mongol–Okhotsk oceanic slab is generally regarded to record bidirectional subduction during the Mesozoic, as judged by the presence of voluminous subduction-related magmatic activity on both sides of the ocean since the Jurassic, from central Mongolia to the present-day Okhotsk Sea (Fig. 17). These widespread plutons consist primarily of granodiorites, monzogranites, syenogranites, biotite granite, and diorites, which formed at active continental margins associated with the bilateral subduction (Tang et al., 2016; Tomurtogoo et al., 2005; Wang et al., 2015b; Wu et al., 2011; Xu et al., 2013). The Mongol–Okhotsk suture belt is one of the most important regions for polymetallic Cu–Mo–Ag–Pb–Zn mineralisation in the CAOB (Lamb and Cox, 1998; Liu et al., 2016; Zeng et al., 2012; Zorin, 1999). Porphyry Mo deposits are a reliable indicator of tectonic setting and related evolutionary processes (Chen et al., 2017). The Late Jurassic–Early Cretaceous (160–130 Ma) Mo deposits are spatially limited to the western Songliao Basin, clustering in the GHR and the western part of the

northern margin of the North China Craton (NNCC), but are absent to the east of the Songliao Basin and in the eastern NNCC. This indicates that the Mo deposits are genetically related to subduction of the Mongol–Okhotsk oceanic plate rather than to rollback of the subducting Paleo-Pacific plate (Chen et al., 2017). Porphyry Mo deposits in the western part of GHR are represented by the Wunugetushan (ca 180 Ma), Dongbulage (ca 158 Ma), Diyanqin'amu (ca 158 Ma), Caosiyao (ca 148 Ma, NNCC), Chalukou (ca 148 Ma), Wulandele (ca 134 Ma), and Xing'a (ca 129 Ma) deposits (Fig. 17), and are associated with highly evolved I-type granitoids interpreted to be closely related to subduction of the Mongol–Okhotsk oceanic plate (Chen et al., 2017; Li et al., 2014; Sun et al., 2014; Wang et al., 2017; Wang et al., 2015b; Wu et al., 2017; Zhang, 2015; Zhang and Li, 2017; Zhang et al., 2016). The Dongbulage porphyry Mo–Pb–Zn polymetallic deposit in southwestern GHR formed during the Late Jurassic. The ore-bearing I-type granite porphyries from the Dongbulage deposit are strongly depleted in HFSEs (e.g., Nb, Ta, and Ti), and enriched in LILEs (e.g., Rb and K) and LREEs. The geochemical characteristics of these rocks suggest that they are arc magmatic rocks and formed in a subduction environment (Barbarin, 1990, 1999; McCulloch and Gamble, 1991). According to geochemical and geochronological data, the highly evolved I-type granite porphyries and associated Mo mineralisation, along with the quartz-monzonites and rhyolites, were emplaced during the Middle to Late Jurassic (ca 173–154 Ma), and formed in the same tectonic setting, related to the southward subduction of the Mongol–Okhotsk Ocean (Fig. 18). The granitic magmas were derived from partial melting of juvenile

lower crust and were intruded into the crust at shallow levels to form the Dongbulage igneous rocks and porphyry Mo deposits.

7. Conclusions (1) The granite porphyries, quartz-monzonites, and rhyolites from the Dongbulage area yield crystallisation ages of 158.7 ± 0.6 to 155.4 ± 1.1 Ma, 166.5 ± 1.3 to 157.8 ± 1.6 Ma, and 172.9 ± 3.0 Ma, respectively. (2) The granite porphyries are highly evolved I-type granites characterised by high SiO2 and K2O contents, enrichment in LILEs, depletion in HREEs, Ba, Nb, Ta, Sr, P, and Ti, and strong negative Eu anomalies, which are consistent with porphyry granitoids and related Mo mineralisation elsewhere in the GHR. Similar to the granite porphyries, the rhyolites are characterised by enrichment in LILEs, depletion in HREEs, Ba, Nb, Ta, Sr, P, and Ti, and strong to moderate negative Eu anomalies. The quartz-monzonites are characterised by high K2O contents, enrichment in LILEs, depletion in HREEs, Nb, Ta, Sr, P, and Ti, and weak negative Eu anomalies. (3) According to field relationships, and geochronological and geochemical data, mineralisation in the Dongbulage deposit is genetically related to the concealed granite porphyry rather than to the quartz-monzonite and rhyolite. (4) Zircon Hf and whole-rock Sr–Nd–Pb isotopic data indicate that the Dongbulage magmatic rocks formed by partial melting of juvenile lower crust. Subsequently, the magmas underwent various degrees of fractional crystallisation and limited crustal contamination during their ascent and emplacement. (5) On the basis of regional geological data and geochemical evidence, we

suggest that the Dongbulage porphyry Mo deposit formed in a subduction setting resulting from southward subduction of the Mongol–Okhotsk oceanic plate.

Acknowledgments This study was financially supported by the National Key Research and Development Program of China (2017YFC0601303) and the National Natural Science Foundation of China (Grant No. 41773030). We are grateful to anonymous reviewers for their critical and helpful comments, which improved the paper. We also thank Prof. Franco Pirajno for his editorial handling. Appreciation is expressed to Prof. Guoxiang Chi from University of Regina for his helpful comments on the paper revision. The manuscript was improved by the thoughtful comments of Dr. Yongjian Kang, Dr. Chenguang Wang, and Dr. Biaoqiang Tai. We thank the Changjian Li, Lingjin Zhang, and Siyao Wang for their help in field work, zircon U-Pb and Hf analyses. We are grateful to Fukun Chen and Ping Xiao for their assistance with whole rock Sr–Nd–Pb isotopic analyses.

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Figure captions Fig. 1 Simplified tectonic map showing the main units of central and eastern Asia and the location of Fig. 2a (modified after Zhou et al., 2011).

Fig. 2 (a) Simplified geotectonic subdivision of NE China (modified after Zhang et al., 2010), showing the location of the southern Great Xing'an Range. (b) Simplified geological map of the southern Great Xing'an Range (modified after Guo et al., 2010), showing the distribution and timing of porphyry Mo–(Cu) and other major deposits. Geochronological data are from Ouyang et al. (2015) and references therein.

Fig. 3 Simplified geological map of the Dongbulage Mo–Zn–Pb deposit (modified after Inner Mongolia Zhongxing Exploration Technology Co., 2010).

Fig. 4 Cross-sections through the Dongbulage Mo–Zn–Pb deposit, showing the spatial relationship between different (meta)sedimentary units, intrusions, and orebodies (modified after Inner Mongolia Sunwell Mining Co., 2017). See Figure 3 for locations.

Fig. 5 Photographs and photomicrographs of hydrothermal alteration features in the Dongbulage deposit. (a) Early quartz–K-feldspar in granite porphyry. (b) Silicification alteration associated with disseminated molybdenite and pyrite. (c) Silicification alteration associated with veined sphalerite, galena, and pyrite. (d) Quartz–sericite alteration, with an alteration assemblage of quartz, sericite, and galena. (e, f) Propylitic zone, with an alteration assemblage of chlorite, calcite, quartz, and pyrite. (g) Quartz coexisting with molybdenite in altered crystal tuff (cross-polarised light).

(h) Sphalerite and quartz coexisting with sericite in altered granite porphyry (cross-polarised light). (i) Sphalerite and quartz coexisting with chlorite in altered crystal tuff (cross-polarised light). (j) Intensive sericite alteration, with early quartz and pyrite in the wall rock (cross-polarised light). (k) Intensive chlorite alteration, with pyrite in the wall rock (cross-polarised light). (l) Intensive calcite alteration, with pyrite in the wall rock (cross-polarised light). Abbreviations: Kfs = K-feldspar, Qtz = quartz, Chl = chlorite, Ser = sericite, Cal = calcite, Mol = molybdenite, Sp = sphalerite, Py = pyrite, Gn = galena.

Fig. 6 Drill-core samples and photomicrographs of the Dongbulage magmatic rocks. (a) Hand-specimen of granite porphyry. (b, c) Disseminated molybdenite in granite porphyry. (d, e) K-feldspar, plagioclase, and quartz phenocrysts in granite porphyry (cross-polarised light). (f) Isolated crystals of molybdenite coexisting with pyrite in altered granite porphyry (reflected light). (g) K-feldspar, quartz, and sphalerite in granite porphyry (plane-polarised light). (h) Sphalerite coexisting with chalcopyrite in altered granite porphyry (reflected light). (i, j) K-feldspar, plagioclase, quartz, and biotite in quartz-monzonite. Feldspars are partially altered to chlorite (cross-polarised light). (k, l) Quartz phenocrysts and fine-grained quartz crystals in rhyolite (cross-polarised light). Abbreviations: Kfs = K-feldspar, Pl = plagioclase, Qtz = quartz, Bt = biotite, Chl = chlorite, Mol = molybdenite, Sp = sphalerite, Py = pyrite, Ccp = chalcopyrite.

Fig. 7 Cathodoluminescence images of representative zircon grains from igneous rocks within the Dongbulage deposit, showing the locations of spots for in situ U–Pb and Hf isotope analyses.

Fig. 8 Concordia diagrams showing U–Pb ages for zircons within igneous rocks from the Dongbulage deposit.

Fig. 9 Geochemical characteristics of the Dongbulage magmatic rocks and the Middle Jurassic to Early Cretaceous (ca 180–130 Ma) mineralised porphyry granitoids from the GHR. (a) Total alkali vs silica (TAS) diagram. (b) K2O vs SiO2 diagram. (c) (Na2O + K2O – CaO) vs SiO2 diagram. (d) A/NK vs A/CNK plot. References for the data are provided in Supplementary Table S6; some altered samples have been removed from the data.

Fig. 10 Chondrite-normalised REE patterns and primitive-mantle-normalised trace-element diagrams for the Dongbulage magmatic rocks and granitic rocks related to porphyry Mo deposits (ca 180–130 Ma; data sources as in Fig. 8) from the GHR.

Fig. 11 Harker diagrams showing: (a–g) Variation in content of selected major oxides (wt%) and trace elements (ppm) against SiO2 (wt%), and (h) Th vs Rb, and (i) Y vs Rb diagrams for the Dongbulage magmatic rocks and mineralised porphyry granitoids (ca 180–130 Ma; data sources as in Fig. 8) from the GHR.

Fig. 12 (a) Ce, (b) Zr, (c) K2O + Na2O vs. 10000 Ga/Al and (d) (K2O + Na2O)/CaO vs. (Zr + Nb + Ce + Y) classification diagrams (Whalen et al., 1987). The compositions of highly-fractionated I-type granites in NE China are after Wu et al. (2003a). FG: Fractionated felsic granite; OGT: unfractionated M-, I-, and S-type granite.

Fig. 13 Ba vs Sr (a) and Rb/Sr vs Sr (b) plots for the Dongbulage magmatic rocks. (a) is after Wu et al. (2003a); (b) is after Yang et al. (2006).

Fig. 14 Plot of εNd(t) vs (87Sr/86Sr)i for the Dongbulage magmatic rocks and granitic rocks related to porphyry Mo deposits in the GHR. References for data are provided in Supplementary Table S6. The two-end-member mixing model used (1) basalt representing the mantle-derived components (87Sr/86Sr = 0.704, Sr = 200 ppm, εNd = +8, Nd = 15 ppm) (Atherton and Petford, 1993); and (2) crustal components (LCC = lower continental crust,

87Sr/86Sr

= 0.708, Sr = 230 ppm, εNd = −15, Nd = 20 ppm;

UCC = upper continental crust, 87Sr/86Sr = 0.740, Sr = 250 ppm, εNd = −12, Nd = 30 ppm) (Audétat, 2010). The end member data are from Wu et al. (2003b).

Fig. 15 Plot of εHf(t) versus U–Pb age (a), histogram of εHf(t), and histogram of TC DM (c) for zircon grains from the Dongbulage magmatic rocks and granitic rocks related to the porphyry Mo deposits in the GHR. Data sources are provided in

Supplementary Table S6.

Fig. 16 Plots of Pb isotopic ratios for the Dongbulage magmatic rocks. (a) 207Pb/204Pb vs

206Pb/204Pb.

(b)

208Pb/204Pb

vs

206Pb/204Pb.

Mantle source reservoirs BSE, DM,

EMI, and EMII are from Zindler and Hart (1986). The Northern Hemisphere Reference Line (NHRL) is from Hart (1984). The evolution line of mantle, orogens, and the lower- and upper-crust reservoirs are from Zartman and Doe (1981). Data sources are provided in Supplementary Table S6.

Fig. 17 Simplified geological map of late Mesozoic (Jurassic–Early Cretaceous) granitoids and mineral deposits in the circum-Mongol–Okhotsk suture belt and adjacent areas of the Great Hinggan Range (modified after Wang et al., 2015a).

Fig. 18 Schematic diagram showing the geodynamic setting of the Dongbulage porphyry Mo deposit in the Great Hinggan Range (modified after Wang et al., 2015b).

Table captions Table 1 Rock type, major mineral contents and accessory minerals of the Dongbulage magmatic rocks. Table 1 Rock type, major mineral contents and accessory minerals of the Dongbulage magmatic rocks.

Rock type

Texture and structure

Major minerals and contents

Accessory minerals

Granite

Porphyritic

Phenocrysts (ratios: 20–30 vol.%):

Apatite,

porphyry

texture,

K-feldspars: 6–9 vol.% (0.3–2 mm,

zircon,

massive

some altered to sericite); plagioclase: 4–

magnetite

structure

6 vol.% (0.5–2.5 mm); quartz: 10–15 vol.% (0.5–3 mm); matrix (ratios: 70–80 vol.%): K-feldspars: 27–30 vol.% (0.03– 0.15 mm); quartz: 30–35 vol.% (0.02– 0.1 mm); plagioclase: 13–15 vol.% (0.05–0.1 mm). (Fig. 5a, b, c, d, e, g)

Quartz-mo

Monzonitic

K-feldspars: 50–55 vol.% (0.03–0.5

Apatite,

nzonite

texture,

mm); quartz: 8–10 vol.% (0.02–0.8

zircon,

massive

mm); plagioclase: 35–40 vol.% (0.05–

magnetite

structure

1.5 mm) ; biotite: <2 vol.% (0.05–0.2 mm). (Fig. 5i, j)

Rhyolite

Porphyritic

quartz: 70–75 vol.% (0.01–0.1 mm);

Apatite,

texture and

alkali feldspar: 15–20 vol.% (0.01–0.05

zircon

felsic texture,

mm); plagioclase: 8–10 vol.% (0.01–

fluidal

0.03 mm). (Fig. 5k, l)

structure

Declarations of interest: none.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Research Highlights



The magmatism in the Dongbulage area occurred in the Middle-Late Jurassic.



The ore-bearing porphyry was crystallized from highly-evolved I-type granitic magmas.



The magmatic rocks were derived from partial melting of the juvenile lower crust.



The deposit was associated with southward subduction of the Mongol–Okhotsk oceanic plate.