Discovery of concealed ore-bodies at the Dongping gold deposit, northern China, revealed by the study of ore-controlling structures

Journal Pre-proofs Discovery of concealed ore-bodies at the Dongping gold deposit, northern China, revealed by the study of ore-controlling structures Xi-An Yang, Jie Wu, Ian M. Coulson, Jinzhang Zhang, Xiaodan Lai, Yabin Zhang, Deru Xu, Jiajun Liu, Guangrong Li, Hualiang Li PII: DOI: Reference:

S0169-1368(19)30668-7 https://doi.org/10.1016/j.oregeorev.2019.103216 OREGEO 103216

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Ore Geology Reviews

Received Date: Revised Date: Accepted Date:

22 July 2019 22 October 2019 6 November 2019

Please cite this article as: X-A. Yang, J. Wu, I.M. Coulson, J. Zhang, X. Lai, Y. Zhang, D. Xu, J. Liu, G. Li, H. Li, Discovery of concealed ore-bodies at the Dongping gold deposit, northern China, revealed by the study of orecontrolling structures, Ore Geology Reviews (2019), doi: https://doi.org/10.1016/j.oregeorev.2019.103216

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Discovery of concealed ore-bodies at the Dongping gold deposit, northern China, revealed by the study of ore-controlling structures Xi-An Yang1,*, Jie Wu2, Ian M. Coulson3, Jinzhang Zhang4, Xiaodan Lai4, Yabin Zhang4, Deru Xu1,**, Jiajun Liu 5,***, Guangrong Li1, Hualiang Li1 1 State

Key Laboratory of Nuclear Resources and Environment, East China University of

Technology, Nanchang 330013, China 2

School of Environment, University of Auckland, Private Bag 92019, Auckland 1142, New

Zealand 3

Solid Earth Studies Laboratory, Department of Geology, University of Regina, Regina,

Saskatchewan S4S 0A2, Canada 4Zinjin 5 State

Mining Group Company Limited, Xiamen, Fujian 361006, China Key Laboratory of Geological Process and Mineral Resources, China University of

Geosciences, Beijing 100083, China

Abstract: The Dongping gold deposit is the largest gold deposit in the Hebei province, China, with proven gold reserves of more than 100 tons. After the main gold ore-bodies were mined out, the Dongping gold deposit faced a shortage in gold reserves with an uncertain future. As a result, we conducted systematic studies on the ore-controlling structures at the Dongping gold deposit, with the aim of finding additional concealed ore-bodies, should these be present. Gold mineralization at Dongping is controlled by two stages of maximum principal compressional stress (1): NW-trending stress that produced the NE- to NW-trending structures, and sub-vertical stress that produced the circular structure, respectively. The sub-vertical compressional stress may have resulted from the emplacement of a concealed intrusive body, which also led to horizontal extensional stress, generating structures surrounding the intrusion. Development of such structures potentially increased the permeability of the local crust, facilitating the mobilisation of gold-bearing hydrothermal fluids that subsequently precipitated 1

along the structures, forming the gold ore-bodies. We propose that the deep part of the Dongping gold deposit at the lithological contact with the concealed intrusive bodies is prospective for the exploration of additional reserves, which is supported by our subsequent drilling and the discovery of concealed gold ore-bodies beneath the deposit.

Keywords: Concealed ore-body; Gold deposit; Dongping; Ore-controlling structure; Stress analysis; Mineral exploration

* Corresponding author at: State Key Laboratory of Nuclear Resources and Environment, East China University of Technology, Nanchang 330013, China ** Corresponding author at: State Key Laboratory of Nuclear Resources and Environment, East China University of Technology, Nanchang 330013, China ***Corresponding author at: State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences, Beijing 100083, China E-mail addresses: [email protected](X.A. Yang), [email protected] (D.R. Xu), [email protected] (J.J. Liu)

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1. Introduction Located in the northern Hebei province, China, the Dongping gold deposit is the largest gold deposit in the province, with over 100 tons of proven gold reserves (Bao et al., 2014). The gold deposit was initially discovered in 1985 (Song and Zhao, 1996), and since mining began some 800 tons of gold ore had been milled per day, prior to 1995. The Zijin Mining Group Company Limited obtained the mining and exploration rights to this deposit in 2006, and increased the production scale to ~2,500 tons of gold ore milled per day, equivalent to an annual gold production of 2,570 kg. The originally projected mine life was about 12 years, but after about 30 years of mining, the main No. 1 gold-quartz veins and No. 70 stockworks had been mined out, leaving the future of mining at the Dongping gold deposit in doubt. Numerous mineral exploration programs have been carried out at the Dongping gold deposit over the past few years, but without success. These included several drilling programs exploring both the rocks at depth under the Dongping gold deposit, and the surrounding surface regions. In total, about 70,000 m including several hundred drill holes were drilled in the region prior to 2015. The drill holes were widely distributed throughout the Dongping gold deposit with a maximum depth of 1,500 m; each hole was targeting concealed gold bodies beneath the existing mine. The drill target generation has, thus, proven to be a challenge. Hosted in the Shuiquangou syenite complex, the Dongping gold deposit was recognized as a new type of gold deposit associated with alkali syenite, by the Golden Army of Chinese People's Armed Police Force. The Dongping is another example of world-class gold deposits that are hosted by high-K intrusions (Muller and Groves, 2019). The gold deposit was named the “Dongping type” by the Geological Society of China in 1992 (Song and Zhao, 1996), a term which has been was widely accepted by subsequent researchers (e.g., Nie, 1998; Bao et al., 2014, 2016). However, recent studies suggest that the genesis of the Dongping gold deposit is complex. For example, the ore-forming processes were controlled by brittle-ductile structures (Lu et al., 1997) or related to magmatic intrusions (Zhang et al., 2012), and the primary ore 3

fluids were derived from Yanshanian granitic magmatism (Fan et al., 2001). In this study, data from remote sensing, ground geophysics, geochemistry, structure and geochronology (Song and Zhao, 1996; Zijin Mining Group Company Limited) have been integrated for the first time to propose a model for the gold mineralization at the Dongping deposit, which will also help to predict the presence and location of concealed ore-bodies. Hence, our model does not only provide new insights into the gold mineralization at Dongping, but it also has important implications for the exploration of deep and concealed mineral deposits in general.

2. Geological background The Dongping gold deposit is situated at the northern margin of the North China Craton, at the junction of the Inner Mongolia Shield and the Yanliao Depression (Mao et al., 2003; Goldfarb et al., 2014). It is located about 10 km south of the Shangyi-Chongli-Chicheng fault belt (SCCFB; Fig. 1a), a ductile to brittle thrust fault that extends along the boundary between the Inner Mongolia Shield and the North China Craton. The SCCFB formed in the Mesoproterozoic, has been reactivated in multiple times, and is currently seismically active (Ma and Zhao, 1999; Hu et al., 2002; Zhang et al., 2007). A series of smaller scale and/or second-order faults were developed along this fault, which are also intruded by granitic, pegmatitic and porphyritic dykes. Gold deposits are hosted by the second-order faults in this area. The regional stratigraphy of this deposit includes the Neoarchean Sanggan Group (south of the SCCFB; Fig. 1b), the Paleoproterozoic Hongqiyingzi Group (north of the SCCFB), the Mesoproterozoic Changcheng Group in the southeast of the gold area, and Jurassic and Quaternary volcano-sedimentary rocks to the south and southeast of the Dongping gold area. The Archean Sanggan metamorphic complex, which is exposed to the south of the Shangyi-Chongli-Chicheng fault, is a metamorphic complex consisting of amphibolite, granulite, and fine-grained paragneisses, generated from a series of mafic to felsic 4

volcaniclastic rocks and clastic sediments. The Hongqiyingzi Group, which is exposed to the north of the fault, consists of marbles, quartzites, and amphibolites, respectively. The Changcheng Group occurs to the southeast of the area, and is composed of marine sedimentary rocks. The Cretaceous Zhangjiakou Formation is a series of volcano–sedimentary rocks comprising rhyolite, rhyodacite, trachyte, and pyroclastic flows; it is widely distributed in this area. The Dongping gold deposit is hosted within the NW-trending Shuiquangou syenite complex, which intrudes metamorphic rocks of the Sanggan Group. The complex is composed of augite-hornblende-alkali feldspar syenite (Fig. 1b), alkali feldspar syenite, and quartz-alkali feldspar syenite, which were emplaced at 400–386 Ma (Miao et al., 2002; Bao et al., 2014). Younger intrusive rocks at Dongping span a wide range of ages (Fig. 1b), and include: (1) the Honghualiang biotite syenogranite (235±2 Ma), (2) the Guzuizi coarse-pegmatitic porphyritic syenogranite (236±2 Ma), (3) the Zhuanzhilian diorite (139.5±0.9 Ma), (4) the Shangshuiquan syenogranite (142.5±1.3 Ma), and (5) the Beizhazi monzogranite (130.5±1.5 Ma) (Jiang et al., 2007; Miao et al., 2002; Li and Bao, 2012). Gold mineralization is proposed to be genetically associated with the syenites (Nie et al., 2004). However, recent age dating on gold-quartz veins from the Dongping gold deposit yields an age of ca. 143 Ma (Cisse et al., 2017), in agreement with the age of the Shangshuiquan alkali granites. This supports the genetic connection between the gold mineralization and the Shangshuiquan alkali granites (Wang et al., 2003; Mao et al., 2003; Nie et al., 2004; Ouyang et al., 2013; Cisse et al., 2017). The Dongping gold deposit occurs near to the southern contact zone between the Shuiquangou syenite complex and the metamorphic rocks. The gold ore-bodies in the upper part of the deposit are gold-quartz veins, whereas those in the lower part are primarily millimeter-wide, pervasive, disseminated stockworks with no continuity at the meter-scale in any direction. Structures at the Dongping gold deposit are dominated by NE-trending faults and NW-trending fractures. The gold-quartz veins are controlled by a series of parallel, 5

commonly en echelon NE-trending faults (Fig. 1c), striking 0–35° NE, dipping to NW at 45–75°, and covering a surface area of more than 50 km2, whereas the stockworks are primarily hosted in NW-trending fractures. The No. 1 gold-quartz veins are the largest concentration of gold-quartz veins, and the No. 70 stockworks are the largest stockworks in the gold deposit. Collectively, these are estimated to contain about 3.3 million tons of gold (grading at 6 g/t on average) and ~9 million tons (grading at 5 g/t on average), respectively (Cisse et al., 2017), accounting for ~80% of the total gold reserves of the Dongping gold deposit (Mao et al., 2003). Primary gold ore-bodies within the deposit, including the No. 1 gold-quartz veins and No. 70 stockworks have recently been mined out. The No. 1 gold-quartz veins strike 20–50° NE with a dip to the NW at angles of 40–50°, and range from tens of metres to 300 metres along strike, tens of meters along the plunge and a few centimeters to tens of centimeters in width (Fig. 2a). The gold-quartz veins are fault-controlled, and there are rare breccias, cemented by the veins. The breccias consist of syenite clasts, a few centimeters to tens of centimeters in size. The No. 70 stockworks mainly consist of intensively disseminated sulfide quartz veinlets, striking 290–340° NW and dipping to the SW at 25–50°. These range from tens of meters to ~100 metres along the strike and few centimeters to several meters in width (Fig. 2b). All of the gold ore-bodies are surrounded by an extensive potassic alteration, comprising quartz, K-feldspar, magnetite, albite, pyrite, sericite, chlorite, and epidote, of which the K-feldspathization (K-feldspar ± magnetite) is the most common alteration type. Gold is generally fine-grained, rarely visible in hand specimen and mainly occurs as native gold, calaverite, electrum, and petzite in quartz and pyrite. The gold ores contain a low proportion (< 3%) of sulfides. Opaque minerals include pyrite, chalcopyrite, galena, sphalerite, covellite, specular hematite and magnetite, while gangue minerals are quartz, microcline, and albite, with subordinate sericite, epidote, chlorite, calcite, kaolinite, and barite (Li and Makovicky, 2001; Cook et al., 2009).

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3. Evidence for deep and concealed magmatic intrusions 3.1 Circular structures controlling the Zhangjiakou-Xuanhua gold belt Three concentric / circular structures are distributed in the Zhangjiakou-Xuanhua area, having resulted from active uplift associated with the Yanshanian igneous event (Niu et al., 2009), triggered by NW subduction of the Izanagi plates beneath eastern China (Zhou et al., 2002). Numerous porphyry-associated multi-metal deposits are located in the two outer circular structures (Fig. 1a). More than 100 gold deposits (occurrences) occur in the inner circular structure, which constitutes the Zhangjiakou-Xuanhua gold belt (ZXGB) and which comprises the following gold deposits: Dongping, Xiaoyingpan, Zhangquanzhuang, Shuijingtun, Hougou, Jinjiazhuang, Zhongshangou, Huangtuliang, and the Zhaojiagou. Each of these gold deposits is distributed within a NW ellipse (Fig. 1b). The ZXGB is controlled by the circular structure and NE- and NW-striking faults (Wang et al., 2007; Li et al., 2012) based upon remote sensing map data (Fig. 3). The geological features of the ZXGB can be summarized as: (1) Archean metamorphic rocks form the core of the ZXGB, with Proterozoic metamorphic rocks in the north, Quaternary formation in the south, and Jurassic formations in the western and eastern segments of the ZXGB (Fig. 1b); (2) Strata and structures in the northern and southern parts of the ZXGB dip to the north and to the south, respectively; the normal faults in the western ZXGB dip to the west, and in the eastern ZXGB to the east, respectively. The Archean metamorphic rocks in the southern ZXGB have a faulted contact with Quaternary rocks, while the Jurassic volcanic sedimentary basins were developed in unconformable contact with the underlying rocks in both the western and eastern parts of the ZXGB; (3) Outcrops of Yanshanian granite surround the ZXGB; (4) The ZXGB forms the highest topography in the northern Hebei province, and the uplifted area is known as the Longguan dome (Shao et al., 2004; Niu et al., 2009). The partly concealed Sandaochuan syenite porphyry only forms rare outcrops in the eastern part of the Dongping gold deposit and it defines a negative gravity anomaly (Zhang, 7

2006). Negative gravity and positive magnetic anomalies are also recorded beneath the ZXGB (Fig. 4). Therefore, concealed intrusive bodies are likely emplaced beneath the ZXGB, and they may well explain the formation of the circular structure controlling the ZXGB. The magma intrusion is interpreted to be coeval with the Yanshanian granite (ca. 143 Ma; Cisse et al., 2017), surrounding the ZXGB (Fig. 1b), and may also have generated the Longguan dome, with its surrounding faults and basins that were later filled by Jurassic volcanic and sedimentary rocks.

3.2 Circular structures controlling the Dongping gold deposit The ZXGB is controlled by the inner circular structure of three concentric / circular structures associated with the Yanshanian magmatic activity (Fig. 1a; Wu, 1995, Wang et al., 2007; Gong et al., 2009; Niu et al., 2009; Lv et al., 2014). The Dongping gold deposit is also controlled by this circular structure, based on remote sensing data (Fig. 5). This is consistent with the observation that numerous gold-quartz veins and magmatic dykes at Dongping consist follow a ring structure (Fig. 1c). The No. 1 gold-quartz veins and the No. 70 stockworks are also controlled by NE-trending faults and NW-trending fractures, respectively (Wu, 2009; Zhang et al., 2012). Thus, we suggest that the Dongping gold deposit is controlled by both the circular structure, and the NE- and NW-trending structures (Fig. 5). Several quartz-trachyte dykes outcrop in the Dongping gold area (Fig. 1c). A specular hematite quartz vein, with cavities and potassic alteration was developed on the hanging wall of a quartz-trachyte dyke in the northern gold area (Fig. 1c; Fig. 6), with alteration minerals consisting of K-feldspar, quartz, and ore minerals that include pyrite, limonite, specular hematite and molybdenite. Additional specular hematite quartz veins with cavities are exposed at an underground level of 1,184 m, in the Dongping gold deposit, and they contain gold grading at 24 g/t (Fig. 7). The alteration minerals in the veins consist of K-feldspar and quartz, while ore minerals comprise pyrite, chalcopyrite and specular hematite. Both the specular 8

hematite quartz veins at the surface and underground, in the gold deposit, have similar mineral assemblages and contain extensional cavities, suggesting that the underground veins may also be associated with the quartz-trachyte dyke. The Laowanggou porphyry Cu-gold-Ag-Pb-Zn deposit is located about 15 km to the east of the Dongping gold deposit (Fig. 1a), and it is a porphyry deposit hosted by the Yanshanian quartz-trachyte. Numerous quartz-trachyte dykes are recorded in this area. Importantly, these dykes are also observed in the area, suggesting that they might be derived from a concealed intrusion beneath the Dongping gold deposit. Based upon earlier studies: (1) Fan et al. (2001) reported that gas inclusions occur in isolation in vein quartz within the gold deposit, with their homogenization temperature of between 372 – 306°C, and salinity of 3.7 – 1.0 wt. % NaCl; stable isotope measurements show during the main episode of gold mineralization, hydrogen and oxygen isotopic ratios of the mineralizing fluids were -70.8 ‰ – 108.4 ‰ and 2.44 ‰ – 4.05 ‰ , respectively. This is consistent with the ore-forming fluids being of magmatic origin (Nie, 1998; Fan et al., 2001; Sun et al., 2013); (2) The distribution pattern of the gold mineralization at Dongping implies that the highest gold grades occur in the deeper parts of the deposit (Fig. 8); (3) A 15-100 m wide porphyritic granite dyke cutting the eastern No. 1 gold-quartz vein, strikes NEE-SWW and contains gold grading of up to 5.96 g/t (Xu et al., 2018); (4) Magnetite ore-bodies are developed near the quartz-trachyte dyke in the northern part of the gold deposit (Fig. 1c); the sulfide-bearing quartz-trachyte dykes intersected in drill core ZK165 contain gold grading at 0.16–0.59 g/t, which indicates that the emplacement of the presumed / concealed intrusive body was probably coeval with the formation of the gold ore-bodies at this deposit; (5) The presence of such intrusive bodies is also supported by geophysical methods, in particular by the apparent resistivity data (Fig. 9), which shows similar patterns to that observed from the outcrop of the Yanshanian syenogranite stock (Song and Zhao, 1996); (6) One of the initial drill holes at Dongping also intersects the Yanshanian syenogranite beneath the gold deposit (Mr. Yabin Zhang, pers. comm., 2014), and the 9

Shangshuiquan syenogranite is exposed in the area southeast of the Dongping deposit (Fig. 1b, 1c), again suggesting the presence of a concealed (Yanshanian) intrusion beneath the Dongping gold deposit.

4. Ore-controlling structures and determination of palaeo stress directions Detailed fieldwork was carried out to investigate the structural characteristics of the Dongping gold deposit, including measuring the attitudes of faults and fractures using a magnetic compass. Stereographic projections of structures compiled with the aid of CAD software were made to determine the compression direction and kinematic features of the structures in the gold deposit. In the Dongping gold deposit, the pegmatite dykes are NE-, NW-, SN- and EW-trending, barren quartz veins are NW-trending, and gold-quartz veins are NE-trending. The pegmatite dykes are evidently crosscut by the NW-trending barren quartz veins and the NE-trending gold-quartz veins, the former of which were also crosscut by the latter (Fig. 10). Therefore, the faults filled by pegmatite dykes, the NW-trending faults and the NE-trending faults, in an order of decreasing age, are assigned to pre-mineralization, and syn-mineralization, respectively. Structures of the pre-mineralization and syn-mineralization are discussed in the following sections.

4.1 Structures of pre-mineralization Several NW-trending faults developed in the Dongping gold deposit, some of which were filled with barren quartz veins that occur mainly as lenses. The veins range from a few centimeters to several meters along strike and plunge, with rough fault surfaces and cavities in the quartz vein that suggest an extensional kinematic regime (Fig. 11). The quartz veins were mined and soon abandoned by the local farmers when they realized that these veins are barren. Some secondary extensional factures also developed in the wall rock of the barren quartz veins (Fig. 12a). The faults hosting the barren quartz veins and these secondary extensional fractures 10

show maximum principal stress (1) strikes at 301° and dips at 23°, implying a regional compressional stress in the NW-SE direction (Fig. 12b).

4.2 Structures of syn-mineralization The NE-trending faults and the NW-trending fractures control the No. 1 gold-quartz veins and No. 70 stockworks, respectively, which are the largest ore-bodies within the Dongping gold deposit. These two types of structures are addressed separately in the following sections.

4.2.1 The NE-trending faults No. 1 gold-quartz veins were hosted in the Shuiquangou syenite complex, and bordered the NE-trending faults. The faults consist of a series of sub-parallel, commonly en echelon faults, and are shown in the cross-section A-B of Figure 2a. A sinistral transpression in pre-mineralization tectonic activity is evident from the observations, including: (1) the flat structural planes, (2) a great number of sub-parallel shear fractures in the syenite complex adjacent to the NE-trending faults, (3) the direction of lenticular wall rock separated by network fractures in the faults, (4) and the gouging along the fault surfaces (Fig. 13a, b, c, d). The network of fractures separating the lenticular wall rock in the faults reflects a compressional stress field for the pre-mineralization episode. The angular breccias are cemented by gold-quartz veins (Fig. 13e, f), indicating an extensional kinematic regime at the time of syn-mineralization. The gold ore-bodies form a series of sub-parallel, commonly en echelon gold quartz vein swarms and quartz-sulfide veinlets within syenite, while the tens of centimeters-wide and a few meters to tens meters-long veins are part of a larger en echelon vein array. The veins are subparallel and spaced a few meters to tens meters apart (Fig. 1c, 2a). These observations suggest that the Dongping gold deposit is controlled by syn-mineralization extensional structures (Wyman et al., 1988). The structural characteristic of the NE-trending faults might result from multiphase tectonic movement. 11

Conjugate joint sets developed in the wall rock (Fig. 14a) or the area adjacent to the NE-trending fault (Fig. 14d). These joint sets are infilled with gold-quartz veins (Fig. 14b), and limonite and native gold (Fig. 14e, f), which suggest they formed pre-mineralization or syn-mineralization. Based upon stereographic projection for the joint sets, the maximum principal stress was determined at the foot wall of the faults, striking at 355° and dipping at 21° (Fig. 14c), and two maximum principal stresses were determined from an adjacent area, striking at 353° and dipping at 48°, and striking at 146° and dipping at 82°, respectively (Fig. 14g, 14h, 15).

4.2.2 The NW-trending fractures Hosted by the Shuiquangou syenite complex, the No. 70 stockworks are controlled by the NW-trending fractures, which were composed of millimeter-wide pervasive fractures with a limited length on the cm-scale. No. 70 stockworks consists of numerous quartz veinlets, of variable length (Fig. 16a, b, c), cavities and breccias in the quartz vein of varying sizes and irregular shapes, each that reflect an extensional kinematic regime for syn-mineralization (Fig. 16d, e). Conjugate joint sets in the No. 70 stockworks are filled with gold-sulfide-quartz, indicating their development pre- or syn-mineralization (Fig. 16f). Calculations show two maximum principal stresses (1) for these joint sets. One strikes at 330° and dips at 20° (Fig. 16g, 17), and the other strikes at 140° and dips at 85° (Fig. 16h).

5. Interpretation of concealed ore-bodies and test drilling results 5.1 Proposed model for gold mineralization As discussed previously, the No. 1 gold-quartz veins are controlled by NE-trending faults, which experienced a switch from a sinistral strike slip movement, pre-mineralization, to an extensional tectonic movement, syn-mineralization. On a stereographic projection, there are 12

two stages of maximum principal stress (1): NW-trending and sub-vertical stresses. A series of subparallel, commonly en echelon, NE-trending faults and numerous sub-parallel shear fractures adjacent to the NE-trending faults may have formed under compressional stress in a NW-SE direction (Fig. 15, Fig. 1c). These transpressional structures became extensional during syn-mineralization, a may be the cause of the sub-vertical compressional stress. The No. 70 stockworks are controlled by the NW-trending fractures that reflect an extensional kinematic regime for syn-mineralization, and display two stages of maximum principal stresses (1): NW-trending and sub-vertical stresses. Similarly, the barren quartz veins that are pre-mineralization are controlled by the NW-trending faults that also reflect an extensional kinematic regime, and show NW-trending maximum principal stress (δ1), implying a regional compressional stress direction of NW-SE (Fig. 17). Thus, the No. 70 stockworks and the barren quartz veins were controlled by the NW-trending structure under an extensional kinematic regime. In contrast to the barren quartz veins, No. 70 stockworks is an economic gold ore-body, which likely resulted from the NW-trending structure, controlling the No. 70 stockworks, having become permeable under extensional kinematic regime attending syn-mineralization. Such an extensional kinematic regime may also be associated with the sub-vertical compressional stress, similar to the No. 1 gold-quartz vein. Li and Santosh (2014) suggest that the regional compressional stress of NW-SE in direction resulted from collision between Siberia and the amalgamated Mongolia-North China blocks during Mesoproterozoic-early Cretaceous (Fig. 1a). Hu et al. (2003) report that NW-trending Zhongshangou-honghuabei-shangshuiquan thrust fault cutting across the southern Dongping gold area formed in the Mesoproterozoic, and was reactivated in multi-stages during the Neoproterozoic, Paleozoic and Mesozoic (Fig. 1b), in response to Paleozoic to early Mesozoic orogenic events on the northern margins of the North China Craton (Zhang and Zhai, 2010). After closure of the Paleo-Asia Ocean in the Late Permian, plate collision produced thrusting and crustal thickening on the northern margin of the North 13

China Craton, which continued throughout the Triassic and Early Jurassic (Xiao et al., 2003). These structures post-date their host Shuiquangou alkaline complex (ca. 394.0 ± 3.2 Ma) but predate the Shangshuiquan alkali granites (ca. 143 Ma, Cisse et al., 2017), which is consistent with their orogenic origin during the Early Paleozoic and Mesozoic (Hu et al., 2002; Zhang et al., 2007). During the orogenic processes, the initial regional compressional stress of NW-SE in direction generated the NE-trending faults and NW-trending fractures (Cook et al., 2009). Subsequently, a switch to an extensional setting accompanying syn-mineralization was most likely related to the sub-vertical compressional stress (1), which was attributed to emplacement of a concealed magmatic intrusion under the Dongping gold deposit, as previously proposed (Zhou et al., 2002; Groves et al., 2003; Zhai et al., 2004; Khomich et al., 2014; Deng and Wang, 2016). The concealed intrusion generated extensional horizontal stress (3). The resulting tensional NE-trending faults and NW-trending fractures became extensional surrounding the concealed magma body (Perfit and Davidson, 2000), and thus increased the permeability in the local dilatant zone, and acted as conduits accommodating the gold mineralization (Yang et al., 2019). Subsequently, the circulation and precipitation of auriferous, silica-bearing, hydrothermal fluids formed the gold-quartz veins and stockworks of the Dongping gold deposit (Fig. 18).

5.2 Drill test results The No. 70 stockworks lies under the No. 1 gold-quartz veins. The main ore-bodies (i.e., No. 1 gold-quartz veins and No. 70 stockworks) are controlled by NE-trending faults and the NW-trending fractures, respectively. During syn-mineralization, both structures were under an extensional kinematic regime, which resulted from the emplacement of a concealed intrusion beneath the Dongping gold deposit. Such extensional structures surrounding the concealed intrusion, may have formed as the magma ascended and was emplaced. Additional gold-quartz veins under the No. 70 stockworks are recorded underground at a level of 1,144 m in the 14

Dongping gold deposit, suggesting that there are perhaps additional concealed ore-bodies under the gold deposit. Thus, we propose renewed exploration for concealed ore-bodies below the Dongping gold deposit. Taking into account the size of the identified ore-bodies, their gold grade and the degree of ore enrichment in the No. 70 stockworks (Fig. 2b), a drill hole, CK1118, was designed with a maximum depth of 1000 m, the location of which is shown on the No. 11 cross-section; this drill was planned to cut beneath the No. 70 stockworks. Drilling stopped at a depth of 898.9 m in response to a sudden influx of water through the faulted strata. It is not clear whether or not these faults are ore-controlling structures. The samples collecting from the drill core evidenced gold mineralization: dozens of stockworks were found with widths of 1–2 m and gold grades of 1-2 g/t (Fig. 19), which were hosted in rocks of the Shuiquangou syenite complex. Two vertically separated stockworks were also discovered in a subsequent drill hole (labelled CK1120 in cross-section No. 11), sited to the east of CK1118 (Fig. 19), which was also hosted in the Shuiquangou syenite complex. An upper gold ore-body of 16.5 m in thickness with gold grading at 2.77 g/t, and a lower one some 6.40 m in thickness, with gold grading at 5.00 g/t were identified. These gold ore-bodies were also intersected by three additional drill holes, shown in the No. 11 cross-section. The stockworks mainly consist of intensively disseminated sulfide quartz veinlets, similar to those of the No. 70 stockworks (Fig. 19).

6. Conclusions (1) The Dongping gold deposit is controlled by a circular structure, and NE- and NW-trending structures. The NE- and NW-trending structures experienced a switch from compressional stress in a NW-SE direction before the onset of mineralization in an extensional kinematic regime. Our data suggest that both the circular structure and the extensional kinematic setting could be related to the emplacement of a concealed intrusion beneath the gold 15

deposit. (2) We propose a new structural model for the deep and concealed orebodies at Dongping. Our study implies that a concealed intrusion has generated ring structures during its emplacement, and gold-bearing hydrothermal fluids subsequently precipitated along these structures, forming the gold ore-bodies. Based on our interpretation we propose further exploration potential for additional concealed orebodies at the Dongping gold deposit. (3) Recent exploration drilling based on our new model has successfully intersected additional concealed ore-bodies beneath the existing Dongping gold mine which significantly contributes to its resource. Acknowledgements This research is financially supported by a scientific research project from the Zijin Mining Group Company Limited (Grant No. 2014001), and the China Scholarship Council (CSC NO. 201908360098). We acknowledge Guichang Song’s contribution to support this study and are indebted to his generous sharing of ideas and thoughts and to his friendship. Junkang Zhao and Jianbiao Fan are thanked for providing geological data, field assistance, and hospitality during fieldwork at the Dongping gold deposit. The authors would also like to thank the Associate Editor, Dr. Daniel Müller, and three anonymous reviewers for their useful comments and constructive reviews, which have significantly improved the manuscript.

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Figure Captions Fig. 1. Geological sketch map of the Dongping gold deposit (modified from Mao et al., 2003; Niu et al., 2009; Bao et al., 2014; Zijin Mining Group Company Limited).

Fig. 2. Geological cross sections of the No. 1 gold-quartz veins and No. 70 stockworks in the Dongping gold deposit (modified from Zijin Mining Group Company Limited).

Fig. 3. The remote sensing map of the Zhangjiakou-Xuanhua gold belt (modified from Wu, 1995; Lv et al., 2014).

Fig. 4. (a) Bouguer gravity anomaly map (derived from Zhang, 2006), and (b) magnetic anomaly map (derived from Song and Zhao, 1996) of the Zhangjiakou-Xuanhua gold belt.

Fig. 5. The remote sensing map of the Dongping gold deposit (modified from Song and Zhao, 1996).

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Fig. 6. (a) Quartz-trachyte dyke in the northern gold area; (b) Specular hematite quartz vein with cavities on the hanging wall of the quartz-trachyte dyke; (c) Molybdenite, and potassic alteration in the hanging wall of the quartz-trachyte dyke, and (d) Limonite in the hanging wall of the quartz-trachyte dyke.

Fig. 7. (a) Specular hematite quartz veins at an underground level of 1,184 m, and (b) Cavities in the specular hematite quartz vein.

Fig. 8. The distribution pattern of the gold mineralization at the Dongping gold deposit (modified from Song and Zhao, 1996; Zijin Mining Group Company Limited).

Fig. 9. Apparent resistivity pattern of the Dongping gold deposit (modified from Zijin Mining Group Company Limited).

Fig. 10. Representative photographs showing the different vein types and their cross-cutting relationships at the Dongping gold deposit.

Fig. 11.

Field photographs of the barren quartz veins related to the pre-mineral stage (a):

Lenticular barren quartz vein and (b) Rough fault surfaces and cavities in a quartz vein.

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Fig. 12. (a) Field photograph showing the characteristics of the pre-mineral faults, and (b) NW-trending maximum principal stress (1) on a stereographic projection for the conjugate joints.

Fig. 13. Field photographs of the NE-trending structures: (a) Flat fault plane; (b) Sub-parallel shear fractures in syenite complex adjacent to the NE-trending faults; (c) Lenticular wall rock partitioned by a network of fractures in the fault; (d) The gouging observed on fault surfaces; (e) Structural lenses in the gold-quartz vein, and (f) The angular breccias cemented by gold-quartz veins.

Fig. 14. Field photographs of the NE-trending structures, and stereographic projections of the conjugate joints: (a) A conjugate joint set developed in the foot wall of the fault; (b) Gold-quartz vein infilled with the conjugate joints; (c) NW-trending maximum principal stress (1) on the stereographic projection for the conjugate joints; (d) A wide range of conjugate joint sets developed adjacent to the NE-trending faults; (e) Limonite on the joint surfaces; (f) Native gold on the joint surfaces; (g) NW-trending maximum principal stress (1) on the stereographic projection for the conjugate joint sets; (h) Sub-vertical maximum principal stress (1) on the stereographic projection.

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Fig. 15. The sinistral compressional NE-trending faults and NW-trending compressional stress.

Fig. 16. Field photographs of the NW-trending fractures, and stereographic projections of the conjugate joint sets: (a) Gold-bearing stockworks; (b) Gold-quartz vein; (c) Gold-bearing stockworks in drill core; (d) Cavities within a gold-quartz vein; (e) Wall rock breccias of varying sizes and irregular shapes in the quartz veins; (f) Several conjugate joint sets developed in the No. 70 stockworks; (g) NW-trending maximum principal stress (1) on the stereographic projection; (h) Sub-vertical maximum principal stress (1) on the stereographic projection.

Fig. 17. The extensional NW-trending faults and NW-trending compressional stress regime.

Fig. 18. Model for the gold mineralization at the Dongping gold deposit.

Fig. 19. Number 11 cross-section through the Dongping gold deposit (modified from Zijin Mining Group Company Limited).

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Highlights: 1. The circular structure could be related to the emplacement of a concealed intrusion; 2. Propose a new structural model for the deep and concealed orebodies at Dongping; 3. Recent exploration drilling based on our new model has successfully intersected additional concealed ore-bodies.

Conflict of interest The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict 26

of interest in connection with the work submitted.

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