Jing-zi He, Zheng-guo Fan, Sheng-qing Xiong, Teng-fei Ge, Xu-zhao Huang, Si-xun Wang

a School of Geophysics and Information Technology, China University of Geosciences, Beijing 100083, China

b China Aero Geophysical Survey and Remote Sensing Center for Natural Resources, Beijing 100083, China

Keywords:Rift zone Aeromagnetic anomaly Gravity anomaly Mafic-ultramafic complex Copper-nickel deposit Geological survey engineering Mineral exploration engineering Beishan Xinjiang

ABSTRACT The Beishan rift zone in Xinjiang Uygur Autonomous Region was formed due to strong activities of faults on the basement of the Tarim continental crust. Despite the fact that many geological research results of the rift zone have been achieved, only a few studies have been conducted on its regional geophysical characteristics. In this paper, the gravity and magnetic anomalies of the rift zone were highlighted through specific data processing of 1∶50000 high-precision aeromagnetic data and gravity data with a grid spacing of 2 km × 2 km. Based on this, the geophysical evidence for the scope and internal structures of the Beishan rift zone was obtained for the first time. The distinct characteristics of magnetic and gravity fields in the areas to the north and south of the Beishan rift zone reveal that deep faults exist between the Beishan rift zone and the geological units on the southern and northern sides. Furthermore, the faults on the two areas contain the bidirectional thrusts and have flower-shaped structures according to the characteristics of the magnetic and gravity fields. The Beishan rift zone can be divided into two tectonomagmatic zones, namely the Zhongposhan-Bijiashan-Cihai-Baishanliang zone (the northern zone)and the Bayiquan-Qixin-Baishan zone (the southern zone). The northern zone can be further subdivided into three comet-shaped anomaly groups (tectonomagmatic areas), while the southern zone can be further subdivided into two tectonomagmatic areas. According to the characteristics of aeromagnetic anomalies and gravity field, 19 mafic-ultramafic complexes were delineated. The known Pobei, Hongshishan, and Qixin complexes are all located within the inferred complexes,with estimates of total explored resources of Ni, Cu, and Au of 3×106 t, 10×103 t and 10 t, respectively. The prospecting of high-grade copper-nickel deposits should focus on the periphery and deep parts of the known and inferred mafic-ultramafic complexes. Among them, the peripheral strata of the complexes specifically have great prospecting potential of large-scale high-grade copper-nickel deposits of magma injection type. Finally, this paper analyzed the application effects of the rapid airborne-ground-drilling synergetic exploration method in the prospecting of copper-nickel deposits in Qixin, Beishan, Xinjiang, which will provide references for further exploration of copper-nickel deposits in Beishan area, Xinjiang.

1. Introduction

The Beishan rift zone is located on the northeastern margin of the Tarim Block. It is rich in copper and nickel resources and has become an important area with great metallogenic potential of copper and nickel deposits (Qin KZ et al., 2011; Li WY et al., 2012; Lou DB et al., 2014, 2018;Tang QY et al., 2015). Owing to its low geological exploration level, previous analyses of its geological structure were primarily conducted based on the results achieved from the 1970s to the late 1980s, including the results of regional geological surveys on different scales and a few scientific research achievements (Gong QS et al., 2003). The exploration of copper-nickel deposits in Beishan area,Xinjiang Uygur Autonomous Region began in the 1980s,followed by successive discovery of some deposits (ore occurrences) such as Poyi super-large nickel deposit,Hongshishan, Poshi, and Luodong large nickel deposits,Bijiashan, and Xuanwoling medium-small nickel deposits(Yang JQ et al., 2002; Jiang CY et al., 2006; Xie W et al.,2011; Li WY et al., 2012; Chen J, 2013; Wang H et al., 2015;Lu HF et al., 2016; Jiao JG et al., 2017; Xue SC et al., 2016,2019), discovering total explored resources of Ni, Cu, and Au estimated to be about 3×106t, 10×103t and 10 t, respectively.After the concept of Beishan rift zone, Xinjiang was firstly proposed (Zhang WS, 1992), the basic geological structure of the rift zone was partially researched, producing fruitful achievements in terms of geotectonic attributes, basement properties, magmatic rock series and their evolution, the distribution of rock- and ore-controlling structures,metallogenic regularity, and the geochronology and geochemistry of mafic-ultramafic rocks (Zhang WS, 1992;Zhang DR, 2000; Gong QS et al., 2003; Mao JW et al, 2006,Wang MC et al, 2007; Lü LS et al., 2007; Su BX et al., 2009,2010; Qin KZ et al., 2011; Jiang HB et al., 2012; Lu HF et al.,2012; Su BX et al, 2013; Bai YL et al., 2004; Feng J et al.,2014; Tang QY et al., 2015; Song XY et al., 2018). In recent years, a great deal of geological work has been carried out in Beishan area, especially the 1∶50000 high-precision aeromagnetic survey across the whole area. All these accumulated abundant geophysical data and provided relatively complete data for further research of the geological structure, exploration, and prospecting of the Beishan rift zone.

Geophysical data contain a large amount of deep metallogenic geological information and can comprehensively characterize underground geological structural and tectonic information. Geophysical prospecting plays an important role in the exploration of magmatic copper-nickel deposits, and many deposits were discovered through further exploration with the guide of geophysical anomalies in early stages (Yao ZS et al., 2014). Previous researches on the Beishan rift zone were mostly carried out from the perspectives of geological tectonics, petromineralogy, and geochemistry but were seldom conducted from geophysical perspective. Furthermore,there are still some scientific problems to be solved at present.For instance, faults have been intermittently exposed in some areas covered by the Quaternary (e.g., Shulehe Fault in the south of the study area is concealed in the Kumutage Desert,making it difficult to accurately delineate the boundaries of the Beishan rift zone during geological mapping). Other problems include the distribution of concealed maficultramafic complexes in the rift zone and the exploration directions of copper-nickel deposits in the zone, especially high-grade copper-nickel deposits. This paper discussed the distribution of fault structures in the Beishan rift zone and the characteristics of the rift zone based on processing and comprehensive analysis of newly obtained 1∶50000 aeromagnetic data and the regional gravity data. Meanwhile,it revealed the characteristics of the geophysical anomalies related to mafic-ultramafic rocks in the rift zone, and inferred the positions of complexes in the rift zone. All these provided clues and bases for the research of regional geological structures, the analysis of metallogenic geological conditions of copper-nickel deposits, and the delineation of prospecting targets in the Beishan rift zone.

2. Geological setting

2.1. Geotectonic characteristics

The Beishan rift zone is located on the northeastern margin of the Tarim Block (Fig. 1) and is one of the secondorder tectonic units in the block (Zhang WS, 1992; Pan GT et al., 2009; Lu HF et al., 2012; Bai YL et al., 2004). It has a primary tectonic line in NE trending, with the Luobupo-Weiya Fault as the northwestern boundary and the Shulehe Deep Fault as the southeastern boundary. It evolved from three large-scale splitting and closing of ∈, O-S, and C-P and migration from south to north on the Archaeozoic and Proterozoic basement of the Tarim Block. As a result, a general tectonic framework of being sandwiched by Kuluketage microblock and Abei-Dunhuang Block was formed (Zhou JY et al., 2000; Wang MC et al., 2007). In terms of geotectonic setting, the study area experienced polycyclic lateral orogeny and vertical accretion as well as complex splicing and strike-slip processes of blocks.

Fig. 1. a-Map of regional location of the study area (modified from Sun T et al., 2013); b-sketch map of geotectonics and distribution of mafic-ultramafic complexes in Eastern Tianshan-Beishan region (modified from Su BX et al., 2013).

2.2. Regional geological tectonic setting

The outcrops in the study area include the strata from Proterozoic to Cenozoic. The Paleoproterozoic Beishan Group constitutes the oldest crystalline basement in the area. The Mesoproterozoic and Neoproterozoic strata comprise the sedimentary cap rocks, including the Dunhuang Group,Changchengian Baihu Group and Yangjibulake Group, and Jixianian Aierjigan Group of the Mesoproterozoic and Sinian Hangeerqiaoke Group of the Neoproterozoic. The Cambrian-Silurian strata are carbonate-siliceous-terrigenous detritus suites, and the Upper Paleozoic strata are littoral clastic suites.The study area features intense and widely distributed magmatic activities and intrusive activities occurring in the Variscan primarily (Carboniferous and Permian intrusive rocks) and the Caledonian secondarily (Silurian intrusive rocks). The rocks in the study area are primarily acidicintermediate and locally basic-ultrabasic. The metamorphism in the study area mainly includes regional metamorphism,followed by dynamic metamorphism. The primary regional tectonic line in the study area is in NE trending and structures such as folds, faults, and ductile shear zones are well developed due to long-term tectonic movement. The tectonic pattern in the study area is mainly controlled by three firstorder faults, namely Kalatage, Luobupo-Weiya and Shulehe deep faults. Second-order faults including Cantoushan-Xiaoqingshan fault, Baidiwa-Yunihe fault, Hongshijing-Maotoushan fault, and Baishan fault are ore-controlling faults within the Beishan rift zone.

2.3. Temporal and spatial distribution of regional coppernickel deposits

The Beishan rift zone is still at a low research level,except its southwestern part (Pobei-Bijiashan area) where more than 10 magmatic sulfide copper-nickel deposits (ore occurrences) have been discovered, such as Hongshishan,Poyi, and Poshi deposits (Fig. 1b). The genesis of these deposits is associated with mafic-ultramafic rocks and they are of magmatic liquation or injection types. The mineral elements of these deposits are dominated by Ni and accompanied by favorable elements such as Cu or Co.Prolonged and large-scale mantle-derived magma intrusion occurred from the Late Carboniferous to Early Permian(260-300 Ma), which is the peak period of large-scale mineralization. For instance, the ages of Luodong, Podong,Poyi, and Poshi rock masses are 273-283 Ma (Li HQ et al.,2006a; Qin KZ, et al., 2011; Xue SC et al., 2016; Jiao JG et al., 2018), the ages of Bijiashan, Hongshishan, and Xuanwoling rock masses are 260.7-286 Ma (Su BX et al.,2010, 2011), and the age of Qixin rock mass is about 286-298 Ma(Xie X, 2018; Xue SC, 2019).

3. Physical characteristics of rocks and ores

The physical characteristics of rocks and ores such as magnetic susceptibility, remanence, and density serve as the premise and important bases of geophysical interpretation.The statistics of physical characteristics of the rocks and ores in Beishan area (dominated by the data measured in this study) are shown in Table 1 and Table 2.

3.1. Magnetic properties of rocks and ores

The ironstone in the study area shows the strongest magnetic properties, with a magnetic susceptibility of up to 2.1 SI. It is followed by the mafic-ultramafic intrusive rocks represented by diabase, gabbro, and pyroxene peridotite,whose magnetic susceptibility is generally about 0.4 SI. Most of the mafic-ultramafic intrusive rocks in the study have high remanence, with the magnetization direction partially opposite to the direction of modern geomagnetic field. For example,the remanence of the gabbro in the Qixin complex has a magnetic inclination of about -50° and a magnetic declination of about 240°, while the modern geomagnetic field in Qixin area has a magnetic inclination of about 61.27° and a magnetic declination of about 0.66°. These indicate that the remanence of the gabbro as the main body of the Qixin complex is almost in the opposite direction of modern geomagnetic field. For volcanic rocks in the study area such as andesite and basalt, their magnetic properties vary greatly.Meanwhile, metamorphic rocks and granite in the study area have weak magnetic properties as a whole. In addition, some schist with basic volcanic rocks as protolith has a high magnetic susceptibility, and some skarn and coarse-grained biotite granite also have strong magnetic properties. In contrast, sedimentary rocks such as sandstone and limestone have the weakest magnetic properties. Copper-nickel deposits in the study area generally occur in ultrabasic intrusive rocks such as pyroxene peridotite, and the magnetic properties of the deposits are primarily similar to those of the ultrabasic intrusive rocks. Therefore, the distribution of volcanic rocks and mafic-ultramafic rocks in the study area can be determined by research of the anomaly characteristics of magnetic field. In this way, the spatial positions of concealed rock masses can be predicted, which will assist the exploration and prospecting of minerals in the study area.

3.2. Density of the rocks and ores

The density of the rocks in the study area becomes higher as the strata of the rocks grow older. Among them, normal sedimentary rocks and metamorphic rocks generally have medium density (2.61-2.84 g/cm3). The density of volcaniclastics is closely related to the basic degree of volcanic cements, and the density of magmatic rocks gradually increases as their basic degree increases.Meanwhile, metal ores such as copper-nickel ores and magnetite have high density. There are two density interfaces between Mesozoic and Palaeozoic strata and between Palaeozoic and Proterozoic strata, with the maximum density difference between Palaeozoic and Proterozoic strata up to 0.45 g/cm3. It should be noted that ultrabasic rocks such asperidotite and pyroxene peridotite suffer strong weathering alteration from the surface to a certain depth and their density correspondingly decreases as a whole, as discovered during the measurement of physical properties of borehole cores. For example, the average density of the pyroxene peridotite in the core of borehole ZK38-3 in Pobei is 2.67 g/cm3at a depth of0-300 m, 2.95 g/cm3at 300-600 m, 3.12 g/cm3at 600-900 m,and 3.20 g/cm3at 900-1200 m.

Table 1. Statistics of physical parameters of rocks and ores in Beishan area, Xinjiang.

Table 2. Statistics of magnetic parameters of rock and ore samples from Beishan area, Xinjiang.

4. Processing and characteristic analysis of gravity and magnetic data

4.1. Processing of gravity and magnetic data

The magnetic survey data used in this paper originated from the aeromagnetic survey results of the program“1∶50000 aeromagnetic survey of the southeastern margin of East Tianshan in Xinjiang” carried out by the China Aero Geophysical Survey and Remote Sensing Center for Natural Resources from 2010 to 2012, and the gravity data used in this paper are sourced from the Bouguer gravity data with a grid spacing of 2 km × 2 km obtained during the program“Potential assessment and synthesis of nationwide key mineral resources in China”. The scale of original gravity survey is 1∶200000 in the Beishan area except for the south of Luanshanzi area, where the scale is 1∶1000000. The Bouguer gravity anomaly map and aeromagnetic ΔTanomaly map of Beishan area were prepared based on the abovementioned data (Fig. 2).

Geophysical data reflect the information on the superposition of all underground inhomogeneous geological bodies (Liu Y et al., 2012). Furthermore, it is necessary to carry out targeted data conversion for the purpose of deep research on underground geological structural and tectonic information of the Beishan rift zone. Therefore, in this paper,the methods of aeromagnetic reduction to the pole (RTP),analytical continuation, vertical derivation, and normalized source strength (NSS) were selected to conduct data conversion, aiming to reduce the influence of oblique magnetization, separate the regional background field and local field, and facilitate the characteristic analysis of fault structures and target geological bodies in rift zones. Only three of them are briefly introduced, namely aeromagnetic RTP, NSS, and calculation of residual gravity anomalies,since most of them are conventional methods.

4.1.1. Aeromagnetic RTP

The center of the study area has a latitude of about 41°N,a longitude of about 92°E, a geomagnetic inclination of about 61.27°, and a magnetic declination of about 0.66°. As shown from the RTP aeromagnetic anomaly map (Fig. 3), the associated negative magnetic anomalies are obviously reduced and the positive magnetic anomalies move northwards and correspond better to the positions of magnetic geological bodies after RTP. Therefore, in this paper, the characteristics of magnetic anomalies were mainly analyzed and explained based on RTP results. However, it should be noted that remanence was not considered during RTP and thus RTP results may suffer deviation in the case that remanence and normal geomagnetic field are not in the same direction.As mentioned above, the remanence direction of maficultramafic intrusive rocks in the study area is obviously different from the normal geomagnetic field direction.Therefore, the influence of remanence should be considered in this case.

4.1.2. NSS

NSS is a rotation invariant derived from magnetic gradient tensor (MGT) matrix of magnetic dipole (Wilson H, 1985;Beiki M et al., 2012; Clark DA, 2012, 2013). In 3D case, NSS is completely independent of magnetization direction for magnetic dipole since NSS is slightly affected by magnetization direction. Wilson H (1985) gave the definition of the dipole as follows:

4.1.3. Calculation of residual gravity anomalies

In this paper, the residual gravity anomalies were extracted by moving averaging window (MAW) method. The residual gravity anomalies extracted using the window with a side length of 30 km (Fig. 5) can fully reflect the underground geological structures and tectonics in the study area such as faults and intrusive rocks.

4.2. Characteristics of gravity and magnetic fields

4.2.1. Characteristics of gravity field

As shown in Fig. 2a, the Bouguer gravity anomalies in the Beishan rift zone are generally distributed in NE trending and obvious banded low Bouguer gravity anomalies are present at the northwestern (F2) and southeastern boundaries (F3) of the rift zone. Within the rift zone, the gravity anomalies are high in the northwest (Baidiwa-Cihai area) and gradually decrease southeastwards, with NE- or NEE-trending alternating positive and negative gravity anomaly belts superimposed on the regional background gravity field. In addition, there are small-scale high gravity anomalies in indistinct directions within the rift zone. As for the areas to the northwest and southeast of the rift zone, the northwestern Central Tianshan Block and Carboniferous basin of Southern Tianshan show large-scale high gravity anomalies, while the southeastern side features low Bouguer gravity anomalies. The basic characteristics of the Bouguer gravity anomalies in the rift zone even remain unchanged at an upward continuation height of 5 km (Fig. 6), except for a slight decrease in the intensity of local anomalies.

Fig. 2. Bouguer gravity anomaly map (a) and aeromagnetic ΔT anomaly map (b) of Beishan area, Xinjiang.

As shown in the map of residual Bouguer gravity anomalies (Fig. 5) and the first vertical derivative map of Bouguer gravity anomalies (Fig. 7), the characteristics of gravity anomalies within the rift zone are obviously different from those in the areas to the northwest and southeast of the rift zone. In detail, the residual Bouguer gravity anomalies within the rift zone are significantly richer and more intensive. They mainly include NE- or NEE-trending banded high gravity anomalies in the areas such as Pobei (Luobupo-Podong-Baidiwa), Bijiashan (Cantoushan-Baiyushan), Cihai-Bailiangshan, Luanshanzi-Qixin, followed by banded NWtrending gravity anomalies in Haiyaquan-Cihai area. In addition, there are multiple cloddy high gravity anomalies in indistinct directions in the areas such as Toudiaoquan,Yutoushan, Baiyushan, and Heihai and low gravity anomalies in areas such as the northern slope of Baishan, the southeast of Cihai, the southwest of Xuanwoling, the south of Hongliujingzi, and Bayiquan. The Bouguer gravity anomalies in the areas to the northwest and southeast of the rift zone are mainly distributed in cloddy or equiaxed shapes in indistinct directions and in a banded shape in NEE trending,respectively.

4.2.2. Characteristics of magnetic field

Fig. 3. RTP aeromagnetic anomaly ΔT map of Beishan area, Xinjiang.

Fig. 4. Aeromagnetic NSS anomaly map of Beishan area, Xinjiang.

Fig. 5. Residual Bouguer gravity anomaly map of Beishan area, Xinjiang.

Fig. 6. Map of Bouguer gravity anomalies at different upward continuation heights in Beishan area, Xinjiang.

Fig. 2b shows that the aeromagnetic ΔTanomalies in the Beishan rift zone mainly consist of banded or blocky positive magnetic anomalies in generally NE trending that are superimposed on a regional low magnetic field. As for the northwestern boundary (F2) of the rift zone, the northeastern section shows an obvious negative magnetic anomaly belt,while the southwestern section serves as the boundary between different magnetic anomaly belts (areas). Meanwhile,the southeastern boundary (F3) of the rift zone exhibits an obvious negative magnetic anomaly belt. The Beishan rift zone is divided into two magnetic field sub-zones with Luotuofeng-Maotoushan-Baishanbeipo as the boundary,namely the northwestern and southeastern aeromagnetic ΔTanomaly areas. The former (Pobei-Bijiashan-Cihai-Bailiangshan area) is mainly characterized by the superimposition of several NE-, NEE-, or EW-trending banded positive magnetic anomalies on a regional low magnetic field, while the latter (Bayiquan-northern Qixin-Heyaquan area) primarily features the superimposition of blocky or irregular positive magnetic anomaly groups (such as Bayiquan, north Qixin, and Heyaquan) on a regional low magnetic field. Another significant characteristic the aeromagnetic ΔTanomalies in the rift zone is the superimposition of multiple elliptical or irregular negative magnetic anomalies on the regional magnetic field, such as Pobei, Bijiashan, Cihai, and Qixin anomalies. The magnetic field outside the Beishan rift zone is as follows. The Carboniferous basin of South Tianshan to the west features blocky high magnetic field and the Central Tianshan Block to the north is an area of a low magnetic field on which banded or linear positive anomalies bulging southwards are superimposed. The high magnetic anomalies in the northernmost area should be caused by the Jueluotage structural belt to the north. Meanwhile, the Tarim Basin to the southeast of the rift zone shows weak positive magnetic anomalies.

As shown from the RTP aeromagnetic ΔTanomalies(Fig. 3), the overall characteristics of magnetic anomalies in the rift zone slightly change after RTP. However, associated negative magnetic anomalies obviously decrease, the positive magnetic anomalies move northwards, the magnetic anomalies show more obvious linear characteristics, and the negative magnetic anomalies reflected by complexes such as Pobei, Bijiashan, Cihai and Qixin are more obvious. The linear anomaly zones and small-scale weak anomalies gradually vanish with an increase in upward continuation height (Fig. 8). At an upward continuation height of 1 km,most linear anomaly belts still exist and are more regular. At an upward continuation height of 5 km, most linear anomaly zones vanish and the northwest of the rift zone (Pobei-Bijiashan-Cihai-Bailiangshan area) appears as an area of a low magnetic field. Meanwhile, at this height, the southeastern part (Bayiquan-northern Qixin-Heyaquan area)shows a high magnetic field and shares similar magnetic field characteristics with the area beyond the southeastern boundary (F3) of the rift zone.

Fig. 7. First vertical derivative map of Bouguer gravityanomalies in Beishan area, Xinjiang.

As shown in the first vertical derivative map of RTP aeromagnetic ΔTanomalies in the Beishan area of Xinjiang(Fig. 9), the characteristics of the anomalies in the Beishan rift zone are significantly different from those in the areas to the northwest and southeast of the rift zone. The anomalies in the rift zone are mainly characterized by NE-, NEE- or EWtrending alternating positive and negative linear magnetic anomalies that are distributed in groups or bands. The Carboniferous basin of South Tianshan to the northwest of the rift zone is mostly an area with a steady magnetic field, while the Central Tianshan Block to the northwest is mainly characterized by alternating positive and negative linear magnetic anomalies distributed in groups and various directions. Dunhuang Block to the southeast of the rift zone mainly shows the characteristics of a gently varying magnetic field.

5. Discussion

5.1. Characteristic analysis of fault structures in Beishan rift zone, Xinjiang

Fig. 8. Map of RTP aeromagnetic ΔT anomalies at different upward continuation heights in Beishan area, Xinjiang.

As indicated by previous geological research results,volcanic activities in an extensional environment in the Beishan rift zone are dominated by fissure eruptions along faults, with volcaniclastic suites and magmatic intrusive rocks developing in the main faults and different horizons near the main faults (Zhang WS, 1992). Most magmatic rocks in the study area have strong magnetic properties, as indicated by the statistics of physical properties of ores and rocks (Table 1).Therefore, linear magnetic anomaly zones or the linear distribution characteristics of magnetic anomaly groups serve as important bases for the identification of fault structures.Based on the principle of identifying fault structures using gravity and magnetic anomalies, 355 main fault structures were inferred from aeromagnetic and gravity data in the study area, including three main boundary faults and six main middle faults (Fig. 10).

5.1.1. Main faults at boundaries

The main boundary faults in the study area include Kalatage Fault (F1), northern Luobupo-Weiya Fault (F2), and Shulehe Fault (F3). Their gravity and magnetic fields are mainly characterized by the fact that they serve as boundaries between the areas with different gravity and magnetic fields.Meanwhile, the characteristics of their gravity and magnetic fields can still be clearly reflected at an upward continuation height of 10 km, indicating that they are crustal faults since they are on a large scale and cut downwards deeply.Therefore, the main boundary faults serve as boundary faults of different tectonic units. Previous researchers have introduced and gained understanding of these faults to different contents (Zhang WS, 1992; Zhao WZ et al., 1992;Chen MJ, 1993; Zhang DR, 2000; Li JY et al., 2006; Wang MC et al., 2007; Xia ZD, 2012). However, the names,locations, and gravity and magnetic anomaly characteristics of these faults were differently described due to the lowaccuracy regional geophysical data surveyed on small scales.For example, the northern Luobupo-Weiya Fault (F2) was previously called Cele-Weiya Fault or Luobuzhuang-Weiya Fault (Zhang DR, 2000).

Fig. 9. First vertical derivative map of RTP aeromagnetic ΔT anomalies in Beishan area, Xinjiang.

Fig. 10. Distribution of faults and mafic-ultramafic complexes inferred from gravity and magnetic data in Beishan area, Xinjiang.

The Kalatage Fault (F1) starts from northeastern Baidingshan in the west and ends at Tuziluke in the east in the shape of a southward convex. It is about 238 km long in the study area, with two ends extending beyond the study area.The northern side of F1has a low magnetic field, on which an arc-shaped linear anomaly belt bulging southwards is superimposed. Meanwhile, it has a gravity field characterized by relatively high gravity values, on which several equiaxed high gravity anomalies are superimposed. The southwestern side of F1is mainly characterized by a large-area high magnetic field and high gravity values, on which several strong-gravity high-anomaly areas in NW or NNW trending are superimposed. The southeastern side of F1has a gravity field characterized by a wide and gentle high-gravity belt in NE trending and a low magnetic field on which NE-trending linear anomaly belt is superimposed. The gravity and magnetic field characteristics of F1can still be distinctly reflected at an upward continuation height of 10 km,indicating that F1is on a large scale and cuts downwards deeply. Therefore, F1is a boundary fault that separates the Central Tianshan Block from the Kulutage Block and Beishan rift zone.

The northern Luobupo-Weiya Fault (F2) is distributed along the Jianshanzi-Daheishan-Heishanliang area in NE trending. It is about 317 km long, with two ends extending beyond the study area (the northeastern end may merge with F1). The western side of F2is a large area with a high magnetic field and a gravity field characterized by a wide and gentle high-gravity belt in NE trending. The northwest side of F2has a low magnetic field, on which an arc-shaped linear anomaly belt bulging southwards is superimposed.Meanwhile, it has a gravity field characterized by relatively high gravity values on which with several equiaxed high gravity anomalies are superimposed. The southeast side of F2has a low magnetic field, on which a NE-trending linear anomaly belt is superimposed. Meanwhile, the gravity field on the southeast side of F2shows a wide and gentle highgravity belt in NE trending. The gravity and magnetic field characteristics of F2can still be clearly reflected at an upward continuation height of 10 km, indicating that F2is on a large scale and cuts downwards deeply. Therefore, F2is a boundary fault that separates the Beishan rift zone from the Central Tianshan Block and Kulutage Block.

The NEE-trending Shulehe Fault (F3) is located within the Kumutage Desert in the southeast corner of the study area. It is about 115 km long, with two ends extending beyond the study area. The northeastern side of F3is an area with linear magnetic anomaly belts in nearly EW trending or cloddy high magnetic fields and high gravity anomaly belts in NE trending. The northwestern side of F3has a gently low magnetic field and a gravity gradient belt. The southeastern side of F3has a linear strong magnetic anomaly belt in NE trending and a high gravity field on which weak-gravity high anomalies are superimposed. The gravity and magnetic field characteristics of F3can still be clearly reflected at an upward continuation height of 10 km, indicating that F3is on a large scale and cuts downwards deeply. Therefore, F3is a boundary fault between the Beishan rift zone and the Abei-Dunhuang Block.

5.1.2. Main middle faults

The main middle faults in the study area include Hongshijing-Heyaquan Fault (F3-1), Cihai-Bailiangshan Fault(F4), Cantoushan-Xiaoqingshan Fault (F5), Luodong-southern Bijiashan Fault (F6), Luotuofeng-Maotoushan-northern slope of Baishan Fault (F7), and Luanshanzi-Heyaquan Fault (F8).Their magnetic fields are mainly characterized by linear and moniliform magnetic anomaly belts or serving as boundaries between different magnetic field areas. Their gravity anomalies mainly involve gravity gradient belts. Their gravity and magnetic field characteristics can still be distinctly reflected at an upward continuation height of 5 km, indicating that they are large-scale basement faults that cut downwards deeply and serve as boundary faults between different tectonic-magmatic active areas in the Baishan rift zone. Some local sections of these faults were previously introduced to a certain extent (Zhang WS, 1992; Chen MJ, 1993; Wang MC et al., 2007; Xia ZD et al., 2012). However, they are yet to be verified with geophysical research results.

The Hongshijing-Heyaquan Fault (F3-1) is located on the northern side of the Shulehe Fault (F3) and within the Kumutage Desert. It is distributed in NEE trending, with a length of about 170 km. Its western end extends beyond the study area and its eastern end intersects with the Shulehe Fault (F3). The northeastern side of F3-1is an area with high magnetic and gravity fields. The southeast side of F3-1is mainly an area with a gently low magnetic field and a low gravity field on which weak-gravity high anomalies are superimposed. In the process of the upward continuation of the gravity and magnetic fields (Figs. 6 , 8), the magnetic and gravity anomalies on the northern side of F3-1do not move southwards in general, while the magnetic and gravity anomalies on the southern side of F3gradually move northwards and tend to gradually approach the magnetic and gravity anomalies on the northern side of F3-1. All these indicate that F3may gradually approach and eventually merged with F3-1in deep parts underground. That is, F3is a major fault and F3-1is one of its important subordinate faults in the north.

The Cihai-Bailiangshan Fault (F4) starts from Cihai in the west and extends beyond the study area in the east. It is distributed in NE trending, and there are long moniliform magnetic anomaly belts and gravity gradient belts near its gravity anomaly peaks. The gravity and magnetic anomalies are all present as gradient belts at an upward continuation height of 5 km. The inferred fault F4is intermittently marked as a fault on the 1∶200000 regional geological mineral map and its position basically coincides with the location of the Hongliuhe Fault introduced by Zhang WS (1992).

The EW-trending Cantoushan-Xiaoqingshan Fault (F5)starts from Cantoushan in the west and extends eastwards until the southern Maanshan through Xiaoqingshan and Bijiashan. Its magnetic anomalies are present as linear positive anomaly belts, and its gravity anomalies appear as gradient belts. The gravity and magnetic field characteristics of F5can still be clearly reflected at an upward continuation height of 5 km. The inferred fault F5is marked as a fault on the 1∶200000 regional geological mineral map. In terms of location, its western section basically coincides with the Hongliuhe-Yigezitage Fault described by Jiang CY et al.(2006), and Tang QY et al. (2015), and the Cantoushan-Xiaoqingshan Fault stated by Xia ZD et al. (2012).

The NE-trending Luodong-southern Bijiashan Fault (F6)starts from Luodong in the south and extends northeastwards until southern Bijiashan through Poshi and Baidiwa. The magnetic and gravity anomalies of F6are characterized by linear positive magnetic anomaly belts and gravity gradient belts, respectively. The gravity and magnetic field characteristics of F6can still be distinctly reflected at an upward continuation height of 5 km. The northern section of the inferred fault F6is basically distributed in the area of the Baidiwa-Yunihe Fault marked on the 1∶200000 regional geological mineral map and described by Xia ZD et al.(2012), and Tang QY et al. (2015).

The Luotuofeng-Maotoushan-Beishanbeipo Fault (F7)starts from the southwest of Luotuofeng in the south and extends eastwards beyond the study area through Daqingshan,Maotoushan, and Baishanbeipo. It spreads in NE and nearly EW-trending successively, showing a quasi-S-shaped distribution pattern. It lies in the middle part of the Beishan rift zone and runs through the study area in a direction almost parallel to faults F2and F3. The magnetic anomalies of F7are present as magnetic field gradient belts, boundaries between areas of different magnetic fields, and moniliform anomaly belts in different sections of F7. The gravity anomalies of F7are mainly characterized by gravity gradient belts. The gravity and magnetic field characteristics of F7can still be obviously reflected at an upward continuation height of 5 km. The section of the inferred fault F7from Maotoushan to Baishanbeipo is basically distributed in the area of the Yunihe Fault marked on the 1∶200000 regional geological mineral map. Meanwhile, the southwestern Maotoushan section of F7is also marked as a fault on the 1∶200000 regional geological mineral map.

The nearly EW-trending Luanshanzi-Heyaquan Fault (F8)starts from Bayiquan Gold Deposit in the west and extends southeastwards to Heyaquan through Luanshanzi and Hongliujingzi. In terms of magnetic field characteristics, it serves as the boundary between areas with different magnetic fields, with moniliform magnetic anomalies being visible in some sections. Meanwhile, it has a gravity field characterized by gravity gradient belts. The gravity and magnetic field characteristics can still be clearly reflected at an upward continuation height of 5 km. The inferred fault F8is marked as a fault on the 1∶200000 regional geological mineral map.

5.1.3. Secondary faults or linear anomaly belts

In this paper, 346 secondary faults or linear anomaly belts were inferred mainly from aeromagnetic anomalies, some of which are consistent with the faults marked on the 1∶200000 regional geological mineral map such as F23, F65, and F75(Baishan Fault; Fig. 10). They are not specially introduced in detail in this paper due to the limitation on paper length.

5.2. Characteristics of Beishan rift zone

5.2.1. Geophysical evidence of Beishan rift zone

Rift valleys are banded down-faulted structural zones that extend from hundreds to thousands of kilometers. As products of crustal extension, they are distributed within continents and oceans where deep mantle materials upwelled. Their main characteristics are as follows. (1) From a planar perspective,their tectonic associations are distributed in complex and regular shapes such as linear, en-echelon, parallel, and fission shapes, or they intersect at different angles. (2) The development of most rift valleys is accompanied by volcanic eruptions from under the crust (long-term and extensive or short-term and local). (3) The boundaries of rift valleys show distinct gravity and magnetic gradient belts, and the gravity and magnetic field characteristics within rift zones differ greatly, depending on the thickness of fillings, the quantity of the sedimentary cap rocks, and the burial depth and volume of intrusive rocks. Based on the previous considerable exploration of geotectonic attributes and evolutionary history,a consensus has been reached on the tectonic attributes of the Beishan area and most researchers argue that the Beishan area is an intracontinental rift valley (Zhang WS, 1992; Cheng SD et al., 2000; Zhou JY et al., 2000).

As shown in the upward continuation maps of gravity and magnetic anomalies (Figs. 6, 8), the northwestern and southeastern sides of the Beishan area show distinct gravity and magnetic gradient belts, which move clearly towards the inside of the Beishan rift zone with an increase in the upward continuation height. Meanwhile, the characteristics of gravity and magnetic fields greatly differ on the two sides. All these indicate that the Beishan area falls within a tectonic unit different from those in the areas to the northwest and southeast of the rift zone and that the faults in the two areas contain the bidirectional thrusts and thus have flower-shaped structures. Local gravity anomalies within the Beishan area are mainly present as NE- or NEE-trending anomaly belts(Fig. 7). Meanwhile, local magnetic anomalies within the Beishan area mainly appear as alternating positive and negative NE- and NEE-trending linear magnetic anomaly belts that are distributed in groups and belts. Especially, there are multiple generally NE-trending negative anomalies in nearly equiaxed shapes (Figs. 3, 4, 9). The characteristics of local gravity and magnetic anomalies in the Beishan area are reflected by complex and regular planar morphologies of tectonic associations such as linear, en-echelon, and parallel shapes. Meanwhile, there are long-term and extensive magmatic activities and intense and generally NE-trending mantle-derived magmatic activities in Beishan area.Therefore, the gravity and magnetic field characteristics indicate that the Beishan area should be an intraplate fault orogen in terms of Paleozoic geotectonic setting.

5.2.2. Boundary locations of Beishan rift zone

The southern boundary of the Beishan rift zone is unanimously considered to be the Shulehe Fault at present,which is concealed in the Kumudage Desert and invisible on the ground surface in the study area. The northern boundary of the Beishan rift zone is still in dispute currently. It is considered to be the deep fault on the southern margin of the Central Tianshan Uplift (Zhang WS, 1992), the northern Cantoushan-northern Xiaoqingshan deep fault (called Sailikesayi strike-slip fault zone regionally; Wang MC et al.,2007), or the Hongliuhe fault (Jiao JG et al., 2017).

The faults such as the Shulehe Fault (F3) and Hongshijing-Heyaquan Fault (F3-1) have different gravity and magnetic anomaly characteristics (Figs. 2-5, 7, 9). However, after upward continuation of their gravity fields, their gravity gradient ranges are enlarged and all of them fall into the same gravity gradient zone (Fig. 6). Meanwhile, the NE-trending strong-magnetic anomalies on the southern side of F3quickly move northwards and thus approach F3-1(Fig. 8), indicating that F3is a thrust fault declining northwards and F3-1is its subordinate fault in the north of the Beishan rift zone.Therefore, in this paper, the southern boundary of the Beishan rift zone is considered to be the Shulehe Fault zone consisting of F3and F3-1(Fig. 10). This is consistent with the current understanding of most researchers but slightly different from the view of Wang MC et al. (2007) that the midwestern and eastern sections of the southern boundary are respectively considered to be the Hongshijing-Fendoujing deep fault and the southern Yantan-western Baishan deep fault (collectively called the Shulehe concealed fault zone regionally).

The eastern section of the Kaladage Fault (F1) inferred in this paper distinctly separates the Beishan rift zone from the northern Central Tianshan Block and the northern Luobupo-Weiya Fault (F2) obviously separates the Beishan rift zone from the western Carboniferous basin of Southern Tianshan(Figs. 4-10). In term of location, F1corresponds to the deep fault on the southern margin of the Central Tianshan Uplift described by Zhang WS (1992), F2corresponds to the Hongliuhe Fault described by Jiao JG et al. (2017), and the southwestern section of F2corresponds to the northern Cantoushan-northern Xiaoqingshan deep fault described by Wang MC (2007). Therefore, this paper concluded that the northern boundary of the Beishan rift zone should be the fault zone composed of the eastern section of F1and F2, which is largely located in northern Luobupo-northern Cantoushannorthern Jianshanzi-Daheishan-southern Xingxingxia area.

5.2.3. Internal structure of Beishan rift zone

Zhang WS (1992) argued that the Beishan rift zone is comprised of three fault activity-induced orogenic belts in nearly EW-trending that are parallel or overlap mutually,namely the Early Paleozoic Hongshishan-Tashui orogenic belt, Carboniferous Zhongposhan-Baishan orogenic belt, and Permian orogenic belt. Among them, the Hongshishan-Tashui orogenic belt is distributed along Hongshishan-Cihai-Baishanliang area and shows a paleotectonic framework of“one uplift and two depressions” consisting of the Tashui fault depression, Fangshankou fault uplift, and Cihai fault depression. The Zhongposhan-Baishan orogenic belt is distributed in an expanse of Zhongposhan, Maotoushang,Qixin, and Baishan areas or in the large southern part of Hongshishan-Tashui orogenic belt, showing a paleotectonic framework of “one uplift and two depressions” consisting of the Zhongposhan fault uplift and the fault depressions on the northern and southern sides of Zhongposhan. The Permian orogenic belt was formed due to the orogeny caused by cracking, rifting, squeeze, and folding on the former fold basement. It consists of two NE-trending branches from south to north, both of which obliquely overlay the Early Paleozoic and Carboniferous orogenic belts.

Based on the characteristics of gravity and magnetic fields, especially the characteristics of aeromagnetic anomalies, this paper argues that the Beishan rift zone should be divided into two tectonomagmatic zones by Luotuofeng-Maotoushan-northern slope of Baishan Fault (F7), namely Zhongposhan-Bijiashan-Cihai-Baishanliang zone (the northern zone) and Bayiquan-Qixin-Baishan zone (the southern zone). The northern zone can be further subdivided into three tectonomagmatic areas (i.e., Zhongposhang-Heihai,Bijiashan-Maanshan, and Cihai-Baishanliang areas), while the southern zone can be further subdivided into two tectonomagmatic areas (i.e., Hongshijing-Bayiquan and Qixin-Baishan areas; Fig. 10).

The magnetic field in the northern zone is mainly characterized by a regional low magnetic field, on which multiple sets of banded (linear) positive magnetic anomalies in NE-, NEE- or EW-trending are superimposed (Figs. 2-4,9). The low magnetic field becomes clearer after upward continuation, which is distinctly different from the southern zone (Fig. 8). The magnetic field in the southern zone mainly features a regional low magnetic field, on which several blocky or irregular groups of positive magnetic anomalies such as Baiyiquan, northern Qixin, and Heyaquan are superimposed. It is greatly increased after upward continuation.

The banded (linear) positive magnetic anomalies in the northern zone is clearly composed of three comet-shaped anomaly groups, namely Zhongposhan-Heihai, Bijiashan-Maanshan, and Cihai-Baishanliang anomaly groups, which successively appear from west to east with the “comet heads”and “comet tails” facing the west and the east, respectively.The Zhongposhan-Heihai anomaly group lies in the westernmost part, with the “comet head” mainly composed of Pobei negative magnetic anomalies and the “comet tail”consisting of several linear magnetic anomalies in nearly NE trending. It extends to the Bijiashan and Baiyushan areas in the east but is destroyed by the Bijiashan-Maanshan anomaly group at Bijiashan. The middle Bijiashan-Maanshan anomaly group lies in the Cantoushan, Xiaoqinshan and Maanshan area, with the “comet head” mainly composed of the negative magnetic anomalies in Bijiashan and the “comet tail”consisting of several linear magnetic anomalies nearly EW trending. The comet tail extends to Cihai in the east and is inserted by the “comet head” of the Cihai-Baishanliang anomaly group. The Cihai-Baishanliang anomaly group lies in the easternmost part among the three comet-shaped anomaly groups. Its “comet head” is the Cihai negative magnetic anomaly, which is induced by mafic-ultramafic intrusive complexes. Its “comet tail” extends to Baishanliang and Toudiaoquan areas in the east. It is composed of several linear magnetic anomalies in nearly NE trending, which are mainly caused by volcanic rocks of fissure eruptions occurring along fault or volcaniclastic rock suites. Therefore, the three cometshaped anomaly groups reflect the three tectonomagmatic areas in the northern zone. The ages of the mafic-ultramafic intrusive rocks of Pobei, Bijiashan, and Cihai are 270-284 Ma,260-286 Ma, and 275 Ma, respectively (Xue SC et al, 2019),showing that mafic-ultramafic intrusive rocks tend to be younger from west to east. This is basically consistent with the appearing sequence and superimposition relationship of the comet-shaped anomaly groups, but distinctly different from results achieved by previous researchers (Zhang WS,1992). This paper argues that the comet-shaped magnetic anomaly groups may be formed due to tectonic-magmatic activities, but their formation mechanism and the regional geotectonic implications they reveal are yet to be further researched.

The southern zone can be subdivided into Hongshijing-Bayiquan and Qixin-Baishan tectonomagmatic areas, with the Luanshanzi-Heyaquan Fault (F8) as the boundary. The former has a magnetic field with banded strong magnetic anomalies in nearly EW trending, which gradually merge after upward continuation and form the strongest magnetic field in the Beishan rift zone. It can be inferred that the characteristics of the magnetic field in the Hongshijing-Bayiquan area is mainly caused by the long-term and extensive intrusive activities of alkaline and meta-alkaline magma. The Qixin-Baishan tectonomagmatic area shows a magnetic field characterized by blocky or irregular positive anomalies and negative magnetic anomalies such as Qixin, Luanshanzi, and Maodong anomalies. These magnetic anomalies have complex and variable trending. They gradually merge after upward continuation. As a result, large-scale northern Qixin and northern Heyaquan positive magnetic anomalies are formed,while the Qixin negative anomaly is still clear. All these indicate that the Qixin-Baishan tectonomagmatic area mainly suffered long-term and extensive intrusive or eruption activities of alkaline and meta-alkaline magma and maficultramafic magma.

5.3. Inferred mafic-ultramafic complexes

5.3.1. Geological and geophysical characteristics of typical mafic-ultramafic complexes

Extensive researches have been carried out on the maficultramafic complexes developing in the Beishan rift zone,Xinjiang. However, they mainly focus on the mafic-ultramafic complexes in the Pobei, Bijiashan, and Qixin areas (Jiang CY et al., 2006, 2012; Xie W et al., 2011; Qin KZ et al, 2011,2012; Lu HF et al., 2012; Jiao JG et al., 2017; Xia ZD et al.,2012; Chai FM et al., 2013; Su BX et al, 2013; Bai YL et al.,2004; Feng J et al., 2014; Tang QY et al., 2015; Xue SC et al,2019). Among them, the Pobei rock mass ranks as the most researched mafic-ultramafic complex. It was also researched as a typical mafic-ultramafic complex in the Beishan rift zone in this paper. Its geological and geophysical characteristics were discussed, based on which the distribution of other complexes in the rift zone was inferred.

The Pobei rock mass was named as such since it is located in the north of Zhongposhan. It is emplaced in metamorphic rocks of the Mesoproterozoic Baihu Group and primarily consists of mafic rocks (i.e., gabbro and olivine gabbro) and a small number of ultramafic rocks (i.e., PI-lherzolite,lherzolite, and peridotite). Among them, Poyi and Poshi copper-nickel deposits are well mineralized. The intrusive and contact relationship between the ultramafic rock mass and gabbro indicates that they are formed due to different stages of magmatic emplacement. From a planar perspective, the Pobei rock mass is distributed in a gourd shape in NE trending (Fig. 11a), with outcrops of about 250 km2. It consists of two interconnected lopoliths of which the long axis is in NE direction spatially.

The density of mafic-ultramafic rocks is higher than that of their surrounding rocks according to the statistics of physical properties. However, the Pobei rock mass does not show distinct anomaly boundaries in the maps of Bouguer gravity and its conversion (Figs. 2, 5, 11b). The possible reason is that the survey scale of the gravity data used in this paper is too small.

In contrast, on the maps of aeromagnetic ΔTanomalies and their conversion, the Pobei rock mass mainly shows a gourd-shaped negative magnetic anomaly in NE trending(“Pobei magnetic anomaly”), on which multiple weak positive magnetic anomalies are superimposed (Figs. 11c-f).According to the statistics of physical properties, the order of magnetic susceptibility is ultramafic intrusive rock ＞basalt＞mafic intrusive rock ＞intermediate-acid intrusive rock.Meanwhile, the magnetic susceptibility of the strata is very low, and the magnetic susceptibility of ultramafic intrusive rock is approximate to that of mafic intrusive rock. The remanence is in the order of mafic intrusive rock ＞ ultramafic intrusive rock ＞ basalt, and the remanence of intermediateacid intrusive rock and strata is very low. Furthermore, special attention should be paid to the direction of the remanence.The magnetic declination and inclination of ultramafic intrusive rocks are largely consistent with those of normal geomagnetic field, which may result in local aeromagnetic ΔTpositive anomalies. Contrarily, the magnetic declination and inclination of mafic intrusive rocks are almost opposite to those of normal geomagnetic field, which may cause local aeromagnetic ΔTnegative anomalies. The ultramafic rocks account for 2%-26% of the entire Pobei rock mass (Qin KZ et al., 2012). Therefore, it can be concluded that the Pobei negative magnetic anomaly is induced by mafic intrusive rocks and the weak positive magnetic anomalies superimposed on the negative magnetic anomaly are caused by ultramafic rocks.

Fig. 11. Characteristics of geology and gravity and magnetic anomalies of Pobei complex. a-geological map (modified from Jiao JG et al.,2017); b-vertical first-order derivation of Bouguer gravity anomalies; c-aeromagnetic ΔT anomalies; d-RTP aeromagnetic ΔT anomalies;e-vertical first-order derivation of RTP aeromagnetic ΔT anomalies; f-RTP aeromagnetic ΔT anomalies at an upward continuation height of 1 km.

As revealed by the comparison between the aeromagnetic ΔTanomaly map (Fig. 11c) and the RTP aeromagnetic ΔTanomaly map (Fig. 11d), the aeromagnetic anomalies move northwards as a whole and the southern boundary of the Pobei magnetic anomaly are clearer after RTP. Meanwhile, the comparison among the aeromagnetic ΔTanomaly map (Fig.11c), the RTP aeromagnetic ΔTanomaly map, (Fig. 11d), and the geological map (Fig. 11a) reveals that the gradient zone of the Pobei magnetic anomaly in Fig. 11d well coincides with the boundary of the Pobei rock mass. Therefore, the equiaxed or elliptoid negative magnetic anomalies on which weak positive magnetic anomalies are superimposed can be used as critical indicators for the identification of mafic-ultramafic complexes.

After aeromagnetic upward continuation (Figs. 8, 11f),the center of the Pobei negative magnetic anomaly gradually moves to the southwest of Changshan. At an upward continuation height of 10 km, the negative magnetic anomalies nearby the Poyi rock mass basically disappear,while those close to the southwest of Changshan are still very clear. As shown by drilling data, the Pobei rock mass extends downward for up to 2400 m, without yet drilling through the bottom (Tang QY et al., 2015). The deepest borehole in Poyi rock mass still remains in the ore-bearing peridotite lithofacies after being drilled down to 2105.7 m. It can be inferred from rock tendency that the ore-bearing lithofacies in the Poyi mafic-ultramafic rock mass can extend for more than 3000 m (Lu HF et al., 2016). Therefore, it can be inferred that the Pobei rock mass can extend downwards for more than 5000 m at the deepest positions and that the magmatic pathways should be near the southwest of Changshan.

5.3.2. Mafic-ultramafic complexes in Beishan rift zone

In this paper, 19 mafic-ultramafic complexes (Fig. 10;Table 3) were inferred from the geological and geophysical characteristics of typical mafic-ultramafic complexes as well as a series of aeromagnetic maps of the Beishan area (Figs. 2a,3, 4, 8). Among them, nine ones have undergone reconnaissance surveys and exploration of mafic-ultramafic magma or copper-nickel deposits.

As indicated by the inferred results, the mafic-ultramafic complexes in the Beishan rift zone are generally distributed in the NE trending. They mainly concentrate in Pobei, Bijiashan,and Qixin areas and are clearly under the control of fault structures (Fig. 10). From a planar perspective, almost every of the complexes is elliptoid, short-rod, or irregular in shape.Meanwhile, they each cover a small area except for the complexes in Cihai, Pobei, Qixin, and the southwest of Cantoushan.

5.4. Analysis of prospecting direction and potential of coppernickel deposits

5.4.1. Prospecting direction of copper-nickel deposits in Beishan area

There are about 12 known copper-nickel deposits in the Beishan rift zone, including two large, one medium, and one small ones-all of which occur in the mafic-ultramafic complexes of Pobei, Luodong, Hongshishan, Xuanwoling,and Qixin. The ore bodies are closely related to the main ultramafic lithofacies (such as peridotite, pyroxene peridotite,and plagioclase pyroxenite) and mainly occur at the bottom of main ultramafic lithofacies or in the mafic lithofacies close to the bottom of the main ultramafic lithofacies in a suspended state (such as gabbro and olivine gabbro; Xie W et al., 2011;Lu HF et al., 2012; Chen J et al., 2013; Wang H et al., 2015;Xue SC et al, 2019). The deposits in Sudbury, Canada have been explored to a depth of 2500 m (Teng JW et al., 2007).Meanwhile, in the Voisey’s Bay deposit with a maximum exploration depth of more than 4000 m, remarkable achievements have been made from exploration at a depth of more than 2000 m (Yao ZS et al., 2014). The deepest borehole in the Poyi rock mass in the Beishan rift zone has been drilled down to 2105.7 m, without yet drilling through the ore-bearing lithofacies. Therefore, it can be inferred that the ore-bearing lithofacies of mafic-ultramafic complexes may extend for over 3000 m along the lithofacies inclination(Lu HF et al., 2016). Therefore, the prospecting in the Beishan rift zone, Xinjiang should focus on the deep parts or periphery of known or inferred mafic-ultramafic complexes.

5.4.2. Prospecting potential of copper-nickel deposits in Beishan area

The Pobei mafic-ultramafic complex is inferred to be 280 km2from aeromagnetic data. About 250 km2of the complex has been explored so far, discovering the large Poyi copper-nickel deposit, medium Poshi copper-nickel deposit,and Posan, Poqi and Podong copper-nickel ore occurrences.The exploration mainly concentrated in the west of the Pobei rock mass. Only a borehole in the Poyi mining area has been drilled down to 2105.7 m, but it still remains in the orebearing peridotite lithofacies. Therefore, it can be inferred that the ore-bearing lithofacies of mafic-ultramafic complexes can extend for over 3000 m along the rock tendency (Lu HF et al.,2016). The aeromagnetic anomalies can still be distinctly reflected at an upward continuation height of 10 km,indicating that the rock mass deeply extends downward.Furthermore, it can be inferred from aeromagnetic data that the root of the Pobei rock mass should be in the eastern part,which is yet to be explored. Therefore, the Pobei maficultramafic complex still has prospecting potential.

The Hongshishan mafic-ultramafic complex is inferred to be 122 km2from aeromagnetic data, most of which is covered by the Quaternary. Up to now, 26 km2of the complex is exposed or has been explored, revealing the large Hongshishan copper-nickel deposit and obtaining an exploration depth of more than 1000 m. However, the deep parts are yet to be completely controlled. The aeromagnetic anomalies can still be visible at an upward continuation height of 10 km, indicating that the rock mass deeply extends downward. Therefore, the Hongshishan mafic-ultramafic complexes still have prospecting potential.

The Qixin mafic-ultramafic complex is inferred to be 269 km2from aeromagnetic data. Reconnaissance survey has been conducted for 128 km2up to now, discovering multiple copper-nickel ore occurrences and one concealed high-grade copper-nickel sulfide ore of magmatic injection type, with drilling depths still less than 500 m. The aeromagnetic anomalies can still be distinctly reflected at an upward continuation height of 10 km, indicating that the rock mass deeply extends downward. Therefore, it can be inferred that the Qixin mafic-ultramafic complex still has prospecting potential for copper-nickel deposits.

In addition, copper-nickel deposits or ore occurrences have also been discovered in mafic-ultramafic complexes with favorable metallogenic geological conditions. such as Luodong, Cantoushan, Xuanwoling, and Bijiashan complexes. However, the extension of the ore bodies in deep parts is yet to be determined. Furthermore, reconnaissance surveys of copper-nickel deposits have not been carried out in the mafic-ultramafic complexes such as Maoxi, Luobei,Maodong, Qidong and Cihai. Therefore, quick and efficient methods are required for further exploration of these complexes.

5.4.3. Analysis of prospecting direction and exploration methods of high-grade copper-nickel deposits

Comprehensive airborne geophysical exploration was conducted as a model in the Qixin area in 2016-2020 based on the delineation of predicted prospecting target areas of copper-nickel deposits in the area. During this period, some ground exploration and borehole drilling were also conducted as a supplement, and thus the quick airborne-ground-drilling synergetic exploration method in the Beishan area was explored. The copper-nickel ore body (Fig. 12e) was revealed by borehole ZK3-2. It is located in the southwest of the periphery of Qixin mafic-ultramafic complex, with felsic hornfels of Palaeoproterozoic Dunhuang Group (Pt1Dh) as the surrounding rocks. The ore body has a vertical attitude, with a strike of about 322° and a dip of about 65°. It inclines towards the southwest, with an apparent thickness of 32.9 m.Meanwhile, burial depth of its roof and floor is 34.9 m and 67.9 m, respectively. However, its scale is yet to be controlled. The grade of Ni is 0.20%-10.99%, with an average of 1.84%, and the average grade of Cu and Co is 0.93% and 0.04%, respectively. Meanwhile, Au and Ag meet the standard of associated useful components according to the analysis of composite samples. Furthermore, the content ofnickel sulfide in the core samples of the borehole ZK3-2(taken at a depth of 34.9-67.9 m) ranges between 72.73% and 84.82%, and thus the copper-nickel ore body is preliminarily determined to be a concealed high-grade copper - nickel sulfide deposit of magmatic injection type.

Table 3. List of mafic-ultramafic complexes in Beishan rift zone, Xinjiang.

The ground residual gravity and aeromagnetic anomalies reflected above the copper-nickel ore body show isolated high anomalies (Figs. 12a, b), and the results of the 1∶10000 airborne time-domain electromagnetic (ATEM) survey show strong electromagnetic anomalies (Fig. 12c). After sampling of electromagnetic signals at the electromagnetic anomalies,the electromagnetic signals of traces No. 0-9 attenuate slowly at an early stage, and thus they are strong and display peaks(Fig. 12c, d). These are typical manifestations of an ultra-low resistivity body. The software Maxwell was used to conduct 1D resistivity imaging through deep conversion of the electromagnetic anomalies (Fig. 12d). The inversion results of apparent resistivity show that the low-resistivity body(inferred to be a high-grade ore body) can extend downward to more than 300 m and expands at a depth of around 200 m.This reveals that ATEM survey can effectively reflect the location, burial depth, scale, and morphology of copper-nickel sulfide ore bodies in the case of copper-nickel ores with a certain scale and grade.

Fig. 12. Exploration results of concealed high-grade copper-nickel deposits in Qixin, Xinjiang. a-RTP residual aeromagnetic anomalies after high-pass filtering (cut-off wavelength: 800 m); b-residual Bouguer-gravity anomalies after high-pass filtering (cut-off wavelength: 800 m);c-electromagnetic anomalies of the first 10 traces obtained from ATEM survey; d-inversion results of apparent resistivity of line AB; e-drilling results

There are two types of copper-nickel deposits mainly in the Beishan area, namely magmatic liquation and magmatic injection types. The magmatic liquation-type deposits mostly have low-grade ore bodies, while the magmatic injection-type deposits have high-grade ore bodies. It is believed that there are high-grade ores the in Beishan area according to the exploration in Qixin, Beishan. Much attention should be paid to copper-nickel sulfide ore bodies of magmatic injection type that occur in surrounding rocks on the margin or at the bottom of mafic-ultramafic complexes and are formed due to squeeze,penetration, and injection along the tectonic fractured zones(e.g. faults and stratigraphic fissures) or parts with weak stress. Most of these ore bodies are high-grade and it is convenient for mining and mineral separation. Therefore, they boast great economic significance. It is feasible to apply airborne gravity and magnetic surveys to delineate the boundaries and surrounding structures of complexes due to their cost saving and high efficiency. Based on this, ATEM survey can be comprehensively used to determine the locations of electromagnetic anomalies. Meanwhile, ground exploration can be conducted as a supplement, followed by drilling for verification. The quick airborne-ground-drilling synergetic prospecting method is effective for prospecting and exploration of high-grade copper-nickel deposits in shallow parts of Beishan area, which will provide important references for further exploration and development of prospecting prospect areas of copper-nickel deposits in the Beishan area.

6. Conclusions

(i) This paper argues that the characteristics of gravity and magnetic fields inside and outside of the Beishan rift zone are different according to the data processing and characteristic analysis of 1∶50000 high-precision aeromagnetic data and gravity data with a grid spacing of 2 km × 2 km. Based on this, the boundaries that separate the Beishan rift zone from external geological units were determined again. It is argued that the northern and southern boundaries of the rift zone should be the fault zone consisting of the eastern section of the Kalatage fault (F1) and northern Luobupo-Weiya Fault(F2) and the Shulehe Fault (F3), respectively. The northern boundary is largely located along the northern Luobuponorthern Cantoushan-northern Jianshanzi-Daheishan-southern Xingxingxia area. Meanwhile, the location of the Shulehe Fault in the study area that is concealed in the Kumutage Desert was drawn based on the geophysical characteristics.

(ii) The principle ore-controlling fault structures and tectonomagmatic zones in the Beishan rift zone were precisely divided and described according to differences in gravity and magnetic fields of the rift zone. The Beishan rift zone was divided into two tectonomagmatic zones, namely the Zhongposhan-Bijiashan-Cihai-Baishanliang zone (the northern zone) and the Bayiquan-Qixin-Baishan zone (the southern zone). Comet-shaped magnetic anomaly groups were discovered in the rift zone, and their formation mechanism and the regional geotectonic implications they reveal are yet to be further researched.

(iii) Typical mafic-ultramafic complexes in Beishan rift zone have the geophysical characteristics of a regional low magnetic field and a regional high gravity field. Nineteen mafic-ultramafic complexes were delineated in the rift zone according to aeromagnetic anomalies and gravity field characteristics. Among them, 10 complexes were identified for the first time.

(iv) This paper analyzed the prospecting potential and direction of copper-nickel deposits in known/inferred maficultramafic complexes based on aeromagnetic data combined with ground geological and drilling data. It is concluded that the deep parts and periphery of the mafic-ultramafic complexes such as Pobei, Hongshishan, Qixin, Cihai, and Maoxi have great prospecting potential.

(v) It is feasible to apply aeromagnetic and gravity surveys to delineate the boundaries and surrounding structures of mafic-ultramafic complexes. Based on this, ATEM survey can be comprehensively used to determine the locations of electromagnetic anomalies. Meanwhile, ground exploration can be conducted as a supplement, followed by drilling for verification. This quick airborne-ground-drilling synergetic prospecting method is effective for the prospecting and exploration of high-grade copper-nickel deposits in shallow parts of Beishan area.

The research in this paper will provide some references for further understanding of the geophysical characteristics,the research on regional geological tectonics, and exploration and prospecting of copper-nickel deposits in Beishan rift zone, Xinjiang.

CRediT authorship contribution statement

Jing-zi He, Zheng-guo Fan and Sheng-qing Xiong conceived of the presented idea. Jing-zi He, Teng-fei Ge and Si-xun Wang wrote the manuscript with support from Xuzhao Huang. Sheng-qing Xiong helped supervise the project.All authors provided critical feedback and helped shape the research, analysis and manuscript.

Declaration of competing interest

The authors declare no conflicts of interest.

Acknowledgment

This study was supported by the National Key Research and Development Program of China (2017YFC0602206) and the projects of the China Geological Survey (DD20160066,DD20190551).