Earthquake Research in China  2020, Vol. 34 Issue (3): 311-327     DOI: 10.19743/j.cnki.0891-4176.202003004
Research Progress of Geophysical Exploration in Karatungk Mine in Northern Xinjiang, China
DU Peixiao1,2, LI Yang1,2, WEI Mengyi1,2, HAN Chunming3, ZHAO Liang1,2, WU Jing1     
1. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;
2. College of Earth Sciences, University of Chinese Academy of Sciences, Beijing 100049, China;
3. Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
Abstract: Karatungk Mine in northern Xinjiang, China, which is a large-scale magmatic Cu-Ni sulfide mine in the Central Asian orogenic belt, has a long history of mining. The mine is located at the merging belt between Altay orogenic belt and Junggar Basin, and has strong tectonic activities. In recent years, mining source detection has become an important target for mineral exploration due to the difficulties in ore body exploitation. In this paper, we systematically summarize the achievements of the geophysical explorations in Karatungk Mine from various aspects, including tectonic backgrounds of the mine, dynamic mechanisms, geophysical characteristics and scientific challenges in the future. Because of the restrictions of observation density and analysis methods, the fine geometrical structure of the mine cannot be completely characterized yet. Therefore, in order to obtain the high-resolution structure and detailed spatial distribution of orebodies, researchers should focus on combining multiple geophysical methods, developing high-resolution imaging methods, and improving petro physical experiments in the future.
Key words: Xinjiang     Karatungk Mine     Geophysical exploration     Detailed shallow crustal structure    

INTRODUCTION

Karatungk Mine, located in the southern margin of Altay orogenic belt in the north of Xinjiang, China, is a large-scale Cu-Ni sulfide mine in the Central Asia orogenic belt (Li Gangzhu et al., 2011). In 2005, the deposit reserve of the mine was predicted to exceed 1.24 million tons (Gong Ying et al., 2005), suggesting that it plays a vital role in China's economic development (Han Chunming et al., 2006; Wang Jianzhong, 2010). The total amount of Cu and Ni nonferrous metals has been proved to be over 940 000 tons since the first geological exploration was conducted in 1980 (Lv Linsu et al., 2007). At present, the mining work in this area is mainly related to three large ore-hosting intrusions, with the mining depth up to 800 m underground (Qin Kezhang et al., 2014). Previous studies indicate that the surface of Karatungk Mine is mostly mafic rock mass, and no large-scale ultramafic rock mass has been observed. They suggest that a large-scale ultramafic rock mass may exist in the deep part of the mine (below 800 m in depth) (Qin Kezhang et al., 2014).

To date, it is rather difficult to discover rich mineral resources in the shallow part (0-800 m) of Karatungk Mine with the large-scale exploitation. As a result, searching for deep concealed deposits is a major challenge, and it is hard to satisfy the forthcoming exploration needs if we only apply traditional geological exploration methods. Considering that the Cu-Ni sulfide orebodies have significant differences in physical properties compared with the surrounding rocks, such as density, magnetic susceptibility, geophysical exploration methods, such as gravity and magnetic exploration, magneto telluric sounding, reflection seismic exploration and surface-hole transient electromagnetic method, have been increasingly implemented in mining work (Malehmir A. et al., 2012; Ferguson I. J. et al., 2016). With the expansion from surface to underground drilling, the research progress of mines has been promoted, and the understanding of Karatungk Cu-Ni sulfide mine has been improved.

The purpose of this paper is to summarize the main research progress of geophysical exploration in Karatungk Mine in recent years. Geological structure background of the mine, the application of geophysical prospecting, such as gravity, magnetic, electric and seismic exploration in the mine and the existing research progress are introduced in this paper. Additionally, we discuss the restrictions of geophysical methods used in recent years in Karatungk Mine, and introduce the geophysical studies we are working on.

1 INTRODUCTION OF THE STRUCTURE AND METALLOGENIC SETTING OF KARATUNGK MINE 1.1 Tectonic Settings and Seismicity

Karatungk Mine is located at the northern part of the Central Asian orogenic belt, the northern margin of the Junggar Plate as well as the eastern part of the Saerbulak-Sasekebasitao synclinorium (Tong Tiegang et al., 2004; Zhou Jianyong et al., 2016). The Junggar Massif is bounded by the deep Ertix-Maynebo fault (Fig. 1(a)) and it borders with the Altay Caledonian orogenic belt to the north. The tectonic structure of the mine includes a series of complex folds and thrust faults with a major strike in NW, and sub-strikes in NE, NNW, and EW directions (Qin Kezhang et al., 2014).

Fig. 1 (a) Geographical location of the Karatungk Mine (Modified from Han Chunming et al., 2007); (b) Geological map of Karatungk orefield showing the distribution of mafic intrusions (Referenced from Qin Kezhang et al., 2014); (c) The distribution of magnetic anomalies and gravity anomalies in Karatungk Mine and its adjacent areas (Referenced from Qin Kezhang et al., 2014)

Fig. 1 shows the major lithology and deposition distribution in Karatungk Mine. The outcropping strata in this area mainly include Devonian, Carboniferous, part of Ordovician, Jurassic, Tertiary and Quaternary. Nanmingshui Formation of Lower Carboniferous is the major stratum in the aforementioned mine, which is the host rock of mafic rock mass. The lithology consists of carbonaceous sedimentary tuffs, argillaceous slate with sedimentary tuffs, and coarse-grained tuffs in gravels, etc. (Yu Xu, 2008).

The tectonic activity in the mine is the intensely active, and the magmatic activity in the vicinity of the mine is also strong. There are three periods in the history of rock intrusion: diorite from early to middle Hercynian, gabbro from middle to late Hercynian, and plagiogranite from early Yanshan, of which the ore-hosting intrusions in Karatungk Mine are primarily magma products from the middle to late Hercynian (Wang Runmin et al., 1993). Rock mass group in the mine is composed of 13 mafic-ultramafic ore-hosting intrusions Y1-Y11 (Fig. 1(b)), G21 and G22 (Fig. 1(c)), respectively (Qin Kezhang et al., 2014). Except for the northwestern end of Y1, Y4 and Y6-Y11 ore-hosting intrusions (hereinafter called Y1, Y4 and Y6-Y11) that are exposed to the surface, other ore-hosting intrusions are all buried underground (Mao Jingwen et al., 2008). The existing geological prospecting studies focus mainly on Y1-Y3, and limited investigations on Y6-Y11 are conducted.

The main sulfide orebodies in Karatungk Mine are pentlandite, pyrrhotite and chalcopyrite according to the available data. There are significant differences in rock physical properties in orebodies, ore-hosting intrusions and surrounding rocks. More specifically, compared with surrounding rocks, Cu-Ni sulfide orebodies have relatively higher magnetism, density, polaribility and lower resistivity, which are known as ' three higher-s and one lower ' geophysical anomalies (Deng Zhenqiu, 1990; Xiao Qibin et al., 2005; Shao Xinglai et al., 2012; Qin Kezhang et al., 2014). Yang Yongqiang et al. (1998) indicate that the density and magnetism of rock mass increase with the increase of orebody host in intrusions, and the magnetism of mineralized rock is obviously higher than that of non-mineralized rock based on the analysis of Y1 (Yang Yongqiang et al., 1998; Jia Nati, 2013). Table 1 shows that Y1 has the largest scale and highest degree of mineralization with almost all rock mineralized, followed by the Y2 and Y3 ore-hosting intrusions (hereafter called Y2 and Y3) with different degrees of mineralization. These three ore-hosting intrusions are main mineral resources previously observed in Karatungk Mine.

Table 1 Basic characteristics of mafic-ultramafic intrusions in the Karatungk ore district (Referenced from Qin Kezhang et al., 2014).

On August 11th, 1931, a M8.0 earthquake occured in Fuyun, Northern Xinjiang, resulting in extensive and severe damage as well as permanent ground deformation in Fuyun and Qinghe areas (Yang Zhang et al., 1980). Karatungk Mine is located between Fuyun and Qinghe, indicating that the surrounding area of the mine is a tectonically and seismically active area. According to the seismic catalogue provided by the China Earthquake Networks Center, the main seismicity statistics around the mine in recent 10 years (2008-2019) are as follows:1 794 earthquakes ≤M1.0, 42 earthquakes ≤M3.0, and the greatest earthquake is M4.7. The seismicity information indicate that it is tectonically active while the ability of seismicity detection is limited in this area.

1.2 Metallogenic Dynamic Mechanism

Researchers hold different views on the metallogenic dynamic mechanism of Karatungk Mine (Fig. 2). Some researchers suggest that plate subduction may be the possible reason for mineralization (Han Chunming et al., 2007; Sillitoe R.H., 1997). It is commonly acknowledged that plate subduction can lead to mantle melting, and the mantle wedge may then generate highly oxidized magma during the local melting process of subduction. Such process may further destabilize the mantle sulfide, thus releasing and transporting copper and gold to the upper crust where they are subsequently mineralized under the following arc-shaped magmatism (Fig. 2(a)). However, some people argue that the metallogenic belt is formed by continental collision (Chen Yanjing, 2013). They indicate that the subducting plate in this process may raise the temperature and pressure, followed by a list of successive processes, including hypabyssal intrusion, metamorphism, and melting. Under the compression and extension transformation system of collision orogeny, the decompression and temperature rising will generate large-scale mineralization (Fig. 2(b)). In addition, some researchers consider that the metallogenic deposits are related to mantle plumes, indicating that there is a geochemical correlation between mafic-ultramafic rocks in the eastern Tianshan, Beishan and Tarim Basin basalts. Meanwhile, the ages of their formation are all 280 Ma, meaning that the continental flood basalts are mostly originated from mantle plume (Fig. 2(c)), Wang Denghong et al., 2007; Qin Kezhang et al., 2011, 2012). Furthermore, the upwelling of thermal materials to the shallow crust caused by mantle plume is the main metallogenic mechanism in Karatungk Mine (Qin Kezhang et al., 2014). The Cu-Ni sulfide orebodies in Karatungk Mine is taken for a high magnesium tholeiitic magma originating from the depleted asthenosphere mantle and lithosphere mantle (Zhang Zhaochong et al., 2003; Jiang Changyi et al., 2009). And the orebodies are considered as the products of upwelling, emplacement, differentiation and crystallization in the context of post-orogenic extension in Early Permian (Han Baofu et al., 2004; Zhang Zuoheng et al., 2008; Han Chunming et al., 2006), during which the local contamination of crustal materials occurred (Tang Zhongli et al., 2006).

Fig. 2 Background of metallogenic dynamics of ore deposits (Modified from Sillitoe R.H., 1997; Hou Zengqian et al., 2011; Xiao Wenjiao et al., 2010).

The primary controversy is whether the genesis of the deposit is in asthenosphere and/or lithosphere, while the characterization of the fine structure in the crust is rare.

2 RESEARCH PROGRESS OF GEOPHYSICAL EXPLORATION IN KARATUNGK MINE

Geophysical exploration methods are more sensitive to changes in the physical properties of rocks and are therefore often used in mineral exploration (Dentith M. et al., 2014). Geophysical methods play a crucial role in the exploration of magmatic Cu-Ni sulfide orebody (Dowsett J.S., 1967; Watts A., 1997). The tectonic movements associated with the magmatic Cu-Ni sulfide mineralization may cause changes in the surrounding geophysical field, resulting in difference in the geophysical properties between the minerals and the surrounding rocks. This provides a theoretical basis for geophysical exploration in the prospecting work.

Magmatic Cu-Ni sulfide orebodies usually include pentlandite, pyrrhotite and chalcopyrite, which have anomalies in many physical properties compared with the surrounding rocks (King A., 2007). The orebodies usually show higher density, higher magnetic properties, higher polarizability and lower resistivity compared with the surrounding rocks, expressing in 'three higher-s and one lower' geophysical characteristics. For the existing geophysical characteristics, scholars have used the 'Gravity-Magnetic-Electric-Seismic' geophysical method for decades to carry out many studies on Karatungk Cu-Ni mine in Xinjiang, and have obtained a series of geophysical achievements (Qin Kezhang et al., 2014). Based on previous studies, we will analyze the geophysical exploration methods used in Karatungk Cu-Ni mine nowadays, and discuss the principles and applications of each method in the prospecting work.

2.1 Gravity Exploration

Gravity exploration, one of the important methods for geophysical prospecting, can infer the geological composition or mineral distribution of the survey area by measuring the gravity anomaly caused by underground density unevenness (Dentith M. et al., 2014; Ren Li et al., 2013). As shown in Table 2, the density can well distinguish sulfide minerals and ore-hosting intrusions (King A., 2007), in which the average density of typical sulfide minerals (such as pentlandite, pyrrhotite, chalcopyrite and magnetite) is above 4.0 g/cm3 and the average density of ore-hosting intrusions is relatively small, most of which are below 3.2 g/cm3 (King A., 2007; Yao Zhuosen et al., 2014). The difference in density between sulfide minerals and ore-hosting intrusions is considered the main reason of high gravity anomalies in sulfide minerals. Therefore, gravity exploration can delineate the Cu-Ni orebodies by directly measuring the response of the high gravity anomaly caused by the density difference between the orebodies and the ore-hosting intrusions.

Table 2 The Ni-Cu-Sulfide Ore Mineral and Host rock Magnetic Susceptibilities, Densities and Electrical Resistivity (Referenced from Telford W.M. et al., 1991; King A., 2007; Yao Zhuosen et al., 2014).

Geophysical prospecting brigade of Xinjiang Geological Bureau of Geological Ministry discovered two gravity anomalies at the east of Y1 in 1980, the length of which are about 1 400-1 600 m and the width of which are about 300-400 m. Although the amplitude of anomalies is relatively small, drilling and exploration results show that the local gravity anomalies are two medium-sized hidden deposits (Zhang Jian et al., 1989; Xiong Guangchu et al., 1996; Deng Zhenqiu et al., 1997). Wang's groups from Chang'an University performed a vertical second derivation of Bouguer gravity anomaly to identify 48 gravity anomalies in the mine in 2012. Most of the gravity anomalies (i.e. Y26, Y1-Y9, G21 and G22) have a good correspondence with known intrusions (Qin Kezhang et al., 2014). Shao's group from Xinjiang Geology and Mineral Technology Development Company performs forward modeling of gravity anomaly, and their results show that after removing the existing intrusions and orebodies, residual gravity anomalies still remain (Qin Kezhang et al., 2014), such as Y2-Y5, Y7-Y9, G21 and G22 anomalies. Zhou Yaoming et al. (2014) also conduct gravity anomaly measurement of the G21 anomaly zone in the mine. Through analysis of the comprehensive anomalous characteristics of gravity and magnetism, it is concluded that the comprehensive anomalies of G21-1 and M21-4 and those of G21-3, G21-4 and M21-3 have the same characteristics. G21-2, M21-1, M21-2 are anomalies with different sources of gravity and magnetism. Drilling analysis of high gravity anomaly G21-2 suggests that the subsequent exploration should focus on the northeast part of G21-2 with high gravity.

Although gravity exploration has obtained many achievements, there are still several challenges in actual mining work. For example, high iron content in the rock can lead to a large density (King A., 2007; Yao Zhuosen et al., 2014), which in turn manifests as a high gravity anomaly. Also low ore content in ore-hosting intrusions cannot help to obtain high gravity anomalies. Considering the multiple solutions in the inversion, gravity exploration has many interference factors and limitations in the prospecting work, and thus could not meet the requirements for better prospecting.

2.2 Magnetic Exploration

Magnetic exploration is a geophysical prospecting method that infers the geological structure, mineral resources, or the distribution law of other detected objects by observing and analyzing magnetic anomalies caused by the magnetic differences of rocks, orebodies, or other detected objects (Yi Qiutian et al., 2015). It can be inferred from Table 2 that the main mineral pyrrhotite in Cu-Ni orebodies has a higher magnetic susceptibility, and the higher its content, the stronger the magnetism of Cu-Ni orebodies (Shao Xinglai, 2012; Yang Yongqiang et al., 1998; Jia Nati, 2013). Surrounding rocks in the mine are mainly rocks with weak magnetism such as sedimentary tuff, mudstone, muddy slate and sandstone (Yu Xu, 2008). The remarkable high magnetic anomaly (Qin Kezhang et al., 2014) makes magnetic exploration an effective way in prospecting research.

The 4th Geological Brigade of Xinjiang Geological Bureau observed obvious magnetic anomalies near the ZK13 hole, which was proved to be a large hidden sulfide Cu-Ni orebody, and named as No.1 ore-hosting intrusions in 1979 (Zhang Jian et al., 1989; Deng Zhenqiu et al., 1997). Zhang Jian et al. (1989) observe a magnetic anomaly group consisting of four small anomalies in the G2 area, and find that there are two industrial orebodies and one diorite body after drilling three of them. Their results point out that the combination of magnetic and geochemical exploration can better prospect the orebodies in shallower depth. If the burial depth of orebodies is larger, the excitation polarization method can be used to evaluate the anomaly. Zhou Yaoming et al. (2014) conduct a 1:5000 high-precision magnetic measurement of the G21 anomaly zone on the periphery of Karatungk Cu-Ni mine, and discover that the magnetic anomaly and gravity anomaly of the G21 anomaly zone correspond to wells. Drilling exploration indicates that the comprehensive anomalies of G21-1 and M21-4 and that of G21-3, G21-4 and M21-3 are not associated with ore-hosting intrusions, which are instead caused by high-density, high-magnetism debris tuff. The high-gravity anomaly of G21-2 is a response of ore-hosting intrusions. Due to the weak magnetic properties of the overburden and quartz diorite in the upper part of the ore-hosting intrusions, this high-gravity anomaly only presents a low magnetism (Zhou Yaoming et al., 2014).

In the actual mining work, we often encounter the phenomenon where the magnetic anomaly of the orebodies and ore-hosting intrusions are usually weak due to its small size and ore content, resulting in some limitations in prospecting work.

2.3 Electrical Exploration

Electrical exploration can determine the location of orebodies through differences in physical properties such as electrical conductivity, dielectric properties, and electrochemical properties between rocks (Cai Zhengbo, 2012). When the electrical properties of underground rock layers and orebodies change in the horizontal and vertical directions, the spatial distribution of electromagnetic fields observed on the ground also changes correspondingly. According to the abnormal characteristics of the spatial distribution of electromagnetic fields (including size, location, shape, burial depth and physical parameters, etc.), it can be inferred whether there are geological structures or orebodies (Yu Hehai, 2009). Table 2 shows that the resistivity of sulfide minerals and ore-hosting intrusions differ by 8 to 9 orders of magnitude, so measuring the electrical conductivity of rocks can effectively determine the content of sulfide minerals (King A., 2007). From the application of resistivity profiling to electromagnetic sounding, electrical exploration has made numerous achievements in research and prospecting of Cu-Ni sulfide mine, especially the recently widely used exploration work represented by the transient electromagnetic method (Qin Kezhang et al., 2014).

Zhang Zhaojing (1996) utilizes the pulsed transient electromagnetic method, the induced electricity in the well, the deep charging method to obtain satisfactory results, which extract the structural form of hidden ore-hosting intrusions and delineate orebodies in the deep part of Karatungk Cu-Ni mine. They believe that there is no high-resistance rock layer between the Y1 and Y2 ore-hosting intrusions, and they are almost connected in one low-resistance tectonic zone. The speculation that massive ore-hosting intrusions may exist in the west of Y2 is verified by a 10 m thick extra-rich massive orebody drilled by the ZK238 borehole in Line 3 (Zhang Zhaojing, 1996). This work also measure the negative anomaly with a large amplitude near the 150 m depth of the ZK17 borehole using the pulsed electromagnetic method in the well. This anomaly basically coincides with the orebody delineated by the actual mining work (Zhang Zhaojing, 1996). In addition, Xu Zhenchao (2003) conducts transient electromagnetic measurements on ground-well zero azimuth of the ZK17 borehole, and indicate that there is a wide range of anomalous bodies with high amplitudes near the depth of 150 m. Later they find a sulfide orebody nearly 20 m thick at that depth (Xu Zhenchao, 2003). Based on the horizontal distribution of minerals determined by gravity and magnetic data, Zhou Yaoming et al. (2014) clearly outline the occurrence of high-resistance bodies in obtained profiles by using transient electromagnetic sounding method, and it is consistent with the actual spatial shape of the rock mass (Zhou Yaoming et al., 2014).

Compared with gravity and magnetic methods, electrical exploration is more flexible and effective in prospecting work, but is still affected by several interference factors. Due to the presence of carbon-bearing strata in the surrounding rocks of Karatung combined with a close resistivity range to that of Cu-Ni sulfide orebodies, when using electrical survey methods such as resistivity profile, electromagnetic sounding and transient electromagnetic, the low-resistance part of the exploration results cannot be accurately interpreted. It is difficult to distinguish the sulfide orebody from the carbon-bearing formation. Meanwhile, due to its limited exploration depth, electrical exploration is not ideal for deeper metal deposits.

2.4 Seismic Exploration Using Artificial Source

Artificial-source seismic exploration refers to the geophysical exploration method for inferring the nature and morphology of underground rock layers by observing and analyzing the propagation of elastic waves generated by artificial sources, namely seismic waves, through the difference in elasticity and density of underground media (Lu Jimeng et al., 2009). Rational use of artificial-source seismic exploration methods can obtain a clearer underground rock structure and lithological boundary, including the physical characteristics of rock density and seismic operation rate. Although gravity, magnetic and electrical methods are able to detect the deep non-ferrous metal deposits, seismic exploration has a significant advantage in resolution. Especially, when single gravity and magnetic anomalies occur, the use of shallow seismic methods to detect bedrock fluctuations can help us study anomalies in detail (Zhang Jian et al., 1989).

Zhou Jianyong et al. (2016) conduct seismic numerical simulation and high-precision seismic reflection research to explore the deep resources of Karatungk Cu-Ni mine. The profile of line 95 shows that the difference in wave impedance between mineralized gabbro and surrounding rocks is larger than that of diorite, so it is easier to identify strong reflections on seismic profiles. They also suggest that an anomaly in the wave amplitude of the 95-line profile has a good correspondence with both the magnetic anomaly of the Bouguer gravity anomaly in the survey area and the first vertical derivative of the anomalous magnetic polarization method. In detail, they initially design a borehole, drill at CDP3386 and then find ore-hosting intrusions with a large thickness within the depths of 686-840 m and 1 060-1 140 m, which are consistent with the interpretation of the profile in Line 95(Zhou Jianyong et al., 2016). Liu Jianxun et al. (2017) take advantage of the technique of seismic exploration and drilling and propose that there are massive mafic intrusions in the deep part of Y3 and Y5. After drilling, gabbro and diorite with a thickness of about 478 m are found under the Y5 ore-hosting intrusions, and their results are partially verified.

Although the artificial-source seismic exploration has improved the detection depth and resolution of orebodies, several restrictions still exist. Previous studies indicate that only when the orebodies are large enough and gently inclined, can the orebodies and ore-hosting intrusions easily be detected and reflected by seismic waves and then be effectively identified. However, it tends to cause the seismic wave energy to propagate downward, making the orebodies difficult to be identified due to the steep-dip distribution in the deep part near the Y2 in Karatungk Cu-Ni mine (Qin Kezhang et al., 2014). Particularly, when orebodies are small, the reflection characteristics in obtained profiles are not easy to be identified, thus the resolution of this method within the capable explorative depth still needs to be improved by more precise instruments and the combination with other methods in order to obtain more accurate interpretation.

2.5 Ambient Noise Tomography Based on a Dense Seismic Array

The short-period dense seismic arrays developed in recent years aim to take advantage of smaller station spacing for the purpose of revealing the detailed crustal structure based on ambient noise tomography, which is also the technique we conduct in Karatungk Mine (Fig. 3). The weak noise signal in a certain directions may be received by two stations aligning along the directions, and the two stations retain the consistency of the signal from specific field source by calculating cross-correlation functions of the records from the two stations and stacking them multiple times, we can extract valid signals in this direction (Weaver R.L., 2005). Then, based on the assumption of diffusion field and the relationship between the cross-correlation functions and the empirical Green functions, information during the seismic wave propagating between the two stations can be obtained, and the underground velocity structure can be retrieved. The resolution of this method depends on the station spacing and the frequency range of surface wave signals in the study area. Therefore, to obtain detailed shallow crustal velocity structures, dense seismic arrays and high-frequency surface wave signals can effectively improve the resolution (Lin Fanchi et al., 2013). Different from the traditional techniques in seismology which are dependent on seismic events, this method overcomes the problem of uneven temporal and spatial distribution of earthquakes. When calculating the cross-correlation function between station pairs, all stations can be regarded as effective sources, and continuous ambient noise can also be used as effective signals. As thus, this method is suitable for Karatungk Cu-Ni mine with weak seismicity (Fig. 3).

Fig. 3 Distribution of short-term dense seismic arrays in the Karatungk Cu-Ni mine Black triangles represent KMA, white areas are ore-hosting intrusions in the mine, and yellow circles indicate seismicity around Karatungk Mine (2000-01-2019-07)

As a methodological attempt, we deploy a two-dimensional dense array (KMA) containing 100 short period seismometers in Karatungk Cu-Ni mine from June 2018 to August 2018. The full aperture of KMA is ~12 km in length and ~3 km in width, and with an average station spacing of ~0.5 km. The yellow circles in Fig. 3 are the seismic activities occurred from January 2000 to July 2019 in the vicinity of the mine. It can be seen that only a handful of seismic events are recorded in the area.At present, we have obtained the velocity structure at a depth of 1.3 km underground in the mine. The horizontal resolution is about 1.1 km, and the vertical resolution is about 0.1 km in the shallow part. The shallow velocity structure corresponds well with the distribution of the known ore-hosting intrusions. In the following process, it is necessary to increase the constraints on the depth deeper than 1 km.

3 CURRENT UNDERSTANDING AND DEFICIENCY

Karatungk Cu-Ni sulfide orebodies are geophysically characterized by higher magnetism, higher density, higher polaribility and lower resistivity, which are known as 'three higher-s and one lower' geophysical anomalies. The density and magnetism of mafic ore-hosting intrusions may increase with the increase of mineralization. Therefore, the combination of gravity and magnetic methods is the most important means of exploration. The gravity anomaly corresponds well to the rock location in the mine. Furthermore, the residual gravity anomalies of large ore-hosting intrusions with higher mineralization are higher, and the anomalies with weak intensity should be verified by borehole data. Obvious magnetic anomalies in a single magnetic exploration often indicate the existence of Cu-Ni orebodies. The characteristics of magnetic anomalies with smaller intensity should be evaluated by the combination of magnetic exploration, geochemical exploration, and inducing polarization methods according to different depths. The combined exploration of gravity and magnetic can effectively delineate the outline of the anomalies in the mine. However, they are easily disturbed by the high density and high magnetic crystal clastic tuff. Besides, the strong magnetism of anomalies can easily be concealed by the quartz diorite in the overburden of ore-hosting intrusions, which may affect the observations.

The application of electromagnetic prospecting and artificial-source seismic exploration has greatly enhanced the exploration depth, and both of them can collect the information of ore-hosting intrusions in depth. Electromagnetic method can be used to distinguish the connection between ore-hosting intrusions and massive orebodies, delineate the anomaly with higher amplitude, and further determine its depth for verification work, which can effectively obtain the depth information of ore-hosting intrusions. In practical work, through the combination of gravity and magnetic methods, the distribution of ore-hosting intrusions can be clearly depicted. Artificial-source seismic exploration can accurately delineate the obvious difference of wave impedance, and infer the ore-hosting intrusions by combining with the magnetic anomaly of the first vertical derivative of Bouguer gravity anomaly and polar magnetic anomaly.

Through the progress of geophysical exploration, it is known that combining multiple explorations of gravity, magnetic, electric and artificial-source seismic exploration can effectively delineate the anomalous bodies compared with the single method. However, in the actual exploration, there are more interference factors in mines, and the gravity and magnetic survey is often affected by bedrock upheaval and depression so that the original high gravity and magnetic anomaly of the ore-hosting intrusions are suppressed and difficult to be observed. Both gravity and magnetic explorations rely on the comprehensive response of anomalies on the surface caused by the difference of density and magnetic susceptibility among orebodies, ore-hosting intrusions and surrounding rocks. They cannot directly distinguish the lithology of abnormal bodies. Therefore, when the content of orebodies in ore-hosting intrusions is too few to be detected, and there are also interference factors of high density and high magnetic susceptibility, some errors may occur in gravity and magnetic measurement consequently. Due to the low wave impedance in the electrical prospecting results of the widespread carbonaceous strata near the surface, which is consistent with Cu-Ni orebodies, it is difficult to interpret the obtained results. In addition to the massive orebodies and the Y1-Y3, the majority of ore-hosting intrusions are small in size, making it difficult to observe the obvious reflection phenomenon in the sections when researchers carry out artificial-source seismic explorations. Some studies have shown that Y2 is steeply inclined in the deep part (Qin Kezhang et al., 2014), which may easily lead to the downward propagation of seismic wave energy. At the same time, the comprehensive geophysical detection method cannot achieve the fusion of multiple physical attributes, but only the unity of the tomography at present. The physical properties of the same rock sample cannot be accurately located and put into practice, suggesting the deficiency of the petrophysical experiments. For example, only few comprehensive experiments with multiple methods have been performed for the same rock sample, and only when the petrophysical bridge is completed can we really achieve the integration of multiple physical properties.

To sum up, due to the great changes in the physical properties of surrounding rocks and carbonaceous strata in the mine, the orebodies cannot accurately reflect the characteristics of "three higher-s and one lower". The effectiveness and accuracy of deep prospecting by "gravity-magnetic-electric-seismic" and other exploration methods need to be improved. At present, the maximum mining depth in Karatungk Mine is about 850 m (Qin Kezhang et al., 2014). Further prospecting targets will focus on concealed orebodies in deeper part. Although seismic reflection has a larger exploration depth, the structural resolution in detectable depth is still far from the expected threshold, and there is some interference in interpretation of obtained profiles. Therefore, how to eliminate the interference factors and improve the exploration resolution is the key problem needing to be solved in the following geophysical research.

4 CONCLUSION AND PROSPECTS

Karatungk Mine is the largest Cu-Ni sulfide mine in Xinjiang, China, which has rich mineral resources and high output. After decades of continuous mining, it is urgent to explore deep mineral resources. In recent years, a variety of geophysical explorations have been applied to explore resources and study structures in the mine, but plenty of interferences and uncertainties are still exist in the practical work and need to be eliminated. Meanwhile, the shallow crustal structure of the mine cannot be described in detail at present, which also restricts our understanding of the spatial distribution of orebodies, the exploitation of mineral resources, and the metallogenic mechanism of the mine.

In order to obtain the high-resolution structure and detailed spatial distribution of orebodies, researchers may focus on combining multiple geophysical methods together, developing high-resolution imaging methods, and improving petro-physical experiments in the future. Also, it is significant for investigating the physical properties of underground materials in the mine. At the same time, the potential geodynamic mechanisms of mineralization in Karatungk Mine can be obtained by combining petrology, mineralogy, early geological data and geochemical research results. In this way, a vital reference can be provided to serve as a guide for further prospecting studies in the mine.

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