1. Introduction
The Deerni copper deposit is one of the largest copper reserves in the Tethys metallogenic belt, with proven copper reserves of 0.556 Mt (Duan et al. Reference Duan, Qian, Huang, Dong, Zhao and Lu2014). This deposit was first discovered by a local herdsman in 1958. In 2003, the Zijin Mining Group Company Limited acquired the exploration and mining rights, commencing mining operations in 2006. After 18 years of extraction, the main copper orebodies are now nearly depleted, making the discovering of new orebodies essential to sustain the copper mining activities at Deerni (Wang et al. Reference Wang, Lu, Zhao, Chen and Chen2008).
Song et al. (Reference Song, Chen and Ren2008) indicated that the carbonaceous Fe-Si rock within the deposit is a hydrothermal sedimentary rock. Based on local-scale geological mapping and lithological profiling, Jiao et al. (Reference Jiao, Huang and Yuan2009) concluded that the copper mineralization at Deerni postdates the ultrabasic rocks. Moreover, Song et al. (Reference Song, Li and Chen2012) interpreted the ferro-siliceous rocks at Deerni as exhalative rocks formed by submarine volcanism, suggesting an association with submarine exhalative-sedimentation. Based on those features, Deerni was classified as a deposit of volcanogenic massive sulphide (VMS) type and subjected to extensive exploration efforts including geochemical and geophysical surveys, as well as exploratory drilling (Zhang, Reference Zhang2014). However, new copper orebodies have yet to be discovered.
Some researchers have proposed a magmatic origin for the Deerni copper deposit. Zhang et al. (Reference Zhang1995, Reference Zhang and Chen1996) suggested that the deposit represents a magmatic sulphide deposit, and the mineralization is contemporaneous with the ultrabasic host rocks and controlled by faulting. Duan (Reference Duan1996, Reference Duan1998) argued that it is a hydrothermal deposit related to the emplacement of Indosinian granite intrusions. They suggested that the ore-forming materials were sourced from both Indosinian granitic magma and Late Paleozoic slate. These researchers also noted that faults bound the deposit and that pyrite occurs sporadically throughout the Indosinian granite, just north of the main Deerni deposit.
The debate over whether the Deerni copper deposit is primarily related to local volcanism or magmatism has persisted for decades. The need to discover new orebodies has revitalized interest in re-examining the source and genesis of the deposit, to refine the prospecting model and guide future exploration efforts.
By integrating geological, geochemical and geophysical data, we propose a new model for the copper mineralization processes at the Deerni copper deposit. Our findings provide new insights into the deposit’s formation and contribute to the development of a more effective exploration strategy for discovering concealed orebodies.
2. Geological background
The Deerni copper deposit is located within the eastern segment of the Animaqin tectonic belt. Intense compression in this belt has resulted in the fragmentation of rock units and the extensive development of complex geological structures (Fig. 1). These fragmented strata form a mélange, with disordered stratigraphic units (Chen et al. Reference Chen, Sun and Pei1999, Reference Chen, Sun, Liu and Pei2000a , Reference Chen, Sun, Pei, Gao, Feng, Zhang and Chen2001). The outcropping rocks in the Deerni copper deposit span Paleozoic, Mesozoic and Cenozoic eras (Fig. 2). The Paleozoic rocks, found in northern Deerni, are in faulted contact with the Deerni ophiolite. These rocks are ∼670 m thick, comprising amphibolite (SHRIMP U-Pb zircon age at 417.11 ± 3.3 Ma, Yang et al. Reference Yang, Xu and Li2005) and interlayered marble, along with banded tremolite marble.
(a) Schematic tectonic map of China (modified from Zhao & Guo, Reference Zhao and Guo2012); (b) Geological sketch map of the eastern part of the Animaqin tectonic belt, NW China (modified from Yang et al. Reference Yang, Xu and Li2005), with the Deerni study area outlined in a red rectangle; (c) A-B profile of the Animaqin tectonic belt (modified from Duan, Reference Duan1991).
Geological sketch map of the Deerni copper deposit (modified from Guo et al. Reference Guo, Zhou, Fan, Wang, Liu and Zhu2024).
The Upper Carboniferous marine clastic and carbonate rocks strike northwest-southeast and dip northeast. The lower portion of this formation is ∼490 m thick and consists of interbedded meta-sandstone, meta-conglomerate, andesite and lenticular limestone. The upper section (∼430 m thick) is characterized by bioclastic and crystalline limestone with thin sandstone beds. This sequence is unconformably overlain by Lower Permian rocks (Yang et al. Reference Yang, Xu and Li2005).
The Lower Permian strata also strike northwest-southeast, with thickness varying from several to tens of metres along strike. They comprise interlayered fine-grained sandstone and slate, as well as lenses of greyish-green meta-andesite and fossiliferous limestone. These strata are interbedded with tectonically dismembered fragments of the Deerni ophiolite, creating a mélange that includes abyssal mudstone and radiolarian chert (Fig. 2). Near the Deerni copper deposit, the Lower Permian slate (>400 m in thickness) is hosted within the ophiolite.
The Upper Permian strata, located to the north of the Upper Carboniferous, consist of interbeds of grey fossiliferous limestone and fine-grained yellow sandstone. Early to Middle Jurassic rocks include sandstone and sandy shale, with carbonaceous shale containing fossils interbedded with limestone lenses and coal beds, which are directly overlain by conglomerate (Yang et al. Reference Yang, Xu and Li2005).
Cretaceous rocks cropping out in the northern part of the study area are composed of amaranth conglomerate, sandy conglomerate and thin layers of very fine-grained sandstone containing fossil plant remains and trace fossils. The conglomerate features detrital grains and rock fragments of limestone, sandstone, slate, andesite and granite, with grains cemented by calcite and micritic calcite (Yang et al. Reference Yang, Xu and Li2005).
The Deerni ultrabasic rocks exhibit characteristics typical of ophiolites, leading to their classification as the Deerni ophiolite (Yang et al. Reference Yang, Shi, Wu, Wang and Robinson2009). This dismembered ophiolite is typically exposed as blocks of varying sizes within a sheared flysch matrix, striking northwest-southeast (Chen et al. Reference Chen, Sun and Pei1999, Reference Chen, Sun, Liu and Pei2000a , Reference Chen, Sun, Pei, Gao, Feng, Zhang and Chen2001, Reference Chen, Sun, Pei, Feng and Zhang2004; Pan et al. Reference Pan, Wang, Li, Yuan, Ji, Yin and Wang2012). The ophiolite extends over ∼80 km with a width of 10 to 20 km (Yang et al. Reference Yang, Shi, Wu, Wang and Robinson2009), consisting of meta-peridotite, basaltic lavas, mafic-ultramafic cumulates and sheeted dykes. The peridotite includes dunite, harzburgite, lherzolite, feldspathic lherzolite and garnet lherzolite. The Deerni ophiolite has been dated to 345.3 ± 7.9 Ma using 40Ar-39Ar method (Chen et al. Reference Chen, Sun, Pei, Gao, Feng, Zhang and Chen2001), slightly older than the zircon SHRIMP U-Pb age of 308.0 ± 4.9 Ma (Yang et al. Reference Yang, Shi, Wu, Wang and Robinson2009). It is considered to represent fragments of subducted oceanic crust, providing evidence for sea-floor spreading during the middle to late Carboniferous, when the Paleo-Tethyan Ocean began to break apart (Yang et al. Reference Yang, Shi, Wu, Wang and Robinson2009).
Several Indosinian granitic intrusions are located along the northern margin of the Deerni ophiolite (Fig. 2). The largest of these is the Deqia granitic complex, with a zircon SHRIMP U-Pb age of 250 ± 20 Ma (Yang et al. Reference Yang, Xu and Li2005). This complex extends 16 km in length and 100 to 200 m in width, consisting primarily of weakly deformed and metamorphosed granite, with minor granodiorite and quartz diorite. It intruded into Paleozoic amphibolite and marble and locally cuts through the Deerni ophiolite. The complex is unconformably overlain by Early to Middle Jurassic sandstone and conglomerate along its northern edge and by Pliocene sedimentary rocks to the west (Yang et al. Reference Yang, Xu and Li2005).
The Deerni copper deposit (latitude: 34°23′7″N, longitude: 100°07′15″E) is situated in the central part of the Deerni ophiolite (Wang & Qin, Reference Wang and Qin1989). The ophiolite serves as the primary host for copper orebodies (Fig. 1c), although some orebodies are also found within Lower Permian slates. The region has experienced significant magmatic activity, leading to the formation of mafic-ultramafic, intermediate-acidic and dike rocks (Guo et al. Reference Guo, Zhou, Fan, Wang, Liu and Zhu2024). Ultramafic rocks are concentrated in the central part of the deposit, forming a belt 70–800 m wide that gradually narrows as it extends beyond the mining area (Tang et al. Reference Tang, Qin and Mao2024). The peridotite has been extensively altered to serpentinite, obscuring its original structure. Slate and phyllite host the ultramafic rocks and occur as xenoliths. The Deerni deposit is intersected by NW-SE and NE-SW faults. Due to the prevalence of ultramafic rocks, fold structures are difficult to discern, though the fold axis appears to align with the strike of the ultramafic rocks. A syncline is adjacent to an anticline on both the southern and northern sides (Fig. 1c), with the orebodies located within the anticlinal core. The anticline structure is prominent between exploration lines 11–26 and 29–37, but becomes less distinct or disappears towards the east and west.
The main orebodies currently being mined include No. 1, 2, 5 and 7 (Fig. 2), located between elevations of 3990 and 4390 m (Fig. 3). These orebodies extend 460–1060 m along strike, with an average width of 88–193 m and depths ranging from 190 to 320 m. The orebodies frequently exhibit boudinage or folding, displaying lenticular shapes, with distinct boundaries between the ore and the wall rocks. Hydrothermal alteration is extensive, characterized by serpentinization, carbonatization, silicification and malachite alteration, as well as magnetite mineralization. Serpentinization and carbonatization are the most common alteration types. Serpentinization involves the replacement of olivine and pyroxene by serpentine, and carbonatization is evident in quartz-calcite veins within the surrounding rocks. Minerals resulting from carbonate alteration include calcite, dolomite and magnesite.
Number 27 cross-section through the Deerni copper deposit (based on unpublished data from the Zijin Mining Group Company Limited, 2015).
The main ore types are classified as massive, vein and breccia ores. Massive ores are primarily composed of sulphides (60–80%), silicates (20–40%) and oxides (5–10%). Veinlet ores, typically 5–30 cm in width, often traverse serpentinite country rocks or massive ores, while breccia ores are characterized by serpentinite clasts cemented by hydrothermal minerals and sulphides. The ores contain 0.64–1.64 wt.% Cu, 0.053–0.17 wt.% Co and 0.65–7.28 wt.% Zn. Ore minerals include pyrite, chalcopyrite, sphalerite, pyrrhotite and magnetite, along with subordinate galena, hengleinite, ilmenite and leucosphenite. Gangue minerals comprise quartz, calcite, rutile, fluorite and zircon. Oxidized ore consists of limonite, tenorite, malachite and azurite, with oxidized ores typically found at the surface and sulphide-rich ores at deep levels.
The ores exhibit a range of textures, including euhedral, anhedral, granular, fractured and pore-filling textures, as well as massive, disseminated, brecciated and veinlet structures. Based on the relationships among ore intersections, structures and mineral assemblages, the Deerni deposit can be categorized into four mineralization stages (Duan, Reference Duan1991).
3. Methods
Comprehensive fieldwork was conducted at the Deerni copper deposit to investigate hydrothermal alterations and the structural characteristics of the deposit.
Fault and fracture attitudes were measured using a compass clinometer. The conjugate joint sets identified in both the wall rocks and copper orebodies were subjected to statistical analysis. Structural geology and CAD software were utilized to create stereographic projections of the measured structures (Yang et al. Reference Yang, Wu, Coulson, Zhang, Lai, Zhang and Xu2020, Reference Yang, Wu, Xu and Ren2023) to determine the compression direction and kinematic features of the structures within the copper deposit. This statistical analysis of the conjugate joint sets provided insights into the compression direction and the ore-controlling structural characteristics at the Deerni copper deposit.
Contour maps serve as effective tools for illustrating the spatial distribution of geological bodies and are widely employed in mineral exploration. They are particularly useful for identifying geochemical and geophysical anomalies, as well as for structural studies (Yang et al. Reference Yang, Wu, Coulson, Zhang, Lai, Zhang and Xu2020, Reference Yang, Wu, Xu and Ren2023). Most drilling activities in the western part of Deerni were completed; so, relevant samples and data from Zijin Mining Group Co., Ltd. were accessed for this study. Contour maps and surface trend maps were generated using Surfer 11 software.
The geographical and geochemical survey of western Deerni has been completed, magnetic, gravity and self-potential anomaly patterns, transient electromagnetic anomaly profile, and distribution pattern of Cu–Pb–Mn–Ni mineralization at western Deerni were obtained from Zijin Mining Group Co., Ltd. for this study.
The original magnetic field dataset covers an exploration area of 18.2 km2, which occupies the vast majority of the Deerni Copper Mine area. The data exist in grid format with a grid size of approximately 100 m × 20 m. The magnetic survey comprises a total of 9,283 measurement stations, covering a cumulative survey line length of 182 km. In addition, seven magnetic survey profiles were completed, containing a total of 692 measurement stations.
In 1992, the Third Brigade of the Non-ferrous Geophysical Team conducted a 1:20,000 gravity survey in the area from western Deerni mine to Zhabenggou. A total of eight gravity anomalies were identified, with some exhibiting considerable scale and broadly coinciding with known orebodies.
During the early exploration phase of the Deerni copper deposit, the self-potential method achieved remarkable success in mineral targeting. In 1990, the Third Brigade of the Non-ferrous Geophysical Team conducted a 1:10,000-scale self-potential survey grid within the mining area and its periphery, covering a cumulative area of 52 km2. A total of 28 self-potential anomalies were identified, with 14 confirmed as mineral-induced through follow-up verification (Wang et al. Reference Wang, Liu, Geng and Lv2007b ). The transient electromagnetic survey of western Deerni has been completed; relevant profiles from Zijin Mining Group Co., Ltd. were accessed for this study. There are nine transient electromagnetic surveying lines in the study area. The dimensions of the working network are 100 m × 100 m (i.e., distances between lines and points are both 100 m; Fig. 2). Due to the large-scale of ore body, shallow burial and intense topographic dissection, the migration conditions of geochemical halos are favourable. In 1990, the Second Brigade of the Non-ferrous Geophysical Team conducted a 1:10,000 scale geochemical survey work from the western Deerni mine to Zhabenggou.
4. Results
4.a. Hydrothermal alterations
Duan (Reference Duan1991) classified the hydrothermal alterations at the Deerni copper deposit into four distinct stages: stage 1 is characterized by serpentinization and carbonatization, with paragenetic minerals of pyrite, chalcopyrite, sphalerite, marcasite, carbonate and talcum (Table 1); stage 2 by serpentinization, carbonatization and magnetite mineralization with magnetite, haematite, cubanite, carbonate and talcum; stage 3 by silicification and carbonatization, with pyrite, chalcopyrite, sphalerite, pyrrhotite, hengleinite, siegenite, quartz and carbonate and stage 4 by primarily carbonatization with pyrite, marcasite and carbonate. In addition, the supergene stage is characterized by malachite mineralization and contains limonite, tenorite, hausmannite, malachite, azurite, covellite, chalcocite, bornite, zigueline, native copper, native gold and chalcanthite.
The paragenetic sequence of mineralization at the Deerni copper deposit (modified from Duan, Reference Duan1991)
A detailed investigation of hydrothermal alterations in the Deerni copper deposit revealed that serpentinization (comprising talcum, antigorite and serpophite) and carbonatization (predominantly calcite, with dolomite and magnesite) occurred throughout the Deerni ophiolite. Serpentinization developed during the hydrothermal stages 1 and 2 and may be overprinted by carbonatization (Fig. 4a). Carbonatization was observed in all four stages and is commonly associated with pre-existing structures (Fig. 4b), appearing as carbonate veinlets along fractures in the ophiolite. These veinlets generally exhibit a lattice-like texture of calcite, with widths ranging from 1 to 20 mm and lengths extending up to several metres.
Outcrop photographs of different types of hydrothermal alteration at the Deerni copper deposit: (a) Serpentinization occurring in the ophiolite on both sides of the copper orebodies; (b) Carbonatization occurring along fracture surfaces on both sides of the copper orebodies; (c) Magnetite occurring as veinlets in fractures within the ophiolite; (d) Silicification mainly occurring in the upper parts of the copper orebodies; (e–f) Malachite principally occurring along fracture surfaces on both sides of the copper orebodies.
Magnetite mineralization, which developed during stage 2, occurs sporadically along fractures within the ophiolitic rocks (Fig. 4c) or as magnetite veinlets and may coexist with malachite.
Silicification emerged during stage 3, with two NW-striking silicified zones identified at the southern slope and the ridge (as a steep cliff) of Mount Deerni. The Deerni copper deposit is located within the silicified zone at the southern slope, where silicification is the most intense in the orebodies and diminishes in the surrounding wall rock. Silicified lenses and bands, along with quartz veinlets, are found in both the ophiolite and slate throughout the upper parts of the copper orebodies (Fig. 4d), measuring several to tens of metres in width and tens to hundreds of metres in length. Silicification mainly comprises quartz deposition along vesicles and fractures, resulting in an earthy to yellow rock surface with a grey to white interior. Along the ridge of Mount Deerni, the extent of silicification in the ophiolite varies along strike. The average Cu grade in the ophiolite is 0.0024 wt.% (Table 2), while the average Cu grade in silicified ophiolites is 0.105 wt.%. Ten silicified ophiolite samples show Cu grades ranging from 0.004 to 0.061 wt.% (Table 2).
The elements content in the wall rocks and copper orebody at the Deerni copper deposit
Malachite mineralization developed during the supergene stage, with joints within the Deerni copper deposit displaying green and blue-green surfaces indicative of malachite and/or azurite copper mineralization. Malachite typically appears along fracture surfaces in altered ophiolite, on either side of the copper orebodies (Fig. 4e and f). It also occurs as patches within the siliceous cap, as stains along the cleavages of magnetite-bearing altered ophiolite, and as coatings on fractured outcrops of granite dykes.
4.b. Ore-controlling structures and determination of paleo stress directions
The Deerni copper deposit is located in the anticlinal core (Fig. 1c) and is clearly influenced by faults, with orebodies bordered along faults in both ophiolites and slates (Table 3). In addition, orebodies were found in gabbro at the Changmahe mineral occurrence, limestone at the Qienugou mineral occurrence, the contact zone between granite and slate at the Zhaheleigou mineral occurrence and volcanic rocks at the Renguoshan mineral occurrence (Duan, Reference Duan1998). This indicates that the Deerni copper deposit is not exclusively associated with ophiolite but is also controlled by faults.
Characteristics of ore-controlling structures at the Deerni copper deposit
The copper orebodies of the Deerni deposit are hosted in both brecciated ophiolite and early Permian slate (Fig. 5a and b). These orebodies strike NW and dip moderately to the northeast (Fig. 5a), displaying clear boundaries with the wall rocks (Table 4). Layered carbonate occurs between the orebody and the wall rocks. The Deerni copper deposit was controlled by faults, evidenced by (1) the presence of copper-mineralized slate fragments and breccias within a single orebody (Fig. 5c), (2) a saw-toothed boundary between copper ore and the wall rocks and (3) copper ore veinlets and malachite mineralization in the wall rock. The striations and step surfaces on the copper orebodies, alongside quartz veins in the hanging wall, suggest that the ore-controlling structures are thrust faults (Fig. 5d).
Field relations for the faulted contact between the copper orebodies and wall rocks at the Deerni copper deposit: (a) The Lower Permian slate forming the hanging wall to the copper orebodies; (b) The ophiolite footwall affected by malachite mineralization; (c) A copper-mineralized slate fragment within a copper orebody; (d) Striations on the surface of strata within the copper orebodies; (e) The attitudes of fractures associated with all structural stages were measured in the wall rocks and plotted on stereographic projections; (f) The attitudes of fractures associated with all structural stages were measured in the copper orebodies and plotted on stereographic projections; (g, h) The compression direction occurring in the wall rocks; (i) The compression direction of the copper orebodies.
The Deerni copper deposit comparing with the Yangla copper deposit
Near the No. 2 orebody, a statistical study of the conjugate joint sets in both the wall rocks and the copper orebodies involved analyzing 300 jointed fabrics to determine the direction of compression. The attitudes of fractures associated with all structural stages were measured and plotted on stereographic projections (Fig. 5e and f). The results indicate that the wall rocks experienced compression along both N-S and SW directions (Fig. 5g and h), and the copper orebodies were predominantly affected by compression along the N-S direction (Fig. 5i).
4.c. Distribution of copper mineralization
A total of 923 samples were collected from drill cores in the western part of Deerni. The spacing of exploration lines was set at 100 m, with varying depths below the surface ranging from 4471 to 3906 m (Fig. 6a and b).
(a) Distribution pattern of copper mineralization in the longitudinal projection of the western part of the Deerni copper deposit. One copper anomaly occurs surrounding the No. 2 copper orebody, and two anomalies are distributed in the western part of the No. 2 copper orebody. (b) Surface trend diagrams of the copper mineralization in the longitudinal projection of the western part of the Deerni copper deposit. A linear low-grade area beneath the No. 2 orebody extends ∼1 km to the west.
5. Discussion
5.a. Proposed model for the Deerni copper mineralization
The boundary between the orebodies and the wall rocks at the Deerni deposit is sharp, contrasting with the gradual transitions typically observed in VMS deposits (Galley et al. Reference Galley, Hannington and Jonasson2007). The hanging wall of the copper orebodies is composed of slate (Fig. 5a), which has undergone silicification and extensive sub-parallel shear fracturing. The footwall is made up of ophiolite, affected by malachite mineralization (Fig. 5b). As a result, the ophiolite serves as the host rock for the copper orebody, overlain by slate.
The Deerni copper deposit is located within the anticlinal core and hosted within the NW-striking faults. This indicates that the Deerni copper deposit was mainly controlled by the NW-striking faults that overprinted the anticline (Figs. 1c and 2). Both the Deerni copper deposit and the Deqia granitic complex, located within the anticlinal core (Fig. 1c), suggest that they are potentially also temporally and genetically related. This is supported by the following evidence. (1) The Yangla copper deposit and the Deerni copper deposit are both located within the Tethys metallogenic belt (Table 4). Recent studies suggest that the Yangla copper deposit is related to the Yangla granodiorite, rejecting its previous classification as a VMS-type deposit (Yang et al. Reference Yang, Liu, Han, Zhang, Luo, Wang and Chen2011, Reference Yang, Liu, Cao, Han, Gao, Wang and Liu2012; Chen et al. Reference Chen, Gu, Cheng, Zheng, Han and Peng2013; Yang et al. Reference Yang, Liu, Han, Jiang and Zhai2014b ; Wei et al. Reference Wei, Chen and He1999). The δ34S values of sulphides from the Deerni deposit (–6.2 ‰ to 6.6 ‰) (Zhang & Li, Reference Zhang and Li2019) are similar to those from the Yangla copper deposit (–6.9 ‰ to 2.5 ‰) (Pan et al. Reference Pan, Wang, Li, Yuan, Ji, Yin and Wang2012), suggesting a similar genesis associated with nearby granitoids (Zhang & Li, Reference Zhang and Li2019). (2) The δ65Cu values from the Deerni deposit (–0.89 ‰ to –0.29 ‰) (Li et al. Reference Li, Chu and Lei2014) are consistent with those from magmatic deposits and igneous rocks, further supporting a magmatic origin (Li et al. Reference Li, Chu and Lei2014; Ikehata & Hirata, Reference Ikehata and Hirata2012; Zhou et al. Reference Zhou, Wang, Yang and Liu2013; Zhao et al. Reference Zhao, Xue, Liu, Symons, Zhao, Yang and Ke2017). (3) The upper parts of the copper orebodies have higher concentrations of Zn, Gd and Ga, and lower concentrations of Co and In, showing a typical magmatic-hydrothermal metallogenic zoning pattern (Duan, Reference Duan1998). (4) The age of formation of copper deposits in the Eastern Kunlun Mountains (240–217 Ma) coincides with the emplacement of Indosinian granitoids, highlighting a close spatial and temporal relationship (Feng et al. Reference Feng, Li, Qu, Du, Wang, Su and Jiang2009, Reference Feng, Wang, Shu, Zhang, Xiao, Liu and Ma2011; He et al. Reference He, Li, Li, Qi and He2009; Wang et al. Reference Wang, Chen, Xie, Li, Tan, Zhang and Wang2013). The Deerni copper deposit’s genesis is likely related to the Indosinian granitoids, dated at ∼250 Ma, located to the north of the deposit (Xia et al. Reference Xia, Wang, Qing, Deng, Carranza, Li and Yu2015).
The Deqia granitic complex was formed around 250 ± 20 Ma (SHRIMP U-Pb zircon age; Yang et al. Reference Yang, Xu and Li2005), indicating that the Deerni copper deposit and the complex likely formed during the closure of the proto-Tethyan Ocean between 250 and 220 Ma (Yang et al. Reference Yang, Liu, Li, Zhai, Yang and Han2013, Reference Yang, Liu, Yang, Han, Sun and Wang2014a , Reference Yang, Liu, Han, Jiang and Zhai2014b ; Li et al. Reference Li, Zhao, Liu, Cao, Yu, Li and Suo2018). The NW-trending Deqia granitic complex intruded along the NW-striking faults, with its eastern part cut by a NE-striking fault, and the deposit is intersected by a NE-striking fault (Fig. 2). Thus, both the Deerni copper deposit and the Deqia granitic complex postdate the NW-striking faults and predate the NE-striking faults.
Two stages of tectonic activity in the Deerni area likely correspond to regional tectonic events. The deposit is located within the Animaqin tectonic belt, which experienced the closure of the proto-Tethyan Ocean during the Indosinian orogeny and the subsequent India-Eurasia collision during the Himalayan Orogeny (Bi et al. Reference Bi, Wang, Wang and Zhu1999). The pre-mineralization NW-striking faults likely resulted from southwest-directed compression associated with the Indosinian closure. Post-mineralization NE-striking faults likely formed from north-south compression linked to the subsequent Himalayan collision (Patriat & Achache, Reference Patriat and Achache1984; Chen et al. Reference Chen, Burchfiel, Liu, King, Royden and Tang2000b ; Bouilho et al. Reference Bouilho, Jagoutz, Hanchar and Dudas2013; Gibbons et al. Reference Gibbons, Zahirovic, Müller, Whittaker and Yatheesh2015).
The malachite mineralization, characteristic of the supergene stage, postdates silicification and magnetite mineralization. Pre-existing fractures in the Late Carboniferous ophiolitic strata and Lower Permian slate likely provided conduits for these fluids, leading to fluid-rock interactions that played a key role in copper mineralization, suggesting that the copper mineralization postdated structural activity affecting the Lower Permian strata in the region.
5.b. Exploration areas
Our study indicates that additional concealed copper orebodies are likely located beneath the silicified caps in both the eastern and western regions of the Deerni copper deposit. Observations from the No. 1 orebody suggest that these orebodies extend eastward, although they are intersected by a normal fault (Fig. 2). However, the depth of these concealed orebodies, ∼300 meters, makes them unsuitable for open-pit mining. Observations from the western Deerni suggest that these concealed copper orebodies may be an extension of the No. 2 orebody (Fig. 2). The prediction has been verified by geophysical and geochemical methods.
Xue (Reference Xue1992) and Liu et al. (Reference Liu, Peng, Wang, Wang and Zhang2019) suggested that magnetic anomalies can be used to detect copper orebodies at the Deerni copper deposit due to the significant difference in magnetic susceptibility between the copper orebodies and the surrounding ophiolitic wall rocks [mean value of 10,400 vs. 310–1100 (4π·10−6 SI)]. A series of strong magnetic anomalies were detected in the Deerni copper area (Fig. 7a, Zijin Mining Group Co., Ltd). The strong magnetic anomalies are perfectly aligned with the NW-SE-trending fault zone. Since the Deerni copper deposit was controlled by the NW-SE trending fault zone, the strong magnetic anomalies are likely caused by either strongly magnetic rocks in the NW-SE-trending fault zone or ore-bearing formations (Holden et al. Reference Holden, Wong, Kovesi, Wedge, Dentith and Bagas2012; Kwan et al. Reference Kwan, Müller, Groves, Legault, Reford, Maacha, Ouadjou, Cao and Liu2025). Comparing the magnetic anomalies with the known Cu orebodies at the Deerni copper deposit, it is clear that the anomalies are not associated with Cu orebodies, but with the ophiolitic host (DeMatties, Reference DeMatties2024).
(a) Magnetic anomaly pattern for the western Deerni orebodies; (b) Gravity anomaly pattern for the western Deerni orebodies; (c) Self-potential anomaly pattern at western Deerni; (d) Distribution pattern of Cu–Pb–Mn–Ni mineralization for western Deerni (Liu et al. Reference Liu, Peng, Wang, Wang and Zhang2019; based on unpublished data from the Zijin Mining Group Company Limited).
Wang et al. (Reference Wang, Chen, Wang and Lu2007a ) suggests that gravity anomalies can effectively identify copper deposits at Deerni due to the density contrast between the copper orebodies (4.52–4.88 g/cm3, average of 4.66 g/cm3) and the surrounding wall rocks (ophiolite and slate, 2.60–2.80 g/cm3) Current mining areas (hosting orebodies No. 1, 2 and 5) all correspond to zones of high positive gravity anomalies (Wang et al. Reference Wang, Chen, Wang and Lu2007a ). A positive gravity anomaly was also detected in western Deerni (Fig. 7b, Zijin Mining Group Co., Ltd), extending south-westward, with a length of ∼750 m, a width of ∼250 m, and covering an area of ∼0.19 km2. The maximum anomaly (>90×10 m/s2) is located at the northeast end. However, the anomaly’s orientation does not align with the dip or strike of the No. 2 orebody, indicating that it may be caused by the surrounding wall rocks or other ore-bearing formations.
Wang et al. (Reference Wang, Liu, Geng and Lv2007b ) suggested that self-potential anomalies are an effective tool for locating concealed orebodies as copper ores in the Deerni deposit contain sulphides. The sulphides significantly increase the electrical conductivity of the ore rocks (10–4 Ω·m) to be several times higher than that of the wall rocks. The known mining areas (orebodies No. 1, No. 2 and No. 5) lie within high self-potential anomaly zones (Wang et al. Reference Wang, Liu, Geng and Lv2007b ; Liu et al. Reference Liu, Xu, Wang, Zhang and Cai2010; Zhang, Reference Zhang2014). In the northwest and central parts of western Deerni, three negative self-potential anomalies were identified (Fig. 7c, Zijin Mining Group Co., Ltd). The first anomaly is elliptical with a northwest-southeast axis, measuring 330 m in length, 100 m in width and covering an area of 0.03 km2, with a maximum amplitude of –240 mV. The second extends north-westward, measuring ∼800 m by ∼100 m, covering ∼0.08 km2, with a maximum amplitude of –640 mV. The third anomaly forms a belt, extending north-westward over 300 m, covering 0.03 km2, with an amplitude of more than –240 mV. The self-potential anomalies align with the strike of the No. 2 orebody, suggesting they are caused by either the orebody itself or nearby concealed copper orebodies.
The No. 1 orebody is located at 20 m below the surface, and the No. 1 orebody generated one transient electromagnetic anomaly 50 m below the surface (Fig. 8a, Zijin Mining Group Co., Ltd). On the transient electromagnetic anomaly profile of western Deerni, two anomalies were detected (Fig. 8b, Zijin Mining Group Co., Ltd). Two deeper anomalies are located at 150 and 300 m below the surface, suggesting that they are likely caused by concealed copper orebodies (Xue et al. Reference Xue, Zhou, Su, Zhang, Yang, Mo and Wu2024).
Transient electromagnetic anomaly profile at Deerni copper deposit. (a) Transient electromagnetic anomaly profile along exploration Line 13 of the Deerni copper deposit; (b) Transient electromagnetic anomaly profile along Line 3 at the western Deerni (based on unpublished data from the Zijin Mining Group Company Limited).
The Cu grade contour diagram (Fig. 6a) reveals relatively ordered isolines for three Cu anomalies. One Cu anomaly surrounds the No. 2 copper orebody, and two additional Cu anomalies are distributed in the western part of this orebody. One surrounds the No. 2 orebody, confirming its association with this orebody. Two additional anomalies are found west of the No. 2 orebody, suggesting the presence of concealed orebodies at depths of 150 and 300 m. These anomalies correspond to the transient electromagnetic anomalies, reinforcing the likelihood of concealed copper deposits in the west of the No. 2 orebody. On the surface trend diagrams of copper mineralization (Fig. 6b), in addition to the three Cu anomalies, an elongated elliptical anomaly with low Cu grades was identified. This elongated elliptical anomaly is located beneath the No. 2 orebody and extends ∼1 km to the west, between elevations of 4200–4000 m, indicating a low-Cu-grade belt extending 1 km westward beneath the No. 2 orebody. Geochemical anomalies for Cu, Pb, Mn and Ni mineralization were identified in western Deerni (Fig. 7d, Zijin Mining Group Co., Ltd). The Cu anomalies overlap with Mn anomalies, extending along the strike and dip of the No. 2 orebody. The Cu anomalies largely coincide with the self-potential anomalies in western Deerni (Fig. 7c, Zijin Mining Group Co., Ltd). Collectively, geological, geophysical and geochemical data point to the presence of concealed copper deposits in western Deerni.
5.c. Drill test results
In the new drilling holes completed at the western Deerni area, concealed copper orebodies were discovered and contributed to the total copper reserve of the deposit. In the number 52 cross-section through western Deerni (Fig. 9), three concealed orebodies were discovered, which are 7.15–13.87 m wide with Cu grade between 1.00 and 1.34 wt.%., adding >10,000 tonnes to the Cu reserve at Deerni. We thus confirm that they represent the westward extension of the No. 2 orebody in the east. Therefore, the copper orebodies at the Deerni deposit are controlled by structures, and the silicified cap of earthy yellow crust at the ground surface is an effective indicator of copper mineralization underground, analogous to a gossan.
Cross-section 52 through western Deerni and its orebodies (based on unpublished data from the Zijin Mining Group Company Limited).
6. Conclusions
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(1) The Deerni copper deposit is controlled by the NW-striking faults. Serpentinization, carbonatization, silicification, malachite alteration and magnetite mineralization have occurred along fractures within the wall rocks and surrounding strata, suggesting that this alteration post-dated structural activity affecting the Lower Permian strata in the region.
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(2) Field investigations, geophysical and geochemical data suggest the presence of copper orebodies in western Deerni.
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(3) The drilling holes intercepted the concealed copper orebodies in the western Deerni area and significantly enhanced the copper reserve at the Deerni deposit.
Acknowledgements
This study was financially supported by the National Natural Science Foundation of China (No. 41830430). We would like to express our gratitude to Wankun Li, Baowen Yan and others for providing geological data and for their assistance and hospitality during fieldwork at the Deerni copper deposit and surrounding areas. We are deeply indebted to Professor Guoxiang Chi for his incisive reviews, valuable comments and suggestions that significantly improved an earlier draft of this manuscript. The authors would also like to thank the associate editor, Dr. Simon Schorn, and anonymous reviewers for their constructive comments, which significantly improved the manuscript.
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