Introduction

Uranium, as the basic material of nuclear industrial facilities, plays an increasingly important role in energy, military and other industries. The types of uranium deposits in China mainly include sandstone type, granite type, volcanic type and carbonsiliceous mudstone type, of which granite type uranium deposits account for 23% of the total reserves1,2. Granite-type uranium deposits are mostly related to Al-rich S-type granite3,4,5, with small part related to A-type granite6. Uranium deposits can be distributed in granite, contact zone and overlying basin, and their uranium are sourced from granite, uranium-rich strata and mantle6,7,8. The metallogenic models of granite-type uranium deposits can be summarized as follows: (1) Uranium leaching in uranium-rich granite caused by mixed atmospheric and basin oxidizing fluids9; (2) Hot spot type uranium mineralization related to deep mantle plumes10; (3) Mantle degassing related to basic magma in the extensional setting11. As a hydrothermal deposit, granite-type uranium deposit is strongly controlled by structure and mainly occurs in deep faults, tectonic conjunctions and activation sites12. Therefore, it is of great significance to clarify the tectonic forms and patterns of granite-type uranium deposits, especially the tectonic superposition state and activation location, and identify the tectonic evolution process to guide the next ore prospecting work12,13,14.

During the early Neoproterozoic period, the Yangtze Plate collided with the Cathaysia Plate to form today’s South China Plate15,16. Subsequently, in the Mesozoic, due to the northwest subduction of the paleo-Pacific Ocean, the South China Plate experienced episodic intracontinental extension and multi-stage magmatic activity, which were accompanied by a large number of metal deposits17,18,19,20. The Taoshan-Zhuguang uranium metallogenic belt, located in the middle of Nanling polymetallic metallogenic Province, South China Plate, is represented by granite-type uranium deposits2,14,21, whose reserves account for more than half of the total reserves of the Taoshan Zhuguang metallogenic belt. Numerous typical deposits such as Lujing, Dongkeng, and Nuanshuitang are hosted within it6,22,23. Previous studies on granite-type uranium deposits in Lujing ore field mainly focus on defining their diagenetic age, magmatic source, ore-controlling structure, and the genesis of the deposit24,25,26. The granite-type uranium deposits in the ore field are controlled by different structures according to different subtypes: the inner-contact-zone-type uranium deposits in the granite are mostly located at the intersection and bending area of secondary faults; The orebodies of outer-contact-zone-type uranium deposits are simultaneously controlled by folds and faults. The overlying-basin-type uranium deposit is mainly produced in the basin-controlling fault and its secondary fault4,13,14. Meanwhile, previous studies on the tectonic evolution of this area focused on NE-trending faulted structures, which are considered to have experienced the tectonic evolution process of three13 or five periods27 from the Late Mesozoic to Cenozoic. However, the structural characteristics of faults in other trends (such as NW, SN) and other periods (such as Caledonian, Indosinian) in this area still remain unclear. At present, a small number of NW-trending ore bodies have been found in the Shabazi uranium depsoit, indicating the ore-bearing potential of the NW-trending structure. But the formation sequence of NW- and NE-trending structures has not been clarified, and the ore-bearing potential of both structures is not clear, which strongly restricts the development of further prospecting work.

Hence, this study selected the Lujing uranium ore field as the study area. Through detailed field geological survey and classical inversion method, the tectonic evolution process of Lujing ore field from Caledonian to Yanshanian has been clarified. Furthermore, combining with the occurrence of orebodies in Lujing and Yangjiaonao mining areas, the rule of tectonic ore control is discussed, and the metallogenic prospect area is delineated, which provides a theoretical basis for the next prospecting work.

Geological background

The Lujing ore field is located in the middle of the Nanling metallogenic belt28, the west margin of the Fujian-Jiangxi post-Caledonian fold belt of the Nanhua active belt between the Yangtze Plate and the Cathaysian Plate13, and the junction of Hunan, Guangdong and Jiangxi provinces (Fig. 1a).

Fig. 1
figure 1

(a) Tectonic map of Southern Hunan-Northern Guangdong area (after29); (b) Geological map of Lujing ore field modified from30. This figure was originally generated in CorelDraw 2020 by Weidong Ren.

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The strata exposed in this region are composed of Sinian siliceous rocks, siliceous slates; Cambrian Xiangnan Formation, Chayuantou Formation and Shuishi Formation, which are dominated by quartz sandstone, sandy slates and muddy slates, with thin coal seams; Ordovocian metamorphic siltstone, banded silty slate, limestone. The Devonian and Carboniferous strata are a set of shallow marine carbonate facies, with lithology including sandstone, conglomerate, limestone and shale (Fig. 1b). The Permian is a swamp lacustrine sedimentary system mainly composed of limestone, quartz sandstone and shale. Among them, the uranium-bearing formations are mainly Lower Cambrian Xiangnan Formation and Middle Cambrian Chaiyuantou Formation13,31.

The magmatic activity in the study area is intense, starting from Caledonian and reaching its peak in Indosinian-Yanshanian, all of which are dominated by medium to acid magmatic intrusions28. The lithologies of each period are Caledonian (granodiorite, quartz diorite, monzogranite), Indosinian (biotite monzonitic granite, two-mica granite, two-mica monzongranite), Yanshanian (biotite granite, monzonitic granite, two-mica granite, potassium granite, granite porphyry) plutons, with lamprophyre, diabase, aplite, granite porphyry, pegmatite dikes. The uranium-producing granite is mainly Indosinian granite27.

The Lujing uranium ore field is controlled by the Suichuan-Shureshui strike-slip fault zone, in which the superimposed zone of the strike-slip fault zone has transverse stretching, forming a pull-apart basin, and further controls the location of the ore deposit32,33,34. The fault and fold structure are developed in this area, wherein the axial direction of the fold was mainly NW, and gradually changed from east to west to EW to NE. The flanks of the fold consist of Cambrian metamorphic basement and are cut by late intrusive pluton. Fault structures are most developed in NE direction, including 5 regional silicified fault zones (QFI−V) and their branches. In addition, a small number of NW, EW and SN-trending faults can be seen27,35. The NE-trending fault zone is the main ore-controlling fault of Lujing uranium ore field, along which the uranium ore deposits are distributed36. The occurrences of ore bodies are consistent with the NE- and NNE-direction ore-containing faults.

Based on the spatial distribution relationship between uranium deposit and granite and the composition, lithology and structural characteristics of ore-bearing rocks, the uranium deposits in Lujing ore field can be classified as follows27,33: Silicate-vein type, produced within the granite, along the QFV fault, such as Xiaguxuan, Yangjiaonao deposits. Alteration rock type, produced in granite, are closely correlated with the acid and alkalic alteration, distributed along QFI fault, such as Gaoxi, Huangfengling deposits. Outer contact zone type, the ore bodies are strongly controlled by structure, such as Shabazi, Lujing deposits.

Materials and methods

In this study, detailed field investigations were conducted in the Lujing region. Over 50 sets of structure (Fault, joints) occurrence in strata (including Sinian, Cambrian, Ordovician, Jurassic, Cretaceous) and over 80 sets of structure (faults and joints) occurrence in granite (Triassic, Jurassic and Cretaceous) were measured in the field (Tables 1 and 2). During the field observation, the outcrops with multiple faults and joints occurrences are carefully observed and the stages of structural development are preliminarily analyzed through their features such as cutting, superposition, displacement, transformation, and tectonic deformation.

Table 1 The occurrence of faults and joints in intrusions in Lujing ore field.
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Table 2 The occurrence of faults and joints in strata in Lujing ore field.
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We systematically collected the occurrences of faults, joints, and scratches in the field outcrops. The occurrences of faults and joints as well as scratches on the faults plane were input into Win-tensor 5.9.2 software to calculate the principal stress axes. Meanwhile, the strike of faults and joints were plotted on rose diagram to show the orientation preferred of these structures. Based on the principle of superposition of structure, the structures were used to conduct stereographic projection and rose map by Orient and Win-tensor software to do the geometric and kinematic analysis37,38. Combined with classical stress inversion method and previous studies conclusions, the tectonic stress field of each period was restored to reveal the formation mechanism and evolution process of the tectonic system in Lujing uranium ore field.

Little structure can be found in Cretaceous strata and granite, which is hard to analysis separately. Therefore, structure occurrence data of Cretaceous strata and granite were discussed together to reveal the tectonic stress field and evolution process of the Late Yanshanian in Lujing ore field.

Result

Tectonic analysis of intrusions

Indosinian intrusions

The Indosinian intrusions are widely exposed in the middle of the Lujing ore field and intrudes into the early Paleozoic strata in the form of complex batholith unconformity. The lithology mainly includes monzonitic granite, biotite granite and two-mica granite, among which monzonitic granite is the most widely exposed, and two-mica granite is exposed in the form of small rock strains in the south and north of the ore field. In addition, a small number of NW-trending Late Indosinian diabase dikes (200 Ma) are found in the northern part of the Yangjiaonao mining area39. A total of 36 groups of joint data and 41 groups of fault data were collected from over 60 locations in the Indosinian intrusions (Table 1). The data results are shown in Fig. 2a, b.

Fig. 2
figure 2

(a) Lower hemispherical equal-area diagram of fault strike in Indosinian pluton; (b) Rose diagram of joint strike in Indosinian intrusions; (c-d) Field photos of structure in Indosinian intrusions.

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As shown in Fig. 2a, b, the strikes of fault include NE, NW and EW directions. The phenomena of traction structure, lenticular bodies and quartz veins along the faults plane indicate that the NE-trending structures consist of both compression and extension stress. Among them, the wide-spreading NE-trending silicate fault belts are the result of Early Yanshanian NW direction compression. In addition, there are some early NE-trending (318°∠62°) extensional faults cut by late NE-trending (173°∠64°) compressional faults (Fig. 2c), which might record the NW direction extension in Late Yanshanian and the following NW-trending compression. The NW direction structures were weakly developed in Indosinian intrusions, which are mainly distributed in the eastern and northern margin. They are compressional faults, with traction structure and quartz veins, indicating Indosinian NE-trending compression (Fig. 2d). The rose diagram of joint strike show that there are two sets of joints: mostly of them are NW-trending (107°-163°) and dip angle of 22°-87°; others are NE direction (191°-261°), with dip angle of 23°-82°; both of them suggest that there are two directions (NE and NW) of stress (Fig. 2b). In summary, Indosinian NE-trending compression, Early Yanshanian NW-trending compression and Late Yanshanian NW-trending extension were recorded in Indosinian intrusions.

Early Yanshanian intrusions

The Early Yanshanian intrusions mostly occurred as granite strains in the south and north of the Lujing ore field, intruding into Indosinian intrusions. The rock type is dominated by medium-fine grained monzogranite and two-mica granite and little biotite granite and coarse-grained monzogranite. In this study, six sets of fault occurrence were collected from six positions in Early Yanshanian intrusions (Table 1). Their strikes are NE direction and dip directions are 136°-164° and 318°-337°, with dip angle of 53°-74° (Fig. 3). The traction structure suggests that the NE-trending faults are compressional, which is the reflection of Early Yanshanian NW-trending compression process.

Fig. 3
figure 3

(a) Lower hemispherical equal-area diagram of fault strike in Early Yanshanian pluton; (b) Field photos of structure in Early Yanshanian intrusions.

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Tectonic analysis of strata

The exposed Caledonian strata in study area are composed of Sinian, Cambrian and Ordovician, located in the northwestern and northeastern part. The rock types include quartz sandstone, sandy slate, carbonaceous slate, muddy shale, etc. Four sets of faults and twenty sets of joints data were collected from 27 locations in Caledonian strata (Table 2).

The joints in Caledonian strata have two strike directions (NW and NE) and dominated by NE-trending, indicating the NW-trending structural stress (Fig. 4a). Reverse faults were observed in Ordovician slate, showing SE-trending dip direction (Fig. 4b). Based on the traction structure in it, these faults belong to compressional reverse faults, might indicating Caledonian NW direction compression. There are also some NW-trending reverse faults (39°∠84°) displaying traction structures, combined with NW-trending anticline in Caledonian strata, indicating the Indosinian NE direction compression (Fig. 4c, d).

Fig. 4
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(a) Rose diagram of joint strike in Caledonian strata; (b-d) Field photos of structure in Caledonian strata.

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The Yanshanian strata include Jurassic Jinji Group, Maziping Group and Cretaceous Fengzhou Basin. Among them, the Jurassic strata located at the northwest of the Lujing ore field, composed of arkose, carbonaceous shale and purple muddy shale. Whereas the Cretaceous strata mainly distributed in the Fengzhou Basin, northern margin of the study area, consisting of red conglomerate and sandstone. Four sets of joints and faults were identified in the field, in which the joints are mainly NW-trending (115°-156°) (Fig. 5a). Meanwhile, NW-trending extensional faults were found in Jurassic strata, and the traction structure in it indicating NE direction extension (Fig. 5b).

Fig. 5
figure 5

(a) Rose diagram of joint strike in Yanshanian strata; (b) Field photos of structure in Yanshanian strata.

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Tectonic analysis of four periods

Caledonian stage: NW-SE compression

The stereonets and rose diagrams of Caledonian structure show that the strikes of faults and foliations in Sinian and Cambrian are mainly NE-trending (10°-88°), then transferred into SN-trending (185°) in Ordovician strata, with varied dip angle (17°-88°) (Fig. 6a, b). In addition, faults are wide-spreading in Early Paleozoic strata in eastern and western side of the Lujing ore field. The Ordovician strata started to bend and thus forming the NE-trending strike anticline, with wings dip angle of 50°-88°. Most of the faults were filled with white quartz veins. The joints in Caledonian strata are mainly NE direction (180°-270°) (Fig. 6). Based on the classical stress inversion method and rose diagram of the joints strikes, it shows that the σ1 approximates NW-SE, σ3 approximates NE-SW. The Caledonian NW-trending compression formed basic structural pattern of the Lujing uranium field.

Fig. 6
figure 6

(a) Lower hemispherical equal-area diagram of structure’s strike; (b)Rose diagram of joint strike in Caledonian period; (c-d) Field photos and forces analysis of structure in Caledonian period.

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Indosinian stage: NE-SW compression

Large amounts of fold transformation were developed in Indosinian in Lujing ore field, represented by Shihuiyao Fold Belt, Chayuan-Dadoubi Fold Belt and Longtou-Shabazi Fold Belt. These folds are compound syncline and strikes northwest. The core is Cambrian Chayuantou Formation, with Sinian, Cambrian Xiangnan Formation and little Ordovician strata as two wings (dip angle 50°-70°), with no involvement of Jurrasic strata, which indicate that these folds are formed during the Indosinian. Furthermore, the southwestern wing displayed inverted anticline secondary folds. The Indosinian intrusions intruded simultaneously, having structural transformation and superposition effect on the Caledonian tectonic deformation (Fig. 7).

Fig. 7
figure 7

(a-b) Lower hemispherical equal-area diagram of fault and foliation line’s strike in Indosinian stage; (c-d) Field photos and forces analysis of structure in Indosinian period.

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During this period, a large number of NE-trending compression foliation also formed in the Indosinian granite. Their strikes are northwestern direction (100°-170°) and dip angles range from 27° to 88° (Fig. 7a, b). In the field, the Indosinian NW-trending fault (236°∠29°) are cut by Early Yanshanian NE-trending fault (100°∠85°) (Fig. 7c). Most foliation and faults show traction structure and white quartz vein filling along the fault/foliation plane, indicating a compression stress (Fig. 7c, d). Meanwhile, the joints’ strikes are mainly NW- and NE-trending, indicating the stress direction are northeast and northwest, respectively (Fig. 8). Above all, all the structure traces suggested that this area underwent NE-trending compression in Indosinian. Based on the rose diagram and classical stress inversion method, the σ1 is inverted to NE-SW, σ3 inverted to NW-SE.

Fig. 8
figure 8

(a) Lower hemispherical equal-area diagram of joint strike in Indosinian stage; (b) Rose diagram of joint strike in Indosinian stage.

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Early Yanshanian stage: NW-SE compression

The structure of this period mainly distributed in the Indosinian and Early Yanshanian strata and intrusions in central and north of the Lujing ore field, dominated by NE-trending reverse fault (dip direction: 113°-164° and 286°-337°, dip angle: 37°-81°) (Fig. 9a, b). The dip angles of those faults are relatively big when closed to Yangjaonao deposit and Suichuan Fault, and relatively small when close to Huangfengling deposit. The Early Yanshanian tectonic activity was superimposed on Caledonian and Indosinian structures, which enhanced the NE-trending structure framework of Lujing ore field.

Fig. 9
figure 9

(a-b) Lower hemispherical equal-area diagram of structure line’s strike in Early Yanshanian stage; (c-d) Field photos and forces analysis of structure in Early Yanshanian stage.

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Meanwhile, the magma activities are gradually strengthened, forming the Early Yanshanian granite strains and little medium-basic dikes. The Early Yanshanian NE-trending faults were filled with white quartz veins and suffered chlorite-hydromica-hematite alteration. These faults are the main ore-controlling structures, represented by QFI−V fault belts (Fig. 9c, d). According to the rose diagram and classical stress inversion method, the σ1 is inverted to NW-SE, σ3 inverted to NE-SW, representing NW-SE compression.

Late Yanshanian stage: NW-SE extension

The structure of Late Yanshanian stage mainly distributed in Late Yanshanian intrusions and Cretaceous strata in central and north of the Lujing ore field. It is dominated by NE-trending extensional normal faults (dip direction 119°-171°, dip angle 13°-84°), which may be formed by NW-SE extension in Late Yanshanian stage (Fig. 10a, b). Meanwhile, the dip angles are increasing from north to south of the Lujing ore field. Combined the rose diagram and classical stress inversion method, the σ1 approximates NE-SW, σ3 approximates NW-SE. Due to the regional extension of this stage, cracks and fractures were produced in the granite, providing upwelling and migrating channel for magma and ore-forming fluids.

Fig. 10
figure 10

(a-b) Lower hemispherical equal-area diagram of structure line’s strike in Early Yanshanian stage; (c-d) Field photos and forces analysis of structure in Early Yanshanian stage.

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Summary

Through the field observation and tectonic stress inversion, the Lujing ore field underwent Caledonian NW-SE compression, followed by Indosinian NE-SW compression, forming large area of the fold belt in the northern part of Lujing ore field (Fig. 11a-c). Subsequently, the Early Yanshanian NW-SE trending compression in Lujing ore field resulted in the main ore-controlling structure (QFI−V fault belts) (Fig. 11d), large amount of uranium deposit forming simultaneously40. Then, the Late Yanshanian NW-SE trending extension modified and superimposed on the previous structures and produced many extensional cracks, which is beneficial for the late-stage uranium mineralization (Fig. 11e).

Fig. 11
figure 11

(a) Simplified geological map in Lujing ore field modified after30; (b-e) Tectonic stress field of different stage in Lujing ore field. This figure was originally generated in CorelDraw 2020 by Weidong Ren.

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Ore-controlling structure and prospecting

The relationship between faults, fluid flow and ore deposition are well known, (e.g. uranium, copper, molybdenite, etc.)20,29,33,41. The Zhuguangshan Complex hosts many uranium fields, such as Lujing, Changjiang, Baishun, etc. However, they show totally different ore-controlling structure framework. The Changjiang uranium field is located in the southern part of the Zhuguangshan Complex, in which the ore-controlling structure is dominated by NNW-trending (almost SN-trending)42. These NNW-trending faults is formed by Yanshanian NWW-SEE extension42. The Baishun uranium field is located at the southeast of the Changjiang field. Its structure is dominated by NE-trending faults, which are also the main ore-controlling structures. Whereas the Lujing uranium field is located in the central part of Zhuguangshan Complex, in which the ore-controlling structures are mainly NE-trending, with minor NW-trending ore-controlling faults. The NE-trending ore controlling structure in Lujing ore field can be represented by the Yangjiaonao ore district and NW-trending structure in Shabazi ore district.

Among them, the NE-trending QFV fault belts is the main ore-controlling structure in Yangjiaonao ore district, which are distributed in Indosinian biotite monzogranite. In this study, we observed and analysed the QFV fault and its branches (QFV−1 and QFV−2 faults), in which the QFV fault is a reverse fault (318°∠74°). Red-brown silicate belt can be seen in the lower part of the QFV fault, bulging at the bottom and vanishing at the top (Fig. 12a). The QFV−1 fault is also a reverse fault (333°∠79°), the upper part is altered granite and lower part is alteration zone. The alteration zone is composed of hematite zone, hydromica-chlorite zone, clay-carbonation zone from inside to outside (Fig. 12b). The QFV−2 fault is a reverse fault of 337°∠74°, and a large amount of fault gouge developed in it (Fig. 12c). In addition, the QFV−2 fault extend to the south of this point, with plentiful torbernite and autunite observed along the extensional fault plane (Fig. 12d). The uranium ore bodies are hosted in the lower part (cataclastic altered rock) of the QFV fault and its branches.

Fig. 12
figure 12

(a) Field photo of the QFV fault; (b) Field photo of the QFV−1 fault; (c) Field photo of the QFV−2 fault; (d) Field photo of autunite and torbernite distributed along the QFV fault plane.

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The above phenomenon indicate that this NE-trending fault is formed during Early Yanshanian NW-SE compression, then transformed by the NW-SE extensional stress in Late Yanshanian stage, thus forming a lot of micro-cracks. Those cracks provided channel for ore forming fluids migration and water-rock reactions occurred to form the alteration zone. Previous studies proposed that the alkalic, silification as well as hematite alteration zones are strongly related to uranium mineralization (Fig. 12b, c)26,27,43. Meanwhile, the calcite S-C-O isotope results suggest that the C is originated by mantle degassing process, while the S is from the Precambrian strata and Indosinian granite44. In addition, the simulations also suggested NW direction extension (forming NE-trending faults) controlled the ore bodies, and the lithologic differences of upper and footwall of the faults as well as the fault dip controlled the distribution of ore bodies35. Therefore, we proposed that: (1) The ore-forming fluids flew along those reactivated NE-trending faults in the depth, reacted with the surrounding rocks to form the alteration belt. The uranium was activated and extracted through this process and form the pitchblende and uraninite. (2) When fluids uplifted to the shallow part, due to the increasing oxygen contents and involvement of meteoric water, pitchblende and uraninite underwent secondary oxidation, crystalizing into those torbernite and autunite along the NE-trending fault plane (Fig. 12d).

The ore bodies in the NW-trending bedding fracture zone were found in the Shabazi and Lihuakai deposits in the north of the Lujing ore field (Fig. 13a), indicating the metallogenic potential of the NW-trending structure. The NW-trending structure in the Shabazi deposit is mainly produced in the Indosinian intrusions, which is formed by the NE-trending compression in the Indosinian period, and is transformed by the NW-trending compression and extension in the Yanshanian period. The intersection area of these NW-trending structures and NE-trending structures provides channels and spaces for the migration of ore-forming fluids and the precipitation of ore-forming materials, which make it an important metallogenic prospecting area. In this paper, some NW-trending structures were newly discovered in the north of Huangfengling, Fengshuxia deposits and Fengzhou basin in Lujing ore field. Among them, the NW-trending extensional faults (65°∠74°) in the late Yanshan period cut across the NE-trending compressive faults (136°∠23°) in the early Yanshan period (Fig. 13b). These NW-trending extensional structures reformed the early NE-trending structures and produced a large number of tensile cleavages, providing space for the precipitation of ore-forming elements. Therefore, this study delineated two metallogenic prospect areas in the northern part of Huangfengling and Fengshuxia deposits in Lujing uranium field, and it is necessary to pay attention to the intersection of NW and NE faults (Fig. 13c). The Jinjiling uranium minerlization found on the northern side of the Huangfengling also confirms this conclusion.

Fig. 13
figure 13

(a) Profile map of the Shabazi ore deposit; (b) Photo shows the NW-trending fault cut across the NE-trending fault in the Late Yanshanian stage; (c) Prospecting area of Lujing ore field (after30). This figure was originally generated in CorelDraw 2020 by Weidong Ren.

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Conclusion

  1. (1)

    The Lujing ore field underwent Caledonian NW-SE compression, Indosinian NE-SW compression, Early Yanshanian NW-SE compression and Late Yanshanian NW-SE extension.

  2. (2)

    The intersection area of NW-trending and NE-trending structures have high uranium mineralization potential. The north of Huangfengling and Fengshuxia were delineated as metallogenic prospect areas.