Deep Structure of Nanling-Xuancheng Ore District, Eastern China: Insights from Integrated Geophysical Exploration

Abstract

As the depth of mineral exploration increases, integrated geophysical methods are increasingly playing a crucial role in prospecting deep structures at the district scale. The Nanling-Xuancheng ore district is the eighth ore district in the middle-lower Yangtze metallogenic belt in China. To reveal the deep structure of the mining district, this study mainly focuses on regional high-precision gravity and magnetic data and integrates the interpretation of magnetotelluric and reflection seismic data from a key area. By using a 2.5D joint inversion method with prior information constraints, new insights into the deep structures, tectonic deformation, and magmatic activity are obtained. Structurally, the Nanling-Xuancheng ore district presents a structural pattern of “two uplifts and two depressions” composed of multi-level thrust-overturned and folds formed by Mesozoic depressions, which has a three-layer structure in the vertical direction (shallower than 10 km). Tectonically, the main faults in the study area trend NW, which intersect with NE-trending and EW-trending faults to form a branching structure from deep to shallow. The fault intersections provide pathways for magma intrusion. The distribution of deep-seated concealed magmatic rocks shows the characteristic pattern of “a primary magma source spawning multiple subsidiary intrusion”.

1. Introduction

The middle-lower Yangtze Fe-Cu metallogenic belt is an important Fe-Cu-Au polymetallic metallogenic belt in eastern China, which was previously thought to consist of seven ore districts [1,2]. The Nanling-Xuancheng ore district is situated in the southeastern wing of the middle-lower Yangtze metallogenic belt’s northeastern section. In recent years, a series of mineral exploration breakthroughs have been made, including the Magushan Cu-Mo deposit, Maweishan Pb-Zn-Ag deposit, Qiaomaishan Cu-W-S-Fe deposit, Tashan Mn deposit, Changshan Pb-Zn-Au deposit, and especially the Chating large-scale Cu-Au deposit. These discoveries made it the eighth ore district in the middle-lower Yangtze metallogenic belt [3,4].

Currently, geological research focuses primarily on the geological characteristics of typical deposits, geochronology of ore-bearing intrusions, regional metallogenic regularities, and tectonic evolution history [5,6,7,8,9,10,11,12]. However, research on deep structural frameworks and deep metallogenic backgrounds is relatively limited due to the fact that this area is mostly covered by Mesozoic and Cenozoic strata, with small rock outcrops. Many basic geological problems remain controversial or lack systematic research. For example, the deep structural framework, the basement nature, and the important fault structures related to the source of metallogenic materials are still unclear. The timing of widespread thrust structures and their control or destruction effects on magmatic-hydrothermal-metallogenesis processes also needs further investigation. Additionally, the distribution range of deep magmatic activity and ore-related volcanic rocks is not clear. These unresolved issues restrict the in-depth development of mineral exploration work, necessitating the use of geophysical tools to clarify deep structures and guide mineral exploration efforts further.

In recent years, with the increase in the depth of mineral exploration, the proportion of geophysical methods in mineral exploration investment has been steadily rising [13,14,15,16,17,18]. The geological targets of geophysical exploration include determining potential ore deposits, understanding strata and structures, and depicting finer details. Due to the large burial depth of deep ore bodies, complex physical properties of geological bodies, and strong interference from various sources, it is becoming increasingly rare to locate target geological bodies using a single method. As a result, the mineral exploration industry has recognized that integrated geophysical exploration and joint inversion are becoming important roles for deep mineral exploration [19,20,21,22,23].

This study provides a comprehensive analysis of integrated geophysical exploration results. We utilized a 2.5D inversion method that incorporates prior information and joint constraints from gravity, magnetic, MT, and seismic data to reveal the deep structure. Our research presents the tectonic framework of the ore district and reveals several new discoveries in basic geology and deep structure. We propose new understandings and comprehensively interpret the spatial distribution and morphological changes of significant rock-controlling and ore-controlling structures, important magmatic rocks, and critical interfaces. Consequently, our understanding of the deep geological processes in the mineralized area has significantly increased, providing further targets and directions for deep exploration.

2. Geological Setting

The Nanling-Xuancheng ore district is geotectonically situated on the northern edge of the Yangtze block and spans two secondary structural units: the Lower Yangtze Depression and the Jiangnan Uplift [24] (Figure 1a). Some scholars believe that the structural composition and evolutionary history of this district are highly complex. Although the two secondary structural units are bounded by the Jiangnan Fault Zone, there is no significant difference in folding characteristics on both sides of the boundary. Therefore, the area from Dongzhi and Guichi South to Nanling Xuancheng is separately divided into a secondary structural unit, namely the Jiangnan Transitional Zone [25]. The Jiangnan Transitional Zone has a dual basement, with the Jiangnan Fault as the boundary; the Kongling-Dongling-style basement is located on the north side, and the Jiangnan-style basement is on the south side. The docking of the basement should occur between the Early Sinian and Late Sinian [26,27,28].

The study district is covered by strata ranging from the Silurian to the Cenozoic period. Since the Mesozoic era, the district has undergone multiple thrusting-deformation events. Firstly, during the Indosinian-Early Yanshanian tectonic period, the Jiangnan intracontinental orogeny formed a series of NE-NEE thrust faults, causing Paleozoic-Triassic strata to be involved in deformation [29]. Secondly, in the Early Cretaceous period, the change in subduction direction of the ancient Pacific Plate caused the most intense thrusting activity in eastern China since the Mesozoic era. The pre-Cretaceous strata were involved in this deformation, and then the study area began to enter an extensional stage [30]. Thirdly, during the Late Cretaceous-Paleogene period, under the background of near E-W compression of the Pacific Plate, thrusting activities caused basin edge reversal and Paleozoic strata were thrust over Upper Cretaceous strata [31,32]. The compression activity lasted for a short time, followed by a relatively long regional extension activity accompanied by strong magmatic activity.

Figure 1. (a) Sketch geological map of magmatic rocks and deposits in the middle-lower Yangtze [3,33], (b) Geological map of the Nanling-Xuancheng ore district showing the location of integrated geophysical profiles [33]. The faults are modified from this study. 1, Chating Cu-Au deposit; 2, Shizishan 3, Changshan Pb-Zn-Au deposit; 4, Qiaomaishan Cu-W-S-Fe deposit; 5, Magushan Cu-Mo deposit; 6, Yaojialing Zn-Au deposit. JNF: Jiangnan Fault; QHF: Qingshuihe-HeWan Fault; SXF: Sanli-Xihe Fault; JLF: Jiulianshan-Liqiao Fault; ZWF: Zhouwang Fault; ZNF: Zhongming-Nanling; KSF: Kunshan Fault; CWF: Chao-hu-Wuhu Fault; MLF: Ma’anshan-Langxi Fault.

Figure 1. (a) Sketch geological map of magmatic rocks and deposits in the middle-lower Yangtze [3,33], (b) Geological map of the Nanling-Xuancheng ore district showing the location of integrated geophysical profiles [33]. The faults are modified from this study. 1, Chating Cu-Au deposit; 2, Shizishan 3, Changshan Pb-Zn-Au deposit; 4, Qiaomaishan Cu-W-S-Fe deposit; 5, Magushan Cu-Mo deposit; 6, Yaojialing Zn-Au deposit. JNF: Jiangnan Fault; QHF: Qingshuihe-HeWan Fault; SXF: Sanli-Xihe Fault; JLF: Jiulianshan-Liqiao Fault; ZWF: Zhouwang Fault; ZNF: Zhongming-Nanling; KSF: Kunshan Fault; CWF: Chao-hu-Wuhu Fault; MLF: Ma’anshan-Langxi Fault.

The magmatic rocks in this district mainly consist of subvolcanic-hypabyssal intrusive rocks and continental volcanic effusive rocks (Figure 1b). The subvolcanic-hypabyssal intrusive rocks are predominantly neutral-acidic, with rock types including pyroxene diorite, granite diorite, and quartz diorite porphyry. Other rock types such as granite porphyry and diorite porphyry are also present, mainly exposed on Magushan and Kunshan thrust/anticlinorium. The continental volcanic effusive rocks belong to the Zhongfencun Formation (K₁z) of the Lower Cretaceous period and can be compared with volcanic rock strata found in volcanic rock basins such as Fanchang, Ningwu, and Lishui. The volcanic rock strata of the district are only sporadically exposed on Henglangshan in Wuhu, Jiulianshan, Jingtingshan, Huangniushan, and Shenxiandong [4,33].

The main body of the ore district is controlled by deep NE faults, and the main structural units are arranged in a NE direction (Figure 1b). The district is bounded to the west by the Qingshuihe-Hewan Fault (QHF), which is adjacent to the Tongling Uplift and Fanchang Basin. To the south is the Zhouwang Fault (ZWF), which runs through the Jiangnan Uplift Belt. Within the ore district, from west to east, there are the Nanling Basin, Jingtingshan-Liqiao Anticlinorium (JL Anticlinorium), Xuancheng Basin, and Magushan Anticlinorium (MY Anticlinorium). The fault network consists of northeastward, northwestward, and east-west faults. NE faults mostly appear as longitudinal faults parallel to structural lines, including the Qingshuihe-Hewan Fault (QHF), Sanli-Xihe Fault (SXF), Jiulianshan-Liqiao Fault (JLF), Kunshan Fault (KSF), and Jiangnan Fault (JNF). The east-west fault is the hidden Zhouwang Fault (ZWF), and the NW fault mostly cuts through early NE faults in a transverse layer, including the Zhongming-Nanling Fault (ZNF), Chaohu-Wuhu Fault (CWF) and Ma’anshan-Langxi Fault (MLF).

3. Data Collection and Processing

3.1. Integrated Geophysical Exploration Strategy

Compared to single methods, integrated geophysical detection has achieved varying degrees of success in mineral exploration, especially when faced with complex geological conditions and the identification of relatively weak anomalous signals [34,35].

Based on the known petrophysical properties, the geological conditions of the area, and the purpose and tasks of this study, we have chosen to use reflection seismic and MT profiles to establish cross-sections, combined with areal gravity and magnetic data, to reveal deep structures. The reflection seismic method can provide high-resolution signals within 5 km [36,37,38], but due to the cost constraints of ore deposit exploration, it can only be deployed in key areas; the gravity and magnetic method can quickly and economically cover the entire exploration area, and the inversion method is relatively mature; the electromagnetic method has been widely used in mineral exploration since 1950s, and can effectively identify deep hidden ore bodies and deep structures related to fractures and broken zones (Figure 2).

3.2. Seismic Reflection Data

We conducted two parallel seismic reflection profiles (Figure 2), running NW-SE and approximately perpendicular to the Jiulianshan-Liqiao anticlinorium. The profiles start from the Chating Cu-Au deposit to the north and cross the Xuancheng Basin and Magushan anticlinorium to the south. To avoid processing issues caused by curved line collection, all profiles were strictly collected along a straight line. The previous seismic profile data came from the Chinese Academy of Geological Sciences [27].

The field seismic reflection data collection used the French-made 428XL seismograph. The measurement work started on 5 November 2018, and was completed on 5 May 2019, lasting for 180 days. Two seismic lines were completed, with 842 production shots and 26 test shots, covering a total length of 40.28 km. Before data collection, two representative test points were selected based on the analysis of surface rock types. The most reasonable excitation factors were chosen by conducting systematic excitation experiments under these two different surface conditions.

The seismic data collection observation system used 4790-10-20-10-4790 (m), with a 20 m trace interval, 60 m shot interval, 480 traces, and 80 coverage times. The well depth for excitation was 16 m for a single well and 5–8 m for a combined well. The explosive charge per single well was 10 kg, and 2–4 kg for a combined well.

This seismic data interpretation was conducted using the PROMAX seismic data processing system. Upon analyzing the original seismic data, it was determined that the data quality was affected by both the surface geological conditions and the deep seismic geological conditions, resulting in a large amount of static correction needed in the original data. To address this issue, various processing techniques were employed based on the characteristics of the original data. These included analyzing the original data to define spatial attributes, field primary static correction, pre-stack noise suppression, amplitude recovery and compensation, deconvolution, velocity analysis, residual static correction, and migration imaging.

• Defining Spatial Attributes

The initial spatial attributes are defined based on the reports and measurement data provided during field data collection. Subsequently, a variety of discrimination methods are employed flexibly, either individually or in combination, for inspection and modification.

• Tomographic Static Correction

Based on an analysis of the initial arrival refracted wave velocity from a single shot and the processing effects, a reference surface elevation of 350 m and a replacement velocity of 4000 m/s are used in this processing.

• Pre-stack Noise Suppression

Considering the frequency range of effective waves, this data processing adheres to the principle of preserving both low-frequency and high-frequency effective information. The pre-stack filtering parameters for shallow parts are set at 5-10-80-90 Hz, while for medium-deep parts they are set at 3-8-70-80 Hz.

• Amplitude Recovery and Compensation

Following amplitude recovery testing, the chosen parameters are a recovery coefficient of 7 db/s and a recovery depth of 4500 s. The recovery effect is satisfactory, catering to both medium-deep parts and shallow energy.

• Deconvolution

For this deconvolution method, surface-consistent predictive deconvolution was chosen. After processing with different deconvolution prediction step lengths, a prediction step length of 32 ms was selected based on a comparative analysis of single-shot spectra.

• Velocity Analysis

In this velocity analysis work, velocities are initially picked at intervals of 1000 m for preliminary stacking. Then, based on the effects seen in stacking profiles and the structural characteristics of the data, timely adjustments and modifications to velocities are made to ensure accurate velocity picking. In areas with structural changes, denser velocity analysis points are used for control.

• Residual Static Correction

During residual static correction, time shift experiments were primarily conducted. In combination with the data frequency range, a time shift within 15 ms was found to be most suitable during the first residual static correction calculation. After two rounds of residual static correction processing, there were significant improvements in both energy intensity and continuity of effective reflection wave phase axes on stacked profiles.

• Migration Imaging

This project employs post-stack time migration processing. Prior to migrating, appropriate denoising and filtering processes are applied to stacked data to obtain a seis-mic stacked time profile with a higher signal-to-noise ratio.

3.3. MT Data

This study involved completing a MT array with a total length of 928 km, a point spacing of 1000 m, and 1044 physical points. Eight sets of V5-2000 systems from Phoenix Geophysics Ltd. in Canada were used for measurement, with an observation frequency band of 320 Hz–0.001 Hz and an observation time of 24 h. Calibration tests and consistency tests were performed on the data collection instruments (MTU-5A) at the start and end of the work, and instrument calibration was performed every month during the construction period to ensure that the MT measurement instruments were in good working condition and to obtain the calibration results at each stage. Observation time tests and electrode distance tests were also conducted at the start of the work, and the remote reference location was selected.

According to the “Technical Regulations for MT Sounding” [39], the MT data quality pass rate of this work was 98.0%. Of the 1044 physical points, 722 points were grade one, accounting for 76.6%; 201 points were grade two, accounting for 21.3%; and 19 points were grade three, accounting for 2%. A total of 31 inspection points were completed, accounting for 3.3%, which met the technical regulations and ensured reliable data.

The MT field data processing used the SSMT2000 and MTEDITOR series software developed by Phoenix Geophysics Ltd. in Canada. Qualitative analysis was performed using error point removal, static shift correction, electrical distortion correction, and other methods [40], and the detailed analysis results are given by [41]. The 26 periods from the 854 stations were inverted with a 3D electromagnetic inversion program (EMinv) using the nonlinear conjugate gradient method (NLCG) [42,43], which has the characteristics of fast inversion iteration speed and good results for complex models.

The inversion grid employs a 172 × 96 × 100 grid, inclusive of 20 air layers. The initial model is a homogeneous half-space with a resistivity of 100 Ω·m. The grid spacing is 1 km, with 12 additional extension grids in the east-west direction, each expanding outward by a factor of 1.5. Similarly, 12 extension grids are added in the north-south direction, each expanding by a factor of 1.5. Vertically, the first layer is 20 m thick, and starting from the second layer, the thickness expands by a factor of 1.2 towards the depth. The frequency is the same as the data collection frequency, with a total of 40 frequency points. The inversion calculation iterates 106 times, taking 151 h, and the final inversion result error RMS converges to 2.9%.

The apparent resistivity data calculated by the 3D inversion model and the measured data fit well overall. The relative distribution of high and low resistance is basically consistent, indicating that the inversion results are reliable and can serve as the basis for further analysis and interpretation. The inversion details are provided in [44].

3.4. Gravity and Magnetic Data

The gravity data are the 1:50,000 scale ground-based collected by the Geological Exploration Technology Institute of Anhui Province from 2011 to 2015, with a total accuracy of ±0.089 mGal for Bouguer gravity anomaly, and the 1:50,000 scale ground gravity data collected by Nanjing Geophysical Survey Center from 2010 to 2013, with a total accuracy of ±0.125 mGal for Bouguer gravity anomaly.

The data collection utilized the LCR-G type gravimeter manufactured in the United States and the CG-5 type gravimeter made in Canada. All raw data underwent a “five unifications” process, which included adopting the 2000 national gravity basic network system; utilizing the 1954 Beijing coordinate system and the 1985 national elevation system; calculating the normal gravity value using the 1980 formula recommended by the International Association of Geodesy (IAG); performing Bouguer correction and inter-mediate layer correction according to the regional gravity survey specification, with a uniform density of 2.67 g/cm³; and conducting terrain correction with a radius of 166.7 km. After processing, the discrete gravity data results were consolidated.

The total number of gravity measurement points is 46,202. Excluding areas such as bodies of water and airports, the average density of the measurement points is 7 points/km². The merged Bouguer anomaly values were gridded using the RGIS2016 software. The data grid spacing is 250 m × 250 m, and the gridding method used is Kriging, shown in the figure (Figure 3a).

The aeromagnetic data collected by the former China Ministry of Geology and Mineral Resources Aero geophysical Brigade from 1984 to 1991, included a flight height of 60–300 m and a total accuracy better than ±3.66 nT (1:50,000 scale). Field flight measurements in the Ningguo and Guangde regions were conducted using the HC-85 type airborne optically pumped magnetometer, manufactured in China. In the Wuhu and Xuancheng regions, data collection was performed using the GB-4 type three-optical-system helium optically pumped magnetometer, also produced in China.

The total number of aeromagnetic measurement points is 58,056. Initially, necessary processing is performed on the aeromagnetic data of the single measurement area. The Geoprobe software from the China Natural Resources Aerial Geophysical and Remote Sensing Center is used to form a grid data of 250 × 250 m, with the interpolation method being the least squares method. Then, based on the accuracy, scale, and time of the single measurement area’s aeromagnetic measurement, methods such as integration, mixing, or suturing are used to stitch the single measurement area’s aeromagnetic grid data together. Subsequently, the central geomagnetic field parameters of the measurement area (magnetization inclination: 45.27°, magnetization deviation angle: −4.22°, total field strength: 46,186 nT) are used for reduction-to-the-pole (RTP) filter to obtain the RTP aeromagnetic map of the study area (Figure 3b).

The gravity anomaly observed on the surface contains the superposition effect of all the density heterogeneities from the surface to the depth. The anomaly density is the difference between the density of the underground geological body and the surrounding rock, also known as the residual density [45]. The anomaly magnetization intensity can be similarly defined. This study uses the spatial domain decreasing radius iterative method for regional-residual anomaly separation of potential field to calculate the residual anomaly, and then performs layer-by-layer separation, downward continuation using the wavenumber domain iterative method, and layer-by-layer inversion on the residual anomaly [45,46], respectively, calculating the spatial characteristics of the density and magnetization intensity of 41 underground layers, with a layer thickness of 200 m (Figure A1 and Figure A2).

3.5. Petrophysics Data

Spatial and temporal changes in petrophysical properties reflect the physical evolution process of Earth’s structure [47]. Petrophysical properties are extremely significant in geological and geophysical research [48]. They serve as the link between geology and geophysics. At the same time, we should also recognize that there may be significant overlap in physical properties between different types of rocks [49], so it is necessary to combine multiple physical properties to distinguish different rocks [50].

In this study, we collected petrophysical measurement data from previous research conducted in the area, specifically from the Geological Exploration Technology Institute of Anhui Province. In addition, we carried out drilling sampling work at the Chating deposit, Magushan deposit, and the neighboring Hehuashan deposit. All petrophysical samples were derived from drill cores, encompassing a total of 30 boreholes (Figure 2). The sampling stratigraphic position, number of samples, and sample identifiers were designed based on the specific conditions of each borehole (as a rule, the number of samples for each stratigraphic set is no less than 30). Given that the majority of drill cores maintain a fixed shape (the radius of the cylindrical body varies according to different drill casings), samples could be directly obtained using a geological hammer and suitably trimmed to ensure as regular a shape as possible (density specimens and resistivity specimens are generally cylindrical). The size of the density samples typically does not exceed two kilograms. Due to instrument limitations, magnetization samples are generally processed into cylinders with a diameter of 25 mm and a length of 22 mm, or cubes with a side length of 20 mm. Resistivity samples are usually taken from cylinders with a diameter less than 90 mm and a length less than 180 mm.

Density measurements in this study were conducted using a JA5003N electronic balance produced in Shanghai. Magnetic measurements were performed using an MS2 magnetic susceptibility meter manufactured in the UK with a dual frequency sensor MS2B, and electrical measurements were carried out using a SCIP sample core measuring instrument produced in Canada. Regardless of whether density or electrical parameters were being measured, samples had to be soaked in untreated subterranean clear water for a certain duration to achieve a water-saturated state, thereby simulating the conditions under which the rock (ore) exists deep underground.

The density of the samples was measured using the drainage method. Some strata samples were relatively loose or had larger voids. Therefore, for these samples, the small bulk density wax sealing drainage method [51] was employed to measure their density. An electronic balance was used to obtain readings by weighing each sample three times during the density measurement process. The specimen was first weighed in air, then weighed after being sealed in wax, and finally weighed in water. A total of 5920 samples were measured, with the mean square error of density measurement being ±2.4 kg/m³.

Susceptibility measurements of the samples were conducted in locations where the magnetic field was stable and temperature variations were minimal. Temperature drift correction could be performed through a sequence of three measurements. The average value of air measurements before and after sample measurement would be deducted from the sample measurement. A total of 3230 samples were measured, with an average relative error of 1.7%.

The resistivity of the samples was measured using a symmetrical quadrupole device. A total of 4235 samples were measured, with an average relative error of 1.53%.

There are clear differences in physical properties among various rocks and major structural layers. These differences serve as the foundation for interpreting geophysical data in this area and for establishing the division table of physical modeling units for this study (Figure 4, Figure 5 and Figure 6).

Based on the comprehensive analysis of physical property results, the range of density values of sedimentary rocks is relatively stable, especially carbonate rocks, which show stable high values, while igneous rocks vary greatly depending on their composition, exposure status, and freshness. The sample magnetization rate increases in the order of carbonate rocks, acidic igneous rocks, clastic rocks, and intermediate alkaline igneous rocks. Although there are slight differences in physical properties between the Lower Yangtze stratigraphic zone and the Jiangnan stratigraphic zone, especially in the Silurian strata, they are divided into two geological units in the inversion.

The strata of the study area are from new to old. Based on the principle of mainly density values and supplemented by magnetism and resistivity, it can be divided into 8 physical layers. For specific quantitative data of each layer, please refer to the comprehensive columnar diagram of physical properties.

• Quaternary loose sediments. Mainly distributed in Nanling Basin and Xuancheng Basin, there are also sporadic distributions in valleys and foothills, characterized by low density, weak magnetism, and low resistance.
• Paleogene-Late Cretaceous clastic rocks. Mainly distributed on the south side of Nanling Basin, there are also sporadic distributions in valleys and foothills, characterized by medium-low density, weak magnetism, and low resistance.
• Early Cretaceous volcanic clastic rocks and tuff. Mainly distributed in Fanchang Volcanic Rock Basin, on both sides of Jingtingshan-Liqiao Anticline at the transition part of the basin. The magnetism varies greatly according to the content of volcanic debris, generally characterized by low density, medium-low magnetism, and secondary low resistance.
• Mesozoic igneous rocks. Due to the large difference in dark mineral content, density values range from medium-high to medium-low. The changes in magnetism and resistivity are also large. The overall physical property changes are wide and appear chaotic. Using magnetism as the main indicator, Mesozoic igneous rocks can be divided into three types: The first type is intermediate intrusive rock bodies with medium-strong magnetism containing more magnetic minerals such as syenite gneiss, gneiss porphyry, quartz diorite, granodiorite, diorite etc. The second type is intermediate-felsic intrusive rocks with weak magnetism and few magnetic minerals mainly composed of quartz and feldspar such as granophyre porphyry orthoclase porphyry biotite granite potassium feldspar granite etc. The third type is subvolcanic rocks with unstable magnetism such as coarse porphyry etc.
• Triassic limestone. Mainly distributed in uplifted areas and outer edges of basins generally characterized by medium-high density low magnetism high resistance.
• Middle-Late Permian clastic rocks. Mainly distributed in uplifted areas and outer edges of basins generally characterized by medium-low density low magnetism medium-low resistance.
• Early Permian-Carboniferous limestone. Mainly distributed in Maoshan Mountain Range outer edge of Daibu Volcanic Rock Basin generally characterized by medium-high density weak magnetism high resistance.
• Devonian-Silurian clastic rocks. The lower Yangtze stratigraphic zone is mainly distributed in Jingtingshan-Liqiao Anticline Tongling Uplift with rock types mainly carbonate rocks and shale overall characterized by high density weak magnetism high resistance; Jiangnan stratigraphic zone is distributed in Magushan Inverted Anticline area with rock types mainly mudstone shale overall characterized by medium density weak magnetism high resistance.

There are clear differences in physical properties among various rocks and major structural layers. These differences serve as the foundation for interpreting geophysical data in this area and for establishing the division table of physical modeling units for this study (Figure 4, Figure 5 and Figure 6).

4. 2.5D Modeling Method

There are currently various methods and strategies for joint inversion, depending on geological conditions and exploration targets [52,53,54,55]. Among these, the iterative collaborative joint inversion strategy is widely used because it can work without changing existing mature software [56,57,58,59]. Additionally, the real-time human-computer interactive inversion process can make better use of geological experts’ understanding, while quickly updating prior information.

We proposed an improved integrated interpretation workflow (Figure 7), aiming to better utilize the comprehensive geophysical data for modeling. Joint inversion of gravity and magnetic profiles involves using mature 2.5D gravity and magnetic anomaly visualization modeling techniques to perform forward and inversion calculations on profile anomalies [60,61,62]. Inversion software used is RGIS 2016 by China Geological Survey [63]. The chosen model is a horizontally finite prism with an arbitrary polygonal cross-section, capable of approximating complex geological bodies. The human-computer interaction inversion work of the gravity and magnetic 2.5D profile is carried out based on prior geological information calibrated by drilling, geological mapping, and geological profile measurement, as well as the geophysical characteristics of the underground 5000 m interpreted by the comprehensive analysis of the reflection seismic profile, the 3D MT inversion results and 3D gravity and magnetic inversion results.

4.1. Integration of Multiple Prior Information

Prior information is diverse and comes in many forms. During the data collection process, it is important to concentrate on materials that either directly provide physical property information or can be transformed into such information. In actual geophysical exploration processes, the collected prior information is often limited and unevenly distributed, resulting in mostly sparse prior information [67]. The priori information can be divided into five categories [68]: the geometric shape and distribution of underground geological bodies, surface rock and mineral sampling, drilling, and geological mapping. Actual physical property parameters can be obtained from these sources. Geological sections and magmatic rocks can be used to interpret the morphological characteristics of hidden magmatic rocks and underground geological bodies, and physical property parameters can be estimated through analysis. Physical property parameters can also be constrained based on large-scale geological blocks.

4.2. Establishment of Initial Model

Based on prior information, an initial geological section is constructed using one of the following methods: Geological experts cut sections on geological maps according to the selected section position to form an initial geological section. They can also assist in field measurements of geological sections and further revise the sections. Comprehensive analysis of existing geological and drilling data is conducted to form a series of geological section maps. Physical property-lithology relationships are then analyzed to divide the mapping modeling geological units and determine the mapping modeling elements. Using physical property data, geological information in geological sections is converted into geophysical models with density and magnetism as initial geological models for modeling.

4.3. Joint Interpretation of Gravity, Magnetic, MT and Seismic Data

The strategy of sequential joint inversion is used to interpret reflection seismic data, followed by MT inversion interpretation, and gravity and magnetic inversion interpretation. Geophysical inversion has multiple solutions [69], and each geophysical exploration method has its own advantages and limitations based on the differences in physical parameters of rocks. Joint inversion can reduce the quantity of “reasonable” result models by comprehensively applying multiple geophysical data in the interpretation process of the same geological body. The obtained results simultaneously fit all geophysical data. Since joint inversion comprehensively uses multiple geophysical methods, it allows each method to fully leverage its advantages while compensating for its limitations, thereby reducing inversion ambiguity and improving the reliability of inversion results [70].

4.4. The 2.5D Interactive Modeling Workflow

The 2.5D gravity-magnetic interactive inversion is a comprehensive geological interpretation process. In the interactive inversion process, 2.5D models should not only fit gravity-magnetic fields but also fully consider reference reflection seismic data, 2D and 3D MT inversions, and 3D gravity-magnetic inversions. The results of each method are given weight to solve different problems according to their advantages.

• Establish the Initial Model

The establishment of the 2.5D initial model was carried out by geological experts who created an initial geological cross-section on the geological map according to the selected cross-section position, which aided in carrying out field-measured geological cross-section geology. The surface was modeled strictly according to the geological units displayed on the geological map, and the borehole location also strictly followed the geological information revealed by the borehole, further correcting the geological cross-section.

• Modeling Unit Division

Modeling unit division was conducted through corresponding analysis of the relationship between physical properties and lithology, merging and dividing the modeling geological units, and determining the mapping modeling elements.

• Single Method Inversion and Interpretation

Single method inversion and interpretation were carried out through 3D inversion of gravity and magnetism, and 3D inversion of MT. Vertical cross-sections were cut out at the main corridor positions (as shown in Figure 1b), and seismic cross-sections were processed and interpreted.

• Correction of Initial Model

The initial model was corrected using 2.5D gravity and magnetic joint inversion software (RGIS 2016). Using MT and seismic results, corrections were made to the thickness of basins and volcanic rocks. This led to the establishment of a deep fault network. The distribution of major stratigraphic interfaces was determined, and gravity and magnetic inversion results were utilized to correct the position of shallow faults and the shape of concealed magmatic bodies.

• Iterative modification

Through full communication with geological and structural experts, the model was corrected to improve geological rationality. New borehole or field observation geological occurrence information was supplemented to further correct the model. During the entire research process, the project team held more than 10 seminars to complete the final geological-geophysical model.

5. Results and Interpretation

5.1. Integrated Profile I

Profile I (Figure 8) is mainly located in the southwest (Figure 1b), and the structural units that pass through from NW to SE are Fanchang Basin, Tongling Uplift, and Nanling Basin. Profile I mainly focuses on the undulation and burial depth of the Fanchang volcanic rock basement, as well as the depth, thickness, and distribution of important strata of volcanic rocks in the basin. Additionally, it focuses on the boundary between the Fanchang volcanic rock basin and the Nanling Basin, as well as the deep and surrounding areas of the Yaojialing Zn-Au deposit.

5.1.1. Fanchang Basin

The Fanchang Basin is a volcanic rock basin characterized by weak magnetism, low density, and low resistivity. Since the Cretaceous period, the tectonic system in eastern China has changed from compression to extension, leading to large-scale magmatic intrusion and volcanic eruption. As a result, volcanic basins such as Luzong, Fanchang, and Ningwu had developed.

The seismic profile (Figure 8e) indicates that the Fanchang basin is shaped like a “dustpan,” with a deep north and shallow south. Within the volcanic rocks, there are almost no continuous reflections, giving them a “transparent” appearance (E1 in Figure 8e). This is due to the complex internal structure of volcanic rocks, small physical property differences, and discontinuous volcanic sedimentation.

The continuous undulating magnetic anomalies at a depth of about 500 m (C1 in Figure 8c) are speculated to be sub-volcanic rock or granite porphyry (F1 in Figure 8f). In the deep part, there is a high-magnetic (C2 in Figure 8c), low-density (B1 in Figure 8b), low-resistivity (D1 in Figure 8d) geological body that is inferred to be a granite basement (F2 in Figure 8f). This basement has a spatial distribution range of 1–18 km and rises to a depth of 1100 m between 8–10 km.

5.1.2. Tongling Uplift

The ZNF bounds the Fanchang basin and Tongling Uplift. This area is characterized by high magnetism, high density, and high resistivity. Scientific deep drilling in the Tongling area showed pink granite below 1775 m, with no penetration beyond 2436.50 m [71]. It is inferred that the high magnetic (C3 in Figure 8c), high resistivity (D2 in Figure 8d), and high-density (B2 in Figure 8b) here indicates the deep granite base (F3 in Figure 8f). The basement’s spatial distribution range is 16–36 km and extends to the deep part of the Nanling Basin.

5.1.3. Nanling Basin

The QHF fault separates the Tongling Uplift from the Nanling Basin. This fault appears as a strong ladder belt on the density section (B3 in Figure 8b), running NE-SW and controlling the western boundary of the Nanling Basin. According to seismic reflection profiles (E2 in Figure 8e), this fault is not only a basin-controlling fault but also a huge thrust fault during the compression period that extends northward or all the way to Ningwu [72].

The Nanling Basin presents no magnetism (C3 in Figure 8c), low density (B4 in Figure 8b), and low resistivity (D3 in Figure 8d), indicating typical sedimentary basin characteristics consistent with the physical characteristics of continental clastic sedimentation. Most of the exposed surface is the K₂ formation with a maximum thickness exceeding 1000 m. The thickness increases gradually from NW to SE and unconformably covers Mesozoic and Paleozoic strata.

5.2. Integrated Profile II

Profile II (Figure 9) passes through the Nanling Basin, Jingtingshan-Liqiao Anticlinorium, Xuancheng Basin, and Magushan Anticlinorium from NW to SE. It is 86 km long and mainly investigates the boundary and basement depth of the Nanling Basin. Additionally, it focuses on the formation mechanism and strata distribution of the Magushan Anticlinorium.

5.2.1. Nanling Basin

The profile represents the structure of the main part of the Nanling Basin. Generally, gravity ladder belts correspond to developed faults, while electrical gradient belts, faulted belts, and distortion belts often indicate the existence of structural elements such as faults or geological body boundaries. At 15 km, the gravity ladder belt (B1 in Figure 9b) and electrical ladder belt appear at the same position (D1 in Figure 9d), which indicates QHF.

The shape of the low-resistivity anomaly (D2 in Figure 9d) clearly depicts the spatial structure of the Nanling Basin (E1 in Figure 9e), which is a “dustpan-shaped” faulted basin with west faulting and east overthrusting. The overlying Quaternary is generally about 10–25 m thick, and the thickness of the Cretaceous gradually increases from SE to NW, reaching up to 3500 m at the thickest.

Below the basin, there are a series of electrical faulted belts (D3 in Figure 9d). Previous seismic exploration showed that the Paleozoic and Triassic strata underlying the Nanling Basin consist of a series of rock slices that thrust northwestward [73]. Therefore, it is inferred that there are a series of faults that thrust northwestward. The boundary fault on the east side of the basin is SXF at 40 km (Figure 9e).

5.2.2. Jingtingshan-Liqiao Anticlinorium

The density section (Figure 9b) shows an overall high gravity (B2 in Figure 9b) caused by the uplift of the basement strata. A local gravity low anomaly (B3 in Figure 9b) indicates a small-scale depression.

The resistivity section (Figure 9d) shows a thin layer of low resistivity in the shallow part (D4 in Figure 9d) and two high resistivity (D5 in Figure 9d) sandwiching a low resistivity in the deep part. The high resistivity is combined with the aeromagnetic results (C1 in Figure 9c), which are inferred to reflect the intrusion of rock bodies (E2 in Figure 9e). The low resistivity reflects the Paleozoic strata and faults.

5.3. Integrated Profile III-1

Profile III-2 (Figure 10) passes through the Nanling Basin, Jingtingshan-Liqiao Anticlinorium, Xuancheng Basin, Magushan Anticlinorium, and the Jiangnan Uplift from NW to SE. Its main focus is on prospecting the properties of the basement, the characteristics of important faults, as well as the surrounding and deep areas of the Magushan deposit.

5.3.1. Jingtingshan-Liqiao Anticlinorium

Most of the surface area of Xinhezhuang is covered by the Quaternary (Figure 10a), while the core of the anticlinorium exposes the S₁g Formation. The wings are composed of the S₁f, S₁m and D Formation. The aeromagnetic anomaly map (Figure 3) presents a complex pattern of positive and negative companions, as well as some small and narrow local anomalies. This indicates the complex underground magmatic activity and the interpenetration and erosion of magmatic rocks.

The resistivity section (Figure 10d) indicates a two-layer structure. The shallow part presents a layered low-resistivity anomaly, while there is a large high-resistivity body in the deep part (D1 in Figure 10d). This body is exposed at a depth of 3000 m and, combined with the high-density (B1 in Figure 10b) presented here, is speculated to be a reflection of the uplift of the Paleozoic strata (F1 in Figure 10f).

5.3.2. Xuancheng Basin

The low-resistivity layer (D2 in Figure 10d) and clear reflection wave (E1 in Figure 10e) characteristics depict the structural features of the basin, which has a west-broken east-superimposed dustpan shape. The reflection seismic profile (Figure 10e) shows that the Kunshan Fault (KSF) controls the NW boundary of the Xuancheng Basin. KSF extends to the deep part, tending to SE, and the most significant reflection wave (E2 in Figure 10e) group on the reflection seismic profile can be seen near 1.2 s to 4 s. The dip angle is steep in the shallow 1 s to 2.5 s section, and slows down from 2.5 s to 4 s.

In the deep part of the basin, there are a series of arc reflections (E3 in Figure 10e) that indicates the deformation structure of the basin basement and the tile shape of reverse thrusting and over thrusting. These strong reflections converge to 42 km in the deep part and suddenly stop. At this point (E4 in Figure 10e), the reflection characteristics are completely different on the NW side (left side of the profile) and southeast side (right side) at a depth of 8 km. At the same time, on the MT inversion profile (Figure 10d), there is a clear low-resistivity zone (D3 in Figure 10d) that cuts through two high-resistivity bodies (D4, D5 in Figure 10d). Based on the regional structural position and previous findings [74], it is speculated that this is a reflection of the Jiangnan fault.

5.3.3. Magushan Anticlinorium

Based on the revealed inverted strata situation of nearby borehole ZK62 and the production state of the exposed rock layers on the surface, it is inferred that the Magushan anticlinorium is an incomplete inverted anticlinorium. The magnetization section (Figure 10c) shows that this is a high magnetic anomaly (C1 in Figure 10c) of a cobblestone-shaped NS distribution, combined with the resistivity profile (Figure 10d) showing that it is a large-scale high-resistivity anomaly (D6 in Figure 10d), reflecting the presence of a granite diorite porphyry body (F2 in Figure 10f). At 52 km, a thin layer of low-resistivity anomaly appears below the high-resistivity anomaly on the surface (D7 in Figure 10d), reflecting the overthrust structure.

The reflection seismic profile (Figure 10e) shows scattered and chaotic reflections (E5 in Figure 10e), some of which face SE, reflecting the squeezing deformation of Paleozoic strata. In the deep part at 52 km, there is a nearly transparent reflection body (E6 in Figure 10e) inclined to the SE. There is also a similar high-resistivity anomaly (D6 in Figure 10d) on the resistivity section, which is speculated to be a reflection of deep magmatic rock invasion. Borehole 4430 reveals the presence of a granite body in the shallow layer. However, from the magnetization section, it is inferred that the deep magmatic rock is not the same nature as the shallow granite body, but rather a syenogranite (F2 in Figure 10f).

5.4. Integrated Profile III-2

The profile III-2 (Figure 11) passes through the Nanling Basin, Jingtingshan-Liqiao Anticlinorium, Xuancheng Basin, and Magushan Anticlinorium from NW to SE. It mainly focuses on the deep and surrounding areas of the Chating Cu-Au deposit, the shape and distribution of the Yaojiata Syenogranite, and the depth and distribution of Permian-Silurian strata.

5.4.1. Jingtingshan-Liqiao Anticlinorium (Chating Deposit)

The northwest side of the Chating deposit (8–18 km) is covered by Quaternary and K₁z formation (Figure 11a). The Chating ore-bearing quartz diorite porphyry has an age of 145–139 Ma, which is roughly consistent with the intrusive rock age of the uplifted area, such as Tongling, Anqing-Guichi, and other districts [75,76].

The high density (B1 in Figure 11b), high magnetic (C1 in Figure 11c), and high resistivity (D1 in Figure 11d) exhibited in the surrounding area of the Chating are reflections of the deep rock formations. The distribution range of the rock body (F1 in Figure 11f) can be delineated by the aeromagnetic anomaly and resistivity section. Based on the statistical characteristics of lithology, it is inferred that the rock body is composed of granite diorite. The highest intrusion of the rock body (F1 in Figure 11f) is located at Shuiyang, with a top interface of about −2.5 km. The top interface gradually decreases southward to the Nanling Basin and northward to the Xuancheng Basin. The resistivity section (Figure 11d) shows a nearly vertical low-resistivity anomaly (D2 in Figure 11d) contained in a large area of high resistivity between 15–18 km, which is consistent with the shape of the Chating ore body hosted in a hidden breccia pipe. This anomaly is reflected on the seismic profile as an upward-wide and downward-narrow tubular transparent reflection zone (E1 in Figure 11e) between 17–18 km.

On the reflection seismic profile (Figure 11e), the reflection characteristics on the southeast side of Chating are more complex, showing a series of arcuate and inclined reflections that reflect the complex deformation structure. The most significant feature is the strong reflection (E2 in Figure 11e) at 18–25 km that is inclined towards the southeast, extending from 1 km to 7 km at a gentle angle. According to the results of Profile I and II, this reflection represents the SXF. The starting and ending points of this reflection are interrupted by transparent reflection areas, reflecting the destruction of later rock bodies on faults. Based on drilling exposure, there is a reversed shape of Triassic strata dipping southeastward here (E3 in Figure 11e), indicating that this area has experienced strong compression deformation and formed tight folds. It is inferred that SXF is an interlayer fault formed during regional compression from SE to NW.

5.4.2. Yaojiata Syenogranite

The Yaojiata syenogranite (F2 in Figure 11f) is extensive and has a deep extension. The rock displays a converging shape inwardly in the deep part, forming a low-value anomaly area on both sides of the high magnetic anomaly (C2 in Figure 11c). The exposed range of the rock generally corresponds to the low density (B2 in Figure 11b). The high-density anomaly on both sides of the rock body corresponds to the uplift of Silurian strata in the core of the anticlinorium.

6. Discussion

6.1. Structure

The district displays a pattern of uplifts and depressions composed of multi-level thrust nappes, folds, and Mesozoic depressions. The structural distribution is mainly oriented NE-SW. The district is characterized by a framework of “two uplifts and two depressions”. Vertically (within 10 km), the district exhibits a three-layer structure. The shallow layer is volcanic rocks, the middle layer is a strongly deformed Paleozoic stratum, and the lower part consists of large-scale magmatic rocks. The faults network developed during three periods of thrusting and folding activities between the three-layer structure creates a channel for magma to invade. Additionally, the detachment and fragmentation zones between each stratum provide corresponding sites for the storage of mineral-containing fluids.

The “two uplifts” are the Jingtingshan-Liqiao Anticlinorium and the Magushan Anticlinorium. The Jingtingshan-Liqiao Anticlinorium is located on the northwest side of the JNF and belongs to the Lower Yangtze stratigraphic zone. The Magushan Anticlinorium is located on the southeast side of the JNF and belongs to the Jiangnan stratigraphic zone. The Paleozoic strata repeat and thicken due to the influence of thrust napping, partially covering the Cretaceous K₁z Formation volcanic rock strata, with a large area of granodiorite at depth. The inverted wing structure forms a series of imbrications and interlayer detachment zones with granite bedrock at depth.

The “two depressions” refer to the Nanling Basin and the Xuancheng Basin, which are relatively stable basins formed during the Cenozoic era. These basins are primarily composed of thick Late Cretaceous sediments. Within the Xuancheng Basin, the maximum thickness of the Cretaceous strata is located in Shuimingqiao, reaching up to 5 km. The Xuancheng Basin is divided into two parts, with Magushan serving as the boundary. The western part of the basin exhibits uneven depth changes and a shallow basin depth. To the east of Magushan, the basin depth is greater and displays relatively shallow characteristics in the north and relatively deep characteristics in the south. This basin can be further divided into multiple local uplifts and depressions, indicating that the Xuancheng Basin has undergone more complex tectonic movements. The overall thickness change of the Nanling Basin is small, with a thicker northwest and thinner southeast. It generally exhibits the characteristics of a dustpan-shaped basin, with a maximum thickness of about 4 km at Huangmu.

6.2. Fault Network

The structural uplifts and depressions within the study area are connected by deep major faults. Integrated geophysical exploration, geological-geophysical inversion reveals the distribution, deep morphology, and properties of the main fault systems.

The fault system is dominated by NE faults on the plane, forming a network of faults that controls the structure of the ore district with NW and EW faults. In the vertical direction (Figure 12), a “dendritic” structure is formed from deep to shallow like Luzong ore district, with a complex interwoven network at the shallow part converging into the deep large crustal fault [21,27]. The intersection of these faults provides a pathway for magma to intrude upward.

• Qingshuihe-Hewan Fault (QHF)

The QHF is both the eastern boundary fault of the Fanchang Basin and the Tongling Uplift, as well as the western boundary fault of the Nanling Basin. It generally trends NE-SW, dips to the SE, with a dip angle of about 40°, and is an early thrust fault that later reversed into a normal fault. Previous deep seismic reflection work suggests that the QHF is a set of crustal-level fault zones, which also serve as the main channel for magma to communicate from deep to the surface [77].

• Sanli-Xihe Fault (SXF)

The SXF is the newly discovered fault in this study, as the SE boundary fault of the Nanling Basin, trending NE. It is a low-angle thrust nappe fault formed by SW-NE thrusting and is concealed beneath the Cretaceous sediments of the Nanling basin. The fault is disrupted by the influence of the east–west ZWF and is greatly affected by deep magmatic intrusion. In areas with higher magmatic intrusion, the fault extends shallower.

• Jiangnan Fault (JNF)

The Jiangnan Deep Fault marks the boundary between the Lower Yangtze and Jiangnan stratigraphic zones [78]. Most scholars believe that it is a large-scale thrust fault zone that has undergone multiple tectonic events [79,80,81]. However, due to the cover, there are few outcrops in the study area and research on the fault is relatively weak.

This study aimed to determine the location and deep extension morphology of the Jiangnan Fault Zone in the study area. The fault generally trends NE-SW from Xuancheng east to Shencun, passing through Nanyi Lake and extending NE. It extends over 8 km deep and exhibits clear signs of multiple activities due to later tectonic movements.

The attitudes of the strata reflections (Figure 12) on both sides of the fault differ significantly, with weak fault plane waves. The dip angle of the deep fault decreases, and the gravity and magnetic anomaly characteristics of the fault zone are apparent, showing NW high and SE low gravity and magnetic field characteristics. It controls the lithology, lithofacies, and other changes of the lower Paleozoic strata in the north and south areas, causing density differences in the lower Paleozoic strata on both sides of the fault.

Previous studies have found that there are multiple faults in the deep Moho surface [27,82]. The results of this exploration reveal that there are physical differences on both sides of the Jiangnan Fault in the upper crust, confirming that the fault is the boundary between the Yangtze Block and the Jiangnan Block, and that the basement on both sides is different. Additionally, the Jiangnan Fault controls nearby magmatic eruptions and intrusive rock activity, such as the Upper Jurassic volcanic rocks that are distributed along the fault in a belt shape. Some intermediate-acidic rock bodies are also distributed along the NW side of the fault.

• Jiulianshan-Liqiao Fault (JLF)

The JLF is the most important thrust nappe interface in the district, extending roughly from Jiulianshan and northward to Xinhezhuang, Chating, and Liqiao, and exhibiting an “S” shaped distribution. The JLF is the southern extension of the Maoshan Fault in Anhui Province [83]. It is generally a low-angle (30°) SE-dipping thrust fault with a large downward extension. On the north side of the fault, K₁z volcanic rocks are exposed, while Paleozoic strata are on the south side, indicating that the Paleozoic strata are covered on top of the Cretaceous volcanic rock strata. The shallow attitude of the fault at Xinhezhuang and Liqiao becomes steeper, about 40°.

• Kunshan Fault (KSF)

The KSF roughly follows the line of Jiulianshan North and Kunshan and acts as the SE boundary fault of the Jingtingshan-Liqiao anticlinorium. It runs parallel to the JLF and is a result of normal faulting after the JLF thrust nappe. The fault is characterized by a high-angle (65°) southeast-dipping normal fault, with the dip angle changing with the trend. The dip angle of the NEE trending section is larger than that of the NE trending section. The fault’s deeper section cuts through the granodiorite bedrock and also influences the intrusive morphology of nearby magmatic rocks.

• Zhouwang Fault (ZWF)

The Zhouwang Fault (ZWF) generally exhibits a near EW trend within the study area, controlling the southern boundary of the Xuancheng Basin. It is characterized by early thrusting and late normal faulting. Within a range of less than 5 km, the fault is manifested as a high-angle normal fault dipping to the NW, but overall, it is a concealed fault. Both reflection seismic and MT surveys show that the ZWF was disrupted by later nappe activity, explaining the lack of surface outcrop evidence during field geological surveys.

• Zhongming-Nanling Fault (ZNF)

The ZNF is the boundary between the Tongling Uplift and the Fanchang Volcanic Basin, trending northwest. There are evident physical differences in the magmatic bedrock on both sides of the fault zone. The intrusion and morphology of the magmatic rocks on the northwest side of this fault are noticeably influenced by it, providing a smooth channel for the ascent and intrusion of magma in the Fanchang Basin.

6.3. Concealed Magmatic Rocks

This study uses physical property statistics and geophysical comprehensive exploration to show that the distribution of magmatic rocks in the ore concentration area has “ a primary magma source spawning multiple subsidiary intrusion “ characteristic. There are significant differences in the lithology of deep hidden magmatic rocks between the uplifted and depressed areas of the ore concentration area, and the intensity and scale of magmatic rock activity show obvious differences and regularity in the region [33]. Ore types and scales change regularly with the intensity and range of magmatic rock activity.

Deep magmatic rocks are found in three areas: the Fanchang-Tongling belt (dominated by diorite, NNE orientation), the Jingtingshan-Liqiao belt (dominated by granodiorite, NE orientation), and the Yaojiata belt (dominated by syenogranite).

• Tongling-Fanchang

Profile I shows that there are more local magnetic anomalies and rapid jumps in the Fanchang Basin on the northwest side (Figure 8c). This corresponds to low density anomalies, with more volcanic and sub-volcanic rocks exposed on the surface. The MT inversion profile (Figure 8d) reveals a geological body with medium to low resistivity at a depth of 1000–1600. On the northwest side of Profile I, a large area of granite is exposed. Based on the magnetic anomaly characteristics and physical properties of the magmatic rock, it is inferred that granite is present on a large scale at depth. The boundary is marked by a ZNF at about 1850. This is a normal fault with a steep southeast dip angle and deep extension. The resistivity section (Figure 8d) shows that granitic magma rises along this fault and dips southeast to the hanging wall of the fault. This indicates that ZNF provides a channel for the rise of granite and volcanic rocks.

Between 1850–3500 (Figure 8), the background values of aeromagnetic anomalies are higher than that of the northwest section, with fewer local magnetic anomalies, slower changes, and wide and gentle anomaly values. This indicates that the deep magmatic rock has a deeper intrusion depth than Fanchang Basin. The deep magmatic rock invades upward into weak zones between stratums, with high and low undulations at the top interface, intruding into strata as rock branches or rock stumps. At the same time, this section corresponds to a wide gravity high anomaly, and the MT inversion profile shows that there is a large range of high-resistivity bodies below a depth of about 300 m. This extends to below 8 km, with conductivity increasing with depth. The characteristics of high magnetic anomaly background, high gravity anomaly, and high resistivity are basically consistent with the physical properties of granodiorite.

The inversion top interface is about 1500 m from the ground surface. According to the range of local magnetic anomaly half-widths, it is judged that the magnetic body is buried deep. This suggests that Tongling’s deep part is occupied by a large magma sea. After reaching a certain height, magma with higher density loses its driving force to rise further and gathers here to form magma sacs to magma chambers. Then it slowly spreads through fractures associated with Indosinian folding structures to strata such as Carboniferous, Permian, Triassic, etc. This provides necessary conditions for alteration and mineralization between magma and carbonate rock strata.

• Xuancheng

The electrical structure of the MT inversion profile on the III-1 section (Figure 10d) indicates a high-resistivity anomaly for the Jingtingshan-Liqiao anticlinorium, with a nearly vertical, elongated elliptical low-resistivity body in the middle. This is consistent with the known location of the Chating deposit, suggesting that the deep part is widely intruded by magma. Based on rock physical property characteristics and regional geological inference, the deep hidden basement is likely granodiorite, with the top interface of the magmatic rock approximately 2000 m from the ground surface.

This depth is deeper than the inferred granodiorite in Tongling’s deep part, as determined by 2.5D gravity-magnetic joint inversion on Profile I (Figure 8). Given that the Jitingshan-Liqiao anticlinorium has exposed granodiorite, granodiorite porphyry, hornblende diorite on the surface, and its geophysical field characteristics are similar to those of Tongling, it is inferred that the deep magmatic activity in Jitingshan-Liqiao anticlinorium is similar to that in Tongling. The deep magmatic rock invades upward into weak zones between stratums, with high and low undulations at the top interface intruding into strata as rock branches or rock stumps of different lithologies.

The Magushan anticlinorium corresponds to a high density and high magnetic anomaly. The resistivity sections (Figure 10d and Figure 11d) show that, from shallow to 7.5 km deep, the Magushan anticlinorium exhibits high-resistivity anomalies that extend to depth, with high-resistivity bodies shrinking to the northwest. The small-scale exposure of granodiorite on the surface and the discovery of copper-molybdenum ore indicates that there is a large magma body in the deep part of Magushan, with the lithology likely being granodiorite.

• Yaojiata

Based on the profiles of Profile III-1 and III-2 (Figure 10 and Figure 11), the Yaojiata area displays obvious low density and high magnetic anomalies, with a good correlation between the anomaly morphology and magmatic rock exposure. The seismic reflection profile indicates that the Yaojiata syenogranite extends northwestward to a maximum burial depth of 6000 m. The MT inversion profile shows a transition from low to high resistivity, with a large area of syenogranite exposed on the surface, which can be compared with the large area of granitic magma exposed in the Fanchang area. Profile II reveals a medium-high magnetic anomaly belt orienting NE near Jiulianshan, with no magmatic rock exposed on the surface, corresponding to a high-density anomaly. The resistivity section shows alternating high and low resistivity anomalies within 5 km, and the deep magmatic rock in Xuancheng is inferred to be granodiorite by 2.5D joint inversion, consistent with the lithology of deep magmatic rocks in the Chating and Tongling areas. The local magnetic anomalies suggest that deep magma intrudes along weak zones to form rock branches penetrating between strata. Due to weaker magmatic activity compared to the Xinhezhuang anticlinorium and Tongling area, the intrusion height of the deep magmatic rock is shallower than both.

7. Conclusions

The ore district presents a tectonic pattern of “two uplifts and two depressions” formed by multi-level thrust nappes, folds, and Mesozoic depressions. They are mainly distributed in the NE-SW direction. The Nanling Basin and Xuancheng Basin are both asymmetric ladle-shaped, and early erupted volcanic rocks were found in the deep part of Nanling Basin, providing direction for further searching for Chating-style deposits in the covered area. In the uplift area, there are three layers of structure vertically (within 10 km), with late erupted volcanic rock strata covering the shallow layer, overlapping tile-like Paleozoic strata that have been thrust and squeezed by reverse thrusting in the middle layer, and large-scale layered magmatic batholith developed in the deep part.

The changes in the tectonic uplift and depression patterns within the study area are connected by deep large faults, which form a “tree branch” fault network that controls the structure with northeastward and east-westward faults, providing a channel for the migration of ore-bearing fluids. The JNF is the most important crustal-level deep large fault in this area. The results of this exploration reveal that there are physical property differences on both sides of the JNF in the upper crust, confirming that the JNF is the boundary between the Yangtze Block and the Jiangnan Block, with different bases on both sides. Additionally, JNF is the main magmatic intrusion channel in this area. JLF is the southern extension of the Maoshan Fault in Anhui Province and is the most important thrust nappe interface in the district. ZWF is shown as a northwestward normal fault in this area, controlling the southern boundary of Xuancheng Basin, and its surface trace was destroyed by later thrusting activities. ZNF is a boundary fault between Fanchang Basin and Tongling Uplift, providing a smooth channel for magmatic rise and emplacement within Fanchang Basin.

The distribution of concealed magmatic rocks in the study area presents a “a primary magma source spawning multiple subsidiary intrusion” characteristic. Within 5 km, there are three levels of rock mass spatial distribution patterns: deep, middle, and shallow; deep magmatic rocks are mainly distributed in three areas: Fanchang-Tongling diorite-dominated NNE-oriented magmatic rock belt, Jingtingshan-Liqiao granodiorite-dominated NE-oriented magmatic rock belt, and Yaojiata nearby syenogranite-dominated magmatic rock belt. Deep layers are mostly large-scale layered batholith, while middle-shallow layers are mostly columnar or mushroom-shaped stocks and stumps that intrude upward into faults and fracture zones.

The integrated geophysical exploration method used in this study finely depicts the district-scale deep structure. Through coupling between fault network-magmatic activity-mineralization space, it provides direction for breakthroughs in mineral exploration. This study provides crucial geophysical information for the next step in constructing a three-dimensional structure of the mining area and explores prospecting techniques suitable for covered areas and deep parts, thereby providing a powerful demonstration for the detection of deep structures in the middle and lower reaches of the Yangtze River.