Integrated Geophysical and Geochemical Surveys for Assessing Mineral Potential in the Xintianling Tungsten Deposit, Nanling Range, South China

Abstract

The Xintianling deposit, a skarn-type tungsten mineralization system in the Nanling Range of South China, presents significant challenges in identifying new high-grade orebodies. This study employs an integrated approach, combining the opposing-coil transient electromagnetic (OCTEM) method with geochemical exploration, to delineate and evaluate concealed mineralization within and beyond the known mining area. High-precision geophysical surveys revealed low-resistivity anomalies along the contact zone between Jurassic granite and the Carboniferous Shidengzi Formation limestone. Integration of these anomalies with geochemical element associations (W-Sn-Fe-Bi and Cu-Mo-As) highlights signatures indicative of tungsten mineralization. The results demonstrate that skarn-type orebodies in the mining area are primarily controlled by the axial planes of N–S-striking anticlines and associated secondary folds, with thick, large orebodies preferentially forming in depressions along the granite roof. Comprehensive analysis of the geophysical and geochemical data identified 15 low-resistivity anomalies in the Shanglongshan–Huanggualing target area, of which 14 are interpreted as potential skarn-type mineralized bodies, thereby delineating three potential exploration targets. This integrated methodology establishes a robust scientific foundation for deep and peripheral prospecting in the mining area and provides methodological guidance for exploring similar skarn-type tungsten deposits.

1. Introduction

The Nanling Range in South China hosts some of the world’s largest tungsten deposits, which formed through complex Mesozoic tectono-magmatic processes that facilitated extensive granitic intrusions and associated mineralization. Among these, the Xintianling tungsten deposit represents a skarn-type system, with proven reserves exceeding 0.33 Mt of WO₃ at an average grade of approximately 0.36% [1,2]. Despite its significance, ongoing mining operations have faced challenges, including orebody instability and high dilution rates, highlighting the need for advancements in identifying thick, high-grade orebodies at depth and along the deposit’s periphery.

To address these challenges, advanced geophysical and geochemical techniques are essential for detecting concealed mineralization [3,4,5]. The opposing-coil transient electromagnetic (OCTEM) method is widely used in engineering surveys [6,7], geohazard prevention [8], and mineral exploration [9,10], where it plays a significant role. The opposing-coil transient electromagnetic (OCTEM) method excels in delineating magmatic rock–strata contact zones and identifying low-resistivity anomalies associated with tungsten-bearing skarn orebodies, thereby guiding deep exploration [11,12]. Complementing this approach, geochemical exploration integrates field observations of lithological features, measurements of geological elements, and rock sampling to reveal trace element signatures in faulted and altered rocks. Statistical analysis of these data extracts geochemical indicators, which are visualized in anomaly maps to inform prospecting [13,14,15].

This study synthesizes metallogenic conditions, ore-controlling factors, and mineralization patterns in the Xintianling mining area with geophysical and geochemical results to develop a comprehensive prospecting model. By correlating anomalies with known deep orebodies, including those confirmed through engineering, we predict locations of concealed orebodies, delineate potential exploration targets, and recommend drilling strategies. This approach not only enhances exploration efficiency in the Xintianling periphery but also provides a transferable framework for similar skarn-type deposits worldwide.

2. Geological Setting

2.1. Regional Geology

The South China Block was formed through the Neoproterozoic collision and amalgamation of the Cathaysia and Yangtze blocks (Figure 1a) [16,17]. During the Mesozoic era, tectonic processes—including the closure of the Paleo-Tethys Ocean, subduction of the Paleo-Pacific Plate, and intracontinental extension—triggered extensive granitic magmatism and the formation of numerous tungsten–tin deposits [1,18,19]. The Nanling Range, located in the central part of the South China Block, is a major global tungsten producer, contributing over 60% of worldwide output and approximately 90% of China’s production [20,21]. The basement consists of low-grade metamorphic Precambrian rocks, overlain by Paleozoic and Mesozoic sedimentary strata [22,23]. Prominent deposits in the region, such as Furong, Huangshaping, Shizhuyuan, Xianghualing, and Yaogangxian, are genetically associated with Mesozoic granites [24,25,26,27,28,29].

The Xintianling deposit is situated in the central Nanling Range of southern Hunan Province, near the northeast-striking Chaling–Chenzhou–Lingwu fault zone (Figure 1b,c) [30]. Exposed strata include Proterozoic to Cambrian schists and metamorphic sandstones, Devonian to Triassic limestones and shales, and Upper Devonian to Tertiary sandstones and siltstones (Figure 1c). The Early Paleozoic intracontinental orogeny generated north-northwest-trending folds and faults under northwest–southeast compression [31,32]. In the Qianlishan–Qitianling area, Triassic to Jurassic granites, Jurassic granitic porphyries, and minor Cretaceous granodiorites are prevalent (Figure 1c) [33]. Tungsten–tin polymetallic mineralization, linked to Mesozoic felsic magmatism, occurred in three principal episodes: Late Triassic (ca. 230–210 Ma), Middle to Late Jurassic (ca. 165–150 Ma), and Early to Middle Cretaceous (ca. 130–80 Ma) [1,34].

Figure 1. (a) Tectonic outline of China. (b) Geologic map of Mesozoic granitoids and tungsten–tin deposits in South China, modified after [2]. (c) Sketch map of granites and tungsten–tin deposits in Southeast Hunan, modified after [35]. SCB = South China Block, CCL fault = Chaling–Chenzhou–Lingwu fault zone.

Figure 1. (a) Tectonic outline of China. (b) Geologic map of Mesozoic granitoids and tungsten–tin deposits in South China, modified after [2]. (c) Sketch map of granites and tungsten–tin deposits in Southeast Hunan, modified after [35]. SCB = South China Block, CCL fault = Chaling–Chenzhou–Lingwu fault zone.

2.2. Deposit Geology

The Xintianling tungsten deposit is located in the northern sector of the Qitianling pluton (Figure 1c). Exposed strata primarily comprise Carboniferous carbonate rocks, with localized Quaternary sediments. These include the Shidengzi Formation (thick-bedded limestone, serving as the primary ore host) [36], Ceshui Formation, Zimenqiao Formation, and Hutian Group (Figure 2). Intense tectonic activity has produced numerous folds and faults, with the northeast-striking Danfengping composite anticline exerting structural control on magma emplacement and mineralization [37]. Predominant faults trend north–south and northeast–southwest, potentially acting as conduits for granitic magma (Figure 2) [38]. Granitic magmatism unfolded in three stages: (1) medium- to coarse-grained porphyritic biotite granite (ca. 165–164 Ma), (2) fine-grained porphyritic biotite granite (ca. 164–157 Ma), and (3) granitic porphyry (ca. 149–143 Ma) [39,40].

The deposit hosts two main orebody types: skarn (predominant) and quartz-vein (subordinate). Skarn orebodies are mainly situated at the contact between porphyritic biotite granite and the Lower Carboniferous Shidengzi Formation limestone, with over 80 bodies identified in endo- and exo-contact zones (endo-contact zones accounting for approximately 90%) [41]. These orebodies extend 100–1300 m in length and 1.00–1.67 m in thickness, exhibiting stratiform, lenticular, or orbicular morphologies, with north-northwest–south-southeast strikes and dips of 15–80° [41,42]. Principal ore minerals are scheelite and molybdenite, accompanied by gangue minerals such as garnet, wollastonite, diopside, tremolite, chlorite, epidote, quartz, fluorite, and calcite. Quartz-vein orebodies, recently discovered, are associated with skarn zones or north–south-striking faults, with thicknesses ranging from 0.05 to 1.5 m. Scheelite is the dominant ore mineral, with quartz, apatite, fluorite, and calcite as gangue. Hydrothermal alteration types include skarnization, greisenization, marbleization, potassic feldspathization, silicification, sericitization, chloritization, epidotization, and hornfelsification.

3. Materials and Methods

Geophysical exploration utilized the OCTEM method, while geochemical investigations employed geochemistry. These approaches were selected based on the project’s objectives and the local geological and geophysical characteristics.

The OCTEM method employs an ungrounded transmitter coil to generate primary pulsed magnetic fields, with the receiver coil measuring secondary eddy-current fields during inter-pulse intervals to infer subsurface resistivity (Figure 3). Its key advantages include high operational efficiency through exclusive secondary-field measurements, enhanced sensitivity to low-resistivity targets within resistive host rocks, optimal target coupling via a coincident-loop configuration that produces strong and geometrically simple anomalies, and integrated profiling and sounding capabilities.

This survey applied the HPTEM-28 system, jointly developed by Central South University and Hunan 5D Geotechnology Co., Ltd. (Changsha, China) [11]. The system mitigates inductive coupling using OCTEM technology, improves lateral resolution through dual-center coupling, and features standardized micro-coil dual magnetic sources, high-sensitivity sensors, 24-bit data acquisition, and high-density sampling. Essential components include the TEM antenna, main control unit, connection cables, a 12 V DC power supply, and an operational PC.

Data processing for the OCTEM surveys was performed using HPTEMDataProcess software HPTEM-28 [11], with the workflow outlined in Figure 4. The procedure included the following steps: (1) Data editing, involving the removal of outliers and noise filtering from raw field data; (2) Data preprocessing, including terrain correction and application of smoothing filters; (3) Qualitative analysis, encompassing parameter evaluation, decay curve classification, and apparent resistivity profiling; (4) Quantitative analysis, comprising forward modeling, iterative model fitting, and quasi-2D inversion; (5) Integrated interpretation, correlating qualitative and quantitative results with established geological constraints.

The integrated interpretation of inverted resistivity sections and geological mapping data—prioritizing geophysical evidence while incorporating geological constraints—focused on: (1) background resistivity values; (2) morphology of low-resistivity anomalies; and (3) anomaly magnitude and gradients to delineate granite contacts and mineralized skarn zones. Granite contacts were identified based on: (1) absolute resistivity values; (2) anomaly geometry; and (3) known geological information. Mineralized skarn zones were inferred from: (1) resistivity curve distortions near contacts and (2) relative low-resistivity zones (<600 Ω·m). Site-specific interpretation criteria were as follows:

(1)

In conjunction with geological data, the upper zone with resistivity of approximately 700 Ω·m is interpreted as Carboniferous limestone, whereas shallow zones with resistivity <300 Ω·m are attributed to Quaternary overburden.

(2)

Zones with uniform resistivity contours ranging from 700–1200 Ω·m are identified as granite.

(3)

Low-resistivity anomalies (<600 Ω·m) at the contact between deep granite and country rocks, along with areas showing resistivity contour depressions or disturbances (relatively lower resistivity), are interpreted as (mineralized) skarn zones.

Geochemistry assesses the mobilization, reaction, and precipitation of ore-forming fluids within fault deformation zones, which is essential for exploring concealed deposits. This method emphasizes geochemical halos associated with faults and the spatial distribution of ore-forming elements, where intensified migration in mineralized zones enables the detection of subtle surface anomalies indicative of deeper orebodies. In this study, the concentrations of 61 elements were determined using four-acid digestion followed by inductively coupled plasma optical emission spectrometry (ICP-OES). Samples were placed in Teflon tubes and digested in three sequential stages with nitric acid, hydrochloric acid, perchloric acid, and hydrofluoric acid. The initial pre-oxidation step with nitric and perchloric acids converted arsenic to its pentavalent form to reduce volatility. Hydrofluoric acid was then added, and the mixture was heated on an electric hot plate; this controlled heating minimized rapid volatilization of hydrofluoric acid during the concurrent digestion of silicates and aluminosilicates. The solution was evaporated to near dryness to eliminate residual hydrofluoric acid, diluted with hydrochloric acid, adjusted to a fixed volume, and analyzed via ICP-OES. Corrections were applied for spectral interferences among elements to obtain accurate results. The analyzed elements included W, Sn, Fe, Bi, Cu, Mo, As, Ce, Ag, and 52 others. Analyses were performed using an Agilent 5110 instrument at Guangzhou ALS Lab Co., Ltd. (Guangzhou, China). Analytical precision was maintained with a relative deviation (RD) of less than 10%. Quality control involved inserting blank samples, duplicates, and standard reference materials into each batch.

The multi-element analytical results were processed using Surfer and SPSS software (SPSS 19). The detailed procedure is as follows:

(1)

Data Import: The multi-element analytical data were imported into Surfer from Excel files. Data columns and grid geometry were defined, with a grid node spacing of 500 × 500 m.

(2)

Data Processing: The workflow encompassed data cleaning, interpolation (using the Kriging method with a linear variogram model in this study), smoothing, and filtering. R-mode cluster analysis and factor analysis were conducted using SPSS.

(3)

Visualization Analysis: Following interpolation, contour maps were generated based on the numerical distributions. Contour intervals were optimized according to element abundances to effectively illustrate the spatial distribution and characteristics of the geochemical data.

(4)

Result Export: Upon completion of processing and analysis, the results were exported as images. This study focused on contour maps for 19 selected elements: As, Be, Bi, Ce, Fe, Li, Mn, Mo, Nb, P, Pb, Sn, Sr, Ti, V, W, Zn, Mg, and Cu.

Three east–west-oriented survey lines (L1, L2, L3), totaling 2593 m in length, were established with a station spacing of 20 m and a line spacing of 500 m. The lines were strategically positioned to perpendicularly cross the inferred subsurface extension of the mineralized contact zone between the Carboniferous limestone and the concealed granite intrusion, which is the primary target for skarn-type mineralization. Minor adjustments to station positions were necessary due to the mountainous terrain interspersed with localized farmland. All stations were precisely georeferenced using real-time kinematic (RTK) GPS (Figure 4). The samples were taken from unweathered surfaces at a typical depth of 20–50 cm below the soil layer to avoid surficial alterations.

4. Results and Discussion

4.1. Geophysical Data Processing and Interpretation

4.1.1. Geophysical Characteristics of the Study Area

Integrated geological surveys and field geophysical data delineate the following lithological units within the OCTEM survey area: Carboniferous limestone, Quaternary residual-slope deposits (comprising clay, sand, and gravel), granite, and skarn. Interpretation of the OCTEM data relies on absolute resistivity values and their spatial variations. Based on existing geological data and measured resistivity values of various rock and ore samples from the area, and using the absolute resistivity values and their relative variation trends from the opposing coils transient electromagnetic (TEM) profiles as the primary criteria, the following interpretations were made: Regions with resistivity greater than 700 Ω·m in the upper section were interpreted as Carboniferous limestone strata, while the lower section was interpreted as granite. Shallow regions with resistivity less than 300 Ω·m were interpreted as Quaternary overburden. Deep regions with uniformly distributed resistivity contours in the 600–800 Ω·m range were interpreted as granite. Within the deep granite, regions with resistivity less than 600 Ω·m, as well as zones exhibiting contour depressions or disturbances (indicating relative low-resistivity anomalies), were interpreted as mineralized skarn zones [11,12].

In summary, target mineralization primarily occurs within skarn zones, which display distinct physical property contrasts with adjacent limestone and granite. Mineralized skarn typically exhibits intermediate resistivity values, transitional between high-resistivity limestone and moderate-resistivity granite, often accompanied by characteristic resistivity contour patterns. These contrasts establish a robust geophysical basis for OCTEM surveys.

4.1.2. Data Interpretation and Analysis

Line L1, extending 800 m east–west, is located on the southern flank of the Xintianling mining area. The survey traverses mountainous terrain with dense vegetation, with smaller station numbers to the east and larger numbers to the west.

Figure 5 illustrates the inverted resistivity section from the OCTEM survey for Line L1. The upper section with resistivity values below 300 Ω·m is interpreted as Quaternary (water-bearing) overburden. A distinct low-resistivity anomaly (0–158 m along the line; elevation 430.5–458.5 m) exhibits clustered characteristics and is interpreted as a water-filled karst cavity, based on integrated geological data. Zones with approximately 700 Ω·m resistivity in the shallow western and eastern sections, extending to depth, are identified as Carboniferous limestone formations displaying a synclinal structure. The lower resistivity zone (~700–1200 Ω·m) is interpreted as granitic intrusion. The limestone–granite contact, marked by a pink line in the figure, descends in elevation from 610 m (west) to 100 m (east), indicating an eastward-dipping discordant intrusive pattern. Four prominent low-resistivity anomalies are identified: L1-1 (400–450 m along the line, elevation 410–450 m); L1-2 (160–300 m along the line, elevation 360–400 m); L1-3 (10–300 m along the line, elevation 260–325 m); and L1-4 (90–310 m along the line, elevation 150–225 m). These anomalies, characterized by clustered low-resistivity patterns or contoured depressions/disturbances, are interpreted as potential skarn-type mineralization zones. Their synclinal, multilayered configuration resembles known orebodies in the district. Additionally, a continuous low-resistivity depression between 150–550 m along the line is interpreted as a fault zone, supported by geological correlations.

Line L2, totaling 993 m in length, is situated on the southern side of the Xintianling mining area. This east–west-trending survey follows a mountain gully in rugged terrain, with smaller station numbers to the east and larger numbers to the west. The area features dense vegetation and flowing water in the gully.

Figure 6 presents the inverted resistivity section from the OCTEM survey for Line L2. The upper section with resistivity values below 300 Ω·m is interpreted as Quaternary (water-bearing) overburden. Shallow western and eastern sections extending to depth, with resistivity around 700 Ω·m, are identified as Carboniferous limestone formations exhibiting an overall synclinal structure. The lower resistivity zone (~700–1200 Ω·m) is interpreted as granite. The limestone–granite contact (marked by a pink line in the figure) descends from 625 m (west) to 180 m (east), reflecting an eastward-dipping, discordant, layered intrusive pattern. Low-resistivity anomalies, interpreted as potential skarn mineralization, include: L2-1 (710–800 m along the line, elevation 410–450 m); L2-2 (360–470 m along the line, elevation 400–425 m); L2-3 (305–425 m along the line, elevation 280–310 m); L2-4 (150–275 m along the line, elevation 255–290 m); and L2-5 (0–130 m along the line, elevation 210–275 m). These zones display clustered low-resistivity anomalies or contour depressions/disturbances, consistent with skarn-type (potentially mineralized) bodies in a synclinal configuration. A continuous low-resistivity depression between 425–725 m along the line is inferred to represent a fault zone, based on geological correlations.

Line L3, with a total length of 800 m, is located on the southern side of the Xintianling mining area. This east–west-trending survey has smaller station numbers on the eastern end and larger numbers on the western end, traversing mountainous terrain with dense vegetation.

Figure 7 shows the inverted resistivity section from the OCTEM survey for Line L3. Upper zones with resistivity values <300 Ω·m are interpreted as Quaternary (water-bearing) overburden. Domains with 700 Ω·m resistivity at shallow depths in the western section and extending from shallow to deeper levels in the eastern section are inferred to represent Carboniferous limestone formations, collectively forming a synclinal structure. Areas with resistivity values of ~700–1200 Ω·m in both shallow and deeper sections of the western and eastern parts are attributed to granite, with the 0–300 m along-line segment (elevation 250–450 m) specifically interpreted as layered granite intruding northward from the southern margin. The granite–limestone contact descends from 640 m elevation (west) to 140 m (east), marked by the pink line in the figure, indicating an east-dipping discordant layered intrusion geometry. Localized low-resistivity anomalies (appearing as clustered zones or contour depressions/distortions) are identified at: L3-1 (475–550 m along the line, elevation 375–405 m); L3-2 (355–455 m along the line, elevation 210–350 m); L3-3 (160–325 m along the line, elevation 200–245 m); L3-4 (0–150 m along the line, elevation 155–225 m); and L3-5 (10–125 m along the line, elevation 280–300 m). Integrated with geological data, these anomalies are interpreted as skarn zones (potentially mineralized), forming a syncline-shaped orebody.

4.2. Geochemical Data Interpretation and Analysis

High-temperature geochemical fields are primarily characterized by W, Sn, Mo, Bi, Fe, Co, Ni, and Ta, which generally align spatially with granite distributions [43]. Intermediate-temperature geochemical fields are defined by Cu, Pb, Zn, and Ag, typically confined to contact zones between granite and country rocks [13]. Structures, particularly faults, exert substantial control on the superimposed distribution of intermediate-temperature elemental assemblages, indicating that mineralization is influenced not only by magmatic activity but also predominantly by fault structures [43]. Consequently, prospecting along ore-controlling faults for related metallic deposits has significant typomorphic value. Low-temperature geochemical fields are marked by As and Sb, controlled by a combination of fault structures, specific stratigraphic units, and late-stage minor magmatic intrusions, which serve as key indicators for mineral exploration [13,43].

Statistical analysis of geochemical parameters was performed for the Shanglongshan–Huanggualing target area. These parameters, including background mean, standard deviation, coefficient of variation, enrichment coefficient, and median, are summarized in

Table 1. Statistical parameters and characteristics of key mineralization-controlling factors in the Shanglongshan–Huanggualing target area (units in ppm).

Element	Maximum	Median	Mean	Standard Deviation	National Background Value	Coefficient of Variation	Enrichment Factor
Ag	1.90	0.50	0.51	0.13	0.08	0.26	6.41
Al	13.95	4.91	4.90	4.03	8.13	0.82	0.60
As	638.00	11.00	34.80	89.87	1.50	2.58	23.20
Ba	950.00	155.00	197.31	196.27	500.00	0.99	0.39
Be	9.50	1.55	2.08	1.98	2.60	0.95	0.80
Bi	9.00	2.00	2.45	1.17	0.48	0.48	5.11
Ca	37.10	0.03	2.25	8.29	3.63	3.68	0.62
Cd	15.50	0.50	0.66	1.44	0.11	2.20	5.97
Ce	230.00	50.00	78.70	42.71	68.00	0.54	1.16
Co	10.00	1.00	1.74	1.57	12.33	0.90	0.14
Cr	241.00	31.00	49.10	41.92	100.00	0.85	0.49
Cu	552.00	3.00	11.72	53.29	21.57	4.55	0.54
Fe	50.00	1.34	3.13	7.00	5.00	2.23	0.63
Ga	40.00	10.00	16.67	8.16	18.00	0.49	0.93
K	5.75	0.79	1.43	1.47	2.59	1.03	0.55
La	120.00	20.00	31.57	27.02	32.00	0.86	0.99
Li	150.00	20.00	34.91	32.73	20.00	0.94	1.75
Mg	12.75	0.08	0.27	1.23	2.09	4.49	0.13
Mn	422.00	67.00	105.81	96.11	678.47	0.91	0.16
Mo	56.00	2.00	2.89	5.36	1.23	1.85	2.35
Na	2.17	0.04	0.20	0.49	2.83	2.42	0.07
Nb	43.00	9.00	14.07	10.62	20.00	0.75	0.70
Ni	65.00	3.00	8.03	11.24	80.00	1.40	0.10
P	910.00	70.00	119.72	149.91	1000.00	1.25	0.12
Pb	2570.00	8.00	40.94	245.68	25.96	6.00	1.58
S	0.22	0.01	0.02	0.03	0.05	1.46	0.42
Sb	13.00	5.00	5.08	0.77	0.74	0.15	6.87
Sc	24.00	4.00	7.41	7.12	16.00	0.96	0.46
Sn	40.00	10.00	10.93	4.20	3.43	0.38	3.19
Sr	920.00	21.00	64.31	141.90	370.00	2.21	0.17
Ta	30.00	10.00	10.46	2.85	2.00	0.27	5.23
Th	80.00	20.00	27.87	17.80	11.92	0.64	2.34
Ti	0.87	0.15	0.24	0.25	0.44	1.05	0.54
U	110.00	10.00	11.30	9.73	2.02	0.86	5.59
V	285.00	19.00	50.56	55.64	79.62	1.10	0.63
W	170.00	10.00	11.67	15.37	0.77	1.32	15.15
Zn	5400.00	11.00	69.74	515.68	1.43	7.39	48.77

. Relative to national average values, the mean concentrations of W, Zn, and As are markedly elevated, signifying a regional high-background field. Similarly, Ag, Bi, Cd, Mo, Nb, Ta, and Li exceed national averages. In contrast, Co, Mg, Mn, and Ti show lower mean values, indicative of a regional low-background field. Elements such as Ce, Ga, and La align with national averages, reflecting a normal regional background. The elevated background levels of W, Zn, As, Ag, Bi, Cd, Mo, Nb, Ta, and Li demonstrate pronounced enrichment, highlighting their roles as critical regional metallogenic factors and prospecting indicators.

Analysis of variation coefficients for ore-forming elements reveals that Cu, Mg, Pb, and Zn are the most mobile in this region, suggesting that their local anomalies can function as effective prospecting indicators. The mean-to-median ratios indicate that mean values exceed medians for nearly all elements, particularly for mesothermal ore-forming elements (Cu, Pb, Zn). This pattern implies that the surface environment is dominated by a mesothermal regime, whereas deeper zones may harbor potential for hypothermal elements (e.g., W). Overall, the integrated interpretation highlights substantial exploration potential for high-temperature mineralization in the study area.

To clarify element association relationships and their correlation strengths, and to comprehensively understand metallogenic enrichment patterns, R-mode cluster analysis was applied to all analytical data based on element correlation coefficients. The results identify W-related associations as W-Sn and W-Sn-Fe-Bi, with Cu-Mo-As serving as potential anomaly indicators (Figure 8). Stable element assemblages include W-Sn, Fe-Bi, Cu-Mo-As, Na-Zn-P, and Be-Nb-Th, with most elements showing strong correlation coefficients (typically >1). The superimposed geochemical field characteristics of these metallogenic elements suggest a complex multi-component system with diverse mineralization types in the region. Accordingly, the primary indicator element associations in the Shanglongshan–Huanggualing target area are W-Sn-Fe-Bi (Figure 9).

Based on anomalies in the W-Sn-Fe-Bi associations, combined with other elemental anomalies, three prospective targets were delineated in the Shanglongshan–Huanggualing area, prioritized as follows: (1) 150–190 m along Line L3, (2) 540–700 m along Line L3, and (3) 700–800 m along Line L2. Notably, the 150–190 m segment of Line L3 exhibits surface alteration traces, rendering it a particularly favorable site for tungsten orebody exploration.

4.3. Integrated Analysis of Geophysical and Geochemical Data and Metallogenic Prediction

Integrated analysis of the three OCTEM survey lines, geochemical exploration data, and geological investigations in the Shanglongshan–Huanggualing target area revealed 15 geophysical anomalies across the profiles. Among these, a clustered low-resistivity anomaly at elevations 430.5–458.5 m (Line L1, 0–158 m) was interpreted as a water-bearing karst cave, constrained by geological evidence. The remaining 14 anomalies, situated at the contact zone between granite and Carboniferous limestone, are inferred to represent potential (mineralized) skarn bodies.

These deep low-resistivity anomalies are primarily concentrated in the southeastern portion of the survey area, with elevations ranging from 155 to 450 m. They indicate a relatively continuous, synclinal, multilayered (mineralized) skarn orebody, with marked stratification evident along the northern Line L1. Based on anomalies in the W-Sn-Fe-Bi and Cu-Mo-As elemental associations, supplemented by other geochemical features, three potential targets were delineated, representing favorable zones for further exploration in the peripheral area (Figure 10).

5. Conclusions

(1)

The Xintianling tungsten deposit is primarily characterized by skarn-type mineralization, structurally controlled by the north–south-trending Danfengping composite anticline and its subsidiary folds. The granite–limestone contact zone represents the most favorable environment for ore formation, with thicker and higher-grade orebodies preferentially developing in depression zones along the pluton roof.

(2)

The opposing-coil transient electromagnetic (OCTEM) method effectively delineated low-resistivity anomalies at the contact zone, exhibiting strong spatial correlation with geochemical anomalies (W-Sn-Fe-Bi and Cu-Mo-As associations). This integrated geophysical–geochemical approach successfully mapped the spatial distribution of mineralization.

(3)

Integrated interpretation identified 15 low-resistivity anomalies, of which 14 are interpreted as prospective mineralization zones. Three potential exploration targets were delineated in the Shanglongshan–Huanggualing sector, offering clear guidance for peripheral prospecting efforts.

(4)

This study demonstrates that the combination of OCTEM and geochemistry provides an effective methodology for exploring concealed skarn-type tungsten deposits, with broad applicability to similar geological settings in the Nanling Range and beyond.