Geophysical Characteristics of Low-Sulfide Epithermal Gold Mineralized Bodies—A Case Study of the Alinghe Mining Area on the South Bank of the Heilong River in China

Abstract

The inability of geophysical methods to directly detect gold ore bodies remains a hot topic in the realm of gold geophysical exploration. Through the analysis of high-precision magnetic method, resistivity and induced polarization methods with the gradient arrays, electrical resistivity tomography, and well logging, combined with the discovery of gold mineralization in exploration trenches and boreholes, it has been found that gold mineralization can be classified into two types: terminal and channel. The terminal-type gold mineralization is marked by a buried depth of less than 30 m, accompanied by varying degrees of silicification and pyritization. In contrast, the channel-type of gold mineralization is buried deeper than 30 m and occurs within structural fractures or volcanic breccia mineralized alteration zones. The resistivity difference constitutes a significant geophysical indicator differentiating these two types of gold mineralization. Both types of gold mineralization are located adjacent to IP anomalies, potentially suggesting characteristics of low-sulfur gold mineralization. After comparing several globally typical epithermal gold deposits, we conclude that the findings presented in this paper encapsulate the geophysical traits of an un-eroded, low-sulfidation epithermal gold deposit. These insights offer a valuable reference for the direct detection of similar gold orebodies using geophysical methods.

1. Introduction

The epithermal gold deposits generally have a metallogenic depth of less than 1–2 km and a metallogenic temperature typically lower than 300 °C. Genetically, they are related to shallow and ultra-shallow intrusive activities as well as terrestrial volcanic and subvolcanic activities. They often develop around multi-stage intruded intermediate and acidic rock masses [1] and can be part of the porphyry metallogenic system [2]. Epithermal gold deposits can be categorized into low-sulfidation, intermediate-sulfidation, and high-sulfidation types. The low-sulfidation type pertains to the near-neutral metallogenic fluid, in contrast to the relatively acidic high-sulfidation type. Globally, epithermal gold deposits are mainly located in the three giant metallogenic domains, namely, the circum-Pacific Metallogenic Belt, the Tethys Metallogenic Belt, and the Laurussia Metallogenic Belt, and possess significant research and economic value [3,4,5].

The genesis of epithermal gold deposits is described as follows: The ore-forming fluids dissolved from porphyry bodies migrate upward along faults or rocks with good permeability, mix with meteoric water, and are replaced and filled in suitable underground metallogenic spaces to form gold deposits. The hydrothermal alteration characteristics include porous silicification, sericite-illite-montmorillonitization, etc., as well as alterations such as alunite and kaolinite [6,7,8,9]. The industrial cut-off grade of gold orebodies is typically within the range of 0.3 to 0.6 g/t [10]. Due to the extremely low gold content, the variance in gold element content does not induce physical parameter differences in rocks and ores, nor is it sufficient to generate conspicuous geophysical anomalies. Hence, direct geophysical exploration of gold orebodies is not feasible. The mineralization process of gold orebodies is significantly controlled by specific geological conditions and the physical-chemical alterations that gold-bearing hydrothermal fluids exert on geological bodies during their migration. This constitutes the main entry point for geophysical exploration of such deposits. The hydrothermal alteration caused by low-temperature epithermal fluids leads to a reduction in the magnetic susceptibility of rocks. Silicification can notably increase the density and resistivity of porous volcanic rocks. Argillization can decrease the resistivity of most volcanic rocks by one to two orders of magnitude. Sulfides (predominantly pyrite) feature low resistivity and high polarizability (chargeability). Therefore, geophysical responses related to mineralization, such as surrounding rocks, faults, alterations, and associated minerals, can be investigated [11,12,13,14,15,16,17]. The electrical resistivity tomography (ERT) method has certain effects in detecting shallow gold mineralized bodies, ore-controlling and ore-hosting structures [18,19]. Currently, certain information regarding epithermal gold deposits can be obtained through geophysical methods to support the optimization of target areas and the deployment of well locations [14,20,21,22]. However, systematic studies from surface geophysics to in-well geophysics have rarely been published, and the geophysical characteristics of low-sulfide epithermal gold orebodies have not been reported.

The Alinghe Mine Area is located on the south bank of the Heilong River in Heilongjiang province, China. It lies within the Duobaoshan–Heihe Cu-Mo-Au-Fe-Zn metallogenic belt. The main types of deposits within the metallogenic belt are porphyry copper deposits and epithermal gold deposits, making it an excellent area for studying epithermal polymetallic deposits [23,24,25,26]. The study area has undergone a systematic ground geophysical exploration. The trenches and verification drill holes, one of which performed logging work at a steep angle, have discovered multiple gold mineralized bodies, providing comprehensive data on the geophysical characteristics of low-sulfidation epithermal gold deposits. This study is the first to divide low-sulfide epithermal gold mineralized bodies into two types based on the combination of polarizability, resistivity, and magnetization anomalies and their geological features. This paper systematically analyzed the geophysical and geological characteristics of the surrounding rocks, hydrothermal alteration zones, and gold mineralized bodies, providing useful exploration for directly using geophysical methods to detect such gold deposits.

2. Geological and Geophysical Characteristics of Alinghe Mining Area

2.1. Geological Features

The Alinghe Mining Area is located on the south bank of the Heilong River in China, also known as the Amur River in Russia. Its tectonic location is in the eastern part of the Xingmeng-Tianshan orogenic system. The Nen River-Balihan fault, which is the separation fault of the Songliao Basin or the west-margin basin control fault, and the Hegenshan-Heihe suture Belt (HHS), which is the convergence belt of the Xing’an Block (XB) and Songnen-Xilinhot Block (SXB), meet in the northwest of the study area (Figure 1) [27,28]. The Duobaoshan–Heihe metallogenic belt is an area with a long metallogenic span and complex mineralization processes, forming the largest porphyry Cu-Mo-Au metallogenic system in Northeast China [29]. In the Alinghe Mining Area, the Lower Cretaceous is mainly composed of the Longjiang Formation (K₁l) andesite and andesitic volcanic breccia, and the Lower Cretaceous Guanghua Formation (K₁gn) rhyolite and rhyitic volcanic breccia. As a part of the Duobaoshan–Heihe metallogenic belt, mineralization belongs to the Late Jurassic-Early Cretaceous volcanic-intrusive magmatic hydrothermal Au-Ag metallogenic series [30].

2.2. Geophysical Characteristics of Rock

In terms of physical parameters work, this study collected previous research results on magnetic susceptibility and electrical resistivity [31,32,33], as well as conducted tests on the magnetic susceptibility, electrical resistivity, and polarizability of some rock and mineral samples. The magnetic susceptibility measurement was conducted using the KT-10 v2 magnetic susceptibility meter designed and produced by Terraplus Canada and Georadis S.R.O. of the Czech Republic. The KT-10 v2 can measure the magnetic susceptibility of uneven rock surfaces. The measurement range of the instrument is 1 × 10⁻⁶ SI to 999 × 10⁻³ SI, and the working frequency is 10 kHz. The electrical measurement was conducted using the SCIP rock electrical parameter measurement instrument produced by GDD Canada. The voltage measurement resolution of the instrument is 1 μV, with an accuracy of 0.2%. The current measurement resolution is 1 nA, with an accuracy of 0.2%. The IP measurement resolution is 1 µV/V, with an accuracy of 0.38%. The measurement results were statistically analyzed, and the geometric mean and arithmetic mean were calculated according to the standard requirements shown in Table 1.

In the above table, the order of rock magnetism from large to small is andesite basalt > intact andesite > broken andesite > andesite volcanic breccia > rhyolite. This result shows that the magnetic susceptibility is roughly positively correlated with the content of dark minerals. In terms of resistivity, the order from large to small is andesite > intact andesite > rhyolite > andesite volcanic breccia > broken andesite, which shows that the resistivity is affected by lithology and production state (degree of fragmentation, porosity, etc.). The results of the rock physical parameters test show that there is a positive correlation between polarizability and magnetic susceptibility in the study area.

Hydrothermal mineralization has a certain effect on physical properties. In terms of magnetism, the magnetic susceptibility of altered andesite and andesite volcanic breccia is significantly lower than that of unaltered andesite and andesite volcanic breccia. It indicates that ore-forming hydrothermal fluid has demagnetized the rocks. In terms of electrical resistivity, the resistivity of silicified andesite is significantly increased compared with that of primary rock. In terms of polarizability, the study area is of low vulcanization type, and the polarizability of the mineralized rocks has increased compared with the original rocks, but it is not significant.

3. Geophysical Methods

Comprehensive geophysical exploration work has been carried out around the existing geochemical gold anomalies in the study area, including the high-precision magnetic method, the resistivity and induced polarization (RIP) method, and electrical resistivity tomography (ERT). According to the comprehensive geological and geophysical research results, exploratory trench (ET) has been conducted. Finally, a comprehensive multi-parameter logging operation was conducted on the borehole BH-1, which has the largest inclination among all the boreholes. The other boreholes could not be logged because of their small dip angle and the accident of the probe pipe getting stuck during the trial logging. It should be noted that due to different measurement methods, the apparent amplitude frequency rate (Fs, equivalent to apparent polarizability) and apparent chargeability respectively measured by RIP and ERT reflect the strength of the induced polarization effect of geological bodies, and their physical nature is the same as that of the apparent polarizability [34,35,36].

3.1. High-Precision Magnetic Method

Magnetic exploration is a geophysical method to analyze and study the distribution form of geologic bodies and orebodies by observing the magnetic field value of rocks at ground height, which is widely used in the field of geological mineral investigation and gold exploration [37,38]. We first carried out a full-coverage high-precision magnetic survey in the geochemical gold anomalies areas, with a scale of 1:5000. The purpose was to identify the magnetic field characteristics of the geochemical gold anomalies areas. The magnetic method was measured by six sets of GSM-19T proton magnetometers produced by GEM of Canada. The measuring accuracy of the instrument reaches 0.1 nT and the absolute accuracy reaches 0.2 nT. The final measuring accuracy is less than 5 nT and meets the specification requirements of high-precision magnetic measurement. The measuring line direction is 90°, and the measuring network is 50 × 20 m. A total of 14,263 observation points were completed.

Data processing mainly includes the steps of daily variation correction, base point correction, height correction, latitude correction, etc.

3.2. Resistivity and IP Measurements with Gradient Array (RIP)

RIP is one of the conventional electrical methods, and two parameters of apparent resistivity (ρs, Ω·m) and apparent polarizability (ηs, %) can be obtained. ρs is generally used to distinguish geological structures and the distribution of different surrounding rocks. ηs is generally applied to identify the distribution of polarizable substances in rocks, which is often related to pyrite generated by mineralization and alteration. The gradient array used in this work is a kind of reconnaissance array that uses a fixed location for the A and B electrodes, which are far apart. Measurements are taken in an area between the current electrodes, and potential differences can be acquired by the receivers between the M and N electrodes (Figure 2). In addition, it usually has a certain relationship with magnetic susceptibility anomalies [39]. The main purpose of RIP is to discover the electrical structure of the magnetic anomaly areas, infer possible mineralized alteration areas, and provide a basis for the subsequent profile geophysical work.

In the field survey, the SQ-5 dual-channel instrument produced by Geosun Hi-Technology Co., Ltd. was used. The instrument is a frequency domain measurement that can obtain the apparent resistivity (ρs) and the apparent amplitude frequency rate (Fs) that has the same physical meaning as the apparent polarizability (ηs). It has been applied well in gold exploration [34]. After testing, the gradient array was used in the field survey. The grid spacing of the observation system was 50 m × 10 m (line spacing × point spacing). The power supply distance (AB) was 800 m, and the measurement distance (MN) was 20 m. A total of 1020 measurement points were completed. Data processing mainly included eliminating abrupt data, conducting appropriate smoothing treatment, and drawing contour maps.

3.3. Electrical Resistivity Tomography (ERT)

The principle of ERT is the same as that of the traditional DC/IP sounding method, and two parameters of apparent resistivity and apparent chargeability (ms) can be obtained. Its characteristic is that dozens to hundreds of electrodes can be deployed at the same time, and the data can be automatically collected, so the information is large, and the work efficiency is high. The reliability of the inversion results for the characterization of underground electrical property and polarizability in a certain depth is much higher than that of the traditional electrical measurement. Its disadvantage is that the detection depth is relatively shallow compared with other electrical methods, usually within 100 m of the depth. The detection depth meets the requirements of the epithermal gold deposits, so this method can be used [18]. The main purpose of implementing ERT is to detect the vertical electrical structure of the alteration area jointly discovered by the magnetic method and RIP.

The instrument used in this ERT fieldwork is the SuperSting-R8 automatic resistivity and IP system produced by AGI Company of the United States. After testing, the measurements are carried out using the Wenner electrode configuration with an electrode spacing of 10 m or a 2D survey. The Wenner array, which is widely employed in electrical resistivity surveying, represents a well-known electrode configuration. In this array, the three spacings among electrodes are maintained equal throughout all measurements, thereby ensuring consistency and facilitating accurate data acquisition. The power supply time was 2 s, and the measurements were repeated twice with the repeated measurements error of less than 2%.

Data processing includes appropriate processing of the mutation points and repeated observation error points. The data are processed and reversed by EarthImager2D. The 2D inversion algorithm uses Occam inversion, also known as Smooth model inversion. In the inversion process, the forward method is the finite element method, the forward solver is Cholesky decomposition, and the boundary conditions are Dirichlet. The initial resistivity inversion model uses a pseudo-cross-section with a maximum mean square error of 3%. The linear induction polarization inversion method is used for chargeability inversion, and both the slippery coefficient and damping coefficient are 10.

3.4. Well Logging

Well logging is the key technology of communication between surface geophysical exploration and geological drilling and provides the basis for the division of underground rock and mineralized layers. If gold ore bodies are detected in the borehole, then by using logging and core testing data, the differences in geophysical properties among the ore bodies, altered rocks, and unaltered surrounding rocks can be studied. Thereby, the geophysical characteristics of the gold ore bodies can be investigated and utilized to guide the ground geophysical exploration work to find new epithermal gold mineralized bodies.

The JHQ-PSK-2 logging system produced by Shangkuang (Hebei) Technology Co., Ltd. was used. Logging items mainly include a variety of logging parameters such as borehole diameter, well fluid resistivity, SP, natural gamma, dual lateral resistivity, polarizability, and magnetic susceptibility logging, etc. The parameters related to this study are as follows:

Dual lateral resistivity logging: two apparent resistivity curves, deep and shallow, can be provided. The deep resistivity curve mainly reflects the resistivity of the original rock formation, as well as the different width, location, and occurrence of fractures in the rock formation [40]. In this study, it can be used to reflect the resistivity of the surrounding rocks after alteration. The shallow resistivity curve mainly reflects the formation resistivity of the well fluid intrusion zone. Deep resistivity curves are used in this work.

Polarizability logging: Polarizability logging refers to the process of measuring the polarizability electromotive force by feeding a current field to the rock formation. The magnitude and variation of polarizability are related to the chemical composition of the constituent rock ore, the mineral structure, and the properties of the solution deep in it. In the rocks rich in pyrite, there will be high polarizability anomalies.

Magnetic susceptibility logging: The iron-cored solenoid in the probe and the rock formation in the borehole form a closed magnetic circuit. Changes in the magnetic permeability of the rock formation will cause changes in self-inductance. This method is used to measure the changes in the magnetic permeability of the rock formation.

The processing of logging data mainly includes depth correction, outlier removal, data smoothing, and interpretation based on curves and crossplots.

4. Analysis of Geophysical Data

4.1. Gold Mineralized Bodies Discovered in Exploratory Trench and Borehole

The discovered gold mineralized bodies include those exposed by exploratory trenches and boreholes (see Figure 1b for locations). The rock and ore samples were tested by the laboratory of Harbin Center for Integrated Natural Resources Survey, China Geological Survey. In general, the highest grade of gold is 21.3 g/t, the sulfide content is low (<0.5%), and it is a low-sulfide type.

The borehole BH-1 was drilled at 320° azimuth and 80° inclination. The actual drilling lithology is mainly andesite, andesitic volcanic breccia, with fracture zone developed, partial silicification, and pyrite phenomenon. Six gold mineralized bodies were delineated within a depth of 100 m. A small amount of pyrite is disseminated, and a small amount of quartz veins are distributed in the altered andesitic volcanic breccia.

The borehole BH-2 was drilled at 320° azimuth and 70° inclination. The actual drilling lithology is mainly andesite, andesitic volcanic breccia, containing a small amount of tuff, and the ore body has silicified quartz veins and stellate pyrite. One ore body was drilled from 44.8 to 48.85 m, and the gold grade reached 21.30 × 10⁻⁶, which is the highest grade gold ore body found in this survey.

The borehole BH-3 was drilled at 320° azimuth and 70° inclination. The actual drilling lithology is andesite, which can be metamorphosed and veined silicification, carbonated and pyrite altered. From 31.35 to 76.25 m, four gold mineralized bodies were discovered, with gold grades ranging from 0.14 to 1.5 × 10⁻⁶.

The borehole BH-4 revealed rock assemblages similar to BH-3, mainly andesite and andesitic volcanic breccia with a small amount of tuff. It was revealed that the primary rocks of many fracture zones are andesite, in the form of fragments, mud, silicified quartz veins, and stellate pyritization. Five gold mineralized bodies were found from 22.2 m to 92.4 m, with gold grades ranging from 0.1–4.54 × 10⁻⁶.

Through exploratory trench, it has been revealed that the average gold grade of the ET-1 sample in the RI-2 area was 4.89 × 10⁻⁶, and the NE-trending gold mineralization alteration zone has been delineated.

4.2. Magnetic Field Characteristics of the Mining Area

First of all, the overall variation characteristics of the magnetic field (ΔT, nT) in the study area (Figure 3) are gentle changes mixed with complex fluctuations, which reflects the distribution of different lithologies. According to the characteristics of the magnetic field distribution in the study area, it can be classified as three anomaly regions: medium-high anomaly region, gentle low anomaly region, and local superimposed anomaly region.

1. Medium-high magnetic anomaly region: It is mainly distributed in the north of the study area, and the field values basically vary within 0 to 800 nT, with low anomaly segmentation mostly in the south and west, and high anomaly segmentation mostly in the north and east. Geological data show that the andesite of Longjiang Formation (K₁lα) is mainly distributed here, which is the main reason for the middle-high anomaly area in this area.

2. Gentle low magnetic anomaly area: mainly distributed in the southern part of the study area, the field value basically changed within −200 to 600 nT. The magnetic field changes were relatively gentle in general, and there were local high and low anomaly areas. Geological data show that the acidic rhyolite of Guanghua Formation (K₁gn) is mainly distributed here, which is the main reason for the gentle low magnetic anomaly in this area.

3. Local superimposed anomaly area: Within the above two major magnetic anomaly areas, the scattered high-value area is the Ganhe Formation basalt (K₁g), which is mainly distributed in the northern part of the working area. There are some differences between this type of magnetic anomaly and the mapping unit. It is speculated that due to the limited outcrop in the coverage area, the reliability of the magnetic anomaly should be greater than that of the mapping results dependent on stone outcrops. The conglomerate rocks of Sunwu Formation (N_1–2S) are mainly distributed in the north and relatively in the low-value area. In the connecting area, the magnetic anomalies present a distinct striped pattern. The positive and negative magnetic anomalies alternate regularly, forming a series of parallel magnetic anomaly stripes. These stripes extend for hundreds of meters or even longer along the northeast direction, with each stripe being several hundred meters wide. The corresponding geological map is the junction of andesite and rhyolite, where all the existing soil gold anomalies are located. The drill holes BH-1 and BH-2, where gold ore bodies were discovered, have the most obvious banded magnetic anomaly areas T1 with alternating high and low values. The low anomaly bands of T1 strike in the northeast direction. The magnetic anomaly area T2 at BH-3 and BH-4 is relatively lower. The surface trench exploration site ET-1 is where gold ore bodies have been discovered. In the analysis of binding physical parameters, the low magnetic anomalies may indicate hydrothermal alteration zones [11]. The highest magnetic field value is 1124 nT, the lowest is −430 nT, resulting in a difference of 1554 nT, indicating that the demagnetization of the hydrothermal solution can reach 1000 nT. This is consistent with the research results of magnetic anomalies in the Gaosongshan gold deposit, which belongs to the Duobaoshan–Heihe metallogenic belt, and the surrounding rock is also andesite and belongs to the low-sulfidation epithermal gold deposits [41]. The magnetic anomaly values generally become lower towards the southeast direction, which may imply that the regional hydrothermal activity is more intense in that direction.

4.3. Electrical Characteristics in the Plane of the Mining Area

The results of apparent resistivity and apparent amplitude frequency rate (equivalent to apparent polarizability) measured by RIP in the study area, along with geological, physical and magnetic measurements, show that the study area has the following characteristics:

1. RI-1 region: The overall trend of the apparent resistivity anomaly is consistent with the overall trend of the magnetic anomaly, and the distribution of magnetic field values is characterized by alternating high and low values trending in the northeast direction. The trend and distribution range of the low apparent resistivity region R1 (Figure 4a) and the magnetic low anomaly region (Figure 3) are basically consistent. The high resistivity anomaly area outside R1 is basically the same as the high magnetic anomaly area. Considering that the lithology in this area is andesite, and the value of the low-resistivity area is less than 200 Ω·m, it is presumed to be a magmatic hydrothermal channel, which has weathered into a water-bearing fracture zone near the surface. The abnormal area P1 (Figure 4b) with a high apparent amplitude frequency rate (equivalent to apparent polarizability) ranges from 2% to 2.9%, the abnormal width is about 100 m, and the length basically runs through the test area without a clear boundary. The anomaly area with high polarizability is located on the southeast side of the andesite fracture zone and is relatively dispersed. The two boreholes (Figure 3) where gold mineralized bodies were found are located in the relatively high polarizability distribution area on the southeast side of the andesite fracture zone, among which the gold ore body discovered by the borehole BH-2 with a grade of 21.30 × 10⁻⁶ is the highest gold grade in this study and is located at the edge of the highly polarized body.

2. RI-2 region: This region is different from RI-1 in that there are high resistivity R2 (Figure 4c) and high polarizability anomaly P2 in the northeast-trending surface (Figure 4d). The exploratory trench (see Figure 1b and Figure 3 for the location) revealed that it is gold-bearing silicified andesite with an average grade of 4.89 × 10⁻⁶ (Figure 5). On the south side of R2 and P2, there is a polarizability anomaly in P3, but its resistivity is relatively low, suggesting that P3 has a different origin from P2. Subsequent boreholes BH-3 and BH-4 encountered gold ore bodies of industrial grade.

The above results show that the combination of geophysical anomalies in the gold ore body in this area is complicated, but it has certain rules, and further surface sounding and logging work should be carried out to identify the genesis and geophysical characteristics of different types of ore bodies.

4.4. Electrical Characteristics in the Vertical Section

In order to determine the vertical distribution pattern of the aforementioned plane geophysical characteristics, we carried out two ERT lines whose positions are shown in Figure 1b. The inversion parameters were configured to a maximum of eight iterations, with the maximum root mean square error (RMS) threshold set at 5%.

In practice, the actual maximum number of iterations was four, and the maximum RMS was 3.11%. The quality of the inversion was good. The 2D inversion results obtained from the ERT-1 survey line indicate that the resistivity within a depth of 30 m in the shallow section of the profile is consistent with the apparent resistivity diagram observed on the RIP method (Figure 4 and Figure 6). This outcome suggests that the apparent resistivity observed on the RIP method primarily reflects a comprehensive representation of a depth up to 30 m within our study area. Along this line, between distances of from 500 to 300 m, there exists an inclined low resistivity zone towards the southeast direction. By integrating geological mapping data, it can be inferred that this low resistivity zone corresponds to water-rich broken andesite (Figure 6a). Furthermore, this low resistivity band aligns with regions exhibiting lower magnetic anomaly values. The extension of this low resistivity band downwards tends to distribute towards the southeast at depths ranging from 20 to 90 m, where local anomalies indicating higher chargeability are relatively steep (Figure 6b). Consequently, it is further deduced that there exists a hydrothermal alteration channel beneath the water-fractured andesite region on the southeastern side, which also supports gold mineralization discoveries made in surface alteration zones located on its southeastern side. Therefore, based on the abnormal combination characterized by low magnetism, low resistivity, and high chargeability; it can be inferred that these areas represent favorable mineralization zones-an inference which is also substantiated by the borehole BH-1 findings (See also in Section 4.1 of this paper, Figure 7).

The strike of the ERT-2 line, whose position is shown in Figure 1b, is consistent with the strike of the resistivity anomaly R2, whose position is shown in Figure 4c. The inversion results show that the depth influence range of resistivity anomaly R2 is not more than 20 m. Through probe ET-1, it is indicated that the lithology of the abnormal combination of high resistivity and high polarizability (chargeability) within 20 m of the surface layer is silicified andesite (Figure 5 and Figure 6c,d). The boreholes BH-3 and BH-4 reveal that the low resistivity-low polarizability combination and the medium resistivity-high polarizability combination, which are located beneath the shallow high resistivity and high polarizability combination, are also gold mineralized bodies with varying grades. The above results indicate that there are abundant gold resources in this area.

4.5. Well Logging Data Analysis

Previous studies on well-logging data in gold deposits mainly focused on the medium to high sulfur type aligning with the distribution of gold ore bodies and pyrite bodies [42,43,44]. However, the well-logging response of low-sulfidation epithermal gold deposits and its comparison with the surface electrical inversion profile have not been publicly disclosed. In this study, we initially performed depth correction and processed abrupt changes in log curves. Subsequently, we analyzed the log response characteristics of various lithologies, alterations, and ore-bearing horizons, referencing core recording data and analyses of gold element content. This comprehensive approach aims to enhance the guidance for surface geophysical prospecting efforts.

4.5.1. Fundamental Characteristics of the Drilling Lithology and Logging Curve of Borehole BH-1

Due to the risk of the logging pipe getting stuck in boreholes with too small a dip angle, only borehole BH-1, which has a large dip angle, was successfully logged during this survey. The lithology encountered by borehole BH-1 according to the lithology catalog results, is relatively simple and can be categorized into six distinct sections (Figure 7):

1. Surface soil (0 to 5.23 m): This section is composed of surface soil, for which no logging curve is available.

2. Andesite (5.23 to 17.75 m): In this interval, the andesite displays a gray-black and gray-green coloration. The magnetic susceptibility values range from 1300 to 1500 × 10⁻⁴ SI, with an average value of 1330 × 10⁻⁴ SI. The resistivity ranges from 310 to 490 Ω·m, with an average value of 435 Ω·m. The polarizability ranges from 0.1% to 0.6%, with an average value of 0.35%. It is characterized by a combination of high susceptibility, high resistivity, and low polarizability.

3. Andesitic volcanic breccia (17.75 to 81.45 m): In this interval, the andesitic volcanic breccia displays a gray-black and gray-green coloration. The origin of the breccia may be volcanic or tectonic. The magnetic susceptibility ranges from 1150 to 1500 × 10⁻⁴ SI, with an average value of 1200 × 10⁻⁴ SI. The resistivity ranges from 100 to 500 Ω·m, with an average value of 190 Ω·m. The polarizability ranges from 0.74 to 3.7%, with an average value of 2.05%. Although the lithology of this section is single, the magnetic susceptibility, resistivity, and polarizability vary according to different laws in depth, which is presumed to be caused by different degrees of fractures, alteration, and pyritization. The gold mineralized bodies drilled are located in this lithologic section and are also the key analysis positions in the crossplot of logging curves below.

4. Andesite (81.45 to 95.27 m): In this interval, the andesite displays a light grayish-green coloration. The magnetic susceptibility ranges from 1200 to 1400 × 10⁻⁴ SI, with an average value of 1320 × 10⁻⁴ SI, and the resistivity ranges from 130 to 220 Ω·m, with an average value of 182 Ω·m. Polarizability values range from 2.12 to 4.5%, with an average of 3.5%. Compared with Section 2, they have the same lithology and similar magnetic susceptibility, but the resistivity of this section is relatively low, which is presumed to be caused by different degrees of silicification.

5. Broken andesite (95.27 to 102.51 m): In this interval, the geological section is a mud-like structural fracture zone. The original rock is andesite, the magnetic susceptibility value is 1150~1300 × 10⁻⁴ SI, and the mean is 1190 × 10⁻⁴ SI. The resistivity ranges from 100 to 170 Ω·m, with an average value of 122 Ω·m. The polarizability ranges from 1.2 to 1.7%, with an average value of 1.5%. This section presents a typical combination of low susceptibility, low resistivity, and low polarizability.

6. Andesite (102.51 to 110 m): In this interval, the andesite displays grain-green with magnetic susceptibility of 1300~1500 × 10⁻⁴ SI, with an average value of 1400 × 10⁻⁴ SI. The resistivity ranges from 350 to 850 Ω·m, with an average value of 680 Ω·m. The polarizability ranges from 1.4 to 2.2%, with an average value of 1.8%. It is a typical combination of high magnetic susceptibility, high resistivity, and low polarizability. Compared with Section 4, although the lithology is the same, the physical parameters combination characteristics of this section indicate that the degree of mineralization and alteration in this section is the least in the borehole.

Crossplot analysis of the borehole BH-1′s log values further reveals the geological-geophysical characteristics of the drilled rock and ore layer.

4.5.2. Geophysical Characteristics of Surrounding Rock

From the resistivity and magnetic susceptibility crossplot of the main lithology sections of surrounding rock (Figure 6a), it can be seen that there are three main lithology types corresponding to three logging parameter combinations:

1. Andesite: The log curve is characterized by high resistivity and high magnetic susceptibility, indicating that it is less affected by hydrothermal.

2. Broken andesite: The log curve is characterized by low resistivity and low magnetic susceptibility, indicating that the broken andesite has low resistivity after containing water and has been demagnetized after being affected by hydrothermal.

3. Andesitic volcanic breccia: The logging curves are characterized by low resistivity and medium magnetic susceptibility, but the overall value and distribution range is slightly higher than the andesite fracture zone.

Correspondingly, the intact andesite shows high magnetic field values in Figure 3 and high resistivity in Figure 4a,c and Figure 6a,c. The broken andesite shows high magnetic susceptibility in Figure 3 and low resistivity in Figure 4a,c and Figure 6a,c. Considering that, generally, rock fracturing will lead to a decrease in resistivity but will not cause a reduction in the magnetic field. Therefore, the decrease in the magnetic susceptibility of the broken andesite may be due to the demagnetization effect of magmatic hydrothermal fluids after fracturing.

These findings indicate that the different structure, lithology, and hydrothermal activity state of andesite have certain regularities.

4.5.3. Geophysical Characteristics of Andesitic Volcanic Breccia

Further analysis of the andesitic volcanic breccia (Figure 8b) shows that the resistivity and magnetic susceptibility are both decreased compared with the unaltered samples. This indicates that the degree of silicification in the drilling alteration zone is weak, and, consequently, the resistivity has not been significantly increased.

4.5.4. Geophysical Characteristics of Gold Mineralized Bodies

It can be seen from the crossplot of geophysical parameters (Figure 8c) of the borehole BH-1′s gold section that the intact andesite without altered mineralization is characterized by a combination of high resistivity, low-medium polarizability, and high magnetic susceptibility. The altered andesitic volcanic breccia is rich in pyrite and contains gold ore bodies, which are characterized by a combination of high polarizability, low resistivity, and low magnetic susceptibility.

The geophysical characteristics of gold ore bodies are clearly classified as two types (Figure 7 and Figure 8c). The relatively shallow (less than 30 m) gold-bearing parts showed a median resistivity distribution (300–500 Ω·m), a median polarizability distribution (0.3–0.6%), and a low-median magnetic susceptibility distribution (1300–1500 × 10⁻⁴ SI). The overall combined characteristics were medium resistivity, medium polarizability, and low-medium magnetic susceptibility. This type of gold ore body is located under the shallow intact andesite (Figure 6a and Figure 7), which is presumed to be caused by the gold-bearing hydrothermal solution encountering the relatively intact andesite or groundwater mixing in the upper part, so it is classified as hydrothermal terminal-type gold mineralized body. A significant feature of hydrothermal terminal gold mineralized bodies in this borehole is that there are obvious anomalous combinations of high resistivity and high magnetic susceptibility above the ore body and gradient variation zones below the anomalous combinations (Figure 6a and Figure 7). This reflects the gradual change of physical properties of surrounding rock caused by hydrothermal solution. The gold ore bodies of silicified andesite type in the RI-2 area have a similar variation trend compared with the channel type due to their spatial location and geophysical combination characteristics, so the gold ore bodies of silicified andesite type in this paper will be classified as the terminal type. The gold ore bodies of silicified andesite type in the RI-2 area were formed in the open space by the gold and silicon hydrothermal fluid overflowing from the underground passage, which was not denuded in the later period. Compared with the gold ore body discovered by the borehole BH-1, the silicified andesite type gold ore body has stronger silicification and pyritization, so the corresponding resistivity and polarizability are also higher.

Therefore, the terminal type can be further subdivided into two cases. The first is the gradual mineralization of the surrounding rock in a closed space, such as the two gold mineralized bodies revealed by the borehole BH-1 (Figure 7). The second type is the gold mineralized body formed in the open space of the surface, such as the high-grade gold mineralized body revealed in the RI-2 area through the exploration trench (Figure 6a,c). Through the ERT profiles and logging, the terminal type is concealed beneath the parallel magnetic anomaly stripes in the T1 magnetic anomaly area in Figure 3 beneath the intact andesite. In the T2 magnetic anomaly area, there are some that are concealed beneath the high magnetic anomaly area and some that are consistent with the low magnetic anomaly, which is consistent with the two types of terminal-type gold mineralized bodies that have been classified previously.

For the relatively deep gold-bearing parts larger than 30 m buried depth, the resistivity distribution is low (100–180 Ω·m), the polarizability distribution is low-median (0.6–2%), and the magnetic susceptibility distribution is low-median (1200–1400 × 10⁻⁴ SI). The overall combined characteristics are low resistivity, low-medium polarizability, and low-medium magnetic susceptibility. Because they are located in the hydrothermal alteration zone (Figure 7), they are classified as hydrothermal channel-type gold mineralized bodies. Compared with the terminal type, its geophysical characteristics mainly lie in its significantly low resistivity, and the low and medium polarizability of the gold ore body is distributed in the high polarizability anomaly side. The ERT profiles and logging results show that all the channel-type gold mineralized bodies are in a concealed state. When there is no terminal-type gold mineralized body present, the parallel magnetic anomaly stripes will be very clear, just like the T1 magnetic anomaly area in Figure 3; otherwise, the magnetic anomaly stripes will be blurry.

In this study, the andesitic volcanic breccia with the highest polarizability is relatively rich in pyrite but has no gold ore body. It is indicated that the gold ore body is located outside the high polarizability anomaly; although the high polarizability anomaly can not directly indicate the gold ore body, it also has important prospecting significance.

5. Discussion

At present, it is not possible to directly explore for gold ore bodies with geophysical methods, but it is possible to indirectly explore for gold by utilizing various geological factors related to gold deposit formation and containing, such as mineralization alteration, surrounding rock, fracture, etc., resulting in changes in the magnetic field, resistivity, and polarizability. The geophysical exploration methods of hydrothermal gold deposits are mainly associated with gold ore bodies and pyrite, which results in high polarizability. However, the low-sulfidation epithermal gold deposits do not have the characteristics of higher polarizability compared with non-ore bodies. The Alinghe Mining Area is an excellent area for the low-sulfidation epithermal gold deposits by geophysical methods: The relatively single lithology, structural conditions, and relatively abundant alteration and mineralization revealed by the surface and borehole in the study area provide a relatively simple geological background and a relatively clear conclusion for the study of the occurrence conditions of epithermal gold ore bodies in this paper, and thus provide a reference for the study of other areas with more complex geological conditions.

The banded low magnetic anomalies in the andesite distribution area reflect the demagnetization of hydrothermal alteration and indicate the existence of the alteration area. Therefore, the exploration of this type of gold deposit should first focus on the area with the same lithology but significantly lower magnetic susceptibility in the geological mapping area to find out whether it is caused by hydrothermal alteration. For the low anomalies caused by the demagnetization of the altered zone detected by the magnetic method, the resistivity characteristics can be further identified by using the RIP measurements with a gradient array, and the distribution of the altered zone can be judged from the plane with multiple parameters, thus narrowing the gold exploration target area. The planar distribution characteristics of low resistivity anomaly and low magnetic anomaly are basically consistent, which is due to different geophysical responses during the same hydrothermal activity process. Because the detection depth of the magnetic method and RIP method is limited and it is difficult to separate vertical anomaly superposition, it can be significantly detected only when the alteration zone has been denuded to the near surface. Andesites have the highest percentage of the 14 large epithermal gold deposits emplaced into different types of volcanic rocks in the circum-Pacific region [3]. The host rocks in this paper are intermediate andesites. The magnetic anomalies within the study area exhibit typical striped characteristics, whereas the altered rhyolite located on the south side of the study area does not possess such characteristics. Therefore, in areas where acidic rocks are distributed, magnetic surveys may be ineffective in identifying alteration zones. In any case, as a fast and cost-effective geophysical method, the magnetic method should be given priority in the exploration of epithermal gold deposits and serve as a reference before the implementation of other geophysical work.

The primary issue with the RIP method is that mineralized alteration invariably leads to a reduction in magnetization, while the situation regarding resistivity is more intricate. For instance, the terminal-type gold mineralization and its host rocks exhibit high resistivity, whereas the channel-type gold mineralization and its host rocks show low resistivity. The terminal-type gold mineralization and its host rocks may transform into low resistivity if they are disrupted by subsequent faulting. Therefore, the RIP results generally cannot be directly interpreted. They must be integrated with ERT data and guided by the correct geological model of the ore deposit in order to obtain a plausible interpretation. Simultaneously, the RIP anomalies that match with the magnetic method will corroborate each other, thereby enhancing the reliability of the identification of the mineralized alteration zones by these two geophysical methods.

The andesite hydrothermal alteration zone where the channel-type gold mineralized bodies are located has the characteristics of banded low magnetic and low resistivity combined anomalies, and their distribution is basically consistent. The low magnetic anomaly reflects the demagnetization caused by hydrothermal alteration. The decrease of magnetic susceptibility of andesite surrounding rock caused by hydrothermal temperature can reach 1000 nT. Therefore, the exploration of gold deposits in this area should first pay attention to the areas with the same lithology but obviously low magnetic susceptibility and accompanied by the same low resistivity on the geological map to find out whether it is caused by hydrothermal alteration.

This study discovered gold mineralized bodies located on the side of high IP anomalies (high polarizability, chargeability, or Fs) using ERT and well logging. This finding is similar to the electrical work conducted at the Hishikari epithermal gold deposit, regarded as one of the world’s richest goldmines: the gold mineralized bodies are located on the upper part of the high polarizability body [11,45]. However, unlike the low resistivity anomaly associated with the Hishikari epithermal gold deposit, we found that there is also a kind of high resistivity anomaly associated with the terminal-type gold mineralization. It can be seen from the ERT lines in different directions that the distribution of gold mineralized bodies has obvious spatial distribution differences, and it is recommended to implement geophysical survey lines that are drilled and aligned with the borehole. The gold mineralized bodies found in this study are inconsistent with the high polarizability region. This may be related to the evolution of pyrite and gold with the decrease in temperature and other physical and chemical conditions of ore-forming fluid [46]. Further study of this scientific problem cannot be solved by geophysical method alone, and it is necessary to carry out rock geochemical research to clarify the relationship between pyrite and gold ore body so as to enhance the guiding significance of using polarizability anomaly to find gold ore body. The spatial distribution difference between the gold mineralized bodies and pyrite may be the geophysical characteristics of the low-sulfidation epithermal gold deposits.

Finally, the existence of the terminal-type gold orebodies in the RI-2 area implies that the study area has not experienced severe later erosion. The Alinghe Mining Area may be located at the terminal of a complete epithermal-porphyry metallogenic system [4]. Therefore, there is great potential for ore prospecting in the deep part of the study area.

6. Conclusions

As far as the current research level is concerned, the following research conclusions have been formed:

1. There is no specific combination of geophysical anomalies in the low-sulfide epithermal gold ore body itself, but, firstly, it is formed in the underground channel of specific structure and lithology and produces demagnetization on the magnetic substances contained in the surrounding rocks. Secondly, silicified and pyritized substances are produced when gold elements are precipitated from the hydrothermal solution, and they follow certain geological and geophysical laws. It is necessary to comprehensively analyze the geophysical characteristics of gold ore body based on the type of gold ore body, the properties of surrounding rock, and its spatial relationship with the alteration zone and mineralization zone, so as to realize the success of spatial distribution prediction and exploration of gold mineralized body.

2. The results of this study indicate that the epithermal gold mineralized bodies can be classified as channel type and terminal type due to their different spatial distribution positions and geophysical characteristics. The terminal-type gold mineralized bodies in the study area are buried less than 30 m, located under the shallow intact andesite that is not affected by the hydrothermal solution or overflow from the surface, and have the characteristics of geophysical combination anomalies with medium resistivity, medium polarizability, and low-medium magnetic susceptibility. The buried depth of the channel type is generally greater than 30 m, located in the andesite volcanic breccia or structural fracture andesite alteration channel, with low resistivity, low-medium polarizability, and low-medium magnetic susceptibility geophysical combination anomaly characteristics. The low and medium polarizability of the gold ore body is distributed on the side of the high polarizability anomaly.

3. The Alinghe Mining Area may be located at the terminal of a complete epithermal-porphyry metallogenic system, and there is a huge potential for ore prospecting in its deep part.

Author Contributions

Conceptualization, Q.Y. and K.W.; methodology, Q.Y. and K.W.; software, Q.Y. and T.S.; validation, T.S., G.T. and Y.J.; formal analysis, Q.Y., H.Y. and K.W.; investigation, K.W., H.C., Y.S. and Q.L.; resources, T.S., Y.J. and H.Y.; data curation, Q.Y. and T.S.; writing—original draft preparation, Q.Y.; writing—review and editing, G.T.; visualization, T.S.; supervision, K.W.; project administration, Q.Y.; funding acquisition, Q.Y. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Table

Figure 1. (a) The structural location map of Alinghe Mining Area (according to Pan et al., 2009 [27]) and (b) the geological map. 1: Nen River-Balihan fault; 2: northwest boundary fault of Songnen basin; 3: Yilan-Yitong fault; EB: Erguna block; XB: Xing’an Block; SXB: Songnen-Xilinhot block; XXS: Xinlin-Xigutu suture zone; HHS: Hegan Mountain-Heihe suture zone.

Figure 2. Sketch map for the gradient arrays used in RIP measurements.

Figure 3. Map of magnetic field ΔT contour and geological boundary in the study area (see Figure 1 for location and names of rocks).

Figure 4. Contour plots of apparent resistivity (a) and apparent amplitude frequency rate (equivalent to apparent polarizability) (b) in the RI-1 region and apparent resistivity (c) and apparent amplitude frequency rate (d) in the RI-2 region in the study area (see Figure 1 for location of RI-1 and RI-2).

Figure 5. Gold-bearing silicified andesite revealed in ET-1.

Figure 6. Inversion and interpretation profiles of ERT-1 (a) resistivity, (b) chargeability and ERT-2, (c) resistivity, and (d) chargeability (see Figure 1 for location).

Figure 7. Main logging curves and results of the borehole BH-1 (the colors in lithology column correspond to the descriptions provided in Section 4.5.1.).

Figure 8. Crossplot of geophysical parameters of andesitic surrounding rock (a), andesitic volcanic breccia (b), gold mineralized bodies, and other meaningful logging sections (c) of the borehole BH-1.

Table 1

Physical parameters statistics of rock and ore body.

Lithology	Magnetic Susceptibility (M.S.)(×10−5 SI)		Resistivity (Ω·m)		Polarizability (%)
	Value Range	Geometric Mean	Value Range	Geometric Mean	Value Range	Arithmetic Mean
(Inact) Andesite	12~1510	170.1	80~4300	609.6	2.5~60	26.9
Broken andesite	35~933	113.7	65~407	182.9	4.6~16.8	10.6
Andesite basalt	2500~3130	2889.4	7000~21,000	11,154.2	11~16	12.7
Andesitic volcanic breccia	8~490	133.9	50~2805	267.6	3.43~29	7.9
rhyolite	0.6~6	3.1	500~993	690.5	4.5~30	18.5
Gold ore body from drilling	10~490	82.3	120~1076	283.4	3.6~63.8	17.2
Gold ore body from exploratory trench	6.5~37.5	19.9	476~2806	789.6	25~50	37.6

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