Dataigou Iron Mine in Benxi, Liaoning Province

Dataigou Iron Mine is located in Qiaotou Town, Benxi City, Liaoning Province, at 16km south of Benxi City, and its administrative division belongs to Qiaotou Town, Pingshan District, Benxi City. The mining area is about 2km away from Qiaotou Railway Station, 4km away from Qiaotou Town and 5km away from Qiaotou Station of Shendan Expressway, and the traffic is very convenient (Figure 2-2- 1).

Figure 2-2- 1 Schematic Diagram of Traffic Location in Dataigou Iron Mine Area

From 1935 to 1938, at the beginning of Japan's invasion of China, the Manchuria Railway Geological Survey and the Manchuria Railway Survey Department, headed by the Japanese, conducted geological surveys of metallic and nonmetallic minerals in this area.

In the early days of the founding of New China (1950 ~ 1960), Shenyang Geological Bureau of the Ministry of Geology, Liaoning Coalfield System and Metallurgical System Geological Team, and Changchun Institute of Geology successively carried out the iron ore survey in the work area. From 1958 to 1959, geological and metallurgical departments carried out 1∶65438+ ten thousand aeromagnetic surveys in the south of Changbai Mountain, which laid the foundation for iron ore prospecting.

During the period of 1970, the Geophysical Exploration Brigade of Liaoning Geological Bureau conducted a ground inspection of Dataigou magnetic anomaly (No.88 aeromagnetic anomaly) in the Qiaotou area of Benxi, and completed the ground magnetic survey of1:65438+100000 km2, inferring that the aeromagnetic anomaly No.88 was caused by Anshan-type magnetite. Through quantitative calculation, the buried depth of the upper edge of the ore body is about 800m, the buried depth of the center is about 2000m, the width is 1600m, the strike length is about 2500m, and the dip angle is nearly vertical. It is suggested to arrange 4 verification holes with a depth of1500m. During the period of 1974, Benxi Geological Brigade successively built three exploration holes for verification, but none of them achieved the purpose of prospecting. The deepest hole ZK 1, the hole depth 12 13.96m, and the final hole is located in sericite phyllite of Langzishan Formation in Liaohe Group. Further inference by the geophysical team members of the team shows that the average buried depth of the anomaly center is about 1 150 m, and the buried depth of the upper edge of the ore body near ZK 1 hole is 1300m.

1973, the second metallurgical aerial survey team conducted an aeromagnetic survey of 1∶25000 in Aberdeen area, and submitted the Aeromagnetic Survey Report of Aberdeen area in February of 1974, and made a survey of1∶ 6500 discovered in 1959. 1973 and 1974, Team 40/kloc-0 of Liaoning Angang Geological Company conducted a general survey and detailed investigation of aeromagnetic anomalies in the Waitoushan-Beitai area, and found a large number of iron deposits and occurrences.

1976, the 2nd Geophysical Exploration Brigade of Metallurgical Geological Combat Command of the Ministry of Metallurgy conducted a comprehensive study on the magnetic anomaly in Dataigou, and completed the data processing of magnetic anomaly measurement of 204.8km2 and gravity profile measurement of 800m× 1 1, and delineated the magnetic anomaly area 10km×5km and intensity. According to the characteristics of gravity and magnetic homology anomaly, it is inferred that the anomaly is caused by Anshan-type iron ore, with a buried depth of1450 m. The south block of the anomaly has the characteristics of high magnetism and high density, and may be a rich ore spot. Two verification holes have been arranged, which were not implemented in that year.

1980 metallurgical geological exploration bureau used imported Japanese new deep-hole drilling rig to drill ZK3 hole at the baseline of the fourth line of Dataigou Iron Mine. Hidden Anshan-type iron ore was seen at the depth of 1525. 15m, but the ore body was not drilled through to172.55m. However, the grade of the ore body was not high, and TFe was generally 65435. Because it was buried too deep, no further work was done.

After a lapse of 20 years, it entered 2 1 century. With the development of national economy, the demand for iron ore resources has increased, and iron ore exploration has been further strengthened, which has brought opportunities for deep prospecting and made new progress in iron ore exploration in this area. In 2005, a project team was set up to organize experts to analyze and study the iron ore resources in Liaoning Province, and Anshan-Benxi-Liaoyang area was selected as the key area to find deep blind ore bodies. Through large-scale geological survey, magnetic survey, verification and screening, the bridgehead area of Benxi was finally selected as the key verification area. In 2006, the project of "Evaluation of Iron Ore in Wujiatai-Sunjiaying Area, Anshan, Liaoning Province" was established by the general survey of land and resources, and the existence of Dataigou iron mine was further confirmed through the deep verification of Benxi magnetic anomaly.

1. Geological Characteristics of Dataigou Iron Mine

Dataigou iron mine is located in the Anshan-Benxi iron ore metallogenic belt, at the southwest end of the NeoArchean Anshan-Benxi volcanic sedimentary basin. Both cherry orchard Formation and Dayugou Formation in this area have iron-bearing deposits, which are the largest Anshan-type iron deposits known in China and the largest resources, and also the concentrated areas of super-large and large iron deposits in China. Qidashan, Dongxishan, Nanfen and Gongchangling have large deposits 10, 2 medium-sized deposits and occurrences 10. As long as the aeromagnetic anomaly area is verified, iron ore can be found in this area, which is an ideal area for finding large iron ore.

The strata exposed in the mining area are mainly Neoproterozoic Xihe Group Diaoyutai Formation, Nanfen Formation and Qiaotou Formation, Sinian Konka Formation, Cambrian Jianchang Formation and Shantou Formation. The iron-bearing rock series of Archean Anshan Group is not exposed on the surface, and the iron-bearing rock series and the top of the ore body are buried below the surface1100 ~1200m. The lithology of horizons (from top to bottom) seen in the completed 17 borehole is summarized as follows:

1) alkali plant formation: limestone mixed with thin siltstone, with a thickness of 27 ~102m;

2) Kangjia Formation: marl and limestone with a thickness of17 ~ 48m;

3) Qiaotou Formation: interbedded with timely sandstone and black shale containing glauconite, with a thickness of about 100 m;

4) Nanfen Formation: egg blue mudstone (gray) and purple mudstone, about 500m thick;

5) Diaoyutai Formation: seasonal sandstone, quartzite and seasonal sandstone mixed with black shale, with a thickness of about 200m;

6) Langzishan Formation of Liaohe Group: silicified marble and chlorite sericite quartz schist with a thickness of 300-700 meters;

7) cherry orchard Formation of Anshan Group: banded magnetite quartzite, hematite quartzite, hematite magnetite quartzite and chlorite schist.

The rock (ore)-rock assemblage characteristics of the iron ore layer seen in each borehole are similar to those of the known iron ore in Anben area, and the ore characteristics and rock inclusion characteristics of Dataigou Iron Mine are similar to those of Qidashan Iron Mine in Anshan. Its horizon should belong to cherry orchard Formation of Anshan Group, which belongs to metamorphic-volcanic-sedimentary iron ore, that is, "Anshan type" iron ore.

At present, there are 20 boreholes in the abnormal center, which control the extension of the ore body to 2,000m, the strike length of the ore body to 2,000m, the buried depth of the top interface of the ore body1100 ~1200m (elevation-900 ~-1000m) and the width to 578. Vertically, the upper part is hematite, the middle part is composite ore and the lower part is magnetite. The ore grade is uniform and continuous, with a grade variation coefficient of 20.2%. The exploration type of deposit belongs to the first exploration type, and the basic control grid is 400m×400m m.

The laboratory beneficiation tests of three ore types show that the ore is easy to beneficiate and has good beneficiation indexes. The recommended beneficiation test flow is: stage grinding-weak magnetic field-strong magnetic field-reverse flotation flow. The grade of iron concentrate can reach above 65%, and the recovery rate is above 70%. The hydrogeological conditions and engineering geological conditions in the mining area are moderately complex. The pre-study on the feasibility of ore deposit mining and dressing engineering shows that it is economical and feasible to carry out large-scale underground mining under the current economic and technical conditions.

The estimated amount of iron ore in Dataigou mining area 15 ~ 4 line is 3,394.93 million tons, and the average grade of ore body is 33.07%. Among them, hematite is 622.93 million tons, composite ore15214.4 million tons and magnetite1250.57 million tons. Among them, (332) resource reserves account for 15%. According to the characteristics of magnetic anomalies, it is predicted that the iron ore resources in the whole mining area can reach 654.38+000 billion t.

Second, geophysical characteristics

(1) Characteristics of aeromagnetic anomalies in this area

The aeromagnetic anomaly of Dataigou1∶ 200,000 has obvious positive and negative anomalies (Figure 2-2-2), with Dataigou as the center, negative anomalies in the north and positive anomalies in the south. The shape of the anomaly is an ellipse in the northwest direction, with obvious anomaly center and high anomaly value (δ t is the highest > > 4000nT). The anomaly is delineated by 1000nT isoline. The main part of the anomaly is oval, and the anomaly is northwest, with a major axis of about 7000 meters and a minor axis of about 4500 meters.

In 1976, the 2nd Geophysical Exploration Brigade of the Metallurgical Geological Combat Command of the Ministry of Metallurgy, according to the characteristics of1∶ 50,000 magnetic anomaly, such as shape, occurrence and buried depth of field source, used the "three-dimensional body selection method" to forward calculate the magnetic anomaly on the computer, and divided the magnetic anomaly into three magnetic anomaly bodies. After giving different morphological parameters and magnetic parameters to each magnetic body, the abnormal value of ground magnetic survey is simulated. When the simulated value and the measured value are within a certain allowable error range, the size of the model represents the size of the magnet. Through forward simulation, the mining area 1 magnet is located in the center of magnetic anomaly in Dataigou, that is, between line 3 and line 12, and the magnetic field intensity of the anomaly center is 3500m~6000nT. It is inferred that the buried depth of the magnet center point is 1755m, the width is 13 15m, and the length is1670 m. Magnet II is located at the northern end of magnet I, distributed between lines 2 and 23, and its southern end overlaps with the middle of magnet I, with abnormal strength of 2000 ~ 6000. It is inferred that the buried depth of the magnet center point is1430m, the width is1266m, the length is 2760m, and the extension depth is 350m ... Magnet III is located at the northwest end of the magnetic anomaly in Dataigou, that is, the line 19 ~ 47, and the magnetic field intensity at the center of the anomaly is1. It is inferred that the buried depth of the magnet center is1352m, the width is 935m, the length is 2563m and the depth is 300m (Figure 2

Figure 2-2-2 Deducing magnetic parameters by simulating the magnetic anomaly isoline δT(nT) in Dataigou iron mine area (according to Du Weiben and Huang Zhongxiang)

The predecessors calculated and analyzed the anomaly through ground inspection, and thought that the anomaly was caused by a magnetic body ("Anshan-style" iron ore). The upper edge was buried about 800m, the width was about 1600m, and the length along the strike was about 2500m m. The anomalies are mainly distributed in Paleozoic strata, Sinian strata and Qingbaikou strata, and it is speculated that there may be Archean Anshan-type iron deposits in the deep part.

(2) Measurement results of physical properties

The determination of physical properties is based on the relevant regulations on the determination of physical properties of rocks and ores in high-precision magnetic survey and electrical survey. There are four parameters in this physical property measurement: magnetic susceptibility κ, remanence Mr, apparent resistivity ρS and apparent polarizability η s. According to the physical property data in this area, the lithology (see table 2-2-2) and ZK00 1 borehole core samples (see table 2-2- 1) collected from the Geological Exploration Institute of Liaoning Metallurgical Geological Exploration Bureau are measured, except for magnetite.

Table 2-2- 1 Physical Parameter Table of Rock (Ore)

Table 2-2-2 Collection of Physical Parameters of Rock (Ore) in Adjacent Areas

The magnetic parameters are measured by Gaussian first position. The average value and variation range of magnetic susceptibility κ and remanence Mr are obtained by calculation. By measuring the electrical parameters, the average values of apparent resistivity ρS and apparent polarizability ηS and their variation ranges are obtained.

There are few previous physical data in Dataigou exploration area. The source of physical parameters in this work comes from two aspects: one is to take a certain number of ore body and surrounding rock samples by drilling and coring in the exploration area for parameter determination (Table 2-2- 1), and the other is to collect the physical data measured in previous geophysical exploration work in neighboring areas. The data source is mainly the Geological Exploration Institute of Liaoning Metallurgical Geological Exploration Bureau (Table 2-2-2).

1. Magnetic parameter characteristics

From the analysis of physical property parameters in Table 2-2- 1 and Table 2-2-2, it can be seen that the changes of magnetic parameters in this well and its adjacent areas are consistent, except for magnetite quartzite, which is strong in magnetism, others are weak or non-magnetic. Therefore, the factors that cause magnetic anomalies in the exploration area are relatively simple, most likely caused by iron ore. This obvious magnetic difference provides an effective geophysical basis for magnetic prospecting for iron ore.

2. Electrical parameter characteristics

Through the analysis of electrical parameters (Table 2-2- 1), it is considered that banded magnetite quartzite and hematite quartzite have obvious characteristics of low resistance and high polarization, and there is little difference in apparent polarizability among other lithology; However, the resistivity varies greatly, with an average range of1843 ~13362 Ω m, showing good electrical differences, which provides a certain geophysical premise for determining the fracture structure shape by electrical (profile) measurement.

Third, the application of geophysical exploration methods and techniques.

(1) high-precision ground magnetic survey

Through the work in 2008 and 2009, 57 high-precision magnetic profiles were completed, with a total length of 40 kilometers, an investigation area of 28.5 square kilometers and 7402 physical points. The purpose of this work is to delineate the scope of magnetic anomalies through surface strong magnetic survey, and provide a basis for further engineering exploration.

1. Magnetic anomaly characteristics

It can be seen from the plane contour map of δ T anomaly (Figure 2-2-2) that the anomaly is oval with a central anomaly value of nearly 6000nT. Based on the contour range of 1500nT, the major axis is about 6000m and the minor axis is 4000m, and the ratio of major axis to minor axis is 3∶2. The abnormal trend is northwest. The two sides of the anomaly are symmetrical, the gradient changes little, there is a negative value in the north, it gradually narrows to the northwest, and the anomaly value gradually decreases.

2. Interpretation and inference of high-precision magnetic survey on the ground

From the analysis of physical property data, except magnetite quartzite and maghemite, other rocks and minerals in this area are weakly magnetic or nonmagnetic, so it is inferred that the magnetic anomaly is caused by iron ore. According to the overall shape of the anomaly and the interpretation results in previous years, it is considered that the magnetic body causing the anomaly is a nearly thick plate body, which is mainly characterized by deep extension and steep occurrence.

In order to understand the characteristics of magnetic bodies, the tangent method is used to quantitatively calculate 132, 140, 148 and 156 sections. Here only the 140 line is described quantitatively (Figure 2-2-3). The calculation results of other sections are shown in the following table (Table 2-2-3). According to the above profile calculation, it can be concluded from the abnormal shape that the average buried depth at the top of the ore body is 1 103m, the average width is 1029m, and the length of the ore body is 1440m.

Fig. 2-2-3 Profile of Calculating Buried Depth of Magnetic Body by δ T Tangent Method on Dataigou Iron Mine 140 Line

Table 2-2-3 Calculation Results of Magnet Tangent Method

(2) Interpretation of ground gravity survey

Gravity data in this area comes from 1976 No.2 Geophysical Exploration Brigade of Metallurgical Geological Combat Command of Ministry of Metallurgy. The gravity profile of 800m× 100m grid 10 was measured in the magnetic anomaly area at the bridgehead. After topographic correction and regional geological background correction, the gravity anomaly is consistent with the magnetic anomaly, and both of them are oval-shaped. They are considered as high-density anomalies with high magnetism (κ and Mr are both high), and are often called gravity and magnetic homologous anomalies (Figure 2-2-4). This shows that there are iron ore bodies in the deep, and the center of gravity and magnetic anomalies is also the center of ore bodies.

Figure 2-2-4 Comparison of Geomagnetic Anomaly and Gravity Anomaly in Dataigou Iron Mine

(3) eh-4 electromagnetic profile measurement

EH-4 continuous conductivity profiler is a Shuang Yuan electromagnetic seismic system jointly developed by EMI and Geometrics. EH-4 is a receiver that uses the earth as a large natural electromagnetic emission source. At high frequency, shallow surface geological information is received, and at low frequency, deep geological information is received. This time, three sections are laid out, namely Line 0, Line 3 and Line 7, and the survey line direction is NE45.

Eh-4 method is used to investigate some areas in this area. According to the spatial distribution of resistivity and geological data, the possible occurrence and spatial distribution characteristics of ore bodies are studied to provide basis for drilling engineering design and construction.

Comprehensive analysis of geological data in this area, drawing geological interpretation results in this area (Figure 2-2-5, Figure 2-2-6, Figure 2-2-7). It shows that this method can well determine the boundary problem of the top surface of the concealed ore body, and puts forward that the technical key to solve this problem is how to determine the resistivity value of the top surface boundary according to the physical property data and the abnormal characteristics of the profile. This paper analyzes the situation through three sections (Line 0, Line 3 and Line 7).

Figure 2-2-5 Comparison of Ore Body Boundary between No.0 Exploration Line Control and EH-4 Inference in Dataigou Iron Mine Area

"Line 0" profile (Figure 2-2-5): In the deep part (> > 1000m), the resistivity gradually decreases in the longitudinal direction and increases in the transverse direction to the northeast, reflecting the possibility of ore bodies. It is unusually wide, with the top gently dipping to the northeast, showing irregular thick plate-like ore body strike, steeply dipping to the south and west, with a width of about 800m m. The inferred boundary is different from the actual drilling control boundary of the ore body.

Vertically, the top interface of the ore body is silicified dolomite marble and grayish white quartz sand, and the rock resistivity is 7918 Ω m and12229 Ω m, respectively, while the resistivity of hematite with 3165 Ω m is relatively low, so there is a high resistance area in the upper part and a low resistance area in the lower part of the section. Similarly, the southwest boundary of the ore body is Archean mixed granite, which is also a region with relatively high resistivity, and should be the boundary of the ore body in the transition zone of high and low resistivity. According to the value of 3500 Ω m, it may be better to judge the boundary between ore and non-ore. The surrounding rock of the northeast boundary of the ore body is chlorite schist, and its resistivity is13362 Ω m, which is also a relatively high resistance area. If the value less than 4500ω·m is used to infer the ore body boundary, it is quite different from the actual ore body boundary. How to determine the boundary resistivity value is the key to infer the accuracy of ore body boundary. In addition, from the distribution pattern of EH-4 continuous conductivity profile curve, if the trend of the curve pattern is horizontal, it means that the vertical resistivity of geological bodies is different, and the geological bodies in this area are nearly horizontal layered distribution; If the distribution is vertical curve, it shows that the difference of geological bodies in horizontal direction is greater than that in vertical direction, reflecting the steep occurrence of geological bodies. For example, the ore-bearing position curve below1300m in the three boreholes ZK003, ZK002 and ZK004 of Line 0 well reflects the existence of deep iron ore bodies (banded magnetite quartzite) (see Figure 2-2-5).

Figure 2-2-6 Comparison Diagram of Ore Body Boundary Controlled by No.3 Exploration Line in Dataigou Iron Mine Area and Exploration Boundary Inferred by EH-4

Figure 2-2-7 Comparison Diagram of Ore Body Boundary Controlled by No.7 Exploration Line in Dataigou Iron Mine Area and Exploration Boundary Inferred by EH-4

The EH-4 continuous conductivity profile of the third line profile (Figure 2-2-6) generally reflects the main boundary of the ore body, and the deep part (below 1000m) reflects that there may be ore body anomalies. The top of the anomaly is almost horizontal and steep in the south and west. The thickness is 800 ~1000m. If the southwest boundary of the ore body is determined according to the value of 2000 ~ 3500 Ω m, the boundary is closer to the actual control position. The south-west boundary on this line is very similar to the south-west boundary on the 0-line section, and the low-resistance area is the distribution position of the ore body boundary. However, the northeast boundary is similar to the 0-line profile, and the resistivity tends to increase gradually. It will be more practical if the ore body boundary is delineated with less than 4500 ω m.

EH-4 section of Line 7 (Figure 2-2-7) has a large lateral change in its shallow part, which clearly reflects the existence of faults. There is a low resistivity anomaly area in the deep, which reflects the suspected ore body anomaly. The top of the anomaly is almost horizontal, slightly inclined to the south and west. It is inferred that the overall thickness of the ore body is less than line 0 and line 3, which is about 600m m. The inferred southwest and northeast boundaries of ore bodies are far from the actual control system. However, the curve characteristics of the low resistivity area (2000 ~ 4000 Ω m) in the southwest boundary can well reflect the ore body boundary, especially the resistivity curve in the east is steep, while the southwest boundary of the ore body has the characteristics of gentle dip. It is concluded that there is a certain gap between the boundary position of ore bodies and the spatial position of actually controlled ore bodies (such as ZK709 and ZK705), which is mainly caused by the strong interference of magnetic field intensity of ore bodies.

Through the above analysis, it is considered that the EH-4 method can well reflect the top boundary of the deep concealed iron ore in Dataigou mining area, and the contour line of 4500ω·m is taken as the dividing line between low resistance and high resistance, and the abnormal overall range of the ore body is delineated by this value, and the boundary of the ore body is further inferred. This method is fast and convenient, and it is of great significance to guide the verification of deep well drilling engineering.

(4) Comprehensive geophysical logging

Select the corresponding probe tube to cooperate with the host computer of JGS- 1B intelligent engineering logging system, and adopt the continuous measurement mode of lowering cable (point distance is 0.5m). In order to check the accuracy of the data, the descent and ascent of each probe are measured once respectively. This work completed 10 drilling holes. The measurement results of the three methods are explained as follows.

1. Three-component magnetic logging

In the cover layer, the value of δ z tends to increase, while the direction and magnitude of δ h and δ t vectors remain unchanged. The δ z at the interface between overburden and ore body is obviously abnormal, which reflects the obvious magnetic interface (the depth is 1 153m), and the direction and mode length of δ h and δ t vectors change. From the characteristics of the curve (Figure 2-2-8), it can be seen that in the magnetite quartzite interval, the abnormal value of δ z is negative and the change is abrupt. Although the sawtooth anomaly is formed, the amplitude is not large, and the direction and magnitude of δ h and δ t are chaotic, showing the characteristics of internal magnetic field change. In the quartzite area of hematite (depth 1746m), the δ z curve changes gently, with an abnormal value of 300 ~ 600 nt, while δ h and δ t remain unchanged.

2. Magnetic susceptibility

It can be seen from the logging curve (Figure 2-2-8) that the magnetic susceptibility in the casing area is stable, and the magnetic susceptibility in the non-mining area varies within the unit range of 400 ~ 1000 Si (κ); On magnetite quartzite, the variation range is 20000~38500SI(κ), and the magnetic susceptibility of hematite quartzite area is 1200 ~ 1500 Si (κ). The magnetic difference between ore body and surrounding rock is obvious, which can be used to classify ore types.

3. Natural gamma rays

Natural gamma logging mainly measures the natural radioactive intensity of strata in borehole. The stratum is composed of different types of rocks, and the rocks are composed of different minerals, and each mineral has different adsorption capacity for radioactivity. Usually, rocks with high content of argillaceous minerals have stronger adsorption capacity and stronger radioactivity. Therefore, the lithology of strata can be compared by layers according to the characteristics of natural logging curves.

Figure 2-2-8 Comprehensive Logging Results of ZK002 in Dataigou Iron Mine

Dataigou mining area is divided into Sinian system, Neoproterozoic Qingbaikou system, Proterozoic Liaohe Group and Neoarchean Anshan Group from top to bottom. From the lithologic characteristics, it is composed of sandstone, shale, marl, marble, schist and iron ore. The highest natural gamma value in the well is black shale, followed by marl, schist, sandstone and marble, and the lowest is iron ore (see Figure 2-2-8), especially the content of banded magnetite and hematite quartzite below 1750m is close to zero.

According to the characteristics of natural gamma logging curve (Figure 2-2-8), the average variation range of natural gamma value of iron ore is 0 ~ 6 API, and the surrounding rock changes greatly and is unstable, with the variation range of 40 ~ 120 API, which is obviously higher than that of ore body. This shows that the radioactivity of Dataigou iron mine is extremely low or does not contain radioactivity, which has little effect on iron ore prospecting.

(5) Main achievements

Through the exploration and geophysical exploration work of Dataigou Iron Mine, through comprehensive research and analysis, a more practical geological interpretation has been made. The distribution characteristics and scope of gravity and magnetoelectric anomalies of iron ore are discussed in space, which provides effective geophysical basis for iron ore exploration in this area and obtains certain geological achievements and understanding. There are mainly the following aspects.

1. Geomagnetic survey

Through high-precision magnetic survey, the location and range of magnetic anomalies in the survey area are delineated in detail. According to the abnormal characteristics, it is qualitatively inferred that it is caused by iron ore bodies, and quantitative calculation is made. It is inferred that the average buried depth at the top of the ore body is 1 103m, the horizontal width is 1029m, and the strike length of the ore body is about 6000m (No.1, 2 and 3).

2.EH-4 electromagnetic profile measurement

The horizontal scale and vertical extension of the deposits in this area show good prospecting prospects and the potential of large and super-large deposits. However, the buried depth of the deposit is large, and the ore body is about 1 100m below the ground.

Eh-4 method in Dataigou mining area can well reflect the top boundary of deep large concealed iron ore, and roughly infer the ore body boundary, which is fast and convenient, and has important guiding significance for the layout of deep drilling engineering. Judging from the measurement results of this EH-4 electromagnetic method, this method also has its limitations: First, the repeatability of the measurement results is poor due to the interference of local climatic conditions; Second, above the strong magnetic ore body, the abnormal area of low resistivity reflected by geomagnetism "drifts" relative to the real position of the ore body, which makes it difficult to determine the real position of the ore body.

In order to change the above two situations, it is suggested to set a known datum point in the work area. Before starting work every day, measure the datum point and record the data as reference value, so as to normalize the data after construction, eliminate interference and correctly infer the geological body. Therefore, when working in the strong magnetic field, we should combine the geomagnetic survey results and make comprehensive analysis and inference.

IV. Verification results

There are many solutions to the interpretation of a geophysical method. In order to accurately provide the target position for drilling layout, the comprehensive geophysical exploration method can well determine the center of the anomaly, determine the boundary and depth extension of the anomaly, and more importantly, accurately analyze the morphological structure of the anomaly.

The main working methods in this area are ground high-precision magnetic survey and magnetotelluric sounding (EH-4). The magnetic anomaly in Dataigou work area was discovered by aeromagnetism in 1990s. Because the ore body is deeply buried, three boreholes were drilled before 2008, and no ore body was found. Through the ground magnetic survey in 2008, it is inferred that the magnetic anomaly is caused by magnetic rocks. Magnetic method can only infer the top buried depth and bottom of ore body, but can not infer the physical property information of geological bodies with different depths; Eh-4 can well reflect the information of different depths of geological bodies. For example, the boundary value of resistivity change is used to divide the boundary range of abnormal body. These two geophysical methods can well determine the shape of concealed geological bodies, reduce drilling expenses and improve the accuracy of prospecting and exploration.

Through surface magnetic survey and magnetotelluric sounding, combined with the geological background of the mining area, a total of 2 1 boreholes were laid out, and no ore bodies were found in the other boreholes, except 309709 and hydrological boreholes. To sum up, comprehensive geophysical prospecting has a good guiding role in finding hidden ore bodies.

(Contributed by: Zhang Mali Tong Wang Changfeng)