Shilu Iron Mine is one of the important iron ore bases in China, with an iron ore grade of over 62%. It is the richest large-scale open-pit iron deposit in China. Shilu Iron Mine was originally developed as a copper mine. According to the Records of Changhua County, in the second year of Chongzhen in the Ming Dynasty (1629), Zhang Sanguang, the magistrate of a county, drove away the miners and announced that private mining of the Yayushan (Shiluling) copper mine was strictly prohibited. During the hundreds of years from Ming Dynasty to Qing Dynasty, Shilu copper mine was mostly privately mined, so mining was prohibited several times.
1933 Hainan Island established Qiongya Industrial Bureau to accept overseas Chinese investment to develop the island's resources. 1935, Qiongya Industrial Bureau sent people to Shiluling to investigate copper mines. It was unexpectedly found that the iron ore here is rich in reserves and high in grade, but it was not mined for various reasons. 1939 February, Japanese imperialist invaders set foot on Qiongdao. In order to realize their strategy of "fighting to support the war" and "supplying on the spot", they immediately sent an investigation team to explore Shilu Iron Mine and ordered Xizhiye Fertilizer Company to invest in large-scale predatory mining. By 1945, when Japan was defeated and surrendered, it plundered more than 3 million tons of iron ore from Hainan (including 2.69 million tons of Tiandu Iron Mine and 690,000 tons of Shilu Iron Mine).
1In August, 946, the former Hainan Iron Mine Preparatory Office of the Resources Committee of the Republic of China was established to take over Hainan Iron Mine, but some of the dismantled mine precision parts were shipped to Vietnam for sale, while others were shipped to Beihai, Guangxi for sale. Hainan Iron Mine, originally thought to be taken over by China people, is still alive and promising, but the result is stagnation. It was not until 1957 after the founding of New China that production resumed.
1957 ~ 1964 Hainan geological brigade carried out supplementary exploration in the mining area, 1957 ~ 1958 95 1 team of the aerial survey brigade of the Geophysical Exploration Bureau of the Ministry of Geology carried out1∶ 65438+/in the mining area. Among them, five anomalies are thought to be caused by concealed iron ore, all of which were discovered by drilling verification. Known proven industrial reserve+prospective reserves are 25.52 million tons, and the average grade of total iron is 46. 27%. There is still a resource crisis in the mining of old mining areas. In 2007-2008, the national crisis mine set up a special exploration project for replacement resources, which was jointly explored by Hainan Provincial Institute of Resource Exploration and Geophysical Exploration Brigade of Guangdong Geological Bureau. Geophysical and geochemical exploration and drilling methods were used to carry out general survey and prospecting in Yi Bei-Hualishan and Nankuang-Chaoyang, and pre-survey was carried out in Jixin, Wulie and Jinniuling areas outside the mining area. According to the geophysical prospecting results and geological conditions, three iron ore prospecting areas have been delineated. 40 million tons of iron ore resources and 20,000 tons of copper and cobalt metal were added.
I. Geological background of the deposit
Shilu iron mine area is located in the west section of Shilu fold belt of South China fold system. Multi-stage tectonic activity and metamorphic-magmatic transformation have formed a structural framework dominated by east-west tectonic-magmatic belt and northeast tectonic-magmatic belt. The regional metallogenic geological structure belongs to Nanling metallogenic belt, which is one of the important metal metallogenic belts in China. Its metallogenic conditions are very superior, and it is an important metallogenic prospect of metallic minerals, nonmetallic minerals and rare rare rare earth minerals in China.
The strata exposed in the mining area mainly include Qingbaikou System and Sinian System (Figure 2-4- 1). The Fe-Co-Cu deposit in the mining area is mainly produced in Qingbaikou formation, which can be divided into six layers according to lithology, among which the first, third, fourth and fifth layers are white or dark gray-gray purple mottled phyllite, quartz schist or siliceous and aluminum rocks such as sericite phyllite and quartzite, generally containing andalusite, and the fifth layer is also sandwiched with a layer of lithic tuff; The second and sixth floors are grayish white-light gray dolomite, diopside tremolite dolomite, diopside tremolite, hematite and quartzite. The sixth layer is the main occurrence rock of iron, cobalt and copper minerals at present. According to the lithologic combination and its relationship with mineralization, it can be subdivided into three sections: the lower section contains cobalt and copper; Middle iron-bearing horizon; The upper dolomite of carbon-bearing phyllite is an iron-free section, which belongs to the ore-bearing layer in dolomite mine. Shilu iron mine is a volcanic-sedimentary metamorphic deposit controlled by strata, and the ore body is layered.
Shilu mining area is a famous large-scale ore concentration area dominated by iron ore in China. Besides iron ore, there are metals such as copper, cobalt, nickel, silver, lead and zinc, and nonmetallic minerals such as dolomite, quartzite, barite, gypsum and sulfur. The main minerals found near the mining area are: iron, copper, lead, zinc, tungsten, tin, gold and other metals, as well as limestone, clay, quartz sand, zirconium, titanium sand and other non-metallic deposits (spots).
Second, the geophysical characteristics of the mining area
(1) magnetic characteristics
The aeromagnetic δ T anomaly (Figure 2-4-2) in the iron ore area strikes nearly east-west. Anomalies are accompanied by positive and negative anomalies, and positive anomalies are located in the south with sparse distribution and small gradient; Negative anomalies are distributed in the north, with dense distribution and large gradient. The highest anomaly intensity is 600nT and the lowest is-900nT, and the negative anomaly is greater than the positive anomaly. Δ t positive anomaly field distribution, its gradient change shape is in good agreement with the complex syncline structure shape of the whole mining area. The distribution characteristics of δ T are closely related to the distribution and structure of ore-bearing rock series in the mining area. The law of magnetic anomalies on iron ore bodies in this area is that anomalies are accompanied by positive and negative anomalies, and negative anomalies appear on the north side of ore bodies. The center of magnetic anomaly deviates from the center of ore body, and the ore body is located between the maximum of positive and negative anomalies, generally near the zero line.
See table 2-4- 1 for the magnetic parameters of rocks and ores in the area. Hematite is strongly magnetic, diorite and structural breccia are weakly magnetic, and other rocks are not magnetic or weakly magnetic.
Figure 2-4- 1 Geological Map of Shilu Iron Mine Area
Table 2-4- 1 core magnetic survey statistics table
sequential
Figure 2-4-2 Aeromagnetic δδT(nT) Anomaly Map of Shilu Iron Mine Area
(2) Electrical characteristics
In 2007, the core resistivity of ZK11KLOC-0/and ZK3 boreholes in the mining area was measured systematically. According to statistics, hematite has the characteristics of low resistance (265 Ω m). The resistivity of dolomite, dolomite limestone, timely sandstone and diorite in most surrounding rocks is 325 ~1500 Ω m, with an average of 700 Ω m, showing the characteristics of medium and high resistivity.
(3) Geological and geophysical model of the deposit
The mining area is a famous large-scale ore concentration area dominated by iron ore in China, accompanied by polymetallic and nonmetallic minerals. According to the results of physical property determination, the resistivity of ore is low, the resistivity of mineralized rocks is relatively low, the electrical properties of various strata are obviously different from those of various mineralized rocks, and the iron ore in the area has strong magnetism, so the geological-geophysical model of this kind of deposit can be determined as the prospecting model of low resistivity and high magnetism.
Third, the application and verification effect of geophysical methods and technologies
(A) the design method and use of the instrument
In 2007-2008, the geophysical prospecting design selected 1 ∶ 1000 magnetic method, CSAMT, TEM and borehole geophysical prospecting, and its main tasks were:
1) Based on high-precision ground magnetic survey and previous gravity and magnetic data, comprehensive interpretation is made to mine deep prospecting information.
2) The main task of controlled source audio-frequency magnetotelluric method is to delineate the strata and structures within the range of 1 depth. The key point is to find out the distribution of the sixth layer of Shilu Formation. When the geological conditions are favorable, it is used to track concealed ore bodies.
3) The main task of transient electromagnetic method is to find and trace hidden ore bodies, which are used to explore the middle and lower carbon-bearing layers (the main occurrence rocks of iron, cobalt and copper minerals) in the sixth layer of Shilu Group, and the exploration depth is 500 ~ 1200m.
4) The main task of three-component magnetic survey in well is to judge the abnormal source and its nature, and infer the depth, direction, location, extension, range and thickness of blind ore.
See table 2-4-2 for the main instruments and equipment invested.
Table 2-4-2 List of main instruments and equipment invested in replacement resource exploration and geophysical exploration in Shilu Iron Mine
(2) Work deployment
In Shilu iron mine area, the depth, edge and periphery of Jixinling, Yi Bei-Hualishan and Nankuang-Chaoyang were surveyed, and the metallogenic geological conditions and ore-controlling factors of iron polymetallic deposits in Shilu area were deeply studied to improve the prospecting effect.
(3) Interpretation and inference of geophysical anomalies
1. Interpretation and inference of controlled source audio magnetotelluric method
1) data processing and inversion effect of controlled source audio magnetotelluric method. Collate and edit Cagnard resistivity and impedance phase data. Because different filtering methods show different results, the lateral distribution of strata and the inversion results of local ore bodies can be highlighted as needed. Fig. 2-4-3 shows the comparison results of two filtering methods for E 1 1 line. It can be seen that in order to highlight the ore body, only through experimental comparison can better results be achieved.
Stereogram of Cagnard resistivity profile.
A. Generally, the high frequency band is of medium and low resistance (tens to hundreds of ohm meters), with uneven distribution, which mainly reflects the Quaternary strata and shallow strata with uneven electrical properties (as shown in Figure 2-4-4 and Figure 2-4-5).
B. In the middle and low frequency band, the low resistivity (tens to hundreds of ohm meters) layer mainly reflects the sixth layer (QnS6) stratum, which is the main ore-bearing stratum, so the resistivity is low; The high resistance layer (hundreds to thousands of ohm meters) is mainly the reflection of the fourth layer (QnS4) and the fifth layer (QnS5).
In the middle and low frequency of C, a closed low-resistance ring is seen, which is concave and is the axis of compound syncline. This is a favorable part of ore-bearing and has important prospecting significance.
D has extremely high resistivity (>10000 Ω m) in low frequency band, which is a reflection of entering the transition zone or near zone.
3) Impedance phase slice stereogram.
A. The phase of middle and high frequency bands is generally higher than 400 mA, and the phase of middle and low frequency bands is lower than 400 mA. This shows that the resistivity of the upper stratum is lower than that of the lower stratum. After entering the low frequency band, the impedance phase drops rapidly and tends to zero or even negative. This is a reflection of entering the transition zone and the near zone.
B. The impedance phase of the main axis of the ore-forming syncline is generally higher than 1000mard, and the middle-low frequency band is higher than the high frequency band, which reflects that the bottom of the syncline axis has low resistivity and is a favorable part for ore-bearing.
Fig. 2-4-3E 1 1 Line inversion results of different filtering methods.
Figure 2-4-4 CSAMT Isofrequency Slice and Isoelevation Resistivity Stereograph
Figure 2-4-5 CSAMT inversion resistivity -400 m elevation plan
4) Inversion of resistivity slice isosurface.
A. Generally, it reflects the geoelectric profile with low resistivity in the upper layer and high resistivity in the lower layer. In addition, the sixth layer (QnS6) and Carboniferous (C 1) show low resistivity, while the fourth layer (QnS4) and the fifth layer (QnS5) show high resistivity.
B. The syncline axis shows low resistance and obviously extends downward in the shape of "pot bottom". Generally speaking, the ore body does not show the lowest resistivity, but the medium-low resistivity (Figure 2-4-5).
2. Transient Electromagnetic Method (TEM) (Wu Zhuo and, 2007)
1) directly reflects the characteristics of transient electromagnetic anomalies of ore bodies. Known ore bodies are located at 30 14 ~ 3064 /E 15, and the ore bodies strike southeast, with a width of about 50 ~ 100 m and a shallow burial depth of about10 ~ 30 m. ..
Fig. 2-4-6 is a TEM voltage profile of line E 15. The response voltage at 30 14 ~ 3064 /E 15 on the profile is strong, and the voltage profile rises abnormally at the second pass after shutdown (6 1.0μs), which fully reflects that the ore body is buried shallowly. Around 30 14 ~ 3064/E 15, the voltage value on the small point side rises slowly, while the voltage value on the large point side decays rapidly. Based on this, it can be inferred that the width of ore body (low resistance body) can extend to the small point direction (west) 1 ~ 2 measuring points, that is, 50 ~ 100.
Figure 2-4-6 E 15 line voltage curve profile
By analyzing the TEM data of the above-mentioned known ore bodies, it can be seen that hematite is a low-resistivity body, and the induced voltage caused by it is very strong and large, showing the abnormal morphological characteristics of "rainbow" on the voltage profile. The shallower the buried depth of ore body, the earlier the induced voltage anomaly appears; On the contrary, the later.
2) Reflect the characteristics of structural transient electromagnetic anomalies. Figure 2-4-7 shows the voltage curve of E 1 1 line. Hole ZK 1 10 1 is located at. 4 150/E 1 1, the hole depth is 487 ~ 670 m, and hematite is found. Both geological data and CSAMT data show that the ore body exposed by ZK101is located in the axis of syncline (point 4050 ~) TEM voltage profile also clearly reflects the structural form of the syncline, which is characterized in that the voltage curve rises at the two wings of the syncline and falls at the axis, showing a "pot bottom" shape, which is particularly prominent in the middle and late period of channel measurement.
Figure 2-4-7 E 1 1 line voltage profile
The abnormal shape of "pot bottom" shown by voltage curve is very similar to syncline structure. The author thinks that this is because there are many sulfides at the bottom of the sixth layer of syncline, and the strata are broken and filled with water to form a thick and low resistivity layer. The low resistance layer on the wing is shallow and the axis is deep. At first, the induced voltage will be abnormal in the shallow low-resistance part (wing), and at the same time, the induced voltage is weak because the induced eddy current does not reach the deep low-resistance layer (shaft).
Due to the particularity of shallow low conductivity, it has a considerable shielding effect, and the transient electromagnetic method sometimes cannot reflect the ore body; But it can reflect the main ore-bearing part of the syncline axis, which is also of guiding significance to the work. According to the above characteristics of transient electromagnetic anomalies, six transient electromagnetic anomalies are delineated.
3. High-precision magnetic measurement results
High-precision ground magnetic survey is consistent with aeromagnetic anomaly. With the improvement of working accuracy, surface magnetic survey shows more detailed and accurate anomaly characteristics, and highlights shallow anomalies and local anomalies. Through compensation smoothing filtering, 100m, 200m, 500m (Figure 2-4-8 ~ Figure 2-4- 10) are extended, which eliminates shallow and surface anomalies, highlights deep anomalies and becomes an associated anomaly body with positive south and negative north. From the anomaly maps of 50m, 100m, 200m and 500m, it can be seen that the higher the extension, the more obvious the general anomaly trend, from northwest to near east and from 500m to near east and west. The location of low-latitude ferromagnetic ore bodies often corresponds to the positive and negative transition zone of magnetic anomalies, indicating that the deep ore bodies are large in scale, wide in scope, nearly east-west distributed, and the buried depth is more than1000 m m.
Figure 2-4-8 Plan of δ T (NT) anomaly in magnetic survey area
Figure 2-4-9 Abnormal Plan of Magnetic Survey Area Δ t (NT) extending 200 meters
Figure 2-4- 10 Abnormal Plan of 500-meter Upper Extension of δ T in Magnetic Survey Area
4. Three-component magnetic logging
Three-component measurement was carried out for deep wells in the survey area, and the salinity of each deep well was determined, which provided conditions for further drilling engineering. * * * A total of 8 borehole surveys have been carried out, all of which have achieved good results.
1)ZK2 logging results. Apparent resistivity logging, magnetic susceptibility logging, three-component magnetic survey in the well and high-precision δ T magnetic survey in the well were carried out. Measurement range: 8.36 ~ 647.3m. There are five ore beds and mineralized beds with a total thickness of 47.10m. 0 ~ 58m is the casing in the well. 142.40 ~ 144.90 m, 2.50m thick, hematite or mineralized layer; 164.90 ~ 203.90m, 39.00m thick, containing magnetic rock (mineralized) layer; 220.90 ~ 224.90 meters, 4.00 meters thick, hematite or mineralized layer; 293.30 ~ 293.90 meters, 0.60 meters thick, thin layer of magnetic mineralization; 310.60 ~ 31.60m thick, 1.00m thin magnetic mineralized layer (Figure 2-4- 1 1).
The magnetic data of the borehole show that the δ z curve is approximately inverted "C" at the depth of 320 ~ 500m. δz =-975 nt at 320m,-2695nt at 4 19m and-916nt at 500m. Δ t curves are all C-shaped. Δ t =-1120.34nt at 260m, Δ t =-2998 at 400m.1nt, and Δ t =-1043nt at 500m. Δ x and Δ y curves are both negative, indicating that the anomaly is in the fourth quadrant.
Fig. 2-4-2-4- 1 1 ZK2 Well Three-component Logging Curve
To sum up, it is preliminarily judged that there is an anomaly of blind ore beside the well in the 320 ~ 500 m interval of this hole. According to the shape and characteristic points of the abnormal curve, it is roughly judged that the buried depth of the center of the abnormal body is equivalent to the well depth of the hole of 400 ~ 420 m and about100 m away from the ZK2 hole; There are anomalies in the southwest of the cave. 200m southwest of the hole is ZK 1 10 1 hole, and the ore section is about 160m.
2)ZK3 logging results. In-hole magnetic susceptibility logging and three-component magnetic survey were carried out (Figure 2-4- 12). Measuring interval:16 ~ 808m. More than 34m is casing.
The main metallogenic or mineralized magnetosphere of this hole is: 582.00 ~ 6 10.50 m, 28.50m629.00 ~ 64 1.00 m, and12.00m; 680.50 ~ 683.50m, with a thickness of 3.00m; 690.50 ~ 700.00 meters, 9.50 meters thick; 706.00~734.50m, thickness 28.50 m801.50 ~ 808.50 m, thickness 7.00m, no obvious curve opening below 800m, judging that there is no big magnetic anomaly in a certain range near the hole bottom.
5. Deep prospecting effect
In this exploration, various geophysical methods, such as ground high-precision magnetic survey, controlled-source audio-frequency magnetotelluric method, transient electromagnetic method and borehole three-component measurement, were used to delineate the spatial state of iron ore bodies, and obvious prospecting results were obtained, which provided valuable information for deep prospecting in the periphery of the mine.
1) The traditional geophysical exploration methods are not effective in deep exploration and are easily disturbed by mine noise. Alternating magnetotelluric method can achieve good detection effect, and the detection depth of CSAMT and TEM methods is above 1000m, which greatly exceeds the detection depth of DC method. In the case of complex terrain conditions, the use of controlled source audio magnetotelluric method and transient electromagnetic method can greatly improve work efficiency. In technical application, it is necessary to determine relevant observation technical measures (such as device parameters, reliability of original data, etc. ) and data processing technology, and the outline of ore body is delineated by inversion technology that highlights local low resistance anomalies.
2) Ground high-precision magnetic survey delineates deep ferromagnetic ore bodies by data processing, filtering and upward extraction of deep metallogenic information, and obtains the buried depth of ore bodies, which has also achieved results. Through the fine measurement of borehole three-component magnetic logging, it will play an important role in the next drilling layout to know whether there are blind ore bodies around the well (citing mine examples).
Fig. 2-4- 12 ZK3 Well Three-component Logging Curve
IV. Verification results
1) From the results, it can be seen that the zero isoline after the magnetic δ T extends 500m upwards, CSAMT shows low-resistance anomaly and high-resistance phase anomaly, and TEM shows abnormal characteristics such as concave and convex, which shows the nature of ore anomalies. The verification results show that the seven boreholes inferred by geophysical exploration are in good condition. This part is undoubtedly the most favorable part for deep prospecting in this survey area.
2) Deep prospecting in crisis mines is a systematic project. Including comprehensive research, method and technology application and engineering verification. An important link in deep prospecting is the application of methods and techniques with the function of "high resolution and deep exploration". Due to the adoption of these new methods and technologies, more information of deep mineralized bodies is obtained, and the outline of ore bodies is delineated, which provides technical means for realizing the spatial positioning of "mineralized bodies".
References and reference materials
Wu Zhuo and. 2007. Application of Transient Electromagnetic Method in Mineral Exploration [J]. South China Earthquake, 27 (3): 26-43
Wu Zhuo and. 2009. Prospecting effect and comprehensive prospecting mode of high-tech geophysical prospecting methods in the periphery of crisis mines-taking XX deposit as an example [J]. Pan-Pearl River Delta, Hong Kong, Macao and Taiwan
Construction of regional geophysical research platform and proceedings of the first academic exchange meeting
(This part is written by Wu Zhuo and)