Among the important metal metallogenic belts in China, there are three metallogenic belts in Xinjiang. The northern part is Altai metallogenic belt, including Altai mountain area and northern and northwestern margin of Junggar, which belongs to the active belt of Altai continental margin of Siberia plate and Junggar microplate area of Kazakhstan plate. Eight key metallogenic prospect areas are divided, striving to become an important copper, nickel, lead, zinc and gold base in China; There are 49 kinds of minerals such as copper, iron, chromium and gold with proven resource reserves, and there are nearly 100 large and medium-sized mines, with iron ore alone reaching 200 million tons. The Middle Tianshan metallogenic belt, located in the hinterland of Central Asia, is an important part of the ancient Asian metallogenic domain, including 18 metallogenic prospect areas; 136 minerals have been discovered, mainly copper, nickel, gold and other metal minerals, and iron ore is listed as one of the main minerals. The southern part is the West Kunlun-Altun metallogenic belt, which is located in the overlapping part of the PaleoAsian metallogenic domain and the Tethys-Himalayan tectonic domain, including11Ⅳ level metallogenic prospect. Mainly concentrated in copper, lead, zinc, iron, tungsten and other minerals, iron ore alone, it is estimated that the new resource reserves will reach 400 million tons.
The harsh natural geographical environment and field working conditions in this vast area restrict the effective application of geophysical and geochemical exploration methods and techniques. There are two unfavorable conditions, favorable conditions 1:① high altitude (3500-6000m), large height difference (above 1000m), complex geological structure, severe terrain cutting in alpine region, and it is impossible to carry out ground geophysical and geochemical exploration methods; (2) According to the existing regulations, "the aeromagnetic flying height generally does not exceed half of the line distance", so it can be seen that this kind of area is an aeromagnetic forbidden zone of1∶ 50,000; ③ The geological and metallogenic conditions are superior, and the regional aeromagnetic anomalies are obvious, so the search for large and medium-sized metal deposits has a good premise, which is highly valued by domestic authorities and prospectors. At present, deep prospecting in Xinjiang is shifting from regional development to middle and low mountain areas (West Tianshan, Kunlun-Altun, etc.). ) has the suitable conditions for comprehensive prospecting.
In this case, experts from Xinjiang Bureau of Geology and Mineral Resources and China Center for Aerial Geophysical Exploration and Remote Sensing of Land and Resources, on the basis of magnetic forward simulation of a large number of typical deposit models, selected typical deposits for comparative flight at different flying heights. The results show that the anomaly reproducibility is good at higher flying altitude. On the basis of the great improvement of the precision, compensation technology and positioning accuracy of airborne magnetic survey instruments, the high-precision airborne magnetic survey in alpine mountain areas1∶ 50,000 will provide important information. From 2007 to 20 10, the Xinjiang Uygur Autonomous Region invested special funds to carry out1:50,000 aeromagnetic surveys in areas with different landforms, geological backgrounds and geophysical conditions, such as Awulal, Karakorum, West Kunlun and Altun-Qimantage, with a cumulative area of 1 1.
I. Comparative test of typical mineral deposits
Comparative flight measurements were made at different heights in Chagangnuoer large volcanic sedimentary iron deposit, Qimantagedimunake large sedimentary metamorphic iron deposit and Panlongfeng skarn iron-copper polymetallic deposit in the western Tianshan Mountains. The results show that (Figure 2-3- 1), under the condition of flying height less than 800m, aeromagnetism has a good reflection ability for large and medium-sized magnetic minerals and magnetic geological bodies with a certain buried depth.
(1) Chagangnuoer Iron Mine
Chagangnuoer iron mine is located in the late Paleozoic Avulale rift zone in the western section of Xitian Mountain, and the ore-bearing rocks are a set of Carboniferous pyroclastic rocks. 1 1 iron ore body is delineated on the surface of the mining area, and copper mineralization is common at the bottom of the iron ore layer. Feizhu orebody is 2835 meters long and 4 1 ~ 120 meters thick, with a maximum dip angle of 320 meters and a total iron grade of 30% ~ 35%. It belongs to marine volcanic sedimentary deposits.
In Chagangnuoer Iron Mine, flight tests were carried out at 400m (controllable minimum flying height), 600m (safe flying height), 800m, 1000m, 1200m and 1500m6, respectively, and the profile orientation was close to the ore body trend (Figure 2-3-/klm) The results show that the background of abnormal areas with different flying heights is stable, and the local anomalies are obvious, with the characteristics of positive southwest and negative northeast. See table 2-3- 1 for specific indicators. The abnormal intensity gradually decreases with altitude, and the attenuation is the fastest from the surface to 400m, from 10000nT to 950nT, and only 42% from 400m to 800m; From 400 meters to 800 meters, the width of zero point increased from 3500 meters to 3750 meters, and the width of half extreme point increased from 1 150 meters to 1500 meters. Combined with the results of quantitative simulation in other areas, there are obvious magnetic anomalies for large and above iron mines under the conditions of flying height of 800 ~ 1000 meters and top buried depth of 400 meters.
Table 2-3- 1 Comparison Table of Test Curves at Different Heights of Chagangnuoer Iron Mine
sequential
Fig. 2-3- 1 aeromagnetic contrast curves of different flying heights in Chagangnuoer Iron Mine
(2) Dimunak Iron Mine
Dimunak iron mine is located in the western Kunlun area and the Qimantage composite trench arc belt on the southern edge of Tarim plate. The main exposed stratum is Tanjianshan Formation of Middle-Upper Ordovician, which is a set of shallow metamorphic clastic rocks, argillaceous rocks and a small amount of volcanic rocks. It is the ore-bearing rock series of sedimentary metamorphic iron ore. There are 37 ore bodies on the surface, mainly Fe36 and Fe37. Among them, the length of Fe37 ore body has been controlled to 2200m, the width is 6 ~ 100m, the cumulative apparent thickness is 18.5 ~ 59m, the grade total iron content is 25.3% ~ 39. 18%, and the dip depth of the main ore body has been controlled to 90 ~ 450m.
The surface magnetic anomaly develops in NW-SE direction, with a length of about 4000m, a width of about100 ~ 500m, and a maximum intensity exceeding100 ~ 500m. Experiments were carried out at four different heights (500 m, 700 m, 900 m and 1200 m) perpendicular to the ore body strike in this mining area (Figure 2-3-2). The local anomaly is clear and the background is stable, which is a positive and negative associated anomaly with positive magnetic anomaly as the main body. At a height of 500m, the strength is 2 10nT, and gradually decreases to 170nT, 120nT, 100nT. The abnormal width does not increase obviously with the increase of flying height.
(3) Panlongfeng iron polymetallic deposit
Panlongfeng iron polymetallic deposit is located in the southeast of Dimunak iron mine. The ore-bearing strata are the Proterozoic Baishahe Formation, and the lithology is biotite plagioclase gneiss, biotite plagioclase amphibole mixed with quartzite and marble. Its original rock is argillaceous sedimentary clastic rock+basic volcanic rock+carbonate rock. The ore-bearing zone of Panlongfeng Iron Mine is divided into five iron mineralization zones (sections): the western section, the middle section, the northern section, the northeast section and the Yichanbada section. The west section (i.e. the experimental area) is 460m long and 30 ~ 120 m wide, which is mainly distributed in the outer contact zone of adamellite, marble and gneiss. The scale is from the mine.
1 ∶ 25000 Surface magnetic survey circled 8 sporadic local anomalies in Panlongfeng iron mine area. Local magnetic anomalies are characterized by sharp, narrow, changeable curves and poor continuity. The scale is generally within 100m, and the maximum intensity is above10000t. Magnetic anomalies are related to iron ore and skarnization. After comparative flight at different altitudes of 400m, 650m and 800m3, the 400m height anomaly is extremely clear, and the characteristics of three superimposed anomalies are obvious, with the highest intensity of180 nt; The anomalies at 650m and 800m are similar in shape, belonging to low and slow anomalies, with a high degree of superposition, which can be identified (Figure 2-3-3). This anomaly is related to ore-bearing geological bodies and contact metasomatic mineralization alteration zones.
Second, the application of methods and technologies
(A) the main technical measures and abnormal decomposition
The aeromagnetic work of Xinjiang mountainous area1∶ 50,000 is undertaken by China Land and Resources Aerogeophysical and Remote Sensing Center. Use a II aircraft to fly roughly along the terrain, equipped with observation systems such as HC-2000 optically pumped magnetometer, SC 1 soft compensation and recorder, GG24 dual constellation GPS navigator and BG3.0 altimeter. Optimize the flight plan (especially the flight direction) according to the topographic characteristics and favorable metallogenic parts. The line spacing is 500m, the sampling rate is 10 times /s, the total accuracy of magnetic survey is better than 3.0nT, and the positioning accuracy is better than10 m.
Figure 2-3-2 Aeromagnetic Contrast Curve of Dimnak Iron Mine at Different Flying Altitude
Flight altitude is the main factor that restricts the popularization of1∶ 50,000 aeromagnetism in alpine mountain areas. Based on the safe flying height of the aircraft, combined with the topographic factors of the working area and the test results, the technical measures of flying height design on the digital topographic map are adopted through special software. The average flying height of the four survey areas is 639 ~ 756 m, and the highest flying height exceeds 1200m, among which the proportion of survey points below 800m is 74% ~ 86%. The aeromagnetic results show that the background magnetic field is clean, the regional magnetic field characteristics are obvious, and the local anomalies are well decomposed. The local anomalies caused by known iron ore and intermediate ultrabasic rocks are clear, and the 1∶ 1 10,000 aeromagnetic anomalies are decomposed in detail, and the anomalies have a good spatial correspondence with known magnetic geological bodies. See Table 2-3-2 for the topographic conditions, flying altitude and anomaly resolution of each survey area.
Table 2-3-2 Abnormal Decomposition Results of Four Aeromagnetic Exploration Areas
(2) Introduction of application results
1. Awulale iron ore-forming belt
The working area is located in Dongbeizhan Iron Mine of Awulale metallogenic belt in the western Tianshan Mountains. Iron deposits (spots) such as Chagangnuoer and Beizhan were discovered in 1970s and 1980s, but they have not yet become the main iron ore belt in Xinjiang. 2 1 century, according to the preliminary investigation results, Chagangnuoer, Zhibo Glacier and Zhanbei iron mines are all large-scale or above, and Songhu iron mine is medium-scale or above, belonging to volcanic rock type. A total of 474 aeromagnetic anomalies were sorted out in the whole region, including 96 anomalies. 2% (456 exceptions) are newly numbered. Through comprehensive analysis and interpretation, there are 10 anomalies caused by known iron mines (points) such as Chagangnuoer, Zhibo Glacier, Zhanbei and Songhu, with large anomaly range, steep gradient and good coincidence, which shows a good prospecting prospect. After anomaly verification, medium-sized and above prospective iron mines such as Dunde, Nixintage and Songhu South have been newly discovered. This is by far the largest metallogenic belt in Xinjiang.
Among them, the local anomaly of Chagangnuoer is obvious, which is a peak anomaly under strong magnetic background, with nearly east-west strike and symmetrical profile curve. Exception length is 1. 6km with a width of 1. 3km, and the strength is 12 15nT. The high background magnetic field is related to intermediate-basic volcanic rocks, and this local anomaly has a good corresponding relationship with the known Chagangnuoer iron mine (Figure 2-3-4).
2. Zankan-Subhas iron ore metallogenic belt
The working area is located in Karakorum Mountain, which is an important iron ore belt discovered and developed by aeromagnetism through regional geological and mineral survey and mineral evaluation in 2 1 century, and has outstanding prospecting prospects. The medium-sized and above prospective iron ores in Zankan, Veteran, Yearick and other places belong to sedimentary metamorphic iron ores. 373 local aeromagnetic anomalies were screened out in the whole region, including 362 newly compiled aeromagnetic anomalies. Among them, 8 anomalies caused by known magnetite such as Zankan and Laohe, and 6 anomalies such as Zankandong, Yearick and Tashaner were preliminarily verified by the ground as magnetite. The aeromagnetic achievements in this area further established the important position of the Zankan-Subhas sedimentary metamorphic iron ore belt.
Among them, Zankan Iron Mine is located in Tashkurgan Block, where the middle-deep metamorphic rock series of Proterozoic Bulunkuole Group (Pt 1B) is exposed (Figure 2-3-5c), which is also the ore-bearing rock series of iron mine. There are three iron mineralization layers and 18 ore bodies in the inner circle of the area, in which M 1 ore body is 3600m long, the controlled dip depth is 200 ~ 400 m, and the thickness of a single engineering ore body is 4. 28m, with an average score of TFe29. Belonging to volcanic sedimentary metamorphic iron ore. On the aeromagnetic Δ T profile (Figure 2-3-5a), there are two spike-shaped strong magnetic anomalies; On the plan of aeromagnetic δT isoline (Figure 2-3-5b), there are two elliptical anomalies with long axes running in the northwest direction, with obvious negative values on the north side (Figure 2-3-5b). Among them, the new C-2008- 1258 anomaly maximum point is located in the valley with an altitude of 4,250 m, the flying height is 900m, and the anomaly intensity is 615 nt; The maximum anomaly point of the new C-2008- 1259 is located at the top of the mountain at an altitude of 4980m, flying below 400m, and the anomaly intensity reaches 1444nT. There are obvious anomalies in Zankan iron mine, and the results show that there is still a good prospecting prospect in the eastern part of the controlled iron mine. After verification in 2008, new ore bodies were discovered.
Figure 2-3-3 Aeromagnetic Contrast Curve of Panlongfeng Iron Mine at Different Flying Altitude
Three. conclusion and suggestion
1) 1∶50000 aeromagnetism is effective and cost-effective. The cost of aeromagnetism is 40% ~ 65% of that of the same scale surface magnetism method. The aeromagnetism is less disturbed by topography and the density of measuring points is high (the space between aeromagnetism points is equivalent to 10 ~ 15m). It is suggested that the aeromagnetic survey of1∶ 50,000 in the middle-high mountain area with an average flying height of less than 800 ~ 1 1,000 m should be taken as an integral part of the national basic geological mapping, which should be uniformly deployed and implemented by stages. At present, it is gradually popularized from strong magnetic field area, variable magnetic field area and important metallogenic belt (such as western Tianshan area in Xinjiang and Kunlun-Altun area).
Figure 2-3-4 Comprehensive Map of Aeromagnetic Anomaly in Chagangnuoer Iron Mine
Figure 2-3-5 Comprehensive Plan of Zankan Magnetite
2) Screening and verification of aeromagnetic anomalies is the key process to deepen the understanding of aeromagnetic anomalies, and the combination of effective methods such as geology, magnetism and geochemical exploration with strong pertinence is the key to making a breakthrough in prospecting. It is the only magic weapon to deepen and develop anomalies by combining aerial geophysical exploration with ground geophysical exploration and geophysical exploration with geology and increasing the workload of verification.
3) In order to ensure the unique geological effect, it is necessary to carry out early metallogenic prediction and target area optimization in the flight area. Special flight plans (such as multi-stage nested flight or oblique flight) are adopted for important prospecting targets to achieve the purpose of reducing abnormal flight height. Therefore, in terms of technical design, it is particularly required to increase the special flight expenses in the budget.
4) In view of the fact that the intensity and range of magnetic field anomalies obtained at high flying altitude become lower and the difficulty of anomaly identification increases, it is necessary to further establish anomaly identification standards in different geological environments, develop special data processing software for middle and high mountain areas, especially downward continuation and weak anomaly extraction software, and attach importance to the identification, extraction and processing technology of single-line anomalies.
5) In view of the special situation that the focus of flight in high mountainous areas is on plateau, mountain and mountainside, the valley and some low mountainous areas are not considered enough (generally less than 25%), and the workload of ground magnetic method should be supplemented as much as possible.
6) Due to the large amount of continuous magnetic survey information, the investment does not increase much. Now there are ground equipment (such as G-858 high-precision proton precession magnetometer) that can be used for continuous positioning measurement. It is suggested that continuous ground magnetic survey should be advocated in exploration technical specifications and commercial geological exploration, the successful experience of aeromagnetism should be absorbed under the premise of limiting linear density, the concept of point distance at different scales should be diluted, the magnetic survey information should be expanded, and the geological effect of magnetic survey application should be improved.
Acknowledgement: The aeromagnetic survey of Xinjiang alpine mountain area1∶ 50,000 was supported by Tian Jianrong, Assistant Chairman of the People's Government of Xinjiang Uygur Autonomous Region, and Dong Lianhui, Chief Engineer of Xinjiang Bureau of Geology and Mineral Resources. Sun, an expert from the Senior Consulting Center of the Ministry of Land and Resources, Xiong Shengqing, chief engineer of China Land and Resources Aviation Geophysical Exploration and Remote Sensing Center, and He Hao, director of China Geological Survey, gave specific technical help. Zheng Guangru, Zhou Daoqing, Song and Xinjiang Geophysical Prospecting Team members have made great efforts. Their joint efforts have promoted the continuous aeromagnetic exploration in Xinjiang alpine mountain area1∶ 50,000, and achieved good prospecting results. Here is just a brief summary of the previous achievements from the technical level, thank you.
References and reference materials
Chen Yuchuan, Liu Dequan, Tang Yanling, et al. 2008. Minerals and metallogenic system of Tianshan Mountain in China [M]. Beijing: Geological Publishing House.
DZ/T 0 142— 1994。 Specification for airborne magnetic survey
Sun, Sun Huanzhen, et al. 2002. Discovery history of China deposit (geophysical and geochemical exploration volume) [M]. Beijing: Geological Publishing House.
Xiong Shengqing, Yao, et al. Aeromagnetic Survey in the Midwest of Qinghai-Tibet Plateau [M]. Beijing: Geological Publishing House.
Zhang, Wang Youbiao, et al. 2006. Metallogenic regularity of main metal minerals in Xinjiang [M]. Beijing: Geological Publishing House.
Zhuang Daoze, Meng Guixiang, Chen. 2004. Preliminary attempt of induced polarization measurement in1∶ 50,000 metallogenic belt [J]. Geological Bulletin, 22 (7): 707-708.
(Contributed by Zhuang Daoze, Zhou Jianxin, Lan Xian)