(Aerogeophysical Remote Sensing Center, Beijing 100083)

On the basis of systematically collecting, sorting out, analyzing and summarizing the previous research results, this study draws lessons from the geological characteristics of known rock gold deposits in neighboring areas, and makes full use of all kinds of existing geoscience information, especially a large number of regional structural information provided by Landsat TM images, to analyze the temporal and spatial distribution characteristics of gold deposits in the study area, so as to provide more detailed information for fine ground prospecting and prospecting prediction.

In terms of method and technology, based on Landsat TM image data, combined with topographic, geological, mineral and geophysical and geochemical data, comprehensive evaluation and prediction are carried out. In addition to extracting abnormal information of altered rocks and ground plants related to copper, gold and other metal sulfide deposits by using image processing system, the methods of variable extraction, transformation, comprehensive analysis and processing of various geoscience information are also explored, that is, with the help of geographic information system, the geological variables related to mineralization are screened by man-machine interactive processing, and the ore-bearing probability values of each geological variable are calculated by maximum likelihood estimation method. Finally, the probability of at least one ore-forming event in each grid area is calculated when the grid density is 4×4 and 1× 1 respectively. This provides quantitative analysis data for regional mineral resources analysis and prospecting target prediction.

I. Overview of the study area

(1) Geographical location and physical geography

The study area is located in the northern section of Xiaoxing 'anling in the east of Heilongjiang Province, and its geographical coordinates are 50 00' north latitude and124 00' east longitude in Heilongjiang Province. The Sino-Russian border is the northern and eastern border of this study area, with a total area of about 54000km2.

This area is located in the northeast border of China and belongs to the alpine forest region. Winter is long, summer is short, the annual average temperature is below zero, and the frost-free period is less than 100 days; The rainy season is from July to August, and snowfall begins in September.

The area is densely forested and swampy, with poor rock exposure conditions. Due to the extremely inconvenient transportation, bad working environment and the limitation of field equipment, it brings great difficulties to image processing, image interpretation and analysis, information extraction and ground inspection and verification.

(2) Geological survey

1. Strata

The exposed strata in this area have been exposed in all dynasties except Archean. In addition, some areas are representative of stratigraphic structure and stratigraphic group.

Proterozoic Xinghua Dukou Group and luoma lake Group are exposed in this area. The Lower Proterozoic Xinghuadu Group is mainly distributed in Xinghua, Hanjiayuanzi, Jiagedaqi and Songlinling in Huma County. From bottom to top, it consists of Xinghua Village Formation, Xing Qiao 'an Formation, Humahe Formation and Xiaogulihe Formation. Its main lithology is homogeneous migmatite, banded migmatite, biotite plagioclase gneiss, biotite amphibole schist, granulite, marble, banded magnetite quartzite and other deep metamorphic rocks. The Upper Proterozoic luoma lake Group is distributed in the Wolegen River, Hekuan River, luoma lake in the upper reaches of Nenjiang River and Tiemaoshan Farm. The main rocks are garnet-bearing adamellite schist, biotite plagioclase granulite, plagioclase gneiss, staurolite adamellite schist and marble interlayer.

The Paleozoic strata in the study area are developed and widely distributed. Mainly a set of excellent geosyncline sedimentary construction. Among them, Cambrian strata are mainly distributed in the northern part of this area, that is, Xinglonggou area. The main lithology is phyllite, sericite, chlorite slate, marble, argillaceous slate, volcanic tuff and lava. In this sedimentary formation, Devonian is a typical formation containing amphibole. Among them, the sodium-rich spilite and amphibole of the Lower Devonian Handaqi Formation, Jinshui Formation and Mishuhe Formation are the main source beds in this area, and most of the placer gold mining activities and the spatial distribution of rock gold spots are consistent with this set of strata.

Except Triassic, Lower Jurassic and Lower Cretaceous strata, Mesozoic strata in other periods are well developed and widely distributed, especially in the west and north of this area. The main characteristics of Jurassic strata are a set of continental sedimentary formations and continental volcanic formations. Cretaceous strata are mainly distributed in some depressions and fault basins, and are a set of inland volcanic-continental clastic coal-bearing formations.

2. Structure

Geotectonically, the study area is located at the intersection of Daxing 'anling and Xiaoxing 'anling fold belts. Although the strata in this area are well developed, it is difficult to restore a complete structural framework in the field because of magmatic activity and the destruction of fault structures, coupled with soil development, serious vegetation coverage and extremely poor stratum exposure conditions. However, through the interpretation and analysis of remote sensing images, we can find a lot of linear structural information. In particular, the identification and discovery of near east-west structural zones make up for the shortcomings of previous conventional methods.

In addition to the above-mentioned east-west structures, the northwest and northeast structures in the area are relatively developed, which are obviously reflected in remote sensing images.

3. Magmatic rocks

Magmatic activities are frequent in the study area, and the rock types are mainly intermediate-acid intrusive rocks and Mesozoic large-scale intermediate-acid continental volcanic eruption rocks. The former occurs mostly in large bedrock, among which the acid intrusive rocks in Variscan period are the most widely distributed, with the largest exposed area and frequent activities, and are most closely related to the main endogenetic metal minerals in the region, such as Duobaoshan copper-molybdenum polymetallic deposit and Sankuanggou skarn iron-copper deposit.

(3) Regional metallogenic geological characteristics

At present, no rock gold deposits with industrial value have been found except the Duobaoshan porphyry copper, molybdenum and gold deposits in the south of the study area. Therefore, this study has to learn from the known metallogenic characteristics of gold deposits in neighboring areas (such as Tuanjiegou large gold deposit in the south), and study the spatial distribution of gold deposits in the area by extrapolation according to geological conditions, sedimentary structure characteristics and metallogenic model. In addition, the information of modern placer gold mining activities and ancient gold mining in this area can provide very important clues for this study.

The existing geological data show that the gold mineralization in the study area is related to the intermediate-acid magmatism in the late Variscan and Yanshanian periods, and is controlled by the sedimentary formation of the upper geosyncline, that is, the Devonian spilite porphyry formation in the south and the lower Proterozoic Xinghua Dukou Group metamorphic rock series in the north belong to the main source beds in this area.

According to the comparison with the known deposits in neighboring areas and the analysis of the known gold spots in this area, it is found that gold mineralization is closely related to regional structure, which directly controls the shape and output position of ore bodies. The known gold mineralization in the study area mainly includes porphyry copper-gold deposits, metamorphic hydrothermal gold deposits and volcanic hydrothermal rock gold deposits. In addition, there are ancient placer gold deposits related to Mesozoic continental sedimentary formation, that is, conglomerate gold deposits. However, the reasons for this type are complicated and not included in the focus of this study.

Alterations related to gold mineralization include silicification, kaolinization, sericitization, limonitization, pyritization and arsenopyrite mineralization.

The gold mineralization in the study area is mainly controlled by the sedimentary formation of the upper geosyncline in the Lower Proterozoic and Paleozoic. In the interactive processing and analysis of GIS, it is found that the spatial distribution of most known ore occurrences and gold mining traces in this area is consistent with the distribution of these two sets of sedimentary formations. This shows that the source bed provides a material source for mineralization, metamorphism or magmatic activity provides a heat source and gas-water hydrothermal solution for mineralization, and structural faults provide a good channel and space for the migration and enrichment of ore-forming materials.

Map 1 geological model of gold mineralization

1-seawater; 2- carbonate rocks; 3- granite; 4- spilite; 5- corneal plaque; 6- Volcanic breccia; 7- clastic rock; 8- gold ore body; 9— ductile shear zone; 10- foliation zone; 1 1- fault

Figure 1 shows the geological model of gold mineralization in this study area. At the early stage of geosyncline development (figure 1a), with the violent subsidence of the earth's crust, large-scale violent submarine volcanic eruption caused a large amount of gold to be distributed in volcanic sedimentary construction to form ore source beds, but the gold content in the strata was far below the industrial grade (except near the crater). However, in the later period of geosyncline development, the sedimentary layer was strongly folded by orogeny and regional metamorphism (Figure 1b). Due to the increase of temperature and pressure, the original dispersed gold is activated, and at the same time, it precipitates and migrates with SiO2 _ 2 and metamorphic hydrothermal solution, and is enriched in a favorable position, that is, the ductile shear zone type or tectonic altered rock type gold deposit is formed. At the same time, in the process of orogeny, large-scale intermediate-acid magmatic intrusion also provided gas-water hydrothermal solution and heat source for mineralization, and further enriched gold in the source bed in magmatic hydrothermal solution, forming high-grade post-magmatic deposits or contact metasomatic deposits in favorable structural parts, while porphyry gold deposits were formed in some small branches or porphyry bodies.

In the late stage of orogeny, mineralization became complicated. On the one hand, it may destroy the early ore bodies, on the other hand, it may further enrich the existing ore bodies. In addition, conglomerate-type gold deposits, that is, ancient placer gold deposits, have been formed in the molasse formations deposited in some piedmont depressions and intermontane basins. The formation mechanism of this kind of deposit is extremely complicated, which mainly depends on the lithofacies paleogeographic environment.

Second, research methods.

(1) remote sensing geological interpretation

In this study, the pseudo-color synthetic images of 4, 5 and 7(R, G and B) bands in Landsat TM image data are mainly used. Because of the extremely developed vegetation and soil in this area, it is very difficult to interpret and analyze the lithology of the image. Nevertheless, the image can still reflect a large number of linear and circular structural features. Many linear structures have not been paid enough attention in previous work. Through comprehensive analysis and research, it is found that the east-west linear structure controls the strata in the study area (especially the gold-bearing strata in the northern region) and the spatial distribution of placer gold and rock gold, while the ring structure reflects the basic characteristics of volcanic mechanism and intrusive rocks to some extent.

1. Interpretation of linear structure

The linear structure of the study area is very obvious in Landsat TM images, and the regional structure is often closely related to the distribution of water system, reflecting the characteristics of structural water system. Mainly manifested as some linear valleys, abnormal linear tones and other image features. The linear structure of this area is described as follows:

(1) east-west linear structure

The east-west structure in the study area is relatively developed, and this conclusion is obtained through remote sensing interpretation and analysis. This structure is the oldest group of structures in this area, and it has become fragmented due to the influence of previous tectonic movements in the later period. Therefore, in remote sensing images, it usually appears as a series of intermittent parallel short-term valleys. This group of structures is obvious in the northern part of the study area and continues to extend eastward, especially in Russia. In previous work, people did not pay enough attention to this group of structures. Through the interpretation of remote sensing images and the comparative analysis of strata and gold mining traces in the region, it is found that the spatial distribution of this group of linear structures is basically consistent with that of Proterozoic strata and closely related to gold mining activities in ancient and modern times.

In addition, there is an east-west structural feature along the line from Heibaoshan to Jinshui in the south of the area. This structural zone is also clearly reflected in aeromagnetic data, especially in the processing results of north-south derivative, which clearly reflects the characteristics of gradient zone. It not only has a certain genetic relationship with Duobaoshan porphyry copper, molybdenum and other polymetallic deposits, but also obviously controls the distribution of quaternary Wudalianchi ancient basalt.

(2) NE-trending linear structure

It is one of the most developed linear structures in this area. It is also the most obvious in remote sensing images, with good continuity. Most of them are linear valleys, angular water systems, connecting lines of water system nodes, linear steep banks and micro-landforms and linear tone anomalies of cutting ridges. In addition, most modern rivers are controlled by this group of structures, such as Nenjiang River, Tahe River, Neiwolegen River and Gala River.

This linear structure controls the stratigraphic deposition, magmatic activity and even the spatial distribution of various metal minerals in this area. The structure of this group is often the dividing line between different geological units and different geological bodies, and the strata and intrusions of different ages have been cut, indicating that its formation age is relatively late.

(3) NW-trending linear structure

The NW-trending structures in this area are relatively developed and widely distributed. Large rivers such as Heilongjiang, Huma River, Wolegen River, Naduli River-Naked River, pilger River-Loach River are all controlled by this group of structures. It should be emphasized that the NW-trending structure obviously controls the endogenetic metal minerals in the study area. Such as Duobaoshan, Sankuanggou, Xiaoduobaoshan, Yubaoshan and other deposits and mineralization are all related to this group structure. In addition, through interactive processing and analysis in GIS, it is found that placer gold, ancient gold mining relics and partial rock gold mineralization in this area are also closely related to this group of linear structures, especially where rivers are distributed along this group of structural zones, such as Dalahan River east of Hanjiayuan.

2. Interpretation of ring structure

The ring structure in the study area is well developed. The manifestations in remote sensing images are also different, such as single-layer circular water system encirclement, multi-layer concentric water system and ridge structure (such as Duobaoshan ring structure), concentric and radial water system * * * together, and ring structure represented by comprehensive signs such as hue. As far as its formation is concerned, it is certain that there are ring structures related to volcanic mechanism, ring structures related to magmatic emplacement, and ring structures formed by ring faults formed by the end of fold structures and structural groups in different directions. At the same time, there are a large number of ring structures of unknown origin, so I won't go into details here.

(1) ring structure related to volcanic mechanism

This kind of ring structure is mostly in this area, especially in the north and west, but the most representative ones are Longyinshan ring structure in the east and Tayuan ring structure in the west.

① Longyinshan ring structure

On the TM false color composite image of Landsat, it presents the characteristics of more than half an oval image, with a four-layer concentric structure, the core and inner ring are dark green, the middle ring is blue-green and the outer ring is yellow. Geologically, the core and inner ring zone are composed of Jurassic neutral volcanic rocks; The central belt consists of Silurian and Devonian. The outer ring belt consists of Ordovician, Carboniferous and granite. In addition, in the survey results of natural heavy sand, there is an abnormal area of cinnabar and gold in this area, which is quite noteworthy.

② Ring structure of tower source

The annular structure is characterized by an annular valley and a nearly radial gully. It consists of Cretaceous subvolcanic rocks, volcanic rocks, Jurassic volcanic rocks to granodiorite and hornblende gabbro.

In addition, the ring image features such as Dabo, Houlle, Shaanxi ring structure and Hayuan ring structure are also very typical.

(2) Ring structure related to intrusive rock mass.

There are many such ring structures. Large-scale structures include Erdaogou ring structure, Xinglong ditch ring structure and Duobaoshan-Woduhe ring structure in Huma County.

① Erdaogou ring structure

The ring structure consists of core mountain, inner ring water system and outer ring water system. The core is composed of Proterozoic granite, and the outside is mainly surrounded by Caledonian granite. There are large placer gold deposits in this ring structure.

② Xinglonggou ring structure

Beixili-Xinglong early Variscan basic rock mass is the core, and Cambrian-Devonian and late Variscan intermediate-acid rock mass form a ring structure with a diameter of about 50 kilometers.

③ Duobaoshan-Woduhe ring structure

It is an annular structure composed of three scattered bodies and a series of radial valleys around them. North to Woduhe Forest Farm, southwest to Baoshan. In the middle and late Variscan, granodiorite, plagiogranite and Baigangyan intruded into Paleozoic, forming a ring structure.

(3) Ring structure formed by tectonic movement

(1) the ring structure of the South Mountain of Betula platyphylla Paizi.

The mountain formed by the Lower Silurian has an obvious turning point, and the strike of rock strata is consistent with the ridge, forming an arc. The peripheral rocks and ridges are also arc-shaped, which can be confirmed as an annular structure composed of syncline turning ends.

② Handa gas ring structure

The eastern part of the ring structure is cut by faults, and there is a ring water system on the other three sides. Several internal faults set off each other, and it is inferred that it is a ring structure bounded by faults.

3. Lithological interpretation

Considering the unfavorable conditions such as dense forests and soil development in this area, lithologic interpretation has not been included in the work focus. However, in areas where this remote sensing technology is difficult to play its special role, some stratigraphic and lithologic characteristics, especially some intermediate-acid intrusion characteristics, can still be identified. For example, the Zinan Mountain area in Hanjiayuan was mistaken for the lower Proterozoic Xinghua Dukou Group stratum in previous mapping. Through the interpretation and analysis of this Landsat image, it is believed that this area belongs to the Variscan granite, which was later confirmed by field investigation. This shows that even in this forest-covered area, remote sensing images can not only provide geologists with a lot of regional structural information, but also reveal some stratigraphic and lithologic characteristics. As Barry S.Siegal et al. (1980) pointed out, the main contribution of remote sensing technology to geology so far is to provide geologists with a lot of useful regional structural information and modify the existing geological map.

(2) Extraction of trace information of ancient gold

During the Seventh Five-Year Plan period, the Geology and Minerals Department of Heilongjiang Province came to the conclusion that rock gold deposits can be directly searched according to the distribution characteristics of placer gold. Therefore, analyzing and studying the spatial distribution characteristics of modern placer gold mining activities and ancient gold mining traces can provide a basis for the evaluation and prediction of regional rock gold deposits. Modern gold mining traces are shown in enhanced Landsat TM images and aerial photographs to varying degrees, while ancient gold mining traces (mainly the remains of placer gold mining activities during the Japanese puppet regime) can only be reflected in aerial photographs and corresponding topographic maps obtained in 1950s.

Figure 2 shows the distribution map of ancient gold mining relics in Xinglonggou-hanjiayuan sub-district. This kind of gold trace information plays a key role in the establishment of regional mineral prediction probability model, variable selection, parameter estimation and probability calculation. In the placer gold mining in the study area, a considerable part of placer metal belongs to residual placer gold, even weathered residual placer gold in the structural fracture zone. Therefore, studying the spatial distribution of this kind of placer gold can provide useful clues for the general survey and prospecting of rock gold.

(3) Comprehensive processing of various geoscience information

Fig. 2 Distribution map of ancient gold mining relics in Hanjiayuan district.

Fig. 3 Research work method box

In this study, in addition to extracting regional structures from remote sensing image data, that is, linear structures and ring structures, combined with conventional geology, minerals, traces of ancient gold mining, heavy sand and geochemical exploration data, the rock gold mineral resources in this area are comprehensively analyzed, evaluated and predicted (Figure 3). Firstly, the above geological information is input into the geographic information system, and then the relationship between various geological variables and the spatial distribution of gold mineralization is studied through human-computer interaction according to the known information of gold deposits, mineralization points and gold mining traces.

The formation of a genetic type of deposit is often the result of superposition of various geological processes. The occurrence, development, evolution and final spatial distribution of mineralization are controlled by many geological factors. However, for various ore-controlling geological factors related to mineralization, whether their respective geological environment and temporal and spatial distribution characteristics, or their contribution to mineralization, mode of action, nature and their combination relationship are extremely complicated (Wang Pinqing, 1990). It is precisely because of the diversity, variability and complexity of geological phenomena and mineralization that uncertainty and inaccuracy exist, that is, randomness and fuzziness. Therefore, it is of great theoretical and practical significance to use mathematical methods such as statistical analysis and probability calculation to deal with and study geological phenomena and metallogenic conditions.

In view of the above discussion and analysis, this study uses the probability model established by the author in the national seventh five-year plan project to predict and analyze the distribution characteristics and laws of gold deposits in the study area. Namely:

Pt(m≥ 1)= 1—Pt(m=0)

Selected Papers on Aerogeophysical Remote Sensing Research

0≤Pt(m≥ 1)≤ 1

In the formula (1), Pt(m≥ 1) is the probability of at least one mineralization event in each grid area; Ni is the number of the I-th statistical unit of geological variables in each grid area, and Pi is the probability constant of each type of geological variables calculated by maximum likelihood estimation method in the process of human-computer interaction.

In order to consider the relationship between linear structures in different directions and gold mineralization in the area, all linear structures in the study area are grouped by direction with reference to the rose diagram of linear structure trend, and their probability constants Pi are estimated by maximum likelihood estimation method.

It should be pointed out that the probability value Pt (m ≥ 1) of at least one gold mineralization in each grid area calculated by equation (1) not only considers the influence of linear structures in different directions and geological variables such as strata, lithology and geochemistry on mineralization, but also numerically determines the spatial distribution characteristics and their combination relations of various ore-controlling geological factors, thus realizing the evaluation of regional metallogenic geological conditions and prospecting. Fig. 4 is the ore-bearing probability isoline and its three-dimensional map of Hanjiayuan district.

Fig. 4 Isogram and 3D display of ore-bearing probability in hanjiayuan district.

Three. Evaluation and prediction of gold deposits

In the whole region, the regional structures near the east-west direction (north) and northeast direction (south) have obvious control over gold mineralization. The placer gold enrichment area is a direct sign of searching for rock gold, because the ore-controlling structure of rock gold directly controls the placer gold enrichment. Therefore, attention should be paid to finding rock gold deposits in placer gold-rich areas.

In the area from Hanjiayuanzi to Xinglonggou in the north, we should focus on finding rocks and gold deposits related to the Lower Proterozoic metamorphic rock series in the ancient depression controlled by the east-west structure.

By comparing the traces of ancient gold mining with the strata in different times and linear structures in different directions in GIS, it is found that the metamorphic rock series of Xinghua Dukou Group in the Lower Proterozoic are usually concentrated in ancient and modern placer gold mining, and often develop east-west structures. Therefore, in such areas, we should focus on finding structural altered rock-type gold deposits related to the regional east-west structural fracture zone, especially the intersection of east-west and northwest structures, which is most conducive to mineralization. In addition, hydrothermal or porphyry gold deposits related to magmatic activities may also be found near intermediate-acid intrusions, such as Hanjiayuanzi to Laodahan. In this kind of area, ancient and modern placer gold mining activities are very active, often where "gold nuggets" or "violent heads" appear, indicating that the moving distance of placer gold is not large, and there is great potential for finding rock gold.

In the southern region, the regional structure in Northeast China not only plays an important role in controlling the gold-bearing sedimentary formation in Paleozoic, but also has a very close relationship with gold mineralization in this area. In particular, it is necessary to look for structural altered rock-type gold deposits and porphyry gold deposits in the areas where the regional structure of the Lower Devonian spilite-amphibole porphyry distribution area is developed. In particular, the intersection of two groups of structures in different directions, namely the northeast regional fault and the local northwest fault, is very beneficial to mineralization, such as Sandaogou-Wudaogou area.

Up to now, the relationship between ring structures, especially those related to magmatic activities, and gold mineralization in this area is not very clear. Although there are traces of ancient and modern placer gold mining activities near Hanjiayuanzi in the north and Jinshuishan in the south, there are few rock and gold deposits, and the relationship between them needs further study.

In terms of prospecting means, besides interpreting and analyzing the regional structure by using remote sensing image data, we should also make full use of the spatial distribution and enrichment law of placer gold to guide the prospecting of rock gold in such forest-covered areas. In addition, the comprehensive analysis should be closely combined with the existing geophysical and geochemical data, and on the basis of comprehensive research, the promising areas should be selected for small-scale exploration, and the enriched areas with deposits should be circled. However, other methods, such as ground geophysical exploration, are not effective in this area where traffic is extremely inconvenient, swamps are everywhere and field positioning is extremely difficult. Because the groundwater level is extremely shallow, it is difficult to carry out the light mountain project in most cases.

Four. conclusion

Although the study area is located in alpine forest area, Landsat TM image data still provide a lot of useful information for remote sensing geological research and prospecting of rock gold. The determination of the east-west structural belt in this area and the establishment of the spatial distribution relationship between this group of structures and old strata, placer gold deposits and rock gold mineralization provide useful clues for further rock gold prospecting. The prospecting model is established, and the intersection of east-west and north-west structures in the northern region and the intersection of north-east and north-west structures in the southern region are favorable prospecting areas.

The identification and extraction of trace information of ancient gold and its application in regional metallogenic prediction have deepened people's understanding of the correlation between rock gold mineralization in the study area and superior geosyncline sedimentary formation in Lower Proterozoic and Paleozoic, and will also promote further investigation and prospecting of rock gold.

On the basis of the above research, combined with geological information such as geology, topography, geophysical and geochemical exploration and heavy sand, the spatial distribution law of rock gold mineralization in this area is comprehensively analyzed, evaluated and predicted by using the improved probability model. In addition, the extraction, exchange, comprehensive processing and analysis methods of various geoscience information are explored.

This study not only summarizes a set of working methods for resource investigation in forest-covered areas using Landsat TM images, but also determines 7 prospecting areas and 3 prospecting targets, which provides important prospecting clues for further searching for rock gold deposits in this area.

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Application of Landsat TM data in gold exploration in forest areas

Wang Pinqing Jiang Deren Meng Fan Jean

(Aerogeophysical Remote Sensing Center, Beijing 100083)

abstract

This paper briefly introduces some results of gold exploration using Landsat TM data in forest areas. By analyzing the linear characteristics displayed in remote sensing images and conventional geological data, several target areas for further investigation are delineated.