Tengchong Mangbang District

The coal-bearing stratum in the study area is the Mangbang Formation (N2m) of Neogene Pliocene, that is, the Neogene Pliocene is the main coal accumulation period in this area.

1. Influence of paleostructure on coal accumulation

(1) Genetic types and basement structural characteristics of coal-bearing basins. The study area is a high-rigidity stable block composed of strata from Mesoproterozoic to Paleozoic. Since Cenozoic, the area has been in a post-orogenic extensional rift environment, and the revival of ancient basement faults in the area has changed from compression to tension, forming three coal-bearing basins: Mangbang, Menglian and Puchuan. All basins are controlled by nearly north-south or north-northeast faults that run through the basin and belong to typical fault basins. This determines that the coal-bearing property of each basin in this area is generally poor. Menglian basin covers an area of 13.5km2, and its basement is composed of Yanshanian Menglian granite, and its basement structure is a wide and gentle syncline structure with nearly north-south strike. The sedimentary center is located in the north-central part of the basin. Pu Chuan basin is arc-shaped with an area of 9.8 square kilometers, located at 20 degrees northeast. The southwest of the basin is relatively open and the northeast is very narrow. The sedimentary center is located in the southwest of the basin, and the basement of the basin is also Yanshanian Menglian granite. Mangbang basin is the largest coal-bearing basin in this area, which is in the shape of a long ladle protruding slightly eastward from north to south and distributed in the east of the study area. The basement of Mangbang Basin is composed of Menglian complex granite and Carboniferous shallow metamorphic dolomite limestone, and the basement in the southeast margin of the basin is mainly metamorphic rocks of Mesoproterozoic Gaoligongshan Group. The basement structure of the basin is an open composite syncline with nearly north-south axis, and there are three secondary depressions: Tuantian, Mangbang-Dabengga and Ganlanzhai.

(2) Tectonic movement in the process of coal accumulation and its coal control function. The influence of tectonic movement on coal accumulation in the study area is mainly manifested in two aspects, one is the influence of synsedimentary fault activity on coal accumulation, and the other is the influence of magmatic activity on coal accumulation. ① Control of synsedimentary structure on coal accumulation. As mentioned above, the three coal-bearing basins in this area are all fault basins, and the nearly north-south faults control the formation and development of each basin. Due to the characteristics of multi-stage activity of the main faults in the north-south direction in the area, the basin was generally in an under-compensated sedimentary environment during the coal accumulation period, and the sediment particle size was coarse, which was not conducive to coal accumulation. The strongest synsedimentary fault activity in this area is in the southeast margin of Mangbang Basin, resulting in a sedimentary thickness of 800 meters, which is mainly composed of light gray layered glutenite with poor coal accumulation. ② Influence of magmatic activity on coal accumulation. The Cenozoic magmatic activities in the area are frequent, mainly basic volcanic eruptions, in which the middle Pliocene and Quaternary are the main eruption periods, and the early and late Pliocene eruptions are small in scale and weak in intensity. The main coal seams in Mangbang Basin mainly occur in the middle and upper part of the lower member of Mangbang Formation. This magmatic activity, which is basically in the same period as the coal accumulation period, leads to a relatively turbulent tectonic environment in the study area, which is obviously not conducive to the occurrence of coal accumulation. However, due to the small range of volcanic eruption at that time, the impact was also local, mainly in the area of Nong Gang Township in the south-central part of the basin.

(3) Coal control analysis of fault structure. The fault structure in this area is dominated by north-south direction, followed by east-west direction, and there are two groups of faults in north-east direction and north-west direction. The formation of Menglian Basin and Pu Chuan Basin and the sedimentary formation of coal-bearing strata are controlled by nearly north-south concealed faults F 1 and F2, respectively. At the early stage of the formation of Mangbang Basin, it was controlled by the western slope fault (F7) of Gaoligongshan Mountain, forming the embryonic form of the basin, and the main control fault for the formation and development of the basin was Longchuanjiang fault (F6). Longchuanjiang fault runs through the whole area along the north-south direction in the east of the basin, and it is speculated that the section is inclined to the west, with a dip angle of more than 70. The fault is a mylonite belt with a width of about 100 m in Shang Yun area in the north of the basin, and it is concealed to the south of Shangyun. Longchuanjiang fault not only controls the formation and development of Mangbang basin, but also obviously controls the distribution of volcanic rocks and craters in the basin. The fault is a basement resurrection fault, which was compressive and torsional in the early stage, and turned into tensile and torsional after Cenozoic, forming Mumba Basin. In Pliocene, Longchuanjiang fault frequently moved up and down, with little difference, resulting in discontinuous deposition in the basin and overlapping layers of overlying ground. Another group of important basement faults in Mangbang Basin are near east-west faults, which are compressive and torsional in the early stage and tensile in the late stage. Later, they revived and cut the above-mentioned north-south faults, and at the same time caused the formation of secondary uplift and depression in the basement of the basin. Three secondary depressions, Tuantian, Mangbang-Dabengga and Ganlanzhai, were formed in Mangbang Basin from south to north, with two secondary uplifts in between. Because the formation of the EW fault is caused by the compression and relaxation of the southern Indian plate, the stress intensity gradually weakens from south to north, and the amplitude of secondary uplift and depression gradually decreases from south to north. This group of faults has the characteristics of multi-stage activity, which controls the lithofacies distribution of coal-bearing strata in the basin. To sum up, the nearly north-south fault controls the formation and development of coal-bearing basins, and also controls the development characteristics of coal-bearing strata in the basins. Near-east-west faults control the secondary rise and fall of the basin, and also control the lithofacies distribution of coal-bearing strata to some extent. In addition, there are two groups of faults in NW and NE directions, which are conjugate torsional faults and also control the lithofacies of coal-bearing strata. Each group of faults at the same level in the area generally has the characteristics of nearly equidistant distribution, which determines that the coal-bearing basin controlled by it and its secondary uplift and depression also have the characteristics of nearly equidistant distribution. For example, the distance between the main faults in the near north-south direction is about 14km, which leads to the equidistance distribution of Mangbang, Menglian, Pu Chuan Basin and Lianghe Basin from east to west 10 ~ 20 km, and the formation time tends to migrate from southwest to northeast. Lianghe Basin includes Miocene Lin Nan Formation and Pliocene Mangbang Formation, while all basins in the study area only have Mangbang Formation. From the structural interpretation map of Mangbang area in Tengchong, it can be seen that the nearly east-west faults are clearly distributed in Mangbang basin, and the spacing is about 15km, which leads to the spacing distribution of Tuantian, Mangbang-Dabangga, Ganlanzhai and other secondary depressions controlled by them being nearly equidistant, and controls the spacing distribution of craters or volcanic mechanism rings formed along F5, F4 and Tengchong areas in the later period, thus correspondingly controlling the coal-bearing facies. Due to the joint action of paleotopography, volcanic eruption center and synsedimentary fault activity, the coal seams in Mangbang Basin are mainly distributed in the areas of Ganlanzhai, Mangbang and Tuantian. See the forecast map of coal-bearing prospect area in Mangbang area of Tengchong.

2. Paleogeography and sedimentary environment

As Mangbang Basin is the focus of this work, the following is only a brief analysis of Mangbang Basin. Rifting occurred along the western margin of Gaoligong Mountain in the early stage of the lower member of Mangbang Formation, which is the initial stage of basin rifting. Alluvial fan sedimentary system is developed along the fault zone. Lithology is mainly gray, taupe conglomerate and glutenite. The sedimentary center is located in Tuantian area, with a thickness of more than 300m m. In the middle period of the lower member of Mangbang Formation, the rift valley of the basin further expanded and the basin scope gradually expanded, forming an alluvial fan-river sedimentary system tract. In addition to the alluvial deposition system at the foothills of the eastern margin of the basin, river deposits such as glutenite, gravelly sandstone, argillaceous siltstone and silty mudstone are developed in most other areas of the basin, and plate-trough cross bedding is developed. In some areas, floodplain swamp facies develops, forming thin coal seams or coal lines, and the coal-bearing property is poor. The sedimentary center is located in Laozhai area, with a thickness of more than 360 meters. The sedimentary thickness in Mangbang area is nearly 300 meters, and the area around Dabinglong and Tuantian is 200 meters. In the late stage of the lower member of Mangbang Formation, the basin subsidence range reached the largest scale, which is a stable period of basin expansion and superposition. The alluvial fan-river-lake sedimentary system tract was formed. During this period, the crust was relatively stable, the water covered was deepened, and lacustrine argillaceous siltstone, mudstone and claystone were widely deposited, forming a large diatomite mine. After that, the lake basin began to shrink shallowly, and a wide range of peat bogs were formed in the transition zone of silt and lake sedimentary system, river sedimentary system and alluvial fan sedimentary system in the shallow part of the lake basin (Figure 5- 1), and the main coal resources in Mangbang Basin were formed at this time. At that time, there were three sedimentary centers, namely the northern part of Ganlanzhai Township, Tian Xin Dam in Mangbang Township and Hanbazhai in Tuantian Township. The maximum sedimentary thickness may be 80m in Tian Xin dam area of Mangbang Township and 60 m in Hanbazhai area. Strong basalt eruption occurred in the middle section of Mangbang Formation, forming a widely distributed basalt marker layer. At this time, the deposition of rivers and lakes is limited to a small range. In the upper member of Mangbang Formation, the basin entered the contraction period, and only local rivers, alluvial fans and lakes were developed. The main lithology is gray granite gravel sandstone, sandstone, siltstone, mudstone and diatomite. Local mudstone is submerged, and only thin coal seam or coal line is formed, which is basically of no industrial value. At the end of Pliocene, the whole area uplifted and most areas suffered denudation. In the Quaternary, the basin entered a period of closure, extinction and reconstruction, and formed the proluvial with destroyed foothills, slope accumulation and river collision.

3. Coal accumulation law

The evolution of lithofacies palaeogeography in coal-bearing basins in this area is generally influenced by crustal movement. Menglian and Pu Chuan basins are small in scale, and their evolution characteristics are not as obvious as those of Mangbang basin. The first two basins are mainly composed of piedmont facies and fluvial facies, while the latter is composed of piedmont facies, fluvial facies and lacustrine facies.

According to Cheng (1994), "Due to the Yanshan movement, the fracture combination of the shallow metamorphic rock belt of Gaoligongshan Group and Menglian rock mass was formed. With the end of the Yanshan movement, the nearly north-south fault group produced east-west tension and gravity settlement, forming the initial prototype of Longchuan River Basin (Mangbang Basin is the southern section). "

According to the metallogenic conditions and prospect prediction of in-situ leachable sandstone uranium deposits in small basins in western Yunnan (1997), in the early stage of the lower member of Mangbang Formation, Himalayan movement was characterized by inherited faults, and the metamorphic rock belt of Gaoligongshan Group lifted Menglian rock mass, which aggravated the tensile collapse of Longchuanjiang fault and formed an asymmetric graben-type fault basin with steep east and slow west. At the same time, due to the relatively slow uplift of Menglian rock mass, a small number of piedmont proluvial appeared in Shang Yun, Wujiazhai and Nongwuzhai in the western margin of the basin. Later, fluvial gravelly sandstone and glutenite were deposited on a large scale in the basin. The thickness of Mangbang area was about 300 meters, that of Dabingnong and Tuantian area was about 200 meters, and that of Laozhai area was as high as 360 meters. At the early stage of the late sub-stage, early basalt eruption occurred in the area from Nongwuzhai to Daxinzhai, forming a local dammed lake. Due to the continuous subsidence of the whole basin, the water depth of the basin increased, and argillaceous siltstone and mudstone were deposited in Ganlanzhai, Mangbangjie-Dabengga, Nong Gang-Tuantian and other places. At this stage, the crust is relatively stable, and peat swamp facies deposits are formed in the above areas. Therefore, the coal accumulation in Mangbang Basin mainly occurs in the areas from Ganlanzhai to Sanjiajie and from Nong Gang to Tuantian.

(2) Delineation of coal-bearing prospect area

Reconstruction of coal measures by late structure. There are two main manifestations of the late tectonic movement in the study area, one is the overall uplift of the crust and the differential uplift between blocks, and the other is the large-scale Neogene to Quaternary intermediate-basic volcanic eruption centered on Tengchong. They have different transformation effects on coal seam and coal quality in this area. After the coal accumulation period, only slight folds occurred in the study area, in which the dip angle of Mangbang Formation in Menglian Basin was less than 20, and the sedimentary center and coal accumulation center of the basin were located in the north-central part, with a coal-bearing area of about 3km2. Mangbang Formation in Pu Chuan Basin tends to the southeast and northeast, with dip angle of 8 ~ 25, generally13 ~15, and the coal-bearing area is only1km2. . Mangbang Formation in Mangbang Basin is a monoclinal structure with gentle dip to the east. The dip angle of the western margin of the basin is15 ~ 20, and the area from the middle to Longchuan River is nearly horizontal. Therefore, the folds in this area have no obvious destructive effect on the coal seam. There is no fault cutting coal seam in Youmenglian basin and Pu Chuan basin, which belongs to simple structural type. The late revival of basement faults in Mangbang basin, such as the local dislocation of coal measures and coal seams caused by nearly north-south and northwest faults, is also a simple structure. Due to the strong uplift of the Quaternary crust in this area, the deep coal measures and coal seams rose to the shallow or surface to varying degrees, resulting in local erosion. Most of the existing minable coal seams are generally open-pit mining. After the coal accumulation period, the volcanic activity in the study area is strong, in which the central part of Mangbang Formation in Mangbang Basin is dominated by basalt, which not only protects the main coal-bearing section and its coal seam in the area, but also has hydrothermal baking effect on the coal seam, which deepens the degree of coal metamorphism. The situation in Menglian and Pu Chuan Basin is similar. Therefore, the coal quality in the study area, especially in the Mangbang Basin, is better than that in the neighboring areas.

Fig. 5- Lithofacies Paleogeographic Map of Mangbang Formation in Early Pliocene in Mangbang Basin +0

Because Menglian Basin and Pu Chuan Basin are small in scope, and the favorable coal-bearing areas in each basin have been surveyed, the delineation of coal-bearing prospect areas is no longer carried out. The following focuses on the delineation of coal-bearing prospective areas in Mangbang Basin. After field investigation and verification, coal seams are exposed in all dam sections from the south of Ganlanzhai to Wuhe Township in Mangbang Basin, which are widely distributed. However, the thickness of coal seam changes greatly and its structure is complex, and the coal quality is mainly lignite. Coal seams are distributed in the east of Majiacun, Zhangjiacun, Mangbang and Hecheng Village, and are still exposed to the east of Longchuan River. At present, the main coal mines are Sanchahe Coal Mine and Wu Hexiang Coal Mine. Sanchahe mining area is about 5 kilometers long from north to south and 4 kilometers wide from east to west, covering an area of about 20 square kilometers. It is the largest coal mine in Tengchong County. Wu Hexiang mining area is 1.5km2, and the proven reserves are about 2 1× 104t, including two minable beds, each with a thickness of 0.4 ~ 0.9m The coal seam occurs in the middle and upper part of the lower member of Mangbang Formation. Main coal seam outcrops are as follows:

Open pit of Mangbang Coal Mine (1). Located at 1.5km to the west of Mangbang Township Government. The total thickness is 4.03m, the structure is 0.3 (0.28) 0.3 (0.27) 0.25 (0.3) 0.52 (0.90) 0.06 (0.20) 0.15 (0.25) 0.20m, and the gangue is fine and coarse sandstone and pure coal. The coal is grayish brown, massive and homogeneous, and the coal and rock composition is mainly dark coal, which belongs to semi-dark briquette. The roof is grayish white coarse sandstone with a thickness of 1.5 ~ 17.7 m, and the floor is light gray-grayish black clayey rock. The formation attitude is 155 ∠ 15.

(2) Wujiazhai coal point. It is located in the north of Mangbang, at 1km to the north of Wujiazhai. The coal seam structure is:1.09 (1.07) 0.31.38) 0.47 m, and the gangue is coarse sandstone. Coal is brown-gray, dull, scaly, uniform in structure and well developed in joints. The composition of coal and rock is mainly dark coal, which belongs to semi-dark briquette and contains carbonized plant trunks. The coal seam contains sandstone lenses, which are unstable, thin and even pinch out in a short distance. The roof is a set of grayish white coarse sandstone, and the floor is coarse sandstone and clayey rock. The stratum occurrence is 100 ∠ 26.

(3) Coal point 500 meters due north of Wujiazhai. Coal seam structure is 0.82 (0.13) 0.03 (0.20) 0.18m, dark brown, dull and scaly. The composition of coal and rock is mainly dark coal, which belongs to semi-dark briquette. The roof is thick layered conglomerate and the floor is mudstone. The formation attitude is 130 ∠ 1 1. Mangbang basin is an asymmetric graben fault basin with poor coal accumulation conditions, unstable coal seam and complex structure. Based on the remote sensing geological interpretation of coalfields, this paper comprehensively analyzes the basement structure, coal-accumulating characteristics and the transformation of coal seam and coal quality by later structures in the basin. It is considered that Ganlanzhai, Mangbangjie-Dabengga and Nong Gang-Tuantian are several sedimentary centers and coal-accumulating centers in the basin. Combined with the analysis of coal seam destruction caused by volcanic eruption in the later period, the Ganlanzhai-Mangbang coal-bearing prospect area and Damengliu-Tuantian coal-bearing prospect area were delineated in Mangbang Basin according to the satellite image anomalies, with the areas of 70.55km2 and 54.39km2 respectively.