Characteristics Of Igneous Intrusions In The Datong Coalfield

The Carboniferous-Permian coal-bearing strata in Datong coalfield was intruded by the large-scale igneous intrusions in Indosinian and Yanshanian periods,which lead to the destruction of the coal seam and decrease of the coal quality,having a serious impact on the safe mining and economic benefits of the coal mine.Studies on the spatial distribution and morphological characteristics of igneous intrusions and coal-igneous intrusions interaction process will have a significant engineering and scientific meaning on safe and efficient mining of coal mines and perfecting the process of coal metamorphism.Through the analysis of the geological exploration data of seven coal mines in Datong coalfield,geological logging and test and analysis of samples,combined with the previous research,the plane and the spatial distribution of the intrusions,morphological characteristics and coal-magma interaction process are studied.The main conclusions have been drawn as follows:Dongzhouyao coal mine is the most seriously intruded by the igneous intrusions in Datong coalfield.According to the drill hole data,about 85%of the igneous intrusion is in the Shan4and No.5 coal seam,the others is in the sandy mudstone between the K₃ sandstone and Shan4coal seam and only one drill hole in the No.8 coal seam exposes igneous intrusion.The differences in the distribution of igneous intrusions(in vertical)can be attributed to the differences in rigidity and homogeneity of the strata and neutral buoyancy.Horizontally,the positions with the largest thickness of igneous intrusions are located in the north of the minefield,and some thick igneous intrusions can be seen in the central and southern coal seams of the minefield,which may be the upward channels of magma.The igneous intrusions close to the channels are mostly distributed in the upper part of the coal seam or the roof of the coal seam forming the direct roof.With the increase of the distance,the igneous intrusions are mostly intruded in the lower part of the coal seam or the floor of the coal seam forming the direct floor.It should be pointed out that faults in the strata will change the lateral distribution of igneous intrusions locally.The four sills in Datong coalfield all present finger-shaped structures composed of several elliptic intrusions,which are formed when low viscosity magma migrates in high viscosity liquid coal seams.Finger structure can be divided into three stages:Finger formation,Finger inflation and Finger coalescence.According to the stages of finger structure,four sills belong to the second and third stages,and the magma flow direction is perpendicular to the section.The viscous-viscous contact between coal and magma and the release of a large amount of gas during the contact leads to the mixture of the coal and igneous intrusion in the contact zone,called peperite structure.Magma intrusion leads to contact metamorphism of the coal seam,and CO₂released from the devolatilization of the coal seam will also lead to the carbonation process of the igneous intrusion in the contact zone,which is manifested in the alteration of mafic minerals into carbonate minerals(dominated by siderite).Because the dike cuts through the strata,the groundwater activity on both sides of the dike increases,which makes albitization of the plagioclase at the edge of the dike,and then forms a large number of kaolinites.Different from the dike,sill has several characteristics including affecting a larger volume of the coal seam,long contact duration,slow temperature reduction and lateral intrusion,which leads to a stronger carbonation process and no serious clavization process.