Distribution characteristics of associated elements in Middle Jurassic coal in the north of Datong coalfield and their geological significance

In order to study the geochemical characteristics and geological significance of associated elements in Middle Jurassic coal of Datong coalfield, the distribution and occurrence of the associated elements in 9 coal seams in the northern Datong coalfield were analyzed by inductively coupled plasma mass spectrometry(ICP-MS), X-ray diffraction(XRD), optical electron microscopy and scanning electron microscopy(SEM). The results show that the Jurassic coal in Datong coalfield has low trace element content, and beneficial elements(Li, Ga) are far lower than the boundary grade, which has no development potential. But the harmful elements such as Co and Zn in No.2 coal, Tl in No.9, and Be in No.11-2 coal are found quite enriched. Their concentration is more than 3 times the average value of the world coal. Its environmental impact deserves attention. The main minerals in coal are kaolinite and quartz. Kaolinite is mainly filled in structural vitrinite, indicating its syngenesis. Quartz often has a good crystal form, indicating its autogenesis. A small amount of pyrite and calcite is filled in the microfissures, indicating the subsequent origin; apatite is related to organic matter. Trace elements Ni and Mo in coal may exist mainly in or-ganic matter in coal, and other elements mainly in inorganic minerals. Si mainly exists in quartz, some Si and Al, Li, Cr, Ga and Nb mainly exist in clay minerals, Fe, Mn, Cd, Zn, and Tl mainly exists in pyrite, Ca, Mg and Co mainly exist in carbonate, P, Be, Sr and Ba mainly exist in apatite, and Rb mainly exists in halite. Vertically, there are two sedimentary cycles in Datong Formation. From the bottom to the top, the water body changes from shallow to deep and the hydrodynamic condition is gradually reduced in each sedimentary cycle, therefore the supply of terrestrial material decreases. This may be related to the strong tectonic movement occurring between the two cycles. Generally speaking, in the coal-forming period, with the development of sedimentation, the input of clastic materials in the coal-accumulating basin decreased gradually, and the active materials increased gradually.