Enrichment Characteristics And Genetic Types Of Trace Elements In The Late Paleozoic Coal From Ordos Basin

Ordos Basin is one of the most important energy basins in China,which is rich in coal resources.In recent years,many metal elements have been found in the Junger coalfield,which has become a hot area of coal geochemical research.However,whether there is enrichment of trace elements in other areas of the basin,and the reasons for the difference of element contents in different areas of the basin need to be further strengthened.It is of great scientific significance to study the mechanism of the enrichment of trace elements in the coal of Ordos Basin and other sedimentary basins and also of great economic and practical value to the exploration and development of coal series mineral resources by strengthening the comparative study of the causes of the enrichment of trace elements in different regions of the basin.In this paper,based on the theoretical knowledge of mineralogy,coalfield geology,mineralogy,coal petrology,coal geochemistry and statistics,and combined the experiments such as optical microscopy,scanning electron microscope equipped with an energy-dispersive X-ray spectrometer(SEM-EDS),high resolution transmission electron microscopy(HRTEM),plasma cryogenic ashing instrument,X-ray diffraction(XRD),X-ray fluorescence spectrometry(XRF),inductively coupled plasma mass spectrometry(ICP-MS)and separation of apatite single mineral,the enrichment degree and spatial and temporal distribution characteristics of trace elements in the Late Paleozoic coals were studied.From the point of "source" and "sink" conditions in the process of migration and enrichment of trace elements in coal,the control of peat forming environment on elements(Ga,Li,REY)in coal was revealed,and the origin of trace elements enrichment in different coalfields was studied in depth.Further,based on the geological characteristics of each coal-bearing tectonic unit and the response of coal geochemical characteristics to the geological evolution of the basin,the complex genetic model of trace element enrichment in coal is established.(1)The types and contents of enriched elements(including slightly enriched elements)in the Late Paleozoic coals of Ordos Basin generally show a trend of high in the north and low in the South and high in the West and low in the east,and the enrichment characteristics of trace elements in different coals of the same coalfield are not uniform.Compared with the world coal average,The No.3 coal in Zhuozishan coalfield on the western margin of the basin is enriched in Li and Th,the No.10 coal is enriched in Li and Cs.In Helanshan coalfield,the No.4 coal is enriched in Li and Pb;In Ningdong coalfield,the No.5 coal is enriched in Li and Th;In Junger coalfield,the No.5 coal is enriched in Be,while the No.6 coal seam is slightly enriched in Li,Th,Sr,In,Ga,Pb and REY;In Hedong coalfield,the No.5 coal seam is enriched in Li;In Weibei Carboniferous-Permian coalfield in southern basin,the coal is enriched in Ga and Li.(2)There are fewer trace elements in Taiyuan Formation coal,which belong to the enrichment degree(CC>5).Only Li and Cs elements are enriched in Zhuozishan coalfield,and Li and Ga elements are enriched in Weibei Carboniferous-Permian coalfield.The coals from the Junger coalfield,Hebaopian mining district in Hedong coalfield and Carboniferous-Permian coalfield in northern Shaanxi did not enrich any elements in general.Although the average content of Ga in Heidaigou coal is 48?ig/g,the content of Ga in other coalmines in Junger Coalfield is not high,thus,the content of Ga in No.6 Coal in Junger Coalfield is not enriched in general.Compared with Taiyuan Formation coal seam,the enrichment of trace elements in Shanxi Formation in the study area increased significantly.The main manifestation is that most of the coalfields containing Shanxi Formation coal seam in the study area are enriched in some elements(except the northern Shaanxi Carboniferous-Permian coalfields),such as Li and Th in the Zhuozishan and Ningdong coalfields,Li,Pb and U in the Helanshan coalfield,Be in the Junger coalfield and Li in the Hedong coalfield.(3)According to the enriched degree(CC>5)in different coal seams,the enriched trace elements in the coal can be divided into four types,namely Li-Th(U)-Pb,Li-Cs,Ga-Li and Rb-Cs assemblages.(4)The results of X-ray diffraction and scanning electron microscopy show that the mineral compositions of low-temperature ash coal sample in the study area are mainly kaolinite,illite,boehmite,calcite,quartz and pyrite.In addition,there are a small amount of chlorite,dolomite,iron dolomite,albite,diaspore,siderite,and rutile,apatite,svanbergite,goyazite and chalcopyrite.The characteristics and occurrence of Nano-minerals in coal were studied by means of high resolution transmission electron microscopy(HRTEM)combined surface scanning,energy spectrum,selected area electron diffraction(SEED)and Fourier transform(FT).The results show that the Nano-minerals in high sulfur coal of Dongpo Coal Mine in southern basin are mainly Si-Al minerals(kaolinite,illite and chlorite mixed layer),S-bearing minerals(pyrite,molybdenite),Ca minerals(calcite,dolomite),quartz and anatase.In addition,through the combination of high-resolution image and Fourier transform,it is further confirmed that Ti element in high Ti kaolinite mainly occurs in the form of anatase coexisting with kaolinite(5)The paleopeat-forming environment of the Junger-Hebaopian mining district,northeastern Ordos Basin,was reconstructed in detail using the nature of the macerals,geochemical indicators,and mineralogical characteristics,and special emphasis was further given to the key role of paleopeat-forming environments in Ga,Li,and REY enrichment by comparing the variations in Ga,Li,and REY contents to the paleopeat-forming environments characteristics.In Junger-Hebaopian mining district,higher REY concentrations mainly occur in coals derived from Neuropteris and Lepidodendron,and the coals formed in relatively alkaline,suboxic to anoxic,brackish or sea water influenced environments with strong hydrodynamic,while Li enrichment occurs in coals formed from sphenopsids and fomed in acidic,brackish water influenced,and relatively anoxic environments with strong hydrodynamic conditions.In addition,an acidic,fresh water,suboxic sedimentary environment with weak hydrodynamic conditions and Neuropteris and Lepidodendron peat-foming vegetation are beneficial to the enrichment and accumulation of Ga and REY.(6)The geochemical characteristics and the mineral association of zircon(c/a>2.5),avanbergite,goyazite,high temperature quartz,apatite(hexagon),and vermicular kaolinite further confirm that the synchronization volcanic ash input during formation of the No.5-2 coal in Dongpo Mine.The variation trend of the types and sizes of these minerals from a volcanic origin in the coal seams,partings,and host rocks indicates that the volcanic activity included multiple eruptions,in a relatively short time interval,that the volcanic material was intermediate-felsic in composition.(7)Based on coal petrology,geochemistry and mineralogy evidence,the genesis of trace element enrichment in the Late Paleozoic coals from Ordos Basin can be divided into the terrigenous clastic supply-peat forming environment dominant in the northeastern margin of the basin,the terrigenous clastic supply-volcanic ash-low temperature hydrothermal fluid dominant in the Weibei Carboniferous-Permian coalfield,terrigenous clastic supply-fault structure-low temperature hydrothermal fluid-coal metamorphism degree dominant in the Western fold thrust belt,and terrigenous clastic supply-low temperature hydrothermal fluid-seawater dominant in the northern Shaanxi Carboniferous-Permian coalfield.(8)On the basis of revealing the response of coal geochemical characteristics to basin evolution,four complex genetic models of trace element enrichment in coals have been established:(1)Terrigenous clastic supply-peat formation environment complex type.In this model,the faults and structures are not developed,and the provenance cause the enrichment of trace elements in the coal of the study area,but the slightly change of peat forming environment leads to the difference of element content in different coal stratification samples.(2)Terrigenous clastic supply-fault structure-low temperature hydrothermal fluid-coal metamorphism degree complex type.In this model,folds and faults are very developed.Although the source may provide enough clastic sources for the study area,the frequent structural changes and erosion lead to the loss of clastic supply,resulting in the source does not ultimately lead to the enrichment of elements,but the widely developed faults structures which provide channels for sulfur-bearing hydrothermal fluids and the increase of coal metamorphism eventually lead to the enrichment of elements in present-day coal in the study area.(3)Terrigenous clastic supply-volcanic ash-low temperature hydrothermal fluid complex type.In this model,the provenance provides the basic terrigenous clastic supply for the study area,which may result in a slight enrichment of elements,but it does not reach the present level of enrichment.However,the input of intermediate-felsic volcanic ash enriched with various trace elements(such as Ga,Li,etc.)and the intrusion of hydrothermal fluids in which minerals or themselves can adsorb trace elements in surrounding rock lead to the secondary enrichment of trace elements in coal.(4)Terrigenous clastic supply-low temperature hydrothermal fluid-seawater influence complex type.In this model,taking No.2,No.4 and No.9 coal in Fugu mining area of monoclinic coal-bearing belt in northern Shaanxi as an example,the dynamic changes of the main factors controlling the enrichment of trace elements in coal during coal formation are emphatically expounded.