Mineral Matter In Lopingian Coals From Eastern Yunnan Province And Its Response To The Regional Geological Evolution

Eastern Yunnan Province is an important coal-reserving area in southwestern China and belongs to the Upper Yangtze coal basin.As a large-scale coal-bearing sedimentary basin,the Upper Yangtze coal-bearing basin has good outcrop and facies preservation.It is a ideal place to research paleoenvironment and geological evolution.As a special carrier of geological information,coal records information on paleoclimate,paleoenvironment,paleogeography,and regional geological evolution.The Late Permian is a relatively special stage in the geological history.It was between the Late-Middle Permian mass extinction?end-Guadalupe mass extinction?and the end-Permian mass extinction.The mantle plume event that occurred at the turn of the Middle-Late Permian was the most important geological event in the southwestern China during the same period.It controlled the tectonic framework of the southwestern China and influenced the geochemical and mineralogical characteristics of the Late Permian coal in the eastern Yunnan Province.The end-Permian mass extinction was the most serious extinction in the geological history,and there are different views on its cause.This study analysis the organic carbon isotope and mineral matter of the late Permian coals to provide new information to this mass extinction.In addition,the collected drillhole samples are analysised in this study.All visible coal seams of the late Permian in the target drillhole were sampled according to macroscopic features or at a certain intervals?5-20cm?.Compared to profile sampling,drillhole samples have better integrity and continuity,providing more accurate geologcal information.X-ray fluorescence spectrometry?XRF?was used to analyze the content of major element oxides in the high-temperature ash?HTA?of the drillhole samples.The coals of this study were enriched in TiO₂,which was mainly affected by the high-titanium basalt in Emeishan.The mass ratio of SiO₂/Al₂O₃ was higher than the average value of coal in China?1.18?.It is mainly influenced by the large amounts of quartz formed by Emeishan basalt weathering and precipitation.The Emeishan basalt is the main source rock,caused the samples in Shugentian Coalfield and the roof and floor samples in Lianying Coalfield have the SiO₂/Al₂O₃ of 3-8,showing basic rock source.The Al₂O₃/TiO₂ of most coal and parting samples in Lianying Coalfield and several samples in Shugentian Coalfield indicate the influence of felsic rocks which may come from the input of mediate-felsic volcanic ash.The content of trace elements in the drillhole samples was analyzed by ICP-MS.Compared with the average value of world coal,the coal in the eastern Yunnan was lightly enriched in Sc,V,Co,Nb,Ta,Zr,Hf,Cu,Zn,Se and REE;Sc,V,Co,Cu and Zn mainly comes from the terrestrial input of Emeishan basalt.Compared to the upper and lower stratified samples,Sc,V,Cr and Co is depleted in LY2-8p,SGT1-11c,which is caused by acidic volcanic ash input.The enrichment of Nb,Ta,Zr,Hf,and REE is caused by the input of alkaline volcanic ash and subsequent hydrothermal-weathering effects.The loss or enrichment of REY content in the benches is also affected by hydrothermal leaching.The Upper Continental Crust?UCC?normalized REY can be used as geochemical indicator.In this study,the positive Eu anomalies were impacted by the Emeishan basalt in the source area,and the Eu negative anomalies were affected by the input of the mediate-acid volcanic ash.In addition,the Li,F,Mo and U are enriched in the coal seams affected by seawater.Powder X-ray diffraction?XRD?,optical microscopy,and scanning electron microscopy with energy spectra?SEM-EDS?are used for mineralogy analysis.The Late Permian coal in Eastern Yunnan contains mainly quartz,clay minerals?mainly kaolinite,I/S mixed clay minerals,chamosite?,calcite,and anatase,with a small amount of illite,dolomite,and marcasite,albite,anorthite,apatite,rutile,bassanite and anhydrite,REY-bearing minerals?monazite,florencite?,zircon?titanium-containing zircon?,phalerite,chalcopyrite,galena,gersdorffite were also detected by SEM.Quartz is the most enriched mineral in coals of this study.It is mainly authigenic,including disseminated quartz and fine quartz particles distributed along the bedding in the organic matter,Authigenic quartz was mainly from the weathered Emeishan basalt fluid.There is a small amount of rounded and relatively large particles from terrestrial source,which also proves that the influence of Emeishan basalt on the coal seam is mainly in the form of solution,supplemented by terrigenous debris.The chamosite is widely distributed in Lianying Coalfield,and its formation is related to the hydrothermal.It is formed by the interaction of the kaolinite with the Fe-Mg-rich hydrothermal fluid in the early stage of diagenesis,or the direct precipitation of Fe-rich siliceous solution;REY-bearing phosphate minerals?florencite,monazite?are considered to be secondary minerals associated with hydrothermal fluids;Sphalerite,galena and gersdorffite,filled with mineral fractures,are also formed by hydrothermal fluids.Cavity-filling carbonate minerals are formed by epigenetic hydrothermal.Vermicular kaolinite,sharp quartz,bamboo-like apatite,self-shaped albite may come from volcanic ash.In the Shugentian coalfield,framboidal and self-shaped pyrite are authigenic,and their formation is affected by seawater.The geochemical and mineralogical characteristics of the Late Permian coal in the Eastern Yunnan Province influenced by the Emeishan mantle plume event,they are reflected in four aspects.Firstly,the basic basalt and the mid-acid rock on the top of ELIP are the main source rocks of the Permian coal.They were fed to the peat bog or coal seam with Fe-Mg-rich siliceous fluids from basalt weathering to form quartz,chamosite,or in the form of terrigenous debris and volcanic debris,forming rounded mineral particles or minerals with sharp corners.Secondly,the accumulation of organic matter in the Late Permian in eastern Yunnan was accompanied by multiple hydrothermal fluids.Hydrothermal intrusion into peat swamps or coal seams formed minerals under certain conditions.During synergism stage,Fe-rich siliceous hydrothermal deposited and formed cell-filling chamosite and quartz.During the epigenetic stage,the hydrothermal fluid invades the coal seam to form cavity-filling carbonate minerals.Furthermore,the invasion of hydrothermal fluids may also alter the existing minerals to form new minerals.The hydrothermal alteration of kaolinite forms chamosite,and the REY-bearing phosphate mineral is a secondary mineral associated with the hydrothermal fluid.The leaching of the hydrothermal fluid can also cause the redistribution of rare earth elements,resulting in the depletion and enrichment of REY.Thirdly,the volcanic ash resulting from the waning activity of the plume termated peat accumulation and formed tonstein or volcanic debris of coal benches;the volcanic ash can directly bring in minerals,such as sharp quartz,self-shaped albite,or can course the recrystallize of minerals after the volcanic ash falls into the peat bog,such as the formation of vermicule kaolinite.Fourthly,the topography?high in west and low in east?that resulted from the ELIP determined the invasion direction of the seawater?from the east to the west?in the southwestern China.From the west to the east are the continental Xuanwei Formation,the transitional Longtan Formation and the marine Wujiaping Formation,the Panxian fault and the Shizong-Guiyang fault were the boundary.The Lianying Coalfield is located in the terrigenous Xuanwei Formation.The Shugentian Coalfield is located in the transitional region,developing the transitional Longtan Formation during the Wuchiapingian and the marine Changxing Formation during the Changhsingian.There are several possible mechanisms that seawater affects the elements in coal:the first was concentration during the life of the plants themselves.The second was concentration by removal of the more soluble material during the decay of the organic matter with some trace elements retained as insoluble hydroxides or due to formation of organo-metallic compounds and the third was post-depositional resulting from reaction with circulating aqueous solutions.When analyzing the influence of seawater on coal,we must distinguish the influence of other factors.In this study,Sr exists in the epigenetic carbonate minerals,so Sr/Ba cannot be used to indicate the influence of seawater.Isotope mass spectrometry was used to analyze the organic carbon isotope,the Pea Dee Belemnite?PDB?is used as the standard to calculate the organic carbon isotope of the 907 drillhole coal sample in the Lianying Coalfield during the Late Permian.At the end of the Late Permian?LY2 coal seam?,?¹³C_org exhibited a negative shift of about 1‰,and the?¹³C_orgvalue moved back to the mean near the Permian Triassic Boundary?PTB?.Evidence of volcanic activity was found in the mineral composition of the LY2 coal seam,and the dyamics of this volcanic activity was related to volcanic arc/orogenic movement,which was different from the volcanic eruption of the Emeishan mantle.It is speculated that volcanic eruptions release CO₂ containing light isotopes into the ocean-atmosphere system,as well as large-scale intrusion of overflow basaltic magma,and interact with organic-rich shales and petroleum-containing evaporites,resulting in the release of CO₂ pyrolysis methane and other gases in the rock.The release of these gases will reduce the?¹³C value of CO₂ in the atmosphere and seawater,and the carbon isotope of coal-forming plants will decrease,which is reflected in the negative shift of the organic carbon isotope value of coal.The negative shift of the oaganc carbon isotope in coal is coupled with the evidence of volcanic activity in the mineral composition and has implications for the mass extinction of the Late Permian.