Geochemistry And Paleo-peat Depositional Environment Of No.11 Coal Seam From Donglutian Mine In Pingshuo District,Ningwu Coalfield

There are several super-enriched lithium,gallium and aluminum deposits in the Carboniferous-Permian coals in Ningwu Basin,but the distribution and enrichment mechanism of these elements have not been completely agreed.In this project,No.11 coal from Donglutian Mine of Pingshuo District,Ningwu basin was selected as the research object,guided by the theories of coal geology,petrology,mineralogy and element geochemistry.By using the Leica microscope,Inductively coupled plasma mass spectrometer(ICP-MS),Inductively coupled plasma emission spectrometer(ICP-OES),A Field Emission Scanning Electron Microscope with an energy dispersive spectrometer(SEM-EDS)、High-resolution transmission electron microscope with an energy dispersive spectrometer(HR-TEM-EDS),Laser denudation plasma mass spectrometry(LA-ICP-MS),Low-temperature ashes(LTA)combined with X-ray diffraction analysis,11 samples of Donglutian mine were tested and analysised.Based on the analysis of major and trace elements content,mineral composition and maceral contents of 11#coal seam in east open-pit,the detrital source and sedimentary microfacies formed in coal seam are discussed,and then the influence of sedimentary environment on trace elements enrichment is established.The results are as follows:(1)The No.11 coal of Donglutian mine is high sulfur,high ash,and high volatile content bituminous coal.There are great differences in volatile content in coal seam.The coal seam is high ash coal,but it is low ash coal at the bottom of coal seam.The macroscopic coal rock characteristics of No.11 coal seam are mainly semi-dark coal,with a small amount of sapropelic coal at the top.The content of vitrinite is 61.6%～91.0%,and the vitrinite is mainly Desmocollinite.Inertinite content ranged from 9.1% to 44.5%,with the most common Fusinite and Semifusinite.(2)Kaolinite and pyrite are widely distributed in the whole coal seam.Quartz,calcite,boehmite,chlorite,anatase and rutile are detected in some layers.Quartz is mainly distributed in the upper part of the coal seam and has the highest content in the roof.Calcite is mainly distributed in the middle and lower part of coal seam.Boehmite is mainly distributed at the bottom of the coal seam.(3)The enriched elements in No.11 coal samples mainly include Li,Zr and Pb.The slightly enriched elements include Cr,Zn,Ga,Sr,Nb,Hf,Ta,Th,U and REY.The element Li in coal may be directly derived from the granitic pegmatite in the north of the basin.The distribution of Zr(Hf)element increases gradually from the bottom to the roof of the coal seam,indicating that the coal seam is gradually affected by terrigenous debris.Zr(Hf)may be derived from the granitic pegmatite in the northern part of the basin together with lithium.(4)The veined calcite in the coal is distributed in the middle and lower part of the coal seam,and REY content in the calcite is unusually high.However,the calcite veins are products of the later stage of coalification,indicating that the secondary distribution of REY may occur in the coal seam.The content of REY in calcite veins is unusually high(average 1236μg/g),which is 27 times that of vitrinite(average 46.2μg/g).The REY in calcite veins is enriched in MREY,while the REY in vitrinite is enriched in HREY,indicating that the REY in calcite veins have no effect on the REY in vitrinite.(5)The coal facies of the Donglutian No.11 coal seam include three types,i.e.,the low moors under the barrier-lagoon,lower delta plain and upper delta plain;Donglutian No.11 coal seam is affected by Marine transgression.The p H,paleosalinity,redox degree and hydrodynamic conditions of marsh water are discussed.Li-Zr-Nb-Ta-Hf is mainly enriched in DLT-11-2,DLT-11-6 and DLT-11-10,which are characterized by similar paleopeat-forming environments,namely,seawater,anoxic to euxinic conditions,strong hydrodynamic conditions,with relatively low sulfur content,the low moors under the barrier-lagoon.Pb-Cu-Ga-Ge-TlREY is mainly enriched in DLT-11-0、DLT-11-4 and DLT-11-8,which are characterized by similar paleopeat-forming environments,namely,seawater,alkaline,oxic or anoxic conditions,weak hydrodynamic conditions,with relatively high sulfur content,lower delta plain.