Enrichment Characteristics And Mechanism Of Associated Strategic Metal Elements In The Coal-Bearing Strata From The Southern Foot Of Western Tianshan

Coals and coal-bearing strata are often rich in rare and rare earth and other strategic metal elements.Study on the comprehensive utilization of rare and rare earth elements in coal-bearing strata is not only of great strategic significance,but also one of the frontier and key subject of our country’s current research.The Ministry of Science and Technology and the National Natural Science Foundation of China have set up major research projects related to this area.The Foreland Basin（Kuqa Basin）on the southern foot of the West Tianshan Mountains is one of the most important coal-bearing areas in southern Xinjiang,with many coal mines distributed.This study selected coal-bearing strata in the western section of the Kuqa Basin as the subject of research.By field structure analysis and geological profile measurement,the deformation characteristics and regional distribution of the coal-bearing strata are analyzed.On this basis,the coal seams,host-rocks and wall-rocks of 5selected coal mines and basin base-rocks in the study area were systematically sampled.The main elements,trace and rare earth elements were tested by XRF and ICP-MS analyses.By XRD analysis,types and ratios of existed clay minerals and percentage of detrital minerals in the coal seams are ascertained.The key elements in clay minerals and detrital minerals in the coal seams are determined by SEM-EDS analyses.By comprehensive analysis of the results,the enrichment characteristics,rules and mechanism of the rare earth and trace elements in the regional coal-bearing strata are discussed and analyzed,and the comprehensive utilization prospect of REY and some other trace elements in the coal-bearing strata is appraised.The main achievements and understanding of this study are as follows:（1）It is considered that the fold deformation of the coal-bearing strata in the study area is weak,mainly occurs as monoclinic strata or open fold,and the strong fold is the result of local thrust in the later stage,with a small distribution range and limited deep depth.The fractures in the study area is very developed,but most of them are very small.The main tectonic activities the coal-bearing strata undergone include the five phases:（1）near north-south compression;（2）near east-west compression;（3）NW compression;（4）NE compression;（5）multi-phase extension.But these tectonic movements have little impact on the overall distribution of the coal-bearing strata.（2）The enrichment characteristics of REY in the coal-bearing strata in the five mining areas are:In Bayili,Bozidun and Pochengzi coal mines,∑REY of host-rocks>∑REY of the coal seams,the∑REY of the host-rocks in these 3 mines are 225.43ppm,202.54ppm and 291.97ppm respectively;∑REY of the coal seams are 86.23ppm,68.03ppm and 99.34ppm respectively.In Kuerguiluke and Kezbulake coal mines,∑REY of the coal seams>∑REY of the host-rocks,∑REY in coal seams are279.81ppm and 211.25ppm,and∑REY of the host-rocks are 143.14ppm and 132.08ppm,respectively.（3）The enrichment characteristics of trace elements enriched in the coals from the five mins are as follows:In Bayali coal mine,the coal seams are enriched in Cs,Rb,Ba,Ga,Zr,Hf,V,Nb,Ta,W and Cr.In Bozidun mine,the trace elements enriched in the coal seams are Rb,Cs,Ba,Zr,Hf and Mn.In Kuerguiluke coal mine,the enriched trace elements in the coal seams are Rb,Cs,Zr,Th,V,Zn,Cr,Mn,Ni,Li,Ba,Sc,Ga and Hf.In Kezbulake coal mine,the trace elements enriched in the coal seams are Li,Rb,Cs,Ba,Cr,Ni and Sc,Ga,Zr,Hf,Th,Sn,V,Nb,Ta,W,P,Cu,Zn,Pb,Sb,Co.In Pochengzi coal mine,the trace elements Cs,Mn,Li,Rb,Sc,Ga,Zr,Hf,V,W,Cu,Pb,and Cr.are enriched in the coal seams.（4）In the 3 coal mines,where the∑REY of host-rocks>the∑REY of the coal seams,the coal seams with low total REY contents shows F-type REY pattern,the REYs in the coal seams mainly come from the source-rocks.The coal seams with high total REY contents shows H-type REY pattern,the REYs in the coal seams except come from the source-rocks,some REYs are from the hydrothermal solution.In the 2coal mines,where the∑REY of the coal seams>the∑REY of host-rocks,the coal seams with low total REY contents shows F-type REY pattern,the REYs in the coal seams mainly come from the source rocks.The coal seams with high total REY contents shows M-type REY pattern,the REYs in the coal seams except come from the source-rocks,some REYs are carried into the coal seams by acidic terrestrial water.（5）The REY in the coal seams in the study area mainly occurs in clay minerals,followed by detrital accessory minerals.Clay minerals enriched in REY include kaolinite,chlorite,and chloritoid.The detrital accessory minerals rich in REY are mainly apatite,a small amount of xenotime,etc.Ga is mainly occurred in aluminum-containing minerals（boehmite or diaspore）,Ga in clay minerals is limited.V is also mainly occurred in aluminum-containing minerals,and is also adsorbed by organic matter.Li occurs in various minerals such as silicate minerals,organic matter and insoluble minerals.Zr mainly occurs in zircon and a small amount occurs in other debris accessory minerals.（6）The coals and the host-rocks in the study area have high REY₂O₃.For example,more than half of the samples in the Kuerguiluk mine have REY₂O₃over 300ppm,and a few samples reach 500-600ppm.Such REY content in coals can achieve a comprehensive industrial-grade after the combustion of the coal.In addition,the key elements in the REY of the study area samples were relatively high,with large REY_defand C_outlvalues.It is considered that the key elements in REY in the coals possess a certain comprehensive utilization prospect.In addition,Ga,Li,V and Zr in coal-bearing strata in study region also have certain comprehensive utilization value.