Enrichment And Differentiation Mechanism Of Minerals And Trace Elements In The Late Permian Coals From Eastern Yunnan And Western Guizhou Province

In this thesis,the abundance,distribution,modes of occurrence,and enrichment mechanism of minerals and trace elements in the Late Permian coals from eastern Yunnan and western Guizhou Province were studied based on the theoretical knowledge of coal geology,mineralogy,coal chemistry,geochemistry,and statistics,as well as the analysis technology of optical microscope,scanning electron microscopy with an energy dispersive X-ray spectrometer?SEM-EDS?,low temperature ashing?LTA?,X-ray diffraction?XRD?,Siroquant quantification system,X-ray fluorescence spectrometry?XRF?,and inductively coupled plasma mass spectrometry?ICP-MS?.The Late Permian coals from Shugentian mine in eastern Yunnan are characterized by super low-medium ash yield and low volatile matter yield,as well as super low-low sulfur content;coals from Yueliangtian mine in western Guizhou are characterized by low-medium ash yield,high volatile matter yield,and super low-medium sulfur content.On the whole,the ash yield,volatile matter yield and total sulfur content of the Shugentian coals are much lower than those of the Yueliangtian coals.The total sulfur content,dominated by pyritic sulfur,is much higher than 1% in both No.19 and No.24 coal from the Yueliangtian mine.The maceral composition of the Shugentian and Yueliangtian coals is generally dominated by the vitrinite group,followed by the inertinite group.The barkinite is the major component of liptinite group in the Yueliangtian coals;however,the liptinite group in the Shugentian coals was not found under the optical microscope due to the high degree of coalification.The results of X-ray diffraction and Siroquant quantitative analyses?XRD+Siroquant?showed that the mineral composition of the Shugentian coals was similar to that of the Yueliangtian coals dominated by quartz and clay minerals?kaolinite,mixed-layer illite/smectite,and chamosite?,followed by anatase and calcite;ankerite and pyrite were distributed only in a few bench samples.In addition,chalcopyrite,sphalerite,barite,zircon,and REE-bearing phosphate minerals?florencite and rhabdophane?were also observed under scanning electron microscopy.According to the vertical variations of minerals in the samples from Shugentian and Yueliangtian mine,the contents of quartz in the coal bench samples are generally higher than those in the roof,floor,and parting samples;however,the trend of the kaolinite is completely opposite to quartz.In contrast to quartz,the contents of anatase in the roof,floor,and parting samples are generally higher than those in the coal bench samples.Calcite is mainly distributed in the coal bench samples.The Late Permian coals from Shugentian and Yueliangtian mine are characterized by abundant authigenic quartz.The optical microscopy and SEM-EDS analyses show that the authigenic quartz has two modes of occurrence:?1?scattered irregular particles distributed in collodetrinite,sometimes along the bedding together with clay minerals or pyrite.The quartz particles are generally fine?about 10?m?.In addition,larger particles of authigenic quartz?about 120?m?with secondary increase phenomenon were also found in the coal bench sample YLT182-7c from Yueliangtian mine.?2?cell-fillings of the structured macerals.Such authigenic quartz either fills alone or coexists with kaolinite or chamosite.According to the modes of occurrence of authigenic quartz,it is deduced that the authigenic quartz was precipitation from Si-rich low-temperature hydrothermal fluid or siliceous solutions derived from weathering of Emeishan basalts in the Kangdian Upland.Compared to the average values for Chinese coals,the contents of SiO₂ in both Shugentian and Yueliangtian coals are high,with the concentration coefficient?CC?of 1.35 and 1.42,respectively.The TiO₂ content is close to or slightly higher than that of Chinese coals.However,the contents of the major-element oxides,including Al+2O₃,Fe₂O₃,MnO,MgO,CaO,Na₂ O,K₂O,and P₂O₅ are generally lower than those in Chinese coals.The value of SiO₂/Al₂O₃ ratio in Shugentian and Yueliangtian coals is significantly higher than that in Chinese coals?1.42?and kaolinite?1.18?,which is mainly due to the large amount of authigenic quartz.In the Shugentian and Yueliangtian coals,the vertical distribution of SiO₂,Al₂O₃ and TiO₂ is similar to each other,the content of which is higher in the roof,floor and parting samples than in the coal bench samples.The high correlation coefficients for TiO₂-SiO₂ and TiO₂-Al₂O₃ indicate that Ti may occur mainly in anatase and clay minerals.According to the similar vertical distribution of Fe₂O₃ and MgO in most coal bench samples,it is inferred that Fe and Mg have common carrier minerals,such as chamosite and ankerite.The vertical distribution of CaO is generally opposite to that of ash yield,and the CaO content is higher in coal bench samples than that in host rocks and parting samples.Compared to the average values for world hard coals,the concentrations of Co,Cu,Zr,Nb and Ta in the Shugentian coal are high,and the concentration coefficients are 2.37,2.21,2.39,2.03 and 2.21,respectively.The trace elements,including V,Co,Cu,Se,Zr,La,Ce,Sm,Eu and Tb,are enriched in the Yueliangtian coals,with the concentration coefficients of 2.68,3,2.97,3.48,2.16,2.04,2.15,2.03,2.48 and 2.06,respectively.The harmful trace elements Se,Hg and Pb are slightly enriched in the No.19 and No.24 coal from Yueliangtian mine,mainly due to the abnormally high concentrations of these elements in the coal bench samples,including YLT19-6c,YLT19-9c,YLT24-8c and YLT24-10 c.It is noted that the concentrations of As and U are relatively high in these four samples.The average concentrations of rare earth elements and yttrium?REY?in the Shugentian and Yueliangtian coals are 101?g/g and 136?g/g,respectively,which are significantly higher than that in the world hard coals?68.6?g/g?.The average REY-oxide?REO?concentrations in the No.5,No.9 coal ash from Shugentian mine and No.18₁ M coal ash from Yueliangtian mine are close to or much higher than the typical REY cut-off-grade?0.1% REO?in coal combustion wastes for byproduct recovery.According to the evaluation method for high-REY coal ash in terms of potential industrial value proposed by Seredin and Dai,most of the partings and coal bench samples can be used as raw materials for extracting REY.In general,the mineral and element combination characteristics in the Late Permian coals from Shugentian and Yueliangtian mine show high similarity.The enrichment and differentiation of minerals and trace elements are mainly affected by the following factors:?1?Sediment-source region influenceThe Kangdian Upland,located to west of the research area,is the primary sediment-source region for the coal basins in southwestern China.The supply of the Kangdian Upland mainly contains the terrigenous detrital materials and siliceous solutions from the weathering of the Emeishan basalt.The abundant authigenic quartz in the Shugentian and Yueliangtian coals were mainly precipitated by the siliceous solution derived from the weathering of the Emeishan basalt.The large quartz particles?>100?m?with a fine psephicity in host rocks and some coal benches provide evidence for terrigenous detrital materials input,as well as kaolinite occurring as thin beds.The geochemical evidences for sediment-source region influence contain the high contents of TiO₂,the transition elements?Sc,V,Cr,Co,Ni,Cu and Zn?,and the high field strength elements?Nb,Ta,Zr and Hf?in the normal sediment host rocks and partings.In addition,the distribution patterns of REY in the normal sediment host rocks and partings from Shugentian and Yueliangtian mine are similar to those of the Emeishan basalts,which are characterized by L-M-type enrichment and distinct positive Eu anomalies.However,the weak negative or no significant Eu anomalies in some normal sediment host rocks and partings indicate that they are also affected by the input of the felsic-intermediate terrigenous materials except for the Emeishan basalts.?2?Felsic volcanic ash inputFour layers of Tonstein,including SGT7U-3p,YLT18₁U-7p,YLT18₁M-6p and YLT19-8p,were found in the coal seams from Shugentian and Yueliangtian mine,the thicknesses of which were 5cm,1cm,2cm,and 5cm,respectively.Each of them has a continuous lateral extent and a clear top and bottom boundary in the coal seam.The mineral compositions of Tonsteins are dominated by clay minerals,with the highest content up to 95.6%.Under the optical microscopy and SEM,volcanic quartz was observed in Tonsteins presenting two modes of occurrence.Most of the volcanic quartz showed sharp edges.Some volcanic quartz had irregular shape or crack due to the high temperature erosion.The zircon particles in Tonsteins occur as well-developed crystal with long prism faces,and sometimes have high-temperature crack or eroded hole.The crystal length to width ratio?c/a?of zircon is much higher than 2.5,which is consistent with the pyroclastic zircon derived from the felsic volcanic ash.Moreover,the elliptical kaolinite aggregates and well-crystallized vermicular kaolinite are also characteristic minerals of Tonsteins.The contents of TiO₂ and transition elements?Sc,V,Cr,Co,Ni,Cu,Zn?in Tonsteins are very low,but the concentrations of Th and U are significantly high.The distribution patterns of REY in the Tonstein samples SGT7U-3p,YLT18₁U-7p and YLT19-8p exhibit strong negative Eu anomalies,which is consistent with the typical characteristics of the felsic Tonsteins found in southwest region.The values of TiO₂/Al2O3 ratio of SGT7U-3p,YLT18₁U-7p,YLT18₁M-6p and YLT19-8p were 0.01,0.03,0.02,and 0.02,respectively.In the plots of Nb/Y-Zr/TiO₂,the points of these four Tonstein samples fell in the Trachyandesite area,indicating that the nature of these Tonseins is similar to Trachyandesite originated from felsic magma,which further confirms their origin of the felsic volcanic ash.?3?Hydrothermal fluid intrusionUnder the SEM observation,chamosite in Shugentian and Yueliangtian coals occurred either as cell-fillings together with quartz or kaolinite or as replacements of kaolinite.According to the modes of occurrence of chamosite,we concluded that it was formed by the interaction of kaolinite and Fe-Mg-rich hydrothermal fluids.The fracture-filling pyrite and calcite were due to the Fe-Ca-rich hydrothermal fluids intrude into the coal seams during the post-diagenetic stage.In addition,chalcopyrite,barite,anatase and REE-bearing phosphate minerals filling in the cells or fractures can also provide evidences for multistage hydrothermal fluids activities.Moreover,the hydrothermal activity lead to the redistribution of some trace elements in coal benches,host rocks,partings and Tonsteins.The relatively high concentrations of As,Se,Hg,Pb and U in the No.19 and No.24 coal from Yueliangtian mine are mainly attributed to the hydrothermal fluids.The high REY concentrations in coal bench samples?YLT18₁U-8c,YLT18₁M-2c,YLT18₁M-7c,YLT182-7c and YLT19-9c?directly underlying the roofs,partings or Tonsteins are probably due to the leaching by hydrothermal fluids or groundwater during coal seam formation.The Eu in the felsic Tonstein YLT18₁M-6p would be expected to have negative anomaly.However,Eu doesn't show any anomalies,probably attributed to the intrusion of high-temperature hydrothermal fluids,which overprinted the negative anomaly inherited from the felsic volcanic ash.In addition,the strong positive Eu anomaly in the coal bench sample YLT18₁M-7c directly underlying the felsic Tonstein also provides evidence for the high-temperature hydrothermal activity.?4?Sedimentary environment impactThe influence of sedimentary environment on mineral matters in coal is mainly represented by seawater.Due to the injection of seawater,the total sulfur contents in the No.19 and No.24 coal from Yueliangtian mine are high,with the highest content up to 4.76%.According to the XRD+Siroquant analyses,a large amount of pyrite was detected in the corresponding coal bench samples.Pyrite shows various modes of occurrence,including framboidal assemblages,massive forms,subhedral to euhedral crystals,and cell-fillings,which are formed by the interaction between SO₄^2- from the sea water and Fe²⁺ from sediment-source region under the action of bacteria during the syndiagenesis.