| Jilin Province is an important coal-reserving area in northeast China,and the Paleogene is the most important period of coal-formation in this Provence,and mainly distributed in eastern Jilin Provence.Hunchun Basin is located in Hunchun City,eastern part of Jilin Provence,which has abundant coal resources in this region.This thesis discusses the mineralogical and geochemical characteristics,as well as enrichment mechanism of mineral matters of the Paleogene coals from Hunchun Basin,based on the corresponding theoretical knowledge of petrology,mineralogy,and geochemistry.The coals from Baliancheng mine are subbituminous rank and have low ash yield and high volatile matter yield;Banshi coals are subbituminous rank and have middle ash yield and high volatile matter yield;as well as Fuqiang coals sre lignite/subbituminous rank and have middle-high ash yield and high volatile matter yield.On the whole,the coals from three mines have very low total sulphur content,and the maceral composition of the coals is generally dominated by the huminite contents,and relatively low liptinite and inertinite contents.The mineral assemblage in the baliancheng,Banshi,and Fuqiang coals was dominated by kaolinite,illite(contain mixed layer illite/smectite),and quartz,with lower proportions of plagioclase(mostly albite),potassium feldspar,and siderite,ankerite and pyrite were detected only in several samples;bassanite was detected only in Fuqiang coal bench samples.In addition,anatase,sphalerite,and barite were also observed under scanning electron microscopy.Quartz in this study has several modes of occurrence:larger and rounded terrigenous clastic quartz;fine and scattered irregular authigenic quartz particles;epigenetic quartz occurs as fracture-fillings;volcanic quartz showed sharp edges or elongated grains.Kaolinite is mainly distributed along the beeding planes,and,to a lesser extent,occurs as disseminated fine particles,fracture-fillings,and locally has vermicular and book-like forms.SEM examination shows that the kinds of feldspar in this study include plagioclase(mostly albite),k-feldspar,and sanidine.In comparison with average values for Chinese coals,the percentagess of K2O,Na2O,and SiO2 in both Baliancheng and Bashi coals are high;the percentages of K2O,SiO2,Na2O,Al2O3,MnO,and MgO are high in Fuqiang coals.In comparison with average values for world low-rank coals,Cs,W,Sb,and Ge are slightly enriched in the Baliancheng coal;the concentrations of As,Sb,Cs,Ge,W,V,and Co in the Banshi coal are high.In addition,more trace elements are enriched in Fuqiang coal,including W,Cs,Sb,Rb,Pb,V,Ta,Ni,F,Li,Sn,Z,Ga,and Th.The mineral and element assemblage characteristics in the Paleogene coal seams from Baliancheng,Banshi,and Fuqiang mine show high similarity.The enrichment of mineral matters in Hunchun coals owing to the four geological factors:1)Sediment-source regionThe Paleogene Hunchun Formation is the mainly coal-bearing sequence in the coalfield,and the Paleozoic metamorphic rocks and Mesozoic volcanic rocks are served as the basement of the coal basin.Combined with the existing geochenical data about the probable source magmatic and metamorphic rocks from the areas surrounding the Hunchun Basin,a comparison between the studied samples and their possible magmatic and metamorphic source rocks with the plots of Al2O3/TiO2 vs.Sr/Y,La/Yb,and Rb/Ti,indicating that the Mesozoic(mostly Lower Cretaceous)volcanic rocks were major suppliers of terrigenous materials into the Hunchun Basin;other significant sources were the Paleozoic granites and associated metamorphic rocks.A minor source is represented by the Paleogene volcaniclastics,including some adakitic rocks characterized by higher Sr/Y and La/Yb ratios.The Upper Continental Crust(UCC)normalized REY can be used as geochemical indicator.In this study,Eu in most studied samples show positive or weakly positive anomalies,as well as the Paleozoic(mostly Permian)intrusive and associated metamorphic rocks and Mesozoic volcanic rocks also have positive Eu anomalies,indicating that the studied sediments most likely inherited this feature from the source rocks.2)Volcanic ash inputThe Hunchun Paleogene coals were affected by volcanic avtivity during the coal-forming period,which the discovery of tonstein layer(BLC-19-2-7p)provided direct evidence for the volcanic ash input in the coal-bearing strata.The thickness of this parting is 3cm,has sharp contacts with the overlying and underlying coal bed and throughout the Baliancheng mine;the mineral compositions of this tonsteins are dominated by clay minerals,contains kaolinite(61.9%)and smectite(22.9).In addition,sharp volcanic quartz,as well as high-temperature mineral sanidine occurs as lenticular,tabular,and irregular particles were observed in this sample.Moreover,the modes of occurrence of kaolinite(vermicular and book-like form)and chloritized biotites are also characteristic minerals of tonsteins.In addition to tonstein layer,sharp volcanic quartz,high content of clay minerals,vermicular kaolinite,and chloritized biotites were also observed in some coal bench samples.3)Hydrothermal fluid intrusionThe samples BS-22A-1c and BLC-19B-3c with the similar geochemical signatures are anomalous,having the highest contents of sulfur,boron,arsenic,pyrite,and quartz.Under the SEM,the authigenic quartz occurs as scattered irregular particles distributed in sample BLC-19B-3c,indicat that the authigenic quartz was precipitation from Si-rich hydrothermal fluid;fracture-fillings quartz and arsenopyrite were found in sample BS-22A-1c,suggesting an epigenetic origin due to hydrothermal activity in this coal bench;riched Fe and Mg hydrothermal fluids altered kaolinite to chamosite,as well as disseminated barite,both indicat their hydrothermal genesis.(4)Acidic waters inpactMost samples of the Hunchun coals are characterized by M-and H-type joint enrichment with a Gd-maximum,this signature is well knowen for acidic natural waters,some of which circulated in coal basins.This conclusion is also further supported by elevated concentrations of B in Hunchun coals.The Ge enrichments that are detected in some individual coal benches from the Hunchun Coalfield were related to tectonic uplift of the old granites and other crystalline rocks surrounding the basin and intensively eroded by a hybrid-hydrothermal-weathering process.The oxidized meteoric waters episodically influenced by volcanic-related hot springs might extract Ge from the source rocks and transport it to the reducing peat swamp environment,then precipitate it as a result of bio-chemical reduction,forming the mineralization disseminated in the coals. |
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