Occurrence Characteristics Of Mineral Matters In Low-rank Coals And Their Implications For Geological Process

As a unique sedimentary rock in the development of geological history,coal is serving as a dignified history book,recording a series of geology events related to biology,physics and geochemistry occurred during the past millions of years.The organic matter of coal,which is formed by coal-forming plants after a series of saprofication,coalification,evolution and polycondensation process,preserved large amount of information on climate and environmental change in the phases of plant growth,death and accumulation respectively;while the coal mineral composition,as the result of different geological process,could not only be used to suggest the coal-forming geological environment,but also to explain the coal seam formation and coalification,as well as to track the historical evolution of the coal basin and some other basic theoretical issues.Thus the coal would potentially be the best research object to be applied the 'uniformitarianism' research method.With the advanced analysis technology,including Inductively Coupled Plasma Mass Spectrometry(ICP-MS),X-ray Fluorecence spectrometry(XRF),Isotope-ratio mass spectrometry(IRMS),X-ray diffraction(XRD),Atomic Absorption Spectroscopy(AAS)and Scanning Electron Microscope in conjunction with X-ray Energy Dispersed Spectrometry(SEM-EDS),and application of Multidisciplinary knowledge of coal petrology,mineralogy and petrology,coal geochemistry,elementary geochemistry and coal geology were applied in this study,based on the basic geological analysis,chemical analysis,statistical analysis and isotope analysis,the author analysised the element distribution and mineral composition of the low-Ge lignite of Shengli Coalfield,Inner Mongolia,and lignite of Mile Basin,Eastern Yunnan,China respectively.This study compares the coals(bitumite)of Guiding Coalfield,Guizhou,and Yanshan Coalfield,Yunnan,and discussed the difference of occurrence modes of elements in the coals of different ranks using the sequential leaching method;the element distribution and differentiation characteristics in the coal combustion products-fly ash have been analyzed to get a better understanding of the relation of element occurrence modes between feeding coal and coal ash;Sequential leaching scheme has been used to quantify the occurrence of the non-mineral inorganic elements;the author proposed the most influential factors for the enrichment of Ge and formation mechanism of the Ge in Wulantuga Ge deposit;based on the mineralogy and element geochemistry,as well as the rare earth elements and their anomalies,the palaeoenvironment has been reconstructed,implying the injection of seawater during the peat accumulation;combined with the sedimentary environment indicated by the palynology and minerals,organic carbon isotopes ?13C in the coal benches though the whole seam of Mile Basin,the impact of macerals of coal,as well as the palaeoenvironment during the peat accumulation processto the organic isotopes have been analyzed;based on data base of the 87Sr/86Sr values of the seawater in individual geological periods,the geologic time of coal deposition and the peat accumulation rate have been deduced and calculated;on the basis of the geological setting of the Mile coal basin,the impact mechanism of seawater input to the peat has also been discussed.As indicated by the proximate and ultimate analysis,low-Ge coal in Shengli Coalfield,and Mile coal have a similar ash yield,and the content of moisture and total sulfur(27.59%and 1.66%,respectively)in low-Ge coal is higher than Mile coal(16.78%and 0.79%respectively),while the content of volatile matter and element C,H,N of the former is distinctly lower than the latter.The sulfur of the low-Ge coal is dominated by organic sulfur with less sulfate sulfur;whist the Mile coal is dominated by sulfate sulfur or having a similar percentage of sulfate and organic sulfur.The huminite average reflectance of the coals is 0.45%and 0.40%respectively.Both of the low-Ge coal and Mile coal are dominated by the huminite group;and the former is relatively riched in inertinite with the abundant of macrinite,while the latter is relatively riched in liptinite with a large amount of resinite,suberinite and cutinite with strong fluorescence.The minerals of the low-Ge coal and Mile coal are mainly quartz,kaolinite,pyrite and gypsum.The abundant of sulfates formed in the low-temperature ashing process,occurred as the main constituents of the low temperature ash yield,have a negative relation with ash yield,indicating the sulfate-forming metal elements is organic associated in the raw(un-ashed)coal.The sequential extraction scheme for the Yanshan and Guiding coals indicates that the highly enriched trace elements V,Cr and Mo are mainly associated with organic matter,and to a lesser extent,some of them occur in the silicates.A small amount of Mo and Cr occur as sulfides in the coal.Se and U mainly occur in the organic matter,and a part of them is closely associated with sulfides.A few of U occurs in the silicate minerals.For the element Cd,it mainly occurs in the sulfide form with a proportion associated with organic matter and silicate minerals.The occurrence modes of the elements in low-Ge coal and Mile coal are slightly different from the Yanshan and Guiding coals.For the low-Ge coal and Mile coal,V has a remarkable correlation to Cr,and the linear dependence relation with ash yield and percentage of SiO2,Al2O3 and K2O indicates the silicates act as the main carriers of V and Cr.Mo is not only closely associated with organic matter and clays,but also associated with sulfides in Mile coal,which resembles the occurrence modes in Guiding and Yanshan coals.Cd is closely associated with clays,and showing no distinct relation with sulfides in low-Ge coal and Mile coal.Both Se and U occur as clays and sulfides in low-Ge coal and Mile coal,while occur in organic matter and sulfides in Guiding and Yanshan coal,showing some differences in the occurrence modes in the coals of different rank,The location of low-Ge coal of present study closely neighbor the Ge ore deposit with 1.2km apart.However the thickness weighted concentration of Ge of the low-Ge coal is only 0.45?g/g,far lower than that in the Ge-rich coal.The Ge-rich coal area,with a total area of 2.2km2,is mainly located in the southwestern of the coalfield,decreasing from southwest to northeast of the coalfield.The spatial distribution of Ge indicates that the decreasing concentration of Ge from southwestern to northeastern of the coalfield is the result of being absorbed by the organic matter in the migration of Ge-rich fluids.In the comparison with Ge ore deposit of Lincang,Yunnan and Pavlov,Russia,although the enrichment of Ge is similarly associated with the leaching of adjacent granite,the migratory direction of the Ge-rich fluids is quite different.For the Ge ore deposit of Shengli Coalfield,the enrichment of Ge occurred in the coal on the margin of coal basinis contributed by the later migration of the Ge-rich solutions,whereas the Ge enrichment of Lincang and Pavlov Ge ore deposits occurred in the joints of different faults,implying the hydrothermal solutionsmigrated vertically along the along fault intersections or similarstauctures,and then were discharged into the peat swamp.Furthermore,the concentration of Ge decreases with the increasing distance from the granite at the bottom,resulting the dome-shaped Ge distribution.The abundant of framboidal pyrite,authigenic gypsum and the varying proportion of sulfur implies the sufficient sulfate is offered by the seawater.The decreasing concentration of pyrite,gypsum and sulfur along the seam depth implies a decreasing trend of seawater input along the seam depth.In addition,the ratio of Ca/Mg?Sr/Ba of the Mile coal suggests a decrease in the water salinity.The variation of Th/U?V/Ni and the concentration of St?Ca?Sr along the seam also implied the decreasing influence of seawater at the peat accumulation process.The coal benches of the lower seam portion possesses the M-H-type feature with distinctly positive La and Gd anomalies,a further reflection of seawater input in the peat accumulation process.The pollens of the Mile coal indicates the coal-forming plants indicates the representing plants of the Oligocene/Miocene phase with characteristics of low and high lands,which were grown in the mountain area with moderately warm with some subtropical characteristics.However the absence of Fagus and Taxodiaceae pollen,which require continuous humid conditions,indicate that these conditions occurred not in all the seasons,implying the dry climate in the coal forming process.The light brown coal layers in interval distribution in the dark brown coal,with less content of huminite macerals but more liptinite macerals and aquatic plant pollens,imply the abundant supply of water in the sediment trap and the dominant herbaceous coal-forming plants with less lignin;while the dark brown coal with high huminite but less liptinite macerals,indicates a lower water lever in the sedimentary zone.The carbon isotope compositions of paleobotany recorded in the coal,was influenced by the CO2 compositions of the atmosphere in the coal-forming stage.Therefore the carbon isotopes ?13C of plant are closely associated with the climate conditions.Heavy organic ?13C in the coal implies dry weather conditions while light ?13C implies humid weather conditions in the coal-forming stage.The similar profile of vegetation index and organic carbon isotopes along the seam indicates the coal-forming plants were dominated by lignin-riched woody plants in the sedimentary zone with high water level in the dry weather condition.The high content of gypsum further confirmed the dry weather condition at the phases with heavy ?13C recorded in the coal.Based on the 87Sr/86Srofplanktonic foraminifers from "Deep Sea Drilling Project Sites"(hole 588C),the range of 87Sr/86Srvalues of the authigenic gypsum in the Mile coal corresponds to Early Miocene seawater.The average 87Sr/86Sr values for the three groups are:the lower group(0.708390)corresponds to 21.7 Ma(+/-0.55 Ma),the middle group(0.708453)to 20.7 Ma(+/-0.56 Ma),and the upper group(0.708575)to 18.7 Ma(+/-0.24 Ma),with the analysis error of +/-0.55 Ma?+/-0.56 Ma ?+/-0.24 Ma respectively.The 87Sr/86Sr analysis indicates the Mile coal,was formed at the early Miocene(21.7 Ma)with an overall span of 3.0 Ma.Considering the coal seam thickness of 27m,the rate of peat accumulation is approximately 0.009 mm/year.A likely hydrological connection between the inland Mile Basin and the remnants of the Tethys Ocean is the Red River-Ailao Shan transform Fault(RRASTFZ)and the joined Xianshuihe-Xiaojiang fault system,which were active in the coal-forming phase,serving as a convenient channel for the transportation of seawater to the coal basin.In addition,the change of subduction direction of Pacific Plate tuUms the east-west-trending fault,including Fuyuan-Mile fault,Xuanwei-XUndian fault and Mile-Shizong faulty into right lateral fault,with the rising of the north region of RRASTFZ(including Mile Basin).On one hand,the rising region contributed to the decreasing influence of seawater on the Ml coal of Mile Basin;on the other hand,the sustaining rising of the region upraises the Mile basin to the current height of 1350m from seawater level.