Enrichment And Differentiation Mechanism Of Minerals And Trace Elements In The Paleogene Coals From Baise Area Of Guangxi Province,China

The mode of occurrence of the mineral matter and the abundance of the elements in Baise coal provide insights into the depositional environments and geological history associated with coal formation.The Baise Coalfield is located in the Baise Basin,southwestern Guangxi Province,southern China.The Baise Basin is a narrow fault-bounded basin developed on a Mesozoic basement,filled with Paleogene terrestrial sediments.Baise basin is an ideal place to research paleoenvironment and geological evolution because the basin has good outcrop and facies preservation.To better understand the origin,occurrence and enrichment mechanism of mineral and trace elements in the studied coals,this thesis reports on the petrological,mineralogical,and geochemical compositions of the Baise coal in the Baise coalfield.Combine the evolution history of the Baise Basin with the experimental results of trace elements and minerals can provided a new acquisition for regional geological history(such as the source region,depositional environments,biological activity,hydrothermal activity,groundwater etc.)from coal geological point of view.Based on lithological compositions,a total of 98 bench samples were collected from the Donghuai coal mine,Dongsun coal mine,Nahuai coal mine and Zhoujing coal mine.The proximate analysis was undertaken following the methods described in standards of ASTM International.X-ray fluorescence spectrometry(XRF)and inductively coupled plasma mass spectrometry(ICP-MS)were used to analyze the percentage of major element oxides and trace elements in the Palaeogene low-rank coal from the Baise Coalfield.A low-temperature oxygen-plasma asher,X-ray diffraction and Siroquant were used to quantitative the percentage of mineral in the study samples.Optical microscopy and scanning electron microscopy with an energy dispersive X-ray spectrometer were used to observe the mineral phases,composition,mo rphology and occurrence.The DH-C and DH-I coals are subbituminous C coal and are characterized by middle ash yield and high volatile matter yield;the DH-D Coal are subbituminous A coal and is characterized by a high ash yield and high volatile matter yield;the DS-F coals are subbituminous B coal and is characterized by a middle ash yield and high volatile matter yield;the NH-A coals are subbituminous A coal and is characterized by a high ash yield and high volatile matter yield;the ZJ-4 and ZJ-5 are subbituminous B coal,low ash yield and high volatile matter yield.The mineral assemblage in the investigated samples are dominated by quartz,illite,kaolinite and chlorite,with lower proportions of anatase and pyrite,siderite,calcite,gypsum,bassanite,anhydrite and jarosite were detected only in several samples.In addition,feldspar,apatite,tourmaline and barite were also observed under scanning electron microscopy.Quartz in this study generally occurs as discrete,angular grains with varying sizes and distributed in organic matter and in clay minerals,indicating a detrital origin;and to a lesser extent,as sharp edges or elongated volcanic quartz grains.The modes of occurrence of the clay minerals as thinly bedded lamellae indicate that they are mostly of detrital origin especially the coarser grained flakes of chlorite and illite-muscovite.However,the occurrence of kaolinite,siderite and apatite in the study samples occurrs as authigenic cavity-fillings or cell-fillings,indicating hydrothermal fluids intrusion.The textural relationship between syngenetic siderite replacing massive pyrite were also found in the studied samples.These explain the changes in acidity in the peat environment during peatification.The depositional environment was weakly acidic at the early stage of peat accumulation when SO₄^2-is totally reduced to S^2-,and iron occurs in surplus,the peat environment changes to neutral to weak alkaline conditions siderite can be formed in the peat.Compared with the average values for Chinese coals,the percentages of SiO₂?Al₂O₃?K₂O and Mg O in both Donghuai Dongsun and Nahuai coals are higher,while the concentrations of Na₂O,Mg O,K₂O and CaO in Zhoujing coals are higher.Compared with average values for world low-rank coals,Donghuai Coal is enriched in Li,V,F,Rb,Sb,Cr,Ni,Zn,Ga,Cu,Cs,Ta and Pb;the concentrations of Li,F,V,Cr,Ni,Rb,Sb,Cs and Hg in Dongsun coal are higher;the concentrations of Cs,Li,Rb,F,V,Cr,Ni,Cu,Zn,Ga,Zr,In,Sb,Hg and Pb in Nahuai coal are higher;the concentrations of V,Ge,Rb,Cd,Ba and Ta in Zhoujing coal are higher;in addition Zhoujing coal is distinctively enriched in Sb and Cs.The elevated Sb and Cs in the coal were derived from the detrital material shedding from the exposed middle Triassic strata,which host notable Au and Sb deposits in the surrounding region.The REY(rare earth elements and Y)distribution patterns for the coal benches,partings,roof and floor strata are generally characterized by enrichment in medium REY and positive Gd anomalies,both of which were due to acidic waters circulating within the coal basin.The high concentration of B in the Palaeogene low-rank coal from the Baise Coalfield is support the conclusion too.