Occurrence And Enrichment Mechanism Of Mineral Matters In The Late Permian Coals From Western Guizhou And Middle Guangxi Provinces,China

This paper discusses the mineralogical and geochemical characteristics of the Late Permian coals from the Yueliangtian coalfield,western Guizhou and Yishan coalfield,middle Guangxi provinces.Based on the knowledge of petrology,mineralogy,element geochemistry,isotope geochemistry,and statistics,the author studies the geological factors responsible for the enrichment and origin of elements and minerals from the studied samples.Most of the Yueliangtian coals are classified as low to middle ash,superlow sulfur,and high volatile bituminous coals.The LL5-K3 coals are classified as low ash,superhigh organic sulfur(SHOS)semianthracite.The CG3-K6 and CG1-K7 coals are classified as high ash,SHOS,low volatile bituminous coals.The geochemical and mineralogical characteristics of the Yishan and Yueliangtian coals are strikingly different,owing to the following geological factors:(1)Sediment-source regionLocated in the west of the Yueliangtian coalfield,the Kangdian Upland supplied substantial terrigenous materials for most of the Late Permian coals in southwestern China.The Kangdian Upland affects the Yueliangtian coals in two ways,first of all,the silica-bearing solutions from the weathering of Emeishan basalt invaded into the peat mire;second,the Upland supplied large amounts of terrigenous detritus for the coal basin,including the basalt and intermediate-felsic rocks detritus.The intermediate-felsic rocks are located in the upper portion of the Emeishan basalt.Quartz,mainly of authigenic origin,is abundant in the Yueliangtian coals The authigenic quartz is precipitated from the silica-bearing solutions from the weathering of basalt in the Kangdian Upland.Besides,the sediment-source region also affects the geochemical characteristics of the Yueliangtian coals.The Al2O3/TiO2 ratio,graph of Nb/Y-Zr/TiO2,concentrations of TiO2,Sc,V,Cr,Co and Ni,REY distribution pattern and anomalies in the Yueliangtian coals indicate that not only the Emeishan basalt but also the intermediate-felsic rocks supply abundant terrigenous detritus for the Yueliangtian coals.The sediment-source region for the Yishan coalfield is the Yunkai Upland,which is composed of the Carboniferous-Permian felsic rocks.The Yunkai Upland is responsible for the Al2O3/TiO2 ratios,concentrations of TiO2,Sc,V,Cr,Co and Ni,and REY distribution pattern and anomalies in the Yishan coals.(2)Multi-character volcanic ash inputAt least two kinds of volcanic ash fell into the Yueliangtian coal basin,forming different kinds of tonstein.Samples YLT6U-3p and YLT12-3p are identified as felsic tonstein,and samples YLT6U-10p and YLT6L-4p are mafic tonstein based on three aspects:a)mineral compositions;b)the occurrences of vermicular kaolinite,high-temperature quartz of sharp-edged and irregular forms,chloritized biotite,and beaded anatase;c)comparing the contents of elements(TiO2,transition elements,high field-strength elements,and REY)in studied samples with those of world clay,Emeishan basalt,tuffs,and tonsteins from the Songzao coalfield,Chongqing.Besides the tonsteins discussed above,volcanic fingerprints are also found in some coal bench and floor samples in the Yueliangtian coalfield.High-temperature quartz,well-crystalized zircon,stripped apatite,and REY minerals(e.g.,florencite)are observed in coal samples.The mineral characteristics suggest small-scale volcanos erupted during the coal-forming period.Consequently,volcanic ash fell into the coal basin.However,the amount of volcanic ash is too small to form tonstein in the Yueliangtian coalfield.Although high-temperature(3 quartz is observed in some coal samples,volcanic fingerprints are not obvisous in the Yishan coalfield.(3)Multi-stage,multi-character hydrothermal solutionsThe Yueliangtian coalfield is invaded by multi-stage,multi-character hydrothermal solutions.For example,quartz vein was precipitated by epigenetic silica-bearing low-temperature hydrothermal solutions.Cell-filling chamosite is formed by two mechanisms.Chamosite filling in the cell alone was precipitated from Fe-rich hydrothermal solutions.In some cases,chamosite fills in the cell with quartz and kaolinite,which was formed by the following process:firstly,the earlier-precipitated kaolinite was invaded by Fe-Mg-rich hydrothermal fluids and generated kaolinite and chamosite during early diagenesis(165-200℃);secondly,kaolinite and chamosite desilicated and generated quartz.Kaolinite and chamosite still remains.Besides,calcite vein,fracture-filling pyrite,REY minerals and anatase in the studied samples all indicate epigenetic hydrothermal solution invasions.Besides the effects on mineralogical characteristics,hydrothermal solutions also have influences on the element redistribution in the Yueliangtian coals.Relative to Ta,Hf,and Th,elements Nb,Zr,and U were easily leached from the upper tonstein,migrating,and adsorbed by the organics in the lower coal seams.Consequently,the Nb/Ta,Zr/Hf,and U/Th ratios are higher in the underlain coal benches than those of the tonsteins.Besides,samples YLT6L-3c and YLT6L-4p are characterized by M-REY enrichment type,with a maximum of 8Gd,which is caused by the circulating acid water(e.g.,pCO2-rich water)in the coal basin.Hydrothermal solutions are also responsible for the mineralogical and geochemical characteristics of the Yishan coals.Multi-stage hydrothermal solutions invaded coalfield and caused different modes of occurrence of minerals.For example,feldspar occurs as veins;there are some cavities on the surface of feldspar,filled by kaolinite or carbonate minerals;buddingtonite aggregations occur at the rim of K-feldspar particles;quartz fills in the fractures.Besides,the enrichment of U-Se-Mo-Re-V in the Yishan coals is also caused by hydrothermal solutions.Fluorite,as carriers of enriched F,occurs as cell-filling in the Yishan coals.The fluorite was probably deposited from F-rich low-temperature(100-200℃)hydrothermal solutions.Calcium is leached from limestone nearby and reacted with F-rich hydrothermal solutions.Consequently,the reaction changed the environment of the coal basin from acid to nearly intermediate where the solubility of fluorine ion is relatively stable and low,which are favorable for the fluorite to precipitate.Furthermore,high-temperature hydrothermal solutions also invaded the Yishan coalfield,especially the Lalang coal mine.As a result,the coal rank increases,Eu exhibits obvious positive anomaly,and elements U-Se-Mo-Re-V are both inorganic and organic associated in the Lalang coals.To the best of our knowledge,it is first time for us to find buddingtonite in coals worldwide.Buddingtonite in the Yishan coals is also formed under high-temperature hydrothermal solutions;it spans from early diagenesis to late coalification to anthracite process.The mechanism for the buddingtonite formations is:organics in the coal released NH4+ under high-temperature hydrothermal solutions and then reacted with pre-existing K-feldspar or albite.Nitrogen isotope data further suggest N-bearing hydrothermal solutions are responsible for the buddingtonite in the Yishan coals.(4)Seawater influenceSeawater have effects on the Yueliangtian coals,especially the YLT6L and YLT12 coals.Compared with other coal seams,the sulfur and pyrite contents in the YLT6L and YLT12 coals are higher;pyrite occurs as framboidal,euhedral cubic and octahedral,cell-filling,nodular and blocky.The sulfur and pyrite characteristics in the Yuliangtian coals indicate that seawater invades coal basin at syngenetic and early diagenesis stages.The Yishan coalfield is deposited on a limited carbonate platform.Consequently,seawater invades the Yishan coals,which are responsible for the enrichment of F and pyrite.