The Occurrence Mode Of Enriched Trace Elements And Nanominerals In A Superhigh-Organic-Sulfur Coal From Ganhe Coalmine,Yanshan Coalfield,Yunnan,China

The occurrence mode of enriched trace elements is one of the frontier topics of coal geology and coal geochemistry.Because it is benefit to understand mechanism of metal deposits and migration and transformation of some potential harmful elements during the process of coal mining.In southwestern China,the Late Permian superhigh-organic-sulfur（SHOS）coals have drawn many researches’ attention,because a variety of valuable and environmental sensitive metal elements are rich in these coals.Therefore,in present paper,based on coal geology,coal petrology,mineralogy,elemental geochemistry,and organic geochemistry,the occurrence mode of enriched trace elements,nanominerals,and some trace elements associated with nanominerals in a SHOS coal from Ganhe coalmine,Yanshan Coalfield,Yunnan,China were investigated by a series of methods: industrial analysis,elemental analysis,coal petrography,X-ray fluorescence spectroscopy（XRF）,inductively coupled plasma mass spectrometry（ICP-MS）,X-raydiffraction（XRD）,scanning electron microscope equipped with energy dispersive spectrometer（SEM-EDS）,high resolution transmission electron microscopy（TEM）and scanning transmission electron microscopy（STEM）coupled with energy-dispersive X-ray spectrometry（EDS）,selected area electron diffraction patterns（SAED）,and fast Fourier transformation（FFT）.The results of this study are as follows:（1）The Late Permian coal of the Ganhe coalmine from Yanshan coalfield,Yunnan,southwestern China,is a superhigh-organic-sulfur（SHOS）coal that has a total sulfur（St）content of 9.32%–12.37%（dry basis）and the organic sulfur content can reach 7.41%–10.5%（dry basis）.The contents of pyritic sulfur（Sp）and sulfate sulfur（Ss）are much lower than So at 1.47% and 0.04% on average,respectively.（2）Combined with XRF,XRD and SEM-EDS,the major minerals in Ganhe coal are muscovite,illite,quartz,pyrite,marcasite,gypsum,and dolomite.Different from No.3 coal,the sanidine and albite were detected by XRD and SEM-EDS in No.4 coal.（3）According to a combination of element geochemistry,coal petrology and mineralogy theory,the sedimentary environment of Ganhe coal was investigated.The Ganhe coal was invaded by seawater in an alkaline reducing environment.There are some differences between the No.3 and 4 coal in regional sedimentary environment: the contents of A_d,Spd and Fe₂O₃ in No.4are higher than those of No.3 coal,while the CaO,K value and U/Th,Ni/Co,V/Ni+V ratios are lower than No.3 coal.To a certain extent,this implies: the No.4 coal may be affected by more groundwater,the environment reduction is relatively weak,the pH value of the aqueous solution is relatively high,and the formation of No.4 coal may be influenced by a volcanic ash component and hydrothermal solution.（4）Compared with the average worldwide and Chinese coals,the Ganhe coal contains significantly higher average contents of Re,Mo,U,Cr,V,and Cd（concentration coefficient（CC）> 10）.Besides,these trace elements are more fasten on No.3 coal.The enrichment of trace elements in No.3 coal may close related with its relatively higher organic matter contents,stronger redox conditions,and higher pH values than No.4 coal.According to the major elements ternary diagram,the Ganhe coal can be divided into four types: partial land-source control type（GH42,GH41-2）,partial reduction control type（GH32-1,GH31-1,GH41-1,and GH45-2）,partial seawater control（GH33,GH31-1,and GH41-1）,and integrated control type（other coal layers）.The highest value of U in the No.3 coal seam is located in GH32-1 which belongs to the partial reduction control type.The highest value of Re,Cr and Cd are located in GH33 which belongs to partial seawater control type.The highest value of Mo and V are located in GH32-2 which belongs to integrated control type.The highest value of Re in No.4 coal is located in GH41-2 which belongs to partial land-source control type.The highest value of Cr,V and Cd are located in GH41-1,Mo and U in GH45-2,they are both belong to partial reduction controltype.Therefore,the controlled factors in No.3 are diverse,but there is no land-source control type.While,the No.4 coal is mainly controlled by reduction environment.（5）According to the correlation analysis results of enriched trace elements（Re,Mo,U,Cr,V,and Cd）and A_d,these trace elements can be divided into three types: weak organic affinity elements（Re and Cd）,medium organic affinity element（Cr and V）and strong organic affinity elements（Mo and U）.（6）Using TEM/STEM-EDS,SAED and FFT,the ultra-fine particles and nanominerals were identified: Al-Si（illite,muscovite,and pyrophyllite）,Fe（pyrite,jarosite,Fe-S-Cr particle,and Fe-Cr-Au particles）,and Ti（Ti-oxides）,Ca（Fe containing dolomite and possible lime）nanominerals with significant concentrations of Cr or V in the Ganhe coal samples.Cr associate with nano illite,muscovite,pyrite,jarosite,Fe-S-Cr particle,Fe-Cr-Au particles,and possible lime;V associate with nano pyrophyllite;Both of Cr and V associate with nano Ti-oxides.Moreover,these nano Ti-oxides contain the most contents of Cr and V（at least in these observed nanominerals in Ganhe coal）.There are also some other nanominerals,such as Mg-Al-Ca particles associated with F,and eremite associated with Ba.To some extent,this finding may explain why Cr and V prefer to associate within macerals rather than the major minerals in Ganhe coal.Because these nano and ultra-fine particles are closely incorporated within macerals and inherent pores,and thus,they can resist the acid and alkali or be retained in the lower-density float-sink fractions.Therefore,although suchelements occur in nanomineral forms,they may appear to have an organic affiliation.