Study On Enrichment Mechanism Of Critical Metals In Coals From The Northeastern Sichuan Basin

In recent years,the demand of world economies for critical metal resources has grown rapidly.However,the reserves of critical metals in conventional deposits are relatively low,and their exploitation and utilization have some limitations.The characteristics of adsorption and reduction barriers caused by organic matter as the main material composition enable coal to enrich a variety of critical elements（e.g.,Li,Ga,Zr,Nb,REE,Th,and U）under the specific geological conditions,then forming the concomitant deposition of coal-hosted critical metals.The Sichuan Basin is a metallogenic prospective area with a high background value of various critical metals.Previous studies on coal-bearing series have focused on the eastern,central,and southern Sichuan Basin,while there are few reports on coal-bearing strata in the northeastern Sichuan Basin.In this study,the coal-bearing strata of the Late Permian Wuchiaping Formation and the Late Triassic Xujiahe Formation in the northeastern Sichuan Basin are selected as the research objects.The abundance,distribution,modes of occurrence,enrichment mechanism,and potential economic value of critical metals in coal-bearing strata from different coalmines have been systematically investigated by using the analytical methods such as coal geology,mineralogy,geochemistry,and metallurgy.1.The enrichment mechanism of critical metals in Late Permian coals from the Tianjia coalmine,northeastern Sichuan BasinThe Late Permian coal from the Tianjia coalmine belongs to a medium-ash and medium-high-sulfur semianthracite coal.The maceral composition is mainly vitrinite,followed by inertinite.The mineral compositions in the coal are primarily represented by clay minerals,carbonate minerals,and pyrite.Anatase,quartz,and florencite occur in minor amounts in most of the coal bench samples.Additionally,some trace minerals such as goyazite,xenotime,kaolinite,and zircon were also observed under the scanning electron microscope with an energy-dispersive X-ray spectrometer despite they below the detection limit of X-ray diffraction.Compared with the mean values of world hard coals,the Tianjia coals are enriched in Mo,Se,V,Zr,and Cd（5<CC<10）,and slightly enriched in Nb,Cu,Ta,Hf,rare earth elements and yttrium（REY）,Sr,In,Hg,Pb,U,Co,W,Ni,and Sc（2<CC<5）.Based on geochemical indicators such as Al₂O₃/Ti O₂ ratios,REY distribution patterns,the abundance of transition element,and the relationships between Al₂O₃/Ti O₂ and Zr/Ti O₂ and Nb/Yb,the source-area compositions of the Tianjia coals are dominantly Neoproterozoic intermediate-mafic igneous rocks from the Hannan Upland and Emeishan basalts from the Kangdian Upland,followed by Neoproterozoic intermediate-mafic igneous rocks from the Micangshan-Dabashan Uplifts and South Qinling orogen.The relationships between Al₂O₃/Ti O₂ and LREE/Al₂O₃,Zr/Al₂O₃,Nb/Al₂O₃,and Ta/Al₂O₃ indicate that the relatively elevated concentrations of critical metals such as LREY,Zr,Nb,and Ta are affected by terrigenous inputs from the potential source regions.Additionally,multi-stage injection of hydrothermal solutions also influenced the re-deposition of critical elements in the coal.The Sr/Ca ratios of the HCl-leachates（median=11.3 mmole/mole）were similar to the ratio of marine carbonate dissolution,and the indications（the high contents of total sulfur and organic sulfur,pyrite of syngenetic origin,high Sr/Ba values,and low Th/U ratios）that the coal seams were influenced by seawater,while the ⁸⁷Sr/⁸⁶Sr values（0.707111-0.707609）in the HCl-leachates were higher than the expected⁸⁷Sr/⁸⁶Sr range of the contemporaneous seawater（0.70700-0.70725）recorded in the Wuchiapingian marine carbonate rocks.This indicates that the carbonate minerals in the coals are of late diagenetic or epigenetic origin,rather than a syngenetic marine origin.The combination of isotopic evolution trends of the coal seam,the petrological characteristics（high vitrinite reflectance and bireflectance）and mineralogical assemblage of NH₄-illite-paragonite and kaolinite-paragonite as well as authigenic minerals such as calcite,ferroan dolomite,chamosite,quartz,anatase,and florencite suggests that the Tianjia coals was likely affected by diagenetic processes such as sediment dewatering with increasing burial depth（i.e.,upward migration of pore fluids from the mudstones interbedded with the coal seams）.2.The enrichment mechanism of critical metals in Late Triassic coals from different coalmines,northeastern Sichuan BasinThe K1 coal from the Caotang coalmine is considered to be a high ash,medium sulfur,low-volatile bituminous coal,K3 coal belongs to medium ash,medium sulfur,semi-anthracite coal;The K2 coal from the Baoguang coalmine is classified as a low ash,high sulfur,anthracite coal,K3 coal is medium ash,super-low sulfur,semi-anthracite coal;The K5 coals from the Gaosheng and Bishan coalmines are belong to high ash,super-low sulfur,low-volatile bituminous coal.The maceral compositions in Late Triassic coals from the Sichuan Basin are characterized by vitrinite,and to a lesser extent,by inertinite.The minerals in K1 coal benches from the Caotang coalmine are mainly illite,calcite,kaolinite,and quartz,while K3 coals are primarily represented by illite and quartz.The minerals of the K2 coal in the Baoguang coalmine are mainly composed by illite,quartz,pyrite,and chamosite,while the illite,quartz,and chamosite are dominant minerals in the K3 coal.The illite,kaolinite,quartz,and calcite are the primary mineralogical compositions of the K5 coal in the Gaosheng and Bishan coalmines.In comparison to the average values of world hard coals,K1 coals from the Caotang coalmine are significantly enriched in Li,enriched in Zr,and slightly enriched in Nb,Ta,F,Hf,Pb,W,Sr,Th,Sc,Sn,Ga,Cd,and Ba;Caotang K3 coals are slightly enriched in Pb,W,Li,F,and Ba;K2 coals from the Baoguang coalmine are slightly enriched in Be,As,Zr,Ta,W,Pb,and Th;Baoguang K3 coals are slightly enriched in Be,Sc,Ga,Rb,Zr,Nb,Sn,Cs,Hf,Ta,W,Pb,and Th;the K5 coal seam from the Gaosheng and Bishan coalmines is dominantly enriched in some critical elements such as Li,Ga,Zr,Nb,Hf,Ta,REY,Th,and U.According to the Al₂O₃/Ti O₂ ratios and the relationships between Al₂O₃/Ti O₂ and Zr/Ti O₂,Nb/Yb,LREE/Al₂O₃,Li/Al₂O₃,Ga/Al₂O₃,Zr/Al₂O₃,Nb/Al₂O₃,and Ta/Al₂O₃,as well as REY distribution patterns,the elevated concentrations of critical elements（Nb,Ta,Zr,Hf,REY,and Ga）in Late Triassic coals from the northeastern Sichuan Basin are mainly related to the contemporaneous felsic-intermediate pyroclastics from the Late Triassic volcanic areas around the Sichuan basin（i.e.,East Kunlun orogen,West Qinling orogen,and East Tibetan Plateau）,while the background values of these critical elements in the coal may be due to the felsic-intermediate terrigenous inputs,which are dominantly derived from the potential sediment source areas（i.e.,southeastern Qinling and Longmenshan orogens,as well as Hannan and Micangshan-Dabashan Uplifts）.The paragonite-kaolinite aggregates with vermicular texture and authigenic chamosite,anatase,calcite,ankerite,quartz,pyrite,barite,sphalerite,and REE-rich minerals,reflect that the Late Triassic coals from the northeastern Sichuan Basin are likely influenced by the multi-stage hydrothermal fluids.These solutions leached the fresh pyroclastic and granitic materials that caused redistribution of critical elements in the coal benches.The Li-bearing chlorites,including cookeite and a chlorite mineral of intermediate composition between cookeite and chamosite,are the dominant host of Li in the K1 coal from the Caotang coalmine.The genetic mechanism of Li-bearing chlorite mineral is formed by the interaction of kaolinite with Li-rich solutions or directly precipitated from Li-bearing solutions during diagenetic or epigenetic processes.3.Preliminary evaluation of the potential economic value of critical metals in coals from the northeastern Sichuan BasinIn considering the comprehensive exploration and extensive utilization of coal ashes,the Late Permian coals from the Tianjia coalmine could be considered to be potential raw materials of critical metals such as REY,Nb,Ta,Zr,Hf,and Ga.The K1coals from the Caotang coalmine having high contents of Li,Ga,Nb,and Ta,similar to those of conventional economic ores,are potential raw sources for the recovery of these critical elements.The Late Triassic K5 coal seam from the Gaosheng and Bishan coalmines also has potential as raw materials for Nb,Ta,Zr,Hf,and Ga recovery.