Enrichment Characteristics And Formation Mechanism Of Lithium,Gallium,and Rare Earth Elements Of Late Permian Coal-bearing Strata Within The Xian’an Coalfield,Guangxi Province

The study of critical metals in coal-bearing sequence has currently become a frontier research hotspot in geology and a new field,a new approach and an important direction for mineral resources exploration,which is due to the fact that the deposits of critical metals in coal-bearing sequence are comparable to or larger than traditional metal deposits in terms of abundance and scale,and can be an important source for increasing reserves of critical metal minerals.The Upper Permian coal-bearing sequence is anomalously enriched in Li,Ga,and REY,but the modes of occurrence,controlled factors and mineralization mechanism of critical metals is relative low investigated,which to a certain extent limits the research and development of critical metals in the coal-bearing sequence in this area.Based on the detailed description of the outcrop and borehole samples of the coal-bearing sequence,as well as systematic sample collection and analysis,this paper aims to identify the lithological composition,mineralogical and geochemical characteristics of the mineralized and non-mineralized layers,reveal the distribution and modes of occurrence of Li,Ga and REY in the mineralized layers,discuss the sources of critical metals,clarify the enrichment process and driving mechanism,and establish the enrichment pattern of Li,Ga and REY.The main conclusions are as follows:There are four Li-Ga-rare-earth element-enriched mineralization layers in the coal-bearing rocks of Heshan Formation in the study area,namely,mineralization layersⅠ,Ⅱ,Ⅲ and Ⅳ,which correspond to the K5,K4,K3 and K2 coal layers of Heshan Formation,respectively.The mineralized layer Ⅰ is usually composed of ferro-aluminite（or missing）-clay rock/bauxite-coal/carbonaceous mudstone from bottom to bottom,and the other mineralized layers are usually composed of clay rock/mudstone-coal/carbonaceous mudstone.The top and bottom of the mineralized layer are argillaceous limestone or limestone,and the coal in the mineralized layer is inferior coal.The high sulfur content in the mineralized layer（mean>2%）indicates the influence of seawater.The mineral assemblage of mineralized layer in the coal-bearing strata can be classified as kaolinite,illite-tobelite,kaolinite-pyrite,illite-calcite-pyrite,illite-cookeite-pyrite,illite-tobelite-kaolinite,kaolinite-quartz,quartz-kaolinite-calcite,illite-tobelite-Kaolinite-pyrophyllite,quartz-kaolinite-pyrite,illite-tobelite-albite and illite-tobelite-quartz-calcite-albite assemblages.In the mineral assemblages,kaolinite and feldspar are mainly of clastic origin,illite/ammonium illite and chlorite are mainly from the transformation of kaolinite,quartz and pyrite are mainly authigenic origin,and calcite is mainly formed by hydrothermal fluid precipitation in late stage.The major elements of mineralized layers Ⅰ,Ⅱ,Ⅲ and Ⅳ are mainly composed of SiO₂ and Al₂O₃,and the UCC-normalized REY distribution pattern shows the MREY and HREY distribution.The average content of lithium in mineralization layers Ⅰ,Ⅱ,Ⅲ and Ⅳ is 1177μg/g,501μg/g,535μg/g and 127μg/g,respectively.The mineralization layer Ⅳ is slightly enriched,while the other mineralization layers are highly enriched.Gallium content is 32μg/g,41μg/g,32μg/g and 22μg/g,respectively;mineralization layer Ⅱ is slightly enriched,and other mineralization layers reach normal level.The contents of REY are 598μg/g,552μg/g,310μg/g and 364μg/g,respectively;mineralization layer Ⅰ,Ⅱ and Ⅳ are slightly enriched,and mineralization layer Ⅲ is at normal level.In lithium-rich samples（Li>500μg/g）,chlorite（possibly cookeite）is the main carrier mineral of lithium,and lithium mainly occurs in mineral lattice rather than as adsorbed ion.In other samples（Li<500μg/g）,lithium mainly occurs in kaolinite or illite/tobelite.Rare earth elements occur mainly in phosphate minerals and clay minerals（e.g.,kaolinite,illite/tobelite,chlorite）.Gallium mainly occurs in diaspore,kaolinite,and illite/tobelite.The clastic materials of the Upper Permian coal-bearing rocks in the study area are mainly derived from the Daxin Upland which provide terrigenous detrital materials as well as the Emeishan mantle plume and the Yuebei magmatic arc which provide volcanic clastic materials.The terrigenous clastic materials from the Daxin Upland are weathered remnants,whose parent materials are partly from the Maokou Formation limestone,and partly from the volcanic ash of Emeishan and Yuebei magmatic arc erupting between the middle and late Permian.These materials are subjected to long-term weathering and leaching and formed bauxitic and clay residue on the top of the Maokou Formation,which provided important terrigenous clastic material for the formation of Late Permian coal-bearing rock strata.Pyroclastic materials include alkaline-hyperalkaline moderate-acid volcanic ash from the Emeishan mantle plume and subalkaline moderate-acid volcanic ash from the northern magmatic arc,which were transported to coal-bearing basins through the atmosphere and provided important pyroclastic materials for the formation of coal-bearing rock strata.The enriched mineralization of lithium,gallium and rare earth elements in the mineralized layers is the result of the input of terrigenous and pyroclastic sources,the acidic fluid/rock interaction in the peating-diagenetic stage and,in a few cases,the input of critical metal-rich fluids.The enrichment and mineralization process of lithium,gallium and rare earth elements in coal-bearing strata can be divided into three stages:（1）the formation stage of critical metal-rich residue.The Dongwu Movement between the middle and late Permian led to the uplift of the Daxin Upland in the west of the study area,which was also accompanied by volcanic activities induced by Emeishan mantle plume and Yuebei magmatic arc.The Maokou Formation limestone and pyroclastic materials are subjected to long-term weathering and leaching and results in removal of some elements（alkali and alkaline elements）and retention of Li,Ga,and REY in weathered residue,which forms bauxitic and clay residue that are enriched in Li,Ga,and REY;（2）formation stage of coal-bearing rock series.At this stage,the remnants from the Daxin Upland and the volcanic activities of the Emeishan and Yuebei magmatic arcs provided important terrigenous and pyroclastic materials for the formation of coal-bearing rock series;（3）transformation stage of mineralized layer.In the paludification and diagenesis stages,a large amount of organic acids are released from coal/carbonaceous mudstone.In the process of acidic fluid/clastic material interaction,lithium,gallium and rare earth elements are easily released from clastic material,accompanied by fluid migration and reprecipitation,resulting in heterogeneous distribution of key metals in the mineralized layer.In some cases,hydrothermal fluid inputs can also lead to heterogeneous distributions of lithium,gallium and rare earth elements.The Li₂O content in mineralized layer Ⅰ,mineralized layer Ⅱ and mineralized layer Ⅲ exceeds the boundary grade,and the REO content,Ga content and （Nb,Ta）₂O₅ content are close to or exceed the boundary grade,therefore,the comprehensive exploration and production of Li-REY-Ga-Nb can be considered;there is no significant exploration and production potential in mineralized layer Ⅳ.