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Abstract: The mineralogical and geochemical characteristics of feed coals and their combustion by-products (fly ash and slag) from Hongyanchi (HONG) and Weihuliang (WEI) coal-fired power plants in Xinjiang Province, Northwest China, were investigated. The enrichment and partitioning behaviour, as well as the leaching potential of the major and trace elements in these combustion by-products were evaluated. Three feed coals from the two power plants (HC, WC1, and WC2) were characterised by medium-moisture contents, low- to medium-ash yields, high volatile matters, and low-S contents as well as low-mineral contents. Except B, Sr, and Ba in WC1 feed coal; P, Ti, V, Cr, Mn, Cu, Zn, Rb, and Ba in WC2 feed coal; and Ti, Mn, Cu, Zn, Rb, Sr, and Ba in the HC feed coal, most major and trace element concentrations were low compared with other Chinese coals and coals worldwide. Most trace element concentrations in the studied fly ashes (HA, WAf, and WAc) were even lower than their minimum and/or 25th percentile (P25) values reported for EU fly ashes, as were the element concentrations in the fly ash leachates. The studied fly ashes were all classified as type F fly ashes, with high glass contents as well as high SiO2/Al2O3 ratios in the glass matrix. Through enrichment factors, mass balance, and partitioning of elements, S, F, Cl, Hg, and B were highly volatile in both PCC plants. Tin, As, Se, Cu, and Zn were also moderately volatile, and they were enriched in fly ashes mainly by volatilisation/condensation. Iron and Mn were slightly enriched in slags, probably due to the high density of Fe–Mn-bearing phases. In the light of the above-mentioned properties, fly ashes and slags from the two power plants in Xinjiang will have a very high potential for a variety of uses, with very low environmental limitations. In summary, this study is demonstrating the high quality of Xinjiang coals with enormous coal reserves, and also the high quality of the combustion by-products that will be produced also in very high amounts in the near future. Major focus is on trace elements. [Copyright &y& Elsevier]

Abstract: The mineralogical and geochemical characteristics of the Late Permian Huayingshan coals, partings, roof and floor strata, as well as lutitic rocks from the Longtan Formation, in Sichuan Province, southwestern China, were investigated in this study. The Huayingshan low volatile bituminous coals are characterized by relatively high mineral contents (mainly kaolinite, quartz, pyrite, and calcite). The concentrations of most trace elements are relatively high compared with other Chinese and world coals. The coals and associated strata show similar element affinities: Al, Ti, Li, Ta, Th, Ga, U, Sn, Sc, Cr, Cu, Rb, Co, and Se probably have an aluminosilicate affinity; Zr, Nb, Hf, Y, REEs, and U probably result from the occurrence of heavy minerals such as zircon; S, Fe, W, and Rb mainly have a sulfide affinity; Ca, Sr, and Mn mainly have an carbonate affinity. The high enrichment in the Huayingshan coal of Zr, Nb, Hf, REEs, Ta, Ga, and Th is caused by a geochemical anomaly in the source of this coal''s detrital material (and probably also by input of volcanic ash), which is influenced by the alkaline volcanic magmas of the Emeishan basalt. The Zr/Hf, Zr/Nb, Zr/La and Zr/Th ratios support the volcanic origin of such a geochemical anomaly. [Copyright &y& Elsevier]

Abstract: The Zhundong coalfield is a very large coal deposit, currently under exploration, and promises to be an important coal mining resource in Western China. The coal reserves amount to 164Gt. At Xiheishan and Wucaiwan, the coal-bearing units are the Badaowan (Lower Jurassic) and, especially, the Xishanyao formations (Middle Jurassic). These contain from one to 31 coal seams (one to 10 workable coals reaching the rank of high volatile A bituminous) with an accumulated workable coal thickness of 20–87m. These recently explored coal reserves are of a high quality and are characterized by low ash, aluminum, iron, sulfur, and trace element contents. The minerals present in this coal are mainly quartz, kaolinite, siderite, and pyrite. The trace element contents in both Xiheishan and Wucaiwan coals are much lower when compared with the usual concentration ranges present in Chinese coals, with the exception of Ba and Sr. Thus, Ba contents in Xiheishan coal are higher than Swaine''s worldwide concentration range. A large number of elements have mainly aluminosilicate affinity, and occur in clay and other detrital coal minerals. Ca and Mn have mainly carbonate affinity; Fe and S have mainly sulphide affinity in Wuchaiwan coal. In Xiheishan, Fe, Mn and Mg have mainly carbonate affinity (siderite), and probably, B, Co, Ni and S have an organic affinity. Thus, the Zhundong coalfield contains coal with a very high quality and very low levels of impurities. The very low ash yields, S, Fe, and trace element contents may be attributed to the sedimentological setting, with intensive peat bog aggradation in a very shallow lake environment with a low detrital supply. The very low sulfur content, the relatively high Ba-sulfate (barite) content and the high siderite occurrence in Xiheishan coal (in contrast to the pyrite occurrence in Wucaiwan coal) may be attributed to the rapid aggradation of peat with the consequent oxidation and leaching of minerals, elements, and degradation of organic components. Under this scenario, sulfate is trapped by the continuous precipitation and accumulation of barium sulfate (very low soluble). [Copyright &y& Elsevier]

Abstract: The geological and geochemical controls of the Ge distribution in the Cretaceous Wulantuga high-Germanium coal deposit in the Shengli coal field, Inner Mongolia are investigated. This paper focuses mainly on the spatial distribution of the Ge contents in coal. The high-Ge coals mainly occur in three splits of the #6 coal in the southwestern part of the Shengli coal field. Mean germanium contents in the coal range from 32 to 820 μg/g, with a mean value of 137 μg/g, on a bulk coal basis (mean of 939 coal samples from 75 boreholes in the #6 coal seam) in an area of 2.2 km2. The highest Ge content occurs SW of #6 coal seam, close to the margins of the coal basin, decreasing with a fan-shaped trend towards NW, the direction of the coal basin. There is an negative correlation between the mean Ge content and the thickness of the coal seam. Different distribution patterns of Ge content were found in vertical profiles. High Ge concentrations may occur in the middle parts of coal seams, at the bottom and/or the top of thick coal seams and inconspicuous variation. A major organic affinity was determined for Ge, with a special enriched in the banded bright and semibright coal. The high-Ge coals and the coalified wood in the sandstone overlaying the #6-1 coal highly enriched in Ge, As, Sb, W, Cs, Tl, Be, and Hg. The Late Jurassic silicified volcanic rocks in the NW of the Ge coal deposit relatively high enriched in Ge, Ga, Sb, As, Cs, Be, Ge and Hg. The correlation coefficients among the elements enriched showed marked variations at close sites in this deposit, suggesting a possible diagenetic origin of the geochemical anomaly. The main Ge anomaly was attributed to early Cretaceous hydrothermal fluids circulating through the fault systems and porous volcanic rocs, probably from the subjacent granitoid rocks. The fault systems, the porous coarse clastic rocks overlying coal seam and the lithotype of coal played an important role in the transport and trapping of Ge. A main diagenetic origin (overlapping a slight synsedimentary accumulation) is deduced for the Ge anomaly. [Copyright &y& Elsevier]

Abstract: The occurrence and distribution of minerals, and major and trace elements of the coal #6-1 from the Cretaceous Wulantuga high-germanium coal deposit in Shengli coal field, Inner Mongolia are investigated. The major mineral constituents of coal #6-1 are quartz (15%), kaolinite (4–5%), illite (1%), pyrite (<1%), feldspars (<1%), gypsum (weathering product, <2%) and traces of chlorite. In addition to these phases, traces of scheelite (CaWO4) and weddellite (an oxalate, CaC2O4·2H2O) were also detected by XRD. On the basis of chemical and mineralogical properties, a clear differentiation was found between the upper and lower coal sections: the lower section with higher sulfide mineral content, and the upper one with low sulfur content and with a higher proportion of mineral phases formed/accumulated under oxidizing conditions (quartz and dolomite). This coal seam is highly enriched in Ge, As, W, and Hg (one to two orders of magnitude higher than the usual worldwide coal concentrations), with high contents of Sb, U, Cs, and Be (one order of magnitude higher than the usual worldwide coal concentrations). The geochemical and mineralogical profile patterns of the coal seam were attributed to the development of a basal reduced marsh environment evolving towards a more oxidizing marsh environment in the upper part of the coal seam. This could be related to the evolution from a high water table low moor marsh environment to a high moor marsh into an open water body with a higher detrital influence at the top of the seam. These syngenetic geochemical characteristics are possibly modified by an unusually intensive hydrothermal alteration causing the formation of oxalate minerals and the enrichment in a series of elements such as Cs, Be, U, As, Se, and Hg. [Copyright &y& Elsevier]

Abstract: A detailed study of the coalification and paleogeothermal evolution of the Stephanian Cevennes basin in southeastern France is presented in this paper. This study includes several parameters: 1) the geometry of the basement, the base of the coal measures and the unconformity beneath the overlying Mesozoic series, 2) the lateral and vertical distribution of the content of volatile matter in the coal beds, 3) the relationship between these parameters and the geological boundaries. This study also addresses qualitative and semi-quantitative parameters of the degree of coalification of organic matter in the same basin. The parameters investigated are: 1) the relief of the basement, 2) the nature and thickness of the overlying Mesozoic series, and 3) the faulting in the basin. In the Cevennes basin two coalification episodes were recognized (early pre-folding and late post-folding), and their ages and intensities have been estimated. The results suggest that coalification was strongly influenced by the faulting and the thickness of the strata overlying the coal seams. The early coalification episode during the Pennsylvanian basin-filling stage was closely related to the syndepositional faulting, and the later coalification episode was affected by subsequent rapid subsidence during the Mesozoic. The ultimate maturity level of the oldest Stephanian coal was achieved by the polyphase and multiformity coalification. [Copyright &y& Elsevier]

Abstract: The concentration of trace elements and their distribution in the late Permian coal in the Heshan coal field, Guangxi Autonomous Region, were analysed in this paper. The late Permian coal of the Heshan mining district was developed in a low energy and shallow, confined carbonate platform. Heshan coal is a low volatile bituminous coal characterized by a high sulphur content, ranging between 2.0% and 8.2%. Compared with the worldwide average content of the trace elements in coal, the content of some trace element in the study coal is markedly high (Bi, Ce, Cr, Cs, Cu, Ga, Hf, Sr, Ta, Th, U, V, W, Y, Zr, La, Mo, Nb and Sc). The trace element associations were investigated by means of intracorrelation analysis. Some elements, such as Cl, F and Sr are found in coal in association with the carbonate minerals. V, Cr, Zn, Mo, Ni and As contents in coal vary significantly amongst the coal samples. They are mainly concentrated in the lower part of the coal #4 upper of Suhe and Lilan mines and the coal #4 lower of Dong mine, and these possibly occurring in minerals such as arsenide and sulphide. The content of U in Heshan coal is high and is mainly concentrated at the upper and the lower parts of the coal seam and it is associated with mineral assemblages with Ba, Mo, V, Ni, Zn, Rb and Cr. Furthermore, La and Ce are highly correlated with those found in phosphate minerals and Pb, Sc, Ga, Th, Y and Sn to those in aluminosilicate minerals. The enrichment of some elements such as V, Cr, Zn, Mo, Ni, Rb as well as total sulphur and iron in the lower part of most coal seams might be associated with the formation of soil horizon before the accumulation of peat in the basin. Some other elements such as Cl, F, Sr and Ca are locally concentrated in the top of specific coal seams as a result of the leaching from overlying carbonates. [Copyright &y& Elsevier]

The occurrence and distribution of major and trace elements have been investigated in two coal-bearing units in the Chonqing mining district (South China): the Late Permian and Late Triassic coals.The Late Permian coals have higher S contents than the Late Triassic coals due to the fixation of pyrite in marine-influenced coal-forming environments. The occurrence of pyrite accounts for the association of a large number of elements (Fe, S, As, Cd, Co, Cu, Mn, Mo, Ni, Pb, Sb, Se, and Zn) with sulphides, as deduced from the analysis of the density fractions. The marine influence is probably also responsible for the organic association of B. The REEs, Zr, Nb, and Hf, are enriched by a factor of 2–3 with respect to the highest levels fixed for the usual worldwide concentration ranges in coal for these elements. The content of these elements in the Late Permian coal is higher by a factor of 5–10 with respect to the Late Triassic coal. Furthermore, other elements, such as Cu, P, Th, U, V, and Y, are relatively enriched with respect to the common range values, with maximum values higher than the usual range or close to the maximum levels in coal. The content of these elements in the Late Permian coal is higher than the Late Triassic coal. These geochemical enrichments are the consequence of the occurrence, in relatively high levels, of phosphate minerals, such as apatite, xenotime, and monazite, as deduced from the study of the density fractions obtained from the bulk coal.The Late Triassic coal has a low sulphur content with a major organic affinity. The trace element contents are low when compared with worldwide ranges for coal. In this coal, the trace element distribution is governed by clay minerals, carbonate minerals, and to a lesser extent, by organic matter and sulphide minerals.Major differences found between late Permian and Triassic coals are probably related to the source rocks, given that the main source rock of the late Permian epicontinental marine basin is the Emeishan basalt formation, characterised by a high phosphate content. [Copyright &y& Elsevier]

The abnormal enrichment of the REY (rare earth elements and yttrium)-Zr (Hf)-Nb (Ta) assemblage in Late Permian coals occurring in some coalfields in SW China has been extensively investigated, but the geological controlling factors of this assemblage are still under debate. This study reports on the mineralogical and geochemical features of coals and noncoal rocks collected from the Yudai and Jinqi coal mines within the Qiandongbei Coalfield in Guizhou Province to elucidate the geological controls on the distribution of REY-Zr (Hf)-Nb (Ta) enrichment horizons. The studied coals show rather consistent clay mineral assemblage of kaolinite, together with varying amounts of quartz, gypsum, pyrite, and calcite. Notably, the coals are anomalously enriched in REY, Nb, Ta, Zr, and Hf and enriched in other incompatible elements (e.g., Ga, Th, and U). Phosphates (e.g., florencite) and, to a lesser extent, zircon are the primary carriers of REY. Zirconium and Hf are mainly incorporated into zircon and then anatase, the latter of which is a predominant carrier of Nb and Ta. The detrital materials originate from a mixture of the erupted felsic volcanic ash resembling a peralkaline rhyolite composition and the eroded materials of Emeishan Large Igneous Province (ELIP) basalts. The studied coals owe their highly enhanced concentrations of the REY-Zr (Hf)-Nb (Ta) assemblage to the input of felsic volcanic ash, wherein the highly acidic aqueous solutions allowed these elements to become mobile and be redistributed. Both geological processes governed the distribution of REY-Zr (Hf)-Nb (Ta) enrichment horizons. The highly acidic solutions were possibly derived from the oxidation of peat bogs and volcanism-related acidic rain. The concentrations of REY, Zr, and Nb in the coal ashes exceed their corresponding cut-off grades; thus, these coals can be considered highly promising raw material sources for the REY-Zr (Hf)-Nb (Ta) assemblage. • The studied coals are significantly enriched in REY-Zr (Hf)-Nb (Ta) assemblage. • Detrital materials are derived from a mixture of felsic volcanic ash and eroded mafic materials. • The joint influence of highly acidic solutions and volcanism governs the REY-Zr (Hf)-Nb (Ta) enrichment horizons. • The volcanic ash erupted into peat bogs is geochemically similar to the peralkaline rhyolite. [ABSTRACT FROM AUTHOR]

The movable-water saturation (MWS) of coal is critical for the flow of coalbed methane (CBM), however, there is a relative lack of research on it, especially of different-scale pores. The relative permeability of coal is another crucial parameter for CBM development due to its strong control on the characteristics of both gas and water production, but it also remains relatively unexplored. Naturally, little attention has been paid to the connections between the MWS and the relative permeability. In this study, an innovative experiment combining water-saturated coal core displacement test with low-field nuclear magnetic resonance (LNMR) was designed to obtain the MWSs of different-scale pores and gas-water relative permeabilities of coals simultaneously. The relative permeability results show that low-medium coals from the Southern Junggar Basin (SJB) were characterized by low MWSs, narrow spans of two-phase flow, and low end-point relative permeabilities to water (K rw *) and gas (K rg *), which were all disadvantageous for CBM development. The calculated MWSs of different-scale pores through the amplitude of T 2 spectrum show that "macropores and fractures (MF)" have the highest MWS, with mesopores following next, "micropores and transition pores (MT)" possess the lowest MWS. Correlation analyses show that the total MWSs and movable porosities both present strong positive correlations with the K a , K rw * , and K rg *. The irreducible water causes a greater obstacle to gas flow than to water flow. The MWSs of different-scale pores all exhibit positive impacts on the coal permeabilities. However, compared with MWSs of MT and MF, the MWSs of mesopores exhibit the strongest correlation with K a , K rw * , and K rg * , indicating the critical role in affecting the fluid flow of coal. Besides, two significant parameters, cleat tortuosity (η) and cleat size distribution index (λ) of SJB coals, were determined. η and λ were rank-dependent. Coal macerals also have impacts on η and λ. • Gas-water relative permeabilities of low-medium rank coals from Southern Junggar Basin were first reported. • The movable-water saturations of different-scale pores and gas-water relative permeabilities were obtained simultaneously. • The impacts of movable-water saturations of different-scale pores on the gas-water relative permeabilities were studied. • The movable spaces in mesopores play a critical role in affecting the fluid flow in coal. [ABSTRACT FROM AUTHOR]

Previous investigations on major and trace elements within the Early Permian coals from Xinjiang Province are very scarce. This paper reports on the geochemistry of the Early Permian coals and associated non-coal rocks in the Jimunai depression, Xinjiang Province, northwestern China. Samples collected from coal exploration boreholes ZK2603, ZK3003, and ZKH1102 as well as the Haerjiao underground coal mine (Nos. HRJ-7 and HRJ-9 Coals) in the Jimunai Depression were analyzed using SEM-EDS, ICP-AES, and ICP-MS. These Early Permian coals reach the rank of semi-anthracite to anthracite and have low-sulfur contents (0.30% on average). In comparison with the average element concentrations of world coals, Zr, Hf, and rare earth elements and Y (REY) are slightly enriched (concentration coefficient (CC) = 2–5, as the ratio of element content in sample vs. that in world coals) and Mn is enriched (CC = 5–10) in the ZK2603, ZK3003, and ZKH1102 coals. Niobium and Ta are highly enriched in the ZK2603 and ZK3003 coals (CC = 10–100). The HRJ-7 and HRJ-9 coals are slightly enriched in Mn (CC = 2–5). The coals have a sediment source region mainly composed of intermediate-felsic compositions from the Saur Volcanic Arc to the south of the Jimunai Depression, which may have provided relatively high geochemical background concentrations of Zr, Hf, Nb, and Ta. Manganese is hosted mainly in ankerite that was derived from epigenetic precipitation from Mn-bearing hydrothermal fluids. Zirconium and Hf in the ZK2603, ZK3003, and ZKH1102 coals are controlled primarily by detrital matter or hydrothermal fluids, the latter of which results in the higher Zr/Hf ratio in the ZK2603 and ZK3003 coals due to the re-distribution of Zr and Hf. Niobium and Ta in the ZK2603 and ZK3003 coals are concentrated chiefly in zircon, and hydrothermal fluid activity caused the release of Nb and Ta from previously-existing inorganic constituents and subsequent incorporation into zircon, resulting in the lower Nb/Ta ratio due to Ta being more easily incorporated into zircon than Nb. REY are hosted primarily within multiple mineral phases (e.g., zircon, phosphates, anatase, clay minerals) and their enrichment results from a combination of detrital matter input and hydrothermal fluid influx, the former of which is evidenced by the occurrence of REY-bearing detrital zircon and anatase, and the latter of which is indicated by the existence of REY-bearing authigenic rhabdophane and anatase and corroded anatase particles, as well as by the positive Gd anomalies and MREY-type or HREY-type REY distribution patterns. • The elevated Zr, Hf, Nb, and Ta are hosted mainly in zircon. • Hydrothermal fluid causes redistribution of Zr, Hf, Nb, Ta, and REY. • The enhanced Mn concentration was derived from epigenetic hydrothermal solution. • Terrigenous material and hydrothermal activity contribute to geochemical anomaly. [ABSTRACT FROM AUTHOR]

This study is the first to provide geochemical and paleoenvironmental information of Jurassic Xishanyao Formation coal in the eastern Junggar Basin, China. Twenty-seven coal core samples were collected from the zk0413 boreholes in Dajing area to determine the geochemical variability. Proximate analyses show that the Xishanyao Formation coal has the medium moisture content (average 13%), special low ash yield (average 6%) and high volatile matter (average 34%), which slightly fluctuate from bottom to top of coal seam. Organic matter in the coal are type III kerogen, with coalification degrees determined as subbituminous A or high volatile bituminous C. All coal samples have poor hydrocarbon generating potential with the yield between 31.93 and 72.36 (average of 43.57) mg HC/g.rock. The primary input of organic matter for coal-formation was higher terrestrial plants, with greater input from algae and micro-organisms and greater role of bacteria in plant degradation higher in the seam. The process of peatification in the lower section of the seam occurred under more acidic and oxic conditions. Coal accumulated in a lacustrine freshwater environment with the rise of the (ground) water table from bottom to top. There is evidence of higher salinity of water within the lower seam and decreased salinity (brackish to fresh water) within the middle of the seam, then an increase to moderate salinity (fresh water to brackish). The increasing stratified water column upward with reduced oxidation in the coal seams may result from the increased temperature in periods of a relatively hot and dry climate. The environment changes demonstrated in this study illustrate the importance of studying combined elemental and organic geochemistry to reconstruct paleoenvironment. [ABSTRACT FROM AUTHOR]

[Display omitted] • The Upper Triassic coal from the Nui Hong open-pit mine is high-organic-sulfur coal. • The Nui Hong coal is promising economic source for strategic metal recovery. • Highly elevated Cs in coal most likely occurs in K-rich tobelite and illite. • Elevated Mo, Sb, U, Ge, and Tl are mainly associated with organic matter in coal. • Enrichment of strategic metals in Nui Hong coal is due to hydrothermal activities. Research on coal-hosted strategic metal ore deposits is an important subject in coal geology and mineral deposit worldwide. However, the related geological characteristics of coals from Vietnam have seldom been investigated, let alone the strategic metals in coals. This paper investigates the mineralogical and geochemical characteristics of the Upper Triassic coal from the Nui Hong open-pit mine, Thai Nguyen Coalfield, NE Vietnam, with emphasis on the enrichment of strategic metals in coal. The Nui Hong coal is low-volatile bituminous with high sulfur organic content. Minerals in the Nui Hong coal is predominantly composed of authigenic quartz, tobelite, pyrite, and calcite, with a small proportion of sphalerite, gypsum and fluorapatite in some coal bench samples. Mo-U-V-Se and Ge-W-Sb-Cs-Tl(-U) strategic metal element assemblages along with As-P toxic element association is significantly enriched in the Nui Hong coal, pointing to a significant economic benefit for Cs, Mo, U, and Ge recovery from the Nui Hong coal. The Ge enrichment in the Nui Hong coal provides a new perspective on prospecting of coal-hosted Ge deposit in high-rank coal. Cesium mostly occurs in K-rich aluminosilicate minerals (tobelite and/or illite), while Mo, Sb, U, Ge, and Tl are predominantly associated with organic matter in the Nui Hong coal. Multistage hydrothermal activities occurring in the marine-influenced S-rich, Fe-poor and clastic-starved anoxic-euxinic coal-forming environment are responsible for the significant enrichment of strategic metal elements in the upper Triassic coals from the Nui Hong open-pit mine, Thai Nguyen Province. [ABSTRACT FROM AUTHOR]

This paper represents the first integrated research on the mineralogical, geochemical and stable isotope characteristics of the Upper Triassic high‑sulfur coals from the Khanh Hoa open-pit mine, Thai Nguyen Coalfield, NE Vietnam. Formed in a marine-influenced carbonate succession, the Khanh Hoa coal is characterized by medium ash yield, low volatile matter yield, and high sulfur content. Minerals in the Khanh Hoa coal are predominantly composed of authigenic calcite in the upper portion; and pyrite, quartz, with minor amounts of tobelite ([(NH 4), K] (Si, Al) 4 Al 2 O 10 (OH) 2), gypsum in the middle and lower portions. The high‑sulfur Khanh Hoa coal is enriched in an As-Mo-Sb-U-Cs-P assemblage, which presents both an organic and inorganic mode of occurrence, and is largely ascribed to hydrothermal fluids. The δ 13C values of the Khanh Hoa coal range from −24.77‰ to −22.80‰ (avg. −23.33‰), reflecting a change in sedimentation pattern (from transgressive to high stand system upwards). The δ 15N values of the Khanh Hoa coal range from 2.41‰ to 3.73‰ (avg. 3.30‰), possibly resulted from influenced by seawater influence and a redox environment. The mineralogical, geochemical and isotopic characteristics reflected that the high‑sulfur Khanh Hoa coal was formed/preserved in marine-influenced anoxic environment, with low detrital input, and strong activities of multi-stage hydrothermal solutions. [ABSTRACT FROM AUTHOR]