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Silicic, mafic and alkali intra-seam tonsteins have been known from SW China for a number of years. This paper reports on the geochemical compositions of coals and tonsteins from three seam sections of the Songzao Coalfield, SW China, and evaluates the geological factors responsible for the chemical characteristics of the coal seams, with emphasis on the influence from different types of volcanic ashes. The roof and floor samples of the Songzao coal seams mostly have high TiO2 contents, consistent with a high TiO2 content in the detrital sediment input from the source region, namely mafic basalts from the Kangdian Upland on the western margin of the coal basin. The coals from the Songzao Coalfield generally have high ash yields and are highly enriched in trace elements including Nb, Ta, Zr, Hf, rare earth elements (REE), Y, Hg and Se; some variation occurs among different seam sections due to input of geochemically different volcanic ash materials. The geochemistry of the Songzao coals has also been affected by the adjacent tonstein/K-bentonite bands. The relatively immobile elements that are enriched in the altered volcanic ashes also tend to be enriched in the adjacent coal plies, possibly due to leaching by groundwaters. The coals near the alkali tonstein bands in the Tonghua and Yuyang sections of the Songzao Coalfield are mostly high in Nb, Ta, Zr, Hf, Th, U, REE and Y. Coal samples overlying the mafic K-bentonite in the Tonghua section are high in V, Cr, Zn and Cu. The Datong coal, which has neither visible tonstein layers nor obvious volcanogenic minerals, has high TiO2, V, Cr, Ni, Cu and Zn concentrations in the intervals between the coal plies affected by mafic and alkaline volcanic ashes. This is consistent with the suggestion that a common source material was supplied to the coal basin, derived from the erosion of mafic basaltic rocks of the Kangdian Upland. Although the Songzao coal is generally a high-sulfur coal, most of the chalcophile trace elements show either poor or negative correlations with total iron sulfide contents. The absence of traditional pyrite-metal associations may reflect wide variations in the concentrations of these elements in individual pyrite/marcasite components, or simply poor retention of these elements in the pyrite/marcasite of the relevant coals. [ABSTRACT FROM AUTHOR]

Some previous studies have investigated the enrichment origin of Sb and Cs in coal, but strong supportive evidence for the sources of the two elements in coal is still absent. The main aims of this study were to provide a better understanding of the geochemical and mineralogical characteristics of the Palaeogene No. 5 Coal from the Zhoujing coal mine, Baise Coalfield, Guangxi Province, southwestern China. The No. 5 Coal has a low rank (0.44% R o,max ; 46.96% volatile matter content), an average ash yield 19.37%, and a medium sulfur content (1.23% on average). The main minerals in the No. 5 Coal are kaolinite, illite, quartz, and pyrite. Bassanite (2CaSO 4 •(H 2 O)) was identified in the low-temperature ashes in varying proportions (2.7% - 37.6%). The mineralogy of the non-coal samples (partings, roof and floor strata) is similar to that of the coals but chlorite is additionally present and pyrite is largely absent in these non-coal samples. The modes of calcite occurrence in the partings indicates a terrigenous origin for this phase. Compared with average values for world low-rank coals, the No. 5 Coal is distinctively enriched in Sb (29.12 μg/g), Cs (9.71 μg/g), and U (15.61 μg/g). Compared with the average values for world clays, the parting and host rock samples have normal concentrations for most trace elements, with an exception of higher Sb (9.93 μg/g). The elevated Sb and Cs in the coal were derived from the detrital material shedding from the exposed middle Triassic strata, which host notable Au and Sb deposits in the surrounding region. The REY (rare earth elements and Y) distribution patterns for the coal benches, partings, roof and floor strata are generally characterized by enrichment in medium REY and positive Gd anomalies, both of which were due to acidic waters circulating within the coal basin. The mineral compositions and their association indicate that the depositional environment was weakly acidic at the early stage of peat accumulation and then changed to neutral or weakly alkaline in the later stage. • The coal investigated is distinctively enriched in Sb (29.12 μg/g) and Cs (9.71 μg/g). • Sb and Cs in the coal were derived from the terrigenous middle Triassic strata hosting Au and Sb deposits. • The medium REY enrichment and positive Gd anomalies were due to the acidic waters circulating within the coal basin. • The depositional environment was acidic at the early stage of peat accumulation. • The depositional environment was changed to neutral or weakly alkaline in the later stage of peat accumulation. [ABSTRACT FROM AUTHOR]

Abstract Coal ash is considered as a promising alternative resource for rare earth elements and yttrium (REE + Y or REY) recovery. Coal ash samples (fly ash and bottom ash), as well as corresponding feed coals, collected from the Luzhou coal-fired power plant in Sichuan, southwestern China were analyzed for their chemical and mineralogical characteristics. The fly ash was further tested for its feasibility of REE extraction. The feed coals are characterized as a low-volatile bituminous, medium‑sulfur, and high-ash coal, sourced from the Guxu coalfield (Late Permian Longtan Formation) in Sichuan, SW China. REE and Y, along with high-field-strength elements Zr(Hf) and Nb(Ta), and Li, F, Sc, V, Cr, Co, Cu, Zn, Ga, Ge, Se, Sr, Cd, In, Sn, Hg, Th, U, are all enriched in the feed coal. The mineralogical compositions of the feed coals are dominated by kaolinite and illite/smectite (I/S) mixed layers, followed by the carbonate minerals including calcite, siderite, ankerite, and minor amounts of anatase and jarosite. The combustion of the feed coals produces a Class F fly ash, characterized by an aluminosilicate composition with a low CaO content. The bottom ash is compositionally similar to the fly ash, except that it is more enriched in high-density elements (Fe and Mn), and more depleted in volatile elements. REY are enriched in both the fly ash and bottom ash, although heavy REY (HREY) are notably more enriched in the fly ash. Mineralogical analysis shows that the fly ash consists of >70% amorphous glass and <30% mineral phases such as mullite, quartz and iron oxides. The bottom ash is composed of ~60% amorphous glass but with a more complex mineralogical composition than the fly ash. Dissolution of the fly ash and bottom ash with 4% hydrofluoric acid (HF) showed that ~90% of all the REY are associated with the amorphous glass in the fly ash, whereas <50% of REY in the bottom ash is contained in the glassy component. An alkaline-acid-combined (NaOH-HCl) sequential leaching process was employed to test the extractability of REY in the Luzhou fly ash. The experimental variables, including extractant concentration, liquid-to-solid ratio, leaching temperature and leaching time, were optimized by an orthogonal array design. The optimal NaOH leaching conditions results in 41.10% of active silica removal from the fly ash and 39.43% of REY enrichment. The HCl leaching of desilicated fly ash achieves 88.15% of REY extraction efficiency under the optimal conditions, which is a dramatic increase as compared to the same leaching of raw fly ash. Even though the extraction efficiency may vary between different coal ashes from various sources, a complete REY extraction procedure from fly ash is suggested for future use. Graphical abstract Unlabelled Image Highlights • REY are enriched in both the fly ash and bottom ash presented in this study. • 90% REY in the fly ash are associated with the amorphous glass. • A NaOH-HCl sequential leaching process was used to test the extractability of REY in the fly ash. [ABSTRACT FROM AUTHOR]

This paper discusses the identification, abundance, modes of occurrence and origin of buddingtonite and associated minerals in three Lopingian coals from the Yishan Coalfield, Guangxi Province, southern China. The coals of this coalfield are preserved within carbonate successions and are characterized by super-high-organic sulphur contents (mostly 6–9%). Three different types of feldspars have been identified in these coals, buddingtonite, albite and minor K-feldspar. Both the albite and K-feldspar in the coal were largely derived from detrital materials of terrigenous origin, although some were authigenically deposited; they were subsequently subjected to corrosion/dissolution by percolating fluids. Buddingtonite was derived largely from K-feldspar via diffusion mechanisms through the substitution of K + by NH 4 + , the latter coming from decomposition of organic matter in the coal, or from changes in the organic matter with rank advance. A small proportion of the buddingtonite was also derived from albite through the replacement of Na + by NH 4 + . [ABSTRACT FROM AUTHOR]

The coal from the Jincheng Coalfield, Qinshui Basin, is a medium- to low-ash yield anthracite. The mineral assemblage in the anthracite is dominated by NH 4 -illite and kaolinite; with varying proportions of pyrophyllite, chlorite, pyrite, and calcite; and minor anatase, rutile, diaspore, florencite, and synchysite. Kaolinite formed during early diagenetic and epigenetic processes has been largely altered to other phyllosilicates by processes associated with the different stages of coal rank advance. The formation of NH 4 -illite in the Jincheng coals is the result of illitization of an early diagenetic kaolinite precursor by interaction of kaolinite with nitrogen derived from organic matter. This reaction may have occurred at the latest stage of bituminization (subbituminous to bituminous transition). Pyrophyllite was produced by the interaction of kaolinite and quartz formed by metamorphic processes associated with anthracitization. Cookeite is relatively common in the studied coals. It may have been derived from the reaction of kaolinite with Li ions, although the source of Li is not yet clearly understood. Minerals in the non-coal samples (partings and floor strata) appear to have responded differently to the metamorphic conditions than those in the adjacent coals. Possible mechanisms for the formation of NH 4 -illite in the Jincheng non-coal rocks at least involve isomorphous substitution of NH 4 for K in the K-illite structure. The well-crystallized K-illite in the non-coal rocks probably formed from detrital K-illite, and wasrecrystallized with the coal rank advance. Absence or minor proportions of pyrophyllite in non-coal samples may be due to a lack of epigenetic kaolinite in the non-coal horizons. Pyrophyllitization occurred during a late stage of metamorphism. The general absence of chlorite in the thick non-coal interval is probably due to the low permeability of that horizon, which resulted in a lack of infiltrating Fe- and Mg-bearing fluids. [ABSTRACT FROM AUTHOR]

The origin and modes of occurrence of rare metals in coal (e.g., rare earth elements, U, Se, Re, Ge, Ga, and Nb) have attracted much attention in recent years because such investigations can provide fundamental information for assessment of their economic significance. This paper investigates the abundance, modes of occurrence, and origin of rare metals in three Lopingian coals (LL5-K3, CG3-K6, and CG1-K7 Coals) from the Yishan Coalfield, Guangxi Province, southern China. The coals of this coalfield are preserved within carbonate successions and are characterized by super-high-organic sulfur (SHOS) contents (mostly 6–9%) and an elemental assemblage enriched in U-Se-Mo-V-Re. Two different hydrothermal solutions were responsible for the geochemical anomalies in the Yishan coals. One was responsible for the U-Se-Mo-V-Re enrichment in the three coal seams while the other was a high-temperature solution responsible for the positive Eu anomalies and the elevated rank ( R o,ran = 2.05%) of the LL5-K3 coal. The U, Se, Mo, V, and Re in the CG3-K6 and CG1-K7 coals mainly have an organic association, but the same elements in the LL5-K3 coal have a mixed organic-inorganic affinity, attributed to high-temperature leaching in the early coalification stage. In addition to highly elevated concentrations of U-Se-Mo-V-Re, the average concentration of oxides of the rare earth elements and Y in the LL5-K3 coal (1387 ppm on average, ash basis) is higher than the cut-off grade for industrial deposits of these elements and thus the coals also have potential as raw materials for the recovery of rare earth elements. [ABSTRACT FROM AUTHOR]

Abstract: Coals from three seam sections in the Songzao Coalfield, SW China, are mainly high-ash, high-sulphur semianthracites. Minerals within the Songzao coals are mainly kaolinite, pyrite (or marcasite in some cases), and quartz, with various proportions of non-kaolinite clay minerals, carbonates, feldspars, and anatase. The illite and mixed-layer illite/smectite (I/S) are Na-rich in some of the Datong coal samples. The I/S in the lower coals of the Datong section is most likely an alteration product of dispersed volcanic ash, due to the availability of necessary ions (e.g. K, Na, and Mg) in the marine-influenced coal swamp. Organically-bound Na, which was expelled from the organic matter with coal rank advance, especially with anthracitization, may have supplied additional Na for the formation of Na-rich illite. Authigenic I/S also occurs in a Tonghua coal ply that is overlain by a mafic bentonite and underlain by an alkali tonstein. Potassium, Na, and Mg for the formation of such I/S were probably derived from the leaching of the adjacent alkali tonstein and mafic bentonite. Although the marine water was also a possible supplier of the alkali elements, authigenic I/S is rare in coal plies that occur further away from the altered volcanic layer. Leaching of the volcanic claystones in the Tonghua coal seam probably led to the formation of relatively abundant anatase and rhabdophane in the underlying coal ply. Fracture-filling REE minerals (probably REE-hydroxides or oxyhydroxides) also occurring in that coal ply crystallized from ascending REE-rich hydrothermal fluids, probably associated with contemporaneous volcanic activity. [Copyright &y& Elsevier]

Determination of the geochemical and mineralogical characteristics of coal can provide useful information on the geological setting and evolution of coal basins and assist in identifying factors that control the distribution of the minerals and elements of interest. The Late Permian coals of southwestern China have received much attention because some of these coals are usually considered to be potential sources of some critical metals and have high concentrations of potentially toxic elements. Mineralogical and geochemical analyses were carried-out on the Late Permian coals from the Kulishu and Sijichun mines of the Zhijin-Nayong Coalfield, Western Guizhou, China. The diagram of Al 2 O 3 /TiO 2 -Nb/Yb shows that the sediment source of the Kulishu and Sijichun coals is dominantly the Emeishan magmatic province with a smaller contribution from a Neoproterozoic metamorphic-granite complex, both of which are the two major components of the Kangdian Upland. The sediment source of the roof and floor rocks is dominantly basaltic volcanics in the Emeishan large igneous province (ELIP), while the partings were probably derived from eruptions of the Emeishan felsic-alkaline magmas and a mixture of ashes derived from a distal volcanic arc. Highly elevated total sulfur contents (dominated by pyritic and organic sulfur), abundant framboidal pyrite, as well as disseminated fine-grained pyrite provide evidence for input of seawater during the peat and coal formation. Paragonite mineralization with enrichment of Li, Nb (Ta), Zr (Hf), REY and Pb in the Sijichun coals and partings of the Kulishu coal are attributed to the intrusion of alkaline magma. The pyrite, quartz, albite, anatase and gypsum occurring as veins-, cleat-fillings and separate well-crystallized aggregates of epigenetic origin in the studied coals are indicative of the influence of hydrothermal fluids from a magmatic source. [ABSTRACT FROM AUTHOR]

The coal-hosted Ge deposit (Early Cretaceous) at Wulantuga, Inner Mongolia, northern China, is one of the largest Ge deposits in the world. Although the origin, modes of occurrence, and spatial distribution of Ge in this deposit have been investigated by several studies, the provenance and pathways of Ge from the source to the deposit are largely unknown. In this study, we investigated the 87Sr/86Sr values in the high-Ge coal of the Wulantuga deposit and the stratigraphically-equivalent low-Ge coal of the No. 1 Pit, both from the Shengli Coalfield, Inner Mongolia, northern China. In addition to an organic association, Sr and Ca in the high-Ge and low-Ge coals also occur in gypsum. The water-soluble Sr (i.e., in gypsum) in the high-Ge coal (Ge concentrations: 45–1170 ppm, 273 ppm on average, whole coal basis) has 87Sr/86Sr values in the range of 0.710411 to 0.710633 (with an average of 0.710478). In contrast, the low-Ge coal (Ge concentrations: up to 4.23 ppm, 0.45 ppm on average, whole coal basis) has lower 87Sr/86Sr values in two sub-groups, a low range of 0.709896–0.71005 (with an average of 0.709736) and a high range of 0.710173–0.710547 (with an average of 0.710329 and dominantly in the range of 0.710173–0.710406). The higher 87Sr/86Sr values in the high-Ge coals indicate that the highly-elevated concentration of Ge is associated with a local source of radiogenic Sr. The enrichment of Ge in the coal is due to several consecutive processes. Surface waters penetrated deeply into granite and dissolved at least partially more soluble phases of the granite. The Ge-rich solution flowed over ground or underground into the Shengli Coalfield and Ge precipitated in the peat, the precursor of the coal investigated in the coalfield. The 87Sr/86Sr values of the water-soluble fraction, the elemental compositions of high-Ge and low-Ge coals, and the geomorphological setting of the coalfield point to the Hercynian granodiorite intrusion on the nearby SE margin of the Shengli Coalfield as the primary source of Ge in the Wulantuga coal-hosted Ge deposit. In addition to serving as an indicator of the mode of formation of the coal-hosted Ge deposit, the 87Sr/86Sr values and their spatial distribution can provide an indication for the hydrological pathways of the mineralising fluid. • The Sr in gypsum in the high-Ge coals has 87Sr/86Sr values from 0.710411 to 0.710633. • The Sr in gypsum in the low-Ge coal has lower 87Sr/86Sr values, 0.709896–0.710547. • The higher 87Sr/86Sr values in the high-Ge coals indicate a grantic Ge source. [ABSTRACT FROM AUTHOR]

• REE-Y-Nb-Ta-Zr-Hf are enriched in the two coal seams (D101 and W407) investigated. • REE-Y-Nb-Ta-Zr-Hf in the D101 coal were dominantly derived from terrigenous region. • Inorganic materials deposited during peat deposit were subjected to groundwater leaching. • Both ashes from the two coals are potential raw sources for REE and Y. Abundances, modes of occurrence, enrichment, and origin of critical elements in coal and coal-bearing sequences have attracted much attention in recent years. Previous investigations have attributed REE-Y-Nb-Ta-Zr-Hf mineralization in coals and in intra- and inter-seam tuffs in coal-bearing sequences to alkali volcanic ash eruptions and subsequent hydrothermal alteration. In this study, two major minable coal seams (D101 and W407) that are located in the lowermost and uppermost Wuchiapingian Longtan Formation in the Liupanshui Coalfield, Guizhou, southwestern China, were investigated in order to better understand the mechanism of REE-Y-Nb-Ta-Zr-Hf enrichment in these coals. Elevated concentrations of REE-Y-Nb-Ta-Zr-Hf in the D101 coal are dominantly due to the alkaline volcanic material forming the top of the Emeishan Large Igneous Province (ELIP), which served as a terrigenous source region for the coal basin and to airborne volcanic ash, which occurs as an intra-seam tonstein within the coal seam. Both of the two source materials were subsequently subjected to groundwater leaching, resulting in redistribution of the rare earth elements and Y (REY), leading to an uneven distribution of REY and Zr along the studied coal-seam sections. The average REY oxides in the D101 and W407 coals are 2682 and 1260 ppm (both on ash basis), respectively, and the outlook coefficients are 0.79 and 1.15 respectively, indicating that both ashes from the two coals are potential raw sources for REY. [ABSTRACT FROM AUTHOR]

• Enrichment of Mo, U, Se, and Re in the two coal benches under the limestone roof is due to marine transgression. • Lithium and REY concentrated in the lowermost coal benches is of hydrothermal in origin. • Light and heavy REE are associated with phosphates and organic matter, respectively. • Lithium, Ga and REY in the Jincheng No. 15 coal are promising for industrial use. The Qinshui Basin is the largest reservoir of commercial coalbed methane in China. The No. 15 seam, one of the major coal seams for coalbed methane exploration in the Jincheng Coalfield, SE Qinshui Basin, has recently been found to be enriched in the critical element Li. This study further investigates the inorganic geochemistry of the No. 15 coal and associated rocks and reveals the enrichment origin of critical elements in these horizons. The Jincheng No. 15 coal is rich in Li, Mo, U, Se, Re, and rare earth elements and Y (REY). Concentrations of Mo, U, Se, and Re are higher in the two topmost coal benches, probably due to marine transgression in the early diagenetic stage after the peat had been established. The enrichment of Li and REY in a few lowermost coal benches is suggested to be derived from hydrothermal fluids that were related to magmatic activity (110–140 Ma) during the Yanshanian Orogeny (Late Jurassic-Early Cretaceous), or a Late Triassic-Early Jurassic abnormal thermal event occurring at 190–215 Ma. Light REY (La, Ce, Pr, and Nd) in the Jincheng No. 15 coal are predominantly associated with phosphate minerals. Heavier REY (Sm to Lu) show a mixed phosphate and organic affinity in the studied coal samples. Non-coal samples are characterized by light REY- and HREY-enrichment, which is probably inherited from the source material in the Trans North China Orogen. Trace elements in the coal can be highly enriched in the ash if the coal is combusted due to the low to medium ash yield of the coal. Critical elements, including Li, Ga, and REY, in the Jincheng No. 15 coal are considered as promising material for industrial recovery and have been previously under-estimated. On the other hand, environmentally sensitive elements including As, Hg, and Pb, while having concentrations within the normal range compared with average worldwide coals, can be highly enriched in the coal combustion residues if the coal is combusted, and thus careful consideration needs to be given to the utilisation of such coals. [ABSTRACT FROM AUTHOR]

• Element geochemistry in coals varies notably in a laterally correlated coal seam. • Various geological factors are responsible for the difference in the elemental geochemistry of the coals. • Some critical metals including REY are redeposited within the coal section as a result of acidic leaching. • The Datanhao coal seam has potential in industrial utilization of certain critical metals. This paper discusses the modes of occurrence of critical elements and their origins in the studied coal and associated rock samples (partings and roof strata) from the Datanhao coal mine, Daqingshan Coalfield, Inner Mongolia, northern China. The studied coals are enriched in trace elements including Zr, Hf, Th, Be, F, Zn, Ga, Nb, Mo, Cd, In, Sn, Ta, Hg, W, Pb, and rare earth elements and Y (REY). Zirconium, Nb, Ta, and Hf are largely associated with anatase in the studied coal and non-coal samples. The main carriers of REY, Th, and U in both coal and non-coal samples are Ca-bearing REE-phosphate minerals, most likely monazite, although U is partly associated with organic matter. Fluorine in the Datanhao coals is mainly associated with kaolinite. Most partings identified as volcanic ash-derived tonsteins from the Datanhao Mine exhibit negative Eu anomalies that, along with the common presence of equant-shaped euhedral volcanic quartz (beta-quartz), indicate a felsic volcanic source located in the Xing'an-Mongolia Orogenic Belt. Other probable sources of clastic material into the basin include the pre-Cambrian granites from the adjacent uplands, which may be responsible for the light rare earth element enrichment of the studied deposits, and associated metamorphic rocks from the adjacent uplands. Owing to acidic hydrothermal fluids, which are most likely related to the Yanshanian (Jurassic to Early Cretaceous) intrusive activity and/or low-temperature basinal waters circulating within the coal-bearing unit during the epigenetic stage, most of the coal benches from the Datanhao mine are also relatively enriched in middle rare earth elements. On the other hand, some coal and non-coal bench samples from the middle Datanhao section are enriched in heavy REE. This, along with the presence of abundant syngenetic or early diagenetic carbonate minerals, indicates deposition in an episodic alkaline environment, the origin of which is mostly probably attributed to evaporation and/or saline water inputs resembling a playa lake environment. As the concentrations of Zr, Hf, Nb, and Ta are all close to or higher than the respective industry cut-off grades or standards for these critical elements, the Datanhao coal seam has potential economic significance for these as an important by-product of the coal mining. [ABSTRACT FROM AUTHOR]