Mineralogy And Geochemistry Of Longtan Formaiton Coals Of Late Permian In The Western Guizhou Province, Southwest China

Based on the analysis of this investigated coals and summarization of previously published literatures, the paper reports the coal chemistry, mineralogy and geochemistry of coals from Late Permian Longtan Formation in the Western Guizhou Province and determines the distribution of coal quality, mineralogical and geochemical characteristics by using the methods of coal chemistry, mineralogy and geochemistry. The distribution and formation of largely enriched elements are confirmed in the present study. The results as follow:1. Coal ChemistryThe Western Guizhou coals are characterized by special moisture content, low-moderate ash yield and volatile matter yield as well as sulfur content. The average ash yield of the Western Guizhou coals is 23%. The ash yield increases toward the basin margin and basin center due to detrital matter input and marine influences on coals. The ash yield displays an increasing-upward distribution from the lower to middle section of the Longtan Formation and a decreasing-upward tendency from the middle to upper portion of the Longtan Formation.The distribution corresponds to the change of seawater level showing transgression-regression-transgression from low to middle to upper portion of the Longtan Formation. Thus, ash yield is markedly influcenced by change of seawater level.The volatile matter yield changes regularly from low-medium to special low to low-medium volatile matter yield from the Western to Middle to Eastern area of Guizhou Province. This is mainly controlled by regional metamorphism. The higher metamorphism of coals in the middle area of Guizhou Province is closely associated with the upper strata thickness of Longtan Formation.The content of pyritic sulfur is the majority of total sulfur content in the Western Guizhou coals. The sulfur content increases regularly from Northwest to Southeast of Guizhou Province. The coals deposited in the terrigenous, paralic delta and marine setting have low, middle-high and high sulfur content, respectively. The sulfur content shows increasing-upward tendency from lower to middle portion of the Longtan Formation and decreasing-upward tendency from middle to upper portion of the Longtan Formation, which is consistent with the change of seawater level. Thus, sulfur content in the Western Guizhou coals is primarily controlled by change of seawater level.2. MineralogyThe mineral matter of these investigated coals is dominated by quartz, pyrite and rectorite, to a lesser extent kaolinite, illite and marcasite, with trace amounts of anatase, montmorillonite, gypsum, palygorskite, chlorite, albite, siderotil and rozenite, which is consistent with previous study showing that the Western Guizhou coals are enriched in quartz and sulfide minerals. The mineral content is different in various coal mining or different coal seams of the same coal mining due to influence of different geological factors.Three mineral associations occur in the coals of the Fenghuangshan and Wenjiaba coal mine of Zhijin mining, which have special low volatile matter yield (8.1% on average), namely, rectorite+pyrite, kaolinite+rectorite+quartz+pyrite and quartz+rectorite associations. The mineral associations, quartz+kaolinite and quartz+illite+ pyrite+marcasite, occur in the coals with low volatile matter yield (18.5% on average) from Liulong coal ming.The quartz, illite, kaolinite, sulfide minerals and anatase contents are higher in the coals close to roof, floor and parting, suggesting that terrigenous detrital matter input plays an important role in mineral distribution. The carbonate minerals occur in these coals in where minerals derived from detrital origin, such as illite and anatase, are low in amount, suggesting that detrital input play an inverse influence on distribution of carbonate minerals. The rectorite is derived from the alteration of kaolinite, suggesting its derivation from metamorphism origin. The formation of pyrite is influenced by degree of detrital input, marine influence and pH condition during the depositon of coal.The two modes of occurrence of illite, detrital origin and syngenetic origin, are observed in these investigated coals. The former contains relatively high titanium content while the latter contains negligible titanium in amount. Rectorite occurs as cleat/fracture infillings or cell lumen infillings. Rectorite together with chlorite occurs as fracture infillings. The pyrite occurs as spherical polycrystalline framboid pyrite, cell lumen-filling pyrite, radial pyrite crystal aggregates, massive pyrite embedded in organic matter, fine-grained anhedral pyrite randomly distributed in organic matter and fracture infillings in the investigated coals. The former three modes of pyrite indicate syngenetic origin while fracture-filling pyrite indicates epigenic origin. Iron-sulphate minerals occur as cell infillings together with illite. This mode of occurrence may represent artefacts produced by oxidation of sulfide species filling cell lumens and thus indicates an epigenetic origin rather than syngenetic origin. The disseminated anhedral to euhedral iron-sulphate crystals are clustered near the cleat/fractures, which may result from permeation of solution along cleat/fractures to lead to alteration of sulfide species to sulphate phase. Gypsum is also clustered near the cleat/fractures and is commonly formed by reaction between calcite and sulfuric acid being produced by oxidation of Fe sulfides. Some gypsum particles, containing Si and A1, may incorporate very small clay mineral praticles (with a size smaller than the field of EDS bean) co-pretipicated with gypsum.3. GeochemistryThe major elements in these investigated coals are dominated by SiO2, Al2O3 and Fe2O3, with exceptions of the No.3 coals in the Liulong coal mine and No.7 coals in the Wenjiaba coal mine, other coal seams are depleted in major elements or are similar to the average of Chinese coal in major elements. The No.3 coals in the Liulong coal mine are enriched in K2O and slightly enriched in TiO2 while the No.7 coal seam in the Wenjiaba coal mine are slightly enriched in K2O. The average of major elements in the Western Guizhou coals is similar to that of Chinese coal, but coals in the Qianbei coalfield are slightly enriched in CaO and MgO and coals in the Xingyi coalfield are slightly enriched in K2O.These elements, Ta, Nb, V, Cr, Co, Cu, Sr, Nb and Ba, are enriched in the No.3 coal of the Liulong coal mine in the Liuzhi mining. These elements, Li, Se, Nb, Mo and Ta are enriched in the No.23 coal and these elements, Se, Nb, Mo, Ta and U, are enriched in the No.27 coal of the Fenghuangshan coal mine in the Zhijin mining. The N0.6 coal and No.7 coal of Wenjiaba coal mine in the Zhijin mining are enriched in Ta and Li as well as Ta and Nb, respectively. The contents of other trace elements are similar to that of Chinese coal. The Western Guizhou coals are enriched in V, Cu, Mo, Sb, Ta, U, As and Hg while other trace elements are depleted or similar in compoarison to that Chinese coal. The different coalfields in the Western Guizhou Province have various trace elements abundances. The coals from Qianbei coalfield are enriched in Cu, Zn and Nb, and from Zhina coalfield are enriched in V, Cu, Nb, Mo, U and Ta, and from Liupanshui coalfield are enriched in V, Ni, Cu, Sb, Ta, U and Hg, as well as from Xingyi coalfield are enriched in Li, V, Cr, Ni, Cu, Sn, Sb, Ta, U, F, As, Mo, Cd and Hg.The aluminosilicate affinities of Be, B, Sc, V, Cr, Co, Ni, Cu, Zn, Ga, Se, Rb, Sr, Y, Zr, Nb, Sn, Cs, Ba, REE, Hf, Ta, Pb, Th and U as well as sulfide affinities of Hg, As and Mo are evidenced in the Liulong coal mine in the Liuzhi mining. The aluminosilicate affinities of Sc, V, Cr, Co, Ni, Zn, Ga, Rb, Sr, Zr, Nb, Sn, Ba, REE, Hf and Ta as well as sulfide affinities of Cu, As, Se, Sb, Pb and Hg are confirmed in the Fenghuangshan coal mine in the Zhijin mining. The aluminosilicate affinities of Be, Sc, V, Cr, Co, Ni, Ga, Rb, Sr, Zr, Nb, Sn, Ba, REE, Hf, Ta and W as well as sulfide affinities of Hg, Pb and Mo are evidenced in the Wenjiaba coal mine in the Zhijin mining. Carbonate affinity of Mn is confirmed in all coals.The elements, Ta, Nb, As, Hg, F and Mo, are highly enriched in the Western Guizhou coals. Nb and Ta are mainly associated with aluminosilicate or heavy minerals. The area where Na and Ta are remarkably enriched is distributed along the Shuicheng-Ziyun synsedimentary fault and is almost overlapped with delta sediment formed near the Liuzhi-shuicheng. This distribution is ascribed to that terrigenous detrital matter is transported along Shuicheng-Ziyun synsedimentary into swamp during the deposition of the Longtan Formation. As, Hg, F and Mo are primarily hosted by sulfide phases. The area where As, Hg, F and Mo are significantly enriched is mainly distributed in Xingyi, Xingren, Anlong and Zhenfeng counties. This area is bounded by Shuicheng-Ziyun synsedimentary fault to Northwest-Southeast, Shizun-Guiyang fault to Northeast-Southwest and Ceheng curved fault. These faults provide convenient path for permeation and transportation of hydrothermal fluids and magma hydrothermal fluids. Thus, tectonic setting determine the area where elements are highly enriched.The enrichments of V, Cr, Co, Ni, Cu, Nb and Ta are controlled by two geological factors, the degree of detrital matter input and nature of Emeishan basalt. The contents of sulfur and associated elements are influenced by three geological factors, the degree of detrital matter input, nature of Emeishan basalt and redox condition. The highly enriched As, Hg, F and Mo are derived from post-depositional hydrothermal fluids.