Mineralogy and geochemistry of Permian coal seams of the Sydney Basin, Australia, and the Songzao Coalfield, SW China

摘要

This study is an investigation of the abundance and modes of occurrence of the mineral matter and trace elements in the Permian coal seams of the Sydney Basin, eastern Australia and the Songzao Coalfield, SW China, as well as the relationships between trace elements and mineral matter components within the different parts of the coal seams. A range of analytical techniques have been used to obtain relevant data, including optical microscopy, electron microscopy/microprobe analyses, quantitative X-ray diffraction, geochemical techniques (ICP-MS/OES, CV-AFS, HG-AFS and Eschka method), and Laser Raman spectroscopy analysis.The Greta coal is a high-volatile bituminous coal and typically contains a high proportion of liptinite. The upper section of the Greta seam has several different indicators of marine influence, such as anomalously low vitrinite reflectance and abundant syngenetic pyrite, in the top part of the seam. Pyrite typically comprises 40 to 56% of the mineral assemblage in the coals from the marine-influenced upper section. In contrast, the mineral matter in the lower section contains minor pyrite, and relatively abundant dawsonite, which may have been formed by reactions between earlier-precipitated kaolinite and Na2CO3- or NaHCO3-bearing fluids. The minerals, including most of the clay minerals, pyrite, siderite and quartz, within most of the Greta coal plies are largely of authigenic origin. Authigenic Na-rich I/S may have been syngenetically precipitated, probably after the peat was accumulated, with abundant Na and relatively minor K ions being supplied by the marine water. Coals from the Great Northern and Bulli seams are mainly high volatile A bituminous and medium volatile bituminous in rank, respectively. The mineral fractions of the coals, especially in the middle parts of the seams, are dominated by authigenic kaolinite with a very low abundance of quartz and carbonate minerals. Apart from tonstein bands in the Great Northern seam, authigenic processes therefore appear to be the dominant mechanism of mineral matter formation in both coal seams. Authigenic K-feldspar also occurs in the lower part of the Great Northern seam, with a variety of unusual modes of occurrence. A late syngenetic low-temperature hydrothermal fluid injection process is suggested for formation of this feldspar component. Both quartz and non-kaolinite clay minerals are also abundant in the lowermost ply of the coal seams, suggesting that the immediate base of the peat bed in each case was made-up of organic matter admixed with the same detrital sediment as supplied to the basin. K-feldspar, which is present in the coals and non-coal bands in the lower metre of the Great Northern seam section, is not present in the Bulli seam. This may reflect deposition of the Bulli seam at a greater distance from the sediment source, which was located in the New England Fold Belt.The coals from the Songzao Coalfield are mainly high ash, high sulphur semianthracites. XRD analysis indicates that minerals within the Songzao coals are mainly kaolinite, pyrite (or marcasite in some cases), and quartz, with minor proportions of carbonates, feldspar, anatase and sulphate minerals. Some of the illite and I/S is Na-rich in some of Datong coal samples. The I/S in the Songzao coals is mainly an alteration product of the original dispersed volcanic ash, due to the availability of necessary ions (e.g. K, Na, 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 and I/S. Authigenic I/S also commonly occurs in a Tonghua coal ply that is overlain by a mafic bentonite and underlain by an alkali tonstein. K, Na and Mg for the formation of the 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 other coals that occur further away from the altered volcanic layers. REE minerals, which occur as fracture infillings in a Tonghua coal sample, were probably crystallized from ascending hydrothermal fluids carrying high REE concentrations, which may in turn have been associated with contemporaneous volcanic activity. Two groups of REE minerals, probably REE-hydroxides or oxyhydroxides, and REE-carbonates, were tentatively identified. Tonstein bands in the Great Northern seam consist essentially of kaolinite. The occurrence of idiomorphic crystals of K-feldspar, which may represent members of the anorthoclase-sanidine series or a sodic sanidine, indicates an acid to intermediate volcanic ash input. Two tonstein and one K-bentonite bands in the Songzao coal seams have kaolinite and I/S as the dominant clay minerals, respectively. The volcanic ash layers in the peat swamp may have been originally converted to smectite, which was in turn altered to I/S and illite during diagenesis and/or rank advance, assuming that necessary ions (e.g. K, Na and Mg) were available from the marine water percolating in the peat swamp. Na-rich I/S may also have been formed in the claystones, with the additional Na probably being released from the organic matter during the coal’s rank advance. The thin tonstein layers were formed, with relevant ions having been largely removed, probably due to a greater leaching efficiency.In the relatively low-ash coals of the Greta, Great Northern, and Bulli seams, the concentrations of most trace elements are lower than that of average worldwide coals. By contrast, the high-ash Songzao coals have relatively high concentrations of most trace elements compared to averages for worldwide coals.In the sulphur-rich Songzao and Greta coals, most of the chalcophile trace elements show either poor or negative correlations with total iron sulphide contents. Only Hg and Se in the Songzao coals and Hg, Tl and As in the Greta coals are positively correlated with iron sulphides, respectively. This may be because the pyrite in the Songzao and Greta coals is mostly of syngenetic origin. Some chalcophile elements are correlated with Al2O3, which most likely indicates a common source. The absence of traditional pyrite-metal associations may reflect wide variations in the concentrations of these elements in individual pyrite/marcasites, or simply poor retention of those elements in the pyrite/marcasite of the relevant coals. In addition to the lithophile elements, chalchophile elements in the Great Northern coals, including Se, Pb and Cu, also appear to be associated with kaolinite, and more likely a common source as well. The geochemistry of the coals has been affected by the adjacent tonstein/bentonite bands. The relatively immobile elements 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 high in Nb, Ta, Hf, Ga, Th, U, and REE. Coal samples overlying the mafic bentonite in the Tonghua section are high in TiO2, V, Cr, Zn and Cu. However, the influence of the acid to intermediate tonstein layers in the Great Northern seam on the geochemistry of the adjacent coals is not as significant as in the Songzao coal seams. Trace elements, such as Li, Th, and U, are relatively high in most of the Great Northern coal plies adjacent to the tonstein bands.