Chemical and mineralogical compositions of silicic, mafic, and alkali tonsteins in the late Permian coals from the Songzao Coalfield, Chongqing, Southwest China
Research Highlights
►Three types of tonsteins (silicic, mafic, and alkali) were identified in coal. ►Mafic tonsteins are high in Sc, V, Cr, Co, and Ni. > Nb, Ta, Zr, Hf, REE, and Ga are enriched in alkali tonsteins. ►The source magmas of tonsteins had an alkali-basalt composition.
Introduction
Tonstein deposits derived from air-borne material of pyroclastic origin in the peat-forming environment have been found in some coal seams (Loughnan, 1971, Loughnan, 1978, Dewison, 1989, Burger et al., 1990, Bohor and Triplehorn, 1993, Ruppert and Moore, 1993, Spears and Lyons, 1995, Hower et al., 1994, Hower et al., 1999, Zhou et al., 1982, Zhou et al., 1989, Zhou et al., 1994a, Zhou et al., 1994b, Zhou et al., 2000, Mardon and Hower, 2004, Lyons et al., 2006, Guerra-Sommer et al., 2008). Understanding the tonsteins in the coal-bearing sequences is both academically and practically significant:
- 1.
Tonsteins typically occur as persistent bands in the host coal seams and have been used in some areas as perfect chrono-stratigraphic markers in order to identify and correlate coal seams in the course of exploration and exploitation activities in the mining industry (Burger and Damberger, 1985, Hill, 1988, Zhou et al., 1989, Burger et al., 1990, Burger et al., 2002, Lyons et al., 1992, Bohor and Triplehorn, 1993, Knight et al., 2000, Ward, 2002);
- 2.
In many cases, tonsteins contain pyrogenic sanidine or zircon that could be used for radiometric age determination, providing an absolute age for chrono-stratigraphic correlation within the globally accepted geologic timetable (Kunk and Rice, 1994, Hess et al., 1999, Triplehorn, 1990, Lyons et al., 2006, Guerra-Sommer et al., 2008);
- 3.
Some tonsteins, typically of alkali origin, may contain valuable trace elements that could be potentially utilized and can be used as indicators in searches for alkali ore deposits (Zhou et al., 2000, Dai et al., 2007); Moreover, Dai et al., 2007, Dai et al., 2010b found a high-Nb containing coal and a Nb(Ta)-Zr(Hf)-REE-Ga polymetallic deposit in the late Permian coal-bearing strata in southwestern China. High-concentrations of Nb, Zr, Ga, and REE (rare earth elements) in both of the high-Nb coal and the polymetallic deposit are derived from alkali volcanic ashes;
- 4.
Intra-seam tonsteins may be incorporated with mined coal products, and if not removed in the preparation plant, become part of the coal when it is used (Ward, 2002).
Igneous rocks (including intrusive and volcanic) can be classified according to chemical or mineralogical parameters. Total alkali-silica content (TAS diagram) for volcanic rock classification is used when modal or mineralogic data are unavailable. Silicic igneous rocks contain a > 63% silica content (e.g., granite and rhyolite), intermediate igneous rocks containing 52–63% SiO2 (e.g., andesite and dacite), basic igneous rocks with 45–52% SiO2 and typically high in Fe and Mg (e.g., gabbro and basalt), ultrabasic igneous rocks with less than 45% silica (e.g, picrite and komatiite), and alkalic igneous rocks with 5–15% alkali (K2O + Na2O) content or with a molar ratio of alkali to silica greater than 1:6 (e.g., phonolite and trachyte) (Le Maitre, 2002, Ma, 2004). Chemical classification also extends to differentiating rocks which are chemically similar according to the TAS diagram, for instance, ultrapotassic, peralkaline, and peraluminous (Le Maitre, 2002). However, neither mineral composition nor TAS diagram for tonstein classification is as reliable as that for intrusive rocks. Zhou et al. (2000) identified silicic and alkali tonsteins in southwestern China using abundances and elemental pairs (e.g., Hf–Ta, Ti–Ta, Ti–V, Hf–Sc, Lu–Hf, and Lu–Th) of incompatible elements. Tonsteins usually have a thickness of 2-7 cm and are mainly of silicic and medium-silicic origin (Addison et al., 1983, Bieg and Burger, 1992, Greb et al., 1999, Burger et al., 2000). As mafic rocks generally do not form tuffs, mafic tonsteins rarely occur in coal-bearing strata around the world and alkali varieties have been reported only by Zhou et al., 2000, Dai et al., 2007 in southwestern China.
Studies by Zhou et al., 1982, Zhou et al., 2000, Burger et al., 2002 showed that tonsteins are widely and well developed in the Late Permian coal-bearing strata of southwestern China, including western Guizhou, eastern Yunnan, Chongqing, and southern Sichuan (Fig. 1). Zhou et al. (2000) suggested that tonsteins in the early stage of the late Permian were mainly of alkali, and the tonsteins in the middle to late stages of the late Permian were of silicic and medium-silicic origin.
Previous studies rarely referred to the magmatic sources of tonsteins (e.g., mantle-, crust-, mantle–crust mixture -derived, or others). In this paper, we describe the geochemical and mineralogical compositions and possible sources of silicic, mafic, and alkali intra-seam tonsteins from the Songzao Coalfield, Chongqing, southwestern China, as well as mafic tuffs underlying the lowermost Late Permian coal-bearing strata. However, the intra-seam partings of terrigenous origin were not discussed in this paper.
Section snippets
Geologic setting
The Songzao coalfield is situated to the southeast of Chongqing and has six mines: Yuyang (YY), Shihao (SH), Fengchun (FC), Songzao (SZ), Tonghua (TH), and Datong (DT) (Fig. 1). The coalfield is 39.5-km long (N–S) and 2–15-km wide (W–E), with a total area of 235.5 km2. The coal reserves of the Songzao coalfield are estimated to be 811 Mt as of 2003 (Li, 2007), accounting for about 42.6% of the total coal reserves in Chongqing, making it one of the richest coalfields in Chongqing.
The coal-bearing
Samples and methods
A total of 35 tonstein partings and four tuff samples were taken from mining faces underground in the six coal mines. Table 1 shows tonstein sample numbers and locations both in coal mines and in coal-seam sections. Note that two parallel samples for the same tonstein layer were collected in some mines, for example, samples DT6-1p1 and DT6-1p2 of mafic tonstein layer in the DT No. 6 coal, DT6-2p1 and DT6-2p2 of silicic tonstein layer in the DT No. 6 coal, and YY11-1p1 and YY11-1p2 of alkali
Petrography and mineralogy of tonsteins and tuffs in the Songzao Coalfield
The thickness of all the tonstein layers is in the range of 2–7 cm and generally is 3–5 cm. Most tonsteins have sharp contacts with the enclosing coal beds and have a relatively continuous lateral extent, not only in the 235.5 km2-Songzao Coalfield, but also in a larger area of about 15 × 104 km2 (Zhou, 1999) covering southern Sichuan Province, Chongqing, eastern Yunnan, and western Guizhou. Their lateral correlation over the large area and the overall similarity of corresponding mineralogical and
Conclusions
- (1)
Three types of intra-seam pyroclastic deposits derived from different sources were identified from the Songzao Coalfield: mafic, silicic, and alkali tonsteins. The alkali and silicic tonsteins were well developed in the early and late stages of the late Permian, respectively. However, the mafic tonsteins are distributed throughout the late Permian coal-bearing section.
- (2)
The clay minerals in most tonsteins are dominated by kaolinite or are mainly composed of mixed layers of illite and smectite.
Acknowledgements
This research was supported by the National Science Fund for Distinguished Young Scholars (No. 40725008) and the National Natural Science Foundation of China (Nos. 40930420 and 40831160520). We are grateful to Mrs. Weiguo Zhang and Yunwei Xing for their assistance in sample collection and preparation.
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