A high-pyrite semianthracite of Late Permian age in the Songzao Coalfield, southwestern China: Mineralogical and geochemical relations with underlying mafic tuffs

• Published in: International journal of coal geology 2010-09, Vol.83 (4), p.430-445
• Main Authors: Dai, Shifeng, Wang, Xibo, Chen, Wenmei, Li, Dahua, Chou, Chen-Lin, Zhou, Yiping, Zhu, Changsheng, Li, Hang, Zhu, Xingwei, Xing, Yunwei, Zhang, Weiguo, Zou, Jianhua
• Format: Article
• Language: English
• Subjects: Chongqing of southwestern China; Coal; Earth sciences; Earth, ocean, space; Exact sciences and technology; Geochemistry; Late Permian coal; Mafic tuffs; Minerals; Sedimentary rocks; Soil and rock geochemistry; Stratigraphy; Trace elements
• ISSN: 0166-5162; 1872-7840
• DOI: 10.1016/j.coal.2010.06.004

The No. 12 Coal (Late Permian) in the Songzao Coalfield, Chongqing, southwestern China, is characteristically high in pyrite and some trace elements. It is uniquely deposited directly above mafic tuff beds. Samples of coal and tuffs have been studied for their mineralogy and geochemistry using inductively coupled plasma-mass spectrometry, X-ray fluorescence, plasma low-temperature ashing plus powder X-ray diffraction, and scanning electron microscopy equipped with energy-dispersive X-ray analysis. The results show that the minerals of the No. 12 Coal are mainly composed of pyrite, clay minerals (kaolinite, chamosite, and illite), ankerite, calcite, and trace amounts of quartz and boehmite. Kaolinite and boehmite were mainly derived from sediment source region of mafic tuffs. Chamosite was formed by the reaction of kaolinite with Fe–Mg-rich fluids during early diagenesis. The high pyrite (S p,d = 8.83%) in the coal was related to marine transgression over peat deposits and abundant Fe derived from the underlying mafic tuff bed. Ankerite and calcite were precipitated from epigenetic fluids. Chemical compositions of incompatible elements indicate that the tuffs were derived from enriched mantle and the source magmas had an alkali-basalt character. Compared to other coals from the Songzao Coalfield and common Chinese coals, the No. 12 Coal has a lower SiO 2/Al 2O 3 (1.13) but a higher Al 2O 3/Na 2O (80.1) value and is significantly enriched in trace elements including Sc (13.5 μg/g), V (121 μg/g), Cr (33.6 μg/g), Co (27.2 μg/g), Ni (83.5 μg/g), Cu (48.5 μg/g), Ga (17.3 μg/g), Y (68.3 μg/g), Zr (444 μg/g), Nb (23.8 μg/g), and REE (392 μg/g on average). Above mineralogical compositions, as well as similar ratios of selected elements (e.g., SiO 2/Al 2O 3 and Al 2O 3/Na 2O) and similar distribution patterns of incompatible elements (e.g., the mantle-normalized diagram for incompatible elements and chondrite-normalized diagram for rare earth elements) of coal and tuff, indicated that enriched trace elements above were largely derived from mafic tuffs, in addition to a minor amount from the Kandian Oldland. ► The No. 12 Coal in the Songzao coalfield of China is a high-pyrite semi anthracite coal.► The No. 12 Coal is high in Sc, V, Cr, Co, Ni, Cu, Ga, Y , Z, Nb, and REE.► Mineral matter in the No. 12 Coal was largely derived from mafic tuffs underlying the coal.► Mafic tuffs were derived from enriched and alkali-basalt mantle.