Japan Geoscience Union Meeting 2015

Presentation information


Symbol S (Solid Earth Sciences) » S-CG Complex & General

[S-CG58] Petrology, Mineralogy and Resource Geology

Mon. May 25, 2015 6:15 PM - 7:30 PM Convention Hall (2F)

Convener:*Akira Miyake(Department of Geology and Mineralogy, Graduate School of Science, Kyoto University), Toshiaki Tsunogae(Faculty of Life and Environmental Sciences (Earth Evolution Sciences), University of Tsukuba), Koichiro Fujinaga(Department of Systems Innovation, School of Engineering, University of Tokyo), Nobutaka Tsuchiya(Department of Geology, Faculty of Education, Iwate University)

6:15 PM - 7:30 PM

[SCG58-P02] Origin of siderite-rich rocks from the Ishikari coalfields of Central Hokkaido, Japan(III)

*Yuki ASANO1, Toshiro MORIKIYO2 (1.Department of Geology, Graduate School of Science Shinsyu University, 2.Department of Geology, Faculty of Science Shinsyu University)

Keywords:carbonate concretions, behavior of the elements, Ishikari coalfield

Ikegami (1958) described mineralogical features of siderite-rich rocks in the Ishikari coalfield at first time. After that, origin of these characteristic rocks were investigated by Matsumoto and Iijima (1981). Asano et al. (2014) measured whole-rock chemical compositions and carbon-oxygen isotope ratios of sideritic rocks, which had not been reported to that time. They reached a new view as to the genesis of siderite, which is different from Matsumoto and Iijima (1981). Subsequently, Asano and Morikiyo (2015) studied Mn/Fe ratios and carbon-oxygen isotope ratios of calcite and siderite in the rocks. On the basis of the chemical data and textural observation, they conclude that crystallization of calcite occurred before the precipitation of siderite. Calcite precipitation is due to the reduction of Fe(OH)3 by carbonaceous matter, which is abundant in the coaly bed.
In this poster presentation, we describe the entire process of sideritic rock formation in the Ishikari coal field.
1.The source of iron of siderite is thought to be dissolved Fe2+ in stream water. The water flows into the lake of meandering river area of plains. Then the water becomes exposed to the oxygen-rich atmosphere. This lead oxidation of dissolved Fe2+ and Mn2+ to Fe(OH)3 and MnO2•nH2O. These solid particles of hydro-oxides were deposited at the bottom of the lake together with FePO4.
2.Most of Fe(OH)3 was contained in the clastic materials with a dispersed fashion. But in some cases, they accumulate at the bottom of a lake horizontally forming thin layers of iron-rich sediments. The chemical compositions of the host clastic matter is similar to the average shale.
3.With the progress of sediment burial, sediments-pore water system became anoxic. Then, Fe(OH)3 deposited within the sediments were reduced to be Fe2+ by the reaction with carbonaceous matter, which was abundant in the sediments. Production of CO2 by the oxidation of carbonaceous matter brought in the precipitation of calcite of lowδ13C value. At the time of decreasing in Eh, MnO2•nH2O is reduced prior to Fe(OH)3 reduction. Because of this, the Mn/Fe ratio of calcite is higher than that of siderite. 4. Since the concentration of SO42- ion of river water is low, the sulfate reduction ceased in an early stage of diagenesis. Then the methane fermentation begins. At this stage, the siderite possessing positive, high δ13C value started to precipitate forming the siderite nodules and thin beds of sideritic rocks.
•Asano, Y., Kusakabe, T., and Morikiyo, T. (2014) Formation of sideritic rocks in the Ishikari coalfield — particularly on the source and precipitation of iron. The 121st Annual Meeting of the Geological Society of Japan Abstracts R9-O-4, p.96
•Asano, Y. & Morikiyo, T. (2015) Formation processes of sideritic rocks in Central Hokkaido Ishikari coalfield (II) —For Calcite coexisting with Siderite. The Annual Meeting of the Sedimentological Society of Japan Abstracts. (Submitted to.)
•Ikegami, S (1958) Preliminary Note on the Sideritic Band (Ironstone) in the Horokabetsu Formation in the ishikari Coal Field., Mineralogical Journal, volume3, No.6, 592-596
•Matsumoto, R. and Iijima, A. (1981) Origin and diagenetic evolution of Ca-Mg-Fe carbonates in some coalfield of Japan. Sedimentology, 28, 239-259.