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Geology and properties of the Kawasaki and Dobuyama bentonite deposits of Zao region in northeastern Japan

¹ Research Center for Deep Geological Environments, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), Central-7, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8567 Japan, ² Geology and Geoinformation Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), 30 Gajung-dong, Yusunggu, Daejeon 305-350, Korea, and ³ Kunimine Industries Co., Ltd., 1-10-5, Iwamoto-cho, Chiyoda-ku, Tokyo, 101-0032 Japan

^* E-mail: takagi-t{at}aist.go.jp

(Received 17 February 2005; revised 21 June 2005)

The Kawasaki and Dobuyama bentonite deposits in northeastern Japan show contrasting properties even though they are only 5 km apart in a sequence of Neogene sedimentary and pyroclastic rocks. The Kawasaki deposit consists of stratiform bentonite layers up to >50 m thick, and its wall rocks are unaltered shallow marine sedimentary rocks. In contrast, the Dobuyama deposit consists of a funnel-shaped ore body 200 m across, and its wall rocks are hydrothermally altered terrestrial rhyolitic pyroclastic rocks. The Kawasaki and Dobuyama bentonites mainly consist of Na-Ca smectite and Ca smectite, respectively, with subordinate opal-CT, quartz and zeolite. The geological occurrences of the deposits and wall-rock properties suggest that the Kawasaki and Dobuyama deposits were probably formed by diagenesis and low-temperature hydrothermal alteration, respectively. The difference in exchangeable cation ratios of the smectite between the two deposits is attributable to the difference in their sedimentary environments and/or burial depth.

KEYWORDS: clay minerals, bentonite, Neogene, diagenesis, hydrothermal alteration, exchangeable cation, supergene process, Japan

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