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

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.