<国立科博専報>厳原町北部地域の白土ならびに陶石鉱床

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Deposits of hakudo (white clay) and toseki (pottery stone) around the town of Izuhara have been known since old times. The clays are being mined as raw material for ceramic and cement manufacture by the Kyoritsu Ceramic Material Co., Ltd., the Onoda Mining Co., Ltd. and others, and are transported to northern Kyushu and Nagoya. Geology of the areas around Izuhara-machi is composed of the Tertiary Taishu Group and the quartz porphyry which either intrudes or is injected into the former. The Taishu Group has partly turned hornfels. The deposits of hakudo and toseki occur in cracks that develop in the quartz porphyry and the Taishu Group with vertical dip and the N30°∿50°W strike. Clays are found also along the contact between the two, or in the hornfelsic part, roughly parallel to the bedding of the group. Where the cracks are concentrated, alteration of the country rock is remarkable and hakudo is rich. Where the cracks are less frequent, the rock has been bleached and silicified, to be called toseki, and gradually grades into the unaltered quartz porphyry. Remarkably altered parts of the Taishu Group have turned reddish purple or brown clay, within which thin layers of hakudo are often intercalated across or parallel to the bedding. Hakudo consists chiefly of hydrated halloysite and kaolinite, mixed with quartz, and occasionally accompanied by oligoclase and mica clay mineral. Constituents of toseki are mostly quartz and oligoclase, accompanied in places by small amounts of hydrated halloysite, kaolinite, montmorillonite, chlorite and mica clay mineral. Formation of the deposits may be due to two different actions; one made pottery stone from the quartz porphyry intruding into the Taishu Group, and the other made white clay that had filled cracks or bedding interspaces and was concentrated in the part with abundant cracks that are roughly oriented in a trend. It is difficult to conclude whether the two actions are ascribed to the same mineralizing magma or not, but the deposits may be a result of acidic hydrothermal mineralization due to the post-volcanic activity. Both toseki and hakudo are cut by black veinlets having a width of 1 to 50 mm. These are a kind of quartz veins associated with some minerals containing manganese, iron and other elements. X-ray powder diffraction, X-ray fluorescence analysis and spectrographic analysis revealed the presence of Zn, Pb and Cu. These veinlets are supposed to have been formed by mineralization after the formation of hakudo, but further study is required. Reserves of workably clays are almost exhausted at present. However, exploitation of subterranean deposits would be expected by means of a precise geological survey for hidden cracks, because hakudo is developed well in the crack-rich parts where toseki also is abundant.
国立科学博物館の論文
1970-10-20

<国立科博専報>厳原町北部地域の白土ならびに陶石鉱床

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