Alteration process at the Murakami clay deposit, Niigata, Japan

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Alteration process in the Murakami deposit, Niigata, has been studied by means of radiometric and fossil age determination of rocks, oxygen and hydrogen isotope analyses of illite, X-ray diffraction method, and field survey.<BR>The tuff, as the host rock of deposit, can be devided into the following five zones defined by the alteration mineral assemblages: pyrophyllite zone(P), illite zone(I), smectite zone(S), illite/smectite mixed layers zone(I/S), and smectite-zeolite zone(SZ). These zones are arranged in this order from the contact to the outer part.<BR>The age determination indicates that the illitization of this area is closely related to an igneous intrusive rock. The intrusive rock consists of biotite rhyolite which occurred at 5-6 Ma. The original host rock is probably a marine-based homogeneous rhyolitic tuff. Diatom fossils from the tuff suggest that the deposition age of the tuff is younger than 7 Ma.<BR>Rock-texture and mineral assemblage of the tuff indicate that hydrothermal alteration has occurred near the contact. The hydrothermal solution probably consists of so-called magmatic water from the intrusive rock and porewater in the tuff. The P, I, and S zones are mainly formed by the reaction between the hydrothermal solution and the tuff. On the other side, I/S and SZ zones are inferred to have formed exclusively by the reaction between the porewater and the tuff under heating by the intrusive rock.
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