1993 Volume 43 Issue 241 Pages 365-373
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.
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.
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.
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.
