Rock Alteration and Formation of Clay Minerals in the Ugusu Silica Deposit, Izu Peninsula, Japan

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Silica deposits of the Ugusu silica mine arising from hydrothermal alteration have been investigated by geological survey and by petrographical, mineralogical and chemical methods. Here, andesite and dacite of the Miocene age and pyroxene andesite of the Pliocene age have been altered to form silica, alunite and argillized rocks containing abound various clay minerals such as pyrophyllite, kaolin, smectite, sericite and chlorite. Zonal arrangements of these minerals are clearly seen in the alteration halos, which can be divided into six successive zones, i.e. chlorite zone (periphery), smectite-sericite zone, kaolin zone, pyrophyllite zone, alunite zone and silica zone (center). Silica minerals occurring in the altered rocks are quartz, cristobalite and tridymite. Quartz predominates in the inner part of the alteration halo, while cristobalite prevails in the outer part. Migration of chemical components between alteration zones and unaltered rocks is shown as changes in their weight contained in unit volume of rock, calculated on the assumption that the volume of rocks remained unchanged during hydrothermal alteration. In the silica zone, SiO_2 increases and almost all other components decrease, as compared with the unaltered rocks. In the alunite zone, Al_2O_3 and K_2O increase and the other components decrease. The behavior of migration of chemical components in the pyrophyllite zone and the kaolin zone is similar to that of the alunite zone, the decrease of silica, however, being less conspicuous. Slight increase of silica is noticed at the outermost part of the smectite zone. In all the alteration zones losses of TiO_2,Fe_2O_3,FeO, MnO, MgO, CaO and Na_2O are noticeable. The most important factors controlling the chemical compositions and mineral associations in the altered rocks are pH , and temperature of the solution ascending through the wall rock. Chemical compositions, especially the amount of alkali metals in the parent rocks have the effects on modes of occurrence of clay minerals. Chemical weathering is conspicuous on the surface of the argillized rocks. Hydrothermal alteration being often accompanied by crystallization of pyrite in altered rocks, groundwater in argillized rocks is enriched with sulfuric acid. Smectite and mica-smectite mixed-layer minerals are transformed to kaolin-smectite mixed-layer minerals or halloysite through chemical weathering by acid ground-water. This transformation is similar to that found in acid clays, indicating that another zonal distribution of clay minerals by chemical weathering overlaps the hydrothermal alteration zones formed earlier.

Rock Alteration and Formation of Clay Minerals in the Ugusu Silica Deposit, Izu Peninsula, Japan

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