Nontronite rocks are found in the altered basalt from Isafushi-mura, Nagasaki prefecture. Near the marcasite deposits of the Isafushi mine, nontronite rock zone is well developed between the kaolin-halloysite-allophane rock and fresh or low altered pyroxene olivine basalt. The nontronite rock includes two essential minerals, yellow or greenish yellow nontronite and white montmorillonite. From the field and microscopic observations of the nontronite rocks, we concluded that the rock under consideration is a replacement-alteration product of the basalts by action of hydrothermal solutions of the epithermal type. It is also believed that the nontronite-rock was formed by low acidic or neutral iron-bearing solutions, derived from the argillized and silicified rock zones.

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Studies on the alterations of country rocks in the ore deposits offer the numerous important, suggestions for the prospecting of ore deposits. Generally, the alteration of wall rock in pneuinatolytic deposits is most significant. However, the country rocks of the hedrothermal deposits are also, altered greatly through the action of residual active solution.
Therefore it is most important to infer the depositing environments by investigating the differentiation of physical and chemical properties in the mineral solution during the alteration process. The writers attempted to study the alteration process by means of dielectric constant method and by other useful methods. From the results of these investigations the horizontal zonings of mineral assemblages were ascertained. The successions of minerals determined are as follows : chlorite, carbonate, montmorillonite, kaolinite, chalcedonic quartz and sulphide from outer zone to inner zone. The predominant members of clay minerals adjacent to the cinnabar vein are kaolinite and montmorillonite. Sericite and chlorite are subordinate members. Judging from the facts that these minerals were successively deposited, by thedecrease of alkalinity and the falling of temperature, the most favourable condition for the deposition of cinnabar may be weak alkaline nature of the solutions which are intimately related to the crystallizations of kaolinite and montmorillonite. As to the mechanism of the deposition of the cinnabar in chalcedonic quartz vein it can be concluded that the ascending acidic solutions containing various kinds of magmatic emanation-gases change their chemical composition to alkaline solutions extracting sodium-metasilicate or aluminate component from the country rock. Carbonate and sericite may crystallize from this strong alkaline solution with pyrite, but cinnabar which dissolves into complex salts may ascend to upper zone with other components and may be deposited with kaolinite and montmorillonite from favourable weak alkaline solution.
The chemical property of residual solution was changed again by the influence of the underground water when it ascended to the upper zone. Then, marcasite and meta-cinnabar crystallized in these acidic solutions with kaolinite. The variation-diagram of dielectric constant then shows the changes of chemical composition, that is the change mineral association was applied to investigate the alteration of wall rock in ore deposit.

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