The Argillization of the Mother Rock of the Marcasite Deposits in the Isafushi Mine, Iki Island

Abstract

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.