GEOCHEMICAL STUDIES ON MINERALIZATION ACCOMPANIED BY TERTIARY GRANITOIDS

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This study aims to search the factors which provide a granitoid is mineralized or barren. For this aim, we selected some mineralized Tertiary granitoids, the Daiyama, Kuromori and Nissho granites, in Northeast Japan, and a barren granitoids, Kumogi granite, in San-in district, and examined their rock texture, bulk rock and mineral chemistry and hydrothermal alteration.From texture of Fe-Ti oxide minerals and chemical composition and zoning of plagiocalse, the Kuromori granite intruded in a high temperature and cooled rapidly. The Daiyama granite intruded at the same time or a slightly later than the Kuromori granites, and cooled slowly with affecting hydrothermal alteration. These results are based on the fact that fine ilmenite-lamellae in magnetite, wide chemical composition ranges of feldspars, enrichment of Rb and K,and development of myrmekite. The hydrothermal alteration occurs strongly in the northern part of the Daiyama granite, resulting in chloritization of mafic minerals, reaching of Sr and formation of vein-type mineralization. The Nissho granite intruded in a more felsic composition and cooled slowly, resulting in a vein-type mineralization in a long temperature range.On the other hand, the Kumogi granite is equi-granular and has rather homogeneous composition, meanin small range of crystallization differentiation. Development of albite-rim in the Kumogi granite is small. It is concluded that the Kumogi granite cooled rapidly in a high oxidized conditions.Detailed examination of mineral texture, mineral and bulk rock chemistry, and hydrothermal alteration can obtain the information of crystallization and mineralization during the magmatic and hydrothermal stages. Adopting these methods to other granitoids, we can obtain necessary and sufficient conditions whether the granitoids are mineralized or barren.