On Some Breccia Dikes with Metallization in the so-called "Green Tuff" Region, Japan

Abstract

The geology of the so-called "Green Tuff" region is mainly composed of altered volcanic and pyroclastic rocks characterized by green colour and sedimentary rocks of Tertiary age. There are many ore deposits, such as Kuroko deposits and vein type deposits, in the region.
The breccia dikes are occasionally found in the ore deposits in some mines, namely, Itomuka, Jokoku, Ainai, Daichi, Furokura, Osarizawa, Takarakura, Hanaoka, Tatemata, Tsuchihata, Sachiu, Obori, Yaso, Ashio, Sawatari and Nakase, in this "Green Tuff" region. They are developed in close connect-ion with the ore bodies.
The results of field and microscopic observations reveal that breccias in the dikes consist of clay slate, black phyllite, chert and acidic plutonic basement rocks accompanied by various rocks of Tertiary formation, and phenocrystic crystals of quartz and small amounts of idiomorphic plagioclase, finegrained quartz, chlorite, epidote and a carbonate mineral occur in the matrix of breccia dikes.
The above fact suggests that the breccia dikes of this type are clearly of intrusive origin, and breccias of basement rocks were brought forth by the intrusion of acidic residual magma which abounds in volatile components.
The breccia dikes are divided into three groups by their formation stage as follows.
(1) Pre-metallization stage-in the Osarizawa and Ashio deposits.
(2) Interval between two metallization stages-in the Furokura, Takarakura and Nakase deposits.
(3) Post-metallization stage-in the Tatemata, Hanaoka and Kosaka deposits.
In the Sachiu and Daichi ore deposits two formation stage of breccia dikes, pre-and post-metallization stages, are discernible.
Some breccia dikes are reliable guides in ore prospecting, and seem to afford a clue for solving the problem of metallization stage in some deposits.