Geochemical Study on the Granite and Ore Fluids in the Tohoku District.

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As a first step to obtain the answer of the problem that a granitoids is mineralized or not, some granitic bodies in the Tohoku district were chosen. The granitic body was examined on the basis of field survey, microscopic observation, modal analysis, and heating/freezing experiment of fluid inclusion. The results obtained are as follows.1, Wasada granodiorite (Higashi-Tagawa-gun, Yamagata Prefecture) : The granodiorite of late Ceata-ceous to early Paleogene age accompanies Cu-Pb-Zn-Mo-Bi-Au-Ag mineralization. According to field servey and microscopic observation, the granodiorite is relatively homogeneous and medium grained. Hornblende is richer in the southeastern part and potash feldspar is richer in the northwestern part of the granodiorite. These results indicate that the northwestern part is solidified later and/or upper part of the granitic body. This is in harmony with the fact that the existence of the ore deposits is abundant and younger in K-Ar age in the northwestern part.2, Nissho granitic complex (Mogami-gun, Yamagata Prefecture) : This granitic complex of Tertiary age accompanies Cu-Pb-Zn mineralization. Quartzdiorite and granodiorite composes the granitic complex and the latter intrude into the former. Because the granodiorite is semi-porphyritic and its peri-pheral part grade into rhyolite, the granitic complex is considered to be intruded into shallow level of the crust. The fluid inclusion in quartz of the granodiorite is composed of three-phase inclusion including NaCl daughter mineral and two-phase inclusion having large different vapor/liquid ratio. NaCl-equivalent concentration is positively correlated with homogenization temperature.These results lead that the boiling occurred accompanied with ascending of the granodiorite in the shallow level, and the fluid released on the boiling is very saline. The ore deposits were formed by the dilution of the saline ore fluid.