Glass and fluid inclusions in quartz of rhyolites, porphyries, and granites from mining areas in Japan were investigated. Rhyolites contain various types of glass inclusions and high density liquid inclusions but they are devoid of highly saline polyphase inclusions. Glass inclusions in rhyolites are generally transparent, indicating rapid cooling. Some of them contain dendritic crystals or small euhedral crystals. Porphyries considered to be genetically related to mineralization generally contain highly saline inclusions besides many liquid inclusions. Porphyries also contain many glass inclusions, but in general they are completely or partially devitrified. Size effect on devitrification is remarkable. Larger glass inclusions are easily devitrified, while smaller inclusions occasionally remain transparent. In most granitic rocks highly saline inclusions are not found, but high density liquid inclusions are common. Small granitic stocks related to mineralization, however, are rich in polyphase inclusions. Since fluid inclusions in igneous rocks are generally very small, the ordinary heating-stage and freezing-stage microscopic methods are generally inapplicable. However, microscopic observation of these fluid inclusions, especially of polyphase inclusions, gives valuable information on the possible change in ore-forming fluids during their migration from the center of mineralization. It is inferred that highly saline fluids represented by poly-phase inclusions would have been released from silicate melts in late magmatic stages, and that dilution of these highly saline fluids would be an important factor controlling the ore deposition.

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