Granitic pegmatite of the Umanotani-Shiroyama quartz-feldspar mine, Shimane Prefecture, southwest Japan.

The evidence presented reveals that the Umanotani-Shiroyama pegmatite deposits are of a typical granitic pegmatite in origin. It includes: (1) close temporal and spatial association of the ilmenite-series Masago granite with pegmatite deposits, (2) macroscopic, graphic intergrowth displayed by quartz and K-feldspar in the ore, (3) melt inclusions trapped in ore quartz and ore K-feldspar, (4) perthites observed in ore K-feldspar, and (5) inheritance of oxygen isotopic signature, especially of quartz, from the surrounding biotite granite. The presence of abundant two-phase (liquid and gas) fluid inclusions trapped in ore quartz and ore K-feldspar, as well as in those of the related igneous rocks, strongly indicates that hydrothermal fluids entered the magmatic-hydrothermal system and circulated through the ore bodies during the later phase of the pegmatite formation and were likely responsible for the formation of microcline in some of the ore K-feldspar and for its striking 18-O isotope depletion.
The evidence presented reveals that the Umanotani-Shiroyama pegmatite deposits are of a typical granitic pegmatite in origin. It includes: (1) close temporal and spatial association of the ilmenite-series Masago granite with pegmatite deposits, (2) macroscopic, graphic intergrowth displayed by quartz and K-feldspar in the ore, (3) melt inclusions trapped in ore quartz and ore K-feldspar, (4) perthites observed in ore K-feldspar, and (5) inheritance of oxygen isotopic signature, especially of quartz, from the surrounding biotite granite. The presence of abundant two-phase (liquid and gas) fluid inclusions trapped in ore quartz and ore K-feldspar, as well as in those of the related igneous rocks, strongly indicates that hydrothermal fluids entered the magmatic-hydrothermal system and circulated through the ore bodies during the later phase of the pegmatite formation and were likely responsible for the formation of microcline in some of the ore K-feldspar and for its striking 18-O isotope depletion.