Granitic Pegmatite of the Umanotani-Shiroyama Quartz-Feldspar Mine, Shimane Prefecture, Southwest Japan

Abstract

The pegmatite deposits at the Umanotani-Shiroyama mine, the largest producer of K-feldspar and quartz in Japan and characterized by a simple mineralogy, dominated by quartz and K-feldspar with minor muscovite and plagioclase, are hosted in the ilmenite-series Masago Granite.The ore zones are classified into the following three zones based on the distance from the host granite inwards: (1) marginal; (2) transitional; and (3) central ore zones. Crystal grain size increases in this order with the resultant occurrence of gigantically grown crystals of quartz in the central ore zone. The evidences presented here reveal that the pegmatite deposits were formed during the latest stage of crystallization and differentiation of the granitic magma being responsible for the Masago Granite. They include:(1) close temporal and spatial association of the granite and the pegmatite deposits (about 95 to 90 Ma), (2) common occurrence of macroscopic “graphic intergrowth" displayed by quartz and K-feldspar in orebodies, (3) common occurrence of melt inclusions trapped in “ore quartz" and “ore K-feldspar", (4) common occurrence of perthites observed in “ore K-feldspar" and (5) inheritance of oxygen isotopic signature, especially of quartz, from the surrounding biotite granite. The δ18O values of quartz, whether in the related igneous rocks or in ores, are almost the same, around +12 ‰, while those of K-feldspar in the igneous rocks is around +11‰ and those in ores are significantly depleted in 18O, about +7 to +8 ‰. It might be due to the difference in the exchange reaction rate of oxygen between quartz and K-feldspar. Abundant two-phase (liquid+gas) fluid inclusions are trapped in “ore quartz" and “ore K-feldspar", as well as in the related igneous rocks. Their presence strongly indicates that hydrothermal fluids, possibly admixture of magmatic fluids released from the Masago Granite and circulating meteoric water, entered to the magmatic-hydrothermal system and circulated through the orebodies mainly during the later phase of the pegmatite formation. Measured homogenization temperatures of two-phase inclusions trapped in “ore quartz" are in the following ranges: 250 to 400 ℃ in the marginal ore zone; 230 to 370 ℃ in the transitional ore zone; 240 to 340 ℃ in the central ore zone. Also suggested is that late circulation of hydrothermal fluids lasted for not so long time as to affect the δ18O values of quartz within the magmatic-hydrothermal system, resulting in escape of the fluids from the system through ubiquitously observed quartz±muscovite veins and fissures.