Studies of mineral assemblages, homogenization temperatures and salinities of fluid inclusions, and hydrogen and oxygen isotopic ratios for the pyrophyllite mineralization of the Nishiyama-higashi deposit, Southwestern Japan, were combined to determine the environment of formation and the origin of the hydrothermal solution responsible for this miner-alization.
Six mineralogic zones were formed in the deposit by a hydrothermal solution passing through an E-W fissure in Cretaceous dacitic volcanic rocks. The constituent minerals of the zones are: Weakly altered zone: quartz ≥ plagioclase, K-feldspar, chlorite, muscovite and pyrite; Zone I: quartz and muscovite ≥ pyrophyllite and hematite; Zone IIa: pyrophyllite and quartz ≥ hematite; Zone IIb: pyrophyllite, kaolinite and quartz ≥ dickite and hematite; Zone III: alunite and quartz ≥ pyrophyllite and kaolinite ≥ muscovite, diaspore and corundum; Zone IV: pyrophyllite, kaolinite and quartz ≥ diaspore; Zone V: corundum, diaspore, quartz, pyrophyllite and kaolinite. The paragenetic sequence of the formation of the deposit from early to late stage is: formation of corundum-diaspore (around 370°C)→formation of diaspore-pyrophyllite (325-280°C)→formation of pyrophyllite-hematite (335-260°C)→formation of kaolinite-dickite-alunite-quartz-hematite (about 190°C). The formation of kaolinite-dickite-alunite-quartz-hematite is associated with boiling of the hydrothermal solution at around 190°C. The salinities of the hydrothermal solution associated with the formation of the Nishiyama-higashi deposit range mainly from 0.0 to 1.1 wt% NaCl equivalent. Si0
2 concentration of the hydrothermal solution was less than 10
-2.3 moles/kg H
2O for the formation of corundum-diaspore, 10
-2.04-10
-1.96 moles/kg H
2O for the formation of diaspore-pyro-phyllite, 10
-2.08 -10
-1.90moles/kg H
2O for the formation of pyrophyllite-hematite, and less than 10
-2.08 moles/kg H
2O for the formation of kaolinite-dickite-alunite-quartz-hematite. The pHs of the hydrothermal solution forming the Weakly al-tered zone, Zone IIb and Zone III were 4.6 - 6.8, 1.6 - 4.5 and 1.6, respectively. The hydrothermal solution forming Zones IIb and III was very acidic. The range of total sulfur concentration of the hydrothermal solution was 10
-2 - 10
-5moles/kg H
2O and the major sulfur species in the solution was HSO
4.
On the basis of the data on hydrogen and oxygen isotopic ratios of pyrophyllite, dickite and quartz, the hydrogen and oxygen isotopic ratios of the hydrothermal solution responsible for the formation of pyrophyllite-hematite are -60‰ and 0‰, respectively. On the other hand, the hydrogen and oxygen isotopic ratios of the hydrothermal solution responsible for the formation of kaolinite-dickite-alunite-quartz-hematite range from -70 to -50‰ and from -7 to -4‰, respectively. The hydrothermal solution for the Nishiyama-higashi pyrophyllite deposit originated from meteoric water in a volcanic environ-ment. As hydrothermal activity progressed, the water/rock ratio and the ratio of meteoric water in the hydrothermal solution increased. The hydrothermal system of the Nishiyama-higashi pyrophyllite deposit was a paleogeothermal system in Creta-ceous time.
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