Fluid inclusions and oxygen isotopes studies reveal the hydrothermal history of quartz vein formation from the Yunosawa Stream-Shirai River conjunction (YS), Yunosawa midstream fumarole zone (YM) and Nagatozawa Stream (NT) near the Toyoha Ag-Pb-Zn deposit.<BR>Vapor-rich primary and pseudosecondary fluid inclusions are dominant both in the YS and NT quartz samples. Some of the vapor-rich inclusions coexist with liquid-rich inclusions. The presence of these vapor-rich and liquid-rich fluid inclusions suggests that boiling occurred during quartz vein formation both at YS and NT. Homogenization temperatures and salinities of primary fluid inclusions are shown as follows:<BR>173-178°C and 6.1-6.7 wt percent NaCl equiv. (YS)<BR>204-251°C and 0.9-1.1 wt percent NaCl equiv. (YM)<BR>152 to 265°C and 0.0-1.1 wt percent NaCl equiv. (NT)<BR>Depth determinations for the inclusions with evidence of boiling are 75-79 m and 130 m below the paleowater table at YS and NT, respectively. The origin of high-salinity fluid inclusions at the YS might be a consequence of locally extreme boiling rather than the steady introduction of a deep brine.<BR>Oxygen isotope values of vein quartz range from 3.0 to 3.4, 2.4-2.8 and 2.5-4.3 permil at YS, YM and NT, respec-tively. Combined with the fluid inclusion data, calculated oxygen isotope values of hydrothermal fluid range from -10.5 to -10.1, -7.0 to -6.5 and -10.4 to -9.6 permil at YS, YM and NT, respectively.<BR>We interpret the data to indicate that the hydrothermal fluids responsible for quartz vein formation at YS YM and NT resulted from a mixing of meteoric groundwater with deep hydrothermal fluids. The fluids both at YS and NT consist of ascended vapor-rich fluids and steam condensates. The ratio of meteoric groundwater to deep fluids at YS and NT could be either the same as or higher than those for Toyoha earlier mineralization.<BR>Since Pliocene (2.4Ma), quartz veins formed through episodic hydrothermal events showing variable temperature and fluid composition both at YS and YM along Yunoswa Fault. Quartz veins at NT formed through a hydrothermal event related to Toyoha's earlier mineralization.