Oxygen isotope measurements of quartz in the tin-mineralized, Permian Mole Granite, New South Wales, support a model of progressive mixing of magmatic water and surface-derived water of meteoric origin in mineralizing hydrothermal circulation systems.The magmatic water, observed as multiphase fluid inclusions in hydrothermal quartz-biotite-orthoclase-topaz-bearing pegmatites, had a high temperature (550 degrees C), high salinity (55-75 wt %), and a delta 18 O SMOW value of about 8.7 per mil. The initial magmatic delta 18 O SMOW value of the granite is estimated to be about 9 per mil.The meteoric water, observed as liquid-rich fluid inclusions in quartz from late veins of quartz-sphalerite-galena, had a low temperature (250 degrees C), low salinity (3-5 wt %), and a delta 18 O SMOW value of-15 per mil. Permian meteoric water is expected to be light isotopically, owing to a near polar (70 degrees ) palcolatitude for the New England fold belt.Quartz from cassiterite-quartz-chlorite-sericite ores has a wide range of delta 18 O values, from -2.8 to +6.7 per mil. This is intermediate between the values for quartz deposited from magmatic and from meteoric waters. The fluid inclusions in these samples have homogenization temperatures of about 300 degrees C and a wide range of salinities (25-35 wt %). The variation in isotopic composition observed for quartz is attributed to a fluctuating isotopic composition of the hydrothermal solutions resulting from variable degrees of mixing of magmatic and meteoric water. Cassiterite has a narrow range of delta 18 O (-5.9--7.2ppm), which suggests precipitation of cassiterite over a narrow range of mixing of the hydrothermal fluids. Three cassiterite-quartz mineral pairs, with textures indicating synchronous crystallization, imply cassiterite-water fractionations of -2.2 per mil at 300 degrees C and -4.1 per mil at 370 degrees C.Hydrogen isotope data are less systematic due to postcrystallization isotopic exchange of hydrous minerals and the presence of younger fluids in secondary fluid inclusions in quartz.