Within the old Hauraki goldfield, the Tui Mine is 130 km NNW of the Taupo volcanic zone and thermal water is found close to the mine. Mineralized quartz veins occur in hydrothermally altered, Tertiary andesites which overlie Mesozoic greywackes. The Tui vein paragenesis is (1) pyrite+chalcopyrite, (2) galena+sphalerite, (3) minor pyrite and chalcopyrite, and (4) minor hematite, barite, cinnabar, kaolinite and carbonates. Stable isotope studies indicate temperatures of formation of 400+ or -80 degrees C (5 sulphide pairs, stage (1)), 320+ or -60 degrees C (18 sulphide pairs, stage (2)) and 150+ or -30 degrees C (barite-water, stage (4)). delta S 34 values of the sulphides range from +5 to -3 per mil (CDT) and show a 2 per mil decrease with time. The vein mineralogy suggests a corresponding decrease in pH and increase in oxidation potential of the ore fluid. Thus late barite with delta S 34 = +16 per mil fixes the delta S 34 (sub sigma S) = +16+ or -1 per mil for the system. Hence at 350 degrees C H 2 S and HSO 4 in the ore fluid would have delta S 34 values of +5 and +24% respectively, indicating a probable magmatic plus greywacke source for the sulphur. Te Aroha thermal water shows an oxygen isotope shift of 4 per mil relative to meteoric water (delta O 18 -5.5 per mil SMOW, delta D -30 per mil SMOW). Waters extracted from sphalerite and quartz fluid inclusions and calculations from isotopic studies of quartz and kaolinite indicate ore fluid values of delta O 18 0+ or -0.5 per mil, delta D -17+ or -1 per mil (stage (2)) and delta O 18 -1.0+ or -0.2 per mil, delta D -24+ or -1 per mil (stage (3)). An ultimate meteoric origin for the ore fluids is postulated. Published Pb isotope data (Cooper and Richards) suggest the Pb to be derived from the greywackes. Many comparisons with present day geothermal systems can be made.