The Tertiary gold deposits of the Hauraki area are predominantly epithermal vein types containing mineral assemblages of quartz, calcite, pyrite, sphalerite, galena, chalcopyrite, acanthite and native gold (electrum)._x000D_
Sulphosalt, telluride (eg hessite), seleniferous sulphide and selenide minerals are important locally._x000D_
Studies of wall rock alteration mineralogy, fluid inclusions in gangue minerals, and sulphur isotopes of sulphide minerals indicate that the ore forming fluids had the following parameters: pH, near neutral at depth but acidic near surface; temperature, predominantly 320 - 270C for base metal dominated assemblages, 280 - 200C for precious metal dominated assemblages and generally below 200C for late stage barite; apparent salinity, six to zero equivalent weight per cent NaCl; and CO2 concentration, generally aroundone mole per cent CO2. There is fluid inclusion evidence for boiling during mineralisation in some deposits._x000D_
Moderate values off S2 and FO2 are indicated for base metal and mixed base metal- precious metal assemblages of most deposits by the predominance of pyrite + chalcopyrite over other iron-bearing phases; primary bornite is absent, whereas pyrrhotite, hematite and magnetite rarely occur in the veins. The largest gold- silver deposits formed from solutions in which the sulphur species in solution were predominantly reduced. Near surface environments and some deep, predominantly base metal, deposits were formed from solutions with approximately equal concentrations of oxidised and reduced sulphur species.