Oxygen and Hydrogen Isotope Study of Veins in the Hishikari Epithermal Deposit, Japan: Origin of Ore-Forming Fluids

Quartz, adularia and clay minerals from low-sulfidation epithermal veins at Hishikari were analyzed for their oxygen and hydrogen stable isotope compositions to establish the source of paleohydrothermal waters. Positive correlation between Au-Ag and adularia: quartz ratio, the presence of quartz pseudomorphs of bladed calcite, and Au-bearing smectite and vermiculite is indicative, but not definitive, evidence for the control of boiling on ore mineralization. Oxygen isotope equilibrium temperatures between quartz and adularia are about 220¦C and average homogenization temperatures of fluid inclusions vary between 190 to 210¦C. The calculated d18OH2O and dDH2O values (from quartz, adularia and early smectite) are consistent with paleohydrothermal fluids that were a mixture of magmatic and meteoric waters, or alternatively, the values could derive from heated meteoric water which had undergone isotope exchange during circulation through basement rock. Overall, d18Oqtz values in veins increase toward the north, most likely reflecting palaeo-fluid flow (source).