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 220C and average homogenization temperatures of
fluid inclusions vary between 190 to 210C. The calculated
18OH2O and DH2O 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, 18Oqtz
values in veins increase toward the north, most likely reflecting palaeo-fluid
flow (source).