The Hishikari Au-Ag Epithermal Deposit, Japan: Oxygen and Hydrogen Isotope Evidence in Determining the Source of Paleohydrothermal Fluids

Abstract

Quartz, adularia, and clay minerals from low-sulfidation epithermal veins at Hishikari (1.3–0.7 Ma) were analyzed for their oxygen and hydrogen stable isotope compositions to establish the source of paleohydrothermal waters. Veins consist predominantly of adularia and quartz, are banded, and exhibit a range of recrystallization and replacement textures indicative of precursor silica polymorphs and platy calcite. Adularia/quartz ratios decrease progressively from older, sulfide-mineral–bearing, outer bands to late-stage, drusy, barren quartz in the central portions of veins. Gold and silver occur predominantly as electrum, but smectite and vermiculite that occur as disseminated zones or thin bands ( 500 ppm) and silver (>150 ppm) concentrations in veins occur within bonanza zones ( 0.5). Positive correlation between Au-Ag and the adularia/quartz ratio, the presence of platy calcite, Au-bearing smectite and vermiculite, and constant vertical elevation of ore zones are permissive evidence that boiling controlled ore deposition. Quartz and adularia in gold- and silver-bearing parts of veins have δ 18O values that range from 7 to 9 per mil and 5 to 6 per mil, respectively. Late-stage drusy quartz usually has lower δ 18O values of 6 to 7 per mil. Gold-bearing smectite and vermiculite have δ 18O values that range from 8 to 13 per mil and δ D values that range from –55 to –85 per mil. Late-stage clay (smectite and kaolinite), a common alteration product of adularia in veins, has δ 18O values (4–11‰) similar to those from early-stage clay but δ D values that are distinctly lower (–90 to –130‰) than early-stage clay. Oxygen isotope equilibrium temperatures between quartz and adularia are 220° to 250°C and 170°C. Average homogenization temperatures of fluid inclusions from early quartz and adularia are about 180° to 220°C. Quartz and adularia results indicate that the ore-forming hydrothermal fluids at Hishikari had δ 18OH2O values that range from about –4 to 0 per mil, 18O enriched compared with present-day meteoric water (–7‰). The Au-bearing smectite and vermiculite have calculated δ 18OH2O and δ DH2O values of 1 to 4 per mil (at temperatures of 200°C) and –65 to –40 per mil, respectively. Calculation of δ 18OH2O variation as a consequence of boiling indicates that closed-system boiling was more likely than open-system boiling. Assuming isotopic equilibrium between columnar adularia and adjacent fine-grained quartz, then the lower calculated equilibrium temperatures of 170°C are consistent with a decrease in pressure causing steam loss and the formation of amorphous silica from fluids that were silica supersaturated. The calculated δ 18OH2O and δ DH2O values (from quartz, adularia, and early smectite) suggest that mineralizing solutions were a mixture of magmatic and meteoric waters. Alternatively, water-rock calculations indicate that these values could derive from heated modern meteoric water that had undergone isotope exchange during circulation through basement rock (water/rock ratio 0.1–3). The overlap in δ D values of magmatic water (–30 to –70‰) and meteoric water (–50‰) at Hishikari makes it impossible to clearly differentiate between these two processes. δ 18Oquartz values of veins have a north-south zonation, and values increase to the north, possibly reflecting a source or fluid flow direction. The latest-stage drusy quartz from the central portions of veins has calculated δ 18OH2O values between –4 and –6 per mil, close to values (–7‰) of present-day meteoric water. Late-stage clays (smectite and kaolinite) have low δ DH2O values (–110 to –75‰) compared with present-day meteoric water (–50‰) that cannot be adequately explained by fluid mixing or water-rock interaction. δ D values of water extracted from fluid inclusions in adularia (–110‰) and quartz (–70‰) at Hishikari are not considered to be reliable for estimating δ DH2O values of paleohydrothermal waters.