Oxygen isotopic composition of vein quartz from the Akenobe mine, southwest Japan and the origin of the ore solution

A preliminary report

Abstract

¹⁸O/ ¹⁶O ratios and homogenization temperatures of fluid inclusions were determined for five quartz samples collected from veins in the Akenobe polymetallic deposits, southwest Japan. The δ ¹⁸O values (relative to SMOW) of quartz range from +0.4 to +1.8‰. The homogenization temperatures of fluid inclusions, which are estimated to be primary, are in a range of 260 to 320°C. δ ¹⁸O values of -8.0 to -4.5‰ are calculated for thermal waters equilibrated with the quartz by utilizing the above data and quartz-water isotopic fractionation. The low δ ¹⁸O values of thermal waters indicate that the waters are primarily of meteoric origin.
A positive correlation between oxygen isotopic composition of thermal waters and homogenization temperatures of fluid inclusions is observed. This correlation suggests mixing of ¹⁸O-poor, low-temperature meteoric water with ¹⁸O-rich, high-temperature magmatic water. If a simple mixing model is assumed, the proportion of meteoric waters in the thermal waters is as high as 80 to 100%. Some modification of isotopic composition of waters may also have been produced by reaction with hot rocks.