The origin of mineralizing fluids responsible for the Hishikari vein-type epithermal Au deposits was studied on the basis of the hydrogen isotopic ratio (delta D) of the inclusion fluid from vein quartz and adularia. The origin of hydrothermal fluids was estimated by combination of the present delta D values and the oxygen isotopic ratios (delta(18)O) previously reported by Shikazono and Nagayama (1993). The water in the fluid inclusions was extracted by means of decrepitation of quartz at 500 degrees C. Hydrogen was obtained by reduction of the collected water with Zn shot at 450 degrees C. The delta D values were determined by mass spectrometer. The delta D values of inclusion fluid obtained from quartz range from -61 to -114 parts per thousand. These are significantly lower than the FD value of the thermal water presently venting from the Hishikari deposits and that of local meteoric water. Hydrogen isotopic fractionation between water and amorphous silica, which might have initially precipitated from the hydrothermal fluids at least partly, is not a probable cause of this isotopic depletion, while some water might have been released from the initial hydrous amorphous silica during recrystallization to quartz observed presently. Thus, a part of ore fluids for the Hishikari deposits is supposed to have been originated from the water having anomalous delta D values of lower than -100 parts per thousand. Such D depletion cannot be caused by simple oxygen-shift of meteoric water or by contribution of magmatic volatiles. The delta D values of water released from the shale samples of the Shimanto-Supergroup, a major host to the Hishikari veins range from -132 to -148 parts per thousand. Therefore, the anomalous delta D values of inclusion fluids from some vein quartz and adularia suggest that the water released from hydrous minerals of the sedimentary basement rocks by dehydration or the groundwater isotopically exchanged with sedimentary rocks at elevated temperatures during circulation, partly contributed to the hydrothermal fluids responsible for the Hishikari deposits.
