THE HISTORY OF VEIN FORMATION DETERMINED BY 40Ar/39Ar DATING OF ADULARIA IN THE HOSEN-1 VEIN AT THE HISHIKARI EPITHERMAL GOLD DEPOSIT, JAPAN

The Hosen-1 vein has a symmetrical banded structure within the Hishikari epithermal gold deposit in Japan. The vein can be divided into four macroscopic bands. We used Ar-40/Ar-39 dating of adularia by incremental step heating to determine the duration of mineralization in the individual bands of the vein and the history of vein formation. Plateau ages were obtained from all adularia samples, and no significant excess Ar-40 is recognized from inverse isochrons. The derived age spectra suggest that thermal overprinting of the epithermal system did not affect the plateau ages, although early bands must have been reheated by the thermal pulses associated with later bands. The Ar-40/Ar-39 ages of adularia indicate that vein formation occurred episodically in distinct periods separated by intervals of about 30,000 to 110,000 yr. The Hosen-1 vein grew in at least six increments at the -20-m level and at least four increments at the +62.5-m level. The total duration of mineralization from the margin to the center of the Hosen-1 vein was about 260,000 yr, during which time the temperature of the hydrothermal fluid decreased from 190 degrees to 210 degrees C to less than 170 degrees C.