Timing and duration of hydrothermal activity in the Oligocene Hamada cauldron, SW Japan: Evidence from K-Ar ages of sericites

Crystallinity, chemical compositions and K-Ar ages of sericites in highly-s ericitized granites and associated fissure-filling veins were examined to d elineate the timing and duration of the hydrothermal activity in the Oligoc ene Hamada cauldron in the San-in district, SW Japan. Sericite separates (< 2 mum) from the highly-sericitized granites consist mainly of 2M(1) polytyp e having high crystallinity and low Kubler indices of 0.22-0.35, while thos e in the fissure-filling veins have lower 2M(1)/1Md ratios and crystallinit y, and high Kubler indices of 0.29-0.35. This suggests that the sericites i n highly-sericitized granites were formed at a higher temperature than the vein sericites. Sericites from the highly-sericitized granites of the Kumogi pluton give K- Ar ages of 30.0 +/-0.7, 30.4 +/-0.7, 30.6 +/-0.7, 30.6 +/-0.7, 32.1 +/-0.7, 32.3 +/-0.7 and 33.0 +/-0.7 Ma (1 sigma), while those of the central pluto ns, 33.8 +/-0.7 and 33.8 +/-0.7 Ma. Sericites in the fissure-filling veins of the Kumogi granite give K-Ar ages of 31.0 +/-0.7, 31.5 +/-0.7, 31.6 +/-0 .7, 31.7 +/-0.7 and 32.3 +/-0.7 Ma. Biotite separates from the fresh Kumogi granite give K-Ar ages of 31.7 +/-0.8, 32.0 +/-0.8, 32.7 +/-0.7 and 33.5 /-0.7 Ma. The K-Ar age data revealed that the hydrothermal alteration began at about 33 Ma and ended by about 30 Ma and that the period of sericite al teration was nearly synchronous with the cooling of the granite intrusions in the Hamada cauldron. Despite intense hydrothermal alteration, the Oligocene granitoids have not accompanied with any economic base metal mineralization. The bulk chemical analyses of sericite separates in the veins indicate that the post-magmatic fluids were originally barren in heavy metals.