2019 Volume 69 Issue 1 Pages 13-21
The Hishikari epithermal gold-silver deposit consists of three deposits, the Honko deposit, the Sanjin and the Yamada deposit. Some of veins in the Honko and the Sanjin deposits terminate at the surface of unconformity between sedimentary basement rocks of the Cretaceous Shimanto Supergroup and volcanic rocks of Quaternary age, named the Hishikari lower andesites, even though the others continue from the basement to the volcanic rocks. In this paper, we discuss the relationship between quartz veins and the surface of unconformity in the Honko and the Sanjin deposits based on the underground development data from -20ML to 110ML. The veins not controlled by the surface of unconformity are widely distributed in the Honko and the Sanjin deposit. They were formed by normal and strike-slip faulting. The stress magnitude of the fault activities related to the formation of these veins would have been small because the fault displacement is relatively small. Most of quartz veins controlled by the surface of unconformity are distributed in the Hosen and Zuisen vein systems and the Sanjin deposit. The strongly argillized Hishikari lower andesites overlying the Shimanto Supergroup formed selectively in the Honko and the Sanjin deposits and the argillized zone seems to have a great effect on the formation of these veins. The strongly argillized layers might have been formed before around 0.9 Ma prior to the occurrences of quartz veins controlled by the surface of unconformity.
