Horizontal strain rate in relation to vein formation of the Hishikari gold deposits, Southern Kyushu, Japan

The Hishikari deposits comprise the Honko (Main), Yamada, and Sanjin deposits. The horizontal strain in the direction (approx. N40°W) normal to the general NE-SW strike of the Hishikari vein system was calculated, based on the measured total vein widths to the distance along three crosscuts. Veins were assumed to accompany no significant fault displacement in the calculation. Veins in the eastern and the middle parts of the Honko-Sanjin deposits spatially occupy 3.2% and 1.3%, respectively, and veins in the Yamada deposit occupy 6.7%. Significant local variation of strain is observed in some areas. Reported K-Ar ages on adularia-quartz veins indicate the duration of vein opening to be 2.6×10⁵ yr in the Honko-Sanjin deposits and 5.9×10⁵ yr in the Yamada deposit. Horizontal strain rates were calculated to be 5.0-12×10^-8 yr^-1 through the Hishikari deposits. The calculated strain rates at the Hishikari deposits are roughly comparable to the regional horizontal strain rate in the Recent. Widespread extensional movement in southern Kyushu seems to have been able to provide sufficient strain for the formation of the Hishikari deposits, rather than contribution of local movements.