1993 Volume 43 Issue 238 Pages 117-127
The Metal Mining Agency of Japan conducted a gold exploration program in the Seta area, Eastern Hokkaido, Japan. In this area, drilling and a comprehensive surface survey including geochemical and geophysical methods were completed.
The gold mineralization in this area is associated with old mercury and kaolinite workings. Judging from its occurrence, it is of, the hot spring type, and is hosted in the Miocene Horokapiribetsugawa Formation and Pliocene Ashoro Formation. The formation age obtained from K-Ar dating of a quartz-adularia vein is 1.2±0.7Ma, which means that the Seta area is the youngest gold mineralized area in the Monbetsu-Kamishihoro Graben.
In surface exposure, there are large clayey alteration areas that can be divided into four zones. The kaolinite zone, smectite zone, interstratified clay zone and zeolite zone are arranged from the center of this area to the margins, in this order. Silicification, acid-leaching and hydrothermal brecciation are sporadically observed and centrally contained within the kaolinite zone. The kaolinite zone has a "mushroom" shape which is widely expressed at shallow levels and gradually shrinks to the margins of the hydrothermal breccia and quartz veins at more deeper levels.
Quartz veins exposed at the surface have small widths of around 20 cm and low gold contents of less than about 2g/t. The width and gold content of these veins increase with depth. A quartz-adularia vein found at 181.2-182.8m depth in drill hole 4MAHB-6, attains 110cm in width with a grade of 15.3g/t gold and 63.3g/t silver. High grade quartz veins, greater than 10g/t gold, occur in the smectite altered andesite beneath the kaolinite zone. This andesite, which is the major host rock of the gold-silver mineralization, lies completely below the surface.
As these veins are located in the smectite zone and show slightly low fluid inclusion homogenization temperatures ranging from 176°to 218°C, it can be inferred that the bonanza gold mineralization exits at more deeper levels.
