Abstract
The Hishikari gold deposits are of an epithermal quartz-adularia vein type, in which two major mineralized areas have so far been confirmed: the Honko and the newly discovered Yamada areas.
The wall rocks in the Honko area are composed of sediments of pre-Neogene Shimanto supergroup and volcanic rocks of Quaternary age, whereas these in the Yamada consist of the latter.
These wall rocks are more or less suffered from hydrothermal alteration in the vicinity of the vein systems. In this paper, four alteration zones are established, based on the mineral assemblage of clay minerals in volcanic rocks and relationship between formation of hydrothermal clay minerals and temperature in active geothermal systems. These are from the center to marginal zone of the mining area: IV quartz-chlorite; III quartz-chlorite (or sericite)/interstratified clay minerals; II quartz-smectite; and I Cristobalite-smectite.
In general, the zone IV occurs at the deepest and central parts and forms the principal alteration zone with which most of the productive veins of the Honko area and the lower half of the Yamada vein system are directly associated.
The zone III and II envelop the mineralized center in the Honko area, while the former is associated with the upper half of the Yamada deposit.
The zone I and least-altered rocks surround these three zones in turn.
In addition to the above, sporadic acidic alteration (quartz-kaolinite and quartz-alunite) is superimposed on them at the higher levels.
