The Ryushoden mine is situated in the so-called "Tohoku Hokkaido Green Tuff Area" in the northernmost parts of Japan. The ore deposits are stratabound disseminated cinnaber deposits which occur in tuffaceous sandstone belonging to the Ryushoden formation of the Miocene Konomai basin.
Prospecting of the Ryushoden mine by the present owner, Hokushin Mining Co., Ltd., was initiated in November 1960, and thus four ore bodies have been discovered. Because of this successful exploration, the mine has become one of the leading mercury producers in Japan. The prospecting program has been carried out through the following four periods.
During the first period (1960-1962), the writers made geological and mineralogical studies on the known ore deposit of small scale and adjacent area. Consequently, the northern extension of No. 1 ore body was discovered in 1961. During the second period (1963-1965), the writers re-examined characters of "F₂ fault" with respect to the No. 1 ore body, and reached a conclusion on the F₂ fault to be of pre-mineralization. This judgment led the writers to the discovery of No. 2 ore body along the F₂ fault in September 1963.
The third period (1966-1968) was spent. to establish the exploration criteria, in summarizing all the available data of geological studies, geochemical and geophysical works, and operation research for drilling span. Some useful indicators was obtained through this work. They are ;
1) to trace the N-S main structure of the basement rocks,
2) to comfirm extension of the known ore deposits along the horizon of the conformable host rocks and also to find of siliceous rocks on the N-S structure,
3) to find gravity anomaly belts on surface,
4) to use geochemical mercury anomaly on drilling core, and
5) to apply operation research for drilling span to decide an area for prospecting and also mining limits.
On the basis of the above procedures, No.3 and No.4 ore bodies were discovered during the final period (1968-1970). The exploration criteria are so far valid for the ore deposits of the Ryushoden mine at this moment. An attension may be called on the criteria to be improved with further development of the new ore bodies.