Geology and Ore Deposits of the Ryushoden Mine, Hokkaido

Abstract

The Ryushoden mine is located at about 6 km southwest of the Monbetsu City in Hokkaido. The studied area is the northwestern margin of the sedimentary basin of the Konomai formation of Miocene in age, which belongs to the so-called green-tuff region in the northeast Hokkaido. The results of the investigation on the mercury deposits of Ryushoden mine are described as follows :
(1) The Ryushoden formation that bears the mercury deposits is correlated to the middle to upper facies of the Konomai formation.
(2) The mercury deposits are mostly of disseminated cinnabar type in the sandstone and conglomerate members of the Ryushoden formation. Some are found as networks or veinlets along the shear zone with NE trend.
(3) The deposits are spotted continuously along the shear zone with N-S trend which was due to the upwarping of the basement rocks.
(4) The scale and quality of the deposits are controlled by shapes of the black siliceous rocks and positions of the green tuffaceous sandstone which constitutes the hanging-wall of the mercury deposits.
(5) The mineral assemblages indicate that the ore solution was initially weakly alkaline and then became weakly acidic, and the crystallization of cinnabar might have taken place at the weakly acidic stage. The cinnabar was dispersed in the present host rocks primarily by vaporization at this stage.
(6) The associated alteration is represented by silicification, carbonatization, argillization (illite-montmorillonite→kaolinite) and dissemination of iron sulphides.
(7) The exploration criteria being valid at present are i) to trace the shear zone of N-S trend which is the most important structural feature of the area, ii) to find the black siliceous rocks which might be an ore-bringer, and also iii) to seak for a permeable bed of the dark grey sandstone capped by the unpermeable green tuffaceous sandstone.