Mechanism of Mineralization and Prospecting Principle of 'Rôseki' Ore Deposits

Abstract

The main components of 'Rôseki' ore are pyrophyllite and quartz, accompanied by kaolin, boehmite, diaspore, corundum or andalusite. In the typical 'Rôseki' ore deposits such as distributed in the Mitsuishi area, Okayama Pref., pyrophyllite is the most dominant of constituent minerals. Such high. temperature minerals as corundum and andalusite are rarely found. The 'Rôseki' ore bodies form a complex ore bed with porous silica ore bodies. The bed is apparently conformably covered by an impermeable sheet or bed, and rather gradually merges into footwall.
The depth at the time of mineralization is estimated to be from n×10² m. to 2 km., and the H₂O pressure to be about n×10² bars. The boehmite-bearing ore may have been formed at temperatures between 100° and 300°C., and the diaspore-bearing ore, 300°-400°C., At the end of mineralization, the temperature may have decreased to a point on the geothermobar as suggested by the presence of gibbsite in Mitsuishi area.
The typical 'Rôseki' ore deposits are connected with fumaroles through shallow type 'Rôseki' deposits or through Ugusu-type alunitesilica deposits and replacement deposits of sulphur. From the common features of those deposits as well as the behaviour of hydrothermal fluid in the geothermal area suchh as at Larderello, Wairakei and Matsukawa, the mechanism of mineralization of 'Rôseki' deposits is. inferred as follows:
(1) There was a combination of an impermeable cover, a reservoir for hydrothermal fluid and a footwall with cracks at the depth of n×10²-2×10³m.(2) The hydrothermal fluid ascending along the crack met with descending ground water at the entrance of the reservoir or a little below, forming a strongly acidic hydrothermal solution which leached the pre-existing rocks, leaving a porous. silica ore. (3) Owing to the thermal convection in the reservoir, the temperature rose to 200-300°C., and the acidity of solution was weakened by dissolving metallic ions. And these resulted in the crystallization of pyrophyllite, kaolin or boehmite to form 'Rôseki' ore bodies.
In order to prospect a hidden 'Rôseki' ore body, the first step will be core drilling to get informations, about the combination of an impermeable cover, a complex ore bed of 'Rôseki' and silica. This will. be followed by exploration by galleries to ascertain the form, tonnage and quality of 'Rôseki' ore deposits. With those ideas, an prospecting has been performed with success at Imazaki, Okayama Pref. The plan at 40m. level, the vertical section subparallel to the general strike and the dip section of the Imazaki deposits are presented as Fig. 3 (a), (b) and (c), respectively.