Ore deposit and exploration at the Matsuki Mine (I)

with special reference to the ore deposit in the M₂ mudstone

Abstract

Matsuki deposit is located in the southwestern part of Odate basin and is known as one of the Kuroko deposits occurring in the acidic pyroclastic rocks and mudstone of Miocene age. Sulphide orebody is mainly composed of yellow ore, pyrite ore and small amount of black ore. There are following three mineralized horizons; the lower part of M₂ member (called as "A" ore deposit), the boundary part of M₂ and T₃ member ("B" ore deposit) and the upper part of T₃ member ("C" ore deposit). "A" ore deposit is composed of several unit ore-bodies which are pan-shaped high-grade yellow ore. "A" ore deposit is divided into the coarse porous orebody and the fine banded one on the basis of modes of occurrence and mineral assemblage. The country rocks of "A" ore deposit are strongly argillized. Clay minerals occurring in this argillized zone are abundant interstratified Mgchlorite/saponite and minor interstratified sericite/montmorillonite and montmorillonite. In "A" ore deposit, a lateral zonal distribution of minerals, sulphide ore zone, gangue minerals zone, black clay zone from center to periphery of the mineralized zone is recognized. Sulphide ore zone is divided into yellow ore sub-zone and black ore sub-zone from center to outside and gangue minerals zone is also divided into barite lens sub-zone and siliceous lens sub-zone from center to outside. Carbonate lenses are sometimes found in the boundary of barite lens subzone and siliceous lens sub-zone. "A" ore deposit is thought to be formed in the unconsolidated M₂ mudstone after deposition of the lower part of M₂ mudstone. The exploration was conducted on the basis of above-mentioned geological and mineralogical investigation and consequently some high-grade orebodies have been found in the lower part of M₂ mudstone.