Mechanism of formation of the Furutobe kuroko ore deposit

Ore deposit and exp'oration at the Furutobe mine(II)

Abstract

Furutobe mine is located in the northern part of Akita Prefecture. The deposits of the mine is known as one of the typical Kuroko deposit occurring in the acidic pyroclastic rocks of Miocene age. Ore deposit of the mine is classified into siliceous ore zone of the lower part and stratiform ore zone of the upper part. Siliceous ore zone is divided into lower siliceous ore zone occurring in rhyolite and upper siliceous ore zone occurring in rhyolitic pyroclastic rocks. Stratiform ore zone consists of following three types of ores. Type A, the typical Kuroko deposit composed of powdery yellow ore, yellow ore, semi-black ore, black ore, ferruginous chert in ascending order. Type B, disseminated ore deposit which is a loose aggregate of sulphide minerals and argillized tuff and Type C, graded ore deposit composed of fragments of ores and rocks. The following processes of mineralization are suggested. Lower siliceous ore zone is fissure-filling type origin. Constituent minerals of this zone were formed from the ascending hydrothermal mineralizing solution. Upper siliceous ore zone was formed in the accumulation of explosion breccia covered by the thin layer of unconsolidated sediments near the sea bottom. Type A deposit of stratiform ore zone is sedimentary ore deposit formed on the sea bottom. Type B was formed in the unconsolidated layer of tuff by filtration and absorption of sediments and clays. On the other hand, Type C is the submarine sliding deposit transported and redeposited along the channel of footwall rocks by local submarine landsliding or slumping during and after mineralization.