The Stock-work Siliceous Ore Deposits at the Kosaka Mine

Abstract

The Kosaka mine located in the northern part of Akita Prefecture has been well known for its typical kuroko mineralizations.
A number of unit ore bodies consisting of kuroko (black ore) and oko (yellow ore) are bedded in form and are underlain by the so-called white rhyolite that forms lava domes of various scales.
In recent years, many keiko (siliceous ore) bodies have been discovered within the white rhyolite. They are stock-work ores having cylindrical shapes as a whole, standing vertically, and grading into the overlying bedded ore bodies. There are two kinds of siliceous ore bodies: zinc-type and copper-type. The zinc-siliceous ore commonly grades into the copper one in lower horizons. The transition is well recognized by the variation in Cu/Pb/Zn or Cu/Pb + Zn ratios in the ores.
The volcanic activity of the white rhyolite is closely related to the kuroko mineralization, and the siliceous ore presumably represents the conduit of ore solutions.