Geology and Ore Deposits of the Furutobe Mine

Abstract

The Furutobe mine, one of the most productive Kuroko deposits in Japan, is located inthe northern part of the so-called "Hokuroku Mineralized Zone" in Northeast Japan. The area is composed of preTertiary, Miocene, and Pleistocene formations. The Miocene formation is subdivided into three stratigraphic units, namely lower, middle and upper formations. All of the ore deposits of the mine are found in the upper part of lower formation which is mainly composed of lavas and pyroclastic rocks of acidic composition. Based on modes of occurrence and mineral assemblages, each ore deposit is divided into several zones. Some deposits are bedded and others are of fissure-filling or dissemination type. As the result of field and microscopic work, the writers conclude that the main parts of these ore deposits were formed by submarine exhalative and hydrothermal sedimentary processes, genetically related to volcanic activities. It is also suggested that the ring-and vent-type structures which are found in the deposits were formed by the mineral solution ascending after the formation of the main sequence of the ore deposits. Later sulfide veins are also recognized.