On the Kuroko-Type Ore Deposits of the Kurosawa Mine, Fukushima Prefecture

Abstract

Ore deposits of the Kurosawa mine comprise six groups of small ore deposits, half of which are composed mainly of gypsum, and the rests are of composite ore bodies of gypsum, iron sulphide ore, yellow ore and black ore. The mine has been exploited for more than 20 years for gypsum at one time and for sulphide ores at another, following the successive discoveries of each different, ore body. Recent monthly production of crude pyritic ores amounts up to 2, 500 tons.
The ore deposits occur in the white gray fine-grained tuff, the uppermost bed, of the Kurosawazawa formation, and are underlain in some cases by rhyolite (R2) lava beds. The ore deposits are overlain by black mudstone. The next upper formation is characterized by dacite and tuffaceous beds with graded bedding and by intercalated basaltic agglomerate, and is in turn overlain by the pyroclastic formation which was deposited by the eruption of rhyolite (R3).
In the Sanjin pyritic ore deposits occur the zone (1) of very fine-grained pyrite and clay, the zone (2) of fine-grained pyrite and less amount of clay, and the zone (3) of medium-grained pyrite in the ascending order. Graine size of pyrite increases and the amount of clay decreases gradually in the ascending order. The zone (2) contains many small nodular balls of massive pyritic ore, while the zone (1) contains less abundant but much larger boulders of pyritic ore.
All ore deposits are deformed and folded irregularly by later tectonic movements, but the successive stratification of gypsum zone, pyrite zone, yellow ore zone and black ore zone in ascending order can be generally recognized in each ore body, Above the ore are locally observed hematite-pyrite rich zones with or without a intervening mudstone or tuff bed. Disseminations and stringers of hematite-siderite-pyrite mineralization are also observed especially in shear zones.
The sericitization is characteristic in the sulphide ore zones, while the chloritization is remarkable in the gypsum zone. The montmorillonitization seems to be restricted to the pyroclastic beds of the upper horizons than the hanging wall black mudstone bed.