Kuroko Deposits Occurring in Mudstone at the Matsuki Mine

Abstract

Kuroko deposits occurring in mudstone at the Matsuki mine, called "A" ore deposits, are mainly composed of very high grade yellow ores. Seven unit orebodies have been found. Sulphide orebodies are divided into the coarse porous and the fine banded orebodies. Both occur within the black clay zone that has originated from the mudstone. The deposits have a regular lateral zoning, that is, the sulphide ore zone, the gangue minerals zone and the black clay zone from its center to periphery. Each zone reveals also the following lateral variations: i) Yellow ore → black ore for the sulphide ore zone and ii) Barite lens → carbonate lens → siliceous lens + nodule for the gangue minerals zone. Fossils of arenaceous foraminifera are found in the interior of orebodies as well as in the hanging-wall and foot-wall of the deposits. The existence of benthonic foraminifera in the mineralized zones and the mode of occurrence of orebodies suggest strongly that "A" ore deposits were formed in the seawater-bearing unconsolidated mudstone, just after sedimentation of the lower half of M2 mudstone. Relatively reduced environment resulted from the weak preceding volcanism may be the main cause to have formed this peculiar deposit as compared with common Kuroko deposits.