On Spilitic Rocks of the Shimokawa Mine and their Genetical Relations to the Ore Deposits

Abstract

The layered cupriferous pyritic deposits of the Shimokawa mine have a characteristically simple geological structure, where the foot-wall consists of thick spilitic and diabasic rocks with numerous thin intercalations of graywackes and slaty rock, and the hanging wall consists of slaty rocks.
These basic rocks were previously regarded as dyke swarms intruding the Hidaka. formation at high angles, and the deposits as epigenetic ones having replaced the sheared slaty rocks near the boundary with the basic rocks.
Recent underground and surface geological studies have revealed that some of the basic rocks are subaqueous lava flows with a pillow structure, and the ore deposits rest on the uppermost spilite flows intercalated with thin sedimentary layers. Immediately after the end of the ore deposition, when no sedimentary cover was existent yet, diabase sheets intruded between the ore body and the foot-wall, often branching into the ore body forming a distinct chilled margin.
These facts clearly indicate that the deposits are one of the most typical syngenetic sedimentary-exhalative deposits originated from a spilitic magma.