小坂鉱山の鉱床群とバラ森火山岩類

抄録

The so-called "green tuff" formations derived from the submarine volcanism in the Miocene epoch are developed in the Kosaka mining district. They are characterized by the acidic effusive volcanics, and are divided into the Baramori volcanic rocks (lower part) and the Akamori volcanic rocks (upper part).
There are several ore deposits in the Kosaka mining district wihch belong to the "Kuroko" type. They are summarized into two types of occurrence : the network-disseminated ore type of pyrite-chal-copyrite (Keiko) in the Baramori volcanic rocks, and the bedded type (including breccia-block type) consisting of three kinds of ores, namely, pyrite-chalcopyrite (Oko), spharelite-galena-barite (Kuroko) and gypsum. They are restricted to the upper most horizon of the Baramori volcanic rocks.
Judging from the following geological facts-1. consistency of ore horizon, 2. absence of structural control of ore bodies by the Akamori volcanic rocks (absence of the cap rock), 3. occasional discordant relation of ore bodies with the Akamori volcanic rocks, 4. weak alteration of the Akamori volcanic rocks as compared with that of the Baramori volcanic rocks-the writer concludes that ore deposits of the Kosaka mine were formed in connection with the Baramori volcanic activities. The Baramori volcanic rocks consist of the lava flow of the Baramori rhyolite and the Motoyama breccia which is locally overlying the Baramori rhyolite. From field and underground observations, it is considered that the Motoyama breccia was derived from the Baramori rhyolite mass by the phreatic explosions.
It is noteworthy that all ore deposits of the Kosaka mine occur only where the Motoyama breccia are accumulated. This fact suggests strongly an intimate relation between the phreatic explosion and the ore deposition. In the previous paper on the Motoyama ore deposits, one of the ore deposits in the Kosaka mine, the writer reported that the condition of the deposition of Kuroko was substantially the same as that of the sedimentation of the Motoyama breccia.
From these facts, the writer considers that the bedded ores were formed syngenetically subsequent to phreatic explosions while network-disseminated ore were being formed in the Baramori volcanic rocks.