On the "Kajika" in "Rhyolite" of the Ashio Mine

Abstract

The "Kajikas", namely, bonanzas in rhyolite of the Ashio Mine, may be divided into the following three types according to the relations of geological structure.
1. Bonanzas have been formed due to replacement of chert blocks, and large xenoliths in rhyolite.
These bonanzas were deposited by the same factors as the Arikoshi bonanzas and the Renkeiji, i.e., chert was replaced by mineralizing solution which ascended along the folding axes and the intersections of shear veins.
2. Bonanzas in the shattered and brecciated portions:
a. Ore shoots are localized by vein intersections ; —Vein intersection is one of the general factors in forming bonanzas. The Honguchi Kajika was formed along the intersections of shear veins and more than two tension fractures connecting with each other.
b. Bonanzas have been formed at shattered and brecciated zones developed frequently near tension fractures.
c. Bonanzas situated in brecciated rock bodies ; —G. ASANO supposed that the pseudoconglomerate, named by him, may have been formed by the local kettle-depression in the very center of rhyolite, but, it is possible that these rock bodies were formed by shattering and brecciation near the intersections of master shears, 2nd order ones, and tension fractures, and then the interspaces of breccias were intruded and filled with rhyolitic pebble dyke.
The bonanzas are formed in such shattered bodies.
d. Bonanzas are formed near the intersections of master shears and shears of second order ; —The veins in Ashio are divided into two groups of shear veins, and one of their directions is N 45°E, and the other N 75°W, and shears of 2nd order were commonly developed from them.
3. Ashio rhyolite mass consists possibly of alternations of flow, tuff breccia and tuff, and most part of the tuff is of welded-tuff type one. Veins deviate or branch into numerous narrow veins in tuff or tuff breccia, and bonanzas ore bodies are widely found in them.