足尾鉱山の流紋岩体の構造と鉱床との関係について(1)

抄録

The funnel or basin shaped rhyolitic complex occupies almost the whole area of the Ashio mining district, and its surface distribution makes an imperfect ellipse-like outline, reaching about 3.3km in its minor axis and about 4.4 km in major axis, and the volcanic activities of these volcanic complex are supposed to have taken place in the late Tertiary period.
The rhyolitic complex is composed of rhyolite massive flow and rhyolitic pyroclastics, such as volcanic breccia, tuff, tuff breccia, lapilli tuff and also welded tuff. They are accumulated alternately, forming nearly horizontal beds. The lower part of the complex consists mainly of rhyolitic lava, intercalating some rhyolitic volcanic breccia and tuff. In the upeer parts are found alternations of tuff breccia, lappili tuff and welded tuff, the last one being most conspicuous. Thus, the volcanic complex of the Ashio mining district seems to form a stratovolcano.
The basal breccia, named by the writer, occurs along the contact surface between the rhyolitic complex and the Palaeozoic sediments or sometimes the late-Mesozoic granodiorite of basement complex, and is composed mainly of angular or subangular fragments of basal rocks, such as chert, clay slate, sandstone, siliceous slate, schalstein and granodiorite. Judging from the mode of occurence, it may be concluded that the basal breccia is not of explosion, but of talus origin, formed on the steep slope of the wall of the basin.
The funnel or basin shaped hollow, filled up with rhyolitic rocks, may represent a caldera, formed by the depression preceding the volcanic explosion. The central vents of this rhyolitic volcano are supposed to be situated near the pyrophyllitized and brecciated zone of Tenguzawa, and the original crater, now indistinct by erosion, might be located also at the same place.
Fissure veins are formed abundantly in rhyolite flow and welded tuff, but scantly in tuff and tuff breccia. Ore shoots of veins are mostly localized in favorable beds such as rhyolite or rhyolitic welded tuff, which lies nearly horizontally, and then the shoots may have a flat elongation. The localization of ore shoots may be attributed mainly to the deflection of each vein according to the different physical characters of each bed. Bonanzas of so-called "Kajika type" are formed sometimes in tuff and tuff breccia, only when the fissure veins swarm closely, though it is not the case where an isolated vein is present.