Geology of the Silica Deposits in the Tamba District

Abstract

In the Tanba district, which occupies the area of thick sedimentary accumulations in the "Chichibu" Palaeozoic geosynclinal terrain, accompanied by some intercalations of "diabase and schalstein", there have been found many silica deposits mined for important refractory raw materails in Japan. The deposits occur sporadically on the surfaces of the submarine flow sheets of the "diabase", and assume thick lenticular forms. Hematite impregnations are usually recognized in the rocks of the upper next horizons of the sheets. The silica rocks are composed of fine-grained red (or green) cherty rock fragments and coarsegrained white matrix made of segregated vein .quartz. Spherical, subspherical, and/or angular fractures define the outline of the fragments of the cherty rocks. From these critical and some other geological occurrences as well as from the petrographic features of the silica rocks, it is concluded that the deposits were formed by the segregation of the vein quartz in the fractured cherts which were precipitated originally as the colloidal silica (with small amounts of iron oxides) from some postvolcanic emanations from the surface of the submarine"diabase"flows. Judging from the structural relations of the deposits, the geologic age of the quartz veins is older than the Mesozoic granite intrusions in the district.