鉱染硫黄鉱床毋岩の変質過程の一考察

抄録

It is well known that the chief alteration products in the country rocks of sulphur deposits are opal, sulphur, kaolin, alunite and iron sulphides. According to the writer's study, almost all country rocks of impregnation sulphur deposits in Japan are two-pyroxene andesites and their pyroclastics. The discussion of the alteration processes leads to the following conclusions:
1) By the action of volcanic gases or solutions, the country rocks, whatever their original appearances may be, are ultimately bleached to whitish colours.
2) By percolating solfataric gases or solutions, materials composing the rock-forming minerals of the country rocks, such as alumina, alkalies, alkali earths and iron, are carried away as soluble sulphates and the spaces from which minerals were removed are filled up with various secondary minerals.
3) At first, colloidal silica derived from the original rocks will occupy the spaces.
4) Or this time, sulphur may fill up the space, if the condition is favourable to deposit sulphur.
5) Alunite is the alteration product of feldspars in the country rocks, and it occurs not only in the original feldspars but also around them. In the process of being carried away as soluble sulphate, some feldspars which are partially altered will be changed to alunite by continuously acting solutions or gases.
6) When gases and solutions percolate into the porphyritic rocks, they will attack and alter first the phenocrysts and then the groundmass. This is the reason porphyritic textures remain in the altered country rocks.
7) Secondary minerals in the altered country rocks are deposited repeatedly at the same alteration stage.