Studies on the Matsuo Sulphur and Iron-Sulphide Deposits (1)

Geology of the Vicinity of the Mine

Abstract

The writer has studied the genesis of the Matsuo sulphur deposits since 1952. In this paper the geology of the mine will be described. Conclusions are briefly summarized as follows.
(1) The district is composed of, going from the lower to the upper, Kitamatagawa formation, Kurosawa formation, Hachimantai formation (the country rock of the deposits), Akagawa lava and agglomerate, Ishiyama lava, Daikokumori agglomerate, Onagane lower and upper lavas, Chausudake lava, Maemoriyama lava and some terrace and alluvial deposits. The Kitamatagawa formation seems to be correlated with the Hashiba formation, which is upper Miocene or lower Pleistocene in age. The ages of the Kurosawa and the Hachimantai formations are still unknown. The rocks overlying these formations are very probably products of Pleistocene volcanic activity. The geological map and columnar section are shown in Fig. 1 and 2.
(2) Except for the basal rocks and the cone-shaped volcanic products, the main part of above-mentioned rock series dip gently toward the east. The semi-lunar low hills of Ishiyame and Kamota seem to have been formed by step faults, and the swampy zones embraced by them are probably due to the presence of underlying soft rocks formed by post-volcanic alteration.
(3) The succession of igneous products in this area is as follows: acidic dacite (Kitamatagawa formation)→more or less basic dacite with quartz-bearing andesite and two-pyroxene andesite (Kurosawa formation)→two-pyroxene andesite (Hachimantai formation, Akagawa and Ishiyama lava)→olivine-bearing two-pyroxene andesite (Daikokumori agglomerate, Onagane, Chausudake and Maemoriyama lava).
(4) After the great eruption of the two-pyroxene andesite, the sulphur and iron-sulphide mineralization took place and continued to the early stage of volcanic activity of the olivine-bearing two-pyroxene andesite.