The Stratigraphic Horizon of the 'Kuroko' Deposits in Hanaoka-Kosaka Area, Green Tuff District of Japan

Abstract

The ore-deposits of the Hanaoka mine and the Kosaka mine, about 18km east of the former, belong to the ‘Kuroko’ type. The ores of the ‘Kuroko’ ore-deposits chiefly consist of four kinds of mineral assemblages: sphalerite-galena-barite, chalcopyrite-pyrite, pyrite, and gypsum. They are generally called black, yellow, sulfide, and gypsum ore respectively. The Hanaoka and the other ‘Kuroko’ ore-deposits have been thought to be massive deposits formed by epithermal replacement. It has become clear in the Hanaoka and some other ‘Kuroko’ore-deposits, however, that the massive form may be boudinage resulting from the folding of the bedded ‘Kuroko’ ore-deposits.
The Hanaoka-Kosaka area belongs to the so-called green tuff region of the Miocene, Tertiary. The stratigraphy of this area can be summarized as follows:
Tobe formation
—unconformity—
Tsutsumizawa formation
Hanaoka formation
—local unconformity—
Hotakizawa formation
—unconformity—
Menaichi formation
—unconformity—
Paleozoic formation
Basic volcanic activity prevails in the Hotakizawa formation and the underlying formations, while acidic volcanic activity is predominant in the Hanaoka formation and the overlying formations. The Hanaoka formation is composed of the Baramori accessory tuff breccia, the Yagaratai acidic lava flow accompanied by intrusive rock facies and an alternation of lapilli tuff and shale in descending order. The first two are developed locally in the uppermost horizon. It seems that the Yagaratai acidic lava flow was extruded in the latest stage of pyroclastic activity in the Hanaoka stage.
All the ore-deposits in the Hanaoka-Kosaka area consist of the ‘Kuroko’ type and the vein type, and which occur only in definite stratigraphic horizons. The former, that is to say, occurs only on the Baramori accessory tuff breccia or directly on the Yagaratai lava flow where it is not accompanied by the tuff breccia, concordant to the bedding of the country rocks. The latter, however, occurs in the Hanaoka formation and the underlying formations across the bedding of the country rocks. The same relationships of the stratigraphic horizons in which ore-deposits of the two types occur have been recently recognized also in other districts.
Judging from these facts and the well known low temperature mineralization in the ‘Kuroko’ type deposits, the writer considers that the ‘Kuroko’ ore-deposits were formed by the material derived from submarine volcanism.