Geologically Shikoku district is located mostly in the Inner zone, and partly in the Outer zone of southwestern Japan. In the Inner zone there occurs no deposit of geological importance, especially with regard to Au, Ag and Cu. On the contrary, many bedded cupriferous pyrite deposits are found in the crystallines schist zone, the Chichibu Paleozoic zone and the Mesozoic zone, which lie on the south of the Median Dislocation Line. The ores of these deposits have respective characteristics in their ore structure, which, in turn, affect the milling process substantially.
In the crystalline schist zone, the ores of the deposits originated in the spotted schist zone are treated with relative ease because of the relatively coarse pyrite which is 0.6 to 0.08 mm in diameter, whereas those of the Nakashichiban anticline, the vicinity of the cape of Sada, the Yakushi anticline, the Mt. Akaboshi anticline, etc., are difficult to treat because of fine-grained pyrite, 0.3 to 0.03mm. As we go southward in the order of the Chichibu Paleozoic zone and the Mesozoic zone, the pyrite gets as fine as 0.2 to 0.02mm and the dressing of the ores becomes more difficult.

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In a previous paper, the writer described the geology and ore deposits, with special references to the, paragenetic relation of ores and altered rocks in some iron, lead-zinc and fluorspar deposits of hydrothermal origin, of northeastern Japan, for the purposes of considering the problems of hydrotherma dolomitization of limestone and the alteration of skarn. In this article, the petrography, chemica analyses, spectrographic analyses, X-ray examinations and differential thermal analyses of limestonee and the dolomitized equivalents in these deposits are given. The results obtained from these studie are summarised as follows:
1) The dolomite-rocks in these deposits are similar to those of hydrothermal origin in foreign countries in respect to megascopic features and physical and chemical characteristics; in some respects, however, distinct differences are recognized, i, e., a) the original rock of hydrothermal dolomite-rocks in these deposits is thermally metamorphosed crystalline limestone; b) the dolomite-rocks are rich in variations of rock types, for instance, there is a highly porous dolomite-rock whose maximum porosity amounts to 36 per cent; dense "marble"-like dolomite-rock with equigranular texture is also found.
2) During dolomitization, somewhat later than the conversion of calcite to dolomite, the formation of apatite in the Akatani mine and the Tide mine, and the formation of Mg(?)-chlorite and chrysotilen in the Sen-nin mine have taken place, together with the introduction of silica.
3) Chemical changes during dolomitization are additions of MgO, FeO, Fe₂O₃, CO₂ and MnO and the. leaching of CaO, AL₂O₃ and SiO₂ in the Akatani mine and the Iide mine. But, in the Sen-nin mine, the later addition of SiO₂ is always marked.
4) In accordance with the changes of major components, spectographic analyses show the additions: of Be and F, and the decreases of Ba, Pb, Sr, Ag, Na and Ti.
5) The dolomitization of limestone wallrock in these deposits is considered to be volume-for-volume replacement, judging from the relict structure of dolomitized limestone. The increase of porosity in the course of dolomitization is, therefore, to be regarded as a result of excessive leaching of lime carbonate by the dolomitizing solution; the pores are "solution cavities".
6) The data of chemical analyses and DTA curves show that the dolomite-rocks in these deposits. consist. largely of low-iron dolomite, but it is noteworthy that, in the Igashima mine, white creamcolored dolomite-rock is composed of ferroan dolomite (para-ankerite).

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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.

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