The Hokuroku district, approximately 40km in W-E length and 20km in N-S, lies within the green-tuff region of the northeastern Japan, where several large "kuroko" deposits, such as Hanaoka, Shakanai, Kosaka and others, are located.
Miocene formations characterized by pyroclastic rocks and violent volcanic activities in the Nishi kurosawa and Onnagawa stage deposited thickly in the central part of the district and thinly in the marginal part.
Detailed geological survey and prospecting by drillings have been carried out for a long time at the Ha-naoka and Kosaka mines, especialy the Miocene volcanism and the stratigraphy of mudstone layers correlated by foraminiferas are investigated by systematic and accurate prospecting methods.
Details of geological structure, stratigraphy and volcanism are given in three areas of Hana-oka, Shinsawa-Shigenai and Kosaka.

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The Toyoha mine, one of the largest mine of silver, lead and zine in Japan, is situated at the western part of Jozankei Spa, about 40km southwest of Sapporo City, Hokkaido.
Tne district under investigation is geologically composed of Miocene series including the Koyana-gizawa, Motoyama, Nagato, Sanbonmata and Ôayama formations in ascending order. These Miocene formations are covered with lavas of dacite and andesite of Quarternary in age. On the tectonic movements of the district in Miocene time, the sequence of events may be summarized as follows : (1) The block movements at the Koyanagizawa and Motoyama stages, which have brought about the fra ctures with a trend of NNE-SSW. (2) The marine transgression resulting from regional subsidence at the Nagato stage. (3) The upheaval movement at the Sanbonmata and Ôeyama stages.
The country rocks of the Toyoha ore deposits consist mainly of epiclastic sediments (sandstone, mudstone and conglomerate) belonging to the Motoyama formation, and pyroclastic rocks including tuff, tuff-breccia and andesitic lava of the Nagato formation.
The Toyoha ore deposits represented by the fissure-filling veins of epithermal character have emplaced in tension cracks, and they may be divided into the two stages of fissuring and mineralization. Namely, the earlier veins are characterized by the trend of E-W and by the abundance of galena, sphalerite and pyrite, and of quartz as a gangue, and the later veins with a trend of NW-SE are characterized by the abundance of manganoan calcite as a gangue.
As a result of detailed geological mapping and structual studies, the following new interpretation for the mechanism of fissure-formation may be possible, and this has been demonstrated by the current explorations.
It is cosidered that the formation of fissures containing ores has been closely related to local volcanism of andesite at the Nagato stage. (1) Andesitic lavas and their pyroclastics have erupted along the fissures trending NNE-SSW which had been formed by block movements at the Koyana-gizawa and Motoyama stages. (2) The depression of older rocks have occurred by these violent volcanism. (3) Sets of steeply dipping tension cracks, together with low-angle shear planes, have been formed around the area of depression. (4) Ore-forming fluids have ascended along the cracks, and vein materials have deposited. (5) Excellent ore veins have been found by current diamond drilling in the virgin areas north of the known mineralized tracts.

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