Toyoha Mine, which is situated in Sapporo City, Hokkaido and belongs to a so-called Western Hokkaido metal mineralized zone, is a epithermal vein deposit the principal minerals of which are galena, sphalerite and pyrite.
The geology of the surrounding area is composed of the Toyoha formation and the Yunosawa formation both of which belong to the miocene and these are intruded by porphylite dikes and quartz-porphyry dikes.
Geological peculiarities of the mine is that it is in a dome-structure at the base of Shakotan Peninsula. In the vicinity of the mine, the existence of wavy foldings with the axis direction of ENE is very conspicuous.
Another regional significance is that the mine areas are in the geothermal area and therefore the rock temperature in the underground workings is considerably high and hot springs are gushing out from fissures along the mineralized veins.
The deposits are made up of galená-sphalerite-rich ores, pyrite rich ores and the ores abundant in rhodochrosite. These deposits cover the such scattered mineralized area, with the width of 3kmfrom south to north, by the length of 5km from east to west, as Iburi, Nagato, Ooezawa, Motoyama, Tundoo, Yunosawa and so forth.
The mineral veins are divided into the one with E-W direction and the other with NW-SE direction. The former is supposed to have been deposited in the earlier stage and the latter in the later stage.
The early mineralization shows strong crystallization and carries galena and sphalerite, while the later mineralization shows crystallized rhodochlocite in large quantities accompanied with galena and sphalerite forming banded structure and with small quantities of chalcopyrite and silver minerals.
The result of X-ray examination on sphalerite gives generally a low lattice constant for sphalerite of earlier stage and a high lattice constant for that of later stage. From the same examination the earlier sphalerite is found to have the low iron content and the later sphalerite to have the high iron content and sometimes some amount of marmatite.
The result of the test on the filling temperatures of liquid inclusions at the mine shows the temperature range of 150-200°C, and 170°C is judged to be the optimum temperature for galena and sphalerite to crystallize.
Based on these geological data 33 drill holes with the total length of 20, 000m have been drilled since 1962 on the Ooezawa mineralized zone and its vicinity with the main targets on the later shearing ore vein within the geological horizon especially recommended to be explored.
This exploration has resulted in discovery of Sooya-hi, Oshima-hi and several other new ore veins and 45, 000 tons per month operation was begun in 1969 and 60, 000 tons per month operation is to be expected in 1971.

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The epithermal ore deposit of Numanoue and Takatama Mine which has been worked mainly for gold and silver is thought to have deposited following continuous igneous activities, while the epithermal ore deposits of Oe and Kinkaseki Mines which have been worked principally for base metals accompanying gold and silver are understood to have deposited after geological structure movements.
In this paper, the writer would present a report for these mines in regard to the following:
i)The location where mineralization occurred.
ii)The relation between first stage mineralization (rock alteration stage) and igneous activities.
iii)The sequence of gangue and ore minerals.
iv)The correlation among various stages of repeated mineralization.
The mineralization appears to have repeated in all of these cases, but judging from the sequence of gangue minerals, ore minerals and minor elements, the mineralization in the cases of Takatamaa and Oe Mines is thought to have repeated from the identical ore magma while in the cases of Numanoue and Kinkaseki it is thought to have repeated from the different one.
In all of these ore deposits, gangue and ore minerals of low, middle and high temperatures occur in paragenetic form showing a xenothermal tendency which is also true in the case of minor elements.
In conducting a more detailed study on the sequence of gangue minerals, ore minerals and minor elements of these ore deposits hereafter, a consideration necessary should be on their geochemical and geophysical conditions.

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The investigation of sulfide systems have been advanced during the past twenty years. As the results it has been made clear various interesting matters about the phase relation among sulfide minerals as follows:
1. Determination of the stable P⋅T field of sulfide minerals.
2. Stable mineral assemblages.
3. The relationship between composition of solid solution and temperatures (application as geothermometer).
4. Existence of minerals being only stable at low temperature or in limited temperature range (existence of secondary minerals formed by solid state reaction).
However, there have been many unsolved problems about sulfide system from a standpoint of interpretation for the natural process of ore deposition. Therefore, the sulfide studies during the last several years tend to cover the experiments mentioned below, presenting phisico-chemical data which are useful for an explanation of ore genesis.
a) Phase equilibrium at low temperature.
b) Influence of sulfur fugacity on the mutual relationship between sulfide minerals, and composition of solid solution.
c) Extension to quaternary or more component sulfide system.
d) Phase relations between sulfide and oxide, carbonate or silicate systems, and their stable mineral assemblages.

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