Exploration at Toyoha Mine

Abstract

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.