The Nebazawa deposits are epithermal, gold and silver-bearing quartz vein type deposits. They occur in E-W trending southward dipping steep fractures in the welded tuff member called Katashinagawa rhyoritic rocks of Paleogene age.
The study of the geology and ore deposits of the mine has disclosed following facts and suggestions which could be useful guides for current and future exploration of the mine.
1) Three stages of mineralization were identified, they are, from older to younger, base metalliferous mineralization (Stage I), auriferous mineralization (Stage II, Ag/Au≥10) and argentiferous mineralization (Stage III, Ag/Au≥100). The latter two mineralizations are important in economic sense.
2) The assay contour is fully affected by low angle east dipping faults called No. 1, No. 2 and No. 3 fault which intersect the veins diagonally. High grade part of the vein is along the hangingwall side of the fault and barren part along the footwall side. This suggests possible important role of these faults as controlling factor of convection system of ground water at the time of ore deposition.
3) Vein fractures are interpreted as high angle reverse type faults initiated by the differential block movement between relatively subsided northern block and southern relatively upheaved block. This block movement seems to be triggered by or related to the formation of N-S trending cauldron subsidence of Miocenc Kinugawa rhyolitic rocks to the east.

View full abstract

The Toyoha mine is located in the geothermal area of Southwest Hokkaido, Japan. The geothermal surveys revealed the presence of very high heat flow areas widely spread over the ore deposit in the Neogene system. The heat flow amounts to 25.7 HFU (heat flow unit), about ten times higher than we expect generally in the Green-tuff region of Japan. Notwithstanding, few hot springs and vapor fumaroles are found around the area and neither prospect tunnels nor drillings have ever disclosed any conspicuous underground hot-water reservoirs.On the basis of the geothermal data obtained from the borehole temperature measurements, it is proved that the temperature distribution has a clear tendency to increase southeastwards and vertically downwards. The source of the high heat flow concerned is presumed as a kind of hot dry rock.
The mining operation will be concentrated on the conspicuously high heat flow areas in the future. Thus it is an important problem how to lower the temperature at the underground working site. For the purpose of obtaining the basic data for the future exploration and development in hot areas, the present study aims to draw a possible limit of exploitation by computing the heat balance in tunnel.

View full abstract

Previous studies of geochemical prospecting in the Hokuroku region have revealed that acid lavas underlying kuroko deposits are in general significantly depleted in Na ₂O in comparison with those on which no kuroko deposits exist. The Na ₂O content of the footwall acid lavas may thus provide a practical geochemical indicator which enables us to delimit a promising area for prospecting around the known kuroko deposits.
The Na ₂O variation in the footwall acid lavas has extensively been examined in the vicinity of the Uchinotai-Uwamuki deposits, Kosaka mine and of the Fukazawa deposits, Hanaoka mine. The following results have been obtained:
1) The most economical threshold to delimit the promising area can be set at Na ₂O = 0.36 wt% in the Uchinotai-Uwamuki area and at Na ₂O = 0.32 wt% in the Fukazawa area.
2) The promising areas thus defined are two or three times as large as the presently known extents of these deposits.
3) Some new orebodies have successfully been discoverd by this method.

View full abstract

Silver orebody has been found in the southeast part of the Mozumi mine. This silver orebody is located in the silicified zone on the top of the South No. 8 lead-zinc orebody, which is composed of normal Mokuji-type ore in the lower part.
Silver minerals recognized under microscope are native silver, argentite, canheldite and small amount of pyrargyrite and silver-bearing tetrahedrite. Tin minerals as cassiterite and stannitea re notably associated with these silver minerals.
By the detailed mapping of the -320 M sublevel, rocks exposed were divided into several geological units and they were checked with chemical analysis. It has turned out that the parts called "grey quartz" have high grade silver content.
Further exploration is still on the way but total about 24, 000 tons of the silver ore reserve containing Ag: 178 g/t, Pb: 0.79% and Zn: 4.3% have been already estimated.
Characters of mineralization of this silver ores are left for further study, but at least it can be said for the exploration that every top of any lead-zinc orebody should be checked for silver mineralization especially in case silicification is recognized.

View full abstract