1979 Volume 29 Issue 155 Pages 131-145
The Tochibora ore deposit of the Kamioka mine is mainly composed of skarn-type lead and zinc orebodies formed by the replacement of tightly folded limestone. Two main types of ore deposits are distinguished. One is what is called "Mokuji", representing pyrometasomatic deposits associated with skarn minerals, and the other is "Shiroji" being mesothermal replacement deposits. The shape and extent of orebodies are controlled by original shape of limestone, complicatedly folded in general. This fact serves as one of the major keys to exploration.
As a result of the recent survey carried out in the lower part of the Tochibora ore deposit, the geological structure and the characteristics of the mineralization have been ascertained as follows.
1) In the lower part of the Tochibora ore deposit, two folding structures called "Inishi No. 3 syncline" and "Inishi No. 10 syncline" can be clearly recognized, as is the case in the upper and middle parts.
2) At -300 m level, the plunge of these synclinal axes is 15°-20 °SW, changing smoothly from 40°-45° SW, which is generally observed in the upper and middle parts.
3) It is evident that the axis of the Inishi No. 10 syncline is reversely plunging to NE at -370 m level after the above-stated smooth change, to form a basin structure.
4) Inishi No. 3 syncline is not clearly recognized between -300 m level and -370 m level.
5) The horizon composing the main series of the Tochibora ore deposit in the lower part is a chain of orebodies of No. 1B-No. 4B-No. 4A-No. 3B-SNo. 1-No. 6B-No. 8C(5)-No. 9-No. 10.
6) Main mineralized zone is situated between the barren skarn (Mokuji) zone and the unmineralized limestone zone.
7) In terms of the Ag: Pb: Zn ratio (so-called π-ratio) of the individual orebodies constituting the Tochibora ore deposit, zonal distribution is clearly recognized; i.e., from west to east, No. 1 type zone (π-Pb<25) No. 2 type zone (5<π-Pb<40)→No. 3 type zone (15<π-Pb<60)→No. 4 type zone (45<π-Pb).
8) The zonal distribution of ore types is also recognized, corresponding with the above zoning of π-ratio; i.e., Cu-bearing Mokuji ore→Pb-Zn Mokuji ore (size of orebodies is small to moderate)→Pb-Zn Mokuji ore (size large)→high-grade Shiroji ore.
Exploration based on the above-mentioned knowledge has been successfully carried out in the lower part of the Tochibora ore deposit, and led to the discovery of new orebodies and of the continuation of known orebodies.
