1980 Volume 30 Issue 161 Pages 137-152
The Ezuri kuroko deposits were discovered near Odate city, northern Honshu, as the result of systematic exploration activities from 1975 to 1977. This report summarizes the processes to the discovery of the deposits.
In the Hokuroku district (ca. 1, 500 km2), northeastern part of Akita Prefecture, large kuroko deposits have been known to concentrate along the eastern and western margins of the Hokuroku basin. Most of the exploration activities in 1960's were, therefore, focused on the peripheries of the basin, and several new kuroko deposits were found successfully. However, the discovery of the Fukazawa deposits in the central area of the basin in 1969 indicated the possibility of finding new deposits in the vast interior of the Hokuroku basin.
An extensive examination of the mode of volcanism and hydrothermal alteration of the dacite lavas below kuroko deposits in the Fukazawa mining area in 1974 and 1975 revealed the following points:
(1) The underlying, pre-mineralization, Neogene dacite lavas distributed widely in the Hokuroku district are discriminated into four types, D 1 lava dome, D 2 lava flow, D 3 lava dome and D4 lava dome on the basis of their mode of occurrence and eruption stage.
(2) Mineralization of the Fukazawa deposits is closely related to the latest D4 lava dome activity.
(3) K2O and MgO contents in the D4 dacite increase toward the ore deposits, while Na2O and CaO show the opposite trend.
(4) Distinct alteration haloes are observed in the country rock dacite around the orebodies with respect to the chemistry and magnetic susceptibility.
Based on the data obtained in the Fukazawa area, the Ezuri area (7 km west of the Fukazawa area) was selected as a promising target and the exploration work commenced in 1975. Firstly, a systematic drilling program was carried out at 400 m interval to investigate the geologic structure. The program disclosed the relatively wide distribution of thick D4 dacite lava domes at the horizon about 200 m below sea level. The drill cores were then examined geochemically, particularly for the contents of alkali-and alkaline-earth metals in the dacites. Presence of the alteration haloes similar to those of the Fukazawa deposits was indicated in the central and southern parts of the explored area. Intensive drilling operations were thus concentrated in these alteration zones and the Iwagami and Ezuri kuroko orebodies were successfully discovered in November, 1975 and July, 1976, respectively.
The confirmed ore reserve is approximately 3 million tons, averaging 1.3 g/t Au, 180 g/t Ag, 0.89 % Cu, 3.3% Pb, and 10.1% Zn. The mine has been in operation since October, 1979.
