1981 Volume 31 Issue 165 Pages 13-25
In the central area of the Hokuroku district, the following alteration envelopes are recognizable in the Neogene sediments which comprise a large amount of acidic volcanic rocks.
(1) Hydrothermal alteration related to kuroko-type mineralization. Alteration zones are arranged from the center to the margin as follows: The chlorite-sericite zone→the mixed layered clay mineral zone→the montmorillonite zone→the analcime zone→the mordenite zone.
(2) Hydrothermal alteration related to vein-type mineralization. Alteration zones are arranged from the center to the margin as follows: The K-feldspar zone→the chlorite-sericite zone→the mixed layered clay mineral zone→the montmorillonite zone.
(3) Hydrothermal alteration and contact metamorphism(?) around quartz diorite intrusive mass. The chlorite-sericite zone alone is recognized.
(4) Burial diagenesis. The fresh glass zone and the clinoptilolite-mordenite zone are recognized.
The alteration envelopes (1) and (2) are pervasive in the whole area. The latter is often superposing upon the former and the rocks of the former often remain as relicts in the latter. A typical example of this superposition is found around Fukazawa kuroko deposit. Judging from the mode of distribution of the alteration zones, both kuroko-type and vein-type hydrothermal activities in this area seem to have occurred along the lines having NW-SE trend.
