Diagenesis and hydrothermal alteration of the neogene rocks in the Aizu district

Abstract

The Neogene formations of the Aizu district have a total thickness of about 3000m, of which volcanic and pyroclastic rocks constitute the dominant part. Structually, the district is made up of eight basins and several upheaval zones, which have resulted from block movements of the basement rocks simultaneous with sedimentation of the Neogene formations. Diagenetic alteration zones of the Neogene acidic volcanics are arranged in the following succession from top to bottom; fresh glass, clinoptilolite-mordenite, analcime, laumontite, and albite-chlorite-sericite zones. In the basins these zones are distributed in subparallel to the stratigraphic boundaries. In the upheaval zones, the zeolite zones are thin or absent, and various types of hydrothermal alteration zone are superimposed on the diagenetic alteration zones.
The Kuroko and Kuroko-type gypsum deposits occur in the Miocene strata, being located in the margins of the basins. Hydrothermal alteration zones around the ore deposits are outwardly arranged as follows: silicified, quartz-sericite, albite-chlorite-sericite, and montmorillonite zones in the floor of the ore depotits; and sericite, sericite-chlorite, and montmorillonite zones. in the roof of the ore deposits. The outer zones of hydrothermal alteration grade outward and upward into the diagenetic alteration zones. Veins of gold, silver, copper, zinc and lead occur in the upheaval zones, and are enclosed by hydrothermal alteration zones. The arrangement of the zones, from center to periphry, is as follows: silicified, quartz-sericite, adularia, and albite-chlorite-sericite zones. Sericite and kaolin deposits also occur in the upheaval zone. The sericite deposits are hydrothermal alteration products caused by the submarine volcanic activities of the Miocene age. On the other hand, kaolin deposits have their origin in the terrestrial volcanic activities of the Miocene and Quaternary ages.