Alteration Zones Surrounding "Kuroko-type" Ore Deposits in Nishiaizu District-Especially the Analcime Zone for an Indicator of Exploration of the Ore Deposits

Abstract

The alteration of various degrees is recognized in the Miocene pyroclastic rocks in Nishiaizu District, Fukushima Prefecture. According to the assemblage of authigenic minerals, the Miocene pyroclastic rocks can be grouped into six zones as follows:
A) Clinoptilolite-mordenite zone,
B) Mordenite zone,
C) Analcime zone,
D) Montmorillonite zone,
E) Chlorite-sericite zone,
F) Gypsum zone.
These zones are distributed surrounding "Kuroko-type" ore deposits, alternately or successively from the gypsum zone or the chlorite-sericite zone in the central part to the clinoptilolite-mordenite zone in the marginal part as shown in 15 cross sections of Figs. 5 to 19. The writers interprete that this zonal distribution of alteration zones genetically relates to the "Kuroko-type" mineralization and later hydrothermal alterations.
Among the alteration zones, the analcime zone which may have been formed by concentration of sodium is a characteristic alteration zone around "Kuroko-type" ore deposits. In fact, typical Kuroko ore deposits are situated in the central part of the area where the analcime zone is distributed and is comparatively thick (Fig. 20). This fact may be usuful for exploration of "Kuroko-type" ore deposits.
Advantages of the analcime zone for an indicator of ore deposits are as follows:
1) Refering the previous works, it may be sure that there is a genetical, relation between the "Kuroko-type" mineralization and the formation of the analcime zone,
2) The determination of analcime by X-ray method is comparatively easier,
3) The analcime zone is distributed in favourable demensions for exploration.