Hydrothermal alterations appearing in the Neogene sediments of Higashi-Aizu mineralization area, northeast Japan.
From 1966 to 1974, a number of drillings were carried by Metallic Mining Agency of Japan for ex ploration of "Kuroko-type" ore deposits in Aizu mineralization area, northeast Japan. As the results, a promising ore body (lower Ohshio Ore Body) was discovered. Futhermore, a large amount of important information on Neogene stratigraphy, geological structure, mineralogy of ore minerals, and hydrothermal alterations relating to the mineralization had been obtained. Though "Kuroko-type" ore deposits in this are area embedded in various stratigraphic horizons ranging from middle to upper Miocene, characteristic authigenic minerals are distributed zonally around them. They are group ed into several zones by a definite assemblage of silicates. A typical arrangement of the zones is as follows: from the nearest to the ore body to the margin, K-feldspar zone• ̈chlorite-sericite zone• ̈mixed layer mineral zone• ̈montmorillonite zone• ̈analcime zone• ̈mordenite zone• ̈clinoptilolite-mordenite zone. The shapes of alteration zones are as like as mashroom in usual, which suggest horizontal extending of hy drothermal solutions in porous sediments. Chemically, pbtasium and magnesium are concentrated in the K-feldspar zone and the chlorite-sericite zone in the central part, while the zeolite zones at the margin are characterized by concentration of sodium and calcium. Thus, the zoning suggests chemical redistributions by hydrothermal solutions which may have moved within each semi-closed space around ore deposits. The temperatures of hydrothermal solution at the center of alteration zones are estimated as 200•‹-250•Ž from fluid inclusion data of ore minerals, while those at the margin are as 40•‹-80•Ž from diagenetic condition data of clinoptilolite and mordenite. Then, horizontal thermal gradients around two ore deposits are tentatively estimated as 4.0-5.2 and 3.0-5.2•Ž/100m.