Further Investigations of a Conversion Series of Dioctahedral Mica/smectites in the Shinzan Hydrothermal Alteration Area, Northeast Japan

-A complete conversion series for mica/smectites was found in a hydrothermal alteration envelope around Kuroko-type ore deposits at the Shinzan area, Akita Prefecture, Northeast Japan. The minerals are an alteration product of volcanic glass of Miocene age and are commonly associated with zeolites and silica minerals. Degrees of ordering of interstratification of the minerals change discontinuously from Reichweite g = 0 (100-55% expandable layers) to g = 1 (45-20% expandable layers), and from g = 1 to g = 2 (<20% expandable layers). This pattern of conversion differs from the behavior of mica/smectites during burial diagenesis which undergo a continuous change in ordering type, and from the behavior of rectorite which displays a constant expandability and ordering (45-55%) over a wide range of conditions. Differences between these minerals were also found in the relationships between expandability and total layer charge, and between expandability and number of non-exchangeable interlayer cations. In mica/smectites from the Shinzan area, chemical changes in the interlayers and tetrahedral and octahedral sites are consistent with a reaction in which K-enrichment and K-fixation in the interlayers are controlled by an increase in negative layer charge. This conversion occurred in response to a steep geothermal gradient and migrating hydrothermal solutions. Key Words--Burial diagenesis, Interstratification, K-fixation, Mica, Rectorite, Smectite. I N T R O D U C T I O N Interstratified dioctahedral mica/smectites can be generated by various processes, e.g., during the diagenetic conversion of montmoril lonite to illite. This reaction has been studied by many workers (Burst, 1969; Perry and Hower, 1970; Weaver and Beck, 1971; Foscolos and Kodama, 1974; Hower et aL, 1976; Srodoff, 1979). The structural and chemical changes of the minerals involved in the reaction are sensitive indicators of the degree of diagenesis. Mica/smectites are also abundant in hydrothermal alteration areas. In Japan, occurrences have been reported mainly from the hydrothermal alteration envelopes surrounding pyrophyllite-sericite ore deposits and Kuroko-type ore deposits. The pyrophyllite-sericite ores (Roseki) commonly contain rectorite-like minerals (Shimoda and Sudo, 1960; Sudo et al., 1962; Brindley and Sandalaki, 1963; Kodama et al., 1969; Matsuda et aL, 1981a, 1981b), whereas the Kurokotype deposits, composed of various sulfides and sulfates are surrounded by hydrothermal alteration aureoles with mica/smectites (Shimoda, 1970, 1972; Shirozu and Higashi, 1972; Higashi, 1974, 1980: Inoue et aL, 1978). In 1978, Inoue et al. reported a complete conversion series of mica/smectites from this type of deposit from the Shinzan area, northeast Japan, where Copyr igh t 9 1983, T h e Clay Minera l s Society the minerals occur as alteration products of felsic volcanic glass. In the present study, the conversion series of mica/ smectites of hydrothermal origin is characterized and compared with a series of mica/smectites of diagenetic origin and with a rectorite series from Roseki-type ore deposits. O C C U R R E N C E OF MICA/SMECTITES IN THE SHINZAN AREA Samples were obtained from eight drillholes in the Shinzan area, Akita Prefecture, as shown in Figure 1. Although a large part of the area is covered by Quaternary sediments, two Miocene formations, which are largely pyroclastic, were drilled in the area. These formations have been subjected to hydrothermal alteration and can be grouped into six zones based on the assemblages of authigenic minerals as listed in Table 1. A zonal arrangement from a sericite-chlorite zone nearest the center o f hydrothermal activity, which is assumed by the exposure of Cu-Pb-Zn impregnation, to a montmoril lonite zone or zeolite zones in the margin is depicted in Figure 2. Interstratified mica/smectires occur extensively between the sericite-chlorite zone and montmoril lonite or zeolite zones. Although the percentage of expandable layers in mica/smectites in-