Hydrothermal experiments involving the conversion of natural clinoptilolite to other zeolites were carried out in the aqueous solutions of NaOH-NaCl and Na₂CO₃-NaCl at various temperatures (100-400°C), pH (5.5 to 13.6) and Na ⁺ concentrations (40 to 45000 ppm). Four mineral phases appeared and two different trends of alteration were recognized: (1) clinoptilolite-mordenite-analcime-albite series and (2) clinoptilolite-mordenite-albite series with increasing temperature. Analcime was formed under the conditions of both higher pH and higher Na ⁺ concentration. With a decrease either in pH or in Na ⁺ concentration, not analcime but albite was formed. The course and degree of alteration vary with temperature, pH and Na ⁺ concentration of the solution. Chemical analyses of residual solution revealed that a large amount of Si ⁴⁺ (>5000 ppm) was released from the reactant of the solution accompanied by the formation of analcime. The experiments with additional silica revealed that the stability field of analcime decreases under higher pH conditions. The experiments in the aqueous solution of Na₂CO₃-NaCl indicate that analcime will be formed under relatively lower pH conditions. Both series of alteration trends correspond to the typical zonal arrangements of Na-zeolites which are commonly observed in the marginal areas of the kuroko-type ore bodies.
The mineralogical zonings of Na-zeolites around the kuroko ore deposits are discussed on the basis of physico-chemical conditions of the solutions as well as the chemical change between liquid and solid phases from the present experiments. The difference in alteration patterns as well as the zonal distribution of Na-zeolites around the kuroko-type ore deposits can be interpreted by the degrees of temperature, Na ⁺ concentration and pH of the solution associated therewith.

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Two types of zoned skarns occur at the Akemidani No. 5 ore body of the Fujigatani mine in the Kuga district, the most important tungusten province in Japan. One is "incrustation skarn", up to 15 m wide, formed around the boundary between a lenticular limestone and its surrounding slate and chert. The other is"vein skarn", up to 2 m wide, developed in the limestone host.
As a basis for the future discussion of metasomatic process, we have made an effort to clarity the characteristics of zoning observed in the vein skarns.
The zonal arrangements of the vein skarn, generally symmetrical, are as follows: from limestone side towards the center, coarse-grained calcite zone, wollastonite zone, garnet-clinopyroxene zone, clinopyroxene-garnet zone, and central zone. As is evidenced by field observations, the central zone was originally a fracture, into which solutions invaded to yield the vein skarns. Preservations of original texture indicate that the inner zone replaced the outer zone towards the limestone side.
Modal and chemical analyses and calculation of components derived from both data indicate the following characteristic tendencies towards the zoning of the vein skarns.
(1) Whilst modal ratios of early skarn minerals, wollastonite, clinopyroxene, garnet, and vesuvianite, are relatively constant in each zone, they change abruptly at the boundaries.
(2) Clinopyroxene do not show a significant variation in composition from the garnet-clinopyroxene zone to the central zone, that is, containing 80 to 90 mol. % hedenbergite, while those in the wollastonite zone are rich in Fe near the garnet-clinopyroxene zone and poor in Fe near the limestone side. The fact that the Fe/Mg ratios of clinopyroxenes in the wollastonite zone is not continuous is due to the partial substitution of the latter by the former, Fe-rich one.
(3) CaO contents decrease from the limestone side towards the central zone, while SiO₂ contents reveal the reverse change. On the other hand, A/2 (AlO _3/2+FeO _3/2) and F (FeO+MnO+MgO) components exhibit the inverse correlation, because they are dependent on the modal composition of garnet and clinopyroxene.

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The metallic mineralization in the Japanese islands can be divided into three groups in view of the ore lead isotope systematics.
(1) The Paleozoic to Mesozoic stratiform (Besshi-type) mineralization is composed of cupriferous iron sulfide ores in which lead is usually a minor constituent (to the level of 10 ¹-10 ² ppm). The leads are isotopically variable, being consistently low in both ²⁰⁷Pb/ ²⁰⁴Pb and ²⁰⁸Pb/ ²⁰⁸Pb ratios as compared to the lead in the "major (conformable) lead orebodies" of similar ages.
(2) The pre-Neogene mineralization excluding the Besshi-type deposits is mostly related to Cretaceous to Paleogene granitoids. Galena is fairly common and forms sizable orebodies in a few skarn-type deposits. Lead isotopic composition is more or less variable regionally, and even locally in some cases, all being, however, low in ²⁰⁶Pb/ ²⁰⁴Pb and high in ²⁰⁸Pb/ ²⁰⁴Pb ratio in comparison with the conformable ore leads.
(3) The Neogene mineralization is represented by the kuroko-and related vein-type mineralization of middle Miocene age, carrying abundant lead as one of the major ore metals. The leads are isotopically rather uniform with consistently low ²⁰⁶Pb/ ²⁰⁴Pb and high ²⁰⁸Pb/ ²⁰⁴Pb ratio as compared to the major lead ore system; 43 samples from 30 localities give average ²⁰⁸Pb/ ²⁰⁴Pb ²⁰⁷Pb/ ²⁰⁴Pb and ²⁰⁸Pb/ ²⁰⁴Pb ratios of 18.471±. 108 (2σ), 15.609±.036, and 38.677±.220, respectively. A remarkably consistent isotopic composition close to this average is observed in the kuroko ores from an extensive area of northeast Japan.
The Besshi-type ore leads except for those of the Mesozoic Taro deposit show an isotopic pattern similar to that of the ocean volcanic leads, strongly suggesting that the major evolutionary environment of these leads has been in the oceanic mantle. The Taro leads show isotopic features close to the major lead ore system and, in this respect, are unique among Japanese ore leads.
The leads of the pre-Neogene and Neogene mineralizations may be identified as crustal lead with respect to their main evolutionary environments. The ²⁰⁷Pb/ ²⁰⁴Pb and ²⁰⁸Pb/ ²⁰⁴Pb ratios of the pre-Neogene leads exhibit a trend in which the relatively high values of these ratios occur in the area where the related granitoids have relatively high initial ⁸⁷Sr/⁸⁶Srr ratios; some Neogene leads seem to follow the same trend. The presence of a relatively old and thick continental crust in the source region of these leads can be deduced.
The available isotopic data for the oceanic sediment lead occurring closer to the Japanese islands (samples near the Mariana volcanic arc and those from the Ryukyu Trench) apparently deviate from the major spectrum of the Pacific sediments data and plot closer to the data for the Japanese Neogene and pre-Neogene ore leads which may approximate the average of Japanese crustal leads. A relatively ²⁰⁸Pb-impoverished and ²⁰⁸Pb-enriched character indicates that the crustal lead around the Japanese islands has been strongly influenced by the lead evolved in the lower continental crust.

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The ferruginous chert rocks which occur in the kuroko field of northern Akita Prefecture as the hanging wall of kuroko deposits were chemically analized. And these chemical data were analized with the principal component analysis that is a kind of multivariate analyses. Consequently, the ferruginous chert which occurs in much nearer places of kuroko deposits was chemically distinguished from those of other places.

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One sample of adularia from the Hosokura Pb-Zn vein-type deposits gives a K-Ar age of 5.8±0.2 Ma. This age is quite younger than that of the host rocks (Hosokura formation) of Daijima to Nishikurozawa (early to middle Miocene).
Two samples of adularia from the different veins of the Sado Au-Ag mine give the K-Ar ages of 14.5±0.5 Ma. and 13.4±0.5 Ma. These ages are 25-15 m. y. younger than those of the host rocks (Odate and Aikawa formation).
Summary of the ages of the vein-type deposits and those of the host rocks in the Green tuff region previously studied, including the present data on the Hosokura and Sado deposits, suggests that almost all of the vein-type mineralization in the Green tuff region of north eastern part of Japan took place after the Nishikurozawa stage (14-16 Ma.). This implies that the ages of the vein-type mineralization were later than and/or similar to that of the Kuroko mineralization.

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