愛媛県砥部地区, 万年および上尾陶石鉱床の生成に関与した熱水変質作用

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Andesite dike and andesite sheet intrude the gravel beds of Kuma Group in the Tobe district, Ehime Prefecture. They are altered to pottery stone by hydrothermal activity. The pottery stone depostits in the Mannen and Uebi areas are typical. The pottery stone deposits of the areas consist generally of abundant quartz with some amounts of clay minerals such as interstratified mica/smectite minerals, tosudite, kaolinite, and so on. Alteration zoning in the deposits is possible on the basis of the clay mineral assemblages. The dike type deposit in the Mannen area shows a zonal arrangement in which the central zone consists of interstratified mica/ smectite minerals with g=3 and P_M>0.9 and the outer zone, the same minerals with g=3-2 and P_M> 0.8 and kaolinite, as the major minerals, where g denotes "Reichweite" and P_M, the probability of occurrence of mica layer. On the other hand, the sheet type deposit in the Uebi area shows a transition of major mineral constituents, from the bottom upward, from tosudite to tosudite plus interstratified mica/smectite minerals with g=1 and PM=0.5 and then to interstratified mica/smectite minerals with same values of g and P_M. Kaolinite is a common constituent in all the bottom, intermediate and top zones and enriched in the hanging sides, in the latter case. The mechanism of hydrothermal alteration which has possibly been related genetically to the formation of pottery stones in the study areas may be explained on the basis of theoretical and empirical considerations as follows : It is a general rule that the stoichiometric solubility of quartz decreases, and that of the usual silicate, oxide and hydroxide minerals increases with the decrease of temperature of hydrothermal systems at a fixed pH. Also, stoichiometric solubility of the usual rock forming minerals is much larger than that of the clay minerals which have the similar chemical composition, so that the latter are probably deposited with dissolution of the former. Therefore, when a hydrothermal solution which has been equilibrated with quartz and other minerals in a host rock is cooled by its ascension, quartz is probably the main product with some clay minerals, and some other minerals often remain as relicts. Glass tends to disappear more rapidly than the crystalline minerals from the path of hydrothermal solutions, because of its relatively high solubility. Adoption of the above reasoning to the pottery stones under the present study may be justified by the presence of idiomorphic quartz particles and various clay minerals in them, together with the observed porous structure of them. Incidental calculation made on the the solution in a model aquifer which is in equilibrium with various minerals indicated that quartz and muscovite are precipitated and feldspars dissolved when the solution composition is changed by temperature drop. Another reaction is assumed to have occurred simultaneouly during the formation of pottery stones in the study areas, as follows : Ground water which had been saturated with oxygen during the movement through the gravel bed diluted the hydrothermal solution and thereby oxidized the sulfides, such as HS- and H_2S, to sulfate. The mixed solution became weakly or strongly acid depending on the extent of the dilution as well as the amount of the sulfides originally present. It is natural to assume that the dilution was limited rather to the outside of the rock body in the case of dike type (Mannen area), and was spread over the whole rock body in the case of sheet type (Uebi area) intrusion. Thus, the difference of the zonal arrangements of clay mineral assemblages observed between the Mannen and Uebi areas can be attributed to the difference of pH of the hydrothermal solutions which were diluted to different degrees by the ground water from the gravel beds, as the stability of clay minerals or their assmblages would depend upon the pH of the parent solutions.
日本粘土学会の論文
1988-12-01

愛媛県砥部地区, 万年および上尾陶石鉱床の生成に関与した熱水変質作用

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