Vol, 17 (1980)

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Category: Vol, 17 (1980)

• Hydrothermal alteration
• granit
• Bor

Authors:Viktor Fiala

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 Abstract: Hydrothermal alteration of boric granite took place under conditions of preservation or creation of functional relationships between SiO₂ and Al₂O₃, FeO, MgO, and between Na₂O–K2O and MgO–H₂O+. The normative mineral composition confirms the extensive deanortitization of plagioclase and shows characteristic quantitative changes, in places also qualitative ones [norm. diopside, norm. nepheline). The material balance of the alteration shows a strong yield of silicon [up to -24% of the original amount], which corresponds to the loss of silicate oxygen, the yield of pre-carbonate calcium [up to -90%] and potassium (up to -80%). Sodium (up to + 100%), magnesium (up to almost + 1000%) and water were brought to a considerable extent. The carried elements may have been temporarily fixed in some newly formed minerals. Other elements were mobilized by the activity of hydrothermal solutions, signs of Fe²⁺ → Fe³⁺ oxidation are also shown. Of the monitored trace elements, barium is the most important, which is regularly recovered from albitized sections, and similarly, rubidium monitored as a K/Rb ratio. Other trace elements are less significant. The hydrothermal damage of the Bor granite was manifested by varying intensity of chlorite-albitic alteration, lined further from the central zone by propylitic altered rock. The product of these changes is (quartz-) chlorotic albitite with the following facies division: Outer alteration zone: sericite-chlorite facies [with leucoxene sub-facies]; Middle alteration zone: sericite-chlorite facies (with carbonate sub-facies) and quartz-epidote facies [with zeolite sub-facies]; Inner alteration zone: quartz-albite facies, albite facies, chlorite-albite facies (with hematite sulfate) and chlorite facies. The hydrothermally altered boron granite has the character of polymetasomatite, which was formed by infiltration and diffuse metasomatic processes during the I., II. and an additional alteration cycle. The discussion of older and younger alteration manifestations enables the formation of an idea of ​​the nature of hydrothermal solutions, whose Eh and pH conditions changed analogously during all alteration cycles (the initially higher Eh decreased at the end of the alteration cycle, the pH changed from acidic to alkaline to quasi-neutral) and they acted on the rock at considerable pressures – in the first two cycles most probably exceeding the corresponding lithostatic pressure. The temperatures of the solutions probably ranged from 400°–270°C, 2500–150°C, and below 100°C. Expressions were postulated to express the intensity of alteration of the Bor granite at the Vítkov locality: potassium-sodium ratio, potassium-rubidium ratio, alkaline alteration coefficient [AKA], ferritic alteration coefficient [FKA], magnalbitic [MMA) and aqualbitic [AMA) alteration modulus.