O występowaniu buł porfirowych z agatami w rejonie Nowego Kościoła (Dolny Śląsk)

Józef Kryza, Jerzy Niśkiewicz


On the occurrence of porphyric balls with agates in the Nowy Kościół region (Lower Silesia)

The occurence of agates in the Sudeten mountains has had a long history (G. Leonhard, 1843; H. Fiedler, 1863; H. Traube, 1888; W. Müller, 1896; I. Kardymowicz, 1957 and B. Bereś, 1958). In the Nowy Kościół region a new locality of agatiferous porphyry balls has been discovered. The agatiferous porphyric globes in this region probably occur (no field works have been carried out there) in the upper part of the porphyry cover and they form a level of a considerable extension. Porphyric balls described by W. Müller (1896) and in the present work belong to this level. The level of agatiferous porphyric globes occurs among massive porphyric rocks and it probably forms a boundary between two covers of porphyry laves. This level would constitute the upper part of the lower cover, while the solid porphyry occuring above forms the base of the upper cover. It is difficult to characterize more precisely the boundary of the level in the base and in the top of the formation, since there have been no good exposures there. The lower boundary seems to be blurred. Some observations point to a gradual transition of solid porphyr into the described level of porphyry globes, whereas the upper boundary is fairly distinct. The stated occurrence of agatiferous pebbles of porphyric globes or their fragments in conglomerates of the upper Rotliegendes and the Lower Zachstein (J. Kryza, 1969) proves that the formation of the globes is connected with volcanic cycle of the Rotliegendes. The formation of agatiferous porphyric globes which occur in the Nowy Kościół region is connected with hydrothermal processes accompanying the Permian vulcanism. The upper parts of the lava covers are the most porous ones and thus they were especially predisposed to generate agate geodes in them. The characteristic, "starlike” shapes of many geodes could also be formed as result of differentiated thickness of lava in various places. It was caused by crystallization of magma around certain centres at the phase of formation of the texture of the porphyric cover. At the same time, the siliceous solutions could be supplied by final hydrothermal processes accompanying the formation of the lower porphyry cover (or, maybe, above all) with hydrothermal processes of the following effusion of lava.

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