Szekessy Vilmos (szerk.): A Magyar Természettudományi Múzeum évkönyve 57. (Budapest 1965)

Csalogovits, I. J.: Geological and petrological conditions of the Szanda - Bercel area

structural line appears in the Chattian clayey series as a closed fault, while in the andésite it forms crushed belts and structures that are suitable for material supply). This is indicated also by chalcedony and calcite capillaries that occur occasionally along the trend of hydrothermal veins in the outcrop of the Chattian series and fill up the microfissures of the „closed lithoclases" in some places. The hydrothermal formations show the following chronology: 1. Siderite-limonite veins with calcite content. Their trend differs from the later ones, their quantity is negligible. These are veins that were found in the andésite quarry of Mt. Bercel: their maximum thickness is 5— 10 cm. Very intense hydroandesitization appears. They are cut through by veins that represent further phases. 2. Veins with montmorillonite material. They show the largest masses, the thickness amounts to 5—6 m. 3. Dusting calcite veins (contain strontium). These occur along the planes that are ripped in the previous formation. Maximum thickness is 1 — 2 m (Mt. Bercel). 4. Wax-opal spots. They occur inside the montmorillonite veins, the structure is colloidal, the amount of this formation insignificant. Maximum thickness if 0.50 m (Szanda, Péterhegy). The latter formation occurs sporadically in andésite also independently (Szanda), but — in conformity with its low formation temperature — causes no hydroandesitization. Judged on the basis of trace element analyses, formations 1 and 2 are of mesoepithermal, formations 3 and 4 of epithermal character. The montmorillonite content and accompanying calcite also indicate the lateral-secretory origin of the postmagmatic formations. The formation of hvdroandesite In connection with the andésites occurring in the area, hydroandesitization can be proved as a result of contact, hydrothermal and weathering processes. 1. Hydroandesitization of hypovulcanitic origin. In the order of magnitude mentioned in connection with contact investigations, a contact-hydroandesite belt is discernible on the edge of andésite bodies that are in contact with sediments: this belt was formed under the effect of the water content of the sediment, during contact-formation. On the basis of analytical data this hydroandesitization can be characterized with slight montmorillonitization and limonitization. Compared with fresh rock, the total amount is similar, which indicates that the limonite material was formed through the oxidation of the original opaque minerals of the rock. Compared with fresh rock, the degree of oxidation is considerably increased (52.2). 2. Endometavulcanitic (hydrothermal) hydroandesites. The formation of hvdroandesite, caused by hydrothermal solutions, is discernible along the hydrothermal veins in varying thickness. Taking the order of hydrothermal formation as a basis it can be concluded that the intensity of decomposition decreased gradually in the later phases of lower temperature, and is in connection with wax-opal material that represents the last generation below the limit of demonstrability. The extent of decomposition is not clearly characterized by the size of the decomposed belt as measured by its distance from the vein: this is a phenomenon connected with local changes, especially tectonic stress on andésite.