Boros István (szerk.): A Magyar Természettudományi Múzeum évkönyve 51. (Budapest 1959)

Kaszanitzky, F.: Genetic relation of ore occurrence in the Western Mátra Mountains, North Eastern Hungary

Altered rocks The rocks of the area have, at the time of the completion of crystallization and thereafter, undergone intense alteration. The tectonic movements subsequent to the solidifying of the andésite types have led to the formation of zones of crushing, and the altering effect of the solutions and vapours rising through the fissure systems of ore formation had a free play. In characterizing the endometavolcanites formed in this way the present author applies the genetical nomenclature introduced by the scientist leading the up-to-date investigation of Hungarian volcanic mountains, Academy Member E. S z á deczky Kar doss. The veins traverse rock formations of intensely variable quality. They have sometimes altered the host rock so intensely that it is very difficult to determine the original nature of the rock. The influence of hydrothermal vapours and solutions has manifested itself in the sericitization, propylitization, calcitization, pyritization, silification and kaolinitization of the rocks. These products of alteration tend to occur jointly. Where the hydrothermal influence was slight, alteration was in the first place due to the incipient degradation (iron loss) of the rock-forming minerals and the decomposition of the magnetite occurring in the ground mass. Where the hydrothermal influence has been more intense, the iron components were entirely leached and thus the rock has become a discolored endoleucovolcanite. Formations of this kind are known from sections of considerable length of the mine galleries. The leaching effect of the ascendent solutions is at times so intense that much of silica is leached out and the hydrovolcanite thus formed is in the end altered into kaolinite. This phenomenon may be best observed in sections of the lode where the gangue is not silicic and carries a great number of horses. Part of the dissolved silica is precipitated at some distance from the vein. There, the cavities of the decomposed porphyric constituents of the host rock are filled by quartz aggregates of minute grain size. There occurs along the veins also metavolcanic propylitization. It is characterized by being restricted to rather narrow zones (0,1—1,0 metre). Hypersthene of the original rock alters into serpentine, augite into chlorite, while plagioclase turns into a kaolinitic aggregate. Propylite shows a gradual transition into the unaltered host rock. In the disclosures of the tectonically more disturbed parts of the area there occur considerable amounts of pseudoagglomerate. This type of rock is taken for true agglomerate at first sight. More thorough investigation reveals, however, that it is an alteration product of an originally homogeneous rock, the alteration of which has reached different degrees. The aggregate of rock chips of different alteration makes the rock appear like a characteristic polymict agglomerate. The greater blocks are generally more or less unaltered, while the smaller ones are already in the leuco- or hydrovolcanite state, respectively. The individual blocks of rock are limited by sharp boundaries : however, under the microscope the continuation of porphyric constituents and even of the feldspar laths of the ground mass across the boundaries is observed. Pseudoagglomerate. is especially common in the northern part of the ore deposit, where it is traversed by the gallery in a total length of about 1200 metres. Therefore, prior to the recognition of the true nature of these "agglomerates" (Szádeczky) the correct interpretation of geological profiles was exceedingly difficult if not