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

impossible. — There occur further in the galleries also pseudotuffs of limited extension. These are formed by the advancing alteration of pseudoagglomerate, accompanied by a steady decrease of grain sizes. These totally altered varieties of rocks consist actually of clay, with indistinct traces of original rock texture. Beside endometavolcanites, there also occur hypovolcanites in the area, at some distance from the hydrothermal veins, frequently separated from the latter by unaltered andésite. These hypovolcanitic rock varieties are independent of hydrothermal activity and occupy a great area. They were formed in the course and after the solidification of the rock, but preceding hydrothermal activity. The regional kaolinitization of rocks is one of the characteristic features of the area. The kaolinitized zones of several sq. kilometres extension comprise original rocks of wide variety. The kaolinitization has not throughout been equally intense. In some places the kaolinitic zones appear along fissure seams only, in a width not exceeding some centimetres. In a more progressed stage kaolinitization involves even large blocks of rock, with the rock left unaltered only at the center. In the kaolinitic hypovolcanites thus formed the dark ingredients turn into chlorite, feldspars into kaolinite, sometimes into sericite. The southern and eastern flank of Cserepes Hill, Nagy Henc, Felső Henc, the upper sections and valleyheads of Nagy-völgy Creek, Bánya Creek, as well as the upper part of Bányabérc consist of such hydrovolcanites. The data of the disclosures indicate that the rocks of the area were, prior to kaolinitization, at least in the propylitic stage of hypovolcanitic alteration. The regional kaolinitization of the rocks has brought about a partial leaching of the silica content of the same. The silicic acid dissolved has after a migration of some distance given rise to the formation of silicoandesites in parts of the propylitized rock. On the hand of microscopic studies, two kinds of silicoandesite could be distinguished : 1. silicified andésite lacking feldspars and melanocratic constituents, 2. feldspar-bearing silicified andésite. The disclosures known at present show that these apparently homogeneous, grey, tobacco-brown, sometimes rusty brown rocks occur in an irregular elliptic spot of about one half sq. kilometre area in the center of the kaolinitized region. In the rocks forming the central part of the ellipse microscopic investigation has revealed the lack of feldspar and melanocratic ingredients. There are not even traces of the original andésite texture left. The ancient ground mass is substituted by a quartz aggregate of fine isometric grains of about equal size (0,05 millimetre in the average), among which there occur groups of quartzes of greater grain size presumably at the site of one-time porphyric ingredients. The interstices of the quartz grains constituting the ground mass are filled by almost totally isotropic opal. The greater quartz grains are of undulatory extinction, the grains of greater aggregates are arranged on either a subparallel or a radial pattern. Some calcite occurs in the form of isolated grains or grain aggregates. The rock type described above can be regarded as an extreme development of feldspar-bearing silicified andésite, which covers a somewhat greater area. The main difference between them is that in the thin sections of the latter one the feldspars are still discernible, although the crystals themselves are mostly altered into an aggregate of kaolinite, calcite and quartz. Melanocrat-