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

of vertical lava bodies — dikes, necks — is subordinate. The space between the lava bodies is filled by agglomerate. There also occur small amounts of tuffs, in a wide range of varieties. The eruption centers are unknown as yet. In all likelihood the simultaneous and repeated eruption of volcanic craters and fissure volcanoes with some monogenic eruptions in between has to be reckoned with. The original volcanic morphology is lacking or, at least, unrecognizable because of intense denudation. The average thickness of the andésite series can be estimated at 600 metres according to J. N o s z k y sen. G. P a n t 6 has recognized the circumstance that the ore-bearing area mostly consists of a rather variable andésite type, older than the main andésite of the Mátra Mountains ; this is the host rock of ore genesis. Panto calls this andésite "inclusion-rich" and the occurrence of ore is connected in his opinion with this kind of rock. — The manifold types of andésite have, partly in consequence of external weathering, partly due to hydrothermal processes, undergone alteration of varying intensity and thus given birth to a number of altered andésite varieties. In the ore area the following andésite types are encountered (in the presumable order of eruption): biotitic hypersthene andésite, andésites lacking melanocratic ingredients, "inclusion-rich" pyroxene andésite, hypersthene andésite, augite andésite, hypersthenic augite andésite. The texture of these rocks is enormously variable. Generally, the recognition of the individual andésite types in the field, regarding the unfavorable conditions of disclosure, is intensely difficult. On the other hand, the data of the mining operations show that the different kinds of andésite occur in irregular alternation, so that the definition of the sequence of eruption is well-nigh impossible. The picture is made even more muddled by the fact that hydrothermal alteration has produced similar rocks from originally different ones. The rock types mentioned may be characterized as follows : 1. The quantity of biotitic hypersthene andésite in the area is subordinate. Its sole occurrence is in a small spot on the southern slope of Cserepes Hill. The rock of medium grain size carries hypersthene, oligoclase and biotite rags as porphyric constituents. Biotite is frequently enclosed by growths of hypersthene. The bulk of the rock consists of the ground mass, containing some scattered microliths of feldspar. The ground mass itself is made up by fields of differing optical orientation. 2. Andésites lacking melanocratic ingredients. This group consists of a number of different andésite types, occurring together within quite small areas. The common feature of these rocks is that they contain traces only of melanocratic ingredients if any at all. At a first glimpse these rocks are thought to be entirely vitreous. Their colour is a light greenish grey. They exhibit conchoidal fracture and sometimes striation. Microscopic investigation reveals a multitude of felsdpar laths of some microns size, arranged in the ground mass in a fluidal pattern. The striation of the rock is due to the local enrichment of these feldspar laths. This type of rock contains no ore particles. Porphyric ingredients are absent, excepting some rare occurrences of feldspar. 3. Glassy andésites with scarce melanocratic ingredients. These apparently vitreous rocks of conchoidal fracture do not essentially differ from those of the foregoing group, except inasmuch as they contain infrequent augite among