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

ic rock-forming minerals are invisible, only some chloritic specks suggest their former presence. The porphyric texture of the original rock was retained to some extent. However, the ground mass consists here, too, mainly of minute quartz grains ; feldspar laths are absent as well as glassy substances. The rock also contains some secondarily formed aggregates of greater quartz crystal grains. The fissures are filled by hosts of calcite and quartz crystals. The greater quartzes are often crushed. Pressure twinning is frequent in calcite, indicating subsequent tectonic strain. In some thin sections aggregates of tiny ankerite crystals were observed. The remnants of the original porphyric texture, which may in some instances be observed, differ from the texture of the young Mátra andésite and resemble that of older andésite. In the Bükkszéi straight tunnel (55 and 90 metres respectively west of the Károly vein) as well as in the main gallery (230 metres north of the Malombérc vein) unaltered and silicified andésite are in tectonic contact. Unaltered andésite overlies the other one along a fault plane of gentle dip. In the Bükkszéi and Péter—Pál straight tunnels as well as in the part east of the Károly vein of the Szaka straight tunnel and in the main gallery it is likewise observed that the ore-bearing veins, accompanied by a relatively narrow zone of alteration, are separated by unaltered andésite of 50 to 150 metres thickness from the silico-andesite masses. As it was already mentioned, the rocks in the ore area have undergone endometavolcanic alteration. This was partly due to hydrothermal agents and, as such, is observed in the host rock immediately touching the veins, while another part, as e. g. pseudoagglomerate, occurs in tectonically active zones. On the other hand, the above-treated regional kaolinization and silification, having acted mainly upon the older members of the andésite body, is considered by the author to belong to the hypovolcanic type of alteration, in the sense of Szádeczky, having been brought about by transvaporization by water vapour. In the knowledge of the sedimentary basement of the volcanic formation it is easy to conceive that the magma, rising through a number of relatively narrow channels, of much greater contact surface than that of a single channel of the same cross section, has broken mainly through sandy to clayey sediment whose water content it has partly assimilated. It seems to be self-evident that the transvaporizatory effect has in the first place acted upon the magma mass risen first, i. e. yielding the older lava flows. This is the reason for the hypovolcanic alteration of older andésites situated at a lower level, while the younger andésites situated above (Mátra andésite) were rather subject to exo- and endometavolcanic alteration. The first 450 metres of the main gallery, some 600 metres distant from the nearest ore occurrence, disclose a peculiar type of rock. The light grey to greenish rock of homogeneous ground mass carries innumerable bubbles of variable size (1 millimetre to 20 centimetres). The ground mass is propylitic, lacking melanocratic ingredients. Porphyric inclusions are likewise absent. The interiors of the smaller bubbles are filled by aggregates of calcite, quartz and pyrite. The greater bubbles are lined by a calcite crust of 2—3 millimetres thickness. Within the same there occurs a filling of calcite, quartz, pyrite and rarely fluorite crystals, forming phenocrysts in some instances. The bubbles, of rotation-ellipsoid shape, are quite close-spaced. The orientation of their long axes, now subparallel, now delineating a variegated pattern of vortices, defines the conditions of flow in the ancient lava. Excepting pyrite, and some limonite