Szekessy Vilmos (szerk.): A Magyar Természettudományi Múzeum évkönyve 62. (Budapest 1970)

Szabó, I. ; Ravasz, Cs.: Investigation of the Middle Triassic volcanics of the Transdanubian Central Mountains, Hungary

poorly rounded nature of pyroclastics exclude the possibility of any transportation by air for great distances or of any redeposition by paleo-currents. Since the magmatites have been explored only in isolated patches, their eruption centres cannot be located in the present stage of knowledge. After falling into sea water, the loose volcanic fragments underwent a halmyrolitic alteration, as discussed above in detail. As shown by stratigraphie, mineralogical and petrographical results, the Middle Triassic alkalic (K) and rhyolitic pyroclastics of the Transdanubian Central Mountains are closely interconnected with the genesis of the bauxite deposits occurring in the Central Mountains territory. The source material of bauxite could not be identified unambiguously as of yet. Various hypotheses suggested clay-type sediments to have been the source of bauxite (BÁRDOSSY); other authors, relying on micromineralogical analyses, suggested the bauxites to derive from metamorphic or igneous rocks, respectively. The present writers consider the Anisian—Ladinian pyroclastic sequence as well as pyroclastic-rich, calcareous dolomitic sediments to have been the source of bauxite. The loose, felspar-rich rock seems to be most conspicuous to account for bauxite origin. A residual sediment, bauxite has reached its present-day composition through the following stages: The above hypothesis is supported by the following considerations: Searching for the source of bauxite in recent years, J. Kiss and I, VÖRÖS attempted to identify the source rock by micromineralogical analyses. Out of the authigenic minerals of the 0.06 mm fraction of bauxite, they determined the following: ilmenite, magnetite, rutile, apatite, zircon, garnet, berill, tourmaline, epidote, zoistite, actinolite, biotite, chlorite, muscovite, corundum, diopside, hematite, anatase, brookite. In addition, mostly limpid quartz grains and carbonized organic detritus were discovered. Most frequent are the opaque minerals, quartz, rutile, zircon and, in some localities, tourmaline. Using the same method, the authors provide further evidence concerning the source rock of the Hungarian bauxites on the basis of the results of investigations restricted to the clastic minerals of bauxite samples from the opencast pit of Gánt, still under exploitation, and from the collection of the Natural History Museum, Budapest. In the bauxite the presence of the following heavy minerals (listed according to their order of frequency) could be shown : ilmenite, magnetite, hematite, zircon, tourmaline, garnet, rutile, hornblende, epidote, zoisite, chlorite, perovskite, metamict zircon (?). The light minerals are represented by quartz, volcanic glass (n < b), sanidine, oligoclase, clay-mineralized volcanic fragments with feldspar laths, chalcedony and, sporadically, microcline. Besides these, epigenic limonite and hematite as well as clay mineral grains, present in 80 to 90%, can be observed. Ilmenite—(magnetite) is represented by idiomorphic crystals with frequent manifestations of magmatic resorption. Zircon can be characterized by well-developed, idiomorphic, colourless or pale-pink crystals, being poorly rounded or completely angular, often fragmentary. Quartz derives from volcanic rocks, being often idiomorphic. The overwhelming majority of quartz crystals do not show undulating extinction. It is a matter of fact, that in the bauxite typical metamorphic minerals are also present in low grain percentage; however, metamorphic minerals of feldspar -*• (glauconite) illite montmorillonite — kaolinite — bauxite.