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  • Author or Editor: György Pantó x
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The age determination method based on microprobe measurements of the Th, U and Pb contents of monazite, and used since the nineties, was applied to Hungarian igneous rocks: the so-called "bostonite" and the granitoids of the Velence Hills and Mecsek Mountains. Measurement results were evaluated by two different methods that produced similar ages; for the sake of precaution the obtained values were mostly accepted by the greater uncertainty domain calculated by linear regression. The age of 117±13 Ma for the "bostonite" proves that this rock is a member of the Lower Cretaceous igneous series. The age of 210±18 Ma obtained for the Nadap granite in the Velence Hills agrees with published data while that of 255±17 Ma obtained for the Kismórágy microgranite in the Mecsek Mountains is younger than previously known.

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The karstic bauxites of Mt. Grebnik, Kosovo, represent a unique deposit of the diasporic bauxites of Upper Cretaceous age in the former Yugoslavia. They are typically transitional between ferruginous bauxites and karstic Ni-Fe ores. Bauxites originated from surrounding Cretaceous weathering crusts on ultramafic rocks and on shale of the diabase-chert formation. Chemical and trace element analyses of the bauxite samples collected along a vertical profile of a typical deposit are presented and discussed. Trace element patterns, including REE and Ni, are typical for karstic bauxites formed in situ, during bauxitization of argillaceous material accumulated in a karstic depression. In connection with this, a progressive enrichment of the REE and Ni toward the bottom of the deposit is well pronounced, with formation of authigenic REE mineral, synchysite-(Nd), and Ni-bearing chlorite on the contact with the footwall limestone.

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The Ni-Fe ores represent the reworked part of the Paleozoic weathering crust on ultramafics. Talcose serpentinite, with a small accumulation of Ni-talc, represents the lowermost part of the destroyed weathering crust. XRD and EPMA study of the ore revealed its complex composition: magnetite is most abundant, with cores of detrital chromite; Ni-chlorite is also abundant; talc, goethite, hematite, maghemite, magnesite and Fe-rich reevesite occur in variable amounts. XRD of the ore revealed weak reflections of reevesite, with the strongest at 7.60 Å. Nine electron probe analyses of this mineral were performed. The richest in nickel gives the following recalculated formula: (Fe2+ 4.03 Ni1.97)6.00 (Fe3+ 11.96 Al0.03 Cr0.01)2.00 CO3 (OH)16 . 4H2O. According to this composition, the mineral is the Fe-analog of reevesite and a new member of the Hydrotalcite Group.

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