On the occurrence of dispersed uraninite in Karkonosze granites

The isotope geochemistry contributed to the rejection of the hypothesis of hydrothermal origin of uranium deposits from Hercynian granitoid massifs. The withdrawal from the classic theories made it necessary to put forward new concepts of the origin of these deposits. The new concepts are primarily based on results of the studies on distribution of uranium in granitoid massifs.The studies have shown that the most perspective areas of occurrence of uranium deposits in several regions such as Central Massif in France, Colorado, or Namibia are connected with the rock massifs where large amounts of uranium are present in the form of uraninite ingrowths in rocks. A high content of uranium in granites of the Karkonosze Mts is well known, so searchings with the use of autoradiographic method were initiated there. The search covered unweathered granite samples from the borehole Karkonosze IG 1. The results obtained confirm the earlier assumptions, as there were found numerous very strongly radioactive microcrystals attaining up to 0.2 mm in size. Hexagonal habit, grey colour in reflected light, isotropy and high radioactivity of these crystals indicate that we are dealing here with a uraninite. The mode of occurrence of the uraninite indicates that it originated during crystallization of the rock. Its occurrence in Karkonosze granites implicates a similarity between Hercynian granitization and mineralization processes taking place in the Sudety Mts and French Central Massif. (author)