Epigenetic‐hydrothermal fluorite veins in a phosphorite deposit from Balaton highland (Pannonian Basin, Hungary): Signatures of a regional fluid flow system in an alpine triassic platform

The middle Anisian extensional tectonics of the Neotethyan realm developed a small, iso‐ lated carbonate platform in the middle part of the Balaton Highland (western Hungary), resulted in the deposition of uranium‐bearing seamount phosphorite on the top of the drowned platform and produced some epigenetic fluorite veins in the Middle Triassic sequence. The stable C‐O isotope data of carbonates are shifted from the typical Triassic carbonate ranges, confirming the epigenetic‐ hydrothermal origin of veining. Primary fluid inclusions in fluorite indicate that these veins were formed from low temperature (85–169 °C) and high salinity NaCl + CaCl2 + H2O type (apparent total salinity: 15.91–22.46 NaCl wt%) hydrothermal fluids, similar to parent fluids of the Alpine‐type Pb‐ Zn deposits. These findings indicate that the Triassic regional fluid circulation systems in the Alpine platform carbonates also affected the area of the Balaton Highland. This is also in agreement with the previously established palinspatic tectonic reconstructions indicating that the Triassic carbonate and basement units in the Balaton Highland area were a part of the Southern Alpine. Similar fluorite veining in phosphorite deposits is also known in the Southern Alpine areas (e.g., Monte San Giorgi, Italy). Raman spectroscopic analyses detected H2 gas in the vapor phase of the fluid inclusions and a defect‐rich fluorite structure in violet to black colored growth zones. This unique phenomenon is assumed to be the result of interaction between the uranium‐rich phosphorite and the parent fluids of the epigenetic fluorite veins.