Shallow level low-sulphidation type epithermal systems in the Regec caldera, Central Tokaj Mts., NE-hungary

Detailed investigation of the Regec caldera in the Central Tokaj Mts. revea led several types of hydrothermal centers of low-sulphidation type formed a t different paleolevels. At the paleosurface, a hydrothermal eruption brecc ia and layered siliceous deposit of opal-C and -CT material with cinnabar a nd anomalous enrichments of Hg and Sb were formed from a hot spring. Silici fied tuff horizons with alunite-kaolinite alteration indicate steam-heated zones. This type of alteration was formed in the near surface zone, probabl y above the paleowater table. Stable isotope data for large-sized alunite c rystals in brecciated tuff may also indicate a magmatic steam origin. In th e deeper zone, around 90-140 m minimal paleodepth, adularia-sericite altera tion with quartz veining in andesite formed with anomalous Sb, As and Ba co ncentrations. Two stages of K-feldspar formation can be recognized: 1. meta somatic K-feldspar, replacing plagioclase phenocrysts, and 2. late adularia in quartz-pyrite-veinlets. Intensive brecciation and adularia formation su ggest a pressure drop and boiling of the mineralizing fluids. Late stage qu artz crystals show frequent homogenization temperatures between 170 and 190 degrees C and a maximum salinity of 3 wt. % NaCl equiv. Stable isotope dat a for quartz crystals suggest that the dominant mineralizing fluid was mete oric water that underwent exchange reaction with the host rock and/or mixed with magmatic water.