Adularia-sericite epithermal gold-containing deposits and occurrences in Bulgaria – an overview

During the last 15-20 years the therm "epithermal deposit" (Lindgren, 1933) reached a significant popularity, especially concerning the gold deposits. Based on both gangues and ore minerals, two types of hydrothermal wall-rock alterations have been defined: acid-sulfate and adularia-sericite (Hayba et aI., 1985; Heald et aI., 1987).
   The adularia-sericite type originates under the activity of neutral solutions (pH 7) and both low temperature and salinity (Heald et aI., 1987; Berger and Henley (1989). It has also been described as low-sulfure (Bonham, 1984) or lowsulfidation type (Hedenquist, 1987; White and Hedenquist, 1995), due to the 2- valence of the sulfur and it presence as H₂S. Mayer and Hemley (1967) show the presence of different types of K-feldspars, depending on the hydrothermal environment.
   The firstly published data on the adulariasericite epithermal type in Bulgaria describes a case of adularization into propylites (Chipchakova, 1966). Later on such terms as: zones of adularization, quartz-adularia zones, quartzadularia and quartz-adularia-sericite metasomatites, adularites and quartz-adularites have been used (Радонова, 1970; Chipchakova, 1974; Велинов et aI., 1977; Янев and Караджова,1976; Велинов et aI., 1987; Кунов,1987; Arnaudova et aI., 1991; etc.). The term "adulariasericite" became to be used by Bulgarian authors since 1990 (Velinov and Nokov, 1991; Nokov and Hristova, 1992; Ноков and Христова, 1994; Kunov et aI., 1994; Кунов and Наков, 1996; etc.).