Formation Environments of Metasomatites and Ores of the Epithermal Gold-silver Deposits

Preore metasomatites of epithermal gold-silver deposits are formed by various types of hydrothermal solutions-from highly acidic and oxidized (argillization, alunitization, secondary quartzites) and moderately acid (sericitization) to rather reduced and alkaline (adularia). The possible processes creating highly acid mineralizing fluids at shallow depths are: (a) the condensation of the acid gas phase of heterogenized fluids (including also the condensation of gaseous HCl, HF and H2SO4 whose water solutions possess properties of maximum boiling azeotropic mixtures); (b) the oxidation of containing SO2 and H2S magmatogene gases by meteoric waters rich in oxygen; and also c) possible disproportionation of sulfur from SO2 to H2S and H2SO4. The epithermal gold-silver deposits of high sulfidation type form at the upper parts of heterophase fluid systems. The major gold-silver and sulfide mineralization (except small amounts of earlier generations of pyrite, enargite, and luzonite) precipitates not synchronously with the acid metasomatites, but essentially later, when the water table of liquid and less acid fluids raise into the realm of ore deposition. At zones of flat subducting ocean crust, the deposits of copper-porphyry type are quite often found beneath the epithermal Au-Ag deposits. However, the former cannot be considered the bottom of the gold-silver ore-forming systems, because they sometimes form much earlier and in closed systems; therefore, they have their own bottom part of the ore-forming system (a zone of K-feldspathization with related molybdenum ore) and the top part (a zone of sericitization with related copper mineralization). In addition, in steep subduction zones typical of the Central West Pacific region, including Khabarovsk and Primorsky krais and Japan, epithermal Au-Ag deposits have no spatial or temporal relations to copper-porphyry deposits due to the nearly complete absence of the latter. There are the alkaline metasomatites that form in the lower, bottom parts of epithermal heterophase fluid systems, creating the major bonanza gold-silver and sulfide mineralization of the adularia (low sulfidation) type in the Russian Far East, western America, and Japan. The term high sulfidation widely used now was created by the artificial combination of the terms high sulfate and high oxidation and is very ambiguous, because it really does not imply sulfidization processes. This type of mineralization would be better named acid sulfate.