The Conditions for Epithermal Mineralization in the Kyplatap Volcanic Field, Central Chukotka

This paper considers the conditions of formation for dominantly silver mineralization in the Kyplatap Volcanic Field (KVF), the Chaun Zone, the Central Chukchi Sector, and the Okhotsk-Chukchi Volcanogenic Belt (OChVB). New data are provided for the composition, distribution, and concentrations of trace elements, including REEs, in ore-bearing metasomatites. Appreciable enrichment of the samples in REEs, the dominance of light lanthanoids over heavy ones, very low Eu/Sm ratios (<< 1), and low slopes of the near-chondrite spectra involving obvious europium minima, are all typical of the KVF epithermal mineralization system. The values of Ce/Ce* vary between small negative and moderate positive values (between 0.81 and 1.38), while Eu/Eu* varies between 0.28 and 0.7. This combination of Ce/Ce* and Eu/Eu* indicates oxidizing conditions that prevailed during mineralization. The results of thermometric and cryometric studies in fluid inclusions in quartz revealed the fact that the ore-forming fluid contained chlorides of Na, Mg, and K, the epithermal mineralization was deposited by homogeneous hydrothermal fluids with salt concentrations (0.3-6.2 wt %-equi. NaCl), fluid density 0.60-0.87 g/cm(3), at temperatures of 203 to 343 degrees C, with the steam pressure varying between 40 and 140 bars. These geochemical and thermometric data corroborate our previous hypothesis, viz., that this KVF mineralization belongs to an intermediate sulfidation epithermal class. These data indicate andesite magmas and meteoric waters as the most likely sources of the fluids. The information reported in this paper is of practical significance for regional predictive metallogenetic constructions, search for and estimates of epithermal Au-Ag deposits.