Geochemical Characteristics and the Ore Potential of the Mutnovsky Geothermal Field, Southern Kamchatka

We provide data on the major and trace element composition of deep waters in the productive reservoirs beneath the Mutnovsky geothermal field confined to the eponymous active volcano in southern Kamchatka. The goal of the present study was to reveal the key role of deep-seated heat carrier in the formation of gold–silver mineralization taking the Mutnovsky geothermal system as an example. Several significant gold–silver ore deposits lie near the system. It has been found that deep waters are mostly alkaline (pH 8.85–9.74) or near-neutral (pH around 6), as well as chloride–sulfate or sulfate–chloride, potassium–sodic or sodium-potassic waters. The salinity varies in the range 260–4465 mg/L. It has been found that salinity is appreciably higher in alkaline waters than in near-neutral. Among the elements reliably detected, the highest concentrations occur for Br, As, Se, Sr, Ba, Li, Ag, and Zn. The suspended particles found in water samples contain compounds such as poorly soluble iodargyrite AgI, naumannite Ag2Se, and electrum (Au, Ag). At present these minerals are formed within the areas of deep-seated vapor-dominated hydrothermal fluids belonging to the Mutnovsky field, forming regions of present-day mineral generation. Arsenic, antimony, selenium, and tellurium occur in alkaline environments in mobile water-soluble forms, and chloride ions favor the mobilization of noble metals, e.g., gold and silver, which tend to make polysulfide and chloride complexes. The mobilization of ore elements increases in alkaline environments owing to the formation of hydroxy complexes, so that for this case alkaline waters have the highest ore potential with regard to noble-metal mineralization.