| Abstract |
The Koshelevsky volcanic massif is one the most promising sites for the development of geothermal power industry in Russia's Kamchatka Krai. Well-known are two modern thermal anomalies – Verkhne (Upper)-Koshelevsky and Nizhne (Lower)-Koshelevsky ones with a total heat output of 75 GW/s which is comparable to a heat efflux of the famous Kamchatka's Valley of Geysers (Vakin et al., 1976). The Nizhne (Lower)-Koshelevsky vapour-dominated geothermal deposit whose capacity equated to electric power amounts to 90 MW (Pisareva, 1987) was identified by 1,500-meter-deep drilling on the western slope of the massif. Hydrothermal discharge at the surface is manifested by gas-vapour jets, condensate waters, separate springs, probably, of deep-seated origin. Research in the 1960s-80s did not resolve all the issues related to the composition and origin of thermal waters of the Koshelevsky volcanic massif. Besides, the last decades were marked by significant changes in the morphology of thermal fields, distribution of discharge areas of hydrothermal waters, etc. All mentioned above has predetermined the timeliness of our work. The waters being discharged within thermal anomalies have a temperature of up to 98 оС and are referred to acid or low acid (рН=3-5.8) sulphate, less often to hydrocarbonate-sulphate ammonium or Ca(Na)-ammonium ones with an elevated content of silicic acid (95-175 mg/l). The total mineral content averagely amounts to 0.6-0.8 g/l and can reach an amount of 2.5 g/l (in water-mud pots). The gas content is dominated by CO2, which is typical for many other Kamchatka's and world systems, H2S, CH4, N2 are also always present. Verkhne (Upper)-Koshelevsky hydrothermal gas-vapour outflows have a higher content of sulphur gases whereas the Nizhne (Lower)-Koshelevsky ones have an elevated content of methane (up to 67% in some drill holes), nitrogen (up to 50%), and presence of a variety of heavy hydrocarbons and presence of oil in vapour condensate (Pozdeev, Nazhalova, 2008). Previously, it was mainly assumed that the hydrothermal gas-vapour outflows of the Koshelevsky deposit were originated under impact of deep (magma-penetrating or mantle ones) fluids which had been confirmed by geodynamic, geological-structural and isotope-geochemical data. Most likely, this hypothesis is relevant and a deep (a mantle one ?) fluid influences the geothermal systems of the Koshelevsky massif. At the same time we focus on the fact that the composition of hydrothermal gas-vapour outflows may be connected with the Neogene volcanogenic-sedimentary rocks of the massif basement. The rocks contain a large amount of organic leftovers with marked presence of petroleum products. Probably, thermal decomposition of the organic matter under high P-T parameters of the environment causes a release of methane, ammonium and boric acid and transition of these components to the solution and to the gaseous state. We estimate that the level of thermal waters in this region may occur at an approximate depth of 2 km below the surface of the Sea of Okhotsk. The formation of a solution boiling zone is accompanied by a release of CO2 in the steam phase, other volatiles, including hydrocarbons, boric and naphthenic acid. A gas-vapour mixture ascends to the earth surface without significant heat losses: a temperature decrease, probably, occurs only due to adiabatic expansion and therefore the temperature of vapour at the thermal fields reaches 120-150 оС. It is still unclear if there is a modern hydrothermal system in the interior of the Koshelevsky volcanic massif or if each large thermal anomaly is formed as a standalone one. This issue is also discussed in the present report. This work has been carried out with a financial support from the Russian Foundation for Fundamental Research (Project 13-05-00262) and Far-Eastern Division of the Russian Academy of Science. |