| Abstract |
Formation of new geothermal anomalies in historical time is a rather rare phenomenon. For the most part, such anomalies are formed due to intensification of magma activity (e.g., heating of active volcanoes' slopes) or hydrothermal processes (emergence of new thermal water springs, fumaroles, funnels from hydrothermal explosions). Thermal anomalies most frequently form within known geothermal fields due to natural reasons, sometimes this occurs due to drilling or field development. In most cases the nature of such phenomena is obvious. Starting increasingly from the 1960s, geological-geophysical, geochemical, geobotanical and other works have been carried out at the Nizhne (Lower)-Koshelevsky vapour-dominated geothermal deposit and at the thermal anomaly of the same name, already known in the 18th century. In 2008, an occurrence of a new thermal anomaly named as Nizhne (Lower)-Koshelevsky New Thermal Field (NKN) was recorded. Vegetation and soil was burnt in an area of 200 х 200 meters, the temperature of soils in the central part of the field reached 100 оС. In 2009-2013, we carried out temperature and geochemical (Hg-metering) areal mapping on this site; composition and properties of diluvial deposits in the most heated part of the field and its outskirts were studied; magnetometry survey and electrical exploration were conducted, i.e. everything needed for monitoring of NKN parameter changes and to answer the question of origin. It was determined that a sudden heating of soils was followed by their rapid cooling. The temperature in the most heated sections decreased from 100 to 40 оС in a 5-year period. Anomalously elevated contents of Hg in diluvial deposits are clearly correlated with positive temperature anomalies. Hydrothermal-metasomatic alteration of rock sections occurred in five years: from weak argillization of sand clay (clay loam) and rock fragments to formation of hydrothermal clays characterized by a high level of Hg concentration. All this testifies in favour of NKN emergence due to inflow of a heat- and chemical ore element-carrying gas-vapour fluid from some depth. A subvertical zone of rocks with reduced resistance which, probably, acts as a channel for the ascending gas-vapour fluid is distinguished at geoelectrical sections. The data acquired cannot unambiguously answer the question about the nature of this phenomenon: NKN formed owing to the development of high-dynamic geothermal processes in the interior of the main thermal anomaly or an influx of a heat-carrying medium is caused by disruption of bore casing in one of exploration drill holes located near the field. In any case, the field formation is connected with the sudden penetration of the gas-vapour heat-carrying medium into the zone of elevated permeability of rocks due to manifestation of some physical mechanism: an earthquake event, hydrothermal explosion in the interior of the hydrothermal system, etc. Further monitoring of changes in NKN physical-chemical parameters may give answers to the questions of great scientific and practical meaning for geothermics: what actual heat and matter transfer rates are in the rocks, how much time is needed for the conversion of pyroclastics into diluvial clays, what possible rates and mechanisms of formations of large geochemical anomalies (Hg, Au, Cu, Zn, etc.) impacting the regional geoecology are. This work has been carried out with a financial support from the Russian Foundation for Fundamental Research (Projects 13-05-00262, 14-05-00708, 14-05-31158) and Far-Eastern Division of the Russian Academy of Science. |