SPECIAL FEATURES OF THE FORMING OF THERMAL WATERS OF THE EASTERN PART OF THE KHANGAY NEOTECTONIC UPLIFT

Khangay neotectonic uplift is a large block of the earth’s crust confined to the area between two sublatitudinal deep faults (Bulnay and Goby-Altay). They are active faults accommodating main compression stresses in contract to the extension existed in the other area of the Khangay uplift. In contrast to continental rift zones of Khangay it is the region of compression. It is area with the increased values of the heat flux. DOI: http://dx.doi.org/10.5564/pmas.v0i4.48 Proceedings of the Mongolian Academy of Sciences 2009 No 4 pp.64-70


Introduction
Khangay neotectonic uplift is a large block of the earth's crust confi ned to the area between two sublatitudinal deep faults (Bulnay and Goby-Altay).They are active faults accommodating main compression stresses in contract to the extension existed in the other area of the Khangay uplift.In contrast to continental rift zones of Khangay it is the region of compression.It is area with the increased values of the heat fl ux [6].
The eastern part of Khangay hydro-geothermal system is one of the most energetically powerful on the quality of the fl uid heat-carrier (temperatures of the geothermal water at ground surface are up to 92°C) with the high yield of thermal sources and on the area of deep magma chamber and high activity of Cenozoic volcanism (Fig. 1).Analysis of the temporal-spatial distributions of the volcanic activities manifested in the area indicated that the most heated mantle, capable of magma generation, remains at present precisely under the eastern -Khangay volcanic area [4].The thermal state of deep subsoil is determined not only by the values of the geothermal parameters obtained during the study of conductive heat fl ux, but also with the intensity of the convective removal of deep heat by the underground fl uids, which are discharged in the form of numerous thermal sources within the zones of active fracturing [6].

Method
We carried out the gas-geochemical and temperature testing on more than 20 thermal water sources of the eastern part of the Khangay neotectonic uplift for the purpose of obtaining the information about the thermal condition of underground hydrosphere.Temperature measurements do not always make it possible to determine the reasons for thermal anomaly and infl uence of exogenous and endogenous factors [5].Estimation of the roles of these factors is possible, if exist data on temperature condition in the thermal water table at the depth.Most precise data on "base temperatures" can be obtained by direct measurements of temperatures in the deep boreholes; however, we do not possess such information.On the other hand, the temperature condition of the water table at depth can be estimated by indirect methods, using hydro-geochemical geothermometers
Silica geothermometer is based on the correlation of temperature with the concentration of Si0 2 ; however, the solubility of silica depends not only on temperature but also on other factors such as forms of silica.V.N.Disler [3] examined the behavior of various forms of silicon both in the water and in minerals for different P-T conditions.Based on the regression curves (Fig 2) obtained in result of the experiment, he classifi ed thermal waters into types.Regression curves shown in fi g 2 are composition and temperature dependent parameters, and so far calculated temperature for each line have been performed with taking account into forms of silicon compounds.

Results and discussion
Data on silica dioxide contents in the water of thermal sources Khangay have been plotted on the solubility curve of α-cristobalite.Then, evaluations of predicted temperatures are made according to Fournier-Trusdel formula for the low-temperature systems of t sur < t boil .The results are shown in table.
The values of temperatures of water table at depth and average geothermal gradient y =25°C/km., accepted for this region [1] make it possible to estimate the depth of formation of hydro-therm.They are within of 4-5 km.
-68 -This is confi rmed by geophysical data of Mordvinova, which revealed powerful heated plume at the large depths caused by the ascending of asthenospheric mantle to earth's crust, but the small thermal bodies at the depths of 4-5 km, which appears in geophysical section have been distinguished, according to the data of teleseismic signals " Mobal 2003" [9].
Predicted temperatures of the formation of hydro-therm at the depth (tabl.)are considerably high, however, during the upwelling, the temperature of the gas-liquid heatcarrier goes down due to mixing with underground water and warming up the surrounding rocks etc.According to the published information these losses usually are within 30 -50 percent.Furthermore for the calculations of the depth of the formation of the hydrotherm, same geothermal gradient used for conductive heat fl ux have been used.We are dealing with the convective fl ow, and consequently the geothermal gradient is higher.
Based on the obtained results, the 6 potential areas have been isolated for further studies with purpose of geothermal energy usage.The vertical electrical sounding have been conducted on two of them (Shivert and Tsenkher) end the promising areas have been contoured.
The geophysical studies (VES) conducted on Shivert area near the basic output of hydro-therm (fi g.3),have shown that the earth crust with the lowered value of resistances is available in the sections of apparent resistances sections of VES 6 and 7 up to the depth of 40 meters.The curve VES 10 is obtained at a distance of 25 m from the acting source.Within a source at a depth of 40 m electrical resistivity is in the range 10-20 om•m (VES 10) and in section 6, the VES increased to 50-100 om•m.This it can testify about the increased mineralization and the temperature of ground water.In 2009 drill hole of 78 m deep had been performed at the section of Shivert, near the basic source.For the selection of the appropriate places for exploration boreholes, the broad data analysis, obtained not only under our expedition, but also under previous exploration work carried out by mineral water division of Mineral resources authority of Mongolia in Ulaanbaatar, have been performed using widely known methods (chemical, gaseous, geothermal, vertical electrical sounding), and also the newest procedures (geochemical geothermometers, isotopic studies, land survey, ICP-MS) [1].
The drilled borehole (2-gr) at chosen places has revealed thermo with the maximum temperature of +66.9°C for this deposit.Schematic hydro-geological and geothermal section the valley of river Shivert have shown in fi g.4.Upwelling thermal waters occurs through the sand-gravel deposit, therefore maximum temperature is achieved at the depth of bedrock -33 m.
The result of the performed works on Shivert deposit, allow assuming the presence of geothermal sources at the depth of up to 1 km with the temperature of gas-aqueous solution of 130-150°C.For determining the precise depth of geothermal source, it is necessary to conduct the additional geophysical survey (gravimetric reconnaissance) penetrating to the depth of 2 km.Vertical electrical sounding is effective for small depths up to 100 m, and teleseismic signals covers too great depths, that is beyond of the earth's crust (tens of kilometers).

Fig. 1 .
Fig.1.Khangay hydro-geothermal system (fragment of the map of the mineral waters of Mongolia M 1:2500000) Centers of upwelling of the nitric siliceous alkaline carbonate sodium hydro-therms: 1-sources, 2-boreholes.Numbers and signs: on top -temperature, °C; in the left -the index of balneological active constituent; in the right -the name of the hot springs; below -fl ow rate or debit, Us.Heat-generating structures: 3-granite plutons; 4-covers of Cenozoic basalts.Water-conducting faults: 5a-observed; 5b-inferred.Boundaries of the Khangay elevation: 6a-expressed in the relief; 6b-conditional.

Fig. 3 .
Fig.3.Section of apparent resistances and the geoelectric section of thermal water deposit of Shivert

Fig. 4 .
Fig.4.Hydro geological and geothermal section of stretch of the valley of river Shivert