Geothermometry and Isotope Geochemistry of CO2-Rich Thermal Waters in Choygan, East Tuva, Russia

Anastasia Shestakova,Natalia Guseva,David Polya,Igor Tokarev,Albina Khvaschevskaya,Yulia Kopylova

doi:10.3390/w10060729

Anastasia Shestakova, Natalia Guseva + Show 4 more

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https://doi.org/10.3390/w10060729

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Abstract

The Choygan area of southern Siberia, Russia hosts a variety of CO2-rich thermal mineral and other waters emerging from springs at temperatures between 7 °C and 39 °C. Chemical analyses of the spring waters (n = 33) were carried out to characterise the waters and determine their origin. A continuum of compositions was observed between relatively lower temperature (7 °C) HCO3-Ca-Na dominated waters with relatively low amount of total dissolved solids (TDS) and high Eh, and higher temperature (39 °C) HCO3-Na-Ca dominated waters with higher TDS and lower Eh—this reflects largely conservative mixing of these components between near surface low temperature, oxidising groundwaters and higher temperature, more reducing thermal waters derived from a deeper geothermal reservoir. Stable isotopic data are consistent with all the water ultimately being derived from meteoric water that has undergone varying degrees of isotopic fractionation following evaporation. The inferred δ18O and δ2H isotopic composition of the unfractionationed meteoric waters is lighter than that expected that of mean annual local precipitation, which together with a strong negative correlation between δ18O and the elevation of the sampled discharging springs, suggests recharge at higher elevations (1600 m to 3000 m; average 2600 m). Reservoir temperature, calculated using geothermometers and an analysis of saturation indices of plausible reservoir minerals, ranged from 70 °C to 100 °C at an inferred depth of 2 to 3 km. Not all chemical components were found to follow conservative mixing behaviour. In particular, (i) the CO2 contents of the waters were highly variable, suggesting either varying degrees of degassing and/or near discharge admixture with air, and (ii) SO4 concentrations in the lower temperature thermal CO2-rich waters were highly variable, suggesting a role of near surface oxidation processes, for example of pyrite, in modifying the concentration of redox sensitive components. Limited δ13C data are consistent with the CO2 predominately being derived from dissolution of metamorphic/igneous carbonate minerals in the reservoir. Based on geological conditions, isotope and chemical data, a conceptual circulation model of the Choygan hydrothermal system is proposed.

Highlights

One current challenge in geochemistry is determining the genesis and the geochemical processes controlling the chemical compositions of mineral waters, which are often generated in complex hydrogeological systems
This study aims to conduct a more detailed survey of the Choygan CO2 -rich thermal waters and to better define the geological and geochemical conditions of formation of the mineral waters and, in particular, the extent and nature of water–rock reaction processes
Thermal CO2-rich mineral waters in the Eastern Sayan Mountainous area in East Tuva (Russia) into the hydrogeological system, where it reacts with primary aluminosilicate minerals that are not as exemplified by springs discharging around the Choygan mineral water natural spa, range in in equilibrium with the waters

Summary

Introduction

One current challenge in geochemistry is determining the genesis and the geochemical processes controlling the chemical compositions of mineral waters, which are often generated in complex hydrogeological systems. These waters, carbon dioxide-rich mineral waters, are widely used for a variety of purposes including geothermal power, carbon dioxide production, heating of shelters and greenhouses as well as bottled water and spas. Better understanding their origins may be helpful to inform sustainable development of these resources. Well known examples are recorded in Turkey [2,3], Iran [1,4], Serbia [5,6], Tunisia [7,8], Mexico [9,10], China [11,12], Korea [13,14,15], Portugal [16,17], Spain [18], Italy [19], France [20,21], Egypt [22,23] and Russia [24,25] amongst other places.

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