هيدروژئوشيمي و منشا سيالات گرمابي منطقه زمين گرمايي سبلان بر پايه يافته هاي ايزوتوپي

Hydrogeochemistry and origin of hydrothermal fluids on the basis of isotopic data in Sabalan geothermal system

شيب زمين گرمايي بالا در اطراف كوه سبلان حاصل رخداد فعاليت هاي پلوتونيكي-ولكانيكي در اين ناحيه بوده كه در اواخر دوره ي ترشيري رخ داده اند. حاصل اين فعاليت ها تشكيل مخروط استراتوولكان سبلان و ديگر مظاهر سطحي مربوط به عهد حاضر است. از قبيل وجود چشمه هاي آب گرم، دودخان ها و منافذ خروج بخار مي باشند. تركيب عمده اين آب ها سديم-كلرايدي، كلرايد-بي كربناتي و كلرايد-سولفاته مي باشد. در اين مطالعه نمونه ها از چشمه هاي آب گرم اطراف سبلان (شمالغرب، شرق و جنوب سبلان) و چاه هاي اكتشافي حفر شده در سايت بهره برداري از انرژي زمين گرمايي سبلان واقع در شمال غرب سبلان نمونه برداري شده و تركيب شيميايي و ايزوتوپي اين نمونه هاي مورد ارزيابي قرار گرفته شده است. آناليز ايزوتوپي از آبهاي گرم چشمه هاي مورد مطالعه نشان داد كه تركيبات ايزوتوپي از 7/5- تا 13/4- پرميل براي δ18O ، از 71/1- تا 78/6- پرميل براي δDو از 0/1 تا 34/6TU براي 3H تغيير مي كنند. مقادير منفي δ18O و δD بدست آمده بواسطه منشاء جوي اين آب ها، واكنش هاي آب-سنگ و حضور كاني هاي رسي مي باشند. محتواي 3H اين آب هاي نيز نشان مي دهد كه سيالاتي نسبتاً با سنين مختلف در درون سيستم زمين گرمايي سبلان در حال چرخش مي باشند. بعضي از اين آبها قبل از انفجارات هسته اي سال 1953 وارد سيستم ژئوترمال شدند، ولي بعضي ديگر بعد از آن وقايع وارد سيستم شده اند.

Extended Abstract Introduction The surface manifestations of geothermal resources such as thermal springs, steam grounds, fumaroles, mud pools, and geysers in a region along with their locality, geologic control, physical environment, temperature, chemistry, and the rate of fluid discharge are the most important features that should be taken into account for geothermal explorations. The preliminary activities in the context of geothermal systems in Iran go back to 1975 when an Italian company (ENEL) carried out comprehensive investigations in geothermal fields in this country. Since then, numerous studies concerning geothermal explorations were done in Iran, and many prospect areas were identified. One of these areas considered to be of prime importance is around the volcanic mount Sabalan in Azerbaijan Block, NW of Iran. In present study, the authors endeavored to assess and discuss the geological characteristics of the geothermal fields around Sabalan volcano with placing special emphasis on geochemical aspects involved in fluids of this field. Materials and Methods Among several cold and hot spring waters in the study area, 48 samples from high fluid flow springs were collected. For sampling of hot springs, the priority was given to those of maximum discharge rate and the highest discharge temperature at the surface. The chemical analyses of these samples for determination of major and trace elements as well as stable isotopes were carried out in hydrogeochemistry lab at Bremen University (Germany). The stable isotope analyses (δD and δ18O) were carried out by using LGR DLT-100 Laser Spectrometer (Los Gatos Research). The analytical precisions for δ18O and δD were ±0.2‰ and ±1‰, respectively. Discussion and Results In the study area, the hot water samples depict temperature and pH range of 9-89ºC and 4.5-8.58, respectively. The total dissolved solids (TDS) vary from 249 to 7006 mg/l. The results obtained from isotopic analyses of 48 cold and hot spring water samples from Sabalan geothermal system were carried out for determination of δ18O and δD, and 3H of these waters (see Table 1). The δ18O values of these samples vary from -13.4‰ to -7.5‰. The δD values range from -77.6‰ to -71.1‰. 3H values vary from 0.1 to 105 TU.