Hydrogeochemistry and hydrothermal alteration of the Hanle geothermal field, Djibouti-East Africa

Öz

This study focuses on the geochemistry and hydrothermal alteration of the Hanle geothermal field (Djibouti-East Africa). Water from five drinking water wells, a hot spring and a steam condensate in the Hanle graben was analyzed for major and minor hydrogeochemical parameters. Analyzed items for water samples (hot spring water, groundwater, and steam condensate) are pH, electric conductivity (EC), SiO2, Cl, SO4, HCO3, F, Na, K, Ca, Mg, Al, Fe, B, As, δD, δ18O. Using the chemical compositions and saturation indexes of the hot springs, the reservoir temperature was estimated at around 120 to 125 °C. Based on the XRD analysis, the clay minerals smectite, chlorite and illite, as well as quartz, calcite, and alunite, were determined as alteration minerals in this region, which represent high temperature environment (>180 °C). The obtained data suggest that the water in the Hanle region was likely formed either by the mixing of groundwater and heated fossil seawater, or by the dissolution of evaporite by heated groundwater.

Anahtar Kelimeler

• Hydrothermal alteration
• Hanle
• Djibouti
• Geochemistry

Hanle jeotermal alanının hidrojeokimyası ve hidrotermal alterasyonu, Cibuti-Doğu Afrika

Öz

Bu çalışmada, Hanle Jeotermal Alanının (Cibuti-Doğu Afrika) jeokimyası ve hidrotermal alterasyonuna dayalı olarak alanın jeotermal potansiyelinin araştırılması amaçlanmıştır. Hanle grabenindeki beş içme suyu kuyusundan, bir kaplıcadan ve bir buhar yoğuşmasından alınan sulardan hidrojeokimyasal analizler yapılmıştır (pH, elektrik iletkenliği (EC)，SiO2, Cl, SO4, HCO3, F, Na, K, Ca, Mg, Al, Fe, B, As, δD, δ18O). Bu suların doygunlukları ve çeşitli jeotermometreler kullanılarak, rezervuar sıcaklığının yaklaşık 120- 125°C olduğu tahmin edilmiştir. XRD çözümlemelerinde yüksek sıcaklık ortamını (>180 °C) temsil eden smektit, klorit ve illit türü kil mineralleri ile kuvars, kalsit ve alunit gibi alterasyon mineralleri belirlenmiştir. Elde edilen veriler, Hanle bölgesindeki suyun ya yeraltı suyu ve ısıtılmış fosil deniz suyunun karışmasıyla ya da ısıtılmış yeraltı suyunun evaporiti çözmesiyle oluşmuş olabileceğini göstermektedir.

Anahtar Kelimeler

• Hidrotermal alterasyon
• Hanle
• Cibuti
• Jeokimya

Kaynakça

1. [1] Khaireh A, Fouad A. "Geothermal Development in Djibouti Republic: A Country Report". Proceedings 4th African Rift Geothermal Conference, Nairobi, Kenya, 21-23 November 2012.
2. [2] Geoffroy L, Le Gall B, Daoud MA, Jalludin M. "Flip-flop detachment tectonics at nascent passive margins in SE Afar". Journal of the Geological Society, 171(5), 689-694, 2014.
3. [3] Corti G, Bastow ID, Keir D, Pagli, C. Rift-Related Morphology of the Afar Depression. Editors: Billi P. Landscapes and Landforms of Ethiopia (Part of the book series: World Geomorphological Landscapes), 251-274, Springer Nature, 2015.
4. [4] Geoffroy L, Bernard Le G, Mohamed D. "Plate break-up geometry in SE-Afar". EGU General Assembly Conference Abstracts. Vienna, Austria, 27 April-02 May 2014.
5. [5] Giggenbach WF, Matsuo S. "Evaluation of results from Second and Third IAVCEI field workshops on volcanic gases, Mt Usu, Japan, and White Island, New Zealand". Applied Geochemistr, 6(2), 125-141. 1991.
6. [6] Giggenbach WF, Minissale AA, Scandiffio G. "Isotopic and chemical assessment of geothermal potential of the Colli Albani area, Latium region, Italy". Applied Geochemistry, 3(5), 475-486, 1988.
7. [7] Arnórsson S. Isotopic and Chemical Techniques in Geothermal Exploration, Development and Use. Sampling Methods, Data Handling, and Interpretation. International Atomic Energy Agency (IAEA), Vienna (Austria), 2000.
8. [8] Arnórsson S, Gunnlaugsson E, Svavarsson H. "The chemistry of geothermal waters in Iceland. III. Chemical geothermometry in geothermal investigations". Geochimica et Cosmochimica Acta, 47(3), 567-577, 1983.

9. [9] Fournier RO, Potter RW. "Magnesium correction to the NaK-Ca chemical geothermometer". Geochimica et Cosmochimica Acta, 43(9), 1543-1550, 1979.
10. [10] Fournier RO, Truesdell AH. "An empirical Na-K-Ca geothermometer for natural waters". Geochimica et Cosmochimica Acta, 37(5), 1255-1275, 1973.
11. [11] Balaban TÖ, Akın H, Tarcan G, Gemici Ü, Çolak M, Karamanderesi İH. “Hydrogeochemical study of the Gümüşköy spa (Aydın) and its vicinity”. Pamukkale University Journal of Engineering Sciences, 22(6), 553-558, 2016.
12. [12] Mehegan JM, Robinson PT, Delaney JR. "Secondary mineralization and hydrothermal alteration in the Reydarfjordur drill core, eastern Iceland". Journal of Geophysical Research: Solid Earth, 87(B8), 6511-6524, 1982.
13. [13] Hedenquist JW, Arribas AR, Eliseo GU. "Exploration for epithermal gold deposits". In: S.G. Hagemann and P.E. Brown (Editors), Gold in 2000: Society of Economic Geology Special Publication, 13, 245-277, 2000.
14. [14] Kristmannsdóttir H. "Alteration of basaltic rocks by hydrothermal-activity at 100-300 C". Developments in Sedimentology, 27, 359-367, 1979.
15. [15] Stakes DS, O’Neil JR. "Mineralogy and stable isotope geochemistry of hydrothermally altered oceanic rocks". Earth and Planetary Science Letters, 57(2), 285-304, 1982.
16. [16] Randive K, Chaudhary S, Dandekar S, Deshmukh K. "Characterization and genesis of the chalcedony occurring within the Deccan lava flows of the LIT hill, Nagpur, India". Journal of Earth System Science, 128, 1-12, 2019.