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

Abstract

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.

Keywords

• Hydrothermal alteration
• Hanle
• Djibouti
• Geochemistry

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

Abstract

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.

Keywords

• Hidrotermal alterasyon
• Hanle
• Cibuti
• Jeokimya

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