Water-rock interaction between basalt and high-salinity fluids in the Asal Rift, Republic of Djibouti

Abstract

Two deep geothermal boreholes were drilled in the Asal Rift in the Republic of Djibouti. The first hole, Asal 1, produced water and steam, temperatures of up to 250°C being recorded at the well head. The second hole, Asal 2, produced minute amounts of geothermal fluid but no vapor, although similar temperatures were measured. The fluid from Asal 1 gives a good representation of the deep geothermal water. This is a NaC̄l brine with a TDS content of 128 g l −1. The isotopic composition ( δ D = −5.5‰ , δ 18 O = −0.6‰ ) is only slightly different from the isotopic composition of seawater. Mineralogic study of cuttings from Asal 2 showed extensive and pervasive rock alteration. The main neoform minerals are calcite and quartz; chlorite, illite, montmorillonite, Fe-hydroxides, albite and adularia are present, together with minor amounts of epidote and prehnite. Calcite and chlorite are systematically present. To a depth of 424 m, alteration is minor and corresponds to a low water/rock, ( W R ) A , ratio of <2.5. Silicification and neoform albite are absent. Neoform quartz appears at a depth of ∼600 m, at a temperature of 175°C. Adularia and albite occur at a depth of 820 m at a temperature of 210°C. Epidote appears at higher temperatures of ∼250°C (1350 m), and propylitic alteration is indicated by the presence of albite, epidote, chlorite and calcite. Oxygen isotope compositions range from + 4.6 to + 12.2‰ for whole-rock samples, from + 7.3 to + 14.3‰ for neoform quartz and from + 7.8 to + 20.4‰ for neoform calcite. δ 18O mineral -values tend to decrease with depth. Quartzwater isotopic temperatures show good agreement with temperatures measured in situ. Calculated water/rock atomic ratios vary from 0.7 in the less altered zone to ⩾50 in zones of greatest circulation. Results for sulfur in sulfide deposits and whole-rock samples indicate that the sulfur of magmatic origin present in the geothermal fluid has been contaminated by sulfur from sulfates of marine origin present in seawater and in the evaporite layers through which the fluid passes.