Hydrochemistry of a complex volcano-sedimentary aquifer using major ions and environmental isotopes data: Dalha basalts aquifer, southwest of Republic of Djibouti

International audienceIn the Republic of Djibouti (Horn of Africa), fractured volcanic aquifers are the main water resources. The country undergoes an arid climate. Alluvial aquifers exist in the wadis (intermittent streams) valleys and, in relation with volcanic aquifers, form complex volcano-sedimentary systems. Due to increasing water demands, groundwater resources are overexploited and require a rigorous management. This paper is focused on the Dalha basalts aquifer, located in the Dikhil area (Southwest of Djibouti). This aquifer is of vital importance for this area. Hydrochemical data and isotopic tracers (O-18 and H-2) were used to identify factors and phenomena governing the groundwater's mineralization. The Piper diagram shows complex water types. Results from multivariate statistical analyses highlight three water families according to their locations: (1) groundwater characterized by low ionic concentrations located at the wadis zones; (2) groundwater characterized by moderate salinity and (3) highly mineralized waters mainly flowing in the eastern and central part of the study area, in volcanic aquifers. Results from scatter plots, especially Na versus Cl and Br versus Cl, suggest that the origin of more saline waters is not from dissolution of halite. The delta O-18 and delta H-2 data indicate that the groundwater flowing in the alluvial aquifer is of meteoric origin and fast percolation of rainwater occurs in the volcanic aquifers. These findings provide a preliminary understanding of the overall functioning of this complex volcano-sedimentary system. Additional investigations (pumping tests, numerical modeling) are in progress to achieve a more comprehensive understanding of this system

Geochemistry of thermal springs around Lake Abhe, Western Djibouti

The Republic of Djibouti, occupying an area of 23,180 km2, falls within the arid zone of East Africa and is located above the 'Horn of Africa', adjacent to the Red Sea. This country has several thermal springs and fumaroles distributed over three regions - Lake Assal, Lake Hanle and Lake Abhe. The most characteristic feature of Lake Abhe is the presence of several linear chains of travertine chimneys. The thermal waters are typical of the Na-Cl type near neutral waters rich in CO2. These waters show an oxygen shift, indicating reservoir temperatures>200°C. The chemical signature of the thermal springs and the geology of the Lake Abhe region are very similar to the Tendaho geothermal area of Ethiopia. The geology, temperature gradient and its proximity to Damah Ale volcano make the Lake Abhe region a potential site for geothermal power development

Caractérisation hydro-climatique, analyse comparative des termes du bilan hydrologique du bassin versant d'Ambouli (République de Djibouti)

International audienceLe bassin versant exoréique de l'Oued Ambouli situé au sud-est de la République de Djibouti, s'étend sur plus de 794 km2 soit 3.5 % de la surface du pays. Il est le plus grand bassin versant du district de Djibouti. Cet oued est soumis à des crues rares et brèves qui causent de lourds dégâts en vies humaines et économiques, accentués par l'expansion urbaine de la ville de Djibouti. Cette étude vise à calculer les différentes composantes du bilan hydrologique. Sur la période 2008-2013, ce bassin a reçu une pluie moyenne annuelle de 93.6 mm; l'évapotranspiration réelle (ETR) sur le bassin versant est estimée à 79.6 mm/an soit 85 % des précipitations moyennes annuelles; l'écoulement total approche un volume de 6.5 × 106 m3/an, soit 3.7 mm/an; par déduction, selon la formule du bilan hydrologique, l'infiltration totale est estimée à 10.3 mm/an. Ces résultats comparés aux résultats obtenus antérieurement, apparaissent comme très différents (principalement une baisse des valeurs). Les données disponibles sont très fragmentées dans le temps et ne couvrent que des périodes trop courtes pour pouvoir prétendre fournir des valeurs de référence pour un bilan hydrologique et permettre un suivi évolutif de celui-ci

The geothermal resources of the Republic of Djibouti — II: geochemical study of the Lake Abhe geothermal field

The Lake Abhe Geothermal Field is located in the South-Western region of the Republic of Djibouti, on the border with Ethiopia. The Lake Abhe geothermal system occurs within a rift basin filled with Pliocene-Quaternary volcanic (mainly basalt) and lacustrine sediments. The thermal water in Lake Abhe geothermal field discharges as hot springs at the bottom of hydrothermal carbonate chimneys distributed along the main faults. Hot springs of Lake Abhe geothermal field as well as ground- and surface waters were sampled and major elements, trace elements, and isotopic (18O/16O, 2H/1H, 3H, 34S/32S, 87Sr/86Sr) compositions were analyzed. Hydrochemical features of the hot springs are dissimilar from those of warm waters: the former are mainly Na–Cl dominated whereas the latter were mostly Na–HCO3–Cl–SO4 and Na–HCO3–Cl. The isotopic composition of sulfur and oxygen in dissolved sulfates suggests equilibrium with anhydrite as the major source of sulfates in the thermal waters. Chemical (mainly Na/K and SiO2), isotope (bisulfate- and anhydrite- water), and multiple mineral equilibrium approaches were applied to estimate the reservoir temperature of the hot springs in the Lake Abhe geothermal field. These different geothermometric approaches estimated a temperature range of the deep geothermal reservoir of 120–160 °C. In spite of the relatively wide range, the three different approaches led to a same mean of about 135 °C. The hot spring and warm borehole waters from the southwestern part of the Republic of Djibouti showed a possible mixing with hydrothermal waters from the local rift. The negligible tritium content and the low deuterium values (δ2H − 10‰)

First evidence of epithermal gold occurrences in the SE Afar Rift, Republic of Djibouti

International audienceThe geology of the Republic of Djibouti, in the SE Afar Triangle, is characterized by intense tectonic and bimodal volcanic activity that began as early as 25-30 Ma. Each magmatic event was accompanied by hydrothermal activity. Mineralization generally occurs as gold-silver bearing chalcedony veins and is associated with felsic volcanism. Eighty samples from mineralized hydrothermal chalcedony, quartz ± carbonate veins and breccias were studied from ten sites representing four major volcanic events that range in age from early Miocene to the present. The most recent veins are controlled by fractures at the edges of grabens established during the last 4 Myr. Gold in excess of 200 ppb is present in 30% of the samples, with values up to 16 ppm. Mineralogical compositions allowed us to identify different types of mineralization corresponding to different depths in the hydrothermal system: (1) surface and subsurface mineralization characterized by carbonate chimneys, gypsum, silica cap and quartz ± carbonate veins that are depleted in metals and Au; (2) shallow banded chalcedony ± adularia veins related to boiling that contain up to 16 ppm Au, occurring as native gold and electrum with pyrite, and tetradymite; (3) quartz veins with sulfides, and (4) epidote alteration in the deepest hydrothermal zones. Samples in which pyrite is enriched in As tend to have a high Au content. The association with bimodal volcanism, the occurrence of adularia and the native Au and electrum in banded chalcedony veins are typical of epithermal systems and confirm that this type of mineralization can occur in a young intracontinental rift system

Retrieving soil surface roughness with the Hapke photometric model: Confrontation with the ground truth

International audienceSurface roughness can be defined as the mean slope angle integrated over all scales from the grain size to the local topography. It controls the energy balance of bare soils, in particular the angular distribution of scattered and emitted radiation. This provides clues to understand the intimate structure and evolution of planetary surfaces over ages. In this article we investigate the capacity of the Hapke photometric model, the most widely used in planetary science, to retrieve surface roughness from multiangular reflectance data. Its performance is still a question at issue and we lack validation experiments comparing model retrievals with ground measurements. To address this issue and to show the potentials and limits of the Hapke model, we compare the mean slope angle determined from very high resolution digital elevation models of volcanic and sedimentary terrains sampled in the Asal-Ghoubbet rift (Republic of Djibouti), to the photometric roughness estimated by model inversion on multiangular reflectance data measured on the ground (Chamelon field goniometer) and from space (Pleiades images). The agreement is good on moderately rough surfaces, in the domain of validity of the Hapke model, and poor on others