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
Abstract:We propose a two-stage tectonic evolution of SE Afar in Djibouti leading to the complex development of highly asymmetric conjugate margins. From c. 8.5 to c. 2 Ma, an early mafic crust developed, associated in the upper crust with synmagmatic growth faults dipping dominantly to the SW. After an erosional stage, a new detachment fault system developed from c. 2 Ma with an opposite sense of motion (i.e. to the NE), during an amagmatic extensional event. In the Asal area, break-up occurred after c. 0.8 Ma along the footwall of an active secondary detachment fault rooted at depth above the lithospheric necking zone. This evolution suggests that flipflop detachment tectonics is developed during extension at passive margins, in connection with the dynamics of the melting mantle and the associated magma plumbing of the crust.
The Republic of Djibouti has an area of 23,000 km2, a coastline 370 km long and a population of 820,000 inhabitants. It experiences an arid climate characterized by high daytime temperatures and low and irregular rainfall (average of 140 mm/year), resulting in continuous periods of drought. These difficult climatic conditions and the absence of perennial surface water have progressively led to an intensive exploitation of groundwater to meet increasing water demands in all sectors (drinking water, agriculture and industries). In coastal areas, seawater intrusion constitutes a significant additional risk of groundwater degradation. This study is focused on the coastal aquifer of Tadjourah which supplies water to the city of Tadjourah, currently comprising 21,000 inhabitants. The main objective of this work is to assess the current resources of this aquifer; its capacity to satisfy, or not, the projected water demands during coming years; and to analyze its vulnerability to seawater intrusion within the frame of climate change. Three RCPs (Representative Concentration Pathway) were used to simulate different climate scenarios up to 2100. The simulated rainfall series allowed to deduce the aquifer recharge up to 2100. The code Seawat was used to model seawater intrusion into the aquifer, using the recharge data deduced from the climate scenarios. The results indicate that the risk of contamination of the Tadjourah coastal aquifer by seawater intrusion is high. The long-term and sustainable exploitation of this aquifer must take into consideration the impact of climate change.
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