Dallol Saline Pan, Danakhil, Ethiopia

The Danakil Depression (aka Dallol Depression) of Ethiopia and Eritrea is an area of intense hydrothermal activity with potash occurrences related to rift magmatism, marine flooding, and deep brine cycling. It's part of the Afar Triple Junction, located in the axial zone of the Afar rift, near the confluence of the East African, Red Sea and Carlsberg rifts. The depression is the northern part of the Afar depression and runs SSE parallel to the Red Sea coast but is some 50–80 km inland and separated from the sea by the Danakil Mountains. It is 185 km long, up to 70 km wide, with a floor more 116 m below sea level in the deeper part of the depression.

The Danakil Depression widens to the south, beginning with a 10 km width in the north and widening up to 70 km in the south. The basin floor in the northern part is the deepest and has elevations as low as 50–120 m below sealevel. A shallow volcano-tectonic barrier behind Mersa Fatma on the coast prevents hydro-graphic recharge, or even substantial seawater seepage, into the current depression from waters of Howakil Bay, which is some 50–60 km WNW of Ito Aichil on the Red Sea coast.

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Dallol (Dalol) saline depression, Ethiopia

In terms of daily temperature, the floor of the Danakil is one of the hottest places on earth; year round the daily temperature is above 34 °C and in summer every day tops 40 °C, with some days topping 50 °C. These high temperatures and a lack of rainfall place it at the hyper-arid end of the world desert spectrum and make it a well-known example of a BWh Köppen climate zone. 

This halite-floored elongate saltpan, known as the Dallol Pan, covers the deepest part of the Danakil Depression over an area some 40 km long and 10 km wide). The pan’s position is asymmetric; it lies near the depression’s western edge, some 5 km from the foot of the escarpment to the Balakia Mountains and the Ethiopian Highlands, but some 50 km from the eastern margin of the depression and today constitutes the deepest continental drainage sump of the Afar depression. The area located northeast of the main modern salt pan is mostly an extensive gypsum plain. It defines a higher less saline lacustrine episode in the basin fill.

Bedded Pleistocene evaporites may underlie the entire Danakil depression, but younger lava flows of the Aden Volcanic Series and alluvium washed in from the surrounding bajada obscure much of the older Pleistocene sedimentary series in the southern part of the depression beyond Lake Assale. A well-defined set of reef outcrops atop the eastern bajada defines a Late Pleistocene marine episode. The region beneath the Dallol pan sediments is considered highly prospective for a variety of potash salts (see Salty Matters Aprile-May 2015, articles 1, 2, 3, 4). 

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Typical evaporite textures in the subsurface of the Dallol depression, Ethiopia. A) Displacive halite in a silty clay matrix at a depth of 45 m B) Alternating mesocrystalline chevron halite and clay interlayers in the Upper Rocksalt Formation, Porosity halite (rocksalt) near the base of the Upper Rocksalt Formation. D) through H) showing representative textures of the various members of the Houston formation. An astersisk indicates members of the “intermediate Unit” that are variably present across a number of wells I) Subaqueous textured halite-anhydrite in core taken from more than 150 m below the surface in Lower Rock Salt Member (some images courtesy of Allana geologists Sam Baldwin and Rhys Cole)

Potash potential

The Danakhil region, especially in the Dallol region of Ethiopia, is world renowned for significant accumulations of potash salts (both muriates and sulphates) and is often cited as a modern example of where potash accumulates today. What is not so well known are the depositional and hydrological dichotomies that control levels of bittern salts in the Pleistocene stratigraphy that is the Danakhil fill. Geological evolution of the potash occurrences in the Dallol saltflat and surrounds highlights the limited significance of Holocene models for potash, when compared to the broader depositional and hydrological spectra preserved in ancient (Pre-Quaternary) evaporite deposits.

What follows is a summary of my current understanding of potash controls in the Dallol depression it expands on the currently published data as summarised in Warren, 2016. The complete set of information on which this summary based on fieldwork by the author during a visit to the region sponsored by BHP Minerals. The parts of the geological story that were given permission to be made public were presented in a series of four Salt Matters articles/blogs on this web site (pdf 1 of 4pdf 2 of 4pdf 3 of 4pdf 4 of 4) and summarised in a recent paper by Bastow et al., 2018.

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Depositional summary of the significance of sedimentary textures in a representative potash-intersecting well, Danakil Depression, Ethiopia.

Surficial geology of the northern Danakil depression shows well-developed outcrop rims defined by an earlier reef and later gypsum phase as well as the extent of the modern halite pan. Each zone is located at successively lower elevations in the depression. Also shown is the position of the Dallol volcanic mound, its fringe of uplifted lake sediments, the associated hydrothermal springs and collapse dolines where carnallite and bischofite are precipitating today.

Away from shallower zones where sylvite forms via alteration, the primary potash interval is mostly as a present as a carnallitite and kainitite association that now dips westward. It is shallow (tens of meters deep) along the eastern side of the depression passing out in buried depths of more than 500 meters further to the south-east, beneath the modern salt pan floor.

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Danakhil depression, Ethiopia. A) Satellite view (Bing image) showing positions of Dallol mound, the salt flat and wells used to construct cross section. a & b refer to hydrothermal karst pools. B) Cross section illustrating the lack of continuity in the various units of the potash stratigraphy above the Kainitite member.

Three styles of Quaternary potash salts in the Danakhil basin

The fact there is not one, but three, associations of potash salts in the Danakhil, each with varying levels of sylvite, carnallite and kainite, has important economic implications for the nature of remobilised potash and the creation of potential potash ores in the Dallol Mound area. Inherent depositional and diagenetic complexity means the visually spectacular sylvite-bischofite pools, that are active at the surface today in the vicinity of the Dallol mound, are process-independent of the other two associations of potash salts. Exploration models must separate potential ore targets from the more regional distribution of primary potash beds (kainitite and carnallitite) and from the bajada associated alteration front that explains the other main style of sylvite occurrence in the Danakhil depression.

Style 1: An initial subaqueous marine-fed setting that deposited widespread bedded potash with significant amounts of potassium sulphates indicating its derivation from seawater with ionic proportions similar to modern seawater bitterns. This bedded, now tilted potash interval is found in subsurface across much of depression. It sits at the top of the lower salt unit, with a deposition transition downward into clastic subaqueous salt textures. Above it, and atop a likely exposure/alteration surface, are the upper halite unit and the interbedded clays and halites, identical to what is accumulating beneath of modern salt pan.

Styles 2 and 3: Remobilised potash salts and brines. There are two main types of remobilised potash; both are related to the circulation of hydrothermal and shallow meteoric fluids that drive alteration fronts whereby groundwaters interact with the original marine-derived potash-sulphate entraining beds to form a variety of potash products including sylvite.


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Mechanisms of potash formation in the Danakhil Depression. A) Primary kainitite carnallitite in drawndown and hydrographically isolated Danakhil rift. B) Hydrothermal carnallite/sylvite in vicinity of Dallol Mound. C) Deep mixing front sylvite via incongruent dissolution of up dip shallow edge of Houston Formation (Warren , 2016).

Style 2: This group of potash salts is tied to the circulation of warm hydrothermal circulation cells in the vicinity of Dallol mound. Importantly, a ground visit quickly shows that Dallol Mound is not a volcanic cone. Rather, it is an anticlinal dome of uplifted and eroded bedded salt, capped and surrounded by hydrothermal crater features typified by karst pools and brine outflows. In the past 50 years, one of these pools formed explosively via the escape of highly pressurised brines, not lavas. Pool and groundwater sump creation is likely related to emplacement of igneous material at depth, into levels with hydrated salts, especially carnallitite and kainitite. As yet there, has been no breakout of volcanic rock material in the Dallol mound area. Instead, the interaction igneous sills and dykes with buried hydrated salt levels drive the release of fluids that can then escape to the surface via a process of hydrofracture to cut across mechanically-weak halites and clays. This same fluid release process may aid in the bed doming and erosion that typifies the Dallol Mound. Ancient equivalents showing fluidisation of hydrated potash zones and alteration of carnallite to sylvite are seen in some Permian salt mines in parts of eastern Germany where Eocene basaltic sills and dykes cut through the Zechstein salts (Schofield et al., 2014).

Style 3: This group of remobilised potash salts/ore occurs in a sylvinite/carnallite band along the western side of the depression as groundwater alteration units developed where deeply circulating saline meteoric interact with potash-rich marine salt beds that were first deposited below the upper halite unit. This set of secondary potash salts is tied to the incongruent dissolution of penecontemporaneous carnallitite and kainitite. This type of sylvite is a potential ore interval (via solution mining) and is now best developed as alteration fronts in the subsurface along the western side of the depression. There, it ties to a hydrochemical interface created by the encroachment of the bajada groundwaters into the western margin of the primary potash salt zone. 

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Hydrothermally-influenced dolines in the saltflat south of the Dallol volcanic mound. A) Regional overview of Dallol volcanic mound and saltflat. B & C detail of potash rich brines occurring in active solution dolines in the Dallol saltflat fed by hydrothermal brines that have dissolved the more soluble portions of uplifted potash-entraining evaporite sediments. Feature C is locally known as Boiling Lake (or Geyser Lake) and is one of the main tourist attractions.

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View north across and pond area with carnallite, bischofite and halite precipitates toward the south west edge of Dallol mound (salt dome) with the metres-high Edge of Black Mountain (uplifted lake halite) in the left mid-ground.

Water chemistry

Water chemistry compiled by Koga and Noda (2006) and published in Detay (2011) shows why some thermal pool waters precipitate carnallite and other bischofite. Based on this published water chemistry and my visit to the pools of the Dallol area, I subdivide the brine chemistry into three groups tied to location. There is one group of waters, the least saline, but still with combined ionic contents above 260,000 mg/l that typify the pore brines of the shallow saltflat sediment. This is a set of Na-Cl-Ca-Mg-SO4 waters, with ambient saltflat temperatures, hosted in pan precipitates dominated by halite with lesser gypsum. Then there are the brines feeding spring mound and seep pools in the interior crater of the Dallol mound. These are hot (≈100-110°C) acidic (ph>1) waters with ionic proportions (Na-Cl-Ca-Mg-SO4) not too dissimilar from the saltflat , although native sulphur, SO4 and Fe-Mn levels are elevated compared to the saltflat giving the halite precipitates their spectacular colouration. The most interesting set of ionic compositions is associated with the various brine-filled dolines of the Black Mountain area (aka Crescent region). These pool waters are thermal (T = 50-70°C) and moderately acidic (pH typically 3-3.5). The most recent pool (formed in a 1926 phreatic fluid eruption) has the highest temperature (110°C) and most elevated salinity (≈426,000 mg/l) of the Black Mountain (Crescent) pool, but with a more moderate pH ≈6.6, compared to the pH ≈ 3.0-3.5 in the nearby carnallitite pools (Figure 25). All the Crescent area brine pools are Cl-Mg-K-Mg-Ca-Na waters with relatively low sulphate levels, hence the precipitation of bischofite and carnallite. These pools have ionic compositions that are distinct from waters in the salt flat sediments or in the Dallol Mound crater and probably reflect an active chemistry, tied to the thermally driven dehydration of potash beds at depth (next section).

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Water chemistry of the Dallol region (replotted and interpreted mineralogically, based on brine chemistry and locations published in Detay, 2011 and Koga and Noda, 2005)

Once again, there is no “one-size-fits-all” model for economic potash understanding (Warren, 2010, 2015). Even in what is probably the youngest known broad-scale marine-fed potash system in the world, the original potash mineralogy and distribution has been altered and locally upgraded via diagenetic interactions with hydrothermal or deep-meteoric fluids. Predicting ore distributions in this, and all potash systems worldwide, requires an understanding of formative process evolution through deep time, and not just the simple application of a layer-cake primary stratigraphic model.

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Dissected and eroded lacustrine sediment exposed on the uplifted margin of the Dallol Mound (credit George Steinmetz)

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