Salty Matters

The Blog is written by me, John Warren. Once every three or four weeks or so I will post an article or two on an evaporite topic that has piqued my interest. On the Saltwork Publications webpage (under "the Works") there is a growing library of pdfs and epubs based on these blogs. These articles on the website have much higher resolution extractable graphics in than in the blog. There is also a link to this set of pdfs and epubs on the home page (www.saltworkconsultants.com).

Danakhil Potash, Ethiopia: Beds of Kainite/Carnallite, Part 2 of 4

John Warren - Wednesday, April 29, 2015

The modern Dallol saltflat described in the previous blog defines the upper part of more than 970 metres of halite-dominated Quaternary evaporites that have accumulated beneath the present salt pan of the Northern Danakhil. The total sequence is made up of interbeds of halite, gypsum, anhydrite and shale with a potash succession separating two thick sequences of halite (Figure 1; Holwerda and Hutchison, 1968; Augustithis, 1980). At depths of more than 35-40 meters, and deepening to the east, this km-thick subcropping Quaternary halite-dominated fill contains one, and perhaps two or more, potash beds. For a more detailed description of the upper part of the fill the reader is referred to the previous blog and Chapter 11 in Warren, 2015.


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 across much of the southern part of the depression beyond Lake Assale). Potash exploration drilling and core recovery is concentrated in the accessible parts of the northern Danakhil rift, where the saltflat facilitates vehicle access, compared with the lava-covered regions south of Lake Assale. The most recent volcanic activity affecting the known potash region was the emplacement of the Dallol Mound, which has driven local uplift of the otherwise subsurface potash section to where it approaches the surface in the immediate vicinity of the mound (Figure 2a).

Away from the Dallol volcanic mound the upper potash bed beneath the saltflat lies at a depth of 38-190 metres. A lower inferred potash bed likely occurs at depth along the eastern end of the saltflat, but this second bed is inferred from high API kicks in gamma logs run in deeper wells, no solid salt was recovered (Holwerda and Hutchison, 1968). The upper proven potash bed is now the target zone for a number of minerals companies currently exploring for potash in the region. Regionally, both potash units dip east, with the deepest indicators of the two units encountered by the drill in a single well on the eastern side of the saltflat at depths of 683 and 930 m, respectively (Figure 2: Holwerda and Hutchison, 1968). The likely Quaternary age of the potash units, the marine brine source, explains the high magnesium content of the potash bittern salts, as modern seawater contains high levels of Mg and SO4.


My study of core that intersected the potash interval and that is sandwiched between the Lower and Upper Halite units shows both the lower and the upper halite units retain pristine sedimentary textures, with features and vertical successions that indicate distinct hydrologies during their deposition (Figure 3). There is no textural evidence of halokinetic recrystallization in halites any of the studied cores and published seismic also indicates consistent dips in the evaporites . Most of the textures in the cored potash interval indicate a subaqueous density-stratified environment, with brine reworking of the upper part of primary kainitite, carnallitite units. Perennial subaqueous, density-stratified brines also typify the hydrology of the Lower Halite unit, albeit with somewhat lower salinities tan those precipitating the bitterns (Figure 3). The brine that precipitating the Upper Rocksalt Formation was shallower and more ephemeral. The following paragraphs summarise my core-based observations and interpretations that led to this interpretation of the evolving brine hydrology.

The Lower Rocksalt Formation (LRF) is dominated by bottom-growth-aligned subaqueous halite textures and lack of siliciclastic detritus, unlike the Upper Rocksalt Formation (Figure 3). Halite textures in the LRF lack porosity and dominated by coarsely crystalline beds made up of cm-scale NaCl-CaSO4 couplets dominated by upward-pointing halite chevrons and mantled by thin CaSO4 layers (Figure 3). This meromictic-holomictic textural association passes up into the upper part of the LRF with cm-scale proportions alternating of less-saline to more-saline episodes of evaporite precipitation decreasing, indicating an “on-average” increasingly shallow subaqueous depositional setting as one approaches the base of the kainitite unit. The combination of bottom-nucleated and cumulate textures in the LRF are near identical to those in the halites in the kainitite interval in the Messinian of Sicily (see later). 

The laminated Kainitite Member is also a subaqueous unit with layered cumulate textures (Figure 3), it was likely deposited on a pelagic bottom beneath a shallow body of marine-fed bittern waters, which never reached carnallite saturation. Above this are the variably present carnallitic Intermediate and Sylvinitite members and the overlying Halite marker beds in turn overlain by the Upper Halite unit. All retain pristine textures indicating mostly subaqueous deposition, soon followed by varying exposure and reaction with shallow phreatic brines moving across the top of Kainitite member. This shallow phreatic brine crossflow drove syndepositional mineral alteration and collapse in the upper part of the kainitite and carnallitite units.


The potash-entraining interval between the URF and LRF is called the Houston Formation has been drilled and cored extensively by explorers in the basin, showing it is consistently between 15 and 40 metres thick (Figure 1). Stratigraphically, it consists of lower Kainitite Member (4-14m thick) atop and in depositional continuity with the LRF (more than 500m thick) (Figure 3). The Kainitite Member is fine-grained, laminated, locally wavy-bedded, containing up to 50% kainite cumulates in a cumulate (non-chevron) halite background, along with small amounts of a white mineral that is likely epsomite. It is overlain by what older literature describes as the “Carnallitite Intermediate unit” (3-25 m thick). More recent potash exploration drilling has shown all the members that constitute the Intermediate Carnallitite Member is not always present within the Dallol depression. Mineralogically is at best considered as variably developed (Figure 3). Its lower part is a layered to laminated carnallite-halite mixture with some kieserite, anhydrite and epsomite. This can pass up or laterally into kainitite with sylvite. Above the Intermediate Member is the 0-10m thick Sylvinite Member containing 20-30% sylvite, along with polyhalite and anhydrite (up to 10%). Typically the sylvinite member shows primary layering disturbed by varying intensities of slumping and dissolution. Often the upper part of a carnallite unit (where present) also shows similar evidence of dissolution and reprecipitation.

Cores through the sylvinite member and parts of the upper carnallitite member sample a range of recrystallization/flow/slump textures, rather than primary horizontal-laminar textures. Beneath the sylvinite member, the variably-present upper carnallitite member contains a varied suite of non-commercial potash minerals that in addition to carnallite include, kieserite, kainite (up to 10% by volume) and polyhalite, along with minor amounts of sylvite. Minor anhydrite is common, while rinneite may occur locally, along with rust-red iron staining. Sylvite is more abundant near the top of the carnallitite member and its proportion decreases downward, perhaps reflecting its groundwater origin. Kainite is the reverse and its proportion increases downward. The sylvinite member and the carnallitite member also show an inverse thickness relationship. Bedding in the carnallitite member is commonly contorted with folded and brecciated horizons interpreted as slumps. The base of the carnallitite member is defined as the level where carnallite forms isolated patches in the kainite before disappearing entirely.

Drilling in the past few years has clearly show that in some parts of the evaporite unit, located nearer to the western side of the basin, the lower and upper carnallite units are separated by thick bischoftite intervals (Figures 2b, 3). The bischofite is layered at a mm-cm scale and with no obvious breaks related to freshening and exposure, implying it too was deposited in a perennially subaqueous or phreatic cavity setting (Pedley et al., in press).

The potash/bischofite interval passes up into a slumped and disturbed halite-dominated unit that is known as the “Marker Beds” because of the co-associated presence of clay lamina and bedded halite, along with traces of potash minerals (Figure 3). This unit then passes up into the massive Upper Rock Salt unit across an unconformity at the top of the halite “Marker Beds.” Bedded, and at times finely laminated cumulate textures in the various magnesian bittern units, are used by many to argue that the kainitite and the lower carnallitite members are primary or syndepositional precipitates.

Three types of potash-barren zones can occur within it and are possibly related to the effects of groundwaters and solution cavity cements within the carnallitite unit, perhaps precipitated before the deposition of the overlying marker halites. Barren zones in the Sylvinite member are regions where: a) the entire sylvinite bed is replaced by a relatively pure stratiform halite, along with dispersed nodules of anhydrite, b) zones up to 23 m thick and composed of pure crystalline halite (karst-fill cements?) that occur patchily within the sylvinite bed and, c) potash-depleted zones defined by coarsely crystalline halite instead of sylvinite. Bedding plane spacing and layering and some slumping styles in the halite in styles a and b are similar to that in the sylvite bed. Contact with throughflushing freshened nearsurface and at-surface waters perhaps created most of the barren zones in the sylvinite. Fluid crossflow may also have formed or reprecipitated sylvite of the upper member, via selective surface or nearsurface leaching of MgCl2 from its carnallite precursor (Holwerda and Hutchison, 1968; Warren 2015). Due to the secondary origin of much of the sylvite in the Sylvinite member, the proportion of sylvite decreases as the proportion of carnallite increases, along with secondary kieserite, polyhalite and kainite.

The kainite member is texturally distinctive and is composed of nearly pure, fine-grained, dense, relatively hard, amber-coloured kainite with ≈ 25% admixed halite (Figure 3). Core study shows the lamina style remained flat-laminar (that is, subaqueous density-stratified with periodic bottom freshening) as the mineralogy passes from the LRF up into the flat-laminated kainitite member (Figure 4: Warren, 2015; Pedley et al., in press). Throughout, the kainitite unit shows a cm-mm scale layering, with no evidence of microkarsting or any exposure of the kainitite depositional surface. That is, the Kainitite Member is a primary depositional unit, like the underlying halite and still retains pristine evidence of its dominantly subaqueous depositional hydrology. The decreased proportion of anhydrite in the Kainitite Member, compared to the underlying LRF, indicates a system that on-average was more saline than the brines that deposited the underlying halite. The preponderance of MgSO4 salts means the Kainitite unit like the underlying LRF formed by the evaporation of seep-supplied seawater.

This situation differs from the present “closed basin” hydrology of the Danakil Depression which typifies the URF and the overlying Holocene succession (Hardie, 1990; Warren, 2015).

Units atop the primary laminated textures of the kainitite, lower carnallitite and bischofite members (where present) tend to show various early-diagenetic secondary textures (Figure 4). It seems much of the sylvinite and upper carnallitite member deposition was in shallow subsurface or at-surface brine ponds subject to groundwater crossflows and floor collapse, possibly aided by seismically-induced pulses of brine crossflow. In addition, this perennial density-stratified brine hydrology was at times of holomixis subject to brine reflux and the brine-displacement backreactions that typify all evaporite deposition, past and present (Warren 2015).

The observation of early ionic mobility in potash zone brines in the Danakil depositional system is also not unusual in any modern or ancient potash deposit. It should not be considered necessarily detrimental to the possibility of an extensive economically exploitable potash zone being present in the Danakil Depression. Interestingly, all the world’s exploited potash deposits, including those in the Devonian of Canada and Belarus, the Perm of the Urals and the potash bed of west Texas, show evidence of syndepositional and shallow burial reworking of potash (Warren, 2015). Early potassium remobilization has created the ore distributions in these and other mined potash depositsTextures and mineralogies in the Upper Rocksalt Formation (URF) define a separate hydrological association to the marine-fed LRF and Houston Formation (Table 4). Compared to the LRF, the URF has much higher levels of depositional porosity, lacks high levels of CaSO4, and has high levels of detrital siliciclastics. This is especially so in its upper part, which shows textural evidence of periodic and ongoing clastic-rich sheetflooding and freshening (Figure 4). It was deposited in a hydrology that evolved up section to become very similar to that active on today’s halite pan surface. The URF contains no evidence of salinities or textures associated with a potash bittern event and is probably not a viable exploration target for solid potash salts.

Above the URF is a clastic unit with significant amounts of, and sometimes beds dominated by, lenticular gypsum and displacive halite. The unit thickens toward the margins of the depression (Figure 2). The widespread presence of diagenetic salts indicates high pore salinities as, or soon after, the saline beds that stack into the clastic unit were deposited. Some of these early diagenetic evaporite textures are spectacular, as seen in the displacive halite recovered in a core from the lower portion of the clastic overburden, some 45 m below the modern pan surface (Figure 3).

What is clear from the textures preserved in the potash-rich Houston formation and the immediately underlying and overlying halites is that they first formed in a subaqueous-dominated marine-fed hydrology (Figure 4), which evolves up section into more ephemeral saltpan hydrologies of today (see the previous blog). The potash interval encapsulated in the Houston formation has primary mineralogical associations that are derived by evaporation of Pleistocene seawater (kainitite, carnallitite). In contrast the sylvite section in the Houston tends to form when these primary mineralogies are altered diagenetically perhaps soon after deposition but, especially, when hydrothermal waters circulated through uplifted beds of the Houston Formation, as is still occurring in the vicinity of the Dallol Volcanic Mound. Or where the chemical/meteoric interface associated with the encroachment of the bajada sediment pile drove incongruent dissolution of carnallite along the updip edge of the Houston Fm (as we shall discuss in the next blog). 

References

Augustithis, S. S., 1980, On the textures and treatment of the sylvinite ore from the Danakili Depression, Salt Plain (Piano del Sale), Tigre, Ethiopia: Chemie der Erde, v. 39, p. 91-95.

Hardie, L. A., 1990, The roles of rifting and hydrothermal CaCl2 brines in the origin of potash evaporites: an hypothesis: American Journal of Science, v. 290, p. 43-106.

Holwerda, J. G., and R. W. Hutchinson, 1968, Potash-bearing evaporites in the Danakil area, Ethiopia: Economic Geology, v. 63, p. 124-150.

Pedley, H. M., J. Neubert, and J. K. Warren, in press, Potash deposits of Africa: African Mineral Deposits, 35TH International Geological Congress (IGC), Capetown (28 August to 4 September 2016).

Warren, J. K., 2015, Evaporites: A compendium (ISBN 978-3-319-13511-3) Released August 2015: Berlin, Springer, 1600 p. 

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