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).

Evaporite interactions with magma Part 2 of 3: Nature of volatile exhalations from saline giants?

John Warren - Saturday, March 16, 2019

 

Introduction

This article discusses general mechanisms of earth-scale volatile entry into the ancient atmosphere during events that involved rapid and widespread heating of saline giants. It develops this notion by looking at whether volumes of volatiles escaping to the atmosphere are enhanced by either the introduction of vast quantities of molten material to a saline giant or the thermal disturbance of that salt basin by bolide impacts. This begins a discussion of the contribution of heated evaporites in two (or three if the Captitanian is counted as a separate event) of the world's five most significant extinction events. It also looks at possible evaporite associations with a substantial bolide impact that marks the end of the Cretaceous. The next article presents the geological details and implications of the various magma-evaporite-volatile associations tied to major extinction events.

As we have seen for evaporite interactions with giant and supergiant volumes of commodities in particular deposits, such as hydrocarbons, base metals (Cu, Pb-Zn and IOCG deposits) evaporites do not form a commodity accumulation. But if evaporites are involved in the accumulation and enrichment processes, the size and strength of the accumulation are much improved. Because of their high reactivity compared to the kinetic stability at and near  thelithosphere's surface across most other lithologies, evaporite act not as creators of enrichment but as facilitators of enrichment (Warren, 2016 Chapters 9, 10, 14, 15 and Salty Matters, March 31, 2017).


End-Permian event

The end-Permian extinction event, colloquially known as the Great Dying, occurred around 252 Ma (million years) ago, and defines the boundary between the Permian and Triassic geologic periods, as well as between the Palaeozoic and Mesozoic eras. It is the Earth's most severe extinction event, when up to 96% of all marine species, 70% of terrestrial vertebrate species disappeared (Table 1, Figure 1). It also involves the only known mass extinction of a number of insect species (≈25%). Some 57% of all biological families and 83% of all genera became extinct. The end-Cretaceous extinction, which marks the demise of dinosaurs, is less severe, although it probably has a stronger hold on the western zeitgeist, while on land, the end-Triassic event marks the ascendancy of the dinosaurs.


Suggested mechanisms driving the end-Permian extinction event include; massive volcanism centred on the Emeishan and Siberian Traps and the ensuing coal or gas fires and explosions, along with a runaway greenhouse effect that was triggered by temperature increases in marine waters (Figure 2). It also may have involved one or more large meteor impact events and a rise in oceanic water temperatures that drove a sudden release of methane from the sea floor due to methane-clathrate dissociation.

The end-Permian event follows on closely from the Capitanian (Emeishan) extinction event when in south China fusulinacean foraminifers and brachiopods lost 82% and 87% of species, respectively (Bond et al., 2015). Proximity in time of the two events may explain why the breadth of the end-Permian extinction event was so severe. The Earth's biota was still recovering from the Emeishan event when the vicissitudes of the End-Permian calamity further decimated the world's biota.

Both the Emeishan and end-Permian extinction events tie to elevated mercury levels in sediments that encompass their respective boundaries (Grasby et al., 2016). Astride both boundaries, the mercury stratigraphy shows relatively constant background values of 0.005–0.010 μg g–1. However, there are notable spikes in Hg concentration over an order of magnitude above background associated with the two extinction events. The Hg/total organic carbon (TOC) ratio shows similar large spikes, indicating that they represent a real increase in Hg loading to the environment. These Hg loading events are associated with enhanced Hg emissions created by the outflows of the Emeishan and end-Permian large igneous province (LIP) magmas.

Interestingly, there is indirect evidence for a synchronous antipodeal impact crater that some argue may have instigated the Siberian volcanism, in much the same way that the end-Cretaceous bolide impact on the Yucatan Peninsula is considered by some to be the antipodeal driver of the Deccan Trap volcanism (von Frese et al., 2009). Other contributing, but likely more gradual tiebacks to the Great Dying, include sea-level variations, increasing oceanic anoxia, increasing aridity tied to the accretion of the Pangean supercontinent, and shifts in ocean circulation driven by climate change (Figure 2).

End-Triassic event

The end-Triassic extinction event, some 201.3 Ma, defines the Triassic-Jurassic boundary. In the oceans, a whole class (conodonts) and 23-34% of marine genera disappeared. On land, all archosaurs other than crocodylomorphs (Sphenosuchia and Crocodyliformes) and Avemetatarsalia (pterosaurs and dinosaurs), some remaining therapsids, and many of the large amphibians became extinct. About 42% of all terrestrial tetrapods went extinct (Figure 3). This event vacated terrestrial ecological niches, allowing the dinosaurs to assume the dominant roles in the Jurassic period. It happened in less than 10,000 years and occurred just before the Pangaean supercontinent started to break apart (Tanner, 2018).


The extinction event marks a floral turnover as well. About 60% of the diverse monosaccate and bisaccate pollen assemblages disappear at the T-J boundary, indicating a significant extinction of plant genera. Early Jurassic pollen assemblages are dominated by Corollina, a new genus that took advantage of the empty niches left by the extinction.

Worldwide the end-Triassic extinction horizon is marked by perturbations in ocean and atmosphere geochemistry, including the global carbon cycle, as expressed by significant fluctuations in carbon isotope ratios (Korte et al., 2019). At this time the Central Atlantic Magmatic Province (CAMP) volcanism triggered environmental changes and likely played a crucial role in this biotic crisis (Schoene et al., 2010). Biostratigraphic and chronostratigraphic studies link the end-Triassic mass extinction with the early phases of CAMP volcanism, and notable mercury enrichments in geographically distributed marine and continental strata are shown to be coeval with the onset of the extrusive emplacement of CAMP (Percival et al. 2017; Marzoli et al., 2018). Sulphuric acid induced atmospheric aerosol clouds from subaerial CAMP volcanism can explain a brief, relatively cool seawater temperature pulse in the mid-paleolatitude Pan-European seaway across the T–J transition. The occurrence of CAMP-induced carbon degassing may explain the overall longterm shift toward much warmer conditions.

End-Cretaceous event

The end-Cretaceous extinction event defines Cretaceous-Tertiary (K–T) boundary, and was a sudden mass extinction event some 66 million years ago. Except for some ectothermic species, such as the leatherback sea turtle and crocodiles, no tetrapods weighing more than 25 kilograms survived. The K-T event marked the end of the Cretaceous period and with it, the entire Mesozoic Era, opening the Cenozoic Era.

A wide range of species perished in the K–T extinction, the best-known being the non-avian dinosaurs. It also destroyed a plethora of other terrestrial organisms, including certain mammals, all pterosaurs, some birds, lizards, insects, and plants. In the oceans, the extinction event killed off plesiosaurs and the giant marine lizards (Mosasauridae) as well as devastating fish, sharks, molluscs (especially ammonites, which became extinct) populations, and many species of plankton. It is estimated that 75% or more of all species on Earth vanished in the end-Cretaceous event.

In its wake, the same extinction event also provided evolutionary opportunities as many groups underwent remarkable adaptive radiation—sudden and prolific divergence into new forms and species within the disrupted and emptied ecological niches. Mammals in particular diversified in the Paleogene, evolving new forms such as horses, whales, bats, and primates. Birds, fish, and perhaps lizards also radiated in newly vacant niches.


In the geologic record, the K–T event is marked by a thin layer of sediment called the K–Pg (Cretaceous - Paleogene) boundary, that is found throughout the world in both marine and terrestrial rocks. The boundary clay shows high levels of the metal iridium and is widely interpreted as indicating the impact of a massive comet or asteroid 10 to 15 km (6 to 9 mi) wide some 66 million years ago (Figure 4a,b). The impact devastated the global environment, mainly through a lingering impact winter, which halted photosynthesis in plants and plankton.

The impact hypothesis, also known as the Alvarez hypothesis (Alvarez et al., 1980), was bolstered by the discovery of the 180-kilometer-wide (112 mi) Chicxulub crater in the Gulf of Mexico in the early 1990s, which provided conclusive evidence that the K–Pg boundary clay represented debris from an asteroid impact. In a 2013 paper, Paul Renne dated the impact at 66.043±0.011 million years ago, based on argon-argon dating (Renne, 2013). He went on to conclude that the main end-Cretaceous mass extinction event occurred within 32,000 years of this date. A 2016 drilling project into the Chicxulub peak ring, confirmed that the peak ring was comprised of granite, likely ejected within minutes from deep in the earth, but the well contained hardly any anhydrite/gypsum, the usual sulphate-containing seafloor rock across the region (Figure 4a, b). As we shall see in part 3, the missing CaSO4 was vaporised in the impact and dispersed as sulphurous aerosols into the atmosphere, causing longer-term deleterious effects on the climate and food chain. Another causal or contributing factors to the end-Cretaceous extinction event may have been the synchronous outflows of the Deccan Traps and other volcanic eruptions, so driving climate change, and possibly sea level change (von Frese et al., 2009).

Volatiles released when cooking saline giants and associated organic-rich sediments

Particular sets of assimilations and metamorphic alterations of evaporites occur within the explosive milieu associated with both igneous interactions and pressurised heating of salts tied to a bolide impact. Any carbonate and organic matter layers present in the saline sequence or adjacent strata generates additional volatiles that will quickly enter the earth's atmosphere. Figure 5 is a schematic of the estimated amount of volatiles released during contact metamorphism of different types of sedimentary rocks in contact with an igneous sill or magma body (after Ganino et al., 2009; Pang et al., 2013). More catastrophic volumes of similar volatile suites enter the atmosphere if a large bolide impacts a region underlain by a saline giant.


Hence, salty interactions must be considered and quantified when attempting to understand earth-scale environmental changes whenever large evaporite masses are caught up in regions of LIP emplacement or bolide impact. In such areas:

  • Basalt and granitoids do not release large volumes of volatiles, as compared to the amounts of volatiles that are released by the heating or assimilation of saliniferous country rock (heat transfer and hydrothermal circulation).
  • Most porous sandstones and organic-lean shales caught up in a contact aureole or consumed in a magma, release water vapour; a release that has little effect on global climate.
  • During desulphation of a magma, gypsum or anhydrite masses are assimilated into a rising magma chamber or the emplacement of a thick sill. If anhydrite beds are consumed (melted and absorbed) by a magma batholith, the reaction releases abundant SO2 constituting up to 47 wt% of the bedded sulphate (Gorman et al., 1984). Direct melting requires high temperatures (≈ 1300- 1400 °C). Such widespread desulphation of thick Devonian anhydrite beds occurred during the emplacement of the supergiant Noril'sk nickel deposit in Siberia (Black et al., 2014; Warren, 2016, Chapter 16).
  • But such elevated temperatures (≈1400°C) are not typical of most contact aureoles where a sill or dyke intrudes anhydritic country rock. However, similar high-volume SO2 releases can proceed at temperatures as low as 615°C if the anhydrite is impure and contains interlayers rich in organics and hydrocarbons (e.g., West and Sutton, 1954; Pang et al., 2013). This is especially so if the interacting calcium sulphate is gypsum (hydrated salt) rather than anhydrite. Experiments by Newton and Manning (2005) demonstrated that the solubility of anhydrite increases enormously with NaCl activity (salinity) in hydrothermal solutions at ≈600 to 800°C (Figure 6).


  • Pure limestone contains large amounts of CO2, but like anhydrite the thermal decomposition of limestone or dolomite into CaO, MgO and CO2 takes place at high temperatures (>950 °C) that are typical when blocks of sedimentary carbonate are assimilated into a magma chamber, but less typical of contact aureoles tied to dykes and sills. Impure limestones can release large amounts of CO2 (up to 29 wt%) during the formation of calc-silicates in the contact aureole at moderate temperatures of 450–500 °C. As early as 1940, Bowen documented the release of CO2 by decarbonisation reactions during progressive metamorphism of siliceous dolomites (Bowen, 1940)
  • Likewise, devolatilization of fine-grained calcareous and saline sedimentary rocks during contact metamorphism directly generates fluids rich in CO2 (i.e., decarbonisation) and SO2 (i.e., desulphatation), which in theory can enter the magmatic system.
  • When heated at a relatively low temperature (<300-400 °C), contact metamorphism and hydrothermal leaching of bituminous halite and organic-carbon-rich saline mudstones releases large volumes of chlorohalogens and methane (Visscher et al., 2004; Beerling et al., 2007). Halocarbon compounds (aka halogenated hydrocarbons) are chemicals in which one or more carbon atoms are linked by covalent bonds with one or more halogen atoms (fluorine, chlorine, bromine or iodine). Methyl chloride (CH3Cl) and methyl bromide (CH3Br) are commonplace halocarbons when a halite-dominant saline giant interacts with igneous sill emplacement. When thermally-derived chlorohalogens enter the upper atmosphere, they tend to be reactive and will degrade ozone.
  • Buring coal and coal gas release abundant CO2. Depending on its grade, coal can ignite at temperatures between 400-530°C. Methane will auto-ignite at temperatures around 550-600°C and in an oxygenated setting produces large volumes of carbon dioxide and water vapour. Flashpoints are much lower than these ignition temperatures.
  • Sulphidic (pyritic) sediments release abundant SO2 when heated at lower temperatures (<400°C).
  • Heating of hydrated salts at moderate temperatures (90-250°C) can release pressurised pulses of hypersaline chloride or sulphate brine, with the dominant ionic proportions dependent on predominant hydrated salt; e.g., carnallite incongruently alters as it releases an MgCl2 brine, gypsum incongruently alters as it releases a Ca-SO4 brine (see part 1). Such pressurised pulses are essential in the generation of explosive breccia pipes sourced at the sill penetration level in the hydrated evaporite interval (discussed in detail for the Siberian Traps in part 3).
  • Getting volatiles into the atmosphere

    When a saline giant is heated during emplacement of a large igneous province (LIP) or during the impact of a large bolide, it and adjacent carbonates and organic-rich mudstones release large volumes of volatiles that can have short and long term harmful effects on the Earth's biosystems (Black et al., 2012, 2014; Jones et al., 2016; Part 3 this series). The volume of volatiles released to the atmosphere by these interactions, especially sulphurous products (SO2, H2S), thermogenic CH4, organohalogens and CO2, are considered primary contributors to three or four of the major extinction events outlined in Figure 1, and perhaps others, as discussed in part 3.

    Height and volume of various volatile injections into the layers of Earth's atmosphere controls the longevity and intensity of climatic effects and are tied to the chemistry of particular volatiles (Figure 7; Textor et al., 2003; Robock, 2000). The low concentration of water in typical modern volcanic plumes results in the formation of relatively dry aggregates entering the atmosphere. More than 99% of these aggregates are frozen because of their fast ascent to low-temperature regions of the atmosphere. With increased salinities, the salinity effect increases the amount of liquid water attaining the stratosphere by one order of magnitude, but the ice phase is still highly dominant. Consequently, the scavenging efficiency for HCl is very low, and only 1% is dissolved in liquid water.


    Scavenging by ice particles via direct gas incorporation during diffusional growth is a significant process for volatile transport. The salinity effect increases the total scavenging efficiency for HCl from about 50% to about 90%. The sulfur-containing gases SO2 and H2S are only slightly soluble in liquid water; however, these gases are incorporated into ice particles in the atmosphere with an efficiency of 10 to 30%. Despite scavenging, more than 25% of the HCl and 80% of the sulphur gases reach the stratosphere during a more intense modern explosive eruption because most of the particles containing these species are typically lifted there by the force of the eruption (Figure 7b).

    Sedimentation of the particles tends to remove the volcanic gases from the stratosphere. Hence, the final quantity of volcanic gases injected in a particular eruption depends on the fate of the particles containing them, which is in turn dependent on the volcanic eruption intensity and environmental conditions at the site of the eruption.

    Today, volcanically-derived SO2 and H2S are the dominant sources for sulphur species in the atmosphere (Jones et al., 2016; Robock, 2000). Conversion of SO2 to aerosols is one of the critical drivers of climatic cooling during recent eruptions (Figure 7a; Robock, 2000). For SO2 to be effective in causing cooling in the atmosphere, escaping hydrogen sulphide quickly oxidises to SO2. Over hours to weeks following its eruptive escape the ongoing reaction of SO2 with atmospheric H2O forms a H2SO4 (sulphuric acid) aerosol, and this is a major cause of the acid rains tied to volcanism (Figure 7a, b).

    Tropospheric sulphate aerosols have an atmospheric lifetime of a couple of weeks due to the rapid incorporation as precipitation into the hydrological cycle (Figure 7b; Robock, 2000). However, if the intensity of the escaping volatile plume is capable of injecting sulphurous material above the tropopause into the stratosphere, then due to the lack of removal by precipitation, the lifetimes of sulphurous aerosols and the associated cooling effects are considerably extended (years rather than weeks: Figure 7a versus 7b).

    Modern eruptions

    World-scale cooling has been observed following a number of modest (by large igneous province standards) volcanic eruptions over the past few centuries (Figure 8; Bond and Wignall, 2014; Sigurdsson, 1990; and references therein). A recent example is provided by the Mount Pinatubo eruption of 1991, which injected 20 megatons of SO2 more than 30 km into the stratosphere. The result was a global temperature decrease approaching 0.5 °C for three years (although this cooling was probably exacerbated contemporaneous Mount Hudson eruption in Chile). One of the largest historical eruptions occurred in 1783-1784 from the Laki fissure in Iceland when a ≈15 km3 volume of basaltic magma was extruded, releasing ≈122 Mt of SO2, 15 Mt of HF, and 7 Mt of HCl. Laki’s eruption columns extended vertically up to 13 km, injecting sulfate aerosols into the upper troposphere and lower stratosphere, where they reacted with atmospheric moisture to produce ≈200 Mt of H2SO4. This aerosol-rich fog hung over the Northern Hemisphere for five months, leading to short-term cooling, and harmful acid rain in both Europe and North America. Additionally, HCl and HF emissions damaged terrestrial life in Iceland and mainland Europe, as this low-level fluorine-rich haze stunted plant growth and acidified soils.

    By causing or aiding in the collapse of food chains during the more intense sulphurous releases involved in the heating of large volumes of anhydrite held in ancient saline giants, vast quantities of acid rain may have killed much of the vegetation on land and photosynthetic organisms in the oceans during the three extinction events discussed in part 3.


    Halocarbons

    For halocarbons to form in a volcanic eruption requires the combination halogens with organic matter/methane or other hydrocarbons. We shall consider the levels and origins of two of the more common halocarbons in today's atmosphere; methyl chloride (CH3Cl) and methyl bromide (CH3Br) although many other species of halogenated hydrocarbons are present both naturally and anthropogenically (Schwandner, 2002; Visscher et al., 2004).

    The average Cl concentration of the Earth has been estimated to be 17 ppm (Worden, 2018 and references therein). Chlorine is the dominant anion in seawater, most modern and ancient evaporite beds and associated brines. Chlorine is present in most igneous rocks at low concentrations with little difference in level shown between granite and basic igneous rocks (both have a Cl- concentration of about 0.02%). However, igneous glass typically has higher Cl concentrations (≈0.08%). Chlorine is concentrated within any residual vapour phase during volcanic eruptions so can be independent of the volatiles created by heating of saline giants. Without the latter, the contribution of volcanically-erupted Cl to the atmosphere is still considerable. For example, the estimated current global volcanic emission of Cl is between 0.4 and 170 mt/year, while individual eruptions can produce hundreds of kilotons of Cl. For example, in 1980, St Helens emitted 670 kt of Cl into the atmosphere.

    In crystalline igneous rocks Br is found at low concentrations, typically <1 ppm in mid-ocean ridge basalts (MORB) (Worden, 2008 and references)). The average Br concentration of the Earth has been estimated to be 0.05 ppm. Chlorine/Bromine ratios are typically between 200 and 1000 in igneous rocks. Bromine is, however, found at relatively high concentrations (up to 300 ppm) in melt inclusions and matrix glass in acid igneous rocks since it is a highly incompatible element that does not easily sit within silicate, oxide or sulphide minerals. Bromine is concentrated within any residual vapour phase during volcanic eruptions. Based on experimentally-derived fractionation factors for halogens in volcanic materials, crustal average halogen concentrations, and measured amounts of Cl emitted from volcanoes, it can be concluded that the contribution of volcanically-erupted Br to the atmosphere is considerable. For example, the estimated current global volcanic emission of Br is between 2.6 and 78 kt while individual eruptions (e.g., St Helens in 1980) can emit 2.4–5.6 kt.

    The hinterlands of sedimentary basins that predominantly enriched in primary igneous rocks will provide only small quantities of Br into the sediment supply but rocks enriched in glass-bearing igneous rocks may supply relatively greater amounts of Br (Worden, 2018). Bromine is found in sedimentary basins as dissolved Br-, in solid solution in halite (NaClxBr1−x), or in less common salts resulting from potash-facies evaporites, such as sylvite. Bromine is also associated with organic-rich sediments, especially in marine settings, including organic-rich mudstone and coal. At a concentration of 65 mg/L, Br- is the second most abundant halogen in modern seawater.

    Organic matter and its more evolved forms –kerogen and hydrocarbons– are typical of most large evaporite basins. Mesohaline carbonates interlayered with anhydrite and halite beds can entrain high levels of organic matter to form high-yield source rocks, while the brine inclusions in some halites contain high amounts of volatile hydrocarbons and pyrobitumens. Evaporite beds composed of anhydrite or halite make excellent seals holding back large volumes of hydrocarbons (for literature documentation of these observations see Warren, 2016, Chapters 9 and 10). In combination, saline giants and their heat-responsive lithologies will contain vast volumes of potential volatiles, including halocarbons.

    Ozone (O3) destruction

    When halocarbons enter the stratosphere, they decimate the ozone layer, allowing harmful levels of ultraviolet (UV) radiation to reach the earth's surface (Figures 7a, 9a). Ozone is destroyed by the entry of a number of free radical catalysts into the stratosphere; today the most important catalysts are the hydroxyl radical (OH), nitric oxide radical (NO), chlorine radical (Cl) and the bromine radical (Br). Each radical is characterised by an unpaired electron in its molecular structure and is thus extremely reactive. All of these radicals have both natural and man-made sources; at present, most of the OH and NO in the stratosphere is naturally occurring, but human activity has drastically increased the levels of chlorine and bromine.

    The elements that form radicals in the stratosphere are found in stable organic compounds, especially halocarbons, which reach the stratosphere without being destroyed in the troposphere due to their low reactivity. Once in the stratosphere, the Cl and Br atoms are released from the parent halocarbon by the action of ultraviolet light.


    Ozone (O3) is a highly reactive molecule that quickly reduces to the more stable oxygen (O2) form with the assistance of a catalyst (radical). Cl and Br atoms destroy ozone molecules through a variety of catalytic cycles. The simplest example of such a reaction is when a chlorine atom reacts with an ozone molecule, taking an oxygen atom to form chlorine monoxide (ClO) and leaving behind an oxygen molecule (O2) (Figure 9b). The ClO can then react with another molecule of ozone, once more releasing the chlorine atom as ClO, so far yielding two molecules of oxygen. This ClO reaction can be repeated until the ClO is flushed from the stratosphere (Figure 9b, Fahey, 2007)

    Thus the overall effect of halocarbons entering the stratosphere is a decrease in the amount of ozone. A single chlorine radical can continuously destroy ozone for up to two years (this the time scale for its transport back down into the troposphere; Figure 7a). But there are other stratopheric reactions that remove CLO from this catalytic cycle by forming reservoir species such as hydrogen chloride (HCl) and chlorine nitrate (ClONO).

    Bromine radicals are even more efficient than chlorine at destroying ozone on a per-atom basis, but at present there is much less bromine than chlorine in the atmosphere. Laboratory studies have shown that fluorine and iodine atoms can participate in similar catalytic cycles. However, fluorine atoms react rapidly with water and methane to form strongly bound HF in the Earth's stratosphere, while organic molecules containing iodine react so quickly in the lower atmosphere that they do not reach the stratosphere in significant quantities.

    Halocarbon concentrations below the tropopause are always higher by several orders of magnitude than in the stratosphere, which contains the seasonally and locally variable ozone layer responsible for absorption of incident solar UV radiation (Schwandner, 2002). Penetration of the tropopause allows the ascent of long-lived halocarbons and today occurs primarily as a result of rising tropical air masses in a Hadley cell, rare turnover events, or large Plinian volcanic eruptions.

    Over the two to three years a chlorine or bromine radical can remain in the stratosphere, it reacts with ozone and converts it to oxygen. It has been estimated that a single chlorine atom can react with an average of 100,000 ozone molecules before it is removed from the catalytic cycle (Figure 8b. Other halocarbon-enabled reactions drive ozone destruction (these catalysts are derived from anthropogenic CFCs and other industrial halocarbons). Over the past half-century, our anthropogenic focus on ozone destruction from industrial chemicals has driven the public's understanding into to the much-needed legislated prevention of the entry of additional industrial halocarbons (especially CFCs) into the stratosphere.


    Implications

    However, there are additional deep-time implications for the health of the Earth's biota when natural events of the past drastically increased the amount of halocarbons entering the stratosphere, along with increased levels of sulphurous volatiles and greenhouse gases. We know modern volcanic exhalations containing relatively high levels of chlorine and bromine. But times of intense magmatic/volcanogenic or bolide heating of evaporites in a saline giant will contribute even greater volumes of halocarbons to the stratospheric levels of the atmosphere (Figure 10). If coals and peats are also present (typically not in the saline portion of the basin's sediment fill), then the heating of these additional organic-rich sediments will contribute even more carbon to the vast volumes of the halocarbons created by heating of the evaporites. Heating reactions in the saline giant and associated deposits can also supply elevated levels of the greenhouse gases CO2 and CH4. Explosive volcanism tied to the emplacement of LIPs in the region of a saline giant or the atmosphere-scale disturbance linked to the impact of a large bolide in an area underlain by a saline giant are efficient mechanisms to move large volumes of halocarbons, sulphurous volatiles and greenhouse gasses to the troposphere. The third article in this series will document the specific evaporite geology that contributed to four of the five major Phanerozoic extinction events (Figure 10).

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    Brine evolution and origins of potash - primary or secondary. Ancient potash ores: Part 3 of 3

    John Warren - Monday, December 31, 2018

    Introduction

    In the previous two articles in this series on potash exploitation, we looked at the production of either MOP or SOP from anthropogenic brine pans in modern saline lake settings. Crystals of interest formed in solar evaporation pans and came out of solution as: 1) Rafts at the air-brine interface, 2) Bottom nucleates or, 3) Syndepositional cements precipitated within a few centimetres of the depositional surface. In most cases, periods of more intense precipitation tended to occur during times of brine cooling, either diurnally or seasonally (sylvite, carnallite and halite are prograde salts). All anthropogenic saline pan deposits examples can be considered as primary precipitates with chemistries tied to surface or very nearsurface brine chemistry.

    In contrast, this article discusses ancient potash deposits where the chemistries and ore textures are responding to ongoing alteration processes in the diagenetic realm. Unlike the modern brine pans where brines chemistries and harvested mineralogies are controllable, at least in part, these ancient deposits show ore purities and distributions related to ongoing natural-process overprints.



    Table 1 lists some modern and ancient potash deposits and prospects by dividing them into Neogene and Pre-Neogene deposits (listing is extracted and compiled from SaltWork® database Version 1.7). The Neogene deposits are associated with a time of MgSO4-enriched seawaters while a majority of the Pre-Neogene deposits straddle times of MgSO4 enrichment and depletion in the ocean waters.

    Incongruent dissolution in burial

    Many primary evaporite salts dissolve congruently in the diagenetic realm; i.e., the composition of the solid and the dissolved solute stoichiometrically match, and the dissolving salt goes entirely into solution (Figure 1a). This situation describes the typical subsurface dissolution of anhydrous evaporite salts such as halite or sylvite. However, some evaporite salts, typically hydrated salts, such as gypsum or carnallite, dissolve incongruently in the diagenetic realm, whereby the composition of the solute in solution does not match that of the solid (Figure 1b). This solubilisation or mineralogical alteration is defined by the transformation of the "primary solid" into a secondary solid phase, typically an anhydrous salt, and the loss of water formerly held in the lattice structure. The resulting solution generally carries ions away in solution.


    More than a century ago, van't Hoff (1912) suggested that much subsurface sylvite is the result of incongruent solution of carnallite yielding sylvite and a Mg-rich solution. According to Braitsch (1971, p. 120), the incongruent alteration (dissolution) of carnallite is perhaps the most crucial process in the alteration of subsurface potash salts and the formation of diagenetic (secondary) sylvite.

    Widespread burial-driven incongruent evaporite reactions in the diagenetic realm include the burial transition of gypsum to anhydrite (reaction 1)

    CaSO4.2H2O --> CaSO4 + 2 H2O ... (1)

    and the in-situ conversion of carnallite to sylvite via the loss of magnesium chloride in solution (reaction 2)

    KMgCl3.6H2O --> KCl + Mg++ + 2Cl- + 6H2O ...(2)

    Typically, a new solid mineral remains, and the related complex solubility equilibrium creates a saline pore water that may, in turn, drive further alteration or dissolution as it leaves the reaction site (Warren, Chapters 2 and 8). Specifically for ancient potash, reaction 2 generates magnesium and chloride in solution and has been used to explain why diagenetic bischofite and dolomite can be found in proximity to newly formed subsurface sylvite. Bischofite is a highly soluble salt and so is metastable in many subsurface settings where incongruent dissolution is deemed to have occurred, including bischofite thermal pool deposits in the Dallol sump in the Danakhil of Ethiopia (Salty Matters, May 1, 2015). In many hydrologically active systems, solid-state bischofite is flushed by ongoing brine crossflow and so help drive the formation of various burial dolomites. Only at high concentrations of MgCl2 can carnallite dissolve without decomposition.

    Laboratory determinations

    In the lab, the decomposition of carnallite in an undersaturated aqueous solution is a well-documented example of incongruent dissolution (Emons and Voigt, 1981; Xia et al., 1993; Hong et al., 1994; Liu et al., 2007; Cheng et al., 2015, 2016). When undersaturated water comes into contact with carnallite, the rhombic carnallite crystals dissolve and, because of the common ion effect, small cubic KCl crystals form in the vicinity of the dissolving carnallite. As time passes, the KCl crystals grow into larger sparry subhedral forms and the carnallite disappears.

    Carnallite’s crystal structure is built of Mg(H2O)6 octahedra, with the K+ ions are situated in the holes of chloride ion packing meshworks, with a structural configuration similar to perovskite lattice types (Voigt, 2015). Potassium in the carnallite lattice can be substituted by other large single-valence ions like NH4+, Rb+, Cs+ or Li(H2O)+, (H3O)+ and Cl- by Br- and I-. These substitutions change the lattice symmetry from orthorhombic in the original carnallite to monoclinic.

    When interpreting the genesis of ancient potash deposits and solutions, the elemental segregation in the lattice means trace element contents of bromide, rubidium and caesium in primary carnallite versus sylvite daughter crystals from incongruent dissolution can provide valuable information. For example, in a study by Wardlaw, (1968), a trace element model was developed for sylvite derived from carnallite that gave for Br and Rb concentration ranges of 0.10–0.90 mg/g and 0.01–0.18 mg/g, respectively. In a later study of sylvite derived by fresh-water leaching of magnesium chloride under isothermal conditions at 25 °C. Cheng et al. (2016), defined a model whereby primary sylvites precipitated from MgSO4-deficient sea water, gave Br and Rb concentration ranges of 2.89–3.54 mg/g and 0.017–0.02 mg/g, respectively (no evaporation occurred at saturation with KCl). In general, they concluded sylvite derived incongruently from carnallite would contain less Br and more Rb than primary sylvite (Figure 2; Cheng et al., 2016).


    Subsurface examples

    The burial-driven mechanism widely cited to explain the incongruent formation of sylvite from carnallite is illustrated in Figure 3 (Koehler et al., 1990). Carnallite precipitating from evaporating seawater at time 1 forms from a solution at 30°C and atmospheric (1 bar) pressure, and so plots as point A, which lies within the carnallite stability field (that is, it sits above the dashed light brown line). With subsequent burial, the pressure increases so that the line defining carnallite-sylvite boundary (solid dark brown line) moves to higher values of K. By time 2, when the pressure is at 1 Kbar (corresponds to a lithostatic load equivalent to 2-3 km depth), the buried carnallite is thermodynamically unstable and so is converting to sylvite + solution (as the plot field now lies in the sylvite + solution field (Figure 3). If equilibrium is maintained the carnallite reacts incongruently to form further sylvite and MgCl2-solution. Thus, provided the temperature does not rise substantially, increasing pressure as a result of burial will favour the breakdown of carnallite to sylvite. However, as burial proceeds, the temperature may become high enough to favour once again the formation of carnallite from sylvite + solution (that is the solution plot point from A moves toward the right-hand side of the figure and back into the carnallite stability field).


    Sylvite, interpreted to have formed from incongruent dissolution of primary carnallite, is reported from the Late Permian Zechstein Formation of Germany (Borchert and Muir, 1964), Late Permian Salado Formation of New Mexico (Adams, 1970), Early Mississippian Windsor Group of Nova Scotia (Evans, 1970), Early Cretaceous Muribeca Formation of Brazil and its equivalents in the Gabon Basin, West Africa (Wardlaw, 1972a, b; Wardlaw and Nicholls, 1972; Szatmari et al., 1979; de Ruiter, 1979), Late Cretaceous of the Maha Sarakham Formation, Khorat Plateau, Thailand and Laos (Hite and Japakasetr, 1979), Pleistocene Houston Formation, Danakil Depression, Ethiopia (Holwerda and Hutchinson, 1968), and Middle Devonian Prairie Formation of western Canada (Schwerdtner, 1964; Wardlaw, 1968) (See Table 1).

    This well-documented literature base supports a long-held notion that there is a problem with sylvite as a primary (first precipitate) marine bittern salt, especially if the mother seawater had ionic proportions similar to those present in modern seawater (see Lowenstein and Spencer, 1990 for an excellent, if 30-year-old, review). We know from numerous evaporation experiments, that sylvite does not crystallise during the evaporation of modern seawater at 25°C, except under metastable equilibrium conditions (Braitsch, 1971; Valyashko, 1972; Hardie, 1984). The sequence of bitterns crystallising from modern seawater bitterns was illustrated in the previous Salty Matters article in this series (see Figure 1 in October 31 2018).

    Across the literature documenting sylvite-carnallite associations in ancient evaporites, the dilemma of primary versus secondary sylvite is generally solved in one of three ways. Historically, many workers interpreted widespread sylvite as a diagenetic mineral formed by the incongruent dissolution of carnallite (Explanation 1). Then there is the interpretation that some sylvite beds, perhaps associated with tachyhydrite, were precipitated in the evaporite bittern part of a basin hydrology that was fed by CaCl2-rich basinal hydrothermal waters (Explanation 2: see Hardie, 1990 for a good discussion    of this mechanism). Then there is the third, and increasingly popular explanation of primary or syndepositional sylvite at particular times in the chemical evolution of the world oceans (MgSO4-depleted oceans).


    Changes in the relative proportions of magnesium, sulphur and calcium in the world’s oceans are well supported by brine inclusion chemistry of co-associated chevron halite (Figure 4). Clearly, there are vast swathes of times in the earth’s past when the chemistry of seawater changed so that MgSO4 levels were lower than today and it was possible that sylvite was a primary marine bittern precipitate (see Lowenstein et al., 2014 for an excellent summary).

    In my opinion, there is good evidence that all three explanations are valid within their relevant geological contexts but, if used exclusively to explain the presence of ancient sylvite, the argument becomes somewhat dogmatic. I would say that that, owing to its high solubility, the various textures and mineralogical associations of carnallite/sylvite and sulphate bitterns found in ancient potash ore beds reflect various and evolving origins. Ambient textures and mineralogies are dependent on how many times and how pervasively in a potash sequence’s geological burial history an evolving and reactive pore brine chemistry came into contact with parts or all of the extent of highly reactive potash beds (Warren, 2000; 2010; 2016).

    In my experience, very few ancient examples of economic potash show layered textures indicating primary precipitation on a brine lake floor, instead, most ancient sylvite ores show evidence of at least one episode of alteration. That is, various forms and textures in potash may dissolve, recrystallise and backreact with each other from the time a potash salt is first precipitated until it is extracted. The observed textural and mineralogical evolution of a potash ore association depends on how open was the hydrology of the potash system at various stages during its burial evolution. The alteration can occur syndepositionally, in brine reflux, or later during flushing by compactional or thermobaric subsurface waters or during re-equilibration tied to uplift and telogenesis. Tectonism (extensional and compactional) during the various stages of a basin’s burial evolution acts as a bellows driving fluid flow within a basin, so forcing and speeding up the focused circulation of potash-altering waters.

     

    A similar, but somewhat less intense, textural evolution tied to incongruent alteration is seen in the burial history of other variably hydrated evaporite salts. For example, CaSO4 can flip-flop from gypsum to anhydrite and back again depending on temperature, pore fluid salinity and the state of uplift/burial. Likewise, with the more complicated double salt polyhalite, there are mineralogical changes related to whether it formed in a MgSO4 enriched or depleted world ocean and the associated chemistry of the syndepositional reflux brines across extensive evaporite platforms (for a more detailed discussion of polyhalite see Salty Matters, July 31, 2018). Kainite-kieserite-carnallite also show evidence of ongoing incongruent interactions. This means that, as in gypsum/anhydrite/polyhalite or kain ite/kieserite sequences, there will be primary and secondary forms of both carnallite and sylvite that can alternate during deposition, during burial and any deep meteoric flushing and then again with uplift. In Quaternary brine factories these same incongruent chemical relationships are what facilitate the production of MOP (sylvite) from a carnallitite feed or SOP from kainite/kieserite/schoenite feed (see articles 1 and 2 in this series).

     

    To document the three end-members of ancient sylvite-carnallite decomposition/precipitation we will look at three examples; 1) Oligocene potash in the Mulhouse Basin where primary sylvite textures are commonplace, 2) Devonian potash ores in western Canada, where multiple secondary stages of alteration are seen, and 3) Igneous-dyke associated sylvite in east Germany where thermally-driven volatisation (incongruent melting) forms sylvite from dehydrated carnallite.


    Oligocene Potash, Mulhouse Basin France

    Moving backwards into deep time, this 34 Ma deposit contains some of the first indications of well preserved primary marine-fed sylvinite (MOP) textures exemplified by laterally-continuous mm-scale alternations of potash and halite layers and lamina (Figure 5a-c). Interestingly, all solid-state potash deposits laid down in the post-Oligocene period contain increased proportions of MgSO4 salts, making them much more difficult to economically mine and process (see Table 1 and Salty Matters, May 12, 2015)

    From 1904 until 2002, potash was conventionally mined in France from the Mulhouse Basin (near Alsace, France). With an area of 400 km2, the Mulhouse Basin is the southernmost of a number of Lower Oligocene evaporite basins that occupied the upper Rhine Graben, which at that time was a narrow adiabatic-arid rift valley (Figure 6a). The graben was a consequence of the collision between European and African plates during the Paleogene. It is part of a larger intracontinental rift system across Western Europe that extended from the North Sea to the Mediterranean Sea, stretching some 300 km from Frankfurt (Germany) in the north, to Basel (Switzerland) in the south, with an average width of 35 km (Cendon et al., 2008). The southern extent of the graben is limited by a system of faults that place Hercynian massifs and Triassic materials into contact with the Paleogene filling. Across the north, a complex system of structures (including salt diapirs) put the basin edges in contact with Triassic, Jurassic and Permian materials. In the region of the evaporite basins, the Paleogene fill of the graben lies directly on the Jurassic basement. The sedimentary filling of this rift sequence is asymmetrical with the deeper parts located at the southwestern and northeastern sides of the Graben (Rouchy, 1997).


    Palaeogeographical reconstructions place the potential marine seaway seepage feed to the north or perhaps also southeast of the Mulhouse Basin, while marginal continental conglomerates tend to preclude any contemporaneous hydrographic connection with Oligocene ocean water (Blanc-Valleron, 1991; Hinsken et al., 2007; Cendon et al., 2008). At the time of its hydrographic isolation, some 34 Ma, the basin was located 40° north of the equator. Total fill of Oligocene lacustrine/marine-fed sediments in the graben is some 1,700m thick. The saline stage is dominated by anhydrite, halite and mudstone. The main saline sequence is underlain by non-evaporitic Eocene continental mudstones, with lacustrine fossils and local anhydrite beds. Evaporite bed continuity in the northern part of the basin is disturbed by (Permian-salt cored) diapiric and or erosional/fault movement. Consequently, these northern basins are not considered suitable for conventional potash mining (Figure 6a).

    The Paleogene fill of the basin is divided into 6 units; a pre-evaporitic series, Lower Salt Group (LSG), Middle Salt Group (MSG), an Upper Salt Group (USG - with potash), Grey Marls Fm., and the Niederroedern Fm (Figure 7; Cendon et al., 2008). The LSG and lower section of the MSG are interpreted as lacustrine in origin, based on the limited palaeontologic and geochemical data. However, based on the presence of Cenozoic marine nannoplankton, shallow water benthic foraminifera, and well-diversified dinocyst assemblages in the fossiliferous zone below Salt IV, Blanc-Valleron (1991) favours a marine influence near the top of the MSG, while recognising the ambiguity of marine proportions with brackish faunas. Many marine-seepage fed brine systems have salinities that allow halotolerant species to flourish in marine-fed basins with no ongoing marine hydrographic connection (Warren, 2011). According to Blanc-Valleron and Schuler (1997), the region experienced a Mediterranean climate with long dry seasons during Salt IV member deposition.


    In detail, the Salt IV member is made up of some 210 m of evaporitic sequence, with two relatively thin potash levels (Ci and Cs). The stratigraphy associated with this potash zone is, from base to top (Figure 7):

    S2 Unit: 11.5 m thick with distinct layers of organic-rich marls, often dolomitic, with dispersed anhydrite layers.

    S1 Unit: 19 m thick, evenly-bedded and made up of alternating metre-scale milky (inclusion-rich) halite layers, with much thinner marls and anhydrite layers. Marls show a sub-millimetric lamination formed by micritic carbonate laminae alternating with clay, quartz, and organic matter-rich laminae. Hofmann et al. (1993a, b) interpreted these couplets as reflecting seasonal variations. Anhydrite occasionally displays remnant swallowtail ghost textures, which suggest that at least part of the anhydrite first precipitated as subaqueous gypsum. Halite shows an abundance of growth-aligned primary chevron textures, along with fluid-inclusion banding suggesting halite was subaqueous and deposited beneath shallow brine sheets (Lowenstein and Spencer, 1990).

    S Unit: Is 3.7 m thick and consists of thin marl layers and anhydrite, similar to the S2 Unit, with a few thin millimetric layers of halite.

    Mi Unit: With a thickness of 6 m, it is mostly halite with similar characteristics to the S1 Unit. Sylvite was detected in one sample, but its presence is probably related to the evolution of interstitial brines (Cendon et al., 2008).

    Ci Unit (“Couche inférieure”): Is formed by 4 m of alternating marls/anhydrite, halite, and sylvite beds (Figure 7).

    The Ti unit consists of alternating beds of halite, marl and anhydrite. The top of the interval is the T unit, which is similar to the S unit and consists of alternating beds of marl and anhydrite. Above this is the Ms or upper Marl, near identical to the lower marl Mi. The Mi is overlain by the upper potash bed (Cs), a thinner, but texturally equivalent, bed compared to the sylvinitic Ci unit.

    Thus, the Oligocene halite section includes two thin, but mined, potash zones: the Couche inferieure (Ci; 3.9m thick), and Couche superieure (Cs; 1.6m thick), both occur within Salt IV of the Upper Salt group (Figures 5, 7).

    Both potash beds are made up of stacked, thin, parallel-sided cm-dm-thick beds (averaging 8 cm thickness), which are in turn constructed of couplets composed of grey-coloured halite overlain by red-coloured sylvite (Figure 5b). Each couplet has a sharp base that separates the basal halite from the sylvite cap of the underlying bed. In some cases, the separation is also marked by bituminous partings. The bottom-most halite in each dm-thick bed consists of halite aggregates with cumulate textures that pass upward into large, but delicate, primary chevrons and cornets. Clusters of this chevron halite swell upward to create a cm-scale hummocky boundary with the overlying sylvite (Figure 5c; Lowenstein and Spencer, 1990).

    The sylvite member of a sylvinite couplet consists of granular aggregates of small transparent halite cubes and rounded grains of red sylvite (with some euhedral sylvite hoppers) infilling the swales in the underlying hummocky halite (Figure 5b,c). The sylvite layer is usually thick enough to bury the highest protuberances of the halite, so that the top of each sylvite layer, and the top of the couplet, is flat. Dissolution pipes and intercrystalline cavities are noticeably absent, although some chevrons show rounded coigns. Intercalated marker beds, formed during times of brine pool freshening, are composed of a finely laminated bituminous shale, with dolomite and anhydrite.

    The sylvite-halite couplets record combinations of unaltered settle-out and bottom-nucleated growth features, indicating primary chemical sediment accumulating in shallow perennial brine pools (Lowenstein and Spencer, 1990). Based on the crystal size, the close association of halites with sylvite layers, their lateral continuity and the manner in which sylvite mantles overlie chevron halites, the sylvites are interpreted as primary precipitates. Sylvite first formed at the air-brine surface or within the uppermost brine mass and then sank to the bottom to form well-sorted accumulations. As sylvite is a prograde salt it, like halite, probably grew during times of cooling of the brine mass (Figure 8a). These times of cooling could have been diurnal (day/night) or weather-front induced changes in the above-brine air temperatures. Similar cumulate sylvite deposits form as ephemeral bottom accumulations on the floor of modern Lake Dabuxum in China during its more saline phases.


    The subsequent mosaic textural overprint seen in many of the Mulhouse sylvite layers was probably produced by postdepositional modification of the crystal boundaries, much in the same way as mosaic halite is formed by recrystallisation of raft and cumulate halite during shallow burial. Temperature-based inclusion studies in both the sylvite and the halites average 63°C, suggesting solar heating of surface brines as precipitation took place (Figure 8b; Lowenstein and Spencer, 1990). Similar high at-surface brine temperatures are not unusual in many modern brine pools, especially those subject to periodic density stratification and heliothermometry (Warren 2016; Chapter 2).

    Mineralogically, potash evaporites in the Mulhouse Basin in the Rhine Graben (also known as the Alsatian (Alsace) or Wittelsheim Potash district) contain sylvite with subordinate carnallite, but lack the abundant MgSO4 salts characteristic of the evaporation of modern seawater. The Rhine graben formed during the Oligocene, via crustal extension, related to mantle upwelling. It was, and is, a continental graben typified by high geothermal gradients along its rift axis. In depositional setting, it is not dissimilar to pree-120,000-year potash fill stage in the Quaternary Danakil Basin or the Dead Sea during deposition of potash salts in the Pliocene Sedom Fm. The role of a high-temperature geothermal inflow in defining the CaCl2 nature of the potash-precipitating brines, versus a derivation from a MgSO4-depleted marine feed, is considered significant in the Rhine Graben deposits, but is poorly understood and still not resolved (Hardie, 1990; Cendón et al., 2008). World ocean chemistry in the Oligocene is on a shoulder between the MgSO4-depleted CaCl2-rich oceans of the Cretaceous and the MgSO4-enriched oceans of the Neogene (Figure 4).


    Cendón et al. (2008) conclude brine reaction processes were the most important factors controlling the major-ion (Mg, Ca, Na, K, SO4, and Cl) evolution of Mulhouse brines (Figure 9a-d). A combined analysis of fluid inclusions in primary textures by Cryo-SEM-EDS with sulphate- d34S, d18O and 87Sr/86Sr isotope ratios revealed likely hydrothermal inputs and recycling of Permian evaporites, particularly during the more advanced stages of evaporation that laid down the Salt IV member. Bromine levels imply an increasingly concentrated brine at that time (Figure 9a). The lower part of the Salt IV (S2 and S1) likely evolved from an initial marine input (Figure 9b-d).

    Throughout, the basin was disconnected from direct marine hydrographic connection and was one of a series of sub-basins formed in an active rift setting, where tectonic variations influenced sub-basin interconnections and chemical signatures of input waters. Sulphate-d34S shows Oligocene marine-like signatures at the base of the Salt IV member (Figure 9c, d). However, enriched sulphate-d18O reveals the importance of synchronous re-oxidation processes.

    As evaporation progressed, other non-marine or marine-modified inputs from neighbouring basins became more important. This is demonstrated by increases in K concentrations in brine inclusions and Br in halite, sulphate isotopes trends, and 87Sr/86Sr ratios (Figure 9b, c). The recycling (dissolution) of previously precipitated evaporites of Permian age was increasingly important with ongoing evaporation. In combination, this chemistry supports the notion of a connection of the Mulhouse Basin with basins situated north of Mulhouse. The brine evolution eventually reached sylvite precipitation. Hence, the chemical signature of the resulting brines is not 100% compatible with global seawater chemistry changes. Instead, the potash phase is tied to a hybrid inflow, with significant but decreasing marine input.

    There was likely an initial marine source, but this occurred within a series of rift-valley basin depressions for which there was no direct hydrographic connection to the open ocean, even at the time the Middle Salt Member (potash-entraining) was first deposited (Cendon et al., 2008). That is, the general hydrological evolution of the primary textured evaporites in the Mulhouse basin sump is better explained as a restricted sub-basin with an initial marine-seepage stage. This gradually changed to ≈ 40% marine source near the beginning of evaporite precipitation, with the rest of hydrological inputs being non-marine. There was a significant contribution of solutes from recycled, in part diapiric, Permian evaporites, likely remobilised by the tectonics driving the formation of the rift valley (Hinsken et al., 2007; Cendon et al., 2008). The general proportion of solutes did not change substantially over the time of evaporite precipitation. However, as the basin restriction increased, the formerly marine inputs changed to continental, diapiric or marine-modified inputs, perhaps fed from neighbouring basins north of Mulhouse basin. As in the Ethiopian Danakhil potash-rift, it is likely brine interactions occurred both during initial and early post-depositional reflux overprinting of the original potash salt beds.


    West Canadian potash (Devonian)

    The Middle Devonian (Givetian) Prairie Evaporite Formation is a widespread potash-entraining halite sequence deposited in the Elk Point Basin, an early intracratonic phase of the Western Canada Sedimentary Basin (WCSB; Chipley and Kyser, 1989). Today, it is the world’s predominant source of MOP fertiliser (Warren, 2016). The flexure that formed the basin and its subsealevel accommodation space was a distal downwarp to, and driven by, the early stages of the Antler Orogeny (Root, 2001). Texturally and geochemically the potash layers in the basin show the effects of multiple alterations and replacements of its potash minerals, especially interactions between sylvite and carnallite in a variably recrystallised halite host.

    Regionally halite constitutes a large portion of the four formations that make up the Devonian Elk Point Group (Figure 10): 1) the Lotsberg (Lower and Upper Lotsberg Salt), 2) the Cold Lake (Cold Lake Salt), 3) the Prairie Evaporite (Whitkow and Leofnard Salt), and 4) the Dawson Bay (Hubbard Evaporite). Today the remnants of the Middle Devonian Prairie Evaporite Formation constitute a bedded unit some 220 metres thick, which lies atop the irregular topography of the platform carbonates of the Winnipegosis Fm. Extensive solutioning of the various salts has given rise to an irregular thickness to the formation and the local absence of salt (Figure 11a).


    The Elk Point Group was deposited within what is termed the Middle Devonian “Elk Point Seaway,” a broad intracratonic sag basin extending from North Dakota and northeastern Montana at its southern extent north through southwestern Manitoba, southern and central Saskatchewan, and eastern to northern Alberta (Figure 11a). Its Pacific coast was near the present Alberta-British Columbia border, and the basin was centred at approximately 10°S latitude. To the north and west the basin was bound by a series of tectonic ridges and arches; but, due to subsequent erosion, the true eastern extent is unknown (Mossop and Shetsen, 1994). In northern Alberta, the Prairie Evaporite is correlated with the Muskeg and Presqu’ile formations (Rogers and Pratt, 2017).

    Hydrographic isolation of the intracratonic basin from its marine connection resulted in the deposition of a drawndown sequence of basinwide (platform-dominant) evaporites with what is a uniquely high volume of preserved potash salts deposited within a clayey halite host. The potash resource in this basin far exceeds that of any other known potash basin in the world.


    Potash geology

    Potash deposits mined in Saskatchewan are all found within the upper 60-70 m of the Prairie Evaporite Formation, at depths of more than 400 to 2750 metres beneath the surface of the Saskatchewan Plains. Within the Prairie Evaporite, there are four main potash-bearing members, in ascending stratigraphic order they are: Esterhazy, White Bear, Belle Plaine and Patience Lake members (Figure 11b). Each member is composed of various combinations of halite, sylvite, sylvinite, and carnallitite, with occurrences of sylvite versus carnallite reliably definable using wireline signatures (once the wireline is calibrated to core or mine control - Figure 12; Fuzesy, 1982).

    The Patience Lake Member is the uppermost Prairie Evaporite member and is separated from the Belle Plaine by 3-12 m of barren halite (Holter, 1972). Its thickness ranges from 0-21 m and averages 12 m, its top 7-14 m is made up halite with clay bands and stratiform sylvite. This is the targeted ore unit in conventional mines in the Saskatoon and Lanigan areas and is the solution-mined target, along with the underlying Belle Plaine Member, at the Mosaic Belle Plaine potash facility. The Belle Plaine member is separated from the Esterhazy by the White Bear Marker beds made up of some 15 m of low-grade halite, clay seams and sylvinite. The Belle Plaine Member is more carnallite-prone than the Patience Lake member (Figure 12). It is the ore unit in the conventional mines at Rocanville and Esterhazy (Figure 11b) where its thickness ranges from 0-18 m and averages around 9 m. In total, the Prairie Evaporite Formation does not contain any significant MgSO4 minerals (kieserite, polyhalite etc.) although some members do contain abundant carnallite. This mineralogy indicates precipitation from a Devonian seawater/brine chemistry somewhat different from today’s, with inherently lower relative proportions of sulphate and lower Mg/Ca ratios (Figure 4).

    The Prairie Evaporite Fm. is nonhalokinetic throughout the basin, it is more than 200 m thick in the potash mining district in Saskatoon and 140 m thick in the Rocanville area to the southeast (Figure 11a; Yang et al., 2009). The Patience Lake member is the main target for conventional mining near Saskatoon. The Esterhazy potash member rises close to the surface in the southeastern part of Saskatchewan near Rocanville and on into Manitoba. This is a region where the Patience Lake Member is thinner or completely dissolved (Figure 11b). Over the area of mineable interest in the Patience Lake Member, centred on Saskatoon, the ore bed currently slopes downward only slightly in a westerly direction, but deepens more strongly to the south at a rate of 3-9 m/km. Mines near Saskatoon are at depths approaching a kilometre and so are nearing the limits of currently economic shaft mining.

    The main shaft for the Colonsay Mine, which took IMC Global Inc. more than five years to complete through a water-saturated sediment column, finally reached the target ore body at a depth of 960 metres. Such depths and a southerly dip to the ore means that the conventional shaft mines near Saskatoon define a narrow WNW-ESE band of conventional mining activity (Figure 11c). To the south potash is recovered from greater depths by solution mining; for example, the Belle Plaine operation leaches potash from the Belle Plaine member at a depth of 1800m.

    The Prairie Evaporite typically thins southwards in the basin; although local thickening occurs where carnallite, not sylvite, is the dominant potash mineral (Worsley and Fuzesy, 1979). The Patience Lake member is mined at the Cory, Allan and Lanigan mines, and the Esterhazy Member is mined in the Rocanville area (Figure 11c). Ore mined from the 2.4 m thick Esterhazy Member in eastern Saskatchewan contain minimal amounts of insolubles (≈1%), but considerable quantities of carnallite (typically 1%, but up to 10%) and this reduces the average KCl grade value to an average of 25% K2O. The converse is true for ore mined from the Patience Lake potash member in western Saskatchewan near Saskatoon, where carnallite is uncommon in the Cory and Allan mines. The mined ore thickness is a 2.74-3.35 metre cut off near the top of the 3.66-4.57metre Prairie Lake potash member. Ore grade is 20-26% K2O and inversely related to thickness (Figure 12). The insoluble content is 4-7%, mostly clay and markedly higher than in the Rocanville mines.


    A typical sylvinite ore zone in the Patience Lake member can be divided into four to six units, based on potash rock-types and clay seams (Figures 12, 13a; M1-M6 of Boys, 1990). Units are mappable and have been correlated throughout the PCS Cory Mine with varying degrees of success, dependent on partial or complete loss of section from dissolution. Potash deposition appears to have been early and related to short-term brine seaway cooling and syndepositional brine reflux. So the potash layering (M1-M6) is cyclic, expressed in the repetitive distribution of hematite and other insoluble minerals (Figure 13). Desiccation polygons, desiccation cracks, subvertical microkarst pits and chevron halite crystals indicate that the Patience Lake member that encompasses the potash ore was deposited in and just beneath a shallow-brine, salt-pan environment (Figure 13b; Boys, 1990; Lowenstein and Spencer, 1990; Brodlyo and Spencer, 1987; pers. obs).

    Clay seams form characteristic thin stratigraphic segregations throughout the potash ore zone(s) of the Prairie Evaporite, as well as disseminated intervals, and constitute about 6% of the ore as mined. For example, the insoluble minerals found in the PCS Cory samples are, in approximate order of decreasing abundance: dolomite, clay [illite, chlorite (including swelling-chlorite/chlorite), and septechlorite, quartz, anhydrite, hematite, and goethite. Clay minerals make up about one-third of the total insolubles: other minor components include: potassium feldspar, hydrocarbons, and sporadic non-diagnostic palynomorphs (Figure 13; Boys, 1990).

    In all mines, the clays tend to occur as long continuous seams or marker layers between the potash zones and are mainly composed of detrital chlorite and illite, along withauthigenic septechlorite, montmorillonite and sepiolite (Mossman et al., 1982; Boys, 1990). Of the two chlorite minerals, septechlorite is the more thermally stable. The septechlorite, sepiolite and vermiculite very likely originated as direct products of settle-out, syndepositional dissolution or early diagenesis under hypersaline conditions from a precursor that was initially eolian dust settling to the bottom of a vast brine seaway. The absence of the otherwise ubiquitous septechlorite from Second Red Beds west of the zero-edge of the evaporite basin supports this concept (Figure 9, 10).


    Potash Textures

    Texturally, at the cm-scale, potash salt beds in the Prairie Evaporite (both carnallitite and sylvinite) lack the lateral continuity seen in primary potash textures in the Oligocene of the Mulhouse Basin (Figure 14). Prairie potash probably first formed as syndepositional secondary precipitates and alteration products at very shallow depths just beneath the sediment surface. These early prograde precipitates were then modified to varying degrees by ongoing fluid flushing in the shallow burial environment. The cyclic depositional distribution of disseminated insolubles as the clay marker beds was possibly due to a combination of source proximity, periodic enrichment during times of brine freshening and the strengthening of the winds blowing detritals out over the brine seaway. Possible intra-potash disconformities, created by dissolution of overlying potash-bearing salt beds, are indicated by an abundance of residual hematite in clay seams with some cutting subvertically into the potash bed. Except in, and near, dissolution levels and collapse features, the subsequent redistribution of insolubles, other than iron oxides, is not significant.

    In general, halite-sylvite (sylvinite) rocks in the Prairie Evaporite ore zones generally show two end member textures; 1) the most common is a recrystallised polygonal mosaic texture with individual crystals ranging from millimetres to centimetres and sylvite grain boundaries outlined by concentrations of blood-red halite (Figure 14a). 2) The other end member texture is a framework of euhedral and subhedral halite cubes enclosed by anhedral crystals of sylvite (Figure 14b). This is very similar to ore textures in the Salado Formation of New Mexico interpreted as early passive precipitates in karstic voids.

    Petrographically, the halite-carnallite (carnallitite) rocks display three distinct textures. Most halite-carnallite rocks contain isolated centimetre-sized cube mosaics of halite enclosed by poikilitic carnallite crystals (Figure 14c); 1) Individual halite cubes are typically clear, with occasional cloudy crystal cores that retain patches of syndepositional growth textures (Lowenstein and Spencer, 1990). 2) The second texture is coarsely crystalline halite-carnallite with equigranular, polygonal mosaic textures. In zones where halite overlies bedded anhydrite, most of the halite is clear with only the occasional crystal showing fluid inclusion banding.

    Bedded halite away from the ore zones generally retains a higher proportion of primary depositional textures typical of halite precipitation in shallow ephemeral saline pans (Figure 14d; Brodylo and Spencer, 1987). Crystalline growth fabrics, mainly remnants of vertically-elongate halite chevrons, are found in 50-90% of the halite from many intervals in the Prairie Evaporite. Many of the chevrons are truncated by irregular patches of clear halite that formed as early diagenetic cements in syndepositional karst.

    In contrast, the halite hosting the potash ore layers lacks well-defined primary textures but is dominated by intergrown mosaics. From the regional petrology and the lower than expected Br levels in halite in the Prairie Evaporite Formation, Schwerdtner (1964), Wardlaw and Watson (1966) and Wardlaw (1968) postulated a series of recrystallisation events forming sylvite after carnallite as a result of periodic flushing by hypersaline solutions. This origin as a secondary precipitate (via incongruent dissolution) is supported by observations of intergrowth and overgrowth textures (McIntosh and Wardlaw, 1968), collapse and dissolution features at various scales and timings (Gendzwill, 1978; Warren 2017), radiometric ages (Baadsgaard, 1987) and palaeomagnetic orientations of the diagenetic hematite linings associated with the emplacement of the potash (Koehler, 1997; Koehler et al., 1997).

    Dating of clear halite crystals in void fills within the ore levels shows that some of the exceptionally coarse and pure secondary halites forming pods in the mined potash horizons likely precipitated during early burial, while other sparry halite void fills formed as late as Pliocene-Pleistocene (Baadsgard, 1987). Even today, alteration and remobilisation of the sylvite and carnallite and the local precipitation of bischofite are ongoing processes, related to the encroachment of the contemporary dissolution edge or the ongoing stoping of chimneys fed by deep artesian circulation (pers obs.).


    Fluid inclusion studies support the notion of primary textures (low formation temperatures in chevron halite in the Prairie evaporite and an associated thermal separation of non-sylvite and sylvite associated halite (Figure 15; Chipley et al., 1990). Most fluid inclusions found in primary, fluid inclusion-banded halite associated with the Prairie potash salts contain sylvite daughter crystals at room temperature or nucleate them on cooling (e.g. halite at 915 and 945 m depth in the Winsal Osler well; Lowenstein and Spencer, 1990). In contrast, no sylvite daughter crystals have been observed in fluid inclusions outlining primary growth textures from chevron halites away from the potash deposits.

    The data illustrated as Figure 15 clearly show that inclusion temperatures in primary halite chevrons are cooler than those in halites collected in intervals nearer the potash levels. Sylvite daughter crystal dissolution temperatures from fluid inclusions in the cloudy centres of halite crystals associated with potash salts are generally warmer (Brodlyo and Spencer, 1987; Lowenstein and Spencer, 1990). Sylvite and carnallite daughter crystal dissolution temperatures from fluid inclusions in fluid inclusion banded halite from bedded halite-carnallite are the hottest. This mineralogically-related temperature schism establishes that potash salts occur in stratigraphic intervals in the halite where syndepositional surface brines were warmer. In the 50° - 70°C temperature range there could be overlap with heliothermal brine lake waters. Even so, these warmer potash temperatures imply parent brines would likely be moving via a shallow reflux drive and are not the result of primary bottom nucleation (in contrast to primary sylvite in the Mulhouse Basin). Whether the initial Prairie reflux potash precipitate was sylvite or carnallite is open to interpretation (Lowenstein and Hardie, 1990).


    Fluid evolution from mineral and isotope chemistry

    Analysis of subsurface waters from various Canadian potash mines and collapse anomalies in the Prairie Evaporite suggest that, after initial potash precipitation, a series of recrystallising fluids accessed the evaporite levels at multiple times throughout the burial history of the Prairie Formation (Chipley, 1995; Koehler, 1997; Koehler et al., 1997). Likewise, the isotope systematics and K-Ar ages of sylvite in both halite and sylvite layers indicate that the Prairie Evaporite was variably recrystallised during fluid overprint events (Table 2; Figure 16). These event ages are all younger than original deposition (≈390Ma) and likely correspond to ages of various tectonic events that influenced subsurface hydrology along the western margin of North America.


    Chemical compositions of inclusion fluids in the Prairie Evaporite, as determined by their thermometric properties, reveal at least two distinct waters played a role in potash formation: a Na-K-Mg-Ca-Cl brine, variably saturated with respect to sylvite and carnallite; and a Na-K-Cl brine (Horita et al., 1996). That is, contemporary inclusion water chemistry is a result in part of ongoing fluid-rock interaction. The ionic proportions in some halite samples are not the result of simple evaporation of seawater to the sylvite bittern stage (Figure 17a; Horita et al., 1996). There is a clear separation of values from chevron halites in samples from the Lanigan and Bredenbury (K-2 area) mines, which plot closer to the concentration trend seen in halite from modern seawater and values from clear or sparry halite. The latter encompass much lower K and higher Br related to fractionation tied to recrystallisation. Likewise, the influence of ongoing halite and potash salt dissolution is evident in the chemistry of shaft and mine waters with mine level waters showing elevated Mg and K values, (Figure 17b; Wittrup and Kyser, 1990; Chipley, 1995). What is more the mine waters of today show  substantial overlap with waters collected more than thirty years ago (Jensen et al., 2006)


    This notion of ongoing fluid-rock interaction controlling the chemistry of mine waters is supported by dD and d18O values of inclusion fluids in both halite and sylvite, which range from -146 to 0‰ and from -17.6 to -3.0‰, respectively (Figure 18). Most of the various preserved isotope values are different from those of evaporated seawater, which should have dD and d18O values near 0‰.

    Furthermore, the dD and d18O values of inclusion fluids are probably not the result of precipitation of the evaporite minerals from a brine that was a mixture of seawater and meteoric water. The low latitude position of the basin during the Middle Devonian (10-15° from the equator), the required lack of meteoric water to precipitate basinwide evaporites, and the expected dD and d18O values of any meteoric water in such a setting, make this an unlikely explanation. Rather, the dD and d18O values of inclusion fluids in the halites reflect ambient and evolving brine chemistries as the fluids in inclusions in the various growth layers were intermittently trapped during the subsurface evolution of the Prairie Formation in the Western Canada Sedimentary Basin. They also suggest that periodic migration of nonmarine subsurface water was a significant component of the crossflowing basinal brines throughout much of the recrystallisation history (Chipley, 1995).

    Prairie carnallite-sylvite alteration over time

    Ongoing alteration of carnallite to sylvite and the reverse reaction means a sylvite-carnallite bed must be capable of gaining or losing fluid at the time of alteration. That is, any reacting potash beds must be permeable at the time of the alteration. By definition, there must be fluid egress to drive incongruent alteration of carnallite to sylvite or fluid ingress to drive the alteration of sylvite to carnallite. There can also be situations in the subsurface where the volume of undersaturated fluid crossflow was sufficient to remove (dissolve) significant quantities of the more soluble evaporite salts. Many authors looking at the Prairie evaporite argue that the fluid access events during the alteration of carnallite to sylvite or the reverse, or the complete leaching of the soluble potash salts was driven by various tectonic events (Figure 16). In the early stages of burial alteration (few tens of metres from the landsurface) the same alteration processes can be driven by varying combinations of brine reflux, prograde precipitation and syndepositional karstification, all driven by changes in brine level and climate, which in turn may not be related to tectonism (Warren, 2016; Chapters 2 and 8 for details).

    In the potash areas of the Western Canada Sedimentary Basin, the notion of 10-100 km lateral continuity is a commonly stated precept for both sylvinite and carnallitite units across the extent of the Prairie Evaporite. But when the actual distribution and scale of units are mapped based on mined intercepts, there are numerous 10-100 metre-scale discontinuities (anomalies) present indicating fluid ingress or egress (Warren, 2017).


    Sometimes ore beds thin and alteration degrades the ore level (Section A-A1-A2), other times these discontinuities can locally enrich sylvite ore grade (B-B1; Figure 19). Discontinuities or salt anomalies are much more widespread in the Prairie evaporite than mentioned in much of the potash literature (Figure 19). Mining for maintenance of ore grade shows that unexpectedly intersecting an anomaly in a sylvite ore zone can have a range of outcomes ranging from the inconsequential to the catastrophic, in part because there is more than one type of salt anomaly or “salt horse" (Warren, 2017).


    Figure 20 summarises what are considered the three most common styles of salt anomaly in the sylvite ore beds of the Prairie Evaporite, namely 1) Washouts, 2) Leach anomalies, 3) Collapse anomalies. These ore bed disturbances and their occurrence styles are in part time-related. Washouts are typically early (eogenetic) and defined as... “salt-filled V- or U-shaped structures, which transect the normal bedded sequence and obliterate the stratigraphy” (Figure 20a; Mackintosh and McVittie, 1983, p. 60). They are typically enriched in, or filled by, insoluble materials in their lower one-third and medium-coarse-grained sparry halite in the upper two thirds. Up to several metres across, when traced laterally they typically pass into halite-cemented paleo-sinks and cavern networks (e.g. Figure 20b). Most washouts likely formed penecontemporaneous to the potash beds they transect, that is, they are preserved examples of synkarst, with infilling of the karst void by a slightly later halite cement. They indicate watertable lowering in a potash-rich saline sump. This leaching was followed soon after by a period of higher watertables and brine saturations, when halite cements occluded the washouts and palaeocaverns. Modern examples of this process typify the edges of subcropping and contemporary evaporite beds, as about the recently exposed edges of the modern Dead Sea. As such, “washouts” tend to indicate relatively early interactions of the potash interval with undersaturated waters, they may even be a part of the syndepositional remobilisation hydrology that focused, and locally enriched, potash ore levels.

    In a leach anomaly zone, the stratabound sylvinite ore zone has been wholly or partially replaced by barren halite, without significantly disturbing the normal stratigraphic sequence (clay marker beds) which tend to continue across the anomaly (Figure 20b). Some loss of volume or local thinning of the stratigraphy is typical in this type of salt anomaly. Typically saucer-shaped, they have diameters ranging from a few metres up to 400m. Less often, they can be linear features that are up to 20 m wide and 1600m long. Leach zones can form penecontemporaneously due to depressions and back-reactions in the ore beds, or by later low-energy infiltration of Na-saturated, K-undersaturated brines. The latter method of formation is also likely on the margins of collapse zones, creating a hybrid situation typically classified as a leach-collapse anomaly (Mackintosh and McVittie, 1983; McIntosh and Wardlaw, 1968).

    Of the three types of salt anomaly illustrated, leach zone processes are the least understood. Historically, when incongruent dissolution was the widely accepted interpretation for loss of unit thickness in the Prairie Evaporite, many leach anomalies were considered metasomatic. Much of the original metasomatic interpretation was based on decades of detailed work in the various salt mines of the German Zechstein Basin. There, in an endemic halokinetic terrane, evaporite textures were considered more akin to metamorphic rocks, and the term metasomatic alteration was commonly used when explaining leach anomalies (Bochert and Muir, 1964, Braitsch, 1971). In the past two decades, general observations of the preservation of primary chevron halite in most bedded evaporites away from the potash layers in the Prairie Evaporite have led to reduced use of notions of widespread metamorphic-like metasomatic or solid-state alteration processes in bedded evaporites. There is just too much preserved primary texture in the bedded salt units adjacent to potash beds to invoke pervasive burial metasomatism of the Prairie Evaporite.


    So how do leach anomalies, as illustrated in Figure 20b, occur in nonhalokinetic settings? One possible explanation is given by the depositional textures documented in anomalies in the Navarra Potash Province (Figure 21). There, the underlying and overlying salt stratigraphy is contiguous, while the intervening sylvite passed laterally into a syndepositional anomaly or “salt horse” created by an irregular topography on the salt pan floor prior to the deposition of onlapping primary sylvinite layers (see Warren 2016, 2017 for detailed discussion)

    On the other hand, in halokinetic situations (which characterises much Zechstein salt) solid-state alteration via inclusion related migration in flowing salt beds is a well-documented set of texture-altering processes (diffusion metasomatism). Most workers in such halokinetic systems would agree that there must have been an original stratiform potassium segregation present during or soon after deposition related to initial precipitation, fractional dissolution and karst-cooling precipitation. But what is controlling potassium distribution now in the Zechstein salts is a recrystallised and remobilised set of textures, which preserve little or no crystal-scale evidence of primary conditions (Warren, 2016; Chapter 6). The complex layering in such deposits may preserve a broad depositional stratigraphy, but the decimetre to metre scale mineral distributions are indications of complex interactions of folds, overfolds, and disaggregation with local flow thickening. We shall return to this discussion of Zechstein potash textures in the next section dealing with devolatisation of hydrated salts such as carnallite. in zones of local heating

    Collapse zones in the Prairie Evaporite are characterized by a loss of recognizable sylvinite ore strata, which is replaced by less saline brecciated, recemented and recrystallized material, with the breccia blocks typically made of the intrasalt or roof lithologies (Figure 20c), so angular fragments of the Second Red beds and dolostones of the Dawson Bay Formation are the most conspicuous components of the collapse features in the Western Canada Sedimentary Basin. When ore dissolution is well developed, all the halite can dissolve, along with the potash salts, and the overlying strata collapse into the cavity (these are classic solution collapse features). Transitional leached zones typically separate the collapsed core from normal bedded potash. Such collapse structures indicate a breach of the ore layers by unsaturated waters, fed either from below or above. For example, in the Western Canada Sedimentary Basin, well-developed collapse structures tend to occur over the edges and top Devonian mud mounds, while in the New Mexico potash zone the collapse zones are related to highs in the underlying Capitan reef trend (Warren, 2017). Leaching fluids may have come from below or above to form collapse structures at any time after initial deposition. When connected to a water source, these are the subsurface features that when intersected can quickly move the mining operation out of the salt into an adjacent aquifer system, a transition that led to flooding in most of the mine-lost operations listed earlier.

    Identifying at the mine scale the set of processes that created a salt anomaly in a sylvite bed also has implications in terms of its likely influence on mine stability whatever decision is made on how to deal with it as part of the ongoing mine operation (Warren, 2016, 2017). Syndepositional karst fills and leach anomalies are least likely to be problematic if penetrated during mining, as the aquifer system that formed them is likely no longer active. In contrast, penetration or removal of the region around a salt-depleted collapse breccia may lead to uncontrollable water inflows and ultimately to the loss of the mine.

    Unfortunately, in terms of production planning, the features of the periphery of a leach anomaly can be similar if not identical to those in the alteration halo that typically forms about the leached edge a collapse zone. The processes of sylvite recrystallisation that define the edge of collapse anomaly can lead to local enrichment in sylvite levels, making these zones surrounding the collapse core attractive extraction targets (Boys 1990, 1993). Boundaries of any alteration halo about a collapse centre are not concentric, but irregular, making the prediction of a feature’s geometry challenging, if not impossible. The safest course of action is to avoid mining salt anomalies, but longwall techniques make this difficult and so they must be identified and dealt with (see Warren 2017).


    Cooking sylvite: Dykes & sills in potash salts

     

    In addition to; 1) primary sylvite and 2) sylvite/carnallite alteration via incongruent transformation in burial, there is a third mode of sylvite formation related to 3) igneous heating driving devolatisation of carnallitite, which can perhaps be considered a form of incongruent melting (Warren, 2016). And so, at a local scale (measured in metres to tens of metres) in potash beds cut by igneous intrusions, there are a number of documented thermally-driven alteration styles and thermal haloes. Most are created by the intrusion of hot doleritic or basaltic dykes and sills into cooler salt masses, or the outflow of extrusive igneous flows over cooler salt beds (Knipping, 1989; Grishina et al, 1992, 1998; Gutsche, 1988; Steinmann et al., 1999; Wall et al., 2010). Hot igneous material interacts with somewhat cooler anhydrous salt masses to create narrow, but distinct, heat and mobile fluid-release envelopes, also reflected in the resulting salt textures. At times, relatively rapid magma emplacement can lead to linear breakout trends outlined by phreatomagmatic explosion craters, as imaged in portions of the North Sea (Wall et al., 2010) and the Danakhil/Dallol potash beds in Ethiopia (Salty Matters, May 1, 2015).

    Based on studies of inclusion chemistry and homogenization temperatures in fluid inclusions in bedded halite near intrusives, it seems that the extent of the influence of a dolerite sill or dyke in bedded salt is marked by fluid-inclusion migration, evidenced by the disappearance of chevron structures and consequent formation of clear halite with a different set of higher-temperature inclusions. Such a migration envelope is well documented in bedded Cambrian halites intruded by end-Permian dolerite dykes in the Tunguska region of Siberia (Grishina et al., 1992).

    Defining h as the thickness of the dolerite intrusion in these salt beds, and d as the distance of the halite from the edge of the intrusion, then the disappearance of chevrons occurs at greater distances above than below the intrusive sill. For d/h < 0.9 below the intrusion, CaCl2, CaCl2, KCl and nCaCl2, mMgCl2 solids occur in association with water-free and liquid-CO2 inclusions, with H2S, SCO and orthorhombic or glassy S8. For a d/h of 0.2-2 above the intrusion, H2S-bearing liquid-CO2 inclusions are typical, with various amounts of water. Thus, as a rule of thumb, an alteration halo extends up to twice the thickness of the dolerite sill above the sill and almost the thickness of the sill below (Figure 22).

    In a series of autoclavation laboratory experiments, Fabricius and Rose-Hampton (1988) found that; 1) at atmospheiric pressure carnallite melts incongruently to sylvite and hydrated MgCl2 at a temperature of 167.5°C. 2) the melting/transformation temperature increase to values in excess of 180°C as the pressure increases (Figure 23).


    A similar situation occurs in the dyke-intruded halite levels exposed in the mines of the Werra-Fulda district of Germany (Steinmann et al., 1999; Schofield, et al., 2014). There the Herfa-Neurode potash mine is located in the Werra-Fulda Basin in the Hessian district of central Germany (Figure 24a). The targeted ore levels consist of the carnallite-rich Kaliflöz Hessen (K1H) and Kaliflöz Thüringen (K1Th) intervals, which form part of the Zechstein 1 (Z1) bedded Werra salt succession (Warren, 2016). In the mine the K1H and K1Th units range in thickness from 2 m to 10 m, are generally subhorizontal and occur at a depth of 650–710 m below the present-day surface.


    In the later Tertiary, basaltic melts intruded these Zechstein evaporites as numerous sub-vertical dykes, but only a few dykes attained the Miocene landsurface. Basaltic melt production was related to regional volcanic activity some 10 to 25 Ma. Basalts exposed in the mine walls, where it cuts non-hydrous units of halite or anhydrite, are typically subvertical dykes, rather than subhorizontal sills. The basalts are phonolitic tephrites, limburgites, basanites and olivine nephelinites. Dyke margins are usually vitrified, forming a microlitic limburgite glass along dyke edges in contact with halite (Figure 24b; Knipping, 1989). At the contact on the evaporite side of the glassy rim, there is a cm-wide carapace of high-temperature salts (mostly anhydrite and ferroan carbonates). Further out, the effect of the high-temperature envelope is denoted by transitions to clear halite, with higher temperature fluid inclusions (Knipping 1989). All of this metre-scale alteration is an anhydrous alteration halo, the halite did not melt (melting temperature of 804°C), rather than migrating, the fluid driving recrystallisation was mostly from entrained brine/gas inclusions. The dolerite/basalt interior of the basaltic dyke is likewise altered and salt soaked, with clear, largely inclusion-free halite typically filling vesicles in the basalt.

    Heating of hydrated (carnallitic) salt layers, adjacent to a dyke or sill, tends to drive off the water of crystallisation (chemical or hydration thixotropy) at much lower temperatures than that at which anhydrous salts, such as halite or anhydrite, thermally melt (Figure 24c; Table 3). For example, in the Fulda region, the thermally-driven release of water of crystallisation within carnallitic beds creates thixotropic or subsurface “peperite” textures as carnallitite alters to sylvinite layers. These are layers where heated water of crystallisation escaped from the hydrated-salt lattice. Dehydration-driven loss of mechanical strength focuses zones of magma entry into particular subhorizontal horizons in the salt mass, wherever hydrated salt layers were present. In contrast, dyke and sill margins are much sharper and narrower in zones of contact with anhydrous salt intervals and the intrusive is sub-vertical to steeply dipping (Figure 24b versus 24c).

    Accordingly, away from the immediate vicinity of the direct thermal aureole, heated and overpressured dehydration waters can enter carnallite halite bed, and drive the creation of extensive soft sediment deformation and peperite textures in hydrated layer (Figure 24c). Mineralogically, sylvite and coarse recrystallised halite dominate the salt fraction in the peperite intervals of the Herfa-Neurode mine. Sylvite in the altered zone is a form of dehydrated carnallite, not a primary-textured salt. Across the Fulda region, such altered zones and deformed units can extend along former carnallite layer to tens or even a hundred or more metres from the dyke feeder. Ultimately, the deformed potash bed passes back out into the unaltered bed, which retains abundant inclusion-rich halite and carnallite (Schofield et al., 2014).

    That is, nearer the basalt dyke, the carnallite is largely transformed into inclusion-poor halite and sylvite, the result of incongruent flushing of warm saline fluids mobilised from the hydrated carnallite crystal lattice as it was heated by dyke emplacement. During Miocene salt alteration/thermal metamorphism in the Fulda region, NaCl-fluids were mixed with fluids and gases originating from thermally-mobilised crystallisation water in the carnallite, as it converted to sylvite. This brine/gas mixture altered the basalts during post-intrusive cooling, an event which numerical models suggest was quite rapid (Knipping, 1989): a dyke of less than 0.5 m thickness probably cooled to temperatures less than 200°C within 14 days of dyke emplacement.

    The contrast in alteration extent between anhydrous and hydrous salt layers shows alteration effects are minimal wherever the emplacement temperature of the magma is below that of the anhydrous salt body as it is next to a basalt dyke. If this is the mechanism driving entry of igneous-related volatiles (gases and liquids) into a salt body, then the distribution of products (including CO2) will be highly inhomogeneous and related to the minerally of the salt unit adjacent to the intrusive. Worldwide, dykes intersecting salt beds tend to widen to become sills in two zones: 1) along evaporite units within the halite mass that contain hydrated salts, such as carnallite or gypsum (Figure 24c) and, 2) where rising magma has ponded and so created laccoliths at the upper or lower halite contact with the adjacent nonsalt strata or against a salt wall (Figure 22 vs 24). The first is a response to a pulse of released water as dyke-driven heating forces the dehydration of hydrated salt layers. The second is a response to the mechanical strength contrast at the salt-nonsalt contact.

    In summary, sylvite formed from a carnallite precursor during Miocene salt alteration/thermal metamorphism in the Fulda region, NaCl-fluids were mixed with fluids originating from thermally-mobilised crystallisation water in the carnallite, as it converted to sylvite. This brine mixture altered the basalts during post-intrusive cooling, an event which numerical models suggest was quite rapid (Knipping, 1989): a dyke of less than 0.5 m thickness probably cooled to temperatures less than 200°C within 14 days of dyke emplacement.

    How do we produce potash salts?

     

    Over this series of three articles focused on current examples of potash production, we have seen there are two main groups of potash minerals currently utilised to make fertiliser, namely, muriate of potash (MOP) and sulphate of potash (SOP). MOP is both mined (generally from a Pre-Neogene sylvinite ore) or produced from brine pans (usually via processing of a carnallitite slurry). In contrast, large volumes of SOP are today produced from brine pans in China and the USA but with only minor production for solid-state ore targets. Historically, SOP was produced from solid-state ores in Sicily, the Ukraine, and Germany but today there are no conventional mines with SOP as the prime output in commercial operation (See Salty Matters, May 12, 2015).

    The MgSO4-enriched chemistry of modern seawater makes the economic production of potash bitterns from a seawater-feed highly challenging. Today, there is no marine-fed plant anywhere in the world producing primary sylvite precipitates. However, sylvite is precipitating from a continental brine feed in salt pans on the Bonneville salt flat, Utah. There, a brine field, drawing shallow pore waters from saltflat sediments, supplies suitably low-MgSO4 inflow chemistry to the concentrator pans. Sylvite also precipitates in solar evaporator pans in Utah that are fed brine circulated through the abandoned workings of the Cane Creek potash mine (Table 1).

    Large-scale production of MOP fertiliser from potash precipitates created in solar evaporation pans is taking place in perennially subaqueous saline pans of the southern Dead Sea and the Qaidam Basin. In the Dead Sea, the feed brine is pumped from the waters of the northern Dead Sea basin, while in the Qaidam sump the feed is from a brine field drawing pore brines with an appropriate mix of river and basinal brine inputs. In both cases, the resulting feed brine to the final concentrators is relatively depleted in magnesium and sulphate. These source bitterns have ionic proportions not unlike seawater in times of ancient MgSO4-depleted oceans. Carnallitite slurries, not sylvinite, are the MOP precipitates in pans in both regions. When feed chemistry of the slurry is low in halite, then the process to recover sylvite is a cold crystallisation technique. When halite impurity levels in the slurry are higher, sylvite is manufactures using a more energy intensive, and hence more expensive, hot crystallisation technique. Similar sulphate-depleted brine chemistry is used in Salar Atacama, where MOP and SOP are recovered as byproducts of the production of lithium carbonate brines.

    Significant volumes of SOP are recovered from a combination of evaporation and cryogenic modification of sulfate-enriched continental brines in pans on the edge of the Great Salt Lake, Utah, and Lop Nur, China. When concentrated and processed, SOP is recovered from the processing of a complex series of Mg-K-SO4 double salts (schoenitic) in the Odgden pans fed brines from the Great Salt Lake. The Lop Nur plant draws and concentrates pore waters from a brine field drawing waters from glauberite-polyhalite-entraining saline lake sediments.

    All the Quaternary saline lake factories supply less than 20% of the world's potash; the majority comes from the conventional mining of sylvinite ores. The world's largest reserves are held in Devonian evaporites of the Prairie Evaporite in the Western Canada Sedimentary Basin. Textures and mineral chemistry show that the greater volume of bedded potash salts in this region is not a primary sylvite precipitate. Rather the ore distribution, although stratiform and defined by a series of clay marker beds, actually preserves the effects of multiple modifications and alterations tied to periodic egress and ingress of basinal waters. Driving mechanisms for episodes of fluid crossflow range from syndepositional leaching and reflux through to tectonic pumping and uplift (telogenesis). Ore distribution and texturing reflect local-scale (10-100 metres) discontinuities and anomalies created by this evolving fluid chemistry. Some alteration episodes are relatively benign in terms of mineralogical modification and bed continuity. Others, generally tied to younger incidents (post early Cretaceous) of undersaturated crossflow and karstification, can have substantial effects on ore continuity and susceptibility to unwanted fluid entry. In contrast, ore textures and bed continuity in the smaller-scale sylvinite ores in the Oligocene Mulhouse Basin, France, indicate a primary ore genesis.

    What makes it economic?

    Across the Quaternary, we need a saline lake brine systems with the appropriate brine proportions, volumes and climate to precipitate the right association of processable potash salts. So far, the price of potash, either MOP or SOP, and the co-associated MgSO4 bitterns, precludes industrial marine-fed brine factories.

    In contrast, to the markedly nonmarine locations of potash recovery from the Quaternary sources, almost all pre-Quaternary potash operations extract product from marine-fed basinwide ore hosts during times of MgSO4-depleted and MgSO4-enriched oceans (Warren, 2016; Chapter 11). This time-based dichotomy in potash ore sources with nonmarine hosts in the Quaternary deposits and marine evaporite hosted ore zones in Miocene deposits and older, reflects a simple lack of basinwide marine deposits and appropriate marine chemistry across the Neogene (Warren, 2010). As for all ancient marine evaporites, the depositional system that deposited ancient marine-fed potash deposits was one to two orders of magnitude larger and the resultant deposits were typically thicker stacks than any Quaternary potash settings. The last such “saline giant” potash system was the Solfifera series in the Sicilian basin, deposited as part of the Mediterranean “salinity crisis,” but these potential ore beds are of the less economically attractive MgSO4- enriched marine potash series.

    So, what are the factors that favour the formation of, and hence exploration for, additional deposits of exploitable ancient potash? First, large MOP solid-state ore sources are all basinwide, not lacustrine deposits. Within the basinwide association, it seems that intracratonic basins host significantly larger reserves of ore, compared to systems that formed in the more tectonically-active plate-edge rift and suture association. This is a reflection of: 1) accessibility – near the shallow current edge of a salt basin, 2) a lack of a halokinetic overprint and, 3) the setup of longterm, stable, edge-dissolution brine hydrologies that typify many intracratonic basins. Known reserves of potash in the Devonian Prairie evaporite in West Canadian Sedimentary Basin (WCSB) are of the order of 50 times that of next largest known deposit, the Permian of the Upper Kama basin, and more than two orders of magnitude larger than any other of the other known exploited deposits (Table 1).

    Part of this difference in the volume of recoverable reserves lies in the fact that the various Canadian potash members in the WCSB are still bedded and flat-lying. Beds have not been broken up or steepened, by any subsequent halokinesis. The only set of processes overprinting and remobilising the various potash salts in the WCSB are related to multiple styles and timings of aquifer encroachment on the potash units, and this probably took place at various times since the potash was first deposited, driven mainly by a combination of hinterland uplift and subrosion. In contrast, most of the other significant potash basins listed in Table 10 have been subjected to ongoing combinations of halokinesis and groundwater encroachments, making these beds much less laterally predictable. In their formative stages, the WCSB potash beds were located a substantial distance from the orogenic belt that drove flexural downwarp and creation of the subsealevel sag depression. Like many other intracratonic basins, the WCSB did not experience significant syndepositional compression or rift-related loading.

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