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Evaporite interactions with magma Part 3 of 3: On-site evaporite and major extinction events?

John Warren - Saturday, April 13, 2019



The previous two articles in this series dealt with heating evaporites, volatiles expelled into the atmosphere, and major biotal extinction events. I argued that short-term heating of a megaevaporite mass during emplacement of a Large Igneous Province (LIP) or heating of evaporities at the site of a large bolide impact, will move vast volumes of sulphurous and halocarbon volatiles, as well as solids, CO2 and CH4 into the earth's upper atmosphere (Figure 1a). The resulting catastrophic climatic effects link in time and probable causes to earth-scale major extinction horizons. (Figure 1b). In this article shall examine how three of the five major Phanerozoic extinction events have an evaporite association, starting with the most intense extinction event of the Phanerozoic; the end-Permian and its link to LIP emplacement into two separate sequences of massive bedded evaporite (Cambrian or Devonian mega-salts) in the Tunguska Basin, Siberia.

End-Permian - Saline interactions during emplacement of Siberian Traps

The Siberian Traps LIP is of significant size (~7 × 106 km2) and total volume (~4 × 106 km3) (Ivanov et al., 2013 and references therein). It is, however, smaller than the Late Cretaceous Deccan Traps and has a volume that is about a half of the Late Triassic Central Atlantic Magmatic Province (CAMP). All three of these continental LIPs are dwarfed by the Early Cretaceous marine Ontong-Java LIP (≈20 × 106 km3). So, it seems that the volume of igneous material in a LIP does not directly relate to the intensity of the extinction event (Figure 1b).

The Siberian Traps include ultramafic alkaline, mafic and felsic rocks that erupted in different proportions within a vast region extending over several thousands of square kilometres across Western and Eastern Siberia (Figure 2a). The Siberian Traps are considered have been emplaced atop a hotspot in a relatively short time frame (≈1 million years), when a large volume of deep mantle-derived igneous material was intruded and erupted at the Permo-Triassic boundary (Burgess et al., 2017).

Trap geology

Near Noril'sk, lava outflows reach thicknesses of over 3 km, while further to the northeast in the Maymecha-Kotuy region, half of the total lava pile is composed of ultramafic rocks including magnesian rich meimechites (Figure 2a). The very high MgO contents (8-40 wt %) of the meimechites in such low-degree melts indicates that the site of initial melting was very deep, as much as 200 km, and either in the lowermost continental lithosphere or in the underlying asthenosphere (Arndt et al., 1995). Melting probably was linked with the arrival of a mantle plume that was in its turn the source of the Siberian basaltic flood volcanism.

Thickness of volcaniclastic material in the Siberian Traps ranges from intercalated layers less than a meter thick on the Putorana Plateau to hundreds of meters near the base of the volcanic sections in the Angara and the Maymecha-Kotuy areas (Figure 2a). The total volume of mafic volcaniclastic material has been estimated at >200,000 km3 or >5% of the total volume of the Siberian Traps (Black et al., 2015). Volcanic rocks of this age are also present in drillcore in the West Siberian Basin (Ivanov et al., 2013).

Magma-sediment and magma-water interactions active during emplacement of the Siberian Traps in the upper lithosphere encompass a variety of heated evaporite interactions: batholith metal-evaporite interactions, lava-water interactions and intense phreatomagmatic explosions via vents and breccia pipes that formed saline-igneous volatile fountains reaching the upper atmosphere. The positions of these fountains are perhaps indicated by vent-related iron-rich diatremes (Figure 2a; Svensen et al., 2009). All these interactions are critical inputs to the End-Permian extinction event that links vast volumes of altered evaporites with the heating mechanisms inherent to Siberian Trap geology.


Evaporite basins (Devonian and Cambrian)

The Siberian Traps region is not only significant because of its vast extent and its deep nickel-prone mantle source, but also in that the immense volumes of igneous rocks that making up the traps were emplaced into two chemically prone saline giants with differing dominant mineralogies and ages; 1) Cambrian mega-halite sediments in the south, with interlayers of hydrated potash salts (mostly carnallitite) and 2) Devonian megasulphates in the north, containing two 50-100m beds of anhydrite (Figures 2b, 5). The interactions with the two types of salt basins, one halite-dominant, the other anhydrite-dominant, gives rise to two distinct meta-evaporite indicator associations. In the North, the interaction of picritic magmas with bedded thick anhydrites formed the supergiant Noril'sk nickel deposit, while in the south the LIP emplacement formed numerous magnetite-rich explosive breccia pipes, sourced at the stratigraphic level of the Cambrian salts (Figure 2b).

Norils'k region & Devonian evaporites

In the northern part of the Tunguska Basin the evaporite sediments hosting the intrusives of the Siberian Traps are a combination of Devonian anhydrites and carbonates, with overlying Carboniferous coals. Trap basalts, now cover this sedimentary sequence (Figure 4a), while sill-like tholeiitic intrusions, varying in composition from subalkaline dolerite to gabbro-dolerite are emplaced in the sediment pile and were part of the feeder system to the flood basalts (Figures 4b, 5, 6).

The region of Devonian evaporites contains the Noril'sk-Talnakh ore deposit, the largest Phanerozoic nickel deposit in the world (Figures 3, 4; Naldrett 2004). In the mine area, ore-bearing gabbroic-dolerites are differentiated, whereby picrite and picritic dolerite are overlain by more felsic differentiates. The Cu-Ni-platinoid mineralisation at Noril'sk forms relatively persistent stratabound horizons of massive sulphides in the lower portions of the three mineralised intrusions (Noril'sk, Talnakh, Kharaelakh), which are made up of segregations and accumulations of pyrrhotite, pentlandite and chalcopyrite (Figures 5, 6).

At the world-scale, the supergiant Permian Noril'sk-Talnakh deposit is an unusual Cu-Ni deposit. It did not form in the Precambrian, and so is unlike almost all the world's other supergiant magmatic nickel-sulphide deposits (Figure 3). It formed at the end of the Palaeozoic and straddles the Permo-Triassic boundary (Black et al., 2014a). Magmatic nickel ores at Noril'sk crystallised outside the influence of the reducing planetary atmosphere that typifies Archaean Ni flood basalt deposits and is not tied to greenstone terranes and the athenospheric transition to more sialic plate-scale conditions. (Figure 3). The high temperatures and near complete assimilation of Devonian sulphate evaporite blocks within the Noril’sk magma mean that this is one of the more enigmatic (“salt is elsewhere”) styles of evaporite-related high-temperature ore deposits (Warren, 2016, Chapter 16). Notions of evaporite assimilation for ore deposits tied to igneous-evaporite interactions are usually only one of multiple possible explanations of a magmatic ore but, in my opinion, for Noril’sk this is the most likely scenario. So, I emphasise the evaporite connection for the Noril’sk-Talnakh deposit in this article. Alternate non-evaporitic orthomagmatic explanations can be found in papers such as Wooden et al., (1992); Lightfoot et al. (1997), and Krivolutskaya (2016). Independent of the mode of nickel-ore fixation, most authors working in the Tunguska Basin agree that the emplacement of the trap intrusives drove the escape of a huge pulse of sediment-derived volatiles into the Earth's atmosphere.

Regional structure of the Noril’sk district is dominated by NNE-NE Permo-Triassic block faulting, which was coeval with magmatic activity. Individual faults may be over 500 km in length with throws of up to a kilometre (Figure 4b; Naldrett, 1997). Mineralised intrusions radiate outward and upward from intrusive centres and penetrate all levels of the overlying sedimentary sequence. Most intrusive centres are associated with prominent block faulting and fault intersections. The main Noril’sk-Kharaelakh fault occurs within the Siberian Platform, but is parallel to the main fault system that defines the boundary between the platform and the nearby Yenisei Trough. The Kharaelakh-Noril’sk fault guided the main upwelling magma body (Figures 4b, 6). Individual sills splay off this fault control and are interlayered with sulphate evaporite beds to can attain lateral lengths of 12 km, widths of 2 km and thicknesses of 30 to 350 m.

Mineralogical compositions of the Devonian sediments interlayered with these sills are of great importance in understanding the geological responses to heating by intrusive igneous sills in the Noril'sk-Talnakh area (Figures 5, 6). Based on their lithological features and paleontological character, the intruded Devonian succession is subdivided into the Yampakhtinsky, Khrebtovsky, Zubovsky, Kureysky, and Razvedochninsky Formations (Lower Devonian), the Manturovsky and Yuktinsky Formations (Middle Devonian), and the Nakohozsky, Kalargonsky and Fokinsky Formations (Upper Devonian) (Figure 5; Krivolutskaya, 2016; Naldrett, 2004). The two main evaporite levels are the Middle Devonian and Lower Devonian anhydrite-dominant successions, both deposited in a subsealevel transitioning rift (Figure 5, 6; Naldrett, 2005; Warren, 2016).

The Yampaktinsky and Khrebtovsky Formations consist of Lower Devonian carbonates interbedded with abundant gypsum (in outcrop) and anhydrite (subsurface), along with some of the oldest lenses of celestite in the area (Figure 5). The total thicknesses of these two CaSO4 units are around 100 and 80 m, respectively. The Lower Devonian Zubovsky Formation is composed of grey-colored dolomitic marls interbedded with argillaceous dolomites, mudstones, and anhydrite with a total thickness of 110–140 m. The Zubovsky Formation unconformably overlies the Lower Devonian Khrebtovsky Formation in the Noril’sk region. The Lower Devonian Kureysky Formation consists of mottled dolomite and calcareous mudstones and marls with rare siltstone and limestone. The thicknesses of all units in the outcrop section remain stratiform and vary within 50–60 m. The contacts with the overlying and underlying formations are conformable.

The Lower Devonian Razvedochninsky Formation is dominated by siltstones, sandstones, and conglomeratic sandstones with a thickness that regionally does not exceed 110–150 m, but reaches 150–235 m in troughs, and decreases sharply to the south until fully wedging out.

The Middle Devonian Manturovsky Formation overlies the eroded Razvedochninsky Formation and consists of a terrigenous-carbonate section with abundant salt-bearing strata, most of which consist of rock salt or brecciated equivalents. This formation’s thickness is 100-210 m but ranges up to 500 m (Figure 6). The Middle Devonian Yuktinsky section is dominated by clastic–carbonate sediments ranging from 12 to 40 m thick, while in the troughs the thickness of interlayered sulphate rocks reaches 55 m. The contacts with the underlying and overlying Middle Devonian Manturovsky deposits are considered comformable. The Upper Devonian Nakokhozsky Formation consists of folded calcium-sulphate-rich variegated shale–carbonate rocks with a thickness of 2–60 m that increases in the troughs to 80–130 m (Figure 5). The Upper Devonian Kalargonsky Formation is characterised by a grey-colored terrigenous-carbonate section that includes dolomites, dolomitic marl, dolomite–limestone, and anhydrite dominate in the basins. This formation’s thickness is 170–270 m. The Kalargonsky Formation unconformably overlies the Middle Devonian Nakokhozsky sediments and the contact is typically a breccia (Figure 5).

The Middle Devonian Fokinsky Formation (as distinct from the mineralised Fokinsky intrusions) consists of evaporite sulphate-rich clastic–carbonate sequences, primarily within the troughs, and anhydrite, dolomitic marls interbedded with limestone lenses of rock salt, and clay–carbonate breccias (Krivolutskaya, 2016). The thickness of this formation is 220–420 m (approximately 500 m in the western part of the Vologochansky Trough).

The Fokinsky Formation is not recognised by all authors working in the region. This disparity in stratigraphic recognition across the region underlines a problem inherent in the litho-stratigraphic descriptions of many bedded evaporite regions worldwide, where it is assumed that a layer-cake stratigraphy/correlation is present pre- and post-intrusion. Thereby the effects of evaporite collapse dissolution, bed wedge-out and possible salt flow are not quantified. In my opinion, sedimentary breccias in such regions are more likely to be diagenetic and laterally discontinuous (see Warren, 2016; Chapter 7).

In summary, the Devonian stratigraphy in the vicinity of the Noril'sk Mine retains significant thicknesses (50-100m) with variations centred on transitions in and out of bedded anhydrite. There is a strong likelihood that the current outcrop geology interpretations under-illustrate former thicknesses of bedded evaporites during to ongoing dissolution, collapse and possible flowage.

The anomalous Phanerozoic age of the Noril’sk-Talnakh ore deposits, compared with the Precambrian ages of other magmatic Ni-Cu deposits, and its relative enrichment in Ni, Cu, Pt and Pd compared with Sudbury and Jinchuan (Figure 3), is thought to reflect the anomalously high volumes of sulphur in the parent magma. Additional sulphur entered the evolving magma chamber via intrusion and assimilation of CaSO4 blocks and associated hydrothermal solutions altering and dissolving adjacent thick-bedded anhydrite successions (Figure 7; Naldrett 1981, 1993, 1997; Pang et al., 2013). Noril'sk-Talnakh's rich sulphur supply contrasts with that of the komatiitic Archaean Cu-Ni deposits, where the sedimentary sulphur supply came from more ubiquitous, less-focused sulphur sources sometimes entrained in widespread sedimentary pyrite (Figure 3). Such pyrite characterises a significant portion of fine-grained sediments accumulated under an anoxic reducing Archaean to Palaeoproterozoic atmosphere.

Abundant crystals of magmatic anhydrite today typify the olivine-bearing (picritic) gabbros in the Kharaelakh intrusion, which is located in the basin stratigraphy at the level of the Devonian anhydrites (Figure 6; Li et al., 2009 Spiridonov, 2010). Along with disseminated sulphides, the anhydrite crystals are characterised by planar boundaries with co-associated olivine and augite. Dihedral angles of ~120°, characteristic of simultaneous crystallisation, are common throughout the anhydrite-augite assemblages. Inclusions of anhydrite in augite and vice-versa are also typical.

Rounded and subrounded sulphide inclusions composed of pyrrhotite, pentlandite, and chalcopyrite, that crystallised from immiscible sulphide liquid droplets in the magma, are commonplace within the magmatic anhydrite crystals and in the contact aureoles (Figure 7). Visual estimates by Li et al. (2009), based on five polished thin sections, indicate that the ratio of anhydrite to sulphide in mineralised samples varies from 0.05 to 0. The observation of abundant wollastonite in contact aureole rocks at this stratigraphic level suggests that reactions such as CaSO4 + SiO2 + H2O = CaSiO3 + H2S + 2O2 occurred, and that sulphate was likely reduced to sulphide before incorporation into the magma (Ripley et al., 2007).

Picritic magmas in mantle plumes can have melt temperatures as high as 1600°C (Hezberg et al., 2007). Assimilation of anhydrite via partial melting of a cooler basaltic magma at shallower depths can be more difficult, owing to the high melting point of pure anhydrite (melt temperatures typically rang between 1360 and 1450°C, although this is significantly lowered in the presence of organics and water). Rather than only melting anhydrite enclosed by picritic magma, additional fluxing mechanisms likely move additional anhydrite-derived sulphur into the melt, either by hydrothermal leaching of sulphate followed by partial reduction, or via a process involving the dissolution of anhydrite during thermochemical sulphate reduction (TSR; Warren, 2016; Chapter 9). The latter process requires heat, anhydrite and organics (generally in the form of hydrocarbons or kerogen).

Some authors use the euhedral outline of anhydrite in mineralised sills, as seen in Figure 7, to argue blocks anhydrite country rock was not assimilated. This is a specious argument as this type of anhydrite was precipitated during cooling of an already sulphur-saturated magma, the euhedral spary outline does not relate to the source of the sulphur, which is more clearly indicated by its sulphur-isotope signature (Figure 8a - also Warren, 2016; Chapter 8).

Isotopic analysis of δ34S in the magmatic anhydrites and associated metal sulphides in the Kharaelakh intrusives require the assimilation of externally-derived high-δ34S sulphur from the adjacent country rock (Figure 8: Ripley et al., 2007, 2010). Where complete sulphate reduction occurred, the δ34S values require mixtures of some 60% anhydrite-derived evaporitic marine sulphur (δ34S values near 20‰), with 40% mantle-derived sulphide (δ34S of 0‰) to produce the required measured magmatic sulphide values ≈12‰ (Figures 8a, b).

The sulphur isotope data and the nature of the sampled contact aureoles suggest intense intracontinental rifting in the Noril’sk region brought deeply-sourced mafic magmas into contact with supracrustal sulphur from evaporitic sulphates at the level of the Kharaelakh intrusion. Sulphur isotope data show the mineralised intervals at Noril’sk are anomalously heavy in δ34S (Figure 8a, b). These data are inconsistent with sulphur derived from mixing of the mantle magma sulphur (δ34Svalues near zero) with sulphur from an evaporitic sulphate source (Godlevski and Grinenko, 1963; Grinenko, 1985; Li et al., 2009; Pang et al., 2012; Black et al., 2014a).

Sulphur isotope values from Paleozoic evaporites vary between +10 and +35‰ (Figure 8b; Claypool et al., 1980). Cambrian evaporites, including the major Irkutsk basin salts in Siberia, are the most 34S-enriched evaporites in the Phanerozoic, with mean δ34SVCDT = +30‰ (Claypool et al., 1980; Black et al., 2014a). Two-member mixing curves between meimechite and anhydrite sulphur (with δ34S = +20 to +35‰) convincingly reproduce the observed δ34S trends for the Noril'sk ores (Figure 8b; Black et al., 2014a).

As the magma rose through the sedimentary cover, it penetrated and assimilated sulphur from extensive Devonian anhydrite layers (Figure 9). Sulphur in calcium sulphate was reduced to sulphide, CaO entered the magma, and iron from the magma reacted with reduced sulphur so that the end result was droplets of immiscible iron sulphide dispersed through the melt (Naldrett and Macdonald, 1980). These droplets acted as collectors for Ni, Cu and the platinum group elements, which are now so enriched in the Noril’sk ores.

Naldrett (1991, 1997, 2005) concluded that prehnite + biotite + anhydrite + carbonate + zeolite + chlorite ± sulphide globules, which typify chromite agglomerations in the picrite of the Noril’sk intrusions, represent remnants of partially assimilated sulphate-rich country rock. Assimilation of anhydrite-rich rocks, coupled with the reduction of sulphate to sulphide, would have introduced considerable oxygen into the silicate melt, which then drove precipitation of chrome-spinel minerals (chromite - FeCr2O4; mangnesiochromite - MgCr2O4). Inclusions of anhydrite-rich material, floating in the magma, would have served as loci for chromite crystallisation, thus giving rise to the association between the agglomerations and the globules. Tarasov (1970) pointed out that Middle Carboniferous coal measures were also assimilated and may have supplied organics that assisted in the reduction of sulphur in the magmas (Figures 6, 7).

Evidence of the assimilation of large volumes of anhydrite and coaly organics into the magma mass has implications beyond the formation of the Noril’sk-Talnakh ore deposits. Li et at. (2009) identified magmatic anhydrite-sulphide assemblages in a subvolcanic intrusion associated with the Siberian Traps. The δ34S values of anhydrite and coexisting sulphide crystals analysed by ion probing are 18‰–22‰ and 9‰–11‰, respectively, are much higher than the anhydrite-contaminated ore values shown in Figure 8). To obtain this level of fractionation means more than 50% of the total sulphur in the intrusion was derived from marine evaporites in the footwall strata. The contaminated magma was highly oxidised and able to dissolve up to one order of magnitude more sulphur than pure mantle-derived basaltic magma. Such sulphur-contaminated magma, when erupted, would have released vast volumes of SO2 into the atmosphere (Black et al., 2012, 2014b). That is, the eruption of the anhydrite-contaminated magma that is the Siberian Traps in the northern Tunguska Basin can help explain the intensity of the end-Permian extinction.

In summary, such igneous - sulphate sediment interaction explains, at least in part: (1) the vast amount of sulphide melt in the Noril’sk-Talnakh ore field; (2) the heavy quasi-anhydrite isotopic composition of sulphur in sideronitic and massive nickel ores; (3) the reduced contents of noble metals in these ores (compared with the drop sulphides that occur toward base of the intrusions and have a likely mantle sulphide source); and (4) the high contents of radiogenic (crustal) osmium in sideronitic and massive ores (Spiridonov, 2010; Walker et al., 1994). In summary, the reserves of the world-class Ni-PGE deposit at Noril’sk-Talnakh, with its anomalous Phanerozoic age, likely reflect a fortuitous occurrence of thick Devoninn anhydrites (ultimate sulphide source) atop an active later set of deep mantle-tapping rift grabens that drove the LIP outlined by the Siberian Traps. Wherever these magmas vented into the Earth's atmosphere they carried significant volumes of sulphurous volatiles.

Cambrian evaporites, potash & breccia pipes

Salt deposits of late Vendian to Early Cambrian age in East Siberia cover an extensive area (ca. 2 million km2) located to the north-west of Lake Baikal with an extent showing it extends across much of the Permian Siberian Traps (Figure 2b). The thickness of this upper Vendian-Lower Cambrian evaporite succession is 2.0–2.5 km in the southern, western, and central parts of the basin, and 1.3–1.5 km in the NE part (Nepa-Vilyui). This saline giant (total volume of upper Vendian–Lower Cambrian evaporites is 785,000 km3; Zharkov, 1984) is characterised by the occurrence of fourteen regional marker carbonate units and 15 salt units (Figure 10; Zharkov, 1984, with references therein). Five major phases of salt deposition are distinguished, namely the late Vendian (Danilovo) and Early Cambrian (Usolye, Belsk, Angara, and Litvintsevo) salt basins (Figure 10a; Zharkov (1984), Kuznetsov and Suchy. (1992).

Average thicknesses of the Cambrian evaporite deposits decreases with time (Figure 10b) as does the area (Figure 10a). The area of the oldest Cambrian basin, the Usolye salt basin is almost 2 million km2, and the average thickness of deposited salt around 200 m (Zharkov, 1984), while the area of the youngest, Litvintsevo salt basin is 0.5 million km2 and the average thickness of its evaporite bed (rock salt and anhydrite) is 50 m (Figure 10; Zharkov, 1984).

Most of the petroleum reservoirs in the region are located in the Cambrian carbonates. The post-Cambrian stratigraphy contains major erosional breaks. As we saw in the Noril'sk discussion, Devonian evaporites are rare in the south but abundant in the north, whereas Ordovician rocks (limestones, marls) are locally abundant in the central parts of the basin. Cambrian salt deposition is interpreted as mostly taking place in a deeper water basin: Petrichenko (1988) concluded that at the termination of halite deposition the final brine depth was 50–260 m, and at the onset of potash deposition it was ≈10–50 m.

Lower Cambrian Angara evaporites host the largest known bedded potash deposit in Russia, which is not yet produced (Figure 11; Garrett, 1995; Warren 2016, Chapter 11). Potash salts occur at the base of the Angara Formation in what is called the sixth halite series (Table 1). This intracratonic potash basin is one of the larger potash-entraining salt sumps in the world, it is several times larger than the Permian Upper Kama deposit and approaches the Prairie Evaporite in aerial extent, but not in lateral continuity, thickness or purity (Figure 11) due in large part to the effects of igneous disburbance.

Plans were made in 1986 under the old Soviet regime to initiate a mining program in a section of this basin called the Nepskoye deposit but were never fully implemented, although some ore was extracted in the mid 1980s (Andreev et al., 1986). The proposed potash development region is located near the towns of Nepa and Ust-Kut (300 km apart) in Irkutsk State. Regionally, the dominant potash mineral is carnallite, but high-grade sylvinite is intersected at depths of 600-1,000 m in beds some 1.5-5 m thick over an area ≈ 1,000 km2 (Garrett, 1995). The lower Bur or K1 bedded potash horizon lies at a depth of 750 - 960 m and is 2-18 m thick (4-6 m in the central area (Figure 11a; Table 1). Two sylvinite zones in this horizon were mapped, with the central one being 16-26 km long and 6-8 km wide (Figure 11a). In the lower horizon (K1) the sylvinite was 1.5-3 m thick, and averaged 15-50% KCl, 0.05-0.5% MgCl2,with 0.5% insolubles. The overlying K2 potash zone (Tunguaka) also entrains several sylvinite beds and is some 679-880 m deep and 2.5-20 m thick. It has a 15-45% KCl content and comparatively low MgCl2 and insoluble contents. This zone represents the major potash reserves of the deposit. In the upper potash beds (K2) the sylvinite strata become more discontinuous, but some reasonably thick, high grade and extensive zones exist (Andreev et al. 1986). The sylvite ore sits in a more regional potash succession composed of a combination of carnallitite and sylvinite (Figure 11b). The broader Nepa potash region as generally mapped in Figure 2 has two interesting characteristics; 1) The igneous trap rocks as defined in the drill-controlled cross sections of Malykh and Geletii (1988) sit below the potash level (Figure 11b), 2) There is a paucity of magnetitic explosive breccia pipes in the Nepa potash region (Figure 2b).

To the south and west, between Irkutsk and Taseyevo some 400 km to the west, other large potash occurrences have been reported in the same general but poorly delineated evaporite basins. For instance, in the Kanak-Taseyevo basin, potash beds (sylvite-carnallite containing 3-24% K2O) have been intersected at depths of 1,240-1,415 m (Garrett, 1995). Potash beds at these depths would require a solution mining methodology, but the at-surface climate would mean either cryogenic pan processing or evaporators, making recovery more difficult and expensive (Warren, 2016; Chapter 11).

Basaltic breccia pipes, Tunguska Basin Siberia

Basalt pipes form a rim to the main basalt body of the Siberian Traps and are genetically linked to trap emplacement (Figure 2b; Polozov et al., 2016). The pipes pierce through all sedimentary strata, even dolerite sills higher in the Permo-Carboniferous portion of the basin stratigraphy, and are considered to be a type of diatreme. Importantly, the basalt pipes with magnetite cores tend to occur across the southern Tunguska Basin, while unmineralised basalt pipes are more widespread (Figure 2b). Some of the basalt pipes bearing magnetite mineralisation are of commercial grade and are mined for their iron ore.

Regionally, it is difficult to estimate the total number of pipes (both “barren” basalt and magnetite-enriched) because repeated glaciations have flattened relief, while thick taiga forest covers significant parts of Siberia. Thus, many pipes are hidden by swampy coniferous forests and so are difficult to map. However, conservative estimates based on prospecting surveys for iron mineralization in the southeern portion of Tunguska Basin, and geological mapping elsewhere, suggest there are more than three hundred magnetite-bearing basalt pipes. This includes 6 large (>100 Mt of iron ores), 14 medium (20–100 Mt) and 19 small <20 Mt) sized iron deposits. All other mineralised basalt pipes are currently of sub-economic grade or underexplored (Polozov et al., 2016).

The magnetite deposits are consistently located in the Tunguska Basin region underlain by Cambrian evaporites and mainly defined by subvertical and cylindrical breccia bodies with magnesio-ferrite and magnetite as the primary ore minerals (Figure 2b). In many ways these deposits are similar to iron oxide, gold and copper (IOGC) deposits worldwide, but are classified in the Russian literature as Angara–Ilim type deposits, named after the two rivers where a large number of iron- mineralised basalt pipes crop out (Soloviev, 2010; Warren 2016, Chapter 16).

Korshunovsky (Korshunovskoye) region, Siberia

This region, in the Irkutsk district, is the eighth largest iron ore producer in Russia, with an annual output of 5 Mt of iron ore concentrate. Across the region, the pipes are sourced in the Cambrian evaporite part of the basin stratigraphy and pierce younger Paleozoic sediments composed of argillites, limestones, marls, siltstones, sandstones and clays of the late Cambrian Lena, Ust'kut, Mamyr and Ordovician Bratsk groups and overlying Early Carboniferous limestone.

We shall focus one of the largest magnesite deposits in the region, the Korshunovshoe (Korshunovsky) magnetite breccia pipe, with an estimated reserve of 1.5 Gt of ore to a depth of 1700 m (Soloviev, 2010; Polozov et al., 2016). It is mined (open pit) and so the interior structures and relationships are well documented (Figure 12). The currently mined pipe is adjacent to another explosion pipe to the immediate south-east, with the mineralised breccias sourced mainly at the level of the Cambrian evaporites (halite, potash and anhydrite; Mazurov et al., 2007). At outcrop and in the pit little evidence, other than secondary textures (dissolution-collapse and brecciation), remains of the primary minerals of the mother saline layer, although remnant, recrystallised evaporite clasts (including halite and anhydrite) typify the mineralised breccia in the lower parts of the pipe (Mazurov et al., 2007, 2018). Textures at the evaporite level in the diatremes are not unlike those seen in regions of Eocene sill interaction with hydrated salts in the Zechstein potash mines of East Germany (Schofield et al., 2014; Warren 2016 and part 1 in this series of Salty Matters articles).

The Korshunovshoe pipe is filled with tuff breccias and fragmentals composed of the surrounding saline country rocks which have undergone considerable metasomatic alteration. They incorporate fragments and larger blocks of sedimentary (60 to 80 vol.%; sandstones, siltstones, limestones, evaporite residues and argillites) and igneous (10 to 40 vol.%; gabbro-dolerites, dolerites and basalts) rocks, cemented by essentially chloritic material as well as by fine-grained carbonate (Figure 13). The central part of the magnetitic diatreme characterised by intense multiple brecciation, with rock fragments in the breccias represented mostly by variably-altered dolerites. They are cemented by a finely-dispersed matrix, entirely replaced by skarn, post-skarn alteration assemblages and iron oxides.

Outside of this zone, intense fracturing has occurred, locally with brecciation in altered sedimentary rocks. The fractures are filled with magnetite, accompanied by chlorite and calcite. Finally, the outermost zone is characterised by weak, predominantly sub-horizontal fractures within sedimentary host rocks, locally replaced by skarns. Steeply-dipping dykes of gabbro-dolerite, dolerite, dolerite-porphyry, and basalt-porphyry are present, both within and outside the breccia pipes, while sub-horizontal dolerite sills occur at depth (Soloviev, 2010; Mazurov et al., 2007, 2018).

Magnetite pipe orebodies at Korshunovshoe are texturally and mineralogically complex (Figures 12, 14) and are composed of: i) Banded masses of metasomatic magnetite that are within, and conformable to saline to calcareous members of the host sedimentary wall rocks (dominantly in dolomitic limestones, marls, calcareous argillites and sandstones with a calcareous or limy matrix, but only to a minor degree in sediments without a saline carbonate component) at a depth of some 700 to 1500 m from the surface; ii) Stock-like, lensoid, layered and columnar bodies of magnetite within the altered pyroclastics of the breccia pipe; and iii) Steeply dipping vein-like masses in zones of intense brecciation and replacement by skarns.

Together these mineralisation styles form two large continuous bodies in the Korshunovshoe pipe (Figure 12). The main deposit has the form of a sub-vertical breccia pipe with plan dimensions of approximately 2400 x 700 m. Mineralisation has been traced by drilling to a depth of 1200 m, and by geophysical data to at least 3 km below the surface (Soloviev, 2010).

The bulk of the ore is associated with brecciation and occurs within sediments, tuffs and igneous rocks and are demonstrably due to the partial replacement and alteration of the host. Massive and banded ores are less well developed. The mineralisation is mostly magnetite (≈82% of iron resources), with minor magno-magnetite, hematite and martite. The main orebody comprises vertically overlapping zones, with variable amounts of hematite and martite in the upper layers, calcite and magnetite in middle layers, and halite and magnetite in lower layers. The magnetite of the upper to middle zone is accompanied by pyroxene, chlorite and minor epidote with lesser amphibole, serpentine, calcite and garnet, and rare quartz, apatite and sphene and occurs as oolites, druses, masses and disseminations. Calcite increases downwards to 20 to 30%. In the lower part of the deposit, halite, amphibole and Mn-magnetite are more abundant. Pyrite, chalcopyrite and pyrrhotite are found throughout. Much of the magnetite is magno-magnetite which contains up to 6% MgO.

Across the region of magnetite breccia pipes, ore is extracted from magmatic diatremes that completely penetrated the highly evaporitic lower Phanerozoic succession (Figure 14; Mazurov et al., 2007, Polozov et al., 2016). Early work on this intrusive magnetite style, which surrounds brecciated diatreme-like pipes, classified it as a skarn association, forming a halo around a set of explosive pipes that accompanied regional trap magmatism (Ivashchenko and Korabel’nikova, 1960).

Characteristic spinel-forsterite magnesian skarns are confined to the overdome parts of large doleritic bodies and are the result of interactions of massive evaporitic and petroliferous dolomites with fluids released from liquid magma (Mazurov et al., 2007). Magnesian skarns of the postmagmatic stage are localised in the marginal parts and on the front (outwedged portions) of doleritic sills, apophyses, and the branches of intrusive bodies hosted at the level of the Cambrian carbonate-evaporite successions (Figure 14). The skarns penetrating the evaporite levels have a banded or layered structure and resemble gravel conglomerates, with carbonate cements. The round fragments (metasomatic pseudo-conglomerates) are composed of globules of disintegrated doleritic porphyrite, completely or partially substituted by zonal magnesian skarns. Their mesostasis is cryptocrystalline, and early phenocrysts of olivine, plagioclase, and pyroxene have undergone dispersion and substitution. Unaltered cores of the metasomatic ‘conglomerate’ are in contact with a fassaite zone, which passes outward into a spinel-fassaite zone and then into a forsterite-magnetite and calciphyre zone.

The geometry of pipe emplacement is broken down into three related styles; i) Root zone, ii) Diatreme zone, iii) Crater zone (Figure 14). The upper crater zone is sometimes complicated by the presence of reworked crater-lacustrine deposits (Polozov et al., 2016). The root zone is typically brecciated with pseudoconglomerated and other saline volatilisation textures described in the previous paragraphs. The root zone can be traced out from the pipe stem as disturbed zones with considerable lateral extents at the level of the Cambrian evaporite beds. Subhorizontal brecciated dolerite “sills” of the Kapaevsk iron deposit were cemented with calcite, magnetite and halite in various ratios and traced down to deep levels close to the root zones in some basalt pipes In the Korshunovsk iron-ore deposit, such a brecciated body extends from the main diatreme pipe some 5 km to the west and 9 km to the south-west (Von der Flaass and Nikulin, 2000).

Although not discussed in terms of a volatilisation mechanism in the published literature, I would argue that the lateral apophsyes are indicative of the former presence of hydrated salt layers, probably carnallitite beds showing similar responses to those seen in the potash mines of East Germany (Shofield et al., 2014; or part 1 in this current series of Salty Matters articles).

The diatreme chimney atop the root zone indicates the rapid rise of a overpressured and upward flowing gas-charged rock mass. Basalt magma served as the ultimate source of iron for the magnetite in the breccia pipes. Extraction of iron from the melt and its transition and accumulation took place in the presence of chlorine-rich fluids, which were formed in the course of thermal decomposition of halite-hosted hydrated salt beds (carnallite). In the later stages of ore formation, some chlorine was fixed in scapolites, while sodium was fixed in albitites and scapolitites (dipyres). In the tuffs of a number of diatremes and paleovolcanoes of the Siberian Platform, native iron can form metal balls in association with moissanites and diamonds (Goryainov et al. 1976). The occurrence of such phases, as well as bitumen in calderas and carbonaceous matter in pisolite tuffs, points to the migration of hydrocarbon fluids through the volcano-tectonic structures (Ryabov et al., 2014).

Hydrocarbons are abundant in the Cambrian and Ordovician sections of the Tunguska Basin, while coals are widespread in the Permo-Carboniferous Tunguska Series sediments (Figure 15). The juxtaposition of a vast volcanic province with its dykes, sills and diatremes interacting with extensive intracratonic saline Cambrian beds containing evaporites sealing substantial oil accumulations and interacting with coal-bearing deposits, likely produced massive quantities of halocarbons along with methane and CO2. Notably, contact metamorphism with hydrothermal systems rich in chlorine, created during pressure dissolution and dehydration of the surrounding evaporites, potentially synthesized large amounts of the organohalogens methyl chloride (CH3Cl) and methyl bromide (CH3Br) (Beerling et al., 2007; Visscher et al., 2004; Svensen et al., 2018).

In terms of rapid transfer of volatiles to the atmosphere, the phreatomagmatic-sediment pipes (diatremes) generated tall, explosive volatile-rich eruption columns, which at times reached the stratosphere (Svensen et al., 2009). Such features simultaneously promote removal of highly soluble volcanic gases, such as HCl and SO2, and potentially deliver large volumes of sulphur, halocarbons water, methane and CO2 to the upper atmosphere (Black et al., 2015).

Timing of trap emplacement

Siberian Traps magmatic activity at the end-Permain is segmented into three distinct emplacement stages (Figure 16; Burgess et al., 2017). Stage 1, beginning just before 252.24±0.1 Ma, was characterised by initial pyroclastic eruptions followed by lava effusion. During this stage, an estimated two-thirds of the total volume of Siberian Traps lavas were emplaced (>1×106 km3). Stage 2 began at 251.907±0.067 Ma, and was characterised by cessation of extrusion and the onset of widespread sill-complex formation. These sills are exposed over a >1.5 × 106 km2 area and form arguably the most aerially extensive continental sill complex on Earth. Intrusive magmatism continued throughout stage 2 with no apparent hiatus. Stage 2 ended at 251.483±0.088 Ma, when extrusion of lavas resumed after an ~420 ka hiatus, marking the beginning of stage 3. Both extrusive and intrusive magmatism continued during stage 3, which lasted until at least 251.354 ± 0.088 Ma, an age defined by the youngest sill dated in the province. A maximum date for the end of stage 3 is estimated at 250.2 ± 0.3 Ma.

Integration of LIP stages with the record of mass extinction and carbon cycle at the Permian-Triassic Global Stratotype Section and Point (GSSP) shows three important relationships (Burgess et al., 2017). (1) Extrusive eruption during stage 1 of Siberian LIP magmatism occurs over the ~300 kyr before the onset of mass extinction at 251.941 ± 0.037 Ma. During this interval, the biosphere and the carbon cycle show little evidence of instability. (2) The onset of stage 2, marked by the oldest Siberian Traps sill, and cessation of lava extrusion, coincides with the beginning of mass extinction and the abrupt (2–18 kyr) negative δ13CPDB excursion immediately preceding the extinction event (Figure 16a). The remainder of LIP stage 2, which is characterised by continued sill emplacement, coincides with broadly declining δ13CPDB values following the mass extinction. (3) Stage 3 in the LIP begins at the inflexion point in δ13CPDB composition, after which the carbon reservoir trends positive, toward pre-extinction values.

Explosive volcanism in the Siberian Traps can be classified in three distinct groups: 1) deep-rooted sediment–magma interactions and pipe eruption where feeder sills are emplaced in evaporites (Cambrian and Devonian country rock), 2) shallower magma-water interactions in areas with abundant groundwater or hydrated salts, and 3) lava flows and lava fountaining during the main stage of effusive volcanism (Jerram et al., 2016a,b). Each stage has a differing set of expressions in terms of the interacting evaporites and the landscape expression of these interactions.

Outcomes of the end-Permian igneous evaporite interplay

A unusual aspect of the Siberian trap eruption compared to many but not all LIPs is the saline and kerogen-rich nature of regional geology in the Siberian platform that interacted with the LIP magmas. The main lithologies of the region are large volumes of Devonian anhydrites in the north, Cambrian halite and hydrated-potash salts in the south, hydrocarbon source rocks and evaporite-sealed hydrocarbons, and coals in the Permo-Carboniferous portions of the stratigraphy sitting directly below the basaltic otflows. Notably, contact metamorphism and the development of hydrothermal systems rich in chlorine (produced from the pressure dissolution and volatilisation of the surrounding evaporites, kerogens, coals and hydrocarbons with evaporite seals) potentially synthesized large amounts of the organohalogens methyl chloride (CH3Cl) and methyl bromide (CH3Br) along with vast volumes of sulphurous gases, CH4 and CO2 (Figure 17).

End-Triassic extinction event - Saline interactions with CAMP magmas

The Central Atlantic Magmatic Province (CAMP) was emplaced at the end of the Triassic (≈201 Ma) in a region created by the tectonic unzipping (rifting-breakup) of the Pangean supercontinent (Figure 18; Marzoli et al., 2018). CAMP extends across the former Pangaea from modern central Brazil northeastward some 5000 km across western Africa, Iberia, and northwestern France, and from Africa westward for 2500 km through eastern and southern North America and as far west as Texas and the Gulf of Mexico (Figure 18 - dashed red line). The Province is composed of basic igneous rocks emplaced in a combination of shallow intrusions and erupted large lava flow fields extending over a land surface area in excess of 10 million km2. During its emplacement, sill intrusions into evaporites are particularly widespread in the vast Amazonas and Solimões intracratonic basins (≈1 ×106km2), representing up to 70% of the total CAMP sill volume (Svensen et al., 2018).

Sedimentary rocks intruded by sills in the Amazonas and Solimões basins include a lower (Ordovician–Mississippian) and upper (Pennsylvanian–Permian) Paleozoic series (Milani and Zalán, 1999). The lower Paleozoic series consists of sandstones and shales, some of which are particularly organic-rich (total organic content up to 8wt.%; Milani and Zalán, 1999; Gonzaga et al., 2000). The upper Paleozoic series is dominated by evaporite and carbonate deposits of varying abundances, interlayered with clastics. Sills are widespread within the upper Paleozoic evaporitic sequence, extending almost continuously from the western margin of the Solimões Basin to the eastern margin of the Amazonas Basin (Fig.19c). Sills within the lower Paleozoic unit are restricted to the eastern part of the Amazonas basin. As illustrated in Fig.19c, high-Ti sills are found only in the lower Paleozoic series. Let's look now at the saline geology of the region and then at the effect its assimilation had on sill geochemistry.

Saline geology

A significant, as yet poorly delineated, set of variable hydrated potash salts and sylvinites occur in bedded halite in the Amazon Basin, Brazil (Figures 19a, 20; Szatmari et al. 1979). The Amazon Basin is about 2,100 km long and 300 km wide, it is an intracratonic sag basin atop an aulacogen between the Guyana and Guaporé cratons (Figure 19b). The basin fill contains a number of stacked mega-sequence cycles (as defined by wireline interpretation) ranging in age from Lower Ordovician (Autaz Mirim Member of Trombetas Formation) to Lower Permian (Figure 19b; Andirá Formation; Gonzaga et al., 2000). The basin has a widespread Upper Cretaceous cover (Alter do Chão Formation) and was affected by widespread tholeiitic magmatic activity at the end-Triassic (e.g.Penatecaua dolerites of the CAMP), making seismic-based hydrocarbon exploration difficult, especially as much of the basin still lies beneath thick tropical jungle. Since the recognition of a widespread igneous overprint of the Palaeozoic sedimentary succession in the 1970s, hydrocarbon exploration efforts have been subdued (Thomaz-Filho et al., 2008). However, in the past few years, SRTM studies are proving useful, in front of seismic surveys and drilling, in the general identification of geological features in the Amazon Basin (Ibanez et al., 2016)

Th Amazonas-Solimoes intracratonic sag basin is developed on the same scale as the Alberta basin of Canada and entrains the Carboniferous (Pennsylvanian, ≈305 Ma) saline Nova Olinda Formation. It is made up of a large laterally extensive set of cyclic evaporite beds, dominated by interbedded combinations of anhydrite, shale and halite (Figures 20c, 21). These evaporites occur within the Carboniferous-Permian megasequence, known as the Tapajós Group, which can be up to 1600m thick (Milani and Zalan, 1999). The lowest part of the megasequence is a blanket of eolian sandstones (Monte Alegre Formation), which is covered by marine-influenced carbonates and evaporites (Itaituba and Nova Olinda Formations, respectively), along with subordinate sandstones and shales (Figure 19c). The Tapajós megacycle is closed by a suite of Permian continental redbeds (Andirá Formation) of Permian age. Subsequent east-west regional extension facilitated a pervasive intrusion of magmatic bodies during the end-Triassic to Early Jurassic (Penatecaua dolerites and equivalents).

Individual halite beds in the Nova Olinda evaporite cycles are 20-80 m thick, while the Nova Olinda Fm. has an average thickness of 900m. Because of the high levels of entrained anhydrite beds in the Nova Olinda Fm., evaporite layers are not halokinetic, but are subject to collapse and flow about the basin margin, especially in areas of intense meteoric dissolution (Figure 20).

Early Petrobras drilling programs conducted in the Amazon Basin from 1953 to 1963, defined the presence of halite but did not appreciate that persistent sylvinite/carnallite beds cap a number of the beds of NaCl in The Nova Olinda Formation. During the late 1960s and 1970s, higher-resolution gamma-ray logging tools were used, along with better mud technology and associated narrower calliper measures. This work identified a number of (0.5 - 2m thick) layers of sylvinite, within the halites (Szatmari et al. 1979). For example, the fifth and seventh depositional cycles define isolated salt sub-basins that accumulated significant potash salts in Fazendinha and Arari regions (Figure 20). KCl contents of these beds are between 28-33% in beds some 2.47-2.65 m thick (Garrett, 1995). The average ore depth at Fazendinha, the larger of the known potash areas, is 1,050m (Figure 20). Much of the halite and potash distribution is controlled by the underlying rift-basin architecture (Figure 19b). Potential potash reserves poorly defined, but are interpreted to be large (Szatmari et al., 1979; Garrett, 1995).

Based on its texture, structure and chemistry, the potash intersection in the Amazon Basin is divided into three distinct zones, called informally, lower (milky or white sylvinite), middle (sulphates) and upper zones (red sylvinite) (Figure 20). The lower zone (milky-white sylvinite zone) contains sylvinite, with halite and subordinate intercalated kieserite and anhydrite beds. The lower potash zone is persistent within the basin and so covers an extensive area, whereas the upper potash zone is patchier. The greater extent of the lower potash zone is perhaps because it is the best isolated from any dissolution driven by circulation of undersaturated pore fluids through the overburden.

The middle zone is composed of a combination of sulphate and chloride salts and is informally termed the sulphate zone. It hosts a variety of K, Mg and sulphate minerals that include a number of hydrated salts. Typical mineral assemblages encompass sylvinite, sylvite, and langbeinite (K2SO4.2MgSO4) as well as the hydrated salts; polyhalite (K2SO4.2MgSO4.2CaSO4.2H2O), kainite (MgSO4.KCl.3H2O) and kieserite (MgSO4.H2O). The sulphate distribution in this unit changes from anhydrite and polyhalite in the west (Fazendinha) to langbeinite and kainite in the east (Faro area). Towards the basin centre, chloride beds replace marginal sulphate beds in the sulphate unit. A gradual increase in potash concentration from west to east is interpreted by Sad et al., 1982, as indicating the inflow direction was from the basin's western boundary.

The upper potash zone consists of coarsely-crystalline red sylvinite, with thin halite and anhydrite laminations. This level includes the best K2O grades drilled so far, averaging 23% K2O (between 33% to 16%). Red sylvinite is interpreted as a second generation product formed diagenetically by incongruent leaching of primary carnallite, but, as yet no carnallite (KCl.MgCl2.6H2O) has been identified in the upper unit.

The potash zone is overlain by impermeable coarsely-crystalline halite, with minor shale intercalations in a zone up to 25 m thick, in turn, overlain by impermeable shale beds some 20 m thick. It is underlain by an impervious, at times sparry, halite interval some 70m thick (Figure 20). At the time it was described (1970s-mid 1980s) little was known of the significance of halite crystal textures in terms of their primary versus diagenetic signatures. Such a study of the nature of the halites enclosing the potash zone in the Amazon basin would aid in the definition of an ore genesis model. We do know that a single potash zone does not extend across the basin. This is seen in a compilation of existing Petrobras wells in the Amazon Basin, which intersect the Nova Olinda Fm. Instead, potash salts accumulated in a series of sumps atop a persistent thick halite unit (Figure 20).

Elevated sulphate content in the potash zone of the Amazon Basin reflects the MgSO4-enriched nature of the world ocean during the Carboniferous. Potentially high levels of sulphate in proximity to adjacent sylvinite ore targets will complicate the processing of potential ore (see Warren 2016, Chapter 11). But in terms of supplying high levels of volatiles during sill intrusions, it is highly likely the various hydrated sulphate salts in the potash zone focused sill emplacement and contributed to elevated levels of halocarbons and sulphurous gases escaping into the earth's atmosphere at the end Triassic. As yet, no phreato-magmatic pipes have been documented in the Nova Olinda, but as the sourcing evaporite unit lie a kilometer beneath the surface and the dense tropical Amazon Jungle, this is not surprising. Increasing future use of STRM data may help solve this (Ibanez et alo., 2016)


Saline sediment-sill interaction

Sills from the Amazonas Basin have previously been described as low-Ti tholeiitic basalts and andesitic basalts De Min et al., 2003), and sills from both basins are generally characterised by a mineral assemblage of clinopyroxene, plagioclase, Fe–Ti oxides, rare olivine and orthopyroxene and accessory quartz-feldspar intergrowths. Recent studies report the presence of high-Ti sills in the eastern part of the Amazonas Basin (Figures 18, 21; Davies et al., 2017; Heimdal et al., 2018, 2019; Marzoli et al., 2018), but no high-Ti occurrences have been observed in the Solimões Basin. 

High-precision U–Pb dates from four dolerites from the Amazonas and Solimões basins overlap in age, with U–Pb ages for low-Ti dolerites of 201.525 ±0.065 (Amazonas Basin) and 201.470 ±0.089 (Solimões Basin), and for high-Ti dolerites in the Amazonas Basin of 201.477 ±0.062 and 201.364 ±0.023 Ma (Figure 18; Davies et al., 2017; Heimdal et al., 2018). This suggests that low-and high-Ti CAMP magmatism were active simultaneously, although low-Ti magmatism likely started earlier.

Detailed studies of CAMP sill geochemistry showing likely assimilation of chloride salts from the Nova Olinda evaporites are published in Heimdal et al., 2019, and summarised in this section. They show the bulk of e dolerites as sampled in the wells, illustrated in Figure 22, are characterised by phenocrysts of clinopyroxene and plagioclase in subophitic to intergranular textures, Fe–Ti oxides, and rare olivine and orthopyroxene. A different mineralogical assemblage (microphenocrysts of alkali-feldspar, quartz, biotite and apatite) is found in small independent domains, localised within the framework of coarser plagioclase and clinopyroxene laths. These fine-grained evolved domains crystallised in late-stage, evolved melt pockets in the interstitial spaces between earlier crystallised coarser grained crystals.

The majority of the studied dolerites are generally evolved tholeiitic basalts and basaltic andesites with low TiO2 concentrations (<2.0 wt.%). Four samples have high TiO2 concentrations (>2.0 wt.%), and are found in the eastern part of the Amazonas Basin (Figure 20a, c).

Whole-rock major and trace element and Sr-Nd isotope geochemistry of both low- and high-Ti sills is similar to that of previously published CAMP rocks from the two magma types. Low-Ti sills show enriched isotopic signatures (143Nd/144Nd201Ma from 0.51215 to 0.51244; 87Sr/86Sr201Ma from 0.70568 to 0.70756), coupled with crustal-like characteristics in the incompatible element patterns (e.g. depletion in Nb and Ta). Unaltered high-Ti samples show more depleted isotopic signatures (143Nd/144Nd201Ma from 0.51260 to 0.51262; 87Sr/86Sr201Maf from 0.70363 to 0.70398).

Low-Ti dolerites from both the Amazonas and Solimões basins contain biotite with extremely high Cl concentrations (up to 4.7 wt.%). They show that there is a strong correlation between host-rock lithology and Cl concentrations in biotite from the dolerites, and interpret this to reflect large-scale crustal contamination of the low-Ti magmas by halite-rich evaporites (Figure 21). The findings of Heimdal et al. (2019) support the hypothesis that sill-evaporite interactions increased volumes of volatile released during the emplacement of CAMP, and underlines the case for the active involvement of this LIP in the end-Triassic extinction event.

End-Cretaceous extinction event - Saline interactions driven by a bolide impact)

About 66 million years ago, at the end of the Cretaceous, one or possibly multiple large asteroids collided with the Earth. Paul Renne dated this impact at 66.043±0.011 million years ago on the Yucatan Peninsula, 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. The bolide produced a crater some 150x180 km in diameter named the Chicxulub impact structure (Figure 23). Worldwide, a record of this event is evidenced by an iridium-enriched interval, in what is now called the Cretaceous-Tertiary Boundary Clay (KTBC) (Alvarez et al., 1980).

Other authors favouring additional bolide impacts at the end of the Cretaceous, such as Lerbekmo (2014) and Chaterjee (1997), have argued that some 40,000 years later, a much larger meteorite struck the shelf of the India-Seychelles continent, which was drifting northward in the southern Indian Ocean, producing a crater, some 450x600 km across, named the Shiva impact (Lerbekmo, 2014; Chaterjee, 1997). If a bolide-related feature, the Shiva crater was split by subsequent plate tectonism and today is not widely recognised by the scientific community as a K-T impact site.

As for any sound scientific hypothesis, there are ongoing arguments for the Chicxulub site being the "smoking gun" for the end-Cretaceous extinction event, many of these arguments and the supporting literature is discussed in (Kring, 2007). I shall focus on the saline geology of the Yucatan impact site, but recognise the arguments of some authors that the Shiva site is closely linked in time with the extrusion of the Deccan Traps. More importantly, voluminous Deccan Traps eruptions and intrusions had likely already degraded the end-Cretaceous atmosphere. A large bolide crashing into an anhydrite saltern in the palaeo-Gulf of Mexico was perhaps the coup de grâce for many already -stressed late Mesozoic communities (Wang et al., 2018)

Saline Geology of the Yucatan site

As it is covered by a Tertiary-age sediment carapace, there are no current evaporite outcrops on the Yucatan Peninsula. However, the region is underlain by thick Cretaceous anhydrite beds and has a nearby giant oil field, Cantarell, reservoired in a carbonate breccia trap possibly related to the impact (Grajales-Nishimura et al., 2000). Ongoing petroleum exploration means a number of exploration wells sample the Cretaceous geology of the Yucatan Peninsula (inset in Figure 24). Regionally, Cretaceous (Albian) saltern anhydrite beds extend from Guatemala, across the Yucatan Peninsula and north possibly to Veracruz. Depositionally similar, back-reef saltern beds typify the early Cretaceous (Albian) Ferry Lake Anhydrite, which extends across the onshore northern, and offshore eastern, Gulf of Mexico (Pittman, 1985; Petty, 1995; Loucks and Longman, 1982).

Pemex wells drilled on the Yucatan Peninsula, penetrate some 1300 –3500 m of bedded Tertiary, Cretaceous, and Jurassic strata (Figure 24; Ward et al., 1995). Palaeozoic metamorphic rocks are intersected at 2418 m in well Y4 and at 3202 m in well Y1. ‘‘Volcanic rock/andesite,’’ now broadly interpreted as an ‘‘impact-melt rock’’ or suevite is intersected in the lower parts of wells Y6 and C1. Based on the well geology there are seven major biostratigraphic-lithostratigraphic units in the Mesozoic section overlying basement rocks in the vicinity of the Chixulub impact site (Units A-F; Ward et al., 1995 and references therein). The regional depositional setting is typical of a Cretaceous carbonate platform, which at times became sufficiently isolated to deposit stacked anhydrite saltern beds in a rudistid back-reef setting (Warren, 2016; Chapter 5).

Unit A consists of red and grey sandstone, shale, and silty dolomite near the base of wells Y1, Y2, and Y4. This unit is Jurassic to Early Cretaceous in age (López Ramos, 1975).

Unit B is predominantly dolomite in its lower part, becomes rich in intercalated anhydrite and dolomite upward. Rock salt was cored in this unit in T1 at 2378–2381 m. Nummoloculina sp. was identified in Y2, suggesting an Albian age.

Unit C is predominantly shallow-water limestone in the lower part, becoming more dolomitic upward. At the base of unit C in wells Y1 and Y2 is a horizon with the large benthic orbitulinid foraminifer Dicyclina schlumbergeri? (Figure 23\4). Nummoloculina (N. heimi?) also occurs in the lower part of this unit in cores Y1 and Y4. Nearer the platform margin (Y4), the upper part of this unit contains a rudist limestone, but in other wells the rocks reflect more restricted depositional environments across the platform interior. Shallow-subtidal to intertidal dolomite makes up most of this section in Y5A, where anhydrite is interlayered with dolomite in the upper parts of the unit in Y1, Y2, and T1. The fossil assemblage indicates an Albian-Cenomanian age for unit C.

Unit D is predominantly somewhat deeper-subtidal limestone and marl, with horizons containing abundant tiny, mainly trochospiral planktic foraminifers as seen in samples from Y1, Y2-Y4, and Y5A.

Unit E consists of shallow-platform limestones with intervals containing abundant small planktic foraminifers. The unit contains rudist-bearing limestones considerd by López Ramos (1975) as Turonian, and a similar age is indicated by the presence of Marginotruncana pseudolinneiana and Dicarinella imbricata in samples from Y1, Y2, Y4, and Y5A.

Unit F consists of dolomitized shallow-platform limestone with benthic foraminifers. Abundant textularid and miliolid foraminifers are at the top of unit F (Fig. 2). The presence of Marginotruncana schneegansi and Globotruncana fornicata in well Y5A suggests a Santonian age for that part of this unit.

Unit G is a thick interval of breccia with abundant sand- to gravel-sized angular to subrounded fragments of dolostone, anhydrite, and minor limestones suspended in a dolomicrite matrix. The poorly-sorted fabric is similar to that of debris-flow deposits. López Ramos (1973) reported marl and limestone intercalations within the thick breccia from 1090 to 1270 m in well C1 (Figs. 1 and 2). In addition, Y4 and Y4 contain dolomite that may separate an upper breccia with rare or no planktic foraminifers from a lower breccia with abundant planktic foraminifers. Core in Y2 is composed of finely crystalline anhydrite, possibly also representing a less disturbed sedimentary layer or anhydrite block within the breccia interval.

Clasts of carbonate rocks in these breccias are fragments of many different kinds of dolostone and limestone, with different diagenetic histories. Anhydrite fragments typically make up 15%–20% of the breccia; much of the anhydrite is composed of tiny angular cleavage splinters. Some breccia layers contain grey-green fragments of altered volcanic ‘‘glass’’ and spherules. Other minor but significant constituents of the breccia are fragments of melt rock and basement as seen in Y6 (1295.5–1299 m), Y6 (1377–1379.5 m), and C1 (1393–1394 m). In addition, Hildebrand et al. (1991) found shocked quartz from Y6 (1208 –1211 m), and Sharpton et al. (1994) reported shock-deformed quartz and feldspar grains and melt inclusions in the dolomite-anhydrite breccia.

Planktic and benthic foraminifers are present in the breccia matrix and include Abathomphalus mayaroensis, Globotrun-canita conica, Rosita patelliformis, Pseudoguembelina palpebra, Racemiguembelina fructicosa, and Hedbergella monmouthensis, which indicate a late Maastrichtian (end-Cretaceous) age for formation of the breccia (Ward et al., 1995).

Climatic outcomes of the Yucatan impact

Widespread Jurassic anhydrites, hydrocarbon reservoirs and source rocks surround the Yucatan impact region; their vaporisation on bolide impact and rapid entry into the upper atmosphere added a good deal to the ensuing climatic mayhem (Figure 25) As we discussed for LIP emplacement, anhydrite decomposes at high temperatures, to form SO2 gas, CaO, and oxygen. Thermodynamic calculation and extrapolation using the free energy of formation of anhydrite and its reaction products as a function of temperature up to 1120°C (Robie et al., 1979), give an equilibrium pressure of 1 bar SO2 over the reaction:

2CaSO4 = 2CaO + 2S02 + O2

at a temperature around 1500°C (Brett, 1992; Yang and Ahrens, 1998). Experimental studies by Rowe et al. (1967) indicate that anhydrite decomposes in an open crucible above 1200°C. Temperatures higher than 1500°C are well in the range of temperatures of material subjected to strong shock in large bolide impacts, and at higher temperatures the equilibrium pressure would be considerably higher. Because the system is open, SO2 and oxygen would escape to the atmosphere as they did in the laboratory crucible of Rowe et al. (1967) and would continue to do so as long as post-impact temperatures were elevated.

Published discussions of the impact site geology all consider anhydrite as the evaporite mineralogy, with minor volumes of halite (well T1 in Figure 24). This lower salinity end of the evaporite series is typical of mega-sulphate settings, worldwide (Warren, 2016; Chapter 5) In addition, there is no evidence for hydrated potash salts in the region and this too is typical of starn salterns in a meg-sulphate basin. There is, however, the additional possibility that not all of the saltern gypsum had converted to anhydrite at the time of the impact. If so, this would have further detabilised and volatised the various lithologies at the site of the impact.

Intercalated carbonates, kerogens and other organic sediments at the collision site contributed additional CO2, CH4, H2O, and halocarbons to the atmosphere, as well as vast quantities of heat and particulates. The following discussion of the various contributors to climatic changes, driven by the Chicxulub impact, is taken mostly from Kring, 2007 (and contained references).

Acid rain; Because the Chicxulub impact occurred in a region with anhydrite, sulphurous vapour was injected into the stratosphere, producing sulphate aerosols and eventually sulphuric acid rain. Estimates of the amount of S liberated vary, consensus ranges from 7.5 × 1016 to 6.0 × 1017 g S, which would have produced 7.7 × 1014 to 6.1 × 1015 mol of sulphuric acid rain. In addition, the earth’s atmosphere was shock-heated by the impact event, producing nitric acid rain as well. Independent of the geology of the impact siter, the earth's atmosphere is heated when pierced by a bolide as the vapour-rich plume expands out from an impact site, and ejected debris rains through the atmosphere. In a Chicxulub-sized impact event, the ejecta debris is, estimated to produce ≈1×1014 mol of NOx in the atmosphere and, thus, ≈1×1015 moles of nitric acid rain. Impact-generated wildfires may have produced an additional ≈3×1015 mol of nitric acid. Sulphuric and nitric acid rain fell over a few months to a few years (Figure 25a).

Wildfires; Evidence of impact-generated fires is recovered from K/T boundary sequences worldwide in the form of fusinite pyrolitic polycyclic aromatic hydrocarbons, carbonised plant debris, and charcoal. The distribution of the fires is still poorly understood and may have had a restricted geographic distribution limited to the vicinity of the impact event, produced not by impact ejecta but by the direct radiation of the impact fireball which had a plasma core with temperatures over 10,000 °C. Several additional parameters influence the outcome (e.g., the trajectory of the impacting object, its speed, and mass of the ejecta). The amount of soot recovered from K/T boundary sediments (imply that the fires released ≈104 GT of CO2, ≈102 GT CH4 and 103 GT CO, which is equal to or larger than the amount of CO2 produced from vapourised target sediments. This likely had a severe effect on the global carbon cycle (Figure 25a).

Dust and aerosols in the atmosphere; Calculations suggest that dust and sulphate aerosols from the impact event, and soot from post-impact wildfires, caused surface temperatures to fall by preventing sunlight from reaching the surface where it was needed for photosynthesis. The base of the marine food chain, composed of photosynthetic plankton, collapsed. Slight increases or decreases in average water temperatures cannot extinguish photosynthetic plankton, nor the presence or absence of organisms higher up the food chain. Photosynthesisers are primarily affected by the availability of their energy source, light. Consequently, the loss of photosynthetic plankton following the Chicxulub impact event is evidence that sunlight was significantly blocked, whether it was by dust, soot, aerosols, or some other agent.

The timescale for particles settling through the atmosphere range from a few hours to approximately a year (Figure 25a, b). The time needed for the bulk of the dust to settle out of the atmosphere is ambiguous, however, because the size distribution of the dust is unclear. Some sites seem to be dominated by spherules ≈250 μm in diameter, which would have settled out of the atmosphere within hours to days. However, if there is a substantial amount of submicron material, then it may remain suspended in the atmosphere for many months. Soot, if it were able to rise into the stratosphere, would have taken similarly long times to settle. Soot that only rose into the troposphere, however, would have been flushed out of the atmosphere promptly by rain.

The dust, aerosols, and soot caused surface cooling after the brief period of atmospheric heating that immediately followed the impact. The magnitude of that cooling is unclear, however, because the opacity generated by the three components is uncertain and their lifetime in the atmosphere is also uncertain. Nonetheless, significant decreases in temperature of several degrees to a few tens of degrees have been proposed for at least short periods. Short-term cooling likely had a severe effect on the global carbon cycle, in what is popularly termed a “nuclear winter’ scenario (Figure 25).

Ozone destruction; Ozone-destroying Cl and Br is produced from the vaporised projectile, vaporised target lithologies, and biomass burning. Over five orders of magnitude more Cl than is needed to destroy today's ozone layer was injected into the stratosphere, compounded by the addition of Br and other reactants. The affect on the ozone layer may have lasted for several years, although it is uncertain how much of an effect it had on surface conditions. Initially, dust, soot, and NO2 may have absorbed ultraviolet radiation, and sulphate aerosols may have scattered the radiation. The settling time of dust was probably rapid relative to the time span of ozone loss, but it may have taken a few years for the aerosols to precipitate.

Greenhouse gases; Water and CO2 were produced from Chicxulub's target lithologies and the projectile, which could have potentially caused greenhouse warming after the dust, aerosols, and soot settled to the ground. Significant CO2, CH, and H2O were added to the atmosphere. Some of these components came directly from target materials. These include carbonates, which liberate CO2 when vaporised, and also includes hydrocarbons, the remainder of which has subsequently migrated into cataclastic dykes beneath the crater and impact breccias deposited along the Campeche Bank (e.g. Cantarell field). Water was liberated from the saturated sedimentary sequence and the overlying ocean (the lesser of the two sources).

The residence times of gases like CO2 are greater than those of dust and sulphate aerosols, so greenhouse warming may have occurred after a period of cooling. Estimates of the magnitude of the heating vary considerably, from an increase of global mean average temperature of 1 to 1.5 °C (based on estimates of CO2 added to the atmosphere by the impact) to ≈7.5 °C (based on measures of fossil leaf stomata).

Local and regional effects; The local and regional effects of the impact were enormous. Tsunamis radiated across the Gulf of Mexico, crashing onto nearby coastlines, and also radiated farther across the proto-Caribbean and Atlantic basins. Tsunamis were 100 to 300 m high when they crashed onto the gulf coast and ripped up sea floor sediments down to water depths of 500 m. The Gulf of Mexico region was also affected by the high-energy deposition of impact ejecta, density currents, and seismically-induced slumping of coastal sediments following magnitude 10 earthquakes. Tsunamis may have penetrated more than 300 km inland. The local landscape (both continental and marine) was buried beneath a layer of impact ejecta that was several hundred meters thick near the impact site and decreased with radial distance. Peak thicknesses along the crater rim may have been 600 to 800 m. Along the Campeche bank, 350 to 600 km from Chicxulub, impact deposits of ≈50 to ≈300 m are logged in the Cantarell boreholes.

Impact events also produce shock waves and air blasts that radiate across the landscape. Wind speeds over 1000 km/h are possible near the impact site, although they decrease with distance from the impact site. The pressure pulse and winds can scour soils and shred vegetation and any animals living in nearby ecosystems. Estimated radii of the area damaged by an air blast range from ≈900 to ≈1800 km.

Significant heat would have been another critical regional effect. Core temperatures in the plume rising from the crater were over 10,000° C, possibly high enough to generate fires out to distances of 1500 to 4000 km. The intense thermal pulse would have been relatively short-lived (5 to 10 min). Additional heating and spontaneous wildfires were ignited when impact ejecta fell through the atmosphere (3 to 4 days; Figure 25a).

The end-Cretaceous bolide impact had both short and long term effects on the Earth's climate and its atmospheric temperatures (Figure 25b). Over hours to days following impact, there was severe atmospheric heating as ejecta rained down through the atmosphere. This was following by a period of weeks to years of cooler temperatures as the atmosphere was polluted by SO2, NOx and soot from the impact preventing sunlight reaching the surface (nuclear winter scenario). Then, across time frame of decades to millennia, after the atmosphere cleared, increased CO2 levels drove a period of global warming. The legacy of the impact and the biotal recovery over the next few hundred thousand years is documented in a recent paper by Lowery et al., 2018. They showed that life reappeared in the basin just years after the impact and a high-productivity ecosystem was established within 30 kyr.

Extinction events intensified by heating evaporites

Evaporite salts are more chemically reactive at earth surface conditions than other sediments. Subsurface evaporites are prone to dissolution, alteration and reprecipition from the time they first precipitated and throughout their subsurface journeys in the diagenetic and metamorphic realms (Warren 2016). The same is true, but perhaps more so, if bedded salts are exposed to a heat source outside the normal geothermal gradient experienced in burial. Additional heat can come for the emplacement of igneous sills, magma bodies or the hot hydrothermal circulation it drives. Or it can come from near instantaneous heating to thousands of degrees associated with a bolide impact. Volatile products that result from this heating, as they enter the earth's atmosphere, can be inimical to life and include vast volumes of halocarbons, SO2, methane and CO2. Methane and CO2 come from kerogens and hydrocarbons stored in intercalated mudstones and limestones while volatilisation of carbonates can supply CO2.

The reactivity of evaporites and the vast volumes of volatiles released explains the intimate association of saline giants, heating and the three most devastating of the five major Phanerozoic extinction events.

Interestingly, two other events on the list of the "big five;" the Emeishan and late Devonian events (Figure 1) also have possible associations with heated evaporites. The Emeishan LIP intersects the edge of the anhydrite-rich Sichuan basin, while the 120km-diam., Late Devonian, Woodleigh bolide impacted the intracratonic Silurian Yaringa Fm. salts (including potash beds) on the coast of West Australia (SaltWork GIS database version 1.8 overlays, Chen et al., 2018; Glikson et al, 2005). But, before definitive conclusions can be made, more work is required to better tie down impact age, actual geographic extent of LIP emplacement, extent of evaporite breccias and evaporite volumes.


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

John Warren - Saturday, March 16, 2019



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.


    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.


    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


    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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