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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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