Evaporites

I define an evaporite as a salt rock that was originally precipitated from a saturated surface or nearsurface brine in hydrologies driven by solar evaporation (Fig. 1.1a). This simple definition encompasses a wide range of chemically precipitated salts and includes alkali earth carbonates (Table 1.1). Some workers restrict the term evaporite to salts formed at the earth’s surface via solar evaporation of hypersaline waters. To underscore the highly reactive nature of evaporites in the sedimentary realm I think of such evaporites as primary, that is, precipitated from a standing body of surface brine and retaining crystallographic evidence of the depositional/hydrological process set where they formed (e.g. bottom-nucleated or current-derived textures). Outside of a few Neogene examples, there are few ancient evaporite beds with textures that are wholly and completely “primary.” Most in the subsurface exhibit “secondary” (burial-related) textures, while remnants that have be uplifted back to the surface show “tertiary” (exhumation-related) overprints (Figs. 1.2 and 1.9).

In Chap.​ 1, an evaporite is defined as a salt rock originally precipitated from a saturated surface or nearsurface brine by hydrologies driven by solar evaporation. There is no assumption as to the origin of the parent brine; it may be marine (thalassic), nonmarine (athalassic) or a hybrid. By implication, there is a need for aridity and for water loss to exceed inflow. This means deposition and diagenesis in evaporites is more climate dependent than in either siliciclastic or carbonate sediments; reaction rates and reversibility are an order of magnitude faster. Suites of precipitated salts follow the geochemical make-up of the parent brine, while primary textures indicate hydrological stability and energy levels of the time of precipitation.

This chapter looks at Quaternary depositional styles depositing saline sediments in ephemeral saline waters, focusing on modern evaporitic mudflats (sabkhas) and saline pans. These two settings encompass that part of the hydrological spectrum dominated by capillary and ephemeral at-surface brine hydrologies. Climatically, these settings tend to occur in arid deserts (Köppen zone: BW) in both coastal and continental interior arid settings where saline groundwaters are found with a metre of the landsurface. A saline watertable may sometimes outcrop for a short period and precipitate a layer of pan evaporites with primary textures before drying up once more. With drying the brine surface sinks into the sediment to become a watertable and the evaporite system switches to capillary evaporation with secondary (very early diagenetic) sabkha salts forming in the sediment host. The other modern hydrological setting where salt beds tend to form is that typified by perennial subaqueous brine hydrologies (primary textures) in coastal and continental settings and this will be the focus of the next chapter.

Ancient shoalwater platform (saltern) and deepwater (slope and basin) evaporites are dominantly subaqueous precipitates, and, as for the mudflats and pan settings discussed in the previous chapter, when we seek equivalence in the Quaternary record we find our marine-fed choices are limited both in number and scale (Fig. 4.1; Table 4.1). If we consider all nonmarine and marine saline perennial water masses in modern systems as suitable for discussion in this chapter then, as well examples of larger saline perennial lakes, such as Great Salt Lake and Lake Urmia, we must include the feeder-edge brine pools to some larger saline pans and lakes, such as pools or moat facies to lakes Magadi and Natron, and some centripetal depressions to the Turkish lakes such as in Acigolu. In Chap.​ 12 subaqueous cryogenic continental examples discussed include Karabogazgol and various mirabilite lakes on the Canadian plains. I also arbitrarily split out of this chapter some examples of the larger continental saline pans with feeder pools. This latter group, including most of the South American salars, were discussed in the preceding chapter. Following a similar reasoning, if the bulk of the Holocene column in a lake is dominated by subaqueous textures, then I include it in this chapter, even though Holocene aggradation has moved uppermost part of the Holocene column into a sabkha setting, as in many modern gypsum-filled coastal salinas in southern and western Australia.

Is the present the key to the past in evaporite studies? It’s part of a broader question that has plagued geologists working with salts since the seventeenth century, when Werner’s Neptunist postulates explained world geology (including igneous strata) as layered precipitates from cooling oceanic waters. And, like Werner’s Scottish antagonists, we are still discussing the merits of strict uniformitarianism. In this chapter I am not questioning the fundamental principle of using present-day process studies to interpret the past. Nor am I questioning the utility of detailed studies of process analogs and the use of physical constants to reliably interpret the past. Let me put it plainly for the creationist community and proponents of “intelligent design,” all the geological evidence clearly shows the earth is not 8,000, but more than 4 billion years old. Biological evolution is fact, it interacts with and responds to the physical environment and has been in operation on the earth’s surface for at least the past 3.45 billion years. There is no need to invoke the supernatural and sky faeries to explain biological evolution and other earth processes, and there is no geological evidence for a worldwide biblical flood of epic proportions drowning mankind and covering all the land.

Many of the world’s larger oil and gas fields occur in halokinetically-influenced structures across many of the world’s salt basins (e.g. Campos Basin, Gulf of Mexico, North Sea, Lower Congo Basin, Santos Basin and Zagros). An understanding of the physics of salt and how salt flow influences tectonics and sedimentation is therefore critical to effective and efficient petroleum exploration (Chap.​ 10). And, I would argue, is also critical to a significant portion of the world’s base and precious metal occurrences (see Chaps. 15 and 16). Halokinetic salt is also a resource in salt, potash, gypsum and nitrate extraction (Chaps. 11 and 12). Salt dome salt has the potential to be employed as a repository for radioactive and other wastes, and can act as a highly efficient seal to sequestered CO₂ (Chap.​ 13).

Given enough time all subsurface evaporites eventually dissolve. Even in the subsurface sedimentary realm there are probably more intervals of dissolution residues than there are beds of salts. The rate of evaporite dissolution changes with temperature and rate and volume of crossflowing undersaturated pore waters (Fig. 7.1) Solubility of the chloride salts consistently increases with increasing temperature (prograde solubility), with the sulphates and the sodium carbonates it initially increases, but can then decrease at higher temperatures (retrograde solubility) Dissolution typically begins in the shallow subsurface as the edges of salt beds that are flushed by meteoric or marine waters and continues deeper in the subsurface, wherever and whenever bed edges are flushed by undersaturated basinal brines. Partial dissolution, whereby crystals of more saline salts (typically halite and bitterns) are flushed, leaves behind residues of the less saline salts (typically gypsum-anhydrite, as in many caprocks).

In Chap.​ 2 we discussed the hydrology of evaporite depositional and early diagenetic (eogenetic) settings, in this chapter we consider diagenetic evolution in subsequent hydrological stages in a sedimentary system containing thick salt, that is, rock-fluid interactions in an evaporite-entraining sequence experiencing ongoing burial, followed by possible uplift (mesogenesis followed by telogenesis. Thick, buried and dissolving evaporites influence subsurface hydrology and formation water chemistries throughout the burial cycle, as they interact with regional and local thermal and pressure regimes. Remobilization of salt and introduction of brine-carried carried components, notably dolomite and anhydrite, can have a significant impacts on reservoir quality, e.g. the Simonette oil field, Swan-Hills Formation (Devonian) Alberta, where late anhydrite and burial dolomite represent up to 50 % of the rock volume (Duggan et al. 2001). Evaporites that enter the metamorphic realm can have an extensive influence consequent rock type and distribution, well after the primary sedimentary and diagenetic salts are long gone (Chap.​ 14).

A hydrocarbon source rock s generally considered to be a finegrained rock that, during its burial and heating, generates and releases enough fluids to form commercial accumulations of oil or gas (Fig. 9.1a). Back in 1981, Kirkland and Evans made the observation that some 50 % of the world’s oil sequestered in carbonate reservoirs may be associated with mesohaline micritic source rocks. Heresy or not, the notion that much of the oil in carbonate reservoirs, sealed by evaporite salts, may have been sourced in earlier less saline, but still related, evaporitic (mesohaline) conditions, is worthy of consideration. The association between mesohaline waters, the accumulation of organic-rich sediments and the evolution of the resulting evaporitic carbonates into source rocks has been noted by many, including: Woolnough (1937), Sloss (1953), Moody (1959), Dembicki et al. (1976), Oehler et al. (1979), Malek-Aslani (1980), Kirkland and Evans (1981), Jones (1984), Hite et al. (1984), Eugster (1985), Sonnenfeld (1985), Ten Haven et al. (1985), Warren (1986), Evans and Kirkland (1988), Busson (1991), Edgell (1991), Beydoun (1993), Benali et al. (1995), Billo (1996), Aizenshtat et al. (1998), Carroll (1998), Schreiber et al. (2001), Love et al. (2007), Schnyder et al. (2009), Warren (2011), Comer (2012).

Even though evaporites constitute less than 2 % of the world’s sedimentary rocks, one-half of the world’s largest oilfields are sealed by evaporites, the other half are sealed by shales (Fig. 10.1; Grunau 1987). Kirkland and Evans (1981) argued that evaporites overlie or seal carbonates containing an estimated 50 % of the world’s known total petroleum reserve. Of the world’s 25 largest gas fields, nine are sealed by evaporites and sixteen by shales and hydrates. Sixteen are capped by Mesozoic seals, 7 by Palaeozoic seals, and only two by Tertiary seals. Fourteen are in the 1,000–2,000 m seal-depth interval, nine in the 2,000–3,000 m interval; and two in the 0–1,000 m interval. As one would expect, more gas fields than oilfields are sealed by Palaeozoic caprocks, and more oilfields than gas fields are sealed by Tertiary caprocks. Surprisingly, the seal depth intervals for the 25 largest oil and gas fields do not differ significantly. However, Grunau argues many “supergiant” gas accumulations below depths of 3,000 m and have either not yet been discovered, or have not yet been put in production (e.g. much of the Khuff-hosted gas in North Dome in the Middle East).

Natural potash evaporites are a typical part of a brine evaporation series, crystallizing at the higher concentration or bittern end, either at the surface (primary salts) or in the shallow subsurface (secondary salts). Today, bedded accumulations of primary potash evaporites are a relatively rare occurrence. Extremely high solubility of most potash salts means they accumulate in highly restricted, some would say highly continental, modern depositional settings (Cendon et al. 2003). Wherever Quaternary potash does occur naturally, as in the playas of the intermontane Qaidam Basin in China and in the Danakil Depression in the Afar Rift of Africa, carnallite, not sylvite, is the dominant potash salt. This has led some to postulate that carnallite is the archetypal primary marine potash phase, while sylvite is a secondary diagenetic mineral formed by incongruent dissolution of carnallite. Others have argued that ancient sylvite was sometimes a primary precipitate, deposited by the cooling of highly saline surface or near surface brines and from seawater with ionic proportions different to those of today (Hardie 1996).

In Chap.​ 11 we focused on potash deposits and concluded that the larger accumulations of potash that dominate the rock record are marine-derived. That is, the larger potash deposits that are conventionally mined across the world accumulated in ancient tectonic (megahalite) basins with no Quaternary counterpart. We shall now discuss various accumulations of other evaporite salts and related products that are exploited as economic resources, namely the borates, Na-carbonates, Na-sulphates, lithium salts and zeolites, along with short considerations of exploited gypsum and halite deposits (Table 12.1). Other than gypsum and halite, they are typically lacustrine precipitates or brine products, formed by the evaporation of waters with nonmarine ionic proportions and in supra-sealevel depositional settings that do have same-scale pre-Quaternary counterparts (Warren 2010). The marine-derived megahalite and megasulphate deposits, which have no same-scale modern counterparts, were the focus of much of the discussion in earlier chapters. Aspects of these halite and gypsum deposits are only mentioned in passing in this chapter, via a discussion of their annual production volumes and uses, but they are the highest ranked deposits in terms of the weight utilized as mineable or extractable natural resources (Table 12.2). Next in terms of extracted volume is soda ash, then the potash salts, then with an order of magnitude less is salt cake and the borate salts.

Salt solution mining is just what it says, the mining of various salts by dissolving them and pumping the resulting brine to the surface where it is concentrated or processed to recover the desired chemical products. Actual dissolution and recovery methodology is predicated on the solubility of the targeted salt, A “rule of thumb” in the solution mining industry is that every 7–8 m³ of freshwater pumped into a cavity will dissolve 1 m³ of halite. Water or undersaturated brine is injected through a purpose-designed well drilled into a salt mass to etch out a void or cavern. The resulting “almost saturated” brine is then extracted for processing. The technique usually targets salts at depths greater than 400–500 m and down to 2,000 m (Fig. 13.1). The current deepest salt solution operation is in the Barradeel concession in northern Netherlands in Zechstein Z2 salts and operating at depths around 2,800 m (Geluk et al. 2007). At depths greater than 2,000 m ongoing salt creep tends to reduce cavern size. Some operating brinefield caverns are as shallow as 150 m, but this can lead to catastrophic chimneying and stoping in sediments above cavity. With deeper operations the landsurface tends to subside into a bowl of subsidence, as it does above many conventional mines. Cavern shape and the upward rise of the cavern roof is today controlled by an inert fluid blanket pumped in and maintained at the top of the zone of active brine creation. Early solution wells did not use this blanket technology. In the 1800’s and extending into first half of last century many brinefield operators perceived surface sinks, collapses and regular abandonment of caved wells as normal, during the operational life of a saltfield. Attitudes in the mining community today, across all types of exploration and production, are much changed.

Evaporite salts can survive well into the metamorphic realm, but are altered, recrystallised or transformed into new minerals. And so, beyond the early greenschist phase, little or nothing of the original sedimentary mineral phase remains with the possible exception of anhydrite (Table 14.1; Spear 1993). Consider “the before and after” situation as a sequence of evaporite-entraining sediments passes into the metamorphic realm. Under a likely “before” regional metamorphism scenario, a typical buried marine evaporite body occupies a continental margin position, is hundreds of metres thick, tens to hundreds of kilometres wide, perhaps with halokinetic geometries and an extensively tilted and faulted overburden. Halite typically makes up more than 60–80 % of the total rock salt volume in a basinwide unit. This halite is likely to be interlayered with anhydrite, bitterns, magnesian carbonates and siliciclastic clays. “After” metamorphism, typically driven by subduction and continent-continent collision, the end product of this once dominantly NaCl body of rock is a series of sodic and magnesian aluminosilicates, magnesites, calc-silicates and marbles with local zones of potassic enrichment. The immediate question is, where did all that salt and the associated volatiles (Cl, CO₂, SO₃) go?

Most subsurface evaporites ultimately dissolve and, through their ongoing dissolution and alteration, can create conditions suitable for metal enrichment and entrapment in subsurface settings ranging from the burial diagenetic through to the metamorphic and igneous realms. Within this framework, this chapter explores relationships between evaporites and lower temperature ore deposits, where ore is defined as a mineral, or an aggregate of minerals, from which a valuable constituent, especially a metal, can be profitably mined or extracted. The next chapter explores the significance of dissolving and altering evaporites in ore deposits formed in the higher temperature igneous and metamorphic realms.

Metalliferous fluid indicators and ore deposits due to direct and indirect interactions between magma and evaporites at a regional scale are neither well documented, nor well understood. Mostly, this is because little or no actual salt remains once these high temperature interactions have run their course. On top of that, some hard-rock geologists with a career working in igneous and metamorphic terranes may not be well versed in sedimentary evaporites or their meta-evaporitic siblings. The term pyrometasomatic encompasses some, but not all, of the types of salt-magma interaction that are the focus of this chapter.