Salty Matters

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Non-solar thick salt masses: Part 2: Oceanic ridge anhydrite and mantle-derived halite

John Warren - Sunday, June 16, 2019

Introduction

The previous article in this discussion of significant salt volumes not created by solar driven evaporation focused on a number of processes that drive crystallisation, namely temperature changes via brine warming (prograde salts) or cooling, especially cryogenesis, as well as brine mixing. In this article, we shall further develop the notion of temperature changes driving salt crystallisation, but now focus into higher-temperature subsurface realms generally flushed by igneous and mantle fluids.

Most of the precipitates can be considered hydrothermal salts, which is a broader descriptor than burial salts (Warren 2016; Chapter 8), that encompasses a higher temperature range compared to the diagenetic realm. One group of such hydrothermal salts, mostly composed of anhydrite, with lesser baryte, typically develop along oceanic seafloor ridges within heated subsurface fractures or at seafloor vents. There seawater-derived hydrothermal waters are heating, mixing, degassing, escaping and ultimately cooling. Active deep seafloor hydrothermal hydrologies create a specific group of sulphide ore deposits known as volcanic-hosted massive sulphide deposits (VHMS), with anhydrite as the primary-salt driving mineralisation.

The other non-solar salt grouping we shall discuss are salting-out precipitates, mostly halite, created when brines reach supercritical temperatures of 400-500°C. Some proponents of this mechanism postulate hydrothermal  halite sources much of the halite in active rifts such as the Red Sea or the Danakhil Depression (Hovland et al., 2006a, b).

Volcanogenic-hosted massive sulphide (VHMS) deposits

Volcanogenic-hosted massive sulphide deposits are forged by thermal circulation of seawater through newly-formed oceanic crust, in close temporal association with submarine volcanism. This milieu is characterised by active hydrothermal circulation and exhalation of metal sulphides, driven by mantle-induced geothermal gradients in oceanic basalt (Piercey et al., 2015). Being hosted in fractured basalts sets apart VHMS deposits from sedimentary exhalative (SedEx) and most sial-hosted Iron-Oxide-Copper-Gold (IOCG) deposits (Warren, 2016; Chapter 16). Hydrothermal anhydrite crystallises within a matrix of submarine volcanics and volcaniclastics via the heating of fissure-bound seawater (Figure 1a).


Anhydrite’s retrograde solubility across a range of salinities means the solubility of anhydrite decreases rapidly with increasing temperature in circulating seawater brines (Figure 1b; Blount and Dickson, 1969). Retrograde solubility also explains why anhydrite is most evident in the upper portions of vent mounds and in black and white “smokers.” Anhydrite's heating response is the opposite of baryte, another typical hydrothermal sulphate precipitate. Simple heating of seawater adjacent to seafloor vents, even without fluid mixing, will precipitate anhydrite, while simple cooling of hydrothermal waters will precipitate baryte. Once buried, hydrothermal calcium sulphate, in the presence of organic matter or hydrocarbons and circulating hydrothermal brines, acts as a sulphur source to create H2S, which then interacts with metal-carrying pore waters to co-precipitate metal sulphides.

Thus hydrothermal anhydrite, or more typically indicators of its former presence, are commonplace within volcanogenic hosted massive sulphide (VHMS) deposits. VHMS deposits usually form in submarine depressions as circulating seawater becomes an ore-forming hydrothermal fluid during interaction with the heated upper crustal rocks. Submarine depressions, especially those created by submarine calderas or by large-scale tectonic activity in median ocean-ridge rift valleys, are favourable sites and are often the home of an endemic chemosynthetic vent biota (Holden et al., 2012).

 

Based on VHMS Kuroko style deposits, fundamental processes typed to hydrothermal circulation and anhydrite distribution in a subduction zone as seen in the Koroko region of Japan, include (Ohmoto, 1996; Ogawa et al., 2007):

 

  • Intrusion of a heat source (typically ≈ 103 km size pluton) into oceanic crust or submarine continental crust causing deep convective circulation of seawater around the pluton (Figure 2). The radius of a typical circulation cell is ≈ 5 km. Temperatures of fluids discharging on to the seafloor increase with time from the ambient seafloor temperature to a typical maximum of ≈ 350° C, and then decrease gradually once more to ambient temperatures, on a time scale of ≈ 100 - 10,000 years. The majority of subsurface sulphide and sulphate mineralization occurs during the waxing stage of hydrothermal activity.

  • Reactions between warm country rocks and downward percolating seawater cause seawater SO4 to precipitate as disseminated and fracture-fill hydrothermal anhydrite in the country rock in areas where internal temperatures are greater than 150°C (Figures 2a, b: Sekko Anhydrite).
  • Reactions of “modified” seawater with higher-temperature rocks during the waning stages of hydrothermal circulation transform this sulphate-depleted “seawater” into metal-rich or H2S-rich ore-forming fluids. Metals are leached from the country rocks, while previously formed hydrothermal CaSO4 is reduced by Fe2+-bearing minerals and organic matter to provide H2S. The combined mass of high-temperature rocks that provide the metals and reduced sulphur in each VHMS marine deposit is typically ≈ 1011 tonnes (≈ 40 km3 in volume). Except for SO2, which produces acid-type alteration in some systems, the roles of magmatic fluids or gases are minor metal and sulphur in most massive sulphide systems.
  • Reactions between the ore-forming fluids and cooler rocks in the discharge zone cause zoned alteration of the rocks and precipitation of ore minerals in stockworkss.
  • Mixing of the ore-forming fluids with local seawater within unconsolidated sediments or on the seafloor can cause precipitation of “primitive ores” with a black ore mineralogy (Figure 2b; Oko Kuroko: sphalerite + galena + pyrite + baryte + anhydrite).
  • Reactions between the “primitive ores” with later and hotter hydrothermal fluids beneath a sulphate-rich thermal blanket cause a transformation of “primitive ores” to “matured ores” that are enriched in chalcopyrite and pyrite, often with a baryte cap in zones of cooling.

  • A mid-oceanic seafloor ridge region with significant documented volumes of anhydrite is located in the sediment-hosted Grimsey hydrothermal field in the Tjörnes fracture zone on the seafloor, north of Iceland (Figure 3a: Kuhn et al., 2003). There an active fracture zone is located at a ridge jump of 75 km, which caused widespread extension of the oceanic crust in this area. Hydrothermal activity in Grimsey field is spread over a 300 m by 1000 m area, at a water depth of 400 m. Active and inactive anhydrite chimneys up to 3 meters high and hydrothermal anhydrite mounds are typical of the seafloor in this area (Figure 3b-f). Clear, metal-depleted shimmering hydrothermal fluids, with temperatures up to 250°C, are venting from active chimneys and fluid inclusion in the precipitated anhydrites show the same homogenisation temperature range (Figure 3g).

    Anhydrite samples collected from Grimsey field average 21.6 wt.% Ca, 1475 ppm Sr and 3.47 wt.% Mg. The average molar Sr/Ca ratio is 3.3x10-3. Sulphur isotopes from anhydrites have typical δ34S seawater values of 22±0.7‰, indicating a seawater source for the SO4. Strontium isotopic ratios average 0.70662±0.00033, suggesting precipitation of anhydrite from a hydrothermal-seawater mixture (Figure 3f). The endmember of the venting hydrothermal fluids, calculated on a Mg-zero basis, contains 59.8 µmol/kg Sr, 13.2 mmol/kg Ca and a 87Sr/86Sr ratio of 0.70634. The average Sr/Ca partition coefficient between the hydrothermal fluids and anhydrite is about 0.67, implying precipitation from a non-evolved fluid. In combination, this suggests anhydrite forms in a zone of mixing between upwelling more deeply-seated hydrothermal fluids and shallowly circulating heated seawater (with a mixing ratio of 40:60). Before and during mixing, seawater is heated to 200-250°C, which drives anhydrite precipitation and the likely formation of an extensive anhydrite-rich zone beneath the seafloor, as in Hokuroko Basin.

    Once hydrothermal circulation slows or stops on a ridge or mound, and the “in-mound” temperature falls below 150°C, and anhydrite in that region tend to dissolve. During inactive periods, the dissolution leads to the collapse of sulphide chimneys and the internal dissolution of mound anhydrite. Additional ongoing disruption by faulting combine, so driving pervasive internal brecciation of the deposit. Through dissolution, former zones of hydrothermal anhydrite evolve into intervals of enhanced porosity and cavities in the mound. Such intervals initiate further fracture and collapse in the adjacent lithologies, which become permeable pathways during later renewed fluid circulation episodes. The alternating “coming and going” role of hydrothermal anhydrite creating precipitation space within the mound hydrology is similar to that of sedimentary evaporites in the sedimentary mineralising systems (Warren, 2016; Chapter 15).

    To form VHMS deposits on the seafloor, through-flushing hydrothermal fluids must transport sufficient amounts of metals and reduced sulphur, each at concentration levels > 1 ppm (Ohmoto, 1996). For a hydrothermal fluid with the salinity of normal seawater (≈0.7m ∑Cl) to be capable of transporting this amount of Cu and other base metals, it must be heated to temperatures > 300°C. Fluids with temperatures above 300°C will boil at pressures >200 bars. Under such conditions, the resulting vapour cannot carry sufficient quantities of metals to form a VHMS deposit. Boiling of a metalliferous hydrothermal brine outflow is prevented when the fluid vents into water that is deep enough to generate sufficient confining pressure. At 350°C, a minimum seawater depth of 1550m is necessary to prevent boiling. If the fluid passes through a sedimentary package where it loses temperature and metals (Cu, Ba) before emanating, the water depth beneath which boiling is prevented is less (≈1375m). Once vented, the turbulent mixing of hot hydrothermal waters with cooler seawater causes rapid precipitation of sulphides and calcium and barium sulphate, which produces the familiar black and white smokers (Blum and Puchelt, 1991).

    In modern oxic oceans, the sulphide-rich hydrothermal mounds are rapidly destroyed after the cessation of the hydrothermal activity (Herzig and Hannington, 1995; Tornos et al., 2015)). When hydrothermal activity at a mound decreases and the hydrothermal fluids cool to below 150 °C, the previously formed vent anhydrite is dissolved (retrograde solubility). This near-surface cooling contributes to the dissolution collapse of the anhydrite supported mound surface, particularly at the mound flanks, and allows additional influx of cold seawater. As mound flank collapse expands the remaining detrital pyritic sand residues are replaced by oxyhydroxides, and copper sulphides tend to be oxidised and replaced by atacamite (Knott et al., 1998). If seafloor weathering continues to completion, all the metal sulphides become oxidized or dissolved. Only those metalliferous VHMS deposits capped by impermeable volcanic, volcaniclastic, or sedimentary deposits soon after formation are preserved due to shielding from the oxidising conditions at the deep seafloor.

    In all cases, VHMS deposit styles of mineralisation, along with associated anhydrite precipitation, are allied to submarine volcanism and hydrothermally-driven circulation of seawater within adjacent deepwater sediments. Hydrothermal anhydrite typifies mineralisation in a variety of tectonic settings and sediment types (more detail on deposit styles and their anhydrites are given in Warren, 2016; Chapter 16):

  • VHMS deposits formed in subduction-related island-arc settings (Kuroko-type deposits; Ogawa et al., 2007);
  • VHMS deposits formed at mid-oceanic or back-arc spreading centres (Grimsey vent field or TAG mound type deposits; Kuhn et al., 2003; Knott et al., 1998);
  • VHMS deposits formed at spreading centres, but, due to the proximity of one or more landmasses, the deposit is sediment-hosted (Besshi-type deposits). This style of deposit shows some affinities with SedEx deposits but, unlike a SedEx deposit, the hydrological drive is linked to igneous intrusion.
  • In all cases, vestiges of the once voluminous anhydrite are a minor component in the cooled brecciated and fractured volcanic pile. This variety of CaSO4, and its variably metalliferous pseudomorphs and breccias, are not associated with solar heating. The occurrence sparry hydrothermal anhydrite as fracture and breccia fill in a labile volcanic pile, always covered with deep ocean sediments, cherts, etc., makes the distinction from sedimentary anhydrite relatively straightforward.

    Halite and a hydrothermal brine's critical temperature

    In terms of a significant volume of non-evaporite salt produced, or the possible ubiquity of a non-solar process contributing large amounts of NaCl, is the possible formation of halite when a brine reaches supercritical temperatures at appropriate depths in tectonically active parts of the earth's crust. Starting in the mid-2000's Hovland et al. (2006a,b) then Hovland and Rueslatten (2009) introduced the concept of substantial volumes of hydrothermal halite precipitating from subsurface brines at supercritical temperatures, especially in the buried hot portions of thermally-active rift basement. Two recent papers Hovland et al. (2018a,b) summarise much of this earlier material and add the notion of serpentinization being sink for chloride and  a driver of halite formation in many evaporite basins. Countering arguments to the notion of a non-evaporite origin for substantial volumes of halite in sedimentary basins are given in Talbot (2008) and Aftabi and Atapour (2018). The notion of the importance of the Wilson cycle in a sedimentary evaporite (megahalite and megasulphate basins) context, rather than a direct igneous-metamorphic source as argued by Hovland, is summarised in Warren (2016, Chapter 5).

    A Hovland model of a non-evaporite source of halite relies on heated subsurface brines becoming supercritical and so transforming a brine to a fluid that does not dissolve but precipitates salt (within specific temperature and pressure ranges). A supercritical fluid is defined as any substance at a temperature and pressure above its critical point; in such a state, it can effuse through solids like a gas, and dissolve materials like a liquid. In addition, close to the critical point, small changes in pressure or temperature result in substantial changes in density. The critical point (CP), also called a critical state, specifies the conditions (temperature, pressure and sometimes composition) at which a phase boundary ceases to exist. At particular pressure/temperature conditions, supercritical water is unable to dissolve/retain common sea salts in solution (Josephson, 1982; Bischoff and Pitzer, 1989; Simoneit 1994; Hovland et al., 2006a).


    When seawater brines are heated in pressure cells in the laboratory, they pass into the supercritical region at a temperature of 405°C and 300 bar pressure (the CP of seawater). A particulate ‘cloud’ then forms via the onset of ‘shock crystallization’ of NaCl and Na2SO4 (Figure 4a). The sudden phase transition occurs as the solubility of the previously dissolved salts declines to near-zero, across a temperature range of only a few degrees, and is associated with a substantial lowering of density (Figure 4b). The resulting solids in the “cloud” consist of amorphous microscopic NaCl and Na2SOparticles with sizes between 10 and 100 mm. The resultant “salting out” can lead to the precipitation of large volumes of subsurface salts in fractures and fissures and perhaps even in the deeper portions of salt structures. The same supercritical conditions improve the ability of brines to carry high volumes of hydrothermal hydrocarbons prior to the onset of supercritical conditions (Josephson, 1982; McDermott et al., 2018). Supercritical water has enhanced solvent capacity for organic compounds and reduced solvation properties for ionic species due to its loss of aqueous hydrogen bonding (Figure 4c; Simoneit, 1994).

    Hovland et al. (2006a, b) predict that some of the large volumes of deep subsurface salt found in the Red Sea, in the Mediterranean Sea and the Danakil depression, formed via the forced magmatically-driven hydrothermal circulation of seawater down to depths where it became supercritical. This salt, they argue, was precipitated deep under-ground via “shock crystallisation” from a supercritical effusive phase and so formed massive accumulations (mostly halite) typically in crustal fractures that facilitated the deep circulation. NaCl then flowed upwards in solution in dense, hot hydrothermal brine plumes, precipitating more solid salt beds upon cooling nearer or on the surface/seafloor. More recently, Scribano et al. (2017) and Hovland et al. (2018a, b) have added the argument that serpentinisation is the dominant source of halite in the Messinian succession of the Mediterranean.

    To date, the Hovland et al. model of hydrothermal sourcing for widespread halite from a supercritical brine source (in active magmatic settings) has not been widely accepted by the geological community (Talbot, 2008; Warren., 2016; Aftabi and Atapour, 2018). To date, no direct indications of the formation of masses of halite formed by this process have been sampled. In contrast to the theories of Hovland et al (2018b), textures in the potash and halite salts in the Danakhil depression are evaporitic with only small volumes of hydrothermal overprint driven by the escape of saline volatiles derived thermal decomposition of hydrated salts. The postulated diapiric structures are not present in seismic, nor are any other buried hydrothermal/halokinetic structures visible in seismic (Bastow et al., 2018; Salty Matters; Warren 2016). Likewise, all the features seen in core and seismic in the Messinian of the Mediterranean are layered with classic sedimentary and halokinetic textures. The seismic across the Red Sea salt structures and the layering in the brine deeps are easily explained by current sedimentary and layered deep seafloor ponded brine (DHAL) models.

    The high temperatures required for supercritical seawater venting mean such sites are rare on the seafloor. The deepest thermal upwelling site where supercritical seafloor conditions are thought to be active just below the upwelling site is the Beebe vent field (Figure 5; Webber et al., 2015; McDermott et al., 2018). At 4960 m below sea level, the vent field sits atop the ultra-slow spreading Mid Cayman Rise and is the world’s deepest known hydrothermal exhalative system. Situated on very thin (2–3 km thick) oceanic crust at an ultraslow spreading centre, this hydrothermal system circulates fluids to depths ≈1.8 km in a basement that is likely to include a mixture of both mafic and ultramafic lithologies (Webber et al., 2015).


    The surface of the active vent field is made up of high temperature (≈401°C) anhydritic ‘‘black smokers’’ that build Cu, Zn and Au-rich sulfide mounds and chimneys (Figure 5a). The vent field is highly gold-rich, with Au values up to 93 pp, with an average Au:Ag ratio of 0.15. Gold precipitation is directly associated with diffuse flow through anhydritic‘‘beehive’’ chimneys. Significant mass-wasting of sulfide material in the vent field, accompanied by changes in metal content results in metalliferous talus and interfingering with deep marine sediment deposits (Figure 5b, c, d, e).

    All the high-temperature endmember fluids venting at the Beebe site show Cl levels that are significantly lower than seawater, with an average endmember concentration of 349 mmol/kg. Due to the lack of a significant sink for Cl within mafic-hosted subsurface circulation pathways, Cl depletions in vent fluids are typically attributed to phase separation (McDermott et al., 2018). Thus, the intrinsic Cl depletions, in conjunction with a seafloor pressure of 496 bar, places the two-phase boundary at 483°C, suggesting that escaping fluids experienced a phase separation at conditions that are both hotter and deeper (higher pressure) than the critical point for seawater at 407°C and 298 bar (Bischoff, 1991).

    During incipient phase separation from supercritical seawater, a small amount of high-salinity brine it thought to condense in the subsurface as a separate phase, so creating the Cl-depleted residual fluid, or vapour phase. The Cl-depleted fluids venting at Beebe are thought to represent this vapour phase. Although a vent fluid of seawater chlorinity is not a supercritical fluid at the conditions of seafloor venting (398°C, 496 bar), the vent fluids indicate sourcing from a supercritical phase owing to their lower chlorinity (Bischoff and Pitzer, 1989).

    Phase relations in the system NaCl-H2O (Bischoff, 1991) can be used to estimate the minimum temperature of phase separation at the Beebe site, based on the chlorinity of the vapour phase and the assumption that phase separation occurs at or below the seafloor. A minimum temperature of 491°C is required to produce the measured Cl concentration of 349 mmol/kg observed in the Beebe Vents endmember fluids. Accordingly, the observed Cl depletion in the high-temperature endmember fluids implies that these fluids must have cooled by at least 90°C prior to venting, and perhaps more (McDermott et al., 2018). For example, if the location of phase separation was 1000 m deeper, then that would require maximum fluid temperatures in the vicinity of separation of 535°C.

    The only salt that can be confused with an evaporite salt at the Beebe site is hydrothermal anhydrite. Any halite or Na2SOderived from seawater reaching its supercritical point is still located in fissures many hundreds of metres below the surface and is as yet unsampled. Likewise, there are no halite-saturated brine ponds on the seafloor and the smoker anhydrite, like much of the metalliferous content, has a low preservation potential as it is being leached back into seawater via galvanic interaction (Webber et al., 2015).

    Herein is the problem for assessing the viability of a Hovlnd-style model for halite. Where is the evidence and the data? Until significant volumes of hydrothermal halite are intersected somewhere on the earth's surface, there is not a working example, only a sophisticated reinterpretation of existing halite occurrences. Modern seawater (rather than an experimental NaCl -H2O system) would give not just halite but also Na2SOat its critical point, where are the volumes or texural and mineralogical indications of these salts, or their brines and alteration haloes? Until there is the physical proof of a working example of substantial hydrothermal halite sourced in supercritical phase separations, I prefer to apply Occam's Razor.


    Halite alteration, renewed deep brine flow and metamorphism

    A source of chlorine-rich hydrothermal fluid (not halite) in the deep subsurface is the recycling of deeply buried sedimentary mega-halite units into the greenschist realm and beyond (Yardley and Graham, 2002). In the metamorphic realm (T>200°C) the derived fluids do not precipitate halite, but a series of meta-evaporite indicator minerals (Table 1). Lewis and Holness (1996) demonstrated that buried salt bodies, subjected to high pressures and elevated temperatures, can acquire a permeability comparable to that of a sand, within what is sometimes called the "Holness zone". This is because the crystalline structure of deeply buried salt (halite) attains dihedral angles between salt crystals of less than 60 degrees, and so creates an impermeable polyhedral meshwork (Figure 6). Such conditions probably begin at the onset of greenschist P-T conditions, whereby highly-saline hot brines form continuous brine stringers around all such altered and recrystallizing salt crystals.


    This polyhedral permeability meshwork allows hot dense brines or hydrocarbons to migrate through salt (Schoenherr et al., 2007a, b) and ultimately dissolve the salt host, releasing a pulse of sodic- and chloride-rich fluid into the metamorphic realm (Warren, 2016; Chapter 14). It is why little or no evidence of solid masses of metamorphosed halite is found in subsurface meta-evaporitic settings where temperatures have exceeded 250 - 300 °C, even though the melting point of halite is 800°C. Given the right subsurface conditions these halite-derived metamorphic brines may evolve into supercritical waters.

    Contrary to conventional geological modelling of salt in diapirs being mostly  impermeable, Hovland et al. (2018a) argue for the formation of salt stocks by hot brines migrating upward through the middle of the salt body; provided that the salt stock is situated within the "Holness zone." This assumes that "Holness zone" flows brine and can also reach subcritical conditions. The inferred rising flow of intrasalt hot brines then reach saturation upon cooling in the upper part of the salt stem, where solid salts are precipitating according to their specific solubility at each particular temperature and pressure interval. The Hovland model thus includes a refining process in the salt stem, where halite, for example, precipitates upon cooling long before calcium and magnesium chloride salts. However, in the Danakhil Depression, where they infer this process is active (Hovland et al., 2018a), the seismic indicates the evaporite mass is bedded and faulted, while the evaporite textures recovered in cores and doline/uplift landforms across the saltflat surface combine to show Holness-zone halokinesis is not segregating the halite, kainite, carnallite, sylvite and bischofite salts that typify the region around the Dallol Mound (Bastow et al., 2018; Warren, 2016, Chapter 11).


    Beyond the greenschist facies and the polyhedral transition of sedimentary/halokinetic halite, metamorphic minerals with an evaporite protolith tend to be enriched in minerals entraining sodium, potassium and magnesium (Figure 7; Table 1; Yardley and Graham, 2002). These metamorphic minerals (meta-evaporites) can entrain high levels of volatiles (Cl, SO3 and CO2) as well as elevated levels of boron, along with high salinity in the associated metamorphic fluids; all indicate their evaporitic protolith (Table 1; Figure 7).

    Sodium tends to come from the dissolution of salts, such as halite, kainite or trona; while magnesium tends to be remobilized from earlier diagenetic minerals, such as reflux dolomites,magnesium-rich evaporitic clays and some potash minerals (Table 1). Boron in tourmalinites may have come from a colemanite/ulexite lacustrine precursor. Once direct evidence of a salty protolith is largely removed via fluid dispersion in burial and ongoing loss of volatiles, the palaeo-evaporite indications are restricted mostly to mineralogic associations, along with an occasional textural relict of a former evaporitic breccia bed, rauwacke or salt weld (Warren, 2016; Chapters 7, 14).

    Evidence of early stages in an evaporite-fed sodic transformation is seen in the sodic phlogopites (phlogopite = magnesian mica) and sodian aluminian talcs in the metapelites of the Tell Atlas in Algeria (Schreyer et al. 1980). Evaporitic sulphate crystals are pseudomorphed in the NaCl-scapolite-dominated sequences of the Cordilleras Beticas of Spain (Gómez-Pugnaire et al. 1994). Rocks of higher temperature and pressure facies, such as the massive stratiform anorthosites in the Grenville Precambrian Province of North America, have been interpreted as possible meta-evaporites (Gresens, 1978), as have the anhydrite-containing Mesoproterozoic calcsilicates in the Oaxacan granulite complex in southern Mexico (Ortega-Gutierrez, 1984) and the pervasive scapolites in the Neoproterozoic Zambesian orogenic belt of Zambia (Hanson et al., 1994). Subsequent work on the Grenville anorthosites, although still allowing for a metasedimentary protolith, has concluded an igneous source of volatiles is more likely (Moecher et al., 1992; Peck and Valley, 2000; Glassly et al., 2010). Defining the likelihood of an evaporite protolith becomes increasingly difficult as the metamorphic grade increases. Once a metamorphic rock enters the granulite facies, its protolith interpretation is typically much more contentious (e.g. the evaporite versus carbonatite interpretations in the Oaxacan granulites, Mexico).

    Hydrothermal gypsum

    Some of the most visually striking examples of hydrothermal gypsum precipitation are in the Naica mine, Chihuahua, Mexico (Figure 8). There several natural caverns, such as Cave of Swords (Cueva de la Espades discovered in 1975) and Cave of Crystals (Cueva de los Cristales discovered in 2000), contain giant, faceted, and transparent single crystals of gypsum as long as 11 m (Figure 9a; García-Ruiz et al., 2007; Garofalo et al., 2010). Crystals in Cueva de los Cristales are the largest documented gypsum crystals in the world. These huge crystals grew slowly at very low supersaturation levels from thermal phreatic waters with temperatures near the gypsum-anhydrite boundary. Gypsum still precipitates today on mine walls.


    According to García-Ruiz et al., 2007, the sulphur and oxygen isotopic compositions of these gypsum crystals are compatible with growth from solutions resulting from dissolution of anhydrite, which was previously precipitated during late hydrothermal mineralisation in a volcanogenic matrix. The chemistry suggests that these megacrystals formed via a self-feeding mechanism, driven by a solution-mediated, anhydrite-gypsum phase transition. Nucleation kinetics calculations based on laboratory data show that this mechanism can account for the formation of these giant crystals, yet only when operating within a very narrow range of temperature of a few degrees as identified by the fluid inclusion values.

    Fluid inclusion analyses show that the giant crystals came from low-salinity solutions at temperatures ≈ 54°C, slightly below the temperature of 58°C where the solubility of anhydrite equals that of gypsum (Figure 9b; García-Ruiz et al., 2007). Van Driessche et al. (2011) argue the slowest gypsum crystal growth in the phreatic cavern occurred when waters were at 55°C. At this temperature, the crystals would take 990,000 years to grow to a diameter of 1 meter. By increasing the temperature in the cave by one degree, to 56° C, the same size crystal could have formed in a little less than half the time, or around 500,000 years. This possible growth rate would work out to about a billionth of a meter of growth per day and is perhaps the slowest growth rate that has ever been measured.


    Garofolo et al., 2010, accept the need for a limited temperature range during precipitation, but argue the precipitating solutions were in part meteorically influenced. Their work focused on Cueva de las Espadas. As for most other hypogenic caves, prior to their analytical work, they assumed that caves of the Naica region lacked a direct connection with the land surface and so gypsum precipitation would be unrelated to climate variation. Yet, utilising a combination of fluid inclusion and pollen spectra data from cave and mine gypsum, they concluded climatic changes occurring at Naica exerted and influence on fluid composition in the Espadas caves, and hence on crystal precipitation and growth.

    Microthermometry and LA-ICP-Mass Spectrometry of fluid inclusions in the gypsum in the Cueva de las Espadas indicate that brine source was a shallow, chemically peculiar, saline fluid (up to 7.7 eq. wt.%NaCl) and that it may have formed via evaporation, during an earlier dry and hot climatic period. In contrast, the fluid of the deeper caves (Cristales) was of lower salinity (≈3.5 eq. wt.% NaCl) and chemically homogeneous, and likely was little affected by evaporation processes. Galofolo et al. (2010) propose that mixing of these two fluids, generated at different depths of the Naica drainage basin, determined the stable supersaturation conditions needed for the gigantic gypsum crystals to grow (Figure 9c). The hydraulic communication between Cueva de las Espadas and the other deep Naica caves controlled fluid mixing. Mixing must have taken place during alternating cycles of warm-dry and fresh-wet climatic periods, which are known to have occurred in the region. Pollen grains from 35 ka-old gypsum crystals from the Cave of Crystals indicates a relatively homogenous catchment basin dominated by a mixed broadleaf wet forest. This suggests precipitation during a fresh-wet climatic period; the debate continues as to whether the gypsum at Naica is a mixing zone or a hydrothermal salt.

    Solar versus nonsolar salts

    This and the previous article show that substantial volumes of various salts (especially retrograde anhydrite) form in the terrestrial subsurface, independent of solar evaporation. Except for some bedded cryogenic salt bodies (e.g., Korabogazgol in Kazakhstan or Noachian lakes on Mars), non-solar evaporation salts tend to nucleate in subsurface fractures, and breccia interspaces in the igneous and metamorphic realm. Crystals  tend to be cavity cements, but can also replace portions of a pre-existing salt mass. On Earth, the most widespread non-solar salt is anhydrite with occurrences ranging from volcanic hosted mid-ocean ridges to Kuroko style deposits in subduction zones. In all cases, the intimate association with submarine volcanics and fissures, where hydrothermally heated seawater once circulated, mean this type of hydrothermal (non solar heating) salt is readily distinguished from sedimentary anhydrite.

    For halite, there is little direct evidence of any massive halite occurrence in outside of sedimentary basins where isolated-sumps of ponded brine were once evaporated. A sophisticated notion theorising hydrothermal halite has been published by Hovland and co-workers (e.g. Hovland et al., 2018a b; Scribano et al., 2017) to explain some halite occurrences in tectonically-active areas. There is little support for this model in the published literature outside of Hovland and co-workers. Thick buried solar halite masses tend form in particular stages of the Wilson Cycle. Once buried, these evaporite masses are mostly impervious, they can flow and dissolve, and on entry into the greenschist realm can become permeable, so feeding large volumes of highly saline brines into the metamorphic and igneous realms. These brines can drive metal accumulations and the formation of characteristic meta-evaporitic minerals and gemstones (Warren, 2016).

    References

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