Oceanic Ridge Anhydrite and Mantle-Derived Halites

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Salty MattersJohn Warren - Sunday June 16, 2019 Non-solar thick salt masses: Part 2: Oceanic ridge anhydrite

Introductionand mantle-derived halites The previous article in this discussion of significant salt volumes Volcanogenic-hosted massive sulphide not created by solar driven evaporation and focused on a num- (VHMS) deposits ber of processes that drive crystallisation, namely; temperature Volcanogenic-hosted massive sulphide deposits are forged by changes via brine warming (prograde salts) or cooling, especial- thermal circulation of seawater through newly-formed oceanic ly cryogenesis, as well as brine mixing. In this article, we shall crust, in close temporal association with submarine volcanism. further develop the notion of temperature changes driving salt This milieu is characterised by active hydrothermal circulation crystallisation, but now focus into higher-temperature subsur- and exhalation of metal sulphides, driven by mantle-induced face realms generally flushed by igneous and mantle fluids. geothermal gradients in oceanic basalt (Piercey et al., 2015). Being hosted in fractured basalts sets apart VHMS deposits Most of the precipitates can be considered hydrothermal salts, from sedimentary exhalative (SedEx) and most sial-hosted Iron- which is a broader descriptor than burial salts (Warren 2016; Oxide-Copper-Gold (IOCG) deposits (Warren, 2016; Chapter Chapter 8), that encompasses a higher temperature range com- 16). Hydrothermal anhydrite crystallises within a matrix of pared to the diagenetic realm. One group of such hydrothermal submarine volcanics and volcaniclastics via the heating of fis- salts, mostly composed of anhydrite, with lesser baryte, typically sure-bound seawater (Figure 1a). develop along oceanic seafloor ridges within heated subsurface fractures or at seafloor vents. There seawater-derived hydrother- Anhydrite’s retrograde solubility across a range of salinities mal waters are heating, mixing, degassing, escaping and ulti- means the solubility of anhydrite decreases rapidly with in- mately cooling. Active deep seafloor hydrothermal hydrologies creasing temperature in circulating seawater brines (Figure 1b; create a specific group of sulphide ore deposits known as volca- Blount and Dickson, 1969). Retrograde solubility also explains nic-hosted massive sulphide deposits (VHMS), with anhydrite why anhydrite is most evident in the upper portions of vent as the primary-salt driving mineralisation. mounds and in black and white “smokers.” Anhydrite's heating response is the opposite of baryte, another typical hydrother- The other non-solar salt grouping we shall discuss are salting-out mal sulphate precipitate. Simple heating of seawater adjacent to precipitates, mostly halite, created when brines reach supercriti- seafloor vents, even without fluid mixing, will precipitate anhy- cal temperatures of 400-500°C. Some proponents of this mech- drite, while simple cooling of hydrothermal waters will precipi- anism postulate this halite sources much of the hydrothermal tate baryte. Once buried, hydrothermal calcium sulphate, in the halite in rifts such as the Red Sea or the Danakhil Depression presence of organic matter or hydrocarbons and circulating hy- (Hovland et al., 2006a, b).

A. Sea oor weathering B. 0.5 Low T (<150°C) 1000 bars Seawater recharge O 2 0.4 O axis alteration NaCl = 7mol extrapolated Pillows Mixing and sulphide deposition (anhydrite dissolution) NaCl = 6mol 0.3 Hydrothermal NaCl = 5mol anhydrite 0.2 NaCl = 4mol in moles/kg H 4 NaCl = 3mol

Dykes 0.1 Mixing and sulphide CaSO NaCl = 1mol NaCl = 2mol Upwelling hydrothermal deposition in H O uids (350°C) 2 MAGMA 100 200 300 400 Temperature °C Figure 1. Hydrothermal anhydrite. A) Formation of hydrothermal anhydrite from heated seawater in oceanic crust. B) Anhydrite solubility as a function of temperature at 1000 bars in H2O and NaCl-H2O solutions of various concentrations. Dashed lines indicate extrapolated values (after Blount and Dickson, 1969).

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drothermal brines, acts as a sulphur source to create H2S, which ed and fracture-fill hydrothermal anhydrite in the country rock in then interacts with metal-carrying pore waters to co-precipitate areas where internal temperatures are greater than 150°C (Fig- metal sulphides. ures 2a, b: Sekko Anhydrite). Thus hydrothermal anhydrite, or more typically indicators of its • Reactions of “modified” seawater with higher-temperature former presence, are commonplace within volcanogenic host- rocks during the waning stages of hydrothermal circulation ed massive sulphide (VHMS) deposits. VHMS deposits usually transform this sulphate-depleted “seawater” into metal-rich or form in submarine depressions as circulating seawater becomes H2S-rich ore-forming fluids. Metals are leached from the coun- an ore-forming hydrothermal fluid during interaction with the try rocks, while previously formed hydrothermal CaSO4 is re- heated upper crustal rocks. Submarine depressions, especially duced by Fe2+-bearing minerals and organic matter to provide those created by submarine calderas or by large-scale tectonic H2S. The combined mass of high-temperature rocks that provide activity in median ocean-ridge rift valleys, are favourable sites the metals and reduced sulphur in each VHMS marine deposit 11 3 and are often the home of an endemic chemosynthetic vent biota is typically ≈ 10 tonnes (≈ 40 km in volume). Except for SO2, (Holden et al., 2012) which produces acid-type alteration in some systems, the roles of magmatic fluids or gases are minor metal and sulphur in most Based on VHMS Kuroko style deposits, fundamental processes massive sulphide systems. typed to hydrothermal circulation and anhydrite distribution in a subduction zone, as seen in the Koroko region of Japan include • Reactions between the ore-forming fluids and cooler rocks in (Ohmoto, 1996; Ogawa et al., 2007): the discharge zone cause zoned alteration of the rocks and precipitation of ore minerals in stockworks. • Intrusion of a heat source (typically ≈ 103 km size pluton) into oceanic crust or submarine continental crust causing deep con- • Mixing of the ore-forming fluids with local seawater within vective circulation of seawater around the pluton (Figure 2). The unconsolidated sediments or on the seafloor can cause precipita- radius of a typical circulation cell is ≈ 5 km. Temperatures of tion of “primitive ores” with a black ore mineralogy (Figure 2b; fluids discharging on to the seafloor increase with time from the Oko Kuroko: sphalerite + galena + pyrite + baryte + anhydrite). ambient seafloor temperature to a typical maximum of ≈ 350° C, • Reactions between the “primitive ores” with later and hotter and then decrease gradually once more to ambient temperatures, hydrothermal fluids beneath a sulphate-rich thermal blanket on a time scale of ≈ 100 - 10,000 years. The majority of sub- cause a transformation of “primitive ores” to “matured ores” that surface sulphide and sulphate mineralization occurs during the are enriched in chalcopyrite and pyrite, often with a baryte cap waxing stage of hydrothermal activity. in zones of cooling. • Reactions between warm country rocks and downward perco- A mid-oceanic seafloor ridge region with significant documented lating seawater cause seawater SO to precipitate as disseminat- 4 volumes of anhydrite is located in the sediment-hosted Grimsey

Type of Nodular Anhydrite Ore Type Size Strontium TER

Sekko or A Massive Zone of baryte and white-ore W sulphide Oko 10cm -

sulphide precipitation (anhydrite SE A Kuroko deposition) Clay 1 Type 3 Type . . >1500 -2000m depth ...... Sulphide ......

. . Sekko ...... Kuroko . . . . 2 Type Sekkoko ...... Y veinlet . . 5cm Baryte TIA R Galena High 2000ppm Massive Large TE R Type 1C Type 20cm VOLCANICS anhydrite Sphalerite alc

Anhydrite T nodules 1B Type 5cm Type 1 Type Sekko Anhydrite

Tu gypsum) (coverts to ALAEOZOIC BASEMENT breccia Chalcopyrite P Magma-derived GRANITIC Tu 1A Type 200ppm

uids ? Low PLUTON 10m Small 0.5cm

Sulphides/baryte Anhydrite Pyrite Gysum

Direction of Anhydrite uid ow Mg-chlorite A. B. mix Ser./Mont. Figure 2. Anhydrite in volcanic-hosted massive sulphide deposits (after Shikazono et al., 1983). A) Hydrothermal circulation that characterises Kuroko deposits. B) Vertical zonation of anhydrite versus sulphide in Kuroko ore (See Warren, 2016, Chapter 16 for more detail) Page 2 www.saltworkconsultants.com

hydrothermal field in the Tjörnes fracture zone on the seafloor, a seawater source for the SO4. Strontium isotopic ratios average north of Iceland (Figure 3a: Kuhn et al., 2003). There an active 0.70662±0.00033, suggesting precipitation of anhydrite from a fracture zone is located at a ridge jump of 75 km, which caused hydrothermal-seawater mixture (Figure 3f). The endmember of widespread extension of the oceanic crust in this area. Hydrothermal the venting hydrothermal fluids, calculated on a Mg-zero basis, activity in Grimsey field is spread over a 300 m by 1000 m area, at contains 59.8 µmol/kg Sr, 13.2 mmol/kg Ca and a 87Sr/86Sr ratio of a water depth of 400 m. Active and inactive anhydrite chimneys up 0.70634. The average Sr/Ca partition coefficient between the hydro- to 3 meters high, and hydrothermal anhydrite mounds, are typical thermal fluids and anhydrite is about 0.67, implying precipitation of the seafloor in this area (Figure 3b-f). Clear, metal-depleted from a non-evolved fluid. In combination, this suggests anhydrite shimmering hydrothermal fluids, with temperatures up to 250°C, forms in a zone of mixing between upwelling more deeply-seated are venting from active chimneys and fluid inclusion in the pre- hydrothermal fluids and shallowly circulating heated seawater cipitated anhydrites show the same homogenisation temperature (with a mixing ratio of 40:60). Before and during mixing, seawater range (Figure 3g). is heated to 200-250°C, which drives anhydrite precipitation and the likely formation of an extensive anhydrite-rich zone beneath Anhydrite samples collected from the Grimsey vent field average the seafloor, as in Hokuroko Basin. 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 vent anhy- Once hydrothermal circulation slows or stops on a ridge or mound, drites have typical δ34S seawater values of 22±0.7‰, indicating and the “in-mound” temperature falls below 150°C, and anhydrite

25°W 20°W 15°W 10°W 5°W 0° Mohns Greenland Eggvin O set Ridge Northern Kolbeinsey Ridge 70°N Medial Jan Mayen Spar O set Kolbeinsey Ridge fracture zone B. C. Southern Kolbeinsey Ridge Tjömes fracture zone

Spreading axis Iceland Fracture zone 65°N Reykjanes Grimsey eld Ridge A. D. E. “Candlestick” chimney 20 (≈250°C) Old anhydrite spire Two-phase uid Recent growth Shimmering water through talus gas bubbles 4. Sea oor Seawater: Seawater: 87Sr/86Sr = 0.709225 87 86 15 Anhydrite+talc 3. Chimney Sr/ Sr = 0.709225 2 m sand and debris talus

Anhydrite precipitation1. from 2. 1. N conductively-heated seawater 10

Phase separation controlling Mixing of 40% upwelling the temperature of the hydrothermal with 60% upwelling hydrothermal heated seawater results in uid uids with 87 86 5 anhydrite precipitation) anhydrite Seawater recharge zone recharge Seawater anhydrite precipitation) anhydrite Seawater recharge zone recharge Seawater Sr/ Sr = 0.70634 (heatong t0 >150°C drive (heatong (heatong t0 >150°C drive (heatong

Deep-seated hydrothermal uid 87Sr/86Sr = 0.702914 - 0.704512 0 Anhydrite-rich Hydrothermal reaction zone about 160 180 200 220 240 260 160 300 1-2 km below sea oor F. G. Grimsey Vent Field TH(°C) Figure 3. Grimsey Hydrothermal Vent field, offshore Iceland (after Kuhn et al., 2003). A) Location map B) Massive and acicular anhydrite from a beehive-structured vent chimney. C) Photomicrograph of anhydrite from an active chimney showing radially fibrous crystals. D) Rectangular anhydrite crystals (scale in C and D: 400 µm; crossed Nicols). E) A spongy talc-like material grows on top of euhedral platey anhydrite. All samples (B-E) are from active chimneys (fluid temperatures about 250 °C). F) Schematic drawing showing the main processes related to Sr-Ca geochemistry. Seawater (87Sr/86Sr = 0.709225) entrained in the seafloor at a shallow depth is heated to >150°C leading to anhydrite precipitation (1). The heating of seawater is either conductively or caused by mixing with upwelling hydrothermal fluid. It is assumed that the latter has equilibrated with sediment or underlying basalt resulting in 87Sr/86Sr = 0.702914 - 0.704512. The temperature of the ascending hydrothermal solution is controlled by phase separation and cannot exceed 250°C at shallow depth. The mixed fluid being further conductively heated to 250°C has 87Sr/86Sr = 0.70634 (2). This fluid rapidly rises to the seafloor (3) and precipitates anhydrite at and beneath the seafloor (4). The circulation portion of this schematic is not to scale. G) Homogenization temperatures of fluid inclusions in anhydrite. Most of the temperatures range between 200 and 280°C and therefore, are consistent with the measured temperature of the venting fluids of 248–250° Page 3 www.saltworkconsultants.com

in that region tend to dissolve. During inactive periods, the disso- en in Warren, 2016; Chapter 16): lution leads to the collapse of sulphide chimneys and the internal • VHMS deposits formed in subduction-related island-arc set- dissolution of mound anhydrite. Additional ongoing disruption tings (Kuroko-type deposits; Ogawa et al., 2007); by faulting combine, so driving pervasive internal brecciation of the deposit. Through dissolution, former zones of hydrothermal • VHMS deposits formed at mid-oceanic or back-arc spreading anhydrite evolve into intervals of enhanced porosity and cavities centres (Grimsey vent field or TAG mound type deposits; Kuhn in the mound. Such intervals initiate further fracture and collapse et al., 2003; Knott et al., 1998); in the adjacent lithologies, which become permeable pathways • VHMS deposits formed at spreading centres, but, due to during later renewed fluid circulation episodes. The alternating the proximity of one or more landmasses, the deposit is sedi- “coming and going” role of hydrothermal anhydrite creating pre- ment-hosted (Besshi-type deposits). This style of deposit shows cipitation space within the mound hydrology is similar to that of some affinities with SedEx deposits but, unlike a SedEx deposit, sedimentary evaporites in the sedimentary mineralising systems the hydrological drive is linked to igneous intrusion. (Warren, 2016; Chapter 15). In all cases, vestiges of the once voluminous anhydrite are a mi- To form VHMS deposits on the seafloor, through-flushing -hy nor component in the cooled brecciated and fractured volcanic drothermal fluids must transport sufficient amounts - ofmet pile. This varuety of CaSO4, and its variably metalliferous pseu- als and reduced sulphur, each at concentration levels > 1 ppm domorphs and breccias, are not associated with solar heating. (Ohmoto, 1996). For a hydrothermal fluid with the salinity of The occurrence of sparry hydrotherma; anhydrite, as fracture normal seawater (≈0.7m ∑Cl) to be capable of transporting this and breccia fill in a labile volcanic pile, always covered with amount of Cu and other base metals, it must be heated to tem- deep ocean sediments, cherts, etc., makes the distinction from peratures > 300°C. Fluids with temperatures above 300°C will sedimentary anhydrite relatively straightforward. boil at pressures >200 bars. Under such conditions, the resulting vapour cannot carry sufficient quantities of metals to form Halite and a hydrothermal brine's critical a VHMS deposit. Boiling of a metalliferous hydrothermal brine temperature outflow is prevented when the fluid vents into water that is deep In terms of a significant volume of non-evaporite salt produced, enough to generate sufficient confining pressure. At 350°C, a or the possible ubiquity of a non-solar process contributing large minimum seawater depth of 1550m is necessary to prevent boil- amounts of NaCl, is the possible formation of halite when a ing. If the fluid passes through a sedimentary package where it brine reaches supercritical temperatures at appropriate depths loses temperature and metals (Cu, Ba) before emanating, the wa- in tectonically active parts of the earth's crust. Starting in the ter depth beneath which boiling is prevented is less (≈1375m). mid-2000's Hovland et al. (2006a,b) then Hovland and Rueslat- Once vented, the turbulent mixing of hot hydrothermal waters ten (2009) introduced the concept of substantial volumes of with cooler seawater causes rapid precipitation of sulphides and hydrothermal halite precipitating from subsurface brines at su- calcium and barium sulphate, which produces the familiar black percritical temperatures, especially in the buried hot portions of and white smokers (Blum and Puchelt, 1991). thermally-active rift basement. Two recent papers Hovland et al. In modern oxic oceans, the sulphide-rich hydrothermal mounds (2018a,b) summarise much of this earlier material and add the are rapidly destroyed after the cessation of the hydrothermal notion of 1serpentinization being a sink for chloride and a driver activity (Herzig and Hannington, 1995; Tornos et al., 2015)). of halite formation in many evaporite basins. Countering argu- When hydrothermal activity at a mound decreases and the hy- ments to the notion of a non-evaporite origin for substantial vol- drothermal fluids cool to below 150 °C, the previously formed umes of halite in sedimentary basins are given in Talbot (2008) vent anhydrite is dissolved (retrograde solubility). This near-sur- and Aftabi and Atapour (2018). The notion of the importance face cooling contributes to the dissolution collapse of the anhy- of the Wilson cycle in a sedimentary evaporite (megahalite and drite supported mound surface, particularly at the mound flanks, megasulphate basins) context, rather than a direct igneous-meta- and allows additional influx of cold seawater. As mound flank morphic source as argued by Hovland, is summarised in Warren collapse expands the remaining detrital pyritic sand residues (2016, Chapter 5). are replaced by oxyhydroxides, and copper sulphides tend to be A Hovland model of a non-evaporite source of halite relies on oxidised and replaced by atacamite (Knott et al., 1998). If sea- heated subsurface brines becoming supercritical and so trans- floor weathering continues to completion, all the metal sulphides forming a brine to a fluid that does not dissolve but precipitates become oxidized or dissolved. Only those metalliferous VHMS salt (within specific temperature and pressure ranges). A super- deposits capped by impermeable volcanic, volcaniclastic, or critical fluid is defined as any substance at a temperature and sedimentary deposits soon after formation are preserved due to pressure above its critical point. In such a state, it can effuse shielding from the oxidising conditions at the deep seafloor. through solids like a gas, and dissolve materials like a liquid. In In all cases, VHMS deposit styles of mineralisation, along with 1 Serpentinite is a rock composed of one or more serpentine group min- associated anhydrite precipitation, are allied to submarine volca- erals, the name originating from the similarity of the texture of the rock to that nism and hydrothermally-driven circulation of seawater within of the skin of a snake. Minerals in this group, which are rich in magnesium and adjacent deepwater sediments. Hydrothermal anhydrite typifies water, light to dark green, greasy looking and slippery feeling, are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock mineralisation in a variety of tectonic settings and sediment from the Earth's mantle. The mineral alteration is particularly important at the sea types (more detail on deposit styles and their anhydrites are giv- floor at tectonic plate boundaries (Wikipedia) Page 4 www.saltworkconsultants.com

1.0 Brine @ 30MPA 100 Phillips et al. 1981 0.8 Solidus MD brine 0.6 MD water 200 250 bar line 0.4 (salting out) 2-phase region

Pressure (bar) Pressure 2-phase Density (gm/cc) 300 region Lower solidus Upper 0.2 solidus 8 pt 400 0.0 200 400 600 800 275 475 675 875 1075 1275 1475 A. Temperature (°C) B. Temperature (kelvin)

100 Normal Near- Super- Super-heated hot critical 50 critical super-critical

(Wt%) water water water water solubility

Hydrocarbon 0 25°C 100 200 300 400 500°C “Salting out” 100

50 (Wt%) solubility Inorganic 0 C. 25°C 100 200 300 400 500°C

Figure 4. Hydrothermal halite derived from supercritical seawater. A) P-T projection of the monovariant solid-liquid-vapour saturation curve (solidus) for the NaCl-water system. Arrows indicate the two points of intersection with the section at 250 bar (defining the lower and upper solidus boundaries). B) Density of water and brine as a function of temperature along the 300 bar isobar. The two-phase region (or “out- salting region”) is indicated by the shaded region with onset indicated by a drastic fall in density over a narrow temperature range. C) Ionic and hydrocarbon solubility in heated water at pressures of 200-300 bars. (A-B after Hovland et al., 2006a, b; C after Josephson, 1982; Simoneit, 1994). addition, close to the critical point, small changes in pressure or sures and perhaps even in the deeper portions of salt structures. temperature result in substantial changes in density. The critical The same supercritical conditions improve the ability of brines point (CP), also called a critical state, specifies the conditions to carry high volumes of hydrothermal hydrocarbons prior to the (temperature, pressure and sometimes composition) at which a onset of supercritical conditions (Josephson, 1982; McDermott phase boundary ceases to exist. At particular pressure/tempera- et al., 2018). Supercritical water has enhanced solvent capacity ture conditions, supercritical water is unable to dissolve/retain for organic compounds and reduced solvation properties for ion- common sea salts in solution (Josephson, 1982; Bischoff and ic species due to its loss of aqueous hydrogen bonding (Figure Pitzer, 1989; Simoneit 1994; Hovland et al., 2006a). 4c; Simoneit, 1994). When seawater brines are heated in pressure cells in the labo- Hovland et al. (2006a, b) predict that some of the large volumes ratory, they pass into the supercritical region at a temperature of deep subsurface salt found in the Red Sea, in the Mediter- of 405°C and 300 bar pressure (the CP of seawater). A partic- ranean Sea and the Danakil depression, formed via the forced ulate ‘cloud’ then forms via the onset of ‘shock crystallization’ magmatically-driven hydrothermal circulation of seawater down of NaCl and Na2SO4 (Figure 4a). The sudden phase transition to depths where it became supercritical. This salt, they argue, occurs as the solubility of the previously dissolved salts declines was precipitated deep under-ground via “shock crystallisation” to near-zero, across a temperature range of only a few degrees, from a supercritical effusive phase and so formed massive accu- and is associated with a substantial lowering of density (Figure mulations (mostly halite) typically in crustal fractures that facili- 4b). The resulting solids in the “cloud” consist of amorphous tated the deep circulation. NaCl then flowed upwards in solution microscopic NaCl and Na2SO4 particles with sizes between 10 in dense, hot hydrothermal brine plumes, precipitating more sol- and 100 mm. The resultant “salting out” can lead to the precip- id salt beds upon cooling nearer or on the surface/seafloor (halite itation of large volumes of subsurface salts in fractures and fis- is a prograde salt). More recently, Scribano et al. (2017) and

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Hovland et al. (2018a, b) have theorised that serpentinisation is 93 pp, with an average Au:Ag ratio of 0.15. Gold precipitation the dominant source of halite in the Messinian succession of the is directly associated with diffuse flow through anhydritic‘‘bee- Mediterranean. hive’’ chimneys. Significant mass-wasting of sulfide material in the vent field, accompanied by changes in metal content results To date, the Hovland et al. model of hydrothermal sourcing for in metalliferous talus and interfingering with deep marine sedi- widespread halite from a supercritical brine source (in active ment deposits (Figure 5b, c, d, e). magmatic settings) has not been widely accepted by the geological community (Talbot, 2008; Warren., 2016; Aftabi and At- All the high-temperature endmember fluids venting at the Beebe apour, 2018). To date, no direct indications of the formation of site show Cl levels that are significantly lower than seawater, masses of halite formed by this process have been sampled. In with an average endmember concentration of 349 mmol/kg. Due 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). A. B. 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 su- C. D. percritical seawater venting mean such sites are rare on the seafloor. The deep- 4850 est thermal upwelling site where supercritical seafloor conditions are thought Beebe vent eld Detrital sump to be active just below the upwelling 4900 site is the Beebe vent field (Figure 5; East Webber et al., 2015; McDermott et 4950 Cu, Zn al., 2018). At 4960 m below sea level, galvanic Graded beds, the vent field sits atop the ultra-slow interaction 5000 ne-grained, spreading Mid Cayman Rise and is the Depth (mbsl) metalliferous world’s deepest known hydrothermal mass wasting sediment exhalative system. Situated on very 5050 thin (2–3 km thick) oceanic crust at Stockwork? an ultraslow spreading centre, this Ma c and ultrama c host hydrothermal system circulates fluids 0 100 200 300 400 500 to depths ≈1.8 km in a basement that E. East Distance (m) West is likely to include a mixture of both mafic and ultramafic lithologies (Web- Figure 5. Beebe Vent Field (after Webber et al., 2015). A) Close-up of beehive style chimneys, white ber et al., 2015). areas are shrimp covered, dark areas are too hot for shrimp to inhabit. Chimneys are 50-100 cm wide. B) Small chimneys growing from an inactive sulphide block made up of amalgamated extinct chimneys. The surface of the active vent field is Image about 2 metres wide. C) Talus slope located immediately below a zone of active and inactive made up of high temperature (≈401°C) chimneys, individual chimneys in blocks are no longer discernible. Base metals are largely leached from material in this zone (10cm laser sights for scale. F) Thick beds in more distal sump positions that anhydritic ‘‘black smokers’’ that build coring shows are composed of graded stacks of turbiditic fine-grained metalliferous sediments, beds Cu, Zn and Au-rich sulfide mounds and drape pillow lavas. E) Schematic cross section showing dominant processes at Beebe vent field. Purple chimneys (Figure 5a). The vent field is indicates solid sulphide mounds, pink indicates draped mound talus undergoing galvanic interaction highly gold-rich, with Au values up to with seawater that is leaching the base metals. Yellow indicates metalliferous turbiditic fine-grained graded beds that are interbedded with talus. Page 6 www.saltworkconsultants.com

to the lack of a significant sink for Cl within Process Example Reference mafic-hosted subsurface circulation pathways, (indicator mineral) Cl depletions in vent fluids are typically at- Scapolitization Betic Cordilleras, southern Torres-Roldan (1978) tributed to phase separation (McDermott et (scapolite) Spain Gómez-Pugnaire et al. (1994) (Na,Ca) [(Al,Si) O ] (Cl,CO ) al., 2018). Thus, the intrinsic Cl depletions, in 4 4 8 3 3 Corella and Staveley Formations, Oliver (1995) conjunction with a seafloor pressure of 496 bar, Mt Isa Block, Australia Stewart (1994) places the two-phase boundary at 483°C, sug- Seve Nappe Complex, northern Svenningsen (1994) gesting that escaping fluids experienced a phase Sweden Frietsch et al., 1997 separation at conditions that are both hotter and Proterozoic Bamble and Engvik et al., 2011 deeper (higher pressure) than the critical point Modum-Kongsberg sectors, South Norway for seawater at 407°C and 298 bar (Bischoff, 1991). Wernecke breccia complex, Yukon Hunt et al. (2005) Canada Kendrick et al. (2008) During incipient phase separation from super- Paleoproterozoic magnesites, Ceara, Parente et al., 2004 critical seawater, a small amount of high-salini- NE Brazil ty brine it thought to condense in the subsurface Granulite complex, Dronning Markl and Piazolo (1998) as a separate phase, so creating the Cl-depleted Maud Land, Antarctica residual fluid, or vapour phase. The Cl-depleted Zambesi Orogenic Belt, Zambia Hanson et al. (1994) fluids venting at Beebe are thought to represent Albitization Proterozoic Paranoa Group, Giuliani et al. (1993) this vapour phase. Although a vent fluid of sea- (albite) Goias, Brazil NaAlSi O water chlorinity is not a supercritical fluid at the 3 8 Olary Block, Australia Cook and Ashley (1992) conditions of seafloor venting (398°C, 496 bar), Heimann et al., 2013 the vent fluids indicate sourcing from a super- Starra Gold deposit, Cloncurry region, Davidson et al., 1994 critical phase owing to their lower chlorinity Australia (Bischoff and Pitzer, 1989). Sodic talcs and/or phlogopites Metapelites, Tell Atlas Algeria Schreyer et al. (1980) Phase relations in the system NaCl-H O (Bis- Manganese-rich talc, Fowler Talc Belt, Ayuso and Brown, 1984 2 New York choff, 1991) can be used to estimate the minimum temperature of phase separation at the Sodic talc- kyanite, Sar-e-Sang, Af- Schreyer and Abraham, 1976 ghanistan Beebe site, based on the chlorinity of the va- Permian evaporites, New Mexico and Bailey, (1949); Evans (1970) pour phase and the assumption that phase Carboniferous evaporites, Nova Scotia separation occurs at or below the seafloor. A minimum temperature of 491°C is required to Phlogopites and pegmatites, southern Morteani et al., 2013 produce the measured Cl concentration of 349 Madagascar mmol/kg observed in the Beebe Vents endmem- Stratiform anorthositization Grenville Series, New York Gresens (1978) ber fluids. Accordingly, the observed Cl deple- (plagioclase) (Na,Ca)Al(Si,Al)Si O tion in the high-temperature endmember fluids 2 8 Oaxacan granulite, Mexico Ortega-Gutierrez (1984) implies that these fluids must have cooled by at Dostal et al. (2004) least 90°C prior to venting, and perhaps more Tourmalinitization Barberton Greenstones, South Africa Byerly and Palmer (1991) (tourmaline) (McDermott et al., 2018). For example, if the Willyama Supergroup, Slack et al. (1989) Na(Mg,Fe) Al (BO ) (Si O ) location of phase separation was 1000 m deep- 3 6 3 3 6 18 Broken Hill Block, Australia Plimer (1988) (OH)4 er, then that would require maximum fluid tem- Liaoning borates, China Peng and Palmer (1995) peratures in the vicinity of separation of 535°C. Minas supergroup, Brazil Trumbull et al., 2018 The only salt that can be confused with an Lapis Lazuli Sar-e-Sang whiteschists, Afghanistan Faryad, 2002 evaporite salt at the Beebe site is hydrothermal (lazurite) Baffin Island, Canada Hogarth and Griffith (1978) anhydrite. Any halite derived from seawater (Na,Ca)8(AlSiO4)6(SO4,S,Cl)2 reaching its supercritical point is still locat- Edwards, New York Hogarth (1979) ed in fissures many hundreds of metres below Columbian emerald Columbia, S. America Giuliani et al. (1995, 2017) the surface and is as yet unsampled. Likewise, (gem-tourmaline) there are no halite-saturated brine ponds on the Tsavorite Bright green to emerald green varieties Feneyrol et al. (2012, 2013) seafloor and the smoker anhydrite, like much (garnet) of vanadium-enriched grossular Ca Al (SiO ) garnet, Tanzania of the metalliferous content, has a low preser- 3 2 4 3 vation potential as it is being leached back into Marble-hosted rubies in e.g. Jegdalek Fm (Afghanistan); Garnier et al., 2008 Southeast Asia Mogok Fm. (Myanmar) seawater via galvanic interaction (Webber et al., 2015). Fe-Ni-Cu sulphide and uranium Munali region, Zambia Evans (2017) ores in evaporite metasediments Eglinger et al. (2014) Herein is the problem for assessing the viabil- Table 1. Examples of some meta-evaporite processes and indicator minerals. Although ity of a Hovlnd-style model for halite. Where this table lists separate examples of indicators, most examples have multiple indications in is the evidence and the data? Until significant the same deposits for example, scapolite and albitite are common co-associated mineral phases (see Warren, 2016, Chapter 14 for details). 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volumes of hydrothermal halite are intersected somewhere on Temperature (°C) the earth's surface, there is not a working example, only a sophisticated reinterpretation of existing halite occurrences. Mod- 0 100 200 300 ern seawater (rather than an experimental NaCl-H2O system) 0 A. Isolated dihedral uid 0 would give not just halite but also Na2SO4 at its critical point, where are the volumes or texural and mineralogical indications of these salts, or their brines and alteration haloes? Until there 20 is the physical proof of a working example of substantial hydro- θ>60° thermal halite sourced in supercritical phase separations, I prefer 1 to apply 2Occam's Razor. 40 Burial Halite alteration, renewed deep brine flow 60 and metamorphism 3 Depth (km) A source of chlorine-rich hydrothermal fluid (not halite) in the 80 deep subsurface is the recycling of deeply buried sedimentary (MPa) Pressure mega-halite units into the greenschist realm and beyond (Yard- θ<60° ley and Graham, 2002). In the metamorphic realm (T>200°C) 100 the derived fluids do not precipitate halite, but a series of me- 4 ta-evaporite indicator minerals (Table 1). Lewis and Holness B. Connected dihedral uid (1996) demonstrated that buried salt bodies, subjected to high 120 pressures and elevated temperatures, can acquire a permeability Figure 6. Effect of dihedral angle on pore connectivity in texturally comparable to that of a sand, within what is sometimes called equilibrated monomineralic and isotopic polycrystalline mosaic halite. the "Holness zone". This is because the crystalline structure of Background shading shows two polyhedral porosity fields and transition deeply buried salt (halite) attains dihedral angles between salt zone calculated for the Ara Salt (black salt or leaking halite plots in crystals of less than 60 degrees, and so creates an impermeable transition zone), Oman. A) Isolated porosity for dihedral angle > 60°. B) Connected polyhedral porosity for dihedral angle < 60° (after Lewis polyhedral meshwork (Figure 6). Such conditions probably be- and Holness, 1996; Schoenherr et al., 2007a,b; Kukla et al. 2011). gin at the onset of greenschist P-T conditions, whereby highly-saline hot brines form continuous brine stringers around all where they infer this process is active (Hovland et al., 2018a), such altered and recrystallizing salt crystals. the seismic indicates the evaporite is bedded and faulted, while the evaporite textures recovered in cores and doline/uplift land- This polyhedral permeability meshwork allows hot dense brines forms across the saltflat surface combine to show Holness-zone or hydrocarbons to migrate through salt (Schoenherr et al., 2007a, halokinesis is not segregating the halite, kainite, carnallite, syl- b) and ultimately dissolve the salt host, releasing a pulse of sod- vite and bischofite salts that typify the region around the Dallol ic- and chloride-rich fluid into the metamorphic realm (Warren, Mound (Bastow et al., 2018; Warren, 2016, Chapter 11). 2016; Chapter 14). It is why little or no evidence of solid masses of metamorphosed halite is found in subsurface meta-evaporitic Beyond the greenschist facies and the polyhedral transition of settings where temperatures have exceeded 250 - 300 °C, even sedimentary/halokinetic halite, metamorphic minerals with an though the melting point of halite is 800°C. Given the right sub- evaporite protolith tend to be enriched in minerals entraining so- surface conditions these halite-derived metamorphic brines may dium, potassium and magnesium (Figure 7; Table 1; Yardley and evolve into supercritical waters. Graham, 2002). These metamorphic minerals (meta-evaporites) can entrain high levels of volatiles (Cl, SO and CO ) as well as Contrary to conventional geological modelling of salt in diapirs 3 2 elevated levels of boron, along with high salinity in the associ- being mostly impermeable, Hovland et al. (2018a) argue for the ated metamorphic fluids; all indicate their evaporitic protolith formation of salt stocks by hot brines migrating upward through (Table 1; Figure 7). the middle of the salt body; provided that the salt stock is situated within the "Holness zone." This assumes that "Holness Sodium tends to come from the dissolution of salts, such as ha- zone" flows brine and can also reach subcritical conditions. The lite, kainite or trona; while magnesium tends to be remobilized inferred rising flow of intrasalt hot brines then reach saturation from earlier diagenetic minerals, such as reflux dolomites,mag- upon cooling in the upper part of the salt stem, where solid salts nesium-rich evaporitic clays and some potash minerals (Table are precipitating according to their specific solubility at each 1). Boron in tourmalinites may have come from a colemanite/ particular temperature and pressure interval. The Hovland mod- ulexite lacustrine precursor. Once direct evidence of a salty pro- el thus includes a refining process in the salt stem, where halite, tolith is largely removed via fluid dispersion in burial and ongo- for example, precipitates upon cooling long before calcium and ing loss of volatiles, the palaeo-evaporite indications are restrict- magnesium chloride salts. However, in the Danakhil Depression, ed mostly to mineralogic associations, along with an occasional textural relict of a former evaporitic breccia bed, rauwacke or 2 Occam's Razor; No more things should be presumed to exist than are salt weld (Warren, 2016; Chapters 7, 14). absolutely necessary, i.e., the fewer assumption an explanation of a phenomenon depends, the better the explanation (William of Ockham (Gulielmus Occamus), Evidence of early stages in an evaporite-fed sodic transformation 1287-1347) is seen in the sodic phlogopites (phlogopite = magnesian mica) Page 8 www.saltworkconsultants.com

NaCl (equiv. wt. %) taining Mesoproterozoic calcsilicates 0 10 20 30 40 50 0 1. Brines in Permian evaporites, Salado Fm, New Mexico in the Oaxacan granulite complex in A. 1. 2. Southern North Sea formation waters 3. southern Mexico (Ortega-Gutierrez, 2. 3. Northern North Sea formation waters 100 5. 4. Central North Sea formation waters 1984) and the pervasive scapolites in 4. 5. Mississippi oil eld brine, sediments above Salt Dome Basin the Neoproterozoic Zambesian oro- 6. 6. Cements in Lower Cretaceous arkoses, oshore Angola 200 7. Thrust belt uids, quartz veins in Mesozoic sediments, Pyrenees genic belt of Zambia (Hanson et al., Approx. halite saturation 8. Columbian emerald-bearing veins in evaporitic metasediments 9. Post-metamorphic Alpine Clefts, central European Alps 1994). Subsequent work on the Gren- dehydrationtrend7. 300 10. Syn-metamorphic quartz veins, Harlech Dome, North Wales ville anorthosites, although still al- 11. Veins in metasediments, Lukmanier Pass, Switzerland lowing for a metasedimentary proto- 10. 9. 8. 12. Damara belt metasediments, Namibia 400 13. Veins in Waterville Fm. metasediments, Maine lith, has concluded an igneous source

Temperature (°C) Temperature 14. Veins in Sangerville Fm. metasediments, Maine 11. 15. Veins in metasediment, Vermont of volatiles is more likely (Moecher 12. 500 13. 15. 16. Veins in dolomitic marble, Campolungo, Italy et al., 1992; Peck and Valley, 2000; 14. 16. 17. Veins in Dalradian metasediments, Connemara, Ireland 18. Eclogite, Dora Maira Massif, Italy Glassly et al., 2010). Defining the 600 19. UHP Eclogite, Dabie Shan, China likelihood of an evaporite protolith 17. 18. dehydration becomes increasingly difficult as the trend A. Encloses all the low salinity elds of formation waters, springs 700 and metamorphic uids hosted by sequences of oceanic or metamorphic grade increases. Once a 19. accretionary prism origin. metamorphic rock enters the granulite Figure 7. Temperature–salinity plot for formation waters and metamorphic fluids hosted by sequences facies, its protolith interpretation is deposited in epeiric seas and at continental margins (after Yardley and Graham, 2002). Boxes reflect the range of data from each of the numbered sources listed, many of the meta-evaporite examples typically much more contentious (e.g. are discussed in Warren 2016. The box, designated A, clearly shows that meta-oceanic and meta-ac- the evaporite versus carbonatite inter- cretionary wedges lack high NaCl/salinity waters. The red arrows show i) the typical dehydration pretations in the Oaxacan granulites, trend in metamorphic successions lacking a widespread evaporite protolith and ii) the high salinity Mexico). rehydration trend within deeply buried successions subject to the elevated temperatures and high salinities created in volatiles released during by eclogite formation (not necessarily evaporitic protolith). Hydrothermal gypsum and sodian aluminian talcs in the metapelites of the Tell Atlas Some of the most visually striking ex- in Algeria (Schreyer et al. 1980). Evaporitic sulphate crystals amples of hydrothermal gypsum precipitation are in the Naica are pseudomorphed in the NaCl-scapolite-dominated sequenc- mine, Chihuahua, Mexico (Figure 8). There several natural caves of the Cordilleras Beticas of Spain (Gómez-Pugnaire et al. erns, such as Cave of Swords (Cueva de la Espades discovered 1994). Rocks of higher temperature and pressure facies, such as in 1975) and Cave of Crystals (Cueva de los Cristales discovered the massive stratiform anorthosites in the Grenville Precambrian in 2000), contain giant, faceted, and transparent single crystals Province of North America, have been interpreted as possible of gypsum as long as 11 m (Figure 9a; García-Ruiz et al., 2007; meta-evaporites (Gresens, 1978), as have the anhydrite-con- 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 com- positions 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-me- diated, anhydrite-gypsum phase transition. Nucleation kinetics calculations based on laboratory data show that this mechanism can account for the forma- Figure 8. These giant gypsum crystal meshworks in the Cueva de los Cristales, Naica region, Mexico tion of these giant crystals, yet only were first discovered in 2000 after water was pumped out of the phreatic cavern system as part of when operating within a very narrow a silver mine's expanding operations (image courtesy of Penn State (https://science.howstuffworks. com/environmental/earth/geology/mexico-giant-crystal-cave.htm). range of temperature of a few degrees Page 9 www.saltworkconsultants.com

as identified by the fluid inclusion values. Cave of Swords (Espadas) Fluid inclusion analyses show that the giant crys- SW NE tals came from low-salinity solutions at tempera- 0 m Original tures ≈ 54°C, slightly below the temperature of phreatic level 58°C where the solubility of anhydrite equals that -130 m of gypsum (Figure 9b; García-Ruiz et al., 2007). Cave of Van Driessche et al. (2011) argue the slowest gyp- Crystals sum crystal growth in the phreatic cavern occurred -290 m Naica fault 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 level in the cave by one degree, to 56° C, the same size Current water crystal could have formed in a little less than half (drawndown to the time, or around 500,000 years. This possible allow mining)

Fault growth rate would work out to about a billionth Gibraltar of a meter of growth per day and is perhaps the -790 m Ore bodies slowest growth rate that has ever been measured. A. 500 m

Garofolo et al., 2010, accept the need for a limited 8

temperature range during precipitation, but argue 6

the precipitating solutions were in part meteori- 4 cally influenced. Their work focused on Cueva de 2 Number las Espadas. As for most other hypogenic caves, 0 prior to their analytical work, they assumed that B. 30 40 50 60 70 caves of the Naica region lacked a direct connec- Temperature (°C) tion with the land surface and so gypsum precipitation would be unrelated to climate variation. 4000 Yet, utilising a combination of fluid inclusion and Espadas high- pollen spectra data from cave and mine gypsum, -1 3000 salinity waters they concluded climatic changes occurring at Nai- ca exerted an influence on fluid composition in the /mg l 2000 Gypsum Ca

Espadas caves, and hence on crystal precipitation a Supersaturation Deep phreatic and growth. M 1000 ixin thermal waters g tren Microthermometry and LA-ICP-Mass Spectrom- d of two uids etry of fluid inclusions in the gypsum in the Cue- 0 va de las Espadas indicate that brine source was 0 400 800 1200 a shallow, chemically peculiar, saline fluid (up to Gypsum -1 aSO4/mg l 7.7 eq. wt.%NaCl) and that it may have formed C. Undersaturation via evaporation, during an earlier dry and hot cli- Figure 9. Naica Mine, Mexico. A) Cross section of Naica mine. Mine exploits a hydro- matic period. In contrast, the fluid of the deeper thermal Pb-Zn-Ag deposit with irregular manto and pipe morphologies entirely enclosed in subhorizontally dipping carbonates. Cavities of gypsum crystals are located in caves (Cristales) was of lower salinity (≈3.5 eq. carbonates close to main and secondary faults. Galleries have been excavated down wt.% NaCl) and chemically homogeneous, and to –760 m, requiring average pumping rate of 55 m3/min to depress groundwater to likely was little affected by evaporation process- –580 m with respect to phreatic level located at −120 m; Naica and Gibraltar faults act es. Galofolo et al. (2010) propose that mixing of as main drains. B) Homogenization temperatures of 31 fluid inclusions showing actual temperature of growth. C) Solubility of gypsum calculated at 55°C and 105 Pa and these two fluids, generated at different depths of measured activities of shallow and deep cave fluids from fluid inclusion data. Mixing at the Naica drainage basin, determined the stable equilibrium between these two fluids in any proportion generates a fluid with a composition supersaturation conditions needed for the gigantic would consistently supersaturated with respect to gypsum, as shown by the position of gypsum crystals to grow (Figure 9c). The hydrau- the mixing curve, indicated by a dashed line, in the gypsum supersaturation field (after lic communication between Cueva de las Espadas García-Ruiz et al., 2007; Garofolo et al., 2010)) and the other deep Naica caves controlled fluid period; the debate continues as to whether the gypsum at Naica mixing. Mixing must have taken place during alternating cycles is a mixing zone or a hydrothermal salt. of warm-dry and fresh-wet climatic periods, which are known to have occurred in the region. Pollen grains from 35 ka-old gyp- Solar versus nonsolar salts sum crystals from the Cave of Crystals indicates a relatively ho- This and the previous article show that substantial volumes of mogenous catchment basin dominated by a mixed broadleaf wet various salts (especially retrograde anhydrite) form in the ter- forest. This suggests precipitation during a fresh-wet climatic restrial subsurface, independent of solar evaporation. Except for some bedded cryogenic salt bodies (e.g., Korabogazgol in Page 10 www.saltworkconsultants.com

Kazakhstan or Noachian lakes on Mars), non-solar evaporation Byerly, G. R., and M. R. Palmer, 1991, Tourmaline mineraliza- salts tend to nucleate in subsurface fractures, and breccia inter- tion in the Barberton greenstone belt, South Africa; early Arche- spaces in the igneous and metamorphic They tend to be cavi- an metasomatism by evaporite-derived boron: Contributions to ty cements, but can also replace portions of a pre-existing salt Mineralogy and Petrology, v. 107, p. 387-402. mass. On Earth, the most widespread non-solar salt is anhydrite Cook, N. D. J., and P. M. Ashley, 1992, Meta-evaporite sequence, with occurrences ranging volcanic hosted mid-ocean ridge to exhalative chemical sediments and associated rocks in the Pro- Kuroko style deposits in subduction zones. In all cases, the in- terozoic Willyama Supergroup, South Australia: implications for timate association with submarine volcanics and fissures where metallogenesis: Precambrian Research, v. 56, p. 211-226. hydrothermally heated seawater once circulated mean this type of salt is readily distinguished from sedimentary anhydrite. Davidson, G. J., and R. R. Large, 1994, Gold metallogeny and the copper-gold association of the Australian Proterozoic: Min- For halite, there is little direct evidence of any massive halite oc- eralium Deposita, v. 29, p. 208 - 223. currence in outside of sedimentary basins where isolated-sumps of ponded brine were once evaporated. A sophisticated notion Dostal, J., J. Keppie, H. Macdonald, and F. Ortega-Gutiérrez, theorising hydrothermal halite has been published by Hovland 2004, Sedimentary Origin of Calcareous Intrusions in the ~1 Ga and co-workers (e.g. Hovland et al., 2018a b; Scribano et al., Oaxacan Complex, Southern Mexico: Tectonic Implications: In- 2017) to explain some halite occurrences in tectonically-active ternational Geology Review, v. 46, p. 528-541. areas. There is little support for this model in the published lit- Eglinger, A., C. Ferraina, A. Tarantola, A.-S. André-Mayer, O. erature outside of Hovland and co-workers. Thick buried solar Vanderhaeghe, M.-C. Boiron, J. Dubessy, A. Richard, and M. halite masses tend form in particular stages of the Wilson Cy- Brouand, 2014, Hypersaline fluids generated by high-grade cle. Once buried, these evaporite masses are mostly impervi- metamorphism of evaporites: fluid inclusion study of uranium ous, they can flow and dissolve, and on entry into the- green occurrences in the Western Zambian Copperbelt: Contributions schist realm can become permeable, so feeding large volumes of to Mineralogy and Petrology, v. 167, p. 1-28. highly saline brines into the metamorphic and igneous realms. These brines can drive metal accumulations and the formation of Engvik, A. K., K. Mezger, S. Wortelkamp, R. Bast, F. Corfu, characteristic meta-evaporitic minerals and gemstones (Warren, A. Korneliussen, P. Ihlen, B. Bingen, and H. Austrheim, 2011, 2016). Metasomatism of gabbro – mineral replacement and element mobilization during the Sveconorwegian metamorphic event: References Journal Of Metamorphic Geology, v. 29, p. 399-423. Aftabi, A., and H. Atapour, 2018, Comment on the papers by Evans, D. M., 2017, Fe-Ni-Cu sulfide-evaporite association at Hovland et al., 2018b, Hovland et al., 2018a “Large salt accu- Munali, Zambia: Society of Geology Applied to Mineral Depos- mulations as a consequence of hydrothermal processes associ- its Biennial Meeting, Québec City, Province de Québec. ated with 'Wilson cycles': A review” (part 1 and 2): Marine and Petroleum Geology, v. 98, p. 890-897. Evans, R., 1970, Genesis of sylvite- and carnallite-bearing rocks from Wallace, Nova Scotia: Third Symposium on Salt, v. 1, p. Ayuso, R. A., and C. E. Brown, 1984, Manganese-rich red tour- 239-245. maline from the Fowler talc belt, New York: Canadian Mineral- ogist, v. 22, p. 327-331. Faryad, S. W., 2002, Metamorphic Conditions and Fluid Com- positions of Scapolite-Bearing Rocks from the Lapis Lazuli De- Bastow, I. D., A. D. Booth, G. Corti, D. Keir, C. Magee, C. A.-L. posit at Sare Sang, Afghanistan: Journal of Petrology, v. 43, p. Jackson, J. Warren, J. Wilkinson, and M. Lascialfari, 2018, The 725-747. development of late-stage continental breakup: Seismic reflec- tion and borehole evidence from the Danakil Depression, Ethio- Feneyrol, J., G. Giuliani, D. Ohnenstetter, A. E. Fallick, J. E. pia: Tectonics, v. 37. Martelat, P. Monié, J. Dubessy, C. Rollion-Bard, E. Le Goff, E. Malisa, A. F. M. Rakotondrazafy, V. Pardieu, T. Kahn, D. Bischoff, J. L., and K. S. Pitzer, 1989, Liquid-vapor relations Ichang'i, E. Venance, N. R. Voarintsoa, M. M. Ranatsenho, C. for the system NaCl-H2O; summary of the P-T-x surface from Simonet, E. Omito, C. Nyamai, and M. Saul, 2013, New aspects 300 degrees to 500° C: American Journal of Science, v. 289, p. and perspectives on tsavorite deposits: Ore Geology Reviews, v. 217-248. 53, p. 1-25. Bischoff, R., 1991, Lithological interpretation of a gas cavern Feneyrol, J., D. Ohnenstetter, G. Giuliani, A. E. Fallick, C. Rol- well in Zechstein evaporites of the Lesum salt dome (Bremen, lion-Bard, J.-L. Robert, and E. P. Malisa, 2012, Evidence of Germany): Oil Gas European Magazine, v. 17, p. 16-19. evaporites in the genesis of the vanadian grossular tsavorite de- Blount, C. W., and F. W. Dickson, 1969, The solubility of anhy- posit in Namalulu, Tanzania: The Canadian Mineralogist, v. 50, p. 745-769. drite (CaSO4) in NaCl-H2O from 100 to 450° C and 1 to 1000 bars: Geochimica et Cosmochimica Acta, v. 33, p. 227-245. Frietsch, R., P. Tuisku, O. Martinsson, and J. Perdahl, 1997, Blum, N., and H. Puchelt, 1991, Sedimentary-hosted polymetal- Early Proterozoic Cu-(Au) and Fe ore deposits associated with lic massive sulphide deposits of the Kebrit and Shaban Deeps, regional NaCl metasomatism in northern Fennoscandinavia: Ore Red Sea.: Mineralium Deposita, v. 26, p. 217-227. Geology Reviews, v. 12, p. 1-34.

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García-Ruiz, J. M., R. Villasuso, C. Ayora, A. Canals, and F. Hogarth, D. D., and W. L. Griffin, 1978, Lapis lazuli from Baffin Otálora, 2007, Formation of natural gypsum megacrystals in Island; a Precambrian meta-evaporite: Lithos, v. 11, p. 37-60. Naica, Mexico: Geology, v. 35, p. 327-330. Holden, J. F., J. A. Breier, K. L. Rogers, M. D. Schulte, and B. M. Garnier, V., G. Giuliani, D. Ohnenstetter, A. E. Fallick, J. Toner, 2012, Biogeochemical Processes at Hydrothermal Vents Dubessy, D. Banks, H. Q. Vinh, T. Lhomme, H. Maluski, A. Microbes and Minerals, Bioenergetics, and Carbon Fluxes: Pecher, K. A. Bakhsh, P. Van Long, P. T. Trinh, and D. Schwarz, Oceanography, v. 25, p. 196-208. 2008, Marble-hosted ruby deposits from Central and Southeast Asia: Towards a new genetic model: Ore Geology Reviews, v. Hovland, M., H. K. Johnsen, and H. Rueslåtten, 2019, Salt-for- 34, p. 169-191. mation in rifting and subduction (Wilson cycles): Reply to Ali- jan Aftabi and Habibeh Atapour on their comments to our two Garofalo, P. S., M. B. Fricker, D. Günther, P. Forti, A.-M. Mer- articles: Marine and Petroleum Geology, v. 100, p. 554-558. curi, M. Loreti, and B. Capaccioni, 2010, Climatic control on the growth of gigantic gypsum crystals within hypogenic caves Hovland, M., T. Kuznetsova, H. Rueslatten, B. Kvamme, H. K. (Naica mine, Mexico)?: Earth and Planetary Science Letters, v. Johnsen, G. E. Fladmark, and A. Hebach, 2006a, Sub-surface 289, p. 560-569. precipitation of salts in supercritical seawater: Basin Research, v. 18, p. 221-230. Giuliani, G., A. Cheilletz, C. Arboleda, V. Carrillo, F. Rueda, and J. H. Baker, 1995, An evaporitic origin of the parent brines Hovland, M., and H. Rueslatten, 2009, Origin and permeability of Colombian emeralds; fluid inclusion and sulphur isotope evi- of deep ocean salts: Geophysical Research Abstracts, v. 11. dence: European Journal of Mineralogy, v. 7, p. 151-165. Hovland, M., and H. Rueslatten, 2009, Origin and permeability Giuliani, G., J. Dubessy, D. Ohnenstetter, D. Banks, Y. Branquet, of deep ocean salts: Geophysical Research Abstracts, v. 11. J. Feneyrol, A. E. Fallick, and J.-E. Martelat, 2017, The role of Hovland, M., H. Rueslåtten, and H. K. Johnsen, 2018a, Large evaporites in the formation of gems during metamorphism of salt accumulations as a consequence of hydrothermal processes carbonate platforms: a review: Mineralium Deposita, p. 1-20. associated with ‘Wilson cycles’: A review Part 1: Towards a new Giuliani, G., G. R. Olivo, O. J. Marini, and D. Michel, 1993, understanding: Marine and Petroleum Geology, v. 92, p. 987- The Santa Rita gold deposit in the Proterozoic Paranoa Group, 1009. 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Talbot, C. J., 2008, Hydrothermal salt--but how much?: Marine and Petroleum Geology, v. 25, p. 191-202. Tornos, F., J. M. Peter, R. Allen, and C. Conde, 2015, Controls on the siting and style of volcanogenic massive sulphide deposits: Ore Geology Reviews, v. 68, p. 142-163. Torres-Roldan, R. L., 1978, Scapolite-bearing and related calc-silicate layers from the Alpujarride Series, (Betic Cordil- leras of southern Spain); a discussion on their origin and some comments: Geologische Rundschau, v. 67, p. 342-355. Trumbull, R. B., G. M. Garda, R. P. Xavier, J. A. D. Cavalca- nti, and M. S. Codeço, 2018, Tourmaline in the Passagem de Mariana gold deposit (Brazil) revisited: major-element, trace-element and B-isotope constraints on metallogenesis: Mineralium Deposita. Van Driessche, A. E. S., J. M. García-Ruíz, K. Tsukamoto, L. D. Patiño-Lopez, and H. Satoh, 2011, Ultraslow growth rates of giant gypsum crystals: Proceedings of the National Academy of Sciences, v. 108, p. 15721. Warren, J. K., 2016, Evaporites: A compendium (ISBN 978-3- 319-13511-3): Berlin, Springer, 1854 p. Webber, A. P., S. Roberts, B. J. Murton, and H. M. R. S., 2015, Geology, sulfide geochemistry and supercritical venting at the Beebe Hydrothermal Vent Field, Cayman Trough: Geochem. Geophys. Geosyst., v. 16, p. 2661–2678. Yardley, B. W. D., and J. T. Graham, 2002, The origins of salinity in metamorphic fluids: Geofluids, v. 2, p. 249-256.

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