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Sulfide Minerals in Hydrothermal Deposits - Elements
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Sulfide Minerals in Hydrothermal Deposits

Hydrothermal ore deposits are large geochemical anomalies of sulfur and metals in the Earth’s crust that have formed at <1 to ~8 km depth. Sulfide minerals in hydrothermal deposits are the primary economic source of metals used by society, which occur as major, minor and trace elements. Sulfides also play a key role during magmatic crystallization in concentrating metals that subsequently may (or may not) be supplied to hydrothermal fluids. Precipitation of sulfides that themselves may have little economic value, like pyrite, may trigger the deposition of more valuable metals (e.g. Au) by destabilizing the metal-bearing sulfur complexes. We review why, where and how sulfide minerals in hydrothermal systems precipitate.

DOI: 10.2113/gselements.13.2.97

Keywords: hydrothermal ore deposits, sulfide minerals, fluids, metal solubility, precipitation

Introduction

Hydrothermal ore deposits are the major source of metals that are needed for our modern society. Magmatic crystallization generates hot aqueous fluids that migrate in the crust and that have the potential to form hydrothermal deposits. Metals can also be transported via heated basinal brines, modified meteoric water and seawater, and by metamorphic fluids. Precipitation of sulfides from hydrothermal fluids at temperatures typically ranging between 500 °C and 100 °C is the major process of base-metal concentration that allows the metals to be mined economically. Sulfide minerals in hydrothermal deposits are the primary source of the base metals Cu, Zn, Pb and a large number of other, lower-concentration metals that include Ag, As, Au, Bi, Cd, Co, Ga, Ge, In, Hg, Mo, Ni, Re, Sb, Se, Sn, Te, and Tl. These metals may form their own sulfides or occur as minor or trace elements in other sulfides and sulfosalts. Subsequent weathering leading to oxidation of pyrite, the most abundant sulfide in hydrothermal deposits, is the main process that generates acidic supergene (secondary) fluids that leach copper and other metals from the original sulfide-bearing hypogene (primary) ore bodies. These supergene-remobilized metals may eventually precipitate in enrichment zones and increase the value of the deposit, particularly copper ore deposits of initially low hypogene grade. On the other hand, some minor and trace elements in sulfides (e.g. As, Cd, Tl and Hg) may decrease the value of the ore concentrate because of their deleterious environmental effects. Acid rock drainage caused by weathering oxidation of sulfide minerals is a major environmental concern in many mines and after the closure of mine sites.

Table 1 and Table 2 summarize the key features of selected sulfide minerals and their hydrothermal ore deposits, and Figure 1 illustrates the variety of sulfide textures in them. The amount (vol%) of sulfide minerals in ores from the different deposit types is highly variable – from <1% in low-grade ores (e.g. porphyry deposits, Fig. 1A) to several 10s % in high-grade ores [e.g. massive sulfide and carbonate-replacement deposits (Figs. 1D, 1E)]. Crosscutting and other textural relationships of sulfide minerals, such as open-space infill (Fig. 1B) or replacement of other minerals (e.g. Figs. 1C, 1F), help researchers to recognize the sequence of sulfide precipitation stages and to trace the evolution of ore-forming fluids.

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Figure 1. Characteristic ore textures in some of the main types of hydrothermal sulfide deposits. (A) Sulfides in stockwork quartz veins or disseminated in the wallrock. From the Atlantida porphyry Cu–Au deposit (Chile). (B) Banded sulfides as open-space filling with gangue minerals, indicating episodic precipitation. From the Laki epithermal polymetallic Pb–Zn vein deposit (Bulgaria). (C) Sulfides associated with quartz, carbonates, and retrograde calc-silicate skarn minerals that replace prograde skarn pyroxene. From the Madan distal Pb–Zn skarn deposit (Bulgaria). (D) Sulfide minerals in the form of fine grained layers and showing syn-sedimentary features. From the Navan sediment–hosted Zn–Pb deposit (Ireland). (E) Fine grained, layered sulfides commonly associated with barite and anhydrite. The base metal sulfides usually postdate the Fe-sulfides. From the Rammelsberg sediment-hosted massive sulfide deposit (Germany). (F) Pyrite and chalcopyrite replacing the host rocks (preserving their original textures) or locally occurring as open-space filling in late veins. From the Punta del Cobre iron oxide–copper–gold (IOCG) deposit (Chile). Abbreviations: bar (barite); bn (bornite); carb (carbonates); cp (chalcopyrite); gn (galena); px (pyroxene); py (pyrite); sp (sphalerite); qtz (quartz).

This article briefly reviews why, where and how sulfide minerals (hereafter ‘sulfides’) precipitate in hydrothermal systems. These precipitation processes are key to understanding the formation of hydrothermal ore deposits – those fascinating and critically important concentration anomalies of sulfur and metals in the Earth’s crust.

Metal abundance in sulfides from hydrothermal ore deposits

Sulfides in hydrothermal ore deposits are of economic interest not only because of their major components, but also because they are the main carriers for some minor and trace metals. Their trace-element components have gained importance in recent times because some elements – Ge, Ga, In, Sn and Bi – are critical for the development of new technologies, everything from cell phones to renewable energy. Now, in situ element analyses of sulfide minerals, using LA–ICP–MS (laser ablation inductively coupled plasma mass spectrometry), provide accurate, rapid and low-cost multi-element data from common minerals, such as pyrite, sphalerite, chalcopyrite, galena and bornite (e.g. Cook et al. 2016).

Trace elements in sulfides occur in different chemical states, being either incorporated into the crystal structure of the major minerals or occurring as nanoparticles of other sulfides or sulfosalts that themselves are either homogeneously or selectively distributed in the host mineral. For example, sphalerite from various hydrothermal deposits may contain from a few ppm up to 1,000 ppm Ge, Ga, Co and Se; up to 10,000 ppm Sn, In, Cu, Ag and Sb; and even higher values for Mn, Cd and Fe (e.g. Cook et al. 2009). Galena can incorporate up to 1,000 ppm Sn, Cu, Cd, Se and Te and up to 10,000 ppm Bi, Sb and Ag (e.g. George et al. 2016). In contrast, chalcopyrite commonly has orders of magnitude lower concentrations of most trace elements, typically less than several 10s ppm (Sn, Ga, Sb, Te, Bi) or 100s ppm (Zn, Mn, Se, In) (George et al. 2016). Pyrite is the most common sulfide in hydrothermal ores and large data sets exist for porphyry, epithermal, orogenic gold, volcanic-hosted massive sulfide (VHMS) and sedimentary rock–hosted deposits (e.g. Large et al. 2014; Franchini et al. 2015). The most abundant elements in pyrite are Co, Ni, Cu, As and Zn, reaching >10,000 ppm levels, followed by Se, Mo, Ag, Sb, Pb (up to 1,000 ppm) and most scarce are Au, Bi, Sn (10s to 100s ppm).

The Role of Magmatic Sulfides as Traps or Source of Metals in Magmatic–Hydrothermal Systems

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Figure 2. Effects of magmatic sulfide saturation on trapping or providing metals in magmatic–hydrothermal systems. (A) Aqueous fluid saturation occurs before sulfide saturation in the magma. Metal content in residual melt increases because metals are not significantly sequestered by the crystallizing silicate ± oxide minerals (a1 → a3) and metals are optimally transferred to the magmatic fluid when saturation occurs. (B) Sulfide saturation occurs before aqueous fluid saturation in the magma. Metal concentrations initially increase until sulfide saturation occurs (b1→b2) then sharply decrease (b3). Transfer of metals to fluid is not optimal because significant proportions of metals have been sequestered by sulfides. (b4) Magmatic sulfides may be destabilized by aqueous fluids exsolved from the same magma in which sulfide saturation previously occurred. (b5) Magmatic sulfides may be destabilized by aqueous fluids exsolved from an underplated mafic magma. (b6) A sulfide-undersaturated mafic magma may recycle the sulfides and release the metals contained therein once the mafic magma becomes saturated in an aqueous fluid.

The magmatic component of the mineralizing fluids in many high- to moderate-temperature (>500 °C to ~150 °C) hydrothermal deposits is sourced in volcanic arcs (e.g. porphyry, skarn, epithermal, and Sn–W vein deposits. These fluids have metal budgets derived (almost) entirely from their parental magma (Cline and Bodnar 1991). A cooling and/or decompressing magma will saturate with respect to an aqueous phase once it reaches the limit of water solubility in the silicate melt, resulting in exsolution of a metal- and volatile (e.g. S, Cl, F, CO2)-bearing aqueous fluid phase (Fig. 2). Other physico-chemical conditions being equal (e.g. pressure, temperature, fluid composition, fluid–melt partition coefficients), the potential of a magma to exsolve a more-or-less metal-rich fluid primarily depends on the initial metal concentration in the silicate melt (Fig. 2): the richer in metals the magma is, the richer in metals will be the fluid exsolved from it. Chalcophile metals in magmatic–hydrothermal systems (e.g. Cu, Au) have a very high affinity not only for fluids (with aqueous fluid/silicate melt partition coefficients of ~101 to 102) (see Pokrovski et al. 2013), but also for sulfide melts (sulfide/silicate melt partition coefficients of 103 to 105). If sulfide melts form during the evolution of a magmatic system, then these tiny sulfide droplets can nearly control the entire budget of chalcophile metals in an evolving magma reservoir. Consequently, the timing of sulfide melt saturation with respect to aqueous fluid in a magma exerts a critical control on the exsolution of metal-rich versus metal-poor aqueous fluids (Candela and Piccoli 2005):

If aqueous fluid saturation occurs before magmatic sulfide saturation, the exsolved fluid will be metal-rich because it will strip metals from a metal-rich magma (Fig. 2A).

If aqueous fluid saturation occurs after magmatic sulfide saturation (Fig. 2B), the fluid will be generated from a melt already depleted in chalcophile metals, which may be unfavourable for the generation of metal-rich magmatic–hydrothermal systems.

The magmatic sulfides themselves can melt and be incorporated into subsequent magmatic pulses, thereby enriching the pulses in metals that can be later exsolved with aqueous fluids. Magmatic recycling of sulfide-rich cumulates in the lower crust (Chiaradia 2014) or upper mantle has been suggested for magmatic systems associated with porphyry deposits in collisional to post-collisional settings (e.g. Richards 2009). Alternatively, subsequent oxidation of magmatic sulfides can also supply metals to aqueous fluids (Fig. 2).

Sulfur and metal content of hydrothermal fluids

In contrast to bulk salinity (NaCl+KCl+CaCl2…), which is reliably determined by microthermometry of fluid inclusions, the concentrations of metals and sulfur in hydrothermal fluids became measurable when in situ methods such as LA–ICP–MS and PIXE were developed in the late 1990s. These methods can accurately analyse individual fluid inclusions trapped in minerals during growth or during post-growth deformation, providing quantitative determinations of the metal content to the ppm level (e.g. Heinrich et al. 2003). In combination with near-infrared microscopy, in situ methods have been extended to the analysis of fluid inclusions in opaque sulfide minerals (Kouzmanov et al. 2010). Quantification of sulfur concentrations in individual fluid inclusions is also possible (Guillong et al. 2008). An extensive literature now exists on the composition of ore-forming fluids from magmatic–hydrothermal systems, including porphyry, skarn and epithermal polymetallic deposits worldwide (see Kouzmanov and Pokrovski 2012). Metal contents of pristine magmatic fluids show patterns that are controlled by the metal abundances in the source magmas and by elevated fluid–melt partition coefficients. As a result, metal concentrations in pristine magmatic fluids typically range between 10s and 1,000s ppm, one to three orders of magnitude higher than the corresponding average metal crustal abundances. Second-order processes, such as separation of aqueous liquid and vapour phases, result in additional enrichment, with the brine phase containing up to several wt% Fe, Cu, Zn, Pb and Mn; thus these metals become major fluid constituents, along with Na and K. Concentrations of other metals of economic interest, such as Mo, As, Sb and Ag, vary between 10s and 100s ppm. Some of these metals, especially those transported as sulfide complexes (e.g. Au), may also be enriched in the vapour phase (Heinrich et al. 2003; Pokrovski et al. 2013). The sulfur content of magmatic–hydrothermal ore-forming fluids varies substantially, reaching wt% concentrations, and so indicating that the major ingredients for sulfide precipitation are available from the hydrothermal fluid itself and do not require an external source.

For VHMS, iron oxide–copper–gold (IOCG), sediment-hosted, and orogenic gold deposits, data pertaining to the mineralizing fluid compositions are still fragmentary. Nevertheless, existing datasets on some sediment-hosted deposits indicate that, despite the two orders of magnitude lower metal contents (10s to 100s ppm Zn and Pb) in basinal brines compared to magma-derived fluids, the reported measured concentrations are much higher than those predicted in the past (e.g. Wilkinson et al. 2009). These new findings sharply changed the understanding of transport mechanisms, of metal deposition efficiency, and of the time span of hydrothermal sulfide ore-forming processes.

Sulfur Sources and Main Reactions

Geological and isotope evidence indicates that sulfur in hydrothermal sulfides can have four sources: (i) segregated from a magma together with water; (ii) leached from sulfides disseminated in rocks (since most magmatic, sedimentary, and metamorphic rocks contain small quantities of sulfides, mainly pyrite and pyrrhotite); (iii) derived from sulfate minerals in evaporites; (iv) derived from seawater sulfate and basinal brines.

Sulfide precipitation can be described by the following schematic reaction (where aq = aqueous, Me = metal, s = solid)

Me2+(aq) + H2S(aq) = MeS(s)+ 2H+(aq)                 (1)

At least three of the four sulfur sources listed above contain sulfur in oxidized form. But, as shown by reaction (1), sulfur must be in its reduced state (S2−) to allow precipitation of sulfides. In addition to biogenic sulfate reduction (Rickard et al. 2017 this issue), there are two other sulfate reduction mechanisms that are key for the precipitation of hydrothermal sulfides.
Magmatic–hydrothermal fluids, depending on their oxidation state, contain different proportions of sulfur species, including H2S, HS, SO2, SO42−, HSO4, and S3. Hydrothermal fluids originating from calc-alkaline magmas are enriched in oxidized sulfur species, to a large extent made up of SO2. In the following disproportionation reaction, which occurs upon fluid cooling, S4+ undergoes both oxidation and reduction and provides the S2− ­necessary to form sulfides and the S6+ necessary to form sulfates such as alunite and anhydrite;

4SO2 + 4H2O = 3SO42− + 6H++ H2S                 (2)

In sedimentary environments in which dissolved sulfate (in basinal brines and other formation waters) encounters hydrocarbons at about 100 °C to 160 °C, sulfate is reduced inorganically by hydrocarbons (a process called ‘thermochemical sulfate reduction’, or TSR). In a simplified form, TSR can be represented as follows:

2H+ + SO42− + CH4 = CO2 + H2S + 2H2O                 (3)

Key Parameters Controlling Sulfide Precipitation in Hydrothermal Systems

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Figure 3. Solubility of metals in hydrothermal fluids in equilibrium (i.e. saturated) with major sulfide minerals as a function of the four parameters (A) temperature, (B) fluid salinity, (C) pH, and (D) H2S concentration. In each case, a number of physical and chemical parameters of the fluid are fixed (as indicated in the figure). Galena solubility, not shown, is slightly lower than that of sphalerite. Abbreviations: hm (hematite); py (pyrite); mt (magnetite). Modified from Kouzmanov and Pokrovski (2012).

Sulfide precipitation from hydrothermal fluids is governed by four main physico-chemical parameters across various ore deposit types: (1) temperature, (2) acidity (pH), (3) salinity, and (4) fO2fS2. Of key importance is the nature of the metal-transporting agents: base metals and Ag are commonly transported as chloride complexes, Mo as (alkali-)oxyhydroxide and possibly oxychloride complexes, and Au mainly as sulfide complexes. Figure 3 shows the influence of these four parameters on the solubility of chalcopyrite, pyrite, sphalerite, molybdenite, argentite, and native gold under conditions typical for a wide range of hydrothermal ore deposits. The solubility of all sulfides decreases with decreasing temperature. For instance, with other parameters constant, a temperature decrease from 350 °C to 250 °C triggers a solubility decrease of two orders of magnitude for Cu and Ag, and one order of magnitude for Zn (Fig. 3A). Similar effects on the solubility of metals transported as chloride complexes are caused by a salinity decrease (base metals and Ag), in contrast to that of Au (Fig. 3B). The solubility of base metals strongly decreases with increasing pH (Fig. 3C) and increasing H2S (Fig. 3D). By contrast, because gold is mainly transported as sulfide complexes, the removal of H2S from the fluid causes Au precipitation, typically through boiling in epithermal deposits and via pyrite formation by a reaction with Fe-bearing wallrock in orogenic-gold deposits.

Although the above four parameters explain most precipitation phenomena, recent data may open new perspectives. The last five years of hydrothermal research were marked by the recognition of a new form of aqueous sulfur in hydrothermal fluids: the trisulfur radical ion, S3 (Pokrovski and Dubessy 2015). The omission of S3 in current models of hydrothermal fluids is due to its very rapid breakdown to sulfate and sulfide in aqueous solutions when the solutions cool below 200–150 °C, which means that S3 in experimental and natural fluid (and melt) samples is not detected at room temperature. Pokrovski and Dubessy (2015) demonstrated the stability of this ion over a wide temperature (T = 200–700 °C) and pressure (P from saturated vapor pressure to ~30 kbar) ranges. Significant amounts of S3 (>10–100 ppm S) may be contained in fluids typical of magmatic–hydrothermal and metamorphic environments, which are characterized by elevated total S concentrations (>1,000 ppm), slightly acidic to neutral pH (3 to 7) and redox conditions enabling the coexistence of sulfate (or sulfur dioxide) and hydrogen sulfide (Fig. 4). Furthermore, Pokrovski et al. (2015) have shown that S3, together with HS, can form stable complexes with Au, enabling transport of this metal in aqueous solutions at 10–100 times higher concentration than by traditional sulfide complexes. By analogy with Au, other sulfur-loving, economically critical metals, such as Mo, Re, and the platinum-group elements, might also form stable complexes with S3. Such complexes would enhance metal mobility and focus the deposition upon destabilization of the radical ion. This hypothesis awaits further study.

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Figure 4. Concentrations of the different forms of sulfur, including the sulfur radical ions, in two typical types of fluids as a function of temperature (T). (A) Magmatic–hydrothermal fluid. (B) Metamorphic fluid. The sulfur species concentrations were predicted using the stability constants of sulfur forms reported in Pokrovski and Dubessy (2015) and in Pokrovski et al. (2015). The concentration of S2 is tentative. The S3, and, potentially, S2 ions may represent a significant part of dissolved sulfur content over a wide TP range. Direct experimental data are currently limited to

Using Sulfide Minerals to Reconstruct Physico-chemical Parameters of Ore Formation

Mineral assemblages can be used to reconstruct the physico-chemical parameters that prevailed during the formation of an ore deposit. The term ‘mineral assemblage’ signifies a group of minerals in chemical equilibrium (Einaudi et al. 2003). The sulfidation state (Fig. 5A) of hydrothermal fluids as a function of log fS2 and temperature determines the stability domains of key sulfide, sulfosalt and oxide minerals (Einaudi et al. 2003).

The typical environments of the main ore deposits types can vary in terms of fS2 and temperature (Fig. 5B). Cross-cutting relationships between successive mineral assemblages allow the sulfidation-state evolution of the hydrothermal system to be traced. Mineralogical and metal zoning of sulfide- and sulfosalt-bearing assemblages can be used for exploration purposes because they may act as prospection guides towards the center of the mineralizing system.

The abundances of minor and trace elements in some sulfides and sulfosalts can also be used as indicators of physical and chemical parameters of ore formation. For example, the iron content of sphalerite strongly depends on temperature and fS2 (Fig. 5A) and has been extensively used in ore deposits research, together with other mineral thermometers and thermobarometers (e.g. Sack and Ebel 2006). Recently, using in situ LA–ICP–MS analyses, Cook et al. (2016) established the partition coefficients for trace elements between different base-metal sulfides, thereby opening new possibilities in the use of these sulfides as tracers of paleo-physicochemical conditions.

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Figure 5. (A) Log fS2 versus temperature diagram, illustrating various sulfidation states (in green) of the hydrothermal fluids, based on stable sulfide mineral assemblages (from Einaudi et al. 2003). FeS (mol%) content of sphalerite is also plotted (in red; from Barton and Toulmin 1964); sphalerite is a common mineral in many hydrothermal ore types and its FeS content can indicate the temperature or sulfur fugacity. (B) Log fS2 versus temperature estimated for various hydrothermal ore deposits. Colours are visualization aids only. Abbreviations: asp (arsenopyrite), bn (bornite), cp (chalcopyrite), cv (covellite), dg (digenite), lo (loellingite), po (pyrrhotite), py (pyrite). Ore deposit abbreviations as in Table 1.

Processes Controlling Sulfide Precipitation in Hydrothermal Ore Deposits

Processes such as cooling, phase separation (boiling), mixing, and fluid–rock interaction are key for sulfide precipitation in hydrothermal systems because they influence the four main parameters of the fluids (temperature, salinity, pH, and fO2fS2). Because sulfide solubility strongly depends on temperature, cooling of hydrothermal fluids is the most efficient process leading to ore deposition. A main cause of cooling is boiling and the removal of the high-heat vapour phase during depressurization events. This causes H2S to fractionate to the vapour phase and so, in turn, metal-sulfide complexes to be destabilized (e.g. responsible for much gold precipitation). Progressive cooling, due to boiling and/or mixing, away from the heat source explains much of the metal zoning observed in different deposit types. For example, at a similar temperature, chalcopyrite is much less soluble than sphalerite or galena (Fig. 3A). This results in metal zoning (at the ore body to deposit scale) of Cu in proximal locations and the deposition of Zn–Pb and Ag–Mn in distal locations, as commonly observed in many types of deposits (porphyry copper, skarn, carbonate-replacement, epithermal, and VHMS).

Mixing between two fluids with contrasting temperature and composition can also cause mineral precipitation, particularly in basinal settings. For example, mixing between hot saline fluids and cold meteoric waters is an efficient mechanism to decrease both the temperature and salinity of metal-bearing fluids, and so favour sulfide precipitation. Mixing of metal-bearing but H2S-poor fluids with fluids rich in H2S can also trigger sulfide precipitation. The key factors in forming a hydrothermal sulfide deposit are not only the availability of metals but also the presence of reduced sulfur.

Metal-bearing hydrothermal fluids are typically not in equilibrium with their host rocks. The resulting water–rock interactions produce alteration in the wallrock and influence the solubility of metal sulfides in the fluid. Neutralization of reactive fluids through interaction with carbonates and feldspars increases the pH, which can lead to sulfide precipitation (e.g. Fig. 3C). Redox conditions are also strongly influenced by fluid–rock interaction (e.g. fluid reduction by Fe2+-bearing silicate minerals or by rocks rich in organic matter, or oxidation by Fe3+-rich detrital continental sediments). Interaction of metal-bearing fluids with rocks containing pyrite is a common sulfide precipitation mechanism for elements like copper that have a high affinity for reduced sulfur (iron from pyrite may in this case re-precipitate as oxide). The opposite process – scavenging of H2S by the host rock via Fe sulfidation reactions – destabilizes metal-sulfide complexes, leading to gold precipitation in orogenic gold deposits.

Selected Examples of Processes Causing Sulfide Precipitation in Hydrothermal Deposits

The analysis of the occurrence of sulfides in porphyry copper, VHMS and sediment-hosted deposits and of the associated precipitation mechanisms helps to unravel the formation of hydrothermal ore deposits.

Porphyry copper deposits are among the largest sulfur anomalies on Earth. These low-grade, high-tonnage deposits are related to shallow intrusions in volcanic arcs, with both metals and sulfur released from the magmas via hydrothermal fluids. Porphyry copper deposits consist of quartz–sulfide stockwork veins and disseminated sulfides in large volumes of hydrothermally altered porphyritic stocks and country rocks (Fig. 1A). In the early stages of porphyry copper deposit formation at depths of 2–3 km, the minerals bornite, chalcopyrite and magnetite precipitate from the magmatic fluid. In subsequent lower temperature stages, the proportion of reduced sulfur increases and pyrite progressively becomes more abundant. At shallow (<1 km), epithermal depths where there is sharp cooling , the sulfides are dominated by intermediate- to very high-sulfidation assemblages of enargite, digenite and covellite (Fig. 5A). In porphyry systems, temperature decrease, accompanied by vapor-liquid separation, as well as fluid–rock interaction and more distal mixing, causes a general mineral, and hence metal, zonation over a scale of a few kilometers: one often observes a core of Cu (± Mo), an intermediate zone of Zn–Pb–Ag veins, and more distal As–Sb (± Au–Hg)-dominated margins.

Volcanic-hosted massive sulfide deposits consist of high-grade, low-tonnage ore bodies, built up by 10s vol% of fine-grained base-metal sulfides (Fig. 1E). The sulfides precipitate at the interface between the oceanic crust and seawater and as replacement assemblages in the subsurface. Cooling and neutralization of hot, reactive and reduced H2S- and metal-bearing fluids with seawater is regarded to be the main process that governs sulfide precipitation in VHMS deposits.

In the sedimentary rock–hosted environments that occur in basins, several deposit styles are related to the flow of basinal brines. In general, stratiform Cu deposits, clastic sediment-hosted massive Pb–Zn ± Cu sulfide (SHMS), and carbonate-related Mississippi Valley type (MVT) Pb–Zn deposits are all formed by fluids of intermediate sulfidation state at temperatures of ~100 °C to 250 °C (Fig. 5B). The SHMS deposits form in passive margins, back-arcs and continental rifts; sulfide precipitation is caused by mixing fluids that ascend along basin margin faults and so vent into basins with anaerobic conditions where they encounter H2S-rich waters. This process is the root of the term ‘sedex’ (‘sedimentary exhalative’), which is frequently applied to SHMS deposits, although researchers now avoid this term because most sulfide precipitation takes place by subsurface replacement of already deposited sediments, where reduced biogenic sulfur is also available as diagenetic pyrite. Fluid mixing is also important in epigenetic base-metal deposits that have been formed by basinal brines on carbonate platforms: the largest MVT deposits form when metal-bearing basinal brines mix with fluids rich in H2S (generated by TSR, Eq. 3) or with fluids containing biogenic H2S in organic matter and/or hydrocarbons. Precipitation is also related to salinity decrease during fluid mixing. Interaction of oxidizing metal-bearing basinal brines with black shales and other sediments containing biogenic reduced sulfur, mostly in the form of framboidal pyrite, is the main process at the origin of stratiform Cu deposits and other sedimentary rock–hosted deposits. As noted above, copper is less soluble than zinc and lead, and, it can only be transported by oxidizing fluids (buffered by hematite), devoid of H2S. The lithologic contact between red beds (oxidized clastic sediments) and black shales controls the stratiform morphology of these copper deposits. Sulfides commonly are zoned with native copper, chalcocite and covellite, bornite, chalcopyrite, sphalerite and galena in a sequence from the oxidized to the reduced H2S-rich part of the redox profile.

Conclusions

The key processes that govern sulfide precipitation in hydrothermal systems are common, to varying degrees, across many ore deposit types, and can be used to formulate genetic models. These models, in turn, can be used to refine methods of exploration for mineral resources (e.g. Sillitoe 2010). Windows of understanding are being opened on the true nature of ore-forming processes via analytical and experimental developments for measuring the metal and sulfur contents (and their speciation) in hydrothermal fluids, via experiments on the stability of metal-bearing sulfur complexes in high TP liquids and vapours, and via determining the trace-element contents and their structural state in sulfide minerals. This improved knowledge contributes to better exploration models and on how to optimize mineral processing. This will help ensure that our technology-hungry society will have a supply of metals long into the future (Arndt et al. 2017) and it will lessen the environmental impact of mining and ore processing operations. All these aspects rely on an understanding of sulfide minerals and their stabilities.

Acknowledgments

We thank J. Hedenquist, D. Smith and Guest Editors Marie Edmonds and Kate Kiseeva for their reviews, which greatly improved the manuscript, and to Jodi Rosso and the Elements staff for the careful editing. We acknowledge financial support from grants SNF 160071, 162415, 165752, Soumet ANR-11-BS56-0009 and RadicalS ANR-16-CE31-0017.

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