The Mineralogy and Geochemistry of Volcanogenic Massive Sulfides and Their Contribution to Human Evolution: Past, Present and Future

Abstract

Volcanogenic Massive Sulfides (VMS) are considered major base (Cu-Zn±Pb) and precious metal (Au and Ag) sources with paramount contribution in the development and evolution of mankind through the ages. They are characterized by variable ore mineralogy and geochemistry, largely attributed to the variety in the geotectonic regime of formation (both divergent and convergent margins) and the variability in the host lithologies. Several VMS types are distinguished depending on the type of volcanism and host-rock lithology. The lens-shaped-to-stratiform bodies composed of fine-grained sulfides, usually accounting for more than 60% of the rock mass, have been exploited since prehistoric times. Recent studies reveal that VMS continue to be formed in deep marine settings and along plate margins on the ocean floor. Besides base and precious metals, nowadays, VMS are considered significant sources of critical and strategic metals, such as Co, Ni, Ga, Ge, In, Bi, As, Sb, Se, Mo, Cd, Sn, Hg, Tl and Bi, particularly after extensive research of the ocean floors in the years following World War II (WWII). Since the late 1970s, the potential of VMS has been further enhanced after the successful deep-sea mining (DSM) pilot tests, with the pipeline-lift mining system considered the most suitable for seabed massive sulfide (SMS) recovery.

1. Introduction

Throughout human history and from the very first steps of mankind in the field of metal exploitation and metallurgical processing, Volcanogenic Massive Sulfides (VMS) have played a paramount role as major resources of base (Cu, Zn and Pb) and precious metals (Au and Ag) ([1,2,3,4] and references therein). The most important characteristics and features of VMS that assisted this very early exploitation include: the high base metal grade (Cu, Zn, Pb) [4,5], the massive ore texture (at least 40% sulfides, yet in most cases higher than 60%) demanding limited pre-enrichment, strong contrast with country rocks (particularly when exposed outcrops have suffered supergene alteration), and the relatively simple mining and extraction [6]. Yet, despite their extensive exploitation since antiquity, the geologic term VMS has only been in use since the early 1970s [7] and covers all massive sulfide mineralizations that are spatially and temporarily associated with sub-marine volcanic activity (see [1,3,8,9,10,11,12,13,14,15,16]).

Other names and synonyms of VMS are Volcanic-Hosted Massive Sulfides (VHMS), Volcano-Sedimentary Massive Sulfides, Volcanic-Associated Massive Sulfides, Volcanic-exhalative Massive Sulfides and Volcanophile Massive Sulfides, although the latter two are not that frequently employed [5]. Volcanogenic MS should not be confused with other massive sulfide ore types, such as Sediment Exhalative—SEDEX, Mississippi Valley Type—MVT, and Irish-type Zn-Pb sulfide ores whose formation is related to sedimentary successions and processes, such as diagenesis and endogenetic fluid circulation ([5,17,18] and references therein). Broken Hill-type massive sulfides also do not fall in the typical VMS classification, as they present intermediate characteristics between the VMS and SEDEX types, with skarn-type characteristics and low S assemblages, and they are associated with medium-to-high grade metamorphic processes [19,20].

Despite the extensive and large-scale exploitation since antiquity, their genetic, spatial and temporal relation with the host lithologies has always been a matter of debate. During the mid-19th century, the accepted consensus involved massive sulfide formation either by fissure-filling and/or hydrothermal replacement, attributing an epigenetic character to the massive sulfide ore (refer to [6]). In the early 20th century, field studies performed in metamorphosed massive sulfide deposits in the U.S.A. and Canada provided the first insights on the syngenetic character of these deposits, namely due to the gradual transition from the massive sulfide ore to the surrounding host lithologies (e.g., [21]). It was not until the mid-20th century and the discovery of mid-ocean ridges and hydrothermal venting on the ocean floor when the exhalative theory of VMS formation was proposed [22,23,24]. These black smokers (also referred to as Seafloor Massive Sulfides—SMS; [25] and references therein) are considered modern analogs of ancient and fossilized VMS deposits, including the TAG field in the Atlantic Ocean [26,27,28] and the Juan de Fuca Ridge in the Pacific Ocean [29]. In fact, VMS are one of the very rare cases where, relative to the vast majority of sulfide ore types, we can witness their formation both in real time and on site.

In recent decades, advances in computational technology have provided many scholars with the necessary tools to model, predict and reconstruct VMS deposits [30,31,32,33,34,35,36,37,38,39,40,41,42,43,44]. Their work revealed that VMS usually form in the temperature range between ~200° and ~400 °C, at water depths greater than 500 m (to prevent boiling), at variable fluid/rock ratios and from hot metal-bearing fluids that vent and discharge at or close to the seafloor during contact with cold seawater (≈4 °C). The ore-forming fluids originate from mixtures of magmatic fluids and seawater at variable ratios that evolve after water-rock interaction, cooling and phase separation [44]. In most cases, VMS are formed over normal faults in extensional basins [45,46] and from hydrothermal convection cells and plumes with a lifespan ranging from below 0.5 Ka to greater than ~0.2 Ma [45], falling within the calculated duration of hydrothermal activity in contemporary (active) black smoker settings [40].

The present review examines the geological, mineralogical and geochemical characteristics of VMS and correlates these features with their impact in the evolution and development of mankind from the earliest phases of human history, in the present, and in the future. It combines all available data on worldwide VMS deposits, examines the proposed classification schemes, discusses the geochemical complexity and variability of VMS, investigates their geological evolution through time, describes their significance in the future development of human society and provides insights on future prospecting.

2. Characteristics of VMS

Volcanogenic massive sulfides present a non-uniform distribution around the world, mainly associated with fossilized or contemporary (active and inactive) deep sea settings associated with plate boundaries ([3,6,47,48]; Figure 1), with massive sulfides considered syngenetic and coeval to the host volcanic, volcaniclastic and siliciclastic rocks [12,49]. A typical VMS structure involves a “mushroom”-shaped ore body developed on the seafloor or sub-seafloor, usually spanning up to a few tens to a few hundred meters laterally and several meters vertically ([50,51,52,53]; Figure 2). The mineralization is stratiform and the upper massive sulfide lens is usually underlain by silica stockworks, known as the stringer zone, which also function as the feeder zones for the overlying massive ore ([3,50]; Figure 2). Concentric and distinctive alteration zoning envelops the feeder zones and the massive ore body [3].

Figure 1. World distribution of fossilized (ancient), active (in situ—contemporary formation) and inactive (in situ—formed in earlier periods) Volcanogenic Massive Sulfides—VMS (data from [6,25,54]).

Figure 1. World distribution of fossilized (ancient), active (in situ—contemporary formation) and inactive (in situ—formed in earlier periods) Volcanogenic Massive Sulfides—VMS (data from [6,25,54]).

The mineralization and geochemistry of VMS ore bodies is also not homogeneous, with the central part (feeder zone) characterized by higher temperature and enriched in Fe and Cu sulfide phases (e.g., pyrite, pyrrhotite, magnetite and chalcopyrite [55]), with the outermost part and the flanks predominated by lower temperature phases such as sphalerite-, galena- and fahlore-type minerals [56]. This characteristic is more prominent in polymetallic ores associated with intermediate to acidic volcanism such as the Bimodal-Felsic and Siliciclastic–Felsic types (former Kuroko-type—refer to Section 3 and Table 1 for details on VMS classification schemes), with the “Oko” (yellow) ore rich in chalcopyrite and pyrite in the center, enveloped by the “Kuroko” (black) ore rich in galena and sphalerite at the flanks [57,58].

Volcanogenic MS also develop large variability in the size of the ore bodies, ranging from a few m to more than 100 m laterally, while the shape may also vary from mushroom-shaped, to stratiform, to irregular (see [59,60]). In most cases, more than one ore body characterizes a VMS deposit, depicting the activity of more than one exhalative center [44,61,62]. At the same time, massive sulfide bodies may be formed either on the seafloor (the most frequent form), or below the seafloor, bearing evidence of the replacement of loose host sediments [3,46,59,63].

Figure 2. Generalized view (off-scale) of an extensional submarine basin with the most prominent locations of VMS formation along high-angle normal faults. (A) Sub-seafloor VMS formation. (B) Seafloor VMS formation (with modifications after [64]).

Figure 2. Generalized view (off-scale) of an extensional submarine basin with the most prominent locations of VMS formation along high-angle normal faults. (A) Sub-seafloor VMS formation. (B) Seafloor VMS formation (with modifications after [64]).

The formation and deposition of VMS have been taking place since the earliest phases of Earth’s history, from the Paleoarchean (3.55 Ga) until the present [3,45], with their temporal distribution, similarly to their spatial distribution, not being uniform. Volcanogenic MS deposits tend to cluster in certain periods of Earth’s evolution, including the late Archean (2.85–2.60 Ga), Paleoproterozoic (2.0–1.7 Ga), Neoproterozoic (900–700 Ma), early Paleozoic (550–450 Ma), Devonian (400–320 Ma) and early Jurassic to present (200–0 Ma) [45]. Deposits dated less than 15 Ma are all related to contemporary ocean settings (extensional/spreading centers and back-arc basins).

The tonnage of VMS is also very variable, ranging from small lenses of less than a metric ton, in most cases dispersed within a larger area, to giant and supergiant deposits with reserves greater than 1000 Mt ([3,5] and references therein). To name a few, the Rio Tinto (approx. 1.5 Bt) and Neves-Corvo (approx. 370 Mt [65]) deposits in the Iberian Pyrite Belt (IPB), the Kholodninskoe in Siberia (approx. 300 Mt), the Gaiskoe deposit in the Urals (approx. 380 Mt [66]) the Windy Craggy and Brunswick No. 12 in Canada (approx. 300 Mt and 230 Mt, respectively) and Ducktown in USA (approx. 163 Mt) are among the largest known VMS deposits in the world.

During the past 50 years, VMS have been extensively studied, resulting in vast corresponding literature. A very important characteristic of VMS is that relative to the majority of sulfide ore deposit types that pinpoint to a specific geologic setting (e.g., replacement type, epithermal, and porphyry copper, to name a few), VMS may form in a diversity of geotectonic environments, including both divergent (i.e., on the seafloor related to mid-ocean ridges) and convergent margins (e.g., arc and back-arc spreading settings), volcanic arcs, rifted continental margins and pull-apart basins [3,5,55,61,67,68,69] (Figure 3). At the same time, the host lithologies, including sedimentary, volcanosedimentary and volcanic rocks, may greatly vary in both composition and geochemistry [70,71], while the ore mineralogy and geochemistry may not only differ between the various VMS types but also between the various ore bodies of a single VMS deposit [59,72,73]. Within the VMS perspective, hydrothermal circulation and convection are prerequisites for sulfide ore formation, regardless of the geotectonic setting. Yet it is very common that this hydrothermal circulation is not always accompanied by volcanism on the seafloor, and in most cases, the host lithologies function as sources of metals through depletion during hydrothermal convection (e.g., [45,63]).

Figure 3. Generalized scheme of the various geotectonic settings of Volcanogenic Massive Sulfide formation (undifferentiated) (with modifications after [3,74]).

Figure 3. Generalized scheme of the various geotectonic settings of Volcanogenic Massive Sulfide formation (undifferentiated) (with modifications after [3,74]).

To conclude, the only features shared between all VMS types are the spatial and/or temporal relation with submarine volcanism and the massive ore texture (regardless of ore mineralogy and geochemistry). It is therefore evident that such discrepancies among the various VMS types and deposits may determine the recoverable commodities, either as products and/or as by-products [5].

3. VMS Classification Schemes

Early attempts on VMS classification began in the 1970s based on the major commodities Pb, Zn and Cu [7]. Later, Cox and Singer [75], in their work on mineral deposit models (USGS) and following the discrimination of Sawkins [76], developed a classification scheme based on host volcanic rock composition and major commodities. They introduced three major VMS sub-groups, including the “Cyprus” (Cu-rich ores associated with mafic volcanic rocks [58,77,78]), “Kuroko” (Pb-Zn-rich associated with felsic-to-intermediate volcanic rocks [79,80,81]) and “Besshi” types (Zn-Cu-rich associated with mafic volcanic and siliciclastic rocks [82,83,84,85]). Ongoing work showed that the aforementioned classification, although accepted among the vast majority of subject-related researchers and professionals, was insufficient as additional data were incorporated in the general VMS descriptive model. Barrie and Hannington [1] proposed a different classification scheme focused solely on host rock composition and indirectly on specific geotectonic settings, since major commodities (Cu, Zn, Pb) largely derive from host rock leaching during convection and do not always correlate with the type of volcanism. Their classification included five sub-groups; Mafic-type composed mainly of mafic volcanic rocks (>75%) with a limited presence of siliciclastic rocks (<10%); Bimodal-Mafic type, comprising more than 50% mafic rocks in the stratigraphy and subordinate siliciclastic rocks; Mafic–Siliciclastic type with equal proportions of mafic and siliciclastic rocks; Bimodal-Felsic type, with more than 50% felsic and less than 15% siliciclastic rocks in the stratigraphy; Bimodal-Siliciclastic type where volcanic and siliciclastic rocks appear in near-equal proportions with felsic volcanic rocks predominating over mafic volcanic rocks. Following the work of Barrie and Hannington [1], several other classification schemes were proposed based either on the same principles (refer to [3]), or on the grades and tonnage of VMS deposits worldwide, thus limiting the number of VMS sub-groups to only three [4]. The latest VMS classification scheme was proposed by Shanks and Koski [86] with subtle differences relative to that of Galley et al. [3] (Table 1).

Table 1. Volcanogenic Massive Sulfides classification schemes and major commodities (the classification scheme used in this review is in bold).

[75]	[1]	[3]	[4]	[86]	Other Sub-Types	Major Commodities
Kuroko- type	Bimodal- Siliciclastic	Felsic– Siliciclastic	Felsic	Siliciclastic–Felsic	Iberian Pyrite Belt-type	Zn-Pb-Cu±(Au, Ag)
	Bimodal-Felsic	Bimodal-Felsic		Bimodal-Felsic		Zn-Pb-Cu±(Au, Ag)
	Bimodal-Mafic	Bimodal-Mafic	Bimodal-Mafic	Bimodal-Mafic	Ural-type, Noranda-type	Cu-Zn-Pb±(Au, Ag)
Besshi- type	Mafic–Siliciclastic	Pelitic–Mafic		Siliciclastic–Mafic	Outokumpu-type	Cu±(Co, Zn, Ni)
Cyprus- type	Mafic	Mafic	Mafic	Mafic–Ultramafic	Atlantic-type (Ultramafic)	Cu-Zn±Au

Table 1. Volcanogenic Massive Sulfides classification schemes and major commodities (the classification scheme used in this review is in bold).

[75]	[1]	[3]	[4]	[86]	Other Sub-Types	Major Commodities
Kuroko- type	Bimodal- Siliciclastic	Felsic– Siliciclastic	Felsic	Siliciclastic–Felsic	Iberian Pyrite Belt-type	Zn-Pb-Cu±(Au, Ag)
	Bimodal-Felsic	Bimodal-Felsic		Bimodal-Felsic		Zn-Pb-Cu±(Au, Ag)
	Bimodal-Mafic	Bimodal-Mafic	Bimodal-Mafic	Bimodal-Mafic	Ural-type, Noranda-type	Cu-Zn-Pb±(Au, Ag)
Besshi- type	Mafic–Siliciclastic	Pelitic–Mafic		Siliciclastic–Mafic	Outokumpu-type	Cu±(Co, Zn, Ni)
Cyprus- type	Mafic	Mafic	Mafic	Mafic–Ultramafic	Atlantic-type (Ultramafic)	Cu-Zn±Au

Besides the aforementioned sub-groups and classification schemes, there are several VMS-style mineralizations that either do not fall in the proposed categories, or present specific features, such as the Iberian Pyrite Belt-type (Siliciclastic–Felsic [87,88,89]), Outokumpu-type (Siliciclastic–Mafic (Ultramafic) with significant contribution of Ultramafic rocks; refer to [90,91,92]), Noranda-type (Bimodal-Mafic; [93,94] and references therein), Atlantic-type (Ultramafic; see [95]) and Ural-type (Bimodal-Mafic [1,96,97]) (Table 1). At the same time, there are VMS that bear characteristics belonging to epithermal-type ores [98,99,100], and in that perspective, Galley et al. [3] have proposed a transitional type between VMS and epithermal-type ores (e.g., Hybrid Bimodal-Felsic type—Eskay Creek, Canada [101]). For this review, the terminology and classification schemes of Shanks and Koski [86] will be employed.

4. Ore Mineralogy

4.1. Hypogene Ore

The consensus regarding the formation of VMS involves volcanism and syngenetic sulfide deposition in deep-sea marine settings [45]. In such settings, massive, stratabound and lens-shaped ore bodies composed of fine to very fine-grained sulfides are formed due to the rapid deposition of sulfides and sulfosalts as soon as hot, metal-bearing fluids react with cold seawater [56]. In general, sulfides and sulfosalts are stable in reducing conditions where fO₂ is very low. Ascending, high-temperature hydrothermal fluids (T > 300 °C) favor the dissolution of metals (mainly as chloride complexes) and sulfur (as non-oxygenated sulfide species including S⁻², HS⁻, and H₂S), resulting in the deposition of metal-bearing sulfide phases as conditions change (most importantly, the sudden drop of temperature). As the system evolves and the temperature drops (T < 150 °C), the formation of oxygenated S species, including SO₄⁻², is favored, thus leading to the closure of the hydrothermal system with the deposition of sulfates, such as gypsum, anhydrite and possibly barite that envelop the massive sulfide ore [102,103,104].

Despite the large diversity in the environment of their formation, all VMS types are characterized by very simple mineralogy with only a handful of sulfide minerals being the major ore phases, and accounting in most cases for more than 90% of the total sulfide ore paragenesis [55,56,64,68,75,87,94,96,105]. These phases include, in various percentages, pyrite, pyrrhotite, chalcopyrite, sphalerite and galena (

Table 2. Most important hypogene ore phases and their relative abundances per VMS type. Data are taken from [56] and references therein.

Major Phases		Minor Phases		Trace Phases
Pyrite	FeS2	Marcasite	FeS2	Acanthite	AgS2
Chalcopyrite	CuFeS2	Cobaltite	(Co, Fe)AsS	Argentite	AgS2
Sphalerite	ZnS	Tennantite	Cu12As4S13	Bismuth	Bi
Galena	PbS	Tetrahedrite	Cu12Sb4S13	Bismuthinite	Bi2S3
Pyrrhotite	Fe1-xS	Magnetite	Fe3O4	Bornite	Cu5FeS4
		Arsenopyrite	FeAsS	Cubanite	CuFe2S3
		Wurtzite	ZnS	Bulangerite	Pb5Sb4S11
				Bournonite	PbCuSbS3
				Bravoite	(Fe, Ni, Co)S2
				Cassiterite	SnO2
				Cinnabar	HgS
				Realgar	AsS
				Freibergite	(Ag, Cu)12(Sb, As)4S13
				Hawleyite	CdS
				Greenockite	CdS
				Germanite	Cu3(Ge, Fe)(S, As)4
				Millerite	NiS
				Pyrargirite	Ag3SbS3
				Pentlandite	(Fe, Ni)9S8
				Electrum	(Au, Ag, Cu)
				Native silver	Ag
				Stannite	Cu2FeSnS4
				Stibnite	Sb2S3
				Native gold	Au
				Au-Ag-Bi tellurides
VMS type	Major		Minor	Trace
Siliciclastic–Felsic	sp, py, gn		cp, tnt, tt, mr, as, sn	Electrum (Au-Ag±Cu alloy), native Ag
Bimodal-Felsic	py, sp, gn		cp, tnt, tt, mr, as	Electrum (Au-Ag±Cu alloy), native Ag
Bimodal-Mafic	py, cp, sp		po, mt, gn	Electrum (Au-Ag±Cu alloy), native Ag
Siliciclastic–Mafic	py, cp		sp, po, mt, co	ha, pt, ml
Mafic–Ultramafic	py, cp		sp, po, mt

Abbreviations: py: pyrite; gn: galena; sp: sphalerite; cp: chalcopyrite; tnt: tennantite; tt: tetrahedrite; po: pyrrhotite, mt: magnetite; as: arsenopyrite; ha: hawleyite; pt: pentlandite; ml: millerite; sn: cassiterite; mr: marcasite; and co: cobaltite.

), with all or some being present in all VMS sub-groups as major ore phases. On the other hand, the number of possible phases present in the massive ore as minor or trace minerals is quite large, and their formation is related to the composition of the magma-related fluids ascending through the crust, and most importantly, the composition and type of the magmatic and/or siliciclastic rocks of the bedrock the hydrothermal fluids circulate through (Table 2) [56,68,96,106,107,108]. Still, there are VMS deposits where the reported minor and trace ore phases (Table 2) are ubiquitous, such as tetrahedrite, stibnite, and realgar at Eskay Creek [101], and stannite and cassiterite at Neves-Corvo (IPB [109]) and Kidd Creek (Abitibi, Canada [110]).

4.2. Supergene Ore

Ongoing tectonics and geological evolution may result in the exposure of VMS on the surface or in near-surface environments, where the massive sulfide ore is exposed to atmospheric conditions and affected by oxygenated, percolating surface waters [56,111]. In oxygenated and water-undersaturated environments, such as those prevailing near the surface, sulfides and sulfosalts are metastable and easily react with oxygen and other oxidizing agents (e.g., Fe⁺³, Fe₂(SO₄⁻²)_3(aq)) [112,113,114], resulting in weathering of the ore bodies. The fine-grained character combined with the presence of significant quantities of Fe-bearing sulfides results in extensive supergene alteration and weathering of the sulfide ore (for details on the weathering of sulfides and sulfosalts, refer to [115,116,117,118]).

In many cases, this ongoing weathering results in the formation of very characteristic weathering surfaces, that range in thickness from a few m to more than 100 m, known as gossans [111,119,120,121]. These gossans are usually characterized by their significant spatial extent and should not be confused with analogous weathering processes taking place on the seafloor following the deposition of hydrothermal massive sulfides [122]. This type of alteration is referred to as “seafloor weathering” and is usually characterized by the formation of low-temperature phases, such as the Cu-chloride atacamite [Cu₂Cl(OH)₃]. Ongoing studies revealed that not all VMS oxidation zones result after surface weathering. For instance, bedded ochres overlying the massive ore at Skouriotissa (Mafic–Ultramafic-type, Cyrpus), which were originally considered a product of surface alteration, are now re-interpreted as submarine gossan [122]. This seafloor weathering, also referred as “halmyrolysis”, is a result of various reactions mediated by oxygenated seawater circulating through the massive ore body along cracks and fractures at ambient temperatures (1–2 °C). Primarily, oxidation occurs at the deposit/seawater interface and gradually moves downward and into the ore body [123].

The gossans developed after atmospheric weathering (oxidation) are easily distinguished by their boxwork texture and colors. To name a few, banded, diamond mesh, triangular, cellular, sponge and colloform textures are common, with the color varying greatly, ranging from red to yellow to brown to black, depending on the total Fe content (usually expressed as Fe₂O₃ wt%). Gossans largely comprise secondary ferric iron phases after oxidation of Fe⁺² present in the lattice of sulfides (mainly pyrite, pyrrhotite, chalcopyrite and Fe-rich sphalerite) and other metal-bearing weathering products, whose geochemistry is directly related to the geochemistry of the primary sulfide ore (

Table 3. Examples of metal-bearing secondary phases of the oxidation zones and gossans of VMS.

Abundant	Chemical Formula	Scarce	Chemical Formula
Hematite	Fe2O3	Ktenasite	(Cu, Zn)5(SO4)2(OH)6 × 6H2O
Goethite	Fe(OH)3	Rosasite	(Cu, Zn)2(CO3)(OH)2
Jarosite	KFe3(SO4)2(OH)6	Schwertmannite	Fe16O16(OH)12(SO4)2
		Hydrocerussite	Pb(CO3)2(OH)2
Common	Chemical Formula	Osarizawaite	Pb(Al, Cu)3(SO4)2(OH)2
Hydronium jarosite	(K, H3O)Fe3(SO4)2(OH)6	Beudantite	PbFe3(AsO4)(SO4)(OH)6
Plumbojarosite	PbFe6(SO4)4(OH)12	Beaverite	Pb(Cu, Fe)3(SO4)2(OH)6
Argentojarosite	AgFe3(SO4)2(OH)6	Linarite	PbCu(SO4)(OH)2
Anglesite	PbSO4	Chalcanthite	CuSO4 × 5H2O
Cerussite	PbCO3	Wroewolfeite	Cu4(SO4)(OH)6 × 2H2O
Smithsonite	ZnCO3	Antlerite	Cu3(SO4)(OH)4
Azurite	Cu3(CO3)2(OH)2	Pyromorphite	Pb5(PO4)3Cl
Malachite	Cu2(CO3)(OH)2	Melanterite	FeSO4 × 7H2O
Atakamite	Cu2Cl(OH)3	Rozenite	FeSO4 × 4H2O
Covellite	CuS	Scorodite	FeAsO4 × 2H2O
Chalcocite	Cu2S	Bukovskyite	Fe2(AsO4)(SO4)(OH) × 7H2O
Cuprite	Cu2O	Butlerite	FeSO4(OH) × 2H2O
Copper	Cu0
Gold	Au0
Silver	Ag0	Detrital	Chemical Formula
Electrum	Au-Ag-Cu alloy	Cassiterite	SnO2
Gypsum	CaSO4 × 2H2O

; e.g., IPB gossans [111]). The Fe⁺³ phases formed a range from rather metastable to strongly stable and insoluble, with the general formation/replacement trend as conditions evolve being as follows:

Jarosite → Schwertmannite → Ferrihydrite → Goethite → Hematite

Early formed Fe⁺³ phases (e.g., sulfates—jarosite species) are considered metastable and insoluble in highly oxidative and acidic environments, whereas as the supergene system evolves and gradually turns less acidic and oxidative and tends to reach a balance with the atmosphere; Fe-oxides and Fe-oxy-hydroxides are the most stable phases (e.g., goethite and hematite). Gossans, mainly due to their distinctive color, were exploited in the past and are still used to this day as indicators of possible underground and unaltered sulfide ores [124]. Moreover, during antiquity and later periods, they were considered very important Fe ore resources.

Gossans are usually underlain by a lower oxidation zone with abundant secondary products (Table 3) and occasionally relics of primary sulfides and sulfosalts extending up to the groundwater table (vadose zone). Oxidation zones resulting from the weathering of VMS tend to develop an extensive variety of secondary species depending on the near-surface system physicochemical conditions and the primary sulfide ore mineralogy and geochemistry. Oxides, carbonates, sulfates, arsenates and phosphates are common, greatly depending on the geochemical behavior of the metals released during weathering [56]. For instance, Pb is considered insoluble in near-surface environments with increasing solubility at very low pH conditions, with anglesite, cerussite, pyromorphite being the most common secondary products [125,126]. On the other hand, Zn and Cd are characterized by their increased solubility in acidic and near-neutral systems, therefore they may be transported as dissolved species at greater distances [125,126] and consequently affect the mineralogy of the oxidation zone.

Below the oxidation zone, the supergene enrichment zone is developed [121,127], and depending on the primary geochemistry of the VMS ore, it may comprise secondary Cu sulfides (e.g., covellite, chalcosite, bornite, digenite) resulting from the contact of percolating Cu-rich fluids with the reducing ground water (e.g., Penokean Volcanic Belt, northern Wisconsin, USA [128]).

A very interesting characteristic of the surface-to-near-surface exposure of VMS is the presence of native metals, such as Cu, Au and Ag, in their oxidation zones and gossans [54,128,129,130], as well as resistant primary phases, such as cassiterite. The latter may be associated with the generally accepted view of historians that the development of the bronze (Cu-Sn±As) alloy was accidental and is largely related to the metallurgical processing of Cu ores rich in Sn. Therefore, it is evident that VMS mineralizations that have been formed since the Paleoarchean, and were later exposed to the surface environment for millions of years, were affected by weathering, thus liberating their metal load and eventually forming native metal ores (e.g., Nubian Shield, Northeast Africa [131]) that were extensively exploited during prehistoric (Neolithic period, Copper Age) and historic times (e.g., IPB [132]).

5. VMS Geochemistry

5.1. Host-Rock Volcanism, Composition and Geochemistry

As stated earlier, VMS are genetically and spatially associated with submarine volcanism in spreading/extensional settings, with the various VMS types (Table 1) associated with specific volcanic, volcaniclastic and siliciclastic rock types [3,5,55,67].

The importance of volcanism in VMS formation, regardless of magma composition, may be summarized as follows [12,71]:

• It acts as heat source for the development of convection systems after mixing, in various ratios, of cold percolating seawater with upwelling hot magmatic fluids [133];
• It confines hydrothermal circulation of the metal-bearing fluids within limited-space zones, particularly along high-angle fault systems characterizing extensional and spreading centers, thereby leading to the deposition of massive sulfides at or near the seafloor;
• It may function as a source of metals, although in most cases, the host lithologies where the hydrothermal fluids circulate through tend to play the most important role in the geochemistry of the sulfide ores deposited [68].

In general, volcanism in spreading centers (associated with both convergent and divergent margins) is a prerequisite for VMS formation, with the VMS-related magmatism taking place at rather shallow crustal levels and near the basin floor (between 1 and 3 km). At these depths, the heat from the generated magma is consistent, thus promoting the development and preservation of a convection system for prolonged periods that may eventually lead to the formation of VMS [12], whereas deeper melts may not be as efficient, since in most cases, the generated magma loses its heating potential while ascending (see [34]).

Piercey [12], in his review on the setting, style, and role of magmatism in the formation of VMS, attempted to distinguish between volcanic rock geochemistry and VMS types and established the vast diversity in volcanic rock geochemistry associated with VMS deposits and mineralizations. For instance, in Mafic VMS-dominated environments (e.g., Mafic–Ultramafic, Siliciclastic–Mafic and Bimodal-Mafic types) the associated volcanic rock types include typical Mid-Ocean Ridge Basalts (MORB; both normal and enriched varieties), Island Arc Tholeiites (IAT), Low-Ti Tholeiites (LOTI) and Boninites (BON). Boninites, IAT and LOTI are mainly related with ophiolite-hosted Mafic–Ultramafic-type VMS and less with the Bimodal-Mafic and the Mafic–Siliciclastic types. On the other hand, MORB, and in particular Back-Arc Basin Basalts (BABB), tend to correlate with Mafic–Ultramafic-type VMS associated with back-arc basins, ophiolites formed in back-arc basin environments, and also Siliciclastic–Mafic VMS types in similar settings.

In evolved geotectonic settings (e.g., continental arc and back-arc environments), where Bimodal-Felsic and Siliciclastic–Felsic VMS formation is favored, limited volumes of MORB and alkali-rich rocks (e.g., Ocean Island Basalts—OIB) contribute to the local stratigraphy. In such settings, the mafic volcanic rocks either form sills overlying the felsic volcanic rock suites, or form dykes that cut the latter (therefore post-dating felsic volcanism in both cases).

Although felsic volcanic and volcaniclastic rocks predominate in the Bimodal-Felsic and Siliciclastic–Felsic VMS types (see Section 3; Table 1), mafic volcanic rocks are also present mainly acting as the “fuel” (heat source) for the development of the hydrothermal system. Moreover, in VMS-forming settings, the volcanic rock stratigraphy usually presents a spatial transition from alkalic basalts (in lower crustal levels) to MORB (in higher crustal levels), largely depicting the transition from early and enriched, lithospheric mantle sources (alkalic) to late and depleted, asthenospheric mantle sources, attributed to the gradual transition from typical rifting to back-arc spreading [134,135,136]. In general, despite the nomenclature of VMS that includes the most prominent volcanic rocks for each type (refer to Section 3; Table 1), mafic volcanism is associated with all VMS types and tends to play a very important role in the development of VMS-related hydrothermal and convection systems, and eventually the deposition of VMS regardless of the geotectonic setting of volcanism.

Due to the large diversity in the geologic environments where VMS form, the host rocks may differ greatly both mineralogically and geochemically [71], with the chondrite and REE normalized spider diagrams of the host volcanic, volcaniclastic and clastic lithologies varying among the VMS sub-types [12]. For instance, mafic volcanic rocks hosting Mafic–Ultramafic-type VMS in mid-ocean ridges tend to develop near-parallel patterns when normalized to primitive mantle values, whereas as the host volcanic rock compositions turn more acidic and/or the geotectonic system becomes more evolved (e.g., back-arc basins in Supra-Subduction Zone settings—SSZ), and enrichment in incompatible elements (negative slopes) is more prominent [109,137,138,139,140,141,142,143].

5.2. Stable and Radiogenic Isotopes in VMS

In recent years, advances in analytical techniques, and in particular ICP-MS, have made possible reliable measurements of both stable and radiogenic isotopes that may further be used in the investigation of VMS (see [13,68,97]). For that matter, isotopic geochemistry may be employed in the identification and distinction of the various geological processes during VMS formation, including the mode of development of hydrothermal convection systems, the degree of contribution of seawater and seafloor sediments, the depth of magma generation acting as a heat source, and most importantly, the sources of metals (see [31,44,64]).

Regarding stable isotopes, the δ³⁴S values are used for the determination of possible S sources for the hydrothermal and ore-bearing fluids (e.g., magmatic, sedimentary, etc), while O (O¹⁸/O¹⁶ ratio) and H (D/H ratio) stable isotopes may be additionally employed in the determination of seafloor sediment contribution and the amount of seawater incorporated during hydrothermal convection and ore formation (see [44,64]). As stated earlier, VMS are characterized by large discrepancies between deep-sea volcanism and the geologic setting of ore formation, which is also depicted in the very large variance in S isotopic signature of VMS deposits, with δ³⁴S values ranging from approximately −5 per mill to more than +15 per mill, with the positive values indicating significant sedimentary S contribution (see [144,145]). Other uncommon stable isotopes used in VMS studies include Fe, Si, Ba, N, B, C, etc. (refer to [146]). For instance, the Siliciclastic–Mafic VMS deposit of Ermioni (Argolis, Greece) reveals significant differences in the S, O and Fe isotope geochemistry between the two most important mine sites (e.g., Roro, Karakasi), depicting subtle differences in the conditions of VMS formation between these two locales [64].

Radiogenic isotopes are largely employed to define the geotectonic setting of VMS formation, the depth of magma development and source of metals (mainly focused on Pb isotopes), while more advanced and sophisticated radiogenic isotopes (e.g., Re-Os) are used for radiochronological analyses. Conventional radiogenic isotopes (e.g., U-Pb, or K-Ar) may prove to be insufficient during radiochronological studies, since in most cases the mineral phases associated with such radiogenic pairs (e.g., zircon, K-mica) are usually found as traces in the volcanic rocks hosting VMS deposits (e.g., Ermioni Siliciclastic-Mafic-type VMS, Argolis, Greece [62]). Moreover, in case such phases are present, they may only provide answers on the age of the host volcanic rocks and only indirect information on the age of the sulfide ore (see [145,147,148]). This issue may be successfully addressed by recent advances in radiogenic isotope geochronology, and the development of more sophisticated dating methods, including Re-Os and Rb-Sr dating in sulfides [149,150], as they provide information on the actual age of the sulfide ore (refer to [145,148,151,152]).

Another major challenge during the study of VMS deposits is the identification and determination of the pulses of hydrothermal circulation, as in many cases the mode and characteristics of VMS formation may change through time. Such cases include the Zn-Pb-Ag deposits of Australia [73], the VMS deposits in the Jinshajiang orogenic belt, southwestern China [72], the Rudny Altai VMS deposits in Siberia [145], and the Siliciclastic–Mafic Ermioni VMS deposit in Argolis, Greece [62,64]. This is a crucial parameter since the precise definition of the ore-forming system(s) not only provides valuable information on other possible deposit types developed in the area under investigation ([5] and references therein) but may also define future exploration strategies.

5.3. Chemical Composition of Ore Phases

Volcanogenic massive sulfides, for many centuries, have been a major resource for Cu and Zn [3] (Figure 4A), until the development of “flotation” in the early 20th century that made the exploitation of low-grade Cu ores (e.g., Porphyry Copper-type) feasible ([153] and references therein). At the same time, VMS are considered minor Pb resources, as Mississippi Valley-type (MVT) and Sediment exhalative (SEDEX) ores are considered the most important ones among massive sulfide deposits [71] (Figure 4A).

In recent years, one of the major objectives in sulfide ore exploration, besides precious metals Au and Ag [154,155], is the trace metal content, and in particular, the critical and strategic metal grades discovered in potentially minable contents [156,157]. In this context, a large number of VMS deposits have been reassessed regarding their critical and strategic metal content, including Co, Ni, Ga, Ge, In, Bi, As, Sb, Se, Mo, Cd, Sn, Hg, Tl and Bi [71,158].

In Table 4, indicative data on the major and trace element geochemistry of bulk VMS ore samples is presented. It is evident that VMS may be characterized by significant contents in precious and various critical and strategic metals (Figure 4), with these contents clearly depicting the varying geotectonic setting and volcanism of VMS formation and the differences in the host rock lithologies. Mafic–Ultramafic and Siliciclastic–Mafic types are associated with hydrothermal circulation and convection through mafic lithologies and tend to produce metal-bearing hydrothermal fluids rich in Cu and Zn (Figure 4A). In Siliclastic–Mafic (Ultramafic) VMS deposits, where the circulation of hydrothermal fluids takes place largely through ultramafic lithologies, the ore may be enriched in Co, Se (Figure 4B,H) and Ni, with ore parageneses including sulfides, such as cobaltite and pentlandite, and tend to resemble the mineralogy of high-temperature Fe-Cu-Co-Ni magmatic sulfides [159,160]. At the same time, the available data shows that Bimodal-Mafic VMS tend to be enriched in Au and Te (Figure 4C,G). In evolved geotectonic settings, hydrothermal circulation and convection occurs within felsic to intermediate volcanic and siliciclastic rocks (e.g., Bimodal-Felsic and Siliciclastic–Felsic types) resulting in polymetallic sulfide ores rich in Zn and Pb, and in many cases Cu, As, Sb, Au and Ag [1,5,89] (Figure 4C–F).

Figure 4. Base, precious and trace metal geochemical diagrams of the various VMS types based on the compiled data reported in Table 4. (A) Cu-Zn-Pb ternary discrimination diagram. (B–H) Box-and-whisker diagrams of selected precious and trace metals (Co, Au, Ag, As, Sb, Te, and Se; all data in ppm; dots stand for outliers and x for average value).

Figure 4. Base, precious and trace metal geochemical diagrams of the various VMS types based on the compiled data reported in Table 4. (A) Cu-Zn-Pb ternary discrimination diagram. (B–H) Box-and-whisker diagrams of selected precious and trace metals (Co, Au, Ag, As, Sb, Te, and Se; all data in ppm; dots stand for outliers and x for average value).

Table 4. Bulk ore geochemistry of VMS deposit types and mineralizations around the world (Cu, Zn, Pb and Sn in wt%, all other elements in ppm).

Deposit Name	Region					VMS Type			Cu	Zn		Pb		Sn		Reference
Hokuroku district	Honsu, Japan					Bimodal-Felsic			1.23	41.20		1.58				[107]
Hokuroku district	Honsu, Japan					Bimodal-Felsic			8.85	0.77		0.28				[107]
Hokuroku district	Honsu, Japan					Bimodal-Felsic			2.01	2.06						[107]
Bisha Deposit	Western Nakfa Terrane, Eritrea					Bimodal-Felsic			0.99	5.72		0.2		0.0006		[161]
Talgan	South Urals, Russia					Bimodal-Felsic			3.2	3.6		0.45				[97]
Oktyabrskoye	South Urals, Russia					Bimodal-Felsic			4.16	2		0.1				[97]
Tash-Tau	South Urals, Russia					Bimodal-Felsic			6.8	2.1		0.25				[97]
Alexandrinskoye	South Urals, Russia					Bimodal-Felsic			2.7	4.6		0.52				[97]
Safyanovskoye	Central Urals, Russia					Bimodal-Felsic			2.2	0.7		0.13				[97]
North Knoll	Iheya Ridge, Okinawa Trough, Japan					Bimodal-Felsic			2.54	31.1		20.2				[162]
North Knoll	Iheya Ridge, Okinawa Trough, Japan					Bimodal-Felsic			4.63	36.3		11.7				[162]
North Knoll	Iheya Ridge, Okinawa Trough, Japan					Bimodal-Felsic			3.46	33.8		10.9				[162]
North Knoll	Iheya Ridge, Okinawa Trough, Japan					Bimodal-Felsic			1.26	36.5		9.14				[162]
North Knoll	Iheya Ridge, Okinawa Trough, Japan					Bimodal-Felsic			3.23	25.9		13.9				[162]
North Knoll	Iheya Ridge, Okinawa Trough, Japan					Bimodal-Felsic			3.06	24.5		15.4				[162]
Sultanovskoye	South Urals, Russia					Bimodal-Mafic			1.4	0.7		0.009				[97]
Yubileynoye	South Urals, Russia					Bimodal-Mafic			1.4	1						[97]
Yaman Kasy	South Urals, Russia					Bimodal-Mafic			2.6	5.6		0.03				[97]
Uzelga	South Urals, Russia					Bimodal-Mafic			1.3	2.6		0.15				[97]
Molodezhnoye	South Urals, Russia					Bimodal-Mafic			2.1	3.3		0.17				[97]
Valentonskoye	North Urals, Russia					Bimodal-Mafic			2.4	3.2		0.24				[97]
Othrys	Thessaly, Greece					Mafic–Ultramafic			1.8	0.12						[159]
Mathiati	Cyprus					Mafic–Ultramafic			0.2	0.26						[159]
Skouriotissa—Apliki	Solea Graben, Cyprus					Mafic–Ultramafic			0.18							[163]
Kokkinoyia—Agrokipia	Mitsero Graben, Cyprus					Mafic–Ultramafic			0.08							[163]
Buribay	South Urals, Russia					Mafic–Ultramafic			3.5	0.5						[97]
Sheikh-Ali	Iran					Mafic–Ultramafic			3.5	0.08						[164]
Sheikh-Ali	Iran					Mafic–Ultramafic			4.8	0.007						[164]
Sheikh-Ali	Iran					Mafic–Ultramafic			0.043	0.052						[164]
Sheikh-Ali	Iran					Mafic–Ultramafic			4.2	0.2						[164]
Sheikh-Ali	Iran					Mafic–Ultramafic			1.54	0.48						[164]
Pacific Ocean	NE Pacific Ridge					Modern black smokers			4.8	0.54						[50,165]
Endeavour Segment (Slab A SZ-FTWZ)	Juan de Fuca Ridge					Modern black smokers			0.1	3.43		1.14				[29]
Endeavour Segment (Slab E SZ-FTWZ)	Juan de Fuca Ridge					Modern black smokers			0.39	8.8		1.26				[29]
Brothers Volcano (sample 57DR-1E)	Southern Kermadec Arc, New Zealand					Modern black smokers			0.21	26.6		0.5		0.0005		[166]
Brothers Volcano (sample 52DR-10)	Southern Kermadec Arc, New Zealand					Modern black smokers			0.0059	0.0085		0.0091		0.0001		[166]
Central Bransfield Strait (Hook Ridge)	Antarctica					Modern black smokers			4.5	18.5		2.6				[167]
Semenov-3 Hydrothermal Field	Mid-Atlantic Ridge					Modern black smokers			0.075	0.0203		0.0072		0.0002		[168]
Semenov-3 Hydrothermal Field	Mid-Atlantic Ridge					Modern black smokers			0.1745	0.0257		0.0085		0.0003		[168]
Karchiga Deposit (Central)	Rudny Altai, Siberia					Siliciclastic–Mafic			2.68	0.2		0.01				[63]
Karchiga Deposit (Northeastern)	Rudny Altai, Siberia					Siliciclastic–Mafic			2.44	0.83						[63]
Ermioni	Argolis, Greece					Siliciclastic–Mafic			4.8	0.15				0.02		[62]
Ermioni	Argolis, Greece					Siliciclastic–Mafic			3.08	0.09						[62]
Ermioni	Argolis, Greece					Siliciclastic–Mafic			0.72	0.6		0.14		0.02		[62]
Outokumpu	Finland					Siliciclastic–Mafic (Ultramafic)			3.8	1.07		0.005		0.013		[95]
Vuonos	Finland					Siliciclastic–Mafic (Ultramafic)			2.45	1.6		0.015		0.014		[95]
Aznalcóllar–Los Frailes (Massive sulfides)	IPB, Spain					Siliciclastic–Felsic (IPB)			0.3	3.8		2.1		0.01		[87,169,170]
Neves-Corvo	IPB, Portugal					Siliciclastic–Felsic (IPB)			2.1	1.2				0.12		[87,171]
Neves-Corvo	IPB, Portugal					Siliciclastic–Felsic (IPB)			1.3	2.9				0.12		[87]
Dergamysh	South Urals, Russia					Ultramafic (Atlantic)			1.3	0.3						[97]
Deposit name	Co	Cd	Ni	Bi	Mo	Se	Au	Ag	As	Sb	Hg	Tl	Te	Ga	Ge	Reference
Hokuroku District		1910		8.5	319		0.5	137	169				0.1	58		[107]
Hokuroku District		45		152	122		1.6	160	5890				35	6.5		[107]
Hokuroku District		103		26	14		0.2	317	22,900				1.0	12		[107]
Bisha Deposit	113.8	176.5			5.5		0.7	47.5	704	28.3	5.5			8.1		[161]
Talgan						15	1.7	44					10			[97]
Oktyabrskoye						30	2.4	24					22			[97]
Tash-Tau							4	58					5			[97]
Alexandrinskoye						8	1.1	37					39			[97]
Safyanovskoye						60	0.5	20					9.2			[97]
North Knoll							0.69	690	296	1000	0.81					[162]
North Knoll							1.75	349	3620	1150	15.1					[162]
North Knoll							1.59	286	3840	2870	19.9					[162]
North Knoll							0.96	310	548	197	0.25					[162]
North Knoll							1.04	290	943	239	3.57					[162]
North Knoll							1.34	865	1100	1360	6.3					[162]
Sultanovskoye						52	1.1	128					38			[97]
Yubileynoye						50	1.6	18					30			[97]
Yaman Kasy						22	3.3	34					325			[97]
Uzelga						110	1.8	34					101			[97]
Molodezhnoye						82	1.6	46					89			[97]
Valentonskoye						22	1.72	26					28			[97]
Othrys	50					5	0.26									[159]
Mathiati	54		33			20	0.8	4								[159]
Skouriotissa—Apliki	299			1.7		662	0.5		119	3.2			8.6			[163]
Kokkinoyia—Agrokipia	116			3.2		47	0.3		671	52			5.4			[163]
Buribay	0.03					50	5.5						20			[97]
Sheikh-Ali	256		19				0.42	34								[164]
Sheikh-Ali	24		37				0.44	28								[164]
Sheikh-Ali	147		23				0.18	10								[164]
Sheikh-Ali	266		31				0.18	16								[164]
Sheikh-Ali	224		24				0.25	21								[164]
Pacific Ocean					200	114	0.8	105	620	32						[50,165]
Endeavour Segment (Slab A SZ-FTWZ)	2	29			139	33		259	670	142		142				[29]
Endeavour Segment (Slab E SZ-FTWZ)	1	189			159	51		173	1020	125		167				[29]
Brothers Volcano (sample 57DR-1E)		590		0.5	55		1.43	430	2660	215	42	150	0.3	0.2		[166]
Brothers Volcano (sample 52DR-10)	750	1.6		5	10			0.7	1630	169	4	80		3.2		[166]
Central Bransfield Strait (Hook Ridge)							0.5		7600	573	57					[167]
Semenov-3 Hydrothermal Field	24	0.47	13	0.12	30	38			148	2.09		6.31	0.08	0.39	0.11	[168]
Semenov-3 Hydrothermal Field	38	0.94	12	0.43	37	44			123	2.45		5.69	0.76	0.39	0.06	[168]
Karchiga Deposit (Central)	280		100	8.5	7.2	57	0.12	6.7								[63]
Karchiga Deposit (Northeastern)	140		65	9.7	5.1	63	0.3	7.1								[63]
Ermioni	0.1		0.01	0.03			0.55	15		0.03						[62]
Ermioni	0.09			0.02			0.43	7		0.02						[62]
Ermioni	0.54		0.01				0.2	0.61								[62]
Outokumpu	2400	28.6	1200	0.72	19	24.2	0.8	8.9	81.6	2.34						[95]
Vuonos	1500	33.5	1300	0.65	17	6.6	0.1	11	74.4	1.84						[95]
Aznalcóllar–Los Frailes (Massive sulfides)							0.4	60								[87,169,170]
Neves-Corvo																[87,171]
Neves-Corvo																[87]
Dergamysh	0.25					54	2	12					16			[97]

Table 4. Bulk ore geochemistry of VMS deposit types and mineralizations around the world (Cu, Zn, Pb and Sn in wt%, all other elements in ppm).

Deposit Name	Region					VMS Type			Cu	Zn		Pb		Sn		Reference
Hokuroku district	Honsu, Japan					Bimodal-Felsic			1.23	41.20		1.58				[107]
Hokuroku district	Honsu, Japan					Bimodal-Felsic			8.85	0.77		0.28				[107]
Hokuroku district	Honsu, Japan					Bimodal-Felsic			2.01	2.06						[107]
Bisha Deposit	Western Nakfa Terrane, Eritrea					Bimodal-Felsic			0.99	5.72		0.2		0.0006		[161]
Talgan	South Urals, Russia					Bimodal-Felsic			3.2	3.6		0.45				[97]
Oktyabrskoye	South Urals, Russia					Bimodal-Felsic			4.16	2		0.1				[97]
Tash-Tau	South Urals, Russia					Bimodal-Felsic			6.8	2.1		0.25				[97]
Alexandrinskoye	South Urals, Russia					Bimodal-Felsic			2.7	4.6		0.52				[97]
Safyanovskoye	Central Urals, Russia					Bimodal-Felsic			2.2	0.7		0.13				[97]
North Knoll	Iheya Ridge, Okinawa Trough, Japan					Bimodal-Felsic			2.54	31.1		20.2				[162]
North Knoll	Iheya Ridge, Okinawa Trough, Japan					Bimodal-Felsic			4.63	36.3		11.7				[162]
North Knoll	Iheya Ridge, Okinawa Trough, Japan					Bimodal-Felsic			3.46	33.8		10.9				[162]
North Knoll	Iheya Ridge, Okinawa Trough, Japan					Bimodal-Felsic			1.26	36.5		9.14				[162]
North Knoll	Iheya Ridge, Okinawa Trough, Japan					Bimodal-Felsic			3.23	25.9		13.9				[162]
North Knoll	Iheya Ridge, Okinawa Trough, Japan					Bimodal-Felsic			3.06	24.5		15.4				[162]
Sultanovskoye	South Urals, Russia					Bimodal-Mafic			1.4	0.7		0.009				[97]
Yubileynoye	South Urals, Russia					Bimodal-Mafic			1.4	1						[97]
Yaman Kasy	South Urals, Russia					Bimodal-Mafic			2.6	5.6		0.03				[97]
Uzelga	South Urals, Russia					Bimodal-Mafic			1.3	2.6		0.15				[97]
Molodezhnoye	South Urals, Russia					Bimodal-Mafic			2.1	3.3		0.17				[97]
Valentonskoye	North Urals, Russia					Bimodal-Mafic			2.4	3.2		0.24				[97]
Othrys	Thessaly, Greece					Mafic–Ultramafic			1.8	0.12						[159]
Mathiati	Cyprus					Mafic–Ultramafic			0.2	0.26						[159]
Skouriotissa—Apliki	Solea Graben, Cyprus					Mafic–Ultramafic			0.18							[163]
Kokkinoyia—Agrokipia	Mitsero Graben, Cyprus					Mafic–Ultramafic			0.08							[163]
Buribay	South Urals, Russia					Mafic–Ultramafic			3.5	0.5						[97]
Sheikh-Ali	Iran					Mafic–Ultramafic			3.5	0.08						[164]
Sheikh-Ali	Iran					Mafic–Ultramafic			4.8	0.007						[164]
Sheikh-Ali	Iran					Mafic–Ultramafic			0.043	0.052						[164]
Sheikh-Ali	Iran					Mafic–Ultramafic			4.2	0.2						[164]
Sheikh-Ali	Iran					Mafic–Ultramafic			1.54	0.48						[164]
Pacific Ocean	NE Pacific Ridge					Modern black smokers			4.8	0.54						[50,165]
Endeavour Segment (Slab A SZ-FTWZ)	Juan de Fuca Ridge					Modern black smokers			0.1	3.43		1.14				[29]
Endeavour Segment (Slab E SZ-FTWZ)	Juan de Fuca Ridge					Modern black smokers			0.39	8.8		1.26				[29]
Brothers Volcano (sample 57DR-1E)	Southern Kermadec Arc, New Zealand					Modern black smokers			0.21	26.6		0.5		0.0005		[166]
Brothers Volcano (sample 52DR-10)	Southern Kermadec Arc, New Zealand					Modern black smokers			0.0059	0.0085		0.0091		0.0001		[166]
Central Bransfield Strait (Hook Ridge)	Antarctica					Modern black smokers			4.5	18.5		2.6				[167]
Semenov-3 Hydrothermal Field	Mid-Atlantic Ridge					Modern black smokers			0.075	0.0203		0.0072		0.0002		[168]
Semenov-3 Hydrothermal Field	Mid-Atlantic Ridge					Modern black smokers			0.1745	0.0257		0.0085		0.0003		[168]
Karchiga Deposit (Central)	Rudny Altai, Siberia					Siliciclastic–Mafic			2.68	0.2		0.01				[63]
Karchiga Deposit (Northeastern)	Rudny Altai, Siberia					Siliciclastic–Mafic			2.44	0.83						[63]
Ermioni	Argolis, Greece					Siliciclastic–Mafic			4.8	0.15				0.02		[62]
Ermioni	Argolis, Greece					Siliciclastic–Mafic			3.08	0.09						[62]
Ermioni	Argolis, Greece					Siliciclastic–Mafic			0.72	0.6		0.14		0.02		[62]
Outokumpu	Finland					Siliciclastic–Mafic (Ultramafic)			3.8	1.07		0.005		0.013		[95]
Vuonos	Finland					Siliciclastic–Mafic (Ultramafic)			2.45	1.6		0.015		0.014		[95]
Aznalcóllar–Los Frailes (Massive sulfides)	IPB, Spain					Siliciclastic–Felsic (IPB)			0.3	3.8		2.1		0.01		[87,169,170]
Neves-Corvo	IPB, Portugal					Siliciclastic–Felsic (IPB)			2.1	1.2				0.12		[87,171]
Neves-Corvo	IPB, Portugal					Siliciclastic–Felsic (IPB)			1.3	2.9				0.12		[87]
Dergamysh	South Urals, Russia					Ultramafic (Atlantic)			1.3	0.3						[97]
Deposit name	Co	Cd	Ni	Bi	Mo	Se	Au	Ag	As	Sb	Hg	Tl	Te	Ga	Ge	Reference
Hokuroku District		1910		8.5	319		0.5	137	169				0.1	58		[107]
Hokuroku District		45		152	122		1.6	160	5890				35	6.5		[107]
Hokuroku District		103		26	14		0.2	317	22,900				1.0	12		[107]
Bisha Deposit	113.8	176.5			5.5		0.7	47.5	704	28.3	5.5			8.1		[161]
Talgan						15	1.7	44					10			[97]
Oktyabrskoye						30	2.4	24					22			[97]
Tash-Tau							4	58					5			[97]
Alexandrinskoye						8	1.1	37					39			[97]
Safyanovskoye						60	0.5	20					9.2			[97]
North Knoll							0.69	690	296	1000	0.81					[162]
North Knoll							1.75	349	3620	1150	15.1					[162]
North Knoll							1.59	286	3840	2870	19.9					[162]
North Knoll							0.96	310	548	197	0.25					[162]
North Knoll							1.04	290	943	239	3.57					[162]
North Knoll							1.34	865	1100	1360	6.3					[162]
Sultanovskoye						52	1.1	128					38			[97]
Yubileynoye						50	1.6	18					30			[97]
Yaman Kasy						22	3.3	34					325			[97]
Uzelga						110	1.8	34					101			[97]
Molodezhnoye						82	1.6	46					89			[97]
Valentonskoye						22	1.72	26					28			[97]
Othrys	50					5	0.26									[159]
Mathiati	54		33			20	0.8	4								[159]
Skouriotissa—Apliki	299			1.7		662	0.5		119	3.2			8.6			[163]
Kokkinoyia—Agrokipia	116			3.2		47	0.3		671	52			5.4			[163]
Buribay	0.03					50	5.5						20			[97]
Sheikh-Ali	256		19				0.42	34								[164]
Sheikh-Ali	24		37				0.44	28								[164]
Sheikh-Ali	147		23				0.18	10								[164]
Sheikh-Ali	266		31				0.18	16								[164]
Sheikh-Ali	224		24				0.25	21								[164]
Pacific Ocean					200	114	0.8	105	620	32						[50,165]
Endeavour Segment (Slab A SZ-FTWZ)	2	29			139	33		259	670	142		142				[29]
Endeavour Segment (Slab E SZ-FTWZ)	1	189			159	51		173	1020	125		167				[29]
Brothers Volcano (sample 57DR-1E)		590		0.5	55		1.43	430	2660	215	42	150	0.3	0.2		[166]
Brothers Volcano (sample 52DR-10)	750	1.6		5	10			0.7	1630	169	4	80		3.2		[166]
Central Bransfield Strait (Hook Ridge)							0.5		7600	573	57					[167]
Semenov-3 Hydrothermal Field	24	0.47	13	0.12	30	38			148	2.09		6.31	0.08	0.39	0.11	[168]
Semenov-3 Hydrothermal Field	38	0.94	12	0.43	37	44			123	2.45		5.69	0.76	0.39	0.06	[168]
Karchiga Deposit (Central)	280		100	8.5	7.2	57	0.12	6.7								[63]
Karchiga Deposit (Northeastern)	140		65	9.7	5.1	63	0.3	7.1								[63]
Ermioni	0.1		0.01	0.03			0.55	15		0.03						[62]
Ermioni	0.09			0.02			0.43	7		0.02						[62]
Ermioni	0.54		0.01				0.2	0.61								[62]
Outokumpu	2400	28.6	1200	0.72	19	24.2	0.8	8.9	81.6	2.34						[95]
Vuonos	1500	33.5	1300	0.65	17	6.6	0.1	11	74.4	1.84						[95]
Aznalcóllar–Los Frailes (Massive sulfides)							0.4	60								[87,169,170]
Neves-Corvo																[87,171]
Neves-Corvo																[87]
Dergamysh	0.25					54	2	12					16			[97]

For this review, spider diagrams were developed with the distribution of selected metals (Zn, Pb, Cu, As, Sb, Ag and Au) normalized to Primitive Mantle and Bulk Continental Crust to correlate and investigate the relative degree of enrichment for the major VMS types (

Table 5. Major, precious and trace metal normalization values employed for bulk VMS ores.

Element	Primitive Mantle	Bulk Continental Crust [173]
Zn (ppm)	50 [173]	80
Sn (ppm)	0.12 [174,175]	2.5
Mo (ppm)	0.063 [176]	1
As (ppm)	100 [176]	1
Sb (ppm)	0.005 [176]	0.2
Pb (ppm)	0.175 [177]	8
Bi (ppb)	10 [173]	60
Cd (ppb)	40 [173]	98
Se (ppm)	0.04 [174,175]	0.05
Ag (ppb)	19 [176]	80
Cu (ppm)	28 [177]	75
Au (ppb)	1.3 [173]	3
Ni (ppm)	2080 [177]	105
Co (ppm)	104 [177]	29
Ga (ppm)	4 [174,175]	18

[173] Taylor and McClennan, 1985; [174] Sun and McDonough, 1989; [175] Joachum et al., 1993; [176] Wolf and Anders, 1980; [177] Hofmann, 1988.

; Figure 5). The order of metals in the spider diagrams is based on their degree of incompatibility, with the most incompatible elements located on the left side of the diagram (order of incompatibility based on the work of Barnes [172]). Despite the limited data, there are certain distinctive enrichment trends for the various VMS types. Firstly, VMS formed in evolved settings (e.g., Bimodal-Felsic) develop the highest enrichment degrees relative to Primitive Mantle and Bulk Continental Crust, reaching up to three orders of magnitude. In particular, the Bimodal-Felsic type shows the highest enrichment in Zn, Sb, Pb and Ag. It is worth mentioning that the Contemporary Black Smokers develop the widest spread among all VMS types (Figure 4), a characteristic largely attributed to the variability of the geological parameters affecting the ore chemistry between the examined localities [NE Pacific Ridge—50; Juan de Fuca Ridge—29; Southern Kermadec Arc, New Zealand—165; Antarctica—166; and Mid-Atlantic Ridge—167; Table 4), while VMS types formed in “juvenile” settings show the lowest degree of enrichment (e.g., Mafic–Ultramafic and Siliciclastic–Mafic), as they are mainly associated with mafic and ultramafic rocks. It is clear that the complexity in the geotectonic setting, volcanism and bedrock lithologies among the various VMS types results in precious, critical and strategic metal grades that vary within a very large range.

The geochemistry of the ore-forming fluids is also very crucial when the mineralogy and mineral chemistry of VMS are examined (

Table 6. Hypogene ore mineral chemistry of VMS deposits and mineralizations around the world (all elements in ppm).

Location	Region		Type			Phase		Analytical Method		Reference
Okinawa	Japan		Bimodal-Felsic			Pyrite		EPMA		[183]
Okinawa	Japan		Bimodal-Felsic			Sphalerite		EPMA		[183]
Okinawa	Japan		Bimodal-Felsic			Orpiment		EPMA		[183]
Keketale (sample 10kk-13-1)	NW China		Bimodal-Felsic			Pyrite (massive)		LA-ICP-MS		[184]
Keketale (sample 10kk-H-1-1)	NW China		Bimodal-Felsic			Pyrite (massive)		LA-ICP-MS		[184]
Keketale (sample kk-09-4-1)	NW China		Bimodal-Felsic			Pyrite (vein)		LA-ICP-MS		[184]
Keketale (sample kk-09-1-1)	NW China		Bimodal-Felsic			Pyrite (vein)		LA-ICP-MS		[184]
Cerro Maimon	Dominican Republic		Bimodal-Mafic			Sphalerite		EPMA		[68]
Cerro Maimon	Dominican Republic		Bimodal-Mafic			Tennantie		EPMA		[68]
San Fernando (upper stratiform)	Cuba		Bimodal-Mafic			Sphalerite		EPMA		[68]
San Fernando (upper stratiform)	Cuba		Bimodal-Mafic			Tennantite		EPMA		[68]
San Fernando (upper stratiform)	Cuba		Bimodal-Mafic			Tetrahedrite		EPMA		[68]
Tanguin Hydrothermal Field	Southern Okinawa Trough, China		Contemporary Black Smokers			Sphalerite (TVG11-2 av.)		EPMA		[185]
Tanguin Hydrothermal Field	Southern Okinawa Trough, China		Contemporary Black Smokers			Sphalerite (ROV11-2 av.)		EPMA		[185]
Tanguin Hydrothermal Field	Southern Okinawa Trough, China		Contemporary Black Smokers			Pyrite (TVG11-2 av.)		EPMA		[185]
Tanguin Hydrothermal field	Southern Okinawa Trough, China		Contemporary Black Smokers			Pyrite (ROV11-2 av.)		EPMA		[185]
Pobeda Hydrothermal Fields	Mid-Atlantic Ridge		Contemporary Black Smokers			Pyrite		LA-ICP-MS		[186]
Pobeda Hydrothermal Fields	Mid-Atlantic Ridge		Contemporary Black Smokers			Pyrite		LA-ICP-MS		[186]
Pobeda Hydrothermal Fields	Mid-Atlantic Ridge		Contemporary Black Smokers			Pyrite		LA-ICP-MS		[186]
Pobeda Hydrothermal Fields	Mid-Atlantic Ridge		Contemporary Black Smokers			Isocubanite + chalcopyrite		LA-ICP-MS		[186]
Pobeda Hydrothermal Fields	Mid-Atlantic Ridge		Contemporary Black Smokers			Isocubanite + chalcopyrite		LA-ICP-MS		[186]
Pobeda Hydrothermal Fields	Mid-Atlantic Ridge		Contemporary Black Smokers			Wurtzite		LA-ICP-MS		[186]
Pobeda Hydrothermal Fields	Mid-Atlantic Ridge		Contemporary Black Smokers			Pyrrhotite		LA-ICP-MS		[186]
Karchiga	Rudny Altai, Siberia		Pelitic–Mafic (Besshi)			Pyrite		EPMA		[63]
Karchiga	Rudny Altai, Siberia		Pelitic–Mafic (Besshi)			Pyrite		EPMA		[63]
Karchiga	Rudny Altai, Siberia		Pelitic–Mafic (Besshi)			Pyrite		EPMA		[63]
Karchiga	Rudny Altai, Siberia		Pelitic–Mafic (Besshi)			Pyrrhotite		EPMA		[63]
Karchiga	Rudny Altai, Siberia		Pelitic–Mafic (Besshi)			Chalcopyrite		EPMA		[63]
Karchiga	Rudny Altai, Siberia		Pelitic–Mafic (Besshi)			Sphalerite		EPMA		[63]
Karchiga	Rudny Altai, Siberia		Pelitic–Mafic (Besshi)			Magnetite		EPMA		[63]
Ermioni	Argolis, Greece		Pelitic–Mafic (Besshi)			Pyrite		EPMA		[62]
Ermioni	Argolis, Greece		Pelitic–Mafic (Besshi)			Pyrite		EPMA		[62]
Ermioni	Argolis, Greece		Pelitic–Mafic (Besshi)			Pyrite		EPMA		[62]
Ermioni	Argolis, Greece		Pelitic–Mafic (Besshi)			Chalcopyrite		EPMA		[62]
Ermioni	Argolis, Greece		Pelitic–Mafic (Besshi)			Chalcopyrite		EPMA		[62]
Alcudia Valley Mineral Field	Eastern Sierra Morena, Spain		Siliciclastic–Felsic (IPB)			Galena (Type B)		XRF (concentrates)		[187]
Alcudia Valley Mineral Field	Eastern Sierra Morena, Spain		Siliciclastic–Felsic (IPB)			Galena (Type C)		XRF (concentrates)		[187]
Alcudia Valley Mineral Field	Eastern Sierra Morena, Spain		Siliciclastic–Felsic (IPB)			Galena (Type D)		XRF (concentrates)		[187]
Alcudia Valley Mineral Field	Eastern Sierra Morena, Spain		Siliciclastic–Felsic (IPB)			Sphalerite (Type B)		XRF (concentrates)		[187]
Alcudia Valley Mineral Field	Eastern Sierra Morena, Spain		Siliciclastic–Felsic (IPB)			Sphalerite (Type C)		XRF (concentrates)		[187]
Alcudia Valley Mineral Field	Eastern Sierra Morena, Spain		Siliciclastic–Felsic (IPB)			Sphalerite (Type D)		XRF (concentrates)		[187]
Lagoa Salgada	IPB, Portugal		Siliciclastic–Felsic (IPB)			Pyrite (LS5-155.7)		EPMA		[188]
Lagoa Salgada	IPB, Portugal		Siliciclastic–Felsic (IPB)			Tetrahedrite (LS1-441.5)		EPMA		[188]
Lagoa Salgada	IPB, Portugal		Siliciclastic–Felsic (IPB)			Sphalerite (LS4-172.7-9)		EPMA		[188]
Wales Group	Prince of Wales Island, Alaska		Unclassified			Pyrite		LA-ICP-MS		[189]
Wales Group	Prince of Wales Island, Alaska		Unclassified			Sphalerite		LA-ICP-MS		[189]
Wales Group	Prince of Wales Island, Alaska		Unclassified			Chalcopyrite		LA-ICP-MS		[189]
Moira Sound unit	Prince of Wales Island, Alaska		Unclassified			Pyrite		LA-ICP-MS		[189]
Moira Sound unit	Prince of Wales Island, Alaska		Unclassified			Sphalerite		LA-ICP-MS		[189]
Moira Sound unit	Prince of Wales Island, Alaska		Unclassified			Chalcopyrite		LA-ICP-MS		[189]
Barrier Islands	Prince of Wales Island, Alaska		Unclassified			Pyrite		LA-ICP-MS		[189]
Kali Kuning-Lerokies orebodies	Wetar Island, Indonesia		Unclassified			Pyrite		EPMA		[190]
Kali Kuning-Lerokies orebodies	Wetar Island, Indonesia		Unclassified			Pyrite		EPMA		[190]
Kali Kuning-Lerokies orebodies	Wetar Island, Indonesia		Unclassified			Galena		EPMA		[190]
Kali Kuning-Lerokies orebodies	Wetar Island, Indonesia		Unclassified			Sphalerite		EPMA		[190]
Kali Kuning-Lerokies orebodies	Wetar Island, Indonesia		Unclassified			Chalcopyrite		EPMA		[190]
Kali Kuning-Lerokies orebodies	Wetar Island, Indonesia		Unclassified			Tennantite		EPMA		[190]
Kali Kuning-Lerokies orebodies	Wetar Island, Indonesia		Unclassified			Tetrahedrite		EPMA		[190]
Location	Cd	Co	Ni	Bi	Mo	Se	Au	Ag	As	Reference
Okinawa	5.7			9.7		321			21	[183]
Okinawa	4010			10.9				59		[183]
Okinawa								926		[183]
Keketale (sample 10kk-13-1)	0.02	255.72	21.09		0.01	3.4		0.02	120	[184]
Keketale (sample 10kk-H-1-1)	0.01	90.6	14.37		0.01	2.82		0.01	1159	[184]
Keketale (sample kk-09-4-1)	0.02	60.45	8.67		0.01	2.37	0.01	0.54	628.4	[184]
Keketale (sample kk-09-1-1)	0.02	222.63	24.74		0.02	2.64	0.04	0.92	842.7	[184]
Cerro Maimon	4100			900		100		300	100	[68]
Cerro Maimon	1900							2700		[68]
San Fernando (upper stratiform)	700			200		400		100	100	[68]
San Fernando (upper stratiform)				4300				8500		[68]
San Fernando (upper stratiform)				3700				48,300	38,600	[68]
Tanguin Hydrothermal Field		270	240			20	90	20	50	[185]
Tanguin Hydrothermal Field		140	150			40	160	10	10	[185]
Tanguin Hydrothermal Field		590	80			30	200	60	530	[185]
Tanguin Hydrothermal Field		700	180			160	110	60	310	[185]
Pobeda Hydrothermal Fields	1	2489	171	4	35	108	0.2	5.6	61	[186]
Pobeda Hydrothermal Fields	4.4	1012	55	1.4	86	53	0.3	17	205	[186]
Pobeda Hydrothermal Fields	0.8	921	50	1	59	52		11	107	[186]
Pobeda Hydrothermal Fields	19	1754	56		3.3	218		20	2.2	[186]
Pobeda Hydrothermal Fields	9.3	1954	65	3.8	41	700	0.3	10	15	[186]
Pobeda Hydrothermal Fields	837	439	6.4		9.3	16	0.4	15	74	[186]
Pobeda Hydrothermal Fields	0.7	0.3	7.5	0.3	8	3.2	0.2	5.7	12	[186]
Karchiga	100	1100	200			100				[63]
Karchiga		1900								[63]
Karchiga	100	3500	100			100				[63]
Karchiga	100	800				200		200		[63]
Karchiga	100	200				300				[63]
Karchiga	3700	1100	800					800		[63]
Karchiga		100								[63]
Ermioni		500				100	100			[62]
Ermioni		200	100			200	300		900	[62]
Ermioni		600				200	200	300		[62]
Ermioni						200		100	100	[62]
Ermioni		100							100	[62]
Alcudia Valley Mineral Field	20.8	3.5		21.5		352	0.012	425		[187]
Alcudia Valley Mineral Field	8.5	3		1.5		79	0.008	480	18.3	[187]
Alcudia Valley Mineral Field	28.6	0.8		3.1		28.4	0.03	436	0.8	[187]
Alcudia Valley Mineral Field	1300	446						29.3	41.3	[187]
Alcudia Valley Mineral Field	735	361						27.2	16.2	[187]
Alcudia Valley Mineral Field	2372	216						16.5	25.7	[187]
Lagoa Salgada				520				170	410	[188]
Lagoa Salgada				740				1620	2360	[188]
Lagoa Salgada	3440					340				[188]
Wales Group	1.07	203	34.5	0.02	2.9	48		0.77	124	[189]
Wales Group	1465	4.56	8.44	0.09	15.4	311		12.7	25.9	[189]
Wales Group	9.86	1.34	12.4	0.13	3.36	239		14.9	10.7	[189]
Moira Sound unit	0.95	21.6	288	0.06	88.8	34.1		2.53	20.8	[189]
Moira Sound unit	1928	9.7	6.85	0.65	5.68	14.6		17.7	15	[189]
Moira Sound unit	8.79	0.56	6.81	0.46	13.3	258		9.23	46.8	[189]
Barrier Islands	1.11	46	132	0.02	12.4	45.8		2.67	1979	[189]
Kali Kuning-Lerokies orebodies						400			67,100	[190]
Kali Kuning-Lerokies orebodies						3800		1900		[190]
Kali Kuning-Lerokies orebodies						100		2600	700	[190]
Kali Kuning-Lerokies orebodies						300				[190]
Kali Kuning-Lerokies orebodies									1000	[190]
Kali Kuning-Lerokies orebodies	700					1300		500		[190]
Kali Kuning-Lerokies orebodies						500		100	70,200	[190]
Location	Sb	Te	Hg	Sn	In	Ga	Ge		Tl	Reference
Okinawa	10.4			11.8						[183]
Okinawa	5.5		19	744		16.3	12.9			[183]
Okinawa	181					254				[183]
Keketale (sample 10kk-13-1)	6.76									[184]
Keketale (sample 10kk-H-1-1)	0.01									[184]
Keketale (sample kk-09-4-1)	1.52									[184]
Keketale (sample kk-09-1-1)	5.16									[184]
Cerro Maimon	800	300								[68]
Cerro Maimon	6200	17,300								[68]
San Fernando (upper stratiform)	200	300	3600							[68]
San Fernando (upper stratiform)	37,600		700							[68]
San Fernando (upper stratiform)										[68]
Tanguin Hydrothermal Field	50		100	30						[185]
Tanguin Hydrothermal Field	80		240	80						[185]
Tanguin Hydrothermal Field	10		50	80						[185]
Tanguin Hydrothermal Field	30		40	30						[185]
Pobeda Hydrothermal Fields	1.9	2		0.9					1.7	[186]
Pobeda Hydrothermal Fields	8.2	0.8		1.6					5.8	[186]
Pobeda Hydrothermal Fields	4.2	0.6		0.8					2.2	[186]
Pobeda Hydrothermal Fields	0.3	11.4		3.9					0.1	[186]
Pobeda Hydrothermal Fields	1.3	33		26					2	[186]
Pobeda Hydrothermal Fields	196	0.4		54					0.3	[186]
Pobeda Hydrothermal Fields	0.2	1		3.3					0.7	[186]
Karchiga										[63]
Karchiga										[63]
Karchiga										[63]
Karchiga										[63]
Karchiga										[63]
Karchiga										[63]
Karchiga										[63]
Ermioni										[62]
Ermioni		100								[62]
Ermioni										[62]
Ermioni		2900								[62]
Ermioni										[62]
Alcudia Valley Mineral Field	553		34							[187]
Alcudia Valley Mineral Field	942		15.7							[187]
Alcudia Valley Mineral Field	809		17.1							[187]
Alcudia Valley Mineral Field	88.8			27.2	76	46	16.2			[187]
Alcudia Valley Mineral Field	66			8.5	2.3	5	54			[187]
Alcudia Valley Mineral Field	65.5			12.7	16.8	4.2	18.8			[187]
Lagoa Salgada					80					[188]
Lagoa Salgada			1160		100					[188]
Lagoa Salgada				90	270					[188]
Wales Group	0.39	1.2	0.67	0.73	0.11				0.08	[189]
Wales Group	3.43	3.97	71.5	11.3	29.2				0.87	[189]
Wales Group	2.11	2.61	2.16	16.9	21.3				0.49	[189]
Moira Sound unit	0.38	0.48	0.34	0.42	0.16				0.09	[189]
Moira Sound unit	0.65	1.49	29.2	5.93	54.5				0.18	[189]
Moira Sound unit	3.3	8.17	1.28	4.48	42.8				1.44	[189]
Barrier Islands	39.2	0.68	2.21	0.96	0.11				46.4	[189]
Kali Kuning-Lerokies orebodies										[190]
Kali Kuning-Lerokies orebodies										[190]
Kali Kuning-Lerokies orebodies	13,100									[190]
Kali Kuning-Lerokies orebodies	300		600							[190]
Kali Kuning-Lerokies orebodies	1200		1400							[190]
Kali Kuning-Lerokies orebodies	39,500									[190]
Kali Kuning-Lerokies orebodies										[190]

). Many trace elements may be incorporated in the lattice of major sulfides (Table 2 and Table 6), thus providing additional potential to the massive sulfide ores exploited. For instance, sphalerite, a major phase in many VMS types, also acts as an important carrier of critical metals, such as Ga, Ge, In, Cd and Hg [178,179,180,181]. There are cases where the trace metal content is abnormally high, with a typical example being the Hg-bearing sphalerite in the Eskay Creek (Hybrid Bimodal-Felsic VMS, Canada). Sphalerite analyses from Eskay Creek show very high Hg content reaching up to 8.4 wt% [182], further supporting the potential of VMS in the exploration of critical and strategic metals. Regarding minor VMS phases, late-stage sulfosalts usually act as scavengers of many precious and critical metals, such as Ag, Bi, Se, Te and Cd. Moreover, pyrite, which in most cases is discarded as waste, may also incorporate several critical metals in its lattice, such as Co, Ni and As, rendering the VMS ore reassessment critical.

There are cases where the host-rock lithology combined with the source of the hydrothermal fluids may result in rather atypical ore parageneses [191]. Such an example is the Siliciclastic–Mafic (Pelitic–Mafic or Besshi-type) deposit of Ermioni (Argolis, Greece), characterized by very simple mineralogy with abundant pyrite (>95% modal), minor chalcopyrite and traces of sphalerite [44,62]. According to Triantafyllidis et al. [62], this atypical and very simple VMS ore mineralogy was the result of the circulation of mixed magmatic (mafic) fluids mixed with seawater through mafic and ultramafic lithologies of the bedrock, with pyrite bearing very low trace element content (barren) in its lattice (e.g., As, Co, Ni, Sn, etc.). Therefore, the seemingly worthless pyrite-rich Ermioni ore was exploited for more than 75 years in the production of sulfuric acid for fertilizers. Although the data are limited, there are distinctive trends on the degree of enrichment of critical metals in specific VMS types and phases (Table 6).

Precious metals Au and Ag develop different trends among the various VMS types. Gold enrichment remains relatively constant among all VMS types, indicating that other factors besides host-rock lithology and volcanism define Au solubility and deposition in VMS deposits, whereas Ag shows the highest degree of enrichment in evolved settings, e.g., the Bimodal-Felsic and Bimodal-Mafic VMS types (Table 4; Figure 4 and Figure 5). Regarding individual trace elements, Co and Ni tend to be enriched in Siliciclastic–Mafic VMS (Figure 4 and Figure 5), mainly incorporated in the lattice of pyrite, and this enrichment is largely related to the mafic and ultramafic lithologies predominating in the bedrock (Table 6). Bismuth and Te, in addition to the Bimodal-Felsic type, also develop increased enrichment in Bimodal-Mafic VMS, closely associated with sulfosalts (tennantite–tetrahedrite) and lesser with sphalerite (Table 6; Figure 4 and Figure 5). Cadmium is enriched in the Bimodal-Mafic, the Siliciclastic–Mafic and the Siliciclastic–Felsic VMS types (Table 4 and Table 6; Figure 4). Although Cd is preferentially incorporated in the lattice of sphalerite, it is also found in significant contents in tennantite–tetrahedrite species (Table 6), and rarely as Cd-sulfides, hawleyite and greenockite (Table 2). Selenium shows the highest degree of enrichment in the Mafic–Ultramafic-type VMS (Table 4), directly related to the composition of the host lithologies, with lower enrichment degrees in all other VMS types. Selenium is mainly incorporated in pyrite and sphalerite and less so in galena and chalcopyrite (Table 6).

All aforementioned features regarding bulk geochemistry, ore mineralogy and mineral chemistry not only prove the variable content in precious and trace metals in all VMS types but also the diversity of sulfides and sulfosalts bearing critical metals, an observation directly linked to the discrepancies in both the geotectonic setting, the composition and type of volcanism, and the host-rock lithology during hydrothermal circulation and eventually VMS formation.

6. Fossilized Versus Modern VMS Deposits

To this day, the exploitation of VMS deposits involves mining terrestrial ore bodies found on land as a result of the various geologic and geotectonic processes that eventually led to the emplacement of VMS above sea level [6]. Despite the importance of VMS in the past, the present, and most importantly the future evolution of mankind, especially when considering the contemporary focus on the exploitation of in situ modern VMS systems (both active and inactive; refer to Section 7), there are certain characteristics that discriminate fossilized and modern VMS deposits that greatly affect the exploitation potential of the latter. Several post-ore processes are involved in the geologic evolution of primary VMS, including (for all or some) diagenesis (both early and late), tectonism, and metamorphism (see [192]), with the end-result being the development of fossilized VMS deposits that differ significantly from their primary counterparts (modern VMS) mainly in shape, texture and mineralogy, and rarely in geochemistry (e.g., Co enrichment of the Siliciclastic–Mafic deposits of the Rudny Altai region; [145] and references therein).

In general, the vast majority of fossilized VMS have been subjected to low-to-high grade metamorphism during emplacement, with the degree of metamorphism directly related to the geotectonic evolution of the VMS-bearing strata (see [192]). The major characteristic of fossilized VMS that discriminate them from modern VMS systems is the coherence of the massive sulfide ore, with the first considered hard and compact and the latter brittle and highly porous. As stated earlier, research on modern VMS settings during recent decades has revealed that they largely comprise loose accumulations of fine-grained sulfides on the seafloor, while they also form sub-seafloor ore bodies with typical penetrating and replacement textures due to circulation of the ore-forming fluids within the host loose sediments or volcanic rocks of the seafloor (see [3,46,59,63]). Seafloor VMS formation is attributed to the collapse of the feeder chimneys that discharge ore-bearing hydrothermal fluids on the sea floor [50,51,52,53], with a typical clastic sandy texture [3]. Other textural features of modern VMS comprise colloform, botryoidal textures and microchimneys, depicting seafloor microbial activity during ore formation, features that may be preserved in evolved fossilized VMS deposits (e.g., the Feitais deposit, Portugal [193]; the Jean Charcot trough, northern New Hebrides, Vanuatu, Solomon Islands [194]; and the Iberian Pyrite Belt [195]).

Despite their geologic evolution, fossilized VMS may still preserve the shape of the primary sulfide ores formed on the seafloor or sub-seafloor (e.g., Murgul deposit, Artvin, NE Turkey [196]). Yet, in most cases the shape of the primary VMS ore bodies is altered, with the most pronounced effect being the development of irregular, folded or thinned ore bodies that follow the stratigraphy of the host rocks (e.g., Hollandaire VMS deposit, Western Australia [197]; Jaguar VMS deposit, Western Australia [198]; Keketale deposit, NW China [184]). Moreover, in intense tectonic settings, the boundary between the lower stringer (feeder) zone and the upper massive sulfide ore usually acts as a detachment plain, resulting in dismembering of the primary VMS structure (e.g., the Siliciclastic–Mafic Ermioni deposit, Argolis, Greece [64]).

Besides coherence and shape, the mineralogy of VMS deposits is also affected by metamorphic and tectonic processes during emplacement. For instance, various generations of pyrite may be identified in VMS deposits (e.g., the Karchiga VMS deposit, Rudny Altai; [63]), which were not part of the primary ore paragenesis and are related to post-ore geologic processes. These phases are easily distinguished based on mineral chemistry and texture (e.g., the Keketale deposit, NW China [184]). As an example, primary colloform pyrite may be replaced by crystalline pyrite (e.g., the Karchiga deposit, Rudny Atai region [63]), whereas coarse-grained and euhedral pyrite may also be associated with metamorphism, mainly after replacement of primary pyrrhotite and/or marcasite, although it is still debatable whether pyrite replaces primary pyrrhotite, or vice versa, as there are many examples where both phenomena are observed (refer to [145] and references therein). Copper, a common trace constituent of mafic volcanic rocks, is characterized by significant mobility during metamorphism and therefore may be found in increased content in metamorphosed VMS deposits hosted in mafic volcanic rocks as a result of the re-distribution of Cu either from the VMS ore body or from the surrounding host lithologies, resulting in ore bodies that are locally enriched in Cu (e.g., the Roro ore body, Ermioni Siliciclastic–Mafic deposit, Argolis, Greece [62]). At the same time, chalcopyrite, the major Cu carrier in VMS (Table 2), is both soft and ductile, and metamorphism may cause changes in its texture. Primary fine-grained chalcopyrite may be subjected to textural changes and transformed into anhedral masses and veins/veinlets developed around harder and brittle phases, such as pyrite, or sulfide clasts (e.g., the Siliciclastic–Mafic Ermioni deposit, Argolis, Greece [62]).

Considering modern VMS systems, the role of early diagenesis is very important when discussing the possibility of deep-sea exploitation. This early diagenesis is largely related to in situ geologic processes that either result in surficial oxidation and/or replacement of primary sulfides by low-temperature phases (refer to Section 4.2 for details). For the vast majority of fossilized VMS, these primary characteristics are usually destroyed due to post-ore evolution and are not accounted for during exploitation. In modern VMS, these features are preserved, including the presence of abundant sulfide weathering products (e.g., iron oxide-hydroxides; Table 3), while at the same time, the massive sulfide ore is also highly porous and brittle, with possible negative effects on mining and ore processing.

To conclude, although the focus of the contemporary debate regarding modern VMS exploitation is largely concentrated on the development of effective mining technologies for deep-sea mining (see Section 7 for details), the ore processing methodologies required are equally important. The particular characteristics of modern VMS systems, including ore texture and mineralogy, that are significantly different relative to fossilized VMS under exploitation, require the development of case-specific processing, enrichment and metallurgical methodologies in order to fully exploit the potential of modern VMS for base, critical and precious metals.

7. VMS Prospecting and Future Exploitation Technologies

During the past millennia, VMS exploitation was focused on terrestrial settings and up to a few hundred meters below the surface, environments generally characterized by easy access and relatively low mining costs [6]. Relative to other sulfide-bearing mineralizations and deposits formed at deep crustal settings that are considered prohibitive for exploitation with contemporary economic and technological standards, VMS are characterized by two major advantages. Firstly, VMS formation is an ongoing process (e.g., the TAG field in the Atlantic Ocean, Atlantis-II Deep in the Red Sea, and the North Pacific and Juan de Fuca Ridges in the Pacific Ocean), with massive sulfides continuously being formed on the seafloor (e.g., black smokers). Secondly, there are many VMS deposits that are still located in their primary setting (in situ) with their size and grades being estimated, thus making deep-sea basins favorable targets.

The potential of deep marine settings for mineral resources is not a recent idea. The earliest exploration attempts took place in the late 19th century (1873–1876) when the expedition of the H.M.S. Challenger discovered manganese nodules on the ocean floor [199,200]. Such endeavors continued in the following decades with the most important milestones being the discoveries of: (i) Metalliferous muds and hot brines in the Red Sea (1963–1965 [201]), (ii) Black Smokers at the East Pacific Rise (1978, 1979 [202]), and (iii) Seabed Massive Sulfides (henceforth SMS) at the Mid-Atlantic Ridge (TAG Field [27]) in 1985 and the Island Arc System at Manus Basin in 1986 [203]. Ongoing research showed that SMS are found in many parts of the ocean floor, particularly along plate margins and spreading centers (Figure 1). The total reserves in modern and hydrothermally active sites are estimated to 650 metric Mt massive sulfides with 10 Mt Cu, 29 Mt Zn, 1 Mt Pb, 33,000 t Ag and 750 t Au [204,205].

Many models and technologies have been proposed and developed since the early 1960s regarding DSM, including the “Submarine-Drag Bucket” [206,207], the “Continuous Line Bucket” [208], the “Shuttle Vessel” and the “Pipeline Lift” mining systems, with the first two primarily developed for the exploitation of manganese nodules in abyssal plains ([209] and references therein). Although a detailed analysis of the characteristics and the advantages and disadvantages of the developed mining models surpasses the objectives of this review, it is important to state that pilot tests have been successful since the late 1970s, primarily in the recovery of manganese nodules. In particular, in 1978, Ocean Management Incorporated (OMI) performed the first successful pilot test on manganese nodule recovery by implementation of the pipeline lift mining system [210].

Due to the inherent characteristics of SMS, including the uneven distribution and thickness of the SMS orebodies combined with the varying seafloor geomorphology, the pipeline lift mining system is largely considered the most suitable, while at the same time, it is also considered the most reliable [209]. Research shows that the height of the water column (usually greater than 2 km and up to 6 km) and the velocity of the ocean currents (hydrodynamic forces) play the most crucial role in the efficiency of all DSM systems [209,211].

The structure of the pipeline lift mining systems comprises the following parts ([205,211]; Figure 6):

• The mining machine that extracts the resources from the seafloor and also performs desilting and crushing of the collected material (slurry);
• The slurry-conveying hose that transports the slurry to the submersed warehouse where it is collected. A buoyancy module is attached to the conveying hose providing the mining machine with the ability to move around within a certain range. The submersed warehouse is usually located a few tens of meters above the sea floor;
• A hard tube (pipeline) connects the submerged warehouse with the mining support vehicle on the sea surface. A slurry pump is attached to the pipeline with objective of transporting the slurry to the support vehicle.

The exploitation of marine resources for each country may be possible in Exclusive Economic Zones (EEZ) and/or Legal Continental Shelves, yet most SMS are formed in deep marine settings far from continental shelves (refer to [25] and references therein). Following the successful pilot tests in the late 1970s, the United Nations Convention on the Law of the Sea in 1982 determined the International Seabed Authority—ISA, with responsibility to develop case-specific legislation regarding DSM [213].

The importance of Deep-Sea Mining (DSM) was clearly expressed by the USGS (United States Geological Survey) stating that by 2065, nearly 40% of the global demand for critical and strategic metals will be covered by DSM, provided the latter is feasible [214]. Yet, despite the significant advantages of DSM, including the absence of physical mine and transport infrastructures [215], it is also very important to take into consideration the specific textural and mineralogical characteristics of modern VMS that differentiate them from fossilized VMS (refer to Section 6 for details). Additionally, and equally important, is to address the environmental impact of such projects. The deep-sea mining of SMS (active and inactive settings; Figure 1) may adversely affect all marine environments, including benthic, bathypelagic, mesopelagic and epipelagic, both in scale and time ([25,216] and references therein).

8. VMS in Human History and Future Perspectives

It is known that mineral resource exploitation goes hand-in-hand with the development and evolution of mankind through millennia. Historically, many populations in the Mediterranean region and around the world have flourished due to metal exploitation and processing, with VMS playing a crucial role in the process [217,218,219]. Historical evidence shows that native metals, mainly Cu and to a lesser extent Ag and Au (“cold” metallurgy), were the first to be exploited during the transition from the Neolithic Age to the Copper Age (also referred to as the Chalcolithic Age) around 3000 BC [220], although there is evidence that in some areas, Cu processing began as early as 5000 BC [221]. In later periods (Bronze Age, ~3000–1200 BC [222] and references therein), metallurgical processing involved both secondary and primary sulfide ores with the objective to produce either native metals, such as metallic copper, or alloys (“hot” metallurgy) with improved characteristics, including bronze (Cu-Sn±As alloy) and brass (Cu-Zn alloy). It is also worth mentioning that although copper processing was considered a rather selective and fine craft, Fe ore processing, which emerged later (Iron Age, ~1200–500 BC) ([223] and references therein) with the objective of producing metallic iron and Fe-bearing alloys (e.g., steel), was a very widespread and common process. Among the various secondary Fe ores mined to produce metallic Fe, gossans formed after supergene alteration of VMS, that are characterized by an abundance of Fe-bearing sulfides, including pyrite, pyrrhotite and chalcopyrite, were also exploited (refer to [111] for Iberian Pyrite Belt gossans).

Regarding VMS exploitation and processing, there is abundant historical evidence proving the contribution of VMS, both as primary and secondary ores, in the development of mankind. For instance, Etruria in northern Italy (Tuscany) was one of the most important Fe ore processing and copper metallurgy sites in the Mediterranean during the 1st Millennium BC ([224] and references therein). Although skarn-type sulfides were also exploited primarily for Sn production, a large number of stratabound Cu-Zn VMS associated with ophiolites were also exploited to produce metallic Cu and associated alloys [219]. The Iberian Pyrite Belt (IPB), the largest VMS province in Europe and one of the largest in the world, hosts large to very large and giant VMS deposits that have been exploited since antiquity ([225] and references therein), while exploitation continues to this day. Local Iberian populations, and later the Romans, exploited both primary and secondary (supergene) ores from 3000 BC with the peak of exploitation and processing taking place during the reign of the Roman Empire in the 2nd and 1st century AD [226]. At the same time, in the eastern Mediterranean, large-scale exploitation was taking place in Cyprus after extensive mining and processing of the Mafic–Ultramafic-type (former Cyprus type; Table 1) VMS deposits at the foot of Troodos Mountain (e.g., Skouriotissa, Mavrovouni, Polys Chrysochous; for details refer to [227,228,229,230,231]).

In later periods, particularly during the Medieval and the Renaissance, the extent of metallic ore exploitation, including VMS, received a significant increase, thus contributing to the development of Western European countries and cultures [232]. It was during the mid-18th century and the Industrial Revolution when sulfide ore exploitation showed an exponential increase, as the demand for base metals to cover industrial needs was further augmented (see [233] for IPB). Up until the late 19th century, among other massive sulfide ores, VMS were considered major base metal resources, as well as significant sources of precious metals, mainly based on past experience. This consensus changed in the early 20th century with the discovery of the “Froth Flotation” method that made processing low-grade sulfide ores, such as porphyry Cu deposits, feasible. Still, advances in instrumental analysis, particularly after WWII, combined with the discovery of hydrothermal venting centers in deep-sea basins and oceans, resulted in a rebirth of the interest in VMS exploration, not only for typical base metals but also for strategic and critical metals found as traces in VMS (see Table 4 and Table 6), whose demand has increased rapidly during the past 60 years.

It is more than clear that mankind’s evolution in the past millennia was closely associated with exploitation of fossilized VMS deposits. Apparently, this will not change in the future, too, as recent developments in computational technologies and exploration techniques could largely contribute to the discovery of new fossilized VMS deposits on land (e.g., the discovery of Cu and Sn ore bodies in 1977, Neves-Corvo VMS deposit, Portuguese part of IPB [8]). Yet, the most significant goal of all VMS exploration and mining schemes involves the exploitation of in situ active and inactive contemporary VMS deposits found in ocean and deep-sea basins (see Section 7 for details), which could meet the future needs of mankind, particularly in critical and strategic metals. Still, one has to consider the particular characteristics of modern VMS that differentiate them from fossilized VMS exploited in terrestrial settings, necessitating the development of case-specific ore-processing methodologies in order to exploit the base, precious and critical metal potential of SMS.

9. Conclusions

Volcanogenic massive sulfides are among the most important sulfide ores formed on the Earth’s crust, and they have greatly contributed to the evolution and development of mankind since prehistoric times. Their most significant features may be summarized as follows:

• They may form in geotectonic settings characterizing both convergent and divergent margins, with the only common features among all VMS types being the spatial and/or temporal relation with submarine volcanism and the massive ore texture, regardless of ore mineralogy and geochemistry;
• Their formation has been occurring since the Paleoarchean (3.55 Ga) and continues to this day in active settings (e.g., black smokers), indicating that the geological processes leading to their formation will also carry on in the future;
• Despite the importance of magma at depth that acts either as a heat source and/or as a source of metals, the host rock lithology and geochemistry play the most important role in the mineralogy and mineral chemistry of the massive sulfide ores deposited;
• Although the various VMS types may point to different geotectonic regimes, a very small number of sulfide phases, such as pyrite, pyrrhotite, chalcopyrite, galena and sphalerite, characterize the main ore mineralogy;
• Due to subtle differences in the setting of VMS formation, the ore geochemistry, including precious and critical metals, may vary within a great range;
• Despite the exploitation of VMS since antiquity, primarily for their base and precious metal content, they are also significant sources of critical and strategic metals highly sought after in recent years, including Co, Ni, Ga, Ge, In, Bi, As, Sb, Se, Mo, Cd, Sn, Hg, Tl and Bi;
• The importance of VMS is not exhausted in terrestrial deposits exploited in the past and present, or in new deposits discovered on land. During the last 60 years, exploration of the oceans has revealed the presence of massive sulfides on the ocean floor (SMS), especially along plate margins, verifying the increased potential of VMS as sources of critical metals that could cover the world’s continuously increasing demands;
• Significant differences characterize fossilized and modern VMS in terms of spatial development, texture and ore mineralogy, features that are largely obscured or destroyed in fossilized VMS. Therefore, specific approaches are required in the development of ore processing methodologies and DSM in order to fully exploit the potential of modern (active and inactive) VMS deposits;
• It is more than clear that VMS will continue to play a crucial role in the future evolution of mankind, not only for base and precious metals, but also towards strategic and critical metals.