April 2017 Issue Table of Contents
Magmatic sulfide ore deposits are products of natural smelting: concentration of immiscible sulfide liquid (‘matte’), enriched in chalcophile elements, derived from silicate magmas (‘slags’). Sulfide ore deposits occupy a spectrum from accumulated pools of matte within small igneous intrusions or lava flows, mined primarily for Ni and Cu, to stratiform layers of weakly disseminated sulfides within large mafic–ultramafic intrusions, mined for platinum-group elements. One of the world’s most valuable deposits, the Platreef in the Bushveld Complex (South Africa) has aspects of both of these end members. Natural matte compositions vary widely between and within deposits, and these compositions are controlled largely by the relative volumes of matte and slag that interact with one another.
Keywords: nickel, sulfide, platinum, layered intrusions, large igneous provinces, magma, igneous petrology
Magmatic sulfide deposits are nature’s smelters. By the same process that has been used since prehistoric times to extract metals from ores, magmatic sulfide ores form by the interaction between immiscible sulfide–oxide liquids (mattes) with silicate magmas (slags). Scavenging of chalcophile elements – Ni, Cu, Au and the platinum-group elements (PGEs) – and the accumulation of the matte component (Fig. 1) has produced some of the world’s most valuable economic metal concentrations (Naldrett 2004). These currently account for ~56% of the world’s Ni production and over 96% of Pt, Pd and the other PGE production (Mudd and Jowitt 2014).
Magmatic Sulfide Deposit Settings
On a deposit scale, magmatic sulfide accumulations are found in a variety of host igneous rock bodies. Broadly, they fall into two major categories: sulfide-rich, exploited primarily for Ni and Cu; and sulfide-poor (typically less than 5% sulfide), dominated by the platinum-group elements (PGEs) and Au.
Sulfide-rich deposits dominated by Ni–Cu can be categorized into three types:
- Sulfide-rich accumulations in small mafic or mafic–ultramafic intrusions, (Figs. 1A, B), usually identifiable as magma conduits (Lightfoot and Evans-Lamswood 2015) and exploited mostly for Ni, Cu and Co. Important examples include Voisey’s Bay (Canada), Jinchuan (China) and the Noril’sk-Talnakh deposits of Siberia (Russia). The PGEs are minor by-products, with the notable exception of the Noril’sk-Talnakh ores where PGEs are unusually abundant.
- Accumulations of sulfide in komatiites, such as the Kambalda and Perseverance deposits in Australia (Barnes 2006), or in ferropicrites, such as the Pechenga deposits in Russia (Hanski et al. 2011). Deposits are hosted in lava flows (Fig. 2A) or shallow subvolcanic intrusions. Exploited dominantly for Ni only.
- Sulfide accumulation beneath an impact-generated crustal melt sheet: the unique example of the Sudbury Ni–Cu–PGE ores (Keays and Lightfoot 2004).
Sulfide-poor deposits dominated by PGEs can be categorized into two types:
- Stratiform accumulations with a few percent disseminated sulfide in cumulates within large layered mafic–ultramafic intrusions, including PGE-enriched “reefs” (Naldrett et al. 2008). Such deposits are typically exploited for PGEs with by-product Ni, Cu and Co. They generally occur as remarkably thin and persistent layers. The best-known example is the Merensky Reef of the Bushveld Complex (Fig. 1C). This exceptional layer is commonly only a few tens of centimetres thick but extends continuously for over 400 km.
- Stratabound sulfide disseminations, commonly PGE-rich, in the marginal rocks of large layered intrusions. An example is the Platreef of the Bushveld Complex (McDonald and Holwell 2011).
The Form of Ore-Hosting Magma Bodies
Stratiform reef-style PGE deposits are exclusively hosted within large sill-like or boat-shaped layered mafic–ultramafic intrusions, usually several kilometres thick. But there is a much greater diversity of form in the magma bodies that host Ni–Cu-dominant, sulfide-rich orebodies (Fig. 2). These latter types all represent the products of magma flowing through restricted conduits or channels, leaving behind an accumulated residue of sulfide liquid and cumulus silicate minerals. These conduits can be feeder tubes or channels within extensive komatiite lava-flow fields (Lesher 1989; Barnes 2006) (Fig. 2A), or feeders to large igneous province volcanism in the form of sill–dike combinations (Fig. 2C) or tube-like conduits (Figs. 2D, E) (Barnes et al. 2016; Lightfoot and Evans-Lamswood 2015). Almost all examples show much larger proportions of sulfide and cumulus silicate minerals (typically olivine) within the flow or intrusion than could have been dissolved in a volume of magma equal to that of the host body. This suggests that magma flowed through the conduit leaving crystals and sulfide liquid behind. Commonly, there is evidence of thermal or thermomechanical erosion, in the form of transgressive footwall troughs beneath komatiite flows (Fig. 2A), or in tube-like or ‘ski-like’ intrusions (chonoliths) that truncate layering within the country rock and often contain partially digested wall-rock fragments (Figs. 2D, E). Such host bodies are usually very small compared to the total volume of magmatism in the province: in the case of the ore-hosting intrusions of the Noril’sk-Talnakh camp, it is about 1 millionth the total volume of the Siberian Trap lavas.
Magmatic sulfide ore deposits generally occur in intracratonic settings, commonly associated with mantle plume activity. Sulfide-rich Ni–Cu-dominant deposits are commonly located close to the margins of ancient Archean cratonic blocks. This is thought to be the result of plume impingement at the base of the craton, with consequent channeling of the magmas into major crustal fault systems around the margins (Begg et al. 2010). A number of Ni–Cu deposits, including those of the Central Asian Orogenic Belt in China, appear to be associated with convergent tectonics and subduction processes rather than with plumes (Li et al. 2012). PGE-dominant deposits in layered intrusions are more commonly located in the interiors of stable cratons.
THE NATURE OF MAGMATIC SULFIDE ORES
Magmatic sulfide ores range in sulfide contents from less than a tenth of a percent in some stratiform PGE ores to 100% sulfide in some Ni–Cu deposits (Fig. 1). Almost all unaltered magmatic sulfide ores, regardless of sulfide mode, have a characteristic assemblage of pyrrhotite–pentlandite–chalcopyrite–platinum-group minerals (PGM). This assemblage formed from the cooling and crystallization of a magma-derived sulfide matte. Natural mattes, consisting predominantly of Fe, Ni, Cu and S, fractionate to form a sequence of phases on cooling. Below ~1,100 °C, (Ni,Fe)S monosulfide solid solution crystallizes to leave a Cu-rich sulfide liquid enriched in Pt, Pd and semi-metals (e.g. Te, Bi, As). At ~900 °C, the Cu-rich liquid crystallizes to intermediate solid solution (approximately CuFeS2) to leave a residual melt progressively enriched in Pt, Pd and semi-metals (Li et al. 1996). This fractionation process takes place on scales from that of individual centimetre-size globules (Fig. 3) to entire orebodies, e.g. the supergiant Cu–PGE rich Oktyabrysky orebody at Talnakh in Siberia (Naldrett 2004). On further cooling to below ~700 °C, monosulfide solid solution breaks down to pyrrhotite and pentlandite (Fig. 3, Fig. 4E), intermediate solid solution to chalcopyrite, and the low-T residual liquid crystallizes Pt- and Pd tellurides, bismuthotellurides and arsenides. The common co-occurrence of magnetite arises from the ability of mattes to also dissolve substantial amounts of FeO (Naldrett 2004).
Sulfide Ore Textures and Evidence for Magmatic Origins
Textural relationships between sulfides and their host silicates are key evidence for their origin (Fig. 4). One of the critical textures, from an historical point of view, is that interpreted by Hawley (1962) as a frozen emulsion of immiscible silicate and sulfide liquids (Fig. 4A). Hawley’s was one of the first papers to argue persuasively for the primary magmatic origin of the Sudbury ores. Other diagnostic magmatic features are (1) net- or matrix textures (Fig. 4B), where sulfides form a continuous 3-D matrix enclosing cumulus silicates; (2) interspinifex ores in komatiites (Fig. 4C), where sulfide occupies the original spaces between dendritic olivine plates (Barnes et al. 2017); (3) sub-spherical globular ores, sometime associated with infilled vesicles (Le Vaillant et al. 2017) (Fig. 4D); (4) breccia textures, where sulfide liquid has percolated through the pore space between wall rock clasts in an intrusion breccia (Fig. 1B).
The other strong line of evidence for magmatic, as opposed to hydrothermal, origins is in the ore chemistry, discussed further below. The suite of elements concentrated in these ores (Ni, Cu, Fe, Se, PGEs) are exactly those that are known from experimental evidence to partition strongly into immiscible sulfide liquids. Other ‘chalcophile’ elements, such as Zn, Pb and Sn, are present only in trace proportions in magmatic sulfide ores, consistent again with their known low partition coefficients (Naldrett 2004).
The great majority of magmatic sulfide deposits form from much the same sequence of three processes: 1) Generation of a sulfide–silicate liquid emulsion; 2) Physical separation of a mixture of sulfide liquid droplets and cumulus silicate minerals from this emulsion; 3) Deposition and coalescence of sulfide liquid in specific sites. In some cases, the final disposition of the ores is influenced by post-deposition migration of coalesced sulfide liquid pools, driven by the balance between surface tension and gravitational forces (Barnes et al. 2017). Tectonic deformation can lead to further modifications due to the differential rheology of solid sulfides and silicates during strain.
Generation of Sulfide Liquids
This fundamental process can happen by a variety of mechanisms.
Partial Melting of Sulfide-Bearing Mantle
Sulfide liquid can be generated at source where the degree of partial melting is low enough to produce melting of the sulfide component of the source but the sulfide liquid does not completely dissolve in the silicate partial melt. The S content of silicate magmas in equilibrium with matte (S content at sulfide liquid saturation, or SCSS) increases with decreasing pressure such that sulfide-saturated magmas generated in the mantle are likely to be undersaturated on arrival in the upper crust (Edmonds and Mather 2017 this issue).
Fractional Crystallization of Silicate Magma
Sulfur behaves as an incompatible element under sulfide-undersaturated conditions. Thus, fractional crystallization causes S content to increase. Sulfide liquation (i.e. nucleation and growth of immiscible sulfide liquid droplets) occurs once the S content of the magma exceeds the SCSS, which itself decreases with decreasing temperature and Fe content. Within cumulate sequences, the first onset of sulfide liquid saturation can generate extremely PGE-enriched mattes, as in the Platinova Reef of the Skaergaard intrusion (Greenland) (Nielsen et al. 2015).
Mixing of Two Magmas Both of which are at or close to Sulfide-Liquid Saturation
This mechanism of mixing two magmas both of which are at or close to sulfide-liquid saturation can give rise to a hybrid magma with transient sulfide supersaturation. This process has been invoked to explain the origin of PGE reefs associated with major magma influxes in large chambers, such as the Merensky Reef of the Bushveld Complex (Campbell et al. 1983).
Incorporation of External Crustal S, Giving Rise to Sulfide ‘Xenomelts’
Addition of external S is regarded as the dominant process in the formation of all komatiite-hosted ores (Lesher 1989) and in the great majority of intrusion-hosted deposits (Ripley and Li 2013). Crustal rocks can have S isotope and S/Se ratio signatures that are very distinct from mantle S. Therefore, any sulfide in an orebody that has a contribution from crustal sources can be traced using their S isotope and S/Se compositions. Examples do exist of deposits with mantle-like S isotope signatures: most notably, the giant Jinchuan deposit (China) (Ripley et al. 2005). But it is likely that, in such cases, extensive equilibration of sulfide with much larger volumes of silicate melt (see below) has caused the final signature to be dominated by the mantle S component. A variety of mechanisms exist for incorporating external S, but direct melting of physically incorporated sulfidic country-rock fragments (xenoliths) to form sulfide ‘xenomelts’ is the fastest and most effective (Robertson et al. 2015).
COMPOSITION OF MAGMATIC SULFIDE ORES
Variability in Ni and Cu Content
The first-order variability in Ni and Cu tenors (tenor = concentration of the metal in 100% sulfide) is related to the composition of the host rocks (Fig. 5A). There is a decrease in the Ni:Cu ratio from values around 20:1 in komatiites, 4:1 in ores associated with komatiitic basalts, and between 0.5:1 to 5:1 in most deposits (both Ni–Cu sulfide-rich type, and reef-style low sulfide PGE type) associated with mafic magmas. Lower Ni:Cu ratios are found in ores where there has been extensive sulfide liquid fractionation, as at Noril’sk-Talnakh, the Sudbury footwall veins (Naldrett et al. 1997) and in disseminated ores associated with advanced fractionation of tholeiitic mafic magmas.
Variability in PGE Content
Platinum group element (PGE) tenors (represented in Fig. 5B by Pd) show a much wider range than those of Ni and Cu. Platinum group element tenors range over nearly 6 orders of magnitude between the most depleted Ni–Cu deposits to the most enriched, reef-style ores. Within each deposit/setting type there is a strong positive correlation between Pt and Pd, and a weaker correlation between Pd and Cu (Fig. 5B) and between Pd and Ni contents, particularly if the reef examples are considered along with the mafic-hosted grouping (Campbell and Naldrett 1979).
Controls on Metal Tenors
The variability in Ni/Cu shown in Figure 5A arises largely from parent magma compositions. Values of Ni/Cu in ores correspond reasonably well with those in the parent magmas themselves (Barnes and Lightfoot 2005), which for mafic host magmas range from high-Mg basalts through to fractionated tholeiites. Nickel becomes depleted in more evolved magmas due to olivine crystallization, whereas Cu becomes enriched due to incompatibility in the major crystallizing silicate phases. Hence, primitive high-T komatiites have very high Ni and low Cu compared with more fractionated mafic magmas that attain progressively lower Ni/Cu. This contrast is reflected in sulfide compositions (Fig. 5A). The most extreme Cu-rich example is the Platinova Reef (PN in Fig. 5) of the Skaergaard Intrusion (Nielsen et al. 2015). Here, sulfide saturation occurred very late in the crystallization history, such that the more compatible Ni had already been depleted from the melt, whereas the more incompatible Cu was strongly enriched. In this case, the magma attained saturation in Cu–Fe sulfide liquid rather than the typical Fe–Ni-dominated sulfide.
The PGE contents of silicate melts are controlled through the variable degree of igneous compatibility of the different PGEs: moderately compatible in the case of Ir, Os and Ru, and strongly incompatible for Pt and Pd. Superimposed on this effect is the very strong tendency of all the PGEs to become strongly depleted during fractional extraction of sulfide liquid, owing to their extreme chalcophile character (Mungall and Brenan 2014). A further consequence of this attribute of the PGEs is their strong susceptibility to mass balance effects.
A major control on sulfide liquid compositions is the mass ratio, R, of silicate to sulfide liquid that react with one another (Campbell and Naldrett 1979). This relationship is expressed as follows:
where Yisul is the final concentration of element i in the sulfide liquid; Xisil is the initial concentration in the silicate liquid; Disul is the partition coefficient between sulfide and silicate melt. Formation of magmatic sulfides is treated here as a batch equilibrium process: a batch of sulfide liquid forms and equilibrates with a given volume of silicate melt. The effect of variations in R is shown on the model curves in Figure 5. Where R for element i is very low compared with Disul, tenors are relatively low; there is relatively little impact of changing D on the content of i in either melt, and tenor depends almost linearly on R. If R is large relative to Disul, then the opposite applies: both silicate and sulfide melt have high metal contents that increase almost linearly with D. The effect operates in natural systems through the wide range in the partition coefficients for the different chalcophile metals: typically, around the low hundreds for Ni, ~1,000 for Cu, and of the order of hundreds of thousands for the PGEs (Kiseeva et al. 2017 this issue). Hence, the extremely chalcophile PGEs are much more susceptible to R factor effects than Ni and Cu, as can be seen in the model curves in Figure 5. Nickel and Cu tenors approach maximum values where R is greater than about 1,000, while the PGE tenors continue to increase almost indefinitely with increasing R owing to their extreme D values. Extremely high silicate:sulfide ratios are necessary to produce the high PGE tenors of reef-style deposits.
Sulfide liquid differentiation can produce significant variability in the proportion of Cu to Ni and of Pt and Pd to Ir, Ru and Os, and is progressively more important in orebodies containing initially higher Cu contents where the melting range of the sulfide component extends to much lower temperatures. This effect produces dispersion at metre- to decimetre scale within orebodies and at the centimetre scale in individual droplets, as seen in Figure 3. But in some very large systems, differentiation is accompanied by physical migration of residual Cu-rich liquid into veins and fractures. At Sudbury, this process generates very high grade Cu–PGE orebodies hundreds of metres below the base of the host magma body (Naldrett et al. 1997).
Case Study: The Platreef of South Africa
The giant layered ultramafic–mafic Bushveld Igneous Complex (South Africa) hosts over three-quarters of the world’s PGE resources in three main deposits: the UG2 chromitite, the Merensky Reef and the Platreef. The former two are archetypal stratiform reef deposits, with a few percent of PGE-rich sulfides associated with chromite and having thicknesses of a few centimetres to a few metres. The Platreef is in the northern part of the complex and is a much thicker orebody (~10–400 m) emplaced as a series of sills with stratabound sulfides present in a package of mostly pyroxenites that rest directly on Archaean–Proterozoic country rocks (Fig. 6). It has been interpreted as the lateral equivalent of the Merensky Reef and as the propagating marginal facies of the Bushveld Complex. The Platreef was probably formed as magma was squeezed out at the edges of the expanding magma chamber (Naldrett et al. 2008). The Platreef is likely to be the main future source of PGEs in the coming decades, making it one of the most economically significant of all known magmatic sulfide ore deposits.
The Platreef displays much complexity due to its multi-stage origin (McDonald and Holwell 2011). Mass independent S isotopes and S/Se ratios give evidence that sulfide saturation was initially triggered by a bulk assimilation event, most likely of pyritic shales at depth (Penniston-Dorland et al. 2008). Contaminated magma with a cargo of dispersed sulfide liquid droplets was then emplaced higher in the crust, at which point interaction with the diverse range of country rocks resulted in further addition of external S, lowering the PGE tenor of the sulfides and modifying the isotopic and PGM compositions to different degrees depending on the country-rock lithology (McDonald and Holwell 2011).
Recent exploration in the northern limb has identified a much more regularly layered sequence, with traceable stratiform mineralization down dip from the surface outcrop of the Platreef, with the addition of thick underlying ultramafic cumulates. This ‘Flatreef’ sequence has many similarities to the Merensky Reef in the rest of the complex and represents a transition from stratabound ‘contact-type’ or ‘marginal’ styles, to stratiform, reef-style mineralization.
The Elephant in the Shield: Sudbury (Canada)
No review of magmatic sulfide ores could ignore the world’s largest known accumulation of magmatic ores: the extraordinary Sudbury impact structure in the southern part of the Canadian Shield, Ontario. In this case, ore formation followed wholesale melting of almost the entire thickness of the crust following a giant bolide impact (Mungall et al. 2004). Sulfide liquid segregated from the resulting melt sheet on subsequent cooling. While many aspects of Sudbury ore genesis are unique, one aspect is highly significant to understanding processes in other deposits: the presence of extensively mobilized veins and dikes of sulfide rich rocks that extend for distances of kilometres below the original base of the melt sheet. These features attest to the extreme physical mobility of sulfide magmas, or possibly sulfide-rich melt emulsions charged with rock fragments, driven by gravity into fracture systems. This is an important clue to the origin of late-stage injections of sulfide-rich breccia-textured ores, common in many intrusion-hosted deposits (Barnes et al. 2016).
Research on magmatic sulfides has made substantial advances over the five decades from the pioneering work of Naldrett and others, particularly in understanding geochemical processes. But a number of fundamental questions remain about physical processes of ore formation.
- Is the addition of external S through assimilation of crustal rocks essential, and by what mechanisms does this addition take place?
- How is sulfide liquid transported in magmas, in what proportions and in what physical form? How far can sulfide liquids be transported from the original site of liquation to the point of deposition?
- What is the physical process of deposition, and to what extent is it governed by mechanical sedimentation versus in-situ chemical deposition of sulfide liquid at the point of nucleation?
Magmatic sulfides continue to be fascinating topics of research, leading to advances in exploration models as well as new insights into magmatic, and even climatic, processes. Applications have been made to meteoritics, in studies of the origin of planetary cores, and into processes of metal and S transport with implications for the origin of porphyry deposits, climate-impacting giant eruptions, and mass extinctions (Mungall et al. 2015; Le Vaillant et al. 2017).
We thank Kate Kiseeva and the Elements editorial team for the kind invitation to contribute to this volume. Anais Pages and Angus MacFarlane reviewed a preliminary draft. Reviews by Tony Naldrett, Ed Ripley and Vera Lorenz greatly improved the clarity of the manuscript. SJB and MLV acknowledge support from the CSIRO Science Leader program. We acknowledge the many colleagues who have contributed data and insights drawn on here.
The image in Figure 3 was collected on the X-ray Fluorescence Mapping beamline of the Australian Synchrotron, Clayton, Victoria, Australia. We acknowledge financial support for this facility from the Science and Industry Endowment Fund (SIEF)
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