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Arndt 2013

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  ©2013 Society of Economic Geologists, Inc. Economic Geology,  v. 108, pp. 1953–1970 The Lithospheric Mantle Plays No Active Role in the Formation of Orthomagmatic Ore Deposits N ICHOLAS  A RNDT † ISTerre UMR 5275 CNRS, Université Grenoble Alpes, BP 53, 38041 Grenoble cedex 9, France  Abstract The hypothesis that the metals in certain orthomagmatic ore deposits come from a source in the subconti-nental lithospheric mantle is evaluated in this paper. According to this hypothesis, parts of the mantle beneath the continents are metasomatically enriched in metals like Ni, Cu, and the platinum group elements (PGE). It is proposed that under some circumstances, these metals are transported into the crust where they become con-centrated in orebodies. An examination of the compositions of xenoliths from the lithospheric mantle reveals little evidence, however, of components that could represent the source of metal-enriched magmas. In addition, the mechanism whereby metals are brought from the source to the surface is very unclear. The lithosphere is the coldest part of the mantle and it only melts under special circumstances. The normal product is a low-degree melt, an alkaline, Si-undersaturated magma of the type that only rarely contains ore deposits. Major magmatic orebodies normally form from high-volume, high-flux magmas that are produced by high-degree melting in deeper, hotter parts of the mantle—in the asthenosphere or a mantle plume. For melting to occur in the lithosphere, rather than in the hotter parts of the mantle, the melting point of the source must be drasti-cally reduced by the presence of volatiles. On the other hand, there is ample evidence that the host magmas of ore deposits were abnormally rich in water or CO 2  as would have been the case if they came from a volatile-rich metasomatized source. Magmas from sublithospheric sources could have interacted with the lithospheric mantle as they ascended toward the surface and they could have picked up some metals through this interac-tion. This process could have contributed to the formation of some ores, a notable example being the PGE deposits in Bushveld Complex. There is ample geological and geochemical evidence, however, that the majority of magmatic deposits form when magmas from sublithosphere sources assimilate material from the continental crust and that the latter process is instrumental in the formation of the deposits. Introduction I N    WHAT  might be called the “standard model” for the for-mation of magmatic sulfide deposits (Naldrett, 1992, 1999, 2004; Lightfoot and Naldrett, 1994; Arndt et al., 2004; Li et al., 2009; Arndt, 2011), magma, usually tholeiitic and resulting from relatively high degrees of melting in the asthenosphere or in a mantle plume, assimilates material from the conti-nental crust, a process that leads to the segregation of sulfide liquid. The chalcophile ore metals partition from the magma into the sulfide liquid, and an ore deposit forms if the sulfide accumulates in sufficient quantity and with a sufficiently high grade. We know within reasonable limits the likely compo-sitions of the magmas that are produced by melting in the upper mantle: these magmas contain moderate to high levels of Ni, Cu, and platinum group elements (PGE; Keays, 1995; Rehkämper et al., 1999) and, given an efficient concentra-tion process, they can produce high-tenor ore. There is some discussion about the exact nature of the ore-forming process, particularly the question of whether sulfur must be added from an external source (Lehmann et al., 2007; Godel et al., 2011), but the general lines of the model, as applied to depos-its such as Norilsk-Talnakh in Russia (Fig. 1; Naldrett, 1992; Lightfoot and Hawkesworth, 1997; Li et al., 2009), Voisey’s Bay in Canada (Li and Naldrett, 1999), Jinchuan in China (Chai and Naldrett, 1992; Tang, 1993; De Waal et al., 2004), and Kambalda in Australia (Lesher and Keays, 2002), are well established. Many recent papers have promoted an alternative hypoth-esis according to which the ore metals in magmatic ore deposits are extracted from a source in the subcontinental lithospheric mantle (SCLM) and not in the asthenosphere or in a plume. The model builds on a concept—well entrenched in the geologic literature—that certain types of continental basalts result from partial melting of metasomatized portions of the SCLM (Hawkesworth et al., 1984b, 1988; Sweeney and  Watkeys, 1990; Hergt et al., 1991; Menzies, 1992b; Lightfoot et al., 1993, 1994; Lassiter and DePaolo, 1997; Shirey, 1997; Jourdan et al., 2007; Zhang et al., 2009). The foundation of this model lies in the trace element and isotopic composi-tion of these magmas, which differ from that of the sublitho-spheric mantle and which, in the opinion of many authors, cannot have resulted from crustal contamination. In the papers advocating an SCLM source of ore metals (e.g., Garuti et al., 1997; Hong et al., 2002; Maier and Barnes, 2004; Fiorentini and Beresford, 2008; Fiorentini et al., 2008; Richardson and Shirey, 2008; Zhang et al., 2008) it is proposed that the metasomatized portions of the SCLM are enriched in chalcophile and highly siderophile elements and that partial melting of this part of the mantle, or contamination within the lithosphere of magma from sublithospheric sources, produces magmas of unusual compositions that are particularly capable of forming magmatic ore deposits (Fig. 2). The evidence cited in support of a metal-enriched source in the SCLM is diverse, ranging from concrete observations such as the mineralogy and chemical compositions of xenoliths from the lithospheric mantle, through to estimation of unusually high metal con-tents or abnormal metal ratios in the parental magmas of ore deposits. A summary of these arguments is given in Table 1. In this paper the arguments that have been advanced to support of an SCLM source of ore metals are evaluated first, 0361-0128/13/4173/1953-18  1953 † E-mail, Nicholas.Arndt@ujf-grenoble.fr  1954  NICHOLAS ARNDT  distinguishing those that provide direct evidence of derivation of metals from the SCLM from those in which a lithospheric source is adopted by default, after evaluation and rejection of other explanations. The SCLM model itself is then critically evaluated, and the range of compositions that have been pro-posed for the mantle beneath the continents are discussed. In this discussion, the emphasis is on models in which the SCLM plays an active role; i.e., as a source of metals and/or volatiles, and those models in which the lithospheric mantle serves only as a passive guide or impediment to the ascent of the mantle source or of magmas derived from this source are excluded (e.g. Begg et al., 2011). The questions of how these reservoirs could have been produced and, crucially, how the metals and other components could have been extracted and transported into the crust are discussed. Finally, the “standard model” is shown to satisfactorily explain most of the features of mag-matic Ni-Cu sulfide deposits. The conclusions that emerge are that most magmatic ore deposits probably develop as magma of normal composition and sublithospheric srcin interacts with continental crust, and that the latter process is instrumental in the formation of the ore deposits.  Arguments Advanced in Support of an SCLM Source of Ore Metals Portions of the SCLM are enriched in chalcophile elements and provide a likely source of metals in ore deposits Several authors (Sharpe, 1982; Sun et al., 1991; Zhang et al., 2008) have proposed that the parental magmas of ore deposits contained high concentrations of ore metals. In many such cases, these magmas are also said to have unusual trace ele-ment or isotopic compositions that have been interpreted to indicate derivation from the SCLM. Perhaps the best-known NorilskKambaldaBushveldComplex Ivrea zone Pechenga LherzRhonda SlaveCraton KamchatkaLihir KaapvaalCraton  Voisey’s Bay  UdachnayaGorgonaReunion F IG .  1. Map showing the localities of ore deposits (in bold) or other geologic units (smaller font) mentioned in the text. metasomatized layer (MM & CM)sub-lithosphere source ofmagma and/or heat  ancientmantle wedge (SC) eclogiteveins (EC)depleted lithosphericmantle (DL) mid- and lower crustalmagma chambers continentalcrust shallow intrusions with ore depositsflood basalts F IG . 2. Sketch showing the possible architecture of the continental litho-sphere. Two alternative models for the formation of magmatic ore deposits are (a) Extraction of metals from a portion of the subcontinental mantle with appropriate composition. Abbreviations: CM = carbonate-metasomatized, MM = melt-metasomatized, SC = subduction component. (b) Derivation of magma from a sublithosphere source (plume or asthenosphere) followed by concentration of metals by processes within magma chambers in the conti-nental crust. ab     LITHOSPHERIC MANTLE: FORMATION OF ORTHOMAGMATIC ORE DEPOSITS 1955and most convincing example is the Bushveld Complex in South Africa whose parental magmas, as estimated from the compositions of peripheral sills or a basal chill zone (Fig. 3), contained unusually high platinum concentrations (Sharpe, 1982; Maier and Barnes, 2004; Wilson, 2012). It is suggested that the high metal contents were crucial to the formation of the PGE ores of, for example, the Merensky Reef and UG2 deposits. Another example is provided by the continental flood basalts that host Ni-Cu-PGE sulfide deposits, like those of the Norilsk-Talnakh region in Russia. These basalts have high PGE contents and near-chondritic ratios of Pt/Pd, and in this respect they differ from basalts of mid-ocean ridges and ocean islands, which contain lower contents of the PGE (Fig. 3). To consider another tectonic setting, many authors argue that the source of Cu or Au in porphyry deposits are extracted from a part of the suprasubduction mantle wedge that was abnormally enriched in these metals (McInnes et al., 1999; Mungall, 2002; Hronsky et al., 2012). And on a broader scale, the notion of a “metallogenic province” is often associ-ated with the hypothesis that these clusters of ore deposits are linked to metal-rich parts of the mantle (Kerrich et al., 2000; Hong et al., 2002; Hronsky et al., 2012).Isotopic data provide intriguing evidence that at least a por-tion of the PGE in the Merensky Reef in the Bushveld were derived from the SCLM. Richardson and Shirey (2008) meas-ured the Re-Os isotope compositions of inclusions in diamonds transported in kimberlites from the Kaapvaal subcontinental lithospheric mantle. They obtained relatively radiogenic iso-tope ratios similar to those previously measured in platinum group minerals (PGM) from the Merensky Reef by Hart and Kinloch (1989) and on this basis argued that the PGE in Bush- veld ores came from an enriched component of the SCLM. It must be noted, however, that (1) in Richardson and Shirey’s model, the bulk of the magma comes from a sublithospheric source that entrains the PGE as it passes through the SCLM, and (2) these data do not necessarily require that the primary magma was abnormally enriched in PGE. As explained in a later section, extensive crustal contamination could account for the radiogenic Os in the Merensky samples, and within-crust processes may have boosted the concentrations of PGE in magmas that initially had only a modest PGE content.In most arguments for an SCLM source of ore metals, ref-erence is made to metasomatism of parts of the subcontinen-tal lithosphere: it is proposed that, in addition to lithophile T ABLE  1. Arguments for a Source of Ore Metals in the Subcontinental Lithospheric MantleArgument Justification ReferencesPortions of the SCLM are enriched in Some parental magmas of ore deposits are enriched in metals; Sharpe (1982), Sun et al. (1991), chalcophile elements and provide a trace-element or isotopic compositions resemble those of an Garuti et al. (1997), Kerrich et al. (2000), likely source of metals in ore deposits SCLM source; metasomatized SCLM is enriched in ore metals Hong et al. (2002), Maier and Barnes (2004), Zhang et al. (2008), Richardson and Shirey (2009), Hronsky et al. (2012)Magma or metals are derived from Magmas that form some ore deposits have near-chondritic Mitchell and Keays (1981), Keays (2012)an S-undersaturated source PGE ratios which requires that they were undersaturated in sulfur; some parts of the SCLM have this characteristicSome Ni-Cu-PGE ores contain The association of minerals such as amphibole and biotite with Fiorentini and Beresford (2008), hydrous minerals from a meta- ore sulfides indicates that the magma was hydrous; trace element Fiorentini et al. (2008)somatized mantle source and isotope data point to a source in metasomatized SCLMDefault arguments The parental magma is not contaminated by continental crust Hawkesworth et al.(1984b), Hergt et al. and its composition differs from that of the asthenosphere or (1991), Lightfoot et al. (1994), Lassiter and plume; therefore the source of magma and/or metals is in DePaolo (1997), Hong et al. (2002), Maier the SCLM and Barnes (2004), Zhang et al. (2008) 048121620 0 10 20 30 40 50 Mgo (wt%)    P   t   (  p  p   b   ) Continental tholeiitesBushveld sillsMantle peridotiteKomatiitesMeimechitesOceanic basaltsBoninites MARID Bushveld sills   0.010.11100 10 20 30 40 50 MgO (wt%)    I  r   (  p  p   b   )  MARID F IG . 3. Platinum and iridium contents, plotted against MgO of various types of volcanic and intrusive rocks and mantle xenoliths. The Pt contents of continental tholeiites, and particularly those of marginal sills of the Bush- veld Complex, are far higher than those in basaltic rocks from oceanic set-tings with similar MgO contents. They are comparable to levels in post 3 Ga komatiites and higher than in mantle peridotites, reflecting the incompatible behavior of Pt and an absence of sulfide in the residue of melting. Ir con-tents of continental tholeiites are also higher than in most oceanic basalts, but lower than in mantle peridotite, reflecting the more compatible behavior of this element. Data from Sharpe (1982), Arndt et al. (2004), Maier and Barnes (2004), Barnes et al. (2010), Maier et al. (2012). MARID = rock composed mainly of mica, amphibole, rutile, ilmenite, and diopside (discussed in text).  1956  NICHOLAS ARNDT  elements and volatiles, the metasomatized regions also became enriched in chalcophile and/or highly siderophile ele-ments. Although the idea is not unreasonable, closer evalu-ation provides little direct evidence that the metasomatized parts of the SCLM are enriched in metals such as Ni, Cu and the PGE. This aspect of the problem is discussed in a later section when the compositions of the diverse components that are known or inferred to exist in the subcontinental litho-spheric mantle are explored. Magma or metals are derived from an S-undersaturated source Mitchell and Keays (1981) and Keays (1995, 2012) argued that the magmas that produce ore deposits are undersatu-rated in sulfide and that these magmas came from a source  with very low sulfide contents such as the depleted peridotite of the SCLM. While it is probably true that most of the mag-mas that yield ore minerals were indeed S undersaturated, this may merely be the consequence of the manner in which the magmas have formed. Wendlandt (1982), Keays (1995), and Mavrogenes and O’Neill (1999) have demonstrated that the solubility of sulfide in mafic or ultramafic magmas decreases with increasing pressure. The magmas parental to ore deposits are produced by melting at great depth, >90 km for basalts and picrites in continental settings. At these depths only a small fraction of sulfur dissolves in the melt, but as the magma rises through the lithosphere, its capacity to dissolve sulfur increases. The consequence is that almost every deep-sourced magma should be S undersaturated when it reaches the crust. An additional process, such as crustal contamina-tion or the addition of S from an external source, is required to induce the segregation of an immiscible sulfide liquid. It therefore seems unnecessary, on this basis alone, to invoke derivation from the SCLM.Furthermore, it is by no means certain that those parts of the SCLM that are invoked as the source of ore-forming mag-mas have very low sulfide contents. It is true that the har-zburgite or dunite that forms the building block of ancient SCLM is highly depleted in S (Lorand et al., 2003; Pearson et al., 2004), but this material is highly refractory and is an unlikely source of partial melt. The more fertile parts affected by metasomatism may or may not be S undersaturated. This issue is discussed later in the paper, following evaluation of the compositions of various components of the SCLM. Some Ni-Cu-PGE ores contain hydrous minerals that could have come from a metasomatized mantle source In several papers the presence of hydrous phases in Ni-Cu-PGE ores has been linked to derivation from a source in metasomatized SCLM. The argument is that metasomatism is caused by the introduction into the mantle of hydrous or other  volatile-bearing fluids and that the presence of volatiles would promote melting and facilitate the extraction of metals from the metasomatized source. Fiorentini and Beresford (2008) noted the presence of hydrous phases and Fe-Ni sulfides in the Valmaggia ultramafic pipe in the Ivrea-Verbano zone of Italy. They invoked a link between hydrous phases (amphi-bole and biotite) in Ni-Cu-PGE ores in the Ivrea region and a hydrated metasomatized source in the SCLM. In another study (Fiorentini   et al., 2008) the same authors proposed that  volatiles in ore-bearing ferropicrites in the Pechenga green-stone belt of Russia came from a source located either in metasomatized subcontinental mantle or in a volatile-bearing plume. To explain how the metals were transported into the continental crust, they proposed (p. 339) that “Metasomatism introduced alkalis, Cu, PGEs and S into the depleted man-tle…. Increased water activity caused the harzburgite to melt, producing volatile-rich sulphide-bearing ultramafic magma that evolved to intrusions that host Ni-Cu-PGE mineraliza-tion.”   In a third paper, Fiorentini et al. (2012) proposed that komatiites from the Ni sulfide belt of the Yilgarn craton in  Western Australia were hydrous and that their degassing may be implicated in the formation of the ore deposits.These ideas are interesting and, particularly in the case of the Pechenga deposits, have considerable merit. However, they are open to the following criticisms:1. In many magmatic deposits hydrous phases are indeed associated with ore sulfides, but the total concentration of these phases is very low. As pointed out by Barnes and Camp-bell (1988), water behaves as an incompatible element and becomes concentrated in the last portion of the silicate liquid to solidify; the hydrous minerals then crystallize from this liq-uid. Sulfide liquid, which remains in the liquid state to tem-peratures below the solidus of the silicate liquid, accumulates together with the last fractions of silicate liquid: the associa-tion between sulfide and hydrous minerals may therefore be largely coincidental and does not imply that water was directly associated with the formation of the ores.2. The mechanism proposed for the transport of metals from the metasomatized zone toward the surface is poorly known, as discussed in a later section. Default arguments for an SCLM source In a great number of papers advocating a lithospheric source of magmas or metals (e.g. (Hawkesworth et al., 1984a; Hergt et al., 1991; Gallagher and Hawkesworth, 1992; Lightfoot et al., 1994; Fedorenko et al., 1996; Molzahn et al., 1996; Las-siter and DePaolo, 1997), the authors first test whether the host magmas have been contaminated by continental crust and conclude that the amount of contamination was negli-gible. They then observe that the compositions of the rocks in question are unlike those of magmas from depleted upper mantle and conclude, with no further ado, that the source was in the SCLM. There are three major problems with this type of argument: First, the criteria used to dismiss crustal contam-ination are based on oversimplified assumptions or are totally invalid; second, even in cases where crustal contamination is not important, the source is not necessarily in the lithosphere but could be a component in a mantle plume or another part of the sublithospheric mantle; third, and most significantly, only rarely is the SCLM model evaluated objectively—instead it is commonly adopted by default after dismissal of other alternatives. Here, the tests that are made of crustal contami-nation models are considered first, and then the SCLM model itself is scrutinized. Crustal contamination should be accompanied by frac- tional crystallization:  If this argument is correct, indices like SiO 2 , Th/Nb or 87 Sr/  86 Sr, which monitor the amount of contamination, should correlate with MgO or Mg numbers,
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