Reviews in Mineralogy & Geochemistry Vol. 81 pp. 489-578, 2016 9 Copyright © Mineralogical Society of America Petrogenesis of the Platinum-Group Minerals Brian O’Driscoll School of Earth, Atmospheric & Environmental Science The University of Manchester Williamson Building, Oxford Road Manchester M13 9PL UK [email protected] José María González-Jiménez Department of Geology and Andean Geothermal Center of Excellence (CEGA) Facultad de Ciencias Físicas y Matemáticas Universidad de Chile Plaza Ercilla #803, Santiago de Chile Chile [email protected] INTRODUCTION The platinum-group minerals (PGM) are a diverse group of minerals that concentrate the platinum-group elements (PGE; Os, Ir, Ru, Rh, Pt, and Pd). At the time of writing, the International Mineralogical Association database includes 135 named discrete PGM phases. Much of our knowledge of the variety and the distribution of these minerals in natural systems comes from ore deposits associated with mafic and ultramafic rocks and their derivatives (see also Barnes and Ripley 2016, this volume). Concentrations of PGM can be found in layered mafic–ultramafic intrusions. Although they don’t typically achieve ore grade status, supra- subduction zone upper mantle (preserved in ophiolite) lithologies (i.e., chromitite [> 60 vol.% Cr-spinel], pyroxenite) characteristically host a diversity of PGM assemblages as well (Becker and Dale 2016, this volume). Occurrences of the PGM in layered intrusions, ophiolites, and several other important settings will all be described in this review. In keeping with the general theme of this volume, the focus of this chapter is on relatively high-temperature (magmatic) settings. This is not a straightforward distinction to make, as PGM assemblages that begin as high-temperature parageneses may be modified at much lower temperatures during metamorphism, hydrothermal processes or surficial weathering (e.g., Hanley 2005). However, the vast majority of the published literature on PGM petrogenesis is based on occurrences from magmatic environments, an understandable bias given the importance of the major ore deposits that occur in some layered mafic–ultramafic intrusions, for example. For that reason, the emphasis of this review will be on high-temperature magmatic settings, with the understanding that lower temperature (sub-solidus; < 600 °C) processes can modify primary PGM assemblages. The geochemical behavior of the platinum-group elements (PGE) in magmatic settings is highly chalcophile and not, as might be expected, highly siderophile. This is because most terrestrial magmatic systems are relatively oxidized, such that native Fe is not stable. A consequence of this is that the PGE have very high partition coefficients for sulfide in magmatic environments. From the high-temperature perspective, the degree of mantle melting and the manner in which magma fractionation proceeds, 1529-6466/16/0081-09$10.00 http://dx.doi.org/10.2138/rmg.2016.81.09 490 O’Driscoll & González-Jiménez whether or not sulfide separation and fractionation has occurred, and the way in which late- stage (metasomatic) alteration has affected the rocks, if at all, are each potentially important processes in determining the PGM assemblage that forms. In particular, the relevance of the separation, accumulation, and crystallization of sulfide liquid (and/or arsenide liquid) in silicate magma systems, with respect to the timing of PGM crystallization, is a crucial issue. The PGE constitute most of the highly siderophile elements (HSE; Os, Ir, Ru, Rh, Pt, Pd, Re, Au). While the HSE have and are being applied to addressing first order problems in Earth and planetary evolution, as outlined throughout this volume, outstanding concerns remain regarding the small-scale siting of the HSE in rocks. Specifically, the control exerted by the different types of PGM on bulk concentrations and relative distributions of the PGE in rock samples is presently an avenue of active investigation. Of critical but relatively unknown importance is the interplay between the high-temperature processes listed above, and the way in which these operate to produce a particular PGM assemblage. The PGE are generally considered to be relatively resistant to geochemical processes that fractionate and modify the lithophile elements in silicate and non-silicate rocks (Walker 2009). However, the petrogenetic links between the PGM and other non-silicate minerals (i.e., oxides such as chromite, base-metal sulfides, arsenides, and antimonides) remain unclear in many natural samples. One reason for this is that some of the PGE in a sample may hypothetically exist in solid solution in minerals such as sulfides or oxides, whilst others form discrete PGM. Progress in unravelling these issues has been made with the advent of high precision microbeam techniques, such as LA-ICP-MS, where trace element concentrations and isotopic information (e.g., 187Os/188Os) can be collected from individual PGM, providing new information on their chronology and petrogenesis (Pearson et al. 2002; Malitch et al. 2003; Ahmed et al. 2006; Shi et al. 2007; Marchesi et al. 2011). Unfortunately, in most relevant natural materials, PGM grain sizes are too small (< 1 μm) for such analyses and uncertainties surrounding PGM crystallization processes persist. The principal goals of this contribution are twofold: (1) to provide an overview of the principal PGM assemblages/parageneses in natural rock samples, including those from terrestrial as well as extraterrestrial environments. This includes highlighting, where possible and/or relevant, how these control whole-rock PGE budgets; (2) to provide the reader with a flavour of the processes (or combination of processes) responsible for the crystallization of PGM in different petrological environments. As noted above, this chapter is principally concerned with synthesizing observations on PGM occurrences that have been well- documented, so the emphasis is on mid-upper crustal mafic–ultramafic intrusions (including layered intrusions) and ophiolitic mantle. However, studies of secondary concentrations of PGM that are considered to be derived from high-temperature source material (i.e., placer deposits associated with ophiolites and zoned Uralian–Alaskan–Aldan-type complexes) have also contributed valuable information to our understanding of high-temperature PGM petrogenesis. For this reason, an appendix that summarizes some of the major findings of placer deposit studies is included with this chapter. The reader is also referred to reviews by Cabri and Feather (1975), Cabri (1981a, 2002) and Brenan and Mungall (2008). PHASE RELATIONS AND ORIGIN OF THE PGM Chemical properties of the PGM The PGE are Group VIII transition metals, together with Fe, Cu, and Co. The PGE have melting points above that of Fe (1665 K), ranging from 1828 K for Pd to 3306 K for Os (Table 1). In their elemental form, Rh, Pd, Ir, and Pt (arranged in increasing Z) are cubic in symmetry (fcc), with Os and Ru being hexagonal (hcp). The electron configurations of the PGE are given in Table 1. They can exist in multiple valence states (0 to +8) and have Petrogenesis of the Platinum-Group Minerals 491 Table 1a. A compilation of some of the common natural PGM discussed in this review, organized according to mineral composition and complexity of bonding with other elements. Melting points, crystal structure and electron configuration details are also provided for the PGE. This table has been adapted and updated from Cabri (1981). Osmium Iridium Ruthenium Melting T (K) 3306 2739 2607 Crystal hcp fcc hcp structure Electron [Xe]4f145d66s2 [Xe]4f145d76s2 [Kr]4d75s1 configuration S Erlichmanite (OsS2) Xingzhongite Laurite (RuS2) [(Pb,Cu,Fe)(Ir,Pt,Rh)2S4] Kashinite (Ir,Rh)2S3 Cuproiridsite CuIr2S4 As Omeiite [(Os,Ru)As2] Iridarsenite [(Ir,Ru)As2] Anduoite [(Ru,Os)As2] Ruthenarsenite [(Ru,Ni)As] AsS Osarsite [(Os,Ru)AsS] Irarsite Ruarsite (RuAsS) [(Ir,Ru,Rh,Pt)AsS] Native metal Osmium Iridium Ruthenium PGE Alloys Ir–Os Os–Ir Pt–Ir Ru–Os–Ir Os–Ir–Ru Chengdeite (Ir3Fe) bonding behavior influenced by their overlapping d-orbitals. Their d-orbitals, in particular, are important for the formation of PGM because they consist of unpaired electrons that can form metal–metal bonds with other PGE, or covalent bonds with electron acceptors such as S. Thus, despite their differences in symmetry, the PGE can exist in solid solution with one another (Cabri 1981a), as well as combining with other siderophiles (e.g., Fe), chalcogenides (e.g., S, Te), and semi-metals (e.g., As, Sb). Native metals (minerals) composed of the PGE do not typically undergo phase transitions within temperature and/or pressure ranges of geological significance (Cabri 1981a) and the principal variable property, in addition to melting temperature (see Table 1), is the compositional range of solid solutions. Berlincourt et al. (1981) provide a detailed synthesis of the phase relations of the PGE. Examples of unary phases include the complete solid solution of Ir, Rh, Pd and Pt with Ni, and the complete solid solution of Os with Ru. More commonly, binary phases are formed between the individual PGE and each other, and elements including As, Bi, Cu, Fe, Hg, In, Ni, Pb, S, Sb, Se, Sn, and Te (see Table 1). Examples of known natural ternary and quaternary phases are also listed in Table 1. The PGE are characterized by their extreme affinity for metal phases, instead of oxides or silicates, and together with Au and Re, are referred to as the highly siderophile elements (HSE); the distribution coefficients of the PGE between metal and silicate are greater than 104 (Jones and Drake 1986; Holzheid et al. 2000; Fortenfant et al. 2003). The siderophilic character of the PGE is manifested by numerous occurrences (see also Brenan et al. 2016, this volume; Lorand and Luguet 2016, this volume), e.g., the high concentrations of PGE in iron meteorites, Fe–Ni metal alloys found in chondrites and the occurrence of many metal alloys as PGM (Table 1). The PGE may also have a tendency to exhibit chalcophile behavior, readily bonding with S, As, and other Group Va and VIa ligands.