Abstract
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 …
Highlights
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)
The petrogenetic links between the PGM and other non-silicate minerals 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)
Platinum-group minerals may be found in carbonaceous chondrites (e.g., Fig. 1a,b), where they occur as refractory metal nuggets (RMN) that have traditionally been considered to be amongst the earliest condensates in the Solar System (Palme 2008)
Summary
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). 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). 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 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 referred to reviews by Cabri and Feather (1975), Cabri (1981a, 2002) and Brenan and Mungall (2008)
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