Petrogenesis of the Platinum-Group Minerals

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 …