The enrichment of platinum group elements (PGEs) in petrology has long been of interest due to their technologically important attributes, notably the ability to increase metallurgical process efficiency. The concentrations of PGEs have therefore been extensively studied in order to elucidate their origin and distribution. In general, PGEs are associated with three types of rocks: mafic, ultramafic and plutonic. Mafic rocks are volcanic and intrusive, and both can contain PGEs in trace amounts. Ultramafic rocks commonly contain higher concentrations of PGEs due to the formation process; solidification of magma depleted in silica leads to more retained PGEs. Granites, pegmatites and plutonic rocks are types of intrusive igneous rocks and generally contain significantly higher levels of PGEs than other intrusive igneous rocks due to fractional crystallization. The mineral characteristics, petrologic environment and provenance can be used to produce meaningful PGE concentration data.
The concentration of PGEs is known to vary from type to type; depleted mantle, which are mantle rocks that have been depleted of many elements during multiple rounds of partial melting, generally contain effectively no PGEs and as such PGE concentrations can be used as a tracer for depleted mantle material. On the other hand, enriched mantle, which are mantle rocks that have been enriched in many elements during multiple rounds of partial melting, can be enriched in PGEs due to the enrichment process. In a plutonic environment, PGEs and other trace elements are generally enriched in minerals associated with fractionation crystallization, such as feldspars and plagioclases. These minerals, which are found in quartz monzodiorite and granodiorite, can be used to track enrichment and depletion of PGEs in the magmatic environment.
On the other hand, heavy mineral deposits, which are sedimentary rocks that have been enriched in dense minerals due to moving and sorting in water, can contain high levels of PGEs due to long term recrystallization and post-mortem events. Alluvial gold deposits, which are accumulations of gold from a variety of sources, are one of the few sedimentary environments where PGEs can be enriched.
PGEs can also be enriched in hydrothermal rocks due to the crystallization process in waters of high temperature. Hydrothermal rocks, which are generally sedimentary rocks that have undergone alteration by hydrothermal fluids and formed mineral deposits, are commonly associated with volcanism and contain naturally high levels of PGEs. These rocks are responsible for forming hydrothermal systems, which can act as both a source and a sink of PGEs. As a source, hydrothermal alteration of existing rocks can lead to PGE enrichment by introducing PGE-enriched fluids in the rock. As a sink, these fluids can be trapped in the rock, leading to an increase in PGE concentrations.
The distribution of PGEs in petrology usually follows a pattern of enrichment and depletion. PGEs are generally depleted in mantle and enriched in plutonic and intrusive rocks. In sedimentary rocks, PGEs can be enriched due to post-mortem events as well as from hydrothermal fluids. This distribution pattern can be used to trace the origin and concentration of PGEs, as well as to infer magmatic and sedimentary processes.
