Description: Geochemical analysis is an essential tool for the confirmation and study of impact structures and the characterization of the various rock types involved (target rocks, impact breccias, melt rocks, etc.). Concentrations and interelement ratios of the platinum-group elements, as well as the osmium and chromium isotope systems, allow quantification of extraterrestrial components and the identification of impactor types in impact deposits. In addition, chemolithostratigraphy can reveal the possible role of impacts in environmental change throughout the geologic record. This article deals predominantly with terrestrial impact structures.

Description: The lateral variation in platinum-group mineral (PGM) assemblages in the magmatic constituents of the Platreef, Bushveld Complex, have been investigated using whole-rock PGE data, scanning electron microscope observations and electron-microprobe data on the PGM. The Platreef comprises a series of mafic–ultramafic sills together with interlayers and xenoliths of metamorphosed sedimentary rocks variably mineralized in PGE, Ni and Cu. In the Central Platreef, the zone with the highest exploitable PGE grade (referred to as PGE reef) occurs at the top of the sequence and is hosted by feldspathic pyroxenite partially replaced by feldspathic harzburgite down-dip. Elevated PGE contents are also related to zones of massive and disseminated chromitites that occur as discontinuous layers within the top reef and at the bottom of the Platreef close to the footwall contact. The harzburgite-hosted reef and chromitites contain PGE mineralization with a high PGE tenor, enriched in Rh and Ru that is taken to indicate proximity to a feeder zone for the Platreef. Textural relationships show that an early succession of laurite ± PGE sulfarsenides ± sperrylite crystallized at a high temperature from a hot magma that was unsaturated with respect to sulfide. Once sulfide saturation was achieved, cooperite and eutectic mixtures of Pt3Fe, Pt sulfide and Fe–Ni–Cu sulfides exsolved first from the sulfide liquid. With decreasing temperature, sperrylite and hollingworthite continued crystallizing across a wide range of conditions from early magmatic to hydrothermal and formed a series of solid solutions and exsolution products. The composition of the PGM assemblages changes along strike within a discrete magmatic layer, and the variation is a function of both the footwall and magmatic rock lithologies. The proportions of Pt–Pd sulfide, Pt–Fe alloy and Rh–Ir–Ru minerals are the most useful in discriminating between different magmatic facies and associated mineralization, whereas Pt–Pd bismuthotellurides are more widespread, and their abundance is dependent on the PGE grade. Early magmatic crystallization of Rh–Ir–Ru-rich PGM distinguishes the PGE profile characteristic of harzburgite and chromitite from the PGE profile typical of the pyroxenite-hosted PGE mineralization that has exsolved from the PGE-rich sulfide liquid.