Résumé
Platinum group elements (PGE) are among the least abundant elements in the earth's crust, and palladium (Pd) generally has a maximum content of 5-10 ppm in currently mined ores. The largest Pd producer to date is the Norilsk-Talnakh deposit (Russia), which produced over 80 tonnes in 2022. Pd, which has a high recycling rate and is essential for the development of catalytic converters and fuel cells in the automotive and hydrogen sectors, has seen a significant increase in demand in recent years, making it a highly critical raw material. Although the main deposits are well identified, the geochemical behavior of palladium remains relatively poorly understood. Most PGEs present in magmatic sulfides are relatively well characterized when they occur as highly concentrated micro- to nanometric nuggets in which PGEs are major elements (PGMs; platinum-group minerals). However, in the case of palladium, pentlandite (Pn; (Fe,Ni)9S8) is known to be the main carrier, but at much lower levels than in PGMs. This raises the question of potential genetic links between Pd-Pn and PGMs. In-depth characterization of the distribution and speciation of palladium may therefore provide new constraints on understanding the behavior of palladium from magmatic liquid sulfides, through the various crystallization stages, to the Pn-PGM assemblage. To address this issue, we propose the detailed study of a magmatic sulfide sample from the Talnakh complex containing mainly an assemblage of cubanite, chalcopyrite and pentlandite. The sample was pre-characterized by SEM and EBSD, then analyzed by mu -XRF and mu -XANES/EXAFS at the Pd K-edge. The results clearly show a bimodal distribution of Pd, with both sub-micron-sized PGM nuggets rich in Pd, and a fairly homogeneous distribution of low-level Pd in Pn, systematically showing depletion at grain rims. In addition, mu -EXAFS directly shows that Pd is present as a substitution in Pn. In contrast to the current state of the art, these observations tend to demonstrate that Pd crystallizes early and synchronously in Pn, suggesting a compatible character with the latter, and then tends to leave the Pn system in favor of the PGM, possibly during interactions with late circulations of metalloid-rich fluids.