Hydrothermal palladium enrichment in podiform chromitites of Calzadilla de los Barros (SW Iberian Peninsula)

Abstract

The serpentinized peridotite massifs of Calzadilla de los Barros (Ossa-Morena zone, Iberian massif) consist of harzburgites and small dunite bodies containing Al-rich chromitite pods. Minor discordant clinopyroxenite veins cut the peridotite massifs. The chromitites show total transformation of the silicate matrix into chlorite and post-magmatic formation of chalcopyrite, millerite, magnetite, and chalcedony. Olivine in harzburgites and dunites is mainly transformed into pargasite. The clinopyroxenites show diopside transformation into epidote, grossular, and titanite and locally into hydrous minerals (chlorite, pargasite, and tremolite) associated with the formation of Cu-Fe-Ni sulfides, suggesting significant hydrothermal alteration. The chondrite-normalized platinum-group elements patterns of the chromitites are very similar to those of the harzburgites and clinopyroxenites, showing an unusual sharply positive slope from Pt to Pd, as well as high Pd/Ir ratios. In the chromitites, the Pd/Ir ratio shows both positive and negative linear correlations with bulk platinum-group element contents. These results suggest that most of the Os, Ir, and Ru were incorporated during the magmatic phase, while Pd and Pt were incorporated during post-magmatic processes of hydrothermal alteration and formation of Cu-Fe-Ni sulfides. This study provides a better understanding of the post-magmatic processes that allow Pd enrichment in ophiolite complexes, and highlights how platinum-group element patterns can provide significant insights into both magmatic and post-magmatic processes.