Textural and compositional evidence for the formation of pentlandite via peritectic reaction: Implications for the distribution of highly siderophile elements

The distribution of highly siderophile elements is used in the study of a wide variety of geological topics, from planet formation and evolution to the formation of ore deposits. Under mantle and crustal conditions, these elements behave as highly chalcophile elements, and pentlandite (Pn) is an important host for most of these elements. Therefore, understanding how Pn forms is important to understanding the processes that control these elements. The classic model for the formation of Pn is that below 650 degrees C, the high-temperature sulfides-monosulfide solid solution (MSS) and intermediate solid solution (ISS)-are no longer stable and exsolve into pyrrhotite (Po), Pn, and chalcopyrite (Ccp). However, Pn has been shown to be the main host of Pd in many ore deposits, and given that Pd is incompatible with both MSS and ISS, this observation is inconsistent with the exsolution model. Furthermore, experimental work has shown that Pn can form by peritectic reaction between MSS and fractionated sulfide liquid. To date, this type of Pn has not been reported in natural samples. In our study of chalcophile-element concentrations in Pn from iconic magmatic Ni-Cu-platinum-group element deposits, we observed three textures of Pn: contact Pn in between Po and Ccp, granular Pn included within Ccp or Po, and flame Pn included within Po. The contact Pn shows zonation in Mo, Rh, Ru, Re, Os, and Ir, with these elements being enriched toward the Po contact and depleted toward the Ccp contact. In some cases, Pd displays a zonation antithetical to that of these elements. In this contribution, we propose that the contact Pn formed via the peritectic reaction described above, and inherited Mo, Ru, Rh, Re, Os, and Ir from the MSS, whereas Pd was contributed from the fractionated sulfide liquid. We expect that this type of Pn should be present wherever MSS and fractionated sulfide liquid remained in contact.