Carbon‑sulfur interactions in the lower continental crust: Implications for Ni-Cu-PGE sulfide ore genesis

Constraining the processes that control the transport and deposition of sulfides in the deep lithosphere is an important step towards expanding mineral exploration search space to exhumed lower crustal rocks. The ultramafic sulfide ore-bearing Valmaggia pipe in the Ivrea-Verbano Zone of NW Italy gives critical insight into metallogenic processes in lower crustal settings. The ~300 m wide metasomatized pipe hosts Fe-Ni-Cu-(PGE) sulfide mineralization that is associated with locally abundant carbonates and hydrous silicates (phlogopite, amphibole). Previous studies highlighted the importance of carbon-rich hydrous fluids in the sulfide ore genesis although the precise mechanisms remain unclear. To elucidate the role of mantle-derived volatiles ( i.e. , S-CO 2 -H 2 O) in the formation of the Valmaggia deposit, we integrate petrographic observations of sulfide ore with electron microprobe and laser ablation time-of-flight ICP-MS analysis. Results show that ore textures vary from semi-massive to disseminated sulfide blebs that are frequently in contact with volatile-rich minerals, most notably carbonates ( e.g. , dolomite, calcite, magnesite), phlogopite and brown amphibole. The carbonates and hydrous silicates exist in various and randomly distributed sizes, textures, orientations, and compositions in relation to sulfides. Additionally, sulfides in relation to metasomatic halos, carbonates and/or hydrous phases show varying compositions. The data show that previously determined mechanisms ( e.g. , bubble-pair sulfide transport) cannot fully explain the variability of volatile orientations, or textural and compositional relationships in any of the studied phases. Consequently, it is argued that the observed variability of the textures and chemical relationships in the pipe resulted from the co-precipitation of carbonates and sulfides from the same fluid/melt. The findings indicate that carbonated metasomatic fluids are intimately associated with the genesis of magmatic sulfide deposits in the lower crust, and that deep-rooted, carbonated metasomatic systems may be promising targets for mineral exploration.