Role of carbon in the formation of Ni-Cu-PGE mineralization in the Valmaggia ultramafic pipe: Insights from laser-ablation-time-of-flight ICP-MS

Understanding the processes that control sulfide transport and deposition in the deep lithosphere is a critical step in the search for ore deposits in lower crustal rocks. The magmatic sulfide ore-bearing Valmaggia pipe in the Ivrea-Verbano Zone (Italy), an exposed cross section of the subcontinental lithospheric mantle and overlying crust, allows for unique insight into ore-forming processes in the lower crust. Sulfides in the pipe are spatially associated with carbonates and hydrous silicates. Previous studies suggested that the sulfides were physically introduced into the pipe via a bubble-pair transport model, i.e., upward flotation of sulfide droplets attached to vapor bubbles. To further evaluate sulfide ore-forming processes, we integrate new petrographic observations with laser-ablation time-of-flight-ICP-MS trace element mapping of sulfide-carbonate-hydrous silicate relationships. We show that carbonates and hydrous silicates exist in varying sizes, compositions, textures and orientations in relation to sulfide phases, and are not restricted to a singular textural or chemical relationship. Similarly, sulfide textures vary and may or may not exhibit rimming by volatile phases. Our data show that bubble-pair transport cannot fully explain the textural relationships between sulfides and volatile-rich minerals, but rather suggest that at least some of the sulfides and carbonates co-precipitated from the same fluid/melt.