Evidence for Two-Stage Sulfide Mineralization in the Giant Xiarihamu Ni-Co Deposit, Northern Tibet

The Xiarihamu Ni-Co sulfide deposit in northern Tibet is the world’s largest orogen-related magmatic sulfide system. Sulfide mineralization is hosted in two spatially juxtaposed but geochemically distinct units (orthopyroxenite and harzburgite), implying contrasting magmatic and ore-forming processes, yet their relationship and detailed magma evolution history in controlling deposit formation remain debated. This study integrates new field observations with zoned orthopyroxene geochemistry, O-Sr-Nd isotopes, and numerical modeling to assess the roles of magma recharge and magma-crust interaction in deposit genesis. Isotopic mixing calculations indicate that both units were derived from a mantle source metasomatized by sediment melts and slab fluids. A sharp contact between two units demonstrates that they originated from discrete magmatic pulses. The parental magma of the pyroxenite assimilated ∼ 10% siliceous crustal material, whereas harzburgite experienced limited contamination, accounting for their isotopic differences. Orthopyroxene oscillatory zoning in Cr-Ca-Fe and δ18O variations record magma recharges, with a critical recharge of primitive influx introducing abundant Ni and Co into shallow magma chamber. Numerical modeling demonstrates that sulfide saturation in both units was triggered by selective assimilation of external sulfur from Precambrian metamorphic basement. Variable magma/sulfide mass ratios explain both the shift of sulfur isotope toward mantle values and the contrasting Ni enrichment in the two units. We propose a refined two-stage sulfide mineralization model for the Xiarihamu deposit, in which distinct magma pulses with independent sulfide saturation and enrichment histories operated within a dynamic conduit system. This model provides new insights into the genesis of orogenic magmatic sulfide systems and exploration targets.