The role of metasomatized lithospheric mantle in generating the Norilsk 1 Ni–Cu–platinum-group element sulfide deposit: Cu isotope evidence

The mechanisms of formation of magmatic Ni–Cu–platinum-group element (PGE) sulfide ore deposits, such as those of the world-class Norilsk–Talnakh mineral system, have been extensively studied, but the source of metals in these systems remains unclear. Potential metal sources are a mantle plume (derived from the lower mantle), subcontinental lithospheric mantle, and crustal materials. To provide insights into the source of metals for the Norilsk–Talnakh system, we integrate new Cu isotope data of chalcopyrite with bulk-rock major-element, trace-element, and PGE–Au contents from the Norilsk 1 deposit in Polar Siberia.

The ⁶⁵Cu/⁶³Cu ratios (reported as δ⁶⁵Cu ratio relative to SRM NIST 976) of disseminated sulfides and massive sulfide samples vary from −0.3 to 0.42 ‰ and − 0.33 to 0.35 ‰, respectively. These values extend to both lower and higher values compared to unmodified upper mantle-derived rocks (δ⁶⁵Cu_mantle from −0.14 to 0.26 ‰). These isotopic signatures cannot be explained by post-magmatic alteration, metamorphic processes, or contamination by upper crustal rocks because they do not correlate with the abundance of secondary minerals, S/Se and Th/Nb ratios, or MgO contents. The variation of δ⁶⁵Cu in disseminated sulfides is consistent with progressive interaction of isotopically light sulfide liquid (δ⁶⁵Cu ≈ −0.5 ‰) with isotopically heavy silicate melt (δ⁶⁵Cu ≈ 0.63 ‰). This isotopic range reflects heterogeneity in the mantle source to the Norilsk 1 deposit. The heavy-δ⁶⁵Cu component may have been inherited from a pyroxenite source in the metasomatized lithospheric mantle beneath the Siberian Craton. After exhaustion of the pyroxenitic component in the source during melting, δ⁶⁵Cu values returned to mantle values, where subsequent magma pulses produced disseminated sulfides with δ⁶⁵Cu values less than 0, the variability of which can be explained by Rayleigh fractionation of the sulfide liquid. The δ⁶⁵Cu values of chalcopyrite in MS are positively correlated with bulk-rock Cu/(Cu + Ni) ratios, consistent with the fractionation of ∼40–60 % monosulfide solid solution from the original sulfide liquid. Our study demonstrates a potential contribution from metasomatized subcontinental lithospheric mantle and a pyroxenitic component to the formation of the Norilsk 1 Ni–Cu–PGE deposit, and highlights the important influence of previous subduction events in modifying and re-enriching the subcontinental lithospheric mantle.