Nickel isotopic variations in Ni–Cu sulfide deposits: from theoretical predictions to natural observations

Extremely light and highly variable δ60/58Ni values have been observed in sulfide-bearing rocks globally, making Ni isotopes a potential tracer for the genesis of magmatic sulfide deposits. However, the underlying mechanisms that cause the large Ni isotopic variation in sulfides remain unclear. In this study, we first conduct first-principles calculations to determine the reduced partition function ratios of 60Ni/58Ni (103lnβ of 60Ni/58Ni) for major Ni-doped sulfide minerals, including chalcopyrite, pentlandite, pyrite, and pyrrhotite. Our calculations demonstrate that the 103lnβ of 60Ni/58Ni values of pyrite and pyrrhotite are sensitive to Ni concentration, and the 103lnβ of 60Ni/58Ni values of sulfides increase in the order chalcopyrite ≈ pyrrhotite < pentlandite < pyrite at natural Ni abundance levels. We then investigate the Ni isotopic variability of sulfide minerals from different types of magmatic Ni–Cu sulfide deposits. Despite the significant Ni isotopic variability observed among the different sulfide minerals, the isotopic differences between pyrrhotite and pentlandite fall along the theoretically calculated equilibrium isotope fractionation line. Chalcopyrite exhibits a progressive enrichment in light Ni isotopes as its Fe/(Fe + Cu) ratio decreases, suggesting that its Ni isotopic composition varies with sulfide liquid fractionation over a wide range of temperatures. Sulfides in the Tulaergen deposit (orogenic setting) have systematically lighter Ni isotopic compositions compared to sulfides in the Hulu (orogenic setting) and Jinchuan (intraplate setting) deposits. These differences may be associated with the different mineralizing processes that operated to form these deposits. The parental magma to the Tulaergen deposit was relatively oxidized, leading to the dissolution of a large amount of isotopically light sulfides in the source of the parent melts, whereas the parental magmas to the Hulu and Jinchuan deposits were comparatively reduced and segregated sulfide liquid prior to emplacement in the crust. Our first-principles calculations and measured Ni isotopic compositions of sulfides in ore samples from magmatic sulfide deposits provide foundational information for the future application of Ni isotopes to the petrogenetic assessment of these economically significant critical metal deposits.