Olivine Ni isotopes of Ni-Cu deposits in northwest China: A potential sulfide segregation indicator in basaltic magmatic systems

Abstract

Sulfide segregation from silicate magmas is essential in forming magmatic sulfide deposits, but a robust Ni isotopic indicator has been lacking. This study investigates the Ni isotopic compositions of olivine and coexisting sulfide from three magmatic Ni-Cu deposits in East Tianshan, northwest China, to reveal the behavior of Ni isotopes during magmatic differentiation and sulfide segregation processes. The δ^60/58Ni values in olivine separates (+0.15 to +0.68 ‰, median 0.40 ‰) are notably higher than typical mantle and basalt values (+0.11 to +0.23 ‰). Decreased Ni at the rim of Fe-Mg unzoned olivine indicates certain degrees of post-crystallization Fe-Ni diffusion between sulfide and olivine. The intragrain diffusion modeling results show that the δ^60/58Ni values in olivine can change up to 0.5 ‰ to 1 ‰ when the diffusional modification of Ni extends to the core. However, the observed thin Ni gradient at the rim and relatively flat Ni profiles in the cores of the olivine samples suggest limited kinetic modifications. Moreover, the δ^60/58Ni values negatively correlate with the Ni content and Ni/Co ratio in olivine. The sulfide separates are isotopically lighter than the coexisting olivine grain, with δ^60/58Ni_olivine – δ^60/58Ni_sulfide values ranging from +0.10 to +0.40 ‰ (median +0.22 ‰). Given limited Ni isotope fractionation between olivine and silicate melt, the observed Ni isotopic differences between olivine and sulfides are attributed to Ni isotope fractionation between silicate melt and sulfide melt at magmatic temperatures, efficiently incorporating isotopically light Ni into the sulfide melt. The sulfide segregation-driven Ni isotope fractionation aligns with the distinct Ni isotopic compositions observed in olivine grains from two highly differentiated mafic units of the Huangshandong deposit with different sulfide evolution histories. Olivine grains from mineralized gabbronorite have higher δ^60/58Ni values (+0.68 ‰, Phase III) than those from sulfide-barren olivine gabbro (δ^60/58Ni = +0.23 ‰, Phase I). Systematic stratigraphic Ni isotope variations in the Huangshanxi olivine from intervals with varying degrees of sulfide segregation and magma replenishment further support this interpretation. These findings demonstrate that Ni isotopes in olivine are reliable for tracing sulfide segregation, provided that diffusional modification is carefully assessed. With limited diffusional modification, the widespread isotopically heavy Ni signature in olivine (δ^60/58Ni > 0.3 ‰) serves as a valuable indicator for sulfide segregation and exploring magmatic Ni-Cu deposits.