Response of Fe-Ni-Cu isotope systematics to sulfide liquid oxygen content: Implications for magmatic sulfide Co enrichment in orogenic settings

The initial silicate melt related to magmatic Ni-Cu deposits located in orogenic settings in China (e.g., East Tianshan orogenic belt, East Kunlun orogenic belt) had a high fO₂ that progressively decreased with continued magmatic evolution. It is still unknown if the sulfide melt that separated from the silicate melt inherited this high fO₂, or even whether oxygen fugacity in sulfide plays an important role in the mineralization processes. In this work we undertook new in situ Fe, Cu, and Ni isotopic analyses of base metal sulfides from the Xiarihamu magmatic sulfide deposit (East Kunlun orogenic belt), and combined this new data with previously published Fe and Cu isotopic results from orogenic and cratonic magmatic sulfide deposits to assess changes in fO₂ in sulfide during sulfide melt evolution, and the role of these processes in metal enrichment.

In the Xiarihamu deposit, pentlandite has a Fe/Ni ratio similar to high-temperature pentlandite (Fe_4.95Ni_4.08S_7.96) found in the upper disseminated ores which host high-temperature maucherite inclusions. These findings indicate a high formation temperature for the sulfide in the upper disseminated ores. Atomic % Fe in pyrrhotite suggests that fO₂ increased during the transition from disseminated mineralized ultramafic rocks (47.2–50.7), through net-textured + massive mineralized ultramafic rocks (47.1–48.1), to disseminated mineralized gabbros (46.9–47.3). Early crystallized, high temperature sulfides in disseminated ores do not display high oxygen fugacity characteristics (high Fe³⁺/ΣFe), whereas in the silicate melt, fO₂ continued to decrease with progressed evolution. Orogenic magmatic sulfide deposits show consistent, uniquely lighter δ⁵⁶Fe and δ⁶⁵Cu in disseminated ores relative to the same sulfides from massive ores. This cannot be explained by crustal contamination and sulfide melt fractionation based on Fe and Cu isotopes. Uncoupled δ⁶²Ni (insensitive to fO₂ variations) and δ⁵⁶Fe, as well as heavy δ⁵⁶Fe and Co enrichment in late crystallized pentlandite (low temperature and high Fe³⁺/ΣFe) from the Xiarihamu and Kalatongke deposits (located in different orogenic belts), suggests that an increase in oxygen fugacity and related Fe³⁺/ΣFe ratios exert control on Co mineralization. Iron, Cu and Ni isotopes in sulfide can be used as indicators of Fe³⁺/ΣFe ratios in magmatic sulfide deposits in an orogenic environment, and changes in the Fe³⁺/ΣFe ratio play a critical role in Co enrichment.