Iron Isotope Compositions of Coexisting Sulfide and Silicate Minerals in Sudbury-Type Ores from the Jinchuan Ni-Cu Sulfide Deposit: A Perspective on Possible Core-Mantle Iron Isotope Fractionation

Many studies have shown that the average iron (Fe) isotope compositions of mantle-derived rocks, mantle peridotite and model mantle are close to those of chondrites. Therefore, it is considered that chondrite values represent the bulk Earth Fe isotope composition. However, this is a brave assumption because nearly 90% of Fe of the Earth is in the core, where its Fe isotope composition is unknown, but it is required to construct bulk Earth Fe isotope composition. We approach the problem by assuming that the Earth's core separation can be approximated in terms of the Sudbury-type Ni-Cu sulfide mineralization, where sulfide-saturated mafic magmas segregate into immiscible sulfide liquid and silicate liquid. Their density/buoyancy controlled stratification and solidification produced net-textured ores above massive ores and below disseminated ores. The coexisting sulfide minerals (pyrrhotite (Po) > pentlandite (Pn) > chalcopyrite (Cp)) and silicate minerals (olivine (Ol) > orthopyroxene (Opx) > clinopyroxene (Cpx)) are expected to hold messages on Fe isotope fractionation between the two liquids before their solidification. We studied the net-textured ores of the Sudbury-type Jinchuan Ni-Cu sulfide deposit. The sulfide minerals show varying delta Fe-56 values (-1.37--0.74 parts per thousand (Po) < 0.09-0.56 parts per thousand (Cp) < 0.53-1.05 parts per thousand (Pn)), but silicate minerals (Ol, Opx, and Cpx) have delta Fe-56 values close to chondrites (delta Fe-56 = -0.01 +/- 0.01 parts per thousand). The heavy delta Fe-56 value (0.52-0.60 parts per thousand) of serpentines may reflect Fe isotopes exchange with the coexisting pyrrhotite with light delta Fe-56. We obtained an equilibrium fractionation factor of Delta Fe-56(silicate-sulfide) approximate to 0.51 parts per thousand between reconstructed silicate liquid (delta Fe-56 approximate to 0.21 parts per thousand) and sulfide liquid (delta Fe-56 approximate to -0.30 parts per thousand), or Delta Fe-56(silicate-sulfide) approximate to 0.36 parts per thousand between the weighted mean bulk-silicate minerals (delta Fe-56([0.70ol,0.25opx,0.05cpx]) = 0.06 parts per thousand) with weighted mean bulk-sulfide minerals (delta Fe-56 approximate to -0.30 parts per thousand). Our study indicates that significant Fe isotope fractionation does take place between silicate and sulfide liquids during the Sudbury-type sulfide mineralization. We hypothesize that significant iron isotope fractionation must have taken place dur