Iron isotope fractionation during sulfide liquid evolution in Cu–PGE mineralization of the Eastern Gabbro, Coldwell Complex, Canada

Abstract

In spite of the fact that metal stable isotopes are not expected to exhibit large equilibrium fractionations at magmatic temperatures, Fe isotope compositions of base-metal sulfides are increasingly being used to characterize the processes that generate and modify Ni–Cu–platinum-group element (PGE) deposits. In the Eastern Gabbro of the Coldwell Complex, Canada, the δ56Fe values (relative to IRMM-014) of pyrrhotite (−1.48‰ to −0.16‰) are consistently negative and exhibit no correlation with style of mineralization, whereas δ56Fe values of chalcopyrite (−0.65‰ to 1.11‰) are largely positive (except for the W Horizon) and decreases in samples from Footwall Zone, through the Main Zone ≈ Four Dams ≈ Sally, to the W Horizon. Source heterogeneity, sulfide segregation, variations in R factor, crustal contamination, hydrothermal fluids, and redox reactions have been ruled out as having had a significant effect on the Fe isotope composition of both pyrrhotite and chalcopyrite. Rather, the positive correlation between δ56FeCcp and the pyrrhotite/chalcopyrite ratio of the mineralized zones indicates that the large range in δ56FeCcp is likely due to variations in the amount of monosulfide solid solution (MSS) that crystallized before intermediate solid solution (ISS). Monosulfide solid solution favors 54Fe, thus the δ56Fe of the residual Cu-rich liquid would be higher than the initial sulfide liquid and variable depending on how much MSS crystallized before ISS. Subsequent to the solidification of MSS, it exsolved pyrrhotite with minor pentlandite, with troilite exsolving from pyrrhotite at lower temperatures. The large range in δ56FePo is likely due to the partitioning of 54Fe and 56Fe among these phases and the variability in the proportion of the phases in the samples. These conclusions have important implications for the applicability of Fe isotopes in base-metal sulfides to characterizing magmatic Ni–Cu–PGE systems as the Fe isotope composition of two of the most dominant base-metal sulfides in these systems, pyrrhotite and chalcopyrite, are not controlled by mineralizing processes, but rather subsolidus processes.