IRON ISOTOPES RECORD FRACTIONATION AND OXIDATION OF SULFIDE LIQUIDS IN THE CU–PGE MINERALIZATION OF THE EASTERN GABBRO, COLDWELL COMPLEX, CANADA

Despite the fact that metal stable isotopes are not expected to exhibit large equilibrium fractionations at magmatic temperatures, variations in δ⁵⁶Fe of up to ~1.8‰ are observed in base-metal sulfides (BMS) in the Cu–PGE mineralization of the Eastern Gabbro, Coldwell Complex, Canada. The δ⁵⁶Fe composition of pyrrhotite (-1.48‰ to -0.16‰) is consistently negative and exhibits no variation with style of mineralization, whereas the δ⁵⁶Fe composition of chalcopyrite (-0.65‰ to 1.11‰) is largely positive (except for the W Horizon) and decreases systematically from Footwall Zone, Main Zone ≈ Four Dams ≈ Sally, to W Horizon. The pyrrhotite/chalcopyrite ratio in these mineralized zones generally decreases in the same order. Because pyrrhotite preferentially sequesters ⁵⁴Fe and crystallizes before chalcopyrite, the correlation between δ⁵⁶Fe_Ccp and pyrrhotite/chalcopyrite ratio likely results from the amount of monosulfide solid solution (MSS) that crystallized prior to intermediate solid solution (ISS). This would have controlled the δ⁵⁶Fe of the residual Cu-rich liquid from which ISS crystallized. The similarity in δ⁵⁶Fe of chalcopyrite in the W Horizon to magmatic pyrrhotite, and the presence of a pyrrhotite-poor and bornite-rich assemblage suggests that only limited MSS crystallized; this is likely the result of oxidation of the sulfide liquid early on in its evolution. The negative correlation observed between δ⁵⁶Fe_Ccp and δ⁶⁵Cu_Ccp is likely due to the simultaneous effects of the extent of MSS crystallization (affects δ⁵⁶Fe), crustal contamination (affects δ⁶⁵Cu), and R factor (affects δ⁶⁵Cu). These conclusions demonstrate that the Fe isotope composition of the two most common BMS in Ni–Cu–PGE systems are not controlled by mineralizing processes, but rather by the evolution of the sulfide liquid.