Cu isotope systematics of conduit-type Cu–PGE mineralization in the Eastern Gabbro, Coldwell Complex, Canada

Abstract

Chalcopyrite from the Cu–PGE sulfide deposits in the Eastern Gabbro, Coldwell Complex, Canada, exhibit a > 2‰ variation in δ65Cu. In the Marathon deposit, the δ65Cu of chalcopyrite increases from the lower Footwall Zone (− 1.49 to − 0.75‰), to the Main Zone (− 1.04 to 0.08‰), to the upper W Horizon (− 0.35 to 1.07‰). In the northern deposits, chalcopyrite at Four Dams and Sally have δ65Cu that range from − 0.08 to 0.47‰ and − 0.59 to − 0.05‰, respectively. Notably, samples from the Marathon deposit with lower chalcopyrite δ65Cu values tend to have higher S/Se and Cu/Pd ratios. Integrated geological and geochemical evidence suggests that secondary hydrothermal alteration and redox processes are unlikely to have been the primary causes of the observed Cu isotope variation. Numerical modeling of δ65Cu–Cu/Pd–S/Se of mineralization in the Eastern Gabbro illustrates three key aspects of Cu isotope behavior in magmatic Ni–Cu–PGE systems. First, R factors less than ~ 10,000 can exhibit significant control on the δ65Cu of sulfides. Second, sulfide liquid–silicate melt fractionation factors for Cu (Δ65Cusul–sil) greater than − 0.5‰ are applicable to Ni–Cu–PGE systems. Third, sulfide segregation exhibits no measurable control on the δ65Cu of sulfides at degrees of fractionation typical of Ni–Cu–PGE systems (< 0.3%). In the Marathon deposit, the range of δ65Cu–S/Se–Cu/Pd is attributed to the addition of Archean sedimentary Cu to a pool of sulfide liquid located at depth, followed by progressive dilution of the contaminated δ65Cu–S/Se signature and decrease in Cu/Pd ratio by influxes of uncontaminated pulses of magma (i.e., increasing R factor), some of which had Cu isotope compositions heavier than the mantle. Variably contaminated and enriched, with respect to Pd, sulfides from this pool were entrained by magma pulses and emplaced to form the Marathon deposit. This contribution demonstrates that Cu isotopes can fractionate at high temperatures and, when combined with other geochemical proxies, can be valuable in characterizing magmatic–post-magmatic processes in Ni–Cu–PGE sulfide deposits and for identifying PGE-rich sulfide deposits.