Geochemical constraints on the origin of sulfide mineralization in the Duke Island Complex, southeastern Alaska

Abstract

Magmatic Cu‐Ni‐PGE sulfide mineralization associated with mafic to ultramafic igneous rocks is rare in convergent plate settings. However, recent exploration in the Duke Island Complex of southeastern Alaska has detected horizons of sulfide mineralization primarily in the olivine clinopyroxenite unit. The composition of the sulfides, recalculated to 100% sulfide, averages 0.48% Ni and 1.4% Cu. The Duke Island Complex was generated by the emplacement of multiple pulses of variably fractionated magma ranging in composition from picrite to ankaramite that originated in deeper staging chambers. The presence of sulfides in the complex indicates that fO2 conditions were more reduced than those commonly ascribed to arc‐related magmas. The mass of sulfide mineralization together with the sulfur isotopic values indicates that external sulfur was added to the parent magmas at staging chambers. Os isotopic data from sulfides and C isotopic compositions of graphite in the olivine clinopyroxenites also indicate that the parental magmas were contaminated by crustal material. However, oxygen isotopic values of clinopyroxene together with the REE data suggest that bulk assimilation of country rocks has not occurred and the contamination was selective, involving C‐ and S‐bearing fluids or graphitic sulfides. The observed enrichment in Cu and S isotopic values of the sulfides is best explained by assimilation of sulfides from metal‐bearing rock types such as black shales or volcanogenic massive sulfide deposits that occur in terranes above the mantle wedge.