THE EVOLUTION OF NI-CU SULFIDE MINERALIZATION IN THE DUKE ISLAND COMPLEX, SOUTHEASTERN ALASKA

The Duke Island Complex in southeastern Alaska is a ~110 Ma old Alaskan-type ultramafic complex exhibiting a crude zonation from a dunite core progressing through layers of wehrlite, olivine clinopyroxenite and hornblende clinopyroxenite towards the margin. Recent exploration has revealed sulfide-rich olivine clinopyroxenite with assemblages comprised of pyrrhotite, chalcopyrite + cubanite and minor pentlandite. Grades are generally low with Cu and Ni less than 0.3 and 0.2 wt. % respectively. Combined Pt + Pd values are locally in excess of 500 ppb. Olivine compositions range from Fo74 in olivine clinopyroxenite to Fo92 in dunite, with both Ni-rich (up to 2700 ppm) and Ni-poor (<100 ppm) populations present. Sulfur isotopic values of minerals in the complex range from -14 to 6 per mil, strongly suggesting that crustal sulfur was involved in sulfide genesis. Os isotopic ratios are also strongly suggestive of the assimilation of crustal rocks (γOs ≈ 1000). However, d¹⁸O values of unaltered olivine and pyroxene range from 5.3 to 5.7 per mil, and indicate that assimilation must have been selective in nature, perhaps involving fluid derived from sedimentary country rocks.

A two-stage process of sulfide saturation in a conduit setting is envisioned. Ni-depleted olivine crystallized at depth in equilibrium with Cu-Ni rich immiscible sulfide liquid. Upon ascent and emplacement of the metal-depleted silicate liquid, additional assimilation of crustal sulfur occurred, leading to the generation of the low-grade assemblage now observed in the complex. Mass balance studies and crystallization modeling using MELTS indicate that the Duke Island Complex formed in a conduit setting with multiple magma inputs. Thus, it is probable that the metal-enriched sulfide liquid that separated from the magma in the first stage and accumulated somewhere along the magmatic conduit system has undergone metal upgrading, as in large deposits such as Noril'sk and Voisey's Bay. However, the batch of sulfide that formed during the second stage, and is now observed in the complex, has undergone little or no increase in tenor.