MINERALOGIC AND SULFUR ISOTOPIC STUDIES OF CU-NI MINERALIZATION IN THE DUKE ISLAND COMPLEX, ALASKA

Alaskan-type complexes are well-known as hosts of sulfide-poor PGE mineralization, but not as hosts of sulfide-rich, Ni-Cu deposits. Recent exploration in the famous Alaskan-type Duke Island Complex in south-eastern Alaska has led to the discovery of a high-sulfide zone with a strike length in excess of 1600m and width up to 650m in ultramafic units. Sulfide mineralization ranges from disseminated (~ 2 vol. %) to locally massive. Sulfide assemblages are dominated by pyrrhotite with minor chalcopyrite and pentlandite in amounts ranging from 1 to 5%. Assays indicate grades up to 2.8% Cu, 0.25% Ni and 1ppm Pt + Pd. The presence of sulfide bearing country rocks and the possibility that the complex served as a feeder to overlying volcanics makes the complex a viable target for ore-grade mineralization.

Olivine in the ultramafic rocks is partially serpentinized. Residual olivine grains range in composition from Fo₇₇ to Fo₈₀. Ni contents in olivine vary from below detection limits to 350ppm. These values are extremely low when compared to those of olivine from MORB, and suggest that either the complex was derived from a Ni-poor source material or (more likely) that Ni had been depleted due to a sulfide segregation event at depth.

δ³⁴S values of the pyrrhotite-rich assemblages range from -15 to 4.6 per mil, with over 70% of the values between -2 and 2 per mil. Although the bulk of the sulfides are consistent with a mantle origin for S, the wide range of δ³⁴S values indicates that an external S component was locally involved in the mineralization. The evidence for sulfur assimilation from country rocks, together with the extremely low Ni content of olivine, suggest that an ore-grade Ni-sulfide mineralization may be present in other parts of the complex.