RE-OS ISOTOPIC STUDIES OF THE DUKE ISLAND COMPLEX, SOUTHEAST ALASKA

The Duke Island Complex is widely known for its exceptional igneous layering structures. The Complex is located in Southeastern Alaska’s Alexander Terrane, and is generally regarded as an Ural-Alaskan- or Alaskan-type complex. The Duke Island Complex, however, lacks the well-developed concentric lithologic zonation characteristic of these intrusive bodies. Crude zonation from dunite to wehrlite to olivine clinopyroxnite to hornblende-magnetite clinopyroxenite is found in part of the Duke Island Complex; however, the Complex generally possesses characteristics more in line with a layered body crystallized from a high-Mg basaltic to ankaramitic liquid. Additionally, the Duke Island Complex is one of very few Alaskan-type complexes to host siginificant sulfide mineralization. δ³⁴S values of sulfide mineralization range from -15‰ to 7.5‰, suggestive of an external crustal sulfur source, but over 80% of the values fall between -2 and +2 ‰( a range that overlaps that of mantle derived S). Re-Os isotope data have been collected from a suite of samples including dunite, sulfide mineralized olivine clinopyroxenite, and sulfidic, graphitic metasedimentary units. Dunite samples are near chondritic, with gamma Os values (110 Ma) generally less than 10, while sulfide-mineralized olivine clinopyroxenites yielded radiogenic values, with gamma Os over 1000. These gamma Os values are similar to those of sulfidic metasedimentary country rocks, suggesting that the addition of country rock-derived S was indeed a key process for sulfide mineralization in the Complex. Our interpretation of these data is that sulfide saturation occurred as a result of crustal contamination after crystallization of the dunite, when clinopyroxene and minor olivine were on the liquidus. Ni depletion in some dunite samples does not correlate with high gamma Os values, and is unlikely to have been the result of contamination involving sulfidic and graphitic country rocks. Fractional crystallization of olivine and chromite can contribute to locally depressed levels of Ni in olivine, but cannot account for the magnitude of observed Ni depletion in olivine.