SULFIDE-SILICATE TEXTURES AND REACTIONS IN THE DUKE ISLAND COMPLEX, SOUTHEASTERN ALASKA

The Duke Island Complex (DIC), located in southeastern Alaska’s Alexander Terrane, is well-known for its exquisite examples of igneous layering. The complex is generally regarded as an Ural-Alaskan intrusion, but the concentric zoning that characterizes most Ural-Alaskan intrusions is poorly developed in the DIC. The intrusion exhibits attributes that are more in line with an origin as a layered body that has crystallized via fractional crystallization of a high-Mg basaltic to ankaramitic liquid. Detailed evaluation of the crystallization of the ultramafic rocks is hampered by the absence of preserved interstitial liquid. Rarely, euhedral olivine is enclosed in larger clinopyroxenes; however, most adjacent crystals in olivine clinoproxenites and dunites meet at 120° triple junctions. Recrystallization is clearly indicated with an absence of preserved interstitial liquid. Only when net-textured interstitial sulfide minerals are present is “trapped liquid” clearly present. Pyrrhotite and chalcopyrite are found in amoeboid textures, embaying both olivine and clinopyroxene. Sulfide inclusions in both olivine and clinopyroxene vary in abundance and morphology, and do not appear to be directly related to the occurrence of sulfide embayments. The presence of these sulfide minerals indicates that a dense immiscible sulfide liquid was retained as olivine and clinopyroxene accumulated, but residual silicate liquid was expelled. Possible interpretations of the silicate-sulfide textures include sulfide inhibition of silicate growth, and dissolution of silicate minerals by reaction with a sulfide-rich interstitial liquid. Textures such as those found in the DIC are extremely uncommon in magmatic Ni-Cu deposits associated with ultramafic rocks. For this reason, an origin related to primary inhibition of silicate mineral growth is considered unlikely. However, the scalloped grain boundaries that typify silicate-sulfide contacts are unlikely to develop during recrystallization. Reactions involving a sulfide melt, residual silicate liquid and either olivine or clinopyroxene are indicated, with buoyant liquid removed from the system.