Sulfide Mineralization in the Duke Island Complex, Alaska: A Unique View into Conduit Processes in the Sub-arc Environment

Indiana University, Bloomington IN

Investigators

Abstract

Intellectual Merit: The Duke Island Complex in southeastern Alaska is a world-famous example of an Ural-Alaska intrusion derived from sub-arc mantle. It is a superb example of igneous crystal accumulation with grain-size layering, inclined/cross layering and features of "soft sediment" deformation. Recent exploration work in the Complex has dispelled claims that Ural-Alaska intrusions are poor hosts for sulfide mineralization, ostensibly due to their oxidized nature. Previous work showed that contamination processes involving graphitic and sulfidic country rocks can lead to reduction of the magma and formation of immiscible sulfide melt. Sulfide saturation appears to have been reached when clinopyroxene was becoming a liquidus mineral - far too late to have permitted the formation of Ni-rich sulfides as Ni would have been sequestered by early-forming olivine. However, questions still remain regarding the accumulation of sulfide in the Complex and the possible importance of multiple stages of sulfide saturation. Although olivine in dunite at Duke Island may be characterized by "normal" Ni contents, examples of Ni-depleted olivine also exist; this suggests that the olivine interacted with, or crystallized in the presence of, sulfide liquid somewhere in the system. The presence of alternating layers of sulfide-bearing and sulfide-deficient olivine clinopyroxenite is also unexplained and may represent either multiple stages of sulfide saturation or involvement of distinct magmas. Sulfide layers are characterized by either euhedral clinopyroxene or granular olivine in a sulfide matrix, or by strongly embayed, amoeboid-type pyroxene and olivine with abundant spherical sulfide inclusions. New geophysical evidence suggests that the Duke Island Complex is part of a "wine-glass" shaped conduit, consistent with the premise that crystals and dense sulfide liquid accumulated in the widened portion of the conduit system. Detailed petrographic, electron microprobe, S-O-Os isotopic, and PGE analyses are proposed to investigate the nature of sulfide saturation in the conduit, and the accumulation of silicate crystals plus sulfide liquid. Results will not only increase understanding of conduit processes in convergent margin settings, and formation of sulfide mineralization in conduit settings in general. Broader Impacts: The proposed research will introduce a graduate student and an undergraduate student to a variety of state-of-the-art analytical techniques. The work also will involve a collaborative effort between company geologists and academic scientists. This research is designed to increase our understanding of a type of sulfide mineral occurrence that has not been well-studied, and that at some point in the future will be important as our national and global need for raw materials increases. The United States has no primary producing Ni mines (although the Eagle deposit in Michigan is scheduled to initiate mining in 2010) at a time when the demand for stainless steel and other alloys is increasing. Resources of the type described in this proposal warrant investigation as they will be relevant to societal development on a sustainable Earth.