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.

Project Report

Our work was designed to better constrain the nature and sequence of igneous events in what is known as the Duke Island Complex, located in southeastern Alaska. In particular our research focused on the origin of metallic (nickel, copper, platinum-group element) mineralization in the Complex, and the specific controls on the genesis of high-sulfide mineralization in a subduction zone setting. In general, in order to form an economic nickel-copper-platinum-group element ore body a large supply of metal-bearing magma is required, along with the addition of sulfur to the magma from pre-existing, rock types (known as country rocks). Once metal and sulfur have bonded in the form of immiscible sulfide liquids or sulfide magma, a suitable collection site, or trap, for the accumulation of the metal sulfide liquid is required. Although subduction zones are the site of intense volcanic activity they have not been productive targets for magmatic nickel-rich sulfide ore bodies. However, because a variety of rock types may be joined to continents as plates collide at subduction (or convergent) zones, it is often the case that at least some of these rock types contain sulfur that could be transferred to passing magma. These rock masses that are joined to the continent at subduction zones are generally referred to as accreted terranes. Hot, nickel-bearing magmas from the mantle in the subduction zone environment may react with sulfur-rich country rocks of the accreted terranes and produce nickel-rich immiscible sulfide liquids.The passage ways for magma coming from the Earth's mantle are referred to as "conduits" and ore bodies generated in such a setting may be called a "conduit-type deposit". Our results have been of broad significance in the study of conduit-related sulfide deposits that have been shown to be a principal exploration target for new deposits of strategic metals (e.g. the Eagle deposit in Michigan and the Voisey’s Bay deposit in Labrador). . Mineralogical and geochemical studies (sulfur, oxygen and rhenium-osmium isotopes, mineral chemistry, and bulk rock major, trace, and platinum-group element analyses) have been utilized to assess the evolution of the mineralized system. Textures developed in rocks of the Duke Island Complex indicate that residual liquid from which primary igneous silicate minerals formed was efficiently expelled during the accumulation of the silicate minerals; the low concentrations of trace elements that normally accumulate in the residual liquid clearly illustrate the lack of residual or "trapped liquid" components in the rocks. Only sulfide-bearing rocks record the preservation of trapped, dense, interstitial sulfide liquid. The extreme density contrast between f the sulfide and silicate liquid likely aided redistribution of silicate and sulfide liquids within the crystal pile. The presence of immiscible sulfide liquid prevented the formation of well-formed silicate minerals; instead minerals with lobate or amoeboid forms resulted. The sulfur and rhenium-osmium isotope measurements clearly indicate that sulfur and osmium were primarily derived from the country rocks. The textures that are developed in the Duke Island Complex show that this contamination involving country rocks occurred before the silicate minerals began to crystallize. Buoyant residual silicate liquid was free to escape upwards in the conduit system but the dense, metal-rich sulfide liquid could not.The conduit environment provides a particularly efficient fluid dynamic mechanism for the separation of metal-rich immiscible sulfide liquid and residual, buoyant silicate liquid. The Duke Island Complex is often referred to as an "Ural-Alaskan" intrusion because it occurs within a belt of intrusions along the Alaskan Panhandle that is similar in form to a belt of igneous rocks that are part of a former subduction zone located within the Ural Mountains of Russia. Uralian bodies supplied much of the world's platinum prior to the development of the South African platinum industry in 1925. For this reason the Alaskan intrusions have been investigated for their platinum potential. However, our work clearly shows that the form of most of the bodies in Alaska and those in the Urals are quite different. The Duke Island Complex lacks the "concentric" zonation of mineral assemblages that are well-developed in the Urals. The Duke Island Complex formed from magma that flowed from the mantle via the conduit system, strongly interacted with sulfur-bearing country rocks, and deposited dense minerals and sulfide liquid in irregularities or widened areas in the conduit. The environment is a good one for the formation of sulfide-rich mineralization, but is distinct from the sulfide poor-platinum-rich environment in which the Uralian bodies formed. Our project required cooperation between personnel from the minerals industry and academia. Graduate students received training in field methods as well as in the operation of state-of-the-art analytical instrumentation. At a time when internationally we are striving to balance the need of natural resources in the growth of developing countries and sustainability, it is paramount that we understand the processes that are responsible for the genesis of diverse forms of metallic natural resources.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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Jennifer Wade
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Indiana University
United States
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