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

智力优势：阿拉斯加东南部的杜克岛杂岩是来自弧下地幔的乌拉尔-阿拉斯加侵入体的世界著名例子。它是一个火成岩晶体堆积的极好例子，具有粒度分层、倾斜/交叉分层和“软沉积”变形的特征。最近在该综合体的勘探工作驳斥了有关乌拉尔-阿拉斯加侵入体不利于硫化物矿化的说法，表面上是因为它们的氧化性质。以前的工作表明，涉及石墨质和硫化物围岩的污染过程可能导致岩浆减少，形成不混溶的硫化物熔体。当单斜辉石成为液相线矿物时，硫化物似乎已经饱和-太晚了，无法形成富镍硫化物，因为镍可能会被早期形成的橄榄石隔离。然而，关于硫化物在复合体中的积累以及多阶段硫化物饱和的可能重要性的问题仍然存在。虽然杜克岛纯橄榄石中的橄榄石可能具有“正常”镍含量的特征，但也存在贫镍橄榄石的例子；这表明橄榄石与系统中某个地方的硫化物液体相互作用或结晶。含硫和缺硫橄榄石单斜辉石岩的交替层的存在也无法解释，可能代表硫化物饱和的多个阶段，也可能涉及不同的岩浆。硫化物层的特征是硫化物基质中的正面体单斜辉石或粒状橄榄石，或强烈侵入的变形型辉石和橄榄石，具有丰富的球状硫化物包裹体。新的地球物理证据表明，杜克岛建筑群是“酒杯”形管道的一部分，这与晶体和稠密的硫化物液体积累在管道系统加宽部分的前提一致。通过详细的岩相学、电子探针、S-O-Os同位素和铂族元素分析，研究了管道中硫化物饱和的性质，以及硅酸盐晶体和硫化物液体的聚集。结果不仅将增加对汇聚边缘环境中的管道过程的了解，以及对管道环境中硫化物矿化形成的总体理解。广泛的影响：拟议的研究将向研究生和本科生介绍各种最先进的分析技术。这项工作还将涉及公司地质学家和学术科学家之间的合作努力。这项研究旨在增加我们对一种硫化物矿物赋存状态的了解，这种类型的硫化物矿物还没有得到很好的研究，而且随着我们国家和全球对原材料需求的增加，这一点在未来的某个时候将变得重要。美国没有主产镍矿(尽管密歇根州的Eagle矿藏计划在2010年开始开采)，而此时对不锈钢和其他合金的需求正在增加。本提案所述类型的资源值得调查，因为它们将与可持续地球上的社会发展有关。

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.