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

知识价值：阿拉斯加东南部的杜克岛是乌拉尔-阿拉斯加入侵的一个世界著名的例子，起源于弧下地幔。它是一个极好的火成岩晶体堆积实例，具有粒度分层、斜/交叉分层和“软沉积”变形特征。最近在该综合体的勘探工作已经消除了乌拉尔-阿拉斯加侵入体不适合硫化物成矿的说法，表面上是由于它们的氧化性质。先前的研究表明，涉及石墨和硫化物岩石的污染过程可以导致岩浆的减少和不混溶硫化物熔体的形成。硫化物饱和度似乎是在斜辉石变成液态矿物时达到的，这对于富镍硫化物的形成来说太晚了，因为镍已经被早期形成的橄榄石隔离了。然而，关于杂岩中硫化物的聚集以及多阶段硫化物饱和的重要性仍然存在疑问。虽然杜克岛的泥质橄榄石可能具有“正常”镍含量的特征，但也存在缺镍橄榄石的例子；这表明橄榄石与系统中的硫化物液体相互作用，或在硫化物液体存在下结晶。含硫化物和缺乏硫化物的橄榄石斜辉石岩交替层的存在也无法解释，可能代表了硫化物饱和的多个阶段或不同岩浆的参与。硫化物层的特征是在硫化物基质中的自形斜辉石或粒状橄榄石，或具有丰富的球形硫化物包裹体的强镶嵌变形虫型辉石和橄榄石。新的地球物理证据表明，杜克岛综合体是一个“酒杯”形管道的一部分，这与晶体和密集硫化物液体积聚在管道系统加宽部分的前提一致。通过详细的岩石学、电子探针、S-O-Os同位素和PGE分析，研究了管道中硫化物饱和度的性质，以及硅酸盐晶体和硫化物液体的富集。研究结果不仅将增加对汇聚边缘环境中管道过程的理解，还将增加对管道环境中硫化物矿化形成的理解。更广泛的影响：拟议的研究将介绍研究生和本科生的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。这项研究的目的是增加我们对一种尚未得到充分研究的硫化物矿物的了解，随着我们国家和全球对原材料需求的增加，这在未来的某个时候将是重要的。美国没有主要的镍矿（尽管密歇根州的Eagle矿床计划在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。本提案中描述的资源类型值得调查，因为它们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.