Causes of Re-Os Isotopic Variations in High-Temperature Sulfide Mineral Assemblages: Insights From the Duluth Complex-Virginia Formation System

智力价值：这个项目将评估解释地壳侵入岩浆中硫化物矿物沉淀的假设--这些假设可能解释与德卢斯侵入杂岩有关的某些重要矿藏的形成。围岩污染被认为是在地幔玄武岩岩浆中产生硫化物饱和的必要条件，要么是通过同化地壳硫，要么是通过同化硅质物质，导致镁铁质岩浆的硫化物溶解度下降。事实证明，Re-Os同位素系统在评估地壳污染在岩浆矿床和一般地幔岩石中的作用方面特别有用。由于Re和Os的亲磷性质，该体系非常适合作为硫化物成矿中壳源金属的指示剂。然而，最近的研究表明，Re-Os同位素系统可能特别容易受到与后来与流体相互作用有关的扰动。S之前对可能的围岩污染物进行的同位素研究表明，这些岩石是与德卢斯杂岩有关的岩浆硫化物矿化的主要硫来源。然而，初步的Re-Os同位素研究表明，接触光环中的硫化物矿物具有非放射性成因Os的特征，而德卢斯的块状硫化物矿化具有放射性成因Os的特征，这表明存在广泛的Os污染。这些岩石中硫化物体系中的极端Re-Os同位素变化反映了必须伴随镁铁质岩浆与地壳岩石相互作用的基本过程。接触光环中的矿脉含有石英和与岩浆硫化物成矿中发现的非常相似的硫化物组合(磁黄铜矿-镍黄铁矿)。为了验证接触晕中Re/Os同位素体系的扰动与含Re-Os流体与硫化物矿物反应有关的假设，对脉状物质进行了流体包裹体、稳定同位素和Re-Os同位素研究，以及空间上与沉积有关的硫化物。这些数据将有助于评估流体的来源和成分，以及它与德卢斯杂岩的岩浆的关系。由于杂岩侵入岩中存在大量围岩包体，保存了岩芯中的低品位矿物组合，因此我们还将能够系统地评价从包体岩芯到变质边缘和周围浸染的含硫化物火成岩的Re-Os和S同位素变化的机制。这项研究的结果将阐明与镁铁质岩浆-地壳相互作用有关的硫化物系统中Re-Os同位素变化的原因，并将在与岩浆硫化物成矿相关的Re-Os同位素系统学评估中具有特殊价值。广泛影响：拟议研究的更广泛方面包括培训研究生和本科生进行系统的野外采样，以及在NSF资助的三个实验室使用最先进的分析设施进行化学和同位素分析。与明尼苏达州自然资源研究所的合作对该项目的成功至关重要。随着第三世界经济的加强，岩浆系统中发现的金属资源的勘探和利用继续增长。了解经常使用的放射性同位素系统如何对涉及岩浆和流体的高温事件作出反应，对于发展未来的勘探模式至关重要。

Intellectual Merit: This project will evaluate hypotheses to explain precipitation of sulfide minerals from crustally intruded magmas - that may explain the formation of certain important ore deposits associated with the Duluth intrusive complex. Country rock contamination has been proposed as a requirement to produce sulfide saturation in mantle-derived basaltic magmas, either through assimilation of crustal sulfur, or by assimilation of siliceous material that results in a decrease in the sulfide solubility of the mafic magma. The Re-Os isotopic system has proven particularly useful in evaluating the role of crustal contamination in magmatic ore deposits and in mantle-derived rocks in general. Because of the chalcophile nature of both Re and Os, the system is well-suited as an indicator of crustally derived metals in sulfide mineralization. However, recent studies have shown that the Re-Os isotopic system may be particularly prone to perturbations related to later interaction with fluids. Previous S isotopic studies of likely country rock contaminants suggest that these rocks were the principal source of sulfur for the magmatic sulfide mineralization associated with the Duluth complex. However, preliminary Re-Os isotopic studies indicate that sulfide minerals in the contact aureole are characterized by unradiogenic Os whereas the massive sulfide mineralization at Duluth is characterized by radiogenic Os - indicative of extensive Os contamination. Extreme Re-Os isotopic variations in the sulfide systems in these rocks are reflective of fundamental processes that must accompany the interaction of mafic magmas with crustal rocks. Veins in the contact aureole contain quartz and a sulfide assemblage (pyrrhotitecubainte-pentlandite) that is very similar to that found in the magmatic sulfide mineralization. In order to test the premise that the perturbation of the Re/Os isotopic system in the contact aureole is related to the reaction between Re-Os-bearing fluids and sulfide minerals, fluid inclusion, stable isotopic, and Re-Os isotopic studies of the vein material, as well as spatially associated sedimentary sulfides, are planned. These data will allow assessment of the origin and composition of the fluid, and its relationship to the magmas of the Duluth Complex. Because large xenoliths of country rocks that preserve low-grade mineral assemblages in their cores are present in the intrusive rocks of the Complex, we will also be able to systematically evaluate the mechanisms responsible for Re-Os and S isotopic variations from xenolith cores through metamorphosed margins and surrounding disseminated sulfide-bearing igneous rocks. The results of this study will shed light on the causes of Re-Os isotopic variations in sulfide systems related to mafic magma-crust interaction, and will be of particular value in the evaluation of Re-Os isotopic systematics associated with magmatic sulfide ore formation.Broader Impacts: Broader aspects of the proposed research include the training of both graduate and undergraduate students in systematic field sampling and in the use of state-of-theart analytical facilities for chemical and isotopic analyses at three NSF-funded laboratories. Cooperation with the Minnesota Natural Resources Research Institute will be essential for the project to succeed. Exploration for, and utilization of, metallic resources found in magmatic systems continues to grow as third-world economies strengthen. Knowledge of how an often utilized radiogenic isotopic system may respond to high-temperature events involving magmas and fluids is essential to the development of future exploration models.