Origin(s) and Significance of Arc-related, Brine-dominated Hydrothermal Alteration, Copiapó Area, Chile

流体流动是地球上地壳中一种自然的、几乎无处不在的过程。它可以从根本上改变地壳的组成，并可以创造多种类型的矿藏。该项目将解决盐水流体高温循环造成的富碱(Na-K-Ca)成分变化(热液蚀变)的起源问题。几种补充方法将应用于智利北部一个特别暴露的地区，该地区包含广泛的富Na-K-Ca蚀变，并在空间上与氧化铁-铜-金(“IOCG”)矿床相关。类似的功能虽然不那么广为人知，但在全球范围内很普遍。尽管IOCG矿床具有重要的经济意义，以及富碱蚀变对全球地球化学循环的可能贡献，但这类热液系统的记录很少，其起源也存在争议。该项目代表了学术界和采矿业(自由港-莫兰铜金矿)之间的合作，以提高我们对这些矿床的基本地质和矿石成因特征的基本了解。这个新项目将检验以下假设：(1)由于卤水循环，浅层地壳的大部分被改变为富碱成分；(2)这些卤水来自沉积或地表来源，而不是来自含水岩浆的结晶；(3)这种蚀变类型与IOCG矿床家族之间存在成因联系。这项工作将从智利北部中生代岩浆弧蚀变的地质测绘开始，以确定热液特征的体积、分布和相对时间。如果是外部成因，这些特征应由区域构造和地层学控制，而不是与特定火成岩的组合(例如，由高含水率的岩浆形成并通常与其他不同类型的成矿系统有关的特征，特别是“斑岩铜”族)。用于确定全岩和同位素(H、B、C、O、S、锶)成分的地球化学分析、矿物组合的岩石学研究以及U-Pb年代学将补充野外工作，提供以下关键文件：(A)成分变化的程度，(B)流体和溶解成分的来源，(C)蚀变的深度、温度和化学条件，以及(D)岩浆事件和热液特征的绝对时间和对比。从预期结果与相关工作相结合，应该可以更好地评估Na-Ca-K热液蚀变的类型、程度和来源，以及从单个侵入体中心到全球弧岩浆活动的尺度上以卤水为主的热液系统对物质通量的影响。关于系统规模过程的推断将是建立更好的矿床规模模型的基础，可用于IOCG矿床的矿产勘探，并应有助于将这些系统与经济上更具生产力的斑岩铜钼金环境区分开来。该项目将支持两名研究生，将补充另一名行业支持的博士项目，并将从事两名继续本科地质学学生的研究。这些学生中有两名是女性，两名属于少数群体。材料将保存在亚利桑那大学的收藏品中；这些收藏品大量用于专业人员的继续教育课程、常规课程和专题研究。

Fluid flow is a natural and nearly ubiquitous process in the Earth's upper crust. It can fundamentally change the composition of the crust and can create many types of mineral deposits. This project will address the origin of alkali-rich (Na-K-Ca) compositional changes (hydrothermal alteration) that were created by high-temperature circulation of saline fluids. Several complementary approaches will be applied to an exceptionally well exposed area in northern Chile that contains widespread Na-K-Ca-rich alteration and has spatially associated iron oxide-copper-gold-rich ("IOCG") ore deposits. Similar features, though less well exposed, are globally widespread. In spite of the economic importance of IOCG deposits and the likely contribution of alkali-rich alteration to global geochemical cycles, these kinds of hydrothermal systems are poorly documented and their origins are controversial. This project represents a collaboration between academia and the mining industry (Freeport-Moran Copper and Gold) to improve our basic understanding of the fundamental geologic and ore genesis characteristics of these deposits. This new project will test the hypotheses: (1) that large portions of the shallow crust are altered to alkali-rich compositions as a result of the circulation of brines, (2) that those brines derive from sedimentary or surficial sources and not from crystallizing hydrous magmas, and (3) that there is a genetic link between this style of alteration and the IOCG family of deposits. The work will begin with geologic mapping of alteration across the Mesozoic magmatic arc of northern Chile to establish the volumes, distribution, and relative timing of hydrothermal features. If of external origin, these features should be controlled by regional structures and stratigraphy rather than by an association with particular igneous rocks (e.g., those that formed from magmas with high water contents and are typically associated with other distinctive types of ore-forming systems, notably the "porphyry copper" family). Geochemical analyses to determine whole rock and isotopic (H, B, C, O, S, Sr) compositions, petrologic studies of mineral assemblages, and U-Pb geochronology will supplement the field work by providing key documentation of: (a) the extent of compositional changes, (b) the source of fluids and dissolved components, (c) the depth, temperature, and chemical conditions of alteration, and (d) the absolute timing and correlation of magmatic events and hydrothermal features. From the anticipated results combined with related work it should be possible to assess better the types, extents, and origins of Na-Ca-K hydrothermal alteration and the consequences for material fluxes in brine-dominated hydrothermal systems at scales from individual intrusives centers to global arc magmatism. Inferences regarding system-scale processes will be fundamental to building better deposit-scale models useful in mineral exploration for IOCG deposits, and should help distinguish these systems from the economically more productive porphyry Cu-Mo-Au environment. This project will support two graduate students, it will complement the PhD project of another who is supported by industry, and it will engage in the research two continuing undergraduate geology students. Two of these students are women and two belong to minority groups. Materials will be archived in collections at the University of Arizona; these collections are heavily used for continuing education courses for professionals, for regular courses and for topical studies.