Contrasting Cenozoic Magmatism and Related Hydrothermal Systems, Western USA

对比新生代岩浆作用和相关热液系统，美国西部

• 批准号: 1447730
• 负责人: John Dilles
• 金额: $ 29.6万
• 依托单位: Oregon State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-03-01 至 2020-02-29
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1447730&HistoricalAwards=false
• 关键词: Contrasting; Cenozoic; Magmatism; Related; Hydrothermal

Part 1:Water- and sulfur-rich andesite and dacite magmas in the Andes, Cascades, Aleutians and other Circum-Pacific convergent-margin tectonic settings are genetically related to magmatic-hydrothermal fluids that produce numerous economic mineral deposits. Porphyry copper-molybdenum-gold mineral deposits are the principal sources of mined copper and molybdenum, and together with epithermal mineral deposits contribute much the world's gold and silver. Annual raw metal production value is more than $25 billion (USA) and and globally $100 billion (global). Many important deposits occur in the western USA, and there remains potential to find more deposits with improved geologic understanding. We plan to study volcanic and granitic rocks that are closely associated with these deposits in the western USA, with the goal of a better understanding of the magmatic processes that produce the hydrothermal fluids (hot, water-rich, and metal-bearing solutions) that are responsible for these mineral deposits. The study will investigate the petrology and geochemistry of Eocene volcanic rocks related to the Carlin gold deposits (100 million ounces of gold contained), Nevada, and Miocene volcanic rocks and granites related to porphyry copper deposits of the western Cascades of the Pacific Northwest. We plan to collect samples of these volcanic and igneous rocks and analyze them for major and trace elements composition, the composition of mineral constituents, the geochemical composition of the trace mineral zircon, and isotopic composition (of strontium and neodymium) of the rock. These data will allow characterization of the composition of the magmas and lead to a better understanding of their origins and what magmatic processes are essential to for large economic mineral deposits. Of particular interest is the understanding of the water content, oxidation state, and the content of ore components such as sulfur, copper, and gold, and how these contents may be governed by contributions from basaltic magmas derived from the upper mantle or from the upper crust. A potential economic outcome is increased understanding of the magmatic processes that form economic hydrothermal mineral deposits. Such knowledge will benefit the US minerals industry by broadening exploration tools. The PI does geology coursework and training of students, many of whom seek employment in exploration, exploitation, and environmental remediation of mineral deposits. The PI annually co-teaches an 8-day field mapping of ore deposits course that has enrolled in the past 10 years 250 professionals from 25 nations and 100 students. Since 2005, the PI has advised 17 graduate students (8 MS, 2 PhD completed) and 7 undergraduate theses, 2 international scholars, and two postdocs (10 of 28 female). The PI has 30 years of success obtaining industry sponsorship, which provides pathways for students to obtain practical experience in the minerals industry and opening the way for future collaborative efforts.Part 2: The planned research builds on and expands our recent NSF-sponsored study that reported SHRIMP-RG ion microprobe analyses of hafnium, titanium and rare earth element abundances of zircon, a nearly ubiquitous and robust trace mineral in crustal magmas (Dilles et al., 2015). Comparison of the compositions of zircons in ore-forming and barren granitic plutons indicate that ore-forming granites crystallized at relatively low temperature and have relatively small negative europium anomalies (Ballard et al., 2001). This small Eu anomaly may result from water-rich silicic magmas that suppress Eu-rich plagioclase crystallization and also lead to large magmatic Sr/Y ratios characteristic of ore-forming intrusions. Second, the small zircon europium anomaly may indicate oxidizing magmatic conditions, and we hypothesize that this partly reflects oxidation due to SO2 degassing or ore fluids from magmas with a relatively low Fe/S ratio. Oxidation of europium and iron in the melt is produced by reduction of magmatic sulfate (S6+) to SO2 (S4+) upon degassing. This interpretation reinforces the important role of oxidized sulfur-rich fluids in porphyry and epithermal mineral deposit formation. Zircon compositions thus may be used to identify ancient magmas that released significant amounts of SO2-rich gases, and regional surveys of zircon composition are potentially a valuable tool for mineral exploration. The new research will extend understanding of zircon geochemistry and magmatic compositions of the important Carlin gold district of northern Nevada, and investigate the Cascade volcanoes of Oregon and Washington which contain relatively few porphyry copper deposits relative to Arizona-New Mexico-Utah-Montana and the Andes of Peru-Chile.

第一部分：安第斯山脉、喀斯喀特山脉、阿留申山脉和其他环太平洋会聚边缘构造环境中富含水和硫的安山岩和英安岩岩浆在成因上与产生许多经济矿藏的岩浆热液流体有关。 斑岩型铜钼金矿床是我国铜、钼的主要来源，与浅成低温热液矿床一起，为世界贡献了大量的金、银。年原金属产值超过250亿美元（美国），全球超过1000亿美元（全球）。 美国西部有许多重要的矿床，随着地质认识的提高，仍有可能发现更多的矿床。 我们计划研究与美国西部这些矿床密切相关的火山岩和花岗岩，目的是更好地了解产生这些矿床的热液流体（热，富含水和含金属的溶液）的岩浆过程。 这项研究将调查与内华达州卡林金矿（含金1亿盎司）有关的始新世火山岩的岩石学和地球化学，以及与太平洋西北部喀斯喀特山脉西部斑岩铜矿有关的中新世火山岩和花岗岩。 我们计划收集这些火山岩和火成岩的样品，并分析它们的主要和微量元素组成，矿物成分的组成，微量矿物锆石的地球化学组成，以及岩石的同位素组成（锶和钕）。 这些数据将有助于确定岩浆成分的特征，并有助于更好地了解岩浆的起源以及什么样的岩浆过程对大型经济矿床至关重要。 特别令人感兴趣的是理解的水含量，氧化态，矿石成分的含量，如硫，铜，金，以及这些内容如何可能是由来自上地幔或上地壳的玄武岩浆的贡献。一个潜在的经济成果是增加对形成经济热液矿床的岩浆过程的了解。 这些知识将通过扩大勘探工具使美国矿业受益。PI做地质学课程和学生的培训，其中许多人在勘探，开采和矿床的环境修复方面寻求就业。PI每年共同教授一个为期8天的矿床实地测绘课程，在过去10年中，该课程招收了来自25个国家的250名专业人员和100名学生。自2005年以来，PI为17名研究生（8名硕士，2名博士完成）和7篇本科论文，2名国际学者和2名博士后（28名女性中的10名）提供咨询。PI在获得行业赞助方面有30年的成功经验，这为学生获得矿业实践经验提供了途径，并为未来的合作努力开辟了道路。这项计划中的研究建立在并扩展了我们最近由NSF赞助的研究的基础上，该研究报告了锆石中铪、钛和稀土元素丰度的SHRIMP-RG离子探针分析，一种在地壳岩浆中几乎无处不在的和坚固的微量矿物（Dilles等人，2015年）的报告。 成矿岩体和无矿岩体中锆石成分的比较表明，成矿花岗岩在相对较低的温度下结晶，并具有相对较小的负铕异常（Ballard等人，2001年）。 这种小的Eu异常可能是由于富水的岩浆抑制了富Eu的斜长石结晶，也导致了成矿侵入体的大的岩浆Sr/Y比值特征。 其次，小的锆石铕异常可能表明氧化岩浆条件，我们推测，这部分反映了氧化由于SO2脱气或矿石流体从岩浆与相对较低的Fe/S比。 熔体中铕和铁的氧化是通过脱气时岩浆硫酸盐（S6+）还原为SO2（S4+）而产生的。 这种解释加强了氧化富硫流体在斑岩和低温热液矿床存款形成中的重要作用。 因此，锆石成分可用于识别释放大量富SO2气体的古老岩浆，锆石成分的区域调查可能是矿产勘探的宝贵工具。 这项新研究将扩大对北方内华达州重要的卡林金矿区的锆石地球化学和岩浆成分的了解，并调查俄勒冈州和华盛顿的喀斯喀特火山，这些火山相对于亚利桑那州-新墨西哥州-犹他州-蒙大拿州和秘鲁-智利的安第斯山脉，含有相对较少的斑岩铜矿。