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与岩浆-热液有关，产生了大量的经济矿藏。斑岩型铜钼金矿床是已开采铜、钼的主要来源，与浅成热液矿床共同贡献了世界上大量的金银。原材料年产值超过250亿美元(美国)和全球1000亿美元(全球)。许多重要的矿藏产于美国西部，随着地质认识的提高，仍有可能发现更多的矿藏。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(热的、富水的和含金属的溶液)的岩浆过程。这项研究将研究与内华达州卡林金矿(含金量1亿盎司)有关的始新世火山岩的岩石学和地球化学，以及与太平洋西北西部喀斯喀兹斑岩铜矿有关的中新世火山岩和花岗岩。我们计划采集这些火山岩和火成岩的样本，并分析它们的主元素和微量元素组成、矿物成分组成、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)。这些数据将有助于描述岩浆的组成，并有助于更好地了解岩浆的来源以及岩浆作用对大型经济矿藏的重要性。特别令人感兴趣的是了解水的含量、氧化状态和矿石成分(如硫、铜和金)的含量，以及这些含量如何受到来自上地幔或上地壳的玄武岩岩浆的影响。一个潜在的经济成果是增加了对形成经济热液矿藏的岩浆过程的了解。这些知识将拓宽勘探工具，使美国矿业受益。PI负责地质学课程和学生培训，许多学生在矿藏的勘探、开采和环境修复方面寻找工作。该协会每年联合教授一门为期8天的矿田测绘课程，在过去10年里，该课程招收了来自25个国家的250名专业人士和100名学生。自2005年以来，PI已经为17名研究生(8名硕士，2名博士)和7名本科生、2名国际学者和2名博士后(28名女性中的10名)提供了咨询。PI已经成功地获得了行业赞助30年，这为学生提供了获得矿产行业实践经验的途径，并为未来的合作努力开辟了道路。第2部分：计划中的研究建立在我们最近由NSF赞助的研究的基础上，并扩展了我们最近报道的SHRIMP-RG离子探针分析锆石的Hf、钛和稀土元素的丰度，锆石是地壳岩浆中几乎无处不在的一种强大的痕量矿物(Dilles等人，2015年)。对成矿和贫瘠花岗岩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，具有相对较小的负Eu异常(Ballard等人，2001年)。这种小的Eu异常可能是富水硅质岩浆抑制了富Eu斜长石结晶的结果，也导致了成矿侵入体特有的高岩浆锶/Y比值。其次，锆石Eu的小异常可能指示了氧化岩浆条件，我们推测这部分反映了SO2脱气或来自相对较低的Fe/S比值的岩浆中的成矿流体的氧化作用。熔体中Eu和Fe的氧化是由于岩浆硫酸盐(S6+)在脱气时还原为SO2(S4+)而产生的。这一解释加强了氧化富硫流体在斑岩和浅成热液矿床形成中的重要作用。因此，锆石成分可以用来识别释放出大量富含SO2气体的古老岩浆，而锆石成分的区域调查可能是矿产勘探的一个有价值的工具。这项新的研究将扩大对内华达州北部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州的喀斯喀特火山，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，这些火山包含的斑岩铜矿相对较少。