Isotopic Geochemistry of Mercury in Hydrothermal Ore Deposits

热液矿床中汞的同位素地球化学

• 批准号: 0106730
• 负责人: Stephen Kesler
• 金额: $ 20.03万
• 依托单位: Regents of the University of Michigan - Ann Arbor
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2001
• 资助国家: 美国
• 起止时间: 2001-07-15 至 2004-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0106730&HistoricalAwards=false
• 关键词: Isotopic; Geochemistry; Mercury; Hydrothermal; Ore

KeslerEAR-0106730This proposal requests funding for a study of the magnitude, possible causes and applications to ore genesis of variations in the isotopic composition of Hg in hydrothermal systems. It is designed to follow up on our recent development of a highly precise and sensitive MC-ICPMS method to measure the isotopic composition of Hg. Hydrothermal systems are the main cause of Hg migration in the upper crust and Hg could undergo both organic and inorganic isotopic fractionation during transport. The main organic process under low-temperature, anaerobic conditions is bacterially mediated production of mono-methylmercury (HgCH3+, MMHg), a critical compound in the cycling and toxicity of Hg in the environment. Inorganic processes such as evaporation, condensation, decomposition, diffusion, oxidation-reduction, precipitation, dissolution, complexation, and ion exchange, might also cause fractionation of Hg isotopes, and will dominate in hydrothermal systems because methylating bacteria are absent. Our goal is to evaluate the relative importance of inorganic fractionation processes on the isotopic composition of Hg in hydrothermal systems and then determine whether Hg isotope compositions can be used as tracers of the source of Hg in these systems. Our preliminary analyses show that Hg from deposits over boiling geothermal systems at Coso (California) and Monte Amiata (Italy) have low 198Hg/204Hg ratios that could have been produced by evaporation. Samples of native Hg and cinnabar from the Idria (Slovenia) deposit have significantly different 198Hg/204Hg ratios that could reflect oxidation-reduction or the thermal decomposition of cinnabar, respectively. Samples from different parts of the Almaden Hg deposit have the same isotopic composition, suggesting that Hg in this large deposit came from a homogeneous source. Finally, 198Hg/204Hg ratios in meteorites (as a proxy for Hg in igneous rocks) and methylmercury in fish (as a proxy for Hg in most organic sediments) differ, suggesting that Hg isotope compositions might distinguish between these two possible sources of Hg in hydrothermal systems. The proposed research will focus on studies of actual mercury mineral deposits, with two main goals. First, we will analyze Hg above and below the boiling zone in active geothermal systems at Coso and the Geysers (California) and in the paleogeothermal system at the Ivanhoe epithermal district, in order to evaluate the importance of boiling to fractionation of mercury isotopes. Second, we will evaluate the use of Hg isotope compositions as tracers by analysis of Hg from ore deposits and possible source rocks in Coast Range Hg belt (California) and Opalite district (Nevada), which represent the full range of possible sedimentary and igneous source rocks for Hg. This work is expected to provide an interpretive framework for Hg isotope studies of active and extinct water-rock systems as well as laying the groundwork for studies seeking to distinguish between natural and anthropogenic sources for Hg in modern accumulations of environmental interest.

该提案要求提供资金，研究热液系统中汞同位素组成变化的幅度、可能的原因及其对矿石成因的应用。它的目的是跟进我们最近开发的一种高精度和灵敏度的MC-ICPMS方法来测量汞的同位素组成。热液系统是上地壳汞迁移的主要原因，汞在迁移过程中可能经历有机和无机同位素分馏。在低温厌氧条件下的主要有机过程是细菌介导的一甲基汞（HgCH 3+，MMHg）的产生，这是汞在环境中循环和毒性的关键化合物。无机过程，如蒸发、冷凝、分解、扩散、氧化还原、沉淀、溶解、络合和离子交换，也可能导致汞同位素分馏，并将在热液系统中占主导地位，因为不存在甲基化细菌。我们的目标是评估的相对重要性，无机分馏过程中的汞同位素组成的热液系统，然后确定是否汞同位素组成可以作为示踪剂的来源，在这些系统中的汞。我们的初步分析表明，来自科索（加州）和蒙特阿米阿塔（意大利）沸腾地热系统的沉积物中的汞具有较低的198 Hg/204 Hg比值，可能是通过蒸发产生的。斯洛文尼亚Idria存款中的自然汞和朱砂样品具有显著不同的198 Hg/204 Hg比值，这可能分别反映了朱砂的氧化还原或热分解。来自Almaden汞存款的不同部分的样品具有相同的同位素组成，这表明汞在这个大的存款来自一个均匀的来源。最后，198 Hg/204 Hg在陨石中的比例（作为火成岩中汞的替代品）和甲基汞在鱼类中的比例（作为大多数有机沉积物中汞的替代品）不同，这表明汞的同位素组成可能区分热液系统中这两种可能的汞来源。拟议的研究将侧重于对实际汞矿床的研究，有两个主要目标。首先，我们将分析汞以上和以下的活跃地热系统在科索和间歇泉（加州）和古地热系统在艾芬豪超热区沸腾带，以评估的重要性沸腾分馏的汞同位素。其次，我们将通过分析来自海岸山脉汞带（加州）和Opalite区（内华达州）的矿床和可能的汞源岩的汞，评估汞同位素组成作为示踪剂的用途，这些矿床和可能的汞源岩代表了汞的全部可能的沉积和火成岩源岩。这项工作有望为活跃和已灭绝水岩系统的汞同位素研究提供一个解释框架，并为在现代环境意义的汞积累中区分汞的自然来源和人为来源的研究奠定基础。