Isotopic Geochemistry of Mercury in Hydrothermal Ore Deposits

KeslerEAR-0106730本提案要求提供资金，用于研究热液系统中汞同位素组成变化的规模、可能的原因及其在矿石成因中的应用。它是为了跟进我们最近开发的一种高精度和高灵敏度的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，汞在迁移过程中可经历有机和无机同位素分馏。在低温、厌氧条件下，主要的有机过程是细菌介导的一甲基汞(HgCH3+，MmHg)的产生，这是环境中汞循环和毒性的关键化合物。蒸发、冷凝、分解、扩散、氧化还原、沉淀、溶解、络合和离子交换等无机过程也可能导致汞同位素的分馏，并将在水热体系中占主导地位，因为没有甲基化细菌。我们的目标是评估无机分馏过程对热液系统中汞的同位素组成的相对重要性，然后确定汞同位素组成是否可以作为这些系统中汞的来源的示踪剂。我们的初步分析表明，加州COSO和意大利蒙特阿米亚塔沸腾地热系统上沉积的汞具有低的198汞/204汞比率，可能是由蒸发产生的。斯洛文尼亚Idria矿床的自然汞和朱砂样品具有明显不同的198Hg/204Hg比值，分别反映了朱砂的氧化还原和热分解作用。阿尔马登汞矿床不同部位的样品具有相同的同位素组成，表明这一大型矿床中的汞来自均一来源。最后，陨石(作为火成岩中汞的替代品)和鱼类中的甲基汞(作为大多数有机沉积物中汞的替代品)中的198Hg/204Hg比值不同，这表明汞同位素组成可能区分了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藏的研究，主要目标有两个。首先，我们将分析COSO和间歇泉(加利福尼亚)活动地热系统中沸腾带以上和沸腾带以下的汞，以及艾芬豪浅热区古地热系统中的汞，以评估沸腾对汞同位素分馏的重要性。其次，我们将通过分析来自海岸山脉汞带(加利福尼亚州)和蛋白石区(内华达州)的矿床和可能的源岩中的汞，来评估汞同位素组成作为示踪剂的使用，这代表了汞可能的沉积和火成岩源岩的全部范围。这项工作预计将为活动和灭绝的水岩系统的汞同位素研究提供一个解释框架，并为在具有环境意义的现代堆积物中区分汞的自然和人为来源的研究奠定基础。

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.