Isotopic Geochemistry of Mercury in Hydrothermal Ore Deposits

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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.