Isotopic Geochemistry of Mercury in Active and Fossil Hydrothermal Systems.

Abstract

Presented here are the first studies of stable Hg isotope geochemistry in hydrothermal systems. A new analytical method for the determination of high precision Hg isotope ratios by cold-vapor multiple-collector inductively-coupled-plasma mass-spectrometry (CV-MC-ICP-MS) was developed and the total range of Hg isotopic compositions measured in natural samples was found to be 5.8 ‰ delta 202Hg (delta 202Hg/198Hg; relative to NIST 3133) or greater than 72 times the analytical precision (± 0.08 ‰, 2 SD) of the method.

The Hg isotopic compositions of samples throughout the vertical extent of two fossil hydrothermal systems in Nevada can be grouped by mineralogy and position; delta 202Hg values at the tops of the systems are lowest in cinnabar-rich sinter and distinct from the higher delta 202Hg values of metacinnabar-rich sinter, deeper veins have delta 202Hg values that are higher than cinnabar-rich sinter. Low delta 202Hg values in cinnabar-rich sinter are most likely due to mass-dependent fractionation that occurred during boiling of the hydrothermal fluid, while the differences between cinnabar and metacinnabar are potentially due to kinetic effects associated with mineral precipitation.

The Hg isotopic compositions of rocks, ore deposits, and spring deposits from the California Coast Ranges were measured. Ore deposits have similar average Hg isotopic compositions that are indistinguishable from averages for the source rocks. This observation suggests that there is little or no isotopic fractionation (<±0.5‰) during release of Hg from source rocks into hydrothermal solutions. Fractionation does appear to take place during transport and concentration of Hg in deposits, especially in their uppermost parts, expressed on the surface as hot springs. Boiling of hydrothermal fluids, separation of a Hg-bearing CO2 vapor or reduction and volatilization of Hg in the near-surface environment are likely the most important processes causing the observed mass-dependent Hg isotope fractionation. This should result in the release of Hg with low delta 202Hg values into the atmosphere from the top of these systems. Estimates of mass balance suggest that residual Hg reservoirs are not measurably enriched in heavy Hg isotopes as a result of this process because only a small amount of Hg (<4%) leaves active ore-forming systems.