HR: 0800h
AN: V51A-0515    [Abstracts]
TI: Mercury Isotope Variations in Hydrothermal Ore Deposits
AU: * Smith, C N
EM: cnsmth@umich.edu
AF: Department of Geological Sciences, University of Michigan , Ann Arbor, MI 48109-1063 United States
AU: Klaue, B
EM: bklaue@umich.edu
AF: Department of Geological Sciences, University of Michigan , Ann Arbor, MI 48109-1063 United States
AU: Kesler, S E
EM: skesler@umich.edu
AF: Department of Geological Sciences, University of Michigan , Ann Arbor, MI 48109-1063 United States
AU: Rytuba, J J
EM: jrytuba@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA 94025
AU: Blum, J D
EM: jdblum@umich.edu
AF: Department of Geological Sciences, University of Michigan , Ann Arbor, MI 48109-1063 United States
AB: The ability to make direct isotopic measurements of ore-forming metals using MC-ICPMS has introduced the possibility for their use as tracers of Hg source in ore deposits and the environment. The isotopic composition of Hg varies by over 5 \permil \delta$^{202}$Hg/$^{198}$Hg (relative to our Almaden Hg standard), more than 50 times the 0.1 \permil (2\sigma ) analytical uncertainty, in a wide variety of hydrothermal ore deposits. This variation could be caused by fractionation related to processes of redox, mineral precipitation, and boiling hydrothermal fluids, among others, that are known to cause large isotopic variations in other stable isotope systems. To test the possibility that Hg isotopes can be used as a tracer of source, we have compared isotopic compositions of ore and possible source rocks in three ore deposit types: epithermal Au-Ag veins and sinter where fluids boiled in the shallow crust; silica-carbonate-type Hg deposits in the California Coast Range, where reduced source fluids mixed with oxygenated groundwater and MVT Pb-Zn deposits, where sulfides are deposited in a basin without redox or boiling. Epithermal ores (-3.1 to +2.5 \permil) and silica-carbonate-type ores (-1.1 to +1.3 \permil) have Hg isotopic compositions that show much larger variations than MVT ores (-0.5 to +0.6 \permil ). The large variations might reflect the effects of fractionation by boiling and oxidation in epithermal and silica-carbonate-type deposits. At the Buckskin National epithermal deposit, unaltered andesite, felsic dike and phyllite basement rock have identical isotopic compositions of -1.3 \permil in the middle of the epithermal ore range, as might be expected if the light and heavy ends of the range were produced by fractionation related to boiling. At the Mayacmas silica-carbonate Hg district, rocks from the Franciscan Complex and Coast Range Ophiolite range from -2.4 to 0.0 \permil compared to a range of -0.2 to +1.3 \permil for ores, as might be expected if kinetic isotope effects related to oxidation concentrated heavy isotopes in the ore. In laboratory experiments, a 1.3 \permil fractionation between vapor and liquid Hg at 20 \deg C, and a 0.1 \permil fractionation between dissolved Hg and HgS in precipitation experiments were measured and further experiments are being conducted to better understand the processes fractionating Hg. The above results suggest that Hg isotopic variability is widespread in hydrothermal systems and may be sufficient to trace Hg source given an understanding of the fractionation mechanisms operating in these systems.
DE: 1040 Isotopic composition/chemistry
DE: 1045 Low-temperature geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting